International Journal of Nanoscience Vol. 10, No. 3 (2011) 433440 # .c World Scienti¯c Publishing Company DOI: 10.1142/S0219581X11008186
OPTICAL AND STRUCTURAL PROPERTIES OF SILVER NANOPARTICLES EMBEDDED IN INDIUM OXIDE FILMS A. A. DAKHEL*,z and F. Z. HENARIy,x *Department of Physics, College of Science University of Bahrain, Kingdom of Bahrain yRoyal College of Surgeons in Ireland Medical University of Bahrain, Kingdom of Bahrain
[email protected] x
[email protected] Received 22 February 2010 Accepted 12 September 2010 Nanoparticles of silver-embedded indium oxide thin ¯lms have been prepared on glass and silicon substrates. Silver concentration were 3 wt.% and 5 wt.% as measured by X-ray °uorescence. X-ray di®raction reveals that indium oxide of these samples remains amorphous even after pre-annealing at 400 C. The optical absorption of the samples manifests the surface plasmon resonance (SPR) phenomena, which varies with Ag content. The Ag nanoparticles radius was estimated with Mie classical theory by using the SPR data analysis. The nonlinear optical properties of ¯lms on glass substrate were investigated using z-scan technique. Under cw excitation the ¯lms exhibit large reverse saturation absorption and negative nonlinearities. The real and imaginary parts of third order susceptibility of the samples were measured and the imaginary part which arise from the change in absorption is found to be dominant. Keywords: Optical properties; indium-silver oxide; Ag-embedded indium oxide; nonlinear properties; z-scan. Pacs Nos.: 61.46.+w78.20.-e, 61.10.NZ, 71.30.
properties of surrounding medium around it.13 The e®ect of a medium type, state, and the nanoparticle physical properties, on the optical properties of the embedding sample and especially the surface plasmon resonance (SPR) signal of the embedded nanoparticles was established. Metal nanoparticles incorporated into semiconductors or insulators have many applications based on their electrical and optical properties such as the SPR and optical nonlinearity. Such properties are useful in the development of data storage media, optical
1. Introduction Experiments show that the optical and electronic properties of materials can be modi¯ed by making them di®erent from those of their corresponding bulk when they are prepared in the form of nanoparticles. Due to the surface volume ratio of the nanoparticle, its physical properties are controlled mainly by the surface atoms rather than atoms inside. Therefore, it is possible to control the optical and electronic properties of the nanoparticles by controlling their shapes, sizes as well as the 433
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A. A. Dakhel & F. Z. Henari
switching, high-speed optical logic devices, chemical gas sensors, etc.49 Silver nanoparticles incorporated in an insulating medium have many applications like the intensi¯cation of luminescence1013 and optical nonlinearity.6,8,14 Silver nanoparticle properties incorporated in di®erent amorphous and crystalline mediums were studied by several authors.1,3,15,16 However, the study of the behavior of silver nanoparticles embedded in indium oxide powder prepared by a chemical method was studied by Singh et al.17 The aim of the ¯rst part of the present work is to prepare thin ¯lms of Ag nanoparticle embedded in amorphous indium oxide thin ¯lms using vacuum evaporation technique and to study their structural and linear optical properties as well as their variation with Ag concentration. In the second part we report on nonlinear measurements of Ag embedded in indium oxide under cw excitation using z-scan technique at wavelengths 488 nm and 514 nm with the output power of 40 W. The nonlinear absorption , nonlinear refractive index , and the third order nonlinear susceptibility ð3Þ were measured. The origins of nonlinear e®ects were discussed.
2. Experimental Details The starting materials were indium (99.99%, Fluka A. G.) and silver metal (from Aldrich Chem. Co.). Molybdenum boat was used for thermal evaporation of the starting metals. The alternating thermal deposition (layer-by-layer) method was used to deposit the starting materials on ultrasonically clean glass and clean silicon wafer substrates held at room temperature in a vacuum chamber of residual oxygen atmosphere of pressure about 1:3 10 2 Pa. The evaporated mass was controlled with a Philips FTM 5 thickness monitor, by which the approximate fraction ratio of (silver to indium) is controlled during preparation. Two groups of di®erent Ag content (S1 and S2) in addition to pure indium oxide (S0) ¯lms were prepared in the present study. The oxidation process was done in an oxygen atmosphere in a temperature-controlled program oven. The heating was carried out successively at 150 C, 200 C, 300 C, and then 400 C for 2 h each. It was observed that Ag impurity prevents crystallization of indium oxide even at 400 C, while pure indium oxide ¯lm crystallizes at 300 C. The ¯nal ¯lm thicknesses were measured after annealing with an MP100-M spectrometer (Mission Peak Optics Inc., USA) to be in the range of 0.180.20 m. The
structures of the prepared ¯lms were studied by the X-ray di®raction (XRD) method using a Philips PW1710 2 system with Cu K radiation (0.15406 nm) and step 0:02 . The study of the elemental content and the measurement of the molar content of silver relative to indium (r) were performed by energy dispersion X-ray °uorescence (EDXRF) technique with Cu K exciting radiation and an Amptek XR-100CR detector. The spectral optical transmittance T ðÞ and re°ectance RðÞ were measured at normal incidence in UVVis NIR spectral region (1901500 nm) with a Shimadzu UV-3600 double beam spectrophotometer.
3. Characterization by X-rays The elemental content and the relative molar fraction ratio (r) of silver to indium in the prepared ¯lms were determined by the well-known XRF microradiographic analysis method.18 The method measures the integral intensity of In L (3.283.48 keV) band and Ag L (2.98 keV) band in reference samples and ¯lm samples. The reference samples were pure thin In2 O3 and Ag ¯lms. Figure 1 shows EDXRF spectrum of the Ag-incorporated indium oxide ¯lms on silicon substrate. The spectrum shows In L -band and Ag L-band. The signals are overlapped due to the limited resolution of the detector. The overlapping was deconvoluted by a program computer. Thus, there is error in measuring the %Ag, which was estimated to be 1%
Fig. 1. The XRF spectrum of S1 and S2 Ag-embedded indium oxide ¯lms grown on Si substrate. The exciting radiation was Cu K of energy 8.047 keV.
Optical and Structural Properties of Silver Nanoparticles Embedded in Indium Oxide Films
435
embedded in nano-In2 O3 medium. This structure must also be deduced by the optical properties study, which will be discussed later. In general, it is clear that Ag ions cannot dope into In2 O3 medium. This might be explained by ionic radii of silver and indium; the ionic radius of Ag þ is 0.126 nm larger to that of ion In 3þ , which is 0.08 nm.22
4. Linear Optical Properties Figure 3 presents the spectral transmittance and re°ectance in the UVVisNIR spectral range (2502600 nm) for pure (S0) and Ag-incorporated indium-oxide ¯lms (S1 and S2). The spectral linear absorption coe±cient ðÞ is calculated by23: Fig. 2. The XRD patterns for pure, S0, and Ag-embedded indium oxide, S1 and S2 ¯lms grown on glass substrates. The radiation used was Cu K -line of wavelength 0.1543 nm.
maximum. The result of calculation of r was about 3 wt.% for S1 and 5 wt.% for S2. The structure of the prepared Ag-incorporated indium oxide ¯lms was investigated by the method of XRD, and the results are shown in Fig. 2. The pattern reveals that indium oxide does not crystallize under annealing at 400 C for both S1 and S2. However, pure indium oxide crystallizes even at 300 C annealing forming highly (111) oriented In2 O3 BCC Ia3 structure of a ¼ 1:010 nm, which is almost the same value as that for bulk oxide 1.0118 nm.19 The absence of any X-ray re°ection from Ag in S1 reveals that it was not crystallized; it aggregated in clusters within the indium-oxide amorphous medium. By increasing the %Ag content, XRD peaks appear due to pure Ag structure (FCC cubic of lattice constant 0.409 nm). A very weak peak identi¯ed as cubic AgO (111) (a ¼ 0:48220) re°ection was observed on the spectrum of S1 and S2. The percent of silver oxide relative to pure silver can be estimated from the ratio of integral intensities of re°ections that appear in S2 pattern, to be about 3.5%. It must be mentioned here that the XRD patterns show a very weak peak at about 30.7 for both S1 and S2, which can be identi¯ed as the (222) re°ection indicative to the beginning of crystallization of the In2 O3 \medium". The nanograin size of In2 O3 medium was 910 nm. The average X-ray grain size (DXR ) of Ag nanograins in S2 was calculated by Scherrer's relation21 to be 42.3 nm. Thus, the structure of S1 consists of Ag clusters while S2 consists of Ag nanograins together with about 3.5% AgO nanograins
¼ ð1=dÞ ln½ð1 RÞ=T ;
ð1Þ
where d is the ¯lm thickness. The experimental data of the transmittance T were corrected relative to identical clean glass substrate, and the re°ectance R and transmittance T used in Eq. (1) were corrected theoretically due to the substrate e®ects according to the method given in Ref. 24. Figure 4 shows the linear optical absorption coe±cient for pure and Ag-incorporated indium oxide samples S1 and S2. As evident from the ¯gure, the absorption band due to surface absorption peak (SPR) appeared in Agincorporated indium oxide. These SPR absorption peaks are attributed to the formation of metal nanoparticles or nanoclusters in the amorphous medium (indium oxide in the present work). These particles have di®erent optical response (due to their small sizes) than the surrounding medium.6,9,25,26
Fig. 3. The normal spectral corrected transmittance and re°ectance for pure, S0, and Ag-embedded indium oxide, S1 and S2 ¯lms grown on glass substrates. The inset shows the relationship ðEÞ 2 versus E for determination of band gaps Eg .
436
A. A. Dakhel & F. Z. Henari Table 1. Linear optical properties: band gap Eg and Ag-SPR peak p , Ag XR grain size (DXR ), and Ag Mie grain size (DMie ) of samples S1 and S2. Sample %.wt Ag Eg (eV) p (nm) DXR (nm) DMie (nm) S0 S1 S2
Fig. 4. The spectral optical linear absorption coe±cient as a function of wavelength, ðÞ for pure, S0, and Ag-embedded indium oxide, S1 and S2 ¯lms grown on glass substrates. The inset shows the ðÞ for S2 showing the deconvolution of the compound SPR band.
The energy position of the SPR peaks depends on the type, state, and properties including the dielectric function of the medium around the Ag nanoparticles in addition to the physical properties of the nanoparticles and the interparticle dipoledipole interaction. The SPR peaks do not exist in Fig. 4 for pure indium oxide ¯lm. The form of the SPR peaks depends on the ¯lm's structure. For sample S1, there is only one peak with a maximum at about 432 nm, which is identical with that previously observed for about 6.0 nm size Ag nanoparticles embedded in silica xerogel27,28 and almost identical with the SPR wavelength of Ag nanoparticles embedded in nanopowder In2 O3 prepared by chemical method (430 nm).17 The Ag SPR wavelength was centered at 429 nm for 5.0 nm Ag nanoparticles incorporated in SnO2 ,29 at 350 nm for 43 nm-nanoparticles in a liquid.30 Authors of works Refs. 31 and 32 have observed the SPR peak at 410 nm for isolated Ag nanoparticles in a colloidal solution, while it was observed at 580 nm for thin Ag ¯lm. Such shift was explained due to dipole dipole interparticle coupling. The SPR peak in S2 has a compound shape, which deconvoluted to be consisting of two overlapped peaks at 437 nm and 563 nm, as shown in the inset of Fig. 4. The ¯rst peak was identi¯ed to be due to nano-Ag shifted slightly to higher wavelength due to increase in %wt Ag content, which leads to decrease in the internanoparticle spacing3133 (identical results were observed in Ag-embedded Eu oxide15) and the other
0.0 3% 5%
3.60 4.20 4.15
— 432 437
— — 42.3
— 2.7 4.3
peak is identi¯ed due to silver oxide. Thus, the oxidation of Ag causes the SPR to shift to higher wavelength, as mentioned in Ref. 6. It is important to mention here that the position of the SPR peak of indium-containing suspensions (indium nanograins' size was 43 nm) was at 350 nm34 or at 400 nm.35 The obtained wavelengths of the SPR peaks (p ) in the present work are given in Table 1. The average size of the Ag nanoparticles is estimated according to the classical Mie theory6: rMie ¼ vF =!1=2 , where rMie is the Mie average radius of metal spheres, vF is the Fermi velocity (1:39 10 6 m/s for Ag), and !1=2 is the full width at half maximum (FWHM) in angular frequency scale, of the absorption SPR peak. This relation assumes that the light-scattering particles are spherical and that there is no interparticle coupling. Therefore, it gives very small values for rMie and thus Mie size, DMir are di®erent from that obtained by X-rays, DXR , as seen in Table 1. However, the obtained Mie size must be proportional to the real size of the scattering nanograins and can be considered as an acceptable practical parameter in this ¯eld. The optical band gap Eg is evaluated according to the well-known relation3537: E ¼ AðE Eg Þ m ;
ð2Þ
where A is a constant and the exponent m is equal to 0.5 or 2 for direct and indirect transitions, respectively. It is observed that the best value suitable for the data of the present work is m ¼ 0:5. Therefore, the plot of ðEÞ 2 versus E, as shown in Fig. 5, gives the values of direct band gaps (Table 1). The obtained value of Eg for pure indium oxide is identical with the known experimental value.38 For Ag-embedded indium oxide, Eg is larger than that of pure indium oxide; same tendency was observed in Ref. 17.
5. Nonlinear Optical Properties The nonlinear absorption and nonlinear refractive index for silver-embedded indium oxide thin ¯lms
Optical and Structural Properties of Silver Nanoparticles Embedded in Indium Oxide Films
437
absorption was observed with the intensity range used in the experiment; therefore, we conclude that the above e®ect is due to the presence of Ag nanoparticles in In2 O3 , which agrees with the absence of the SPR in the linear absorption spectrum of indium oxide as mentioned above. The normalized transmission for the open z-scan is given by39: Top ðzÞ ¼ 1 ’ð1 þ xÞ 1 ;
Fig. 5. Open aperture z-scan response for Ag-embedded indium oxide (S2) solid circles under 488 nm and hollow triangles for pure indium-oxide ¯lm on a glass substrate. The line corresponds to a ¯t to Eq. (3) to the data.
were measured using z-scan technique.39 The z-scan technique o®ers the possibility of distinguishing between these two mechanisms even if they are present simultaneously. The technique relies on the fact that the intensity varies along the axis of the convex lens and it is maximum at the focus. Hence, by shifting the sample through the focus, the intensity dependence can be measured as a change of transmission. The nonlinear refraction can be measured by observing the spot size variation at the plane of the ¯nite aperture/detector combination. In the z-scan procedure, the transmission of the sample was measured with and without an aperture in the far ¯eld of the lens as the sample moved through the focal point. This enables the nonlinear refractive index (closed aperture) to be separated from that of the nonlinear absorption (open aperture). The z-scan was performed with cw Argon laser at wavelengths 488 and 514 nm and at the intensities, ranging from 2.53:3 10 8 W=m 2 . Figure 5 shows the normalized transmittance without an aperture as a function of the distance along the lens axis z, for Ag-embedded indium oxide ¯lm S2 at wavelength 488 nm. The transmission is symmetric with respect to the focus (z ¼ 0), where it has a minimum transmission. This demonstrates that the ¯lm exhibits reverse saturation absorption, RSA (optical limiting). To establish that the e®ect seen is due to Ag nanoparticles, the z-scan (open aperture) was performed also with a pure indiumoxide ¯lm on glass substrate, where no nonlinear
ð3Þ
where x ¼ z=z0 (with zo ¼ w 20 =) is the di®raction length of the Gaussian pffiffiffi beam and wo is the beam's waist, ’ ¼ Io l=2 2 and l ¼ ð1 expðo dÞÞ=O with O is a linear absorption coe±cient, d is the sample thickness, l is the e®ective thickness of the sample, Io is the intensity of the laser beam at the focus, and is the nonlinear coe±cient. The values of nonlinear absorption coe±cient, calculated by ¯tting Eq. (3) to experimental data at 488 nm and 514 nm for S1 and S2 samples are given in Table 2. These values are higher by two orders than the reported value for C60 and C70 fullerenes at 514 nm, cw excitation40 and in the same order of the reported value for Ag nanoparticles41 and for CdS nanoparticles.42 The imaginary part of the third order nonð3Þ linearities I is related to nonlinear absorption coe±cient by ð2Þ
I ¼
n 2 "0 c ; 2
ð4Þ
where n is the linear refractive index (n ¼ 1:8Þ, is the wavelength, "0 is the permittivity of free space, and c is the velocity of light. The experimentally ð3Þ determined values of I for S1 and S2 samples are shown in Table 2. The normalized transmittance through closed aperture for Ag-embedded indium oxide S1 at 514 nm is shown in Fig. 6. The peak valley con¯guration is a signature of negative refractive nonlinearity. The nonsymmetric height of peak and valley indicates that the nonlinear absorption that gave rise to refractive index changes through KramersKronig relation. The normalized transmission is given by43: 4’x T ðzÞ ¼ 1 ð1 þ x 2 Þð9 þ x 2 Þ 2 ð3 þ x 2 Þ ; ð5Þ ð1 þ x 2 Þð9 þ x 2 Þ
438
A. A. Dakhel & F. Z. Henari Table 2.
Nonlinear-optical measurements of samples S1 and S2. ð3Þ
ð3Þ
Sample
(nm)
(cm 1 )
(m/W)
I (esu)
(m 2 /W)
R (esu)
S1 (3%)
514 488
18339 21427
1:21 10 3 2:13 10 3
8:00 10 5 1:03 10 4
1:20 10 10 5:24 10 11
1:45 10 4 6:42 10 5
S2 (5%)
514 488
25119 25305
1:41 10 2 1:72 10 2
2:02 10 3 8:62 10 4
2:22 10 10 4:13 10 10
2:58 10 4 5:04 10 4
where ¼ kIl and k ¼ 2=. is a nonlinear refractive index, and is given by ¼ Il. The values of nonlinear refractive index, calculated by ¯tting Eq. (5) to experimental data are given in Table 2. The ¯tting was performed by keeping the same value of ’ as that obtained in Eq. (3). The values reported here are higher by two orders than the reported value for Au and Ag colloids.44,45 The di®erences may rise to larger thermal contribution in the case of the colloidal solutions. The real part of the third order nonlinearities ð3Þ R is given by ð3Þ
R ¼ 2n 2 "0 c :
ð6Þ ð3Þ
The experimentally determined values of R are shown in Table 2. By comparing the values of the imaginary and real nonlinearities, one can conclude ð3Þ ð3Þ ð3Þ that I > R , i.e., I which gives rise to the absorption change is dominant, and this can be seen from Fig. 6, where the valley is much larger than the ð3Þ ð3Þ peak. The values of the I and R nonlinearities of
Ag-incorporated indium oxide reported here are of the same order of magnitude as the nonlinearities of materials such as chalcogenide glasses,46 and polycrystalline silicon.47 Note that there is an increasing trend for the values of , , and ð3Þ as the concentration of the sliver particles increases. This may be attributed to the fact that as the concentration increases, more sliver particles are excited resulting in an enhanced e®ect. From the physical point of view, the mechanism responsible for nonlinear absorption is the spectral range used in the experiment in reverse saturation absorption as mentioned above. The reverse saturation absorption of Ag-incorporated indium oxide ¯lms arises from the interband transition from 4d to 5s states via two-photon absorption. The excitation wavelengths used in the experiment were 488 nm and 514 nm corresponding to the photon energies 2.6 eV and 2.4 eV, respectively, which are smaller than the band gap energy 4.2 eV measured above. In this case, the only possible transition would be through a two-photon absorption that leads to direct pumping of electrons to the plasmon state. The nonlinear refractive index may arise from the thermal e®ect, which may arise from the use of cw laser beam. Thermal e®ect leads to thermalizing hot electrons, and subsequent dissipation of their energy leads to increase is the surrounding temperature which results in the refractive index change.
6. Conclusions
Fig. 6. Closed aperture z-scan response for Ag-embedded indium oxides (S1) under 514 nm; the points represent the measurement data; the line corresponds to a ¯t to Eq. (5).
X-ray di®raction reveals that the prepared nanoparticle Ag-embedded indium oxide samples remains amorphous even after pre-annealing at 400 C. Optical absorption study shows that Ag nanoparticles display a SPR band. Silver nanoparticles' Mie radius was estimated to be in the range 2.74.3 nm. It was observed that the SPR peak position is redshifted by increasing Ag content in the indium oxide matrix due to interparticle coupling e®ect on SPR. Nonlinear absorption coe±cients and
Optical and Structural Properties of Silver Nanoparticles Embedded in Indium Oxide Films
nonlinear refractive indices in Ag-incorporated indium oxide were measured using z-scan technique, under cw excitation. Two-photon absorption mechanism was used to explain the observed reverse saturation. The negative sign of nonlinear refractive index data is indicative of the dominance of thermal lensing. The result of the present work is important from physical point of view and technical applications in optoelectronic devices and applications related to colloidal particle properties like °uorescence and nonlinear optics.
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