N. John DiNardo

Early applications of High Resolution Electron Energy Loss Spectroscopy (HREELS) for the study of ultrathin polymer films and polymer-metal interfaces provided important information on bonding and electronic structure through analysis of sequentially-measured vibrational and electronic excitation spectra in the near-surface region. However, detailed analyses of vibrational spectra were limited by energy resolution >40 cm−1 which made it difficult to separate close-lying vibrational modes. Recent developments in HREEL spectrometers has made it possible to achieve <8 cm−1 energy resolution, and along with the ability to excite both optically-active and optically-forbidden transitions, virtually all the vibrational modes of complex polymer ultrathin films can be distinguished. Then the analysis of surface functionalities and interfacial bonding proceeds by comparison with reference infrared, Raman, and uv-vis spectra of molecular analogs. In this paper, we present brief highlights in the development of HREELS for the study of polymer systems and discuss our recent studies of polyaniline ultrathin films deposited on metal surfaces. These studies have (1) identified substrate-dependent effects in initial bonding and growth of vapor-deposited polyaniline oligomers, (2) shown evidence for polymerization at the surface, (3) found relatively high local conductivity in protonated films after ex-situ doping, and (4) provided new insights in the conductivity mechanisms themselves. Finally, prospects and limitations of HREELS as applied to polymer systems are noted. Early applications of High Resolution Electron Energy Loss Spectroscopy for the study of ultrathin polymer films and polymer-metal interfaces provided important information on bonding and electronic structure through analysis of sequentially-measured vibrational and electronic excitation spectra in the near-surface region. Thin film polymers are used in a host of applications ranging from protective coatings to insulating layers in microelectronics devices. Ultrathin films deposited on conducting substrates can usually conduct well so that charging effects are minimized. Vapor-deposited ultrathin poly aniline films grown on silver(110) and gold can be compared with those grown on copper(110). No oligomer-substrate losses are observed on the silver and gold surfaces suggesting that the amino groups react only weakly with these surfaces. Significantly, similar trends in the transfer of intensity of the CH out-of-plane modes from monosubstituted rings to paradisubstituted rings are observed indicating the occurrence of a polymerization reaction.

The absolute saturation coverage of Cs adsorbed on the Si(1 1 1)-7×7 surface at room temperature has been measured using Rutherford backscattering spectrometry. At saturation, coverage is 0.51±0.02 monolayers (ML) where 1 ML=7.83×10 14 atoms/cm 2. This value constrains proposed ordered structural models by placing a strict upper limit on the absolute coverage of the first Cs layer adsorbed on Si(1 0 0) at room temperature. Proposed structural models with lower coverages of ordered Cs must include substantial disordered components.

The physical and chemical properties of polyaniline thin films produced by vapor deposition of aniline tetramers on a Cu(110) substrate have been investigated using high-resolution electron energy loss spectroscopy (HREELS). HREELS spectra showed that aniline tetramers are stable on the Cu(110) surface and do not react to form longer chains. Tetramer films doped with HCl exhibited HREEL spectra with features characteristic of a semiconductor. The physical properties of the tetramer films have also been compared to those of films grown by exposing Cu(110) to aniline oligomers produced via pyrolysis of bulk emeraldine. Those films exhibit thickness-dependent changes related to reactivity of the vapor with the surface and metallic spectral features upon doping with HCl.

This paper focuses on several approaches in the characterization and modification of thin films made possible by recent experimental advances. The structural and electronic properties of two model systems are considered as examples: a-tC thin films grown by pulsed laser deposition (PLD) and polyaniline thin films grown by vapor deposition. First, scanning probe microscopies and X-ray scattering are used to investigate the structural aspects of a-tC films as a function of PLD growth conditions. The possible connection of nanoscale surface modification and characterization with electron emission properties is discussed. Second, the results of inelastic scattering spectroscopy and other surface techniques are discussed to obtain information on both interfacial aspects of the growth of polyaniline thin films and microscopic and macroscopic aspects of electrical conductivity upon doping. Comparisons are made with other studies that address properties of analogous crystalline systems as appropriate. A brief assessment of the broader problem of analyzing these systems are given.

Bioactive materials such as calcium phosphate ceramics and bioactive glasses enhance bone tissue formation and then bond to bone tissue. In our work, we question what particular surface feature or features of bioactive materials are responsible for the bone tissue response. In this study we have uncoupled surface charge from surface chemistry, energy, and topography and have examined osteoblast adhesion to titanium surfaces of varying surface charge. We have shown that a negative surface charge promotes osteoblast adhesion by approximately 60% over a neutral surface and that conversely, a positive surface charge inhibits osteoblast adhesion by about 20%. Continued examination of surface characteristics that control cellular responses are warranted with the eventual goal of applying those desirable surface characteristics to any structural biomaterial for bone implant or tissue engineering applications.

Ultra-thin films of polyaniline on Ag(110) were characterized using high-resolution electron energy loss spectroscopy (HREELS) in the vibrational and electronic loss regimes. The films were grown in situ by chemical vapor deposition using an emeraldine source. Initially, short polyaniline oligomers adsorb on the surface followed by polymerization to form longer polyaniline chains. The loss spectra in the far-infrared region indicate that the films become highly conducting upon protonation. In addition, the role of the substrate in the initial stages of polymer film growth is demonstrated by comparison with polyaniline on Cu and Au surfaces.

Amorphous carbon (a-C) films grow via energetic processes such as pulsed-laser deposition (PLD). The cold-cathode electron emission properties of a-C are promising for flat-panel display and vacuum microelectronics technologies. These ultrahard films consist of a mixture of 3-fold and 4-fold coordinated carbon atoms, resulting in an amorphous material with 'diamond-like' properties. We study the structures of a-C films grown at room temperature as a function of PLD energetics using x-ray reflectivity, Raman spectroscopy, high- resolution transmission electron microscopy, and Rutherford backscattering spectrometry. While an understanding of the electron emission mechanism in a-C films remains elusive, the onset of emission is typically preceded by 'conditioning' where the material is stressed by an applied electric field. To simulate conditioning assess its effect, we use the spatially-localized field and current of a scanning tunneling microscope tip. Scanning force microscopy shows that conditioning alters surface morphology and electronic structure. Spatially-resolved electron energy loss spectroscopy indicates that the predominant bonding configuration changes from predominantly 4-fold to 3-fold coordination.