Enhancement of thermoelectric performance in two-dimensional materials: A review of recent progress

A thermal gradient not only generates a heat current but could also result in a usable voltage difference if applied to a suitable material. Conversely, it is possible to use voltage differences to drive heat currents. The effects related to the mutual conversion of temperature and voltage differences are known as the thermoelectric (TE) effects. TE materials have been used for energy harvesting and solid-state cooling applications for decades. Nevertheless, their performances need significant enhancements to overcome the growing demands for applications. The challenge is that a good TE material needs to have a high electrical conductivity like in a metal, a high Seebeck coefficient like in an insulator, and a low thermal conductivity like in a glass. A number of strategies have been proposed to meet these diverse demands. Dimension reduction is a promising route to enhance the efficiencies of conventional TE materials, and it was proposed long before the innovation of 2D materials. The emergence of 2D materials has stimulated intensive research into their TE properties, with numerous approaches proposed to enhance their performance. Here, we review the recent literature on the TE properties of 2D materials by focusing on the employed enhancement strategies. In addition to the effects of quantum confinement, strategies such as nanostructuring, strain engineering, edge/surface functionalization, creating defects/vacancies, and doping are discussed thoroughly. Approaches to incorporate the intrinsic electronic properties, to utilize the materials with intrinsically low lattice thermal conductivity, and proposed techniques to further lower thermal conductivity are also reviewed. Lastly, the properties of emerging 2D materials with potentially high TE efficiencies are summarized.