Deconstructing the governing dissipative phenomena in the nanoscale

An expression describing the controlling parameters involved in short range nanoscale dissipation is proposed and supported by simulations and experimental findings. The expression is deconstructed into the geometrical, dynamic, chemical and mechanical properties of the system. In atomic force microscopy these are translated into 1) tip radius and tip-sample deformation, 2) resonant frequency and oscillation amplitude and 3) hysteretic and viscous dissipation. The latter are characteristic parameters defining the chemical and mechanical properties of the tip-sample system. Long range processes are also discussed and footprints are identified in experiments conducted on mica and silicon samples. The present methodology can be exploited to validate or invalidate nanoscale dissipative models by comparing predictions with experimental observables

Elucidation of the wettability of graphene through a multi-length-scale investigation approach

Univocal conclusions around the wettability of graphene exposed to environmental conditions remain elusive despite the recent efforts of several research groups. The main discrepancy rests on the question of whether a graphene monolayer (GML) is transparent or not to water and more generally what the role is that the substrate plays in determining the degree of wetting of the GML. In this work, we investigate the water transparency of GML by means of a multi-length-scale approach. We complement traditional static contact angle measurements and environmental scanning electron microscopy experiments with atomic force microscopy based force spectroscopy to assess the role that intermolecular interactions play in determining the wetting of GML. To gain deeper insight into the wetting transparency issue, we perform experiments on inert metals, such as gold and platinum, covered or not covered by GML. The comparison of the results obtained for different systems (i.e. GML covered and uncovered inert metals), provides unambiguous evidence that supports the non-wetting transparency theory of GML. This work aims to assist the development of technologies based on graphene-water interaction, such as graphitic membranes for water separation processes

Ion exchange and DNA molecular dip sticks: studying the nanoscale surface wetting of muscovite mica

Mica is an abundant crystal mineral that has important and interesting bulk and surface properties for a variety of applications. These properties arise from its anisotropic structure, in which layers of aluminum silicate, 1 nm thick, are ionically bonded together, typically with K+ ions. The surface properties of mica can be varied through ion exchange with the exposed lattice sites. In this study, the effect of kinetics on ion exchange with nickel ions (Ni2+) and its influence on surface water accumulation as a function of time has been investigated. Mica was ion-exchanged for 30 s or 5 min for a range of Ni2+ concentrations (i.e., 1.0-20.0 mM), and its surface properties were measured for up to 96 h after incubation in a controlled environment. The nanoscale physicochemical properties of nickel-functionalized muscovite mica (Ni-mica) were investigated by reconstructing the conservative force profile between an atomic force microscopy (AFM) tip and the surface. This information provides a hint of the surface water accumulation and enables details of the spatial and temporal variations in surface properties due to the ion-mediated adsorption of water to be elucidated. Variations in the water-layer accumulation were confirmed using noncontact AFM imaging under ambient conditions and DNA molecules as "molecular dip sticks". It was found that the surface properties were largely independent of the incubating concentration but did depend on the incubation time during ion exchange and the aging time. For the longer incubation time of 5 min, the water-layer accumulation remained constant at around ∼1.5 nm deep, whereas for the short incubation time of 30 s, the accumulation was initially subnanometer but grew with aging time and converged to a similar final value after 96 h. The extracted force of adhesion (FAD) also showed the same trends, where reduced values of FAD indicated increased screening of the van der Waals interaction through thicker water layers

A nanoscopic approach to studying evolution in graphene wettability

The equilibrium state of graphene surfaces exposed to ambient conditions is of significant importance for applications from electronics to anti-corrosion layers and from a fundamental perspective. The environmental exposure influences macroscopic properties of the graphene surface, as recent studies on wettability reported. However, these studies are controversial for two reasons: firstly, time dependency of graphene wettability complicates comparison of results from different groups. Secondly it's inherently difficult to understand the underlying physical phenomena by means of macroscopic measurements alone. Here we study the evolution of the wettability of graphene monolayers on copper by exposing samples to controlled ambient conditions. Static contact angle measurements reveal that the graphene undergoes a transition in its wettability from slightly hydrophilic to hydrophobic over timescales of the order of hours. To gain further insight, we apply Fourier transform infrared spectroscopy and a recently developed dynamic atomic force microscopy-based force spectroscopy to probe the sample surface at the nanoscale. This novel nanoscopic approach allows us to demonstrate that the transition in wettability of graphene is induced by the adsorption of water molecules from the environment in combination with hydrocarbon contamination. Furthermore, our nanoscale observations challenge the wetting transparency theory of graphene after exposure to the environment.AJM acknowledges support for his studentship from the EPSRC doctoral training grant. AV acknowledges project MAT2012-38319 from MINECO, Government of Spain.Peer Reviewe

Size Dependent Transitions in Nanoscale Dissipation

The irreversible loss of energy that occurs when a nanoscale tip vibrates over a surface can be monitored and quantified in amplitude modulation atomic force microscopy (AM AFM). Furthermore, two distinct dissipative processes can be identified and related to viscous and hysteretic forces respectively. Here, experimental evidence of a transition from viscous to hysteretic prevalent dissipation during mechanical contact is provided as the size of the tip increases from a few nm to 10 nm or more. Long range dissipation, defined as distances for which mechanical contact does not occur, is also investigated and related to capillary interactions. Experiments conducted on freshly cleaved mica samples show that energy dissipation increases with tip size and relative humidity in the long-range before mechanical contact occurs. Long- and short-range interactions are discussed in terms of observables both experimentally and by numerically integrating the equation of motion

Fabrication and morphology tuning of graphene oxide nanoscrolls

Here we report the synthesis of graphene oxide nanoscrolls (GONS) with tunable dimensions via low and high frequency ultrasound solution processing techniques. GONS can be visualized as a graphene oxide (GO) sheet rolled into a spiral-wound structure and represent an alternative to traditional carbon nano-morphologies. The scrolling process is initiated by the ultrasound treatment which provides the scrolling activation energy for the formation of GONS. The GO and GONS dimensions are observed to be a function of ultrasound frequency, power density, and irradiation time. Ultrasonication increases GO and GONS C–C bonding likely due to in situ thermal reduction at the cavitating bubble–water interface. The GO area and GONS length are governed by two mechanisms; rapid oxygen defect site cleavage and slow cavitation mediated scission. Structural characterization indicates that GONS with tube and cone geometries can be formed with both narrow and wide dimensions in an industrial-scale time window. This work paves the way for GONS implementation for a variety of applications such as adsorptive and capacitive processes.United States. Dept. of Defense (National Defense Science and Engineering Graduate Fellowship (NDSEG) Program)United States. Army Research Office (contract W911NF-07-D-0004)National Science Foundation (U.S.) (NSF award number ECS-0335765

The Mendeleev-Meyer force project

et al.Here we present the Mendeleev-Meyer Force Project which aims at tabulating all materials and substances in a fashion similar to the periodic table. The goal is to group and tabulate substances using nanoscale force footprints rather than atomic number or electronic configuration as in the periodic table. The process is divided into: (1) acquiring nanoscale force data from materials, (2) parameterizing the raw data into standardized input features to generate a library, (3) feeding the standardized library into an algorithm to generate, enhance or exploit a model to identify a material or property. We propose producing databases mimicking the Materials Genome Initiative, the Medical Literature Analysis and Retrieval System Online (MEDLARS) or the PRoteomics IDEntifications database (PRIDE) and making these searchable online via search engines mimicking Pubmed or the PRIDE web interface. A prototype exploiting deep learning algorithms, i.e. multilayer neural networks, is presented.The authors would like to thank ADNOC for supporting this work.Peer Reviewe

A nanoscopic approach to studying evolution in graphene wettability

The equilibrium state of graphene surfaces exposed to ambient conditions is of significant importance for applications from electronics to anti-corrosion layers and from a fundamental perspective. The environmental exposure influences macroscopic properties of the graphene surface, as recent studies on wettability reported. However, these studies are controversial for two reasons: firstly, time dependency of graphene wettability complicates comparison of results from different groups. Secondly it’s inherently difficult to understand the underlying physical phenomena by means of macroscopic measurements alone. Here we study the evolution of the wettability of graphene monolayers on copper by exposing samples to controlled ambient conditions. Static contact angle measurements reveal that the graphene undergoes a transition in its wettability from slightly hydrophilic to hydrophobic over timescales of the order of hours. To gain further insight, we apply Fourier transform infrared spectroscopy and a recently developed dynamic atomic force microscopy-based force spectroscopy to probe the sample surface at the nanoscale. This novel nanoscopic approach allows us to demonstrate that the transition in wettability of graphene is induced by the adsorption of water molecules from the environment in combination with hydrocarbon contamination. Furthermore, our nanoscale observations challenge the wetting transparency theory of graphene after exposure to the environment

Revealing Amphiphilic Nanodomains of Anti-Biofouling Polymer Coatings

Undesired bacterial adhesion and biofilm formation on wetted surfaces leads to significant economic and environmental costs in various industries. Amphiphilic coatings with molecular hydrophilic and hydrophobic patches can mitigate such biofouling effectively in an environmentally friendly manner. The coatings are synthesized by copolymerizing (Hydroxyethyl)­methacrylate and perfluorodecylacrylate via initiated chemical vapor deposition (iCVD). In previous studies, the size of the patches was estimated to be ∼1.4–1.75 nm by fitting protein adsorption data to a theoretical model. However, no direct observations of the molecular heterogeneity exist and therefore the origin of the fouling resistance of amphiphilic coatings remains unclear. Here, the amphiphilic nature is investigated by amplitude modulation atomic force microscopy (AM-AFM). High-resolution images obtained by penetrating and oscillating the AFM tip under the naturally present water layer with sub-nanometer amplitudes reveal, for the first time, the existence of amphiphilic nanodomains (1-2 nm²). Compositional heterogeneity at the nanoscale is further corroborated by a statistical analysis on the data obtained with dynamic AM-AFM force spectroscopy. Variations in the long range attractive forces, responsible for water affinity, are also identified. These nanoscopic results on the polymers wettability are also confirmed by contact angle measurements (i.e., static and dynamic). The unprecedented ability to visualize the amphiphilic nanodomains as well as sub-nanometer crystalline structures provides strong evidence for the existence of previously postulated nanostructures, and sheds light on the underlying antifouling mechanism of amphiphilic chemistry