Formation and characterization of Group IV semiconductor nanowires.

To enable the application to next-generation devices of semiconductor nanowires (NWs), it is important to control their formation and tune their functionality by doping and the use of heterojunctions. In this paper, we introduce formation and the characterization methods of nanowires, focusing on our research results. We describe a top-down method of controlling the size and alignment of nanowires that shows advantages over bottom-up growth methods. The latter technique causes damage to the nanowire surfaces, requiring defect removal after the NW formation process. We show various methods of evaluating the bonding state and electrical activity of impurities in NWs. If an impurity is doped in a NW, mobility decreases due to the scattering that it causes. As a strategy for solving this problem, we describe research into core-shell nanowires, in which Si and Ge heterojunctions are formed in the diameter direction inside the NW. This structure can separate the impurity-doped region from the carrier transport region, promising as a channel for the new ultimate high-mobility transistor.

Binder-free boron-doped Si nanowires toward the enhancement of lithium-ion capacitor.

Lithium-ion capacitors (LICs) are next-generation electrochemical storage devices that combine the benefits of both supercapacitors and lithium-ion batteries. Silicon materials have attracted attention for the development of high-performance LICs owing to their high theoretical capacity and low delithiation potential (∼0.5 V versus Li/Li+). However, sluggish ion diffusion has severely restricted the development of LICs. Herein, a binder-free anode of boron-doped silicon nanowires (B-doped SiNWs) on a copper substrate was reported as an anode for LICs. B-doping could significantly improve the conductivity of the SiNW anode, which could enhance electron/ion transfer in LICs. As expected, the B-doped SiNWs//Li half-cell delivered a higher initial discharge capacity of 454 mAh g-1with excellent cycle stability (capacity retention of 96% after 100 cycles). Furthermore, the near-lithium reaction plateau of Si endows the LICs with a high voltage window (1.5-4.2 V), and the as-fabricated B-doped SiNWs//AC LIC possesses the maximum energy density value of 155.8 Wh kg-1at a battery-inaccessible power density of 275 W kg-1. This study provides a new strategy for using Si-based composites to develop high-performance LIC.

Photosensitizer Encryption with Aggregation Enhanced Singlet Oxygen Production.

Chromophores that generate singlet oxygen (1O2) in water are essential to developing noninvasive disease treatments using photodynamic therapy (PDT). A facile approach for formation of stable colloidal nanoparticles of 1O2 photosensitizers, which exhibit aggregation enhanced 1O2 generation in water toward applications as PDT agents, is reported. Chromophore encryption within a fuchsonarene macrocyclic scaffold insulates the photosensitizer from aggregation induced deactivation pathways, enabling a higher chromophore density than typical 1O2 generating nanoparticles. Aggregation enhanced 1O2 generation in water is observed, and variation in molecular structure allows for regulation of the physical properties of the nanoparticles which ultimately affects the 1O2 generation. In vitro activity and the ability of the particles to pass through the cell membrane into the cytoplasm is demonstrated using confocal fluorescence microscopy with HeLa cells. Photosensitizer encryption in rigid macrocycles, such as fuchsonarenes, offers new prospects for the production of biocompatible nanoarchitectures for applications involving 1O2 generation.

Defect control and Si/Ge core-shell heterojunction formation on silicon nanowire surfaces formed using the top-down method.

Control of surface defects and impurity doping are important keys to realizing devices that use semiconductor nanowires (NWs). As a structure capable of suppressing impurity scattering, p-Si/i (intrinsic)-Ge core-shell NWs with radial heterojunctions inside the NWs were formed. When forming NWs using a top-down method, the positions of the NWs can be controlled, but their surface is damaged. When heat treatment for repairing surface damage is performed, the surface roughness of the NWs closely depends on the kind of atmospheric gas. Oxidation and chemical etching prior to shell formation removes the surface damaged layer on p-SiNWs and simultaneously achieves a reduction in the diameter of the NWs. Finally, hole gas accumulation, which is important for suppressing impurity scattering, can be observed in the i-Ge layers of p-Si/i-Ge core-shell NWs.

ZnO/Ge core-shell nanowires and Ge nanotubes fabricated by chemical vapor deposition and wet etching.

One-dimensional germanium (Ge)-related nanostructures including core-shell nanowires and nanotubes with high specific surface area show enhanced performance in energy storage and electronic devices, and their structural control is important for further improving their performance and stability. In this work, we fabricated vertically formed ZnO/Ge core-shell nanowires with different shell thicknesses. The dependence of morphology, crystallinity, and internal stress of the nanowires on the shell growth time and temperature was investigated. By applying the wet-etching method to the ZnO/Ge core-shell heterojunction nanowires, we demonstrated the Ge nanotube fabrication and stress relaxation in Ge after ZnO core removal.

Au-Sn Catalyzed Growth of Ge1-xSnx Nanowires: Growth Direction, Crystallinity, and Sn Incorporation.

Ge1-xSnx nanowires (NWs) have been a focus of research attention for their potential in realizing next-generation Si-compatible electronic and optoelectronic devices. To control the growth of NWs and increase their Sn content, the growth mechanism needs to be understood. The use of Au-Sn alloy catalysts instead of Au catalysts allows an easier understanding of Ge1-xSnx NW growth, and the effects of Sn at different concentrations in catalysts on growth direction, Sn incorporation, and crystallinity of Ge1-xSnx NWs can be clarified. High Sn content in Au-Sn alloy catalysts favors ⟨110⟩-oriented NW growth and high Sn incorporation in NWs. The higher Sn content in Au-Sn alloy catalysts also improves the crystallinity of NWs.

Three-dimensional radial junction solar cell based on ordered silicon nanowires.

Highly ordered silicon nanowires (SiNWs) were fabricated by nanoimprint lithography and Bosch etching methods. A polycrystalline silicon shell was grown to form a radial p-n junction. To enhance its anti-reflection properties and conductivity, a thin ITO layer was deposited on the SiNWs solar cell, then a micro-grid electrode was introduced to minimize the metal areas to maximize carrier collection. Finally, shorter nanowires were used to reduce surface recombination and achieve an efficiency of 10.5%. This work is expected to show some possible techniques to improve the performance of silicon nanostructure solar cell.

Functionalization of Silicon Nanostructures for Energy-Related Applications.

Silicon (Si) is used in various application fields such as solar cells and electric devices. Functionalization of Si nanostructures is one way to further improve the properties of these devices such as these. This Review summarizes recent results of solar cell and Li-ion battery applications using Si-related nanostructures. In solar cell applications, the light trapping effect is increased and the carrier recombination rate is decreased due to the short carrier collection path achieved by radially constructed p-n junction in Si nanowires, resulting in higher power conversion efficiency. The nonradiative energy transfer effect created by nanocrystalline Si is a novel way of improving solar cell properties. Si-related nanostructures are also anticipated as new anode materials with higher capacity in Li-ion batteries. Si-related nanocomposite materials which show densely packed microparticle structures agglomerated with small nanoparticles are described here as a promising challenge. These unique structures show higher capacity and longer cycle properties.

Surface-Enhanced Raman Spectroscopy of Ammonium Nitrate Using Al Structures, Fabricated by Laser Processing of AlN Ceramic.

This work presents results on laser-induced surface structuring of AlN ceramic and its application in Surface-Enhanced Raman Spectroscopy (SERS). The laser processing is performed by nanosecond pulses in air and vacuum. Depending on the processing conditions, different surface morphology can be obtained. The ablation process is realized by ceramic decomposition as the formation of an aluminium layer is detected. The efficiency of the fabricated structures as active substrates in SERS is estimated by the ability of the detection of ammonium nitrate (NH4NO3). It is conducted for Raman spectrometer systems that operate at wavelengths of 514 and 785 nm where the most common commercial systems work. The obtained structures contribute to enhancement of the Raman signal at both wavelengths, as the efficiency is higher for excitation at 514 nm. The limit of detection (LOD) of ammonium nitrate is estimated to be below the maximum allowed value in drinking water. The analysis of the obtained results was based on the calculations of the near field enhancement at different conditions based on Finite Difference Time Domain simulation and the extinction spectra calculations based on Generalized Mie scattering theory. The structures considered in these simulations were taken from the SEM images of the real samples. The oxidation issue of the ablated surface was studied by X-ray photoelectron spectroscopy. The presented results indicated that laser structuring of AlN ceramics is a way for fabrication of Al structures with specific near-field properties that can be used for the detection of substances with high social impact.

Functionalized aluminum-catalyzed silicon nanowire formation and radial junction photovoltaic devices.

Vertical-oriented silicon nanowire (SiNW) arrays with shaped smooth, nanodot-, or NW-structured surfaces offer many desirable advantages for advanced device applications. In this study, these functionalized SiNW formations were simplified by ex situ preparation of an aluminum (Al) catalyst along with optimization of the substrate temperature and time during vapor-liquid-solid chemical vapor deposition as a one-step process. SiNW-based photovoltaic cells were demonstrated with minimized NW surface defects through NW surface modification, opening a new path for the development of versatile Al-catalyzed SiNWs as a material of choice for on-chip integration in future nanotechnologies.

Conversion of Amorphous Carbon on Silicon Nanostructures into Similar Shaped Semi-Crystalline Graphene Sheets.

Graphene sheets displaying partial crystallinity and nanowire structures were formed on a silicon substrate with silicon nanowires by utilizing an amorphous carbon source. The carbon source was deposited onto the silicon nanostructured substrate by breaking down a polymer precursor and was crystallized by a nickel catalyst during relatively low temperature inert gas annealing. The resulting free-standing graphene-based material can remain on the substrate surface after catalyst removal or can be removed as a separate film. The film is flexible, continuous, and closely mimics the silicon nanostructure. This follows research on similar solid carbon precursor derived semi-crystalline graphene synthesis procedures and applies it to complex silicon nanostructures. This work examined the progression of the carbon, finding that it migrates through the thin film catalyst and forms the graphene only on the other side, and that the process can successfully be used to form 3D shaped graphene films. Semi-crystalline graphene has the possible application of being flexible transparent electrodes, and the 3D shaping opens the possibility of more complex configurations and applications.

Growth and characterization of InP ringlike quantum-dot molecules grown by solid-source molecular beam epitaxy.

In this paper, we have studied the fabrication of InP ringlike quantum-dot molecules on GaAs(001) substrate grown by solid-source molecular beam epitaxy using droplet epitaxy technique and the effect of In deposition rate on the physical and optical properties of InP ringlike quantum-dot molecules. The In deposition rate is varied from 0.2 ML/s to 0.4, 0.8 and 1.6 ML/s. The surface morphology and cross-section were examined by ex-situ atomic force microscope and transmission electron microscope, respectively. The increasing of In deposition rate results in the decreasing of outer and inner diameters of InP ringlike quantum-dot molecules and height of InP quantum dots but increases the InP quantum dot and ringlike quantum-dot molecule densities. The photoluminescence peaks of InP ringlike quantum-dot molecules are blue-shifted and FWHM is narrower when In deposition rate is bigger.

Adjustable metal particle grid formed through upward directed solid-state dewetting using silicon nanowires.

Sub-micron sized metal particles were formed through the annealing of sputtered metal thin films on silicon nanowires (SiNWs). During high-temperature annealing, the cylindrical SiNW structures induce the solid-state dewetting behavior to consistently move up the SiNW sides and form partial-spherical particles with uniform sizes on the nanowire tops. By adjusting the size parameters of the SiNW substrate and the metal thin film, the particles can be adjusted in size and layout along an array. This contrasts with the random dewetted particles seen on planar surfaces, and known movement towards pitted nanostructures. Ag, Au, Cu, and Ni have shown equivalent particle formation behavior and some alloying is also shown to be possible. These results open a path for a well-controlled and consistent method of metal particle formation at the nano to micro-scale and offer some insight on metal particle dewetting mechanisms. Suggested applications for the resulting regular particle grids include plasmonic sensors such as SERS.

On-site growth method of 3D structured multi-layered graphene on silicon nanowires.

An experimental method is described in which a orderly 3D array of graphene sheets is grown to conform to the shape of an underlying nanowire (NW) substrate that remains on-site. The procedure uses a sacrificial nickel catalyst-based CVD growth process that is capable of producing graphene onto an insulating SiO2 substrate. Nano-imprint silicon NWs serve both as the scaffolding for the catalyst and as the final underlying substrate. The graphene is polycrystalline and multi-layered as expected from this nickel catalyzed growth method. This presents a novel and quick method that can be used to produce conductive graphene sheets in precise shapes and configurations seen in complex device applications but which are difficult to produce with current transfer methods. The geometry of the nanostructured substrate itself contributes to the on-site growth method by making it difficult for the graphene to wash off during wet etching. The SiNWs used in this research have increased surface area and a light trapping effect that, in combination with the graphene, can be used in future sensor and photovoltaic device applications.

Interfacial intermixing of Ge/Si core-shell nanowires by thermal annealing.

Ge/Si core-shell nanowires (NWs) have huge potential for the realization of high mobility channels in NW field-effect transistors. Thermal annealing is a crucial process for optimizing electrical properties in many applications because it affects the NWs' morphology, crystallinity, dopant activation, and interface intermixing. In this study, we investigated the structural transformation of core-shell NWs at the interface and their thermal stability. The intermixing of Ge and Si atoms at the interface closely depends on, and is enhanced by, the temperature and pressure during annealing, while no intermixing occurred at pressures lower than 6 × 10-6 Pa.

Silicon Nanotubes Fabricated by Wet Chemical Etching of ZnO/Si Core-Shell Nanowires.

Silicon nanotubes (SiNTs) have garnered a great deal of interest for both their synthesis and their potential for application to high-capacity energy storage, biosensors, and selective transport. In this study, we report a convenient and low-cost route to the fabrication of vertically aligned SiNTs via a wet-etching process that enables the control of the wall thickness of SiNTs by varying the gas flux and growth temperature. Transmission electron microscopy (TEM) characterization showed the resultant SiNTs to have an amorphous nature and a hexagonal hollow core. These SiNTs can be crystallized by thermal annealing.

Surface-Enhanced Raman Spectroscopy (SERS) of Neonicotinoid Insecticide Thiacloprid Assisted by Silver and Gold Nanostructures.

This study expresses our results on surface-enhanced Raman spectroscopy (SERS) analyses of neonicotinoid insecticide thiacloprid, i.e., Calypso 480 SC, in quantities much smaller than usually applied in the agricultural medicine. Advanced Ag and Au nanostructures created by the thermal deposition technique on Al2O3 ceramic were applied as active substrates for SERS analyses. The minimum concentration of thiacloprid detected was 380 µM and the enhancement factor was estimated to be about 3 × 103. The intensity of the SERS peaks increased by an order of magnitude after pulsed laser annealing of the films and formation of nanoparticle arrays and the enhancement factor reached ≈104, respectively. The proposed study has direct bearing on the environment and human health by detection of small amounts or residue of harmful pollutants using a relatively cheap and easy method to produce active SERS substrates.

Marimo-Bead-Supported Core-Shell Nanocomposites of Titanium Nitride and Chromium-Doped Titanium Dioxide as a Highly Efficient Water-Floatable Green Photocatalyst.

The release of untreated industrial wastewater creates a hazardous impact on the environment. In this regard, the development of an environmentally friendly catalyst is of paramount importance. Here, we report a highly efficient and reusable core-shell TiN/SiO2/Cr-TiO2 (TSCT) photocatalyst that is composed of SiO2-cladded titanium nitride (TiN) nanoparticles (NPs) decorated with Cr-doped TiO2 NPs for the removal of organic contaminants from water. The TiN NPs serve as the main light absorber component with excellent visible-light absorption along with Cr-TiO2 NPs. The TSCT shows remarkable improvement in the photodecomposition of methylene blue (MB) over Cr-TiO2 and TiO2 NPs. An efficient structural design is proposed by adopting calcium alginate beads (P-Marimo beads) as a transparent scaffold for supporting our TSCT, which floats nature on the water surface and realizes easy handling as well as excellent reusability for multipurpose water purification. Surprisingly, our TSCT is found to keep its catalytic activity even after the illumination is turned off. Our proposed P-Marimo-encapsulated TSCT can be utilized as an excellent green photocatalyst with high photocatalytic performance, good recyclability, and easy handling.

Nanomolecular singlet oxygen photosensitizers based on hemiquinonoid-resorcinarenes, the fuchsonarenes.

Singlet oxygen sensitization involving a class of hemiquinonoid-substituted resorcinarenes prepared from the corresponding 3,5-di-t-butyl-4-hydroxyphenyl-substituted resorcinarenes is reported. Based on variation in the molecular structures, quantum yields comparable with that of the well-known photosensitizing compound meso-tetraphenylporphyrin were obtained for the octabenzyloxy-substituted double hemiquinonoid resorcinarene reported herein. The following classes of compounds were studied: benzyloxy-substituted resorcinarenes, acetyloxy-substituted resorcinarenes and acetyloxy-substituted pyrogallarenes. Single crystal X-ray crystallographic analyses revealed structural variations in the compounds with conformation (i.e., rctt, rccc, rcct) having some influence on the identity of hemiquinonoid product available. Multiplicity of hemiquinonoid group affects singlet oxygen quantum yield with those doubly substituted being more active than those containing a single hemiquinone. Compounds reported here lacking hemiquinonoid groups are inactive as photosensitizers. The term 'fuchsonarene' (fuchson + arene of resorcinarene) is proposed for use to classify the compounds.

Controlling Catalyst-Free Formation and Hole Gas Accumulation by Fabricating Si/Ge Core-Shell and Si/Ge/Si Core-Double Shell Nanowires.

The catalyst-free formation of silicon (Si) and germanium (Ge) core-shell and core-double shell nanowires (NWs) was studied for use as building blocks of high electron (hole) mobility transistors (HEMTs). Vertically aligned p-type Si (p-Si)/intrinsic Ge (i-Ge) core-shell NWs and p-Si/i-Ge/p-Si core-double shell NWs with uniform diameters were formed by combining nanoimprint lithography, Bosch etching, and chemical vapor deposition. The boron (B) doping process was used to prepare p-Si NWs. The hole gas accumulation could be reliably detected from the i-Ge shell region in the p-Si/i-Ge core-shell NW and p-Si/i-Ge/p-Si core-double shell NW arrays through the Fano resonance effect, showing that core-shell NW heterostructures can suppress impurity scattering and act as high-mobility transistor channels. This provides the possibility for the future creation of vertical high-speed transistors.