Tin disulfide is attractive as a potential visible-light photocatalyst because its elemental components are cheap, abundant and environmentally benign. As a 2-dimensional semiconductor, SnS2 can undergo exfoliation to form atomic layer sheets that provide high surface areas of photoactive material. In order to facilitate the deployment of this exciting material in industrial processes and electrolytic cells, single crystals of phase pure SnS2 are synthesised and analysed with modern spectroscopic techniques to ascertain the values of relevant semiconductor properties. An electron affinity of 4.16 eV, ionisation potential of 6.44 eV and work function of 4.81 eV are found. The temperature dependent band gap is also reported for this material for the first time. We confirm the valence band is formed predominately by a mixture S 3p and Sn 5s, while the conduction band consists of a mixture of Sn 5s and 5p orbitals and comment on the agreement between experiment and theory for values of band gaps
Transmission electron microscopy (TEM) and scanning electron microscope cathodoluminescence (CL) have been used to determine the influence of edge and screw dislocations on the light emitting properties of InxGa1−xN quantum wells. TEM is used to locate and identify the nature of dislocations. CL on the same samples is used to determine the spatial variation of the luminescence. A direct correlation of CL maps with TEM has been established, showing that threading edge dislocations act as nonradiative recombination centers with an associated minority carrier diffusion length of 200 nm. Threading dislocations of screw and mixed type were found to be associated with surface pits which were also nonradiative in the quantum well (QW) emission, but owing to the absence of QW growth on the pit facets. The contributions of edge and screw/mixed dislocations to the reduction of the QW emission are quantified, and the wider significance of these results is discussed.
Hexagonal boron nitride (h-BN) has been predicted to exhibit an in-plane thermal conductivity as high as~550 W m −1 K −1 at room temperature, making it a promising thermal management material. However, current experimental results (220-420 W m −1 K −1) have been well below the prediction. Here, we report on the modulation of h-BN thermal conductivity by controlling the B isotope concentration. For monoisotopic 10 B h-BN, an in-plane thermal conductivity as high as 585 W m −1 K −1 is measured at room temperature,~80% higher than that of h-BN with a disordered isotope concentration (52%:48% mixture of 10 B and 11 B). The temperature-dependent thermal conductivities of monoisotopic h-BN agree well with first principles calculations including only intrinsic phonon-phonon scattering. Our results illustrate the potential to achieve high thermal conductivity in h-BN and control its thermal conductivity, opening avenues for the wide application of h-BN as a next-generation thin-film material for thermal management, metamaterials and metadevices.
A combination of transmission electron microscopy imaging and diffraction techniques is used to characterize crystal defects in homoepitaxial GaN thin films. The Burgers vectors of dislocations is established by combining large-angle convergent beam electron diffraction and conventional diffraction contrast techniques. It is shown that dislocations with Burgers vectors c, a, and c+a are present. Evidence is presented that dislocation segments lying in the interfacial plane are dissociated on a fine scale. The significance of the observations for understanding homoepitaxial growth of GaN is discussed.
Off-axis electron holography in a transmission electron microscope is used to examine the charge on threading edge dislocations in n-GaN (0001). It is shown that the crystal inner potential is reduced within 10 nm of the dislocation consistent with a negatively charged core. The results can be explained by a simple unscreened potential due to a core charge of about 4 x 10(7) electrons cm (-1). The origin of this charge is discussed. The application of the method to other types of dislocation is also considered.
Arrays of dislocation free uniform
Ga-polar GaN columns have been
realized on patterned SiO
x
/GaN/sapphire
templates by metal organic vapor phase epitaxy using a continuous
growth mode. The key parameters and the physical principles of growth
of Ga-polar GaN three-dimensional columns are identified, and their
potential for manipulating the growth process is discussed. High aspect
ratio columns have been achieved using silane during the growth, leading
to n-type columns. The vertical growth rate increases with increasing
silane flow. In a core–shell columnar LED structure, the shells
of InGaN/GaN multi quantum wells and p-GaN have been realized on a
core of n-doped GaN column. Cathodoluminescence gives insight into
the inner structure of these core–shell LED structures.
Correlations between the population
of deep trap states in an array
of TiO2 nanotubes (NT) and the dynamic photocurrent responses
under supra-band-gap illumination are investigated. Ordered arrays
of TiO2 NT of 10 μm length, 125 nm inner diameter,
and 12 nm wall thickness featuring strong anatase character were obtained
by anodization of Ti in ethylene glycol solution containing NH4F. Cyclic voltammograms at pH 10 show the characteristic responses
for nanostructured TiO2 electrodes, in particular a sharp
cathodic peak as the electron density in the film increases. These
responses are associated with the population of deep trap states with
an average value of 5 × 104 electrons per NT. Dynamic
photocurrent measurements clearly show that the characteristic rise
time of the photocurrent increases as the potential is increased above
the onset region for charging deep trap states. At potentials in which
deep trap states are fully depopulated in the dark, photocurrent rise
time approaches values just below 1 s, which is more than 3 orders
of magnitude slower than the estimated RC time constant.
The occupancy of the deep trap states under photostationary conditions
is a fraction of the density of states estimated from voltammetric
responses. These findings are discussed in the context of current
views about trap states in high surface area TiO2 electrodes.
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