Energy Band Dispersion Research Articles

Monte Carlo simulation of electron transport in a graphene diode with a linear energy band dispersion

Electron transport and energy relaxation in a 100-nm channel n+-n-n+ monolayer graphene diode were studied by using semiclassical Monte Carlo particle simulations. A diode with a conventional parabolic band and an identical geometry and scattering process was also analyzed in an attempt to confirm that the characteristic transport properties originated from the linear energy band structure. We took into account two scattering mechanisms: isotropic elastic scattering and inelastic phonon emission. The carrier velocity distributions in the two diodes show remarkable differences reflecting their band dispersions. Electron velocity in the monolayer graphene diode is high in the channel region and remains almost constant until the energy relaxation begins. Inelastic scattering does not reduce electron velocity so severely, whereas elastic scattering significantly decreases it through backscattering of hot electrons with high kinetic energy. Elastic scattering also degrades the ballisticity and the drain current; however, increasing the inelastic scattering offsets these effects. We found that elastic scattering should be suppressed to improve the performance of graphene devices.

Microscopic Mechanism of 1/f Noise in Graphene: Role of Energy Band Dispersion

A distinctive feature of single-layer graphene is the linearly dispersive energy bands, which in the case of multilayer graphene become parabolic. A simple electrical transport-based probe to differentiate between these two band structures will be immensely valuable, particularly when quantum Hall measurements are difficult, such as in chemically synthesized graphene nanoribbons. Here we show that the flicker noise, or the 1/f noise, in electrical resistance is a sensitive and robust probe to the band structure of graphene. At low temperatures, the dependence of noise magnitude on the carrier density was found to be opposite for the linear and parabolic bands. We explain our data with a comprehensive theoretical model that clarifies several puzzling issues concerning the microscopic origin of flicker noise in graphene field-effect transistors (GraFET).

Green Eu 2+-doped Ba 3Si 6O 12N 2 phosphor for white light-emitting diodes: Synthesis, characterization and theoretical simulation

The Eu 2+-doped Ba 3Si 6O 12N 2 green phosphor (Eu x Ba 3− x Si 6O 12N 2) was synthesized by a conventional solid state reaction method. It could be efficiently excited by UV-blue light (250–470 nm) and shows a single intense broadband emission (480–580 nm). The phosphor has a concentration quenching effect at x=0.20 and a systematic red-shift in emission wavelength with increasing Eu 2+ concentration. High quantum efficiency and suitable excitation range make it match well with the emission of near-UV LEDs or blue LEDs. First-principles calculations indicate that Ba 3Si 6O 12N 2:Eu 2+ phosphor exhibits a direct band gap, and low band energy dispersion, leading to a high luminescence intensity. The origin of the experimental absorption peaks is clearly identified based on the analysis of the density of states (DOS) and absorption spectra. The photoluminescence properties are related to the transition between 4f levels of Eu and 5d levels of both Eu and Ba atoms. The 5d energy level of Ba plays an important role in the photoluminescence of Ba 3Si 6O 12N 2:Eu 2+ phosphor. The high quantum efficiency and long-wavelength excitation are mainly attributed to the existence of Ba atoms. Our results give a new explanation of photoluminescence properties and could direct future designation of novel phosphors for white light LED.

Electronic States of Organic Semiconductors and Their Interfaces: Unraveling Electrical Conduction

This article describes recent progress in organic semiconductor research with ultraviolet photoelectron spectroscopy (UPS). Origins of the energy level alignment at organic-conductor/dielectrics interfaces and the charge mobility in organic semiconductor systems are the most important subjects to be elucidated. Direct experimental detection of very low electronic density-of-states in the band gap is necessary to unravel mysteries of the energy level alignment. Experimental measurements of the energy band dispersion give information about the intermolecular interaction and the effective mass of transporting hole, and experimental detection of the highest occupied molecular orbital (HOMO) holemolecular vibration coupling is indispensable to comprehend impacts of the coupling on the hole transport. In this article, we summarize our recent challenges on UPS measurements of organic semiconductors, which give the band dispersion of the HOMO, the HOMO hole-vibration coupling and the band-gap states appearing near the interface.

First-principles simulations of the electronic density of states for superionic Ag 2CdI 4 crystals

Energy band dispersion calculations have been performed for Ag 2CdI 4 superionic within a framework of local density approximation (Perdew–Zunger parameterization) exploiting the first-principles CASTEP computer code. The ab-initio electronic structure simulations were performed for both ( I4̅ and I4̅2 m) types of ε-Ag 2CdI 4 crystalline structures. Principal optical functions as well as the density of electronic states in the spectral range of inter-band optical transitions (2.5 eV–20 eV) were determined. Theoretically calculated absorption coefficients derived from the obtained band structure are compared with appropriate experimental data.

Ab initio study of electronic structures of InAs and GaSb nanowires along various crystallographic orientations

InAs and GaSb nanowires oriented along different crystallographic axes—the [0 0 1], [1 0 1] and [1 1 1] directions of zinc-blende structure—have been studied utilizing a first-principles derived nonlocal screened atomic pseudopotential theory, to investigate the band structure, polarization ratio and effective masses of these semiconductor nanowires and their dependences on the wire lateral size and axis orientation. The band energy dispersion over entire Brillouin zone and orbital energy are determined and found to exhibit different characteristics for three types of wires. There is an explicit dispersion hump in the conduction bands of [0 0 1] nanowires with two larger diameters and [1 0 1] nanowires with the smallest diameter considered. Moreover, the [1 1 1] nanowires are shown to exhibit very different orbital energy for the maximum valence state at the zone-boundary point, compared with [0 0 1] and [1 0 1] nanowires. These differences present significant and detailed insight for experimental determination of the band structure in InAs and GaSb nanowires. Furthermore, we study the polarization ratio of these nanowires for different orientations. Our calculation results indicate that, for the same lateral size, the [1 1 1] nanowires give extraordinarily higher polarization ratio compared to nanowires along the other two directions, and at the same time have larger band-edge photoluminescence transition intensity. Consequently, the [1 1 1] nanowires are predicted to be better suitable for optoelectronic applications. We also significantly find that polarization ratio and transition intensity displays different varying trend of dependence on lateral size of nanowires. Specially, the calculated polarization ratio is shown to increase with the decreasing size, which is opposite to the behavior displayed by the optical transition intensity. The predicted polarization ratios of [1 0 1] and [1 1 1] nanowires for 10.6 Å diameter approach the limit of 100%. In addition, the electron and hole masses for InAs and GaSb nanowires with different crystallographic axes have been calculated. For the [1 0 1] and [1 1 1] oriented nanowires, the hole masses are predicted to be around 0.1–0.2 m 0, which are notably smaller than the values (∼0.5 m 0) along the same direction for their bulk counterparts. Thus, we demonstrates an inspired possibility of obtaining a high hole mobility in nanowires that is not available in bulk. The small hole mobility is interpreted as to be associated with the strong electronic band mixing in nanowires.

Electronic and Magnetic Structures in O/Cr(001) Surface from Angle-Resolved Photoemission Spectroscopy

We measured the angle-resolved photoemission spectra of Cr(001) surface with and without oxygen adsorption to study the surface and interfacial electronic structures in relation to its magnetism. The experimental results show that the surface states and Cr 3 d –O 2 p interfacial state are found near the Fermi level, and they are responsible to the surface ferromagnetism of Cr(001). The energy band dispersion of this interfacial state indicates the odd symmetry with respect to the mirror plane (010), and originates from the 3 d x y –2 p y antibonding state. It was found that the fully occupied antibonding state leading to the negative surface relaxation results in the enhancement of magnetic moments in O/Cr(001). This magneto-volume effect also realized that the energy band width of O/Cr(001) is at most 50% less than that of clean Cr(001) surface.

Energy band dispersion in photoemission spectra of argon clusters

Using photoemission we have investigated free argon clusters from a supersonic nozzle expansion in the photon energy range from threshold up to 28 eV. Measurements were performed both at high resolution with a hemispherical electrostatic energy analyser and at lower resolution with a magnetic bottle device. The latter experiments were performed for various mean cluster sizes. In addition to the ≈1.5 eV broad 3p-derived valence band seen in previous work, there is a sharper feature at ≈15 eV binding energy. Surprisingly for non-oriented clusters, this peak shifts smoothly in binding energy over the narrow photon energy range 15.5–17.7 eV, indicating energy band dispersion. The onset of this bulk band-like behaviour could be determined from the cluster size dependence.

Geometric optics of Bloch waves in a chiral and dissipative medium

We present a geometric optics theory for the transport of quantum particles (or classical waves) in a chiral and dissipative periodic crystal subject to slowly varying perturbations in space and time. Taking account of some properties of particles and media neglected in previous theory, we find important additional terms in the equations of motion of particles. The (energy) current density field, which traces the geometric optics rays, is not only governed by the Bloch band energy dispersion but also involves there additional fields. These are the angular momentum of the particle, the dissipation dipole density, and various geometric gauge fields in the extended phase space spanned by space-time and its reciprocal, momentum and frequency. For simplicity, the theory is presented using light propagation in photonic crystals.

Antiferromagnetic Phase Transition in Ordered FePt3 Investigated by Angle-Resolved Photoemission Spectroscopy

The electronic band structure of ordered FePt 3 (110) single crystal was studied by angle-resolved photoemission spectroscopy with synchrotron radiation. The energy band dispersions along the X –Γ– X and R – M – R directions of a simple cubic Brillouin zone were successfully observed at room temperature. Along the latter direction, band folding due to antiferromagnetic (AF) ordering was clearly observed below the Néel temperature ( T N2 ) of 100 K. Through comparison with theoretical calculations, (1/2, 0, 0)-type AF ordering or a mixture of the (1/2, 0, 0)-type and (1/2, 1/2, 0)-type AF orderings was concluded to occur below T N2 . These magnetic orderings can be understood from the viewpoint of the nesting between the electron and hole pockets.

Momentum-dependent resonant inelastic X-ray scattering at the Si K edge of 3C-SiC: A theoretical study on a relation between spectra and valence band dispersion

Abstract We theoretically demonstrate that a resonant inelastic X-ray scattering (RIXS) with a sizable momentum transfer can be utilized to study valence band dispersion for broad band materials. We take RIXS at the Si K edge of 3C-SiC as a typical example. The RIXS spectra are calculated by systematically changing the transferred momentum, an incident photon polarization and an incident photonenergy, on the basis of an ab initio calculation. We find that the spectra depend heavily on both the transferred momentum and the incident photon polarization, and the peaks in the spectra correspond to the energies of the valence bands. We conclude that the information on the energy dispersion of valence bands can be extracted from the transferred momentum dependence of the RIXS spectra. These findings lead to further application for RIXS when investigating the band structure of broad band materials.

Electronic and optical properties of single-walled carbon nanotubes under a uniform transverse electric field: A first-principles study

A systematic study on the effect of a transverse electric field on the optical properties of carbon nanotubes has been performed using density-functional calculations. The band gaps of the zigzag tubes decrease significantly as the electric field is increased. In the case of armchair tubes, the metallic band energy dispersion relation develops multivalleys at the Fermi level at high fields. The electric-field-induced changes in band dispersions lead to changes in optical properties as manifested in the dielectric functions. Some of the changes are not obvious. For example, a significant field-induced change in band gap can lead to small changes in dielectric functions in some special cases. Our results show that the armchair carbon nanotubes can be a promising material for electro-optical modulation device applications.

Electronic structure of bis(benzo)pentathienoacene in gas and solid phase: ultraviolet photoemission spectroscopy and energy band calculation

Ultraviolet photoemission spectroscopic measurements of bis (benzo)pentathienoacene were carried out in gas and solid phase. For the measurements of solid phase, vacuum deposited films in both amorphous and crystalline phase were prepared on different substrates of HOPG and polycrystalline Au, respectively. The adiabatic ionization energies were determined to be 6.84, 5.32, and 5.08 eV, for gas, amorphous, and crystalline phases, respectively. The spectral lineshapes were interpreted with the aid of the density functional calculations for both isolated molecule and single-crystal structure. The calculated electronic structures were further analyzed in terms of the energy band dispersion and the transport properties of charge carriers.

Electronic properties of iron arsenic high temperature superconductors revealed by angle resolved photoemission spectroscopy (ARPES)

We present an overview of the electronic properties of iron arsenic high temperature superconductors with emphasis on low energy band dispersion, Fermi surface and superconducting gap. ARPES data is compared with full-potential linearized plane wave (FLAPW) calculations. We focus on single layer NdFeAsO 0.9F 0.1 (R1111) and two layer Ba 1− x K x Fe 2As 2 (B122) compounds. We find general similarities between experimental data and calculations in terms of character of Fermi surface pockets, and overall band dispersion. We also find a number of differences in details of the shape and size of the Fermi surfaces as well as the exact energy location of the bands, which indicate that magnetic interaction and ordering significantly affects the electronic properties of these materials. The Fermi surface consists of several hole pockets centered at Γ and electron pockets located in zone corners. The size and shape of the Fermi surface changes significantly with doping. Emergence of a coherent peak below the critical temperature T c and diminished spectral weight at the chemical potential above T c closely resembles the spectral characteristics of the cuprates, however the nodeless superconducting gap clearly excludes the possibility of d-wave order parameter. Instead it points to s-wave or extended s-wave symmetry of the order parameter.

High-Resolution Angle-Resolved Photoemission Study of the Al(100) Single Crystal

The free-electron-like surface-derived electronic state in Al(100) has been examined in detail by high-resolution angle-resolved photoemission spectroscopy (ARPES). We observed a kink structure in the energy band-dispersion at the energy of ∼ -40 meV, which is derived from the electron-phonon interaction. Based on the quantitative analyses of the ARPES line shapes, we have obtained the imaginary and real parts of the self-energy. The electron-phonon coupling parameter has been evaluated to be λel-ph = 0.67, which is much larger than the electron-electron coupling parameter λel-el = 0.06. [DOI: 10.1380/ejssnt.2009.57]

Electronic structure of disjoint diradical 4,4′-bis(1,2,3,5-dithiadiazolyl) thin films

The electronic structure of the thiazyl diradical, 4,4'-bis(1,2,3,5-dithiadiazolyl) (BDTDA), has been investigated by ultraviolet photoemission spectroscopy. Stacked BDTDA dimers showed an energy band dispersion of about 0.3 eV for the highest occupied molecular orbital in the direction of the surface normal of the BDTDA solid film. The pi-orbital overlap between the stacked dimers therefore evolves into a quasi one-dimensional energy band along the dimer stacking direction.

Energy band and electron-vibration coupling in organic thin films: photoelectron spectroscopy as a powerful tool for studying the charge transport

This article describes the origins of the width of the highest occupied molecular orbital (HOMO) state observed in the ultraviolet photoemission spectra (UPS) of thin organic semiconductor films. Although much research has been performed on the electrical properties of organic devices, a lot of crucial problems still remain. Among these problems, the charge mobility in organic semiconductor systems is one the most important subjects to be elucidated. In order to discuss the mobility, it is essential to understand both the intermolecular interaction and the electron-molecular vibration coupling. Experimental measurements of the energy band dispersion give information about the intermolecular interaction, and experimental detection of the HOMO hole-vibration coupling is indispensable to comprehend impacts of the electron-vibration coupling on the hole transport. Since most of the information is hidden behind the finite bandwidth of the HOMO, only careful UPS measurements can provide information on these important phenomena related to charge carrier dynamics. In this article, we summarize our recent challenges on UPS measurements of organic thin films, which give the band dispersion of the HOMO and the HOMO hole-vibration coupling, and discuss the origins of the UPS bandwidth that relates to the charge carrier dynamics.

Dynamical properties and electronic structure of (LaLi)TiO3 conductors

A computer simulation by a molecular dynamics method is performed to study the properties of structure and Li ion diffusion in La4/3 − xLi\(_{3x}\Box_{2/3-2x}\) Ti2O6 =(LaLi)TiO3 =LLTO, which is the perovskite-type Li ion conductor. In the low Li concentration, Li ions conduct a two-dimensional motion, while Li ions diffuse a three-dimensional motion in the high Li ion concentration. The partial distribution function for Li–Ti and the diffusion paths of Li ions suggest that Li ions stay for a long time at off-site positions, which are 2.7A away from a body-centered Ti ion. The Li ion concentration dependence to the conductivity σ is in approximate agreement with experiments. The energy band dispersion and the density of states are calculated using the linear-muffin-tin-orbital method. The energy contour map shows the stable position of Li ions is off center of the vacant La sites. Both calculations suggest that the stable position of Li ions is off center of the vacant La sites.

Fundamental interface properties in OFETs: Bonding, structure and function of molecular adsorbate layers on solid surfaces

AbstractThe functionality of organic field effect transistors resides in their interfaces. Highly ordered molecular adsorbate layers provide an excellent tool to study the relevant physical properties of these interfaces. In this article we focus on molecules at the organic/metal interface. We find that their geometry and electronic structure are very responsive to influences from the substrate and the molecular environment, while interface structures evolve cooperatively in the sway of interfacial and intermolecular interactions and can thus be extremely complex. Although our results cannot be transferred directly to real devices, a number of intriguing physical effects emerge that could play an important role in (nanoscale) OFETs, e.g. an anomalously strong energy band dispersion at the organic/metal interface, a remarkable plasticity of the electronic structure of molecules in ultra‐thin films, and Kondo physics in a non‐magnetic molecular wire. Finally, we report the realization of a mechanically gated single‐molecule transistor that allows investigating the transport physics in a molecular wire as a function of contact properties. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic structure at highly ordered organic/metal interfaces: Pentacene on Cu(110)

The electronic structure at highly ordered pentacene monolayer prepared on Cu(110) substrate was studied by angle-resolved ultraviolet photoemission spectroscopy. The valence-level photoemission line shape showed the evidences of (i) formation of the interface states and (ii) two-dimensional energy-band dispersion of the resultant interface states. The lattice constant deduced from the observed energy-band dispersion is consistent with the reported one based on the low-energy electron diffraction experiments. Thus, the observed energy-band dispersion can be ascribed to the in-plane intermolecular energy-band dispersion in the pentacene monolayer on Cu(110). These phenomena may originate from the hybridization between the molecular orbital and the wave function of the substrate surface. Furthermore, work-function change of about $\ensuremath{-}0.9\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ by adsorption of pentacene was observed from the shift of the secondary-electron cutoff. Such a decrease of the work function indicates the formation of a dipole layer at the interface with the molecule positively charged. This direction is opposite to the naive expectation from the electron transfer from the substrate to the molecule, which was suggested from the previous work of core-level photoemission spectroscopy [McDonald et al., Surf. Sci. 600, 1909 (2006)]. This unexpected result may originate from the charge redistribution at the interface due to the induced image charge in the metal and the push back of electrons spilled out from the metal surface by the adsorbed molecules, which may overwhelm the effect of electron transfer.