Lu Yang, one of the study’s co-authors, explained that diamonds are considered high-performance electronic and photonic materials due to their ultra-high thermal conductivity, exceptional electric charge carrier mobility, high breakdown strength and ultra-wide bandgap.
Bandgap is a key property in semiconductors, and wide bandgap allows the operation of high-power or high-frequency devices.
However, the large bandgap and tight crystal structure of diamonds make it difficult to “dope,” a common way to modulate semiconductors’ electronic properties during production. This hampers diamonds’ industrial application in electronic and optoelectronic devices.
A potential alternative is by “strain engineering”, that is, to apply a very large lattice strain to change the electronic band structure and associated functional properties, something Lu and his collaborators discovered was possible to do with nanoscale diamonds. This finding allowed the scientists to continue researching how to develop functional diamond devices.
The team firstly microfabricated single-crystalline diamond samples from solid diamond single crystals. The samples were in bridge-like shape – about one micrometre long and 300 nanometres wide, with both ends wider for gripping. The diamond bridges were then uniaxially stretched in a well-controlled manner within an electron microscope. Under cycles of continuous and controllable loading-unloading of quantitative tensile tests, the diamond bridges demonstrated a highly uniform, large elastic deformation of about 7.5% strain across the whole gauge section of the specimen, rather than deforming at a localized area in bending. And they recovered their original shape after unloading.
The team then performed density functional theory calculations to estimate the impact of elastic straining from 0 to 12% on diamonds’ electronic properties. The simulation results indicated that the bandgap of diamonds generally decreased as the tensile strain increased, with the largest bandgap reduction rate down from about 5 eV to 3 eV at around 9% strain along with a specific crystalline orientation.
Their calculation results also showed that the bandgap could change from indirect to direct with the tensile strains larger than 9% along another crystalline orientation. Direct bandgap in semiconductors means an electron can directly emit a photon, allowing many optoelectronic applications with higher efficiency.
These findings are an early step in achieving deep elastic strain engineering of microfabricated diamonds.
“I believe a new era for diamonds is ahead of us,” Lu said in a media statement.
