Time-resolved STM

The occurrence of both reaction pathways has been demonstrated in recent time-resolved STM investigations (Kim and Wintterlin, 2004), which also showed that the reaction takes place at random positions, because the O and CO diffusion barriers are comparable to the activation energy for the CO + O reaction. The most recent theoretical calculations support these findings fully they even suggest that the dominating reaction is between the adsorbed CO and O (Reuter et al., 2004). 

The estimated intrinsic timescale for tunnehng across the jnnction between the sample and the tip has been estimated to be of the order 10 fs (10 s) or less. It was suggested that, in order for the time-resolved STM to be usefnl, it mnst be possible to relate the shape of the time-resolved current to the time dependence of the nnderlying processes. 

Figure 23 Time-resolved STM images of the ID chains formed by the ligand shown in (d) and C0CI2. The white arrows indicate a given ID network that disassembles over time. (Reproduced from Ref. 85. Wiley-VCH, 2007.)...

A high concentration and mobility of vacancies in the Cu(lOO) surface at room temperature was confirmed by time-resolved STM studies. Surface alloys involving small amounts of In [19, 20] or Co [21] allow one to make use of the guest metal atoms as tracers. Quantitative evaluation of the frequencies of their various short- and long-distance jumps confirmed vacancy diffusion as lateral transport mechanism (Figure 12.3) [19-21]. 

To elucidate the mechanism of metal UPD, a number of studies on Cu and Ag UPD have been performed over the years comprising phenomenological, CV-based studies and microscopic investigations with STM both ex situ and in situ [38,39, 42, 43, 202, 204, 210-214]. While some features, such as the formation of UPD islands, were commonly reported for various systems (different thiols and metals, that is, Ag and Cu) differing interpretations were given with respect to the details such as formation, extension or height, possibly due to the sometimes difficult interpretation of data that, furthermore, can vary with the details of the system and the experimental conditions applied. Some of the issues could be resolved in a recent study on high-quality aromatic SAMs where the UPD process could be extremely slowed down to allow time-resolved in-situ studies [43]. 

A procedure has been described to perform/fli t time-resolved experiments with STM." As already seen, STM is the only method that can be carried ont based on localized quantum mechanical tunnehng of electrons between the sample and the tip. This procedure offers an observed resolntion of the molecules in the three-dimensional domain. The fonrth dimension has been snggested to be also possible by this method, which relates to the atomic timescale. 

Summary. Time-resolved, atomic-scale, in-situ STM studies of phase formation at metal electrode surfaces are described. Examples include structural phase transitions within the electrode surface layer and in anionic or metallic adsorbate layers as well as metal deposition and dissolution processes. 

In this paper recent results of our in-situ STM studies on the structure of bare and adsorbate-covered electrode surfaces are summarized. In particular, we discuss transitions between different phases on these surfaces, which often proceed via nucleation and growth processes. This includes structural transitions in the electrode surface layer, phase transitions in adsorbate layers, electrodeposition processes, and dynamical fluctuations at the metal-electrolyte interfiice under equilibrium. We show that in-situ STM provides a valuable tool for time-resolved, atomic-scale studies of such processes. For experimental details and for in-depth discussions the reader is referred to the original literature. 

Time-resolved in situ STM experiments [360] performed in the so-called x-t mode (one scanline is recorded as a function of time) revealed first structural details of these phase formation processes. Figure 18 indicates that... 

Kinetic Aspects The kinetics of 2D phase formation and dissolution of organic adlayers were mostly studied by i—t, q —t or C-t single or multiple potential step experiments, and analyzed on the basis of macroscopic models according to strategies described in Chapter 3.3.3. Only rather recently, modern in situ techniques such as STM [20, 201, 453, 478, 479, 484, 487, 488] and time-resolved infrared spectroscopy (SEIRAS) [475,476] were applied to study structural aspects of these phase transitions at a molecular or atomistic level. 

Single and multiple potential step experiments demonstrated that the macrokinetics of the formation of the phy-sisorbed uracil film represents a first-order phase transition and follows the exponential law of nucleation (cf. Eq. (34)) in combination with surface diffusion-controlled growth [183]. In situ STM [20, 478, 479] and time-resolved SEIRAS studies [475] suggest that these processes are strongly related to the formation/breaking of uracil-water and water-water hydrogen bonds within the Helmholtz region. 

Pioneering approaches were developed, on the basis of time-resolved in situ STM, SEIRAS, and SXS-studies, to explore structural details of dynamic processes involved in phase transitions in adlayers and substrate surfaces, and to relate these results to macroldnetic models of nucleation and growth. 

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