Intermolecular interactions and substrate effects for an adamantane monolayer on a Au(111) surface

We study theoretically and experimentally the infrared (IR) spectrum of an adamantane monolayer on a Au(111) surface. Using a STM-based IR spectroscopy technique (IRSTM) we are able to measure both the nanoscale structure of an adamantane monolayer on Au(111) as well as its infrared spectrum, while DFT-based ab initio calculations allow us to interpret the microscopic vibrational dynamics revealed by our measurements. We find that the IR spectrum of an adamantane monolayer on Au(111) is substantially modified with respect to the gas-phase IR spectrum. The first modification is caused by the adamantane-adamantane interaction due to monolayer packing, and it reduces the IR intensity of the 2912 cm${}^{-1}$ peak (gas phase) by a factor of 3.5. The second modification originates from the adamantane-gold interaction, and it increases the IR intensity of the 2938 cm${}^{-1}$ peak (gas phase) by a factor of 2.6 and reduces its frequency by 276 cm${}^{-1}$. We expect that the techniques described here can be used for an independent estimate of substrate effects and intermolecular interactions in other diamondoid molecules and for other metallic substrates.

Figure 1

(a) Model of molecular structure of an adamantane molecule. Hydrogen and carbon atoms are represented by white and gray spheres, respectively. (b) STM topography of a self-assembled island of adamantane molecules on a Au(111) surface (sample bias $V_{\mathrm{sample}}=-1.0$ V, current setpoint $I=100$ pA, temperature $T=13$ K). (c) Schematic picture of alignment of the molecules on the Au(111) surface. Gold atoms in the topmost layer, second layer, and third layer are represented by gold, blue, and green color spheres of different shades. Black lines show the supercell of the $\sqrt{7}\times \sqrt{7}$ molecular alignment. Uppercase letters indicate the location of adsorption sites: A for atop site, B for bridge site, and H for hollow site. Top view of an adamantane in the hollow-atop geometry is also illustrated (the center of the molecule is in the hollow site and the three bottom hydrogen atoms are in the atop site). The three bottom hydrogen atoms can be seen at the bottom of Fig. 1a, but not be seen in Fig. 1c.

Figure 2

The green line shows the experimentally observed spectrum (averaged over 15 sweeps) of 0.8 ML of adamantane on a Au(111) surface with gold baseline signal subtracted. The IR absorption peaks are seen at $2846\pm 2$ and $2912\pm 1$ ${\mathrm{cm}}^{-1}$. The vertical black dashed lines show the IR peak positions of an adamantane molecule in the gas phase as listed in Table (2859, 2912, 2938 ${\mathrm{cm}}^{-1}$).

Figure 3

Eigendisplacement vectors of (a) ${\omega }_1$, (b) ${\omega }_2$, (c) ${\omega }_4$, (d) ${\omega }_5$, (e) ${\omega }_6$, and (f) ${\omega }_7$ modes of an isolated adamantane molecule (notation is from Table ). Gray and white spheres represent carbon and hydrogen atoms, respectively. Blue arrows indicate the displacements of the atoms (mostly hydrogen atoms). Some of the atoms and eigendisplacements are overlapping since the molecule is shown from a high-symmetry direction.

Figure 4

Calculated IR spectrum of an isolated adamantane molecule (black dashed line), an adamantane monolayer (blue dotted line), and an adamantane monolayer on Au(111) (red solid line). The IR spectra are displayed in arbitrary units (chosen so that the intensity of the IR peak around 2650 cm${}^{-1}$ equals 1.0). The black vertical line in the figure separates lower and higher frequency regimes (there are no IR active modes in the intermediate regime between 2730 and 2840 cm${}^{-1}$). Here we show only the IR spectra close to the C-H stretching mode frequencies (the closest IR mode not corresponding to C-H stretching is below 1500 cm${}^{-1}$).

Figure 5

Interpolated IR spectra between the single molecule adamantane (solid black line) and the adamantane monolayer (solid blue line). The dashed ($\lambda =0.2$), dotted ($\lambda =0.4$), and chain ($\lambda =0.8$) lines are interpolated spectra between two cases (using techniques from Sec. 3b). The arrows indicate the peaks evolution from the isolated molecule case to the monolayer case.

Figure 6

Phonon eigendisplacement vectors of (a) ${\omega }_1^{\mathrm{M}}$, (b) ${\omega }_2^{\mathrm{M}}$, (c) ${\omega }_3^{\mathrm{M}}$, (d) ${\omega }_4^{\mathrm{M}}$, (e) ${\omega }_5^{\mathrm{M}}$, and (f) ${\omega }_6^{\mathrm{M}}$ modes of an adamantane monolayer without a gold substrate.

Figure 7

Interpolated IR spectra going from an isolated molecular monolayer to a monolayer on Au(111). The blue solid line shows the IR spectrum of the isolated molecular monolayer, while the red solid line shows the IR spectrum of the monolayer on the Au(111) substrate. The dashed ($\lambda =0.1$), dotted ($\lambda =0.2$), and chain ($\lambda =0.4$) lines show an interpolated spectra between the two cases. The arrow shows the transition from ${\omega }_1^{\mathrm{M}}$ to ${\omega }_1^{\mathrm{Au}}$. We also indicate the peak position of the relatively weak ${\omega }_2^{\mathrm{Au}}$ mode. The severely redshifted peak ${\omega }_6^{\mathrm{Au}}$ is not shown in this figure (see left panel of Fig. 4.)

Figure 8

Eigendisplacement vectors of (a) ${\omega }_1^{\mathrm{Au}}$, (b) ${\omega }_2^{\mathrm{Au}}$, (c) ${\omega }_3^{\mathrm{Au}}$, (d) ${\omega }_4^{\mathrm{Au}}$, (e) ${\omega }_5^{\mathrm{Au}}$, and (f) ${\omega }_6^{\mathrm{Au}}$ modes of an adamantane monolayer on the Au(111) surface. Yellow spheres illustrate Au atoms in the topmost layer of the Au(111) surface.

Figure 9

Uncorrected (blue dashed line) and corrected (red solid line) theoretical IR spectra and experimentally observed IR spectrum (green solid line). The frequency region between 2730 and 2840 cm${}^{-1}$ is not shown. The vertical scale is chosen so that the theoretical and experimental peaks around 2850 cm${}^{-1}$ have almost the same height. Left and right vertical axes correspond to theoretical and experimental IR intensity. Corrected theoretical values of ${\omega }_1^{\mathrm{Au}}$, ${\omega }_2^{\mathrm{Au}}$, ${\omega }_4^{\mathrm{Au}}$, ${\omega }_5^{\mathrm{Au}}$, and ${\omega }_6^{\mathrm{Au}}$ modes are 2914, 2851, 2893, 2869, and 2644 cm${}^{-1}$, respectively.

Figure 10

Summary of our main results. There are three IR active C-H stretching modes in the isolated (gas phase) adamantane molecule (black dashed line). The interaction between the neighboring adamantane molecules in the monolayer (packing effect) reduces the IR intensity of the central IR active mode by a factor of 3.5. The interaction between the monolayer and the Au(111) surface (Au substrate effect) reduces the highest frequency gas phase mode by 276 cm${}^{-1}$, and it increases its IR intensity by a factor of 2.6. The remaining third IR active gas phase mode is affected both by the Au(111) substrate and by the packing effect. See Sec. 4b for more detailed analysis. The calculated IR spectrum of an adamantane monolayer on Au(111) is shown with a red line, while the experimental spectrum is shown with a green line.