Highly Anisotropic and Robust Excitons in Monolayer Black Phosphorus (1411.1695v1)

Abstract: Semi-metallic graphene and semiconducting monolayer transition metal dichalcogenides (TMDCs) are the two-dimensional (2D) materials most intensively studied in recent years. Recently, black phosphorus emerged as a promising new 2D material due to its widely tunable and direct bandgap, high carrier mobility and remarkable in-plane anisotropic electrical, optical and phonon properties. However, current progress is primarily limited to its thin-film form, and its unique properties at the truly 2D quantum confinement have yet to be demonstrated. Here, we reveal highly anisotropic and tightly bound excitons in monolayer black phosphorus using polarization-resolved photoluminescence measurements at room temperature. We show that regardless of the excitation laser polarization, the emitted light from the monolayer is linearly polarized along the light effective mass direction and centers around 1.3 eV, a clear signature of emission from highly anisotropic bright excitons. In addition, photoluminescence excitation spectroscopy suggests a quasiparticle bandgap of 2.2 eV, from which we estimate an exciton binding energy of around 0.9 eV, consistent with theoretical results based on first-principles. The experimental observation of highly anisotropic, bright excitons with exceedingly large binding energy not only opens avenues for the future explorations of many-electron effects in this unusual 2D material, but also suggests a promising future in optoelectronic devices such as on-chip infrared light sources.

• The paper demonstrates that monolayer black phosphorus exhibits highly anisotropic excitons with a robust binding energy of approximately 0.9 eV, verified through polarization-resolved spectroscopy and first-principles calculations.
• Experimental measurements reveal a quasi-particle band gap of about 2.2 eV and a primary photoluminescence peak at 1.3 eV with strong linear polarization along the x-direction.
• The findings highlight monolayer BP's potential for advanced optoelectronic devices, leveraging its unique anisotropic excitonic behavior for polarized infrared emission.

Highly Anisotropic and Robust Excitons in Monolayer Black Phosphorus

The paper introduces an experimental investigation into the optical properties of monolayer black phosphorus (BP), focusing on the characterization of excitonic features within this promising 2D semiconductor material. Leveraging polarization-resolved photoluminescence (PL) and photoluminescence excitation spectroscopy (PLE), combined with first-principles calculations, the paper elucidates the highly anisotropic nature and robust exciton binding energy characterizing monolayer BP.

Monolayer black phosphorus is distinguishable from its multilayered counterpart due to the pronounced anisotropic properties it exhibits at the quantum-confined two-dimensional limit. Unlike other two-dimensional materials such as graphene and transition metal dichalcogenides (TMDCs), monolayer BP demonstrates unique electrical, optical, and phononic anisotropies due to its orthorhombic lattice structure belonging to the D2h point group. The investigation conducted in the paper revealed that the thickness of a monolayer BP flake is approximately 0.7 nm—a measurement corroborated by atomic force microscopy and Raman spectroscopy.

The paper demonstrated that monolayer BP exhibits photoluminescence dominated by highly anisotropic excitons with the primary emission centered at approximately 1.3 eV. Notably, the emitted light possesses a strong linear polarization along the x-direction regardless of the excitation light polarization, a remarkable indication of the material’s in-plane anisotropic excitonic nature. The team's first-principles calculations further support this observation by demonstrating a robust exciton binding energy of around 0.9 eV, which is consonant with theoretically predicted values.

The experimental results suggest the presence of highly anisotropic and tightly bound excitons resulting from the Coulomb interactions that occur within the structure of monolayer BP. These interactions prefer binding in the relatively flat y-direction due to the different mobilities along the x and y directions.

Photoluminescence emission remains predominantly polarized along the x-direction, further corroborating the calculated anisotropic band dispersions. At the same time, photoluminescence excitation spectroscopy unveils a quasi-particle band gap of approximately 2.2 eV for monolayer BP, further leading to an estimated exciton binding energy of 0.88 ± 0.12 eV.

This high degree of anisotropy in excitonic behavior impacts potential applications for monolayer BP in developing future optoelectronic devices. Notably, the material's remarkable excitonic properties parallel those observed in quasi-one-dimensional materials such as carbon nanotubes, reinforcing the potential for implementation in polarized infrared light emission and advanced optical communication platforms.

The results underline the distinct properties of monolayer black phosphorus and reflect significant advancements in realizing devices powered by such highly anisotropic 2D materials. The observations present potential implications for extending the functionality of monolayer BP in electronics and tailored optoelectronic applications, offering a gateway to innovations in infrared photonics and beyond.

Ultimately, by expanding the comprehension of monolayer black phosphorus's fundamental excitonic features, this research potentially paves way for creating novel device paradigms leveraging this material's inherently anisotropic nature. Moving forward, further exploration of many-electron effects and various environmental influences on monolayer BP may offer even deeper insights into optimizing its characteristics for practical implementation.