Nano-mechanical oscillations in a single-C 60 transistor

Over the last decade, electron transport through quantum dots has attracted considerable attention from the scientific and engineering community. The electronic motion through these structures is strongly modified by single-electron charging and energy level quantization1,2. Recently, much effort has been directed toward extending these studies to chemical nanostructures, such as molecules3-8, nanocrystals9-13, and nanotubes14-17. Here we report for the first time the fabrication of single-molecule transistors based on individual C60 molecules. Transport measurements of single-C60 transistors provide evidence for coupling between the center-of-mass motion of C60 and single-electron hopping18, a novel conduction mechanism that has not been observed in previous quantum-dot studies. This coupling manifests itself as quantized nano-mechanical oscillations of C60 against the gold surface. The frequency of this oscillation is determined to be around 1.2 THz, in excellent agreement with a simple theoretical estimate based on van der Waals and electrostatic interactions between C60 and gold electrodes. Single-C60 transistors were prepared by depositing a dilute toluene solution of C60 onto a pair of connected gold electrodes fabricated using e-beam lithography. A break-junction technique 11 was then used to create a ~1 nm gap between these electrodes by the process of electromigration. In a significant fraction of these devices, the conductance of the junction after initial breaking is substantially enhanced compared to devices with no C60 deposited, indicating that C 60 molecules reside in the junction. The entire structure was defined on a SiO 2 insulating layer on top of a degenerately doped silicon wafer which serves as a gate electrode that modulates the electrostatic potential of C 60. A schematic diagram of an idealized single-C 60 transistor is shown in the lower inset of Fig. 1. Figure 1 presents representative current-voltage ( I-V) curves obtained from a single-C 60 transistor at different gate voltages ( Vg). The device exhibited strongly suppressed conductance near zero bias voltage followed by step-like current jumps at higher voltages. The voltage width of the zero-conductance region (conductance gap) could be changed in a reversible manner by changing Vg. In ten devices prepared from separate fabrication runs, the conductance gap could be reduced to zero by adjusting Vg , although the gate voltage at which the conductance gap closed ( Vc) varied from device to device. Figures 2 and 3 show two-dimensional plots of differential conductance ( , 9) as a function of both V and Vg for four different devices. Peaks in , 9, which correspond to the step-like features in Fig. 1, show up as lines in these plots. As seen clearly in Figs. 2 and 3, the size of the conductance gap and the , 9 peak positions evolve smoothly as Vg is varied. As the gate voltage was varied farther away from Vc in both positive and negative directions, the conductance gap continued to widen and exceeded | V|  mV in some devices. Many , 9 peaks outside the conductance gap are also observed. The Vg-dependent features described above were not observed in devices when C 60 was not deposited on the electrodes. In addition, many different devices exhibited similar conductance characteristics, as shown in Fig. 2. Furthermore, the observed behavior is consistent with a single nanometer-sized object bridging two electrodes, as explained in detail below1. Although C60 could not be imaged directly in these devices due to its small size (~7 Å in diameter), these experimental observations indicate that individual C 60 molecules are responsible for the conductance features observed in the experiment. The global patterns observed in Figs. 1 3 can be understood using ideas borrowed from the Coulomb blockade model developed for the analysis of quantum-dot transport 1. The conductance gap observed in the data is a consequence of the finite energy required to add (remove) an electron to (from) C 60. This energy cost arises from the combined effect of single-electron charging of C 60 and the quantized excitation spectrum of the C 60-transistor system. The maximum observed gap in the experiment indicates that the charging energy of the C 60 molecule in this geometry can exceed 150 meV. The conductance gap changes reversibly as a function of Vg because more positive gate voltage stabilizes an additional electron on C60. The conductance gap disappears at Vg = Vc where the electrochemical potential becomes identical for two different C 60 charge states. When the gate voltage traverses Vc in the positive direction, the equilibrium number of charges on C 60 changes by one electron from n 60 C to ( )1 + n 60 C , where n designates the number of charges on C 60. It is determined by both Vg and the local electrochemical environment, that is the work function of the metal electrode and the local charge