Enhancement of upconversion photoluminescence in phosphor nanoparticle thin films using metallic nanoantennas fabricated by colloidal lithography

rsc.li/materials-advances Materials Advances rsc.li/materials-advances Volume 01 Number 1 February 2020 Pages 001-200 ISSN 2633-5409 PAPER XXXXXXX XXXXXXX et al. xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Materials Advances Accepted Manuscript This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. View Article Online View Journal This article can be cited before page numbers have been issued, to do this please use: T. T. Ngo, J. M. Viaña, M. Romero, M. E. Calvo, G. Lozano and H. Míguez, Mater. Adv., 2023, DOI: 10.1039/D3MA00775H. Enhancement of upconversion photoluminescence in phosphor nanoparticle thin films using metallic nanoantennas fabricated by colloidal lithography Thi Tuyen Ngo†, Jose M. Viaña†, Manuel Romero, Mauricio E. Calvo, Gabriel Lozano* and Hernán Míguez* Institute of Materials Science of Seville, Spanish National Research Council – University of Seville, Américo Vespucio, 49. 41092 Seville, Spain. † Authors with equal contribution. * Corresponding authors: [email protected]; [email protected] Abstract Lanthanide-doped upconversion nanoparticles (UCNPs), as multifunctional light sources, are finding utility in diverse applications ranging from biotechnology to light harvesting. However, the main challenge in realizing their full potential lies in achieving bright and efficient photon upconversion (UC). In this study, we present a novel approach to fabricate an array of gold nanoantennas arranged in a hexagonal lattice using a simple and inexpensive colloidal lithography technique, and demonstrate a significant enhancement of UC photoluminescence (UCPL) by up to 35-fold through plasmonenhanced photoexcitation and emission. To elucidate the underlying physical mechanisms responsible for the observed UCPL enhancement, we provide a comprehensive theoretical and experimental characterization, including a detailed photophysical description and numerical simulations of the spatial electric field distribution. Our results shed light on the fundamental principles governing the enhanced UCNPs and pave the way for their potential applications in photonic devices. Introduction Lanthanide-doped upconversion nanoparticles (UCNPs) have attracted much attention due to their nonlinear character, which enables the conversion of low-energy photons into higher-energy ones via energy transfer between the long-lived and ladder-like energy levels of lanthanide ions.1–3 This intriguing phenomenon endows these nanomaterials with tremendous potential for applications in various fields, including biotechnology,4–8 thermometry,9,10 light harvesting,11,12 or anti-counterfeiting.13–15 However, the low efficiency of nonlinear processes and the small absorption cross section of dopants limit the brightness of UCNPs and pose a challenge to the development of the UC-based technology.16–18 In recent years, several strategies have been developed to overcome this limitation. The most successful ones can be divided into those that optimize intrinsic material properties, such as composition and crystalline structure, including surface modification,19–21 doping level,3,22 or core/shell architecture ,22,23 and those that tune the optical environment of the UC materials.24 Among the latter, localized surface plasmon resonances (LSPRs) have been demonstrated to be an excellent choice for modulating the emission of UC materials.25–29 LSPRs arise from the collective oscillations of free Page 1 of 13 Materials Advances Materials Advances Accepted Manuscript Open Access Article. Published on 02 November 2023. Downloaded on 11/3/2023 10:27:47 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D3MA00775H electrons on the metal surface under illumination, resulting in a significant enhancement of the electric field intensity in the vicinity of the resonant NPs.30 Such an enhanced electric field can drastically alter the photoexcitation or the emission dynamics if the LSPR modes match the excitation or emission bands of nearby emitters.26 Indeed, Ag nanocavities or gold NPs supporting an LSPR mode overlapping with the emission of UCNPs have enabled an enhancement of the radiative decay rate, thereby improving the UCPL intensity.31,32 It has also been shown that metallic nanostructures resonant with the excitation bands of UCNPs give rise to increased UCPL due to absorption enhancement.33–35 In all approaches, controlling the spectral position of the LSPRs is the key point to achieve UCPL enhancement. This can be achieved by tuning the material properties and the dimensions of the individual plasmon nanoparticles.36–38 For example, core36,37 and core/shell38–40 metallic nanorods have been carefully synthesized to support LSPRs tuned to the excitation or emission of UCNPs, resulting in several-fold enhancement of UCPL. An alternative route to tailoring LSPRs relies on the use of lithographic techniques. Indeed, a variety of lithographic techniques have been used for this purpose, including laser interference,34 nanoimprint,33,41 electron beam42–45 or a combination of lithography, chemical etching,35,46 and combined with thermal evaporation.33,34 Although, these techniques allow the fabrication of nanostructures with fine precision, they have the disadvantage of requiring costly steps. Herein, we present a facile method to fabricate an array of regular gold nanoantennas arranged in a hexagonal lattice to enhance the UC emission of NaYF4:Yb3+,Er3+ (abbreviated as NaYF) NPs. The method involves the use of a monolayer of selfassembled polystyrene (PS) spheres, as an evaporation mask for metal deposition and the subsequent removal of the PS layer by a mechanical process. Note that similar masked deposition has previously been used to prepare irregular and random metal nanoparticle arrays.29 Our approach combines the advantages of self-assembly and thermal evaporation as simple, inexpensive, reproducible and large scale, and allows its combination with thin layers of NAYF NPs, the workhorse material for UC. By controlling the dimensions of the mask, the LSPRs of the gold nanostructures are tuned to match both the excitation and emission bands of the UCNPs. As a result, we demonstrate a large UCPL enhancement, up to 20 and 35 times for green and red emission bands of Er3+ cations, respectively. The underlying mechanism is revealed by a detailed photophysical characterization together with numerical simulations. Our analysis confirms that the presence of resonant gold NPs significantly enhances the NaYF absorption and tunes the UC emission. Plasmon-mediated excitation accelerates the filling of the Er3+ energy levels and increases the radiative decay rate of Er3+ transitions. Despite its simplicity, our approach provides an effective way to improve the brightness of UC thin films and highlights the potential of colloidal lithography to develop bright, large-area UC coatings that will be of interest for applications in imaging, sensing, or security. Results and discussion Page 2 of 13Materials Advances Materials Advances Accepted Manuscript Open Access Article. Published on 02 November 2023. Downloaded on 11/3/2023 10:27:47 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D3MA00775H Metallic nanoparticle array coated with UCNPs. First an array of gold nanostructures is fabricated by colloidal lithography, which involves three main steps as described in the Methods section and illustrated in Figure 1a. Briefly, a colloidal mask consisting of an ordered monolayer of submicron PS spheres is prepared using a wedge evaporation technique. Convective assembly methods have proven useful in directing the assembly of nanoparticles from dilute suspensions as the dispersant is removed. In the particular case of monodisperse beads, the evaporation of the dispersant forces the nanoparticles to arrange in the meniscus formed between substrate, dispersion and air. In fact, careful design of the meniscus curvature during drying using a wedge-shaped cell allows the assembly of polymer beads into cm2-sized ordered monolayers. By controlling the humidity, temperature and concentration of the suspension, an optimal evaporation and deposition rate of the spheres is achieved, resulting in a monolayer of PS spheres arranged in a hexagonal lattice -see Figure 1a.I-. A 100 nm layer of gold is then deposited over such a monolayer by thermal evaporation. Removal of the PS spheres by a mechanical process results in a periodic array of gold nanostructures arranged in a honeycomb lattice, with nanoparticles located at the vertices of each hexagon in the lattice, as shown in Figure 1a.II. Figure 1b displays an atomic force microscope (AFM) image of the array of gold nanostructures, which allows the size and shape of the gold nanoparticles and their periodic distribution to be accurately estimated. In fact, the average height of the gold nanostructures corresponds to ~100 nm, which is consistent with the thickness of the evaporated gold layer-. Notice that a similar method can be used to fabricate large-scale arrays of silver nanoparticles. Finally, a UC phosphor thin film is deposited over the metal array by spin-coating a suspension containing NaYF NPs and poly(acrylic acid) (PAA) - see Figure 1a.III-. NaYF NPs were synthesized in an inert atmosphere, see the Methods section and the Supplementary Information (Figure S1) for details. They are uniform and show a rod-like morphology with an average size of (50±2) nm x (35±2) nm. The addition of PAA increases the viscosity of the dispersion and facilitates the processing of homogeneous coatings. Figure 1c shows scanning electron microscope (SEM) images of the gold array before and after deposition of a UCNP film with a thickness of ~100 nm. In particular, the left side of Figure 1c shows a top view of the array, while the right side displays a top view of the same array coated with a NaYF-PAA film (referred to as Au/UC). The NaYF NPs exhibit uniform coverage of the space between the metal nanoparticles (see also Figure S2). It is noteworthy that unlike the UCNP film, the gold nanoparticle array does not completely cover the glass substrate. Thus, the area of the UC film without gold nanoparticles serves as a reference (referred to as UC) in the photophysical characterization that we show next. Page 3 of 13 Materials Advances Materials Advances Accepted Manuscript Open Access Article. Published on 02 November 2023. Downloaded on 11/3/2023 10:27:47 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D3MA00775H Figure 1. (a) Schematic description of the fabrication process of the gold nanoparticle array coated with UCNPs. It includes I) the preparation of a colloidal mask made of a polystyrene sphere monolayer, II) 100 nm metal evaporation and mask removal, and III) the deposition of a UC coating. (b) Atomic force microscope (AFM) image of the gold nanoparticle array. (c) Backscattered electron (BSE) scanning electron microscope (SEM) images of the top view of gold nanoparticles without (left side) and with (right side) the UC coating. Spectroscopic characterization. We measure the ballistic transmittance of the samples, see Figure 2a. The UC coating (black long dashed line) on glass is fairly transparent, with ballistic transmittance values above ~85% in a wide spectral range between 400 and 1200 nm. The metal nanostructure array shows a reduced transmittance (~75%) (black short dashed line) with a pronounced dip between ~700 and ~1200 nm. The lowest transmittance value (~60%) is observed at ~950 nm, close to the excitation wavelength used to pump the UCNPs (980 nm, marked as a grey dashed line). In addition, the transmittance also features a shallower dip between ~500 and ~700 nm, overlapping with the green and red emission bands of Er3+ -see grey bands in Figure 1a-. The black solid line in Figure 2a represents the transmittance of the UCNP layer deposited over the metal array. The presence of a thin film alters the refractive index environment of the metal nanostructures, causing a slight shift in the features observed in the transmittance spectrum. In addition, it can be seen that the modes that are supported by the antennas determine the response of the UC film when it is deposited on them. Numerical simulations. To investigate the physical origin of the resonances, we perform finite difference time domain (FDTD) simulations under plane wave illumination. The calculated reflectance of the metal array is presented in Fig. S3. We calculate the Page 4 of 13Materials Advances Materials Advances Accepted Manuscript Open Access Article. Published on 02 November 2023. Downloaded on 11/3/2023 10:27:47 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D3MA00775H integrated intensity enhancement (IIE) for a given wavelength , as the ratio between (𝜆) the electric field intensity in the volume occupied by the UCNPs in the presence and in the absence of the gold array, using the following equation: 𝐼𝐼𝐸 ( 𝜆 ) = ∫ | 𝐸 𝐴𝑢/𝑈𝐶 | 2 ( 𝜆, 𝒓 ) | 𝐸 0,𝐴𝑢/𝑈𝐶 | 2 ( 𝜆, 𝒓 ) 𝑑 𝒓 ∫ | 𝐸 𝑟𝑒𝑓 | 2 ( 𝜆, 𝒓 ) | 𝐸 0,𝑟𝑒𝑓 | 2 ( 𝜆, 𝒓 ) 𝑑 𝒓 ( 𝐸𝑞. 1 ) Where r is a position in space, and are the electric field intensities in | 𝐸 𝐴𝑢/𝑈𝐶 | 2 | 𝐸 𝑟𝑒𝑓 | 2 the Au/UC film and the UC reference, respectively, while are | 𝐸 0,𝐴𝑢/𝑈𝐶 | 2 and | 𝐸 0,𝑟𝑒𝑓 | 2 the incident electric field intensities of the illumination source in each case, respectively. Figure 2b presents the IIE as a function of the excitation wavelength ( ). A pronounced 𝜆 𝑒𝑥 peak is observed at ~950 nm, which we attribute to the excitation of an LSPR in the gold nanoparticles.33,34 Nevertheless, the full width at half maximum at resonance for the calculation is smaller than that observed in the transmittance measurements. We ascribe this discrepancy to inhomogeneities in the size and shape of the fabricated gold nanostructures. Figure 2c displays the spatial distribution of the electric field intensity simulated at = 980 nm. A strong electric field intensity is observed around the edges 𝜆 𝑒𝑥 of the gold nanoparticles, with the field mostly concentrated at their vertices, as expected for an LSPR. The metal nanoparticles in the array allow the efficient exchange of nearfield and far-field energy at the nanoscale, acting as individual optical antennas, or simply nanoantennas. Although the calculations show hot spots of more than 100-fold field intensity, the integral over the thin film volume gives a 7-fold IIE at ~980 nm, which overlaps with the of the UCNPs. Similar behavior is observed for the electric field 𝜆 𝑒𝑥 intensity calculated at the emission wavelengths ( ) of the UCNPs, but with lower IIE 𝜆 𝑒𝑚 values (less than 2-fold). In particular, calculations show moderate IIE in the visible with ~1.5-fold in the red (~700 nm) and ~1.2-fold in the green (~540 nm) -see Fig. S4. Thus, our simulations indicate that the main contribution to the overall UCPL enhancement associated with our nanoantennas is expected to come from a resonant excitation. Figure 2. (a) Experimental transmittance of a UC coating over glass (black long dashed line), a gold nanoantenna array (black short dashed line) and a UC coating over a gold nanoantenna array (black solid line. The emission bands of NaYF UCNPs are included as grey bands. The excitation wavelength (980 nm) is also shown as a grey dashed line. (b) Calculated integrated intensity enhancement (IIE) for the UC coating over a gold Page 5 of 13 Materials Advances Materials Advances Accepted Manuscript Open Access Article. Published on 02 November 2023. Downloaded on 11/3/2023 10:27:47 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D3MA00775H nanoantenna array. (c) Simulated electric field intensity for the same system at 980 nm. Two sections are shown: the top panel displays a XY plane (z=0 nm), while the bottom panel shows an XZ plane intersecting two of the nanoantennas. UCPL enhancement assessment. To study the effect of the LSPR supported by the nanoantennas on the emission properties of UCNPs, we use a 980 nm laser, which coincides with the 2F7/2 to 2F5/2 transition of Yb3+ ions.31,47 Upon excitation, 980 nm light is absorbed by Yb3+ and energy is transferred to Er3+, resulting in green and red emission -see Fig. 3a-. These emission bands are centered at ~541 and ~655 nm, respectively, and are associated with the 4S3/2, 2H11/2, and 4F9/2 transitions to the 4I15/2 ground state of Er3+ - see Fig. S5-.48,49 In the presence of nanoantennas, the UCPL intensity is significantly improved compared to that of a reference film, up to ~10and ~20-fold for green and red emission, respectively. Metal nanoparticle-supported LSPRs can couple with the excitation and the emission of the UCNPs as discussed above, resulting in LSPRmediated UCPL enhancement at both and .25,34,50. In the low excitation power 𝜆 𝑒𝑥 𝜆 𝑒𝑚 regime, the UCPL is proportional to the square of the fraction of light absorbed by the UCNPs at .33,51 Also, in view of the simulations, according to the reciprocity 𝜆 𝑒𝑥 theorem,52 a further, albeit small, enhancement of the UCPL is expected at . As a 𝜆 𝑒𝑚 result, the UCPL enhancement is mainly due to plasmon-mediated resonant photoexcitation and is greater for the red band than for the green band. To analyze its nonlinear behavior, we monitor the UCPL intensity as a function of the excitation power for UC films deposited on a glass substrate and over the nanoantennas. Although the UCPL follows a quadratic dependence (see Fig. S6),31,47,53,54 a slight reduction of the slope is observed in the presence of the metal nanoparticles, since the LSPR accelerates the energy transfer from the 2F5/2 energy level of the Yb3+ sensitizers to the 4I11/2 of the Er3+ emitters.31 In addition, the UCPL enhancement decreases with the excitation power density. Specifically, Fig. 3b shows that at low power densities the UCPL enhancement for the red emission band reaches values as high as ~35-fold at ~5 W/cm2, but decreases to ~20-fold at ~55 W/cm2. Similar behavior is observed for the green band, but with lower factors because the LSPR-mediated UCPL enhancement is lower for the green band. Interestingly, it has been reported that Er3+ emission in the NaYF4 host matrix is not thermally stable, with UCPL intensity decreasing with temperature,9 an effect that may be exacerbated in the presence of nanoantennas due to the temperature rise induced by metal absorption, which may explain the decrease in UCPL enhancement with power. Further analysis shows that the green to red intensity ratio is also affected by the pump power.22,55–57 Although still under debate,56,57 its origin may lay in the different pathways to populate the 4F9/2 energy level of Er3+. At high power, red emission is favored, resulting in a decrease of the green/red ratio. Furthermore, for a given excitation power, the green to red intensity ratio is lower in the presence of the nanoantennas, since the LSPR-mediated enhancement is higher for the red band as discussed above (see Fig. S7 for more details). Time-dependent UCPL characterization. To gain more insight into the underlying mechanism of plasmon-mediated UCPL enhancement, we perform time-dependent Page 6 of 13Materials Advances Materials Advances Accepted Manuscript Open Access Article. Published on 02 November 2023. Downloaded on 11/3/2023 10:27:47 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D3MA00775H measurements. Using a pulsed 980 nm laser as excitation source, we monitor the green and red emission bands at 541 and 655 nm, respectively, associated with the 4S3/2 and 4F9/2 transitions to the 4I15/2 ground state of Er3+. The time dependence of the UCPL intensity can be described using the following equation, which has already been proven useful for describing the PL decay of Er3+ transitions in the NaYF4 matrix:58,59 𝐼 ( 𝑡 ) = ∫ 𝑡 ― ∞ 𝐼𝑅𝐹(𝑡 ― 𝑡 ′ )( ― 𝐴 𝑟𝑖𝑠𝑒 𝑒 ― 𝑡 ′ 𝜏 𝑟𝑖𝑠𝑒 + 𝐴 𝑑𝑒𝑐𝑎𝑦 𝑒 ― 𝑡 ′ 𝜏 𝑑𝑒𝑐𝑎𝑦 ) 𝑑𝑡 ′ ( 𝐸𝑞. 2 ) where is the time required to populate the excited energy level and corresponds 𝜏 𝑟𝑖𝑠𝑒 𝜏 𝑑𝑒𝑐𝑎𝑦 to the lifetime of the transition.56 The fitting constants Arise and Adecay must be greater than 0. Eq. 2 also includes the instrumental response function (IRF) to account for the temporal width of the pulsed laser. The fitting results are shown in Figs. 3c-d and Table 1. The residuals of the fits are displayed in Figure S8. The UC reference features a of 52 µs 𝜏 𝑟𝑖𝑠𝑒 (100 µs) to populate the 4S3/2 (4F9/2) energy level associated with the green (red) emission, respectively. The difference in rise time is due to the different ways of populating these energy levels (see Fig. S5). In fact, the green emission results from the pathway 4I15/2 → 4I11/2 → 4F7/2 → 2H11/2 → 4S3/2 → 4I15/2, while there are two possible pathways associated with the red emission, which are: 4I15/2 → 4I11/2 → 4F7/2 → 2H11/2 → 4S3/2 → 4F9/2 → 4I15/2 and 4I15/2 → 4I11/2 → 4I13/2 → 4F9/2 → 4I15/2.56,57 The is 140 µs (330 µs) for the green 𝜏 𝑑𝑒𝑐𝑎𝑦 (red) transition, in agreement with previous reports.49,59,60 The presence of the nanoantennas enables a reduction in and for both transitions, as shown in Figs. 𝜏 𝑟𝑖𝑠𝑒 𝜏 𝑑𝑒𝑐𝑎𝑦 3c-d and Table 1. Such a rate enhancement has been previously observed in similar NaYF UCNPs coupled to a layer of gold nanorods.36 Our results indicate that the rise time for the green (red) transition decreases from 52 µs (100 µs) to 37 µs (33 µs). The excited energy level is populated faster when the UCNPs are deposited over the metal nanostructures due to an enhanced Yb3+ excitation rate, which is consistent with the large IIE calculated for ~980 nm and the enhanced energy transfer between the 2F5/2 energy level of Yb3+ to the 4I11/2 of Er3+.31,61 Also, the metallic nanostructures allow a lifetime reduction.34,37,62 The reduces from 140 µs (330 µs) to 130 µs (240 µs) for the green 𝜏 𝑑𝑒𝑐𝑎𝑦 (red) transition -see Table 1-, associated with the fast decay channels typically introduced by metals. The lifetime reduction along with the larger UCPL intensity observed indicate an enhanced radiative decay rate mediated by the coupling between UCNPs and nanoantennas. Finally, we would like to mention that we have extended our study to Ag nanostructures and obtained similar results (see Fig. S9). We observe an enhancement of the UCPL as a result of the coupling between UC emission and plasmon resonances supported by either Au or Ag nanoantennas. Ag nanostructures support LSPRs that overlap with both the excitation and emission bands of UCNPs. As a result, the UCPL enhancement reaches values up to ~ 50 (80) times for high (low) power density for the red emission band, higher than that observed for Au due to a more effective excitation enhancement. Our results highlight the potential of colloidal lithography to fabricate metallic nanostructures and develop bright, large-area UC coatings in a simple and costeffective manner. Page 7 of 13 Materials Advances Materials Advances Accepted Manuscript Open Access Article. Published on 02 November 2023. Downloaded on 11/3/2023 10:27:47 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D3MA00775H Figure 3. (a) UCPL intensity spectra of a reference film (black solid line) and that of the same film over a gold nanoantenna array (grey dashed line) at 53.2 W/cm2 excitation. UCPL intensity is normalized to that of the reference film at 655 nm. (b) UCPL enhancement of green (integrated between 510-575 nm) and red (integrated between 630-695 nm) bands at different excitation powers. A continuous 980 nm laser is used for excitation. (c-d) UCPL decay of the reference film (circle symbols) and that of the same film over a gold nanoantenna array (square symbols) monitored at 541 (c) and 655 nm (d). Solid lines are the fits. The instrumental response function is included as a grey dashed line. A pulse 980 nm laser is used for excitation. Samples λ (nm) τrise (µs) τdecay (µs) UC reference 52.0 140 Au/UC 541 36.9 130 UC reference 100 330 Au/UC 655 32.8 240 Table 1. Fitting parameters obtained from the UCPL curves shown in Figures 3c and 3d. Conclusions We have demonstrated a simple and inexpensive method, based on colloidal lithography, for fabricating a nanoantenna array to enhance the emission of upconverting phosphor nanoparticles. By carefully choosing the shape and size of gold nanostructures, it is possible to engineer LSPRs to match both the excitation and emission bands of NaYF4:Yb3+,Er3+ nanoparticles. At resonance, the electric field intensity in the volume surrounding the metal is significantly increased, which greatly enhances the absorption and emission of nearby UCNPs. As a result, we demonstrate a 20and 35-fold UCPL Page 8 of 13Materials Advances Materials Advances Accepted Manuscript Open Access Article. Published on 02 November 2023. Downloaded on 11/3/2023 10:27:47 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D3MA00775H