(159b) Plasmon-Enhanced Energy Transfer and Other Photophysical Effects in Doped-Lanthanide Nanocrystals

Upconversion of infrared radiation into visible light has been investigated for applications in biological imaging and photovoltaics. However, low conversion efficiency due to small absorption cross-section for infrared light (Yb³⁺), and slow rate of energy transfer (to Er³⁺ states) has prevented application of upconversion photoluminescence (UPL) for diffuse sunlight or imaging tissue samples. Here, we utilize resonant surface plasmon polaritons (SPP) waves to enhance UPL in doped-lanthanide nanocrystals. Our analysis indicates that SPP waves not only enhance the electromagnetic field, and hence weak Purcell effect, but also increases the rate of resonant energy transfer from Yb³⁺ to Er³⁺ ions by 6 fold. While we do observe strong metal mediated quenching (14 fold) of green fluorescence on flat metal surfaces, the nanostructured metal is resonant in the infrared, and hence enhances the nanocrystal UPL. This strong columbic effect on energy transfer can have important implications for other fluorescent and excitonic systems too. We also investigated the effect of surface plasmons on color tuning and other optoelectronic properties in upconverting nanoparticles. SPP strongly affected the emission color in different-codoped lanthanide UCNPs by tuning the energy transfer between lanthanide ions (Er, Yb and Tm here), depending on the energy overlap between resonant SPP modes and multiple energy states in respective ions. This selective enhancement of resonant energy transfer provides valuable physical insights and useful design principles for electronically coupling different upconverting nanoparticles with matched semiconductor devices and plasmonic absorbers. To test the role of upconversion enhancement in optoelectronics, upconversion enhanced photocurrent was measured on 2D semiconductor nanosheets coupled with UCNPs. The semiconductor 2D nanosheets were nominally transparent to infrared radiation, and no photocurrent is observed on illumination of sub-bandgap radiation. However, strongly enhanced photocurrent was observed in these 2D nanosheets using 980 nm light irradiation, which can enable new applications in development of ultrathin room-temperature infrared detectors and high-resolution imaging systems.