Showing papers on "Band gap published in 1993"

Acoustic band structure of periodic elastic composites.

Abstract: We present the first full band-structure calculations for periodic, elastic composites. For transverse polarization of the vibrations we obtain a ``phononic'' band gap which extends throughout the Brillouin zone. A complete acoustic gap or a low density of states should have important consequences for the suppression of zero-point motion and for the localization of phonons, and may lead to improvements in transducers and in the creation of a vibrationless environment.

Colloidal Semiconductor Q‐Particles: Chemistry in the Transition Region Between Solid State and Molecules

Abstract: In semiconductor particles of nanometer size, a gradual transition from solid-state to molecular structure occurs as the particle size decreases. Consequently, a splitting of the energy bands into discrete, quantized levels occurs. Particles that exhibit these quantization effects are often called “Q-particles” or, generally, quantized material. The optical, electronic and catalytic properties of Q-particles drastically differ from those of the corresponding macrocrystalline substance. The band gap, a substance-specific quantity in macrocrystalline materials, increases by several electron volts in Q-particles with decreasing particle size. In Q-particles there are approximately as many molecules on the surface as in the interior of the particle. Therefore, the nature of the surface as well as the particle size is also largely responsible for the physico-chemical properties of the particle. Q-particles of many materials can be prepared in the form of colloidal solutions or embedded in porous matrices and are stable over a long period of time. In sandwich colloids, in which Q-particles of different materials are coupled, as well as in porous semiconductor electrodes containing Q-particles in the pores, very efficient primary charge separation is observed. As a result, sandwich colloids have greatly enhanced photocatalytic activity relative to the individual particles, while electrodes modified with Q-particles show high photocurrents. This article deals with the size quantization effect, the synthesis and characterization of Q-particles, as well as with the spectroscopic, electrochemical, and electron-microscopic investigation of these particles.

Quantum confinement in size-selected, surface-oxidized silicon nanocrystals.

Abstract: The dynamics and spectroscopy of silicon nanocrystals that emit at visible wavelengths were analyzed. Size-selective precipitation and size-exclusion chromatography cleanly separate the silicon nanocrystals from larger crystallites and aggregates and provide direct evidence for quantum confinement in luminescence. Measured quantum yields are as high as 50 percent at low temperature, principally as a result of efficient oxide passivation. Despite a 0.9—electron-volt shift of the band gap to higher energy, the nanocrystals behave fundamentally as indirect gap materials with low oscillator strength.

Chalcopyrite/defect chalcopyrite heterojunctions on the basis of CuInSe2

Abstract: A new model for the formation of heterojunctions in polycrystalline CuInSe2 thin films on the basis of surface analysis experiments is presented. In situ photoemission measurements of CuInSe2 clearly show the existence of an In‐rich n‐type surface layer on samples relevant for solar‐cell devices. Furthermore, this layer has been identified as an ordered vacancy compound (OVC) with a band gap of about 1.3 eV. The previous model of the CuInSe2/CdS solar cell with a p‐n heterojunction between p‐type CuInSe2 and n‐type CdS is replaced by the model of a chalcopyrite/defect chalcopyrite heterojunction between p‐type bulk CuInSe2 and the In‐rich n‐type OVC. The existence of this junction was proven directly by evaporating an ohmic metal contact onto the surface n‐type layer and measuring the spectral quantum efficiency and electron‐beam‐induced current of this device. The band offsets of CuInSe2‐based devices have been determined.

Electronic States of Carbon Nanotubes

Abstract: The π-electron states of carbon nanotubes (CN's) in magnetic fields are calculated in the effective-mass theory. A sensitive change of CN from metal to semiconductor depending on its structure is well reproduced. The band gap is inversely proportional to the tube diameter and exhibits a drastic change as a function of magnetic flux passing through the cylinder with period c h / e due to the Aharonov-Bohm effect. In a magnetic field perpendicular to the tube axis, the band-gap is reduced strongly and the energy spectra approach those of a graphite sheet.

Quantum confinement in Si nanocrystals

Abstract: The electronic structure of nanocrystalline Si which shows visible photoluminescence is calculated using the density-functional approach for finite structures. Except for geometry this is the same theory as for first-principles band structures of semiconductors and other solids. Our results for clusters ranging up to 706 Si atoms suggest that the band gap scales linearly with ${\mathit{L}}^{\mathrm{\ensuremath{-}}1}$, where L is the cluster diameter. For such clusters it is found that dipole transitions across the gap are symmetry allowed. The finite structures thus show a direct band gap which is considerably larger than the one of bulk silicon. For larger clusters we find a strong decrease of oscillator strength, consistent with the occurrence of the indirect gap in the bulk limit.

Synthesis and Characterization of Surface-Modified Colloidal CdTe Quantum Dots

Abstract: The controlled synthesis of quantized colloidal CdTe nanocrystals (in aqueous solutions) with narrow size distributions and stabilized against rapid oxidation was achieved by capping the quantum dot particles with 3-mercapto-1,2-propanediol. Nanocrystals (i.e., quantum dots) with mean diameters of 20, 25, 35, and 40 A were produced. Optical absorption spectra showed strong excitonic peaks at the smallest size; the absorption coefficient was shown to follow an inverse cube dependence on particle diameter, while the extinction coefficient per particle remained constant. The quantum yield for photoluminescence increased with decreasing particle size and reached 20% at 20 A. The valence band edges of the CdTe quantum dots were determined by pulse radiolysis experiments (hole injection from oxidizing radicals); the bandgaps were estimated from pulse radiolysis data (redox potentials of hole and electron injecting radicals) and from the optical spectra. The dependence of the CdTe bandgap on quantum dot size was found to be much weaker than predicted by the effective mass approximation; this result is consistent with recently published theoretical calculations by several groups. 36 refs., 5 figs., 1 tab.

Interlayer Tunneling and Gap Anisotropy in High-Temperature Superconductors

Abstract: A quantitative analysis of a recent model of high-temperature superconductors based on an interlayer tunneling mechanism is presented. This model can account well for the observed magnitudes of the high transition temperatures in these materials and implies a gap that does not change sign, can be substantially anisotropic, and has the same symmetry as the crystal. The experimental consequences explored so far are consistent with the observations.

Microstructure and optical properties of free-standing porous silicon films: Size dependence of absorption spectra in Si nanometer-sized crystallites.

Abstract: We have studied the microstructure and optical properties of free-standing porous Si thin films fabricated by electrochemical anodization. Raman-spectroscopy and transmission-electron-microscopy examinations show that Si crystallite spheres with diameters of several nanometers are dispersed in the amorphous phase. The blueshift of the optical-absorption spectrum is observed for decreasing average diameter of the Si crystallites. However, there is no clear size dependence of the peak energy of the broad photoluminescence (PL) spectrum. Spectroscopic analysis strongly suggests that the photogeneration of carriers occurs in the c-Si core, whose band gap is modified by the quantum-confinement effect, while the strong PL comes from the near-surface region of small crystallites.

Photonic band gaps and localization

Abstract: Localization, Diffusion, and Correlation: The Localization of Light S. John. The Speed of Diffusing Light B.A. van Tiggelen, A. Lagendijk. Diffusion of Classical Waves in Random Media C.M. Soukoulis, et al. Accurate Measurement of Backscattered Light from Random Media P.N. den Outer, et al. Photonic Band Gaps: Photonic Band Structure E. Yablonovitch. Photonic Gaps for Electromagnetic Waves in Periodic Dielectric Structures K.M. Ho, et al. Plane-Wave Calculation of Photonic Band Structure K.M. Leung. Measurements of Localization and Photonic Band Gap Systems in Two Dimensions S. Schultz, D.R. Smith. Wave Propagation in Random Media: Localization Transition in Anisotropic and Inhomogenous Systems P. Sheng, Z.Q. Zhang. Statistical Inversion of Stratified Media from Acoustic Pulses Scattered at Two Angles of Incidence B. White, et al. 30 additional articles. Index.

Radiation properties of a planar antenna on a photonic-crystal substrate

Abstract: The photonic crystal is investigated as a substrate material for planar antennas in the microwave and millimeter-wave bands. Experimental results are presented for a bow-tie antenna on a (111)-oriented face-centered-cubic photonic-crystal substrate with a band gap between approximately 13 and 16 GHz. When driven at 13.2 GHz, the antenna radiates predominantly into the air rather than into the substrate. This suggests that highly efficient planar antennas can be made on photonic-crystal regions fabricated in semiconductor substrates such as GaAs.

Band gap energy and band lineup of III-V alloy semiconductors incorporating nitrogen and boron

Abstract: The band gap energy and band lineup of 15 binary, 42 ternary and 39 quaternary III-V alloy semiconductors composed of (B, Al, Ga, In)(N, P, As, Sb) are calculated by mean of the dielectric method of Van Vechten and the Harrison model, respectively. The alloys including N are predicted to have negative band gap energy in most of the compositional range due to the large electro-negativity of nitrogen atom. Possible material combinations for a lattice-matched doubleheterostructure for blue-light-emitting devices are discussed. The chart presented here will be useful in designing blue-to-ultraviolet light emitters with and without strain in the devices

Quantum well intermixing

Abstract: Intermixing the wells and barriers of quantum well structures generally results in an increase in the band gap and is accompanied by changes in the refractive index. A range of techniques, based on impurity diffusion, dielectric capping and laser annealing has been developed to enhance the quantum well intermixing (QWI) rate in selected areas of a wafer; such processes offer the prospect of a powerful and relatively simple fabrication route for integrating optoelectronic devices and for forming photonic integrated circuits (PICS). Recent progress in QWI techniques is reviewed, concentrating on processes which are compatible with PIC applications, in particular the achievement of low optical propagation losses.

Identification of radiative transitions in highly porous silicon

Abstract: The authors report time-resolved photoluminescence spectroscopy of highly porous silicon. Their results show that the luminescence is due to localized quantum-confined excitons in undulating crystalline silicon wires. The resonantly excited photoluminescence spectrum exhibits satellite structure due to momentum-conserving phonons of crystalline silicon. This provides a clear signature of the crystalline-silicon electronic band structure. The spin states of the localized exciton are split by the electron-hole exchange interaction. This splitting is manifested both in the strong dependence of the luminescence lifetime on temperature, and as an energy gap in the resonantly excited photoluminescence spectrum. The experimental splitting is in good agreement with the value calculated for a localized exciton in crystalline silicon.

Electronic Structure of Materials

Abstract: The diatomic molecule from the finite to the infinite into two and three dimensions band gaps - origins and consequences s-p bonding - a case study in silicon free electron theory properties of free electron metals the transition metals structural stability of compounds introduction to modern quantitative theory where band theory breaks down set problems sample examination questions.

Trapped holes on titania colloids studied by electron paramagnetic resonance

Abstract: An electron paramagnetic resonance (EPR) study of paramagnetic species formed on band gap irradiation of TiO 2 colloids, aqueous suspensions of anatase (Degussa P-25), and rutile (Aldrich) powders is reported. The EPR signals (observed at 6-200 K) of the trapped hole in all of these systems exhibit similar properties. Holes produced by band gap irradiation of a TiO 2 colloid move from the oxygen lattice to the surface and are trapped directly on oxygen atoms bound to surface Ti IV atoms

Quasiparticle band structure of AlN and GaN.

Abstract: The ab initio pseudopotential method within the local-density approximation and the quasiparticle approach have been used to investigate the electronic properties of AlN and GaN in the wurtzite and zinc-blende structures. The quasiparticle band-structure energies are calculated using a model dielectric matrix for the evaluation of the electron self-energy. For this calculation, good agreement with the experimental results for the minimum band gaps in the wurtzite structure is obtained. In the zinc-blende structure we predict that AlN will be an indirect (\ensuremath{\Gamma} to X) wide band-gap semiconductor (4.9 eV) and that GaN will have a direct gap of 3.1 eV at \ensuremath{\Gamma} in good agreement with recent absorption experiments on cubic GaN (3.2--3.3 eV). A discussion of the direct versus indirect gap as well as other differences in electronic structure between the wurtzite and zinc-blende phases is presented. Other properties of quasiparticle excitations are predicted in this work and remain to be confirmed by experiment.

Photonic band structure and defects in one and two dimensions

Abstract: We present an experimental and numerical study of electromagnetic wave propagation in one-dimensional (1D) and two-dimensional (2D) systems composed of periodic arrays of dielectric scatterers. We demonstrate that there are regions of frequency for which the waves are exponentially attenuated for all propagation directions. These regions correspond to band gaps in the calculated band structure, and such systems are termed photonic band-gap (PBG) structures. Removal of a single scatterer from a PBG structure produces a highly localized defect mode, for which the energy density decays exponentially away from the defect origin. Energy-density measurements of defect modes are presented. The experiments were conducted at 6–20 GHz, but we suggest that they may be scaled to infrared frequencies. Analytic and numerical solutions for the band structure and the defect states in 1D structures are derived. Applications of 2D PBG structures are briefly discussed.

Electronic structure of double‐layer graphene tubules

Abstract: The electronic structure of coaxial, graphene double‐layer tubules is predicted for various combinations of metallic and insulating constituent inner and outer monolayers, depending on the diameter and chirality of the tubule. For the examples chosen, some of the energy bands of the inner and outer tubules are coupled to each other by commensurate interlayer interactions. Nevertheless, because of symmetry, the energy bands of metallic monolayer tubules remain metallic even after interlayer interactions are considered. The possible implications of these results on molecular metal‐insulator devices are discussed.

Linear and Nonlinear Optical Properties of TeO2 Glass

Abstract: A thin plate of TeO 2 glass of 5.0 × 4.0 × 0.25 mm 3 size, which was large enough for various optical measurements, was obtained by a rapid quenching method. The linear refractive index was measured as a function of wavelength from 486.1 to 1000 nm. The refractive index at 486.1 nm was as high as 2.239. The optical energy band gap was estimated as 3.37 eV from the optical absorption spectrum. The third-order nonlinear optical susceptibility, χ (3) , was determined by the third-harmonic generation (THG) method. The χ (3) value was as high as 1.4 × 10 −12 esu, about 50 times as large as that of SiO 2 glass. The results are discussed based on Lines' model in which an influence of cationic empty d-orbitals on the nonlinear properties was taken into account

Unconventional charge gap formation in FeSi

Abstract: We report the first infrared and optical measurements of the absolute conductivity of FeSi Above 200 K the ac conductivity of FeSi resembles that of a dirty metal, while at low temperature most of the conductivity below 60 meV is depleted, consistent with an energy gap of that magnitude Thus in contrast to what is expected from a picture based on simple thermal activation, the gap disappears at a temperature which is low relative to its size Moreover, the spectral weight lost below the gap does not reappear just above the gap, but is instead distributed over an energy range of order eV

Quantum confinement in semiconductor heterostructure nanometer-size particles.

Abstract: Nanometer-size semiconductor particles coated with another semiconductor can exhibit unusual and interesting phenomena associated with the redistribution of the electron and hole wave functions. Using the band offsets and effective masses, the overlap of the electron and hole wave functions can be altered by changing the core radius of the particles. The theory can incorporate multiple shells, band bending, and charge effects. An efficient method for solving the equations is given.

Electron Localization in Metallic Quasicrystals

Abstract: Bulk icosahedral-quasicrystalline aluminum-palladium-rhenium alloys of high structural quality and thermal stability are found to exhibit low-temperature electrical resistivities that are four orders of magnitude larger than those found in disordered metals and metallic glasses. Experiments suggest that these quasiperiodic alloys, which have a semimetallic electron density, are insulators at low temperature. The findings are discussed in light of theories on electron localization and band-gap formation in ordered metallic systems.

Polyfuran: a new synthetic approach and electronic properties

Abstract: Polyfuran films have been synthesized by electropolymerization of terfuran and investigated as a function of film preparation conditions in both the doped and undoped state. These Films have been characterized by infrared spectroscopy, scanning electron microscopy, optical absorption, X-ray photoelectron spectroscopy, dc conductivity, and electron spin resonance measurements. The electrolyte anion (dopant) used for the preparation of these films heavily influences the π-conjugated system of the polymer backbone. Structural disorder and doping level depends on the nature of the electrolyte anion. CF 3 SO 3 - was found to be the best dopant and to cause the least structural disorder or furan ring opening

Conformational effects in poly(p-phenylene vinylene)s revealed by low-temperature site-selective fluorescence

Abstract: Low-temperature site-selective fluorescence (SSF) spectroscopy is employed to study morphological effects on the conformation of poly(p-phenylene vinylene) (PPV) and its phenyl-substituted, soluble derivative poly(phenylphenylenevinylene) (PPPV). Samples of PPV prepared as spin-coated thin films and stretch-aligned free-standing films, and samples of PPPV prepared as cast films and as blends with poly(methylmethacrylate) and polycarbonate have been studied. The results that the authors present are considered with the notion that each polymer sample consists of an array of ordered chain segments whose average length reflects the perfection of the local structure. The statistical distribution of the segment lengths is responsible for inhomogeneous broadening of the optical spectra (absorption and emission). The dominant electronic excitation created by photoexcitation across the pi - pi * energy gap is a singlet exciton that can execute a random walk among the chain segments. SSF spectroscopy allows the authors to distinguish the contributions to the apparent fluorescence Stokes shift that arise from energy relaxation through excitation migration (spectral diffusion) and from structural relaxation of the polymer chain (self-localization). The structural contribution to the Stokes shift approaches zero in well aligned PPV and reaches values of up to 500 cm-1 in highly disordered PPPV films. The SSF method also provides a means of assessing the extent of phase separation that occurs in PPPV blends.

First-principles calculation of the structural, electronic, and vibrational properties of gallium nitride and aluminum nitride

Abstract: First-principles pseudopotential calculations have been performed on GaN and AlN in the wurtzite and zinc-blende structures. The mixed-basis approach is employed due to the localized nature of the valence charge density in these materials. In the stress calculation within the mixed-basis set, a correction term is introduced to the stress expression in order to make it consistent with the pressure given by the total-energy calculations. The lattice constants in the wurtzite structure are in good agreement with the experimental data. The band gap appears to be direct except for zinc-blende AlN, which has the conduction-band minimum at the X point. The effective mass of the electron is found to be nearly isotropic for both wurtzite GaN and AlN. The agreement of the optical \ensuremath{\Gamma}-phonon frequencies with the Raman experimental data is excellent for wurtzite GaN and good for wurtzite AlN, except for ${\mathit{A}}_{1}$--transverse-optical (${\mathit{A}}_{1}$-TO) mode. The calculated ${\mathit{A}}_{1}$-TO mode frequency of AlN is 11% smaller than the experimental value. Both GaN and AlN are found to have the wurtzite structure in the ground state.

Application of the self-interaction correction to transition-metal oxides.

Abstract: We have implemented the self-interaction-corrected local-spin-density approximation within the standard linear-muffin-tin-orbital--atomic-sphere-approximation band-structure method making use of a unified Hamiltonian concept. We have used this ab initio band-structure scheme to study the electronic structure of MnO, FeO, CoO, NiO, and CuO. We find them to be wide-gap insulators, where the top of the valence band, of predominantly oxygen p character, shows a substantial hybridization with the metal d states.