Anisotropic Nonradiative Recombination of Carriers in a Few-Layered MoS₂ Probed by Mid-IR Ultrafast Spectroscopy

Abstract

Futuristic optoelectronic devices could be replaced by promising extremely thin layered transition-metal dichalcogenides (TMDCs) like MoS 2 . Often the efficiencies of TMDCs devices directly depend on the nonradiative decay dynamics of the carriers. The energy dissipation pathways are mainly mediated by defect states, and these detailed mechanisms are not entirely understood. Only low-energy mid-IR probes can reveal the carrier population dynamics near the band edge and the defect states. With ultrafast visible pump and mid-IR probe spectroscopy, carrier dynamics on chemical vapor deposition grown 3-to 4-layered MoS 2 system is presented. We optically pumped this system with energies at near bandgap 1.9 eV and at continuum 2.5 eV separately, while the excited nonequilibrium dynamics of carriers are probed with both 0.62 and 0.31 eV mid-IR pulses. Contrasting carrier dynamics are revealed at these probe wavelengths, and their dynamics is discussed in detail. Also, pump-power-dependent dynamics and polarization anisotropy at both pump wavelengths and at both probe wavelengths are presented. In all the cases, we obtained the carrier−carrier scattering time in the order of 2 to 3 ps and the carrier recombination time in the order of tens of picoseconds. Interestingly for the first time, we report the polarization anisotropic slow decay time constants when probed at 0.62 eV, which is not present for a 0.31 eV probe energy. Our experiments gave an intricate understanding of the defect-mediated carrier dynamics in a 3-to 4-layered MoS 2 system, where such defect states dictate the optoelectronic properties of TMDs.