Diamond is one of the hardest substances in the world but tiny diamond needles a few hundred nanometres long are quite bendy, according to new research.
Though diamond “is commonly believed to be undeformable”, says Subra Suresh of Nanyang Technological University in Singapore, one of the leaders of the international team behind the discovery, the nano-needles can be stretched by as much as 9% before breaking.
“This is beautifully done,” says Steven Prawer, a physicist who heads up an advanced materials science group at the University of Melbourne in Australia. “It’s difficult to stretch diamond, but when you bend a needle one side is compressed and the other is stretched. It’s a very elegant way of measuring the elastic properties of diamond.”
Diamond owes its enormous strength to its atomic structure: a crystalline array of carbon atoms arranged in a cubic lattice. The bond between two carbon atoms is extremely difficult to break by force, making diamond very hard, or by chemical reaction, making it chemically inert.
The hardness and inertness make diamond useful in biotechnology, as it can take a beating and won’t produce unwanted reactions in the body. It also has electronic properties that mean it could be used as a semiconductor in circuitry such as silicon, and with the right treatment it can also function as a sensor for changes in magnetic fields and temperatures as well as exotic quantum quantities such as spin densities.
As a result, manipulating diamond at miniature scales has been a very live area of research around the world for decades, and new information about basic properties – such as how much it can be bent – is highly sought after.
The current research brought together an international team of scientists alongside Suresh: Wenjun Zhang and Yang Lu and colleagues from the City University of Hong Kong and Ming Dao and others from the Nanomechanics Laboratory at MIT in the US. (Lu makes a point of highlighting the contributions of Amit Banerjee, Hongti Zhang and Daniel Bernoulli, the co-first authors of the paper.)
To bend diamonds, the researchers first used hot hydrocarbon vapour to condense a thin diamond coating onto a silicon surface, like fog condensing on a windowpane in a steamy bathroom. They then etched away much of the film to leave a miniature forest of conical diamond needles, each a few hundred nanometres tall and about 100 nanometres wide at the base.
The next step was testing the strength of the needles, which involved gently prodding them with a slightly larger probe and recording electron microscope footage to see what happened while measuring how much force was applied. Finally, the team performed detailed computer simulations and discovered – happily – that their measurements lined up well with the theoretical calculations.
“It’s delightful see how well the theory and the experiment match,” says Prawer. “The two really come together.”
The key finding was that the diamond needles could be stretched by as much as 9%, which corresponds to a tensile strength close to 90 Gigapascals – around 100 times that of stainless steel. This was due to the high purity of the diamond, and smoothness of the needles, according to the authors, as there were no imperfections that could trigger cracking.
“What we discovered is a general physical phenomenon,” says Suresh, adding that the flexibility is largely a “size effect” of working with such tiny structures.
Flexible nano-diamonds will be more durable than expected, according Suresh, which means that they would also be more robust and cost effective to use for many applications. He gives a few examples, including diamond nano-needle patches for delivering drugs or other molecules into cells and tiny quantum sensors that use nitrogen atoms embedded in diamond.
Another effect of bending diamond is to minutely adjust the amount of space between the carbon atoms. This affects the amount of energy held by the electrons orbiting the atoms, which could mean “revolutionary changes in diamond’s electronic and optical properties”.
The bendiness was not entirely surprising, says Prawer, citing his own research from 2012 that showed how creating a graphite bubble inside a diamond could create a significant bulge at the surface. “We’ve seen it before,” he says, “but it is a beautiful demonstration.”
The result is another reminder, if one were needed, that the world is different at the nanoscale. “Even for super-hard, brittle crystalline materials like diamond,” says Suresh.