ACKNOWLEDGMENT This was supported by National Science Foundation (NSF) through the grant MRI-RAPID. Part of this work was performed at the Center for Nanoscale Systems, Harvard University, a member of the National Nanotechnology Infrastructure Network, which is supported by the NSF under the award number ECS-0335765. R. R. acknowledges fellowship support from Battelle’s National Security Global Business with Dr. Robert Carnes as the Director of Internal Research and Development. The authors would to thank Nenad Miljkovic, Sean C. O’Hern and Prof. Rohit Karnik, Department of Mechanical Engineering, Massachusetts Institute of Technology and Emil Song and Prof. K. L. Wang, Department of Electrical Engineering, University of California, Los Angeles for their help during experiments. This work has been published in the Nano Letters and can be found online at DOI: 10.1021/nl304647t Substrate Effect on the Wettability of Graphene Rishi Raj 1 , Shalabh C. Maroo 2 , and Evelyn N. Wang 1 1 Mechanical Engineering, MIT, Cambridge, MA 02139, USA 2 Mechanical and Aerospace Engineering, Syracuse University, NY 13244, USA ABSTRACT Experiments, molecular dynamics simulations, and theory demonstrate that the wettability of graphene coated substrate closely matches the wettability of graphite • Test samples: Mono-/bi-/tri-layered graphene sheets on Cu, SiO 2 , and glass • Test fluid: De-ionized water • Experimental observations: • MD and theoretical results agree with advancing contact angles measurements • Advancing contact angle represents the surface energy of graphene coatings • Large interlayer spacing responsible for negligible substrate effect on wettability • High contact angle hysteresis (16° − 37°) • Advancing contact angle independent of number of graphene layers • Receding contact angle influenced by defects in graphene EXPERIMENTS CHARACTERIZATION MOLECULAR DYNAMICS (MD) BEFORE PINNING AFTER NO RESIDUE AFTER COMPLETE EVAPORATION DROPWISE CONDENSATION • AFM measurements confirmed that the roughness ratio was negligible (~1.004) • Visualization of the contact line revealed local contact pinning on such smooth surfaces • Raman spectroscopy measurements visualized holes (solid arrows) in transferred graphene coatings which act as heterogeneity on these otherwise homogenous coating resulting in the observed hysteresis • Negligible substrate effect was apparent on the contact angle of water on monolayer graphene, both for Cu and SiO 2 • The simulated contact angles match with the experimental advancing contact angles • The discrepancy between the experimental and simulated values for monolayer graphene on Cu is attributed to the partial oxidation of Cu underneath the graphene grain boundaries • MD and experimental advancing angle values compare well with the 3-9 potential • Evaluation of solid-liquid equilibrium distance critical for the correct estimation of contact angle • The large interlayer spacing in graphene based systems minimize the effect of substrate ANALYSIS MD 3-9 POTENTIAL CONTINUUM • Redistribution of discrete graphene layers as continuum via integration is inaccurate • As a result, continuum models were found to be inaccurate for graphene based systems • Validated through contact angle, adsorption, exfoliation and cleavage energy calculations CONCLUSIONS • Dynamic contact angle experiments were performed on three bare substrates (Cu, SiO 2 , and Glass), nine graphene samples (mono-,bi-,tri-layered graphene), and graphite (HOPG) • Negligible substrate effect was apparent on the advancing contact angle of water on few layer graphene for all substrates • The advancing contact angle with a monolayer graphene matched those on HOPG • Significant contact angle hysteresis was observed on all samples • Transferred graphene coatings demonstrated maximum contact angle hysteresis • Condensation experiments with monolayer graphene on SiO 2 substrate were performed in the enclosed chamber of an Environmental Scanning Electron Microscope • Dropwise condensation confirmed the hydrophobic nature of monolayer graphene coatings • Contact angle of 85° − 92° was observed during the droplet growth phase • Significant contact line pinning time and hysteresis were also observed during these experiments Monolayer graphene on SiO 2 • Substrate has negligible effect on the wettability of graphene • Advancing contact angle is representative of the actual graphene coating • Receding contact angle is affected by defects in as grown and transferred graphene coatings • Continuum models invalid due to the large interlayer spacing in graphene based substrates 10 μm
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ACKNOWLEDGMENT This was supported by National Science Foundation (NSF) through the grant MRI-RAPID. Part of this work was performed at the Center for Nanoscale Systems, Harvard University, a member of the
National Nanotechnology Infrastructure Network, which is supported by the NSF under the award number ECS-0335765. R. R. acknowledges fellowship support from Battelle’s National Security Global
Business with Dr. Robert Carnes as the Director of Internal Research and Development. The authors would to thank Nenad Miljkovic, Sean C. O’Hern and Prof. Rohit Karnik, Department of Mechanical
Engineering, Massachusetts Institute of Technology and Emil Song and Prof. K. L. Wang, Department of Electrical Engineering, University of California, Los Angeles for their help during experiments. This
work has been published in the Nano Letters and can be found online at DOI: 10.1021/nl304647t
Substrate Effect on the Wettability of Graphene
Rishi Raj1, Shalabh C. Maroo2, and Evelyn N. Wang1
1Mechanical Engineering, MIT, Cambridge, MA 02139, USA 2Mechanical and Aerospace Engineering, Syracuse University, NY 13244, USA
ABSTRACT
Experiments, molecular dynamics simulations, and theory demonstrate that the wettability of graphene coated substrate closely matches the wettability of graphite
• Test samples: Mono-/bi-/tri-layered graphene sheets on Cu, SiO2, and glass
• Test fluid: De-ionized water
• Experimental observations:
• MD and theoretical results agree with advancing contact angles measurements
• Advancing contact angle represents the surface energy of graphene coatings
• Large interlayer spacing responsible for negligible substrate effect on wettability
• High contact angle hysteresis (16° − 37°)
• Advancing contact angle independent of number of graphene layers
• Receding contact angle influenced by defects in graphene
EXPERIMENTS
CHARACTERIZATION
MOLECULAR DYNAMICS (MD)
BEFORE PINNING AFTER
NO RESIDUE AFTER
COMPLETE EVAPORATION
DROPWISE CONDENSATION
• AFM measurements confirmed that the roughness ratio was negligible (~1.004)
• Visualization of the contact line revealed local contact pinning on such smooth surfaces
• Raman spectroscopy measurements
visualized holes (solid arrows) in
transferred graphene coatings which
act as heterogeneity on these
otherwise homogenous coating
resulting in the observed hysteresis
• Negligible substrate effect was apparent on the contact angle of water on monolayer graphene,
both for Cu and SiO2
• The simulated contact angles match with the experimental advancing contact angles
• The discrepancy between the experimental and simulated values for monolayer graphene on
Cu is attributed to the partial oxidation of Cu underneath the graphene grain boundaries
• MD and experimental advancing angle values compare well with the 3-9 potential
• Evaluation of solid-liquid equilibrium distance critical for the correct estimation of contact angle
• The large interlayer spacing in graphene based systems minimize the effect of substrate
ANALYSIS
MD 3-9 POTENTIAL CONTINUUM
• Redistribution of discrete graphene layers as continuum via integration is inaccurate
• As a result, continuum models were found to be inaccurate for graphene based systems
• Validated through contact angle, adsorption, exfoliation and cleavage energy calculations
CONCLUSIONS
• Dynamic contact angle experiments were performed on three bare substrates (Cu, SiO2, and
Glass), nine graphene samples (mono-,bi-,tri-layered graphene), and graphite (HOPG)
• Negligible substrate effect was apparent on the advancing contact angle of water on few layer
graphene for all substrates
• The advancing contact angle with a monolayer graphene matched those on HOPG
• Significant contact angle hysteresis was observed on all samples
• Transferred graphene coatings demonstrated maximum contact angle hysteresis
• Condensation experiments with monolayer graphene on
SiO2 substrate were performed in the enclosed chamber
of an Environmental Scanning Electron Microscope
• Dropwise condensation confirmed the hydrophobic nature
of monolayer graphene coatings
• Contact angle of 85° − 92° was observed during the
droplet growth phase
• Significant contact line pinning time and hysteresis were
also observed during these experiments Monolayer graphene on SiO2
• Substrate has negligible effect on the wettability of graphene
• Advancing contact angle is representative of the actual graphene coating
• Receding contact angle is affected by defects in as grown and transferred graphene coatings
• Continuum models invalid due to the large interlayer spacing in graphene based substrates 10 µm
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