The adsorption of benzene, naphthalene, and anthracene on Cu(111) and its vicinal Cu(221) and Cu(443) surfaces has been studied by means of thermal desorption spectra (TDS) and low energy electron diffraction. For each of the studied acenes (C4n+2H2n+4, n=1,2,3) two distinct binding states were observed in the TDS corresponding to adsorption on terraces and at step edges. The binding energies of both states are found to increase linearly with the number of aromatic rings n. In contrast to the Cu(111) surface where these acenes form disordered adlayers only, ordered films of naphthalene and anthracene with an alignment of the molecular axis along the steps could be prepared on the vicinal surfaces.
The adsorption of alkanethiols [CH3(CH2) n - 1SH] of various chain lengths (n = 1, 2, 4, 7, 10) on a Cu(100) surface has been studied by means of low-energy electron diffraction, He atom scattering , thermal desorption spectroscopy, and X-ray photoelectron spectroscopy. Highly ordered thiolate films were prepared by vapor-phase deposition which formed, at room temperature, an intermediate (2 × 2) phase and, upon further exposure, a c(6 × 2) saturation structure. An extended XPS analysis unveiled that, in contrast to the intermediate phase, the saturation phase is metastable and decomposes upon partial desorption of alkyl chains into a previously unobserved thermodynamically stable mixture of thiolate and sulfide species under retention of its c(6 × 2) structure. As a consequence, the density of the alkyl chains is significantly reduced, demonstrating that such alkanethiolate monolayers on copper are not as densely packed as in the case of gold.
The adsorption kinetics of heptanethiol [CH3(CH2)6SH] on Cu(110) and the ordering of monolayer films prepared by ultrahigh vacuum vapor deposition have been studied by means of thermal-desorption spectroscopy, X-ray photoelectron spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy. Below 200 K, heptanethiol is found to grow in multilayers for which a molecular binding energy of 63 kJ/mol was determined. The formation of the chemisorbed monolayer takes place via a physisorbed precursor. Heating the chemisorbed layer leads to a dissociative desorption of the alkyl chain with an enthalpy of 117 kJ/mol. In contrast to the rather disordered physisorbed overlayers, several superstructures for the various coverages have been observed for the chemisorbed phase. Heating the saturated monolayer to 350 K results in a well-ordered ( ) structure.
The adsorption of dodecanethiol [CH3(CH2)11SH] films on Cu(110) by vapor deposition under ultrahigh vacuum conditions has been studied by means of thermal desorption spectroscopy, scanning tunneling microscopy, X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction with a special emphasis on the structural changes accompanying the transition from a physisorbed monolayer to a chemisorbed saturation structure. Adsorption at 110 K leads to the formation of an ordered physisorbed layer with flat-lying thiol molecules. Upon room-temperature deposition, initially an ordered pinstripe phase is formed which may be a molecular double layer. This layer transforms with time into a stable saturation structure of upright-tilted thiolates in a local c(2 × 2) arrangement that exhibits a long-range c(12 × 16) modulation, attributed to a Moiré pattern. The XPS measurements show that the roomtemperature saturation structure contains a fraction of sulfide species formed by partial decomposition and desorption of alkyl chains. At 400 K, the thiolate monolayer desorbs dissociatively, eventually resulting in a p(5 × 2) sulfur structure.
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