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Langmuir 2000, 16, 4358-4361
The Influence of Steps on the Orientation of Copper Phthalocyanine Monolayers on Au(111) I. Chizhov,† G. Scoles,‡ and A. Kahn*,† Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, Department of Chemistry, Princeton University, Princeton, New Jersey 08544, and Princeton Materials Institute, Princeton University, Princeton, New Jersey 08544 Received December 10, 1999 The structure of monolayer films of copper phthalocyanine (CuPc) deposited on a Au(111) surface in ultrahigh vacuum is studied using scanning tunneling microscopy and low-energy electron diffraction. CuPc molecules adsorb with their molecular plane parallel to the surface and form a highly ordered overlayer with a square unit cell. On terraces wider than ∼15 nm, the orientation of the monolayer is determined by the underlying substrate and the sides of CuPc square unit cells coincide very closely with the [112 h ] and [11 h 0] directions of the Au(111) surface. On narrow terraces, the sides of the CuPc unit cells are aligned along the step edges. This effect is explained in terms of the maximization of coverage which favors the formation of the CuPc domains where the molecules are aligned parallel to the step edges.
Phthalocyanines (including copper phthalocyanine (CuPc), Figure 1) are technologically important compounds for organic electronic devices such as thin-film transistors and solar cells.1 Their stability and ease of deposition in thin films have also made them model systems to study the processes of molecular adsorption and ordering and to investigate issues related to intramolecular image contrast in scanning tunneling microscopy (STM).2 Various groups have reported on the growth of CuPc on a number of substrates3-10 and concluded that a key prerequisite for obtaining molecular order was the use of relatively inert substrates allowing sufficient molecular surface mobility.4,7-10 On the other hand, some of these studies also concluded that high surface mobility could cause difficulties in obtaining meaningful high-resolution STM images at low ( ∼15 nm), the orientation of the film is defined by the substrate structure and the sides of the CuPc unit cells are oriented within (2° with of the [112h ] and [11h 0] directions of the Au(111) surface. On narrow terraces (W < ∼15 nm), the molecular rows of the CuPc domains are parallel to the step edges, which is explained by the tendency of the film-substrate system to maximize the surface coverage. For CuPc coverage below and above 1 monolayer, no molecular structure can be reproducibly resolved by the STM. All the evidence collected here is consistent with a CuPc/Au(111) interaction which depends very weakly on the translational coordinates of the molecule on the surface. Acknowledgment. This work was supported by the MRSEC program of the National Science Foundation (DMR-9809483). Helpful discussions with Dr. M. Gerstenberg are gratefully acknowledged. LA9916225
