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Showing papers on "Band gap published in 2006"


Journal ArticleDOI
TL;DR: The authors' ab initio calculations show that the origin of energy gaps for GNRs with armchair shaped edges arises from both quantum confinement and the crucial effect of the edges, which differs from the results of simple tight-binding calculations or solutions of the Dirac's equation based on them.
Abstract: Based on a first-principles approach, we present scaling rules for the band gaps of graphene nanoribbons (GNRs) as a function of their widths. The GNRs considered have either armchair or zigzag shaped edges on both sides with hydrogen passivation. Both varieties of ribbons are shown to have band gaps. This differs from the results of simple tight-binding calculations or solutions of the Dirac's equation based on them. Our ab initio calculations show that the origin of energy gaps for GNRs with armchair shaped edges arises from both quantum confinement and the crucial effect of the edges. For GNRs with zigzag shaped edges, gaps appear because of a staggered sublattice potential on the hexagonal lattice due to edge magnetization. The rich gap structure for ribbons with armchair shaped edges is further obtained analytically including edge effects. These results reproduce our ab initio calculation results very well.

4,471 citations


Journal ArticleDOI
TL;DR: According to the extrapolated inverse power law obtained in this work, armchair carbon nanoribbons of widths larger than 8 nm will present a maximum band gap of 0.3 eV, while for ribbons with a width of 80 nm the maximum possible band gap is 0.05 eV.
Abstract: We present a systematic density functional theory study of the electronic properties, optical spectra, and relative thermodynamic stability of semiconducting graphene nanoribbons. We consider ribbons with different edge nature including bare and hydrogen-terminated ribbons, several crystallographic orientations, and widths up to 3 nm. Our results can be extrapolated to wider ribbons providing a qualitative way of determining the electronic properties of ribbons with widths of practical significance. We predict that in order to produce materials with band gaps similar to Ge or InN, the width of the ribbons must be between 2 and 3 nm. If larger bang gap ribbons are needed (like Si, InP, or GaAs), their width must be reduced to 1-2 nm. According to the extrapolated inverse power law obtained in this work, armchair carbon nanoribbons of widths larger than 8 nm will present a maximum band gap of 0.3 eV, while for ribbons with a width of 80 nm the maximum possible band gap is 0.05 eV. For chiral nanoribbons the band gap oscillations rapidly vanish as a function of the chiral angle indicating that a careful design of their crystallographic nature is an essential ingredient for controlling their electronic properties. Optical excitations show important differences between ribbons with and without hydrogen termination and are found to be sensitive to the carbon nanoribbon width. This should provide a practical way of revealing information on their size and the nature of their edges.

1,557 citations


Journal ArticleDOI
TL;DR: In this paper, a tight-binding model is used to calculate the band structure of bilayer graphene in the presence of a potential difference between the layers that opens a gap between the conduction and valence bands.
Abstract: A tight-binding model is used to calculate the band structure of bilayer graphene in the presence of a potential difference between the layers that opens a gap $\ensuremath{\Delta}$ between the conduction and valence bands. In particular, a self-consistent Hartree approximation is used to describe imperfect screening of an external gate, employed primarily to control the density $n$ of electrons on the bilayer, resulting in a potential difference between the layers and a density dependent gap $\ensuremath{\Delta}(n)$. We discuss the influence of a finite asymmetry gap $\ensuremath{\Delta}(0)$ at zero excess density, caused by the screening of an additional transverse electric field, on observations of the quantum Hall effect.

1,104 citations


Journal ArticleDOI
TL;DR: Impedance spectroscopy was applied to investigate the characteristics of dye-sensitized nanostructured TiO2 solar cells with high efficiencies of light to electricity conversion of 11.1% and 10.2%, allowing a separate analysis of the contribution of different resistive processes to the overall conversion efficiency.
Abstract: Impedance spectroscopy was applied to investigate the characteristics of dye-sensitized nanostructured TiO 2 solar cells (DSC) with high efficiencies of light to electricity conversion of 11.1% and 10.2%. The different parameters, that is, chemical capacitance, steady-state transport resistance, transient diffusion coefficient, and charge-transfer (recombination) resistance, have been interpreted in a unified and consistent framework, in which an exponential distribution of the localized states in the TiO 2 band gap plays a central role. The temperature variation of the chemical diffusion coefficient dependence on the Fermi-level position has been observed consistently with the standard multiple trapping model of electron transport in disordered semiconductors. A Tafel dependence of the recombination resistance dependence on bias potential has been rationalized in terms of the charge transfer from a distribution of surface states using the Marcus model of electron transfer. The current-potential curve of the solar cells has been independently constructed from the impedance parameters, allowing a separate analysis of the contribution of different resistive processes to the overall conversion efficiency.

1,049 citations


Journal ArticleDOI
Nick Serpone1
TL;DR: It is argued that the red-shift of the absorption edge is in fact due to formation of the color centers, and that while band gap narrowing is not an unknown occurrence in semiconductor physics it does necessitate heavy doping of the metal oxide semiconductor, thereby producing materials that may have completely different chemical compositions from that of TiO(2) with totally different band gap electronic structures.
Abstract: Second-generation TiO2-xDx photocatalysts doped with either anions (N, C, and S mostly) or cations have recently been shown to have their absorption edge red-shifted to lower energies (longer wavelengths), thus enhancing photonic efficiencies of photoassisted surface redox reactions. Some of the studies have proposed that this red-shift is caused by a narrowing of the band gap of pristine TiO2 (e.g., anatase, Ebg = 3.2 eV; absorption edge ca. 387 nm), while others have suggested the appearance of intragap localized states of the dopants. By contrast, a recent study by Kuznetsov and Serpone (J. Phys. Chem. B, in press) has proposed that the commonality in all these doped titanias rests with formation of oxygen vacancies and the advent of color centers (e.g., F, F+, F++, and Ti3+) that absorb the visible light radiation. This article reexamines the various claims and argues that the red-shift of the absorption edge is in fact due to formation of the color centers, and that while band gap narrowing is not an...

999 citations


Journal ArticleDOI
TL;DR: These results provide a characterization of the electronic states associated with N impurities in TiO2 and, for the first time, a picture of the processes occurring in the solid under irradiation with visible light.
Abstract: Nitrogen-doped titanium dioxide (N-TiO2), a photocatalytic material active in visible light, has been investigated by a combined experimental and theoretical approach. The material contains single-atom nitrogen impurities that form either diamagnetic (Nb-) or paramagnetic (Nb•) bulk centers. Both types of Nb centers give rise to localized states in the band gap of the oxide. The relative abundance of these species depends on the oxidation state of the solid, as, upon reduction, electron transfer from Ti3+ ions to Nb• results in the formation of Ti4+ and Nb-. EPR spectra measured under irradiation show that Nb centers are responsible for visible light absorption with promotion of electrons from the band gap localized states to the conduction band or to surface-adsorbed electron scavengers. These results provide a characterization of the electronic states associated with N impurities in TiO2 and, for the first time, a picture of the processes occurring in the solid under irradiation with visible light.

820 citations


Journal ArticleDOI
TL;DR: In this article, the electro-optical and photocatalytic properties of the synthesized TiO2 nanoparticles were studied along with several commercially available ultra-fine TiO 2 particles (e.g., 3.8-5.7nm).
Abstract: Anatase TiO2 nanocrystallines (17–29 nm) were successfully synthesized by the metal–organic chemical vapor deposition method (MOCVD). Moderate manipulation of system parameters of MOCVD can control the particle size. The electro-optical and photocatalytic properties of the synthesized TiO2 nanoparticles were studied along with several commercially available ultra-fine TiO2 particles (e.g., 3.8–5.7 nm). The band gap of the TiO2 crystallines was determined using the transformed diffuse reflectance technique according to the Kubelka–Munk theory. Results showed that the band gap of TiO2 monotonically decreased from 3.239 to 3.173 eV when the particle size decreased from 29 to 17 nm and then increased from 3.173 to 3.289 eV as the particle size decreased from 17 to 3.8 nm. The results of band gap change as a function of particle size agreed well with what was predicted by the Brus’ equation, i.e., the effective mass model (EMM). However, results of the photocatalytic oxidation of 2-chlorophenol (2-CP), showed that the smaller the particle size, the faster the degradation rate. This is attributed in part to the combined effect of band gap change relative to the spectrum of the light source and the specific surface area (or particle size) of the photocatalysts. The change of band gap due to particle size represents only a small optical absorption window with respect to the total spectrum of the light source, i.e., from 380 to 400 nm versus >280 nm. Consequently, the gain in optical property of the larger particles was severely compromised by their decrease in specific surface area. Our results clearly indicated the importance of specific surface area in controlling the photocatalytic reactivity of photocatalysts. Results also showed that the secondary particle size grew with time due mainly to particle aggregation. The photocatalytic rate constants decreased exponentially with increase in primary particle size. Primary particle size alone is able to predict the photocatalytic rate as it is closely related to the electro-optical properties of photocatalysts.

768 citations


Journal ArticleDOI
TL;DR: In this article, the electronic structure of benzene on graphite (0001) is computed using the $GW$ approximation for the electron self-energy, and the benzene quasiparticle energy gap is predicted to be 7.2 eV, substantially reduced from its calculated gas phase value of 10.5 eV.
Abstract: The electronic structure of benzene on graphite (0001) is computed using the $GW$ approximation for the electron self-energy. The benzene quasiparticle energy gap is predicted to be 7.2 eV on graphite, substantially reduced from its calculated gas-phase value of 10.5 eV. This decrease is caused by a change in electronic correlation energy, an effect completely absent from the corresponding Kohn-Sham gap. For weakly coupled molecules, this correlation energy change can be described as a surface polarization effect. A classical image potential model illustrates the impact for other conjugated molecules on graphite.

730 citations


Journal ArticleDOI
TL;DR: Nitrogen doping-induced changes in the electronic properties, defect formation, and surface structure of TiO2 rutile(110) and anatase(101) single crystals were investigated and thermal instability may degrade the catalyst during applications.
Abstract: Nitrogen doping-induced changes in the electronic properties, defect formation, and surface structure of TiO2 rutile(110) and anatase(101) single crystals were investigated. No band gap narrowing is observed, but N doping induces localized N 2p states within the band gap just above the valence band. N is present in a N(III) valence state, which facilitates the formation of oxygen vacancies and Ti 3d band gap states at elevated temperatures. The increased O vacancy formation triggers the 1 x 2 reconstruction of the rutile (110) surface. This thermal instability may degrade the catalyst during applications.

639 citations


Journal ArticleDOI
TL;DR: In this article, the band offsets of various gate dielectrics including HfO2, Al2O3, Gd2O 3, Si3N4, and SiO2 on III-V semiconductors such as GaAs, InAs, GaSb, and GaN have been calculated using the method of charge neutrality levels.
Abstract: III-V semiconductors have high mobility and will be used in field effect transistors with the appropriate gate dielectric. The dielectrics must have band offsets over 1eV to inhibit leakage. The band offsets of various gate dielectrics including HfO2, Al2O3, Gd2O3, Si3N4, and SiO2 on III-V semiconductors such as GaAs, InAs, GaSb, and GaN have been calculated using the method of charge neutrality levels. Generally, the conduction band offsets are found to be over 1eV, so they should inhibit leakage for these dielectrics. On the other hand, SrTiO3 has minimal conduction band offset. The valence band offsets are also reasonably large, except for Si nitride on GaN and Sc2O3 on GaN which are 0.6–0.8eV. There is reasonable agreement with experiment where it exists, although the GaAs:SrTiO3 case is even worse in experiment.

632 citations


Journal ArticleDOI
TL;DR: In this article, the potential of poly(3-hexylthiophene) and 6,6-phenyl C61-butyric acid methyl ester (PCBM) polymeric solar cells was explored.
Abstract: We present model calculations to explore the potential of polymer/fullerene bulk heterojunction solar cells. As a starting point, devices based on poly(3-hexylthiophene) and 6,6-phenyl C61-butyric acid methyl ester (PCBM), reaching 3.5% efficiency, are modeled. Lowering the polymeric band gap will lead to a device efficiency exceeding 6%. Tuning the electronic levels of PCBM in such a way that less energy is lost in the electron transfer process enhances the efficiency to values in excess of 8%. Ultimately, with an optimized level tuning, band gap, and balanced mobilities polymeric solar cells can reach power conversion efficiencies approaching 11%.

Journal Article
TL;DR: The benzene quasiparticle energy gap is predicted to be 7.2 eV on graphite, substantially reduced from its calculated gas-phase value of 10.5 eV, an effect completely absent from the corresponding Kohn-Sham gap.
Abstract: The electronic structure of benzene on graphite (0001) is computed using the $GW$ approximation for the electron self-energy. The benzene quasiparticle energy gap is predicted to be 7.2 eV on graphite, substantially reduced from its calculated gas-phase value of 10.5 eV. This decrease is caused by a change in electronic correlation energy, an effect completely absent from the corresponding Kohn-Sham gap. For weakly coupled molecules, this correlation energy change can be described as a surface polarization effect. A classical image potential model illustrates the impact for other conjugated molecules on graphite.

Journal ArticleDOI
TL;DR: In this article, two sub-band-gap energy photons are absorbed simultaneously in an intermediate-band solar cell and one photon produces an optical transition from the intermediate to the conduction band while the second pumps an electron from the valence band to the intermediate band.
Abstract: We present intermediate-band solar cells manufactured using quantum dot technology that show for the first time the production of photocurrent when two sub-band-gap energy photons are absorbed simultaneously. One photon produces an optical transition from the intermediate-band to the conduction band while the second pumps an electron from the valence band to the intermediate-band. The detection of this two-photon absorption process is essential to verify the principles of operation of the intermediate-band solar cell. The phenomenon is the cornerstone physical principle that ultimately allows the production of photocurrent in a solar cell by below band gap photon absorption, without degradation of its output voltage.

Journal ArticleDOI
TL;DR: In this article, a polycrystalline thin films of tin sulphide have been synthesized using spray pyrolysis, which had resistivities ∼30 cm with an optical energy band gap (E g ) of 1.32 eV.

Journal ArticleDOI
TL;DR: It is experimentally demonstrated that using semiconductor nanocrystals the authors can reduce this energy loss to a nearly absolute minimum allowed by energy conservation by producing multiple excitons per single photon.
Abstract: The performance of photovoltaic and photochemical devices is directly linked to the efficiency with which absorbed photons are converted into electron−hole pairs (excitons). A usual assumption is that one photon produces a single exciton, while the photon energy in the excess of the material's energy gap (the gap that separates the conduction from the valence band) is wasted as heat. Here we experimentally demonstrate that using semiconductor nanocrystals we can reduce this energy loss to a nearly absolute minimum allowed by energy conservation by producing multiple excitons per single photon. Specifically, we generate seven excitons from a photon with an energy of 7.8 energy gaps, which corresponds to only ∼10% energy loss, while in the normal scenario (one photon produces one exciton) ∼90% of the photon energy would be dissipated as heat. Such large yields of charge carriers (photon-to-exciton conversion efficiency up to 700%) has the potential to dramatically increase the performance of photovoltaic ce...

Journal ArticleDOI
TL;DR: In this paper, the generalized gradient approximation (GGA) with empirical self-interaction corrections was applied to correct for the overestimation of covalency intrinsic to GGA-DFT calculations.
Abstract: Density-functional theory (DFT) calculations of intrinsic point defect properties in zinc oxide were performed in order to remedy the influence of finite-size effects and the improper description of the band structure The generalized gradient approximation (GGA) with empirical self-interaction corrections $(\mathrm{GGA}+U)$ was applied to correct for the overestimation of covalency intrinsic to GGA-DFT calculations Elastic as well as electrostatic image interactions were accounted for by application of extensive finite-size scaling and compensating charge corrections Size-corrected formation enthalpies and volumes as well as their charge state dependence have been deduced Our results partly confirm earlier calculations but reveal a larger number of transition levels: (1) For both the zinc interstitial as well as the oxygen vacancy, transition levels are close to the conduction band minimum (2) The zinc vacancy shows a transition rather close to the valence band maximum and another one near the middle of the calculated band gap (3) For the oxygen interstitials, transition levels occur both near the valence band maximum and the conduction band minimum

Journal ArticleDOI
TL;DR: In this paper, a series of Cd 1 - x Zn x S ( x = 0 -0.92) photocatalysts were prepared by coprecipitation method and were calcined at 723 K under N 2 atmosphere.

Journal ArticleDOI
TL;DR: The k-space structure of the lowest conduction band of LiZnSb is analyzed in detail, and excellent thermoelectric properties are expected for this material.
Abstract: An automated band structure calculation based on the inorganic crystal structure database and the augmented plane wave method for electronic structure calculations is presented. Using a rigid band approach and semiclassic Boltzmann theory the band structures are analyzed and a large number of compounds are screened for potential interesting thermoelectric properties. We thereby propose LiZnSb as a potential new thermoelectric material. The k-space structure of the lowest conduction band of LiZnSb is analyzed in detail, and excellent thermoelectric properties are expected for this material. Furthermore the lattice dynamics are calculated, and anisotropic lattice thermal conduction is predicted.

Journal ArticleDOI
TL;DR: The propagation of acoustic waves in a phononic crystal slab consisting of piezoelectric inclusions placed periodically in an isotropic host material is analyzed and it is observed that the band gaps of a phononics crystal slab are distinct from those of bulk acoustic waves propagating in the plane of an infinite two-dimensional phononic Crystal.
Abstract: The propagation of acoustic waves in a phononic crystal slab consisting of piezoelectric inclusions placed periodically in an isotropic host material is analyzed. Numerical examples are obtained for a square lattice of quartz cylinders embedded in an epoxy matrix. It is found that several complete band gaps with a variable bandwidth exist for elastic waves of any polarization and incidence. In addition to the filling fraction, it is found that a key parameter for the existence and the width of these complete band gaps is the ratio of the slab thickness, d, to the lattice period, a. Especially, we have explored how these absolute band gaps close up as the parameter d/a increases. Significantly, it is observed that the band gaps of a phononic crystal slab are distinct from those of bulk acoustic waves propagating in the plane of an infinite two-dimensional phononic crystal with the same composition. The band gaps of the slab are strongly affected by the presence of cutoff frequency modes that cannot be excited in infinite media.

Journal ArticleDOI
TL;DR: In this article, the structural, optical and electrical properties of Co3O4 thin films have been investigated by means of X-ray diffraction, scanning electron micrograph (SEM), optical absorption and electrical resistivity measurements.

Journal ArticleDOI
TL;DR: In this paper, the authors compared the properties of carbon nanoribbons and carbon nanotubes and found that the low-and high-field mobilities of the carbon nanobbons are significantly higher than those of silicon and thus are better suited for low power applications.
Abstract: Electronic properties of graphene (carbon) nanoribbons are studied and compared to those of carbon nanotubes. The nanoribbons are found to have qualitatively similar electron band structure which depends on chirality but with a significantly narrower band gap. The low- and high-field mobilities of the nanoribbons are evaluated and found to be higher than those of carbon nanotubes for the same unit cell but lower at matched band gap or carrier concentration. Due to the inverse relationship between mobility and band gap, it is concluded that graphene nanoribbons operated as field-effect transistors must have band gaps <0.5eV to achieve mobilities significantly higher than those of silicon and thus may be better suited for low power applications.

Journal ArticleDOI
TL;DR: The all-electron GW approximation energy band gap of bulk hexagonal boron nitride is shown to be of indirect type and a detailed analysis of the excitonic structures within the band gap shows that the low-lying excitons belong to the Frenkel class and are tightly confined within the layers.
Abstract: The all-electron $GW$ approximation energy band gap of bulk hexagonal boron nitride is shown to be of indirect type. The resulting computed in-plane polarized optical spectrum, obtained by solving the Bethe-Salpeter equation for the electron-hole two-particle Green function, is in excellent agreement with experiment and has a strong anisotropy compared to out-of-plane polarized spectrum. A detailed analysis of the excitonic structures within the band gap shows that the low-lying excitons belong to the Frenkel class and are tightly confined within the layers. The calculated exciton binding energy is much larger than that obtained by Watanabe et al. [Nat. Mater. 3, 404 (2004).] based on a Wannier model assuming $h\mathrm{\text{\ensuremath{-}}}\mathrm{BN}$ to be a direct-band-gap semiconductor.

Journal ArticleDOI
TL;DR: In this article, perovskite-type polycrystalline BiFeO3 (BFO) nanowires were synthesized using the anodized alumina template technique.
Abstract: Perovskite-type polycrystalline BiFeO3 (BFO) nanowires (∼50nm in diameter and ∼5μm in length) were synthesized using the anodized alumina template technique. An energy band gap of ∼2.5eV was determined from the UV-visible diffuse reflectance spectrum, and its photocatalytic ability to produce O2 was revealed under UV irradiation. Weak ferromagnetism at room temperature and superparamagnetism at low temperature were observed for the BFO nanowires, different from the antiferromagnetic order in bulk BFO, reflecting the significant size effects on the magnetic ordering of BFO.

Journal ArticleDOI
TL;DR: In this paper, the size-dependent band gap of colloidal quantum dots is calculated using a recently developed method that predicts accurately the eigenstates and eigenenergies of nanostructures by utilizing the adiabatic theorem of quantum mechanics.
Abstract: The size-dependent band gap of semiconductor quantum dots is a well-known and widely studied quantum confinement effect. In order to understand the size-dependent band gap, different theoretical approaches have been adopted, including the effective-mass approximation with infinite or finite confinement potentials, the tight-binding method, the linear combination of atomic orbitals method, and the empirical pseudopotential method. In the present work we calculate the size-dependent band gap of colloidal quantum dots using a recently developed method that predicts accurately the eigenstates and eigenenergies of nanostructures by utilizing the adiabatic theorem of quantum mechanics. We have studied various semiconductor (CdS, CdSe, CdTe, PbSe, InP, and InAs) quantum dots in different matrices. The theoretical predictions are, in most cases, in good agreement with the corresponding experimental data. In addition, our results indicate that the height of the finite-depth well confining potential is independent ...

Journal ArticleDOI
TL;DR: Extrapolation of HOMO-LUMO gaps at the B3LYP/6-31G(d) level of theory predict accurately (within 0.1-0.2 eV) the band gaps of conjugated polymers only when long (at least 20-mer) pi-conjugated oligomers are used for the extrapolation.

Journal ArticleDOI
TL;DR: A complete surface acoustic wave band gap is found experimentally in a two-dimensional square-lattice piezoelectric phononic crystal etched in lithium niobate, in good agreement with theoretical predictions.
Abstract: A complete surface acoustic wave band gap is found experimentally in a two-dimensional square-lattice piezoelectric phononic crystal etched in lithium niobate. Propagation in the phononic crystal is studied by direct generation and detection of surface waves using interdigital transducers. The complete band gap extends from 203 to 226 MHZ, in good agreement with theoretical predictions. Near the upper edge of the complete band gap, it is observed that radiation to the bulk of the substrate dominates. This observation is explained by introducing the concept of the sound line.

Journal ArticleDOI
TL;DR: The pyrolytic synthesis of homogeneously alloyed CdS(x)Se(1-x) nanocrystals in all proportions is reported, yielding a bowing constant of 0.29, in agreement with bulk values.
Abstract: Alloy nanocrystals provide an additional degree of freedom in selecting desirable properties for nanoscale engineering because their physical and optical properties depend on both size and composition. We report the pyrolytic synthesis of homogeneously alloyed CdSxSe1-x nanocrystals in all proportions. The nanocrystals are characterized using UV−visible absorption spectroscopy, transmission electron microscopy, X-ray diffractrometry, and Rutherford backscattering spectrometry to determine precisely structure, size, and composition. The dependence of band gap on nanocrystal size and composition is elucidated, yielding a bowing constant of 0.29, in agreement with bulk values. In addition, the morphology of the resultant nanocrystals can be altered by changing the reaction conditions, generating structures ranging from homogeneous, spherical nanocrystals to one-dimensional gradient nanorods.

Journal ArticleDOI
TL;DR: The optical detection of DNA hybridization on the surface of solution suspended single-walled carbon nanotubes (SWNTs) through a SWNT band gap fluorescence modulation is demonstrated, enabling selective detection of specific DNA sequences.
Abstract: We demonstrate the optical detection of DNA hybridization on the surface of solution suspended single-walled carbon nanotubes (SWNTs) through a SWNT band gap fluorescence modulation. Hybridization of a 24-mer oligonucleotide sequence with its complement produces a hypsochromic shift of 2 meV, with a detection sensitivity of 6 nM. The energy shift is modeled by correlating the surface coverage of DNA on SWNT to the exciton binding energy, yielding an estimated initial fractional coverage of 0.25 and a final coverage of 0.5. Hybridization on the nanotube surface is confirmed using Forster resonance energy transfer of fluorophore-labeled DNA oligonucleotides. This detection is enabled through a new technique to suspend SWNTs using adsorption of single-stranded DNA and subsequent removal of free DNA from solution. While the kinetics of free DNA hybridization are relatively fast (<10 min), the kinetics of the process on SWNTs are slower under comparable conditions, reaching steady state after 13 h at 25 degrees C. A second-order kinetic model yields a rate constant of k = 4.33 x 10(5) (M h)(-1). This optical, selective detection of specific DNA sequences may have applications in the life sciences and medicine as in vitro or in vivo detectors of oligonucleotides.

Journal ArticleDOI
TL;DR: ZnO nanocrystals doped with Cd, Mg, Mn, and Fe ions were obtained by thermolysis of a family of metal cupferrates and are sufficiently defect-free to exhibit band edge luminescence.
Abstract: ZnO nanocrystals doped with Cd, Mg, Mn, and Fe ions were obtained by thermolysis of a family of metal cupferrates. The nanocrystals were characterized by X-ray diffraction, energy-dispersive X-ray analysis, transmission electron microscopy, UV-visible, luminescence, and excitation spectroscopy. The band gap of the nanocrystals can be tuned in the range of 2.9-3.8 eV by the use of the dopants. In most cases, the nanocrystals are sufficiently defect-free to exhibit band edge luminescence.

Journal ArticleDOI
TL;DR: In this paper, the structural and electronic properties of wurtzite MgO, ZnO, and CdO were investigated using first-principles methods based on density functional theory within the local density approximation (LDA).
Abstract: Using first-principles methods based on density functional theory within the local density approximation (LDA) we calculate the structural and electronic properties of wurtzite MgO, ZnO, and CdO, and discuss their similarities and dissimilarities with the corresponding Group-III nitrides AlN, GaN, and InN. We treat the semicore $d$ states of Zn, Cd, Ga, and In explicitly as valence states in a pseudopotential approach, investigate the effects of including on-site Coulomb interaction for Zn, Cd, Ga, and In semicore $d$ states within the $\mathrm{LDA}+U$ method, and propose a novel approach to calculate the parameter $U$. Our results show that the $\mathrm{LDA}+U$ approach systematically improves the LDA band gap by indirectly acting on both the valence-band maximum and conduction-band minimum. We also discuss the effects of the on-site Coulomb interaction on structural parameters and absolute deformation potentials of ZnO, CdO, GaN, and InN.