Font size:
Print
Quantum Dots and the Rise of 2D Metals
Scientists make strange 2D metals sought for future technologies
Context: In the ever-evolving world of materials science, few discoveries have had as profound an impact as the creation of quantum dots — tiny semiconductors only a few nanometres wide.
More on News
- These minuscule particles are used in LED lighting, medical diagnostics, semiconductor manufacturing, and solar panels, thanks to a peculiar quantum phenomenon called quantum confinement.
- Their small size belies their immense influence, earning the scientists who developed a quick and reliable method to produce them the 2023 Nobel Prize in Chemistry.
The Science Behind the Dot
- Quantum confinement explains the extraordinary properties of quantum dots.
- In conventional electrical systems like the copper wires in your house, electrons move freely, spreading out and gaining energy smoothly. But inside a quantum dot, the spatial constraints are so tight that electrons can’t roam freely.
- While they are no longer confined to individual atoms, the limited space means they can only occupy specific energy levels, much like electrons in atoms.
- Think of it this way: in a copper wire, electrons can sit wherever they like — like choosing any seat in an empty theater. In a quantum dot, only specific rows and seats are available, and all the rest are blocked. Because of this behaviour, the entire quantum dot begins to behave like a “giant atom”.
Beyond the Dot: Enter 2D Materials
- Quantum dots are classified as zero-dimensional materials because their electrons are confined so tightly they essentially occupy a point in space.
- This contrasts with 1D and 2D materials, where electrons can move in one or two directions, respectively.
- Graphene, for instance, is a 2D material composed of a single sheet of carbon atoms in a hexagonal pattern. The electrons in graphene move only in two dimensions, and as a result, behave in strange ways — sometimes even as if they don’t have mass.
- The unique behaviours arising from quantum confinement have massive real-world implications. That’s why scientists have been striving for over a decade to create 2D metals, materials that could offer exotic properties for futuristic technologies. Yet they’ve been hitting a wall.
The 2D Metal Challenge
- Creating 2D metals has proven notoriously difficult. That’s because metal atoms prefer to bond in three dimensions. Unlike carbon, which can maintain strong bonds in a single layer, metals naturally form thick, bonded clusters.
- Researchers have tried numerous approaches — from vapour deposition to physically pressing and slicing metal layers — but the results have often been only a few nanometres thick and far from the atomic-level thinness they need.
- Another hurdle has been surface interactions. Metal atoms exposed to air easily oxidise, forming unwanted compounds. Moreover, most metal sheets produced so far have uneven surfaces and unstable structures. But the potential is too great to ignore.
- 2D metals like bismuth and tin are predicted to be topological insulators, materials that conduct electricity only along their edges. Such materials could lead to faster, more efficient computers and sensors for medicine and defence.
A Breakthrough from China
- A new study published in Nature by a team from the Beijing National Laboratory for Condensed Matter Physics, University of Chinese Academy of Sciences, and Songshan Lake Materials Laboratory could represent a turning point.
- Their method for creating 2D sheets of metals like bismuth, gallium, indium, tin, and lead is surprisingly straightforward, though made possible by years of technological advancement. Here’s how it works:
- Start with a pure metal powder (e.g., bismuth).
- Place it on a sapphire plate coated with a single layer of molybdenum disulphide (MoS₂) — the bottom “anvil.”
- Heat the anvil to melt the metal powder into a droplet.
- Cover the droplet with a second MoS₂-coated sapphire plate — the top anvil.
- Twist the top anvil slightly and press both together with a pressure of 200 million pascals (Pa), keeping the setup intact until it cools.
- Peel off the ultra-thin metal sheet that has formed.
- The result? A bismuth sheet is just 6.3 Å thick, or about two atoms deep — thin enough to confine electrons to two dimensions.
| The choice of MoS₂ and sapphire was crucial. MoS₂ has a Young’s modulus of 430 billion Pa and sapphire, 300 billion Pa — making them rigid enough to apply the intense pressure. Moreover, both materials have smooth surfaces and chemically inert atoms, which prevent unwanted bonding with the metal atoms. |
Exotic Properties and New Possibilities
- This 2D bismuth displayed a strong field effect — meaning its conductivity could be tuned with an electric field — and a nonlinear Hall effect, where an applied electric field generated a perpendicular voltage. These effects are hallmarks of 2D metals and do not appear in 3D metal forms.
- In a commentary accompanying the paper, Javier Sanchez-Yamagishi, a condensed-matter physicist at the University of California, Irvine, noted that the team’s use of centimetre-scale sapphire plates coated with MoS₂ might be the key to achieving true atomic thinness.
- While similar efforts have produced ultra-thin crystals of bismuth and gold nanocrystals before, this method represents a “substantial improvement” in simplicity and scalability.
- He also highlighted future avenues, such as:
- Creating multi-metal 2D alloys,
- Engineering room-temperature topological insulators more reliably,
- Scaling up the process to make larger-area sheets.
- Perhaps most excitingly, Sanchez-Yamagishi remarked that very little is known about the electronic behaviour of these new 2D metals. Their stability and larger size open doors to integrate them into real-world electronic and photonic devices.
Looking Ahead
- Just as the Nobel-winning breakthrough in quantum dot synthesis revolutionised electronics, displays, and diagnostics, this new method of creating atomically thin 2D metals could set the stage for the next technological revolution.
- From faster computers to ultra-sensitive sensors, the possibilities are vast. And like all great scientific discoveries, this one began by reimagining the very fabric of the materials we thought we knew.
