Graphene has attracted a lot of interest due to its excellent properties and potential for practical applications. Graphene has superior electrical conductance and mechanical properties, though its lack of an intrinsic bandgap limits its applications. Recently the thermoelectric power (TEP), described by the Seebeck Coefficient (S), of this material has been studied in order to apply its thermoelectric properties to photodetection or energy harvesting applications. Additionally, because the TEP depends on the material’s band structure, it is a powerful tool to help characterize the electronic structure of a material. Graphene has been shown to have improved characteristics when it is suspended from the substrate. Despite this, there has been little research into the thermoelectric properties of suspended graphene.
The TEP is the ability of a material to convert a temperature gradient into a voltage. Efficient power generation is shown by the figure of merit, ZT = σS2T/K, where σ is the electrical conductance, T is the temperature, K is the thermal conductance, and S = ΔV/ΔT. A high ZT value indicates a higher performance of the thermoelectric material. To study and improve the thermoelectric properties of graphene in this research, an established suspended device structure is used for graphene grown via chemical vapor deposition (CVD).
Graphene was grown via a CVD process by placing a copper substrate into a low pressure furnace with an Ar, H2 mixture and ramping the temperature to 1000C. Then CH4 was introduced for 10 minutes, after which the furnace was allowed to cool. The graphene was then spin-coated with PMMA before etching the substrate with ammonium persulfate. The graphene was then wet transferred onto a pre-patterned low stress SiNx wafer. The wafer was then patterned again such that a platinum heater and gold contacts were added, and that the graphene and islands became suspended.
Fabricated suspended devices with graphene were then studied using a closed circuit refrigeration system under high vacuum. The device is first calibrated for the R-T correlation of the platinum resistors on each island for use as thermometers. This process is done by using an I-V slope to find the resistance at each temperature point. A temperature gradient can be created and measured across the islands by using a relatively large current on one of the resistors. Measuring the resulting voltage across the graphene at the same time allows S to be found. The electrical conductance of graphene can also be found using the same device structure. Preliminary studies in this research have shown excellent thermal insulation of the device indicating that the heat is being transferred via thermal conduction instead of radiation. Finally this research will discuss results from gold nanoparticle covered graphene samples to see if any improvements on S or ZT occur.