A novel technique known as remote epitaxy has been developed to improve the production of crystalline films. This method involves inserting an intermediate layer, typically made of graphene or another material, between the substrate and the crystals during the growth process. After the epitaxy is completed, the entire assembly is treated with a chemical solution that dissolves the intermediate layer, allowing for the preservation of the crystalline film. While effective, this approach is costly, challenging to scale, and time-consuming. Researchers at MIT sought a more efficient solution by growing crystals directly on the substrate without the use of intermediate layers, aiming for a non-stick effect similar to that of a frying pan but at an atomic level.
Weakening the bonds
In their exploration, the researchers identified that the element preventing the crystalline films from adhering to the substrates was lead, not Teflon. During previous experiments on various films, the team discovered that a specific material, PMN-PT (lead magnesium niobate-lead titanate), could easily detach from the substrate while maintaining an atomically smooth surface.
The presence of lead atoms in PMN-PT played a crucial role by weakening the covalent bonds between the films and the substrates, which prevented electrons from transitioning through the interface of the two materials. According to researcher Zhang, “We just had to exert a bit of stress to induce a crack at the interface between the film and the substrate and we could realize the liftoff. Very simple—we could remove these films within a second.”
Beyond its non-stick properties, PMN-PT revealed additional capabilities, particularly its outstanding pyroelectric characteristics. Upon realizing the potential to produce and easily separate PMN-PT films, the researchers aimed to create a more advanced device: a cooling-free, far-infrared radiation detector. Zhang noted, “We were trying to achieve performance comparable with cooled detectors.”
The resulting detector was constructed from 100 individual PMN-PT films, each just 10 nanometers thick and approximately 60 square microns in size, strategically transferred onto a silicon chip. This assembly formed a 100-pixel infrared sensor. Subsequent tests showed that the sensor outperformed leading night vision systems and demonstrated sensitivity to radiation across the full infrared spectrum, in contrast to mercury cadmium telluride detectors, which only respond to a limited range of wavelengths.
