Narrow Energy Band Research Articles

Titania Nanotube Derived Titanium Nitride Nano-cluster for Visible Light Driven Water Splitting

Herein, titanium nitride (TN) nano cluster was synthesized through NH3 nitrification of titania nanotubes. The effect of nitrification temperature on the formation of TN was investigated by characterizations of X-ray diffractometer, Transmission electron microscopy, Diffuse reflectance UV–vis spectra, X-ray photoelectron spectroscopy, etc. The TN-900, nitrificated at 900 °C, demonstrated high hydrogen (H2) yield in visible light-induced water splitting owing to a narrow energy band gap, unique nano-cluster structure, reasonable surface area and excellent electrical conductivity, which promoted light harvesting, reactant adsorption, and photo-electron–hole separation.

An energy-resolved atomic scanning probe

We propose a method to probe the local density of states (LDOS) of atomic systems that provides both spatial and energy resolution. The method combines atomic and tunneling techniques to supply a simple, yet quantitative and operational, definition of the LDOS for both interacting and non-interacting systems: it is the rate at which particles can be siphoned from the system of interest by a narrow energy band of non-interacting states contacted locally to the many-body system of interest. Ultracold atoms in optical lattices are a natural platform for implementing this broad concept to visualize the energy and spatial dependence of the atom density in interacting, inhomogeneous lattices. This includes models of strongly correlated condensed matter systems, as well as ones with non-trivial topologies.

Experimental investigation of charge transfer, charge extraction, and charge carrier concentration in P3HT:PBD-DT-DPP:PC70BM ternary blend photovoltaics

A sensitizer consisting of D-A structured polymer (PBD-DT-DPP) is assessed with P3HT:PC70BM solar cell. This polymer possesses a narrow energy band gap, which complements in absorption of P3HT:PC70BM binary blend. Use of PBD-DT-DPP as a sensitizer in the ternary blend OPV is reported earlier, which showed enhanced power conversion efficiency (PCE) in the conventional device structure. In this work, charge transfer and charge transport properties of the ternary blend (inverted device) for its improved performance are discussed in detail. Ternary blend system demonstrated an improved device performance (∼3.35%) as compared with the binary of P3HT:PC70BM devices (∼2.58%). Ternary blend devices showed enhanced short circuit current density (JSC) of ∼4 mA/cm2 relative to P3HT:PC70BM. To understand the reason for the decrease in fill factor (FF), mobility analysis using charge extraction with increasing linear voltage (CELIV) measurements were carried out. However, there is not much change in the open circuit voltage (VOC). Results indicate increased bimolecular recombination for the ternary blend, suggesting increased disorder in the morphology of ternary blend films. Time-resolved microwave conductivity (TRMC) measurement was utilized to understand the photoconductance properties of the ternary blend films. Interestingly, TRMC results showed binary PBD-DT-DPP:P3HT and PBD-DT-DPP:PC70BM mixtures possess similar transient photo-conductance to that of P3HT:PC70BM. Further, photoluminescence (PL) measurements were carried out to probe the charge transfer process in the blend systems. Capacitance-voltage results showed enhanced carrier concentration in ternary system. The detailed analysis showed that ternary system helps in improving the charge generation and charge transport and hence improved device performance.

Ionic liquid‐assisted synthesis of porous BiOBr microspheres with enhanced visible light photocatalytic performance

Porous BiOBr microspheres have been successfully synthesized through an ethylene glycol‐assisted solvothermal method using reactive ionic liquids as templates and reactants during the synthetic process. The obtained samples were characterized using X‐ray diffraction, scanning and transmission electron microscopies and X‐ray photoelectron, energy‐dispersive X‐ray, Fourier transform infrared, diffuse reflectance and photoluminescence spectroscopies. The photocatalytic activities were evaluated using the degradation of rhodamine B and tetracycline under visible light irradiation. The photocatalytic efficiency of BiOBr photocatalyst synthesized using single‐molecule ionic liquid as template is better than that of BiOBr obtained using polyionic liquid as template. A possible photocatalytic mechanism is also provisionally proposed. And the degradation rate using BiOBr‐IL‐1 was 2.74, 1.24 and 4.84 times higher than that using BiOBr‐IL‐2, BiOBr‐IL‐3 and BiOBr‐KBr. The larger surface area and narrower energy band gap of BiOBr‐IL‐1 could improve the visible light harvesting ability and facilitate the separation of photogenerated electron–hole pairs and the photocatalysis process. This study affords a facile way to develop such novel photocatalysts with special morphology using ionic liquids.

Narrow-band single-photon emission through selective aryl functionalization of zigzag carbon nanotubes.

The introduction of sp3 defects into single-walled carbon nanotubes through covalent functionalization can generate new light-emitting states and thus dramatically expand their optical functionality. This may open up routes to enhanced imaging, photon upconversion, and room-temperature single-photon emission at telecom wavelengths. However, a significant challenge in harnessing this potential is that the nominally simple reaction chemistry of nanotube functionalization introduces a broad diversity of emitting states. Precisely defining a narrow band of emission energies necessitates constraining these states, which requires extreme selectivity in molecular binding configuration on the nanotube surface. We show here that such selectivity can be obtained through aryl functionalization of so-called 'zigzag' nanotube structures to achieve a threefold narrowing in emission bandwidth. Accompanying density functional theory modelling reveals that, because of the associated structural symmetry, the defect states become degenerate, thus limiting emission energies to a single narrow band. We show that this behaviour can only result from a predominant selectivity for ortho binding configurations of the aryl groups on the nanotube lattice.

Characteristics of Two-episode Emission Patterns in Fermi Long Gamma-Ray Bursts

Abstract Two-episode emission components separated by quiescent gaps in the prompt emission of gamma-ray bursts (GRBs) have been observed in the Swift era, but there is a lack of spectral information due to the narrow energy band of the Swift/Burst Alert Telescope. In this paper, a systematic analysis of the spectral and temporal properties of the prompt emission of 101 Fermi/Gamma-ray Burst Monitor detected long GRBs show the existence of two-episode emission components in the light curves, with quiescent times of up to hundreds of seconds. We focus on investigating the differences of those two emission episodes. We find that the light curves of the two emission components exhibit different behavior, e.g., a soft emission component that either precedes or follows the main prompt emission or that the intensity of the two emission episodes are comparable with each other. No statistically significant correlation in the duration of the two emission episodes can be claimed. We define a new parameter ε as the ratio of the peak flux of the first and second emission episodes and find that a higher ε corresponds to a larger fluence. The preferred spectral model in our analysis is a cutoff power-law model for most GRBs. The distribution of E p for episodes I and II range from tens of keV to 1000 keV with a lognormal fit and there are no significant differences between them. Moreover, we do not find significant relationships between ε and E p for the two emission episodes. Those results suggest that these two-episode emission components likely share the same physical origin.

Enhanced quasiparticle lifetime in a superconductor by selective blocking of recombination phonons with a phononic crystal

When quasiparticles in a BCS superconductor recombine into Cooper pairs, phonons are emitted within a narrow band of energies above the pairing energy at 2$\Delta$. We show that a phonon bandgap restricting the escape of recombination phonons from a superconductor can increase the quasiparticle recombination lifetime by more than an order of magnitude. A phonon bandgap can be realized and matched to the recombination energy with a phononic crystal, a periodically-patterned dielectric membrane. We discuss in detail the non-equilibrium quasiparticle and phonon distributions that arise in a superconductor due to a phonon bandgap and a pair-breaking photon signal. Although intrinsically a non-equilibrium effect, the lifetime enhancement in the small-signal regime is remarkably similar to an estimate from an equilibrium formulation. The equilibrium estimate closely follows $\exp(\Omega_{bg}/k_BT_b)$, where $\Omega_{bg}$ is the phonon bandgap energy bandwidth above 2$\Delta$, and $T_b$ is the phonon bath temperature of the coupled electron-phonon system. We discuss the impact of a phononic bandgap on the performance of superconducting devices, and propose a superconducting microwave resonator circuit to measure the enhancement in the quasiparticle lifetime.

LDA + DMFT and LDA + U study of the electronic and magnetic properties of DyFeSi

Considering the strong correlation between 4f electrons in the rare earths has major effect on the calculated electronic and magnetic properties of compounds including rare earths, as neglecting this strong correlation in density functional theory (DFT) leads to the failing DFT for a proper description of mentioned properties. In this study, to consider the strong correlation between 4f electrons in Dy atom of DyFeSi compound, local Coulomb repulsion between 4f electrons, U, are treated both statically as in the LDA + U approximation and dynamically as in the dynamical mean-field theory in the Hubbard-I approximation (LDA + DMFT[HI]). In this regards, we apply first principle density functional calculation by using full potential linear muffin-tin orbital (FP-LMTO) method to present a comparative study between the calculated electronic and magnetic properties of DyFeSi within LDA + U and LDA + DMFT[HI] approaches and show that the LDA + DMFT[HI] approach presents so much better results compared to the LDA + U approach. In the magnetic properties we find that the calculation within LDA + DMFT[HI] reproduces the Dy magnetic moments in accordance to the observations and standard model of rare earth with the best accuracy. In the electronic properties we show that LDA + U just produces narrow energy bands of 4f-electrons and it is not able to produce atomic multiples features of 4f valance bands, while LDA + DMFT[HI] approach shows peaks caused by the formation of atomic multiples of the Dy atoms in DyFeSi compound.

Exploring the spectral variability of the Seyfert 1.5 galaxy Markarian 530 with Suzaku

A 2012 Suzaku observation of the Seyfert 1.5 galaxy Markarian 530 was analysed and found to exhibit two distinct modes of variability, which were found to be independent from one another. Firstly, the spectrum undergoes a smooth transition from a soft to a hard spectrum. Secondly, the spectrum displays more rapid variability seemingly confined to a very narrow energy band (~ 1-3 keV). Three physical models (blurred reflection, partial covering, and soft Comptonisation) were explored to characterise the average spectrum of the observation as well as the spectral state change. All three models were found to fit the average spectrum and the spectral changes equally well. The more rapid variability appears as two cycles of a sinusoidal function, but we cannot attribute this to periodic variability. The Fe K-alpha band exhibits a narrow 6.4 keV emission line consistent with an origin from the distant torus. In addition, features blueward of the neutral iron line are consistent with emission from He-like and H-like iron that could be originating from the highly ionised layer of the torus, but a broad Gaussian profile at ~ 6.7 keV also fits the spectrum well.

Visible light-driven photocatalytically active g-C3N4 material for enhanced generation of H2O2

Reduced g-C3N4 material was prepared by a thermal treatment of g-C3N4 in presence of NaBH4 under N2 atmosphere. The prepared catalyst material was characterized by using elemental analyzer, FTIR and XPS and the analysis showed that the reduction treatment created nitrogen vacancies followed by a formation of functional group CN owing to a break-up reaction in the pyridine nitride of a s-triazine-C3N4. The findings of UV–vis DRS and DFT calculation revealed that the formed functional group CN results in a narrowed energy band gap owing to positive shift in the conduction band as well as valence band. The downshift observed in the valence band level made the catalyst material with a feature of visible light-driven water oxidation capacity, that was confirmed by the electron and hole sacrifice and OH trapping-EPR techniques. The intermediate energy level within the band gap of g-C3N4 originated from the vacancies caused an extended absorption, especially to the visible region. The analysis of PL emission spectrum confirmed that the reduction treatment could facilitate the spatial separation of photo-excited electron and hole, and enhance the charge transfer as well. RDE studies showed that the selective production of H2O2 by two-electron reduction of O2 was a predominant reaction step using the reduced g-C3N4. The reduced g-C3N4 prepared at 370 °C exhibited an efficient visible light driven catalytic performance on H2O2 production (170 μmol/L h−1) from pure H2O and O2 at ambient atmosphere in the absence of organic electron donors. The solar-to-H2O2 chemical conversion efficiency and apparent quantum yield approached to ∼0.26%, ∼4.3%, respectively. In addition, the experimental results obtained on recycling of the prepared g-C3N4 evidenced the photocatalytic stability of the material.

Distinguishing quantum dot-like localized states from quantum well-like extended states across the exciton emission line in a quantum well

We have closely examined the emission spectrum at the heavy-hole exciton resonance in a high-quality GaAs multi-quantum well sample using picosecond excitation-correlation photoluminescence (ECPL) spectroscopy. Dynamics of the ECPL signal at low and high energy sides of the excitonic photoluminescence (PL) peak show complementary behavior. The ECPL signal is positive (negative) below (above) the PL peak and it changes sign within a narrow band of energy lying between the excitonic absorption and emission peaks. The energy at which this sign change takes place is interpreted as the excitonic mobility edge as it separates localized excitons in quantum dot-like states from mobile excitons in quantum well-like states.

Mg doping in BiFeO3: An advantage over pure BiFeO3 having enhanced ferroelectric and optical properties for opto-electronic device applications

ABSTRACTMg doped bismuth ferrite (BiFeO3; BFO) ceramics named as Bi1−xMgxFeO3 (BMO), with x = 0, 0.1, 0.15 and 0.2 were synthesized by solid state reaction routes. The structural characterization of the ceramics suggested that there was a slight distortion in the rhombohedral structure of BFO in the doped ceramics. The microstructural analysis revealed that the ceramics were distributed uniformly and homogeneously in Mg doped bismuth ferrite (BMO) as compared to that of pure BFO. The dielectric measurement showed that BMO possessed an enhanced dielectric and electrical properties than that of pure BFO. The ferroelectric measurements signified that BMO possessed comparatively high remanent polarization and lower coercive field than that of pure BFO indicating its (BMO's) enhanced ferroelectric properties. The optical properties illustrated that BMO had comparatively narrower optical energy band gap as compared to that of pure BFO. Hence Mg doped BFO (BMO) having good dielectric, electrical, ferroelectric and narrow band gap could have broader applications in opto-electronic devices than that of pure BFO.

Heating without heat: Thermodynamics of passive energy filters between finite systems.

Passive filters allowing the exchange of particles in a narrow band of energy are currently used in microrefrigerators and energy transducers. In this Rapid Communication, we analyze their thermal properties using linear irreversible thermodynamics and kinetic theory, and discuss a striking phenomenon: the possibility of simultaneously increasing or decreasing the temperatures of two systems without any supply of energy. This occurs when the filter induces a flow of particles whose energy is between the average energies of the two systems. Here we show that this selective transfer of particles does not need the action of any sort of Maxwell demon and can be carried out by passive filters without compromising the second law of thermodynamics. This phenomenon allows us to design cycles between two reservoirs at temperatures T_{1}<T_{2} that are able to reach temperatures below T_{1} or above T_{2}.

Anderson metal–insulator transition in doped polar cuprates

In this paper, we investigate the possibility of the Anderson transition for polarons, different dopants, and impurities in doped cuprates. We have developed the continuum theory of carrier self-trapping in a deformable lattice and near different dopants (impurities), the appropriate variational methods and tight-binding model for studying the formation of the localized hole states and the narrow energy bands of large polarons and dopants (impurities) in hole-doped cuprates $$\mathrm{La}_{2-x}\mathrm{Sr}_{x}\mathrm{CuO}_{4}$$ (LSCO) and $$\mathrm{YBa}_{2}\mathrm{Cu}_{3}\mathrm{O}_{7-\delta }$$ (YBCO). To develop the quantitative theory of the Anderson localization and derive the adequate quantitative criteria for the Anderson metal–insulator transition (MIT) in doped materials, we have studied localized states of non-interacting hole carriers in crystalline solids and metal–insulator transitions (MITs) caused by disorder in distribution of large polarons, dopants (or impurities). It is suggested that the large polarons, dopants, and impurities from different superlattices with different degrees of disorder. We have considered a crystalline array of potential wells with the depth $$U = U_{0}$$ in the absence of disorder and developed the approximate methods for the quantitative estimation of the possible changes (or fluctuations) of the depth U of these potential wells between the limits $$U_{0}-V_{0}/2<U<U_{0}+V_{0}/2$$ in the presence of disorder, where $$V_{0}$$ is a random potential which is determined as the amplitude of the change of U due to random distribution of large polarons and dopants (impurities). Using the uncertainty principle, we have obtained the new and more adequate criteria for the Anderson transition in doped polar materials and analyzed the applicability of these criteria to the hole-doped cuprates both in the absence and in the presence of polaronic effects. We have shown that our theoretical results on Anderson MIT obtained by taking into account polaronic effects are in quantitative agreement with the well-established experimental results on MITs in hole-doped cuprates LSCO and YBCO.

A room-temperature heptazine core discotic liquid crystal

ABSTRACTHeptazine is the fundamental structural and functional unit of graphitic carbon nitrides and has a π-conjugated planar symmetry with semiconducting properties. Molecular self-assembly is one of the key factors to drive the electronic behaviour of π-conjugated organic molecules. To enhance the semiconductivity, heptazine is decorated at its active sites with 2,3,5-tris(dodecyloxy)aniline to get well-organised columnar packing. A novel heptazine-core room-temperature discotic liquid crystal (HDLC) with hexagonal columnar geometry is discovered. The molecular structure and mesomorphic properties of HDLC are investigated by 1H-NMR, 13C-NMR, IR, polarised optical microscopy, differential scanning calorimetry and X-ray diffraction. Photophysical and electrochemical properties of HDLC are studied by UV–vis/fluorescence spectroscopy and cyclic voltammetry (CV), respectively. The energy band gap of HDLC estimated from CV at room temperature is 1.63 eV, which is much narrower than the previously reported band gap for heptazine derivatives. This decrease in the energy band can be attributed to the particular designing of HDLC for columnar packing. As a columnar liquid crystal providing a smooth path to the charge transport, HDLC with such a narrow energy band gap may find applications in organic electronics.

Self-dual quasiperiodic systems with power-law hopping

We introduce and explore a family of self-dual models of single-particle motion in quasiperiodic potentials, with hopping amplitudes that fall off as a power law with exponent $p$. These models are generalizations of the familiar Aubry-Andre model. For large enough $p$, their static properties are similar to those of the Aubry-Andre model, although the low-frequency conductivity in the localized phase is sensitive to $p$. For $p \leq 2.1$ the Aubry-Andre localization transition splits into three transitions; two distinct intermediate regimes with both localized and delocalized states appear near the self-dual point of the Aubry-Andre model. In the intermediate regimes, the density of states is singular continuous in much of the spectrum, and is approximately self-similar: states form narrow energy bands, which are divided into yet narrower sub-bands; we find no clear sign of a mobility edge. When $p < 1$, localized states are not stable in random potentials; in the present model, however, tightly localized states are present for relatively large systems. We discuss the frequency-dependence and strong sample-to-sample fluctuations of the low-frequency optical conductivity, although a suitably generalized version of Mott's law is recovered when the power-law is slowly decaying. We present evidence that many of these features persist in models that are away from self-duality.

Adjustable band position of strontium titanate by doping-free solvents effect and its correlation with photodegradation performance

Strontium titanate (STO) with shape of nanosheet, polyhedron and nanosphere is prepared by one-pot solvothermal method using ethanolamine (EA), diethanolamine (DA) and ethylene glycol (EG), respectively. Upon the different crystalline size and morphology of STO, effect of solvents on nucleation and growth is discussed. In this case, energy band and flat band potential of samples are measured by UV–Vis diffuser reflectance (DRS) and Mott-Schottky (MS). Results show narrow energy band (Eg) 2.68, 2.87 and 3.17 eV of as-synthesized pure STOs (EA-STO, DA-STO and EG-STO) compared with empirical STO. Simultaneously, valence band shows 0.55, 1.80 and 2.21 eV respectively. Rhodamine B (RhB) and methyle orange (MO) are selected as anionic and cationic target contaminants. Correlation of photodegradation performance and valence band is depicted.

Optimisation of laser driven proton beams by an innovative target scheme

Proton beams driven by the target normal sheath acceleration mechanism exhibit large divergence and a broad energy distribution with low particle number at high energy. Such undesirable characteristics of the beam can be controlled and optimised by employing a recently developed helical coil technique, which exploits the transient self-charging of solid targets irradiated by intense laser pulses. Highly chromatic focusing of the broadband proton beams was achieved by employing this technique at the TARANIS laser system, where the selected energy slice was tuned by varying the pitch of the coil. A quasi-collimated, narrow energy band proton beam of ∼ 107 particles at 10 MeV was achieved, through a combination of focussing, energy selection and in-situ post-acceleration. This technique may provide a platform for a generation of compact, all-optical ion accelerators.

Improving Visible Light-Absorptivity and Photoelectric Conversion Efficiency of a TiO2 Nanotube Anode Film by Sensitization with Bi2O3 Nanoparticles

This study presents a novel visible light-active TiO2 nanotube anode film by sensitization with Bi2O3 nanoparticles. The uniform incorporation of Bi2O3 contributes to largely enhancing the solar light absorption and photoelectric conversion efficiency of TiO2 nanotubes. Due to the energy level difference between Bi2O3 and TiO2, the built-in electric field is suggested to be formed in the Bi2O3 sensitized TiO2 hybrid, which effectively separates the photo-generated electron-hole pairs and hence improves the photocatalytic activity. It is also found that the photoelectric conversion efficiency of Bi2O3 sensitized TiO2 nanotubes is not in direct proportion with the content of the sensitizer, Bi2O3, which should be carefully controlled to realize excellent photoelectrical properties. With a narrower energy band gap relative to TiO2, the sensitizer Bi2O3 can efficiently harvest the solar energy to generate electrons and holes, while TiO2 collects and transports the charge carriers. The new-type visible light-sensitive photocatalyst presented in this paper will shed light on sensitizing many other wide-band-gap semiconductors for improving solar photocatalysis, and on understanding the visible light-driven photocatalysis through narrow-band-gap semiconductor coupling.

The mySpot beamline at BESSY II

mySpot beamline is used to provide stable beam especially tuned for the mySpot experiment. Depending on the experiment requirements, different optical devices are used. The schematic view shows two different configurations, one tuned for low divergence, and one for narrow energy band width, as required for the scattering and spectroscopy experiments respectively. Since the goal of the experiment is to provide several methods at the same time, beamline properties can be tuned to provide the optimal beam for a given combination of experiments. Total intensity, divergence, energy resolution, high harmonics suppression, and stability in scans can be tuned to match the requirements (Erko &amp; Zizak, 2009).