The STM Experiment

The initial stages of the STM experiment require the positioning of the tip in proximity of the surface such that a tunnelling current can be detected this often means moving the tip by several micrometres or even millimetres. The piezoelectric materials used for scanning are not suitable for this initial approach and most instruments therefore contain a second coarse positioning driver frequently this is also a piezoelectric material in a stick-slip kind of design.27... 

The surface states play an important role in the STM experiments. On Si and Ge surfaces, near the Fermi level, the surface states usually dominate the surface charge density, which are what STM is probing. The tips are usually made of d band metals, W, Pt, and Ir. Surface states have a strong presence on the tips and often dominate tunneling current. 

Many, if not most, of the perfect surfaces studied by STM have also been studied by first-principles calculations with adequate accuracy. A fast growing field in theoretical surface physics is in first-principles calculations of the surfaces with adsorbates. A recent review of this field in given by Feibelman (1990). As the STM experiments are moving rapidly to the study of adsorbates as well, a direct comparison between the experimental observations and the theoretical predictions becomes practical and desirable. 

Fig. 16.11. The two structures in the top row were the subject of speculation by the previous authors, which are easy to understand. However, various studies showed that under certain circumstances, neither of these two structures can explain the observed phenomena. The STM experiment of Boland (1990) unambiguously identified a third structure, as shown in the bottom of...

STM measurements on molecular junctions in solution have been realized by groups in Miami [66], Lyngby [34] and Liverpool [33, 67]. Performing the STM experiments in a liquid environment provides a way to combine well-established electrochemical techniques with in situ electronic STM characterization of a single or a small number of molecules. One particular advantage of this approach is the option to use an electrochemical reference electrode as a third electrode (gate) in the setup in addition to the tip and substrate electrodes (source and drain). Such a setup closely resembles a (three-terminal) transistor setup, as the third electrode can be used to manipulate the transmission properties of the junction molecules by applying a potential between the substrate and the reference electrode. 

Fig.5 Schematic presentation of the STM experiment. The tunneling current is utilized for distance control of an electrode above the sample surface and maps spatially resolved the geometric and electronic properties of surfaces...

In the light of these results, it can be concluded that the Al deposition in [EMIM]TFSA behaves on the nanoscale more or less as in the first generation ionic liquids published earlier [12, 13], but it is much easier to keep the quality of the STM experiments as the liquids are per se water free. With [Pyi,4] TFSA the behavior is more complicated. Figure 9.8 shows a set of STM images on Au(lll) in the upper phase of the [Pyi JTFS A/AICI3 mixture at room temperature at the open... 

For the STM experiments described below, the pressure at which just half of the clusters reach the FTM was found and recorded. A previously unexposed substrate was then rotated into the position previously occupied by the FTM and exposed for a measured time to this cluster flux. This procedure eliminates codeposition of clusters and uncondensed atoms and slows the clusters to thermal speeds before depositing them on a substrate. After several samples are prepared, the MECS and the vacuum system are shut down and bled up to atmospheric pressure and the samples are removed for analysis. No attempt was made to prevent the supported clusters from adsorbing gas phase contaminants. 

The STM s roots lie in electron vacuum tunneling spectroscopy. In the context of measuring electronic properties, it is more correctly described as a spectrometer, for it is the electronic properties of the surface that are being probed in the STM experiment. The correspondence between the electronic and topographic properties is responsible for the microscope label. 

O.OIM HCIO4 containing 0.005 M Pb " or Tr. Commercial Ag(lll) electrodes were prepared by mechanical polishing (diamond polish of decreasing grain size), followed by chemical chromate polishing. The electrode was transferred luider electrolyte cover first into a conventional electrochemical cell for test voltammetric measurements, then transferred into the electrolytic STM cell. The STM measurements were performed in a commercial Nanoscope II instrument equipped with a homebuilt electrolytic cell [3]. Electrochemically etched Pt/Ir tunneling tips insulated laterally with Apiezon wax were used for the STM experiments. 

Fig. 5 Two of the configurations found for the Cgo molecule on the Si(l 11) surface a this structure would correspond to the large molecules type observed in the STM experiments b this structure would correspond to experimentally more stable small molecules type. In the latter structure two of the adatoms have broken their bonds with the Si atoms in the surface, allowing the formation of two extra bonds highlighted in the figure). Estimated binding energies, charge transfers to the fullerene, and height of the molecule center of mass over the substrate are also indicated. Reproduced with permission from [179]...

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