1. Introduction

Studies on rare earth (RE)-doped UC phosphors have grown rapidly owing to their wide applications in solid-state lasers, three-dimensional flat-panel displays, optical fiber-based telecommunications, low-intensity infrared (IR) imaging, and especially bioprobes and bioimages [1–6]. Compared with the traditionally used biological labels such as various organic dyes, metal complexes, and semiconductor quantum dots, RE-doped fluoride NCs possess excellent luminescent characteristics (large Stoke’s shift, sharp emission peaks, and long fluorescence lifetime, etc), low toxicity, high thermal stability, long durability, neglectable photobleaching, etc [7]. These characters are attractive for biological imaging applications with multicolor or time-resolved detection [8–10].

Yb³⁺/Er³⁺ doped NCs, with visible UC emissions under 980 nm excitation, have been widely studied both in vitro and in vivo [11, 12]. However, due to the shallow penetration of visible emission light of imaging probes, clinical applications of optical imaging were confront with the conundrum of imaging of deep tissue [13, 14]. To overcome this problem, more attentions have been paid to the biological applications of Yb³⁺/Tm³⁺-doped NCs with a NIR UC emission at 800 nm under 980 nm excitation. The excitation and emission lights both locate within the NIR spectral range of 750-1000 nm (NIR-NIR UC emission), which is considered the “window of optical transparency” for biological tissues with a relatively high tissue penetration depth [15–17]. This feature allows high contrast in vitro and in vivo optical bioimaging of deep tissue, as both light attenuation and scattering are significantly reduced in the NIR spectral range and the autofluorescence of cells and tissues is absent under the conditions of UC excitation and emission. Several methods have been used to enhance the intensity of 800 nm luminescence such as increasing the content of Yb³⁺ ions [18] and designing core/shell structure [16], etc. Consequently, the development of NIR probes with higher efficiency is more attractive.

The UC processes have been extensively studied in RE-doped fluorides. It is worthwhile to point out that specific UC emissions in NCs can be artificially devised by using different combinations of RE ions at different concentrations [13, 19, 20]. Not only that, UC emissions can be enhanced or quenched by nonradiative ET from one RE ion to another ion. The ion–pair interactions, referred as ET, have been studied widely in the materials doped with Er³⁺, Eu³⁺, Tb³⁺, Ho³⁺, and Tm³⁺ ions [21–24]. For example, white UC fluorescent and 2.0 µm fluorescent emissions in Yb^/Ho/Tm tridoped crystals have been extensively investigated under 980 nm excitation [25–28]. The relevant mechanisms associated with the UC processes have been discussed detailedly. However, few results have been reported on the improved NIR 800 nm UC luminescence of Tm³⁺ in Yb/Ho/Tm tridoped NaYF₄ NCs pumped by a conventional 980 nm NIR light.

In this letter, we presented an observation of enhanced NIR ³H₄→³H₆ transition of Tm³⁺ ions in Yb/Ho/Tm tridoped NaYF₄ nanorods (NRs) under 980 nm excitation. Owing to the Ho³⁺ and Tm³⁺ ions dopants possess closely spaced energy levels, the complex ET processes between them decreased most emissions of Ho³⁺ and Tm³⁺ ions. The distinctly enhanced NIR 800 nm emissions under 980 nm excitation were investigated and the relevant mechanisms associated with the UC processes are discussed here in detail.

2. Experimental

In a typical preparation, 0.6-g NaOH was dissolved in a solution containing oleic acid, ethanol and deionized water (10/5/4, v/v). Then NaF, Y(NO₃)₃·6H₂O, Yb(NO₃)₃·6H₂O, Tm(NO₃)₃·6H₂O and Ho(NO₃)₃·6H₂O were added into the solution under vigorous stirring. The mixture was agitated for 30 min and then transferred into a 50-mL autoclave, sealed, and treated at 180 °C for 19 h. Subsequently, the mixture was allowed to cool to room temperature, and the powder were obtained by centrifuge, rinse and drying [29].

3. Results and discussion

3.1 Structure and morphology of NaYF₄:Yb³⁺/Ho³⁺/Tm³⁺ NCs

To identify the crystallization phase, x-ray-diffraction (XRD) analysis was carried out with a powder diffractometer (Model Rigaku RU-200b), using Ni-filtered $CuK\alpha$ radiation ($\lambda =1.5406$ Å). The size and the morphology were characterized by field-emission scanning-electron microscopy (FE-SEM) (Hitchi S-4800). UC emission spectra of the samples were recorded with a fluorescence spectrophotometer (Hitachi F-4500). A power-adjustable laser diode (980 nm, 0-2W) with a lens making the beam parallel was employed as the UC excitation source. The spectra were all recorded under the same conditions (emission slit is 1.0 nm, high voltage of the photomultiplier tube is 400 V, and the excitation power density is about 200 W/cm²).

The XRD pattern depicted in Fig. 1 confirmed the existence of both cubic (JCPDS card 77-2042) and hexagonal phase NaYF₄ (JCPDS card 16-0334); in addition, small amounts of NaF were also found in the XRD patterns of the as-prepared NRs. These elementary results indicated that the NaF content has effects not only on the morphology and size but also on the crystal structure of the as-synthesized NRs. The inset in Fig. 1 displays the FE-SEM image of the sample. It is clear that the diameter of NRs is about 100-120 nm and the length is about 700 nm.

 

Fig. 1 XRD and SEM patterns of sample NaYF₄: Yb/Tm/Ho tri-doped NRs with a small amount of cubic NaYF₄ (denoted as the red word) and excessive NaF (labeled at the patterns).

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3.2 Enhanced NIR 800 nm UC emissions of Tm³⁺ ions under NIR 980 nm excitation

Figure 2(a) shows the dependence of the NIR 800 nm UC luminescence spectra of Tm³⁺ ions on the Ho³⁺ ion concentration in NaYF₄: Yb³⁺/Ho³⁺/Tm³⁺ NRs under 980 nm excitation. For discernible distinction between the spectra at the different doping concentration of Ho³⁺, luminescence integral intensities corresponding to spectra were shown in Fig. 2(b). With the increase of the Ho³⁺ ion concentration (mol% from 0% to 3.5%), the luminescence intensities at λ = 800 nm first increase and reach their maximum value at 2%, and then decrease. As can be seen clearly in Fig. 2(b), the luminescence intensity at λ = 800 nm for the NaYF₄: 20%Yb³⁺/2%Ho³⁺/0.5%Tm³⁺ NRs is about 3 times higher than that of NaYF₄: 20%Yb³⁺/0.5%Tm³⁺ NRs. The increased luminescence intensity undoubtedly originates from the sensitization effect of the Ho³⁺ ions.

 

Fig. 2 Under 980 nm excitation, UC emission spectra (a) and luminescence integral intensities corresponding to spectra (b) of NaYF₄: 0.2 mol% Yb³⁺/ x mol% Ho³⁺/0.005 mol% Tm³⁺ at the different Ho³⁺ ion concentrations (x mol% from 0 mol % to 3.5 mol %) recorded at the same conditions. Excitation power density is 200 W/cm².

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Under 980 nm excitation, the NaYF₄: Yb³⁺/Ho³⁺/Tm³⁺, NaYF₄: Yb³⁺/Tm³⁺ and NaYF₄: Yb³⁺/Ho³⁺ NRs emitted IR-to-UV UC fluorescence, as shown in Fig. 3 . Most emissions of Tm³⁺ and Ho³⁺ ions correspond well to what observed with Yb³⁺/Tm³⁺ and Yb³⁺/Ho³⁺ codoped UC crystals [30, 31]. For discernible distinction between emissions of co-doped and tri-doped NaYF₄ NRs, their spectra were shown in Fig. 3(a) and Fig. 3(b), respectively. Figure 3(a) shows the UC luminescence spectra of NaYF₄: Yb³⁺/Ho³⁺/Tm³⁺ and NaYF₄: Yb³⁺/Tm³⁺ NRs. Figure 3(b) shows the UC luminescence spectra of NaYF₄: Yb³⁺/Ho³⁺/Tm³⁺ and NaYF₄: Yb³⁺/Ho³⁺ NRs. Owing to the Ho³⁺ and Tm³⁺ ions dopants possess closely spaced energy levels, the changes of peaks intensity corresponding to different levels are different due to the complex ET processes as shown in Fig. 3.

 

Fig. 3 Under 980 nm excitation, UC emission spectra of NaYF₄: Yb³⁺/Ho³⁺/Tm³⁺, NaYF₄: Yb³⁺/Tm³⁺ (a) and NaYF₄: Yb³⁺/Ho³⁺ (b) NRs recorded at the same conditions. Excitation power density is 200 W/cm².

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3.3. UC and ET mechanisms for Tm³⁺ and Ho³⁺ions

Figure 4 shows the energy level diagrams of Yb³⁺, Tm³⁺, Ho³⁺ and as well as the possible upconverted process [30, 32]. Under NIR laser excitation, Yb³⁺ ions continuously absorbed 980 nm photons and transfer the energy to populate the states of ³H₅, ³F₂ (³F₃), ¹G₄, ¹D₂, ¹I₆, ³P_J of Tm³⁺ and ⁵I₆, ⁵F₂/⁵F₄, ⁵F₂/³F₂/⁵G₂ of Ho³⁺ in turn, as described in Ref [31]. As mentioned above, the most important mechanism for the enhancement of 800 nm luminescence is the ET between excited Tm³⁺ and Ho³⁺ ions. Three possible ET processes should be considered for their appropriate energy matching: ET1 ³F₄ → ³H₆ (Tm³⁺): ⁵I₈ → ⁵I₇ (Ho³⁺), ET2 ⁵I₆ → ⁵I₈ (Ho³⁺): ³H₆ → ³H₅ (Tm³⁺), and ET3 ⁵I₄ → ⁵I₈ (Ho³⁺): ³H₆ → ³H₄ (Tm³⁺). All the three ET processes are responsible for improving the population of ³H₄ level of Tm³⁺ as follows. The population of ⁵I₇ level of Ho³⁺ is improved while the population of ³F₄ level of Tm³⁺ is decreased through ET1 process. The ⁵I₇ level of Ho³⁺ has more energy to absorb a photon from Yb³⁺ to populate the ⁵F₅ level of Ho³⁺. This can be confirmed from the undiminished spectra of ⁵F₅ → ⁵I₈ (Ho³⁺) transition before and after doping of Tm³⁺ in Fig. 3(b). Then, most of the population of ⁵F₅ level of Ho³⁺ relax rapidly to the ⁵I₄ level. Some of Tm³⁺ in the ³H₄ state can be populated through the ET3, and a small proportion of Ho³⁺ in ⁵I₄ level relax rapidly to the ⁵I₅ level which will decreased the population process ⁵I₅ → ⁵F_3,2 (Ho³⁺). The intensity of ⁵S₂/⁵F₄ → ⁵I₈ (Ho³⁺) emission decreased as shown in Fig. 3(b). Otherwise, the population of ³H₅ state of Tm³⁺ can be increased and that of ⁵I₆ state of Ho³⁺ can be decreased through ET2, resulting in the enhancement of 800 nm emission. Especially, the intensity of ¹D₂ → ³H₆ emission were quenched much compared to that of ¹G₄ → ³H₆ emission of Tm³⁺ as shown in Fig. 3(a), owing to the abundant levels of Ho³⁺ around the energy height of ¹D₂ level of Tm³⁺ named as ET4.

 

Fig. 4 Energy level diagrams of Yb³⁺, Tm³⁺, and Ho³⁺ ions and possible UC and ET processes under 980 nm excitation.

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To understand the ET processes well, we investigated the excitation power dependence of UC luminescence intensities. For an unsaturated UC process, the integrated UC luminescence intensity I_f is proportional to Pⁿ [33], where P is the pumping laser power, and n is the number of laser photons required in populating the upper emitting state. Figure 5 shows the typical pump-power dependence of UC luminescence of NaYF₄: 20%Yb³⁺, 2%Ho³⁺, 0.5%Tm³⁺. The values of photon number n were 1.93 ± 0.04 for 642.2 nm (⁵F₅ → ⁵I₈ of Ho³⁺) and 1.98 ± 0.01 for 800 nm (³H₄ → ³H₆ of Tm³⁺) emissions, respectively, indicating that these transitions were of two-photon UC processes. Power dependence analyses illustrate that the ³H₄ level of Tm³⁺ has the same multi-photon UC character with the ⁵F₅ and ⁵I₄ levels of Ho³⁺ ions and confirm that it is populated by the ETs from the corresponding levels of Ho³⁺ ions.

 

Fig. 5 Excitation power dependence of UC luminescence in NaYF₄: 20%Yb³⁺, 2%Ho³⁺, 0.5%Tm³⁺ NRs under 980 nm excitation.

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4. Conclusions

In conclusion, Yb³⁺/Ho³⁺/Tm³⁺ tridoped NaYF₄ NRs were successfully synthesized using a simple wet-chemical route. Particularly, the introduction of Ho³⁺ into the Tm³⁺/Yb³⁺ co-doped NaYF₄ NRs resulted in a significant increase in UC efficiency of 800 nm emission of Tm³⁺, which was suitable for all the advanced synthesis methods of ultrasmall (with size of less than 10 nm) monodisperse NCs. We offer a fast, simple, inexpensive, and highly reproducible approach for the synthesis of NIR-NIR UC NCs.