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At least 415 records · Page 23Linked to original sources
arXiv2013

Conversion of multilayer graphene into continuous ultrathin sp3-bonded carbon films on metal surfaces

The conversion of multilayer graphenes into sp^3-bonded carbon films on metal surfaces (through hydrogenation or fluorination of the outer surface of the top graphene layer) is indicated through first-principles computations. The main driving force for this conversion is the hybridization between carbon sp^3 orbitals and metal surface dz^2 orbitals. The induced electronic gap states in the carbon layers are confined in a region within 0.5 nm of the metal surface. Whether the conversion occurs depend on the fraction of hydrogenated (fluorinated) C atoms and on the number of stacked graphene layers. In the analysis of the Eliashberg spectral functions for the sp^3 carbon films on diamagnetic metals, the strong covalent metal-sp^3 carbon bonds induce soft phonon modes that predominantly contribute to large electron-phonon couplings, suggesting the possibility of phonon-mediated superconductivity. Our results suggest a route to experimental realization of large-area ultrathin sp^3-bonded carbon films on metal surfaces.

cond-mat.mtrl-sci↗
arXiv2013

A new approach to simulation of limiting photoconversion efficiency of tandem solar cells

We develop a new approach to calculate the obtainable limit of photoconversion efficiency of tandem solar cells (SCs) and applied it to SCs with both vertical and lateral designs at AM0 and AM1.5 conditions. To get the maximum efficiency, only radiative recombination has been considered using typical radiative recombination parameters of the direct band gap III-V semiconductors, and explicit energy dependence of light absorption. When simulating the efficiency, we selfconsistently took into account the fact that the amount of the heat dissipated by SC decreases as the number of current-matched sub-cells increases. As the operating SCs temperature decreases both the open-circuit voltage and the photoconversion efficiency increase. It is shown that the above effect is especially strong for SCs operating under AM0 conditions. As the number of subcells is increased, narrowing the spectral range for each subcell, the photocurrent is additionally reduced due to the energy dependent light absorption, the factor generally ignored in the standard approaches. Application of our formalism results in a maximum in the theoretical dependence of the efficiency on the number of subcells, which was indeed observed experimentally. Besides agreement with experiment, our theoretical results are also close to other efficiencies calculated using detailed balance based approaches.

cond-mat.mtrl-sci↗
arXiv2013

Thermomechanics of hydrogen storage in metallic hydrides: modeling and analysis

A thermodynamically consistent mathematical model for hydrogen adsorption in metal hydrides is proposed. Beside hydrogen diffusion, the model accounts for phase transformation accompanied by hysteresis, swelling, temperature and heat transfer, strain, and stress. We prove existence of solutions of the ensuing system of partial differential equations by a carefully-designed, semi-implicit approximation scheme. A generalization for a drift-diffusion of multi-component ionized "gas" is outlined, too.

math-ph↗
arXiv2013

A template method for carbon nanotube production from sugar water

The methods of carbon nanotubes (CNTs) growth inside nanoporous materials are among the best candidates to fabricate CNTs with controlled geometrical parameters. We report obtaining CNTs by pyrolysis of sugar water and show that this chemical reaction is better suited for various applications than other known template methods. The diameter of the obtained CNTs, their alignment, and the distance between them can be controlled by choosing the bendable nanoporous template (chrysotile). Electron microscopy shows good crystalline quality of the obtained approximately 10 μm long CNTs after the template etching. Quasi-metallic transport is observed in this material after annealing CNTs under inert atmosphere.

cond-mat.mtrl-sci↗
arXiv2013

Silicon nanostructures toxicity. An ab inito approach

Silicon nanoparticles are widely used in the medical area and until now they have not manifested toxicological effects in humans beings. In order to understand the physical properties that determine their low-toxicity, we perform ab initio computational simulations of silicon nanoclusters, pure, p-doped and hollow structured. The topological and electronic properties obtained pointed out to the number of dangling bonds and electronic density as fundamental parameters to locate active sites and directly related to toxicity, besides the size and surface chemistry of the silicon nanoparticle.

cond-mat.mtrl-sci↗
arXiv2013

Defective graphene as promising anode material for Na-ion battery and Ca-ion battery

We have investigated adsorption of Na and Ca on graphene with divacancy (DV) and Stone-Wales (SW) defect. Our results show that adsorption is not possible on pristine graphene. However, their adsorption on defective sheet is energetically favorable. The enhanced adsorption can be attributed to the increased charge transfer between adatoms and underlying defective sheet. With the increase in defect density until certain possible limit, maximum percentage of adsorption also increases giving higher battery capacity. For maximum possible DV defect, we can achieve maximum capacity of 1459 mAh/g for Na-ion batteries (NIBs) and 2900 mAh/g for Ca-ion batteries (CIBs). For graphene full of SW defect, we find the maximum capacity of NIBs and CIBs is around 1071 mAh/g and 2142 mAh/g respectively. Our results will help create better anode materials with much higher capacity and better cycling performance for NIBs and CIBs.

physics.chem-ph↗
arXiv2013

Lattice and polarizability mediated spin activity in EuTiO_3

EuTiO_3 is shown to exhibit novel strong spin-charge-lattice coupling deep in the paramagnetic phase. Its existence is evidenced by an, until now, unknown response of the paramagnetic susceptibility at temperatures exceeding the structural phase transition temperature T_S = 282K. The "extra" features in the susceptibility follow the rotational soft zone boundary mode temperature dependence above and below T_S. The theoretical modeling consistently reproduces this behavior and provides reasoning for the stabilization of the soft optic mode other than quantum fluctuations.

cond-mat.mtrl-sci↗
arXiv2013

Si nano-VINe: Electrical Percolation in Quantum-Confined nc-Si:SiO2 Systems

Nanostructures, especially of silicon, are of paramount importance for new generation solar cell applications. The key material requirements for solar cells are good electrical conductivity, confinement of excitons, and a tunable band-gap that allows for tandem arrangements to absorb a maximally large portion of the solar spectrum. However, to date, existing Si-based nanostructures have addressed these requirements individually. The difficulty is that these requirements are entangled in complex ways, so an effort to improve one can lead to a major trade-offs in the others. In this study, we report on a novel silicon nanostructure, where the key advance is that we have managed to satisfy all the abovementioned requirements simultaneously: We demonstrate that the excitons are confined, the bandgap is tunable, and the electrons can freely travel along the nc-Si network.

cond-mat.mtrl-sci↗
arXiv2013

Optical conductivity spectra of the rattling phonons and charge carriers in type-VIII clathrate Ba$_8$Ga$_{16}$Sn$_{30}$

We have investigated optical conductivity spectra of $n$- and $p$-type Ba$_8$Ga $_{16}$Sn$_{30}$ ($α$-BGS) with type-VIII clathrate structure, at temperatures from 296\,K down to 6\,K with a terahertz time-domain spectrometer (0.2\,-\,2.5\,THz). The continuous spectra contributed from charge carriers are dispersive in this frequency range and also temperature- and carrier type-dependent. The Drude-Smith model taking multiple-scatterings of charge carriers into account well reproduces those data. The relaxation rate of the $n$-type carriers decreases more sharply than that in the $p$-type material, suggesting that a stronger electron-phonon interaction may exist in the $n$-type than in the $p$-type. On the other hand, the localized infrared-active modes observed at 1.3\,THz and 1.7\,THz, identified as the rattling phonons of the Ba$^{2+}$ ion's quasi-on-center vibrations, become soft and broad significantly with decreasing temperature as well as observed in type-I BGS and BGG (Ba$_8$Ga$_{16}$Ge$_{30}$) clathrates. The softening in the $n$-type is smaller by about 30% than in the $p$-type, whereas the linewidth brodening is almost the same independently on the carrier type. The difference in the softening is discussed with a scenario where the interaction of rattling phonons with carriers can modify the anharmonic potential of the guest ions. The anomalous broadening at low temepratures is also discussed by the impurity-scattering model presented for a rattling-phonon system strongly hybridized with acoustic cage phonons.

cond-mat.mtrl-sci↗
arXiv2013

Local sheet conductivity and sheet current density mapping using a single scanning voltage probe

We demonstrate how a single scanning voltage probe can be used to map the local conductivity and current density in a thin film with no a priori knowledge of the geometry of the electrical contacts. With state-of-the-art scanning voltage probes, under appropriate conditions such mapping should be possible down to nanometer scales. The technique requires two non-colinear voltage scans. When only one voltage map is available, determination of the conductivity is not possible because the solution to the governing equation is not unique. The only restriction on the technique is that the sheet conductivity is a local function of position.

cond-mat.mtrl-sci↗
arXiv2013

Epitaxial aluminum contacts to InAs nanowires

We report a method for making epitaxial superconducting contacts to semiconducting nanowires. The temperature and gate characteristics demonstrate barrier-free electrical contact, and the properties in the superconducting state are investigated at low temperature. Half-covering aluminum contacts are realized without the need of lithography and we demonstrate how to controllably insert high-band gap layers in the interface region. These developments are relevant to hybrid superconductor-nanowire devices that support Majorana zero energy states.

cond-mat.mtrl-sci↗
arXiv2013

The Sub-bandgap Photoconductivity in InGaAs:ErAs Nanocomposites

The photoconductions of ultrafast InGaAs:ErAs nanocomposites at low temperatures were investigated. The parabolic Tauc edge as well as the exponential Urbach tail are identified in the absorption spectrum. The Tauc edge supports that the density of states at the bottom of conduction band is proportional to the square root of energy. The Urbach edge is attributed to interband transition caused by smooth microscopic internal fields. The square root of mean-squared internal fields, whose distribution is Gaussian, is found in the order of $10^{5}$V/cm, agreeing very well with the theoretical predictions by Esser (B. ~ Esser, Phys. stat. sol. (b), vol. 51, 735 (1972)).

cond-mat.mtrl-sci↗
arXiv2013

Irreversible Temperature Quenching and Antiquenching of Photoluminescence of ZnS/CdS:Mn/ZnS Quantum Well Quantum Dots

An experimental observation on irreversible thermal quenching and antiquenching behavior is reported for photoluminescence (PL) of ZnS/CdS:Mn/ZnS quantum well quantum dots (QWQDs) prepared with a reverse micelle method. The dual-color emissions, a blue emission band centered at 430 nm and a Mn2+ 4T1 -> 6A1 orange emission peak at 600 nm, were found to have different dependences of emission intensity on temperature in the range of 8-290 K. Depending on Mn2+ doping concentration, they can both show strong antiquenching behavior in a certain temperature range in addition to the usual thermal quenching when lowering down temperature while it is very prominent for orange emission but weaker for blue emission. Moreover, the antiquenching behavior is weakened with rise of temperature, giving rise to a hysteretic PL temperature dependence for the QWQDs.

cond-mat.mtrl-sci↗
arXiv2013

On the Effect of Micro-alloying on the Mechanical Properties of Metallic Glasses

"Micro-alloying", referring to the addition of small concentration of a foreign metal to a given metallic glass, was used extensively in recent years to attempt to improve the mechanical properties of the latter. The results are haphazard and nonsystematic. In this paper we provide a microscopic theory of the effect of micro-alloying, exposing the delicate consequences of this procedure and the large parameter space which needs to be controlled. In particular we consider two very similar models which exhibit opposite trends for the change of the shear modulus, and explain the origins of the difference as displayed in the different microscopic structure and properties.

cond-mat.mtrl-sci↗
arXiv2013

Silicon-Vacancy Color Centers in Nanodiamonds: Cathodoluminescence Imaging Marker in the Near Infrared

We demonstrate that nanodiamonds fabricated to incorporate silicon-vacancy (Si-V) color centers provide bright, spectrally narrow, and stable cathodoluminescence (CL) in the near-infrared. Si-V color centers containing nanodiamonds are promising as non-bleaching optical markers for correlated CL and secondary electron microscopy, including applications to nanoscale bioimaging.

cond-mat.mtrl-sci↗
arXiv2013

Understanding H-defect complexes in ZnO

From state-of-the-art density-functional calculations using hybrid functionals we show that, persistent $n$-type conductivity in ZnO is due to defect complexes formed between H with intrinsic and extrinsic defects. H exhibits cationic, anionic, and electrically-inactive character on interacting with defects in ZnO. The electrically-inactive molecular hydrogen can contribute to $n$-type conductivity in ZnO by activating deep donor levels into shallow levels. By calculating local vibrational mode frequencies, we have identified origins of many H-related Raman and infra-red frequencies and thus confirmed the amphoteric behavior of H.

cond-mat.mtrl-sci↗
arXiv2013

High values of Ferroelectric polarization and magnetic susceptibility in BFO nanorods

Remarkably high values of polarization as well as a significant magnetic susceptibility have been observed in multiferroic Bismuth Ferrite (BFO) in the form of nanorods protruding out. These were developed on porous Anodised Alumina (AAO) templates using wet chemical technique. Diameters of nanorods are in the range of 20-100 nm. The high values of polarization and magnetic susceptibility are attributed to the BFO nanorod structures giving rise to the directionality. There is no leakage current in P-E loop examined at various frequencies. Magnetocapacitance measurements reflect a significant enhancement in magnetoelectric coupling also.

cond-mat.mtrl-sci↗