Thiol films on noble metal surfaces attract considerable interest due to their ability of facile self-assembly from the solution phase. 1 Films of only monomolecular thickness can modify the electronic, physical, and chemical properties of the underlying substrate dramatically. This offers powerful opportunities for fundamental studies of electron transport, 2 single-molecule devices (e.g., tunneling diodes 3 or transistor 4 ), control of surface wettability, 5 etc. The formation of thiol films is driven predominantly by strong substrate-sulfur interactions. At saturation coverage, the result is a layer of molecules that stand close to upright on the surface. For alkanethiols on Au(111), ( 3 x 3)R30°and related superlattices were inferred. [6][7][8] In a solution environment, it is difficult to follow the initial stages of thiol chemisorption because of their high surface mobility prior to formation of a dense film and the presence of the surrounding solution. Vacuum deposition of thiols allows the study of low coverages, and a large variety of different alkanethiols patterns have been reported. [9][10][11][12] To the best knowledge of the authors, low coverages of arenethiols have not been addressed so far, although arenethiols have much larger potential for electronic applications than alkanethiols. This study uses thiophenol (TP) and, and pentafluoro-substituted (5FTP) analogues as model compounds for arenethiol film formation and explores the impact of a slight variation of arenethiol size and substituent electronegativity (EN) on the films' structural properties. We studied a broad range of coverages and found the most dramatic effects at incomplete films, where the molecules aggregate into isolated islands that are separated by empty terraces.We used two home-built STM systems that were operated in UHV (<10 -10 Torr) at cryogenic temperatures (15 or 81 K). Multiple cryopanels enclosed the STMs in order to minimize drift and sample contamination. We used a Cu(111) single crystal as a substrate. Sample preparation involved cycles of sputtering (Ar, 1.5 keV) and annealing (600 K). All arenethiol coverages were prepared by backfilling the chamber to a pressure of ∼10 -9 Torr and (if necessary, multiple cycles of) sample exposure for ∼15 s.We observed spontaneous formation of the superlattices in Figure 1 at 81K, which suggests that they may form transiently during the deposition of larger coverages. Figure 1a,b shows STM images of isolated CTP and TP molecules at 15 K after hydrogen abstraction. The molecules adsorb flat on the surface, and their image has a depression that we associate with the position of the thiol group. Using lateral manipulation 13 and coadsorption of CO for registry, 14 we find that both the S and the halogen atoms occupy Cu(111) hollow sites. The overall shape of FTP, 5FTP, and BTP is similar to Figure 1a,b and their size scales with the dimension of the substituent(s). Figure 1c-f shows a clean Cu(111) surface and islands of the parasubstituted molecules. Fourier transformation (FT) ...

We present data on the coverage and nearest-neighbor dependences of the diffusion of CO on Cu(111) by time-lapsed scanning tunneling microscope (STM) imaging. Most notable is a maximum in diffusivity of CO at a local coverage of one molecule per 20 substrate atoms and a repulsion between CO molecules upon approach closer than three adsites, which in combination with a less pronounced increase in potential energy at the diffusion transition state, leads to rapid diffusion of CO molecules around one another. We propose a new method of evaluating STM-based diffusion data that provides all parameters necessary for the modeling of the dynamics of an adsorbate population.

We investigated the adsorption of 2,5-di-chloro-thio-phenol ͑DCTP͒ on Cu͑111͒ at 15 K and the formation of the thiolate upon electronic and thermal excitation. Initially, the sulfur atom of DCTP adsorbs at an on-top site and the molecule is able to rotate through six almost identical surface orientations. Attachment or removal of electrons from anywhere within the molecule at several hundred mV bias leads to the abstraction of the hydrogen atom from the thiol group in a nonthermal one-electron process with perfect selectivity. The resultant thiolate is locked into position on the surface.

A systematic study of the dehydrogenation of substituted thiophenols by controlled charge injection from the tip of a scanning tunneling microscope (STM) reveals a pronounced dependence of the reaction yield on the position and the chemical nature of the substituent. We evaluate the dehydrogenation rate of para-halosubstituted species within a linear free energy relationship, namely the Hammett equation. The resultant value of 1.4 can faithfully predict the reaction rates of molecules that are meta-halo-substituted or para-methyl-substituted. The positive sign of suggests a negatively charged transition state at the core of the STM-induced process, and the magnitude of the value indicates that the presence of the substrate does not preclude substantial substituent effects. The applicability of the Hammett equation to singlemolecule chemistry offers facile prediction of the rate of STMbased single-molecule chemistry in a field, which so far has been addressed by focusing on involved quantum-mechanical modeling of its underlying processes.Hammett equation ͉ scanning tunneling microscopy ͉ single-molecule chemistry T he groundbreaking work of the groups of Eigler (1), Avouris (2), Gimzewski (3), Weiss (4), Rieder (5), and, most recently, Ho (6), has converted the scanning tunneling microscope (STM) from an imaging instrument (7) to a versatile tool capable (i) of control of chemical reactions (8) and (ii) of spectroscopic identification of their reactants and products (9). With the availability of commercial instrumentation for these procedures (10), the need for facile prediction of STM-based singlemolecule chemistry becomes imminent. In conventional organic chemistry, linear free energy relationships such as the Hammett equation † have proven to be powerful tools for the prediction of the reactivity of a broad range of compounds (13-17). Here, we present the transfer of the Hammett equation to the realm where individual molecules are excited one at a time to form specific chemical bonds.The Hammett equation (Eq. 1) predicts the impact of the substituent X on the rate (or equilibrium) constant, k, of the reaction Z 3 ZЈ, where Z is a reactive group attached to the same aromatic ring as X.The value describes how a specific substituent affects the acidity of benzoic acid in water (i.e., Z ϭ COOH, ZЈ ϭ COO Ϫ ). These values can be found tabulated in the literature; for the purpose of this manuscript, they are assumed to be universal constants. The value indicates how susceptible a reaction Z 3 ZЈ is to substitution at the aromatic ring. It compares Z's susceptibility to the susceptibility of benzoic acid in water, which has by definition a value of 1. Because of its simplicity and strong predictive power, the Hammett equation found applications all over organic chemistry, and their number is still growing.We explore the validity of a linear free energy relationship similar to the Hammett equation at the realm of singlemolecule chemistry by a systematic study of thiophenols (TP) rather than benzoic acids, i.e., Z ϭ SH...

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