Tuning Enhancement Efficiency of Multiple Emissive Centers in

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Tuning Enhancement Efficiency of Multiple Emissive Centres in Graphene Quantum Dots by Core-Shell Plasmonic Nanoparticles Shujun Wang, Ashleigh Clapper, Peng Chen, Lianzhou Wang, Igor Aharonovich, Dayong Jin, and Qin Li J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02550 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 2, 2017

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The Journal of Physical Chemistry Letters

Tuning Enhancement Efficiency of Multiple Emissive Centres in Graphene Quantum Dots by Core-Shell Plasmonic Nanoparticles Shujun Wang1,2*, Ashleigh Clapper 2, Peng Chen3, Lianzhou Wang3, Igor Aharonovich4, Dayong Jin4, Qin Li1,2* 1. Queensland Miro- and Nanotechnology Centre, Griffith University, Nathan, QLD 4111, Australia 2. School of Engineering (Environmental), Griffith University, Nathan, QLD 4111, Australia 3. Nanomaterials Centre, School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, QLD 4072, Australia 4. Institute for Biomedical Materials and Devices (IBMD), Faculty of Science, University of Technology Sydney, Ultimo, NSW 2007, Australia Abstract: Graphene quantum dots (GQDs) are emerging luminescent nanomaterials for energy, bioimaging and optoelectronic applications. However, unlike conventional fluorophores, GQDs contain multiple emissive centres that results in a complex interaction with external electromagnetic fields. Here we utilize core-shell plasmonic nanoparticles to simultaneously enhance and modulate the photoluminescence (PL) intensities and spectral profiles of GQDs. By analysing the spectral profiles, we show that the emissive centres are highly influenced by the proximity to the metal particles. Under optimal spacer thickness of 25 nm, the overall PL displays a 4 fold enhancement compared to a pristine GQD. However, detailed lifetime measurements indicate the presence of mid-gap states that act as the bottleneck for further enhancement. Our results offer new perspectives for fundamental understanding

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and new design of functional luminescent materials (e. g. GQDs, graphene oxide, carbon dots) for imaging, sensing and light harvesting.

Key words: Graphene quantum dots; fluorescence enhancement; localized surface plasmon resonance; time resolved photoluminescence spectroscopy Graphene quantum dots (GQDs) referring to graphene fragments with lateral size smaller than 100nm

1

possess distinguishing properties1-5 from their parent material graphene. Due to their versatile photoluminescence (PL) and excellent biocompatibility6, GQDs have been regarded as one highly promising candidate for advanced applications including bio-imaging, optical sensing

and

optoelectronics7-8. However, one of the shortcomings of GQDs with respect to the expected applications especially for bio-imaging and PL-based sensing is the low quantum yield (QY). The QY of GQDs varies dramatically as a function of their synthesis method, surface conditions and chemical compositions. So far, majority of the synthesized GQDs exhibit QYs lower than 10%. Hence, there has been an increasing interest in modifying GQDs to improve their QYs, including chemical or photochemical reduction9-10, passivation11 and chemical functionalization12. However, these techniques only offer a marginal improvement over the QY meanwhile suffering from poor freedom of control and involvement of highly toxic chemicals. A promising pathway to increase the luminescence from emitters is the use of metallic nanoparticles which provides localized surface plasmon resonance (LSPR)

13-15

. Specifically, LSPR can modify the

electromagnetic environment surrounding the metallic nanoparticles, thereby affecting the optical properties of molecules or nanoparticles located in the proximity of the particles. Hitherto, LSPR has


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been proven as an efficient technique for enhancement of PL of different dye molecules16-19 and nanoparticle fluorophores20-24. In these studies, investigations have been centred on the morphology engineering of metallic nanoparticles (NPs) or nanocomposite structures for optimal enhancement efficiency. However, such a mainstream perspective treats a fluorophore with multiple emission centres as no different from a fluorophore with just one emission centre (e. g. dipole-like molecular fluorophores). Hence, it overlooks the in-depth information on the interactions of the individual emission centres with the plasmonic field, which is important for fundamental understanding of fluorescence mechanism as well as effective design of functional fluorophores for real life applications. In this study, we demonstrate that in addition to enhancement of the overall intensity, LSPR also alters the spectral profiles of PL of GQDs. This change of spectral profiles was correlated to the variant and tunable enhancement efficiencies of the photoluminescence components from different emissive centres in GQDs under LSPR. PL deconvolution and time-resolved PL spectroscopy were the major tools used for understanding the interactions between the plasmon field and GQDs that have multiple emissive centres. Figure 1 schematically represents the procedures of sample preparation in our study. We adopted silver nanoparticles as the plasmonic metal particles. The silver nanoparticles were synthesized from a polyol process25, with an average diameter of 30 nm (Figure S 1 a). The silica shells that serve as dielectric spacers were coated using a procedure described elsewhere26. Its thickness was tuned via the loading of precursor tetraethoxysilane (TEOS) as displayed in the HRTEM images at the bottom of Figure 1, showing shell thickness from left to right 7nm, 14nm, 20nm, 25nm and 65nm (More TEM and HRTEM images of core-shell and shell particles are available in the supporting information, Figure S2). GQDs (Figure S 1b ) in this study were synthesized via hydrothermal cutting of carbon nano sheets as developed previously12. The GQDs possess a relative quantum yield of ~0.01 as measured by adopting quinine sulphite (QY=0.54) as the reference.



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Figure 1. Schematic flowchart representation of the process of preparing nanoparticles in this study: silver nanoparticles (Ag NP) with an average diameter of 30nm were coated with silica shells via hydrolysis of tetraethoxysilane (TEOS) resulting Ag@SiO2 core-shell particles, the thickness of shell could be gradually tuned via the loading of TEOS (as shown by the bottom HRTEM images, shell thickness from left to right-7nm, 14nm, 20nm, 25nm and 65nm. All scale bars indicate 10nm); Removal of the Ag cores by addition of NaCl created SiO2 shell particles; Both the as prepared core-shell and shell particles are negatively charged (as indicated by the greenish colour shade), therefore, Poly (allylamine hydrochloride) (PAH) was applied to alter the surface to be positively charged (as indicated by the orangish colour shade) to facilitate the loading of negatively charged GQDs; The shell particles loading with GQDs (GQDs@SiO2) were created as the control samples of their core-shell counterparts (GQDs@SiO2@Ag).


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The as-synthesized Ag@SiO2 (i. e. Core-shell) particles are negatively charged (as indicated by the greenish colour shade in Figure 1). As the GQDs are also negatively charged due to the existence of oxygenated functional groups12, the surface charge of the Ag@SiO2 particles are altered by Poly(allylamine hydrochloride) (PAH) to facilitate the loading of GQDs to the Ag@SiO2 particles. The success of surface charge alteration was confirmed with measurement of zeta potentials as presented in Figure S 3 a. In order to reveal the modification of PL by LSPR, control samples with only silica shells were also synthesized by adding NaCl to the Ag@SiO2 particles for removal of Ag cores. As the loading of GQDs on the silica shells (for both SiO2 shell and Ag@SiO2 core-shell particles) is facilitated by the electrostatic attraction between negatively charged GQDs and positively charged silica shells, to make sure the loading of GQDs are comparable, we monitored the zeta potentials of the particles before and after removal of the silver core and confirmed that the silver removal does not impact the zeta potential in any significant way as shown by the comparison of equilibrium Zeta potentials (Figure. S 3 b) . Figure 2 a~c present the PL spectra of GQDs@SiO2@Ag with the silica shell thickness of 14, 25 and 37 nm, respectively. Enhancement of PL intensity is apparent in the three hybrid particles. However, for the sample with a shell thickness of 7nm, we did not achieve meaningful PL increase as shown in Figure S 4. In addition to the increased PL intensity, we also noticed that the shapes of the spectra differ from the ones recorded from the control samples. This difference becomes more obvious after the normalization of spectra as shown in the inserted graphs in Figure 2 (a-c): the prominent shoulders of the spectra at the high energy side (i. e. blue side) become less apparent upon interaction with the LSPR of silver nanoparticles and the spectra are also slightly red shifted too. We believe that underlying this spectral change is the dissimilar sensitivities of individual PL components from different emissive centres in the GQDs to the plasmon field of Ag particles. To confirm this assumption, we systematically evaluated the spectra via a deconvolution procedure.



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Figure 2. Enhancement of PL of GQDs with SLPR: a-c, the PL spectra (excitation wavelength=370nm) with and without the presence of Ag core for particles with shell thickness 14nm, 25nm and 37nm respectively. Inserts are normalized spectra showing change of spectral profiles caused by higher enhancement efficiency on the red side than on the blue side of the spectra; d, the PL mechanism of GQDs, depicting the overall PL of GQDs is a combination of PL components from four types of electron transitions, σ*-n and π*-n transitions dominated by the functional groups, π*- π transition of the aromatic cores and π*-midgap states-π transitions; e and f, the efficiency of enhancement as a function of shell thickness for overall PL (e) and each individual PL component (f) derived from PL deconvolution: both e and f show maximum enhancement efficiency achieved with a shell thickness of 25nm. e also reveals that the PL component from π*-midgap states-π transitions residing on the red side of the spectra have the highest enhancement efficiency among all the PL components which is consistent to the shape change of the PL spectra as shown in a-c. With respect to the deconvolution, the spectra data as acquired experimentally were transformed into energy domain with Jacobian and lineshape corrections applied to the PL intensity (see supporting


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information for the details of PL spectra deconvolution). The deconvolution was first conducted on the PL spectrum of the as synthesized GQDs. According to previous study, the PL of GQDs originates from three types of emissive centres and four types of electronic transitions: σ*-n and π*-n transitions dominated by the functional groups, π*- π transition of the aromatic cores and π*-midgap states-π transitions dominated by the structural deformation induced by both defects and functional groups (Figure 2 d)27. Accordingly, we applied four Gaussian components and obtained perfect fitting. The deconvolution reveals that the dominant emissive centres for the overall PL of GQD are π*- π, π*-n and π*-midgap states-π transition (Figure S 6), whereas σ*-n transition has the least contribution. The peak positions and maximum width at half maximum (MWHF) of four Gaussian components were maintained throughout the following deconvolutions of the PL of GQDs@SiO2 and GQDs@SiO2@Ag particles (Figure S 7 and Table S 1). After the deconvolution, the results were reversely transformed back to wavelength domain for further analysis (Figure S 8). The enhancement factors of the overall PL and individual components were then calculated and plotted as a function of shell thickness in Figure 2 e and f, respectively. Figure 2 e exhibits the enhancement factors of the overall PL derived from both maxima of PL intensity and integration of PL intensity (i. e. area under spectra). The maximum enhancement factors occurred at a shell thickness of 25 nm for both cases but with increase in intensity maxima (~4.5 folds) higher than the intensity integration (~4 folds), which is consistent with previous theoretical prediction that optimal enhancement of PL by LSPR of similar plasmonic particles are likely to be accomplished within a distance range of 20~30nm from the surface of the plasmonic particles16. Figure 2 f shows the enhancement of each individual PL transition derived from integration of PL intensity. The highest enhancement efficiency is on the PL component from π*-midgap states-π transition for all shell thickness and the rest of the PL components have varied enhancement efficiencies. It is worth noting that the PL components from σ*-n and π*-n transitions of hybrid GQDs@SiO2@Ag particles with 7nm shell is in fact weakened by the presence of Ag (i. e.



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enhancement factors < 1) as shown in Figure 2 f. This is due to energy transfer from these two transitions of GQDs to Ag, which usually takes place in the distance range less than 10nm28-29. As well known, matching plasmon resonance of metal particles to the emission spectrum of a fluorophore and maintaining optimum distance between the two are essential for achieving efficient enhancement of its PL with LSPR16. To understand the enhancement mechanism further, we plot the positions of the individual PL components of the GQD together with the plasmon resonance (Figure 3a). As is shown in Figure 2 f, the highest enhancement efficiency is on the PL component from π*-midgap states-π transition. However, the plasmon band does not match well with π*-midgap states-π transition in contrast to σ*-n, π*-π and with π*-n transitions. This is due to the fact that enhancement of π*-midgap states-π transitions relies on the enhancement of the emission rate of π*-π transition. Therefore, as depicted in Figure 3 b, at non-optimal distances, the enhancement of π*-π transition (blue arrows) is significantly offset by electron drainage (i. e. relaxation) to π*-midgap states-π transitions (orange arrows). Hence, the enhancement of PL from π*-midgap states-π transitions (yellow spectra in ‘PL’ panel in Figure 3 b) is more prominent than PL from π*-π transition (blue spectra in ‘PL’ panel in Figure 3 b). Only at the optimal distance, the highest enhancement of emission rate of π*-π transition ensures efficient enhancement of both PL components as shown in Figure 2 f. To further confirm above findings, we carried out fluorescence lifetime measurements, through which we can study the modification of the decay parameters of GQDs with the presence of Ag nanoparticles. The measurement identified three lifetime components: a fast one less than 1ns (τ1), a medium one around 3~4ns (τ2) and a slow one (10~12ns) in the raw GQDs as well as control samples (see supporting information for details).

The three components are associated with the aforementioned electronic

transitions- τ1 corresponding σ*-n and π*-n transitions, τ2 corresponding to π*- π transition and τ3 corresponding to π*-midgap-π transitions27. As well known, the lifetime values could be correlated to the main decay parameters through Equation 1 ~ 4: the quantum yield (Q0) of a fluorophore is correlated to


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Figure 3. Overlapping of individual PL components of GQD with plasmon band of Ag (a) and schematic representation of the dependence of π*-midgap states-π transitions on π*-π transitions during the LSPR enhancement of PL of GQD (b): PL component from π*-midgap states-π transitions does not match well with plasmon band of Ag in comparison to other PL components (particularly PL component from π*-π transition) as revealed by a, however, as shown in b π*-midgap states-π transitions (indicated by orange arrows) withdraw electrons (indicated by the grey wavy arrows) from π*-π transition (indicated by blue arrows) resulting the higher enhancement efficiency of the corresponding PL component but constraining the enhancement of the PL component from π*-π transition.



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the rates of radiative decay (Γ) and non-radiative decay (knr) as depicted by Eq. 1; The PL lifetime is the inverse of the overall relaxation rate (Γ+knr) as shown with Eq. 2; The presence of LSPR normally does not change the rate of non-radiative decay (knr), however, it will modify the rate of radiative decay by a certain factor17. If we denote this factor of modification as m and the quantum yield and lifetime of the fluorophore upon the interaction with LSPR as Qp and τp respectively, Qp and τp can be depicted by Eq. 3 and Eq. 4 respectively.

ܳ଴ =

୻

(1)

୻ା௞೙ೝ

ଵ

߬଴ = ୻ା௞

(2) ೙ೝ

௠୻

ܳ௣ = ௠୻ା௞

(3) ೙ೝ

ଵ

߬௣ = ௠୻ା௞

(4) ೙ೝ

Based on Eq. 1~ 4, one can obtain Eq. 5 (see supporting information for details). For a given fluorophore, we can treat Γ, τ0, and knr as constants, therefore, the ratio Qp/Q0 is only determined by the ratio of τp /τ0. The smaller τp /τ0 is, the larger the Qp/Q0. ொ೛ ொబ

ଵ

= ୻ఛ − బ

௞೙ೝ ఛ೛ ୻ ఛబ

(5)

Figure 4a presents the τp /τ0 of the three lifetime components (i. e. τ1, τ2 and τ3) for samples with shell thickness of 7 nm, 25 nm and 37 nm. The plot clearly shows that among the three lifetime components, τ2 is reduced by the SLPR for shell thicknesses 25 nm and 37 nm and the lifetime reduction is the most significant for a shell thickness of 25 nm; τ1 shares a similar tendency with τ2 but with a much smaller extent of reduction; the change of τ3 is negligible (τp /τ0 ~1) indicating that LSPR can barely affect this lifetime component. The τp /τ0 trend is consistent with the difference in enhancement efficiencies of


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different emission centres observed in the static PL (Figure 2 f): the deepest lifetime decrease for τ1 and τ2 both occurred at a shell thickness of 25 nm (as shown in Figure 4 a) corresponding to the highest PL enhancement efficiencies observed at the same shell thickness in Figure 2 f. Moreover, the different sensitivities of the three lifetime components to LSPR also agree well with the overlapping of silver plasmon band to the emission components as shown in Figure 3a: τ2 (corresponding to π*- π transition) possesses the highest sensitivity to LSPR among the three lifetime components, which is in accordance to the best match of π*- π transition band with the measured plasmon band of silver nanoparticles; the significant reduction of τ2 (τp /τ0 down to 0.6) suggests that π*- π transition is effectively enhanced by the plasmon band, which, however, is not directly translated to its associated PL due to the electron drainage to the midgap states. Secondly, the fact that the change of τ3 (corresponding to π*-midgap-π transitions) is negligible confirms the enhancement of the PL induced by π*-midgap-π transitions is not directly caused by the coupling to plasmon band due to minimal overlap between the plasmon resonance and the resonance of this transition, however, it is caused by the increased number of electrons arrived at the midgap states as a result of electron drainage from enhanced π*- π transitions. In addition to the analysis based on reduction of PL lifetime, we also studied the PL enhancement process by calculating the change of different parameters in Eq 1~4 including Γ, knr, m and Q of the individual emission centres. As π*-midgap-π transitions (corresponding to τ3) are subject to π*-π transition (corresponding to τ2) and τ3 is not modified by the presence of Ag particles, the calculation were conducted for τ1 and τ2. The calculation was carried out based on samples with a shell thickness of 25nm by applying Eq 1~4. Although the relative quantum yield of the overall PL of the raw GQD is around 0.01 as previously mentioned, which is determined based on the number of photons detected by the fluorescence spectrometer. However, the exact quantum yields of the individual emission centres, especially the ‘absolute quantum yields’ that is defined by the number of photons emitted by the emission centres, are unknown. Therefore, we calculated the enhancement factors by arbitrarily varying the original



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Figure 4. Analysis of PL decay based on the time correlated single photon counting (TCSPC): a, change of PL lifetime as a function of shell thickness (τ0=PL lifetime of GQDs@SiO2, τp = PL lifetime of the GQDs@SiO2@Ag), a value lower than 1 is necessary for PL enhancement and the lower the value, the higher the enhancement as suggested by Eq. 5; b and c, as calculated decay parameters by varying original absolute quantum yields (Q0) in 0.001~0.9 for emission centres represented by τ1 (b, corresponding to σ*-n and π*-n transitions) and τ2 (c, corresponding to π*-π) respectively-main graphs show the enhancement factors of absolute quantum yields (Qp/Q0) and enhancement factors of emission rate (m), insets in b and c exhibit the original radiative decay rates (Γ), the modified decay rates (mΓ ) and the nonradiative decay rates (knr) absolute quantum yields (Q0) of emission centres in the range 0.001~0.9. The detail of the calculation can be found in the supporting information. Figures 4 b and c present the main results of the calculation. Firstly, Figure 4 b and c exhibit the ascalculated enhancement factors of absolute PL quantum yields (Qp/Q0) as a function of Q0. In general, regardless of Q0, the Qp/Q0 of τ2 is always greater than that of τ1 at each Q0. Moreover, as explained before, the enhancement of PL under SLPR is achieved via increase of radiative decay rate from Γ to mΓ. As shown in the insets of Figure 4 b and c, regardless of Q0, the radiative decay rates for both τ1 and τ2 are increased but the enhancement of radiative decay rate of τ2 is much more efficient than τ1. Both enhancement of quantum yields and radiative decay rates are consistent with the best match of PL component of π*-π transition to plasmon band of silver nanoparticles.


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In particular, Table 1 lists the calculated values for emission centres dominant by π*-π transitions at Q0=0.01 (i. e. the quantum yield of raw GQD). The enhanced quantum yield of emission centre represented by τ2 is 0.39, a 39.4-fold increase in total, which is significantly higher than the enhance factor of ~3.4 experimentally achieved. This result suggests the enhancement of PL component from π*-π transition could be much more prominent if without the presence of midgap states. Hence, from the calculation of decay parameters, we learnt the general trend of enhancement of PL emission that is independent of the quantum yields : 1) the PL enhancement is realized by enhancement of radiative decay rates; 2) the enhancement of radiative decay rate of π*-π is more efficient than the σ*-n and π*-n transitions, however; 3) the existence of midgap states counteracts the enhancement of decay rate of π*-π which restrained the overall enhancement efficiency. Table.1 Summary of calculated decay parameters for emission centres dominant by π*-π transitions at Q0=0.01 Samples

τ2(ns)

Γ

Knr 6

m 8

Q

GQD GQD@SiO2

3.38 3.12

2.96E×10 2.93×10 3.21E×106 3.18×108

GQD@SiO2@Ag

1.91

2.07E×108 3.18×108 64.39 0.39

Qp/Q0

0.01 0.01 39.41

To summarize, we provided a detailed mechanism to explain the emission enhancement of the various emission centres in GQDs by designing and engineering hybrid GQDs@SiO2@Ag nanoparticles. Our results suggest that under each of the plasmonic environments determined by the thickness of the silica spacer, the photoluminescence components from the different emissive centres have different enhancement efficiencies resulting the change of PL spectra profiles. In addition, we have achieved a large emission enhancement of over 4 folds. However, the PL lifetime measurements indicate the presence of mid-gap states act as the bottleneck preventing more significant enhancement to be achieved. Our results are important for understanding the fundamental emission mechanisms of GQDs and their future use for a variety of applications in energy, bioimaging, sensing and optoelectronics.



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Supporting Information. Experimental details, TEM morphology, Zeta potential, Procedures for PL deconvolution, PL lifetime and details on calculation of decay parameters. Corresponding Authors *Email: [email protected]. *Email: [email protected]. Acknowledgements S.W. acknowledges the support of a Griffith Publication Assistance Scholarship (PAS). Q.L. and S. W. wish to thank the support from Australian Research Council (DP160104089). The authors are grateful for the support of Centre of Microscopy and Microanalysis (CMM) at the University of Queensland for acquiring SEM and TEM images. References: (1) Ponomarenko, L. A.; Schedin, F.; Katsnelson, M. I.; Yang, R.; Hill, E. W.; Novoselov, K. S.; Geim, A. K. Chaotic Dirac Billiard in Graphene Quantum Dots. Science 2008, 320, 356-358. (2) Ritter, K. A.; Lyding, J. W. The Influence of Edge Structure on the Electronic Properties of Graphene Quantum Dots and Nanoribbons. Nat. Mater. 2009, 8, 235-242. (3) Wang, W. L.; Meng, S.; Kaxiras, E. Graphene Nanoflakes with Large Spin. Nano Lett. 2008, 8, 241-245. (4) Zhang, Z. Z.; Chang, K. Tuning of Energy Levels and Optical Properties of Graphene Quantum Dots. Phys. Rev. B 2008, 77. (5) Shi, H. Q.; Barnard, A. S.; Snook, I. K. High Throughput Theory and Simulation of Nanomaterials: Exploring the Stability and Electronic Properties of Nanographene. J. Mater. Chem. 2012, 22, 1811918123. (6) Wang, S.; Cole, I. S.; Li, Q. The Toxicity of Graphene Quantum Dots. Rsc Adv. 2016, 6, 8986789878. (7) Li, L. L.; Wu, G. H.; Yang, G. H.; Peng, J.; Zhao, J. W.; Zhu, J. J. Focusing on Luminescent Graphene Quantum Dots: Current Status and Future Perspectives. Nanoscale 2013, 5, 4015-4039. (8) Shen, J.; Zhu, Y.; Yang, X.; Li, C. Graphene Quantum Dots: Emergent Nanolights for Bioimaging, Sensors, Catalysis and Photovoltaic Devices. Chem. Commun. 2012, 48, 3686-99. (9) Feng, Y. Q.; Zhao, J. P.; Yan, X. B.; Tang, F. L.; Xue, Q. J. Enhancement in the Fluorescence of Graphene Quantum Dots by Hydrazine Hydrate Reduction. Carbon 2014, 66, 334-339. (10) Sun, H. J.; Wu, L.; Gao, N.; Ren, J. S.; Qu, X. G. Improvement of Photoluminescence of Graphene Quantum Dots with a Biocompatible Photochemical Reduction Pathway and Its Bioimaging Application. Acs Appl. Mater. Inter. 2013, 5, 1174-1179.


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(11) Shen, J. H.; Zhu, Y. H.; Yang, X. L.; Zong, J.; Zhang, J. M.; Li, C. Z. One-Pot Hydrothermal Synthesis of Graphene Quantum Dots Surface-Passivated by Polyethylene Glycol and Their Photoelectric Conversion under near-Infrared Light. New J. Chem. 2012, 36, 97-101. (12) Wang, S.; Lemon, Z.; Cole, I. S.; Li, Q. Tailoring the Edges of Graphene Quantum Dots to Establish Localized Pi-Pi Interactions with Aromatic Molecules. Rsc Adv. 2015, 5, 41248-41254. (13) Nicoletti, O.; de la Pena, F.; Leary, R. K.; Holland, D. J.; Ducati, C.; Midgley, P. A. ThreeDimensional Imaging of Localized Surface Plasmon Resonances of Metal Nanoparticles. Nature 2013, 502, 80-84. (14) Zhou, W.; Odom, T. W. Tunable Subradiant Lattice Plasmons by out-of-Plane Dipolar Interactions. Nat. Nanotechnol. 2011, 6, 423-427. (15) Zeng, S. W.; Baillargeat, D.; Ho, H. P.; Yong, K. T. Nanomaterials Enhanced Surface Plasmon Resonance for Biological and Chemical Sensing Applications. Chem. Soc. Rev. 2014, 43, 3426-3452. (16) Tovmachenko, O. G.; Graf, C.; van den Heuvel, D. J.; van Blaaderen, A.; Gerritsen, H. C. Fluorescence Enhancement by Metal-Core/Silica-Shell Nanoparticles. Adv. Mater. 2006, 18, 91-95. (17) Bardhan, R.; Grady, N. K.; Cole, J. R.; Joshi, A.; Halas, N. J. Fluorescence Enhancement by Au Nanostructures: Nanoshells and Nanorods. Acs Nano 2009, 3, 744-752. (18) Yang, J. P.; Zhang, F.; Chen, Y. R.; Qian, S.; Hu, P.; Li, W.; Deng, Y. H.; Fang, Y.; Han, L.; Luqman, M.; Zhao, D. Y. Core-Shell Ag@SiO2@mSiO2 Mesoporous Nanocarriers for Metal-Enhanced Fluorescence. Chem. Commun. 2011, 47, 11618-11620. (19) Lessard-Viger, M.; Rioux, M.; Rainville, L.; Boudreau, D. Fret Enhancement in Multilayer CoreShell Nanoparticles. Nano Lett. 2009, 9, 3066-3071. (20) Ray, K.; Badugu, R.; Lakowicz, J. R. Metal-Enhanced Fluorescence from Cdte Nanocrystals: A Single-Molecule Fluorescence Study. J. Am. Chem. Soc. 2006, 128, 8998-8999. (21) Li, C. Y.; Zhu, Y. H.; Wang, S. W.; Zhang, X. Q.; Yang, X. L.; Li, C. Z. Enhanced Fluorescence of Graphene Oxide by Well-Controlled Au@SiO2 Core-Shell Nanoparticles. J. Fluoresc. 2014, 24, 137141. (22) Omidvar, A.; RashidianVaziri, M. R.; Jaleh, B.; Shabestari, N. P.; Noroozi, M. Metal-Enhanced Fluorescence of Graphene Oxide by Palladium Nanoparticles in the Blue-Green Part of the Spectrum. Chinese Phys. B 2016, 25. (23) Li, C. Y.; Zhu, Y. H.; Zhang, X. Q.; Yang, X. L.; Li, C. Z. Metal-Enhanced Fluorescence of Carbon Dots Adsorbed Ag@SiO2 Core-Shell Nanoparticles. Rsc Adv. 2012, 2, 1765-1768. (24) Deng, W.; Jin, D. Y.; Drozdowicz-Tomsia, K.; Yuan, J. L.; Wu, J.; Goldys, E. M. Ultrabright EuDoped Plasmonic Ag@SiO2 Nanostructures: Time-Gated Bioprobes with Single Particle Sensitivity and Negligible Background. Adv. Mater. 2011, 23, 4649-4654. (25) Silvert, P. Y.; HerreraUrbina, R.; TekaiaElhsissen, K. Preparation of Colloidal Silver Dispersions by the Polyol Process .2. Mechanism of Particle Formation. .J Mater. Chem. 1997, 7, 293-299. (26) Graf, C.; Vossen, D. L. J.; Imhof, A.; van Blaaderen, A. A General Method to Coat Colloidal Particles with Silica. Langmuir 2003, 19, 6693-6700. (27) Wang, S.; Cole, I. S.; Zhao, D. Y.; Li, Q. The Dual Roles of Functional Groups in the Photoluminescence of Graphene Quantum Dots. Nanoscale 2016, 8, 7449-7458. (28) Roy, R.; Hohng, S.; Ha, T. A Practical Guide to Single-Molecule Fret. Nat. Methods 2008, 5, 507516. (29) Li, M.; Cushing, S. K.; Wang, Q. Y.; Shi, X. D.; Hornak, L. A.; Hong, Z. L.; Wu, N. Q. SizeDependent Energy Transfer between Cdse/Zns Quantum Dots and Gold Nanoparticles. J. Phys. Chem. Lett. 2011, 2, 2125-2129.



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Figure 1. Schematic flowchart representation of the process of preparing nanoparticles in this study: silver nanoparticles (Ag NP) with an average diameter of 30nm were coated with silica shells via hydrolysis of tetraethoxysilane (TEOS) resulting Ag@SiO2 core-shell particles, the thickness of shell could be gradually tuned via the loading of TEOS (as shown by the bottom HRTEM images, shell thickness from left to right7nm, 14nm, 20nm, 25nm and 65nm. All scale bars indicate 10nm); Removal of the Ag cores by addition of NaCl created SiO2 shell particles; Both the as prepared core-shell and shell particles are negatively charged (as indicated by the greenish colour shade), therefore, Poly (allylamine hydrochloride) (PAH) was applied to alter the surface to be positively charged (as indicated by the orangish colour shade) to facilitate the loading of negatively charged GQDs; The shell particles loading with GQDs (GQDs@SiO2) were created as the control samples of their core-shell counterparts (GQDs@SiO2@Ag).



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Figure 2. Enhancement of PL of GQDs with SLPR: a-c, the PL spectra (excitation wavelength=370nm) with and without the presence of Ag core for particles with shell thickness 14nm, 25nm and 37nm respectively, Inserts are normalized spectra showing change of spectral profiles caused by higher enhancement efficiency on the red side than on the blue side of the spectra; d, the PL mechanism of GQDs, depicting the overall PL of GQDs is a combination of PL components from four types of electron transitions, σ*-n and π*-n transitions dominated by the functional groups, π*- π transition of the aromatic cores and π*-midgap statesπ transitions; e and f, the efficiency of enhancement as a function of shell thickness for overall PL (e) and each individual PL component (f) derived from PL deconvolution: both e and f show maximum enhancement efficiency achieved with a shell thickness of 25nm, e also reveals that the PL component from π*-midgap states-π transitions residing on the red side of the spectra have the highest enhancement efficiency among all the PL components which is consistent to the shape change of the PL spectra as shown in a-c. 171x86mm (300 x 300 DPI)


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Figure 3. Overlapping of individual PL components of GQD with plasmon band of Ag (a) and schematic representation of the dependence of π*-midgap states-π transitions on π*-π transitions during the LSPR enhancement of PL of GQD (b): PL component from π*-midgap states-π transitions does not match well with plasmon band of Ag in comparison to other PL components (particularly PL component from π*-π transition) as revealed by a, however, as shown in b π*-midgap states-π transitions (indicated by orange arrows) withdraw electrons (indicated by the grey wavy arrows) from π*-π transition (indicated by blue arrows) resulting the higher enhancement efficiency of the corresponding PL component but constraining the enhancement of the PL component from π*-π transition.



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Figure 4. Analysis of PL decay based on the time correlated single photon counting (TCSPC): a, change of PL lifetime as a function of shell thickness (τ0=PL lifetime of GQDs@SiO2, τp = PL lifetime of the GQDs@SiO2@Ag), a value lower than 1 is necessary for PL enhancement and the lower the value, the higher the enhancement as suggested by Eq. 5; b and c, as calculated decay parameters by varying original absolute quantum yields (Q0) in 0.001~0.9 for emission centres represented by τ1 (b, corresponding to σ*n and π*-n transitions) and τ2 (c, corresponding to π*-π) respectively-main graphs show the enhancement factors of absolute quantum yields (Qp/Q0) and enhancement factors of emission rate (m), insets in b and c exhibit the original radiative decay rates (Γ), the modified decay rates (mΓ ) and the nonradiative decay rates (knr) 101x25mm (300 x 300 DPI)


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Tuning Enhancement Efficiency of Multiple Emissive Centers in

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