Ge Nanowires Research Articles

In Situ Focused Ion Beam-Scanning Electron Microscope Study of Crack and Nanopore Formation in Germanium Particle during (De)Lithiation

Germanium (Ge) has been considered a promising anode active material, due to its high capacity, low voltage, fast lithium ion diffusion, and high electrical conductivity. However, Ge based anode materials suffer from pulverization due to a large volume change, up to 300%. Although pulverization issues can be alleviated by nano-structuring the Ge-based anode materials, recent studies have shown that nanometer-sized pores are still formed in the nano-structured electrode due to vacancy-mediated diffusion, which could lead to pulverization during repeated cycling. In recent years, the reaction mechanism of the Ge anode has been studied by using different in situ and operando characterization methods such as X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). For instance, Lim et al. proposed the phase transformations of Ge that occur during lithiation and delithiation.1 They also studied the effect of cycling rate on Ge phase transformation during the charge/discharge process and proposed a mechanism to explain the C‐rate dependence phase transformation phenomenon. However, it is still necessary to correlate the microstructural evolution of the Ge electrode and its electrochemical performance. To this end, some advanced imaging technologies have been used to monitor the microstructural evolution of the Ge electrode during cycling such as transmission electron microscopy (TEM)2 and transmission X-ray microscopy (TXM).3 In situ TEM studies have shown the volume expansion during lithiation and formation of nanopores during delithiation in Ge nanowires in real time. But due to the small size of the sample used in the TEM, it is difficult to control the current rate. In situ TXM can obtain two dimensional (2D) or three dimensional (3D) microstructures of Ge particles during cycling with a large field of view. However, the resolution of TXM is not sufficient for monitoring nanometer-sized structures, such as nanopores. In this study, we developed an approach to build a single particle battery in the chamber of focused-ion beam-scanning electron microscope (FIB-SEM) to monitor the microstructural evolution of a single Ge micro-particle during cycling. The experiment was performed within a ZEISS Nvision FIB-SEM at the Center for Nanoscale Materials, Argonne National Laboratory. A Ge particle was attached to the tungsten probe by carbon coating using FIB deposition as positive electrode. The lithium metal was placed on the top of SEM stage as negative electrode. One drop of ionic liquid electrolyte (ILE) was placed on the top of lithium metal. The ILE was made by dissolving the lithium salt, lithium bis (trifluoromethylsulfonyl) imide (LiTFSI), in a solvent of 1-butyl-1-methylpyrrolidinium bis (trifluoromethyl -sulfonyl) imide (P14TFSI). The tungsten probe and the SEM stage were connected to a Keithley 6430 sub-femtoamp remote sourcemeter. Galvanostatic mode was used in all electrochemical cycling. The particle was immersed in the ILE drop during cycling and lifted out for imaging at different states of charge. The particle was polished by FIB before imaging to remove ILE on the surface. This study reveals that germanium electrodes with low and high cycling rates have better microstructure integrity, which leads to better cycling performance. The nanopores tend to aggregate into large porous structures during cycling which leads to particle pulverization and capacity fading of the electrode. 1. Lim, L. Y.; Fan, S.; Hng, H. H.; Toney, M. F., Storage Capacity and Cycling Stability in Ge Anodes: Relationship of Anode Structure and Cycling Rate. Advanced Energy Materials 2015, 5 (15), 1500599. 2. Liu, X. H.; Huang, S.; Picraux, S. T.; Li, J.; Zhu, T.; Huang, J. Y., Reversible Nanopore Formation in Ge Nanowires during Lithiation-Delithiation Cycling: An In Situ Transmission Electron Microscopy Study. Nano Letters 2011, 11 (9), 3991-3997. 3.Weker, J. N.; Liu, N.; Misra, S.; Andrews, J. C.; Cui, Y.; Toney, M. F., In situ nanotomography and operando transmission X-ray microscopy of micron-sized Ge particles. Energy & Environmental Science 2014, 7 (8), 2771-2777.

Free standing Si (Ge) nanowire/Cu nanowire composites as lithium ion battery anodes

Silicon (Si) and germanium (Ge) can alloy with lithium to achieve high theoretical specific capacities (3579 mAh/g for Li15Si4 and 1384 mAh/g for Li15Ge4), which are several times greater than commercial graphite (372 mAh/g). However, they exhibit significant volume and crystal structural change during cycling processes, leading to structural pulverization and loss of electrical conduction paths between individual active materials and current collector. We report a non-carbon based free standing electrodes fabricated by directly mixing Si and Ge nanowires with Cu nanowires as lithium ion battery anodes. The one dimensional nanostructure of Si (Ge) and Cu provide good charge transport along their length and many contact points were formed between NWs after annealing process. The nanowire composites have several advantages compared to the conventional electrode that slurry materials are coated on a metal foil. For example, the weight is lighter than conventional electrodes because it does not need a current collector and is fabricated without any additives, and binders. Moreover, the space among the nanowires can accommodate the volume contraction of Si (Ge) during alloying and dealloying process and enhance the electrolyte penetration.

Surface oxide modification enables super-linear photoresponse in a single Germanium nanowire photodetector

We report observation of a super-linear photocurrent dependence on intensity of illumination in a single Ge nanowire (NW) of diameter ~60 nm. The super-linear dependence was enabled by a controlled and short duration vacuum annealing of the prefabricated NW photodetector to 400 °C. The short annealing leads to substantial overall performance enhancement of the broad band (300 nm–1100 nm) photo-detector such as reaching a Responsivity of ~0.6 × 107 A/W and a Specific Detectivity ~1014cmHz1/2/W. A combination of low frequency (<10 Hz) 1/f noise spectroscopy as well as X-Ray Photoelectron Spectroscopy showed that reduction of sub-oxide content in the surface oxide layer of a Ge NW leads to reduction of states with slow kinetics. That in turn prompts the onset of super-linear behavior along with enhancement of photodetector figures of merit.

Consistent description of mesoscopic transport: Case study of current-dependent magnetoconductance in single GaN:Ge nanowires

The so called phase-coherence length ${l}_{\ensuremath{\varphi}}$ and its relation to the geometrical dimensions of a sample determine the electronic transport regime. Different approaches are established for extracting ${l}_{\ensuremath{\varphi}}$ from magnetotransport data of mesoscopic systems and need to be cross-checked by using experimental data on the same model system in order to ensure an overall consistent theoretical description. Suitable model systems for testing this consistency are single GaN:Ge nanowires. Their magnetoconductance at low temperatures exhibits universal conductance fluctuations as well as weak localization effects. We find that the values of ${l}_{\ensuremath{\varphi}}$ obtained from the established analysis of the magnitude of the conductance fluctuations rms($\mathrm{\ensuremath{\Delta}}G$) decrease with increasing measurement current, whereas the corresponding values of ${l}_{\ensuremath{\varphi}}$ determined by the analyses of the correlation field ${B}_{\text{C}}$ and the weak localization effect yield the same value for ${l}_{\ensuremath{\varphi}}$ independent of the measurement current used. We apply and modify the existing theoretical framework for bias-dependent differential conductance fluctuations, in order to explain the decrease of the conductance fluctuations rms($\mathrm{\ensuremath{\Delta}}G$) with increasing current density in our dc measurements. This leads to the same values of ${l}_{\ensuremath{\varphi}}$ independent of the analysis approach applied to the same set of data.

Observation of Phonon Propagation in Germanium Nanowires Using Femtosecond Pump–Probe Microscopy

The excited-state dynamics in individual Ge nanowires (NWs) are imaged using ultrafast pump–probe microscopy with high spatial (∼600 nm) and temporal (∼500 fs) resolution. Photoexcitation of the NW by a focused femtosecond laser pulse promotes electrons from the valence band to the conduction band within a 400 nm segment of the 20–30 μm long NW. The localized excitation is then probed by a delayed femtosecond pulse, whose position with respect to the pump pulse is precisely controlled. The pump–probe signals contain contributions from free carriers, thermal excitation, and impulsively excited acoustic phonons, the latter of which are detected in the time domain as a coherent oscillation in the pump–probe signal. Examination of an ensemble of NWs with diameters (d) ranging from 50 to 300 nm shows that the coherence frequency is inversely proportional to d, consistent with excitation of a radial breathing mode. In addition, experiments performed with spatially separated pump and probe beams show that the vibrational motion is not confined to the excitation region but, rather, spreads as much as 2.5 μm along the NW axis during the first 3 ns after excitation. Spatially separated pump–probe microscopy also reveals the creation of a longitudinally propagating wave that travels along the NW at ∼6500 m/s or ∼20% faster than the speed of sound in bulk Ge.

A First-Principles Study on the Oxygen Adsorption and Interface Characteristics with a-GeO2 of Ge[001] Nanowire

First-principles calculations were performed to study the atomic and the electronic structures of the oxygen adsorption on the sidewalls of Ge[001] nanowires. For several possible adsorption sites, the adsorption energy of atomic oxygen, as well as the atomic configuration and electronic properties of the adsorbed structure, were examined. The adsorption stability is not much affected by the nanowire diameter for diameters ranging from 0.8 nm to 2 nm while the electronic structures like band gap and gap states, vary with the nanowire diameter. The interface between the Ge nanowire and amorphous GeO2 (a-GeO2) was also investigated. The interface region is composed of Ge suboxides like those between Ge bulk slab and a-GeO2. The valence band offsets between Ge nanowire and a-GeO2 decreases considerably as the diameter of nanowire shrinks while the conduction band offsets show less dependence on the diameter of the nanowire.

Si and Ge allotrope heterostructured nanowires: experimental evaluation of the thermal conductivity reduction

We have studied the thermal conductivity of Ge and Si allotrope heterostructured nanowires (NWs) synthesized by phase transformation. The NWs are composed of successive hexagonal 2H and cubic diamond 3C crystal phases along the 〈111〉 axis. Using 3ω-scanning thermal microscopy on NWs embedded in a silica matrix, we present the first experimental evidence of thermal conductivity reduction in such allotrope 2H/3C heterostructured NWs. In Ge heterostructured 2H/3C NWs, similarly to homogeneous 3C NWs, we show a thermal conductivity reduction when the NW diameter decreases. In addition, in Si and Ge NWs, we observe a reduced thermal conductivity due to the heterostructuration 2H/3C. We evidence that the temperature of phase transformation, which influences the size and the number of 2H domains, can constitute an efficient parameter to tune the thermal conductivity.

Mobility model based on piezoresistance coefficients for Ge 3D transistor

The conventional first order piezoresistance model has commonly been used to describe carrier mobility enhancement for low levels of process induced stress in Complementary Metal-Oxide-Semiconductor Field Effect Transistor (CMOS) technology. However, many reports show it failing to describe the nonlinear behavior observed at high levels of stress. In this paper, mobility model based on the modified piezoresistance model with nine stress-independent piezoresistance coefficients is proposed such that a mobility model can be applied correctly to calculate the strain-induced carrier mobility changes. Hence, the overall accuracy is improved compared to the conventional piezoresistance (PR) model. Its validation is confirmed with the results from TCAD simulations of carrier mobility for Ge Fin Field Effect Transistors (FinFET) and nanowire transistors.

Re-Imaging the Lithium-Ion Battery Configuration for Enhanced Thermodynamic Stability and Performance

The ever-growing problems of global warming, pollution and the depletion of fossil fuels have propelled Lithium-ion batteries (LiBs) to the forefront of the initiative for clean, renewable and sustainable energy. However, the current state-of-the-art LiB technology is not sufficiently equipped to handle adverse climatic conditions. Typical LiBs consist of a lithiated transition metal oxide cathode and a graphitic anode. Excessively low temperatures mitigate the functionality of the conventional LiCoO2-Graphite cell, rendering it non-operational. Conversely, elevated temperatures accelerate capacity fade through electrolyte degradation. Despite its low-cost and ubiquity, graphite exhibits a meagre gravimetric capacity of 372mAhg-1 along with poor performance under high current densities1. Moreover, at low cycling temperatures, graphitic anodes exhibit poor conductivity, overcharging and metallic lithium plating2. Recently, group-IV semiconducting nanomaterials have appeared as auspicious candidates to replace the archetypal graphitic anodes. Germanium, in particular, has a gravimetric capacity of 1384mAg-1, a fourfold increase to that of graphite. This, in conjunction with the excellent electronic properties of Germanium, highlights the huge potential of Ge-based anodes for high performance LiBs. Moreover, employing nanostructured anodes allows for substantially improved high current performance through larger contact areas and better conductivity through shorter diffusion lengths. Herein, we present comparative analysis of graphite-based and germanium nanowire-based LiBs operating over a wide temperature range (-40°C to +40°C ) for different electrolyte compositions. Preliminary results have highlighted the substantially superior performance of Germanium nanowire (NW) anodes over graphite anodes, becoming more pronounced at lower temperatures (Fig S1). Ge NWs were synthesised using a simple, solvent-free vapour-solid-solid (VSS) synthetic approach. Ge NWs were grown on a stainless steel current collector with a 2nm coating of Cu. Cyclic voltammetry and galvanostatic cycling of the cells have allowed us to quantify the performance of these cells over a range of temperatures. Moreover, SEM and TEM have allowed us to track changes in the morphology of the anodic material, with cycling. Electrochemical Impedance Spectroscopy (EIS) of LiCoO2-Germanium full cells has revealed that, at low temperatures, the conductivity of the traditional carbonate-based electrolyte is severely diminished; increasing concerns over electrolyte freezing and cell degradation. Conversely, galvanostatic cycling reveals substantial capacity fade for high temperatures (>40°C), indicative of electrolyte decomposition. Enhancing the thermal stability of LiBs is critical in advancing the commercial viability of LiBs operating at elevated temperatures. The high temperature performance of LiBs is limited by the instability and the susceptibility of the electrolyte to degradation at high temperatures. The traditional LiPF6 electrolyte becomes rapidly unstable at elevated temperatures (>40°C )3, decomposing into toxic by-products like POF3 and HF. Recently, mixtures of ionic liquids (ILs) such as FSI, TFSI and Poly(PPDA) have emerged as potential replacements for the traditional carbonated-based electrolyte4. Moreover, investigatory studies by Xu et al.5 found that lithium bis(oxolato)borate (LiBOB) has appeared as a promising candidate to replace the traditional LiPF6 electrolyte salt, owing to increased thermodynamic stability along with a wider electrochemical stability window6. The low-cost of graphite-based LiBs is of little consolation when considering their drastically diminished performance in harsh climates. Electrolytes that can sufficiently function at low temperatures and remain thermodynamically stable at high temperatures is a requisite in developing the optimal LiB configuration for adequate functionality over a wide temperature range. A dual effort of improving both the anode and the electrolyte's functionality, over a wide temperature range, will go a long way in advancing the field of LiBs. Cheng, X. B.; Zhang, R.; Zhao, C. Z.; Wei, F.; Zhang, J. G.; Zhang, Q., A review of solid electrolyte interphases on lithium metal anode. Advanced Science 2016, 3 (3), 1500213.Smart, M.; Ratnakumar, B.; Surampudi, S.; Wang, Y.; Zhang, X.; Greenbaum, S.; Hightower, A.; Ahn, C.; Fultz, B., Irreversible capacities of graphite in low‐temperature electrolytes for lithium‐ion batteries. Journal of The Electrochemical Society 1999, 146 (11), 3963-3969.Krause, L. J.; Lamanna, W.; Summerfield, J.; Engle, M.; Korba, G.; Loch, R.; Atanasoski, R., Corrosion of aluminum at high voltages in non-aqueous electrolytes containing perfluoroalkylsulfonyl imides; new lithium salts for lithium-ion cells. Journal of power sources 1997, 68 (2), 320-325.Kerner, M.; Johansson, P., Pyrrolidinium FSI and TFSI-Based Polymerized Ionic Liquids as Electrolytes for High-Temperature Lithium-Ion Batteries. Batteries 2018, 4 (1), 10.Xu, K.; Zhang, S.; Jow, T. R.; Xu, W.; Angell, C. A., LiBOB as salt for lithium-ion batteries: a possible solution for high temperature operation. Electrochemical and Solid-State Letters 2002, 5 (1), A26-A29.Xu, W.; Angell, C. A., Weakly coordinating anions, and the exceptional conductivity of their nonaqueous solutions. Electrochemical and Solid-State Letters 2001, 4 (1), E1-E4. Figure 1

One-Step Fabrication of GeSn Branched Nanowires

We report for the first time the self-catalyzed, single-step growth of branched GeSn nanostructures by a vapor–liquid–solid mechanism. These typical GeSn nanostructures consist of ⟨111⟩-oriented, Sn-rich (∼8 atom %) GeSn “branches” grown epitaxially on GeSn “trunks”, with a Sn content of ∼4 atom %. The trunks were seeded from Au0.80Ag0.20 nanoparticles followed by the catalytic growth of secondary branches (diameter ∼ 50 nm) from the excess of Sn on the sidewalls of the trunks, as determined by high-resolution electron microscopy and energy-dispersive X-ray analysis. The nanowires, with ⟨111⟩-directed GeSn branches oriented at ∼70° to the trunks, have no apparent defects or change in crystal structure at the trunk–branch interface; structural quality is retained at the interface with epitaxial crystallographic relation. The electrochemical performance of these highly ordered GeSn nanostructures was explored as a potential anode material for Li-ion batteries, due to their high surface-to-volume ratio and increased charge carrier pathways. The unique structure of the branched nanowires led to high specific capacities comparable to, or greater than, those of conventional Ge nanowire anode materials and Ge1–xSnx nanocrystals.

Structural Analysis and Electronic Properties of Unpassivated and H-Passivated Germanium Nanowires for Different Growth Directions

The structural analysis and electronic properties of unpassivated and hydrogen (H)-passivated germanium (Ge) nanowires (NWs) oriented along , , , and directions using a sp3 tight binding model have been investigated in this paper. It has been observed that the unit cell and complete structures of a GeNW is different for different growth directions, therefore, the total number of Ge atoms and H atoms within each unit cell and complete structure, length of unit cell, and diameter of a GeNW are changed with the change in their growth direction. Further, the electronic properties of both the unpassivated and H-passivated GeNWs have been investigated with the help of their band structures obtained for different growth directions. The band structures show that the unpassivated -oriented and -oriented GeNWs are indirect band gap semiconductors with band gap magnitude of 0.40 eV and 1.49 eV while the unpassivated -oriented GeNW is metallic because of zero band gap and -oriented GeNW is a direct band gap semiconductor with a very small band gap magnitude of 0.22 eV. Further, it has been observed that the nature of a GeNW is changed from indirect to direct when their entire surface dangling bonds are passivated with H atoms, and it has been investigated that the H-passivated GeNWs oriented along , , , directions are direct band gap semiconductors with band gap magnitude of 3.66 eV, 2.73 eV, 2.96 eV, 2.51 eV which are larger than the band gaps of an unpassivated GeNWs. Finally, it has been observed from these results that the H-passivated GeNWs are useful for designing novel nanoelectronic and optoelectronic devices.

Tunable Core-Shell Nanowire Active Material for High Capacity Li-Ion Battery Anodes Comprised of PECVD Deposited aSi on Directly Grown Ge Nanowires.

Herein, we report the formation of core@shell nanowires (NWs) comprised of crystalline germanium NW cores with amorphous silicon shells (Ge@aSi) and their performance as a high capacity Li-ion battery anode material. The Ge NWs were synthesized directly from the current collector in a solvent vapor growth (SVG) system and used as hosts for the deposition of the Si shells via a plasma-enhanced chemical vapor deposition (PECVD) process utilizing an expanding thermal plasma (ETP) source. The secondary deposition allows for the preparation of Ge@aSi core@shell structures with tunable Ge/Si ratios (2:1 and 1:1) and superior gravimetric and areal capacities, relative to pure Ge. The binder-free anodes exhibited discharge capacities of up to 2066 mAh/g and retained capacities of 1455 mAh/g after 150 cycles (for the 1:1 ratio). The 2:1 ratio showed a minimal ∼5% fade in capacity between the 20th and 150th cycles. Ex situ microscopy revealed a complete restructuring of the active material to an interconnected Si1- xGe x morphology due to repeated lithiation and delithiation. In full-cell testing, a prelithiation step counteracted first cycle Li consumption and resulted in a 2-fold improvement to the capacity of the prelithiated cell versus the unconditioned full-cells. Remarkable rate capability was also delivered where capacities of 750 mAh/g were observed at a rate of 10 C.

Prediction of Stable and High-Performance Charge Transport in Zigzag Tellurene Nanoribbons

The 2-D materials possess application perspective in semiconductor logic devices owing to their bonding free surface carrier transport. However, the scaling transistor width-induced edge issues have significantly limited their surface transport within the lateral direction, leading to effective mass increasing, and bandgap fluctuations. Therefore, it is important to find a novel 2-D material with both excellent surface transport and immunity to edge effect. In this paper, inspired by the recently discovered tellurene and its unique Te-Te bonds, the edge effect on the transport characteristics of tellurene is studied theoretically in the form of tellurene nanoribbons (TNRs). Among the four edge types, three of them exhibit semiconducting characteristic owing to the unique bonding property of Te. Simulation results demonstrate that the tetragonal edges (TEs) in zigzag TNRs (ZTNRs) almost have no influence on surface transport, resulting in minimized bandgap and effective mass variations, compared to that of graphene nanoribbons, and Si and Ge nanowires. Transport behaviors of the TE-ZTNRs are investigated using a metal–oxide–semiconductor transistor model and a multiscale simulation flow, with the calculated current densities exceeding $\textsf {1}$ mA/ $\mu \text{m}$ and the on–off ratios over $10^{\textsf {11}}$ . Based on the results, an explicit prediction that the TE-ZTNRs are competitive candidates for nanoscale transistor materials is proposed.

Growth of heterojunctions in Si–Ge alloy nanowires by altering AuGeSi eutectic composition using an approach based on thermal oxidation

Understanding the growth mechanism of heterojunctions in silicon–germanium alloy (Si–Ge) nanowires is helpful for designing adequate physical properties in the material for device applications. We examine the formation of the heterojunction in low Ge-content Si–Ge nanowires by an approach of thermal oxidation, which produces an atomically abrupt interface with an obvious concentration change. Forming heterojunctions in Si–Ge nanowires by this approach involves more complicated reaction routes than direct growth of heterojunction nanowires using the vapor–liquid–solid method. At the beginning of the oxidation process, the AuGeSi eutectic liquid at the nanowire tip significantly etches the Si–Ge alloy nanowires. Selective oxidation of Si results in a change of the relative amount of Ge to Si in the eutectic liquid, which further modulates the solubility of Ge and Si atoms. The compositional variation in the Au–Ge–Si ternary alloy system during the oxidation process accounts for the observed concentration profile in the heterojunction nanowire. The thermal oxidation approach is applied on a low Ge-content Si–Ge thin film that is coated with Au nanoparticles. Si–Ge nanodots, which exhibit a higher Ge concentration, are precipitated epitaxially in the film, as a result of compositional modulation in the AuGeSi eutectic liquid.

Influence of Structure on Electronic Charge Transport in 3D Ge Nanowire Networks in an Alumina Matrix

We demonstrate formation of material consisting of three-dimensional Germanium nanowire network embedded in an insulating alumina matrix. A wide range of such nanowire networks is produced using a simple magnetron sputtering deposition process. We are able to vary the network parameters including its geometry as well as the length and width of the nanowires. The charge transport in these materials is shown to be related to the nanowire surface per unit volume of the material, α. For low values of α, transport is characterized by space charge limited conduction and a drift of carriers in the extended states with intermittent trapping-detrapping in the localized states. For large values of α, charge transport occurs through hopping between localized electronic states, similar to observations in disorder-dominated arrays of quantum dots. A crossover between these two mechanisms is observed for the intermediate values of α. Our results are understood in terms of an almost linear scaling of the characteristic trap energy with changes in the nanowire network parameters.

In Situ Transmission Electron Microscopy Analysis of Aluminum-Germanium Nanowire Solid-State Reaction.

To fully exploit the potential of semiconducting nanowires for devices, high quality electrical contacts are of paramount importance. This work presents a detailed in situ transmission electron microscopy (TEM) study of a very promising type of NW contact where aluminum metal enters the germanium semiconducting nanowire to form an extremely abrupt and clean axial metal–semiconductor interface. We study this solid-state reaction between the aluminum contact and germanium nanowire in situ in the TEM using two different local heating methods. Following the reaction interface of the intrusion of Al in the Ge nanowire shows that at temperatures between 250 and 330 °C the position of the interface as a function of time is well fitted by a square root function, indicating that the reaction rate is limited by a diffusion process. Combining both chemical analysis and electron diffraction we find that the Ge of the nanowire core is completely exchanged by the entering Al atoms that form a monocrystalline nanowire with the usual face-centered cubic structure of Al, where the nanowire dimensions are inherited from the initial Ge nanowire. Model-based chemical mapping by energy dispersive X-ray spectroscopy (EDX) characterization reveals the three-dimensional chemical cross-section of the transformed nanowire with an Al core, surrounded by a thin pure Ge (∼2 nm), Al2O3 (∼3 nm), and Ge containing Al2O3 (∼1 nm) layer, respectively. The presence of Ge containing shells around the Al core indicates that Ge diffuses back into the metal reservoir by surface diffusion, which was confirmed by the detection of Ge atoms in the Al metal line by EDX analysis. Fitting a diffusion equation to the kinetic data allows the extraction of the diffusion coefficient at two different temperatures, which shows a good agreement with diffusion coefficients from literature for self-diffusion of Al.

Thermal conductivity measurement and analysis of Ge-SixGe1−x core–shell nanowires

We measured the thermal conductivity (κ) of two different Ge-SixGe1−x core–shell (CS) nanowires (NWs) with diameters of 40–50 nm and found to be 11.1 and 12.0 W m−1 K−1, respectively which were close to those of the Ge NW at 300 K. We also analyzed the measurement results using the modified Debye–Callaway model. Thermal analysis indicated that the decrease in the κ of the CS NWs resulted from dislocations at the CS interfaces, and predicted that the Ge NW has a rougher surface than the CS NWs because of oxygen protrusion through the oxidation front.

One-Step, Catalyst-Free, Scalable in Situ Synthesis of Single-Crystal Aluminum Nanowires in Confined Graphene Space.

Nanowires have a wide range of applications, such as transparent electrodes, Li-ion battery anodes, light-emitting diodes, solar cells, and electronic devices. Currently, aluminum (Al) nanowires can be synthesized by thermally induced substitution of germanium (Ge) nanowires, chemical vapor deposition on other metal substrates, and template-assisted growth methods. However, there are still challenges in fabricating extremely high-purity nanowires, large-scale manufacturing, and simplifying the synthesis process and conditions. Here, we report for the first time that single-crystal Al nanowires can be one-step, in situ synthesized on a reduced graphene oxide (RGO) substrate on a large scale without using any catalysts. Through a simple high temperature treatment process, commercial micro-sized Al powders in RGO film were transformed into a single-crystal Al nanowire with an average length of 1.2 μm and an average diameter of 18 nm. The possible formation mechanism of the single-crystal Al nanowires is proposed as follows: hot aluminum atoms eject from the pristine aluminum/alumina core/shell structure of Al powders when they build up enough energy from the thermal stress under high temperature and confined space conditions, which is supported by both experimental and computational results. The method introduced here can be extended to allow the synthesis of one-dimensional highly reactive materials, like alkali metal nanowires, in confined spaces.

Enhancing the performance of germanium nanowire anodes for Li-ion batteries by direct growth on textured copper.

Herein, textured Cu foil is presented as an attractive current collector substrate for directly grown Ge nanowire (NW) anodes. Compared to planar stainless steel (SS) current collectors, textured Cu led to an increase in achievable mass loading, removal of the requirement for a catalyst deposition step, improved adhesion of the active material and dramatically enhanced capacity retention. When SS and textured Cu foil based anodes with similar areal loadings (∼1.4 mA h cm-2) were compared, the capacity after 250 cycles for textured Cu was 2.7 times higher than the SS anode, illustrating the key role of the current collector.

Highlighting the Importance of Full-Cell Testing for High Performance Anode Materials Comprising Li Alloying Nanowires

Herein, the electrochemical performance of directly grown Ge nanowire anodes in full-cell Li-ion configurations (using lithium cobalt oxide cathodes) are examined. The impacts of voltage window, anode/cathode balancing and anode preconditioning are assessed. The cells had a useable upper cutoff of 3.9 V, with a higher voltage cutoff of 4.2 V shown by SEM analysis to lead to Li plating on the anode surface. The rate performance of Ge NW anodes was shown to be boosted within full-cells compared to half-cells, meaning that existing studies may underestimate the rate performance of alloying mode anode materials if they are only based on half-cell investigations. The capacity retention of the full-cells is lower compared to equivalent half-cells due to progressive consumption of cyclable Li. This phenomenon is demonstrated using a parallel anode and cathode delithiation approach that could be extended to other full-cell systems. The findings stress the importance of testing promising anode materials within full-cell configurations, to identify specific capacity fade mechanisms that are not relevant to half-cells and aid the development of higher energy density storage systems.