Small Diameter Nanowires Research Articles

Monte Carlo simulation of thermal conductivity of Si nanowire: An investigation on the phonon confinement effect on the thermal transport

Thermal conductivity of Si nanowire is calculated by applying Monte Carlo (MC) simulation of 110 nm, 37 nm, and 22 nm wire diameter. To study the thermal conductivity of both thick and thin nanowires different phonon group velocity is used in the simulation. This change in the phonon velocity for small diameter nanowire is due to the phonon confinement effect, which decreases the slope of phonon acoustic modes. Very good agreement with previously reported experimental value is obtained for all nanowire diameters. Another investigation by using average relaxation time approximation is also discussed along with the Monte Carlo simulation.

Computing Raman and infrared wavenumbers of nanostructures: application to silicon nanowires

The characterization of nanostructures with spectroscopic methods is a fundamental tool in nanoscience. For novel nanostructures, the interpretation of spectral features is a challenging task. To address this issue, we present the “Symmetry‐Filtered Molecular Dynamics (SFMD)” method to calculate Raman and infrared wavenumbers from molecular dynamics (MD) simulations, employing only the symmetry of the atomic structure. Explicit and expensive calculations of the electric polarizability or the dipole moment are not required. Therefore, our method can be easily used with any standard MD software. On the basis of the density functional tight‐binding method for the MD simulations, we apply our method to bulk silicon and small‐diameter hydrogen‐passivated silicon nanowires. For bulk silicon, we study the wavenumber shift of the Raman peak with temperature and obtain results that are in good agreement with experiments. We further show that thermal lattice expansion is a minor effect (22%) and that temperature‐driven anharmonic effects (78%) are the main contributions to that wavenumber shift. By analyzing the bond lengths of different silicon nanowires, we found that surface stress manifests as a 0.37% shortening of bonds only in the outermost silicon layer. We further analyzed the diameter‐dependent wavenumber shift of a Raman peak in silicon nanowires. We found that the main contribution to the wavenumber shift comes from the phonon confinement effect and surface stress leads to an additional shift of 9–22%. Our results indicate that our method is able to produce quantitative results that can be compared with experiments. We propose our method to be used for the understanding of Raman and infrared spectra of novel bulk and nanostructures. Copyright © 2012 John Wiley & Sons, Ltd.

Kinetic Competition Model and Size-Dependent Phase Selection in 1-D Nanostructures

The first phase selection and the phase formation sequence between metal and silicon (Si) couples are indispensably significant to microelectronics. With increasing scaling of device dimension to nano regime, established thermodynamic and kinetic models in bulk and thin film fail to apply in 1-D nanostructures. Herein, we present an unique size-dependent first phase formation sequence in 1-D nanostructures, with Ni-Si as the model system. Interfacial-limited phase which forms the last in thin film, NiSi(2), appears as the dominant first phase at 300-800 °C due to the elimination of continuous grain boundaries in 1-D silicides. On the other hand, θ-Ni(2)Si, the most competitive diffusion-limited phase takes over NiSi(2) and wins out as the first phase in small diameter nanowires at 800 °C. Kinetic parameters extracted from in situ transmission electron microscope studies and a modified kinetic growth competition model quantitatively explain this observation. An estimated critical diameter from the model agrees reasonably well with observations.

Mechanical Model of Cell Membrane Penetration by Vertical Nanowires

For many therapeutic and scientific applications, direct access into a cell's interior is the key for delivering various biomolecules to alter cell behavior or for intracellular assays. However, the lipid membrane presents a challenging barrier that prevents biomolecular species from entering the cytosol.The recent discovery of cell viability despite penetration by vertical nanowires (NW) has opened new avenues for direct intracellular access. Cells are found to be impaled onto small diameter nanowires without application of any external force. Vertical NW arrays have been reported to serve as a universal and efficient platform for introducing RNA, DNA, proteins and peptides into a broad range of cell types.However, the cell membrane penetration mechanism by vertical nanowires is still unknown. Several recent experiments have shown that the penetration efficiency is greatly reduced with increasing NW diameter. Understanding the penetration mechanism is the key to optimizing the design of nanowires and developing advanced devices based on this technique.In this work a mechanical model is developed to predict cell membrane penetration by vertical nanowires. The tension and strain on cell membranes due to indentation by nanowire are calculated using a model of axisymmetric deformation of elastic membrane indented by a probe with a hemispherical tip. The critical membrane rupture conditions are determined under different failure criteria, either tension or strain. The effects of NW radius, NW aspect ratio and cell membrane stiffness on penetration are investigated based on the mechanical model. Our results provide a practical guide to designing nanowires for applications in cell membrane penetration.

Thermal Conductivity of Ge and Ge–Si Core–Shell Nanowires in the Phonon Confinement Regime

Heterostructure core-shell semiconductor nanowires (NWs) have attracted tremendous interest recently due to their remarkable properties and potential applications as building blocks for nanodevices. Among their unique traits, thermal properties would play a significant role in thermal management of future heterostructure NW-based nanoelectronics, nanophotonics, and energy conversion devices, yet have been explored much less than others. Similar to their electronic counterparts, phonon spectrum and thermal transport properties could be modified by confinement effects and the acoustic mismatch at the core-shell interface in small diameter NWs (<20 nm). However, fundamental thermal measurement on thin core shell NWs has been challenging due to their small size and their expected low thermal conductivity (κ). Herein, we have developed an experimental technique with drastically improved sensitivity capable of measuring thermal conductance values down to ∼10 pW/K. Thermal conductivities of Ge and Ge-Si core-shell NWs with diameters less than 20 nm have been measured. Comparing the experimental data with Boltzmann transport models reveals that thermal conductivities of the sub-20 nm diameter NWs are further suppressed by the phonon confinement effect beyond the diffusive boundary scattering limit. Interestingly, core-shell NWs exhibit different temperature dependence in κ and show a lower κ from 300 to 388 K compared to Ge NWs, indicating the important effect of the core-shell interface on phonon transport, consistent with recent molecular dynamics studies. Our results could open up applications of Ge-Si core shell NWs for nanostructured thermoelectrics, as well as a new realm of tuning thermal conductivity by "phononic engineering".

Heat Transfer across the Interface between Nanoscale Solids and Gas

When solid materials and devices scale down in size, heat transfer from the active region to the gas environment becomes increasingly significant. We show that the heat transfer coefficient across the solid-gas interface behaves very differently when the size of the solid is reduced to the nanoscale, such as that of a single nanowire. Unlike for macroscopic solids, the coefficient is strongly pressure dependent above ∼10 Torr, and at lower pressures it is much higher than predictions of the kinetic gas theory. The heat transfer coefficient was measured between a single, free-standing VO(2) nanowire and surrounding air using laser thermography, where the temperature distribution along the VO(2) nanowire was determined by imaging its domain structure of metal-insulator phase transition. The one-dimensional domain structure along the nanowire results from the balance between heat generation by the focused laser and heat dissipation to the substrate as well as to the surrounding gas, and thus serves as a nanoscale power-meter and thermometer. We quantified the heat loss rate across the nanowire-air interface, and found that it dominates over all other heat dissipation channels for small-diameter nanowires near ambient pressure. As the heat transfer across the solid-gas interface is nearly independent of the chemical identity of the solid, the results reveal a general scaling relationship for gaseous heat dissipation from nanostructures of all solid materials, which is applicable to nanoscale electronic and thermal devices exposed to gaseous environments.

In situ observation of morphological change in CdTe nano- and submicron wires

We report growth and characterization of CdTe wires 30–400 nm in diameter by thevapor–liquid–solid technique. Individual nanowires were placed on a movable piezotube,which allowed three-dimensional motion toward a scanning tunneling microscope(STM). A bias was applied to the STM tip in contact with the nanowire, andthe morphological changes due to Joule heating were observed in situ using atransmission electron microscope (TEM) in real time. For thick CdTe wires (d > ∼ 150 nm), the process results in the growth of superfine nanowires (SFNWs) of 2–4 nm diameteron the surface of the wire. Smaller diameter nanowires, in contrast, disintegrateunder the applied bias before the complete evolution of SFNWs on the surface.

Enhancement of irradiation-induced defect production in Si nanowires

We performed classical molecular dynamics simulations of defect production in small-diameter hexagonal Si nanowires under Ar ion irradiation. Using irradiation energies of 30 eV to 10 keV, we find that for low energies the defect production in the nanowires may be enhanced by as much as a factor of 3 in comparison to bulk Si due to the large surface-to-volume ratio of the systems. Conversely, at higher energies the increased transmission of ions causes a significant decrease in defect production.

Spatially Resolved Plasmonically Enhanced Photocurrent from Au Nanoparticles on a Si Nanowire

Semiconducting nanowires have been demonstrated as promising light-harvesting units with enhanced absorption compared to bulk films of equivalent volume. However, for small diameter nanowires, the ultrahigh aspect ratio constrains the absorption to be polarization selective by responding primarily to the transverse magnetic (TM) light. While this effect is useful for polarization-sensitive optoelectronic devices, practical light-harvesting applications demand efficient light absorption in both TM and transverse electric (TE) light. In this study, we engineer the polarization sensitivity and the charge carrier generation in a 50 nm Si nanowire by decorating the surface with plasmonic Au nanoparticles. Using scanning photocurrent microscopy (SPCM) with a tunable wavelength laser, we spatially and spectrally resolve the local enhancement in the TE photocurrent resulting from the plasmonic near-field response of individual nanoparticles and the broad-band enhancement due to surface-enhanced absorption. These results provide guidance to the development and the optimization of nanowire-nanoparticle light-harvesting systems.

Stability of Donor-Pair Defects in Si1–xGex Alloy Nanowires

We perform density-functional calculations to investigate the defect properties of group-V elements (P, As, Sb) in Si, Ge, and Si1–xGex alloy nanowires. In all nanowires, P dopants have a tendency to form donor-pair defects, which consist of two dopants at the first nearest distance, when the wire diameter decreases below a critical value. The quantum confinement and chemical bonding effects play a role in stabilizing donor-pair defects against isolated substitutional donors. As the donor-pair defect has a deep level in the band gap, which is electrically inactive, the doping efficiency is reduced in small-diameter nanowires. As the Ge concentration increases, the formation of the donor-pair defect becomes more favorable, lowering further the doping efficiency. On the other hand, with As and Sb dopants, which have the larger atomic radii, the formation of donor-pair defects is suppressed due to large strain energies. Uniaxial compressive strain also reduces the stability of donor-pair defects and thereby ...

Characteristics of Progressive Damage of ZnO Nanowires during Contact Sliding under Relatively Low Loads

ZnO nanowires have received much interest owing to their particular structural and piezoelectric properties. For widespread application of ZnO nanowires in various nanotechnologies, the mechanical reliability of the nanowires should be assessed. In this paper, the damage characteristics of vertically grown ZnO nanowires due to contact sliding against a 2 mm diameter steel ball under relatively low loads were investigated. Frictional behavior and wear characteristics of the specimens were assessed. Furthermore, contact sliding tests were performed inside an SEM to monitor the progression of damage of the nanowires. It was found that the friction coefficient was about 0.35 under all loads while the damage characteristics of the nanowires were quite different for each load. The large diameter nanowires tended to fracture earlier than the small diameter nanowires. Wear tests performed inside the SEM confirmed the surface damage characteristics observed during the friction tests.

Surface Effects on the Atomic and Electronic Structure of Unpassivated GaAs Nanowires

On the basis of accurate ab initio calculations, we propose a model for predicting the stability of III-V nanowires (NW) having different side walls and ridge configurations. The model allows us to obtain the NW formation energies by performing calculations only on relatively "small" systems, small diameter NWs and flat surfaces, to extract the contributions to the stability of each structural motif. Despite the idea illustrated here for the case of hexagonally shaped GaAs NWs grown along the [111]/[0001] direction, the method can also be applied generally to other differently shaped and oriented III-V NWs. The model shows that NW surfaces (side walls plus ridges) mainly determine the NW stability, so the changes to the surface structure (e.g., induced by defects or different growth conditions) would modify the final NW structure in a remarkable way. We find that wurtzite and zinc blende nanowires have similar energies over a wide diameter range, thus explaining the observed polytypism. Furthermore, new more stable ridge reconstructions are proposed for zinc blende nanowires. The surface-related structural motifs also have clear fingerprints on the NW electronic structure. We find that the more stable nanowires are all semiconducting. The band gaps are ruled by surface states and do not follow the trend mandated by the quantum confinement effect. Small diameter wurtzite nanowires have an indirect band gap, but for some of them, an indirect to direct transition can be foreseen to occur at larger diameters. Surface states have a larger impact on the zinc blende NW band gaps than on the wurtzite NW ones. Zinc blende nanowire band gaps reduce significantly with increasing nanowire radius, reaching the bulk value at a diameter of about 30 Å. The surface structure and the high surface related DOS below the conduction band are going to affect the nanowire dopant incorporation and efficiency when doping is carried out during the NW growth.

Germanium Nanowire Synthesis from Fluorothiolate-Capped Gold Nanoparticles in Supercritical Carbon Dioxide

Ge nanowires seeded from Au nanoparticles capped with fluorothiolate ligands were synthesized in supercritical carbon dioxide (sc-CO2) by the thermal decomposition of diphenylgermane (DPG) at a temperature of 380 °C and a pressure of 25.7 MPa. Both perfluorinated and semifluorinated capped Au nanoparticles acted as effective catalysts for growing Ge nanowires, with mean diameters of 11 nm (σ = 2.8) and 14 nm (σ = 3.5), respectively. The mean diameter of the Ge nanowires grown from the fluorous-capped Au nanoparticles were considerably smaller than those synthesized from dodecanethiol-capped nanoparticles in sc-toluene, under the same reaction conditions, i.e., 28 nm and σ = 10.3. Differences in the ligand conformations on the surface of the Au nanoparticles and phase separation of the fluorocarbon/CO2 and hydrocarbon/toluene systems gave rise to greater steric stabilization of the fluorous-capped Au nanoparticles in CO2, resulting in small diameter nanowires with a relatively narrow size distribution. Ele...

The Benefits of HCl in the Growth of Silicon Nanowires by Chemical Vapour Deposition: Growth of Small Diameter Nanowires and Controlled Facet Evolution

AbstractThe effects of hydrogen chloride (HCl) on the growth of silicon nanowires by Chemical Vapour Deposition are investigated. HCl is found to enable the growth of small diameter gold-catalyzed silicon nanowires while reducing the kink occurrence. Specific growth sequences are presented in order to obtain a one-to-one ratio of nanowire growth versus gold colloidal seed. Other growth sequences using HCl are illustrated but achieve mixed results concerning the nanowire structure and the surface state.

Growth mechanism and diameter control of well-aligned small-diameter ZnO nanowirearrays synthesized by a catalyst-free thermal evaporation method

Well-aligned small-diameter ZnO nanowire arrays have been synthesized on Si wafers pre-coated withc-oriented ZnO thin films by a catalyst-free thermal evaporation method. The morphology ofthe products has been found to be greatly affected by the oxygen flow rate. Theself-catalyzed VLS mechanism is proposed to interpret the growth of the ZnO nanowiresand the change in the product morphology. Classical nucleation theory is employed toanalyze the growth process of the ZnO nanowires, and Zn vapor supersaturation isproposed to be a key factor to affect the diameter and the areal density of the ZnOnanowires. The average diameter of the ZnO nanowires can be finely controlled in the rangeof 12–31 nm by controlling the oxygen flow rate and hence the Zn vapor supersaturation.This result is consistent with our theoretical prediction. X-ray photoelectron spectroscopymeasurements were performed to characterize the stoichiometry of nanowires withvarious average diameters. It also revealed that there are many OH species on thenanowire surface. Photoluminescence measurements showed that the deep-levelemission of the nanowires with average diameter of 12 nm peaks in the yellowregion, which may be attributed to the presence of OH species and surface effects.

Integrated silicon nanowire diodes and the effects of gold doping from the growth catalyst

We report on integrated, silicon single-nanowire diodes. Gold catalyst templates, defined by lithography, controlled the location of nanowires grown with a vapor-liquid-solid mechanism. The nanowire growth, by atmospheric-pressure chemical vapor deposition, used SiCl4 diluted in H2 on (100) n-type silicon substrates. Postgrowth oxidation and wet etching reduced the nanowire diameters and removed unintentional small diameter nanowires. Spin-on glass isolated the nanowire tips from the substrate, which were then contacted with aluminum. Current-voltage measurements show rectification and ideality factors consistent with pn junction diodes. However, the gold catalyzed nanowires have much higher than expected hole concentrations that cannot be explained by behaviors reported for gold diffused into silicon.

New Insights on the Nanoparticle Growth Mechanism in the Citrate Reduction of Gold(III) Salt: Formation of the Au Nanowire Intermediate and Its Nonlinear Optical Properties

The synthesis of gold nanospheres via the reduction of AuCl4- salt by sodium citrate has been revisited, with focus on the mechanism of particle growth. In stark contrast to the widely accepted LaMer nucleation-growth model, the initially formed nanoclusters of about 5 nm diameter self-assemble to form an extensive network of nanowires. The diameter of the nanowires progressively increases in size, and at the same time the connected network is fragmented into small segments before the final spherical particles are formed. We put forward a modified nanocrystal growth mechanism, which is able to explain a number of other experimental observations. By arresting the reduction process by rapid cooling, the 5 nm diameter nanowires could be isolated and were found to be very stable. The nonlinear optical properties of the Au nanowires were investigated and discussed. Highly connected, small diameter nanowires of highly conductive metals may be useful for future electronic applications, e.g., conductive polymer−metal composites with low metal loading.

Diameter dependent transport properties of gallium nitride nanowire field effect transistors

The authors report transport property measurements of individual GaN nanowire field effect transistors and the correlation of the electron mobilities with the existence of grain boundaries in these nanowires. Room temperature field effect electron mobilities as high as 319cm2V−1s−1 were obtained for the 200nm diameter nanowires. Mobilities calculated from these reliable nanowire field effect transistors indicated that the surface scattering plays a dominant role in smaller diameter nanowires, whereas for intermediate diameter devices transport is dominated by grain boundary scattering. Reduction of the mobility with decreasing diameter of nanowires can be explained using “continuous surface” model.

Structural stability, electronic properties, and quantum conductivity of small-diameter silicon nanowires

Structures and energetics of various types of silicon nanowires have been investigated using quantum molecular dynamics simulations to determine the most stable forms. The tetrahedral type nanowires oriented in the ⟨111⟩ direction are found to be the most stable. The stability of the cagelike nanowires is determined to lie somewhere between this and tetrahedral nanowires oriented in other directions. Furthermore, their electrical conducting properties are found to be better than those of tetrahedral nanowires, suggesting useful molecular electronic applications.

Uniformly Sized Gold Nanoparticles Derived from PS-b-P2VP Block Copolymer Templates for the Controllable Synthesis of Si Nanowires

A monolayer of gold-containing surface micelles has been produced by spin-coating solution micelles formed by the self-assembly of the gold-modified polystyrene-b-poly(2-vinylpyridine) block copolymer in toluene. After oxygen plasma removed the block copolymer template, highly ordered and uniformly sized nanoparticles have been generated. Unlike other published methods that require reduction treatments to form gold nanoparticles in the zero-valent state, these as-synthesized nanoparticles are in form of metallic gold. These gold nanoparticles have been demonstrated to be an excellent catalyst system for growing small-diameter silicon nanowires. The uniformly sized gold nanoparticles have promoted the controllable synthesis of silicon nanowires with a narrow diameter distribution. Because of the ability to form a monolayer of surface micelles with a high degree of order, evenly distributed gold nanoparticles have been produced on a surface. As a result, uniformly distributed, high-density silicon nanowires have been generated. The process described herein is fully compatible with existing semiconductor processing techniques and can be readily integrated into device fabrication.