Figure 1

(a) A double-barrier tunnel junction. (b) Schematic representation for the resonant electron tunneling into the anion vibronic states of a single molecule (for positive sample bias). (c) $dI/dV$ spectrum of a naphthalocyanine (NPc) molecule adsorbed on the oxide surface grown on NiAl(110). The tunneling gap was set with 2.0 V sample bias and 100 pA tunneling current. The bias modulation was 7 mV (rms) at 327 Hz. Insets show a schematic of NPc-molecule and constant-current (17 pA) STM images ($4\mathrm{nm}\times 4\mathrm{nm}$) with sample bias of $-1.35\mathrm{V}$ (left), $-1.2\mathrm{V}$ (center), and 1.2 V (right). Images are not filtered. Dotted line within the right-hand image represents the positions of $d^{2}I/d{V}^2$ mapping shown in Fig. 2.Reuse & Permissions

Figure 2

$d^{2}I/d{V}^2$ spectroscopic mapping taken across a NPc molecule at positions equally spaced from point A to J, as shown in the inset of Fig. 1c. The bottom curve represents the background signal over the oxide in the vicinity of the molecule. The set point for the tunneling gap was 1.3 V and 60 pA. The spectra are offset for clarity.Reuse & Permissions

Figure 3

Curve fittings of the $d^{2}I/d{V}^2$ spectra (a) F (in the center) and (b) J (on the lower left lobe) with a series of equally- spaced Gaussian functions. The resolved peaks are grouped into 4 progressions. The first progression is composed of the first, fifth, ninth, and thirteenth peaks. Second progression: second, sixth, and tenth peaks. Third progression: third, seventh, and eleventh peaks. Fourth progression: fourth, eighth, and twelfth peaks (see text for detail). These progressions are indicated by the blue, magenta, green, and orange lines, respectively (color online). The summation of the fitted peaks is the curve following most closely the data points (gray line in color online). Insets show the spatial distribution of the peak intensity for the first (first, fifth, and ninth peaks, lower panel) and third (third and seventh peaks, upper panel) progressions. The topographic trace of the molecule at the set point for mapping is also shown in the upper inset (dashed curve, $\sim 0.5\mathrm{nm}$ in height).Reuse & Permissions

Figure 4

Schematic representation of the progression of progressions for a NPc molecule. The principal vibronic states (${\upsilon }_1$) are denoted by thick lines, from which the energies for the other vibrational modes (${\upsilon }_2$, ${\upsilon }_3$, ${\upsilon }_4$) are measured. The second excited states for ${\upsilon }_2$, ${\upsilon }_3$, and ${\upsilon }_4$ vibrations are not resolved in the experiment. With increasing sample bias, the Fermi level of the tip aligns with each vibronic substate one after the other, and the tunneling probability increases. The corresponding spectral features in $dI/dV$ and $d^{2}I/d{V}^2$ spectra are indicated on the right.Reuse & Permissions