Search

Single crystals of ThO2, UO2, and their solid solutions, UxTh1–xO2, have been obtained through various hydrothermal growth conditions. This technique offers the better of two other growth processes: (i) single crystal purity as by photochemical growth of nanocrystals; and (ii) large/bulk sizes as obtained by the arc melt method. The band gap of the UxTh1–xO2 single crystal solid solution, along with the luminescence transition, have been characterized. The occupied and unoccupied structures are determined using ultraviolet and inverse photoemission spectroscopy and the electronic band gap was measured to be 3–4 eV. The strain of incorporating U into the ThO2 is analyzed through Vegard's law. In this crystal there are defect and impurity sites, likely arising from the kinetic growth process, giving rise to a similar yet slightly different optical gap evident with cathodoluminescence spectroscopy. There is a major luminescence feature spanning the range from 3.18 to 4.96 eV (250–390 nm) with a maximum at 4.09 eV (303 nm), corresponding with the measured electronic band gap. In this paper, the electronic properties of a solid solution U0.22Th0.78O2 are measured and interpreted compared to the pure actinide oxides, ThO2 and UO2.

Single crystals of thorium dioxide ThO2, grown by the hydrothermal growth technique, have been investigated by ultraviolet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES), and L3, M3, M4, and M5 X-ray absorption near edge spectroscopy (XANES). The experimental band gap for large single crystals has been determined to be 6 eV to 7 eV, from UPS and IPES, in line with expectations. The combined UPS and IPES, place the Fermi level near the conduction band minimum, making these crystals n-type, with extensive band tailing, suggesting an optical gap in the region of 4.8 eV for excitations from occupied to unoccupied edge states. Hybridization between the Th 6d/5f bands with O 2p is strongly implicated. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The local structure of Mn-doped Li2B4O7(001) was investigated using extended X-ray absorption fine structure (EXAFS) at the Mn K edge and electron paramagnetic resonance (EPR). The location of the Mn dopant in a lithium tetraborate crystal is consistent with occupation of a site with strong oxygen coordination. The MnO bond lengths are similar to those observed with Mn doping of the icosahedral based boron carbide where Mn is in a substitutional dopant in one of the cage sites. From EXAFS, the manganese does not appear to greatly alter the overall tetragonal form of lithium tetraborate, with the dopant most likely substituting for one of the two B sites and with placement of some of the Mn in a Li site still possible. The EPR spectra agree with the literature examined resolving multiple Mn species in the crystal lattice.

Gd/Ni compounds with different Gd concentration have been investigated with constant initial state spectroscopy (CIS) of photoemission utilizing the light of synchrotron radiation. The photoemission cross-sections of Gd 5d and 4f states show a strong resonance in CIS spectra at the Gd 5p absorption edge. The screenings of Gd ions by Ni 3d conduction electrons results in a dramatic suppression of the 5p 6 4f 7 5d 1 6s 2 to 5p 5 4f 7 5d 2 6s 2 resonant enhancement of photoemission valence band intensities. This decrease in the super-Coster―Kronig photoemission resonant enhancement of the valence band can be related to the hybridized Gd 5d and Ni 3d bands near the Fermi level, consistent with performed first-principles calculations of local densities of states.

Time-of-flight mass analysis with multi-stop coincidence detection was used to study the multi-cation ionic fragmentation of the closo carborane cage molecule closo -1,2-orthocarborane (C2B10H12) following inner-shell excitation in or above the B 1s regime. Electron ion coincidence spectra reveal the cationic products which are formed after core level excitation. Distinct changes in fragmentation pattern are observed as a function of excitation energy. Photoelectron–photoion–photoion coincidence (PEPIPICO) spectroscopy was used to study the dominant fission routes in the core level excitation regime. Series of ion pairs are identified, where asymmetric fission dominates, leading to ion pairs of different mass. Suitable fission and fragmentation mechanisms are discussed. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

There is interplay between intra-atomic orbital hybridization and extra-atomic hybridization in various gadolinium systems, which affects magnetic coupling and electron itinerancy (localization). The results do not always follow expectation. The experimental band structure of thin Gd(0001) films, grown on the Mo(112) surface, along the Γ-M does not agree with expectations even qualitatively. In particular, the dispersion of the gadolinium band, with strong 5d weight near 2 eV binding energy provides considerable evidence to support the case for 4f-5d hybridization, with increasing 5d localization. On the other hand, there is also evidence of extra-atomic Gd 4f hybridization leading band dispersion in the occupied 4f levels in Gd 2 O 3 .

The possible origins of a low density of defect states within the highest occupied molecular orbital to lowest unoccupied molecular orbital gap are suggested for regioregular poly(3-hexylthiophene). A number of “chemical” defects, impurities, and structural defects could contribute to features in photoemission for regioregular poly(3-hexylthiophene), observed within the highest occupied molecular orbital to lowest unoccupied molecular orbital gap of regioregular poly(3-hexylthiophene). POLYM. ENG. SCI., 47:1359–1364, 2007. © 2007 Society of Plastics Engineers

We fi nd evidence for a gradual change in the electronic properties of spin coated poly(3-hexylthio-phene) thin fi lms with temperature. The conduction properties appears to be mediated by hopping conduction dominated by a low density of defects states within the highest occupied molecular or-bital to lowest unoc cupied molecular orbital gap, not by a change in band gap. The photoemission and transport measure ments indicate a loss of charge mobility or carrier concentration occurs with decreasing temperature, while the molecular confi guration (through chain conjugation) also changes with temperature. The defects states identifi ed by photoemission (–1.7 eV) as well as others contrib-uting to the transport properties are believed to be very heterogeneous along the polymer backbone. 1 Introduction For conducting polymers, changes in the relative orientation of aromatic rings and/or overall back-bone confi guration with temperature, are commonly observed [1–4]. Such structural changes lead to re-duced π–π intermolecular interactions and/or create defects that can have a profound effect on conductiv-ity and π-band localization in the highest occupied molecular orbitals. In addition to temperature-induced structural defects, are temperature-independent chemical defects, such as missing hydrogens on aliphatic pendant groups and/or impurities. “Chemical” defects in the polymer chain may induce hole carriers im-parting a p-type character to the semiconductor resulting in an effectively smaller optical band gap, rela-tive to the highest occupied to lowest unoccupied molecular orbital (HOMO–LUMO) gap, as noted else-where [5]. If chemical defects play an important role in the conductivity of a polymer, then dramatic changes in conductivity should occur with decreasing temperature as the carrier concentration and/or mobility de-creases. Decreases in temperature can lead a “freezing” of carriers, i.e. carriers become trapped, or there is an insuffi cient thermal activation of carriers. This decrease in carriers or carrier mobility effectively amounts to a semiconductor to insulator transition (an increasingly better dielectric) or the classic tem-perature behavior of a semiconductor mediated by hopping conductivity. The resulting increase in insu-lating character should occur at a distinctly different transition temperature than the transitions related to the tilting of the aromatic rings of the polymer backbone (i.e., structural defects). Such distinct tran

Both the experimental and theoretical band structure of the W(112) surface are presented, with the theoretical band structure calculated by the film–LAPW (linearized augmented plane waves) method. The results permit one to compare the W(112) and Mo(112) band structures and illuminate the similarities. It is found that for the W(112) surface the main photoemission features combine contributions from both the surface and bulk, as has been previously noted for Mo(112). The main differences between the electronic structures of the furrowed W(112) and Mo(112) surfaces are seen in the width of occupied bands. The differences are attributed to the extent of localization of valence 4d and 5s electrons in Mo and 5d and 6s electrons in W. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Heteroepitaxial coupling at complex oxide interfaces presents a powerful tool for engineering the charge degree of freedom in strongly correlated materials, which can be utilized to achieve tailored functionalities that are inaccessible in the bulk form. Here, the charge-transfer effect between two strongly correlated oxides, Sm0.5 Nd0.5 NiO3 (SNNO) and La0.67 Sr0.33 MnO3 (LSMO), is exploited to realize a giant enhancement of the ferroelectric field effect in a prototype Mott field-effect transistor. By switching the polarization field of a ferroelectric Pb(Zr,Ti)O3 (PZT) gate, nonvolatile resistance modulation in the Mott transistors with single-layer SNNO and bilayer SNNO/LSMO channels is induced. For the same channel thickness, the bilayer channels exhibit up to two orders of magnitude higher resistance-switching ratio at 300 K, which is attributed to the intricate interplay between the charge screening at the PZT/SNNO interface and the charge transfer at the SNNO/LSMO interface. X-ray absorption spectroscopy and X-ray photoelectron spectroscopy studies of SNNO/LSMO heterostructures reveal about 0.1 electron per 2D unit cell transferred between the interfacial Mn and Ni layers, which is corroborated by first-principles density functional theory calculations. The study points to an effective strategy to design functional complex oxide interfaces for developing high-performance nanoelectronic and spintronic applications.

The electronic structure near to the Fermi level of the colossal magnetoresistance (CMR) perovskite manganite materials. La 0.65 A 0.35 MnO 3 (A = Ca, Sr, Ba), has been studied using both photoemission and inverse photoemission spectroscopy. The electronic structure for all three materials is very similar and consistent with an Mn-O terminal layer regardless of dopant. Small differences in the electronic structure among the materials are, however, observed. The observed band gap is not significant for La 0.65 Ca 0.35 MnO 3 and La 0.65 Ba 0.35 MnO 3 while there is a gap, about 1.5 eV, for La 0.65 Sr 0.35 MnO 3 . There is a shift to higher binding energies of the extensively hybridized Mn-O Δ 5 (e) bands for the surface (the surface on the Mn-O plane with C 4v symmetry) and t 2g bands for the bulk in the valence band spectra with increasing atomic number or atomic radius of dopants, approximately to 5.8, 6.8, and 7.8 eV for La 0.65 Ca 0.35 MnO 3 , La 0.65 Sr 0.35 MnO 3 , and La 0.65 Ba 0.35 MnO 3 . respectively. The O-Mn-O terminal layer in these materials seems to be much more defect free than is the case for La 0.9 Ca 0.1 MnO 3 . where the Ca-O terminal layer appears to be rich in defects.

This article reviews the surface adsorption of p-benzoquinonemonoimine zwitterions from the class of N-alkyldiaminoresorcinones (or 4,6-bis-dialkylaminobenzene-1,3-diones, i.e. C6H2(···NHR)2(···O)2), where R = H, R = C2H5, n-C4H9), on a variety of conducting surfaces. These small molecules with a large electrical dipole exhibit molecular orientation and packing on surfaces depending on the deposition methodology (from solution or from the vapor) as well as the substrate. What is very clear from the investigations of this class of molecules is that inter-molecular interaction is not simply driven by dipolar interactions alone, but frontier orbital symmetry as well. This is illustrated by the reversible adsorption of di-iodobenzene on molecular films of (6Z)-4-(butylamino)-6-(butyliminio)-3-oxocyclohexa-1,4-dien-1-olate C6H2(···NHR)2(···O)2 where R = n-C4H9, where it is clear that absorption is isomer specific.

Surfaces are often different materials, and typically have a different electronic structure from the bulk and since the dawn of surface science, surface-localized electronic states, surface states, have been extensively studied and investigated with growing accuracy. Of particular importance to spintronics are magnetic surface states. Interfaces will play a very important role in many spintronics devices, yet the interface properties are often ignored, poorly understood or badly characterized. For many nominally half metal materials, materials that in some ground state calculations exhibit 100% spin polarization, the magnetic surface states may significantly reduce the effective spin polarization. We review the magnetic surface states of several well known and often highly touted high spin polarized materials such as NiMnSb, Fe 3 O 4 , CoS 2 and CrO 2 . Finally, we summarize surface state measurements of magnetoelectric antiferromagnets Cr 2 O 3 , which has electrically controllable net surface spins, a major complication to the study of CrO 2 by photoemission.

We have investigated one and two monolayers of Hg adsorbed on Cu(100) by angle-resolved photoemission. The electronic structure of one monolayer of Hg on Cu(100) is very similar to that of the free atom. Two monolayers of Hg on Cu(100) has an electronic structure that closely resembles that of bulk Hg. These results are compared to those obtained for Hg overlayers on Ag(100). We have also observed that the shallow 5d core levels of the first monolayer of Hg shift by 0.38 ± 0.05 eV to greater binding energy following the adsorption of the second monolayer of Hg.

The electronic structures and magnetic properties of many rare-earth monopnictides are reviewed in this article. Possible candidate materials for spintronics devices from the rare-earth monopnictide family, i.e. high polarization (nominally half-metallic) ferromagnets and antiferromagnets, are identified. We attempt to provide a unified picture of the electronic properties of these strongly correlated systems. The relative merits of several ab initio theoretical methods, useful in the study of the rare-earth monopnictides, are discussed. We present our current understanding of the possible half-metallicity, semiconductor-metal transitions, and magnetic orderings in the rare-earth monopnictides. Finally, we propose some potential strategies to improve the magnetic and electronic properties of these candidate materials for spintronics devices.

The most commonly used method for the fabrication of arrays of isolated magnetic features is lithography [1–3], but recently more advanced methods have been introduced. These techniques include fabrication using chemical vapor deposition in a scanning tunneling microscope [4], an atomic force micro-scope [5], or a scanning transmission microscope [6]. Here we report the fabrication and investigation of patterned nickel fi lms using a different and somewhat uncon-ventional approach: laser assisted organometallic chemical vapor deposition (OMCVD). The process involved in this technique is the photolysis of organometallic molecules under exposure to a certain radiation with a specifi c wave-length. The main advantages are that this method uses a single-step fabrication process, the chemical contamination of the deposited fi lms is very low, and that this technique is compatible with ultra-high vacuum technologies. The features grow with rhombohedrally distorted f.c.c. (111) texture and exhibit in-plane anisotropy.

The overlayer structures formed with halogen adsorption on an Fe(110) surface have been investigated using LEED. The LEED patterns are described using variable, non-coincident nets of halogen adatoms. The adlayer nets rotate and contract with increasing halogen coverage. The structural changes within the adlayer are consistent with an adatom model developed for the Cl2/Br2/I2 Fe(110) systems. The internuclear spacings and geometry of the halogen overlayers on Fe(110) are shown to be consistent with previous work on Fe(110).