(Invited) Transition Metal Dichalcogenide Monolayers As Quantum Wells
Authors: Takenobu Taishi 
Published: February 2020

Recently, transition metal dichalcogenide (TMDC) monolayers, such as molybdenum disulfide (MoS2) and tungsten diselenide (WSe2), have attracted strong attention as novel two-dimensional (2D) semiconducting materials due to their large bandgap (1−2 eV) and excellent transport properties. Moreover, the thickness of monolayer TMDCs is less than 1 nm, which is one of the thinnest materials, and it leads to strong confinement effects, resulting in large binding energy of exciton (> 100 meV) and formation of charged excitons. Particularly, due to their layered structure, there are no dangling-bond states on the surface of TMDC monolayers and it could be an ideal quantum well. Here, we will report the optical and thermoelectric properties of TMDC monolayers as quantum wells.
In this study, we fabricated ion-gel-gated EDLTs using large-area TMDC monolayers, MoS2 and WSe2, grown by chemical vapor deposition [1-8]. The Fermi level of TMDCs can be continuously shifted by applying gate voltage, and we can induce both hole and electron transport in these devices. The hole mobility of WSe2 can be enhanced up to 90 cm2/Vs at high carrier density of 1014 cm-2, whereas the MoS2 showed electron mobility of 60 cm2/Vs.
Using EDLT technique, we investigated the electric field modulation spectroscopy of TMDC monolayers and the clear signature of quantum well, such as quantum confined Stark effect, was observed. Very importantly, it is well-clarified that, in quantum wells, the bandgap reduction by electric field is simply proportional to width of quantum well. Therefore, in ultimately thin TMDC monolayers, we need extremely strong electric field to realize obvious spectral modulation, which is very difficult for conventional field-effect transistors with oxide dielectric layers such as SiO2 and Al2O3. To break this limitation, we used EDLT technique and firstly observed the obvious evidence of quantum confined Stark effect.
Seebeck coefficient, which is the proportional constant to the voltage generation against induced temperature gradient, is a significant factor to design thermoelectric materials. Importantly, according to the Mott equation, Seebeck coefficient is proportional to the energy derivative of the electronic density of states at around Fermi energy and, therefore, low-dimensional structures/materials are suitable for thermoelectric applications. We also investigated Fermi energy dependence of Seebeck coefficient in TMDC monolayers using EDLT technique. As the results, we observed clear enhancement in thermoelectric properties due to the low-dimensional effect.
[1] J. Pu, L.-J. Li, T. Takenobu, et al., Nano Lett. 12, 4013 (2012).
[2] J.-K. Huang, T. Takenobu, L.-J. Li, et al., ACS Nano. 8, 923 (2014).
[3] J. Pu, L.-J. Li, T. Takenobu, et al., Appl. Phys. Lett., 103, 23505 (2013).
[4] J. Pu, L.-J. Li and T. Takenobu, Phys. Chem. Chem. Phys., 10.1039/C3CP55270E (2014).
[5] Y.-H. Chang, T. Takenobu, L.-J. Li, et al., ACS Nano. 8, 8582 (2014).
[6] C.-H. Chen, T. Takenobu, L.-J. Li, et al., 2D Materials, 1, 034001 (2014).
[7] L. Chu, T. Takenobu, G. Eda , et al., Scientific Reports, 4, 7293 (2014)
[8] K. Funahashi, L.-J. Li, T. Takenobu, et al., Jpn. J. Appl. Phys. 54, 06FF06 (2015).