A joint research team led by City University of Hong Kong (CityU) has demonstrated for the first time the large, uniform tensile elastic straining of microfabricated diamond arrays through a nanomechanical approach. This stretching of the diamond lattice changes the bandgap, allowing photon emission to open up the potential of strained diamond in photonics, and quantum information technologies.
The research was co-led by Dr Lu Yang, Associate Professor in the Department of Mechanical Engineering (MNE) at CityU and researchers from Massachusetts Institute of Technology (MIT) and Harbin Institute of Technology (HIT).
“This is the first time showing the extremely large, uniform elasticity of diamond by tensile experiments. Our findings demonstrate the possibility of developing electronic devices through ‘deep elastic strain engineering’ of microfabricated diamond structures,” said Lu.
However nanoscale diamond can be elastically bent with large local strain, and the latest study showed how this phenomenon can be used for developing functional diamond devices.
“I believe a new era for diamond is ahead of us,” he said.
Diamond is well established as a high-performance electronic and photonic material due to its ultra-high thermal conductivity, exceptional electric charge carrier mobility, high breakdown strength and ultra-wide bandgap for high-power or high-frequency devices. “That’s why diamond can be considered as ‘Mount Everest’ of electronic materials, possessing all these excellent properties,” Dr Lu said.
The large bandgap and tight crystal structure of diamond make it difficult to dope to create a semiconductor. One potential alternative is strain engineering, using a very large lattice strain to modify the electronic band structure and associated functional properties, but this has been challenging as a result of the strength of the diamond lattice.
The team firstly microfabricated single-crystalline diamond samples from a 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.
Further optimizing the sample geometry using the American Society for Testing and Materials (ASTM) standard allowed a maximum uniform tensile strain of up to 9.7%, and is close to the theoretical elastic limit of diamond. More importantly, this showed elastic straining of the microfabricated diamond arrays.
The team then performed density functional theory (DFT) calculations to estimate the impact of elastic straining from 0 to 12% on the diamond’s electronic properties. The simulation results indicated that the bandgap of diamond 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 a specific crystalline orientation. The team performed an electron energy-loss spectroscopy analysis on a pre-strained diamond sample and verified this bandgap decreasing trend.
Their calculation results also showed that, interestingly, the bandgap could change from indirect to direct with the tensile strains larger than 9% along another crystalline orientation. Direct bandgap in semi-conductor 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. Using the nanomechanical approach, the team demonstrated that the diamond’s band structure can be changed, and more importantly, these changes can be continuous and reversible, allowing different applications, from micro/nanoelectromechanical systems (MEMS/NEMS), strain-engineered transistors, to novel optoelectronic and quantum technologies.
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