6. Conclusions and Future Work

6.1. Conclusions

Helium atom reflectivity has been demonstrated to be a powerful surface analysis tool for the determination of the energetics and kinetics of adsorption. With the ability to quantify surface coverage with high-sensitivity during deposition, several experimentally challenging adsorption systems have been examined.

The physisorption of hydrocarbons on Au(111) has been used to develop experimental procedure and data analysis techniques for the interpretation of specular reflectivity data. For systems where Langmuir adsorption is a valid assumption, equations have been derived which enable determination of sticking coefficients to complement adsorption energy measurements.

For the physisorption of hydrocarbons or alkanethiols on Au(111), the adsorption energy has been shown to be proportional to bulk molecular properties (e.g. heat of vaporization) and to be predictable through the use of an additive model based on the composition and structure of the molecule. From the analysis of the adsorption energy of the linear hydrocarbons, the end atoms of the molecule appear to be responsible for a larger fraction (in proportion) of the adsorption energy than the atoms located at the center of the molecule.

While hydrocarbons are only capable of physisorption on Au(111), alkanethiols and dialkyldisulfides also chemisorb with an adsorption energy of 124 kJ/mol. This adsorption energy is independent of chain length for all 1-alkanethiols studied; however, it is reduced in the case of sterically hindered thiols (e.g. tert-butyl thiol). Chemisorption has been observed to proceed by a precursor-mediated process with an activation barrier that is also independent of chain length. However, the rate of chemisorption (and the pre-exponential factor) depends on the number of carbon atoms in the chain.

With the ability to generate large fluxes of vibrationally excited molecules, the activated adsorption of methane on Pt(111) and Ni(111) has also been explored in this thesis. Although an average of 10% of the methane flux was vibrationally excited in the 2₃ asymmetric stretching mode, the rate of adsorption on Pt(111) was observed to be unaffected by laser excitation. If the suggestions contained in the literature that the adsorption rate of methane on transition metal surfaces depends on vibrational excitation are correct, our results imply that the effect of the vibration is highly mode specific as the overtone of the asymmetric stretching mode used by this experiment may be less effective for the promotion of adsorption than, for instance, the bending modes.

6.2. Future Work

6.2.1. Physisorption Measurements

Although the UHV apparatus was primarily designed to perform adsorption experiments with laser-excited molecules, more basic adsorption problems can be studied with the high sensitivity and real-time capability of helium atom reflectivity. At present, most adsorption studies reported in the literature (where adsorbate populations are quantified by TPD, LEED, or AES after deposition) are insensitive to the presence of transient, disordered physisorbed molecules. While stable coverages of ordered physisorbed species can be readily prepared and observed, these molecules will desorb before detection by traditional techniques during adsorption experiments performed at higher surface temperatures. Since these physisorbed molecules potentially act as precursors for chemisorption by increasing the residence time of the molecule on the surface, a better understanding of the physisorption process and ordering behavior of physisorbed molecules is essential for complete characterization of the substrate/adsorbate interaction.

Several systems have been observed to chemisorb through a precursor-mediated process based on measurements of the dependence of the rate of chemisorption on surface temperature (e.g. O₂ on Pt(111), CO and CO₂ on Ni(100)^,, and N₂ on W(100)). Although the rate of chemisorption has been measured by TPD or AES, these systems have not been studied by helium atom reflectivity. With the ability to determine the population of physisorbed and chemisorbed molecules (Section 4.3.) as a function of flux and exposure, the effect of increased residence time and the cooperative presence of adjacent molecules can be identified.

Adsorbate/substrate systems with coverage-dependent adsorption behavior are particularly well-suited for helium atom reflectivity studies. In studies of O₂ on Pt(100), Guo et al. have demonstrated the ability of this technique to detect changes in ordering mechanism and island size which directly correlate with observed changes in the sticking coefficient. Previously studied with microcalorimetry, the adsorption system of NO on Ni(100) displays a gradual transition from molecular adsorption to dissociative adsorption at a coverage of 0.10­0.15 ML. This transition could be detected by helium atom scattering as an increase in the rate of specular decay due to the generation of two adsorbates for each dissociatively adsorbing molecule. By systematic variation of flux and surface temperature, the effect of ordering on the energetics of the adsorption process could be explored.

6.2.2. Collision-Induced Desorption (CID) of Self-Assembled Monolayers

In Chapter 4, temperature programmed desorption (TPD) measurements of the adsorption energies of SAMs formed from alkanethiols demonstrated that the activation energy for desorption of the physisorbed species could be increased above that of the chemisorbed species by increasing the chain length of the alkanethiol adsorbates beyond 14 carbon atoms. Although this behavior could be used to determine if the adsorbate species was an alkylthiolate or a dialkyldisulfide (by detection of a doubling of the adsorption energy due to dimerization), reactions of adsorbates during the temperature ramp of TPD would prevent measurement of the desorption of the equilibrium species.

By performing collision-induced desorption (CID) with high-energy noble gas atoms, it is believed that the surface species can be preserved during desorption. By using this technique, two sources of information will be available. First, the surface species will be desorbed intact for analysis by mass spectrometry. And second, the binding energy of the surface species can be measured by the determination of the threshold incident energy of probe gas. The combination of the two observations will be able to provide additional evidence to determine the presence of the sulfur-sulfur bond between adjacent thiolates.

Modifications to the existing apparatus to perform CID experiments would be relatively minor. A helium beam seeded with argon or xenon could be used to direct the probe atoms to the surface and to provide information about the surface coverage by helium atom reflectivity. A new nozzle would be required to significantly increase the available range of translational energies. Bolometric measurement of the energy of the beam would be able to determine the translational energy for each nozzle temperature and gas mixture. For detection of desorbing species, differential pumping of the mass spectrometer will also improve sensitivity.

6.2.3. Laser-Excitation Experiments: Alloy Surfaces

New surfaces for study have been generated by Holmblad, et al. by the deposition of gold on Ni(111) surfaces. Although bulk phase diagrams do not predict the formation of stable Au-Ni alloy phases, evaporative deposition of the gold followed by annealing at 773 K was able to generate a surface alloy of varying gold concentration. Below 0.3 ML, the surface structure remains unchanged, with gold substituting into the Ni(111) lattice. However, at higher levels of alloying, disruption of the lattice periodicity results in the formation of triangular superstructures with a unit cell which is determined by the concentration of gold atoms on the surface.

When the gold-nickel alloy surfaces were exposed to methane gas, the rate of adsorption decreased with increasing gold concentration at the surface. This behavior indicated that the presence of gold atoms was deactivating sites for adsorption of methane. A model which related the rate of adsorption to the number of active sites (nickel atoms which were surrounded by six nickel atoms) showed a good correlation.

Using the external resonant cavity system, the surface specificity of the chemisorption process can be investigated on custom alloy surfaces. In a study of the vibrational effects on the dissociative adsorption of hydrogen and metals, Darling and Holloway have shown that the choice of substrate is critical in determining the potential energy surface of the reaction. In quantum mechanical simulations, the dissociation of hydrogen on copper and iron surfaces was observed to proceed across a late barrier, while the dissociation on nickel and palladium occurred through an early barrier. Since the alloy composition of the surface may shift the location of the barrier to methane chemisorption, it may be possible to alter the efficacy of vibrational energy for the promotion of adsorption by systematic variation of the concentration and elemental choice of the doping. Additional rate effects due to coadsorption with electropositive (sodium, potassium) or electronegative (sulfur, oxygen) atoms could also be investigated. Unfortunately, since this series of alloys was unexpected, this new class of materials is just now being explored.

References