Self-Organized Conductive Gratings of Au Nanostripe Dimers Enable Tunable Plasmonic Activity
, Giuseppe Della Valle 2 and Francesco Buatier de Mongeot 1,*Round 1
Reviewer 1 Report
I had the opportunity to read this manuscript titled "Self-organized conductive gratings of Au nanostripe dimers enable tunable plasmonic activity". In this work plasmonic nanostructure was prepared by a novel maskless nanofabrication method. Also characterisation data has been presented in the form of AFM, SEM. Optical measurements were taken to show transmission properties. I like this simple in nature fabrication process and convinced by the data presented. I recommend the publication of this work.
Author Response
We thank the referee for reading the manuscript and we are glad for his/her positive comments on this work.
Reviewer 2 Report
The authors presented a self-organized method to fabricate plasmonic nanostripes of Au and Au-SiO2-Au dimers over large area with homogeneity and desired optical and electrical properties. The properties can be tuned via adjusting the structural parameters. This is a very interesting topic and can be potentially deployed in many applications. I would recommend publication given the following comments are addressed properly.
Please provide critical parameters, tools and samples (company, part number, etc.) in the fabrication procedure. Does the ion beam irradiating the sample have a tilted angle in order to create asymmetric tilt out of plane? Otherwise, please explain why the wrinkles are asymmetric? Add scale bars in Fig. 1f and 1g and Fig. 2e. Fig. 1b, x axis is in unit of nm. I doubt it. Should be um instead? For high homogeneity, it is hard to tell just from the optical micrograph in Fig. 1c. Fig. 1c only shows that this technique is able to fabricate such large area. It is necessary to quantify some critical parameters (such as period, height, angles) at several different locations distributed over the entire cm^2 sample surface. In Figs. 2a and 2b, why are there transmission peaks for TE at transmission valleys for TM? Based on Figs. 2a and 2b, if the film works as a wire grid polarizer, its extinction ratio can’t be high. Please calculate its extinction ratio. What is the yellow band in Fig. 2b? Remove it if necessary. In all other figures, the nanostripes are oriented vertically, but in Fig. 2c, it is horizontal. Please rotate the schematic 90 degrees to be consistent with other figures. In Fig. 3d, why dimer transmission (red) drops so much compared to the other two? There is no description for Fig. 4f in its figure caption. Line 3 on page 5, is silica thickness 22nm or 24nm (see caption for Fig. 4b)? If allowed, the authors may consider using sub-sections for Section 2: Results and discussion. Otherwise, it is now lengthy and the structure of the manuscript is less clear.Author Response
We thank the referee for carefully reading the manuscript and for giving useful comments. Based on his/her considerations we have changed the manuscript in order to better clarify the results. In the following we provide point by point answer to the referee comments and we report the manuscript changes.
Author Response File: 
Author Response.docx
Reviewer 3 Report
A welcome follow up to the authors recent papers on the subject showing some further advances made. Whilst there is much of interest to report and it can be said that the title is apt for the paper, my principle dispute is with the claims of the self-organized method leading to "precise control of the nanoscale morphology" and "high nanofabrication control". This is at odds with their appraisal of that achieved by conventional lithographic methods, i.e. "despite the high degree of control provided by the lithographic methods" and their description of their structures as being "quasi-one dimensional (1D) nanopatterns". They are quasi 1D, and they are orderly but they cannot be called precision controlled nor obtained through highly controlled nanofabrication as conventional methods do this far better and even then they are far from perfect. In relation to this a welcome addition would be a suggestion and/or plan how to make them more 1D and less quasi 1D.
Better explanation of the red and yellow lines n Figure 1 would help and all images should have scale bars. The English is quite good although some errors were missed such as "which enable the confinement" instead of "which enables the confinement".
Author Response
We thank the referee for carefully reading the manuscript and for his/her comments. We agree that the sentence ‘precise control on the nanoscale morphology’ can be misleading recalling the degree of control provided by the lithographic methods. The nanofabrication approach shown in this paper has an intrinsic degree of disorder which is due to the self-organized nature of the patterning. However, we demonstrate an easy way to tailor the nanoscale morphology of the nanostripes (e.g. width or height) by simply changing macroscopic parameters such as the Au incidence angle on the surface and/or the Au dose. Additionally, the morphology of the rippled template could be modified by changing the ion irradiation conditions, as described in detail in Ref. [30]. Under this condition we expect a further change of the morphology of the plasmonic structures confined on the glass templates. In this perspective it is evident how this self-organized method allows a good control on the nanoscale morphology, compatible with its nature.
In order to better address this point we have modified few sentences in the manuscript as follows, avoiding the term ‘precise control’ and ‘high nanofabrication control’.
Page 2, line 12: “The possibility to effectively tailor the nanoscale morphology thus achieving a fine tuning of the optical response in a cost-effective way, combined with the superior plasmonic near-field confinement and light scattering properties, opens a variety of applications for these templates in optical spectroscopy, biosensing, optical microscopy, and non-linear optics.”
Page 4, line 6: “The control enabled by the self-organized nanofabrication process allowed us to develop a multi-step approach for increasing the complexity of the plasmonic nanoantennas, while still preserving the large area homogeneity of the sample.”
Page 5, line 23: “This self-organized approach enables to tailor the nanoscale morphology of the nanostripes, tuning the plasmonic response over a broadband VIS and Near-IR spectral range.”
Page 5, line 29: “The nanofabrication control characteristic of this self-organized approach has been exploited for developing a multi-step growth of quasi-1D arrays of Au-SiO2-Au nanodimers characterized by hybridized plasmon resonances with superior near field confinement capabilities.”
- Better explanation of the red and yellow lines n Figure 1 would help and all images should have scale bars. The English is quite good although some errors were missed such as "which enable the confinement" instead of "which enables the confinement".
We thank the referee for the comment. The red scale lines in Fig.1 (f,g) of the revised manuscript are the scale bars of the SEM images which read 500 nm and 1 um respectively. In order to clarify this we have used a single color for scale bars and the caption at page 6, line 5 has been modified as follows:
“(f,g) SEM images of the tilted Au nanostripe arrays grown under condition of panel (d,e), respectively. The red scale bars in panel (f,g) correspond to 500 nm and 1 µm, respectively.”
Author Response File: Author Response.docx
Reviewer 4 Report
The manuscript is not a review, it is a research paper devoted to study of quasi-1D gold nanostripe arrays fabricated by the recently reported experimental method. The approach is based on the treatment of the glass substrates near the glass transition temperature by the defocused ion beam, leading to a formation of nanoscale ripples. Further vacuum deposition of gold at oblique angle allows one to get the nanostripe arrays on a large substrate area (a few cm^2). The authors report the optical dichroism of the metal arrays and the electrical conductivity, proving the formation of the continuous metal nanowires across the surface. The possibility to control the aspect ratio (height/width) of the nanostripes and, thus, the optical properties, is demonstrated.
The authors study the optical properties of dimer nanostripes using experiment and FEM simulations. The two extinction peaks in the spectrum are explained by the plasmon hybridization. The near field distribution for the two hybridized modes are presented. The simulations satisfactorily support the experimental data. The paper is clear and easy to follow. Based on the above, I recommend the manuscript for the publication in Applied Sciences.
There are a few minor issues, which I would like authors to take care about prior to publication:
The size of the scale bars in Figure 1 (f,g) is not provided The caption of Figure 2 should be corrected: there should be (d,e) instead of (c,d) in line 4 on page 7; ‘NIR’; instead of ‘Near’ in line 6 on the page 7 In the right panel of Fig.2(b) one of the green lines should be dashed In Figure 4(b) the green and the black line are supposed to correspond to the structures, which differ only in thickness of the silica layer. Therefore, according to the hybridization theory, reducing the thickness of the dielectric layer between gold stripes, should lead to the higher peak splitting (the high-energy mode should move to shorter wavelength, and the low-energy mode – to the longer wavelengths). However, the authors observe the red-shift of the high-energy mode (black curve), and this is not commented in the text.
Author Response
We thank the referee for carefully reading the manuscript and for giving a positive evaluation of our work. We have considered his/her useful comments, providing point by point answer in the following.
- The size of the scale bars in Figure 1 (f,g) is not provided
We thank the referees for the comments. In the revised version we have indicated the scale bars values of 500 nm (Fig. 1f) and 1µm (Fig. 1g) in the figure caption.
-The caption of Figure 2 should be corrected: there should be (d,e) instead of (c,d) in line 4 on page 7;
The caption has been corrected.
-‘NIR’; instead of ‘Near’ in line 6 on the page 7
We have indicated the acronym NIR.
- In the right panel of Fig.2(b) one of the green lines should be dashed
The plot has been changed in the revised version.
- In Figure 4(b) the green and the black line are supposed to correspond to the structures, which differ only in thickness of the silica layer. Therefore, according to the hybridization theory, reducing the thickness of the dielectric layer between gold stripes, should lead to the higher peak splitting (the high-energy mode should move to shorter wavelength, and the low-energy mode – to the longer wavelengths). However, the authors observe the red-shift of the high-energy mode (black curve), and this is not commented in the text.
We agree with the referee that the observed red shift of the high energy mode when the thickness of the silica is reduced is not in accordance with the hybridization model.
We attribute this effect to a change of the refractive index of the silica layer in the case of the thinner film (black curve) with respect to the case of the thicker films (red and green curves). This effect is attributed to a change of the stoichiometry of the silica layer which in turn contributes to a redshift of the dipolar plasmon modes. Is possible that a slightly different oxidation condition occurred during deposition of the ultra-thin layer by Radio-Frequency (RF) sputtering or immediately after deposition when the base pressure in the UHV chamber was in the range of 10-6 mbar.
In order to address this issue we have added the following sentence at page 5, line 15: “An unexpected red shift of the ED mode from 550 nm to 580 nm wavelength is also observed when the thickness of the silica is reduced to 22 nm which is not in accordance with the pure hybridization model. We attribute this small spectral shift to a change (reduction) of the refractive index of the silica layer in the case of the thinner film, due to a slight difference in the stoichiometry obtained by RF sputtering deposition.”
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
The authors addressed the previous comments satisfactorily. It can be accepted for publication.
