Types Of Electronic States Research Articles

Controlling the surface chemistry towards unsaturated carbon-carbon bonds over non-oxide transition metal ceramics: Trends in adsorption and distortion energies and adsorbate “activation”

Controlling the surface chemistry of catalysts towards unsaturated carbon-carbon bonds is vital to achieving many contemporary chemical transformations. Transition metal (TM) ceramics are promising materials that exhibit lower and tunable surface reactivity towards C=C bonds. Herein, we have employed an analysis of the adsorption of benzene over a large suite of TM ceramic and pure metal surfaces to benchmark a large selection of materials to isolate trends. Results showed a wide range of surface reactivity as a function of the constituent elements, surface composition, TM-to-p-element ratio, surface ensemble size, electronic structure, etc. The 1st row TM ceramics exhibited lower surface reactivity and more stable surface composition than the 2nd/3rd TM ceramics suggesting that orbital-size match could be a simple marker for predicting general surface chemistry and surface composition stability in TM solid compounds. The TM-to-p-element ratio also generally dictated metal-like or markedly-modified reactivity of the constituent elements at the ceramic surface. Electronic analysis further quantified the correlation between the density and type of electronic states near the Fermi level and the reactivity of surface-lattice elements. Structural and energetic analysis of the adsorbed benzene indicated the activation of the adsorbate could be better quantified beyond simple adsorption energy.

Static polarizabilities for excited states within the spin-conserving and spin-flipping equation-of-motion coupled-cluster singles and doubles formalism: Theory, implementation, and benchmarks.

We present the theory and implementation for calculating static polarizabilities within the equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) framework for electronically excited states and its spin-flip variant. We evaluate the second derivatives of the EOM-CCSD Lagrangian with respect to electric-field perturbations. The relaxation of reference molecular orbitals is not included. In our approach, the wave function amplitudes satisfy the 2n + 1 rule and the amplitude-response Lagrange multipliers satisfy the 2n + 2 rule. The new implementation is validated against finite-field and CCSD response-theory calculations of the excited-state polarizabilities of pyrimidine and s-tetrazine. We use the new method to compute static polarizabilities of different types of electronic states (valence, charge-transfer, singlets, and triplets) in open- and closed-shell systems (uracil, p-nitroaniline, methylene, and p-benzyne). We also present an alternative approach for calculating excited-state static polarizabilities as expectation values by using the EOM-CCSD wave functions and energies in the polarizability expression for an exact state. We find that this computationally less demanding approach may show differences up to ∼30% relative to the excited-state polarizabilities computed using the analytic-derivative formalism.

Optical probe for surface and subsurface defects induced by ion bombardment.

We demonstrate that reflectance difference spectroscopy (RDS) is sensitive to defects induced by ion bombardment, located either in the topmost layer or in the subsurface region. Most importantly, these two kinds of defects can be spectrally discriminated, since the corresponding signatures in the RD spectrum arise from perturbations of different types of electronic states: The defects in the topmost surface layer mainly lead to a quenching of the optical anisotropy related to surface states, whereas the subsurface defects strongly affect the optical anisotropy originating from transitions between surface-modified bulk electronic states. Consequently, RDS can be used to simultaneously monitor the defects in the topmost surface layer and in the subsurface region in-situ during ion bombardment and thermal annealing.Characteristic RD spectra and the corresponding STM images for a Cu(110) substrate before and after healing of the subsurface defects.

Ultrafast Exciton Dynamics in Colloidal CdSe/CdS Octapod Shaped Nanocrystals

We studied the carrier dynamics in colloidal octapod-shaped cadmium selenide/cadmium sulfide (CdSe/CdS) nanocrystals in the solution phase via pump–probe optical techniques with subpicosecond resolution. We could resolve bleaching from two different types of electronic states having distinct dynamics and assigned them to states delocalized in the pods and mildly localized in the core based on the good agreement of energies found with effective mass modeling. Contrary to other CdSe/CdS core/shell nanocrystals, such a mild localization has geometrical origins as the best agreement was found for negligible conduction band offset. Moreover, even though the large surface of the CdSe/CdS heterointerface results into a weak signature of electron trapping in the bleaching spectrum, we found that a relevant fraction of electrons do remain delocalized in pod states for long times and are thus available for diffusion in photovoltaic applications where the highly branched geometry is expected to advantageously yield to effective percolation in dense assemblies.

Ab initiostudy of phonon-induced dephasing of plasmon excitations in silver quantum dots

Phonon-induced pure dephasing of electronic excitations in silver quantum dots (QDs) is investigated with ab initio molecular dynamics at ambient and low temperatures. Three types of electronic states are studied corresponding to bulk, surface, and plasmon excitations. The electron-phonon coupling is strongest for bulk states and decreases for surface and plasmon states. The plasmon states dephase within 30--40 fs, which is consistent with the recent experiments [M. Z. Liu, M. Pelton, and P. Guyot-Sionnest, Phys. Rev. B 79, 035418 (2009)]. The dephasing time shows weak dependence on the QD size but changes significantly with temperature. The bulk, surface, and plasmon states couple primarily to low-frequency acoustic phonons.

Topological bound states

In the tight binding approximation, it has been shown that a peculiar type of electronic states localized near the topology change point exists in branching molecules and quantum conductors. Bound states of this type exist both below and above the allowed band, i.e., for both electrons and holes; this property fundamentally differentiates these states from the bound states formed in the minimum of the potential energy. The damping decrement of the wavefunction is independent of the band parameters and is an invariant determined by the characteristic of topology. The tunnel interaction between the topological bound states significantly determines the change in the electronic spectrum of the molecular systems in configuration transitions.

Deep traps and optical properties of partially strain-relaxed InGaAs/GaAs heterostructures

The electrical and optical properties of the lattice-mismatched InGaAs/GaAs heterostructures, with partial strain relaxation, were studied by deep level transient spectroscopy (DLTS) and photoreflectance (PR) spectroscopy, respectively. In the samples one of deep electron traps, revealed by DLTS, was ascribed to misfit dislocations at the interface, for which capture kinetics, concentration depth profiles and the type of electronic states have been specified. Room temperature PR spectroscopy was used for analysing the effect of residual strain on the optical response from the samples, i.e. interband transitions and the valence band splitting.

Calculations of spin-induced transport in ferromagnets

Based on first-principles density functional calculations, a general approach for determining and analyzing the degree of spin polarization $(P)$ in ferromagnets is presented. The approach employs the so-called tetrahedron method to evaluate the Fermi surface integrations of $P$ in both ballistic and diffusive regimes. The validity of the method is examined by comparing the calculated $P$ values for Fe and Ni with the experiment. The method is shown to yield highly accurate results with minimal computational effort. Within our approach, it is also possible to systematically analyze the contributions of various types of electronic states to the spin-induced transport. As a case study, the transport properties of the soft-ferromagnet $\mathrm{Ce}\mathrm{Mn}{\mathrm{Ni}}_{4}$ are investigated in order to explain the origin of the existing difference between the experimental and theoretical values of $P$ in this intermetallic compound.

Capture kinetics at dislocation-related deep levels in III-V heterostructures

Lattice-mismatch induced defects have been studied by means of deep-level transient spectroscopy (DLTS) in two types of GaAs-based heterostructures, GaAs/InGaAs and GaAs/GaAsSb, and in a GaN-based laser-diode heterostructure grown on a bulk GaN substrate. DLTS measurements, carried out with p-n junctions formed near the heterostructure interfaces, revealed three deep-level traps in the investigated structures. which have been related to dislocations on the grounds of their logarithmic kinetics for capture of charge carriers into the trap states. An electron trap at E C - 0.64 eV and a hole trap at E V + 0.67 eV, found in the GaAs-based structures, have been attributed, respectively, to threading and misfit dislocations. An electron trap at E C - 0.23 eV, found in the GaN-based structure, has been tentatively related to dislocations in a p-type layer of the structure. Detailed investigation of the dependence of DLTS-line amplitude and its shape on the filling time of the traps with charge carriers allowed us to specify the type of electronic states associated with the traps.

Electronic States and Schottky Barrier Height at Metal/MgO(100) Interfaces

Using an ab initio total energy approach, we study the electronic structure of metal/MgO(100) interfaces. By considering simple and transition metals, different adsorption sites and different interface separations, we analyze the influence of the character of metal and of the detailed interfacial atomic structure. We calculate the interface density of states, electron transfer, electric dipole, and the Schottky barrier height. We characterize three types of electronic states: states due to chemical bonding which appear at well defined energies, conventional metal-induced gap states associated to a smooth density of states in the MgO gap region, and metal band distortions due to polarization by the electrostatic field of the ionic substrate. We point out that, with respect to the extended Schottky limit, the interface formation yields an electric dipole mainly determined by the substrate characteristics. Indeed, the metal-dependent contributions (interfacial states and electron transfer) remain small with respect to the metal polarization induced by the substrate electrostatic field.

Colossal magnetoresistance manganites: A new approach

Manganites of the LA1−x Ca x MnO3 family show a variety of new and poorly understood electronic, magnetic and structural effects. Here we outline a new approach recently proposed by us, where we argue that due to strong Jahn-Teller (JT) coupling with phonons the twofold degeneratee g states at the Mn sites dynamically reorganize themselves into localised, JT polaronsl with exponentially small inter-site hopping, and band-like, nonpolaronic statesb, leading to anew 2-band model for manganites which includes strong Coulomb and Hund’s couplings. We also discuss some results from a dynamical mean-field theory treatment of the model which explains quantitatively a wide variety of experimental results, including insulator-metal transitions and CMR, in terms of the influence of physical conditions on the relative energies and occupation of thel andb states. We argue that this microscopic coexistence of the two types of electronic states, and their relative occupation and spatial correlation is the key to manganite physics.

Intermetallic bonds and midgap interface states at epitaxial Al/GaAs(001) junctions

Using first-principles pseudopotential calculations, we have investigated the nature of the electronic states with energies within the semiconductor band gap of abrupt, defect-free As-terminated Al/GaAs(001) junctions. While bondinglike/antibondinglike semiconductor evanescent states occur near the valenceband/conduction-band edges, the semiconductor midgap region is characterized by a different type of electronic states, not accounted for by commonly accepted models. These states, which correspond to intermetallic bonds between the outermost Ga cations of the semiconductor and Al atoms of the metal occur near the Fermi energy. They are localized at the interface and are located around the $\mathbf{J}$ point of the Brillouin zone. These new interface states derive from an interaction between localized states of the Al(001) surface and bulk GaAs conduction-band states, mediated by localized states of the unreconstructed As-terminated GaAs(001) surface.

Two Types of Electronic States in One-Dimensional Crystals of Finite Length

Exact and general results on the electronic states in one-dimensional crystals bounded at τ and τ+ L, where L= Na, N is a positive integer, and a is the potential period, are presented. Corresponding to each energy band of the Bloch wave, there are N−1 states in the finite crystal whose energies are dependent on L but not on τ and map the energy band exactly. There is always one and only one electronic state corresponding to each band gap of the Bloch wave, whose energy is dependent on τ but not on L. This state is either a constant-energy confined band-edge state or a surface state in the band gap.

Identification of a new type of electronic state in the magnetoresistive orthomanganites

PEROVSKITES of composition Ln1–xAxMnO3 (where Ln and A are rare-earth and alkaline-earth elements respectively) have become the focus of scientific and technological interest because of their extraordinary electronic and magnetic properties. For example, compositions with x = 0.3 exhibit an unusual combination of ferromagnetism and good electrical conductivity1; moreover, the observation2 of a 'colossal' magnetoresistive response near the Curie temperature (Tc), which can be changed dramatically by altering the strength of the Mn–O–Mn interactions3,15, points to potential practical applications for these materials. Recent resistivity studies4 of these materials have suggested that the accepted 'tight-binding' band description5 of the conduction electrons below Tc may be inadequate and that strong interactions between the electrons and dynamic local lattice distortions associated with the Mn sites may be important. Here we report resistivity and thermopower measurements at high pressure, which indicate a more exotic origin for the electronic properties of these materials. We find that below Tc the conduction electrons lose their localized character and condense into extended electronic states that exhibit no energy dispersion. We attribute the formation of this new state to the strong coupling of the conduction electrons to cooperative oxygen vibrations along the Mn–O bond axes.

Relaxation of electronic excitations in YAG and YAP crystals

One- and two-photon spectroscopy is used for the investigation of the nature of the lowest electronic excitations and their relaxation in YAG and YAP crystals. The coexistence of two types of electronic states (the metastable excitons and the electron-hole continuum) having different relaxation ways has been observed in the exciton absorption region. The peculiarities of the electronic state spectrum observed are supposed to be connected with the complexity of the electron structure of the oxides investigated.

EQUIVALENCE IN GEOMETRIC SCALING OF TRANSPORT PROPERTIES AND WAVEFUNCTIONS

The transport properties and wavefunction behave identically with respect to multifractal scaling. To establish the above statement, we carried out multifractal analysis on normalised transmittance and normalised wavefunction of two types of electron states, namely the resonance state and the localised state of a one-dimensional Anderson model.

Interference photoconductivity and photoelectromagnetic effect in amorphous silicon.

The existence of the so-called interference photoconductivity (PC) and photoelectromagnetic (PEM) effects in the investigated thin films of a-Si:H,F is shown. These effects appear due to interference of radiation in a sample. In this paper, the fitting of interference PC and PEM responses to theoretical relationships is presented. Values of carrier lifetime, diffusion length, and surface recombination velocity have been estimated. Spectral dependences of individual quantum efficiency coefficients for PC and PEM effects can give information about energy distribution and type of electronic states in the investigated samples. Influence of radiation intensity on optical and recombination parameters of the material is shown.

Mechanisms of Schottky-barrier formation in metal–semiconductor contacts

During the formation of metal–semiconductor contacts two principally different types of electronic states are effective in determining the surface position of the Fermi level within the semiconductor band gap. These are, first, adatom-related surface states of donor character and, second, the continuum of adsorbate-induced interface states which result from the tailing of the metal wave functions into the virtual gap states of the semiconductor band structure. The first type of state is observed at submonolayer coverages either after depositions at low temperatures or even at room temperature when a cation exchange occurs. Each metal adatom contributes one of those surface donors and their energy levels are linearly correlated with the first ionization energies of the metal atoms. The second type of state exists at interfaces under metallic islands and continuous metallic films. The charge transferred across the interface by these tails of the metal wave functions is explained by the difference in electronegativities of metal and semiconductor in analogy to the concept of the partial ionic character of covalent bonds.

Unified theory of point–defect electronic states, core excitons, and intrinsic electronic states at semiconductor surfaces

A simple unified theory has been developed for predicting three different types of electronic states at semiconductor surfaces: (i) point–defect states, (ii) core excitons, and (iii) intrinsic surface states. The predictions are in agreement with experiment for each class of states. The theory indicates that Fermi level pinning at III–V semiconductor interfaces is usually caused by surface antisite defects.

The stabilization of silicon surfaces using silicon nitride

The silicon-silicon nitride system was investigated and three types of electronic states were found: interface states (concentration similar 1010 cm−2), shallow nitride traps (similar 1011 cm−2) and deep nitride traps (similar 1012 cm−2). The deep traps were of both types and a form of compensation was found to occur; thus by applying fields in excess of a certain threshold field (similar, equals1·75 MV cm−1) either a positive or negative charge could be fixed in the nitride. A model for the deep traps is presented, based on stacking faults in local crystallized areas of the amorphous nitride film. A hysteresis effect was observed at all biases and temperatures, its magnitude remaining sensibly constant. This effect is explained in terms of the shallow nitride traps.At room temperature the apparent concentration of surface states (this term is taken to include interface states and shallow and deep nitride traps) was found to vary as ln f but at liquid-nitrogen temperature it was found to be constant. An explanation of the frequency and temperature dependence of the trapped carriers is presented.Preliminary work on the d.c. characteristics of the nitride films indicated that a Frenkel-Poole mechanism of charge transfer occurs at room temperature.