Coexistence of Different Structural Phases in Thioaromatic Monolayers on Au(111)

Abstract

Self-assembled monolayers formed by adsorption of 4-methyl-4‘-mercaptobiphenyl, CH3-(C6H4)2−SH (BPT), on Au(111) have been studied using scanning tunneling microscopy. The results show that with increasing coverage (resulting from longer immersion times) BPT forms a series of different structural phases with different molecular arrangements, closely resembling the behavior reported previously for n-alkanethiolate adlayers. For short immersion times, striped structures are observed (α and β), where the molecules are orientated with their axes parallel to the surface. Longer immersion times yield additional phases, namely, an ordered χ-phase, where the molecules are proposed to be oriented with their molecular axis tilted away from the surface, a disordered δ phase, and, finally, a densely packed (2√3 × √3) ε phase where the molecular axes are orientated almost upright. Unexpectedly, for BPT after adsorption small islands are seen on the Au substrate instead of the etch pits commonly observed after formation of organothiolate adlayers.

Structure of Mercaptobiphenyl Monolayers on Mercury

“…2 In this phase, the molecular long axis is aligned parallel to the surface, with one phenyl ring lying flat on the surface, and the other rotated (around the molecular long axis) at an angle away from it. Although this phase was not found in the STM study of liquid-adsorbed MMB on Au(111) by Azzam et al, 32 their striped phase yields 72 Å 2 /molecule, not too far from the present A 0 . In this phase, the molecules are found to lie flat on the surface.…”

“…Moreover, on Au the molecular axis of MMB was argued to be not fully parallel to the surface, but tilted up from it by a small angle, presumably by the need to make room for the lower section of the dihedrally rotated ring but also because of the strong attraction exerted on the terminal thiol group by the Au atoms of the surface. 2,32 Such a tilt is likely to be present here as well for the very same reasons. If this is indeed the case, the XR-observed layer thickness would be a combined result of the up tilt of the molecule from the surface and the non-zero dihedral rotation angle of the rings.…”

“…X-ray measurements 2 on the lying-down phase of MMB on Au(111) reveal a striped phase having a long-range in-plane crystalline order with a distorted rectangular unit cell. A recent detailed STM study 32 at this coverage range resolved two different striped phases, with slightly different long-range ordered structures. On mercury, however, no GID peaks are observed for the corresponding phase of lying-down molecules, indicating that the coherence length of any order that may exist within the monolayer cannot exceed very few tens of angstroms, as for a liquid or an amorphous solid.…”

“…This phase converts to the hexagonal phase at longer immersion times. 32 This structure is in marked contrast with the equilibrium phase of MMB on mercury, discussed above, which has a rectangular cell structure of 5.56 Å × 7.84 Å, and thus a (similar) area per molecule of 21.84 Å 2 /molecule. The A x values of MMB on both Au and Hg are close to the value of 22.1 Å 2 /molecule derived from the 3D crystal structure of biphenyl, which has a unit cell of 5.51 Å × 8.04 Å in the (x,y) plane.…”

“…This, again, is in contrast with alkanethiols on Au(111) which show long-range ordered striped phases 41 akin to those observed for MMB on Au. 2,32 The discussion leads, therefore, to the not very surprising conclusion that the subphase's structure, or lack thereof, plays a major role in determining the structure of the lying-down monolayer: for the long-range ordered Au subphase, a commensurate order (at least in one direction) is epitaxially induced in the organic overlayer, while for the disordered (or short-range ordered) liquid Hg interface, a similarly disordered (or short-range ordered) layer is obtained.…”