Graphene -a monolayer of carbon atoms densely packed into a hexagonal lattice (1)-

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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. ArticleThe discovery of graphene in 2004 [1] has led to many advances in solid state physics. Research into this new material is fuelled by interest in fundamental physics as the quantum Hall effect has been observed in graphene at room temperature [2] and electrons within graphene behave as massless dirac fermions, mimicking relativistic particles [3]. Graphene has been suggested as a candidate for a wide variety of applications in electronics (due to its ballistic transport at room temperature) and composite materials [2]. It is the first experimental realisation of a truly 2-dimensional material.To date there have been no studies published on graphene at high pressure. This is surprising in view of the huge interest in the mechanical properties of graphene [4-9] motivated particularly by its possible applications in nanoelectronics [4,5]. Strain monitoring is of critical importance [10,11] in this field. It should be of particular relevance in the case of graphene due to the predicted dependence of electronic bandgap on strain [12], and also due to the fact that some of the materials related to graphene are intrinsically stressed due to the presence of the substrate, for example graphene grown epitaxially on SiC [13]. The possibility of using graphene as an ultrasensitive strain sensor has also been suggested [9]. Study of graphene at high

Silicon solar cells containing boron and oxygen are one of the most rapidly growing forms of electricity generation. However, they suffer from significant degradation during the initial stages of use. This problem has been studied for 40 years resulting in over 250 research publications. Despite this, there is no consensus regarding the microscopic nature of the defect reactions responsible. In this paper, we present compelling evidence of the mechanism of degradation. We observe, using deep level transient spectroscopy and photoluminescence, under the action of light or injected carriers, the conversion of a deep boron-di-oxygen-related donor state into a shallow acceptor which correlates with the change in the lifetime of minority carriers in the silicon. Using ab initio modeling, we propose structures of the BsO2 defect which match the experimental findings. We put forward the hypothesis that the dominant recombination process associated with the degradation is trap-assisted Auger recombination. This assignment is supported by the observation of above bandgap luminescence due to hot carriers resulting from the Auger process.

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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 nanocluster/crystal growth. From this we obtain incidental, yet accurate estimates for the silicon surface energy and a Tolman length, d % 0.15 6 0.1 nm using the Laplace-Young relation.

Results available in the literature on minority carrier trapping and light‐induced degradation (LID) effects in silicon materials containing boron and oxygen atoms are briefly reviewed. Special attention is paid to the phenomena associated with “deep” electron traps (J. A. Hornbeck and J. R. Haynes, Phys. Rev. 1955, 97, 311) and the recently reported results that have linked LID with the transformation of a defect consisting of a substitutional boron atom and an oxygen dimer (BsO2) from a configuration with a deep donor state into a recombination active configuration associated with a shallow acceptor state (M. Vaqueiro‐Contreras et al., J. Appl. Phys. 2019, 125, 185704). It is shown that the BsO2 complex is a defect with negative‐U properties, and it is responsible for minority carrier trapping and persistent photoconductivity in nondegraded Si:B+O samples and solar cells. It is argued that the “deep” electron traps observed by Hornbeck and Haynes are the precursors of the “slow” forming shallow acceptor defects, which are responsible for the dominant LID in boron‐doped Czochralski silicon (Cz‐Si) crystals. Both the deep and shallow defects are BsO2 complexes, transformations between charge states and atomic configurations of which account for the observed electron trapping and LID phenomena.

The THz spectrum lies between microwaves and the mid-infrared, a region that remains largely unexplored mainly due to the bottleneck issue of lacking compact, solid state, emitters and detectors. Here, we report on a novel asymmetric-nanochannel device, known as the self-switching device, which can operate at frequencies up to 2.5 THz for temperature up to 150 K. This is, to our knowledge, not only the simplest diode but also the quickest acting electronic nanodevice reported to date. The radiation was generated by the free electron laser FELIX (Netherlands). The dependences of the device efficiency as a function of the electric bias, radiation intensity, radiation frequency and temperature are reported.