2D-crystal inks and devices: graphene is just the start!

The class of 2-Dimensional (2D) atomic crystals started with graphene − a monolayer of carbon atoms arranged into a hexagonal lattice. The discovery of graphene opened a floodgate for many other 2D crystals, obtained by exfoliating their 3D layered counterparts. Nearly a dozen atomically thin materials have been demonstrated so far, but the class of 2D materials is very large if one considers the existence of hundreds of layered materials. Graphene stands out due to its unique electronic structure, which allows ballistic transport on a micron scale under ambient conditions. Furthermore, it is the strongest material available to us, its conductivity is millions of times higher than copper and it has very high thermal conductivity. The 2D thickness of graphene allows for maximum electrostatic control, optical transparency, sensitivity and mechanical flexibility. The other 2D crystals carry a wide range of interesting complementary properties. Of particular interest are: hexagonal-Boron Nitride (hBN), a semiconductor with a large band gap (~6eV) with excellent chemical and thermal stability, and Transition Metal Dichalcogenides (TMDCs), which have an array of electronic properties ranging from semiconducting to metallic, from charge density waves to superconducting, depending on their exact composition, electronic density, geometry, and thickness. For example, single-layer Molybdenum Disulfide (MoS2) is a semiconductor with a direct gap of ~1.8 eV ideal for optoelectronics; Niobium Selenide (NbSe) is a superconductor with critical temperature of 7.2K; Bismuth Telluride (BiTe) is a topological insulator, etc.

 Nobel prize medal printed with graphene ink on paper

Nobel prize medal printed with graphene ink on paper

2D crystals, being characterized by weak inter-layer interactions, can be easily combined in one stack with atomic precision, similar to “LEGO bricks”. The LEGO tower made in such way is called a “heterostructure”. Heterostructures play a crucial role in technology: for example semiconductor lasers, light-emitting diodes and fast electronic switches are all based on this concept. However, traditionally the choice of materials to be used for heterostructure fabrication has been limited to those that can be grown (typically by molecular beam epitaxy) one on top of another, thus limiting the types of heterostructures that can be prepared. In contrast, the family of 2D crystals allows us to make heterostructures of arbitrary complexity, which exploit the unique properties of 2D crystals, giving rise to better performance and multi-functionalities compared to heterostructures made with traditional methods.

2D-crystal based heterostructures are typically made by using 2D-material produced by micro-mechanical exfoliation. Photodetectors, tunneling transistors and light emitters have been demonstrated. However, the fabrication of such devices is not mass scalable, therefore such heterostructures still remain a lab product. One of the methods that could bring such heterostructures from the lab into real products is based on solution-processed 2D materials, which allows mass scalable and simple technique such as inkjet printing to fabricate 2D materials based heterostructures.

 Array of fully printed heterostructures on plastic

Array of fully printed heterostructures on plastic

However, both solution processing and inkjet printing require the ink to have specific physical properties. Because of that, the most used 2D-material based inks rely on the use of organic solvents, such as n-methyl-2-pyrrolidone (NMP). Water-based 2D material inks have been also demonstrated but they are not ideal for printed electronics as they typically have a low concentration of 2D crystals (<0.1 weight %) and contain a high amount of residual surfactant, which is difficult to remove.

In our work we develop an inkjet printable water-based ink by carefully engineering the components of the ink: first, a water based 2D material formulation is made using traditional chemical-exfoliation methods, and then its composition is tuned by adding tiny amounts of compounds to tune the physical parameters of the ink to achieve optimum printability. In addition, a binder is added to the ink in order to minimize the remixing of different 2D materials when printed one on top of the other in the fabrication of the heterostructure. Existing inks did show a very strong re-dispersion of the material at the interfaces of the heterostructure, giving rise to poor device reliability and performance.

 Schematic of an all-printed logic memory made from 2D materials

Schematic of an all-printed logic memory made from 2D materials

With this optimized 2D-material based ink, we have been able to fabricate an heterostructure of 2D-materials only by using an inkjet printer. We have demonstrated a wide range of devices, from arrays of photodetectors to logic memories (i.e. devices able to store information in the form of a binary code). With these optimized inks, we are now able to exploit the full potential of inkjet printing for 2D materials. In particular we can now fabricate arrays of devices and to engineer every device in the array ad-hoc in order to sense or store a particular type of information.

We expect this technology to be useful in several area, for example in packaging (e.g. smart labels and coding) where lighter, cheaper and easy to integrate electronic components are strongly needed.

Prof Cinzia Casiraghi, from the Graphene Centre, University of Manchester, will be presenting this work in greater detail at the IMI Europe Inkjet Ink Development Conference in Lausanne, Switzerland on 15-16 March 2017.