Vapor-Solid Growth of High Optical Quality MoS2 Monolayers With Near-Unity Valley Polarization (1302.5758v1)

Abstract: Monolayers of transition metal dichalcogenides (TMDCs) are atomically thin direct-gap semiconductors with potential applications in nanoelectronics, optoelectronics, and electrochemical sensing. Recent theoretical and experimental efforts suggest that they are ideal systems for exploiting the valley degrees of freedom of Bloch electrons. For example, Dirac valley polarization has been demonstrated in mechanically exfoliated monolayer MoS2 samples by polarization-resolved photoluminescence, although polarization has rarely been seen at room temperature. Here we report a new method for synthesizing high optical quality monolayer MoS2 single crystals up to 25 microns in size on a variety of standard insulating substrates (SiO2, sapphire and glass) using a catalyst-free vapor-solid growth mechanism. The technique is simple and reliable, and the optical quality of the crystals is extremely high, as demonstrated by the fact that the valley polarization approaches unity at 30 K and persists at 35% even at room temperature, suggesting a virtual absence of defects. This will allow greatly improved optoelectronic TMDC monolayer devices to be fabricated and studied routinely.

• The paper introduces a catalyst-free vapor-solid technique to produce high-quality MoS₂ monolayers with lateral dimensions up to 25 microns.
• The study confirms superior optical and structural integrity through AFM and Raman spectroscopy, showing consistent features akin to exfoliated samples.
• The authors achieve near-unity valley polarization at low temperatures and promising polarization at room temperature, underscoring potential for scalable valleytronic applications.

Vapor-Solid Growth of High Optical Quality MoS₂ Monolayers

This paper presents a significant advancement in the synthesis of monolayer molybdenum disulfide (MoS₂) via a catalyst-free vapor-solid (VS) growth method. The authors successfully produce high optical quality monolayer crystals on various insulating substrates, including SiO₂, sapphire, and glass, achieving nearly unity valley polarization—a crucial parameter for valleytronics applications—at low temperatures.

Synthesis Methodology

The paper introduces a straightforward method using physical vapor transport of MoS₂ powder in an argon carrier gas environment. The process yields monolayer flakes with lateral dimensions of up to 25 microns, forming equilibrated triangular shapes indicative of single-crystal quality. A critical advantage of this approach is the lack of catalyst use, simplifying the synthesis procedure and integration into device fabrication.

Optical and Structural Characterization

Characterization through atomic force microscopy and Raman spectroscopy confirms the superior quality of the monolayers. The morphological uniformity and atomic flatness, critical for optoelectronic performance, are validated by showing consistent Raman spectral features across different substrates. The Raman peak positions and separations align well with those observed in exfoliated samples, indicating compatible crystal structuring while highlighting improved scalability with the VS method.

Valley Polarization and Photoluminescence

The paper focuses on the valley polarization of these monolayer MoS₂ crystals, which is pivotal in valleytronics. The valley polarization approaches near unity at 30 K and maintains a value of 35% at room temperature. This result underscores the very low defect density of the synthesized monolayers, as high optical quality is often hampered by defect-induced intervalley scattering. These findings suggest that this growth technique could lead to reliable and reproducible TMDC monolayer device applications, as these high polarization levels are generally challenging to attain at higher temperatures with mechanically exfoliated samples.

Implications and Future Directions

The results have profound implications for the development of valleytronic devices, potentially leading to their integration into practical applications at room temperature. This method addresses a significant gap between scalable production and maintaining high crystal quality, making it promising for industrial applications where large-area, high-performance TMDC monolayers are required.

Future research could extend the application of this VS growth technique to other transition metal dichalcogenides (TMDCs), exploring their potential in various applications and the further development of valley physics. Moreover, continued optimization of growth conditions could improve monolayer uniformity and scalability, addressing challenges related to crystal size restriction and defect reduction.

In summary, the vapor-solid growth method presented in this paper signifies a notable progression towards scalable production of high-quality MoS₂ monolayers, providing a foundation for further exploration in valleytronics and 2D material-based optoelectronics. The high degree of valley polarization achieved at both low and room temperatures marks a significant stride toward practical valleytronic applications, potentially influencing the future landscape of nanotechnology and semiconductor physics.