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Printing two-dimensional functional materials is one of the current hot spots in materials science. However, so far, this has proven to be problematic. A paper published in the "Advanced Materials" magazine demonstrated a novel and versatile method for room temperature printing and coating of these industrially important materials.
Structure and film formation of polymerized two-dimensional materials. (a) Left: Graphene dispersion aggregated in NMP (1 vol.%), which can form a gel. Right: A transparent film obtained from the same dispersion after homogenization (low shear treatment). Scale bar: 1 cm. (b) In the dynamic shear rheometer, the aggregate dispersion is treated with low shear by the low-speed rotation of the parallel plate. Curve (I): the degree of aggregation and dispersion obtained. During the acceleration phase, a huge unrecoverable drop in viscosity indicates an orderly structure. Curves (II) and (III): Repeat the same test on the same ink, rest for 30 minutes in between. Curve (IV): The same ink is processed by three-roll milling. (c) Amplitude scanning test of vdW ink made of different 2D materials with different solid content, confirming their gel structure. (D) (Left) Aggregate dispersion and (right) Schematic diagram of the internal structure of the gel obtained after shearing the aggregate dispersion. (e) Binary X-ray tomogram of freeze-dried graphene vdW ink (scale bar: 40 µm) and (f) SEM image (scale bar: 2 µm). Image source: Abdolhosseinzadeh, S et al. Advanced Materials
Traditionally, additives have been used to solve the processing challenges inherent in manufacturing materials. However, this can cause problems. Additives can significantly reduce the electronic properties of the material. The high temperature post-deposition process is used to remove additives, but this complicates the manufacturing process and limits the available materials, thereby eliminating the possibility of using heat-sensitive materials.
To meet these challenges, it is necessary to produce additive-free inks that do not require post-processing. Now, the research that has been proposed has developed a general strategy to achieve this goal by taking advantage of the unique properties of 2D materials.
The 2D material family is very large, hundreds of materials have been discovered, and more materials are expected. They display a wide range of chemical and physical properties. Two-dimensional materials can be synthesized by solution-based methods or collected from layered crystals suspended in liquid. Both synthesis methods produce a suspension of two-dimensional nanosheets, which can then be further processed into printable inks.
The concentration of nanoplatelets in the suspension has an upper limit, beyond which they will become unstable. However, high-concentration ink is necessary to form a network of percolating particles. Regardless of the concentration, the suspension is thermodynamically unstable.
Particles reduce their surface energy by aggregation. Minimizing the difference in surface energy between the solvent and the material will reduce this settling rate. In addition, the solvent may not be suitable for subsequent processing.
In traditional inks, additives such as binders and surfactants are used to solve problems and make functional inks. A commercial example is graphene: a large amount of polymer binder is required to process screen printing materials.
Post-processing is helpful, but can cause problems such as cracking and delamination. Similar problems exist in selective heat treatments, including laser or microwave-based printing methods.
(i) A schematic diagram of two nanosheets approaching each other to reassemble. (jl) Digital photos of terpineol-graphene vdW inks in three concentrations for (j) flexographic printing (0.7 vol.%), (k) screen printing (2 vol.%) and (l) Extrusion printing (4 vol.%)). Image source: Abdolhosseinzadeh, S et al. Advanced Materials
The team eliminated the need for additives and post-processing. The additive-free process uses van der Waals interactions between particles in the material to create a type of ink called vdW ink. By manipulating this characteristic, the solvent can be dispersed in the interconnected network of two-dimensional materials.
This improves the range of solvent selection, which is currently limited by factors related to dispersibility.
In addition, the inter-sheet vdW attractive force controls the flow behavior of the ink and the mechanical properties of the resulting film. The study discussed in detail the structure, film-forming behavior and rheological properties of the ink.
It also demonstrated the potential for large-scale production and formulation of inks for commercially viable high-throughput methods and their room temperature applications.
This process overcomes current challenges and can prevent undesirable consequences when printing 2D materials. Able to facilitate room temperature printing methods, these inks provide a unique method.
In order to make a space-filled gel, the nanosheets must be in close enough contact with each other to take advantage of van der Waals interaction and aggregation. The particles are firmly bonded together, and the film and substrate are bonded together.
In order to create an ink with stable flow rate behavior that can be printed or coated on a substrate, a short and gentle shear is required through a process including three-roll milling.
The flattening and alignment of the nanosheets is manifested as a significant and irreversible drop in the viscosity level, thereby forming a gel that can be synthesized and printed at room temperature without any additional additives or post-processing chemicals. Drying the ink after deposition will produce a uniform and continuous film.
Application of vdW ink. (a) From left to right: 1. Slit-coated propylene glycol-graphene vdW ink on PET substrate (substrate size: DIN-A5). 2, 3. 0.7 vol.% Terpineol-IPA-graphene vdW ink (Terpineol: IPA 80:20) gravure printing and flexographic printing on glossy photo paper and PET substrate (respectively). 4. Extrusion printing 8 vol.% NMP-WSe2 vdW ink on PET substrate (scale bar: 1 cm). Image source: Abdolhosseinzadeh, S et al. Advanced Materials
The project solved several challenges. For example, depending on the coating or printing method, the film may encounter problems such as increased surface roughness, thickness variations, and discontinuities. In screen printing, the mesh of the cell walls will cause less or no ink deposits in some places.
With a sufficiently long recovery time, these problems will alleviate, but if it takes too long, the ink will spread uncontrollably and lose fidelity. The flow rate of vdW ink can be adjusted to meet the requirements of traditional printing methods. Other challenges were discussed and explored.
The potential of vdW ink is huge. An important commercial potential demonstrated in the study is the room-temperature screen printing of field-effect transistors based entirely on two-dimensional materials. Although challenges remain, these inks may provide solutions to common problems in the field of printable 2D functional materials.
Abdolhosseinzadeh, S et al. (2021) General method for room temperature printing and coating of 2D materials [Online] Advanced Materials | onlinelibrary.wiley.com. Available at:
https://onlinelibrary.wiley.com/doi/abs/10.1002/adma.202103660
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Reg Davey is a freelance writer and editor based in Nottingham, UK. Writing for news medicine represents a fusion of various interests and fields in which he has been interested and involved for many years, including microbiology, biomedical sciences, and environmental sciences.
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