Synthesis of WS₂ Ultrathin Films by Magnetron Sputtering Followed by Sulfurization in a Confined Space

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors report very thorough characterization of WS2 layers produced by sputtering of W or WS2 substrates. After deposition these layers are exposed to S vapor in a tube furnace to affect sulfurization. Results are most promising for forming the precursor film by sputtering a W target. Whereas the title states that the synthesis of ultrathin films is “optimized”, they still are far from flat and uniform. However, the preferential alignment of the (00L) planes for films produced by sulfurization of W-sputtered films suggests that further optimization should yield more uniform multilayer films. Since considerable variation in film structure is noted across the substrate, much of the variability may be related to non-uniformity in the sputtered flux. The authors might want to address how this flux could be made more uniform. Alternatively, can the surface chemistry of the substrate be altered to favor layer-by-layer growth rather than the formation of islands as indicated in the SEM images in Fig. 6? The easiest to try would be to strip the native oxide by dipping the Si wafer into HF prior to deposition. A second alternative would be to first form a WSi2 layer by sputtering and annealing and then sputtering again before sulfurization.

The manuscript is well written and flows logically. At Line 124, change to “a film thickness”. In the methods section the authors should describe how the native oxide layer on the Si substrate was cleaned prior to deposition.

Author Response

Reviewer 1

The authors report very thorough characterization of WS2 layers produced by sputtering of W or WS2 substrates. After deposition these layers are exposed to S vapor in a tube furnace to affect sulfurization. Results are most promising for forming the precursor film by sputtering a W target.

Dear Reviewer,

We appreciate your time and attention to give useful suggestions for improving our work. Below are our answers to your questions.

1. Whereas the title states that the synthesis of ultrathin films is “optimized”, they still are far from flat and uniform.

Answer: We agree with your comment. However, it is worth mentioning that an optimization was made since we also did other sulfurization experiments, similar with those found in the literature that uses very large amounts of sulfur (i.e. 2 g), that are not shown in the manuscript. We encountered the following difficulties:

1. Due to the high concentration of sulfur vapors, they condensed on the exhaust valve, clogged and created overpressure, that caused the flanges on the quartz tube to jump, losing the inert atmosphere;
2. If the concentration of sulfur vapors is reduced, the precursors are not completely sulfurized;
3. When the adequate concentration of sulfur vapors was found, the W precursor was chemically etched by sulfur and almost nothing remains on the substrate.

Our solution to these challenges was to conduct sulfurization within a confined graphite box, significantly reducing the amount of sulfur needed and mitigating the issues encountered in open systems.

Text changes: (page 1, Title) Synthesis of WS₂ Ultrathin Films by Magnetron Sputtering followed by Sulfurization in a Confined Space

2. However, the preferential alignment of the (00L) planes for films produced by sulfurization of W-sputtered films suggests that further optimization should yield more uniform multilayer films. Since considerable variation in film structure is noted across the substrate, much of the variability may be related to non-uniformity in the sputtered flux. The authors might want to address how this flux could be made more uniform.

Answer: According to the work of Wu et al. [C.-R. Wu, T.-W. Chu, K.-C. Chen, S.-Y. Lin, Preparation of Large-area Vertical 2D Crystal Hetero-structures Through the Sulfurization of Transition Metal Films for Device Fabrication, JoVE (2017) 56494. https://doi.org/10.3791/56494.] this non-uniformity in thickness of the sulfurized W precursor is due to the oxide (WO_x) layer, formed on the surface of the W precursor when it comes into contact with the ambient air. During the sulfurization treatment, the uniform oxide layer segregates into islands because this phenomenon is faster than the formation of WS₂, especially when the sulfur vapor concentration is low. The solution should be the use of a rich sulfur atmosphere. Unfortunately, a larger amount of sulfur (than what was reported in the paper) cannot be introduced into the graphite box. We are now searching for a technical solution.

We attempted to mitigate the issues related to non-uniformity in the sputtered flux by employing a very low deposition rate (~0.005 nm/s) during magnetron sputtering, which resulted in uniformly deposited W films with roughness below 1 nm. This approach was aimed at ensuring a uniform initial layer, anticipating that the subsequent sulfurization step would benefit from this uniformity. However, achieving a more uniform sulfurized film remains a challenge.

3. Alternatively, can the surface chemistry of the substrate be altered to favor layer-by-layer growth rather than the formation of islands as indicated in the SEM images in Fig. 6? The easiest to try would be to strip the native oxide by dipping the Si wafer into HF prior to deposition. A second alternative would be to first form a WSi2 layer by sputtering and annealing and then sputtering again before sulfurization.

Answer: Thank you for suggesting innovative approaches to enhance the layer-by-layer growth of WS₂ films. The suggestion to strip the native oxide by dipping the Si wafer into HF prior to deposition is an excellent point. We plan to incorporate this step into our process to see if it promotes more uniform layer growth. This could potentially provide a cleaner surface for the initial deposition, which might favor the desired growth mode. Additionally, your proposal to form a WSi2 layer through sputtering and annealing, followed by a second sputtering step before sulfurization, is an interesting approach. While we observed the formation of Si-W bonds in the case of the WS2 precursor, indicating some level of interaction with the substrate, the resulting improvement was not as significant as hoped.

Moreover, we also plan the adjustment of the sputtering setup to allow for sulfurization within the setup itself. This would prevent the exposure of the W film to air, potentially avoiding the formation of tungsten oxide. Even if we aim to avoid the use of toxic gases, the use of H₂S may provide a pathway to achieving more uniform films.

4. The manuscript is well written and flows logically. At Line 124, change to “a film thickness”.

Answer: We corrected all the typos that we found in the manuscript.

Text changes: The changes are highlighted in yellow in the revised manuscript.

5. In the methods section the authors should describe how the native oxide layer on the Si substrate was cleaned prior to deposition.

Answer: We added the details in the Materials and Methods section

Text changes: (page 3, line 121) The silicon substrates with a native SiO₂ layer were cleaned consecutively with acetone (20 min), isopropyl alcohol (20 min) and DI-water.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The submitted manuscript entitled ‘Optimized Synthesis of WS₂ Ultrathin Films on Si Substrates using Magnetron Sputtering followed by Sulfurization’ describes the synthesis of tungsten disulfide films followed by general characterization, including XRR, XRD, SEM, XPS, and Raman scattering spectroscopy. While the letter seems sound and clear, I fail to find any novelty in its contents. Moreover, I find the title a little bit misleading as there is no optimization of any synthesis steps. The authors used a relatively common route of the synthesis (a very similar process as in some cited references, e.g., ref. 19). However, they failed to provide new insights into the topic.

My additional comments on the manuscript are the following.

In section 2.2, Sulfurization process for pr^W and pr^WS2 layers, there are certain ambiguities I would like to address. First, the authors state that the oxygen was purged from the sealed quartz tube prior to the introduction of argon. Here, I’m not sure what they mean by this step. Do the authors mean the air (not the oxygen) was removed by pumping? In the same section, the amount of sulfur of 25 mg was used. I wonder if this amount has some specific justification; in other words, why exactly 25 mg of S? In fact, I would expect (and again, as the title evokes) that there will be some optimization (e.g., different amounts of sulfur, different thicknesses of individual samples, etc.).

In section 2.3., Characterization of the precursor and sulfurized films, the authors emphasize the meticulous analysis of samples (even twice). While the reader does not doubt the time-consuming experiments regarding the characterization of samples, there are numerous papers on the topic that provide more valuable information. In other words, very few results give new additions to the research of transition metals dichalcogenides itself – although one realizes this may be the limitation of available characterization techniques and resources suitable for such research.

In conclusion, I do not recommend the manuscript for publication in Surfaces journal due to its lack of originality and novelty.

Author Response

Reviewer 2

The submitted manuscript entitled ‘Optimized Synthesis of WS2 Ultrathin Films on Si Substrates using Magnetron Sputtering followed by Sulfurization’ describes the synthesis of tungsten disulfide films followed by general characterization, including XRR, XRD, SEM, XPS, and Raman scattering spectroscopy. While the letter seems sound and clear, I fail to find any novelty in its contents. Moreover, I find the title a little bit misleading as there is no optimization of any synthesis steps. The authors used a relatively common route of the synthesis (a very similar process as in some cited references, e.g., ref. 19). However, they failed to provide new insights into the topic.

Dear Reviewer,

Thank you sincerely for your thorough review and constructive feedback. We appreciate the opportunity to clarify and highlight the novel contributions of our work. Below are our responses, incorporating suggested textual changes to the manuscript to reflect these clarifications.

Answer: Our study introduces several innovative aspects as follows:

1. We obtained WS₂ films directly on Si\SiO₂ substrates, unlike many studies that require transfer, potentially introducing defects. The studies found in literature, as well as the reference indicated by the reviewer, use a sapphire substrate to grow the film, afterwards being transferred on Si\SiO₂ substrates to build devices.
2. We use a new sulfurization process, in a confined space. While in all the literature papers, sulfur vapors are produced in a cooler area of the furnace and are carried out by an Argon flow to the precursor films, in our case, the sulfur powder and the precursor films are held together, in a confined space at the same temperature. This significantly reduces the quantity of necessary sulfur during the process. In literature, 1 - 2.5 g of S is used, while in our report 25 mg of S were enough. This quantity is at least 40 times smaller, which avoids the formation of toxic gases such as SO₂ during sulfurization, marking a significant improvement in environmental and safety aspects.
3. We used an ultra-low deposition rate (i.e. ~0.005 nm/s) that can lead to uniform ultra-thin films, as compared with the reference indicated by the reviewer where a deposition rate of 5 nm/s was used in order to obtain a film of ~25 nm
4. Our method does not require toxic gases such as H₂S for sulfurization, enhancing the safety and environmental friendliness of the synthesis process, as opposite of the study from the reference indicated by the reviewer.
5. This study is the first to compare the use of both W and WS₂ precursors under the same experimental conditions, to our knowledge, providing insights into their effects on the films properties.

Regarding the optimization of the sulfurization process, as we also replied to Reviewer 1, we also did other sulfurization experiments, similar with those found in the literature that uses very large amounts of sulfur (i.e. 2 g), that are not shown in the manuscript. We encountered the following difficulties:

1. Due to the high concentration of sulfur vapors, they condensed on the exhaust valve, clogged and created overpressure, that caused the flanges on the quartz tube to jump, losing the inert atmosphere;
2. If the concentration of sulfur vapors is reduced, the precursors are not completely sulfurized;
3. When the adequate concentration of sulfur vapors was found, the W precursor was chemically etched by sulfur and almost nothing remains on the substrate.

The saving solution was doing the sulfurization in a graphite box.

We agree that our process still needs improvements, but the results are promising and we believe that they are worth to be shown for discussion and further advancements.

Text changes: (page 1, line 2): Synthesis of WS₂ Ultrathin Films by Magnetron Sputtering followed by Sulfurization in a Confined Space

(page 1, line 12): In the quest for advanced materials suitable for next-generation electronic and optoelectronic applications, tungsten disulfide (WS₂) ultrathin films have emerged as promising candidates due to their unique properties. However, obtaining WS₂ directly on the desired substrate, eliminating the need of transfer which produces additional defects, possess many challenges. This paper aims to explore the synthesis of WS₂ ultrathin films via physical vapor deposition (PVD) followed by sulfurization in a confined space, addressing the challenge of film formation for practical applications. Precursor layers of tungsten and WS₂ were deposited by RF magnetron sputtering. Subsequent sulfurization treatments were conducted in a small, closed graphite box, to produce WS₂ films.

(page 3, line 106): This study meticulously investigates the synthesis and properties of WS₂ ultra-thin films, using PVD followed by sulfurization in a confined space utilizing two distinct types of precursors, namely W and WS₂. The use of a small, closed graphite box for the sulfurization significantly reduces the quantity of necessary sulfur during the process Through a detailed exploration that encompasses deposition, sulfurization and comprehensive characterization, we elucidate the complexity of fabricating WS₂ films. The tungsten disulfide ultra-thin films are obtained directly on Si substrates, eliminating the necessity of transfer which produces additional defects. Moreover, no toxic gases such as H₂S are used during the fabrication process. This work provides important insights into the production of WS₂ ultra-thin films.

1. My additional comments on the manuscript are the following. In section 2.2, Sulfurization process for prW and prWS2 layers, there are certain ambiguities I would like to address. First, the authors state that the oxygen was purged from the sealed quartz tube prior to the introduction of argon. Here, I’m not sure what they mean by this step. Do the authors mean the air (not the oxygen) was removed by pumping?

Answer: Thank you for pointing out the need for clarity in our description of the sulfurization process. Indeed, by stating "oxygen was purged," our intention was to communicate the removal of atmospheric air, which naturally includes oxygen, to prepare for a controlled argon environment essential for the sulfurization process. The correct procedure, involved evacuating the sealed quartz tube to remove air. This step was performed three times to ensure a thorough evacuation before introducing a constant flow of argon gas at 50 sccm, establishing the inert atmosphere required for our experiments.

Text changes: (page 3, line 139): The air was removed from the sealed quartz tube by pumping, and a constant argon flow of 50 sccm was introduced.

2. In the same section, the amount of sulfur of 25 mg was used. I wonder if this amount has some specific justification; in other words, why exactly 25 mg of S? In fact, I would expect (and again, as the title evokes) that there will be some optimization (e.g., different amounts of sulfur, different thicknesses of individual samples, etc.).

Answer: The selection of 25 mg of sulfur was determined by the physical constraints of our experimental setup, particularly the capacity of the graphite box used in the sulfurization process. We found that this amount represents the maximum that can be introduced, as higher quantities led to increase vapor pressure that would remove the box lid. We also tried alternative methods, including the use of 2 g of sulfur in a quartz boat, which did not yield successful results under our conditions. We acknowledge the reviewer's point regarding the optimization of experimental parameters, such as sulfur amount and sample thickness. We plan to address these variables in future studies to further refine our understanding and optimization of the sulfurization process.

3. In section 2.3., Characterization of the precursor and sulfurized films, the authors emphasize the meticulous analysis of samples (even twice). While the reader does not doubt the time-consuming experiments regarding the characterization of samples, there are numerous papers on the topic that provide more valuable information. In other words, very few results give new additions to the research of transition metals dichalcogenides itself – although one realizes this may be the limitation of available characterization techniques and resources suitable for such research.

Answer: We acknowledge that our findings might not surpass the existing literature on transition metal dichalcogenides. However, our work introduces a novel sulfurization process characterized by minimal sulfur usage, which represents a significant step towards more sustainable and efficient fabrication methods for these materials. Our results, while perhaps modest in groundbreaking revelations, highlights the potential of this new method as a promising direction for future research in the field.

4. In conclusion, I do not recommend the manuscript for publication in Surfaces journal due to its lack of originality and novelty.

Answer: We respect the reviewer's perspective regarding the perceived originality and novelty of our manuscript. However, we kindly request a reevaluation of our work in light of the detailed responses and clarifications provided above. We believe these elements collectively enhance the manuscript's novelty and align well with the research scope of. We are hopeful that these considerations will result in a reconsideration of our manuscript for publication in the Surfaces journal.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors investigated ultra thin WS2 film grown by PVD followed by a sulfurization process. All data are looked good. Here are several suggestions.

1. Since the films thickness is so thin, EDS in FESEM includes very deep signal. It produced a large error for those thin films. Why not the authors calculate the composition of the thin films through XPS data, which was already done.

2. It's very strange for the unavailable  XPS data of sulfurized prWS2 sample, since the GIXRD showed the difraction peak of WS2. Because the XPS measurement is very sensitive for the surface atoms.

3. Several superscripts of 'prw' and 'prWS2' in Line No. 256, 275 should be corrected.

Author Response

Dear Reviewer,

Thank you for your time and comments. Attached are our responses.

Best regards,

Alin Velea

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

I want to thank the authors for the improvements/changes in the manuscript based on my previous review. I believe the authors provided sufficient additional information to reevaluate my decision. I would also like to acknowledge that the authors changed the title of the manuscript. Therefore, I find the current form of a paper acceptable for publishing in Surfaces journal.