Effect of Halo Substitution on the Geometry of Arenethiol Films on Cu(111)

Abstract

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) ...

The local adsorption geometry of benzenethiolate on Cu(100)

“…On Cu(110), it appears that the benzenethiolate may lie flat at low coverage but stands up at high coverages [12]. This same behavior may be inferred for Cu(111): an early photoemission investigation of a saturation coverage at room temperature concludes that the molecules stand up [9], while a very recent STM (scanning tunneling microscopy) investigation at 15 K shows clear evidence for a lying-down species at low coverage [10]. Indeed, this STM study indicates that at cryogenic temperatures the highest achievable coverage may correspond to a lying-down state, but it is probable that further adsorption to higher coverage is kinetically hindered at this low temperature.…”

“…Ultra-high vacuum surface-science studies of the interaction of benzenethiol with metal surfaces seem to be limited mainly to Cu ((111) [9,10], (110) [11,12], (410) [13], (100) [14]) and Au ((111) [15], (110) [16 ]) noble metal surfaces, although a number of investigations on the more reactive transition metal surfaces, including Ni(111) [17], 3 Ni(100) [18], Mo(110) [19,20], Rh(111) [21], are available in the literature, for which the motivation is more related to desulfurization catalysis. On all of these surfaces there is clear evidence for deprotonation of the thiol to produce the thiolate; on the transition metal surfaces this typically occurs even by adsorption at ~100 K, with further decomposition or reaction occurring below room temperature, but on the noble metal surfaces the thiolate appears to be stable at room temperature.…”

Bicomponent Supramolecular Architectures at the Vacuum–Solid Interface

“…149 The change of substituent modifies the electronegativity of the ending group, which is found to accommodate the intermolecular interaction via the molecular quadrupole moment.…”

Electrons, Photons, and Force: Quantitative Single-Molecule Measurements from Physics to Biology

“…Conversely, a 2D STM lattice measurement shows heterogeneous features but may or may not provide information about the locations of nuclei within the molecules, depending on molecular orientation. 55,57 …”