Delocalization Of Electronic States Research Articles

Observing the Role of Electron Delocalization in Electronic Transport by Incorporating Actinides into Ligated Metal-Chalcogenide Superatoms.

Since delocalization of electronic states is a prerequisite for exerting unique electron transport properties, early actinides (An) with highly delocalized 5f/6d orbitals are natural candidates. However, given the experimental difficulties of such radioactive compounds and the complex relativistic effects in theoretical studies, understanding the electronic structure and bonding of actinides is underdeveloped on the periodic table. A further challenge is the very complicated electronic structures encountered in the confinement of actinides, as vividly illustrated by the weakly radioactive Th(Thorium)-encapsulated metal chalcogenide clusters, Th@Co6Te8L6 (L = PH3, PMe3, PEt3). Here we report the electronic structure and the electron transport properties of the Th@Co6Te8L6 clusters and compare them with those of the hollow Co6Te8L6 clusters using the nonequilibrium Green's function combined with relativistic density functional theory (NEGF-DFT). We found that the equilibrium conductance in Th@Co6Te8(PH3)6 (0.76 G0) has been greatly improved over that in Co6Te8(PH3)6 (0.03 G0), which has also been verified under an applied different bias voltage. The covalent bonding character between 6d (Th) and 3d (Co) atomic orbitals resulting from steric confinement is the source of the performance enhancement and a most important factor governing the accessibility of such 5f/6d orbitals. The results are of significance to the rapidly developing field of molecular nanoelectronics.

Vacancy‐Ordered Superstructure‐Induced Delocalized States Enable Superior Sodium Ion Storage

AbstractTransition metal sulfides (TMSs) are perceived as competitive candidate anodes for sodium‐ion batteries (SIBs) on account of their high capacity and admirable reversibility. However, a majority of TMSs suffer from huge volume expansion and poor kinetics so they cannot achieve durable and fast Na+ storage. Herein, a cation vacancy‐ordered Cr2/3S is fabricated by extracting quantitative Cr atoms from nickel arsenide type CrS via sulfur “atomic pump,” that is, sulfur and particular sit Cr atoms are bonded to extend a new structure. The ordered vacancies not only construct a loosely‐packed crystal structure but also induce delocalized electron states of Cr atoms, hence effectively accelerating Na+ diffusion and releasing volume strain to enable high‐rate and longevous SIBs. The new Cr2/3S anode presents a high reversible capacity of 544 mAh g−1 after 100 cycles at 1 A g−1 and excellent high‐rate performance of ≈100% capacity retention after 7000 cycles at 20 A g−1. Subsequent in situ and ex situ characterizations reveal the Na+ storage mechanism of Cr2/3S. The proposed cation exaction strategy through an innovative sulfur “atomic pump” can be an efficient way to achieve loosely‐packed structure material for large‐capacity and fast‐kinetics anodes.

Understanding the electronic pi-system of 2D covalent organic frameworks with Wannier functions

We investigate a family of hexagonal 2D covalent organic frameworks (COFs) with phenyl and biphenyl spacer units and different chemical linker species. Chemical trends are elucidated and attributed to microscopic properties of the pi-electron-system spanned by atomic p_z-orbitals. We systematically investigate the electronic structure, delocalization of electronic states, effects of disorder, bond torsion, and doping, and correlate these with variable pi-conjugation and nucleus-independent chemical shift (NICS) aromaticity. Molecular orbitals are obtained from maximally localized Wannier functions that have sigma- and pi-character, forming distinct sigma- and pi-bands for all valence states. The Wannier-orbital description goes beyond simple tight-binding models and enables a detailed understanding of the electronic topology, effective electronic coupling and delocalization. It is shown that a meaningful comparison between COFs with different chemical elements can only be made by examining the entire pi-electron system, while a comparison of individual bands (e.g., bands near the Fermi energy) can be a insufficient to derive general design rules for linker and spacer monomer selection. We further identify delocalized states that are spread across tens or hundreds of pores of the 2D COFs and analyze their robustness against structural and energetic disorders like out-of-plane rotations of molecular fragments, different strength of energetic disorder and energetic shifts due to chemical doping.

Photoelectron spectra explain how ammonia solutions become metallic

The gradual emergence of delocalized electron states in lithium–ammonia solutions underlies their transition.

Steplike spectral distribution of photoelectrons at the percolation threshold in heavily p -doped GaAs

We study the origin of the step-like shoulder on the high energy side of the low temperature photoluminescence spectrum of heavily $p$-doped GaAs. We show experimentally that it is controlled by the Fermi-Dirac distribution of the holes and by the energy distribution of the photoexcited electrons showing a sharp step-like dependence. This step is attributed to the percolation threshold in the conduction band separating localized from delocalized electron states. A comprehensive set of optical techniques based on spin orientation of electrons, namely the Hanle effect, time- and polarization-resolved photoluminescence, as well as transient pump-probe Faraday rotation are used for these studies. We identify two different electron ensembles with substantially different spin lifetimes of 20 and 280~ps, limited by the lifetime of the electrons. Their spin relaxation times are longer than 2~ns. The relative contribution of short- and long-lived photoexcited electrons to the emission spectrum changes abruptly at the high-energy photoluminescence step-like tail. For energies above the percolation threshold the electron states are empty due to fast energy relaxation, while for lower energies the relaxation is suppressed and the majority of photoelectrons populate these states.

Effect of wavefunction delocalization on shift current generation

We derive upper bounds on the magnitude of shift photocurrent generation of materials in two limiting cases: the flat-band limit of almost-isolated systems such as molecular crystals, and the wide-band limit of one-dimensional or quasi-one-dimensional materials such as ferroelectric polymers or other materials with chain-like motifs. These bounds relate the magnitudes of the shift current bulk photovoltaic effect to materials parameters. In both cases, we find that ratio of electron hopping amplitudes to the band gap plays a vital role in maximizing the amount of nonlinear response. Furthermore, by using the Wannier function formalism, we quantify the effect of long-range hopping amplitudes, showing how delocalization of electronic states gives rise to larger photocurrents. These results inform the design and selection of new materials for large shift current generation.

Point Group Symmetry Analysis of the Electronic Structure of Bare and Protected Metal Nanocrystals.

The electronic structures of a variety of experimentally identified gold and silver nanoclusters from 20 to 246 atoms, either unprotected or protected by several types of ligands, are characterized by using point group specific symmetry analysis. The delocalized electron states around the HOMO–LUMO energy gap, originating from the metal s-electrons in the cluster core, show symmetry characteristics according to the point group that describes best the atomic arrangement of the core. This indicates strong effects of the lattice structure and overall shape of the metal core to the electronic structure, which cannot be captured by the conventional analysis based on identification of spherical angular momentum shells in the “superatom” model. The symmetry analysis discussed in this paper is free from any restrictions regarding shape or structure of the metal core, and is shown to be superior to the conventional spherical harmonics analysis for any symmetry that is lower than Ih. As an immediate application, we also demonstrate that it is possible to reach considerable savings in computational time by using the symmetry information inside a conventional linear-response calculation for the optical absorption spectrum of the Ag55 cluster anion, without any loss in accuracy of the computed spectrum. Our work demonstrates an efficient way to analyze the electronic structure of nonspherical, but atomically ordered nanocrystals and ligand-protected clusters with nanocrystal metal cores, and it can be viewed as the generalization of the superatom model demonstrated for spherical shapes 10 years ago (Walter, M.; et al. Proc. Natl. Acad. Sci. U. S. A.2008, 105, 9157−916218599443).

Enhanced Superconductivity in Restacked TaS2 Nanosheets.

Since interface superconductivity was discovered at the interface between two insulating layers LaAlO3 and SrTiO3, such interface-induced superconducting systems have been a research hotspot in superconductivity. Here, we report homogeneous interfaces formed by stacking chemically exfoliated monolayer TaS2 nanosheets randomly. Enhanced superconductivity of Tc = 3 K is observed, compared with 0.8 K of parent 2H-TaS2. The measurement of heat capacity shows the increase of electronic specific-heat coefficient γ of restacked TaS2 nanosheets compared to parent 2H-TaS2 crystals. Density functional theory calculations indicate that increase and delocalization of electron states near the Fermi surface due to the homogeneous interfaces effects could account for the enhanced superconductivity.

Characteristic electron transport on pyridine-linked molecular devices with graphene nanoribbons electrodes and gold electrodes

The electrodes in the molecular devices play a crucial role in creating functional organic electronic devices. We employed the first-principles calculations to investigate the conjugated pyridine-terminated molecule-4, 4[Formula: see text]-vinylenedipyridine attached to monolayer zigzag graphene nanoribbons (ZGNRs) and Au electrodes. Results show that the ZGNRs-based device exhibits excellent electrical properties. It has larger equilibrium conductance or stronger transmission capacity due to higher strength of individual channel and stronger delocalization of electronic states at the Fermi level. The transmission of two devices near the Fermi level is influenced by resonant electron transport through the discrete energy and the edge states. Comparatively, different from the rising trend of gold-based device with the increasing voltage, the current of the device with ZGNRs electrodes changes in a completely different way with the augment of the applied voltage, exhibiting a negative differential resistance effect unexpectedly. The changing trends of the current through two devices are elucidated by the evolution of the transmission peak nearest to the Fermi level.

Microscopic models for charge-noise-induced dephasing of solid-state qubits

Several experiments have shown qubit coherence decay of the form $exp[\ensuremath{-}{(t/{T}_{2})}^{\ensuremath{\alpha}}]$ due to environmental charge-noise fluctuations. We present a microscopic description for temperature dependencies of the parameters ${T}_{2}$ and $\ensuremath{\alpha}$. Our description is appropriate to qubits in semiconductors interacting with spurious two-level charge fluctuators coupled to a thermal bath. We find distinct power-law dependencies of ${T}_{2}$ and $\ensuremath{\alpha}$ on temperature depending on the nature of the interaction of the fluctuators with the associated bath. We consider fluctuator dynamics induced by first- and second-order tunneling with a continuum of delocalized electron states. We also study one- and two-phonon processes for fluctuators in either GaAs or Si. These results can be used to identify dominant charge-dephasing mechanisms and suppress them.

Delocalization of electronic states in graphene oxide stabilised mesoporous silica nanoparticles revealed using photoluminescence

Mesoporous silica aerogel obtained through a sol–gel process when surface treated with 2-D graphene oxide sheets reveals many fascinating features particularly, leading to multiple coordination complexes based on amide linkages.

Kosterlitz–Thouless transition in disordered two-dimensional topological insulators

The disorder-driven metal–insulator transition in the quantum spin Hall systems is studied by scaling analysis of the Thouless conductance g. Below a critical disorder strength, the conductance is independent of the sample size M, an indication of critically delocalized electron states. The calculated beta function β = dlng/dlnM indicates that the metal–insulator transition is of Kosterlitz–Thouless (KT) type, which is characterized by binding and unbinding of vortex–antivortex pairs of the local currents. The KT-like metal–insulator transition is a basic characteristic of the quantum spin Hall state, being independent of the time-reversal symmetry.

Effect of Anderson Localization on Auger Destruction of DNA

The effect of Anderson localization in DNA on the Auger destruction by the Coulombic explosion at ionized radiation has been theoretically discussed in the present work. The theory of Auger destruction of DNA has been modified taking into account the localized and delocalized electron states in DNA owing to the correlated disorder in a sequence of nucleotides. According to the modified theoretical model of Auger destruction, the dominant ratio of delocalized states to localized states in exon compared to intron results in stronger radiation resistance of exons to ionized irradiation causing the Auger-cascade process than the radiation resistance of introns.

Theory of intermolecular electron transfer in nanoscale biological structures

Macromolecular biological systems performing directed electron transfer are nano-sized structures. The distance between carrier molecules (cofactors), which represent practically isolated electron localization centers, reaches tens of angstroms. The electron transfer theory based on the concept of delocalized electron states, which is conventionally used in biophysics, is unable to adequately interpret the results of concrete observations in many cases. On the basis of the theory of electronic transitions in the case of localized states, developed in the physics of disordered matter, a mechanism of long-distance electron transfer in biological systems is suggested. The molecular relaxation of the microenvironment of electron localization centers that accompanies the electron transfer process is also considered.

Nuclear quantum effects in electronically adiabatic quantum time correlation functions: Application to the absorption spectrum of a hydrated electron

A general formalism for introducing nuclear quantum effects in the expression of the quantum time correlation function of an operator in a multilevel electronic system is presented in the adiabatic limit. The final formula includes the nuclear quantum time correlation functions of the operator matrix elements, of the energy gap, and their cross terms. These quantities can be inferred and evaluated from their classical analogs obtained by mixed quantum-classical molecular dynamics simulations. The formalism is applied to the absorption spectrum of a hydrated electron, expressed in terms of the time correlation function of the dipole operator in the ground electronic state. We find that both static and dynamic nuclear quantum effects distinctly influence the shape of the absorption spectrum, especially its high energy tail related to transitions to delocalized electron states. Their inclusion does improve significantly the agreement between theory and experiment for both the low and high frequency edges of the spectrum. It does not appear sufficient, however, to resolve persistent deviations in the slow Lorentzian-like decay part of the spectrum in the intermediate 2-3 eV region.

Optical Properties and van der Waals - London Dispersion Interactions of Polystyrene Determined by Vacuum Ultraviolet Spectroscopy and Spectroscopic Ellipsometry

The interband optical properties of polystyrene in the vacuum ultraviolet (VUV) region have been investigated using combined spectroscopic ellipsometry and VUV spectroscopy. Over the range 1.5–32 eV, the optical properties exhibit electronic transitions we assign to three groupings, E1, E2, and E3, corresponding to a hierarchy of interband transitions of aromatic (π → π*), non-bonding (n → π*, n → σ*), and saturated (σ → σ*) orbitals. In polystyrene there are strong features in the interband transitions arising from the side-chain π bonding of the aromatic ring consisting of a shoulder at 5.8 eV (E1′) and a peak at 6.3 eV (E1), and from the σ bonding of the C–C backbone at 12 eV (E3′) and 17.1 eV (E3). These E3 transitions have characteristic critical point line shapes associated with one-dimensionally delocalized electron states in the polymer backbone. A small shoulder at 9.9 eV (E2) is associated with excitations possibly from residual monomer or impurities. Knowledge of the valence electronic excitations of a material provides the necessary optical properties to calculate the van der Waals–London dispersion interactions using Lifshitz quantum electrodynamics theory and full spectral optical properties. Hamaker constants and the van der Waals–London dispersion component of the surface free energy for polystyrene were determined. These Lifshitz results were compared to the total surface free energy of polystyrene, polarity, and dispersive component of the surface free energy as determined from contact angle measurements with two liquids, and with literature values. The Lifshitz approach, using full spectral Hamaker constants, is a more direct determination of the van der Waals–London dispersion component of the surface free energy of polystyrene than other methods.

Adsorption-induced constraint on delocalization of electron states in an Au chain on NiAl(110)

We have carried out a density functional study of the localized constraint on the delocalized, unoccupied resonance states in a mono atomic Au chain on a NiAl(110) surface by adsorption of a CO molecule on one of the adatoms in the chain. This constraint was observed recently by scanning tunnelling microscopy and spectroscopy. The repulsive interaction of the occupied 5${\sigma}$ molecular state with the unoccupied, resonance state in a Au adatom in the chain is found to break the degeneracy of the Au adatom-induced resonance states and to disrupt their delocalization in the chain.

Theoretical study of the polymer molecules (TeO2)n as model systems for the local structure in TeO2 glass

Stable (TeO2)n polymers (n<7) in the form of chains, rings and three-dimensional entities are studied through the ab initio B3LYP calculations including their Raman spectra, polarizabilities and hyperpolarizabilities. The 3D-polymers are found to be the most stable, and can be proposed as the model fragments of TeO2 glass. According to the calculations, the huge hyperpolarizability of TeO2-based compounds is likely to be related to the essentially delocalized electron states (inherent to polymerized systems), and not to the electron lone pairs of tellurium atoms.

Optical polarization of nuclear spins in GaAs

The photon energy, irradiation time, and sample temperature dependences of laser-enhanced NMR spectra from semi-insulating GaAs at 9.4 T were measured. These data were used to test the existing model for the mechanism of optical nuclear polarization---namely, optical excitation of shallow donor states and subsequent spin diffusion of polarization throughout the bulk. Features of the present results, such as their uniqueness to semi-insulating GaAs and pumping data obtained via laser irradiation above the band gap, are inconsistent with this model. An additional model is proposed that includes localized paramagnetic centers as storage sites for electron spin polarization, but relies upon delocalized electron states to accomplish the bulk nuclear spin polarizations. The delocalized states could be free excitons.

Non-linear electrical response in a charge/orbital ordered Pr 0.63Ca 0.37MnO 3 crystal: the charge density wave scenario

Non-linear conduction in a CO manganese oxide Pr 0.63Ca 0.37MnO 3 is reported. This kind of behavior is usually interpreted as a current-induced breakdown of the CO state. We propose in this report to compare this feature of the coexistence of localized and delocalized electron states with what occurs in the Charge Density Waves systems in which the same non-linearity is observed. In such a framework the non-linear conduction arises from the motion of the CDW condensate which carries a net electrical current.