Near-infrared emission upconversion

Wang, F. and Liu, X.G (2008) Upconversion multicolor fine-tuning visible to near-infrared emission from lanthanide-doped NaYF4 nanoparticles. Journal of the American Chemical Society, 130, 5642-5643. 

Under excitation of a 980 nm laser diode, a dazzling green spot from both samples could be clearly seen by the naked eye. Figure 3 gives the typical upconversion luminescent emission bands. Green and near-infrared emission bands were detected in both samples, which were very similar to fluorescent emissions. Especially, the red emission band was measured around 667 nm associated with the F5—> lg transition. Furthermore, the ultra-violet and blue upconversion emissions can also be measured. The ultra-violet emissions in the ranges of 381-394 nm and 409-428 nm are assigned to the G4—> lg and G5— Ig transitions, respectively. The blue emission between 473 and 500 nm... 

Pedroni, M., PiccineUi, F., Passuello, T., Gioarola, M., Mariotto, G., Polizzi, S., et al., 2011. Lanthanide doped upconverting colloidal CaF2 nanoparticles prepared by a single-step hydrothermal method toward efficient materials with near infrared-to-near infrared upconversion emission. Nanoscale 3, 1456—1460. 

Comparisons between these two metals have been carried out on complexes with donor-acceptor Schiff bases. The new species show efficient absorption in the orange-red part of the spectrum and room-temperature near-infrared (NIR) phosphorescence. Particularly, Pt(ii) complexes possess phosphorescence quantum yields (O) of 0.1, but the emission of the respective Pd(ii) complexes is less efficient (OaO.Ol). The Pd(ii) and Pt(ii) complexes are demonstrated to be efficient sensitizers in triplet-triplet annihilation-based upconversion systems. 

The term upconversion describes an effect [1] related to the emission of anti-Stokes fluorescence in the visible spectral range following excitation of certain (doped) luminophores in the near infrared (NIR). It mainly occurs with rare-earth doped solids, but also with doped transition-metal systems and combinations of both [2, 3], and relies on the sequential absorption of two or more NIR photons by the dopants. Following its discovery [1] it has been extensively studied for bulk materials both theoretically and in context with uses in solid-state lasers, infrared quantum counters, lighting or displays, and physical sensors, for example [4, 5]. Substantial efforts also have been made to prepare nanoscale materials that show more efficient upconversion emission. Meanwhile, numerous protocols are available for making nanoparticles, nanorods, nanoplates, and nanotubes. These include thermal decomposition, co-precipitation, solvothermal synthesis, combustion, and sol-gel processes [6], synthesis in liquid-solid-solutions [7, 8], and ionothermal synthesis [9]. Nanocrystal materials include oxides of zirconium and titanium, the fluorides, oxides, phosphates, oxysulfates, and oxyfluoiides of the trivalent lanthanides (Ln ), and similar compounds that may additionally contain alkaline earth ions. Wang and Liu [6] have recently reviewed the theory of upconversion and the common materials and methods used. 

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