Self-catalyzed Growth Research Articles

Gradients of Be-dopant concentration in self-catalyzed GaAs nanowires

Effective and controllable doping is instrumental for enabling the use of III–V semiconductor nanowires (NWs) in practical electronics and optoelectronics applications. To this end, dopants are incorporated during self-catalyzed growth via vapor–liquid–solid mechanism through the catalyst droplet or by vapor–solid mechanism of the sidewall growth. The interplay of these mechanisms together with the competition between axial elongation and radial growth of NWs can result in dopant concentration gradients along the NW axis. Here, we report an investigation of Be-doped p-type GaAs NWs grown by the self-catalyzed method on lithography-free Si/SiOx templates. The influence of dopant incorporation on the structural properties of the NWs is analyzed by scanning and transmission electron microscopy. By combining spatially resolved Raman spectroscopy and transport characterization, we are able to estimate the carrier concentration, mobility and resistivity on single-NW level. We show that Be dopants are incorporated predominantly by vapor–solid mechanism for low Be flux, while the relative contribution of vapor–liquid–solid incorporation is increased for higher Be flux, resulting in axial dopant gradients that depend on the nominal doping level.

The construction of Mycobacterium tuberculosis 16S rDNA MSPQC sensor based on Exonuclease III-assisted cyclic signal amplification

Tuberculosis caused by Mycobacterium tuberculosis (M. tuberculosis) remains one of the most serious infectious diseases all over the world. The key to reduce the spread and mortality rate of tuberculosis is to develop faster and more sensitive approaches for detection of M. tuberculosis. However, current detection methods can not meet the requirements of rapid clinical M. tuberculosis detection in terms of detection time. Herein, a new 16S rDNA multichannel series piezoelectric quartz crystal (MSPQC) sensor based on Exonuclease III (Exo III)-aided target recycling has been developed for rapid detection of M. tuberculosis. The specific 16S rDNA fragment of M. tuberculosis was used as biomarker, DNA capture probes complementary to the biomarker were designed and modified on the surface of AuNPs. The Exo III which could recognise hybrid duplexes and selectively digest DNA capture probe was used to assist digestion cycle by digesting DNA capture probe and releasing the intact target fragment. After all DNA probes loading on the surface of AuNPs were removed, the surface of AuNPs was exposed and conductive connection was formed between the nanogap network electrode by self-catalytic growth of exposed AuNPs in the glucose and HAuCl4 solution. This resulted in sensitive response of M. tuberculosis sensor and M. tuberculosis was detected by recording this response. The limit of detection (LOD) of the method was 20 CFU/mL and the detection time was less than 3 h. It was expected to be widely used in detection methods of M. tuberculosis.

In-situ solid-state growth of N, S codoped carbon nanotubes encapsulating metal sulfides for high-efficient-stable sodium ion storage

A self-catalyzed solid-state growth method was developed for the synthesis of N, S codoped carbon nanotubes (NS-CNTs) in-situ encapsulating strings of metal sulfide nanocrystals. In this facile one-pot synthesis, the cross-linking reactions at the initial stage enable a well dispersion of metal ions (further metal sulfide clusters) in the chelate complex. By using the surrounding solid carbon as feedstocks, the metal sulfide clusters on the surface can catalyze the continuous growth of ultralong NS-CNTs in-situ encapsulating strings of metal sulfide nanocrystals. The representative Ni3S2-encapsulated NS-CNTs can deliver reversible capacitances of 500 mAh g−1 (@0.03 A/g) and 463 mAh g−1 (@0.2 A/g) when investigated as anode materials in sodium-ion batteries. In particular, an outstanding initial Coulombic efficiency of 93% is achieved at a current density of 200 mA g−1 together with a long-term stability of over 300 cycles, attributing to the unique core-shell nanostructures with long tubes of over 10 μm and rich N, S co-dopants. Meanwhile, the generated ultrathin solid electrolyte interphases (SEI), consisting of robust inorganics and highly reversible species, such as polyethers, -CFx and carbonate, could successfully afford high stable and efficient sodium ion storage.

AlGaN nano-shell structure on a GaN nanorod formed with the pulsed MOCVD growth

An AlGaN/GaN multi-shell structure on a GaN nanorod (NR) is formed by using the self-catalytic pulsed growth process of metalorganic chemical vapor deposition with Ga and Al/N supplies in the first and second half-cycles, respectively. With Al supply, a thin AlGaN layer is precipitated near the end of a growth cycle to form the AlGaN/GaN structure. Because of the lower chemical potential for GaN nucleation, when compared with AlN, a GaN layer is first deposited in a growth cycle. AlGaN is not precipitated until the AlN nucleation probability becomes higher when the catalytic Ga droplet is almost exhausted. Because the Al adatoms on the NR sidewalls hinder the upward migration of Ga adatoms for contributing to the Ga droplet at the NR top, the size of the Ga droplet decreases along growth cycle leading to the decrease of GaN layer thickness at the top until a steady state is reached. In this process, the slant facet of an NR changes from the (1–102)-plane into (1–101)-plane. To interpret the observed growth behaviors, formulations are derived for theoretically modeling the AlN nucleation probability, NR height increment in each growth cycle, and the time of exhausting the Ga droplet in a cycle.

Growth mechanism identification of sputtered single crystalline bismuth nanowire

Single crystalline bismuth nanowire is recently considered as one of the most attractive low dimensional materials for the exploration of exotic higher-order topological properties. However, its growth mechanism by sputtering, which is regarded as one of the most cost-effective and simplest method, is still unrevealed. In this work, a bismuth nanowire growth model based on surface diffusion driven by chemical potential difference among crystal facets is proposed for sputtering method. The morphology evolution of bismuth nanowires is captured for the first time, and three corresponding growth stages are clearly discriminated. The possible self-catalyzed, stress-driven, and screw dislocation-driven nanowire growth mechanisms are precluded separately based on the theoretical and experimental data. The thoroughly understanding of growth mechanism is fundamental and necessary for fabrication of size-controllable bismuth nanowires and may pave the way for the preparation of other low dimensional materials and the investigation of novel physics.

Nanowire Growths, and Mechanisms of These Growths for Developing Novel Nanomaterials.

Nanowire nanoelectronics depends largely on nanowire materials characteristics. The nanowires are grown by a number of different mechanisms. A comparative and comprehensive investigation of nanowires by these mechanisms has been carried out to address the nanowire characteristics best suited to nanoelectronics. The development and optimization of nanowire processing techniques for viable nanowire nanoelectronics have also been elucidated. The fundamentals of the growth mechanisms that lead to the best possible nanowires have been evaluated, as well. Among various mechanisms, the vapor-liquid-solid (VLS) mechanism is the oldest. It is most widely employed mechanism for nanowire growth. Challenges to the VLS nanowires have been discussed. Attributes of nanowires by the self-catalytic growth (SCG) have been explored. The details of the growth conditions of the SCG mechanism have been explained. Photoluminescence characteristics of nanowires by various mechanisms have been compared. This comparison sheds light on the relative strengths of these mechanisms. Control of growth directions, in situ arrays of nanowire self-organization, vertically aligned positioning, nanowire quality, nanowire contamination; seed/substrate mismatch interface states, etc. are the keys to nanowire applicability to nanoelectronics, which have been described. The fundamentals of various issues such as self-organization, growth criteria, Ostward ripening, Stranski-Krastanov growth mode, etc. have been articulated. Defects, dislocations, and stacking faults are very detrimental for nanotechnology. Relative strengths and weaknesses of various mechanisms based on nanowire defects, dislocations, and stacking faults have been elaborated.

Fundamental aspects to localize self-catalyzed III-V nanowires on silicon

III-V semiconductor nanowires deterministically placed on top of silicon electronic platform would open many avenues in silicon-based photonics, quantum technologies and energy harvesting. For this to become a reality, gold-free site-selected growth is necessary. Here, we propose a mechanism which gives a clear route for maximizing the nanowire yield in the self-catalyzed growth fashion. It is widely accepted that growth of nanowires occurs on a layer-by-layer basis, starting at the triple-phase line. Contrary to common understanding, we find that vertical growth of nanowires starts at the oxide-substrate line interface, forming a ring-like structure several layers thick. This is granted by optimizing the diameter/height aspect ratio and cylindrical symmetry of holes, which impacts the diffusion flux of the group V element through the well-positioned group III droplet. This work provides clear grounds for realistic integration of III-Vs on silicon and for the organized growth of nanowires in other material systems.

Microlens-Enhanced Substrate Patterning and MBE Growth of GaP Nanowires

In this paper we demonstrate the results on selective area growth of GaP nanowires via self-catalyzed growth method using molecular beam epitaxy (MBE) technique on patterned Si(111) substrates. The pattern fabrication method on a base of the photolithography process over an array of microspherical lenses has been studied theoretically and then optimized in order to obtain the nanostructures with controlled morphology. It was found that the positive resist thickness corresponding to the best achievable resolution in the subwavelength region is 250 nm in case of 1.5 μm silica spheres and excitation with 365 nm LED. The silica growth mask for selective epitaxy was fabricated. The ordered array of GaP nanowires was synthesized with MBE. Large scale ordering and selectivity of the growth technique is demonstrated.

Two Methods of Calculation Ternary Nanowire Composition

The growth of ternary nanowires via the vapor-liquid-solid mechanism is one of the possible ways of band gap engineering. However, the dependence of ternary nanowire composition on growth parameters is still unclear. We compare two theoretical approaches to ternary nanowire composition determined either by the formation of the critical island or by the step-flow growth of the monolayer. For typical parameters of gold-catalyzed growth, these two models give similar results. In the case of self-catalyzed growth, the step-determined model predicts higher In content in the NW than the island-determined.

Analysis of recombination processes in polytype gallium arsenide nanowires

Here we investigate recombination in polytype gallium arsenide (GaAs) nanowires (NWs) for photovoltaic applications, through photoluminescence studies coupled with rate equation analysis. Polytype NWs exhibit switching between zinc-blende (ZB) and wurtzite (WZ) crystal phases along the wire due to rotational twinning during self-catalyzed growth. When photons are absorbed in polytype NWs, electrons and holes separate: in the simplest case, electrons quickly thermalize to the band-edge of the ZB phase, while holes thermalize to the band-edge of the WZ phase, recombining indirectly in space across the type-II offset. The recombination mechanisms of this system are investigated experimentally through time-resolved photoluminescence (TRPL) at liquid helium temperature, and time-integrated photoluminescence (TIPL) at various temperatures, for the baseline case of AlGaAs capped GaAs NWs. The effects of the surface recombination on sub-bandgap transitions are also investigated using Al2O3 and no capping at the surface. We infer that carriers quickly thermalize to the spatially closest, lowest energy level, where they radiatively recombine across a sub-bandgap energy gap at a slower radiative rate than band-to-band. We use a rate equation model to investigate different configurations of polytype defects along the wire, including the effects of the surface and temperature, which compares well with experiment considering spatially indirect recombination between different polytypes, and defect-related recombination due to twin planes and other defects.

Growth of foreign-catalyst-free vertical InAs/InSb heterostructure nanowires on Si (1 1 1) substrate by MOCVD

In this study, growth of Au-free InAs/InSb vertical heterostructure (HS) nanowires (NWs) on highly lattice mismatched Si (1 1 1) substrate by metal organic chemical vapor deposition (MOCVD) is demonstrated. Careful selections of InSb growth parameters lead to In-rich growth regime such that direct impingement of growth precursors on top of grown InAs NW (self-assembled) stems result in axial InSb HSs nucleation on InAs stems. The observed indium (In) droplet on InSb HS NW tip shows that InSb HS growth followed a self-catalyzed growth mechanism. InSb HS axial growth rate and morphology are controlled by growth temperature and growth time. A small change in the growth parameters significantly affects In particle accumulation, particle size and its chemical composition, and thus affects the InSb nucleation on the stem. Especially, it is found that (i) growth time of less 10 min yields vertical InSb HS NW on the top of the InAs stem, (ii) as the growth time increases above 10 min, the growth turned to be InSb wrapped InAs/InSb core-shell HS due to coaxial lateral growth. High resolution transmission electron microscopy (HRTEM) study reveals that all the InSb HSs exhibit pure zinc-blende (ZB) crystal structure. The results presented here provide significant guidelines for external catalyst-free InAs/InSb HS NW growth using MOCVD on silicon (Si) substrate for various optoelectronics and electronics applications.

A brief review on the growth mechanism of CuO nanowires via thermal oxidation

Abstract

Self-catalytic VLS growth one dimensional layered GaSe nanobelts for high performance photodetectors

Layered GaSe is an important binary semiconductor because of its anisotropic crystallography characteristics, wide band gap and attractive optical and electrical properties. Here, we report the self-catalytic growth of GaSe nanobelts via a simple chemical vapor deposition method. The quality of the as-grown nanobelts have been confirmed by transmission electron microscope and photoluminescence spectra. The field-effect transistors based on these individual GaSe nanobelts have been fabricated and show p-type semiconducting behaviors and on-off ratio of 106. The GaSe nanobelts not only show good responsivity of 164.4 A W−1, but also exhibit excellent stability and reliability, which are superior to their bulk counterparts. These results make GaSe nanobelt a promising optoelectronic material in integrated electronic/optoelectronic devices.

Self-catalyzed VLS growth of PbSe wires with significant suppression of the VS process

Controllable growth of high-quality PbSe wires with strong mid-infrared emission was achieved with significant suppression of the vapor–solid-grown cubes.

Theoretical modeling of the self-catalyzed nanowire growth: nucleation- and adsorption-limited regimes

In this paper we study theoretically influence of group 3 and group 5 deposition rates on A3B5 nanowires (NWs) self-catalyzed vapor–liquid–solid (VLS) growth. Our self-consistent approach allows numerical calculation of chemical potential difference of the growth species, group 5 atomic concentration in the droplet and NW elongation rate for two-component system depending on deposition rates. It is found that chemical potential difference tends to saturation with increasing group 5 deposition rate (J5), while increase of group 3 deposition rate (J3) leads to its logarithmic growth. Two growth regimes are distinguished depending on relation between the deposition rates: high J3 and low J5 leads to adsorption-limited NW growth and in the opposite case nucleation-limited regime occurs. Nanowire elongation rate increases linearly with group 3 deposition rate in nucleation-limited regime. Further increase of the deposition rate leads to saturation of the growth rate in adsorption-limited regime. On the contrary, elongation rate grows linearly with group 5 deposition rate in adsorption-limited regime with slower linear growth in nucleation-limited regime.

Straight Indium Antimonide Nanowires with Twinning Superlattices via a Solution Route.

Indium antimonide (InSb) enables diverse applications in electronics and optoelectronics. However, to date, there has not been a report on the synthesis of InSb nanowires (NWs) via a solution-phase strategy. Here, we demonstrate for the first time the preparation of high-quality InSb NWs with twinning superlattices from a mild solution-phase synthetic environment from the reaction of commercial triphenylantimony with tris(2,4-pentanedionato)-indium(III). This reaction occurs at low temperatures from 165 to 195 °C (optimized at ∼180 °C), which is the lowest temperature reported for the growth of InSb NWs to date. Investigations reveal that the InSb NWs are grown via a solution-liquid-solid (SLS) mechanism due to the catalysis of the initially formed indium droplets in the mild solution-phase reaction system. The twinning superlattices in the InSb NWs are determined with a pseudoperiodic length of ∼42 monolayers, which result from an oscillating self-catalytic growth related to the periodical fluctuation between reduction rate of In and Sb sources in the route. The optical pump-terahertz probe spectroscopic measurement suggests that the InSb NWs have potential for applications in high-speed optoelectronic nanodevices.

Investigation of growth parameters influence on Au-catalyzed ITO nanowires by electron beam evaporation method

Indium tin oxide (ITO) nanowires (NWs) were synthesized via Gold (Au) catalyst assisted Vapor–Liquid–Solid (VLS) growth mechanism by electron beam evaporation method (EBE). Effects of growth parameters such as evaporation rate of ITO, oxygen partial pressure, and different types of substrates on the growth of ITO NWs were studied. Experimental investigations show that the Au-catalyzed VLS growth mechanism was dominating over self-catalytic VLS growth mechanism at lower evaporation rates of ITO. Self-catalytic VLS growth was dominating over Au-catalyzed VLS growth at the higher ITO evaporation rates of 6 Å/sec and above. Furthermore, change of growth mechanism was explained by considering incubation time and diameter of the catalyst particles. It was observed that the NWs growth enhanced at lower oxygen partial pressures and completely seized at higher oxygen partial pressure of 6×10−4 mbar and above. The complete absence of NWs growth at higher oxygen partial was explained by considering the growth rates of VLS and VS deposition. It was found that, the ITO NWs growth is independent of type of the substrate used. Understanding the growth mechanism transition and growth behaviour with deposition parameters are novel findings for the present report. In addition to the above, optical and electrical properties of ITO NW films were also studied. These ITO NWs films will be useful in antireflection, transparent conducting oxide coating applications. Moreover, these ITO NWs films may improve the performance of the various optoelectronic devices.

Decoupling the Two Roles of Ga Droplets in the Self-Catalyzed Growth of GaAs Nanowires on SiOx/Si(111) Substrates

Liquid Ga droplets play a double role in the self-catalyzed growth of GaAs nanowires on Si(111) substrates covered with a native SiOx layer: they induce the formation of nanosized holes in SiOx and then drive the uniaxial nanowire growth directly onto the underlying Si. The independent control of the two mechanisms is a prerequisite for mastering the growth of nanowires, but it is challenging in a conventional growth procedure where they both take place under the same droplets. To that end, we have developed an in situ procedure where the Ga droplets used for the formation of SiOx holes are removed before new Ga droplets drive the growth of GaAs nanowires. In that way, it was possible to study the interaction between Ga droplets and SiOx, to create holes in SiOx with controlled number density and size, and, finally, to grow GaAs nanowires only within those holes. Our results show unprecedented control of the nanowire nucleation with unique possibilities: (1) deliberate control of the number density of nan...

A Two-Step Growth Pathway for High Sb Incorporation in GaAsSb Nanowires in the Telecommunication Wavelength Range

Self-catalyzed growth of axial GaAs1−xSbx nanowire (NW) arrays with bandgap tuning corresponding to the telecommunication wavelength of 1.3 µm poses a challenge, as the growth mechanism for axial configuration is primarily thermodynamically driven by the vapor-liquid-solid growth process. A systematic study carried out on the effects of group V/III beam equivalent (BEP) ratios and substrate temperature (Tsub) on the chemical composition in NWs and NW density revealed the efficacy of a two-step growth temperature sequence (initiating the growth at relatively higher Tsub = 620 °C and then continuing the growth at lower Tsub) as a promising approach for obtaining high-density NWs at higher Sb compositions. The dependence of the Sb composition in the NWs on the growth parameters investigated has been explained by an analytical relationship between the effective vapor composition and NW composition using relevant kinetic parameters. A two-step growth approach along with a gradual variation in Ga-BEP for offsetting the consumption of the droplets has been explored to realize long NWs with homogeneous Sb composition up to 34 at.% and photoluminescence emission reaching 1.3 µm at room temperature.

Self-catalyzed core-shell GaAs/GaNAs nanowires grown on patterned Si (111) by gas-source molecular beam epitaxy

We report structural studies on the epitaxial growth of GaAs/GaNAs core-shell nanowires (NWs) on patterned Si (111) substrates by self-catalyzed selective area growth using Gas-Source Molecular Beam Epitaxy. Epitaxial growth conditions were obtained using a combination of dry and time-sensitive wet etching of the SiO2 growth mask and native SiO2 layer, respectively. We found that higher growth temperatures resulted in a higher yield for the epitaxial growth of patterned self-catalyzed GaAs NWs on Si with an optimal temperature of 690 °C. The GaNAs shell growth at 500 °C was found to be conformal and maintained an epitaxial and dislocation-free interface with both the Si substrate and the GaAs nanowire. The micro-photoluminescence (μ-PL) measurement at 6 K revealed two bands peaking at 1.45 and 1.17 eV, which could be emission from the GaAs core and GaNAs shell. Transmission electron microscopy showed the zincblende crystal structure of GaAs and GaAs/GaNAs core-shell NWs with minimal twinning near the base of the GaAs nanowires and at the tips of the GaAs/GaNAs core/shell nanowires. This study illustrates the feasibility of the epitaxial growth of patterned GaAs with dilute nitride shells on Si substrates, which would have potential for Si-friendly intermediate band solar cells and telecom emitters.