Thiolate‐Induced Metal Adatom Trapping at Solid–Liquid Interfaces

Abstract

Organosulfur compounds, in particular thiolates, are one of the most important classes of surface-active species and have very diverse applications. On the one hand, they are extensively employed in molecular self-assembly, for which they may be considered as the archetypal system, with wide ranging applications in nanoscience (for example molecular electronics, immobilization of biomolecules, and stabilization of nanoparticles). [1][2][3] On the other hand, organosulfur species have been empirically developed and used as additives for a long time in various areas of chemical engineering, including corrosion inhibition and modern galvanic plating, such as the damascene plating process used for the formation of copper interconnects on ultra large-scale integrated microchips. [4] While the structure of ordered self-assembled thiolate monolayers has been studied in great detail, [1][2][3] much less is known on thiolate adsorbates at low surface coverage, where these species are highly mobile on the metal surface at room temperature. Clarifying the dynamic behavior of thiolates in the low-coverage regime is of great importance for understanding the elementary mechanisms of both molecular selfassembly as well as their influence on the surface chemistry in additive applications, where the surface coverages likewise are often well below saturation densities. Atomic-scale studies of thiolates at low coverage, performed in ultrahigh vacuum (UHV) at cryogenic temperatures, revealed a much more complex behavior than previously anticipated, involving pronounced interactions with metal adatoms. [5][6][7] Herein,we present studies on the surface dynamics at solid-liquid interfaces and room temperature; that is, under conditions typically employed in applications, for the most simple organosulfur adsorbate, methyl thiolate, on Cu(100) electrode surfaces in 0.01m HCl solution. The use of this system not only is interesting in view of the importance of thiol-bound species in an electrochemical environment (for example in copper electroplating), but also allows the dynamic behavior to be influenced by the applied potential, thus providing additional information on the molecular mechanisms. As will be shown by our quantitative in situ high-speed scanning tunneling microscopy (video-STM) studies, interactions with metal adatoms also play a significant role under these conditions, which may have important implications for the surface chemistry of these species.In the studied potential range, the chloride coadsorbate forms a well-ordered square c(22) lattice on the Cu(100) electrode surface (lattice spacing a 0 = 3.6 ), which is clearly visible in high-resolution STM images. [8,9] Upon addition of dimethyl disulfide to the solution, distinct isolated adsorbates gradually emerge on the surface, which occupy sites of the c(22) lattice (Figure 1 a). Based on their small size and their similar appearance in the STM images as adsorbed sulfide, [10,11] these species are identified as methyl thiolate adsorbates (CH 3 S ad ), formed by dissoc...

Thiolate-induced surface reconstruction of Cu(1 0 0) electrodes studied by in situ Video-STM

“…This is similar to our previous data for CH 3 S ad adsorbed on the Cu(1 0 0) surface, where repulsive CH 3 S ad -CH 3 S ad interactions of 22 to 33 meV were measured [18,19]. The arrangement in alternating CH 3 S sp -Cu chains suggests effective attractive interactions, but in view of the complex interface structure with different species in the metal surface and the adsorbate plane the origin of these interactions is difficult to access.…”

“…At more positive potentials the STM studies revealed a coexistence of those adlayers with the (kinetically slowly formed) Cu reconstruction. This potential dependent phase behavior is very different from that observed in our extensive Video-STM studies of adsorbed methyl thiolate on Cu(1 0 0) in the potential range À0.40 to À0.20 V. Here, no indication for surface reconstruction was found, but the Cu surface was covered by a mixed CH 3 S ad /Cl ad adlayer with c(2 Â 2) structure or, at higher CH 3-S ad coverages, a coexistence of this c(2 Â 2) phase with islands of a c(6 Â 2) CH 3 S ad adlayer [18,19]. Formation of the Cu reconstruction exclusively occurred at potentials of À0.6 V or lower, with the speed of the reconstruction process strongly increasing towards more negative potentials.…”

“…After that, 1-7 ll of a 7 lM aqueous solution of dimethyl disulfide (Sigma-Aldrich, 99% purity) were added to the STM cell (total volume %1 ml), followed by a waiting time of at least 30 min. As shown in our previous publications [18,19], this procedure results in the dissociative formation of irreversibly adsorbed methyl thiolate with average (potential-independent) surface coverages between 2% and 20%, depending on the added amount. Direct STM observations of the adsorbates verify that the thiolate coverage does not change over the experiment.…”

“…Adsorbate-induced reconstruction involving massive surface restructuring has been found by high resolution structure-sensitive methods, in particular scanning tunneling microscopy (STM), for a large number of adsorbate-substrate systems, both under ultrahigh vacuum (UHV) conditions and in electrochemical environment [1][2][3][4]. An important and extensively studied class of such systems is sulfur and sulfur-containing molecular adsorbates on noble metal single crystal surfaces, which exhibit very unusual and complex surface reconstructions [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19].…”

“…We have recently reported a detailed study of methyl thiolate dynamics on Cu(1 0 0) electrodes in HCl solution using in situ STM with high temporal resolution (electrochemical Video-STM) [18,19]. By employing this technique detailed data on the adsorbate dynamics and phase formation were obtained.…”