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(Color online) TEM image of the , Er nanocrystals (data adapted from the previous work published in Ref. 7 ). The average diameter of the nanocrystals is . The uniform distribution of the nanocrystals might have resulted from the long chain oleylamine ligand on the crystal surface that can prevent aggregation. Inset is the SAED pattern of the : 20% Yb, 2% Er nanocrystals observed by TEM; the spotty polycrystalline diffraction rings correspond to the (100), (110), (111), (201), (311), and (321) planes of the hexagonal lattice.
(Color online) (a) Structure of single layer NIR photodetectors. The MEHPPV/nanocrystals films were prepared by spin casting from the composite solutions on ITO substrates. The devices were fabricated with the architecture of ITO/composite/Al by thermally evaporating Al electrode on the spin-casted thin films. (b) Schematic band diagram of NIR photodetectors showing the Fermi levels and work functions of the ITO and the Al electrodes, and the HOMO-LUMO levels of MEHPPV.
(Color online) (a) Dependence of the photocurrent curves of a NIR photodetector device on the incident laser power . (b) The photocurrent of the NIR photodetector at bias as a function of the incident laser power. The dashed line is a linear fit of the data for guiding the eyes.
(a) The NIR-to-visible upconversion fluorescence spectra of the , Er nanocrystals. The three emission peaks at 522.5, 541.5, and correspond to the , , and transitions of Er, respectively. (b) Response of the device under a visible light excitation of bias. The inset of Fig. 4(b) shows the absorbance of the upconversion fluorescence nanocrystals as a function of the wavelength of NIR excitation near , indicating that the NIR from can be effectively upconverted.
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