Abstract: Graphene quantum dots are known to exhibit tunable photoluminescence (PL) through manipulation of edge functionality under various synthesis conditions. Here, we report observation of excitation dependent anomalous m-n type fingerprint PL transition in synthesized amino functionalized graphene quantum dots (5-7 nm). The effect of band-to-band π*-π and interstate to band n-π induced transitions led to effective multicolor emission under changeable excitation wavelength in the functionalized system. A reasonable assertion that equi-coupling of π*-π and n-π transitions activated the heterogeneous dual mode cyan emission was made upon observation of the PL spectra. Furthermore, investigation of incremented dimensional scaling through facile synthesis of amino functionalized quantum graphene flakes (20-30 nm) revealed it had negligible effect on the modulated PL pattern. Moreover, an effort was made to trace the origin of excitation dependent tunable heterogeneous photoluminescence through the framework of energy band diagram hypothesis and first principles analysis. Ab initio results suggested formation of an interband state as a manifestation of p orbital hybridization between C-N atoms at the edge sites. Therefore comprehensive theoretical and experimental analysis revealed that newly created energy levels can exist as an interband within the energy gap in functionalized graphene quantum structures yielding excitation dependent tunable PL for optoelectronic applications.

Abstract: ConspectusElectrides, in which electrons occupy interstitial regions in the crystal and behave as anions, appear as new phases for many elements (and compounds) under high pressure. We propose a unified theory of high pressure electrides (HPEs) by treating electrons in the interstitial sites as filling the quantized orbitals of the interstitial space enclosed by the surrounding atom cores, generating what we call an interstitial quasi-atom, ISQ.With increasing pressure, the energies of the valence orbitals of atoms increase more significantly than the ISQ levels, due to repulsion, exclusion by the atom cores, effectively giving the valence electrons less room in which to move. At a high enough pressure, which depends on the element and its orbitals, the frontier atomic electron may become higher in energy than the ISQ, resulting in electron transfer to the interstitial space and the formation of an HPE.By using a He lattice model to compress (with minimal orbital interaction at moderate pressures between ...

Abstract: The use of a density functional theory methodology with on-site corrections (DFT + U) has been repeatedly shown to give an improved description of localised d and f states over those predicted with a standard DFT approach. However, the localisation of electrons also carries with it the problem of metastability, due to the possible occupation of different orbitals and different locations. This study details the use of an occupation matrix control methodology for simulating localised d and f states with a plane-wave DFT + U approach which allows the user to control both the site and orbital localisation. This approach is tested for orbital occupation using octahedral and tetrahedral Ti(III) and Ce(III) carbonyl clusters and for orbital and site location using the periodic systems anatase-TiO2 and CeO2. The periodic cells are tested by the addition of an electron and through the formation of a neutral oxygen vacancy (leaving two electrons to localise). These test systems allow the successful study of orbital degeneracies, the presence of metastable states and the importance of controlling the site of localisation within the cell, and it highlights the use an occupation matrix control methodology can have in electronic structure calculations.

Abstract: Spin-orbit coupling has been conjectured to play a key role in the low-energy electronic structure of Sr2RuO4. By using circularly polarized light combined with spin- and angle-resolved photoemission spectroscopy, we directly measure the value of the effective spin-orbit coupling to be 130 � 30 meV. This is even larger than theoretically predicted and comparable to the energy splitting of the dxy and dxz;yz orbitals around the Fermi surface, resulting in a strongly momentum-dependent entanglement of spin and orbital character in the electronic wavefunction. As demonstrated by the spin expectation value h sk · s−ki calculated for a pair of electrons with zero total momentum, the classification of the Cooper pairs in terms of pure singlets or triplets fundamentally breaks down, necessitating a description of the unconventional superconducting state of Sr2RuO4 in terms of these newly found spin-orbital entangled eigenstates.

Abstract: The metal L-edge (2p → 3d) X-ray absorption spectra are affected by a number of different interactions: electron-electron repulsion, spin-orbit coupling, and charge transfer between metal and ligands, which makes the simulation of spectra challenging The core restricted active space (RAS) method is an accurate and flexible approach that can be used to calculate X-ray spectra of a wide range of medium-sized systems without any symmetry constraints Here, the applicability of the method is tested in detail by simulating three ferric (3d5) model systems with well-known electronic structure, viz, atomic Fe3+, high-spin [FeCl6]3− with ligand donor bonding, and low-spin [Fe(CN)6]3− that also has metal backbonding For these systems, the performance of the core RAS method, which does not require any system-dependent parameters, is comparable to that of the commonly used semi-empirical charge-transfer multiplet model It handles orbitally degenerate ground states, accurately describes metal-ligand interactions, and includes both single and multiple excitations The results are sensitive to the choice of orbitals in the active space and this sensitivity can be used to assign spectral features A method has also been developed to analyze the calculated X-ray spectra using a chemically intuitive molecular orbital picture

Abstract: For transmission of electrons through a π system, when the Landauer theory of molecular conductance is viewed from a molecular orbital (MO) perspective, there obtains a simple perturbation theoretic dependence, due to Yoshizawa and Tada, on a) the product of the orbital coefficients at the sites of electrode attachment, and b) the MO energies. The frontier orbitals consistently and simply indicate high or low transmission, even if other orbitals may contribute. This formalism, with its consequent reinforcement and/or interference of conductance, accounts for the (previously explained) difference in direct vs. cross conjugated transmission across an ethylene, as well as the comparative ON/OFF ratios in the experimentally investigated dimethyldihydropyrene and dithienylethene-type single-molecule switches. A strong dependence of the conductance on the site of attachment of the electrodes in a π system is an immediate extrapolation; the theory then predicts that for some specified sites the switching behavior will be inverted; i.e. the “open” molecular form of the switch will be more conductive.

Abstract: Evolution of electronic structures, properties and stabilities of neutral and cationic molybdenum encapsulated germanium clusters (Mo@Gen, n = 1 to 20) has been investigated using the linear combination of atomic orbital density functional theory method with effective core potential. From the variation of different thermodynamic and chemical parameters of the ground state clusters during the growth process, the stability and electronic structures of the clusters is explained. From the study of the distance-dependent nucleus-independent chemical shifts (NICS), we found that Mo@Ge12 with hexagonal prism-like structure is the most stable isomer and possesses strong aromatic character. Density of states (DOS) plots of different clusters is then discussed to explain the role of d-orbitals of the Mo atom in hybridization. Quenching of the magnetic moment of the Mo atom with increase in the size of the cluster is also discussed. Finally, the validity of the 18-electron counting rule is applied to further explain the stability of the metallo-inorganic magic cluster Mo@Ge12 and the possibility of Mo-based cluster-assembled materials is discussed.

Abstract: We verify the theory of nonadiabatic ionization of degenerate valence orbitals in strong circularly polarized laser fields by numerically solving the two-dimensional time-dependent Schrodinger equation for an effective one-electron potential of neon. The numerically calculated ionization ratios of the and orbitals agree well with the theoretical results (i.e., the counter-rotating electron tunnels more easily). However, for strong laser pulses and low laser frequencies, the adiabatic laser-dressed orbitals play an important role. In a Floquet treatment of a three-level model, we find that in this regime the ionization ratio of initial and orbitals depends strongly on the orbital energy order of valence s and orbitals. We also show that the emission angles of valence and electrons are different and should be observable in attoclock experiments.

Abstract: One of the main characteristics of the new family of two-dimensional crystals of semiconducting transition metal dichalcogenides (TMD) is the strong spin-orbit interaction, which makes them very promising for future applications in spintronics and valleytronics devices. Here we present a detailed study of the effect of spin-orbit coupling (SOC) on the band structure of single-layer and bulk TMDs, including explicitly the role of the chalcogen orbitals and their hybridization with the transition metal atoms. To this aim, we combine density functional theory (DFT) calculations with a Slater-Koster tight-binding model. Whereas most of the previous tight-binding models have been restricted to the K and K' points of the Brillouin zone (BZ), here we consider the effect of SOC in the whole BZ, and the results are compared to the band structure obtained by DFT methods. The tight-binding model is used to analyze the effect of SOC in the band structure, considering separately the contributions from the transition metal and the chalcogen atoms. Finally, we present a scenario where, in the case of strong SOC, the spin/orbital/valley entanglement at the minimum of the conduction band at Q can be probed and be of experimental interest in the most common cases of electron-doping reported for this family of compounds.

Abstract: Understanding the bonding between silicon and transition metals is valuable for devising strategies for incorporating magnetic species into silicon. CrSi12 is the standard example of a cluster whose apparent high stability has been explained by the 18-electron rule. We critically examine the bonding and nature of stability of CrSi12 and show that its electronic structure does not conform to the 18-electron rule. Through theoretical studies, we find that CrSi12 has 16 effective valence electrons assigned to the Cr atom and an unoccupied 3dz(2) orbital. We demonstrate that the cluster's apparent stability is rooted in a crystal field-like splitting of the 3d orbitals analogous to that of square planar complexes. CrSi14 is shown to follow the 18-electron rule and exhibits all conventional markers characteristic of a magic cluster.

Abstract: Evolution in the atomic structure, bonding characteristics, stability, and the spin magnetic moment of neutral and cationic AgnV clusters has been investigated using first-principles density functional approach with gradient corrected functional. It is shown that at small sizes, the V 4s states hybridize with Ag states to form 1S and 1P like superatomic orbitals, whereas the 3d states are localized on V giving the V atom an effective valence of 1 or 2. Starting from Ag8V+, the V 3d states begin to participate in the bonding by hybridizing with the nearly free electron gas to form 1D superatomic orbitals increasing the V atom effective valence toward 5. For the cationic clusters, this changing valence results in three shell closures that lead to stable species. These occur for cationic clusters containing 5, 7, and 14 Ag atoms. The first two stable species correspond to filled 1S and 1P shells in two and three dimensions with a valence of 2 for V, whereas the closure at 14 Ag atoms correspond to filled 1S,...

Abstract: Recent theories of charge density wave (CDW) order in high temperature superconductors have predicted a primarily d CDW orbital symmetry. Here, we report on the orbital symmetry of CDW order in the canonical cuprate superconductors La1.875Ba0.125CuO4 (LBCO) and YBa2Cu3O6.67 (YBCO), using resonant soft x-ray scattering and a model mapped to the CDW orbital symmetry. From measurements sensitive to the O sublattice, we conclude that LBCO has predominantly s' CDW orbital symmetry, in contrast to the d orbital symmetry recently reported in other cuprates. Additionally, we show for YBCO that the CDW orbital symmetry differs along the a and b crystal axes and that these both differ from LBCO. This work highlights CDW orbital symmetry as an additional key property that distinguishes the different cuprate families. We discuss how the CDW symmetry may be related to the "1/8--anomaly" and to static spin ordering.

Abstract: We present a combined experimental–theoretical demonstration of the energy spectrum and exchange coupling of an isolated donor pair in a silicon nanotransistor. The molecular hybridization of the atomic orbitals leads to an enhancement of the one- and two-electron binding energies and charging energy with respect to the single donor case, a desirable feature for quantum electronic devices. Our hydrogen molecule-like model based on a multivalley central-cell corrected effective mass theory incorporating a full configuration interaction treatment of the 2-electron spectrum matches the measured data for an arsenic diatomic molecule with interatomic distance R = 2.3 ± 0.5 nm.

Abstract: We report the development of a quantum mechanics/molecular mechanics free energy perturbation (QM/MM-FEP) method for modeling chemical reactions at metal-water interfaces. This novel solvation scheme combines planewave density function theory (DFT), periodic electrostatic embedded cluster method (PEECM) calculations using Gaussian-type orbitals, and classical molecular dynamics (MD) simulations to obtain a free energy description of a complex metal-water system. We derive a potential of mean force (PMF) of the reaction system within the QM/MM framework. A fixed-size, finite ensemble of MM conformations is used to permit precise evaluation of the PMF of QM coordinates and its gradient defined within this ensemble. Local conformations of adsorbed reaction moieties are optimized using sequential MD-sampling and QM-optimization steps. An approximate reaction coordinate is constructed using a number of interpolated states and the free energy difference between adjacent states is calculated using the QM/MM-FEP method. By avoiding on-the-fly QM calculations and by circumventing the challenges associated with statistical averaging during MD sampling, a computational speedup of multiple orders of magnitude is realized. The method is systematically validated against the results of ab initio QM calculations and demonstrated for C-C cleavage in double-dehydrogenated ethylene glycol on a Pt (111) model surface.

Abstract: The concept of aromaticity has been advanced beyond the framework of organic chemistry, and multiple aromaticity (σ, π, and δ) has been observed to account for the highly symmetric structures or unusual stability of the clusters. In the present study, the electronic structures and chemical bonding of small monolanthanum boride clusters are investigated using photoelectron imaging spectroscopy and first principles electronic structure calculations. Accurate electron affinities of 1.32 ± 0.04 and 1.13 ± 0.06 eV for the neutral LaB2 and LaB3 clusters are obtained by the vibrationally-resolved photoelectron spectra of the LaB2− and LaB3− clusters, respectively. It is shown that LaB2− and LaB3 exhibit enhanced stability in their respective cluster series, as evidenced from the calculated removal energies and HOMO–LUMO gaps. Molecular orbital analysis discloses that these two clusters possess doubly aromatic characters (σ and π), responsible for their enhanced stability. Interestingly, unlike conventional σ-, π-, and δ-aromaticity formed by the delocalization of unhybridized p or d orbitals, the σ and π delocalized molecular orbitals shown here are formed through the effective overlap between the 5d atomic orbital of the La atom and the p orbitals of the remaining boron atoms, representing an intriguing d–p hybridized aromaticity.

Abstract: The breaking of orbital degeneracy on a transition metal cation and the resulting unequal electronic occupations of these orbitals provide a powerful lever over electron density and spin ordering inmetal oxides. Here, we use ab initio calculations to show that reversibly modulating the orbital populations on Mn atoms can be achieved at ferroelectric/manganite interfaces by the presence of ferroelectric polarization on the nanoscale. The change in orbital occupation can be as large as 10%, greatly exceeding that of bulk manganites. This reversible orbital splitting is in large part controlled by the propagation of ferroelectric polar displacements into the interfacial region, a structural motif absent in the bulk and unique to the interface. We use epitaxial thin film growth and scanning transmission electron microscopy to verify this key interfacial polar distortion and discuss the potential of reversible control of orbital polarization via nanoscale ferroelectrics.

Abstract: We present an extensive study of electron transport (ET) in several crystal forms of phenyl-C61-butyric acid methyl ester (PCBM) and 1-thienyl-C61-butyric acid methyl ester (ThCBM) fullerene derivatives. Our calculations are based on a localized representation of the electronic states. Orbital couplings, site energies and reorganization energies have been calculated using various density functional and semi-empirical techniques and used within the Landau–Zener, Marcus and Marcus–Levich–Jortner expressions to evaluate electron transfer rates. Electron mobilities have been then estimated by kinetic Monte Carlo (KMC) simulations. The adiabaticity of electron transfer directions within the different crystal structures has also been verified using the Landau–Zener expression. Finally, the role of low energy virtual orbitals of the fullerene molecules has been investigated using charge transport networks of increasing complexities. Our results show that these molecules may form one-, two- or three-dimensional percolation networks and that their higher energy orbitals often participate in ET. The highest mobility values were obtained for the crystal structure of ThCBM and are comparable to experimental values.

Abstract: Using density functional theory (DFT)-based calculations, we explore the extent to which achievable modes of atomic layering can tune the near-gap electronic structure of tin- and lead-halide perovskites with applications in dye-sensitized solar cells. We show that regardless of how atomic layering is achieved---whether by the growth of layered inorganic phases such as the Ruddlesden-Popper series, hybrid perovskites connected by organic linker molecules, or layered perovskite heterostructures---their band gaps can similarly be widened by several tenths of an eV or more. Because these classes of compounds are known to have band gaps spanning much of the visible region of the solar spectrum, the ability to controllably tune their near-gap electronic structure could further optimize their performance in solar energy conversion applications. Throughout this work, trends in band gap are explained based on the effects of atomic layering and quantum confinement on the character and energy of band-edge crystal orbitals.

Abstract: One of the key parameters determining the rate of electron transfer is reorganization energy, an energy associated with geometry change during electron/hole transfer between molecules. To achieve efficient electron transfer, molecules with small reorganization energy are pursued, but the design guidelines remain elusive. It has been shown that a π-conjugated organic molecule with strong local nonbonding character in frontier orbitals may have small internal reorganization energy (λ). To explore how one can introduce such character in frontier orbitals so as to design high-performance materials, in this study we employed fragment molecular orbital analysis and pairing theorem to understand why the frontier orbital of phenalenyl radical had perfect local nonbonding character. The principles learned from phenalenyl radical lead to the design of various closed-shell π-conjugated skeletons with small λ. Functionalization of these skeletons afforded potential n-type materials with small λ (<100 meV) and large e...

Abstract: The structures of neutral cobalt-doped silicon clusters have been assigned by a combined experimental and theoretical study. Size-selective infrared spectra of neutral SinCo (n = 10–12) clusters are measured using a tunable IR–UV two-color ionization scheme. The experimental infrared spectra are compared with calculated spectra of low-energy structures predicted at the B3P86 level of theory. It is shown that the SinCo (n = 10–12) clusters have endohedral caged structures, where the silicon frameworks prefer double-layered structures encapsulating the Co atom. Electronic structure analysis indicates that the clusters are stabilized by an ionic interaction between the Co dopant atom and the silicon cage due to the charge transfer from the silicon valence sp orbitals to the cobalt 3d orbitals. Strong hybridization between the Co dopant atom and the silicon host quenches the local magnetic moment on the encapsulated Co atom.

Abstract: One of the most important and accurate approaches for studying interacting many-electron correlated systems is the quantum renormalization group (RG) procedure, upon which we propose a concept in the framework of natural orbitals so that we can generalize the RG into general orbital space. To demonstrate its effectiveness, we apply this new approach on a quantum cluster-impurity model with four impurities in comparison with the exact diagonalization method. We also find a shortcut to dramatically improving this RG procedure. A profound impact on the cluster dynamical mean-field theory and multi-impurity/orbital Kondo problems is further discussed.

Abstract: A number of small TiGen− (n = 2–6) clusters were investigated using anion photoelectron spectroscopy and density functional theory calculations. Their structures were determined by comparison of the theoretical vertical detachment energies and simulated spectra of the low-lying isomers with the experimental results. The most stable structure of TiGen− (n = 2–6) clusters can be considered as a Ti atom substituting one of the Ge atoms in the corresponding Gen+1 cluster or a Ti atom capping a Gen cluster. The HOMOs of TiGen− (n = 2–6) clusters are mainly localized on the Ti atom and the Ge atoms interacting directly with the Ti atom.

Abstract: The nature of the stereochemically active lone pair has long been a matter for debate. Here, by application of our recently developed orbital selective external potential (OSEP) method, we have studied the microscopic mechanism of stereochemically active lone pairs in various compounds. The OSEP method allows us to shift the energy level of a specific atomic orbital, therefore is helpful to identify unambiguously the role of this orbital in the chemical and physical properties of the system we are interested in. Our numerical results, with compelling proofs, demonstrate that the on-site mixing of the cation valence s orbital with the nominally empty p orbitals of the same subshell is crucial to the formation of a lone pair, whereas the anion p orbital has only a small effect. Our detailed investigation of Sn and Pb monochalcogenides shows that structures of these systems have significant effects on lone pairs. In return, the formation of lone pairs, which can be controlled by our OSEP method, could result in structural instabilities of Sn and Pb monochalcogenides.

Abstract: Our theoretical examination of second- and third-harmonic generation from metal-based nanostructures predicts that nonlocal and quantum-tunneling phenomena can significantly exceed expectations based solely on local, classical electromagnetism Mindful that the diameter of typical transition-metal atoms is approximately 3 \AA{}, we adopt a theoretical model that treats nanometer-size features and/or subnanometer-size gaps or spacers by taking into account (i) the limits imposed by atomic size to fulfill the requirements of continuum electrodynamics, (ii) spillage of the nearly free electron cloud into the surrounding vacuum, and (iii) the increased probability of quantum tunneling as objects are placed in close proximity Our approach also includes the treatment of bound charges, which add crucial, dynamical components to the dielectric constant that are neglected in the conventional hydrodynamic model, especially in the visible and UV ranges, where interband transitions are important The model attempts to inject into the classical electrodynamic picture a simple, perhaps more realistic description of the metal surface by incorporating a thin patina of free electrons that screens an internal, polarizable medium