Showing papers on "Band gap published in 2004"
TL;DR: In this paper, the authors showed that the photogenerated excitons are usually not split by the built-in electric field, which arises from differences in the electrode work functions.
Abstract: Conjugated polymers are attractive semiconductors for photovoltaic cells because they are strong absorbers and can be deposited on flexible substrates at low cost. Cells made with a single polymer and two electrodes tend to be inefficient because the photogenerated excitons are usually not split by the built-in electric field, which arises from differences in the electrode work functions. The efficiency can be increased by splitting the excitons at an interface between two semiconductors with offset energy levels. Power conversion efficiencies of almost 4% have been achieved by blending polymers with electron-accepting materials such as C60 derivatives, cadmium selenide, and titanium dioxide. We predict that efficiencies higher than 10% can be achieved by optimizing the cell's architecture to promote efficient exciton splitting and charge transport and by reducing the band gap of the polymer so that a larger fraction of the solar spectrum can be absorbed.
2,132 citations
TL;DR: It is demonstrated for the first time that impact ionization (II) (the inverse of Auger recombination) occurs with very high efficiency in semiconductor nanocrystals (NCs) and can be used to considerably increase the power conversion efficiency of NC-based solar cells.
Abstract: We demonstrate for the first time that impact ionization (II) (the inverse of Auger recombination) occurs with very high efficiency in semiconductor nanocrystals (NCs). Interband optical excitation of PbSe NCs at low pump intensities, for which less than one exciton is initially generated per NC on average, results in the formation of two or more excitons (carrier multiplication) when pump photon energies are more than 3 times the NC band gap energy. The generation of multiexcitons from a single photon absorption event is observed to take place on an ultrafast (picosecond) time scale and occurs with up to 100% efficiency depending upon the excess energy of the absorbed photon. Efficient II in NCs can be used to considerably increase the power conversion efficiency of NC-based solar cells.
1,732 citations
TL;DR: It was suggested that the specific surface nanostructure was effective for the suppression of recombination between photogenerated electrons and holes and for the separation of H(2) evolution sites from oxidation reaction sites.
Abstract: (AgIn)xZn2(1-x)S2 solid solutions between ZnS photocatalyst with a wide band gap and AgInS2 with a narrow band gap showed photocatalytic activities for H2 evolution from aqueous solutions containing sacrificial reagents, SO32- and S2-, under visible-light irradiation (λ ≥ 420 nm) even without Pt cocatalysts. Loading of the Pt cocatalysts improved the photocatalytic activity. Pt (3 wt %)-loaded (AgIn)0.22Zn1.56S2 with a 2.3 eV band gap showed the highest activity for H2 evolution, and the apparent quantum yield at 420 nm amounted to 20%. H2 gas evolved at a rate of 3.3 L m-2·h-1 under irradiation using a solar simulator (AM 1.5). The diffuse reflection and the photoluminescence spectra of the solid solutions shifted monotonically to a long wavelength side as the ratio of AgInS2 to ZnS increased in the solid solutions. The photocatalytic H2 evolution depended on the compositions as well as the photophysical properties. The dependence of the photophysical and photocatalytic properties upon the composition wa...
758 citations
TL;DR: Wave propagation was found to depend dramatically on both frequency and incident direction, which leads to very large negative refraction, which can be used to focus a diverging ultrasonic beam into a narrow focal spot with a large focal depth.
Abstract: We present a combined experimental and theoretical study of phonon focusing phenomena in a pass band above the complete band gap in a 3D phononic crystal Wave propagation was found to depend dramatically on both frequency and incident direction This propagation anisotropy leads to very large negative refraction, which can be used to focus a diverging ultrasonic beam into a narrow focal spot with a large focal depth The experimental field patterns are well explained using a Fourier imaging technique, based on the 3D equifrequency surfaces calculated from multiple scattering theory
549 citations
TL;DR: It was concluded that the primary factor for the enhanced activity of combustion-synthesized catalyst is a larger amount of surface hydroxyl groups and a lowered band gap.
Abstract: Nanocrystalline TiO2 was synthesized by the solution combustion method using titanyl nitrate and various fuels such as glycine, hexamethylenetetramine, and oxalyldihydrazide. These catalysts are active under visible light, have optical absorption wavelengths below 600 nm, and show superior photocatalytic activity for the degradation of methylene blue and phenol under UV and solar conditions compared to commercial TiO2, Degussa P-25. The higher photocatalytic activity is attributed to the structure of the catalyst. Various studies such as X-ray diffraction, Raman spectroscopy, Brunauer−Emmett−Teller surface area, thermogravimetric−differential thermal analysis, FT-IR spectroscopy, NMR, UV−vis spectroscopy, and surface acidity measurements were conducted. It was concluded that the primary factor for the enhanced activity of combustion-synthesized catalyst is a larger amount of surface hydroxyl groups and a lowered band gap. The lower band gap can be attributed to the carbon inclusion into the TiO2 giving Ti...
518 citations
TL;DR: In this paper, the authors presented low-temperature electronic structure imaging studies of a lightly hole-doped copper oxide: Ca2-xNaxCuO2Cl2.
Abstract: The phase diagram of hole-doped copper oxides shows four different electronic phases existing at zero temperature. Familiar among these are the Mott insulator, high-transition-temperature superconductor and metallic phases. A fourth phase, of unknown identity, occurs at light doping along the zero-temperature bound of the 'pseudogap' regime. This regime is rich in peculiar electronic phenomena, prompting numerous proposals that it contains some form of hidden electronic order. Here we present low-temperature electronic structure imaging studies of a lightly hole-doped copper oxide: Ca2-xNaxCuO2Cl2. Tunnelling spectroscopy (at energies |E| > 100 meV) reveals electron extraction probabilities greatly exceeding those for injection, as anticipated for a doped Mott insulator. However, for |E| < 100 meV, the spectrum exhibits a V-shaped energy gap centred on E = 0. States within this gap undergo intense spatial modulations, with the spatial correlations of a four CuO2-unit-cell square 'checkerboard', independent of energy. Intricate atomic-scale electronic structure variations also exist within the checkerboard. These data are consistent with an unanticipated crystalline electronic state, possibly the hidden electronic order, existing in the zero-temperature pseudogap regime of Ca2-xNaxCuO2Cl2.
505 citations
TL;DR: The lack of difference in the band gap energies for titania nanosheet electrodes with different numbers of layers suggests that a Nanosheet is electronically isolated in multilayer assemblies without affecting the electronic state of neighboring nanosheets.
Abstract: Electrochemical and photoelectrochemical studies were conducted on self-assembled multilayer films of titania nanosheets on a conductive ITO substrate. Cyclic voltammogram (CV) curves indicated that the titania nanosheet electrode underwent insertion/extraction of Li+ ions into/from the nanosheet galleries, associated with reduction/oxidation of Ti4+/Ti3+. These processes accompanied reversible changes in UV−vis absorption of the titania nanosheet electrodes. Applying a negative bias of −1.3 V (vs Ag/Ag+) and lower brought about absorption reduction where the wavelength is shorter than 323 nm, and vice versa, indicating a flat-band potential of (approximately) −1.3 V and a band gap energy of 3.84 eV. Photocurrents were generated from the titania nanosheet electrodes under a positive bias. The onset potential for photocurrent generation from the titania nanosheet electrodes was around −1.27 V, and the band gap energy estimated from the photocurrent action spectra was 3.82 eV, in excellent agreement with th...
493 citations
TL;DR: An intrinsic surface electron accumulation layer is found to exist and is explained in terms of a particularly low Gamma-point conduction band minimum in wurtzite InN, which produces charged donor-type surface states with associated downward band bending.
Abstract: The electronic structure of clean InN(0001) surfaces has been investigated by high-resolution electron-energy-loss spectroscopy of the conduction band electron plasmon excitations. An intrinsic surface electron accumulation layer is found to exist and is explained in terms of a particularly low Gamma-point conduction band minimum in wurtzite InN. As a result, surface Fermi level pinning high in the conduction band in the vicinity of the Gamma point, but near the average midgap energy, produces charged donor-type surface states with associated downward band bending. Semiclassical dielectric theory simulations of the energy-loss spectra and charge-profile calculations indicate a surface state density of 2.5 (+/-0.2)x10(13) cm(-2) and a surface Fermi level of 1.64+/-0.10 eV above the valence band maximum.
472 citations
TL;DR: In this article, the structural, electronic, and optical properties of hydrogen-passivated silicon nanowires along [110] and [111] directions with diameter d up to 4.2 nm from first principles were investigated.
Abstract: We investigate the structural, electronic, and optical properties of hydrogen-passivated silicon nanowires along [110] and [111] directions with diameter d up to 4.2 nm from first principles. The size and orientation dependence of the band gap is investigated and the local-density gap is corrected with the GW approximation. Quantum confinement becomes significant for d<2.2 nm, where the dielectric function exhibits strong anisotropy and new low-energy absorption peaks start to appear in the imaginary part of the dielectric function for polarization along the wire axis.
458 citations
TL;DR: Electrochemical and spectroscopic properties of a well-characterized, synthetically accessible, 1.1 nm diam Au nanoparticle, Au(38), where PhC(2)S is phenylethylthiolate, are described and a working model of the energy level structure is presented.
Abstract: This paper describes electrochemical and spectroscopic properties of a well-characterized, synthetically accessible, 1.1 nm diam Au nanoparticle, Au(38)(PhC(2)S)(24), where PhC(2)S is phenylethylthiolate. Properties of other Au(38) nanoparticles made by exchanging the monolayer ligands with different thiolate ligands are also described. Voltammetry of the Au(38) nanoparticles in CH(2)Cl(2) reveals a 1.62 V energy gap between the first one-electron oxidation and the first reduction. Based on a charging energy correction of ca. 0.29 V, the indicated HOMO-LUMO gap energy is ca. 1.33 eV. At low energies, the optical absorbance spectrum includes peaks at 675 nm (1.84 eV) and 770 nm (1.61 eV) and an absorbance edge at ca. 1.33 eV that gives an optical HOMO-LUMO gap energy that is consistent with the electrochemical estimate. The absorbance at lowest energy is bleached upon electrochemical depletion of the HOMO level. The complete voltammetry contains two separated doublets of oxidation waves, indicating two distinct molecular orbitals, and two reduction steps. The ligand-exchanged nanoparticle Au(38)(PEG(135)S)(13)(PhC(2)S)(11), where PEG(135)S is -SCH(2)CH(2)OCH(2)CH(2)OCH(3), exhibits a broad (1.77-0.89 eV) near-IR photoluminescence band resolvable into maxima at 902 nm (1.38 eV) and 1025 nm (1.2 eV). Much of the photoluminescence occurs at energies less than the HOMO-LUMO gap energy. A working model of the energy level structure of the Au(38) nanoparticle is presented.
416 citations
TL;DR: In this paper, exact analytic expressions for the valence band effective masses in the spin-orbit, and an automated fitting algorithm are used to produce improved parameter sets for Si and Ge at room temperature.
Abstract: Exact, analytic expressions for the valence band effective masses in the spin-orbit, ${\mathrm{sp}}^{3}{d}^{5}{s}^{*}$ empirical tight-binding model are derived. These expressions together with an automated fitting algorithm are used to produce improved parameter sets for Si and Ge at room temperature. Detailed examinations of the analytic effective-mass expressions reveal critical capabilities and limitations of this model in reproducing simultaneously certain gaps and effective masses. The [110] masses are shown to be completely determined by the [100] and [111] masses despite the introduction of $d$ orbitals into the basis.
TL;DR: In this article, the state of the art in using electrodeposition to prepare chalcopyrite absorber layers in thin film solar cells is reviewed and the manufacturability of the corresponding process to large areas is presented.
TL;DR: In this article, the average crystal structure of BaTaO2N is a cubic perovskite, with a Ta−O/N distance of 2.056 A. The optical band gaps are estimated from diffuse reflectance spectra as follows:
Abstract: The syntheses, crystal structures, electrical properties, and optical absorbance spectra of six perovskite oxynitrides, AMO2N (A = Ba, Sr, Ca; M = Ta, Nb) have been investigated. The average crystal structure of BaTaO2N is a cubic perovskite, with a Ta−O/N distance of 2.056 A. SrTaO2N and CaTaO2N are distorted by octahedral tilting, showing noticeably smaller Ta−O/N distances of approximately 2.02 A. Electron diffraction studies of BaTaO2N are consistent with the simple cubic perovskite crystal structure determined using X-ray powder diffraction methods. Each of the niobium oxynitrides is isostructural with its tantalum analogue, though the Nb−O/N distances are observed to be slightly longer. The optical band gaps are estimated from diffuse reflectance spectra as follows: BaTaO2N, 1.8 eV; SrTaO2N, 2.1 eV; CaTaO2N, 2.4 eV; BaNbO2N, 1.8 eV; SrNbO2N, 1.9 eV; CaNbO2N, 2.1 eV. Impedance spectroscopy was carried out on sintered pellets of the ATaO2N and BaNbO2N to investigate the dielectric and electrical tran...
TL;DR: In this paper, a NiO (0.2 wt %)/NaTaO3:La (2%) photocatalyst with a 4.1-eV band gap showed high activity for water splitting.
Abstract: Photocatalysts for water splitting developed by the present authors are reviewed. A NiO (0.2 wt %)/NaTaO3:La (2%) photocatalyst with a 4.1-eV band gap showed high activity for water splitting into ...
01 Jan 2004
TL;DR: In this article, a detailed study of RF-MBE growth conditions for obtaining high-quality InN films is presented, and the full widths at half maximum (FWHMs) of ω-mode X-ray diffraction (XRD), ω 2θ mode XRD and E2 (highfrequency)phonon-mode peaks in the Raman scattering spectrum of the grown layer were 236.7 arcsec, 28.9 arcsec and 3.7 cm -1, respectively.
Abstract: The fundamental band gap of InN has been thought to be about 1.9 eV for a long time. Recent developments of metalorganic vapor phase epitaxy (MOVPE) and RF-molecular beam epitaxy (RF-MBE) growth technologies have made it possible to obtain highquality InN films. A lot of experimental results have been presented very recently, suggesting that the true band-gap energy of InN should be less than 1.0 eV. In this paper, we review the results of the detailed study of RF-MBE growth conditions for obtaining high-quality InN films. The full widths at half maximum (FWHMs) of ω-mode X-ray diffraction (XRD), ω –2θ mode XRD and E2 (high-frequency)phonon-mode peaks in the Raman scattering spectrum of the grown layer were 236.7 arcsec, 28.9 arcsec and 3.7 cm -1 , respectively.
TL;DR: Differential pulse voltammetry (DPV) of TOPO-capped CdTe nanoparticles (NPs) in dichloromethane and a mixture of benzene and acetonitrile showed two anodic and one cathodic peaks of the NPs themselves and an additional anodic peak resulting from the oxidation of reduced NPs.
Abstract: Differential pulse voltammetry (DPV) of TOPO-capped CdTe nanoparticles (NPs) in dichloromethane and a mixture of benzene and acetonitrile showed two anodic and one cathodic peaks of the NPs themselves and an additional anodic peak resulting from the oxidation of reduced NPs. The electrochemical band gap (∼2.1 eV) between the first anodic and cathodic DPV peaks was close to the value (2 eV) obtained from the absorption spectrum. Electrogenerated chemiluminescence (ECL) of CdTe NPs was highly intense for scans into the negative potential region in dichloromethane. The fact that the ECL peak occurs at about the same wavelength as the band-edge photoluminescence (PL) peak indicates that, in contrast to CdSe NPs, the CdTe NPs as synthesized had no deep surface traps that can cause a substantially red shifted ECL.
TL;DR: In this article, the density of states of Li x MPO 4 ( x = 0, 1 and M=Fe, Mn) has been determined with the ab initio generalized gradient approximation (GGA)+U method, appropriate for these correlated electron systems.
TL;DR: The participation of the normally inert A- site cation in the electronic structure near the Fermi level can be considered an inductive effect, as it utilizes substitution on the A-site to directly modify the electronic structures of the SnO(3)(2)(-) framework.
Abstract: Experimental and computational studies were performed to understand the electronic structure of ternary perovskites (ASnO3, A = Ca, Sr, Ba, Cd), pyrochlores (RE2Sn2O7, RE = Y, La, Lu; Cd2Sb2O7), and defect pyrochlore oxides (Ag2Sb2O6) containing the main group ions Sn4+ and Sb5+. In all compounds, the lowest energy states in the conduction band arise primarily from the antibonding Sn/Sb 5s−O 2p interaction. In the alkaline-earth stannate perovskites (BaSnO3, SrSnO3, and CaSnO3) the conduction bandwidth decreases strongly in response to the octahedral tilting distortion triggered by the decreasing size of the alkaline-earth cation. This in turn leads to a corresponding increase in the band gap from 3.1 eV in BaSnO3 to 4.4 eV in CaSnO3. The band gap of CdSnO3 is relatively small (3.0 eV) considering the large octahedral tilting distortion. The origin of this apparent anomaly is the mixing between the empty Cd 5s orbitals and the antibonding Sn 5s−O 2p states. This mixing leads to a widening of the conductio...
TL;DR: This work proposes that the previously observed bleaching of the CNT absorbance spectrum at low pH is most likely a consequence of the oxidation of the nanotubes by oxygen, and demonstrates facile oxidation and reduction of CNTs.
Abstract: UV/vis/NIR absorbance spectra were used to monitor electron transfer between small-molecule redox reagents and carbon nanotubes (CNTs). The oxidation of (6, 5)-enriched nanotubes in water with K(2)Ir(Cl)(6) reveals a valence electron density of 0.2-0.4 e(-)/100 carbon atoms and a reduction potential of approximately 800 mV versus NHE. The reduction potential of CNTs is found to increase with increasing band gap and to decrease with the introduction of an anionic dispersant. In light of this newly revealed redox chemistry of CNTs, we propose that the previously observed bleaching of the CNT absorbance spectrum at low pH is most likely a consequence of the oxidation of the nanotubes by oxygen. These results demonstrate facile oxidation and reduction of CNTs, provide a way to quantify the population of valence electrons, and point to possible applications of CNT in the catalysis of redox reactions.
Patent•
14 Sep 2004TL;DR: In this article, the authors presented a nitride semiconductor device consisting of an active layer of a quantum well structure, a conductive clad layer and a second conductive layer, which was made of the quaternary n-drone semiconductor InAlGaN having a lattice constant equal to or larger than the active layer.
Abstract: The present invention provides a nitride semiconductor device comprising an active layer of a quantum well structure, a first conductive clad layer and a second conductive clad layer. The first conductive clad layer is made of the quaternary nitride semiconductor InAlGaN having a lattice constant equal to or larger than that of the active layer and includes a first nitride semiconductor layer having an energy band gap larger than that of the active layer, a second nitride semiconductor layer having an energy band gap smaller than that of the first nitride semiconductor layer and a third nitride semiconductor layer having an energy band gap larger than that of the second nitride semiconductor layer, sequentially closer to the active layer.
TL;DR: In this article, the electronic structure of bulk ZnO was analyzed using the full potential linearized augmented plane wave method using fatbands, density of states and partial density-of-states.
Abstract: In the present work, we report the synthesis of high quality ZnO nanocrystals with sharp absorption edges in four different sizes, namely 3.0, 3.5, 4.7 and 5.4 nm, characterized by X-ray and electron diffraction, as well as transmission electron microscopy. The bandgaps of these samples, in conjunction with further data from the published literature, exhibit a systematic dependence on the nanocrystal size. In absence of any prior reliable theoretical results in the literature to understand this dependence quantitatively, we have analyzed for the first time, the electronic structure of bulk ZnO obtained from the full potential linearized augmented plane wave method using fatbands, density of states and partial density of states. The crystal orbital Hamiltonian population is obtained from linearized Muffin-Tin orbital band structure calculations to understand the range of hopping interactions relevant for an accurate description of the electronic structure. Using these analyses, a realistic tight binding model is proposed. Based on this model, we calculate the variation of the bandgap with the size of ZnO nanocrystals. These theoretical results agree well with all available data over the entire range of sizes, establishing the effectiveness of this approach.
TL;DR: In this article, the authors derived explicit formulations of the plane harmonic bulk wave and the surface wave dispersion relations in a general phononic structure based on the plane wave expansion method.
Abstract: Successful application of photonic crystals has led recently to a rapidly growing interest in the analogous acoustic effects in periodic elastic structures called phononic crystals. This study is aimed at developing a theory for two-dimensional phononic crystal consisting of materials with general anisotropy. Explicit formulations of the plane harmonic bulk wave and the surface wave dispersion relations in such a general phononic structure are derived based on the plane wave expansion method. Two-dimensional phononic structures with either the square or the hexagonal lattice are considered in the numerical examples. Band gap characteristics of the phononic structures with different anisotropic background materials (isotropic, cubic, hexagonal, and orthorhombic) are calculated and discussed.
TL;DR: In this paper, microstructured silicon surfaces with femtosecond laser irradiation in the presence of SF6 were shown to display strong absorption of infrared radiation at energies below the band gap of crystalline silicon.
Abstract: We microstructured silicon surfaces with femtosecond laser irradiation in the presence of SF6. These surfaces display strong absorption of infrared radiation at energies below the band gap of crystalline silicon. We report the dependence of this below-band gap absorption on microstructuring conditions (laser fluence, number of laser pulses, and background pressure of SF6) along with structural and chemical characterization of the material. Significant amounts of sulfur are incorporated into the silicon over a wide range of microstructuring conditions; the sulfur is embedded in a disordered nanocrystalline layer less than 1 μm thick that covers the microstructures. The most likely mechanism for the below-band gap absorption is the formation of a band of sulfur impurity states overlapping the silicon band edge, reducing the band gap from 1.1 eV to approximately 0.4 eV.
TL;DR: The semiempirical quantum-chemical (ZINDO) method can assign the characteristic bands of the "camel-back" absorption spectrum to one charge transfer state at lower energy localized on the TBT unit, and one delocalized excitonic state at higher energy corresponding to the pi-conjugated electron system.
Abstract: We calculate the electronic states of the low bandgap polyfluorene-based copolymer DiO-PFDTBT, which consists of alternating 9,9-dioctyl-9H-fluorene and 4,7-di-thiophen-2-ylbenzo[1,2,5]thiadiazole (TBT) units, and compare with the steady-state absorption, emission, and excitation spectrum. Using the semiempirical quantum-chemical (ZINDO) method we can assign the characteristic bands of the “camel-back” absorption spectrum to one charge transfer state at lower energy localized on the TBT unit, and one delocalized excitonic state at higher energy corresponding to the π-conjugated electron system. Additional “dark” charge transfer states in the gap between these bands have been revealed. Calculations are also made on the red light emitting polyfluorene-based copolymer poly(fluorene-co-benzothiadiazole) (F8BT), which contains benzo[1,2,5]thiadiazole instead of TBT. The nature of the electronic states in F8BT and DiO-PFDTBT are found to be qualitatively the same.
TL;DR: Fluorescence emission, electron paramagnetic resonance, and X-ray absorption spectroscopy provide evidence for the presence of Mn in the interior as well as on the surface of the nanocrystals of Mn-doped ZnO.
Abstract: We report the synthesis and characterization of several sizes of Mn-doped ZnO nanocrystals, both in the free-standing and the capped particle forms. The sizes of these nanocrystals could be controlled by capping them with polyvinylpyrollidone under different synthesis conditions and were estimated by X-ray diffraction and transmission electron microscopy. The absorption properties of PVP-capped Mn-doped ZnO exhibit an interesting variation of the band gap with the concentration of Mn. Fluorescence emission, electron paramagnetic resonance, and X-ray absorption spectroscopy provide evidence for the presence of Mn in the interior as well as on the surface of the nanocrystals.
TL;DR: In this paper, a cascaded energy transfer structure made of semiconductor nanocrystals is proposed, where optical excitations efficiently transfer along the band gap gradient and are finally captured by the largest nanocrystal.
Abstract: We propose a cascaded energy transfer structure made of semiconductor nanocrystals. Funnel-like band gap profiles are realized applying layer-by-layer assembly to CdTe nanocrystals of distinct sizes. Optical excitations efficiently transfer along the band gap gradient and are finally captured by the largest nanocrystals. The photoluminescence yield from the center layer is surprisingly high. The stepwise passing on of excitation energy avoids hot carriers, and the funnel structure recycles otherwise trapped and lost electron−hole pairs.
TL;DR: Yellow SrTiO3 powders codoped with nitrogen and lanthanum (STO:N,La) were studied as visible light photocatalysts and showed high activity due to the decrease in the oxygen vacancies, which acted as electron-hole recombination centers and codoping with La3+ and N3- ions maintained the charge balance.
Abstract: Yellow SrTiO3 powders codoped with nitrogen and lanthanum (STO:N,La) were studied as visible light photocatalysts. The crystal phase of STO:N,La exhibited a pure perovskite phase, and O and Sr sites atoms were substitutionally doped with N and La atoms, respectively. The first principle calculation of STO:N,La indicated that the edge of the N(2p) band is situated above the valence band, which consisted of O(2p) orbitals, and the La orbitals did not exist in the band gap of SrTiO3. STO:N,La exhibited a higher oxidation activity of gaseous 2-propanol under vis illumination than SrTiO3 doped only with nitrogen (STO:N). The high activity of STO:N,La was due to the decrease in the oxygen vacancies, which acted as electron-hole recombination centers, because codoping with La3+ and N3- ions maintained the charge balance. The optimum doping density of N and La for visible light activity was 0.5%, and STO:N,La(0.5%) had an activity under UV illumination similar to pure SrTiO3.
TL;DR: Evidence of the influence of the Aharonov-Bohm phase on the band gap of a solid is reported, confirming theoretical predictions that the band structure of a single-walled carbon nanotube is dependent on the magnetic flux ϕ threading the tube.
Abstract: We report interband magneto-optical spectra for single-walled carbon nanotubes in high magnetic fields up to 45 tesla, confirming theoretical predictions that the band structure of a single-walled carbon nanotube is dependent on the magnetic flux ϕ threading the tube. We have observed field-induced optical anisotropy as well as red shifts and splittings of absorption and photoluminescence peaks. The amounts of shifts and splittings depend on the value of ϕ/ϕ and are quantitatively consistent with theories based on the Aharonov-Bohm effect. These results represent evidence of the influence of the Aharonov-Bohm phase on the band gap of a solid.
TL;DR: In this paper, the lattice constants of pure and Mg doped ZnO thin films were investigated using an x-ray diffractometer (XRD) to investigate the structural properties of the thin films, and the peak position of (002) orientation was found to shift from 34.39° to 34.55°.
Abstract: Pure and Mg doped ZnO thin films were deposited at 400 °C on glass substrates by pulsed laser deposition. An x-ray diffractometer (XRD) was used to investigate the structural properties of the thin films. It is found that all the thin films have a preferred (002) orientation. The peak position of (002) orientation is found to shift from 34.39° to 34.55°. The lattice constants of ZnO thin films were also obtained from XRD data. It is found that, with the increase of the dopant concentration, the lattice constant a decreases from 3.25 to 3.23 A, and c decreases from 5.20 to 5.16 A. From the spectrophotometer transmittance data, the band gap energies of the thin films were calculated by a linear fitting process. The band gap energy of Mg doped ZnO thin film increases with increasing dopant concentration. In photoluminescence (PL) spectra, two PL emission peaks are found in pure ZnO thin films, one is the near band edge (NBE) emission at 3.28 eV, and the other is green-yellow-red emission at around 2.4 eV. However, with the increase of the dopants, no green-yellow-red emissions are found in PL of Mg doped ZnO thin films. The NBE emission has a blueshift compared with that of pure ZnO thin film (as much as 0.12 eV). As time goes on, NBE emission in pure ZnO thin film is enhanced, and the green-yellow-red emissions disappear.
TL;DR: Spectroelectrochemical experiments reveal that compounds T1-T4 are stable electrochromic systems which change their color reversibly from colorless in the neutral state to colored (from red to purple color; approximately 500-600 nm) in the oxidized state.
Abstract: This paper describes the strategy toward novel monodisperse, well-defined, star-shaped oligofluorenes with a central truxene core and from monofluorene to quaterfluorene arms. Introduction of solubilizing n-hexyl groups at both fluorene and truxene moieties results in highly soluble, intrinsically two-dimensional nanosized macromolecules T1-T4. The radius for the largest oligomer of ca. 3.9 nm represents one of the largest known star-shaped conjugated systems. Cyclic voltammetry experiments reveal reversible or quasi-reversible oxidation and reduction processes (Eox = +0.74 to 0.80 V, Ered = -2.66 to 2.80 eV vs Fc/Fc+), demonstrating excellent electrochemical stability toward both p- and n-doping, while the band gaps of the oligomers are quite high (EgCV = 3.20-3.40 eV). Close band gaps of 3.05-3.29 eV have been estimated from the electron absorption spectra. These star-shaped macromolecules demonstrate good thermal stability (up to 400-420 degrees C) and improved glass transition temperatures with an increase in length of the oligofluorene arms (from Tg = 63 degrees C for T1 to 116 degrees C for T4) and show very efficient blue photoluminescence (lambdaPL = 398-422 nm) in both solution (PhiPL = 70-86%) and solid state (PhiPL = 43-60%). Spectroelectrochemical experiments reveal that compounds T1-T4 are stable electrochromic systems which change their color reversibly from colorless in the neutral state (approximately 340-400 nm) to colored (from red to purple color; approximately 500-600 nm) in the oxidized state.