Growth Of Silicon Nanowires Research Articles

Electric field induced slanting growth of silicon nanowires with enhanced hydrophobic property

The etching process of Si nanowires under DC electric field was studied in this work. Interestingly, the growth direction of silicon nanowires became slanting when applied with DC electric intensity of 600V/m, which greatly influenced the surface wettability ascribed to the variation of surface morphologies. The contact angle of slant Si nanowire was enhanced compared with vertical growth Si nanowire (132.4° vs. 86.8°).

Focused ion beam-assisted fabrication of soft high-aspect ratio silicon nanowire atomic force microscopy probes

In this study, high-aspect ratio silicon nanowire (SiNW) – modified atomic force microscopy (AFM) probes are fabricated using focused ion beam (FIB) microfabrication technology and vapor-solid-solid synthesis. Commercially available soft silicon nitride probes are used for localized nanowire growth yielding soft high-aspect ratio AFM probes. The SiNW-modified cantilevers are used here for imaging in PeakForce Tappingۛ (PFT) mode, which offers high force control along with valuable information about tip-sample adhesion. A platinum catalyst, deposited accurately at a truncated AFM tip by ion beam-induced deposition (IBID), was used for localized nanowire synthesis. It could be shown that the deposition of a thin silicon dioxide layer prior to the catalyst deposition resulted in controlled SiNW growth on silicon as well as silicon nitride probes. In addition, a FIB-based method for post-growth alignment of the fabricated SiNW tips is presented, which allows tilt-compensation specifically tailored to the specifications of the used AFM instrumentation. To demonstrate the capability of such soft, high-aspect ratio AFM probes, optical gratings fabricated in GaAs and silver halide fibers were imaged in PFT mode. Additionally, the mechanical stability of these high-aspect AFM probes was evaluated on a sapphire substrate.

In-situ Mueller matrix ellipsometry of silicon nanowires grown by plasma-enhanced vapor-liquid-solid method for radial junction solar cells

In-situ Mueller matrix spectroscopic ellipsometry was applied for monitoring the silicon nanowire growth by plasma-enhanced vapor-liquid-solid method. The technique is proposed as a real-time, non-destructive, and non-invasive characterization of the deposition process in a plasma-enhanced chemical vapor deposition reactor. The data have been taken by spectrally resolved Mueller matrix ellipsometer every 1min during the 8–10min long nanowire growth process. We have developed an easy-to-apply optical model to fit the experimental data, which enables to study the evolution of the parameters of the structure during initial stages of the growth. The first results provide information about the effective deposition rate determined from the linear increase of the deposited silicon volume with the deposition time.

Controlling the geometrical orientation of hot-wire chemical vapor process grown silicon nanowires

In this work, the effect of chamber pressure on the morphology of hot wire chemical vapor processed silicon nanowires (SiNWs) using Sn as catalyst has been studied. It is observed that their geometrical orientation can be controlled as per requirement by adjusting the growth pressure. SiNWs synthesized at low pressure of 0.67Pa grow preferentially perpendicular to the substrate. If the pressure is increased to 2.7Pa, SiNWs become tilted to the substrate and have bending type structure with random distribution. Further increase in the chamber pressure to 4Pa very few wires are seen to grow and at 5.3Pa no-growth of SiNWs is observed. Transmission electron microscopy study shows that the straight SiNWs have crystalline structure whereas the bent ones show polycrystalline structure.

In-situ catalyst mediated growth and self-doped silicon nanowires for use in nanowire solar cells

We report the growth of self-doped silicon nanowires (SiNWs), using gallium and bismuth catalysts, by employing a vapour-liquid-solid method. This enables the formation of both p- and n-doped nanowires without the use of expensive and toxic gases that are conventionally used and opens a new route to a simplified and cost effective process for doping SiNWs. The chosen catalysts have the lowest eutectic temperature available for the growth of silicon nanowires. The growth of self-doped SiNWs, for the fabrication of photovoltaic cells, has been demonstrated for the first time in this study.

Growth Mechanisms of Inductively-Coupled Plasma Torch Synthesized Silicon Nanowires and their associated photoluminescence properties.

Ultra-thin Silicon Nanowires (SiNWs) were produced by means of an industrial inductively-coupled plasma (ICP) based process. Two families of SiNWs have been identified, namely long SiNWs (up to 2–3 micron in length) and shorter ones (~100 nm). SiNWs were found to consist of a Si core (with diameter as thin as 2 nm) and a silica shell, of which the thickness varies from 5 to 20 nm. By combining advanced transmission electron microscopy (TEM) techniques, we demonstrate that the growth of the long SiNWs occurred via the Oxide Assisted Growth (OAG) mechanism, while the Vapor Liquid Solid (VLS) mechanism is responsible for the growth of shorter ones. Energy filtered TEM analyses revealed, in some cases, the existence of chapelet-like Si nanocrystals embedded in an otherwise silica nanowire. Such nanostructures are believed to result from the exposure of some OAG SiNWs to high temperatures prevailing inside the reactor. Finally, the intense photoluminescence (PL) of these ICP-grown SiNWs in the 620–950 nm spectral range is a clear indication of the occurrence of quantum confinement. Such a PL emission is in accordance with the TEM results which revealed that the size of nanostructures are indeed below the exciton Bohr radius of silicon.

Performance enhancement of micro-supercapacitor by coating of graphene on silicon nanowires at room temperature

Fabrication of high performance micro-supercapacitor (μ-SC) based on Silicon nanowires (SiNWs) at low temperatures by simple, effective and a scalable technique is always desired. In this regard, the performance of the μ-SC was greatly improved by employing a novel architecture developed by coating SiNWs with Graphene (rGO) in the present work. This coating of Graphene was deposited by an easy and efficient technique, Electrophoretic Deposition (EPD), at the room temperature with no intricate steps. These rGO coated SiNWs electrodes show as much as four times larger capacitance (240 μF/cm2) than the bare SiNWs (60 μF/cm2). Moreover, the key advantage of the architecture is that both rGO and SiNWs structurally complement each other enhancing the electrode performance - EPD deposited rGO fills the empty space between SiNWs by making a bridge type structure, increasing the surface area of the electrodes whereas SiNWs prevent restacking and aggregation of the rGO nano-sheets and also provide support to rGO nano-sheets to stand vertically over the substrate. One more notable bonus of using EPD technique is that the wires are selectively coated. SiNWs were synthesized at a temperature of 320 °C by the Hot-wire assisted SiNW growth technique and deposition of Graphene oxide (GO) and its reduction into reduced Graphene oxide (rGO) was carried out at the room temperature. The results suggest that rGO coated SiNWs could be a promising candidate as an electrode for on-chip integrable μ-SC application.

Plasma-Assisted Growth of Silicon Nanowires by Sn Catalyst: Step-by-Step Observation.

A comprehensive study of the silicon nanowire growth process has been carried out. Silicon nanowires were grown by plasma-assisted-vapor-solid method using tin as a catalyst. We have focused on the evolution of the silicon nanowire density, morphology, and crystallinity. For the first time, the initial growth stage, which determines the nanowire (NW) density and growth direction, has been observed step by step. We provide direct evidence of the merging of Sn catalyst droplets and the formation of Si nanowires during the first 10 s of growth. We found that the density of Sn droplets decreases from ~9000 Sn droplets/μm2 to 2000 droplets/μm2 after just 10 s of growth. Moreover, the long and straight nanowire density decreases from 170/μm2 after 2 min of growth to less than 10/μm2 after 90 min. This strong reduction in nanowire density is accompanied by an evolution of their morphology from cylindrical to conical, then to bend conical, and finally, to a bend inverted conical shape. Moreover, the changes in the crystalline structure of nanowires are from (i) monocrystalline to (ii) monocrystalline core/defective crystalline shell and then to (iii) monocrystalline core/defective crystalline shell/amorphous shell. The evolutions of NW properties have been explained in detail.

Power Response of a Planar Thermoelectric Microgenerator Based on Silicon Nanowires at Different Convection Regimes

Abstract A thermoelectric microgenerator based on multiple silicon nanowire arrays is fabricated and its performance evaluated for different convection regimes. Mature silicon microfabrication technology is used to fabricate the device structure. As a post-process, a bottom-up approach is used to grow silicon nanowires by a VLS-CVD mechanism. The thermal design of the microgenerator features a thermally isolated silicon platform which is connected to the bulk silicon rim through several arrays of silicon nanowires. Simulations are carried out to evaluate the need of an external heat sink to improve the thermal gradient seen by the nanowires and the power output of the microgenerator. Results show a significant improvement with a heat sink raising the thermal gradient from 3 K to approximately 100 K when the external temperature gradient is 300 K. Experimental measurements with different convection regimes also show a radical improvement on the power output comparing natural convection and two different forced convection regimes. The first forced convection regime is a broad airflow from a commercial CPU fan placed on top of the device, while the second (air jet forced convection) uses a syringe to focus the airflow from the compressed air line to the platform. The maximum output power for a natural convection regime is 2.2 nW for a hotplate temperature of 200 °C, while the air jet forced convection regime generates up to 700 nW, which correspond to 35 µW/cm2 considering a device footprint of 2 mm2.

Direct synthesis of electrically integrated silicon nanowires forming 3D electrodes

Arrays composed of vertically aligned nanoscale electrodes are under discussion for various biological and analytical applications including extra‐ and intracellular measurements, intracellular drug delivery, and cellular interfacing. Based on their nanoscale morphology, conductive nanowires are a preferred electrode material but device fabrication remains a key challenge. In this article, we present a straightforward assembly strategy for directly connected and quasi‐vertically aligned silicon nanowires (SiNWs) that could lead to well‐defined electrode arrays in future. Our strategy exploits certain features of platinum‐catalyzed SiNW growth and simultaneously interlinks the SiNW synthesis and the formation of the electrical contacts using a single metallization film. Our assembly strategy relies fully on conventional photolithography and can cope with various electrode designs and configurations considering single‐nanowire electrode readouts.

The Synergetic Catalytic Effect of Molten Gallium in the Presence of Hydrogen and Nitrogen Plasmas: Study and Applications

The synergetic effect of plasma and a metal catalyst is rising as an alternative to synthesize a great variety of materials - from gases to nanomaterials - that take place under highly energy consuming reactions. More specifically, this study will have an impact in the growth of silicon nanowires (Si NWs) as anodes for Li-ion batteries and the production of bulk gallium nitride. Our group has been able to synthesize Si NWs at temperatures as low as 200 C and in recent efforts we have been able to grow densely packed Si NWs (d∽20 nm) uniformly on stainless steel with areas as big as 1.5 x 5 cm. Highly oriented GaN flakes (as big as 1.25 cm x 1.25 cm) were also synthesized on quartz substrates using a plasma assisted catalytic method. The X-ray diffraction spectrum of the GaN film shows only (0002) and (0004) planes of wurtzite GaN, indicating good c-plane orientation. We have demonstrated a basic concept in which crystals self-orient with respect to each other if the growth takes place on molten gallium due to the enhanced rotational and translational mobility. To gain insights into these mechanisms we have studied the interaction of hydrogen and nitrogen plasmas with low melting point metals, primarily with gallium. Absorption/desorption experiments as well as theoretical-computational calculations were performed. Experiments have shown an increment of adsorbed gaseous species into the molten metal in the presence of plasmas; furthermore, Density Functional Theory (DFT) calculations suggest a strong interaction between atomic hydrogen and molten gallium that is described as a high absorption on the surface, rapid diffusion and a steady state concentration of the gas inside the bulk. Recent results suggest that the synergetic effect of molten gallium in the presence of hydrogen/nitrogen plasmas could have a transformational impact in the way energy-intensive processes are carried out. This presentation will highlight the novel aspects of molten metal plasma catalysis, future directions and applications. Acknowledgments The authors acknowledge the financial support from NSF EPSCoR Grant 1355438. References “Plasma Catalysis: Synergistic Effects at the Nanoscale.” E. C. Neyts, K. Ostrikov, M. K. Sunkara, and A. Bogaerts. Chemical Reviews 2015 115 (24), 13408-13446 DOI: 10.1021/acs.chemrev.5b00362 ‘Low Temperature Synthesis of Silicon Nanowire Arrays” M.L. Carreon, J.B. Jasinski and M.K. Sunkara. 2014 Mater. Res. Express 1 045006 DOI: http://dx.doi.org/10.1088/2053-1591/1/4/045006

Incubation behavior of silicon nanowire growth investigated by laser-assisted rapid heating

We investigate the early stage of silicon nanowire growth by the vapor-liquid-solid mechanism using laser-localized heating combined with ex-situ chemical mapping analysis by energy-filtered transmission electron microscopy. By achieving fast heating and cooling times, we can precisely determine the nucleation times for nanowire growth. We find that the silicon nanowire nucleation process occurs on a time scale of ∼10 ms, i.e., orders of magnitude faster than the times reported in investigations using furnace processes. The rate-limiting step for silicon nanowire growth at temperatures in the vicinity of the eutectic temperature is found to be the gas reaction and/or the silicon crystal growth process, whereas at higher temperatures it is the rate of silicon diffusion through the molten catalyst that dictates the nucleation kinetics.

Aluminum-catalyzed silicon nanowires: Growth methods, properties, and applications

Metal-mediated vapor-liquid-solid (VLS) growth is a promising approach for the fabrication of silicon nanowires, although residual metal incorporation into the nanowires during growth can adversely impact electronic properties particularly when metals such as gold and copper are utilized. Aluminum, which acts as a shallow acceptor in silicon, is therefore of significant interest for the growth of p-type silicon nanowires but has presented challenges due to its propensity for oxidation. This paper summarizes the key aspects of aluminum-catalyzed nanowire growth along with wire properties and device results. In the first section, aluminum-catalyzed nanowire growth is discussed with a specific emphasis on methods to mitigate aluminum oxide formation. Next, the influence of growth parameters such as growth temperature, precursor partial pressure, and hydrogen partial pressure on nanowire morphology is discussed, followed by a brief review of the growth of templated and patterned arrays of nanowires. Aluminum incorporation into the nanowires is then discussed in detail, including measurements of the aluminum concentration within wires using atom probe tomography and assessment of electrical properties by four point resistance measurements. Finally, the use of aluminum-catalyzed VLS growth for device fabrication is reviewed including results on single-wire radial p-n junction solar cells and planar solar cells fabricated with nanowire/nanopyramid texturing.

Pressure-induced formation of highly controlled branched silicon nanowires suitable for broadband absorption

We report the realization of branched silicon nanowires (SiNWs) by means of controlling the pressure during vapor–liquid–solid growth. Gold nanoparticles migrated from the main catalyst droplet act as seeds for the secondary growth of SiNWs across the trunk sidewall. To have gold nanoparticles migration as well as fewer kinks in the trunks, it is crucial to gradually reduce the pressure, which results in fully covered branched SiNWs. Transmission electron microscopy confirmed the crystallinity of the Si nano-branches by SAED analysis. From optical spectroscopy, it is observed that the light absorption is around 98 % over a wide range of wavelengths from 300 to 600 nm and drops to 90 % for higher wavelengths. It is shown that high absorption is due to the excitation of Mie resonances and waveguide modes in Si nanostructures. In order to confirm the presence of these optical modes, Raman spectroscopy has been used. Raman spectrum of branched SiNWs indicates higher enhancement factor compared to plain SiNWs.

Growth of epitaxial silicon nanowires on a Si substrate by a metal-catalyst-free process.

The growth of epitaxial Si nanowires by a metal-catalyst-free process has been investigated as an alternative to the more common metal-catalyzed vapor–liquid–solid process. The well-aligned Si nanowires are successfully grown on a (111)-oriented Si substrate without any metal catalysts by a thermal treatment using silicon sulfide as a Si source at approximately 1200 °C. The needle-shaped Si nanowires, which have a core–shell structure that consists of a single-crystalline Si core along the <111> direction consistent with the substrate direction and a surface coating of silicon oxide, are grown by a metal-catalyst-free process. In this process, the silicon sulfide in the liquid phase facilitates the nucleation and nanowire growth. In contrast, oxygen-rich nanowires that consist of crystalline Si at the tip and lumpy silicon oxide on the body are observed in a sample grown at 1300 °C, which disturbs the epitaxial growth of Si nanowires.

Insights into gold-catalyzed plasma-assisted CVD growth of silicon nanowires

Understanding and controlling effectively the behavior of metal catalyst droplets during the Vapor-Liquid-Solid growth of nanowires are crucial for their applications. In this work, silicon nanowires are produced by plasma-assisted Chemical Vapor Deposition using gold as a catalyst. The influence of hydrogen plasma on nanowire growth is investigated experimentally and theoretically. Interestingly, in contrast to conventional chemical vapor deposition, the growth rate of silicon nanowires shows a decrease as a function of their diameters, which is consistent with the incorporation of silicon via sidewall diffusion. We show that Ostwald ripening of catalyst droplets during nanowire growth is inhibited in the presence of a hydrogen plasma. However, when the plasma is off, the diffusion of Au atoms on the nanowire sidewall can take place. Based on this observation, we have developed a convenient method to grow silicon nanotrees.

Silicon nanowires arrays for visible light driven photocatalytic degradation of rose bengal dye

The uniform growth of aligned silicon nanowires (SiNWs) on p-type silicon (Si) wafer were obtained by metal-assisted chemical etching (MCE) technique. MCE was performed using aqueous solution of AgNO3 and HF for Ag deposition and etching respectively. The morphological characterizations showed well-aligned and uniform growth of SiNWs with the average diameter of ~80–100 nm. The grown SiNWs were applied as effective photocatalyst for the photocatalytic degradation of rose bengal (RB) dye under the light illumination. The high degradation rate of ~96 % within ~90 min over the surface of SiNWs was observed for the degradation of RB dye. The enhanced photocatalytic activity might due to the increase of electron trapping and reduction in the recombination rate.

Electrochemical preparation of silicon nanowires from porous Ni/SiO2 blocks in molten CaCl2

Silicon nanowires (SiNWs) with diameter distributions ranging from 80 to 350 nm were prepared by electrochemical reduction of Ni/SiO2 in molten CaCl2. The effect of the content of nickel additives on the morphology of produced silicon was investigated. Large quantities of SiNWs are obtained by the electrochemical reduction of Ni/SiO2 blocks with SiO2 to Ni molar ratio of 20 and 10. Nickel additives repress the growth of irregular branches and promote longitudinal growth of SiNWs. Wire morphologies and surfaces are influenced by the electrolysis temperature. SiNWs become thicker with the increase of the electrolysis temperature. The optimum temperature to prepare single crystal SiNWs with high aspect ratio and extraordinary surface quality seems to be 1173 K. The amorphous layer of the silicon nanowire is thinner compared to the SiNWs obtained from the pure SiO2 pellets. The produced SiNWs show a photoluminescence emission peak at about 758 nm at room temperature. This work demonstrates the potentiality for the electrochemical reduction process to obtain large quantities of SiNWs with high quality. Silicon nanowires (SiNWs) were prepared by the electrochemical reduction of Ni/SiO2 blocks in molten CaCl2. The effects of the content of nickel additives and the electrolysis temperature on the morphology of produced silicon were investigated. Nickel additives promote the oriented growth of SiNWs and the SiNWs become thicker with the increase of electrolysis temperature.

Direct growth of Si nanowires on flexible organic substrates

A key characteristic of semiconductor nanowires (NWs) is that they grow on any substrate that can withstand the growth conditions, paving the way for their use in flexible electronics. We report on the direct growth of crystalline silicon nanowires on polyimide substrates. The Si NWs are grown by plasma-enhanced chemical vapor deposition, which allows the growth to proceed at temperatures low enough to be compatible with plastic substrates (350 °C), where gold or indium are used as growth seeds. In is particularly interesting as the seed not only because it leads to a better NW crystal quality but also because it overcomes a core problem induced by the use of Au in silicon processing, i.e. Au creates deep carrier traps when incorporated in the nanowires.

Chemically Etched Silicon Nanowires As Anodes for Lithium-Ion Batteries

Lithium ion batteries are becoming increasingly prevalent as they exhibit high specific energy, a long cycle-life, a low self-discharge rate and can operate in a wide range of temperatures.1 The limitation is that carbonaceous anodes do not provide high enough specific capacities for small scale applications of Li-ion batteries. LiC6 has a theoretical capacity of 372 mAh/g while Li15Si4, an alloy formed at room temperature and low voltage, has a theoretical capacity of 3,579 mAh/g.2,3 Unfortunately, a volume expansion of roughly 270% occurs in silicon upon lithiation and results in fracturing of the electrode particles and rapid loss of capacity with cycling.2 Silicon nanowires experience the same volume expansion, but are able to accommodate the expansion better which may improve cyclability. Commonly, silicon nanowires are grown epitaxial with a metallic impurity and a gas containing the reactive species. This method, known as vapor-liquid-solid mechanism is slow and expensive since it requires a clean room, high temperatures and metallic impurities such as gold.4Etching is the other method for producing nanowires. Etching is accomplished by first removing native oxides and then dispersing metallic nanoparticles onto the surface of a silicon wafer. The silver nanoparticles are oxidized and the cation reacts locally with silicon to remove it from the surface. This results in uniform single crystalline nanowires made from the top down. This research will discuss the difference in performance of bulk silicon, grown silicon nanowires and etched silicon nanowires as anodes for lithium-ion batteries. The anodes are tested in an electrochemical half-cell and put through cyclic voltammetry and charge-discharge cycling. The cells are disassembled and the physical effects of cycling are examined with a scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). Cyclic voltammetry is used to determine the voltages at which insertion and desertion of lithium into silicon occurs. Then charge-discharge cycling is employed to test the performance of the cell over a number of cycles. Since silicon exhibits fracturing as a result of its large volume expansion SEM/EDS is used to examine the damage that cycling has on the electrodes and the overall chemical composition of each component. The chemical composition helps to identify solid electrolyte interphase formation and confirms that silicon is the bulk material in the system. The bulk silicon wafers experienced rapid deterioration after just a few cycles. Both the etched nanowires and the wafers in Figure 1 exhibited fracturing, but it was more prominent in the bulk wafer than in the nanowires. There was also a clear difference in the capacity of the nanowires versus the bulk wafer. Charge-discharge cycling in Figure 2 revealed that silicon wafers had a capacity around 20 mAh/g while the silicon nanowires were reaching capacities over 2000 mAh/g. While there was a rapid loss in capacity seen in the nanowires the capacity after 20 cycles was still 25 times higher than the bulk silicon. These results clearly show that the silicon nanowires are capable of handling the large volume expansion and maintain their capacity more effectively than bulk silicon wafers. The capacity loss is more drastic, but the nanowires still maintain a higher capacity than the bulk silicon wafers and traditional carbonaceous anodes currently in use. Comparison with grown silicon nanowires will also be discussed, but etched nanowires are the preferred anode since they have economical and production time advantages over the commonly grown nanowires. 1Dahn, J.; Ehrlich, G.; Reddy, T. McGraw Hill, New York: 2011. 2 Beattie, S. D.; Larcher, D.; Morcretty, M.; Simon, B.; Tarascon, J.M.J.; Electrochem. Soc. 2008, 155, A158. 3 Hatchard, T. D.; Dahn, J.R.J.; Electrochem Soc. 2004, 151, A838. 4 Wagner, R.S.; Ellis, W.C. Appl. Phys. Lett. 1964, 4, 89. 5 Smith, Z.R.; Smith, R.L.; Collins, S.D. Electrochim. Acta 2013, 92,139. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND2015-10771 A Figure 1