Building three dimensional structures from two dimensional materials, novel approaches for the creation of van der Waals heterostructures

We have recently developed a novel process for the fabrication of van der Waals heterostructures, a materials system which has only recently been conceived and fabricated. Our method is inexpensive, generic and scalable. The focus of this project is to improve our understanding of the science underpinning the approach and investigate novel heterostructures produced via this method.

Van der Waals heterostructures (vdWHSs) are layered materials derived from monolayer two-dimensional solids such as graphene and its non-carbon analogues (such as the transition metal dichalcogenides, black phosphorous etc.). These ultra-thin three-dimensional structures can be built up by stacking units made up of one or more layers of different two-dimensional materials in a nanoscale equivalent to a layer cake. The resulting architectures are novel 'smart' materials which combine the individual, outstanding properties of the various two-dimensional building blocks, enabling the creation of new functionality ranging from photodetectors and light-absorbing coatings to flexible electronic devices. vdWHSs offer great advantages over conventional semiconductor heterostructures which are limited in their materials combinations due to the need to match the lattice structure of each layer precisely - in vdWHSs the weak van der Waals interactions between layers and strong intra-layer bonding fully relaxes the requirement that layers be commensurate in structure and can produce clean, defect-free interfaces using a wide palette of two-dimensional materials.

Current methods of producing vdWHSs are expensive, time consuming and difficult to scale. The first vdWHSs were produced by an extension of the 'Scotch tape' technique, by which individual layers of material are peeled off by hand, and positioned manually with micron-scale precision, a laborious approach which is impossible to scale. More recently, some advances have been made with chemical vapour deposition (CVD) approaches to the growth of these structures, but they are still limited in the range of solids accessible and the quality of the heterostructure produced. Moreover, the thermal budget makes CVD approaches expensive and difficult to scale. Our liquid suspension deposition (LSD) approach, which has been proven in preliminary studies [1], involves the precise and controlled assembly of vdWHsS from easily produced liquid phase suspensions [2] on almost any materials surface with limited (and potentially no) pre-treatment. The physics of the LSD approach is not yet fully understood and work is still required to optimise the quality of the structures produced and widen the range of materials employed [3]. Addressing these issues would be the core activity of the proposed PhD project, providing a platform for industrial collaboration and future funding applications. The low-cost, rapid and scalable production of vdWHSs will revolutionise their availability and enable incorporation into manufactured goods.



References

[1] A.G.M. Mathieson, M. Zulqurnain, M. Szablewski and M.R.C. Hunt (unpublished, in preparation).

[2] 'Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids' K.R. Paton et al. Nature Materials 13 (2014) 624
[3] 'Progress, Challenges and Opportunities in Two-Dimensional Materials Beyond Graphene' S.Z. Butler et al. ACS Nano 7 (2013) 2898-2926