The preparation and characterization of nanoparticle Ag–SiO2 composite films with super low refractive index

Accepted Manuscript The preparation and characterization of nanoparticle Ag-SiO2 composite films with super low refractive index Yu-Long Sun, Jian Wang, Shou-Yi Li, Xin He, Cheng-Wei Wang PII: DOI: Reference:

S0749-6036(14)00178-5 http://dx.doi.org/10.1016/j.spmi.2014.05.019 YSPMI 3273

To appear in:

Superlattices and Microstructures

Received Date: Revised Date: Accepted Date:

3 March 2014 4 May 2014 6 May 2014

Please cite this article as: Y-L. Sun, J. Wang, S-Y. Li, X. He, C-W. Wang, The preparation and characterization of nanoparticle Ag-SiO2 composite films with super low refractive index, Superlattices and Microstructures (2014), doi: http://dx.doi.org/10.1016/j.spmi.2014.05.019

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The preparation and characterization of nanoparticle Ag-SiO2 composite films with super low refractive index Yu-Long Sun, Jian Wang*, Shou-Yi Li, Xin He, Cheng-Wei Wang∗ Key Laboratory of Atomic and Molecular Physics & Functional Materials of Gansu Province, College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, P. R. China Abstract The spin coating technique is used to prepare the porous Ag-SiO2 composite films, and the subsequent annealing process makes the nanoparticle Ag-SiO2 composite films formed. As the increase of the annealing temperature from 300

to 500

, the

size of Ag-SiO2 composite nanoparticles gradually decreases. The SEM, TEM and XRD measurements of Ag-SiO2 composite films indicate that the single crystal metal nanoparticles are uniformly embedded into the SiO2 particles, forming Ag-SiO2 composite nanoparticles. The SPR absorption spectra of the Ag-SiO2 composite films show the content of metal Ag increases with the increase of annealing temperature, but when the annealing temperature increases to 500

, the SPR absorption is not

observed on the absorption spectra although the total absorption of Ag-SiO2 composite films is enhanced. Furthermore, the optical constants of Ag-SiO2 composite films have been determined based on the measurement of ellipsometric parameters. It is more important the refractive index lower than 1 is obtained after annealing process of Ag-SiO2 composite films under 300 and 400 ∗

, but the extinction coefficient is low,

Corresponding author. Tel: +86 9317971503 Fax: +86-9317971503. E-mail address: [email protected] (Wang CW) [email protected] (Wang J)

which makes the film have low absorption. Keywords: nanoparticle films; Ag-SiO2 composite; annealing process; super low refractive index; 1. Introduction Noble metal nanoparticles (especially silver and gold) have novel optical properties, such as localized surface plasmon resonance (LSPR), surface-enhanced Raman scattering (SERS), superlow refractive index [1], and so on. They are conducive to various technological application, plasmonic lasers [2] photocatalysts [3-5], solar cells [6,7], light-emitting diodes [8], optical switches, photonic crystal devices [9,10] and waveguides etc. Significantly, most of these applications require various shape and very small size of metal nanoparticels. However, the surface state of single metal nanoparticle with so small size is unsteady, and easily passivated so that the working life of the related devices with the metal nanoparticle greatly decreases. Obviously, the metal nanoparticle is embedded into the dielectric medium or dispersed into solution, it can not only impede the passivation of surface state and its aggregation, but also can form composite films which may have more rich physical properties. Of course, the synthesis and application of composite films embedded by the metal nanoparticles have been widely studied. A large amount of

metal nanoparticels were

deposited into nanopores of anodic aluminium oxidation or titania nanotube arrays through electrochemical deposition technique to form metallic composite films and be applied as catalysts or into solar cells [6,7,11]. Many works have indicated the metallic nanoparticle composite has enormous specific surface so as to enhance

greatly the photoelectric conversion efficiency of catalysts or solar cells. Furthermore, the optical constants, such as refractive index, extinction coefficient, are necessary parameters of optical films for the application and design of photoelectric devices. Especially, the photonic crystals (PCs) are intresting structures and have vast potential for the applications in photoelectric devices. However, for the sake of obtainment of perfect photonic band gaps or photonic defect states, the photonic crystal needs large refractive index contrast between the adjacent mediums. Certainly, in order to increase the refractive index contrast of materials, one way is to further search for the transparent materials with higher refractive index, it is clear that this has become more difficult. Obviously, the superlow refractive index is a perfect choice [1]. Generally, the spin coating is a simple and effective method to synthesize films, and has been widely used to prepare the nanoparticle or nanoporous films [12]. Therefore, the spin coating is used to prepare the porous and nanopaticle Ag-SiO2 composite films. The subsequent characterizations of morphology, crystal phase and optical absorption are used to verify the prepared film is nanopaticle Ag-SiO2 composite. The ellipsometry is used to study the modulation of annealling temperature on the optical constants so as to obtain superlow refractive index. This work would be beneficial to various photoelectric applications. 2. Experimental section 2.1 Synthesis of Ag-SiO2 Sol A 100 ml beaker was filled with Ethanol(purity>99%), Tetraethyl orthosilicate

(TEOS,99.9%) and nitric acid (HNO3,65-68%) under vigorous magnetic stirring at room temperature for 1.5h. HNO3 was added to adjust the pH of the sol to pH = 3 which facilitates hydrolysis for TEOS. Then, the transparent solution was formed after the deionized water and ethanol were added the mixed solution and stirred 10 min. The prepared solution was ageing at room temperature for 48 h until the sol becomes transparent yellowish solution. Lastly, silver nitrate and amylaceum were added to the mixed solution under ultrasound concussion at temperature of 60 for 1.5h, clear and precipitate-free aqueous sols were obtained. 2.2 Preparation of Ag-SiO2 composite films Prior to spin coating, the glass substrates (15mm×15mm) were rinsed multiple times with acetones, ethanol and distilled water, dried with air. Then the synthesized sol was coated on the rinsed glass substrates by spin coating (MIDAS SPIN-1200D) at 4000 rpm. The resultant films were further dried at room temperature for 10 h in air and then annealed in an open air furnace at 300, 400 and 500

for 2 h.

2.3 Characterization The surface and cross section morphologies of the prepared Ag-SiO2 and pure SiO2 films were studied by using the scanning electron microscope (SEM) (JEOL JSM-6704F). Energy Dispersive Spectrometer attached with the SEM has been used for the elemental analysis of samples. The transmission electron microscopy (TEM) was used to investigate the microstructure and crystal phase structure of silver nanoparticles embedded into the Ag-SiO2 films. The crystalline structures of the films were further analyzed by the X-ray diffractometer (XRD, Rigaku RINT2000) with

wavelength of Cu Ka (40kV×40mA) radiation, and their absorption spectra were measured by using Lambda 900 spectrophotometer (Perkin Elmer) for the wavelength of 300 to 1000 nm. The optical images of samples were obtained by the optical digital camera (Sony Dsc-T20). Lastly, a spectroscopic Beaglehole Picometer ellipsometer was used to study the optical properties of the Ag-SiO2 composite films, and the ellipsometric data were fitted to obtain the refractive index. 3. Results and discussion The pure SiO2 sol and Ag-SiO2 sol were deposited on the glass substrate by using spin coating technique and dried at room temperature, then their surface morphology were measured. Interestingly, on the smooth surface of pure SiO2 film, some pits appear (Seen in Fig. 1(a)). Similarly, the pit also appears on the film surface obtained by using the Ag-SiO2 sol, but its distribution becomes more closely, shown in Fig. 1(b). The diameter of the pit is nonuniform, and the average value is about 77.8nm. The depth of the pit can be obtained from the cross-section of films, as shown by the Fig. 1(d), is about 90.6 nm. Furthermore, it can also be observed that some nanoparticles are embedded in the Ag-SiO2 film, shown by the small circles in Fig. 1(b) and Fig. 1(c) which is high resolution SEM image corresponding to the Fig. 1(b), and they distribute uniformly in the Ag-SiO2 film, but they are not observed in the pure SiO2 film. For the further verifying the existence of the metal Ag in the Ag-SiO2 film, the energy dispersive spectrometer (EDS) analysis of Ag-SiO2 film excited by an electron beam (20 keV) were performed and the typical uncertainty of EDS measurement is about 2%. The results are shown in Fig.1(e), the typical peaks of

both Si and Ag elements are observed at 1.682 kev (Si) and 2.951 kev (Ag), respectively. It is illustrated that the element of Ag exists in the Ag-SiO2 film in the form of ion and elemental silver. Obviously, the SiO2 and Ag-SiO2 films with porous surface can be easily formed by using the simple spin coating technique, and this is surely beneficial to various photoelectric applications. For the formation of the porous surface, there are two reasons. Fristly, the SiO2 and Ag-SiO2 sol contain a great deal of air bubbles, after they are deposited on substrate by the spin coating and dried in air, the bubble-like pits are formed. Secondly, compared with the SiO2 film, the pits on the Ag-SiO2 film are denser. The reason is followings. In synthesized SiO2 sol, there are many ions of SiO- and electrophilic Ag+. Because the ion of Ag+ is light and easily adsorbed around the SiO- ion [13], the more pits are formed after a great deal of Ag+ ions move to SiO- ions and are adsorbed with the volatilization of solvent in process of spin coating. Generally, anealling process is an effective treatment to the film, would have great effects on the surface morphology, crystal phase of the film, and so on. Fig. 2(a) are the SEM image of the Ag-SiO2 film annealed under 300

for 2 h in atmosphere

environment, the inset is high resolution SEM image of its corresponding cross section. It can be easily seen that a great deal of nanoparticles are distributed on the film surface, the shape of large most particles is rectangular, and some small sphere nanoparticles exist among these rectangular nanoparticles. As the annealing temperature increases to 400

, most big rectangular nanoparticles gradully vanish

and more small sphere nanoparticles are formed and distributed on the film surface,

shown in Fig. 2(b). When the annealling temperature is further increased to 500 ℃, as shown in Fig. 2(c), the film surface is completely covered by smaller nanoparticles, they also disperse uniformly into the film (seeing the corresponding inset). Meantime, the film corresponding thickness is also obtained, and is 406.7nm 400.9nm 380.7nm, respectively. Fig. 2(d-f) show the size distribution of nanoparticles in the film annealed under 300, 400 and 500

, the size of nanoparticles become smaller, and its

distribution become narrower with the icrease of annealling temperature. Obviously, the Ag-SiO2 nanoparticle film can be easily prepared by the spining coating technique and subsequent annealling process. However, as above mentioned, the element of Ag exists in the Ag-SiO2 film in the form of ion and elemental silver. Thus, to further make clear the existing form of the element Ag in the annealed Ag-SiO2 film, the transmission electron microscope (TEM) analysis to the Ag-SiO2 film annealed under 300

was made, as shown in Fig. 3(a), a

great deal of nanoparticles uniformly disperse into the SiO2 film. The inset is the high resolution TEM image of the small nanoparticles, showing that the small nanoparticles are single crystal particles, the lattice contant is 0.24 nm, corespongding to crystal plane (111) of the elemental silver, and the average size of small nanoparticles is about 11.6 nm. Furthermore, the selected area electron diffraction (SAED) was made to analyze the crystal phase structure of area b without nanoparticles (Fig. 3(b)) and area c including small nanoparticles (Fig. 3(c)). It is found that the film without nanoparticles is amorphous, and the film including small nanoparticles is composite structure with amorphous and single crystal. Obviously,

the silver nanoparticles are embedded into SiO2. Fig. 4 shows the the X-ray diffraction (XRD) pattern of the Ag-SiO2 film annealed under 300

, all peaks

assigned to diffraction from the (111), (200) and (220) crystal planes of metal silver are observed, and their intensity is gradully enhanced with the increase of annealling temperature to 400

, but then decreases with the further increase of annealling

temperature. So, when the Ag-SiO2 film was annealed under 300

, the Ag-SiO2

composite film is formed, and the content of single crystal silver gradully increases with the increase of annealling temperature and then decreases when the annealling temperature increases to 500 . When the nobel metal Ag nanoparticle was formed and embedded into dielectric medium, it will exibit unique optical properties, surface plasmon resonance absorption (SPR), this is not only used to verify the existence of the metal Ag nanoparticles in the film, but also has great potential applications. Fig. 5 is the absorption spectra of the SiO2 film, the as-prepared Ag-SiO2 film dried in room temperature, and the annealed Ag-SiO2 film under 300, 400, and 500 . Obviously, the SPR peaks of metal Ag nanoparticles are observed in absorption spectra of all Ag-SiO2 films, showing that the Ag nanoparticles surely exist in the as-prepared Ag-SiO2 film only dried in room temperature, as the annealing temperature increases, the SPR absorption gradully is enhanced and shifts to short wavelength, it is also found from the sample surface colour (see the inset). This illustrates the volume ratio of Ag nanoparticles increases and the shape and size are also changed in the film. However, when the annealing temperature reaches to 500

, although the as-prepared absoption is still enhanced,

the SPR peaks are not observed. Combining the XRD results of Ag-SiO2 films, it is concluded that the metal Ag nanoparticles are aggregated. Thus the Ag-SiO2 composite films with the well dispersed Ag nanoparticles can be prepared through the annealing process under the temperature lower than 500

.

The effective optical constants of composite films are very important parameteres for the photoelectric application. The ellipsometry is an effective method to determine the effective optical constants. In ellipsometry, the variation of the amplitude and the phase difference between the perpendicular (p) and the parallel (s) components of the reflected light polarized with respect to the plane of incidence are measured. In general, reflection causes a change in the relative phase of p and s waves and in the ratio of their amplitudes. The effect of reflection is measured by the two quantities, ψ (which measures the amplitude ratio) and ∆ (which measures the relative phase change). These are given by the expression

ρ=

rp rs

= tanψ eiΔ

Where rp and rs are the reflection coefficients for the p and s component of the waves respectively. The measured ellipsometry spectra are then fitted with theoretically generated spectra assuming a realistic sample structure. The present samples were prepared by spin-coating Ag-SiO2 sol on a silicon wafer. Thus, a three layered sample structure: Si/Ag-SiO2/air would be the most realistic assumption for the theoretical fit. In case of composite layers, consisting of Ag and SiO2, the calculation for the effective optical constants has been done using the Bruggeman Effective Medium Approximation (BEMA) model [14], which describes a composite

of aggregated phases or a random mixture microstructure. In the BEMA model the effective medium acts as the host following the expression: fm

ε m - ε eff ε -ε + f d d eff = 0 ε m + 2ε eff ε d - 2ε eff

Where εeff, εm and εd are the dielectric functions of the composite medium, the metal and the dielectric and fm and fd are the volume fraction of the metal and the dielectric respectively. Assuming the above sample structure and using the BEMA model, the theoretical rp and rs values are calculated and hence ψ and ∆ spectra are then generated using the standard Fresnel’s relations for thin film structures. The measured ellipsometric spectra are fitted by minimizing the mean-square error (MSE) [15] between the measured and calculated values of the ellipsometric parameters give by : MSE =

1 2K - M

k

[(ψ mod -ψ exp ∑ j j ) j =1

2

2 + (Δ mod - Δ exp j j ) ]

where K is the number of (ψ , Δ) pairs, M is the number of the model parameters, and the superscripts mod and exp refer to model and experimental values, respectively. Fig. 5(a and b) show the experimental Ψ and ∆ for the films annealed under various temperatures along with the best fit theoretical curves over the wavelength range of 400–800 nm. The sample structures obtained for the films annealed under different temperatures from the best fit are also shown by the inset table in the figure 5(a). It is evident that the model shows good agreement with the experimental results of ψ and ∆ spectra. With the increase of annealing temperature, the fitted film

thickness gradually decreases from 407 nm to 385 nm, which are agreed well with the values obtained by the SEM. The silver content in the composite films increases from 2.19% to 5.66%, then decreases to 4.29% when the annealing temperature reaches to 500 , but the void content of the composite films decreases greatly from 46.82% to 5.69%. Furthermore, it is also seen that the silver content has a pronounced effect on the relative phase and amplitude of the reflected light of the composite films. For the as-prepared Ag-SiO2 films, the relative phase change (Ψ) and amplitude change (∆) vary approximated linearly with the wavelength due to little silver content. But as the annealing temperature increases, a narrow peak centered about 500 nm on both ψ and

∆ spectra is observed, this is mainly caused by the SPR absorption of increased Ag nanoparticles. With the further increase of annealing temperature to 500 , the narrow peak is gradually weaken. This is mainly attributed to the aggregation of Ag nanoparticles and the change of the porosity. Based on the best fit of ellipsometric analysis, the ψ and ∆ spectra are then inverted to get the refractive index (n) and extinction coefficient (k) of the Ag-SiO2 composite films which are shown in Fig. 6(a) and (b) respectively. Obviously, near the surface plasmon band centered at about 500 nm, the refractive indexes of Ag-SiO2 composite films annealed under 300 and 400

show an anomalous dispersion and

this is consistent with the Kramers-Kronig relation [15, 16]. Also, it is more important the refractive index is decreased to 0.44 near to the value of silver bulk, but the extinction coefficient k has a low value which insures the Ag-SiO2 composite films is transparent. This would be beneficial to the design of photonic crystal devices.

4. Conclusion The porous SiO2 and Ag-SiO2 films were easily prepared by using the simple spin coating technique, then after the annealing process under 300, 400 and 500 , the nanoparticle Ag-SiO2 composite films were formed, and the nanoparticles of single crystal metal Ag are embedded uniformly into the SiO2 particles to form Ag-SiO2 composite particles. As the annealing temperature increases, the size of Ag-SiO2 composite particles gradually decreases, and the SPR absorption of the Ag-SiO2 composite films is gradually enhanced. Furthermore, the optical constants of Ag-SiO2 composite films were determined based on the measurement of ellipsometric parameters. It is more important the refractive index lower than 1 is obtained after annealing process of Ag-SiO2 composite films under 300 and 400

, but the

extinction coefficient is also very low, which makes the composite films have low absorption. This works would be beneficial to various photoelectric applications.

Acknowledgements The Authors are grateful to the support of the National Natural Science Foundation of China (Grant Nos. 11264034 and 11364036), the Natural Science Foundation of Gansu Province of China (Grant No. 1208RJZA197), the Foundation of Key Laboratory of Polymer Materials of Gansu Province (Grant No. KF-09-02), and the Foundation

of

Northwest

Normal

University

NWNU-LKQN-11-28,NWNU-LKQN-12-10).

of

China

(Grant

Nos.

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Figures Caption Figure 1 Typical low resolution SEM images of pure SiO2 film (a), Ag-SiO2 film (b) dried at room temperature in air, high resolution SEM image of Ag-SiO2 film (c) and its corresponding cross-section (d), EDS spectra (e). Figure 2 The SEM images of Ag-SiO2 films annealed at 300 500

(a), 400

(b) and

(c), respectively, for 2 h in atmosphere environment. The insets are their

corresponding cross-sections. (d), (e) and (f) are their corresponding distribution of particle size. Figure 3 TEM images of Ag-SiO2 film (a) and SAED images of Ag-SiO2 film taken from area b (b) and c (c). The inset shows a HRTEM image of Ag nanoparticles. Figure 4. XRD patterns of pure SiO2 (a), Ag–SiO2 thin films dried at room temperature (b) and annealed at 300

(c), 400

(d) and 500

(e) for 2 h in air,

respectively. Figure 5 UV–vis absorption spectra of pure SiO2 (a), as-prepared Ag-SiO2 film (b), annealed at 300 (c), 400

(d) and 500

(e), respectively, and the inset are their

corresponding optical images. Figure 6 Variations of Ψ (a) and ∆ (b) with the wavelength of the incident radiation

for the as-prepared film (1) and for the films annealed at 300 500

(2), 400

(3) and

(4), respectively. Inset shows their corresponding structure parameters obtained

from the best fit. Figure 7 Variations of refractive index n (a), and extinction coefficient k (b) with wavelength for the as-prepared film and for the films annealed at 300 , 400 500

, respectively.

and

Highlights 1. The nanoparticle Ag-SiO2 composites were formed by annealing process of coatings. 2. The Ag nanoparticles are uniformly embedded into the SiO2 particles. 3. The optical constants of Ag-SiO2 composites were determined by using ellipsometry. 4. The refractive index of the Ag-SiO2 film is lower than 1.