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Peer-Review Record

Low-Temperature Vapor-Solid Growth of ZnO Nanowhiskers for Electron Field Emission

Coatings 2019, 9(11), 698; https://doi.org/10.3390/coatings9110698
Reviewer 1: Anonymous
Reviewer 2: Anonymous
Reviewer 3: Anonymous
Coatings 2019, 9(11), 698; https://doi.org/10.3390/coatings9110698
Received: 23 August 2019 / Revised: 1 October 2019 / Accepted: 22 October 2019 / Published: 25 October 2019
(This article belongs to the Special Issue 1D, 2D and 3D ZnO: Synthesis, Characterization and Applications)

Round 1

Reviewer 1 Report

It is a research work on the optimization of the synthesis of the ZnO nanowires to obtain such with the highest aspect ratio. I have several comments that should be addressed:

  1. L. 81. „at least 10 nanowires”. Measuring only 10 or more (but how many in reality) nanowires, the authors assume that these nanostructures are monodispersive and exhibit a constant aspect ratio. What about the average dimensions and the aspect ratio assessed in case of 100 nanowires? Are they the same as those given in the text?
  2. L. 120. “up to 100 nm”. It would be better to give a range or the average value along with the standard deviation (and the number of the nanowires taken for the calculations).
  3. L. 131. The aspect ratio should be given.
  4. L. 132. The aspect ratios reported in refs. 18, 22-24 should also be given.
  5. L. 146. Why was the temperature of 550 degrees was taken instead of 580 degrees, which was recognized in the previous paragraph to give the highest aspect ratio?
  6. L. 165. This paragraph should be illustrated by the appropriate dimensions of the nanowires that were changed due to the changes of the experimental conditions.
  7. L. 184-186, 191, 196, 233-234, 236, 265-267. The number of the nanostructures taken for the calculation of their average dimensions should be given.
  8. L. 254 and the related text. The authors found the experimental conditions that provided the synthesis of the ZnO nanowires of the highest aspect ratio. However, it was not mentioned by them that they also obtained the nanostructures having one of the highest lengths, i.e., 3000-4900 nm, and diameters, i.e., 340-410 nm. Were such high lengths and diameters of these structures appropriate considering the FE properties? Can these wires (with such relatively high lengths and diameters) still be called nanowires?

Author Response

We thank the referee for carefully reviewing our manuscript.

In fact, we had a lot of dimension data  in a previous draft of our manuscript. However, during internal review, we decided to remove these number to allow for a more easy readability. Anyhow, the referee is totally right to ask for the dimensions and aspect ratios determined. To keep the reading flow, we added a long table (Table 1) in the Supplemental Materials which presents the different process parameters and which dimension have been extracted from the SEM images. We believe that this table is a good compromise between presenting all facts/numbers versus the text flow. 

Please find attached a point-by-point reply to the comments

  1. 81. „at least 10 nanowires”. Measuring only 10 or more (but how many in reality) nanowires, the authors assume that these nanostructures are monodispersive and exhibit a constant aspect ratio. What about the average dimensions and the aspect ratio assessed in case of 100 nanowires? Are they the same as those given in the text?

Indeed, this is a justified question. We analyzed 57 nanowires (sample of Fig 7c which were later tested for electron field emission properties) and compared that values to the previously measured series (Table SI1 highlighted line N=57 and line before N=17). We believe that our initial selection of N=17 represents in general the polydispersity of the nanowire quite well. It was never our aim to hide the polydispersity; thus, our selection for at least 10 nanowires was done in a way that all sizes and length were mirrored.

  1. 120. “up to 100 nm”. It would be better to give a range or the average value along with the standard deviation (and the number of the nanowires taken for the calculations).

We changed the sentence to the following: At temperatures above 650 °C (Figure 3e), a rough, polycrystalline film of ZnO is formed again in which only isolated, individual nanowires with length and diameters of 78 ± 28 nm and 86 ± 31 nm and visible surface facets developed.

The number of measured nanowire is given in Table S1.

  1. 131. The aspect ratio should be given.
  2. 132. The aspect ratios reported in refs. 18, 22-24 should also be given.

We believe that mentioning the aspect ratios at that point could cause some irritation by a reader because at that point only the morphology as a function of growth temperature is discussed. However, the referee is right that we should discuss our results more in detail to the literature. We decided to add an additional paragraph in the intermediated summary section (3.6):

Nanowires can be grown in optimized conditions with aspect ratios from 7.7 to 16.0 depending on the substrate type. It was previously reported in the literature that nanowires with similar aspect ratios of 4.4 [23], 50.0 [22], and 67.0 [21] have been fabricated by using zinc acetylacetonate and oxygen as precursors. As already discussed above, different experimental setups as well as substrate types and higher gas flows might have caused a slightly different growth behavior and about that nanowires with increased dimensions and aspect ratios, compared to the structures produced in this work, could be fabricated.

(Please note by introducing additional references and moving paragraphs within the manuscript, the number of ordering is different in the revised manuscript).

  1. 146. Why was the temperature of 550 degrees was taken instead of 580 degrees, which was recognized in the previous paragraph to give the highest aspect ratio?

Thank you very much pointing this out. In fact, we initially did the substrate test (at being slightly off the optimal conditions) first and then optimized in finer steps the optimal conditions. We now changed the paragraph first substrate position and then temperature to keep a logical order.

  1. 165. This paragraph should be illustrated by the appropriate dimensions of the nanowires that were changed due to the changes of the experimental conditions.

We elaborated a bit more on the change of the surface tips:

The tip angles decreased from 90.0 ± 13.5° (25 sccm O2 flow) to 72.5 ± 7.7° and 65.8 ± 12.4° by increasing the oxygen flow to 50 sccm and 83 sccm, respectively.

  1. 184-186, 191, 196, 233-234, 236, 265-267. The number of the nanostructures taken for the calculation of their average dimensions should be given.

Now all given in Table S1

  1. 254 and the related text. The authors found the experimental conditions that provided the synthesis of the ZnO nanowires of the highest aspect ratio. However, it was not mentioned by them that they also obtained the nanostructures having one of the highest lengths, i.e., 3000-4900 nm, and diameters, i.e., 340-410 nm.

Thank you very much for this comment. We now clearly state with in section 3.4:

The longest nanowires achieved in our setup within 20 h growth and an initial precursor amount of 3.5 g reached a length of 8.216 ± 1.34 µm  and a diameter of 721 ± 81 nm leading to an aspect ratio of 11.4 (Figure SI2). 

Were such high lengths and diameters of these structures appropriate considering the FE properties?

Another really helpful comment by the referee. For non-experts of electron field emission our motivation why exactly using this nanowires could be somehow unclear. By introducing the following sentence in section 3.6:

These nanowire devices are promising candidates for subsequent electron field emission measurements due to their sharp tips and their aspect ratio of about 10 leading to a local electrical field enhancement. 

we hope we could state clearly and directly why we are favouring these kind of structures.

Can these wires (with such relatively high lengths and diameters) still be called nanowires?

This is a rather good question about definition and nomenclature of nanostructures. We just use the standard conventions as also used in other fields and publications. Here just a selection of citations from other publications which show that this size range is still be seen as "nano" within the community.

CrystEngComm, 2010,12, 2675–2678 (DOI: 10.1039/c005318j)
"As-obtained individualnanowires (Fig. 1C) maintain an average width of 385 106 nm withlengths of up to several microns."
"his protocol yields smooth, monodisperse nanorodswith average diameters of800 nm and lengths of up to 3 microns."

Materials 2015, 8, 4565-4581; (DOI:10.3390/ma8074565)
"The maximum achievable short-circuit current density (Jsc) was estimated, which is shown in Figure 7, for 400-nm-diameter Si NW arrays with various periods (400–800 nm)."

Phys. Status Solidi C 6, No. 9, 2053–2055 (2009) (DOI: 10.1002/pssc.200881758)
"The lengths of the examined nanowires were between 5 µm and 15 µm, while their diameters varied from 100nm nm to 400 nm."

Scientific Reportsvolume 7, Article number: 3011 (2017) (DOI: 10.1038/s41598-017-03588-x)
"The Young’s moduli of the SiC NWs with a wide range of NW diameters (215–400 nm) were measured"

Scientific Reports volume 6, Article number: 39389 (2016) (DOI: 10.1038/srep39389)
"The LBO nanowires possess diameters ranging from 200 to 800 nm and lengths of up to 200 μm"

 

Reviewer 2 Report


The manuscript discusses the growth of ZnO nanowires from a zinc acetylacetonate hydrate powder and oxygen gas. The study is performed while varying temperature, oxygen flowrate and location of substrate. Then various substrates are explored.

This is well organized study and well written manuscript that is well suited for publication in Coatings. I like the fact that the findings are discussed in light of earlier works and and other approaches to grow ZnO nanowires.

I have only a few minor comments on the manuscript:

  1. i) Use Pa instead of mbar for pressure unit.
  2. ii) Maybe a schematic of the boat and the substrate position and maybe all of the heating zones is helpful. This should then appear in Section 2.1. This is particularly helpful for understanding the study shown in Figure 2.

iii) Remove "of them" in line 29

 

Author Response

We thank the reviewer for her/his positive comments.

Please find below a point-by-point answering of all comments:

  1. i) Use Pa instead of mbar for pressure unit.

Thank you very much. In fact, we should use SI units and change.

  1. ii) Maybe a schematic of the boat and the substrate position and maybe all of the heating zones is helpful. This should then appear in Section 2.1. This is particularly helpful for understanding the study shown in Figure 2.

Thank you very much for this advice. We implemented an additional figure (now Figure 1) which includes a schmese of the vapor-solid growth as well as a schematic of the used oven and the precursor and substrate boats.

iii) Remove "of them" in line 29

done

Reviewer 3 Report

This work by Zierold et al. presents the research about the low temperature vapor-solid growth of ZnO nanowires in a three-zone tube furnace by utilizing zinc acetylacetonate hydrate and oxygen as precursors for electron field emission applications. They presented a comprehensive study of ZnO nanowire array growth as a function of the process parameters - temperature, substrate type and position, gas flow as well as zinc precursor amount and growth time. The manuscript is well written overall, and includes sufficient experimental results. Therefore, I recommend that the manuscript be considered for the publication after minor revision in “Coatings” which is understood as a technical journal devoted to the science and engineering of coatings, thin and thick films, surfaces and interfaces.

The following issues need to be addressed before the final publication.

In the caption, ‘/’ is used to indicate unit of the parameter. However, the ‘/’ symbol is confused with the general divide symbol. This makes it difficult for readers to understand the name of each axis in Figure 8, particularly. It is recommended that the symbol ‘/’ used for unit be replaced by ‘[ ]’in every Figure. Figure 1: No indication is found for (f) in the caption. The decimal point on the axis of every graph should be represented as ‘.’ rather than ‘,’ . The SEM image in Figure 4 are listed in the order of the amount of zinc precursor, except for (f) and (g). It is recommend the position of (f) and (g) be switched each other. One of the popular synthesis routes for ZnO nanowires is the hydrothermal growth method (“Angew. Chem. Int. Ed. 2003, 42, 3031–3034”, “Nano Lett. 2011, 11, 2, 666-671”, “Langmuir, 2012, 28, 10, 4787-4792”), by which ZnO nanowires can be synthesized at much lower temperature (<100’C) and more easily. The authors need to emphasize what are the benefits or advantages of the suggested vapor-solid synthesis method. The authors also need to mention what are the advantages of using zinc acetylacetonate hydrate as Zn source over using any other precursors.

Author Response

We thank the referee for evaluating our manuscript in such a positive way. We are grateful for the comments which improved our manuscript further:

Please find a detailed point-by-point reply below:

In the caption, ‘/’ is used to indicate unit of the parameter. However, the ‘/’ symbol is confused with the general divide symbol. This makes it difficult for readers to understand the name of each axis in Figure 8, particularly. It is recommended that the symbol ‘/’ used for unit be replaced by ‘[ ]’in every Figure.

Personally we prefer the "/" to indicate the unit of parameter. However, we totally agree with the referee that this might be misleading in Fig 8 (new Figure 9). To be consistent we replaced "/" by "[]" in every figure.

Figure 1: No indication is found for (f) in the caption.

We fixed that. Please note, due to some rearrangements and a new figure, this now Figure 3.

The decimal point on the axis of every graph should be represented as ‘.’ rather than ‘,’ .

Thank you -done!

The SEM image in Figure 4 are listed in the order of the amount of zinc precursor, except for (f) and (g). It is recommend the position of (f) and (g) be switched each other.

Good point - we switched the subfigures and the corresponding text links.

One of the popular synthesis routes for ZnO nanowires is the hydrothermal growth method (“Angew. Chem. Int. Ed. 2003, 42, 3031–3034”, “Nano Lett. 2011, 11, 2, 666-671”, “Langmuir, 2012, 28, 10, 4787-4792”), by which ZnO nanowires can be synthesized at much lower temperature (<100’C) and more easily. The authors need to emphasize what are the benefits or advantages of the suggested vapor-solid synthesis method.

We just focused on vapor-transport method discussion but the referee is totally right. We thus rearranged the introduction part a bit and introduced an additional paragraph:

Although ZnO nanowire fabrication is possible at temperatures below 100 °C and without catalyst by hydrothermal synthesis, there are several advantages for using vapor-solid growth, such as a higher purity of the nanowires, a single step process with only two precursor compounds (zinc precursor powder and oxygen gas), and the formation of sharp tips which are beneficial for the electron field emission application.

An additional argument (not mentioned in the text) is the drying and rinsing process. Later we prepared these nanowires on freestanding membranes which could be potentially applied in MALDI-TOF mass spectrometry detectors. If we would use wet chemical synthesis routes, the additional drying & rinsing process is a potential risk/danger for breaking this supporting membrane.

The authors also need to mention what are the advantages of using zinc acetylacetonate hydrate as Zn source over using any other precursors.

To clarify that, we inserted an additional sentence:

The vaporization of zinc acetylacetonate occurs at much lower temperatures (75-135 °C) than the temperatures required for vaporizing Zn or ZnO powder (above 550 °C) [22,23]. 

Round 2

Reviewer 1 Report

I have to admit that the authors very well addressed all the comments raised at the first stage of the reviewing. In my opinion, all the changes made in the text of the revised version of their article and the replies to my questions and doubts are well suited. I recommend this article for the publication. Well done.  


 

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