Confinement Effect in Thermoelectric Properties of Two-Dimensional Materials

Thermoelectric (TE) materials, or materials that can generate an electrical energy from temperature gradient, are promising for renewable energy technology. One fundamental aspect in the TE research is the demand to maximize the TE power-factor, PF = Sσ, by having as large Seebeck coefficient (S) and electrical conductivity (σ) as possible. In the early 90s, Hicks and Dresselhaus proposed the PF enhancement by using low-dimensional materials, in which electrons are confined in certain directions and they move freely in the other directions. This quantum effect is known as the confinement length (L) effect, in which L is the thickness or diameter of the two-dimensional (2D) or one-dimensional materials, respectively. However, a key challenge is to understand the critical value of L, at which the PF can be significantly enhanced. Recently, we reevaluated the confinement theory of the low-dimensional materials to solve this issue. We showed that electrons are fully confined only when L is smaller than an intrinsic length Λ, the so-called thermal de Broglie wavelength, which depends on the materials and can be experimentally measured. Monolayer 2D materials naturally satisfy the condition of L < Λ since their confinement length is ∼ 1 nm, while their thermal de Broglie wavelength is ∼ 5-10 nm. Therefore, they could be a good candidate for TE materials. In this review article, we first review the TE materials with low dimensions. Then, we show the basic concept of the confinement effect and the consequence of such an effect. Finally, based on this effect, we turn our attention to the progress achieved recently in the TE properties of the 2D materials such as monolayer InSe, GaN electron gas, and SrTiO3 superlattices.