Strain Tuning of Localized Neutral and Charged Excitons in a Monolayer Semiconductor

One promising path for the development of optoelectronic devices is the integration of two-dimensional (2D) materials with functional substrates. Transition metal dichalcogenides, such as tungsten diselenide (WSe₂), have attracted considerable interests due to their strong light-matter interaction. In this work, we show the strain tuning of localized neutral and charged excitons in a monolayer WSe₂ by using the inverse piezoelectric effect from a single crystal piezoelectric substrate. An out-of-plane electric field applied to the piezoelectric substrate, depending on the direction, can generate either tensile or compressive strain.<br/>We used the commercially available (001) orientated piezoelectric substrate, (PbMg_0.33Nb_0.67)<i>_1-x</i>: (PbTiO₃)<i>_x</i>; <i>x</i> = 0.29-0.32 (PMNT-PT), for this study. After pre-patterning of electrodes by using electron beam lithography, we transferred the monolayer WSe₂ on the substrate, followed by boron nitride (BN) and graphite layers to form a top-gated field effect transistor. We noticed that previous reports on strain tuning mainly focused on neutral excitons [1,2]. To the best of our knowledge, independent strain manipulation and electric control have not been realized in the layered materials. While the localized charged excitons, which can be optically initialized as quantum emitters, could provide more possibilities for applications in optoelectronics and quantum information sciences.<br/>We carried out the measurements in a closed cycle cryostat with a base temperature of ~3.6 K. We first characterized the localized neutral excitons, which generally exhibit negligible hysteresis and a maximum energy shift of ~4.5 meV in the range of -200 to +200 V. The localized charged excitons, on the other hand, show a much larger hysteresis. Particularly, the difference for both sweeping directions at 0 V reaches more than 2 meV. We furthermore measured the polarization and observed a hysteresis loop as well. Such hysteresis in polarization and energy are absent in the neutral excitons. We thus hypothesized that the extra charge in the localized charged exciton couples to the ferroelectric dipole moment in the substrate. Although being known as piezoelectric, the PMN-PT substrate is actually a relaxor ferroelectric with weak ferroelectricity.<br/>Our results demonstrate that 2D materials can be combined with functional ferroelectric substrates to produce highly tunable optoelectronic devices. The capacity to manipulate localized excitons in WSe₂ dynamically creates new opportunities for novel optoelectronic devices.<br/><br/>[1] C. Chakraborty, A. Mukherjee, H. Moon, K. Konthasinghe, L. Qiu, W. Hou, T. Peña, C. Watson, S.M. Wu, D. Englund, and N. Vamivakas, <i>Optica</i>, <b>7</b>(6), pp.580-585 (2020).<br/>[2] O. Iff, D. Tedeschi, J. Martín-Sánchez, M. Moczala-Dusanowska, S. Tongay, K. Yumigeta, J. Taboada-Gutiérrez, M. Savaresi, A. Rastelli, P. Alonso-González, and S. Hofling, <i>Nano letters</i>, <b>19</b> (10), pp.6931-6936 (2019).