Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

Recent years have seen a rapid growth of interest by the scientific and engineering communities in the thermal properties of materials. Heat removal has become a crucial issue for continuing progress in the electronic industry, and thermal conduction in low-dimensional structures has revealed truly intriguing features. Carbon allotropes and their derivatives occupy a unique place in terms of their ability to conduct heat. The room-temperature thermal conductivity of carbon materials span an extraordinary large range--of over five orders of magnitude--from the lowest in amorphous carbons to the highest in graphene and carbon nanotubes. Here, I review the thermal properties of carbon materials focusing on recent results for graphene, carbon nanotubes and nanostructured carbon materials with different degrees of disorder. Special attention is given to the unusual size dependence of heat conduction in two-dimensional crystals and, specifically, in graphene. I also describe the prospects of applications of graphene and carbon materials for thermal management of electronics.

Thermal properties of graphene and nanostructured carbon materials

The dye-sensitized solar cells (DSC) provides a technically and economically credible alternative concept to present day p–n junction photovoltaic devices. In contrast to the conventional systems where the semiconductor assume both the task of light absorption and charge carrier transport the two functions are separated here. Light is absorbed by a sensitizer, which is anchored to the surface of a wide band semiconductor. Charge separation takes place at the interface via photo-induced electron injection from the dye into the conduction band of the solid. Carriers are transported in the conduction band of the semiconductor to the charge collector. The use of sensitizers having a broad absorption band in conjunction with oxide films of nanocrstalline morphology permits to harvest a large fraction of sunlight. Nearly quantitative conversion of incident photon into electric current is achieved over a large spectral range extending from the UV to the near IR region. Overall solar (standard AM 1.5) to current conversion efficiencies (IPCE) over 10% have been reached. There are good prospects to produce these cells at lower cost than conventional devices. Here we present the current state of the field, discuss new concepts of the dye-sensitized nanocrystalline solar cell (DSC) including heterojunction variants and analyze the perspectives for the future development of the technology.

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Dye-sensitized Solar Cells

Recent years witnessed a rapid growth of interest of scientific and engineering communities to thermal properties of materials. Carbon allotropes and derivatives occupy a unique place in terms of their ability to conduct heat. The room-temperature thermal conductivity of carbon materials span an extraordinary large range – of over five orders of magnitude – from the lowest in amorphous carbons to the highest in graphene and carbon nanotubes. I review thermal and thermoelectric properties of carbon materials focusing on recent results for graphene, carbon nanotubes and nanostructured carbon materials with different degrees of disorder. A special attention is given to the unusual size dependence of heat conduction in two-dimensional crystals and, specifically, in graphene. I also describe prospects of applications of graphene and carbon materials for thermal management of electronics.

Thermal Properties of Graphene, Carbon Nanotubes and Nanostructured Carbon Materials

The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

The main directions of studies of materials with negative index of refraction, also called left-handed or metamaterials, are reviewed. First, the physics of the phenomenon of negative refraction and the history of this scientific branch are outlined. Then recent results of studies of photonic crystals that exhibit negative refraction are discussed. In the third part numerical methods for the simulation of negative index material configurations and of metamaterials that exhibit negative index properties are presented. The advantages and the shortages of existing computer packages are analyzed. Finally, details of the fabrication of different kinds of metamaterials are given. This includes composite metamaterials, photonic crystals, and transmission line metamaterials for different wavelengths namely radio frequencies, microwaves, terahertz, infrared, and visible light. Furthermore, some examples of practical applications of metamaterials are presented.

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Negative Refractive Index Materials

Structural data have been obtained for the photosensitizer bis[(4,4‘-carboxy-2,2‘-bipyridine)(thiocyanato)]ruthenium(II) (1) via X-ray diffraction analysis. Crystals of 1 are triclinic, a = 11.4663(4) A, b = 12.5897(5) A, c = 18.9329(7) A, α = 75.238(2)°, β = 89.611(2)°, γ = 66.446(2)°, space group P1, Z = 2, refinement to R = 0.0809, Rw = 0.0950 for 4045 observed reflections. Structural models of sensitizer molecules anchoring to the TiO2 anatase surface and models of close-packed sensitizer monolayers with different anchoring types have been built by using experimental geometry of known organic Ti complexes and the X-ray structure of sensitizer 1. On the basis of a simple one-dimensional tight-binding model, it was suggested that possible modifications of TiO2/sensitizer interface could enhance interfacial transparency for injected electrons.

Structure of Organic/Inorganic Interface in Assembled Materials Comprising Molecular Components. Crystal Structure of the Sensitizer Bis[(4,4‘-carboxy-2,2‘-bipyridine)(thiocyanato)]ruthenium(II)

The temperature dependence of the thermal conductivity of porous, nanostructured alumina has been measured. It is shown that this dependence can be described with a single parameter, the value of which depends on the intergrain boundary structure and is independent of the grain size and porosity.

High-temperature thermal conductivity of porous Al2O3 nanostructures

Phonon propagation in the disordered nanostructures at a high (about the Debye temperature or higher) temperature is considered. Scattering at the grain boundaries is assumed to be the main mechanism restricting the thermal conductivity. Influence of the structure (the grain size and its dispersion, the pore diameter and their volume concentration, and the intergrain interface structure) as well as temperature on the thermal conductivity is discussed.

High-temperature phonon thermal conductivity of nanostructures

Successful high-temperature photonic structure design requires a thorough theoretical understanding in several disciplines In this review, we examine many theoretical questions, as well as published examples of these structures Research on this topic started with studies of Bragg mirrors, and progressed rapidly as a result of developments in the field of photonic crystals The practical applications, which have benefited most from this research, have been ceramic thermal barrier coatings for gas turbines, and selective emitters for thermophotovoltaic applications Materials and fabrication research has allowed for the construction of one-, two- and three-dimensional structures which can operate under extreme temperatures in excess of 1000 � C Boundary condition approaches can be used to extract the transmission spectra of regular and irregular photonic structures In particular, it is possible to estimate the propagation of waves in unorganized granular materials and in materials with large pores, where the average size exceeds the mean photon wavelength Further, we examine the relationship between structure disorder, photon mean free path, and radiation heat transfer Finally, we outline various numerical techniques used for solving photonic structures, including time and frequency domain techniques, discretization methods, and semi-empirical approaches By means of an example, we discuss practical steps helpful for the optimization of such structures A design example for high-temperature photonic structure with low layer count is presented

Leonid Braginsky

Papers

Negative Refractive Index Materials

Structure of Organic/Inorganic Interface in Assembled Materials Comprising Molecular Components. Crystal Structure of the Sensitizer Bis[(4,4‘-carboxy-2,2‘-bipyridine)(thiocyanato)]ruthenium(II)

High-temperature thermal conductivity of porous Al2O3 nanostructures

High-temperature phonon thermal conductivity of nanostructures

High-Temperature Photonic Structures. Thermal Barrier Coatings, Infrared Sources and Other Applications