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Metal nanowires show promise in a broad range of applications and can be fabricated via a number of methods, such as vapor–liquid–solid process and template-based electrodeposition. However, the synthesis of Al nanowires (NWs) is still challenging from the stable alumina substrate. In this work, the Ni-catalyzed fabrication of Al NWs has been realized using various Al2O3 substrates. The growth dynamics of Al NWs on Ni/Al2O3 was studied using in situ transmission electron microscopy (TEM). The effect of alumina structures, compositions, and growth temperature were investigated. The growth of Al NWs correlates with the Na addition to the alumina support. Since no eutectic mixture of nickel aluminide was formed, a mechanism of Ni-catalyzed reduction of Al2O3 for Al NWs growth has been proposed instead of the vapor–liquid–solid mechanism. The key insights reported here are not restricted to Ni-catalyzed Al NWs growth but can be extended to understanding the dynamic change and catalytic performance of Ni/Al2O3 under working conditions. [ABSTRACT FROM AUTHOR]

A stretch-release strategy is proposed to analyze the problem of surface energy-induced stress fields in nanocrystals, which is resolved into a stretch sub-problem and a release sub-problem using the superposition principle. The surface effect of silicon nanowires with hexagonal cross-sections is analyzed by the proposed method. The severe stress concentration near the triple junctions of the wire surfaces and the large shear stress on the plane {111} is quantified, which provides a solid mechanical explanation for the kink phenomena in growth transition from direction 〈111〉 to 〈112〉 observed in experiments. Different from the conventional view of negligible surface effect for bulk material, we found that there exists a size-independent part of the surface effect on the stress in the order of tens or hundreds of mega Pascal, which corresponds to the stretch-induced biaxial stress in the surface layer and the shape influence of the geometry of nanocrystals. This size-independent part could well explain the size-independent kinking phenomenon during the growth of silicon nanowires. [ABSTRACT FROM AUTHOR]

We discuss the conversion of a pure screw dislocation into a helical (single helix) dislocation through absorption of vacancies. These helical dislocations having some edge and mostly screw character can pair up through attractive interaction to form a double helix. This attractive interaction between two helices is derived through the edge component of mixed dislocations and from the presence of vacancy jogs. Recent results on the growth of GeS [] and PbS [] nanowires with Eshelby twist showed large disagreements between experimental and theoretical Burgers vectors for these crystal structures, which can be resolved by introducing the double helix of screw dislocations.

Abstract Growth of high-quality III–V nanowires at a low cost for optoelectronic and electronic applications is a long-term pursuit of research. Still, controlled synthesis of III–V nanowires using chemical vapor deposition method is challenge and lack theory guidance. Here, we show the growth of InP and GaP nanowires in a large area with a high density using a vacuum chemical vapor deposition method. It is revealed that high growth temperature is required to avoid oxide formation and increase the crystal purity of InP nanowires. Introduction of a small amount of Ga into the reactor leads to the formation of GaP nanowires instead of ternary InGaP nanowires. Thermodynamic calculation within the calculation of phase diagrams (CALPHAD) approach is applied to explain this novel growth phenomenon. Composition and driving force calculations of the solidification process demonstrate that only 1 at.% of Ga in the catalyst is enough to tune the nanowire formation from InP to GaP, since GaP nucleation shows a much larger driving force. The combined thermodynamic studies together with III–V nanowire growth studies provide an excellent example to guide the nanowire growth.

Bottom-up growth offers precise control over the structure and geometry of semiconductor nanowires (NWs), enabling a wide range of possible shapes and seamless heterostructures for applications in nanophotonics and electronics. The most common vapor-liquid-solid (VLS) growth method features a complex interaction between the liquid metal catalyst droplet and the anisotropic structure of the crystalline NW, and the growth is mainly orchestrated by the triple-phase line (TPL). Despite the intrinsic mismatch between the droplet and the NW symmetries, its discussion has been largely avoided because of its complexity, which has led to the situation when multiple observed phenomena such as NW axial asymmetry or the oscillating truncation at the TPL still lack detailed explanation. The introduction of an electric field control of the droplet has opened even more questions, which cannot be answered without properly addressing three-dimensional (3D) structure and morphology of the NW and the droplet. This work describes the details of electric-field-controlled VLS growth of germanium (Ge) NWs using environmental transmission electron microscopy (ETEM). We perform TEM tomography of the droplet-NW system during an unperturbed growth, then track its evolution while modulating the bias potential. Using 3D finite element method (FEM) modeling and crystallographic considerations, we provide a detailed and consistent mechanism for VLS growth, which naturally explains the observed asymmetries and features of a growing NW based on its crystal structure. Our findings provide a solid framework for the fabrication of complex 3D semiconductor nanostructures with ultimate control over their morphology.

We discuss the conversion of a pure screw dislocation into a helical (single helix) dislocation through absorption of vacancies. These helical dislocations having some edge and mostly screw character can pair up through attractive interaction to form a double helix. This attractive interaction between two helices is derived through the edge component of mixed dislocations and from the presence of vacancy jogs. Recent results on the growth of GeS [1] and PbS [3] nanowires with Eshelby twist showed large disagreements between experimental and theoretical Burgers vectors for these crystal structures, which can be resolved by introducing the double helix of screw dislocations. Single helix and double helix of screw dislocations provide continuous growth steps for all crystalline structures. Thus, nanostructures materials can be grown without any need for catalysts. These dislocations also introduce Eshelby twist and create novel nanostructured materials with unique properties. [ABSTRACT FROM AUTHOR]

Growth of high-quality III–V nanowires at a low cost for optoelectronic and electronic applications is a long-term pursuit of research. Still, controlled synthesis of III–V nanowires using chemical vapor deposition method is challenge and lack theory guidance. Here, we show the growth of InP and GaP nanowires in a large area with a high density using a vacuum chemical vapor deposition method. It is revealed that high growth temperature is required to avoid oxide formation and increase the crystal purity of InP nanowires. Introduction of a small amount of Ga into the reactor leads to the formation of GaP nanowires instead of ternary InGaP nanowires. Thermodynamic calculation within the calculation of phase diagrams (CALPHAD) approach is applied to explain this novel growth phenomenon. Composition and driving force calculations of the solidification process demonstrate that only 1 at.% of Ga in the catalyst is enough to tune the nanowire formation from InP to GaP, since GaP nucleation shows a much larger driving force. The combined thermodynamic studies together with III–V nanowire growth studies provide an excellent example to guide the nanowire growth. [ABSTRACT FROM AUTHOR]

Herein, the self‐assembled formation of AlN nanowires (NWs) by molecular beam epitaxy on sputtered TiN films on sapphire is demonstrated. This choice of substrate allows growth at an exceptionally high temperature of 1180 °C. In contrast to previous reports, the NWs are well separated and do not suffer from pronounced coalescence. This achievement is explained by sufficient Al adatom diffusion on the substrate and the NW sidewalls. The high crystalline quality of the NWs is evidenced by the observation of near‐band‐edge emission in the cathodoluminescence spectrum. The key factor for the low NW coalescence is the TiN film, which spectroscopic ellipsometry and Raman spectroscopy indicate to be stoichiometric. Its metallic nature will be beneficial for optoelectronic devices using these NWs as the basis for (Al,Ga)N/AlN heterostructures emitting in the deep ultraviolet spectral range. [ABSTRACT FROM AUTHOR]

Photoelectrical mechanism of single Indium Oxide nanowires have been investigated using current-voltage characteristics measurements varying with temperature. The fabricated In 2 O 3 nanowires show high photosensitivity up to 140 at room temperature. It is shown that the photosensitivity of In 2 O 3 nanowires decreases by increasing the temperature due to shorter carrier life time, higher dark conductance and lower electron mobility at higher temperatures. The significantly enhanced photoconduction of single-In 2 O 3 nanowires observed under UV illumination is attributed mainly to the transition from defect levels, which are introduced by the oxygen vacancies that are mostly present on the surface of the nanowires. [ABSTRACT FROM AUTHOR]

Gases that are widely used in research and industry have a significant effect on both the configuration of solid materials and the evolution of reactive systems. Traditional studies on gas-solid interactions have mostly been static and post-mortem and unsatisfactory for elucidating the real active states during the reactions. Recent developments of controlled-atmosphere transmission electron microscopy (TEM) have led to impressive progress towards the simulation of real-world reaction environments, allowing the atomic-scale recording of dynamic events. In this review, on the basis of the in situ research of our group, we outline the principles and features of the controlled-atmosphere TEM techniques and summarize the significant recent progress in the research activities on gas-solid interactions, including nanowire growth, catalysis, and metal failure. Additionally, the challenges and opportunities in the real-time observations on such platform are discussed. [ABSTRACT FROM AUTHOR]

Selective area epitaxy is a promising approach to define nanowire networks for topological quantum computing. However, it is challenging to concurrently engineer nanowire morphology, for carrier confinement, and precision doping, to tune carrier density. We report a strategy to promote Si dopant incorporation and suppress dopant diffusion in remote doped InGaAs nanowires templated by GaAs nanomembrane networks. Growth of a dilute AlGaAs layer following doping of the GaAs nanomembrane induces incorporation of Si that otherwise segregates to the growth surface, enabling precise control of the spacing between the Si donors and the undoped InGaAs channel; a simple model captures the influence of Al on the Si incorporation rate. Finite element modeling confirms that a high electron density is produced in the channel.

Nanocrystal facets evolution is critical for designing nanomaterial morphology and controlling their properties. In this work, we report a unique high-energy crystal facets evolution phenomenon at the tips of wurtzite zinc oxide nanowires (NWs). As the zinc vapor supersaturation decreased at the NW deposition region, the NW tip facets evolved from the (0001) surface to the {101̅3} surface and subsequently to the {112̅2} surface and eventually back to the flat (0001) surface. A series of NW tip morphology was observed in accordance to the different combinations of exposed facets. Exposure of the high-energy facets was attributed to the fluctuation of the energy barriers for the formation of different crystal facets during the layer-by-layer growth of the NW tip. The energy barrier differences between these crystal facets were quantified from the surface area ratios as a function of supersaturation. On the basis of the experimental observation and kinetics analysis, we argue that at appropriate deposition conditions exposure of the crystal facets at NW growth front is not merely determined by the surface energy. Instead, the NW may choose to expose the facets with minimal formation energy barrier, which can be determined by the Ehrlich-Schwoebel barrier variation. This empirical law for the NW tip facet formation was in analogy to the Ostwald-Lussac law of phase transformation, which brings a new insight toward nanostructure design and controlled synthesis. [ABSTRACT FROM AUTHOR]

Evidence for the influence of Si(001)-(2 × 1) surface reconstruction on the elongation direction of CoSi 2 flat islands is discussed in this paper. Step height analysis of these flat islands shows that flat island heights, H A , follow discrete values of N A such that H A = mN A + c , where N A = 1, 2, 3, …, m is equivalent to the number of monoatomic step height (1.4 Å) of the Si(001) surface, and c is the initial island height when N A = 0. The N A values were found to be correlated to the flat island elongation direction with respect to the (2 × 1) dimer rows. For a given terrace, the preferred elongation direction of these flat islands is parallel to the Si dimer rows. As a result, orthogonally elongated islands are clearly resolved on adjacent terraces, which are separated by monoatomic steps. The endotaxial growth of these flat islands is thus also influenced by the anisotropic adatom diffusion due to (2 × 1) surface reconstruction. [ABSTRACT FROM AUTHOR]

We present a theoretical analysis of vertical versus radial growth of InP nanowires obtained by selective area metalorganic molecular beam epitaxy at temperatures from 400 to 480 °C. It is shown that the nanowire formation is controlled by surface diffusion of indium adatoms whose concentration on the sidewalls determines the local radial growth rate. Two models for the radial growth rate are considered, with linear (step flow) and exponential (two-dimensional nucleation) dependences on the adatom concentration. The linear model reproduces fairly well the vertical elongation rate and the observed shapes of the upper nanowire parts, while the non-linear shapes at the nanowire bottom are described correctly only within the exponential model. We estimate the effective migration lengths of indium, which are of the order of several hundreds of nanometers between 400 and 450 °C. The migration length increases up to 2200 nm at 480 °C so that radial growth is completely suppressed at this high growth temperature. [ABSTRACT FROM AUTHOR]

Various approaches are described to synthesize Mn-doped Ge NWs using UHV-CVD growth via the VLS mechanism. We explored in-situ doping using an organo-metallic precursor (TCMn) and tested different catalyst seeds such as Au, Au-Mn or Ge-Mn. All in-situ attempts failed to incorporate Mn atoms into the NW. Moreover we observed that the use of TCMn and/or Au-Mn catalysts have a crippling effect on the growth kinetics. Finally, ex-situ doping with ion implantation is right now considered as a doping alternative. We report on preliminary results aiming at testing the radiation resistance of Ge NWs embedded in a removable silica matrix. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Crystalline Ga2O3 nanowires are synthesized via an Au-catalyzed, as well as a self-catalyzed, growth by a low-temperature metal-organic (MO)CVD process using tBu3Ga as a novel Ga source. Morphology, elemental composition, and crystallinity of the resulting nanowires are analyzed by scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and selected area electron diffraction (SAED). Photoluminescence (PL) spectra of the Ga2O3 nanowires show efficient defect-luminescence in the visible and UV ranges with blue and green emission peaks at 430nm and 512 nm, respectively, at room temperature. [ABSTRACT FROM AUTHOR]

Highly controlled particle-assisted growth of semiconductor nanowires has been performed for many years, and a number of novel nanowire-based devices have been demonstrated. Full control of the epitaxial growth is required to optimize the performance of devices, and gold seed particles are known to provide the most controlled growth. Successful nanowire growth from gold particles generated and deposited by various different methods has been reported, but no investigation has yet been performed to compare the effects of gold particle generation and deposition methods on nanowire growth. In this article we present a direct comparative study of the effect of the gold particle creation and deposition methods on nanowire growth characteristics and nanowire crystal structure, and investigate the limitations of the different generation and deposition methods used.[Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

This paper discusses the electronic transport properties of nanowire field-effect transistors (NW-FETs). Four different device concepts are studied in detail: Schottky-barrier NW-FETs with metallic source and drain contacts, conventional-type NW-FETs with doped NW segments as source and drain electrodes, and, finally, two new concepts that enable steep turn-on characteristics, namely, NW impact ionization FETs and tunnel NW-FETs. As it turns out, NW-FETs are, to a large extent, determined by the device geometry, the dimensionality of the electronic transport, and the way of making contacts to the NW. Analytical as well as simulation results are compared with experimental data to explain the various factors impacting the electronic transport in NW-FETs. [ABSTRACT FROM AUTHOR]

Abstract: Silicon-based nanotechnology is highly promising since it is compatible with conventional silicon integrated technique. To date silicon nanowires have been synthesized by varying experimental conditions and a wide range of electronic nanodevices have been demonstrated. A key challenge facing the device realization is the elaboration of a self-assembly nanotechnology enabling the formation of nanoobjects with preset shape and size, crystalline structure, chemical composition, and consequently, physical and chemical properties. To integrate nanodevices into conventional silicon chips, a spatial location and density distribution of nanowires on a chip should be controlled as well. To locate nanowires on desired places with a specific distribution, one should commonly use nanolithography. In this paper we describe a new possibility and its practical realization on silicon for metal-enhanced growth of nanowires with a self-arrangement over the substrate. The proposed physical and mathematical models of the effect is a thermo-stimulated analogue of Liesegang pattern theory. Results of the modeling fit satisfactorily a geometry and nanowire size distribution inside the structure. [Copyright &y& Elsevier]