Density Of The States Research Articles

Indium doping induced modification of the structural, optical and electrical properties of nanocrystalline CdSe thin films

Nanocrystalline thin films of CdSe:In 1% and CdSe:In 5% are prepared by thermal evaporation using the Inert Gas Condensation method. The effects of different doping concentrations of In have been investigated. X-ray diffraction, optical absorption, Photoluminescence (PL) and electrical techniques are used to characterize the films. X-ray diffraction measurements indicate that the nc-CdSe:In 1% and nc-CdSe:In 5% thin films possess hexagonal structure. The crystallinity of the films and the grain size decreases as the In doping concentration increases. The results of PL measurements indicate that the PL intensity decreases as the In doping concentration increases. The optical band gap values increase as the In doping concentration increases. The dc electrical conductivity increases from undoped nc-CdSe to nc-CdSe:In 1% and decreases for nc-CdSe:In 5%. The density of the states near Fermi level N(EF), degree of disorder (To), hopping distance (R) and hopping energy (W) near the Fermi level are calculated using dc conductivity measurements at low temperatures. The effect of In doping concentration on these parameters is discussed. Hall measurements are performed on nc-CdSe, nc-CdSe:In 1% and nc-CdSe:In 5% to calculate the carrier concentration, carrier type and mobility of charge carriers.

Interaction of pyrazinamide drug functionalized carbon and boron nitride nanotubes with pncA protein: a molecular dynamics and density functional approach

First principles calculations are performed to study the noncovalent functionalization of single-wall carbon nanotubes (SWCNTs) and boron nitride nanotubes (BNNTs) with pyrazinamide (PZA), an antitubercular chemotherapeutic, providing a detailed insight into the nanotube structure and electronic properties prior to and after functionalization. The simulations highlight the modification in electronic structure of both SWCNTs and BNNTs with the enhancement of electronic states within the valence and conduction band regions of the PZA–SWCNT system, significant lowering of the energy gap and the presence of new dispersionless bands in the PZA–BNNT system. Depending on the chirality of the nanotubes, PZA exhibits a preferential selectivity for adsorption, which is further affirmed from the band structure, density of the states, total projected density of the states and frontier orbital analysis with a charge transfer between PZA and a nanotube. A molecular docking simulation suggests that SWCNTs facilitate the targeted release of PZA within the protein through a lock and key mechanism. The intermolecular interaction between PZA–SWCNT and pncA was compared using a molecular dynamics simulation, and by observing any structural change induced within the system due to the interactions. Thus, functionalization of SWCNTs and BNNTs with PZA mediated by π–π stacking is an effective method to engineer nanotubes’ electronic properties and band gaps for applications pertinent to tuberculosis chemotherapy.

Effect of the shape of a nano-object on quantum-size states

In this paper, we propose an original functional method that makes it easy to determine the effect of any deviation in the shape of a nano-object from the well-studied shape (e.g., spherical) on the quantum characteristics of charge localized inside the nano-object. The maximum dimension of the object is determined by the magnitude of influence of quantum-size effects on quantum states of charge, and is limited by 100 nm. This method is ideologically similar to the perturbation theory, but the perturbation of the surface shape, rather than the potential, is used. Unlike the well-known variational methods of theoretical physics, this method is based on the assumption that the physical quantity is a functional of surface shape. Using the method developed, we present the quantum-size state of charges for two different complex shapes of nano-objects. The results from analyzing the quantum-size states of charge in the nano-objects with a deformed spherical shape indicated that the shape perturbations have a larger effect on the probability density of locating a particle inside the nano-object than on the surface energy spectrum and quantum density of the states.

Characterization of electronic structure in dielectric materials by making use of the secondary electron emission

The methodology of characterizing electronic structure in dielectric materials will be presented in detail. Energy distribution of the electrons emitted from dielectric materials by the Auger neutralization of ions is measured and rescaled for Auger self-convolution, which is restructured from the energy distribution of the emitted electrons. The Fourier transform is very effective for obtaining the density of states from the Auger self-convolution. The MgO layer is tested as an example of this new measurement scheme. The density of states in the valence band of the MgO layer is studied by measuring the energy distribution of the emitted electrons for MgO crystal with three different orientations of (111), (100) and (110). The characteristic energy of ɛ0 corresponding to the peak density of the states in the band is determined, showing that the (111) orientation has a shallow characteristic energy ɛ0 = 7.4 eV, whereas the (110) orientation has a deep characteristic energy ɛ0 = 9.6 eV, consistent with the observed coefficient γ of the secondary electron emission for MgO crystal. Electronic structure in new functional nano-films spayed over MgO layer is also characterized. It is therefore demonstrated that secondary electron emission by the Auger neutralization of ions is highly instrumental for the determination of the density of states in the valence band of dielectric materials. This method simultaneously determines the valence band structure and the coefficient γ of the secondary electron emission, which plays the most important role in the electrical breakdown phenomena.

Phonon-assisted tunneling through a double quantum dot system

Electron transport through a double quantum dot system is studied, taking into account the electron–phonon interaction. The Keldysh nonequilibrium Green function formalism is used to compute the current and transmission coefficient of the system. The influence of the electron–phonon interaction, interdot tunneling and temperature on the density of states and current is analyzed. Results show that the transmission coefficient is a function of the bias in the presence of the electron–phonon interaction. We found that although the electron–phonon interaction results in the appearance of side peaks in the conductance at low temperatures, these disappear at high temperatures. In addition, the temperature influences the shape of the density of the states.

Computational Study of the Vibrational Structure of the Ammonia Molecule Adsorbed on the fcc (111) Transition Metal Surfaces

We have computationally studied adsorption and vibrational energy levels of the ammonia molecule adsorbed on the fcc (111) transition metal surfaces Ni(111), Cu(111), Rh(111), Pd(111), Ag(111), Ir(111), Pt(111), and Au(111). Vibrational Hamiltonians are obtained by combining an exact kinetic energy operator for the isolated ammonia molecule with plane-wave density functional theory (DFT) potential energy surfaces. The resulting eigenvalue problems are solved variationally. This procedure gives us the anharmonic vibrational energy levels of the adsorbed ammonia molecule. The local density of the states (LDOS) analysis reveals that ammonia adsorbs to all studied surfaces through its lone pair orbital. It makes the symmetric bending potential asymmetrical around the planar structure, quenches inversion splittings, and blue shifts the symmetric bend wavenumber, in agreement with experimental observations. In this work, it has been observed that the magnitude of this blue shift depends almost linearly on the a...

First principles study of the adsorption of a NO molecule on N-doped anatase nanoparticles

The adsorption of a NO molecule on 72 atom N-doped TiO2 nanoparticles has been studied by first principles calculations. Two types of adsorption are considered in the calculations. In one type of the adsorption, the NO molecule forms one bond with the particle, while in the other type of adsorption, the NO molecule forms two bonds with the particle. The second type of adsorption is more energetic favorable. The adsorption energies, bond lengths, density of the states (DOSs), and the difference of the charge density are calculated to investigate the adsorption.In the adsorption process, the unpaired electron of the NO molecule transfers to the empty state of the particle, making the Fermi levels lower. As a result, the electrons of the N-doped system occupy lower energy states, making the system energy lower than that of the undoped particle. Since the adsorption of a NO molecule on N-doped nanoparticles is stronger than that on undoped particles, N-doped particles can adsorb more NO molecules on their surfaces than the undoped particles do. Meanwhile, there are more adsorption sites on the N-doped particles, on which the adsorption energies are much higher than that of the undoped particle, some of them are even higher than the highest adsorption energy of the undoped particle. It suggests that N-doped particles are more active and they can adsorb more small toxic gas molecules in the air. So, the doping method can be used to remove NO molecules for the air pollution control through the surface adsorption strategy.

Distribution of localized states from fine analysis of electron spin resonance spectra of organic semiconductors: Physical meaning and methodology

We develop an analytical method for the processing of electron spin resonance (ESR) spectra. The goal is to obtain the distributions of trapped carriers over both their degree of localization and their binding energy in semiconductor crystals or films composed of regularly aligned organic molecules [Phys. Rev. Lett. 104, 056602 (2010)]. Our method has two steps. We first carry out a fine analysis of the shape of the ESR spectra due to the trapped carriers; this reveals the distribution of the trap density of the states over the degree of localization. This analysis is based on the reasonable assumption that the linewidth of the trapped carriers is predetermined by their degree of localization because of the hyperfine mechanism. We then transform the distribution over the degree of localization into a distribution over the binding energies. The transformation uses the relationships between the binding energies and the localization parameters of the trapped carriers. The particular relation for the system under study is obtained by the Holstein model for trapped polarons using a diagrammatic Monte Carlo analysis. We illustrate the application of the method to pentacene organic thin-film transistors.

THE INFLUENCE OF ALPHA-IRRADIATION ON THE SILICON MOS –TRANSISTORS

This work aims at the study ofalfa-particle-induced changes in the characteristics and parameters of silicon MOS-transistors with poly-silicon gate. The influence of alfa-irradiation on the threshold voltage Uth, transconductanse S and output resistance Ri of MOS transistors was studied. It was found that alfa-irradiation is accompanied by Uth decrease with irradiation time. The physical mechanisms of radiationaldefec-tcreation are analysed. The result of the observed changes of the parameters are interpreted within the model of the radiation-induced charges: positive in the volume undergate insulator and on the surface stages of the volume Si-SiO2. The possibility of the participation of the ions of hydrogen in the formation positive built-in charge in the SiO2 is discussed. The specialities of the behavior of density of the states Dit on the border Si-SiO2 with the time of irradiation isanalised.

Theoretical Analysis of the DOS and Band Structural Properties of Ti<sub>1-X</sub>Sn<sub>x</sub>O<sub>2</sub> Solid Solutions

The band structure and density of states (DOS) of Ti1-xSnxO2 solid solutions with x=0, 1/8, 1/4, 1/2 and 1 were investigated by means of the first-principle calculations based on density functional theory. The result indicated that band gap and Fermi level of TiO2-SnO2 vary continuously from those of pure TiO2 to those of Sn content increasing. In addition, the DOS moves towards low energy and the bang gap is broadened with growing value of x. The wide band gap and the low density of the states in the conduction band result in the enhancement of photoactivity in Ti1-xSnxO2.

Nitrogen doping of graphene and its effect on quantum capacitance, and a new insight on the enhanced capacitance of N-doped carbon

Many researchers have used nitrogen (N) as a dopant and/or N-containing functional groups to enhance the capacitance of carbon electrodes of electrical double layer (EDL) capacitors. However, the physical mechanism(s) giving rise to the interfacial capacitance of the N-containing carbon electrodes is not well understood. Here, we show that the area-normalized capacitance of lightly N-doped activated graphene with similar porous structure increased from 6 μF cm−2 to 22 μF cm−2 with 0 at%, and 2.3 at% N-doping, respectively. The quantum capacitance of pristine single layer graphene and various N-doped graphene was measured and a trend of upwards shifts of the Dirac Point with increasing N concentration was observed. The increase in bulk capacitance with increasing N concentration, and the increase of the quantum capacitance in the N-doped monolayer graphene versus pristine monolayer graphene suggests that the increase in the EDL type of capacitance of many, if not all, N-doped carbon electrodes studied to date, is primarily due to the modification of the electronic structure of the graphene by the N dopant. It was further found that the quantum capacitance is closely related to the N dopant concentration and N-doping provides an effective way to increase the density of the states of monolayer graphene.

The effects of Mo doping on 0.3Li[Li0.33Mn0.67]O2·0.7Li[Ni0.5Co0.2Mn0.3]O2 cathode material

Mo doped Li excess transition metal oxides formulated as 0.3Li[Li(0.33)Mn(0.67)]O(2)·0.7Li[Ni(0.5-x)Co(0.2)Mn(0.3-x)Mo(2x)]O(2) were synthesized using the co-precipitation process. The effects of the substitution of Ni and Mn with Mo were investigated for the density of the states, the structure, cycling stability, rate performance and thermal stability by tools such as first principle calculations, synchrotron X-ray diffraction, field-emission SEM, solid state (7)Li MAS nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), elemental mapping by scanning TEM (STEM), inductively coupled plasma atomic emission spectrometry (ICP-AES) and a differential scanning calorimeter (DSC). It was confirmed that high valence Mo(6+) doping of the Li-excess manganese-nickel-cobalt layered oxide in the transition metal enhanced the structural stability and electrochemical performance. This increase was due to strong Mo-O hybridization inducing weak Ni-O hybridization, which may reduce O(2) evolution, and metallic behavior resulting in a diminishing cell resistance.

Atomic and Electronic Properties of Realizable Size Single-Crystal GaN Nanotubes by First Principles

We studied the diameter and wall thickness dependent atomic and electronic properties of practical size single-crystal GaN nanotubes using first principle calculations. Single-crystal GaN nanotubes are similar to the hexagonal GaN nanowires, grown in the [0001] direction with [10-10] facets, except there is an axial hexagonal void in them. We first demonstrated that the atomic and electronic properties of these tubes are mainly determined by the thickness of their wurtzite walls; and their diameters have negligible effects. Then, considering the individual walls of GaN nanotubes in two-dimensional slab calculations we examine the bond distances, formation energy, band gap, effective electron mass and the evolution of electronic density of the states as a function of thickness for unsaturated and hydrogen-saturated slabs of GaN. Calculations revealed that the unsaturated dangling bonds at the surfaces induce defect states in the band gap region of unsaturated tubes. Therefore, regardless of diameter and wall thickness, their band gaps are always smaller than that of the bulk GaN. However, the band gaps of the hydrogen-saturated tubes are found to be amplified with respect to bulk GaN. The amplification in the band gaps as a function of wall thickness in the range of 5.6-16.9 A and 16.9-28.1 A scales with a factor of 1/d(0.9281) and 1/d(1.769), respectively. Our results show that, regardless of diameter, hydrogen saturated single-crystal GaN tubes with the wall thickness as small as 28.1 A would be stable and they would have a noticeably larger band gap with respect to the band gap of bulk GaN.

First-principle study of NO adsorption on the LaFeO 3 (0 1 0) surface

The adsorption of NO molecule on the LaFeO 3 (0 1 0) surface was studied using first-principle calculations based on density functional theory. The calculated results indicate that the Fe-top site is the most favorable for NO adsorption. The N–O bond length, Mulliken charge, and the N–O vibration frequency of the NO molecule are discussed after adsorption. The analysis results of the density of the states show that when NO is adsorbed with the Fe–NO configuration, the bonding mechanism is mainly from the interaction between the NO and the Fe d orbit.

Probing the band structure of hydrogen-free amorphous carbon and the effect of nitrogen incorporation

Amorphous carbon and carbon nitride bottom gate thin film transistors have been fabricated, which show bulk carrier field effect mobilities around 10 −3 (cm 2 V −1 s −1) which is orders of magnitude higher than the previously reported values with p-channel devices at high electric fields between source and drain. The incorporation of nitrogen atoms into the amorphous carbon films deposited by pulsed laser deposition was studied using a wide range of techniques in order to understand the role of nitrogen in the conduction mechanism at high fields. The density of the states (DOS) was measured with several techniques such as electron energy loss spectroscopy, scanning tunnelling spectroscopy, and ultraviolet photoelectron spectroscopy, whereas the joint density of states (JDOS), corresponding to the transitions of electrons from the valence to the conduction bands, were obtained by optical transmittance and photothermal deflection spectroscopy. These measurements when combined can provide unparallel data on the shape and magnitude of the energy band states which are crucial to the understanding of the materials properties and thus opto-electronic applications for these thin films. In this report, the conduction mechanism will be discussed with a band diagram drawn based on the experimentally obtained DOS and JDOS measurements.

First-principles calculations on electronic structures of Fe-vacancy-codoped TiO 2 anatase (1 0 1) surface

The electronic structures of Fe-doped TiO 2 anatase (1 0 1) surfaces have been investigated by all spin-polarized density functional theory (DFT) plane-wave pseudopotential method. The general gradient approximation (GGA)+U (Hubbard coefficient) method has been adopted to describe the exchange-correlation effects. Through the density functional calculations for the formation energies of various configurations, the complex of a substitutional Fe plus an O vacancy was found to form easily in the most range of O chemical potential. The calculated density of the states of the system of Fe-doped surface with a surface oxygen vacancy shows a band gap narrowing from 2.8 to 1.9 eV comparing with the pure surface due to the synergistic effects of surface Fe impurities with O vacancies. The system processes high visible light sensitivity and photocatalytic ability by decreasing extrinsic absorption energy. By comparing the partial DOS of some O and Ti atoms lying in the outermost and bottom layers of Fe-doped surfaces, it was found that the influence of Fe impurities on the electronic structure of the system is localized.

Distribution of electron density of states in allowed bands and interband absorption in amorphous semiconductors

Different types of electronic transitions and the corresponding spectral characteristics of the absorption coefficient of amorphous semiconductors, where the energy of absorbed photons exceeds the mobility band gap, have been investigated. Partial spectral characteristics of the absorption coefficient and, correspondingly, the distributions of the electron density of the states involved in these optical transitions are obtained for the case where the electron densities of states in the allowed bands and the tails of these bands change with energy according to the power and exponential laws, respectively. The conditions for the occurrence of a peak in the spectral characteristic of the interband absorption coefficient are determined.

Constrained particle filter approach to approximate the arrival cost in Moving Horizon Estimation

Moving Horizon Estimation (MHE) is an efficient state estimation method used for nonlinear systems. Since MHE is optimization-based it provides a good framework to handle bounds and constraints when they are required to obtain good state and parameter estimates. Recent research in this area has been directed to develop computationally efficient algorithms for on-line application. However, an open issue in MHE is related to the approximation of the so-called arrival cost and of the parameters associated with it. The arrival cost is very important since it provides a means to incorporate information from the previous measurements to the current state estimate. It is difficult to calculate the true value of the arrival cost; therefore approximation techniques are commonly applied. The conventional method is to use the Extended Kalman Filter (EKF) to approximate the covariance matrix at the beginning of the prediction horizon. This approximation method assumes that the state estimation error is Gaussian. However, when state estimates are bounded or the system is nonlinear, the distribution of the estimation error becomes non-Gaussian. This introduces errors in the arrival cost term which can be mitigated by using longer horizon lengths. This measure, however, significantly increases the size of the nonlinear optimization problem that needs to be solved on-line at each sampling time. Recently, particle filters and related methods have become popular filtering methods that are based on Monte-Carlo simulations. In this way they implement an optimal recursive Bayesian Filter that takes advantage of particle statistics to determine the probability density properties of the states. In the present work, we exploit the features of these sampling-based methods to approximate the arrival cost parameters in the MHE formulation. Also, we show a way to construct an estimate of the log-likelihood of the conditional density of the states using a Particle Filter (PF), which can be used as an approximation of the arrival cost. In both cases, because particles are being propagated through the nonlinear system, the assumption of Gaussianity of the state estimation error can be dropped. Here we developed and tested EKF and eight different types of sample based filters for updating the arrival cost parameters in the weighted 2-norm approach (see Table 1 for the full list). We compare the use of constrained and unconstrained filters, and note that when bounds are required the constrained particle filters give a better approximation of the arrival cost parameters that improve the performance of MHE. Moreover, we also used PF concepts to directly approximate the negative of the log-likelihood of the conditional density using unconstrained and constrained particle filters to update the importance distribution. Also, we show that a benefit of having a better approximation of the arrival cost is that the horizon length required for the MHE can be significantly smaller than when using the conventional MHE approach. This is illustrated by simulation studies done on benchmark problems proposed in the state estimation literature.

Temperature-dependent ac conductivity and dielectric response of vanadium doped CaCu3Ti4O12 ceramic

Successful incorporation of vanadium dopant within the giant dielectric material CaCu 3Ti 4O12 (CCTO) through a conventional solid-state sintering process is achieved and its influence on the dielectric as well as electrical properties as a function of temperature and frequency is reported here. Proper crystalline phase formation together with dopant induced lattice constant shrinkage was confirmed through X-ray diffraction. The temperature dependence of the dielectric constant at different constant frequencies was investigated. We infer that the correlated barrier hopping (CBH) model is dominant in the conduction mechanism of the ceramic as per the temperature-dependent ac conductivity measurements. The electronic parameters such as density of the states at the Fermi level, N(Ef) and hopping distance, Rω of the ceramic were also calculated using this model.

Interface-dependent magnetic anisotropy of Fe/BaTiO3: A first principles study

Using first principles calculations, we investigated interface structure effects on the magnetic properties of the Fe/BaTiO3 system. On the BaO-terminated surface, a Fe monolayer is formed as two Fe atoms are adsorbed on the top sites of Ba and O in the (1×1) surface unit and a Fe monolayer (ML) is formed on the TiO2-terminated surface as two Fe atoms are adsorbed on the two O top sites. The magnetic anisotropy energy of Fe was higher on the TiO2–terminated surface (1.5 eV) than on the BaO-terminated surface (0.5 eV). The decomposed electron density of the states showed that the stronger hybridization of Fe with the TiO2 layer than with the BaO layer is the most important reason for the higher magnetic anisotropy energy.