Accessible and Inexpensive Parameter Testing Platform for Adhesive Removal in Mechanical Exfoliation Procedures

Comments 1: This paper illustrates one modular system that can peel off 2D materials. This method controls the key parameters of angles or speed to control the exfoliations to improve yields. However, from my point of view, the key issue of this paper is lacking enough improving evidence between the peel-off in the traditional way and this new way. For example, besides Figure 7 of optical images, to illustrate and show differences among different conditions, at least AFM needs to be done.  The experimental design and scientific soundness are not enough for our journal. 

Response 1:  Thank you for pointing this out. We have revised to better describe how our proposed system is different in purpose from those intending to simply automate the exfoliation process but rather allow for rigorous study of its key factors. 

To more properly emphasize this point, the title has been revised slightly to:

Platform for Accessible and Inexpensive Parameter Testing for Adhesive Removal in Mechanical Exfoliation Procedures   

The end of section 2.1 on page 3 has been revised to the following:

Very few articles offering specifications for automated systems for the mechanical exfoliation deposition step have been published, and those that have been preprinted or published each require individual expensive and custom machined components (many in excess of US$2,000 each), do not address the normalization of peel speeds across different angles, or combine many variables at once, making quantitative study of individual factors difficult[8,9,16]. This system presented in this publication is not designed to simply automate production of high quality flakes, but instead to allow isolated analysis of the factors that improve deposition of high quality flakes and provide a low-cost, accessible solution for researchers and educators

We also agree that the optical images

were lacking to show quality of 2D materials and have added Raman spectroscopy data in Figure 7.  

Comments 1: Only when the contact point between the substrate and the tape has the same height as the moving stage, the contact angle is ϴ. As the exfoliation proceeds, the height of the contact point will change so the contact angle will also change. The author should assess the angle error during the exfoliation process using different angles.

Response 1: Thank you for pointing this out, we agree this is important to cover in the manuscript. We have added a portion addressing this angle error during testing.  

We have added the following to the first paragraph section 2.4 on page 7:

As the tape is peeled from the substrate, a slight difference in height is created between the moving stage and peel height. To compensate for this angle error, the moving stage and substrate mounting stage are placed sufficiently far to keep the change in angle less than 1° over the course of a peel. The graphene samples used for testing were 10mm, meaning approximately 580mm or greater distance to keep angle error within the tolerance range. The modular table mounts of the system and 600mm thread screw length are designed to offer flexibility to allow similar compensation for larger area samples.

Comments 2: The authors do not provide any information related to the crystal used for exfoliation. If the size, thickness, or quality of the crystals used for different exfoliations are different, the comparison will be in vain.

Response 2: We completely agree and believe we should provide more information about the base materials for the testing that was done. 

We have added the following information to the first paragraph of Section 3 on page 7:

Bulk crystal used for exfoliation was SPI Supplies Grade-1 HOPG 10x10x1mm with each sample prepared using the same exfoliation process. Samples were exfoliated 3 times on clean sections of a piece of mother tape then exfoliated a fourth time onto a fresh piece of daughter tape. 

Comments 3:  Normally, a slow speed will leave more samples and residual glues on the substrate, and this was also supported by Figures a and b. However, an opposite conclusion will be obtained by comparing Figures c and d. The author should provide reasonable explanations.

Response 3: Thank you so much for identifying this. We agree it is important to address the conclusions that could be obtained with the figures.

We have emphasized the typical behavior in the caption for Figure 6:

Differing densities of deposition at different peel angles and speeds. Results of fast 5 mm/sec and slow 0.01 mm/sec peels at 90° and 180° angles onto a single substrate. Dark blue areas represent few layer graphene flakes while lighter yellow portions are bulk graphite. Teal areas represent adhesive residue which is often more prominent on slower peels. Image was taken using a mosaic of 50x magnification optical microscopy images.

And added the following paragraph to Section 3.1 on page 9:

In most cases, slower peel speeds are expected to have larger amounts of residue present, however, the 180°  results in Figure 6 could be interpreted to suggest the opposite. This is possibly due to uneven contact with the hotplate during the heat annealing step of the procedure. This specific test was conducted using a single whole 100mm SiO2 wafer to normalize the substrate comparison between each sample. During normal procedures, smaller 25mmx25mm wafers are often used instead which are not expected to have difficulty with even heat distribution.   

Comments 4: Following comment 3, the authors only use one material, one substrate, and do a one-time experiment to get some conclusions. The authors should try more speeds, more angles, more materials, and repeat some times to solidify their conclusions.

Response 4:  Thank you for providing this feedback. We strongly agree that a more comprehensive study of more angles would further confirm the value of the proposed system,  and so we are in the process of conducting such a study for a follow-on publication. We have tried to keep the scope of this paper limited to the proof of the system’s function as an instrument and platform for accurate testing rather than the conclusions gathered through parametric study. 

To more properly emphasize this point, the title has been revised slightly to:

Platform for Accessible and Inexpensive Parameter Testing for Adhesive Removal in Mechanical Exfoliation Procedures   

We have added some additional data from another run at a different test angle in Figure 7 to bolster existing claims.

Comments 5: Following comment 4, the author did not provide any useful insights into angles and speeds used for material exfoliation since all researchers in this research field know that 180o and low speed are better than 90o and high speed, respectively. The authors should use this system to explore more. For example, they can try to exfoliate a very large monolayer crystal to prove the system is very powerful and useful.

Response 5:  We really appreciate this comment as it confirms we are researching a topic of significant interest. Similar to our response to Comment 4, we have limited the test results to demonstration of the utility of the system as a platform for parameter control and data collection. We agree that continuing research to more broadly investigate applications of the system would be of value yet are beyond the scope of this paper.

We have, however, added optical images of some examples of the system’s exfoliated monolayer flakes and their Raman Spectroscopy characterization data in Figure 7 to expand on our initial demonstration of the platform.  

Comments 1: Please discuss in more detail the difference between the existing two-dimensional materials, to which materials your techniques are applied the best and why?

Response 1: Thank you so much for this comment and your interest in our work. Our technique was developed specifically targeting the optimization for deposition of graphene, as this is the basis material for the devices we are working on currently fabricating. Specifically, we are investigating superconducting devices utilizing Magic Angle Twisted Bilayer Graphene (ref: Nature 556, 43–50 (2018). https://doi.org/10.1038/nature26160).

Since mechanical exfoliation procedures are mostly similar between other 2D materials, the system proposed in our manuscript is capable of being applied just as easily for investigation of other popular materials such as, WSe2 and MoS2. We believe that virtually any material that utilizes the popular “Scotch Tape” method of mechanical exfoliation can better investigated with a tool such as our proposed system.

Comments 2: The paper contains only two equations relating connecting actuator speed and substrate peel speed, so it would be also good to elaborate on this mathematical model a little more.

Response 2: Yes, thank you we did have additional computations to relate the actuator and substrate peel speed but they are mostly straightforward conversions for simple mechanical functions of the system such as converting the stepper motor’s revolutions to the pitch of the thread screw. 

The details of these computations were drafted but left out of the publication as it was evaluated to detract from the more important aspect which is the angle correction that is often overlooked in other similar work. The additional mechanical modeling behaviors are available through the software available on the linked Github page in the manuscript although some are abstracted into existing imported library functions.   

Comments 3:  This paper could become more interesting for a broader readership if the authors discuss how the zero or small band gap --  the most specific properties of  graphene and other two-dimensional Dirac materials --  could be modified by circular polarized itradiation shown in https://journals.aps.org/prb/abstract/10.1103/PhysRevB.99.205135 and some other similar publications.

Response 3: Thank you so much for this suggestion. We are very interested in including specifics of the electrical properties of graphene in a follow-on publication utilizing this system for a larger study. 

For this current publication, we are prioritizing the scope of this manuscript on the proof of the system’s capability as a data collection and parametric investigation platform for the purposes of mechanical exfoliation. 

To more properly emphasize this point, the title has been revised slightly to:

Platform for Accessible and Inexpensive Parameter Testing for Adhesive Removal in Mechanical Exfoliation Procedures   

Comments 1: Only tape exfoliation process is not the most tricky and labor-heavy part of this process. A lot of efforts are also devoted to the characterization of 2D materials in terms of optical microscope (OM), AFM, or Raman process. To really prove this work is worthy of publication, the authors must elaborate on, for instance, how to locate or autonomous with 2D selecting process. If only just using mechanical-assisted 2D tape splitting, random exfoliated 2D with varying quality and thickness remains on a big number of locations on substrates, and heavy human works are still needed to select the 2D after this process.

Response 1: We appreciate this comment very much and we agree that the characterization of 2D material flakes is another laborious and time-consuming procedure to occur after the mechanical exfoliation procedure. We also agree it is necessary to elaborate on the existing software and tools we employed for flake identification and characterization. 

We have revised Section 1 Paragraph 3 on page 2 with the following:

In this publication, an instrument and methodology are introduced that offer the ability to test and isolate specific conditions that have a large effect on yield as well as automate control specific parameters during one of the most time-consuming portions of the procedure. 

We have added further detail in the last paragraph of section 3 on page 7:

Deposited flakes were examined using a Horiba Xplora Plus video microscope's software mosaic feature to automatically scan high resolution images of sample surface areas. Optical microscopy images of the results, each demonstrated speed and angle combination, are shown in Figure 6. A quick qualitative analysis can show differences in deposition densities when varying peel angle and speed between a relatively fast pull of 5 mm/s and slow pull of 0.01mm/s. Images were manually searched for areas of higher density flakes and were further investigated via optical microscope to determine individual flake surface area and number of layers via Raman spectroscopy, shown in Figure 7. 

Comments 2: Related to the above, as for Table 1, the vendor and prices are typically not within the interest of a research paper, as anyhow this is an academic paper not a patent. Instead, the reliability, and how the quality of the resultant 2D are the focused.

(1) The authors need to list another table, comparing this work with other massive already published papers on the autonomous/ or machine-assisted transfer tools to compare. Key parameters, like the yield, speed, resultant 2D flake sizes, and the uniformity or layer-number controllability, need to be properly and thoroughly benchmarked with other published references.

(2) This designed setups must be compared and benchmarked with other similar publications to highlight the novelty and advantages of this proposed work. For instance, precise MEMS-based 2D transfer and twisted rotation tools are already proposed (Ref: Nature 632, 1038-1044, 2024) with good performance.

Response 2: 

(1)

Thank you for catching this, we agree we must better establish this system compared to existing automated tools that have been published. We have revised to better describe how our proposed system is different in purpose and addresses some shortcomings when trying to use existing systems for that purpose. 

To more properly emphasize this point, the title has been revised slightly to:

Platform for Accessible and Inexpensive Parameter Testing for Adhesive Removal in Mechanical Exfoliation Procedures   

(2)

The precise MEMS-based 2D transfer and twisting tools provided in Ref: Nature 632, 1038-1044, 2024 address a different procedure, specifically the manipulation of flakes for fabrication after they are already deposited onto a desired substrate. Many other publications exist to address that but the topic largely differs from our system’s purpose as we are addressing the exfoliation and deposition yields of graphene flakes onto the initial target substrate before manipulation. Other systems we have found that most similarly address our research either exist only in preprints and press releases or have large differences in their execution that do not meet our system’s goal. We have tried to clarify this in our revision.

The section 2.1 paragraph 3 on page 3 has been revised to emphasize these differences:

Very few articles offering specifications automated systems for the mechanical exfoliation deposition step have been published, and those that have been preprinted or published each require individual expensive and custom machined components (many in excess of US$2,000 each), do not address the normalization of peel speeds across different angles, or combine many variables at once, making quantitative study of individual factors difficult[8,9,16]. This system presented in this publication is not designed to simply automate production of high quality flakes, but instead to allow isolated analysis of the factors that improve deposition of high quality flakes and provide a low-cost, accessible solution for researchers and educators.

Comments 3:  In background, advanced 2D CVD techniques providing grain-boundaries-free 2D sheets (see Ref: Nature 614, 88–94, 2023), and van der Waals transfer approaches that has high impact on 2D materials performance should be added.

Response 3: 

Thank you so much for bringing this other publication to our attention, we very much agree that the latest developments in other techniques should be addressed properly. We have made sure in the background section to address the progress that has been made with reduction in grain boundaries using chemical vapor deposition. Additionally, We have made minor additions to emphasize our study utilizes graphene as the primary material of focus during our testing. 

We have also revised the Background section paragraph 1 on page 2 to the following:

While alternative methods can offer scalability or more control of key parameters, the adverse effects they introduce can compromise the quality of the resulting material, making mechanical exfoliation still preferable for certain applications requiring pristine, high-quality flakes. The liquid phase exfoliation technique often introduces defects and contaminants through liquid contact. Chemical Vapor Deposition (CVD) of graphene allows for relatively precise control of parameters involved in the crystalline growth during deposition of graphene, but other publications have shown CVD to produce grain boundaries which degrade the electrochemical properties of the material[10-12]. Although recent advances in this area with other materials, such as WSe2 and MoS2, suggest the CVD method may become a compelling alternative for graphene in the future. [13].

The abstract has been revised with the following:

“Mechanical exfoliation of two-dimensional (2D) materials using adhesive tape is a widely used method for producing high-quality single-layer graphene flakes. However, this technique is time-consuming, with low yields and inconsistent results due to process variations and human error. This paper introduces a modular system designed to rigorously test and optimize the conditions for 2D material deposition focusing on graphene.

Comments 4: More explanations & justifications must be made for Fig. 2 & Fig. 3. For example, will some cases happen in this setup? e.g. like the 2Ds already transferred on substrate can be taken away by the repeating tape rolling process? According to current description, the tool looks lack certain intelligence for the transfer process and the transfer results can be also highly random or not very repeatable/ layer-number controllable.

Response 4: We completely agree with this point and believe additional information should be added further describing the operation cases of the system. A large issue with mechanically exfoliated transfer methods is they lack an intelligent way of maximizing yields in the transfer process making it inherently highly random. The tape is not rolled in our case but rather removed a single time very slowly depositing material onto the substrate surface. The process normally relies on the use of a skilled technician who slowly peels the tape by hand making testing the parameters of the peeling process difficult to control and measure quantitatively.

We have added the following to the end of section 2.2 paragraph 4 on page 4:  

Operation of the system covers the adhesive removal step of the mechanical exfoliation procedure. Samples are cleaned and prepared using the standard procedure exfoliation until the samples are heat annealed onto the substrate. After, samples are then mounted to the angled substrate mounting stage. The system is then homed to a fixed position at the start of each test and the peel angle and speed is defined via software for speed correction and normalization. Another piece of adhesive tape is attached between the moving stage and loose end of tape attached to the substrate. When the start command is issued, the system automatically peels the tape from the substrate at a precisely controlled speed and angle providing a consistent, repeatable peel.

Comments 5: Fig. 5 is not informative. More annotations and descriptions are needed.

Response 5:  This is a very useful comment, thank you so much for pointing this out. 

We have combined Figure 5 with Figure 4 to provide better comparative context then added the following caption to Figure 4:

Angled Substrate mounting blocks and locking base. (a) Examples of other modeled
substrate mounting blocks listed from left-to-right top-to-bottom 120◦ 105◦ 90◦ 60◦ 45◦ and 30◦.
Angles of any incremental degree can be modeled, 3D printed, and interchanged into the system.
(b) Diagram depicts the correlation between actuator displacement and peel distance along with
an example of a 45◦ block slotted into the interlocking base. Peel angles are measured from the
horizontal plane and are notated as θ.

Comments 6: Some fundamental contents are insufficient at current manuscript. For example, the tool execution speed is almost a pure mechanical parameter and very straightforward to manage (as for Table 2). What is the relation of this to the quality and yield etc. of the 2D transfer? The authors should focus on meaningful parameters and direct 2D transfer performance parameters instead of indirect ones like the speed. 

Other meaningful table summary like for which speed, how many repeating cycles, at what angles, etc. can get optimal 2D transfer results can be listed instead for meaningful discussions.

Response 6: We very much agree that the focal point of this publication should be data associated with the actual deposited material. We have reorganized the results section to focus on the deposited results and data associated at the beginning. 

We have moved the Table 2 to the end of the results section and to emphasize the difficulties associated with managing mechanical parameters if data is to be compared:

Accuracy of speed signals is critical when performing analysis of very slow peels as error can compound with each delay cycle of the stepper motor. To properly normalize data compared between different test runs, the speed error of the system must be validated across a wide range to ensure that a similar level of accuracy is achieved in different ranges. Compensation of peel speeds at different angles is also necessary to enable comparison of speed data between runs. The system's accuracy was validated using a stopwatch modified to start and stop based on limit switch inputs placed at a fixed interval on the linear guide rail. The system was measured by the amount of time to travel a fixed distance of 200mm over several orders of magnitude of speeds. The system demonstrated a relative speed error of less than 0.7% across all tested speeds, ranging from 5 mm/sec to 0.001 mm/sec. The results of the speed testing are shown in Table 2.

Comments 7: Actually the OM in Fig. 7 dose not convey sufficient meaningful information. Really useful 2D exfoliation or transfer processes should at least show the layer number of the resultant 2D flakes. Based on Fig. 7, readers are unable to tell the quality of this exfoliation process.

Critical 2D characterizations, at least one, such as AFM showing flake thickness, or location-resolved Raman spectroscope of the different 2D flakes, or the SHG or PL signals that give info on the 2D layers and quality, must be given in revised manuscript for the readers to evaluate the quality of this exfoliation processes.

Response 7: Thank you for bringing this up, our paper could be improved through more data associated with the resultant 2D flakes. 

We have added Raman spectra data for a deposited single layer and multilayer flake in Figure 7 with the following caption:

Examples of Raman spectra evaluation of flakes obtained from the system during a slow 0.01 mm/sec peel at a 45° angle. Image and data was taken using a 100x magnification lens on a Horiba Xplora Plus. Single Layer Graphene (SLG) is characterized by a higher intensity 2D peak relative to the G peak. Multi-Layer Graphene (MLG) is identified by the 2D peak position and intensity relative to the G peak [14].