Orbital-selective single molecule excitation and spectroscopy based on plasmon-exciton coupling

The electronic excitation of molecules triggers diverse phenomena such as luminescence and photovoltaic effects, which are the bases of various energy-converting devices. Understanding and control of the excitations at the single-molecule level are long standing targets, however, they have been hampered by the limited spatial resolution in optical probing techniques. Here we investigate the electronic excitation of a single molecule with sub-molecular precision using a localised plasmon at the tip apex of a scanning tunnelling microscope (STM) as an excitation probe. Coherent energy transfer between the plasmon and molecular excitons is discovered when the plasmon is located in the proximity of isolated molecules, which is corroborated by a theoretical analysis. The polarised plasmonic field enables selective excitation of an electronic transition between anisotropic frontier molecular orbitals. Our findings have established the foundation of a novel single-molecule spectroscopy with STM, providing an integrated platform for real-space investigation of localised excited states.


SUMMARY PARAGRAPH:
The electronic excitation of molecules triggers diverse phenomena such as luminescence and photovoltaic effects, which are the bases of various energy-converting devices. Understanding and control of the excitations at the single-molecule level are long standing targets [1][2][3][4] , however, they have been hampered by the limited spatial resolution in optical probing techniques. Here we investigate the electronic excitation of a single molecule with sub-molecular precision using a localised plasmon at the tip apex of a scanning tunnelling microscope (STM) 5 as an excitation probe. Coherent energy transfer between the plasmon and molecular excitons 2,6-8 is discovered when the plasmon is located in the proximity of isolated molecules, which is corroborated by a theoretical analysis. The polarised plasmonic field enables selective excitation of an electronic transition between anisotropic frontier molecular orbitals. Our findings have established the foundation of a novel single-molecule spectroscopy with STM, providing an integrated platform for realspace investigation of localised excited states.
Single-molecule spectroscopy employing luminescence and Raman scattering has been established [9][10][11][12] and become indispensable in various research fields such as quantum physics, physical chemistry, and biophysics. The single-molecule sensitivity in luminescence and Raman spectroscopies relies on the fact that these processes involve energy conversions, so the spectroscopic signals can be separated from the excitation source. This is not the case for absorption spectroscopy [1][2][3][4] which is the primary technique to directly investigate the excitation process. Because absorbance is measured as attenuation of the incident light at a given frequency, the absorption signal competes with the huge shot-noise of the incident photons 13 .
This signal isolation problem and the limited spatial resolution in absorption spectroscopy makes it particularly difficult to investigate the excitations at the single molecule level.
In contrast to the conventional techniques using external light sources, scanning tunnelling luminescence (STL) spectroscopy, where luminescence is induced by the atomically localised tunnelling current of STM, provides a unique way to investigate local optical properties of matters 5,12,[14][15][16][17][18][19] . A previous STL study reports that the energy of the localised plasmon in the tunnelling junction of an STM is absorbed by molecules located close to the STM tip 20 . Although the number of molecules interacting with the plasmon was not determined in the study, the pioneering work showed that the near-field excitation of molecules is possible in the STM, opening up an opportunity for revealing the molecular excitations with an unprecedented precision. Figure 1a illustrates the design of the experiment to investigate the detail of near-field interaction between the localised plasmon and molecular excitons. The tunnelling current of the STM excites the localised plasmon which then interacts with the molecule through the plasmon-exciton coupling, and the photons emitted from this coupled system are detected. The strength of the plasmon-exciton coupling is finely tuned by changing the lateral distance between the molecule and the location of the localised plasmon with the angstrom precision. Figure 1b shows a topographic STM image of the sample. Since the electronic decoupling of a molecule from a metallic substrate is necessary to identify the intrinsic optical properties of the molecule 11 , free-base phthalocyanine (H2Pc) was deposited on an ultrathin NaCl(100) film 21 grown on Ag(111) (results of scanning tunnelling spectroscopy measurement are shown in Supplementary Fig. 1). We define the coordinates around the molecule as shown in Fig. 1c.
A series of luminescence spectra induced by the tunnelling current were obtained near an H2Pc/NaCl as a function of the lateral distance of the STM tip from the molecular centre (Fig. 1d). When the tip is placed far from the molecule ( ≥ 4 nm, # 1, 2), the luminescence spectrum is dominated by a broadband emission, which is attributed to the radiative decay of the localised plasmon 5 . Remarkably, dip structures emerge at 1.81 and 1.92 eV in the broad spectrum when the tip is positioned close to the molecule (r = 1.4-3 nm, # 3-11), and the spectral features becomes prominent as the tip is closer to the molecule. When the STM tip reaches at the edge of the molecule where direct excitation of the molecule by carrier injection turns into possible, intensive molecular luminescence is observed (Fig. 1e). The luminescence peaks at 1.81 and 1.92 eV are attributed to transitions from the first and second singlet excited states of H2Pc, so-called the Qx and the Qy states 22-25 , respectively (detailed peak assignment is given in Supplementary Fig. 3). Figure 2a shows two representative STL spectra measured with tip positions far from and close to the molecule. We define the STL spectrum measured with the tip located far from the molecule as the excitation source spectrum I0, and that measured close to the molecule as I(r,θ). As shown in Fig. 2b, the ratio spectrum I/I0 clearly reveals the change in the spectral shape resulting from the plasmon-exciton coupling.
The larger dip at 1.81 eV has an asymmetric feature, and the smaller dip at 1.92 eV has a more complex shape.
The origin of the dip structures is investigated based on the theory of STL using the nonequilibrium Green's function method 7,8 . Figure 2c shows calculated STL spectra with the plasmon-exciton coupling ℏg of 0 and 10 meV, and Fig. 2d displays the I/I0 spectrum. When ℏg = 10 meV, the energy of localised plasmons is absorbed by the electronic transitions of the molecule through the plasmon-exciton coupling, which leads to the dip structures at 1.81 and 1.92 eV. The asymmetric spectral profile is explained by a quantum mechanical interference effect 2,6-8 . The energy of localised plasmons is absorbed by the molecule, and then the energy of the excited molecule is re-emitted into localised plasmons. These processes interfere with each other, and the constructive and destructive interference of these processes leads to enhancement and suppression of the energy transfer, resulting in asymmetric spectral shapes 7,8 . These dynamic processes in the coupled plasmon-exciton system are schematically summarised in Fig. 2e. Figure 3a shows the θ dependence of the excitation I/I0 signal (θ is defined in Fig. 1c). When the STM tip is on the x-axis (θ = 0º), the spectrum only shows the Qx dip at 1.81 eV and a small dip at 1.90 eV (indicated by an arrow). This is reminiscent of a typical absorption spectrum having a mirror symmetry with the corresponding luminescence spectrum 24 . Indeed, the small dip at 1.90 eV can be attributed to the transition from the ground state to a vibrationally excited state of the Qx state, because the dip is located at the symmetric position about the band origin of the Qx state (1.81 eV) with the small vibronic peak at 1.72 eV observed in the electroluminescence spectrum (Fig. 1e). When θ = 90º, the Qx dips disappear and the spectrum displays only the Qy dips around 1.92 eV with a rather complicated dip structure. The congested spectral feature has been explained by state-mixing between the Qy state and vibrationally excited Qx state [22][23][24] . If the tip is placed between the xand y-axes (θ = 45º), both the Qx and Qy dips appear. transition is the major component of the second (first) singlet excited state, inversely 25 . This remarkable assignment is fully consistent with our results. As shown in Fig. 3a, the second singlet excited state (the Qy state) is exclusively detected when the direction of ∥ ⃗⃗⃗⃗ ( ) is aligned with the y-axis of H2Pc (θ = 90º), in which only the electric dipole transition between the HOMO and LUMO is allowed by the selection rule. In addition, the HOMO−LUMO transition in the x-direction is prohibited by the selection rule. This is because both MOs have the same (odd) parity in x (Fig. 3b), and the Qy state is never detected when ∥ ⃗⃗⃗⃗ ( ) is aligned with the x-axis (θ = 0º).
We note that the I/I0 spectrum reproduces the following features seen in the typical absorption spectrum of H2Pc 21,23,29,30 : the signal at 1.81 eV (Qx) is stronger than that at 1.92 eV (Qy), and the energy separation of 110 meV between the two dips agrees with the reported value (106-126 meV which varies depending on the environment surrounding H2Pc) 24 measured at low temperature with the matrix isolation method.
Therefore, it is concluded that the I/I0 spectrum allows us to measure the excitation or absorption characteristics of a single molecule with a molecular scale (~2 nm) spatial resolution.
One important requirement for a spectroscopy is the capability to differentiate molecules. Figure 4a shows an STM image, where H2Pc and magnesium phthalocyanine (MgPc) are co-deposited on a 3ML-NaCl film on Ag(111). The adsorption structures of MgPc and H2Pc on the NaCl film were previously investigated and reported elsewhere 27 . I spectra of MgPc and H2Pc as well as an I0 spectrum on 3ML-NaCl were measured using the same STM tip (Fig. 4b), and analysed to provide the I/I0 spectra (upper curves in Fig. 4c and 4d). The I/I0 spectra of MgPc shows only one doubly-degenerate molecular resonance at 1.89 eV (Q state), which is between the Qx (1.81 eV) and Qy (1.92 eV) states of H2Pc. This observation is consistent with the four-fold molecular symmetry and the previously results of conventional spectroscopy 28 . The apparently symmetric dip structure in the I/I0 spectrum of MgPc is reasonable in the coupled plasmonexciton system 2,6-8 where the interference condition depends on the relative energy of the plasmon and exciton 6 .
The STL spectra measured on the molecules are also presented in Fig. 4c and 4d (lower curves). While the I/I0 spectra of the two molecules have similar signal intensities, the intensity of STL fluorescence of MgPc is surprisingly weak; the integrated photon intensity of the Q fluorescence peak of MgPc is almost one-order weaker than that of Qx fluorescence peak of H2Pc. The similar intensities in the I/I0 spectra are straightforward to understand. Since the Q state of MgPc and the Qx state H2Pc have similar magnitudes of transition dipole moment ⃗ 29 , the strengths of the plasmon-exciton coupling ⃗ • ⃗⃗ which dominate the I/I0 spectrum should also be similar. In contrast, precise explanation for the very different intensities in STL fluorescence is not an easy task. This is because the fluorescence intensity is determined by several factors including the probability of creating an exciton by a tunnelling electron, and the rates of fluorescence and competing non-radiative processes. Thus, it can be concluded that I/I0 spectrum provides fundamental information about the molecular excitations and STL fluorescence spectrum reflects the complicated energy and charge dynamics in the excited states.
We have revealed the details of plasmon-exciton coupling and demonstrated the direct analysis of singlemolecule excitation. As shown in Supplementary Fig. 4, the energy of the localised plasmon is tunable from the near-infrared to the visible range, thus our method is expected to be widely applicable to various molecules, clusters, and local defects, even including non-luminescent objects. More importantly, the developed technique in combination with STL has established the foundation of an absorption/emission spectroscopy with the unprecedented sensitivity and spatial resolution, which will certainly expand our Preparation of the sample and the tip.
The Ag(111) surface was cleaned by repeated cycles of Ar + ion sputtering and annealing. The deposition of NaCl was performed using a home-made evaporator heated to 850 K onto Ag(111) held at room temperature. H2Pc and MgPc were deposited onto the NaCl-covered Ag(111) at 4.7-10 K cooled in the STM head using a commercial three-cell evaporator (Kentax) heated to 575 K for H2Pc and 625 K for MgPc.
The STM tips were prepared by electrochemical etching of a silver wire and conditioned by controlledindentation and voltage pulse on the Ag(111) surface.

STL measurement.
The STM stage was designed to be equipped with two optical lenses (each covered a solid angle of about 0.5 sr). The emitted light was collimated with the lens and directed out of the UHV chamber, where it was refocused onto a grating spectrometer (Acton, SpectraPro 2300i) with a charge-coupled-device photon detector (Princeton, Spec10) cooled with liquid nitrogen. All the optical spectra (except for figures in Supplementary Information) were measured using a grating with 300 gr/mm, and the spectral resolution of the optical system was better than 1 nm, which corresponds to an energy resolution better than 0.003 eV at 1.80 eV. The peak energies presented in this paper are rounded to two decimal places. The STL spectra are not corrected for the optical throughput of the detection system.

Theoretical calculations.
We