Upconversion (UC) is a nonlinear process in which two, or more, long wavelength photons are converted to a shorter wavelength photon. This process is based on sequential absorption of two or more photons, involving metastable, long lived intermediate energy states, thus is not restricted to upconversion of coherent laser radiation as a non-coherent process. Hence, requirements for UC processes are long lived excited states, a ladder like arrangement of energy levels and a mechanism inhibiting cooling of the hot charge carrier. UC holds great promise for bioimaging, enabling spatially resolved imaging in a scattering specimen and for photovoltaic devices as a mean to surpass the Shockley-Queisser efficiency limit.
Here, we present a novel luminescence upconversion nano-system based on colloidal semiconductor double quantum dots, consisting of a NIR-emitting component and a visible emitting component separated by a tunneling barrier in a spherical onion-like geometry. These dual near-infrared and visible emitting core/shell/shell PbSe/CdSe/CdS nanocrystals are shown to upconvert a broad range of NIR wavelengths to visible emission at room temperature, covering a spectral range where there are practically no alternative upconversion systems. The synthesis is a three-step process, which enables versatility and tunability of both the visible emission color and the NIR absorption edge. Using this method one can achieve a range of desired upconverted emission peak positions with a suitable NIR band gap. The physical mechanism for upconversion in this structure, as well as possible extensions and improvements will be discussed. 1
(1) Teitelboim, A.; Oron, D. ACS Nano 2015, acsnano.5b05329.
Ayelet Teitelboim and Dan Oron, "Broadband near-infrared to visible upconversion in quantum dot-quantum well heterostructures
(Conference Presentation)," Proc. SPIE 9919, Nanophotonic Materials XIII, 99190F (Presented at SPIE Nanoscience + Engineering: August 30, 2016; Published: 7 November 2016); https://doi.org/10.1117/12.2237387.5159519605001.
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