Steady-state photoluminescence and UC photoluminescence measurement: All relative PL and UCPL measurements were conducted on the set up shown in SI Figure . The samples were illuminated with an 808 nm continuous wave laser at an angle of ~45 degrees. The emission from the samples was collected using a high NA lens and fiber-coupled spectrometer (QE pro high-performance, Ocean Insight). The fiber coupling to the detector included a filter holder. A 750 nm short pass filter (Thorlabs) was used to filter the laser out of the UCPL spectra. An 850 nm long pass filter (Thorlabs) was used to filter the laser out of the PbS QD PL spectra.