InGaAlN and II–VI Systems for Blue–Green Light-Emitting Devices

Recent progress in research on InGaAlN and II–VI systems has been remarkable, and this paper reviews the properties of these materials from the viewpoint of making reliable light-emitting devices. The growth of these materials by molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE) is also reviewed, and the current status of both materials is reviewed with respect to light-emitting devices.     

Cr-doped II–VI materials for lasers and nonlinear optics

The paper reviews material, spectroscopic, laser and nonlinear optical properties of wide-band Cr²⁺-doped II–VI materials. The strong revival of research interest in these materials is explained by the announcement of the extremely efficient (up to >70% slope efficiency) laser operation at room-temperature. With up to 11 W of average output power, one can achieve super broad tunability (up to 1100 nm between 2 and 3.1 μm) in narrow-line continuous-wave operation and 4 ps pulses at 400 mW in mode-locked regime. Directly diode-pumping and lasers based on ceramic active media have been demonstrated, allowing development of cost-efficient compact tunable and mode-locked lasers, with a possibility to generate few-optical cycle pulses. In this wavelength range the Cr²⁺-doped lasers proved to be viable competitors to the conventional semiconductor lasers or more complex laser systems, based on frequency conversion techniques in such applications as medicine, trace gas monitoring, remote sensing, spectroscopy, metrology, optical radars, optical communications, and all-optical switching. In contrast to conventional dielectric laser materials the Cr²⁺-doped II–VI compounds combine properties of semiconductors with that of the traditionally used dielectric active media. The semiconductor nature determines strong nonlinear optical response of these materials, giving rise to charge transport and photorefractive-like phenomena, harmonic generation and parametric processes, and self-focusing effects of various origins. This calls for a considerable modification of the mode-locking techniques and reconsideration of the existing theories, which should finally enable generation of few-optical cycle pulses directly from the laser oscillator in the mid-infrared. In this connection a number of important new aspects are being discussed, such as contribution of cascaded second-order nonlinearity and Raman processes to the third-order nonlinearity, its dispersion and anisotropy, and others.     

Preparation and optical properties of sol–gel derived photo-patternable organic–inorganic hybrid films for optical waveguide applications

Photo-patternable TiO₂/organically modified silane hybrid films were prepared by combining a low-temperature sol–gel technique with a spinning–coating process. A ridge waveguide pattern was fabricated by ultraviolet light irradiation through a mask placed contact with the hybrid film in direct. Optical properties and photochemical activity of the hybrid film, including refractive index, thickness, propagation mode, and propagation loss, were studied and monitored by a prism coupling technique and Fourier transform infrared spectroscopy. The change of transmittance with exposure time was also observed by ultraviolet–visible spectroscopy. These results indicate that the hybrid film is potential application for fabrication of photonic devices by ultraviolet light irradiation. The structure of ridge waveguide pattern was characterized and studied by scanning electron microscope and surface profiler. The fabrication process of the as-prepared photosensitive hybrid film as compared with traditional binary mask has a great amount of advantages of cost-effective, simple, and smooth surface over non-photosensitive material methods.     

Structural,elastic, electronic and optical properties of new layered semiconductor BaGa2P2

We report the results of a detailed first-principles based density functional theory study of the structural, elastic, electronic and optical properties of a recently synthesized layered semiconductor BaGa₂P₂. The optimized structural parameters are in excellent agreement with the experimental structural findings, which validates the used theoretical method. The single crystal and polycrystalline elastic constants are numerically estimated using the strain–stress method and Voigt–Reuss–Hill approximations. Predicted values of the elastic constants suggest that the considered material is mechanically stable, brittle and very soft material. The three-dimensional surface and its planar projections of Young’s modulus are visualized to illustrate the elastic anisotropy. It is found that Young’s modulus of BaGa₂P₂ show strong dependence on the crystallographic directions. Band structure calculation reveals that BaGa₂P₂ is a direct energy band gap semiconductor. The effective masses of electrons and holes at the minimum of the conduction band and maximum of the valence band are numerically estimated. The density of state, charge density distribution and charge transfers are calculated and analyzed to determine the chemical bonding nature. Dielectric function, refractive index, extinction coefficient, absorption coefficient, reflectivity and electron-loss energy function spectra are computed for a wide photon energy range up to 20 eV. Calculated optical spectra exhibit a noticeable anisotropy.     

Synthesis and characterization of Y-shape electron donor–acceptor type organic dyes for dye-sensitized solar cells

Three Y-shape organic dyes, (Z)-3-(5-(3,5-bis(4-(9H-carbazol-9-yl)styryl)-4-methoxyphenyl)thiophen-2-yl)-2-cyanoacrylic acid (OD-1), (Z)-3-(5′-(3,5-bis(4-(9H-carbazol-9-yl)styryl)-4-methoxyphenyl)-2,2′-bithiophen-5-yl)-2-cyanoacrylic acid (OD-2) and (Z)-3-(5′-(3,5-bis(4-(9H-carbazol-9-yl)styryl)-4-methoxyphenyl)-3,4′-4″-trithiophenyl-5-yl)-2-cyanoacrylic acid (OD-3) were synthesized and used as sensitizers in nanocrystalline dye-sensitized solar cells (DSSCs). The introduction of the bis(carbazolylstyryl) units as an electron donor group and oligothiophene units as a both electron donors and π-spacers increased the conjugation length of the sensitizers and thus improved their molar absorption coefficient and light harvesting efficiency. DSSCs with the configuration of SnO₂:F/TiO₂/organic dye/liquid electrolyte/Pt devices were fabricated using these OD-1, OD-2 and OD-3 as a sensitizers. Among the devices, the DSSC composed of OD-3 exhibited highest power conversion efficiency of 3.03% under AM1.5G (100 mW cm⁻²).     

First-principles studies of the electronic and elastic properties of metal nitrides XN (X = Sc, Ti, V, Cr, Zr, Nb)

Electronic and elastic properties of a series of the transition metal ion mononitrides (ScN, TiN, VN, CrN, ZrN, NbN) have been modeled in the framework of ab initio plane wave spin-polarized calculations using the generalized gradient and local density approximations. The calculated band structures are typical for metallic compounds, except for ScN, whose band structure is that one of the gapless semiconductor. Strongly delocalized d states of transition metal ions are spread over a wide region of about 10-12 eV and are strongly hybridized with the nitrogen 2p states. Among the considered nitrides, only CrN exhibits a clear difference between the spin-up and spin-down states, which would manifest itself in magnetic properties. The overall appearance of the calculated cross-sections of the electron density difference changes drastically when going from Sc to Nb in the considered series of compounds. For the first time the calculated tensors of the elastic constants and elastic compliance constants were used for the analysis and visualization of the directional dependence of the Young’s moduli. It was shown that ScN and VN can be characterized as more or less elastically isotropic materials, whereas in TiN, CrN, ZrN, and NbN the Young’s moduli vary significantly in different directions. The maximal values of the Young’s moduli are along the crystallographic axes, the minimal values are along the bisector direction in the coordinate planes; the difference between them in the case of CrN exceeds one order of magnitude. In addition, pressure dependence of the “metal - nitrogen” distance was modeled.     

Simulation and analysis of the capacitance–voltage characteristics of the δ-doped semiconductors

In this paper, a self-consistent model to simulate the general characteristics of one-dimensional semiconductor structures is demonstrated. During calculation, possible quantum effects and the distributions of both electrons and holes are all considered. In this model, a continuity equation is solved to calculate the distribution of free electrons and holes. The possible quantum wells are sought using the Schrödinger equation. The overall charge density and potential are obtained self-consistently by an iteration scheme. The C–V characteristics of the δ-doped structures are simulated and then compared with those of practical samples. By comparing with these δ-doped samples, the effective numbers of dopant atoms can be precisely determined. For these highly doped samples, it is found that the activation rates are only about half. This finding can be verified by Hall measurements which confirms the accuracy in this study.     

Optoelectronic properties of a perylene substituted (cholesteryl)benzoateethynylene co-polymer

A perylene cholesteryl-benzoateethynylene co-polymer was synthesized by Sonogashira reaction and characterized by NMR, UV–Vis, static and dynamic fluorescence spectroscopy and cyclic voltammetry. The optical and electrochemical properties in solution are consistent with photoinduced energy transfer from the electron donor conjugated backbone to the electron acceptor perylene substituent. Photovoltaic properties are indeed found, even if the performance of the solar cells is quite low due to the formation of aggregates. The incorporation of (6,6)-phenyl C61–butyric acid methyl ester (PCBM), however, increases by an order of magnitude the efficiency of the prototype (from 10⁻⁴ to 10⁻³%) due to both better phase mixing and improved electrical continuity as supported by Atomic Force Microscopy (AFM) and Electrical Force Microscopy (EFM) studies.     

Low-temperature synthesis and optical properties of wurtzite ZnS nanowires

Single-crystal wurtzite ZnS nanowires have been synthesized via a facile solution route with polyethylene glycol-400 as inducing template at low temperature (170 °C). The as-prepared products have been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and energy dispersive X-ray analysis (EDX). Raman and photoluminescence spectrum (PL) were used to investigate the optical properties of ZnS nanowires. The strong emission peak centered at 322 nm in PL spectrum could be attributed to the band to band transitions.     

Synthesis and properties of new fluorene-based polyimides containing trifluoromethyl and isopropyl substituents

A new fluorene-based diamine monomer containing pendant trifluoromethyl and isopropyl substituents, 9,9-bis[4-(4-amino-2-trifluoromethylphenoxy)-3-isopropylphenyl]fluorene (3), was successfully synthesized through a three-steps reaction using 2-isopropylphenol and 9-fluorenone as starting materials. Then it was employed to prepare a series of fluorinated polyimides (5a–5d) with various commercial aromatic dianhydrides via a one-step high-temperature polycondensation. These polymers could be quickly dissolved in some strong polar organic solvents, such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, and N,N-dimethylformamide, chloroform and dichloromethane at room temperature. Flexible and transparent films can be obtained conveniently by solution-casting. Their films exhibited high thermal stability with glass transition temperatures in the range of 239–287 °C, and good optical transparency with the cutoff wavelength in the range of 326–363 nm. They also revealed low dielectric constants with the values in the range of 2.65–2.84 at 1 MHz measured for their capacitance. In comparison with some trifluoromethyl-substituted polyimides without the isopropyl substituents, these polyimides showed better solubility and lower dielectric constants and cutoff wavelength.     

Optoelectronic properties of graphene thin films prepared by thermal reduction of graphene oxide

Graphene thin films have been prepared by thermal reduction of graphene oxide. Raising the reduction temperature results in a red-shift of the G peak in Raman spectra. The reduction temperature turns out to strongly affect the morphology of the prepared graphene film. Photoluminescence (PL) results show that the band gap of graphene can be tuned by varying the reduction temperature. The thermal reduction process has been optimized in an effort to minimize the formation of wrinkles/folds on the graphene surface leading to enhanced PL and Raman peak intensities and reduced electrical sheet resistance.     

Electronic and optical properties of the group IV doped copper gallium chalcopyrites

Ternary chalcopyrite semiconductors are well known as promising materials for photovoltaic applications. The substitution of group IV atoms in sulfur and selenium copper gallium chalcopyrites GaCuX₂, with X = S and Se, could have important implications either for photovoltaic or spintronic applications. To better understand these effects, we have made first-principles calculations on the doped chalcopyrites with group IV atoms, with different concentrations and geometries. Many of these materials have an intermediate band that modifies the optical properties of the host semiconductor. The sub-gap absorptions have also been determined using the absorption coefficients.     

Single crystalline Co3O4: Synthesis and optical properties

Crystals of Co₃O₄ have been prepared from thermal decomposition of molecular precursors derived from salicylic acid and cobalt (II) acetate or chloride at 500 °C. A cubic phase Co₃O₄ micro- and nanocrystals have been obtained. The as-synthesized products were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The images of electron microscopes showed octahedral crystals of Co₃O₄. The volume and polarizability of the optimized structures of molecular precursors have been calculated and related to the particle size. The optical band gap of the obtained crystals has been measured. The results indicated two optical band gaps with values 2.65 and 2.95 eV for (E_g1) (E_g2), respectively.     

Optical and other physical characteristics of Ge–Se–Cd thin films

Amorphous Ge₂₀Se_80−xCd_x thin films with different compositions (x = 0, 2.5, 5, 7.5 and 10 at.%) were deposited onto glass substrates by thermal evaporation. The reflection spectra, R(λ), of the films at normal incidence were obtained in the spectral region from 400 to 2500 nm. Based on the use of the maxima and minima of the interference fringes, a straightforward analysis proposed by Minkov has been applied to derive the optical constants and the film thickness for the Ge₂₀Se_80−xCd_x thin films. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple and DiDomenico model. Tauc relation for the allowed non-direct transition describes the optical transition in the studied films. With increasing cadmium content the refractive index increases while the optical band gap decreases. The optical band gap decreases from 2 to 1.5 eV with increasing cadmium content from 0 to 10 at.%. The chemical-bond approach has been applied successfully to obtain the excess of Se–Se homopolar bonds and the cohesive energy of the Ge₂₀Se_80−xCd_x system.     

Structural and optical properties of vanadium pentoxide sol–gel films

The V₂O₅ films were obtained using sol–gel procedure. The composition and mesostructure of the layers were investigated with the UV and Raman spectroscopy, as well as with electron microscopy. We showed that the changes in the properties of thin layers accompanying the variation of film thickness are connected with the changes in the microstructure of the film rather than with changes in its composition. The thin V₂O₅ layers obtained in the present study are composed of disordered clusters; their mean size is 4–13 nm.