Approaching the Intrinsic Photoluminescence Linewidth in Transition Metal Dichalcogenide Monolayers (1702.05857v1)

Abstract: Excitonic states in monolayer transition metal dichalcogenides (TMDCs) have been the subject of extensive recent interest. Their intrinsic properties can, however, be obscured due to the influence of inhomogeneity in the external environment. Here we report methods for fabricating high quality TMDC monolayers with narrow photoluminescence (PL) linewidth approaching the intrinsic limit. We find that encapsulation in hexagonal boron nitride (h-BN) sharply reduces the PL linewidth, and that passivation of the oxide substrate by an alkyl monolayer further decreases the linewidth and also minimizes the charged exciton (trion) peak. The combination of these sample preparation methods results in much reduced spatial variation in the PL emission, with a full-width-at-half-maximum as low as 1.7 meV. Analysis of the PL line shape yields a homogeneous width of 1.43$\pm$0.08 meV and inhomogeneous broadening of 1.1$\pm$0.3 meV.

• The paper shows that combining h-BN encapsulation with substrate passivation reduces PL linewidths to as low as 1.7 meV, approaching intrinsic limits.
• The methodology employs hexagonal boron nitride and alkyl substrate passivation to mitigate environmental and inhomogeneous broadening in TMDC monolayers.
• The findings enhance excitonic performance, paving the way for advanced optoelectronic applications such as quantum computing and valleytronics.

Approaching the Intrinsic Photoluminescence Linewidth in Transition Metal Dichalcogenide Monolayers

The paper investigates the intrinsic photoluminescence (PL) properties of transition metal dichalcogenide (TMDC) monolayers, focusing on reducing extrinsic broadening influences to approach intrinsic linewidth. TMDCs like MoSe$_2$ are highly prized for their two-dimensional structure which leads to exceptional excitonic states and potential for optoelectronic applications. However, achieving optical properties that are primarily dictated by intrinsic factors remains a challenge due to environmental inhomogeneities affecting these atomically thin materials.

The research details methodologies to fabricate high-quality TMDC monolayers with nearly intrinsic PL linewidths. Two key techniques are highlighted: encapsulating the monolayers in hexagonal boron nitride (h-BN) and passivating the oxide substrate with an alkyl monolayer. These methods synergistically decrease PL linewidths and align the trion to neutral exciton intensity ratio, thereby reducing charged exciton peaks.

Key findings are represented by full-width-at-half-maximum (FWHM) figures, where an FWHM as low as 1.7 meV was achieved, proximal to the intrinsic limit. Detailed analysis revealed homogeneous linewidths of 1.43±0.08 meV, juxtaposed against inhomogeneous broadening of 1.1±0.3 meV. This indicates the narrowing of linewidths through environmental and substrate passivation. The paper found substantial narrowing of emissive peaks using h-BN encapsulation and substrate passivation. Monolayers on untreated substrates showed linewidths averaging 9.8±2.8 meV, which contrasts significantly with the 2.0±0.2 meV of monolayers on passivated and encapsulated substrates.

The work probes the photophysical properties central to TMDC's optical performance, offering insights into electron-phonon interactions and intrinsic radiative lifetimes by using PL linewidths as markers of material quality. The authors examined lateral spatial variation across samples using hyperspectral imaging, identifying significant variation in exciton and trion presence in non-passivated samples versus the relative uniformity seen post-encapsulation and passivation.

The observed improvements can drive performance in applications including quantum computing and valleytronics by enhancing material properties inherent to TMDC monolayers. Future explorations might examine the effects of different substrate materials and extend these methods across various 2D materials to universalize such preparation techniques. The paper contributes valuable methodologies for advancing research in 2D materials, potentially helping refine the optical applications of TMDCs and providing pathways for more extended paper into exciton-based phenomena within these nanomaterials.

In summary, the paper underscores the potential of h-BN encapsulation and substrate passivation for approaching intrinsic PL linewidths in TMDC monolayers while critically reducing disorder-induced broadening. These methodologies highlight significant steps forward in achieving the optical quality necessary for leveraging 2D materials in advanced applications.