Physical consequences of the equivalence of conditions for the steady-state growth of nanowires and the nucleation on triple phase line

A new theoretical model describing the steady-state growth and crystalline structure of semiconductor nanowires (NWs) is proposed and its physical consequences are considered. It is demonstrated that the Nebol’sin-Shchetinin condition (nonwetting of the NW side surface by the liquid drop) necessary for the steady-state growth of NWs according to the vapor-liquid-solid (VLS) mechanism is equivalent to the Glas condition of nucleation on the triple phase line for the monocentric NW growth. An energy criterion for the steady-state growth of NWs is formulated in the general case of faceted NW side surface. Effective surface energies are found that determine the activation barrier for nucleation at the NW top. Based on the proposed model, the issue of determining the III–V semiconductor NW crystal structure (cubic zinc blende type versus hexagonal wurtzite type) is considered. In particular, it is shown that a decrease in the surface energy of a catalyst must lead to the predominant formation of a cubic phase, which is confirmed by experimental data on the growth of GaAs nanowires according to the VLS mechanism with Au and Ga catalysts.     

Semi-transparent,flexible, and electrically conductive silicon mesh by capillarity-driven welding of vapor-liquid-solid-grown nanowires over large areas

Bottom-up synthesis of semiconductor nanowires (NWs) by the vapor-liquid-solid (VLS) mechanism has enabled diverse technological applications for these nanomaterials. Unlike metallic NWs, however, it has been challenging to form large-area interconnected NW networks. Here, we generate centimeter-scale meshes of mechanically and electrically interconnected Si NWs by sequentially growing, collapsing, and joining the NWs using a capillarity-driven welding mechanism. We fabricate meshes from VLS-grown NWs ranging in diameter from 20 to 100 nm and find that the meshes are three-dimensional with a thickness ranging from ~ 1 to ~ 10 microns depending on the NW diameter. Optical extinction measurements reveal that the networks are semi-transparent with a color that depends on the absorption and scattering characteristics of individual NWs. Moreover, active voltage contrast imaging of both centimeter- and micron-scale meshes reveals widespread electrical connectivity. Using a sacrificial layer, we demonstrate that the mesh can be liberated from the growth substrate, yielding a highly flexible and transparent film. Electrical transport measurements both on the growth substrate and on liberated, flexible films reveal electrical conduction across a centimeter scale with a sheet resistance of ~ 160–180 kΩ/square that does not change significantly upon bending. Given the ability to encode complex functionality in semiconductor NWs through the VLS process, we believe these meshes of networked NWs could find application as neuromorphic memory, electrode scaffolds, and bioelectronic interfaces.

     

Lateral, Ge, nanowire growth on SiO2

Vapor-liquid-solid (VLS) nanowires (NWs) typically grow in [111] directions. Previously, the authors have demonstrated guided Si NW growth, engineering the VLS NWs to grow in a [110] direction against a SiO(2) surface. In this work, the authors demonstrate guided high-quality Ge nanowire growth against a SiO(2) surface in the substrate plane to bridge between two Si mesas. The authors explore the interfaces between a Ge NW and the two Si device-layer mesas and report high-quality, epitaxial interfaces between the Ge NW and both Si mesas.     

Nucleation‐Controlled Distributed Plasticity in Penta‐twinned Silver Nanowires

A unique size‐dependent strain hardening mechanism, that achieves both high strength and ductility, is demonstrated for penta‐twinned Ag nanowires (NWs) through a combined experimental‐computational approach. Thin Ag NWs are found to deform via the surface nucleation of stacking fault decahedrons (SFDs) in multiple plastic zones distributed along the NW. Twin boundaries lead to the formation of SFD chains that locally harden the NW and promote subsequent nucleation of SFDs at other locations. Due to surface undulations, chain reactions of SFD arrays are activated at stress concentrations and terminated as local stress decreases, revealing insensitivity to defects imparted by the twin structures. Thick NWs exhibit lower flow stress and number of distributed plastic zones due to the onset of necking accompanied by more complex dislocation structures.     

Tunable catalytic alloying eliminates stacking faults in compound semiconductor nanowires

Planar defects in compound (III-V and II-VI) semiconductor nanowires (NWs), such as twin and stacking faults, are universally formed during the catalytic NW growth, and they detrimentally act as static disorders against coherent electron transport and light emissions. Here we report a simple synthetic route for planar-defect free II-VI NWs by tunable alloying, i.e. Cd(1-x)Zn(x)Te NWs (0 ≤ x ≤ 1). It is discovered that the eutectic alloying of Cd and Zn in Au catalysts immediately alleviates interfacial instability during the catalytic growth by the surface energy minimization and forms homogeneous zinc blende crystals as opposed to unwanted zinc blende/wurtzite mixtures. As a direct consequence of the tunable alloying, we demonstrated that intrinsic energy band gap modulation in Cd(1-x)Zn(x)Te NWs can exploit the tunable spectral and temporal responses in light detection and emission in the full visible range.     

Unravelling a solution-based formation of single-crystalline kinked wurtzite nanowires: The case of MnSe

The search for a novel strategy to sculpt semiconductor nanowires (NWs) at the atomistic scale is crucial for the development of new paradigms in optics,electronics,and spintronics.Thus far,the fabrication of single-crystalline kinked semiconductor NWs has been achieved mainly through the vapor-liquid-solid growth technique.In this study,we developed a new strategy for sculpting single-crystalline kinked wurtzite (WZ) MnSe NWs by triggering the nonpolar axial-oriented growth,thereby switching—at the atomistic scale—the NW growth orientation along the nonpolar axes in a facile solution-based procedure.This presents substantial challenges owing to the dominant polar c axis growth in the solution-based synthesis of one-dimensional WZ nanocrystals.More significantly,the ability to continuously switch the nonpolar axial-growth orientation allowed us to craft the kinking landscape of types 150°,120°,90°,and 60°.A probabilistic analysis of kinked MnSe NWs reveals the correlations of the synergy and interplay between these two sets of nonpolar axial growth-orientation switching,which determine the actual kinked motifs.Furthermore,discriminating the side-facet structures of the kinked NWs significantly strengthened the spatially selected interaction of Au nanoparticles.We envisage that such a facile solution-based strategy can be useful for synthesizing other single-crystalline kinked WZ-type transition-metal dichalcogenide NWs to develop novel functional materials with finely tuned properties.     

Shape-controlled Au particles for InAs nanowire growth

We present a study of InAs nanowire (NW) growth with shape-controlled Au seed particles. In comparison to more conventional spherical particles, the highly faceted, shaped Au particles are found to enhance the initial growth kinetics of InAs NWs at identical growth conditions. Analysis of the NWs after growth by transmission electron microscopy and energy-dispersive spectroscopy suggests that while In diffuses into the bulk of the shaped Au particles, in accordance with the vapor-liquid-solid (VLS) growth mechanism, the surface faceting is preserved. A key difference is that the shaped Au particles are characterized by a thicker In shell on their surfaces than the spherical Au particles, indicating that increased adsorption of In leads to the observed growth rate enhancement. On the basis of these results, we propose that our picture of VLS growth in regards to liquefaction and droplet formation is incomplete and that the initial particle morphology can be used to tailor NW growth.     

Sb-Mediated Tuning of Growth- and Exciton Dynamics in Entirely Catalyst-Free GaAsSb Nanowires

Vapor-liquid-solid (VLS) growth is the mainstream method in realizing advanced semiconductor nanowires (NWs), as widely applied to many III-V compounds. It is exclusively explored also for antimony (Sb) compounds, such as the relevant GaAsSb-based NW materials, although morphological inhomogeneities, phase segregation, and limitations in the supersaturation due to Sb strongly inhibit their growth dynamics. Fundamental advances are now reported here via entirely catalyst-free GaAsSb NWs, where particularly the Sb-mediated effects on the NW growth dynamics and physical properties are investigated in this novel growth regime. Remarkably, depending on GaAsSb composition and nature of the growth surface, both surfactant and anti-surfactant action is found, as seen by transitions between growth acceleration and deceleration characteristics. For threshold Sb-contents up to 3–4%, adatom diffusion lengths are increased ≈sevenfold compared to Sb-free GaAs NWs, evidencing the significant surfactant effect. Furthermore, microstructural analysis reveals unique Sb-mediated transitions in compositional structure, as well as substantial reduction in twin defect density, ≈tenfold over only small compositional range (1.5–6% Sb), exhibiting much larger dynamics as found in VLS-type GaAsSb NWs. The effect of such extended twin-free domains is corroborated by ≈threefold increases in exciton lifetime (≈4.5 ns) due to enlarged electron-hole pair separation in these phase-pure NWs.     

Comparison of the top-down and bottom-up approach to fabricate nanowire-based silicon/germanium heterostructures

Silicon nanowires (NWs) and vertical nanowire-based Si/Ge heterostructures are expected to be building blocks for future devices, e.g. field-effect transistors or thermoelectric elements. In principle two approaches can be applied to synthesise these NWs: the ‘bottom-up’ and the ‘top-down’ approach. The most common method for the former is the vapour-liquid-solid (VLS) mechanism which can also be applied to grow NWs by molecular beam epitaxy (MBE). Although MBE allows a precise growth control under highly reproducible conditions, the general nature of the growth process via a eutectic droplet prevents the synthesis of heterostructures with sharp interfaces and high Ge concentrations. We compare the VLS NW growth with two different top-down methods: The first is a combination of colloidal lithography and metal-assisted wet chemical etching, which is an inexpensive and fast method and results in large arrays of homogenous Si NWs with adjustable diameters down to 50 nm. The second top-down method combines the growth of Si/Ge superlattices by MBE with electron beam lithography and reactive ion etching. Again, large and homogeneous arrays of NWs were created, this time with a diameter of 40 nm and the Si/Ge superlattice inside.     

Influence of O contamination and Au cluster properties on the structural features of Si nanowires

Silicon nanowires (Si NWs) are the emerging nanostructures for future nanodevices. In this work we have prepared them by electron beam evaporation (EBE) through the vapor-liquid-solid (VLS) mechanism. We discuss the growth of epitaxial NWs by EBE and the possibility to control their orientation and length by changing the experimental conditions. Moreover, the effects of the surface contamination and of the Au cluster preparation on the NWs structural properties and density will be discussed. We demonstrate that any O contamination has to be avoided since just a very thin native SiO₂ layer stops ad-atom diffusion on the surface and inhibits the NWs growth. Au cluster preparation has a determinant role too: by varying the procedure for their formation, it is possible to change NWs density and length. In particular, we observed that by evaporating Au on the heated substrate, the droplets active to promote NW growth are immediately formed and a faster growth process starts. The growth rate is about a factor of 4 times higher than in the sample where the Au is evaporated at room temperature and the cluster formed after a subsequent thermal annealing. On the contrary, the slower process allows the atom arrangement and ordering in an epitaxial manner, and a precise control of NW orientation can be achieved.     

High output nanogenerator based on assembly of GaN nanowires

GaN nanowires (NWs) were synthesized through a vapor-liquid-solid (VLS) process. Based on structural analysis, the c-axis of the NW was confirmed to be perpendicular to the growth direction. Nanogenerators (NGs) fabricated by rational assembly of the GaN NWs produced an output voltage up to 1.2 V and output current density of 0.16 μA cm?2. The measured performance of the GaN NGs was consistent with the calculations using finite element analysis (FEA).     

Growth of Silver Nanowires from Controlled Silver Chloride Seeds and Their Application for Fluorescence Enhancement Based on Localized Surface Plasmon Resonance

A “Polyol” method has granted low‐cost and facile process‐controllability for silver‐nanowire (Ag‐NW) synthesis. Although homogenous and heterogeneous nucleation and growth during Ag‐NW synthesis are possible using polyol methods, heterogeneous nucleation and growth of Ag NW guarantees highly selective growth of nanostructures using silver chloride (AgCl) seeds, which provides a stable source of chloride ions (Cl?) and thermodynamic reversibility. In this paper, a microdroplet has been adopted to synthesize uniform AgCl seeds with different diameter that are used for seed‐mediated Ag‐NW synthesis. The concentration of two precursors (AgNO₃ and NaCl) in the droplets is modulated to produce different sizes of AgCl seeds, which determines the diameter and length of Ag NWs. The process of the seed‐mediated growth of Ag NWs has been monitored by observing the peak shift in the time‐resolved UV–vis extinction spectrum. Furthermore, the distinct plasmonic property of Ag NWs for transverse and longitudinal localized‐surface‐plasmon‐resonance (LSPR)‐mediated fluorescence enhancement is utilized. The high aspect ratio and sharp tips work as simple antennas that induce the enhanced fluorescence emission intensity of a fluorophore, which can be applied in the fields of biological tissue imaging and therapy.     

Assessing the minimum diameter of nanowhiskers

The issue of the minimum diameter of nanodimensional whiskers (NWs) which can be achieved under given deposition conditions is considered within the framework of a kinetic model describing the NW growth according to the vapor-liquid-solid (VLS) mechanism. Based on the results of numerical modeling, it is shown that the minimum NW diameter can be determined not only by the Gibbs-Thomson dimensional effect, but also by the kinetic factors controlling growth on the substrate surface. The results of numerical calculations of the growth rate of NWs with various diameters are presented.     

Mass Transport Determined Silica Nanowires Growth on Spherical Photonic Crystals with Nanostructure‐Enabled Functionalities

A robust and facile method has been developed to obtain directional growth of silica nanowires (SiO₂NWs) by regulating mass transport of silicon monoxide (SiO) vapor. SiO₂NWs are grown by vapor–liquid–solid (VLS) process on a surface of gold‐covered spherical photonic crystals (SPCs) annealed at high temperature in an inert gas atmosphere in the vicinity of a SiO source. The SPCs are prepared from droplet confined colloidal self‐assembly. SiO₂NW morphology is governed by diffusion‐reaction process of SiO vapor, whereby directional growth of SiO₂NWs toward the low SiO concentration is obtained at locations with a high SiO concentration gradient, while random growth is observed at locations with a low SiO concentration gradient. Growth of NWs parallel to the supporting substrate surface is of great importance for various applications, and this is the first demonstration of surface‐parallel growth by controlling mass transport. This controllable NW morphology enables production of SPCs covered with a large number of NWs, showing multilevel micro‐nano feature and high specific surface area for potential applications in superwetting surfaces, oil/water separation, microreactors, and scaffolds. In addition, the controllable photonic stop band properties of this hybrid structure of SPCs enable the potential applications in photocatalysis, sensing, and light harvesting.     

Growth of doped silicon nanowires by pulsed laser deposition and their analysis by electron beam induced current imaging

Doped silicon nanowires (NWs) were epitaxially grown on silicon substrates by pulsed laser deposition following a vapour-liquid-solid process, in which dopants together with silicon atoms were introduced into the gas phase by laser ablation of lightly and highly doped silicon target material. p-n or p(++)-p junctions located at the NW-silicon substrate interfaces were thus realized. To detect these junctions and visualize them the electron beam induced current technique and two-point probe current-voltage measurements were used, based on nanoprobing individual silicon NWs in a scanning electron microscope. Successful silicon NW doping by pulsed laser deposition of doped target material could experimentally be demonstrated. This doping strategy compared to the commonly used doping from the gas phase during chemical vapour deposition is evaluated essentially with a view to potentially overcoming the limitations of chemical vapour deposition doping, which shows doping inhomogeneities between the top and bottom of the NW as well as between the core and shell of NWs and structural lattice defects, especially when high doping levels are envisaged. The pulsed laser deposition doping technique yields homogeneously doped NWs and the doping level can be controlled by the choice of the target material. As a further benefit, this doping procedure does not require the use of poisonous gases and may be applied to grow not only silicon NWs but also other kinds of doped semiconductor NWs, e.g. group III nitrides or arsenides.     

III-V nanowire growth mechanism: V/III ratio and temperature effects

We have studied the dependence of Au-assisted InAs nanowire (NW) growth on InAs(111)B substrates as a function of substrate temperature and input V/III precursor ratio using organometallic vapor-phase epitaxy. Temperature-dependent growth was observed within certain temperature windows that are highly dependent on input V/III ratios. This dependence was found to be a direct consequence of the drop in NW nucleation and growth rate with increasing V/III ratio at a constant growth temperature due to depletion of indium at the NW growth sites. The growth rate was found to be determined by the local V/III ratio, which is dependent on the input precursor flow rates, growth temperature, and substrate decomposition. These studies advance understanding of the key processes involved in III-V NW growth, support the general validity of the vapor-liquid-solid growth mechanism for III-V NWs, and improve rational control over their growth morphology.     

Effect of carbon tetrabromide on the morphology of GaAs nanowires

Carbon is a commonly used p-type dopant in planar III-V semiconductors, however its use in nanowire (NW) growth has been much less reported. In this work we show that the morphology of gold assisted GaAs NWs can be strongly modified by the presence of CBr(4) vapor during growth by metalorganic vapor phase epitaxy. GaAs NWs were grown under conditions which result in strong tapering and lateral growth at low growth temperatures by the use of triethylgallium (TEGa) instead of the more usual precursor, trimethylgallium (TMGa). Under these conditions, NWs grown in the presence of CBr(4) exhibit higher axial and lower radial growth rates, and negligible tapering compared with NWs grown in the absence of CBr(4) under the same conditions. We attribute this primarily to the suppression of the 2d growth rate by CBr(4), which enhances the axial growth rate of the nanowires. NWs grown with CBr(4) show stacking-fault-free zincblende structure, while the NWs grown without CBr(4) show a high density of stacking faults. This work underlines the striking effects which precursor chemistry can have on nanowire morphology.     

In situ TEM observation of heterogeneous phase transition of a constrained single-crystalline Ag2Te nanowire

Laterally epitaxial single crystalline Ag2Te nanowires (NWs) are synthesized on sapphire substrates by the vapor transport method. We observed the phase transitions of these Ag2Te NWs via in situ transmission electron microscopy (TEM) after covering them with Pt layers. The constrained NW shows phase transition from monoclinic to a body-centered cubic (bcc) structure near the interfaces, which is ascribed to the thermal stress caused by differences in the thermal expansion coefficients. Furthermore, we observed the nucleation and growth of bcc phase penetrating into the face-centered cubic matrix at 200 °C by high-resolution TEM in real time. Our results would provide valuable insight into how compressive stresses imposed by overlayers affect behaviors of nanodevices.     

Synthesis and properties of single-crystal FeSi nanowires

We report for the first time the chemical synthesis of free-standing single-crystal nanowires (NWs) of FeSi, the only transition-metal Kondo insulator and the host structure for ferromagnetic semiconductor Fe(x)Co(1-x)Si. Straight and smooth FeSi nanowires are produced on silicon substrates covered with a thin layer of silicon oxide through the decomposition of the single-source organometallic precursor trans-Fe(SiCl3)2(CO)4 in a simple chemical vapor deposition process. Unlike typical vapor-liquid-solid (VLS) NW growth, FeSi NWs form without the addition of metal catalysts, have no catalyst tips, and depend strongly on the surface employed. X-ray spectroscopy verifies the identity and the room-temperature metallic nature of FeSi NWs. Room-temperature electrical transport measurements using NW devices show an average resistivity of 210 micro Omega cm, similar to the value for bulk FeSi. Investigations into the low-temperature physical properties of the first one-dimensional Kondo insulator and the possible new NW growth mechanism are underway. This unique synthetic approach to FeSi NWs will be generally applicable to many other transition-metal silicides.