Ultraflat transfer method for graphene surface force balance
(Nanowerk Spotlight) For the past several decades, the surface force balance (SFB) – also called surface force apparatus – has provided pioneering measurements of surface and colloidal forces in liquids such as electrostatic surface forces, van der Waals forces, and solvation forces.
Until now, the SFB required mica sheets as the substrate for measurements. This was the only material available in a truly (atomically) smooth state over centimeter-scale areas as well as being optically transparent as required for the optical interferometry. Mica is an insulator, and so no electrochemical or electrode-controlled measurements were possible.
By replacing the mica sheets with graphene, electrically conducting and atomically smooth surfaces for the measurement of surface forces have now been created.
In order to create transparent conductive lenses for surface force balance (SFB) measurements using graphene, macroscopic samples of ultra flat graphene is essential. While 'full coverage' CVD graphene on copper has become a standard material in the community and is commercially available, its quality unfortunately does not suffice for SFB work due to sample roughness or tears.
Moreover, standard transfer methods currently routinely employed in most graphene laboratories generally induce ripples, folds, and significant amounts of polymer residue. Such defects can influence the performance of the graphene.
A new transfer method, developed at the University of Oxford, allows the ultraflat transfer of macroscopic graphene flakes (cm2) (root-mean-square roughness of 0.19 nm) that are free from polymer residues over macroscopic areas (>1 cm2).
The team, led by Professors Nicole Grobert and Susan Perkin, developed a novel double-transfer procedure by modifying the conventional polymer transfer method with an additional transfer step using freshly cleaved mica as a perfectly clean and flat support.
(a) Illustration of a surface force balance lens structure detailing the five-layer interferometer setup consisting of epoxy-graphene-film-graphene-epoxy. (b and c) Two sets of graphene lens surfaces showing the detrimental effect of positive protrusions on finding a contact point. (Reprinted with permission by American Chemical Society) (click on image to enlarge)
This research, initiated through the EPSRC Inspire project, combined the two groups' in-depth expertise in SFB work, nanomaterials production and processing techniques.
"The new graphene transfer procedure devised and reported by us creates atomically smooth graphene films (with only ‘negative protrusions’ which do not obscure interaction measurements), transparent to visible light, and ideal for surface force measurements with electrochemical control," says Jude Britton, a PhD Student in her final year working on 2D nanomaterials synthesis and aberration corrected electron microscopy, and first author of the paper.
Chemical vapor deposition is one of the most viable technique to generate large areas of graphene. A major drawback of this technique however, is the transfer of the graphene to other substrates which is a necessary step before graphene can be fully exploited. Other 2D materials grown by CVD face similar issues.
"Our new technique allows us to overcome this bottleneck and with the transfer issue being solved we are one step closer to exploiting these exciting materials for a large number of applications that can be envisaged for 2D materials," explains Dr. Nico Cousens, a researcher in Perkin's group and co-first author of the paper.
Grobert points out that this work opens the way to many possibilities in terms of measurement of surface interactions: "Van der Waals forces, electrostatic interactions, self-assembly, friction, can all be detected at very high resolution using this new graphene SFB. The particular properties of graphene are interesting in themselves – for example how do the van der Waals and electrostatic interactions between graphene sheets differ from bulk graphite? – but the setup will also act as a model for electrode processes in general. This new approach opens up a plethora of possibilities to study a wide range of 2D nanomaterials and combinations of the same."