B. Winter

Growth and thermal decomposition of ultrathin ion‐beam deposited C:H films

Several monolayers thick hydrogenated carbon films, C:H, were prepared by ion beam deposition from hydrocarbon process gases onto Pt and monolayer C covered Pt single crystal surfaces and investigated with Auger electron and thermal desorption spectroscopies in an UHV environment. Efficient deposition was achieved at ion energies in the 160–300 eV range. The deposited thickness and H/C ratio of the films depend on both, target temperature and H/C ratio of the process gas. It is shown that the C monolayer is crucial for efficient on‐top deposition. Irrespective of the process gas used for deposition, the films grow as a C network and assume a constant H/C ratio at thicknesses greater than ∼ 3 monolayers. The H/C ratio of the films scale with the H content of the hydrocarbon process gas, a H/C ratio of 0.4 was obtained for ethane at 350 K substrate temperature. Upon thermally activated decomposition the films release molecular hydrogen as the major gaseous species and various hydrocarbons as minority specie...

Radiation induced degradation and surface charging of organic thin films in ultraviolet photoemission spectroscopy

Abstract The effect of vacuum ultraviolet radiation on the valence electronic structure of the electroluminescent organic materials p-sexiphenyl (6P) and tris-(8-hydroxy quinoline) aluminum (Alq 3 ) was investigated by ultraviolet photoemission spectroscopy (UPS). The intense radiation of an undulator at the storage ring BESSY II (Berlin) caused a loss of conjugation in 6P, evidenced by a decrease in intensity of delocalized π-orbitals in the UPS spectra. Depending on the degree of film degradation, surface charging was observed for both materials. It is shown that by illuminating the sample with laser light that can be absorbed by the organic/metal substrate system the surface charging could be compensated. Thus, the generation of free charge carriers by optical means appears to be a useful substitute for the use of an electron flood-gun in photoemission experiments, whenever sensitive samples could suffer from irradiation with electrons.

Mechanism of chemical erosion of sputter‐deposited C:H films

The mechanism of thermally activated chemical erosion of sputter‐deposited C:H films of a few atomic layer thickness is investigated using thermal desorption spectroscopy. Methane, CH3 radicals, and various C2Hj species of molecular and radical nature desorb as gaseous products above 600 K competitively to H2. C‐CiHj bond breaking is determined to be the rate limiting step of hydrocarbon production. The reaction is of first order with respect to CiHj precursors in the films with a distribution of activation energies, 56±5 kcal/mol for methane production. CH3 radical desorption occurs predominantly from the very surface of the C:H films.

Hydrogenation of amorphous C : H surfaces by thermal H (D) atoms

C : H films of several monolayers thickness have been prepared by ion beam deposition of ethane ions of 160 eV kinetic energy at Pt surfaces covered with a monolayer of graphite. These C : H films typically exhibit a HC ratio of 0.5 and contain about equal amounts of carbon atoms in the sp2 (graphitic) and sp3 (diamond) hybridization states. The hybridization states of surface CHx groups have been characterized by means of vibrational spectroscopy (HREELS). The spectra revealed the presence of aliphatic sp3-CHx (x = 1, 2, 3), graphitic sp2-CH and terminal sp-CH groups at the surface of C : H films deposited at 350 K. Thermal treatment of the films successively destroys sp (T≈500 K), sp3 (T≈850 K), and sp2 (T = 1150 K) related CH groups, which is paralleled by the evolution of hydrogen and hydrocarbon species from the film. Exposing these films to thermal H (D) atoms at 350 K causes hydrogenation of sp and sp2 hybridized CH groups resulting in sp3-CHx group formation. The same phenomena are observed at surfaces of C : D films deposited from fully deuterated ethane. The results provide a microscopic description of the primary reaction steps of graphite erosion by H atoms and allow for an understanding of rate determining steps in low pressure chemical vapor deposition of diamond or diamond-like carbon.

The origin of reduced chemical erosion of graphite based materials induced by boron doping

Abstract The effect of boron doping on the chemical bonding in ultrathin amorphous hydrogenated carbon (C:H) films was investigated with surface sensitive spectroscopies. Undoped C:H films contain about equal amounts of carbon atoms in sp 3 and sp 2 hybridization states. Boron doping suppresses the formation of sp 2 C centers, thereby reducing the films graphitic constituents. Therefore, with increasing boron concentration, film exhibit enhanced capacity for H as compared to undoped films. At heated films the fraction of chemical erosion species with respect to released molecular hydrogen is reduced in B doped films. Due to the specific C/H chemistry in sp 3 dominated networks, chemical erosion and hydrogen release of heated B doped films occurs at lower temperature and in a narrower temperature range than at undoped films. The results at the model films are in excellent agreement with those reported for bulk boronized graphites and explain at a microscopic level B doping effects on erosion.

Anisotropy in ordered sexithiophene thin films studied by angle-resolved photoemission using combined laser and synchrotron radiation

We present angle-resolved photoemission (PE) spectra of ordered multilayer sexithiophene (6T) films, 200 nm thick, grown on a Au(110) single crystal. However, the measurement of sharp and nonshifted PE spectral features from the low-conducting organic material is only possible if the positive surface charge, generated in the PE process, is fully compensated. We have accomplished this by simultaneous laser irradiation. On the basis of the resulting data we found that for these thick films the 6T molecules are preferentially oriented with their long axes nearly normal to the surface.

Elementary steps of the interaction of C: H film surfaces with thermal H/D atoms

Abstract The interaction of thermal H and D atoms with thin ion-beam deposited C: H films were studied with vibrational (HREELS) and Auger electron spectroscopy. At surfaces which exhibit aromatic sp2 CH groups a hydrogenation reaction towards sp3 is observed with a cross-section of 4.5 A2. At fully hydrogenated C: H surfaces with solely sp3 CH groups present, H abstraction towards molecular HD occurs at D impact with an initial cross-section of 0.05 A2. The magnitude of both cross-sections is in line with an Eley-Rideal mechanism. As a radical C atom is produced by the H abstraction reaction, a thermally activated de-excitation via a hydrocarbon radical split-off is initiated which causes chemical erosion of the C: H film at a yield of about 0.01 C per incoming H around 600 K.

HREEL spectroscopy of thin ion beam deposited C:H(D) films

Abstract Several monolayers thick C:H films, H/C ratio ca 0.5, have been prepared by ion beam deposition of hydrocarbons at a Pt(111)/C carrier. HREEL spectra of films deposited at 350 K exhibit a rich structure in the Cue5f8H stretch region around 3000 cm−1 and fingerprint region below 1600 cm−1. The spectra confirm the presence of CHx groups at carbon in sp, sp2, and sp3 hybridization states in a graphitic network. Vibrational spectra of deuterated films exhibit a normal isotope shift. The nature of the deposited ion: ethane, ethylene, acetylene, benzene, has a negligible effect on the vibrational structure of the C:H films. Annealing of the films up to 1250 K successively destroys sp, sp3, and sp2 carbon related CH groups. Although the films are not crystalline, the angular distribution of the elastically scattered electrons is considerably peaked in the specular direction. The primary energy dependence of the ν(Cue5f8H) vibration losses exhibits a resonance at 5 eV which is considered as due to a marked impact mechanism contribution to the respective spectral features.

Investigation of azobenzene side group orientation in polymer surface relief gratings by means of photoelectron spectroscopy

The molecular orientation of azobenzene side groups in polymer films before (nonpatterned) and after (patterned) development of a surface relief grating has been investigated by photoelectron spectroscopy using synchrotron radiation. The photoemission spectra obtained for 60–100 eV photons of a patterned and a nonpatterned surface are similar when the polarization vector of the synchrotron light is parallel to the grating vector. However, for perpendicular excitation, considerable spectral intensity differences can be observed for 9–14 eV electron binding energy. The observed changes are attributed to the formation of well-oriented azobenzenes at the surface.

The influence of boron doping on the structure and thermal decomposition of ultrathin C/B:H films

Few monolayers thick hydrogenated carbon films doped with boron (C/B:H) were prepared and investigated in an ultrahigh‐vacuum environment by high‐resolution electron energy loss, Auger electron, electron energy loss, and thermal desorption/decomposition spectroscopies with specific emphasis on their chemical erosion behavior as compared to their undoped C:H counterparts. Films of thicknesses ranging from 1 to about 10 monolayers, with a maximum B/C ratio of 0.5, were grown by ion‐beam deposition at room temperature on a carrier consisting of a Pt(100) single‐crystal surface covered with a graphite monolayer. The process gas used was a mixture of ethane and trimethylboron of varied compositions. While at zero boron concentration the films exhibit a graphiticlike structure with about equal amounts of carbon atoms in the sp2 and sp3 hybridization state, with increasing boron concentration the film structure becomes increasingly sp3 dominated. This is evidenced by decreasing HREELS loss intensities of the vib...