Publications

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.

Misfit layered compounds (MLC) with the composition (LaS)(1.15)TaS2 (for simplicity denoted as LaS-TaS2) and LaS-NbS2 were prepared and studied in the past. Nanotubes of LaS-TaS2 could be easily synthesized, while tubular structure of the LaS-NbS2 were found to be rather rare in the product. To understand this riddle, quaternary alloys of LaS-NbxTa(1-x)S2 with ascending Nb concentration were prepared herein in the form of nanotubes (and platelets). Not surprisingly, the concentration of these quaternary nanotubes shrank (and the relative density of platelets increased) with increasing Nb content in the precursor. The structure and chemical composition of such nanotubes was elucidated by electron microscopy. Conceivably, the TaS2 in the MLC compounds LnS-TaS2 (Ln = lanthanide atom) crystallizes in the 2H polytype. High resolution transmission electron microscopy showed however that, invariably, MLC nanotubes prepared from 80 at% Nb content in the precursor belonged to the 1T polytype. Raman spectroscopy of individual tubes revealed that up to 60 at% Nb, they obey the standard model of MLC, while higher Nb lead to large deviations, which are discussed in brief. The analysis indicated also that such nanotubes do not exhibit the pattern assigned to charge density wave transition so typical for binary 1T-TaS2. The prospect for revealing interesting quasi-1D behavior of such quaternary nanotubes is also briefly discussed.

The surface-guided growth of horizontal nanowires (NWs) allows assembly and alignment of the NWs on the substrate during the synthesis, thus eliminating the need for additional processes after growth. One of the major advantages of guided growth over postgrowth assembly is the control on the NWs direction, crystallographic orientation, and position. In this study, we use the guided growth approach to synthesize high-quality, single-crystal, aligned horizontal ZnS NWs on flat and faceted sapphire surfaces, and show how the crystal planes of the different substrates affects the crystal structure and orientation of the NWs. We also show initial results of the effect of Cu doping on their photoluminescence. Such high-quality aligned ZnS NWs can potentially be assembled as key components in phosphorescent displays and markers due to their unique optical properties. The ZnS NWs have either wurtzite or zinc-blende structure depending on the substrate orientations and contain intrinsic point defects such as sulfur vacancies, which are common in this material. The crystallographic orientations are consistent with those of guided NWs from other semiconductor materials, demonstrating the generality of the guided growth phenomenon. The successfully grown ZnS NWs and the Cu doping are the first step toward the fabrication of optoelectronic devices based on ZnS nanostructures.

1D core-shell heterojunction nanostructures have great potential for high-performance, compact optoelectronic devices owing to their high interface area to volume ratio, yet their bottom-up assembly toward scalable fabrication remains a challenge. Here the site-controlled growth of aligned CdS-CdSe core-shell nanowalls is reported by a combination of surface-guided vaporliquid-solid horizontal growth and selective-area vapor-solid epitaxial growth, and their integration into photodetectors at wafer-scale without postgrowth transfer, alignment, or selective shell-etching steps. The photocurrent response of these nanowalls is reduced to 200 ns with a gain of up to 3.8 x 10(3) and a photoresponsivity of 1.2 x 10(3) A W-1, the fastest response at such a high gain ever reported for photodetectors based on compound semiconductor nanostructures. The simultaneous achievement of sub-microsecond response and high-gain photocurrent is attributed to the virtues of both the epitaxial CdS-CdSe heterojunction and the enhanced charge-separation efficiency of the core-shell nanowall geometry. Surface-guided nanostructures are promising templates for wafer-scale fabrication of self-aligned core-shell nanostructures toward scalable fabrication of high-performance compact photodetectors from the bottom-up.

We report here a unique and efficient methodology for the surface functionalization of closed-cage inorganic fullerene-like (IF) nanoparticles and inorganic nanotubes (INTs) composed of two-dimensional nanomaterials of transition-metal chalcogenides (MS
₂; M = W or Mo). The first step is the physical coverage of these robust inorganic materials with monodispersed and dense monolayers of gold, silver, and palladium nanoparticles. The structural continuity at the interface between the IF/INT and the metallic nanoparticles is investigated. Lattice matching between these nanocrystalline materials and strong chemical affinity lead to efficient binding of the metallic nanoparticles onto the outer sulfide layer of the MS
₂-based structures. It is shown that this functionalization results in narrowing of the IF/INT optical band gap, increased work function, and improved surface-enhanced Raman scattering. In the second step, functionalization of the surface-bound nanoparticles is carried out by a ligand-exchange reaction. This ligand exchange involving the tetraoctylammonium bromide capping layer and an alkyl thiol enhances the solubility (∼10×) of the otherwise nearly insoluble materials in organic solvents. The scope of this method is further demonstrated by introducing a ruthenium(II) polypyridyl complex on the surface of the surface-bound AuNPs to generate fluorescent multicomponent materials.

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.

Self-assembly of inorganic nanoparticles has been used to prepare hundreds of different colloidal crystals, but almost invariably with the restriction that the particles must be densely packed. Here, we show that non-close-packed nanoparticle arrays can be fabricated through the selective removal of one of two components comprising binary nanoparticle superlattices. First, a variety of binary nanoparticle superlattices were prepared at the liquid-air interface, including several arrangements that were previously unknown. Molecular dynamics simulations revealed the particular role of the liquid in templating the formation of superlattices not achievable through self-assembly in bulk solution. Second, upon stabilization, all of these binary superlattices could be transformed into distinct "nanoallotropes"-nanoporous materials having the same chemical composition but differing in their nanoscale architectures.

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.

The organization of nanowires on surfaces remains a major obstacle toward their large-scale integration into functional devices. Surface-material interactions have been used, with different materials and substrates, to guide horizontal nanowires during their growth into well-organized assemblies, but the only guided nanowire heterostructures reported so far are axial and not radial. Here, we demonstrate the guided growth of horizontal core-shell nanowires, specifically of ZnSe@ZnTe, with control over their crystal phase and crystallographic orientations. We exploit the directional control of the guided growth for the parallel production of multiple radial p-n heterojunctions and probe their optoelectronic properties. The devices exhibit a rectifying behavior with photovoltaic characteristics upon illumination. Guided nanowire heterostructures enable the bottom-up assembly of complex semiconductor structures with controlled electronic and optoelectronic properties.

New types of core-shell nanoparticles are reported: Pb@GaS fullerene-like and nanotubular structures, achieved via the continuously high reactor temperatures and ultra-hot strong-gradient annealing environments created by highly concentrated sunlight. Structural and chemical characterizations suggest a formation mechanism where vaporized Pb condenses into nanoparticles that are stabilized as they become covered by molten GaS, the ensuing crystallization of which creates the outer layers. Hollow-core GaS fullerene-like nanoparticles and nanotubes were also observed among the products, demonstrating that a single solar procedure can generate a variety of core-shell and hollow nanostructures. The proposed formation mechanisms can account for their relative abundance and the characterization data.

Iron formations deposited in marine settings during the Precambrian represent large sinks of iron and silica, and have been used to reconstruct environmental conditions at the time of their formation. However, the observed mineralogy in iron formations, which consists of iron oxides, silicates, carbonates and sulfides, is generally thought to have arisen from diagenesis of one or more mineral precursors. Ferric iron hydroxides and ferrous carbonates and silicates have been identified as prime candidates. Here we investigate the potential role of green rust, a ferrous-ferric hydroxy salt, in the genesis of iron formations. Our laboratory experiments show that green rust readily forms in early seawater-analogue solutions, as predicted by thermodynamic calculations, and that it ages into minerals observed in iron formations. Dynamic models of the iron cycle further indicate that green rust would have precipitated near the iron redoxcline, and it is expected that when the green rust sank it transformed into stable phases within the water column and sediments. We suggest, therefore, that the precipitation and transformation of green rust was a key process in the iron cycle, and that the interaction of green rust with various elements should be included in any consideration of Precambrian biogeochemical cycles.

We report here how the crystallinity of AuNPs and the choice of binding sites of molecular cross-linkers control their aggregation. The combination of different binding moieties (N-oxides, Ar_F-I) and the reactivity of the particles' facets allow control over the organization and crystallinity of the AuNP assemblies.

We report the synthesis and supporting density-functional-theory computations for a closed-cage, misfit layered-compound superstructure from PbS-SnS₂, generated by highly concentrated sunlight from a precursor mixture of Pb, SnS₂, and graphite. The unique reactor conditions created in our solar furnace are found to be particularly conducive to the formation of these nanomaterials. Detailed structural and chemical characterization revealed a spontaneous inside-out formation mechanism, with a broad range of nonhollow fullerene-like structures starting at a diameter of ∼20 nm and a wall thickness of ∼5 layers. The computations also reveal a counterintuitive charge transfer pathway from the SnS₂ layers to the PbS layers, which indicates that, in contrast to binary-layered compounds where it is principally van der Waals forces that hold the layers together, polar forces appear to be as important in stabilizing superstructures of misfit layered compounds. (Figure Presented).

Perfectly aligned horizontal ZnSe nanowires are obtained by guided growth, and easily integrated into high-performance blue-UV photodetectors. Their crystal phase and crystallographic orientation are controlled by the epitaxial relations with six different sapphire planes. Guided growth paves the way for the large-scale integration of nanowires into optoelectronic devices.

Nanotubes that are formed from layered materials have emerged to be exciting one-dimensional materials in the last two decades due to their remarkable structures and properties. Misfit layered compounds (MLC) can be produced from alternating assemblies of two different molecular slabs with different periodicities with the general formula [(MX)_1+x]_m[TX₂]_n (or more simply MS-TS₂), where M is Sn, Pb, Bi, Sb, rare earths, T is Sn, Nb, Ta, Ti, V, Cr, and so on, and X is S, Se. The presence of misfit stresses between adjacent layers in MLC provides a driving force for curling of the layers that acts in addition to the elimination of dangling bonds. The combination of these two independent forces leads to the synthesis of misfit layered nanotubes, which are newcomers to the broad field of one-dimensional nanostructures and nanotubes. The synthesis, characterization, and microscopic details of misfit layered nanotubes are discussed, and directions for future research are presented. (Figure Presented).

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.

The guided growth of horizontal nanowires has so far been demonstrated on a limited number of substrates. In most cases, the nanowires are covalently bonded to the substrate where they grow and cannot be transferred to other substrates. Here we demonstrate the guided growth of well-aligned horizontal GaN nanowires on quartz and their subsequent transfer to silicon wafers by selective etching of the quartz while maintaining their alignment. The guided growth was observed on different planes of quartz with varying degrees of alignment. We characterized the crystallographic orientations of the nanowires and proposed a new mechanism of "dynamic graphoepitaxy" for their guided growth on quartz. The transfer of the guided nanowires enabled the fabrication of back-gated field-effect transistors from aligned nanowire arrays on oxidized silicon wafers and the production of crossbar arrays. The guided growth of transferrable nanowires opens up the possibility of massively parallel integration of nanowires into functional systems on virtually any desired substrate.

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.

WS ₂ nanostructures hold structural characteristics which suggest they will be suitable for heterogeneous catalysis in the hydrodesulfurization (HDS) process. In this work, WS ₂ nanotubes (INT-WS ₂) were coated with cobalt nanoparticles using electroless plating method. Prior to cobalt deposition, the nanotubes surface was activated using palladium seeding process. The deposited cobalt nanoparticles had hcp crystal structure and formed non-uniform layer on the nanotubes surface. The catalytic reactivity of the produced cobalt coated nanotubes toward thiophene decomposition was characterized by an atmospheric flow reactor. The coated nanotubes revealed good catalytic reactivity toward thiophene mineralization. Further, the adsorption kinetics of thiophene on coated INT-WS ₂ was studied by thermal desorption spectroscopy (TDS). The cobalt coated system was found to be more catalytically active than the pristine INT-WS ₂ system. This result is promising since further optimization of the nanofabrication process of the catalyst should increase the conversion rates even further.

Photon antibunching is ubiquitously observed in light emitted from quantum systems but is usually associated only with the lowest excited state of the emitter. Here, we devise a fluorophore that upon photoexcitation emits in either one of two distinct colors but exhibits strong antibunching between the two. This work demonstrates the possibility of creating room-temperature quantum emitters with higher complexity than effective two level systems via colloidal synthesis.

Roll 'em up, move 'em out: The growth of SnS ₂ and SnS ₂/SnS nanotubes and nanoscrolls with ordered superstructures is promoted by the relaxation of stress between adjacent SnS ₂ and SnS layers. Partial decomposition of the SnS ₂ precursor to more sulfur-deficient SnS was manifested in the exfoliation of layers and scrolling. The presence of the two main structures (see picture) was confirmed by HRTEM and Raman spectroscopy.

The large-scale assembly of nanowires with controlled orientation on surfaces remains one challenge preventing their integration into practical devices. We report the vapor-liquid-solid growth of aligned, millimeter-long, horizontal GaN nanowires with controlled crystallographic orientations on different planes of sapphire. The growth directions, crystallographic orientation, and faceting of the nanowires vary with each surface orientation, as determined by their epitaxial relationship with the substrate, as well as by a graphoepitaxial effect that guides their growth along surface steps and grooves. Despite their interaction with the surface, these horizontally grown nanowires display few structural defects, exhibiting optical and electronic properties comparable to those of vertically grown nanowires. This paves the way to highly controlled nanowire structures with potential applications not available by other means.

Thermoplastic nanocomposites, based on high-density polyethylene, polyamide 6, polyamide 66, poly(butylene terephthalate), or polycarbonate and containing multiwalled carbon nanotubes (CNTs), were compounded with either neat CNTs or commercial CNT master batches and injection-molded for the evaluation of their electrical, mechanical, and thermal properties. The nanocomposites reached a percolation threshold within CNT concentrations of 2-5 wt %; however, the mechanical properties of the host polymers were affected. For some nanocomposites, better properties were achieved with neat CNTs, whereas for others, master batches were better. Then, polycarbonate and poly(butylene terephthalate), both with a CNT concentration of 3 wt %, were injection-molded with a screening design of experiments (DOE) to evaluate the effects of the processing parameters on the properties of the nanocomposites. Although only a 10-run screening DOE was performed, such effects were clearly observed. The volume resistivity was significantly dependent on the working temperature and varied up to 4 orders of magnitude. Other properties were also dependent on the processing parameters, albeit in a less pronounced fashion. Transmission electron microscopy indicated that conductive samples formed a percolation network, whereas nonconductive samples did not. In conclusion, injection-molding parameters have a significant impact on the properties of polymer/CNT nanocomposites, and these parameters should be optimized to yield the best results. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 70-78, 2011

Aversatile scheme for the preparation of nanoparticle (NP) multilayers is presented. The method is based on the step-by-step assembly of NPs and bishydroxamate disulfide ligand molecules by means of metal-organic coordination using easily synthesized tetraoctylammonium bromide (TOAB)-stabilized gold NPs. The assembly of NP multilayers was carried out via a Zr(IV)-coordinated sandwich arrangement of the hydroxamate ligands on Au and glass surfaces. The latter were precoated with electrolessly deposited Au clusters to enable binding of the first NP layer. The new method avoids the need to perform elaborate colloid reactions to prepare the NP building blocks. Au NP monolayer and multilayer films prepared in this manner were characterized by UV-vis spectroscopy, atomic force microscopy (AFM), and cross-sectional transmission electron microscopy (TEM), showing a regular growth of NP layers. The use of coordination chemistry as the binding motif between repeat layers allows for the convenient assembly of hybrid nanostructures comprising molecular and NP components. This was demonstrated by the construction of Au NP multilayers with controlled spacing from the surface or between two NP layers. Drying the samples during or after the construction process induces NP aggregation and changes in the film morphology and optical properties.

The crystalline perfection of wurtzite InAs nanowires grown by the Vapor-Liquid-Solid Molecular Beam Epitaxy technique in combination with careful fabrication of nanowire-based FET devices allowed us to observe a variety of phenomena associated with mesoscopic coherent transport. When the single nanowire channel is nearly pinched-off the Coulomb blockade conductance oscillations exhibit well-pronounced Kondo effect approaching the conductance unitary limit. At some gate voltages the breaking of odd-even parity of the Kondo effect related to the formation of the triplet ground state is observed. At higher gate voltages when the channel is open we observe the Fabry-Perot type conductance oscillations. The length of the Fabry-Perot electron resonator deduced from the period of the oscillations is in agreement with the physical length of the nanowire device.

Low concentrations (0.1-1% of the Zn concentration) of Sb ions in an alkaline ZnO chemical bath deposition solution were found to lead to pronounced changes in the ZnO film morphology. The tapered nanorods obtained in the absence of Sb become flat-topped and more closely packed when Sb is present, and the nanorod diameter changes, the direction of these changes depending on other bath parameters. An initial compact layer that was comprised of very small nanocrystals is formed. Sb was found to be present in the films, mostly in the compact initial layer. We postulate that initial Sb-rich nuclei promote ZnO nucleation but retard subsequent ZnO crystal growth. As the Sb concentration (relative to the Zn) in the bath drops, ZnO nanorods can then grow, but with altered morphology because of preferential adsorption of the low levels of Sb in solution on (002) ZnO crystal faces. These films were found to be very suitable for solid state semiconductor-sensitized solar cells. Such cells normally require a separate deposition of a blocking compact layer before the nanoporous layer. The initial in situ compact layer in our films is very efficient for this purpose, giving much more reproducible performance.

We report on observation of coherent electron transport in suspended high-quality InAs nanowire-based devices. The InAs nanowires were grown by low-temperature gold-assisted vapor-liquid-solid molecular-beam-epitaxy. The high quality of the nanowires was achieved by removing the typically found stacking faults and reducing possibility of Au incorporation. Minimizing substrate-induced scattering in the device was achieved by suspending the nanowires over predefined grooves. Coherent transport involving more than a single one-dimensional mode transport was observed in the experiment and manifested by Fabry-Pérot conductance oscillations. The length of the Fabry-Pérot interferometer, deduced from the period of the conductance oscillations, was found to be close to the physical length of the device. The high oscillations visibility imply nearly ballistic electron transport through the nanowire.

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.

"Figure Presented" Stolen identity: The molecular geometries of a series of cross-linkers that bear between one and four pyridyl moieties are expressed in the optical properties of AuNP assemblies (see picture). TEM analysis indicates that the molecular-level structural differences of the cross-linkers are also transferred at the submicrometer level in the formation of the AuNP assemblies.

Magnetic susceptibility measurements of WO3 crystals hydrogen doped on the surface suggest 2D local non-percolated superconductivity with an onset temperature of 120 K. Observed zero field cooled vs. field cooled magnetization response is characteristic of type II superconductivity. The diamagnetic response at the critical temperature is field dependent, and is suppressed by a magnetic field of similar to 1700 Oe.

Multiwall WS₂ nanotube templates were used as hosts to prepare core-shell Pbl₂@WS₂ nanotubes by a capillarywetting method. Conformal growth of Pbl₂ layers on the inner wall of the relatively wide WS₂ nanotubes (i.d. ca.10 nm) leads to nanotubular structures which were not previously observed in narrow carbon nanotube templates. Image simulation after structural modeling (see picture) showed good agreement with the experimental HRTEM image.

The growth of thin GaAs and InAs nanowires (NWs) was studied both theoretically and experimentally. InAs and GaAs NWs grown by the Vapor Liquid Solid (VLS) method were studied by a high resolution transmission electron microscope (HRTEM). The wurtzite (wz) structure is dominant in such wires with occasional stacking faults (SFs) resulting from the intermixing of zinc-blended (zb) stacking. Growth direction is 〈0001〉 and 〈111〉 for the wz and the zb type sections of the NW, respectively. Nevertheless, we find that for NWs thinner than ca 100 Å, the wz/zb SFs do not appear. Using ab initio methods, we studied the stability of the structure of the NWs; in particular the competition between wz and zb phases. We have found that for the diameters of up to 50 Å the most stable NWs adopt the wz 〈0001〉 structure. For NWs with the diameter larger than 100 Å the free energies of wz and zb become nearly equal, which explains the occasional occurrence of SFs observed in as grown NWs.

Inorganic fullerene-like (IF) Mo_1-xRe_xS₂ and W_1-xRe_xS₂ nanoparticles have been synthesized by a gasphase reaction involving the respective metal halides with H₂S. The IF-Mo(W)_1-xRe_xS₂ nanoparticles, contain - ing up to 5% Re, were characterized by a variety of experimental techniques. Analyses of the X-ray powder diffraction and different electron microscopy techniques show that the Re is doped in the MoS₂ host lattice. Interestingly, Re-dop ed MoS₂ nanotubes are present as well, although in small quantities (∼5%). XPS results confirm the nanoparticles to be more n-type arising from the effect of Re doping. Additionally, density-functional tight-binding (DFTB) calculations support the observed n-type behavior.

The microstructure and composition of the interfacial layer between chemically deposited PbSe and GaAs substrates were studied using high-resolution transmission electron microscopy (HRTEM), Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS) and energy-filtered TEM. The thickness of the interfacial layer varied significantly from direct contact of the film with the substrate to 5 nm in the thickest regions. The results established the presence of a discontinuous, amorphous intermediate layer of Ga(2)O(3) at the PbSe/GaAs interface. Copyright (C) 2008 John Wiley & Sons, Ltd.

Inorganic fullerene-like (IF) nanoparticles and nanotubes of WS₂ were synthesized by a gas phase reaction starting from WCl_n (n = 4, 5, 6) and H₂S. The effect of the various metal chloride precursors on the formation of the products was investigated during the course of the study. Various parameters have been studied to understand the growth and formation of the IF-WS₂ nanoparticles and nanotubes. The parameters that have been studied include flow rates of the various carrier gases, heating of the precursor metal chlorides and the temperature at which the reactions were carried out. The best set of conditions wherein maximum yields of the high quality pure-phase IF-WS₂ nanoparticles and nanotubes are obtained have been identified. A detailed growth mechanism has been outlined to understand the course of formation of the various products of WS₂.

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.

Charcoal produced in natural fires is widespread, but surprisingly little is known about its structure and stability. TEM and electron energy loss spectroscopy (EELS) were used to characterize the organized graphitelike microcrystallites and amorphous nonorganized phases of modern charcoal that had been produced in natural fires. In addition, a semiordered structure was identified in two modern charcoal samples. Fossilized charcoal contains fewer graphite-like microcrystallites than modern samples. EELS spectra confirmed that the dominant structure in fossilized charcoal is amorphous carbon. EELS measurements also revealed that only the nonorganized phase contains oxygen, which indicates that the degradation of the fossilized charcoal structure occurs mainly through oxidation processes. The few graphite-like microcrystallites found in fossilized charcoal were composed of onion-like structures that are probably less prone to oxidation owing to their rounded structures.

It is well accepted by now that nanoparticles of inorganic layered compounds form closed-cage structures (IF). In particular closed-cage nanoparticles of metal dihalides, like NiCl₂, CdCl₂ and CdI₂ were shown to produce such structures in the past. In the present report IF-NiBr₂ polyhedra and quasi-spherical structures were obtained by the evaporation/recrystallization technique as well as by laser ablation. When the nanoclusters were formed in humid atmosphere, nickel perbromate hydrate [Ni(BrO₄)₂(H₂O)₆] polyhedra and short tubules were produced, as a result of a reaction with water. Nanooctahedra of NiBr₂ were found occasionally in the irradiated soot. The reoccurrence of this structure in the IF family suggests that it is a generic one. Consistent with previous observations, this study showed that formation of the IF materials stabilized the material under the electron-beam irradiation. The growth mechanism of these nanostructures is briefly discussed.

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.

We show how simple mechanical agitation of precipitated CdSe quantum dot aggregates causes partially reversible color changes (clearly visible to the eye) in the absorption spectrum of the CdSe (about 4 nm size). The color changes, which are due to changes in size quantization, are not accompanied by change in quantum dot size. This phenomenon is explained by partial deaggregation of the precipitates, leading to reduced charge overlap between neighboring dots. Shaking was shown to result in a looser aggregate structure. It is suggested that CdSO ₃ particles (an initial product of the CdSe formation reaction) act as weak bridges between CdSe quantum dots, mediating the interparticle interactions and allowing the deaggregation to occur on shaking.

Bi₂Se₃ nanorods have been synthesized through a simple hydrothermal reduction approach. The nanorods formed were ≈10nm in diameter and 100-200nm in length. XRD characterization suggested that the product consisted of the hexagonal phase of pure Bi₂Se₃. EDX and XPS studies further confirmed the composition and purity of the product. A possible mechanism for the reaction is proposed, where Bi₂Se ₃ microsheets are presumed to be the intermediate for the formation of the nanorods. The effect of solvent on the morphology of the final product is discussed, where, in the presence of aprotic solvent DMF, nanoparticle formation is observed. A bandgap of 2.25eV is observed from the UV-visible absorption spectra.

Motivated by the discovery of the C₆₀ molecule (buckminsterfullerene), the search for inorganic counterparts of this closed-cage nanostructure started in 1992 with the discovery of nested fullerene-like nanoparticles of WS₂. Inorganic fullerene-like (IF) materials have since been found in numerous two-dimensional compounds and are available in a variety of shapes that offer major applications such as in lubricants and nanocomposites. Various synthetic methodologies have been employed to achieve the right conditions for the constricted or templated growth needed for the occurrence of this new phase. In this study, IF-TaS₂ is produced from a volatile chloride precursor in the gas phase and in small yield by room temperature laser ablation both in argon and in liquid CS ₂. For the gas-phase reaction, a high yield of IF nanoparticles was obtained between 400 and 600°C with a low concentration of the precursor gas. The average size and the yield of the IF-TaS₂ nanoparticles decrease with temperature. Above 600°C, IF nanoparticles were found in low yields and at sizes below 20 nm. The stability of the IF nanoparticles produced by the gas-phase reaction is discussed in the light of two existing theoretical models. Laser ablation in argon leads to IF nanoparticles filled with clusters of TaS₂. Agglomeration of the nanoparticles can be avoided by laser ablation in liquid CS₂.

Metal nanoparticle nanotubes (NPNTs) have been introduced by us as a new class of template-synthesized, nanoparticle-based nanotubes possessing unique features such as room-temperature preparation, highly corrugated wall structure, electrical conductivity, mechanical stability, and defined optical absorbance. The nanotubes are prepared by passing a citrate-stabilized metal (Au, Ag) colloid solution through the pores of an aminosilane-modified nanoporous alumina membrane. The nanoparticles (NPs) aggregate, forming multilayers on the pore walls, and undergo spontaneous room-temperature coalescence to afford solid, porous, multiwall metallic nanotubes. Self-sustained NPNTs are obtained by membrane dissolution. It is shown that the nanotubes are formed in two stages, i.e., NP accumulation and initial coalescence in the wet stage, and final solidification upon drying, both crucial to their formation. The NPNT synthetic scheme is extended here to the construction of composite NPNTs, i.e., formation of bimetallic Au-Pd NPNTs using a mixed colloid solution. High-resolution transmission electron microscopy (HRTEM) of single-metal and composite NPNTs indicates actual coalescence and creation of metallic interfaces between individual NPs, with lattice continuation that extends into the NP bulk.

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.

Cesium oxides are materials of great interest to the photodetection industry because of their relatively low work function (∼1 eV). Used mainly as coating films for photoemissive devices, they provide high wavelength thresholds and high photocurrents. However, they are unstable, air-sensitive, and hygroscopic, rendering them short-lived and limiting their applications. Although the technology of these devices is highly developed, their characterization on the micro- and nanoscale suffers from their poor chemical stability and poor crystallinity. In the present study, cesium oxides were synthesized from the elements and were characterized using a combination of chemical and structural analysis techniques. Because the reaction products were extremely sensitive to humidity, sample analysis without atmospheric exposure was essential, and techniques were developed for the transfer of the samples to the measurements systems. Extensive data obtained from X-ray energy-dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), X-ray diffraction (XRD), and Raman microscopy were obtained. Raman spectra with bands at 103, 742, and 1134 cm^-1 strongly confirmed the presence of the oxide, peroxide, and Superoxide ions, respectively, as well as the absence of carbonate as an impurity. The A_1g mode of Cs₂O was detected as an anti-Stokes band at 103 cm^-1. This study provides further insight into the reactivity of the various cesium oxides.

Inorganic fullerene-like structures (IF) of the layered hafnium sulfide, Hf₂S, have been synthesized by the laser ablation of HfS₃ in tert-butyl disulfide medium. Apart from the Hf₂S IFs exhibiting quasi-spherical as well as faceted nested-shell geometries, quasi-spherical nanoparticles of HfS were observed by this means. Whereas Hf₂S has anti-NbS₂ structure with S layers sandwiched between two Hf layers, HfS has the nonlayered WC-type structure. The nanoparticles of HfS show excess sulfur in the core, and they do not possess closed-shell geometry. The mechanism of formation of these nanoparticles has also been discussed.

Tin disulfide pellets were laser ablated in an inert gas atmosphere, and closed cage fullerene-like (IF) nanoparticles were produced. The nanoparticles had various polyhedra and short tubular structures. Some of these forms contained a periodic pattern of fringes resulting in a superstructure. These patterns could be assigned to a superlattice created by periodic stacking of layered SnS₂ and SnS. Such superlattices are reminiscent of misfit layer compounds, which are known to form tubular morphologies. This mechanism adds up to the established mechanism for IF formation, namely, the annihilation of reactive dangling bonds at the periphery of the nanoparticles. Additionally, it suggests that one of the driving forces to form tubules in misfit compounds is the annihilation of dangling bonds at the rim of the layered structure.

It was shown previously that nanoparticles of layered comppunds form closed-cage and nanotubular structures. These nanostructures received the generic name of inorganic fullerene-like (IF) materials. In particular nanoparticles of metal dihalides, like NiCl₂ and CdCl₂, were shown to form such closed-cage structures in the past. In the present report faceted IF-CdI₂ nanoparticles encapsulating a Cd core are reported. These nanoparticles were obtained using electron beam irradiation of the source powder of CdI₂. The faceted nanoparticles exhibit one of two morphologies: hexagonal or elongated rectangular characters. The nanoparticles can be considered as core (Cd)-shell (CdI₂) nanostructures. Consistent with previous observations, this study shows that the seamless structure of the IF materials can stabilize phases, which are otherwise unstable Under the electron-beam irradiation. The growth mechanism of these nanostructures is briefly discussed.

A new method of laser-induced lithography for direct writing of carbon on a glass surface is described, in which deposition occurs from a transparent precursor solution. At the glass-solution interface where the laser spot is focused, a micro-explosion process takes place, leading to the deposition of pure carbon on the glass surface. Transmission electron microscopy (TEM) analysis shows two distinct co-existing phases. The dominant one shows a mottled morphology with diffraction typical of cubic (sp³) diamond. The other region shows an ordered array of graphene sheets with diffraction pattern typical of sp²-bonded carbon. The sp³ crystallites range in size from 9 to 30 Å and are scattered randomly throughout the sample. A UV Raman spectrum shows a broad band at the location of the expected diamond peak, together with a peak corresponding to the graphite region. We conclude that the patterned carbon is composed of a mixture of nanocrystalline sp³ and sp² carbon forms.

Semiconductor nanocrystals can be epitaxially electrodeposited onto single-crystal substrates. The lateral size of the nanocrystals was previously shown to be controlled mainly by the lattice mismatch between the substrate and semiconductor. Here we show that, although the lateral dimensions of the nanocrystals are only slightly dependent on the current density and temperature of deposition, the vertical dimension is strongly dependent on these parameters. This allows control of the shape and aspect ratio of the nanocrystals, from spherical at high deposition current to taller crystals with a shape between columnar and square pyramidal with rounded tops at low currents. The shape of the taller crystals is explained by considering the reduced role of kinetic factors at low current density. Cross-sectional TEM is used to image the nanoparticle shape, while photoelectrochemical spectral measurements allow approximate band gap values to be estimated for the various nanocrystals.

The magnetization of an ensemble of isolated lead grains of sizes ranging from 4 to 1000 nm is measured. A sharp disappearance of the Meissner effect with a lowering of the grain size is observed for the smaller grains. This is a direct observation by magnetization measurement of the occurrence of a critical particle size for superconductivity, which is consistent with Andersons criterion.

Recently, it has been established that WS₂ and MoS₂ nanoparticles (inorganic fullerene-like, IF) mixed with oil, and impregnated into porous matrix of powdered materials appear to enhance the tribological properties of mating surfaces in definite loading range in comparison to typical metal dichalcogenide solid lubricants. The main results have been obtained under relatively low pressures. It is important to evaluate the tribological properties of IF when the concentrated contact is obtained. The effect of the IF in oil was studied using pin-on-disk tester in the regime of mixed lubrication. The interaction between the full film and the asperity contact fractions has been considered and the time evolution of the friction force was evaluated. The states of the mating surfaces and the nanoparticles before and after the friction test were studied by transmission electron microscopy (TEM), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). It was established that the IF nanoparticles mixed with oil allow to decrease the fraction of straight asperity contact under mixed lubrication regime and thus improve their tribological properties. TEM analysis showed that the shape of the IF nanoparticles is preserved under low loads. It was found that some of the IF nanoparticles were damaged after the friction at the maximal load of 420 N. The IF nanopaticles appear to form a protective film allowing increased load capacity of the rubbed pairs. The role of the IF solid lubricants as a part of a third body is discussed in this work. The mechanism of friction and wear of the IF nanoparticles are discussed.

Elliptically shaped (Pb_1-xCd_x)S nanoparticles (NPs) of average size 2.3 × 2.9 nm (minor axis × major axis) have been prepared via reaction of a solid [oligo(p-phenylene-ethynylene) dicarboxylate]Pb_0.9Cd_0.1 salt matrix, with gaseous H₂S. A significantly long emission lifetime, with multi-exponential behavior, is detected in time-resolved photoluminescence measurements, substantially different from the decay patterns of pure PbS and CdS NPs within the same organic matrix. Evidence for the co-existence of Cd and Pb within the same particle is provided by light-induced X-ray photoelectron spectroscopy.

(matrix presented) Palladium 15-20 nm particles stabilized by a Keggin-type polyoxometalate were prepared by reduction of K₅PPdW₁₁O₃₉ with H₂. The nanoparticles were shown to be effective catalysts for Suzuki-, Heck-, and Stille-type carbon-carbon coupling and carbon-nitrogen coupling reactions of bromoarenes in aqueous media. Chloroarenes were also reactive in reaction media without solvent.

The synthesis of multigram portions of WS₂ nanotubes and large foils consisting of aligned nanotubes was presented. Nanotubes and fullerene-like nanoparticles of WS₂ were observed by annealing very thin films of tungsten in a H₂S atmosphere. It is found that a slow in-diffusion of the sulfur and out-diffusion of the oxygen leads to a conversion of the oxide core into a multilayer WS₂ nanotube.

Layered metal disulfides - MoS2 and WS2 in the form of fullerene-like (IF) nanoparticles and in the form of platelets (crystallites of the 2H polytype) have been intercalated by exposure to alkali metal (potassium and sodium) vapor using a two-zone transport method. The composition of the intercalated systems was established using X-ray energy dispersive spectrometer (EDS) and X-ray photoelectron spectroscopy (XPS). X-ray powder diffraction (XRD) analysis and transmission electron microscopy (TEM) of the samples, which were not exposed to the ambient atmosphere, showed that they suffered little change in their lattice parameters. On the other hand, after exposure to ambient atmosphere, substantial increase in the interplanar spacing (3-5 Angstrom) was observed for the intercalated phases. Insertion of one to two water molecules per intercalated metal atom was suggested as a possible explanation for this large expansion along the c-axis. The modifications in magnetic and transport properties of the intercalated materials were investigated, and are believed to occur via charge transfer from the alkali metal to the conduction band of the host lattice. Restacking of the MS2 layers after prolonged exposure to the atmosphere and recovery of the pristine compound properties were observed as a result of deintercalation of the metal atoms.

NbS₂ nanoparticles with a closed cage structure (inorganic fullerene-like phase - IF) were synthesized by reaction between NbCl₅ vapor and H₂S gas in a reducing atmosphere at 400°C, and subsequent annealing under a H₂S/H₂ atmosphere at 550°C. Following the synthesis, the nanoparticles were found to have a large excess of Nb; they were agglomerated; highly dislocated and enfolded by amorphous material. After annealing, most of the amorphous material crystallized into closed NbS₂ shells, and the nanoparticles appeared to be much more faceted. Transmission electron microscopy revealed that the interlayer spacing (c/n) of the annealed particles had decreased. They were also agglomerated, being connected to neighboring nanoparticles through their outer NbS₂ layers after annealing. In related experiments Nb₂O₃ nanofibers, which organize into a nanoflower superstructure, and also NbS₂ nanofibers were synthesized under somewhat different growth conditions.

Recently, pure phases of hollow multi-walled WS₂ nanotubes were prepared from a surface reaction with reduced tungsten oxide nanowhiskers. During the process, which starts with WO_3-x nanoparticles and finishes with WS₂ nanotubes, particular care has been devoted to the evolution of the tungsten oxide cores once the first encapsulating WS₂ layer has been formed. The reduced tungsten oxide phases were previously studied by a combination of techniques, including high-resolution transmission electron microscopy and X-ray diffraction. In the present study, Raman spectroscopy combined with the previous two techniques is used to give further detail concerning the structure of the reduced oxide phases. This study sheds some further light on the reduction process of the tungsten suboxide phases and the growth mechanism of oxide nanowhiskers and subsequently WS₂ nanotubes from quasi-isotropic tungsten oxide nanoparticles.

The absolute orientations of the amphiphilic molecules α-hydroxy ω-bromo alcohols BrC_nH_2n OH, n= 21, 22, and the alkyl hydroxy esters C_mH_2m+1COO(CH₂)_nOH, m = 14, 15, n = 10, in crystalline monolayer forms on water have been determined, the former by grazing incidence X-ray diffraction (GIXD) and the latter by sum frequency generation (SFG). The assignment was made for the alkyl hydroxy esters by establishing the polar angle between the terminal CH₃-C bond and the normal to the plane of the monolayer; for the bromo alcohols the assignment was made by a determination of the two-dimensional crystal structure via X-ray structure factor calculations. The SFG results are in agreement with reported GIXD and lattice energy analyses of the alkyl hydroxy esters m = 19, n = 9, 10. These studies have further revealed the absolute orientation of the alcohol C-OH bonds at the water surface, which in turn can be correlated with the ice-nucleating behavior of the monolayers on supercooled water drops in terms of the odd and even values of n.

CdS and Cd_1-xMn_xS nanoparticles arranged in patterns within acrylamide were prepared via the topotactic reaction of the corresponding metal thioalkanoates with gaseous ammonia. The organization of the particles was improved in some systems and induced in others in the presence of alkanoates that act as site-directing nucleating centers.

The preparation of lead sulfide and cadmium sulfide quantum size particles arranged in periodic layers within organic matrices is described. Superlattices of the particles have been generated in crystals or thin films of long chain amphiphilic acids by topotactic gas-solid reactions of the lead or cadmium salts, that pack in layer structures, with H₂S gas. X-ray powder diffraction and transmission electron microscopy reveal that the crystalline order of the reactant has been partially retained in the organic-inorganic composite. This approach is demonstrated by examples of organic acids bearing an aromatic ring along the hydrocarbon chain, such as derivatives of phenyl propionic and benzoic acids.

Monolayers of the long-chain alcohols on water promote nucleation of ice. In order to determine the minimum size of crystalline alcohol monolayer domains that induce ice nucleation, we reduced their size in two distinct ways. One approach encompassed embedding the pure hydrocarbon alcohol, C_nH_2n+1OH(n = 20, 31), into a matrix of an immiscible monolayer of perfluoro alcohol C₁₀F₂₁C₂H₄OH, which is inert as an ice nucleator. The second set of experiments involved the introduction of random defects into the monolayer crystalline domains of C₂₉H₅₉OH through the use of the fully miscible guest alcohol molecule, C₃₁H₆₃OH, as additive. By these methods we estimated that ∼450 water molecules are necessary to form a stable ice cluster at the onset of induced ice nucleation at a temperature just below 0°C.

Tailor-made auxiliaries for the control of nucleation and growth of molecular crystals may be classified into two broad categories: inhibitors and promoters. Tailor-made inhibitors of crystal growth can be used for a variety of purposes, which include morphological engineering and etching, reduction of crystal symmetry, assignment of absolute structure of chiral molecules and polar crystals, elucidation of the effect of solvent on crystal growth, and crystallization of a desired polymorph. As for crystal growth promoters, monolayers of amphiphilic molecules on water have been used to induce the growth of a variety of three-dimensional crystals at the monolayer-solution interface by means of structural match, molecular complementarity or electrostatic interaction. A particular focus is made on the induced nucleation of ice by monolayers of water-insoluble aliphatic alcohols. The two-dimensional crystalline structures of such monolayers have been studied by grazing incidence X-ray diffraction. It has become possible to monitor, by this method, the growth, dissolution and structure of self-aggregated crystalline nonolayers, and indeed multilayers, affected by the interaction of solvent molecules in the aqueous. suphase with the amphiphilic headgroups, and by the use of tailor-made amphiphilic additives.

Spreading of the bolaamphiphile alpha,omega-docosanediol, HO(CH2)(22)OH, on water results in spontaneous aggregation into embryonic 3-D crystallites. These crystallites have been analyzed by cryo-transmission electron microscopy, scanning force microscopy, X-ray reflectivity, and external reflection Fourier transform infrared spectroscopy. The molecules are aligned in an almost perpendicular fashion in a rectangular cell, and the crystallites are about five molecular layers thick.

The structure of an uncompressed monolayer of C₃₁H₆₃OH, which induces freezing of supercooled water drops at ∼-2°C, was studied by grazing incidence X-ray diffraction in the temperature range +6°C to freezing of the water subphase. As the water was cooled, the hydrocarbon chains underwent a change in tilt angle. The monolayer preserved its structure on the epitaxially-grown surface of hexagonal ice (after freezing) and, after the ice was melted, subsequent to the temperature being raised above 0°C. The coherence length of the observed ice crystallites parallel to the interface was about 25 Å, which is in agreement with the extent of match between the lattice of the monolayer and the ab lattice of ice. This yields some information on the critical size of the ice nucleus bound to the monolayer, just below 0°C.

Monolayers of aliphatic long chain alcohols (C_nH_2n+1OH) were found to induce nucleation of ice at temperatures approaching O°C in contrast to watersoluble alcohols which are efficient antifreeze agents. The corresponding fatty acids and alcohols with bulky hydrophobic groups, induce ice nucleation at temperatures lower by as much as 12°C. The freezing point induced by the amphiphilic alcohols was found to be sensitive not only to area per molecule but also to chain length and parity, reaching higher temperatures for monolayers with n odd. Grazing incidence Xray diffraction studies performed on some of these alcohols at the air/water interface (at 5°C and at zero pressure), demonstrated the formation of crystalline twodimensional clusters with a distorted hexagonal cell whose dimensions resemble those of hexagonal ice. The catalysis of ice nucleation by these alcohol monolayers may be rationalized in terms of the structural match between the monolayer domains and the ab layer of hexagonal ice.