Asymmetric relief of surface stress induced by a chiral adsorbate: Alaninate adsorption on Cu(110)

Using first-principles calculations, we explore symmetry breaking in the surface stress tensor induced upon adsorption of chiral species on an achiral metallic substrate, namely, alaninate on Cu(110). The stress sensitivity to coverage and adsorption geometry is studied in the known $\left(2\times 3\right)$ phase and other related reconstructions. We find that alaninate relieves stress in the Cu(110) surface by electron transfer from the substrate to the molecule. In a low coverage regime, $\Theta =1/6$, the principal stresses are found to deviate in opposite directions by an angle $\alpha \sim 15°$ from the substrate mirror plane directions, $\left[1\overline{1}0\right]$, depending on the chirality of the alaninate adsorption footprint. In medium-high coverage regimes, $\Theta \ge 0.3$, the observed asymmetries are weaker, consistent with the presence of H bonds lying close to $\left[1\overline{1}0\right]$. The studied $\Theta =1/3$ enantiopure models show the same asymmetry orientation with $\alpha \sim 5°$. Thus, stress relief anisotropy is diagnostic of the molecular chirality in this case. We also find that the substrate intrinsic tensile stress is relieved at $\Theta \gtrsim 0.25$, when the adsorbates form the onset of a H-bound network. At higher coverages, the system is under net compressive stress.

Figure 1

(Color online) $\left(2\times 3\right)$ unit cell of $\Theta =1/6$ models. The color code for the atoms is $\text{white}=\text{H}$, $\text{gray}=\text{C}$, $\text{blue (left)}=\text{N}$, and $\text{red (right)}=\text{O}$. The three point binding footprint in each case is schematically represented by dashed triangles. The left-hand side inset shows the sign convention adopted here for a positive $\alpha$ value, with the eigenvector ${\widehat{v}}_1$ depicted as a dashed line.

Figure 2

(Color online) $\Theta =1/3$ enantiopure models. The solid rectangle shows the $\left(2\times 3\right)$ unit cell and footprints are indicated with dashed triangles.

Figure 3

(Color online) $\Theta =1/3$ racemic models. The solid rectangle shows the $\left(2\times 3\right)$ unit cell and footprints are indicated with dashed triangles.

Figure 4

(Color online) RrR model. The pseudo-$\left(2\times 3\right)$ cell is shown in white and the real oblique cell in black.

Figure 5

Projected DOS on the alaninate species (solid line) and on the $d$ and $s$ orbitals of the Cu(1–3) atoms (dashed and dashed-dotted lines, respectively) in the xR model. The surface Fermi level is at $E_F=0$. The dotted lines show the gas-phase alanine discrete energy levels, also with $E_F=0$.

Figure 6

(Color online) Labeling of Cu atoms in the $\Theta =1/3$ models unit cell.

Figure 7

Compilation of the major principal stress difference, $\Delta {\sigma }_1$, as a function of coverage $\Theta$ for the 17 models studied in this work. Enantiopure and racemic models are represented by circles and triangles, respectively. A linear scaling is observed up to $\Theta =0.3$. The horizontal line indicates when tensile stress along $\left[1\overline{1}0\right]$ in clean Cu(110) would be relieved.