Model Of Interband Transitions Research Articles

A study of mechanism on infrared photoresponse in three-dimensional single-walled carbon nanotubes

Two fundamental models have been formulated to explain the origin of the photoresponse in the electrical conductivity of SWNT films for an illumination of infrared (IR): the interband transition model (band model) and the bolometric model. However, the photoresponse of SWNTs has generated considerable debate, with various studies leading to different interpretations of the origin of photoconductivity. We fabricated three-dimensional SWNTs (3D-SWNTs) suspended between SiO2 pillars grown on flat quartz to suggest the origin of the photoresponse of SWNTs. From the measurements of photoconductivity of our samples under three conditions using a continuous Xe lamp, one IR laser pulse, and a repeated IR laser pulse, we demonstrated that two response modes coexist.

Interband plasmons in supported graphene on metal substrates: Theory and experiments

We present a theoretical modeling of the energy-loss spectroscopy data for monolayer graphene (MLG) supported by Pt(111), Ru(0001) and Ni(111) substrates. To reproduce the experimental loss function, we have used a two-dimensional, two-fluid hydrodynamic model for interband transitions of graphene's π and σ electrons and an empirical Drude–Lorentz model in the local approximation for metal substrates. The electronic response from the visible to the ultraviolet frequency range has been nicely reproduced for MLG/Pt(111) and MLG/Ru(0001). For graphene nanodomes on Ru(0001), the loss function of valleys shows an additional mode at 8–9 eV. By contrast, this models fails for the case of MLG/Ni(111), presumably due to the strong hybridization between the π states of graphene and the d bands of Ni, which is not accounted for in the model.

Discrete states and carrier-phonon scattering in quantum dot population dynamics.

The influence of the growth conditions of multilayer CdTe/ZnTe quantum dots (QDs) on Si substrate upon their carrier dynamics is studied using intensity integration and broadening photoluminescence. The unusual temperature dependence of the line broadening is explained using a model for interband transitions that involves a lowest discrete electronic state (1Se) with different discrete hole states (1S3/2 and 2S3/2) and a 1P transition. These transitions are expected to play a critical role in both the thermally activated energy and the line broadening of the QDs. We also demonstrate that a thermally activated transition between two different states occurs with band low-temperature quenching, with values separated by 5.8–16 meV. The main nonradiative process is thermal escape assisted by carrier scattering via emission of longitudinal phonons through the hole states at high temperature, with an average energy of 19.3–20.2 meV.

Theoretical modeling of experimental HREEL spectra for supported graphene

Abstract We present a simple model for the resonant feature designated as the π plasmon in single-layer graphene supported by a metal substrate, which is excited via high-resolution electron energy loss spectroscopy (HREELS) in the off-specular scattering geometry. Using a two-dimensional, two-fluid hydrodynamic model for interband transitions of graphene's π and σ electrons and an empirical Drude–Lorentz model for the metal in the local approximation enables us to reproduce, at the qualitative and semi-quantitative levels, the typical experimental features of the HREEL spectra in the visible to the ultraviolet frequency range.

Comparative study of tetragonal Cu 2In 7Se 11.5 and trigonal CuIn 5Se 8 by spectroscopic ellipsometry

A comparative study of chalcopyrite-related Cu 2In 7Se 11.5 and CuIn 5Se 8 crystals by spectroscopic ellipsometry is presented. Their complex dielectric function ɛ( ω) = ɛ 1( ω) + iɛ 2( ω) has been determined in the 0.8–4.7 eV photon energy range. The spectral dependence of ɛ 1( ω) and ɛ 2( ω) as well as the complex refractive index, the absorption coefficient and the normal-incidence reflectivity for tetragonal Cu 2In 7Se 11.5 and trigonal CuIn 5Se 8 crystals have been modelled using the Adachi's model for interband transitions. The results are in a good agreement with the experimental data over the entire range of photon energies. The model parameters, including the energies corresponding to the lowest direct gap ( E 0) and higher critical points ( E 1 A , E 1 B ), have been determined using the simulated annealing algorithm. The results show that the tetragonal CuIn 3Se 5, the tetragonal Cu 2In 7Se 11.5 and the trigonal CuIn 5Se 8 crystals belong to different phases of the Cu 4In 2 n Se 2+3 n system, n = 6, n = 7 and n = 10, respectively.

Optical properties of AlxGa1−xN alloy

The optical response of the AlxGa1−xN alloy has been analyzed using a simplified model of interband transitions (E≤10 eV). The present model reveals distinct structures at energies of the E0, E1, F1, and E0′ critical points. The Cauchy−Lorentz expression popularly used for the modeling of the three-dimensional M0 and saddle-point excitonic transitions does not satisfy the Kramers−Krönig requirements but does so if its form is properly modified. As a result, excellent agreement is achieved between the experimental and calculated ε(E) spectra over the entire range of photon energies. Dielectric-related optical constants, such as the complex refractive index, absorption coefficient, and normal-incidence reflectivity, of the AlxGa1−xN alloy are also presented. The high-frequency and static dielectric constants of the AlxGa1−xN alloy are determined to be ε∞(x)=5.27−1.07x and εs(x)=9.28−1.45x, respectively.

Fundamental optical spectra of ferroelectric NaNO2

Polarized spectra of complete complexes for fundamental optical functions of the NaNO2 ferroelectric at 77K in the region of 4–24 eV for E ‖ a, E ‖ b, and E ‖ c are calculated. For the first time, integral polarized curves of dielectric permeability and specific volumetric losses of electrons are factorized into elementary transverse and longitudinal components of transfer bands and their main parameters are determined. For the calculations, the experimental reflection spectra, methods of Kramer-Kronig, and joint Argand diagrams are used. On the basis of theoretical calculations for the zones and dielectric permeability, a scheme of the nature and localization of the obtained bands for the components of separation of ɛ2 is proposed in accordance with the model of interband transitions.

The synthesis and fabrication of horizontally aligned single-walled carbon nanotubessuspended across wide trenches for infrared detecting application

Horizontally aligned single-walled carbon nanotubes (SWNTs) were directly grown across trenches producedon SiO2/Si substrate, forming suspended aligned SWNTs over the trenches, and werefabricated with simple methods for infrared (IR) detection application.The horizontally aligned SWNTs can grow across trenches as wide as40 µm. The detector based on such SWNTs shows sharp and remarkable responses to theON/OFF state changes of the IR lamp and IR radiation with different frequencies. Theresistance of the SWNTs decreases about 22.86% under IR radiation with powerless than 4 mW. The detector photoresponse to IR radiation of 120 Hz at roomtemperature could be easily observed. The response time constant is only about500 µs, which is more than two orders of magnitude shorter than that of the reported devicesbased on SWNT films and SWNT–polymer composites. This was achieved due to theabsence of intertube junctions in our horizontally aligned SWNTs which exist in SWNTbundles and films and may reduce the signal conduction speed and the IR photoresponseperformance. Instead of the bolometric model, the interband transition model is suggestedto be the dominating origin of the photoresponse of our SWNT-based detectorconfiguration.

Optical constants of CuGa5Se8 crystals

Spectral dependence of the real ε1(ω) and imaginary ε2(ω) parts of the complex dielectric function, complex refractive index, absorption coefficient, and normal-incidence reflectivity of CuGa5Se8 crystals with slightly different Cu contents are modeled using Adachi’s model for interband transitions. The results are in good agreement with the experimental data over the entire range of photon energies. The model parameters are determined using the simulated annealing algorithm.

Light Emission from Porous Silicon Photoetched in Aqueous Alkali Salt Solutions

An interband-transition model was applied to explain the emission mechanism in porous silicon (PSi) fabricated by photoetching in aqueous HF and salt (NaF and KF) solutions. The HF-formed samples show a yellow photoluminescence band at ∼2 eV. The salt-formed samples, on the other hand, show an ultraviolet (UV) emission peak at ∼3.3 eV with a spectral width of ∼0.1 eV, together with a broad emission band at ∼2.7 eV. The broad emission bands at ∼2 and ∼2.7 eV can be explained by the quantum-mechanical confinement effect, i.e., a relaxation of the momentum conservation at and above the indirect-absorption edge (supra-EgID emission). This effect may also lead to a change in the E1 critical point (CP) from the two-dimensional M0 to the zero-dimensional CP with decreasing nanocrystalline size. The change in the E1-CP dimensionality makes possible an emission in the UV region with a narrow spectral width.

Influence ofMgOoverlayers on the electronic states of bctCo(001)thin films grown on bccFe(001)∕GaAs(001)

The spin polarization of the valence band electronic states of strained bcc $\mathrm{Co}(001)$ and $\mathrm{MgO}∕\mathrm{Co}(001)$ thin films grown onto a bcc $\mathrm{Fe}(001)$ seed layer on $\mathrm{GaAs}(001)$ are investigated by employing spin-resolved photoemission spectroscopy. The experimental results are compared with the calculated energy band structure of bcc and bct $\mathrm{Co}(001)$, and discussed in the framework of the interband transition model, which allows one to ascribe the observed spectral features to bands of given spin and spatial symmetry. In contrast to the positive spin polarization observed at the $\mathrm{MgO}∕\mathrm{Fe}(001)$ interface, a large negative spin polarization of the electronic states at the Fermi level is observed for the $\mathrm{MgO}∕\mathrm{Co}∕\mathrm{Fe}∕\mathrm{GaAs}(001)$ system. Such a large negative spin polarization is attributed to a change in the energy band structure at the bct Co/bcc $\mathrm{Fe}(001)$ interface.

Modelling the optical constants of cubic ZnS in the 0–20 eV spectralregion

We have analysed the complex dielectric-function spectraε(E) = ε1 (E) + i ε2 (E)of cubic (c-)ZnS in the full spectral range (E = 0–20 eV) using aclassical harmonic oscillator and a simplified interband transition model. The experimentalε(E)spectra reveal the reststrahlen band, distinct critical-point structures and cationd-band excitations in the spectra. The critical points are assigned to specific points inthe Brillouin zone with the aid of the band-structure calculation. They areE0 doublet at ∼ 3.8 eV;E1 at ∼ 6.4 eV;E2 at ∼ 7.0 eV;E2 + δ at ∼ 7.4 eV;E0 ′ at ∼ 7.9 eV andE1 ′ at ∼ 9.4 eV.Excellent agreement is also achieved between the modelled and experimental ε(E)spectra over the entire range of photon energies. The sum rules are usedto extract more detailed information. The high-frequency and staticdielectric constants of c-ZnS are determined to be ε∞ = 5.1 and εs = 8.0,respectively. Dielectric-related optical constants, such as the complex refractiveindex, absorption coefficient and normal-incidence reflectivity, of c-ZnS are alsopresented.

Second-order magneto-optic effects in anisotropic thin films

For films with in-plane anisotropy, second-order magneto-optic effects proportional to the square of the spin–orbit interaction can manifest themselves. We report typical measurements of the second-order magneto-optic Kerr effect in Fe, Co, and NiFe thin films with in-plane anisotropy and compare to theoretical predictions based on an interband transition model. We conclude that intraband transitions must play a role at 1.96 eV. The second-order magneto-optic effects provide interesting insights into the magnetization reversal process, and also describe the dielectric tensor of a ferromagnetic metal. Furthermore, since the second-order magneto-optic effect is connected with the asymmetry of the magneto-optic hysteresis loop, which arises as a result of the coherent rotation of the magnetization vector, we interpret such asymmetries, which have been previously reported in the literature.

Electrochromic vanadium–pentoxide–based films: Structural, electrochemical, and optical properties

Crystalline VO2.5 films were prepared by reactive dc magnetron sputtering followed by annealing posttreatment. Lithium was inserted electrochemically from an electrolyte so that LixVO2.5 (0<x<1.5) was formed. The evolution of the electromotive force (EMF) was recorded during Li intercalation. For lithiated samples, structure by x-ray diffraction, optical properties by spectrophotometry in the 0.3–2.5 μm range, infrared reflectance by spectrophotometry in the 400–1200 cm−1 interval, and the mechanical stress by beam deflectometry were studied. Changes in lattice parameters, phonon spectra, and stress levels gave a consistent picture of the structural evolution. Measurements of optical absorption and EMF were interpreted within a conceptual model of interband transitions between the O 2p band and a split V 3d band. Durability and reversibility of the Li intercalation/deintercalation were verified by potentiodynamic measurements. For comparison, the properties of highly disordered films were also measured.

Analysis of spectroscopic-ellipsometry and thermoreflectance spectra of Si

Spectroscopic-ellipsometry (SE) and thermoreflectance (TR) spectra of bulk single-crystalline Si are measured. Measurements are made on the same sample in the 2.5–6.0 eV range at room temperature. These data are analyzed on the basis of a simplified model of interband transitions, named the model dielectric function. Results are in satisfactory agreement with the experimental data over the entire range of photon energies. The finding definitely links the temperature-induced change in the dielectric function (TR) to the first derivative of the dielectric function (SE).

Calculation of optical properties and self-energy shifts for ferromagnetic Ni, Co and Fe

The diagonal and off-diagonal elements of the optical conductivity tensors of ferromagnetic Ni, Co and Fe have been calculated using the direct interband transition model from the self-consistent spin-polarized band structures. At low photon energies (<3 eV), most of the interband transitions making peaks in both the diagonal and off-diagonal elements involve the localized 3d character in the minority-spin bands with the majority-spin bands only contributing structureless backgrounds. At higher photon energies, both the majority- and minority-spin bands construct separate peaks with the minority-spin peak being located at higher energies. The energy differences between the two peaks for Ni, Co and Fe are 0.3 eV, 0.8 eV and 1.5 eV respectively, which agree well with the spin-exchange splittings determined from angle-resolved photoemission measurements. For Ni and Co significant differences have been found in energy positions of the interband peaks between the calculated and experimental spectra. An empirical self-energy correction model has been applied, in which the sizes of the energy shifts of the excited-state quasiparticle states from those of the ground state are proportional to the density of d character in each state and the energy difference between each state and the Fermi level. The self-energy-corrected spectra for Ni and Co have been brought into good agreement with the experimental spectra.

Optical Properties of (AlxGa1-x)0.5In0.5P Quaternary Alloys

The dielectric function spectra ε(E) of (Alx Ga1-x )0.5In0.5P quaternary alloys lattice-matched to GaAs have been measured at room temperature in the 1.2–5.5-eV photon-energy range using spectroscopic ellipsometry (SE). The measured SE spectra show distinct structures at energies of the E0, E1 and E2 critical points. These spectra are analyzed on the basis of a simplified model of interband transitions. This model enables us to obtain the, optical response in (Alx Ga1-x )0.5In0.5P alloys of arbitrary composition x and photon energy E=\\hbarω. Results are presented for the complex refractive index (n*=n+ ik), absorption coefficient (α) and normal-incidence reflectivity (R) of these alloys.

Optical Constants of Cubic ZnS

The real (?1) and imaginary (?2) parts of the dielectric function of cubic ZnS have been measured by spectroscopic ellipsometry in the 1.2-5.6 eV photon-energy range at room temperature. The measured dielectric-function spectra reveal distinct structures at energies of E0/(E0+?0) critical points. These data are analyzed on the basis of a simplified model of interband transitions. Results are in satisfactory agreement with the experimental data over the entire range of photon energies. Dielectric-related optical constants of cubic ZnS, such as the complex refractive index (n*=n+ik), the absorption coefficient (?) and the normal-incidence reflectivity (R), are presented and analyzed.

Optical Constants of In1-xGaxSb Ternary Alloys: Experiment and Modeling

The optical response of In1-xGaxSb ternary alloys in the 1.5-5.4-eV photon-energy range at room temperature is measured by spectroscopic ellipsometry (SE). The measured SE data show distinct structures at energies of the E1, E1+Δ1, E0′, E0′+Δ0′, E2 and E1′ critical points. These spectra are analyzed on the basis of a simplified model of interband transitions. The model proposed here enables us to obtain the optical response in In1-xGaxSb alloys of arbitrary composition (x) and photon energy (E=\\hbarω). Results of the surface-treatment effect on the pseudodielectric functions are also presented.

Analysis of Optical Constants for Sputter-Deposited InSb Films Based on the Interband-Transition Model

The dielectric-function spectra for InSb films rf-sputter-deposited on (0001) and (011̄0) sapphire substrates have been analyzed by using a simplified interband transition model. The film deposited on (0001) sapphire is epitaxial, while the film on (011̄0) sapphire is polycrystalline in structural properties. The model based on the Kramers-Kronig transformation includes the E0, E0+Δ0, E1, E1+Δ1, E0′, E0′+Δ0′, E2, E1′ gaps and indirect gap (EgL) as the main dispersion mechanisms. The analysis suggests that the sputter-deposited InSb films include a small and a large number of void networks in the film medium for epitaxial and polycrystalline films, respectively. Dielectric-function-related optical constants, such as the refractive index and extinction coefficient, of the InSb films are also presented and analyzed.