Atomic-scale mapping of hydrophobic layers on graphene and few-layer MoS2 and WSe2 in water

By Manuel R. Uhlig, Daniel Martin-Jimenez & Ricardo Garcia from Materials Science Factory, Instituto de Ciencia de Materiales de Madrid, Madrid, Spain
Published in Materials
Atomic-scale mapping of hydrophobic layers on graphene and few-layer MoS2 and WSe2 in water

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1. Could you briefly outline the key findings of your paper?

R. We have developed a force microscope-based method (3D-AFM) to study the interactions of liquid water with 2D materials surfaces. The method provides atomically-resolved images and videos of the three-dimensional organization of water molecules and solutes at the 2D materials-liquid water interface. The interface is characterized by a layered structure formed by hydrophobic layers.  In fact, water molecules are expelled from the vicinity of the surface.


2. What is your role in this work?

R. Manuel Uhlig optimized the performance of the instrument by improving the sensitivity and the acquisition rate. He performed all the experiments involving 2D materials.  Daniel Martin-Jimenez performed some back up measurements on mica. He made decisive contributions during the design and development of our first 3D-AFM microscope. Ricardo Garcia generated the idea and planned the experiments.


3. What was the genesis of this paper?  How did you come to this particular problem?

Ricardo Garcia: Two independent factors have generated this paper. Our group develops high resolution force microscopes to understand how liquid water interacts with surfaces, in particular, biomolecular surfaces and interfaces. Some of those interfaces are characterized by the presence of both hydrophilic and hydrophobic regions. While there is many information at the atomic scale about the organization of water on hydrophilic surfaces much less is known about   the structure and organization of liquid water in the vicinity of hydrophobic regions. At this point, the 2D materials came into play. We have some expertise on 2D materials because in a different scientific activity, we are applying scanning probe lithography for making nanoelectronic devices with those materials. From the nanolithography  experiments we knew that 2D materials offer a wide variety of atomically flat hydrophobic surfaces. We use them as model hydrophobic surfaces to understand the interaction with water.. This is a step to address the problem on less ideal interfaces, from the topographic point of view, such as biomolecular surfaces. Our results are also very relevant for any application of 2D materials that involve the interaction with aqueous solutions. For example in  development biosensing and biomedical applications.


4. What is the most empowering implication of your results?

R. Our results show that the water molecules are expelled from the vicinity of a 2D materials surface.  The water layer starts about 2 nm from solid surface. The gap between the water layer and the 2D materials surface is occupied by several molecular-size hydrophobic layers. The molecules forming the hydrophobic layers come from the original liquid water. We remark that with independence of the purification process, any liquid water that is equilibrated with ambient air will contain some dissolved gases (N2) and airborne molecules. Those molecules are incorporated into the hydrophobic layers. We propose that the formation of hydrophobic layers near the surface is a universal property that applies to any extended hydrophobic surface immersed in liquid water.


5. How have 2D materials been uniquely instrumental to enabling these results?

R. Indeed, they have greatly facilitated the experiments and interpretation of the data. The relative easiness to prepare clean 2D and few-layer materials surfaces makes them excellent models to study the interaction and organization of water on hydrophobic regions.


6. Can you describe the main challenges associated to the preparation of this manuscript? Any anecdotes you’d like to share with us?

R. We liked the way the editor of NCOMMS handled the manuscript, from the selection of the reviewers to the speed on how the manuscript was processed.


7. Anything that stroke you as particularly surprising, unexpectedly pleasant/unpleasant during the peer review process?

R. We like the rigour (and speed) and of the peer review process. The three reviewers provide comprehensive and prompt responses. Initially, one of the reviewers was highly critical with our findings and recommended the rejection of the manuscript. He/She suggested  some additional experiments to be included in a future version. We entered in a sort of brainstorming process. In a few days, Manuel Uhlig came with a clever experiment to address the most critical question. The new data changed the reviewer’s opinion.  At the end, the three reviewers  have enthusiastically supported the publication of the manuscript. 


8. What is your favourite 2D paper published in 2016/2017, and why?

Ricardo Garcia:  I would like to mention a paper from 2018. Jarillo-Herrero group Nature paper about the discover of  superconductivity of graphene bilayers. It is just fascinating to show that by twisting the angle between adjacent graphene layers and pressing them together one can get a  superconductor.


9. Which is the development in the field of 2D materials that you would like to see in the next 10 years?

Ricardo Garcia:  I would like to see a product based on 2D materials that reaches the general public.


10. And now, what’s next?

Manuel Uhlig:

It is great to see the final article after all the time we spent in developing our 3D AFM, the software and the experiments. For me this was in particular an important step towards completing my PhD thesis. When I joined Ricardo’s group three years ago, 3D AFM was still a young technique. Based on the initial work of Daniel Martin-Jimenez, that was vital for the project, we continued optimizing our Amplitude Modulation 3D AFM over time and adapted it to our needs. It has turned into a user-friendly technique that we are now applying routinely to very different and also less ideal samples. While studying the 2D materials all of us have learned a lot and we are working already at follow-up ideas that came up during the writing of the article. In a general context, I see 3D AFM as a very promising technique with a growing number of users and applications, and I am quite happy to be part of this community.

Ricardo Garcia: First, we would like to take a moment to enjoy the publication. Second, we are curious about the reaction of the 2D materials and physical chemistry communities. After that, we plan to study the organization of water on more complex interfaces. 

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