Find more about our research by clicking the link below to Dr. Hui Zhao's google scholar page. This gives you access to a list of all publications written by Dr. Zhao and research that he has collaborated on.

Google Scholar

Selected Publications

  • Exciton formation in monolayer transition metal dichalcogenides

    Abstract: Two-dimensional transition metal dichalcogenides provide a unique platform to study excitons in confined structures. Recently, several important aspects of excitons in these materials have been investigated in detail. However, the formation process of excitons from free carriers has yet to be understood. Here we report time-resolved measurements on the exciton formation process in monolayer samples of MoS2, MoSe2, WS2, and WSe2. The free electron–hole pairs, injected by an ultrashort laser pulse, immediately induce a transient absorption signal of a probe pulse tuned to the exciton resonance. The signal quickly drops by about a factor of two within 1 ps and is followed by a slower decay process. In contrast, when excitons are resonantly injected, the fast decay component is absent. Based both on its excitation excess energy and intensity dependence, this fast decay process is attributed to the formation of excitons from the electron–hole pairs. This interpretation is also consistent with a model that shows how free electron–hole pairs can be about twice more effective than excitons in altering the exciton absorption strength. From our measurements and analysis of our results, we determined that the exciton formation times in these monolayers to be shorter than 1 ps.

  • Ultrafast Laser Spectroscopy of Two-Dimensional Materials Beyond Graphene

    Abstract: Starting with the discovery of graphene in 2004, the interest in two-dimensional materials since then has been exponentially growing. Across many disciplines, their exceptional electrical, chemical, thermal, and optical properties have drawn considerable attention that has created an entire field within a decade of their discovery. Driven by the mechanical exfoliation technique that allows for the quick exploration of these two-dimensional materials and their novel devices, joint efforts have been made in order to understand and exploit their potential, consequently leading to the development of their large-scale growth. This review focuses on recent studies using ultrafast laser spectroscopy that have revealed the photocarrier dynamics in two-dimensional materials and laid the foundation of their behavior. We provide a brief introduction on ultrafast laser spectroscopy, discuss several aspects of the photocarrier dynamics, and conclude with our perspective on future developments.

  • Ultrafast hole transfer from monolayer ReS2 to thin-film F8ZnPc

    Abstract: We report a combined experimental and computational study on the charge transfer properties of heterostructures formed by monolayer 1T′-ReS2 and fluorinated zinc phthalocyanine, F8ZnPc. Two-dimensional (2D) ReS2 monolayer flakes were exfoliated from bulk crystals, and an F8ZnPc film with a thickness of 4 nm was thermally deposited on ReS2. Density functional theory shows that the two materials form a type-II band alignment. In transient absorption measurements, photocarriers are selectively excited in ReS2 and monitored in F8ZnPc. We found that holes in ReS2 can transfer to F8ZnPc on a timescale shorter than 0.35 ps. The transferred holes have a long lifetime in F8ZnPc on the order of 1 ns, confirming the lack of electron transfer. The efficient charge transfer from a 1T′ transition metal dichalcogenide monolayer to an organic semiconductor illustrates the feasibility of developing 2D/organic heterostructures with in-plane anisotropic electronic and optoelectronic properties.

Federally Funded Publications (Open Access)

An all-optical approach to control interlayer charge transfer and interlayer exciton dynamics is demonstrated using transition metal dichalcogenide heterostructures of MoSe2/MoS2 and MoSe2/WS2 as examples. In the three-pulse pump-probe experiments, a control pulse injects photocarriers and produces an electric field due to the interlayer charge separation. A pump pulse excites new carriers and their dynamics under the influence of the control-injected carriers and field is time-resolved by measuring differential reflectance of a third probe pulse. In MoSe2/MoS2, the interlayer electron transfer time from MoSe2 to MoS2 decreases with increasing the control-injected carrier density and electric field. The dipole moment of the interlayer excitons is reduced by the control pulse, which can be utilized for ultrafast and all-optical control of interlayer excitons. The recombination of the interlayer excitons is enhanced by the control pulse, which increases the spatial overlap of the electron and hole wave functions. The effect of the control pulse on the interlayer excitons is confirmed in the MoSe2/WS2 heterostructure, although its effect on the electron transfer time is not resolvable. These results reveal useful information to understand and control interlayer charge transfer dynamics and provide tools for ultrafast manipulation of electrons in van der Waals heterostructures.

Find the full paper here (PDF).