Abstract

Using an electromigration procedure which we recently developed, we generate 10 nm-scale single-wall carbon nanotube quantum dot (SWCNT-QD) transistors. Because these devices are so short, we can explore fundamental mesoscopic physics, engineer tuneable nanoelectromechanical systems (NEMS) and create ultra-short transistors. These dramatic effects arise from enhanced electron-vibron and QD-lead
coupling in short devices.

Contrary to what has been observed in longer SWCNT devices, we observe strong electron-hole asymmetry, due to charge doping from the metallic leads. This asymmetry manifests itself as a striking difference between electron and hole charging energies (up to a factor of 3), and their conductance (0D to 1D transport). The magnitude of this asymmetry depends on the length of the SWCNT. Suspended SWCNTs can strongly couple to their electrostatic environment through the bending mode, and act as NEMS sensors. Shorter NEMS have higher frequencies and therefore higher sensitivity. By creating very short devices, we observe self-actuated bending mode frequencies up to approximately 280 GHz, and tune this frequency by electrostatic strain. We clearly resolve the first and second harmonic of the bending resonance and extract their effective coupling lambda on the order of one.

In high conductance devices, we observe strong electron-electron interactions, with Kondo temperatures up to Tk approximately 28 K, and use these interactions to resolve the energy spectrum of the QD. In devices combining Kondo and bending oscillations, we measure a reduction in charging energy, to the point of complete suppression.
This is, to our knowledge, the first time this effect has been observed in molecular transistors.