Physics

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In this paper, a fluence-dependent antireflection performance is presented from ion-beam fabricated nanofaceted-Si surfaces. It is also demonstrated that these nanofacets are capable of producing room temperature ultra-violet and blue photoluminescence which can be attributed to inter-band transitions of the localized excitonic states of different Si-O bonds at the Si/SiOx interface. Time-resolved photoluminescence measurements further confirm defect-induced radiative emission from the surface of silicon nanofacets. It is observed that the spectral characteristics remain unchanged, except an enhancement in the photoluminescence intensity with increasing ion-fluence. The increase in photoluminescence intensity by orders of magnitude stronger than that of a planar Si substrate is due to higher absorption of incident photons by nanofaceted structures. © 2014 AIP Publishing LLC.

World University root node

(SoNS) School of Natural Sciences

Fulltext

Journal of Applied Physics

Tailoring room temperature photoluminescence of antireflective silicon nanofacets

The lithium storage mechanism in molybdenum disulfide (MoS2) has been comprehensively investigated as the existing conversion-based storage mechanism is unable to explain the reason behind its high practical capacity, high polarization losses, and the change in the discharge profile after the 1st charge-discharge cycle. To resolve these issues and to gain a deeper understanding of MoS2-based Li-ion batteries, for the first time, we have studied the reaction mechanism of the MoS2 anode using various experimental techniques such as XRD, Raman spectroscopy, electrochemical impedance spectroscopy, XANES, and EXAFS, as well as ab initio density functional theory based calculations. On the basis of the results presented here, and in line with some experimental findings, we find that the reaction of MoS2 with Li is not as simple as with usual metal oxide based conversion reactions, but that the pathway of the conversion reaction changes after the first discharge process. In the first discharge process, lithiation is initiated by a limited intercalation process, followed by a conversion reaction that produces molybdenum nanoparticles (Mo) and lithium sulfide (Li 2S). Whereas, unlike oxide-based conversion materials, MoS 2 does not transverse back during the delithiation process. Indeed, instead of MoS2 formation, we identified the presence of polysulfur after the complete cycle. In consecutive cycles, polysulfur reacts with lithium and forms Li2S/Li2S2, and this Li-S reaction is found to be highly reversible in nature and the only source of the high practical capacity observed in this electrode. To validate our experimental findings, an atomic scale ab initio computational study was also carried out, which likewise suggests that Li first intercalates between the MoS2 layers but that after a certain concentration, it reacts with MoS2 to form Li2S. The calculations also support the non-reversibility of the conversion reaction, by showing that Mo + Li2S formation is energetically more favorable than the re-formation of MoS2 + Li. © 2014 the Partner Organisations.

Nanoscale

An experimental and computational study to understand the lithium storage mechanism in molybdenum disulfide

Due to the fact that much of the world's best solar resources are inversely correlated with population centers, significant motivation exists for developing technology which can deliver reliable and autonomous conversion of sunlight into electricity. Thermoelectric generators are gaining incremental ground in this area since they do not require moving parts and work well in remote locations. Thermoelectric materials have been extensively used in space satellites, automobiles, and, more recently, in solar thermal applications as power generators, known as solar thermoelectric generators (STEG). STEG systems are gaining significant interest in both concentrated and non-concentrated systems and have been employed in hybrid configurations with solar thermal and photovoltaic systems. In this article, the key developments in the field of thermoelectric materials and on-going research work on STEG design conducted by various researchers to date are critically reviewed. Finally, we highlight the strategic research directions being undertaken to make highly efficient thermoelectric materials for developing a cost-effective STEG system, which could serve to bring this technology towards commercial readiness. This journal is © the Partner Organisations 2014.

RSC Advances

Recent advances in thermoelectric materials and solar thermoelectric generators-a critical review

Room temperature irradiation of InSb(111) by 60keV Ar+-ions at normal (0°) and oblique (60°) angles of incidence led to the formation of nanoporous structure in the high fluence regime of 1×1017 to 3×1018 ions cm-2. While a porous layer comprising of a network of interconnected nanofibers was generated by normal ion incidence, evolution of plate-like structures was observed for obliquely incident ions. Systematic studies of composition and structure using energy dispersive x-ray spectroscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, Raman mapping, grazing incidence x-ray diffraction, and cross-sectional transmission electron microscopy revealed a high degree of oxidation of the ion-induced microstructures with the presence of In2O3 and Sb2O3 phases and presence of nanocrystallites within the nanoporous structures. The observed structural evolution was understood in terms of processes driven by ion-induced defect accumulation within InSb. © 2014 AIP Publishing LLC.

60 keV Ar+-ion induced modification of microstructural, compositional, and vibrational properties of InSb

The sum of independent Wishart matrices, taken from distributions with unequal covariance matrices, plays a crucial role in multivariate statistics, and has applications in the fields of quantitative finance and telecommunication. However, analytical results concerning the corresponding eigenvalue statistics have remained unavailable, even for the sum of two Wishart matrices. This can be attributed to the complicated and rotationally noninvariant nature of the matrix distribution that makes extracting the information about eigenvalues a nontrivial task. Using a generalization of the Harish-Chandra-Itzykson-Zuber integral, we find exact solution to this problem for the complex Wishart case when one of the covariance matrices is proportional to the identity matrix, while the other is arbitrary. We derive exact and compact expressions for the joint probability density and marginal density of eigenvalues. The analytical results are compared with numerical simulations and we find perfect agreement. © EPLA, 2014.

Eigenvalue statistics for the sum of two complex Wishart matrices

Scattering is a ubiquitous phenomenon which is observed in a variety of physical systems which span a wide range of length scales. The scattering matrix is the key quantity which provides a complete description of the scattering process. The universal features of scattering in chaotic systems is most generally modeled by the Heidelberg approach which introduces stochasticity to the scattering matrix at the level of the Hamiltonian describing the scattering center. The statistics of the scattering matrix is obtained by averaging over the ensemble of random Hamiltonians of appropriate symmetry. We derive exact results for the distributions of the real and imaginary parts of the off-diagonal scattering matrix elements applicable to orthogonally-invariant and unitarily-invariant Hamiltonians, thereby solving a long standing problem. © 2013 Elsevier Inc.

Annals of Physics

Distributions of off-diagonal scattering matrix elements: Exact results

Lack of control on the chirality or diameter of single-wall carbon nanotubes (SWCNTs) during synthesis is a major impediment in the path of their widespread commercialization. We demonstrate that the humble technique of catalytic chemical vapor deposition of methane, without any sophisticated catalyst preparation, can provide significant control on the diameter of the synthesized SWCNTs. The catalyst used is a solid solution of the bimetals Fe-Mo or Co-Mo in MgO. The radial breathing modes (RBMs) in the Raman spectra of SWCNTs were used to find out the diameters. Kataura plot along with RBMs was used to study the chirality of the tubes. High concentration of the catalysts (Co:Mo:MgO = 1:0.5:15 and Fe:Mo:MgO = 1:0.5:30) resulted in high yields. However, most of these carbonaceous materials were impurities. Reducing the concentration not only improved the purity and crystallinity (I D /I G ratio ∼0.1), but most importantly reduced the diameter spread of the SWCNTs. Majority of the SWCNTs grown using the low concentration catalysts (Co:Mo:MgO = 1:0.5:300 and Fe:Mo:MgO = 1:0.5:200) were estimated to have diameters lying between 1.13 and 1.65 nm. This narrowing of diameter spread happened for both Fe and Co catalyst systems and depended only on the concentration of the catalyst. © 2014 Elsevier B.V. All rights reserved.

Applied Surface Science

Diameter control of single wall carbon nanotubes synthesized using chemical vapor deposition

The accumulation of water and hydrocarbons molecules on pure Au smooth surfaces were monitored during 100 eV electron bombardment at various beam current levels. Our studies showed that these low energy electrons could accelerate the physical adsorption processes of the gaseous contaminant molecules on the mirror surface. The 100 eV electron beam was used to provide a rough simulation of the secondary electrons generated during the interaction between the EUV beam at 13.5 nm wavelength and the mirror surface in an EUVL device. The adsorption enhancement phenomenon was explained in accordance with Langmuir's Adsorption model by the increase of the sticking coefficient of adsorbed molecules onto the mirror surface. We have also shown that a positive biasing of the top of mirror surface can be used for preventing the secondary electron emission from the mirror surface. © 2013 Elsevier B.V.

Electron irradiation-enhanced water and hydrocarbon adsorption in EUV lithography devices

GaSb(1 0 0) samples were irradiated with 60 keV Ar + -ions at normal incidence for fluences in the range of 7 × 10 16 to 3 × 10 18 ions cm -2 at room temperature, showing gradual evolution of a porous surface layer containing interconnected nanofibers. In particular, fluence dependent formation of patches on the nanoporous layer is observed by scanning electron microscopy. Combined results of grazing incidence x-ray diffraction and transmission electron microscopy reveal the presence of nanocrystallites in the porous structures. Compositional analysis by x-ray photoelectron spectroscopy indicates the development of oxide phases, mainly Ga 2 O 3 and Sb 2 O 3 where the former increases with fluence. We have proposed a model addressing a competition between ion-induced-defect driven growth of the nanoporous layer and redeposition of sputtered target atoms on the growing layer. © 2014 Elsevier B.V.

Evolution of porous network in GaSb under normally incident 60 keV Ar + -ion irradiation

We study the physics of Kaluza-Klein (KK) top quarks in the framework of a non-minimal Universal Extra Dimension (nmUED) with an orbifolded (S 1 /Z 2) flat extra spatial dimension in the presence of brane-localized terms (BLTs). In general, BLTs affect the masses and the couplings of the KK excitations in a non-trivial way including those for the KK top quarks. On top of that, BLTs also influence the mixing of the top quark chiral states at each KK level and trigger mixings among excitations from different levels with identical KK parity (even or odd). The latter phenomenon of mixing of KK levels is not present in the popular UED scenario known as the minimal UED (mUED) at the tree level. Of particular interest are the mixings among the KK top quarks from level '0' and level '2' (driven by the mass of the Standard Model (SM) top quark). These open up new production modes in the form of single production of a KK top quark and the possibility of its direct decays to SM particles leading to rather characteristic signals at the colliders. Experimental constraints and the restrictions they impose on the nmUED parameter space are discussed. The scenario is implemented in MadGraph 5 by including the quark, lepton, the gauge-boson and the Higgs sectors up to the second KK level. A few benchmark scenarios are chosen for preliminary studies of the decay patterns of the KK top quarks and their production rates at the LHC in various different modes. Recast of existing experimental analyzes in scenarios having similar states is found to be not so straightforward for the KK top quarks of the nmUED scenario under consideration. © 2014 The Author(s).