There is a big problem with quantum technology — it’s tiny. The distinctive properties that exist at the subatomic scale usually disappear at macroscopic scales, making it difficult to harness their superior sensing and communication capabilities for real-world applications, like optical systems and advanced computing. Now, however, an international team led by physicists at Penn State and Columbia University has developed a novel approach to maintain special quantum characteristics, even in three-dimensional (3D) materials.
The researchers recently published their findings in Nature Materials.
“Although the functionalities displayed by two dimensional (2D) materials are vast and their potential is revolutionary, maintaining their superior properties beyond the 2D limit remains a formidable challenge,” said First Author Yinming Shao, Assistant Professor of Physics at Penn State, explaining that such materials are typically crystals that are only one atom thick and can be applied in a variety of fashions, including for flexible electronics, energy storage and quantum technologies. “Realization, understanding and control of nanoscale confinement are, thus, crucial for both exploration of quantum physics and future quantum technologies.”
The team examined quasiparticles known as excitons, which have unique optical properties and can carry energy without an electrical charge, in a semiconductor material. Semiconductors — which are ubiquitous across computers, phones and other electronics — conduct electricity under certain conditions and inhibit it under others. Excitons are produced when light hits a semiconductor, energizing an electron to jump to the next energy level. The resulting excited electron and the hole it left are jointly referred to as an exciton. Excitons occur homogenously across typical 3D semiconductors, like silicon.
Here is an exclusive Tech Briefs interview, edited for length and clarity, with Shao.
Tech Briefs: What was the biggest technical challenge you faced while developing this novel approach?
Shao: I think experiment-wise it was not so difficult. It’s a combination of new materials and new ways of measuring them, and also new ideas of how to explain the results that really is more challenging. The experimental approach we use is pretty standard: optical spectroscopy, reductance, those are standard. We do need to cool it down to cryogenic temperatures, which is not unusual in such research, but obviously is very cold compared to the normal temperatures you'll see in your life.
Tech Briefs: What was the catalyst for this project?
Shao: Professor [of Chemistry at Columbia University and Co-Author] Xavier Roy has been a pioneer on this material for a few years. That's one reason we know of this material and started looking at its properties, but then we found something more interesting.
It's a nice example of a collaboration in the sense that, usually, these kinds of discoveries don't happen individually at a single place. Other people may also find it around the same time. Usually, that leads to competition between different groups. They're trying to publish as soon as possible and then become rivals. But, in this case, we and another group in Germany, talked to the same researchers, who happen to know the both of us.
Tech Briefs: Can you please explain in simple terms how it works?
Shao: These 2D materials are from graphene, they’re maybe 21 years old. That’s when people first realized you can exfoliate the 2D material from a crystal and then stabilize it in certain conditions. Then, about 14 years ago, a new kind of 2D transition metal was discovered. It is made of two atoms, usually tungsten and sulfide; those materials are semiconductors, which are typically very important parts of our daily electronics. These new 2D semiconductors also have the interesting property that they can absorb light in a typically very strong energy range — the visible range.
So, in principle, you can think of using this for solar cells or photo detectors, something like that. But the problem is that the nice properties of these 2D semiconductors quickly go away once you have layers of this material.
So, if you really want to use these nice 2D properties, you're going to be in trouble because you have to figure out a way to maintain the small layers while still scaling it up to a bulk format. That's a key challenge that we address in our work, in the sense that we found a new way to maintain the nice 2D excitonic properties but still keep it in a bulk form.
Tech Briefs: Do you have any plans for further research, work, etc.? If not, what are your next steps?
Shao: We will try to add more and more of the properties that we use in electronics and other systems, and try to put it on a 2D platform. So, solar cells are one example. But this material is virtually new, maybe four years old. But it's very nice because it's also very stable in ambient conditions. Many of the new materials, when they are first discovered, just stay in the lab in a very controlled environment. Once they are exposed to air, they quickly decompose. So, it's good for scientific discovery, but it's very challenging to work with in real life.
However, we found that chromium sulfide bromide, is very stable in ambient conditions. That already is a huge benefit compared to many other candidates, because now people don't have to put it into a very well-controlled environment.
By applying a small strain, you can also change its optical response. But then the spin properties of this compound — because it's magnetic, it has some spin resonance in the gigabit range. People also demonstrated that there's some coupling between the gigabit range spin dynamics and the optical frequency properties. This kind of crosstalk or coupling, is very promising because you can imagine in the future, when people may be working on quantum communications, they could potentially incorporate this material into their designs, into their circuits, and then we could add our optical control to it. So, to us, it's really a very versatile platform to explore all the different diverse phenomena of this material.
There are still many things in this system our community as a whole is still trying to explore.
Tech Briefs: Is there anything else you'd like to add that I didn't touch upon?
Shao: This work is a collaboration with many different groups. I was at Columbia when this work was done, and now I moved to Penn State, and then there's also still a collaboration going on. The other group working on the same system we have a joint paper with from Germany is using the crystals as grown by a Czech Republic group. So, it's basically two different crystals, two different measurements in two different countries. It's a demonstration of how robust, how solid, this observation is.
