Steven W. Robey

Striped domains at the pentacene:C60 interface

Scanning tunneling microscopy observations of the initial growth stages of the first layer of pentacene on a monolayer film of C60 on Ag(111) are presented. Pentacene films nucleate and grow with molecules standing up at the pentacene:C60 interface similar to thin film phases observed on weakly interacting substrates such as SiO2. Unlike reported thin film pentacene phases, those on 1 monolayer C60/Ag(111) exhibit a striated morphology with domains of 4 nm nominal width. This long range pattern of periodic pentacene displacements relative to the substrate is the response to stress induced in the pentacene film by its interaction with the rigid C60 support.

C60 cluster formation at interfaces with pentacene thin-film phases

The C60-thin film pentacene interface was investigated using scanning tunneling microscopy, atomic force microscopy, and ultraviolet photoemission spectroscopy. C60 deposition on a multilayer pentacene film (standing) yields an interface dominated by C60 clusters, regardless of the underlying substrate. Three-dimensional cluster growth dominates due to weak interactions with the underlying Pn. C60 cluster size and density on sequential Pn layers suggest an Ehrlich–Schwoebel-type barrier at Pn layer boundaries. Cluster formation reduces the C60 lowest unoccupied molecular orbital–Pn highest occupied molecular orbital (HOMO) separation, while increasing the respective HOMO-HOMO offset. Heterostructure fabrication protocols can alter interface morphology and induce band shifts on the order of 0.3 eV.

C60-Pentacene Network Formation by 2-D Co-Crystallization

We report experiments highlighting the mechanistic role of mobile pentacene precursors in the formation of a network C(60)-pentacene co-crystalline structure on Ag(111). This co-crystalline arrangement was first observed by low temperature scanning tunneling microscopy (STM) by Zhang et al. (Zhang, H. L.; Chen, W.; Huang, H.; Chen, L.; Wee, A. T. S. J. Am. Chem. Soc. 2008, 130, 2720-2721). We now show that this structure forms readily at room temperature from a two-dimensional (2-D) mixture. Pentacene, evaporated onto Ag(111) to coverages of 0.4-1.0 ML, produces a two-dimensional (2-D) gas. Subsequently deposited C(60) molecules combine with the pentacene 2-D gas to generate a network structure, consisting of chains of close-packed C(60) molecules, spaced by individual C(60) linkers and 1 nm x 2.5 nm pores containing individual pentacene molecules. Spontaneous formation of this stoichiometric (C(60))(4)-pentacene network from a range of excess pentacene surface coverage (0.4 to 1.0 ML) indicates a self-limiting assembly process. We refine the structure model for this phase and discuss the generality of this co-crystallization mechanism.

Resonant photoelectron spectroscopy studies of BaTiO3 and related mixed oxides

Abstract Synchrotron radiation-based ultraviolet photoelectron spectroscopy (UPS was used to investigate the electronic structure of BaTiO 3 and related mixed oxides. In particular, resonant photoelectron effects at the Ti 3 p threshold provide a picture of the contribution of the Ti 3 d states to the predominantly O 2 p valence band, and thus produce information on Ti 3 d O 2 p hybridization. This is used to determine the TiO hybridization in BaTiO 3 and comparisons are made to the Ti partial density of states obtained from first principles calculations of the band structure of BaTiO 3 . We have also investigated the effects of substitution of Nd for Sr in SrTiO 3 to form the mixed oxide Nd 1 − x Sr x TiO 3 . These studies illuminate the effects of changing crystal structure and bandfilling on Ti 3 d O 2 p hybridization.

Reactive coevaporation of DyBaCuO superconducting films: The segregation of bulk impurities on annealed MgO(100) substrates

This article reports results from a study on the effect bulk impurities in MgO substrates may have when substrates are annealed to improve film microstructure and transport properties. DyBaCuO thin films are prepared by molecular beam reactive coevaporation on both annealed and unannealed MgO(100). While predeposition annealing has led to improved film quality for some groups, under our conditions just the opposite occurs. We point out the existence of Ca segregation to the MgO surface as a factor that needs investigation because Ca is a common contaminant in commercially supplied MgO. Atomic force microscope images show decoration of annealed substrates with submicron size particles that may be composed of the segregated Ca. These particles seem to underlie the distinct differences seen in the surface microstructure of films on annealed and unannealed substrates. In the case of annealed substrates a more random microstructure is promoted. Under identical growth conditions, outgrowths present in films on ...

Electronic structure of high-Tc superconductors studied using photoelectron spectroscopy

Abstract Fundamental information about the structure of the valence band and the chemical valence states of the various constituents of the LaSrCu, YBaCu, BiSrCaCu and TlCaBaCu oxides have been obtained using photoelectron spectroscopy. These results show that the one-electron theories do not adequately describe the electronic structure of these superconductors. The atomic origins of the features observed in the valence bands have been investigated by studying photoemission resonances and changes in excitation cross-sections with photon energy. Results to date suggest that these materials have varying densities of states at the Fermi level, valence bands composed of O 2p and Cu 3d states, and display no significant changes in the band structure associated with the superconducting behaviour when the temperature is lowered below T c . In addition, the complex surface chemistry of these oxides make it essential to study the surface stoichiometry and the interaction of simple molecules. O 2 and CO are found to interact only weakly with the surfaces of the materials studied to date, while the H 2 O and CO 2 react strongly, forming hydroxides and carbonates.

Substitution-induced mid-gap states in RxBa1−xTiO3−δ whereR = Y, La, and Nd

Changes induced in the electronic structure of BaTiO3 by substitutions of R = Y, La or Nd for Ba to form the mixed oxides RxBa1−xTiO3−δ have been investigated using ultraviolet photoelectron spectroscopy. Substitution of formally R+3 ions for Ba+2 leads to the introduction of filled states in the band gap that are shown by resonant photoemission measurements to have significant Ti 3d character, consistent with a Mott-Hubbard insulator description for these oxides. It is suggested that the dominant factor is electron-electron correlation and this leads to the estimatesU∼Δ∼3 eV for this system, whereU is the correlation energy for the 3d and Δ is the charge transfer energy. Changes are observed in the photoelectron spectral shape for these states as a function of increasing substitution in the Nd system and discussed in the context of the opening of the Hubbard gap.

Time-resolved photoemission studies of exciton dissociation in organic photovoltaics

In the drive to improve efficiencies to market-penetration levels, organic photovoltaics would benefit from the introduction of new organic electronic materials in the form of nonfullerene acceptors, allowing the greater tunability of electronic levels and improved optical absorption. Efforts to incorporate new acceptors have produced limited results although the improvements have recently shown steady progress. Possible barriers to the use of nonfullerene acceptors include the formation of unfavorable morphologies in nonfullerene systems and/or favorable excitation/carrier delocalization in fullerenes, both factors that may be overcome through advanced synthesis. More problematic are suggestions that the molecular excited states of fullerenes lead to greatly enhanced dissociation of the initial S1 exciton in the donor material. This would suggest a fundamental limitation on the use of many nonfullerene acceptors. The authors tested this suggestion using time-resolved two-photon photoemission to directly ...

C60 chain phases on ZnPc/Ag(111) surfaces: Supramolecular organization driven by competing interactions.

Serpentine chain C60 phases were observed in scanning tunneling microscopy (STM) images of C60 layers on zinc phthalocyanine (ZnPc) or pentacene covered Ag(111) and Au(111) surfaces. This low-density, quasi-one-dimensional organization contrasts starkly with the close-packed hexagonal phases observed for C60 layers on bare metal substrates. STM was employed to perform a detailed investigation of these chain structures for C60/ZnPc/Ag(111) heterolayers. Motivated by the similarity of these chain phases, and the chain and stripe organization occurring in dipole-fluid systems, we investigated a model based on competing van der Waals attractions and electrostatic repulsions between C60 molecules as an explanation for the driving force behind these monolayer phases. Density functional theory (DFT) calculations revealed significant charge transfer to C60 from the Ag(111) substrate, through the intervening ZnPc layer, inducing electrostatic interactions between C60 molecules. Molecular dynamics simulations performed with attractive van der Waals interactions plus repulsive dipole-dipole interactions reproduced the C60 chain phases with dipole magnitudes consistent with DFT calculations.

Initial etching of GaAs (001) during H2 plasma cleaning

The initial etching of GaAs (001) during remote H2 plasma etching for oxide removal was examined using atomic force microscopy, coupled with in situ reflection high energy diffraction and Auger spectroscopy. Localized etching of the GaAs surface is particularly evident for etch temperatures below ∼650 K and leads to the formation of etch pits 5–10 nm deep and 20–50 nm in diameter. Etch pit formation was found to occur well before complete oxide removal. The development of etch pits is influenced by surface mis-cut steps, possibly due to increased hydrogen incorporation into the near surface at surface defects such as steps.