Local Electronic Properties Research Articles

Electron Spectroscopy of Single Quantum Objects To Directly Correlate the Local Structure to Their Electronic Transport and Optical Properties.

Physical property of a single quantum object is governed by its precise atomic arrangement. The direct correlation of localized physical properties with the atomic structures has been therefore strongly desired but still limited in the theoretical studies. Here, we have successfully examined the localized electronic properties of individual carbon nanotubes by means of high-resolution electron energy-loss spectroscopy combined with high-resolution transmission electron microscopy. Well-separated sharp peaks at the carbon K(1s) absorption edge and in the valence-loss spectra are obtained from a single freestanding carbon nanotube with the local chiral index and unambiguously identified as the transitions between the van Hove singularities. The spectra features clearly vary upon the different areas even in the individual carbon nanotube. Variations in interband transitions, plasmonic behaviors, and unoccupied electronic structures are clearly attributed to the local irregular atomic arrangement such as topological defect and/or elastic bond stretching.

Isotopic transient studies of sodium promotion of Pt/Al2O3 for the water–gas shift reaction

The promotional effect of sodium on Pt/Al2O3 catalysts for the water–gas shift (WGS) reaction was investigated with operando FTIR during steady-state and isotopic transient experiments. The highest turnover frequency (TOF) promotion per surface Pt occurred on a 0.82wt.% Pt/Al2O3 catalyst with a 30:1molar ratio of Na:Pt. This catalyst exhibited a TOF of 0.35s−1 at 250°C and 7% CO, 11% H2O, 9% CO2, and 37% H2 compared to 0.43×10−2s−1 on Pt/Al2O3. Operando IR revealed that Na addition modifies CO adsorption on Pt to create more strongly bound, multiply-bonded CO species at 1768cm−1 and 1697cm−1 compared to predominantly linear CO on Na-free Pt/Al2O3 at 2060cm−1. Transient experiments with 12CO/13CO isotope switches showed that the number of carbon-containing active intermediates increased from less than 1% of the surface Pt for Na-free Pt/Al2O3 to nearly 100% of the surface Pt for 20Na:Pt/Al2O3, which indicates that only a small fraction of the Pt surface on the Na-free samples participates in the WGS reaction. From time-resolved IR spectra during transient WGS with 12CO/13CO and 12CO2/13CO2 isotope switches, we propose that surface formate species are spectators for all Na-promoted Pt/Al2O3 catalysts under these WGS conditions. The results suggest that Na promotes Pt/Al2O3 for WGS by modifying the local electronic properties of Pt and creating new H2O activation sites, which provide greater availability of surface OH/H species to react with nearly all CO on the metallic Pt surface through a non-formate pathway.

Operando and Time-Resolved X-Ray Absorption Spectroscopy for the Study of Photoelectrode Architectures

Photoelectrocatalytic processes (e.g. water splitting) are particularly interesting as a method of solar to chemical energy conversion. The catalytic process is driven by transient changes in the semiconductor material’s properties, such as the metal oxidation state, the local reconstruction of catalytic sites or charge recombination processes. Here, we report a novel experimental technique for performing pump and probe time-resolved hard X-ray absorption spectroscopy in electrochemical conditions under UV–vis irradiation: pump and probe XAS enables the study of processes occurring in pico- to microseconds time scale by probing the local electronic properties of a selected atom. The described technique was developed using α-Fe2O3/IrOx as a model photoelectrode.

Linear and Branched PEIs (Polyethylenimines) and Their Property Space.

A chemical property space defines the adaptability of a molecule to changing conditions and its interaction with other molecular systems determining a pharmacological response. Within a congeneric molecular series (compounds with the same derivatization algorithm and thus the same brute formula) the chemical properties vary in a monotonic manner, i.e., congeneric compounds share the same chemical property space. The chemical property space is a key component in molecular design, where some building blocks are functionalized, i.e., derivatized, and eventually self-assembled in more complex systems, such as enzyme-ligand systems, of which (physico-chemical) properties/bioactivity may be predicted by QSPR/QSAR (quantitative structure-property/activity relationship) studies. The system structure is determined by the binding type (temporal/permanent; electrostatic/covalent) and is reflected in its local electronic (and/or magnetic) properties. Such nano-systems play the role of molecular devices, important in nano-medicine. In the present article, the behavior of polyethylenimine (PEI) macromolecules (linear LPEI and branched BPEI, respectively) with respect to the glucose oxidase enzyme GOx is described in terms of their (interacting) energy, geometry and topology, in an attempt to find the best shape and size of PEIs to be useful for a chosen (nanochemistry) purpose.

Large Electronic Anisotropy and Enhanced Chemical Activity of Highly Rippled Phosphorene

We investigate the electronic structure and chemical activity of rippled phosphorene induced by large compressive strains via first-principles calculation. It is found that phosphorene is extraordinarily bendable, enabling the accommodation of ripples with large curvatures. Such highly rippled phosphorene shows a strong anisotropy in electronic properties. For ripples along the armchair direction, the band gap changes from 0.84 to 0.51 eV for the compressive strain up to -20% and further compression shows no significant effect, for ripples along the zigzag direction, semiconductor to metal transition occurs. Within the rippled phosphorene, the local electronic properties, such as the modulated band gap and the alignments of frontier orbitals, are found to be highly spatially dependent, which may be used for modulating the injection and confinement of carriers for optical and photovoltaic applications. The examination of the interaction of a physisorbed NO molecule with the rippled phosphorene under different compressive strains shows that the chemical activities of the phosphorene are significantly enhanced at the top and bottom peaks of the ripples, indicated by the enhanced adsorption and charge transfer between them. All these features can be ascribed to the effect of curvatures, which modifies the orbital coupling between atoms at the ripple peaks.

Room-temperature local ferromagnetism and its nanoscale expansion in the ferromagnetic semiconductor Ge(1-x)Fex.

We investigate the local electronic structure and magnetic properties of the group-IV-based ferromagnetic semiconductor, Ge1−xFex (GeFe), using soft X-ray magnetic circular dichroism. Our results show that the doped Fe 3d electrons are strongly hybridized with the Ge 4p states, and have a large orbital magnetic moment relative to the spin magnetic moment; i.e., morb/mspin ≈ 0.1. We find that nanoscale local ferromagnetic regions, which are formed through ferromagnetic exchange interactions in the high-Fe-content regions of the GeFe films, exist even at room temperature, well above the Curie temperature of 20–100 K. We observe the intriguing nanoscale expansion of the local ferromagnetic regions with decreasing temperature, followed by a transition of the entire film into a ferromagnetic state at the Curie temperature.

Three-dimensional interaction force and tunneling current spectroscopy of point defects on rutile TiO2(110)

The extent to which point defects affect the local chemical reactivity and electronic properties of an oxide surface was evaluated with picometer resolution in all three spatial dimensions using simultaneous atomic force/scanning tunneling microscopy measurements performed on the (110) face of rutile TiO2. Oxygen atoms were imaged as protrusions in both data channels, corresponding to a rarely observed imaging mode for this prototypical metal oxide surface. Three-dimensional spectroscopy of interaction forces and tunneling currents was performed on individual surface and subsurface defects as a function of tip-sample distance. An interstitial defect assigned to a subsurface hydrogen atom is found to have a distinct effect on the local density of electronic states on the surface, but no detectable influence on the tip-sample interaction force. Meanwhile, spectroscopic data acquired on an oxygen vacancy highlight the role of the probe tip in chemical reactivity measurements.

Atmospheric doping effects in epitaxial graphene: correlation of local and global electrical studies

We directly correlate the local (20 nm scale) and global electronic properties of a device containing mono-, bi- and tri-layer epitaxial graphene (EG) domains on 6H–SiC(0001) by simultaneously performing local surface potential measurements using Kelvin probe force microscopy and global transport measurements. Using well-controlled environmental conditions we investigate the doping effects of N2, O2, water vapour and NO2 at concentrations representative of the ambient air. We show that presence of O2, water vapour and NO2 leads to p-doping of all EG domains. However, the thicker layers of EG are significantly less affected. Furthermore, we demonstrate that the general consensus of O2 and water vapour present in ambient air providing majority of the p-doping to graphene is a common misconception. We experimentally show that even the combined effect of O2, water vapour, and NO2 at concentrations higher than typically present in the atmosphere does not fully replicate p-doping from ambient air. Thus, for EG gas sensors it is essential to consider naturally occurring environmental effects and properly separate them from those coming from targeted species.

Resistive Switching Mechanisms on TaOx and SrRuO3 Thin-Film Surfaces Probed by Scanning Tunneling Microscopy

The local electronic properties of tantalum oxide (TaOx, 2 ≤ x ≤ 2.5) and strontium ruthenate (SrRuO3) thin-film surfaces were studied under the influence of electric fields induced by a scanning tunneling microscope (STM) tip. The switching between different redox states in both oxides is achieved without the need for physical electrical contact by controlling the magnitude and polarity of the applied voltage between the STM tip and the sample surface. We demonstrate for TaOx films that two switching mechanisms operate. Reduced tantalum oxide shows resistive switching due to the formation of metallic Ta, but partial oxidation of the samples changes the switching mechanism to one mediated mainly by oxygen vacancies. For SrRuO3, we found that the switching mechanism depends on the polarity of the applied voltage and involves formation, annihilation, and migration of oxygen vacancies. Although TaOx and SrRuO3 differ significantly in their electronic and structural properties, the resistive switching mechanisms could be elaborated based on STM measurements, proving the general capability of this method for studying resistive switching phenomena in different classes of transition metal oxides.

Theoretical study of global and local reactivities of coumarin and its hydroxylated derivatives

Coumarins are bioactive substances of the benzo‐α‐pyrone family, which have shown antioxidant, antiviral, anti‐inflammatory and antitumor activities, among others. 7‐Hydroxycoumarin and 6,7‐dihydroxycoumarin (esculetin) are two coumarin derivatives that have been reported to exhibit antitumor activity, but the action mechanism underlying this activity remains unknown. In this work, to elucidate this mechanism, a theoretical study of the local and global electronic reactivity properties of a series of hydroxylated and dihydroxylated coumarin derivatives with possible antitumor action is performed using Density Functional Theory in aqueous solution. The substitution by one or two hydroxyl groups in the benzene ring of coumarin produces better charge‐donor than charge‐acceptor compounds. All the studied compounds are generally stable in water and exhibit permanent polarization in the solvent. With one hydroxyl substitution, 7‐hydroxycoumarin is the most polar and polarizable derivative, whereas 5,7‐dihydroxycoumarin is the most polar and polarizable compound with two hydroxyl substitutions. 5,7‐Dihydroxycoumarin is suggested to possess antitumor activity. © 2016 Wiley Periodicals, Inc.

Disclosing the electronic structure and optical properties of Ag4V2O7crystals: experimental and theoretical insights

This work combines experimental and computational techniques to exploit the best of both methodologies and thus provide a tool for understanding the geometry, cluster coordination, electronic structure, and optical properties of Ag4V2O7 crystals. This tool has been developed to create structural models and enable a much more complete and rapid refinement of 3D (three dimensional) structures from limited data, aiding the determination of structure–property relationships and providing critical input for materials simulations. Ag4V2O7 crystals were synthesized by a simple precipitation method from an aqueous solution at 30 °C for 10 min. The obtained crystals were characterized by X-ray diffraction (XRD) and Rietveld refinement, and micro-Raman spectroscopy. The crystal morphology was determined by field emission scanning electron microscopy (FESEM), and the optical properties were investigated by ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy and photoluminescence (PL) measurements. The geometric and electronic structures of Ag4V2O7 crystals were investigated by first-principles quantum-mechanical calculations based on the density functional theory. The theoretical results agree with the experimental data to confirm that Ag4V2O7 crystals have an orthorhombic structure, and that the building blocks of the lattice comprise two types of V clusters, [VO4] and [VO5], and two types of Ag clusters, [AgO5] and [AgO6]. This type of fundamental studies, which combine multiple experimental methods and first-principles calculations, have been proved valuable in obtaining a basic understanding of the local structure, bonding, band gap, and electronic and optical properties of Ag4V2O7 crystals.

Local charge states in hexagonal boron nitride with Stone-Wales defects.

A Stone-Wales (SW) defect is the simplest topological defect in graphene-like materials and can be potentially employed to design electronic devices . In this paper, we have systematically investigated the formation, structural, and electronic properties of the neutral and charged SW defects in hexagonal boron nitride (BN) using first-principles calculations. The transition states and energy barrier for the formation of SW defects demonstrate that the defected BN is stable. Our calculations show that there are two in-gap defect levels, which originate from the asymmetrical pentagon-heptagon pairs. The local defect configurations and electronic properties are sensitive to their charge states induced by the defect levels. The electronic band structures show that the negative and positive charged defects are mainly determined by shifting the conduction band minimum (CBM) and valence band maximum (VBM) respectively, and the SW-defected BN can realize -1 and +1 spin-polarized charge states. The effects of carbon (C) substitution on neutral and charged SW-defected BN have also been studied. Our results indicate that the C substitution of B in BN is in favour of the formation of SW defects. Structural and electronic calculations show rich charge-dependent properties of C substitutions in SW-defected BN, thus our theoretical study is important for various applications in the design of BN nanostructure-based devices.

Na(+) doping induced changes in the reduction and charge transport characteristics of Al2O3-stabilized, CuO-based materials for CO2 capture.

Chemical looping combustion (CLC) and chemical looping with oxygen uncoupling (CLOU) are emerging CO2 capture technologies that could reduce appreciably the costs associated with the capture of CO2. In CLC and CLOU, the oxygen required to combust a hydrocarbon is provided by a solid oxygen carrier. Among the transition metal oxides typically considered for CLC and CLOU, copper oxide (CuO) stands out owing to its high oxygen carrying capacity, exothermic reduction reactions and fast reduction kinetics. However, the low Tammann (sintering) temperature of CuO is a serious drawback. In this context, it has been proposed to support CuO on high Tammann temperature and low cost alumina (Al2O3), thus, reducing the morphological changes occurring over multiple CLC or CLOU redox cycles and stabilizing, in turn, the high activity of CuO. However, in CuO-Al2O3 systems, phase stabilization and avoiding the formation of the CuAl2O4 spinel is key to obtaining a material with a high redox stability and activity. Here, we report a Na(+) doping strategy to phase stabilize Al2O3-supported CuO, yielding in turn an inexpensive material with a high redox stability and CO2 capture efficiency. We also demonstrate that doping CuO-Al2O3 with Na(+) improves the oxygen uncoupling characteristics and coke resistance of the oxygen carriers. Utilizing in situ and ex situ X-ray absorption spectroscopy (XAS), the local structure of Cu and the reduction pathways of CuO were determined as a function of the Na(+) content and cycle number. Finally, using 4-point conductivity measurements, we confirm that doping of Al2O3-supported CuO with Na(+) lowers the activation energy for charge transport explaining conclusively the improved redox characteristics of the new oxygen carriers developed.

Radiation response of multi-quantum well solar cells: Electron-beam-induced current analysis

Solar cells utilizing multi-quantum well (MQW) structures are considered promising candidate materials for space applications. An open question is how well these structures can resist the impact of particle irradiation. The aim of this work is to provide feedback about the radiation response of In0.01Ga0.99As solar cells grown on Ge with MQWs incorporated within the i-region of the device. In particular, the local electronic transport properties of the MQW i-regions of solar cells subjected to electron and proton irradiation were evaluated experimentally using the electron beam induced current (EBIC) technique. The change in carrier collection distribution across the MQW i-region was analyzed using a 2D EBIC diffusion model in conjunction with numerical modeling of the electrical field distribution. Both experimental and simulated findings show carrier removal and type conversion from n- to p-type in MQW i-region at a displacement damage dose as low as ∼6.06–9.88 × 109 MeV/g. This leads to a redistribution of the electric field and significant degradation in charge carrier collection.

Electronic aspects of formation and properties of local structures around Mn in Cd1−xMnxTe1−ySey

Local electronic and structural features around Mn in Cd1−xMnxTe0.97Se0.03 (x = 0.02; 0.05; 0.1; y = 0.03) were studied by means of X-ray Absorption Fine Structure (XAFS) techniques. Manganese ions with an average valence 2+, are found to be well incorporated into the host CdTe lattice, with clear preference for Te atoms as the first neighbors. However, Mn and Te are found to form two essentially different types of bonds, one short, strong and directional (cubic MnTe-alike bond), and three much longer, predominantly ionic in nature (hexagonal MnTe-alike bonds), thereby distorting the tetrahedral coordination around Mn. The origin of peculiar Mn–Te bonds distribution and details of their nature and strength are further elaborated by employing the first principle electronic structure calculations. That way a thorough insight in impact of the Mn–Te bond length variation on the electronic structure of the compound is obtained. The relations established between the local structures and electronic properties offer a reliable procedure for detailed analysis of the structural and electronic consequences of the 3d-transition metals (TM) incorporation in II–VI semiconductor host. Clear distinction between various influences makes the procedure easily adoptable also to the studies of TM impurities in other semiconductors.

First principles study of Ca in BaTiO3 and Bi0.5Na0.5TiO3

BaTiO3–Bi0.5Na0.5TiO3 is one of the promising candidates as a high-temperature relaxor with a high Curie temperature and several preferred dielectric characteristics. It has been found experimentally for a long time that adding calcium to BaTiO3–Bi0.5Na0.5TiO3 improves its temperature characteristic of the capacitance [J. Electron. Mater. 39, 2471]. In this study, Calcium (Ca) defects in perovskite BaTiO3 and Bi0.5Na0.5TiO3 have been studied based on first-principles calculations. In both BaTiO3 and Bi0.5Na0.5TiO3, our calculations showed that Ca atom energetically prefers to substitute for the cations, that is Ba, Bi, Na and Ti, depending on the growth conditions. In most cases, Ca predominantly substitutes on the A-site without providing additional electrical carriers (serve as either neutral defects or self-compensating defects). The growth conditions where Ca can be forced to substitute for B-site (with limited amount) and the conditions where Ca can be forced to serve as an acceptor are identified. Details of the local structures, formation energies and electronic properties of these Ca defects are reported.

Comprehensive studies of structural, electronic and magnetic properties of Zn0.95Co0.05O nanopowders

X-ray absorption (XANES, EXAFS, XMCD) and photoelectron (XPS) spectroscopic techniques were employed to study local structural, electronic and magnetic properties of Zn0.95Co0.05O nanopowders. The substitutional Co2+ ions are incorporated in ZnO lattice at regular Zn sites and the sample is characterized by high structural order. There was no sign of ferromagnetic ordering of Co magnetic moments and the sample is in paramagnetic state at all temperatures down to 5K. The possible connection of the structural defects with the absence of ferromagnetism is discussed on the basis of theoretical calculations of the O K-edge absorption spectra.

Whispering-gallery mode quantum resonators in graphene

Recently, a research group from the Center for Nanoscale Science and Technology at the National Institute of Standards and Technology (NIST), and the Massachusetts Institute of Technology in the United States has demonstrated a new type of quantum electro-optic phenomenon, whispering-gallery mode resonators.1 The resonators are generated by a scanning tunneling microscope (STM) in proximity to graphene devices (Figure 1). On the basis of the quantum effect of electron tunneling, STM is a powerful technique to investigate the local electronic properties of both metallic and semiconducting systems with atomic resolution. Graphene, the most acclaimed material of the last decade, has enabled new horizons for STM research. The graphene surface can be directly probed by the scanning tip, whereas remaining chemically stable and clean even exposure to ambient air for days. Charged carriers in graphene can be readily tuned from holes to electrons using an external gate electrode. Furthermore, the charge carriers in graphene, often called Dirac particles, behave like electromagnetic waves, setting the stage for graphene to realize quantum electro-optic phenomena such as Veselago lensing2 and Klein tunneling.3

Raman spectroscopy as probe of nanometre-scale strain variations in graphene.

Confocal Raman spectroscopy has emerged as a major, versatile workhorse for the non-invasive characterization of graphene. Although it is successfully used to determine the number of layers, the quality of edges, and the effects of strain, doping and disorder, the nature of the experimentally observed broadening of the most prominent Raman 2D line has remained unclear. Here we show that the observed 2D line width contains valuable information on strain variations in graphene on length scales far below the laser spot size, that is, on the nanometre-scale. This finding is highly relevant as it has been shown recently that such nanometre-scaled strain variations limit the carrier mobility in high-quality graphene devices. Consequently, the 2D line width is a good and easily accessible quantity for classifying the crystalline quality, nanometre-scale flatness as well as local electronic properties of graphene, all important for future scientific and industrial applications.

Low Pressure CO2 Hydrogenation to Methanol over Gold Nanoparticles Activated on a CeO(x)/TiO2 Interface.

Capture and recycling of CO2 into valuable chemicals such as alcohols could help mitigate its emissions into the atmosphere. Due to its inert nature, the activation of CO2 is a critical step in improving the overall reaction kinetics during its chemical conversion. Although pure gold is an inert noble metal and cannot catalyze hydrogenation reactions, it can be activated when deposited as nanoparticles on the appropriate oxide support. In this combined experimental and theoretical study, it is shown that an electronic polarization at the metal-oxide interface of Au nanoparticles anchored and stabilized on a CeO(x)/TiO2 substrate generates active centers for CO2 adsorption and its low pressure hydrogenation, leading to a higher selectivity toward methanol. This study illustrates the importance of localized electronic properties and structure in catalysis for achieving higher alcohol selectivity from CO2 hydrogenation.