Pentacene monolayer

Figure 15.3 Pentacene growth on Ag and graphite, (a) Side view of the film structure of pentacene on Ag(l 11). A flat lying pentacene monolayer acts as a growth template, (b) Side view of the growth of pentacene on graphite. The stmcture resembles the growth on Ag, but it is not clear whether a pentacene wetting layer is present.

Fritz, S. E. et ah. Structural characterization of a pentacene monolayer on an amorphous Si02 substrate with grazing incidence x-ray diffraction, J. Am. Chem. Soc., 126, 4084, 2004. 

Fritz, S.E. et al.. Structural characterization of a pentacene monolayer on an amorphous... 

FIGURE 4.1.7 (a) GIXD pattern (bottom of (a)) for a pentacene monolayer and a diffraction pattern (top of (a)) calculated for an energy-minimized crystal structure model based on the GIXD lattice parameters and the (001) layer motif of huUc pentacene as the starting point (b) Normal views of the ab planes of bulk pentacene and the model monolayer structures (a and d) and the respective side views (b and e, e and f). The z-axis is the normal to the ab plane. (From Fritz, S.E. et al., J. Am. Chem. Soc. 126, 4084, 2004. With permission.)... 

Figure 4.3.14 illustrates AO in EFM images for pentacene monolayer islands before any charge injection as a function of The AO-Vgg curve varies as Vppi/ and the curve is slightly shifted by a linear component. From a second-order polynomical fit of the AO-Egg curve, an effective surface charge density of the ordered pentacene island is estimated as 2(X) charges/pm. Conversely, EFM analysis for disordered pentacene islands shows ... 

Heim, T., Lmimouni, K., and VuiUaume, D., Ambipolar charge injection and transport in a single pentacene monolayer island. Nano Lett. 4, 2145-2150, 2004. 

Pratontep, S., Nuesch, R, Zuppiroli, L., and Brinkmann, M., Comparison between nucleation of pentacene monolayer islands on polymeric and inorganic subshates, Phys. Rev. B, 72, 085211, 2005. 

Fig. 3 Pentacene grown by supersonic molecular beam deposition to form near monolayer p-type FETs with thiolate monolayer modified Au source and drain contacts (a) visualized by atomic force microscopy and with well-behaved (b) /d-Eds and (c) -Eg characteristics...

Park B-N, Seo S, Evans PG (2007) Channel formation in single-monolayer pentacene thin film transistors. J Phys D 40 3506-3511... 

Asadi K, Wu Y, Gholamrezaie F, Rudolf P, Blom PWM (2009) Single-layer pentacene field-effect transistors using electrodes modified with self-assembled monolayers. Adv Mater 21 4109 114... 

Some improvement was observed with pentacene deposited on top of silane layers, but it was also observed that the silane deposition is not easy to control, and side-reactions often result in rough layers with considerable unreacted content remaining. An alternative approach relies on application of self-assembled monolayers which mimic vapor-deposited silanes [32, 33] on the dielectric interface of the organic devices. Figure 2.6 shows an overview of the different surface-treatment application methods discussed in this section. 

Fig. 2.8. Normal (left) and side views (center, right) of the ab planes of bulk pentacene and the proposed monolayer structures. The monolayer views were constructed from a model based on grazing-incidence X-ray diffraction data [24].

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... 

Note, that in the case of bismuth the growth of upright oriented pentacene molecules has been observed - even in the very first monolayer [47]. This orientation has been explained [47] by the rather small density of states near the Fermi level of this semi-metal leading to a rather small molecule-substrate interaction which becomes comparable or even weaker than the mutual molecular interaction and thus parallels the situation observed for pentacene deposited on inert substrates such as Si02 [48]. 

In order to exclude the appearance of rotational domains in the initial stage of growth we have used a Cu(llO) substrate and have studied the growth of pentacene films on this substrate quite extensively. In fact, a highly ordered submonolayer and a saturated monolayer phase are found which exhibit a uniform alignment of the flat lying molecules with their long axes orientated parallel to the (110 )-azimuth direction of the substrate [46, 52]. 

Figure 11.6 Evolution of pentacene films on Cu(l 10). After completion of the first monolayer revealing a predominant (6.5 X 2) phase and occasionally a coexisting c(13 X 2) phase (a) an intermediate phase A is formed whereas for thickness above 2 nm the molecules continue in an upright orientation a-dopting the well known thin film phase B (b).

Figure 11.7 Stmcture and morphology of pentacene films grown on Au(l 11). (a) SEM micrograph of 2 nm grown at room temperature together with (b) STM data (U= -2Y, I = 15 pA) showing the stmcture of the first monolayer (indicated by I) between the islands formed (indicated by II). A similar morphology was also obtained for films grown at various conditions, (c) 30 nm (15 nm/min). The resulting islands reveal characteristic orientations relative to the substrate as shown schematically in (d).

D. Kafer and G. Witte, Evolution of pentacene films on Ag(l 11) Growth beyond the first monolayer, Chem. Phys. Lett. 442,376-383 (2007). 

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