  Phase-change materials (PCMs), which are well-established in optical and
random-access memories, are increasingly studied for emerging topics such as
brain-inspired computing and active photonics. These applications take
advantage of the pronounced reflectivity and resistivity changes that accompany
the structural transition in PCMs from their amorphous to crystalline state.
However, PCMs are typically fabricated as thin films via sputtering, which is
costly, requires advanced equipment, and limits the sample and device design.
Here, we investigate a simpler and more flexi-ble approach for applications in
tunable photonics: the use of sub-10 nm colloidal PCM nanoparticles (NPs). We
report the optical properties of amorphous and crystalline germanium telluride
(GeTe) NP thin films from the infrared to the ultraviolet spectral range. Using
spectroscopic ellipsometry with support from cross-sectional scanning electron
microscopy, atomic force microscopy, and absorption spectroscopy, we extract
refractive indices n, extinction coefficients k, and band gaps Eg and compare
to values known for sputtered GeTe thin films. We find a decrease of n and k
and an increase of Eg for NP-based GeTe films, yielding insights into
size-dependent property changes for nanoscale PCMs. Furthermore, our results
reveal the suitability of GeTe NPs for tunable photonics in the near-infrared
and visible spectral range. Finally, we studied sample reproducibility and
aging of our NP films. We found that the colloidally-prepared PCM thin films
were stable for at least two months stored under nitrogen, further supporting
the great promise of these materials in applications.
