Silicon contamination halves graphene performance

Australian researchers have revealed how to fully harness the potential of graphene, the so-called ‘supermaterial’ that was anticipated to transform the electronics industry upon its discovery.

As well as being the strongest material ever tested, graphene is also flexible, transparent and able to conduct heat and electricity 10 times better than copper. After graphene research won the Nobel Prize for Physics in 2010, it was hailed as a transformative material for flexible electronics, computer chips, solar panels, water filters and biosensors; however, its performance since then has been mixed and industry adoption slow.

Seeking answers to this mystery, a research team led by RMIT University inspected commercially available graphene samples, atom by atom, with a state-of-the-art scanning transition electron microscope. The results, published in the journal Nature Communications, revealed that high levels of silicon contamination were the root cause, with massive impacts on the material’s performance.

Testing showed that silicon present in natural graphite, the raw material used to make graphene, was not being fully removed when processed. Co-lead researcher Dr Dorna Esrafilzadeh said, “We believe this contamination is at the heart of many seemingly inconsistent reports on the properties of graphene and perhaps many other atomically thin two-dimensional (2D) materials.”

The testing not only identified these impurities but also demonstrated the major influence they have on performance, with contaminated material performing up to 50% worse when tested as electrodes.

“This level of inconsistency may have stymied the emergence of major industry applications for graphene-based systems,” Dr Esrafilzadeh said. “But it’s also preventing the development of regulatory frameworks governing the implementation of such layered nanomaterials, which are destined to become the backbone of next-generation devices.”

The two-dimensional property of graphene sheeting, which is only one atom thick, makes it ideal for electricity storage and new sensor technologies that rely on high surface area. The study reveals how that 2D property is also graphene’s Achilles heel, by making it extra vulnerable to surface contamination.

The good news is that pure graphene can still be produced using high-purity graphite, with the researchers demonstrating that such material performed extraordinarily well when used to build a supercapacitator. When tested, the device’s capacity to hold electrical charge was the biggest so far recorded for graphene — and within sight of the material’s predicted theoretical capacity.

The team then used pure graphene to build a versatile humidity sensor with the highest sensitivity and the lowest limit of detection ever reported. These findings constitute a milestone for the complete understanding of atomically thin two-dimensional materials and their successful integration within high-performance commercial devices.

“Graphene was billed as being transformative, but has so far failed to make a significant commercial impact, as have some similar 2D nanomaterials,” Dr Esrafilzadeh said. “Now we know why it has not been performing as promised, and what needs to be done to harness its full potential.”

Image caption: Schematic representation of the available surface area of graphene for molecular interaction: a) pure surface vs b) contaminated surface. The red spheres represent molecules that can interact with the surface, while the orange rafts represent the contaminants. Image courtesy of the study authors under CC BY 4.0

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