Localized Gap States Research Articles

Electronic states and curved surface effect of silicon quantum dots

The calculation results show that the bonding energy and electronic states of silicon quantum dots (Si QDs) are different on various curved surfaces (CS), for example, a Si-O-Si bridge bond on curved surface provides the localized levels in band gap and its bonding energy is shallower than that on facet. Curved surface breaks symmetrical shape of silicon quantum dots on which some bonds can produce localized electronic states in band gap. The red-shifting of photoluminescence spectra on smaller silicon quantum dots can be explained by CS effect. In CS effect, surface curvature is determined by the shape of Si QDs or silicon nanostructures, which is independent of their sizes. The CS effect has the interesting fundamental physical properties in nanophysics as that of quantum confinement effect.

Self-assembly mechanisms of short atomic chains on single-layer graphene and boron nitride

Nucleation and growth mechanisms of short chains of carbon atoms on single-layer, hexagonal boron nitride (h-BN), and short BN chains on graphene are investigated using first-principles plane wave calculations. Our analysis starts with the adsorption of a single carbon ad-atom and examines its migrations. Once a C$_2$ nucleates on h-BN, the insertion of each additional carbon at its close proximity causes a short segment of carbon atomic chain to grow by one atom at at a time in a quaint way: The existing chain leaves its initial position and subsequently is attached from its bottom end to the top of the carbon ad-atom. The electronic, magnetic and structural properties of these chains vertically adsorbed to h-BN depend on the number of carbon atoms in the chain, such that they exhibit an even-odd disparity. An individual carbon chain can also modify the electronic structure with localized states in the wide band gap of h-BN. As a reverse situation we examined the growth of short BN atomic chains on graphene, which attribute diverse properties depending on whether B or N is the atom bound to the substrate. These results together with ab-initio molecular dynamics simulations of the growth process reveal the interesting self-assembly behavior of the grown chains. Furthermore, we find that these atomic chains enhance the chemical activity of h-BN and graphene sheets by creating active sites for the bonding of various ad-atoms and can act as pillars between two and multiple sheets of these honeycomb structures leaving wider spacing between them to achieve high capacity storage of specific molecules.

Hydrogen passivation of polycrystalline silicon thin films

The influence of post-hydrogenation on the electrical and optical properties of solid phase crystallized polycrystalline silicon (poly-Si) was examined. The passivation of grain-boundary defects was measured as a function of the passivation time. The silicon dangling-bond concentration decreases with increasing passivation time due to the formation of Si-H complexes. In addition, large H-stabilized platelet-like clusters are generated. The influence of H on the electrical properties was investigated using temperature dependent conductivity and Hall-effect measurements. For poly-Si on Corning glass, the dark conductivity decreases upon hydrogenation, while it increases when the samples are fabricated on silicon-nitride covered Borofloat glass. Hall-effect measurements reveal that for poly-Si on Corning glass the hole concentration and the mobility decrease upon post-hydrogenation, while a pronounced increase is observed for poly-Si on silicon-nitride covered Borofloat glass. This indicates the formation of localized states in the band gap, which is supported by sub band-gap absorption measurments. The results are discussed in terms of hydrogen-induced defect passivation and generation mechanisms.

Activation of silicon quantum dots for emission

The emission of silicon quantum dots is weak when their surface is passivated well. Oxygen or nitrogen on the surface of silicon quantum dots can break the passivation to form localized electronic states in the band gap to generate active centers where stronger emission occurs. From this point of view, we can build up radiative matter for emission. Emissions of various wavelengths can be obtained by controlling the surface bonds of silicon quantum dots. Our experimental results demonstrate that annealing is important in the treatment of the activation, and stimulated emissions at about 600 and 700 nm take place on active silicon quantum dots.

Making Photo-selective TiO2Materials by Cation–Anion Codoping: From Structure and Electronic Properties to Photoactivity

Photoselective oxidation yielding high-added value chemicals appears as a green novel process with potential to be explored. In this study we combine spectroscopic XPS (N 1s and O 1s) and multiwavelength Raman data with density functional theory calculations to explore the structural and electronic properties of W,N-codoped TiO2 anatase surfaces and interpret the outstanding photocatalytic properties of such a system in partial oxidation reactions. Theoretical calculations allow us to examine several substitutional and N-interstitial configurations at different concentrations of the W,N dopants (similar to those experimentally found), as well as their interaction with structural point defects: Ti cation vacant sites and surface wolframyl species that are required to compensate the extra charge of the W6+ and N-containing anions. The joint use of theoretical and experimental XPS and Raman tools renders key structural information of W,N-codoped microcrystalline TiO2 solids. The incorporation of N at substitutional positions of anatase with the concomitant presence of W═O species introduces localized states in the band gap, a result that is critical in interpreting the chemical behavior of the solids. The combination of the electronic and geometric structural information leads to a simple mechanism that rationalizes the experimentally observed photoactivity and selectivity in partial oxidation reactions.

Recombination Through Different Types of Localized States in Organic Solar Cells

AbstractRecombination in bulk heterojunction solar cells is explored by observing the result of prolonged white light illumination, thermal annealing to high temperature, and chemical doping. Measurements of the photocurrent spectral response, the steady state photocurrent‐voltage characteristics, transient photoconductivity and the dark forward bias current on polymer:fullerene solar cells provide information about the density of states and the electronic properties. Illumination generates deep localized states in the interface gap, which act as recombination centers and also increase the diode ideality factor. Annealing induces both nanostructural and electronic changes. The coarsening of the domain structure reduces the probability that excitons reach the interfaces and also reduces the charge transfer absorption. At the same time annealing broadens the exponential band tails and increases the recombination rate. Doping introduces shallow states near the fullerene conduction band, which also act as recombination centers. The results show that recombination is through localized states of different character, depending on the circumstances.

Spectroscopic Studies and Luminescence Spectra of Dy2O3 Doped Lead Phosphate Glasses

Dy3 + -doped lead phosphate glasses were prepared by a melt quenching technique and investigated through Infrared absorption spectra (IR), photoluminescence (PL), and UV-Visible optical absorption measurements (UV-Vis). The luminescence spectra show two intense bands at 483 and 575 nm, which are attributed to 4F\(_{9/2}\to^{6}\)H15/2 (blue) and 4F\(_{9/2} \to ^{6}\)H13/2 (yellow) transitions, respectively. The optical spectra data was used to evaluate the values of indirect allowed transitions. It was found that the optical band gap increases with Dy2O3 content. Variation in optical gap energy with the variation in localized state tails, confirms the theories for localized states in the energy gap of amorphous semiconductors. The characteristic infrared absorption bands of these glasses due to the stretching and bending vibrations were identified and analyzed by the increasing of the Dy2O3 content. This fact allowed us to identify the specific structural units which appear in these glasses.

A correlation between structural and optical properties of cobalt oxide nanoparticles for various annealing conditions

Two stable phases of cobalt oxide nanoparticles of controlled sizes have been synthesized using environmentally friendly inorganic precursor. Structural characterization using X-ray diffraction (XRD) shows a single-phase spinal Co3O4 structure up to annealing temperature of 800 °C and a mixed phase of Co3O4 and CoO particles for T>900 °C. Single-phase CoO nanoparticles are also obtained by annealing the particles at a temperature >900 °C and cooling in inert atmosphere. Average macro- and micro-strain were estimated using XRD data. Macrostrain was found to be the minimum for particles annealed at 600 °C, whereas microstrain was found to decrease with increasing annealing temperature up to 900 °C. A correlation between the density of localized states (DOS) in the band gap and strain is expected because the origin of both strain and DOS are defects and bond length distortions. Sub-gap absorption measurement and model calculations have been used for the determination of DOS. For cobalt oxide nanoparticle samples we find a correlation between estimated strain and density of states in the band gap.

Optical absorption and X-ray luminescence of glassy lithium tetraborate activated by terbium oxide

It has been demonstrated on the basis of absorption spectra measurements that the activation of glassy lithium tetraborate by terbium results in the formation of band of localized electronic states in the pseudo-band gap, which correspond to the energy levels of rare earth element ions. As follows from the analysis of the X-ray luminescence spectra, the activator ion plays the most important role in the emergence of luminescent properties of glassy lithium tetraborate. It has been established that X-ray luminescence in glassy lithium tetraborate activated by terbium is mostly realized due to transitions from excited 5D3- and 5D4-states to the levels of spin-orbital splitting of the main 7FJ state of Tb3+ ion.

Selecting of modes in nano-laser of silicon quantum dots

In a nano-laser of Si quantum dots (QD), the smaller QD fabricated by nanosecond pulse laser can form the pumping level tuned by the quantum confinement (QC) effect. Coupling between the active centers formed by localized states of surface bonds and the two-dimensional (2D) photonic crystal used to select model can produce a sharp peak at 2.076eV in the nano-laser. It is interesting to make a comparison between the localized electronic states in gap due to defect formed by surface bonds and the localized photonic states in gap of photonic band due to defect of 2D photonic crystal.

Tuning electronic structure and photocatalytic properties by Ag incorporated on (0 0 1) surface of anatase TiO2

The incorporation of Ag on (001) surface of anatase TiO2 has been systematically investigated by means of density functional theory to understand the Ag effects on the electronic structure and photocatalytic properties in Ag/TiO2 composites. Several possible adsorptional, substitutional and interstitial sites with two different Ag concentrations at surface and subsurface layers are examined. Our results about stability of various Ag-incorporated (001) surfaces indicate that the adsorption site is favorable neglect of the oxygen conditions and the substitution site becomes more stable under the oxygen-rich condition, but it becomes difficult to incorporate Ag onto the surface with high concentration, especially for substitution sites in the limited range of oxygen chemical potential. The adsorption of Ag introduces gap states near or below the conduction band minimum (CBM) and the Fermi level locates near or in the conduction band, which can act as photo-generated electron trap centers and inhibit the recombination of electron–hole pairs. The electron transitions from the impurity level to the level above the Fermi level may be responsible for the small visible light absorption peak in experiment. Substitution Ag introduces some localized gap states, while the Fermi level is pinned near the top of valence band, and the impurity states can trap the hole to suppress the recombination of photo-generated carriers. For the interstitial Ag in surface, the Fermi level locates at the bottom of conduction band, and the partial occupied states may also act as electron trap centers which can improve the photocatalytic efficiency.

Growth and Characterization of Indium-Doped Zinc Oxide Thin Films Prepared by Sol-Gel Method

by change in the In content, especially at the In content of 3 at.%. The values of direct band gap were decreased with increase in the In content. The width of localized states in the optical band gap of the indium-doped ZnO thin films were changed with In content and the Urbach energy (EU) was changed inversely with optical band gap of the indium-doped ZnO thin films.

Determination of the Density of States on N-type Ptcdi-c13 Organic Thin-film Semiconductor

ABSTRACTIn this paper we study the density of states in n-type N,N’-ditridecylperylene-3,4,9,10-tetracarboxylic diimide organic semiconductor using two different methods. The first one is based on the temperature dependence of the channel conductance in field-effect transistors. The second one is based on the subgap optical absorption coefficient measured using the Photothermal Deflection Spectroscopy technique. Both techniques allow estimating the distribution of localized states in the band gap of the semiconductor.

Effects of Zn impurities on the electronic properties of Pr doped CaTiO 3

Microcosmic investigations of weak red-emitting materials are crucial for their further development and application. In this work, we have focused on the band structures and electronic properties of Pr mono- and (Zn, Pr) co-doped CaTiO 3 using density functional theory. Zn substitution for Ca or Ti tends to form clusters energetically with Pr substituting for Ca in CaTiO 3. In Pr mono-doped CaTiO 3, the O 2p→Ti 3d transition in CaTiO 3 host corresponds to the centered 330 nm excitation spectra. The gap states above the valence band of ∼1.30 eV and ∼2.06 eV are hybridized by Pr 4f, O 2p and Ti 3d orbitals. They are mainly due to Pr 4f orbitals in CaTiO 3:Pr. The former gap level is related to red emission at 614 nm due to 1D 2→ 3H 4 transition of Pr 3+ activator. The latter is related to the excitation spectra centered at 380 nm due to the low-lying Pr-to-mental intervalence charge transfer transitions (Pr 3+–O 2−–Ti 4+⇌Pr 4+–O 2−–Ti 3+). The band structures of (Zn, Pr) co-doped CaTiO 3 keep the similar gap levels to those in Pr mono-doped CaTiO 3. The incorporation of Zn brings out the two stronger localized gap states, which are hybridized by Pr 4f, O 2p and Ti 3d orbitals, in comparison with those in Pr mono-doped CaTiO 3. Therefore, when Zn impurities are added into Pr doped CaTiO 3, the present calculations visualize the two enhanced levels and the distorted structures around Pr.

An all-silicon laser by coupling between electronic localized states and defect states of photonic crystal

In a nano-laser of Si quantum dots (QD), the smaller QD fabricated by nanosecond pulse laser can form the pumping level tuned by the quantum confinement (QC) effect. Coupling between the active centers formed by localized states of surface bonds and the two-dimensional (2D) photonic crystal is used to select model in the nano-laser. The experimental demonstration is reported in which the peaks of stimulated emission at about 600nm and 700nm were observed on the Si QD prepared in oxygen after annealing which improves the stimulated emission. It is interesting to make a comparison between the localized electronic states in gap due to defect formed by surface bonds and the localized photonic states in gap of photonic band due to defect of 2D photonic crystal.

COMPACT CAPACITANCE MODEL FOR PRINTED THIN FILM TRANSISTORS WITH NON-IDEAL CONTACTS

We report on a compact capacitance model that accurately describes both the gate length dependent and the gate length independent frequency dispersion observed in C-V curves for printed thin film transistors with non-ideal contacts. We also show that modeling the drain current with two adjacent subthreshold regions (instead of just one in the previous RPI TFT model) is needed to match the measured current-voltage characteristics. We show that Elmore model which accounts for channel length transit time is not sufficient for describing the frequency dispersion in C-V curves for printed TFTs and present the new Variable Dispersion Model (VDM). VDM reproduces the experimentally observed gate length independent dispersion arising due to the finite time of the electron exchange between the localized states in the mobility gap and the states above the mobility edge in amorphous semiconductors. The combined Elmore-VDM model has been implemented in AIM-Spice and showed good agreement with measured C-V data.

Gap states in insulating LaMnPO1−xFx(x=0–0.3)

Infrared transmission and electrical resistivity measurements reveal that single crystals of LaMnPO${}_{1\ensuremath{-}x}$F${}_{x}$ ($x\ensuremath{\leqslant}0.28$) are insulating. The optical gap obtained from transmission measurements is nearly unaffected by doping, decreasing only slightly from 1.3 eV in undoped LaMnPO to 1.1 eV for $x=0.04$. The activation gaps obtained from electrical resistivity measurements are smaller by at least an order of magnitude, signaling the presence of states within the optical gap. At low temperatures, the resistivity is described well by variable range hopping conduction between these localized gap states. Analysis of the hopping conduction suggests that the gap states become slightly more delocalized with fluorine content, although metallic conduction is not observed even for fluorine concentrations as large as $x=0.28$.

Optical Absorption of Polyvinyl Alcohol Films Doped with Nickel Chloride

Films of pure and doped Polyvinyl alcohol (PVA) with different concentration of Nickel Chloride (NiCl2) were prepared using the casting technique, in order to investigate the effect of NiCl2 additions on the optical properties of PVA host. The dispersion studies of pure PVA film and PVA films doped with NiCl2 were studied using complex refractive index in the wavelength range 190-1100 nm. The absorption spectral analysis showed that the optical energy band is due to the direct and indirect allowed optical transitions. The optical energy band gap Egopt of the films decreases with increasing NiCl2 contents, while the Urbach energy ∆Et called the width of localized states in the optical band gap decreases from 0.7414 to 0.1891 eV. The reductions of optical energy band gap were observed for all samples after annealing at temperatures 70oC for three hours.

Superconductivity in boron carbide? Clarification by low-temperature MIR/FIR spectra

The electronic structure and phonon density of B13B2 boron carbide calculated by Calandra et al (2004 Phys. Rev. B 69 224505) defines this compound as metallic, and the authors predict superconductivity with TCs up to 36.7 K. Their results are affected by the same deficiencies as former band structure calculations on boron carbides based on hypothetical crystal structures deviating significantly from the real ones. We present optical mid IR/far IR (MIR/FIR) spectra of boron carbide with compositions between B4.3C and B10.37C, evidencing semiconducting behaviour at least down to 30 K. There is no indication of superconductivity. The spectra yield new information on numerous localized gap states close to the valence band edge.

Graphene nanoribbons with smooth edges behave as quantum wires

Graphene nanoribbons with perfect edges are predicted to exhibit interesting electronic and spintronic properties, notably quantum-confined bandgaps and magnetic edge states. However, so far, graphene nanoribbons produced by lithography have had rough edges, as well as low-temperature transport characteristics dominated by defects (mainly variable range hopping between localized states in a transport gap near the Dirac point). Here, we report that one- and two-layer nanoribbon quantum dots made by unzipping carbon nanotubes exhibit well-defined quantum transport phenomena, including Coulomb blockade, the Kondo effect, clear excited states up to ∼20meV, and inelastic co-tunnelling. Together with the signatures of intrinsic quantum-confined bandgaps and high conductivities, our data indicate that the nanoribbons behave as clean quantum wires at low temperatures, and are not dominated by defects.