Solid Nanowires Research Articles

Synthesis of ZnO nanowire and ZnO/CeO2 solid solution nanowire by bio-morphing and its characterization

ZnO nanowire (NW), ZnO/CeO2 solid solution nanowire (SSNW) and composite nanowire (CNW) were resulted by varying the proportions of CeO2, synthesized by bio-morphing. The nano sized cellulosic fibrils present inside the micro fiber bundles of banana (Musa Balbisiana Colla) pseudo stem were used as bio-morphing template. The high aspect ratio of NW imitates the internal morphology of the cellulosic micro fiber bundles (CMFB) used. Raw CMFB and synthesized NWs morphology, composition, crystallinity, band gap energy and functionalities were examined by FESEM, EDX, TEM, XRD, UV–visible DRS and FTIR respectively. The formation mechanism of NW within the natural CMFB also proposed with the help of TG-DTA. Reaction kinetics and mechanism of dye degradation of Direct Red dye (DRD) was investigated under UV and solar light irradiation, the effects of DRD concentration, catalyst dosage and pH were also explored using the synthesized SSNW. The degradation studies substantiated the enhanced photocatalytic degradation under solar light.

Sub-Poissonian length distributions of vapor–liquid–solid nanowires induced by nucleation antibunching

Herein, we present an analytic length distribution of vapor–liquid–solid nanowires growing without the nucleation delays of the very first monolayer, by the direct impingement of material from vapor, and in the absence of desorption. We show that nucleation antibunching narrows the Poissonian length distribution to a time-independent asymptotic shape. The obtained distribution depends on the sole control parameter describing the effect of antibunching, and has a variance of rather than spreading infinitely with time. A good agreement is found between the analytic shapes obtained within the continuum growth theory and Green’s function of the discrete rate equations describing the length statistics of nanowires. Overall, this narrowing effect can be used to improve the size homogeneity of III–V and other semiconductor nanowires grown under the appropriate conditions.

Refinement of Nucleation Theory for Vapor–Liquid–Solid Nanowires

Features of nucleation of two-dimensional islands in vapor–liquid–solid nanowires determine the nanowire growth rates, morphologies, and crystal phases and thus are central for controlling the physical properties of such structures. Herein, we present a development of nucleation theory that accurately accounts for the droplet depletion effect on the island formation energy. We show that the latter modifies itself to a curve with a maximum at a critical size and a minimum at a stationary size, which determines the size to which the island can actually grow without a refill from vapor. This effect is general and should occur in any nucleation from an isolated nanoparticle. For III–V nanowires, the depletion is extremely sensitive to the concentration of the group V atoms in the droplet and is more pronounced for smaller nanowire radii and lower concentrations. Whenever the stationary size is smaller than that of the facet, the droplet depletion decreases the amplitude of the chemical potential oscillations....

Development of Growth Theory for Vapor–Liquid–Solid Nanowires: Contact Angle, Truncated Facets, and Crystal Phase

This article tries to circumvent the fundamental uncertainty in the contact angle of droplets catalyzing vapor–liquid–solid nanowires. Based entirely on surface energetic grounds, our model predicts the following behavior. The initial contact angle of the surface droplets is smaller than for developed nanowires. At the beginning, the energetically preferred growth configuration is inward tapered nonwetted side facets such that the contact angle increases and the radius decreases with length. Such isotropic growth continues until a steady state contact angle is reached, which is usually larger than 90° (111° for the plausible parameters of Au-catalyzed GaAs nanowires) but smaller than the Nebol’sin–Shchetinin angle. This value determines the contact angle for stable vertical nanowires at a fixed liquid volume. If the latter keeps increasing for kinetic reasons, the contact angle also increases to the second critical point (128° for our parameters) where vertical interface is transitioned to wetted truncate...

Highly Efficient Solid Gear-Shaped Silicon Nanowire for Solar Energy Harvesting

Silicon nano-wire (SiNW) is a viable solution to harvest large amount of photons from the sun. In this letter, novel solid gear-shaped SiNW is presented and particle swarm optimization technique is used to optimize its geometrical parameters. The defined optimization goal was to increase the absorption capabilities of the proposed SiNW at the maximum wavelength emission of the sun, $\lambda =500$ nm. For the sake of fabrication flexibility, we have also introduced the complementary of the proposed optimized SiNW with the same optimized dimensions. The optimal SiNW and its complementary structure with 1- $\mu \text{m}$ -thick Si substrate offer ultimate efficiencies of about 50.21% and 45.12% employing 3-D-finite-difference time-domain method. It is worth noting that the ultimate efficiencies of the proposed SiNWs are higher than that of the previous work of slanting SiNWs and funnel-shaped SiNW.

Template-Assisted Electrodeposition of Fe-Ni-Co Nanowires: Effects of Electrolyte pH and Sodium Lauryl Sulfate

The influence of pH and sodium lauryl sulfate (SLS) electrolyte composition on the potentiostatic, template-assisted electrodeposition of Fe-Ni-Co alloy nanowires into alumina nanoporous membranes with a large accompanying hydrogen side reaction is presented. Solid nanowires and tubular, composite tips were observed and the average alloy composition was characterized. The growth of nanowires was insensitive to SLS at pH values of 2.0, but was a strong function of SLS at a low pH of 0.5. Anomalous codeposition was evident and the Fe-rich alloy nanowires were independent of SLS concentration at pH values above 0.5. The change in deposition rate and alloy composition was attributed to changes in adsorbed species: metal ion intermediates, hydrogen ion, hydrogen gas bubbles and SLS.

High-performance solid-state on-chip supercapacitors based on Si nanowires coated with ruthenium oxide via atomic layer deposition

Abstract Solid-state on-chip supercapacitors based on ruthenium oxide coated silicon nanowires were fabricated using a process that is compatible with silicon integrated circuit processing. Ordered arrays of silicon nanowires were fabricated using metal-assisted anodic etching (MAAE). Atomic layer deposition (ALD) was used to form a uniform coating of ruthenium oxide on high-aspect-ratio silicon nanowires at a moderate temperature of 290 °C. Coated nanowire electrodes were studied using cyclic voltammetry and charge-discharge tests in a neutral Na2SO4 electrolyte, and a specific capacitance of 19 mFcm−2 was achieved at 5 mVs−1. Solid state nanowire capacitors were then fabricated with symmetric face to face nanowire arrays separated by a polymer-based electrolyte. This device exhibited a specific capacitance as high as 6.5 mFcm−2 at 2 mVs−1. The full device was tested over 10000 cycles under galvanostatic charge-discharge at 0.4 mAcm−2, and showed a retention of 92% of the specific capacitance. The specific capacitance was found to scale with the total nanowire surface area, as controlled by controlling the aspect ratios of the wires. The solid state nanowire-based device also achieved high specific energies without sacrificing power performance.

Uniform phosphorus doping of untapered germanium nanowires

One of the major challenges in the growth of vapor–liquid–solid (VLS) nanowires is the control of dopant incorporation in the structures. In this work, we study the n-type doping and morphology of nanowires grown by chemical vapor deposition when HCl is introduced. We obtain fully untapered nanowires with a growth temperature up to 410 °C and measure their resistivity using the 4-probe technique to be 2.0 mΩ cm. We perform energy dispersive x-ray measurements showing a concentration of dopants in the (5–7) × 1018 cm−3 range, being radially and axially uniform. The combination of these two measurements shows that the mobility is the same as for bulk germanium, demonstrating that the VLS mechanism has no detrimental effect for the electron transport in these nanowires.

Studies of Silicon Nanowires with Different Parameters — By PECVD

One-dimensional nanostructures such as nanowires have a wide range of applications. Silicon is the best competitive material for the carbon nanotubes (CNTs). Carbon and silicon have some similar and peculiar properties. Silicon nanowires (SiNWs) were synthesized using plasma enhanced chemical vapor deposition (PECVD) on p-Si (111) wafer. Gold is used as a catalyst for the growth of the SiNWs. Based on our fundamental understanding of vapor–liquid–solid (VLS) nanowire growth mechanism, different levels of growth controls have been achieved. Gold catalyst deposited and annealed at different temperatures with different thicknesses (450[Formula: see text]C, 500[Formula: see text]C and 550[Formula: see text]C, 600[Formula: see text]C, 650[Formula: see text]C for 4[Formula: see text]min and 8[Formula: see text]min and 3[Formula: see text]nm, 5[Formula: see text]nm, 30[Formula: see text]nm Au thickness). SiNW grown by PECVD with different carrier gases varies with flow rate. We observed the different dimensions of Si nanowires by FESEM and optimized the growth parameters to get the vertical aligned and singular Si nanowires. Optical phonon of the Si nanowires and crystallinity nature were identified by Raman spectral studies.

Nanowire formation is preceded by nanotube growth in templated electrodeposition of cobalt hybrid nanostructures

Cobalt fluted nanowires, novel nanostructures with a diameter of 200 nm consisting of a solid nanowire base and a thin, nanotubular flute shape, were grown in track-etched polycarbonate membranes via templated electrodeposition. The structures were characterized electrochemically via cyclic voltammetry, chronoamperometry, and charge stripping, and structurally via scanning electron microscopy, transmission electron microscopy, and focused ion beam cross-sectioning. Electrochemical and structural analysis reveals details of their deposition kinetics, structure, and morphology, and indicate possible mechanisms for their formation and control. These unique structures provide inspiration for an array of possible applications in electronics, photonics, and other fields.

In Situ and Ex Situ TEM Study of Lithiation Behaviours of Porous Silicon Nanostructures.

In this work, we study the lithiation behaviours of both porous silicon (Si) nanoparticles and porous Si nanowires by in situ and ex situ transmission electron microscopy (TEM) and compare them with solid Si nanoparticles and nanowires. The in situ TEM observation reveals that the critical fracture diameter of porous Si particles reaches up to 1.52 μm, which is much larger than the previously reported 150 nm for crystalline Si nanoparticles and 870 nm for amorphous Si nanoparticles. After full lithiation, solid Si nanoparticles and nanowires transform to crystalline Li15Si4 phase while porous Si nanoparticles and nanowires transform to amorphous LixSi phase, which is due to the effect of domain size on the stability of Li15Si4 as revealed by the first-principle molecular dynamic simulation. Ex situ TEM characterization is conducted to further investigate the structural evolution of porous and solid Si nanoparticles during the cycling process, which confirms that the porous Si nanoparticles exhibit better capability to suppress pore evolution than solid Si nanoparticles. The investigation of structural evolution and phase transition of porous Si nanoparticles and nanowires during the lithiation process reveal that they are more desirable as lithium-ion battery anode materials than solid Si nanoparticles and nanowires.

Effect of porous layer engineered with acid vapor etching on optical properties of solid silicon nanowire arrays

In this paper, we report, for the first time, an investigative study involving the engineering of lightly doped porous silicon nanowire arrays (pSiNWs) by exposing solid silicon nanowire arrays (SiNWs) to an acid vapor emanating from HF/HNO3 hot solution. SEM and TEM images exhibit vertically distributed SiNW arrays on the whole silicon (Si) surface with relatively smooth surface sidewalls. By submitting the SiNW arrays to Acid Vapor Etching (AVE), they become porous with a substantial decrease in their densities and lengths. Increasing etching duration leads to a higher porosity without affecting the wire diameter which remains almost constant nearly 100nm. Exceeding a critical etching duration, a porous structure is observed superseding the SiNW structure. The morphological characterizations have been correlated to the optical properties. We note a blue shift of the strong visible photoluminescence (PL) bands after AVE treatment due to the decrease of the silicon quantum dots diameter (Si-QDs). UV–Visible measurement shows a decrease of the total reflectivity by 5% after AVE treatment.

Design of asymmetric nanovoid resonator for silicon-based single-nanowire solar absorbers

Single semiconductor nanowires offer an attractive option for low-cost and high-efficiency with diverse applications in sensing, photodetecting, and light-harvesting. One of the crucial goals for the single-nanowire-based devices is to find the most effective strategies to realize broadband and high optical absorption. We demonstrate in this study that integrating a nanovoid resonator into the single nanowire can substantially enhance the absorption to sunlight by exciting highly strengthened optical resonances in the nanovoid-deformed nanowire cavity. Without considering the factor of the reduced photoactive material, the photocurrent density of the single-nanowire solar absorber under the asymmetrical nanovoid design can be improved by ~37.5% over that based on the conventional solid nanowire devices. Moreover, the high absorption can be well preserved by either filling the nanovoid with low-index dielectric or introducing the dielectric substrate. This study advances our understanding on how the sunlight can be effectively manipulated by a nanovoid in high-index semiconductor nanostructures and provides an inspiring strategy to realize highly efficient photovoltaic devices.

Hydrogen Generation using non-polar coaxial InGaN/GaN Multiple Quantum Well Structure Formed on Hollow n-GaN Nanowires.

This article demonstrates for the first time to the best of our knowledge, the merits of InGaN/GaN multiple quantum wells (MQWs) grown on hollow n-GaN nanowires (NWs) as a plausible alternative for stable photoelectrochemical water splitting and efficient hydrogen generation. These hollow nanowires are achieved by a growth method rather not by conventional etching process. Therefore this approach becomes simplistic yet most effective. We believe relatively low Ga flux during the selective area growth (SAG) aids the hollow nanowire to grow. To compare the optoelectronic properties, simultaneously solid nanowires are also studied. In this present communication, we exhibit that lower thermal conductivity of hollow n-GaN NWs affects the material quality of InGaN/GaN MQWs by limiting In diffusion. As a result of this improvement in material quality and structural properties, photocurrent and photosensitivity are enhanced compared to the structures grown on solid n-GaN NWs. An incident photon-to-current efficiency (IPCE) of around ~33.3% is recorded at 365 nm wavelength for hollow NWs. We believe that multiple reflections of incident light inside the hollow n-GaN NWs assists in producing a larger amount of electron hole pairs in the active region. As a result the rate of hydrogen generation is also increased.

Electrolytic Formation of Crystalline Silicon/Germanium Alloy Nanotubes and Hollow Particles with Enhanced Lithium-Storage Properties.

Crystalline silicon(Si)/germanium(Ge) alloy nanotubes and hollow particles are synthesized for the first time through a one-pot electrolytic process. The morphology of these alloy structures can be easily tailored from nanotubes to hollow particles by varying the overpotential during the electro-reduction reaction. The continuous solid diffusion governed by the nanoscale Kirkendall effect results in the formation of inner void in the alloy particles. Benefitting from the compositional and structural advantages, these SiGe alloy nanotubes exhibit much enhanced lithium-storage performance compared with the individual solid Si and Ge nanowires as the anode material for lithium-ion batteries.

Electrolytic Formation of Crystalline Silicon/Germanium Alloy Nanotubes and Hollow Particles with Enhanced Lithium‐Storage Properties

AbstractCrystalline silicon(Si)/germanium(Ge) alloy nanotubes and hollow particles are synthesized for the first time through a one‐pot electrolytic process. The morphology of these alloy structures can be easily tailored from nanotubes to hollow particles by varying the overpotential during the electro‐reduction reaction. The continuous solid diffusion governed by the nanoscale Kirkendall effect results in the formation of inner void in the alloy particles. Benefitting from the compositional and structural advantages, these SiGe alloy nanotubes exhibit much enhanced lithium‐storage performance compared with the individual solid Si and Ge nanowires as the anode material for lithium‐ion batteries.

Spontaneous, Defect-Free Kinking via Capillary Instability during Vapor-Liquid-Solid Nanowire Growth.

Kinking, a common anomaly in nanowire (NW) vapor-liquid-solid (VLS) growth, represents a sudden change of the wire's axial growth orientation. This study focuses on defect-free kinking during germanium NW VLS growth, after nucleation on a Ge (111) single crystal substrate, using Au-Ge catalyst liquid droplets of defined size. Statistical analysis of the fraction of kinked NWs reveals the dependence of kinking probability on the wire diameter and the growth temperature. The morphologies of kinked Ge NWs studied by electron microscopy show two distinct, defect-free, kinking modes, whose underlying mechanisms are explained with the help of 3D multiphase field simulations. Type I kinking, in which the growth axis changes from vertical [111] to ⟨110⟩, was observed in Ge NWs with a nominal diameter of ∼ 20 nm. This size coincides with a critical diameter at which a spontaneous transition from ⟨111⟩ to ⟨110⟩ growth occurs in the phase field simulations. Larger diameter NWs only exhibit Type II kinking, in which the growth axis changes from vertical [111] directly to an inclined ⟨111⟩ axis during the initial stages of wire growth. This is caused by an error in sidewall facet development, which produces a shrinkage in the area of the (111) growth facet with increasing NW length, causing an instability of the Au-Ge liquid droplet at the tip of the NW.

Fully Analytical Description for the Composition of Ternary Vapor–Liquid–Solid Nanowires

We present a fully analytical model that is capable of describing the steady state growth rates and compositions of Au-catalyzed ternary III–V nanowires growing from a quaternary alloy. We investigate some general features of this complex growth process and find out the reasons for the nanowire composition being different from the vapor content. The solid compositions are mapped out versus the vapor composition, nanowire radius, and V/III flux ratio in different regimes, both for group III- and V-based ternaries. We show how the nanowire composition can be tuned to a desired value by the group V flux and other growth parameters. Overall, our simple approach seems to be relevant to underline the major trends in the compositional control within vapor–liquid–solid III–V nanowires and may serve as the first step toward a more comprehensive understanding of quaternary solutions and ternary nanostructures.

Porous Silicon Nanoneedles By Metal Assisted Chemical Etch for Intracellular Sensing and Delivery

Nanoneedles display great potential as a facile, high-throughput, minimally invasive strategy for intracellular investigation due to their low-cost, versatility and ease of use[1]. Nanoneedles can interface efficiently with cells, with minimal effects on their survival and proliferation. Both sensing of the intracellular milieu and delivery to the cell cytosol can be achieved by nanoneedles with higher accuracy and lower invasiveness that currently established methods. Metal assisted chemical etch (MACE) is emerging as a low-cost, high-throughput, versatile strategy for the synthesis of high aspect ratio silicon nanostructures, including nanowires and nanoneedles[2]. MACE allows fine control over the geometry and porous structure of nanoneedles by tuning a limited number of etch parameters. Here we will review the latest advancements in the development of porous silicon nanoneedles by metal assisted chemical etch and their applications for intracellular delivery. An initial systematic exploration of the MACE phase space indicates that controlling substrate resistivity, concentration of H2O2 and HF and catalytic metal determines the formation of solid and porous nanowires as well as electropolished surfaces[3]. By developing a process combining MACE with semiconductor processing it is possible to realize conical nanoneedles (nN) with controlled length, apical and basal diameter, density and porosity[4]. These nanoneedles can interface with cells either from the bottom (nN-B), by seeding the cells over them or from the top (nN-T) by pressing them over the cells. In both instances the nanoneedles interface with the cell cytosol without inducing toxicity and enable intracellular delivery and sensing. The cell-nanoneedle interface evolves over time, with cells progressively descending along the needles, which come in close proximity to the nucleus, deforming it[5]. Eventually the either the nanoneedles degrade (nN-B) or they are forcefully removed (nN-B) and cytosolic interfacing is lost. For the duration of the interfacing the nanoneedles are capable of both delivering payload to the cell and sensing the intracellular environment, regardless of interfacing strategy. Nanoneedles can efficiently deliver DNA and siRNA payloads to over 90% of the cells being interfaced, capable respectively of expressing and silencing genes[4]. The delivery of hVEFG165 DNA to the muscle of mouse in vivo results in the sustained expression of the human VEGF protein for over a week, leading to the formation of new blood vessels resulting in a six-fold increase in local blood perfusion. Similarly nanoneedles can mediate the delivery of nanoparticles to the cell cytosol, bypassing the endolysosomal system[5]. Such direct delivery enables the presentation of quantum dots within cells both in culture and in a small animal model. Differently from topical administration of nanoparticles, their nanoinjection results in highly localized and prolonged retention for up to 100h. The nanoinjection delivery mechanism is highly biocompatible and minimally invasive. Nanoinjection to skin, ear and muscle of mouse did not display any sign of alteration to tissue structure at the micro and nanoscale, neither on the impacted surface nor in the bulk of the tissue. Furthermore no sign of acute or chronic inflammation, such as cell necrosis, infiltration of immune cells or formation of granulation tissue, were observed in the context of nanoinjection[4]. In summary, mesoporous silicon nanoneedles demonstrate the ability to efficiently deliver a broad range of therapeutic agents within cells, both in vitro and in vivo, with minimal invasiveness and elevated biocompatibility, enabling highly localized treatment. Such ability testifies to the potential of nanoneedles for clinical translation in the context of tissue engineering and the treatment of chronic, degenerative diseases.

(Invited) New Synthesis Approaches to Nanostructured Complex Functional Metal Oxides

Metal oxide materials can display unique electronic, structural, and optical properties when scaled down to nanometer dimensions and find utility in many applications such as ferroelectrics and piezoelectrics, superconductors, dielectrics, heterogeneous catalysis, photocatalysis, fuel cells, and batteries. To this end, the ability to fabricate or synthesize functional oxides in a variety of nanostructured geometries is important. In this presentation, three example systems will be presented detailing new synthetic approaches to obtaining metal oxides with advanced nanostructured morphologies. The effect of the nanostructured morphology on the structure and properties of the materials will also be discussed. Recently, there has been much interest in developing synthesis methods to obtain nanostructures with higher order geometries such as branching or hyperbranching. The ability to synthesize metal oxides in hierarchical structures can allow for new properties or phenomena not observable in bulk morphologies. In the first example, the solution-phase synthesis of cubic perovskite potassium lanthanum titanate into orthogonal nanostructures with hyperbranched and hexapod morphology will be discussed. A facile hydrothermal reaction without using a template, catalyst, substrate, or structure-directing agent was employed. The properties of the hyperbranched materials as water splitting photocatalysts for hydrogen generation will be discussed. In the second example, our efforts towards understanding the materials chemistry of nanostructured solid state Li-ion conductors, which are attractive as safer alternatives to liquid organic electrolytes, will be presented. Our group has developed electrospinning methods for synthesizing titanate and zirconate-based Li-ion conducting ceramics into nanowire morphologies. Some of the improved properties we have observed in these solid electrolyte nanowires, such as improved sintering characteristics, phase stability, and Li ionic conductivity, will be discussed. Finally, our efforts towards the synthesis of functional 2D nanosheets comprising Li-ion battery cathode materials such as LiCoO2 and LiNi1/3Mn1/3Co1/3O2 will be presented. 2D materials have attracted a great deal of attention for their unique electrical and magnetic properties, but may also play important roles in energy storage applications. Many conventional battery materials have layered structures, and hence can be readily exfoliated into 2D nanosheet materials. The high surface area and short ionic diffusion distances in the 2D nanosheets may improve the charging/discharging rates and result in more lithium insertion or surface adsorption. Furthermore, hybrid electrode materials comprised of layers of different cathode materials may be possible by reassembling different nanosheets. However, the synthesis of Li-rich materials into 2D nanosheets is challenging due to the strong interlayer electrostatic interactions and binding by protons during osmotic swelling processes, which can block Li sites. Our modified osmotic swelling approach circumvents these problems and allows for the synthesis of 2D nanosheets from Li-rich materials that can be restacked and display the expected electrochemical properties of bulk counterparts. Our work is a firm step forward for improving understanding of osmotic swelling processes for the synthesis of nanosheets from complex metal oxides as well as the design and fabrication of high performance hybrid electrodes for energy storage applications.