Kenneth W. Nebesny

Valence and core photoelectron spectroscopy of C60, buckminsterfullerene

Abstract The MgKα X-ray (XPS) and He(I)/He(II) ultraviolet (UPS) photoelectron spectra of several thin films of C 60 (one to three monolayers) prepared by vapor deposition on gold are reported. The core XPS spectrum shows a single narrow carbon 1s ionization at 285.1 eV. The valence UPS spectra show very sharp valence ionization bands, comparable to gas phase spectra, with wide separations between the lowest ionization energy features. The ionizations are consistent with theoretical calculations based on the highly symmetric truncated icosahedral structure. The first vertical ionization energy relative to the vacuum level is estimated to be 7.6 ± 0.2 eV from these solid state measurements.

Photoemission spectroscopy of LiF coated Al and Pt electrodes

Thin lithium fluoride (LiF) interlayers between the low work function electrode and the electron transport layer in organic light emitting diodes (OLED) result in improved device performance. We investigated the electronic structure of LiF coated Al and Pt electrodes by x-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). Thin LiF films were grown in several steps onto Ar+ sputtered Al and Pt foils. After each growth step the surfaces were characterized in situ by XPS and UPS measurements. After evaluating band bending, work function and valence band offset for both samples, their band lineups were determined. Our measurements indicate that despite the insulating character of LiF in both samples, band bending is present in the LiF layer. The difference in band bending between the samples allows the conclusion that the driving force for the development of the band bending results from the contact potential between the metal and the LiF overlayer. The band bending is most...

Observation of strong band bending in perylene tetracarboxylic dianhydride thin films grown on SnS2

Perylene tetracarboxylic dianhydride (PTCDA) thin films were grown in several steps on tin disulfide (SnS2) single crystals and characterized by combined x-ray and ultraviolet photoemission spectroscopy (XPS), (UPS) in order to characterize the frontier orbital line-up and the interface dipole at their interface. Due to the large difference between the work functions of PTCDA (4.26 eV) and SnS2 (5.09 eV) this experiment represents a model system for the investigation of band bending related phenomena in organic semiconductor heterojunctions. Our results show that the equilibration between the Fermi levels of both materials in contact is achieved almost solely by band bending (bulk charge redistribution) in the PTCDA layer. No significant interface dipole was detected which means that the PTCDA molecular orbitals and the SnS2 bands align at the vacuum level corresponding to the electron affinity rule. Our experiments clearly demonstrate the importance of an additional XPS measurement which (in most cases) ...

Determination of frontier orbital alignment and band bending at an organic semiconductor heterointerface by combined x-ray and ultraviolet photoemission measurements

The alignment of the highest occupied molecular orbitals (HOMO) at the tris (8-hydroxy quinoline) aluminum (Alq3)/N,N′-di-(3-methylphenyl)-N,N′diphenyl-4,4′-diaminobiphenyl (TPD) heterojunction, used in organic light-emitting diodes (OLED), was determined by growing a TPD layer in several steps on a thick Alq3 substrate layer. After each growth step the sample was characterized in situ by x-ray and ultraviolet photoemission spectroscopy. The offset of the HOMO maxima at the interface was determined to be −0.13 eV from Alq3 to TPD. By including the known HOMO–lowest occupied molecular orbital (LUMO) gaps for both molecules into the evaluation, the offset of the LUMO minima was determined to be −0.33 eV from Alq3 to TPD. These values are consistent with previous assumptions that this interface represents a higher barrier for electron injection from Alq3 to TPD than for hole injection from TPD to Alq3.

Evaluation of transparent carbon nanotube networks of homogeneous electronic type

In this report, we present a description of the optical and electronic properties of as-deposited, annealed, and chemically treated single-walled carbon nanotube (SWNT) films showing metallic or semiconducting behavior. As-deposited and annealed semiconducting SWNT films were significantly less conductive than metallic SWNT films; however, chemical treatment of semiconducting SWNT films resulted in sheet resistance values as low as 60 Omega x sq(-1) in comparison to 76 Omega x sq(-1) for similarly processed metallic SWNT films. We conclude that the greater improvement of electrical conductivity observed in the semiconducting SWNT film results from the difference in the density of available electronic states between metallic and semiconducting SWNTs. A corroborative investigation of the change in surface work function and the chemical composition of SWNT films, as revealed by X-ray photoelectron spectroscopy, is provided to support these conclusions and to give new perspective to the formation of electronically homogeneous SWNT networks.

Large molecule epitaxy on single crystal metals, insulators and single crystal and MBE-grown layered semiconductors

We review the packing structures for a series of aromatic hydrocarbons, deposited by vacuum deposition methods as ordered monolayers→→multilayers, on a variety of metal, semiconductor and insulator surfaces. New results are presented for the adsorption of monolayers of perylenes, phthalocyanines, coronene, and pentacenes on the Cu(100) surface, along with the implications of these studies for the formation of ordered multilayers of these molecular systems. Aromatic molecules without heteroatoms appear to pack in a flat-lying motif, and exhibit approximately hexagonal close packing, even on a substrate with four-fold symmetry such as Cu(100). In general, aromatic systems whose bulk structures lend themselves to layer-by-layer growth during vacuum deposition appear to be the best candidates for ordered multilayer growth.

Orientation and structure of monolayer →→ multilayer phthalocyanine thin films on layered semiconductor (MoS2 and SnS2) surfaces

Thin films of both chloroindium and copper pthalocyanines have been vacuum deposited onto metal dichalcogenide surfaces such as MoS2 and SnS2, with ordering achieved for these four‐fold symmetric molecules ranging from below monolayer to multilayers. Reflection high‐energy electron diffraction suggests that square lattice geometries are adopted for low coverages of each phthalocyanine (Pc), but with multiple domains. Low‐energy electron diffraction confirms the presence of three square lattice domains, each domain rotated by 60° with respect to the other. Basal plane defects, and especially terrace sites in the metal dichalcogenide surface, are implicated as the nucleation sites for the growth of these square lattice domains. Optical spectroscopies have been used to characterize submonolayer to multilayer deposits of chloroindium phthalocyanine on SnS2 thin films, where the packing geometries of the adjacent Pcs cause perceptible changes in the position and width of the absorbance band in the visible/near...

Complexation of electroconducting polypyrrole with copper

Abstract A copper complex of polypyrrole is obtained when the polymer perchlorate is treated with a Cu(II)-containing alkaline solution. The X-ray photoelectron spectrum of the resulting polymer material shows that the copper atoms are coordinated to pyrrole nitrogen. The ESR signal of copper ions and that of the pyrrole rings are observed independently of each other; there is no appreciable magnetic interaction between the two paramagnetic species. The room-temperature conductivity is 4 × 10−4 S cm− and the temperature dependence shows semiconductive behavior with an activation energy of 0.11 eV. The degradation of conducting polypyrrole in aqueous solution is suppressed by the presence of copper: the stabilization is attributed to CuN bond formation.

The Ionizations of C 60 in the Gas Phase and in Thin Solid Films.

The high-resolution He I photoelectron spectrum of C 60 in the gas phase is reported and compared with the photoelectron spectrum of C 60 as a thin film prepared by vapor deposition (one to three monolayers) on gold. The spectra show low valence ionization bands that are very sharp and well-separated for a molecule of this size, consistent with the highly symmetric truncated icosahedral structure and theoretical calculations. The total band widths of the valence ionizations from the thin film samples are comparable to those from the gas phase species, showing that the electronic interactions between the molecules and with the surface do not significantly influence these measurements of the molecular electronic structure. The gas phase photoelectron spectra also show vibrational fine structure in the first and second ionization bands with spacings that are consistent with the two totally symmetric vibrational modes of C 60 . The first vertical ionization energy relative to the vacuum level is determined to be 7.61 ± 0.02 eV from these gas phase measurements.

Optical constants of the noble metals determined by reflection electron energy loss spectroscopy

Abstract The optical constants for Cu, Ag, and Au determined by reflection electron energy loss spectroscopy (REELS) using a data treatment method described in the previous paper in this Journal are reported. The purpose of this study is to further test the data treatment method on less free-electron-like metals. The method involves removal of surface effects by deconvolution of data taken with low-energy electrons (200 eV) from data taken with higher energy electrons (1500 eV). The low- and high-energy electron experimental spectra are discussed along with the resulting optical constants. It is found that, although the surface loss contribution to the high-energy electron data is not readily apparent, its contribution does affect the results of the Kramers-Kronig analysis used to determine the complex dielectric function. Thus, some account must taken of the surface loss contribution in order to obtain quantitative information about the optical constants. Along with surface effects, plural-scattering contributions to the spectral response are also removed using deconvolution procedures. Comparison of these results to literature values demonstrates that the data treatment method is successful in closely approximating the optical constants for Cu, Ag, and Au.