Synthesis , Tunable Multicolor Output , and High Pure Red Upconversion Emission of Lanthanide-Doped Lu 2 O 3 Nanosheets

Yb and Ln (Ln = Er, Ho) codoped Lu 2 O 3 square nanocubic sheets were successfully synthesized via a facile hydrothermal method followed by a subsequent dehydration process. The crystal phase, morphology, and composition of hydroxide precursors and target oxides were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), and energy-dispersive X-ray spectroscope (EDS). Results present the as-prepared Lu 2 O 3 crystallized in cubic phase, and the monodispersed square nanosheets were maintained both in hydroxide and oxides. Moreover, under 980 nm laser diode (LD) excitation, multicolor output from red to yellow was realized by codoped different lanthanide ions in Lu 2 O 3 . It is noteworthy that high pure strong red upconversion emission with red to green ratio of 443.3 of Er-containing nanocrystals was obtained, which is beneficial for in vivo optical bioimaging.


Introduction
In the past few decades, lanthanide-doped upconversion phosphors have received considerable attention due to their application in display systems [1], optical processing sensors [2], solid-state lasers [3], fluorescent detection, and label of biomolecules [4].More importantly, as the applications in biological assays and medical imaging, the conventional semiconductor quantum dots were limited due to cytotoxicity, photobleaching, photodamage and low signal-to-noise ratio for excitation with short wavelength (usually ultraviolet and visible light) [5,6].Upconversion nanocrystals (UCNCs), which overcome those problems and absorb long wavelength (usually near infrared (NIR) light) and emit short wavelength, were believed to be a new kind of fluorescence materials [7,8].Besides, UCNCs have low radiation damage, chemical stability, and deep penetration of NIR excitation source compared with conventional fluorescent materials [9,10].
It is well known that the luminescence property of UCNCs is affected by a combination of different host/activator, doping concentrations, nanocrystal size, shape, and the coatings [11][12][13].Therefore, it is important to select appropriate host matrixes for achieving excellent luminescence property.Lutetium oxides are one of the excellent host matrix for ionizing radiation detection and X-ray phosphors because of their unique properties: high atomic number of Lu (71) and high density (9.42 g/cm −3 ) [14,15].Besides, the ionic radius of Lu 3+ (1.117 Å) is closer to the radius of Yb 3+ (1.125 Å) than that of other lanthanide ions [16], which may improve the stability of UCNCs while doping high concentration Yb 3+ .More importantly, the energy levels of Lu, especially the 4f orbital, make Lu-containing structure of stronger luminescence intensity than Y-containing crystal [17][18][19], which was known as very efficient phosphors.
As a potential host matrix, lutetium oxides have received many research interests.Studies on the synthesis of Lu 2 O 3 with controlled size and various morphologies have been extensively carried out [20][21][22].In addition, the luminescence properties of Lu-containing nanocrystals, for example, multicolor output manipulation by changing the reaction temperature, doping different lanthanide ions, and tuning the concentration of sensitizer or activator ions, have been widely investigated [23][24][25].However, most of those researches are about lutetium fluorides, and the study on lanthanide ionsdoped Lu 2 O 3 was still limited.Furthermore, the lutetium oxide nanocubic sheets have rarely been mentioned.
Herein, we have prepared Yb 3+ , Ln 3+ (Ln = Er, Ho) codoped Lu 2 O 3 via a simple and green hydrothermal method followed by a dehydration process [26].Moreover, under the excitation of 980 nm laser diode, strong eye-visible red and yellow lights were observed.And the mechanism of the pure red upconversion emission was discussed in detail.

Experimental Section
Rare earth nitrates RE(NO 3 ) 3 ⋅6H 2 O (RE = Lu, Yb, Er, Ho, 99.99%) were purchased from Sinopharm Chemical Reagent Co., Ltd (China).All other materials are of analytical grade and used directly without further purification.( = 0.04) was prepared by using a facile hydrothermal method followed by a dehydration process [26].As for the synthesis of Lu 2 O 3 :Yb 3+ , Er 3+ , the corresponding amount of rare earth nitrate (total amount: 1 mmol) was thoroughly mixed with 20 mL distilled water, then NaOH solution with [OH − ] = 5 mol/L was gradually added to obtain the hydroxide precipitates and adjust the pH to 14 under strong stirring.After being stirred 10 mins, the mixed solution was transferred into autoclaves to carry out a hydrothermal treatment at 180 ∘ C for about 12 h, and after cooling down to room temperature the hydroxide precursor was purified by centrifugation, washed with distilled water several times, and then dried at 80 ∘ C. At last, the corresponding rare earth-doped oxides were prepared successfully through the subsequent dehydration at 800 ∘ C for about 4 hours.The Yb/Ho codoped Lu 2 O 3 was prepared in a similar procedure.

Characterization.
Power X-ray diffraction (XRD) patterns of the samples were measured with D/max 2500/PC diffractometer using Cu-K radiation ( = 1.5406Å) at 40 kV and 250 mA.The morphology and composition of the samples were recorded on a FE-SEM equipped by an EDS system (FEI NanoSEM 450).The upconversion spectra were recorded using R-500 spectrophotometer under an unfocused 980 nm LD excitation at room temperature, and the corresponding digital photographs were taken by a digital camera under 980 nm infrared irradiation.More information about morphology and composition can be obtained by FE-SEM observation and EDS analysis.Figure 2 shows the FE-SEM images of precursor Lu(OH) 3 (Figures 2(a) and 2(b)) and Lu 2 O 3 :Yb 3+ , Ho 3+ (Figures 2(c) and 2(d)).As shown in FE-SEM images, all of these two samples present square nanosheets structure and no obvious difference between the two samples in the morphology is founded, implying the samples have well thermal stability.The corresponding EDS (Figure 2(e)) shows the main elements components are Lu, O, Yb, and Ho, implying the Yb and Ho ions are doped into the host matrix successfully.3(a) demonstrates the upconversion luminescence spectra of Yb-Er codoped Lu 2 O 3 nanocrystals.Under the excitation of 980 nm laser diode, an intense red emission band centered at 655 nm and a very weaker green emission area are observed.After magnifying 350 times (left inset in Figure 3(a)), the green emission area includes two weak bands centered at 526 nm and 542 nm, respectively.Moreover, the red to green (R/G) ratio is measured to a very large value of 443.3, resulting in a pure red upconversion emission, which is also verified by the digital photograph (right inset of Figure 3(a)) and CIE chromaticity coordinates ( = 0.6312,  = 0.33346) illustrated in Figure 3(b).When codoped Yb-Ho ions to Lu 2 O 3 nanocrystals, the as-prepared structure has both intense green and red emissions (Figure 3(c)).Therefore, bright eye-visible yellow light was observed (inset of Figures 3(c) and 3(d)), differing from the red light seen from the Yb-Er codoped samples.Therefore, the multicolor tuning from red to yellow can be achieved by simply doping different activator ions such as Er 3+ and Ho 3+ .More importantly, the pure red upconversion emission with large R/G ratio of 443.3 was realized, which is more beneficial for optical bioimaging for deep tissue penetration.To reveal the possible upconversion mechanism, a simplified energy level diagram with indicated pathways is presented (Figure 4).As shown, the two weak green emissions of Yb-Er co-doped samples are assigned to the 2 H 11/2 → 4 I 15/2 (526 nm) and 4 S 3/2 → 4 I 15/2 (542 nm) transitions of Er 3+ , and the red emission is attributed to the 4 F 9/2 → 4 I 15/2 transition of Er 3+ .The high intensity ratio of the red to green emission can be explained from Figure 4.One reason of the red emission enhancement is that energy transfer from sensitizer ion Yb 3+ at 2 F 5/2 state to activator ion Er 3+ through 4 I 15/2 → 4 I 11/2 transition, and then perform a nonradiative decay, which increase the number of Er 3+ at 4 I 13/2 level, at last, energy transfer from Yb 3+ at 2 F 5/2 state to Er 3+ through 4 I 13/2 → 2 F 9/2 .Another reason is the efficient cross-relaxation (CR) process of Er 3+ : 4 I 11/2 + 4 F 7/2 → 4 F 9/2 + 4 F 9/2 illustrated in Figure 4.The CR process strengthens the red band and weakens the green emission because most of the ions at 4 F 7/2 state transfer to 4 F 9/2 directly bypassing the 2 H 11/2 and 4 S 3/2 , which contribute to the green emission [23][24][25].

Upconversion Luminescent Properties. Figure
On the other hand, the green and red upconversion emissions of Ho containing sample are mainly attributed to the transitions 5 F 4 → 5 I 8 (541 nm), 5 S 2 → 5 I 8 (551 nm), 5 F 5 → 5 I 8 (640-678 nm) of Ho 3+ (Figure 4).As illustrated in Figure 4, all the processes need two-photo energy transfer; the red emission state 5 F 5 is attributed to phonon-assisted energy transfer: 5 I 7 (Ho 3+ ) → 5 F 5 (Ho 3+ ) and non-radiative decay from 5 S 2 .When doping high concentration Yb 3+ (20%), all the three emissions are enhanced and lead to an intense eye-visible yellow light with CIE chromaticity coordinates at  = 0.419,  = 0.5318.

Conclusion
In summary, monodispersed Lu 2 O 3 co-doped with Yb 3+ and Ln 3+ (Ln = Er, Ho) square nanocubic sheets were successfully fabricated with a simple hydrothermal method.The upconverted luminescence properties of as-prepared samples were well investigated under excitation of 980 nm laser diode.By changing the doped ions from Er to Ho, the upconversion emissions transform from eye-visible red to bright yellow light.Moreover, high pure red upconversion emission was obtained in Yb/Er codoped sample, which is suitable for high contrast optical bioimaging with absence of autofluorescence owing to the low tissue absorption at red light area (655 nm).
3.1.Phase and Morphology.Figure 1 shows the XRD patterns of Lu 2 O 3 :Yb 3+ , Ho 3+ and the standard cubic phase Lu 2 O 3 (JCPDS no.86-2475) for comparison.As demonstrated, all diffraction peaks of Yb-Ho codoped Lu 2 O 3 are perfectly indexed to the pure cubic phase Lu 2 O 3 and no other impurity peaks are detected, implying the high purity cubic phase structure is obtained via the facile hydrothermal approach.Moreover, compared with the standard diffraction peaks, there are very little deviations because the radius of main doped ion Yb 3+ (1.125 Å) is very close to that of Lu 3+ (1.117 Å) [16]. 2 O 3 : Yb 3+ , Ho 3+Figure 1: XRD patterns of Yb 3+ and Ho 3+ codoped Lu 2 O 3 powder obtained via a hydrothermal approach (black line); the red lines indicate the standard data of pure cubic Lu 2 O 3 structure (JCPDS: 86-2475).