A High-Sensitivity SPR Sensor with Bimetal/Silicon/Two-Dimensional Material Structure: A Theoretical Analysis

Round 1

Reviewer 1 Report

The paper entitled  “A high-sensitivity SPR sensor with bimetal/silicon/two-dimen-2 sional material structure” by Tang et Al. could represent a step forward in the realization and study about SPR sensors. 

I read carefully the work and I suggest the following modifications/ improvements in the paper. The following comment can increase the overall paper quality and could offer to the reader an interesting point of view about the sensors realization.

1) The introduction suffers of an incomplete introduction about other recent results in realizing sensors, in the best of my knowledge, I suggest, in line 45, to introduce the possibility to use metamaterials and metastructures placed on them to excite SPP and BPP modes as reported by Strangi et all. in the following review  doi.org/10.1515/nanoph-2020-0466

 

2) Also here, line 48, this configuration is largely used in the Strangi's work, indeed you can find papers referred to this in the same review named above. 

 

3) At the end of the introduction I think that could be a missed opportunity to not cite a recent work of Caputo et Al. where they demonstrate a possibility to fabricate all dielectric metastructures with high resolution. On the best of my knowledge it sould be an incredible step forward for the realization of dielectric coupler able to excite SPP and BPP in metamaterials structures like yours. I wormly suggest you to discuss this great opportunity in this work.  Paper “10.1002/adma.202008644”

 

4) Figure 1, it is convinient to report the size of the illustred simulation, I mean about the size of the unit cell. 

 

5) Captions of figure 2, figure 3, figure 4a and 4c and figure 6 need to report the wavelenght used for this analysis. I know that the wavelenght is defined in line 57 but it is too far to the figure, and the reader prefers to read it close to the picture.

 

6) The work seems initially an experimental work, but then, reading it, I discovered that it is just a numerical work. In this case I prefer to read just in the abstract or in the introduction that it is a numerical study, for example you can use here and there “a numerical implementation devoted to design and optimize a novel SPR sensors” or sentences similar to this one. Therefore, I kindly ask you use where it is possible the word “numerical” just to avoid misunderstanding in the reader mind.   

6) One more thing, I would like to see a spectral graph (wavelength from 400 nm to 900 nm) where the best case reports the entire reflectance curve for the water refractive index (1.333) and the changed one (1.333 + Del-tan).  After that, you can calculate the sensitivity as S=Deltan/DeltaLambda, where the DeltaLambda is like your DeltaTheta. After that, you can compare if your system is more sensible in terms of angle or spectral sensitivity. This evaluation offers an incredible stage for your work that can show how to think a new class of bio-sensors that go forward to the classical vision that are currently considered. 

Author Response

Thank you for your comments. We have responded to these suggestions one by one in the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

in this paper the authors illustrate the design of a SPR sensor based on Au/Ag/Si/2D-material. The optimization of the multilayer stack and the choice among different 2D materials enable an improvement in the sensor performances.

The topic discussed here, thin film SPR sensors, has been extensively investigated during the last 2 decades and a large number of papers have been published both on theoretical and experimental aspects. This manuscript is limited to simulations on multi-materials design and even if the obtained performances are not bad I cannot significant advancement with respect to the state-of-the-art.

Anyway, the manuscript could be a good technical paper once improved and corrected on some aspects.

1.considering that the paper discusses on a so extensive topic a better introduction is needed. the authors cited only 27 papers and this does not allow to have a clear view of the current state-of-the-art on plasmonic sensors. the choice to cite review papers 20 years old is rather unfortunate. Alternative better choices are:

DOI: 10.1016/B978-0-323-49778-7.00006-0

DOI: 10.1007/s00216-012-6624-0

doi.org/10.1515/nanoph-2017-0022

doi.org/10.1063/1.4959982

moreover the authors should mention the best performing plasmonic sensors reported in literature

(doi: 10.1038/nmat4609;

DOI:doi.org/10.1038/nmat2546;

doi: 10.1039/c9nh00168a)

2.In the introduction the authors say "Compared with Ag, Au has better biological affinity, chemical stability and higher sensitivity [13,14]. Ag has a sharper resonance spectrum and 30 higher detection accuracy [15]," 

this sounds weird. Au and Ag do not have sensitivity or resonance, but an SPR sensor prepared with these materials can have.

3. figure 2 illustrates the simulated R for different configurations. looking at the figure Au50nm has the best performance, also with a R close to 0 (experimentally not possible, but desiderated considering the high contrast that can be achieved if a 0%R slightly changes). 

Then figure 3 reports some curves and it's clear that for thickness above 50nm the sensitivity does not change. anyway, high thickness means low transmission. the choice of 50nm seems to be not well justified. can it be 60nm ? or 45nm? or 55nm? what does it change?

4. the use of Si to better confine the field is not new, here the authors report on some optimization and they show that 6 nm of Si is the best choice, anyway looking at figure 5 I cannot see a significant improvement in Au/Ag/Si with respect to Ag/Si.

5. graphene, MXene and MoS2  are then tested as "cap-layer" to improve even more the sensitivity. being high-index materials I would like to know what happen if they are used without Si. the high index in 2D materials should enable to confine the field and maybe the performaces can be comparable. finally the authors claim an improvement in performance by a factor of 2 with respect to similar platforms but it should be interesting to know the improvement with respect to standard SPR sensors (both with prism or with grating coupling). for example in doi.org/10.1364/OE.19.013164  the authors demonstrated a factor of 4 in sensitivity improvement with respect to a Au SPR sensors just by using porous metal as alternative material.

Author Response

Thank you for your comments. We have responded to these suggestions one by one in the attachment. 

Author Response File: Author Response.docx

Reviewer 3 Report

P. Zhang et al. numerically have investigated a multilayer Au/Ag/Si/2D-material SPR sensor and show that such architecture might exhibit enhanced sensitivity in comparison to traditional SPR bimetallic sensor.

Although similar architectures have been extensively investigated in the literature the simple approach based on the inversion of the typical Ag/Au bilayer brings a substantial improvement in the sensitivity of the sensor, thus in my opinion the study might be valuable for experts in the field if the results are backed up by a proper analysis.

The presented results concerning the Au/Ag/Si architecture are convincing and properly presented, however, the part referring to the 2D material seems to be only partial and incomplete.

The authors show that an increase in the number of graphene layers from 1 to 2 improves the sensitivity of the sensor at the optimal Si thickness. Why authors do not investigate a higher number of graphene layers at the same conditions when lowering the thickness of Si leads to a decrease in the sensor performance.

Authors should conduct a similar analysis for the considered 2-D materials showing how the sensitivity of the sensor depends on the number of layers of the additional material.

Adding lossy material into the SPR system usually results in the decrease of FOM of the sensor. Authors should also include this parameter in the final discussion of the achieved results.

Author Response

Thank you for your comments. We have responded to these suggestions one by one in the attachment. 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Dear Authors, 

I appreciate the changes that you performed in the main text.

I am extremely confident that the section entitled " Sensing characteristics in wavelength interrogation " furnished an important point of view on sensing exploiting the well-know resonant shift approach that is generally used/saw in literature. 

I found that other reviewers suggested other "hot" points that increase the paper overall merit. 

I support the publication of this work on MDPI Photonics.

Reviewer 2 Report

the authors have significantly improved the manuscript

I recommend the publication.

 

Reviewer 3 Report

The authors improved the manuscript significantly especially the part referring to the influence of 2D-materials on the multilayer SPR sensor performance. In my opinion, the work in the present form can be considered for publication in the Photonics journal. 

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.