Recent decades have seen such an investment in silicon-wafer manufacturing and processing techniques that the technology will continue to play a dominant role for at least the next 10 years. This means that, even though new 2D materials are increasingly recognised as superior for many applications, the best way of exploiting them in the near term is to integrate them into existing silicon-based approaches.

An inexpensive and relatively simple way to derive the 2D materials suitable for optoelectronic devices is to separate them from the bulk using liquid-phase exfoliation. At high enough concentrations, the resulting nanosheet dispersions behave like liquid crystals, and flow-induced stress can cause them to adopt the nematic phase, in which suspended crystals share a common orientation.

Depositing these aligned nanoparticles on a substrate results in a highly ordered thin film that reflects the low-dimensionality of the constituent 2D material, and such an arrangement is generally preferable to the less well ordered films that are produced from isotropic dispersions. As Baldycheva pointed out in her talk, however, keeping nanosheets suspended in liquid allows a whole new set of possibilities.

Liquid assets

The method of integration put forward by Baldycheva and collaborators involves the fabrication of a microfluidic layer on the back face of a silicon-on-insulator (SOI) photonic chip. The microfluidic channels are themselves formed from silicon, and are compatible with standard complementary metal–oxide–semiconductor (CMOS) manufacturing processes.

By introducing fluid-dispersed graphene or graphene-oxide flakes into these channels, Baldycheva's group formed 3D optical structures that can be reconfigured using electric fields. As a demonstration of the principle, Baldycheva presented an example developed by her team in which a nanosheet dispersion in a central reservoir acted as an omni-directional grating for four convergent optical interconnects. The same method could be applied to manufacture many other light-manipulating devices, such as tunable optical filters and nanoantenna phased arrays.

Other 2D materials might lend themselves to even more uses. "We intend to continue investigating applications of fluid-dispersed nanosheets for silicon photonics," Baldycheva told "This will involve synthesizing and characterizing dispersions of new materials, integration into novel devices, and demonstration of in situ applicability of the materials."