Lawrence H. Dubois

Molecular ordering of organosulfur compounds on Au(111) and Au(100): Adsorption from solution and in ultrahigh vacuum

Low‐energy electron diffraction and reflection‐absorption infrared spectroscopy were used to study the monolayers formed by the adsorption of n‐alkane thiols [HS(CH2)mCH3] on both (111) and (100) single‐crystal gold substrates. Samples were prepared by dosing either from solution (m=15, 17, 18, and 21) or in ultrahigh vacuum (m=0–9). On Au(111), ordered surface structures are obtained which can be indexed as (n√3×√3)R30°, where n varies from 1 to 6. On Au(100), the adsorption of short chain thiols leads to the formation of a c(2×2) overlayer while the longer chain homologs show additional diffraction spot splittings. It is also found that chain length influences both the character of the diffraction seen and perturbs the reactive sticking probability of molecules dosed in UHV. Infrared studies reveal that the polymethylene chains of the monolayers formed on Au(100) are comprised of nearly all‐trans conformations and are less canted than the comparable structures formed on Au(111). A simple model is propos...

Studies of the temperature-dependent phase behavior of long chain n-alkyl thiol monolayers on gold

The influences of temperature on the infrared spectra of a docosylthiol (C22) monolayer on a gold substrate are reported. The data suggest that this adsorbate exhibits a complex phase state at 300 K in which gauche conformations concentrated at the chain termina figure prominently. The detailed temperature dependence of the spectral band intensities is found to be very similar in many regards to that of bulk hydrocarbon crystals. At low temperature, band splittings are observed which suggest the lowest energy structure for this adsorbate is one containing two chains per unit cell. A simple lattice argument, consistent with steric models and low‐energy electron diffraction data, is presented to explain these observations.

Infrared Studies of the Surface and Gas Phase Reactions Leading to the Growth of Titanium Nitride Thin Films from Tetrakis(dimethylamido)titanium and Ammonia

The reaction of tetrakis(dimethylamido)titanium (Ti(NMe 2 ) 4 ) with ammonia has been studied in the gas phase and on titanium disilicide, aluminum, and copper surfaces using infrared spectroscopy. In the gas phase the main product of this reaction, dimethylamine, forms rapidly even at 300 K, and a fine yellow powder is deposited on the windows of the IR cell. Under ultrahigh vacuum conditions there is no reaction between Ti(NMe 2 ) 4 and NH 3 on any of the three surfaces studied at temperature-between 300 and 650 K

VIBRATIONAL ENERGY RELAXATION OF A POLYATOMIC ADSORBATE ON A METAL SURFACE: METHYL THIOLATE (CH3S) ON AG(111)

The lifetime of the first excited level of the symmetric C‐H stretching mode of methyl thiolate (CH3S) bonded to Ag(1 1 1) is measured by populating the level with a picosecond infrared pulse and probing the population by transient sum frequency generation spectroscopy. The population transient shows a biexponential decay across the experimental temperature range from 110 to 380 K. The fast decay component has a lifetime of 2.5–3 ps at all temperatures. The slow relaxation component lifetime varies from 55 ps at 380 K to 90 ps at 110 K. Neither relaxation component shows decay rates that are compatible with direct energy transfer to phonons or electron‐hole pairs of the metal substrate. Both relaxation components are instead assigned to intramolecular energy transfer to excited vibrational levels of other vibrational modes of the molecule.

Step effects in the thermal decomposition of methanol on Pt(111)

Abstract Methanol decomposition to adsorbed CO and hydrogen on Pt(111) was observed using time-resolved high resolution electron energy loss spectroscopy. No reaction was seen at temperatures ≲ 180 K, the molecular methanol desorption temperature. Between 180 and 200 K the decomposition reaction is controlled by the small amount of defects present on our carefully prepared sample. This was principally demonstrated by the fact that only 1–2% of a monolayer of CO was formed regardless of the initial methanol coverage. A steady state reaction run between 200 and 240 K showed only a low rate of CO formation (only a few percent of a monolayer even after many langmuir exposure of methanol). At a 300 K surface temperature, decomposition proceeds much faster, with about 10% of a monolayer of CO being formed. The origin of this temperature dependence is attributed to the ability of the product H and CO to diffuse away from sites where they were initially formed. Using a He seeded methanol beam, there was no apparent incident energy dependence for the methanol decomposition reaction.

Complex CO–potassium interactions on Cu(100): An electron energy loss, thermal desorption, and work function study

A detailed examination of the adsorption states of carbon monoxide on a potassium‐dosed Cu(100) surface is reported. High resolution electron energy loss spectroscopy (EELS) studies on surfaces with a variety of potassium precoverages indicate the presence of at least seven different C–O vibrational modes ranging in frequency from 900 to 2090 cm−1 with as many as five modes observed simultaneously. Thermal desorption spectroscopy (TDS) demonstrates at least five states of CO coadsorbed with potassium with desorption maxima occuring over a range from 120 to 600 K. Low resolution EELS and work function changes are discussed as well. The complexity of the high resolution EELS and TDS measurements precludes models involving only one or two types of potassium–CO interactions. In this system we suggest that potassium influences CO adsorption by, (1) indirectly increasing 2π* backbonding, (2) indirectly changing bonding sites, and (3) effecting a direct interaction of adsorbed CO with one, two, or possibly three...

Selectivity and Copper Chemical Vapor Deposition

The question of selectivity (on metal vs. insulator) in the growth of copper thin films from organometallic precursors is addressed by studying the adsorption of Cu(hfac)(vtms) (where hfac=1, 1,1,5,5,5-hexafluoroacetylacetonate and vtms=vinyltrimethylsilane) on high surface area silica using transmission infrared spectroscopy. No deposition is observed on dehydrated SiO 2 . In the presence of adsorbed, hydrogen bonded OH (OD) groups (from the dissociative adsorption of either water or ethanol-d 6 ), thin film growth is nucleated and selectivity is lost

Model studies of low temperature titanium nitride thin film growth

Abstract The reaction of tetrakis(dimethylamido)titanium [Ti(NMe2)4, tdmat] and ammonia to yield titanium nitride (TiN) thin films and gas phase dimethylamine (HNMe2) is discussed. Film growth is dominated by gas phase reactions which lead to the formation of large, oligomeric clusters of the form: These decompose on the surface of the growing film a temperatures ≳450 K. By isotopically labelling the ammonia with 15N or deuterium, we conclude that both the nitrogen in a clean TiN film and the nitrogen bound hydrogen (deuterium) in the gaseous dimethylamine product are derived exclusively from ammonia. Evidence for carbon contamination through the intermediacy of a TiNC metallocycle is provided. Our observations are compared with the known thermal chemistry of titanium dialkylamides in solution.

Vibrational spectra of atomic adsorbates: Carbon, oxygen, and sulfur on Rh (100)

High resolution electron energy loss spectroscopy (EELS), low‐energy electron diffraction (LEED), and Auger electron spectroscopy (AES) were used to study the adsorption of ordered carbon, oxygen, and sulfur overlayers on a Rh (100) single crystal surface at 300 K. Each adsorption system was characterized by a single vibrational frequency; νRh–C=535, νRh–O=403, and νRh–S=313 cm−1. The observed high resolution EELS spectra of all of these species is consistent with the bonding of adsorbed atoms in the highest coordination site on the Rh (100) surface. During the course of these experiments, modes corresponding to rhodium surface phonons (181, 272 cm−1) and to adsorbed carbon monoxide (bridged: 358, ∼1920; linear: 428, ∼2030 cm−1), boron oxides (740, 950, 1405 cm−1), and disulfur (585 cm−1) were identified. High resolution EELS is clearly a powerful tool for studying the latter two species since they have not been detected by other spectroscopic techniques.

Aluminum thin film growth by the thermal decomposition of triethylamine alane

Abstract Triethylamine alane (TEAA) decomposes on an Al(111) single crystal surface at temperatures above ∼310 K to yield pure aluminum thin films, liberating hydrogen and triethylamine into the gas phase. Aluminum deposition is epitaxial and clean (no carbon or nitrogen containing species are observed by Auger electron spectroscopy). The film growth rate is limited by the rate of the recombinative desorption of hydrogen from the surface. These results are compared to similar data from experiments on the surface-mediated thermal decomposition of trimethylamine alane.