Structural investigation of caffeine monolayers on Au(111)

The asymmetric and achiral character of caffeine (${\mathrm{C}}_8{\mathrm{H}}_{10}{\mathrm{N}}_4{\mathrm{O}}_2$) leads to two on-surface chiralities which has an impact on its on-surface formation. An analysis of its on-surface behavior reveals new insights of its crystallite growth. In this study the structural formation of caffeine monolayers on a Au(111) surface was analyzed by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. The monolayers were prepared by molecular beam epitaxy (MBE) and analyzed at room temperature. Caffeine molecules self-assemble in a quasihexagonal phase on Au(111) similar to the high-temperature $\alpha$ phase. Two mirrored hexagonal domains are present with respect to the surface. Within the XPS measurements, no strong surface interaction was found. Therefore, a theoretical analysis of a hypothetical free-standing caffeine monolayer structure was performed by ab initio simulations. We found that a caffeine monolayer with three molecules per unit cell is preferable to one with just a single molecule, as could be expected from the LEED pattern.

Figure 1

Molecular structure of caffeine (${\mathrm{C}}_8{\mathrm{H}}_{10}{\mathrm{N}}_4{\mathrm{O}}_2$) and assignment of numbers to atoms within the molecule.

Figure 2

STM images of caffeine adsorbed on Au(111). (a) After heating the substrate at $80{}^{\circ }\mathrm{C}$ for $10\min$ the caffeine film of $\sim 1.3\mathrm{ML}$ coverage is found homogeneous with some islands on top $(500\times 500\mathrm{nm},10\mathrm{pA},-2.0\mathrm{V})$. (b) Close-up view of the surface with hexagonal caffeine monolayer formation on Au(111). An angle of $10\pm 3^{\circ }$ was observed between the substrate $\left[1\overline{1}0\right]$ direction and the caffeine $\left[5\overline{4}\overline{1}\right]$ direction. $(36\times 36\mathrm{nm},17\mathrm{pA},-1.0\mathrm{V})$.

Figure 3

(a) STM image of a caffeine monolayer on Au(111) with two mirrored film domains separated by a substrate step edge $(19\times 19\mathrm{nm},16\mathrm{pA},-1.0\mathrm{V})$. (b) Fast Fourier transform corresponding to (a) showing 12 spots. (c) Domain border of a dense caffeine monolayer on the same substrate terrace $(28\times 28\mathrm{nm},17\mathrm{pA},-1.0\mathrm{V})$. (d) STM image of a caffeine monolayer on Au(111) $(11\times 11\mathrm{nm},15\mathrm{pA},-1.0\mathrm{V})$.

Figure 4

(a) LEED pattern of the caffeine monolayer formation on Au(111) recorded at an electron energy of $E_{\text{kin}}=20\mathrm{eV}$. On top of the LEED image the LEEDpat simulation of a $\left(\begin{array}{cc}6& 4\\ 2& 6\end{array}\right)$ or $2\sqrt{7}$ and a $\left(\begin{array}{cc}10/3& 2/3\\ 8/3& 10/3\end{array}\right)$ or $\frac{2}{3}\sqrt{21}$ unit cell is printed in red and its mirrored domain in blue. The green circles represent a diffraction simulation performed with the ab initio simulated surface structure where the spot intensity corresponds to the simulated diffraction intensity. (b) Structure model of one domain with single caffeine molecules represented in red and the Au substrate in yellow. One $\frac{2}{3}\sqrt{21}$ and a $2\sqrt{7}$ unit cell are shown in blue and black, respectively.

Figure 5

Discretized pairwise interactions of caffeine molecules in a free-standing monolayer in two chiral arrangements (a) and (b). In the bottom right corner of both plots the in-plane geometry of a molecule, relative to the centered fixed molecule, is shown with the corresponding arrow in the center of the pyrimidine ring. The arrows represent the energetically most favorable in-plane rotation of the second caffeine molecule at each respective location. The insets visualize the interaction energies for all different rotations at the energetically most favorable positions in each of the three energetically favorable areas. (a) Pair interaction energies for both molecules with the same on-surface chirality. (b) Pair interaction energy results for different surface chirality.

Figure 6

Geometry-optimized monolayer of caffeine with three molecules in a $\left(\begin{array}{cc}6& 4\\ 2& 6\end{array}\right)$ or $2\sqrt{7}$ unit cell on Au(111). The three molecules within a unit cell are aligned in the structure that yields the best energy per area (rather than per molecule), balancing out repulsive interactions due to tight packing with attractive intermolecular interactions. The three molecular orientations are marked by letters A, B, and C. The surface basis vectors $v_1$ and $v_2$ are indicated. The radii used for the atoms shown are 1.2 times the respective covalent radii.

Figure 7

C $1s$ core-level XPS of caffeine on Au(111) recorded at a photon energy of $h\nu =340\mathrm{eV}$ and at an emission angle of $\mathrm{\Theta }=0^{\circ }$. The assignment of carbon atoms A, B, C, and D is indicated by green circles. In the bottom panel, the simulated XP spectrum of the caffeine monolayer is shown.

Figure 8

N $1s$ core-level XPS of caffeine on Au(111) recorded at a photon energy of $h\nu =500\mathrm{eV}$ and at an emission angle of $\mathrm{\Theta }=0^{\circ }$. The simulated XP spectrum of the caffeine monolayer is shown in the bottom panel.

Figure 9

O $1s$ core-level XPS of caffeine on Au(111) recorded at a photon energy of $h\nu =650\mathrm{eV}$ and at an emission angle of $\mathrm{\Theta }=0^{\circ }$. The bottom panel shows the simulated XP spectrum of the caffeine monolayer.