Liquid-Based Ternary Colloidal Nanoparticles for Tunable Phase Change Optics

Phase change memory (PCM) materials have gained major interest and revival since their utility in optical data storage discs. This is due to the tunable contrast in material properties such as complex refractive index and electrical conductivity between amorphous and crystalline states. The most heavily studied PCM materials fall in the pseudo-binary tie line of GeTe and Sb₂Te₃. Aside from Sb, doping GeTe with elements such as Pb, Bi, Cu, and Sn has been shown to improve power efficiency, induce faster crystallization, and enhance phase-contrast during switching.<br/>Currently, vacuum technology is heavily used for the fabrication of PCM. Despite having the advantages of producing high-quality thin film and good stoichiometry control, vacuum technology has limited combinatorial throughput and flexibility for new materials discovery. Colloidal inorganic nanoparticle chemistry has shown great promise for versatility in synthesizing new materials and cost-effective translation to device applications through liquid-based fabrication routes.<br/>Here, we report a facile hot injection synthesis approach to prepare a library of ternary nanoparticles, M_xGe_1-xTe (where M is Sn, Cu, Pb, Bi), which are promising for PCM applications. We then investigate two material systems, namely SnGeTe (SGT) and CuGeTe (CGT), in more details. We utilize the chemical tunability of the synthesis to achieve varied compositions and phases in the SGT and CGT nanoparticles. Using high-temperature <i>in-situ</i> XRD experiments we find that increasing the Sn-doping lowers the crystallization temperature while Cu shows the opposite with respect to stoichiometric GeTe nanoparticles. We explain these observations from the viewpoint of local atomic arrangements of SGT and CGT nanoparticles, using X-ray absorption spectroscopy. Furthermore, we present thin-film fabrication through an inorganic ligand-exchange route and spin coating, which we characterize for optical properties in the amorphous and crystalline state using ellipsometry. From this we identify a reflectivity contrast and that SGT and CGT show the most pronounced difference in the refractive index of 0.9 at 1070 nm and 0.51 at 1490 nm respectively. Finally, we demonstrate an optical utility by fabricating a simple reflective stack with SGT nanoparticle thin film sandwiched between ITO layers of tunable thickness on Al mirror. We discuss the change in reflection spectra and optical contrast of the stack in the context of reflective display applications. Our work paves the way to simple fabrication of other photonic PCM applications, including near-IR metalenses, tunable filters, and beam steerer.