The ability to pattern graphene films has produced a much easier fabrication route for graphene-based elements in many industrial applications, especially optoelectronics. However, the patterning process itself allows many problems to arise, which range from contamination to substrate damage, and has prompted alternative patterning approaches to be considered.
A team of researchers from China have developed a novel method to pattern CVD-grown graphene using a magnetic-assisted ultraviolet (UV) ozonation method with a stencil mask.
The ability to pattern graphene films has been a big step for towards the industrial scale production of graphene-based components. Aside from building a specific pattern through bottom-up nucleation growth approaches, top-down methods are very common methods with photolithography and electron-beam lithography being the most common.
Whilst these techniques are widely used for the resolution ability, they do experience a long list of problems such as organic contamination, linewidth resolution and substrate damage. As patterned graphene films show great promise as component materials in optoelectronic devices, such problems have presented a major hurdle and new ways of patterning graphene have been tested to minimise these undesirable effects.
There are many methods and techniques that have been trialled and tested for the patterning on graphene but have never quite made a substantial difference towards the commercial viability of the product(s).
The research team have utilized a magnetic-assisted ultraviolet (UV) ozonation method using a ferromagnetic stencil mask to produce the desired pattern.
UV ozonation methods have previously been tried in the past, but with little effect. UV ozonation is a milder oxidation method compared to other oxygen-based methods, such as oxygen plasma treatments, but in the past, has shown to be too weak to pattern CVD-grown graphene films.
However, it was shown to cut graphene oxide into strips under ultrasonic irradiation and thermal annealing, which prompted the researchers to attempt other applied UV ozone methods.
The method uses a vertical magnetic field under the irradiation of a xenon lamp. The process photochemically dissociates oxygen molecules to generate paramagnetic oxygen radicals which become magnetised an attracted to the inhomogeneous magnetic field.
Under a directionally applied magnetic field, the random motions of the radicals become converted into directional motions and the quality of the patterning is significantly enhanced. To define the pattern, the researchers used a ferromagnetic steel mask.
The researchers used a home-designed a UV ozonation vacuum machine containing a 1.5 kV xenon lamp, with tuneable light source distances and magnetic fields. Upon irradiation, the oxygen molecules dissociated to the O-3p ground state producing radicals.
These radicals interacted with the graphene films to produce a mixture of carbon monoxide, carbon dioxide and ozone gases- i.e. reactive products.
The paramagnetic oxygen molecules were found to have a two-times greater molar magnetic susceptibility than the diamagnetic reactive products. So, when the magnetic field was applied, the difference in magnetic susceptibility caused the oxygen molecules to direct themselves towards the graphene sheet, patterning it, whilst the other reactive products were left unaffected.
An optical microscope (Leica DM 4000), scanning electron microscope (SEM) (Zeiss Ultra Plus), confocal micro-Raman spectroscopy (Senterra R200-L) and X-ray photoelectron spectroscopy (XPS) (Kratos Axis UltraDLD spectrometer) were used to characterise the patterned graphene film.
The method had the ability to produce patterned graphene microstructures with a line width of 29 µm and lateral dimensions of less than 4 µm- a remarkable achievement compared to similar efforts and previous attempts.
The patterned graphene film was found to possess minimal issues, of which the tuneable nature of the custom-built machine could alter the properties and further eliminate any problems. The sheets also showed a high sample-to-sample consistency and reproducibility, two key factors for commercial considerations.
Both the technique and results currently show great commercial potential, especially for the production of graphene field-effect transistor (FET) arrays, where such components could be implemented into new and current opto-electronic devices.
The research shows a great new commercially viable route towards producing resist-free, substrate non-damaging and cost effective microscale patterning of graphene films.
“Patterning Graphene Film by Magnetic-assisted UV Ozonation”- Wu Y., et al, Scientific Reports, 2017, DOI: 10.1038/srep46583
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