Dependence Of Energy Gap Research Articles

Biomolecule Detection in High Ionic Strength With Enhanced Sensitivity Using Graphene Nanogrid FET Immunosensor

Detecting charges in high ionic strength solutions is a challenge due to the inherent problem of ionic screening in small Debye length. There have been some efforts to improve the sensing in high ionic strength buffer but the sensitivity and detection limit is poor. To address this challenge, we report high frequency sensing using smooth graphene nanogrid FET immunosensor. These sensors have certain intrinsic advantages: firstly, in the nanopores, the biomolecules usually reside within a distance shorter than the original pore radius which statistically raises the charge transfer probability between the biomolecules and the surface even at high ionic strength of the buffer and secondly, such a graphene nanostructure has a size dependent effective energy gap which increases the transconductance of the device. These features coupled with high frequency sensing have enabled detection down to 0.1 fMHep-B in a high ionic strength buffer (150mM) at 8 MHz with a sensitivity as high as 445.45%. This is an enhancement by more than 18 times compared to the existing reports.

Energy gaps, effective masses and ionicity of AlxGa1−xSb ternary semiconductor alloys

A pseudopotential calculation of the electronic structure of Al[Formula: see text]Ga[Formula: see text]Sb ternary alloys in the zinc-blende structure has been performed. The compositional dependence of energy gaps, electron and heavy hole effective masses and ionicity of the material system of interest have been examined and discussed. Special attention has been given to the effect of the alloy disorder on the direct ([Formula: see text]–[Formula: see text]) bandgap energy. It is found that all features of interest vary monotonically with increasing the Al concentration [Formula: see text]. Besides, bandgap bowing parameters and extent of the direct-to-indirect bandgap transition have been determined. Our findings agree generally well with the data reported in the literature. Trends in ionicity are found to be consistent with the Phillips ionicity scale.

All-Optical Switching of Two Continuous Waves in Few Layer Bismuthene Based on Spatial Cross-Phase Modulation

Bismuthene, the last and heaviest group-VA elemental two-dimensional material, has received tremendous interests owing to its advantages in electronic-transport, semimetallic bonding, and intrinsic spin-orbit coupling. However, light–bismuthene interaction is relatively less investigated. Herein, sonochemical exfoliation approach had been employed to deliver a successful synthesis of few-layer bismuthene with an average thickness of ∼3 nm and a lateral size of ∼0.2 μm. The corresponding band structure from mono- to sextuple-layer had been therotically calculated and it was found that bismuthene possesses a thickness dependent energy gap from almost zero to 0.55 eV, suggesting that bismuthene may also find unique applications from terahertz, mid-infrared toward infrared regime. The nonlinear optical absorption and refraction parameters had been well characterized by laser Z-scan and spatial phase modulation measurement techniques, respectively. By taking advantage of its strong nonlinear refraction effect,...

First-principles study of band structures of anthracene and tetracene under pressure

The pressure effect up to 30 GPa on the structural and electronic properties of crystalline anthracene and tetracene were studied within the framework of density functional theory with van der Waals interactions. Lattice parameters, bulk modulus and cohesive energy were calculated. The pressure dependence of interplanar angles, which describe the relative packing of molecules in the crystal, has been calculated. The calculated electronic band structures demonstrate the dependence of energy gap under pressure. Energy gap in crystalline anthracene monotonously decreases with increasing pressure. The tetracene energy gap decrease from 2.39 to ∼0 eV at 26 GPa. Further increase of pressure subsequently increases energy gap.

Superconducting correlations induced by charge ordering in cuprate superconductors and Fermi-arc formation

We have developed a generalized electronic phase separation model of high-temperature cuprate superconductors that links the two distinct energy scales of the superconducting (SC) and pseudogap (PG) phases via a charge-density-wave (CDW) state. We show that simulated electronic-density modulations resembling the charge order (CO) modulations detected in cuprates intertwine the SC and charge orders by localizing charge and providing the energy scale for a spatially periodic SC attractive potential. Bulk superconductivity is achieved with the inclusion of Josephson coupling between nanoscale domains of intertwined fluctuating CDW and SC orders, and local SC phase fluctuations give rise to the Fermi arcs along the nodal directions of the SC gap. We demonstrate the validity of the model by reproducing the hole-doping dependence of the PG onset temperature $T^*$, and the SC transition temperature $T_c$ of ${\rm YBa_2Cu_3O_y}$ and ${\rm Bi_{2-y}Pb_ySr_{2-z}La_zCuO_{6+\delta}}$. The results show that the periodicity of the CDW order is controlled by the PG energy scale, and the hole-doping dependence of the SC energy gap is controlled by the charge ordering free energy.

Electronic Excited State Lifetimes of Anionic Water Clusters: Dependence on Charge Solvation Motif.

An ongoing controversy about water cluster anions concerns the electron-binding motif, whether the charge center is localized at the surface or within the cluster interior. Here, mixed quantum-classical dynamics simulations have been carried out for a wide range of cluster sizes (n ≤ 1000) for (H2O)n- and (D2O)n-, based on a nonequilibrium first-order rate constant approach. The computed data are in good general agreement with time-resolved photoelectron imaging results (n ≤ 200). The analysis reveals that, for surface state electrons, the cluster size dependence of the excited state electronic energy gap and the magnitude of the nonadiabatic couplings have compensating influences on the excited state lifetimes: the excited state lifetime for surface states reaches a minimum for n ∼ 150 and then increases for larger clusters. It is concluded that the electron resides in a surface-localized motif in all of these measured clusters, dominating at least up to n = 200.

The Boundary Effect on the Energy Gap of High Tc Superconductors

Spatial dependence of the pairing potential across the thickness of the superconducting CuO2 planes in high Tc superconductors (HTSC) copper oxides is found by using the Ginzburg-Landau-Gor’kov (GLG) theory. The potential turned out to be significantly suppressed due to an effect of non-superconducting layers, which separate the CuO2 planes. The effect leads also to the reduction of the critical temperature of these superconductors. The temperature dependence of the effective energy gap was calculated in this work.

Temperature dependent energy gap shifts of single color center in diamond based on modified Varshni equation

Although the diamond has the similar structure (diamond cubic) and symmetry (Oh) as many other conventional semiconductor materials, the extraordinary large Debye temperature and small lattice expansion coefficient make the temperature dependence of energy gaps in diamond to be totally different from those in other semiconductors. Here we propose a modified Varshni formula to describe the temperature dependent zero field splitting and zero phonon line of the nitrogen vacancy center in diamond. The shift of energy gaps shows a T4 scaling at low temperature and a T2 scaling at high temperature. The crossover between these two regimes is determined by two positive parameters in the modified formula. This empirical formula can give an excellent prediction of the temperature dependent energy gaps up to 700K. We experimentally verify this temperature dependent behavior with single negatively charged nitrogen vacancy centers in diamond. These observations demonstrate the leading role of electron-acoustic phonon interaction in the temperature dependent shift of energy gaps in diamond. This empirical equation can benefit the diamond-based applications such as quantum sensing and computing.

Temperature shift of intraband absorption peak in tunnel-coupled QW structure

An experimental study of the intersubband light absorption by the 100-period GaAs/Al0.25Ga0.75As double quantum well heterostructure doped with silicon is reported and interpreted. Small temperature redshift of the 1–3 intersubband absorption peak is detected. Numerical calculations of the absorption coefficient including self-consistent Hartree calculations of the bottom of the conduction band show good agreement with the observed phenomena. The temperature dependence of energy gap of the material and the depolarization shift should be accounted for to explain the shift.

Quantum confinement effect in low temperature grown homo-epitaxial GaN nanowall network by laser assisted molecular beam epitaxy

Vertically well-aligned homo-epitaxial GaN nanowall network (NWN) was grown on metal organic chemical vapor deposited 3.5 μm thick GaN (0001) on c-sapphire by laser assisted molecular beam epitaxy (LMBE). The honeycomb GaN NWN with wall width of 8–12 nm and pore size of ∼100–200 nm were obtained at a low growth temperature of 600 °C. The optical properties measured by photoluminescence (PL) spectroscopy showed a blue-shift of ∼100 meV near band edge (NBE) emission for GaN NWN. In addition, temperature dependent (90–300 K) PL measurements of GaN NWN were investigated which follow the Varshni's temperature dependent energy gap relation. The structural and optical properties of pristine and wet-etched LMBE grown homo-epitaxial GaN thin film at 500 °C were also investigated and compared with the GaN NWN structures. X-ray photoelectron spectroscopy study confirms Ga rich nature of GaN nanowalls along with significant decrease in surface band bending in NWN compared to film. The KOH wet etched of GaN NWN showed that the wall width decreased to 6–10 nm and exhibited a blue-shift of 120 meV in the NBE in PL emission confirming the realization of quantum size effect in the GaN NWN structure. The low temperature grown GaN NWN structures can be integrated with light emitting diodes and solar cells for the enhancement of light extraction and device efficiency.

Spin and valley dependent line-type resonant peaks in electrically and magnetically modulated silicene quantum structures

A barrier with a tunable spin-valley dependent energy gap in silicene could be used as a spin and valley filter. Meanwhile, special resonant modes in unique quantum structure can act as energy filters. Hence we investigate valley and spin transport properties in the potential silicene quantum structures, i.e., single ferromagnetic barrier, single electromagnetic barrier and double electric barriers. Our quantum transport calculation indicates that quantum devices of high accuracy and efficiency (100% polarization), based on modulated silicene quantum structures, can be designed for valley, spin and energy filtering. These intriguing features are revealed by the spin, valley dependent line-type resonant peaks. In addition, line-type peaks in different structure depend on spin and valley diversely. The filter we proposed is controllable by electric gating.

Temperature effect on the structural stabilities and electronic properties of X22H28 (X=C, Si and Ge) nanocrystals: A first-principles study

Based on ab initio molecular dynamic simulations, we have theoretically investigated the structural stabilities and electronic properties of X22H28 (X=C, Si, and Ge) nanocrystals, as a function of temperature with consideration of vibrational entropy effects. To compare the relative stabilities of X22H28 isomers, the vibration free energies are obtained according to the calculated phonon spectrum, where the typical modes are shown to be dominant to the structural stabilities. In addition, there is a significant gap reduction as the temperature increases from 0 K to 300 K, where the decrements are 0.2 /0.5 /0.6eV for C/Si/Ge nanocrystals, respectively. The dependence of energy gap on the variance of bond length is also analyzed according to the corresponding atomic attributions to the HOMO and LUMO levels.

Energy spectrum of surface electrons over a 3He – 4He solution with a spatially non-uniform distribution of the light isotope

The energy gap between the ground and first excited energy levels of surface electrons deposited over a dilute 3Не – 4Не solution is evaluated. Two spatial distributions of 3Не atoms near the free surface solution are considered. One consists of a thin though macroscopic 3Не film and in the other the 3Не concentration varies continuously from the surface inside the liquid. The energy gap is calculated as a function of the parameters of the 3Не spatial distribution for these distributions. It is shown that the energy gap dependence on the distribution parameters allows using measurements of intersubband transitions of the surface electrons to determine the 3Не concentration distribution and, in principle, the nature of the spatial distribution of the light isotope near the surface of the solution.

Electronic structure calculation of Si1−xSnx compound alloy using interacting quasi‐band theory

We investigate energy band structures of the Si1−xSnx compound alloy in the zincblende structure using interacting quasi‐band (IQB) theory. We first extend the IQB theory for four‐element compounds and subsequently calculate the electronic structures of the virtual Si1−xSnxSi1−ySny alloy where x = y. Diagonalizing a 20 × 20 non‐Hermitian Hamiltonian matrix using sp3s* empirical tight‐binding model with parameters obtained for the Si, Sn, and SiSn (zincblende) crystals, we obtain the electronic energy spectrum of the Si1−xSnx alloy for arbitrary x. Comparing the band structures, we reveal that the indirect–direct gap crossover in Si1−xSnx alloys occurs around x = 0.67 with Eg = 0.87 eV.Calculated Sn concentration dependence of energy gaps in Si1−xSnx alloy.

Concentration and temperature dependent double energy gap characteristic properties of hexagonal YMnO3–xBiFeO3 films

The study of hexagonal YMnO3–xBiFeO3 () (YBMF x) films can realize the modulation of h-YMO films, especially for the significant modulation of crystal orientation and surface. The choice of the solvent and solution, and the co-doping of Fe and Bi are two important facts of realizing the modulation. The results of x-ray diffraction and atomic force microscopy measurements indicated that there are the internal relations of the lattice orientation and the surface morphology for highly c-axis-orientation (HCO) and no preferred orientation (NPO), respectively. X-ray photoelectron spectroscopy confirmed that manganese ions show a valence of +3 in YMnO3 film. The peak positions of the Mn 2p level move to higher energy with increasing concentration. Furthermore, there is an optimal doping concentration of for the YBMF x () films by taking the shapes of P–E loops, the values of 2Pr and 2Ec into consideration. The transmittance spectra at room temperature pointed out that the double energy gaps (∼1.25 eV and 3.3 eV) corresponded to significant absorption edges, the band gap (Eg1) and subband gap (Eg2), respectively. The different dependency of Eg1 at the lower temperature can be explained by the ionized impurity scattering. A critical temperature (∼100 K) of Eg2 can be attributed to the relaxation of antiferromagnetic phase transformation. The abnormal shrinkage of Eg2 below 100 K can be explained by the pinning effect of composite domain walls and the accumulation of discrete oxygen vacancies at composite domain walls. The present study sheds light on the understanding of the electronic band structure, antiferromagnetic phase transformation and their relation in h-YMO. Moreover, it contributes to the verification and improvement of Katsufuji’s model (Katsufuji et al 2001 Phys. Rev. B 64 104419).

The importance of temperature dependent energy gap in the understanding of high temperature thermoelectric properties

In this work, we show the importance of temperature dependent energy band gap, Eg(T), in understanding the high temperature thermoelectric (TE) properties of material by considering LaCoO3 (LCO) and ZnV2O4 (ZVO) compounds as a case study. For the fix value of band gap, Eg, deviation in the values of α has been observed above 360 K and 400 K for LCO and ZVO compounds, respectively. These deviation can be overcomed by consideration of temperature dependent band gap. The change in used value of Eg with respect to temperature is ∼4 times larger than that of In As. This large temperature dependence variation in Eg can be attributed to decrement in the effective on-site Coulomb interaction due to lattice expansion. At 600 K, the value of ZT for n and p-doped, LCO is ∼0.35 which suggest that it can be used as a potential material for TE device. This work clearly suggest that one should consider the temperature dependent band gap in predicting the high temperature TE properties of insulating materials.

Intermolecular Vibrational Energy Transfers in Melts and Solutions

Resonant and nonresonant intermolecular vibrational energy transfers in Gdm-SCN/KSCN=1/1, GdmSCN/KS13CN=1/1 and GdmSCN/KS13C15N=1/1 mixed crystals in melts and in aqueous solutions are studied with the two dimensional infrared spectroscopy. The energy transfers in the samples are slower with a larger energy donor/acceptor gap, independent of the Raman spectra. The energy gap dependences of the nonresonant energy transfers cannot be described by the phonon compensation mechanism. Instead, the experimental energy gap dependences can be quantitatively described by the dephasing mechanism. Temperature dependences of resonant and nonresonant energy transfer rates in the melts are also consistent with the prediction of the dephasing mechanism. The series of results suggest that the dephasing mechanism can be dominant not only in solutions, but also in melts (pure liquids without solvents), only if the molecular motions (translations and rotations) are much faster than the nonresonant energy transfer processes.

Temperature dependence of energy gap of Ge1−xSnx alloys with x < 0.11 studied by photoreflectance

Compressively strained Ge1−xSnx layers of various Sn content grown on Ge/Si templates have been studied by photoreflectance in 15–295 K temperature range. It has been confirmed that the direct optical transition between heavy-hole band and conduction band shifts to lower energy upon incorporation of Sn atoms. The shift of the transition energy in that temperature range has been found to be 68–91 meV which is slightly higher than the energy shift of direct transitions in Ge over the same temperature range. Varshni and Bose–Einstein coefficients for compressively strained Ge1−xSnx have been extracted from the analysis of the direct transitions and compared with parameters from literature. In addition the heavy- and light-hole related direct optical transitions in tensile strained Ge buffer layers have been measured and analyzed.

High Frequency Sensing with Enhanced Sensitivity in Graphene Nanogrid FET Immunosensor

:-FET based charge detection sensing mechanism with various nanostructures has many advantages like label free detection, femtomolar sensitivity and compatibility with electronic interface. However detecting charges in high ionic strength solution is a challenge due to the inherent problem of ionic screening beyond the Debye length.To overcome this effect, high frequency signal has been applied in single walled carbon nanotube structures which has enabled detection of biotin in presence of 100mM buffer solution [1]. But the limit of detection achieved is not satisfactory. In this work we superimpose a high frequency ac signal in the range of 100KHz to 10MHz across the drain and source terminal in a graphene nanogrid structure and applied it for detection of IgG-protein A binding in sub femtomolar regime. The graphene nanogrid structure has been developed on a nanoporous silicon oxide template as shown in Fig.1 and is an interconnected grid structure of bilayer reduced graphene oxide(RGO) which is composed of long and narrow strips alternating with series combination of short and narrow strips on planar regions and within pores.The long and short strips are interconnected during the deposition stage and hence the structure is expected to have fewer edges. Such a structure is expected to possess certain advantages in addition to the improved binding efficiency of the biomolecules due to large surface area to volume ratio. Firstly, in the nanopores, the biomolecules usually reside within a distance shorter than the original pore radius which statistically raises the charge transfer probability between the biomolecules and the surface even at high ionic strength of buffer. As a consequence, the heterogeneous charge transfer gets facilitated and the electrode potential reaches equilibrium faster for a given electrochemical system [2]. Secondly, such a graphene nanostructure has a size dependent effective energy gap which increases the transconductance of the device [3]. Due to the absence of edges, the conductance of the fabricated smooth nanogrid structure will be contributed by every section of the device and hence all the antigen molecules that get captured at any location contribute towards the change in conductance. Also there is a probable localization effect of the charge modulation after antigen capture within the corresponding quantum dots, which can result in an enhanced change of the overall carrier transport. Fabrication of the nanoporous silicon oxide template has been carried out by anodic etching of p-type silicon<100> wafers of 10–20 Ω-cm resistivity in a double pond electrochemical bath for 30 minute using a constant current density of 2.35 mA/cm2 with an electrolyte mixture of 48% hydrofluoric acid and dimethyl sulfoxide in the ratio of 1:9 by volume. The nanopores formed are of about 30nm thickness and 100 nm length. The silicon substrates have been oxidized for subsequent graphene deposition. Graphene has been deposited by electrophoretic method[4] and after that antibody has been immobilized through poly-l-lysine (PLL). Raman spectroscopic measurements have been performed for characterization of deposited graphene[Fig:2] and the spectra of the graphene shows three Raman bands at 1195,1590 and 2670 cm-1that assigned to the well-documented D,G, and 2D bands respectively in RGO. The bilayer thickness has been confirmed by the presence of a symmetric 2D band and 2D/G intensity ratio of around 1.4. The two lateral electrodes used for RGO deposition act as the drain and source electrodes. Graphene based FET has been realized with a platinum top gate electrode suspended in buffer. Protein A has been selected as the antigen and its different concentration has been listed using 100mM buffer in the range of 100KHz to 10MHz. An ac voltage of amplitude 20 mV has been fed between the drain-source terminal. The gate potential has been fixed at 0.2 V. It has been observed [Fig:3] that the drain current (Id ) decreases with frequency for 100mM which may be attributed to the slow decay of the surface potential resulting in effective increase of the Debye length and hence the capacitive impedance. We further plotted the sensitivity [Fig.:4], calculated as the fractional change in Id before and after Protein A attachment. A peak in sensitivity has been observed at a frequency of around 8MHz which corresponds to the optimum interaction of the antigen molecules with the graphene nanostructure. The intrinsic advantages of the graphene nanogrids(as discussed above) enable detection down to 0.1 fg/ml antigen in a high ionic strength buffer which has significant impact in point-of care applications. Figure 1

Non-BCS Temperature Dependence of Energy Gap in Thin Film Electron-Doped Cuprates

We investigate the dependence of the energy gap ($G$) on the temperature ($T$) for the electron-doped high-temperature superconductors. The following compounds, in the form of the thin films, have been taken into consideration: $\rm La_{2-x}Ce_xCuO_{4}$ (LCCO), $\rm Pr_{2-x}Ce_xCuO_{4}$ (PCCO), and $\rm Nd_{2-x}Ce_xCuO_4$ (NCCO). It was found that $G\left(T\right)$ deviates from the BCS prediction more, if a concentration of cerium assumes the lower values. For the lowest concentration (in the case of LCCO and NCCO), the function $G\left(T\right)$ is not quite like the BCS curve, which is connected with the existence of the residual Nernst region. Next, it has been pointed out that the NCCO superconductor becomes structurally unstable for the maximum concentration of cerium, which is leading to the anomalous dependence of the energy gap on the temperature and the induction of the wide Nernst region.