InSb-based self-assembled quantum dots (SAQDs) are very promising for the mid-infrared (3-5 μm) optical range. We have analysed the electronic structure and optical properties of InAs x Sb 1-x /InAs dots. In this paper, we present the results of the modelling of electronic structure and optical properties from photoluminescence (PL) measurement for InAs x Sb 1-x /InAs SAQDs, focusing on the effects of SAQD morphology and composition. In particular, we analyse the electronic structure of InAs x Sb 1-x /InAs SAQD of various shapes, aspect ratios and compositions. We also suggest a method of assessing the geometry and composition of InAs x Sb 1-x /InAs quantum dots using their optical spectra and limited microscopy information. The calculated transition energies agree well with the experimental results. The results show that the geometry of the dot can be estimated from the optical spectra if the composition is known, and vice versa.

Strong resonant enhancement of Raman scattering on photonic resonance was observed in GaAsP semiconductor nanowires. The enhancement allowed for detailed studies of the surface phonon polariton (SPhP) scattering peak on individual nanowires. In particular, for the first time, the effect of the nanowire cross section shape on SPhP properties has been investigated. It was found that the cross section flattening induces a strong polarisation and a spectral shift of SPhPs supported by such nanowire. The assisting numerical simulations allowed to link the induced polarisation effect to a splitting of the resonant HE11 mode in the flattened nanowire. The observed spectral shift of SPhP has been also theoretically reproduced in elliptical approximation for the flattened cross section. The obtained results pave a ground for engineering of SPhP polarisation response and accurate spectral control of SPhPs in applications utilising the nanowire morphology.

Surface optical phonons are normally considered as subtle and poorly reproducible features in the Raman spectra of nanostructured semiconductors, from which little or no information about the sample can be extracted. The present study demonstrates the potential for changing this situation. For a common type of GaAs semiconductor nanowire (NW), we have shown that due to a combination of size-resonant light concentration, tapered shape and favourable scattering geometry, the surface phonon polariton (SPhP) Raman signal can be enhanced by orders of magnitude. The high signal gain enables routine detailed characterisation of the SPhP peak on an individual NW level, revealing its polarisation properties and spectral shift under variation of the dielectric environment. This detailed characterisation was conducted using very low excitation power density despite high absorption of the excitation light in the NW material. The findings provide an effective way to use SPhP Raman scattering in the characterisation of dielectric NWs and the prospect of developing novel surface sensors.

This paper describes the development of a novel method of producing nanoporous polymeric membranes in a cost-effective and reproducible manner. The novelty of the technique hinges on the exploitation of a new type of sacrificial material & structures - self-assembled arrays of magnetic nanoparticles. The arrays are obtained through application of an external magnetic field to a thin layer of colloidal solution of superparamagnetic nanoparticles in a polymerizable monomer; this is followed by photopolymerisation. The resulting columnar structures form the pore templates which when selectively etched away leave an array of nanopores spanning across the polymeric film. The morphological characterisation of the nanopores by scanning electron microscopy and ionic conductivity revealed a very unusual sponge-like pore morphology. The applications which would benefit from the specific pore morphology and arrayed manufacturing are discussed.

The recently proposed highly efficient route of pyridine-catalysed CO2 reduction to methanol was explored on platinum electrodes at high CO2 pressure. At 55 bars of CO2, the bulk electrolysis in both potentiostatic and galvanostatic regimes resulted in methanol production with Faradaic yield up to 10% for the first 5-10 C/cm of charge passed. For longer electrolysis, the methanol concentration failed to increase proportionally, being limited to sub-ppm levels irrespective of biasing conditions and pyridine concentration. This limitation cannot be removed by electrode reactivation and/or pre-electrolysis and appears to be an inherent feature of the reduction process. In agreement with bulk electrolysis findings, the CV analysis supported by simulation indicated that hydrogen evolution is still the dominant electrode reaction in pyridine-containing electrolyte, even at excess CO2 concentration in the electrolyte. No prominent contribution from either a direct or coupled CO2 reduction was found. The results obtained suggest that the CO2 to methanol reduction is a transient process that is largely decoupled from the electrode charge transfer. This is the peer reviewed version of the following article: S. I. Rybchenko, D. Touhami, J. D. Wadhawan, S. K. Haywood, ChemSusChem 2016, 9, 1660, which has been published in final form at http://dx.doi.org/10.1002/ cssc.201600267. This article may be used for non-commercial purposes in accordance With Wiley Terms and Conditions for self-archiving.

The recently proposed highly efficient route of pyridine-catalyzed CO2 reduction to methanol was explored on platinum electrodes at high CO2 pressure. At 55bar (5.5MPa) of CO2, the bulk electrolysis in both potentiostatic and galvanostatic regimes resulted in methanol production with Faradaic yields of up to 10% for the first 5-10Ccm(-2) of charge passed. For longer electrolysis, the methanol concentration failed to increase proportionally and was limited to sub-ppm levels irrespective of biasing conditions and pyridine concentration. This limitation cannot be removed by electrode reactivation and/or pre-electrolysis and appears to be an inherent feature of the reduction process. In agreement with bulk electrolysis findings, the CV analysis supported by simulation indicated that hydrogen evolution is still the dominant electrode reaction in pyridine-containing electrolyte solution, even with an excess CO2 concentration in the solution. No prominent contribution from either a direct or coupled CO2 reduction was found. The results obtained suggest that the reduction of CO2 to methanol is a transient process that is largely decoupled from the electrode charge transfer.

This paper reports on word recognition experiments using a silent speech interface based on magnetic sensing of articulator movements. A magnetic field was generated by permanent magnet pellets fixed to relevant speech articulators. Magnetic field sensors mounted on a wearable frame measured the fluctuations of the magnetic field during speech articulation. These sensor data were used in place of conventional acoustic features for the training of hidden Markov models. Both small vocabulary isolated word recognition and connected digit recognition experiments are presented. Their results demonstrate the ability of the system to capture phonetic detail at a level that is surprising for a device without any direct access to voicing information.

Structural parameters of InGaAs/GaAs self-assembled quantum dots (SAQDs), which were grown using In-flush technique, were deduced using optical spectroscopy combined with computer modeling. The results are in excellent agreement with the experimental data obtained from transmission electron microscopy. The developed approach suggests a promising alternative to structural characterization methods for SAQDs.

Intraband absorption in quantum-dot-in-a-well (DWELL) mid-infrared photodetectors is investigated using photocurrent spectroscopy and computationally cost-effective modeling linked to experimental data. The DWELL systems are challenging for modeling the electronic structure, which involves both discrete levels and the continuum energy spectrum. We show that the latter can be successfully approximated by a quasi-continuum in a large three-dimensional (3D) “quantum box” in which the electronic structure is calculated in the effective mass approximation using the finite element method. Experimental and simulated spectra show good agreement with each other, which justifies using the modeling for analysis of the experimental data. In particular, the origin of the peaks and the dot parameters, such as composition are deduced. Effects of dot composition and shape on the intraband absorption spectra are also predicted. Our model proves to be a useful tool in designing and analyzing advanced DWELL structures for any realistic 3D quantum dot geometry.

In previous publications, a silent speech interface based on permanent-magnetic articulography (PMA) has been introduced and evaluated using standard automatic speech recognition techniques. However, word recognition is a task that is computationally expensive and introduces a significant time delay between speech articulation and generation of the acoustic signal. This paper investigates a direct synthesis approach where control parameters for parametric speech synthesis are generated directly from the sensor data of the silent speech interface, without an intermediate lexical representation. Users of such a device would not be tied to the limited vocabulary of a word-based recogniser and could therefore express themselves more freely. This paper presents a feasibility study that investigates whether it is possible to infer speech synthesis parameters from PMA sensor data

There are a number of situations where individuals wish to communicate verbally but are unable to use conventional means-so called 'silent speech'. These include speakers in noisy and covert situations as well as patients who have lost their voice as a result of a laryngectomy or similar procedure. This paper focuses on those who are unable to speak following a laryngectomy and assesses the possibility of speech recognition based on a magnetic implant/sensors system. Permanent magnets are placed on the tongue and lips and the changes in magnetic field resulting from movement during speech are monitored using a set of magnetic sensors. The sensor signals are compared to sets of pre-recorded templates using the dynamic time warping (DTW) method, and the best match is identified. Experimental trials are reported for subjects with intact larynx, typically using 500-1000 utterances used for speaker dependant training and testing. It is shown that recognition rates of over 90% are achievable for vocabularies of at least 57 isolated words: sufficient to drive command-and-control applications.

Much effort has been committed to development of quantum-dot-based infrared photodetectors owing to their potential for normal-incidence absorption and low dark current. Quantum-dot-in-well structures offer additional advantages, such as better wavelength tunability and improved carrier collection. This system presents a challenge for modeling of electronic structure, as it requires solution for a complex system (quantum dot plus quantum well) with both discrete levels and the continuum energy spectrum. The Green's function method, mostly used for such problems, has very high computational cost. Here we use the Finite Element Method to model intraband absorption spectra of quantum-dot-in-well structures within the effective mass approximation.

Colloidal nanoparticles are very popular as building blocks of functional arrays for electronic and optical applications. However, there is a problem in achieving electrical conductivity in such nanoarrays due to their molecular shells. These shells, which are inherent to colloidal particles, physically separate the nanoparticles in an array and act as very effective insulators. Post-assembly thinning of the shells is therefore required to enhance the array conductivity to a sensible value. Here, we introduce a conceptually new approach to the thinning, using compressive stress applied to the array by the supporting matrix. The stress arises from polymerization-induced shrinkage of the matrix as an integral step during device assembly. Using arrays of oleic-acid-covered magnetite nanoparticles in conjunction with an HDDA-polymer (HDDA: 1,6-hexanediol diacrylate) matrix, we have achieved a significant steady current in the array along with an unprecedented value of the magnetoresistance. Our results serve as a proof-of-concept for other colloidal nanoparticles.

InSb-based self-assembled quantum dots are very promising for the midinfrared (3-5 mu m) optical range. We have analyzed the effect of geometry and composition on the electronic structure and optical spectra of InAsxSb1-x/InAs dots. The calculated transition energies agree well with the available experimental data. The results show that the geometry of the dot can be estimated from the optical spectra if the composition is known, and vice versa.

We investigate the long-standing controversy surrounding modeling of the electronic spectra of InAs/GaSb short-period superlattices (SPSLs). Most commonly, such modeling for semiconductor heterostructures is based on the k center dot p method. However, this method has so far failed to predict the band structure for type-II InAs/GaSb SPSLs. Instead, it has systematically overestimated the energy gap between the electron and heavy-hole minibands, which led to the suggestion that the k center dot p method is inadequate for these heterostructures. Our results show that the physical origin of the discrepancy between modeling and experimental results may be the graded and asymmetric InAs/GaSb interface profile. We have performed band-structure modeling within the k center dot p method using a realistic interface profile based on experimental observations. Our calculations show good agreement with experimental data, both from our own measurements and from the published literature.

Effect of lattice-mismatch-induced strain on Γ-, X- and L-conduction-band edges in III–V self-assembled quantum dots has been calculated. The misfit strain is shown to strongly affect the band edges, leading to a possibility of Γ-L and Γ-X crossover. The Γ-L crossover is predicted for realistic self-assembled InSb/GaSb (001) dots, in which the lowest interband transition is from the L-valley state. Available experimental PL data were found to be in good agreement with the crossover phenomenon.

We address the occurrence of conduction-band crossover in III-V self-assembled quantum dots solely due to misfit strain. Band structure analysis in terms of standard deformation-potential theory shows that Gamma-X crossover can occur in the dot, while both Gamma-X and Gamma-L crossovers are possible in the matrix at the interface. Crossover changes the nature of the fundamental band gap in the heterostructure, which may dramatically affect the optical properties. The implications of this are studied for a realistic InSb/GaSb (001) heterostructure, where Gamma-L crossover renders the ground-state optical transition indirect in k space. Our calculations and photoluminescence data are in remarkable agreement.

We have studied the effect of shape on the strain-modified electron∕hole confinement potential in zinc-blende quantum dots (QDs), using standard deformation potential theory and an anisotropic continuum-elasticity approximation. Calculations were performed for a variety of shapes of InAs∕GaAs (001) QDs. Our results show that the essential features of the confinement potential are determined primarily by one geometric parameter, i.e., the aspect ratio, being insensitive to other details of the QD shape. The underlying trends in strain distribution are also revealed. Our results suggest that a simple analytical model, based on the oblate-ellipsoid shape and isotropic elasticity approximation, adequately describes the major features of the strain-modified confinement potential for a wide range of self-assembled III-V (zinc-blende) QDs.

Room temperature photoluminescence is reported from GaInAsSbP pentanary alloys grown by liquid phase epitaxy on GaSb. The epitaxial layers exhibited emission in the midinfrared between 3 and 4μm. Investigation of the structural and photoluminescence properties revealed localization effects associated with potential fluctuations in the pentanary alloy arising from compositional modulation.