Delocalized Band States Research Articles

Nanotube-Like Electronic States in [5,5]-C90 Fullertube Molecules.

Fullertubes, that is, fullerenes consisting of a carbon nanotube moiety capped by hemifullerene ends, are emerging carbon nanomaterials whose properties show both fullerene and carbon nanotube (CNT) traits. Albeit it may be expected that their electronic states show a certain resemblance to those of the extended nanotube, such a correlation has not yet been found or described. Here it shows a scanning tunneling microscopy (STM) and spectroscopy (STS) characterization of the adsorption, self-assembly, and electronic structure of 2D arrays of [5,5]-C90 fullertube molecules on two different noble metal surfaces, Ag(111) and Au(111). The results demonstrate that the shape of the molecular orbitals of the adsorbed fullertubes corresponds closely to those expected for isolated species on the grounds of density functional theory calculations. Moreover, a comparison between the electronic density profiles in the bands of the extended [5,5]-CNT and in the molecules reveals that some of the frontier orbitals of the fullertube molecules can be described as the result of the quantum confinement imposed by the hemifullerene caps to the delocalized band states in the extended CNT. The results thus provide a conceptual framework for the rational design of custom fullertube molecules and can potentially become a cornerstone in the understanding of these new carbon nanoforms.

Peculiar bond characters of fivefold coordinated octet compound crystals.

The present work exemplifies complementary perspectives offered by the band and bond pictures of solids, with an emphasis on the chemical intuition pertaining to the latter, especially in the presence of interfaces. The modern computational method of constructing a unique set of maximally localized Wannier functions from delocalized band states imparts new interpretations to the familiar concept of chemical bonds in the context of crystalline solids. By bridging the band and bond pictures using advanced computational tools, we reveal for the first time the unusual bond characters of a long-predicted fivefold coordinated structure of binary octet compounds ANB8−N consisting of AA′ stacked planar AB honeycombs. While the isolated monolayer retains the familiar pz–π bonding in a honeycomb framework as in graphene and hexagonal boron nitride, the bulk foregoes in-plane π bonding and embraces out-of-plane ⋯A–B–A–B⋯ chain bonding via overlapping pz orbitals. Not only does the chemical intuition gained by invoking the bond picture clarify the chemical nature of the fivefold coordination, but it also facilely explains a salient discrepancy in theoretical predictions in otherwise sound ample experimental evidence in the form of epitaxial thin films, paving the way towards rational synthesis of such thin films for optoelectronic applications. On the other hand, we show that the conduction band minimum, important in determining the electrical and optical properties, is a distinctly extended state that can only be properly described within the band picture.

Molecular Control of Heterogeneous Electrocatalysis through Graphite Conjugation.

The efficient interconversion of electrical and chemical energy requires catalysts capable of accelerating multielectron reactions at or near electrified interfaces. These reactions can be performed at metallic surface sites on heterogeneous electrocatalysts or through redox mediation at molecular electrocatalysts. The relative ease of synthesis and characterization for homogeneous catalysts has allowed for molecular-level control over the active site and permitted systematic tuning of activity and selectivity. Similar control is difficult to achieve with heterogeneous electrocatalysts, because they typically exhibit a distribution of active site geometries and local electronic structures, which are challenging to modify with molecular precision. However, metallic heterogeneous electrocatalysts benefit from a continuum of electronic states that distribute the redox burden of multielectron transformations, enabling more efficient catalysis. We envisioned that we could combine the attractive properties of molecular and heterogeneous catalysts by integrating tunable molecular active sites into the delocalized band states of a conductive solid. The Surendranath group has developed a class of electrocatalysts in which molecules are strongly electronically coupled to graphitic electrodes through a conductive, aromatic pyrazine linkage such that they behave like metallic surface active sites. In this Account, we discuss the dual role of these graphite-conjugated catalysts (GCCs) as a platform with which to answer molecular-level questions of metallic active sites and as a tool with which to fundamentally alter the mechanism and enhance the performance of molecular active sites. We begin by describing the electrochemical and spectroscopic studies that demonstrated that GCC sites behave like metallic active sites rather than simply as redox mediators attached to electrode surfaces. We then discuss how electrochemical studies of a series of graphite-conjugated acids enabled the construction of a molecular model for the thermochemistry of proton-coupled electron transfer reactions at GCC sites based on the pKa of the molecular analogue of the conjugated site and the potential of zero free charge of the electrode. In the final section, we discuss the effects of graphite conjugation on the mechanism and rate of oxygen reduction, hydrogen evolution, and carbon dioxide reduction catalysis across four different GCC platforms involving N-heterocycle, organometallic, and metalloporphyrin active sites. We discuss how molecular-level tuning at graphite-conjugated active sites directly correlates to changes in catalytic activity for the oxygen reduction reaction. We demonstrate that graphite-conjugated porphyrins show enhanced catalytic oxygen reduction activity over amide-linked porphyrins. Lastly, we describe how catalysis at graphite-conjugated sites proceeds through mechanisms involving concerted electron transfer and substrate activation, in stark contrast to the mechanisms observed for molecular analogues. Overall, we showcase how GCCs provide a rich platform for controlling heterogeneous catalysis at the molecular level.

Influence of Transition Elements Dopants on Physical Properties of Zinc Oxide Nanofilms

Pure and doped zinc oxide nanofilms were deposited onto glass substrates by the sol–gel method. Doping can change structural, optical and magnetic properties of matter. In this research, transition elements (Fe, Co, Ni and Mn) were chosen as dopants because of their ferromagnetic property at room temperature. Structural, optical and magnetic properties of pure and doped zinc oxide were experimentally investigated. The band gap and absorption edge of nanofilms were determined, and the optical constants of films were calculated by the Cauchy formula. Transmittance spectrum, XRD diffraction and hysteresis loop were measured, and spontaneous and saturation magnetic moment of samples was calculated. Results of our measurements show Mn-doped samples are paramagnetic and the others are ferromagnetic, and band gap variation does not follow a specific pattern. Controlling dopant concentrations improves matter properties, so different Mn and Ni concentrations on ZnO nanolayers were prepared and investigated for finding out the effect of different concentrations of dopants on ZnO properties. In the case of different Mn concentrations, the band gap was decreased in low doping (Mn 3%), and this is due to the exchange interaction between the localized d shell electrons of the magnetic ions and the delocalized band states. Results of investigating different concentrations of Ni indicate that low Ni concentration slightly increases the band gap and high concentration of Ni leads to a decrease in the band gap. This phenomenon is due to the magnetic property of Ni and quantum confinement that can increase the band gap.

Physical-Based Analytical Model of Amorphous InGaZnO TFTs Including Deep, Tail, and Free States

Amorphous InGaZnO (a-IGZO) is a candidate material for thin-film transistors (TFTs) owing to its large electron mobility and good uniformity over large area. a-IGZO TFTs drain current models are essential for further pushing both a-IGZO TFTs technology and circuit design. In this paper, we propose a simple physical-based and analytical model of the drain current of a-IGZO TFTs. The model is valid in both non degenerate and degenerate conduction and it accounts for deep interface states, tail localized states, and free delocalized band states. The model is validated with the measurements of both coplanar and staggered a-IGZO TFTs. It provides key physical and material parameters of the transistor and, owing to its class $C^{\infty }$ formulation, it can be straightforwardly implemented in circuit simulators.

Manifestation of Anomalous Weak Space-Charge-Density Acentricity for a Tl4HgBr6 Single Crystal.

Density functional theory (DFT) calculations within the concept of the MBJ+U+SO (modified Becke-Johnson potential + U + spin orbit) approach were performed for a Tl4HgBr6 single crystal for the first time assuming weak noncentrosymmetry (space group P4nc). Excellent agreement was achieved between the calculated and experimental band-gap-energy magnitudes as well as the density of electronic states measured by the X-ray photoelectron spectroscopy method. It is a very principal result because usually the DFT calculations underestimate the energy-gap values. In the present study, we carry out calculations of the optical properties (absorption coefficient, real and imaginary parts of the dielectric function, electron energy-loss spectrum, refractive index, extinction coefficient, and optical reflectivity dispersions). It has been established that the principal origin of the observed weak acentricity is determined by delocalized band states at the top of the valence band originating from the p states of the Br atoms.

Application of photoreflectance to advanced multilayer structures for photovoltaics

Photoreflectance (PR) is a convenient characterization tool able to reveal optoelectronic properties of semiconductor materials and structures. It is a simple non-destructive and contactless technique which can be used in air at room temperature. We will present experimental results of the characterization carried out by means of PR on different types of advanced photovoltaic (PV) structures, including quantum-dot-based prototypes of intermediate band solar cells, quantum-well structures, highly mismatched alloys, and III–V-based multi-junction devices, thereby demonstrating the suitability of PR as a powerful diagnostic tool. Examples will be given to illustrate the value of this spectroscopic technique for PV including (i) the analysis of the PR spectra in search of critical points associated to absorption onsets; (ii) distinguishing signatures related to quantum confinement from those originating from delocalized band states; (iii) determining the intensity of the electric field related to built-in potentials at interfaces according to the Franz–Keldysh (FK) theory; and (v) determining the nature of different oscillatory PR signals among those ascribed to FK-oscillations, interferometric and photorefractive effects. The aim is to attract the interest of researchers in the field of PV to modulation spectroscopies, as they can be helpful in the analysis of their devices.

Electronic structure of the strongly hybridized ferromagnetCeFe2

We report on results from high-energy spectroscopic measurements on CeFe2, a system of particular interest due to its anomalous ferromagnetism with an unusually low Curie temperature and small magnetization compared to the other rare earth-iron Laves phase compounds. Our experimental results indicate very strong hybridization of the Ce 4f states with the delocalized band states, mainly the Fe 3d states. In the interpretation and analysis of our measured spectra, we have made use of two different theoretical approaches: The first one is based on the Anderson impurity model, with surface contributions explicitly taken into account. The second method consists of band-structure calculations for bulk CeFe2. The analysis based on the Anderson impurity model gives calculated spectra in good agreement with the whole range of measured spectra, and reveals that the Ce 4f -- Fe 3d hybridization is considerably reduced at the surface, resulting in even stronger hybridization in the bulk than previously thought. The band-structure calculations are ab initio full-potential linear muffin-tin orbital calculations within the local-spin-density approximation of the density functional. The Ce 4f electrons were treated as itinerant band electrons. Interestingly, the Ce 4f partial density of states obtained from the band-structure calculations also agree well with the experimental spectra concerning both the 4f peak position and the 4f bandwidth, if the surface effects are properly taken into account. In addition, results, notably the partial spin magnetic moments, from the band-structure calculations are discussed in some detail and compared to experimental findings and earlier calculations.

Dissociative process in Auger-electron spectra of alloys

The dissociative-decay broadening of the final two-hole bound state of the Auger-transition, arising from its mixing with delocalized band states with one hole in ‘A’ band and one hole in ‘B’ band in ‘AB’ alloy, was proposed by Bennett et al. to explain the excessive width of the two-hole bound state [Phys. Rev. B, 27 (1983) 2194]. The many-body effect of the dissociative process is discussed in detail, using a simple model band and Green’s function method. The hole–hole interaction U empirically determined from the Auger-electron spectrum, could be affected by the energy shift of the two-hole bound state by the virtual dissociative process. The interplay between the localization and delocalization of the two-hole state by two different mechanisms is discussed.

Electron-assisted exciton transfer and long-lived electrons and holes in GaAs/AlxGa1-xAs quantum wells.

Long-lived (up to ${10}^{\mathrm{\ensuremath{-}}3}$ s), separately localized electrons and holes are revealed in undoped photoexcited GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells at T=2 K by their effect on localized excitons. The luminescence spectrum of the resonantly photoexcited (e1:hh1)1S excitons is modified when additional electron-hole pairs are generated, and this modulation decays a very long time after the pair photogeneration terminates. We explain the effect by a localized electron (hole)-exciton interaction that accelerates the exciton transfer from weakly into strongly localized states of the spatially fluctuating in-plane potential. The excess exciton energy is imparted to the interacting electron (hole). This results in an exciton population redistribution and excitation of the localized carriers into delocalized band states. The electron-assisted exciton transfer rate between interface islands that localize the particles is estimated. We find that even under a low excitation level the estimated rate is comparable to the exciton recombination, as well as to the phonon-assisted exciton transfer rate. In addition, we show that the radiative recombination of closely localized electrons and holes (geminate pairs) gives rise to the LO-phonon photoluminescence sideband with a characteristic decay time of ${10}^{\mathrm{\ensuremath{-}}7}$ s. \textcopyright{} 1996 The American Physical Society.

Theoretical Investigation of XPS, UPS, and BIS in Rare‐Earth Compounds — Thulium Monochalcogenides

AbstractThe investigation of electron spectra in thulium monosulphide and monoselenide is based on the calculations of relative intensities for multiplet transitions fN → fN±1 in the intermediate coupling using the results of self‐consistent band calculations as a zero approximation of perturbation theory. Correct interpretation of all experimental data in rare‐earth compounds is shown to be actual only in the framework of the common calculational model of excitations from highly localized 4f‐states to delocalized band states.

Theoretical estimates of the sheet resistance of Gaussian [formula omitted] ion-implanted layers in semiconductors: phosphorus in silicon : L. M. Scarfone and J. D. Chlipala. Solid-St. Electron.22, 559 (1979)

Photoreflectance (PR) is a convenient characterization tool able to reveal optoelectronic properties of semiconductor materials and structures. It is a simple non-destructive and contactless technique which can be used in air at room temperature. We will present experimental results of the characterization carried out by means of PR on different types of advanced photovoltaic (PV) structures, including quantum-dot-based prototypes of intermediate band solar cells, quantum-well structures, highly mismatched alloys, and III–V-based multi-junction devices, thereby demonstrating the suitability of PR as a powerful diagnostic tool. Examples will be given to illustrate the value of this spectroscopic technique for PV including (i) the analysis of the PR spectra in search of critical points associated to absorption onsets; (ii) distinguishing signatures related to quantum confinement from those originating from delocalized band states; (iii) determining the intensity of the electric field related to built-in potentials at interfaces according to the Franz–Keldysh (FK) theory; and (v) determining the nature of different oscillatory PR signals among those ascribed to FK-oscillations, interferometric and photorefractive effects. The aim is to attract the interest of researchers in the field of PV to modulation spectroscopies, as they can be helpful in the analysis of their devices.

The decay of localized states into delocalized band states. Theory and preliminary experimental investigations using optically detected electron spin coherence

A technique for measuring the decay of localized excited triplet states into the delocalized band states of a solid utilizing the optical detection of electron spin coherence is described. Specifically, the use of optically detected electron spin locking for measuring kinetic phenomena in the presence of fluctuating triplet spin sublevel populations is presented and illustrated. In addition, preliminary experimental data is reported which yields the temperature dependence of the phonon assisted promotion of a localized isotopic site to an energy equal to that associated with the energies of the host exciton band. Finally, the data is analyzed in terms of a kinetic model which relates the localized state's rate of promotion and radiationless decay into the exciton band states to explicit features of the exciton and phonon band structure.

Coherent energy migration in solids. I. Band-trap equilibria at Boltzmann and non-Boltzmann temperatures

A model is presented which relates the dynamics of energy migration in crystals to the mechanism by which thermal equilibrium between delocalized band states and localized trap states is achieved. Central to this model is the requirement that coherent energy migration must be the dominant mode of migration at low temperatures in order to achieve Boltzmann equilibrium between band and trap states within the lifetime of the excited electronic state. Second, a stochastic model for detrapping is developed which is based on an irreversible radiationless relaxation process of a phonon-trap intermediate into the density of delocalized band states. Explicit account is taken of phonon-trap interactions in the formation of the excited trap intermediate. Further, the relation between detrapping and the ability of a crystal to achieve thermal equilibrium within the excited-state lifetime is developed and applied to one-dimensional crystals. Experimental results on molecular crystals representing examples of one-dimensional exciton bands are also presented. Specifically, the temperature dependence of phosphorescence from excited triplet trap states is interpreted in terms of the above considerations. From these experiments one can obtain both the sign of the intermolecular interaction and the dispersion of the first excited triplet band in addition to an estimate of the coherence length associated with exciton migration in the Frenkel limit. Finally, some new and unique methods for studying energy migration are presented which utilize optically detected magnetic-resonance techniques in zero field. They include experiments based on the measurement of electron-spin coherence in the rotating frame and the relationship of the spin coherence to the various rate processes important in trap-exciton interactions.

On the electronic phase transitions in the lower oxides of vanadium

The phase transitions found in the lower oxides of vanadium are discussed in terms of a model recently proposed by Fr?hlich, in which Coulomb correlations play a dominant role: below the temperature of the phase transition, Tt, they suppress the band motion of the d electrons in favour of localization, whilst as T -> Tt they cause the thermal ionization of these electrons (out of their localized states into delocalized band states) to become a co-operative process, through screening effects; at Tt, the population of the d band increases catastrophically leading to a first-order phase transition into the metallic state. Experimental evidence, collected from VO, V2O3 and VO2, which tends to support the mechanisms of the model is presented. The VO2 phase transition is investigated in detail and the following conclusions reached: the crystallographic transformation which occurs at Tt(similar, equals 340 ?K) can be regarded as a consequence of the a priori electronic transition; for T Tt, all the d electrons move with very low mobility (similar 0?1 cm2 v-1 s-1) in a degenerate t2g sub-band, of width about 1 ev, which is split off from the fivefold degenerate d band by the internal crystal field of the anion sublattice - the system is now metallic and the observed magnetic susceptibility attributed to Kubo temperature-independent paramagnetism and spin paramagnetism of the d conduction electrons, which because of the high effective mass (m* similar, equals 40m) of the latter is temperature dependent. Finally, the thermodynamics of the transition is briefly discussed.