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Photophysical color tuning for photon upconverting nanoparticles Qi -C. Sun, Yuchen Ding, and Prashant Nagpal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09284 • Publication Date (Web): 10 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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ACS Applied Materials & Interfaces

Photophysical color tuning for photon upconverting nanoparticles

Qi-C. Sun1,2,‡, Yuchen Ding1,2,3,‡, Prashant Nagpal1,3,4,*

1Department

of Chemical and Biochemical Engineering, University of Colorado Boulder, Boulder, CO 80309, United States

2

Renewable and Sustainable Energy Institute (RASEI), University of Colorado Boulder, Boulder, CO 80309, United States

3Department

of Chemistry, University of Colorado Boulder, Boulder, CO 80309, United States

4Materials

Science and Engineering, University of Colorado Boulder, Boulder, CO 80303, United States

ACS Paragon Plus Environment

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KEYWORDS: surface plasmon polaritons, photon upconversion, color tuning, energy transfer, photophysics, optoelectronics

ABSTRACT: Generating a single higher-energy photon from several photons with lower energy, or photon upconversion, can have useful applications in renwable energy and biological imaging. However, efficient utilization of sub-bandgap infrared radiation in these optoelectronic devices requires high upconversion efficiency and precise tunability of emitted visible light. While several studies have utilized chemical doping to tune the color of upconverted light, low upconversion-efficiency can limit their applicability. In this study, color tuning of upconversion photoluminescence (UPL) is successfully realized by modulating the photophysics using surface plasmon polaritons. Using absorption of nearinfrared light in Yb3+ ions, the occupation of different energy states and the relative energy transfer rate (from Yb3+ to Er3+ and Tm3+ dopants here) can be simultaneously tuned, with a complete shift in color emission using a chromaticity diagram. Furthermore, the efficacy of color tuning by transforming upconverted light (well matched to their enhanced bandedge absorption) into photocurrent is also demonstrated by using ultrathin 2D


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semiconductor nanosheets. Therefore, photophysical color tuning and integration of these precisely tuned upconverting nanoparticles with ultrathin semiconductors can pave the way for designed metal nanostructures for highly-efficient utilization of low-intensity sub-bandgap infrared radiation in optoelectronic devices.

1. Introduction Utilization of low-intensity infrared radiation in optoelectronic devices like photovoltaics and near-infrared photodetectors can have important implications for biological imaging and renewable energy.1–3 Like most multiphoton processes, increasing the intensity of the incident radiation can enhance the efficiencies of upconversion photoluminescence.4 However, an alternative for practical applications with diffuse sunlight or scattering bioimaging systems is utilizing cheaply fabricated designed nanoscale metallic structures for the generation of surface plasmon polaritons. Furthermore, challenges in efficient utilization of infrared radiation in optoelectronic devices include precise tuning of emitted visible light with the bandgap of semiconductor photocell. This challenge is further exemplified in monolayers thick 2D semiconductor nanosheets, where extraordinary


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optical absorption at the excitonic bandgap/bandedge leads to strong absorption and conversion of incident light into photogenerated charge carriers, but a slight mismatch in the color of the emitted light can lead to very small absorption and transmission of the incident/upconverted radiation. Upconversion photoluminescence (UPL) has been well-investigated to generate visible emission from near-infrared excitation by doping with lanthanide ions. Through controlling the relative concentration of lanthanide dopants, the emission tuning of different colors were obtained by several research groups in recent years,5–11 which has been largely applied in fields such as photonics, solar cells, biological imaging, and therapeutics.2,12– 17

In addition, doped-lanthanide upconverting nanoparticles can also be used as a model

system to investigate several important electronic processes including (non-radiative and radiative) decay mechanisms, dopant-assisted quenching, and energy transfer.1,3,14,16,18 Here we show a series of synthesized UCNPs (Figure 1a, S1) and photophysical color tuning using surface plasmon. While infrared absorption of dominant Yb3+ can be improved simultaneously through weak electromagnetic or Purcell effect on the ultrasmooth gold pyramid substrate with SPP generation, the enhancement of energy


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transfer to minority dopants Er3+ or Tm3+,2,3,18 quenching, and enhancement of nonradiative relaxation, relative completion of energy transfer between ions provides an important opportunity to tune the color of emitted light by modulating the photophysics in these

lanthanide

dopants.

Steady

state

and

time-resolved

upconverted

photoluminescence measurements, spectrally-resolved confocal imaging, and single nanosheet current-sensing atomic force microscopy (CS-AFM) measurements were taken, along with our proposed theoretical model to deconvolute different photophysical effects. We also investigated the conversion of upconverted light directly into electric current using CS-AFM as a single nanoparticle photodetection device with UCNPs coated MoS2 and CdSe nanosheets.

2. Results and Discussion To understand the relative photon upconversion and photophysics in as-synthesized nanoparticles, different lanthanide doped UCNPs (Figure 1b) were photoexcited on a glass substrate, using an infrared diode laser (980 nm). A series of upconverted emissions in excited Tm3+ (1D23F4 (blue: 450 nm), 1G43H6 (blue: 480 nm), 1G43F4


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(red: 660 nm), and 3F23H6 (red: 690 nm)); and Er3+ (2H11/24I15/2 (green: 520 nm), 4S

4 3/2 I15/2

(green: 540 nm), and 4F9/24I15/2 (red: 650 nm)) were observed.3,5,19 Based

on the experimental results and reported literature,5–11 we outlined the energy diagram for Er3+, Tm3+, and Yb3+ codoped system, as shown in Figure 1c. Two emission peaks of Tm3+ below 750 nm were missing because of the optical filter used during these measurements (see experimental section). In order to understand the detailed photophysical processes, we performed the plasmon-enhanced UPL experiments on the gold pyramid substrate for different lanthanide doping UCNPs. Pattern periodicity was selected according to the ~980 nm surface plasmon polaritons (SPP) which improves infrared absorption of the Yb3+ donor and energy transfer to Er3+ and Tm3+ acceptors through Coulomb coupling. This strong Coulombic effects on near-field electronic and photophysical phenomenon can be explained by the resonance between the incident light waves and the large charge fluctuations produced by SPP waves. Using UCNPs coated on the gold pyramid substrate (with resonant SPP waves for uniform plasmon-enhancement), the spatially-resolved multispectral UPL data were obtained and carefully analyzed (Figure S3). The blue


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(Figure 2a, b, c), green (Figure 2d, e, f) and red (Figure 2g, h, i) UPL 2D, 3D mapping and spatially resolved line intensity of NaYF4: 20% Yb3+, 4% Er3+, 1% Tm3+ nanoparticles on a gold pyramid substrate using 980 nm incident light, which indicated uniform enhancement along the substrate. Spatially-resolved line intensity was also obtained for the other five samples (Figure S4, 5) to obtain the values of tip-to-bottom ratios (Figure 3b, c), which were used for further analysis. The spectra of six typical UCNPs on the gold pyramid (Figure 3a) showed that the blue and green emission got quenched and the red emission got enhanced compared to Figure 1b. Frequency-dependent upconversion enhancement corresponding to different rate of radiative emission from Er3+ and Tm3 showed higher blue and green emission enhancement compared to red (Figure 3b, c) due to the combination of enhancement and quenching effects (Supporting Information). The lower blue tip-to-bottom ratio compared to the green one in Figure 3b indicates less quenching on the flat gold for Tm3+ compared to Er3+ because Er3+ and Tm3+ have the same enhancement, which was indicated by the similar red bottom-to-tip ratios of Er3+, Tm3+ (1%) codoped UCNPs (Figure 3b) compared to the one of Tm3+ (1%) doped UCNPs (Figure 3c) due to Purcell enhancement. The lower blue tip-to-bottom ratio in Figure 3c


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compared to the blue one in Figure 3b can be understood by the combination of less quenching on the flat gold for Tm3+ and more energy transfer from Tm3+ to Er3+ as shown in Figure 4b. To quantify different photophysical rates in photoexcitation, energy transfer, recombination (radiative and non-radiative) and different Er3+ states occupation, a model describing the above photophysical processes was proposed to fit our spatially resolved multispectral UPL data (Supporting Information).3,18 Since Tm3+ and Er3+ have the same energy enhancement based on the observation in Figure 3b, we can successfully separate each Er3+ energy level from series of Er3+, Tm3+ (1%) codoped UCNPs. We also conducted experimental measurements on 3% Er3+ doped UCNPs (Figure S7, S8) to reveal the significance of each process. To understand how the generated SPP wave will affect the rate of energy transfer from Yb3+ to Er3+, we applied the same analysis from our previous work to the UPL data, and obtained an underestimated enhancement of energy transfer (plasmon-enhanced at 980 nm) rate for series of Er3+, Tm3+ (1%) codoped UCNPs as shown in Figure 4a (detailed analysis available in Supplementary Information). Despite of the ~6 times plasmon-enhanced energy transfer rate, other non-resonnant


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energy transfer in Er3+ either remain unchanged or counterbalanced by quenching or phonon relaxation, as suggested by our analysis. Therefore, a minimum of 6-times enhancement of resonant energy transfer (from Yb3+ to Er3+) can be realized to decouple these photophysical processes in this model UPL system. Our analysis also showed that both energy enhancement and quenching increased with increasing doping concentration (Figure 4a), which was consistent with the bottom-to-tip ratios as shown in Figure 3b. To understand the results, we estimated the fraction of energy transfer to Tm3+ using the spectra on glass and gold pyramid substrates (Figure 1b, 3a, S9, and Table S1, S2), respectively. Here we roughly estimated the fraction energy transfer to Tm3+ using the total intensity of each ion. Figure 4b showed that both fraction of energy transfer to Tm3+ on the glass and gold pyramid substrates decreased with increasing Er3+ concentration. In other words, the energy transfer to Er3+ became more efficient at the high concentration, which possibly increased the energy distribution in each level in Er3+ itself resulted in the energy enhancement and quenching increase with increasing Er concentration.


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In order to understand this result, we introduced the theory for resonance energy transfer from the aspect of both classical and quantum mechanics. The rate of energy transfer (kT) between the donor (Yb3+) and acceptor (Er3+ or Tm3+) is given by,20

𝑘𝑇(𝑟) =

𝑄 𝐷 𝜅2 𝐴

∞ ∫ 𝐹 (𝜆)𝜀𝐴(𝜆)𝜆4𝑑𝜆, 𝜏𝐷𝑟6 𝑁𝑛4 0 𝐷

(1)

where, r: donor-acceptor distance, QD : quantum yield of donor in the absence of acceptor, κ2: factor accounting for the relative orientation of the donor-acceptor dipoles (κ2 ~ 2/3), τD: donor lifetime in the absence of acceptor, A: constant, N: Avogadro’s number, n: medium’s refractive index, FD(λ): corrected donor fluorescence intensity from λ to λ + Δλ, εA(λ): acceptor extinction coefficient at λ. In this relationship, only the distance and the overlap integral can affect the energy transfer rate, and other parameters should be constant for light doped NaYF4. Because different Er3+ doping UCNPs have similar size and monodispersity as shown in Figure 1a and S1, the higher Er3+ concentration with the fixed Yb3+ concentration (20%) in the ~38 nm UCNPs can reduce the donor (Yb3+)-acceptor (Er3+) distance (r) according to the probability theory. Thus, the energy transfer rate should increase with the increase of Er3+ concentration according to


10

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Equation 1, which matches the results from our analysis as shown in Figure 4a. Since higher Er3+ doping concentration with a higher energy transfer rate should absorb more energy from the donor (Yb3+), the decrease of the fraction of energy transfer to Tm3+ with the increase of Er3+ concentration on both glass and gold pyramid substrates in Figure 4b is reasonable. Another interesting result in Figure 4b is that more energy transferred to Tm3+ as samples on the gold pyramid substrate compared to the same UCNPs on the glass substrate, which can be understood by the increase of the overlap integral in Equation 1 through the SPP coupling. As shown in Figure S2b, broad infrared-absorbing (~980 nm) gold puramid substrate not only increases the SPP coupling for the donor (Yb3+), but also increases it for the acceptors (Tm3+, Er3+). Since one photon process in Tm3+ (3H63H5, Figure 1c) never interacts with 980 nm photons when the samples on the glass substrate, the SPP coupling should lead more energy transfer to Tm3+ as the samples on the gold pyramid substrate, which is resulted of the high fraction of energy transfer to Tm3+ in Figure 4b.


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The other effect of lanthanide doping and SPP is the color tuning. Figure 4c directly showed the color change of difference UCNPs solution under a 980 diode laser. The color of UCNPs on the glass substrate was estimated using the spectra in Figure 1b in order to understand the SPP color tuning. Transformations from spectra to colors are straight forward and given as, 𝑋,𝑌,𝑍 = 𝑘∫𝑃(𝜆)𝛼𝑖(𝜆)𝑑𝜆.21 Where, X, Y, Z: standard CIE primaries, αi, (i = X, Y, Z): color-matching functions, P(λ): power distribution as a function of λ, and 4𝑋

9𝑋

k: constant for self-luminous bodies. Using the relations, 𝑢′ = 𝑋 + 15𝑌 + 3𝑍 and 𝑣′ = 𝑋 + 15𝑌 + 3𝑍 , we calculated the emission color for each samples as shown in Figure 4d. The photographic and the simulated colors for each sample were almost identical because there were no extra effects for both solution and on the glass substrate. This gave us the confidence to estimate the color tuning by SPP enhancement as shown in Figure 4e. Due to the selective enhancement of red emissions and quenching of blue and green emissions by the SPP, the color of all samples was tuned to red region. To test the upconversion enhanced photovoltaics, we first measured optoelectronic properties of two 3-layered nanosheets, MoS2 and CdSe (Figure S10) using the setup as


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shown in Figure 5a. Single nanosheet current-voltage relationship was measured under dark and monochromatic light-imlluminated conditions using CS-AFM, and the photocurrents (normalized with light intensity) (Figure 5b, c) were expected to mimic the absorbance (insets of Figure 5b, c) of the nanosheets. Secondly, we measured enhanced photocurrents on two nanosheets with different UCNPs to test the idea of upconversion enhanced photovoltaics. As increased the laser diode power, we observed a continuous increase of photocurrents for all samples. Therefore, these results point to possible pathways to improve the photovoltaic effect from these ultrathin 2D nanomaterials using UCNPs. Moreover, we found the slopes were 1.8 for 3% Er3+, 2.0 for 2% Er3+, 1% Tm3+, and 2.7 for 3% Tm3+ doped UCNPs on MoS2 nanosheets, and 2.9 for 3% Tm3+ on CdSe nanosheets, demonstrating mostly the multiphoton (two for Er3+ and three for Tm3+) excitation processes shown in the energy diagram in Figure 1c and the result of Figure S8. To estimate the quantum yield (QY) given as, 𝑄𝑌 = 𝑃

𝐼𝑈𝐶𝑁𝑃 𝑙𝑎𝑠𝑒𝑟∑𝑐𝑗

∙ 𝑆𝑗

, where 𝐼𝑈𝐶𝑁𝑃 is the

photocurrent on nanosheets coated UCNPs by deducting the dark current, 𝑃𝑙𝑎𝑠𝑒𝑟 = 164.9 𝑚𝑊 is the highest power of the laser diode, 𝑐𝑗 is the intensity fraction at j-th peak


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in Figure 1b, 𝑆𝑗 is values in Figure 5b, c at j-th peak position. We estimated QY (0.0095% for 3% Tm3+, 0.011% for 2% Er3+, 1% Tm3+, and 0.015% for 3% Er3+ UCNPs on MoS2 nanosheets, and 0.054% for 3% Tm3+ on CdSe nanosheets) using photocurrent results (Figure 5b-e), which were in good agreement with the literature values.19 Considering SPP enhancement (100X), we can easily obtain decent quantum yield of upconversion enhanced photovoltaics as the device coupled with SPP structure for future investigation.

3. Conclusion In conclusion, we found that not only doping can tune the color of UPL, but also SPP can affect the color emission in lanthanide-doped UCNPs. Using a combination of steadystate UPL, spatially-resolved multiphoton confocal measurements, and the theoretical modeling, we have demonstrated an enhancement of UPL caused by a combined effects of energy transfer rate increase and weak-Purcell enhancement. The energy transfer rate increased with increasing Er3+ doping concentration because the fraction of energy transfer to Erbium increased combined with the increase of energy transfer rate of Er3+. Therefore, appropriate conditions for coupling the desired photophysical processes to the


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plasmon modes are required. The upconversion enhanced photovoltaics was tested on 2D nanosheets. These findings can benefit renewable energy applications of UPL, and pave the way to optimally design other excitonic and fluorescent systems including excitonic and organic photovoltaics.

4. Experimental Section

Nanoparticle synthesis and Gold pyramid fabrication: Thermal decomposition method was used for the synthesis of co-doped lanthanide upconversion nanoparticles.22 The gold pyramid plasmonic substrates were fabricated on a patterned silicon wafer by selflimited anisotropic etching with KOH (details available in the Supplementary Information).23,24 Further experimental details were provided in the Supplementary Information.

Physical characterization: Tranmission electron micrographs (TEM) of the UCNPs (drop-cast on carbon-coated grids) were taken on a Philips CM100 electron microscope (100 kV acceleration voltage). The steady-state upconverted fluorescence spectrum was collected using a fiber spectrometer (Ocean Optics USB4000, ~0.1 nm resolution at 980


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nm). Confocal imaging for analyzing the plasmon-enhancement effects on UPL was performed with a 980 nm femtosecond excitation pulse (140 fs, 80 MHz) using a tunable (680-1080 nm) Ti:Sapphire oscillator (Chamelon Ultra-II, Coherent). The UPL was collected by epi-detection using various interference filters to remove the light from the excitation laser beam.3,18

ASSOCIATED CONTENTS

Supporting Information. Detailed experimental methods, additional figures, tables, and analysis were included. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author *Email: [email protected]

Author Contributions


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‡These authors contributed equally.

ACKNOWLEDGEMENTS

The authors acknowledge financial support for this work from the National Science Foundation (NSF) CAREER Award CBET-1351281. The authors would like to thank Dr. Haridas Mundoor and Prof. Ivan Smalyukh for help with multiphoton confocal measurements, and Dr. Vivek Singh for help with current sensing AFM.


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FIGURES

Figure 1. Room temperature upconversion emission spectra and simplified energy diagram. (a) TEM images of representative UCNPs with size distribution (collected from several images). Note that, the host and donor are NaYF4 and 20%Yb3+, and the scale bar is 100 nm for both TEM images. (b) Upconversion emission spectra of six nanoparticles on glass substrates. (c) A Yb3+-sensitized Er3+ and Tm3+ system energystate diagram showing the upconversion excitation and visible emission schemes.


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Upconversion energy transfer, radiative, and multiphoton processes are indicated with dashed, solid, and curly arrows. Enhancement of near infrared absorption (mainly 980 nm photons, in Yb3+) and improvement of subsequent energy transfer to Er3+ and Tm3+ (dashed lines) results in higher efficiency of UPL.


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Figure 2. Confocal images of for blue, red and green emissions of NaYF4: 20% Yb3+, 4% Er3+, 1% Tm3+ nanoparticles on the gold pyramid substrate. (a, d, g) 2D confocal images for blue, green, and red emissions of NaYF4: 20% Yb3+, 4% Er3+, 1% Tm3+ nanoparticles on the gold pyramid substrate. (b, e, h) 3D confocal scan images of blue, green, and red


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emissions of UCNPs on the gold pyramid substrate. (c, f, i) Spatially resolved line intensity for blue, green, and red emissions on the gold pyramid substrate, using the 3D image.


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Figure 3. Tip-to-bottom ratios using spatially resolved line intensity for blue, green, red emissions on the gold pyramid substrate. (a) Upconversion emission spectra of six nanoparticles on gold pyramid substrates. (b) Blue, green, and red tip-to-bottom ratios (TBR) as a function of Er3+ concentration for NaYF4: Yb3+, Er3+, Tm3+ nanoparticles on gold pyramid substrates. (c) Blue and red tip-to-bottom ratios (TBR) as a function of Tm3+ concentration for NaYF4: Yb3+, Tm3+ nanoparticles on gold pyramid substrates.


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Figure 4. Photophysics and color tuning of upconversion nanoparticles. (a) Energy transfer enhancement and quenching as a function of Er3+ concentration for NaYF4: Yb3+, Er3+, Tm3+ nanoparticles on gold pyramid substrates. (b) Fraction of energy transfer to Tm3+ as a function of Er3+ concentration for NaYF4: Yb3+, Er3+, Tm3+ nanoparticles on glass and gold pyramid substrates (c) Luminescent photos showing corresponding


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colloidal solutions. (d) CIE1976 chromaticity diagram of corresponding nanoparticles on glass substrates. Insets: the real color of 3% Er3+ and 3% Tm3+ nanoparticles. (e) CIE1976 chromaticity diagram of corresponding nanoparticles on gold pyramid substrates.


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Figure 5. CS-AFM measurements of optoelectronic properties of MoS2 and CdSe nanosheets, and their behaviors using UCNPs excited by the 980 nm laser. (a) Schematic design of optoelectronic characterization. (b) Photocurrent as a function of wavelength


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for MoS2 nanosheets. Inset: Absorbance of MoS2 nanosheet. (c) Photocurrent as a function of wavelength for CdSe nanosheets. Inset: Absorbance of MoS2 nanosheet. (d) Photocurrent as a function of 980 nm laser power for corresponding nanoparticles on MoS2 nanosheets. The slopes indicate two-photon or three-photon process. (e) Photocurrent as a function of 980 nm laser power for corresponding nanoparticles on CdSe nanosheet. The slope indicates a three-photon process.


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