Nanowires, nanorods and axial heterostructures of silicon and germanium synthesised in a solvent vapour system

This thesis describes the formation of Si and Ge nanostructures using two growth systems. Chapter 3 details the controlled growth of Ge nanowires and nanorods using a surfactant-free hotplate based growth method. Control over the aspect ratio of the Ge nanostructures is achieved by varying the concentration and reactivity of the organometallic precursor. This route used Cu3Ge catalyst seeds which were formed in situ from bulk copper foil. This approach was significantly expanded in Chapter 4 to allow the formation of Ge nanowires over centimetre squared areas. The use of a thin thermally evaporated Cu layer as catalyst source led to high density nanowire growth on substrates which were ideally suited to direct incorporation as Li-ion battery anodes. The optimised nanowire covered substrates showed excellent capacities of 1040 mAhrg-1 after 500 charge/discharge cycles. Focus then shifted to the use of a glassware-based solvent vapour growth system. Here, dense Ge and Si nanowire mats were formed directly on stainless steel using an abundant catalyst material, Sn (Chapter 5). These nanowires are promising candidates for Li-ion storage applications as both the nanowire and catalyst add to device capacity. Capacity figures of 1078 mAhrg-1 for Si and 1000 mAhrg-1 for Ge were noted after 50 cycles (with both the masses of the Li-active Sn seed and nanowire segment taken into consideration). Finally, Sn was shown to be a suitable catalyst for the formation of compositionally abrupt Si/Ge heterostructure nanowires (Chapter 6) using the same solvent vapour growth system detailed in Chapter 5. Compositional abruptness was facilitated by the extremely low solubilities of Si and Ge in the Sn catalyst material while control over the length of the Si and Ge segments was achieved by controlling the reactivity of the respective precursors and associated reaction conditions

Synthesis of silicon-germanium axial nanowire heterostructures in a solvent vapor growth system using indium and tin catalysts

Here we describe a relatively facile synthetic protocol for the formation of Si-Ge and Si-Ge-Si1-xGex axial nanowire heterostructures. The wires are grown directly on substrates with an evaporated catalytic layer placed in the vapour zone of a high boiling point solvent with the silicon and germanium precursors injected as liquids sequentially. We show that these heterostructures can be formed using either indium or tin as the catalyst seeds which form in situ during the thermal anneal. There is a direct correlation between growth time and segment length allowing good control over the wire composition. The formation of axial heterostructures of Si-Ge-Si1-xGex nanowires using a triple injection is further discussed with the alloyed Si1-xGex third component formed due to residual Ge precursor and its greater reactivity in comparison to silicon. It was found that the degree of tapering at each hetero-interface varied with both the catalyst type and composition of the NW. The report shows the versatility of the solvent vapour growth system for the formation of complex Si-Ge NW heterostructures

A Rapid, Solvent-Free Protocol for the Synthesis of Germanium Nanowire Lithium-Ion Anodes with a Long Cycle Life and High Rate Capability

A rapid synthetic protocol for the formation of high-performance Ge nanowire-based Li-ion battery anodes is reported. The nanowires are formed in high density by the solvent-free liquid deposition of a Ge precursor directly onto a heated stainless steel substrate under inert conditions. The novel growth system exploits the in situ formation of discrete Cu₃Ge catalyst seeds from 1 nm thermally evaporated Cu layers. As the nanowires were grown from a suitable current collector, the electrodes could be used directly without binders in lithium-ion half cells. Electrochemical testing showed remarkable capacity retention with 866 mAh/g achieved after 1900 charge/discharge cycles and a Coulombic efficiency of 99.7%. The nanowire-based anodes also showed high-rate stability with discharge capacities of 800 mAh/g when cycled at a rate of 10C

Atomically Abrupt Silicon–Germanium Axial Heterostructure Nanowires Synthesized in a Solvent Vapor Growth System

The growth of Si/Ge axial heterostructure nanowires in high yield using a versatile wet chemical approach is reported. Heterostructure growth is achieved using the vapor zone of a high boiling point solvent as a reaction medium with an evaporated tin layer as the catalyst. The low solubility of Si and Ge within the Sn catalyst allows the formation of extremely abrupt heterojunctions of the order of just 1–2 atomic planes between the Si and Ge nanowire segments. The compositional abruptness was confirmed using aberration corrected scanning transmission electron microscopy and atomic level electron energy loss spectroscopy. Additional analysis focused on the role of crystallographic defects in determining interfacial abruptness and the preferential incorporation of metal catalyst atoms near twin defects in the nanowires

Synthesis of Tin Catalyzed Silicon and Germanium Nanowires in a Solvent–Vapor System and Optimization of the Seed/Nanowire Interface for Dual Lithium Cycling

Silicon and germanium nanowires are grown in high density directly from a tin layer evaporated on stainless steel. The nanowires are formed in low cost glassware apparatus using the vapor phase of a high boiling point organic solvent as the growth medium. HRTEM, DFSTEM, EELS, and EDX analysis show the NWs are single crystalline with predominant ⟨111⟩ growth directions. Investigation of the seed/nanowire interface shows that in the case of Si an amorphous carbon interlayer occurs that can be removed by modifying the growth conditions. Electrochemical data shows that both the tin metal catalyst and the semiconductor nanowire reversibly cycle with lithium when the interface between the crystalline phases of the metal and semiconductor is abrupt. The dually active nanowire arrays were shown to exhibit capacities greater than 1000 mAh g^–1 after 50 charge/discharge cycles

Growth of crystalline copper silicide nanowires in high yield within a high boiling point solvent system

Here, we report the formation of high density arrays of Cu15Si4 nanowires using a high boiling point organic solvent based method. The reactions were carried out using Cu foil substrates as the Cu source with nanowire growth dependent upon the prior formation of Cu15Si4 crystallites on the surface. The method shows that simple Si delivery to metal foil can be used to grow high densities of suicide nanowires with a tight diameter spread at reaction temperatures of 460 degrees C. The nanowires were characterized by high-resolution transmission electron microscopy (HRTEM), high-resolution scanning electron microscopy (HRSEM), and X-ray photoelectron spectroscopy (XPS), and electrical analysis showed that they possess low resistivities

High-performance germanium nanowire-based lithium-ion battery anodes extending over 1000 cycles through in situ formation of a continuous porous network

Here we report the formation of high-performance and high-capacity lithium-ion battery anodes from high-density germanium nanowire arrays grown directly from the current collector. The anodes retain capacities of similar to 900 mAh/g after 1100 cycles with excellent rate performance characteristics, even at very high discharge rates of 20-100C. We show by an ex situ high-resolution transmission electron microscopy and high-resolution scanning electron microscopy study that this performance can be attributed to the complete restructuring of the nanowires that occurs within the first 100 cycles to form a continuous porous network that is mechanically robust. Once formed, this restructured anode retains a remarkably stable capacity with a drop of only 0.01% per cycle thereafter. As this approach encompasses a low energy processing method where all the material is electrochemically active and binder free, the extended cycle life and rate performance characteristics demonstrated makes these anodes highly attractive for the most demanding lithium-ion applications such as long-range battery electric vehicles

Growth of Crystalline Copper Silicide Nanowires in High Yield within a High Boiling Point Solvent System

Here, we report the formation of high density arrays of Cu₁₅Si₄ nanowires using a high boiling point organic solvent based method. The reactions were carried out using Cu foil substrates as the Cu source with nanowire growth dependent upon the prior formation of Cu₁₅Si₄ crystallites on the surface. The method shows that simple Si delivery to metal foil can be used to grow high densities of silicide nanowires with a tight diameter spread at reaction temperatures of 460 °C. The nanowires were characterized by high-resolution transmission electron microscopy (HRTEM), high-resolution scanning electron microscopy (HRSEM), and X-ray photoelectron spectroscopy (XPS), and electrical analysis showed that they possess low resistivities

High-Performance Germanium Nanowire-Based Lithium-Ion Battery Anodes Extending over 1000 Cycles Through in Situ Formation of a Continuous Porous Network

Here we report the formation of high-performance and high-capacity lithium-ion battery anodes from high-density germanium nanowire arrays grown directly from the current collector. The anodes retain capacities of ∼900 mAh/g after 1100 cycles with excellent rate performance characteristics, even at very high discharge rates of 20–100C. We show by an ex situ high-resolution transmission electron microscopy and high-resolution scanning electron microscopy study that this performance can be attributed to the complete restructuring of the nanowires that occurs within the first 100 cycles to form a continuous porous network that is mechanically robust. Once formed, this restructured anode retains a remarkably stable capacity with a drop of only 0.01% per cycle thereafter. As this approach encompasses a low energy processing method where all the material is electrochemically active and binder free, the extended cycle life and rate performance characteristics demonstrated makes these anodes highly attractive for the most demanding lithium-ion applications such as long-range battery electric vehicles

High Density Growth of Indium seeded Silicon Nanowires in the Vapor phase of a High Boiling Point Solvent

Herein, we describe the growth of Si nanowires (NWs) in the vapor phase of an organic solvent medium on various substrates (Si, glass, and stainless steel) upon which an indium layer was evaporated. Variation of the reaction time allowed NW length and density to be controlled. The NWs grew via a predominantly root-seeded mechanism with discrete In catalyst seeds formed from the evaporated layer. The NWs and substrates were characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). The suitability of the indium seeded wires as anode components in Li batteries was probed using cyclic voltammetric (CV) measurements. The route represents a versatile, glassware-based method for the formation of Si NWs directly on a variety of substrates