Molecular ordering of glycine on Cu(100): The $p$(2$\times$4) superstructure

Glycine molecules deposited on a Cu(100) surface give rise to an anisotropic free-electron-like (FEL) electronic dispersion in its $p(2\times 4)$ superstructure, as reported in recent experiments [K. Kanazawa et al., Phys. Rev. Lett. 99, 216102 (2007); J. Am. Chem. Soc. 129, 740 (2007)]. Using density functional theory and exhaustively calculating sixteen possible structures, we have determined the molecular arrangement that can give the experimentally observed FEL behavior. Eight configurations, among the sixteen, were not investigated before in the literature and one of them (denoted Str-3) is able to provide the FEL behavior in excellent agreement with the experiments. In addition, the particular configuration Str-3 satisfies other criteria of the observed $p(2\times 4)$ superstructure, e.g., chirality and cleavable orientation.

Figure 1

Four molecular configurations. (a)–(c) The configurations previously proposed and investigated, i.e., (a) $c(2\times 4)$ (Str-g in this work), (b) $p(2\times 4)$ (Str-d), and (c) $p(2\times 4)$-pg (Str-h). (d) A new configuration proposed in this work, in which the in-plane molecular orientation is roughly perpendicular to that in (a)–(c). The red solid line in (a) denotes $\left[\overline{1}30\right]$, a direction included in the $\langle 3\overline{1}0\rangle$ direction.

Figure 2

According to O-O orientation, O-N direction, the relative position of equivalent atoms, and the chirality, there are $2^4=16$ configurations that should be considered, including eight previously considered configurations (Str-a to Str-h) and eight newly proposed configurations (Str-1 to Str-8).

Figure 3

Calculated band structures along [110] (blue lower-half-filled circle) and [$\overline{1}10$] (red upper-half-filled circle) of Str-d (a), Str-h (b), and Str-3 (c). Red and blue shaded areas present the experimentally measured curves with a broadening of 0.04 eV for the clarity of plotting. Both colors are consistent throughout the paper, i.e., blue for [110] and red for [$\overline{1}10$]. The first Brillouin zone together with three $k$ points (G, X, Y) were shown in (a) inset.

Figure 4

Experimental STM image of the $p(2\times 4)$ superstructure [Fig. 2(e) in Ref. 13] (left) and simulated STM images for configurations Str-3 (middle left), Str-d (middle right), and Str-h (right), respectively. The sample bias in the simulation is $-100$ mV, the same as that in the experiment.