Terahertz photoconductivity and photocarrier dynamics in few-layer hBN/WS2 van der Waals heterostructure laminates

Van der Waals (vdW) heterostructures is a rapidly emerging field that promises to produce on-demand properties for novel optoelectronic devices. Assembly of dissimilar two-dimensional atomic crystals in the vdW heterostructure enables unique features and properties which are fundamentally different from individual 2-dimensional (2D) crystals. Currently, most growth and fabrication methods prohibit large scale, micron-thick and robust heterostructures. An alternative approach is the one based on liquid phase exfoliation allowing the possibility of scalable thin films and composites. Such thin films have inherent and predicted advantages: they can display new behaviors due to their extremely high surface area and, as free-standing laminates, can be manipulated with mixing of nanosheets and other materials for novel device attributes. We use the aforementioned route to prepare spray-coated and few microns thick WS2 and hBN/WS2 heterostructure laminates. A combination of photoluminescence and transmission electron microscope measurements show that, despite the disordered layer stacking inherent to the fabrication process, the laminates preserve the few layer optical response. In particular, the hBN/WS2 heterostructure laminates exhibit a 3-layer average distribution. Using optical pump-terahertz probe (OPTP) measurement to access the photocarrier dynamics and photoconductivity, we study and compare the photocarrier dynamics and photoconductivity of pure WS2 and hBN/WS2 samples. The hBN/WS2 samples show an unusual response that is different from what has been previously reported for pure transition metal dichalcogenides. After photoexcitation, instead of a monotonic decay as in pure WS2, an initial fast decay is followed by a rise of the negative differential terahertz (THz) transmission dominating the dynamics for the following 50 ps. By analyzing the time resolved THz complex photoconductivity, we attribute this effect to the presence of free carriers as well as dipoles at the hBN/WS2 interfaces. As previously reported in hBN/Graphene heterostructures, interfacial dipoles can form along with free carriers at the instant of photoexcitation. Whereas free carriers cause a decrease in the transient THz transmission due to Coulomb screening, dipoles can provide an increase in the pump-induced change in transmission. In terms of complex photoconductivity, free carriers have both real and imaginary components while dipoles probed off-resonance provide almost a purely imaginary response. Our results provide a deeper understanding of the photoconductive response of large van der Waals heterostructure laminates fabricated by liquid phase exfoliation, and will enable their use in future optoelectronic applications.