Enhanced up-conversion luminescence from NaYF₄:Yb,Er nanocrystals by Gd³⁺ ions induced phase transformation and plasmonic Au nanosphere arrays

Abstract

NaYF₄:Yb,Er nanocrystals with different concentrations of Gd³⁺ ions are prepared via a hydrothermal method. With increasing Gd³⁺ dopant, up-conversion (UC) emission is gradually enhanced and the strongest UC emission is improved by 50-fold for a sample with 15% Gd³⁺ doping compared with the one without Gd³⁺ incorporation. However, further increasing the Gd³⁺ dopant causes the reduction of UC emission which can be ascribed to the increased surface state recombination. Moreover, the self-assembly technique is used to fabricate Au nanosphere arrays with diameters of 300 nm to enhance the UC luminescence. It is found that the luminescence intensity is increased by 2-fold due to the surface plasmon resonance effect which is confirmed by the absorption spectra.

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Introduction

Up-conversion (UC) luminescence from lanthanide-doped nanocrystals has attracted great interest in recent years since it can be applied to many kinds of devices, such as solar cells,^1,2 display devices³ as well as biological sensors.^4–6 Usually, NaYF₄ is used as a host material for UC emission due to its low phonon energy (∼350 cm⁻¹), which can minimize non-radiative loss.⁷ It was reported that the UC emission was strongly dependent on the crystal phase (cubic and hexagonal) and the emission from NaYF₄ nanocrystals with a hexagonal phase was stronger than that from sample with a cubic phase.⁸ Up to now, the internal quantum efficiency of the UC phosphors is about 7% and the external quantum efficiency is further reduced by the low absorption cross section of the Yb³⁺ sensitizer, which is still low from the viewpoint of actual applications.^9,10 Therefore, it is currently an open question to enhance the UC luminescence efficiency. Recently, the enhancement of UC emission via the structural modification,^11–14 photon management^15–17 or the localized surface plasmonic resonance effect^18–24 has been achieved. Paudel et al.²⁵ fabricated engineered plasmonic Au nanoarrays by use of electron beam lithography technique and found that UC emission from a thin film of PMMA embedded with β-NaYF₄:Yb,Er nanocrystals was significantly enhanced. The finite-difference time-domain (FDTD) simulation results revealed that Au nanoarrays could provide an approximately 11-fold amplification on excitation intensity when compared to a smooth Au thin film substrate. Saboktakin et al.²⁶ also designed a tunable plasmonic nanohole array resonant at the excitation wavelength of β-NaYF₄:Yb,Er nanocrystals by electron beam lithography and found the frequency-dependent luminescence enhancement of UC emission up to 35-fold.

It was reported that the up-conversion nanocrystals prepared in the high-boiling solvents above 300 °C exhibited a good fluorescence properties.^8,27 However, by using hydrothermal method to prepare small-sized pure β-NaYF₄ nanocrystals which emit strong UC luminescence usually need high reaction temperature about 300 °C and long reaction time (up to several days), hazardous coordinating solvents and air protection etc. In our present work, we used the simple and cheap hydrothermal method to fabricate the β-NaYF₄ nanocrystals doped by Gd³⁺ ions at lower temperature and shorter reaction time without requiring air protection condition. Gd³⁺ ions are incorporated into the NaYF₄:Yb,Er nanocrystals during the preparation process and it is confirmed that the crystal phase is gradually changed from cubic to hexagonal structure with increasing Gd³⁺ ions. The UC emission intensity is also enhanced with the change of crystal phase and the strongest UC emission is achieved for sample with Gd³⁺ ions doping ratio of 15%. In order to further enhance the UC emission, we then introduced the plasmonic nanostructures into the NaYF₄ nanocrystals doped by Gd³⁺ ions. Instead of the expensive e-beam lithography technique, nanosphere lithography technique is used to get 2-dimensional (2D) Au nanoarrays by using self-assembly polystyrene (PS) sphere array as a mask. By coating the NaYF₄:Yb,Er film incorporated 15% Gd³⁺ ions on the 2D Au nanoarrays, the UC emission intensity is further enhanced due to the coupling of plasmonic modes of Au nanoarrays with the absorption peak of Yb³⁺ ions.

Experiment

NaYF₄:Yb,Er nanocrystals were synthesized by a hydrothermal method. Briefly, 1.5 g of NaOH was dissolved in 7.5 mL of deionized water, then mixed with 25 mL of ethanol and 25 mL of oleic acid under stirring. 10 mL of RE(NO₃)₃ (0.2 M, RE = Y, Yb, Er and Gd. Y(NO₃)₃ : Yb(NO₃)₃ : Er(NO₃)₃ : Gd(NO₃)₃ = (80 − X) : 18 : 2 : X mol%, X is the concentration of Gd³⁺ ions) and 5 mL of NH₄F (2 M) were added to the resulting mixture. The concentration of the Er³⁺ and Yb³⁺ ions is constant for all samples whereas the Y³⁺ ions are replaced by Gd³⁺ ions. Then the solution was transferred into a 100 mL of Teflon-lined autoclave and heated at 215 °C for 2 h, which was a little bit higher than that in the previous work.¹² The nanocrystals were washed with water and ethanol several times and collected by centrifugation. Finally, the nanocrystals were dry at 60 °C for 12 h and re-dispersed into ethanol to form an aqueous dispersion and were ready for use.

A monolayer of PS nanospheres with diameter of 300 nm was coated on the p-Si substrates by using the self-assembly technique, as described in our previous work.^28,29 Then a thin Au film was deposited on Si/PS substrates using magnetron sputtering to form Au nanoarrays. Finally NaYF₄:Yb,Er nanocrystals were dropped on the Au nanoarrays. The morphologies and structures of formed samples were characterized by field emission scanning electron microscopy (FE-SEM, Sigma), transmission electron microscopy (TEM, Tecnai G2 F20) and X-ray powder diffractometer (XRD). UC luminescence spectra were measured by use of a fluorescence spectrophotometer (Edinburgh Photonics, FLS980) equipped with a 980 nm diode laser with tunable power of 0–1000 mW.

Results and discussions

Fig. 1(a) and (b) show XRD patterns and FE-SEM images for the NaYF₄:Yb,Er nanocrystals with different concentration of Gd³⁺ ions respectively. These samples are labeled as “GdX (X = 0, 15, 30, 60 mol%)” and this naming convention is used throughout the remainder of this manuscript. As shown in Fig. 1(a), the main strong diffraction peaks of the Gd0 and Gd5 samples are in well agreement with results for the α-NaYF₄ phase (JCPDS card, no. 77-2042), while the small residual peaks are consistent with β-NaYF₄ phase (JCPDS card, no. 16-0334). It indicates that the main phase of Gd0 and Gd5 samples is cubic. As shown in Fig. 1(b), many spherical-like particles with a few rods can be identified in FE-SEM images, the α-NaYF₄ has isotropic unit cell structure, resulting in an isotropic growth of particles³⁰ and therefore the spherical-like particles can be defined as the α-NaYF₄ nanocrystals. However, the XRD pattern for Gd15 sample is quite different from that of Gd0 and Gd5 samples. The main diffraction peaks are related to NaYF₄ with hexagonal phase while the peaks relate to cubic phase become weak. Correspondingly, the nanorods with 80–100 nm in width, 400–700 nm in length can be clearly observed in FE-SEM image of Gd15 sample as shown in Fig. 1(b). From the TEM image, we can find some nanocrystals with size of 30 nm attached on the nanorod, as shown in Fig. 2(a). Fig. 2(b) shows high resolution TEM image of a nanorod for the Gd15 sample. The inter-planar spacing of a nanorod is about 0.515 nm, indicating the β-NaYF₄ [100] crystalline orientation, which is well agreement with the results measured by the X-ray diffraction, as shown in Fig. 1(a). As discussed above, the nanoparticles should be the NaYF₄ with cubic phase and the nanorods are β-NaYF₄ with hexagonal structures. With further increasing Gd³⁺ ions, the peaks related to cubic-phase NaYF₄ are nearly disappeared and only XRD peaks associated with hexagonal phase NaYF₄ can be identified which indicates that the formation of pure β-NaYF₄ (hexagonal phase) nanocrystals. Correspondingly, FE-SEM images exhibit the decreased average size of nanorods with gradually increasing the Gd³⁺ ions. Furthermore, we have measured the EDX pattern of Gd15 sample to reveal the existence of doped elemental Gd, the existence of Gd in the NaYF₄ nanocrystals can be clearly indicted, as shown in Fig. S1.†

Fig. 1 (a) XRD patterns of NaYF₄:Yb,Er nanocrystals prepared with different concentration of Gd³⁺ dopant ions. The standard data of α-NaYF₄ (JCPDS card, no. 77-2042) and β-NaYF₄ (JCPDS card, no. 16-0334) are shown as references. (b) FE-SEM images of Gd0, Gd5, Gd15, Gd30 and Gd60 samples respectively.

Fig. 2 (a) TEM image of Gd15 sample. (b) High resolution TEM image of a nanorod.

The samples with different concentration of Gd³⁺ dopant ions exhibit distinctively different XRD patterns, which confirm that increasing the concentration of Gd³⁺ ions can induce the transformation of NaYF₄ nanocrystals from cubic phase to hexagonal phase. The phase transformation can be explained as the ordered array of F²⁻ ions with two types of relatively low-symmetry cation sites selectively occupied by Na⁺ and RE³⁺ ions, resulting in significant electron cloud distortion of the cations to accommodate the structural change. Gd³⁺ ions with large ionic radii exhibit a high tendency towards electron cloud distortion owing to increased dipole polarizability, they tend to favor the hexagonal structures.¹² Furthermore, it is found that increasing Gd³⁺ dopant concentration from 15% to 60%, nanorods gradually decrease in size. It can be partly attributed to the strong effect of the Gd³⁺ dopant ions on crystal growth rate through surface charge modification. Owing to an increase in charge repulsion, the change of electron charge density on the surface of the small-sized nanocrystals can substantially slow the diffusion of negatively charged F²⁻ ions to the surface, which can result in a tunable reduction of the NaYF₄ nanocrystal size.¹² Another possible reason may due to the fast nucleation of seeds with the introduction of Gd³⁺ ions. The fast nucleation can also cause the small size of formed nanoparticles in the end. The further work is needed to clarify this point clearly.

The UC emission spectra of the Gd0, Gd5, Gd15, Gd20, Gd30, Gd60 samples are shown in Fig. 3(a) under the excitation of 980 nm laser with power of 656 mW. Three major emission bands at 521 nm (green light), 539 nm (green light), and 654 nm (red light) can be detected, which are assigned to the ²H_11/2 to ⁴I_15/2, ⁴S_3/2 to ⁴I_15/2, and ⁴F_9/2 to ⁴I_15/2 transitions of Er³⁺ ions, respectively.³¹ It is found that the six samples have the same emission peaks yet with quite different emission intensities. The UC emission can be clearly observed by the naked eyes as shown in inset of Fig. 3(a). It is found that the UC emission intensities are first increased with incorporated Gd³⁺ ions and then decreased with further adding Gd³⁺ ions.

Fig. 3 (a) UC emission spectra of NaYF₄:Yb,Er nanocrystals with different concentration of Gd³⁺ dopant ions dispersed in ethanol solution under 980 nm laser diode excitation, inset (a) is luminescence photograph of the Gd15 sample. (b) The integrated emission intensity of the emission spectra over wavelength in the range 500–700 nm versus dopant concentration of Gd³⁺ (0–60 mol%).

The integrated emission intensity at emission wavelength from 500–700 nm is plotted as a function of incorporated Gd³⁺ ions in Fig. 3(b). The Gd15 sample shows the strongest emission which is enhanced by 50-fold compared with that of sample without Gd³⁺ ions (Gd0 sample). It is found the gradual changes in UC emission intensities due to the phase transformation. The UC emission is indeed enhanced significantly with formation of hexagonal-phase NaYF₄ nanocrystals. However, the UC emission is decreased with further increasing the Gd³⁺ ions (>15%) although the pure β-NaYF₄ can be formed at that stage. The possible reason may be related to the increasing surface states due to the reduction of nanocrystals size and the formation of defect states due to the large amount of Gd³⁺ incorporated with the NaYF₄ crystals, which increased the non-radiative recombination probably to suppress the UC emission. It needs further investigation in our future work.

To further clarify the UC process of the NaYF₄:Yb,Er nanocrystals, log–log plots of the UC emission intensities of Gd15 sample as a function of pump power under 980 nm laser excitation are shown in Fig. 4(a). The UC emission intensity generally has a nonlinear dependence on the excitation power density, which can be described as³²

I_em ∝ (P_pump)ⁿ

I_em is the emission intensity, P_pump is the power of pump laser, n is the number of the excitation photons required to produce the UC emission. The value n can be deduced by fitting log–log plot for the UC emission at 521 nm (²H_11/2 to ⁴I_15/2), 539 nm (⁴S_3/2 to ⁴I_15/2), and 654 nm (⁴F_9/2 to ⁴I_15/2) respectively. The deduced n value is 1.8, 1.8, and 1.9 as shown in Fig. 4(a), which indicates a two-photon UC process for our samples. As shown in Fig. 4(b), the mechanisms of the UC emissions can be summarized based on our experimented results. The electron of Yb³⁺ is first excited from ²F_7/2 to ²F_5/2 level in NaYF₄:Yb,Er nanocrystals under 980 nm laser excitation, which can promote Er³⁺ ion from ⁴I_15/2 level to the ⁴I_11/2 level due to the energy transfer process. Then, a second 980 nm photon transferred by the adjacent Yb³⁺ ions can excite Er³⁺ ions from ⁴F_11/2 to ⁴F_7/2 by the two-photon process. Finally, the Er³⁺ ion can relax non-radiatively to the ²H_11/2, ⁴S_3/2 and ⁴F_9/2 levels. Then they return to the ground state ⁴I_15/2, emitting 521 nm, 539 nm, and 654 nm photons respectively.

Fig. 4 (a) Log–log plots of the UC emission intensity versus pump power for Gd15 sample under 980 nm laser diode excitation. (b) Schematic energy level diagram of the UC emission mechanisms for NaYF₄:Yb,Er nanocrystals.

In order to further enhance the UC emission intensity of NaYF₄:Yb,Er nanocrystals, we studied effect of the metallic nanosphere structures on the UC emission process. The 2D Au nanoarray structures were fabricated by evaporating Au thin film on the self-assembly formed monolayer polystyrene (PS) nano-sphere arrays. Fig. 5(a) is the morphology of formed PS nanosphere arrays. The diameter of PS nanosphere is 300 nm. It shows the ordered and closed packed PS nanosphere arrays. Fig. 5(b) shows the cross-sectional SEM image of Au coated nanosphere arrays. It is shown that the surface morphology is kept after Au evaporating which demonstrates the formation of 2D periodic Au nanoarray structures. In the inset of Fig. 5(b), the cross-sectional SEM image of Au nanoarrays shows that the Au film is sputtered both on PS spheres and on the flat silicon surface via the exposed regions between the PS nanospheres. Therefore we can measure the thickness of Au film on the flat silicon surface which is about 15 nm as shown in the inset of Fig. 5(b). After formation of Au nanoarrays, the NaYF₄:Yb,Er nanocrystals with incorporation 15% Gd³⁺ ions (which show the strongest UC emission before) were dropped on the Au nanoarrays and the structure diagram of sample is illustrated in Fig. 5(c). Fig. 5(d) is cross-sectional FE-SEM image of NaYF₄:Yb,Er nanocrystals on Au nanoarrays. It is shown that the thickness of NaYF₄:Yb,Er film is about 500 nm. And Fig. 5(e) shows the compact film containing nanocrystals are coated on the Au nanoarrays. More details of these FE-SEM images can be seen in Fig. S2.†

Fig. 5 (a) FE-SEM image of 300 nm PS sphere nanoarrays on Si substrate. (b) Cross-sectional FE-SEM image of 300 nm Au nanoarrays on Si substrate. (c) The structure diagram of NaYF₄:Yb,Er/Au nanoarrays/Si. (d) and (e) Cross-sectional and top-view FE-SEM images of NaYF₄:Yb,Er nanocrystals on Au nanoarrays, respectively.

The UC emission spectra are shown in Fig. 6 for sample on Au arrays. For comparison, the spectra collected from the sample without Au arrays (flat Si) and bare PS spheres are also given. It is found that the emission at 521 nm, 539 nm and 654 nm can be detected for all the samples and the emission intensities for three emission bands are all enhanced obviously by introducing Au nanoarrays. The emission intensities of the bare PS spheres are similar to that of flat Si substrate. However, the integrated emission intensity introducing Au nanoarrays is almost 2-fold stronger than that of reference samples.

Fig. 6 UC emission spectra of NaYF₄:Yb,Er nanocrystals on Au nanoarrays.

To understand the enhancement by using Au nanoarrays, the reflectance spectra of Au nanoarrays on Si wafer as well as the flat Si wafer and bare PS spheres on Si wafer were measured. As shown in Fig. 7, a reflectance valley centered at 956 nm can be observed for 300 nm Au nanoarrays, which means the resonance mode at this spectra range.¹⁹ The reflection spectra indicates that the plasmonic resonant mode is centered at 956 nm, which is well consistent with the absorption energy level of sensitizer of Yb³⁺ ions in NaYF₄ nanocrystals. It is believed that the existence of Au nanoarrays can significantly enhance the excitation efficiency due to the coupling of plasmonic modes (∼956 nm) and the absorption band of Yb³⁺ ions (∼980 nm) which act as the sensitizer in NaYF₄ nanocrystals. As a consequent, the UC emission intensities are in turn enhanced as revealed in our experimental results. Therefore, the enhancement of UC emission with introducing 300 nm-periodic Au nanoarrays can be attributed to the effectively coupling of plasmonic modes with the absorption band of Yb³⁺ ions which increases the excitation rate by local field enhancement effect.

Fig. 7 Reflection spectra of flat Si, 300 nm PS sphere arrays and 300 nm Au nanoarrays.

Conclusions

In conclusion, we have prepared NaYF₄:Yb,Er nanocrystals with different concentration of Gd³⁺ ions via hydrothermal method. The UC luminescence intensity of NaYF₄:Yb,Er nanocrystals is significantly enhanced by Gd³⁺ ions due to the formation of hexagonal-phase NaYF₄ nanocrystals. The Gd15 sample has the strongest UC emission intensity and the enhancement is reached more than 50-fold. Moreover, 2D Au nanosphere arrays are fabricated by using the nanosphere lithography technique and the UC emission from NaYF₄:Yb,Er nanocrystals is further improved due to the coupling of localize surface plasmonic resonant effect. Our results not only deepen our understanding of the UC emission process from nanocrystals, but also provide a cheap approach for making efficient thin-film UC materials, which have a great potential in the future device applications.

Acknowledgements

This work was supported by “973 program” (2013CB632101) and NSFC (No. 11274155), “333 project” of Jiangsu Province (BRA2015284) and PAPD.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra20382e

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