Single Crystalline GeSe Van Der Waals Ribbons With Uniform Layer Stacking, High Carrier Mobility, and Adjustable Edge Morphology.

Performance of the group IV monochalcogenide GeSe in solar cells, electronic, and optoelectronic devices is expected to improve when high-quality single crystalline material is used rather than polycrystalline films. Crystalline flakes represent an attractive alternative to bulk single crystals as their synthesis may be developed to be scalable, faster, and with higher overall yield. However, large - and especially large and thin - single crystal flakes are notoriously hard to synthesize. Here it is demonstrated that vapor-liquid-solid growth combined with direct lateral vapor-solid incorporation produces high-quality single crystalline GeSe ribbons with tens of micrometers size and controllable thickness. Electron microscopy shows that the ribbons exhibit perfect equilibrium (AB) van der Waals stacking order without extended defects across the entire thickness, in contrast to the conventional case of substrate-supported flakes where material is added via layer-by-layer nucleation and growth on the basal plane. Electrical measurements show anisotropic transport and a high Hall mobility of 85 cm2 V-1 s-1, on par with the best single crystals to date. Growth from mixed GeSe and SnSe vapors, finally, yields ribbons with unchanged structure and composition but with jagged edges, promising for applications that rely on ample chemically active edge sites, such as catalysis or photocatalysis.

Composition and optical properties of (In, Ga)As nanowires grown by group-III-assisted molecular beam epitaxy.

(In, Ga) alloy droplets are used to catalyse the growth of (In, Ga)As nanowires by molecular beam epitaxy on Si(111) substrates. The composition, morphology and optical properties of these nanowires can be tuned by the employed elemental fluxes. To incorporate more than 10% of In, a high In/(In+Ga) flux ratio above 0.7 is required. We report a maximum In content of almost 30% in bulk (In, Ga)As nanowires for an In/(In+Ga) flux ratio of 0.8. However, with increasing In/(In+Ga) flux ratio, the nanowire length and diameter are notably reduced. Using photoluminescence and cathodoluminescence spectroscopy on nanowires covered by a passivating (In, Al)As shell, two luminescence bands are observed. A significant segment of the nanowires shows homogeneous emission, with a wavelength corresponding to the In content in this segment, while the consumption of the catalyst droplet leads to a spectrally-shifted emission band at the top of the nanowires. The (In,Ga)As nanowires studied in this work provide a new approach for the integration of infrared emitters on Si platforms.

Composition of Vapor-Liquid-Solid III-V Ternary Nanowires Based on Group-III Intermix.

Compositional control in III-V ternary nanowires grown by the vapor-liquid-solid method is essential for bandgap engineering and the design of functional nanowire nano-heterostructures. Herein, we present rather general theoretical considerations and derive explicit forms of the stationary vapor-solid and liquid-solid distributions of vapor-liquid-solid III-V ternary nanowires based on group-III intermix. It is shown that the vapor-solid distribution of such nanowires is kinetically controlled, while the liquid-solid distribution is in equilibrium or nucleation-limited. For a more technologically important vapor-solid distribution connecting nanowire composition with vapor composition, the kinetic suppression of miscibility gaps at a growth temperature is possible, while miscibility gaps (and generally strong non-linearity of the compositional curves) always remain in the equilibrium liquid-solid distribution. We analyze the available experimental data on the compositions of the vapor-liquid-solid AlxGa1-xAs, InxGa1-xAs, InxGa1-xP, and InxGa1-xN nanowires, which are very well described within the model. Overall, the developed approach circumvents uncertainty in choosing the relevant compositional model (close-to-equilibrium or kinetic), eliminates unknown parameters in the vapor-solid distribution of vapor-liquid-solid nanowires based on group-III intermix, and should be useful for the precise compositional tuning of such nanowires.

Self-Catalytic Ternary Compounds for Efficient Synthesis of High-Quality Boron Nitride Nanotubes.

The large-scale synthesis of high-quality boron nitride nanotubes (BNNTs) has attracted considerable interests due to their applications in nanocomposites, thermal management, and so on. Despite decades of development, efficient preparation of high-quality BNNTs, which relies on the effective design of precursors and catalysts and deep insights into the catalytic mechanisms, is still urgently needed. Here, a self-catalytic process is designed to grow high-quality BNNTs using ternary W-B-Li compounds. W-B-Li compounds provide boron source and catalyst for BNNTs growth. High-quality BNNTs are successfully obtained via this approach. Density functional theory-based molecular dynamics (DFT-MD) simulations demonstrate that the Li intercalation into the lattice of W2 B5 promotes the formation of W-B-Li liquid and facilitates the compound evaporation for efficient BNNTs growth. This work demonstrates a high-efficient self-catalytic growth of high-quality BNNTs via ternary W-B-Li compounds, providing a new understanding of high-quality BNNTs growth.

Dynamics of Monolayer Growth in Vapor-Liquid-Solid GaAs Nanowires Based on Surface Energy Minimization.

The vapor-liquid-solid growth of III-V nanowires proceeds via the mononuclear regime, where only one island nucleates in each nanowire monolayer. The expansion of the monolayer is governed by the surface energetics depending on the monolayer size. Here, we study theoretically the role of surface energy in determining the monolayer morphology at a given coverage. The optimal monolayer configuration is obtained by minimizing the surface energy at different coverages for a set of energetic constants relevant for GaAs nanowires. In contrast to what has been assumed so far in the growth modeling of III-V nanowires, we find that the monolayer expansion may not be a continuous process. Rather, some portions of the already formed monolayer may dissolve on one of its sides, with simultaneous growth proceeding on the other side. These results are important for fundamental understanding of vapor-liquid-solid growth at the atomic level and have potential impacts on the statistics within the nanowire ensembles, crystal phase, and doping properties of III-V nanowires.

Calculation of Hole Concentrations in Zn Doped GaAs Nanowires.

We have previously demonstrated that we can grow p-type GaAs nanowires using Zn doping during gold catalyzed growth with aerotaxy. In this investigation, we show how to calculate the hole concentrations in such nanowires. We base the calculations on the Zhang-Northrup defect formation energy. Using density functional theory, we calculate the energy of the defect, a Zn atom on a Ga site, using a supercell approach. The chemical potentials of Zn and Ga in the liquid catalyst particle are calculated from a thermodynamically assessed database including Au, Zn, Ga, and As. These quantities together with the chemical potential of the carriers enable us to calculate the hole concentration in the nanowires self-consistently. We validate our theoretical results against aerotaxy grown GaAs nanowires where we have varied the hole concentration by varying the Zn/Ga ratio in the aerotaxy growth.

Oscillations of As Concentration and Electron-to-Hole Ratio in Si-Doped GaAs Nanowires.

III-V nanowires grown by the vapor-liquid-solid method often show self-regulated oscillations of group V concentration in a catalyst droplet over the monolayer growth cycle. We investigate theoretically how this effect influences the electron-to-hole ratio in Si-doped GaAs nanowires. Several factors influencing the As depletion in the vapor-liquid-solid nanowire growth are considered, including the time-scale separation between the steps of island growth and refill, the "stopping effect" at very low As concentrations, and the maximum As concentration at nucleation and desorption. It is shown that the As depletion effect is stronger for slower nanowire elongation rates and faster for island growth relative to refill. Larger concentration oscillations suppress the electron-to-hole ratio and substantially enhance the tendency for the p-type Si doping of GaAs nanowires, which is a typical picture in molecular beam epitaxy. The oscillations become weaker and may finally disappear in vapor deposition techniques such as hydride vapor phase epitaxy, where the n-type Si doping of GaAs nanowires is more easily achievable.

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 (SiO2 NWs) by regulating mass transport of silicon monoxide (SiO) vapor. SiO2 NWs 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. SiO2 NW morphology is governed by diffusion-reaction process of SiO vapor, whereby directional growth of SiO2 NWs 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.

Synthesis of Multiwall Boron Nitride (BN) Nanotubes by a PVD Method Based on Vapor-Liquid-Solid Growth.

Multiwall boron nitride (BN) nanotubes were synthesized by a novel physical vapor deposition (PVD) method, in which the BN nanotubes grow on a compact substrate composed of AlN, γ-Al2O3, Y2O3, and carbon powders. The obtained BN nanotubes assemble in an orderly manner with a typical length of over one millimeter and a diameter of one-hundred nanometers. The hollow multiwall tubes have a spherical tip, which is presumed to be a liquid drop at the synthesis temperature, indicating the vapor-liquid-solid (VLS) growth mechanism.

Impact of N Incorporation on VLS Growth of GaP(N) Nanowires Utilizing UDMH.

III-V nanowires (NWs) possess great potential for use in future semiconductor technology. Alloying with dilute amounts of nitrogen provides further flexibility in tuning their material properties. In this study, we report on successful in situ nitrogen incorporation into GaP(N) NWs during growth via the Au-catalyzed vapor-liquid-solid (VLS) mechanism. The impact of the nitrogen precursur unsymmetrical dimethyl hydrazine (UDMH) on morphology was found to be overall beneficial as it strongly reduces tapering. Analysis of the crystal structure of NWs with and without N reveals zinc blende structure with an intermediate amount of stacking faults (SF). Interestingly, N incorporation leads to segments completely free of SFs, which are related to dislocations transverse to the growth direction.

Investigation of growth parameters influence on self-catalyzed ITO nanowires by high RF-power sputtering.

ITO nanowires have been successfully fabricated using a radio-frequency sputtering technique with a high RF-power of 250W. The fabrication of the ITO nanowires has been optimized through the study of oxygen flow rates, temperatures and RF-power. The difference in the morphology of the ITO nanowires prepared by using a new target and a used target has been first observed and the mechanism for the difference has been discussed in detail. A hollow structure and air voids within the nanowires are formed during the process of the nanowire growth. The ITO nanowires fabricated by this method has demonstrated good conductivity (15Ω/sq) and a transmittance of more than 64% at a wavelength longer than 550nm after annealing. Furthermore, detailed microstructure studies show that the ITO nanowires exhibit a large number of oxygen vacancies. As a result, it is expected that they can be useful for the fabrication of gas sensor devices.

Self-assembly of silicon nanowires studied by advanced transmission electron microscopy.

Scanning transmission electron microscopy (STEM) was successfully applied to the analysis of silicon nanowires (SiNWs) that were self-assembled during an inductively coupled plasma (ICP) process. The ICP-synthesized SiNWs were found to present a Si-SiO2 core-shell structure and length varying from ≈100 nm to 2-3 µm. The shorter SiNWs (maximum length ≈300 nm) were generally found to possess a nanoparticle at their tip. STEM energy dispersive X-ray (EDX) spectroscopy combined with electron tomography performed on these nanostructures revealed that they contain iron, clearly demonstrating that the short ICP-synthesized SiNWs grew via an iron-catalyzed vapor-liquid-solid (VLS) mechanism within the plasma reactor. Both the STEM tomography and STEM-EDX analysis contributed to gain further insight into the self-assembly process. In the long-term, this approach might be used to optimize the synthesis of VLS-grown SiNWs via ICP as a competitive technique to the well-established bottom-up approaches used for the production of thin SiNWs.