Excitons in atomically thin transition metal dichalcogenides (1707.05863v2)

Abstract: Atomically thin materials such as graphene and monolayer transition metal dichalcogenides (TMDs) exhibit remarkable physical properties resulting from their reduced dimensionality and crystal symmetry. The family of semiconducting transition metal dichalcogenides is an especially promising platform for fundamental studies of two-dimensional (2D) systems, with potential applications in optoelectronics and valleytronics due to their direct band gap in the monolayer limit and highly efficient light-matter coupling. A crystal lattice with broken inversion symmetry combined with strong spin-orbit interactions leads to a unique combination of the spin and valley degrees of freedom. In addition, the 2D character of the monolayers and weak dielectric screening from the environment yield a significant enhancement of the Coulomb interaction. The resulting formation of bound electron-hole pairs, or excitons, dominates the optical and spin properties of the material. Here we review recent progress in our understanding of the excitonic properties in monolayer TMDs and lay out future challenges. We focus on the consequences of the strong direct and exchange Coulomb interaction, discuss exciton-light interaction and effects of other carriers and excitons on electron-hole pairs in TMDs. Finally, the impact on valley polarization is described and the tuning of the energies and polarization observed in applied electric and magnetic fields is summarized.

• The paper demonstrates that strong Coulomb interactions in TMD monolayers produce unusually large exciton binding energies.
• It utilizes absorption, photoluminescence, and two-photon excitation techniques to resolve optical transitions and characterize excitonic states.
• The findings imply that controlling exciton dynamics in TMDs can significantly boost optoelectronic and valleytronic device performance.

Excitons in Atomically Thin Transition Metal Dichalcogenides

The paper "Excitons in Atomically Thin Transition Metal Dichalcogenides" focuses on the optical properties of atomically thin transition metal dichalcogenide (TMD) monolayers, exploring their promising applications in optoelectronics and valleytronics. TMDs, such as monolayer MoS$_2$, MoSe$_2$, and WS$_2$, are notable due to their two-dimensional (2D) nature, significant spin-orbit interactions, and strong Coulomb interactions that lead to the formation of excitons—bound states of electrons and holes.

Understanding Excitons in TMDs

In TMD monolayers, the excitons demonstrate prominent characteristics due to the materials' reduced dimensionality and unique electronic structure:

1. Large Exciton Binding Energies: TMDs exhibit strong Coulomb interactions, resulting from reduced dielectric screening. This leads to substantial exciton binding energies, typically on the order of hundreds of meV, which are much larger than those in traditional semiconductors like GaAs.
2. Direct Band Gap Nature: In the monolayer limit, TMDs transform into direct band gap semiconductors with band extrema located at the $K^+$ and $K^-$ points of the Brillouin zone. This characteristic is significant for efficient light emission, as evidenced in various device prototypes for phototransistors, sensors, and logic circuits.
3. Optical Transitions and Spin-Valley Coupling: The presence of strong spin-orbit coupling results in spin-split valence and conduction bands, directly affecting the optical selection rules, where $\sigma^+$ and $\sigma^-$ polarized light facilitate interband transitions at $K^+$ and $K^-$ valleys, respectively.

Experimental Techniques and Findings

Several experimental techniques illustrate the nature of excitons in TMDs:

• Absorption and Photoluminescence (PL): These techniques confirm the optical band gaps and reveal excitonic peaks.
• Two-Photon Photoluminescence Excitation (2P-PLE): This technique has confirmed the $2p$ exciton states, further supporting binding energy estimations.
• Scanning Tunneling Spectroscopy (STS): Provides insights into the free-particle band gap energies by measuring local density of states, revealing the large exciton binding energies.

Challenges and Future Directions

Despite significant progress in understanding and characterizing excitons in TMDs, several challenges remain:

• Effect of Dielectric Environment: The profound influence of substrate material and encapsulation on exciton binding energy demands further exploration to manipulate excitonic properties actively.
• Dark vs. Bright Excitons: The paper delineates between spin-allowed bright excitons and spin-forbidden dark excitons. Studies on the potential of dark excitons, which show longer coherence times, could further enhance device applications.
• Valley Polarization Dynamics: Refinement of valley polarization control mechanisms, particularly in the context of applications beyond low temperatures, remains an area ripe for exploration.

Practical Implications and Speculations

The research into excitonic properties of TMDs presents several far-reaching implications. Practically, TMDs could lead to next-generation optoelectronic devices with enhanced efficiency due to their strong light-matter interaction. The capabilities for manipulating spin and valley degrees of freedom also propose novel opportunities in quantum computing and information storage. Theoretical developments could fuel advances in understanding many-body interactions in 2D materials, informing the synthesis of new material systems with tailored electronic properties.

The significant interest and ongoing research into TMD excitonics suggest that further exploration will continue to uncover their potential, both in enhancing our understanding of fundamental physics and in advancing practical technological applications.