Surface Energy Engineering for Tunable Wettability through Controlled Synthesis of MoS2 (1403.2948v1)

Abstract: MoS2 is one of the important members of transition metal dichalogenides which is emerging as a potential 2D atomically thin layered material for low power electronic and opto-electronic applications. However, for MoS2 a critical fundamental question of significant importance is how the surface energy and hence the wettability is altered in nanoscale -- in particular, the role of crystal quality in low dimensions. Present work reports the synthesis of large area MoS2 films on insulating substrates with different surface morphology via vapor phase deposition by varying the growth temperatures. The crystallinity of the samples is examined by transmission electron microscopy and Raman spectroscopy. From contact angle measurements, it is possible to correlate the wettability with crystallinity at nanoscale. The specific surface energy for few layers thick MoS2 is estimated to be around 46.5 mJ/m2. Our results shed light on the MoS2-water interaction which is significant for developing important devices based on MoS2 coated surfaces for micro-fluidic applications.

• The paper demonstrates that vapor deposition of MoS2 enables control over film crystallinity and wettability through precise temperature variation.
• High-resolution HRTEM and Raman spectroscopy reveal that lower temperatures yield quasi-crystalline, edge-terminated structures, while higher temperatures produce a well-defined honeycomb lattice.
• Surface energy estimation using Neumann’s equation shows crystalline MoS2 films have energy levels similar to graphene, highlighting their potential in electronic and optoelectronic devices.

Surface Energy Engineering for Tunable Wettability through Controlled Synthesis of MoS$_2$

This paper presents a rigorous paper on the synthesis and characterization of molybdenum disulfide (MoS$_2$) thin films for potential applications in electronic and optoelectronic devices. The investigation focuses on understanding the influence of surface energy and wettability, primarily affected by the crystallinity and surface morphology of the synthesized MoS$_2$ films under varying growth conditions.

Synthesis Methodology

MoS$_2$ is a prominent member of the transition metal dichalcogenides (TMDCs), recognized for its promising electronic and optical properties. The paper employs vapor phase deposition for the synthesis of MoS$_2$ films on insulating substrates, manipulating the growth temperatures to observe how these changes influence the films' characteristics. The surface morphology and crystallinity of these films were thoroughly examined using high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and Raman spectroscopy.

Key Findings

1. Crystallinity and Morphology: The paper provides detailed HRTEM and Raman spectroscopy evidence showing that MoS$_2$ films exhibit varied crystallinity levels depending on the synthesis temperature. Films grown at lower temperatures (e.g., 550 °C) were found to possess edge-terminated structures with quasi-crystalline nature, whereas high-temperature growth (e.g., 900 °C) resulted in well-crystalline MoS$_2$ with a substantial honeycomb lattice structure.
2. Wettability and Surface Energy: The wettability of these films was assessed through static contact angle measurements. The films demonstrated a transition from hydrophilic to hydrophobic characteristics corresponding to an increase in their synthesis temperature, indicating a direct correlation between crystallinity and wettability. Specifically, films grown at 550 °C showed hydrophilic properties (contact angle ~23.8°), while those synthesized at 900 °C displayed hydrophobic properties (contact angle ~91.6°).
3. Surface Energy Estimation: Utilizing Neumann's equation of state, the specific surface energy for highly crystalline MoS$_2$ films was approximated to be 46.5 mJ/m², which closely parallels that of graphene films.

Implications and Future Prospects

The implications of these findings are multifaceted. The ability to tune the wettability of MoS$_2$ films through controlled synthesis advances their feasibility for diverse applications in microfluidics and surface coatings, where specific hydrophobic or hydrophilic characteristics are desired. The demonstration of a relationship between synthesis conditions and surface properties provides a critical pathway for optimizing the material's performance in electronic and optoelectronic devices.

Theoretically, this work enriches the understanding of TMDCs' surface interaction dynamics, particularly in non-bulk configurations. From a practical standpoint, techniques and insights from this paper could be pivotal for the fabrication of future nanostructured devices, paving the way for tailored material properties through controlled synthesis.

Prospective research may explore expanding the synthesis techniques to other TMDCs under varied environmental conditions to further validate the observed correlation between temperature, crystallinity, and wettability. Moreover, integrating these insights with computational modeling could deepen the comprehension of the mechanisms governing surface interactions at the nanoscale, offering refined control over material specification for targeted applications.