Researchers
have discovered a way to transform the electronic properties of
nanoscale
needles of diamond. Courtesy: MIT News
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, the researchers say.
Their
findings, which are based on simulations, calculations, and previous
experimental results, were published on October 5, 2020, in the Proceedings of
the National Academy of Sciences. The paper is by MIT Professor Ju Li and
graduate student Zhe Shi; Principal Research Scientist Ming Dao; Professor Subra
Suresh, who is president of Nanyang Technological University in Singapore as
well as former dean of engineering and Vannevar Bush Professor Emeritus at MIT;
and Evgenii Tsymbalov and Alexander Shapeev at the Skolkovo Institute of
Science and Technology in Moscow.
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. Li and his collaborators 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 team led by Suresh, Dao, and Yang Lu from the City
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.
“We found
that it’s possible to reduce the bandgap from 5.6 electron volts all the way to
zero,†Li says. “The point of this is that if you can change continuously from
5.6 to 0 electron volts, then you cover all the range of bandgaps. Through
strain engineering, you can make diamond have the bandgap of silicon, which is
most widely used as a semiconductor, or gallium nitride, which is used for LEDs.
You can even have it become an infrared detector or detect a whole range of
light all the way from the infrared to the ultraviolet part of the spectrum.â€
“The
ability to engineer and design electrical conductivity in diamond without
changing its chemical composition and stability offers unprecedented
flexibility to custom-design its functions,†says Suresh. “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.â€
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.
One
constraint, which required not only the right amount of strain but also the
right orientation of the diamond’s crystalline lattice, was to prevent the
strain from causing the atomic configuration to cross a tipping point and
turning into graphite, the soft material used in pencils.
The
process can also make diamond into two types of semiconductors, either “directâ€
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, Li
suggests, such as for highly sensitive quantum-based detectors that use defects
and dopant atoms in a diamond. “Using strain, we can control the emission and
absorption levels of these point defects,†he says, allowing novel ways of
controlling their electronic and nuclear quantum states.
But given
the great variety of conditions made possible by the different dimensions of
strain variations, Li says, “if we have a particular application in mind, then
we could optimize toward that application target. And what is nice about the
elastic straining approach is that it is dynamic,†so that it can be
continuously varied in real time as needed.
This
early-stage proof-of-concept work is not yet at the point where they can begin
to design practical devices, the researchers say, 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.