Infrared Spectroscopy of Molecular Submonolayers on Surfaces by Infrared Scanning Tunneling Microscopy: Tetramantane on Au(111)

We have developed a new scanning-tunneling-microscopy–based spectroscopy technique to characterize infrared (IR) absorption of submonolayers of molecules on conducting crystals. The technique employs a scanning tunneling microscope as a precise detector to measure the expansion of a molecule-decorated crystal that is irradiated by IR light from a tunable laser source. Using this technique, we obtain the IR absorption spectra of [121]tetramantane and [123]tetramantane on Au(111). Significant differences between the IR spectra for these two isomers show the power of this new technique to differentiate chemical structures even when single-molecule-resolved scanning tunneling microscopy (STM) images look quite similar. Furthermore, the new technique was found to yield significantly better spectral resolution than STM-based inelastic electron tunneling spectroscopy, and to allow determination of optical absorption cross sections. Compared to IR spectroscopy of bulk tetramantane powders, infrared scanning tunneling microscopy (IRSTM) spectra reveal narrower and blueshifted vibrational peaks for an ordered tetramantane adlayer. Differences between bulk and surface tetramantane vibrational spectra are explained via molecule-molecule interactions.

Figure 1

(a) STM topography of [121]tetramantane molecules on Au(111) ($V_{\mathrm{sample}}=1.0\mathrm{V}$, $I=50\mathrm{pA}$, $T=13\mathrm{K}$). The molecular lattice has an oblique structure with lattice constants $|\mathbf{a}|=11.1\pm 0.1\AA$, $|\mathbf{b}|=8.3\pm 0.1\AA$, and an interior angle of $59°\pm 1°$. The inset shows a model of [121]tetramantane (gray and white balls represent carbon and hydrogen atoms). (b) STM topography of [123]tetramantane on a Au(111) surface ($V_{\mathrm{sample}}=-2.0\mathrm{V}$, $I=100\mathrm{pA}$, $T=13\mathrm{K}$). Within experimental resolution, [123]tetramantane molecules form a hexagonal structure with $|\mathbf{a}|=|\mathbf{b}|=9.8\pm 0.1\AA$. The inset shows a model of $P$ [123]tetramantane [our [123]tetramantane/Au(111) samples were prepared from a racemic mixture containing $P$ and $M$ enantiomers].

Figure 2

Experimental setup combining a tunable IR laser and a UHV STM. The IR light intensity is stabilized by a feedback loop consisting of an acousto-optical modulator (AOM), a beam splitter (BS), and a diode detector. The IR light was directed into the STM as described in the text. A lock-in amplifier and chopper were used for ac measurements.

Figure 3

(a) Differential ac surface expansion due to absorption of modulated IR light, measured via lock-in amplifier for constant-current STM $Z$ signal (chopper modulation frequency $\sim 13\mathrm{Hz}$, lock-in amplifier time constant 3 s). The black (lower) and red (upper) lines show ac surface expansion of bare gold and [121]tetramantane-decorated gold, respectively. (b) dc surface expansion of bare gold (black lower line) and [121]tetramantane-decorated gold (red upper line) as a function of incident IR frequency, measured via constant-current STM $Z$ signal.

Figure 4

dc surface expansion due to molecular IR absorption as a function of incident IR frequency. (a) The black solid line shows the average dc surface expansion of [121]tetramantane/Au(111) (five sweeps) with gold baseline signal subtracted. The blue line (with a single broad peak) shows an STM $d^{2}I/d{V}^2$ spectrum of [121]tetramantane/Au(111) from Ref. 17. (b) The black solid line shows the average dc surface expansion of [123]tetramantane/Au(111) (twelve sweeps) with gold baseline signal subtracted. The red dashed lines are (a) six-peak and (b) five-peak Lorentzian fits to the experimental data.