Fabrication of Low Electrical Percolation Threshold Multi-Walled Carbon Nanotube Sensors Using Magnetic Patterning

Round 1

Reviewer 1 Report

I tried to evaluate the manuscript in terms of originality, novelty and language appropriateness. The manuscript was academically well written and the materials were properly demonstrated. The used language was quite well and it can be published after clarifying the following comments:

1- The authors claim that they placed PDMS/MWCNT mixtures in acrylic molds, on top of the magnetic patterns for 15 hours. However, as the PDMS is waiting in RT, it begins to cure and the viscosity increases with time. Can the authors explain why they selected 15 hours and did they evaluate the effect of time on the patterning quality

2- Page 4, line 141 Is "Keythley" right?

Author Response

Reviewer #1

I tried to evaluate the manuscript in terms of originality, novelty and language appropriateness. The manuscript was academically well written and the materials were properly demonstrated. The used language was quite well and it can be published after clarifying the following comments:

• The authors claim that they placed PDMS/MWCNT mixtures in acrylic molds, on top of the magnetic patterns for 15 hours. However, as the PDMS is waiting in RT, it begins to cure and the viscosity increases with time. Can the authors explain why they selected 15 hours and did they evaluate the effect of time on the patterning quality

Dear Reviewer, we appreciate your time reading and providing feedback on our manuscript. The selected time was based on preliminary tests. According to the datasheet, it takes 48 hours for the PDMS to be completely cured, however, in the first stage, we do not need it to be completely cured. The objective was to ensure that the PDMS was solid enough to prevent the dislocation of the MWCNT particles in case the magnetic field was removed before the final curing process. The final curing process was done in an oven at 110 °C for 30 minutes without magnetic fields. The objective of the final curing was to make sure that the PDMS had good mechanical properties, without affecting the magnetic orientation of the particles. In conclusion, if the room temperature curing time is too short, the PDMS will be too fluid, and the particles will move after removing the magnetic field. If the curing process is too fast, there is not enough time for the particles to settle in the right position, affecting the patterning quality and increasing the electrical resistance of the electrodes. We added some comments regarding this in the revised version of the manuscript.

• Page 4, line 141 Is "Keythley" right? You are right; we made this correction.

Dear Reviewer, we are confident to have addressed all your concerns, and we would appreciate your feedback once more. If you believe we overlooked any issues, please let us know.

Our kind regards,

David Esteves

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript of Nelson Durães et al. studies multi-walled carbon nanotubes based sensors using different magnetic patternings. Although the manuscript is interesting, there are some weaknesses regarding the magnetic characterization and properties that must be clarified before considering the work for publication:

- Section 3.1 is repeated. Please, label the second section as section 3.2

- In section 3.1, line 3, authors affirm that "The SQUID analysis showed that the MWCNT have a ferroelectric behaviour at lower fields (<6000 Oe) and a diamagnetic behaviour at higher fields". Form the point of view of this reviewer, Figure 4 does not give that information. Firstly, there is not a "ferroelectric" behaviour there, but a ferromagnetic. And the diamagnetic behaviour can not start at 6000 Oe, as this is the limit of the magnetic field represented in the figure.

- Authors do not explain why the diamagnetic behaviour apperas in the MWCNT. Actually, the behaviour of the hysteresis loop is not described. Why that behaviour? Why at certain magnetic field, the magnetization start decreasing?

- Does Figure 4 (b) represent a MWCNT/PDMS composite with magnetic patterning or with random orientation? Why did you limit your measurements to the 5 wt.% composite? Why did you not measure the hysteresis loop for a higher range of MWCNT composites?

- Why Figure 4 (b) has a stronger diamagnetic behaviour than Figure 4 (a)

- In section 3.1, line 5, authors affirm that "The measured coercivity (Hc), retentivity (Mr) and saturation magnetisation (Ms) of the MWCNT particles at 300 K were 113.48 Oe, 0.1318 emu/g, and 0.9361 emu/g, respectively". As the Figure 4 is represented, it does not seem that there is any hysteresis. A zoom of Figure 4 (a) represented in an inset would help to determine those values from the loop.

- I highly recommend to revise the english of the manuscript

As conclusions, authors must rewrite section 3.1 with the mentioned corretions before considering for publication.

Author Response

Reviewer #2:

The manuscript of Nelson Durães et al. studies multi-walled carbon nanotubes based sensors using different magnetic patternings. Although the manuscript is interesting, there are some weaknesses regarding the magnetic characterization and properties that must be clarified before considering the work for publication:

- Section 3.1 is repeated. Please, label the second section as section 3.2

 Dear Reviewer, we appreciate your time and patience in reading and analysing our manuscript. Thanks for this observation we made this correction.

- In section 3.1, line 3, authors affirm that "The SQUID analysis showed that the MWCNT have a ferroelectric behaviour at lower fields (<6000 Oe) and a diamagnetic behaviour at higher fields". Form the point of view of this reviewer, Figure 4 does not give that information. Firstly, there is not a "ferroelectric" behaviour there, but a ferromagnetic. And the diamagnetic behaviour can not start at 6000 Oe, as this is the limit of the magnetic field represented in the figure.

We added a zoomed image for each sample to make it easier to analyse the magnetization curve. We changed figure 4, rearranged the section, and now it is figure 5.

Analysing the magnetisation curve (Figure 5) it is noticeable that ferromagnetism dominates at lower fields (between -6000 and 6000 Oe) and diamagnetism dominates at higher fields (between 6000 and 60000). Even though diamagnetism also affects the sample at lower fields, ferromagnetism dominates until it saturates. Then at fields between 6000 and 50000 Oe the diamagnetic property dominates. Not because it is more intense at higher fields but because magnetic saturation is achieved, while the diamagnetic effect keeps changing linearly with the magnetic field.

- Authors do not explain why the diamagnetic behaviour appears in the MWCNT. Actually, the behaviour of the hysteresis loop is not described. Why that behaviour? Why at certain magnetic field, the magnetization starts decreasing?

From our understanding, the MWCNT particles and the nanocomposite present both ferromagnetism and diamagnetism. However, after the ferromagnetic saturation (between 6000 and 60000 Oe) the diamagnetism keeps changing linearly with the field. The diamagnetic property could be associated with the different materials that were identified by SEM-EDS, it could be due to the magnetic properties of MWCNT, or the catalysts (alumina or Al4C3). The ferromagnetic response could be related to the presence of iron, which is a ferromagnetic material that was also identified by SEM-EDS. Accordingly, with the bibliography presented in the manuscript, a similar behaviour was presented for the MWCNT. The authors concluded that the diamagnetism was related to the multi-walled nanotubes, and the ferromagnetism was due to the presence of iron nanoparticles derived from the fabrication process.  In another example, it was shown that the MWCNT together with the catalyst materials presented a dominant ferromagnetic behaviour, and the catalyst materials were diamagnetic. We added some comments regarding this in the revised version of the manuscript.

- Does Figure 4 (b) represent a MWCNT/PDMS composite with magnetic patterning or with random orientation? Why did you limit your measurements to the 5 wt.% composite? Why did you not measure the hysteresis loop for a higher range of MWCNT composites?

Figure 5 c) and d) (before it was figure 4 b)), show the SQUID analysis for a random distribution of 5 wt%. MWCNT/PDMS. From our tests, we concluded that at 5 wt.% the electrical resistance is similar for both samples with and without magnetic orientation. At this weight percentage, the sample has too much MWCNT, and it is impossible to see any magnetic patterning on the sample. We assume that at higher concentrations the magnetic patterning stops working because the conductive path is already formed even without any applied magnetic field. Due to this reason, we stopped increasing the MWCNT concentration, and therefore we did not test samples above 5 wt%. The reason why we just tested the 5 wt%. composite in SQUID, was because it was the sample with the most weight percentage of MWCNT, and we wanted to understand if the PDMS could have any impact on the magnetic properties when compared with the MWCNT particles. From the measurements, we were able to understand that, as expected, the ferromagnetism is smaller for the composite due to the smaller amount of MWCNT, however, the behaviour is relatable with the particles. The sample that was used had the MWCNT randomly distributed, to be comparable with the randomly distributed MWCNT particles. This topic was already changed and clarified in the revised version of the manuscript.

- Why Figure 4 (b) has a stronger diamagnetic behaviour than Figure 4 (a)

It is indeed noticeable that the composite has a stronger diamagnetic behaviour than the MWCNT particles. This increase in the diamagnetic behaviour could be associated with the PDMS matrix. In the work of Hamidi et al., it was possible to observe a similar behaviour. When the authors tested samples with and without the PDMS matrix, a dominant diamagnetism was also present for higher fields, after the ferromagnetic saturation of the particles. We also added some comments regarding this in the revised version of the manuscript.

- In section 3.1, line 5, authors affirm that "The measured coercivity (Hc), retentivity (Mr) and saturation magnetisation (Ms) of the MWCNT particles at 300 K were 113.48 Oe, 0.1318 emu/g, and 0.9361 emu/g, respectively". As the Figure 4 is represented, it does not seem that there is any hysteresis. A zoom of Figure 4 (a) represented in an inset would help to determine those values from the loop.

We changed Figure 4, which is now figure 5, and we added a zoom with the dominant ferromagnetic effect for each sample.

- I highly recommend to revise the english of the manuscript

We took your advice, and the English was revised.

As conclusions, authors must rewrite section 3.1 with the mentioned corretions before considering for publication.

Dear Reviewer, to conclude, section 3.1 was rewritten according to our corrections. We are confident to have addressed all your concerns, and we would appreciate your feedback once more. If you believe we overlooked any issues, please let us know.

Our kind regards,

David Esteves

Author Response File: Author Response.docx

Reviewer 3 Report

This manuscript reports a new low-cost fabrication technique for developing low electrical percolation threshold sensors based on magnetically oriented MWCNT/PDMS nanocomposites. The research design is appropriate and the data is very informative. The manuscript is well written except for a few text errors. For example, the texts in Line 292 and Line 325 are same, and it seems that it should be “triboelectric sensors” in Line 325. Therefore,  I suggest to accept after minor revision.

Author Response

Reviewer #3:

This manuscript reports a new low-cost fabrication technique for developing low electrical percolation threshold sensors based on magnetically oriented MWCNT/PDMS nanocomposites. The research design is appropriate and the data is very informative. The manuscript is well written except for a few text errors. For example, the texts in Line 292 and Line 325 are same, and it seems that it should be “triboelectric sensors” in Line 325. Therefore, I suggest to accept after minor revision.

Dear Reviewer, we are confident to have addressed all your concerns (text errors have been corrected), and we would appreciate your feedback once more. If you believe we overlooked any issues, please let us know.

Our kind regards,

David Esteves

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Authors have properly amended the manuscript. I can now recommend for its publication