Assembling 2D nanosheets into van der Waals thin films for large-area electronics

We here define and generalize the concept and applicability of van der Waals thin films constructed from assembling diverse 2D nanosheets (metallic, semiconducting, insulating), to enable a unique class of flexible thin-film electronic devices.
Assembling 2D nanosheets into van der Waals thin films for large-area electronics

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Towards solution-processable thin-film electronics

To deliver emerging applications based on large-area flexible and wearable devices, such as next-generation displays, electronic paper, and health monitoring systems, in the past two decades, the solution-processable organic semiconductors have offered an appealing path to semiconductor thin films on flexible substrates using low-temperature and scalable processes such as inkjet printing. A most exciting advantage of the solution-based fabrication processes is the potential for high-throughput "roll-to-roll" continuous productions which may enable cost-effective manufacturing, an essential step towards practical large-area electronic applications. But, organic semiconductors are facing their own challenges, including the rather low electronic performance (typical carrier mobility < 1-10 cm2·V-1·s-1, despite higher mobility > 10 cm2·V-1·s-1 was reported), and intrinsic structural instability and performance sensitivity to ambient environment (air, moisture, heat, etc.), which creates critical roadblocks to practical applications.

Solution-processable inorganic nanostructures

An alternate approach to the next-generation large-area flexible electronics is made possible with the solution-dispersible inorganic nanostructures. With reduced dimensions at the nanoscale, those inorganic nanostructures, which are difficult to be dissolved or processed in the bulk form, can now be dispersed and solution processed like organic molecular materials. Meanwhile, inorganic crystalline nanostructures may inherit the superior electronic performance and robust material/performance stability from their bulk counterpart. To this end, solution-processable inorganic crystalline nanostructures may represent a unique class of materials that combine the merits of both organic and inorganic structures. Over the past two decades, solution-processable 0D quantum dots and 1D nanowires have been extensively studied for constructing thin-film electronics, with many wonderful device demonstrations been realized already.

How about using 2D van der Waals thin films for large-area electronics?

Here, we explored the application of solution-processable 2D nanosheets in thin-film electronics. The 2D nanosheets derived from layered crystals (such as graphene and MoS2) are intrinsically free of surface dangling bonds and thus interfacial chemical disorders that are typically seen in most 0D or 1D nanostructures originating from the termination of bulk crystal lattice, surface ligands/species, and misalignment among randomly oriented nanocrystals. When assembled into continuous thin films, the dangling-bond-free 2D nanosheets tile on each other with broad-area plane-to-plane contacts and atomically clean van der Waals (vdW) interface. Such vdW interface is free of disordered chemical bonds and thus can promise optimum charge transport across the grain boundaries, as demonstrated in our previous studies (Nature 2018, 562, 254-258; Nano Lett. 2014, 14, 6547−6553). To capture the nature and merit of vdW interactions among the tiled 2D nanosheets, we define the resulting thin films as 2D van der Waals thin films.

In our most recent Perspective article, we examined the potential and challenges of creating large-area electronics based on these vdW thin films. In particular, we have

1. introduced the benefits of vdW thin films, including (1) significantly lowered density of grain boundary and thus interfacial trapping states, (2) formation of compact and dense thin films with a full surface coverage, (3) broad-area plane-to-plane vdW contacts between neighbouring nanosheets with dangling-bond-free clean interfaces and nearly negligible transport barrier at grain boundary, and (4) exceptional mechanical strength as the plane-to-plane vdW force scales with 1/d3 (d is the gap distance between nanosheets ).

2. assessed the solution-phase formulation of 2D nanosheet inks, evaluated the performance of currently available nanosheets, and discussed the requirements of nanosheet inks for constructing high-performance electronic devices.

3. described the potential techniques for the scalable assembly of large-area vdW thin films using the 2D nanosheets building blocks.

4. explored the construction of large-area flexible electronic devices by integrating diverse vdW thin films with distinct functionalities (i.e. metals, semiconductors, and insulators) and reviewed the current achievements in vdW thin-film electronics. 

5. pointed out some of the key fundamental and practical challenges that need to be addressed before transforming the vdW thin film concept into practical technologies, and prospected a few potential ideas or ways of solving those problems.

Within this Perspective article, we hope to define and generalize the concept and applicability of vdW thin films constructed from 2D nanosheets, to enable a series of thin-film electronic materials (metallic, semiconducting, insulating, etc.) with nearly ideal dangling-bond-free 2D/2D vdW interfaces and thus excellent electronic properties. It may define a new and robust material platform for high-performance, low-cost, solution-processable flexible devices.

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