A paper published in the Science journal on April 20, 2018, describes an astonishing finding by an international team of researchers that diamond, the hardest known naturally occurring material in the world, is also flexible when formed into nanoscale needles. The team included Professor Subra Suresh, President of Nanyang Technological University, Singapore (NTU Singapore).
The researchers showed that it is possible to bend and stretch diamond nano-needles - thinner than a human hair strand by approximately a 1000 times - up to 9% before returning to their original state upon removal of pressure.
Bulk diamond, in sizes that can be seen simply by the naked eye, would be anticipated to stretch by much lesser than 1%. Other characteristically strong and brittle materials exhibit a similar lack of deformability and an attempt to flex them resulted in breaking of the materials.
The researchers expect that their discovery might find innovative applications in ultra-strength nanostructures, optoelectronic devices, data storage, drug delivery, and bioimaging and biosensing. Moreover, using elastic strains induced by mechanical deformation, for instance, bending, opens up possibilities to modify optical, magnetic, electrical, and other physical characteristics.
The discovery has been reported on April 20, 2018, in the Science journal and was made by an interdisciplinary team, whose senior author is Professor Subra Suresh, President and also Distinguished University Professor at NTU Singapore. Other corresponding authors of the study are Professor Yang Lu and Professor Wenjun Zhang from the City University of Hong Kong, Dr. Ming Dao from the Massachusetts Institute of Technology (MIT) in the United States, along with other co-authors from Hong Kong, the United States, and South Korea.
A diamond probe was used to put pressure on the sides of the diamond nano-needles, grown through a unique process known as chemical vapor deposition and etched into final shape. The researchers used a scanning electron microscope to ‘video record’ the process instantaneously. They measured the extent to which each needle could bend without being fractured.
Our results were so surprising that we had to run the experiments again under different conditions just to confirm them. We also performed detailed computer simulations of the actual specimens and bending experiments to measure and determine the maximum tensile stress and strain that the diamond nano-needles could withstand before breaking.
This work also demonstrates that what is usually not possible at the macroscopic and microscopic scales can occur at the nano-scale where the entire specimen consists of only dozens or hundreds of atoms, and where the surface to volume ratio is large.
Professor Subra Suresh
The researchers ran hundreds of computer simulations in tandem with their experiments to learn and elucidate how the diamond needles endured large elastic strains, while brittle materials generally stretched less than 1%.
After two years of careful iterations between simulations and real-time experiments, we now know that the deformed shape of a bent nano-needle is the key in determining its maximum tensile strain achieved. The controlled bending deformation also enables precise control and on-the-fly alterations of the maximum strain in the nano-needle below its fracture limit.
Dr. Ming Dao
Earlier theoretical analyses had established that when elastic strain surpasses 1%, quantum mechanical calculations denote considerable changes in chemical or physical properties. The prospect of introducing elastic strains in diamond by flexing it up to 9% opens up new doors for tweaking its electronic properties. Furthermore, this phenomenon could be adapted to customize light-emitting, electrical, magnetic, optical, thermal, and mechanical properties for designing sophisticated materials for a range of applications.
Besides demonstrating up to 9% tensile stretch in single crystal diamonds, Professor Suresh and his colleagues also demonstrated that polycrystalline diamond nano-needles, where each needle is made up of several nano-size crystals or grains of diamond, have the ability to resist a reversible, elastic stretch of up to 4% before being fractured.
When it is possible to change maximum flexibility to between 0% and 9% in nano-diamonds in real-time, there is great potential to probe extraordinary material properties.
Some of the specific potential applications of the nano-diamonds are designing of superior ultra-small biosensors for enhanced performance. Another specifically noteworthy application area is the nitrogen-vacancy (NV) emission centers in diamond which are highly sensitive to spin densities, ion concentrations, temperatures, and magnetic fields. As changes in elastic strains are susceptible to magnetic fields, they can be potentially used in fields such as data storage, where data could be encoded into diamonds by using lasers.
Owing to their biocompatibility, diamonds could also be valuable for delivering drugs into cells where strong yet flexible nano-needles are required.
In biosensing applications, NV could also be used in nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI) to realize even better accuracy and resolutions in addition to three-dimensional imaging of biomolecules and complex nanostructures.
This breakthrough paves new avenues for producing innovative diamond architectures for a variety of functional applications in devices, imaging, biomedicine, materials science and engineering, and micro-testing, besides mechanical applications.
Together with Drs Subra Suresh, Yang Lu, Weijun Zhang, and Ming Dao, the other authors of the study are Amit Banerjee (lead author), Hongti Zhang (co-lead author), Muk-Fung Yuen, Jianbin Liu, and Jian Lu from the City University of Hong Kong; Daniel Bernoulli (co-lead author) from MIT; Jichen Dong from the Institute for Basic Science, Ulsan, Korea; and Feng Ding from the Ulsan Institute of Science and Technology, Korea.
A diamond nano-needle being bent by a diamond probe before it bounces back like a rubber tip. (Video credit: NTU Singapore; MIT; City University of Hong Kong; Institute for Basic Science, Korea; and the Institute of Science and Technology, Korea)