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Potential Profile (potential + profile)
Selected AbstractsModified Gaussian-2 level investigation of the identity ion-pair SN2 reactions of lithium halide and methyl halide with inversion and retention mechanismsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 4 2004Yi Ren Abstract Identity ion-pair SN2 reactions LiX + CH3X , XCH3 + LiX (X = F, Cl, Br, and I) have been investigated in the gas phase and in solution at the level of the modified Gaussian-2 theory. Two possible reaction mechanisms, inversion and retention, are discussed. The reaction barriers relative to the complexes for the inversion mechanism [,H(inv)] are found to be much higher than the corresponding values for the gas phase anionic SN2 reactions, decreasing in the following order: F (263.6 kJ mol,1) > Cl (203.3 kJ mol,1) > Br (174.7 kJ mol,1) > I (150.7 kJ mol,1). The barrier gaps between the two mechanisms [,H (ret) , ,H (inv)] increase in the order F (,62.7 kJ mol,1) < Cl (4.4 kJ mol,1) < Br (24.9 kJ mol,1) < I (45.1 kJ mol,1). Thus, the retention mechanism is energetically favorable for fluorine and the inversion mechanism is favored for other halogens, in contrast to the anionic SN2 reactions at carbon where the inversion reaction channel is much more favorable for all of the halogens. The stabilization energies for the dipole,dipole complexes CH3X · · · LiX (,Hcomp) are found to be similar for the entire set of systems with X = F, Cl, Br, and I, ranging from 53.4 kJ mol,1 for I up to 58.9 kJ mol,1 for F. The polarizable continuum model (PCM) has been used to evaluate the direct solvent effects on the energetics of the anionic and ion-pair SN2 reactions. The energetic profiles are found to be still double-well shaped for most of the ion-pair SN2 reactions in the solution, but the potential profile for reaction LiI + CH3I is predicted to be unimodal in the protic solvent. Good correlations between central barriers [,H (inv)] with the geometric looseness of the inversion transition state %C,X,, the dissociation energies of the C,X bond (DC,X) and Li,X bond (DLi,X) are observed, respectively. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 461,467, 2004 [source] Direct evidence for group III atoms migration in aged 980 nm InGaAs/AlGaAs pump lasersPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2003M. Bettiati Abstract Low Temperature,Spectrally Resolved Cathodo-Luminescence (LT-SRCL) and Cathodo-Luminescence Imaging (CLI) have been used to study the effects of high temperature and high current density aging on 980 nm InGaAs/AlGaAs pump lasers. The aged lasers show a characteristic evolution of the luminescence band related to a thin (, 25 nm) InGaP etch-stop layer: a blue-shift of several meV is observed, together with a rather strong increase in the luminescence intensity. The blue-shift is related to an increase in the disorder of the InGaP alloy while the strong increase of the intensity may result from a different localization of the injected carriers due to a modification of the potential profile. A possible microscopic mechanism for the disordering of the InGaP layer is discussed, based on the migration of either p-type impurities (Zn, in this case) or excess group III interstitials. [source] Polarization effects in 2-DEG and 2-DHG AlGaN/AlN/GaN multi-heterostructures measured by electron holographyPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 7 2010Q. Y. Wei Abstract The electrostatic potential profiles and charge distributions in modulation-doped n-type and p-type AlGaN/AlN/GaN heterostructures have been measured by electron holography with high spatial resolution. For n-type two-dimensional electron gas structure a negative curvature and for p-type two-dimensional hole gas structure a positive curvature in the potential profile at the AlN/GaN interface were observed, which demonstrated the accumulation of two-dimensional carriers. The measured electrostatic potential profiles were also compared with the calculated band diagram in the heterostructures. [source] Interband transmission in armchair graphene ribbons with a step-like profile of potential energy: Relevance to Klein's tunnelingPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 11-12 2009Lyuba Malysheva Abstract Three principal results concerning graphene-based wires and their ambipolar behavior are presented. First, it is the exact expression of the transmission coefficient for armchair graphene wires described by the tight-binding Hamiltonian with the step-like change U of site energies. Second, the exact relation between the energy of incident electrons or holes and potential U at which there is no backscattering for the given mode of the transverse motion. Third, the range of relevance of Klein's formula describing the motion of relativistic particles in the same potential profile is established. Analysis of newly derived results shows that physics of interband transitions at constant energy in graphene wires is richer than it was believed. [source] Analysis of electrical properties of insulators (Si3N4, SiO2, AlN, and Al2O3)/0.5 nm Si3N4/AlGaN/GaN heterostructuresPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2007Narihiko Maeda Abstract The electrical properties in AlGaN/GaN heterostructures with Si- and Al-based insulators (Si3N4, SiO2, AlN, and Al2O3) have been examined and analyzed. By insulators deposition, significant increase in the two-dimensional electron gas (2DEG) density (Ns) was observed with the order of Ns(Al2O3) > Ns(AlN) , Ns(SiO2) > Ns(Si3N4) > N0 (N0: Ns without insulators). As the result, the decrease in the sheet resistance (R) was observed; the smallest order of R was R(Al2O3) < R(AlN) < R(Si3N4) < R0 , R(SiO2) (R0: R without insulator). The insulators deposition effect has thus been shown to be significant and different among insulators. The increase in Ns was analyzed in terms of the change in the potential profile, and the observed differences in Ns among insulators have been interpreted. The band engineering including insulators is indispensable in understanding and designing AlGaN/GaN HFETs, since insulators are commonly used for the surface passivation as well as for the gate insulators, and the insulators deposition is to alter the essential device parameters such as the source resistance. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Nonuniform Nanowire Doping Profiles Revealed by Quantitative Scanning Photocurrent MicroscopyADVANCED MATERIALS, Issue 30 2009Jonathan E. Allen Scanning photocurrent microscopy (SPCM) is used in semiconductor nanowire devices to establish quantitative potential profiles correlated with nonuniformities in electrical resistivity. Surface doping leads to a nonuniform axial photocurrent (a). Surface etching improves the uniformity of the local photocurrent (b) and reduces the radial and axial carrier concentration gradients (c, blue curve after etching). [source] | ||||||||||