How to Bend Diamonds

Flexible nanodiamonds promise to open up novel optical and electronic properties

“Experiment (left) and simulation (right) of a diamond nanoneedle being bent by the side surface of a diamond tip, showing ultralarge and reversible elastic deformation.”
Image: Yang Lu, Amit Banerjee, Daniel Bernoulli, Hongti Zhang, Ming Dao and Subra Suresh

It’s commonly known that diamonds are the hardest natural material. However, with that hardness comes brittleness: they may be hard but they’re not very flexible.

Now an international team of researchers has demonstrated that diamonds, which are commonly believed to be inflexible, can be bent and stretched significantly. The researchers showed that the maximum tensile elastic strain of a diamond can reach nearly 9 percent, close to the theoretical limit of the material.

The researchers believe that these enhanced mechanical properties make nanodiamonds much more durable than expected, and therefore could lead to applications that involve mechanical loading, making them candidates for applications such as diamond needle-based intracellular delivery. But it is what this flexiblity does to diamonds’ optical and electrical properties may prove to be the most significant in the long run.

In research described in the journal Science, scientists from the City University of Hong Kong, China, the Massachusetts Institute of Technology, Ulsan National Institute of Science and Technology (UNIST) in South Korea and Nanyang Technological University in Singapore produced nanoscale diameter (about 100-300nm) diamond needles using reactive ion etching (RIE) of CVD (chemical vapor deposition)-grown diamond thin films.

The resulting material demonstrates that the mechanical properties of even hard, brittle crystalline materials like diamonds can be fundamentally changed when their sizes are reduced to nanometer-length scale.

While the purity of the internal crystalline structure and surface smoothness of the material is important, the key to achieving such flexibility in diamonds is actually the “size”—the dimension/diameter of the nanodiamond needles, according to Yang Lu, an associate professor at the City University of Hong Kong.

Flexibility on its own is great for using diamonds in applications that require more mechanical loading, but there is also a quantum mechanical effect that comes with this bending.

Previous theoretical studies showed that when elastic strains exceed 1%, quantum mechanical calculations indicate significant physical and/or chemical material property alterations due to the changes in energy band gap structures, according to Ming Dao, principal investigator and director at MIT’s Nanomechanics Lab, who was a co-author of the research.

“These property alterations may include significant changes in mechanical, thermal, optical, magnetic, electrical, electronic and chemical reaction properties, and could be used to design advanced materials for various applications through the emerging ‘elastic strain engineering’,” said Dao. “When maximum elastic strains can be changed in real-time between 0 to 9% in nanodiamonds, there is a lot of potential for exploring unprecedented material properties.”

After two years of careful iterations between simulations and real-time experiments, Dao said that he and his colleagues have managed to streamline a nanomechanical process that can precisely control and quantify the maximum amount of elastic deformation within the nanodiamonds.  The resulting method enables accurate control and on-the-fly alterations of the maximum strain in the nanoneedle below its fracture limit. This also means, of course, that its electronic properties can be changed on-the-fly as well.

While this level of elasticity in the nanodiamonds can change their band gap structures, incorporating impurities into the severely strained lattice of the nanodiamonds may lead to revolutionary changes in diamond’s electronic and optical properties, according to Yu. “This could open up a lot of novel optoelectronics applications for diamonds, such as more powerful or colorful laser or maser (microwave amplification by stimulated emission of radiation),” said Yu.

The first step toward commercialization of these applications will require researchers to microfabricate diamond nanostructures in well-defined geometries and crystalline structures.

For optoelectronics applications, another challenge is to quantify and control the local optical/electronic property changes, in real time, for a single diamond nanostructure under elastic straining, according to Lu.

Lu added: “We will work with physicists and electrical engineers to explore the optoelectronics applications of the elastic nanodiamond structures, and we may even may find its applications in the emerging diamond-based quantum computing technologies.”

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