Direct Observation of Layer-Dependent Electronic Structure in Phosphorene

Abstract: Phosphorene, a single atomic layer of black phosphorus, has recently emerged as a new twodimensional (2D) material that holds promise for electronic and photonic technology. Here we experimentally demonstrate that the electronic structure of few-layer phosphorene varies significantly with the number of layers, in good agreement with theoretical predictions. The interband optical transitions cover a wide, technologically important spectrum range from visible to mid-infrared. In addition, we observe strong photoluminescence in few-layer phosphorene at energies that match well with the absorption edge, indicating they are direct bandgap semiconductors. The strongly layer-dependent electronic structure of phosphorene, in combination with its high electrical mobility, gives it distinct advantages over other twodimensional materials in electronic and opto-electronic applications.

• The paper demonstrates that absorption spectroscopy reveals significant variations in electronic band structure as phosphorene layer thickness changes.
• It reports precise bandgap values of 1.73 eV, 1.15 eV, and 0.83 eV for monolayer, bilayer, and trilayer phosphorene, respectively.
• The findings provide a robust framework for tuning electronic properties in phosphorene, paving the way for advanced nanoelectronic and optoelectronic applications.

Direct Observation of Layer-Dependent Electronic Structure in Phosphorene

The presented paper provides a comprehensive experimental study on the electronic structure of few-layer phosphorene, a two-dimensional (2D) material derived from black phosphorus. The work delineates the variation in electronic band structure as a function of phosphorene layer thickness, corroborating theoretical predictions with experimental evidence. The research utilized absorption spectroscopy to determine the optical bandgap of monolayer and few-layer phosphorene while demonstrating significant layer-dependent changes in electronic properties.

Phosphorene's electronic structure is notably sensitive to its layer count. The study reports bandgap values of 1.73 eV, 1.15 eV, and 0.83 eV for monolayer, bilayer, and trilayer phosphorene, respectively, with the bandgap narrowing further toward the bulk value of 0.35 eV. The use of high-quality samples on sapphire substrates, coupled with hexagonal boron nitride (hBN) capping, ensured minimal degradation and facilitated reliable optical measurements. This approach enabled direct insight into the band structure that previous photoluminescence studies could not achieve due to their susceptibility to defects and impurities.

The researchers employed optical reflectance spectroscopy at 77 K to assess the polarization-resolved reflection spectra, revealing anisotropy in optical absorption due to phosphorene's unique crystal structure. The experimental data effectively aligns with a one-dimensional tight-binding model, which describes the evolution of optical resonances and confirms the presence of direct bandgaps throughout varying phosphorene layers.

Furthermore, optical absorption analysis demonstrated additional spectral resonances attributed to transitions between higher quantum well-like subbands induced by interlayer interactions. Ab initio GW Bethe Salpeter equation (GW-BSE) calculations supported these findings, revealing minor exciton binding energies due to the increased screening from hBN encapsulation. The calculated optical bandgaps closely agree with experimental results, offering a robust predictive framework for electronic configurations in few-layer phosphorene.

The study's implications are substantial for future electronic and optoelectronic applications. The ability to fine-tune the electronic properties of phosphorene via layer manipulation, electrostatic fields, and mechanical strain opens promising pathways for the development of versatile, high-mobility devices operating across a broad spectral range, from infrared to visible light. These qualities render phosphorene a viable candidate for new-generation electronic components and systems, bridging critical gaps in current material technology limitations.

In conclusion, the paper provides an essential experimental benchmark for phosphorene's electronic properties, validating theoretical models with empirical data. The pronounced layer-dependent behavior elucidated here reflects the material's potential to transform applications in nanoelectronics, optoelectronics, and beyond. Such research sets the stage for further exploration into phosphorene's material characteristics and device implementations.