The Murray Group (IMOD; UPenn), in collaboration with the Osuji Group (UPenn), have developed a new ligand system that stabilizes nanoparticles in a liquid crystal matrix to enable the preparation of uniform, well-dispersed, and highly stable liquid crystal / nanoparticle mixtures, setting the stage for development of advanced optoelectronic applications.

Liquid crystals are an unusual state of matter that has properties between those of a conventional liquid and those of a solid crystal. For example, it may flow in the same way a liquid does, but the constituent molecules, even though they are flowing, maintain a solid crystal-like alignment. These uncommon physical properties have made liquid crystals the subject of focused and maintained interest for a number of applications, in particular those that exploit their optical properties, such as liquid crystal displays (LCDs).

The properties of liquid crystals can be manipulated and fine-tuned through the introduction of dopants in sub-stoichiometric amounts. Such dopants can be thought of as Sergeants, with the liquid crystal molecules acting as Soldiers. The Sergeants act as leaders; the physical properties of the dopants impact and influence whole regions of the liquid crystal “Soldiers” and change the bulk properties of the material.

Of particular interest in optoelectronics research is the use of nanoparticles as a dopant. However, the stable integration of nanoparticles into a liquid crystal matrix is not so simple. Under typical solvent conditions a nanoparticle needs to be stabilized by the attachment of organic ligands to its surface. While the design of these stabilizing organic ligands has been figured out for traditional isotropic solvents (such as toluene or chloroform), these do not work for the unusual liquid crystal phase, and the mixture collapses.

This report, that describes collaborative work between the Murray Group (IMOD; UPenn) and the Osuji Group (UPenn), addresses this need. A range of new organic ligands were designed and synthesized and tested for their ability to stabilize nanoparticles in a liquid crystal “solvent”. It was found that a special class of ligands, so-called dendritic branched ligands, have the right cone-type shape and physical properties to effectively stabilize the nanoparticle Sergeants in the liquid crystal Soldier phase.

The ability to efficiently and reliably prepare a stable nanoparticle-doped liquid crystal mixture enables investigation of the properties and potential applications of this previously inaccessible class of materials. The design of these ligands was developed with generalization in mind – the modular approach means that this can be adapted to a range of different liquid crystal or nanoparticle structures.