Scientists demonstrate flexibility of diamond nanoscale needles
An international interdisciplinary team of scientists  that includes Prof Subra Suresh, President of Nanyang Technological University, Singapore (NTU Singapore), demonstrated that diamond, the world’s hardest natural material, is also flexible when made into nanoscale needles. This discovery may lead to new applications in bioimaging and biosensing, drug delivery, data storage, opto-electronic devices and ultra-strength nanostructures.
Up to 9% tensile stretch
The diamond nano-needles - about a thousand times thinner than a strand of human hair - can be bent and stretched up to nine per cent, before bouncing back to their original state when pressure is removed. This is significantly greater than the flexibility of bulk diamond in sizes visible to the naked eye. The latter is expected to stretch by well below one per cent. Other strong and brittle materials also usually break when attempts are made to flex them.
The nano-needles were grown through a special process called chemical vapour deposition and etched into final shape. The team used a diamond probe to put pressure on the sides of the diamond nano-needles, recording the process in real time using a scanning electron microscope. The researchers measured how much each needle could bend before it fractured.
“Our results were so surprising that we had to run the experiments again under different conditions just to confirm them,” said Prof Suresh.
Prof Suresh explained that this this is a demonstration 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.”
The team ran hundreds of detailed computer simulations alongside their experimental tests to understand and explain how the diamond needles underwent large elastic strains. They learnt after two years of iterations between simulations and real-time experiments 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.
In addition to showing up to 9% tensile stretch in single crystal diamonds, Prof Suresh and his collaborators also showed that polycrystalline diamond nano-needles, where each needle comprises many nano-size grains or crystals of diamond, can withstand a reversible, elastic stretch of up to 4% before breaking.
When elastic strain exceeds one percent, quantum mechanical calculations in previous theoretical studies indicate significant physical or chemical property changes. Hence, this demonstrated ability of introducing elastic strains in diamond by flexing it up to 9% provides opportunities for fine-tuning its electronic properties. This phenomenon could also be used to tailor mechanical, thermal, optical, magnetic, electrical, and light-emitting properties to design advanced materials for various applications.
The nano-diamonds could help design of better ultra-small biosensors for greater performance.
Another application area of particular significance is the nitrogen-vacancy (NV) emission centres in diamond. The NV centre consists of a nearest-neighbor pair of a nitrogen atom, which substitutes for a carbon atom, and a lattice vacancy. It is one of the point effects in diamond, which are Imperfections in the crystal lattice of diamond. NV Centres are extremely sensitive to magnetic fields, temperatures, ion concentrations and spin densities. Since changes in elastic strains are sensitive to magnetic fields, potential applications could include such fields as data storage where lasers could encode data into diamonds.
In biosensing applications, NV could also be used in Magnetic Resonance Imaging (MRI) or Nuclear Magnetic Resonance (NMR) to achieve even higher accuracy and resolutions, as well as 3-dimensional imaging for complex nanostructures and biomolecules.
As diamonds are biocompatible, they could also be useful for drug delivery into cells where strong yet flexible nano-needles are needed.
This discovery also shows new pathways for producing novel diamond architectures for mechanical applications, as well as a variety of functional applications in devices, biomedicine, imaging, micro-testing, and materials science and engineering.
 The findings were published on 20 April in the journal Science. The interdisciplinary team included Prof Subra Suresh, President and also Distinguished University Professor at NTU Singapore, as senior author. Other corresponding authors include Prof Yang Lu and Prof Wenjun Zhang from the City University of Hong Kong, Dr Ming Dao from the Massachusetts Institute of Technology (MIT) in United States, with other co-authors from Hong Kong, United States and South Korea.
In addition to Drs. Subra Suresh, Yang Lu, Weijun Zhang and Ming Dao, the list of authors includes: 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.