posted on 2018-06-12, 00:00authored byMilan Palei, Vincenzo Caligiuri, Stefan Kudera, Roman Krahne
Colloidal nanocrystals
are a promising fluorescent class of materials whose spontaneous emission
features can be tuned over a broad spectral range via their composition,
geometry, and size. However, toward embedding nanocrystal films in
elaborated device geometries, one significant drawback is the sensitivity
of their emission properties on further fabrication processes like
lithography, metal or oxide deposition, etc. In this work, we demonstrate
how bright-emitting and robust thin films can be obtained by combining
nanocrystal deposition from solutions via spin coating with subsequent
atomic layer deposition of alumina. For the resulting composite films,
the layer thickness can be controlled on the nanoscale and their refractive
index can be finely tuned by the amount of deposited alumina. Ellipsometry
is used to measure the real and imaginary part of the dielectric permittivity,
which gives direct access to the wavelength dependent refractive index
and absorbance of the film. Detailed analysis of the photophysics
of thin films of core–shell nanocrystals with different shapes
and different shell thicknesses allows to correlate the behavior of
the photoluminescence and of the decay lifetime to the changes in
the nonradiative rate that are induced by the alumina deposition.
We show that the photoemission properties of such composite films
are stable in wavelength and intensity over several months and that
the photoluminescence completely recovers from heating processes up
to 240 °C. The latter is particularly interesting since it demonstrates
robustness to the typical heat treatment that is needed in several
process steps like resist-based lithography and deposition by thermal
or electron beam evaporation of metals or oxides.