M. P. Halsall

Control of Graphene's Properties by Reversible Hydrogenation: Evidence for Graphane

Although graphite is known as one of the most chemically inert materials, we have found that graphene, a single atomic plane of graphite, can react with atomic hydrogen, which transforms this highly conductive zero-overlap semimetal into an insulator. Transmission electron microscopy reveals that the obtained graphene derivative (graphane) is crystalline and retains the hexagonal lattice, but its period becomes markedly shorter than that of graphene. The reaction with hydrogen is reversible, so that the original metallic state, the lattice spacing, and even the quantum Hall effect can be restored by annealing. Our work illustrates the concept of graphene as a robust atomic-scale scaffold on the basis of which new two-dimensional crystals with designed electronic and other properties can be created by attaching other atoms and molecules.

High-pressure Raman spectroscopy of graphene

In-situ high pressure Raman spectroscopy is used to study monolayer, bilayer and few-layer graphene samples supported on silicon in a diamond anvil cell to 3.5 GPa. The results show that monolayer graphene adheres to the silicon substrate under compressive stress. A clear trend in this behaviour as a function of graphene sample thickness is observed. We also study unsupported graphene samples in a diamond anvil cell to 8 GPa, and show that the properties of graphene under compression are intrinsically similar to graphite. Our results demonstrate the differing effects of uniaxial and biaxial strain on the electronic bandstructure.

Probing the phonon confinement in ultrasmall silicon nanocrystals reveals a size-dependent surface energy

We validate for the first time the phenomenological phonon confinement model (PCM) of H. Richter, Z. P. Wang, and L. Ley [Solid State Commun. 39, 625 (1981)] for silicon nanostructures on the sub-3 nm length scale. By invoking a PCM that incorporates the measured size distribution, as determined from cross-sectional transmission electron microscopy (X-TEM) images, we are able to accurately replicate the measured Raman line shape, which gives physical meaning to its evolution with high temperature annealing and removes the uncertainty in determining the confining length scale. The ability of our model to explain the presence of a background scattering spectrum implies the existence of a secondary population of extremely small (sub-nm), amorphous silicon nanoclusters which are not visible in the X-TEM images. Furthermore, the inclusion of an additional fitting parameter, which takes into account the observed peak shift, can be explained by a size-dependent interfacial stress that is minimized by the nanoclu...

CdS/CdSe intrinsic Stark superlattices

Strained layer superlattices of wurtzite CdS/CdSe have been grown on (111)A GaAs substrates by metalorganic chemical vapor deposition and their optical properties studied by photoluminescence spectroscopy. It is shown that the superlattice layers contain giant strain‐induced piezoelectric fields exceeding 2×108 V m−1. These fields are similar to those reported for (111) orientated III–V superlattices, but an order of magnitude greater. The recombination energies from a series of samples provide evidence for a type II conduction band offset of 0.23±0.10 eV (the electron wells being in the CdS), with the band structure heavily modified by the internal electric fields. In addition, the photoluminescence peak emission energy shows a strong dependence on the excitation power. This is interpreted as further evidence for the effect of internal fields. We conclude that this system shows new effects not previously observed in II–VI compound superlattices. The large band‐gap tunability and the space‐charge effects ...

Photoluminescence of wide bandgap II–VI superlattices

Abstract Photoluminescence (PL) and PL decay measurements are used to characterize typical samples of wide gap II–VI strained layer superlattices (SLSs). The results show that good quality material is obtained for SLSs of ZnSe/ZnS, CdSe/CdS (hex), ZnSe/CdSe and ZnS/CdS using atmospheric pressure MOCVD. Quantum confinement in SLSs is confirmed by the peak shift, temperature dependence and decay characteristics of the exciton luminescence. The PL intensity at low temperatures (

Electron diffraction and Raman studies of the effect of substrate misorientation on ordering in the AlGaInP system

We report a systematic study of the effect of substrate orientation on the ordering in the AlGaInP system, including the Al0.52In0.48P lattice-matched ternary case. Four AlGaInP/GaAs/AlInP samples were grown by metalorganic vapor phase epitaxy under identical growth conditions on [100] substrates orientated 0°, 2°, 10° and 15° either towards the [110] or the [111] axis. The ordering in both the AlInP and the AlGaInP layers was studied by electron diffraction and the Raman scattering technique. The tendency for ordering decreased with increasing misorientation and is less for (AlxGa1−x)0.52In0.48P than for AlInP. The AlInP was found to spontaneously order even when grown after a completely disordered AlGaInP layer. The Raman results show features correlated to the electron diffraction results and hence we conclude that this technique constitutes a reliable nondestructive means of characterizing this system.

Growth and assessment of CdS and CdSe layers produced on GaAs by metalorganic chemical vapour deposition

Abstract CdS and CdSe layers have been grown successfully on GaAs by MOCVD. The surface morphology, RHEED patterns and photo-luminescence properties of the layers were studied for different substrate orientations. It was found that the CdS grew hexagonally on the (111)A substrate face, whilst the layers produced on (100), (110) and (111)B faces were a mixture of cubic and hexagonal phases. Layers of CdSe grown on GaAs were also a mixture of cubic and hexagonal phases for substrate faces other than (111)A, for which the layer was predominantly cubic. The change in photoluminescence with growth temperature was investigated and for CdS the best growth temperature, as measured by the width of the bound exciton emission, was 350 ° C. No evidence of large scale diffusion of Ga or As from the substrate was found.

Acceptor binding energy in δ-doped GaAs/AlAs multiple-quantum wells

A series of Be δ-doped GaAs/AlAs multiple-quantum wells with the doping at the well center were grown by molecular beam epitaxy. The photoluminescence spectra were measured at 4, 20, 40, 80, 120, and 200 K, respectively. The two-hole transitions of the acceptor-bound exciton from the ground state, 1S3/2(Γ6), to the first-excited state, 2S3/2(Γ6), have been clearly observed and the acceptor binding energy measured. A variational calculation is presented to obtain the acceptor binding energy as a function of well width. It is found that the experimental results are in good agreement with the theory.

Ga2Te3 and tellurium interfacial layers in ZnTe/GaSb heterostructures studied by Raman scattering

Raman spectroscopy of ZnTe layers grown by molecular beam epitaxy on (100) GaSb is reported. When the laser excitation is above the band gap of the ZnTe, scattering is observed only from the ZnTe LO mode and overtones. With excitation below the ZnTe band gap, a series of low frequency peaks is observed. By comparison with bulk data these peaks are identified as originating from Ga2Te3 and Te present at the GaSb/ZnTe interface. We conclude that the presence of this interface material may degrade the layer quality and give rise to the anomalously large strain previously reported for such epilayers.

Hydrogenation of Graphene by Reaction at High Pressure and High Temperature

The chemical reaction between hydrogen and purely sp(2)-bonded graphene to form graphenes purely sp(3)-bonded analogue, graphane, potentially allows the synthesis of a much wider variety of novel two-dimensional materials by opening a pathway to the application of conventional chemistry methods in graphene. Graphene is currently hydrogenated by exposure to atomic hydrogen in a vacuum, but these methods have not yielded a complete conversion of graphene to graphane, even with graphene exposed to hydrogen on both sides of the lattice. By heating graphene in molecular hydrogen under compression to modest high pressure in a diamond anvil cell (2.6-5.0 GPa), we are able to react graphene with hydrogen and propose a method whereby fully hydrogenated graphane may be synthesized for the first time.