Publications

We present a Raman study of the collective vibrations arising from the homogeneous bundling of single-walled carbon nanotubes and analyze the dependence of their vibrational coupling on the tube diameter using two systems, single-walled carbon nanotube coils and a monochiral carbon nanotube film. We report on two breathing-like modes for quasi-infinite bundles, compared to the single radial breathing mode characteristic for isolated tubes. The exciton-phonon coupling in these modes is probed with resonant Raman spectroscopy, revealing the same resonance energy for both breathing-like peaks. Our experimental findings align well with previously reported theoretical studies, demonstrating a 1/d scaling for all modes, as well as confirming the relative shift of the modes dependent on intertube interaction. These vibrations provide insight into the role of intertube lattice dynamics in two-dimensional THz-range phononic crystals.

The simultaneous control of the orientation and position of organic semiconductor nanowires remains a major challenge when integrating them into monolithic devices. In this study, tris(8-hydroxyquinoline)aluminum(III) (Alq3) molecules are self-assembled into single-crystalline nanowires with consistent orientation and predictable positions by selective-area graphoepitaxial growth. The nanowire orientation is determined by parallel nanogrooves on a periodically modified faceted sapphire surface, and the position is simultaneously defined using a shadow mask. Computational fluid dynamics simulations showed that the mass flow field over the sapphire surface is tailored by the mask, resulting in preferential nanowire nucleation around the hole centers and leaving sufficient free space for the subsequent growth. Accordingly, the number, length, and density of the nanowires can be controlled by adjusting the mask layout. The good alignment and predictable positions of these nanowires facilitated their subsequent device integration, eliminating laborious assembly steps and potential damage after nanowire growth. Measurements from an in situ integrated two-terminal device based on the Alq3 nanowires revealed that the nanowires exhibit a remarkable negative differential resistance and fast photoresponse in the UV region. Overall, selective-area graphoepitaxial growth provides a versatile protocol for fabricating site- and orientation-controlled organic semiconductor nanowires for the monolithic fabrication of nanowire-based devices.A versatile method for growing orientation-controlled organic nanowires with predictable positions is proposed by combining the selective shadow effect of microsized holes with the graphoepitaxial mechanism along periodic nanofeatures. Simulations confirmed that the selective-area graphoepitaxial growth is driven by the hole-tailored mass-flow field. An in-situ integrated device based on Alq3 nanowires exhibited an intriguing negative differential resistance and fast photoresponse.image

Strain relaxation mechanisms during epitaxial growth of core-shell nanostructures play a key role in determining their morphologies, crystal structure and properties. To unveil those mechanisms, we perform atomic-scale aberration-corrected scanning transmission electron microscopy studies on planar core-shell ZnSe@ZnTe nanowires on α-Al2O3 substrates. The core morphology affects the shell structure involving plane bending and the formation of low-angle polar boundaries. The origin of this phenomenon and its consequences on the electronic band structure are discussed. We further use monochromated valence electron energy-loss spectroscopy to obtain spatially resolved band-gap maps of the heterostructure with sub-nanometer spatial resolution. A decrease in band-gap energy at highly strained core-shell interfacial regions is found, along with a switch from direct to indirect band-gap. These findings represent an advance in the sub-nanometer-scale understanding of the interplay between structure and electronic properties associated with highly mismatched semiconductor heterostructures, especially with those related to the planar growth of heterostructured nanowire networks.

Polar materials display a series of interesting and widely exploited properties owing to the inherent coupling between their fixed electric dipole and any action that involves a change in their charge distribution. Among these properties are piezoelectricity, ferroelectricity, pyroelectricity, and the bulk photovoltaic effect. Here we report the observation of a related property in this series, where an external electric field applied parallel or anti-parallel to the polar axis of a crystal leads to an increase or decrease in its second-order nonlinear optical response, respectively. This property of electric-field-modulated second-harmonic generation (EFM-SHG) is observed here in nanowires of the polar crystal ZnO, and is exploited as an analytical tool to directly determine by optical means the absolute direction of their polarity, which in turn provides important information about their epitaxy and growth mechanism. EFM-SHG may be observed in any type of polar nanostructures and used to map the absolute polarity of materials at the nanoscale.

The interest in metal halide perovskites has grown as impressive results have been shown in solar cells, light emitting devices, and scintillators, but this class of materials have a complex crystal structure that is only partially understood. In particular, the dynamics of the nanoscale ferroelastic domains in metal halide perovskites remains difficult to study. An ideal in situ imaging method for ferroelastic domains requires a challenging combination of high spatial resolution and long penetration depth. Here, we demonstrate in situ temperature-dependent imaging of ferroelastic domains in a single nanowire of metal halide perovskite, CsPbBr3. Scanning X-ray diffraction with a 60 nm beam was used to retrieve local structural properties for temperatures up to 140 °C. We observed a single Bragg peak at room temperature, but at 80 °C, four new Bragg peaks appeared, originating in different real-space domains. The domains were arranged in periodic stripes in the center and with a hatched pattern close to the edges. Reciprocal space mapping at 80 °C was used to quantify the local strain and lattice tilts, revealing the ferroelastic nature of the domains. The domains display a partial stability to further temperature changes. Our results show the dynamics of nanoscale ferroelastic domain formation within a single-crystal perovskite nanostructure, which is important both for the fundamental understanding of these materials and for the development of perovskite-based devices.

The challenge of nanowire assembly is still one of the major obstacles toward their efficient integration into functional systems. One strategy to overcome this obstacle is the guided growth approach, in which the growth of in-plane nanowires is guided by epitaxial and graphoepitaxial relations with the substrate to yield dense arrays of aligned nanowires. This method relies on crystalline substrates which are generally expensive and incompatible with silicon-based technologies. In this work, we expand the guided growth approach into noncrystalline substrates and demonstrate the guided growth of horizontal nanowires along straight and arbitrarily shaped amorphous nanolithographic open guides on silicon wafers. Nanoimprint lithography is used as a high-throughput method for the fabrication of the high-resolution guiding features. We first grow five different semiconductor materials (GaN, ZnSe, CdS, ZnTe, and ZnO) along straight ridges and trenches, demonstrating the generality of this method. Through crystallographic analysis we find that despite the absence of any epitaxial relations with the substrate, the nanowires grow as single crystals in preferred crystallographic orientations. To further expand the guided growth approach beyond straight nanowires, GaN and ZnSe were grown also along curved and kinked configurations to form different shapes, including sinusoidal and zigzag-shaped nanowires. Photoluminescence and cathodoluminescence were used as noninvasive tools to characterize the sine wave-shaped nanowires. We discuss the similarities and differences between in-plane nanowires grown by epitaxy/graphoepitaxy and artificial epitaxy in terms of generality, morphology, crystallinity, and optical properties.

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.

Tri-gate transistors offer better performance than planar transistors by exerting additional gate control over a channel from two lateral sides of semiconductor nanowalls (or ``fins''). Here we report the bottom-up assembly of aligned CdS nanowalls by a simultaneous combination of horizontal catalytic vapor-liquid-solid growth and vertical facet-selective noncatalytic vapor-solid growth and their parallel integration into tri-gate transistors and photodetectors at wafer scale (cm(2)) without postgrowth transfer or alignment steps. These tri-gate transistors act as enhancement-mode transistors with an on/off current ratio on the order of 10(8), 4 orders of magnitude higher than the best results ever reported for planar enhancement-mode CdS transistors. The response time of the photodetector is reduced to the submicrosecond level, 1 order of magnitude shorter than the best results ever reported for photodetectors made of bottom-up semiconductor nanostructures. Guided semiconductor nanowalls open new opportunities for high-performance 3D nanodevices assembled from the bottom up.

Energy dissipation is a fundamental process governing the dynamics of physical, chemical and biological systems. It is also one of the main characteristics that distinguish quantum from classical phenomena. In particular, in condensed matter physics, scattering mechanisms, loss of quantum information or breakdown of topological protection are deeply rooted in the intricate details of how and where the dissipation occurs. Yet the microscopic behaviour of a system is usually not formulated in terms of dissipation because energy dissipation is not a readily measurable quantity on the micrometre scale. Although nanoscale thermometry has gained much recent interest(1-15), existing thermal imaging methods are not sensitive enough for the study of quantum systems and are also unsuitable for the low-temperature operation that is required. Here we report a nano-thermometer based on a superconducting quantum interference device with a diameter of less than 50 nanometres that resides at the apex of a sharp pipette: it provides scanning cryogenic thermal sensing that is four orders of magnitude more sensitive than previous devices-below 1 mu K Hz(-1/2). This non contact, non-invasive thermometry allows thermal imaging of very low intensity, nanoscale energy dissipation down to the fundamental Landauer limit(16-18) of 40 femtowatts for continuous readout of a single qubit at one gigahertz at 4.2 kelvin. These advances enable the observation of changes in dissipation due to single-electron charging of individual quantum dots in carbon nanotubes. They also reveal a dissipation mechanism attributable to resonant localized states in graphene encapsulated within hexagonal boron nitride, opening the door to direct thermal imaging of nanoscale dissipation processes in quantum matter.

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.

Owing to their mechanically tunable electronic properties, carbon nanotubes (CNTs) have been widely studied as potential components for nanoelectromechanical systems (NEMS); however, the mechanical properties of multiwall CNTs are often limited by the weak shear interactions between the graphitic layers. Boron nitride nanotubes (BNNTs) exhibit a strong interlayer mechanical coupling, but their high electrical resistance limits their use as electromechanical transducers. Can the outstanding mechanical properties of BNNTs be combined with the electromechanical properties of CNTs in one hybrid structure? Here, we report the first experimental study of boron carbonitride nanotube (BCNNT) mechanics and electromechanics. We found that the hybrid BCNNTs are up to five times torsionally stiffer and stronger than CNTs, thereby retaining to a large extent the ultrahigh torsional stiffness of BNNTs. At the same time, we show that the electrical response of BCNNTs to torsion is 1 to 2 orders of magnitude higher than that of CNTs. These results demonstrate that BCNNTs could be especially attractive building blocks for NEMS.

We report the guided growth of highly coherent, horizontal GaN nanowires (NWs) on atomically flat singular SiC (0001) and on periodically stepped vicinal SiC (0001) substrates. On singular SiC (0001) the NWs. grow in six symmetry-equivalent directions, while on vicinal SiC (0001) the NWs grow only in the two directions parallel to the atomic step edges. All of the NWs have the same epitaxial relations with the substrate on both singular and vicinal (0001). Owing to the low mismatch (similar to 3.4%) with the substrate, the NWs grow highly coherent, with a much lower density of misfit dislocations than previously observed on sapphire. This is also the first observation of NW VLS growth along atomic steps. Epitaxially coherent guided NWs have potential uses in many fields, including high-power electronics, light-emitting diodes (LEDs), and laser diodes.

We report the experimental and theoretical study of boron nitride nanotube (BNNT) torsional mechanics. We show that BNNTs exhibit a much stronger mechanical interlayer coupling than carbon nanotubes (CNTs). This feature makes BNNTs up to 1 order of magnitude stiffer and stronger than CNTs. We attribute this interlayer locking to the faceted nature of BNNTs, arising from the polarity of the B-N bond. This property makes BNNTs superior candidates to replace CNTs in nanoelectromechanical systems (NEMS), fibers, and nanocomposites.

In this work, we have studied the resonance Raman spectra of 17 single-wall carbon nanotube (SWNT) serpentines, using a 532 nm laser. Each serpentine consists of a series of straight, parallel, and regularly spaced segments, connected by alternating U-turns formed on top of crystalline miscut quartz. The interaction between the SWNT and the substrate affects the properties of nanotubes, generating different behaviors of the G-band. We observed variations in the G-band frequency along the SWNTs, which can be organized into three different groups, as a result of different types of interaction with the substrate, generated during the deposition of the SWNTs.

The organization of carbon nanotubes into well-defined straight or curved geometries and arrays on surfaces is a critical prerequisite for their integration into nanocircuits and a variety of functional nanosystems. We review the recent development of a new approach to carbon nanotube organization based on self-organized growth directed by well-defined crystal surfaces, or "nanotube epitaxy". We identify three different modes of surface-directed growth, namely by atomic rows, atomic steps, and nanofacets. Particular emphasis is given here to the combinations of such surface-directed growth with external forces-like those exerted by an electric field or gas flow-for the creation of well-defined complex geometries, including crossbar architectures, serpentines, and coils.

We experimentally observed atomic-scale torsional stick-slip behavior in individual nanotubes of tungsten disulfide (WS₂). When an external torque is applied to a WS₂ nanotube, all its walls initially stick and twist together, until a critical torsion angle, at which the outer wall slips and twists around the inner walls, further undergoing a series of stick-slip torque oscillations. We present a theoretical model based on density-functional-based tight-binding calculations, which explains the torsional stick-slip behavior in terms of a competition between the effects of the in-plane shear stiffness of the WS₂ walls and the interwall friction arising from the atomic corrugation of the interaction between adjacent WS₂ walls.

Crossbar arrays of single-wall carbon nanotubes are produced spontaneously in a single step of chemical vapor deposition by simultaneous graphoepitaxy along faceted nanosteps and field-directed growth, perpendicular to each other. The two alignment mechanisms take place selectively on miscut C-plane sapphire and patterned amorphous SiO₂ islands, respectively, without mutual interference, producing dense nanotube grids, with up to 12 junctions per square micrometer. This one-step method of orthogonal self-assembly may open up new possibilities for nanotube circuit integration.

Single-wall carbon nanotubes grow along self-assembled nanosteps of annealed miscut C-plane sapphire. Depending on the miscut orientation and annealing conditions, graphoepitaxy leads to the formation of either unprecedentedly straight and parallel nanotubes, with angular deviations as small as ±0.5°, or to wavy nanotubes loosely conformal to sawtooth-shaped faceted nanosteps.

Mind the step! Single-wall carbon nanotubes grow along the atomic steps of vicinal α-Al₂O₃ surfaces to give highly aligned arrays of nanometer-wide wires on a dielectric material. The nanotubes (see blue background image) reproduce the atomic features of the surface including steps, facets, and kinks (see model). The direction and morphology of the atomic steps can be controlled by the crystal miscut.

Modification of a glass support with triethoxy propylaminosilane yields an active interface for the assembly of Au colloids. The colloids are imaged by AFM using a low applied load (0.5-0.7 nN). The lateral Au-colloid dimensions, 33 +/- 3 nm, deviate from the particle dimensions determined by TEM (19 +/- 2 nm) and absorption spectroscopy (15 nm). This deviation is attributed to the intrinsic curvature of the AFM tip. Application of higher loads on the tip (3 nN) results in the sweeping of Au colloids from the monolayer. The An colloid monolayer is etched in the presence of CN-. The etching proceeds by the initial coincidental etching of Au particles followed by the kinetically favored etching of particles at the edges of the etched domains. This provides means for the micro machining and the chemical manipulation of Au colloids of controlled spatial arrangement. (C) 1999 Elsevier Science S.A. All rights reserved.

Herein, we report the first covalent modification of single-walled carbon nanotubes1 (SWNTs) to create high-resolution, chemically sensitive probe microscopy tips. Carboxylic acid groups at the open ends of SWNTs were coupled to amines to create additional probes with basic or hydrophobic functionality. Force titrations recorded between the ends of the SWNT tips and hydroxy-terminated self-assembled monolayers (SAMs) confirmed the chemical sensitivity and robustness of these SWNT tips. In addition, images recorded on patterned SAM and partial bilayer surfaces have demonstrated chemically sensitive imaging with nanometer-scale resolution. These studies show that well-defined covalent chemistry can be exploited to create functionalized SWNT probes that have the potential for true molecular-resolution, chemically sensitive imaging.

TiO2 powders (Degussa P-25) were modified by a bipyridinium V2+ monolayer. The resulting modified V2+-TiO2 photocatalyst reveals enhanced activities for the light-induced degradation of the series of organic compounds, 1,4-dimethoxybenzene (1), 1,2-dimethoxybenzene (veratrole) (2), indole (3) and eosin (4), exhibiting electron donor properties. Photodegradation of p-chlorobenzaldehyde (5) which lacks electron donor properties is inhibited in the presence of the modified photocatalyst V2+-TiO2, whereas it degrades in the presence of unmodified TiO2. The enhanced photodegradation of the series of electron-rich compounds is attributed to the formation of a supramolecular donor-acceptor complex with the bipyridinium units at the semiconductor surface. Concentration of the organic materials at the s.c. surface leads to effective utilization of photogenerated O-2(.-) and effective mineralization.

The photoinduced electron transfer from eosin, Eo(2-) (1), and ethyl eosin, EoEt(-) (2), to Fe(CN)(6)(3-) is examined in AOT reverse micelles in heptane. For a microheterogeneous system having a water-to-surfactant molar ratio w = 30, the lifetime of the photogenerated redox products in the system that includes EoEt(-) is ca. 300-fold longer than in the photosystem that includes Eo(2-): tau = 4.3 mu s for Eo(2-) and tau = 1400 mu s for EoEt(-). Stabilization of the redox products against recombination in the system containing EoEt(-) is attributed to the extraction of the hydrophobic oxidized photoproduct (2)EoEt(.) from the water pool of the reverse micelles to the continuous oil phase. Photoinduced electron transfer from Eo(2-) to Fe(CN)(6)(3-) in the reverse micelles has been quantitatively analyzed by assuming a Poisson distribution of the quencher over the reverse micelles. Kinetic analysis of the transients allowed determination of the quencher distribution, micelle concentration [m] 1.44 x 10(-4) M, and water-pool diameter 2R = 82 Angstrom. The kinetics of photoinduced electron transfer from EoEt(-) to Fe(CN)(6)(3-) could be analyzed in terms of a similar quencher distribution. Detailed kinetic analysis revealed that, in the Eo(2-)/Fe(CN)(6)(3-) reverse-micellar photosystem, photoinduced electron transfer is followed by a fast intramicellar recombination. In the EoEt(-)/Fe(CN)(6)(3-) photosystem, fast escape of the neutral oxidized species (2)EoEt(.) from the reverse micelle competes with the intramicellar recombination (escape efficiency: theta(esc) = 0.52), leading to separation of the redox products. The separated photoproducts undergo a slow secondary recombination. A kinetic model for the overall photochemical processes is presented, and kinetic equations for the photoinduced electron transfer in the reverse micelles followed by intramicellar recombination and escape are provided.

Ultrasmall TiO2 particles (2R = 9.0 +/- 0.5 Angstrom) were generated in situ in a water-in-oil (w/o) microemulsion composed of water, cetyldimethylbenzylammonium chloride (CDBA), and benzene, by controlled hydrolysis of TiCl4. Effective photosensitization of the TiO2 particles by Ru(II) tris(bipyridine), Ru(bpy)(3)(2+), proceeds in the w/o microemulsion. The photosensitization of TiO2 was studied by following the emission decay of excited RU(bpy)(3)(2+) at different TiO2 concentrations. The emission decay curves of excited Ru(bpy)(3)(2+) were analyzed by assuming a Poisson distribution of the TiO2 particles over the reverse micelles and realizing that excited Ru(bpy)(3)(2+) photosensitizes TiO2 by two photosensitization pathways: (a) intramicellar electron injection quenching (k(q)) and (b) particle incorporation by intermicellar exchange (k(c)) followed by fast intramicellar quenching. The derived values of intramicellar electron injection and intermicellar exchange rate constants were kg = 7 X 10(6) s(-1) and 1.4 x 10(9) M(-1) s(-1), respectively. The kinetic analysis allowed to estimate the mean agglomeration number of the TiO2 particles to be 11 +/- 2, which corresponds to a particle diameter of 9.0 +/- 0.5 Angstrom, Effects of the water-pool size on the TiO2 particle structure and photosensitization process were also investigated. As the water-pool size increases, the intermicellar exchange rate constant is enhanced but the intramicellar electron injection rate is unaffected. The agglomeration number is higher in larger water pools.

The new tris(6,6'-oligoethyleneglycol-3,3'-bipyridazine)Ru(II)-dichlorides 1, 2 form binuclear metal complexes with alkali and alkaline earth ions with high selectivity and these complexes show increased fluorescence quantum yields and lifetimes.