Metal oxide bulk doping transition metals

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. 

PL measurements for the mechanistic studies (72-75). From detailed studies on n-Ti02 and that doped with transition metal ions such as Cr, we proposed a new mechanism that the reaction in acidic solutions is initiated by the oxidation of bulk OH species, not surface OH species 14). However, we later found a paper (76) reporting that Cr ions contained in Ti02 as an impurity emitted a PL band accidentally at the same wavelength (840 nm) as that observed by us till then. Also, a criticism to our conclusion was reported (77) probably due to confusion of the interpretation of the PL band. Thus it had become necessary to re-investigate our mechanism in detail, and we started systematic studies 18,19). The purpose of the present paper is to give confirmative experimental evidence to our new mechanism. 

UV-vis spectroscopy shows that the doping elements present in the form of oxoanions. The absorption bands are due to ligand-metal charge transition. In all cases UV bands were found to correspond to polyoxoanions M Oy ". Fig. 4 gives the exemple of Mb doped samples which demonstrate a band characteristic for a niobate compound compared with the bulk oxide. 

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