Controllable preparation and upconversion luminescence properties of Tm3+/Ho3+ doped NaErF4@NaYF4 core-shell nanocrystals with bright red emission
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摘要: 为了获得具有红光发射的上转换纳米材料,以实现深层生物成像的应用,采用热分解法制备了一系列Tm3+、Ho3+掺杂NaErF4@NaYF4核壳上转换纳米晶,并对其形貌、结构和发光性能进行了表征。结果表明:所制备NaErF4、NaErF4∶Tm3+和NaErF4∶Ho3+裸核均为六方相结构,呈现良好的球形形貌,平均粒径分别为28.01 nm、23.19 nm、27.89 nm。包覆NaYF4惰性壳层后,样品晶型没有改变,形貌变为短棒状,平均长度增大至37.82 nm、38.51 nm、42.65 nm。在980 nm近红外光激发下,由于Er3+的4F9/2→4I15/2跃迁,所制备NaErF4、NaErF4∶Tm3+和NaErF4∶Ho3+裸核均呈现明显的红光发射,且包覆NaYF4惰性壳层后,发光强度和荧光寿命都明显增加,特别是NaErF4@NaYF4核壳样品的发光强度约是NaErF4裸核的1787倍,荧光寿命达到2.04 ms。此外,与NaErF4@NaYF4纳米棒相比,NaErF4∶(Tm3+, Ho3+)@NaYF4体系中的Tm3+、Ho3+充当了能量捕获中心且与Er3+之间发生能量传递,使其具有更大的红绿发射峰比值(R/G),发光颜色更趋近于红色,与色坐标(CIE)颜色区域相一致。最后,根据发光强度与激发功率的关系,详细分析了上转换纳米晶的发光增强机制及可能存在的能量传递过程。Abstract: In order to obtain upconversion nanomaterials with bright red emission for deep bioimaging applications, a series of Tm3+, Ho3+ doped NaErF4@NaYF4 core-shell upconversion nanocrystals were prepared by thermal decomposition method, and their morphology, structure and luminescence properties were characterized. The results show that NaErF4, NaErF4∶Tm3+ and NaErF4∶Ho3+ bare core all exhibit hexagonal phase structure and good spherical morphology, with average sizes of 28.01 nm 23.19 nm, and 27.89 nm, respectively. After coating NaYF4 inert shell, the crystal phase remains unchanged, but the morphology became short rod-like, and the average length increases to 37.82 nm, 38.51 nm, 42.65 nm. Under the excitation of 980 nm near-infrared light, NaErF4, NaErF4∶Tm3+ and NaErF4∶Ho3+ show obvious red emission due to the 4F9/2→4I15/2 transition of Er3+, and the luminescence intensity and lifetime increase significantly after coating NaYF4 inert shell. In particular, the luminescence intensity of NaErF4@NaYF4 core-shell sample is about 1787 times that of NaErF4 bare core, and the lifetime is 2.04 ms. In addition, compared with NaErF4@NaYF4, the Tm3+, Ho3+ ions in the NaErF4∶(Tm3+, Ho3+)@NaYF4 system act as energy trapping centers and generate energy transfer with Er3+, resulting in a larger red-green emission peak ratio (R/G), and the luminescence color is closer to red, which is consistent with the chromaticity coordinate (CIE) color region. Finally, according to the relationship between luminescence intensity and excitation power, the upconversion luminescence enhanced mechanism and the possible energy transfer process are analyzed in detail.
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Key words:
- upconversion luminescence /
- NaErF4 nanocrystal /
- red emission /
- core-shell /
- doping
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图 1 NaErF4 ((a), (g))、NaErF4∶Tm3+ ((b), (h))、NaErF4∶Ho3+ ((c), (i))、NaErF4@NaYF4 ((d), (j))、NaErF4∶Tm3+@NaYF4 ((e), (k))、NaErF4∶Ho3+@NaYF4 ((f), (l))纳米粒子的HRTEM图像和尺寸分布图
Figure 1. HRTEM images and size distributions of NaErF4 ((a), (g)), NaErF4∶Tm3+ ((b), (h)), NaErF4∶Ho3+ ((c), (i)), NaErF4@NaYF4 ((d), (j)), NaErF4∶Tm3+@NaYF4 ((e), (k)), NaErF4∶Ho3+@NaYF4 ((f), (l)) nanoparticles
d—Average diameter
图 5 980 nm激发下NaErF4@NaYF4 (a)、NaErF4∶Tm3+@NaYF4 (b)、NaErF4∶Ho3+@NaYF4 (c)核壳样品与对应裸核的上转换发射光谱对比图;不同核壳样品的上转换发射光谱(d)和红绿发射峰强度比值(R/G)图(e)
Figure 5. Comparison of emission spectra between bare core and core-shell samples under 980 nm excitation: (a) NaErF4@NaYF4; (b) NaErF4∶Tm3+@NaYF4; (c) NaErF4∶Ho3+@NaYF4; Emission spectra (d) and the ratio of red to green emission intensity (R/G) (e) of different core-shell samples
图 8 980 nm激发(Ex)下NaErF4、NaErF4∶Tm3+、NaErF4∶Ho3+裸核(a)和NaErF4@NaYF4、NaErF4∶Tm3+@NaYF4、NaErF4∶Ho3+@NaYF4核壳纳米粒子中653 nm发射(Em) (b)的荧光衰减曲线
Figure 8. Decay curves of 653 nm emission (Em) in NaErF4, NaErF4∶Tm3+, NaErF4∶Ho3+ core (a), NaErF4@NaYF4, NaErF4∶Tm3+@NaYF4, NaErF4∶Ho3+@NaYF4 core-shell nanoparticles under 980 nm excitation (Ex) (b)
τ—Lifetime
图 9 980 nm激发下NaErF4、NaErF4∶Tm3+、NaErF4∶Ho3+裸核(a)及NaErF4@NaYF4、NaErF4∶Tm3+@NaYF4、NaErF4∶Ho3+@NaYF4核壳(b)纳米粒子的色坐标图
Figure 9. Chromaticity coordinate (CIE) diagram of NaErF4, NaErF4∶Tm3+, NaErF4∶Ho3+ bare core (a) and NaErF4@NaYF4, NaErF4∶Tm3+@NaYF4, NaErF4∶Ho3+@NaYF4 core-shell (b) nanoparticles under 980 nm excitation
Tc—Color temperature
表 1 NaErF4、NaErF4∶Tm3+、NaErF4∶Ho3+及NaErF4@NaYF4、NaErF4∶Tm3+@NaYF4、NaErF4∶Ho3+@NaYF4样品的色坐标参数
Table 1. CIE parameters of NaErF4, NaErF4∶Tm3+, NaErF4∶Ho3+, NaErF4@NaYF4, NaErF4∶Tm3+@NaYF4 and NaErF4∶Ho3+@NaYF4 samples
No. Sample CIE (x, y) CCT/K Color 1 NaErF4 (0.4608, 0.4652) 5067 Yellow 2 NaErF4∶0.5%Tm3+ (0.6649, 0.3123) 2962 Red 3 NaErF4∶0.5%Ho3+ (0.4638, 0.4969) 5103 Green-yellow 4 NaErF4@NaYF4 (0.4711, 0.4984) 5083 Green-yellow 5 NaErF4∶0.5%Tm3+@NaYF4 (0.6138, 0.3764) 4080 Orange-red 6 NaErF4∶0.5%Ho3+@NaYF4 (0.5109, 0.4719) 4908 Orange Note: CCT—Color correlated temperature. -
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