Synthesis of Er 3+ /yb 3+ Codoped Namnf 3 Nanocubes with Single-band Red Upconversion Luminescence †

We have developed a facile low-temperature synthetic method for the preparation of NaMnF 3 nanocubes with Er 3+ and Yb 3+ ions homogeneously incorporated in the host lattice. The effects of the reaction temperature, and the volume ratio between ethanol and DI water on the morphology of NaMnF 3 nanocubes are systematically investigated. The NaMnF 3 nanocubes can be produced in the low temperature range (25–80 C), and the higher reaction temperature (80 C) is favorable for the formation of a smooth surface. The formation of NaMnF 3 nanocubes strongly depends on the ethanol solvent. The morphology and single-phase of the obtained samples could be well maintained by controlling the doping concentration (Yb 3+ # 20 mol%).


Introduction
2][3] Especially as a biological labeling material, UC uorescent labels show very low background light as a result of their unique uorescence properties and high detection limits compared with their traditional counterparts, such as organic dyes and quantum dots. 4,5The most efficient UC phosphor currently known is based on Er 3+ ion in combination with Yb 3+ ion as a sensitizer, which exhibits a green emission ($550 nm) as well as a red emission ($660 nm). 6The red emission is of technological importance since it is located in the "optical transmission window" of biological tissues, which has the minimum absorption of tissues and the maximum penetration depth. 7On the other hand, the green emission cannot effectively penetrate the deep tissue and may also cause many unwanted side effects that will reduce the sensitivity of the imaging. 8Therefore, avoiding the green emission and achieving strong and single-band red emission from Er 3+ -Yb 3+ couple is eagerly demanded for the development of high-sensitivity and high-specicity probes for bioimaging.
6][17][18][19] Hence, much effort has been dedicated on controlling the size and shape of the particles. 10,20However, in most cases, the geometry of Mn 2+ -based nanostructures obtained by conventional hydro/solvo-thermal method is spherical, and the synthesis of non-spherical nanostructures still suffers from extra technological difficulties. 11,13,14n addition, the previously reported approaches still suffer from problems including complicated experimental conditions, tedious procedures, and high reaction temperatures ($160 C). 10,12 Hence, from safety and energy-saving viewpoints, it is highly desirable to develop a novel low-temperature solution-phase synthesis protocol to manipulate the morphology of Mn 2+ -based nanostructures.
In the present work, we have developed a straightforward wet-chemical approach to fabricate uniform and monodispersed Er 3+ /Yb 3+ codoped NaMnF 3 nanocubes.The effects of the reaction temperature, and the volume ratio between ethanol and DI water on the morphology of NaMnF 3 nanocubes are systematically investigated.We examine the structural and UC luminescence properties of the NaMnF 3 :Er 3+ /Yb 3+ nanocubes as RSC Advances COMMUNICATION a function of dopant concentrations of Er 3+ /Yb 3+ (1-3 : 5-20 mol%).The UC luminescence properties of as-prepared nanocubes are compared with those of hexagonal-phase NaYF 4 with the same dopant concentrations.

Sample preparation
NaF (99%), MnCl 2 $4H 2 O (99%), YbCl 3 $6H 2 O (99.9%), ErCl 3 $6H 2 O (99.9%), and absolute ethanol were purchased from Sigma-Aldrich and were used as starting materials without further purication.DI water is used as solvent for the above chemicals to prepare stock solution.The strategy for synthesizing Er 3+ /Yb 3+ codoped NaMnF 3 nanocubes is schematically depicted in Scheme 1.In a typical synthesis process, NaMnF 3 doped with 2 mol% Er 3+ and 20 mol% Yb 3+ was synthesized as follows: 3.12 mL of 0.2 M MnCl 2 $4H 2 O, 0.8 mL of 0.2 M YbCl 3 $6H 2 O and 0.08 mL of 0.2 M ErCl 3 $6H 2 O, and 4 mL of 0.6 M NaF were sequentially added to a beaker containing 24 mL of absolute ethanol under vigorous stirring.The reaction temperatures were set to be room temperature (25 C), 50 C and 80 C, according to the experiment requirements.The nal products were collected by means of centrifugation, washed with DI water for several times.

Characterization
The crystal structure of prepared products was analyzed by an X-ray powder diffractometer (Rigaku-TTR/S2) using CuKa radiation (l ¼ 1.54056 Å).The size and morphology of the products were examined by using a eld emission scanning electron microscope (FE-SEM, JSM-6700F at an acceleration voltage of 5 kV) equipped with an energy dispersive X-ray spectroscope (EDX, Horiba 7593-H model).The UC luminescence spectra were recorded using a uorescence spectrophotometer (Horiba Jobin Yvon FluoroLog3) in conjunction with a 980 nm laser as the excitation source.All measurements were performed at room temperature.

Characterizations of structure and morphology
The synthesis of NaMnF 3 materials is performed in various methods to study the effects of the experiment parameters such as reaction temperature, solvent and dopant.Fig. 1a presents the XRD patterns of NaMnF 3 host materials synthesized at various reaction temperatures.It can be seen that all the diffraction peaks of the samples correspond to the NaMnF 3 crystal (JCPDS standard card no.18-1224).The similar diffraction patterns of all samples reveal that the NaMnF 3 crystal can be formed in the temperature range of 25-80 C. The sharp and strong peaks of NaMnF 3 crystals suggest high crystallinity of the obtained samples.The XRD patterns of NaMnF 3 : 2 mol% Er 3+ , (10-30) mol% Yb 3+ phosphors are also shown in Fig. 1b.It is evidenced that the crystal structure keeps the same until the Yb 3+ concentration reaches 20 mol%, indicating that doped elements have been effectively doped into the host lattice.It is notable that an impurity phase is developed for the NaMnF 3 : 2 mol% Er 3+ , 30 mol% Yb 3+ sample, which can be assigned to the Na 5 Yb 9 F 32 crystal (JCPDS standard card no.27-1426).
The morphology of NaMnF 3 host obtained at different reaction temperatures is characterized by SEM.From the lowresolution SEM images (Fig. 2a-c), uniform and monodispersed nanocubes with an average size around 900 nm can Scheme 1 Schematic illustration of the fabrication strategy for Er 3+ /Yb 3+ codoped NaMnF 3 nanocubes.be obtained in the reaction temperature range of 25-80 C. As revealed by the magnied SEM image (Fig. 2d), the surfaces of nanocubes are very rough, and full of cracks are observed when the reaction is carried out at room temperature.With the increase of reaction temperature to 50 C, the cracks are gradually disappeared (Fig. 2e), and nally, very smooth surface over the whole particle is obtained at 80 C (Fig. 2f).The morphology of NaMnF 3 host is also investigated by doping various amounts of rare-earth ions.As shown in Fig. 3a and b, the morphologies of Er 3+ /Yb 3+ codoped NaMnF 3 nanocrystals are kept well until the Yb 3+ doping concentration reaches 20 mol%.However, for the higher Yb 3+ doping (30 mol%), besides the nanocubes, the coexistence of nanoparticles with the size of 100 nm is observed (Fig. 3c).It is conrmed that these nanoparticles are responsible for the impurity phase shown in Fig. 1b, which indicates that the excessive Yb 3+ ions in solution prefer to react with NaF to form Na 5 Yb 9 F 32 crystal, rather than doped into NaMnF 3 host.Based on the both XRD and SEM results, doping Yb 3+ ion lower than 20 mol% is essential to preserve the single-phase and morphology of obtained samples.
Reaction solvent is another critical factor for the growth of nanocrystals, which can inuence the reaction rate of crystal formation and further determine the phase and morphology of the nal products. 21,22To evaluate the effect of reaction solvent on the formation of obtained samples, a set of NaMnF 3 nanocrystals are fabricated in the mixed solutions of ethanol (ET) and DI water (DW).The sum amount of ethanol and DI water was xed to 24 mL, and the ET/DW volume ratio was varied to 0 : 24 mL, 8 : 16 mL, 16 : 8 mL, and 24 : 0 mL.As shown in Fig. 4a, with the solvent of DI water, the irregularly-shaped and strongly-aggregated large blocks (several micrometers) as well as nanoparticles ($100 nm) can be produced.With the addition of ethanol (8 mL) into solvent, the formation of aggregated micro-clusters and micro-hexahedrons with large size distributions are conrmed (Fig. 4b).In the solvent with 16 mL ethanol, the micro-clusters are disappeared, and irregular hexahedrons with the size range of 1-2 mm are observed (Fig. 4c).When the reaction is carried out in absolute ethanol, monodispersed nanocubes with a size of about 900 nm can be obtained (Fig. 4d).The results reveal that the introduction of ethanol in reaction system can effectively prevent agglomeration and stimulate the growth into NaMnY 3 nanocubic assemblies.
4][25] For instance, it was reported that the size of BaSO 4 is reduced from 85 nm to 54 nm with the increase of ethanol percentage in ethanol-water mixed solvent from 30% to 70%.The effect of water and ethanol amount on the morphology of NaMnY 3 could be attributed to the solvent interactions with the precursors, manganese chloride and sodium uoride. 26It is well known that water has higher degree of porosity than ethanol.Increasing the ET/DW ratio will decrease the solvent polarity and the interfacial energy with the particles, which prevents the aggregation of the particles due to water swelling effect and makes the system more homogeneous. 27,28On the other hand, the reason for the formation of cubic particles may lie in an unusual inherent characteristic of NaMnF 3 . 29The ethanol solvent with relatively longer chain than water may change the order of the free energies of different facets through their interactions with the specic facets of NaMnF 3 crystals. 30This alternation may signicantly affect the relative growth rates of different facets and lead to the crystals with cubic morphology.

Upconversion luminescence properties
Fig. 5a and b show the room-temperature UC emission spectra of NaMnF 3 nanocubes doped with various concentrations of Er 3+ and Yb 3+ ions.In comparison with Er 3+ /Yb 3+ codoped routine rare-earth based uoride nanocrystals which typically exhibit multiple-band emissions in the visible spectral region, a single-band emission in the spectral range of 640-690 nm is detected for all the NaMnF 3 nanocubes doped with different amount of Er 3+ /Yb 3+ (1-3 : 5-20 mol%) upon excitation at 980 nm, which is assigned to the 4 F 9/2 / 4 I 15/2 transition of Er 3+ ions.It should be noted that though single-band red emission has been realized in several host materials, it is still challenging to obtain red emission with the high chromatic purity in NaMnF 3 host. 31,32In NaMnF 3 nanocubes, the red-to-green intensity ratios in all samples are larger than 40, which indicate that the present materials are favorable for applications in deep-tissue bioimaging (Fig. S1 †).In addition, the full width at half maximum (FWHM) of the red-emitting band is measured to be 27 nm, which is comparable to that for KMnF 3 :Yb 3+ /Er 3+ nanocrystals (20 nm), but is narrower than the red emission bands of ZrO 2 :Yb 3+ /Er 3+ nanocrystals (42 nm) or Y 2 O 3 :Yb 3+ /Er 3+ nanocrystals (75 nm). 10,33,34o gain more information on the UC mechanism, the pumping power dependence of UC luminescence intensity is studied.For an unsaturated UC process, the UC emission intensity (I) increases in proportion to the excitation power (P) according to the power law I f P x , and generally, the measured slope of x is indicative of an upconversion process, which involves at least n photons, where n is the smallest integer greater than x or equal to x if x is an integer. 35,36Fig. 5c shows the log-log plots of the red luminescence intensity in 2 mol% Er 3+ /20 mol% Yb 3+ doped NaMnF 3 nanocubes as a function of excitation intensity at 980 nm.The result indicates that, a slope n value of 1.83 is obtained for the red emission band, indicating that two-photon processes are involved for generating the UC emissions in the present sample.It is also noteworthy that, the single-band feature of Er 3+ /Yb 3+ codoped NaMnF 3 can be remained well in the broad excitation power density range of 7.5-20 mW cm À2 (Fig. S2 †).
According to the energy matching and quadratic dependence on excitation power, the possible UC mechanisms for the singleband red emission are discussed based on the simplied energy levels of Er 3+ , Yb 3+ and Mn 2+ ions.As illustrated in Fig. 5d, the Er 3+ ion can be rstly excited to the 4 I 11/2 state through an energy transfer process from a Yb 3+ ion, and then further jumped to the 4 F 7/2 state by absorbing the energy from another Yb 3+ ion.Then Er 3+ ion can be nonradiatively relaxed to two lower levels, 2 H 11/2 and 4 S 3/2 , resulting in the green ( 2 H 11/2 / 4 I 15/2 and 4 S 3/2 / 4 I 15/2 ) UC emissions, and even further relaxed to 4 F 9/2 level to generate red ( 4 F 9/2 / 4 I 15/2 ) emission.However, with the presence of large amount of Mn 2+ ions in NaMnF 3 host, the interaction between Er 3+ and Mn 2+ plays an important role on modifying the UC emissions.Due to the close proximity and excellent overlap of energy levels of the Er 3+ and Mn 2+ ions in the host lattices, a nonradiative energy transfer from the 2 H 11/2 and 4 S 3/2 levels of Er 3+ to the 4 T 1 level of Mn 2+ , which is followed by back-energy transfer to the 4 F 9/2 level of Er 3+ . 10,31The large red-to-green intensity ratios in all samples suggests that an extremely efficient exchange-energy transfer process occurs between the Er 3+ and Mn 2+ ions.
As is known, the luminescence intensity from rare-earth ions strongly depends on the doping level, and the proper doping is indispensable to achieve maximum intensity. 9,37The UC luminescence intensities are compared between the NaMnF 3 nanocubes doped with various concentrations of Er 3+ and Yb 3+ ions (Fig. S3 †).In the condition of 20 mol% Yb 3+ in NaMnF 3 nanocubes, the sample with Er 3+ : 2 mol% irradiates brightest red luminescence, which indicates that the further increase of Er 3+ concentration does not benet luminescence intensity.In the previous publication, Du et al. reported that, the optimum Er 3+ concentration in Ca 0.65 La 0.35 F 2.35 host should be 2 mol%, which is consistent with our results. 38On the other hand, with 2 mol% Er 3+ concentration, the red emission increases with the Yb 3+ concentration increases from 5 mol% to 15 mol%, and further increase of Yb 3+ concentration results in the decrease of red This journal is © The Royal Society of Chemistry 2014 emission due to concentration quenching effect. 20The above comparative studies suggest that our NaMnF 3 :Er 3+ /Yb 3+ nanocrystals fabricated by present synthetic procedure own the strongest single-band emission feature at the dopant concentrations of Er 3+ (2 mol%) and Yb 3+ (15 mol%).
It should be noted that, in the previously published paper, Zhang et al. synthesized Er 3+ /Yb 3+ doped NaMnF 3 nanocrystals in the mixed solvents of 1-octadecene and oleic acid at 300 C, and the strongest emission appears at Er 3+ (25 mol%) and Yb 3+ (25 mol%). 11In contrast, in our samples fabricated in ethanol at 80 C, the dopant ions (Er 3+ + Yb 3+ ) cannot reach very high level (#22%), since the excessive Yb 3+ ions in ethanol prefer to react with NaF to form Na 5 Yb 9 F 32 crystal, rather than doped into NaMnF 3 host.The dopant ions can occupy the Mn 2+ sites in the host crystals.It is reasonable to assume that, some Mn 2+ sites are favourable for dopant ions when fabricated by our method.With the increase of reaction temperature, the dopants may occupy more Mn 2+ sites.Therefore, the distance and interaction between dopants are different in samples fabricated by different methods, which results in distinct luminescence properties.
Among the investigated uorides, hexagonal-phase NaYF 4 is known as one of the most efficient host lattices for both downconversion and UC processes. 39,40To evaluate the luminescence efficiency of NaMnF 3 host, a reference sample, hexagonal-phase NaYF 4 doped with 2 mol% Er 3+ and 15 mol% Yb 3+ has been prepared by a modied version of the procedure described previously (Fig. S4 †). 41,42Fig. 6 shows the comparison of UC luminescence spectra of NaYF 4 and NaMnF 3 doped with 2 mol% Er 3+ and 15 mol% Yb 3+ ions, respectively.The same amounts of samples are measured at the same experimental condition.Under the 980 nm excitation, the NaYF 4 sample shows multi-peak emissions in the green and red regions,  though pure red emission is detected from NaMnF 3 sample.Despite the emission difference, the red emission intensity of NaMnF 3 :Er 3+ /Yb 3+ is 1.4 times stronger and overall (green-plusred) emissions are 1.2 times greater than those of NaYF 4 :Er 3+ / Yb 3+ sample, indicating that NaMnF 3 is a promising host material for deep tissue bioimaging.Such a red-emission enhancement should mostly arise from the efficient crossrelaxation of energy between Mn 2+ and Er 3+ ions.

Conclusions
In summary, we have demonstrated the fabrication of uniform and monodispersed NaMnF 3 nanocubes by a facile lowtemperature solution-based method at ambient conditions.It is revealed that the proper controlling of the reaction temperature and solvent is critical for the formation of NaMnF 3 nanocubes.Though the NaMnF 3 nanocubes can be formed in the temperature range of 25-80 C, higher temperature is favorable to obtain uniform and smooth products.The ethanol solvent is essential for the formation of NaMnY 3 nanocubic assemblies.Doping Yb 3+ ion lower than 20 mol% is required to preserve the single-phase and morphology of obtained materials.As a result of efficient energy transfer between the dopant Er 3+ ion and host Mn 2+ ion, remarkably pure red UC emissions were generated in the dopant concentration ranges of Er 3+ /Yb 3+ (1-3 : 5-20 mol%).The strongest red emission in these Er 3+ / Yb 3+ doped nanocrystals has been realized at the dopant concentrations of Er 3+ (2 mol%) and Yb 3+ (15 mol%).The achieved red emission is 1.4 times stronger and overall (green-plusred) emissions are 1.2 times greater than those of NaYF 4 :Er 3+ / Yb 3+ phosphor.