Non-equilibrium electron dynamics in semiconducting transition metal
Jožef Stefan International Postgraduate School, Ljubljana
Invitation and abstract [PDF]
In the talk I will discuss equilibrium and non-equilibrium optical properties of two-dimensional materials, in particular transition metal dichalcogenides (TMDs) such as MoS2 and WSe2. After an introduction to TMD materials and their photophysics as well as the experimental methods, the first first part of the talk addresses the apparent conflict between strongly bound excitons on the one hand and efficient photovoltaics and sensitive photodetectors on the other. I use continuous wave photomodulation and femtosecond pump-probe spectroscopy to identify the spectral features of photogenerated charges and trace their dynamics, starting with their generation either by direct impulsive excitation into the charge continuum or via exciton dissociation.
But the origin of the femtosecond transient absorption changes can have an alternative interpretation. It can be formed not due to population dynamics but due to band gap renormalization and peak shift. I will discuss influence of many-body interaction on early pump-probe signal in the second part of the talk.
In the last part of the talk I present the attempt to develop metrics for characterization of the doping level of two-dimensional materials. The investigation of the three main Raman peaks of WSe2 in a field effect transistor with ionic liquid gating, which allows to control the doping level over a wide range was done. I track the positions and intensities of these peaks as a function of doping level and find several combinations – peak distances or intensity ratios that can be used as metrics for fast characterization.