In situ microscopy
Charge and heat transport in graphene nanostructures depend critically on their atomistic details. We combine cryogenic transport measurements with a wide range of in situ methods, including transmission electron microscopy and scanning thermal microscopy, to investigate the microscopic origins of effects such as Fabri-Pérot interference, Peltier heating/cooling and Franck-Condon blockade.
Transmission electron microscopy Graphene-based nanoscale quantum devices are an ideal platform for molecular electronics, advanced sensing, and understanding quantum phenomena. A key challenge to their realization is the understanding of the atomic and molecular configuration of the system and particularly how this influences the stability, sensitivity, and capability of the devices. To address this challenge, we have developed a high-yield process to fabricate suspended graphene nanodevices compatible with low-voltage aberration-corrected scanning transmission electron microscopy (AC-STEM) and to correlate atomic-scale characterization with quantum transport measurements obtained from the same devices.
Scanning thermal microscopy Nanostructuring can strongly alter the thermoelectric properties of materials. We use a scanning thermal microscope (SThM) to map the spatial dependence of nanoscale heating and cooling resulting from the electrical current flowing through an active device. By heating the SThM tip and measuring the voltage drop over the global contacts, we can also map the thermovoltage of the same device. Both the Seebeck and Peltier coefficients can be mapped with a lateral resolution of tens of nanometres.
Quantum transport in nanoscale electronic devices
Electron transfer is perhaps the most ubiquitous molecular process, and has received a constant attention for nearly a century. Single-molecule transistors provide an exciting platform for studying electron transfer mechanism under conditions that are inaccessible in other experimental setups, and on a scale of individual molecules. We study charge transport through atomic- and molecular-scale transistors to uncover the quantum mechanical nature underlying charge and heat transport.
Quantum interference A key feature of electron transport through atomic- and molecular-scale nanostructures is the appearance of transport resonances associated with quantum interference. Examples include Breit–Wigner resonances, multipath Fabry–Pérot resonances, and Fano resonances. We study quantum interference in electroburnt graphene nanoconstrictions, top-down fabricated silicon nanowires, and single-molecule transistors.
Electron-phonon coupling Coupling between electronic and vibrational degrees of freedom in single-molecule devices can lead to transport properties very different from those of metal/semiconductor nanostructures. Charge transfer can excite vibrational modes, or phonons, and strong electron–phonon coupling leads to suppression of tunnel current at low bias voltages – so-called Franck-Condon blockade. Analogous to exciton transport in vivo (which has attracted a great deal of attention in the nascent field of quantum biology) vibrational interactions in molecular systems can also significantly enhance the efficiency of charge transport.
Heat-to-electricity conversion Conventional heat engines convert a temperature difference into mechanical work. Similarly, molecular heat engines use quantum transport to turn a thermal energy into electrical power. Nanoscale heat engines are small and do not have any moving parts, therefore they are ideal for low-power applications. We can measure the thermoelectric power conversion of a single molecule or graphene quantum dot device. The energy conversion rates achieved in these devices can be tuned close to the theoretical limit by carefully engineering the electronic energy levels, thus providing a viable pathway towards on-chip cooling and energy harvesting for quantum technologies.
Graphene-based molecular biosensing
Nanoscale biosensor technologies hold the promise of revolutionizing techniques ranging from biological interfaces to rapid pathogen detection to enabling DNA data storage. With high accuracy, sensitivity, and affordability, these sensors are predicted to drive a shift to personalized medicine and rapid diagnostics in real-time anywhere in the world. We are utilizing graphene as the active component for scalable tunneling sequencing methods.