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Upconversion luminescence quenching mechanism of single Au nanoparticles decorated NaYF4: Yb3+, Er3+ hexagonal disk
Materials Research Bulletin 121 (2020) 110613
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Upconversion luminescence quenching mechanism of single Au nanoparticles decorated NaYF4: Yb3+, Er3+ hexagonal disk
T
⁎
Enjie Hea, , Junjun Yub, Chi Wangc, Yi Jiangd, Xuzhong Zuoa, Bing Xua, Jun Wene, Yanfu Qina, Zhaojin Wangf a
School of Electrical and Electronic Engineering, Anhui Science and Technology University, Bengbu, 340303, China School of Mechanical Engineering, Anhui Science and Technology University, Fengyang, 233100, China c School of Physics and Information Technology, Shaanxi Normal University, Xi’an, 710119, China d School of Life and Health Sciences, Anhui Science and Technology University, Fengyang, 233100, China e School of Physics and Electronic Engineering, Anqing Normal University, Anqing, 246011, China f College of Physics and Optoelectronics Technology, Baoji University of Arts and Sciences, Baoji, 721016, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Upconversion luminescence quenching Surface plasmon resonance Cavity quantum optics Nonradiative energy transfer
Different amounts of Au nanoparticles decorated NaYF4: Yb3+, Er3+ hexagonal disks were synthesized by using a hydrothermal strategy. In comparsion with single NaYF4: Yb3+, Er3+ hexagonal disk, the average upconversion quenching factors for the green and red upconversion emission bands of single Au nanoparticles decorated hexagonal disk increased obviously with the increasing of the amounts of uploaded Au nanoparticles. The local field distribution accompanied with the enhancement factors were obtained by finite element method. Based on the cavity quantum optics theory, a quantitative analysis for the Au nanoparticles induced quenching was addressed. The results show that the presence of the higher mode Purcell factors induced nonradiative energy transfer from the upconverter to the Au nanoparticles, which contributed to the quenching significantly. The modest weakening of local fields only played a minor role. It could offer a novel theory guidance for the seeking of the novel plasmonic nanostructure-upconverter hybrid system.
1. Introduction In recent years, rare-earth doped upconversion nanomaterials have shown considerable potentials in innovative fields, including narrowband near-infrared photodetection, super-resolution nanoscopy, encryption keys and full-color 3D displays due to their advantages of large anti-Stokes shifts, sharp emission peaks, multiple emission bands, long luminescence lifetime, high chemical stability and free background [1–6]. However, the low upconversion luminescence (UCL) efficiency induced by their low excitation efficiency, limited excitation wavelength tunability, high surface-to-volume ratio, and multiple nonradiative channels severely restricts their practical applications [2,6,7]. So far, there are many strategies that have been employed to enhance the UCL efficiency, including core-shell or hollow structure design, ion codoping, optical microring resonance, dielectric superlensing modulation, and surface plasmon coupling etc [8–17]. Surface plasmon coupling strategy has become one of the most important scientific interests for UCL enhancement, as it can touch off strong localized electromagnetic field in dimensions below the
⁎
diffraction limit [18,19]. The enhancement mechanisms are generally considered to be the surface plasmon resonance (SPR) effect of metal or semiconductor nanostructures induced excitation and emission enhancements, except a few other effects [20,21]. Employing the Mie theory and the exact electrodynamic theory, Stefan etc demonstrated that the UCL of NaYF4: Er3+ could be increased in proximity of a 200 nm Au nanosphere due to the enhancements of excitation and emission fields [22]. Using a thorough analysis of rate equations, Wounjhang etc showed that the Au nanograting induced UCL enhancement were ascribed to the excitation enhancement in the strong excitation regime, and both the excitation and Förster energy transfer (ET) enhancements in the weak excitation regime [23]. Song etc proposed a model to explain the interaction mechanism between semiconductive Cu2-xS plasmon nanoparticles (NPs) and NaYF4: Yb3+, Er3+ nanocrystals (NCs), i.e., the plasmon scatting effect at low excitation power and electron diffusion via two-photon interband transition at high excitation power [24]. With the help of time-resolved measurements and finite-difference time-domain simulations, Zeng’s group confirmed that both excitation and emission processes of UCL of single
Corresponding author. E-mail address: [email protected] (E. He).
https://doi.org/10.1016/j.materresbull.2019.110613 Received 23 May 2019; Received in revised form 3 September 2019; Accepted 3 September 2019 Available online 06 September 2019 0025-5408/ © 2019 Elsevier Ltd. All rights reserved.
Materials Research Bulletin 121 (2020) 110613
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NaYF4: Yb3+, Er3+, Mn2+ NCs were accelerated by the SPR effect of single Au nanorod [25]. Reuven’s team exhibited a kind of cascaded plasmon enhanced UCL of a single upconversion NPs trapped in a metal nanoaperture optical tweezer, and argued that the excitation enhancement was more significant than the emission enhancement [26]. Li etc explored the effect of cavity plasmonic mode on the UCL of NaLuF4: Yb3+, Er3+@SiO2 core-shell nanosphere capped with Ag hemisphere, and found that the Purcell effect played an important role in the upconversion process [27]. Although many efforts have been devoted to disclosing the specific regulation mechanism of SPR induced UCL enhancement, the UCL quenching originated from the nonradiative ET from upconverter to plasmonic metal nanostructure has been rarely reported [28,29], let alone for the specific quenching mechanism. In fact, the exploration of specific quenching mechanism is as important as the discussion of enhancement mechanism. As it can not only ascertain the reason of quenching, but also offer theory guidance for the seeking of novel plasmonic nanostructure-upconverter hybrid system, and the acquirement of high luminescence efficiency. In the research, we presented our experimental results on the SPR of inhomogeneously distributed Au NPs induced UCL quenching in single NaYF4: Yb3+, Er3+ hexagonal disk, and addressed the thorough analysis based on the cavity quantum optics theory. Our quantitative exploration showed that the quenching was mainly caused by the nonradiative ET originated from the higher mode Purcell factors in the Au NPs decorated NaYF4: Yb3+, Er3+ hexagonal disk resonant cavity, and the weakening of excitation and emission fields only played a secondary role in the quenching process.
as A400NYF and A600NYF, respectively. 2.4. Instruments and measurements X-ray power diffraction (XRD) patterns were obtained with a D8ADVANCE diffractometer (with Davinci design) with graphite monochromatic Cu Kα radiation (λ = 1.5418 Å). Transmission electron microscopy (TEM) measurements were conducted by using a JEM2100 F electron microscope (accelerating voltage: 200 kV) equipped with a charge coupled device (CCD) camera (Gatan Inc.). The element mappings were acquired by an Oxford, INCA, EDX micro analyzer (Model: 6853-H) attached to the TEM. Fourier transform infrared (FTIR) spectra were obtained by a Thermo Fisher NICOLET 380 infrared spectrometer using KBr pellets. Absorption spectra were collected by using a Shimadzu UV-3600Plus UV-Vis–NIR scanning spectrophotometer in the wavelength range of 200–1200 nm. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Fisher ESCALAB 250Xi instrument with photoelectrons generated by a nonmonochromatic AlKα irradiation (1486.6 eV) source at a residual gas pressure of below 5 × 10−9 Torr. A 980 nm diode laser (Changchun New Industries Optoelectronics Technology Co., Ltd.) was employed as the excitation source. Laser confocal microscopy was realized by an OLYMPUS BX51 optical microscope. The focal spot diameter was about 1 μm. A SP 2750i monochromator (ACTON, spectral resolution: 0.008 nm) equipped with a PIXIS 100 CCD (ACTON) and a PD471 photomultiplier tube (PMT, ACTON) was used for spectra acquisition. A Tektronix TDS 5104B digital storage oscilloscope and a boxcar (SRS) were used for luminescence decay profile collection. Proper notch filters were placed in front of the monochromator to block the scattering light. All the measurements were conducted at room temperature. The electric field distributions were simulated by the finite element method (FEM) with the commercial COMSOL Multiphysics 4.3 software package (RF module). Data of Au was given by Johnson and Christy, and data of NaYF4 was given by Sokolov [31,32].
2. Experiments and simulations 2.1. Materials Y(NO3)3·6H2O (99.8%), Yb(NO3)3·5H2O (99.9%), Er(NO3)3·6H2O (99.8%) and HAuCl4·3H2O (≥49.0% Au basis) were supplied by SigmaAldrich Co. NaF (≥98.0%), Na3C6H5O7·2H2O (≥99.0%) and NaNO3 (≥99.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd (China). All chemicals were used without further purification. Deionized water was used throughout experiments.
3. Results and discussion 3.1. Sample characterization The XRD patterns of NaYF4: Yb3+, Er3+ and Au NPs decorated NaYF4: Yb3+, Er3+ hexagonal disks were recorded to verify the crystalline phases and composites. In Fig. 1, the main diffraction peaks of NYF, A200NYF, A400NYF and A600NYF were well-defined, which could be well indexed with the hexagonal phase NaYF4 (JCPDS No. 160334). It indicated that both of them had good crystallinity. Different from the diffraction peaks of sample NYF, two additional weak peaks located at 2θ = 38.300 and 44.600 that indexed well with JCPDS of Au
2.2. Preparation of NaYF4: 20% Yb3+, 2% Er3+ hexagonal disks NaYF4: 20% Yb3+, 2% Er3+ hexagonal disks (simplified as NYF) were synthesized via the suggested hydrothermal method with a minor modification [30]. Firstly, 0.8425 g Na3C6H5O7·2H2O was thoroughly dissolved into 22.5 mL deionized water. Then, a mixture of 0.39 mL Y (NO3)3 (0.5 M), 0.10 mL Yb(NO3)3 (0.5 M) and 0.10 mL Er(NO3)3 (0.05 M) was added into the above solution, and stirred for 20 min. Thereafter, 6.4476 g NaNO3 and 0.1824 g NaF were added orderly and stirred for 120 min. The obtained precursor solution was then sealed into a 35 mL Teflon-lined autoclave, and kept the temperature at 180 ℃ for 9 h. The target product was separated centrifugally, washed with deionized water for several times and dried at 60 ℃ for 48 h. 2.3. Preparation of Au NPs decorated NaYF4: 20% Yb3+, 2% Er3+ hexagonal disks Typical Au NPs decorated NaYF4: 20% Yb3+, 2% Er3+ hexagonal disks (Au200@NaYF4: Yb3+, Er3+, simplified as A200NYF) was prepared by using Na3C6H5O7·2H2O as coupling agent and reductant [30]. Firstly, the hexagonal disks (0.25 mmol) were re-dispersed in an 8 mL Na3C6H5O7 solution (0.06 M) by stirring. Then, a 200 μL HAuCl4 solution (0.025 M) was added dropwise into the mixed solution. Thereafter, the mixture was stirred at 100 ℃ for 15 min. The lilac power was obtained by centrifuging and washing with deionized water, and drying at 60 ℃ for 48 h. Similarly, the other two kinds of hexagonal disks obtained by reducing 400 μL and 600 μL HAuCl4 solutions were simplified
Fig. 1. XRD patterns of NYF, A200NYF, A400NYF and A600NYF. 2
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Fig. 2. Typical dark field TEM images of single hexagonal disks form (a) NYF and (b) A400NYF, and the HRTEM images of corresponding red square regions in (c) Fig. 2(a) and (d) Fig. 2(b). (The inset in Fig. 2(c) shows the magnification of the red square region in Fig. 2(a)). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Obviously, the amount of uploaded Au NPs with various sizes increased with the increasing of added volume of HAuCl4 solution, which was consistent with the XRD result. Typical dark field scanning transmission electron microscopy (STEM) image with elemental mappings of sample A400NYF were presented in Fig. 4. It could be seen that the Na, Y, F, Yb, and Er elements were distributed homogeneously in the hexagonal disk (Fig. 4(b)(f)). The spatial distribution of Au element was inhomogeneous (Fig. 4(g)), and in accordance with the result presented in Fig. 4(a). It further confirmed the successful conjugation of Au NPs with hexagonal disk. Representative FTIR spectra of sample NYF and A400NYF were presented in Fig. 5. The weak vibrating peaks of CeC stretching at 1119 cm−1 and 1190 cm−1, and −OH wagging at 611 cm−1 in the spectrum of NaYF4: Yb3+, Er3+ were essentially invisible in that for sample A400NYF. Similar phenomena have also been observed for the samples of A200NYF and A600NYF. It provides another powerful evidence for the conjugation of Au NPs with hexagonal disk. The chemical compositions of as-prepared samples were further inspected by XPS analysis. As observed in Fig. 6(a), the fully scanned spectra of the samples of NYF and A400NYF indicates that Na, F, Y, Yb, Er, C, and O elements existed in both samples, while Au element only existed in
(No. 01-1172) appeared in the diffraction patterns of other samples, and their intensities increased with the increasing of the added HAuCl4 solution. It illustrated that the increasing amount of Au NPs appeared in the samples of A200NYF, A400NYF and A600NYF orderly. The morphology and microstructure of as-prepared samples were inspected by TEM. Typical single NaYF4: Yb3+, Er3+ hexagonal disk was shown in Fig. 2(a), which exhibited a perfect hexagonal disk structure with side length and thickness approximate 600 nm and 70 nm (inset of Fig. 2(c)) correspondingly. The clear lattice fringes in HRTEM image of Fig. 2(c) (the magnification of red square region in Fig. 2(a)) demonstrated the high crystalline nature with an interplanar spacing of 3.0237 Å, corresponding to the (110) crystal plane of βNaYF4 (JCPDS No. 16-0334). Typical single Au NPs decorated NaYF4: Yb3+, Er3+ hexagonal disk (from sample A400NYF) was also presented in Fig. 2(b). Some small particles and a few clusters were observed on the front and side of hexagonal disk, which indicated the successful conjugation of Au NPs with hexagonal disk. It could be confirmed by the clear lattice fringes for the magnification of red square region in Fig. 2(b) (Fig. 2(d), interplanar spacing: 1.9987 Å), which was in good agreement with the (200) interplanar spacing of cubic Au (JCPDS No. 01-1172). Representative TEM images of single Au NPs decorated hexagonal disks from A200NYF, A400NYF and A600NYF were shown in
Fig. 3. Representative TEM images of single Au NPs decorated hexagonal disks of (a) A200NYF, (b) A400NYF and A600NYF. 3
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Fig. 4. (a) Typical STEM image of single Au NPs decorated hexagonal disk (sample A400NYF), and corresponding element mappings of (b) Na, (c) Y, (d) F, (e) Yb, (f) Er, and (g) Au elements, respectively.
sample A400NYF (Fig. 6(b)), which agreed well with the XRD analysis. When the sample changed from NYF to A400NYF, a 0.30 eV displacement of O 1s towards to low energy was observed, which was probably ascribed to the substitution of −OH by -O-Au for the conjunction of Au NPs. To inspect the SPR absorption properties of as-prepared samples, the UV–vis-NIR absorption spectra were collected, as shown in Fig. 7. No obvious absorption peak was observed in sample NYF, except for the weak peak originated from the 2F7/2→2F5/2 absorption of Yb3+. All the absorption bands of Au NPs decorated samples were broad and strong bands extending from 400 nm to 1200 nm, which was largely different from the absorption of sample NYF, and could be ascribed to the strong plasmon coupling among those adjacent Au NPs.
3.2. Upconversion luminescence quenching Fig. 8 showed the average UCL spectra of single and different amounts Au NPs decorated NaYF4: Yb3+, Er3+ hexagonal disks
Fig. 5. FTIR spectra of NYF and A400NYF. 4
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Fig. 6. XPS spectra of NYF and A400NYF: (a) fully scanned, (b) Au 4f and (c) O 1s high-resolution spectra.
Fig. 7. UV–vis-NIR absorption spectra of NYF and A400NYF.
Fig. 8. Average UCL spectra of single and and different amounts Au NPs decorated NaYF4: Yb3+, Er3+ hexagonal disks (λexc = 980 nm, each spectrum is the average value obtained from 20 measurements of 20 different hexagonal disks under the same excitation condition).
(λ = 980 nm). Typical green and red emission bands originated from 2 H11/2/4S3/2→4I15/2 (green, 514.40–566.82 nm) and 4F9/2→4I15/2 (red, 641.92–679.94 nm) transitions of Er3+ were observed. Generally speaking, the absorption of Er3+ around 980 nm could be ignored, as the absorption cross-section was very small. As shown in Fig. 9, the 5
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Fig. 11(a) was employed to simulate the local electric field around the hexagonal disk. For the sake of quantitative description, a rectangular coordinate system was constructed (the middle part of Fig. 11(a)), in which the x-axis oriented along the diagonal line of regular hexagon, and the z-axis was perpendicular to the plane of regular hexagon. Additionally, the polarization direction of excitation light was parallel to the x-axis (the right part of Fig. 11(a)). As shown in Fig. 11(b)-(m), the typical local electric field strength and the region of electric enhancement increased gradually with the increase of z value. However, the average electric field strength in the single hexagonal disk was probably smaller than that obtained in single NaYF4: Yb3+, Er3+ hexagonal disk, as the maximum of colour bar was slightly bigger than 1. To inspect the average electric field regulation effect and the UCL quenching mechanism, various average electric field enhancement factors for the samples of A200NYF, A400NYF, and A600NYF were calculated by integral over the volume of each hexagonal disk, as shown in Table 1. It should be noted that each enhancement factor in Table 1 was obtained by averaging the enhancement factors of 20 different hexagonal disks from the same sample. Obviously, the average electric field intensity in the hexagonal disk decreased slightly with the presence of Au NPs (Table 1). Furthermore, the more amount Au NPs attached on the hexagonal disk, the more obvious quenching was obtained. Although UCL quenching instead of enhancement in our system was presented, the implicit quenching mechanism could also make sense to suppress the UCL quenching, and provide theoretical guidance for pursuing the greater SPR induced UCL enhancement, which could satisfy practical application. In our previous exploration [33], a novel strategy based on cavity quantum optics theory [34] was employed to explain the Au nanolayer decorated AAO template induced downconversion luminescence (DCL) enhancement and UCL quenching successfully. To disclose the current quenching mechanism, a similar strategy has been adopted by using a optical resonant cavity (example: simulation in Fig. 11(a) for Fig. 4(a)). Similarly, the current quenching can also be ascribed to the interaction between luminescent rare-earth ion and vacuum fluctuation of electric field in the cavity. The interaction intensity is influenced by the photon decay rate of the cavity κ , the non-resonant decay rate γ , and the luminescent rare-earth ion-photon coupling parameter g0 . Although the strong absorption has been presented in the wavelength ranges of green and red emission bands (Figs. 7 and 8), the local electric field only decreased slightly (Fig. 11 and Table 1). It indicates that the g0 was much smaller than the larger of κ and γ , and the luminescent rare-earth ion-photon interaction can be regarded as a kind of weak coupling [35]. Based on the Purcell effect and the dipole approximation, the theoretical UCL quenching factor Fque can be denoted as follows,
Fig. 9. Schematic energy level diagram and UCL mechanism with corresponding ET processes (ET1, ET2, ET3 and ET4 represent the corresponding ET from Er3+ to Au).
population of 2H11/2/4S3/2 was induced by two sequential ET processes of (2F5/2, 4I15/2)→(2F7/2, 4I11/2) and (2F5/2, 4I11/2)→(2F7/2, 4F7/2), and subsequent nonradiative relaxation process of 4F7/2→2H11/2/4S3/2. The 4 F9/2 state had two population ways. One was the 2H11/2/4S3/2→4F9/2 nonradiative relaxation, the other was consisted by the sequential (2F5/ 4 2 4 4 4 2, I15/2)→( F7/2, I11/2) ET, I11/2→ I13/2 nonradiative relaxation, and (2F5/2, 4I13/2)→(2F7/2, 4F9/2) ET. In comparison with pure NaYF4: Yb3+, Er3+ hexagonal disk, obvious UCL quenching in Au NPs decorated samples were observed. For the green emission bands, the quenching factors (Fque ) were calculated to be 2.61, 4.60 and 8.35 for the samples of A200NYF, A400NYF and A600NYF respectively. The obtained counterparts for the red emission band were 3.52, 6.22, and 11.56, which were larger than those for the green emission band. Obviously, the uploaded Au NPs induced the UCL quenching, and the quenching factor increased with the increase of uploaded Au NPs. To visualize the quenching effect, typical single particle UCL microscopic imaging photographs of sample NYF and A400NYF were taken, as shown in Fig. 10. By combining with the coordinate values of color bars, we could easily find that the integrated intensity of each emission band reduced significantly after additional Au NPs modification, and the quenching of red emission band was more obvious, which was consistent with the spectroscopic analysis. General speaking, there were many factors induce UCL quenching. In our system, the influence of the morphology and size of hexagonal disk could be ignored, as the differences of morphology and size between samples with and without Au NPs were even smaller, and each spectrum was the average of 20 measurements for 20 different hexagonal disks. The infrared absorption strength and position of main peaks in Fig. 5 nearly kept unchanged, which indicated that the quenching was not caused by the introduction of additional chemical groups. Furthermore, all the excitation conditions in the system, including spot size, excitation power, integral time and slit width were invariant, thus these factors could be ruled out. An obvious broad absorption band ranged from 400 to 1200 nm induced by irregularly distributed Au NPs appeared in each absorption spectrum of A200NYF, A400NYF and A600NYF (Fig. 7). We could ascribe the quenching to the SPR absorption of those Au NPs, as the direct contact between Au NPs and hexagonal disk probably induced the weakening of the local electric field in hexagonal disk. FEM was employed to explore the possible local electric field variation induced by Au NPs through numerical simulation. Take one of typical hexagonal disk from sample A400NYF (Fig. 4(a)) as an example. Based on the position and size distributions of attached Au NPs, two same combinations of 21 Au spheres with 27 nm diameter, 16 Au spheres with 21 nm diameter, and 16 Au spheres with 12 nm diameter were selected to attach on the topside and backside of single hexagonal disk, and 32 Au spheres sized in 27 nm were attached on its lateral side. The suggested specific geometric model shown in the left part of
1+ Fque =
(
EAu (vexc ) 4 ⎡ ⋅ 1 E0 (vexc ) ⎢
)
⎣
Kr Kr + Knr
+
(
∞
∑l = 1 Fl (vem )
EAu (vem ) 2 Δωcav − R ⎤ ⋅ Δω E0 (vem ) cav − R + Δωcav − NR ⎥
)
(1)
⎦
in which EAu (vexc ) and E0 (vexc ) are the electric field intensities at the frequency of excitation light and the positions of luminescent ions included in hexagonal disks with and without Au NPs, respectively. EAu (vem) and E0 (vem) are the counterparts at the frequency of emission light (typical corresponding wavelengths of 538.2 nm and 660.4 nm for the green and red emission bands orderly). The factors of and
(
EAu (vem ) 2 Eou (vem )
)
(
EAu (vexc ) 4 Eou (vexc )
)
(Table 1) are obtained from the simulations suggested
above, and the latter is called the Purcell factor. The spectral line-width of the resonant cavity Δωcav can be decomposed into the radiative component Δωcav − R and the nonradiative component Δωcav − NR , thus the Δωcav − R that indicates the nonradiative loss of the Purcell factor Δωcav − R + Δωcav − NR factor should be included in Eq. (1). For various resonances, the value Δωcav − R is estimated to be 0.1 to 3 [36]. In current system, of Δωcav − R + Δωcav − NR the absorption spectra of Au NPs decorated NaYF4: Yb3+, Er3+ hexagonal disks are very broad (Fig. 7), which means that the nonradiative 6
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Fig. 10. Typical single particle UCL microscopic imaging photographs, including (a) green and (b) red emission bands of NYF, and (c) green and (d) red emission bands of A400NYF (λexc = 980 nm). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
influence on the UCL quenching, the UCL evolution curves were collected, as shown in Fig. 12. Obviously, in comparison with the hexagonal disk without Au NPs, the rise time lengthened and the decay time shortened in the samples with Au NPs, and the variations of the two time values were more significant in the sample decorated with more Au NPs (Fig. 12). The lengthening of rise time indicates the slowing down of ET processes, including (2F5/2, 4I15/2)→(2F7/2, 4I11/2), (2F5/2, 4I11/2)→(2F7/2, 4F7/2) and (2F5/2, 4I13/2)→(2F7/2, 4F9/2), which was caused by the additional nonradiative ET processes (ET3 and ET4 in Fig. 9) induced depopulation of 2F5/2 and 4I11/2 states. However, the contribution of the depopulation to the UCL quenching was quite limited, as the local electric field was a little changed under the excitation of 980 nm laser (Table 1). The shortening of the decay time illustrated the increase of decay rates of 2H11/2/4S3/2 and 4F9/2 excited states, which was ascribed to the additional nonradiative ET processes presented in Fig. 9 (ET1 and ET2). It should be noted that these two nonradiative processes were mainly originated from the emergence of the higher mode Purcell factors in Au NPs decorated NaYF4: Yb3+, Er3+ hexagonal disk, rather than the SPR of Au NPs induced slight electric field quenching at the frequency of emission band (Table 1). In a word, the observed UCL quenching was mainly induced by the higher mode Purcell factors induced additional nonradiative ET in Au NPs decorated NaYF4: Yb3+, Er3+ hexagonal disks, and the slight weakening of emission and excitation fields only played a minor role.
spectral line-width of the resonant cavity is very large, and the value of Δωcav − R can be reasonably considered as minimum 0.1. As the Δωcav − R + Δωcav − NR higher modes of the Purcell factor cannot receive or send photons from or to external far-field, interaction between the excited rare-earth ion and the higher mode Purcell factor of Fl (vem) (l = 2: quadrupole, l = 3: octopole, …) results in UCL quenching with a factor of ∞ K 1 + K +rK ∑l = 1 Fl (vem) , in which Kr and Knr are radiative and nonr nr radiative decay rates of luminescent rare-earth ions in free space, respectively. For rare-earth ion luminescence, the quenching factor of ∞ K 1 + K +rK ∑l = 1 Fl (vem) is very important, as Kr is close to 1. Kr + Knr r nr Employed the empirical parameters suggested above Δωcav − R Kr = 01, K + K = 1) and the average electric field en( Δω cav − R + Δωcav − NR r nr hancement factors in Table 1, the summations of the higher mode ∞ Purcell factors ∑l = 1 Fl (vem) for the green emission band were calculated to be 1.754, 3.330 and 3.732 for A200NYF, A400NYF and A600NYF respectively, and the counterparts for the red emission band were 2.749, 4.904, and 5.570. It indicates that the nonradiative ET originated from interaction between the excited rare-earth ion and the higher mode Purcell factor played an important role in the UCL quenching process. Furthermore, the quenching of red emission in each Au NPs included sample should be more obvious than that for the green ∞ emission, as the value of ∑l = 1 Fl (vem) for the red emission band is obviously bigger than that for the green counterpart. Additionally, whether for the green emission or for the red emission, the more the amount ∞ of uploaded Au NPs is, the bigger the value of ∑l = 1 Fl (vem) is. It shows that the more significant nonradiative ET (from Er3+ to plasmonic Au NPs, suggested in Fig. 9) in the sample included more Au NPs. To verify the occurrence of the suggested nonradiative ET and its
4. Conclusion In conclusion, pure and different amount Au NPs decorated NaYF4: 7
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Fig. 11. Suggested model for the representative single hexagonal disk of A400NYF and distributions of the ratio of electric field intensity in the single hexagonal disk with Au NPs (Fig. 4(a), from sample A400NYF) to that without Au NPs. (a) Left: A hexagonal disk with a side length of 600 nm and a thickness of 70 nm. A combination of 21 Au spheres with 27 nm diameter, 16 Au spheres with 21 nm diameter, and 16 Au spheres with 12 nm diameter is attached on each regular hexagon side of the hexagonal disk. Middle: the suggested rectangular coordinate system. Right: the polarization direction of excitation light. Simulated electric field distributions on (b) z = 0 (λexc = 538.2 nm), (c) z = 17.5 nm (λexc = 538.2 nm), (d) z = 52.5 nm (λexc = 538.2 nm), (e) z = 70 nm (λexc = 538.2 nm), (f) z = 0 (λexc = 660.4 nm), (g) z = 17.5 nm (λexc = 660.4 nm), (h) z = 52.5 nm (λexc = 660.4 nm), (i) z = 70 nm (λexc = 660.4 nm), (j) z = 0 (λexc = 980 nm), (k) z = 17.5 nm (λexc = 980 nm), (l) z = 52.5 nm (λexc = 980 nm), and (m) z = 70 nm (λexc = 980 nm) planes respectively.
Yb3+, Er3+ hexagonal disk was synthesized by a simple hydrothermal strategy. The remarkable UCL quenching was observed in single hexagonal disk for the green and red emission bands. Employed the cavity quantum optics theory, the resonant cavity consisted by irregularly distributed Au NPs on the hexagonal disk was constructed. Based on the local field simulation and luminescence dynamics analysis, the quenching mechanism was discussed. The results indicates that the nonradiative ET originated from the presence of the higher mode Purcell factors mainly contributed to the quenching while the slight weakening of excitation and emission fields only exerted a minor influence on the quenching. The exploration can supply a novel perspective for the seeking of the rare earth doped luminescent materialsmetal nanostructure hybrid system with the high UCL efficiency.
Table 1 Various average electric field enhancement factors obtained under different excitation wavelengths (E0 and EAu are the average electric field intensities in single NaYF4: Yb3+, Er3+ and single Au NPs decorated NaYF4: Yb3+, Er3+ hexagonal disks, respectively. They are obtained by averaging the enhancement factors of 20 different hexagonal disks of the same sample). λexc (nm)
Sample
( )
( )
( )
538.2
A200NYF A400NYF A600NYF A200NYF A400NYF A600NYF A200NYF A400NYF A600NYF
0.826 0.810 0.767 0.893 0.872 0.795 0.989 0.957 0.836
0.713 0.684 0.614 0.813 0.774 0.644 0.988 0.924 0.715
0.587 0.536 0.432 0.711 0.639 0.449 0.985 0.881 0.534
660.4
980.0
EAu E0
EAu 2 E0
EAu 4 E0
Acknowledgements This work was supported by the Natural Science Foundation of Anhui (No.1808085MA09), the Natural Science Major Project for Universities in Anhui (No. KJ2018A0526, KJ2018A0529), the National 8
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Fig. 12. Normalized UCL evolution curves of single hexagonal disk from NYF, A200NYF, A400NYF, and A600NYF. They are monitored at (a) 538.23 nm and (b) 660.93 nm (λexc = 980 nm, the obtained average rise and decay times are listed in the inserted tables).
Natural Science Foundation of China (No. 51702002, 11604002), and the Stable Talent Project of AHSTU (No. DQWD201603, DQYJ201703).
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Upconversion luminescence quenching mechanism of single Au nanoparticles decorated NaYF4: Yb3+, Er3+ hexagonal disk
T
⁎
Enjie Hea, , Junjun Yub, Chi Wangc, Yi Jiangd, Xuzhong Zuoa, Bing Xua, Jun Wene, Yanfu Qina, Zhaojin Wangf a
School of Electrical and Electronic Engineering, Anhui Science and Technology University, Bengbu, 340303, China School of Mechanical Engineering, Anhui Science and Technology University, Fengyang, 233100, China c School of Physics and Information Technology, Shaanxi Normal University, Xi’an, 710119, China d School of Life and Health Sciences, Anhui Science and Technology University, Fengyang, 233100, China e School of Physics and Electronic Engineering, Anqing Normal University, Anqing, 246011, China f College of Physics and Optoelectronics Technology, Baoji University of Arts and Sciences, Baoji, 721016, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Upconversion luminescence quenching Surface plasmon resonance Cavity quantum optics Nonradiative energy transfer
Different amounts of Au nanoparticles decorated NaYF4: Yb3+, Er3+ hexagonal disks were synthesized by using a hydrothermal strategy. In comparsion with single NaYF4: Yb3+, Er3+ hexagonal disk, the average upconversion quenching factors for the green and red upconversion emission bands of single Au nanoparticles decorated hexagonal disk increased obviously with the increasing of the amounts of uploaded Au nanoparticles. The local field distribution accompanied with the enhancement factors were obtained by finite element method. Based on the cavity quantum optics theory, a quantitative analysis for the Au nanoparticles induced quenching was addressed. The results show that the presence of the higher mode Purcell factors induced nonradiative energy transfer from the upconverter to the Au nanoparticles, which contributed to the quenching significantly. The modest weakening of local fields only played a minor role. It could offer a novel theory guidance for the seeking of the novel plasmonic nanostructure-upconverter hybrid system.
1. Introduction In recent years, rare-earth doped upconversion nanomaterials have shown considerable potentials in innovative fields, including narrowband near-infrared photodetection, super-resolution nanoscopy, encryption keys and full-color 3D displays due to their advantages of large anti-Stokes shifts, sharp emission peaks, multiple emission bands, long luminescence lifetime, high chemical stability and free background [1–6]. However, the low upconversion luminescence (UCL) efficiency induced by their low excitation efficiency, limited excitation wavelength tunability, high surface-to-volume ratio, and multiple nonradiative channels severely restricts their practical applications [2,6,7]. So far, there are many strategies that have been employed to enhance the UCL efficiency, including core-shell or hollow structure design, ion codoping, optical microring resonance, dielectric superlensing modulation, and surface plasmon coupling etc [8–17]. Surface plasmon coupling strategy has become one of the most important scientific interests for UCL enhancement, as it can touch off strong localized electromagnetic field in dimensions below the
⁎
diffraction limit [18,19]. The enhancement mechanisms are generally considered to be the surface plasmon resonance (SPR) effect of metal or semiconductor nanostructures induced excitation and emission enhancements, except a few other effects [20,21]. Employing the Mie theory and the exact electrodynamic theory, Stefan etc demonstrated that the UCL of NaYF4: Er3+ could be increased in proximity of a 200 nm Au nanosphere due to the enhancements of excitation and emission fields [22]. Using a thorough analysis of rate equations, Wounjhang etc showed that the Au nanograting induced UCL enhancement were ascribed to the excitation enhancement in the strong excitation regime, and both the excitation and Förster energy transfer (ET) enhancements in the weak excitation regime [23]. Song etc proposed a model to explain the interaction mechanism between semiconductive Cu2-xS plasmon nanoparticles (NPs) and NaYF4: Yb3+, Er3+ nanocrystals (NCs), i.e., the plasmon scatting effect at low excitation power and electron diffusion via two-photon interband transition at high excitation power [24]. With the help of time-resolved measurements and finite-difference time-domain simulations, Zeng’s group confirmed that both excitation and emission processes of UCL of single
Corresponding author. E-mail address: [email protected] (E. He).
https://doi.org/10.1016/j.materresbull.2019.110613 Received 23 May 2019; Received in revised form 3 September 2019; Accepted 3 September 2019 Available online 06 September 2019 0025-5408/ © 2019 Elsevier Ltd. All rights reserved.
Materials Research Bulletin 121 (2020) 110613
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NaYF4: Yb3+, Er3+, Mn2+ NCs were accelerated by the SPR effect of single Au nanorod [25]. Reuven’s team exhibited a kind of cascaded plasmon enhanced UCL of a single upconversion NPs trapped in a metal nanoaperture optical tweezer, and argued that the excitation enhancement was more significant than the emission enhancement [26]. Li etc explored the effect of cavity plasmonic mode on the UCL of NaLuF4: Yb3+, Er3+@SiO2 core-shell nanosphere capped with Ag hemisphere, and found that the Purcell effect played an important role in the upconversion process [27]. Although many efforts have been devoted to disclosing the specific regulation mechanism of SPR induced UCL enhancement, the UCL quenching originated from the nonradiative ET from upconverter to plasmonic metal nanostructure has been rarely reported [28,29], let alone for the specific quenching mechanism. In fact, the exploration of specific quenching mechanism is as important as the discussion of enhancement mechanism. As it can not only ascertain the reason of quenching, but also offer theory guidance for the seeking of novel plasmonic nanostructure-upconverter hybrid system, and the acquirement of high luminescence efficiency. In the research, we presented our experimental results on the SPR of inhomogeneously distributed Au NPs induced UCL quenching in single NaYF4: Yb3+, Er3+ hexagonal disk, and addressed the thorough analysis based on the cavity quantum optics theory. Our quantitative exploration showed that the quenching was mainly caused by the nonradiative ET originated from the higher mode Purcell factors in the Au NPs decorated NaYF4: Yb3+, Er3+ hexagonal disk resonant cavity, and the weakening of excitation and emission fields only played a secondary role in the quenching process.
as A400NYF and A600NYF, respectively. 2.4. Instruments and measurements X-ray power diffraction (XRD) patterns were obtained with a D8ADVANCE diffractometer (with Davinci design) with graphite monochromatic Cu Kα radiation (λ = 1.5418 Å). Transmission electron microscopy (TEM) measurements were conducted by using a JEM2100 F electron microscope (accelerating voltage: 200 kV) equipped with a charge coupled device (CCD) camera (Gatan Inc.). The element mappings were acquired by an Oxford, INCA, EDX micro analyzer (Model: 6853-H) attached to the TEM. Fourier transform infrared (FTIR) spectra were obtained by a Thermo Fisher NICOLET 380 infrared spectrometer using KBr pellets. Absorption spectra were collected by using a Shimadzu UV-3600Plus UV-Vis–NIR scanning spectrophotometer in the wavelength range of 200–1200 nm. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Fisher ESCALAB 250Xi instrument with photoelectrons generated by a nonmonochromatic AlKα irradiation (1486.6 eV) source at a residual gas pressure of below 5 × 10−9 Torr. A 980 nm diode laser (Changchun New Industries Optoelectronics Technology Co., Ltd.) was employed as the excitation source. Laser confocal microscopy was realized by an OLYMPUS BX51 optical microscope. The focal spot diameter was about 1 μm. A SP 2750i monochromator (ACTON, spectral resolution: 0.008 nm) equipped with a PIXIS 100 CCD (ACTON) and a PD471 photomultiplier tube (PMT, ACTON) was used for spectra acquisition. A Tektronix TDS 5104B digital storage oscilloscope and a boxcar (SRS) were used for luminescence decay profile collection. Proper notch filters were placed in front of the monochromator to block the scattering light. All the measurements were conducted at room temperature. The electric field distributions were simulated by the finite element method (FEM) with the commercial COMSOL Multiphysics 4.3 software package (RF module). Data of Au was given by Johnson and Christy, and data of NaYF4 was given by Sokolov [31,32].
2. Experiments and simulations 2.1. Materials Y(NO3)3·6H2O (99.8%), Yb(NO3)3·5H2O (99.9%), Er(NO3)3·6H2O (99.8%) and HAuCl4·3H2O (≥49.0% Au basis) were supplied by SigmaAldrich Co. NaF (≥98.0%), Na3C6H5O7·2H2O (≥99.0%) and NaNO3 (≥99.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd (China). All chemicals were used without further purification. Deionized water was used throughout experiments.
3. Results and discussion 3.1. Sample characterization The XRD patterns of NaYF4: Yb3+, Er3+ and Au NPs decorated NaYF4: Yb3+, Er3+ hexagonal disks were recorded to verify the crystalline phases and composites. In Fig. 1, the main diffraction peaks of NYF, A200NYF, A400NYF and A600NYF were well-defined, which could be well indexed with the hexagonal phase NaYF4 (JCPDS No. 160334). It indicated that both of them had good crystallinity. Different from the diffraction peaks of sample NYF, two additional weak peaks located at 2θ = 38.300 and 44.600 that indexed well with JCPDS of Au
2.2. Preparation of NaYF4: 20% Yb3+, 2% Er3+ hexagonal disks NaYF4: 20% Yb3+, 2% Er3+ hexagonal disks (simplified as NYF) were synthesized via the suggested hydrothermal method with a minor modification [30]. Firstly, 0.8425 g Na3C6H5O7·2H2O was thoroughly dissolved into 22.5 mL deionized water. Then, a mixture of 0.39 mL Y (NO3)3 (0.5 M), 0.10 mL Yb(NO3)3 (0.5 M) and 0.10 mL Er(NO3)3 (0.05 M) was added into the above solution, and stirred for 20 min. Thereafter, 6.4476 g NaNO3 and 0.1824 g NaF were added orderly and stirred for 120 min. The obtained precursor solution was then sealed into a 35 mL Teflon-lined autoclave, and kept the temperature at 180 ℃ for 9 h. The target product was separated centrifugally, washed with deionized water for several times and dried at 60 ℃ for 48 h. 2.3. Preparation of Au NPs decorated NaYF4: 20% Yb3+, 2% Er3+ hexagonal disks Typical Au NPs decorated NaYF4: 20% Yb3+, 2% Er3+ hexagonal disks (Au200@NaYF4: Yb3+, Er3+, simplified as A200NYF) was prepared by using Na3C6H5O7·2H2O as coupling agent and reductant [30]. Firstly, the hexagonal disks (0.25 mmol) were re-dispersed in an 8 mL Na3C6H5O7 solution (0.06 M) by stirring. Then, a 200 μL HAuCl4 solution (0.025 M) was added dropwise into the mixed solution. Thereafter, the mixture was stirred at 100 ℃ for 15 min. The lilac power was obtained by centrifuging and washing with deionized water, and drying at 60 ℃ for 48 h. Similarly, the other two kinds of hexagonal disks obtained by reducing 400 μL and 600 μL HAuCl4 solutions were simplified
Fig. 1. XRD patterns of NYF, A200NYF, A400NYF and A600NYF. 2
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Fig. 2. Typical dark field TEM images of single hexagonal disks form (a) NYF and (b) A400NYF, and the HRTEM images of corresponding red square regions in (c) Fig. 2(a) and (d) Fig. 2(b). (The inset in Fig. 2(c) shows the magnification of the red square region in Fig. 2(a)). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Obviously, the amount of uploaded Au NPs with various sizes increased with the increasing of added volume of HAuCl4 solution, which was consistent with the XRD result. Typical dark field scanning transmission electron microscopy (STEM) image with elemental mappings of sample A400NYF were presented in Fig. 4. It could be seen that the Na, Y, F, Yb, and Er elements were distributed homogeneously in the hexagonal disk (Fig. 4(b)(f)). The spatial distribution of Au element was inhomogeneous (Fig. 4(g)), and in accordance with the result presented in Fig. 4(a). It further confirmed the successful conjugation of Au NPs with hexagonal disk. Representative FTIR spectra of sample NYF and A400NYF were presented in Fig. 5. The weak vibrating peaks of CeC stretching at 1119 cm−1 and 1190 cm−1, and −OH wagging at 611 cm−1 in the spectrum of NaYF4: Yb3+, Er3+ were essentially invisible in that for sample A400NYF. Similar phenomena have also been observed for the samples of A200NYF and A600NYF. It provides another powerful evidence for the conjugation of Au NPs with hexagonal disk. The chemical compositions of as-prepared samples were further inspected by XPS analysis. As observed in Fig. 6(a), the fully scanned spectra of the samples of NYF and A400NYF indicates that Na, F, Y, Yb, Er, C, and O elements existed in both samples, while Au element only existed in
(No. 01-1172) appeared in the diffraction patterns of other samples, and their intensities increased with the increasing of the added HAuCl4 solution. It illustrated that the increasing amount of Au NPs appeared in the samples of A200NYF, A400NYF and A600NYF orderly. The morphology and microstructure of as-prepared samples were inspected by TEM. Typical single NaYF4: Yb3+, Er3+ hexagonal disk was shown in Fig. 2(a), which exhibited a perfect hexagonal disk structure with side length and thickness approximate 600 nm and 70 nm (inset of Fig. 2(c)) correspondingly. The clear lattice fringes in HRTEM image of Fig. 2(c) (the magnification of red square region in Fig. 2(a)) demonstrated the high crystalline nature with an interplanar spacing of 3.0237 Å, corresponding to the (110) crystal plane of βNaYF4 (JCPDS No. 16-0334). Typical single Au NPs decorated NaYF4: Yb3+, Er3+ hexagonal disk (from sample A400NYF) was also presented in Fig. 2(b). Some small particles and a few clusters were observed on the front and side of hexagonal disk, which indicated the successful conjugation of Au NPs with hexagonal disk. It could be confirmed by the clear lattice fringes for the magnification of red square region in Fig. 2(b) (Fig. 2(d), interplanar spacing: 1.9987 Å), which was in good agreement with the (200) interplanar spacing of cubic Au (JCPDS No. 01-1172). Representative TEM images of single Au NPs decorated hexagonal disks from A200NYF, A400NYF and A600NYF were shown in
Fig. 3. Representative TEM images of single Au NPs decorated hexagonal disks of (a) A200NYF, (b) A400NYF and A600NYF. 3
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Fig. 4. (a) Typical STEM image of single Au NPs decorated hexagonal disk (sample A400NYF), and corresponding element mappings of (b) Na, (c) Y, (d) F, (e) Yb, (f) Er, and (g) Au elements, respectively.
sample A400NYF (Fig. 6(b)), which agreed well with the XRD analysis. When the sample changed from NYF to A400NYF, a 0.30 eV displacement of O 1s towards to low energy was observed, which was probably ascribed to the substitution of −OH by -O-Au for the conjunction of Au NPs. To inspect the SPR absorption properties of as-prepared samples, the UV–vis-NIR absorption spectra were collected, as shown in Fig. 7. No obvious absorption peak was observed in sample NYF, except for the weak peak originated from the 2F7/2→2F5/2 absorption of Yb3+. All the absorption bands of Au NPs decorated samples were broad and strong bands extending from 400 nm to 1200 nm, which was largely different from the absorption of sample NYF, and could be ascribed to the strong plasmon coupling among those adjacent Au NPs.
3.2. Upconversion luminescence quenching Fig. 8 showed the average UCL spectra of single and different amounts Au NPs decorated NaYF4: Yb3+, Er3+ hexagonal disks
Fig. 5. FTIR spectra of NYF and A400NYF. 4
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Fig. 6. XPS spectra of NYF and A400NYF: (a) fully scanned, (b) Au 4f and (c) O 1s high-resolution spectra.
Fig. 7. UV–vis-NIR absorption spectra of NYF and A400NYF.
Fig. 8. Average UCL spectra of single and and different amounts Au NPs decorated NaYF4: Yb3+, Er3+ hexagonal disks (λexc = 980 nm, each spectrum is the average value obtained from 20 measurements of 20 different hexagonal disks under the same excitation condition).
(λ = 980 nm). Typical green and red emission bands originated from 2 H11/2/4S3/2→4I15/2 (green, 514.40–566.82 nm) and 4F9/2→4I15/2 (red, 641.92–679.94 nm) transitions of Er3+ were observed. Generally speaking, the absorption of Er3+ around 980 nm could be ignored, as the absorption cross-section was very small. As shown in Fig. 9, the 5
Materials Research Bulletin 121 (2020) 110613
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Fig. 11(a) was employed to simulate the local electric field around the hexagonal disk. For the sake of quantitative description, a rectangular coordinate system was constructed (the middle part of Fig. 11(a)), in which the x-axis oriented along the diagonal line of regular hexagon, and the z-axis was perpendicular to the plane of regular hexagon. Additionally, the polarization direction of excitation light was parallel to the x-axis (the right part of Fig. 11(a)). As shown in Fig. 11(b)-(m), the typical local electric field strength and the region of electric enhancement increased gradually with the increase of z value. However, the average electric field strength in the single hexagonal disk was probably smaller than that obtained in single NaYF4: Yb3+, Er3+ hexagonal disk, as the maximum of colour bar was slightly bigger than 1. To inspect the average electric field regulation effect and the UCL quenching mechanism, various average electric field enhancement factors for the samples of A200NYF, A400NYF, and A600NYF were calculated by integral over the volume of each hexagonal disk, as shown in Table 1. It should be noted that each enhancement factor in Table 1 was obtained by averaging the enhancement factors of 20 different hexagonal disks from the same sample. Obviously, the average electric field intensity in the hexagonal disk decreased slightly with the presence of Au NPs (Table 1). Furthermore, the more amount Au NPs attached on the hexagonal disk, the more obvious quenching was obtained. Although UCL quenching instead of enhancement in our system was presented, the implicit quenching mechanism could also make sense to suppress the UCL quenching, and provide theoretical guidance for pursuing the greater SPR induced UCL enhancement, which could satisfy practical application. In our previous exploration [33], a novel strategy based on cavity quantum optics theory [34] was employed to explain the Au nanolayer decorated AAO template induced downconversion luminescence (DCL) enhancement and UCL quenching successfully. To disclose the current quenching mechanism, a similar strategy has been adopted by using a optical resonant cavity (example: simulation in Fig. 11(a) for Fig. 4(a)). Similarly, the current quenching can also be ascribed to the interaction between luminescent rare-earth ion and vacuum fluctuation of electric field in the cavity. The interaction intensity is influenced by the photon decay rate of the cavity κ , the non-resonant decay rate γ , and the luminescent rare-earth ion-photon coupling parameter g0 . Although the strong absorption has been presented in the wavelength ranges of green and red emission bands (Figs. 7 and 8), the local electric field only decreased slightly (Fig. 11 and Table 1). It indicates that the g0 was much smaller than the larger of κ and γ , and the luminescent rare-earth ion-photon interaction can be regarded as a kind of weak coupling [35]. Based on the Purcell effect and the dipole approximation, the theoretical UCL quenching factor Fque can be denoted as follows,
Fig. 9. Schematic energy level diagram and UCL mechanism with corresponding ET processes (ET1, ET2, ET3 and ET4 represent the corresponding ET from Er3+ to Au).
population of 2H11/2/4S3/2 was induced by two sequential ET processes of (2F5/2, 4I15/2)→(2F7/2, 4I11/2) and (2F5/2, 4I11/2)→(2F7/2, 4F7/2), and subsequent nonradiative relaxation process of 4F7/2→2H11/2/4S3/2. The 4 F9/2 state had two population ways. One was the 2H11/2/4S3/2→4F9/2 nonradiative relaxation, the other was consisted by the sequential (2F5/ 4 2 4 4 4 2, I15/2)→( F7/2, I11/2) ET, I11/2→ I13/2 nonradiative relaxation, and (2F5/2, 4I13/2)→(2F7/2, 4F9/2) ET. In comparison with pure NaYF4: Yb3+, Er3+ hexagonal disk, obvious UCL quenching in Au NPs decorated samples were observed. For the green emission bands, the quenching factors (Fque ) were calculated to be 2.61, 4.60 and 8.35 for the samples of A200NYF, A400NYF and A600NYF respectively. The obtained counterparts for the red emission band were 3.52, 6.22, and 11.56, which were larger than those for the green emission band. Obviously, the uploaded Au NPs induced the UCL quenching, and the quenching factor increased with the increase of uploaded Au NPs. To visualize the quenching effect, typical single particle UCL microscopic imaging photographs of sample NYF and A400NYF were taken, as shown in Fig. 10. By combining with the coordinate values of color bars, we could easily find that the integrated intensity of each emission band reduced significantly after additional Au NPs modification, and the quenching of red emission band was more obvious, which was consistent with the spectroscopic analysis. General speaking, there were many factors induce UCL quenching. In our system, the influence of the morphology and size of hexagonal disk could be ignored, as the differences of morphology and size between samples with and without Au NPs were even smaller, and each spectrum was the average of 20 measurements for 20 different hexagonal disks. The infrared absorption strength and position of main peaks in Fig. 5 nearly kept unchanged, which indicated that the quenching was not caused by the introduction of additional chemical groups. Furthermore, all the excitation conditions in the system, including spot size, excitation power, integral time and slit width were invariant, thus these factors could be ruled out. An obvious broad absorption band ranged from 400 to 1200 nm induced by irregularly distributed Au NPs appeared in each absorption spectrum of A200NYF, A400NYF and A600NYF (Fig. 7). We could ascribe the quenching to the SPR absorption of those Au NPs, as the direct contact between Au NPs and hexagonal disk probably induced the weakening of the local electric field in hexagonal disk. FEM was employed to explore the possible local electric field variation induced by Au NPs through numerical simulation. Take one of typical hexagonal disk from sample A400NYF (Fig. 4(a)) as an example. Based on the position and size distributions of attached Au NPs, two same combinations of 21 Au spheres with 27 nm diameter, 16 Au spheres with 21 nm diameter, and 16 Au spheres with 12 nm diameter were selected to attach on the topside and backside of single hexagonal disk, and 32 Au spheres sized in 27 nm were attached on its lateral side. The suggested specific geometric model shown in the left part of
1+ Fque =
(
EAu (vexc ) 4 ⎡ ⋅ 1 E0 (vexc ) ⎢
)
⎣
Kr Kr + Knr
+
(
∞
∑l = 1 Fl (vem )
EAu (vem ) 2 Δωcav − R ⎤ ⋅ Δω E0 (vem ) cav − R + Δωcav − NR ⎥
)
(1)
⎦
in which EAu (vexc ) and E0 (vexc ) are the electric field intensities at the frequency of excitation light and the positions of luminescent ions included in hexagonal disks with and without Au NPs, respectively. EAu (vem) and E0 (vem) are the counterparts at the frequency of emission light (typical corresponding wavelengths of 538.2 nm and 660.4 nm for the green and red emission bands orderly). The factors of and
(
EAu (vem ) 2 Eou (vem )
)
(
EAu (vexc ) 4 Eou (vexc )
)
(Table 1) are obtained from the simulations suggested
above, and the latter is called the Purcell factor. The spectral line-width of the resonant cavity Δωcav can be decomposed into the radiative component Δωcav − R and the nonradiative component Δωcav − NR , thus the Δωcav − R that indicates the nonradiative loss of the Purcell factor Δωcav − R + Δωcav − NR factor should be included in Eq. (1). For various resonances, the value Δωcav − R is estimated to be 0.1 to 3 [36]. In current system, of Δωcav − R + Δωcav − NR the absorption spectra of Au NPs decorated NaYF4: Yb3+, Er3+ hexagonal disks are very broad (Fig. 7), which means that the nonradiative 6
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Fig. 10. Typical single particle UCL microscopic imaging photographs, including (a) green and (b) red emission bands of NYF, and (c) green and (d) red emission bands of A400NYF (λexc = 980 nm). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
influence on the UCL quenching, the UCL evolution curves were collected, as shown in Fig. 12. Obviously, in comparison with the hexagonal disk without Au NPs, the rise time lengthened and the decay time shortened in the samples with Au NPs, and the variations of the two time values were more significant in the sample decorated with more Au NPs (Fig. 12). The lengthening of rise time indicates the slowing down of ET processes, including (2F5/2, 4I15/2)→(2F7/2, 4I11/2), (2F5/2, 4I11/2)→(2F7/2, 4F7/2) and (2F5/2, 4I13/2)→(2F7/2, 4F9/2), which was caused by the additional nonradiative ET processes (ET3 and ET4 in Fig. 9) induced depopulation of 2F5/2 and 4I11/2 states. However, the contribution of the depopulation to the UCL quenching was quite limited, as the local electric field was a little changed under the excitation of 980 nm laser (Table 1). The shortening of the decay time illustrated the increase of decay rates of 2H11/2/4S3/2 and 4F9/2 excited states, which was ascribed to the additional nonradiative ET processes presented in Fig. 9 (ET1 and ET2). It should be noted that these two nonradiative processes were mainly originated from the emergence of the higher mode Purcell factors in Au NPs decorated NaYF4: Yb3+, Er3+ hexagonal disk, rather than the SPR of Au NPs induced slight electric field quenching at the frequency of emission band (Table 1). In a word, the observed UCL quenching was mainly induced by the higher mode Purcell factors induced additional nonradiative ET in Au NPs decorated NaYF4: Yb3+, Er3+ hexagonal disks, and the slight weakening of emission and excitation fields only played a minor role.
spectral line-width of the resonant cavity is very large, and the value of Δωcav − R can be reasonably considered as minimum 0.1. As the Δωcav − R + Δωcav − NR higher modes of the Purcell factor cannot receive or send photons from or to external far-field, interaction between the excited rare-earth ion and the higher mode Purcell factor of Fl (vem) (l = 2: quadrupole, l = 3: octopole, …) results in UCL quenching with a factor of ∞ K 1 + K +rK ∑l = 1 Fl (vem) , in which Kr and Knr are radiative and nonr nr radiative decay rates of luminescent rare-earth ions in free space, respectively. For rare-earth ion luminescence, the quenching factor of ∞ K 1 + K +rK ∑l = 1 Fl (vem) is very important, as Kr is close to 1. Kr + Knr r nr Employed the empirical parameters suggested above Δωcav − R Kr = 01, K + K = 1) and the average electric field en( Δω cav − R + Δωcav − NR r nr hancement factors in Table 1, the summations of the higher mode ∞ Purcell factors ∑l = 1 Fl (vem) for the green emission band were calculated to be 1.754, 3.330 and 3.732 for A200NYF, A400NYF and A600NYF respectively, and the counterparts for the red emission band were 2.749, 4.904, and 5.570. It indicates that the nonradiative ET originated from interaction between the excited rare-earth ion and the higher mode Purcell factor played an important role in the UCL quenching process. Furthermore, the quenching of red emission in each Au NPs included sample should be more obvious than that for the green ∞ emission, as the value of ∑l = 1 Fl (vem) for the red emission band is obviously bigger than that for the green counterpart. Additionally, whether for the green emission or for the red emission, the more the amount ∞ of uploaded Au NPs is, the bigger the value of ∑l = 1 Fl (vem) is. It shows that the more significant nonradiative ET (from Er3+ to plasmonic Au NPs, suggested in Fig. 9) in the sample included more Au NPs. To verify the occurrence of the suggested nonradiative ET and its
4. Conclusion In conclusion, pure and different amount Au NPs decorated NaYF4: 7
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Fig. 11. Suggested model for the representative single hexagonal disk of A400NYF and distributions of the ratio of electric field intensity in the single hexagonal disk with Au NPs (Fig. 4(a), from sample A400NYF) to that without Au NPs. (a) Left: A hexagonal disk with a side length of 600 nm and a thickness of 70 nm. A combination of 21 Au spheres with 27 nm diameter, 16 Au spheres with 21 nm diameter, and 16 Au spheres with 12 nm diameter is attached on each regular hexagon side of the hexagonal disk. Middle: the suggested rectangular coordinate system. Right: the polarization direction of excitation light. Simulated electric field distributions on (b) z = 0 (λexc = 538.2 nm), (c) z = 17.5 nm (λexc = 538.2 nm), (d) z = 52.5 nm (λexc = 538.2 nm), (e) z = 70 nm (λexc = 538.2 nm), (f) z = 0 (λexc = 660.4 nm), (g) z = 17.5 nm (λexc = 660.4 nm), (h) z = 52.5 nm (λexc = 660.4 nm), (i) z = 70 nm (λexc = 660.4 nm), (j) z = 0 (λexc = 980 nm), (k) z = 17.5 nm (λexc = 980 nm), (l) z = 52.5 nm (λexc = 980 nm), and (m) z = 70 nm (λexc = 980 nm) planes respectively.
Yb3+, Er3+ hexagonal disk was synthesized by a simple hydrothermal strategy. The remarkable UCL quenching was observed in single hexagonal disk for the green and red emission bands. Employed the cavity quantum optics theory, the resonant cavity consisted by irregularly distributed Au NPs on the hexagonal disk was constructed. Based on the local field simulation and luminescence dynamics analysis, the quenching mechanism was discussed. The results indicates that the nonradiative ET originated from the presence of the higher mode Purcell factors mainly contributed to the quenching while the slight weakening of excitation and emission fields only exerted a minor influence on the quenching. The exploration can supply a novel perspective for the seeking of the rare earth doped luminescent materialsmetal nanostructure hybrid system with the high UCL efficiency.
Table 1 Various average electric field enhancement factors obtained under different excitation wavelengths (E0 and EAu are the average electric field intensities in single NaYF4: Yb3+, Er3+ and single Au NPs decorated NaYF4: Yb3+, Er3+ hexagonal disks, respectively. They are obtained by averaging the enhancement factors of 20 different hexagonal disks of the same sample). λexc (nm)
Sample
( )
( )
( )
538.2
A200NYF A400NYF A600NYF A200NYF A400NYF A600NYF A200NYF A400NYF A600NYF
0.826 0.810 0.767 0.893 0.872 0.795 0.989 0.957 0.836
0.713 0.684 0.614 0.813 0.774 0.644 0.988 0.924 0.715
0.587 0.536 0.432 0.711 0.639 0.449 0.985 0.881 0.534
660.4
980.0
EAu E0
EAu 2 E0
EAu 4 E0
Acknowledgements This work was supported by the Natural Science Foundation of Anhui (No.1808085MA09), the Natural Science Major Project for Universities in Anhui (No. KJ2018A0526, KJ2018A0529), the National 8
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Fig. 12. Normalized UCL evolution curves of single hexagonal disk from NYF, A200NYF, A400NYF, and A600NYF. They are monitored at (a) 538.23 nm and (b) 660.93 nm (λexc = 980 nm, the obtained average rise and decay times are listed in the inserted tables).
Natural Science Foundation of China (No. 51702002, 11604002), and the Stable Talent Project of AHSTU (No. DQWD201603, DQYJ201703).
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