Wetting translucency of graphene

Scalable graphene coatings for enhanced condensation heat transfer

Water vapor condensation is commonly observed in nature and routinely used as an effective means of transferring heat with dropwise condensation on nonwetting surfaces exhibiting heat transfer improvement compared to filmwise condensation on wetting surfaces. However, state-of-the-art techniques to promote dropwise condensation rely on functional hydrophobic coatings that either have challenges with chemical stability or are so thick that any potential heat transfer improvement is negated due to the added thermal resistance of the coating. In this work, we show the effectiveness of ultrathin scalable chemical vapor deposited (CVD) graphene coatings to promote dropwise condensation while offering robust chemical stability and maintaining low thermal resistance. Heat transfer enhancements of 4× were demonstrated compared to filmwise condensation, and the robustness of these CVD coatings was superior to typical hydrophobic monolayer coatings. Our results indicate that graphene is a promising surface coating to promote dropwise condensation of water in industrial conditions with the potential for scalable application via CVD.

Atomically thin epitaxial template for organic crystal growth using graphene with controlled surface wettability

A two-dimensional epitaxial growth template for organic semiconductors was developed using a new method for transferring clean graphene sheets onto a substrate with controlled surface wettability. The introduction of a sacrificial graphene layer between a patterned polymeric supporting layer and a monolayer graphene sheet enabled the crack-free and residue-free transfer of free-standing monolayer graphene onto arbitrary substrates. The clean graphene template clearly induced the quasi-epitaxial growth of crystalline organic semiconductors with lying-down molecular orientation while maintaining the "wetting transparency", which allowed the transmission of the interaction between organic molecules and the underlying substrate. Consequently, the growth mode and corresponding morphology of the organic semiconductors on graphene templates exhibited distinctive dependence on the substrate hydrophobicity with clear transition from lateral to vertical growth mode on hydrophilic substrates, which originated from the high surface energy of the exposed crystallographic planes of the organic semiconductors on graphene. The optical properties of the pentacene layer, especially the diffusion of the exciton, also showed a strong dependency on the corresponding morphological evolution. Furthermore, the effect of pentacene-substrate interaction was systematically investigated by gradually increasing the number of graphene layers. These results suggested that the combination of a clean graphene surface and a suitable underlying substrate could serve as an atomically thin growth template to engineer the interaction between organic molecules and aromatic graphene network, thereby paving the way for effectively and conveniently tuning the semiconductor layer morphologies in devices prepared using graphene.

Wetting of mono and few-layered WS2 and MoS2 films supported on Si/SiO2 substrates

The recent interest and excitement in graphene has also opened up a pandora's box of other two-dimensional (2D) materials and material combinations which are now beginning to come to the fore. One family of these emerging 2D materials is transition metal dichalcogenides (TMDs). So far there is very limited understanding on the wetting behavior of "monolayer" TMD materials. In this study, we synthesized large-area, continuous monolayer tungsten disulfide (WS2) and molybdenum disulfide (MoS2) films on SiO2/Si substrates by the thermal reduction and sulfurization of WO3 and MO3 thin films. The monolayer TMD films displayed an advancing water contact angle of ∼83° as compared to ∼90° for the bulk material. We also prepared bilayer and trilayer WS2 films and studied the transition of the water contact angle with increasing number of layers. The advancing water contact angle increased to ∼85° for the bilayer and then to ∼90° for the trilayer film. Beyond three layers, there was no significant change in the measured water contact angle. This type of wetting transition indicates that water interacts to some extent with the underlying silica substrate through the monolayer TMD sheet. The experimentally observed wetting transition with numbers of TMD layers lies in-between the predictions of one continuum model that considers only van der Waals attractions and another model that considers only dipole-dipole interactions. We also explored wetting as a function of aging. A clean single-layer WS2 film (without airborne contaminants) was shown to be strongly hydrophilic with an advancing water contact angle of ∼70°. However, over time, the sample ages as hydrocarbons and water present in air adsorb onto the clean WS2 sheet. After ∼7 days, the aging process is completed and the advancing water contact angle of the aged single-layer WS2 film stabilizes at ∼83°. These results suggest that clean (i.e., nonaged) monolayer TMDs are hydrophilic materials. We further show that substitution of sulfur atoms by oxygen in the lattice of aged monolayer WS2 and MoS2 films can be used to generate well-defined 'hydrophobic-hydrophilic' patterns that preferentially accumulate and create microdrop arrays on the surface during water condensation and evaporation experiments.

Direct epitaxial CVD synthesis of tungsten disulfide on epitaxial and CVD graphene

Direct epitaxial growth of WS₂ isolated crystals and WS₂ continuous films onto epitaxial- and CVD-graphene providing a homogeneous and narrow PL peak.

van der Waals screening by single-layer graphene and molybdenum disulfide

A sharp tip of atomic force microscope is employed to probe van der Waals forces of a silicon oxide substrate with adhered graphene. Experimental results obtained in the range of distances from 3 to 20 nm indicate that single-, double-, and triple-layer graphenes screen the van der Waals forces of the substrate. Fluorination of graphene, which makes it electrically insulating, lifts the screening in the single-layer graphene. The van der Waals force from graphene determined per layer decreases with the number of layers. In addition, increased hole doping of graphene increases the force. Finally, we also demonstrate screening of the van der Waals forces of the silicon oxide substrate by single- and double-layer molybdenum disulfide.

Convex nanobending at a moving contact line: the missing mesoscopic link in dynamic wetting

The morphological information on the very front of a spreading liquid is fundamental to our understanding of dynamic wetting. Debate has lasted for years concerning the nanoscopic local angles and the transition from them to the macroscopic counterpart, θ(D). This study of nonvolatile liquids analyzes the interface profile near the advancing contact line using an advanced atomic force microscopy. The interface is found following the macroscopic profile until bending in a convex profile around 20 nm from the substrate. This shoe-tip-like feature is common in partially wetting while absent for completely wetting, and its curvature varies with advancing speed. The observation ends the long-standing debate about the nanoscopic contact angles and their speed dependency. The convex nanobending provides a mesoscopic link and effectively complicates the dynamic wetting behaviors.

Spectroscopic investigation of the wettability of multilayer graphene using highly ordered pyrolytic graphite as a model material

We report the intrinsic water contact angle (WCA) of multilayer graphene, explore different methods of cleaning multilayer graphene, and evaluate the efficiency of those methods on the basis of spectroscopic analysis. Highly ordered pyrolytic graphite (HOPG) was used as a model material system to study the wettability of the multilayer graphene surface by WCA measurements. A WCA value of 45° ± 3° was measured for a clean HOPG surface, which can serve as the intrinsic WCA for multilayer graphene. A 1 min plasma treatment (100 W) decreased the WCA to 6°, owing to the creation of surface defects and functionalization by oxygen-containing groups. Molecular dynamics simulations of water droplets on the HOPG surface with or without the oxygen-containing defect sites confirmed the experimental results. Heat treatment at near atmospheric pressure and wet chemical cleaning methods using hydrofluoric acid and chloroform did not change the WCA significantly. Low-pressure, high-temperature annealing under argon and hydrogen reduced the WCA to 54°, close to the intrinsic WCA of HOPG. Raman spectroscopy and atomic force microscopy did not show any significant change for the HOPG surface after this treatment, confirming low-pressure, high-temperature annealing as an effective technique to clean multilayer graphene without damaging the surface. Time-of-flight secondary ion mass spectrometry indicated the existence of hydrocarbon species on the surface of the HOPG sample that was exposed to air for <5 min and the absence of these impurities in the bulk. X-ray photoelectron spectroscopy analyses of the sample surfaces after the different cleaning techniques were performed to correlate the WCA to the surface chemistry. X-ray photoelectron spectroscopy results revealed that the WCA value changed drastically, depending on the amounts of oxygen-containing and hydrocarbon-containing groups on the surface.

Nanoscale wetting and fouling resistance of functionalized surfaces: a computational approach

A computational modeling methodology has been developed and employed to characterize the nanoscale wettability and antifouling properties of functionalized hard and deformable surfaces, with a specific focus on poly(ethylene glycol) grafted substrates and their resistance to graphitic carbons. Empirical evidence suggests that the antifouling behavior of polyethylene PEG is associated with two main mechanisms: steric repulsions and hydration via formation of a structured water layer. However, there is also little attention paid to the contribution of steric repulsion vs surface hydration. We examine these two mechanisms through a combination of in silico contact angle and force measurements at the nanoscale level. We investigate the properties of the grafted functional chains and the underlying substrate, responsible for resisting surface deposition of graphitic contaminants in aqueous solution. Our results reveal that the fouling-release efficiency is enhanced when PEG chains are grafted onto hard hydrophilic substrates such as silica in contrast to deformable polymer substrates where surface modifications are effectively mitigated during interfacial contact with a hard contaminant. We conclude that the contribution of steric repulsion vs surface hydration to the antifouling ability of surfaces is strongly dependent on the nanoscale structure and deformability of the substrate. This generic method can be applied to examine individual contribution of steric repulsions and surface hydration to antifouling performance of grafted chains.

Microwetting of supported graphene on hydrophobic surfaces revealed by polymerized interfacial femtodroplets

Understanding the wettability of graphene is the crucial step toward the design and control of graphene-based surface in contact with liquids. In this work, the static microwettability of a supported single layer graphene (SLG) immersed in water or alcoholic aqueous solutions is revealed by the morphological characterization of the polymerized interfacial femtoliter droplets. As expected, the contact angle of the femtoliter droplets on the SLG in water is in between that on the underlying silanized silicon and that on graphite (HOPG). However, the wettability of femtoliter droplets on the SLG demonstrates a unique dependence on the compositions of the surrounding liquid medium: Their contact angle on SLG becomes much larger than that on both graphite and on silanized silicon, once short-chain alcohol molecules are present in the surrounding medium. To account for this finding, we hypothesize two scenarios to rationalize the effect of alcohol on the microwettability on SLG. The understanding elucidated in this study may allow for improved control of the interaction between graphene and the surrounding liquid environment and facilitate applications in which graphene is in contact with liquids, such as in microfluidics and in lab-on-chip systems.

Wetting-transparent graphene films for hydrophobic water-harvesting surfaces

Wetting-transparent graphene films grown in situ by chemical vapor deposition on hydrophobic (roughened) copper surfaces offer excellent resistance to copper corrosion while maintaining the intrinsic hydrophobicity of the surface, enabling superior performance for water-harvesting applications.

Surface energy engineering for tunable wettability through controlled synthesis of MoS2

MoS2 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/m(2). 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.

3-Dimensional atomic scale structure of the ionic liquid-graphite interface elucidated by AM-AFM and quantum chemical simulations

In situ amplitude modulated atomic force microscopy (AM-AFM) and quantum chemical simulations are used to resolve the structure of the highly ordered pyrolytic graphite (HOPG)-bulk propylammonium nitrate (PAN) interface with resolution comparable with that achieved for frozen ionic liquid (IL) monolayers using STM. This is the first time that (a) molecular resolution images of bulk IL-solid interfaces have been achieved, (b) the lateral structure of the IL graphite interface has been imaged for any IL, (c) AM-AFM has elucidated molecular level structure immersed in a viscous liquid and (d) it has been demonstrated that the IL structure at solid surfaces is a consequence of both thermodynamic and kinetic effects. The lateral structure of the PAN-graphite interface is highly ordered and consists of remarkably well-defined domains of a rhomboidal superstructure composed of propylammonium cations preferentially aligned along two of the three directions in the underlying graphite lattice. The nanostructure is primarily determined by the cation. Van der Waals interactions between the propylammonium chains and the surface mean that the cation is enriched in the surface layer, and is much less mobile than the anion. The presence of a heterogeneous lateral structure at an ionic liquid-solid interface has wide ranging ramifications for ionic liquid applications, including lubrication, capacitive charge storage and electrodeposition.

Exohedral physisorption of ambient moisture scales non-monotonically with fiber proximity in aligned carbon nanotube arrays

Here we present a study on the presence of physisorbed water on the surface of aligned carbon nanotubes (CNTs) in ambient conditions, where the wet CNT array mass can be more than 200% larger than that of dry CNTs, and modeling indicates that a water layer >5 nm thick can be present on the outer CNT surface. The experimentally observed nonlinear and non-monotonic dependence of the mass of adsorbed water on the CNT packing (volume fraction) originates from two competing modes. Physisorbed water cannot be neglected in the design and fabrication of materials and devices using nanowires/nanofibers, especially CNTs, and further experimental and ab initio studies on the influence of defects on the surface energies of CNTs, and nanowires/nanofibers in general, are necessary to understand the underlying physics and chemistry that govern this system.