Amplifying Excitation-Power Sensitivity of Photon Upconversion in a

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Amplifying Excitation-Power Sensitivity of Photon Upconversion in a NaYbF:Ho Nanostructure for Direct Visualization of Electromagnetic Hotspots 4

Bing Chen, Yong Liu, Yao Xiao, Xian Chen, Yang Li, Mingyu Li, Xvsheng Qiao, Xianping Fan, and Feng Wang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02210 • Publication Date (Web): 15 Nov 2016 Downloaded from http://pubs.acs.org on November 18, 2016

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The Journal of Physical Chemistry Letters

Amplifying Excitation-Power Sensitivity of Photon Upconversion in a NaYbF4:Ho Nanostructure for Direct Visualization of Electromagnetic Hotspots Bing Chen,†, ‡ Yong Liu,§ Yao Xiao,† Xian Chen,‡ Yang Li,§ Mingyu Li,§ Xvsheng Qiao,† Xianping Fan,*,† and Feng Wang*,‡,# †

State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

‡

Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong SAR, China §

State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, Hangzhou 310027, China

#

City Universities of Hong Kong Shenzhen Research Institute, Shenzhen 518057, China

ABSTRACT: Controlling excitation power is the most convenient approach to dynamically tuning upconversion that is essential for a variety of studies. However, this approach suffers from a significant constraint due to insensitive response of most upconversion systems to excitation power. Here we present a study of amplifying excitation power-sensitivity of upconversion in Ho3+ ions through the use of a NaYbF4 host. Mechanistic investigation reveals that the sensitive response of Ho3+ upconversion to excitation power stems from maximal use of the incident energy enabled by concentrated Yb3+ sensitizers. This allows us to sensitively tune the red-to-green emission intensity ratio from 0.37 to 5.19 by increasing the excitation power from 1.25 to 46.25 W cm-2, which represents a 5.6-fold amplification of the tunability (from 0.19 to 0.49) offered by Yb/Ho (19/1 mol%) co-doped NaYF4. Our results highlight that the excitation-power sensitive upconversion emission can be exploited to experimentally visualize electromagnetic hotspots.

Table of Contents artwork

SYNOPSIS: A NaYF4@NaYbF4:Ho@NaYF4 nanoparticle is exploited to provide sensitive response of upconversion emission color to excitation power, enabling an exciting opportunity for direct visualization of electromagnetic hotspots.

Dynamical multicolor tuning of photon upconversion in an invariable nanomaterial is important for applications in diverse fields encompassing information technology,[1-4] biomedicine,[5-10] as well as imaging[11-17] and display.[18-21] Through the design of chemical composition and the use of core-shell nanostructure, recent efforts have made available an array of upconversion systems to produce temporally resolvable emissions by adjusting the excitation variables such as excitation pulse width[22] and wavelength[23-28] and by incorporating electric,[29-32] magnetic,[33-

35]

or plasmon modulations.[36] Despite the capability of dynamical control over upconversion emission in a wide spectral range, these approaches generally require expensive instrument and complicated experimental setup.

Excitation power represents another appealing factor that can be harnessed to dynamically control upconversion emissions by using basic instrument needed to excite upconversion. Identified as early as the discovery upconversion, excitation power dependent emission has been observed and deliberately created in a rather wide diversi-

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in good agreement with the lattice spacing in the (100) planes of hexagonal phase NaYF4 (0.515 nm, Joint Committee on Powder Diffraction Standards file number 160334).

Figure 1. a) Typical TEM image of the as-synthesized NaYF4@NaYbF4:1%Ho@NaYF4 core-shell-shell nanoparticles. b) HAADF scanning TEM image of the nanoparticles highlighting the core-shellshell structure. c) High-resolution TEM images of a nanoparticle revealing single crystalline nature and a d-spacing of 0.51 nm in the [100] crystalline plane. d, e) Normalized emission spectra of the NaYF4@NaYbF4:Ho (1%)@NaYF4 and the NaYF4@NaYF4:Yb/Ho (19%/1%)@NaYF4 nanoparticles under 980 nm excitation of varying powers. Insets are luminescence photos of the corresponding samples.

ty of materials.[37, 41-45] However, the emission profiles of most upconversion materials are relatively insensitive to excitation power. For example, a significant variation of excitation power by more than two orders of magnitude may be required to switch the upconversion emission colors.[38, 40, 45] In addition, the emission colors may be concurrently affected by aggregation of nanoparticles.[46-48] The problems have largely constrained further progress in utilizing excitation power-modulated multicolor tuning. Here we present observation and mechanistic investigation of dramatic amplification of excitation-power sensitivity of Ho3+ upconversion in NaYbF4 nanoparticles. We find that a slight variation in excitation power can induced distinct color change insusceptible to interference caused by aggregation of nanoparticles. The sensitive and exclusive response of the upconversion emission to excitation power creates an exciting opportunity for visualizing electromagnetic hotspots.[49, 50] The NaYbF4:Ho (1 mol%) nanoparticles in our study were synthesized by seed-mediated growth on preformed NaYF4 nanoparticles,[51-56] as NaYbF4 alone tend to grow into big particles unsuitable for many investigations.[57-59] We also applied an inert NaYF4 protection shell, which enables efficient upconversion processes in the Yb-based host by eliminating dissipation of the excitation energies (Figures S1-S3, Supporting Information). Figure 1a shows typical transmission electron microscopy (TEM) image of the as-synthesized nanoparticles with a uniform morphology of short hexagonal prism. High-angle annular dark-field (HAADF) scanning TEM image in Figure 1b shows clear contrasts between the constituent layers, confirming the formation of a core-shell-shell structure. High-resolution TEM image (Figure 1c) further reveals single-crystalline structure of the nanoparticles and lattice fringes with observed d-spacing of 0.51 nm, which is

Figure 1d depicts upconversion emission spectra of the NaYF4@NaYbF4:Ho (1 mol%)@NaYF4 nanoparticles under 980 nm excitation. The spectra all consist of characteristic emission peaks that can be assigned to 5F3 → 5I8 (488 nm), 5 F4, 5S2 → 5I8 (543 nm), 5F5 → 5I8 (648 nm), and 5F4, 5S2 → 5 I7 (750 nm) transitions of Ho3+, respectively. With increasing excitation power from 1.25 to 46.25 W cm-2, the red-to-green emission intensity ratio (R/G ratio) is largely increased from 0.37 to 5.19, corresponding to a clear color change from green to yellow and to red. In stark contrast, control experiment using a NaYF4 host co-doped with Yb/Ho (19/1 mol%) only offered a marginal increase in the R/G ratio from 0.19 to 0.49 within the same excitation power range, rendering no distinct color change (Figure 1e). We attribute the pump power dependent multicolor emission to dissimilar excitation rates for populating the 5 S2 and 5F5 states. Since the red and green emissions are governed by different power laws, the 5F5 state is primarily populated through the 5I7 → 5F5 transition rather than the 5 S2 → 5F5 relaxation (Figure 2a). In measurements of rise curve versus increasing pump power, we detected a clear sharpening of the rise edge for the 5S2 state (Figure 2b). By contrast, the rise edge for the 5F5 state is essentially unaffected by change of pump power in the same range (Figure 2c). The comparison suggests that the probability of the 5I6 → 5S2 transition is appreciably higher than that of the 5I7 → 5F5 transition under Yb3+ sensitization, probably owing to the 5F4 manifold that is thermally coupled to the 5 S2 state and accepts the energy of Yb3+ as well. As formulated by Pollnau,[62] an upconversion process can be partially saturated when successive excitation competes with linear decay for depopulating the intermediate state. The pump power induced color change is therefore explained by preferential saturation of the green emission due to the high 5I6 →5S2 excitation rate. By recording the slope of emission intensity versus pump power in a doublelogarithmic representation (Figure 2, d and e), we detected a smaller slope for the green emission relative to that for the red, especially at a high Yb3+ concentration. The results confirm that the green upconversion process is more readily saturated. The higher excitation-power sensitivity obtained in the NaYbF4 host with respect to that in the NaYF4 is due to an enhanced excitation rate that accelerates saturation of the green emissions. A high Yb3+ content in the host lattice offers a high capacity to sustain the excitation energy and concurrently initiates a fast energy migration process that facilitates energy delivery to the Ho3+ activators. At lower Yb3+ concentrations, reduced amounts of excitation energy are captured by the Ho3+ activators, leading to relatively inefficient excitation processes (Figure S4, Supporting Information).


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Figure 3. a) Luminescence decay curves of Ho3+ emission at 543 nm as a function Yb3+ concentration in the NaYF4@NaYF4:Yb/Ho (x%/1%)@NaYF4 nanoparticles at a fixed excitation power of 0.6 W. b) R/G ratio of Ho3+ emission versus Yb3+ concentration in the NaYF4@NaYF4:Yb/Ho (x%/1%)@NaYF4 nanoparticles by 980 nm excitation at different pump powers. Note that the solid lines are intended to guide the eye. c) Double logarithmic plots of pump power versus upconversion emission intensity of Ho3+ at 543 nm and 648 nm in the NaYF4@NaYF4:Yb/Ho (79%/x%)@NaYF4 nanoparticles incorporated with varying concentrations of Ho3+.

The enhancement of excitation rates at high Yb3+ concentrations is backed by time-resolved spectroscopic studies. The results reveal a sharpening of the rise edge for the 5S2 state (Figure 3a) with increasing Yb3+ concentration, while the rise curves for the 5F5 state are essentially independent of the Yb3+ concentration (Figure S5, Supporting Information). In agreement with the timeresolved measurements, we also observed a growing R/G ratio with increasing Yb3+ concentration at a fixed excitation power (Figure 3b). The R/G ratio grows faster at higher pump powers, which is in line with high capacity of concentrated Yb3+ sensitizers to make maximal use of intense pump powers. The efficient energy transfer upconversion process achieved in the NaYbF4 lattice is largely attributed to the core−shell−shell structural design, which eliminates energy dissipation to surface quenching centers.[57] In the absence of the outermost protection shell of NaYF4, the overall emission intensity drops markedly and the color response to excitation power can hardly be detected (Figure S6, Supporting Information). It is worth noting that back-energy-transfer from Ho3+ to Yb3+ is unlikely responsible for the change of R/G ratio as we detected no appreciable shortening of the Ho3+ lifetime with increasing Yb3+ concentration (Figure 3a). In order to verify the saturation effect on the sensitive color tuning, we further assessed a set of NaYF4@NaYF4:Yb/Ho@NaYF4 nanoparticles incorporated with increasing concentrations of Ho3+ activators (1 mol%, 2 mol%, and 5 mol%) (Figure S7, Supporting Information). The Yb3+ concentrations were fixed at relatively low value of 79 mol% to ensure a uniform Yb3+ content in these samples. The elevated concentrations of Ho3+ ions are designed to diminish the amount of energy that can be received by individual activators, thereby relieving the saturation effect. As anticipated, the double-logarithmic

Figure 4. a) Optical micrographs of the NaYF4@NaYbF4:Ho (1%)@NaYF4 nanoparticles dispersed on a smooth glass slide, showing independence of the emission colour on aggregation state of the nanoparticles. b) SEM image of a gold nanopillar array deposited on a silicon substrate. Inset: Figure 2. a) Simplified energy level structures showing the proposed Electric field distribution around a single nanopillar relative to the incienergy transfer mechanism for populating the 5S2 and 5F5 states in Ho3+ dent light. c) Optical micrographs of the upconversion nanoparticles activators. b, c) Luminescence decay curves of Ho3+ emission in the dispersed on the substrate shown in b). NaYF4@NaYbF4:Ho (1%)@NaYF4 nanoparticles at 543 nm and 648 nm as a function of pump power. d, e) Double logarithmic plots of pump power versus upconversion emission intensity of Ho3+ at 543 nm and 648 nm in the NaYF4@NaYbF4:Ho (1%)@NaYF4 and the NaYF4:Yb/Ho (19%/1%)@NaYF4 nanoparticles, respectively.

plot of pump power and emission intensity yields a clear increase in n values for the green emission from 0.57 (1 mol% Ho3+) to 1.03 (2 mol% Ho3+) and to 1.30 (5 mol% Ho3+) (Figure 3c). In agreement with the relieved saturation effect, the emission color becomes less sensitive to excitation power (Figure S8, Supporting Information). It is worth noting that the selection of lanthanide dopants and host materials is essential for attaining a high excitation-power sensitivity of photon upconversion. For example, substitution of Er3+ for Ho3+ leads to rather close emission colors in a wide excitation power range (1.25– 46.25 W cm-2) (Figure S9 a-c, Supporting Information). The phenomenon is ascribed to similar excitation rates for populating the green and red emitting states of Er3+ as supported by time-resolved spectroscopic investigations (Figure S9 d and e, Supporting Information). In addition, the upconversion emission color of Ho3+ become less sensitive to excitation power when the host material is replaced by a LiYbF4:Ho@LiLuF4 nanoparticle (Figure S10 ac, Supporting Information), owing to altered electron transition probabilities of Ho3+ in a LiYbF4 lattice (Figure S10 d and e, Supporting Information). In a further set of experiments, we assessed resistance of the excitation power controlled multicolor emission to particle aggregations by drop coating NaYF4@NaYbF4:Ho (1%)@NaYF4 nanoparticles on a glass slide. Figure 4a reveals that the emission colors are predominantly determined by excitation power and hardly affected by aggregation state of the nanoparticles, although the area comprising agminate nanoparticles appears brighter. The sensitive and exclusive color response to excitation power enables a convenient means to experimentally detecting locally enhanced electromagnetic hotspots, which was largely reliant on theoretical simulations in previous studies.[21, 50] As proof-of-concept experiments, we fabricated a silicon substrate comprising a gold nanopillar array (Figure 4b), which create electromagnetic hotspots at tips of



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the pillars.[49, 63, 64] By applying the upconversion nanoparticles on the substrate, clear color patches highlighting the hotspots are developed under excitation of a 980 nm diode laser (Figure 4c). The direct visualization of electromagnetic hotspots should provide experiment evidence for understanding luminescence enhancement in complex nanostructures. In conclusion, we demonstrate that the excitation dynamics of Ho3+ ions can be rationally manipulated through control of dopant concentration of Yb3+ sensitizers. A high concentration ratio of Yb3+ and Ho3+ was discovered to expedite saturation of the green emission process, leading to sensitive color change from green to yellow and to red by minor variation of excitation power. Accordingly, we have established a colorimetric technique for direct visualization of electromagnetic hotspots. We believe that the discoveries described here will enhance our ability to precise control of color emission and increase the impact of upconversion in diverse fields including optical sensing and security printing.

EXPERIMENTAL METHOD Synthesis of Nanoparticles. We synthesized the lanthanide-doped core-shell nanoparticles through a layer-bylayer growth process.[57] The detailed synthesis procedure is provided in the supporting information. Fabrication of Au nanopillar on a silicon substrate. A silicon substrate was coated with a silica film (5 μm) using plasma enhanced chemical vapor deposition (PECVD) and then with an Au film (100 nm) by a sputtering process. A 520 nm-thick PMMA layer was spin coated on the Au film. A square-lattice comprising 360 nm round holes with a period of 720 nm was generated on the PMMA layer by electron-beam lithography (Raith150 Two system). A 70 nm-thick gold film was deposited on the patterned PMMA layer by sputtering. The gold pillar array was formed by the lift-off method. Finite-difference time-domain simulations. The distribution of electric field in the gold nanopillar structure was simulated by three-dimensional finite-difference time domain (3D-FDTD) method. The incident light (980 nm) propagates along the z direction perpendicular to the substrate. The perfect matching layer (PML) was used at z-axis boundaries, while periodic was used at x- and y-axis boundaries in the simulation region. The electric field distribution was normalized to the incident light (980 nm). Physical Measurements. Transmission electron microscopy (TEM) was performed on an FEI/Philips Tecnai 12 BioTWIN transmission electron microscope operating at an acceleration voltage of 120 kV. High-resolution TEM image HAADF-STEM image were recorded on an FEI aberration-corrected Titan G2 80-200 Chemi transmission electron microscope operated at 200 kV. SEM images were obtained from a Hitachi S-4800 field emission scanning electron microscope. Optical micrographs were recorded using an advanced research microscope (ECLIPSE Ni-U, Nikon) equipped with a high-definition color cam-

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era (DS-Ri2, Nikon). The emission spectra were recorded with an F-4600 spectrophotometer (Hitachi), in conjunction with a 980 nm diode laser as the excitation sources (continuous wave, 1.25−46.25 W cm-2). Lifetime measurements were performed on an FLS900 fluorescence spectrometer (Edinburgh Instruments) equipped with a pulsed laser (980 nm, 50 Hz) as the excitation source. All spectra were collected at room temperature under identical experimental settings unless otherwise stated.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Research Grants Council of Hong Kong (CityU 11208215), the National Natural Science Foundation of China (Nos. 21573185, 51332008, and 61535010), the public project of the Science and Technology Department of Zhejiang Province (No. 2014C31030), and Zhejiang Provincial Natural Science Foundation of China (No. LY16F050005).

Supporting Information Supporting Information Avaliable: Additional experimental details and Figures S1−S10.

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