Large Relaxation Energies Research Articles

The electronic and optical properties of Al interstitial defects in KH2PO4 crystal: First principles study

KH2PO4 (KDP) crystals are widely used as frequency multipliers in inertial confinement controlled fusion ignition devices. The electronic and optical properties of KDP crystals with Al interstitial defects based on first principles are investigated. The results show that neutral and +3 charged Al interstitial can exist stably in the crystal. The +3 charged defect produces a larger lattice distortion. Bond length varies from −18.78 % to 43.71 % in the paraelectric phase(P-KDP) and from −25.57 % to 66.09 % in the ferroelectric phase (F-KDP). At the same time, +3 charged Al interstitial may also cause HO bond to break, resulting in the generation of H interstitials, which may compensate for the H vacancies in as-grown KDP. This may make the laser damage threshold slightly increase when KDP crystal is doped with +3 charged Al ion at low concentration. The optical spectra of Al interstitial are calculated, and the results show that in the paraelectric and ferroelectric phases, the optical transition of Al ions generally produces a large lattice relaxation energy, such as the lattice relaxation energy of up to 4.00 eV in the +2 to +3 charged defect transition in the paraelectric phase. This shows that the optical transition of Al interstitial will cause serious damage to the crystal. In the paraelectric phase, the absorption peaks of optical transitions found in the ultraviolet and visible regions are consistent with experiments. The absorption peak of the +1 to +2 charged defect transition is at 260 nm, which is very close to the absorption peak at 270 nm measured by experiments. The analysis of the Huang-Rys factors and transition displacement further confirms the conclusion that the optical transition of the defect charged state will cause serious crystal damage.

Computational study of K vacancy with an H interstitial defect in [formula omitted] crystal

The electronic structures, defect formations, defect transition levels and optical properties for VK+Hi defect models in KDP crystals have been studied based on DFT. Lattice dynamics methods give the most reasonable compensation mechanism for VK, namely compensation with the third-nearest Hi neighbor from VK for the paraelectric (PE) phase and compensation with the fourth-nearest neighbor Hi from VK for the ferroelectric (FE) phase. The defect formation energies indicate that the (VK+Hi)x (The superscript represents the charged state, the ‘x’ represents neutral, ‘’’ represents −1 charge state and ‘.’ represents +1 charge state.) is the main defect type in this kind of defect cluster and a self-trapped electron is located at Hi in the (VK + Hi)′ system. For (VK+Hi)x system, one electron is accommodated in VK. There is not a new defect state in the band gap. The Hi bonds with the O ion (0.99 Å) form a hydroxyl. For (VK + Hi)′ system, the hydroxyl is broken, the Hi exists in an atomic form and introduces new defect states in the band gap. As the large relaxation energy leads to a large Huang-Rhys factor, the Stokes red shifts will significantly affect the optical properties. A broad ultraviolet (UV) absorption band and emission band range from UV to visible are obtained originating from the defect cluster. We believe that the far-violet absorption peaks (167 nm and 179 nm) caused by defect cluster VK+Hi can significantly impact the optical damage threshold of the KDP.

Optimize energy storage performance of NaNbO3 ceramics by multivariable control strategy

NaNbO3 ceramics have broad application prospects in ultra-high power electronic systems, electromagnetic pulses, and other applications. In this study, (1-y)[(1-x)NaNbO3-xSmMg0.5Zr0.5O3]-y(Bi0.5Na0.5)0.7Sr0.3TiO3 (x = 0.06, 0.08 and 0.10)(y = 0.30, 0.35, 0.40 and 0.45) ceramics were fabricated. The effects of the incorporation of SmMg0.5Zr0.5O3 (SMZ) and (Bi0.5Na0.5)0.7Sr0.3TiO3 (BNST) on the energy storage performance, impedance, actual charge and discharge capacity, phase structure and microstructure of the ceramic samples were analyzed. Finally, when x = 0.08, and y = 0.35, the recoverable energy storage density of the sample was 5.8 J/cm3 and a high energy storage efficiency of 88 % was obtained at 545 kV/cm. In addition, the ceramic maintains a stable actual charge and discharge capacity and an extremely fast discharge time (16.1 ns) in a wide temperature range (20–160 °C). These excellent energy-storage performances may be due to the fine grain size and large relaxation and activation energies.

Unveiling surface stability and oxygen diffusion of rare‐earth zirconate pyrochlores by density functional theory

AbstractThe surface structures, bond variations, and segregation of oxygen vacancies play crucial roles in the structural stability and functionality of nanocrystalline rare‐earth zirconate pyrochlores. In this work, the stabilities of (1 0 0), (1 1 0), and (1 1 1) surfaces of pyrochlore A2Zr2O7 (A = La, Ce, Pr, Nd, Pm, Sm, Eu, or Gd) are investigated by first‐principles calculations. Surface reconstruction occurs on (1 1 0) surface with a transition of ZrO6 octahedron to ZrO4 tetrahedron, leading to their large relaxation energies. In combination with the small amount of broken bonds during the surface formation process, the (1 1 0) surfaces are identified having the lowest surface formation energies than the (1 0 0) and (1 1 1) surfaces. Moreover, the reconstructed (1 1 0) surface has characteristics of the segregation of oxygen vacancies. The surface oxygen vacancies have the low migration barriers (<1.2 eV), which are comparable with those in bulk and ensure the long‐distance diffusion of oxygen vacancies in A2Zr2O7. These discoveries provide fundamental insight to the surface structure and related oxygen vacancy behavior, which are expected to guide the optimization of the surface related properties for nanocrystalline rare‐earth zirconates.

Segregation and Migration of the Oxygen Vacancies in the Σ3 (111) Tilt Grain Boundaries of Ceria

In nanocrystalline materials, defect–grain boundary (GB) interactions play a key role in determining structural stability, as well as size-dependent ionic, electronic, magnetic, and chemical properties. In this study, using density functional theory, we systematically investigated the segregation and migration of oxygen vacancies at the Σ3 [110]/(111) grain boundary of ceria. Three oxygen layers near the GB are predicted to be segregation sites for oxygen vacancies. Moreover, the presence of oxygen vacancies stabilizes this tilt GB at a low Fermi level and/or under oxygen-poor conditions. An atomic strain model is proposed to rationalize the layer dependency of the relaxation energy for a 2+-charged oxygen vacancy. The structural origin of the large relaxation energies at layers 1 and 2 was determined to be free-volume space, which induces ion relaxation toward the GB. Our results not only pave the way for improving oxygen transport near GBs in ceria, but also provide important insights into the engineer...

Optical characterization of colloidal CdSe quantum dots in endothelial progenitor cells

We have quantitatively analyzed the confocal spectra of colloidal quantum dots (QDs) in rat endothelial progenitor cells (EPCs) by using Leica TCS SP5 Confocal Microscopy System. Comparison of the confocal spectra of QDs located inside and outside EPCs revealed that the interaction between the QDs and EPCs effectively reduces the radius of the exciton confinement inside the QDs so that the excitonic energy increases and the QD fluorescence peak blueshifts. Furthermore, the EPC environment surrounding the QDs shields the QDs so that the excitation of the QDs inside the cells is relatively weak, whereas the QDs outside the cells can be highly excited. At high excitations, the occupation of the ground excitonic state in the QD outside the cells becomes saturated and high-energy states excited, resulting in a large relaxation energy and a broad fluorescence peak. This permits, in concept, to use QD biomarkers to monitor EPCs by characterizing QD fluorescence spectra.

Electronic Structure of Unsaturated V2O5(001) and (100) Surfaces: Ab Initio Density Functional Theory Studies

Vanadium oxides based materials are well known to play an active role as catalysts in many chemical processes of technological importance like for example hydrocarbon oxidation reactions or selective catalytic reduction of NO x in the presence of ammonia. Usually the (010) surface is pointed out as the most important, however one has to underline that other low-indices surfaces are by far less studied. In the present study the electronic structure of V2O5(001) and (100) surfaces are determined by ab initio DFT methods using gradient-corrected RPBE exchange-correlation functional. As models of surface sections different embedded V14O45H20, V14O44H18, and V21O65H25 clusters are considered for the (001) surface and V12O40H20, V14O46H22, V16O52H24 for the (100) surface. Detailed analyses of the electronic structure of each cluster are performed using charge density distributions, Mayer bond orders, electrostatic potential maps, character of frontier orbitals, and density of states (total as well as partial, atom projected). Results of the calculations show that overall negative charge of the surface oxygen sites scales with their coordination independent of the surface orientation. Terminal oxygen O(1) is charged the least negatively while doubly coordinated atoms –O(2) and Oe(2) have charge twice as large. This indicates that bridging (for (001) and (100) netplanes) and edging (only for (001) netplane) oxygen sites are more nucleophilic than terminal vanadyl sites, which becomes important in view of the reactivity of the different sites for surface chemical reactions. Vanadium atoms present at these surfaces are positively charged (electrophilic) and may play a role of electron acceptors. The unsaturated surfaces show a strong tendency to surface relaxation that manifest by large relaxation energies.

Bonding mechanism and relaxation energy of [formula omitted] ( [formula omitted]): First-principles study

Using first-principles calculations, we have studied the electronic structures, the relaxation energy and the bonding mechanism of SrBi 2 B 2 O 9 ( B = Ta , Nb ). Unlike the case of prototype perovskites, the covalent bonding between cations and oxygen of SrBi 2 B 2 O 9 ( B = Ta , Nb ) does not directly stabilize their ferroelectric phase through reducing their total energy. Whereas, the ferroelectric stability of SBN and SBT is determined by a large relaxation energy derived from a larger distortion along with their phase transition. The origin of the spontaneous polarization of the materials is also briefly discussed in our present work.

Thickness‐Dependent Structural Evolutions and Growth Models in Relation to Carrier Transport Properties in Polycrystalline Pentacene Thin Films

AbstractThickness‐dependent crystal structure, surface morphology, surface energy, and molecular structure and microstructure of a series of polycrystalline pentacene films with different film thickness ranging from several monolayers to the several hundred nanometers have been investigated using X‐ray diffraction (XRD), atomic force microscopy (AFM), contact angle meter, and Raman spectroscopy. XRD studies indicate that thin film polymorphs transformation behaviours are from the orthorhombic phase to the thin‐film phase and then to the triclinic bulk phase as measured by the increased tilt angle (θtilt) of the pentacene molecule from the c‐axis toward the a‐axis. We propose a growth model that rationalizes the θtilt increased along with increasing film thickness in terms of grain size and surface energy varying with film growth using AFM combined with contact angle measurements. The vibrational characterizations of pentacene molecules in different thickness films were investigated by Raman spectroscopy compared to density functional theory calculations of an isolated molecule. In combination with XRD and AFM the method enables us to distinguish the molecular microstructures in different thin film polymorphs. We proposed a methodology to probe the microscopic parameters determining the carrier transport properties based on Davydov splitting and the characteristics of aromatic C–C stretching modes in Raman spectra. When compared to the triclinic bulk phase at a high thickness, we suggest that the first few monolayer structures located at the dielectric surface could have inferior carrier transport properties due to weak intermolecular interactions, large molecular relaxation energy, and more grain boundaries.

Electronic structure and reactivity of defect MoS 2: I. Relative stabilities of clusters and edges, and electronic surface states

The defect structure of 2H-MoS 2 is examined by density functional methods, and the results are compared with experiment where available. The defects considered range from edge structures to clusters of decreasing size to the single MoS 2 molecule. Stability of the edges is found to be in the order (1 0 1 ̄ x ) (most stable) > (1 2 ̄ 1 x ) > (1 2 ̄ 1 0) > (1 0 1 ̄ 0), where the inclination index x=3 or 4. A large relaxation energy is associated with the reconstruction of the edge from ideal geometry of a cut through the 2H-MoS 2 crystal: for the (1 0 1 ̄ x ) edge, energy of 0.63 eV per MoS 2 molecular formula is released upon a concerted movement of exposed S atoms that increases the coordination of the edge Mo atoms from 4 to 5. The electronic band structure calculation identifies surface states, which have a metallic character for the (1 0 1 ̄ x ) edge and a narrow gap semiconducting character for the (1 2 ̄ 1 x) edge. For reference purposes, periodic band structure calculation of a single 2-D sheet of MoS 2 by the LCAO DFT method used in this work yields practically identical results to earlier LAPW DFT calculations of Park et al. [J. Chem. Phys. 111 (1999) 1636]. The HOMO of the (1 0 1 ̄ x ) edge is a surface state that penetrates beyond the first layer of edge Mo atoms. The effective mass of an electron in this state is calculated to be 1.9 m e, compared to 4.1 m e for a hole in the HOMO of the 2-D sheet. Clusters of (MoS 2) n ( n=7, 3, 2, 1) also show large relaxation energies from the 2H-MoS 2 geometry, with the main tendency to increase the coordination of the exposed Mo atoms by inward movement. The difference in energies of the spin singlet and triplet states increases from near zero for (MoS 2) 7 to 0.53 eV for the single MoS 2 molecule, in excellent agreement with the data for MoS 2 in frozen argon matrix reported by Liang and Andrews [J. Phys. Chem. A 106 (2002) 6945].

Controlled p doping of the hole-transport molecular material N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine with tetrafluorotetracyanoquinodimethane

We investigate p-type doping of the hole-transport organic molecular material N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) with tetrafluorotetracyanoquinodimethane (F4-TCNQ) using direct and inverse photoemission spectroscopy, contact potential difference measurements, and in situ current–voltage (I–V) measurements. The close match between the ionization energy of α-NPD and the electron affinity of F4-TCNQ leads to an efficient charge transfer between highest occupied molecular orbital of the host and lowest occupied molecular orbital of the dopant. The Fermi level moves down towards the valence states by 0.62 eV in the 0.5% doped film with respect to the undoped film, and a narrow space charge layer (∼60 Å) forms at the interface with Au. Hole injection in the doped devices increases by several orders of magnitude due to tunneling through the depletion region. The large relaxation energy of the ionized α-NPD molecule limits the movement of the Fermi level and, ultimately, the hole injection.

Vibrational Predissociation Spectroscopic and Ab Initio Theoretical Studies on Protonated Ethylenediamine−(Water)3 Complex

Structures, interactions, and vibrations of an intramolecular hydrogen bond (IHB)-containing protonated ion(water) 3 complex, g-enH + (H 2 O) 3 (g-enH + = gauche-protonated ethylenediamine) and t-enH + (H 2 O) 3 (t-enH + = trans-protonated ethylenediamine) with no IHB were investigated by combined vibrational predissociation spectroscopy and ab initio calculations. The observed vibrational feature at 3552 cm - 1 (signature of cyclic ion-(water) 3 structure), consistent with ab initio results on the stabilities of structural isomers, suggested the IHB-assisted bicyclic structure for g-enH + (H 2 O) 3 , Ab initio many-body analyses further indicated that the greater stability of bicyclic isomer versus monocyclic and bicyclic tripod isomers originates from the destabilization of the latter by the large relaxation energies and repulsive nonadditive interactions. The close correlation between the observed and calculated vibrational spectra for g-enH + (H 2 O) 3 revealed the coexistence of bicyclic isomers and monocyclic open isomers in our beam, consistent with the trend of the calculated Gibbs free energies at 150 K.

Theory of emission state of tris(8-quinolinolato)aluminum and its related compounds

Fluorescence of fac-AlQ3 (Q=8-quinolinolato), mer-AlQ3, mer-Al(mQ)3 (mQ=4-methyl-8-quinolinolato), and BeQ2 were investigated with electronic structure calculations. The molecular structure of the first singlet excited state (the emission state) was optimized with the ab initio “configuration interaction with single excitations” (CIS) method. Ab initio CIS and semiempirical “Zerner’s intermediate neglect of differential overlap” (ZINDO) methods were used to calculate the emission energies (ΔE) and also the corresponding absorption energies. Although the ab initio CIS method overestimated the experimental value of ΔE by 1.09–1.16 eV, the ZINDO method reproduced it to a reasonable accuracy (within 0.26 eV). The optimized excited-state structure has an interesting feature in that one of the equivalent ligands distorts appreciably, while the thers keep their ground-state structures. As a result the highest occupied and the lowest unoccupied molecular orbitals (HOMO and LUMO) are localized on the distorted ligand. Since the excited state is characterized as the HOMO–LUMO transition, the emission from AlQ3 (and its analogues) directly reflects that of the ligand. In order to analyze this intrigung excited-state structure, exciton transfer theory was applied, where the exciton coupling between ligands and the structural relaxation of the ligand upon excitation were taken into account. By examining these two factors for BeQ2, it is shown that the exciton localization results from weak exciton coupling and/or large structural relaxation energy.

Density-matrix renormalization-group calculations of excited states of linear polyenes

We present density-matrix renormalization-group calculations of the Pariser-Parr-Pople-Peierls model of linear polyenes within the adiabatic approximation. We calculate the vertical and relaxed transition energies, and relaxed geometries for various excitations on long chains. The triplet $(1{}^{3}{B}_{u}^{+})$ and even-parity singlet $(2{}^{1}{A}_{g}^{+})$ states have a 2-soliton and 4-soliton forms, respectively, both with large relaxation energies. The dipole-allowed $(1{}^{1}{B}_{u}^{\ensuremath{-}})$ state forms an exciton-polaron, and has a very small relaxation energy. The relaxed energy of the $2{}^{1}{A}_{g}^{+}$ state lies below that of the $1{}^{1}{B}_{u}^{\ensuremath{-}}$ state. We observe an attraction between the soliton-antisoliton pairs in the $2{}^{1}{A}_{g}^{+}$ state. The calculated excitation energies agree well with the observed values for polyene oligomers; the agreement with polyacetylene thin films is less good, and we comment on the possible sources of the discrepancies. The photoinduced absorption is interpreted. The spin-spin correlation function shows that the unpaired spins coincide with the geometrical soliton positions. We study the roles of electron-electron interactions and electron-lattice coupling in determining the excitation energies and soliton structures. Electronic interactions play the key role in determining the ground-state dimerization and the excited-state transition energies.

Electron-Lattice Relaxation, and Soliton Structures and Their Interactions in Polyenes

Density matrix renormalization group calculations of a suitably parametrized model of long polyenes (polyacetylene oligomers), which incorporate both long range Coulomb interactions and adiabatic lattice relaxation, are presented. The ${1}^{3}{B}_{u}^{+}$ and ${2}^{1}{A}_{g}^{+}$ states are found to have a 2-soliton and a 4-soliton form, respectively, both with large relaxation energies. The ${1}^{1}{B}_{u}^{\ensuremath{-}}$ state forms an exciton polaron and has a very small relaxation energy. The relaxed energy of the ${2}^{1}{A}_{g}^{+}$ state lies below that of the ${1}^{1}{B}_{u}^{\ensuremath{-}}$ state. The soliton/antisoliton pairs are bound.

Vacancy Properties In 5d Bcc Transition Metals: Ab Initio Study At Finite Electron Temperature

ABSTRACTThe self-diffusion constants for the monovacancy mechanism in the 5d transition-metals with bcc structure (β-Hf, Ta and W) are investigated by first-principles pseudopotential calculations within the framework of the Local Density Functional Theory. The formation and migration energies, calculated for relaxed configurations using supercells containing 27 and 54 atomic sites, are in quite good agreement with experimental data in Ta and W, with a discrepancy lower than 10 %. Preliminary results in β-Hf using smaller supercells suggest very large relaxation energies. The effect of finite electron-temperature is shown to be quite important, and very different from one element to the other: the electron contribution to the activation entropy is negative in Ta and positive in W, reaching respectively −2 kB and 2 kB at the melting temperature. Using simple estimates for the attempt frequencies and the vibrational formation entropies, the calculated self-diffusion coefficient is in exceptional agreement with experiments in W, and clearly reproduces an accelerated diffusivity in Ta.

Spectroscopy and dynamics of jet-cooled hydrazines and ammonia. I. Single-photon absorption and ionization spectra

Electronic-state properties of hydrazine (N2H4), monomethyl hydrazine (MMH), unsymmetrical dimethyl hydrazine (UDMH), and ammonia were investigated by single-photon vacuum ultraviolet (vuv) absorption and photoionization spectroscopy in a molecular-beam apparatus. The photoionization spectrum of NH3 shows a sharp threshold at a value of 10.16 eV in excellent agreement with previous measurements. The ionization thresholds of the hydrazines are very broad reflecting the large (nearly 2 eV) structural relaxation energy for conversion from a C2 pyramidal gauche neutral geometry to a D2h planar ion geometry. The measured threshold photoionization potentials are 8.32 eV (N2H4), 8.05 eV (MMH), and 7.87 eV (UDMH). The vuv absorption spectra of the hydrazines are also broad, which is to be contrasted with the well-resolved spectrum of NH3. The band shapes can be attributed to planarization of the Rydberg excited states in analogy to the ion structure. A molecular-orbital model is developed to provide understanding of the excited states in terms of their symmetries, energy ordering, and selection rules, as a function of geometry.

Violent oxidation of lithium-containing aluminum alloys in liquid oxygen

A strong exothermic and quite well known thermite reaction involving aluminum, oxygen and transition metals (Fe, Cr, Ni, etc.) has apparently been initiated during impact testing of Alcoa aluminum alloy #2090 in liquid oxygen at NASA-MSFC. In some instances, this reaction, essentially an oxidation process, has been so intense that the Inconel 718 cup containing the aluminum alloy disk and associated impacter has melted raising certain safety concerns in the use of this alloy. Reaction products as well as the test specimen surfaces have been studied with surface science techniques like XPS/ESCA, SIMS and AES. Typically, in order to initiate the thermite reaction a temperature of approximately 1000°C is necessary. The mechanism responsible for this oxidation is of great interest. The analysis of the reaction products together with a theoretical analysis, including digital modeling has been pursued. There is strong evidence that the large relaxation energy of the aluminum oxide coating, formed during the aluminum alloy cleaning process, is causing a highly localized energy release during fracture or lattice deformation which is enhancing the oxidation process to a runaway condition. The presence of alkali atoms (Li) enhances the likelihood and intensity of the oxidation reaction. The details of the surface studies will be discussed.

Characterization of deep levels in CdTe by photo-EPR and related techniques

In photo-EPR, the EPR signature of microscopically identifiable defects is used to monitor light-induced electronic transitions. In this paper we summarize photo-EPR investigations of group IV donors, 3d transition metal impurities and complexes in CdTe. Energy levels and Huang-Rhys factors are derived for the 3d impurities by fitting a model calculation to the photo-ionization cross-section which is derived from constant photo-EPR. Comparison with results for other II–VI compounds confirm host-independent relative zero-phonon level positions for different 3d metal impurities and large lattice relaxation energies.

Electronic structure of organometallic compounds. Part 28. Photoelectron spectra of transition-metal-1-methylborinato complexes in the outer valence region. Semiempirical model calculations based on Green's function formalism

The electronic structures of the 1-methylborinato complexes of V(CO)/sub 4/ (1), Mn(CO)/sub 3/ (2), Co(CO)/sub 2/ (3), and Co(C/sub 4/H/sub 4/) (4) have been studied by means of INDO calculations and their He I photoelectron (PE) spectra. The calculated rotational barriers and the orbital sequences of 1-3 have been compared with their isovalent (C/sub 5/H/sub 5/)M(CO)/sub n/ species. The measured ionization energies are discussed on the basis of those calculated by means of the Green's function formalism. It if found that Koopmans' theorem is only valid in the case of 1. Significant differences between the filling scheme of the canonical MO's in the ground state and the sequence of the ionic states are demonstrated for 2-4. It is shown that large relaxation energies are partially compensated by pair-relaxation contributions (i.e., many-body correction). 63 references, 10 figures, 5 tables.