Remarkable Enhancement of Upconversion Luminescence on Cap

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Remarkable enhancement of upconversion luminescence on CapAg/PMMA ordered platform and trademark anti-counterfeiting He Wang, Mengchao Li, Ze Yin, Tianxiang Zhang, Xu Chen, Donglei Zhou, Jinyang Zhu, Wen Xu, Hai-Ning Cui, and Hongwei Song ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10015 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 2, 2017

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ACS Applied Materials & Interfaces

Remarkable enhancement of upconversion luminescence on Cap-Ag/PMMA ordered platform and trademark anti-counterfeiting

He Wang†,‡, Mengchao Li†,‡, Ze Yin†, Tianxiang Zhang†, Xu Chen†, Donglei Zhou†, Jinyang Zhu†,‡, Wen Xu†, Haining Cui*,‡,,Hongwei Song*,†

AUTHOR ADRESS †State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, P. R. China. E-mail: [email protected] ‡College of Physics, Jilin University, 2699 Qianjin Street, Changchun 130012, China E-mail: [email protected]

KEYWORDS Upconversion, surface plasmon, periodic silver, photonic crystal, nanoparticles, dielectric

ACS Paragon Plus Environment

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ABSTRACT

High brightness of upconversion luminescence (UCL) for thinner layer of upconversion nanoparticles (UCNPs) is significant for routine and applications of effective trademark anticounterfeiting technology. In this work, efficient UCL of NaYF4:Yb3+,Er3+/Tm3+, was realized by combining a Ta2O5 dielectric layer on the cyclical island silver films supported by PMMA opal photonic crystals (OPCs). The synergistic modulation of localized surface plasmon resonance (LSPR) and photonic crystal effect results in the significant improvement of local electromagnetic field and optimum upconversion enhancement of hundreds folds. Furthermore, colorful pattern nano-printing has been applied to this composite and used for trademark anticounterfeiting. The combination of angle-dependent photonic crystal effect and infrared to visible UCL represents a more advanced anti-counterfeiting technique.

1.

INTRODUCTION In recent years, upconversion materials, which can transform multiple low energy photons

to a single high energy photon through an anti-Stokes process, have attracted a great deal of attention due to their unique photometric characteristics and application potentials in the field of biosensing, biological imaging, infrared photodynamical therapeutics, solar cells and etc.1-8 Among various upconversion phosphors, the relatively effective rare earth fluoride inorganic compounds like NaREF4 are widely applied. So far, NaYF4 stands out from numerous host matrixes for green and red luminescence by codoping of Yb3+, Er3+ ions, which arises from relatively efficient excitation of Yb3+, energy transfer from Yb3+ to Er3+ and emissions of Er3+. However, their low quantum efficiency and small excitation cross-section limited their applications, especially for upconversion nanoparticles (UCNPs). Therefore, various methods


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have been adopted to boost the luminescent intensity/efficiency of UCNPs, including changing the host lattices/dopants, designing core-shell structure, or coupling with noble metal nanostructures etc.9-12 Among various external approaches, the combination of photonic crystal effect and metal’s surface plasmon effect has been proved to be a efficient tool to boost the excitation light capture and UCL extracting, and it can enhance upconversion luminescence (UCL) to three-orders.13,14 Polymethyl methacrylate photonic crystals (PCs) could also modulate either the emitter wavelength or emission wavelength to enhance the electromagnetic field through far-field coupling.15-19 In addition, it also provides a cyclical island structure on its surface. Otherwise, the electromagnetic enhancement is usually prompted by surface plasmon resonance (SPR), and plasmonic modulation of noble metal nanostructures (MNPs) is intensively investigated owing to their structure-dependent chemical properties and applications. Generally, plasmonic nanostructures can increase the radiative recombination rate of emitters through near-field coupling. The periodic cap metallic nanostructures can transform the construction into a more efficient platform.20,21 In this work, we represent a simple and effective way to significantly enhance UCL of UCNPs by utilizing the dielectric layer on the cyclical island silver films supported by periodic PC structure. The PMMA PCs/Cap-Ag/Ta2O5/NaYF4:Yb3+, Er3+ (PMIU) laminated films are designed to obtain enhanced UCL. And, the detailed experimental and theoretical analysis demonstrated that the array platform has outstanding optical and photonic performances compared to amorphous silver nanofilm without array structure. The hybrid structure provides an excellent platform to enhance the UCL of RE-doped UCNPs, through using semiconductor oxide as isolation layer, the cap noble metal as plasma resonance layer, and PCs as excitation field


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coupling and template layer. Consequently, the hybrids with ultrastrong UCL are constructed by nano-printing technique on flexible substrates and used for infrared anti-counterfeiting. 2.

Experimental The Cap-Ag film was made by the mask template method. Firstly, monodispersed polymethyl

methacrylate latex spheres (PMMA) of different sizes were synthesized.22 Secondly, ultra-thin Cap-Ag films were deposited by thermal evaporation on the surface of PCs. The deposition process was performed at room temperature (~25°C) and high vacuum (~8×10-4Pa). Ag films were deposited by a tungsten boat using silver particles, and the deposited rate was controlled at 0.15Å/s. Thirdly, the isolation layer-Ta2O5 film was deposited on PCs/Cap-Ag substrates using physical vapor deposition (PVD) magnetron sputtering technique by using a pure Ta target (99.99%) with 250mm distance from the substrate. It should be noted that pre-sputtering was adopted to remove the oxide layer on the surface of Ta target in an argon atmosphere. Then, the vacuum degree of the chamber was less than 5×10-4 Torr. The flow rate of argon gas and oxygen gas and alternating current (AC) power were 22.5sccm, 2.5sccm and 100W, respectively. Using a typical solvothermal procedure, NaYF4: 20% Yb3+, 2% Er3+ plate-like nanocrystals were prepared.23 Finally, the pattern was printed where the blue fluorescent nanoparticles were NaYF4: 20% Yb3+, 0.2% Tm3+, and the red ones were NaYF4: 80% Yb3+, 2% Er3+. Under the action of capillary force of the liquid, the NaYF4:Yb3+, Er3+ UCNPs were self-organized on the surface of Cap-Ag or glass substrates in the evaporation process. 3. RESULTS AND DISCUSSION 3.1 Characterization of PMIU


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Schematic illustration structure of the PMIU is shown in Scheme 1(a), which consists of PMMA opal photonic crystals (OPCs), Ag caps layer, Ta2O5 isolate layer and NaYF4: Yb3+, Er3+ layer in turn. They were prepared by the self-assembly of PMMA spheres, vacuum evaporation, magnetic sputtering, and solvent self-assembly of the UCNPs, respectively, as is illustrated in Scheme 1(b). Here, the reflection light path was used to measure the UCL of the UCNPs, which was shown in Scheme 1(c), and the optical circuit remained unchanged in the measurements.

Scheme 1(a) Schematic illustration of the PMIU. (b) Schematic illustration of the fabrication process of PMIU composite film. (c) Schematic of the UCL reflection measurement illuminating with 980 nm. The SEM images record the morphology of PMMA PCs and PMMA PCs/Cap-Ag (Fig. 1(af)). The PCs are densely packed in cubic close packing (FCC) with 111 planes paralleling to the basement.24 The Ag films germinate on the surface of the opal template and change with its ups


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and downs to form and shape bonnet Ag structure. It can be found that all Ag films are bumpy and inerratic (see Fig. 1(b)-(f)). The morphology of silver is different with the change of Ag thickness, and relies on the species of substrates.23, 24As the deposited thickness of Ag from 2nm to 20nm, clear small silver particles are observed to adhere to the undulating surface of PCs and discontinuous cap structure occurs. While the deposited thickness of Ag increases to 40nm, the cap-Ag film changes to a continuous undulating structure. When further increasing thickness of Ag, the top of the silver film becomes flat and level. It should be highlighted that all the hybrids films of NaYF4 and PMIU film show the similar crystal structure, and the photonic stop band (PSB) of all the PCs locate around 990 nm, as shown in Fig. 1(d).As we can see from Fig. 1(g)(h), the UCNPs are covered on the surface of PCs/Cap-Ag/Ta2O5 (PMI) film, and no particles fall below. As a dielectric layer, Ta2O5 layer plays a role in supporting UCNPs and isolates the direct contact between Cap-Ag and UCNPs, which can suppress the nonradiative transitions from UCNPs to metal nanostructure due to the possible energy transfer.

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The reason for

choosing Ta2O5 as isolation layer is its high transmittance and its refractive index (2.1) being close to the refractive index of NaYF4 (1.95) among the common optical thin films, which can suppress total reflection from the spacer to NaYF4layer. 29


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Fig. 1(a) SEM image of PMMA PCs, (b-f)SEM images of PCs/Cap-Ag with thickness of 2-55 nm: (b) 2nm, (c) 7nm, (d) 20nm, (e) 40nm, (f) 55nm. (g-h) SEM images of PMIU and the corresponding cross-section image, The excitation spectra (i) and diffuse reflection spectra (j)of PCs, dense Ag film and PCs/Cap-Ag films. (k) The scatter to absorption ratio of above samples. The extinction and the diffuse reflectance, and the scatter to absorption ratio were measured as shown in Fig. 1(i-j). After being deposited with silver, the central locations of the extinction spectra rarely change. (see the Fig. 1(i)) The extinction value increases with the increasing thickness of Ag films, and PSB of PCs/ Cap-Ag films becomes shallow. The PSB disappears while it forms a continuous film with the increasing thickness of Ag. In other words, the increase of the thickness leads to the continuity of the silver structure, which makes the photonic crystals lose control of light and results in the reduction of the amplification factor of the upconversion material by the long-range modulation of PCs to the optical field. The rule of discontinuous noble metal nano film is beneficial to the increase of upconversion intensity. 30 The diffuse reflectance spectra were tested, as shown in Fig. 1(j). When the Ag thickness is only 2nm, the sample surface morphology presents the aggregation of silver particles, diffuse


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reflection peak at 400nm. With the increase of thickness, the silver particles gather further to form lager silver grains, and the diffuse reflection appears a red shift. It begins to form cap shape silver structure which the diffuse reflection appears two peaks as the Ag thickness reaches 20nm. The two peaks are 374 nm (Cap-Ag) and 432nm (Ag particles), respectively. The silver cap scattering peak strengthens with the further increase of the thickness, while the surface begins to form a continuous silver cap structure, the PCs located in the bottom would be no control of light. Previous studies have shown that large scattering and small extinction are beneficial to the fluorescence enhancement of nanophosphors, because scattering of metal is helpful of radiative transitions of surrounding emitters, while extinction of metal leads to the improvement of nonradiative transitions.31 Fig. 1(k) shows the silver scatter to absorption ratio versus the Ag thickness. It can be concluded that the strongest scatter to absorption ratio occurs with 20nm Cap-Ag thickness. 3.2 UCL enhancement of the PMIU film To better understand the enhancement of UCL for the PMIU hybrid film, the schematic of UC populating and emission processes of NaYF4: Yb3+, Er3+ UCNPs are proposed under 980 nm excitation (Fig. 2(a)).

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Firstly, the electrons of Yb ions are excited from 2F7/2 to 2F5/2 level

illuminating with 980 nm, which can pump Er ions from 4I15/2 to 4I11/2 level due to the first-step energy transfer (ET) process. A part of electrons on 4I11/2 will nonradiatively relax to 4I13/2 on the surface of UCNPs. Then, another electron absorbed by the adjacent Yb ions can excite Er3+ ions from 4I11/2 to 4F7/2 or from 4I13/2 to 4F9/2 by the second-step ET process. And then, the Er3+ ions on 4

F7/2 relax non-radiatively to 2H11/2 and 4S3/2 level, generating green-emissions of 2H11/2, 4S3/2-

4

I15/2, while the Er3+ ionson4F9/2 radiate to the ground state, generating 4F9/2-4I15/2 transitions. It


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should be noted that both 2H11/2, 4S3/2-4I15/2 and 4F9/2-4I15/2 transitions originate from the twophoton process.32

Fig. 2(a) Schematic of upconversion populating and emission processes for NaYF4:Yb3+, Er3+. (b) The UCL spectra of NaYF4:Yb3+, Er3+ (~10nm) and PMIU films with the excitation power of 73 W/cm2. (c) The enhancement factor of PMIUs films versus Cap-Ag thickness. (d) The upconversion enhancement factor versus photonic stop band of PMIU films. (e) EF versus UCNPs thickness and size. (f) The enhancement factor versus excitation power density of PMIU film under 980-nm excitation. (g) Decay time constants of UCNPs in glass, dense Ag and PMI films. The emission spectra of NaYF4: Yb3+, Er3+ nanofilm and PMIU nanofilm are recorded in Fig. 2(b). It indicates that in the PMIU hybrid, the emission intensity of Er3+ is highly improved in


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comparison to that directly deposited on the glass. Next, the upconversion enhancement factor (EF) was calculated by the ratio of upconversion fluorescence intensity of UCNPs on some substrates (such as: dense Ag, or PMI) to the corresponding signals of UCNPs on a reference glass substrate according to the following formula:

EF=

(1)

where IPL and IRef are PL intensities. Actually, EF is strongly dependent on the thickness of Ag, the location of PSB of PCs, and the thickness/particle size of NaYF4: Yb3+, Er3+(See Fig. 2(c-e)). Here, the definition of the enhancement factor is the intensity ratio of Er3+ emissions of the PMIU hybrids relative to NaYF4: Yb3+, Er3+ film under the same experimental conditions (the excitation power density is fixed at 73W/cm2 all the time). The results in Fig. 2(c) show that the composite films with regular Cap-Ag structure have stronger magnification than dense silver film. The EF increases firstly, then decreases quickly as the thickness of Cap-Ag film increases and the optimum thickness is 20nm. The highest EF is 145 fold as the thickness of Ag is 20 nm. This is in consistent with the result of extinction to absorption ratio, originating from the special morphology of the composite structure. It should be noted that in the experimental above the NaYF4 thickness for all the samples was fixed at 150nm. The EFs varied with the shop band location of PCs shown in Fig. 2(d). The stop band was controlled by adjusting the size of photonic crystal sphere in this experiment. (see Fig. S1) In contrast to the continuous silver film, the EF of PMIU nanofilm increases considerably, depending on the location of PSB.(see Fig. S2) When the PSB is coupled with the excited light (980nm), optimum EF is obtained, to be 145 fold (the thickness of NaYF4 is 150nm, and the thickness of Cap-Ag is 20nm). As the stop band of PCs is gradually far from the wavelength of excitation light, the EF has a significant reduction.18


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This result definitely demonstrates that excitation field enhancement may be the dominant mechanism for the enhancement of UCL, which will be proved further by the measurement of luminescent dynamics and finite difference time domain (FDTD) calculation on local electromagnetic field distribution. Fig. 2(e) displays that the EF decreases exponentially with the increase of the thickness of NaYF4: Yb3+, Er3+ layer as well as the original size of the UCNPs. Interestingly, a ～231 fold enhancement is obtained under 73 W/cm2 980 nm excitation light when the thickness of UCNPs is controlled at 8nm (see AFM image shown in Fig. S3(a)). The result of thickness-dependent EF is very reasonable, because the effective interaction distance of surface plasmon resonance is within several ten nanometer to hundreds nanometers. Improved thickness of NaYF4: Yb3+, Er3+ will gradually decrease the interaction of metal NPs to the UCNPs.33 As to the size-dependent behavior of NaYF4: Yb3+, Er3+UCNPs, may be attributed to improved dispersion properties with the decreasing particle size of NaYF4: Yb3+, Er3+, which leads to sufficient approach between the UCNPs and the Ag/PMMA PCs hybrid. It should be noted that according to previous literatures, the size change of the UCNPs consisting of the NaYF4: Yb3+, Er3+ layer might also induce the thickness change, affecting the enhancement result of UCL. According to the Beer–Lambert law, we measure the thickness of the films to characterize the relative thickness of these UCNPs on different substrates (Seeing supporting information). It can be obtained that the relative thickness of NaYF4 in different prepared samples is almost the same, as listed in Table 1. And, the thickness of UCNPs films with different sizes can be adjusted by controlling the number of UCNPs in precursor solution.31 Table 1Relative thickness of the NaYF4 layer prepared with different sizes of NaYF4 UCNPs on the glass substrate (D1) and PMI (D2) and the ratio of D2/D1. Here the thickness of the 10 nm NaYF4 film was normalized


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10 (nm)

21 (nm)

29 (nm)

40 (nm)

55 (nm)

D1 (NaYF4 film)

1

0.993

1.002

0.997

0.997

D2 (PMIU)

1.051

1.026

1.075

1.024

1.029

D2/D1

1.051

1.033

1.073

1.027

1.032

Actually, the enhancement factor strongly depends on the power density of excitation light as well as the emission wavelength, as shown in Fig. 2(f). The enhancement factor decreases as the excitation power increases for all transitions and the 145-fold optimum enhancement factor of visible UCL is obtained under 980nm excitation (73W/cm2 excitation power density), owing to the enhanced thermal effect and saturation effect induced by illumination of lights. Indeed, the surface temperature of the sample increases when the excitation power increases. (Fig. S5(c)) The local thermal effect is obvious and nonradiative transition is enhanced. Thus, the radiation transition is suppressed and the amplification factor decreases. In order to clarify the fact above better, Fig. S5(a-b) record the excitation power dependence of the emission intensity of UCNPs. Because of the saturation effect, the slopes in ln-ln plot (IUCL∝Pn) are less than 2, originating from the competition between linear decay and upconversion processes for the depletion of the intermediate excited states.31 It can be concluded that the saturation effect in the PMIU is easier to happen than it in the UCNPs films, which also leads enhancement factor to decrease as the excitation power density increases. The essential result is caused by the improved local excitation electromagnetic field modulated by Cap-Ag and PCs effect. In general, the interaction between the local surface plasmon resonance and excitation electromagnetic field leads to the enhancement of local excitation field strength.28 To further demonstrate this, the decay dynamics in different samples are measured by 980nm excitation


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light, as shown in Fig. 2(g) and more detailed decay curves of UCNPs on different substrates were presented in Fig. S6. It can be concluded that the variation for the 4S3/2-4I15/2 and 4F9/2-4I15/2 of Er3+ decay time constants for the pure NaYF4: Yb3+, Er3+ film and PMIU hybrids film(990 nm stop band) is less than 13%. This indicates that the considerable enhancement is mainly originated from the interaction between SPR or OPCs and excitation electromagnetic field, rather than emission electromagnetic field. In general, the luminescence quenching would happen through the non-radiative energy transfer from luminescence centers to Cap-Ag, which causes a decrease in the total decay rate. As is well known, this process is efficient within~10 nm, while the NaYF4 thickness is much larger than the non-radiative energy transfer distance. Furthermore, the detected total decay rate is an average result of all NaYF4, so it is reasonable that the small variation of total decay rate is obtained. 3.3 Local electromagnetic field distribution of PCs/Cap-Ag film Considering the effective distance of precious metal plasma, the local electromagnetic distribution based on FDTD is calculated, as shown in Fig.3. In the calculation, the dimension for computation is set to 10×10 array in x-y plane, and 10 layers in a plane. Actually, we carried on the corresponding layer simulation on reflectance spectra of 10-30 layers OPCs and observed that the location and depth of stop band for the OPCs had no obvious change when the OPCs surpassed 10 layers (see Fig. S7). In addition, an unpolarized plane wave was introduced normally to the plane with the boundary condition of the perfectly matched layer (PML) in a 3D box, as shown in Fig. 3(a).


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Fig. 3(a) 3D model for the 3D FDTD simulation. (b-f) Simulated local electromagnetic field distribution of PCs/Cap-Ag with different Ag thickness under 980nm incident light excitation. (b) 2 nm, (c) 7 nm, (d) 20 nm, (e) 40 nm, (f) 55 nm. Here, |E|2/|E0|2 is defined as the enhancement of local electromagnetic field, where |E| and |E0| represent the amplification of the local electromagnetic field and the incident electromagnetic field, respectively. As shown in the figure, the SPR effect of silver nanoparticles leads to a strong and highly localized electric field around silver nanoparticles, inducing the enhancement of the excitation field.34 In the former section, we experimentally studied the influence of silver thickness on upconversion enhancement. Here, we targeted the theoretical calculation with FDTD (Fig. 3(b-f)).It can be concluded that the electromagnetic field increases firstly as the thickness of silver increases on the surface of PC. And then, the electromagnetic field decreases


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further with increase of the Ag thickness. In addition, the local electromagnetic field varies largely at different spots. The hot spots usually locate surrounding the gaps of the opal photonic crystals for all the samples. At hot spots, the maximum field amplification factor approaches to 2.3×103 fold when the thickness of Ag is 20nm. The reason for this result is that the different thickness of Ag has diverse surface structures, causing disparate material scattering and absorption. Then, it leads amplification of light field is discrepant. In order to observe the amplification of the field more intuitively and compare with the experimental results, we calculated the average amplification of the electromagnetic field in the region of the image (|E| ⁄|E | ). Optimally, the average electromagnetic field increases about 13.71 folds when the thickness of Ag is 20nm. Considering a two-photon UC emission, the upconversion intensity can increase at most 13.712 folds (187.96 folds) owing to the amplification of excitation field. The results of theoretical calculation are in consistent with the results in Fig. 2(c).


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Fig. 4 Simulated local electromagnetic field distribution versus the photonic stop band of PCs under 980nm incident light excitation(the thickness of Ag is fixed at 20nm, and the stop band is at (a) 544nm, (b) 660nm, (c) 760nm, (d) 823nm, (e) 980nm). (f) Dependence of average electromagnetic field of the samples on photonic stop band. Next, simulated local electromagnetic field distribution of PMI with different stop band of PCs is shown in Fig. 4(a-e). It is obvious that various photonic crystals have different amplification effects on 980nm incident light. The composite film with photonic stop band of 980 nm has the largest amplification effect. The shorter stop band is, the smaller the amplification of light field is. This is also consistent with the experimental results above (see Fig. 2(d)). In addition, the maximum field of electromagnetic amplification of the material locates always between two silver hats, regardless of the wavelength of photonic stop band.


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Fig. 5(a-i) Simulated local electromagnetic field distribution of PCs/Cap-Ag (20nm) along the zdirection above Cap-Ag. The distances are (a) 0, (b) 10 nm, (c) 20 nm, (d) 30 nm, (e) 40 nm, (f) 50 nm, (g) 60 nm, (h) 70 nm, (i) 80 nm. (j) Theoretical simulation of field amplification versus the distance of UCNPs and Cap-Ag, the distance variable is the distance between the center of Ag curved surface and UCNPs. Incident: Experimental measurement (EF) of enhancement factor results versus the thickness of Ta2O5 layer (T-Ta2O5). It is clearly visible from the diagram that the enhancement of E-field depends on the change of the z-direction for PMIU films. Combined with the result provided in the Fig. 5(a-i), therefore, it can be considered that the electromagnetic enhancement increases firstly then decreases with the increase of operating distance-z, and the best range is between about5-15 nm.35,36 In order to make it more intuitive, the magnification bar is unified to 200 times. It can be observed that the aperture radius is 201nm, which is the same as the radius of PCs. The location of the aperture is between cap silver and cap silver which are not connected, where maximum action of the field amplification is the noble metal action gap. It indicates that the maximum electromagnetic


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enhancement locates in the plane where the distance above the upper surface of PCs/Cap-Ag is 10nm. The reason for this result is the effective surface plasmon localized effect, and the effective distance is usually the order of magnitude of nanometers. With the increase of the operating distance, the amplification of the surface plasmon field is exponentially attenuated. The quenching effect would appear when the action distance is too close. Fig. 5(j) indicates that after intense attenuation in optical magnification, a state of shock appears. This phenomenon is caused by light field interference into opal photonic crystals with silver cap layer and the oscillation cycle is 500 nm. Actually, we also experimentally studied the upconversion enhancement in the PMIU films by adjust the isolation layer (Ta2O5) thickness, as shown in the inset. We only observed the exponential decrease of EF with the distance, but did not observe the oscillation of enhancement factor. The reason may be that the distance of the luminescent centers to the PCs/Cap-Ag film varies greatly, leading to the oscillation of luminescent centers with different distances offsets each other. 3.4 Anti-counterfeiting with PMIU composite film In this paper, we applied nano-printing technique to print patterns of UCNPs based on the enlargement of Ag Cap/PMMA PCs hybrids. First we transferred the prepared PCs thin films onto the PET substrate, and then prepared the subsequent films on this substrate. The transfer process was to tilt the glass film into an aqueous solution at a certain angle and extremely slowly teared the photonic crystal film down from the glass substrate through the tension of the water surface. Then, transferred it down and carried on with the PET film very slowly. The structural test of the flexible substrate was first conducted (see Fig. S9). The result indicates that after bending 50 times, the ordered structure has not been destroyed, and the transmission properties of the films nearly reserved as a constant. Finally, the 3D printing results obtained are presented


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in the Fig. 6(a)-(b), which is a direct imaging of the UCNPs on a flexible substrate and the luminescent imaging of the composite film under the pumping of 980nm excitation light (10W/cm2). As can be seen from the Fig. 6(a), the mosaic pattern on the UCNPs was printed by diluted black ink, which demonstrated various colors as the hybrid was bended, which is a typical feature of three-dimensional PCs. The luminescent patterns in different regions in Fig. S10 demonstrate different colors, blue, green and red. Noted that the green patterns were prepared by NaYF4: 20% Yb3+, 2% Er3+, the red ones were NaYF4: 80% Yb3+, 2% Er3+ and the blue ones were NaYF4: 20% Yb3+, 0.2% Tm3+. Among them, the UCL intensity of red light was enhanced 151 folds, the green light was enhanced 135 folds, while the blue light was enhanced 205 folds. (NaYF4: 20% Yb3+, 0.2% Tm3+, see Fig. S11). It should be also highlighted that in the same experimental conditions, the EF for NaYF4: Yb3+, Tm3+ is larger than that for NaYF4: Yb3+, Er3+, because the blue emission for Tm3+ is a three-photon transition process, while the green and red emission for Er3+ is a two-photon transition process.33 In addition, the time stability was measured in Fig. S12. As can be seen from the results, upconversion fluorescence intensity of nanoparticles composite films has no change. And, this technology was also applied to clothes icon anti-counterfeiting, as shown in Fig. 6(b). The trademark pattern in the picture can be clearly observed and the image has obvious photonic crystal color in different observation angles under the day light.


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Fig. 6 Digital photo of trademark figure under the daylight. (a) Curving mosaic pattern on the PMIU/PET film. (b) Clothes icon anti-counterfeiting. 4. CONCLUSIONS Overall, PMIU hybrid films (PMMA PCs/Cap-Ag/Ta2O5/NaYF4:Yb3+,Er3+) were fabricated by utilizing the dielectric layer on the cyclical island silver films supported by periodic OPC structure. The hybrids demonstrate an optimum upconversion enhancement of hundred folds for the overall intensity of Er3+. This designed platform relies on the synergetic interaction of the localized surface plasmon resonance of noble metals and photon localization effect of the PMMA opal photonic crystals. It is the excitation electromagnetics field enhancement dominating the considerable luminescent enhancement, which is supported by FDTD calculations and the luminescent dynamics. And more, this complex hybrid with ultrastrong UCL was easily constructed on the flexible substrate through nano-printing ink technique and successfully applied to multi-color luminescent imaging anti-countering under the irradiation of infrared light. This work demonstrates that the combination of LSPR effect and PC effect is a powerful technique to enhance local electromagnetic field, and has important application in technology security.


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SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website. Detailed experimental procedures, characterization, as well as the supporting data. AUTHOR CONTRIBUTIONS He Wang conducted the preparation and characterization of the samples and wrote this paper. Haining Cui gave valuable suggestions throughout this work. Hongwei Song supervised the project, and gave precious advices on the experimental selection and the analysis of result about preparation of materials, and revised the manuscript. Wen Xu gave the advices on the experimental scheme, Mengchao Li, Ze Yin, Tianxiang Zhang, Donglei Zhou and Jinyang Zhu undertook the characteristics of some samples. All the results were discussed and commented by all authors on the manuscript at all stages. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Haining Cui) *E-mail: [email protected] (Hongwei Song) Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by the National Key Research and Development Program (2016YFC0207101),the Major State Basic Research Development Program of China (973 Program) (NO.2014CB643506,2014CB921302), the National Natural Science Foundation of China (Grant NO. 11374127,11674126, 11674127, 11504131, 61674067),the Jilin Province Natural Science Foundation of China (NO. 20150520090JH, 20170101170JC), the Jilin Province Science Fund for Excellent Young Scholars (No. 20170520129JH, 20170520111JH), and the Challenge Cup Fund of Jilin University (No. 450060497067).

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TOC GRAPHICS


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Remarkable Enhancement of Upconversion Luminescence on Cap

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