Low-temperature STM

We want to point out that many analytical apparatus originally designed for room temperature have been modified to allow operation at low temperature with much better performances. An example is the STM (scanning tunnelling microscope) [1,2] which allows to image the morphology of a surface. Spectacular demonstrations have been provided of what could be done with a 4K STM [2,3], Today several low-temperature STM of different design are in operation [4-12],... 

This work shows the exceptional physics that can be done with a STM operated at cryogenic temperatures and the availability of STMs working down to liquid helium temperature opens broad avenues of research in the coming years. No doubt that among the many future scientific experiments accessible with low temperature STMs, the real-space electronic characterization of the metal-superconductor transition in /c-phases of BEDT-TTF salts, because Tc > 4 K, as well as the study of magnetic ordering in MOMs, will certainly occupy a relevant position. 

Fig. 12.5. STM with spring motion demagnifler. The design, due to Smith and Binnig, is used for low-temperature STM experiments. (Reproduced from Smith and Binnig, 1986, with permission.)...

Other Compounds Recently, low-temperature STM has been used [315] to characterize variously structured submonolayers and near monolayers of benzene on Au(lll). 

Using low-temperature STM operating at 4 K, the contribution of the tip to T could be determined in the case of Fe adsorbed on Pt(ll 1) because the tip could be positioned on a defined point for a long time. After deconvolution the true distribution of states was obtained (see M. F. Crommie, G.P. Lutz, D.M. Eigler, Phys. Rev. B 48 2851 (1993). The main features of the distribution were however resoived without deconvolution. 

Fig. 10 Low temperature STM image of Cso on Au(lll) recorded using a bias of 1.5 V. This excerpt, from the original image of a (7x7) superstructure, shows a strip of eight molecules in which the orientation changes progressively from left to right. Reprinted figure with permission from G SchuU and R Bemdt, Phys. Rev. Lett. 99, 226105 (2007) [37]. Copyright (2007) by the American Physical Society...

Tanaka and co-workers [178] have introduced poly(3-hexylthiophene-2,5-diyl) (PHT) into ultrahigh vacuum by pulse injection onto Cu(lll) surfaces. With low temperature STM (92K) they could observe the polymer chain with high resolution and study details of curving and kinks in the chain (Fig. 10.27c). 

Generally, in the first monolayer on (coinage) metals pentacene and other planar aromatic molecules were found to adsorb with their aromatic ring plane parallel to the substrate. A detailed discussion of the molecule/metal interaction probed by means of low temperature STM can be found in Chapter 12 by Soubatch et al. The preference for such a planar adsorption geometry can be... 

As a way out of this dilemma, we have developed a two-step approach based on commensurate, highly ordered molecular layers on metal surfaces. In step 1, the powerful armoury of surface science is employed to characterise the structural and electronic properties of the metal-molecule contact. For the model molecule PTCDA, some of the results of these experiments have in fact been described in Sections 12.2 and 12.3. Once step 1 has been completed, the tip of a low-temperature STM is used to establish a covalent contact with an individual molecule of the highly ordered monolayer (step 2). Because of the excellent imaging properties of the STM, it is in fact possible to select the part of the molecule which is contacted with very high accuracy. 

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. 

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