Towards a 2D excitonic diode

November 28, 2022

The growth of lateral heterostructures with two different TMD materials is important for the fabrication of new strictly planar devices. Using tip-enhanced optical spectroscopies allowing sub-40 nm spatial resolution, it is shown the nonsymmetric diode-like behavior of a MoSe₂/WSe₂ lateral heterostructure. Thus, excitons created under the tip in the WeSe₂ part can migrate through the junction and recombine in the MoSe₂ area. Contrary, excitons created in the MoSe₂ part are blocked at the interface due to the energy barrier.

Monolayers of transition metal dichalcogenides (TMD) are extensively studied for their unique optoelectronic properties. TMD heterostructures were initially obtained by stacking two monolayers of two different TMDs, for instance MoSe₂ and WSe₂. Recently, pure 2D lateral heterostructures (see fig.) have been obtained by chemical vapor deposition by Tuchanin’s group in Iena, then transferred on a substrate encapsulated between h-BN layers, and characterized in the framework of an international collaboration, including researchers from LPCNO and CEMES in Toulouse.

At CEMES, the heterojunction was studied by tip-enhanced Raman spectroscopy (TERS) and tip-enhanced photoluminescence spectroscopy (TEPL), allowing a spatial resolution of 40 nm, about 10 times better than with a standard confocal microscope. The excitation laser is focused on the silver-coated tip, locally enhancing the electric near-field, hence the creation of excitons (an electron-hole pair) in the TMD layer. The PL due to electron-hole pair recombination and the Raman signal are detected and analyzed in each point of the sample. In fact, the sample is raster scanned while recording spectra sequentially with the tip in contact with the surface, and 30 nm above the surface. The near-field contribution is then obtained by taking the difference between both recorded signals.

The TERS mappings delimit the very sharp boundary between both materials with a resolution of about 40 nm, equivalent to the tip diameter at its apex. The TEPL spectra are fitted using a combination of the exciton contributions of MoSe₂ and WSe₂. The first important result is that a MoSe₂ contribution can be detected when the tip is on the WSe₂ side, proving that excitons created even far from the interface (up to 400 nm) can diffuse and recombine in the MoSe₂ region. In contrary, when the tip is in the MoSe₂ region, even close to the interface, no WSe₂ contribution is ever detected.

This asymmetric exciton transport behavior, schematically shown in the figure, proves that the lateral heterojunction acts as an excitonic diode, with unidirectional transfer of excitons from WSe₂ to MoSe₂.

This work was partly funded by projects ANR HiLight (ANR-19-CE24-0020-01) and EUR NanoX 2DLight (ANR-17-EURE-0009). Tip enhanced optical spectroscopies were performed during H. Lamsaadi’s master internship.

Publication:
Exciton spectroscopy and unidirectional transport in MoSe₂-WSe₂ lateral heterostructures encapsulated in hexagonal boron nitride
D. Beret, I. Paradisanos, H. Lamsaadi, Z. Gan, E. Najafidehagani, A. George, T. Lehnert, J. Biskupek, U. Kaiser, S. Shree, A. Estrada-Real, D. Lagarde, X. Marie, P. renucci, K. Watanabe, T. Taniguchi, S. Weber, V. Paillard, L. Lombez, J.-M. Poumirol, A. Turchanin, and B. Urbaszek.
npj 2D Materials and Applications volume 6, 84 (2022)
https://doi.org/10.1038/s41699-022-00354-0

Contacts:
Jean-Marie Poumirol – jean-marie.poumirol[at]cemes.fr
Vincent Paillard – vincent.paillard[at]cemes.fr