Nanoparticles exhibiting localized surface plasmon resonances (LSPR) are valuable tools traditionally used in a wide field of applications including sensing, imaging, biodiagnostics and medical therapy. Plasmonics in semiconductor nanocrystals is of special interest because of the tunability of the carrier densities in semiconductors, and the possibility to couple the plasmonic resonances to quantum confined excitonic transitions. Here, colloidal Cu_2–xSe nanocrystals were synthesized, whose composition was shown by Rutherford backscattering analysis and electron dispersive X-ray spectroscopy, to exhibit Cu deficiency. The latter results in p-type doping causing LSPRs, in the present case around a wavelength of 1100 nm, closely matching the indirect band gap of Cu_2–xSe. By partial exchange of the organic ligands to specific electron trapping or donating species the LSPR is fine-tuned to exhibit blue or red shifts, in total up to 200 nm. This tuning not only provides a convenient tool for post synthetic adjustments of LSPRs to specific target wavelength but the sensitive dependence of the resonance wavelength on surface charges makes these nanocrystals also interesting for sensing applications, to detect analytes dressed by functional groups.

Metal halide perovskites are revolutionizing X‐ray detection through a combination of low cost, solution processing, favorable optoelectronic properties, and high stopping power for high‐energy ionizing radiation. While perovskite single crystals and polycrystalline wafers are considered direct X‐ray converters, most medical X‐ray applications are based on scintillators that shift high‐energy radiation into the visible. Several materials are on the market, but demonstrations based on CsPbBr₃ nanocrystals, possibly embedded in a matrix material or combined with organic molecules as luminescent species, highlight their competitiveness with established scintillators in terms of radioluminescence yield and transient behavior. Major hurdles that perovskite nanocrystal scintillators must overcome are environmental stability and toxicity. While there are still few examples of high‐performance lead‐free perovskite nanocrystal scintillators, microcrystalline perovskites are emerging with promising properties, reduced toxicity, and significant Stokes shifts to avoid reabsorption of emission in thick films. Thus, the near future of perovskite nanocrystal scintillator materials will primarily be the adoption of recipes for materials with proven properties in microcrystalline form. The nanocrystal colloidal solutions will facilitate the large‐scale printing of homogeneous and scattering‐free films to obtain high contrast and spatial resolution X‐ray images by scintillation.