Activation Energy Of Electrons Research Articles

Effect of Magnetic Field on Phosphorus Centre in Diamond

This work focuses on a sample of diamond thin film which was grown epitaxially on polished {111} surface of synthetic type Ib diamond crystals. Phosphorus doping was achieved during growth using phosphine in the reactant gas ([P]/[C] = 500 ppm, [P] = 3 × 1018 cm—3). Hall effect measurements show n-type conductivity and thermal activation energy of free electrons of 610 ± 10 meV. The sample was characterised by Fourier Transformed Infrared (FTIR) spectroscopy at 1.8 K under a magnetic field (0 to 13 Tesla) parallel to the {111} direction. The evolution of the 1S → 2P+/— absorption peak shows an increase of the FWHM (full width at half maximum) proportionally to the magnetic field. The results suggest a linear Zeeman splitting. For a magnetic field of 13 T the absorption peak splits into 1S → 2P+ and 2P—. This splitting confirms the attribution of this peak to a transition to the 2P+/— excited state of phosphorus. A value for the transverse effective mass of the electron mt = 0.31m0 has been deduced from the splitting.

An MD/QM Study of the Chorismate Mutase-Catalyzed Claisen Rearrangement Reaction

The reaction path for the rearrangement of chorismate to prephenate, catalyzed by chorismate mutase, has been calculated with ab initio quantum chemistry. The calculation of the reaction path is initiated from two catalytically competent conformations of the enzyme that are selected from an X-ray structure and from a snapshot of a molecular dynamics simulation leveraged from the X-ray structure. The quantum calculations employ effective fragment potentials (EFPs) to model the interaction of the protein active site with the substrate in the quantum Hamiltonian. The ability to leverage the X-ray structure into a range of protein conformations abstracted from molecular dynamics simulations to be further analyzed using ab initio methods is demonstrated. Ab initio optimized enzyme−substrate complexes for the oxabicyclic transition state analogue (TSA) and the product, prephenate, compare well with the X-ray structures. We predict the geometry of the active site complex with the reactant, chorismate, and the transition state for the pericyclic reaction. We identify two residues as critical to catalysis, glu78 and tyr108. Binding of the cyclohexadienyl ring's C4−OH substituent of chorismate to the carboxylate of glu78 activates the breaking ether bond. The tyr108 residue is essential in providing the appropriate orientation of the transition-state fragments within the active site to ensure that prephenate is formed. The calculated electronic activation energies for both enzyme conformations studied are lower than the reaction barrier obtained experimentally. The large kinetic isotope effect (KIE) for O18 in the ether bond agrees qualitatively with the experimental value that suggests a very polarized transition state.

Diffusion processes at photoemission from lithium into its passivating layer

The pulse laser technique was employed to study the kinetics of the diffusion processes which follow electron photoinjection from metallic lithium into the solid electrolyte passivating film on the lithium ∣ solution interface. Solutions of LiClO 4, LiAsF 6, and LiPF 6 in the mixed solvents propylene carbonate+dimethoxyethane and ethylene carbonate+diethyl carbonate were used. A systematic comparison of the experimental photopotential relaxation curves recorded under various experimental conditions (temperature, film thickness and conductivity) with the corresponding theoretical equations was performed. The electron emission length (about 1 nm) has been estimated. The electron diffusion coefficient and activation energy of diffusion calculated from out transient photopotential curves have been found to be close to these values related to the ionic carriers in the solid electrolyte layer. A possible mechanism of photoelectron transfer in the solid electrolyte layer on lithium is discussed.

Modern valence-bond description of chemical reaction mechanisms: the 1,3-dipolar addition of diazomethane to ethene.

The electronic mechanism for the gas-phase concerted 1,3-dipolar cycloaddition of diazomethane (CH2N2) to ethene (C2H4) is described through spin-coupled (SC) calculations at a sequence of geometries along the intrinsic reaction coordinate obtained at the MP2/6-31G(d) level of theory. It is shown that the bonding rearrangements occurring during the course of this reaction follow a heterolytic pattern, characterized by the movement of three well-identifiable orbital pairs, which are initially responsible for the pi bond in ethene and the C-N pi bond and one of the N-N pi bonds in diazomethane and are retained throughout the entire reaction path from reactants to product. Taken together with our previous SC study of the electronic mechanism of the 1,3-dipolar cycloaddition of fulminic acid (HCNO) to ethyne (C2H2) (Theor. Chim. Acc. 1998, 100, 222), the results of the present work suggest strongly that most gas-phase concerted 1,3-dipolar cycloaddition reactions can be expected to follow a heterolytic mechanism of this type, which does not involve an aromatic transition state. The more conventional aspects of the gas-phase concerted 1,3-dipolar cycloaddition of diazomethane to ethene, including optimized transition structure geometry, electronic activation energy, activation barrier corrected for zero-point energies, standard enthalpy, entropy and Gibbs free energy of activation, have been calculated at the HF/6-31G(d), B3LYP/6-31G(d), MP2/6-31G(d), MP2/6-31G(d,p), QCISD/6-31G(d) and CCD/6-31G(d) levels of theory. We also report the CCD/6-311++G(2d, 2p)//CCD/6-31G(d), MP4(SDTQ)/6-311++G(2d,2p)//CCD/6-31G(d) and CCSD(T)/6-311++G(2d, 2p)//CCD/6-31G(d) electronic activation energies.

Enhancement of electrical conductivity and emission stability of oxide cathodes using Ni addition

An investigation has been carried out into the use of conductive phase additions to enhance the conductivity and emission behavior of the oxide cathode coating as used in CRTs. Electrical and emission characteristics have been studied for various additions of filamentary nickel (Ni) added to the sprayed strontium-barium carbonate precursors prior to spray deposition, followed by conventional thermal conversion and activation processes in vacuum. The conductivity and the electronic activation energy have been studied as a function of temperature in the range 300 to 1250 K, during conversion and activation processes allowing the conduction behavior to be compared to conventional materials. The conduction behavior has been found to change as a function of heat-treatment temperature as the conduction paths develop and subsequently evolve in the microstructure of the resultant composite coating during conversion, activation and subsequent aging/service life conditions, with metallic-dominated conduction at temperatures below 850 K and pore conduction mechanisms dominating at higher temperatures. The emission characteristics immediately after conversion are impaired by the Ni addition, however, the long-term emission characteristics show improvement with the conductive phase.

Numerical analysis of a mechanism of electrical conductivity of substance in the middle and lower mantle

The method proposed is based on the Arrenius’ equation, that links electrical conductivity of solids with an enthalpy of activation, is studied and well-known distributions of temperature, pressure and electrical conductivity in the mantle. Formulas for the quantitative estimation of parameters, that characterize energy state of a lattice of the mantle, are received. Dependence of electrical conductivity on activation energy of free electron or ion, mobility of which has a power dependence on temperature, and of polaron with an exponential dependence on temperature was studied. The analysis of results has reveled that an electrical conductivity in the middle and in the lower mantle is realized by a change of activation energy of a polaron.

Conduction mechanism of hydrogenated nanocrystalline silicon films

A heteroquantum dot (HQD) tunneling model for the conduction mechanism of hydrogenated nanocrystalline silicon films is proposed. The main features of the HQD model follow. (i) The micrograins and their amorphous counterparts have very different band gaps and band structures; as a result, they form a heterojunctionlike structure in interface regions where the band offset dramatically reduces the activation energy of electrons and the micrograins act as a quantum dot (QD). (ii) In the presence of an external field, activated electrons have ballistic properties in the QD's and quantum tunneling processes through the interface barriers. Based on this model, an equation for the conductivity of nc-Si:H films is deduced, and the calculated results are in good agreement with the experimental results. Using nc-Si:H thin layers a resonant tunneling diode with quantum characteristics was fabricated.

Growth and Characterization of Phosphorus Doped n-Type Diamond Thin Films

Phosphorus (P) doped diamond thin films were grown by microwave plasma-assisted chemical vapor deposition (CVD) using phosphine (PH3) as a dopant source. The addition of PH3 in the gas ambient significantly influenced the optimal growth condition for diamond. The {111} surface of diamond was found to be the best surface as substrate for the growth of P-doped diamond. At a methane to hydrogen ratio of 0.15% and a substrate temperature of 950 °C, smooth {111} oriented homoepitaxial diamond thin films are obtained even with the addition of PH3 up to 1000 ppm to CH4. The epitaxial films grown with PH3/CH4 = 600 to 20000 ppm have shown clear n-type conduction as confirmed by Hall effect measurements over a wide temperature range. The activation energy of electrons was about 0.55 eV deduced from the temperature dependence of the carrier concentration. For the lightly doped sample, the Hall mobility of 28 cm2/Vs has been obtained in a temperature range of about 370 to 700 K.

Electronic structure and activation energy of hydrogen in NEG alloy using nonlinear response theory

Electronic structure of hydrogen in NEG alloy [Zr0.70V0.246Fe0.054] is calculated by using nonlinear response theory [Kohn and Sham,Phys. Rev. A140,1133 (1965)]. The configurational energy is calculated by assuming the ideal hcp structure for NEG alloy. The calculated configurational energy predicts that hydrogen prefers octahedral (0)-site in NEG alloy.s-Type shallow bound state of energy -1.580 x 10-5 Ryd. suggests that hydrogen does not form NEG hydride and it stays as a free ion in NEG alloy. This conclusion confirms the prediction of Tripathiet al.

Carbon fiber polymer-matrix structural composite as a semiconductor and concept of optoelectronic and electronic devices made from it

An epoxy-matrix composite with continuous crossply carbon fibers was found to be a semiconductor in the through-thickness direction, with a tunable energy gap of - eV (infrared). The higher the pressure during composite fabrication by lamination, the higher the interlaminar stress and the greater the energy gap, which is the activation energy for electron jumping from one lamina to the adjacent one in the composite. The semiconducting behavior involves the contact electrical resistivity between adjacent laminae in the composite decreasing reversibly with increasing temperature. The concept of optoelectronic and electronic devices made from carbon fiber polymer-matrix composites is provided. Devices include solar cells, light emitting diodes, lasers, infrared detectors and transistors. Thus, a new dimension is added to smart structures and a new field of electronics (`structural electronics') is born.

Hydrogen permeability in (CeO 2) 0.9(CaO) 0.1 at high temperatures

It was found that hydrogen permeated the bulk of the fluorite-type (CeO 2) 0.9(CaO) 0.1 at 1075–1800 K. Two tube shaped specimens of different lengths were prepared. The hydrogen source gas was H 2(7.8%)+N 2 which was humidified at 293 K and supplied to the outside of specimens. Ar gas was let flow to the inside of specimens and the permeated hydrogen in the outlet Ar gas was analyzed by a gas chromatograph. Measurements were made at the same conditions on the specimens to eliminate the hydrogen permeated through Pt rings, which were used to fix the specimen, the alumina tube and disk, and the alumina supporting materials. The permeability, J H, in log( J H/mol h −1 cm −1) was almost proportional to 1/ T ( T: temperature). The gradient of the line changes at 1525 K. The activation energy, E, of conductive protons or electrons was estimated from the Arrhenius' relation, log J H= A− E/2.303 kT. A is constant and E is the activation energy, which is 1.07±0.03 eV above 1525 K and 0.73±0.06 eV below 1525 K, respectively. The electronic activation energies, which were determined from the oxygen permeability, were higher than the above values, and they might be the activation energy of protons. There is a possibility that (CeO 2) 0.9(CaO) 0.1 is an electron–proton mixed conductor under H 2–H 2O atmosphere at high temperature.

Ab Initio Study of the Regiochemistry of 1,3-Dipolar Cycloadditions. Reactions of Diazomethane and Formonitrile Oxide with Ethene, Propene, Acrylonitrile, and Methyl Vinyl Ether.

Structures and energetics of reactants and transition structures of the cycloadditions of diazomethane (DZM) and formonitrile oxide (FNO) with ethene (ET), propene (PR), acrylonitrile (ACN), and methyl vinyl ether (MVE) have been investigated with the use of ab initio molecular orbital calculations. The reaction of acetonitrile oxide (MNO) with acrylonitrile has been also included for comparisons. Structure optimizations were performed at the RHF/6-31G(d) and density functional B3LYP/6-31G(d) levels of approximation. Single-point electronic energies were computed up to the MP4SDTQ/6-31G(d) level. Kinetic contributions to activation enthalpies and entropies were computed at the RHF/6-31G(d) level. Transition structures of ethene cycloadditions (prototype reactions) were also checked with the MP2/6-31G(d) approximation. Solvent effects were introduced both at a semiempirical level (AMSOL) and at an ab initio level using the Pisa model (interlocking spheres) and the IPCM procedure (isodensity surface polarized continuum model). Electronic activation energies are found to be very sensitive to the treatment of electron correlation and failed to converge to values unaffected by further theoretical improvements: indeed, the inclusion of full fourth-order correlation (MP4) decreases the activation energies by 5-10 kcal/mol with respect to the preceding level of correlation (MP3). Anyway, activation free enthalpies and entropies of the reactions under study appear to be close to the experimental values available for this class of reactions. Still in agreement with experimental observations is the effect of solvent polarity on the reaction rates. Theoretical regioselectivity is less sensitive to the level of calculation, although the inclusion of electron correlation, both with the Moeller-Plesset technique and the use of the density functional theory, is able to reverse the regiochemical predictions obtained with RHF energies for the reactions of nitrile oxides with acrylonitrile. This explains why the frontier orbital theory, which is based on uncorrelated HF-wave functions, cannot arrive at the correct prediction of the regiochemistry in these cases. Calculated solvent effects appear to influence the regiochemistry of 1,3-dipolar cycloaddition, but in general, they reinforce the prediction obtained in vacuo.

Mechanism of Addition of Fluoromethyl Radicals to Fluoroethylenes

The self-consistent charge and configuration method for subsystems (SCCCMS), charge sensitivity analysis, and our new scheme for energy partitioning were applied to discuss the mechanism of addition of fluoromethyl radicals to fluoroethylenes. A hybrid density functional method, i.e., the combination of Becke's three-parameter functional and the Perdew/Wang 91 gradient-corrected functional, was used in the calculations with the 6-31G* basis set. Charge sensitivity characteristics (chemical potentials and hardnesses) of the reactants have been obtained. Chemical potentials clearly showed that alkenes act as donors of electrons (bases), while radicals act as acceptors (acids). The systems “electronic activation energies” were decomposed into deformation, electrostatic, polarization, charge-transfer, and exchange components. Steric effect was identified with the sum of deformation and exchange energies, while polar effect was related to the sum of electrostatic, charge-transfer, and polarization contributions. A correlation between the magnitude of polar effect and inter-reactant distance in transition states was found. It has been demonstrated that the anomaly in regioselectivity in methyl or fluoromethyl radical addition to trifluoroethylene is understandable as an electrostatically controlled process.

Metastability of Oxygen Donors in AlGaN

Experimental and theoretical evidence is presented for the metastability of oxygen donors in AlxGa12xN. As the aluminum content increases, Hall effect measurements reveal an increase in the electron activation energy, consistent with the emergence of a deep DX level from the conduction band. Persistent photoconductivity is observed in Al0.39Ga0.61N:O at temperatures below 150 K after exposure to light, with an optical threshold energy of 1.3 eV. A configuration coordinate diagram is obtained from first-principles calculations and yields values for the capture barrier, emission barrier, and optical threshold which are in good agreement with the experimental results. [S0031-9007(98)05950-X]

Growth and characterization of phosphorus doped n-type diamond thin films

Abstract n -Type semiconducting diamond thin films were successfully grown by microwave plasma CVD using phosphine (PH 3 ) as a dopant source. The addition of PH 3 in the gas ambient strongly influences the optimal growth condition for diamond. The {111} surface of diamond was found to be the best surface as a substrate for the growth of phosphorous (P) doped diamond. At a methane to hydrogen ratio of 0.15% and a substrate temperature of 950 °C, smooth {111} oriented homoepitaxial diamond thin films are obtained even with the addition of PH 3 up to 1000 ppm to CH 4 . The epitaxial films grown with PH 3 /CH 4 : 600–20 000 ppm have shown clear n -type conduction by Hall effect measurements over a wide temperature range. An activation energy of electron of 0.46 eV was deduced from the temperature dependence of the carrier concentration. For the best (600 ppm) sample, the Hall mobility of 28 cm 2 /V-s has been obtained at a temperature range around 400–600 K.

Evidence for Oxygen DX Centers in AlGaN

ABSTRACTExperimental and theoretical evidence is presented for oxygen DX centers in AlxGa1−xN. As the aluminum content increases, Hall effect measurements reveal an increase in the electron activation energy, consistent with the emergence of a deep DX level from the conduction band. Persistent photoconductivity is observed in Al0 39Ga0. 61N:O at temperatures below 150 K after exposure to light, with an optical threshold energy of 1.3 eV, in excellent agreement with first-principles calculations. Unlike oxygen, silicon does not exhibit DX-like behavior, in agreement with previous theoretical predictions.

Transient Photocurrent Studies on Amorphous andβ-Rhombohedral Boron

Carrier transport properties in amorphous andβ-rhombohedral boron were studied by the time of flight method. The activation energy for holes inβ-rhombohedral boron was estimated to be 240 meV. This activation energy corresponds to the depth of hole traps originating from B12, that is, the intrinsic acceptor level. Dispersive (non-Gaussian) transient photocurrents were observed both for electrons and holes in amorphous boron. The activation energy for electrons and holes were estimated to be about 20 and 85 meV, respectively. The average hopping distance of holes was evaluated from the electric field dependence of the photocurrent to be 11 Å. These activation energies and hopping distances are small compared with those of other amorphous semiconductors. The results suggest that the excited carriers propagate by hopping between localized states within a certain narrow band.

Electron structure and activation energy of hydrogen in α-Zr using nonlinear response theory

The electron structure of hydrogen in hcp Zr is calculated by using self-consistent nonlinear screening theory. The host-ion contribution is included through the spherical solid model potential (SSMP). The resulting charge density and scattering phase shifts are used to calculate the activation energy and residual resistivity of hydrogen in α-Zr matrix. The calculated activation energy 0·285 eV is found in reasonably good agreement with experimental value 0·3 eV. The estimated residual resistivity 0·53 μΩ cm/at% for Zr-H system using the scattering phase shifts agrees reasonably well with the observed value 0·27 μΩ cm/at%. The calculated configurational energy shows that hydrogen prefers tetrahedral(T)-sites over octahedral(O)-sites in α-Zr. The strong binding energy of electron-proton suggests that hydrogen forms zirconium hydride.

Mixed conduction in charge transfer materials based on aromatic diamines and iodine having different mole ratios

Mixed ionic and electronic conduction in charge transfer materials based on aromatic diamine electron donors (benzidine,o-tolidine (3,3′-dimethyl benzidine) andN,N′-diphenylbenzidine) with iodine in different mole ratios has been reported. The current-voltage and capacitance-voltage curves as a function of time and temperature have been obtained to determine the role of ionic conduction in electronic conductors based on charge transfer complexes. The compositional dependence of ionic transport numbers, diffusion parameters, ionic(σ i) and electronic(σ e) conductivities and thermal activation energies has been studied. Temperature and frequency dependence of AC impedance and related parameters have been studied to learn about the electrical conduction behaviour in these non-stoichiometric charge transfer materials. An electrochemical mechanism has been proposed to account for the ionic conduction in some of these complexes.

Thermoluminescence of agro-food products interpreted with a solid-state physical band model

The application of ionizing γ-radiation in the case of agricultural and food-industrial products necessitates the exact detection of radiation-induced changes, according to international protocols. Thermoluminescence (TL) method can be used not only to prove the radiation treatment of foodstuffs but to study the structure of these materials.Low- and high-temperature measurements were carried out on protein-containing products and spices, moreover on isolated silicate minerals. The change of TL intensity as functions of absorbed γ-dose and storage time, the shape of TL curve with several linear heating rates were investigated.TL processes were interpreted with a solid-state physical (i.e.organic semiconductor) band model. It was found that each TL curve was composed of second-order curves. The main parameters (activation energy and frequency factor) of electron traps responsible for the phenomenon were determined by the methods of different heating rates and curve fitting.