Directed synthesis of germanium oxide nanowires by vapor-liquid-solid oxidation

We report on the directed synthesis of germanium oxide (GeO(x)) nanowires (NWs) by locally catalyzed thermal oxidation of aligned arrays of gold catalyst-tipped germanium NWs. During oxygen anneals conducted above the Au-Ge binary eutectic temperature (T?>?361?°C), one-dimensional oxidation of as-grown Ge NWs occurs by diffusion of Ge through the Au-Ge catalyst droplet, in the presence of an oxygen containing ambient. Elongated GeO(x) wires grow from the liquid catalyst tip, consuming the adjoining Ge NWs as they grow. The oxide NWs' diameter is dictated by the catalyst diameter and their alignment generally parallels that of the growth direction of the initial Ge NWs. Growth rate comparisons reveal a substantial oxidation rate enhancement in the presence of the Au catalyst. Statistical analysis of GeO(x) nanowire growth by ex?situ transmission electron microscopy and scanning electron microscopy suggests a transition from an initial, diameter-dependent kinetic regime, to diameter-independent wire growth. This behavior suggests the existence of an incubation time for GeO(x) NW nucleation at the start of vapor-liquid-solid oxidation.     

Highly sensitive ZnO nanowire CO sensors with the adsorption of Au nanoparticles

In this study, the growth of high density single-crystalline ZnO nanowires on patterned ZnO:Ga/SiO(2)/Si templates was reported. We also adsorbed Au nanoparticles onto nanowire surfaces and fabricated ZnO nanowire CO sensors. With 50?ppm CO gas, it was found that we could enhance the device sensitivities at 350?°C from 4.2% to 46.5% by the adsorption of Au nanoparticles. It was also found that measured sensitivities were around 30%, 37%, 46.5% and 53% when concentration of the injected CO gas was 5, 20, 50 and 100?ppm, respectively.     

The growth mechanism for silicon oxide nanowires synthesized from an Au nanoparticle/polyimide/Si thin film stack

During pyrolysis of polyimide (PI) thin film, amorphous silicon oxide nanowires (SiO(x)NWs) were produced on a large scale through heat treatment of an Au nanoparticle/PI/Si thin film stack at 1000?°C. It was shown that carbonization of the PI film preceded the nucleation of the SiO(x)NWs. The formation of the SiO(x)NWs was sustained by the oxygen derived from carbonization of the polyimide thin film while Si was provided from the substrate. Au nanoparticles promoted the SiO(x)NW growth by inducing localized melting of the Si substrate and by catalyzing the nanowire growth.     

Fabrication of multilayered Ge nanocrystals by magnetron sputtering and annealing

Multilayered Ge nanocrystals embedded in Si and Ge oxide films have been fabricated on Si?substrate by a (SiO(2)+Ge)/(SiO(2)+GeO(2)) superlattice approach, using an rf magnetron sputtering technique with a Ge+SiO(2) composite target and subsequent thermal annealing in N(2) ambient at 750?°C for 5?min. X-ray diffraction (XRD) measurements indicated the formation of Ge nanocrystals with an average size estimated to be 9.8?nm. Raman scattering spectra showed a peak of the Ge-Ge vibrational mode shifted downwards to 298.8?cm(-1), which was caused by quantum confinement of phonons in the Ge nanocrystals. X-ray photoemission spectroscopy (XPS) analysis demonstrated that the Ge chemical state is mainly Ge(0) in the (SiO(2)+Ge) layer and Ge(4+) in the (SiO(2)+GeO(2)) layer in the superlattice structure. Transmission electron microscopy (TEM) revealed that Ge nanocrystals were confined in (SiO(2)+Ge) layers, and had good crystallinity. This superlattice approach significantly improved both the size uniformity of Ge nanocrystals and their uniformity of spacing on the 'Z' growth direction compared with the conventional Ge-ncs fabrication method using a single and thick SiO(2) matrix film.     

Influence of hydrogen on thermally induced phase separation in GeO/SiO2 multilayers

The influence of the annealing atmosphere on the temperature induced phase separation of Ge oxide in GeO(x)/SiO(2) multilayers (x≈1), leading to size controlled growth of Ge nanocrystals, is explored by means of x-ray absorption spectroscopy at the Ge K-edge. Ge sub-oxides contained in the as-deposited multilayers diminish with increasing annealing temperature, showing complete phase separation at approximately 450?°C using inert N(2) ambient. The use of reducing H(2) in the annealing atmosphere influences the phase separation even at an early stage of the disproportionation. In particular, the temperature regime where the phase separation occurs is lowered by at least 50?°C. At temperatures above 400?°C the sublayer composition, and thus the density of the Ge nanocrystals, can be altered by making use of the reduction of GeO(2) by H(2).     

Chemical control of semiconductor nanowire kinking and superstructure

We show that methylgermane (GeH(3)CH(3)) can induce a transition from 111 to 110 oriented growth during the vapor-liquid-solid synthesis of Ge nanowires. This hydride-based chemistry is subsequently leveraged to rationally fabricate kinking superstructures based on combinations of 111 and 110 segments. The addition of GeH(3)CH(3) also eliminates sidewall tapering and enables Ge nanowire growth at temperatures exceeding 475 °C, which greatly expands the process window and opens new avenues to create Si/Ge heterostructures.     

Selective growth of silica nanowires using an Au catalyst for optical recognition of interleukin-10

The vapor-liquid-solid (VLS) growth procedure has been extended for the selective growth of silica nanowires on SiO(2) layer by using Au as a catalyst. The nanowires were grown in an open tube furnace at 1100?°C for 60?min using Ar as a carrier gas. The average diameter of these bottom-up nucleated wires was found to be 200?nm. Transmission electron microscopy analysis indicates the amorphous nature of these nanoscale wires and suggests an Si-silica heterostructure. The localized silica nanowires have been used as an immunoassay template in the detection of interleukin-10 which is a lung cancer biomarker. Such a nanostructured platform offered a tenfold enhancement in the optical response, aiding the recognition of IL-10 in comparison to a bare silica substrate. The role of nanowires in the immunoassay was verified through the quenching behavior in the photoluminescence (PL) spectra. Two orders of reduction in PL intensity have been observed after completion of the immunoassay with significant quenching after executing every step of the protocol. The potential of this site-specific growth of silica nanowires on SiO(2) as a multi-modal biosensing platform has been discussed.     

Template-Free Preparation of Crystalline Ge Nanowire Film Electrodes via an Electrochemical Liquid-Liquid-Solid Process in Water at Ambient Pressure and Temperature for Energy Storage

The direct electrodeposition of crystalline germanium (Ge) nanowire film electrodes from an aqueous solution of dissolved GeO(2) using discrete 'flux' nanoparticles capable of dissolving Ge(s) has been demonstrated. Electrodeposition of Ge at inert electrode substrates decorated with small (<100 nm), discrete indium (In) nanoparticles resulted in crystalline Ge nanowire films with definable nanowire diameters and densities without the need for a physical or chemical template. The Ge nanowires exhibited strong polycrystalline character as-deposited, with approximate crystallite dimensions of 20 nm and a mixed orientation of the crystallites along the length of the nanowire. Energy dispersive spectroscopic elemental mapping of individual Ge nanowires showed that the In nanoparticles remained at the base of each nanowire, indicating good electrical communication between the Ge nanowire and the underlying conductive support. As-deposited Ge nanowire films prepared on Cu supports were used without further processing as Li(+) battery anodes. Cycling studies performed at 1 C (1624 mA g(-1)) indicated the native Ge nanowire films supported stable discharge capacities at the level of 973 mA h g(-1), higher than analogous Ge nanowire film electrodes prepared through an energy-intensive vapor-liquid-solid nanowire growth process. The cumulative data show that ec-LLS is a viable method for directly preparing a functional, high-activity nanomaterials-based device component. The work presented here is a step toward the realization of simple processes that make fully functional energy conversion/storage technologies based on crystalline inorganic semiconductors entirely through benchtop, aqueous chemistry and electrochemistry without time- or energy-intensive process steps.     

Correlation between the performance and microstructure of Ti/Al/Ti/Au Ohmic contacts to p-type silicon nanowires

Understanding the electrical and microstructural aspects of contact formation at nanoscale is essential for the realization of low-resistance metallization suitable for the next generation of nanowire based devices. In this study, we present detailed electrical and microstructural characteristics of Ti/Al/Ti/Au metal contacts to p-type Si nanowires (SiNWs) annealed at various temperatures. Focused ion beam cross-sectioning techniques and scanning transmission electron microscopy (STEM) were used to determine the microstructure of the source/drain metal contacts of working SiNW field-effect transistors (FETs) annealed for 30 s in the 450-850?°C temperature range in inert atmosphere. Formation of titanium silicides is observed at the metal/semiconductor interface after the 750?°C anneal. Extensive Si out-diffusion from the nanowire after the 750?°C anneal led to Kirkendall void formation. Annealing at 850?°C led to almost complete out-diffusion of Si from the nanowire core. Devices with 550?°C annealed contacts had linear electrical characteristics; whereas the devices annealed at 750?°C had the best characteristics in terms of linearity, symmetric behavior, and yield. Devices annealed at 850?°C had poor yield, which can be directly attributed to the microstructure of the contact region observed in STEM.     

Ge-Si-O phase separation and Ge nanocrystal growth in Ge:SiO(x)/SiO(2) multilayers--a new dc magnetron approach

Ge:SiO(x)/SiO(2) multilayers are fabricated using a new reactive dc magnetron sputtering approach. The influence of the multilayer stoichiometry on the ternary Ge-Si-O phase separation and the subsequent size-controlled Ge nanocrystal formation is explored by means of x-ray absorption spectroscopy, x-ray diffraction, electron microscopy and Raman spectroscopy. The ternary system Ge-Si-O reveals complete Ge-O phase separation at 400?°C which does not differ significantly to the binary Ge-O system. Ge nanocrystals of < 5?nm size are generated after subsequent annealing below 700?°C. It is shown that Ge oxides contained in the as-deposited multilayers are reduced by a surrounding unsaturated silica matrix. A stoichiometric regime was found where almost no GeO(2) is present after annealing. Thus, the Ge nanocrystals become completely embedded in a stoichiometric silica matrix favouring the use for photovoltaic applications.     

Diameter-dependent composition of vapor-liquid-solid grown Si(1-x)Ge(x) nanowires

Diameter-dependent compositions of Si(1-x)Ge(x) nanowires grown by a vapor-liquid-solid mechanism using SiH(4) and GeH(4) precursors are studied by transmission electron microscopy and X-ray energy dispersive spectroscopy. For the growth conditions studied, the Ge concentration in Si(1-x)Ge(x) nanowires shows a strong dependence on nanowire diameter, with the Ge concentration decreasing with decreasing nanowire diameter below approximately 50 nm. The size-dependent nature of Ge concentration in Si(1-x)Ge(x) NWs is strongly suggestive of Gibbs-Thomson effects and highlights another important phenomenon in nanowire growth.     

Fabrication of reliable semiconductor nanowires by controlling crystalline structure

One-dimensional SnO(2) nanomaterials with wide bandgap characteristics are attractive for flexible and/or transparent displays and high-performance nano-electronics. In this study, the crystallinity of SnO(2) nanowires was regulated by controlling their growth temperatures. Moreover, the correlation of the crystallinity of nanowires with optical and electrical characteristics was analyzed. When SnO(2) nanowires were grown at temperatures below 900?°C, they showed various growth directions and abnormal discontinuity in their crystal structures. On the other hand, most nanowires grown at 950?°C exhibited a regular growth trend in the direction of [100]. In addition, the low temperature photoluminescence measurement revealed that the higher growth temperatures of nanowires gradually decreased the 500 nm peak rather than the 620 nm peak. The former peak is derived from the surface defect related to the shallow energy level and affects nanowire surface states. Owing to crystallinity and defects, the threshold voltage range (maximum-minimum) of SnO(2) nanowire transistors was 1.5 V at 850?°C, 1.1 V at 900?°C, and 0.5 V at 950?°C, with dispersion characteristics dramatically decreased. This study successfully demonstrated the effects of nanowire crystallinity on optical and electrical characteristics. It also suggested that the optical and electrical characteristics of nanowire transistors could be regulated by controlling their growth temperatures in the course of producing SnO(2) nanowires.     

Growth characteristics of GaAs nanowires obtained by selective area metal-organic vapour-phase epitaxy

GaAs nanowires were selectively grown by metal-organic vapour-phase epitaxy within a SiO(2) mask window pattern fabricated on a GaAs(111)B substrate surface. The nanowires were 100-3000?nm in height and 50-300?nm in diameter. The height decreased as the mask window diameter was increased or the growth temperature was increased from 700 to 800?°C. The dependence of the nanowire height on the mask window diameter was compared with a calculation, which indicated that the height was inversely proportional to the mask window diameter. This suggests that the migration of growth species on the nanowire side surface plays a major role. Tetrahedral GaAs grew at an early stage of nanowire growth but became hexagonal as the growth process continued. The calculated change in Gibbs free energy for nucleation growth of the crystals indicated that tetrahedra were energetically more favourable than hexagons. Transmission and scanning electron microscopy analyses of a GaAs nanowire showed that many twins developed along the [Formula: see text] B direction, suggesting that twins had something to do with the evolution of the nanowire shape from tetrahedron to hexagon.     

Gold nanowires fabricated by immersion plating

The growth mechanism of oriented Au nanowires fabricated by immersion plating was investigated. Both n-type crystal Si (c-Si) and amorphous Si (a-Si) with an electron-beam (E-beam) patterned resist nanotrench were immersed into the plating bath HAuCl(4)/HF. For the Au nanowires fabricated on c-Si, voids, nanograins, and clusters were observed at various plating conditions, time and temperature. The voids were often found in the center of the Au nanowires due to there being fewer nucleation sites on the c-Si surface. However, Au can easily nucleate on the surface of a-Si and form continuous Au nanowires with grain sizes about 10-50?nm. The resistivities of Au nanowires with width 105?nm fabricated on a-Si are about 4.4-6.5?μΩ?cm. After annealing at 200?°C for 30?min in N(2) ambient, the resistivities are lowered to about 3.0-3.9?μΩ?cm, measured in an atomic force microscope (AFM) in contact mode. The grain size of Au is in the range of ～50-100?nm. A scanning electron microscope (SEM) examination and grazing incident x-ray diffraction (GIXRD) analysis were also carried out to study the morphology and crystalline structure of the Au nanowires.     

Diameter control of tungsten oxide nanowires as grown by thermal evaporation

Tungsten oxide nanowires with controllable diameter were synthesized on Si substrates by thermal evaporation of tungsten trioxide powder in a tube furnace. Depending on the temperature of the source (900-1000?°C), tungsten oxide W(18)O(49) nanowires with diameters ranging from 10 to 100?nm are obtained with high yield. The exponential dependence of the nanowire diameter on the source temperature leads to an energy of about 2.0?eV. The growth process is discussed; it is believed to be a kinetic effect.     

Temperature conditions for GaAs nanowire formation by Au-assisted molecular beam epitaxy

Molecular beam epitaxial growth of GaAs nanowires using Au particles as a catalyst was investigated. Prior to the growth during annealing, Au alloyed with Ga coming from the GaAs substrate, and melted. Phase transitions of the resulting particles were observed in?situ by reflection high-energy electron diffraction (RHEED). The temperature domain in which GaAs nanowire growth is possible was determined. The lower limit of this domain (320?°C) is close to the observed catalyst solidification temperature. Below this temperature, the catalyst is buried by GaAs growth. Above the higher limit (620?°C), the catalyst segregates on the surface with no significant nanowire formation. Inside this domain, the influence of growth temperature on the nanowire morphology and crystalline structure was investigated in detail by scanning electron microscopy and transmission electron microscopy. The correlation of the nanowire morphology with the RHEED patterns observed during the growth was established. Wurtzite GaAs was found to be the dominant crystal structure of the wires.     

Lateral, Ge, nanowire growth on SiO2

Vapor-liquid-solid (VLS) nanowires (NWs) typically grow in [111] directions. Previously, the authors have demonstrated guided Si NW growth, engineering the VLS NWs to grow in a [110] direction against a SiO(2) surface. In this work, the authors demonstrate guided high-quality Ge nanowire growth against a SiO(2) surface in the substrate plane to bridge between two Si mesas. The authors explore the interfaces between a Ge NW and the two Si device-layer mesas and report high-quality, epitaxial interfaces between the Ge NW and both Si mesas.     

Synthesis and characterization of cadmium telluride nanowire

CdTe nanowires with controlled composition were cathodically electrodeposited using track-etched polycarbonate membrane as scaffolds and their material and electrical properties were systematically investigated. As-deposited CdTe nanowires show nanocrystalline cubic phase structures with grain sizes of up to 60 nm. The dark-field images of nanowires reveal that the crystallinity of nanowires was greatly improved from nanocrystalline to a few single crystals within nanowires upon annealing at 200?°C for 6?h in a reducing environment (5%?H(2)+95%?N(2)). For electrical characterization, a single CdTe nanowire was assembled across microfabricated gold electrodes using the drop-casting method. In addition to an increase in grain size, the electrical resistivity of an annealed single nanowire (a few 10(5)?Ω?cm) was one order of magnitude greater than in an as-deposited nanowire, indicating that crystallinity of nanowires improved and defects within nanowires were reduced during annealing. By controlling the dopants levels (e.g.?Te content of nanowires), the resistivity of nanowires was varied from 10(4) to 10(0)?Ω?cm. Current-voltage (I-V) characteristics of nanowires indicated the presence of Schottky barriers at both ends of the Au/CdTe interface. Temperature-dependent I-V measurements show that the electron transport mode was determined by a thermally activated component at T>-50?°C and a temperature-independent component below -50?°C. Under optical illumination, the single CdTe nanowire exhibited enhanced conductance.     

Temperature-dependent growth of germanium oxide and silicon oxide based nanostructures, aligned silicon oxide nanowire assemblies, and silicon oxide microtubes

We demonstrate the temperature-dependent growth of germanium oxide and silicon oxide based composite nanostructures (multiple nanojunctions of Ge nanowires and SiO(x) nanowires, Ge-filled SiO(2) nanotubes, Ge/SiO(2) coaxial nanocables, and a variety of interesting micrometer-sized structures), aligned SiO(x) nanowire assemblies, and SiO(x) microtubes. The structures were characterized by SEM, TEM, energy-dispersive X-ray spectroscopy, and electron diffraction. The combination of laser ablation of a germanium target and thermal evaoporation of silicon monoxide powders resulted in the formation of Ge and SiO(x) species in a carrier gas; the nano/micro-sized structures grow by either a Ge-catalyzed vapor-liquid-solid or a Ge-nanowire-templated vapor-solid process.     

Stacked Ge nanocrystals with ultrathin SiO₂ separation layers

The aim of this work is the tailored growth of Ge nanocrystals (NCs) in (GeO(x)/SiO(2)) multilayers (ML) for photovoltaic applications. For this purpose the fabrication of regularly stacked Ge NCs separated by ultrathin SiO(2) layers is essential to enable charge carrier transport by direct tunnelling. In this paper we report on the fabrication of (GeO(x)/SiO(2))(50) multilayer stacks via reactive dc magnetron sputtering and Ge NCs formation after subsequent annealing. It is shown that magnetron sputtering allows us to deposit very regular ML stacks with a total thickness of about 300 nm, characterized by ultrathin (down to 1 nm) and very smooth (roughness ～ 0.6 nm) SiO(2) separation layers. A main challenge is to keep these properties for a thermal budget necessary to form Ge NCs. For this reason, the temperature dependence of phase separation. Ge crystallization and ML morphology was investigated by Rutherford backscattering, x-ray scattering, Raman spectroscopy and electron microscopy. The formation of size confined Ge NCs of about 5 nm after annealing of only 550?°C is confirmed. This low thermal budget ensures the suppression of GeO emanation and multilayer stability. Spectroscopic ellipsometry was applied to determine the optical Ge NC bandgap to (1.65 ± 0.5) eV.