Researchers at Australia’s Monash University have demonstrated a counterintuitive way to protect atomically thin electronics: adding vibrations to reduce vibration issues. The team’s trick is to “squeeze” a thin droplet of liquid gallium on graphene devices, which are then painted with a protective coating of glass — gallium oxide.
It’s the first time such a strategy to reduce the resistance due to thermal vibrations has been demonstrated in a graphene device. The oxide is remarkably thin — fewer than 100 atoms — yet covers centimeter-wide scales; this could make it applicable for industrial large-scale fabrication.
“The oxide not only enhances and protects our devices when we first transfer it, but also later, during subsequent processing and fabrication,” said co-author Dr. Semonti Bhattacharyya.
The glass film that forms on the surface of droplets of liquid gallium metal is more than 5,000 times thinner than a human hair, but it can be reliably “printed” from the surface of the liquid metal to form uniform continuous layers over centimeter-sized areas.
The team showed that gallium oxide protected the graphene from surface damage by testing their devices with industrial growth tools. Depositing another oxide layer damaged only the uncovered areas of graphene, while the areas that were covered by gallium-oxide retained their qualities.
The vibrations of materials due to heat, which cause electrical resistance in materials, are called phonons. The phonons cause the solid-state atoms to oscillate, and flowing electrons bounce off these oscillations and change their direction — casuing electrical resistance.
The thermal vibrations of the carbon atoms in graphene itself cause remarkably little resistance, but the one-atom-thick nature of graphene means that thermal vibrations in surrounding materials can have a large effect on electrons in graphene. These are the predominant cause of electrical resistance in graphene at room temperature. As temperatures heat up, more phonons are excited — increasing the resistance by scattering electrons.
“You can think of this scenario as a fence,” said Matt Gebert, lead author, PhD candidate, Monash University. “The fence (the 2D graphene) is affected by the actions of neighbors on both sides (the insulating materials on either side of graphene). One neighbor might have a clean environment on their side of the fence (a good insulator, with few phonons) but the other neighbor might have an overgrown garden that damages the fence (a bad insulator with strong phonons).
“So in the end, your fence (graphene) doesn’t serve the purpose it was intended to, perhaps not even forming a complete fence (electronic circuit) anymore.”
To investigate gallium oxide’s protective qualities, the team mechanically transferred large areas onto graphene devices.
“Surprisingly, adding the layer of Ga2O3 glass reduces the electrical resistance in graphene that is due to phonon scattering,” said Gebert. “This is counter-intuitive, because by adding this material, you are introducing additional phonons. So you might think: the more phonons, the higher we would expect resistance to be!”
The work could be used to identify better-performing hybrid materials at room temperature for 2D electronics.
Here is a Tech Briefs interview — edited for clarity and length — with Gebert and Bhattacharyya.
Tech Briefs: What inspired the research?
Bhattacharyya: I started at the lab in 2017 as a postdoc. We have a group of researchers, called Fleet, which is Future Low-Energy Electronic Technology. I was getting introduced to different labs, and then at the same time I was invited to visit a scientist’s lab at MIT. And just before that, they had published their paper, where they first demonstrated the technique of making very thin, large-scale oxide materials using this touch-printing technique. They demonstrated the technique in front of us, and then we discussed what we can do with it.
We were discussing different types of ideas. And then I thought, ‘OK, it'll be very interesting if we can integrate it with graphene. These materials all have high dielectric constant, these are oxides, amorphous, very thin, so all these can be very good as top dielectric.’
Then I met Matthew Gebert, who was looking for a project for his honors. We decided it was a great opportunity and started working on it. This provides us an opportunity of making oxides with a very low-cost, easy technique.
Tech Briefs: To what other kind of electronics applications have you applied this technology?
Bhattacharyya: We are talking about different kinds of possibilities where this method can be used, and all of these are somehow related to transistors or 2D materials. For example, there is a 2021 paper that showed gallium oxide can be a very nice, encapsulating layer, and it can preserve the optical characteristics of this, which was not possible with earlier deposition techniques.
Also, an advanced materials group in China showed that antimony dioxide can be used for scalable encapsulation by using a very similar method. They can use it for encapsulating 2D materials, which can degrade in air. And, another group in Australia showed that gallium oxide can be used as a very thin dielectric on top.
Tech Briefs: What was the biggest technical challenge you faced?
Gebert: When we first started trying to make devices, we’d find that when we put the gallium oxide metal on top of our devices to try and deposit this layer, the metal would stick around and eat up the device or eat up the contacts we’d made to the device.
The metal itself can sometimes be a little bit intrusive and sticky and damage what’s going on; you want to keep it preserved well. What we needed to do was develop an intermediate step to be able to transfer this nanosheet from the oxide material. What we ended up doing was, we developed a polymer step where you end up depositing the oxide material onto a polymer first before you transfer it.
It turned out that made it much more applicable and useful to be able to make these big devices. You could choose a nice area of this gallium oxide. We make these devices by hand, so they’re quite well-handcrafted. What we ended up being able to do was show that you could make centimeter scales of this oxide and get a nice, clean area. We ended up having to work with another group in Canberra (Australia), and they used it for their paper as well.
I ended up being able to use a hot knife to cut the polymer but keep the sheet very well preserved. You could cut out very select areas and be able to transfer it onto these devices without any of the gallium metal left — a super-clean, large area. That was our biggest technical challenge: Being able to get that process working without destroying our devices. That’s really what made this work.
Tech Briefs: Please explain in simple terms how the technology works.
Gebert: For the liquid metal transfer, you take a liquid metal, in this case gallium. Gallium is a liquid, very close to room temperature, around 30 °C. The surface of the liquid metal ends up interacting with oxygen in the air, just naturally. It forms a very thin layer of oxide material on its surface, and that reaction is limited; there’s only so much oxygen that will penetrate the surface. You end up getting a very thin nanosheet, naturally, on these metal oxides.
You can take that liquid metal droplet and manipulate it. That way you can deposit that nanometer-scale oxide that forms on its surface. What we wanted to do was take that oxide and not take the liquid metal underneath it. That’s basically what the technology is; you can create incredibly large nanosheets of material.
What a dielectric does is it basically makes impurities invisible on the surface, that's what that charged scattering is. The vibrational side of things: It’s a bit odd. Graphene is a very sensitive material to what its environment is. It picks up the vibrations of neighboring materials, and they reduce its conductivity. It’s not as ideal when it comes into contact with some materials.
When we add gallium oxide, it screens out the vibrations in the material that was already on the graphene. We take graphene on some device to begin with, and we put the gallium oxide on top of it, and that gallium oxide screens the vibrations — it stops the vibrations in the second material. It also then dominates and ends up giving its own vibrations, but they’re much smaller compared to that underlying material.
Tech Briefs: What’s the next step with regards to your research and testing?
Gebert: There are two aspects in which this work can progress. The next thing that we could do is take out gallium oxide and use it as a gated structure. In this work, we’ve shown its properties in contact to graphene and how it works well and how it protects graphene over large areas. I think the next step would be putting metal on top of that and using it as a very close proximity switch, so you can show it’s got some different properties.
The alternative to that, what you’d want to do is find a slightly different material. In gallium oxide, what we showed is that just below room temperature it works incredibly well, and it improves your device by removing these vibrations. However, at room temperature it will probably add a little bit of extra vibrations than what you’d normally get.
The method is sound, but I think if you use a slightly different material with slightly better properties, you’d find that you can reduce vibrations at room temperature. The versatility of the liquid metal idea is that you can add different metals into your liquid droplet and design what oxide forms on the surface. So you actually have a lot of control over what sort of material you want to be able to design when you print it on top of these liquid metals.
Tech Briefs: Do you have any advice for researchers aiming to bring their ideas to market?
Gebert: Our research is quite fundamental, but it’s already clear what sort of things we appeal to when we’re looking at the practicality of this research. One of the unique selling points for us is to identify what sort of space does our work fit into that other areas don’t. You want to be able to identify end users or certain cases where your work is going to be set apart from other existing work.
In our work, in particular, we’ve been able to make this nanosheet over incredible scales — centimeter sort of scales. That’s done by physically depositing the material and in a soft way; other methods of creating such thin layers can be destructive. You can do damage by having to spray high-energy atoms onto surfaces. This way, comparatively, is very unique. It reduces a lot of the problems that people might have because of damage to their surfaces.