Ultrafast photocarrier dynamics in hybrid van der Waals heterostructures

Two-dimensional materials, as well as other non-2D materials in thin-film form, have been the topic of current extensive research. On top of that, due to their interaction, combining different materials to form heterostructures can result in other desirable properties from materials not found in nature. This work presents results for the study of photocarrier dynamics of hybrid van der Waals. The kind of heterostructures studied in this dissertation are type II, which results in electrons and holes being separated in different layered materials, due to the relative difference in energy minimas. The charge transfer mechanism is a fundamental process in these heterostructures, but a complete understanding is still lacking. To expand the possible charge transfer mechanisms and dynamics, we extend the systems typically used at these interfaces (heterobilayers), after detailing the techniques used to quantify charge transfer. We report a combined experimental and theoretical study of the effect of energy valleys on interlayer charge transfer in transition metal dichalcogenide heterostructures. By transient absorption measurements, we find that the electrons excited in the K valley of 4L WSe$_2$ can scatter to its $\Lambda_{min}$ valley, followed by their relatively slow transfer to the K valley of MoS$_2$ with a time constant of 14 ps. This transfer time is at least one order of magnitude slower than those of K-K charge transfer observed in all 1L/1L transition metal dichalcogenide heterostructures previously studied. These results provide experimental ingredients for understanding the interlayer charge transfer mechanisms, reveal the important role of the energy valley and momentum conservation in this process, demonstrate a method to control the charge transfer rate, and offer basic parameters for such heterostructures. Furthermore, we also construct a new kind of interface by using flourinated zinc phthalocyanine, F$_8$ZnPc with 1\textit{T'}-ReS$_2$. Two-dimensional (2D) ReS$_2$ monolayer flakes were exfoliated from bulk crystals, and an F$_8$ZnPc film with a thickness of 4 nm was thermally deposited on ReS$_2$. Density functional theory calculations done by our collaborator show that the two materials form a type-II band alignment. In transient absorption measurements, photocarriers are selectively excited in ReS$_2$ and monitored in F$_8$ZnPc. We found that holes in Re$_2$ can transfer to F$_8$ZnPc on a timescale shorter than 0.35 ps. The transferred holes have a long lifetime in F$_8$ZnPc on the order of 1 ns, confirming the lack of electron transfer. The efficient charge transfer from a 1\textit{T'} transition metal dichalcogenide monolayer to an organic semiconductor illustrates the feasibility of developing 2D/organic heterostructures with in-plane anisotropic electronic and optoelectronic properties. Lastly, a hybrid van der Walls multilayer heterostructure formed by monolayer graphene, few-layer transition metal dichalcogenides, and organic semiconductor of F$_8$ZnPc was fabricated. Transient absorption microscopy measurements are performed to study photocarrier dynamics. In heterostructures of F$_8$ZnPc/few- layer-MoS$_2$/graphene, electrons excited in F$_8$ZnPc can transfer to graphene and thus be separated from the holes that reside in F$_8$ZnPc. By increasing MoS$_2$ thickness, these electrons acquire long recombination lifetimes of over 100 ps and high mobility of 2800 cm$^{-2}$V$^{-1}$s$^{-1}$. Graphene doping with mobile holes is also demonstrated with WS$_8$ as the middle layers. These artificial heterostructures with novel photocarrier properties can improve the performance of graphene-based optoelectronic devices.