Recent progress in enhancing the figure of merit for thermoelectric materials has relied on a range of innovative strategies, including reduction of the dimensionality, use of complex bulk materials, or introduction of impurity states in the bulk phase. These advances, based on modifications to three-dimensional (3D) crystalline materials, have been enabled by a deep understanding of the key thermoelectric (TE) properties such as thermal and electronic transport, and the impact of structural and chemical changes on these properties, in turn providing new design strategies for high-efficiency TE materials. In contrast to the case of 3D materials, two-dimensional (2D) materials such as graphene, single-layer boron nitride, or molybdenum disulfide have received little attention regarding their potential for thermoelectric applications.

Given the fact that 2D materials are “all-surface,” any modification to their environment or chemical functionalization will be expected to have enormous impact on properties, an appealing attribute when faced with such constrained optimization problems as in the case of thermoelectrics. Chemical functionalization, in particular, plays a key role for both thermal and electronic transport in 2D materials because either all or nearly all atoms in the system can be functionalized, leading to large modifications of charge carriers and phonon modes; this is in contrast to the 3D case, where surface chemical functionalization has a much smaller impact on transport. … In this work, we consider the prospect of patterned graphene nanoroads as an efficient thermoelectric material.

—Kim and Grossman (2015)

(a) Schematic of 2D thermoelectric based on patterned graphene nanoroads with an armchair-type boundary. Yellow shaded area indicates the patterned region. (b) H-functionalized graphene (HG) for 6 Å pattern width (top) and C5-functionalized graphene (C5G) (bottom). C and H atoms are represented by gray and yellow spheres, respectively. Pink contour plot represents charge density of the conduction band in each sample. Green and blue spheres indicate the C sites where C5 chains are attached on the up and down side of graphene, respectively. (c) Density of states (DOS) per unit cell of graphene, HG and C5G. Credit: ACS, Kim and Grossman. Click to enlarge.

The patterning introduces quantum confinement in pure (un-functionalized) graphene by reducing its dimensionality from 2D to quasi-1D, with the periodic functionalization lines forming arrays of parallel graphene regions. Confinement in the material leads to an improvement in the power factor by increasing the density of states at the Fermi level.

The MIT team investigated the thermoelectric properties of patterned graphene nanoroads functionalized with H and C5 chains using both classical and quantum mechanical calculations.

Using ab initio electronic structure calculations and the Boltzmann transport approach, they determined the role of such functionalization on the electrical transport properties, including the electrical conductivity (σ) and Seebeck coefficient. Combined with computed thermal conductivities, this enabled them to predict the full ZT in these materials.

The results showed that ZT can reach values as high as 3 at room temperature, and demonstrated the potential for controllably tuning the properties of 2D materials for thermoelectric applications.

Resources

• Jeong Yun Kim and Jeffrey C. Grossman (2015) “High-Efficiency Thermoelectrics with Functionalized Graphene” Nano Letters doi: 10.1021/nl504257q