Organic ω-(4’-methyl-1,1’-biphenyl-4-alkanethiol) molecules (CH3-C6H4-C6H4-(CH2)n-SH, BPn) form well ordered selfassembled monolayers (SAM) structures on metal surfaces. Characterizing the ordering and packing behaviour of BPn is essential for the understanding of the electronic and optical properties. BPn structures are predominantly characterized by STM. The topographical contrast yield periodic structures. The distances and alignment between periodically repeating bright or dark intensity protrusions can be measured with respect to the underlying substrate structure. These experimentally determined unit cells were usually attributed to commensurate superlattices. However, the accuracy depends on the visibility of reference structures, which often cannot be observed simultaneously with atomic resolution leading to an ongoing discussion about different unit cells of BPn on Au(111).
LEED should allow a reliable determination of the unit cell and its lattice vectors. Therefore, we studied the adsorption of BP3 and BP4 on Au(111) surfaces using a micro channel plate (MCP) LEED system, which permits a more gentle examination with reduced electron beam current in the pA regime. The samples were exposed at room temperature to BPn molecules by gas phase deposition and subsequently annealed. For BP3 a (2√3 x √3) structure has been proposed, based on a diffuse LEED pattern (Azzam et. al., Langmuir 19 (2003) 8262) apparently in agreement with topographical STM pictures. In contrast, we observed a sharp (2√3 x √91) LEED pattern which can be related to 6 slightly distorted (2√3 x √3) units. For BP4 a number of coexisting phases have been observed depending on the heat treatment. Annealing to 365 K leads to a (6 x 2√3) LEED pattern (α-phase). Further annealing to 390 K results in a complete transition to the β-phase characterized by a (√3 x 5√3) superlattice. Finally, we observe predominantly a (8 x √3) structure (γ-phase) after annealing to 395 K. These LEED structures will be compared with recent STM structures showing that the topographical contrast does not necessarily allow a straightforward unit cell determination.
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