Identification of excitons, trions and biexcitons in single-layer WS2 (1507.01342v3)

Abstract: Single-layer WS$_2$ is a direct-gap semiconductor showing strong excitonic photoluminescence features in the visible spectral range. Here, we present temperature-dependent photoluminescence measurements on mechanically exfoliated single-layer WS$_2$, revealing the existence of neutral and charged excitons at low temperatures as well as at room temperature. By applying a gate voltage, we can electrically control the ratio of excitons and trions and assert a residual n-type doping of our samples. At high excitation densities and low temperatures, an additional peak at energies below the trion dominates the photoluminescence, which we identify as biexciton emission.

• The paper reveals distinct exciton and trion emissions at room temperature through temperature-dependent photoluminescence measurements.
• The paper demonstrates that gate-dependent measurements shift the exciton-trion energy separation, indicating the impact of intrinsic doping and Fermi energy.
• The paper identifies a transition from defect-bound exciton to dominant biexciton emission under increased excitation, suggesting novel optoelectronic applications.

Identification of Excitons, Trions, and Biexcitons in Single-layer WS$_2$

The paper presented addresses the intriguing properties of single-layer WS$_2$, a direct-gap semiconductor with notable excitonic features discernible through photoluminescence (PL) across various temperatures. The researchers investigate the binding characteristics of excitons, trions, and biexcitons using temperature-dependent PL measurements, alongside gate- and power-dependent studies, to elucidate the intricate excitonic phenomena observable in mechanically exfoliated WS$_2$ layers.

Key Findings

Central to the findings is the identification of exciton (X) and trion (X$^-$) emission peaks within the PL spectra, detectable distinctly even at room temperature. Notably, the paper confirms a separation of these peaks at higher temperature ranges, enhancing the understanding of excitonic stability within the material. Trions, observed to be negatively charged, suggest a residual intrinsic doping in WS$_2$ samples. The precise nature of these quasiparticles is further confirmed by gate-dependent measurements, which shift the exciton-trion energy separation relative to the applied gate voltage, thereby validating the presence of Fermi energy as a dominant influence.

The work also identifies a low-energy emission peak observable at reduced temperatures, which the authors attribute to a superposition of defect-bound exciton and biexciton emission. Power-dependent measurements demonstrate significant shifts in PL intensity, with the low-energy peak transitioning from defect-bound emission to dominant biexciton-related features at higher excitation densities, indicating a secondary formation mechanism sensitive to pump energy levels.

Methodological Approach

The methodology utilized for sample preparation emphasizes the careful mechanical exfoliation of WS$_2$, ensuring minimal substrate interaction, which contributes to the distinct excitonic features reported. The authors utilize a combination of low-temperature, gate-dependent, and helicity-resolved PL measurements to extract detailed energy-level information and assess the impact of external fields on excitonic behavior.

Implications and Future Directions

The paper's results offer significant implications for the development of transition metal dichalcogenide (TMDC) materials in optoelectronic applications, where control over excitonic states can lead to enhanced device functionality. Furthermore, the observed biexciton emission suggests potential applications in valleytronics, where the manipulation of coupled electron-hole pairs (excitons and trions) could be harnessed.

Future research could focus on examining the kinetics of exciton and biexciton formation in similar TMDC systems, using advanced spectroscopic methods such as time-resolved PL to gain further insights into these quasi-particles' dynamic properties. Variability in intrinsic doping and substrate influences also warrant additional exploration to standardize WS$_2$ samples in experimental applications.

Conclusion

The identification of excitons, trions, and biexcitons in single-layer WS$_2$ adds to the growing body of knowledge surrounding TMDC materials, offering new insights into their nuanced electronic and optical characteristics. The multi-faceted approach combining temperature, gate, and power-dependent analysis underscores the complexity of excitonic interactions, while contributing valuable data that may guide future technological advancements in 2D material applications.