Efficient manganese luminescence induced by Ce3+-Mn2+ energy transfer in rare earth fluoride and phosphate nanocrystals

Manganese materials with attractive optical properties have been proposed for applications in such areas as photonics, light-emitting diodes, and bioimaging. In this paper, we have demonstrated multicolor Mn2+ luminescence in the visible region by controlling Ce3+-Mn2+ energy transfer in rare earth nanocrystals [NCs]. CeF3 and CePO4 NCs doped with Mn2+ have been prepared and can be well dispersed in aqueous solutions. Under ultraviolet light excitation, both the CeF3:Mn and CePO4:Mn NCs exhibit Mn2+ luminescence, yet their output colors are green and orange, respectively. By optimizing Mn2+ doping concentrations, Mn2+ luminescence quantum efficiency and Ce3+-Mn2+ energy transfer efficiency can respectively reach 14% and 60% in the CeF3:Mn NCs.


Introduction
The preparation of fluorescent nanomaterials continues to be actively pursued in the past decades. The potentially broad applicability and high technological promise of the fluorescent nanomaterials arise from their intrinsically intriguing optical properties, which are expected to pale their bulk counterparts [1][2][3][4]. Particularly, controllable energy transfer in the nanomaterials has been receiving great interest because it leads luminescence signals to outstanding selectivity and high sensitivity, which are important factors for optoelectronics and optical sensors [5].
Great efforts have been devoted to Mn 2+ -doped semiconductor nanocrystals [NCs] due to their efficient sensitized luminescence [6,7]. When incorporating Mn 2+ ions in a quantum-confined semiconductor particle, the Mn 2+ ions can act as recombination centers for the excited electron-hole pairs and result in characteristic Mn 2+ ( 4 T 1 -6 A 1 )-based fluorescence. Compared with the undoped materials, the Mn 2+ -doped semiconductor NCs often have higher fluorescence efficiency, better photochemical stability, and prolonged fluorescence lifetime. Therefore, such Mn 2+ -doped NCs have recently been proposed as bioimaging agents [8,9] and recombination centers in electroluminescent devices [10,11]. They may even find applications in future spin-based information processing devices [12,13] and have been examined as models for magnetic polarons [14]. Moreover, as emission centers, Mn 2+ ions can be used for the synthesis of long persistent phosphors [15,16], and white-light ultraviolet light-emitting diodes [17], when doped in inorganic host materials (such as silicate, aluminate, and fluoride).
Rare earth ions (such as Ce 3+ and Eu 2+ ) have been commonly used as sensitizers to improve Mn 2+ fluorescence efficiency in bulk materials [18][19][20]. Typically, the efficient room temperature [RT] luminescence were reported in the Mn 2+ , Ce 3+ co-doped CaF 2 single crystal and other matrixes, which were assigned to the energy transfer from the Ce 3+ sensitizers to the Mn 2+ acceptors through an electric quadrupole short-range interaction in the formed Ce 3+ -Mn 2+ clusters [18]. However, a portion of isolated Ce 3+ and Mn 2+ ions which are randomly dispersed in the host usually causes a low Ce 3+ -Mn 2+ energy transfer efficiency.
In this work, we have synthesized the CeF 3 :Mn and CePO 4 :Mn NCs and investigated the Ce-Mn energy transfer in these representative rare earth NCs. Upon UV light excitation, both the CeF 3 :Mn and CePO 4 :Mn show bright Mn 2+ luminescence in the visible region. Their fluorescence output colors, however, are quite different owing to different host crystal structures. The optimum Mn 2+ doping concentration has been found at which the Mn 2+ luminescence quantum efficiency and Ce 3+ -Mn 2+ energy transfer efficiency peak at 14% and 60% in the CeF 3 :Mn NCs, respectively.

Synthesis of CeF 3 :Mn nanocrystals
CeF 3 NCs were synthesized using a modified method reported previously [21]. In a typical procedure, x mL of 0.2 M MnCl 2 and (0.2x) mL of 0.2 M CeCl 3 were added to 15 mL of ethanol with 5 mL of PEI solution (5 wt.%).
After stirring for 30 min, an appropriate amount of NH 4 F was charged. The well-agitated solution was then transferred to a Teflon-lined autoclave and subsequently heated at 200°C for 2 h. After cooling down, the product was isolated by centrifugation, washed with ethanol and deionized water several times, and dried in vacuum.

Synthesis of CePO 4 :Mn nanocrystals
In a typical procedure, x mL of 0.2 M MnCl 2 and (12-x) mL of 0.2 M CeCl 3 were mixed. The mixture was agitated for 10 min, then charged with 5 mL of 0.5 M H 3 PO4, and eventually placed under ultrasonic irradiation for 2 h. All ultrasonic irradiations were performed in a water bath with an ultrasonic generator (100 W, 40 kHz; Kunshan Ultrasonic Instrument Co., Shanghai, China). The particles were obtained by centrifugation, washed with ethanol and deionized water several times, and dried in vacuum.

Physical and optical measurements
The transmission electron microscopy [TEM] measurements were carried out on a JEOL 2010 HT transmission electron microscope (operated at 200 kV). X-ray diffraction [XRD] analyses were performed on a Bruker D8-advance X-ray diffractometer with Cu Kα irradiation (l = 1.5406 Å). The absorption spectra were obtained with a Varian Cary 5000 UV/Vis/NIR spectrophotometer. The photoluminescence [PL] and PL excitation [PLE] spectra were recorded by a Hitachi F-4500 fluorescence spectrophotometer with a Xe lamp as the excitation source.

Results and discussion
Morphology and structure Both the CeF 3 :Mn and the CePO 4 :Mn NCs were synthesized by effective hydrothermal processes. The prepared CeF 3 :Mn NCs are shaped as hexagonal plates with average sizes of~25 nm, as shown by the TEM image in Figure 1a. Figure 1b demonstrates CePO 4 :Mn nanowires with an average diameter of~8 nm and an average length of~400 nm. Figure 2 shows XRD spectra of CeF 3 :Mn and CePO 4 : Mn NCs. The XRD pattern of the CeF 3 :Mn NCs shows that all the peak positions are in good agreement with the literature data of the hexagonal CeF 3 crystal, and the peak positions exhibited by the CePO 4 :Mn NCs are well indexed in accord with the hexagonal CePO 4 crystal, revealing high crystallinity of these two kinds of products.

Absorption spectra
As shown in Figure 3, the CeF 3 :Mn NCs exhibit four absorption peaks located at 248, 235, 218, and 205 nm, which are attributed to the electronic transitions from the ground state to different 5d states of the Ce 3+ ions. The above absorption peaks' wavelength of the CeF 3 :Mn NCs are in good agreement with those reported for   [23]. The two bands are overlapped because the excited state is strongly split by the crystal field [24]. We note that the Mn 2+ 6 A 1g (S)-4 E g (D) and 6 A 1g (S)-4 T 2g (D) absorption transitions from 310 to 350 nm [18] in these NCs are not obvious due to the much weaker Mn 2+ absorption ability and low Mn 2+ /Ce 3+ ratio in the host. Figure 4a schematically depicts the Ce 3+ -Mn 2+ energy transfer process in the CeF 3 :Mn NCs, which efficiently induces a bright green luminescence under UV irradiation at RT. The RT PL emission spectra (with excitation wavelength l ex = 260 nm) of the CeF 3 :10%Mn NCs contain not only the strong Mn 2+ emission at 498 nm but also the Ce 3+ emission at 325 nm. As known, the Mn 2+ 6 A 1g (S)-4 E g (D) and 6 A 1g (S)-4 T 2g (D) absorption transition is respectively at 325 and 340 nm [18]; both of these absorption bands are overlapped by the Ce 3+ emission. This overlap facilitates the energy transfer from Ce 3+ to Mn 2+ , resulting in the characteristic 4 T 1g (G)-6 A 1g (S) emission of Mn 2+ [25,26]. Such Ce 3+ -Mn 2+ energy transfer is induced by the electric dipole-quadrupole interaction between the Ce 3+ sensitizers and Mn 2+ acceptors [19]. Furthermore, in Figure 4a, only the RT excitation peak ascribed to the Ce 3+ 4f-5d transition can be observed at 260 nm, while the Mn 2+ characteristic peaks cannot be witnessed because the Mn 2+ absorption transitions are forbidden by spin and parity for electric dipole radiation as T > 200 K [27]. Since the RT Mn 2+ luminescence is very difficult to be found in the transition-metal concentrated materials like MnF 2 [27], the Ce 3+ -Mn 2+ energy transfer offers an efficient route for obtaining Mn 2+ RT luminescence in nanomaterials.

Photoluminescence properties
Similarly, the Ce 3+ -Mn 2+ energy transfer process in the CePO 4 :10%Mn NCs triggers an orange luminescence under UV irradiation (Figure 4b). The emission spectra of the CePO 4 :Mn upon excitation at 260 nm contain both the Ce 3+ emission at 355 nm and the Mn 2+ orange emission around 575 nm arising from the 4 T 1g (G)- 6   (S) transition of Mn 2+ . As known, the luminescence output color of the Mn 2+ ions is strongly dependent on the coordination environment of the host lattice, such as the strength of the ligand field and the coordination number. The green emission of Mn 2+ ions at about 500 nm is usually obtained in a weak crystal field environment where Mn 2+ is usually four or eightfold [27,28]. In contrast, the CePO4 NCs have a monazite structure in which the dopant ions are probably ninefold and in a stronger crystal field environment [29]. Thus, the orange emission can be observed in our synthesized CePO 4 :Mn NCs. We note that the CePO 4 :Mn NCs synthesized are rodlike particles whose shape is greatly different from the platelike CeF 3 :Mn NCs due to the different growth behavior. To eliminate the influence of the particle shape on the luminescence output color of Mn 2+ ions, we have further synthesized rodlike hexagonal phase NaYF 4 :Ce,Mn NCs using our established method [21] in which the Ce 3+ -Mn 2+ energy transfer also results in green Mn 2+ luminescence at 500 nm (data not shown).

Quantum efficiency and energy transfer efficiency
The Mn 2+ luminescence quantum efficiency (h QE ) was determined by comparing the Mn 2+ emission intensity of the CeF 3 :Mn aqueous solution with a solution of quinine bisulfate in 0.5 M H 2 SO 4 with approximately the same absorption at an excitation wavelength of 260 nm [30]. It is important that all the sample solutions were sufficiently diluted (absorption value of 0.03 at 260 nm) to minimize the possible effects of reabsorption and other concentration effects [31]. The h QE of the CeF 3 :Mn NCs increases significantly and reaches 14% as the doped Mn 2+ molar concentration increases to 2%. The decreased h QE at Ce 3+ concentrations above 2% is probably due to the increased Mn 2+ ↔Mn 2+ energy migration which weakens the Ce 3+ -Mn 2+ energy transfer. We note that the highest h QE we obtained is similar to that of the Ce, Tb co-doped LaF 3 NCs reported previously [32]. The Ce 3+ -Mn 2+ energy transfer efficiency (h ET ) was estimated from the emission intensity ratio I Mn /(I Ce + I Mn ) when the sample solutions were sufficiently diluted and the energy loss caused by the re-absorption effects between different particles could be neglected [31,33]. As shown in Figure 5a, a high h ET of 60% is observed in the CeF 3 :Mn NCs while the Mn 2+ doping concentration is over 10%. We note that the I Mn is much weaker than the I Ce in the previously reported Mn,Ce co-doped CaF 2 and other bulk materials because of a portion of randomly dispersed Ce 3+ and Mn 2+ ions beyond the interaction distance for the short-range energy transfer [19,34]. In our CeF 3 :Mn NCs, the Ce 3+ -Mn 2+ clusters are easily formed and result in the efficient Ce 3+ -Mn 2+ energy transfer. By using the method discussed above, we have also investigated the h QE and h ET of the CePO 4 :Mn 2+ NCs in the presence of different Mn 2+ concentrations (Figure 5b). Upon doping with the increasing concentrations of Mn 2+ , both the h QE and h ET increase firstly, and the h QE reaches the peak at 0.6% when the Mn 2+ doping concentration is 10%. It is worth noting that both the h QE and h ET in the CeF 3 :Mn NCs are higher than those in the CePO 4 :Mn NCs. Compared with phosphates, fluorides normally have lower vibrational energies, which can decrease the quenching of the excited state of rare earth ions [35] and result in higher quantum efficiency. Besides, the energy transfer efficiency between the sensitizers and acceptors is influenced greatly by the interaction distance of these dopant ions [19,36]. Here, the less energy transfer efficiency in CePO 4 :Mn is probably attributed to the larger interaction distance between the Ce 3+ and Mn 2+ ions. A further increase of the quantum efficiency and energy transfer efficiency is possible by applying an undoped inorganic shell as a protective layer.

Conclusions
The sensitized Mn 2+ luminescence has been realized based on the Ce 3+ -Mn 2+ energy transfer in the prepared Mn 2+ -doped rare earth NCs. The 4 T 1g (G)-6 A 1g (S) characteristic emission of Mn 2+ reveals green luminescence in CeF 3 :Mn and orange luminescence in CePO 4 :Mn, resulting from the crystal field differences of these two hosts. We worked out that the highest Mn 2+ luminescence quantum efficiency can reach 14% and 0.6% in the CeF 3 :Mn and CePO 4 NCs, respectively. Our results may find applications in the manipulations of the Ce 3+ -Mn 2+ energy transfer for redox switches [37] and broadly impact areas such as photonics, light-emitting diodes, and bioimaging based on manganese materials.