Variable temperature STM

All the STM results from our group presented in this chapter employed the variable temperature STM, with tips made by electrochemical etching of tungsten wire. For noncontact AFM (NC-AFM), we employ commercial conducting silicon cantilevers with force constants of approximately 2-14 rn 1 and resonant frequencies of approximately 60-350kHz (Nanosensors and Mikromasch). The NC-AFM images we present here were recorded in collaboration with Professor Onishi at Kobe University and employed a UHV JEOL (JSPM-4500A) microscope. 

As already discussed in Chapter 1, this kind of mixed valence salt becomes conductive due to the transfer of one electron from two BEDT-TTF molecules to the anion layers. However, at the surface, the charge can become unbalanced, resulting is an incomplete CT. This leads to differentiated surface vs. bulk nesting vectors and to the existence of surface CDWs (Ishida et al, 1999). The Peierls transition has also been observed on the a -planes of single crystals of TTF-TCNQ with a variable temperature STM (Wang et al, 2003) and will be discussed in Section 6.1. 

The real-space characterization of the CDW-induced modulation of a 2D surface lattice can be ideally performed with variable temperature STMs. The temperature-dependent modulation can be classified according to the HFW model introduced in Section 4.2 taking the ideal 1x1 surface structure as the reference lattice (as and bs) and the projected CDW-modified structure as the overlayer system (uo and bo). In the case of TTF-TCNQ = a and h = b and for the images taken at 63 K... 

Fig. 11 High pressure, high temperature STM. Sections A-UHV sample manipulator with indirect heating and cooling, B-variable temperature STM, C-magnetically coupled linearrotary transfer arm, D-optical access for analyzers. E-pneumatic air legs and suspension frame, F-turbo molecular pump, G-ion pump. ...

In summary, we have shown that the stability of binding sites, diffusion and rotational barriers can be extracted straightforwardly from real time variable temperature STM experiments. Moreover, we... 

The first qualitative observation of vacancy-induced motion of embedded atoms was published in 1997 by Flores et al. [20], Using STM, an unusual, low mobility of embedded Mn atoms in Cu(0 0 1) was observed. Flores et al. argued that this could only be consistent with a vacancy-mediated diffusion mechanism. Upper and lower limits for the jump rate were established in the low-coverage limit and reasonable agreement was obtained between the experimentally observed diffusion coefficient and a theoretical estimate based on vacancy-mediated diffusion. That same year it was proposed that the diffusion of vacancies is the dominant mechanism in the decay of adatom islands on Cu(00 1) [36], which was also backed up by ab initio calculations [37]. After that, studies were performed on the vacancy-mediated diffusion of embedded In atoms [21-23] and Pd atoms [24] in the same surface. The deployment of a high-speed variable temperature STM in the case of embedded In and an atom-tracker STM in the case of Pd, allowed for a detailed quantitative investigation of the vacancy-mediated diffusion process by examining in detail both the jump frequency as well as the displacement statistics. Experimental details of both setups have been published elsewhere [34,35]. A review of the quantitative results from these studies is presented in the next subsections. 

Today low-temperature STMs at liquid He temperature, variable-temperature STMs offering cooling and annealing, high-speed-STMs allowing movie shots, and liquid-phase STMs for the study of electrolytic surface reactions have been developed and are commercially available. 

Our laboratories are currently equipped with three UHV Omicron microscopes, a variable-temperature scanning tunneling microscope (STM), a room-temperature atomic force microscope (AFM)/STM, and a low-temperature liquid helium bath cryostat STM, all of which are currently driven by Omicron Scala software and electronics. 

STM instruments have evolved from a complicated homebuilt instrument sensitive to vibrations into a compact, rigid, stable variable-temperature microscope, available commercially at fairly low cost. The availability of the instrumentation for STM has led to a tremendous diversification of the application of the technique, and new areas of applications are constantly being developed. 

STM measurements can be employed to systematically study tracer diffusion by following adsorbate migration at the atomic level in situ. This was achieved for a variety of systems, including adsorbed gas atoms and molecules at metal surfaces [20]. Moreover, STM allows for detailed investigation of the bonding geometries and 2-D rotational motions. The temperature dependence is conveniently investigated by variable-temperature... 

Imaging Au on TiO2(110) at elevated pressures with variable temperature and pressure STM... 

The surface was imaged using a variable temperature Oxford Instruments STM and details of this instrument can be found elsewhere [15]. The STM is contained within a UHV chamber which is equipped with similar facilities as described for the molecular beam system. Up to 550 K the thermal drift is sufficiently small such that the same area can be scanned... 

The final electrical connections to the STM can be done with copper wires. A small amount of helium is used as an exchange gas to anchor the temperature of the whole assembly to the cryogenic fluid. The body of the STM can be made out of copper, which will respond quickly to temperature changes for variable temperature measurements and provide a uniform temperature environment for the tunnel junction. One has to estimate the differential thermal contraction of the component parts to make sure that a tunnel junction separation set at room temperature is sufficiently large to prevent tip crash on cooling. Other materials like Macor or Invar , which closely match the thermal expansion properties of the piezoelectric transducers, are used as well but take more time to thermally stabilize. Some references are given in [6.30-6.43]... 

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