Ali Ashraf

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.

Heterogeneous, three-dimensional texturing of graphene.

We report a single-step strategy to achieve heterogeneous, three-dimensional (3D) texturing of graphene and graphite by using a thermally activated shape-memory polymer substrate. Uniform arrays of graphene crumples can be created on the centimeter scale by controlling simple thermal processing parameters without compromising the electrical properties of graphene. In addition, we show the capability to selectively pattern crumples from otherwise flat graphene and graphene/graphite in a localized manner, which has not been previously achievable using other methods. Finally, we demonstrate 3D crumpled graphene field-effect transistor arrays in a solution-gated configuration. The presented approach has the capability to conform onto arbitrary 3D surfaces, a necessary prerequisite for adaptive electronics, and will enable facile large-scale topography engineering of not only graphene but also other thin-film and 2D materials in the future.

Doping-Induced Tunable Wettability and Adhesion of Graphene.

We report that substrate doping-induced charge carrier density modulation leads to the tunable wettability and adhesion of graphene. Graphenes water contact angle changes by as much as 13° as a result of a 300 meV change in doping level. Upon either n- or p-type doping with subsurface polyelectrolytes, graphene exhibits increased hydrophilicity. Adhesion force measurements using a hydrophobic self-assembled monolayer-coated atomic force microscopy probe reveal enhanced attraction toward undoped graphene, consistent with wettability modulation. This doping-induced wettability modulation is also achieved via a lateral metal-graphene heterojunction or subsurface metal doping. Combined first-principles and atomistic calculations show that doping modulates the binding energy between water and graphene and thus increases its hydrophilicity. Our study suggests that the doping-induced modulation of the charge carrier density in graphene influences its wettability and adhesion [corrected]. This opens up unique and new opportunities for the tunable wettability and adhesion of graphene for advanced coating materials and transducers.

Rapid Stencil Mask Fabrication Enabled One-Step Polymer-Free Graphene Patterning and Direct Transfer for Flexible Graphene Devices

We report a one-step polymer-free approach to patterning graphene using a stencil mask and oxygen plasma reactive-ion etching, with a subsequent polymer-free direct transfer for flexible graphene devices. Our stencil mask is fabricated via a subtractive, laser cutting manufacturing technique, followed by lamination of stencil mask onto graphene grown on Cu foil for patterning. Subsequently, micro-sized graphene features of various shapes are patterned via reactive-ion etching. The integrity of our graphene after patterning is confirmed by Raman spectroscopy. We further demonstrate the rapid prototyping capability of a stretchable, crumpled graphene strain sensor and patterned graphene condensation channels for potential applications in sensing and heat transfer, respectively. We further demonstrate that the polymer-free approach for both patterning and transfer to flexible substrates allows the realization of cleaner graphene features as confirmed by water contact angle measurements. We believe that our new method promotes rapid, facile fabrication of cleaner graphene devices, and can be extended to other two dimensional materials in the future.

Assembly and Densification of Nanowire Arrays via Shrinkage

Chemically synthesized semiconductor nanowires (NWs) have demonstrated substantial promise for nanoelectronics, nanoenergy, and nanobiotechnology, but the lack of an effective and controllable assembly process has limited the wide adoption of NWs in these areas. Here we demonstrate a facile, robust, and controllable approach to assembling and densifying a parallel array of NWs using shrinkable shape memory polymers. Using thermal-induced shrinkage of polystyrene, we were able to successfully assemble and densify NW arrays up to close-packing and, furthermore, achieve tunable density (up to ∼300% amplification of density) by controlling the shrinkage process. We also demonstrate scalable assembly and densification of NWs on a 2.5 × 6 inch scale to explore the manufacturability of the shrink-induced assembly process. Finally, we demonstrate the successful transfer of the shrink-assembled NW arrays onto various 2-dimensional and 3-dimensional substrates without compromising the integrity of NW assembly and density.

Hierarchical, Dual-Scale Structures of Atomically Thin MoS2 for Tunable Wetting

Molybdenum disulfide (MoS2), a well-known solid lubricant for low friction surface coatings, has recently drawn attention as an analogue two-dimensional (2D) material beyond graphene. When patterned to produce vertically grown, nanoflower-structures, MoS2 shows promise as a functional material for hydrogen evolution catalysis systems, electrodes for alkali metal-ion batteries, and field-emission arrays. Whereas the wettability of graphene has been substantially investigated, that of MoS2 structures, especially nanoflowers, has remained relatively unexplored despite MoS2 nanoflowers potential in future applications. Here, we demonstrate that the wettability of MoS2 can be controlled by multiscale modulation of surface roughness through (1) tuning of the nanoflower structures by chemical vapor deposition synthesis and (2) tuning of microscale topography via mechanical strain. This multiscale modulation offers broadened tunability (80-155°) compared to single-scale tuning (90-130°). In addition, surface adhesion, determined from contact angle hysteresis (CAH), can also be tuned by multiscale surface roughness modulation, where the CAH is changed in range of 20-40°. Finally, the wettability of crumpled MoS2 nanoflowers can be dynamically and reversibly controlled through applied strain (∼115-150° with 0-200% strain), and remains robust over 1000 strain cycles. These studies on the tunable wettability of MoS2 will contribute to future MoS2-based applications, such as tunable wettability coatings for desalination and hydrogen evolution.

Three-dimensional, flexible graphene bioelectronics.

We report 3-dimensional (3D) graphene-based biosensors fabricated via 3D transfer of monolithic graphene-graphite structures. This mechanically flexible all-carbon structure is a prospective candidate for intimate 3D interfacing with biological systems. Monolithic graphene-graphite structures were synthesized using low pressure chemical vapor deposition (LPCVD) process relying on the heterostructured metal catalyst layers. Nonplanar substrates and wet-transfer method were used with a thin Au film as a transfer layer to achieve the 3D graphene structure. Instead of the typical wet-etching method, vapor-phase etching was performed to minimize the delamination of the graphene while removing the transfer layer. We believe that the monolithic graphene-graphite synthesis combined with the conformal 3D transfer will pave the way for the 3D conformal sensing capability as well as the intracellular recording of living cells in the future.

Correction to Doping-Induced Tunable Wettability and Adhesion of Graphene.

I Note Added in Proof, we cited a relevant paper published on June 1, 2016 (Nano Lett., 2016, 16 (7), pp 4447−4453. DOI: 10.1021/acs.nanolett.6b01594) by Hong et al. In order to fully acknowledge this recent work that reports conclusions that are similar to our independently performed work, we would like to (1) remove our claim in reference to “for the first time” from the paragraphs referenced below, and (2) add the sentences below discussing the work of Hong et al. to our paper. In the abstract and conclusion paragraphs, “for the first time” is removed to read as below, respectively: Our study suggests that the doping-induced modulation of the charge carrier density in graphene influences its wettability and adhesion. In conclusion, we used experimental, analytical, and simulation studies to demonstrate that the WCA of graphene can be modulated by doping. In the first paragraph before the last sentence, we would like to add the sentences below: We have also become aware of the work by Hong et al. during the proof stage of our paper. Our work independently demonstrates and further supports similar findings reported in Hong et al., showing doping-induced wettability modulation of graphene.