Light-Induced Subnanometric Modulation of a Single-Molecule Electron Source

Single-molecule electron sources of fullerenes driven via constant electric fields, approximately 1 nm in size, produce peculiar emission patterns, such as a cross or a two-leaf pattern. By illuminating the electron sources with femtosecond light pulses, we discovered that largely modulated emission patterns appeared from single molecules. Our simulations revealed that emission patterns, which have been an intractable question for over seven decades, represent single-molecule molecular orbitals. Furthermore, the observed modulations originated from variations of single-molecule molecular orbitals, practically achieving the subnanometric optical modulation of an electron source.

Figure 1

Conceptual diagram of ultrafast electron emissions from a nano-object by irradiating a light pulse (a) and optical control of emission sites (b). Red solid circles indicate localized optical fields. (c) Conceptual diagram of spatial modulation of an electron source using resonance electron emissions through a molecule. Blue lines indicate the LDOS of the corresponding molecular orbitals, $\rho$. (d) Schematic of electron emissions from single ${\mathrm{C}}_{60}$ protrusions situated on a molecular layer formed on a sharp metallic tip. (e) Potential energy diagram of possible electron emission mechanisms from a single molecule due to dc electric fields and laser pulse irradiation. In (d) and (e), the green (red) arrows indicate the paths of electrons induced by dc electric fields (laser pulse irradiation) in real space and a potential configuration.

Figure 2

(a),(c) Observed electron emission patterns from single molecules without (upper panel) and with (lower panel) irradiation of femtosecond light pulses. Tip voltages of the upper and lower panels are (a) $-800$ and $-600$ V, and (b) $-2800$ and $-2200$ V. The laser fluence was $0.35\mathrm{mJ}/{\mathrm{cm}}^2$. (b) and (d) Simulations corresponding to the observations in (a) and (c), respectively. They are done under dc electric fields of $5.14\mathrm{V}/\mathrm{nm}$ [17]. The front views of the molecular orbitals of the main contributors to each emission pattern are also shown in (b) and (d). The molecular energy levels of orbitals M1, M2, M3, M4, and M5 are $-0.912$, $-0.340$, 2.571, 2.580, and 2.204 eV, respectively. (e) The sequential changes in emission patterns with and without laser excitation. Tip voltage and laser fluence are shown with each pattern. The polarization vector was parallel to the tip axis in (a) and (c), but in (e), the vector was rotated by 45° from the tip axis in a clockwise direction when the laser beam propagated toward the tip apex.

Figure 3

(a)–(d) Variations of observed electron emission patterns from a variety of single molecules (upper panels) and simulated results from a single fullerene molecule (lower panels) via dc electric fields. Tip voltages are (a) $-3000$ V, (b) $-2600$ V, (c) $-3000$ V, and (d) $-4200$ V. In the observed images, the shadow images from the metallic mesh, as can be seen in the emission patterns in Fig. 1, were removed using a fast Fourier transform filter. Simulations are done under dc electric fields of $6.17\mathrm{V}/\mathrm{nm}$ for (a) and (c), and $5.14\mathrm{V}/\mathrm{nm}$ for (b) and (d). (e) The geometry of ${\mathrm{C}}_{60}$ used in the simulation together with the $X$ and $Y$ vectors of the Cartesian coordinate axes. The vector of the $z$ axis is directed towards the readers (not shown). The electric fields were applied along the $z$ axis so that the electrons were emitted toward the reader. (f) Conceptual diagram for a change of molecular orbitals to be observed in emission patterns from a single molecule.