Interlayer exciton dynamics in a dichalcogenide monolayer heterostructure (1703.00379v1)

Abstract: In heterostructures consisting of different transition-metal dichalcogenide monolayers, a staggered band alignment can occur, leading to rapid charge separation of optically generated electron-hole pairs into opposite monolayers. These spatially separated electron-hole pairs are Coulomb-coupled and form interlayer excitons. Here, we study these interlayer excitons in a heterostructure consisting of MoSe$_2$ and WSe$_2$ monolayers using photoluminescence spectroscopy. We observe a non-trivial temperature dependence of the linewidth and the peak energy of the interlayer exciton, including an unusually strong initial redshift of the transition with temperature, as well as a pronounced blueshift of the emission energy with increasing excitation power. By combining these observations with time-resolved photoluminescence measurements, we are able to explain the observed behavior as a combination of interlayer exciton diffusion and dipolar, repulsive exciton-exciton interaction.

Interlayer Exciton Dynamics in MoSe$_2$-WSe$_2$ Heterostructures

The research paper investigates the dynamics of interlayer excitons within a heterostructure composed of molybdenum diselenide (MoSe$_2$) and tungsten diselenide (WSe$_2$) monolayers. This paper explores phenomenological aspects of exciton behavior, focusing on their stability and interactions as modulated by temperature and excitation power, highlighting the unique properties conferred by two-dimensional transition metal dichalcogenide (TMDC) materials.

Experimental Setup and Observations

The authors employed photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy, alongside time-resolved photoluminescence (TRPL) techniques, to gain insights into the exciton phenomena. In TMDC heterostructures, a staggered band alignment facilitates the separation of optically generated electron-hole pairs across layers, leading to the formation of interlayer excitons with notable binding energies and extended radiative lifetimes. These characteristics contrast starkly with intralayer excitons in TMDC monolayers, which recombine in sub-picosecond timeframes.

The heterostructure samples were manufactured using a deterministic transfer method, leading to the overlap of MoSe$_2$ and WSe$_2$ monolayers. These heterostructures were subjected to annealing to ameliorate interlayer coupling, essential for the robustness of excitonic phenomenon studied here.

Temperature and Power Dependence

The paper reveals that interlayer excitons exhibit a nontrivial dependence on temperature and excitation power. At temperatures ranging from 4.5 K to 40 K, a pronounced redshift of approximately 10 meV occurs, whereas between 40 K and 60 K, a conspicuous blueshift is observed. The linewidth changes are similarly complex, nearly doubling between 4.5 K and 40 K and witnessing a peak around 60 K. A similar non-linear energy shift as a function of temperature has been documented in disordered semiconductor systems, suggesting an analogy in underlying mechanics, likely driven by exciton diffusion and interaction with localized states.

Power-dependent studies underscored a blueshift of the excitonic emission peak at increased excitation densities. This phenomenon was determined to emanate from dipolar exciton-exciton interactions. The repulsive forces borne of exciton density augment the energy landscape, further affirming the interaction's role in the observable blueshift.

Implications and Future Directions

This research significantly contributes to the understanding of exciton behavior in TMDC heterostructures, providing evidence for diffusion and interaction-induced energy shifts that challenge traditional interpretations. The identification of stable interlayer excitons at room temperature paves the way for novel applications in optoelectronics, particularly where long-lived excitons are advantageous. Furthermore, the paper's findings regarding exciton-exciton interaction open avenues for the exploration of quantum phenomena, possibly leveraging excitonic condensation and the facilitation of valleytronic applications.

Exciton dynamics within such heterostructures offer compelling perspectives on charge transport phenomena, with substantial implications for the development of new electronic and photonic devices. This paper's insights also bolster the robustness of TMDC materials as platforms for fundamental physics research, likely influencing future theoretical models and experimental inquiries into two-dimensional material heterojunctions.