Researchers at Peking University have tuned diamond's ultraviolet light emission by bending single-crystal nanoribbons, shifting the material's bandgap from 5.30 eV to 4.80 eV across a strain difference of about 1.9% — all without introducing a single dopant atom. The work, published in Physical Review Letters (Yuxuan Zhang et al., 2026, DOI: 10.1103/gcf5-chmf), demonstrates that a strain gradient, not uniform strain, can reshape both the electronic bands and the phonons that govern how this wide-bandgap semiconductor emits light.

The result matters because diamond has long resisted conventional semiconductor engineering. It combines an ultrawide bandgap, high carrier mobility, exceptional thermal conductivity, deep-ultraviolet emission and stable single-photon sources — a profile that makes it attractive for next-generation electronics and optoelectronics. But doping it is notoriously difficult.

"Many dopants bind charge carriers so tightly that they cannot be readily released at room temperature," said Lin Yang of Peking University, the study's corresponding author.

Elastic strain engineering sidesteps that problem. Stretching or compressing a crystal lattice changes atomic spacing, which shifts the energy bands and therefore the bandgap. Bulk diamond is brittle, but prior studies established that nanoscale diamond can withstand elastic strains up to roughly 10%. The Peking University team worked well inside that envelope.

The researchers started with an undoped single-crystal diamond grown by chemical vapor deposition, then carved it into nanoribbons with a focused ion beam while preserving the continuous crystal. A microprobe bent each ribbon, and platinum fixings anchored the ends so the deformation held. Peak strain reached 1.49%.

"When a ribbon bends, the outer side stretches while the inner side compresses. This changes the spacing between atoms differently across the ribbon," Yang explained. "A thin ribbon can bend sharply with relatively small stretching or compression across its thickness."

That geometry produces a strain gradient — a continuous variation from compression on the inside to tension on the outside — something conventional uniform-strain approaches do not create.

Measuring bands and phonons

The team characterized the bent ribbons with scanning transmission electron microscopy combined with electron energy loss spectroscopy (STEM-EELS), which maps local bandgap and phonon energies from the energy lost by electrons passing through the sample. They then compared the measurements against first-principles band-structure calculations and molecular dynamics simulations of phonon behavior under strain gradients.

The calculations traced the 0.5 eV bandgap narrowing to the conduction band shifting down under tension while the valence band stayed nearly flat.

Because diamond is an indirect semiconductor, its electrons need phonons — packets of collective atomic vibration — to supply the momentum change required for photon emission. The strain gradient reached those phonons too. Phonon energies dropped by up to about 7 meV from the compressed to the stretched region, and the emission peak shifted from 4.93 eV on the stretched side to 5.01 eV on the compressed side.

Brightness changed as well. Cathodoluminescence measurements at 80 K showed emission 36% weaker on the compressed side, which the researchers attribute to the higher phonon energies there: at that temperature, fewer high-energy phonons are thermally available to assist emission.

The gradient also broadened the spectrum. In unstrained diamond, a phonon of given momentum has one well-defined energy; under a strain gradient, the varying atomic spacing lets the same phonon span a range of energies. The emission peak consequently widened to about 0.70 eV, versus less than 0.20 eV in unstrained diamond.

"Together, these changes open additional pathways for light emission over a wider range of photon energies, contributing to the broader spectrum we observed," said Yang.

From lab bench to device

A practical device remains some distance away. The current experiment used an electron beam to excite the ribbon and a microprobe to bend it.

"Moving toward a practical device requires integrating the diamond nanoribbon with a compact actuator that can precisely and repeatedly control its deformation," Yang said. "Our demonstration used an electron beam, so an important next step is to investigate whether similar control can be achieved with optical pumping or electrical injection."

The team plans to design strain gradients that tailor specific emission spectra and to track how emission responds as deformation changes over time. Yang also sees a broader materials question worth answering: "Extending this approach to other wide-bandgap semiconductors would help determine how broadly the mechanism applies and identify suitable materials for mechanically tunable photonic devices."