From: City University of Hong Kong
December 31, 2020 -- Diamond is the
hardest material in nature. But out of many expectations, it also has great
potential as an excellent electronic material. 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 the nanomechanical approach. Their findings have shown the potential of
strained diamonds as prime candidates for advanced functional devices in
microelectronics, 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). Their findings have been recently published in
the scientific journal Science, titled "Achieving large
uniform tensile elasticity in microfabricated diamond."
"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 Dr Lu.
Diamond: "Mount Everest" of
electronic materials
Well known for its hardness, industrial
applications of diamonds are usually cutting, drilling, or grinding. But
diamond is also considered 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.
Bandgap is a key property in semi-conductor, and wide bandgap allows operation
of 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.
However, the large bandgap and tight
crystal structure of diamond make it difficult to "dope," a common
way to modulate the semi-conductors' electronic properties during production,
hence hampering the diamond's industrial application in electronic and
optoelectronic devices. A potential alternative is by "strain
engineering," that is to apply very large lattice strain, to change the
electronic band structure and associated functional properties. But it was
considered as "impossible" for diamond due to its extremely high
hardness.
Then in 2018, Dr Lu and his
collaborators discovered that, surprisingly, nanoscale diamond can be
elastically bent with unexpected large local strain. This discovery suggests
the change of physical properties in diamond through elastic strain engineering
can be possible. Based on this, the latest study showed how this phenomenon can
be utilized for developing functional diamond devices.
Uniform tensile straining across the
sample
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 (See image: Tensile straining of
diamond bridges). 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.
By further optimizing the sample
geometry using the American Society for Testing and Materials (ASTM) standard,
they achieved a maximum uniform tensile strain of up to 9.7%, which even
surpassed the maximum local value in the 2018 study, and was close to the
theoretical elastic limit of diamond. More importantly, to demonstrate the
strained diamond device concept, the team also realized elastic straining of
microfabricated diamond arrays.
Tuning the bandgap by elastic strains
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. By
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. "I believe a new era for
diamond is ahead of us," said Dr Lu.
The research at CityU was funded by the
Hong Kong Research Grants Council and the National Natural Science Foundation
of China.
Journal Reference:
- Chaoqun
Dang, et al. Achieving large uniform tensile elasticity in
microfabricated diamond. Science, Jan 1st, 2021 DOI: 10.1126/science.abc4174
No comments:
Post a Comment