Liquid-phase exfoliation of nanosheets from bulk materials is generally achieved by high-energy processes like ultrasonication, coupled with chemical treatments to prevent reaggregation. Such methods can be effective and inexpensive, but monolayers usually make up no more than a few per cent of the yield, and residual chemical additives can compromise the properties of the resulting products.

Novel solutions

An alternative procedure reported by Howard allows complete dissolution of single-layer nanosheets without the need for agitation or chemical surfactants, resulting in pure, high-quality 2D materials. This achievement by Howard and collaborators is significant given that the nanomaterials with the most potential for optoelectronic and photonic applications are typically poorly soluble.

Key to the approach is the processing of the bulk precursor by intercalating alkali metal atoms between its layers. Howard and colleagues successfully prepared more than a dozen materials in this way – including graphite, molybdenum disulphide and molybdenum diselenide. Exposing the resulting layered salt to polar, aprotic solvents for two weeks – without stirring or centrifugation – was enough to produce a homogenous solution of alkali metal cations and nanosheet anions.

Dropping and drying the solution onto a substrate resulted in the formation of "coffee ring" deposits of the desired materials, which the researchers characterized using high-speed atomic force microscopy (HS-AFM). This technique allowed measurements to be made at 10,000 sites rather than the few hundred possible by conventional AFM, and showed that although overlapping agglomerations of restacked nanosheets were present in the rings, peripheral regions exhibited flat, exclusively single-layer nanosheets with good areal coverage. The researchers also observed that these nanosheets frequently retained their hexagonal form, confirming that the low-energy solution method pulls layers apart gently.

Upon initial solvent contact, it was found that the 2D materials dissolved as few-layer sheets, and in this state electrostatic forces between these particles could cause hexagonal 2D nanosheets to self-assemble on the substrate. Furthermore, because the dissolved 2D materials are negatively charged, Howard and his team could also use electroplating to deposit high-quality, low-dimensional layers over large areas with micron precision.

Environmentally friendly approach

DZP Technologies also prints nanolayers from liquid media, but the research and products presented by Stoeva relate to aqueous dispersions rather than nanomaterials dissolved in organic solvents. This has some advantages in that water-based formulations, being non-toxic and environmentally benign, are easier to work with and require no costly storage or transportation considerations. Water-based nanoparticle inks can also be used where some other solvents would be forbidden – in applications in which organic solvents would interact with other layers in the device, for example.

DZP Technologies already offers aqueous dispersions of graphene commercially, but recent research has focused on preparing suspensions of molybdenum disulphide and tungsten disulphide. Transition metal dichalcogenides such as these have optical and electronic properties that can be modified by altering the layer thickness and stacking arrangement, and seem more promising than graphene for some applications. Examples given by Stoeva as being of particular interest to her company are solution-processable flexible optoelectronics, transparent electronics and printed photodetectors and phototransistors.

The dispersions formulated by the company rely on a proprietary process developed in-house, and typically involve up to 15 separate ingredients including the functional material and a range of special additives. These dispersions are stable, with a shelf-life longer than 12 months, and produce nanoparticle films with narrow particle-size distributions and free from cracks and aggregations. The challenge addressed by DZP Technologies is in ensuring that the obtained films retain their useful electronic and optical properties despite the complex formulation.