Molybdenum Oxide Functional Passivation of Aluminum Dimers for Enhancing Optical-Field and Environmental Stability

Round 1

Reviewer 1 Report

In this paper, “Molybdenum oxide functional passivation of aluminum dimers for enhancing optical field and environmental stability”, the authors present an experimental and numerical study on the coating of Al plasmonic nanostructures through a conformal layer of high refractive index molybdenum oxide. Based on the obtained results, the authors claimed that the outcomes can be useful for various applications, including but not limited to: sensing, catalysis, etc. Overall, this manuscript has a strong potential for another review round after applying the issues and addressing the shortcomings listed below:

1-The authors should polish/revise some grammatical mistakes and typos along the manuscript. I invite the authors to read their manuscript carefully and make the required changes where necessary.

2-In the Introduction section, while discussing recent developments in the field of plasmonic NPs, the following works should also be considered and cited to give a more general view to the possible readers of the work: [(i) Monolithic metal dimer-on-film structure: new plasmonic properties introduced by the underlying metal, Nano Letters 20, 2087-2093 (2020); (ii) Gold nano-island platforms for localized surface plasmon resonance sensing: a short review, Molecules 25, 4661 (2020)].

3-For the FDTD calculations, what was the boundary condition setting(s) of the simulation environment? Please explain.

4-Please provide H-field enhancement version of Figure 2 (just for the review purposes).

5-Please increase size of the graphs in Figure 4.

 

 

 

Author Response

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Reviewer 2 Report

The authors investigated the effects on coating Al plasmonic nanostructures with a conformal layer of high refractive index molybdenum oxide. They showed that with this thin coating, the gap within the Al dimers can be narrowed down to sub-5 nm, leading to large field enhancement and confinement. The authors provided sufficient details on the numerical and experimental processes and this manuscript is well organized. This idea is interesting and will find many potential applications since it can greatly reduce the cost of producing dimers with extremely narrow gaps for achieving large field enhancement. I believe the results are correct. However, the authors should address the following questions before this manuscript can be considered for publication.

 

1.     Since the near field enhancement lack experimental support, I suggest the authors to provide experimental support for the simulation on the extinction spectra instead. By providing good agreement between the simulated and measured extinction spectra, the authors are in better position to convince the authors that the numerically obtained field enhancements are reliable.

2.     Following the above question, it is better to add “measured” or “simulated” in the figure captions as well as in the main text in order to distinguish which results are obtained from measurement and which are from simulations.

3.     On the near-field intensity or enhancement, I suggest to use and plot |E/E0|^2 rather than the amplitude |E/E0|. Additionally, “(a.u.)” should be removed from the y label “Field Enhancement”. Can the authors also provide the side-views of the fields?

4.     On Fig. 3, can the authors describe also in another way? That is, what is the percentage of the narrowed gap compared with bare dimers on the field enhancement (in terms of |E/E0|^2)? I believe the smaller field enhancement is the cost of the low-cost fabrication strategy, and it can determine whether this strategy satisfies the requirements in many applications.

Author Response

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Reviewer 3 Report

Type of manuscript: Article

Title: Molybdenum Oxide Functional Passivation of Aluminum Dimers for

Enhancing Optical Field and Environmental Stability

Journal: Photonics

 

This manuscript reported the coating effects of high index dielectric material such as molybdenum oxide, on the aluminium based plasmonic dimers. Experimental and numerical studies were performed to investigate the EM field strength enhancement and spectral redshift of the extinction spectrum as the coating thickness increased for different polarizations of incident light. Authors also addressed potential application of controlled dielectric coating thickness for nano-gap engineering down to sub-5nm gap. In addition, the manuscript reported durability and stability of the plasmonic dimers enhanced by coating such oxide materials. I believe the manuscript delivers interesting results and discussion and it can be accepted for publication after a number of points are addressed as shown in the following:

1.      Authors needs to clarify how the nano-gap narrows as a result of thicker coating of high index dielectric such as MoOx.

2.      It would be much better to provide a comparison between plasmonic dimers with narrowed nano-gap and plasmonic structure with nano-gap filled with aluminum which may look like quasi-plasmonic nanorod, particularly in terms of an extinction spectrum. This is because of the following assumption: as the nano-gap narrows, the two NDs couple more strongly, resulting in red shift of extinction for longitudinal polarization. This narrowing makes the dimer towards nano-rod like structure where its longitudinal resonance shows redder resonance than the corresponding counterpart transverse resonance.

3.      Authors need to break down main texts into more number of paragraphs for easier reading.

4.      On the 4th line from the subheading of Introduction, authors need to use a different expression for “light-matter interaction” because it broadly covers photophysical phenomena including SERS.

Author Response

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Reviewer 4 Report

 

In their work, Lorenzo and co-workers present an experimental and numerical study on the coating of Al nanodimers through a conformal layer of molybdenum oxide. The effect of the thin coating is to help gap narrowing down to the sub-5 nm range, where large electromagnetic field enhancement and confinement can be achieved. I partially agree with the Authors that the solution represents an alternative to more complex and challenging lithographic approaches used to achieve sub-10 nm gap sizes. In fact, the gap is reduced, but this approach does not allow to have a real and clean gap between two metallic surfaces. Putting material (in this case an oxide) between the two structures does not really provide the same effect as reducing the gap size, since the field enhancement scales exponentially, and in this case it does not (blue and red curves in Figure 3). Nevertheless, I agree with the Authors that the presence of the oxide is beneficial for the long-term stability. The article is well written and can be suitable for publication in Photonics. Before giving a final opinion, I would like the Authors to address the following comments.

1.      A detailed analysis of the field confinement is absent. The Authors must quantify how much the field confinement is affected by narrowing the gap using the oxide and provide a curve showing that this value scales (or not) with the gap size.  This might also explain the broadening of the dimer resonance at large wavelengths by decreasing the gap size (I am referring to Figure 1b).

2.      The Authors must provide the side view of the dimers in the simulations of the near field. I expect that the oxide has been deposited everywhere, not just around the dimers. How does this affect the overall performance of the system?

3.      The Authors claim that their system can be used for many applications, such as sensing, surface-enhanced Raman spectroscopy and catalysis. Now, I do not expect them to perform SERS or catalysis processes. But they should provide at least a simulation showing how much the sensitivity of their system changes with the gap size and compare it with a bare Al dimer where the gap is changed “manually”, that is without the presence of the oxide.

4.      The authors should provide more details on how they have extracted the values of the oxides thickness by providing calibration curves. The wrong knowledge of this parameter can change the ellipsometry results extracting the refractive index.

5.      The introduction to the topic, which is very broad and has been extensively studied by many groups, is a bit poor. Some important references are missing, and the topic should be contextualized better in the introduction, providing more references on plasmonic dimers and their applications. Just few examples (but the Authors should look for more): J. Am. Chem. Soc. 2021, 143, 23, 8631–8638, J. Mater. Chem. C, 2018,6, 9607-9614, Nanoscale, 2016, 8, 10576-10581 and Opt. Express, 2016, 24, 5, 4801-4811.

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

In its current form, the revised manuscript is suitable for publication.

Reviewer 2 Report

I believe the authors have satisfactorily addressed the concerns raised by the reviewers.