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Inorganic layered compounds (2D-materials), particularly transition metal dichalcogenide (TMDC), are the focus of intensive research in recent years. Shortly after the discovery of carbon nanotubes (CNTs) in 1991, it was hypothesized that nanostructures of 2D-materials can also fold and seam forming, thereby nanotubes (NTs). Indeed, nanotubes (and fullerene-like nanoparticles) of WS₂ and subsequently from MoS₂ were reported shortly after CNT. However, TMDC nanotubes received much less attention than CNT until recently, likely because they cannot be easily produced as single wall nanotubes with well-defined chiral angles. Nonetheless, NTs from inorganic layered compounds have become a fertile field of research in recent years. Much progress has been achieved in the high-temperature synthesis of TMDC nanotubes of different kinds, as well as their characterization and the study of their properties and potential applications. Their multiwall structure is found to be a blessing rather than a curse, leading to intriguing observations. This concise minireview is dedicated to the recent progress in the research of TMDC nanotubes. After reviewing the progress in their synthesis and structural characterization, their contributions to the research fields of energy conversion and storage, polymer nanocomposites, andunique optoelectronic devices are being reviewed. These studies suggest numerous potential applications for TMDC nanotubes in various technologies, which are briefly discussed.

Multiwall WS₂ nanotubes have been synthesized from W₁₈O₄₉ nanowhiskers in substantial amounts for more than a decade. The established growth model is based on the \u201csurface-inward\u201d mechanism, whereby the high-temperature reaction with H₂S starts on the nanowhisker surface, and the oxide-to-sulfide conversion progresses inward until hollow-core multiwall WS₂ nanotubes are obtained. In the present work, an upgraded in situ SEM μReactor with H₂ and H₂S sources has been conceived to study the growth mechanism in detail. A hitherto undescribed growth mechanism, named \u201creceding oxide core\u201d, which complements the \u201csurface-inward\u201d model, is observed and kinetically evaluated. Initially, the nanowhisker is passivated by several WS₂ layers via the surface-inward reaction. At this point, the diffusion of H₂S through the already existing outer layers becomes exceedingly sluggish, and the surface-inward reaction is slowed down appreciably. Subsequently, the tungsten suboxide core is anisotropically volatilized within the core close to its tips. The oxide vapors within the core lead to its partial out-diffusion, partially forming a cavity that expands with reaction time. Additionally, the oxide vapors react with the internalized H₂S gas, forming fresh WS₂ layers in the cavity of the nascent nanotube. The rate of the receding oxide core mode increases with temperatures above 900 °C. The growth of nanotubes in the atmospheric pressure flow reactor is carried out as well, showing that the proposed growth model (receding oxide core) is also relevant under regular reaction parameters. The current study comprehensively explains the WS₂ nanotube growth mechanism, combining the known model with contemporary insight.

Uranium is a high-value energy element, yet also poses an appreciable environmental burden. The demand for a straightforward, low energy, and environmentally friendly method for encapsulating uranium species can be beneficial for long-term storage of spent uranium fuel and a host of other applications. Leveraging on the low melting point (60 °C) of uranyl nitrate hexahydrate and nanocapillary effect, a uranium compound is entrapped in the hollow core of WS₂ nanotubes. Followingly, the product is reduced at elevated temperatures in a hydrogen atmosphere. Nanocrystalline UO₂ nanoparticles anchor within the WS₂ nanotube lumen are obtained through this procedure. Such methodology can find utilization in the processing of spent nuclear fuel or other highly active radionuclides as well as a fuel for deep space missions. Moreover, the low melting temperatures of different heavy metal-nitrate hydrates, pave the way for their encapsulation within the hollow core of the WS₂ nanotubes, as demonstrated herein.

The inclusion of tungsten disulphide nanotubes (WS₂ NTs) in chitosan, plasticized with glycerol, facilitates the formation of a polyelectrolyte complex. The glycerol interrupts the intramolecular hydrogen bonding between chitosan chains allowing positively charged protonated amines of chitosan to form a complex with negatively charged oxygen ions chemisorbed to the tungsten atoms in defects. These interactions, with the unique mechanical and chemical properties of WS₂ NTs, result in a chitosan film with superior properties relative to unfilled chitosan. Even at low WS₂ NT loadings (≤1 wt%), the Young's modulus (E) increases by 59%, tensile strength (σ) by 40% and tensile toughness by 74%, compared to neat chitosan, without sacrificing ductility. Addition of highly dispersed WS₂ NTs significantly improves the gas barrier properties of chitosan, with a 50% reduction in oxygen permeability, while the addition of both glycerol and WS₂ NTs to chitosan effectively reduces the carbon dioxide permeability by 80% and the water vapor transmission rate by 90%. The intrinsic antimicrobial efficacy of chitosan against both Gram-positive and Gram-negative bacteria is enhanced on inclusion of WS₂ NTs. Polyelectrolyte complexation of WS₂ NTs and glycerol-plasticized chitosan provides a cost-effective, sustainable route to biodegradable films with desirable mechanical, gas barrier properties, and antimicrobial efficacy suitable for food packaging applications.

The synthesis of complex new nanostructures is challenging but also bears the potential for observing new physiochemical properties and offers unique applications in the long run. High-temperature synthesis of ternary WSe2xS2(1-x) (denoted as WSSe) nanotubes in a pure phase and in substantial quantities is particularly challenging, requiring a unique reactor design and control over several parameters, simultaneously. Here, the growth of WSSe nanotubes with the composition 0 ≤ x

The dimensional limit of ferroelectricity has been long explored. The critical contravention is that the downscaling of ferroelectricity leads to a loss of polarization. This work demonstrates a zero-dimensional ferroelectricity by the atomic sliding at the restrained van der Waals interface of crossed tungsten disufilde nanotubes. The developed zero-dimensional ferroelectric diode in this work presents not only non-volatile resistive memory, but also the programmable photovoltaic effect at the visible band. Benefiting from the intrinsic dimensional limitation, the zero-dimensional ferroelectric diode allows electrical operation at an ultra-low current. By breaking through the critical size of depolarization, this work demonstrates the ultimately downscaled interfacial ferroelectricity of zero-dimensional, and contributes to a branch of devices that integrates zero-dimensional ferroelectric memory, nano electro-mechanical system, and programmable photovoltaics in one.Down-scaled ferroelectricity normally diminishes due to the arising depolarization field. Here, the authors realize a 0D ferroelectric diode device taking advantage of the sliding at the van der Waals interface by the two crossed tungsten disulfide nanotubes.

Functionalisation of nanofillers is required for the promotion of strong interfacial interactions with polymers and is essential as a route for the preparation of (nano)composites with superior mechanical properties. Tungsten disulphide nanotubes (WS₂ NTs) were functionalized using (3-aminopropyl) triethoxysilane (APTES) for preparation of composites with poly(lactic acid) (PLA). The WS₂ NTs : APTES ratios used were 1 : 1, 1 : 2 and 1 : 4 WS₂ NTs : APTES. The APTES formed siloxane networks bound to the NTs via surface oxygen and carbon moieties adsorbed on the WS₂ NTs surface, detected by X-ray photoelectron spectroscopy (XPS) studies and chemical mapping using energy dispersive X-ray spectroscopy in the scanning transmission electron microscope (STEM-EDS). The successful silane modification of the WS2 NTs was clearly evident with both significant peak shifting by as much as 60 cm(-1) for Si-O-Si vibrations (FTIR) and peak broadening of the A(1g) band in the Raman spectra of the WS2 NTs. The evolution of new bands was also observed and are associated with Si-CH₂-CH₂ and, symmetric and assymetric -NH³⁺ deformation modes (FTIR). Further evidence for functionalization was obtained from zeta potential measurements as there was a change in surface charge from negative for pure WS₂ NTs to positive for APTES modified WS₂ NTs. Additionally, the thermal stability of APTES was shifted to much higher temperatures as it was bound to the WS₂ NTs. The APTES modified WS2 NTs were organophilic and readily dispersed in PLA, while presence of the pendant amine and hydroxyl groups resulted in strong interfacial interactions with the polymer matrix. The inclusion of as little as 0.5 wt% WS2 NTs modified with 2.0 wt% APTES resulted in an increase of 600% in both the elongation at break (a measure of ductility) and the tensile toughness relative to neat PLA, without impacting the stiffness or strength of the polymer.

The birth of nanoscience more than 50 years ago fueled the renaissance in layered materials research leading to many materials discoveries with unprecedented scientific and technological impacts. Following the early reports on carbon fullerenes and nanotubes, the discovery of inorganic one-dimensional (1D) nanotubes and zero-dimensional (0D) fullerenes created a major playground for new physicochemical observations. The meteoric rise of two-dimensional (2D) materials in concert set off outstanding advances in the synthesis and manipulation of layered materials with atomic precision. This review identifies new directions in materials science that emerge through integrating the two layered systems2D with inorganic 1D and 0D. Summarizing the key developments in the two distinct nanomaterials families, we highlight preliminary instances of integrating them into functional nanostructures. A few gedankenexperiments regarding prospective applications of the integrated system are then introduced to stimulate further experimental and theoretical investigations that can potentially result in unforeseen scientific observations. Graphical abstract: [Figure not available: see fulltext.]

The synthesis of fundamentally small MoS₂ nanotubes and nanocones(horns) that have proven elusive in prior studies has been achieved via ablation of a precursor mixture of crystallites of MoS₂ + MoO₃ by highly concentrated solar radiation. The special far-from-equilibrium conditions achieved in the solar furnace prove conducive to the generation of these singular nanostructures. Extensive electron microscopy and characterization results (transmission electron microscopy (TEM), electron diffraction (ED), X-ray diffraction (XRD), scanning TEM (STEM), and high angle annular dark field (HAADF)) reveal a range of nanoparticle shapes and sizes based on which reaction mechanisms are proposed. Molecular dynamics simulations indicate that the sizable thermal fluctuations intrinsically produced in the high-temperature solar reactor soften the MoS₂ nanostructures, yielding corrugated layers that favor nanostructures with only a few layers, in agreement with the experimental observations.

Even though WS₂nanotubes (NTs-WS₂) have great potential as anode materials for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) thanks to their unusual layered structure, their conductivity and cycling stability are far from satisfactory. To tackle these issues, carbon-coated WS₂(NTs-WS₂@C) nanocomposites were prepared through a facile synthesis method that involved precipitating a carbon precursor (20% sucrose) on WS₂nanotubes, followed by annealing treatment under an argon environment. Thanks to the presence of highly conductive and mechanically robust carbon on the outer surface, NTs-WS₂@C nanocomposites show improved electrochemical performance compared with bare NTs-WS₂. After 60 cycles at 80 mA g^-1current density, the cells display high capacities of 305 mAh g^-1in LIBs and 152 mAh g^-1in SIBs, respectively. As the current density increases to 600 mA g^-1, it provides specific capacities of 209 and 115 mAh g^-1, correspondingly. The enhanced electrochemical performance in LIBs and SIBs is primarily attributed to the synergistic effects of the tubular architecture of WS₂, carbon network and stable nanocomposite structure, which can effectively constrain volume variation during the metal ions intercalation/deintercalation processes.

Nanotubes of transition metal dichalcogenides such as WS2 and MoS2 offer unique quasi-1D properties and numerous potential applications. Replacing sulfur by selenium would yield ternary WS2(1x)Se2x (0 ≤ x ≤ 1; WSSe) nanotubes, which are expected to reveal strong modulation in their absorption edge as a function of selenium content, x Se. Solid WO2.72 oxide nanowhiskers were employed as a sacrificial template to gain a high yield of the nanotubes with a rather uniform size distribution. Though sulfur and selenium belong to the same period, their chemical reactivity with oxide nanowhiskers differed appreciably. Here, the closed ampoule technique was utilized to achieve the completion of the solidvapor reaction in short time scales instead of the conventional flow reactor method. The structure and chemical composition of the nanotubes were analyzed in detail. X-ray and electron diffractions indicated a systematic modulation of the WSSe lattice upon increasing the selenium content. Detailed chemical mapping showed that the sulfur and selenium atoms are distributed in random positions on the anion lattice site of the nanotubes. The optical excitonic features and absorption edges of the WSSe nanotubes do not vary linearly with the composition x Se, which was further confirmed by density functional theory calculations. The WSSe nanotubes were shown to exhibit strong lightmatter interactions forming excitonpolariton quasiparticles, which was corroborated by finite-difference time-domain simulations. Transient absorption analysis permitted following the excited state dynamics and elucidating the mechanism of the strong coupling. Thus, nanotubes of the ternary WSSe alloys offer strong band gap tunability, which would be useful for multispectral vision devices and other optoelectronic applications.

Exploring the behavior of nanocrystals with varying shapes and sizes under high pressure is crucial to understanding the relationship between the morphology and properties of nanomaterials. In this study, we investigated the compression behaviors of WS2 nanotubes (NT−WS2) and fullerene-like nanoparticles (IF−WS2) by in situ high-pressure X-ray diffraction (XRD) and Raman spectroscopy. It was found that the bulk modulus of NT−WS2 is (81.7 GPa), which is approximately twice as large as that of IF-WS2 (46.3 GPa). This might be attributed to the fact that IF−WS2 with larger d-spacing along the c−axis and higher defect density are more compressible under isotropic pressure than NT−WS2. Thus, the slender NT−WS2 possess a more stable crystal structure than the IF-WS2. Our findings reveal that the effects of morphology and size play crucial roles in determining the high-pressure properties of WS2 nanoparticles, and provide significant insight into the relationship between structure and properties.

Among the different methods for orienting liquid crystal (LC) molecules, adding nanoparticles into the matrix of the substrate material towards modifying its surface, is actively pursued. In this context, the influence of the nanoparticle content on the texture of the surface of polymer film used as the substrate for the LC orientation is of particular interest. Thus, in the current paper, WS₂ nanotubes were used to dope the polyimide (PI) substrate-film in order to modify and control its surface morphology/roughness and properties. The modified organic surface structure is applied in order to achieve a new means for controlling the orientation of the LC molecules. This tool adds to the classical methods for controlling the orientation of the LC molecules, such as the display technique.

Radiation curing (RC) thermosets offer a variety of advantages, such as curing on demand, low viscosity, good adhesion to many surfaces, high modulus, good appearance of final coating, zero volatile organic compounds, and more. Alongside the advantages, it has some drawbacks like high brittleness, limited curing of thick and opaque formulations. This review outlines the basic chemistries and developments that were carried out through the years related to radical RC of acrylates and cationic RC of epoxies and their nanocomposites. In addition, commercially available monomers, photoinitiators will be presented, in addition to the attributes of the curing processes and future developments.

Asymmetric two-dimensional (2D) structures (often named Janus), like SeMoS and their nanotubes, have tremendous scope in material chemistry, nanophotonics, and nanoelectronics due to a lack of inversion symmetry and time-reversal symmetry. The synthesis of these structures is fundamentally difficult owing to the entropy-driven randomized distribution of chalcogens. Indeed, no Janus nanotubes were experimentally prepared, so far. Serendipitously, a family of asymmetric misfit layer superstructures (tubes and flakes), including LaX-TaX2 (where X = S/Se), were synthesized by high-temperature chemical vapor transport reaction in which the Se binds exclusively to the Ta atoms and La binds to S atoms rather than the anticipated random distribution.With increasing Se concentration, the LaS-TaX2 misfit structure gradually transformed into a new LaS-TaSe2-TaSe2 superstructure. No misfit structures were found for xSe = 1. These counterintuitive results shed light on the chemical selectivity and stability of misfit compounds and 2D alloys, in general. The lack of inversion symmetry in these asymmetric compounds induces very large local electrical dipoles. The loss of inversion and time-reversal symmetries in the chiral nanotubes offers intriguing physical observations and applications.

Abstract: Tungsten disulfide polycrystalline microfibers were successfully synthesized by a process involving electrospinning, calcination, and sulfidation steps. We used an aqueous solution of silicotungstic acid (H₄SiW₁₂O₄₀) and polyvinyl alcohol as precursors for the synthesis of composite fibers by the needle-less electrospinning technique. The obtained green composite fibers (av. diam. 460 nm) were converted by calcination in air to tungsten oxide WO₃ fibers with traces of SiO₂ and a smaller diameter (av. diam. 335 nm). The heat treatment of the WO₃ fibers under flowing H₂/H₂S/N₂ stream led to conversion to tungsten disulfide WS₂ with retention of the fibrous morphology (av. diam. 196 nm). Characterization of the intermediate and final fibers was performed by the XRD, SEM, TEM, HAADF STEM EDS, elemental analyses ICP-OES, and IR spectroscopy methods. Graphical abstract: [Figure not available: see fulltext.]

Analyzing naturally occurring layered alumo(magnesium)silicates (asbestos) minerals, Pauling proposed in 1930 that the strain in compounds with asymmetric structure, like kaolinite (hallyosite), lends itself for folding of the layers. Once transmission electron microscopy gained sufficient resolution in 1950, chrysotile and hallyosites nanotubes were discovered fully confirming this early hypothesis. Following the discovery of carbon fullerene (C₆₀) by Kroto and Smalley and carbon nanotubes by Iijima, Tenne proposed a new mechanism for the formation of nanotubes from inorganic compounds with layered structure via seaming of their chemically reactive rims. Early on, nanotubes of WS₂ and subsequently MoS₂, GaSe and BN were found experimentally and in silico and many others followed over the years. Here, different mechanisms for the formation of nanotubes from inorganic compounds with layered structure are analyzed with a few examples. Few potential applications of such nanotubes are briefly discussed, as well. Finally, several frontiers and scientific challenges in this field in the years ahead are presented

Misfit layered compounds (MLCs) have been studied in the literature for the last 40 years. They are generally made of an alternating sequence of two monolayers, a distorted rocksalt structure, and a hexagonal layered compound. In a typical MLC, the c-axis is common to the two monolayers and so is one of the axes in the layer plan. However, the two compounds are non-commensurate along at least one axis, and the ratio between the two axes is an irrational number making the MLC a non-stoichiometric compound. The two main families of MLC are those based on metal dichalcogenides and CoO2 as the hexagonal layered compound. Traditionally, ternary MLCs were prepared and studied, but some quaternary and multinary MLC minerals have been known for many years. Over the last few years, interest in MLCs with four and even larger number of atoms has grown. Doping or alloying of a ternary MLC permits precise control of the charge carrier density and hence the electrical, thermoelectric, catalytic, and optical properties of such compounds. In this short review, some of these developments will be discussed with the main emphasis put on quaternary MLC nanotubes belonging to the chalcogenide series. The synthesis, structural characterization, and some of their properties are considered. Some recent developments in quaternary cobaltite MLCs and recent studies on exfoliated MLCs are discussed as well.

The non-stoichiometric misfit layered compounds (MLC) of the general formula ((MX)(1+y))(m)(TX2)(n) (abbreviated herein as MX-TX2) have been investigated quite extensively over the last 30 years. Here MX is a atomic slab of a material with distorted rocksalt structure and TX2 is a layered compound with hexagonal (octahedral) coordination between the metal T atom and the chalcogen X atom. Recognizing the mismatch between the two (MX and TX2) sublattices, nanotubes from the MLC of different compositions were described in the past. In particular, semimetallic nanotubes belonging to the family LnX-TaX2 with Ln = rare earth atom and X = S, Se, Te have been studied in the past. While some of them, like LaS-TaS2 were obtained with moderately high yields, others like YbS-TaS2 were scarce. In the present study, a new strategy for promoting the yield of such MLC nanotubes by alloying the LaS sublattice with another Ln atom is proposed. Detailed transmission electron microscopy investigation of the (mixed) Ln(x)La((1-x))S-TaS2 (Ln = Pr, Sm, Ho, Yb) nanotubes show clearly that the substituting Ln atom resides in the rocksalt LaS sublattice of the nanotubes. Raman measurements show distinct differences between mixed tubes with open-shell (Pr, Sm, Ho) and close-shell (La, Yb) rare-earth atoms. Density functional calculations show that the interplay between two important factors determine the enhanced stability of the mixed nanotubes- the size and electronic structure of the substituting rare-earth atom. The smaller is the substituting rare-earth atom (larger Z number), the more dissimilar it is to the original La atom. This dissimilarity enhances the incommensurability between the Ln(x)La((1-x))5 and the TS2 subunits, promoting thereby the stability of the mixed MLC. However, the electronic structure of the Ln atom was found to play a more significant role. The MLC lattice of the LaS-TaS2 is electron-rich and consequently the 4d(z)(2) level of Ta is full. The unoccupied 4f levels of the substituent open-shell atoms (Pr, Sm, Ho), which are positioned below the Fermi level, serve as electron acceptors. Consequently, the Ln substitution is found to enhance the stability of the mixed lattice and nanotubes thereof. This strategy can be employed for enhancing the yield of these and other misfit nanotubes using different substituents of the right size and energy profile.

Proton exchange membranes with high through-plane proton conductivity are a critical component of high-performance fuel cells, electrolyzers, and batteries. However, isotropically distributed proton-conducting channel structures of current membranes present a limitation. Herein, a proton exchange membrane with straight proton-conducting channels aligned in the thickness direction is fabricated, achieved by magnetic field-induced alignment of proton-conductive, paramagnetic, and one-dimensional (1D) tungsten disulfide nanotubes (pms-WS2) distributed in a perfluorinated sulfonic acid (Nafion) membrane. The pms-WS2 nanotubes feature straight WS2 nanotubes as a core, a polystyrenesulfonate (PSS) skin layer, and surface-decorated Fe3O4 nanoparticles. A molecular dynamics simulation suggests that straight proton-conducting channels are constructed at the interface of Nafion/pms-WS2 due to densely populated sulfonic acids. Spectroscopic investigation and magnetization measurements verify the through-plane alignment of pms-WS2 under a weak through-plane magnetic field (0.035 T) during the removal of solvent from the membrane cast. Compared with a recast Nafion membrane with the same thickness, the through-plane aligned composite membrane exhibits 69% higher proton conductivity and 51% higher power performance in a proton exchange membrane fuel cell, demonstrating its efficacy. The through-plane alignment of a proton-conductive inorganic 1D material promises improved power performance of advanced electrochemical devices.

We present the analysis of a family of nanotubes (NTs) based on the quaternary misfit layered compound (MLC) YxLa1-xS-TaS2. The NTs were successfully synthesized within the whole range of possible compositions via the chemical vapor transport technique. In-depth analysis of the NTs using electron microscopy and spectroscopy proves the in-phase (partial) substitution of La by Y in the (La,Y)S subsystem and reveals structural changes compared to the previously reported LaS-TaS2 MLC-NTs. The observed structure can be linked to the slightly different lattice parameters of LaS and YS. Raman spectroscopy and infrared transmission measurements reveal the tunability of the plasmonic and vibrational properties. Density-functional theory calculations showed that the YxLa1-xS-TaS2 MLCs are stable in all compositions. Moreover, the calculations indicated that substitution of La by Sc atoms is electronically not favorable, which explains our failed attempt to synthesize these MLC and NTs thereof.

Based on the analysis of spectral parameters, orientation angle, and the time dependency of the reaction and relaxation of the media composed of the nematic liquid crystal mesophase and WS2 nanoparticles, the correlation associated with the formation of intermolecular interaction between CN-group of the liquid crystal and WS2 nanoparticle has been established. The correlation was supported by experiments and partially by quantumchemical modeling.

Strong coupling of electric transition dipoles with optical or plasmonic resonators modifies their light-matter interaction and, therefore, their optical spectra. Semiconducting WS2 nanotubes intrinsically provide the dipoles through their excitonic resonances, and the optical cavity via their cylindrical shape. We investigate the nonequilibrium light-matter interaction in WS2 nanotubes in the time domain using femtosecond transient extinction spectroscopy. We develop a phenomenological coupled oscillator model with time-dependent parameters to describe the transient extinction spectra, allowing us to extract the underlying nonequilibrium electron dynamics. We find that the exciton and trion resonances shift due to many-body effects of the photogenerated charge carriers and their population dynamics on the femto- A nd picosecond timescale. Our results show that the time-dependent phenomenological model quantitatively reproduces the nonequilibrium optical response of strongly coupled systems.

The photovoltaic effect in traditional pn junctionswhere a p-type material (with an excess of holes) abuts an n-type material (with an excess of electrons)involves the light-induced creation of electronhole pairs and their subsequent separation, generating a current. This photovoltaic effect is particularly important for environmentally benign energy harvesting, and its efficiency has been increased dramatically, almost reaching the theoretical limit1. Further progress is anticipated by making use of the bulk photovoltaic effect (BPVE)2, which does not require a junction and occurs only in crystals with broken inversion symmetry3. However, the practical implementation of the BPVE is hampered by its low efficiency in existing materials410. Semiconductors with reduced dimensionality2 or a smaller bandgap4,5 have been suggested to be more efficient. Transition-metal dichalcogenides (TMDs) are exemplary small-bandgap, two-dimensional semiconductors11,12 in which various effects have been observed by breaking the inversion symmetry inherent in their bulk crystals1315, but the BPVE has not been investigated. Here we report the discovery of the BPVE in devices based on tungsten disulfide, a member of the TMD family. We find that systematically reducing the crystal symmetry beyond mere broken inversion symmetrymoving from a two-dimensional monolayer to a nanotube with polar propertiesgreatly enhances the BPVE. The photocurrent density thus generated is orders of magnitude larger than that of other BPVE materials. Our findings highlight not only the potential of TMD-based nanomaterials, but also more generally the importance of crystal symmetry reduction in enhancing the efficiency of converting solar to electric power.

Advanced nanomaterials play a prominent role in nanoscience and nanotechnology developments, opening new frontiers in these areas. Among these nanomaterials, due to their unique characteristics and enhanced chemical and physical properties, inorganic nanotubes have been considered one of the most interesting nanostructures. In recent years, important progress has been achieved in the production and study of these nanomaterials, including boron nitride, transition metal dichalcogenide nanotubular structures, misfit-based nanotubes and other hybrid/doped nanotubular objects. This review is devoted to the in-depth analysis of recent studies on the synthesis, atomic structures, properties and applications of inorganic nanotubes and related nanostructures. Particular attention is paid to the growth mechanism of these nanomaterials. This is a crucial point for the challenges ahead related to the mass production of high-quality defect-free nanotubes for a variety of applications.

Transition-metal dichalcogenides (TMDs) are exemplary low-dimensional semiconductors. By breaking the inversion symmetry inherent in their bulk crystals, various exotic physical phenomena have been realized. We studied electrical responses under light illumination in such noncentrosymmetric TMDs belonging to a few different point groups, and observed a large photovoltaic effect induced by the reduction of the crystal symmetry.

Transition metal dichalcogenide nanotubes are fascinating platforms for the research of superconductivity due to their unique dimensionalities and geometries. Here we report the diameter dependence of superconductivity in individual WS2 nanotubes. The superconductivity is realized by electrochemical doping via the ionic gating technique in which the diameter of the nanotube is estimated from the periodic oscillating magnetoresistance, known as the LittleParks effect. The critical temperature of superconductivity displays an unexpected linear behavior as a function of the inverse diameter, that is, the curvature of the nanotube. The present results are an important step in understanding the microscopic mechanism of superconductivity in a nanotube, opening up a new way of superconductivity in crystalline nanostructures.

Herein, the use of highly concentrated sunlight for materials science research is reviewed. Specific research directions include: (1) the generation of inorganic nanostructures, some of which had eluded experimental realization with conventional synthetic processes, and (2) elucidating the processes governing the degradation of organic and perovskite-based photovoltaic materials and devices, along with accelerated assessment of their stability. Both approaches employ solar concentrators capable of producing flux densities exceeding those of terrestrial solar radiation by up to three orders of magnitude, and are geared toward either creating extensive ultrahot reactor conditions conducive to the rapid, safe synthesis of unusual nanomaterials or judiciously interrogating photovoltaic devices.

Transition metal dichalcogenide materials have recently been shown to exhibit a variety of intriguing optical and electronic phenomena. Focusing on the optical properties of semiconducting WS2 nanotubes, we show here that these nanostructures exhibit strong light-matter interaction and form exciton-polaritons. Namely, these nanotubes act as quasi 1-D polaritonic nano-systems and sustain both excitonic features and cavity modes in the visible-near infrared range. This ability to confine light to subwavelength dimensions under ambient conditions is induced by the high refractive index of tungsten disulfide. Using "finite-difference time-domain'' (FDTD) simulations we investigate the interactions between the excitons and the cavity mode and their effect on the extinction spectrum of these nanostructures. The results of FDTD simulations agree well with the experimental findings as well as with a phenomenological coupled oscillator model which suggests a high Rabi splitting of similar to 280 meV. These findings open up possibilities for developing new concepts in nanotube-based photonic devices.

In this paper, we demonstrate the formation of hybrid nanostructures consisting of two distinctive components mainly in a one-to-one ratio. Thermolysis of inorganic nanotubes (INT) and closed-cage, inorganic fullerene-like (IF) nanoparticles decorated with a dense coating of metallic nanoparticles (M = Au, Ag, Pd) results in migration of relatively small NPs or surface-enhanced diffusion of atoms or clusters, generating larger particles (ripening). AuNP growth on the surface of INTs has been captured in real time using in situ electron microscopy measurements. Reaction of the AuNP-decorated INTs with an alkylthiol results in a chemically induced NP fusion process at room temperature. The NPs do not dissociate from the surfaces of the INTs and IFs, but for proximate IFs we observed fusion between AuNPs originating from different IFs.

Screening effect in finite-length carbon nanotubes (CNT) and their agglomerates hinders significantly the electromagnetic interaction in composite materials. Screening effect is strong in the microwave range, and it decreases with increasing frequency resulting in a strong frequency dependence of the effective conductivity of the composite. Since screening effect is rather small in the terahertz range, the effective conductivity in this range is determined directly by the intrinsic conductivity of the inclusions. The ratio of the microwave to terahertz effective conductivities was proposed as a parameter to estimate how effectively carbon nanotube inclusions contribute to the electromagnetic performance of composite materials in the microwave range. CNT film was considered as a material where maximal possible interaction of the CNTs with EM field occurs. Single-walled CNT films and CNT-based composite materials, as well as hybrid film comprising mixtures of WS
₂ nanotubes and CNTs were fabricated and measured in the microwave and terahertz ranges. The electromagnetic field interaction with the inclusions has been estimated for all the samples fabricated.

Films combining hydroxyapatite (HA) with minute amounts (ca. 1 weight %) of (rhenium doped) fullerene-like MoS2 (IF) nanoparticles were deposited onto porous titanium substrate through electrophoretic process (EPD). The films were analyzed by scanning electron microscopy (SEM), X-ray diffraction and Raman spectroscopy. The SEM analysis showed relatively uniform coatings of the HA + IF on the titanium substrate. Chemical composition analysis using energy dispersive X-ray spectroscopy (EDS) of the coatings revealed the presence of calcium phosphate minerals like hydroxyapatite, as a majority phase. Tribological tests were undertaken showing that the IF nanoparticles endow the HA film very low friction and wear characteristics. Such films could be of interest for various medical technologies. Means for improving the adhesion of the film to the underlying substrate and its fracture toughness, without compromising its biocompatibility are discussed at the end.

We report here a new methodology for the formation of freestanding nanotubes composed of individual gold nanoparticles (NPs) cross-linked by coordination complexes or porphyrin molecules using WS
₂ nanotubes (INT-WS
₂) as a template. Our method consists of three steps: (i) coverage of these robust inorganic materials with monodispersed and dense monolayers of gold NPs, (ii) formation of a molecular AuNP network by exposing these decorated tubes to solutions containing a ruthenium polypyridyl complex or meso-tetra(4-pyridyl)porphyrin, and (iii) removal of the INT-WS
₂ template with a hydrogen peroxide solution. Nanoindentation of the template-free AuNP tubes with atomic force microscopy indicates a radial elastic modulus of 4 GPa. The template-free molecular AuNP tubes are characterized using scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, and micro-Raman spectroscopy. The methodology provides a convenient and scalable strategy for the realization of molecular AuNP tubes with a defined length and diameter, depending on the dimensions of the template.

We have synthesized quaternary chalcogenide-based misfit nanotubes LnS(Se)-TaS₂(Se) (Ln = La, Ce, Nd, and Ho). None of the compounds described here were reported in the literature as a bulk compound. The characterization of these nanotubes, at the atomic level, has been developed via different transmission electron microscopy techniques, including high-resolution scanning transmission electron microscopy, electron diffraction, and electron energy-loss spectroscopy. In particular, quantification at sub-nanometer scale was achieved by acquiring high-quality electron energy-loss spectra at high energy (∼between 1000 and 2500 eV). Remarkably, the sulfur was found to reside primarily in the distorted rocksalt LnS lattice, while the Se is associated with the hexagonal TaSe₂ site. Consequently, these quaternary misfit layered compounds in the form of nanostructures possess a double superstructure of La/Ta and S/Se with the same periodicity. In addition, the interlayer spacing between the layers and the interatomic distances within the layer vary systematically in the nanotubes, showing clear reduction when going from the lightest (La atom) to the heaviest (Ho) atom. Amorphous layers, of different nature, were observed at the surface of the nanotubes. For La-based NTs, the thin external amorphous layer (inferior to 10 nm) can be ascribed to a Se deficiency. Contrarily, for Ho-based NTs, the thick amorphous layer (between 10 and 20 nm) is clearly ascribed to oxidation. All of these findings helped us to understand the atomic structure of these new compounds and nanotubes thereof.

The nature of the interface between a reinforcement and the matrix is crucial in determining the degree of dispersion and mechanical properties. Controlling the interface is particularly important when 2D materials are used as reinforcements, as their flat nature eliminates bonding mechanisms such as mechanical lock-in. For example, we have previously shown that unfunctionalised graphene in an epoxy matrix has just a tenth of the interfacial strength of a carbon fibre. Herein, we have developed chemical functionalization of two-dimensional (2D) materials as a route to control their interface with thermosets, thermoplastics and elastomers. One well-established method for functionalizing graphene is the use of aryl diazonium chemistry. This functional step is typically applied on as-made powder, however we have found that graphene can be simultaneously functionalized and exfoliated from graphite by electroreduction. This one-step process prevents aggregation, increases thin-flake yield and leads to high quality dispersions. However, similarly structured inorganic 2D materials such as transition metal dichalcogenides (TMDs) have generally been considered to be chemically inert, with few studies into the covalent modification and functionalization of them. Previously, it was believed that only the 1T phase of TMDs could be functionalised by aryl diazonium due to the difference in electronic structure. However, in this work we demonstrate that the more stable and naturally occurring 2H-phase can be successfully functionalised via aryl diazonium chemistry. We have measured the interfacial strength of these functionalized materials within different matrices using Raman mapping of in-situ mechanical tests of single flakes and applying a simple shear lag model. Bulk composites were then produced with flakes processing different aspect ratios to prove that these micromechanics approaches can be translated to the macroscale.

Keywords: Chemistry, Multidisciplinary

The study of inorganic nanometer-scale materials with hollow closed-cage structures, such as inorganic fullerene-like (IF) nanostructures and inorganic nanotubes (INTs), is a rapidly growing field. Numerous kinds of IF nanostructures and INTs were synthesized for a variety of applications, particularly for lubrication, functional coatings, and reinforcement of polymer matrices. To date, such nanostructures have been synthesized mostly by heating a transition metal or oxide thereof in the presence of precursor gases, which are however toxic and hazardous. In this context, one frontier of research in this field is the development of new avenues for the green synthesis of IF structures and INTs, directly from the bulk of layered compounds. In the present work, we demonstrate a simple roomerature and environmentally friendly approach for the synthesis of IF nanostructures and INTs via ultrashort-pulse laser ablation of a mixture of transition-metal dichalcogenides in bulk form mixed with Pb/PbO, in ambient air. The method can be considered as a synergy of photothermally and photochemically induced chemical transformations. The ultrafast-laser-induced excitation of the material, complemented with the formation of extended hot annealing regions in the presence of the metal catalyst, facilitates the formation of different nanostructures. Being fast, easy, and material-independent, our method offers new opportunities for the synthesis of IF nanostructures and INTs from different bulk metal chalcogenide compounds. On the basis of the capabilities of laser technology as well, this method could advantageously be further developed into a versatile tool for the simultaneous growth and patterning of such nanostructures in preselected positions for a variety of applications.

Nanoparticles, and more specifically gold nanoparticles (AuNPs), have attracted much scientific and technological interest in the last few decades. Their popularity is attributed to their unique optical, catalytic, electrical and magnetic properties when compared with the bulk. However, one of the main problems with AuNPs is their long-term stability. Two-dimensional materials like MoS₂ (WS₂) are semiconductors that exhibit a combination of properties which make them suitable for electronic, optical and (photo)catalytic devices. Few-layer MoS₂ (WS₂) nanoparticles (NPs), and in particular single-layer ones, show intriguing optical and electrical properties which are very different from those of the bulk compounds. Here we demonstrate the synthesis of AuNPs sheathed by a single layer of MoS₂ (WS₂), i.e. a core-shell nanostructure (AuNP@1L-MoS₂). The hybrid NPs exhibit optical properties that are different from those of either constituent and are amenable for modulation via their chemistry, offering a myriad of applications.

Owing to their outstanding variety of properties, nanoparticles attracted considerable interest for developing a new age of novel polymer nanocomposites and adhesives. Among them, inorganic nanotubes (INTs) of tungsten disulfide (WS₂) and carbon nanotubes (CNTs) have been identified as unique candidates for many industrial applications by virtue of their superior mechanical, thermal and tribological properties. In this work, a comparative study between INT-WS₂ and multi-walled carbon nanotubes (MWCNTs) regarding the properties of structural polyurethane (PU) adhesives is presented. Specifically, we evaluate the thermomechanical properties, chemical and micro-phase structure, bonded joints performance in lap shear and peel modes, and fracture mechanisms, with aiming to highlight some of the differences between these nanotubes. The added content spanned over the range of 0.3, 0.6, 0.9 and 1.2 wt% of INT-WS₂, while CNTs are incorporated at the same volumetric fractions (e.g., 0.1, 0.2, 0.3 and 0.4 wt%). According to the results of the tests, in all measured aspects, INT-WS₂ outperformed the carbon counterparts. With regards to the thermomechanical attributes, the hard segment Tg of the PU increased by 14 °C at 0.6 wt% INTs, whereas CNTs recorded only modest improvement of 8 °C at the nanocomposite loaded with 0.4 wt%. The structural bonding performance indicates an improvement of 117% and 38% in the shear and peel strengths at 0.9 and 1.2 wt% INTs, respectively, while CNTs show moderate changes. Interestingly, the changes are also reflected by the interplay between the hard and soft segments of the PU, and different fracture mechanisms operating within the nanocomposites. At any rate, the combined attributes of >20 MPa in the shear strength and approximately 2 N/mm in (average) peel strength, targets the PU/INT nanoadhesives as promising alternative for conventional epoxy systems which has similar shear strength with inferior peel strength.

Chirality of materials are known to affect optical, magnetic and electric properties, causing a variety of nontrivial phenomena such as circular dichiroism for chiral molecules, magnetic Skyrmions in chiral magnets and nonreciprocal carrier transport in chiral conductors. On the other hand, effect of chirality on superconducting transport has not been known. Here we report the nonreciprocity of superconductivity-unambiguous evidence of superconductivity reflecting chiral structure in which the forward and backward supercurrent flows are not equivalent because of inversion symmetry breaking. Such superconductivity is realized via ionic gating in individual chiral nanotubes of tungsten disulfide. The nonreciprocal signal is significantly enhanced in the superconducting state, being associated with unprecedented quantum Little-Parks oscillations originating from the interference of supercurrent along the circumference of the nanotube. The present results indicate that the nonreciprocity is a viable approach toward the superconductors with chiral or noncentrosymmetric structures.

Inorganic fullerene (IF)-like nanoparticles and inorganic nanotubes (INT) form a relatively new class of nanomaterials. They are generically produced from layered (2D) materials, which enable formation of stable, closed, hollow structures in the nanodomain. Taking advantage of the structural analogy between graphite and inorganic 2D compounds, researchers demonstrated that nanoparticles of WS₂ (Tenne et al. 1992), MoS₂ (Feldman et al. 1995, Margulis et al. 1993), BN (Chopra et al. 1995, Golberg et al. 1998), NiCl₂ (Rosenfeld Hacohen et al. 1998), and the like become unstable in the platelet (bulk) form and spontaneously assemble into hollow seamless structures, such as multiwall quasi-spherical spheres, nanooctahedra (Parilla et al. 1999), or nanotubes. The research on these nanostructures brought about major advances in chemistry and nanotechnology, and pointed to many possible applications.

The simple process of a liquid wetting a solid surface is controlled by a plethora of factors - surface texture, liquid droplet size and shape, energetics of both liquid and solid surfaces, as well as their interface. Studying these events at the nanoscale provides insights into the molecular basis of wetting. Nanotube wetting studies are particularly challenging due to their unique shape and small size. Nonetheless, the success of nanotubes, particularly inorganic ones, as fillers in composite materials makes it essential to understand how common liquids wet them. Here, we present a comprehensive wetting study of individual tungsten disulfide nanotubes by water. We reveal the nature of interaction at the inert outer wall and show that remarkably high wetting forces are attained on small, open-ended nanotubes due to capillary aspiration into the hollow core. This study provides a theoretical and experimental paradigm for this intricate problem.

The relationship between structure and properties has been followed for different nanoscale forms of tungsten disulfide (2H-WS₂) namely exfoliated monolayer and few-layer nanoplatelets, and nanotubes. The similarities and differences between these nanostructured materials have been examined using a combination of optical microscopy, scanning and high-resolution transmission electron microscopy and atomic force microscopy. Photoluminescence and Raman spectroscopy have also been used to distinguish between monolayer and few-layer material. Strain induced phonon shifts have been followed from the changes in the positions of the A_1g and E_2g¹ Raman bands during uniaxial deformation. This has been modelled for monolayer using density functional theory with excellent agreement between the measured and predicted behaviour. It has been found that as the number of WS₂ layers increases for few-layer crystals or nanotubes, the A_1g mode hardens whereas the E_2g¹ mode softens. This is believed to be due to the A_1g mode, which involves out of plane atomic movements, being constrained by the increasing number of WS₂ layers whereas easy sliding reduces stress transfer to the individual layers for the E_2g¹ mode, involving only in-plane vibrations. This finding has enabled the anomalous phonon shift behaviour in earlier pressure measurements on WS₂ to be resolved, as well as similar effects in other transition metal dichalcogenides, such as molybdenum disulfide, to be explained.

We present Raman spectra of misfit layer (PbS)(1.14)NbS2 nanotubes and lead intercalated NbS2 nanotubes. They represent interesting model systems to investigate the nature of interlayer interaction in layered materials. A direct correlation to the Raman modes of the parent 2H-NbS2 compound exists, but some modes are seen drastically upshifted in frequency in the misfit layer and intercalated compound while others remain almost unchanged. On the basis of the Raman spectroscopic investigations and with the help of supporting calculations we examine different interlayer bonding mechanisms and contribute to the discussion as to why these frequency shifts occur.

Polyvinyl alcohol (PVA) is a biocompatible, semi-crystalline and water soluble polymer with moderate tensile properties. To improve the thermal and mechanical properties of PVA, as well as to reduce water uptake, structural modification by glutaric acid (GA) and nanoparticle reinforcement by tungsten disulphide nanotubes (WSNTs) were used to prepare PVA based composites. We observed a significant drop in the water uptake of GA crosslinked PVA, an indication of the formation of a network. Fourier transform infrared spectroscopy was applied to confirm the presence of covalent bonds formed during the crosslinking. Crosslinked PVA and composites are found to have higher thermal stability and mechanical properties compared to their un-crosslinked counterparts. Tensile tests show that the presence of WSNTs increases the strength (up to 25%), modulus (up to 120%) and toughness (up to 80%) of the pristine as well as the crosslinked PVA. (C) 2016 Elsevier Ltd. All rights reserved.

DC and AC electrical properties of hybrid films, consisting of carbon nanotubes and tungsten disulfide nanotubes (and fullerene like nanoparticles) were studied within the 2 - 300 K temperature range and over the 20 Hz - 1 MHz frequency range. The temperature dependences of the resistance R(T) exhibit behavior typical for the fluctuation-induced tunneling model in the intermediate temperature range. Analysis of the dependences of real and imaginary components of the impedance on the frequency (Z(f ) and Z(f )) demonstrates the rising role of the contact barriers between carbon nanotubes inside hybrid films, consisting of the carbon nanotubes and inorganic tungsten disulfide nanotubes as the temperature was decreased. The active component of the impedance was found to prevail in the AC electrical properties of the hybrid films, consisting of multiwall carbon nanotubes and WS₂ nanoparticles over the entire available temperature range.

The process engineering of Mg alloys is nevertheless compromised due to their relatively poor mechanical properties. To address some of these difficulties, metal-matrix composites (MMCs) of Mg-alloys have been investigated for sometime. In the present work, small amounts (up to 1 wt%) of WS2 nanotubes are mixed with Mg-alloy (AZ31) using melt-stirring process above 700 degrees C. The new MMC nano-composites exhibit much superior mechanical properties vis a vis the pristine alloy. Metallographic investigation demonstrates that the average grain size has been reduced in inverse proportion to the added amounts of nanotubes up to 1 wt%. Physical considerations suggest that the main mechanism responsible for the reinforcement effect lies in the mismatch between the thermal expansion coefficients of the metal and the nanotubes. This mismatch induced large density of dislocations in the grain boundaries in the vicinity of the nanotube-matrix interface, which obstruct the crack propagation. (C) 2015 Elsevier B.V. All rights reserved.

Silica aerogels are unique solids with extremely high porosity (>99.5% air by volume), transparency and low density. With their high surface area and thermal resistivity they make excellent heat insulators. However, due to their fine structure they are fragile and therefore impractical for structural applications. To improve their mechanical properties we suggest incorporation of minute amounts of tungsten disulfide nanotubes. Nanotubes of tungsten disulfide are known for their high mechanical strength, strain and thermal stability. Adding some 0.11 wt% of tungsten disulfide nanotubes to a variety of polymers clearly lead to substantial improvement in their mechanical properties. The current study reports the preparation of silica aerogels compounded with small amounts of tungsten disulfide nanotubes. Three-point bending and uniaxial compression tests of the composite aerogel revealed 85% and 23% improvement in the composite material toughness, respectively.

The incorporation of nanostructures into nanoelectronic and nanoelectromechanical systems is a long sought-after goal. In the present article, we report the first torsional electromechanical measurements of pure inorganic nanotubes. The WS₂ nanotubes exhibited a complex and reproducible electrical response to mechanical deformation. We combined these measurements with density-functional-tight-binding calculations to understand the interplay between mechanical deformation, specifically torsion and tension, and electrical properties of WS₂ nanotubes. This yielded the understanding that the electrical response to mechanical deformation may span several orders of magnitude on one hand and detect several modes of mechanical deformation simultaneously on the other. These results demonstrate that inorganic nanotubes could thus be attractive building blocks for nanoelectromechanical systems such as highly sensitive nanometric motion sensors.

Electrochemical generation of hydrogen by non-precious metal electrocatalysts at a lower overpotential is a focus area of research directed towards sustainable energy. The exorbitant costs associated with Pt-based catalysts is the major bottleneck associated with commercial-scale hydrogen generation. Strategies for the synthesis of cost-effective and stable catalysts are thus key for a prospective 'hydrogen economy'. In this report, we highlight a novel and general strategy to enhance the electrochemical activity of molybdenum disulfide (MoS₂) in a fullerene structure (IF-). In particular, pristine (undoped) and rhenium-doped nanoparticles of MoS₂ with fullerene-like structures (IF-MoS₂) were studied, and their performance as catalysts for the hydrogen evolution reaction (HER) was compared to that of 2H-MoS₂ particles (platelets). The current density of the IF-MoS₂ was higher by one order of magnitude than that of few-layer (FL-) MoS₂, due to the enhanced density of the edge sites. Furthermore, Re doping of as low as 100 ppm in IF-MoS₂ decreased the onset potential by 60-80 mV and increased the activity by 60 times compared with that of the FL-MoS₂. The combined synergistic effect of Re doping and the IF structure not only changes the intrinsic nature of the MoS₂ but also increases its reactivity. This strategy highlights the potential use of the IF structure and Re doping in electrocatalytic hydrogen evolution using MoS₂-based catalysts.

Inorganic, fullerene-like (IF) nanoparticles of tungsten disulfide (WS2) and molybdenum disulfide (MoS2) have been synthesised in the past and their useful tribological properties have been studied quite extensively. Rhenium doping of such nanoparticles was also reported and studied to some extent. Herein, further studies of the physico-chemical properties are reported in connection to lubrication under mild loads. Due to their self-repelling character, the doped nanoparticles form tessellated monolayers on certain substrates. When mixed in small amounts, the IF nanoparticles lead to a reduction in the viscosity of water-based gels. Furthermore, the added nanoparticles lead to a precipitous reduction in traction force (friction) under mild loads. The lubrication mechanism of such nanoparticles is briefly discussed.

Impaired salivary gland (SG) function leading to oral diseases is relatively common with no adequate solution. Previously, tissue engineering of SG had been proposed to overcome this morbidity, however, not yet clinically available. Multiwall inorganic (tungsten disulfide [WS₂]) nanotubes (INT-WS₂) and fullerene-like nanoparticles (IF-WS₂) have many potential medical applications. A yet unexplored venue application is their interaction with SG, and therefore, our aim was to test the biocompatibility of INT/IF-WS₂ with the A5 and rat submandibular cells (RSC) SG cells. The cells were cultured and subjected after 1 day to different concentrations of INT-WS₂ and were compared to control groups. Growth curves, trypan blue viability test, and carboxyfluorescein succinimidyl ester (CFSE) proliferation assay were obtained. Furthermore, cells morphology and interaction with the nanoparticles were observed by light microscopy, scanning electron microscopy and transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy. The results showed no significant differences in growth curves, proliferation kinetics, and viability between the groups compared. Moreover, no alterations were observed in the cell morphology. Interestingly, TEM images indicated that the nanoparticles are uptaken by the cells and accumulate in cytoplasmic vesicles. These results suggest promising future medical applications for these nanoparticles.

The synthesis of nanotubes from layered compounds has generated substantial scientific interest. "Misfit" layered compounds (MLCs) of the general formula [(MX)_1+x]_m[TX₂]_n, where M can include Pb, Sb, rare earths; T=Cr, Nb, and X=S, Se can form layered structures, even though each sub-system alone is not necessarily a layered or a stable compound. A simple chemical method is used to synthesize these complex nanotubes from lanthanide-based misfit compounds. Quaternary nanotubular structures formed by partial substitution of the lanthanide atom in nanotubes by other elements are also confirmed. The driving force and mechanism of formation of these nanotubes is investigated by systematic temperature and time-dependent studies. A stress-inducement mechanism is proposed to explain the formation of the nanotubes. The resulting materials may find applications in fields that include thermoelectrics, light emitters, and catalysis and address fundamental physical issues in low dimensions. When two systems meet: Interactions between two systems comprising different in-plane periodicities can result in misfit layered compounds even if each subsystem alone is not a layered or stable compound. A combination of two independent stimuli, namely the incommensurability of the misfit lattice (often leading to folding and scrolling) and the reactivity of the layer rim atoms, provides a new strategy to synthesize misfit nanotubes.

Theoretical analysis of experimental data on unzipping multilayered WS ₂ nanotubes by consequent intercalation of lithium atoms and 1-octanethiol molecules [C. Nethravathi, et al., ACS Nano, 2013, 7, 7311] is presented. The radial expansion of the tube was described using continuum thin-walled cylinder approximation with parameters evaluated from ab initio calculations. Assuming that the attractive driving force of the 1-octanethiol molecule is its reaction with the intercalated Li ions ab initio calculations of a 1-octanethiol molecule bonding with Li⁺ were carried out. In addition, the non-chemical interactions of the 1-octanethiol dipole with an array of positive point charges representing Li⁺ were taken into account. Comparing between the energy gain from these interactions and the elastic strain energy of the nanotube allows us to evaluate a value for the tube wall deformation after the implantation of 1-octanethiol molecules. The ab initio molecular dynamics simulation confirmed our estimates and demonstrated that a strained WS₂ nanotube, with a decent concentration of 1-octanethiol molecules, should indeed be unzipped into the WS₂ nanoribbon.

Tubular structures of the MS-TaS₂ with (M = Pb, Sn, Sb, Bi) misfit layered compounds are reported. The lattice mismatch between the alternating MS and TaS₂ layers leads to a variety of chiral tubular structures. Such tubular structures are studied via scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). For the PbS-TaS₂ and SnS-TaS ₂ tubules, relative in-plane orientations as well as folding vectors of the two subsystems can be determined. However, almost ring-like SAED patterns are obtained for SbS-TaS₂ nanotubes precluding exact determination of the relative in plane orientation. Also, very complex diffraction patterns were obtained for BiS-TaS₂ nanotubes.

A new two-step procedure for the synthesis of MoS₂ nanotubes using lead as a growth promoter is reported. In the first step, molybdenum suboxide nanowhiskers containing a small amount of lead atoms were created by exposing a pressed MoS₂+Pb mixture to highly compressed shock-heated argon gas, with estimated temperatures exceeding 9900 K. In the second step, these molybdenum suboxide nanowhiskers served as templates for the sulfidization of the oxide into MoS₂ nanotubes (by using H₂S gas in a reducing atmosphere at 820 °C). Unlike the case of WS₂ nanotubes, the synthesis of a pure phase of MoS₂ nanotubes from molybdenum oxide has proven challenging, due mostly to the volatile nature of the latter at the high requisite reaction temperatures (>800 °C). In contrast, the nature and apparent reaction mechanism of the method reported herein are amenable to future scale-up. The high-temperature shockwave system should also facilitate the synthesis of new nanostructures from other layered materials.

The optical and electronic properties of suspensions of inorganic fullerene-like nanoparticles of MoS₂ are studied through light absorption and zeta-potential measurements and compared to those of the corresponding microscopic platelets. The total extinction measurements show that, in addition to excitonic peaks and the indirect band gap transition, a new peak is observed at 700-800 nm. This spectral peak has not been reported previously for MoS₂. Comparison of the total extinction and decoupled absorption spectrum indicates that this peak largely originates from scattering. Furthermore, the dependence of this peak on nanoparticle size, shape, and surface charge, as well as solvent refractive index, suggests that this transition arises from a plasmon resonance.

Carbon fullerenes and nanotubes revolutionized understandingof the reactivity of nanoscale compounds. Subsequently, our group and others discovered analogous inorganic compounds with hollow, closed nanostructures. Such inorganic nanostructures offer many applications, particularly in the energy and electronics industries.One way to create inorganic nanostructures is via misfit layer-ed compounds (MLC), which are stacks of alternating two-dimensional molecular slabs, typically held together via weak van der Waals forces. They contain "misfits" in their a-b plane structures that can make them unstable, leading to collapse of the slabs into tubular nanostructures. For example, metal chalcogenide MLCs of the general formula (MX)_1+y/ TX₂ (M = Sn, Pb, Bi, Sb, and other rare earths; T = Sn, Ti, V, Cr, Nb, Ta, etc.; X = S or Se) consist of a superstructure of alternating layers where the MX unit belongs to a (distorted NaCl) orthorhombic symmetry group (O), the TX₂ layer possesses trigonal (T) or octahedral symmetry, and the two layers are held together via both van der Waals and polar forces. A misfit in the a axis or both a and b axes of the two sublattices may lead to the formation of nanostructures as the lattices relax via scrolling. Previous research has also shown that the abundance of atoms with dangling bonds in the rims makes nanoparticles of compounds with layered structure unstable in the planar form, and they tend to fold into hollow closed structures such as nanotubes.This Account shows that combining these two triggers, misfits and dangling bond annihilation in the slab rims, leads to new kinds of nanotubes from MLCs. In particular, we report the structure of two new types of nanotubes from misfits, namely, the SnS/SnS₂ and PbS/NbS₂ series. To decipher the complex structures of these nanotubes, we use a range of methods: high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), selected area electron diffraction (SAED) analyses, scanning electron microscopy (SEM), and Cs-corrected scanning transmission electron microscopy (STEM) in the high-angle annular dark-field mode (HAADF). In both new types, the lattice mismatch between the two alternating sublayers dictates the relative layer-stacking order and leads to a variety of chiral tubular structures. In particular, the incommensuration (a type of misfit) of the SnS₂/SnS system in both the (in plane) a and b directions leads to a variety of relative in-plane orientation and stacking orders along the common c-axis. Thus the SnS/SnS₂ nanotubes form superstructures with the sequence O-T and O-T-T, and mixtures thereof. We also report nanotubes of the misfit layered compound (PbS)_1.14NbS₂, and of NbS ₂ intercalated with Pb atoms, with the chemical formula PbNbS ₂. Thus, the possibility to use two kinds of folding mechanisms jointly offers a new apparatus for the synthesis of unique 1-D nanostructures of great complexity and a potentially large diversity of physicochemical properties.

Next-generation catalysts for water splitting are crucial towards a renewable hydrogen economy. MoS₂ and WS₂ represent earth-abundant, noble metal cathode alternatives with high catalytic activity at edge sites. One challenge in their development is to nanostructure these materials in order to achieve increased performance through the creation of additional edge sites. In this work, we demonstrate a simple route to form nanostructured-WS₂ using sonochemical exfoliation to break interlayer and intralayer bonds in WS₂ nanotubes. The resulting few-layer nanoflakes are ∼100 nm wide with a high density of edge sites. WS₂ nanoflakes are utilized as cathodes for the hydrogen evolution reaction (HER) and exhibit superior performance to WS₂ nanotubes and bulk particles, with a lower onset potential, shallower Tafel slope and increased current density. Future work may employ ultra-small nanoflakes, dopant atoms, or graphene hybrids to further improve electrocatalytic activity. [Figure not available: see fulltext.]

Recently large amounts of inorganic nanotubes (INT) and inorganic fullerene-like (IF) nanoparticles of WS₂ became available and methods for their dispersion in different media were developed. In the present work the tribological properties of epoxy composite compounded with tungsten disulfide particles of different sizes and morphologies, including quasi-spherical IF nanoparticles, one-dimensional INT as well as micron-size platelets (2H) were investigated. The coefficient of friction and wear loss were measured under dry contact conditions using different tribological rigs. Remarkable reduction in wear and also friction (under high load) was demonstrated for the IF/INT epoxy nanocomposite. The reduced wear is attributed in general to the reinforcement of the polymer matrix by nanoparticles and the simultaneous reduction of the epoxy brittleness. Contrarily, the friction of the neat epoxy sample and epoxy mixed with platelets was accompanied with strong wear and transfer of a polymer film onto the rubbed surfaces. These results are consistent with the recently reported improvements in the fracture toughness, peel and shear strength of the epoxy-nanoparticles (IF/INT) composites.

Insertion of endoscopes and other medical devices into the human body are ubiquitous, especially among aged males. The applied force for the insertion/extraction of the device from the urethra must overcome endoscope-surface-human-tissue interactions. In daily practice a gel is applied on the endoscope surface, in order to facilitate its entry into the urethra, providing also for local anesthesia. In the present work, a new solid-state lubricant has been added to the gel, in order to reduce the metal-urethra interaction and alleviate the potential damage to the epithelial tissue. For that purpose, a urethra model was designed and fabricated, which allowed a quantitative assessment of the applied force for extraction of the endoscope from a soft polymer-based ring. It is shown that the addition of MoS₂ nanoparticles with fullerene-like structure (IF-MoS₂) and in particular rhenium-doped nanoparticles (Re:IF-MoS₂) to Esracain gel applied on the metal-lead reduced the friction substantially. The Re:IF-MoS ₂ showed better results than the undoped fullerene-like nanoparticles and both performed better than the gel alone. The mechanism of friction reduction is attributed to fullerenes' ability to roll and act as a separator between the active parts of the model.

We report the first transistor based on inorganic nanotubes exhibiting mobility values of up to 50 cm² V^-1 s^-1 for an individual WS₂ nanotube. The current-carrying capacity of these nanotubes was surprisingly high with respect to other low-dimensional materials, with current density at least 2.4 × 10⁸ A cm^-2. These results demonstrate that inorganic nanotubes are promising building blocks for high-performance electronic applications.

We report a reasonably high yield (∼50%) synthesis of silicon carbide (SiC) nanowires from silicon oxides and carbon in vacuum, by novel solar and lamp photothermal ablation methods that obviate the need for catalysis, and allow relatively short reaction times (∼10 min) in a nominally one-step process that does not involve toxic reagents. The one-dimensional core/shell β-SiC/SiO_x nanostructures - characterized by SEM, TEM, HRTEM, SAED, XRD and EDS - are typically several microns long, with core and outer diameters of about 10 and 30 nm, respectively. HRTEM revealed additional distinctive nanoscale structures that also shed light on the formation pathways.

We investigated the infrared vibrational properties of pristine and Re-substituted MoS₂ nanoparticles and analyzed the extracted phonon lifetimes in terms of multiple scattering events. Our measurements reveal both size- and doping-dependent changes that we attribute to grain boundary scattering and charge and mass effects, respectively. By contrast, Born charge is affected only by size. These findings illustrate the utility of reaching beyond traditional bulk semiconductors and quantum dots to explore how doping and confinement impact carrier-phonon interactions in low-dimensional semiconducting nanomaterials.

We investigated the optical properties of rhenium-doped MoS₂ nanoparticles and compared our findings with the pristine and bulk analogues. Our measurements reveal that confinement softens the exciton positions and reduces spin-orbit coupling, whereas doping has the opposite effect. We model the carrier-induced exciton blue shift in terms of the Burstein-Moss effect. These findings are important for understanding doping and finite length scale effects in low-dimensional nanoscale materials.

Mo₂C nanoparticles encapsulated within MoS₂ inorganic fullerene-like nanoparticles and nanotubes were produced by carbothermal reaction at 1200-1300 °C inside a vertical induction furnace. The particles were analyzed using various electron microscopy techniques and complementary methods.

Inorganic fullerene-like (IF) nanoparticles (NP) and inorganic nanotubes (INT) of layered compounds, such as WS₂, have been of particular interest due to their unique structural characteristics. Recently, the catalytic decomposition of thiophene using INT of WS₂ decorated with Co NP was demonstrated. This finding also suggests that these materials could be also suitable for the photocatalytic treatment of pollutants in wastewaters. In the present work, the photocatalytic decomposition of methyl orange (MO) in aqueous solution using Co-coated INT-WS₂ as well as other NP was investigated. The photocatalytic reactivity under visible light illumination of this photocatalyst was measured and compared with that of various IF and INT and TiO₂ (P25). The Co NP-coated INT-WS₂ exhibited the highest photodegradation of MO among the studied NP. The significant enhancement in the photoactivity of the hybrid nanostructure can be attributed to the combination of the metallic Co NP and the semiconducting WS₂ nanotubes. The hybrid nanostructure enables the efficient light absorption by the INT and the subsequent charge separation of the hybrid semiconductormetal NP under visible light illumination. In addition, Raman spectroscopy technique was used to verify that the MO was decomposed by Co-coated nanotubes and not adsorbed in large amounts on the hybrid NP surface.

A reactivity screening of new nano-hydrodesulfurization (HDS) catalysts was conducted using an ambient pressure flow reactor as well as ultra-high vacuum kinetics techniques. Thiophene was used as a probe molecule. Clean multiwall WS₂ nanotubes (INT-WS₂ ) as well as Ni- and Cocoated INT-WS₂ were considered. In addition, undoped MoS₂ and Re-doped nanoparticles with fullerene-like structures were studied. Commercial Ni and Co HDS catalysts from Haldor Topsoe (Denmark) as well as "nano MoS₂" from Impex Corp. (USA) were considered as reference materials. The lab-synthesized and commercial systems broke down thiophene into quite similar non-sulfur containing products, as identified by a gas chromatograph. The Ni and Co promoted catalysts showed similar thiophene conversion rates. Although the commercial catalysts had larger thiophene conversion rates than the laboratory-synthesized systems, the Re-doped nano-HDS catalyst showed quite low rates of formation of H₂S, an undesirable by-product.

A new procedure for the synthesis of MoS ₂ nanotubes is reported, and additionally demonstrated for MoSe ₂, WS ₂, and WSe ₂. Highly concentrated sunlight creates continuous high temperatures, strong temperature gradients, and extended hot annealing regions, which, together with a metallic (Pb) catalyst, are conducive to the formation of different inorganic nanotubes. Structural characterization (including atomic resolution images) reveals a three-step reaction mechanism. In the first step, MoS ₂ platelets react with water-air residues, decompose by intense solar irradiation, and are converted to molybdenum oxide. Subsequently, the hot annealing environment leads to the growth of Pb-stabilized MoO _3-x nanowhiskers. Shortly afterward, the surface of the MoO _3-x starts to react with the sulfur vapor supplied by the decomposition of nearby MoS ₂ platelets and becomes enveloped by MoS ₂ layers. Finally, the molybdenum oxide core is gradually transformed into MoS ₂ nanotubes. These findings augur well for similar syntheses of as yet unattained nanotubes from other metal chalcogenides.

Tubular structures of the SnS ₂/SnS misfit compound, which are currently prepared in large amounts, are reported. The lattice mismatch between the two alternating sublayers of SnS ₂ and SnS leads to a variety of chiral tubular structures. Such tubular structures are studied via high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). The diversity of the structures manifests itself through different stacking orders of SnS ₂ and SnS layers along their common c-axis and their relative in-plane orientation. Folding vectors and chiral angles of both subsystems can be determined.

We propose that a photodetector based on nanotubes formed from layered structure may have a faster response than nanowires or nanobelts. The layered compound tungsten disulfide (WS ₂) can absorb visible and near-infrared lights. We fabricated photodetectors based on individual WS ₂ nanotubes. The photodetectors exhibited a remarkable response to excitation with 633 and 785 nm light. The nanotube-based photodetectors exhibited short rise and decay times of a few hundred μs, high on/off ratio, and high spectral responsivity and external quantum efficiency. Our results imply that WS ₂ nanotubes are prospective candidates for high-performance nanoscale optoelectronic devices.

The electrostatic effects in tribological systems have been studied in the past, especially with regards to data storage media. Nanoparticles (NP) of WS2 and MoS2 with fullerene-like structure (IF) have been studied in the past and showed very good tribological behavior. Being semiconductors, their electrical properties can be controlled by, e.g., substituting the lattice Mo (W) atoms with Re (n-type conductivity) and Nb (p-type conductivity) atoms. In this study doping of IF-MoS2, and to a lesser degree IF-WS ₂, NP with small amounts (

Metallic films impregnated with fullerene-like-WS2 (MoS2) nanoparticles were fabricated on stainless steel and Ti-Ni substrates using galvanic and electroless deposition. The coatings were obtained from aqueous suspensions containing the metallic salts as well as the dispersed nanoparticles. Tribological tests showed that the films have low friction and wear. Such coatings could be useful for numerous civilian and defense-related applications.

The phenomenon of a partial 2H→1T phase transition within multiwalled WS ₂ nanotubes under substitutional rhenium doping is discovered by means of high-resolution transmission electron microscopy. Using density-functional tight-binding calculations for the related MoS ₂ compound, we consider a possible origin of this phase transition, which was known formerly only for WS ₂ and MoS ₂ intercalated by alkali metals. An interplay between the stability of layered or nanotubular forms of 2H and 1T allotropes is found to be intimately related with their electronic structures and electro-donating ability of an impurity.

It is now possible to synthesize inorganic nanotubes (INT) of WS2 in a pure phase and substantial amounts. Due to their crystalline perfection, they are characterized by excellent mechanical behavior. This study is dedicated to the investigation of the effect of INT-WS2 on the mechanical, thermal, adhesion and tribological properties of epoxy based nanocomposites. Various concentrations up to 1.0 wt% of the INT were added to the epoxy matrix. First a method was developed to mix the nanotubes in the epoxy resin matrix. A combination of both magnetic stirring and ultrasonic mixing was used. The adhesion, fracture toughness and strain energy release rate were studied. The INT-WS2 were found to significantly improve all these properties. The wear of the nanotubes-reinforced epoxy was eight-times lower than that of pure epoxy. These results suggest numerous applications.

In this work chromium-rich coatings impregnated with fullerene-like (IF)-WS₂ nanoparticles were deposited on stainless steel substrates. The coatings were obtained from a trivalent chromium bath at pH 2 by galvanostatic electrodeposition. Zinc and cobalt salts were added to the aqueous solution in small amounts serving as cationic growth promoters. Photodeposition of tin-palladium nanoparticles was used as seeding enhancer for the co-deposition of the fullerene-like nanoparticles. The coatings were characterized by a number of techniques and were found to show a decreasing gradient of the IF nanoparticles towards the film-substrate interface. Tribological tests showed that in contrast to the substrate and the pure metal coating, the IF-containing films exhibit low friction and wear.

We report on the synthesis of inorganic fullerene-like molybdenum disulfide (MoS₂) nanoparticles by pulsed laser ablation (PLA) in water. The final products were characterized by scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and resonance Raman spectroscopy, etc. Cell viability studies show that the as-prepared MoS₂ nanoparticles have good solubility and biocompatibility, which may show a great potential in various biomedical applications. It is shown that the technique of PLA in water also provides a green and convenient method to synthesize novel nanomaterials, especially for biocompatible nanomaterials.

The growth mechanism of WS₂ nanotubes is briefly discussed. Two distinct growth mechanisms can be delineated, leading to somewhat different products: 1) thick (50-150 nm) and very long (20-50 microns and above) nanotubes consisting of many (> 20) layers, and 2) slender (20-25 nm) nanotubes with 5-10 layers. The synthesis of large amounts of pure WS₂ nanotubes belonging to the first category in the large-scale fluidized-bed reactor is described. Characterization of the nanotubes, which grow catalyst-free by a number of analytical techniques, is reported. The nanotubes reveal highly crystalline order, suggesting very good mechanical behavior and numerous applications, especially in the field of nanocomposites.

A nanostructure, being either an Inorganic Fullerene-like (IF) nanostructure or an Inorganic Nanotube (INT), A1-x-Bx-chalcogenide are described. A being a metal or transition metal or an alloy of metals and/or transition metals, B being a metal or transition metal B different from that of A and x being ≤0.3. A process for their manufacture and their use for modifying the electronic character of A-chalcogenide are described.

Inorganic layered materials can form hollow multilayered polyhedral nanoparticles. The size of these multi-wall quasi-spherical structures varies from 4 to 300 nm. These materials exhibit excellent tribological and wear-resisting properties. Measuring and evaluating the stiffness of individual nanoparticle is a non-trivial problem. The current paper presents an in situ technique for stiffness measurements of individual WS₂ nanoparticles which are 80 nm or larger using a high resolution scanning electron microscope (HRSEM). Conducting the experiments in the HRSEM allows elucidation of the compression failure strength and the elastic behavior of such nanoparticles under uniaxial compression.

New materials and techniques pertaining to the synthesis of inorganic nanotubes have been ever increasing since the initiation of the field in 1992. Recently, WS2 nanotubes, which are produced now in large amounts, were filled with molten lead iodide salt by a capillary wetting process, resulting in PbI₂@WS₂ core-shell nanotubes. This work features progress in the synthesis of new core-shell nanotubes, including Bil₃@WS ₂ nanotubes produced in a similar same manner. In addition, two new techniques for obtaining core-shell nanotubes are presented. The first is via electron-beam irradiation, i.e., in situ synthesis within a transmission electron microscope. This synthesis results in SbI₃ nanotubes, observed either in a hollow core Of WS₂ ones (Sbl₃@WS ₂ nanotubes), or atop of them (WS₂@Sbl₃ nanotubes). The second technique involves a gaseous phase reaction, where the layered product employs WS₂ nanotubes as nucleation sites. In this case, the MoS₂ layers most often cover the WS₂ nanotube, resulting in WS₂@MoS₂ core-shell nanotubes. Notably, superstructures of the form MoS₂@WS₂ are occasionally obtained. Using a semi-empirical model, it is shown that the Pbl₂ nanotubes become stable within the core of MoS2 nanotubes only above a critical core diameter of the host (>12 nm); below this diameter the PbI₂ crystallizes as nanowires. These model calculations are in agreement with the current experimental observations, providing further support to the growth mechanism of such core-shell nanotubes.

Nanoparticles of inorganic compounds with layered (2D) structures, like graphite and MoS₂, were shown to be unstable in the planar from and fold on themselves forming seamless hollow structures like multiwall nanotubes and fullerene-like nanoparticles. The present concise tutorial review reports on the salient developments in this field over the last several years. Numerous applications for such nanophases have been proposed, like solid lubricants, ultra-strong nanocomposites, catalysts, etc.

WS₂ belongs to a family of layered metal dichalcogenide compounds that are known to form cylindrical (inorganic nanotubes-INT) and polyhedral nanostructures - onion or nested fullerene-like (IF) particles. The outermost layers of these IF nanoparticles can be peeled under shear stress, thus IF nanoparticles have been studied for their use as solid lubricants. However, the IF nanoparticles tend to agglomerate, presumably because of surface structural defects induced by elastic strain and curvature, a fact that has a deleterious effect on their tribological properties. In the present work, chemical modification of the IF-WS₂ surface with alkyl-silane molecules is reported. The surface-modified IF nanoparticles display improved dispersion in oil-based suspensions. The alkyl-silane coating reduces the IF-WS₂ nanoparticles' tendency to agglomerate and consequently improves the long-term tribological behavior of oil formulated with the IF additive.

Using a new quartz-made reactor, large amounts of fullerene-like (IF) MoS ₂ nanoparticles were synthesized by reacting MoO ₃ vapor with H ₂S in a reducing atmosphere. The nanoparticles were found to be of high crystalline order; with an average size of 70 nm and consist of more than 30 closed shells. Extensive tribological testing of the nanoparticles in two types of synthetic oils- poly-alpha olefins (PAO)- was carried out and compared to that of bulk (2H platelets) MoS ₂ and IF-WS ₂. These tests indicated that under high pressure and relatively low humidity, the IF-MoS ₂ exhibited a friction coefficient as low as 0.03 and the smallest wear rate of the measured systems. However, its performance was found to be lower in comparison to IF-WS ₂ after 2500 cycles, due probably to its inferior chemical stability. This study indicates that the tribological performance of the IF nanoparticles depends strongly on their crystalline order and size.

Inorganic fullerene-like WS₂ and MoS₂ nanoparticles have been synthesized using exclusively solid precursors, by reaction of the corresponding metal oxide nanopowder, sulfur and a hydrogen-releasing agent (NaBH₄ or LiAlH₄), achieved either by conventional furnace heating up to a 900 °C or by photothermal ablation at far higher temperatures driven by highly concentrated white light. In contrast to the established syntheses that require toxic and hazardous gases, working solely with solid precursors permits relatively safer reactor conditions conducive to industrial scale-up.

In this article a comparison between inorganic nanoparticles with hollow closed structure and the carbon fullerenes and nanotubes is undertaken. First, the structural evolution of inorganic fullerene-like (IF) nanoparticles of MoS₂ as a function of their size is examined in some detail and compared to that of carbon and BN fullerenes. It is shown that hollow closed structures of MoS₂ are stable above 3 nm (app 103 atoms). In the range of 3-8 nm (103-105) nanooctahedra with metallic character are the most stable form of MoS₂ Semiconducting nanotubes and quasispherical IF nano-particles become the stable-most form beyond that size and the bulk (platelets) are stable above about 0.2 μm. The stability of inorganic nanotubes is also discussed. The scaling-up of the synthesis of IF-WS ₂ and the very recent successful synthesis of large, amounts of pure WS₂ nanotubes are briefly described. The stability of IF and INT of MoS₂ (WS₂) under pressure and that of carbon is also discussed. Applications of the IF-WS₂ as superior solid lubricants, which lead to their recent commercialization, is demonstrated.

We report on the first nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) investigation of inorganic fullerene-like MoS ₂ nanoparticles. Spectra of bulk 2H-MoS₂ samples have also been measured for comparison. The similarity between the measured quadrupole coupling constants and chemical shielding anisotropy parameters for bulk and fullerene-like MoS₂ reflects the nearly identical local crystalline environments of the Mo atoms in these two materials. EPR measurements show that fullerene-like MoS₂ exhibits a larger density of dangling bonds carrying unpaired electrons, indicative of them having a more defective structure than the bulk sample. The latter observation explains the increase in the spin-lattice relaxation rate observed in the NMR measurements for this sample in comparison with the bulk 2H- MoS₂ ones.

Although graphite, with its anisotropic two-dimensional lattice, is the stable form of carbon under ambient conditions, on nanometre length scales it forms zero- and one-dimensional structures, namely fullerenes and nanotubes, respectively. This virtue is not limited to carbon and, in recent years, fullerene-like structures and nanotubes have been made from numerous compounds with layered two-dimensional structures. Furthermore, crystalline and polycrystalline nanotubes of pure elements and compounds with quasi-isotropic (three-dimensional) unit cells have also been synthesized, usually by making use of solid templates. These fi ndings open up vast opportunities for the synthesis and study of new kinds of nanostructures with properties that may differ signifi cantly from the corresponding bulk materials. Various potential applications have been proposed for the inorganic nanotubes and the fullerene-like phases. Fullerene-like nanoparticles have been shown to exhibit excellent solid lubrication behaviour, suggesting many applications in, for example, the automotive and aerospace industries, home appliances, and recently for medical technology. Various other potential applications, in catalysis, rechargeable batteries, drug delivery, solar cells and electronics have also been proposed.

The growth mechanism of WS 2 nanotubes in the large-scale fluidized-bed reactor is studied in greater detail. This study and careful parameterization of the conditions within the reactor lead to the synthesis of large amounts (50-100 g/batch) of pure nanotubes, which appear as a fluffy powder, and (400-500 g/batch) of nanotubes/ nanoplatelets mixture (50:50), where nanotubes usually coming in bundles. The two products are obtained simultaneously in the same reaction but are collected in different zones of the reactor, in a reproducible fashion. The characterization of the nanotubes, which grow catalyst-free, by a number of analytical techniques is reported. The majority of the nanotubes range from 10 to 50 micron in length and 20-180 nm in diameter. The nanotubes reveal highly crystalline order, suggesting very good mechanical behavior with numerous applications.

A new type of composite metal-nanoparticle coating that significantly reduces the friction force of various surfaces, particularly archwires in orthodontic applications, is demonstrated. The coating is based on electrodeposited Ni film impregnated with inorganic fullerene-like nanospheres of tungsten disulphide. The first encouraging tests have shown reduction of up to 60% of the friction force between coated rectangular archwires and self-ligating brackets in comparison with uncoated archwires. The coating not only significantly reduces friction of commercial archwires but also maintains this low value of friction for the duration of the tests in comparison to archwires coated with nickel film without the nanoparticles. The coated surfaces of the wires were examined by scanning electron microscopy equipped with energy dispersive analyzer and by x-ray powder diffraction methods before and after the friction tests. Using these analyses, it was possible to qualitatively estimate the state of the Ni+IF-WS₂ coating before and after the friction test compared to Ni coated wires without IF-WS₂.

The characterization of nanostructures down to the atomic scale is essential to understand some physical properties. Such a characterization is possible today using direct imaging methods such as aberration-corrected high-resolution transmission electron microscopy (HRTEM), when iteratively backed by advanced modeling produced by theoretical structure calculations and image calculations. Aberration-corrected HRTEM is therefore extremely useful for investigating low-dimensional structures, such as inorganic fullerene-like particles and inorganic nanotubes. The atomic arrangement in these nanostructures can lead to new insights into the growth mechanism or physical properties, where imminent commercial applications are unfolding. This article will focus on two structures that are symmetric and reproducible. The first structure that will be dealt with is the smallest stable symmetric closed-cage structure in the inorganic system, a MoS₂ nanooctahedron. It is investigated by means of aberration-corrected microscopy which allowed validating the suggested DFTB-MD model. It will be shown that structures diverging from the energetically most stable structures are present in the laser ablated soot and that the alignment of the different shells is parallel, unlike the bulk material where the alignment is antiparallel. These findings correspond well with the high-energy synthetic route and they provide more insight into the growth mechanism. The second structure studied is WS ₂ nanotubes, which have already been shown to have a unique structure with very desirable mechanical properties. The joint HRTEM study combined with modeling reveals new information regarding the chirality of the different shells and provides a better understanding of their growth mechanism.

The recent progress in intercalation of nanotubes and fullerene-like nanoparticles with various metals was reviewed. Nanotubes from layered materials and also those from non layered compounds were discussed. While nanotubes from layered compounds are generically crystalline those of non layered compounds have in most cases a polycrystalline structure. Different intercalation routes were presented and their relative merits and pitfalls were discussed. Furthermore, the changes in the structural and physical properties of the nanoparticles which accompany the intercalation reaction were described. Special attention was paid to the implementation of the nanotubes as a potential electrode material in lithium based batteries.

The temporal chemical changes occurring at the surface of fullerene-like (IF) nanoparticles of WS₂ were investigated using X-ray photo-electron spectroscopy (XPS) and compared to those of bulk powder (2H) of the same material. It is possible to follow the long term (surface oxidation and carbonization) occurring at defects on the outermost surface (0001) layer of the fullerene-like nanoparticles. Similar but perhaps more distinctive changes are observed on the prismatic (hk0) surfaces of the 2H powder. Vacuum annealing is shown to remove most of these changes and bring the surface close to its stoichiometric composition. In accordance with previous measurements, further evidence is obtained for the existence of water molecules which are entrapped in the hollow core and interstitial defects of the fullerene-like nanoparticles during the synthesis. They are also shown to be removed by the vacuum annealing process. Chemically resolved electrical measurements (CREM) in the XPS show that the vacuum annealed IF samples become more intrinsic. Finally, tribological measurements show that the vacuum annealed IF samples perform better as an additive to oil than the non-annealed IF samples and the bulk (2H) platelets powder.

The mechanical properties of individual WS2 nanotubes were investigated and directly related to their atomic structure details by in situ transmission electron microscope measurements. A brittle mode deformation was observed in bending tests of short (ca. 1 mu m in length) multilayer nanotubes. This mode can be related to the atomic structure of their shells. In addition, longer nanotubes (6-7 mu m in length) were deformed in situ scanning electron microscope, but no plastic deformation was detected. A "sword-in-sheath" fracture mechanism was revealed in tensile loading of a nanotube, and the sliding of inner shells inside the outermost shell was imaged "on-line". Furthermore, bending modulus of 217 GPa was obtained from measurements of the electric-field-induced resonance of these nanotubes.

We present the advancement of electron tomography for three-dimensional structure reconstruction of fullerene-like particles toward atomic-scale resolution. The three-dimensional reconstruction of nested molybdenum disulfide nanooctahedra is achieved by the combination of low voltage operation of the electron microscope with aberration-corrected phase contrast imaging. The method enables the study of defects and irregularities in the three-dimensional structure of individual fullerene-like particles on the scale of 2-3 Å. Control over shape, size, and atomic architecture is a key issue in synthesis and design of functional nanoparticles. Transmission electron microscopy (TEM) is the primary technique to characterize materials down to the atomic level, albeit the images are two-dimensional projections of the studied objects. Recent advancements in aberration-corrected TEM have demonstrated single atom sensitivity for light elements at subångström resolution. Yet, the resolution of tomographic schemes for three-dimensional structure reconstruction has not surpassed 1 nm³, preventing it from becoming a powerful tool for characterization in the physical sciences on the atomic scale. Here we demonstrate that negative spherical aberration imaging at low acceleration voltage enables tomography down to the atomic scale at reduced radiation damage. First experimental data on the three-dimensional reconstruction of nested molybdenum disulfide nanooctahedra is presented. The method is applicable to the analysis of the atomic architecture of a wide range of nanostructures where strong electron channeling is absent, in particular to carbon fullerenes and inorganic fullerenes.

Solar ablation creates the sharp radiative and temperature gradients, as well as the high-temperature annealing environment, that favor nanomaterial syntheses. Using highly concentrated sunlight, we generated fullerene-like MoS₂, ranging from single-walled nanotubes and closed-cage structures to their larger multi-walled counterparts. TEM, HRTEM and EDS unambiguously established the nanostructures, some achieving fundamentally minimum sizes predicted by molecular structural theory. Irradiation of MoS₂ and powdered mixtures of MoS₂ + SiO₂ in evacuated quartz ampoules also generated nanofibers and nanospheres of amorphous SiO₂: the first production of SiO₂ nanostructures purely from quartz. Also, solar ablation of MoS₂ + SiO mixtures produced nanowires and nanospheres of crystalline Si.

In this paper, we report on the synthesis and applications of semiconducting nanostructures. Nanostructures of interest were zinc oxide (ZnO) nanowires and tungsten disulfide WS₂ nanotubes where transistors/phototransistors and photovoltaic (PV) energy conversion cells have been fabricated. ZnO nanowires were grown with both high- and low-temperature approaches, depending on the application. Individual ZnO nanowire side-gated transistors revealed excellent performance with a field-effect mobility of 928 cm² V·s. ZnO networks were proposed for large-area macroelectronic devices as a less lithographically intense alternative to individual nanowire transistors where mobility values in excess of 20 cm² /V·s have been achieved. Flexible PV devices utilizing ZnO nanowires as electron acceptors and for photoinduced charge separation and transport have been presented. Phototransistors were fabricated using individual WS₂ nanotubes, where clear sensitivity to visible light has been observed. The results presented here simply reveal the potential use of inorganic nanowires/tubes for various optoelectronic devices.

The temporal chemical changes occurring at the surface of fullerence-like (IF) nanoparticles of WS2 were investigated using X-ray photoelectron spectroscopy (XPS) and compared to those of bulk powder (2H) of the same material. It is possible to follow the long term (surface oxidation and carbonization) occurring at defects on the outermost surface (0001) layer of the fullerene-like nanoparticles. Similar but perhaps more distinctive changes are observed oil the prismatic (hk0) surfaces of the 2H powder. Vacuum annealing is shown to remove most of these changes and bring the surface close to its stoichiometric composition. In accordance with previous measurements, further evidence is obtained for the existence of water molecules which are entrapped in the hollow core and interstitial defects of the fullerene-like nanoparticles during the synthesis. They are also shown to be removed by the vacuum annealing process. Chemically resolved electrical measurements (CREM) in the XPS show that the vacuum annealed IF samples become more intrinsic. Finally, tribological measurements show that the vacuum annealed IF samples perform better as an additive to oil than the non-annealed IF samples and the bulk (2H) platelets powder.

Following the discovery of fullerenes (C₆₀) and carbon nanotubes, it was shown that nanoparticles of inorganic layered compounds, like WS₂ and MoS₂, are unstable in the planar form and they form closed cage structures with polyhedral or nanotubular shapes. Although initially the method of synthesis for the formation of such closed caged structures and nanotubes involved starting from the respective oxides, it is now well established that the gas-phase synthetic route (using metal chlorides, carbonyls etc) provides an alternative which is suitable for the synthesis of very many closed caged structures and nanotubes hitherto unknown. Various issues with this method of synthesis, including its fundamentals, mechanism, and the properties of the inorganic fullerene-like structures produced are reviewed, together with some possible applications.

Fullerene-like WS ₂ (MoS ₂) nanoparticles (IF) have been studied in the past as solid lubricants. Using the tribological ball-on-flat experiments, it was shown that the size of the aggregates and their distribution determine the penetration and entrapping of the IF nanoparticles at the interface. It is expected that the wedge clearance at the inlet of the contact, i.e., the oblique-angle entrance to the contact zone between the two mating tribological surfaces, as well as the average surface roughness, can limit the supply of the lubricant into the interface in, e.g., the block-on-ring experiment. In the present series of experiments, the Stribeck curve was designed first using a linear loading scheme and pure oil. It was concluded that a wedge clearance (oblique-angle) in the inlet of the contact zone leads to entrapment of the IF nanoparticles and their compaction, which hamper the supply of the fluid lubricant into the interface. A ball-on-flat and flat block-on-ring friction devices with wedge clearance in the inlet of the contact can distort the efficacy of IF. Procedures for improving the supply of the IF nanoparticles to the contact zone and improving thereby their efficacy are considered.

IF-MO_1-xNb_xS₂ nanoparticles have been synthesized by a vapor-phase reaction involving the respective metal halides with H₂S. The IF-MO_1-xNb_xS₂ nanoparticles, containing up to 25% Nb, were characterized by a variety of experimental techniques. Analysis of the powder X-ray powder diffraction, X-ray photoelectron spectroscopy, and different electron microscopy techniques shows that the majority of the Nb atoms are organized as nanosheets of NbS₂ within the MoS₂ host lattice. Most of the remaining Nb atoms (3%) are interspersed individually and randomly in the MoS₂ host lattice. Very few Nb atoms, if any, are intercalated between the MoS₂ layers. A sub-nanometer film of niobium oxide seems to encoat the majority of the nanoparticles. X-ray photoelectron spectroscopy in the chemically resolved electrical measurement mode (CREM) and scanning probe microscopy measurements of individual nanoparticles show that the mixed IF nanoparticles are metallic independent of the substitution pattern of the Nb atoms in the lattice of MoS₂ (whereas unsubstituted IF-MoS₂ nanoparticles are semiconducting). Furthermore the IF-MO_1-xNb_xS₂ nanoparticles are found to exhibit interesting single electron tunneling effects at low temperatures.

To investigate phonon confinement in nanoscale metal dichalcogenides, we measured the low-temperature specific heat of layered and nanoparticle WS ₂. Below 9 K, the specific heat of the nanoparticles deviates from that of the bulk counterpart. Further, it deviates from the usual T³ dependence below 4 K due to finite size effects that eliminate long wavelength acoustic phonons and interparticle-motion entropy. This separation of nanoscale effects from T³ dependence can be modeled by assuming that the phonon density of states is flexible, changing with size and shape. We invoke relationships between the low-temperature T³ phonon term, Young's modulus, and friction coefficient to assess the difference in the tribological properties. On the basis of this analysis, we conclude that the improved lubrication properties of the nanoparticles are extrinsic.

Composite coatings of Co + fullerene-like WS₂ nanoparticles on stainless steel substrate were obtained through electroless deposition, using DMAB (dimethyl borane complex, 97%) as the reducing agent, and by electroplating in acidic solution. Phase analysis results show that the coatings consist of Co and the fullerene-like WS₂ nanoparticles alone. Tribological measurements show reduced wear and friction of the composite coatings as compared with the pure cobalt film or the stainless steel substrate.

MoS₂ nanooctahedra are believed to be the smallest stable closed-cage structures of MoS₂, i.e., the genuine inorganic fullerenes. Here a combination of experiments and density functional tight binding calculations with molecular dynamics annealing are used to elucidate the structures and electronic properties of octahedral MoS₂ fullerenes. Through the use of these calculations MoS₂ octahedra were found to be stable beyond n_MO > 100 but with the loss of 12 sulfur atoms in the six corners. In contrast to bulk and nanotubular MoS₂, which are semiconductors, the Fermi level of the nanooctahedra is situated within the band, thus making them metallic-like. A model is used for extending the calculations to much larger sizes. These model calculations show that, in agreement with experiment, the multiwall nanooctahedra are stable over a limited size range of 10⁴-10⁵ atoms, whereupon they are converted into multiwall MoS₂ nanoparticles with a quasi-spherical shape. On the experimental side, targets of MoS₂ and MoSe₂ were laser-ablated and analyzed mostly through transmission electron microscopy. This analysis shows that, in qualitative agreement with the theoretical analysis, multilayer nanooctahedra of MoS₂ with 1000-25 000 atoms (Mo + S) are stable. Furthermore, this and previous work show that beyond ∼10⁵ atoms fullerene-like structures with quasi-spherical forms and 30-100 layers become stable. Laser-ablated WS₂ samples yielded much less faceted and sometimes spherically symmetric nanocages.

A simple, inexpensive, and reproducible photothermal procedure for synthesizing IF-Cs₂O nanoparticles with immensely concentrated sunlight was demonstrated. The solar-driven synthesis of IF-Cs₂O was performed in evacuated quartz ampoules that contained 3R-Cs₂-O crystallites under continuous irradiation with a concentrated solar power of 2.0-7.7 W, and periods ranging from 30 to 840 s. Samples were collected from the deposit that gathered on the quartz ampoule during the solar irradiation for TEM analysis. The interaction of the beam with the materials led to thermal motion, charging, desorption of trace quantities, or possible chemical changes. The results of the electron energy loss spectroscopy and imaging with a Gatan imaging filters proved that only cesium and oxygen were present in the nanoparticles.

Inorganic fullerene-like ( IF) nanoparticles of MoS2 were synthesized using gas-phase reaction starting from MoCl5 and H2S. The IF-MoS2 nanoparticles are spherical and in some cases faceted with diameters in general ranging between 20 and 80 nm. The IF-MoS2 nanoparticles have large hollow cores, filled in some cases with amorphous material. Various parameters have been investigated to understand the growth and formation of the IF-MoS2 nanoparticles. The parameters that have been studied include flow rates of the various carrier gases, temperature at which the reaction was carried out, time of the reaction and heating of the precursor material. The best set of conditions wherein maximum yields of the IF-MoS2 nanoparticles are obtained have been identified. Additionally, annealing the as-obtained samples or heating them in a mixture of H-2 along with H2S improves the crystallinity and reduces the amorphous material filling in the core. Apart from the fullerene-like nanoparticles under certain experimental conditions nanotubes of MoS2 have also been obtained nonetheless in small yields.

Electrical resistivity and Hall effect measurements of pellets compacted from fullerene-like WS2 nanoparticles (IF-WS2) and bulk 2H-WS2 powder were carried out using the van der Pauw method over a wide temperature range. In addition IF-WS2 pellets were annealed at elevated temperatures under vacuum in a specially designed system. Arrhenius plots for the conductivities of the WS2 samples (2H, IF and IF+annealing) exhibit marked uprise of partial derivative In (sigma T-1)/partial derivative T-1 with temperature. The resistivity of the non-annealed IF-WS2 pellets is higher by 2-8 orders of magnitude than that of 2H-WS2 pellets, whereas the resistivity of the annealed IF pellets is higher than that of the non-annealed ones. Hall Effect measurements at 300 K show p-type conductivity and similar carrier concentration for both types of materials. The carrier mobility of 2H-WS2 platelets is found to be in the range of the reported values. However, IF-WS2 pellets have shown an unusually low mobility for a semiconducting material. The experimental data was found to be in a good agreement with a model used for analyzing the conductivity of polycrystalline semiconductors, which takes into consideration fluctuations of the barrier heights among the different nanoparticles as well as within a single nanoparticle boundary. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Current-voltage characteristics measured using STM on fullerene-like WS₂ nanoparticles show zero-bias current and contain segments in which the tunneling current flows opposite to the applied bias voltage. In addition, negative differential conductance peaks emerge in these reversed current segments, and the characteristics are hysteretic with respect to the change in the voltage sweep direction. Such unusual features resemble those appearing in cyclic voltammograms, but are uniquely observed here in tunneling spectra measured in vacuum, as well as in ambient and dry atmosphere conditions. This behavior is attributed to tunneling-driven electrochemical processes.

Uneven malaligned teeth are a problem afflicting large numbers of people, having significant economic and societal repercussions. Sliding a tooth along an archwire during orthodontic treatment involves a frictional type of force which resists this movement, causing a number of adverse effects. First, using excessive orthodontic force, leads to unwanted movements of the anchor teeth and increasing the risk of damage to the roots of the teeth. Furthermore, the frictional force is distributed unevenly between the archwire and the brackets interface, leading to strong adhesion between the wire and the bracket's corner. This force-asymmetry causes lengthening of treatment and frequent visits for fine-tuning of the orthodontic appliances. Despite numerous efforts to lower the friction, no satisfactory solution to this issue has been obtained. In the present work a self-lubricating metal coating containing fullerene-like WS₂ (IF) nanoparticles is demonstrated. Such coatings significantly reduce archwire friction, and may alleviate the adverse complications. Moreover, a number of other medical applications of the self-lubricating coatings are foreseen.

The mechanical properties of materials and particularly the strength are greatly affected by the presence of defects; therefore, the theoretical strength (≈10% of the Young's modulus) is not generally achievable for macroscopic objects. On the contrary, nanotubes, which are almost defect-free, should achieve the theoretical strength that would be reflected in superior mechanical properties. In this study, both tensile tests and buckling experiments of individual WS₂ nanotubes were carried out in a high-resolution scanning electron microscope. Tensile tests of MoS₂ nanotubes were simulated by means of a density-functional tight-binding-based molecular dynamics scheme as well. The combination of these studies provides a microscopic picture of the nature of the fracture process, giving insight to the strength and flexibility of the WS₂ nanotubes (tensile strength of ≈16 GPa). Fracture analysis with recently proposed models indicates that the strength of such nanotubes is governed by a small number of defects. A fraction of the nanotubes attained the theoretical strength indicating absence of defects.

A new class of inorganic nanostructures with a closed cage structure has been discovered. These inorganic fullerenes are multi-walled (i.e. onion-like) spherical particles which can also be wrapped to form nanotubes. They are extremely effective as solid lubricants, particularly under extreme conditions. The development and initial applications of these materials are reviewed.

TiS2 nanoparticles with nested fullerene-like structure (IF) 60-120 nm in size consisting of up to 100 concentric molecular layers and having quite a perfectly spherical shape were obtained by reacting TiCl4 and H2S using first a horizontal and subsequently a vertical reactor. The proposed growth mechanism of these nanoparticles, i.e. nucleation and growth, is radically different from the one proposed for the growth of the fullerene-like WS2 from the respective oxide nanoparticles. It was found that adding 1-2% IF-TiS2 improves the behavior of lubricating oil substantially. The improved performance of the additive was attributed to the nearly spherical shape of the nanoparticles which promotes rolling friction.

We obtained a Raman signal from an individual WS2 nanotube mounted on an atomic force microscopy cantilever tip. We discuss the implications for simultaneous investigations of the mechanical properties of WS2 nanotubes by combining different experimental methods. From the orientation dependence of this nanotube's resonant Raman intensity, we estimate the ratio of the perpendicular to parallel polarizabilities αXXαZZ0.16. We compare the WS2 nanotube with single-walled carbon nanotubes and expect a similarly strong depolarization effect for multiwalled carbon nanotubes.

We present the results from an experimental study of the magnetotransport of superconducting wires of amorphous indium-oxide having widths in the range 40-120 nm. We find that, below the superconducting transition temperature, the wires exhibit clear, reproducible, oscillations in their resistance as a function of magnetic field. The oscillations are reminiscent of those that underlie the operation of a superconducting quantum interference device.

(Figure Presented) Fullerene-like Cs₂O nanoparticles were prepared by laser ablation of 3R-Cs₂O powder in evacuated quartz ampoules. The Cs₂O closed cages, such as the faceted nanoparticle shown in the picture, are remarkably stable as compared with the corresponding extremely unstable but technically important bulk compound, which makes them potentially useful in applications involving cesium oxide coatings, for example, photoemissive devices and catalytic converters.

Recently, the behavior of inorganic fullerene-like (IF) WS₂ nanoparticles in the interface of steel-on-steel pair has been analyzed. It was shown that originally when the gap between the contact surfaces is smaller than the size of the IF nanoparticles, there is no effect of the nanoparticles on the friction force. During the test stiff IF nanoparticles can plough the surface of hard steel samples and penetrate into the interface under friction. Molecular sheets of WS₂ from the delaminated IF nanoparticles, which reside in the valleys of the rough surfaces cover the contact spots and thus decrease the number of adhered spots at the transition to seizure. The goal of the present work was to study the behavior of IF nanoparticles in the interface of ceramic surfaces. The friction tests were performed using a ball-on-flat device. A silicon nitride ball was slid against an alumina flat with maximum contact pressure close to 2 GPa. SEM, TEM and AFM techniques have been used in order to assess the behavior of IF nanoparticles in the interface. The behavior of IF nanoparticles in the much harder ceramic interfaces was found to be appreciably different from the steel pair. The pristine IF nanoparticles are damaged in the inlet of the contact during the first few cycles and thin shells of broken nanoparticles gradually cover the middle range of the contact surface. Different modes of deformation and destruction of the IF nanoparticles are exhibited when going from the middle to edge area of the contact. While aggregates of the pristine nanoparticles are formed at the edge of the contact, thin shells of broken IF nanoparticles are observed in the middle area where contact pressure is maximum. Mechanical stability and damage of IF nanoparticles in the ceramic interface are discussed.

Nanoparticles of WS₂ and MoS₂ with a closed cage structure (fullerene-like) that are termed IF phases are synthesized in large amounts in a pure form. These nanoparticles were shown to play a favorable role as solid lubricants under severe conditions where fluids are unable to support the heavy load and are squeezed away from the contact area. Various tribological scenarios are presented for these superior solid lubricants, demonstrating the large scale potential for applications of these materials. The mechanism of action of these solid lubricants is briefly discussed. Various other potential applications of IF phases for nanocomposites with high impact resistance; in rechargeable batteries and in optical devices are discussed in short.

Fullerene-like WS₂ (MoS₂) nanoparticles (IF) have been studied in the past. Their efficacy as additives for lubrication fluids has been demonstrated. Recently, IF-WS₂ nanoparticles were confined inside a porous and densified bronze-graphite matrix, prepared by powder metallurgy (PM) technique. Substantial reduction in both friction and wear, and an increase in the critical load were observed. Novel applications of IF nanoparticles as development of polymer nanocomposites, burnishing, and friction of ceramic materials under severe contact conditions were presented. Polymer nanococomposites with embedded IF nanoparticles exhibited excellent tribological properties. The main friction mechanism was the transfer of the IF nanoparticles from the surface of the polymer composite to the surface of the mating steel disk leading to a reduction in both the friction coefficient and wear rate of the contact surfaces. This is an abstract presented at the World Tribology Congress III (Washington, DC 9/12-16/2005).

Synthetic methods for the production of multiwall WS₂ nanotubes by sulfidizing WO₃ nanoparticles have been described and further progress is underway. A fluidized-bed reactor for the synthesis of ≤ 0.50 kg/day of fullerene-like WS₂ nanoparticles has been established, and the scaling-up of the synthesis to 100 kg/day is under way. The detailed mechanisms for the synthesis of fullerene-like WS₂ and MoS₂ nanoparticles and nanotubes of these compounds have been elucidated. Testing programs have indicated the usefulness of the fullerene-like WS₂ (MoS₂) as solid lubricants in various products and applications, e.g., automotive, machining, aerospace, electronics, medical and numerous other kinds of industries. This technology was licensed to NanoMaterials, which is involved in many collaborative development programs. Novel applications of inorganic nanotubes in the fields of microelectronics; Li rechargeable batteries; medical and opto-electronic are presented. This is an abstract of a paper presented at the AIChE Annual Meeting and Fall Showcase (Cincinnati, OH 10/30/2005-11/4/2005).

When dispersed in a synthetic polyalphaolefin (PAO) base oil, inorganic fullerene-like (IF-MS₂) nanoparticles of metal dichalcogenides (IF-MoS₂, IF-WS₂, IF-NbS₂) lead to a significant reduction of both friction and wear under boundary lubrication. The effect of the contact pressure on the tribological properties of IF nanoparticles is particularly interesting. Results show that the higher is the pressure, the lower is the friction coefficient. The effect of the concentration shows that, even used at a low concentration (0.1%wt), IF-MS₂ is able to decrease friction (0.05) compared to base oil only (0.08). A steady state friction coefficient of 0.04 was reached with IF-WS₂ at 1%wt in PAO. Friction-induced transformation of the IF-MS₂ nanoparticles into H-MS₂ single sheets was evidenced by High Resolution Transmission Electron Microscopy (HRTEM). Some of these superimposed sheets are found in incommensurate positions, thus possibly explaining the very low friction coefficient of 0.04 obtained with IF-WS₂. In-situ Raman spectroscopy was performed during a friction test to follow this structural modification. The lubrication mechanism of IF-MS₂ is very similar to a "drug delivery system". A very low concentration of additives is sufficient and the activation is obtained by the opening of the nested structure, like in certain micellar structures. Furthermore, no chemical reaction is required to obtain interesting properties. Thus, fullerene-like nanoparticles are active at the very beginning of the test and even at ambient and low temperature.

Adding guest atoms to inorganic nanotubes, known as 'doping', influences their room-temperature magnetic properties - properties that could be exploited in 'spintronic' devices and computer memory.

A study of the tribological behavior of nested inorganic fullerene-like (IF) nanoparticles of WS₂, as a potential additive to base oils is presented. Friction measurement results obtained from three different test rigs over a wide range of normal loads and sliding velocities are shown. Stribeck curves are used to reveal the lubrication regimes where the IF are most effective. It is found that the addition of IF-WS₂ nanoparticles to the base oils results in up to 50% reduction in friction coefficient in the mixed lubrication regime. The mechanism of improved friction and wear behavior with the IF additive is discussed.

Inorganic fullerene-like materials have been identified as being of potentially utmost importance for many industrial applications. MoS₂ and WS₂ hollow nanoparticles have been identified as strong candidates for tribological applications such as solid lubricants. The main goal of this work was to evaluate the mechanical properties of solid lubricant particles in ensemble under hydrostatic pressure. The behavior of nanopowders under compression has been described on the basis of constitutive models of continuum mechanics. The model will be applied to an isotropic compaction of copper (well-studied medium), fullerene-like (IF-WS₂) nanoparticles and a natural powder of 2H-WS₂ platelets. The morphology of individual nanoparticles and nanoparticle ensembles will be examined and discussed. Another aspect of this work was to study the applicability and limitations of the proposed constitutive model for the understanding of the tribological behavior of solid lubricant nanoparticles. Compression with the maximal pressure (500 MPa) showed that the shape of the IF nanoparticles is preserved. The dominant mechanism of damage was found to be the delamination or peeling-off of the external sheets of hollow nanoparticles. Strong destruction of 2H-WS₂ platelets was observed under compression.