NSEF Forum Award Presentation: Functionalization of Fine Particles By Atomic/Molecular Layer Deposition (ALD/MLD)

Authors: 
Weimer, A. W., University of Colorado at Boulder

The functionalization of fine primary particles, including nanoparticles, is easily carried out using sequential self-limiting surface reactions in a fluidized bed reactor.  The self-limiting reactions result in the deposition of atomic or molecular layers, i.e. ALD or MLD.  This functionalization process, referred to as Particle ALD/MLD, can be used to deposit conformal and pinhole-free films of refractory oxides, non-oxides, metals, and hybrid polymer-based materials, among others.  It is also possible to deposit conformal porous or semi-continuous films where partial coverage is preferred instead.  Fluidized bed reactors are well suited for large scale operations.  

In this process, the particles are normally fluidized under reduced pressure conditions using an inert gas, although atmospheric pressure operation can also be carried out.  Precursor doses can be delivered to the bed of particles sequentially and, in most cases, can be utilized at nearly 100% efficiency without precursor breakthrough and loss.   The progress of the coating process can be monitored continuously using an in-line downstream mass spectrometer.  The ability to use precursors with efficiencies approaching 100% opens the door for a unique opportunity to utilize precursors that previously might have been considered to be too expensive.  Fluidized beds containing particles comprising hundreds of thousands, even millions of m^2 of surface area can be coated efficiently.  The dose times depend on the amount of surface area required to be coated and the flow rate of the precursor into the fluidized bed.  Although dose times could potentially be very long for coating millions of m^2 of particles, i.e. minutes to hours, the efficient use of the precursor to place nearly perfect films on primary particles makes Particle ALD/MLD the low cost and potentially only process that can cost-effectively functionalize high surface area ultra-fine dry particles.

Particle ALD/MLD has been demonstrated to place nanofilms on primary nanoparticles as small as 10 nm as well as on nanotubes having surface areas approaching 1000 m2/g and within the porous structure of polymeric materials having porosity near 95%.  Physical, optical, electrical, and magnetic properties of the particles can be controlled in order to passivate, activate, or is some manner functionalize the particles.  Current applications of interest include passivation of phosphors and nanocrystals used in LEDs, improved cycling of Li-ion batteries, and high-activity sintering resistant catalysts among others.