A large amount of work world-wide has been directed towards obtaining an understanding of the fundamental characteristics of porous Si. Much progress has been made following the demonstration in 1990 that highly porous material could emit very efficient visible photoluminescence at room temperature. Since that time, all features of the structural, optical and electronic properties of the material have been subjected to in-depth scrutiny. It is the purpose of the present review to survey the work which has been carried out and to detail the level of understanding which has been attained. The key importance of crystalline Si nanostructures in determining the behaviour of porous Si is highlighted. The fabrication of solid-state electroluminescent devices is a prominent goal of many studies and the impressive progress in this area is described.

The structural and luminescence properties of porous silicon

We have studied the electron transport in ${\text{SiO}}_{2}(\text{Co})/\text{GaAs}$ and ${\text{SiO}}_{2}(\text{Co})/\text{Si}$ heterostructures, where the ${\text{SiO}}_{2}(\text{Co})$ structure is the granular ${\text{SiO}}_{2}$ film with Co nanoparticles. In ${\text{SiO}}_{2}(\text{Co})/\text{GaAs}$ heterostructures giant magnetoresistance effect is observed. The effect has positive values, is expressed when electrons are injected from the granular film into the GaAs semiconductor, and has the temperature-peak-type character. The temperature location of the effect depends on the Co concentration and can be shifted by the applied electrical field. For the ${\text{SiO}}_{2}(\text{Co})/\text{GaAs}$ heterostructure with $71\text{ }\text{at}\text{.}\text{ }%$ Co the magnetoresistance reaches 1000 $({10}^{5}%)$ at room temperature. On the contrary, for ${\text{SiO}}_{2}(\text{Co})/\text{Si}$ heterostructures magnetoresistance values are very small (4%) and for ${\text{SiO}}_{2}(\text{Co})$ films the magnetoresistance has an opposite value. High values of the magnetoresistance effect in ${\text{SiO}}_{2}(\text{Co})/\text{GaAs}$ heterostructures have been explained by magnetic field-controlled process of impact ionization. The spin-dependent potential barrier is formed in the accumulation electron layer in the semiconductor near the interface. The impact ionization induced by injected electrons produces holes, which move, are accumulated in the region of the potential barrier and lower the barrier height. The height decrease grows the electron current flowing through the barrier and leads to the enhancement of the avalanche. Due to the formed positive feedback small variations in the barrier height lead to great changes in the current. The applied magnetic field increases the height and reduces the transparency of the barrier. As a result, small feedback reduction suppresses the onset of the impact ionization. The spin-dependent potential barrier is formed by the exchange interaction between electrons in the accumulation electron layer in the semiconductor and $d$ electrons of Co. Existence of spin-polarized localized electron states in the accumulation layer results in the temperature-peak-type character of the barrier, which is manifested in the magnetoresistance effect. Spin injectors and efficient magnetic sensors on the base of ferromagnet/semiconductor heterostructures with holes traps and quantum wells containing spin-polarized localized electrons in the semiconductor at the interface are considered.

Giant magnetoresistance in semiconductor/granular film heterostructures with cobalt nanoparticles

The spectroscopy and dynamic behavior of the self-assembled, Soret-excited zinc tetraphenylporphyrin (ZnTPP) plus fullerene (C60) model system in solution has been examined using steady state fluorescence quenching, nanosecond time-correlated single photon counting, picosecond fluorescence upconversion, and picosecond transient absorption methods. Evidence of ground state complexation is presented. Steady-state quenching of the S2 and S1 fluorescence of ZnTPP by C60 reveals that the quenching processes only occur in the excited complexes, are ultrafast, and proceed at different rates in the two states. Only uncomplexed ZnTPP is observed by fluorescence lifetime methods; the locally excited complexes are either dark or, more likely, rapidly relax to products that do not radiate strongly. Both short-range (Dexter) energy transfer and electron transfer relaxation mechanisms are evaluated. Picosecond transient absorption data obtained from the subtle differences between the spectra of Soret-excited ZnTPP with...

Photophysics of untethered ZnTPP-fullerene complexes in solution.

Photoinduced charge transfer (CT) is a fundamental process that determines the overall energy conversion efficiency of organic solar photovoltaic cells (OPVs). This review focuses on the advantages and pitfalls of theoretical/computational methods available to describe CT excitations in donor–acceptor (D–A) complexes. Studies of porphyrin–fullerene constructs as model D–A systems will be used to illustrate progress in this area.

Computational studies of charge transfer in organic solar photovoltaic cells: A review

We have studied losses of microwave electromagnetic radiation in granular amorphous hydrogenated carbon films a-C:H(Co) containing cobalt nanoparticles at cobalt concentrations below the percolation threshold and in the percolation threshold region. The difference between the experimental values of the losses in granular structures sputtered on corundum-based substrates and those on aramide tissues is observed. It is found that this is determined by the difference in localization sizes of cluster electron states in a-C:H(Co) films. We have found that losses grow with increasing concentration of cobalt nanoparticles and with increasing frequency. The frequency dependence of losses is determined by the fast spin relaxation of nanoparticles. The anisotropy of losses in a-C:H(Co) covers on aramide tissues is observed.

Microwave properties of granular amorphous carbon films with cobalt nanoparticles

Amorphous hydrogenated carbon films with varied copper concentration were prepared by dc co-sputtering of graphite and copper targets in an argon-hydrogen atmosphere. The relative atomic concentrations of carbon, copper, and oxygen were determined using proton Rutherford backscattering and the method of nuclear reactions. The dc conductivity of the films was studied in the in-plane and transverse geometries. The conductivity data are discussed in terms of the model of a medium in the form of a dielectric matrix containing two types of conducting inclusions in the form of graphite-like and copper nanoclusters.

Elemental composition and electrical properties of (a-C:H):Cu films prepared by magnetron sputtering

In this work a novel material, fullerene-Zn(II) tetraphenylporphyrin, is obtained and studied using UV-Vis-NIR absorption and Raman scattering methods. C-60-ZnTPP thin films were produced by vacuum ...

Optical and Vibrational Properties of Thin Film Fullerene‐Zn(II) Tetraphenylporphyrin Complexes

Scanning tunneling spectroscopy was used to study a-C:H and a-C:(H, Cu) films under atmospheric conditions; these films were formed on semiconductor (Si) and metallic (Cr/Si) substrates using dc magnetron sputtering of graphite or graphite/copper targets. The local density of electron states was determined from normalized differential tunneling conductance with the aim of probing the individual sp
2-phase clusters. The well-defined valence-band edge and the varying (i.e., dependent on the scanning coordinate) shape of the distribution of the density of electron states within the conduction band are characteristic of the a-C:H films; also, the largest experimental value of the band gap in these films is ∼3 eV; finally, the tendency towards the stable position of the Fermi level at a level of ∼1 eV above the valence-band top is observed in a-C:H films. The a-C:(H, Cu) films are homogeneous with respect to the local density of electron states, which is accounted for by the formation of a homogeneous surface layer in the course of growth.

Scanning tunneling spectroscopy of a-C:H and a-C:(H, Cu) films prepared by magnetron sputtering

Growth behavior of a-C:H and a-C:H〈Cu〉 films produced by the magnetron sputtering of a composite target consisting of graphite and copper plates in an argon-hydrogen atmosphere was studied by infrared spectroscopy, scanning electron microscopy, and ellipsometry. The introduction of copper into amorphous hydrogenated carbon films was shown to cause no marked changes in the carbon-hydrogen bonds in the matrix. In the a-C:H〈Cu〉 films ∼2 µm thick, a thin uniform layer (∼1000 A) was found to adjoin the substrate; closer to the free surface, the layer acquires a columnar texture with columns oriented from the substrate to the surface. The results of ellipsometry measurements were analyzed in terms of a two-layer film model.

L. V. Sharonova

Papers

Elemental composition and electrical properties of (a-C:H):Cu films prepared by magnetron sputtering

Optical and Vibrational Properties of Thin Film Fullerene‐Zn(II) Tetraphenylporphyrin Complexes

Scanning tunneling spectroscopy of a-C:H and a-C:(H, Cu) films prepared by magnetron sputtering

Growth of a-C:H and a-C:H〈Cu〉 films produced by magnetron sputtering

Diamond-like films formed by laser evaporation and multilayer structures based on them