Scalable Large-Area 2D-MoS2/Silicon-Nanowire Heterostructures for Enhancing Energy Storage Applications.

Two-dimensional (2D) transition-metal dichalcogenides have shown great potential for energy storage applications owing to their interlayer spacing, large surface area-to-volume ratio, superior electrical properties, and chemical compatibility. Further, increasing the surface area of such materials can lead to enhanced electrical, chemical, and optical response for energy storage and generation applications. Vertical silicon nanowires (SiNWs), also known as black-Si, are an ideal substrate for 2D material growth to produce high surface-area heterostructures, owing to their ultrahigh aspect ratio. Achieving this using an industrially scalable method paves the way for next-generation energy storage devices, enabling them to enter commercialization. This work demonstrates large surface area, commercially scalable, hybrid MoS2/SiNW heterostructures, as confirmed by Raman spectroscopy, with high tunability of the MoS2 layers down to the monolayer scale and conformal MoS2 growth, parallel to the silicon nanowires, as verified by transmission electron microscopy (TEM). This has been achieved using a two-step atomic layer deposition (ALD) process, allowing MoS2 to be grown directly onto the silicon nanowires without any damage to the substrate. The ALD cycle number accurately defines the layer number from monolayer to bulk. Introducing an ALD alumina (Al2O3) interface at the MoS2/SiNW boundary results in enhanced MoS2 quality and uniformity, demonstrated by an order of magnitude reduction in the B/A exciton photoluminescence (PL) intensity ratio to 0.3 and a reduction of the corresponding layer number. This high-quality layered growth on alumina can be utilized in applications such as for interfacial layers in high-capacity batteries or for photocathodes for water splitting. The alumina-free 100 ALD cycle heterostructures demonstrated no diminishing quality effects, lending themselves well to applications that require direct electrical contact with silicon and benefit from more layers, such as electrodes for high-capacity ion batteries.

Characterising the broadband, wide-angle reflectance properties of black silicon surfaces for photovoltaic applications.

Black silicon nanotextures offer significant optical performance improvements when applied to crystalline silicon solar cells. Coupled with conventional pyramidal textures, to create so-called hybrid black silicon, these benefits are shown to be further enhanced. Presented here is a comprehensive analysis of different variations of this texture, coupled with typical anti-reflectance schemes such as coated pyramids, with a view to the significance of this on subsequent, real-world, solar energy generation. The study uses an angle-resolved spectrophometry system to characterise and compare the optical properties of these surface textures in terms of reflectance versus wavelength and incident angle, with and without encapsulant layers. This analysis, coupled with time-resolved, location specific irradiance data, leads to a new figure-of-merit, the weighted reflectivity, with which to compare surface textures for use in solar cells. Weighted reflectivity for an encapsulated solar cell surface, averaged over a year, for a Southampton, UK, location is calculated to be 7.6% for hybrid black silicon, compared to 10.6% for traditional random pyramids with a thin film anti-reflective coating.

A Fullerene-Platinum Complex for Direct Functional Patterning of Single Metal Atom-Embedded Carbon Nanostructures.

The development of patterning materials ("resists") at the nanoscale involves two distinct trends: one is toward high sensitivity and resolution for miniaturization, the other aims at functionalization of the resists to realize bottom-up construction of distinct nanoarchitectures. Patterning of carbon nanostructures, a seemingly ideal application for organic functional resists, has been highly reliant on complicated pattern transfer processes because of a lack of patternable precursors. Herein, we present a fullerene-metal coordination complex as a fabrication material for direct functional patterning of sub-10 nm metal-containing carbon structures. The attachment of one platinum atom per fullerene molecule not only leads to significant improvement of sensitivity and resolution but also enables stable atomic dispersion of the platinum ions within the carbon matrix, which may gain fundamentally new interest in functional patterning of hierarchical carbon nanostructures.

Efficient light harvesting in hybrid quantum dot-interdigitated back contact solar cells via resonant energy transfer and luminescent downshifting.

In this paper, we propose a hybrid quantum dot (QD)/solar cell configuration to improve performance of interdigitated back contact (IBC) silicon solar cells, resulting in 39.5% relative boost in the short-circuit current (JSC) through efficient utilisation of resonant energy transfer (RET) and luminescent downshifting (LDS). A uniform layer of CdSe1-xSx/ZnS quantum dots is deposited onto the AlOx surface passivation layer of the IBC solar cell. QD hybridization is found to cause a broadband improvement in the solar cell external quantum efficiency. Enhancement over the QD absorption wavelength range is shown to result from LDS. This is confirmed by significant boosts in the solar cell internal quantum efficiency (IQE) due to the presence of QDs. Enhancement over the red and near-infrared spectral range is shown to result from the anti-reflection properties of the QD layer coating. A study on the effect of QD layer thickness on solar cell performance was performed and an optimised QD layer thickness was determined. Time-resolved photoluminescence (TRPL) spectroscopy was used to investigate the photoluminescence dynamics of the QD layer as a function of AlOx spacer layer thickness. RET can be evoked between the QD and Si layers for very thin AlOx spacer layers, with RET efficiencies of up to 15%. In the conventional LDS architecture, down-converters are deposited on the surface of an optimised anti-reflection layer, providing relatively narrowband enhancement, whereas the QDs in our hybrid architecture provide optical enhancement over the broadband wavelength range, by simultaneously utilising LDS, RET-mediated carrier injection, and antireflection effects, resulting in up to 40% improvement in the power conversion efficiency (PCE). Low-cost synthesis of QDs and simple device integration provide a cost-effective solution for boosting solar cell performance.

A Single-Step Route to Single-Crystal Molybdenum Disulphide (MoS2) Monolayer domains.

We report a simple, single-cycle synthetic method for forming highly-crystalline, micron-sized monolayer domains of phase-pure MoS2. This method combines liquid chemistry with discrete, layer-by-layer deposition from a novel Mo precursor. Single-crystalline MoS2 with domain sizes up to 100 µm have been obtained and characterised by optical and electron microscopy as well as Raman and photoluminescence spectroscopy.

Charged particle single nanometre manufacturing.

Following a brief historical summary of the way in which electron beam lithography developed out of the scanning electron microscope, three state-of-the-art charged-particle beam nanopatterning technologies are considered. All three have been the subject of a recently completed European Union Project entitled "Single Nanometre Manufacturing: Beyond CMOS". Scanning helium ion beam lithography has the advantages of virtually zero proximity effect, nanoscale patterning capability and high sensitivity in combination with a novel fullerene resist based on the sub-nanometre C60 molecule. The shot noise-limited minimum linewidth achieved to date is 6 nm. The second technology, focused electron induced processing (FEBIP), uses a nozzle-dispensed precursor gas either to etch or to deposit patterns on the nanometre scale without the need for resist. The process has potential for high throughput enhancement using multiple electron beams and a system employing up to 196 beams is under development based on a commercial SEM platform. Among its potential applications is the manufacture of templates for nanoimprint lithography, NIL. This is also a target application for the third and final charged particle technology, viz. field emission electron scanning probe lithography, FE-eSPL. This has been developed out of scanning tunneling microscopy using lower-energy electrons (tens of electronvolts rather than the tens of kiloelectronvolts of the other techniques). It has the considerable advantage of being employed without the need for a vacuum system, in ambient air and is capable of sub-10 nm patterning using either developable resists or a self-developing mode applicable for many polymeric resists, which is preferred. Like FEBIP it is potentially capable of massive parallelization for applications requiring high throughput.

Silicon slot fin waveguide on bonded double-SOI for a low-power accumulation modulator fabricated by an anisotropic wet etching technique.

We propose a new low VπL, fully-crystalline, accumulation modulator design based on a thin horizontal gate oxide slot fin waveguide, on bonded double Silicon-on-Insulator (SOI). A combination of anisotropic wet etching and the mirrored crystal alignment of the top and bottom SOI layers allows us for the first time to selectively pattern the bottom layer from above. Simulations presented herein show a VπL = 0.17Vcm. Fin-waveguides and passive Mach-Zehnder Interferometer (MZI) devices with fin-waveguide phase shifters have been fabricated, with the fin-waveguides having a transmission loss of 5.8dB/mm and a 13.5nm thick internal gate oxide slot.

Wide-field broadband extreme ultraviolet transmission ptychography using a high-harmonic source: publisher's note.

This note amends the list of funders in a recent Letter [Opt. Lett.41, 1317 (2016)OPLEDP0146-959210.1364/OL.41.001317].

Wide-field broadband extreme ultraviolet transmission ptychography using a high-harmonic source.

High-harmonic generation (HHG) provides a laboratory-scale source of coherent radiation ideally suited to lensless coherent diffractive imaging (CDI) in the EUV and x-ray spectral region. Here we demonstrate transmission extreme ultraviolet (EUV) ptychography, a scanning variant of CDI, using radiation at a wavelength around 29 nm from an HHG source. Image resolution is diffraction-limited at 54 nm and fields of view up to ∼100 µm are demonstrated. These results demonstrate the potential for wide-field, high-resolution, laboratory-scale EUV imaging using HHG-based sources with potential application in biological imaging or EUV lithography pellicle inspection.

A nanoporous gold membrane for sensing applications.

Design and fabrication of three-dimensionally structured, gold membranes containing hexagonally close-packed microcavities with nanopores in the base, are described. Our aim is to create a nanoporous structure with localized enhancement of the fluorescence or Raman scattering at, and in the nanopore when excited with light of approximately 600 nm, with a view to provide sensitive detection of biomolecules. A range of geometries of the nanopore integrated into hexagonally close-packed assemblies of gold micro-cavities was first evaluated theoretically. The optimal size and shape of the nanopore in a single microcavity were then considered to provide the highest localized plasmon enhancement (of fluorescence or Raman scattering) at the very center of the nanopore for a bioanalyte traversing through. The optimized design was established to be a 1200 nm diameter cavity of 600 nm depth with a 50 nm square nanopore with rounded corners in the base. A gold 3D-structured membrane containing these sized microcavities with the integrated nanopore was successfully fabricated and 'proof of concept' Raman scattering experiments are described.

Dopant profiling based on scanning electron and helium ion microscopy.

In this paper, we evaluate and compare doping contrast generated inside the scanning electron microscope (SEM) and scanning helium ion microscope (SHIM). Specialised energy-filtering techniques are often required to produce strong doping contrast to map donor distributions using the secondary electron (SE) signal in the SEM. However, strong doping contrast can be obtained from n-type regions in the SHIM, even without energy-filtering. This SHIM technique is more sensitive than the SEM to donor density changes above its sensitivity threshold, i.e. of the order of 10(16) or 10(17)donorscm(-3) respectively on specimens with or without a p-n junction; its sensitivity limit is well above 2×10(17)acceptorscm(-3) on specimens with or without a p-n junction. Good correlation is found between the widths and slopes of experimentally measured doping contrast profiles of thin p-layers and the calculated widths and slopes of the potential energy distributions across these layers, at a depth of 1 to 3nm and 5 to 10nm below the surface in the SHIM and the SEM respectively. This is consistent with the mean escape depth of SEs in silicon being about 1.8nm and 7nm in the SHIM and SEM respectively, and we conclude that short escape depth, low energy SE signals are most suitable for donor profiling.

Design and fabrication of a 3D-structured gold film with nanopores for local electric field enhancement in the pore.

Three-dimensionally structured gold membrane films with nanopores of defined, periodic geometries are designed and fabricated to provide the spatially localised enhancement of electric fields by manipulation of the plasmons inside nanopores. Square nanopores of different size and orientation relative to the pyramid are considered for films in aqueous and air environments, which allow for control of the position of electric fields within the structure. Designs suitable for use with 780 nm light were created. Here, periodic pyramidal cavities produced by potassium hydroxide etching to the {111} planes of (100) silicon substrates are used as templates for creating a periodic, pyramidal structured, free-standing thin gold film. Consistent with the findings from the theoretical studies, a nano-sized hole of 50 nm square was milled through the gold film at a specific location in the cavity to provide electric field control which can subsequently used for enhancement of fluorescence or Raman scattering of molecules in the nanopore.

Ordered mesoporous silica films with pores oriented perpendicular to a titanium nitride substrate.

The production of thin mesoporous silica films with small (∼2-3 nm) pores oriented perpendicular to a titanium nitride growth surface is demonstrated using two methods. These are the growth from a Stöber silica solution with surfactant ordering at the surface of the electrode, and electrochemically assisted growth from an acidic sol achieved by polarisation of the electrode surface. The thickness, pore order and pore size that can be achieved with these two methods is contrasted. A number of methods to vary the pore size by using different surfactants and swelling agents are explored. The advantage of applying these growth methods on titanium nitride surfaces is that it provides access to a wider electrochemical window for nanowire growth and sensor applications with non-aqueous electrolytes whilst retaining good film growth and adhesion properties.

Reflectance properties of silicon moth-eyes in response to variations in angle of incidence, polarisation and azimuth orientation.

We report a study of the optical properties of silicon moth-eye structures using a custom-made fully automated broadband spectroscopic reflectometry system (goniometer). This measurement system is able to measure specular reflectance as a function of wavelength, polar incidence angle and azimuth orientation angle, from normal to near-parallel polar incidence angle. The system uses a linear polarized broadband super-continuum laser light source. It is shown that a moth-eye structure composed of a regular array of protruding silicon rods, with finite sidewall angle reduces reflectance and sensitivity to incident wavelength in comparison to truly cylindrical rods with perpendicular sidewalls. It is also shown that moth-eye structures have omnidirectional reflectance properties in response to azimuth orientation of the sample. The importance of applying the reflectometer setup to study the optical properties of solar cell antireflective structures is highlighted.

Ultrafast nonlinear control of progressively loaded, single plasmonic nanoantennas fabricated using helium ion milling.

We demonstrate milling of partial antenna gaps and narrow conducting bridges with nanometer precision using a helium ion beam microscope. Single particle spectroscopy shows large shifts in the plasmonic mode spectrum of the milled antennas, associated with the transition from capacitive to conductive gap loading. A conducting bridge of nanometer height is found sufficient to shift the antenna from the capacitive to the conductive coupling regime, in agreement with circuit theory. Picosecond pump-probe spectroscopy reveals an enhanced nonlinear response for partially milled antennas, reaching an optimum value for an intermediate bridge height. Our results show that manipulation of the antenna load can be used to increase the nonlinear response of plasmonic antennas.

Suppression of backscattered diffraction from sub-wavelength 'moth-eye' arrays.

The eyes and wings of some species of moth are covered with arrays of nanoscale features that dramatically reduce reflection of light. There have been multiple examples where this approach has been adapted for use in antireflection and antiglare technologies with the fabrication of artificial moth-eye surfaces. In this work, the suppression of iridescence caused by the diffraction of light from such artificial regular moth-eye arrays at high angles of incidence is achieved with the use of a new tiled domain design, inspired by the arrangement of features on natural moth-eye surfaces. This bio-mimetic pillar architecture contains high optical rotational symmetry and can achieve high levels of diffraction order power reduction. For example, a tiled design fabricated in silicon and consisting of domains with 9 different orientations of the traditional hexagonal array exhibited a ~96% reduction in the intensity of the -1 diffraction order. It is suggested natural moth-eye surfaces have evolved a tiled domain structure as it confers efficient antireflection whilst avoiding problems with high angle diffraction. This combination of antireflection and stealth properties increases chances of survival by reducing the risk of the insect being spotted by a predator. Furthermore, the tiled domain design could lead to more effective artificial moth-eye arrays for antiglare and stealth applications.

Ionoluminescence in the helium ion microscope.

Ionoluminescence (IL) is the emission of light from a material due to excitation by an ion beam. In this work, a helium ion microscope (HIM) has been used in conjunction with a luminescence detection system to characterize IL from materials in an analogous way to how cathodoluminescence (CL) is characterized in a scanning electron microscope (SEM). A survey of the helium ion beam induced IL characteristics, including images and spectra, of a variety of materials known to exhibit CL in an SEM is presented. Direct band-gap semiconductors that luminesce strongly in the SEM are found not do so in the HIM, possibly due to defect-related nonradiative pathways created by the ion beam. Other materials do, however, exhibit IL, including a cerium-doped garnet sample, quantum dots, and rare-earth doped LaPO4 nanocrystals. These emissions are a result of transitions between f electron states or transitions across size dependent band gaps. In all these samples, IL is found to decay with exposure to the beam, fitting well to double exponential functions. In an exploration of the potential of this technique for biological tagging applications, imaging with the IL emitted by rare-earth doped LaPO4 nanocrystals, simultaneously with secondary electron imaging, is demonstrated at a range of magnifications.

Helium ion microscopy of Lepidoptera scales.

In this report, helium ion microscopy (HIM) is used to study the micro and nanostructures responsible for structural color in the wings of two species of Lepidotera from the Papilionidae family: Papilio ulysses (Blue Mountain Butterfly) and Parides sesostris (Emerald-patched Cattleheart). Electronic charging of uncoated scales from the wings of these butterflies, due to the incident ion beam, is successfully neutralized, leading to images displaying a large depth-of-field and a high level of surface detail, which would normally be obscured by traditional coating methods used for scanning electron microscopy (SEM). The images are compared with those from variable pressure SEM, demonstrating the superiority of HIM at high magnifications. In addition, the large depth-of-field capabilities of HIM are exploited through the creation of stereo pairs that allows the exploration of the third dimension. Furthermore, the extraction of quantitative height information which matches well with cross-sectional transmission electron microscopy measurements from the literature is demonstrated.

Imaging the bulk nanoscale morphology of organic solar cell blends using helium ion microscopy.

We use helium ion microscopy (HeIM) to image the nanostructure of poly(3-hexylthiophene)/[6,6]-phenyl-C(61)-butric acid methyl ester (P3HT/PCBM) blend thin-films. Specifically, we study a blend thin-film subject to a thermal anneal at 140 °C and use a plasma-etching technique to gain access to the bulk of the blend thin-films. We observe a domain structure within the bulk of the film that is not apparent at the film-surface and tentatively identify a network of slightly elongated PCBM domains having a spatial periodicity of (20 ± 4) nm a length of (12 ± 8) nm.

Solar energy harvesting in the epicuticle of the oriental hornet (Vespa orientalis).

The Oriental hornet worker correlates its digging activity with solar insolation. Solar radiation passes through the epicuticle, which exhibits a grating-like structure, and continues to pass through layers of the exo-endocuticle until it is absorbed by the pigment melanin in the brown-colored cuticle or xanthopterin in the yellow-colored cuticle. The correlation between digging activity and the ability of the cuticle to absorb part of the solar radiation implies that the Oriental hornet may harvest parts of the solar radiation. In this study, we explore this intriguing possibility by analyzing the biophysical properties of the cuticle. We use rigorous coupled wave analysis simulations to show that the cuticle surfaces are structured to reduced reflectance and act as diffraction gratings to trap light and increase the amount absorbed in the cuticle. A dye-sensitized solar cell (DSSC) was constructed in order to show the ability of xanthopterin to serve as a light-harvesting molecule.