M. Folman

Low temperature XPS studies of NO and N2O adsorption on Al(100)

Abstract The adsorption of NO and N 2 O on clean and oxygen saturated Al(100) single crystal was studied at low temperatures (down to 80 K) by means of XP spectroscopy. It has been shown that the adsorption of NO and N 2 O is dissociative as well as associative. In addition, upon adsorption of NO, formation of N 2 O(a) is observed; this being due to the catalytic activity of the metal substrate. The initial sticking probability coefficients of NO and N 2 O at 80 K are 1 and 0.0016, respectively. The negative charge transfer from the Al atom to the adsorbate molecules is 0.51 e − for N 2 O(a) and 0.54 e − for NO(a).

Predesorption of CO from the sodium chloride (100) surface: Study by the complex coordinate method

The rotational predesorption resonances of CO adsorbed on NaCl(100) were calculated by the quantum mechanical complex coordinate method, the CO bond length being varied adiabatically. It is shown that the internal vibrational excitation of adsorbed CO by an infrared laser does not have a strong effect on the resonance positions and lifetimes, i.e., predesorption states, and the desorption of CO from the NaCl(100) surface is probably a rotational predissociation phenomenon.

Deuterium adsorption-desorption from diamond (100) single crystal surfaces studied by TPD

Abstract Due to the crucial role of hydrogen in diamond CVD process the adsorption-desorption of hydrogen on different diamond surface planes has been studied thoroughly in recent years. Contradictory results and explanations have been reported in the literature regarding the mechanisms and dynamics of the above system. Deuterium adsorption-desorption from a Di(100) surface was investigated by TPD and LEED. It was found that hydrogen plasma treatment results in a well-defined reconstructed Di(100)-(2 × 1):H surface for which a high quality LEED pattern was observed at primary electron beam energy as low as 28 eV. Annealing of the diamond (100) hydrogenated surface at 1200 C results in hydrogen desorption. The hydrogen-free Di(100)-(2 × 1) surface displays charging effects under low energy electron bombardment. Activated low-coverage deuterium adsorption on the Di(100) surface was performed in situ. The results show that deuterium desorbs from a single adsorption site, α 1 , on the (100) diamond surface. Desorption from these sites obeys first order kinetics. Simulation of the TPD spectra yields K = 10 13 s −1 and E act = 88 kcal/mol for the deuterium desorption from well-defined surface. Repeated deuterium adsorption-thermal desorption cycles cause surface degradation which results in additional peaks in the TPD spectrum. A possible mechanism to the first order desorption kinetics is suggested.

Induced infrared absorption in H2, D2, and HD adsorbed on high surface area NaCl and Csl

Abstract Induced infrared absorptions in H2, HD, and D2 adsorbed on NaCl and CsI high surface area films were studied at 20°K. For H2 adsorbed on NaCl an absorption band was obtained at 4112 cm−1 whereas for H2, HD, and D2 adsorbed on CsI absorptions at 4132 cm−1, 3602 cm−1, 2961 cm−1 were found. In all cases the frequencies of induced absorptions were shifted to lower values as compared with gas phase frequencies known from Raman spectra. The isosteric heat of adsorption of H2 on NaCl was 1300 cal/mole as found from isotherms recorded at 77°K and 63°K. Adsorptions potentials were calculated for different adsorption sites and orientations of H2 on 100 surface plane of NaCl. It appeared that the relatively high heat of adsorption was obtained owing to a large contribution from quadrupole-field gradient interaction. Spectral shifts were calculated using the perturbation method and anharmonic wave functions of the adsorbate. As perturbation the adsorption potential was employed and was expanded as a power series of the vibration coordinate. Good agreement with the experimental Δv value was found.

A study of O2 adsorption on α-CuAl(100) surfaces of different Al concentrations by means of AES and XPS

Abstract The adsorption of O 2 on α-CuAl(100) for small concentrations of aluminum has been studied at room temperature by means of AES and XPS. A strong long range influence of aluminum on the adsorption processes as compared to pure Cu(100) has been found, the surface composition of the alloys has been studied and different aspects of adsorption processes have been addressed. The enhanced rate of oxygen adsorption on the alloy is explained on the basis of a long range influence of aluminum atoms on copper sites.

Thermal-programmed desorption (TPD) of deuterium from Di(111) surface: presence of two adsorption states

Abstract Deuterium desorption from diamond (111) surface was investigated by thermal-programmed desorption (TPD), electron energy loss spectroscopy (EELS) and low energy electron diffraction (LEED). Annealing of diamond (111) deuterated surface results in (2 × 2)/(2 × 1) reconstruction. Activated deuterium adsorption on Di(111) surface, pre-treated in MW hydrogen plasma, was performed in situ for doses in the 0.14–30 L range (1 L  10 −6 Torr × s). Within this dose-range the (2 × 2)/(2 × 1) reconstruction did not revert into the (1 × 1) structure. The deuterium coverage for the highest adsorption dose (30 L) was estimated as ∼0.5 ML. It was found that deuterium desorbs out of two distinct adsorption sites, β 1 and β 2 from the reconstructed (111) diamond surface. No thermal-induced redistribution of deuterium between the β 1 and β 2 adsorption sites was observed. Based on the TPD peak shapes and their dependence on coverage, a first order kinetics was assumed for the desorption process. The β 1 and β 2 adsorption states were attributed to monohydride (C–D) formation on Di(111) planes and diamond edges, respectively. The desorption kinetic parameters were calculated with the following results: β 1 (high temperature TPD peak): K = 5 × 10 12 s −1 , E act = 80 kcal mol −1 and β 2 (low temperature TPD peak): K = 5 × 10 12 s −1 ; E act = 67 kcal mol −1 .

Annealing of ion beam amorphized diamond surfaces studied by in situ electron spectroscopy

Abstract In the present work the effect of annealing an amorphized diamond (100) surface by 0.5 keV Ar irradiation was investigated in situ using electron spectroscopy. The recovery of the damaged layer to an sp2, graphitic-like, carbon network structure was traced by the effect of heat treatment in the range 300–1200 K on the C(KLL) line shape, electron energy loss (EEL) and secondary electron emission (SEE) spectra. The analysis suggests that the graphitization of the damaged layer is a thermally activated process. From a quantitative analysis of the EEL spectra measured as a function of annealing time at 600 K, the graphitization process of the damaged layer was found to follow a first-order kinetic process with activation energy of approximately 178 kJ mol−1.

Effects of deuterium adsorption-desorption on the state of diamond: surface degradation and stabilization of sp3 bonded carbon

Abstract Changes on the (100) diamond surface due to cyclic adsorption and desorption of deuterium were investigated by electron energy loss spectroscopy (EELS) and low energy electron diffraction (LEED) and the effects of surface degradation are discussed. After several deuterium adsorption-desorption thermal cycles it is determined that the diamond surface gradually degrades. This is reflected in the smearing of characteristic losses, the appearance of a loss peak at about 6 eV and a broad loss peak centered at 30 eV. The reconstructed surface, as observed by LEED, fades. It is suggested that this surface degradation results in an sp2 bonded carbon layer. EELS measurements taken as a function of primary energy show that the degradation is localized on the uppermost surface layers. Deuterium adsorption on the degraded surface results in a decrease in intensity of the 6 eV loss peak which is associated with conversion of sp2 bonded carbon to sp3 hybridization. Thermal desorption of deuterium from the degraded surface results in the reappearance of the 6 eV loss peak, suggesting reversible formation of sp2 bonded carbon on deuterium desorption. It is determined that exposure of a degraded diamond (100) surface to a microwave hydrogen plasma at 900 °C results in characteristic EELS features, and half-order LEED spots showing that this treatment leads to a (2 × 2) (2 × 1) reconstructed surface.

Kinetics of the initial stages of CVD diamond growth on non-diamond substrates: surface catalytic effects and homoepitaxy

Abstract Recently, we have reported that the density of chemical vapour deposition (CVD) diamond particles obtained on non-diamond substrates abraded with a diamond slurry can be enhanced by a few orders of magnitude by abrasion with a mixed metal/diamond slurry. In this work, we present growth kinetics studies of isolated diamond crystallites (prior to coalescence into a continuous film), which are formed at the initial stages of deposition on metal/diamond and diamond-pretreated silicon. The metals used were Ti, Fe and Cu. It has been found that the temperature dependence of the diamond growth rates on differently pretreated substrates can be described by an Arrhenius plot. From these plots, it has been concluded that at the initial stages of deposition the activation energy for CVD diamond formation, as observed for diamond-abraded substrates, can be substantially decreased as a result of substrate surface pretreatment with a mixed metal/diamond slurry. However, after coalescence of the crystallites into a continuous film, the activation energies for metal/diamond and diamond-pretreated substrates were found to be similar. Based on micro-Raman measurements it has been found that at the initial stages of deposition, the quality of diamond that was grown on the metal/diamond abraded substrates is higher than that deposited on the diamond-abraded ones. In contrast, after a long deposition time, the quality of diamond grown on the differently pretreated substrates was found to be similar. It is suggested that the observed effects are associated with the metals catalytic activity towards the hydrogenation of unsaturated hydrocarbons, namely enhancing the concentration of sp 3 -bonded hydrocarbon fragments. At the initial stage of deposition, before a stable substrate is formed, this enhancement leads to an increase in the diamond growth rate, lowering of the activation energy for its homoepitaxial growth on diamond residues left by pretreatment, and improvement of its quality.

AES and XPS studies of no adsorption on Al(100) single crystal

The adsorption of NO on Al(100) at room temperature was investigated by means of AES and XPS. Results were compared with those obtained on adsorption of O 2 on Al(100) and for nitrogen implanted Al. The adsorption was found to be dissociative. Three states in the adsorption process were distinguished: (1) dissociative chemisorption, (2) formation of an oxide and nitride layer and (3) saturation of the adsorbent surface. The thickness of the adlayer was estimated from AES line intensities. The chemical stability of the adlayer is considered and charge transfer to the oxygen and nitrogen adatoms is estimated.