Transport in single molecule transistors
Yu, Lam H.
Doctor of Philosophy
As the size of a physical system decreases toward the nanoscale, quantum mechanical effects such as the discretization of energy levels and the interactions of the electronic spins become readily observable. To understand what happens when an isolated quantum mechanical object, such as an individual molecule, is coupled to a classical object, such as a macroscopic piece of metal, is one of the goals of modern condensed matter physics. The central question of our research is: How do the degrees of freedom of a single molecule (both electronic and mechanical) interact with an electrostatic environment under a constrained geometry? We have chosen to answer this question by looking at the electronic transport through single molecule transistors (SMTs), nanometer-scale transistors in which charge transport occurs through individual molecular states. We use an electromigration technique to fabricate SMTs based on C60 and transition metal coordination complexes (TMCCs). In these devices, the molecule of interest is constrained between two metallic electrodes which act as reservoirs of electrons and energy. At low temperatures, each transistor acts as a single-electron device in the Coulomb blockade regime. Our experimental results suggest that the vibrational modes of the molecules contribute to the transport characteristics of the SMTs. From measurements of the differential conductance of these devices, we observe direct tunneling features that are consistent with vibrational excitations of the molecules. In the TMCC-based SMTs, we also observe inelastic cotunneling features that correspond energetically to vibrational excitations of the molecule, as determined by Raman and infrared spectroscopy. This is a form of gate-modulated inelastic tunneling spectroscopy. In some of the SMTs we observe conductance features characteristic of the Kondo effect, a coherent many-body state comprising an unpaired spin on the molecule coupled by electronic correlation effects to the conduction electrons of the leads. The inferred Kondo temperature in these devices typically exceeds 50 K. In TMCC-based SMTs that exhibit the Kondo effect we observe unusual transport characteristics that deviate from the simplest model of Kondo physics in single electron devices. We suggest possible mechanisms, including strong intramolecular exchange and electron-phonon interaction, that may explain the observed deviation.
Condensed matter physics