Trans. Nonferrous Met. Soc. China 22(2012) 373-379

Synthesis and upconversion luminescence of Lu₂O₃:Yb³⁺,Tm³⁺ nanocrystals

LI Li^{1, 2}, CAO Xue-qin², ZHANG You¹, GUO Chang-xin²

1. College of Mathematics and Physics, Chongqing University of Posts and Telecommunications,Chongqing 400065, China;

2. Department of Physics, University of Science and Technology of China, Hefei 230026, China

Received 21 January 2011; accepted 22 September 2011

Abstract: Lutetium oxide nanocrystals codoped with Tm³⁺ and Yb³⁺ were synthesized by the reverse-like co-precipitation method, using ammonium hydrogen carbonate as precipitant. Effects of the Tm³⁺, Yb³⁺ molar fractions and calcination temperature on the structural and upconversion luminescent properties of the Lu₂O₃ nanocrystals were investigated. The XRD results show that all the prepared nanocrystals can be readily indexed to pure cubic phase of Lu₂O₃ and indicate good crystallinity. The experimental results show that concentration quenching occurs when the mole fraction of Tm³⁺ is above 0.2%. The optimal Tm³⁺ and Yb³⁺ doped molar fractions are 0.2% and 2%, respectively. The strong blue (490 nm) and the weak red (653 nm) emissions from the prepared nanocrystals were observed under 980 nm laser excitation, and attributed to the ¹G₄→³H₆ and ¹G₄→³F₄ transitions of Tm³⁺, respectively. Power-dependent study reveals that the ¹G₄ levels of Tm³⁺ can be populated by three-step energy transfer process. The upconversion emission intensities of 490 nm and 653 nm increase gradually with the increase of calcination temperature. The enhancement of the upconversion luminescence is suggested to be the consequence of reducing number of OH^- groups and the enlarged nanocrystal size.

Key words: Lu₂O₃:Yb³⁺,Tm³⁺ nanocrystal; co-precipitation method; upconversion luminescence

1 Introduction

In  recent  years,  upconversion  luminescence  of lanthanide-doped nanocrystals have attracted much attention due to their unique optical properties  and  potential  applications  in  such  fields  as infrared to visible upconversion lasers, fibre amplifiers, color displays, optical  communications and  biological  ?uorescencelabels [1-3]. Many trivalent rare earth ions, such as Er³⁺ [4], Tm³⁺ [5] and Pr³⁺ [6], are doped as emission and absorption centres in these materials. Among the rare earth ions, Yb³⁺ becomes the most suitable sensitizer ion because of its special energy levels and long excited level lifetime. Tm³⁺ is a promising optical activator that opens the possibility for simultaneous  blue  and  ultraviolet  emission  of  laser  action  and  various  applications [7]. When a rare earth-doped optical device is developed, the host material is a very important factor to be considered. The host material with low phonon energy can result in a reduction of the multiphonon relaxation and help efficient upconversion occur. Therefore, choosing an appropriate host is very important for obtaining highly efficient upconversion luminescence. Rare earth oxides, as promising hosts for upconversion, possess relatively low phonon energy, high chemical durability and favorable physical properties [8]. During the past few years, extensive studies have been concentrated on the upconversion luminescence of Y₂O₃ host [9, 10]. The sesquioxide Lu₂O₃ is isostructural to Y₂O₃ and crystallizes in a cubic bixbyite structure. Lutetium could be a more favorable cation for RE³⁺ ions dopant emission [11]. Additionally, Yb³⁺ (86 pm) and Tm³⁺ (87 pm) have ionic radii similar to Lu³⁺ (85 pm), in favor of the dopant substitution. Therefore, Lu₂O₃ is an ideal host for the upconversion luminescent materials.

Co-precipitation method is one of the most promising techniques because of its advantages, such as the relatively simple synthetic route, low cost and ease of mass production. Although there are only some reports on the upconversion of rare earth ions-doped Lu₂O₃nanocrystals [12-14], there is no systematic study on the effect of Tm³⁺ content and calcination temperature on the structural and upconversion luminescent properties of Lu₂O₃:Yb³⁺,Tm³⁺ nanocrystals. In this work, lutetium oxide nanocrystals codoped with Tm³⁺ and Yb³⁺ were synthesized by a reverse-strike co-precipitation method. The effects of Tm³⁺, Yb³⁺ molar fractions and calcination temperature on the structure and the upconversion emission intensity of the Lu₂O₃:Yb³⁺,Tm³⁺ nanocrystals were investigated. The upconversion mechanisms were also discussed.

2 Experimental

Lu₂O₃ nanocrystals codoped with 2% Yb³⁺ (molar fraction, the same below) and 0.04%, 0.2%, 0.5%, 1%, and 2% Tm³⁺ were prepared by the reverse-like co-precipitation method, respectively. High pure Lu₂O₃ (99.99%), Yb₂O₃ (99.99%) and Tm₂O₃ (99.99%) were used as starting materials and ammonium hydrogen carbonate (NH₄HCO₃, A. R., 1 mol/L solution) was precipitant. Aqueous nitrate solutions were prepared by dissolving Lu₂O₃, Yb₂O₃ and Tm₂O₃ in HNO₃ (A. R.) and distilled water under stirring and heating. The respective nitrate solutions were mixed with an appropriate proportion. The mixed salt solution was then added drop by drop to the excess NH₄HCO₃ solution at a speed of 2 mL/min under vigorous stirring at room temperature. After the process of titration, the precipitate was aged for 2 h with agitation by a electric stirrer to make the reaction proceed sufficiently. After filtering and washing with distilled water and anhydrous ethanol several times, the precipitates were dried at 80 °C for 12 h in an oven. The precipitates were further calcined in air at 900 °C for 2 h to obtain nanocrystals. Moreover, Lu₂O₃ nanocrystals codoped with 0.2% Tm³⁺ and 1%, 2%, 4%, 6%, 10%, and 15% Yb³⁺ were synthesized by the same procedure, respectively.

To investigate the effect of calcination temperature on the upconversion luminescence of Lu₂O₃:2% Yb, 0.2% Tm nanocrystals, the nanocrystals were obtained by calcining the precipitates at 800, 900, 1000 and 1100 °C for 2 h, respectively, using the same procedure.

The crystal structures were analyzed by a MXPAHF rotating anode X-ray diffractometer with Cu K_a radiation (l=0.154056 nm). The morphology of nanocrystals was examined on a FEI-Sirion 200 field-emission scanning electron microscope. The FT-IR transmittance spectra were recorded in the range of 400-4000 cm^-1 on a Magna-IR 750 Fourier transformation infrared spectrometer. The upconversion luminescence spectrum was detected by R955 (Hamamatsu) from 400 to 700 nm, using an 980 nm diode laser Module (K98D08m-30W, China) as the excitation source. The excitation power was 455 mW. All spectra were collected at room temperature.

3 Results and discussion

3.1 Influence of Tm³⁺ molar fraction on structural and upconversion luminescent properties for Lu₂O₃:2%Yb³⁺, xTm³⁺ nanocrystals

Figure 1 shows the XRD patterns of Lu₂O₃ nanocrystals doped with 2% Yb³⁺ and x Tm³⁺(x=0.04%, 0.2%, 0.5%, 1%, 2%) and calcined at 900 °C and the data of JCPDS card No. 43-1021 for Lu₂O₃ were used as a reference. The patterns indicate that the nanocrystals were crystallized as cubic Lu₂O₃ with spatial group Ia3(206). No additional peaks of other phases have been found, indicating that the RE³⁺ ions are effectively doped into the host lattice. The high intensity of the diffraction peaks indicates good crystallinity of the nanocrystals. The average crystalline sizes can be estimated by using Scherrer equation [15]:

                                 (1)

where l stands for the wavelength of Cu K_a radiation; b is the corrected full width at half maximum (FWHM) of the diffraction peak; q denotes the diffraction angle; 0.89 is the characteristic of spherical particle. Using the Scherrer equation, the estimated crystalline size of Lu₂O₃: Yb³⁺, Tm³⁺ nanocrystals is 40-60 nm.

Fig. 1 XRD patterns of Lu₂O₃ nanocrystals doped with 2% Yb³⁺ and 0.04%, 0.2%, 0.5%, 1%, 2% Tm³⁺ and calcined at 900 °C and compared with standard pattern of JCPDS 43-1021

Figure 2 presents the typical FE-SEM image of Lu₂O₃:2% Yb³⁺, 0.2% Tm³⁺ nanocrystals calcined at 900 °C. As shown in Fig. 2, the obtained Lu₂O₃:2% Yb³⁺, 0.2% Tm³⁺ nanocrystals are aggregated, and have nearly spherical shape and an average diameter of 60 nm, which are consistent with the XRD results. The similar    results were also observed for other Lu₂O₃:Yb³⁺, Tm³⁺ nanocrystals with various contents of Tm³⁺ (not shown here).

Fig. 2 FE-SEM image of Lu₂O₃:2% Yb³⁺, 0.2% Tm³⁺ nanocrystals

The typical upconversion spectrum of the Lu₂O₃:2% Yb³⁺, 0.2% Tm³⁺ nanocrystals calcined at 900 °C and excited by 980 nm diode laser excitation at room temperature is shown in Fig. 3. The strong blue emission around 490 nm easily seen by naked eye is assigned to the ¹G₄→³H₆ transition of Tm³⁺. Weak red emission around 653 nm is attributed to the ¹G₄→³F₄ transition. The experimental results show that the upconversion emission intensity ratio of the blue to red is approximately equal to 12 for the Lu₂O₃:2% Yb³⁺, 0.2% Tm³⁺nanocrystals.

Fig. 3 Upconversion emission spectrum of Lu₂O₃:2% Yb³⁺, 0.2% Tm³⁺ nanocrystals calcined at 900 °C under 980 nm excitation

Figure 4 shows the upconversion emission intensity of Lu₂O₃ nanocrystals doped with 2% Yb³⁺and xTm³⁺ and calcined at 900 °C as a function of Tm³⁺ content. As shown in Fig. 4, with increasing the Tm³⁺ content, the upconversion emission intensity increases at first, reaching a maximum value with 0.2% Tm³⁺ and then gradually decreases at higher doping concentrations. Concentration quenching occurs above 0.2% dopant content. When Tm³⁺ content is high, the self-quenching or cross-relaxation mechanisms between Tm³⁺ ions becomes active [16-18]. The energy transfer processes can be described as follows:

which can depopulate the¹G₄ levels but populate the ³H₅ levels, leading to an increase in the extent of non-radiative transitions. Moreover, host may transfer the energy to lattice defects discussed later, which may trap energy, resulting in concentration quenching.

Fig. 4 Upconversion emission intensity of Lu₂O₃ nanocrystals doped with 2% Yb³⁺ and xTm³⁺ and calcined at 900 °C as function of Tm³⁺ mole fraction (The red emission intensity of 653 nm is magnified by a factor of 5)

3.2 Influence of Yb³⁺ molar fraction on structural and upconversion luminescent properties for Lu₂O₃:xYb³⁺, 0.2%Tm³⁺ nanocrystals

Figure 5 shows the XRD patterns of Lu₂O₃ nanocrystals doped with 0.2% Tm³⁺ and x Yb³⁺(x=1%, 2%, 4%, 6%, 10%, 15%) and calcined at 900 °C. The XRD results show that the diffracting peak positions of all the Lu₂O₃:xYb³⁺,0.2% Tm³⁺ nanocrystals are consistent with the standard powder diffraction pattern of Lu₂O₃ (JCPDS 43-1021). The crystalline size of Lu₂O₃: xYb³⁺, 0.2% Tm³⁺ nanocrystals is 40-60 nm using the Scherrer equation.

Figure 6 shows the upconversion emission intensity of Lu₂O₃ nanocrystals doped with 0.2% Tm³⁺ and xYb³⁺ and calcined at 900 °C as a function of Yb³⁺ content. It is noticed that the emission intensities of 490 nm and 653 nm first increase and approach a maximum at a   doping content of 2% and then decrease with the increase of Yb³⁺ content. It is obvious that with the increase of Yb³⁺ content, more energy is transferred from Yb³⁺ to Tm³⁺. However, when Yb³⁺ ions are heavily doped, many factors such as increased amount of impunities, concentration-quenching of Yb³⁺, energy back transfer from Tm³⁺ to Yb³⁺ can be described as, which effectively reduces upconversion emission intensity.

Fig. 5 XRD patterns of Lu₂O₃ nanocrystals doped with 0.2% Tm³⁺ and 1%, 2%, 4%, 6%, 10%, 15% Yb³⁺ and calcined at 900 °C and compared with standard pattern of JCPDS 43-1021

Fig. 6 Upconversion emission intensity of Lu₂O₃ nanocrystals doped with 0.2% Tm³⁺ and xYb³⁺ as function of Yb³⁺ mole fraction (The red emission intensity of 653 nm is magnified by a factor of 5)

3.3 Influence of calcination temperature on structural and upconversion luminescent properties for Lu₂O₃:2% Yb³⁺, 0.2% Tm³⁺ nanocrystals

Figure 7 shows the XRD patterns of Lu₂O₃:2% Yb³⁺, 0.2%Tm³⁺ nanocrystals calcined at various temperatures. The  results  reveal  that  all  the  prepared  Lu₂O₃ nanocrystals are of cubic structure, and the diffracting peak positions are well consistent with the standard powder diffraction pattern of Lu₂O₃ (JCPDS 43-1021). The average crystalline size can be estimated by using Scherrer equation. In cubic crystal lattice [19], the lattice constant can be calculated from Eq. (2).

                            (2)

where d, a, and ( h k l) are the interplanar distance, lattice constant and crystal indices (Miller indices), respectively. The strongest peaks (222) at 2q=29.775, 29.812, 29.807, 29.806° and d=3.0010, 2.9962, 2.9967, 2.9961 ? were used to calculate the average crystallite size (D) and lattice constants (a) of the Lu₂O₃: 2% Yb³⁺, 0.2% Tm³⁺ nanocrystals, respectively. Table1 presents the calculated crystallite sizes and lattice constants of Lu₂O₃:2% Yb³⁺, 0.2% Tm³⁺ nanocrystals at different calcination temperatures. The lower the calcination temperature is, the smaller the crystallite sizes are. The crystallite size increases from 32 to 70 nm with the increase of calcination temperatures from 800 to 1100 °C. The lattice constants are compatible with the literature values (10.390 ?) from the standard card (JCPDS 43-1021).

Fig. 7 XRD patterns of Lu₂O₃:2% Yb³⁺,0.2%Tm³⁺ nanocrystals calcined at various temperatures and compared with standard pattern of JCPDS 43-1021

Table 1 Calculated crystallite sizes and lattice constants of Lu₂O₃:2%Yb³⁺,0.2%Tm³⁺ nanocrystals calcined at various temperatures

Figure 8 shows the FT-IR spectra of Lu₂O₃:2%Yb³⁺, 0.2% Tm³⁺ nanocrystals calcined at various temperatures. The data show that the bands around 490 and 580 cm^-1 are assigned to the Lu—O vibration of cubic Lu₂O₃ [20]. The absorption bands of OH^- (around 3400 cm^-1) become weaker with the increase of calcination temperature. OH^- groups with high vibration frequency will increase the nonradiative relaxation rate and hence decrease upconversion efficiency [21]. This indicates that the enhanced upconversion intensity may come from the reducing of OH^- groups, which are located on the surface of nanocrystals. From Table 1, it is known that the crystallite size of Lu₂O₃:2% Yb³⁺, 0.2% Tm³⁺  nanocrystals increases with increasing the calcination temperature. The decrease of surface-to-volume ratio can reduce the OH^- groups on the surface of nanocrystals. Therefore, the reducing of OH^- groups is suggested to be relevant to the enhancement of the upconversion emission.

Fig. 8 FT-IR spectra of Lu₂O₃:2% Yb³⁺, 0.2% Tm³⁺ nanocrystals calcined at various temperatures

Figure 9 shows the influence of calcination temperature on the upconversion emission intensity of 490 nm and 653 nm under 980 nm laser excitation. It can be seen that the emission intensities of 490 nm and 653 nm become stronger gradually with the increase of calcination temperature. The increased nanocrystal size can also contribute to the upconversion emission enhancement, since the larger nanocrystal size has less defects [22]. Larger Lu₂O₃:2% Yb³⁺, 0.2% Tm³⁺ nanocrystals have lower surface-to-volume ratio which leads to lower surface scattering of the excitation light by the surface of the radiated Lu₂O₃ nanocrystals. As a result, the Lu₂O₃:2% Yb³⁺, 0.2% Tm³⁺ nanocrystals with larger diameter can absorb more efficiently the excitation light than Lu₂O₃:2% Yb³⁺, 0.2% Tm³⁺ nanocrystals with smaller diameter. It can be seen from Fig. 9 that the enhanced upconversion intensity may be due to the reduction of OH^- groups [23]. Therefore, the enhancement of the upconversion luminescence is suggested to be the consequence of reducing number of OH^- groups and the enlarged nanocrystal size.

Fig. 9 Effect of calcination temperature on upconversion emission intensity of Lu₂O₃:2% Yb³⁺, 0.2% Tm³⁺ nanocrystals (The red emission intensity of 653 nm is magnified by a factor of 5)

3.4 Upconversion mechanisms

The energy level diagram of Tm³⁺ and Yb³⁺ shown in Fig.10 can be used to explain the upconversion mechanism. The pump photons of 980 nm laser only excite Yb³⁺ ion, because Tm³⁺ ion has no matched excited level above its ground state. The upconversion emission intensity depends on the population of ¹G₄ levels for Tm³⁺, which can be populated by three-step process [24]. Under 980 nm laser excitation, the ¹G₄ excited state can be populated by a three-step energy transfer (ET) process. Firstly, the ²F_7/2→²F_5/2 transition of Yb³⁺ occurs due to absorption of 980 nm photons. Then, Yb³⁺ in the ²F_5/2 level transfers its energy to Tm³⁺, which is excited to the ³H₅ level and relaxes nonradiatively to the ³F₄ level. Tm³⁺ in the ³F₄ level can be excited to the ³F₂ level by energy transfer process after which nonradiative relaxation to ³H₄ occurs. Subsequently, Yb³⁺ in the ²F_5/2 level transfers its energy to Tm³⁺ and excites Tm³⁺ at ³H₄ level to ¹G₄ level. Therefore, the Tm³⁺ decays radiatively to the ³H₆ ground state and the ³F₄ metastable state generating strong blue and weak red emissions around 490 nm and 653 nm from the ¹G₄ levels, respectively.

Fig. 10 Energy level diagram of Tm³⁺ and Yb³⁺ with excitation and emission process scheme (The full arrows pointing upwards represent energy absorption; full arrows pointing downwards represent emission; dotted arrows pointing downwards represent multiphonon relaxations (nonradiative decay); the diagonal  line without arrows represents energy back transfer; dash-dotted arrows indicate energy transfer processes, respectively.)

The upconversion efficiency is principally governed by nonradiative processes in materials, which are dependent on the energy gap separating the upper and lower levels as well as the highest phonon energy in the materials. The multiphoton nonradiative decay rate can be expressed as [25]

               (3)

where T is the absolute temperature; W_n(T) is the rate at temperature T; W₀(0) is the rate at 0 K; k is the Boltzmann constant; n=ΔE/(hv), n accounts for the number of phonons involved in the sensitizer excitation; hν is the maximum phonon energy of the host matrix; ?E is the energy gap involved. According to the energy gap law, multiphoton relaxation is dominant when n<5 [26]. The energy gap of Tm³⁺ between the levels ¹G₄ and the ³F₂ is  approximately equal to 5840 cm^-1, which corresponds to 9 phonons of the highest phonon energy (the maximum phonon energy of 600 cm^{-1 in our experiments), so the probability of non-radiative transition from the level 1G}₄ to the level ³F₂ can be negligible. This indicates that high population at the level ¹G₄ can be achieved.

To further understand the upconversion mechanisms, pump power (P_pump) dependences on the upconversion emission intensities (I_up) of Lu₂O₃:2% Yb³⁺,0.2% Tm³⁺ nanocrystals calcinated at 900 °C are shown in Fig. 11. According to the relationship between  and , where n is the number of photons involved in the pump process [27], the typical upconversion luminescent intensity I_up depends on the incident pump power P_pump. The upconversion emission intensities of Lu₂O₃:2% Yb³⁺, 0.2% Tm³⁺nanocrystals were integrated at 490 and 653 nm for various pump powers. A plot of ln I versus ln P yields a straight line with slope n. On the basis of the experimental data in Fig. 11, the obtained n values are 2.597 and 2.712. The two slopes are close to 3, which indicates that the blue and weak red upconversion luminescence is indeed a three-photon upconversion process.

Fig. 11 Relationship between lnI and lnP of Lu₂O₃:2% Yb³⁺, 0.2% Tm³⁺ nanocrystals under 980 nm excitation

4 Conclusions

1) The Lu₂O₃ nanocrystals codoped with 2% Yb³⁺ and 0.04%, 0.2%, 0.5%, 1%, 2% Tm³⁺ were successfully synthesized by the reverse-like co-precipitation method. The further investigation reveals that the upconversion emission intensity depends on the Tm³⁺ and Yb³⁺ contents and calcination temperature.

2) The experimental results show that all the prepared Lu₂O₃:2% Yb³⁺, 0.2% Tm³⁺ nanocrystals can be readily indexed to pure cubic phase of Lu₂O₃. The strong blue and weak red emissions from the prepared nanocrystals were observed, and attributed to the ¹G₄→³H₆ and ¹G₄ →³F₄ transitions of Tm³⁺ ion, respectively. Concentration quenching occurs when the content of Tm³⁺ is above 0.2%. Power-dependent study reveals that the ¹G₄ level can be populated by three-step energy transfer process.

3) The upconversion emission intensities of 490 nm and 653 nm increase gradually with the increase of calcination temperature. The enhancement of the upconversion luminescence is suggested to be the consequence of reducing number of OH^- groups and the enlarged nanocrystal size.

4) The data presented in this study might provide useful information for further development of upconversion ceramic laser associated with the¹G₄→³H₆ transitions of Tm³⁺ ions.

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Lu₂O₃:Yb³⁺,Tm³⁺  纳米晶的制备和上转换发光性能

李 丽^{1, 2}, 曹雪琴², 张 友¹, 郭常新²

1. 重庆邮电大学 数理学院，重庆 400065；2. 中国科学技术大学 物理系，合肥 230026

摘  要：采用碳酸氢铵(NH₄HCO₃)为沉淀剂，用共沉淀法制备Yb³⁺ 和Tm³⁺ 共掺杂的Lu₂O₃:Yb³⁺,Tm³⁺纳米晶。研究Tm³⁺ 摩尔分数、Yb³⁺摩尔分数和煅烧温度对Lu₂O₃:Yb³⁺,Tm³⁺ 纳米晶的结构和上转换发光性能的影响。结果表明：所制备的纳米晶具有纯的Lu₂O₃相，结晶性较好。当掺杂的Tm³⁺浓度超过0.2%(摩尔分数)时，出现浓度淬灭效应。Tm³⁺ 和Yb³⁺的最佳掺杂比分别为0.2%和2%(摩尔分数)。在980 nm半导体激光器的激发下，样品发射出蓝光(490 nm)和红光(653 nm)，分别对应Tm³⁺的¹G₄→³H₆ 和¹G₄→³F₄跃迁。发射强度与激发功率的关系表明，Tm³⁺ 的¹G₄能级布居是三光子能量传递过程。随着煅烧温度的升高，上转换发光强度增强，这主要是因为随着温度的升高纳米晶表面的OH^-减少和纳米晶尺寸增大。

关键词：Lu₂O₃:Yb³⁺,Tm³⁺纳米晶；共沉淀法；上转换发光

(Edited by LI Xiang-qun)

Foundation item: Projects (10704090, 10774140, 11047147) supported by the National Natural Science Foundation of China; Projects (KJ090514, KJTD201016) supported by the Natural Science Foundation of Chongqing Municipal Education Commission, China

Corresponding author: LI Li; Tel: +86-23-62471721; E-mail: lilic@cqupt.edu.cn; lilic@mail.ustc.edu.cn

DOI: 10.1016/S1003-6326(11)61186-2