Origin of polytype formation in VLS-grown Ge nanowires through defect generation and nanowire kinking

Solution processable Si/Ge heterostructure NWs enabling anode mass reduction for practical full-cell Li-ion batteries

Here, we report the solution phase synthesis of axial heterostructure Si and Ge (hSG) nanowires (NWs). The NWs were grown in a high boiling point solvent from a low-cost Sn powder to achieve a powder form product which represents an attractive route from lab-scale to commercial application. Slurry processed anodes of the NWs were investigated in half-cell (versus Li-foil) and full-cell (versus NMC811) configurations of a lithium ion battery (LIB). The hSG NW anodes yielded capacities of 1040 mA h g-1 after 150 cycles which corresponds to a 2.8 times increase compared to a standard graphite (372 mA h g-1) anode. Given the impressive specific and areal capacities of the hSG anodes, a full-cell test against a high areal capacity NMC811 cathode was examined. In full-cell configuration, use of the hSG anode resulted in a massive anode mass reduction of 50.7% compared to a standard graphite anode. The structural evolution of the hSG NW anodes into an alloyed SiGe porous mesh network was also investigated using STEM, EDX and Raman spectroscopy as a function of cycle number to fully elucidate the lithiation/delithiation mechanism of the promising anode material.

Growth and analysis of the tetragonal (ST12) germanium nanowires

New semiconducting materials, such as state-of-the-art alloys, engineered composites and allotropes of well-established materials can demonstrate unique physical properties and generate wide possibilities for a vast range of applications. Here we demonstrate, for the first time, the fabrication of a metastable allotrope of Ge, tetragonal germanium (ST12-Ge), in nanowire form. Nanowires were grown in a solvothermal-like single-pot method using supercritical toluene as a solvent, at moderate temperatures (290-330 °C) and a pressure of ∼48 bar. One-dimensional (1D) nanostructures of ST12-Ge were achieved via a self-seeded vapour-liquid-solid (VLS)-like paradigm, with the aid of an in situ formed amorphous carbonaceous layer. The ST12 phase of Ge nanowires is governed by the formation of this carbonaceous structure on the surface of the nanowires and the creation of Ge-C bonds. The crystalline phase and structure of the ST12-Ge nanowires were confirmed by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. The nanowires produced displayed a high aspect ratio, with a very narrow mean diameter of 9.0 ± 1.4 nm, and lengths beyond 4 μm. The ST12-Ge nanowire allotrope was found to have a profound effect on the intensity of the light emission and the directness of the bandgap, as confirmed by a temperature-dependent photoluminescence study.

Crystal phase evolution in kinked GaN nanowires

Seed-catalysed growth has been proved to be an ideal method to selectively tune the crystal structure of III-V nanowires along its growth axis. However, few results on relevant nitride NWs have been reported. In this study, we demonstrate the growth of epitaxial kinked wurtzite (WZ)/zinc-blende (ZB) heterostructure GaN NW arrays under the oxygen rich condition using hydride vapour-liquid-solid vapour phase epitaxy (VLS-HVPE). The typical GaN crystal includes WZ and ZB phases throughout the whole NW structure. A detailed structural analysis indicates that a stacking faults free zone was occasionally observed near the NW tips and in the relatively long kinked 〈11-23〉 directions segments (>200 nm). Furthermore, some NWs (<5%) develop phase boundaries, resulting in kinking and crystal phase evolution. A layer-by-layer growth mode was proposed to explain the crystal phase evolution along the phase boundaries. This study provides new insights into the controlled growth of wurtzite (WZ)/zinc-blende (ZB) heterostructure GaN NW.

Template-free electrochemical synthesis of Cd/CdTe core/shell nanowires and CdTe nanotubes

Template-free electrodeposition was utilized to prepare Cd/CdTe nanowires and CdTe nanotubes for the first time, where the formation of one-dimensional structures was due to the highly anisotropic crystal structure and the Kirkendall effect.

Phonon transport across crystal-phase interfaces and twin boundaries in semiconducting nanowires

We combine state-of-the-art Green's-function methods and nonequilibrium molecular dynamics calculations to study phonon transport across the unconventional interfaces that make up crystal-phase and twinning superlattices in nanowires. We focus on two of their most paradigmatic building blocks: cubic (diamond/zinc blende) and hexagonal (lonsdaleite/wurtzite) polytypes of the same group-IV or III-V material. Specifically, we consider InP, GaP and Si, and both the twin boundaries between rotated cubic segments and the crystal-phase boundaries between different phases. We reveal the atomic-scale mechanisms that give rise to phonon scattering in these interfaces, quantify their thermal boundary resistance and illustrate the failure of common phenomenological models in predicting those features. In particular, we show that twin boundaries have a small but finite interface thermal resistance that can only be understood in terms of a fully atomistic picture.

Direct evidence of 2H hexagonal Si in Si nanowires

Hexagonal Si (2H polytype) has attracted great interest because of its unique physical properties and wide range of potential applications. For example, it might be used in heterojunctions based on hexagonal and cubic Si. Although hexagonal Si has been reported in Si nanowires, its existence is doubted because structural defects of diamond cubic Si can produce structural signals similar to those attributed to hexagonal Si. Here, through the use of atomic resolution high-angle annular dark-field scanning transmission electron microscopy imaging, we unambiguously report the coherent intergrowth of diamond cubic (3C polytype) and 2H hexagonal Si in Si nanowires grown by chemical vapor deposition. A model describing the intergrowth of 3C and 2H Si is proposed and the reasons for the generation of 2H Si are discussed in detail.

Synthesis of Polytypic Gallium Phosphide and Gallium Arsenide Nanowires and Their Application as Photodetectors

One-dimensional semiconductor nanowires often contain polytypic structures, owing to the co-existence of different crystal phases. Therefore, understanding the properties of polytypic structures is of paramount importance for many promising applications in high-performance nanodevices. Herein, we synthesized nanowires of typical III-V semiconductors, namely, gallium phosphide and gallium arsenide by using the chemical vapor transport method. The growth directions ([111] and [211]) could be switched by changing the experimental conditions, such as H₂ gas flow; thus, various polytypic structures were produced simultaneously in a controlled manner. The nanobeam electron diffraction technique was employed to obtain strain mapping of the nanowires by visualizing the polytypic structures along the [111] direction. Micro-Raman spectra for individual nanowires were collected, confirming the presence of wurtzite phase in the polytypic nanowires. Further, we fabricated the photodetectors using the single nanowires, and the polytypic structures are shown to decrease the photosensitivity. Our systematic analysis provides important insight into the polytypic structures of nanowires.

Crystalline, Phononic, and Electronic Properties of Heterostructured Polytypic Ge Nanowires by Raman Spectroscopy

Semiconducting nanowires (NWs) offer the unprecedented opportunity to host different crystal phases in a nanostructure, which enables the formation of polytypic heterostructures where the material composition is unchanged. This characteristic boosts the potential of polytypic heterostructured NWs for optoelectronic and phononic applications. In this work, we investigate cubic Ge NWs where small (∼20 nm) hexagonal domains are formed due to a strain-induced phase transformation. By combining a nondestructive optical technique (Raman spectroscopy) with density-functional theory (DFT) calculations, we assess the phonon properties of hexagonal Ge, determine the crystal phase variations along the NW axis, and, quite remarkably, reconstruct the relative orientation of the two polytypes. Moreover, we provide information on the electronic band alignment of the heterostructure at points of the Brillouin zone different from the one (Γ) where the direct band gap recombination in hexagonal Ge takes place. We demonstrate the versatility of Raman spectroscopy and show that it can be used to determine the main crystalline, phononic, and electronic properties of the most challenging type of heterostructure (a polytypic, nanoscale heterostructure with constant material composition). The general procedure that we establish can be applied to several types of heterostructures.

Reduction of Thermal Conductivity in Nanowires by Combined Engineering of Crystal Phase and Isotope Disorder

Nanowires are a versatile platform to investigate and harness phonon and thermal transport phenomena in nanoscale systems. With this perspective, we demonstrate herein the use of crystal phase and mass disorder as effective degrees of freedom to manipulate the behavior of phonons and control the flow of local heat in silicon nanowires. The investigated nanowires consist of isotopically pure and isotopically mixed nanowires bearing either a pure diamond cubic or a cubic-rhombohedral polytypic crystal phase. The nanowires with tailor-made isotopic compositions were grown using isotopically enriched silane precursors ²⁸SiH₄, ²⁹SiH₄, and ³⁰SiH₄ with purities better than 99.9%. The analysis of polytypic nanowires revealed ordered and modulated inclusions of lamellar rhombohedral silicon phases toward the center in otherwise diamond-cubic lattice with negligible interphase biaxial strain. Raman nanothermometry was employed to investigate the rate at which the local temperature of single suspended nanowires evolves in response to locally generated heat. Our analysis shows that the lattice thermal conductivity in nanowires can be tuned over a broad range by combining the effects of isotope disorder and the nature and degree of polytypism on phonon scattering. We found that the thermal conductivity can be reduced by up to ∼40% relative to that of isotopically pure nanowires, with the lowest value being recorded for the rhombohedral phase in isotopically mixed ²⁸Si _x³⁰Si_{1- x} nanowires with composition close to the highest mass disorder ( x ∼ 0.5). These results shed new light on the fundamentals of nanoscale thermal transport and lay the groundwork to design innovative phononic devices.

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.

Nanowire Kinking Modulates Doping Profiles by Reshaping the Liquid-Solid Growth Interface

Dopants modify the electronic properties of semiconductors, including their susceptibility to etching. In semiconductor nanowires doped during growth by the vapor-liquid-solid (VLS) process, it has been shown that nanofaceting of the liquid-solid growth interface influences strongly the radial distribution of dopants. Hence, the combination of facet-dependent doping and dopant selective etching provides a means to tune simultaneously the electronic properties and morphologies of nanowires. Using atom-probe tomography, we investigated the boron dopant distribution in Au catalyzed VLS grown silicon nanowires, which regularly kink between equivalent ⟨112⟩ directions. Segments alternate between radially uniform and nonuniform doping profiles, which we attribute to switching between a concave and convex faceted liquid-solid interface. Dopant selective etching was used to reveal and correlate the shape of the growth interface with the observed anisotropic doping.

Polarity and growth directions in Sn-seeded GaSb nanowires

We here investigate the growth mechanism of Sn-seeded GaSb nanowires and demonstrate how the seed particle and its dynamics at the growth interface of the nanowire determine the polarity, as well as the formation of structural defects. We use aberration-corrected scanning transmission electron microscopy imaging methodologies to study the interrelationship between the structural properties, i.e. polarity, growth mechanism, and formation of inclined twin boundaries in pairs. Moreover, the optical properties of the Sn-seeded GaSb nanowires are examined. Their photoluminescence response is compared with one of their Au-seeded counterparts, suggesting the incorporation of Sn atoms from the seed particles into the nanowires.

Crystal Phase Effects in Si Nanowire Polytypes and Their Homojunctions

Recent experimental investigations have confirmed the possibility to synthesize and exploit polytypism in group IV nanowires. Driven by this promising evidence, we use first-principles methods based on density functional theory and many-body perturbation theory to investigate the electronic and optical properties of hexagonal-diamond and cubic-diamond Si NWs as well as their homojunctions. We show that hexagonal-diamond NWs are characterized by a more pronounced quantum confinement effect than cubic-diamond NWs. Furthermore, they absorb more light in the visible region with respect to cubic-diamond ones and, for most of the studied diameters, they are direct band gap materials. The study of the homojunctions reveals that the diameter has a crucial effect on the band alignment at the interface. In particular, at small diameters the band-offset is type-I whereas at experimentally relevant sizes the offset turns up to be of type-II. These findings highlight intriguing possibilities to modulate electron and hole separations as well as electronic and optical properties by simply modifying the crystal phase and the size of the junction.

Spontaneous, Defect-Free Kinking via Capillary Instability during Vapor-Liquid-Solid Nanowire Growth

Kinking, a common anomaly in nanowire (NW) vapor-liquid-solid (VLS) growth, represents a sudden change of the wire's axial growth orientation. This study focuses on defect-free kinking during germanium NW VLS growth, after nucleation on a Ge (111) single crystal substrate, using Au-Ge catalyst liquid droplets of defined size. Statistical analysis of the fraction of kinked NWs reveals the dependence of kinking probability on the wire diameter and the growth temperature. The morphologies of kinked Ge NWs studied by electron microscopy show two distinct, defect-free, kinking modes, whose underlying mechanisms are explained with the help of 3D multiphase field simulations. Type I kinking, in which the growth axis changes from vertical [111] to ⟨110⟩, was observed in Ge NWs with a nominal diameter of ∼ 20 nm. This size coincides with a critical diameter at which a spontaneous transition from ⟨111⟩ to ⟨110⟩ growth occurs in the phase field simulations. Larger diameter NWs only exhibit Type II kinking, in which the growth axis changes from vertical [111] directly to an inclined ⟨111⟩ axis during the initial stages of wire growth. This is caused by an error in sidewall facet development, which produces a shrinkage in the area of the (111) growth facet with increasing NW length, causing an instability of the Au-Ge liquid droplet at the tip of the NW.

One-dimensional Growth of Zinc Crystals on a Liquid Surface

The catalyst-free growth of nanocrystals on various substrates at room temperature has been a long-standing goal in the development of material science. We report the growth of one-dimensional zinc nanocrystals on silicone oil surfaces by thermal evaporation method at room temperature (20 ± 2 °C). Uniform zinc nanorods with tunable size can be obtained. The typical length and width of the nanorods are 250-500 nm and 20-40 nm, respectively. The growth mechanism can be attributed to the effect of the liquid substrate and the preferential growth direction of the crystals. This result provides a novel and simple way to fabricate the precursors (zinc crystals) for preparation of Zn-based semiconductors and other metallic crystals on liquid substrates.

Probing Thermal Flux in Twinned Ge Nanowires through Raman Spectroscopy

We report a noninvasive optical technique based on micro-Raman spectroscopy to study the temperature-dependent phonon behavior of normal (nondefective) and twinned germanium nanowires (Ge-NWs). We studied thermophysical properties of Ge-NWs from Raman spectra, measured by varying excitation laser power at ambient condition. We derived the laser-induced temperature rise during Raman measurements by analyzing the Raman peak position for both the NWs, and for a comparative study we performed the same for bulk Ge. The frequency of the Ge-Ge phonon mode softens for all the samples with the increase in temperature, and the first-order temperature coefficient (χT) for defected NWs is found to be higher than normal NWs and bulk. We demonstrated that apart from the size, the lamellar twinning and polytype phase drastically affect the heat transport properties of NWs.

Tunable Polarity in a III-V Nanowire by Droplet Wetting and Surface Energy Engineering

Controllable axial switching of polarity in GaAs nanowires with minimal tapering and perfect twin-free ZB structure based on the fundamental understanding of nanowire growth and kinking mechanism is presented. The polarity of the bottom segment is confirmed to be (111)A by atomically resolved scanning transmission electron microscopy.

Controlling the morphology, composition and crystal structure in gold-seeded GaAs(1-x)Sb(x) nanowires

While III-V binary nanowires are now well controlled and their growth mechanisms reasonably well understood, growing ternary nanowires, including controlling their morphology, composition and crystal structure remains a challenge. However, understanding and control of ternary alloys is of fundamental interest and critical to enable a new class of nanowire devices. Here, we report on the progress in understanding the complex growth behaviour of gold-seeded GaAs1-xSbx nanowires grown by metalorganic vapour phase epitaxy. The competition between As and Sb atoms for incorporation into the growing crystal leads to a tunability of the Sb content over a broad range (x varies from 0.09 to 0.6), solely by changing the AsH3 flow. In contrast, changing TMSb flow is more effective in affecting the morphology and crystal structure of the nanowires. Inclined faults are found in some of these nanowires and directly related to the kinking of the nanowires and controlled by TMSb flow. Combined with the observed sharp increase of wetting angle between the Au seed and nanowire, the formation of inclined faults are attributed to the Au seed being dislodged from the growth front to wet the sidewalls of the nanowires, and are related to the surfactant role of Sb. The insights provided by this study should benefit future device applications relying on taper- and twin-free ternary antimonide III-V nanowire alloys and their heterostructures.

Germanium microflower-on-nanostem as a high-performance lithium ion battery electrode

We demonstrate a new design of Ge-based electrodes comprising three-dimensional (3-D) spherical microflowers containing crystalline nanorod networks on sturdy 1-D nanostems directly grown on a metallic current collector by facile thermal evaporation. The Ge nanorod networks were observed to self-replicate their tetrahedron structures and form a diamond cubic lattice-like inner network. After etching and subsequent carbon coating, the treated Ge nanostructures provide good electrical conductivity and are resistant to gradual deterioration, resulting in superior electrochemical performance as anode materials for LIBs, with a charge capacity retention of 96% after 100 cycles and a high specific capacity of 1360 mA h g(-1) at 1 C and a high-rate capability with reversible capacities of 1080 and 850 mA h g(-1) at the rates of 5 and 10 C, respectively. The improved electrochemical performance can be attributed to the fast electron transport and good strain accommodation of the carbon-filled Ge microflower-on-nanostem hybrid electrode.

Novel heterostructured Ge nanowires based on polytype transformation

We report on a strain-induced phase transformation in Ge nanowires under external shear stresses. The resulted polytype heterostructure may have great potential for photonics and thermoelectric applications. ⟨111⟩-oriented Ge nanowires with standard diamond structure (3C) undergo a phase transformation toward the hexagonal diamond phase referred as the 2H-allotrope. The phase transformation occurs heterogeneously on shear bands along the length of the nanowire. The structure meets the common phenomenological criteria of a martensitic phase transformation. This point is discussed to initiate an on going debate on the transformation mechanisms. The process results in unprecedented quasiperiodic heterostructures 3C/2H along the Ge nanowire. The thermal stability of those 2H domains is also studied under annealing up to 650 °C by in situ TEM.

Interplay between defect propagation and surface hydrogen in silicon nanowire kinking superstructures

Semiconductor nanowire kinking superstructures, particularly those with long-range structural coherence, remain difficult to fabricate. Here, we combine high-resolution electron microscopy with operando infrared spectroscopy to show why this is the case for Si nanowires and, in doing so, reveal the interplay between defect propagation and surface chemistry during ⟨211⟩ → ⟨111⟩ and ⟨211⟩ → ⟨211⟩ kinking. Our experiments show that adsorbed hydrogen atoms are responsible for selecting ⟨211⟩-oriented growth and indicate that a twin boundary imparts structural coherence. The twin boundary, only continuous at ⟨211⟩ → ⟨211⟩ kinks, reduces the symmetry of the trijunction and limits the number of degenerate directions available to the nanowire. These findings constitute a general approach for rationally engineering kinking superstructures and also provide important insight into the role of surface chemical bonding during vapor-liquid-solid synthesis.

Direct comparison on the structural and optical properties of metal-catalytic and self-catalytic assisted gallium nitride (GaN) nanowires by chemical vapor deposition

The structural and optical properties of GaN nanowires (NWs) grown by catalytic and self-catalytic-assisted vapor liquid solid approach using chemical vapor deposition (CVD) are reported.

Formation of stacking faults and the screw dislocation-driven growth: a case study of aluminum nitride nanowires

Stacking faults are an important class of crystal defects commonly observed in nanostructures of close packed crystal structures. They can bridge the transition between hexagonal wurtzite (WZ) and cubic zinc blende (ZB) phases, with the most known example represented by the "nanowire (NW) twinning superlattice". Understanding the formation mechanisms of stacking faults is crucial to better control them and thus enhance the capability of tailoring physical properties of nanomaterials through defect engineering. Here we provide a different perspective to the formation of stacking faults associated with the screw dislocation-driven growth mechanism of nanomaterials. With the use of NWs of WZ aluminum nitride (AlN) grown by a high-temperature nitridation method as the model system, dislocation-driven growth was first confirmed by transmission electron microscopy (TEM). Meanwhile numerous stacking faults and associated partial dislocations were also observed and identified to be the Type I stacking faults and the Frank partial dislocations, respectively, using high-resolution TEM. In contrast, AlN NWs obtained by rapid quenching after growth displayed no stacking faults or partial dislocations; instead many of them had voids that were associated with the dislocation-driven growth. On the basis of these observations, we suggest a formation mechanism of stacking faults that originate from dislocation voids during the cooling process in the syntheses. Similar stacking fault features were also observed in other NWs with WZ structure, such as cadmium sulfide (CdS) and zinc oxide (ZnO).