As energy storage devices, supercapacitors that are also called electrochemical capacitors possess high power density, excellent reversibility and long cycle life. The recent boom in electronic devices with different functions in transparent LED displays, stretchable electronic systems and artificial skin has increased the demand for supercapacitors to move towards light, thin, integrated macro- and micro-devices with transparent, flexible, stretchable, compressible and/or wearable abilities. The successful fabrication of such supercapacitors depends mainly on the preparation of innovative electrode materials and the design of unconventional supercapacitor configurations. Tremendous research efforts have been recently made to design and construct innovative nanocarbon-based electrode materials and supercapacitors with unconventional configurations. We review here recent developments in supercapacitors from nanocarbon-based electrode materials to device configurations. The advances in nanocarbon-based electrode materials mainly include the assembly technologies of macroscopic nanostructured electrodes with different dimensions of carbon nanotubes/nanofibers, graphene, mesoporous carbon, activated carbon, and their composites. The electrodes with macroscopic nanostructured carbon-based materials overcome the issues of low conductivity, poor mechanical properties, and limited dimensions that are faced by conventional methods. The configurational design of advanced supercapacitor devices is presented with six types of unconventional supercapacitor devices: flexible, micro-, stretchable, compressible, transparent and fiber supercapacitors. Such supercapacitors display unique configurations and excellent electrochemical performance at different states such as bending, stretching, compressing and/or folding. For example, all-solid-state simplified supercapacitors that are based on nanostructured graphene composite paper are able to maintain 95% of the original capacity at a 180° folding state. The progress made so far will guide further developments in the structural design of nanocarbon-based electrode materials and the configurational diversity of supercapacitor devices. Future developments and prospects in the controllable assembly of macroscopic nanostructured electrodes and the innovation of unconventional supercapacitor configurations are also discussed. This should shed light on the R&D of supercapacitors.

Unconventional supercapacitors from nanocarbon-based electrode materials to device configurations

Applications of the non-line-of-sight vapor deposition techniques, such as chemical vapor deposition (CVD) and atomic layer deposition (ALD), offer unique opportunities to produce well-defined high surface area current collectors, thin films or various nanostructures of active (ion-storage) materials, protective coatings, solid electrolytes and improved separators. These features hold significant promise for solving emerging issues in advanced LiBs and supercapacitors. This paper reviews recent developments and applications of CVD and ALD for these energy storage devices, providing selected examples and outlining critical challenges for further exploration.

Chemical vapor deposition and atomic layer deposition for advanced lithium ion batteries and supercapacitors

© 2016 IOP Publishing Ltd. Accurately measuring and controlling the electrical properties of semiconductor nanowires is of paramount importance in the development of novel nanowire-based devices. In light of this, terahertz (THz) conductivity spectroscopy has emerged as an ideal non-contact technique for probing nanowire electrical conductivity and is showing tremendous value in the targeted development of nanowire devices. THz spectroscopic measurements of nanowires enable charge carrier lifetimes, mobilities, dopant concentrations and surface recombination velocities to be measured with high accuracy and high throughput in a contact-free fashion. This review spans seminal and recent studies of the electronic properties of nanowires using THz spectroscopy. A didactic description of THz time-domain spectroscopy, optical pump-THz probe spectroscopy, and their application to nanowires is included. We review a variety of technologically important nanowire materials, including GaAs, InAs, InP, GaN and InN nanowires, Si and Ge nanowires, ZnO nanowires, nanowire heterostructures, doped nanowires and modulation-doped nanowires. Finally, we discuss how THz measurements are guiding the development of nanowire-based devices, with the example of single-nanowire photoconductive THz receivers.

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A review of the electrical properties of semiconductor nanowires: insights gained from terahertz conductivity spectroscopy

One-dimensional semiconductor nanostructures (nanowires (NWs), nanotubes, nanopillars, nanorods, etc) based photodetectors (PDs) have been gaining traction in the research community due to their ease of synthesis and unique optical, mechanical, electrical, and thermal properties Specifically, the physics and technology of NW PDs offer numerous insights and opportunities for nanoscale optoelectronics, photovoltaics, plasmonics, and emerging negative index metamaterials devices The successful integration of these NW PDs on CMOS-compatible substrates and various low-cost substrates via direct growth and transfer-printing techniques would further enhance and facilitate the adaptation of this technology module in the semiconductor foundries In this paper, we review the unique advantages of NW-based PDs, current device integration schemes and practical strategies, recent device demonstrations in lateral and vertical process integration with methods to incorporate NWs in PDs via direct growth (nanoepitaxy) methods and transfer-printing methods, and discuss the numerous technical design challenges In particular, we present an ultrafast surface-illuminated PD with 114-ps full-width at half-maximum (FWHM), edge-illuminated novel waveguide PDs, and some novel concepts of light trapping to provide a full-length discussion on the topics of: 1) low-resistance contact and interfaces for NW integration; 2) high-speed design and impedance matching; and 3) CMOS-compatible mass-manufacturable device fabrication Finally, we offer a brief outlook into the future opportunities of NW PDs for consumer and military application

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A Perspective on Nanowire Photodetectors: Current Status, Future Challenges, and Opportunities

Silicon, arguably the most important technological semiconductor, is predicted to exhibit a range of new and interesting properties when grown in the hexagonal crystal structure. To obtain pure hexagonal silicon is a great challenge because it naturally crystallizes in the cubic structure. Here, we demonstrate the fabrication of pure and stable hexagonal silicon evidenced by structural characterization. In our approach, we transfer the hexagonal crystal structure from a template hexagonal gallium phosphide nanowire to an epitaxially grown silicon shell, such that hexagonal silicon is formed. The typical ABABAB... stacking of the hexagonal structure is shown by aberration-corrected imaging in transmission electron microscopy. In addition, X-ray diffraction measurements show the high crystalline purity of the material. We show that this material is stable up to 9 GPa pressure. With this development, we open the way for exploring its optical, electrical, superconducting, and mechanical properties.

Hexagonal silicon realized

Ultra-dense and highly doped SiNWs for micro-supercapacitors electrodes

Undoped silicon nanowire (Si NW) field-effect transistors (FETs) with a back-gate configuration have been fabricated and characterized. A thick (200 nm) Si3N4 layer was used as a gate insulator and a p++ silicon substrate as a back gate. Si NWs have been grown by the chemical vapour deposition method using the vapour–liquid–solid mechanism and gold as a catalyst. Metallic contacts have been deposited using Ni/Al (80 nm/120 nm) and characterized before and after an optimized annealing step at 400 °C, which resulted in a great decrease in the contact resistance due to the newly formed nickel silicide/Si interface at source and drain. These optimized devices show a good hole mobility of around 200 cm2 V−1 s−1, in the same range as the bulk material, with a good ON current density of about 28 kA cm−2. Finally, hysteretic behaviour of NW channel conductance is discussed to explain the importance of NW surface passivation.

https://iopscience.iop.org/article/10.1088/0268-1242/26/8/085020/pdf

High-performance silicon nanowire field-effect transistor with silicided contacts

Recent publications have reported the presence of hexagonal phases in Si nanowires. Most of these reports were based on 'odd' diffraction patterns and HRTEM images—'odd' means that these images and diffraction patterns could not be obtained on perfect silicon crystals in the classical diamond cubic structure. We analyze the origin of these 'odd' patterns and images by studying the case of various Si nanowires grown using either Ni or Au as catalysts in combination with P or Al doping. Two models could explain the experimental results: (i) the presence of a hexagonal phase or (ii) the presence of defects that we call 'hidden' defects because they cannot be directly observed in most images. We show that in many cases one direction of observation is not sufficient to distinguish between the two models. Several directions of observations have to be used. Secondly, conventional TEM images, i.e. bright-field two-beam and dark-field images, are of great value in the identification of 'hidden' defects. In addition, slices of nanowires perpendicular to the growth axis can be very useful. In the studied nanowires no hexagonal phase with long range order is found and the 'odd' images and diffraction patterns are mostly due to planar defects causing superposition of different crystal grains. Finally, we show that in Raman experiments the defect-rich NWs can give rise to a Raman peak shifted to 504–511 cm⁻¹ with respect to the Si bulk peak at 520 cm⁻¹, indicating that Raman cannot be used to identify a hexagonal phase.

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Hidden defects in silicon nanowires

We demonstrate in this paper the possibility to vertically integrate SiGe nanowires in order to use them as vertical channel for field-effect transistors (FETs). We report a threshold voltage close to 3.9 V, an ION/IOFF ratio of 104. The subthreshold slope was estimated to be around 0.9 V/decade and explained by a high traps density at the nanowire core/oxide shell interface with an estimated density of interface traps Dit ∼ 1.2 × 1013 cm−2 eV−1. Comparisons are made with both vertical Si and horizontal SiGe FETs performances.

Vertically integrated silicon-germanium nanowire field-effect transistor

Nanowires are unique objects to explore quasi-one-dimensional electronic systems. In order to obtain quantum-confined excitons in silicon nanowires, we strongly oxidized such nanowires and obtained core-shell silicon-silicon dioxide structures. Low-temperature photoluminescence measurements were performed after each oxidation step to link the band-gap energy to the dimensions of the nanowires. Surprisingly, the electronic band gap first decreases prior to increasing at quantum dimensions (the silicon nanowires, once oxidized, are stressed by the silicon dioxide shell). The controlled nanowire oxidation allowed us to develop an empirical model to estimate the strain-induced gap energy shift. With this model, we extracted the quantum energy as a function of the diameters of smaller nanowires, and its diameter dependence is in agreement with tight-binding calculations.

Pascal Gentile

Papers

Ultra-dense and highly doped SiNWs for micro-supercapacitors electrodes

High-performance silicon nanowire field-effect transistor with silicided contacts

Hidden defects in silicon nanowires

Vertically integrated silicon-germanium nanowire field-effect transistor

Quantum confinement effects and strain-induced band-gap energy shifts in core-shell Si-SiO 2 nanowires