[show abstract][hide abstract] ABSTRACT: 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. KEYWORDS: silicon nanowires, indium, Li ion anode material ■ INTRODUCTION Si nanowires (NWs) have attracted considerable research interest because of their suitability for a wide range of emerging applications from transistor gate channels in the semiconductor industry to high density light trapping antennae in photo-voltaics. 1−3 More recently, Si NWs have been developed as a viable candidate material for Li ion battery anodes due their ability to withstand the volume expansion associated with Li cycling (not possible in bulk), allowing the high specific capacity (4200 mAhr g −1) to be exploited. 4 Practical integration of Si NWs in energy conversion and storage applications requires their formation in high density and at low cost if grid competitive devices are to be realized. A wide range of metal catalyzed protocols have been developed in conventional chemical vapor deposition (CVD) processes, which facilitate Si NW growth by vapor−liquid− solid (VLS) or vapor−solid−solid (VSS) mechanisms. 5−7 These reports can largely be divided into three general categories based on the state of the NW catalyst. Type A catalytic materials possess a single eutectic point with high Si solubility (typically >10%) and include Ag, 8 Al, 9 and the archetypal NW catalyst, Au. 10,11 Type B catalysts (e.g., In, Ga) 12−15 also show one dominant eutectic point; however, the composition is typically <1% Si whereas type C catalysts are silicide forming metals with complex phase diagrams that allow either VLS or VSS growth depending on the reaction temperature. 16 Indium in particular is an attractive type B catalyst for Si NW growth as its melting point, of just 156.6 °C, facilitates low reaction temperatures, while the incorporation of In atoms into the NW lattice can also be used to impart p-type doping. 17−19 The incorporation of dopants also increase the electrical conductivity of the NWs, which can improve their suitability for Li ion cells due to enhanced charge/discharge rates versus their undoped analogues. 20 While vacuum based CVD processes allow precise tuning of dimensions 21 and composition 22 and doping of the nanostruc-tures, 23 more recently developed organic solvent based syntheses have gained attention for their potential to produce high density NWs at low cost. 24−26 The challenge is that the boiling point of conventional solvents provides for a very narrow temperature window (typically <400 °C), limiting this technique mainly to compound semiconductors that nucleate at lower temperatures. 27 This obstacle can be overcome by operating within pressurized supercritical fluid systems, 25,28,29 or alternatively, by selecting a suitable metal catalyst and conducting the NW growth within a high boiling point organic solvent (HBS). 30,31 While Ge NWs can be routinely nucleated in HBS systems, 30,32 Si NW growth is more difficult and the only success to date involved the use of a highly reactive
Chemistry of Materials 05/2012; · 8.24 Impact Factor
[show abstract][hide abstract] ABSTRACT: Vertical nanorod assembly over three dimensions is shown to result in the formation of Moiré interference patterns arising from rotational offsets between respective monolayer sheets. Six distinct patterns are observed in HRTEM and angular dark-field STEM (DF-STEM) images, allowing the exact angle of rotation to be determined from their respective size and repeat order. At large rotation angles approaching 30°, the aperiodicity in the structure of the nanorod supercrystals becomes apparent, resulting in 12-fold ordering characteristics of a quasicrystal. The rotational offsets are further elucidated from Fourier transform and small angle electron diffraction, allowing interpretation of several multilayers when combined with DF-STEM and SEM. Pattern formation owing to angular rotation is differentiated from those occurring from a lateral shift, providing an important insight into the complex multilayered structures in assembled rods that may have an impact on their collective electronic or photonic properties. We also show how random tetrapods when present at low concentrations in colloidal nanorod solutions act as termination points for 2D sheet crystallization, impacting the size and shape of the resultant assemblies. The occurrence of Moiré patterns in rod assemblies demonstrates the extraordinary order achievable in their assembly and offers a nondestructive technique to precisely map the placement of each nanorod in this important nanoarchitecture.
[show abstract][hide abstract] ABSTRACT: Germanium nanowires (Ge NWs) have found significant emerging applications spanning the solar, semiconductor, and storage industries. 1À5 The combination of high mobility and a large Bohr excitonic radius makes Ge NWs the semiconductor material of choice either for next generation, on-chip gate architec-tures or as pÀn junction absorber arrays in photovoltaics. 1,2 Ge NWs have shown promise in Li battery anodes as a result of their ability to withstand volume expansion on lithium insertion coupled with high theoretical capacities (1600 mA h/g) and increased room temperature diffusivity (compared to Si). 3 The conventional seeding protocol where a metal nanoparticle forms a eutectic melt with the semiconductor has several advantages in forming NWs, particularly for discrete applications, as precise control over diameter and, in some cases, length is possible. 4À6 The metal seed acts as a sink for the growth species either as a liquid 7 (vapor liquid solid (VLS)) or as a solid 8À10 (vapor solid solid (VSS)) with work showing that the preferred orientations and defects in the metal seed can be transferred to the semiconductor NW. 11 Metal seeded growth has shifted toward solid catalysts consisting of Cu, 12 Ni, 13,14 and Fe 15 in an effort to limit the dif-fusion of metal atoms into the NWs associated with the use of Au, which can severely impact the electrical properties of the NWs. 16,17 The emergence of storage and photovoltaic applica-tions places new demands on synthetic protocols for Ge NWs, with production directly from the current collector, in high yield and low cost, desirable. In directly seeded growth, high density is difficult to achieve, as temperature driven agglomeration can limit the number of catalytic sites available while also leading to a diameter spread in the NWs formed. 18À20 The emergence of self-catalytic 15,21À25 growth systems for Ge NW formation has allowed high yield growth, often from low cost precursors. 26 Using the correct conditions, spontaneous NW formation with defect free morphologies and tight diameter distributions have been achieved. 27 Self-catalytic approaches (without the direct incorporation of discrete nanoparticle seeds) have also been successfully extended to the formation of Ge NWs directly on Fe, 15 Ta, and W substrates. 25 The most interesting candidate for NW growth is copper, because of its use as a current collector in lithium ion batteries. While VSS seeding from copper nanocrystals is known, direct growth from bulk copper would be very attractive, as it potentially offers a route toward binder free cells at low cost, with sufficient gravimetric density for commer-cial viability. 28À30 Here, we present the highly dense growth of Ge NWs in a self-catalyzed process through the thermal decomposition of diphe-nylgermane (DPG) on copper foil in the vapor phase of a high boiling point solvent. The NWs are grown without the incor-poration of discrete metal nanoparticles. Rather, we show that the in situ formation of Cu 3 Ge acts as a catalyst for the formation of extremely dense NW mats with a very low diameter variation. This is the first direct observation of metal germanide tips in a self-induced, VSS process from bulk metals, offering important
Chemistry of Materials 10/2011; · 8.24 Impact Factor
[show abstract][hide abstract] ABSTRACT: Silicon nanocrystals were synthesized at high temperatures and high pressures by the thermolysis of diphenylsilane using a combination of supercritical carbon dioxide and phosphonic acid surfactants. Size and shape evolution from pseudo-spherical silicon nanocrystals to well-faceted tetrahedral-shaped silicon crystals with edge lengths in the range of 30-400 nm were observed with sequentially decreasing surfactant chain lengths. The silicon nanocrystals were characterized by transmission electron microscopy (TEM), energy-dispersive x-ray spectroscopy (EDX), x-ray diffraction (XRD), photoluminescence (PL), scanning electron microscopy (SEM) and Raman scattering spectroscopy.