Nonlinear Optical Investigations in Nine-Atom Silver Quantum Clusters

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Nonlinear Optical Investigations in 9-Atom Silver Quantum Clusters and Graphitic Carbon Nitride Nanosheets Kishore Sridharan, Perumbilavil Sreekanth, Tae Joo Park, and Reji Philip J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b02372 • Publication Date (Web): 18 Jun 2015 Downloaded from http://pubs.acs.org on July 1, 2015

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The Journal of Physical Chemistry

Nonlinear Optical Investigations in 9-Atom Silver Quantum Clusters and Graphitic Carbon Nitride Nanosheets Kishore Sridharan†, P. Sreekanth‡, Tae Joo Park †, and Reji Philip*,‡ †

Department of Materials Science and Engineering, Hanyang University, Ansan 426-791,

Republic of Korea ‡

Ultrafast and Nonlinear Optics Lab, Light and Matter Physics Group, Raman Research Institute,

Bangalore 560080, India

ACS Paragon Plus Environment

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ABSTRACT

Absorption saturation due to surface plasmon resonance affects the optical limiting efficiency of metal nanoparticles (NPs) by raising the limiting threshold to higher laser fluences. It has been shown that in gold, compared to the larger NPs, smaller quantum clusters (QCs) exhibit better optical limiting with lower limiting thresholds due to the absence of absorption saturation. Here we report optical limiting properties of two novel materials; namely nine-atom silver (Ag9) QCs and graphitic carbon nitride (GCN) nanosheets. The relatively large nonlinear absorption of Ag9 QCs compared to Ag NPs is revealed from open aperture Z-scan measurements carried out using 532 nm, 5 ns laser pulses. Optical nonlinearity in the QCs arises mostly from free carrier absorption and a relatively weak saturable absorption. The superior limiting efficiency of Ag9 QCs is complemented by excellent chemical stability, which makes silver quantum clusters ideal candidates for optical limiting applications. The two-dimensional sheet like structure of GCN is ideal for grafting metals and semiconductors, and we show that even though the nonlinearity of pristine GCN is low, it can be improved substantially by grafting lightly with Ag9 QCs.

Keywords: Ag9 quantum clusters, graphitic carbon nitride, optical nonlinearity, surface plasmon resonance, quantum confinement


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INTRODUCTION Metal nanoparticles (NPs), especially gold and silver, exhibit optical absorption bands created by the collective oscillation of electrons occupying states near the Fermi level, leading to a phenomenon called surface plasmon resonance (SPR).1 The SPR effect has been well explored in metal nanostructures of various sizes and shapes, and it has found applications in several important fields such as optics, nano-electronics, catalysis, sensing and biomedicine.2 Similarly, the linear and nonlinear optical (NLO) properties of metal NPs have attracted significant research attention, and due to their marked NLO behavior they have potential applications in optical information processing,3 optical data storage,4 all-optical switching,5 and optical limiting.6-7 Metal NPs are known to exhibit third-order nonlinearity with fast response times (a few picoseconds).8 The nonlinear optical response of metal NPs exhibits either saturable absorption (SA) or reverse saturable absorption (RSA) when excited near the plasmon resonance.3 While SA is seen at relatively low laser fluences, RSA becomes prominent at relatively high fluences. Graphene Oxide also behaves in a similar fashion when excited near resonance.9 SA is useful for passive Q-switching of laser systems, while RSA can be utilized for the construction of optical limiters for the protection of delicate optical instruments and human eyes from intense laser beams.10-12 In an optical limiter, the optical transmittance is substantially reduced as the input light intensity or fluence is increased.13 Sridharan et al.

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has reported a

reduction of SA in silver (Ag) NPs doped with transition metals such as Ni and Fe, resulting in an enhanced optical limiting efficiency. Ideally, an optical limiting device should uniformly show RSA from low to high laser fluences at the studied wavelength, and SA should be negligible (C60 excited at 532 nm using ns laser pulses is an example).


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Noble metal quantum clusters (QCs) are a special class of materials with sizes ranging from subnanometer to 2 nm, containing tens of atoms protected by an organic ligand, exhibiting distinctly different optical, electronic and chemical properties in comparison to their nanoparticle (NP, size > 5 nm) and bulk forms. Interest in the research and development of few-atom metal QCs is on the rise at present due to the convergence of atomic and nanoparticle properties at this size regime.15 Typically, metal QCs are named along with the protecting ligand group, for example Au144(SR)60, where Au144 is the number of gold atoms in the cluster, and (SR)60 is the 2phenylethanethiolate ligand (SR). Since the size of the metal QCs reach the de Broglie wavelength of the electron at the Fermi energy of the metal (EFermi = 5.53 eV and 5.49 eV for Au and Ag, respectively), plasmon excitation is hindered and SPR becomes absent.16 Philip et al. 17 has reported that due to the absence of plasmon resonance, a significant enhancement of the optical limiting efficiency occurs in atomic clusters of gold compared to Au NPs. Therefore it will be worthwhile to investigate atomic clusters of silver – which is another noble metal – to see whether similar behavior can be elicited. Graphitic carbon nitride (g-C3N4, hereafter referred as GCN) is a material of significant research interest for applications in heterogeneous catalysis, energy conversion & storage.18-20 GCN is well recognized as a stable allotrope of carbon nitride, and as a metal-free visible light active (bandgap, Eg = 2.7 eV) photocatalyst. It is a highly stable material like titanium oxide, and its two dimensional sheet-like structure is ideal for grafting metal or semiconductor nanoparticles on its surface. To the best of our knowledge, there are no reports yet on the optical limiting properties of pristine and/or grafted GCN. In the present work we investigate the nonlinear optical absorption of Ag9 QCs, Ag NPs and GCN at the visible spectral wavelength of 532 nm, using 5 ns laser pulses. We show that Ag9


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QCs exhibit enhanced optical limiting in comparison to Ag NPs, essentially because of the absence of surface plasmon resonance in them. Atomic silver clusters being highly stable, these can be potentially used for the protection of human eyes and sensitive detectors from harmful laser radiation. We further demonstrate that the weak nonlinearity of pristine GCN can be substantially enhanced by grafting lightly with Ag9 QCs, while grafting with Ag NPs yields a moderate enhancement. Donor-acceptor complexes are formed when the composites are photoexcited, and nonlinearity enhancement occurs due to both energy and electron transfer mechanisms. EXPERIMENTAL Synthesis of Ag9 QCs-GCN. Ag9 QCs prepared by a simple solid-state synthetic approach21 were reduced on an ethanolic solution of pre-synthesized GCN obtained by the pyrolysis of urea in a sealed crucible.22 Typically, an orange color powder was obtained by grinding 47 mg of AgNO3 with 187 mg of H2MSA and a final brown color product was obtained by grinding the orange color powder with 50 mg of NaBH4. A reddish brown colloidal solution (Ag9 QCs solution) was obtained by the dropwise addition of 15 ml of DI water to the brown color product. GCN solution was prepared by ultrasonic dispersion of 20 mg of pre-synthesized GCN in 20 ml of ethanol. 2 ml of Ag9 QCs solution was slowly added in drops to the GCN solution under vigorous stirring resulting in the immediate precipitation of Ag9 QCs. The Ag9 QCs obtained through this process had a composition of Ag9(H2MSA)7. The final product (hereafter referred to as Ag9 QCs-GCN) which settled at the bottom of the beaker was washed well with excess ethanol.


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Synthesis of Ag NPs-GCN. For synthesizing Ag NPs-GCN composite, 20 mg of GCN was first dispersed in 50 ml DI water by ultrasonication. 2.1 mg of AgNO3 (0.25 mM) and 3.7 mg of trisodium citrate (0.25 mM) were added to this solution under agitation. After 5 min, under vigorous magnetic stirring, 1.5 ml of freshly prepared NaBH4 (0.01 M) solution was added in drops. The color of the solution changed at once to yellow indicating the formation of Ag-sols. The final product (hereafter referred to as Ag NPs-GCN) was collected by repeated centrifugation using excess DI water. Characterization. Powder X-ray diffraction (XRD) patterns of the samples were recorded using an X-ray diffractometer (Rigaku-Dmax 2500) with Cu-Kα radiation (λ = 0.15405 nm, 40 kV, 100 mA). The sample morphology, chemical composition and orientation were studied by transmission electron microscopy (TEM) using a FEI Technai G2 F20 (200 kV). Samples for TEM were prepared by dropping the sample solution dissolved in ethanol onto a carbon-coated copper grid. Solid-state UV-Vis measurements were performed on a JASCO V-550 UV-Visible spectrophotometer equipped with an integrating sphere (JASCO ISV-469), using a dedicated powder sample holder (JASCO PSH-001). UV-Vis absorption spectra of the liquid samples were measured using the same spectrophotometer. Nonlinear transmission measurements. Nonlinear optical transmission of the samples was measured by the open aperture Z-scan technique.23 Here the laser beam was focused by a converging lens, and the sample transmission was measured for different positions with respect to the focal point. An Nd:YAG laser (MiniLite, Continuum) emitting 5 ns laser pulses at the single-shot mode, at the second harmonic wavelength of 532 nm, was used for excitation. A plano-convex lens of 10.75 cm focal length was used for focusing the laser to a spot radius of about 18 µm. Uniform dispersions of the samples (Ag9 QCs, Ag NPs, GCN, Ag9 QCs-GCN and


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Ag NPs-GCN) obtained by ultrasonication, taken in 1 mm thick glass cuvettes, were used for the measurements. The sample was mounted on a programmable linear translation stage and moved along the beam axis from one side of the focal point (z=0) to the other in small steps of 200 microns each. The transmission T (z) was measured at each step. Since the sample experiences a different incident fluence at each position as it is translated along the z direction, any nonlinearity in transmission will be revealed by this measurement. Depending on the nonlinearity shown by each sample, laser pulse energies ranging from 10 µJ to 150 µJ were used in the measurements as appropriate. Laser energy reaching the sample (input) and exiting the sample (output) were measured using two pyroelectric energy probes (RjP 735, Laser Probe Inc.). The experiment was automated using a LabVIEW program. A plot of T(z) vs. z gives the Z-scan curve, from which the nature of the absorptive optical nonlinearity can be determined. RESULTS AND DISCUSSION Structure, morphology and chemical composition analysis. Measured XRD patterns of Ag NPs, Ag9 QCs, GCN, Ag9 QCs-GCN, and Ag NPs-GCN are shown in Figure 1. For the GCN, Ag9 QCs-GCN and Ag NPs-GCN samples, the XRD peak formed at 2θ = 27.4o can be indexed to the (002) plane of GCN. In both Ag9 QCs-GCN and Ag NPs-GCN, no significant diffraction peaks corresponding to Ag are observed owing to the small loading of Ag9 QCs and Ag NPs in GCN. The intense XRD peaks seen for as-synthesized Ag NPs can be indexed to the face-centred cubic (FCC) structure of Ag (JCPDS card # 04-04783). Ag9 QCs do not exhibit peaks of similar strength, but from a magnified plot (see inset of Figure 1) a peak at 2θ = 38o can be clearly seen, which corresponds to the (111) plane of Ag.21


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TEM and HAADF-STEM (high angle annular dark field scanning transmission electron microscopy) images of the as-synthesized Ag9(H2MSA)7 QCs shown in Figure 2a and b, respectively, appear as tiny dots. Owing to the high intensity of the electron beam falling over a small area as shown in Figure 2c, ligand protected clusters coalesce and form nanoparticles.24 The lattice spacing (d) value of 0.24 nm estimated from Figure 2c can be indexed to the (111) plane of Ag. Energy dispersive X-ray spectra (EDS) of Ag9(H2MSA)7 QCs (see Figure S1) was acquired from the area marked in the HAADF-STEM image (see inset of Figure S1). The elemental peaks from the EDS spectrum corresponding to Ag, S and Na confirm the formation of Ag9 QCs. On the other hand, Figures 2d, 2e and 2f show the TEM, HAADF-TEM and HRTEM images of GCN. The sheet-like morphology seen from TEM images, and the EDS spectrum of GCN (see Figure S2) with elemental peaks corresponding to C and N acquired from the area marked in the HAADF-STEM image (inset of Figure S2), confirm the formation of GCN. Figures 3a and 3b depict the TEM and HAADF-STEM images of Ag9 QCs-GCN. Ag9 QCs are not visible in the TEM image shown in Figure 3a, owing to the small percentage loading and their extremely small size. On the contrary, Figure 3b shows the presence of tiny dot like Ag9 clusters (highlighted using circles) which appear to be perfectly grafted on GCN. Similarly, Figure 3c shows the TEM image of Ag NPs-GCN in which the Ag NPs are not clearly visible owing to the small fraction of particle loading and its small size. But, the presence of Ag NPs grafted on GCN is clearly evident from the HAADF-TEM image shown in Figure 3d. EDS analysis shown in Figure S3 confirms the formation of Ag9 QCs-GCN and Ag NPs-GCN composites without impurity.


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Linear optical properties. Linear optical properties tend to change with size and shape for nanoscale materials.25-26 UV-Vis optical absorption spectra of Ag9 QCs and Ag NPs are shown in Figure 4a, the inset of which shows color photographs of Ag9 QCs and Ag NPs in solution. The formation of Ag NPs is confirmed from the bright yellow solution which exhibits an SPR peak at ~390 nm, while the dark solution of Ag9 QCs exhibits an absorption spectrum resulting from the quantum confinement effect, which is practically devoid of the SPR signature.21 Figure 4b shows the optical absorbance spectra of GCN, Ag9 QCs-GCN and Ag NPs-GCN samples measured by diffuse reflectance spectroscopy. Compared to pristine GCN, both Ag NPs-GCN and Ag9 QCsGCN exhibit improved visible light absorptivity owing to the presence of silver. Nonlinear optical properties. Optical limiting properties of the samples were studied using the open aperture Z-scan technique employing 5 ns laser pulses at 532 nm.23 Samples were dispersed in ethanol to the appropriate dilution such that when taken in glass cuvettes of 1 mm path length they had a linear transmission of 70%. The measured open aperture Z-scan curves and the corresponding optical limiting curves calculated from the Z-scan data, for GCN and Ag9 QCsGCN composites, are plotted in Figures 5a to 5c. In comparison to Ag9 QCs the nonlinearity of GCN is weak. However, when Ag9 QCs are lightly grafted to GCN the optical limiting efficiency of the resulting composite is substantially improved. For example, while an input laser pulse energy of 50 µJ was sufficient for measuring limiting in Ag9 QCs and Ag9 QCs-GCN, it had to be raised to 150 µJ for getting a similar limiting efficiency in pristine GCN (GCN does not show unambiguous nonlinearity at 50 µJ, see Figure S4). Such improvement in optical limiting has been

reported

previously for metal

oxide-CNT

hybrids,27

and Se-Te/C

core-shell

nanostructures.26 Figures 6a to 6c show the open aperture Z-scans and the corresponding fluence dependent nonlinear transmissions obtained for Ag NPs, GCN and Ag NPs-GCN respectively.


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Absorption saturation of the SPR band of Ag NPs6 is clearly visible in the Z-scans (except for pristine GCN) in the form of two humps flanking the central valley. In this case also the nonlinearity of the composite is enhanced compared to that of pristine GCN, but it is not as pronounced as in the case of Ag9 QCs-GCN. For example, from Figures 5 and 6 we can estimate that for an input laser fluence of 5 J/cm2, the percentage enhancement in limiting of GCN is 21.2% with Ag9 QCs grafting, and 4% with Ag NPs grafting. We find that the nonlinear transmission of the samples given in Figures 5 and 6 can be best modelled numerically by a propagation equation which involves excited state absorption and relatively weak absorption saturation, given by 28 dF  ασ  2 = −α F −  F dz ′  2hν 

(1)

where σ is the excited state absorption cross section, F is the input laser fluence, z’ is the sample path length, h is the Planck’s constant, and ν is the laser frequency given by c/λ where c is the light velocity and λ is the wavelength. α is the absorption coefficient which is given by α (F) = α0/(1+F/Fs) where Fs is the saturation fluence. Since Fs is given by hν/2σ0, it is possible to calculate σ0, the ground state absorption cross section, from the value of Fs (which is particularly advantageous in the case of samples whose molar concentrations are not directly known, as in the present case). Numerically calculated values of Fs and σ are given in Table 1. As discussed below, excited state absorption due to free carriers is the major cause of the observed nonlinearity.

Mechanism of optical nonlinearity. Plasmon excitation is hindered in Ag9 QCs because their size is close to the de Broglie wavelength of the electron at the Fermi energy of metallic silver


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(EFermi = 5.49 eV). The absence of SPR is confirmed from the broad absorption spectrum of Ag9 QCs shown in Fig. 4a. Due to strong quantum confinement, absorption peaks will be formed at 450, 479, 625 and 886 nm in Ag9(H2MSA)7 QCs, which can be related to the Kohn-Sham molecular orbitals and ascribed to various electron transitions from the HOMO (highest occupied molecular orbital) to the LUMO (lowest unoccupied molecular orbital) (Figure 7a).21 The first set of peaks between 886 to 625 nm (1.4-2 eV) can be attributed to the HOMO-LUMO transitions or intraband (sp-sp) transition, while the peaks between 479 to 450 nm (2.58-2.75 eV) are due to mixed intraband (sp-sp) and interband (d-sp) transitions. Theoretical studies suggest that molecular orbitals of Ag9 QCs near the Fermi level will mostly have sp character since the d-band is situated more than 3 eV below the Fermi level.29,30 When Ag9 QCs are excited with nanosecond laser pulses at 532 nm, intraband and interband transitions will generate free carriers in the LUMO level. These free carriers have a tendency to absorb additional photons through a phonon assisted process, and therefore, a strong optical limiting (decrease in sample transmittance) is facilitated through this free carrier absorption phenomenon.17 On the other hand, in the case of Ag NPs, the absorption spectrum (Fig. 4a) shows SPR band at ~390 nm, which arises from the collective oscillation of the free electrons in the conduction band that occupy energy states near the Fermi level. Since the specific heats of the conduction electrons are very small, they can be easily elevated to temperatures of several hundred degrees by photoexciting the plasmon band.31-33 As a result the Fermi-Dirac electron distribution gets modified, since part of the one-electron levels below the Fermi level are emptied and part of those above are occupied. This leads to a transient modification of the dielectric constant and hence a transient redistribution of the equilibrium plasmon band, which often results in a transient decrease in absorption.6, 34 This decrease in absorption (which appears as two humps in


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the Z-scan as seen in Figure 6) is sometimes referred to as “plasmon band bleach” in literature. However, as the input light fluence is further increased interband and intraband transitions gain prominence, eventually leading to substantial free carrier absorption and optical limiting behavior. In the case of composites (Ag9 QCs-GCN and Ag NPs-GCN) an intersystem excitation transfer mechanism, typically via energy and electron transfer processes, contributes to enhancing the free carrier population and hence the limiting action. From the overlap of the absorption spectra it is obvious that resonant energy transfer is more likely in Ag NPs-GCN. On the other hand for smaller nanoparticles (size < 2 nm) and atomic clusters where spectral overlap is insufficient, excited state intersystem coupling of energy 35 and electron transfer may be dominant. The ability of GCN to photo-catalytically reduce and oxidize water in the presence of sacrificial electron donors and acceptors, respectively, is well known.36 Since Ag9 QCs are anionic in character, they can efficiently accept electrons from GCN. The relatively higher photoluminescence quenching observed in Ag9 QCs-GCN (see Figure S5) also indicates that excited electrons in GCN are more efficiently transferred to Ag9 QCs compared to Ag NPs. This charge transfer can be explained if the composites are considered to form donor-acceptor complexes upon photo-excitation (Figure 7b). It has been recently reported that a donor-acceptor ionic complex containing positively charged porphyrin (donor) and negatively charged Au NPs and graphene oxide (acceptor) exhibits photoluminescence quenching and enhanced optical limiting.

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Our results also are

similar in that Ag9 QCs-GCN exhibit higher photoluminescence quenching and higher excited state absorption cross section (2.0 × 10-19 cm2) in comparison to Ag NPs-GCN (4.0 × 10-20 cm2).

CONCLUSION


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In conclusion, Ag9 quantum clusters, Ag nanoparticles, GCN, and their composites have been prepared through a simple chemical approach. The structure, morphology and chemical composition of the as-synthesized samples are determined from XRD, TEM and EDS analysis. The optical absorption spectrum of Ag9 QCs is significantly different from that of Ag NPs due to the quantum confinement effect. Open aperture Z-scan measurements of Ag NPs reveal the presence of absorption saturation due to SPR, but the same is absent in Ag9 atomic QCs, owing to which there is a substantial enhancement in its optical limiting efficiency. The nonlinear optical response of pristine GCN is weak, but it is enhanced in Ag NPs-GCN and substantially improved in Ag9 QCs-GCN, despite the fact that Ag loading in the composites is small. Photoexcited GCN composites can be visualized as donor-acceptor complexes, and the enhanced electron transfer from GCN to Ag9 QCs-GCN leads to enhanced free carrier absorption and optical limiting. Unlike other involved approaches employed to prepare Au and Ag QCs, the present solid-state route is a cost-effective and time saving approach for the preparation of stable Ag9 QCs, which can be easily transferred on to glass or plastic substrates to construct optical limiters for device applications. Finally, the optical nonlinearity of GCN is substantially enhanced by grafting with Ag9 QCs to yield a stable material with photocatalytic and optical limiting properties, with potential applications in energy conversion and storage as well.

ASSOCIATED CONTENT EDS data of Ag9 QCs, GCN, Ag9 QCs-GCN, Ag NPs-GCN, open aperture z-scan data of GCN measured using an input laser pulse energy of 50 µJ, and PL spectra of the samples. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION


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*Corresponding author Email: [email protected]; Fax: +91-80-236104392; Tel: +91-80-2310122 (x 479)

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FIGURES

Figure 1. XRD patterns of the synthesized samples. Inset shows a magnified view of the pattern measured for Ag9 QCs. Pristine GCN and the composites have nearly identical patterns, showing that silver loading in the composites is small.


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Figure 2. (a, b) TEM and HAADF-STEM images of ligand protected Ag9(H2MSA)7 quantum clusters. (c) HRTEM image of the Ag9(H2MSA)7 QCs, depicting the formation of nanoparticles due to prolonged electron beam irradiation. (d, e, f) TEM, HAADF-STEM and HRTEM images of graphitic carbon nitride (GCN).


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Figure 3. (a, c) TEM images of Ag9 QCs-GCN and Ag NPs-GCN. (b, d) HAADF-STEM images of Ag9 QCs-GCN and Ag NPs-GCN, which clearly indicate the presence of Ag QCs (tiny dots inside the circles) and Ag NPs grafted on GCN.


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Figure 4. (a) UV-Vis absorption spectra of Ag9 QCs and Ag NPs. Inset shows photographs of Ag9 QCs (dark solution) and Ag NPs (bright yellow solution). Absorbance given on the left yaxis is for Ag9 QCs, while that on the right y-axis is for Ag NPs. Surface plasmon resonance is prominent in the spectrum of Ag NPs. (b) UV-Vis absorption spectra of GCN, Ag9 QCs-GCN and Ag NPs-GCN measured through diffuse reflectance spectroscopy.


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Figure 5. Normalized transmittance plotted against input laser fluence for (a) Ag9 QCs, (b) GCN, and (c) Ag9 QCs-GCN, for excitation at 532 nm using 5 ns laser pulses. Open aperture Z-scans are shown in the insets. Data shown in (a) and (c) are measured using laser pulse energy of 50 µJ, while the data shown in (b) is measured using a higher energy of 150 µJ. Solid curves are numerical fits to Equation 1.


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Figure 6. Normalized transmittance plotted against input fluence for (a) Ag NPs, (b) GCN, and (c) Ag NPs-GCN, for excitation at 532 nm using 5 ns laser pulses. Open aperture Z-scans are shown in the insets. Saturable absorption owing to the SPR band is clearly visible in the Z-scan curves of Ag NPs in the form of two small humps flanking the central valley. Data shown in (a) and (c) are measured using laser pulse energy of 20 and 50 µJ respectively, while the data shown in (b) is measured using a higher energy of 100 µJ. Solid curves are numerical fits to Equation 1.


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Figure 7. (a) Energy level diagram for Ag9 cluster. Absorption at 886 nm and 625 nm can be attributed to the (sp-sp) interband transition. Absorption at 479 nm and 450 nm are due to intraband (sp-sp) as well as interband (d-sp) transitions. (b) The composites form donor-acceptor complexes upon excitation by the laser beam, facilitating electron transfer.


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TABLE

Table 1. Absorption cross sections and saturation fluences calculated for the samples, for 5 nanosecond excitation at 532 nm. The nonlinear transmission behavior will depend on the values of σ/σ0 and Fs. Sample

Linear Laser Pulse Transmission Energy at 532 nm (microjoules) (%)

Excited state absorption cross section, σ

Saturation Fluence, Fs (J/cm2)

(cm2)

Ground state absorption cross section, σ0

σ/σ0

(cm2)

Ag9 QCs

70

50

1.8 × 10-19

10

1.9 × 10-20

9.5

Ag NPs

70

50

1.0 × 10-19

1.7

1.1 × 10-19

0.9

GCN

70

100

4.0 × 10-20

10

1.9 × 10-20

2.1

50

1.6 × 10-19

9.5

2.0 × 10-20

8.0

50

7.0 × 10-20

10

1.9 × 10-20

3.7

Ag9 QCs- 70 GCN Ag NPs- 70 GCN


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TABLE OF CONTENTS


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Figure 1. XRD patterns of the synthesized samples. Inset shows the XRD pattern of Ag9 QCs plotted separately. 221x184mm (300 x 300 DPI)



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Figure 2. (a, b) TEM and HAADF-STEM images of ligand protected Ag9(H2MSA)7 quantum clusters. (c) HRTEM image of the Ag9(H2MSA)7 QCs, depicting the formation of nanoparticles owing to prolonged electron beam irradiation. (d, e, f) TEM, HAADF-STEM and HRTEM images of graphitic carbon nitride (GCN). 728x482mm (96 x 96 DPI)


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Figure 7. (a) Energy level diagram for Ag9 cluster. Absorption at 886 nm and 625 nm can be attributed to the (sp-sp) interband transition. Absorption at 479 nm and 450 nm are due to intraband (sp-sp) as well as interband (d-sp) transitions. (b) The composites form donor-acceptor complexes upon excitation by the laser beam, facilitating electron transfer. 777x341mm (96 x 96 DPI)


Nonlinear Optical Investigations in Nine-Atom Silver Quantum Clusters

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