Single-molecule Transport

Single-molecule transport spectroscopy reveals rich physics of quantum dots. In our lab we fabricate nanoscale field-effect transistors (FETs) where patterned graphene serves as leads bridged by a single molecule (quantum dot). The electrostatic environment experienced by the single molecule can be tuned by applied gate- and bias voltages, that respectively alter the discrete chemical potentials of the single molecule and the leads. In this way, we can electrostatically align the chemical potentials of the device to allow an electric current to pass through the single molecule, or bring them away from alignment to block the current. The current-voltage characteristics of the device therefore depend on the properties of the quantum-dot channel, which means that single-molecule FETs are devices that can perform spectroscopy on its own channel through charge transport.

 

 

With the help of our dilution refrigerator, our devices can be cooled down to ~20 mK which reduces the electronic thermal noise to a level where we can resolve individual quantum states of the single molecule. A superconducting vector-rotate magnet is integrated with the dilution refrigerator and enables the application of vector magnetic fields of up to 6 Tesla. This allows us to investigate the magnetic properties of single molecules with single-electronic-spin sensitivity. With this setup we can study static quantum properties of single-molecule FETs, e.g. addition energies, ground- and excited states, electron-vibron coupling, magnetic effects and spin-filling. By integrating microwaves, we could add electron spin resonance to our experimental toolbox, which would expand our spectroscopical range to include dynamic quantum properties of our device under testing. Spin-lattice relaxation time (T1) and spin-dephasing time (T2) are dynamic properties that become accessible through microwave techniques.

Our collaborators at The Max Planck Institute for Polymer Science in Mainz provides us with molecularly precise graphene nanoribbons (GNRs) which we can incorporate into single-molecule FETs and study using transport spectroscopy. These GNRs are special because their fabrication method solves the problem of edge disorder in GNRs thorugh bottom-up chemical synthesis which yields molecular GNRs with well-defined edges and side groups. Very few transport experiments have been carried out on single GNRs with low disorder mainly due to the fabrication issues, but a vast amount of theoretical works predict a range of interesting features such as magnetic edge states. With access to molecular GNRs we are now in a unique position to study the quantum physics of GNRs unperturbed by noise sourcing from edge disorder.