A new technique produces perovskite nanocrystals right where they’re needed, so the exceedingly
delicate materials can be integrated into nanoscale devices.
Halide perovskites are a family of materials that have attracted attention for their superior
optoelectronic properties and potential applications in devices such as high-performance solar
cells, light-emitting diodes, and lasers.
These materials have largely been implemented into thin-film or micron-sized device applications.
Precisely integrating these materials at the nanoscale could open up even more remarkable
applications, like on-chip light sources, photodetectors, and memristors.
However, achieving this integration has remained challenging because this delicate material
can be damaged by conventional fabrication and patterning techniques.
To overcome this hurdle, MIT researchers created a technique that allows individual halide
perovskite nanocrystals to be grown on-site where needed with precise control over location,
to within less than 50 nanometers.
The size of the nanocrystals can also be precisely controlled through this technique, which is
important because size affects their characteristics.
Since the material is grown locally with the desired features, conventional lithographic
patterning steps that could introduce damage are not needed.
The technique is also scalable, versatile, and compatible with conventional fabrication
steps, so it can enable the nanocrystals to be integrated into functional nanoscale devices.
The researchers used this to fabricate arrays of nanoscale light-emitting diodes (nanoLEDs)
— tiny crystals that emit light when electrically activated.
Such arrays could have applications in optical communication and computing, lensless microscopes,
new types of quantum light sources, and high-density, high-resolution displays for augmented and
virtual reality.
The researcher says that as their work shows, it is critical to develop new engineering
frameworks for integration of nanomaterials into functional nanodevices.
By moving past the traditional boundaries of nanofabrication, materials engineering,
and device design, these techniques can allow them to manipulate matter at the extreme nanoscale
dimensions, helping them realize unconventional device platforms important to addressing emerging
technological needs.
The research is published in the journal Nature Communications.
Integrating halide perovskites into on-chip nanoscale devices is extremely difficult using
conventional nanoscale fabrication techniques.
In one approach, a thin film of fragile perovskites may be patterned using lithographic processes,
which require solvents that may damage the material.
In another approach, smaller crystals are first formed in solution and then picked and
placed from solution in the desired pattern.
In both cases there is a lack of control, resolution, and integration capability, which
limits how the material can be extended to nanodevices.
Instead, the research team developed an approach to “grow” halide perovskite crystals in
precise locations directly onto the desired surface where the nanodevice will then be
fabricated.
Core to their process is to localize the solution that is used in the nanocrystal growth.
To do so, they create a nanoscale template with small wells that contain the chemical
process through which crystals grow.
They modify the surface of the template and the inside of the wells, controlling a property
known as “wettability” so a solution containing perovskite material won’t pool on the template
surface and will be confined inside the wells.
They apply a solution containing halide perovskite growth material to the template and, as the
solvent evaporates, the material grows and forms a tiny crystal in each well.
The researchers found that the shape of the wells plays a critical role in controlling
the nanocrystal positioning.
If square wells are used, due to the influence of nanoscale forces, the crystals have an
equal chance of being placed in each of the well’s four corners.
For some applications, that might be good enough, but for others, it is necessary to
have a higher precision in the nanocrystal placement.
By changing the shape of the well, the researchers were able to engineer these nanoscale forces
in such a way that a crystal is preferentially placed in the desired location.
As the solvent evaporates inside the well, the nanocrystal experiences a pressure gradient
that creates a directional force, with the exact direction being determined using the
well’s asymmetric shape.
This allows the researchers to have very high precision, not only in growth, but also in
the placement of these nanocrystals.
They also found they could control the size of the crystal that forms inside a well.
Changing the size of the wells to allow more or less growth solution inside generates larger
or smaller crystals.
They demonstrated the effectiveness of their technique by fabricating precise arrays of
nanoLEDs.
In this approach, each nanocrystal is made into a nanopixel which emits light.
These high-density nanoLED arrays could be used for on-chip optical communication and
computing, quantum light sources, microscopy, and high-resolution displays for augmented
and virtual reality applications.
In the future, the researchers want to explore more potential applications for these tiny
light sources.
They also want to test the limits of how small these devices can be, and work to effectively
incorporate them into quantum systems.
Beyond nanoscale light sources, the process also opens up other opportunities for developing
halide perovskite-based on-chip nanodevices.
Their technique also provides an easier way for researchers to study materials at the
individual nanocrystal level, which they hope will inspire others to conduct additional
studies on these and other unique materials.