Professor Jiwoong Park and his team at the University of Chicago have made a material that is crystalline in the X-Y direction, but amorphous in the Z direction. It therefore has the unique thermal properties that it is an excellent transmitter of heat in the x-y plane but a thermal insulator in the z plane.

Tech Briefs: How did you get started on this research?

Professor Jiwoong Park: This project started three or four years ago with our curiosity about a material we had made. If you look at any material, let's say metal or glass, it has similar structural properties in all three directions. If it's a crystal, it's crystalline in all three directions. If it's amorphous, like glass, then it's amorphous in all three directions.

Our group had grown a very thin two-dimensional crystal that can be made on a fairly large scale. We started wondering if you stacked them one by one, whether it would behave like a crystal in the vertical direction.

Prof. Park

That question arose because you cannot align all the items perfectly, atom by atom, although within each layer the structure will be crystalline. So, effectively what we were making was crystalline in the X-Y direction, but amorphous in the Z direction. We realized that the properties would be different in the vertical direction, and we were thinking that it would be the thermal property that would change dramatically.

Tech Briefs: I’m trying to visualize how this could be used in practice. First of all, how large an area of this material did you, or can you, make?

Park: We can make it into literally wafer scale or larger because although we start with individual very thin material that is grown about 10 centimeters in size, we could increase that. Since we stack them one by one, depending on the need, we could make it arbitrarily larger. Of course, that would take a longer time. To do that, we would probably need to automate the whole thing, but in principle it could be any thickness that we would like. In our experiment, we went up to 20 or 30 layers, which amounts to 10 to 20 nanometers. There could be other processes that would make it a lot easier to make it larger.

Tech Briefs: How do you make this material?

Park: The first step is to grow the individual layer. We use a technique called metal organic chemical vapor deposition, which is a fancy way of saying cooking. We have a substrate, we introduce precursors, meaning the ingredient atoms and molecules, and then we control the temperature. We usually heat it up, then bring it to low pressure and the molecule breaks down, releasing the atoms that we want — in this case molybdenum and sulfur. They react on the surface of the substrate and form a mono-layer crystal.

Tech Briefs: How do you stack them?

Park: To begin with, these are two-dimensional materials, meaning that they are connected laterally, they are not bonded to the bottom surface. We had previously developed a technique to peel this with some new material, so we can peel it and then we move it to another one and then pull both of them together and then we just repeat. So, it’s peeling and stacking repeatedly.

Photo:, Daniel Spacek, Pavel Jirak, Chalmers University

Tech Briefs: Does this process currently exist on a commercial scale or would something new have to be developed?

Park: In principle, this can be done at a commercial scale. We are working on a method that automates the whole process.

Tech Briefs: Do you picture this on the scale of integrating it into a chip or a larger electronic assembly?

Park: Introducing any new step or ingredient to a CMOS process, is a really daunting task because you need to consider so many processes that happen before and after. Even the idea of transferring anything to a substrate during chip building is a tricky business. Since we have demonstrated that this extremely anisotropic material can be made, we are in conversation with the industry to think about a way to integrate it, but it's at a very early stage.

But what we can say, is that it's a new material that takes heat in the X-Y plane so that you can get rid of hot spots but insulates the heat from going in a certain direction. It’s an early stage but we are optimistic.

Tech Briefs: That's interesting. I imagine that on a larger scale it could be mounted on a mechanical structure, possibly inside an iPhone or something like that.

Park: I do have to think about it more because the big question is always how to separate heat producing parts from heat-sensitive parts. One has to do something to put them together in a tight space. For those types of applications, what we are making will be very useful.

Tech Briefs: I guess where you’re removing the heat you would move it over to some sort of heat sink or something that would dissipate it.

Park: Yes, that’s right.

Tech Briefs: I can’t wait to see it.

Park: It was even surprising to us, how anisotropic this material was.

Tech Briefs: How close are you to being able to implement this in an actual product?

Park: I really don't know at this point. One thing that I'm working on now is to make the anisotropy much larger. Right now, the ratio of the conductive part to the insulating part is about 1000. If we can make it 10,000 or 100,000, it will become more and more appealing. On top of that, realizing the same properties with different materials will give industry much more opportunity to think about using this for a variety of different processes. At the current stage, we want to make this platform more general so that more people can pick up on it.

Tech Briefs: It sounds to me like a major innovation, a brand-new material that people can start thinking about.

Park: I agree with that — I've been pretty excited about this. The one thing that was surprising to us is that it was originally thought of as a great electronic material because it's a semiconductor. It was very satisfying for us that we thought of a new, different, use for this material — using it for a heat conductor was not something that people thought about. I'm very happy that our initial curiosity led to something quite surprising.

An edited version of this interview appeared in the February 2022 issue of Tech Briefs.