S. K. Haywood

Room temperature photoluminescence at 4.5μm from InAsN

Nitrogen incorporation in InAsN epilayers grown by radio-frequency plasma-assisted molecular beam epitaxy was investigated as a function of growth conditions. Reduced growth rate, growth temperature, and arsenic flux significantly enhance the nitrogen incorporation. Optimal growth conditions allowed us to obtain high quality InAsN with nitrogen composition of up to 2.5%. The epilayers exhibit intense 4K photoluminescence (PL) with double-peak features, which were attributed to free carrier recombination and localized carrier recombination. Strong room temperature PL emission up to a wavelength of 4.5μm is obtained.

Empowering the smart grid: can redox batteries be matched to renewable energy systems for energy storage?

The charging of an undivided cerium–zinc redox battery by various current waveforms some of which mimic the output of renewable energy (solar, wind, tidal, biofuel burning) to electricity transducers is considered in this work, where the battery operates through diffusion-only conditions, and is discharged galvanostatically. Under reasonable assumption, the mathematical model developed enables the observation that the performance characteristic of the cells charged with a constant power input differentiates between the various current–charge waveforms, with cell geometry and electrode kinetics playing subtle, but significant, roles; in particular, high efficiency is observed for sunlight-charged batteries which are thin and suffer no corrosion of the sacrificial electrode, and which have already experienced a charge–discharge cycle. The performance characteristics of the systems are interpreted in the light of consequences for smart grid realisation, and indicate that, for a constant power input, the most matched renewable is biofuel burning with a current output that linearly increases with time.

Study of Pyridine-Mediated Electrochemical Reduction of CO2 to Methanol at High CO2 Pressure.

The recently proposed highly efficient route of pyridine-catalyzed CO2 reduction to methanol was explored on platinum electrodes at high CO2 pressure. At 55 bar (5.5 MPa) 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 5-10 C cm(-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.

Importance of aspect ratio over shape in determining the quantization potential of self-assembled zinc-blende III-V quantum dots

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.

Strained arrays of colloidal nanoparticles: conductance and magnetoresistance enhancement

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.

Modeling and analysis of intraband absorption in quantum-dot-in-well mid-infrared photodetectors

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 a...

The structural parameters of self-assembled quantum dots determined from the optical spectra

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.

Modeling of intraband absorption for quantum dot-in-well structures with low computational cost

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 Greens 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.

L-Band-Related Interband Transition in InSb/GaSb Self-Assembled Quantum Dots

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.

Intersubband Absorption from 2–7 μm using Strain‐compensated Double‐barrier InGaAs Multiquantum Wells

We report observation of strong room temperature electron intersubband absorption in strain‐compensated In0.84Ga0.16As/AlAs/In0.52Al0.48As double barrier quantum wells grown on InP substrates. Multiple Γ‐Γ intersubband transitions have been observed across a wide range of the mid infrared spectrum (2–7 μm) in three structures of differing InGaAs well width and therefore with a differing net strain. From the multiple Γ‐Γ intersubband transitions observed in the 8 nm well, it is inferred that the electron effective masses and nonparabolicity parameters for the first two subbands differ significantly from each other. For the range k ∼ 5 × 106 – 6 × 106 cm−1, the difference in subband parameters results in a spread in transition energy of about twice the value calculated for the corresponding GaAs/AlGaAs quantum well.