Charge transport and density of trap states in balanced high mobility ambipolar organic thin-film transistors

We report on charge transport and density of trap states (trap DOS) in ambipolar diketopyrrolopyrrole-benzothiadiazole copolymer thin-film transistors. This semiconductor possesses high electron and hole field-effect mobilities of up to 0.6 cm 2/V-s. Temperature and gate-bias dependent field-effect mobility measurements are employed to extract the activation energies and trap DOS to understand its unique high mobility balanced ambipolar charge transport properties. The symmetry between the electron and hole transport characteristics, parameters and activation energies is remarkable. We believe that our work is the first charge transport study of an ambipolar organic/polymer based field-effect transistor with room temperature mobility higher than 0.1 cm 2/V-s in both electrons and holes.

First-principles analysis of electron correlation, spin ordering and phonons in the normal state of FeSe1-x

We present first-principles density-functional-theory-based calculations to determine the effects of the strength of on-site electron correlation, magnetic ordering, pressure and Se vacancies on phonon frequencies and electronic structure of FeSe1-x. The theoretical equilibrium structure (lattice parameters) of FeSe depends sensitively on the value of the Hubbard parameter U of on-site correlation and magnetic ordering. Our results suggest that there is a competition between different antiferromagnetic states due to comparable magnetic exchange couplings between first- and second-neighbor Fe sites. As a result, a short range order of stripe antiferromagnetic type is shown to be relevant to the normal state of FeSe at low temperature. We show that there is a strong spin-phonon coupling in FeSe (comparable to its superconducting transition temperature) as reflected in large changes in the frequencies of certain phonons with different magnetic ordering, which is used to explain the observed hardening of a Raman-active phonon at temperatures (similar to 100 K) where magnetic ordering sets in. The symmetry of the stripe antiferromagnetic phase permits an induced stress with orthorhombic symmetry, leading to orthorhombic strain as a secondary order parameter at the temperature of magnetic ordering. The presence of Se vacancies in FeSe gives rise to a large peak in the density of states near the Fermi energy, which could enhance the superconducting transition temperature within the BCS-like picture.

Vibrational spectroscopy and density functional theory analysis of 3-O-caffeoylquinic acid

Density functional theory (DFT) calculations are being performed to investigate the geometric, vibrational, and electronic properties of the chlorogenic acid isomer 3-CQA (1R,3R,4S,5R)-3-{(2E)-3-(3,4-dihydroxyphenyl)prop-2-enoyl]oxy}-1,4, 5-trihydroxycyclohexanecarboxylic acid), a major phenolic compound in coffee. DFT calculations with the 6-311G(d,p) basis set produce very good results. The electrostatic potential mapped onto an isodensity surface has been obtained. A natural bond orbital analysis (NBO) has been performed in order to study intramolecular bonding, interactions among bonds, and delocalization of unpaired electrons. HOMO-LUMO studies give insights into the interaction of the molecule with other species. The calculated HOMO and LUMO energies indicate that a charge transfer occurs within the molecule. (C) 2012 Elsevier B.V. All rights reserved.

Performance analysis of boron nitride embedded armchair graphene nanoribbon metal-oxide-semiconductor field effect transistor with Stone Wales defects

We study the performance of a hybrid Graphene-Boron Nitride armchair nanoribbon (a-GNR-BN) n-MOSFET at its ballistic transport limit. We consider three geometric configurations 3p, 3p + 1, and 3p + 2 of a-GNR-BN with BN atoms embedded on either side (2, 4, and 6 BN) on the GNR. Material properties like band gap, effective mass, and density of states of these H-passivated structures are evaluated using the Density Functional Theory. Using these material parameters, self-consistent Poisson-Schrodinger simulations are carried out under the Non Equilibrium Green's Function formalism to calculate the ballistic n-MOSFET device characteristics. For a hybrid nanoribbon of width similar to 5 nm, the simulated ON current is found to be in the range of 265 mu A-280 mu A with an ON/OFF ratio 7.1 x 10(6)-7.4 x 10(6) for a V-DD = 0.68 V corresponding to 10 nm technology node. We further study the impact of randomly distributed Stone Wales (SW) defects in these hybrid structures and only 2.5% degradation of ON current is observed for SW defect density of 3.18%. (C) 2014 AIP Publishing LLC.

An analysis on overall crack-number-density of short-fatigue-cracks

The evolution of dispersed short-fatigue-cracks is analysed based on the equilibrium of crack-number-density (CND). By separating the mean value and the stochastic fluctuation of local CND, the equilibrium equation of overall CND is derived. Comparing with the mean-field equilibrium equation, the equilibrium equation of overall CND has different forms in the expression of crack-nucleation-rate or crack-growth-rate. The simulation results are compared with experimental measurements showing the stochastic analyses provide consistent tendency with experiments. The discrepancy in simulation results between overall CND and mean-field CND is discussed.

I. Electronic states of heavily-doped molecular crystals--naphthalene. I. Theoretical. II. Experimental. II. Lowest singlet and triplet excited states of pyrazine

PART I

The energy spectrum of heavily-doped molecular crystals was treated in the Green’s function formulation. The mixed crystal Green’s function was obtained by averaging over all possible impurity distributions. The resulting Green’s function, which takes the form of an infinite perturbation expansion, was further approximated by a closed form suitable for numerical calculations. The density-of-states functions and optical spectra for binary mixtures of normal naphthalene and deuterated naphthalene were calculated using the pure crystal density-of-state functions. The results showed that when the trap depth is large, two separate energy bands persist, but when the trap depth is small only a single band exists. Furthermore, in the former case it was found that the intensities of the outer Davydov bands are enhanced whereas the inner bands are weakened. Comparisons with previous theoretical calculations and experimental results are also made.

PART II

The energy states and optical spectra of heavily-doped mixed crystals are investigated. Studies are made for the following binary systems: (1) naphthalene-h₈ and d₈, (2) naphthalene--h₈ and αd₄, and (3) naphthalene--h₈ and βd₁, corresponding to strong, medium and weak perturbations. In addition to ordinary absorption spectra at 4˚K, band-to-band transitions at both 4˚K and 77˚K are also analyzed with emphasis on their relations to cooperative excitation and overall density-of-states functions for mixed crystals. It is found that the theoretical calculations presented in a previous paper agree generally with experiments except for cluster states observed in system (1) at lower guest concentrations. These features are discussed semi-quantitatively. As to the intermolecular interaction parameters, it is found that experimental results compare favorably with calculations based on experimental density-of-states functions but not with those based on octopole interactions or charge-transfer interactions. Previous experimental results of Sheka and the theoretical model of Broude and Rashba are also compared with present investigations.

PART III

The phosphorescence, fluorescence and absorption spectra of pyrazine-h₄ and d₄ have been obtained at 4˚K in a benzene matrix. For comparison, those of the isotopically mixed crystal pyrazine-h₄ in d₄ were also taken. All these spectra show extremely sharp and well-resolved lines and reveal detailed vibronic structure.

The analysis of the weak fluorescence spectrum resolves the long-disputed question of whether one or two transitions are involved in the near-ultraviolet absorption of pyrazine. The “mirror-image relationship” between absorption and emission shows that the lowest singlet state is an allowed transition, properly designated as ¹B_3u ← ¹A_1g. The forbidden component ¹B_2g, predicted by both “exciton” and MO theories to be below the allowed component, must lie higher. Its exact location still remains uncertain.

The phosphorescence spectrum when compared with the excitation phosphorescence spectra, indicates that the lowest triplet state is also symmetry allowed, showing a strong 0-0 band and a “mirror-image relationship” between absorption and emission. In accordance with previous work, the triplet state is designated as ³B_3u.

The vibronic structure of the phosphorescence spectrum is very complicated. Previous work on the analysis of this spectrum all concluded that a long progression of v_6a exists. Under the high resolution attainable in our work, the supposed v_6a progression proves to have a composite triplet structure, starting from the second member of the progression. Not only is the v_9a hydrogen-bending mode present as shown by the appearance of the C-D bending mode in the d₄ spectrum, but a band of 1207 cm^-1 in the pyrazine in benzene system and 1231 cm^-1 in the mixed crystal system is also observed. This band is assigned as 2v_6b and of a_1g symmetry. Its anonymously strong intensity in the phosphorescence spectrum is interpreted as due to the Fermi resonance with the 2v_6a and v_9a band.

To help resolve the present controversy over the crystal phosphorescence spectrum of pyrazine, detailed vibrational analyses of the emission spectra were made. The fluorescence spectrum has essentially the same vibronic structure as the phosphorescence spectrum.

Density of photon states in dye-doped chiral nematic liquid crystal cells in the presence of losses and gain

We calculate the density of photon states (DOS) of the normal modes in dye-doped chiral nematic liquid crystal (LC) cells in the presence of various loss mechanisms. Losses and gain are incorporated into the transmission characteristics through the introduction of a small imaginary part in the dielectric constant perpendicular and along the director, for which we assume no frequency dispersion. Theoretical results are presented on the DOS in the region of the photonic band gap for a range of values of the loss coefficient and different values of the optical anisotropy. The obtained values of the DOS at the photonic band gap edges predict a reversal of the dominant modes in the structure. Our results are found to be in good agreement with the experimentally obtained excitation thresholds in chiral nematic LC lasers. The behavior of the DOS is also discussed for amplifying LC cells providing additional insight to the lasing mechanism of these structures. © 2012 American Physical Society.

Density of photon states in dye-doped chiral nematic liquid crystal cells in the presence of losses and gain.

We calculate the density of photon states (DOS) of the normal modes in dye-doped chiral nematic liquid crystal (LC) cells in the presence of various loss mechanisms. Losses and gain are incorporated into the transmission characteristics through the introduction of a small imaginary part in the dielectric constant perpendicular and along the director, for which we assume no frequency dispersion. Theoretical results are presented on the DOS in the region of the photonic band gap for a range of values of the loss coefficient and different values of the optical anisotropy. The obtained values of the DOS at the photonic band gap edges predict a reversal of the dominant modes in the structure. Our results are found to be in good agreement with the experimentally obtained excitation thresholds in chiral nematic LC lasers. The behavior of the DOS is also discussed for amplifying LC cells providing additional insight to the lasing mechanism of these structures.

Theoretical analysis of characteristics of GaxIn1-xNyAs1-y/GaAs quantum well lasers with different intermediate layers

Based on the band anticrossing model, the effects of the strain-compensated layer and the strain-mediated layer on the band structure, gain and differential gain of GaInNAs/GaAs quantum well lasers have been investigated. The results show that the GaNAs barrier has a disadvantage in increasing the density of states in the conduction band. Meanwhile, the multilayer quantum wells need higher transparency carrier density than the GaInNAs/GaAs single quantum well with the same wavelength. However, they help to suppress the degradation of the differential gain. The calculation also shows that from the viewpoint of band structure, the strain-compensated structure and the strain-mediated structure have similar features.

Theoretical analysis of modal gain in p-doped 1.3 mu m InAs/GaAs quantum dot lasers

A theoretical study of modal gain in p-doped 1.3 mu m InAs/GaAs quantum dot (QD) lasers is presented. The expression of modal gain is derived, which includes an effective ratio that describes how many QDs contribute to the modal gain. The calculated results indicate that the modal gain with the effective ratio is much smaller than that without the effective ratio. The calculated maximum modal gain is is a good agreement with the experimental data. Furthermore, QDs with lower height or smaller aspect ratio are beneficial in achieving a larger maximum modal gain that leads to lower threshold current density and higher differential modal gain. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Reactivity of the 4d transition metals toward N hydrogenation and NH dissociation: A DFT-based HSAB analysis

The density functional theory (DFT) based hard-soft acid-base (HSAB) reactivity indices, including the electrophilicity index, have been successfully applied to many areas of molecular chemistry. In this work we test the applicability of such an approach to fundamental surface chemistry. We have considered, as prototypical surface reactions, both the hydrogenation of atomic nitrogen and the dissociative adsorption of the NH molecular radical. By use of a DFT methodology, the minimum energy reaction pathways, and corresponding reaction barriers, of the above reactions over Zr(001), Nb(110), Mo(110), Tc(001), Ru(001), Rh(111), and Pd(111) have been determined. By consideration of the chemical potential and chemical hardness of the surface metal atoms, and the principle of electronegativity equalization, it is found that the charge transferred to the NH radical during the process of dissociative adsorption correlates very well with that determined by Mulliken population analysis. Furthermore, it is found that the stability of the NH/surface transition state complex relates directly to this charge transfer and that the trend in transition state stability predicted by a HSAB; treatment correlates very strongly with that determined by DFT calculations. With regards to N hydrogenation, we find that during the course of the reaction, H loses cohesion to the surface, as it must migrate from a 3-fold hollow site to either a bridge or top site, to react with N. Partial density of states (PDOS) and Mulliken population analysis reveal that this loss of bonding is accompanied by charge transfer from H to the surface metal atoms. Moreover, by simple modeling, we show that the reaction barriers are directly proportional to this mandatory charge transfer. Indeed, it is found that the reaction barriers correlate very well with the electrophilicity index of the metal atoms.

Density functional study of the C atom adsorption on the α-Fe 2O3 (001) surface

The adsorption of C atoms on the α-Fe₂O₃ (001) surface was studied based on density function theory (DFT), in which the exchange-correlation potential was chosen as the PBE (Perdew, Burke and Ernzerhof) generalized gradient approximation (GGA) with a plane wave basis set. Upon the optimization on different adsorption sites with coverage of 1/20 and 1/5 ML, it was found that the adsorption of C atoms on the α-Fe ₂O₃ (001) surface was chemical adsorption. The coverage can affect the adsorption behavior greatly. Under low coverage, the most stable adsorption geometry lied on the bridged site with the adsorption energy of about 3.22 eV; however, under high coverage, it located at the top site with the energy change of 8.79 eV. Strong chemical reaction has occurred between the C and O atoms at this site. The density of states and population analysis showed that the s, p orbitals of C and p orbital of O give the most contribution to the adsorption bonding. During the adsorption process, O atom shares the electrons with C, and C can only affect the outermost and subsurface layers of α-Fe₂O₃; the third layer can not be affected obviously. Copyright © 2008 Chinese Journal of Structural Chemistry.

Application of abrupt cut-off models in the analysis of the capacitance spectra of conjugated polymer devices

In this paper, a detailed study of the capacitance spectra obtained from Au/doped-polyaniline/Al structures in the frequency domain (0.05 Hz-10 MHz), and at different temperatures (150-340 K) is carried out. The capacitance spectra behavior in semiconductors can be appropriately described by using abrupt cut-off models, since they assume that the electronic gap states that can follow the ac modulation have response times varying rapidly with a certain abscissa, which is dependent on both temperature and frequency. Two models based on the abrupt cut-off concept, formerly developed to describe inorganic semiconductor devices, have been used to analyze the capacitance spectra of devices based on doped polyaniline (PANI), which is a well-known polymeric semiconductor with innumerous potential technological applications. The application of these models allowed the determination of significant parameters, such as Debye length (approximate to 20 nm), position of bulk Fermi level (approximate to 320 meV) and associated density of states (approximate to 2x10(18) eV(-1) cm(-3)), width of the space charge region (approximate to 70 nm), built-in potential (approximate to 780 meV), and the gap states` distribution.

Joint Experimental and Theoretical Analysis of Order Disorder Effects in Cubic BaZrO(3) Assembled Nanoparticles under Decaoctahedral Shape

Periodic first-principles calculations based on density functional theory at the B3LYP level has been carried out to investigate the photoluminescence (PL) emission of BaZrO(3) assembled nanoparticles at room temperature. The defect created in the nanocrystals and their resultant electronic features lead to a diversification of electronic recombination within the BaZrO(3) band gap. Its optical phenomena are discussed in the light of photoluminescence emission at the green-yellow region around 570 nm. The theoretical model for displaced atoms and/or angular changes leads to the breaking of the local symmetry, which is based on the refined structure provided by Rietveld methodology. For each situation a band structure, charge mapping, and density of states were built and analyzed. X-ray diffraction (XRD) patterns, UV-vis measurements, and field emission scanning electron microscopy (FE-SEM) images are essential for a full evaluation of the crystal structure and morphology.

Theoretical analysis of the energy levels induced by oxygen vacancies and the doping process (Co, Cu and Zn) on SnO2 (110) surface models

Density functional calculation at B3LYP level was employed to study the surface oxygen vacancies and the doping process of Co, Cu and Zn on SnO2 (110) surface models. Large clusters, based on (SnO2)(15) models, were selected to simulate the oxidized (Sn15O30), half-reduced (Sn15O29) and the reduced (Sn15O28) surfaces. The doping process was considered on the reduced surfaces: Sn13Co2O28, Sn13Cu2O28 and Sn13Zn2O28. The results are analyzed and discussed based on a calculation of the energy levels along the bulk band gap region, determined by a projection of the monoelectron level structure on to the atomic basis set and by the density of states. This procedure enables one to distinguish the states coming from the bulk, the oxygen vacancies and the doping process, on passing from an oxidized to a reduced surface, missing bridge oxygen atoms generate electronic levels along the band gap region, associated with 5s/5p of four-/five-fold Sn and 2p of in-plane O centers located on the exposed surface, which is in agreement with previous theoretical and experimental investigations. The formation energy of one and two oxygen vacancies is 3.0 and 3.9 eV, respectively. (C) 2001 Elsevier B.V. B.V. All rights reserved.