Research on Tunable SPR Sensors Based on WS2 and Graphene Hybrid Nanosheets
Round 1
Reviewer 1 Report
This paper concerns SPR sensor based on WS2 and graphene hybrid nanosheets. I believe that the obtained results justify publication of the paper in Photonics. However, the authors need to revise the paper with respect to the comments.
1.
The authors show proposed model of the sensor in Fig. 1. The sensing medium is usually at the top of the sensor. However, the sensing medium is sandwiched between WS2/graphene. The authors need to state on how to use the sensor in detail.
2.
The authors show the relation between sensor sensitivity and number of ws2 layers in Fig. 6. When number of ws2 layer is 2 the sensitivity of the sensor reaches the muximum sensitivity. However, the readership would like to know the reason of this. Therefore, the authors need to state the reason why the sensitivity of the sensor is changed by number of ws2 layers.
Similarly, this is the same for graphene in Figure 7.
3.
The authors set thickness of sensing medium with 400 nm. As a sensor, it is better to be able to sense with a less of sensing medium. Did the authors consider how to reduce this size? The author should add an opinion on this matter.
4.
At the line 260, the authors refer Fig. 7 (c) and (d). This is mistake for Fig. 7 (a) and (b). The authors need to reviced it.
5.
In Table 4, the name of the proposed sensor is missing graphene. The authors need to reviced it.
Author Response
Please see the attachment
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Recommendation: Publish after minor revision.
Comments:
The authors presented an in-depth study of applying WS2 and graphene as plasmonic layers to improve the sensitivity of constructed SPR sensors, as well as the dependency between layer and device sensitivity. Clear and plausible results were presented, and based on the overall merit of the manuscript, the reviewer would be happy to recommend it to publish in Photonics addressing just a few minor corrections and comments.
1. As this review article covers two far-related classes of materials for their use in chemical sensing applications, the current title could lead to a slight deviation from the main topic which is chemical sensing. Maybe changing the sequence of chemical sensing to the front could lead this manuscript to a more suitable audience, such as “A Comprehensive Review on Recent Advances and Future Challenges in the field of chemical sensing Using Ionic Liquids and Metal-Organic Frameworks”
2. The authors need to work on the English language: better expressions and typo corrections are needed in several places. (e.g., Figure 1, incidengce light; line 74, MoS2/MoSe2/WS2/Se2[20], should it be WSe2?; Table 4, materials composition of ref[28], Graphen was a typo; etc.)
3. The characterizations of both graphene and WS2 were missing. From the current form of the manuscript, the reviewer could not determine the morphology or quality of both 2D material layers. Although the authors reported the study of the device performance with respect to changing the number of layers of both 2D materials, it is important to know the quality of these materials (i.e., synthetic method, domain size, the extent of defects, etc.). Such factors are important for the application of 2D materials, as they could significantly influence the properties of 2D materials (e.g., bandgap, migration rate). Feasible characterizations include Raman for defects, AFM for morphology, and TEM for diffraction lattice. Such characterizations are highly recommended, as even commercially available 2D materials are not guaranteed high quality, therefore a further characterization is needed. The authors specified that the graphene and WS2 can be prepared by CVD (line 119), but CVD needs high temperature, which indicates the 2D materials are not directly grown on the Ag surface using CVD, it’ll be either plasma-enhanced CVD (PECVD) or transferred on Ag substrate after CVD growth. Either way, the material will have a significant number of defects, which need to be characterized to support the layer-dependent study conducted in this manuscript.
4. Please pay attention to descriptions and consistency. For example, in line 144, FWHM was defined as half-width height but was defined as half-height width in line 146.
5. What could be a plausible reason for 2 layers of WS2 outperforming single-layer WS2 in terms of sensitivity? What do you expect if you use another wavelength of incidence light? Does it significantly decrease the sensitivity? And does it make 2-layer WS2 no longer the optimal number of layers for the best sensitivity? My gut feeling is that 2-layer WS2 has a better suitable bandgap for the 633 nm incidence light, therefore if incidence light wavelength changes, the optimal number of layers should change as well. Providing absorption spectra of 1/2/3/4/5-layer WS2 material might answer this question and add significant merit to this manuscript.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
The manuscript by Liu et al. presents a comprehensive study of SPR sensors based on WS2/graphene/Ag sandwiched symmetric structures. In the study, the authors investigated different parameters and systematically compared their influences in the sensing behaviours (critical parameters). The introduction of 2D materials into SPR sensor can indeed improve the device effciency but there are still lots of improvements required for practical applications. The manuscript was written in a relatively good manner though many parts would require large modifications and language improvement is strongly recommended. At current stage, I would recommend a major revision. Here are some of my comments:
# 1 As shown in Figure 1, there is a certain sequence order of graphene and WS2. Graphene is the layer directly contactnig the medium. Would the order influence the calculated sensitivity? What will be the difference?
# 2 The comparison made in Figure 2a is not fair. Why for Au, the authors used a single layer, while for Ag, symmetric (double) layers are used? Though I agree with the authors that Au might not produce sharp dip like Ag but the comparison here can not demonstrate this claim.
# 3 Following the above comment, why the authros proposed to use the symmetric (double) layer structure? Is there any reason for this (to generate two resonance dips)? How would be the advantages comparing with a single layer?
# 4 Regarding Figure 3 and following figrues, it is a bit confusing to use the term FWHM. As shown in Figure 3a, the spectrum is plotted in a reflectivity versus angle way. Why the FWHM has the unit of nm? I suppose it should be degree? In addition, since the experiment is using a single wavelength laser, what could be the origin of the FWHM as large as 3.75 nm?
#5 Though in the introduction part, the authros presented the previous progress of this field using 2D materials. I still could not understand the role of 2D materials in this device (not optoelectronic properties). Are they just a surrounding material with contrasting refractive index for silver? I would ask the authors to add more details about this which should include the explanation or claim in the literature.
#6 WS2 seems a semiconductor. Would the light excite electron/hole pairs in the material? How would this influence the overall properties? Will the material emit light after absorption (photoluminescence)? Will the structure re-absorb the emitted photons?
#7 For the refractive index of WS2, the authros thought the index has no independence of layer numbers. However, as far as I know, with the increase of layer number, WS2 can be tuned from a direct semiconductor to an indirect semiconductor. I believe this could lead to some variations in refractive index. Could the authors discuss or explain about this factor?
#8 WS2 and graphene can sometimes form heterjunction and this could lead to some changes in the electrical and optical propeties at the interface. How would this influence the SPR sensing? Could the authors comment on this?
#9 There are some other minor things listed here (not limited to these).
- In the first paragraph, “… reflected light cause …” cause should be because.
- “But the shorcoming is that they are easy to oxidize…” oxidize here would be “to be oxidized”.
- “Figure 1. Shows its structure of it”. The dot after Figure 1 should be removed.
- In the text explaining Figure 2b and c. “Optical distribution” is not a correct term.
- The equations used in the manuscript should have references.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The authors have addressed the comments that I raised in my prior review. I believe that this paper is suitable for publication in Photonics.
Reviewer 3 Report
I think the authors have resolved all my concerns and I have no further comment on the manuscript. I suggest its publication at its current form.
