**4.2 Transmission electron microscope (TEM)**

The morphology, size distribution, and selected area electron diffraction (SAED) circular patterns of silver nanoparticles were exhibited in **Figure 2**. From **Figure 3(a–c)**, the Ag NPs are nanospheres in shape, crystalline, and size is about 7 nm respectively. The planes (111), (200), (220), and (311) from the SAED pattern were exhibiting FCC structure from **Figure 3(d)**.

#### **4.3 UV-Vis absorption spectroscopy**

UV–Vis linear absorption of silver nanoparticles with 240–800 nm range areas displayed in **Figure 4**. The absorption spectra were measured at various concentrations of silver nanoparticles added to 3 ml of distilled water and silver nanoparticles with Eu and Sm complexes at various concentrations.

UV–visible absorption spectrum of various concentrations are (0.16, 0.33, 1.0, 1.60, 2.32, 3.32, 6.64 and 9.96 μM) of silver nanoparticles as depicted in **Figure 3(a)**. The band at 262 nm is becoming strong as increase the concentration because the number of organic compounds absorption is increasing, which helps to reduce the silver ions. The plasmonic band of silver nanoparticles at 460 nm is increasing with an increase in the concentration from 0.16 μM to 9.96 μM. The inset shows the absorption of silver NPs is increasing linearly at 460 nm i.e. the nanoparticles are not agglomerating with concentration.

The absorption spectrum of europium complex with various concentrations (0.13, 0.20, 0.26, 0.33, 0.99, 1.98 and 3.3 μM) as displayed in **Figure 4(b)**. The linear absorption peaks at 262 nm and 342 nm corresponding to π ! π\* transition *Green Synthesis of Metal Nanostructures and Its Nonlinear Optical Properties DOI: http://dx.doi.org/10.5772/intechopen.99449*

#### **Figure 3.**

*(a) TEM figures (b) high-resolution TEM (HRTEM) image (c) particle size distribution and (d) SAED circular patterns of silver nanoparticles.*

due to complex absorbance and n ! π\* transition of europium and occurrence of the shoulder at (1.99 and 3.3 μM) higher concentrations due to strong interaction among the europium molecules [38].

The absorption spectrum of europium complex (0.13 μM) with various concentrations (0.16, 0.33, 1.0, 1.60, 2.32, 3.32, and 6.64 μM) of silver nanoparticles (Ag NPs) and the absorption bands appeared at 262 nm, 342 nm, and 460 nm as seen in **Figure 4(c)**. The absorption band in the range of 300–400 nm is shifted to red slightly because the interaction of silver nanoparticles and europium molecules i.e. the Plasmon field of silver nanoparticles is influencing the europium ions but the surface plasmonic peak of Ag NPs at 460 nm is not altering. The inset shows a linear increase in absorption with silver NPs, suggesting that nanoparticles are not agglomerating in the complex solution.

Absorption spectrum of samarium complex (1.45 μM) with various concentrations (0.16, 0.33, 1.0, 1.60, 2.32 and 3.32 μM) of silver nanoparticles as depicted in **Figure 5**. The absorption band at 265 nm corresponds to the π ! π\* transition of the samarium complex and biological components in the extract, and bands at 342 nm and 460 nm correspond to the n ! π\* transition of samarium and plasmonic peak (SPR) of silver nanoparticles. Samarium complex concentration (1.45 μM) is more as compared with silver nanoparticles so the bands of samarium (265 nm and 342 nm) are predominating the SPR.

#### **4.4 Fourier transform infrared (FTIR) technique**

FT-IR spectrum displays the *Raphanussativus* leaf extract used for biosynthesis of Ag NPs in **Figure 6**. The Silver suspension was loaded on a potassium bromide (KBr) pellet and dried. The FT-IR peaks of silver nanoparticles appeared at

#### **Figure 4.**

*UV–visible spectra of (a) silver nanoparticles with various concentrations: a) 0.16, b) 0.33, c) 1.0, d) 1.60, e) 2.32, f) 3.32, g) 6.64 and h) 9.96 μM. Inset displays the linear absorption of silver nanoparticles at 460 nm with increasing silver. (b) Various concentrations of europium ions in ethanol are: i) 0.13, ii) 0.20, iii) 0.26, iv) 0.33, v) 0.99, vi) 1.98 and vii) 3.3 μM. (c) Various concentrations of Ag NPs with 0.13 μM europium solution: A*<sup>0</sup> *) 0.16, b*<sup>0</sup> *) 0.33, c*<sup>0</sup> *) 1.0, d*<sup>0</sup> *) 1.60, e*<sup>0</sup> *) 2.32, f*<sup>0</sup> *) 3.32 and g*<sup>0</sup> *) 6.64 μM. Inset figure exhibits the absorption of silver nanoparticles at different concentrations in (0.13 μM) europium solution at 460 nm.*

3315 cm�<sup>1</sup> indicate hydroxyl –OH stretching; 2929 cm�<sup>1</sup> and 2834 cm�<sup>1</sup> assigned to stretching C-H modes of methyl groups; 1636 cm�<sup>1</sup> assigned to carbonyl (–C=C) stretching; 1382 cm�<sup>1</sup> attributed to –C-O stretching mode of water-soluble organic components like polyphenols, alkaloids, and flavonoids in *Raphanussativus* extract; 1040 cm�<sup>1</sup> indicate the C-O alcoholic stretching group. These results concluded that the organic compounds of the extract are responsible for making nanoparticles [39–42].

### **4.5 Photoluminescence studies**

The excitation spectra of europium complex (λem = 614 nm) at various concentrations (0.03, 0.07, 0.13, 0.20, 0.26, 0.66, 1.99 and 3.3 μM) as shown in **Figure 7**. The bands in the range 250–400 nm are due to π ! π\* transitions of Eu. At very low concentrations of europium (0.03 μM and 0.07 μM), the broadband appears at 340 nm. As increase the concentration (0.13–0.66 μM), the new band appeared at 270 nm and 340 nm band splits into two bands which results, strong interactions among the europium complex in the solution phase and depend on the

*Green Synthesis of Metal Nanostructures and Its Nonlinear Optical Properties DOI: http://dx.doi.org/10.5772/intechopen.99449*

**Figure 5.**

*Linear absorption spectrum of samarium complex (1.45 μM) with various concentrations are a) 0.166 b) 0.33 c) 1.0 d) 1.60 e) 2.32 f) 3.32 and g) 9.96 μM of Ag NPs.*

**Figure 6.** *FT-IR Spectrum of* Raphanussativus *leaf extract used for biosynthesized silver nanoparticles.*

concentration of europium. As increase the concentration further (1.99 μM and 3.3 μM), 270 nm and 340 nm bands shifted to the blue region, and the 270 nm band has vanished. Even for further concentrations, the 370 nm band shifted to the red region, which indicates that the interaction among europium ions enhanced.

The emission spectra of the europium complex excited with 262 nm as shown in **Figure 8**. The band at 614 nm (<sup>5</sup> D0 ! <sup>7</sup> F2) is a hypersensitive electric-dipole

**Figure 7.** *Excitation spectra of europium complex (λem = 614 nm) at various concentrations (0.03, 0.07, 0.13, 0.20, 0.26, 0.66, 1.99 and 3.3 μM).*

#### **Figure 8.**

*Emission spectra of europium complex (λexc = 262 nm). The inset figure displays the <sup>5</sup> D0* ! *<sup>7</sup> F2 transition on various silver nanoparticle in europium complex (0.13 μM).*

transition, a dominant peak. The magnetic dipole transitions are at 577 nm and 590 nm (<sup>5</sup> D0 ! <sup>7</sup> F0 and <sup>5</sup> D0 ! <sup>7</sup> F1). Inset displays the figure of concentration of silver nanoparticles with the intensity of electric dipole transition (<sup>5</sup> D0 ! <sup>7</sup> F2) in *Green Synthesis of Metal Nanostructures and Its Nonlinear Optical Properties DOI: http://dx.doi.org/10.5772/intechopen.99449*

europium complex. As increase, the concentration of silver nanoparticles, affect the ligand field surrounding the europium ions, consequently enhance the electric dipole transition rate. The <sup>5</sup> D0 ! <sup>7</sup> F2 transition enhanced its intensity 25 times up to the 6.64 μM of silver nanoparticle and quenching slowly exceeding 6.64 μM in 0.13 μM europium solution. Hence, we emphasize the effect of silver on the europium luminescence emission intensity of electric dipole transition (<sup>5</sup> D0 ! <sup>7</sup> F2).

The emission spectra of europium (λexc = 350 nm) with various (0.16, 0.33, 1.0, 1.60, 2.32, 3.32 and 6.64 μM) of silver nanoparticles as seen in **Figure 9**. The luminescence emission intensity of <sup>5</sup> D0 ! <sup>7</sup> F2 transition starts enhanced and maximum at 1.6 μM of silver NPs and gets quenched for further increase the silver in europium (0.13 μM) and the enhancement factor is �5. In the spectra of excitation, Ag NPS cannot intensify the europium ions excitation at 262 nm and 350 nm, nanoparticles absorption (400–550 nm) is uncertain at excitation wavelengths, although it enormously affects the luminescence centers of europium ions in the emission. At excitation wavelength around absorption of silver (350 nm), the intensity of the electric field increased certain times induce an intensification in luminescence intensity by a few hundred times. The enhancement of luminescence intensity because of the overlap of europium emission and the scattering of nanoparticles which bank on the gap of NPS [33, 43, 44]. The distance of NPS reduces, the scattering at 612 nm overlay on the emission enlarges appearing in the increment of luminescence intensity. Similarly, the intensity decreases with the distance of nanoparticles increases. The luminescence increment was observed only at 0.13 μM concentration of europium and at other concentrations of 0.10, and 0.20 μM, the luminescence is quenching. That means, the enhancement of luminescence intensity purely depends on the concentration, distance, size, and shape of nanoparticles.

#### **Figure 9.**

*Emission spectra of europium with various silver nanoparticles: a) 0.16, b) 0.33, c) 1.0, d) 1.60, e) 2.32, f) 3.32 and g) 6.64 μM excited with 350 nm. Inset figure shows dependence of luminesce intensity with silver concentration. Eu concentrations are (a) 0.1 (b) 0.13 and (c) 0.2 μM.*

At 260 nm excitation, the enhancement factor is high as compared with 350 nm excitation, which is caused by the biological components in extract and ligand enhance the luminescence effectively and the overlap of emission europium and scattering of silver, luminescence efficiency will be high.

To understand the luminescence enhancement alteration on various lanthanides, we also obtained the emission of the samarium complex. The emission spectra of samarium with different concentrations of silver nanoparticles (0.16, 0.33, 1.0, 1.60, 2.32, 3.32, and 6.64 μM) at 350 nm excitation as can be seen in **Figure 10**. The inset picture exhibits the luminescence emission versus silver concentration with varying samarium concentrations (1.32, 1.45, and 1.58 μM). The luminescence intensity is noticed at 1.45 μM of samarium. The transitions at 645 nm (4 G5/2 ! <sup>6</sup> H9/2) electric dipole, 566 nm (4 G5/2 ! <sup>6</sup> H5/2), and 602 nm (4 G5/2 ! <sup>6</sup> H7/2) are magnetic dipole transitions. The enhancement factor of electric dipole transition is 7.4 at 2.32 μM of silver. For more increases in silver, the enhancement factor decreases [45, 46]. The enhancement factors for magnetic dipole transitions at 566 nm are 1.9 and 602 nm is 5.2. The scattering can alter by altering the distance of nanoparticles. Hence, the change in luminescence intensity overlay on the scattering and emission spectra as in the case of europium. So this is the reason for emission intensity quenched at 1.32 μM and 1.58 μM of samarium as depicted in the inset.

#### **4.6 Nonlinear optical properties**

Nonlinear absorption coefficients were obtained by using the Z-Scan technique with Nd: YAG laser, repetition rate 10 Hz, 30 ps pulses with 532 nm, and Ti: Sapphire laser, 1 kHz, 110 fs pulses with 800 nm. The nonlinear absorption studies of the biosynthesized silver nanoparticles solution were studied using an open aperture Z-scan set up (**Figure 11**) [29, 47]. In the Z-scan setup, the Gaussian

#### **Figure 10.**

*Emission spectra of samarium 1.45 μM with silver nanoparticles: a) 0.16, b) 0.33, c) 1.0, d) 1.60, e) 2.32, f) 3.32 and g) 6.64 μM excited with 350 nm. Inset shows the dependence PL intensity on silver concentration at various samarium concentrations (a) 1.32 (b) 1.45 and (c) 1.58 μM.*

*Green Synthesis of Metal Nanostructures and Its Nonlinear Optical Properties DOI: http://dx.doi.org/10.5772/intechopen.99449*

**Figure 11.** *Z-scan experimental set up a) open aperture b) closed aperture.*

profile of the laser beam is concentrated with the lens. The silver solution in a 1 mm thickness quartz cuvette is moving along the Z-direction through-beam-focused direction. A focus point, the silver sample undergoes maximal intensity and slowly reduce from the focal point in both directions and the *f*/40 configuration is operated here. The width of the sample undergoes less than the Rayleigh range (3 mm). For beam, shaping apertures are used, and to change the intensity of laser neutral density filters are used. The experimental data is measured by examining the sample along with the focus and saving the data by boxcar averager (model SRS 250) with an analog-to-digital (ADC) card to a computer. The absorption coefficient (*α*2) for open aperture Z scan is measured by fitting the transmittance equation

$$T\_{OA(2PA)} = 1 - \frac{a\_2 I\_0 L\_{\epsilon\mathcal{Y}}}{2^{3/2} \left(1 + z^2 / z\_0^2\right)}\tag{1}$$

Where, I0 - intensity at focus on the sample, z - sample position, λ - laser wavelength, z0 = πω<sup>0</sup> 2 /λ is Rayleigh range, ω<sup>0</sup> - beam waist at the focus (Z = 0), *α*2 nonlinear absorption coefficient, and Leff - effective path length is Leff <sup>¼</sup> <sup>1</sup>‐<sup>e</sup> ‐*α* 0*L <sup>α</sup>*<sup>0</sup> ,Lsample length, *α*<sup>0</sup> is the linear absorption coefficient.

**Figure 12(a)** displays the Z-scan data of open aperture and varying input intensities of silver nanoparticles. The symbols are the experimental data, those are fitting theoretically (solid curves) using Eq. 1. These curves showed reverse saturable absorption (RSA) behavior in biosynthesized silver nanoparticles, which are ascribed to the excitations from the plasmonic band to the free carrier absorption band of silver nanoparticles and two-photon absorption (TPA) from the ground state. We determined <sup>α</sup><sup>2</sup> is 10.2 � <sup>10</sup>�<sup>9</sup> cm<sup>2</sup> /W at various intensities 1.2 GW/cm<sup>2</sup> – 3.9 GW/cm<sup>2</sup> . These coefficients are not altering much at input intensities.

Optical limiting experimental curves of silver nanoparticles as shown in **Figure 12(b)**. The threshold optical limiting value is 4 mJ/cm<sup>2</sup> . Nonlinear scattering is not observed so optical limiting is due to two-photon absorption (TPA) and excited-state absorption (ESA) from SPR.

Similarly, **Figure 13(a)** illustrates the open aperture Z-scan data measured for silver nanoparticles with a wavelength of 800 nm at different input intensities from

**Figure 12.** *(a) Z-scan of silver nanoparticles with various input intensities and (b) optical limiting curve with 532 nm.*

**Figure 13.** *(a) Z-scan of silver nanoparticles with various input intensities and (b) optical limiting curve with 800 nm.*

4.7x1011–7.7x10<sup>11</sup> W/cm<sup>2</sup> . Solid lines give the fits theoretically acquired by Eq. (2). RSA behavior of open aperture data gives the two-photon absorption (TPA). The two-photon absorption coefficients are 1.6x10<sup>9</sup> – 3.8x10<sup>9</sup> cm<sup>2</sup> /W measured from theoretical fitting. The optical limiting data of silver nanoparticles with femtosecond laser is shown in **Figure 13(b)**. The optical limiting threshold value is 1.2 mJ/cm<sup>2</sup> . So bio-reduced silver nanoparticles displaying good optical limiting behavior in both regimes and silver nanoparticles can behave as broadband optical limiters [29, 48].

## **4.7 Degenerate four-wave mixing technique (DFWM)**

By using Ti: Sapphire laser with 800 nm, the third-order nonlinear susceptibility was obtained. Carbon disulfide (CS2) has taken reference to the same input powers to measure third-order nonlinear susceptibility χ(3) of silver nanoparticles using Degenerate Four Wave Mixing set up (**Figure 14**). The temporal profile of biosynthesized silver nanoparticles is shown in **Figure 15**. The cubit fit of the DFWM signal provides the nature of the third-order susceptibility. The inset picture gives the slope of silver nanoparticles, it is approximately 4. It shows that the DFWM signal has the contribution of two-photon absorption due to the electronic polarizability of the ground state alone. The χ(3) was obtained as 2.95 X 10<sup>14</sup> esu for silver nanoparticles by the following equation

*Green Synthesis of Metal Nanostructures and Its Nonlinear Optical Properties DOI: http://dx.doi.org/10.5772/intechopen.99449*

**Figure 14.** *Four wave mixing experimental set up.*

#### **Figure 15.**

*Temporal response of DFWM signal of silver nanoparticles with 0.9 mg/l. inset picture gives the DFWM signal versus input intensity at zero delays. The solid line gives linear fit.*

*Nonlinear Optics - Nonlinear Nanophotonics and Novel Materials for Nonlinear Optics*

$$\chi^{(3)}\_{\text{sample}} = \left(\frac{n\_{\text{sample}}}{n\_{\text{ref}}}\right)^2 \left(\frac{I\_{\text{sample}}}{I\_{\text{ref}}}\right)^{1/2} \left(\frac{L\_{\text{ref}}}{L\_{\text{sample}}}\right) aL\_{\text{sample}} \left(\frac{e^{\frac{aL\_{\text{sample}}}{2}}}{1 - e^{-aL\_{\text{sample}}}}\right) \chi^{(3)}\_{\text{ref}} \tag{2}$$

Where L - path length of the sample, n - refractive index, I - DFWM signal intensity, and α - absorption coefficient.

The second-order hyperpolarizability (γ) is measured by the equation.

$$\chi\_{sample}^{(3)} = T^4 \left[ \mathcal{N}\_{solvent} \chi\_{solvent} + \mathcal{N}\_{sample} \chi\_{sample} \right] \tag{3}$$

T - local field factor, *<sup>T</sup>* <sup>¼</sup> *<sup>n</sup>*2þ<sup>2</sup> 3*:*

Where, n - refractive index, N- number density of the solvent and the sample, and γ is the second-order hyperpolarizability. The Number density N can be written as *N* ¼ *N*0*Cs=*1000

Where *N*0-Avogadro number and Cs -concentration of the solution

Second-order hyperpolarizability is obtained as 2.1 X10�<sup>32</sup> esu for silver nanoparticles. The nonlinear refractive index is measured from χ (3) as 6.57 x 10�<sup>16</sup> cm2 /W. For reproducibility, the experiment was repeated twicely [49].

#### **4.8 Decay measurements**

The decay curve of europium ions (<sup>5</sup> D0 level) in various concentrations of silver nanoparticles was obtained with 350 nm excitation. The decay profile of 0.13 μM of europium ions with silver nanoparticles (1.60 μM) was measured by monitoring the 5 D0 ! <sup>7</sup> F2 transition at 612 nm as shown in **Figure 16(a)**. The decay curves are having single exponential behavior. Average decay time (τ) is obtained by below equation

$$
\tau = \frac{\int t I(t) dt}{\int I(t) dt} \tag{4}
$$

From the data, it is clear that the lifetime increase with the concentration of Ag NPs (1.60 μM) then decreases rapidly for further increase the silver at a particular concentration of europium. The lifetime was increased from 275 μs to 361 μs, from 0 to 1.60 μM concentration of Ag NPs, then decreased lifetime for further increase the nanoparticles. The alteration of a lifetime follows the same tendency as the emission

#### **Figure 16.**

*Fluorescence decay rate of (a) europium and (b) samarium complexes monitoring at 612 nm and at 645 nm bands excited with 350 nm in 1.60 and 2.32 μM of silver nanoparticles respectively.*
