Growth per cycle of alumina atomic layer deposition on nano- and micro-powders

Abstract

Core–shell powders consisting of a tungsten particle core and thin alumina shell have been synthesized using atomic layer deposition in a rotary reactor. Standard atomic layer deposition of trimethylaluminum/water at 150 °C utilizing a microdosing technique was performed on four different batches of powder with different average particle sizes. The particle size of the powders studied ranges from ∼25 to 1500 nm. The high mass-thickness contrast between alumina and tungsten in transmission electron microscopy images demonstrates that the particle core/shell interface is abrupt. This allows for the uncomplicated measurement of alumina thickness and therefore the accurate determination of growth per cycle. In agreement with prior works, the highest growth per cycle of ∼2 Å/cycle occurred on the batch of powder with the smallest average particle size and the growth per cycle decreased with increasing average particle size of a powder batch. However, the growth per cycle of the alumina film on an individual particle in a batch is shown to be independent of the size of an individual particle, and therefore, a powder batch which consists of particles size spanning orders of magnitude has constant shell thickness on all particles. This uniformity of thickness on different particle sizes in a particular batch is determined to be due to the difficulty of removing residual water molecules from the powder during the purging cycle of the atomic layer deposition (ALD) process. Therefore, rotary ALD on a single batch of powder with wide particle size distribution provides the same shell thickness regardless of individual particle size, which may have positive implications for particle ALD applications where the shell thickness determines critical parameters, such as particle passivation and manipulation of optical properties.