MIT researchers have discovered a way to transform the electronic properties of nanoscale needles
of diamond.
Long known as the hardest of all natural materials, diamonds are also exceptional thermal conductors
and electrical insulators.
Now, researchers have discovered a way to tweak tiny needles of diamond in a controlled
way to transform their electronic properties, dialing them from insulating, through semiconducting,
all the way to highly conductive, or metallic.
This can be induced dynamically and reversed at will, with no degradation of the diamond
material.
The research, though still at an early proof-of-concept stage, may open up a wide array of potential
applications, including new kinds of broadband solar cells, highly efficient LEDs and power
electronics, and new optical devices or quantum sensors.
Their findings, which are based on simulations, calculations, and previous experimental results,
are reported in the journal Proceedings of the National Academy of Sciences.
The team used a combination of quantum mechanical calculations, analyses of mechanical deformation,
and machine learning to demonstrate that the phenomenon, long theorized as a possibility,
really can occur in nanosized diamond.
The concept of straining a semiconductor material such as silicon to improve its performance
found applications in the microelectronics industry more than two decades ago.
However, that approach entailed small strains on the order of about 1 percent.
The researchers have spent years developing the concept of elastic strain engineering.
This is based on the ability to cause significant changes in the electrical, optical, thermal,
and other properties of materials simply by deforming them — putting them under moderate
to large mechanical strain, enough to alter the geometric arrangement of atoms in the
material’s crystal lattice, but without disrupting that lattice.
In a major advance in 2018, a research team from the Polytechnic University of Hong Kong
showed that tiny needles of diamond, just a few hundred nanometers across, could be
bent without fracture at room temperature to large strains.
They were able to repeatedly bend these nanoneedles to tensile strain as much as 10 percent; the
needles can then return intact to their original shape.
Key to this work is a property known as bandgap, which essentially determines how readily electrons
can move through a material.
This property is thus key to the material’s electrical conductivity.
Diamond normally has a very wide bandgap of 5.6 electron volts, meaning that it is a strong
electrical insulator that electrons do not move through readily.
In their latest simulations, the researchers show that diamond’s bandgap can be gradually,
continuously, and reversibly changed, providing a wide range of electrical properties, from
insulator through semiconductor to metal.
They found that it’s possible to reduce the bandgap from 5.6 electron volts all the
way to zero.
The methods demonstrated in this work could be applied to a broad range of other semiconductor
materials of technological interest in mechanical, microelectronics, biomedical, energy and photonics
applications, through strain engineering.
So, for example, a single tiny piece of diamond, bent so that it has a gradient of strain across
it, could become a solar cell capable of capturing all frequencies of light on a single device
— something that currently can only be achieved through tandem devices that couple different
kinds of solar cell materials together in layers to combine their different absorption
bands.
These might someday be used as broad-spectrum photodetectors for industrial or scientific
applications.
The process can also make diamond into two types of semiconductors, either “direct”
or “indirect” bandgap semiconductors, depending on the intended application.
For solar cells, for example, direct bandgaps provide a much more efficient collection of
energy from light, allowing them to be much thinner than materials such as silicon, whose
indirect bandgap requires a much longer pathway to collect a photon’s energy.
The process could be relevant for a wide variety of potential applications, such as for highly
sensitive quantum-based detectors that use defects and dopant atoms in a diamond.
This early-stage proof-of-concept work is not yet at the point where they can begin
to design practical devices, but with the ongoing research they expect that practical
applications could be possible, partly because of promising work being done around the world
on the growth of homogeneous diamond materials.