Probing Intermolecular Coupled Vibrations between Two Molecules

Intermolecular interactions can induce energy shifts and coupling of molecular vibrations. However, the detection of intermolecular coupled vibrations has not been reported at the single molecule level. Here we detected an intermolecular coupled vibration between two CO molecules, one on the surface and another on the tip within the gap of a subkelvin scanning tunneling microscope, and analyzed the results by density functional calculations. We attribute the evolution of the energy and intensity of this coupled vibration as a function of tip-sample distance to the tilting and orbital alignment of the two CO molecules.

Figure 1

STM topographies and vibrational IETS of CO on Ag(110). (a) STM constant current topography ($45\AA \times 45\AA$) of CO molecules adsorbed on Ag(110) surface scanned with a bare Ag tip, the tunneling set point is 0.1 nA and 10 mV. (b) and (c) Constant current topographies of the same area with a CO-terminated tip at 0.1 and $0.5\mathrm{nA}/10\mathrm{mV}$. The CO molecule transferred to the tip is indicated with the magenta arrow in (a). (d) $d^{2}I/d{V}^2$ STM-IETS for a bare Ag tip over Ag(110) as background (upper curves in black), Ag tip over $\mathrm{CO}/\mathrm{Ag}(110)$ (upper curves in red), CO tip over Ag(110) (middle curves in blue) and CO tip over $\mathrm{CO}/\mathrm{Ag}(110)$ (bottom curves in green). The schematic diagrams of the three tunneling junctions are displayed in the insets. The magnified ($\times 5$) IETS feature near 35 meV is obtained with finer bias step size and more averaging passes. For all spectra: sample bias modulation is $2{\mathrm{mV}}_{\mathrm{rms}}$ at 311.11 Hz, tunneling gap set with $0.17\mathrm{nA}/10\mathrm{mV}$ for the Ag tip and $0.1\mathrm{nA}/10\mathrm{mV}$ for the CO tip.

Figure 2

$Z$-dependent STM-IETS. (a) A series of STM-IETS spectra at different tip-sample distances taken with the CO tip over the center of another CO adsorbed on Ag(110). The spectra are multiplied by different scale factors as indicated on the left side and offset vertically for better visualization. Sample bias modulation was $1.5{\mathrm{mV}}_{\mathrm{rms}}$ at 311.11 Hz. (b) Schematic diagram of the CO tip over CO on Ag(110). $Z_0$ is the tunneling gap defined by $I=80\mathrm{pA}$ and $V=10\mathrm{mV}$. For each spectrum, the tip feedback was turned off at $Z_0$, then the CO tip was advanced (positive values) or retracted (negative values) by $\mathrm{\Delta }Z$ before making the measurement. (c) The vibrational energies extracted from (a) versus $\mathrm{\Delta }Z$. Error bars of energy reflect the uncertainty from Gaussian peak fitting; error bars of $\mathrm{\Delta }Z$ (about 0.03 Å) reflect the tip height drift during the measurement and uncertainty of STM scanner $\mathrm{\Delta }Z$ calibration. The higher energy mode ${\upsilon }_{\beta }$ is the out-of-phase coupled hindered translation as illustrated in the inset.

Figure 3

(a) Calculated energy of coupled vibrational mode ${\upsilon }_9$ (magenta curve) and the angle between $\overrightarrow{{\mathit{O}}_s{\mathit{O}}_t}$ and $(\overrightarrow{{\nu }_t^O}-\overrightarrow{{\nu }_s^O})$ (light blue curve) versus tip-sample distance $Z$. The energy reaches a maximum at $Z=7.25\AA$. In the meantime, the $\overrightarrow{{\mathit{O}}_s{\mathit{O}}_t}$ is aligned with $(\overrightarrow{{\nu }_t^O}-\overrightarrow{{\nu }_s^O})$ and the angle between them approaches 0°. (b) Schematic diagram of the coupled mode eigenvector at $Z=7.25\AA$; $Z$ is defined as the vertical distance from the protruding Ag atom on the tip and the Ag surface layer before structure relaxation in the calculation. (c) $Z$ dependence of distances between pairs of atoms in the two CO molecules.

Figure 4

(a) The relative conductance change $\mathrm{\Delta }\sigma /\sigma$ as a function of $\mathrm{\Delta }Z$ for mode ${\upsilon }_{\beta }$. The vibrational energies are also shown to reveal the correlation between energy and IETS intensity. Three coupling regimes specified by i, ii, and iii are defined based on the IETS intensity of ${\upsilon }_{\beta }$ mode. Regime ii corresponds to lowest IETS intensity. (b) Schematic diagrams illustrating the interplay between the CO tilting angle and $\pi$ orbital alignment of the two CO molecules in the junction. The opposite phases of the CO orbitals are indicted by open and gridded circles. The spatial distribution of the ${\upsilon }_{\beta }$ mode deformation potential $\delta V_{\text{vib}}$ is depicted by color palette. Red and blue highlight the phase differences of $\delta V_{\text{vib}}$, which reflect the local symmetry of this out-of-phase coupled vibration.