Synthetic Crystal Research Center, Shanghai Institute of Ceramics
Key Laboratory of Transparent Opto-functional Inorganic Materials, CAS
TWO-DIMENSIONAL SEMICONDUCTORS FOR OPTOELECTRONICS
Femtosecond Optics Laboratory, MIREA – Russian Technological University, Moscow, Russia
Over the past decade, graphene and other 2D materials have attracted much attention in both fundamental studies and potential applications due to their extraordinary properties. The mostly used photoactive 2D materials, transition metal dichalcogenides (TMDs) play a critical role in the creation of 2D optoelectronic devices based on single layers themselves, heterojunctions or in combination with other 2D materials.
Monolayers were commercial fabricated (SixCarbon Technology) by chemical vapor deposition technique on SiO2(285nm)/Si substrate. Optical confocal microscope was used to obtain reflectivity, luminescence and optical second harmonic images, which were complemented by atomic force microscopy. To measure the relaxation time for hot photocarriers in monolayer, the two-color pump-probe experiment was performed at room temperature using the amplified Ti-sapphire laser and frequency doubler. The differential reflectivity of a WSe2 monolayer was obtained and fitted within the two-time relaxation model which is commonly used for direct band semiconductors. The relaxation time constants were found to be slightly dependent on wavelength and to be of the order of magnitude of t1~1.5 ps, and t2~10 ps. Resonant behavior of the transient reflectivity amplitudes was observed at 0.83 eV. This value doubled coincides with the luminescence peak position of 1.67 eV. This means that two photon process takes place for the probe in the pump-probe experiment.
The model was suggested to describe the obtained results based on the spectral dependence of time evolution of complex refractive index for semiconductor with photoexcited carriers. Numerical calculation of the reflectivity dependence on carrier concentration along with the calculation of the temporal dependence of carrier concentration allows to simulate time dependence of reflectivity.