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Fluorescence modulation of graphene quantum dots near structured silver nanofilms Weon-Sik Chae, Jungheum Yun, Sang-Hyeon Nam, Sang-Geul Lee, Won-Geun Yang, Hyewon Yoon, Minsu Park, and Seokwoo Jeon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19524 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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ACS Applied Materials & Interfaces

Fluorescence modulation of graphene quantum dots near structured silver nanofilms Weon-Sik Chae,†,* Jungheum Yun,‡ Sang-Hyeon Nam,§ Sang-Geul Lee,† Won-Geun Yang,† Hyewon Yoon,§ Minsu Park,§ Seokwoo Jeon§,* † Daegu

Center, Korea Basic Science Institute, Daegu 41566, Republic of Korea

‡

Surface Technology Division Korea Institute of Materials Science, Changwon, Gyeongnam 51508, Republic of Korea

§ Department

of Materials Science and Engineering, KAIST Institute for the Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, South Korea

KEYWORDS. Silver nanofilms, Graphene quantum dot, Fluorescence enhancement, Fluorescence lifetime, Surface plasmon ABSTRACT. Here, we study the plasmonic metal-enhanced fluorescence properties of blueemitting graphene quantum dots (GQD) and green-emitting graphene oxide quantum dots (GOQD) using fluorescence lifetime imaging microscopy. Reactive ion sputtered silver (Ag) on zinc oxide (ZnO) thin films deposited on silicon (Si) wafers are used as the substrates. The morphology of the sputtered Ag gradually changes from nanoislands, via and elongated network and a continuous film with nanoholes, to a continuous film with increasing sputtering time. The fluorescence properties of GQD and GOQD on the Ag are modulated in terms of the intensities and lifetimes as the morphology of the Ag layers changes. Although both GQD and GOQD show similar fluorescence modulation on the Ag nanofilms, the fluorescence of GQD is enhanced, while that of GOQD is quenched due to the charge transfer process from GOQD to ZnO. Moreover, the GQD and GOQD exhibit different fluorescence lifetimes due to the effect of their electronic configurations. The theoretical calculation explains that the fluorescence amplification on the Ag nanofilms can largely be attributed to the enhanced absorption mechanism arising from accumulated optical fields around nanogaps and nanovoids in the Ag nanofilms.

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1. Introduction As light irradiates a metal surface, the collective free electron oscillation (i.e., surface plasmon) that occurs on the metal surface is known to be a quantized phenomenon and can induce unique optical property for the adjacent molecular system.1,2 When the metal has a dimension approaching the nanometer scale, the surface plasmon oscillation occupies a major portion of the optical transition in the visible region depending on the size, shape, and composition.3 The speed of light is slowed down on a metal surface with a high dielectric constant. An electromagnetic field can additionally be accumulated on a roughened surface.4,5 The density of the optical field can affect the absorption transition of adjacent molecules, typically increasing absorption and the scattering probability.6−8 By now, plasmon metal nanomaterials with many different shapes and morphologies

have

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synthesized

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fabricated

using

solution

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evaporation,13−15 lithography,16,17 and templating routes.18−20 In particular, plasmon metals with patterned surfaces, sharp tips, and nanopores have shown extraordinarily high optical field accumulation on their surfaces.6,21−24 Compared to surface-enhanced Raman scattering (SERS), with a long history of more than thirty years, metal-enhanced fluorescence (MEF) studies have recently attracted much attention in basic science and multidisciplinary research areas.25−29 In the early stage, Geddes et al. reported the MEF property on Ag island films, with outstanding fluorescence enhancement.30−32 Recently, MEF studies have been conducted with more deliberately fabricated metal materials, such as nanodot arrays,33,34 nanowires,28,29 nanohole arrays,35−37 and nanoporous materials.38,39 Since fluorescence spectroscopy can detect molecules with single molecular level sensitivity, many recent studies have focused on applying fluorescence imaging and super-high-sensitive detection, applications as well as fundamental MEF physics. Currently, special fluorescent


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molecules have been designed and used as probe dyes for specific chemical binding for target cell imaging.40 Interestingly, carbon nanoparticles and graphene quantum dots (GQD) have attracted increasing attention due to the unique advantages. GQDs are known to have low toxicity, reasonable emission yield, and good biocompatibility, which make them useful in the ranged fields of applications including cell imaging.41−43 Recently, several groups have reported notable results on stem cell and deep-tissue imaging using GQD.44,45 However, the emission quantum yield (QY) of GQD is still as low as approximately ~10%41,46−48 while some carbon nanoparticles showed above 50% QY, with a controversial issue regarding photostability.49 Therefore, it is obviously meaningful to enhance and to control the fluorescence of GQD by utilizing the MEF phenomenon. In this study, we examine the metal-induced fluorescence properties of GQD on structured Ag nanofilms by employing two types of GQD (GQD and GOQD) using time- and space-resolved fluorescence confocal microscopy. As the nanomorphology of the Ag films gradually changes, the fluorescence properties are interestingly modulated both in the intensities and in the lifetimes. In the following sections, we carefully inspect and explain the observed unique near-field fluorescence behavior in terms of the electronic states of the GQD and the optical fields of the Ag nanofilms.

2. Experimental details 2.1. Fabrication of Ag nanofilms. Ag was deposited on 20-nm-thick ZnO-coated Si wafers to fabricate high-quality uniform nanofilm morphologies, and the ZnO film was deposited using a ZnO target (Williams Advanced Materials Inc.) of 99.999% purity on a Si wafer prior to the Ag deposition.13 The detailed deposition conditions of internal pressure, radio frequency power, and


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target distance were well described in the previous literature.13 Ag nanofilms of different thicknesses (1−12 nm) were deposited on the ZnO films in the same system via a dc reactive sputtering process with a dc power of 50 W (0.13 W cm−2 ) using a 4 in. Ag target (Williams Advanced Materials Inc.).13 The Ag deposition was carried out at 3 mTorr by supplying pure Ar gas at a flow rate of 45 sccm. 2.2. Synthesis of GQD and GOQD. GQD and GOQD were fabricated from graphite intercalation compounds (GICs) and graphite oxides (GOs), respectively, via previously reported methods.50–52 To prepare GQD from GICs, 20 mg of graphite powder and 300 mg of potassium sodium tartrate tetrahydrate (KNaC4H4O6•4H2O) were mixed and ground using a pestle. The mixture was heated at 250°C in a heating mantle overnight to form GICs and cooled down. The GICs were then exfoliated in 30 mL of water with ultrasonication for 3 h to produce GQD. The as-prepared GQD was then centrifuged using 10,000 MWCO (molecular weight cut off) centrifugal microfilters to obtain homogeneously dispersed GQD of < 5 nm in size. Finally, the GQD aqueous solution was dialyzed with a 3.5 kDa dialysis membrane filter for 3 days to remove residual salts. To prepare GOQD from GOs, 75 mL of sulfuric acid (H2SO4), 25 mL of nitric acid (HNO3), and 200 mg of GO powder were mixed in a two-necked round flask. The mixture was then heated to 100°C for 24 h with vigorous stirring in a silicone oil bath and then cooled down. After adding water, the mixture was centrifuged at 10,000 rpm for 30 min. Finally, the precipitates were dialyzed to adjust the pH (removing acids) of the GOQD aqueous solution with a 3.5 kDa dialysis membrane filter until the solution was completely neutralized.


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2.3. Material characterizations. Nanostructured surface morphology imaging of the Ag nanofilms was performed using an UHR FE-SEM (S-5500, Hitachi Co.) at the Jeonju Center of the Korea Basic Science Institute (Jeonju, Republic of Korea). X-ray diffraction patterns were obtained in the range 30−90° (2θ) using a diffractometer (Empyrean, PANalytical) with Cu-Kα radiation (1.5406 Å). Atomic force microscopy (AFM) imaging was performed on a scanning probe microscope (NX20, Park systems). Reflectance spectrum in the ultraviolet-visible (UVVis) region was obtained using a spectrophotometer (Varian, Cary 5G). Steady-state fluorescence spectra were measured using a spectrophotometer (F-7000, Hitachi). 2.4. Fluorescence lifetime imaging. Fluorescence lifetime imaging (FLIM) was carried out using an inverted-type scanning confocal microscope (MicroTime-200, Picoquant, Germany) with a 40× (air) objective (see Supporting Information, Scheme S-1). Single-mode pulsed diode lasers (375 and 470 nm with a pulse width of ~30 ps) were used as excitation sources. A dichroic mirror (Z375RDC, AHF), a long-pass filter (HQ405lp, AHF), a 50 µm pinhole, a band-pass filter, and an avalanche photodiode detector (PDM series, MPD) were used to collect emissions from GQD under 375 nm laser irradiation. A dichroic mirror (490 DCXR, AHF), a long-pass filter (HQ500lp, AHF), a 50 µm pinhole, and a single photon avalanche diode (PDM series, MPD) were used to collect emissions from GOQD under 470 nm laser irradiation. The timecorrelated single-photon counting (TCSPC) technique was used to count fluorescence photons near the surface of the Ag nanofilms. FLIM images consisting of 200 × 200 pixels were recorded using the time-tagged time-resolved (TTTR) data acquisition route. The acquisition time was set to 1 ms for each pixel during image scanning. The photon counting rate was kept under 1% of the excitation rate. Exponential fitting for the obtained fluorescence decays was performed using the Symphotime-64 software (version 5.3) with a multi-exponential decay model.


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2.5. FDTD simulation. Electric field intensities and distributions were numerically calculated by finite-differential time-domain (FDTD, Lumerical Solutions Ltd.) simulation. Scanning electron microscopy (SEM) images of Ag nanoparticles deposited at thickness of 3 nm and 6 nm were directly imported to model the real particle shapes. Each modeling area was set to 250 nm × 170 nm with a periodic geometry. The Ag nanoparticles were assumed to be located on the 20 nm ZnO film surface, which was deposited on a Si wafer. The entire simulation domain was filled with a 1 nm cubic mesh. For the material data, we used inherent Palik data in FDTD as then, and k values of Au and Si. The optical parameters of zinc oxide were imported from prior measurements.53 A plane wave source with a wavelength from 375 nm to 470 nm was set to model the experimental light source. The electric field distribution was monitored at 1 nm below the top of the nanoparticles.

3. Results and discussion The scanning electron microscopy images in Figure 1 show the morphological changes of the silver nanofilms grown by the reactive sputtering process on ZnO (20 nm)-coated Si wafers with increasing sputtering time. As the film thickness increases, the morphology of the Ag layer is gradually changed from nanoislands to a continuous film. The Ag forms nanoislands (1−3 nm) in the initial state, which then change via an elongated network (4−6 nm) to a continuous film with nanohole morphology (8−10 nm) as the sputtering time increases. Beyond 12 nm of thickness, the grown Ag nanofilm has a continuous film morphology, although a slightly roughened surface is observed.


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Figure 1. SEM images of the Ag nanofilms grown by the reactive sputtering process on ZnO (20 nm) substrates: (a-h) 1, 2, 3, 4, 6, 8, 10, and 12 nm in thickness. The scale bar is 50 nm.

The crystallographic characteristics of the films were examined by X-ray diffraction analysis. As the thickness increases, the observed diffractions show increased intensity and decreased full width at half maximum, which is attributed to the increasing crystallite size (see Supporting Information, Figure S1). Moreover, the diffraction result indicates that the sputtered Ag films have a face-centered cubic (fcc) phase (JCPDS-ICDD No. 87-720) without any detectable impurity of the oxidized form. The total diffuse reflectance spectra of the films were measured in the ultraviolet-visible range using an integrating sphere (see Supporting Information, Figure S-2). As the Ag film thickness


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increases, reflection in the 400-700 nm range increases. The observed low reflection of the thinner Ag less than 10 nm is attributed to the enhanced absorption and scattering of the incident light around Ag island particles and structured nanofilms.13,54,55 The absorption spectra of the as-synthesized GQD and GOQD were also captured in the UVVis region, showing gradually increased absorption from the visible to the UV region (Figure 2). The GQD shows absorption onset at approximately 400 nm, and the GOQD has extended absorption beyond 500 nm. The GQD emits a blue color near the absorption band-edge, while the GOQD emits a green color. It has been previously reported that the green emission originates from intermediate electronic trapped states below the conduction bands.48,56 Upon the laser irradiation, the photo-excited carriers are easily trapped in the intermediate trapped states and then recombine to give red-shifted emissions (inset of Figure 2).

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Figure 2. UV-Vis absorption spectra of the GQD (solid line) and GOQD (dashed line) in aqueous solution. Inset is the fluorescence spectra of the corresponding blue-emitting GQD and greenemitting GOQD.


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To understand the metal-induced fluorescence modulation of the GQD and GOQD near the Ag nanofilms, we performed FDTD simulation for the uniquely structured 3 and 6 nm thickness Ag nanofilms (Figure 3 and Figure S-3 in Supporting Information). As shown in Figure 1, the 3 nm thickness Ag film has a nanoisland morphology, and the 6 nm thickness Ag film has elongated network morphology. The calculated results show distinctively different features in the optical field enhancement. The 3 nm thickness Ag sample occupies a notably higher population in the extended optical field scale of 1−3 compared to the 6 nm thickness Ag film, with the major population slightly above 1. In the close view of the simulations, it clearly appears that optical field enhancement occurs at the nanogaps among the nanoislands in the case of the 3 nm thickness Ag film. However, the 6 nm thickness Ag film shows enhanced optical fields at the void space in the elongated Ag network morphology. From the electron microscopy image

Figure 3. FDTD simulations of the Ag nanofilms with film thicknesses of (a, b) 3 and (c, d) 6 nm. The wavelengths of the excitation light are 375 and 470 nm.


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Figure 4. (a) Fluorescence lifetime images of the GQD and GOQD near Ag nanofilms on ZnO substrates (image dimension: 40 µm×80 µm). The excitation wavelengths are 375 and 470 nm for the GQD and GOQD, respectively. (b) Fluorescence photon counting rates of the blue GQD (square) and GOQD (circle) near Ag nanofilms on ZnO-coated Si substrates. The dotted lines indicate the photon counting rates of the bulk GQD and bulk GOQD solutions as control references. (c) Fluorescence lifetimes of the GQD (square) and GOQD (circle) near Ag nanofilms on ZnO-coated Si substrates. The dotted lines indicate the average fluorescence lifetimes of the bulk GQD and bulk GOQD solutions (GQD and GOQD in aqueous solution) as control references. Triangle symbols present the photon counting rate and fluorescence lifetime of the GQD when excited by a 470 nm laser. The ‘0’ in x-axis means bare ZnO without Ag.


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analysis, the Ag filled fraction in the two-dimensional space is estimated to be 0.46 and 0.70 for the 3 and 6 nm thickness Ag films, respectively. Considering the Ag filled fraction, the 3 nm Ag film seems to have more frequent chances to form narrow nanogaps, offering higher probability to excite GQD and GOQD. Fluorescence modulation in terms of the intensities and the lifetimes near the Ag nanofilms was studied using the blue-emitting GQD and the green-emitting GOQD. Figure 4a shows the fluorescence lifetime images of GQD and GOQD near the structured Ag nanofilms with thicknesses of 1−12 nm. Figure 4b summarizes the fluorescence photon counting rates (i.e., fluorescence intensities) as a function of the thickness of the Ag nanofilm. Interestingly, the fluorescence intensities of both GQD and GOQD are modulated with increasing film thickness. For GQD, as the film thickness increases, the fluorescence intensities gradually increase and are maximized on the 3 nm Ag nanofilm. The intensities decrease a little in the range from 3 nm to 8 nm and then recover again with increasing film thickness. It can also be observed that the overall fluorescence intensities near the Ag are higher than that of bulk GQD since the Ag nanofilms positively affect the amplification of the fluorescence intensity of the vicinal GQD. In the case of GOQD, the overall fluorescence intensities are considerably quenched compared to that of bulk GOQD regardless of the Ag film thickness. However, the GOQD on the Ag nanofilms also exhibits a similar tendency as in the case of the GQD in terms of the fluorescence intensity modulating with the increase of Ag nanofilm thickness, which is maximized on the 3 nm Ag nanofilm and minimized on the 6 nm Ag nanofilm. For the simple neat Ag films with a thickness beyond 10 nm, the enhanced and recovered fluorescence can be attributed to the roughened surface of the Ag films.6,21


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Fluorescence lifetimes estimated from the FLIM images are also discussed for correlation with the fluorescence intensities as a function of the Ag film thickness (Figure 4c). The average fluorescence lifetime of the GQD conserves its bulk lifetime (5.01 ns) similarly near the bare ZnO and the Ag nanofilms, regardless of the Ag film thickness. In the case of the GOQD, however, the bulk fluorescence lifetime (3.95 ns) is slightly reduced to 3.93 ns on the bare ZnOcoated Si wafer. The fluorescence lifetime is further reduced on the Ag-coated ZnO substrate and minimized (3.20 ns) on the 3 nm-thick Ag nanofilm. With further increase of the Ag film thickness, the observed fluorescence lifetime is gradually recovered to its lifetime of the bulk GOQD (in aqueous solution). Meanwhile, the fluorescence intensity is somewhat recovered on the 3~4 nm-thick Ag films. For the GOQD near the bare ZnO film, we observed emission quenching and slight lifetime reduction. It is known that the conduction band of ZnO lies ~0.5 eV lower than that of GQD.57 The underlying trapped states of GOQD can be easily coupled with the conduction band of ZnO. Therefore, this fluorescence quenching is responsible for the non-radiative charge transfer process from the trapped states of the GOQD to the conduction band of the ZnO. On the other hand, regarding the observed no lifetime change of the GQD near ZnO surface, it seems that the electronic band coupling is quite negligible between the conduction bands of ZnO and blueemitting GQD. As the Ag nanofilm thickness increases, the observed fluorescence lifetime of the GOQD is notably reduced on the 2~6 nm-thick Ag films. Regarding the fluorescence intensity and lifetime reduction on Ag nanofilms, the GOQD seems to experience dynamic fluorescence quenching. Previously, it was reported that dynamic fluorescence quenching can be activated through nonradiative pathways from the fluorophore to metal nanoparticles.58,59 Correspondingly, the


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abundant deeply trapped states from the oxygenous functional groups of the GOQD may provide efficient non-radiative pathways for photo-excited carriers. Furthermore, we carefully studied the fluorescence properties of GQD by exciting the weak absorption tail with the 470 nm laser. The emission appears in green spectral region (see Figures S-4 in Supporting Information). The observed fluorescence is quenched and the lifetime is reduced with varying Ag film thickness, similar to the GOQD (Figure 4b and 4c). We consider that the weak absorption tail in the longer wavelength of GQD is attributed to the presence of oxidized impurity in a small quantity. To inspect the photo-excited carriers’ dynamics, fluorescence lifetime sub-components and their contribution are carefully analyzed as a function of the Ag film thickness (Figure 5 and Figures S-5 and S-7 in Supporting Information). The blue-emitting GQD has three subcomponent lifetimes that show negligible lifetime modulation with varying Ag film thickness. The assignment of each lifetime components does not mean exactly separated photo-physical process, but somewhat overlapped each other in such the broadened fluorescence. Therefore, we carefully assigned the short-lived component to direct recombination including non-radiative processes, and the two long-lived components are assigned to recombinations through defectinduced trap states and/or localized ‘subdomain’ emission center in GQDs.48,50 In the case of the green-emitting GOQD, the fluorescence lifetimes consist of three or four lifetime subcomponents. One notable point is that an ultrafast lifetime component (~60 ps) is newly observed for the 1~10 nm thickness Ag film. The contribution (A1) of the fast sub-component to the total fluorescence becomes the major component by reducing the contributions of the other sub-component lifetimes. Therefore, it is considered that this ultrafast lifetime component is attributed to a new metal-coupled recombination pathway near Ag. This A1 contribution is


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maximized up to 64% on the 3 nm Ag film, which interestingly agrees with the localized fluorescence recovery for the GOQD on the same thickness. The detailed fluorescence lifetime parameters are also summarized in Table S-1 (in Supporting Information). According to the theory of MEF, the fluorescence intensity can be enhanced through both absorption enhancement and an increased radiative process near metal nanostructures.25,31 In this study, for the blue-emitting GQD, the fluorescence enhancement is estimated to be 63 and 28% on the 3 and 6 nm thickness Ag nanofilms, respectively (Figure 4b). Regarding the nonmodulating fluorescence lifetime behavior and the accumulated optical field distribution near the

Figure 5. (a, c) Fluorescence lifetime sub-components and (b, d) amplitude analysis of the blueemitting GQD and green-emitting GOQD near the Ag nanofilms as a function of film thickness. The excitation wavelengths are 375 and 470 nm for the GQD and GOQD, respectively. The ‘0’ in x-axis means bare ZnO without Ag.


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nanogaps in the Ag nanofilms, the observed fluorescence enhancement of the blue-emitting GQD is dominantly attributed to the absorption enhancement process. On the other hand, the green-emitting GOQD experiences more complex processes near Ag nanofilms. In addition to the absorption enhancement by the Ag nanogap, increased radiative (enhancing) and nonradiative (quenching) processes inevitably compete during the relaxation of the photo-excited carriers. The fluorescence quenching of GOQD might be attributed to the charge transfer process from the underlying trapped states of GOQD to the conduction band of the ZnO. The suggested electronic transitions are carefully presented in Scheme 1. Finally, it is noteworthy that the 3 nmthick Ag nanofilm exhibits maximized fluorescence enhancement for both GQD and GOQD, owing to its maximized nanogap population.

Scheme 1. Illustration of the electronic transitions of the GQD and GOQD near Ag deposited on the ZnO substrate when excited by (a) 375 and (b) 470 nm, respectively.


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We additionally performed fluorescence lifetime sub-component analysis for the GQD and GOQD at 470 and 375 nm, respectively (Figures S-6 and S-8−10 in Supporting Information). The results show severe emission quenching due to the defective impurity (GQD) and thermalization (exciton-photon coupling) in the case of high-energy excitation (GOQD), eventually preventing efficient MEF near metal nanostructures in off-resonance excitations.

4. Conclusions We studied fluorescence modulations in terms of the intensities and in the lifetimes of the GQD and GOQD near Ag nanofilms. The reactive sputtering process provided high quality Ag nanofilms with uniquely structured nanomorphology changing from nanoislands, via an elongated network and a continuous film with nanoholes, to a continuous film with increasing sputtering time. The fluorescence modulating behavior was examined for both the blue-emitting GQD and the green-emitting GOQD on the structured Ag nanofilms. The fluorescence enhancement was maximized with the blue-emitting GQD on a 3 nm-thick Ag film and minimized on 6−8 nm Ag nanofilms. The fluorescence of the green-emitting GOQD was significantly quenched near the Ag nanofilms on ZnO possibly through the charge transfer of photo-excited carriers from GOQD to ZnO. Based on the theoretical calculation, nanogaps and nanovoids can accumulate electromagnetic fields therein. A high population of nanogaps in the 3 nm-thick Ag film enabled the most positive environment to enhance absorption transition; therefore, the fluorescence was also enhanced. Consequently, it is revealed that both the morphology of the plasmonic metal and the electronic states configuration of GQD or GOQD are important factors and should cooperate with each other to maximize metal-enhanced fluorescence.


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ASSOCIATED CONTENT Supporting Information. Details about FLIM measurement and additional results (X-ray diffractions, reflectance spectra, FDTD simulation, and fluorescence decay analysis). This material is available free of charge on the ACS Publications website at DOI:

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF2015R1D1A1A01058935), and partially supported by the Center for Advanced Soft-Electronics funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (CASE2013M3A6A5073173) and a KBSI grant (C38230).


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List of Figure Captions Figure 1. SEM images of the Ag nanofilms grown by the reactive sputtering process on ZnO (20 nm) substrates: (a-h) 1, 2, 3, 4, 6, 8, 10, and 12 nm in thickness. The scale bar is 50 nm. Figure 2. UV-Vis absorption spectra of the GQD (solid line) and GOQD (dashed line) in aqueous solution. Inset is the fluorescence spectra of the corresponding blue-emitting GQD and greenemitting GOQD. Figure 3. FDTD simulations of the Ag nanofilms with film thicknesses of (a, b) 3 and (c, d) 6 nm. The wavelengths of the excitation light are 375 and 470 nm. Figure 4. (a) Fluorescence lifetime images of the GQD and GOQD near Ag nanofilms on ZnO substrates (image dimension: 40 µm×80 µm). The excitation wavelengths are 375 and 470 nm for the GQD and GOQD, respectively. (b) Fluorescence photon counting rates of the blue GQD (square) and GOQD (circle) near Ag nanofilms on ZnO-coated Si substrates. The dotted lines indicate the photon counting rates of the bulk GQD and GOQD solutions as control references. (c) Fluorescence lifetimes of the GQD (square) and GOQD (circle) near Ag nanofilms on ZnOcoated Si substrates. The dotted lines indicate the average fluorescence lifetimes of the bulk GQD and GOQD solutions (GQD and GOQD in aqueous solution) as control references. Triangle symbols present the photon counting rate and fluorescence lifetime of the GQD when excited by a 470 nm laser. The ‘0’ in x-axis means bare ZnO without Ag. Figure 5. (a, c) Fluorescence lifetime sub-components and (b, d) amplitude analysis of the blueemitting GQD and green-emitting GOQD near the Ag nanofilms as a function of film thickness.


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The excitation wavelengths are 375 and 470 nm for the GQD and GOQD, respectively. The ‘0’ in x-axis means bare ZnO without Ag.

List of Scheme Caption Scheme 1. Illustration of the electronic transitions of the GQD and GOQD near Ag deposited on the ZnO substrate when excited by (a) 375 and (b) 470 nm, respectively.


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Table of Contents (TOC) artwork


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