Abstract
AbstractMonolayers of transition metal dichalcogenides are semiconducting materials which offer many prospects in optoelectronics. A monolayer of molybdenum disulfide (MoS2) has a direct bandgap of 1.88 eV. Hence, when excited with optical photon energies below its bandgap, no photocarriers are generated and a monolayer of MoS2 is not of much use in either photovoltaics or photodetection. Here, we demonstrate that large size MoS2 monolayer sandwiched between two graphene layers makes this heterostructure optically active well below the band gap of MoS2. An ultrafast optical pump‐THz probe experiment reveals in real‐time, transfer of carriers between graphene and MoS2 monolayer upon photoexcitation with photon energies down to 0.5 eV. It also helps to unravel an unprecedented enhancement in the broadband transient THz response of this tri‐layer material system. We propose possible mechanism which can account for this phenomenon. Such specially designed heterostructures, which can be easily built around different transition metal dichalcogenide monolayers, will considerably broaden the scope for modern optoelectronic applications at THz bandwidth.
Highlights
Suitability of optical materials for applications in photovoltaics and photodetection relies on the efficient conversion of light photons into free electron-hole pairs
We find that the MoS2-like contribution in the transient THz conductivity of the heterostructure systematically reduces with the decreasing optical excitation photon energy and it becomes negligible at ~0.45 eV
To strengthen the above observation regarding the THz response of GrMoS2Gr excited below the MoS2-bandgap and get more details before discussing the possible mechanism for this effect, we present here additional experimental results obtained at excitation photon energies much smaller than 1.55 eV
Summary
Suitability of optical materials for applications in photovoltaics and photodetection relies on the efficient conversion of light photons into free electron-hole pairs. In this regard the limiting processes related to carrier-carrier scattering, heat generation through phonon emission by hot carriers and the electron-hole recombination, are important to be known as precisely as possible. In real devices, the latter two processes must be reduced to enhance the conversion of light energy into free carriers only. Novel promising optoelectronic devices using quantum materials such as graphene and transition metal dichalcogenides (TMDs), and their heterostructures have been designed
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