Stress and strain: artistic representation of the device used to stretch the diamond samples. In real life the clear pieces of diamond are about 5 micron tall. (Courtesy: City University of Hong Kong)
Normally brittle diamond has been stretched by almost 10% by an international team of scientists in China. Modeling done by the team suggests that this stretching significantly alters the electronic properties of the material, which could lead to new applications for diamond such as LEDs and variable-color lasers.
As well as shining brilliantly in jewelry, diamond has long fascinated scientists for its range of extraordinary properties. Diamond is the hardest material on Earth, has very high charge-carrier mobility and is almost totally transparent. It also has a high thermal conductivity and a very high dielectric breakdown strength – a combination that would be useful in high-power electronic devices.
Unfortunately, diamond has several properties that make it unsuitable for electronic applications. It has a very large indirect band gap of 5.47 eV, giving it very few free charge carriers. Moreover, indirect band gap semiconductors do not interact efficiently with light via electronic transitions, so diamond cannot be used as a light source or a solar cell.
Strain engineering
The electronic properties of semiconductors can be altered by adding impurity atoms (doping), but this is very difficult in diamond because of the rigidity of its carbon lattice. An alternative to doping is strain engineering, which involves changing the atomic spacing in a lattice. The processor in every modern PC, for example, contains strained silicon with electron or hole mobility that has been altered by strain engineering.
“In silicon – which we’re also working on – if you apply just a 1% strain you see a huge change in electric mobility,” explains joint team leader Yang Lu of the City University of Hong Kong. In the new research, the team looked at whether strain engineering could help with diamonds.
Theoretical calculations suggest that the diamond lattice should withstand strains up to about 12%. In practice however, when real diamonds are stressed, the strain becomes concentrated at defects in the lattice, causing the diamonds to break rather than stretch.
High elasticity
In 2018, Lu and colleagues etched nanoscale diamonds and, by bending them, demonstrated that diamonds could show high elasticity: “We discovered this, but the straining method can’t be well characterized,” explains Lu. “Since then, a lot of other groups have done follow up work – mostly computational – predicting band gap change, metallization and even superconductivity.” Testing these predictions, however, requires the precise characterization of the strain.
In their new research, Lu and colleagues used advanced fabrication processes to produce micron-scale, T-shaped samples of single-crystal diamond, which they stretched using a custom-made mechanical gripper (see figure). They measured different Young’s moduli along different crystallographic axes, achieving a maximum strain of 9.7�fore the diamond fractured. At lower values, the lattice returned to its original configuration when the strain was removed.
“We managed to microfabricate diamond into the shape and geometry that we want,” says Lu. “This is just an illustration of the device concept. We can scale up.”
Direct band gap
Experimental limitations prevented the researchers from directly measuring the effects of stretching on the electronic band structure of their samples. However, calculations done using density functional theory and their experimental data suggest that along one crystalline axis, the band gap falls from above 5 eV to around 3 eV at around 9% strain. Along another axis, the band gap changes from indirect to direct.
Lu’s group is now looking at several possible applications for stretched diamond including making lasers of different colors. “Intrinsically, diamond has a wide bandgap thus would turn out ultraviolet radiation light,” he says. “Now imagine if we have a tendency to stretch the diamond, it goes from ultraviolet to violet, and if we stretch it more it goes from violet to red.”
