Germanium Nanowire Growth Below the Eutectic Temperature

Nucleation-Limited Kinetics of GaAs Nanostructures Grown by Selective Area Epitaxy: Implications for Shape Engineering in Optoelectronics Devices

The growth kinetics of vertical III-V nanowires (NWs) were clarified long ago. The increasing aspect ratio of NWs results in an increase in the surface area, which, in turn, enhances the material collection. The group III adatom diffusion from the NW sidewalls to the top sustains a superlinear growth regime. In this work, we report on the growth of different GaAs nanostructures by selective area MOVPE on GaAs (111)B substrates. We show that the opening dimensions and geometry qualitatively alter the morphology and height evolution of the structures. We compare the time evolution of vertical GaAs NWs stemming from circular holes and horizontal GaAs nanomembranes (NMs) growing from one-dimensional (1D) rectangular slits on the same substrate. While NW heights grow exponentially with time, NMs surprisingly exhibit sublinear kinetics. The absence of visible atomic steps on the top facets of both NWs and NMs suggests layer-by-layer growth in the mononuclear mode. We interpret these observations within a self-consistent growth model, which links the diffusion flux of Ga adatoms to the position- and shape-dependent nucleation rate on top of NWs and NMs. Specifically, the island nucleation rate is lower on top of the NMs than that on the NWs, resulting in the total diffusion flux being directed from the top facet to the sidewalls. This gives a sublinear height evolution for the NMs. These results open innovative perspectives for shape engineering of III-V nanostructures and new avenues for the design of optoelectronics and photonic devices.

Local Heating Induced Single-Crystalline Phase Control in Electrochemical Synthesis of Nanomaterials

Development of synthetic strategies selectively yielding single crystals is desired owing to the facet-dependent chemical reactivities. Recent advances in electrochemical materials synthesis yielded nanomaterials that are surfactant-free, however, typically in polycrystalline forms. In this work, an electrochemical synthetic strategy selectively yielding single-crystalline nanoparticles by implementation of surface-selective heating of the working electrode is developed. Single crystals of copper, silver, gold, and platinum are afforded, and the crystallinity verified by electron diffraction and chemical reactivity studies. Notably, Cu (100) surface prepared by electrochemical synthesis yielded high single product selectivity when applied to electrochemical CO2 reduction catalysis.

Thermal Stability and Crystallization Processes of Pd78Au4Si18 Thin Films Visualized via In Situ TEM

Amorphous alloys or metallic glasses (MGs) thin films have attracted extensive attention in various fields due to their unique functional properties. Here, we use in situ heating transmission electron microscopy (TEM) to investigate the thermal stability and crystallization behavior of Pd-Au-Si thin films prepared by a pulsed laser deposition (PLD) method. Upon heating treatment inside a TEM, we trace the structural changes in the Pd-Au-Si thin films through directly recording high-resolution images and diffraction patterns at different temperatures. TEM observations reveal that the Pd-Au-Si thin films started to nucleate with small crystalline embryos uniformly distributed in the glassy matrix upon approaching the glass transition temperature Tg=625K, and subsequently, the growth of crystalline nuclei into sub-10 nm Pd-Si nanocrystals commenced. Upon further increasing the temperature to 673K, the thin films transformed to micro-sized patches of stacking-faulty lamellae that further crystallized into Pd9Si2 and Pd3Si intermetallic compounds. Interestingly, with prolonged thermal heating at elevated temperatures, the Pd9Si2 transformed to Pd3Si. Simultaneously, the solute Au atoms initially dissolved in glassy alloys and eventually precipitated out of the Pd9Si2 and Pd3Si intermetallics, forming nearly spherical Au nanocrystals. Our TEM results reveal the unique thermal stability and crystallization processes of the PLD-prepared Pd-Au-Si thin films as well as demonstrate a possibility of producing a large quantity of pure nanocrystals out of amorphous solids for various applications.

Presolidification in Eutectic Droplets

Evidence of presolidification, the counterpart to premelting, is reported. Near the eutectic temperature, T_{C}, the propagation direction of thermal gradient driven motion of eutectic Ge-Pt droplets on Ge(110) is determined by presolidification. Well above T_{C}, the micron-sized droplets move towards the hottest location at the substrate, irrespective of crystalline direction. At 90 K above T_{C}, a strong, unanticipated preference for propagation along the substrate [001] azimuth suddenly emerges, which is attributed to presolidification at the liquid-solid interface. The propagation along [001] is accompanied by a distinct change in shape from compact to elongated along [001].

Single-step fabrication of fibrous Si/Sn composite nanowire anodes by high-pressure He plasma sputtering for high-capacity Li-ion batteries

To realize high-capacity Si anodes for next-generation Li-ion batteries, Si/Sn nanowires were fabricated in a single-step procedure using He plasma sputtering at a high pressure of 100-500 mTorr without substrate heating. The Si/Sn nanowires consisted of an amorphous Si core and a crystalline Sn shell. Si/Sn composite nanowire films formed a spider-web-like network structure, a rod-like structure, or an aggregated structure of nanowires and nanoparticles depending on the conditions used in the plasma process. Anodes prepared with Si/Sn nanowire films with the spider-web-like network structure and the aggregated structure of nanowires and nanoparticles showed a high Li-storage capacity of 1219 and 977 mAh/g, respectively, for the initial 54 cycles at a C-rate of 0.01, and a capacity of 644 and 580 mAh/g, respectively, after 135 cycles at a C-rate of 0.1. The developed plasma sputtering process enabled us to form a binder-free high-capacity Si/Sn-nanowire anode via a simple single-step procedure.

About Some Fundamental Aspects of the Growth Mechanism Vapor-Liquid-Solid Nanowires

This study provides the formation of semiconductor nanowires (NWs) with a singular facet and a curved end surface by the vapor-liquid-solid (VLS) process that is analyzed and explained in details. Given the evidence, it is confirmed that the wettability of a liquid catalyst droplet on a crystal surface and the contact angle between the droplet and crystal play an essential role in the VLS process of NWs development. It is shown that for the VLS mechanism, the formation of NWs depends on the reduction in activation barrier to crystallization caused by the release of surplus-free energy by a spheroidizing drop in the region of the triple junction during the process of lowering surface area. This decreases the necessary supersaturation for the development of NW vertex facets at a fixed growth rate. The source of the extra free energy that drives the catalyst droplet movement during the steady-state development of NWs is the droplet’s outer surface. During the formation of NWs, those angles of inclination of the lateral surface NWs and droplet contact are obtained at which the solid/vapor, solid/liquid, and liquid/vapor interfaces experience the smallest increase in free energy. The wetting hysteresis is demonstrated to occur at the vertex of NWs, and the contact angle of a catalyst droplet may be regarded as an independent and fully-fledged thermodynamic parameter of the system’s state.

Tuning the Liquid-Vapour Interface of VLS Epitaxy for Creating Novel Semiconductor Nanostructures

Controlling the morphology and composition of semiconductor nano- and micro-structures is crucial for fundamental studies and applications. Here, Si-Ge semiconductor nanostructures were fabricated using photolithographically defined micro-crucibles on Si substrates. Interestingly, the nanostructure morphology and composition of these structures are strongly dependent on the size of the liquid-vapour interface (i.e., the opening of the micro-crucible) in the CVD deposition step of Ge. In particular, Ge crystallites nucleate in micro-crucibles with larger opening sizes (3.74-4.73 μm²), while no such crystallites are found in micro-crucibles with smaller openings of 1.15 μm². This interface area tuning also results in the formation of unique semiconductor nanostructures: lateral nano-trees (for smaller openings) and nano-rods (for larger openings). Further TEM imaging reveals that these nanostructures have an epitaxial relationship with the underlying Si substrate. This geometrical dependence on the micro-scale vapour-liquid-solid (VLS) nucleation and growth is explained within a dedicated model, where the incubation time for the VLS Ge nucleation is inversely proportional to the opening size. The geometric effect on the VLS nucleation can be used for the fine tuning of the morphology and composition of different lateral nano- and micro-structures by simply changing the area of the liquid-vapour interface.

DFT Analysis of Hole Qubits Spin State in Germanium Thin Layer

Due to the presence of a strong spin–orbit interaction, hole qubits in germanium are increasingly being considered as candidates for quantum computing. These objects make it possible to create electrically controlled logic gates with the basic properties of scalability, a reasonable quantum error correction, and the necessary speed of operation. In this paper, using the methods of quantum-mechanical calculations and considering the non-collinear magnetic interactions, the quantum states of the system 2D structure of Ge in the presence of even and odd numbers of holes were investigated. The spatial localizations of hole states were calculated, favorable quantum states were revealed, and the magnetic structural characteristics of the system were analyzed.

Regulated Dynamics with Two Monolayer Steps in Vapor-Solid-Solid Growth of Nanowires

The growth of ZnTe nanowires and ZnTe-CdTe nanowire heterostructures is studied by in situ transmission electron microscopy. We describe the shape and the change of shape of the solid gold nanoparticle during vapor-solid-solid growth. We show the balance between one monolayer and two monolayer steps, which characterizes the vapor-liquid-solid and vapor-solid-solid growth modes of ZnTe. We discuss the likely role of the mismatch strain and lattice coincidence between gold and ZnTe on the predominance of two monolayer steps during vapor-solid-solid growth and on the subsequent self-regulation of the step dynamics. Finally, the formation of an interface between CdTe and ZnTe is described.

Nanostructured Ge and GeSn films by high-pressure He plasma sputtering for high-capacity Li ion battery anodes

We fabricated nanostructured Ge and GeSn films using He radio-frequency magnetron plasma sputtering deposition. Monodisperse amorphous Ge and GeSn nanoparticles of 30-40 nm size were arranged without aggregation by off-axis sputtering deposition in the high He-gas-pressure range of 0.1 Torr. The Ge film porosity was over 30%. We tested the charge/discharge cycle performance of Li-ion batteries with nanostructured Ge and GeSn anodes. The Ge anode with a dispersed arrangement of nanoparticles showed a Li-storage capacity of 565 mAh/g after the 60th cycle. The capacity retention was markedly improved by the addition of 3 at% Sn in Ge anode. The GeSn anode (3 at% Sn) achieved a higher capacity of 1128 mAh/g after 60 cycles with 92% capacity retention. Precise control of the nano-morphology and electrical characteristics by a single step procedure using low temperature plasma is effective for stable cycling of high-capacity Ge anodes.

Bottom-up nanoscale patterning and selective deposition on silicon nanowires

We demonstrate a bottom-up process for programming the deposition of coaxial thin films aligned to the underlying dopant profile of semiconductor nanowires. Our process synergistically combines three distinct methods-vapor-liquid-solid nanowire growth, selective coaxial lithography via etching of surfaces (SCALES), and area-selective atomic layer deposition (AS-ALD)-into a cohesive whole. Here, we study ZrO₂on Si nanowires as a model system. Si nanowires are first grown with an axially modulated n-Si/i-Si dopant profile. SCALES then yields coaxial poly(methyl methacrylate) (PMMA) masks on the n-Si regions. Subsequent AS-ALD of ZrO₂occurs on the exposed i-Si regions and not on those masked by PMMA. We show the spatial relationship between nanowire dopant profile, PMMA masks, and ZrO₂films, confirming the programmability of the process. The nanoscale resolution of our process coupled with the plethora of available AS-ALD chemistries promises a range of future opportunities to generate structurally complex nanoscale materials and electronic devices using entirely bottom-up methods.

Nanowire-based sensor electronics for chemical and biological applications

Detection and recognition of chemical and biological species via sensor electronics are important not only for various sensing applications but also for fundamental scientific understanding. In the past two decades, sensor devices using one-dimensional (1D) nanowires have emerged as promising and powerful platforms for electrical detection of chemical species and biologically relevant molecules due to their superior sensing performance, long-term stability, and ultra-low power consumption. This paper presents a comprehensive overview of the recent progress and achievements in 1D nanowire synthesis, working principles of nanowire-based sensors, and the applications of nanowire-based sensor electronics in chemical and biological analytes detection and recognition. In addition, some critical issues that hinder the practical applications of 1D nanowire-based sensor electronics, including device reproducibility and selectivity, stability, and power consumption, will be highlighted. Finally, challenges, perspectives, and opportunities for developing advanced and innovative nanowire-based sensor electronics in chemical and biological applications are featured.

Dissecting Biological and Synthetic Soft-Hard Interfaces for Tissue-Like Systems

Soft and hard materials at interfaces exhibit mismatched behaviors, such as mismatched chemical or biochemical reactivity, mechanical response, and environmental adaptability. Leveraging or mitigating these differences can yield interfacial processes difficult to achieve, or inapplicable, in pure soft or pure hard phases. Exploration of interfacial mismatches and their associated (bio)chemical, mechanical, or other physical processes may yield numerous opportunities in both fundamental studies and applications, in a manner similar to that of semiconductor heterojunctions and their contribution to solid-state physics and the semiconductor industry over the past few decades. In this review, we explore the fundamental chemical roles and principles involved in designing these interfaces, such as the (bio)chemical evolution of adaptive or buffer zones. We discuss the spectroscopic, microscopic, (bio)chemical, and computational tools required to uncover the chemical processes in these confined or hidden soft-hard interfaces. We propose a soft-hard interaction framework and use it to discuss soft-hard interfacial processes in multiple systems and across several spatiotemporal scales, focusing on tissue-like materials and devices. We end this review by proposing several new scientific and engineering approaches to leveraging the soft-hard interfacial processes involved in biointerfacing composites and exploring new applications for these composites.

Vapor-solid-solid growth dynamics in GaAs nanowires

Semiconductor nanowires are promising material systems for coming-of-age nanotechnology. The usage of the vapor-solid-solid (VSS) route, where the catalyst used for promoting axial growth of nanowires is a solid, offers certain advantages compared to the common vapor-liquid-solid (VLS) route (using a liquid catalyst). The VSS growth of group-IV elemental nanowires has been investigated by other groups in situ during growth in a transmission electron microscope (TEM). Though it is known that compound nanowire growth has different dynamics compared to elemental semiconductors, the layer growth dynamics of VSS growth of compound nanowires have not been studied yet. Here we investigate for the first time controlled VSS growth of compound nanowires by in situ microscopy, using Au-seeded GaAs as a model system. The ledge-flow growth kinetics and dynamics at the wire-catalyst interface are studied and compared for liquid and solid catalysts under similar growth conditions. Here the temperature and thermal history of the system are manipulated to control the catalyst phase. In the first experiment discussed here we reduce the growth temperature in steps to solidify the initially liquid catalyst, and compare the dynamics between VLS and VSS growth observed at slightly different temperatures. In the second experiment we exploit thermal hysteresis of the system to obtain both VLS and VSS at the same temperature. The VSS growth rate is comparable or slightly slower than the VLS growth rate. Unlike in the VLS case, during VSS growth we frequently observe that a new layer starts before the previous layer is completely grown, i.e., 'multilayer growth'. Understanding the VSS growth mode enables better control of nanowire properties by widening the range of usable nanowire growth parameters.

Growth, phase transition, and island motion of Au on Ge(111)

Using low energy electron microscopy, Au on Ge(111) is determined to follow a Stranski-Krastanov growth mode consisting of a single layer up to one monolayer (ML), followed by three-dimensional Au-Ge alloy droplets. Near 600 °C, we report the first observation of a reversible first-order phase transition that occurs from the (3 × 3)R30° phase to a (1 × 1) phase, which has a coverage of 0.367 ML. The transition gradually occurs through a coexistence region with a temperature range of about 2 °C and weakly depends on coverage, varying from 640 °C at 1 ML down to 580 °C at 0.8 ML. The phase transition is accompanied by phase fluctuations of small domains or the fluctuations of phase boundaries of large domains. At coverage >1 ML and above 250 °C, the 3D droplets move with stick-slip hopping behavior that has previously been explained by dissolution of Ge at step edges into the alloy droplet, which then comes to concentration and thermal equilibrium via the island motion.

A Review of Self-Seeded Germanium Nanowires: Synthesis, Growth Mechanisms and Potential Applications

Ge nanowires are playing a big role in the development of new functional microelectronic modules, such as gate-all-around field-effect transistor devices, on-chip lasers and photodetectors. The widely used three-phase bottom-up growth method utilising a foreign catalyst metal or metalloid is by far the most popular for Ge nanowire growth. However, to fully utilise the potential of Ge nanowires, it is important to explore and understand alternative and functional growth paradigms such as self-seeded nanowire growth, where nanowire growth is usually directed by the in situ-formed catalysts of the growth material, i.e., Ge in this case. Additionally, it is important to understand how the self-seeded nanowires can benefit the device application of nanomaterials as the additional metal seeding can influence electron and phonon transport, and the electronic band structure in the nanomaterials. Here, we review recent advances in the growth and application of self-seeded Ge and Ge-based binary alloy (GeSn) nanowires. Different fabrication methods for growing self-seeded Ge nanowires are delineated and correlated with metal seeded growth. This review also highlights the requirement and advantage of self-seeded growth approach for Ge nanomaterials in the potential applications in energy storage and nanoelectronic devices.

Eutectics: formation, properties, and applications

Various eutectic systems have been proposed and studied over the past few decades. Most of the studies have focused on three typical types of eutectics: eutectic metals, eutectic salts, and deep eutectic solvents. On the one hand, they are all eutectic systems, and their eutectic principle is the same. On the other hand, they are representative of metals, inorganic salts, and organic substances, respectively. They have applications in almost all fields related to chemistry. Their different but overlapping applications stem from their very different properties. In addition, the proposal of new eutectic systems has greatly boosted the development of cross-field research involving chemistry, materials, engineering, and energy. The goal of this review is to provide a comprehensive overview of these typical eutectics and describe task-specific strategies to address growing demands.

Quantifying Real-Time Sample Temperature Under the Gas Environment in the Transmission Electron Microscope Using a Novel MEMS Heater

Accurate control and measurement of real-time sample temperature are critical for the understanding and interpretation of the experimental results from in situ heating experiments inside environmental transmission electron microscope (ETEM). However, quantifying the real-time sample temperature remains a challenging task for commercial in situ TEM heating devices, especially under gas conditions. In this work, we developed a home-made micro-electrical-mechanical-system (MEMS) heater with unprecedented small temperature gradient and thermal drift, which not only enables the temperature evolution caused by gas injection to be measured in real-time but also makes the key heat dissipation path easier to model to theoretically understand and predict the temperature decrease. A new parameter termed as “gas cooling ability (H)”, determined purely by the physical properties of the gas, can be used to compare and predict the gas-induced temperature decrease by different gases. Our findings can act as a reference for predicting the real temperature for in situ heating experiments without closed-loop temperature sensing capabilities in the gas environment, as well as all gas-related heating systems.

Direct growth of germanium nanowires on glass

We report a detailed characterization of Ge NWs directly grown on glass by a MOVPE system, showing how different growth parameters can affect the final outcome and comparing NWs grown on a monocrystalline Ge(111) substrate with NWs grown on amorphous glass. Our experimental results indicate that the choice of the substrate does not affect any of the relevant morphological, crystallographic or electrical properties of Ge NWs. Lengths are in the 20-30 micrometer range with minimal tapering, while growth rates are very similar to to NWs grown on Ge(111); TEM and Raman characterization show a very good crystallinity of measured nanostructures. We have also analyzed the growth process on glass and we were able to reach a conclusion on the specific growth mechanism for Ge NWs on amorphous substrates. Our findings demonstrate that glass is a valid option as cheap substrate for the mass production of these nanostructures.

In-situ reflectometry to monitor locally-catalyzed initiation and growth of nanowire assemblies

We investigate in-situ laser reflectometry for measuring the axial growth rate in chemical vapor deposition of assemblies of well-aligned vertical germanium nanowires grown epitaxially on single crystal substrates. Finite difference frequency domain optical simulations were performed in order to facilitate quantitative analysis and interpretation of the measured reflectivity data. The results show an insensitivity of the reflected intensity oscillation period to nanowire diameter and density within the range of experimental conditions investigated. Compared to previous quantitative in-situ measurements performed on III-V nanowire arrays, which showed two distinct rate regimes, we observe a constant, steady-state nanowire growth rate. Furthermore, we show that the measured reflectivity decay can be used to determine the germanium nanowire nucleation time with good precision. This technique provides an avenue to monitor growth of nanowires in a variety of materials systems and growth conditions.

In Situ TEM Observations of Discharging/Charging of Solid-State Lithium-Sulfur Batteries at High Temperatures

Understanding the structural evolution of Li₂ S upon operation of lithium-sulfur (Li-S) batteries is inadequate and a complete decomposition of Li₂ S during charge is difficult. Whether it is the low electronic conductivity or the low ionic conductivity of Li₂ S that inhibits its decomposition is under debate. Furthermore, the decomposition pathway of Li₂ S is also unclear. Herein, an in situ transmission electron microscopy (TEM) technique implemented with a microelectromechanical systems (MEMS) heating device is used to study the precipitation and decomposition of Li₂ S at high temperatures. It is revealed that Li₂ S transformed from an amorphous/nanocrystalline to polycrystalline state with proceeding of the electrochemical lithiation at room temperature (RT), and the precipitation of Li₂ S is more complete at elevated temperatures than at RT. Moreover, the decomposition of Li₂ S that is difficult to achieve at RT becomes facile with increased Li⁺ ion conduction at high temperatures. These results indicate that Li⁺ ion diffusion in Li₂ S dominates its reversibility in the solid-state Li-S batteries. This work not only demonstrates the powerful capabilities of combining in situ TEM with a MEMS heating device to explore the basic science in energy storage materials at high temperatures but also introduces the factor of temperature to boost battery performance.

Planar Growth, Integration, and Applications of Semiconducting Nanowires

Silicon and other inorganic semiconductor nanowires (NWs) have been extensively investigated in the last two decades for constructing high-performance nanoelectronics, sensors, and optoelectronics. For many of these applications, these tiny building blocks have to be integrated into the existing planar electronic platform, where precise location, orientation, and layout controls are indispensable. In the advent of More-than-Moore's era, there are also emerging demands for a programmable growth engineering of the geometry, composition, and line-shape of NWs on planar or out-of-plane 3D sidewall surfaces. Here, the critical technologies established for synthesis, transferring, and assembly of NWs upon planar surface are examined; then, the recent progress of in-plane growth of horizontal NWs directly upon crystalline or patterned substrates, constrained by using nanochannels, an epitaxial interface, or amorphous thin film precursors is discussed. Finally, the unique capabilities of planar growth of NWs in achieving precise guided growth control, programmable geometry, composition, and line-shape engineering are reviewed, followed by their latest device applications in building high-performance field-effect transistors, photodetectors, stretchable electronics, and 3D stacked-channel integration.

In situ TEM observation of the vapor-solid-solid growth of <001[combining macron]> InAs nanowires

In situ transmission electron microscopy characterization is a powerful method in investigating the growth mechanism of catalyst-induced semiconductor nanowires. By providing direct evidence on the crystal growth at the atomic level, a real-time in situ heating investigation was carried out on Au-catalyzed <001[combining macron]> InAs nanowires. It was found that the Au catalyst maintained itself in the solid form during the nanowire growth, and maintained a fixed epitaxial relationship with its underlying InAs nanowire, indicating the vapor-solid-solid mechanism. Importantly, the growth of <001[combining macron]> InAs nanowires through a layer-by-layer manner at the catalyst/nanowire interface is evident. This study provides direct insights into the vapor-solid-solid growth and clarified the growth mechanism of <001[combining macron]> III-V nanowires, which provides pathways in controlling the growth of <001[combining macron]> semiconductor nanowires.

The Limits of Electromechanical Coupling in Highly-Tensile Strained Germanium

Speculations regarding electronic and photonic properties of strained germanium (Ge) have perpetually put it into contention for next-generation devices since the start of the information age. Here, the electromechanical coupling of <111> Ge nanowires (NWs) is reported from unstrained conditions to the ultimate tensile strength. Under tensile strain, the conductivity of the NW is enhanced exponentially, reaching an enhancement factor of ∼130 at ∼3.5% of strain. Under strains larger than ∼2.5%, the electrical properties of Ge also exhibit a dependence on the electric field. The conductivity can be further enhanced by ∼2.2× with a high bias condition at ∼3.5% of strain. Cyclic loading tests confirm that the observed electromechanical responses are repeatable, reversible, and related to the changing electronic band structure. These tests reveal the excellent prospects for utilizing strained Ge NWs in photodetector or piezoelectronic transistor applications, but significant challenges remain to realize strict direct band gap devices.

Development of Growth Theory for Vapor–Liquid–Solid Nanowires: Wetting Scenario, Front Curvature, Growth Angle, Linear Tension, and Radial Instability

In this paper, we report that under wetting conditions (or modes) of nanowire (NW) growth, when a nonplanar crystallization front emerges under a catalyst droplet, a shift in the three-phase line (TPL) of the vapor–liquid–crystal interface occurs under thermodynamically stable conditions when the angle with respect to the droplet surface, termed the growth angle, is fixed. The growth angle of the NWs is determined not from a geometrical perspective but on the basis of the physical aspects of the processes occurring around the TPL, revealing a size dependence caused by the influence of linear tension of the three-phase contact of a vapor–liquid crystal. The observed radial periodic instability of the NWs is described according to the size dependence of the thermodynamic growth angle, which induces negative feedback in the system. Under the influence of linear tension and positive feedback, the tips or needles of NWs can be formed.

High-quality epitaxial wurtzite structured InAs nanosheets grown in MBE

In this study, we have grown epitaxial wurtzite structured InAs nanosheets using Au catalysts on a GaAs{111}_B substrate by molecular beam epitaxy. Through detailed electron microscopy characterization studies on grown nanosheets, it was found that these wurtzite structured InAs nanosheets grew epitaxially on the GaAs{111}_B substrate, with {0001[combining macron]} catalyst/nanosheet interfaces and extensive {112[combining macron]0} surfaces. It was anticipated that the epitaxially grown InAs nanosheet can be triggered by a high supersaturation in catalysts, leading to an inclined growth leaving the substrate surface, and driven by the small lattice mismatch between the nanosheets and the substrate, with the orientation relationship of (0001[combining macron])_InAs//(112[combining macron])_GaAs. This study provides insights into achieving epitaxial free-standing III-V nanosheet growth.

Free-Standing InAs Nanobelts Driven by Polarity in MBE

In this study, we demonstrated the Au-catalyzed growth of free-standing defect-free zinc-blende structured InAs nanobelts on the GaAs {111}_B substrate by molecular beam epitaxy. Through detailed morphological, chemical, and structural characterizations using advanced electron microscopy, it was found that the nanobelts grew along the ⟨001̅⟩ direction, induced by Au catalysts via vapor-solid-solid mechanism, with features of {001̅} catalyst/nanobelt interfaces and extensive {11̅0} surfaces. The formation of the belt-shaped morphology of our nanostructures resulted from a faster lateral growth rate along the ±[110] direction than that along the ±[11̅0] direction, driven by polarity. This study provides insights into understanding the growth of free-standing zinc-blende structured <001̅> InAs nanobelts.

Atomic Transport in Au-Ge Droplets: Brownian and Electromigration Dynamics

The deposition of Au on Ge(111)-sqrt[3]×sqrt[3]-Au above the eutectic temperature results in the formation of AuGe liquid droplets that reach the liquidus composition by digging a hole in the Ge substrate. The combination of low-energy electron microscopy and atomic force microscopy measurements shows that AuGe droplets randomly migrate or electromigrate under an applied electric current dragging their underneath hole. The droplet motion is due to a mass transport phenomenon based on Ge dissolution at the droplet front and Ge crystallization at its rear. At high temperature the mass transport is limited by attachment or detachment at the solid-liquid interface and the activation energy is 1.05±0.3  eV. At low temperature the effective activation energy increases as a function of the droplet radius. This behavior is attributed to the nucleation of 2D layers at the faceted liquid-solid interface.

In situ analysis of catalyst composition during gold catalyzed GaAs nanowire growth

Semiconductor nanowires offer the opportunity to incorporate novel structures and functionality into electronic and optoelectronic devices. A clear understanding of the nanowire growth mechanism is essential for well-controlled growth of structures with desired properties, but the understanding is currently limited by a lack of empirical measurements of important parameters during growth, such as catalyst particle composition. However, this is difficult to accurately determine by investigating post-growth. We report direct in situ measurement of the catalyst composition during nanowire growth for the first time. We study Au-seeded GaAs nanowires inside an electron microscope as they grow and measure the catalyst composition using X-ray energy dispersive spectroscopy. The Ga content in the catalyst during growth increases with both temperature and Ga precursor flux.

Semiconductor nanowires are promising materials for miniaturized devices, but a thorough understanding of their growth mechanism is necessary for controlled synthesis. Here, the authors use in situ spectroscopy and microscopy to measure the composition of the catalyst droplet as a function of different growth parameters during Au-seeded GaAs nanowire growth.

Au-Sn Catalyzed Growth of Ge1-xSnx Nanowires: Growth Direction, Crystallinity, and Sn Incorporation

Ge_1-xSn_x nanowires (NWs) have been a focus of research attention for their potential in realizing next-generation Si-compatible electronic and optoelectronic devices. To control the growth of NWs and increase their Sn content, the growth mechanism needs to be understood. The use of Au-Sn alloy catalysts instead of Au catalysts allows an easier understanding of Ge_1-xSn_x NW growth, and the effects of Sn at different concentrations in catalysts on growth direction, Sn incorporation, and crystallinity of Ge_1-xSn_x NWs can be clarified. High Sn content in Au-Sn alloy catalysts favors ⟨110⟩-oriented NW growth and high Sn incorporation in NWs. The higher Sn content in Au-Sn alloy catalysts also improves the crystallinity of NWs.

Nanowire Electronics: From Nanoscale to Macroscale

Semiconductor nanowires have attracted extensive interest as one of the best-defined classes of nanoscale building blocks for the bottom-up assembly of functional electronic and optoelectronic devices over the past two decades. The article provides a comprehensive review of the continuing efforts in exploring semiconductor nanowires for the assembly of functional nanoscale electronics and macroelectronics. Specifically, we start with a brief overview of the synthetic control of various semiconductor nanowires and nanowire heterostructures with precisely controlled physical dimension, chemical composition, heterostructure interface, and electronic properties to define the material foundation for nanowire electronics. We then summarize a series of assembly strategies developed for creating well-ordered nanowire arrays with controlled spatial position, orientation, and density, which are essential for constructing increasingly complex electronic devices and circuits from synthetic semiconductor nanowires. Next, we review the fundamental electronic properties and various single nanowire transistor concepts. Combining the designable electronic properties and controllable assembly approaches, we then discuss a series of nanoscale devices and integrated circuits assembled from nanowire building blocks, as well as a unique design of solution-processable nanowire thin-film transistors for high-performance large-area flexible electronics. Last, we conclude with a brief perspective on the standing challenges and future opportunities.

Kinetics of Au-Ga Droplet Mediated Decomposition of GaAs Nanowires

Particle-assisted III-V semiconductor nanowire growth and applications thereof have been studied extensively. However, the stability of nanowires in contact with the particle and the particle chemical composition as a function of temperature remain largely unknown. In this work, we use in situ transmission electron microscopy to investigate the interface between a Au-Ga particle and the top facet of an ⟨1̅1̅1̅⟩-oriented GaAs nanowire grown via the vapor-liquid-solid process. We observed a thermally activated bilayer-by-bilayer removal of the GaAs facet in contact with the liquid particle during annealing between 300 and 420 °C in vacuum. Interestingly, the GaAs-removal rates initially depend on the thermal history of the sample and are time-invariant at later times. In situ X-ray energy dispersive spectroscopy was also used to determine that the Ga content in the particle at any given temperature remains constant over extended periods of time and increases with increasing temperature from 300 to 400 °C. We attribute the observed phenomena to droplet-assisted decomposition of GaAs at a rate that is controlled by the amount of Ga in the droplet. We suggest that the observed transients in removal rates are a direct consequence of time-dependent changes in the Ga content. Our results provide new insights into the role of droplet composition on the thermal stability of GaAs nanowires and complement the existing knowledge on the factors influencing nanowire growth. Moreover, understanding the nanowire stability and decomposition is important for improving processing protocols for the successful fabrication and sustained operation of nanowire-based devices.

Highly Effective GeNi Alloy Contact Diffusion Barrier for BiSbTe Long-Term Thermal Exposure

A GeNi alloy diffusion barrier for contacts on bismuth antimony telluride is proposed. Multiple gold contact diffusion barriers were tested at different thermal aging conditions in air and reducing atmospheres. Among all diffusion barriers, the GeNi alloy barrier shows the best performance for bulk samples with no substantial degradation of the contact resistance, no contact color change, and no change of thermoelectric properties. We observed D _Au-GeNi = (9.8 ± 2.7) × 10^-20 m²/s within the GeNi alloy barrier, which is 4 times smaller than D _Au-BiSbTe. The presence of the initial Ge layer also proves to be effective in reducing nickel diffusion yielding D _Ni-BiSbTe = (8.57 ± 0.49) × 10^-19 m²/s. During GeNi alloy formation, Ge diffusion into BiSbTe produces GeTe, which apparently blocks the van der Waals gaps eliminating Au and Ni fast diffusion pathways. Thermal aging of BiSbTe nanowires shows that Au and Ni diffusion degrades the thermoelectric power factor, whereas the GeNi alloy barrier sample is mostly preserved. The GeNi alloy barrier is a reliable solution to long-term thermal applications of BiTe-based materials.

Direct Vapor-Liquid-Solid Synthesis of All-Inorganic Perovskite Nanowires for High-Performance Electronics and Optoelectronics

Controlled synthesis of lead halide perovskite (LHP) nanostructures not only benefits fundamental research but also offers promise for applications. Among many synthesis techniques, although catalytic vapor-liquid-solid (VLS) growth is recognized as an effective route to achieve high-quality nanostructures, until now, there is no detailed report on VLS grown LHP nanomaterials due to the emerging challenges in perovskite synthesis. Here, we develop a direct VLS growth for single-crystalline all-inorganic lead halide perovskite ( i.e., CsPbX₃; X = Cl, Br, or I) nanowires (NWs). These NWs exhibit high-performance photodetection with the responsivity exceeding 4489 A/W and detectivity over 7.9 × 10¹² Jones toward the visible light regime. Field-effect transistors (FET) based on individual CsPbX₃ NWs are also fabricated, where they show the superior hole mobility of up to 3.05 cm²/(V s), higher than other all-inorganic LHP devices. This work provides important guidelines for the further improvement of these perovskite nanostructures for utilizations.

Vapor Phase Growth of Semiconductor Nanowires: Key Developments and Open Questions

Nanowires are filamentary crystals with a tailored diameter that can be obtained using a plethora of different synthesis techniques. In this review, we focus on the vapor phase, highlighting the most influential achievements along with a historical perspective. Starting with the discovery of VLS, we feature the variety of structures and materials that can be synthesized in the nanowire form. We then move on to establish distinct features such as the three-dimensional heterostructure/doping design and polytypism. We summarize the status quo of the growth mechanisms, recently confirmed by in situ electron microscopy experiments and defining common ground between the different synthesis techniques. We then propose a selection of remaining defects, starting from what we know and going toward what is still to be learned. We believe this review will serve as a reference for neophytes but also as an insight for experts in an effort to bring open questions under a new light.

Growth of Ga2O3 Nanowires via Cu-As-Ga Ternary Phase Diagram

Currently, it is challenging to develop new catalysts for semiconductor nanowires (NWs) growth in a complementary-metal-oxide-semiconductor (CMOS) compatible manner via a vapor-liquid-solid (VLS) mechanism. In this study, chemically synthesized Cu2O nano cubes are adopted as the catalyst for single crystalline β-Ga2O3 NWs growth in chemical vapor deposition. The growth temperature is optimized to be 750 to 800 °C. The NW diameter is controlled by tuning the sizes of Cu2O cubes in the 20 to 100 nm range with a bandgap of ~4.85 eV as measured by ultraviolet-visible absorption spectroscopy. More importantly, the catalyst tip is found to be Cu5As2, which is distinguished from those Au-catalyzed Au-Ga alloys. After a comprehensive phase diagram investigation, the β-Ga2O3 NWs are proposed to be grown by the ternary phase of Cu-As-Ga diffusing Ga into the growth frontier of the NW, where Ga react with residual oxygen to form the NWs. Afterward, Ga diminishes after growth since Ga would be the smallest component in the ternary alloy. All these results show the importance of the catalyst choice for CMOS compatible NW growth and also the potency of the ternary phase catalyst growth mode in other semiconductor NWs synthesis.

Surface Crystallization of Liquid Au-Si and Its Impact on Catalysis

In situ transmission electron microscopy reveals that an atomically thin crystalline phase at the surface of liquid Au-Si is stable over an unexpectedly wide range of conditions. By measuring the surface structure as a function of liquid temperature and composition, a simple thermodynamic model is developed to explain the stability of the ordered phase. The presence of surface ordering plays a key role in the pathway by which the Au-Si eutectic solidifies and also dramatically affects the catalytic properties of the liquid, explaining the anomalously slow growth kinetics of Si nanowires at low temperature. A strategy to control the presence of the surface phase is discussed, using it as a tool in designing strategies for nanostructure growth.

Nanowire morphology and particle phase control by tuning the In concentration of the foreign metal nanoparticle

Controllable particle assisted growth (PAG) of III-V nanowires is today almost exclusively done with Au, Ga or In nanoparticles, whereas other metals often yield nanowires with uncontrolled growth directions. To improve the control of the initial growth direction in PAG, independent of choice of metal, we propose to initiate nanowire growth from a group-III-rich foreign metal particle. For III-V nanowire growth, the group III concentration of the particle can be made to increase or decrease with the relative supply of group III and group V material, which can be used to promote the liquid phase that is necessary for vapor-liquid-solid growth. In this paper, 30 nm Pd nanoparticles are used to develop growth conditions for In-rich PAG of InAs nanowires. The particle size evolution for different growth times and V/III ratios is correlated with changes in nanowire density and morphology. In addition, we demonstrate In-rich Co, Pd, Pt and Rh nanoparticles and optimized In-rich PAG from Au and Pd seeds. The Au and Pd seeded nanowires are remarkably similar and by tuning the particle composition we trigger a morphological change. The vertical nanowire morphology is associated with In-rich nanoparticles that contain a liquid phase. The curly nanowire morphology, with random growth directions have an In concentration less than or equal to that of the most In rich compound of the seed metal-In system.

Catalyst-free growth of zinc nanocrystals with various morphologies on ionic liquid surfaces

We report a catalyst-free growth of zinc nanocrystals with various morphologies on ionic liquid surfaces at room temperature.

Hollow Rims from Water Drop Evaporation on Salt Substrates

We report on the observation of thin salt shells that form at the periphery of evaporating pure water drops on salt. Shell shapes range from rings of inclined walls to hollow toroidal rims. We interpret this phenomenon as a consequence of a molecular coffee-stain effect by which the dissolved salt is advected toward the pinned contact line where an increased evaporation takes place. The subsequent salt supersaturation in the vicinity of the triple line drives the crystallization of the shell at the liquid-air interface. This interpretation is supported by a simple model for shell growth.

Solution-based low-temperature synthesis of germanium nanorods and nanowires

ABSTRACT

The Ga-assisted formation of Ge nanorods and nanowires in solution has been demonstrated and a catalytic activity of the Ga seeds was observed. The synthesis of anisotropic single-crystalline Ge nanostructures was achieved at temperatures as low as 170 °C. Gallium not only serves as nucleation seed but is also incorporated in the Ge nanowires in higher concentrations than its thermodynamic solubility limit.

GRAPHICAL ABSTRACT

In Situ Investigation of Defect-Free Copper Nanowire Growth

The fabrication and placement of high purity nanometals, such as one-dimensional copper (Cu) nanowires, for interconnection in integrated devices have been among the most important technological developments in recent years. Structural stability and oxidation prevention have been the key issues, and the defect control in Cu nanowire growth has been found to be important. Here, we report the synthesis of defect-free single-crystalline Cu nanowires by controlling the surface-assisted heterogeneous nucleation of Cu atomic layering on the graphite-like loop of an amorphous carbon (a-C) lacey film surface. Without a metal-catalyst or induced defects, the high quality Cu nanowires formed with high aspect ratio and high growth rate of 578 nm/s. The dynamic study of the growth of heterogeneous nanowires was conducted in situ with a high-resolution transmission electron microscope. The study illuminates the new mechanism by heterogeneous nucleation control and laying the groundwork for better understanding of heterosurface-assisted nucleation of defect-free Cu nanowire on a-C lacey film.

Atomic-Scale Choreography of Vapor-Liquid-Solid Nanowire Growth

Functional materials and devices require nanoscale control of morphology, crystal structure, and composition. Vapor-liquid-solid (VLS) crystal growth and its related growth modes enable the synthesis of 1D nanostructures, commonly called "nanowires", where the necessary nanoscale heterogeneity can be encoded axially. During the VLS process, a seed particle collects atoms and directs the nucleation of crystalline material. Modulating the delivery of growth species or conditions permits compositional and/or structural encoding. A range of materials and devices (e.g., for electronics, photonics, thermal transport, and bioprobes) have been produced by VLS growth, but plenty of challenges remain: many desirable structures cannot currently be made, and even for those structures that can be made, the parameter window-in terms of, e.g., temperatures and pressures-is often narrow. Moreover, we are quite far from ab initio determination of which growth conditions should be used or even if a desired structure is fundamentally achievable within the VLS framework. To fully understand the challenges and promises of VLS growth, the governing physicochemical processes must be explored and understood at the atomic scale. This final level of detail is being unraveled with the help of in situ characterization techniques. The picture that is emerging is of a highly dynamical process with several deeply interconnected and highly fundamental components that are difficult to detect with postgrowth ex situ interrogation. For example, recent in situ microscopy and spectroscopy studies have shown that the growth front can undergo cyclical reshaping involving dissolution as well as crystallization and that the state of the nanowire surface, which changes with growth conditions as a result of a competition between adsorption and desorption of passivating species, plays a crucial role in determining the transport to/from and the stability of the seed particle. The available in situ observations currently constitute a somewhat disparate list, but if they can be connected to each other and to the outstanding challenges, they promise meaningful advances in our understanding of VLS growth. In this Account, we review the state of the art regarding the atomic-scale thermodynamic and kinetic phenomena that control VLS growth. Rather than cataloging all of the outstanding contributions to the field, we give priority to in situ observations that have revealed unexpected effects as well as those that hint at incongruities in our current knowledge. As such, our discussion should be viewed as an opportunity to gain deeper understanding and control of the fundamental processes at play, which will be crucial in future scale-up efforts and expansion to completely new materials systems and application areas.

Low temperature solution synthesis of silicon, germanium and Si–Ge axial heterostructures in nanorod and nanowire form

Herein, we report the formation of silicon, germanium and more complex Si–Si_xGe_1−x and Si–Ge axial 1D heterostructures, at low temperatures in solution. The incorporation of a reducing agent into the reaction is shown to be effective to lower precursor decomposition temperatures.

Hydrophobic mediated growth of galvanic-nanobuds from germanium nanowires for a highly tunable SERS substrate

A SERS substrate is fabricated through a scalable process exhibiting suitable hotspot distribution, shelf life, tunability, and biological applications.

Role of Precursor-Conversion Chemistry in the Crystal-Phase Control of Catalytically Grown Colloidal Semiconductor Quantum Wires

Crystal-phase control is one of the most challenging problems in nanowire growth. We demonstrate that, in the solution-phase catalyzed growth of colloidal cadmium telluride (CdTe) quantum wires (QWs), the crystal phase can be controlled by manipulating the reaction chemistry of the Cd precursors and tri-n-octylphosphine telluride (TOPTe) to favor the production of either a CdTe solute or Te, which consequently determines the composition and (liquid or solid) state of the Bi_xCd_yTe_z catalyst nanoparticles. Growth of single-phase (e.g., wurtzite) QWs is achieved only from solid catalysts (y ≪ z) that enable the solution-solid-solid growth of the QWs, whereas the liquid catalysts (y ≈ z) fulfill the solution-liquid-solid growth of the polytypic QWs. Factors that affect the precursor-conversion chemistry are systematically accounted for, which are correlated with a kinetic study of the composition and state of the catalyst nanoparticles to understand the mechanism. This work reveals the role of the precursor-reaction chemistry in the crystal-phase control of catalytically grown colloidal QWs, opening the possibility of growing phase-pure QWs of other compositions.

Formation of Self-Connected Si0.8Ge0.2 Lateral Nanowires and Pyramids on Rib-Patterned Si(1 1 10) Substrate

In this work, Si_0.8Ge_0.2 is deposited onto the rib-patterned Si (1 1 10) template oriented in the [1 -1 0] direction. Atomic force microscopy (AFM) reveals that the rib sidewalls reshape into pyramid-covered (0 0 1) and smooth {1 1 3} facets, respectively, while the {1 0 5} facets-bounded lateral SiGe nanowires dominate the rib top along the [5 5 -1] direction. At both the rib shoulder sites and the pyramid vacancy sites, self-connecting occurs between the meeting nanowire and pyramids to form elongated huts, which are driven by the minimization of the total energy density according to the finite-element simulations results. These results suggest a convenient solution to form lateral SiGe nanowires covering multi-faceted surfaces on the patterned template.