Intrinsic Energy Gap Research Articles

Ultrabright Dibenzofluoran-Based Polymer Dots with NIR-IIa Emission Maxima and Unusual Large Stokes Shifts for 3D Rotational Stereo Imaging.

Emerging organic molecules with emissions in the second near-infrared (NIR-II) region are garnering significant attention. Unfortunately, achieving accountable organic emission intensity over the NIR-IIa (1300nm) region faces challenges due to the intrinsic energy gap law. Up to the current stage, all reported organic NIR-IIa emitters belong to polymethine-based dyes with small Stokes shifts (<50nm) and low quantum yield (QY; ≤0.015%). However, such polymethines have proved to cause self-absorption with constrained emission brightness, limiting advanced development in deep-tissue imaging. Here a new NIR-IIa scaffold based on rigid and highly conjugated dibenzofluoran core terminated by amino-containing moieties that reveal emission peaks of 1230-1305nm is designed. The QY is at least 10 times higher than all synthesized or reported NIR-IIa polymethines with extraordinarily large Stokes shifts of 370-446nm. DBF-BJ is further prepared as a polymer dot to demonstrate its in vivo 3D stereo imaging of mouse vasculature with a 1400nm long-pass filter.

Superior energy storage efficiency through tailoring relaxor behavior and band energy gap in KNN-based ferroelectric ceramic capacitors

With the increasing demand of high-power and pulsed power electronic devices, environmental-friendly potassium sodium niobate ((Na[Formula: see text]K[Formula: see text])NbO3, KNN) ceramic-based capacitors have attracted much attention in recent years owning to the boosted energy storage density ([Formula: see text]). Nevertheless, the dielectric loss also increases as the external electric field increases, which will generate much dissipated energy and raise the temperature of ceramic capacitors. Thus, an effective strategy is proposed to enhance the energy storage efficiency ([Formula: see text]) via tailoring relaxor behavior and bad gap energy in the ferroelectric 0.9(Na[Formula: see text]K[Formula: see text])-NbO3–0.1Bi(Zn[Formula: see text](Nb[Formula: see text]Ta[Formula: see text])[Formula: see text])O3 ceramics. On the one hand, the more diverse ions in the B-sites owing to introducing the Ta could further disturb the long-range ferroelectric polar order to form the short–range polar nanoregions (PNRs), resulting in the high [Formula: see text]. On the other hand, the introduction of Ta ions could boost the intrinsic band energy gap and thus improve the [Formula: see text]. As a result, high [Formula: see text] of 3.29 J/cm3 and ultrahigh [Formula: see text] of 90.1% at the high external electric field of 310 kV/cm are achieved in [Formula: see text] = 0.5 sample. These results reveal that the KNN-based ceramics are promising lead-free candidate for high-power electronic devices.

Recent Progress of Novel Organic Near‐Infrared‐Emitting Materials

Organic near‐infrared (NIR) emissive materials integrating the intrinsic NIR light advantages and organic semiconductor characters have generated immense scientific interest in fundamental science and practical application, including optical communications, night vision, surveillance, and biomedical applications. Nevertheless, the intrinsic energy gap law engenders a huge challenge to simultaneously meet longer emission wavelength and higher luminescence efficiency for organic NIR emitters. Herein, it is suggested that the purposefully molecular design and crystal engineering for novel organic NIR materials are urgently studied and explored. Herein, recent advances in novel organic NIR materials, mainly focusing on the hybridized local and charge transfer (HLCT) compounds, thermally activated delayed fluorescence (TADF) emitters, CT cocrystals, neutral π‐radicals, and excited‐state intramolecular proton transfer (ESIPT)‐active materials, are summarized. It is hoped that this perspective can afford a new insight into the fine design and synthesis of the desired organic NIR emitters.

Modulation of Thermally Stable Photoluminescence in Cr3+-Based Near-Infrared Phosphors

Broadband near-infrared (NIR) light sources based on phosphor-converted light-emitting diodes (pc-LEDs) are desirable for various photonics applications, while developing thermally stable NIR phosphors remains a great challenge. Increasing the temperature accelerates the severe nonradiative relaxation process gorverned by the intrinsic energy gap law, which further suspends the efficient low-energy emission of Cr3+ emitters in the inorganic lattice. To address this rule, several state-of-the-art strategies have been put forward in this perspective to modulate the critical law from the viewpoints of (1) crystal structure design, (2) defect engineering, (3) strengthened rigidity, and (4) energy transfer. This perspective suggests avenues for exploring novel broadband NIR phosphors with high thermal stability and will also stimulate further studies on NIR spectroscopy for high-power applications.

Atomic Simulations of (8,0)CNT-Graphene by SCC-DFTB Algorithm

Self-consistent density functional tight binding (SCC-DFTB) approaches were used to study optimized structures, energy, differential charge density, and Mülliken populations for the (8,0) carbon nanotubes (CNTs) connected to the graphene having different topology defects. Based on the calculations, nine seamless (8,0)CNT-graphenes were selected. For these connected systems, geometric configurations of the graphene and nanotubes were characterized, and the nearest neighbor length of C-C atoms and average length were obtained. The intrinsic energy, energy gap, and chemical potential were analyzed, and they presented apparent differences for different connection modes. Differential charge densities of these connection modes were analyzed to present covalent bonds between the atoms. We have also thoroughly analyzed the Mülliken charge transfer among the C atoms at the junctions.

Unveiling quasiparticle dynamics of topological insulators through Bayesian modelling

Quasiparticle - a key concept to describe interacting particles - characterizes electron-electron interaction in metals (Fermi liquid) and electron pairing in superconductors. While this concept essentially relies on the simplification of hard-to-solve many-body problem into one-particle picture and residual effects, a difficulty in disentangling many-body effects from experimental quasiparticle signature sometimes hinders unveiling intrinsic low-energy dynamics, as highlighted by the fierce controversy on the origin of Dirac-band anomaly in graphene and dispersion kink in high-temperature superconductors. Here, we propose an approach to solve this fundamental problem - the Bayesian modelling of quasiparticles. We have chosen a topological insulator TlBi(S,Se)2 as a model system to formulate an inverse problem of quasiparticle spectra with semiparametric Bayesian analysis, and successfully extracted one-particle and many-body characteristics, i.e. the intrinsic energy gap and unusual lifetime in Dirac-quasiparticle bands. Our approach is widely applicable to clarify the quasiparticle dynamics of quantum materials.

Atomic simulations of bamboo-like N-doped CNTs with spaced nitrogen and carbon atoms by DFTB algorithm

Density functional tight binding (DFTB) simulations were performed to study structures, energy, and Mülliken populations for Zigzag type (n,0) (n = 3–14) bamboo-like nitrogen carbon nanotubes with spaced packing patterns. After performing self-consistant-charge DFTB calculations, geometric parameters of bamboo-type nitrogen carbon nanotubes were obtained, including the nearest neighbor length of CC or NN atoms, the average CN length, the diameters of C or N rings, the C-C-N angle and the C-N-N angle of the six-atom ring parallel to the tube axis, the C-C-C angle and the N-N-N angle perpendicular to the tube axis. The intrinsic energy，energy gap, and chemical potential were analyzed, and they present apparent differences compared with their pure carbon counterparts. We have thoroughly analyzed the charges transfer of the C and N atoms for the bamboo-like nitrogen‑carbon nanotubes. Mülliken populations present different prospects on these C or N atoms as the chiral index increases, especially for these relative small indexes.

Ultrastable AgBiS2 Hollow Nanospheres with Cancer Cell-Specific Cytotoxicity for Multimodal Tumor Therapy.

Specific cytotoxicity for catalytic nanomedicine triggered by the tumor microenvironment (TME) has attracted increasing interest. In this work, we prepared AgBiS2 hollow nanospheres with narrow bandgaps via rapid precipitation in a weakly polar solvent, which lowered the intrinsic energy gap for the active production of highly reactive hydroxyl radicals (•OH), especially in the TME. The as-prepared AgBiS2 hollow nanospheres exhibited enhanced optical absorption and high photothermal conversion efficiency (44.2%). In addition, the hollow structured AgBiS2 nanospheres were found to have a peroxidase-mimicking feature to induce cancer cell-specific cytotoxicity while exhibiting negligible cytotoxicity toward normal cells, which might be attributed to the efficient production of highly reactive •OH originating from the overexpression H2O2 in the TME caused by surface catalysis. In particular, the cancer cell-specific cytotoxicity of the nanospheres was greatly enhanced both in vitro and in vivo upon irradiation with a near-infrared (NIR) laser (808 nm). The above-mentioned features of the hollow structured AgBiS2 will make it a promising candidate for tumor therapy.

The effects of various molecular weight of passivator on the photoluminescence properties of graphene quantum dots

Here, we realize manipulation of photoluminescence (PL) intensity of GQDs through various molecular weight of surface passivator. With the increase of passivator molecular weight, only the PL intensity increases. However, the peak positions and half-peak width of spectral line remain unchanged. Our results reveal that surface passivation with larger molecular weight only weakens the chemical activity of graphene quantum dots (GQDs) and the π-π stacking interaction between GQDs, but exerts less impact on GQDs intrinsic energy gap. Therefore, the weaker π-π stacking interaction and chemical activity induce the weaker non-radiative recombination, leading to enhanced fluorescence from radiative recombination.

Flux Superperiods and Periodicity Transitions in Quantum Hall Interferometers

For strongly screened Coulomb interactions, quantum Hall interferometers can operate in a novel regime: the intrinsic energy gap can be larger than the charging energy, and addition of flux quanta can occur without adding quasiparticles. We show that flux superperiods are possible and reconcile their appearance with the Byers-Yang theorem. We explain that the observation of anyonic statistical phases is possible by tuning to the transition from a regime with constant chemical potential to a regime with constant particle density, where a flux superperiod changes to a periodicity with one flux quantum at a critical magnetic field strength.

Engineering zero modes, Fano resonance, and Tamm surface states in the waveguide-array realization of the modified Su-Schrieffer-Heeger model.

In this article, we developed a generalized coupled-mode theory for mixing an isolated state with a continuum having an intrinsic energy gap, which dubbed as "the bound states in the gapped continuum" (BIGC). We investigated the mixture interaction by mimicking the Su-Schrieffer-Heeger model in an optical coupled waveguide array (WA), and presented a unified engineering mechanism for topologically-protected zero modes, Fano resonance, and Tamm surface states, even though those phenomena are diverse in topological insulators, atomic physics and semiconductors, respectively. By tuning the on-site potential and coupling strength of the isolated state, we found the unified operating characteristics for zero modes, Fano resonance, and Tamm states, with demonstrating their localization, transmission spectra, and distinct evolution dynamics explicitly. As an extension for triple-modes coupling, two special sandwich-like configurations are studied: the isolated-continuous-isolated and continuous-isolated-continuous configurations lead to adiabatic eliminations and domain walls, respectively, revealing possible applications and wide connections in many fields of physics and optics.

Effect of Intrinsic Layer Energy Gap and Thicknesses Optimization on the Efficiency of p-i-n Amorphous Silicon Solar Cell

morphous silicon solar cells with single p-i-n layer were grown on 10 cm 2 ITO coated glass substrates. Fabrication process done by using 13.56 MHz RF-Plasma Enhanced Chemical Vapor Deposition (PECVD). Hydrogen flow on the deposition process is widely known to enable the passivation of the dangling bond on Silicon bonds. The passivation of dangling bond affects the band gap of each layer and cell performance in the absorption of photon. In the deposition process of intrinsic layer, SiH 4 gas flow is set constant at 2.5 sccm, while variation is done in hydrogen gas flow at 0 sccm – 90 sccm. Energy gaps obtained for p-layer and n-layer are 2.0 eV and 2.2 eV at thickness 64 nm and 36 nm respectively. Optimizations have been done for intrinsic layer which band gaps are 1.4 eV, 1.6 eV and 1.9 eV at thickness 400 nm. The solar cell efficiency was increased from 4.8% to 5.64% based on the band gaps variety. In addition, i-layer thicknesses were also varied from 400 nm, 500 nm and 600 nm. Thicknesses variation shows an increase of 5.78% in the solar cell efficiency.

Poor electronic screening in lightly doped Mott insulators observed with scanning tunneling microscopy

The effective Mott gap measured by scanning tunneling microscopy (STM) in the lightly doped Mott insulator $(\rm{Sr}_{1 -x}\rm{La}_x)_2\rm{IrO}_4$ differs greatly from values reported by photoemission and optical experiments. Here, we show that this is a consequence of the poor electronic screening of the tip-induced electric field in this material. Such effects are well known from STM experiments on semiconductors, and go under the name of tip-induced band bending (TIBB). We show that this phenomenon also exists in the lightly doped Mott insulator $(\rm{Sr}_{1 -x}\rm{La}_x)_2\rm{IrO}_4$ and that, at doping concentrations of $x\leq 4 \%$, it causes the measured energy gap in the sample density of states to be bigger than the one measured with other techniques. We develop a model able to retrieve the intrinsic energy gap leading to a value which is in rough agreement with other experiments, bridging the apparent contradiction. At doping $x \approx 5 \%$ we further observe circular features in the conductance layers that point to the emergence of a significant density of free carriers in this doping range, and to the presence of a small concentration of donor atoms. We illustrate the importance of considering the presence of TIBB when doing STM experiments on correlated-electron systems and discuss the similarities and differences between STM measurements on semiconductors and lightly doped Mott insulators.

Extrinsic Rashba spin–orbit coupling effect on silicene spin polarized field effect transistors

Regarding the spin field effect transistor (spin FET) challenges such as mismatch effect in spin injection and insufficient spin life time, we propose a silicene based device which can be a promising candidate to overcome some of those problems. Using non-equilibrium Green’s function method, we investigate the spin-dependent conductance in a zigzag silicene nanoribbon connected to two magnetized leads which are supposed to be either in parallel or anti-parallel configurations. For both configurations, a controllable spin current can be obtained when the Rashba effect is present; thus, we can have a spin filter device. In addition, for anti-parallel configuration, in the absence of Rashba effect, there is an intrinsic energy gap in the system (OFF-state); while, in the presence of Rashba effect, electrons with flipped spin can pass through the channel and make the ON-state. The current voltage (I–V) characteristics which can be tuned by changing the gate voltage or Rashba strength, are studied. More importantly, reducing the mismatch conductivity as well as energy consumption make the silicene based spin FET more efficient relative to the spin FET based on two-dimensional electron gas proposed by Datta and Das. Also, we show that, at the same conditions, the current and ratio of silicene based spin FET are significantly greater than that of the graphene based one.

The influence of proximity induced ferromagnetism, superconductivity and Fermi-velocity on evolution of Berry phase in Bi2Se3 topological insulator

Bi2Se3 is a well known 3D-topological insulator (TI) with a non-trivial Berry phase of attributed to the topology of the band structure. The Berry phase shows non-topological deviations from in the presence of a perturbation that destroys time reversal symmetry and gives rise to a quantum system with massive Dirac fermions and finite band gap. Such a band gap opening is achieved on account of the exchange field of a ferromagnet or the intrinsic energy gap of a superconductor that influences the topological insulator surface states by virtue of the proximity effect. In this work the Berry phase of such gapped systems with massive Dirac fermions is considered. Additionally, it is shown that the Berry phase for such a system also depends on the Fermi-velocity of the surface states which can be tuned as a function of the TI film thickness.

Spin-semiconducting properties in silicene nanoribbons.

We have investigated the relative stabilities and electronic properties of silicene nanoribbons with sawtooth edges (SSiNRs) by first-principles calculations. The SSiNR is more stable than the zigzag silicene nanoribbon (ZSiNR) and has a ferromagnetic ground state with an intrinsic energy gap between majority and minority spin-polarized bands, which shows that SSiNR is a spin-semiconductor. Under an external transverse electric field, the energy gap decreases and even vanishes. Meanwhile, the charge densities of the two edge bands near the Fermi level become spatially separated at different edges. We find also that the electric field-induced features can be achieved by a suitable uniaxial compressive strain. This can be understood from the effect of the Wilson transition. At last, the electronic structures of SSiNRs tuned by electric field and strain together are studied, showing that a small tensile strain makes the SSiNRs more sensitive to the electric field. These results suggest that the electric field or/and strain modulated SSiNRs have potential applications in silicon-based spintronic nanodevices.

Electrospun Cu-, W- and Fe-doped TiO2 nanofibres for photocatalytic degradation of rhodamine 6G

Titanium dioxide (TiO2), thanks to its interesting properties as non-toxicity, low cost and high chemical stability, has been extensively investigated for several applications in which, following light absorption, the generated charges can be usefully applied, as for photovoltaic systems and photocatalytic devices. However, due to the wide intrinsic energy gap of TiO2 (between 3.0 and 3.2 eV, depending on the crystalline structure), only a small fraction of the solar spectrum can be used to promote the electron–hole pair formation. In order to reduce the energy gap and, therefore, increase the fraction of the solar spectrum that can be absorbed, different approaches have been used, mainly by doping TiO2 with metals or anions. In this work we will present our results on the deposition, by means of the electrospinning technique, of TiO2 nanofibres doped with metals (iron, copper and tungsten). These nanofibres exhibit morphology at bundle structure composed by sheaths of about 300-nm diameter and core filled with 20-nm-thick fibrils. Metal-doped TiO2 nanofibres have been characterized by SEM, TEM, EDX, XPS and XRD for morphological, chemical and structural analyses, respectively. The effect of the metal concentration on the photocatalytic properties of these nanofibres have been investigated through the study of the degradation of a pollutant model (rhodamine 6G) by using visible lamp and sunlight as light source.

Potential and pH dependence of photocurrent transients for boron-doped diamond electrodes in aqueous electrolyte

Using illumination at energies below the intrinsic diamond energy gap, photocurrent transients have been recorded for boron-doped diamond (BDD) as an electrode in an aqueous electrolyte of 0.1M KH2PO4. The commercially-supplied BDD was in the form of a free-standing, polycrystalline film grown by chemical vapour deposition (CVD), with a boron acceptor concentration of ≥1020cm−3. The effects of mechanical polishing of the BDD, of electrochemical hydrogen evolution and of electrochemical oxygen evolution (in 0.1M KH2PO4), on the potential dependence of the photocurrent transients have been examined. Measurements of the cathodic photocurrent at light switch-on have been used to determine the photocurrent onset potential as a measure of the flatband potential. Comparison with and between related literature observations has shown broad agreement across considerably varying BDD/electrolyte systems. The flatband potential shifted positively following electrochemical oxygen evolution, indicating the formation of oxygen-containing groups on the diamond surface, these increasing the potential drop across the Helmholtz layer. For the electrochemically oxidised electrode, the cathodic photocurrent transient at a fixed potential changed reproducibly with changing solution pH, owing to the participation of the oxygen-containing surface groups in acid–base equilibrium with the solution. This clear demonstration of BDD as a photoelectrochemical pH sensor is in principle extendable to mapping the spatial variation in pH across a BDD surface by use of a focussed light spot.

Oxygen Surface Functionalization of Graphene Nanoribbons for Transport Gap Engineering

We numerically investigate the impact of epoxide adsorbates on the transport properties of graphene nanoribbons with width varying from a few nanometers to 15 nm. For the wider ribbons, a scaling analysis of conductance properties is performed for adsorbate density ranging from 0.1% to 0.5%. Oxygen atoms introduce a large electron-hole transport asymmetry with mean free paths changing by up to 1 order of magnitude, depending on the hole or electron nature of charge carriers. The opening of a transport gap on the electron side for GNRs as wide as 15 nm could be further exploited to control current flow and achieve larger ON/OFF ratios, despite the initially small intrinsic energy gap. The effect of the adsorbates in narrow ribbons is also investigated by full ab initio calculations to explore the limit of ultimate downsized systems. In this case, the inhomogeneous distribution of adsorbates and their interplay with the ribbon edge are found to play an important role.

Electron Tunneling Spectroscopy of SmB&lt;SUB&gt;6&lt;/SUB&gt; Studied by &lt;I&gt;In Situ&lt;/I&gt; Nano-Break-Junction Method

Tunneling spectra of intermediate-valence semiconductor SmB6 are reported for in-situ break junctions, being able to make nano-scale planar tunnel junctions. The electron tunneling using break junction method is a powerful probe of the intrinsic energy gap. The investigated tunneling conductance dI/dV curves are mostly reproducible and symmetric with respect to the applied voltage. Two kinds of characteristic energy gaps are observed at 2E(d) = 20 mV and 2E(a) = 9 mV, which coincides well with those previously studied by point-contact spectroscopy and the activation energy fitted by our electrical resistivity data. The positions of the gap structures are independent of the zero-bias conductance, implying no additional voltage drop induced by the break junctions. The small anomaly at the activation energy 2E(a) indicates a relatively low density of in-gap states. Furthermore, the results of magnetic properties reveal the ratio of Sm2+:Sm3+ = 3.7:6.3 and the antiferromagnetic nature at high temperature.