Observation of tightly bound trions in monolayer MoS2

Abstract: Two-dimensional (2D) atomic crystals, such as graphene and transition-metal dichalcogenides, have emerged as a new class of materials with remarkable physical properties. In contrast to graphene, monolayer MoS2 is a non-centrosymmetric material with a direct energy gap. Strong photoluminescence, a current on-off ratio exceeding 10^8 in field-effect transistors, and efficient valley and spin control by optical helicity have recently been demonstrated in this material. Here we report the spectroscopic identification in doped monolayer MoS2 of tightly bound negative trions, a quasi-particle composed of two electrons and a hole. These quasi-particles, which can be created with valley and spin polarized holes, have no analogue in other semiconducting materials. They also possess a large binding energy (~ 20 meV), rendering them significant even at room temperature. Our results open up new avenues both for fundamental studies of many-body interactions and for opto-electronic and valleytronic applications in 2D atomic crystals.

• The paper reveals that monolayer MoS₂ hosts negative trions composed of two electrons and one hole with a binding energy around 20 meV.
• It employs FET-controlled doping and photoluminescence spectroscopy to observe power-law spectral shifts that confirm strong Coulomb interactions in a 2D system.
• The findings indicate that the robust trionic states in MoS₂ can drive advancements in optoelectronic and valleytronic devices through enhanced many-body interactions.

Insights into Tightly Bound Trions in Monolayer MoS₂

The paper provides a comprehensive study on the spectroscopic identification of tightly bound negative trions in the monolayer form of molybdenum disulfide, MoS₂. These quasiparticles are composed of two electrons and one hole, a structure that is unprecedented in other materials, and are characterized by a significant binding energy of approximately 20 meV. The significance of this trionic state, even at room temperature, positions monolayer MoS₂ as an exciting candidate for both theoretical investigations of many-body interactions and practical applications in optoelectronics and valleytronics.

Key Observations and Numerical Results

The research identifies that the trion binding energy in monolayer MoS₂ is nearly an order of magnitude larger than those observed in conventional quasi-two-dimensional systems, such as quantum wells. This large binding energy is attributed to the strong Coulomb interactions accentuated in monolayer MoS₂ due to reduced dielectric screening in two-dimensional gapped crystals and the relatively heavy carrier band masses associated with the Mo d-manifolds.

Methodological Approach

The researchers employ field-effect transistors (FETs) to systematically vary the doping density, analyzing the optical response of monolayer MoS₂ using absorption and photoluminescence (PL) spectroscopy. Conducted at both low (10 K) and room temperatures, these experiments reveal notable shifts in the optical spectrum, marking the presence of trions as excess electrons bind with photoexcited electron-hole pairs under increasing doping conditions. The study effectively delineates these transitions using power-law spectral dependencies predicted for doped 2D semiconductors, thus providing empirical validation for the observed trion features.

Theoretical and Practical Implications

The presence and stability of trions at finite temperatures indicate the potential of MoS₂ monolayers in applications requiring robust trionic states. The unique properties demonstrated, including high-spin polarization and valley specificity, underline significant prospects for developing advanced optoelectronic devices. The research opens an avenue for enhanced manipulation of optical excitation through trion dynamics, which could be harnessed in future energy-harvesting technologies.

Speculation on Future Developments

The high trion binding energy implies that monolayer MoS₂ is ideally suited for further exploration of many-body phenomena, including carrier multiplication and potential Wigner crystallization. The dynamic control of trion properties through external electric fields or optical methods could lead to novel device concepts, pushing the boundaries of current optoelectronic applications.

In summary, this paper enriches the understanding of quasiparticle dynamics in 2D materials by illustrating the distinct properties and ramifications of trionic states in monolayer MoS₂. The insights gained from this study lay the groundwork for future research focused on exploiting these findings in practical technologies and advancing theoretical models of many-body systems in low-dimensional materials.