Photoluminescence Of Mn²⁺ Ions And The Aggregates

Luminescent materials activated by rare earth（Ln） ions and transition metal（TM） ions have attracted extensive interests due to their wide applications in modern lighting, color display, anti-counterfeiting, laser materials and biological imaging. However, Ln³⁺ ions activated materials usually present multiple, fixed emission peaks with narrow emission bandwidths, which inevitably restrict their applications. On the contrary, transition metal ion Mn²⁺ is a significant activator for the Stokes photoluminescence, long-persistent luminescence（LPL） and upconversion（UC） luminescence due to the unique advantages including broad single-band and wavelength-tunable emission, which can make up for the weaknesses of Ln³⁺ ions. In addition, selective regulation of luminescence activated by Ln³⁺ ions can be obtained by controlling the relative energy position between ⁴T₁（⁴G） state of Mn²⁺ and excited states of Ln³⁺. For a long time, however, the investigation on Mn²⁺ luminescence are almost restricted to the isolated Mn²⁺ ions and that on the luminescence induced by activator aggregation between two Mn²⁺ ions are rarely reported. Thus in this dissertation, the tunable photoluminescence and long persistent luminescence tailored by the super-exchange interactions between Mn²⁺ ions, room-temperature Mn²⁺ tunable UC luminescence and selectively enhanced up- and down-conversion emissions of Er³⁺ via Yb³⁺-Mn²⁺ dimer sensitizing are investigated in detail. The specific research results in the dissertation can be summarized as follows:（1） A series of Mn²⁺ single-doped CaO samples were synthesized by high temperature solid-state reaction process. Tunable single-band（⁶00 nm） to dual-bands（⁶00/660 nm） visible emissions have been demonstrated via controlling the doping concentration of Mn²⁺ in one crystallographic site. Based on the investigations of photoluminescence excitation and emission spectra, luminescence decay lifetimes, crystal structure and density functional theory（DFT） calculations, it can be concluded that the emission bands at ⁶00 and ⁶60 nm should originate from the ⁴T₁（⁴G）â†'⁶A₁（⁶S） transitions of isolated Mn²⁺ ions and ⁶A₁（⁶S）⁴T₁（⁴G）â†'⁶A₁（⁶S）⁶A₁（⁶S） transitions of super-exchange coupled Mn²⁺-Mn²⁺ dimers, respectively. In addition, CaO:Mn²⁺ samples also show LPL phenomenon, indicating that Mn²⁺-Mn²⁺ dimers can also be acted as emission center to generate LPL in heavily Mn²⁺-doped CaO.（2） RT tunable green to orange UC luminescence can be obtained from CaO:Yb³⁺,Mn²⁺ with different concentrations of Mn²⁺ ions upon 980 nm laser excitation. The emission spectra consists of a green band（508 nm） and wavelength-tunable orange band, which are originating from the cooperative luminescence of Yb³⁺-Yb³⁺ pairs and 〉 transition of exchange-coupled Yb³⁺-Mn²⁺ dimers, respectively.（3） A series of MgGa₂O₄:Er³⁺,Mn²⁺ and MgGa₂O₄:Er³⁺,Yb³⁺,Mn²⁺ samples were prepared by high temperature solid-state reaction method. Under 980 nm LD excitation, selectively enhanced green UC emission（533 nm） and near-infrared downconversion emission（1550 nm） of Er³⁺ can be obtained by Yb³⁺-Mn²⁺ dimer sensitizing, namely, the energy transfer from 丨²F_7/2,⁴T₁（⁴G）〉 state of Yb³⁺-Mn²⁺ dimer to ²H_11/2 state of Er³⁺.