We have produced ultrathin epitaxial graphite films which show remarkable 2D electron gas (2DEG) behavior. The films, composed of typically three graphene sheets, were grown by thermal decomposition on the (0001) surface of 6H−SiC, and characterized by surface science techniques. The low-temperature conductance spans a range of localization regimes according to the structural state (square resistance 1.5 kΩ to 225 kΩ at 4 K, with positive magnetoconductance). Low-resistance samples show characteristics of weak localization in two dimensions, from which we estimate elastic and inelastic mean free paths. At low field, the Hall resistance is linear up to 4.5 T, which is well-explained by n-type carriers of density 1012 cm-2 per graphene sheet. The most highly ordered sample exhibits Shubnikov−de Haas oscillations that correspond to nonlinearities observed in the Hall resistance, indicating a potential new quantum Hall system. We show that the high-mobility films can be patterned via conventional lithographic...

Ultrathin epitaxial graphite: 2D electron gas properties and a route toward graphene-based nanoelectronics.

A critical review of light-driven interfacial charge-transfer reactions of transition-metal compounds anchored to mesoporous, nanocrystalline TiO2 (anatase) thin films is described. The review highlights molecular insights into metal-to-ligand charge transfer (MLCT) excited states, mechanisms of interfacial charge separation, inter- and intra-molecular electron transfer, and interfacial charge-recombination processes that have been garnered through various spectroscopic and electrochemical techniques. The relevance of these processes to optimization of solar-energy-conversion efficiencies is discussed (483 references).

Photodriven heterogeneous charge transfer with transition-metal compounds anchored to TiO2 semiconductor surfaces

Impedance spectroscopy was applied to investigate the characteristics of dye-sensitized nanostructured TiO 2 solar cells (DSC) with high efficiencies of light to electricity conversion of 11.1% and 10.2%. The different parameters, that is, chemical capacitance, steady-state transport resistance, transient diffusion coefficient, and charge-transfer (recombination) resistance, have been interpreted in a unified and consistent framework, in which an exponential distribution of the localized states in the TiO 2 band gap plays a central role. The temperature variation of the chemical diffusion coefficient dependence on the Fermi-level position has been observed consistently with the standard multiple trapping model of electron transport in disordered semiconductors. A Tafel dependence of the recombination resistance dependence on bias potential has been rationalized in terms of the charge transfer from a distribution of surface states using the Marcus model of electron transfer. The current-potential curve of the solar cells has been independently constructed from the impedance parameters, allowing a separate analysis of the contribution of different resistive processes to the overall conversion efficiency.

Characteristics of high efficiency dye-sensitized solar cells.

Humidity measurement is one of the most significant issues in various areas of applications such as instrumentation, automated systems, agriculture, climatology and GIS. Numerous sorts of humidity sensors fabricated and developed for industrial and laboratory applications are reviewed and presented in this article. The survey frequently concentrates on the RH sensors based upon their organic and inorganic functional materials, e.g., porous ceramics (semiconductors), polymers, ceramic/polymer and electrolytes, as well as conduction mechanism and fabrication technologies. A significant aim of this review is to provide a distinct categorization pursuant to state of the art humidity sensor types, principles of work, sensing substances, transduction mechanisms, and production technologies. Furthermore, performance characteristics of the different humidity sensors such as electrical and statistical data will be detailed and gives an added value to the report. By comparison of overall prospects of the sensors it was revealed that there are still drawbacks as to efficiency of sensing elements and conduction values. The flexibility offered by thick film and thin film processes either in the preparation of materials or in the choice of shape and size of the sensor structure provides advantages over other technologies. These ceramic sensors show faster response than other types.

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Humidity Sensors Principle, Mechanism, and Fabrication Technologies: A Comprehensive Review

Electrons in nanostructured TiO2 solar cells: Transport, recombination and photovoltaic properties

Impedance spectroscopy measurements are performed on a sintered network of TiO2 nanoparticles at different humidity A transition from nearly normal to dispersive transport is observed at a certain frequency depending on the humidity The low-frequency conductivity depends on the humidity by power law with an exponent of 173, while the low-frequency dielectric constant remains constant The results can be explained by hopping proton conduction over the percolation threshold We propose that chemisorbed water molecules mainly supply the protons and the hopping sites are provided by physisorbed water molecules

Effect of humidity on the ac conductivity of nanoporous TiO2

A thermoelectric nanocomposite of Bi0.5Sb1.5Te3 with the C60 fullerene molecules has been synthesized and studied. The fullerene molecules provide the phonons blocking, reducing the lattice thermal conductivity. The reduction of the electrical conductivity is much less than the heat conductivity reduction at a low fullerene content. Therefore, the thermoelectric figure of merit increases up to 25% at 305 K. The thermoelectric properties of the synthesized materials have been analyzed theoretically in the frame of the Boltzmann equation approach. The calculations predict the optimal carrier concentration and C60 content for a maximal value of the thermoelectric figure of merit in a wide temperature range.

Thermoelectric properties of Bi0.5Sb1.5Te3/C60 nanocomposites

Results on the density of sates of nanostructured TiO2 as a function of particle size and temperature are reported. In TiO2 nanoparticles with a mean diameter 10 nm, the density of states (DOS) is strongly temperature-dependent, indicating a rearrangement of the bandgap states in which the exponential energy parameter (width of the distribution) increases from 0.080 to 50 °C. For nanoparticles with mean diameters of 20 and 30 nm the DOS is much closer to an exponential distribution, and is much less sensitive to temperature variations. It is suggested that nanometer confinement has a significant influence on the density of electronic states for 10-nm particles, while band tailing is similar to that occurring in bulk semiconductors for the larger particles.

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Properties of the electronic density of states in TiO 2 nanoparticles surrounded with aqueous electrolyte

The crystal structure of Sn3P4, a long-known tin phosphide, has been determined. It crystallizes in the trigonal space group R3m with unit-cell parameters a = 4.4315(1) A and c = 28.393(1) A (Z = 3). The crystal structure of Sn3P4 is disordered. It consists of alternating layers of phosphorus and tin atoms that are combined into five-layer blocks and propagate along the c-axis. The major structural feature is the disordered orientation of the P24– dumbbells, which link the tin atoms. The latter possess two types of coordination. One third of the tin atoms reside inside the block and are octahedrally coordinated by phosphorus atoms. The rest of the tin atoms confine the block and possess a [3 + 3] environment made up of three close phosphorus neighbors and three rather distant (3.4 A) tin atoms of the adjacent block. The coordination of the tin atoms is confirmed by 119Sn Mossbauer spectroscopy. The electron diffraction and high-resolution electron microscopy data reveal ordered regions at the microscopic...

http://ematweb.cmi.ua.ac.be/emat/pdf/1670.pdf

Highly Disordered Crystal Structure and Thermoelectric Properties of Sn3P4

Photovoltage signals were observed at ultrathin metal oxide $({\mathrm{TiO}}_{2},{\mathrm{Cu}}_{2}\mathrm{O},\mathrm{}\mathrm{ZnO})/$ metal structures by transient and spectral photovoltage (PV) techniques. The sign, the spectral behavior and the time-dependent relaxation of the PV are determined by the nature of the traps in the metal oxide layers. At lower temperatures, the relaxation of the PV signal in ${\mathrm{TiO}}_{2}$ layers is controlled by recombination due to the overlap of the wave functions of the spatially separated electrons and holes. At higher temperatures, thermal emission accelerates the recombination process. The Bohr radius of trapped holes, the tail of the exponential approximation of electronic states distribution above the valence band, the density of states at the valence band edge were obtained for ${\mathrm{TiO}}_{2}$ layers by using the proposed model of trap limited PV. The concept of trap limited PV gives a general tool for the investigation of excess carrier separation in ultrathin metal oxide or semiconductor layers with trap states.

Vladimir G. Kytin

Papers

Effect of humidity on the ac conductivity of nanoporous TiO2

Thermoelectric properties of Bi0.5Sb1.5Te3/C60 nanocomposites

Properties of the electronic density of states in TiO 2 nanoparticles surrounded with aqueous electrolyte

Highly Disordered Crystal Structure and Thermoelectric Properties of Sn3P4

Trap-limited photovoltage in ultrathin metal oxide layers