Manjunath Rajagopal1 Krishna Valavala1 Jangyup Son1 Sunphil Kim1 Arend Van Der Zhande1 Sanjiv Sinha1

1, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States

Two-dimensional materials such as MoS2 have been shown to exhibit large Seebeck coefficient and are promising candidates for thermoelectric energy conversion [1]. The ab-initio calculations predict a high intrinsic thermoelectric power factor if the substrate effects are excluded [2]. But the overall power factor is typically limited by its low electrical conductivity. This is possibly due to localized states arising from the impurities and the adsorbates introduced from the substrate [2]. In this work, we measure Seebeck coefficient and electrical conductance of CVD-grown monolayer MoS2 on Si/SiO2 substrates [3]. We observe hopping transport from the localized states introduced by the substrate at low temperatures and low electrostatic doping. To reduce the observed defect states, we use 2D hexagonal boron nitride (hBN) as a substrate because it provides less charge inhomogeneity and atomically flat surface. The CVD grown MoS2 monolayer is placed on top of hBN using a dry transfer printing process and a PDMS stamp [4]. We observe an enhanced thermoelectric power factor from MoS2 on hBN substrate over Si/SiO2. However, we still find the localized states to be dominating the extended states from Seebeck measurements at low temperatures and doping. We attribute these defect states observed in hBN/MoS2 to arise from the dry transfer printing process that could introduce strain heterogeneity. We systematically characterize the defect states through electrical measurements on similarly transfer printed monolayer MoS2 on Si/SiO2 substrates. This work advances the understanding of substrate effects on transfer printed 2D materials for improved thermoelectric performance.
1. Wu, Jing, et al. Nano letters 14.5 (2014): 2730-2734.
2. Babaei, Hasan, J. M. Khodadadi, and Sanjiv Sinha. Applied Physics Letters 105.19 (2014): 193901.
3. Lee, Gwan-Hyoung, et al. ACS nano 7.9 (2013): 7931-7936.
4. Lee, Chul-Ho, et al. Nature nanotechnology 9.9 (2014): 676-681.