Viewing the Interior of a Single Molecule: Vibronically Resolved Photon Imaging at Submolecular Resolution

We report the spatial imaging of the photon transition probability of a single molecule at submolecular resolution. Photon imaging of a ringlike pattern is further resolved as two orthogonal vibronic transitions after incorporating spectral selectivity. A theoretical model and the calculated intensity images reveal that the transition probability is dominated by the symmetry of the positions of the tip and the transition dipole moment. This imaging technique enables the probing of the electronic and optical properties in the interior of a single molecule.

Figure 1

Topographic images (top, Z) and photon images (bottom, P) taken simultaneously. Image sizes and resolutions are labeled at the top of each figure. Same color pallets are used from (A) to (C) but the indicated absolute values of both Z and P are only for (C). All images are acquired under 2.2 V and 0.2 nA. Integration time of the APD at each pixel is $0.1\mathrm{s}$ in (A) and $0.2\mathrm{s}$ in (B) and (C). The dots and numbers in (C) indicate the positions where the spectra in Fig. 2 were taken.

Figure 2

(A) Vibronically resolved emission spectra taken at five positions inside the MgP molecule [$P1\mathrm{\text{−}}P4$ and center; positions are indicated in Fig. 1c], oxide background ($\times 5$), and metal background (NiAl). Colors are used to distinguish type-I (blue) and type-II (red) spectra in the online edition. All spectra are taken under 2.3 V bias and 0.25 nA tunneling current with $300\mathrm{s}$ accumulation time. (B),(C) Photon images of individual vibronic peaks taken under 2.3 V bias and 0.4 nA tunneling current. The size for all the images is $35\AA \times 35\AA$. Photon image of type-I peak from 750 to 763 nm (B) shows orthogonal symmetry compare to that of type-II transition from 766 to 778 nm (C). CCD integration time at each pixel is $1.0\mathrm{s}$. (D),(E) Top and side view of the molecular structure of MgP. Its orientation is aligned to the photon images in (B) and (C). I and II label the positions of type-I and type-II emission, respectively.

Figure 3

$dI/dV$ spectra, excitation spectra, and the illustrations of the LUMO-HOMO transitions mechanism. (A) $dI/dV$ spectra at $P1$, $P2$, and center of the MgP as indicated in Fig. 1c. All the color codings are the same as Fig. 2. (B) Photon excitation spectra as a function of bias voltage. Notice that the oxide background emission reaches its maximum at 1.7 V. This slightly affects the determination of the LUMO onset. (C)–(D) Schematic illustrations of LUMO-HOMO transition. (C) At this bias voltage, the HOMO is being lifted above the Fermi level by the electrostatic potential and becomes an empty state. (D) The vibronic transition occurs when the bias is high enough to inject electrons into LUMO ($>1.8\mathrm{V}$).

Figure 4

(A),(B) Mechanism for generating a net perpendicular dipole moment, when the transition dipole is parallel to the surface. If the tip is centered over the molecule, the contribution from the two point dipole illustrated in (A) cancel, whereas they do not when the tip is off center, as shown in (B). (C),(D) The simulated intensity images of light emission from the two orthogonal LUMO states. The intensity is maximized when tip is off from the center of the molecule.