Presence Of Midgap States Research Articles

Effect of calcination atmosphere on structural, optical and photocatalytic activity of TiO2/SnS2 core-shell nanostructures in the reduction of aqueous Cr(VI) to Cr(III)

Conversion of Cr(VI) to Cr(III) in mitigating pollution of water bodies is of significant importance to public health due to the fact that Cr(VI) is known to be a potent carcinogen, while Cr(III) is relatively low in toxicity. Photocatalytic approaches are considered as important means to achieve this reduction. Here, TiO2/SnS2 core-shell nanostructures have been produced using a single-step hydrothermal method and its photocatalytic activity is tested for the reduction of aqueous Cr(VI). The structural and optical properties of the as-synthesized products are characterized by XRD, HRTEM, Raman, FTIR, XPS and DRS techniques. The present work reveals that by calcining the core-shell nanoparticles in Ar atmosphere a defective Ti3O5 phase is formed as the core with low band gap, and hence, offers improved light absorption in the visible range. However, its photoactivity was found to be lower than that of the core-shell nanoparticles annealed in oxidizing atmosphere. The observed lower photoreduction was due to the presence of midgap states which acted as recombination centres and hence, reduced the photocatalytic activity.

Target power influence on optical and electrical properties of amorphous titanium oxide deposited by reactive grid-assisted magnetron sputtering

Titanium oxide (TiOx) thin films were deposited by reactive grid-assisted magnetron sputtering on glass substrates as function of the target power. Films were characterized by mechanical profilometry, optical spectrophotometry, four-probe method, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) in order to investigate the effect of the target power on electrical, structural and electronic properties. The electrical properties were measured after deposition and one year later to evaluate the stability of the sheet resistance. Data for suboxides show the decreasing of the optical gap, sheet resistance and density of electronic states in conduction and/or valence bands, and increasing of the optical gap entropy, optical absorption and film thickness as the target power increases. The absorption coefficient indicates the presence of mid-gap states in suboxides, inducing the redshift of the absorption onset. The TiO2 reference sample presents only a single transition in the ultraviolet part. XRD show amorphous structure for all samples. Ti2p and O1s spectra from XPS indicate an oxidized surface with a small contribution of Ti3+ states in either titanium suboxide or dioxide, where the surfaces with the highest concentrations of hydroxyl radicals presented the best stability of the sheet resistance after one year.

Facile One-Pot Two-Step Synthesis of Novel in Situ Selenium-Doped Carbon Nitride Nanosheet Photocatalysts for Highly Enhanced Solar Fuel Production from CO2

Novel in situ Se-doped carbon nitride (C3N4) nanosheets are synthesized for the first time by a facile one-pot two-step strategy. The as-prepared Se-doped C3N4 nanosheets show a high visible light driven formic acid production rate of 1.001 × 105 μmol gcat–1 h–1. It represents the highest value reported in literature for formic acid production from CO2 by visible light harvesting C3N4 photocatalysts. The exceptional photocatalytic activity is largely attributed to selenium-doped conjugated system besides porous nanosheet morphology of the photocatalyst. The presence of midgap state in carbon nitride (−0.35 V vs standard hydrogen electrode) due to Se doping is found to significantly extend the light absorption up to 570 nm thereby further enhancing light harvesting ability of the photocatalyst. The current work not only provides a facile and scalable route to synthesize highly efficient Se-doped carbon nitride (C3N4) nanosheet photocatalysts but also sheds light on devising new avenues for rational design ...

Gate-tunable transport characteristics of Bi2S3 nanowire transistors

Electrical transport and resistance noise spectroscopy measurements are performed on individual, single crystalline Bi2S3 nanowires in the field-effect geometry. The nanowires exhibit n-type conduction and device characteristics such as activation energy, ON/OFF ratio, and mobility are calculated over a temperature range of 120–320 K and at several bias values. The noise magnitude is measured between 0.01 and 5 Hz at several gate voltages as the device turns from it's OFF to ON state. The presence of mid-gap states which act as charge traps within the band gap can potentially explain the observed transport characteristics. Sulfur vacancies are the likely origin of these mid-gap states which makes Bi2S3 nanowires appealing for defect engineering as a means to enhance its optoelectronic properties and also to better understand the important role of defects in nanoscale semiconductors.

Tuning Enhancement Efficiency of Multiple Emissive Centers in Graphene Quantum Dots by Core-Shell Plasmonic Nanoparticles.

Graphene quantum dots (GQDs) are emerging luminescent nanomaterials for energy, bioimaging, and optoelectronic applications. However, unlike conventional fluorophores, GQDs contain multiple emissive centers that result in a complex interaction with external electromagnetic fields. Here we utilize core-shell plasmonic nanoparticles to simultaneously enhance and modulate the photoluminescence (PL) intensities and spectral profiles of GQDs. By analyzing the spectral profiles, we show that the emissive centers are highly influenced by the proximity to the metal particles. Under optimal spacer thickness of 25 nm, the overall PL displays a four-fold enhancement compared with a pristine GQD. However, detailed lifetime measurements indicate the presence of midgap states that act as the bottleneck for further enhancement. Our results offer new perspectives for fundamental understanding and new design of functional luminescent materials (e.g., GQDs, graphene oxide, carbon dots) for imaging, sensing, and light harvesting.

Photoluminescence in Functionalized/Doped Graphene Quantum Dots: Role of Surface States

Photoluminescence in Functionalized/Doped Graphene Quantum Dots: Role of Surface States It has been shown that functionalization/doping of Graphene Quantum Dots (GQDs) can lead to further tuning of their already existing band gap. We have successfully functionalized GQDs (synthesized by hydrothermal process) with oxygen, hydrogen, fluorine, and doped with nitrogen using capacitively coupled plasma of respective gas as evidenced by Raman and X-ray photo emission spectroscopy (XPS). Room temperature photoluminescence (PL) of each functionalized GQD shows distinctive features and can be explained as due to charge transfers between the functional groups/dopants and GQDs as well as presence of mid gap states as a result of functionalization and doping. An energy diagram model is proposed to elucidate the PL modulation resulting from functionalization and doping.

Large spectral weight transfer in optical conductivity of SrTiO3 induced by intrinsic vacancies

The optical conductivity (σ1) of SrTiO3 for various vacancies has been systematically studied using a combination of ultraviolet-vacuum ultraviolet reflectivity and spectroscopic ellipsometry. For cation (Ti) vacancies, σ1 shows large spectral weight transfer over a wide range of energy from as high as 35 eV to as low as 0.5 eV and the presence of mid-gap states, suggesting that strong correlations play an important role. Meanwhile, for anion (O) vacancies, σ1 shows changes from 7.4 eV up to 35 eV.

Midgap states, Raman scattering, glass homogeneity, percolative rigidity and stress transitions in chalcogenides

AbstractRaman scattering from binary GexSe100 − x glasses excited using 1064 nm radiation display vibrational modes whose linewidths significantly exceed those observed using 647 nm radiation. In these glasses, 1064 nm radiation excites midgap states while 647 nm radiation excites conduction band tail states. Presence of midgap states in glasses, ascribed to coordination defects, is responsible for vibrational mode broadening that smears glass compositional variation of mode frequency using 1064 nm radiation but not with the 647 nm radiation. In the latter, mode‐frequency variation of Corner‐ (CS) and Edge‐sharing (ES) tetrahedral units, in specially prepared homogeneous glasses, display thresholds near the rigidity (x = 19.5%) and stress (x = 26.0%) transitions, opening an intermediate phase (IP) that correlates well with the reversibility window (blue region) observed in calorimetric measurements (figure on the right). magnified imageCompositional trends in the non‐reversing enthalpy of relaxation at Tg, ΔHnr(x), in binary GexSe100 − x glasses examined in 1997, 2009, and 2011. The reversibility window (blue region) progressively sharpens to acquire a square‐well like behaviour as melts/glasses homogenize. Figure taken from Ref. 1.

THIN FILM SILICON SOLAR CELLS UNDER MODERATE CONCENTRATION

ABSTRACTThere are only very few reports on the effects of concentration in thin film silicon-based solar cells. Due to the presence of midgap states, a fast decline in fill factor was observed in earlier work. However, with the advent of more stable and lower defect density protocrystalline silicon materials as well as high quality micro-/nanocrystalline silicon materials, it is worth revisiting the performance of cells with these absorber layers under moderately concentrated sunlight. We determined the behavior of the external J-V parameters of pre-stabilized substrate-type (n-i-p) amorphous and microcrystalline solar cells under moderate concentrations, between 1 sun and 21 suns, while maintaining the cell temperature at 25oC. It was found that the cell efficiency of both the amorphous and the microcrystalline cells increased with moderate concentration, showing an optimum at approximately 2 suns. Furthermore, the enhancement in efficiency for the microcrystalline cells was larger than for the amorphous cells. We show that the Voc’s up to 0.63 V can be reached in microcrystalline cells while FF’s only decrease by 9%. The effects have also been computed using the device simulator ASA, showing qualitative agreement.

Scanning Tunneling Microscopy Characterization of the Electrical Properties of Wrinkles in Exfoliated Graphene Monolayers

We report on the scanning tunneling microscopy study of a new class of corrugations in exfoliated monolayer graphene sheets, that is, wrinkles approximately 10 nm in width and approximately 3 nm in height. We found such corrugations to be ubiquitous in graphene and have distinctly different properties when compared to other regions of graphene. In particular, a "three-for-six" triangular pattern of atoms is exclusively and consistently observed on wrinkles, suggesting the local curvature of the wrinkle provides a sufficient perturbation to break the 6-fold symmetry of the graphene lattice. Through scanning tunneling spectroscopy, we further demonstrate that the wrinkles have lower electrical conductance and are characterized by the presence of midgap states, which is in agreement with recent theoretical predictions. The observed wrinkles are likely important for understanding the electrical properties of graphene.

Ac Josephson effect in superconductingd-wave junctions

We study theoretically the ac Josephson effect in superconducting planar d-wave junctions. The insulating barrier assumed to be present between the two superconductors may have arbitrary strength. Many properties of this system depend on the orientation of the d-wave superconductor: we calculate the ac components of the Josephson current. In some arrangements there is substantial negative differential conductance due to the presence of mid-gap states. We study how robust these features are to finite temperature and also comment on how the calculated current-voltage curves compare with experiments. For some other configurations (for small barrier strength) we find zero-bias conductance peaks due to multiple Andreev reflections through midgap states. Moreover, the odd ac components are strongly suppressed and even absent in some arrangements. This absence will lead to a doubling of the Josephson frequency. All these features are due to the d-wave order parameter changing sign when rotated $90^{\circ}$. Recently, there have been several theoretical reports on parallel current in the d-wave case for both the stationary Josephson junction and for the normal metal-superconductor junction. Also in our case there may appear current density parallel to the junction, and we present a few examples when this takes place. Finally, we give a fairly complete account of the method used and also discuss how numerical calculations should be performed in order to produce current-voltage curves.

Spin-Peierls transition in an incommensurately modulated system

Under certain conditions uniform magnetic chains are known to be unstable for a lattice dimerization modulating the exchange integral, i.e. the spin-Peierls (SP) transition. In sufficiently high fields a SP system enters a magnetic discommensuration (MD) state incommensurate with the underlying lattice, characterized by the presence of midgap states. The question is raised, what happens in those magnetic systems that are not uniform from the beginning. Based on experimental evidence obtained in triclinic DMM-TCNQ 2, it will be argued that in zero field there is an instability for a lattice modulation, that approaches the dimerization wave vector via a multiple of the original modulation vector. In sufficiently high fields the so created midgap band becomes magnetic field dependent, like the MD-phase of a regular SP-system.

A Polar State in a Slab as a Soluble Model of p-Wave Fermi Superfluid in Finite Geometry

A p-wave polar state whose order parameter extends perpendicular to the slab wall is analyzed in order to study the size effect in p-wave Fermi superfluids. It is shown that an order parameter profile essentialy the same as that of the half unit cell of the soliton lattice in the continuum model of polyacetylene is an analytic self-consistent solution of the gap equation. The presence of midgap states in the energy spectrum gives novel features to this model superfluid.