The Lu Yang team from City University of Hong Kong publishes "Science" again: Climbing the "Mount Everest" in the field of electronic materials—

Diamond is known for its hardness, and its industrial applications are usually cutting, drilling or grinding
.
But it is not only the hardest material in nature, it also unexpectedly possesses ultra-high thermal conductivity, dielectric breakdown strength, carrier mobility, and ultra-wide band gap
.
Thanks to these excellent characteristics, diamond is called the "Mount Everest" of electronic materials, and has high hopes in the direction of optoelectronic applications
.
But also because of the large energy band gap and tight crystal structure, diamond is difficult to "doping" (a common method for adjusting the electronic properties of semiconductors).
Want to "top Everest"-the industrial application of diamond in optoelectronic devices is still Difficulties
.
For materials that are difficult to dope, a potential solution is to apply a huge lattice strain to change the electronic band structure and related properties
.
But because of the extremely high hardness of diamond, this method was once considered ineffective
.
? Until 2018, Dr.
Yang Lu of the City University of Hong Kong and his collaborators discovered for the first time that nano-scale diamonds can produce super-large elastic strains, with local tensile elastic strains reaching 9% or even higher
.
This surprising discovery shows that it is possible to change the physical properties of diamond through elastic strain engineering (ESE)
.
?? On January 1, 2020, Professor Lu Yang, Alice Hu's team from City University of Hong Kong, Professor Zhu Jiaqi from Harbin Institute of Technology, and Professor Li Ju from Massachusetts Institute of Technology, for the first time demonstrated the uniform depth of the microcrystalline diamond array through the nanomechanics method.
Elastic strain
.
The super-large, highly controllable elastic strain can fundamentally change the band structure of diamond, reducing the band gap by up to 2eV through calculation
.
This discovery shows that the deep elastic strain engineering of the finely processed diamond structure makes stretchable diamond promising for the next generation of microelectronics, photonics and quantum information technology
.
The work was published on "Science" as "Achieving large uniform tensile elasticity in microfabricated diamond"
.
?? Uniform tensile strain Figure1.
Loading and unloading tensile experiment along the [101] direction? The team first microfabricated a single crystal diamond sample from a solid diamond single crystal
.
These samples are bridge-shaped, about one micron long, 300 nanometers wide, and wider at both ends for easy clamping
.
Under the continuous controlled loading and unloading quantitative tensile test cycle, the diamond bridge exhibits highly uniform and super-large elastic deformation (approximately 7.
5% strain) across the entire section, instead of deforming in the local area of ​​bending
.
After uninstalling, they completely restored their original shape
.
By using the American Society for Testing and Materials (ASTM) standards to further optimize the sample geometry, a maximum uniform tensile strain of up to 9.
7% was finally obtained, which even exceeded the maximum local value in the 2018 study, and was close to the theoretical elastic limit of diamond
.
In order to demonstrate the feasibility of strained diamond as a device, the author also micromachined the diamond bridge array.
The array can be completely restored to its original shape after a uniform strain of 5.
8%.
The finite element analysis also confirmed the uniform elasticity of the diamond array.
Strain distribution
.
? Adjust the band gap by elastic strain Figure 2.
[100], [101] and [111] oriented diamond statistical tensile results? The author summarized all the tensile strength of [100], [101] and [111] oriented diamond samples The strength of the experimental data, the experiment proved that the sample can reach 6.
5 ~ 8.
2% of the sample's wide elastic strain in three different directions, and the recovery is complete
.
As the experiment approached a uniform elastic strain of 10%, the authors performed density functional calculations (DFT) to evaluate the effect of elastic strain from 0 to 12% on the electronic properties of diamond
.
The simulation results show that the band gap of diamond generally decreases with the increase of tensile strain.
At a strain of about 9%, the band gap along a specific crystal orientation can be reduced from about 5 eV to about 3 eV at most
.
The author also performed an electron energy loss spectroscopy analysis on the sample, which further confirmed this trend of band gap reduction.

.
? The large and uniform elastic strain should drive the band gap change.
Compared with the other two directions, the strain along the [101] direction will cause a larger band gap reduction
.
After more in-depth calculations, when the tensile strain along the [111] direction is greater than 9%, the indirect band gap will be converted into a direct band gap, which means that there is no need to release or absorb phonons during the electronic transition.
The device will have higher efficiency
.
? Summarizing these findings is an early step to realize the deep elastic strain engineering of micro-machined diamond
.
Through the nanomechanics method, the author proved that the band structure of diamond can be changed, and more importantly, these changes can be continuous and reversible
.
The micron-sized single crystal diamond bridge structure is very suitable for the scale of electromechanical systems (MEMS/NEMS), strain engineering transistors, and novel optoelectronic and quantum device arrays
.
Professor Lu Yang said: "I believe that the new era of diamonds is right in front of us
.
"