Growth of Unidirectional Molecular Rows of Cysteine on $\mathrm{A}\mathrm{u}(110)\mathrm{\text{−}}(1\times 2)$ Driven by Adsorbate-Induced Surface Rearrangements

Using scanning tunneling microscopy we have studied the nucleation and growth of unidirectional molecular rows upon adsorption of the amino acid cysteine onto the anisotropic $\mathrm{A}\mathrm{u}(110)\mathrm{\text{−}}(1\times 2)$ surface under ultrahigh vacuum conditions. By modeling a large variety of possible molecular adsorption geometries using density-functional theory calculations, we find that in the optimum, lowest energy configuration, no significant intermolecular interactions exist along the growth direction. Instead the driving force for formation of the unidirectional molecular rows is an adsorbate-induced surface rearrangement, providing favorable adsorption sites for the molecules.

Figure 1

Large-scale STM image showing coexisting molecular structures of d cysteine on a $\mathrm{A}\mathrm{u}(110)\mathrm{\text{−}}(1\times 2)$ surface ($425\times 425\AA$, constant-current mode $I=-0.1\mathrm{n}\mathrm{A}$, $V=-1.2\mathrm{V}$ applied at the sample). The molecular double-row structure (1) is terminated at ascending (A) or descending (D) step edges or at defects on the terraces. Inset: a close-up of a double-row formed from d cysteine molecules ($40\times 40\AA$). The unit cell of this structure is depicted in black.

Figure 2

Selected adsorption structures considered in DFT calculations. (a) Bulk-truncated $\mathrm{A}\mathrm{u}(110)\mathrm{\text{−}}(1\times 1)$ surface. (b) The most stable d cysteine double-row structure identified. (c) “Zigzag” structure where each molecule forms hydrogen bonds with two molecules in the adjacent row. (d) A similar structure as in (b), but with the S adsorption site on one row shifted along [$1\overline{1}0$]. (e) Missing-row reconstructed $\mathrm{A}\mathrm{u}(110)\mathrm{\text{−}}(1\times 2)$ surface. (f) Optimum structure identified on the $\mathrm{A}\mathrm{u}(110)\mathrm{\text{−}}(1\times 2)$ surface. The arrows indicate the energy balance for the reaction $\mathrm{A}\mathrm{u}(110)+2\mathrm{\text{cysteine}}\to {\mathrm{H}}_2+2\mathrm{\text{cysteinate}}/\mathrm{A}\mathrm{u}(110)$. For (b)–(d), the final formation energies (shown in the upper right corners of the models) are obtained by adding the 0.40 eV required to lift the missing-row reconstruction.

Figure 3

Most stable cysteine double-row structure, as obtained from the DFT calculations, superimposed onto an STM image. Note that also adjacent double rows, as observed occasionally in Fig. 1, are consistent with this model.

Figure 4

Gold vacancy formation energies: (a) 0.90 eV for initiating a double row on the terrace, (b) 0.40 eV for continuing the growth of a cysteine double row, and (c) 0.68 eV for creating a four Au atom vacancy site for the isolated cysteine dimer of Ref. 15. These energies result from appropriate summations of the energy costs of 0.31, 0.14, 0.12, and 0.11 eV for the removal of the first through fourth gold atom, respectively, in a close-packed gold row [obtained from DFT calculations in a $(6\times 1)$ surface unit cell].