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Photochemical upconversion of near-infrared light from below the silicon bandgap


Photochemical upconversion is a strategy for converting infrared light into more energetic, visible light, with potential applications ranging from biological imaging and drug delivery to photovoltaics and photocatalysis. Although systems have been developed for upconverting light from photon energies in the near-infrared, upconversion from below the silicon bandgap has been out of reach. Here, we demonstrate an upconversion composition using PbS semiconductor nanocrystal sensitizers that absorb photons below the bandgap of silicon and populate violanthrone triplet states below the singlet oxygen energy. The triplet-state violanthrone chromophores luminesce in the visible spectrum following energy delivery from two singlet oxygen molecules. By incorporating organic chromophores as ligands onto the PbS nanocrystals to improve energy transfer, we demonstrate that violanthrone upconverts in the absence of oxygen by the triplet–triplet annihilation mechanism. The change in mechanism is shown by exploiting the magnetic field effect on triplet–triplet interactions.

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Fig. 1: A cartoon showing the energy flow and mechanism of nanocrystal-sensitized PUC.
Fig. 2: Upconversion below the silicon bandgap.
Fig. 3: Absorption and emission spectra of the PbS nanocrystals, TTCA ligands and V79.
Fig. 4: Energy transfer kinetics in the PbS NC/TTCA/V79 PUC system.
Fig. 5: The effect of oxygen on PUC with V79.

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Data availability

The data that support the findings of this study are available from the corresponding author on reasonable request.


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This work was supported by the Australian Research Council (Centre of Excellence in Exciton Science CE170100026). The research used facilities supported by Microscopy Australia at the Electron Microscope Unit (EMU) and the Solid State and Elemental Analysis Unit within the Mark Wainwright Analytical Centre (MWAC) at UNSW Sydney.

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Authors and Affiliations



E.M.G., S.K.K.P., S.N. and T.I. performed the optical measurements and analysed the data. S.H. and Z.L.T. synthesized the PbS material. J.E.A. and A.J.P. synthesized the TTCA ligands. J.H.C. performed theoretical analysis of the magnetic field effects. S.C. and R.D.T. performed the TEM measurements. T.W.S. conceived the experiments and wrote the manuscript.

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Correspondence to Timothy W. Schmidt.

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Supplementary Figs. 1–9, Tables 1–3 and text.

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Gholizadeh, E.M., Prasad, S.K.K., Teh, Z.L. et al. Photochemical upconversion of near-infrared light from below the silicon bandgap. Nat. Photonics 14, 585–590 (2020).

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