Probing Excitonic Dark States in Single-layer Tungsten Disulfide (1403.5568v1)

Abstract: Transition metal dichalcogenide (TMDC) monolayer has recently emerged as an important two-dimensional semiconductor with promising potentials for electronic and optoelectronic devices. Unlike semi-metallic graphene, layered TMDC has a sizable band gap. More interestingly, when thinned down to a monolayer, TMDC transforms from an indirect bandgap to a direct bandgap semiconductor, exhibiting a number of intriguing optical phenomena such as valley selective circular dichroism, doping dependent charged excitons, and strong photocurrent responses. However, the fundamental mechanism underlying such a strong light-matter interaction is still under intensive investigation. The observed optical resonance was initially considered to be band-to-band transitions. In contrast, first-principle calculations predicted a much larger quasiparticle band gap size and an optical response that is dominated by excitonic effects. Here, we report experimental evidence of the exciton dominance mechanism by discovering a series of excitonic dark states in single-layer WS2 using two-photon excitation spectroscopy. In combination with GW-BSE theory, we find the excitons are Wannier excitons in nature but possess extraordinarily large binding energy (~0.7 eV), leading to a quasiparticle band gap of 2.7 eV. These strongly bound exciton states are observed stable even at room temperature. We reveal an exciton series in significant deviation from hydrogen models, with a novel inverse energy dependence on the orbital angular momentum. These excitonic energy levels are experimentally found robust against environmental perturbations. The discovery of excitonic dark states and exceptionally large binding energy not only sheds light on the importance of many-electron effects in this two-dimensional gapped system, but also holds exciting potentials for the device application of TMDC monolayers and their heterostructures.

• The paper demonstrates that two-photon spectroscopy and GW-BSE modeling reveal excitons with binding energies around 0.7 eV in WS2 monolayers.
• It finds that the 1s-2p and 1s-3p energy separations deviate from 2D hydrogen model predictions due to spatial-dependent dielectric screening.
• The study implies that robust excitonic states in WS2 may impact next-generation optoelectronic devices and quantum systems.

Analysis of Excitonic Dark States in Monolayer Tungsten Disulfide

The paper presents a rigorous paper on excitonic dark states in single-layer tungsten disulfide (WS$_2$), revealing critical insights into the excitonic effects within two-dimensional transition metal dichalcogenides (TMDCs). Through a combination of two-photon excitation spectroscopy and advanced theoretical modeling using the GW plus Bethe-Salpeter equation (GW-BSE) approach, the research dismantles previous assumptions about band-to-band transitions, highlighting exciton dominance within these structures.

The authors focused on WS$_2$ monolayers, characterized by a direct bandgap, and utilized two-photon spectroscopy to probe the normally inaccessible excitonic dark states. The paper discovered excitons with unusually large binding energies, approximately 0.7 eV, significantly greater than those in conventional semiconductors, indicating a quasiparticle band gap of 2.7 eV. These findings contrast with previous reports of smaller bandgap sizes and emphasize the dominant role of excitonic states, fully confirmed by GW-BSE calculations.

Key experimental observations included two-photon resonances at 2.28 eV and 2.48 eV, corresponding to the 2p and 3p excited states of excitons, respectively. The 1s-2p and 1s-3p energy separations exhibited substantial deviations from the 2D hydrogen model predictions, suggesting novel energy-level behavior attributed to the spatial-dependent dielectric screening in monolayer TMDCs. The paper posits that this anti-screening effect arises from reduced dimensionality and specific characteristics of TMDCs and is instrumental in causing the peculiar excitonic series.

One of the critical implications explored is the robust nature of the excitonic states against environmental perturbations. The report presents strong evidence that these excitonic properties are intrinsic to the monolayer structure, with minimal shifts observed under varying dielectric environments and temperatures. This stability underpins potential applications in fields ranging from optoelectronics to sensors, where resilience to external conditions is paramount.

The research contributes to the broader theoretical framework by highlighting the importance of many-electron effects in understanding light-matter interactions in 2D systems. The experimental techniques coupled with high-fidelity computational models provide a template for exploring excitonic phenomena across other TMDCs and similar two-dimensional heterostructures.

From a practical perspective, this work catalyzes further exploration into exploiting WS$_2$ and similar materials in next-generation optoelectronic devices. The identified excitonic properties have the potential to revolutionize the understanding of photonic processes at the atomic scale and drive innovation in device engineering regarding power efficiency, miniaturization, and integration capabilities.

Future developments in this area may center on refining the control of exciton properties through electrostatic gating, mechanical strain, or hybrid structure formation. These advancements could enhance the performance and utility in applications such as ultrafast optical switches, high-sensitivity photodetectors, and quantum information systems.

Overall, the paper emerges as a detailed account contributing fundamental insight into the excitonic interactions in monolayer TMDCs, with both deep theoretical analyses and significant pragmatic implications.