(Invited) Thermoelectric Studies of Graphene Antidot Lattices and Beyond
Authors: Hao Qing 
Published: February 2020

Pristine graphene has poor thermoelectric performance due to its low Seebeck coefficient S and ultra-high thermal conductivity k. Various approaches have been explored to improve this low Seebeck coefficient, mainly by opening a band gap within semimetal graphene. Among these, one effective method is to pattern periodic nanoscale or sub-1-nm pores (antidots) across graphene to form the so-called graphene antidot lattices (GALs). In GALs, a geometry-dependent band gap can be opened to dramatically increase S. Antidots also strongly scatter the phonons, leading to a significantly reduced k. The combination of these two phenomena results in a high thermoelectric figure of merit (ZT).

Despite numerous theoretical studies on GALs, the only comprehensive experimental study was carried out on GALs fabricated from monolayer or bilayer graphene grown by chemical vapor deposition [1]. In this recent work, the electrical measurements (300-520 K) were performed on a SiO2/Si substrate using a commercial system (TEP-600, Seepel Instrument, Korea), while k measurements (300-450 K) were performed with two-laser Raman thermometry for suspended GALs. In the reported data, however, inconsistency still existed in measured S and estimated band gaps for various GALs. In addition, the in-air Raman measurements had uncertainties due to a) the difficulty in determining the exact laser power absorbed by a sample, b) the influence of air conduction and convection, and c) the strong thermal nonequilibrium between electrons, optical phonons and acoustic phonons under laser heating [2].

In this work, GALs with ~10 nm patterns are systematically measured on SiC, SiO2 and hexagonal boron nitride (h-BN) substrates for the three thermoelectric properties. The measurements are performed at 77-300 K to reveal the low-temperature performance of GALs. For the SiO2 and h-BN substrates, a gate voltage is further applied to tune the electrical properties. In contrast with aforementioned electrical measurements using a commercial setup, nanosized probes are fabricated on GALs to accurately read the temperature and voltage. When the electron mean free paths are mostly restricted by the ultrafine nanoporous structures, the substrate influence on the electrical properties becomes weak so that the measured electrical properties slightly vary for different substrates. In addition, the thermal measurements are also carried out on suspended GALs using a T-bridge microdevice employed for previous graphene measurements [3]. Beyond GALs, other representative 2D materials with/without nanopores are measured for their thermoelectric properties. Discussions will be further given for the electron/phonon modeling of general 2D nanoporous materials [4, 5].

References:

[1] J. Oh, H. Yoo, J. Choi, J.Y. Kim, D.S. Lee, M.J. Kim, J.-C. Lee, W.N. Kim, J.C. Grossman, J.H. Park, S.-S. Lee, H. Kim, J.G. Son, Nano Energy, 35 (2017) 26-35.

[2] A.K. Vallabhaneni, D. Singh, H. Bao, J. Murthy, X. Ruan, Physical Review B, 93 (2016) 125432.

[3] W. Jang, W. Bao, L. Jing, C. Lau, C. Dames, Applied Physics Letters, 103 (2013) 133102.

[4] Q. Hao, Y. Xiao, H. Zhao, Journal of Applied Physics, 120 (2016) 065101.

[5] Q. Hao, H. Zhao, D. Xu, Journal of Applied Physics, 121 (2017) 094308.