MRS Bulletin Materials News Podcast

Sophia Chen of MRS Bulletin interviews Dan Walkup of the National Institute of Standards and Technlogy about an unusual concentric quantum dot structure created in graphene. Read the abstract in Physical Review B .
Transcript
SOPHIA CHEN: Physicist Dan Walkup has a mystery on his hands. Working at the National Institute of Standards and Technology in Gaithersburg, Maryland, his team has engineered a strange phenomenon in the 2D material graphene using a scanning tunneling microscope, or STM. They created the phenomenon by accident playing around with the STM, whose very sharp tip manipulates single atoms on a material. In the graphene it created a quantum dot (QD).

DAN WALKUP: Historically we weren’t trying to study coupled QDs per se. We were trying to figure out how to tune the properties of the graphene with STM tip. In that way we came eventually to this QD study.

SC: You can visualize the QD as an island in the graphene, where electric charges are confined and isolated from the rest of the material. At the QD, negative electrons gather around positive electron holes. They can also do the charge inverse of this, where the positive holes go around a negatively charged nucleus. From this, you might get the sense why QDs are sometimes known as artificial atoms. Like atoms, quantum dots consist of one type of charge going around a nucleus of the opposite charge. The researchers have taken the graphene, stuck it on a substrate of hexagonal boron nitride, and manipulated the electric charges with the STM inside these two materials to create the QD.

DW: We create a little pocket of charge in the hexagonal boron nitride, and that charge pocket attracts oppositely charged electrons in the graphene and makes a little charge pocket in the graphene, which becomes a QD.

SC: But this isn’t your garden variety QD. The geometry of this particular island has never been seen in graphene. By using the STM and applying a strong magnetic field to the material, Walkup’s team has made a nested QD, one island of charge stacked on the other. From overhead it looks like a bulls’ eye, with one island of charge at the center, and another forming a ring around it.

DW: The two dots are like the two tiers of this wedding cake.

SC: They’re two concentric quantum dots: one dot is in the center and the other dot is the ring around the first. These two structures are distinct quantum dots because electrons from one island are generally confined to that island. Walkup’s team ran some experiments in which they added electrons to each quantum dot. They did this by applying a voltage to the back of the material, causing electrons to move toward each dot. The researchers can then monitor where the electrons go using the scanning tunneling microscope. And what they found was puzzling. They found that as they added electrons to either of the two quantum dots, they behaved in a way that can’t be explained by accepted models of quantum dot physics. Walkup says you would expect the two dots to repel each other as you add electrons to them, since negative charges repel each other. But the inner dot only cared about its own charge. It did not care about the charge of the outer dot. Whereas the outer dot responded to the combined charge of both dots. They want to figure out why.

DW: Part of this paper is an open invitation to the theorists in the world to figure out why it is this way instead of some other way.

SC: A better understanding of the basic physics of this bizarre quantum dot configuration could help the development of applications such as quantum computing, in which information is stored in the way quantum dots share electrons. This work was published in a recent issue of Physical Review B.

As part of the MRS Communications 10th Anniversary event, Gopal Rao, Chief Editor for Technical Content at MRS, interviews David Morse, Executive Vice President and Chief Technology Officer at Corning, about research, development, and innovations at Corning. They discuss Corning’s contributions to addressing the COVID-19 pandemic, the company’s latest version of Gorilla glass, and Corning’s R&D efforts in ceramics as well as the role of industrial R&D labs in the research enterprise.

In this podcast episode, MRS Bulletin’s Laura Leay interviews Michael Dickey from North Carolina State University about his work manipulating liquid gallium. When submerged in an aqueous solution, liquid gallium will form a sphere. When fed by gravity through a thin nozzle that is surrounded by aqueous solution, it will instead flow into the shape of a wire. Passing an electrical current through the liquid metal wire means that a magnetic field is created, which means the wire can be shaped using external magnets, following the Lorentz force. This research was published in a recent issue of the Proceedings of the National Academy of Sciences (doi:10.1073/pnas.2117535119).

Prachi Patel of MRS Bulletin interviews Carmel Majidi of Carnegie Mellon University about utilizing atom transfer radical polymerization to create liquid metal–polymer hybrid materials with high stability, excellent dispersibility, and tunable mechanical and optical properties. Read the article in Nature Nanotechnology.

Transcript
PATEL: Rubbers and plastics have snuck into hundreds of products we use every day. They have excellent mechanical properties for these applications. But they are terrible at transporting heat and electricity. That’s a problem if you are an engineer who’s trying to make soft robots or artificial skin, like Carmel Majidi of Carnegie Mellon University. He is trying to create new kinds of composites by tailoring the electrical and thermal properties of polymers. He and his colleagues recently found a way to do this by filling polymers with nanoscale droplets made of liquid metals.

MAJIDI: And the liquid metal itself is an alloy of gallium and indium. These are two metals that are solid at room temperature but when you mix them together they form this eutectic this liquid that has certain nice properties. It’s important that they’re liquid because that allows the droplets to deform with the surrounding rubber as the rubber stretches.
PATEL: Up until now, researchers have tried to make such composites by using mixers that are a bit like kitchen blenders. You essentially throw in the liquid metal alloy along with the molten rubber or plastic. The metal breaks into tiny droplets that disperse through the polymer. But Majidi worked with chemistry professor Krzysztof Matyjaszewski to develop a new technique. They first break up the liquid metal into nanodroplets using high-frequency sound waves. But then, instead of adding the liquid metal to the polymer, they grow the polymer on the tiny metal droplets, like a coating.

MAJIDI: You basically start with your nanoscale droplets of liquid metal and then pretty much atom by atom or monomer by monomer you grow these polymer chains from the surface of these droplets—almost like the rays of the sun kind of emanating out. We’re working with these you know gallium indium liquid metal alloys. It wasn’t really obvious whether his polymerization technique would apply to this you know very different and somewhat unique class of materials. So he gave it a shot and it turned out it worked you know pretty much the same way it’s worked for a lot of the other types of metal nanoparticles that he’s worked with. We were delighted that it did.

PATEL: The technique, called atom transfer radical polymerization, or ATRP—gives very fine control on the length of the polymer chains so the droplets get evenly suspended in the composite. Plus, it enables the researchers to suspend these droplets in a much wider range of polymers and materials systems than was previously possible. Until now, the researchers could use commercially available materials like silicone rubbers and polyurethanes.

MAJIDI: The ability to suspend these liquid metal nanodroplets in virtually any kind of polymer or kind of matrix material opens the door for kind of a wide range of functionalities. A lot of the materials we have been exploring and will continue to explore, they could be used in say wearable or even implantable applications. So we could work with polymers that are biocompatible. We’ve been looking at different types of polyacrylates. Those are materials that are generally kind of popular in engineering. Everything from adhesives to 3D printing. And again using ATRP we’ve been able to show you can suspend liquid metal droplets in those materials. I mean if you just think about just generally where are plastics used and where are rubbers used, there’s a huge spectrum out there.

Omar Fabián of MRS Bulletin interviews Alireza Dolatshahi-Pirouz of the Technical University of Denmark about the use of silk to fabricate eco-friendly electronics. Read the article in Advanced Science.

Transcript
FABIÁN: We have an electronic waste problem. While the development of recyclable plastics has helped curb that problem, currently only about 15% of e-waste is actually recycled. So how can we make a bigger dent? Materials researchers from Denmark are looking to the silkworm for answers.

ALIREZA DOLATSHAHI-PIROUZ: Who can do it better than Mother Nature, right?

FABIÁN: That’s Prof. Alireza Dolatshahi-Pirouz. His research team at the Technical University of Denmark is developing a new class of thin-film electronics they’ve dubbed “fleco-ionics.” That’s short for flexible, eco-friendly electronics. And they’re doing it using cocoons woven by silkworms.

DOLATSHAHI-PIROUZ: Silk is one of the strongest materials out there. It has strength that is many times stronger than steel. It’s cheap. It’s readily available in nature. It’s biodegradable. It’s green. It has electronic properties. It is an ionic conductor.

FABIÁN: But silk alone isn’t enough. Films cast from silk fibers are unstable in water. Their unwieldy protein structure, a mixture of random coils and sheets, makes for bad, water-permeable electrodes. To remedy that, a second ingredient is needed, namely, laponite. The nanosized disks that make up this natural ceramic iron out the silk fibers—like pouring hockey pucks on a plate of spaghetti. The result is a water-tight film. And because the disks carry charge of their own, they actually improve the fibers’ ionic conductivity.

DOLATSHAHI-PIROUZ: So it’s pretty amazing, right? You have something that doesn’t work, and then you add something to it, and then suddenly it works. And you get other properties along the way as well.

FABIÁN: Among the most valuable of those properties are low cost and flexibility. Because although electrodes made of gold, copper, or even carbon nanotubes might show higher conductivity, the team’s silk-nanoclay films are much cheaper and able to wrap around almost any curved surface.

DOLATSHAHI-PIROUZ: That’s not something you typically relate with ordinary electronics. Ordinary electronics are expensive, they are rigid. They consume a lot of power. This does not do that. So that’s why I would say we have something pretty fantastic in our hands at the moment.

FABIÁN: As a proof of concept, the researchers have fashioned the hybrid films into wearable electrodes able to track movement throughout the body, such as the flexing of the elbow or the fiddling of the digits. And that could make for interesting applications down the road.

DOLATSHAHI-PIROUZ: We have plans to use this concept inside a glove to develop an electronic glove. An electronic glove, which I think is the exciting thing about the application right now, would entail to have these small thin films inside a glove, and they would then be connected to an amplifier and a wireless unit that can transmit these signals wirelessly to, let’s say, a computer, or a mobile phone, or a portable device. So you have this glove on your hand that is kind of like sending data to the physician so you can, in real time, monitor whether you’re doing these exercises properly or not.

FABIÁN: This concept of an e-glove isn’t new. But the approach is. Co-opting natural materials like silk for advanced electronics applications could help cut cost, time, and, perhaps most importantly, the mountains of electronic waste we generate each year.

DOLATSHAHI-PIROUZ: We need to think simple. Why do we want to do old, complicated chemical syntheses that takes months and years to optimize when we can be smart and look into nature.