Electrical and Mechanical Properties of Graphene
Graphene is an exciting new atomically-thin two-dimensional (2D) system of carbon atoms organized in a hexagonal lattice structure. This "wonder material" has been extensively studied in the last few years since it's first isolation in 2004. Its rapid rise to popularity in scientific and technological communities can be attributed to a number of its exceptional propertiess. In this thesis I will present several topics including fabrication of graphene devices, electrical and mechanical properties of graphene.
I will start with a brief introduction of electronic transport in nanosclae system including quantum Hall effect, followed by a discussion of fundamental electrical and mechanical properties of graphene. Next I will describe how graphene devices are produced: from the famous "mechnical exfoliation" to our innovative "scratching exfoliation" method, together with the traditional lithography fabrication for graphene devices. We also developed a lithography-free technique for making electrical contacts to suspended graphene devices. Most of the suspended devices presented in this thesis are fabricated by this technique.
Graphene has remarkable electrical properties thanks to its crystal and band structures. In Chapter 3, I will first focus on proximity-induced superconductivity in graphene Josephson transistors. In this section we investigate electronic transport in single layer graphene coupled to superconducting electrodes. We observe significant suppression in the critical current Ic and large variation in the product IcRn in comparison to theoretic prediction; both phenomena can be satisfactorily accounted for by premature switching in underdamped Josephson junctions.
Another focus of our studies is quantum Hall effect and many body physics in graphene in suspended bilayer and trilayer graphene. We demenstrate that symmetry breaking of the first 3 Landau levels and fractional quantum Hall states are observed in both bilayer and trilayer suspended graphene devices. A surprising finding in these systems is the observation of insulating states in both suspended bilayer and trilayer graphene devices, which arises from electronic interactions. In bilayer graphene, we observe a phase transition between the single-particle metallic state and the interaction-induced insulating state in ultra-clean BLG, which can be tuned by temperature, disorder, charge density n and perpendicular electric field E. In trilayer graphene we demonstrate dramatically different transport properties arising from the different stacking orders, and an unexpected spontaneous gap opening in charge neutral ABC-stacked trilayer graphene.
One of graphene's unique properties is that it is nature's thinnest elastic membrane with exceptional mechanical properties. In chapter 7 I will describe the first direct observation and controlled creation of one- and two-dimensional periodic ripples in suspended graphene sheets, using both spontaneously and thermally generated strains. We are able to control ripple orientation, wavelength and amplitude by controlling boundary conditions and exploiting graphene's negative thermal expansion coefficient, which we measure to be much larger than that of graphite. In addition, we also study the morphological change of suspended graphene sheets by apply gate voltages, which is a simple and direct method to strain and buckle graphene.
Our experimental results contribute to the fundamental understanding of electrical and mechanical properties of graphene, and may have important implications for future graphene based applications.