J. Biener

H atom impact induced chemical erosion reaction at C:H film surfaces

Abstract C:H film surfaces which are subjected to a flux of thermal H atoms erode chemically via hydrocarbon, probably methyl, production. At the present H flux the erosion reaction is effective above 400 K and below 700 K, with a maximum around 600 K. The erosion efficiency at this temperature is ≈ 0.01 C atom per incoming H. A kinetic analysis of the erosion reaction and competing hydrogenation and dehydrogenation surface reactions under impact ofH reveals an activation energy of ≈37 kcal mol for the H atom impact induced erosion. As the efficiency of the erosion reaction depends on the incoming H flux, it may contribute as an important reaction in low-pressure diamond synthesis.

A surface reaction with atoms: Hydrogenation of sp‐ and sp2‐hybridized carbon by thermal H(D) atoms

Hydrogen containing carbon (C:H) films have been prepared by ion beam deposition of 160 eV ethane ions on a carrier consisting of a Pt(111) single crystal surface covered with a monolayer of graphite. Several monolayers thick films deposited at 350 K contain hydrogen bound to sp, sp2, and sp3 hybridized carbon. Vibrational spectroscopy reveals that thermal D(H) atoms directed to the C:H film surface hydrogenate unsaturated CH groups at the surface and transform sp and sp2 hybridized carbon to sp3. The observed reaction mechanism explains the microscopic processes in chemical erosion of graphite by hydrogen atoms which is a crucial reaction step in the low pressure synthesis of diamondlike carbon.

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...

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.

Spectroscopic identification of CH species in C:H films using HREELS

High resolution electron energy loss spectroscopy (HREELS) was used to analyze the vibrational spectra of ethane ion beam deposited C:H films. The spectral features observed in the CH stretching mode region around 3000 cm−1 give a clear indication of H bonding to sp, aromatic sp2, and sp3 hybridized carbon. In the low frequency region below 1700 cm−1, C=C stretching, aromatic C=C stretching, CC skeleton, and CH bending modes are observed. The development of spectral characteristics upon thermal treatment of the films in temperature range 600 to 1200 K suggests a stability sequence CHn(sp2, aromatic) > CHn(sp3 > CH(sp). The temperature dependent evolution of hydrogen and hydrocarbon species from thermally treated C:H films is a consequence of the stability regimes of CHn groups in the films.

D(H) atom impact induced Eley-Rideal hydrogen abstraction reaction towards HD at fully hydrogenated C:H(D) film surfaces

Abstract Ultrathin C:H(D) films were produced by ion beam deposition at a suitable target and fully hydrogenated at the surface by H(D) atoms. Vibrational spectroscopy revealed that thermal D(H) atoms impinging at these fully hydrogenated surfaces initiate a hydrogen recombination reaction towards HD thereby dehydrogenating the C:H(D) film surface. The reaction proceeds via an Eley-Rideal mechanism with an apparent cross section of 0.05 A2. An isotope effect is absent in line with this mechanism. The present investigation, for the first time, directly identifies the hydrogen abstraction reaction instrumental in low-pressure diamond synthesis and related processes.

H/D exchange reaction at graphitic CH groups by thermal H(D) atoms

Abstract Ultrathin C:H films were produced by hydrocarbon ion-beam deposition at Pt(100) single crystal surfaces covered with a monolayer of graphite. Temperature and thermal H(D) impact induced effects were studied with HREELS and AES. C:H film surfaces which exhibit only aromatic sp 2 CH groups, HCCH, get deuterated at a temperature independent rate with an apparent cross section of about 1.3 A 2 . After sufficient D exposure at 350 K eventually only sp 3 CHD groups cover the surface as deduced from the CH(D) stretch mode structure in HREEL spectra. At 600 K the radicalic intermediate surface HDCCH· groups generated by D addition to sp 2 groups can split off H or D through thermal activation, thereby transforming the group back to sp 2 . Thereafter in HREEL spectra both, sp 2 and sp 3 contributions were measured in the CH(D) stretch frequency regions. Due to a kinetic isotope effect the surface H is preferentially removed by this reaction. C:H film surfaces under impact of H atoms erode chemically via thermally activated release of hydrocarbon species. As expected, an isotope effect was also observed in the erosion reaction proceeding from the radicalic intermediate.

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.

INTERACTION OF D(H) ATOMS WITH PHYSISORBED BENZENE AND (1,4)-DIMETHYL-CYCLOHEXANE : HYDROGENATION AND H ABSTRACTION

Benzene and (1,4)‐dimethyl‐cyclohexane monolayers were physisorbed on graphite covered Pt(111) surfaces. Exposure of benzene monolayers at 125 K to D atoms (1700 K) initially hydrogenates sp2 hybridized C atoms with a cross section of ca. 8 A2 producing C6H6D intermediates. Additional D atom reactions either transform this intermediate via a second hydrogenation reaction to cyclohexadiene‐d2, C6H6D2, or restore benzene, C6H5D, via H abstraction. Once the aromaticity is broken, successive hydrogenation of the diene occurs rapidly generating the saturated cyclohexane‐d6, C6H6D6. The C6H5D reaction product can undergo further H/D exchange reactions and, at any level of deuteration, the benzene species might get hydrogenated. Monolayers of the saturated hydrocarbon (1,4)‐dimethyl‐cyclohexane (DMCH) that are exposed to D atoms produce deuterated DMCH via successive abstraction/hydrogenation reactions. Thermal desorption mass spectra revealed that H atoms at the ring were exchanged with an apparent cross sectio...