The multiphoton effect based on two-photon or three-photon technology is receiving increasing attention due to its comprehensive applications in biomedicine, photovoltaics, optical telecommunications, etc.^{[1–10" target="_self" style="display: inline;">–10]}. Similar to other nonlinear optical technologies, the common multiphoton effect has very low efficiency to upconvert the energy of excited photons, as the intermediate levels are virtual^[11]. In comparison, lanthanide-doped upconversion nanoparticles (UCNPs) can effectively convert low-energy photons (always infrared light) into ultraviolet, visible, or near-infrared (NIR) photons, owing to their ladder-like system of energy levels^[12]. Until now, owing to their high efficiency, the most frequently used UCNPs have been hexagonal β-NaYF4 nanoparticles (NPs) doped with Yb3+/RE3+ (RE=Er, Tm, Ho)^[13,14]. Upon laser excitation at 980 nm, these UCNPs emit visible light by a two-photon process.

For biomedical applications, it is often important for the excitation light to penetrate far into the sample; however, the penetration depth is limited by Rayleigh scattering, owing to the effect on the beam quality. Since Rayleigh scattering scales as λ−4, long-wavelength multiphoton excitation seems promising for biomedical applications, e.g., in vivo biological imaging and photo dynamics therapy (PDT) of deep tumors. It has been shown that an optimum wavelength window, owing to scattering and absorption in tissue, lies close to the telecom band^[15]. Recently, Er3+ ions have been considered to be a promising choice, owing to their strong absorption at ∼1500nm, which corresponds to the energy transfer from the I13/24 energy level to the I15/24 energy level^[16]. LiYF4:Er3+ NPs were first reported by Chen et al.^[17] to exhibit multicolor emission under 1490 nm laser excitation by three-photon upconversion (UC) process. Subsequently, NaYF4:Er3+ UCNPs excited at 1550 or 1523 nm were reported^[18,19], and it was found that the emission intensity could be largely enhanced by the inert NaYF4 shell^[20]. These UCNPs irradiated almost multicolor light containing ∼540 nm (green), ∼660 nm (red), and ∼800/980nm NIR wavelengths. However, red emission is widely considered to be the most promising candidate for deep tissues, owing to its penetration capabilities^[21,22]. Additionally, the most frequently used photosensitizer in PDT applications strongly absorbs in the red light region^[23].

Many studies have investigated UCNPs with pure red emission under a 980 nm laser excitation by a two-photon UC process^{[24–29" target="_self" style="display: inline;">–29]}, but no studies were published on NPs upconverting from the telecom band to red light by a three-photon process. In this work, Er3+-doped Na3ZrF7 NPs were synthesized by following a procedure that had been previously used^[30]. Upon 1480 nm laser excitation, these UCNPs irradiated pure red light centered at 660 nm in the visible light region. The effect of the Er3+ ion concentration on the emission intensity of Er3+-doped Na3ZrF7 NPs was investigated. The mechanism behind the pure red emission was explained by considering the cross-relaxation effect.

Figure 1(a) shows the X-ray diffraction (XRD) patterns of Er3+-doped Na3ZrF7 NPs with different Er3+ ion concentrations. The patterns revealed that all the as-synthesized NPs had a pure tetragonal phase structure (according to the standard Na3ZrF7 host lattice of JCPDS No. 12-0562). No other phase was detected. This result indicated that the dopant Er3+ ions completely replaced Zr4+ ions in the Na3ZrF7 lattice. The typical transmission electron microscope (TEM) morphology of Na3ZrF7:Er3+5% NPs is shown in Fig. 1(b). The as-synthesized Na3ZrF7:Er3+5% NPs were monodispersed and tetragonal and exhibited a size of ∼22nm. Notably, the concentration of Er3+ ions had a negligible effect on the size of the Er3+-doped Na3ZrF7 NPs.

Fig. 1. (a) XRD patterns of Er3+-doped Na3ZrF7 NPs with different Er3+ concentrations, and (b) typical TEM image of Na3ZrF7:Er3+5% NPs.

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Figure 2 shows the UC emission spectra of the synthesized Er3+-doped Na3ZrF7 NPs under 980 and 1480 nm laser excitations. In our previous report, we confirmed that Er/Yb co-doped Na3ZrF7 NPs displayed a single red emission under 980 nm laser excitation. However, the Er3+-doped Na3ZrF7 NPs displayed multiband emission under 980 nm laser excitation even at the Er3+ ion concentration of 20%, as shown in Fig. 2(b). Upon 1480 nm laser excitation, the Er3+-doped Na3ZrF7 NPs displayed pure red emission (∼660nm, corresponding to F9/24→I15/24) in the visible light region, whereas the green emission corresponding to the energy transfer from the S3/24 level to the I15/24 level almost disappeared, as shown in Fig. 2(a). A weak emission at ∼820nm (corresponding to I9/24→I15/24) and a strong emission at ∼980nm (corresponding to I11/24→I15/24) were observed in the NIR region. The emission intensity of Er3+-doped Na3ZrF7 NPs was significantly affected by the concentration of Er3+ ions. The emission intensity of the Na3ZrF7:Er3+2% NPs was very weak, whereas the Na3ZrF7:Er3+5% NPs displayed the strongest emission.

Fig. 2. Spectra of Er3+-doped Na3ZrF7 NPs with different Er3+ concentrations excited by (a) 1480 and (b) 980 nm lasers. The insets are the photographs of the Na3ZrF7:5%Er3+ NPs dispersed in hexane.

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To investigate the influence of the Er3+ concentration on the emission intensity of Na3ZrF7:Er NPs, the decay profiles of the S3/24→F15/24 and F9/24→F15/24 transitions of Er3+ ions at 540 and 655 nm, respectively, were also measured under 1480 nm laser excitation, as shown in Fig. 3. The effective lifetime τm is given by the following formula: τ=∫0+∞tI(t)dt∫0+∞I(t)dt,(1)where I(t) is the intensity of the emission at time t. The effective decay times were 73.7, 54.3, and 31.6 μs for the S3/24 states and 580.9, 534.3, and 415.6 μs for the F9/24 states of Er3+ ions in Na3ZrF7 NPs with Er3+ concentrations of 5%, 10%, and 20%, respectively. The decay curves of the Na3ZrF7:Er3+2% NPs were ignored, as the signal was very low and could be hardly detected. Clearly, the emission intensity of the Na3ZrF7 NPs was closely related to the decay times: as the decay time decreased, the emission intensity of the Na3ZrF7 NPs significantly declined. The short lifetime was mainly attributed to the appearance of defect-related quenching effects. When trivalent Er3+ ions replaced tetravalent Zr4+ ions, extra Na+ ions and F− vacancies were formed in the Na3ZrF7 matrix to re-balance the charge. These defects resulted in quenching effects that reduced the emission intensity of the Na3ZrF7:Er NPs. By increasing the Er3+ concentration in the Na3ZrF7:Er NPs, the defect-related quenching effects were enhanced, and the emission intensity was reduced. The very weak emission of Na3ZrF7:Er3+2% NPs was mainly related to two aspects: weak absorption to the excited photons, and weak energy transfer between neighboring Er3+ ions, due to the low Er3+ concentration.

Fig. 3. Decay profiles of transitions of Er3+ ions under 1480 nm laser excitation: (a) S3/24→I15/24 at 540 nm and (b) F9/24→I15/24 at 655 nm.

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Figure 4 shows the Er3+ ion energy levels and the proposed energy transfer UC mechanisms under the 1480 and 980 nm laser excitations. As shown in Fig. 4(a), under 1480 nm laser irradiation, the excitation of Er3+ ions from the ground I15/24 state to the I13/24 state occurs through a ground state absorption (GSA) process, followed by energy transfer to the neighboring Er3+ ions, which are then promoted to the higher I9/24 and H11/24 states. The emission centered at 820 nm [shown in Fig. 2(a)] was generated from the radiative decay from the I9/24 state to the I15/24 state by two-photon emission. Simultaneously, nonradiative relaxations occur from the I9/24 state to the I11/24 state, which produce the emission centered at 980 nm [Fig. 2(a)]. The radiative decay from the H11/24 and S3/24 state to the I15/24 state by two-photon emission could produce the emissions at 525 and 540 nm (green emission). The disappearance of the green emission in the spectrum of Na3ZrF7:Er NPs [Fig. 2(a)] was attributed to the cross relaxation between Er3+ ions. Notably, Er3+ clusters can easily form in the Na3ZrF7 matrix because of the large mismatch between Zr4+ ions and Er3+ ions, which reduced the distance between Er3+ ions and enhanced the cross relaxation between Er3+ ions [S3/24→F9/24(3121cm−1):I9/24→F9/24 (2928cm−1); Fig. 4(a)]. Owing to the cross-relaxation effect, the red emission (660 nm) was enhanced, whereas the green and 800 nm emissions were reduced, resulting in pure red emission in the visible light regions of the Na3ZrF7:Er NP spectra [Fig. 2(a)]. When the Na3ZrF7:Er NPs are excited by the 980 nm laser, excitation of the Er3+ ions from the ground I15/24 state to the H11/24 state through a GSA process occurs, followed by a transfer of energy to the F7/24 state of neighboring Er3+ ions. The energy of the F7/24 state is partly transferred to the F9/24 state by the cross relaxation between Er3+ ions [F7/24→F9/24(5190cm−1):I11/24→F9/24 (5030cm−1)] to enhance the red emission. Meanwhile, nonradiative relaxation occurs from the F7/24 state to the H11/24 and S3/24 states, producing green emissions centered at 525 and 540 nm. This mechanism explains exactly why the Na3ZrF7:Er NPs exhibit pure red emission under 1480 nm laser excitation, whereas they show strong red emission and weak green emission under 980 nm laser excitation.

Fig. 4. Diagram of UC via energy transfer UC processes between two Er3+ ions under (a) 1480 and (b) 980 nm laser excitation.

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Monodisperse Na3ZrF7:Er NPs with different Er3+ ion concentrations are synthesized at 260°C. A negligible effect of the Er3+ concentration on the Na3ZrF7:Er NP microstructure and sizes is observed. These Na3ZrF7:Er NPs irradiate pure red emission in the visible light region under 1480 nm laser excitation. The emission intensity is influenced significantly by the Er3+ ion concentration. The suppression of the emission intensity with the increase in the Er3+ ion concentration in the Na3ZrF7:Er NPs results from the defect-related quenching effect, which emerges when tetravalent Zr4+ ions are replaced by trivalent Er3+ ions. The pure red emission of the Na3ZrF7:Er NPs under 1480 nm excitation is related to the cross relaxation [S3/24→F9/24(3121cm−1):I9/24→F9/24 (2928cm−1)] between Er3+ ions, which becomes closer to each other owing to the formation of Er3+ clusters.