A Nanocar and Rotor in One Molecule

Abstract

Depending on its adsorption conformation on the Au(111) surface, a zwitterionic single-molecule machine works in two different ways under bias voltage pulses. It is a unidirectional rotor while anchored on the surface. It is a fast-drivable molecule vehicle (nanocar) while physisorbed. By tuning the surface coverage, the conformation of the molecule can be selected to be either rotor or nanocar. The inelastic tunneling excitation producing the movement is investigated in the same experimental conditions for both the unidirectional rotation of the rotor and the directed movement of the nanocar.

Thermal with Electronic Excitation for the Unidirectional Rotation of a Molecule on a Surface

“…The obtained experimental yield, or action spectrum 22 (Y = R(I/e)), is plotted in Figure 4c. For V > 400 mV, we confirm the nearly constant yield of about 5 × 10 −10 rotations/electron previously reported, 15 which is assigned to the onset of the C−H stretch vibrational mode at 370 meV. This vibration is the principal energy entry port to trigger a one-step rotation event in the higher bias voltage range.…”

“…Note that the entry port for the electron tunneling allowing a one-step rotation is at the methyl group on the opposite side of the Au–O bond, where repositioning of the tip after each pulse is necessary. Since DMNI-P is a chiral with the fixed Au–O anchoring point (Figure , namely, a “left DMNI-P”), we observed an opposite rotational direction with right DMNI-P . At these tunneling conditions, a single rotation step of 60° always in the same direction is reproducibly (99% of the cases from 195 pulses on different rotors) observed within 5 s. Such reaction time is extracted plotting the tip height over time during the pulse ( z ( t ), see Figure S2, Supporting Information).…”

“…At a temperature of 5 K, the isolated DMNI-P molecules in known to reliably rotate in one direction by a bias voltage of V ≥ 350 mV applied for a few seconds. 15 This specific molecule-rotor therefore represents a well-suitable model system to investigate the onset of the unidirectional rotation by combining thermal and low-voltage tunneling electronic excitations. As presented in Figure 2, at T = 5 K, a series of voltage pulses (typically V = 500 mV, I = 250 pA, t = 10 s) produce the controlled unidirectional rotation of the molecule in steps of 60°a round the anchoring position at the oxygen atom.…”

“…Compared to other single-molecule rotors, 3,12,23 this molecule strongly chemisorbs at low molecular coverage on Au(111), making it impossible to induce any lateral movement on the surface. 15 In the STM tunnel junction, the needed structural asymmetry is carried internally by the molecule-rotor itself. At a temperature of 5 K, the isolated DMNI-P molecules in known to reliably rotate in one direction by a bias voltage of V ≥ 350 mV applied for a few seconds.…”

“…At each electron transfer event, a quantum mixing of the virtual cationic ground state and the anionic excited electronic states occurs, with a very short femtosecond quantum occupation time . This experiment was followed by several others, where chiral or asymmetric molecules adsorbed on a surface were investigated aiming at reaching unidirectional rotation by inelastic electron tunneling. ,,− …”