Abstract
1 Introduction
There are many heats that are being thrown away around us. About 42% of Japan's exhaust heat is below 200 ℃, and recovery of these low-temperature exhaust heat is hardly progressing. This is because the places where low-temperature exhaust heat is emitted are dispersed in homes, subways, cars, and so on. Inorganic materials such as Bi-Te series that have been used for thermoelectric power generation are rare, expensive, and have no flexibility. Therefore, we focused on graphene, which is a carbon material with a two-dimensional structure. Graphene is a lightweight, flexible material that exhibits unique electrical properties near room temperature. Thermoelectric devices have improved performance by alternately connecting p-type and n-type semiconductor elements. However, since graphene is p-type doped in the atmosphere, there are few reports of stable realization of n-type in graphene.
In this study, the thermoelectric properties of high quality graphene synthesized by plasma CVD were evaluated. In addition, we tried to make it n-type by performing metal intercalation on bilayer graphene.
2 Experiments and results
2-1 Synthesis and transfer of graphene
Bilayer graphene was synthesized on copper foil by plasma CVD method. Next, the copper foil was etched to transfer graphene to the PET substrate. In addition, graphene with a maximum of 20 layers was prepared by repeating transfer. The graphene of each layer number prepared was evaluated by a Raman spectroscope (HORIBA, XploRA). The laser wavelength used was 532 nm. The number of graphene layers was calculated from the transmittance measured using a haze meter (Nippon Denshoku Industries, NDH5000).
2-2 Evaluation of thermoelectric properties of graphene
The Seebeck coefficient and electric conductivity of bilayer and 20-layer graphene were measured with a thermoelectric property evaluation device (ADVANCE RIKO, ZEM-3) (Figure). The Seebeck coefficient was 70 μV / K and the PF was 2.96 mW / mk2 when the sample temperature was 376 K in bilayer graphene. This is a high value compared to the reported organic thermoelectric materials 1). The increase of the number of layers contributed to the improvement of electric conductivity, but did not change the Seebeck coefficient. Further, it was found that the graphene produced was p-type because the Seebeck coefficient had a positive value.
2-3 Elemental intercalation into bilayer graphene
An attempt was made to intercalate K for electron doping of graphene. The substrate used was SiO2 / Si, and it was soaked in a 20 mol% KOH solution for 20 minutes immediately before the step of transferring graphene to the substrate 2). Similarly, we tried to intercalate Ca by infiltrating it in a 0.06 mol% Ca(OH)2 solution. The doping states of these samples were evaluated by SEM / EDX. In addition, a field effect transistor using graphene as a channel was manufactured by a lithography process in order to evaluate the electrical characteristics. In the future, the electrical characteristics of graphene will be measured and pn determination will be performed.
1) Takao Ishida, Surface Technology, Special Feature: Energy Harvesting, 67, 334-337 (2016)
2) T. Yamada, Y. Okigawa, M, Hasegawa, Appl. Phys. Lett. 112, 04310 5 (2018)
Figure 1