Title Goes Here

Effect of the Annealing Temperature on the Growth of the Silver Nanoparticles Synthesized by Physical Route

Manvendra Singh Gangwar1, a) and Pratima Agarwal1, 2, b)

1Department of Physics, Indian Institute of Technology Guwahati, Guwahati, Assam, India-781039 2Centre for Energy, Indian Institute of Technology Guwahati, Guwahati, Assam, India-781039

a) Corresponding author: manvendrasingh@iitg.ac.inb) pratima@iitg.ac.in

Abstract. Silver nanoparticles have attracted the attention due to their chemical stability, catalytic activity, localized surface plasma resonance, and high conductivity. Silver thin films with a thickness of ~21 nm were deposited on corning glass substrate using radio frequency magnetron sputtering system. The deposited silver films were subsequently annealed to grow silver nanoparticles in the temperature range of 200 to 500 with the difference of 100 under a vacuum (Pressure ~10-6 mbar) for fixed duration of 60 minutes. Composition and surface morphology of silver nanoparticles on the glass substrate were determined using field emission scanning electron microscope (FESEM) and atomic force microscopy (AFM) and energy dispersive spectroscopy (EDS). The results obtained from this study showed that by annealing at higher temperature, the flat silver thin films changed into rough silver thin films and further increment in temperature changed the silver thin films into silver nanoparticles. To study optical properties of Ag nanoparticles, the absorbance and diffuse reflectance spectra were also measured using UV–Vis–NIR spectroscopy.

iNtroduction

Metal nanoparticles (NPs) have received considerable attention due to their unique characteristics and a wide range of application [1-3]. Among the different metal nanostructures, noble metal nanoparticles have become more popular due to their superior properties [4]. Silver thin films have found widespread technological applications in optical devices [5, 6], photovoltaic devices [7], sensors and electronics [8] and Plasmonics devices [9], due to their optical, electrical, sensing, catalytic and antibacterial properties [10]. The properties and applications strongly depend on the morphology, orientation and dimensions of Ag nanostructures. The compatibility of Ag with underlying substrate materials is important for device performance and reliability [11]. There are many techniques and methods for the deposition of Ag thin films such as chemical vapor deposition, electrochemical deposition, thermal evaporation, pulsed laser deposition, sputtering, chemical bath deposition [12-17]. Alford et al [18] studied the influence of annealing on the thermal stability of Ag thin films on underlying substrates and found that the elevated temperatures can affect the microstructure and surface diffusivity of Ag films. Samavat et al [19] used an electron beam for the deposition Ag films and investigated the influence of annealing on morphology and optical properties. The results show that the size of nanoparticle becomes larger by increasing annealing temperature. Khan et al [20] studied the thermal properties of Ag films deposited by dip-coating method. The dominant weight loss was observed at 200°C -300°C temperature range which indicates that these Ag films are thermally unstable in this range. Razak et al [21] synthesized Ag/TiO2 thin films by sol-gel spin coating and investigated the influence of annealing. The thin films were found to be more stable with minimum cracks and pull out on higher temperatures. The compatibility and stability of thin films are highly dependent on annealing temperatures for possible production of uniformly distributed silver nanoparticle of regular shape and size. Our motivation in this work is to introduce an alternative method to produce the Ag nanoparticles of regular shape and size uniformly distributed over the surface for the light trapping application for thin film solar cells. For this purpose, we fabricated flat Ag thin films (approx. 21 nm thick) on glass substrate using radio frequency magnetron sputtering system. To produce the Ag nanoparticles, we annealed the flat Ag films at different annealing temperatures for 60 minutes. Their morphological and optical properties were analyzed using the FESEM images, EDS and UV–Vis–NIR spectroscopy. We also calculated the size, counts of Ag nanoparticles and surface area coverage by the Ag nanoparticles using FESEM images at 30 KX magnification through image J software.

Experimental DETAILS Deposition of Ag precursor thin films

The silver precursor thin films were deposited on corning 1737 glass silicon substrate using RF magnetron sputtering technique. Deposition time for Ag precursor thin films was varied from 30 secs to 120 secs. We have chosen Argon flow rate (AFR) of 5 sccm, RF power of 40 W, substrate temperature of 50, process pressure of 5 mbar, and electrode separation of 6.5 cm. Thickness of deposited Ag precursors films for deposition time of 30 sec, 60 sec, 90 sec,and 120 secs are measured by stylus profilometer and listed in table:

TABLE 1. Thickness measurement of Ag precursor films deposited at different deposition time

Deposition time

(Sec)

AFR

(sccm)

Process Pressure (mbar)

Substrate Temperature ()

RF Power (Watt)

Thickness (nm)

30

60

90

120

5

5

50

40

12 ± 2

21 ± 1

34 ± 3

50 ± 3

Annealing of Ag precursor thin films to grow Ag NPs

Ag precursor thin films deposited over corning 1737 glass were annealed in inert atmosphere at base pressure mbar using Radio frequency plasma enhanced chemical vapour deposition system (RF PECVD). As deposited Ag thin films were annealed at 400 for 1hour. After studying the effect of the deposition time on Ag NPs we found that Ag NPs formed for the Ag film deposited for 60 sec time were more uniformly distributed with nearly circular shape. Thus we used Ag precursor thin films deposited for 60 sec for further synthesis of NPs with variation in annealing temperature from 200 to 500 keeping the annealing time fixed at 1 hour.

RESULT AND DISCUSSION

To study the shape and size of Ag NPs, Field Emission scanning electron microscopy (FESEM) was done. SEM images of Ag NPs formed for 21 nm thick Ag precursor films annealed for 1 hour under four different temperatures of 200, 300, 400, and 500 are shown in Fig.1. In the low temperature regime, the formation of irregular nanoclusters were observed. Well-defined and ellipsoidal NPs were obtained above the 300. With further increase of annealing temperature up to 400, aggregation process of NPs initiated, the size uniformity of the NPs increased and the number of the small particles decreased. The NPs smaller than 100 nm are approximately round for annealing at 400 whereas bigger particles tend to be elongated. The roundness of NPs is notably increased for annealing temperature of 400.

The solid state dewetting (SSD) process produces particle ensembles with broad distribution of size. Therefore, the analysis and its correlation with optical properties requires statistical approaches commonly realized by histograms of NPs size. For the low annealing temperature, the formation of Ag nanoparticles was not observed due to the absence of Ag NPs and histogram are not available. Histograms reveals that for 400 annealing temperature the size of most of the NPs are between 10- 50 nm and few NPs are size of greater than 100 nm but for the 500 annealing temperature sufficient number of large NPs (greater than 100 nm) are observed.

(d) 500

(c) 400

(b) 300

(a) 200

FIGURE 1. SEM images of Ag NPs on corning glass formed from 21 nm thick Ag precursor film annealed for 1 hour under four different annealing temperature on 200 nm scale. (a) 200, (b) 300, (c) 400 and (d) 500.

FIGURE 2. The histogram of NPs’ size (longitudinal diameter) on corning glass formed from 21 nm thick Ag precursor film annealed for 1 hour under different annealing temperatures.

Surface morphological study of the Ag NPs synthesized at different annealing temperatures for 60 minute fixed annealing time was done by using AFM. 10 area has been taken for AFM imaging in non-contact mode. The root mean square (rms) value of roughness are 2.37 nm, 1.02 nm, 13.56 nm and 12.89 nm at annealing temperature of 200, 300, 400, and 500 respectively. A drastic change in surface morphology has been found between the annealing temperature ranges from 300 to 400 in form of roughness due to the formation of Ag NPs.

(a) 200

(b) 300

(d) 500

(c) 400

FIGURE 3. AFM images of Ag NPs on corning glass formed from 21 nm thick Ag precursor film annealed for 1 hour under four different annealing temperature. (a) 200, (b) 300, (c) 400 and (d) 500.

Compositional analysis of Ag NPs on the glass substrate were studied using EDS. EDS spectra of Ag NPs at four different temperatures for 60 minute fixed annealing time are shown in Fig.4. Other elements like Aluminum, Silicon and oxygen are also present in EDS spectra due to the glass substrate. It was observed that with increase in annealing temperature from 200 to 500, the weight percentage of Ag has decreased from 21.1% to 12.5% and weight percentage of other elements increased. It is due to the formation of Ag NPs at higher temperature (above 300) and consequences the contribution of the substrate has increased.

(d) 500

(c) 400

(b) 300

(a) 200

FIGURE 4. EDS spectra of Ag NPs on corning glass formed from 21 nm thick Ag precursor film annealed for 1 hour under four different annealing temperature on 200 nm scale. (a) 200, (b) 300, (c) 400 and (d) 500.

To study the optical properties of Ag NPs, absorbance and diffuse reflectance spectrum are recorded for the wavelength range 200-1500 nm and 200-800 nm. These are shown in Fig. 5

(a) 200

(b) 200

FIGURE 5. (a) UV-Vis-NIR absorbance and (b) Diffuse reflectance spectra of Ag NPs formed from 21 nm thick Ag precursor film annealed for 1 hour under four different annealing temperatures.

With the increase of annealing temperature, the diffuse reflectance and the absorbance increased for the wavelength range 300-600 nm and 400-600 nm. It is due to the aggregation process of Ag NPs, which initiates at higher temperature (above 300) and consequences the size uniformity of NPs increases and the number of small particle decreases. The increase in number of large particles can contribute to the more scattering and consequently enhancement in absorbance and the diffuse reflectance.

Conclusions

In this work, we studied the effect of annealing temperature on the growth of Ag NPs. The formation of irregular nanocluster was observed in the low annealing temperature regime. Well-defined and nearly circular NPs were obtained while annealed above 300. With further increase of the annealing temperature upto 400 aggregation process of NPs initiates due to which size uniformity of the NPs increases and the number of small particles decreases.

ACKNOWLEDGMENTS

We acknowledge the Central Instrument Facility (CIF) IIT Guwahati for FESEM and Department of Physics for Stylus Profilometer and UV-Vis-NIR Spectroscopy.

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