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 [1–4]. 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 Mn2+-doped semiconductor nanocrystals [NCs] due to their efficient sensitized luminescence [6, 7]. When incorporating Mn2+ ions in a quantum-confined semiconductor particle, the Mn2+ ions can act as recombination centers for the excited electron-hole pairs and result in characteristic Mn2+ (4T1-6A1)-based fluorescence. Compared with the undoped materials, the Mn2+-doped semiconductor NCs often have higher fluorescence efficiency, better photochemical stability, and prolonged fluorescence lifetime. Therefore, such Mn2+-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, Mn2+ 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 Ce3+ and Eu2+) have been commonly used as sensitizers to improve Mn2+ fluorescence efficiency in bulk materials [18–20]. Typically, the efficient room temperature [RT] luminescence were reported in the Mn2+, Ce3+ co-doped CaF2 single crystal and other matrixes, which were assigned to the energy transfer from the Ce3+ sensitizers to the Mn2+ acceptors through an electric quadrupole short-range interaction in the formed Ce3+-Mn2+ clusters [18]. However, a portion of isolated Ce3+ and Mn2+ ions which are randomly dispersed in the host usually causes a low Ce3+-Mn2+ energy transfer efficiency.

In this work, we have synthesized the CeF3:Mn and CePO4:Mn NCs and investigated the Ce-Mn energy transfer in these representative rare earth NCs. Upon UV light excitation, both the CeF3:Mn and CePO4:Mn show bright Mn2+ luminescence in the visible region. Their fluorescence output colors, however, are quite different owing to different host crystal structures. The optimum Mn2+ doping concentration has been found at which the Mn2+ luminescence quantum efficiency and Ce3+-Mn2+ energy transfer efficiency peak at 14% and 60% in the CeF3:Mn NCs, respectively.

Experimental section

Materials

Reagents MnCl2 (>99%), TbCl3 (>99%), CeCl3 (>99%), NH4F (>99%), and H3PO4 (>85%) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). Polyethylenimine [PEI] (branched polymer (-NHCH2CH2-) x (-N(CH2CH2NH2)CH2CH2-) y ) was purchased from Sigma-Aldrich (St. Louis, MO, USA). All reagents were used as received without further purification.

Synthesis of CeF3:Mn nanocrystals

CeF3 NCs were synthesized using a modified method reported previously [21]. In a typical procedure, x mL of 0.2 M MnCl2 and (0.2 - x) mL of 0.2 M CeCl3 were added to 15 mL of ethanol with 5 mL of PEI solution (5 wt.%). After stirring for 30 min, an appropriate amount of NH4F 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 CePO4:Mn nanocrystals

In a typical procedure, x mL of 0.2 M MnCl2 and (12 - x) mL of 0.2 M CeCl3 were mixed. The mixture was agitated for 10 min, then charged with 5 mL of 0.5 M H3PO4, 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 (λ = 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 CeF3:Mn and the CePO4:Mn NCs were synthesized by effective hydrothermal processes. The prepared CeF3: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 CePO4:Mn nanowires with an average diameter of ~8 nm and an average length of ~400 nm.

Figure 1
figure 1

TEM images. TEM images of CeF3:Mn (a) and CePO4:Mn (b) NCs.

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Figure 2 shows XRD spectra of CeF3:Mn and CePO4:Mn NCs. The XRD pattern of the CeF3:Mn NCs shows that all the peak positions are in good agreement with the literature data of the hexagonal CeF3 crystal, and the peak positions exhibited by the CePO4:Mn NCs are well indexed in accord with the hexagonal CePO4 crystal, revealing high crystallinity of these two kinds of products.

Figure 2
figure 2

XRD spectra. XRD spectra of CeF3:Mn and CePO4:Mn NCs.

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Absorption spectra

As shown in Figure 3, the CeF3: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 Ce3+ ions. The above absorption peaks' wavelength of the CeF3:Mn NCs are in good agreement with those reported for CeF3 bulk crystals [22]. The CePO4:Mn NCs exhibit two absorption bands with peaks at 256 and 273 nm [23]. The two bands are overlapped because the excited state is strongly split by the crystal field [24]. We note that the Mn2+ 6A1g(S)-4Eg(D) and 6A1g(S)-4T2g(D) absorption transitions from 310 to 350 nm [18] in these NCs are not obvious due to the much weaker Mn2+ absorption ability and low Mn2+/Ce3+ ratio in the host.

Figure 3
figure 3

Absorption spectra attributed to electronic transitions. Absorption spectra of CeF3:Mn and CePO4:Mn NCs.

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Photoluminescence properties

Figure 4a schematically depicts the Ce3+-Mn2+ energy transfer process in the CeF3:Mn NCs, which efficiently induces a bright green luminescence under UV irradiation at RT. The RT PL emission spectra (with excitation wavelength λ ex = 260 nm) of the CeF3:10%Mn NCs contain not only the strong Mn2+ emission at 498 nm but also the Ce3+ emission at 325 nm. As known, the Mn2+ 6A1g(S)-4Eg(D) and 6A1g(S)-4T2g(D) absorption transition is respectively at 325 and 340 nm [18]; both of these absorption bands are overlapped by the Ce3+ emission. This overlap facilitates the energy transfer from Ce3+ to Mn2+, resulting in the characteristic 4T1g(G)-6A1g(S) emission of Mn2+[25, 26]. Such Ce3+-Mn2+ energy transfer is induced by the electric dipole-quadrupole interaction between the Ce3+ sensitizers and Mn2+ acceptors [19]. Furthermore, in Figure 4a, only the RT excitation peak ascribed to the Ce3+ 4f-5d transition can be observed at 260 nm, while the Mn2+ characteristic peaks cannot be witnessed because the Mn2+ absorption transitions are forbidden by spin and parity for electric dipole radiation as T > 200 K [27]. Since the RT Mn2+ luminescence is very difficult to be found in the transition-metal concentrated materials like MnF2[27], the Ce3+-Mn2+ energy transfer offers an efficient route for obtaining Mn2+ RT luminescence in nanomaterials.

Figure 4
figure 4

PLE and PL spectra. PLE and PL spectra of CeF3:Mn (a) and CePO4:Mn (b) NCs.

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Similarly, the Ce3+-Mn2+ energy transfer process in the CePO4:10%Mn NCs triggers an orange luminescence under UV irradiation (Figure 4b). The emission spectra of the CePO4:Mn upon excitation at 260 nm contain both the Ce3+ emission at 355 nm and the Mn2+ orange emission around 575 nm arising from the 4T1g(G)- 6A1g(S) transition of Mn2+. As known, the luminescence output color of the Mn2+ 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 Mn2+ ions at about 500 nm is usually obtained in a weak crystal field environment where Mn2+ 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 CePO4:Mn NCs. We note that the CePO4:Mn NCs synthesized are rodlike particles whose shape is greatly different from the platelike CeF3:Mn NCs due to the different growth behavior. To eliminate the influence of the particle shape on the luminescence output color of Mn2+ ions, we have further synthesized rodlike hexagonal phase NaYF4:Ce,Mn NCs using our established method [21] in which the Ce3+-Mn2+ energy transfer also results in green Mn2+ luminescence at 500 nm (data not shown).

Quantum efficiency and energy transfer efficiency

The Mn2+ luminescence quantum efficiency (η QE) was determined by comparing the Mn2+ emission intensity of the CeF3:Mn aqueous solution with a solution of quinine bisulfate in 0.5 M H2SO4 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 η QE of the CeF3:Mn NCs increases significantly and reaches 14% as the doped Mn2+ molar concentration increases to 2%. The decreased η QE at Ce3+ concentrations above 2% is probably due to the increased Mn2+↔Mn2+ energy migration which weakens the Ce3+-Mn2+ energy transfer. We note that the highest η QE we obtained is similar to that of the Ce, Tb co-doped LaF3 NCs reported previously [32].

The Ce3+-Mn2+ energy transfer efficiency (η 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 η ET of 60% is observed in the CeF3:Mn NCs while the Mn2+ 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 CaF2 and other bulk materials because of a portion of randomly dispersed Ce3+ and Mn2+ ions beyond the interaction distance for the short-range energy transfer [19, 34]. In our CeF3:Mn NCs, the Ce3+-Mn2+ clusters are easily formed and result in the efficient Ce3+-Mn2+ energy transfer.

Figure 5
figure 5

Investigated η QE and η ET . Mn2+ luminescence quantum efficiency (η QE) and Ce3+-Mn2+ energy transfer efficiency (η ET) vs. molar percent of Mn2+ in CeF3:Mn (a) and CePO4:Mn NCs (b).

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By using the method discussed above, we have also investigated the η QE and η ET of the CePO4:Mn2+ NCs in the presence of different Mn2+ concentrations (Figure 5b). Upon doping with the increasing concentrations of Mn2+, both the η QE and η ET increase firstly, and the η QE reaches the peak at 0.6% when the Mn2+ doping concentration is 10%. It is worth noting that both the η QE and η ET in the CeF3:Mn NCs are higher than those in the CePO4: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 CePO4:Mn is probably attributed to the larger interaction distance between the Ce3+ and Mn2+ 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 Mn2+ luminescence has been realized based on the Ce3+-Mn2+ energy transfer in the prepared Mn2+-doped rare earth NCs. The 4T1g(G)-6A1g(S) characteristic emission of Mn2+ reveals green luminescence in CeF3:Mn and orange luminescence in CePO4:Mn, resulting from the crystal field differences of these two hosts. We worked out that the highest Mn2+ luminescence quantum efficiency can reach 14% and 0.6% in the CeF3:Mn and CePO4 NCs, respectively. Our results may find applications in the manipulations of the Ce3+-Mn2+ energy transfer for redox switches [37] and broadly impact areas such as photonics, light-emitting diodes, and bioimaging based on manganese materials.