Showing papers on "Band gap published in 1999"

Single-Mode Photonic Band Gap Guidance of Light in Air.

Abstract: The confinement of light within a hollow core (a large air hole) in a silica-air photonic crystal fiber is demonstrated Only certain wavelength bands are confined and guided down the fiber, each band corresponding to the presence of a full two-dimensional band gap in the photonic crystal cladding Single-mode vacuum waveguides have a multitude of potential applications from ultrahigh-power transmission to the guiding of cold atoms

Electronic States and Luminescence in Porous Silicon Quantum Dots: The Role of Oxygen

Abstract: Depending on the size, the photoluminescence (PL) of silicon quantum dots present in porous silicon can be tuned from the near infrared to the ultraviolet when the surface is passivated with Si-H bonds. After exposure to oxygen, the PL shifts to the red by as much as 1 eV. This shift and the changes in PL intensity and decay time, show that both quantum confinement and surface passivation determine the electronic states of silicon quantum dots. A theoretical model in which new electronic states appear in the band gap of the smaller quantum dots when a Si-O bond is formed, is in good agreement with experiments. This result clarifies the controversy regarding the PL mechanisms in porous silicon.

A Novel Aqueous Process for Preparation of Crystal Form-Controlled and Highly Crystalline BiVO4 Powder from Layered Vanadates at Room Temperature and Its Photocatalytic and Photophysical Properties

Abstract: BiVO4 photocatalysts for O2 evolution, which work under visible light irradiation, were prepared by an aqueous process. The BiVO4 photocatalysts were obtained by the reaction of layered potassium vanadate powder (KV3O8 and K3V5O14) with Bi(NO3)3 for 3 days in aqueous media at room temperature. Highly crystalline monoclinic and tetragonal BiVO4 were selectively synthesized by changing the ratio of vanadium to bismuth in the starting materials. X-ray diffraction and scanning electron microscopy measurements showed that the monoclinic BiVO4 was formed via a tetragonal BiVO4 intermediate. Tetragonal BiVO4 with a 2.9 eV band gap mainly possessed an ultraviolet absorption band while monoclinic BiVO4 with a 2.4 eV band gap had a characteristic visible light absorption band in addition to the UV band. The UV bands observed in the tetragonal and monoclinic BiVO4 were assigned to the band transition from O2p to V3d whereas the visible light absorption was due to the transition from a valence band formed by Bi6s or ...

Band Anticrossing in GaInNAs Alloys

Abstract: We present evidence for a strong interaction between the conduction band and a narrow resonant band formed by nitrogen states in $\mathrm{Ga}{}_{1\ensuremath{-}x}\mathrm{In}{}_{x}\mathrm{N}{}_{y}\mathrm{As}{}_{1\ensuremath{-}y}$ alloys. The interaction leads to a splitting of the conduction band into two subbands and a reduction of the fundamental band gap. An anticrossing of the extended states of the conduction band of the $\mathrm{Ga}{}_{1\ensuremath{-}x}\mathrm{In}{}_{x}\mathrm{As}$ matrix and the localized nitrogen resonant states is used to model the interaction. Optical transitions associated with the energy minima of the two subbands and the characteristic anticrossing behavior of the transitions under applied hydrostatic pressure have been unambiguously observed using photomodulation spectroscopy. The experimental results are in excellent quantitative agreement with the model.

Guided modes in photonic crystal slabs

Abstract: We analyze the properties of two-dimensionally periodic dielectric structures that have a band gap for propagation in a plane and that use index guiding to confine light in the third dimension. Such structures are more amenable to fabrication than photonic crystals with full three-dimensional band gaps, but retain or approximate many of the latter's desirable properties. We show how traditional band-structure analysis can be adapted to slab systems in the context of several representative structures, and describe the unique features that arise in this framework compared to ordinary photonic crystals.

Low-Voltage Organic Transistors on Plastic Comprising High-Dielectric Constant Gate Insulators

Abstract: The gate bias dependence of the field-effect mobility in pentacene-based insulated gate field-effect transistors (IGFETs) was interpreted on the basis of the interaction of charge carriers with localized trap levels in the band gap. This understanding was used to design and fabricate IGFETs with mobility of more than 0.3 square centimeter per volt per second and current modulation of 10 5 , with the use of amorphous metal oxide gate insulators. These values were obtained at operating voltage ranges as low as 5 volts, which are much smaller than previously reported results. An all-room-temperature fabrication process sequence was used, which enabled the demonstration of high-performance organic IGFETs on transparent plastic substrates, at low operating voltages for organic devices.

Estimation of the thermal band gap of a semiconductor from Seebeck measurements

Abstract: It is shown that the magnitude of the Seebeck coefficient of a semiconductor has a maximum value that is close to one-half the energy gap divided by eT. An expression for the position of the Fermi level at which the Seebeck coefficient has a maximum or minimum value is derived, with account taken of the mobility and effective mass ratios. It is concluded that measurement of the Seebeck coefficient as a function of temperature on any novel semiconductor is one of the simplest ways of estimating its band gap.

An oxide-diluted magnetic semiconductor: Mn-doped ZnO

Abstract: Epitaxial thin films of an oxide-diluted magnetic semiconductor, Mn-doped ZnO, were fabricated by pulsed-laser deposition technique. Solubility of Mn into ZnO exceeds thermal equilibrium limit as a result of nonequilibrium film growth process. As Mn content is increased, the lattice constants of both a and c axes of wurtzite Zn1−xMnxO films (x<0.35) increase and the band gap expands although considerable in-gap absorption develops. Itinerant electrons over 1019 cm−3 can be doped into the Zn1−xMnxO films by Al doping, in contrast to low carrier density in the other II–VI diluted magnetic semiconductors. The temperature dependence of the resistivity is almost metallic and considerable magnetoresistance is observed at low temperatures.

Exact Dimer Ground State and Quantized Magnetization Plateaus in the Two-Dimensional Spin System SrCu 2 ( BO 3 ) 2

Abstract: Magnetic susceptibility, Cu NQR, and high-filed magnetization have been measured in polycrystalline $\mathrm{SrCu}{}_{2}(\mathrm{BO}{}_{3}{)}_{2}$ having a two-dimensional (2D) orthogonal network of Cu dimers. This cuprate provides a new class of 2D spin-gap system $(\ensuremath{\Delta}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}30\mathrm{K})$ in which the ground state can be solved ``exactly.'' Furthermore, in the magnetization, two plateaus corresponding to $\frac{1}{4}$ and $\frac{1}{8}$ of the full Cu moment were first observed for 2D quantum spin systems.

InGaAsN solar cells with 1.0 eV band gap, lattice matched to GaAs

Abstract: The design, growth by metal-organic chemical vapor deposition, and processing of an In{sub 0.07}Ga{sub 0.93}As{sub 0.98}N{sub 0.02} solar Al, with 1.0 ev bandgap, lattice matched to GaAs is described. The hole diffusion length in annealed, n-type InGaAsN is 0.6-0.8 pm, and solar cell internal quantum efficiencies > 70% arc obwined. Optical studies indicate that defects or impurities, from InGAsN doping and nitrogen incorporation, limit solar cell performance.

Ultrafast white-light continuum generation and self-focusing in transparent condensed media

Abstract: We report an investigation of white-light continuum generation and self-focusing by 140-fs Ti:sapphire laser pulses in extended transparent media. It is found that continuum generation is triggered by self-focusing and that both phenomena depend on the medium’s bandgap. There is a bandgap threshold for continuum generation. Above that threshold the continuum’s width increases with increasing bandgap. Furthermore, the beam’s self-focal diameter is discontinuous across the threshold. To explain the observations a mechanism is proposed that involves multiphoton excitation of electrons into the conduction band at the self-focus; the generated free electrons cause spectral superbroadening and limit the self-focal diameter. The continuum beam’s surprisingly low divergence is then investigated and explained in terms of a Kerr lensing effect.

H2 or O2 Evolution from Aqueous Solutions on Layered Oxide Photocatalysts Consisting of Bi3+ with 6s2 Configuration and d0 Transition Metal Ions

Abstract: Bi2W2O9, Bi2WO6, and Bi3TiNbO9 consisting of layered structure with perovskite slabs interleaved with Bi2O2 layers showed photocatalytic activities for H2 evolution from an aqueous methanol solution and O2 evolution from an aqueous silver nitrate solution. Bi2WO6 with the Aurivillius structure and a 2.8 eV band gap was active for the O2 evolution reaction under visible light irradiation (λ > 420 nm).

The influence of transition metal doping on the physical and photocatalytic properties of titania

Abstract: Titanium dioxide is one of the most efficient photocatalysts for the detoxification of organically charged waste water. However, this material suffers from the drawback of poor absorption properties because of a band gap of 3.2 eV. Thus, wavelengths shorter than 400 nm are needed for light induced generation of electron–hole pairs. Therefore, doping with transition metal ions is interesting for inducing a batho-chromic shift of the band gap. However, this doping changes other physical properties such as lifetime of electron–hole pairs and adsorption characteristics. This paper deals with doping titania by Cr 3+ and Mo 5+ ions. We want to show the complex interactions between variations in lifetime of charge carriers, adsorption properties and photocatalytic behaviour.

Optical and Electronic Properties of Si Nanoclusters Synthesized in Inverse Micelles

Abstract: Highly crystalline, size-selected silicon (Si) nanocrystals in the size range 2-10 nm were grown in inverse micelles and their optical absorption and photoluminescence (PL) properties were studied. High resolution TEM and electron diffraction results show that these nanocrystals retain their cubic diamond stuctures down to sizes {approximately}4 nm in diameter, and optical absorption data suggest that this structure and bulk-like properties are retained down to the smallest sizes produced ({approximately}1.8 nm diameter containing about 150 Si atoms). High pressure liquid chromatography techniques with on-line optical and electrical diagnostics were developed to purify and separate the clusters into pure, monodisperse populations. The optical absorption revealed features associated with both the indirect and direct bandgap transitions, and these transitions exhibited different quantum confinement effects. The indirect bandgap shifts from 1.1 eV in the bulk to {approximately}2.1 eV for nanocrystals {approximately}2 nm in diameter and the direct transition at r(l_"X - r15) blue shifts by 0.4 eV from its 3.4 eV bulk value over the same size range. Tailorable, visible, room temperature PL in the range 700-350 nm (1.8 - 3.5 eV) was observed from these nanocrystals. The most intense PL was in the violet region of the spectrum ({approximately}400 nm) and is attributed to direct electron-hole recombination. Other less intense PL peaks are attributed to surface state and to indirect bandgap recombination. The results are compared to earlier work on Si clusters grown by other techniques and to the predictions of various model calculations. Currently, the wide variations in the theoretical predictions of the various models along with considerable uncertainties in experimental size determination for clusters less than 3-4 nm, make it difficult to select among competing models.

Band-gap change of carbon nanotubes: Effect of small uniaxial and torsional strain

Abstract: We use a simple picture based on the $\ensuremath{\pi}$ electron approximation to study the band-gap variation of carbon nanotubes with uniaxial and torsional strain. We find (i) that the magnitude of slope of band gap versus strain has an almost universal behavior that depends on the chiral angle, (ii) that the sign of slope depends on the value of $(n\ensuremath{-}m)\mathrm{mod}3,$ and (iii) a novel change in sign of the slope of band gap versus uniaxial strain arising from a change in the value of the quantum number corresponding to the minimum band gap. Four orbital calculations are also presented to show that the $\ensuremath{\pi}$ orbital results are valid.

Femtosecond IR Study of Excited-State Relaxation and Electron-Injection Dynamics of Ru(dcbpy)2(NCS)2 in Solution and on Nanocrystalline TiO2 and Al2O3 Thin Films

Abstract: The photophysics and electron injection dynamics of Ru(dcbpy)2(NCS)2 [dcbpy = (4,4‘-dicarboxy-2,2‘-bipyridine)] (or Ru N3) in solution and adsorbed on nanocrystalline Al2O3 and TiO2 thin films were studied by femtosecond mid-IR spectroscopy. For Ru N3 in ethanol after 400 nm excitation, the long-lived metal-to-ligand charge transfer (3MLCT) excited state with CN stretching bands at 2040 cm-1 was formed in less than 100 fs. No further decay of the excited-state absorption was observed within 1 ns consistent with the previously known 59 ns lifetime. For Ru N3 absorbed on Al2O3, an insulating substrate, the 3MLCT state was also formed in less than 100 fs. In contrast to Ru N3 in ethanol, this excited state decayed by 50% within 1 ns via multiple exponential decay while no ground-state recovery was observed. This decay is attributed to electron transfer to surface states in the band gap of Al2O3 nanoparticles. For Ru N3 adsorbed onto the surface of TiO2, the transient mid-IR signal was dominated by the IR abs...

Effects of Na on the electrical and structural properties of CuInSe2

Abstract: We found theoretically that Na has three effects on CuInSe2: (1) If available in stoichiometric quantities, Na will replace Cu, forming a more stable NaInSe2 compound having a larger band gap (higher open-circuit voltage) and a (112)tetra morphology. The ensuing alloy NaxCu1−xInSe2 has, however, a positive mixing enthalpy, so NaInSe2 will phase separate, forming precipitates. (2) When available in small quantities, Na will form defect on Cu site and In site. Na on Cu site does not create electric levels in the band gap, while Na on In site creates acceptor levels that are shallower than CuIn. The formation energy of Na(InCu) is very exothermic, therefore, the major effect of Na is the elimination of the InCu defects and the resulting increase of the effective hole densities. The quenching of InCu as well as VCu by Na reduces the stability of the (2VCu−+InCu2+), thus suppressing the formation of the “Ordered Defect Compounds.” (3) Na on the surface of CuInSe2 is known to catalyze the dissociation of O2 int...

Excitonic structure and absorption coefficient measurements of ZnO single crystal epitaxial films deposited by pulsed laser deposition

Abstract: The optical properties of high quality single crystal epitaxial zinc oxide thin films grown by pulsed laser deposition on c-plane sapphire substrates were studied. It was found that annealing the films in oxygen dramatically improved the optical and electrical properties. The absorption coefficient, band gap, and exciton binding energies were determined by transmission measurements and photoluminescence. In both the annealed and the as-deposited films excitonic absorption features were observed at both room temperature and 77 K. In the annealed films the excitonic absorption peaks were substantially sharper and deep level photoluminescence was suppressed.

Improving Optical and Charge Separation Properties of Nanocrystalline TiO2 by Surface Modification with Vitamin C

Abstract: The structural and electrochemical properties of nanoparticles were found to be different from those of the corresponding bulk semiconductors. Due to the specific binding of modifiers to “corner defects”, the optical properties of small titania particles were red shifted 1.6 eV compared to unmodified nanocrystallites. It was found using electron paramagnetic resonance (EPR) that, as with organic charge transfer superconductors, these novel nanocrystallites operate with a charge-transfer mechanism, and exhibit semiconducting properties through both constituents (large band gap semiconductor and organic modifier). The EPR spectra were consistent with hole trapping on the surface modifier and electron trapping on shallow interstitial and partially delocalized Ti sites. These systems have an important feature in that charge pairs are instantaneously separated into two phasesthe holes on the donating organic modifier and the electrons in the conduction band of TiO2.

Coupling of InGaN quantum-well photoluminescence to silver surface plasmons

Abstract: The coincidence in excitation energy between surface plasmons on silver and the GaN band gap is exploited to couple the semiconductor spontaneous emission into the metal surface plasmons. A 3-nm InGaN/GaN quantum well (QW) is positioned 12 nm from an 8-nm silver layer, well within the surface plasmon fringing field depth. A spectrally sharp photoluminescence dip, by a factor \ensuremath{\approx}55, indicates that electron-hole energy is being rapidly transferred to plasmon excitation, due to the spatial overlap between the semiconductor QW and the surface plasmon electric field. Thus, spontaneous emission into surface plasmons is \ensuremath{\approx}55 times faster than normal spontaneous emission from InGaN quantum wells. If efficient antenna structures can be incorporated into the metal film, there could be a corresponding increase in external light emission efficiency.

Thickness Dependence of the Optical Properties of Ordered Silica-Air and Air-Polymer Photonic Crystals

Abstract: We report observations of the optical stop band of periodic planar arrays of submicron silica spheres, and of macroporous polymers grown from these silica templates. The stop-band width and peak attenuation depend on the number of layers and on the dielectric contrast between the spheres and the interstitial regions, both of which are experimentally controlled. The results are compared to the predictions of the scalar wave approximation. This is the first systematic study of the thickness dependence of the stop band in colloidal photonic band gap structures.

Spectroscopic and structural characterization of electrochemically grown ZnO quantum dots

Abstract: We report a novel method for the synthesis of stable, OH free zinc oxide quantum dots, using an electrochemical route. The optical properties of these quantum dots were studied at room temperature, by taking the optical absorption and luminescence spectra. The band gap luminescence is predominant in ZnO quantum dots synthesized by the present technique, while the green defect induced luminescence, typical of ZnO, is strongly quenched. The role of defects in photoluminescence emission is discussed.

Electrical and mechanical properties of distorted carbon nanotubes

Abstract: We have calculated the effects of structural distortions of armchair carbon nanotubes on their electronic and electrical properties. We found that the bending of the nanotubes decreases their transmission function in certain energy ranges and leads to an increased electrical resistance. Electronic structure calculations show that these energy ranges contain localized states with significant $\ensuremath{\sigma}\ensuremath{-}\ensuremath{\pi}$ hybridization resulting from the increased curvature produced by bending. Twisting strongly affects the electronic structure of nanotubes (NTs). Normally metallic armchair $(n,n)$ NT's develop a band gap which initially scales linearly with twisting angle and then reaches a constant value. This saturation is associated with a structural transition to a flattened helical structure. The computed values of the twisting energy and of the band gap are strongly affected by allowing structural relaxation in the twisted structures. Finally, our calculations show that the large contact resistances observed for single-wall NT's are likely due to the weak coupling of the NT to the metal in side bonded NT-metal configurations.

Cu2O: Electrodeposition and Characterization

Abstract: Well-defined polycrystalline Cu2O was electrodeposited on transparent conducting substrates from alkaline Cu(II) lactate solutions. The morphology of the layers could be controlled by choosing the appropriate experimental conditions; the pH and temperature of the deposition solution were especially important. The growth of the oxide involves a thermally activated process, for which an activation energy of 0.8 eV was found. A mechanism for the deposition is proposed. The optical absorption of the electrodeposited oxide in the visible range was much weaker than expected for a direct semiconductor with a band gap of 2.0 eV. Finally, we show that it is possible to grow Cu2O inside an n-type TiO2 nanoporous matrix.

Red light emission by photoluminescence and electroluminescence from Eu-doped GaN

Abstract: Visible light emission has been obtained at room temperature by photoluminescence (PL) and electroluminescence (EL) from Eu-doped GaN thin films. The GaN was grown by molecular beam epitaxy on Si substrates using solid sources (for Ga and Eu) and a plasma source for N2. X-ray diffraction shows the GaN:Eu to be a wurtzitic single crystal film. Above GaN band gap photoexcitation with a He–Cd laser at 325 nm resulted in strong red emission. Observed Eu3+ PL transitions consist of a dominant narrow red line at 621 nm and several weaker emission lines were found within the green through red (543 to 663 nm) range. Below band gap PL by Ar laser pumping at 488 nm also resulted in red emission, but with an order of magnitude lower intensity. EL was obtained through use of transparent indium–tin–oxide contacts to the GaN:Eu film. Intense red emission is observed in EL operation, with a spectrum similar to that seen in PL. The dominant red line observed in PL and EL has been identified as the Eu3+ 4f shell transitio...

Significantly improved performance of MOSFETs on silicon carbide using the 15R-SiC polytype

Abstract: Recent studies regarding MOSFETs on SiC reveal that 4H-SiC devices suffer from a low inversion layer mobility, while in 6H-SiC, despite a higher channel mobility the bulk mobility parallel to the c-axis is too low, making this polytype unattractive for power devices. This work presents experimental mobility data of MOSFETs fabricated on different polytypes as well as capacitance-voltage (C-V) measurements of corresponding n-type MOS structures which give evidence that the low inversion channel mobility in 4H-SiC is caused by a high density of SiC-SiO/sub 2/ interface states close to the conduction band. These defects are believed to be inherent to all SiC polytypes and energetically pinned at around 2.9 eV above the valence band edge. Thus, for polytypes with band gaps smaller than 4H-SiC like 6H-SiC and 15R-SiC, the majority of these states will become resonant with the conduction band at room temperature or above, thus remarkably suppressing their negative effect on the channel mobility. In order to realize high performance power MOSFETs the results reveal that 15R-SiC is the best candidate among all currently accessible SiC polytypes.

Surface Synthesis of Zinc Sulfide Nanoparticles on Silica Microspheres: Sonochemical Preparation, Characterization, and Optical Properties

Abstract: Ultrasonic irradiation of a slurry of amorphous silica microspheres, zinc acetate, and thioacetamide in an aqueous medium for 3 h under ambient air yields zinc sulfide coated on silica. The powder X-ray diffraction of the initial zinc sulfide−silica (ZSS) powder yields diffraction peaks corresponding to the ZnS phase. The TEM image of ZSS shows that the porous ZnS nanoparticles (diameter 1−5 nm) coated the silica (SiO2) surface as thin layers or nanoclusters, depending on the reactant concentration. Infrared spectroscopy illustrates the structural changes that occurred in the siloxane network and surface silanol groups of SiO2 upon the ultrasonic deposition of ZnS. The optical absorption of porous ZnS shows a broad band at around 610 nm, ascribed to unusual surface state transition. The absorption energy of the surface state transition is lower than the band gap of the ZnS particles and probably stems from the dangling surface bonds or defects. On the other hand, the ZSS does not show the surface state tr...

Nitrogen-Activated Transitions, Level Repulsion, and Band Gap Reduction in GaAs 1-x N x with x < 0.03

Abstract: We report electroreflectance spectra for a series of GaAs{sub 1{minus}x} N{sub x} samples with x{lt}0.03 . For all samples, the fundamental band gap transition (E{sub 0}) and the transition from the spin-orbit split-off valence band (E{sub 0}+{Delta}{sub 0}) are observed. For samples with x{ge}0.008 , an additional transition (E{sub +}) is observed. With increasing nitrogen content, the increase in E{sub +} is linear in, and nearly equal to, the band gap reduction indicative of a nitrogen-induced level repulsion. The directly observed E{sub +} transition may arise from either a nitrogen-related resonant level or a disorder-activated indirect transition. {copyright} {ital 1999} {ital The American Physical Society}