2D materials with enhanced thermoelectric (TE) properties

Abstract

Graphene, one of the most well-studied two-dimensional (2D) materials, has been identified as a prospective thermoelectric (TE) material. Computational studies have shown that graphene can possibly achieve extraordinary TE effect even though it has yet to be showcased experimentally. Lower than expected TE effect is caused by the presence of random defects and grain boundaries introduced during growth. These defects can influence the electrical properties of the monolayer carbon network negatively. As such, there are limited development in the area of graphene TE device research.

To overcome the ill effects of poor control exercised during graphene growth, a methodology concerning defect density manipulation is discussed. The presence of oxygen molecules in the methane precursor has been identified as the main source of contaminant. Oxygen could oxidize methane to become methanol and formaldehyde. During graphene growth, these molecules are incorporated into the graphene superlattice. The amount of defects commensurate with the amount of oxygen present.

While low defect graphene is generally favoured for most applications, highly defective graphene can realize structures with different properties. Examples are when defective graphene is functionalized with ultraviolet photoreduced metal nanoparticles to form p-type graphene-Au, graphene-Ag and graphene-Pt, the associated sheet resistances improved by a factor of 5.26, 1.43 and 1.33, respectively. In particular, an 80% decrease in sheet resistance was reported in Au-graphene system. It translates to a more than three times increase in thermoelectric power factor.

Besides graphene, n-type materials, MoS2 and Bi2O2Se devices are fabricated. In this case, they are grown via cold wall and hot wall CVD methods, respectively. The materials demonstrated a large Seebeck coefficient of 300 V/K and 555 V/K at room temperature, respectively. Finally, both n- and p- type will be paired up to form a working thermocouple. With a suitable absorber, the formation of a thermopile could be used for thermal detection purpose.

This thesis paves ways for the development of 2D material based thermal detection system with performance far exceeding that of current systems. The final setup will showcase a nano-sized detector system. The system can be incorporated at relatively low cost, onto platforms that enable remote sensing with minimal human intervention.