Scientists make strange 2D metals sought for future technologies

A quantum dot is a type of semiconductor that’s only a few nanometres wide. It has a wide range of applications, including in LED lighting, medical diagnostics, printing, semiconductor fabrication, and solar panels. They’re very small but they’ve had a big impact on our world as we know it. This is why the people who found a quick, reliable way to make quantum dots were awarded the Nobel Prize for chemistry in 2023.

Quantum dots get their curious but powerful abilities from a phenomenon called quantum confinement. When you throw a switch, a bulb comes on. This is because electrons flow from a power source to the bulb through copper wires. Because the wires are fairly thick (from an electron’s perspective) and very long, the electrons aren’t tightly packed in and move freely. But in a quantum dot, there isn’t much space and the electrons are relatively more close to each other. So even though they’re free to move around the entire quantum dot, and not be confined to their atoms, their movement is still restricted.

In this situation, the amount of energy each electron can have changes. In a copper wire in your house’s circuit, if an electron gains some extra energy in some way, it can simply move around faster. But in a quantum dot there’s nowhere to go, so the electrons can’t simply acquire more energy even if, say, you increase the voltage on the dot. Instead, the electrons can have only specific amounts of energy each. This is exactly how electrons in an atom behave: they have limited energy levels. It’s like they’re in a movie hall. The copper-wire electrons are free to fill any seats they like. But in an atom, some rows are closed off and in the other rows, only specific seats are available. Because all the electrons in a quantum dot behave in this way, the dot itself behaves like a giant atom.

The materials not in 3D

The restrictions the electrons feel because they’re so packed in is said to be due to quantum confinement. A material is described as 1D or 2D depending on how much it confines its electrons. A quantum dot is considered to be a zero-dimensional material: while its electrons can technically move in three dimensions, the volume available is so small that it might as well be a point in space.

Likewise, graphene is a famous 2D material: it consists of a single sheet of carbon atoms bonded to each other in a hexagonal pattern. The electrons in this sheet can only move around in two dimensions, thus 2D. As a result they behave as if they don’t have mass, for example, giving rise to properties not seen in other materials.

The unusual material properties quantum confinement gives rise to are clearly of great real-world value. This is why scientists have also been trying to create 2D metals — but they’ve been running into a thorny problem.

If one graphene sheet is placed above another, the two sheets will develop weak links between them called a van der Waals interaction. They’re very weak bonds: they can keep the sheets from drifting apart but if you tug even one sheet just a little, the interaction will break and allow the sheets to be separated.

The scientists who discovered graphene also found that by attaching some cellophane tape on graphite, then pulling it in one smooth motion, they could get a few layers of graphene to come off with the tape.

Really, really flat metals

This wouldn’t have been possible if carbon had been a metal. The problem with a metal atom is that it likes to bond with all the same atoms around itself. Put differently, the atom readily forms bonds in 3D. Forcing it to form bonds only in 2D is very difficult. This is why materials scientists have been trying for a decade to create 2D metals using different techniques, to no avail. They’ve tried carefully depositing metal atoms on a substrate, sandwiching metal slices between a 2D material and a substrate, even hammering metal pieces down.

They’ve only been able to manage metal sheets a few nanometres thick. This isn’t good enough: atomically thin sheets are 10-times thinner, at best a few angstroms (Å) deep. Scientists have also found the surface of these materials to be uneven and that often the metal atoms interact with oxygen in the atmosphere to form oxide compounds.

Yet they’ve been motivated to keep going because 2D metals are expected to have highly unique properties that can be exploited for next-generation technologies, including super-sensitive sensors with applications ranging from medicine to the military. 2D bismuth and tin in particular are expected to be exotic materials called topological insulators, conducting electric currents only along their edges, not anywhere else. In such a state, the material can become magnetised in small islands — a phenomenon physicists have said can be exploited to make faster computers of the future.

A high-pressure sandwich

Now, if a study published recently in Nature is to be believed, there may finally be light at the end of the 2D tunnel. A team of scientists from the Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, the University of Chinese Academy of Sciences (both in Beijing), and Songshan Lake Materials Laboratory (Dongguan) has reported a way to produce 2D sheets of bismuth, gallium, indium, tin, and lead. The team’s technique isn’t complicated either — although that’s partly because the necessary technologies have taken a long time to get to their current advanced state.

It goes roughly like this: (i) Create a pure powder of a metal, say bismuth. (ii) Lay it on a plate made of sapphire on top of which a single layer of molybdenum disulphide (MoS₂) has been deposited. This is the bottom anvil. (iii) As the bottom anvil is heated, the metal powder on top of it will melt and spread out. (iv) The droplet is overlaid with the top anvil, which also consists of a single MoS₂ layer pasted on a sapphire substrate. At this point the droplet is sandwiched between two layers of MoS₂, which in turn are sandwiched between two layers of sapphire. (v) The top anvil is twisted by a small angle and then the two anvils are pressed together. The pressure is kept up until the anvils have cooled to room temperature, then removed. (vi) The smooshed sheet of metal is peeled off.

According to the team, the bismuth sheet was just 6.3 Å thick — a depth of roughly two atoms and sufficient for electrons in the metal to be confined in 2D.

The use of MoS₂ and sapphire wasn’t accidental. MoS₂ has a Young’s modulus — the amount of force required to deform it — of 430 billion pascal (Pa) and sapphire, of 300 billion Pa. That’s more than a million-times the atmospheric pressure at sea level. The squeeze the scientists applied to make 2D bismuth was ‘just’ 200 million Pa. Both MoS₂ and sapphire also have smooth surfaces, which means their atoms don’t try to bond with the bismuth atoms near them.

The researchers also found the bismuth sheet thus produced exhibit a strong field effect and a nonlinear Hall effect. A field effect means how well the sheet conducts electricity can be changed by applying an external electric field. The nonlinear Hall effect was more peculiar: when an electric field was applied, the bismuth sheet acquired a voltage in the perpendicular direction. Both the strong field effect and the nonlinear Hall effect occur in 2D metals, not in 3D metals.

To change the world

The new effort is “not the first to grow thin crystals between layers of van der Waals materials. In the past year, there have been reports of single-atom-thick graphene nano-ribbons grown between layers of hexagonal boron nitride, and of gold nanocrystals just a few nanometres thick grown between flakes of MoS₂,” University of California, Irvine, condensed-matter physics researcher Javier Sanchez-Yamagishi wrote in a commentary accompanying the paper. “My own group has also produced ultra-thin crystals of bismuth by squeezing the metal between layers of hexagonal boron nitride, although the minimum thickness of our crystals was 5 nanometres.”

“A key difference between our method and that of Zhao and colleagues is that they used large (centimetre-scale) sapphires covered with MoS₂, which might be crucial for making atomically thin metals,” he added.

Sanchez-Yamagishi also wrote that the new technique represents a “substantial improvement over what can be made using more expensive and complex techniques”. Since it’s a first attempt, more opportunities as well as new challenges await. For example, researchers can look for ways to use the technique to make 2D sheets composed of multiple metal species, not just one.

For another, the geometric arrangement of bismuth atoms in the 2D sheet the team made allows it to become a topological insulator only in particular conditions. Future research can improve the technique to make room-temperature topological insulators in a more reliable way — just the way the 2023 chemistry Nobel Prize laureates changed the world when they discovered a simple, reliable way to make quantum dots. Yet another opportunity is to refit the procedure to make 2D metals of larger area.

Ultimately, scientists stand to learn more about 2D metals themselves, especially hitherto unknown properties. “Even less is known about the electronic properties of the other 2D metals prepared in the study,” Sanchez-Yamagishi wrote. “The stability and large sizes of these materials open up many possibilities for integrating them with other materials and for making new electrical or photonic devices.”