Vibronic Spectroscopy with Submolecular Resolution from STM-Induced Electroluminescence

A scanning tunneling microscope is used to generate the electroluminescence of phthalocyanine molecules deposited on $\mathrm{NaCl}/\mathrm{Ag}(111)$. Photon spectra reveal an intense emission line at $\approx 1.9\mathrm{eV}$ that corresponds to the fluorescence of the molecules, and a series of weaker redshifted lines. Based on a comparison with Raman spectra acquired on macroscopic molecular crystals, these spectroscopic features can be associated with the vibrational modes of the molecules and provide a detailed chemical fingerprint of the probed species. Maps of the vibronic features reveal submolecularly resolved structures whose patterns are related to the symmetry of the probed vibrational modes.

Figure 1

(a) Sketch of the STM-induced emission experiment. (b) STM image ($I=30\mathrm{pA}$, $V=-2.5\mathrm{V}$) of a single ZnPc molecule (lower left), a dimer (lower right) and a linear tetramer (top right) of ZnPc molecules deposited on $\mathrm{NaCl}/\mathrm{Ag}(111)$. (c) STM-induced light emission spectrum (black line) of the ZnPc linear tetramer [$I=0.75\mathrm{nA}$, $V=-2.5\mathrm{V}$, $t=300\mathrm{s}$, tip located at the white arrow in (b)]. The right part of the spectrum is represented magnified by 50 and compared to a resonance Raman spectra acquired on a ZnPc crystal at room temperature and in air (excitation wavelength 632.8 nm). The raw (smoothed) data appear in gray (black) in the magnified spectrum. (d) Frequency shifts (in ${\mathrm{cm}}^{-1}$) of the vibronic peaks in the STM-LE and Raman spectra displayed in (c), in the fluorescence spectra of ZnPc molecules trapped in frozen Argon matrices taken from Ref. [16] and assignment to vibrational modes of the ZnPc with their respective symmetry based on DFT calculations.

Figure 2

(a) STM images ($I=30\mathrm{pA}$, $V=-2.5\mathrm{V}$) and (b) STM-LE spectra acquired for a single molecule, a dimer, a trimer and a tetramer of molecules deposited on $\mathrm{NaCl}/\mathrm{Ag}(111)$ ($V=-2.5\mathrm{V}$). The white arrows in (a) mark the location of the tip during the acquisition of the optical spectra. The peak width in the case of the tetramer is only 60% larger than our spectral resolution; consequently its FWHM is slightly overestimated. The panels (c) and (d) show the low energy vibrational features of the same spectra as a function of the wavelength (c) and as a function of the shift from the 0-0 lines (d).

Figure 3

(a) STM-induced light emission spectra ($V=-2.5\mathrm{V}$) acquired for three positions of the tip [coloured marks in (b)] with respect to a molecular trimer. The spectra are vertically shifted for clarity. (b) Constant height STM image of the trimer ($5.1\times 2.7{\mathrm{nm}}^2$, $V=-2.5\mathrm{V}$) and its scaled model. Normalized photon intensity maps for (c) the 0-0 emission and for (d) the 1332 and (e) $1507{\mathrm{cm}}^{-1}$ peaks ($5.1\times 2.7{\mathrm{nm}}^2$, $V=-2.5\mathrm{V}$, $\mathrm{time}/\mathrm{pixel}=10\mathrm{s}$. The photon maps were slightly processed with a Gaussian smoothing algorithm.)

Figure 4

Sketches (a) of the diodelike emission mechanism and (b) of the energy transfer mechanism. (c) $dI/dV$ spectra (in log scale) acquired on a molecular dimer for different tip-molecule distances $\mathrm{\Delta }z$. $\mathrm{\Delta }z=0$ corresponds to a current set point of $I=7.5\mathrm{pA}$ at $V=-2.5\mathrm{V}$. (d) Light spectra acquired on the same dimer as a function of voltage ($I=150\mathrm{pA}$, $t=180\mathrm{s}$). The spectra are vertically shifted for clarity.