Paradisanos Ioannis, Wang Gang, Alexeev Evgeny M, Cadore Alisson R, Marie Xavier, Ferrari Andrea C, Glazov Mikhail M, Urbaszek Bernhard
Université de Toulouse, INSA-CNRS-UPS, LPCNO, 135 Avenue Rangueil, Toulouse, 31077, France.
Cambridge Graphene Centre, University of Cambridge, Cambridge, CB3 0FA, UK.
Nat Commun. 2021 Jan 22;12(1):538. doi: 10.1038/s41467-020-20244-7.
Energy relaxation of photo-excited charge carriers is of significant fundamental interest and crucial for the performance of monolayer transition metal dichalcogenides in optoelectronics. The primary stages of carrier relaxation affect a plethora of subsequent physical mechanisms. Here we measure light scattering and emission in tungsten diselenide monolayers close to the laser excitation energy (down to ~0.6 meV). We reveal a series of periodic maxima in the hot photoluminescence intensity, stemming from energy states higher than the A-exciton state. We find a period ~15 meV for 7 peaks below (Stokes) and 5 peaks above (anti-Stokes) the laser excitation energy, with a strong temperature dependence. These are assigned to phonon cascades, whereby carriers undergo phonon-induced transitions between real states above the free-carrier gap with a probability of radiative recombination at each step. We infer that intermediate states in the conduction band at the Λ-valley of the Brillouin zone participate in the cascade process of tungsten diselenide monolayers. This provides a fundamental understanding of the first stages of carrier-phonon interaction, useful for optoelectronic applications of layered semiconductors.
光激发电荷载流子的能量弛豫具有重大的基础研究意义，并且对于单层过渡金属二硫属化物在光电子学中的性能至关重要。载流子弛豫的主要阶段会影响大量后续的物理机制。在此，我们测量了接近激光激发能量（低至约0.6毫电子伏特）的二硒化钨单层中的光散射和发射。我们揭示了热致发光强度中的一系列周期性最大值，其源于高于A激子态的能量状态。我们发现，在低于（斯托克斯）激光激发能量处有7个峰、高于（反斯托克斯）激光激发能量处有5个峰，其周期约为15毫电子伏特，且具有强烈的温度依赖性。这些被归因于声子级联，即载流子在自由载流子能隙之上的真实状态之间经历声子诱导的跃迁，且在每一步都有辐射复合的概率。我们推断，布里渊区Λ谷处导带中的中间态参与了二硒化钨单层的级联过程。这为载流子 - 声子相互作用的初始阶段提供了基本理解，对层状半导体的光电子应用很有用。
