Proton-assisted growth of ultra-flat graphene films
Authors: Qian-Qian Yuan
Wang Chen
Xiaoxiang Xi
Yi Zhang
Junyu Zong
Shao-Chun Li
Xuedong Xie
Di Wang
Dongjing Lin
Guowen Yuan
Yong Wang
Jie Xu
Hang Zheng
Libo Gao
Xianlei Huang
Jian Sun
Journal: Nature (2020)
Discipline: Multidisciplinary
Keywords: Graphene films; chemical vapour deposition; wrinkles; proton-assisted method; ultra-flat
Affiliations: National Laboratory of Solid State Microstructures and School of Physics, Nanjing University, Nanjing, China., Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing, China., National Laboratory of Solid State Microstructures and School of Physics, Nanjing University, Nanjing, China. ., Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing, China. .

Abstract:  Graphene films grown by chemical vapour deposition have unusual physical and chemical properties that offer promise for applications such as flexible electronics and high-frequency transistors1-10. However, wrinkles invariably form during growth because of the strong coupling to the substrate, and these limit the large-scale homogeneity of the film1-4,11,12. Here we develop a proton-assisted method of chemical vapour deposition to grow ultra-flat graphene films that are wrinkle-free. Our method of proton penetration13-17 and recombination to form hydrogen can also reduce the wrinkles formed during traditional chemical vapour deposition of graphene. Some of the wrinkles disappear entirely, owing to the decoupling of van der Waals interactions and possibly an increase in distance from the growth surface. The electronic band structure of the as-grown graphene films shows a V-shaped Dirac cone and a linear dispersion relation within the atomic plane or across an atomic step, confirming the decoupling from the substrate. The ultra-flat nature of the graphene films ensures that their surfaces are easy to clean after a wet transfer process. A robust quantum Hall effect appears even at room temperature in a device with a linewidth of 100 micrometres. Graphene films grown by proton-assisted chemical vapour deposition should largely retain their intrinsic performance, and our method should be easily generalizable to other nanomaterials for strain and doping engineering. 

Retracted: No

PubPeer comments:
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 1. Until now, I still do not know who you are? 2. You sent all the same comments through emailing to Nature editor, who ask me to response it in Sep 2020. I wrote a 20 page response letter. Then, Nature editor send the comments and my response letter to other three referees, and they all think your comments are unreasonable. 3. The comments come from that you do not understand the background about plasma to graphene, permeability of graphene, transport measurements of graphene. You seems to be familar to the thermal CVD only, but the background in study is far more than the thermal CVD. Plasma enhanced CVD is far more complicated than the thermal CVD. You should read more background about this firstly. 4. I and my collaborators are trying to devolop more important works based on this paper, and some of them will be published in 2021. This study lays a foundation to the latter series of works. 5. Most of your concerns are the contents in the latter works. For the first paper, I think it is not neccesary to refine every detail. But in the latter work, I or other groups will continue to perfect it. 6. In summary, if you are still angry about it (maybe this study affects your carreer), you can submit a comment to Nature, I will be glad to response it again. Then, I and other readers will know your name and why you are angry about this study, but not a simple academic exchange. , 
 Dear Dr. Gao, Sincere questions or concerns about scientific data are not less valid because they are raised anonymously. You seem very eager to know the identity of the group who posted their concerns in #1, but your response seems to not include actual replies to their questions. Instead of telling the commenters to read more background, why do you not answer their questions and give them that background? I will admit that I know nothing about graphene, but I do admire scientists who are able to answer serious questions and explain what they did without being condescending. The Concerned Graphene Researchers seem to raise valid concerns and I would love to see your answers here. Could you also take a look at Extended Figure 8D, pleases? The 430 and 360dC plots look more similar to each other than expected. I have marked them with blue boxes.  , 
 #2  I wrote a 20 page response letter. Fantastic. It should not take more than a minute to paste that response here. Why would you wish to hide from readers of the above criticism a response to it that convinced a Nature editor and three referees?  ... you can submit a comment to Nature... As you are well aware, Nature is where scientific debate goes to die. , 
 **Reposting #1 for better clarity** Chemical vapor deposition (CVD) synthesis of graphene on copper can generate high-quality films over large areas, but the large thermal expansion mismatch with the copper causes wrinkling of the graphene upon cooling from the high growth temperature. These wrinkles lead to inhomogeneity of properties and can trap impurities, and have thus emerged as a major obstacle for use of CVD graphene in applications. Therefore, we were excited when a recent paper in Nature (January 2020) reported a new technique could eliminate these wrinkles. However, this paper, titled u201cProton-assisted growth of ultra-flat graphene filmsu201d, falls far short of substantiating most of its central claims and contains numerous errors. We feel that it is important to point out the many flaws in this work to alert researchers in graphene synthesis and applications that the critical challenge of wrinkles should not yet be considered a solved problem. **The Main Claims that have not been Explained** This paper proposes a method of removing wrinkles from graphene that are always produced during growth at high temperatures on metallic films or substrates (due to the mismatch in the coefficients of thermal expansion or CTEs between the graphene and the metal). The authors claim that an inductively coupled plasma (ICP) treatment at ~400-650 u00b0C of a pre-grown wrinkled u2018graphene-on-Cu Ni film-on-sapphire substrateu2019 un-wrinkles it. The authors say that H+ ions generated by the ICP are reduced to H2 at the metal-graphene interface, and generate H2 bubbles that help u201cde-coupleu201d the graphene from the metal. But (1) why do the H+ ions migrate to the interface in the first place?**(a)** (2) Or, why how do these H2 bubbles help in this de-coupling process? (3) And, if the graphene is already decoupled from the metal why is a process required later to delaminate it from the metal substrate **(P209)****(b)**? (4) And, most importantly, how and why does this u201cde-couplingu201d (mostly) un-wrinkle the wrinkled film? These are events that are neither expected nor obvious or self-explanatory and should have been further elucidated by the authors. **Footnotes** (a) Please note that the simulation results in **Figure 1b** and **Extended Data Figure 1** merely illustrate how the van der Waals interactions with the metal and the graphene may be reduced when H2 bubbles are present at the interface. (b) A u2018transfer protocolu2019 involving etching of the copper substrate was required to separate the graphene films from the metal and has been described later, in the u201cMethodsu201d section. At this point, the u201cde-coupledu201d films should simply be physically adhering to the substrate and capable of being floated away by introducing a suitable solvent. **(CONTINUED)** , 
 **Continuation of Repost of #1 (for better clarity)**  **CONTINUED** , 
 **Continuation of Repost of #1 (for better clarity)** **Issues with the Figures and Data (c)** Now, if one were to argue that the authors are simply reporting their observations, these, as represented by the figures, do not offer much clarity either. Firstly, the sequence of events is generally hard to follow. Two types of graphene films were synthesized, thermally at 1050 u00b0C or using a plasma at 650 u00b0C (**Methods, P209**). They have been subjected to u201cICP-decouplingu201d at 400 or 650 u00b0C, that may be followed by a UHV annealing step at 400 u00b0C. It has not been clearly explained as to which type of film undergoes what processing steps, either in the main text or the figure captions, making it extremely confusing for the reader to understand which image comes from which film and at what point in the process. Some additional examples of issues with the data and figures are described below. u2022tWrinkles before and after the ICP treatment It has been observed with in-situ LEEM that graphene wrinkles may disappear at high temperatures and re-appear again when cooled down to room temperature. Thus, the ex-situ AFM images shown in **Figures 2c-2e** are inconclusive. Small wrinkles will disappear when heated to 650 u00b0C and re-form at other places when cooled down to room temperature (RT), even without the treatment of ICP. Interestingly, the authors mention this in the caption of **Extended Data Figure 5**: u201cPartly released wrinkles when heated to 650 u00b0C, with weak fstrain and strong fcouplingu201d. Relevant control experiments should have been provided. The authors have not described any methodology as to how the number and sizes of wrinkles and bubbles that they mention were monitored. Are statements such as u201cAfter ICP at 650 u00b0C, most wrinkles are reduced, and some have been eliminated.u201d (**P205**) based on statistical analysis? Or were specific regions tracked? Ideally, before and after images of specific regions, or, statistical analysis of wrinkling before and after the ICP treatment, should have been provided. **Figures 1 and 2 **do not match. The schematic in **Figure 1b**, supposedly representative of the process, shows wrinkles along steps on the metal surface u2018flattening outu2019. However, the AFM image in **Figure 2d** does not show this happening: there is a wrinkle right across the steps. Snapshots 1 and 2 depict the figures from the paper discussed above.   **Footnotes** (c) Please note that u201csnapshotsu201d are parts of figures of the paper that have been copied directly for the readeru2019s convenience. The text in italics are comments by us. **CONTINUED** , 
 **Continuation of Repost of #1 (for better clarity)** u2022t**The Graphene-Metal Distance and Nano-Bubbles** De-coupling of the graphene from metal means a lower van der Waals interaction and increased separation between them. This is shown in the simulations in **Figure 1b** and **Extended Data Figure 1**. In line with this, P205 contains the following statement: u201cAfter ultrahigh vacuum (UHV, 10u22126 Pa) annealing at 400 u00b0C, the bubbles are enlarged, but more wrinkles have disappeared completely **(Figure 2f**).u201d However, in the caption of **Extended Data Figure 8c**, a reduction in the metal-graphene distance is mentioned: u201cThe thickness is decreased by about 5 u00c5 after this UHV annealingu201d. One wonders which is the correct statement here, if either? It has been mentioned that the u2018bright dotsu2019 observed in AFM images (**Figure 2, and, P205**) are nano-sized graphene bubbles: u201cThere are also numerous nano-sized bubblesu201d. But, how were these dots identified as nano-bubbles? Based on the experience of a variety of graphene growers around the world, the bright dots are more likely to be the contaminant particles, which are frequently observed on the surface of CVD-grown graphene when it is synthesized inside reactors made of different materials (unless particular steps are taken to u201cshieldu201d the graphene from the walls of the reactor). Moreover, these bright dots are also observed in un-treated graphene (**Figure 2c**) and He-ICP treated graphene (**Extended Data Figure 4g**) where, according to this explanation, these nano-bubbles should not be present. Again, based on the assumption that the bright dots are indeed nano-sized bubbles, the later statement u201cThe enlarging and increasing bubbles indicate the existence of H2 molecules, formed by re-bonding of atomic hydrogenu201d **(P205**) is unsupported by any hard evidence. Additionally, de-coupling of the film would mean the creation of space between the substrate and the film. This raises the question as to why some H2 gas remains trapped as bubbles if the film is decoupled from the substrate! **CONTINUED** , 
 **Continuation of Repost of #1 (for better clarity)** u2022t**Device performance and measurements** Incorrect conclusions have been arrived at about device performance: u201cThe ultra-flat and easy-clean surface of the graphene strongly indicates homogeneous performance. Here, the robustness of the quantum Hall effects (QHE) is measuredu201d (**P207**). The transport measurements shown here are very much dependent on the substrate material and the transfer process (including specific chemicals and treatments used) and are not meaningful for inferring about the flatness or lack of flatness of graphene film(s) on the copper substrate(s). The statement (**P207**) is incorrect: u201cThe robust QHE in the ultra-flat films indicates ballistic transport properties, and the performances of all of these graphene films should be much closer to their extraordinary intrinsic nature than seen in wrinkled films (more of this has been discussed in **Extended Data Figures 10bu2013d**).u201d Ballistic transport occurs when the electronic mean free path is longer than the size of the sample. The measured room-temperature mobility of 4900 cm2 Vs is roughly a factor of 10 below the intrinsic phonon limit for graphene, and implies an electronic mean free path of order 100 nm u2014 1000 times smaller than the sample size! Observation of QHE u2014 in any material u2014 does not imply ballistic transport. In short, the data provided by the authors simply do not provide adequate evidence for the main claim of the paper, that is, ICP treatment of wrinkled graphene films on metal substrates can reduce the wrinkling. **Other Incorrect Statements** u2022tThe second sentence in the first paragraph of the paper (**P204**) states that the performance of CVD graphene is still not equivalent to exfoliated graphene. This is simply not true as many of the works from the Stampfer and Hone groups have repeatedly shown. u2022tThe statement on **P204** u201cCVD processes usually need high temperature ( 800 u00b0C) to decompose a gaseous carbon source and there are plenty of atomic hydrogens, protons and electrons in the reaction atmosphere.u201d is incorrect. Thermal heating (typically 800 u2013 1100 u00b0C during graphene synthesis) does not ionize gases or produce protons and electrons. u2022tAgain, on **P204**, the statement u201cThe graphene is impermeable to atomic hydrogen, but the hydrogen can transform it into graphane (Cu2013H bonds) at room temperature (RT)u201d is incomplete. No details about the conditions under which this might be applicable or supporting references have been provided. Why would it transform into graphane at room temperature? **An Odd Thing** Though the graphene films discussed have been synthesized on Cu-Ni(111) films, the Cu-Ni(111) films have been referred to as Cu(111) films throughout! Only if one looks at the Methods section (P209) one comes to know that these are actually Cu-Ni(111) films. **Final Thoughts** Although there are claims in this paper that it has solved an important problem, that of eliminating wrinkling in CVD grown graphene, it fails to provide sufficient data or rigorous analysis to convince the reader of the results presented. **References** N Diaye, A. T. et al. In situ observation of stress relaxation in epitaxial graphene. New J Phys 11, doi:Artn 11305610.1088 1367-2630 11 11 113056 (2009). Lim, C. H., Nesladek, M. Loh, K. P. Observing high-pressure chemistry in graphene bubbles. Angew Chem Int Ed Engl 53, 215-219, doi:10.1002 anie.201308682 (2014); Lim, C. H. et al. A hydrothermal anvil made of graphene nanobubbles on diamond. Nat Commun 4, 1556, doi:10.1038 ncomms2579 (2013). Banszerus, L. et al. Ultrahigh-mobility graphene devices from chemical vapor deposition on reusable copper. Sci Adv 1, e1500222, doi:10.1126 sciadv.1500222 (2015); Banszerus, L. et al. Ballistic Transport Exceeding 28 mum in CVD Grown Graphene. Nano Lett 16, 1387-1391, doi:10.1021 acs.nanolett.5b04840 (2016); Drogeler, M. et al. Dry-transferred CVD graphene for inverted spin valve devices. Appl Phys Lett 111, doi:Artn 152402 10.1063 1.5000545 (2017); Petrone, N. et al. Chemical vapor deposition-derived graphene with electrical performance of exfoliated graphene. Nano Lett 12, 2751-2756, doi:10.1021 nl204481s (2012). **(END of #1)** , 
 **Continuation of Repost of #1 (for better clarity)** u2022t**Raman Data** It has been stated that u201cThe higher compressive strains of these ICPu2013CVD graphene films are consistent with the absence of wrinklesu201d (**P205**). However, when the wrinkled graphene film is de-coupled from the metal, un-wrinkled at ~650 u00b0C and then, cooled back to room temperature, it should not experience any compressive stress as it is no longer coupled to the copper substrate! Hence, it is difficult to understand the reason behind this observed higher compressive stress in these films. Similarly, it is impossible to understand the u2018strain-couplingu2019 schematic in **Extended Data Figure 5**. Hydrogen functionalization of graphene is well-known (d). Hence, it should be clarified as to why no increase in the intensity of the Raman D peak was observed (**Extended Data Figure 3**) during ICP processing (owing to graphene functionalization by reaction with hydrogen gas, free radicals or ions). Additionally, the paper describes the formation of nano-bubbles where the trapped hydrogen gas is under significantly higher pressures: high pressure chemistry of graphene with several reagents have been reported. Such high pressure reactions of graphene and hydrogen may cause an enhancement of the Raman D peak. Finally, the metallic Cu-Ni substrate may play a catalytic role in such reactions. All these aspects should also be clarified.  **Footnotes** (d) For example, thermal etching of graphene with H2 gas has been observed at high temperatures (~700 u2013 1000 u00b0C). Again, graphene can be etched with hydrogen plasma, at much lower temperatures(~RT and above). See: (i) Gao, L. et al. Repeated growth and bubbling transfer of graphene with millimetre-size single-crystal grains using platinum. Nat Commun 3, 699, doi:10.1038 ncomms1702 (2012); (ii) Xie, L., Jiao, L. Dai, H. Selective etching of graphene edges by hydrogen plasma. J Am Chem Soc 132, 14751-14753, doi:10.1021 ja107071g (2010); (iii) Hug, D. et al. Anisotropic etching of graphite and graphene in a remote hydrogen plasma. NPJ 2D Materials and Applications 1, doi:10.1038 s41699-017-0021-7 (2017); (iv) Diankov, G., Neumann, M. Goldhaber-Gordon, D. Extreme monolayer-selectivity of hydrogen-plasma reactions with graphene. ACS Nano 7, 1324-1332, doi:10.1021 nn304903m (2013); (v) Shi, Z. et al. Patterning graphene with zigzag edges by self-aligned anisotropic etching. Adv Mater 23, 3061-3065, doi:10.1002 adma.201100633 (2011); (vi) Elias, D. C. et al. Control of graphene s properties by reversible hydrogenation: evidence for graphane. Science 323, 610-613, doi:10.1126 science.1167130 (2009). **CONTINUED** ,