Investigating the Possible Origin of Raman Bands in Defective sp2/sp3 Carbons below 900 cm−1: Phonon Density of States or Double Resonance Mechanism at Play?
, Gilles Cartry 1, Luc Lajaunie 2, Raul Arenal 3,4,5 and Josephus Gerardus Buijnsters 6Round 1
Reviewer 1 Report
The manuscript entitled: “Origin of Raman bands below 900 cm-1 in defective carbons: phonon density of states or double resonance mechanism?” by Cedric Pardanaud, Gilles Cartry, Luc Lajaunie, Raul Arenal and Josephus Gerardus reports on a Raman study of three different carbon samples: CVD diamond films, hydrogenated amorphous carbon (H:aC) exposed to RF plasma.
Samples have been grown by varying fabrication parameters and have been investigated by means of Raman spectroscopy. The work focuses on the low-medium frequency range i.e. 400-900 cm^-1 of the Raman spectrum.
Samples with different degree of defects have been analysed. In particular the intensity at 400 cm^-1 have been investigated. The ratio between this peak and the G peak is plotted with the sp2 content measured by EELS.
Although a huge work has been done on Raman spectroscopy of carbon films, this work is somehow interesting since it focuses on a spectral region less investigated. Some points should be addresed before considering the paper for publication as listed below:
1) the ratio of the peak at 400 cm-1 and the G peak is used even when there is hardly a peak as in the case of HOPG at 250 eV. How general is such approach?
2) It is not very clear what is HPDOS/HG in Figure 5b
3) The main conclusion is that The paper does not provide enough and robust evidence of the origin of the peaks at low frequency. In this sense the title is quite misleading.
4) I suggest the following works could be considered in the discussion:
A.C. Ferrari, A. Libassi, B. K. Tanner, V. Stolojan, J. Yuan, L. M. Brown, S. E. Rodil, B. Kleinsorge, and J. Robertson Phys. Rev. B 62, 11089 2000
C.S. Casari, A. Li Bassi, A. Baserga, L. Ravagnan, P. Piseri, C. Lenardi, M. Tommasini, A. Milani, D. Fazzi, C.E. Bottani, P. Milani Low-frequency modes in the Raman spectrum of sp-sp2 nanostructured carbon Physical Review B 77, 195444 (2008)
5) EELS data should be provided at least in the supplementary information
Author Response
The manuscript entitled: “Origin of Raman bands below 900 cm-1 in defective carbons: phonon density of states or double resonance mechanism?” by Cedric Pardanaud, Gilles Cartry, Luc Lajaunie, Raul Arenal and Josephus Gerardus reports on a Raman study of three different carbon samples: CVD diamond films, hydrogenated amorphous carbon (H:aC) exposed to RF plasma.
Samples have been grown by varying fabrication parameters and have been investigated by means of Raman spectroscopy. The work focuses on the low-medium frequency range i.e. 400-900 cm^-1 of the Raman spectrum.
Samples with different degree of defects have been analysed. In particular the intensity at 400 cm^-1 have been investigated. The ratio between this peak and the G peak is plotted with the sp2 content measured by EELS.
Although a huge work has been done on Raman spectroscopy of carbon films, this work is somehow interesting since it focuses on a spectral region less investigated. Some points should be addresed before considering the paper for publication as listed below:
We thank the reviewer for the constructive comments that helped us to enhance the quality of our manuscript. We reply point by point below, with yellow colors used to spot the changes made in the revised paper.
1) the ratio of the peak at 400 cm-1 and the G peak is used even when there is hardly a peak as in the case of HOPG at 250 eV. How general is such approach?
Answer: We agree that in some cases the Raman bands are not very intense (the worst case is, as pointed out by reviewer 1, HOPG subjected to 250 eV ions). However, we selected experimentally the good signal to noise ratio to make these bands visible on the grouped spectra. In the case of HOPG/250 eV, this is a problem of the previous figure display only (i.e., the improper selection of dashed purple color line plots). For example for HOPG/400eV, the bands are better seen in Figure 4-e, with a display which is different from Figure 4-b. Note that, if we add the data for HOPG/250 eV to Figure 4-e, this figure will be less clear (too many spectra, whereas the message of figures d, e and f is about multiwavelength). Then, as units are arbitrary for the intensity axis of Figure 4-b, we decided to multiply the intensity of some spectra in order for the bands to be better seen.
There is also a physical reason why the bands are less intense for HOPG/250 eV than for HOPG/400 eV. This is because the implantation depth is lower for 250 eV ions than for 400 eV ions (10 and 15 nm, respectively). The material volume of implanted graphite is thus significantly less extensive for HOPG/250 eV than for HOPG/400 eV.
Revision: We added two comments: “ Implantation depth are estimated to be ~10 nm for 250 eV and ~15 nm for 400 eV (see [17] for more details).” in the experimental section (line 132), and “ The spectral data in (d), (e), and (f) corresponding to the HOPG implanted with 400 eV deuterium ions have been rescaled for better visualization. ”in the caption of Figure 4.
2) It is not very clear what is HPDOS/HG in Figure 5b
Answer: we would like to thank the reviewer for his/her sharp observation. It should have been removed from the submitted manuscript version but unfortunately this was forgotten.
Revision: The corresponding item has been rewritten as “H400/HG” (caption of figure 5). Figure 5 has been modified accordingly as well.
3) The main conclusion is that The paper does not provide enough and robust evidence of the origin of the peaks at low frequency. In this sense the title is quite misleading.
Answer: The aim of the paper is not to definitively conclude on the subject, as defects created in our HOPG samples have not been characterized by other techniques than Raman spectroscopy and quantum calculations have not been performed for such complex systems. The aim is to evidence the effect, as a starting point for future studies. We agree with the reviewer that that the title did not reflect this idea correctly, except with the question mark.
Revision: We changed the title accordingly.
4) I suggest the following works could be considered in the discussion:
A.C. Ferrari, A. Libassi, B. K. Tanner, V. Stolojan, J. Yuan, L. M. Brown, S. E. Rodil, B. Kleinsorge, and J. Robertson Phys. Rev. B 62, 11089 2000
C.S. Casari, A. Li Bassi, A. Baserga, L. Ravagnan, P. Piseri, C. Lenardi, M. Tommasini, A. Milani, D. Fazzi, C.E. Bottani, P. Milani Low-frequency modes in the Raman spectrum of sp-sp2 nanostructured carbon Physical Review B 77, 195444 (2008)
Answer: we would like to thank the reviewer for the suggested references. The first reference was added to the Introduction. We added the second reference to the Introduction and a comment when discussing the use of the 400 cm-1 band to stress the reader that for sp rich materials a band was also found at 400 cm-1 in the literature.
Revision: [Ref 1] was added in the sentence: “Energy Electron Loss Spectroscopy (EELS), very usefull to obtain sp2/sp3 ratio [30, 31] has been used for amorphous carbon and diamond films analyzed in this study” (line 92) ,page 3. The sentences corresponding to [Ref2] “and bending modes of sp hybridized linear structures [18] “ and “Last comment about the use of this band: one should take care about systematically using its intensity as for sp rich materials, a band was found lying at the same frequency [18] “ were respectively added page 2 and 10..
5) EELS data should be provided at least in the supplementary information
Answer + revision: EELS data were added to the supplementary information (new Figure S3).
Reviewer 2 Report
The article analyzes Raman spectrum of different type carbon films (diamond, a-C:H, HOPG). It is interesting research bringing up new ideas how to probe defects in the carbon based samples.
The minor comments to the well prepared manuscript would be as follows:
1) There are minor typing errors through the text (i.e. invesitgate, etc.)
2) The intensity of the bands of interest is quite low, what is the reproducibility of the results? Is it really the band related to the defects in carbon or signal background. (Fig. 4)
Author Response
Reviewer 2:
The article analyzes Raman spectrum of different type carbon films (diamond, a-C:H, HOPG). It is interesting research bringing up new ideas how to probe defects in the carbon based samples.
The minor comments to the well prepared manuscript would be as follows:
We thank the author for the supportive comment and remarks. Changes made in the text are highlighted in green.
1) There are minor typing errors through the text (i.e. invesitgate, etc.)
Answer: We would like to thank the reviewer for the critical reading of our manuscript. We have corrected for the minor typing errors in the revised manuscript.
Revision: among others, “invesitgate” was changed to “investigate” (Page 2).
2) The intensity of the bands of interest is quite low, what is the reproducibility of the results? Is it really the band related to the defects in carbon or signal background. (Fig. 4)
Answer: we would like to thank the reviewer for the questions. When a sloped background is superimposed to Raman signatures, this is always a question we have to handle with. To disentangle the physical origins of a spectroscopic signature (i.e. from the background, being electronic or not), using at least two different lasers is a good approach. If the signal to be interpreted still has the same band position when changing the laser wavelength, it means it is due to a Raman effect. If the signature changes its band position by several hundreds of cm-1 when the spectrum is expressed as a Raman shift, then the signature is caused by some other process (e.g., luminescence). In the case of HOPG, the bands of interest below 900 cm-1 do not come from some luminescence related processes as they are found nearly at the same frequency when changing the wavelength of the laser from 325 to 633 nm. For diamond and amorphous carbons, this is also the case (see Figures 4b and c). However, we agree that the background could be tilted , which could induce some uncertainty on the value of the band intensity/height retrieved. For that reason, we added a comment in the text.
Revision: the following comment was added to the revised text (page 10): “This is however qualitative as band height are obtained after background substraction, which could introduce error bars if this operation is not well done when backgrounds are very slopy (see figure S2-b for example).”.
Round 2
Reviewer 1 Report
The authors have answered to the major points and they have submitted a revised version, accordingly. The manuscript can be accepted for publciation.
