Improved characteristics of near-band-edge and deep-level emissions from ZnO nanorod arrays by atomic-layer-deposited Al₂O₃ and ZnO shell layers

Optical properties changes in core-shell nanowires result from "annealing" in ALD reactor

Allan Thomas   (2012-01-10 09:40)  University of Arkansas - Little Rock

We have also recently published similar work along the lines of this publication, and based on our results and understanding, suggest the authors here may be misinterpreting their results.

See these links for our article if desired: URL: http://link.aip.org/link/?JVA/30/01A116, DOI: 10.1116/1.3660389

The authors in this work find that both alumina and ZnO shells can increase the near band edge (NBE) intensity in the photoluminescence (PL) of ZnO nanowires. They find that this improvement tends to scale with the shell thickness, meaning thicker shells lead to a further increase in the improved NBE intensity.

We feel these actual PL results are not incorrect, but rather attributed to the wrong underlying mechanism.

Note that the ZnO nanowires in this work were deposited by a low temperature hydrothermal process in aqueous solution at 95 degrees C. Then, the as-grown nanowires were coated with ALD shells of alumina or ZnO at a process temperature of 180 degrees C.

The authors indicate that the uncoated, as-grown ZnO nanowires as well as the ALD coated core-shells were then compared with respect to their PL properties. This is likely not a correct comparison because during the ALD shell deposition process, the ZnO nanowires (grown at 95 degrees C) will be in the ALD reactor at a much higher temperature of 180 degrees C, and in a nitrogen ambient environment at relatively moderate vacuum conditions.

Such conditions will almost undoubtedly improve the optical properties of the low temperature grown ZnO nanowires, even without a separate shell layer being deposited on top of them. See our recent paper mentioned above for a comparison of the effects of moderate annealing on the PL properties of solution-grown ZnO nanowires.

We find that, due to their very low temperature growth, the optical properties of the ZnO nanowires are significantly improved by moderate annealing, even in air. Such results should be similar for the conditions in the ALD reactor and possibly even better due to the absence of oxygen.

In our study mentioned above, we made sure to make an appropriate reference sample that did NOT obtain an ALD coating, but nonetheless was placed in the ALD reactor so that it was also "annealed" like the nanowire samples that DID obtain a shell coating.

This type of reference sample allows for an appropriate comparison to see the effects of the ALD shell layers ALONE, not just the effects of the annealing that occurs during the ALD deposition.

Most likely, the results in this work are due to the effects of annealing not the ALD shell material. The reason the thickness of the ALD shell appears to cause further optical properties changes is related to the "annealing" time - thicker ALD shells require a longer deposition time, therefore the annealing effects will be further shown.

This is also likely the reason both alumina and ZnO shells created similar results in this work - the shell material is actually not responsible but rather the time in the ALD reactor is about the same so the annealing effects are similar.

Note that in our study we did NOT observe any enhancement in the PL of the as-deposited core-shells, rather only after post-growth annealing of the core-shells do we see effects due to the various shell materials. These effects are indeed associated with the shells alone.

Please note that we can only comment on this article based on our experience and similar research into this area. Also, we are basing these comments on the material available in the paper, assuming the authors did not leave out any pertinent details of their experimental methods.

Competing interests

The only competing interests we may have are associated with the fact that we also recently published a very similar study in another journal related to this work. Still, we are not suggesting the results in this article are specifically incorrect, rather the underlying mechanism is overlooked and given a likely incorrect interpretation.

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