Crystalline Orientation Identification of Phosphorene Using Polarized Raman Spectroscopy without Analyzer
Round 1
Reviewer 1 Report
- Brief summary
Authors propose the new, simple method of orientation identification for the Raman spectroscopy of polarized light. In the new method, analyzer is removed from the Raman measurement system, which makes the measurement easier and simpler. Authors compare the Raman spectra of polarized light for black phosphorus between the conventional and new system and concluded that the new method is effective for the polarized Raman spectroscopy.
-Broad comments
The Raman measurement system proposed by authors are too simple and the results seem inevitable, and thus I slightly doubt novelty of the new Raman measurement system. However, this method has a potential to give a strong message to the research area of the polarized Raman spectroscopy which is a hot topic now. I would like to recommend this manuscript for the publication from Applied Sciences.
The manuscript is easy to read and kind for the readers to understand the concept of new Raman measurement system. I think the present form is almost sufficient for the publication, but for the improvement of the manuscript, I have some questions and comments to the authors as follows.
Questions and comments
Authors show a lot of results of Raman measurements to compare the PP and NA (Figs. 5-9). However the discussion part of these results are few in the manuscript. If possible, it is better to discuss more about the comparison of PP and NA.
Regarding comment 1, in Fig. 7(b) the result of PP, the polarization appears not only 0 deg, but also small polarization to 90 deg. This does not appear in NA and same tendency is seen in Fig. 9(b). Is there the reason of this result?
In Figs 5-9, all the caption of (c) is "Ag1 at PP", but I think it should be "Ag1 at NA". Is it correct?
Author Response
Q: Authors show a lot of results of Raman measurements to compare the PP and NA (Figs. 5-9). However the discussion part of these results are few in the manuscript. If possible, it is better to discuss more about the comparison of PP and NA.
A: Thank reviewer for this constructive suggest. In the revised manuscript, the Raman results in Fig.5-Fig.9 were discussed further, labeled in yellow, by carefully comparing the results of PP with those of NA.
Q: Regarding comment 1, in Fig. 7(b) the result of PP, the polarization appears not only 0 deg, but also small polarization to 90 deg. This does not appear in NA and same tendency is seen in Fig. 9(b). Is there the reason of this result?
A: It is really an important phenomenon. In either the Fig.7(b) or the Fig.9(b), the experimental data of Ag2 showed a secondary maximum at the direction vertical to that of the primary maximum. Moreover, the primary maximum was much larger than the secondary one. Quite a few existing experiments, not only in this work but also some by other groups, detected such a phenomenon similar to that in the Fig.7(b) or the Fig.9(b), which showed this phenomenon of Ag2 might, probably but not inevitably, appear under PP configuration only. It was discussed in the reference [17], which regarded it was induced by the birefringence relatively obvious under the PP configuration and could be well described by using the Eq.4. While under the NA configuration, according to the Eq.6, the polarization-intensity curve seemed to be a peanut-like shape with only one maximum direction.
Q: In Figs 5-9, all the caption of (c) is "Ag1 at PP", but I think it should be "Ag1 at NA". Is it correct?
A: In Figs 5-9, all the caption of (c) should be "Ag1 at NA", not "Ag1 at PP". We had rectified these mistakes (labeled in yellow) in the revised manuscript. Thank the reviewer for the great help and hard work.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors provide a comparison of polarized Raman spectroscopy of black phosphorus with and without an analyzer in front of the detector. In my opinion they show convincingly that if the only aim is to learn the crystal orientation of a BP flake, this can be done reliably without using an analyzer. Furthermore, they show that this conclusion holds true for several different flake thicknesses. I think these conclusions will be useful results for the community of BP researchers.
The technical criticism I have is in the layer number identification. The authors extract a B2g - A2g frequency difference for layer number assignment based on Ref. 20, but in Fig. 4b, this difference clearly depends on whether the parallel-polarized or non-analyzer Raman configurations are used. In particular, the non-analyzer approach appears to increase the difference, which would result in an underestimate of the layer number. The authors do not specify which method they used to extract the different. This point should be clarified, and the authors should introduce a cautionary note that the calibration between layer number and frequency difference used in the traditional, parallel-polarized configuration may be different from that in the authors' simpler method.
Author Response
Q: The technical criticism I have is in the layer number identification. The authors extract a B2g - A2g frequency difference for layer number assignment based on Ref. 20, but in Fig. 4b, this difference clearly depends on whether the parallel-polarized or non-analyzer Raman configurations are used. In particular, the non-analyzer approach appears to increase the difference, which would result in an underestimate of the layer number. The authors do not specify which method they used to extract the different. This point should be clarified, and the authors should introduce a cautionary note that the calibration between layer number and frequency difference used in the traditional, parallel-polarized configuration may be different from that in the authors' simpler method.
A: Thank a lot. It is really a professional and constructive comment. Even the kernel topic of this paper was the identification of crystalline orientation, the authors noticed the problem of the layer number identification gradually. In the original manuscript, the layer numbers of all the samples were identified by using the method similar to that in the Ref. 20 (as the Fig.A1 shows). The B2g - Ag2 wavenumber difference, Dω, from the first set of Raman data (those detected under NA Raman configuration when the polarized direction of the incident laser is zero) was used to conclude the layer number by using the following identifying criterion: 1 layer if Dω≥ 30.3 cm-1, 2 layers if 30.3 cm-1 > Dω ≥ 29.3 cm-1, 3 layers if 29.3 cm-1 > Dω ≥ 28.4 cm-1, 4 layers if 28.4 cm-1 > Dω ≥ 27.8 cm-1 and > 4 layers if 27.8 cm-1.
Just as the reviewer pointed out however, the wavenumber difference used for identifying the layer number clearly depended on whether the PP or NA Raman configurations were used (as the Fig.A2 shows). Meanwhile, the low intensity and high dispersion of Raman information lead to poor reliability of single measurement on a single sampling spot. Therefore, we improved the identification method, which still applied the criterion above but used the average value of all the 37 data (at polarization direction from 0° to 360° with 10° rotating step) under NA Raman configuration. Using this method, the layer numbers of the samples were revised. We also compared the results under the two Raman configurations and found that, if using the same identifying criterion on the data under different polarized Raman configurations, the results may be inconsistent (as the Fig.A3 shows). This proved the idea of the reviewer: the wavenumber difference used for identifying the layer number clearly depended on whether the PP or NA Raman configurations were used. Therefore, there is a requirement of further investigations on the accurate identification of layer number under different polarized Raman configurations, and the wavenumber-polarization-layernumber relativity of phosphorene as well, by applying some in-situ Raman-AFM instrument, such as WiTec Alph300ra. We are going to promote this work immediately. All the authors thank the reviewer for this suggestive advice.
Fig. A1 Peak frequency of Ag2 (top) and B2g (bottom) Raman modes (right vertical axis) and the difference between them (left vertical axis) with changing thickness [20]
Fig.A2 Wavenumbers of B2g - Ag2 under PP and NA configurations
Fig.A3 Dω under PP and NA configurations
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The authors have clarified my only technical concern, and in my opinion the manuscript is now suitable for publication in Applied Sciences.
