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Zinc porphyrin solar cell dyes with donor-π-acceptor architectures combine light absorber (π), electron-donor, and electron-acceptor moieties inside a single molecule with atomic precision. The donor-π-acceptor design promotes the separation of charge carriers following optical excitation. Here, we probe the excited-state electronic structure within such molecules by combining time-resolved X-ray absorption spectroscopy at the N K-edge with first-principles time-dependent density functional theory (TD-DFT) calculations. Customized Zn porphyrins with strong-donor triphenylamine groups or weak-donor tri-tert-butylbenzene groups were synthesized. Energetically well-separated N K-edge absorption features simultaneously probe the excited-state electronic structure from the perspectives of the macrocycle and triphenylamine N atoms. New absorption transitions between the macrocycle N atoms and the excited-state HOMO vacancy are observed, and the triphenylamine associated absorption feature blue-shifts, consistent with partial oxidation of the donor groups in the excited state.

The energies of the frontier orbitals and their electron–hole binding energy are key quantities in organic electronics, such as organic light-emitting diodes (OLEDs) and solar cells. A method is developed for obtaining these quantities from ultraviolet photoelectron spectroscopy (UPS) and optical absorption spectroscopy, combined with calculations of the ionization energy (IE) and the electron affinity (EA). This method provides a more efficient and accurate determination of the lowest unoccupied molecular orbital (LUMO) together with the exciton binding energy. The concept is extended to element-sensitive core-level spectroscopies, such as X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS), in order to obtain core exciton binding energies. Two prototypical organic molecules for OLED applications are used as examples, i.e., N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPD) and dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HAT:CN). These...

Tantalus began as a 240 MeV electron storage ring that served as a testbed for the next-generation accelerator (which eventually would become Fermilab) [1, 2]. It was to be injected from the 40 MeV FFAG synchrotron that was already operating. Before its completion, funding from the AEC was discontinued. During the time it was in financial limbo, a report from an NAS study group suggested using Tantalus as a source of VUV synchrotron radiation for spectroscopy and possibly other techniques. The AFOSR eventually funded the completion and early use of Tantalus.

The complex mixture of conformational states exhibited by oligo(ethylene glycol)-terminated alkanethiols on Ag and Au surfaces is explored by polarization-dependent X-ray absorption spectroscopy. Three self-assembled monolayers (SAMs) with known helical or all-trans conformations are used as references to characterize a SAM with unknown conformations. This case study is used as a prototype for developing a systematic framework to extract the conformations of SAMs from the polarization dependence of several orbitals. In the case at hand, these are associated with the C-H/Rydberg bonds of the alkane, the C-H/Rydberg bonds of ethylene glycol, and the C-C bonds of the backbone. The C-H/Rydberg orbitals of the alkane and ethylene glycol are distinguished via the chemical shift of the corresponding C 1s core levels.

The atomic-scale origin of the unusually high performance and long-term stability of wurtzite p-GaN oriented nanowire arrays is revealed. Nitrogen termination of both the polar (0001¯) top face and the nonpolar (101¯0) side faces of the nanowires is essential for long-term stability and high efficiency. Such a distinct atomic configuration ensures not only stability against (photo) oxidation in air and in water/electrolyte but, as importantly, also provides the necessary overall reverse crystal polarization needed for efficient hole extraction in p-GaN.

X-ray absorption spectroscopy (XAS) is combined with density functional theory (DFT) to determine the orbitals of one- and two-dimensional carbon π-systems (lycopene, beta-carotene, retinal, retinol, retinoic acid, coronene, triphenylene). Considerable fine structure is observed for the transition from the C 1s level to the lowest unoccupied molecular orbital (LUMO) and explained by DFT. The wave functions of the one-dimensional chain molecules display the node structure of a vibrating string. The XAS transition energy is decomposed into contributions from the C 1s core level, the π* final state, and the electron–hole interaction. For the latter, we develop a simple model that accurately represents a full Δ-self-consistent field (ΔSCF) calculation. The distortion of the LUMO because of its interaction with the C 1s hole is investigated. These results illustrate the electronic states of prototypical π-bonded carbon structures with low-dimensional character, such as those used in molecular complexes for solar cells, confined graphene structures, and molecular wires., This research was supported by the NSF through the University of Wisconsin Materials Research Science and Engineering Center (DMR-1121288). We also acknowledge support by the DOE (BES) for the synchrotron radiation experiments at the Advanced Light Source (ALS) under the Contracts DE-AC02-05CH11231 (ALS) and DE-SC0006931 (endstation). J.M.G.L. acknowledges support from the Spanish Ministry of Economy and Competitiveness under Projects FIS2012-30996 and FIS2013-46159-C3-1-P, Grupos Consolidados (IT578-13), and COST Action MP1306 (EUSpec).

Monolayer-thickness two-dimensional layers of α,ω-dihexylsexithiophene (α,ω-DH6T) exhibit field-effect hole mobility of up to 0.032 cm2 V−1 s−1, higher than previously reported for monolayers of other small-molecule organic semiconductors. In situ measurements during deposition show that the source-drain current saturates rapidly after the percolation of monolayer-high islands, indicating that the electrical properties of α,ω-DH6T transistors are largely determined by the first molecular monolayer. The α,ω-DH6T monolayer consists of crystalline islands in which the long axes of molecules are oriented approximately perpendicular to the plane of the substrate surface. In-plane lattice constants measured using synchrotron grazing-incidence diffraction are larger in monolayer-thickness films than the in-plane lattice constants of several-monolayer films and of previously reported thick-film structures. Near-edge X-ray absorption fine structure spectroscopy (NEXAFS) reveals that the larger in-plane lattice constant of single-monolayer films arises from a larger tilt of the molecular axis away from the surface normal. NEXAFS spectra at the C 1s and S 2p edges are consistent with a high degree of molecular alignment and with the local symmetry imposed by the thiophene ring. The high mobility of holes in α,ω-DH6T monolayers can be attributed to the reduction of hole scattering associated with the isolation of the thiophene core from the interface by terminal hexyl chains.

A brief overview of the three elements of photon-in photon-out spectroscopy is given, i.e., X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES), and resonant inelastic X-ray scattering (RIXS). Key features are emphasized, such as element selectivity and the ability to study electronic states in dilute systems, at buried interfaces, and in a liquid environment. Characteristic applications in the areas of nanomaterials, biomaterials, and photovoltaic materials illustrate these capabilities.

The prospects for converting angle-resolved photoemission data from reciprocal space to real space are evaluated. Examples are given for one- and two-dimensional systems, where wave functions have been obtained for quantum well states confined between steps and for planar molecules adsorbed at a surface. The extension to three-dimensional systems via variable photon energies is discussed. The uncertainty relation between position and momentum makes such a capability complementary to imaging in real space by scanning tunneling microscopy. Angle-resolved photoemission provides access to the internal structure of an electronic orbital and penetrates below the surface.

This study addresses the need for methods that validate the surface chemistry leading to the immobilization of biomolecules and provide information about the resulting structural configurations. We report on the use of near-edge X-ray absorption fine structure spectroscopy (NEXAFS) to characterize a widely employed immobilization chemistry that leads to the covalent attachment of a biologically relevant oligopeptide to a surface. The oligopeptide used in this study is a kinase substrate of the epidermal growth factor receptor (EGFR), a protein that is a common target for cancer therapeutics. By observing changes in the pi* and sigma* orbitals of specific nitrogen and carbon atoms (amide, imide, carbonyl), we are able to follow the sequential reactions leading to immobilization of the oligopeptide. We also show that it is possible to use NEXAFS to extend this characterization method to submonolayer densities that are relevant to biological assays. Such an element-specific chemical characterization of small peptides on surfaces fills an unmet need and establishes NEXAFS as useful technique for characterizing the immobilization of small biomolecules on surfaces.

We report direct measurements of changes in the conduction-band structure of ultrathin silicon nanomembranes with quantum confinement. Confinement lifts the 6-fold-degeneracy of the bulk-silicon conduction-band minimum (CBM), Delta, and two inequivalent sub-band ladders, Delta(2) and Delta(4), form. We show that even very small surface roughness smears the nominally steplike features in the density of states (DOS) due to these sub-bands. We obtain the energy splitting between Delta(2) and Delta(4) and their shift with respect to the bulk value directly from the 2p(3/2)--Delta transition in X-ray absorption. The measured dependence of the sub-band splitting and the shift of their weighted average on degree of confinement is in excellent agreement with theory, for both Si(001) and Si(110).

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 .

Random copolymer brushes of poly(styrene-r-methyl methacrylate) (PS-r-PMMA) have previously been used to control the wetting behavior of poly(styrene-b-methyl methacrylate) (PS-b-PMMA) block copolymer. Here we demonstrate that the use of random copolymer may be generalized to other block copolymer systems. A series of poly(styrene-r-2-vinylpyridine-r-hydroxyethyl methacrylate) (PS-r-P2VP-r-PHEMA) random copolymers was synthesized and evaluated as brush layers to control the orientation of domains in thin films of poly(styrene-b-2-vinylpyridine) (PS-b-P2VP) block copolymer. On nonpreferential brushes, thin films of PS-b-P2VP had a mixed orientation of domains consisting of a perpendicular orientation of lamellar domains near the copolymer−substrate interface and a parallel orientation of lamellar domains at free surface because of the lower surface energy of the PS block. The composition window of brushes that induce vertical orientation of domains near the substrate ranges from 47.8% styrene to 57.0% styr...

The uniaxial chain alignment of low molecular weight poly(9,9-bis(ethylhexyl)fluorene-2,7-diyl) (PF2/6) in thin films cast atop rubbed polyimide templates is characterized by polarized optical absorption spectroscopy, carbon K near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and grazing incidence X-ray diffraction (GIXRD) in response to thermal annealing and increasing film thickness (ca. 15−150 nm). The highest overall levels of uniaxial alignment are obtained in the thinnest PF2/6 films. However, the orientation of chains at the top surface after thermal annealing is nearly thickness independent despite a large drop in the maximum optical dichroic ratio as the film thickness increases. The kinetics of chain alignment on heating are strongly film thickness dependent and only weakly correlated with a crossover to a nematic liquid crystal state. All told these data support a structural model in which there is a graded morphology such that the top and bottom surfaces exhibit appreciable plan...

The ability to immobilize proteins with high binding capacities on surfaces while maintaining their activity is critical for protein microarrays and other biotechnological applications. We employed poly(acrylic acid) (PAA) brushes as templates to immobilize ribonuclease A (RNase A), which is commonly used to remove RNA from plasmid DNA preparations. The brushes are grown by surface-anchored atom-transfer radical polymerization (ATRP) initiators. RNase A was immobilized by both covalent esterification and a high binding capacity metal-ion complexation method to PAA brushes. The polymer brushes immobilized 30 times more enzyme compared to self-assembled monolayers. As the thickness of the brush increases, the surface density of the RNase A increases monotonically. The immobilization was investigated by ellipsometry, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The activity of the immobilized RNase A was determined using UV absorbance. As much as 11.0 microg/cm(2) of RNase A was bound to PAA brushes by metal-ion complexation compared to 5.8 microg/cm(2) by covalent immobilization which is 30 and 16 times the estimated mass bound in a monolayer. The calculated diffusion coefficient D was 0.63 x 10(-14) cm(2)/s for metal-ion complexation and 0.71 x 10(-14) cm(2)/s for covalent immobilization. Similar values of D indicate that the binding kinetics is similar, but the thermodynamic equilibrium coverage varies with the binding chemistry. Immobilization kinetics and thermodynamics were characterized by ellipsometry for both methods. A maximum relative activity of 0.70-0.80 was reached between five and nine monolayers of the immobilized enzyme. However, the relative activity for covalent immobilization was greater than that of metal-ion complexation. Covalent esterification resulted in similar temperature dependence as free enzyme, whereas metal-ion complexation showed no temperature dependence indicating a significant change in conformation.

This work has three main themes: (1) fabricate atomically precise nanostructures at surfaces, particularly nanowires consisting of atom chains; (2) explore the behavior of one-dimensional electrons in atomic chains; (3) find the fundamental limits of data storage using an atomic scale memory. Semiconductor surfaces lend themselves towards self-assembly, because the broken covalent bonds create elaborate reconstruction patterns to minimize the surface energy. An example is the large 7 × 7 unit cell on Si(1 1 1), which can be used as building block. On semiconductors, the surface electrons completely de-couple from the substrate, as long as their energy lies in the band gap. Angle-resolved photoemission reveals surprising features, such as a fractional band filling and a spin-splitting at a non-magnetic surface. An interesting by-product is a memory structure with self-assembled tracks that are five atom rows wide and store a bit by the presence or absence of a single silicon atom. This toy memory is used to test the fundamental limits of data storage and to see how storage on silicon compares to storage in DNA.

Surface science has made great strides towards tailoring surface properties via self-assembly of nanoscale molecular adsorbates. It is now possible to functionalize surfaces with complex biomolecules such as DNA and proteins. This brief overview shows how NEXAFS (near edge X-ray absorption fine structure spectroscopy) can be used to characterize the assembly of biological molecules at surfaces in atom- and orbital-specific fashion. To illustrate the range of applications, we begin with simple self-assembled monolayers (SAMs), proceed to SAMs with customized terminal groups, and finish with DNA oligonucleotides and Ribonuclease A, a small protein containing 124 amino acids. The N 1s absorption edge is particularly useful for characterizing DNA and proteins because it selectively interrogates the π* orbitals in nucleobases and the peptide bonds in proteins. Information about the orientation of molecular orbitals is obtained from the polarization dependence. Quantitative NEXAFS models explain the polarization dependence in terms of molecular orientation and structure.Key words: NEXAFS, bio-interfaces, ribonuclease A, immobilization, orientation.

In recent years, it has become possible to create well-ordered semiconductor surfaces with metallic surface states by using self-assembly of metal atoms. Since these states lie in the band gap of the semiconductor, they completely decouple from the substrate. In addition to two-dimensional structures it is possible to obtain arrays of one-dimensional atomic chains, which may be viewed as the ultimate nanowires. The dimensionality can be varied systematically by using vicinal surfaces with variable step spacing. Angle-resolved photoemission and scanning tunnelling spectroscopy reveal surprising features, such as a fractional band filling, nanoscale phase separation into doped and undoped chain segments, and a spin-splitting at a non-magnetic surface. Prospects for one-dimensional electron gas physics in atomic chains are discussed.

X-rays from synchrotron radiation enable incisive spectroscopic techniques which speed up the discovery of new materials for photovoltaics and photoelectrochemistry. A particularly useful method is X-ray absorption spectroscopy (XAS), which probes empty electronic states. XAS is element- and bond-specific, with the additional capability of determining the bond orientation. Close feedback from density functional calculations makes it possible to discover and exploit systematic trends in the electronic properties. Case studies are presented, such as solar cells that combine an absorber with an electron donor and an acceptor in one molecular complex and nanowire arrays serving as photoanodes for water splitting. In addition to the energy levels the lifetimes of the charge carriers play an essential role in device performance. A new generation of laser-like X-ray sources will make it possible to follow the fate of excited charge carriers traveling across a molecular complex or through a device structure in real time.

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence., An extensive database of spectroscopic properties of molecules from ab initio calculations is used to design molecular complexes for use in tandem solar cells that convert two photons into a single electron-hole pair, thereby increasing the output voltage while covering a wider spectral range. Three different architectures are considered: the first two involve a complex consisting of two dye molecules with appropriately matched frontier orbitals, connected by a molecular diode. Optimized combinations of dye molecules are determined by taking advantage of our computational database of the structural and energetic properties of several thousand porphyrin dyes. The third design is a molecular analogy of the intermediate band solar cell, and involves a single dye molecule with strong intersystem crossing to ensure a long lifetime of the intermediate state. Based on the calculated energy levels and molecular orbitals, energy diagrams are presented for the individual steps in the operation of such tandem solar cells. We find that theoretical open circuit voltages of up to 1.8 V can be achieved using these tandem designs. Questions about the practical implementation of prototypical devices, such as the synthesis of the tandem molecules and potential loss mechanisms, are addressed., KBØ and KST would like to thank the Danish Council for Independent Research's DFF-Sapere Aude program (grant no. 11-1051390) for financial support. JMGL acknowledges support from the Spanish Ministry of Economy and Competitiveness under Projects FIS2010-21282-C02-01 and FIS2012-30996 and through Ramon y Cajal grant RYC-2011-07782. FJH acknowledges funding by the Department of Energy, Basic Energy Sciences, Contract no. DE-SC0006931. AR acknowledges financial support from the European Research Council Advanced Grant DYNamo (ERC-2010-AdG-267374), Spanish Grant (FIS2013-46159-C3-1-P), Grupos Consolidados UPV/EHU del Gobierno Vasco (IT578-13) and European Community FP7 project CRONOS (Grant number 280879-2).

We have studied the electronic properties of 3C- and 4H-SiC with X-ray absorption (XAS). Particular emphasis is placed on the conduction bands because they exhibit larger differences between the various SiC polytypes than valence bands. XAS spectra at the Si2p and C1s edges provide projections onto Si3d, 4s and C2p conduction band states. We explain the observed differences in the Si L2,3 XAS data to arise from transition into dispersive bands which occur at the M and K point of the hexagonal Brillouin zone. The XAS data are sensitive to a difference in the dispersion of the two lowest conduction bands. For 3C-SiC the dispersion is larger than for 4H-SiC in agreement with theory. We compare the XAS data at the Si L edge with CFS and CIS spectra and find that the SiLVV Auger is dominant.

Ribonuclease A (RNase A) is immobilized on silver surfaces in oriented and random form via self-assembled monolayers (SAMs) of alkanethiols. The immobilization process is characterized step-by-step using chemically selective near-edge X-ray absorption fine structure spectroscopy (NEXAFS) at the C, N, and S K-edges. Causes of imperfect immobilization are pinpointed, such as oxidation and partial desorption of the alkanethiol SAMs and incomplete coverage. The orientation of the protein layer manifests itself in an 18% polarization dependence of the NEXAFS signal from the N 1s to pi* transition of the peptide bond, which is not seen for a random orientation. The S 1s to C-S sigma* transition exhibits an even larger polarization dependence of 41%, which is reduced to 5% for a random orientation. A quantitative model is developed that explains the sign and magnitude of the polarization dependence at both edges. The results demonstrate that NEXAFS is able to characterize surface reactions during the immobilization of proteins and to provide insight into their orientations on surfaces.

We present new two-photon molecular architectures for photovoltaics where atomic precision can be obtained by synthetic chemistry., An extensive database of spectroscopic properties of molecules from ab initio calculations is used to design molecular complexes for use in tandem solar cells that convert two photons into a single electron–hole pair, thereby increasing the output voltage while covering a wider spectral range. Three different architectures are considered: the first two involve a complex consisting of two dye molecules with appropriately matched frontier orbitals, connected by a molecular diode. Optimized combinations of dye molecules are determined by taking advantage of our computational database of the structural and energetic properties of several thousand porphyrin dyes. The third design is a molecular analogy of the intermediate band solar cell, and involves a single dye molecule with strong intersystem crossing to ensure a long lifetime of the intermediate state. Based on the calculated energy levels and molecular orbitals, energy diagrams are presented for the individual steps in the operation of such tandem solar cells. We find that theoretical open circuit voltages of up to 1.8 V can be achieved using these tandem designs. Questions about the practical implementation of prototypical devices, such as the synthesis of the tandem molecules and potential loss mechanisms, are addressed.

Self-assembled atomic chains can be triggered at stepped Si(111) surfaces by adding sub-monolayer amounts of metals, such as gold, silver, platinum, alkali metals, alkaline earths, and rare earths. A common feature of all these structures is the honeycomb chain, a graphitic strip of Si atoms at the step edge that is lattice matched in the direction parallel to the edge but completely mismatched perpendicular to it. This honeycomb chain drives one-dimensional surface reconstructions even on the flat Si(111) surface, breaking its three-fold symmetry. Particularly interesting are metallic chain structures, such as those induced by gold. The Au atoms are locked rigidly to the Si substrate but the electrons near the Fermi level completely decouple from the substrate because they lie in the band gap of silicon. The electronic structure of one-dimensional electrons is predicted to be qualitatively different from that of higher dimensions, since electrons cannot avoid each other when moving along the same line. The single-electron picture has to be abandoned, making way for collective excitations, such as spinons and holons, where the spins and charges of electrons become separated. Although such excitations have yet to be confirmed definitively, the band structure seen in angle-resoled photoemission exhibits a variety of unusual features, such as a fractional electron count and a doublet of nearly half-filled bands. Chains of tunable spins can be created with rare earths. The dimensionality can be controlled by adjusting the step spacing with intra- and inter-chain coupling ratios from 10:1 to >70:1. Thus, metal-induced chain structures on stepped silicon provide a versatile class of low-dimensional materials for approaching the one-dimensional limit and exploring the exotic electronic states predicted for one dimension.

A self-assembled monolayer of ferrocene-capped alkanethiols is produced in two stable oxidation states of Fe (Fe2+ and Fe3+). These oxidation states are detected with sub-monolayer sensitivity by the fine structure of the Fe2p absorption edge. The native Fe2+ ferrocene layer is converted chemically to Fe3+, and the Fe3+ layer can be transformed back to Fe2+ or possibly Fe0 by irradiation with soft X-rays. The results have implications on switching mechanisms in molecular electronics.

It's not exactly Russian roulette, but scheduling October events outdoors is not risk-free, even in usually sunny California. An overflow crowd or more than 400 registered users, ALS staff, and vendors enjoyed a full indoor program featuring science highlights and work-shops spread over two and a half days from October 18 to October 20, 2004. However, a major storm, heralding the onset of the San Francisco Bay Area rainy season, posed a few weather challenges for the events on the ALS patio. Users' Executive Committee chair Dennis Lindle (University of Nevada, Las Vegas) started the meeting off with a warm welcome to new Berkeley Lab Director Stephen Chu. Coming from a laser-based science background, Chu confessed he was not yet intimately familiar with the ALS but was already hearing that it is one of the best facilities for users to do great science.

The structure of self-assembled monolayers presenting aromatic rings at a surface is studied by near edge x-ray absorption fine structure spectroscopy (NEXAFS). Fluorine substitution at asymmetric positions in the aromatic rings is used to generate a layer of dipoles at the surface of the monolayer. We find that fluorine substituted aromatic rings are more ordered than unsubstituted aromatic rings by a factor of two based on the polarization dependence of the lowest C 1s to pi* transition, which is associated with transitions from phenyl carbons attached to hydrogens. This result is consistent with the influence of dipole-dipole interactions and quadrupolar interactions between the aromatic groups due to the substitution of fluorine atoms. The work also serves to illustrate how subtle variations in the orientation of an end group of a self-assembled monolayer can be determined by using NEXAFS.

It is shown how atomically precise step arrays can be fabricated on vicinal Si(111). Decoration with metal atoms produces atomic chains that serve as the ultimate nanowires for the study of electrons between one and two dimensions. Chain structures are formed by a large variety of metal atoms with different chemical properties. They range from alkali and alkaline earth metals to noble metals and include transition metals (Pt) and rare earths (Gd, Ho). As a first step toward functionalizing such surfaces, we demonstrate how NEXAFS spectroscopy characterizes organic layers in atom- and orbital-specific fashion, with examples ranging from alkanes with customized end groups to DNA oligonucleotides.

Scanning tunneling microscopy images of the Si(1 1 1)5×2–Au surfaces exhibit irregularly distributed protrusions of atomic size. They are identified as silicon adatoms by evaporating small amounts of silicon and gold onto the reconstructed surface. While extra silicon increases the density of protrusions, extra gold transforms the reconstruction partially into a Si(1 1 1)√3×√3 reconstruction.

Synchrotron radiation-based x-ray photoemission spectroscopy was used to study the surface Fermi level position within the band gap for thin metal overlayers of Au, Al, Ni, Ti, Pt, and Pd on n–GaN and p–GaN. Nonequilibrium effects were taken into account by measuring the Fermi edge of the metal overlayer. There are two different behaviors observed for the six metals studied. For Au, Ti, and Pt, the surface Fermi level lies about 0.5-eV higher in the gap for n-type than for p-type GaN. For Ni, Al, and Pd, the surface Fermi level position is independent of doping, but varies from one metal to the other. Results for Ni, Pd, and Al fit a modified Schottky–Mott theory, while Au, Ti, and Pt demonstrate a more complex behavior. Atomic force microscopy was used along with photoemission to investigate the growth mode of each metal on the GaN surface.

Arrays of atomic wires can now be synthesized by self-assembly at stepped surfaces. They provide opportunities for exploring electrons in one dimension and to test predictions of exotic properties, such as spin charge separation in a Luttinger liquid.

High-quality vicinal Si(1 1 1) surfaces are used as templates to create single domains of the Si(1 1 1)3 � 1-Ca reconstruction which exhibit atomic chains parallel to Si steps. Scanning tunneling microscope images support the formation of honeycomb chains of Si atoms, rather than zigzag chains proposed in earlier models. Angle-resolved photoemission is used to map out the dispersion of valence band states parallel and perpendicular to the chains. A gap of � 0.9 eV is found below the Fermi level with both in-plane and out-of-plane polarization of the synchrotron light. The observed semiconducting behavior suggests that the honeycomb chain channel model proposed for the alkaliinduced 3 � 1 reconstruction be modified for divalent alkaline earths, e.g. a 3 � 2 structure with 1/6 monolayer coverage. 2002 Elsevier Science B.V. All rights reserved.

A one-dimensional 5 � 2 reconstruction of Gd is observed on Si(1 1 1), which can be stabilized in single domain form on vicinal surfaces. It contains zig-zag rows of occupied orbitals alternating with rows of unoccupied orbitals, as inferred from spectroscopic scanning tunneling microscopy. The large magnetic moment of the Gd 4f electrons creates atomic chains with a magnetic coupling. 2001 Published byElsevier Science B.V.

Polarization-dependent near edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to determine the ordering of octadecyltrichlorosilane (OTS) molecules in self-assembled (SA) films on Si/SiOx. Coverages of adsorbed material for different SA films were determined by integration of the NEXAFS signal due entirely to the C 1s absorption. The structures and coverages of the SA films deposited by chemisorption from solution in toluene were determined for two different deposition conditions: (1) open to the atmosphere and (2) water- and oxygen-free conditions inside a drybox. SA films of OTS deposited under ambient air conditions grew as islands on the substrate and were well-ordered as indicated by a strong polarization-dependence in the NEXAFS spectra. For depositions performed in the drybox, ordered or disordered films grew uniformly over the substrate. Ordered SA films deposited in the drybox exhibited higher average tilt angles and lower coverages of adsorbed material than SA films deposited u...

This work follows upon the recent demonstration of the photocatalytic conversion of N2 to NH3 in aqueous solutions via H-terminated, polycrystalline diamond electrodes. X-ray absorption spectroscopy serves as an atom- and bond-specific probe of the local bonding of oxygen and nitrogen species at the surface of the electrodes. Ozone treatment of the electrodes creates a sharp C1s transition to an unoccupied surface state at 2.5 eV below the onset of the transitions to the bulk conduction band. A similar transition occurs at 2.6 eV below the onset after ammonia treatment. It is surprising to obtain such a well-defined surface state on complex, real-life electrode materials, which suggests a characteristic local bonding configuration. An extensive comparison with spectra from reference molecules reveals keto groups (C=O) at the ozone-treated surface, with the possibility of additional tertiary alcohols (C-OH). The electron–hole interaction is taken into account for estimating the absolute energy of the surfa...

Recently, single-walled carbon nanotubes as well as graphene functionalized with azobenzene chromophores have drawn attention for applications in optoelectronics due to their ability to undergo cis–trans isomerization when exposed to light. The electronic properties of the nanocarbon materials at these unconventional interfaces can be tailored by gaining structural insight into the organic monolayers at the molecular level. In this work, we use polarization-dependent X-ray absorption spectroscopy to probe the orientation of three chromophores on graphene, all identical except for their terminal groups. All three terminal groups (methyl, nitro, and nitrile) are well-oriented, with a tilt angle of about 30° from the substrate for the shared azobenzene group. Density functional theory calculations are in good agreement with experimental results and give two similar, stable configurations for the orientation of these molecules on graphene.

Polarization-dependent X-ray absorption spectroscopy is combined with density functional calculations and atomic multiplet calculations to determine the crystal field parameters 10Dq, Ds, and Dt of transition metal phthalocyanines and octaethylporphyrins (Mn, Fe, Co, Ni). The polarization dependence facilitates the assignment of the multiplets in terms of in-plane and out-of-plane orbitals and avoids ambiguities. Crystal field values from density functional calculations provide starting values close to the optimum fit of the data. The resulting systematics of the crystal field can be used for optimizing electron-hole separation in dye-sensitized solar cells.

The step edges on the Si(553)-Au surface undergo a 1 x 3 reconstruction at low temperature which has recently been interpreted theoretically as the x3 ordering of spin-polarized silicon atoms at the edges of the graphitic Si nanowires on this vicinal surface. This predicted magnetic ground state has a clear spectroscopic signature - a silicon step-edge state at 0.5 eV above the Fermi level - that arises from strong exchange splitting and hence would not occur without spin polarization. Here we report spatially resolved scanning tunneling spectroscopy data for these nanowires. At low temperature we find an unoccupied state at 0.5 eV above every third step edge silicon atom, in excellent agreement with the spin-polarized ground state predicted theoretically. This spin-polarized state survives up to room temperature where the position of the spins rapidly fluctuates among all Si step-edge sites.

One-dimensional electron systems can now be synthesized at stepped surfaces by self-assembly of atomic and molecular chains. A wide variety of adsorbate and substrate combinations provides opportunities for systematically tailoring electronic properties, such as the intra-chain and inter-chain coupling, the electron count, magnetic moment and the Coulomb interaction. Angle-resolved photoemission with synchrotron radiation is an ideal probe to reveal the complete set of quantum numbers for electrons at an ordered surface, i.e. energy, momentum parallel to the surface, spin and point group symmetry. Interesting electronic features are discussed, such as spin-charge separation in a Luttinger liquid, charge density waves, the Peierls gap, mixed dimensionality and one-dimensional quantum well states.

The growth mode of Au on stepped Si(111)7×7 surfaces is determined by scanning tunneling microscopy, with the goal of providing a continuous gold layer that replicates the step morphology. Functionalization with gold allows attaching organic and biomolecules via thiol groups (e.g., alkanes and DNA). On clean Si(111), gold grows in the Stranski–Krastanov mode and produces islands with a size comparable to the step spacing. A Ti wetting layer produces smooth Au films that preserve the step topography down to a scale of a few nanometers.

Alkanes and DNA are adsorbed via thiol groups on ultrathin gold films that have been templated on stepped Si(111)7×7 surfaces. The orientation of the adsorbed molecules is determined by polarization-dependent near edge x-ray absorption fine structure spectroscopy from the C 1s and N 1s core levels. An anisotropy in the polar distribution is found which is consistent with models in which the molecules have a preferred tilt angle with respect to normal but a random azimuthal distribution. In order to obtain maximum coverage and an oriented overlayer it is necessary to deposit more than 3 nm of Au (12 monolayers).

The electron wave function at Cu(1 1 1) vicinal surfaces switches between two qualitatively different regimes, i.e. from terrace modulation at small miscut angles to step modulation for large miscuts. This is deduced in angle resolved photoemission experiments from the photon energy dependent shift of the surface band. In this work we examine the critical miscut of 7° (17 A terrace width) at which the transition is observed. The analysis of the data within a diffraction-like framework is consistent with a mixed terrace–step modulation. The mechanism that triggers the switch-over from terrace to step modulation of the surface state is discussed.

The photoemission cross section of quantum well states in thin Cu(1 0 0) films on Co(1 0 0) has been analyzed in normal emission as a function of photon energy. Assuming conservation of the perpendicular wave vector k ⊥ , the cross section maxima can be assigned to direct transitions from the (initial) quantum well state to bulk-like final states. The value of k ⊥ is obtained from the sharp k ⊥ distribution of specific quantum well states, which is centered about discretized k ⊥ values. Our data are compared with free-electron-like models and a low energy electron diffraction calculation of high-energy states in bulk Cu.

Nanostructures at surfaces and interfaces are a fertile testing ground for bringing the idea of ‘tailored solids’ towards reality. Electronic properties can be controlled systematically by confinement or by interface effects. The presence of a single crystal substrate allows for the self-assembly of highly regular nanoobjects, such as stripes and strings of dots with sizes of about 10 nm. Using silicon as substrate facilitates the electronic integration of nanodevices into micro-electronics. We speculate how such structures might evolve into future devices, such as data storage arrays with densities of Terabits/cm 2 and self-assembled

Resonant inelastic X-ray scattering (RIXS) has been observed in graphite and hexagonal boron nitride (hBN) above and below their K edges. Below the core threshold, inelastic-loss features are observed, which disperse linearly with excitation energy, but as the excitation goes above the core binding energy, nonlinear dispersive effects are observed in graphite but not in hBN. We show that these two effects, which have previously been thought of as separate processes, i.e. resonant X-ray Raman scattering (below threshold) and RIXS (above threshold), are in fact described by the same physics of coherent fluorescence. Very good agreement between experiment and simulated RIXS is achieved using a simple one-electron framework. The role core-excitons play in the RIXS process is examined in finer detail, by using narrow-band excitation. Our results indicate that core-hole effects play a minor role in the RIXS observed from graphite but are more pronounced in hBN.

We report on the orientation of nematic liquid crystals supported on highly regular, periodic, nanometerscale structures formed from atomic steps on the surface of miscut silicon crystals thermally annealed in ultrahigh vacuum (UHV). Prior to removal from UHV, the silicon templates were passivated by reaction with methanol, acetone, or hydrogen and characterized by ultraviolet photoelectron spectroscopy using synchrotron radiation. In contrast to silicon substrates prepared in the past by wet chemical methods and substrates prepared by oblique deposition of metal and metal oxide films, the passivated templates prepared in UHV have highly regular single and bunched atomic steps that can be observed by scanning tunneling microscopy. Three independent measurements of the orientation of nematic liquid crystals of 4-cyano4′-pentylbiphenyl (5CB) on these templates revealed the alignment of 5CB to be parallel to the atomic step edges on samples with periodic single atomic steps (0.3 nm height, 15 nm width) as well as bunched atomic steps (5 nm height, 70 nm width). Because the nanometer-scale structures of these surfaces are well-defined and readily manipulated on spatial scales comparable to the sizes of a range of biological species, these surfaces may be useful as templates for the detection of the binding of biological species through changes in the orientation of liquid crystals.

The orbital character, orientation, and magnetic polarization of the $\mathrm{O}\ensuremath{-}2p$ unoccupied states near the Fermi level ${(E}_{F})$ in ${\mathrm{CrO}}_{2}$ was determined using polarization-dependent x-ray absorption spectroscopy and x-ray magnetic circular dichroism from high-quality, single-crystal films. A sharp peak observed just above ${E}_{F}$ is excited only by the electric-field vector $(\mathbf{E})$ normal to the tetragonal c axis, characteristic of a narrow band $(\ensuremath{\approx}0.7$ eV bandwidth) constituted from $\mathrm{O}\ensuremath{-}2p$ orbitals perpendicular to c $(\mathrm{O}\ensuremath{-}{2p}_{y})$ hybridized with Cr ${3d}_{\mathrm{xz}\ensuremath{-}\mathrm{yz}}{t}_{2g}$ states. By comparison with band-structure and configuration-interaction cluster calculations our results support a model of ${\mathrm{CrO}}_{2}$ as a half-metallic ferromagnet with large exchange-splitting energy $({\ensuremath{\Delta}}_{\mathrm{exch}\ensuremath{-}\mathrm{split}}\ensuremath{\approx}3.0\mathrm{eV})$ and substantial correlation effects.

The localization of Na impurities at the buried heterojunction of CdS/Cu(In,Ga)Se2 thin film solar cells has been studied by photoelectron spectroscopy and X-ray emission spectroscopy. This combination of a surface- and a bulk-sensitive technique allows to identify the localization of impurities at a buried interface in a non-destructive, semi-quantitative, and element-specific way. We compare samples with increasing CdS-overlayer thickness on (a) a CIGS film with nominal Na content and (b) a Na-rich CIGS film. The data clearly indicate a self-limitation of the Na content at this interface. The consequences are discussed in view of the possibility to tailor the electronic structure of the buried heterojunction by controlling the nominal Na content in the CIGS film.