Our work pertaining to the thickness-dependent (number of ALD cycles) bandgap modulation of TiO2 on silicon wafers was highlighted in an earlier article.
In this most recent development, we have demonstrated the same blue shift in bandgap for ZnO films deposited on spherical particles. The ability to deposit quantum-confined nanostructures on substrates ranging from flat surfaces to nanoparticles and nanotubes opens up a range of possibilities for device makers.
We were able to directly measure the film crystallinity with respect to film thickness, which gave us a better understanding of the crystal phase evolution behaviour in ALD films. We showed that particle ALD offers the ability to deposit controlled thickness films with very good control over the crystallite size, independent of line of sight, and on bulk quantities of particles at a time.
With the effective precursor utilization offered by operating ALD processes in a fluidized bed reactor, the cost to functionalize surfaces using ALD techniques goes way down. Even in large-scale batch reactors, the common stereotype that ALD processes are expensive does not necessarily apply. The next step, however, is the development of semi-continuous industrial-scale processes that leverage automated load/unload steps for high-throughput operation. We have increasingly heard that thick CVD-based films are often used for particle passivation/functionalization, when the true need is for a pinhole-free alternative.
Particle ALD offers the opportunity to deposit these non-porous ultra-thin films on particle surfaces, and also allows for the coating of much smaller particles. Particle ALD films have been successfully grown on both nanoparticle surfaces and conformally throughout the inner walls of nanoporous particles. This is a scalable technique that can deposit conformal, quantum-confined films on bulk quantities of particles without line-of-sight dependence. This work provides a key building block to enable the incorporation of quantum-based devices in a variety of industries.