Letter pubs.acs.org/NanoLett
Surface Energy Engineering for Tunable Wettability through Controlled Synthesis of MoS2 Anand P. S. Gaur,† Satyaprakash Sahoo,*,† Majid Ahmadi,† Saroj P. Dash,‡ Maxime J.-F. Guinel,†,§ and Ram S. Katiyar*,† †
Department of Physics and Institute for Functional Nanomaterials, University of Puerto Rico, San Juan, Puerto Rico 00931, United States ‡ Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-41296, Göteborg, Sweden § Department of Chemistry, College of Natural Sciences, University of Puerto Rico, P.O. Box 70377, San Juan, Puerto Rico 00936-8377, United States S Supporting Information *
ABSTRACT: MoS 2 is an important member of the transition metal dichalcogenides that is emerging as a potential 2D atomically thin layered material for low power electronic and optoelectronic applications. However, for MoS2 a critical fundamental question of significant importance is how the surface energy and hence the wettability is altered at the nanoscale in particular, the role of crystallinity and orientation. This work reports on the synthesis of large area MoS2 thin films on insulating substrates (SiO2/Si and Al2O3) with different surface morphology via vapor phase deposition by varying the growth temperatures. The samples were examined using transmission electron microscopy and Raman spectroscopy. From contact angle measurements, it is possible to correlate the wettability with crystallinity at the nanoscale. The specific surface energy for few layers MoS2 is estimated to be about 46.5 mJ/m2. Moreover a layer thickness-dependent wettability study suggests that the lower the thickness is, the higher the contact angle will be. Our results shed light on the MoS2−water interaction that is important for the development of devices based on MoS2 coated surfaces for microfluidic applications. KEYWORDS: MoS2, wettability, contact angle, defects, chemical vapor deposition, Neumann’s method
S
atomically layered MoS2 is extremely important from both fundamental and application points of view. Because of the weak van der Waals interlayer forces that exist between two consecutive layers in MoS2, it is apparently easier for low force shearing. Thus, it is an excellent candidate that has been used as solid lubricant in tribological applications. Interestingly, their tribological properties vary in vacuum and in other media. MoS2 has tribological properties that deteriorate in water vapor medium due to absorbed oxygen or introduced defects due to elevated temperature at contact point in tribological experiments.22 Recent studies have demonstrated that hollow concentric MoS2 nanospheres exhibit excellent tribological properties.23 Earlier reports on wetting properties of MoS2 reveal its hydrophobic nature where experiments were performed using the MoS2 powder samples. But for nanoelectronics applications, it is extremely important to understand wetting properties of exposed surface. It has been also demonstrated that the presence of humidity can alter the MoS2 transistor characteristics.24 Moreover, 2D materials are equally important for
ince the discovery of graphene, two-dimensional (2D) materials have been receiving tremendous research interest. Although graphene exhibits excellent physical properties,1−6 its application in future electronic devices is limited by its zero bandgap.7,8 In developments parallel to that of graphene, transition metal dichalcogenides (TMDC) abbreviated as MX2, where M is a metal and X is chalcogen (S, Se, or Te), are emerging as alternative 2D materials for various applications.6,9−13 In each layer of TMDC, the M and X atoms are bonded covalently in a prismatic arrangement, and layers are held together by van der Waals forces,14 making it possible to obtain a single layer by mechanical or liquid exfoliation methods.15,16 Among these TMDCs, recently monolayer MoS2 has been studied extensively due to its excellent electronic and optical properties.17−19 For instance, in bulk it has an indirect band gap (1.2 eV) whereas it changes to direct band gap in monolayer (1.8 eV).20 Kim et al. have demonstrated excellent room-temperature mobility in monolayer MoS2 for its application in low-power nanoelectronics and optoelectronics.21 Although there is considerable progress in understanding the electronic, optical, and spintronics properties of MoS2, less efforts were made to understand the wettability of MoS2 by studying the water−MoS2 interaction particularly along edges and basal planes. The wettability properties of © 2014 American Chemical Society
Received: March 24, 2014 Revised: June 11, 2014 Published: July 30, 2014 4314
dx.doi.org/10.1021/nl501106v | Nano Lett. 2014, 14, 4314−4321
Nano Letters
Letter
that surface contamination due to adsorption can affect the wettability; in view of this, all the samples were heat treated at 200 °C for 30 min in argon prior to measurements. Result and Discussion. SEM and AFM images showed the films are smooth and continuous with measured roughness and thickness
