New properties discovered in diamond semiconductors

Diamond, often celebrated for its unmatched hardness and transparency, has emerged as an exceptional material for high-power electronics and next-generation quantum optics. Diamond can be engineered to be as electrically conductive as a metal, by introducing impurities such as the element boron.

Researchers from Case Western Reserve University and the University of Illinois Urbana-Champaign have now discovered another interesting property in diamonds with added boron, known as boron-doped diamonds. Their findings could pave the way for new types of biomedical and quantum optical devices — faster, more efficient and capable of processing information in ways that classical technologies cannot. Their results have been published in Nature Communications.

The researchers found that boron-doped diamonds exhibit plasmons — waves of electrons that move when light hits them — allowing electric fields to be controlled and enhanced on a nanometre scale. This is important for advanced biosensors, nanoscale optical devices and for improving solar cells and quantum devices. Previously, boron-doped diamonds were known to conduct electricity and become superconductors, but not to have plasmonic properties. Unlike metals or even other doped semiconductors, boron-doped diamonds remain optically clear.

“Diamond continues to shine,” said Giuseppe Strangi, professor of physics at Case Western Reserve, “both literally and as a beacon for scientific and technological innovation. As we step further into the era of quantum computing and communication, discoveries like this bring us closer to harnessing the full potential of materials at their most fundamental level.”

“Understanding how doping affects the optical response of semiconductors like diamond changes our understanding of these materials,” said Mohan Sankaran, professor of nuclear, plasma and radiological engineering at Illinois Grainger College of Engineering.

Plasmonic materials, which affect light at the nanoscale, have captivated humans for centuries, even before their scientific principles were understood. The vibrant colours in medieval stained-glass windows result from metal nanoparticles embedded in the glass. When light passes through, these particles generate plasmons that produce specific colours. Gold nanoparticles appear ruby red, while silver nanoparticles display a vibrant yellow. This ancient art highlights the interaction between light and matter, inspiring modern advancements in nanotechnology and optics.

Diamonds, composed of transparent crystals of the element carbon, can be synthesised with small amounts of boron, adjacent to carbon on the periodic table. Boron contains one less electron than carbon, allowing it to accept electrons. Boron essentially opens up a periodic electronic ‘hole’ in the material that increases the material’s ability to conduct current. The boron-doped diamond lattice remains transparent, with a blue hue. (The famous Hope Diamond is blue because it contains small amounts of boron).

Because of its other unique properties — it’s also chemically inert and biologically compatible — boron-doped diamond could potentially be used in contexts that other materials could not, such as for medical imaging or high-sensitivity biochips or molecular sensors.

“Not only is its uniqueness critical in changing our way of thinking about the origin of plasmons, but being able to use diamond itself as a plasmonic platform could benefit physicists and chemists working in a range of fields,” said Souvik Bhattacharya, one of the paper’s authors. “We are very excited to explore the applications of this phenomenon in diamond.”

Diamonds synthesised at low pressure were pioneered at Case Western Reserve (then Case Institute of Technology) in 1968 by faculty member John Angus, who died in 2023. Angus was also the first to report on the electrical conductivity of diamond-doping with boron.

Image credit: iStock.com/spawns