Photoluminescence of freestanding single- and few-layer MoS2 (1311.5824v2)

Abstract: We present a photoluminescence study of freestanding and Si/SiO2 supported single- and few-layer MoS2. The single-layer exciton peak (A) is only observed in freestanding MoS2. The photoluminescence of supported single-layer MoS2 is instead originating from the A- (trion) peak as the MoS2 is n-type doped from the substrate. In bilayer MoS2, the van der Waals interaction with the substrate is decreasing the indirect band gap energy by up to ~ 80 meV. Furthermore, the photoluminescence spectra of suspended MoS2 can be influenced by interference effects.

Photoluminescence Properties of Freestanding vs. Supported MoS$_2$ Monolayers

The paper presents an in-depth photoluminescence analysis of freestanding and silicon dioxide-supported molybdenum disulfide (MoS$_2$) layers, proposing significant insights into the effects of substrate interactions on the optical properties of two-dimensional materials. MoS$_2$, an indirect semiconductor in its bulk form, transitions to a direct bandgap semiconductor at the monolayer level, a property that has pivotal implications for next-generation optoelectronic and nanoelectronic devices.

The paper delineates a comprehensive exploration of photoluminescence in both single- and few-layer MoS$_2$. A salient conclusion is the distinct photoluminescence behavior in freestanding versus supported MoS$_2$ monolayers. Freestanding MoS$_2$ predominantly exhibits the A exciton peak, along with a notable presence of the A$^-$ trion peak. This observation contrasts with supported single-layer MoS$_2$, where the trion (A$^-$) is dominant due to n-type doping induced by substrate interactions. Consequently, photoluminescence associated with single-layer MoS$_2$ on Si/SiO$_2$ likely stems from substrate-mediated doping effects, a hypothesis which reshapes our understanding of previously reported photoluminescence data in this context.

The interaction between the MoS$_2$ layers and the substrate is also shown to affect the few-layer MoS$_2$. In bilayer MoS$_2$, and other multilayer configurations, substrate-induced van der Waals interactions lower the indirect I peak energy by approximately up to 80 meV. This impact diminishes as the layer count increases, suggesting a reduced influence of the substrate on thicker MoS$_2$ films, which is critical for understanding multi-layered heterostructures in practical applications.

The paper employs a methodical approach, utilizing Raman and photoluminescence spectroscopy to differentiate between the effects of substrate interactions and intrinsic photonic properties. Density Functional Theory (DFT) calculations are employed to rule out strain and dielectric environment effects as primary causes of the observed photoluminescence variations. These calculations provide evidence that supports the conclusion that band structure sensitivity, particularly near the $\Gamma$ point, may explain the blue shift observed in the photoluminescence associated with the freestanding configurations.

The implications of these findings are substantial for the design and fabrication of optoelectronic devices based on 2D materials, highlighting the critical role that substrate interactions play in determining the electronic and optical properties of MoS$_2$. Future research might explore exploring the modulation of such interactions through engineered substrates or controlled doping, providing pathways to tune the electronic properties tailored for specific applications.

Ultimately, this research underscores the complexity of photoluminescence phenomena in 2D semiconductors and fosters a more nuanced understanding of MoS$_2$ that will likely propel advancements in material science and nanotechnology further.