Beyond graphene: 2D materials and 3D heterostructures

This is a guest post by Thanasis Georgiou, the Director of MoS2 Crystals, a Manchester based MoS2 Crystals and flakes provider.

Graphene has mesmerized the labs of hundreds of research groups worldwide. With its exceptional set of properties it is widely expected that it would find applications in a variety of areas. However, looking back on the work of the Nobel Laureates, their highly cited PNAS publication was entitled Two-dimensional atomic crystals(Novoselov et al., 2005). Indeed, graphene was just the first of many different crystalline materials that can be exfoliated and investigated. Graphene justifiably attracted such intense research due to its exotic Dirac-cone nature of its electronic spectrum.

While graphene is seen as an important candidate for post-silicon electronics, pristine graphene does not have a band gap, which is much-required for switching OFF efficiently a transistor. As such, graphene itself cannot be used as the conductive channel in a typical field-effect transistor. 

This is where the semiconducting MoS2 becomes interesting. MoS2 is a group VI transition metal dichalcogenide and has a similar structure to graphite; it consists of tightly bound layers of MoS2 vertically held together by weak van der Waals forces.  As such, it can be easily exfoliated and investigated. Already, there has been a significant amount of work carried out in this field and MoS2 turns out to be a very interesting system in its own right.

First, single-layer MoS2 has distinct Raman signature, which makes it fairly straight-forward to identify mono- from multi-layers (Lee et al., 2010). Second, it can be used to establish high ON/OFF transistors, reaching 100 million, by using atomic layer deposition to form a HfO2 top gate . (Radisavljevic, Radenovic, Brivio, Giacometti, & Kis, 2011). Third, single layer MoS2 can also be used for new valleytronic devices, where charge carriers get trapped in a minimum or maximum in the valence or conduction band, respectively (Mak, He, Shan, & Heinz, 2012; Zeng, Dai, Yao, Xiao, & Cui, 2012). It remains to be seen what other novel devices could be fabricated for this material.

Even more fascinating are designer materials, where different 2D crystals are combined in a vertical stack, thus fabricating heterostructures, which may be tailored for a specific application. The first such structures to be investigated where graphene-hexagonal boron nitride-graphene. Indeed, such structures already present interesting physical phenomena like quantum tunnelling, metal-insulator transition, coulomb drag (Manchester group, 2011-12). These vertical heterostructures can also be seen as a field-effect transistor, where the transport through the insulating h-BN or semiconducting spacer (MoS2) is modulated by the change of the apparent height and width of the spacer, due to a shift in the Fermi level of graphene (Britnell et al., 2012). Growth of these heterostructures has also been demonstrated with MoS2 being epitaxially grown on graphene (Shi et al., 2012).

While these MoS2 and heterostructures are very interesting physical systems, the isolation of both single layer MoS2 and the assembly of the heterostructures still remains a tedious task.

MoS2 Crystals is a new start-up which provides researchers with solution in pursuing research surrounding two-dimensional systems. While there are plenty of companies who concentrate in mass-production and scalable methods of producing flakes, at MoS2 Crystals we specialize on producing products of the highest quality that intended strictly for research.. MoS2 crystals is maintained by Thanasis Georgiou, a member of the Manchester group and as such remains in close proximity with the latest ground-breaking research, which is in turn transferred to its customers. 

Specifically it offers:

• MoS2 flakes and heterostructures with graphene and h-BN: While many groups already have expertise with exfoliating graphene, still, large single-layer flakes of MoS2 are not readily obtained. Heterostructures based on different materials are still not easy to fabricate. MoS2 Crystals provides large single-layer flakes of MoS2 on the substrate of your choice and assembles customised heterostructures in order to meet customer requirements.

• MoS2 natural crystals: Research groups that already have the expertise in exfoliating materials can take advantage of our MoS2 crystals, which are specifically chosen for exfoliation.

• MoS2 dispersions*: We offer MoS2 dispersions in a variety of solvents. (*Coming soon). 

References

Biritnell, L., Gorbachev, R. V., Jalil, R., Belle, B. D., Schedin, F., Mishchenko, a, Georgiou, T., et al. (2012). Field-effect tunneling transistor based on vertical graphene heterostructures. Science (New York, N.Y.), 335(6071), 94750. doi:10.1126/science.1218461

Lee, C., Yan, H., Brus, L. E., Heinz, T. F., Hone, J., & Ryu, S. (2010). Anomalous lattice vibrations of single- and few-layer MoS2. ACS nano, 4(5), 2695700. doi:10.1021/nn1003937

Mak, K. F., He, K., Shan, J., & Heinz, T. F. (2012). Control of valley polarization in monolayer MoS(2) by optical helicity. Nature nanotechnology, (June), 15. doi:10.1038/nnano.2012.96

Novoselov, K. S., Jiang, D., Schedin, F., Booth, T. J., Katsnelson, M. I., Morozov, S. V., & Geim, A. K. (2005). Two-dimensional atomic crystals. Proceedings of the National Academy of Sciences of the United States of America, 102(30), 104513. doi:10.1073/pnas.0502848102

Radisavljevic, B., Radenovic, A., Brivio, J., Giacometti, V., & Kis, A. (2011). Single-layer MoS2 transistors. Nature nanotechnology, 6(3), 14750. doi:10.1038/nnano.2010.279

Shi, Y., Zhou, W., Lu, A.-Y., Fang, W., Lee, Y.-H., Hsu, A. L., Kim, S. M., et al. (2012). van der Waals Epitaxy of MoS(2) Layers Using Graphene As Growth Templates. Nano letters, 12(6), 278491. doi:10.1021/nl204562j

Zeng, H., Dai, J., Yao, W., Xiao, D., & Cui, X. (2012). Valley polarization in MoS(2) monolayers by optical pumping. Nature nanotechnology. doi:10.1038/nnano.2012.95