Array of diamond nano-bridges.  Credit: Dang Chaoqun, City University of Hong Kong
Array of diamond nano-bridges. Credit: Dang Chaoqun, City University of Hong Kong

Diamond boasts a range of outstanding properties from the mechanical to the electronic, but its large bandgap makes its manipulation by doping a challenge for optoelectronic and electronic applications. A technique known as strain engineering, in which a material is deformed to induce changes to its bandgap, can be used as an alternative to doping. Diamond, despite its great strength, can deform elastically at the nanoscale [Banerjee et al., Science 360 (2018) 300–302]. Now the researchers who first demonstrated this remarkable nanoscale deformation behavior have shown that much larger diamond structures can be elastically deformed in a uniform and controlled way too [Dang et al., Science 371 (2021) 76–78,].

"In our previous research, we discovered that, at the nanoscale, diamond can be elastically deformed at an unprecedented magnitude, suggesting the possibility of tuning diamond’s properties with large strains, explains Yang Lu of City University of Hong Kong, who led the work. It was considered ‘impossible’ for bulk diamond because of its extremely high hardness and brittleness.

In their latest work, the researchers at National Changhua University of Education, Harbin Institute of Technology, National Chiao Tung University, Southern University of Science and Technology, Lawrence Berkeley National Laboratory, University of California, Berkeley, and Massachusetts Institute of Technology fabricated arrays of diamond “bridges” 1 µm by 100 nm. Single crystals of diamond grown using microwave-assisted chemical vapor deposition were carved into arrays of bridges using a focused ion beam (FIB). The team then used a home-made gripper to stretch the diamond bridges in a controlled and uniform manner within electron microscopes, both scanning and transmission electron instruments, so that observations could be made at the same time.

Uniform and well-controlled straining of diamond structures is needed for device applications, points out Lu. So this time we microfabricated single-crystalline diamond micro-bridge structures and achieved sample-wide uniform elastic strains under uniaxial tension straining at room temperature.

Large strains of up to 9.7% were achieved in the diamond bridges, approaching the theoretical limit for the material. After removing the strain, the diamond bridges rebound back to their original configuration. Beyond the maximum strain, however, the bridges fractured in a typical brittle way. In combination with these mechanical experiments, the researchers also undertook density functional theory (DFT) calculations to estimate the effect of elastic strain on the electronic properties of diamond. The simulations indicate that diamond’s bandgap should decrease with increasing strain, from around 5–3 eV at 9% strain along the [101] crystal direction. These predictions agree well with the researchers’ electron energy-loss spectroscopy (EELS) measurements.

“Of course, precisely characterizing bandgap changes in strained diamond remains very challenging and requires more comprehensive studies with synergic efforts from researchers from different fields, including physicists and electrical engineers,” points out Lu.

Nevertheless, the researchers are confident that bandgap engineering of strained diamond is feasible and could be extremely useful in future microelectronics. While in this study the team only achieved>9% strains in the [101] direction, larger strains could be possible along the [111] direction. If this level of strain could be reached in the [111] direction, a transition from an indirect to direct bandgap could be possible.

This is the first demonstration of the extremely large, uniform elastic straining of diamond by tensile experiments, says Lu. Our findings pave the way for 'deep elastic strain engineering' of functional device arrays based on microfabricated diamonds for photonics, electronics, and quantum information technologies.

Lu and his colleagues are confident that their arrays of FIB-fabricated diamond micro-bridges could be potentially scaled up for large-scale integration into semiconductor devices. The results are a promising indication that diamond’s band structure can be modulated in a continuous and reversible manner, enabling bandgap engineering for a range of applications from micro/nanoelectromechanical systems (MEMS/NEMS), strain-engineered transistors, to novel optoelectronic and quantum technologies.

Yury Gogotsi, Charles T. and Ruth M. Bach Distinguished University Professor and Director of the A.J. Drexel Nanomaterials Institute at Drexel University, agrees that strain engineering of diamond’s electronic properties could find applications in MEMS devices for photonics, electronics, and other technologies.

Diamond is the hardest known material [and] one would not expect it to deform elastically to a significant extent – it’s not a rubber band, he points out. However, it has been known for a while that diamond can be deformed, both plastically and elastically, when in a single-crystal state. This study shows that this stiffest and hardest of materials can stretch – like rubber – when fabricated into ~100 nanometer-thin films.

The reversible changes in the electronic structure of diamond reported by the researchers in this work are significant, believes Gogotsi, and reflect a major step forward in controlling the electronic properties of this wide bandgap semiconductor.

“Microelectronics based on these strained diamonds won't be ready for prime-time any day soon, but … we believe a new era for diamond is ahead of us,” adds Lu.

This article was originally published in Nano Today 37 (2021) 101112.