PTCDA Ag

Figure 12.14 Simulation of the PTCDA/Ag(l 11) bond cleavage by tip retraction (calculated by local density approximation of density functional theory) [41].

In the light of the PTCDA/Au(l 11) contact distance, the PTCDA/Ag(l 11) contact really looks like a chemisorptive one. To analyse the bonding more closely, the results of density functional calculations, of which several are... 

On the basis of experiment and calculations, we can draw up the following picture of the PTCDA/Ag(lll) interaction (Figure 12.7). The molecule interacts with the Ag surface via its two functionalities. (1) The 7i-system hybridises with states Ifom the Ag 5s band and accepts charge from the metal. This is an extended interaction. (2) The carboxylic oxygen atoms form local bonds... 

Although the structure of the PTCDA/Ag(l 11) interface is dominated by the molecule-substrate interaction (cf. the commensurate interface structure), the interaction between molecules does play an important role for the structural details and the energetics at the interface. On the one hand, there is the attractive electrostatic interaction between molecules. Because of this interaction PTCDA molecules always cluster in two-dimensional islands. However, at surface temperatures below 150 K, these islands do not yet exhibit the familiar herringbone structure [35] since the electrostatic interaction does not exhibit sufficient directional specificity, a considerable degree of structural disorder prevails, in spite of a clear propensity of the molecules to arrange in a T-like... 

The above example illustrates the eomplex interplay between adsorbate-substrate and adsorbate-adsorbate interaetions for PTCDA/Ag(l 11). On the one hand, the ordering implies the formation of intermoleeular hydrogen bonds and thus requires the weakening of the moleeule-substrate bond, but on the other hand it also enables all the moleeules to adsorb at their preferred bridge sites, i.e. it optimises the molecule-substrate interaetion. 

But why does pulling the molecule gate the transport This has to do with the specific properties of PTCDA/ Ag(l 11) that have been discussed in Section 12.2. As the molecule is peeled off the surface, the metal-molecule 7i-bond is gradually cleaved. The schematic in Figure 12.5 shows that this dehybridisation must shift the former LUMO back up in energy and sharpens it. Resonant transport then enhances the slope of the I-V curves at low biases and thus yields the transistor-like output curves of Figure 12.13. ... 

PTCDA represents a further example of non-planarity. When chemisorbed on an Ag(l 11) surface the carboxylic oxygen atoms become 0.018 nm closer to the... 

A ML can be simply defined as a one-molecule thick 2D film, but the molecular surface density has to be defined for each molecule-substrate system because it depends on the shape, size and relative orientation of the molecules. To clarify this point let us consider the examples of PTCDA and Ceo on the Ag(l 11) surface. The surface density of the substrate is 1.4 x 10 atoms cm , which is usually defined as 1 ML as a reference limit. The surface density of the (102) plane of PTCDA, the cleavage plane, is 8.4 x 10 and 8.3 x 10 cm (molecules cm ) for the monocliiuc a and polymorphs, respectively. Therefore, full coverage corresponds to 0.02 ML according to this definition. On the other hand, the surface density of a full hexagonal layer of closed-packed Ceo molecules corresponding to the (111) plane in the fcc-Ceo crystal is 1.2 x 10 " cm . Thus, Ceo would fully cover the Ag(l 11) surface at a coverage of 0.09 ML. However, other authors define 1 ML as... 

The comparison of PTCDA with its parent perylene molecule is extremely interesting. For perylene MLs on Ag(l 11), electron diffraction suggests an orientational liquid, in which the molecules are positionally ordered in an incommensurate close-packed superlattice but orientationally disordered and mobile. The same activated Raman peaks as for PTCDA are observed but they are, however, orders of magnitude weaker, indicating that, while a molecular reaction centre may still exist in the perylene backbone, its residual activity would be too small for the molecule to recognize a preferred site. 

PTCDA films OMBD-grown on Ag(lll) single crystals show a transition from layer-by-layer growth with smooth and small-grained morphology towards the... 

Hirose Y, Kahn A, Aristov V, Soukiassian P, Bulovic V, Forrest SR (1996) Chemistry and electronic properties of metal-organic semiconductor interfaces Al, Ti, In, Sn, Ag, and Au on PTCDA. Phys Rev B 54 13748... 

Figure 12.3 Submolecular STM contrast of two PTCDA molecules adsorbed on the Ag(l 11) surface.

Figure 12.6 Schematic representation of the distortion of PTCDA on Ag(lll). The structure has been calculated by density functional theory in the local density approximation [33].

Figure 12.7 Schematic representation of the bonding interaction of PTCDA with Ag(l 11) [27, 33].

Figure 13.6 Raman spectra for In (5 nm), Ag (4,5 nm) and Mg (5 nm) coverages on 15 nm thick PTCDA films, compared with the spectrum of the bare PTCDA film in the spectral region of the internal breathing mode (left) and in the spectral region of C-H deformation and C=C stretching modes (right).

Figure 13.10 Ratio between enhancement factor of the B mode (1243 cm in PTCDA and 1246 cm in DiMe-PTCDI) and of the C-C stretching Ag mode (1572 cm- in PTCDA and 1570 cm- in DiMe-PTCDI) as a function of the metal coverage.

Figure 13.11 Raman spectra of 15 nm thick PTCDA films covered with Ag, In and Mg in the region of the external modes.

Figure 9.7 Different bonding distances and adsorption geometries of PTCDA on Au(l 11), Ag(l 11) and Cu(l 11) as measured by XSW [20-22]. From Ref [28] with permission.

Figure 9.8 shows anti-Bragg oscillations for PTCDA deposition on Ag(l 11) at different substrate temperatures clearly demonstrating (1) the decay of crystallinity and therefore growth oscillations at low substrate temperatures, and... 

B. Krause, Growth and Structure of the Organic Molecule PTCDA on Ag(l 11), Ph.D. thesis, Universitat Stuttgart (2002). 

There are, however, a few planar aromatic molecules with different bulk structures which do show the existence of such orientational precursors. One of these exceptions is PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) which has been extensively studied in the past and is commonly considered as a prototype system for organic thin film growth [62, 63]. Especially on Ag(l 11) an epitaxial growth of well-ordered PTCDA films was observed where molecules adopt the same arrangement as in the (102)-plane of the bulk structure of PTCDA [62]. An examination of this (102)-plane reveals that this... 

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