Raman 2D Peak Line Shape in Epigraphene on SiC

Round 1

Reviewer 1 Report

The authors reported the 2D peak lineshape in epitaxial graphene on SiC grown at different experimental conditions. While the topic is interesting, the following issues have to be addressed before publication:

The morphology of the graphene at different growth conditions have to be provided, such as SEM, AFM plus Raman imaging.

There is some difference in the temperature used for different growth conditions. Can the authors explain why?

The authors need to discuss more on the previous examples on graphene/SiC systems studying on the homogeneous and/or inhomogeneous broadening, especially on the ‘Discussion’ part. Additionally, more discussions are needed to compare the results of the three different growth conditions.

There is some inconsistence for the references, for instance, page number is missing.

Author Response

Dear referee,

we thank for your comments. Our response is following:

1) The morphology of the graphene at different growth conditions have to be provided, such as SEM, AFM plus Raman imaging.

We have additionally measured AFM and LFM (lateral force microscopy). Since the conclusions from LFM and SEM are similar (number of graphene layers/different friction/different material) we do not think it is neccessary to measure also SEM. Please, see our revised manuscript for the new AFM/LFM data and the discussion.

2) There is some difference in the temperature used for different growth conditions. Can the authors explain why?

We have explained this issue in more detail in our revised manuscript. Please, see the reviesed Materials and Methods.

3) The authors need to discuss more on the previous examples on graphene/SiC systems studying on the homogeneous and/or inhomogeneous broadening, especially on the ‘Discussion’ part. Additionally, more discussions are needed to compare the results of the three different growth conditions.

We discuss differences between the different growth conditions in the revised manuscript. Please, see the discussion of AFM and LFM. We have also extended discussion of the homogeneous and/or inhomogeneous broadening. Please, see the revised Discussion section. 

4) There is some inconsistence for the references, for instance, page number is missing.

We thank referee for this comment. We have fixed all page numbers. 

Reviewer 2 Report

In this research the authors analyzed in details a 2D peak lineshape of epitaxial graphene grown on SiC in high vacuum, argon and graphene prepared by hydrogen intercalation from the buffer layer on a silicon face of SiC. They revealed that the Voigt lineshape describes the 2D peak lineshape better. They interpreted the inhomogeneous 2D peak broadening observed in epitaxial graphene grown on SiC in high vacuum and argon by a submicron length scale strain variation. The hydrogen intercalated buffer layer is shown to have the smallest homogeneous and inhomogeneous 2D peak broadening. The analysis suggested by authors can be used to further optimize graphene growth.

My comments:

The manuscript can be accepted after misprint correction.

Line 9; 72; 121; 122; 125; 126; 128; after Eq.3; Figure 3 caption – “broadenning”.  Right – “broadening”.

Line 54 – “acuired”.  Right – “acquired”.

Line 126 – “lenght”.  Right – “length”.

Author Response

Dear Referee,

we appreciate your time and expertise, and, we thank you for your comments. We have corrected all the typographical errors.

Reviewer 3 Report

In this work the authors explore the lineshape of the Raman 2D peak of epitaxial graphene on SiC and the physical origin of their components. The work is serious and well conducted and presented and provides interesting results to the research community working on graphene and more particularly on the Raman characterization of graphene based systems. In the following I provide a number of  requests or recommendations which could help to improve the manuscript and its impact.

1) Title. The title should replace “2D peak” by “Raman 2D peak” so all readers really know what it is about. I will recommend to be more explicit on the title, as for instance: “Raman 2D peak lineshape in epitaxial graphene on SiC and the study of the inhomogeneous contributions”

2)Abstract.  In the abstract it is said “The inhomogeneous broadening is attributed to the spatial variations of the charge density and strain on the sub-micrometer length scales” but the most important result is that for the author’s study samples it is mostly given by inhomogenous strain variations at the submicrometer scale. This should be in my opinion mentioned in the abstract.

3) Introduction. The introduction is good. I recommend some improvements:

3.1) It is written “there is little known about the mutual effect of the homogeneous and inhomogeneous
30 broadening”. I think that “mutual” is not the most convenient adjective, as the two effects combine but do not influence each other. In this sentence and the others following I will recommend to replace “mutual” by “combined”

3.2) The authors discuss physical effects on the 2D line broadening in graphene and bilayer graphene. The effect of biaxial strain and charge doping on the splitting of the 2D line components of bilayer graphene was shown in [J. Nicolle et al, Nano Letters 11, 3564 (2011). This article should be mentioned.

3.3) It is said “We show here that the Voigt broadening describes the 2D peak lineshape better than the Lorentzian broadening.” I would recommend replacing “broadening”, the two times by “profile” in this sentence.

4) Materials and methods

4.1) Page 2. Line 47. The authors write “The hydrogen intercalated buffer [34,35], so called quasi free-standing
 monolayer graphene (QFMLG) was grown in two steps.” I suppose they want to say “The hydrogen intercalated buffer [34,35], leads to the so called quasi free-standing8 monolayer graphene (QFMLG) was grown in two steps.”.

4.2) Line 54 “acuired" needs to be replaced by “acquired”

5) Results

5.1) I think that a figure comparing the full Raman spectrum of the 3 studied samples would be helpful or at least a description of their major differences concerning the other Raman peaks (G and D band in particular).

5.2) Page 2. Line 38. Replace “resspect" by “respect”

5.3) Figure 3. I recommend to use the same vertical scales for the b) and c) panels to facilitate their comparison. The authors may then want to enhance the vertical to horizontal aspect of the panels if they want to maintain an optimum visualization.

5.4) The origin of Eq. 8 and 9. (associated references or studies) with the parameters used is not provided. This should be included.

5.5) The color label in the main panel of Figure 4 where it is given the corresponding color for the QFMLG, SLG-argon and SLG-vacuum samples is almost invisible. Could the authors either increase the size of their points for the labels or explicit that colors in the Figure legend?

5.6) The authors write “These deviations from the normal strain distribution can be reduced by defocusing the laser spot. Defocusing will also reduce the Raman signal, and, it will also cause broader 2D peak.”. It is not clear if the authors are proposing this or if they have verified this effect. Could they please be more explicit?

Author Response

Dear referee,

we thank you for your time and expertise and we respond to all your comments as follows:

1) Title. The title should replace “2D peak” by “Raman 2D peak” so all readers really know what it is about. I will recommend to be more explicit on the title, as for instance: “Raman 2D peak lineshape in epitaxial graphene on SiC and the study of the inhomogeneous contributions”

We have changed the title to be more specific.

2)Abstract.  In the abstract it is said “The inhomogeneous broadening is attributed to the spatial variations of the charge density and strain on the sub-micrometer length scales” but the most important result is that for the author’s study samples it is mostly given by inhomogenous strain variations at the submicrometer scale. This should be in my opinion mentioned in the abstract.

We agree this was inconsistent statement in the abstract. We have changed the abstract accordingly.

3) Introduction. The introduction is good. I recommend some improvements:

3.1) It is written “there is little known about the mutual effect of the homogeneous and inhomogeneous
30 broadening”. I think that “mutual” is not the most convenient adjective, as the two effects combine but do not influence each other. In this sentence and the others following I will recommend to replace “mutual” by “combined”

We agree with referee. We changed the "mutual" by "combined".

3.2) The authors discuss physical effects on the 2D line broadening in graphene and bilayer graphene. The effect of biaxial strain and charge doping on the splitting of the 2D line components of bilayer graphene was shown in [J. Nicolle et al, Nano Letters 11, 3564 (2011). This article should be mentioned.

We thank referee for mentioning the work of Nicolle. We were not aware of this work. We added the reference in the introduction section.

3.3) It is said “We show here that the Voigt broadening describes the 2D peak lineshape better than the Lorentzian broadening.” I would recommend replacing “broadening”, the two times by “profile” in this sentence.

We thank for this comment. We agree the word "broadening" was used too many times. We replaced it by "profile".

4) Materials and methods

4.1) Page 2. Line 47. The authors write “The hydrogen intercalated buffer [34,35], so called quasi free-standing
 monolayer graphene (QFMLG) was grown in two steps.” I suppose they want to say “The hydrogen intercalated buffer [34,35], leads to the so called quasi free-standing8 monolayer graphene (QFMLG) was grown in two steps.”.

We have reformulated the sentence. Please see the revised manuscript.

4.2) Line 54 “acuired" needs to be replaced by “acquired”

We corrected this typographical error.

5) Results

5.1) I think that a figure comparing the full Raman spectrum of the 3 studied samples would be helpful or at least a description of their major differences concerning the other Raman peaks (G and D band in particular).

We included a new Fig.1 comparing the typical Raman spectra of all three samples. 

5.2) Page 2. Line 38. Replace “resspect" by “respect”

We corrected this typographical error.

5.3) Figure 3. I recommend to use the same vertical scales for the b) and c) panels to facilitate their comparison. The authors may then want to enhance the vertical to horizontal aspect of the panels if they want to maintain an optimum visualization.

We apologize but we do not agree with referee here. We prefer to display the data inf Fig.3 as they are displayed now. There would be too much of unused white space if we follow referee's suggestion.

5.4) The origin of Eq. 8 and 9. (associated references or studies) with the parameters used is not provided. This should be included.

We thank referee for pointing out this weak point of our manuscript. We have extensively improved the references for Eq. 8 and 9 and we have also improved significantly the discussion. Please, see the revised manuscript.

5.5) The color label in the main panel of Figure 4 where it is given the corresponding color for the QFMLG, SLG-argon and SLG-vacuum samples is almost invisible. Could the authors either increase the size of their points for the labels or explicit that colors in the Figure legend?

We thank for mentioning the visibility issue. We enlarged the point size in the legend of Fig.4 (currently Fig. 7 in the revised manuscript).

5.6) The authors write “These deviations from the normal strain distribution can be reduced by defocusing the laser spot. Defocusing will also reduce the Raman signal, and, it will also cause broader 2D peak.”. It is not clear if the authors are proposing this or if they have verified this effect. Could they please be more explicit?

This was our suggestion. We did not verify it since it is rather obvious consequence of defocusing the laser spot size. We have explicitly stated this in the discussion now, that this is our suggestion only. 

Round 2

Reviewer 1 Report

The authors have made relevant changes accordingly, therefore it can be accepted at the present form.