copper codoped silica thin films

Nand%uduredlhtmhk. Vol. 8. No. 8.pp. 1149-1156.1997 EbwiaSoimc8Ltd 0 1998Ada Mbrllulgia sm. FvintedinuxusA. .ulrightsd 0963-9773/97 $17.00+ .lm

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SURFACE PLASMON ABSORPTION CHARACTERISTICS AND NONLINEAR OPTICAL PROPERTIES OF SILVER/COPPER CODOPED SILICA THIN FILMS Ji-Hye GUI&, Lee Chae, Sung-Jim Kim, and Miuyuug Lee* Department of Chemistry, Ewha Womans University, Seoul 120-750,Korea (Accepted February 12,1998) Abstmct - Ag-Cu nanocrystal mixture having the molar ratio of 1 :I, I :3, and I :5 was prepared in thinfilm SiOz using the sol-gel method and high temperature heat treatment (5OOoC). The high resot’utionTIM mictvgraph shows the particle size is distributed broadly covering I-IO nm with the mranimumat 4-5 nm. The absorption spectra were simulated using the bulk AglCu dielectric constants by accommo&ting the nanocrystal size, and were compared with the measured absorption spectra. The thirdorder nonlinear optical properties of those nanocluster mixtures have been studied by the degenerate four wave mixing technique. The measured third-order nonlinear susceptibilities, $3). were on the order of 10V8- lo7 esu. @D!J8 Acta Metallurgica Inc.

1. INTRODUCTION Noble metal nanoparticles embedded in dielectric matrices have a great potential for nonlinear optical devices due to their large nonlinear optical nonlinearity in the surface plasmon absorption region. So far, the most attention has been focused on the pure fomt of noble metals. Recently, experimental studies move on to the mixture of metal particles or alloy form that reveal new interesting optical phenomena, including change of surface plasmon abso@on characteristics and nonlinear optical properties (l-4). There have been two reports on the optical pmperties of SioZ thin film doped with Ag and Cu nanoclusters. Magruderand coworkers implanted Ag and Cu ions on silica and observed formation of Ag-Cu nanocluster“alloy” (l-2). They attributed the formation of 14g-Cualloy to sequential doping of energetic Ag and Cu ions. De et al. used the sol-gel method, to co-dope Ag and Cu nanoparticlesin silica thin fihn (3). In their sample, it was observed that Ag and Cu do not form alloy, but exist as independent Ag and Cu nanoparticle mixtures. This work concerns understandingthe opticalproperties of metal nanocluster mixture in the microscopic level. Ag/Cu nanocluster mixture was pmpared in S0.2 thin fti by the sol-gel metbodatthevariousmolarratioof 1:1,1:3,and1:5. Thenanoclustersizeandfihnthicknesswere characterized by high resolution TEM. We simulated the cptical absorption spectra of those samples as a function of nanocluster size and composition ratio. The simulated spectra were then compared to tlhe recorded absorptionspectra in the 300-800nm wavelength region with which the 1149

1150

J-H GWAK,L CHAE, SJ KIM AND

M LEE

whole surface plasmon absorptionofAg and Cu particle is covered. In addition to the linearoptical property, the nonlinear optical property of the nanoclustermixture was studied by the degenerate four wave mixing technique in backward geometry. 2. EXPERIMENTAL METHODS Ag and Cu nanocrystal mixture in Si@ was prepared by the method used by De et al. with slightmodification(3). Insteadof 1:1,1:2,and 1:3,weusedAg/Cumolarmtioof l:l, 1:3,and 1:5, named as lAglCu, lAg3Cu. and lAgSCu,respectively. Moreover,the heat treatment temperature was lowered to 500°C rather than 700°C in order to make the particle size smaller and to maintain more homogeneous size distribution. TheTEM micrograph was obtained by an Hitachi analytical transmission electron microscope (model H-9000)that has 0.18 nm point resolution and 0.1 nm lateral resolution. The TFM picture was prepared in the following way. Two4x4 mm plates were prepared by cutting the slide glass (25 x 25 mm) on which Ag-Cu/Si@ thin film was coated. The slide glass plates were glued using epoxy in the way of coated sides facing each other. The same size of two dummy silicon wafers was also prepared and each was pasted on the outsides of the sample. Then, the coated area was centered on the cross-sectionof 3 mm diameter which was cut through to make a cylinder type, using a cutting machine. A 100 pm thick disk was prepared by cutting the cylinder parallel to the cross section and was dimpled by thinning. After polishing the dimpled area by alumina solution, final thinning to have electron transparency was achieved by ion milling with a singleAr gun at 5 keV. In this way of sample preparation for TEM, it is possible to measure the film thickness as well as the particle size. The nonlinear optical properties of the sample were measured by a standard degenerate four wave mixing (DFWM) setup (5), equipped with a Q-switched Nd:YAGlaser that generates the frequency doubledbeam (532nm)of3OOmJ,8nspulsesat 1OHz.Thepulseenergywasattenuated by the factor of ten to avoid sample damage and saturation,and was split out into three beams that consist of the forward, backward, and probe beam. The forward and backward beam have equal intensity, but the probe beam intensity was 5% of both beams. The diffracted signal detected by a calibrated photodiode was amplified and every hundred shots were averaged by a storage oscilloscope. Pure CS2liquid contained in a 1mm thick quartzoptical cell was used as a reference. The xc31value of 1.7 x 1@12esu was used for CS2 (6). 3. RESULTS AND DISCUSSION The size distribution was obtained by high resolution TEM. Figure 1 shows TEM micrograph of the lAglCu sample. Thepicture shows clearly the dual coating nature of the sample. It also shows that the second coating gives better homogeneity across tbe film. In the first coating the interfacial layer between the sol-gel film and the glass substrate produces somewhat larger metal particles. The crystal size was counted and its distribution was shown in Figure 2. The nanocrystal size appears to be l-10 nm, having the maximum at around 4-5 nm. The nanocrystal mixture, prepared by sol-gel process, was previously reported by De et al. (3). Their data show much larger size distribution and the distribution was bimodal. That is because they used higher processing temperature (7ooOC).Welowered the processing temperatureto preparenarrower size distribution with smaller particle size, which expectedly give an ensemble of better physical property.

SILWCOPPER

CO-DOPED SILKSTHINFILM

1151

Figure 1. TEM micrographof Ag/Cu nanocrystal mixture

TABLE 1 rsical Constants of Ag and Cu Used to Simulate Absorption Specea. Parameter

Silver

COPPer

Electron density (1022 cu~-~) Electron mass ( 1U3t kg) Plasma frequency ( lo-l6 se&) Fermi velocity (108 cm WC-‘) Relaxation time ( 1t314 set)

5.86 9.01 1.38 1.39 3.68

8.47 9.32 1.61 1.57 2.48

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J-H GWAK, L CME, !3J KIM ANOM LEE

25i-

20l-

15

10

5

0

1

2

3

4

5

6

7

8

9

IO 11

Particle diameter (nm) Figure 2. Particle size distribution of Ag/Cu nanocrystal mixture.

By the TEM data, it was not possible to distinguish the individual size distribution of Ag and Cu particle. According to the Mie scattering theory, the absorption spectrum of a nanoparticle can be calculated by applying the quasi-static approximation. That is, in the UV-vis spectral region, the Mie theory is good for the particle size of less than ten nanometer. The absorption coefficient of a metal particle is both contributed by the interband transition and surface plasmon resonance that canbecharacterizedbythemetaldielectricconstante(o)=&t(o)+i&2(o)in whichoistheangular frequency of the electromagnetic wave. The absorption spectrum of spherical particles is then given by A = (NI / 2.303)18x V &2(o)~,~/Ll(k[(~t(o) + 2~,)~ + EZ(O)~]

I

HI

where A is the absorbance, N the particle number density, 1the pathlengtb, V the particle volume, h the wavelength, and em the matrix dielectric constant, respectively. In deriving this equation, only the dipole term was considered. In general the dielectric constant of a metal is described by additive contribution from conduction electrons and bound electrons. That is, E(O) = &t(e)) + &z(o) = [At + Bt] + i[A2 + Bz]

PI

SILVER/COPPER

CO-DOPED SILICA THINFauns

1153

where A is due to conduction electrons, and B due to bound electrons of the metal. For nanoparticles, the value of A is different from bulk, because of the free electron confinement. On the other hand, the value of B is independent of the particle size. A is calculated by At= 1 -o~~/(&+u,~)

131

and A2 = 01~odo(&+

00~)

r41

with the plasma frequency o = (4me2/m*)‘n in which N and m* are electron density and the effective mass of conduction electrons, respectively. The collision frequency 00 is the inverse of the relaxation time for bulk. As the particle size decreases, the collision frequency increases due to electron confmement in a sphere:

where VFis the Fermi velocity and R is the particle size. The simulated spectra of nanoparticles were obtained in the following way. Since Bt and B2 values are independent of particle size, those values are calculated from the bulk dielectric constant data: A1 and A2 of Ag and Cu were first calculated from the known value of the physical constants of the bulk metal given in Table 1. By subtracting out the A values from the real and imaginary part of the metal dielectric constant at a given wavelength, the B values are obtained as a function of wavelength. Then, for a nanoparticle having the radius of R, At and A2 are calculated using eq. [3-51. The metal dielectric constant of the particle with radius R is obtained by adding A and B at each wavelength. The simulated absorption spectrum of silver andcopper mixture are obtained by summing that of individual metal particles with taking into account the molar ratio. Figure 3 shows the simulated absorption spectra of lAglCu, lAg3Cu, 1AgSCu sample having the particle diameter of 1,4, and 10 nm. In Figure 4 the measured ahsorption spectra of lAglCu, lAg3Cu, 1AgSCuare plotted. The recorded spectra are similar to the simulated spectra with the particle diameter of 4 nm, in terms of the peak position and the isobeatic point. Of course, they cannot be the same because of the broad size distribution of the prepared sample. The nonlinear optical coefficient of Ag-Cu nanocrystal mixture sample was obtained by the degenerate four wave mixing. By measuring the diffraction efficiency of the sample and thalt of the reference (CS2). the xs %alue of the sample is obtained using the following equation

Is1’2

X SC3) -=---_

ns2

I,

xr(3)

nr2

1s Ir1’2

ln(l/

T)

T1’2(1 -T)

WI

where n is the linear refractive index, 1 the pathlength, and I the diffraction efficiency. The transmittance T is given by exp[-aQ in which a is the absorption coefficient and 1 the sample thickness. The subscripts, sand r, refer to the sample and thereference,respectively. Table 2 shows the calculated xc3) values of Ag-Cu mixture sample. The result shows that the nonlinear optical coefficient slightly increases as the Cu fraction in nanocrystal content increases. It should be mentioned that the nonlinear response in this case is not purely electronic in origin, but contains thermal contrilbution, because nanosecond pulses were used in the experiment. The ~(~1value has

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J-H GWAK,L CHAE, SJ KIMANDM LE

D=l.Onm

Wavelength Figure 3. Simulated absorption spectra of AgjCu mixture. been measured as (4.6-10.6) x 10e8esu, depending on the composition. Using the same type of measurement, the nonlinear coefficient of pure Cu nanoparticle, prepared by ion-implantation, was previously measured as 5.6 x 10s8esu. Magruder et al. first measured the nonlim~ optical properties of AgCu sample which is prepared by the ion-implantationmethod. By using the z-scan method, they obtained the nonlinear refractive index (nz) value of (1.0-1.6) x lo-l3 m2/W at 575 nm. This cotresponds to x(3)value of (4.1-6.5)x low8esu. The nonlinear optical properties of Ag-Cu mixture, prepared by sol-gelprocess, wasalso measuredby Haglund andcoworkers. Using the z-scan method, they obtained the n2 value of the order of lo-l3 m2/Win tbe wavelength range of 570-5% nm for lAglCu. It should be mentioned, however, that the nz value obtained by the

!SILVE~CO~PER

1155

CO-DOPED SILICA THINFws

I .8 Ag:Cu=l :1 -----Ag:Cu=1:3 _.....-___ *g:cu=,.5

I.6 1

0.8 0.6

300

400

500

600

700

800

Wavelength Figure 4. Measured absorption spectra of Ag/Cu nanocrystal mixture.

TABLE 2 Third-OrderNonlinear Optical Coeffkient xc3)of Ag-Cu Nanoclusters Doped in Si@ Thin Film Absorbance at 532 nm

x(3’ (lo+) esu)

0.244

1.m 0.14

0.298

2.21f 0.27

0.311

2.66f 0.46

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J-H &AK,

L &AE,

SJ ffi~AND

M i_EE

z-scan technique is related to the real part of x c3),while the DFWM measures the absolute value of xt3). It seems that the nonlinear optical coefficient of Ag-Cu nanocrystal mixture in Sio2 lies on the order of lo-* esu. At 532-5% nm region, the nonlinearity of the composite arises from copper nanoparticles, without being much affected by the presence of silver. ACKNOWLEDGMENTS This work has been financially supportedby a grantfrom Ewha Womans University and by the academic research fund of Ministry of Education, Republic of Korea. REFERENCES 1. 2. 3. 4. 5. 6.

MagruderIII,R.H.,Wittig,J.E.,andzuhr,R.A.,JournalofNon-CrystallineSolids. 1993,163,162. Magruder III, R.H., OsborneJr.,D.H. and Zulu, R.A., Journal of Non-Crystalline Solidr, 1994, 176,299. De,G.,Tapfer,L.,Catalano,M.,Battaglin,G.,Caccavae,F.,Gonella,F.,Mazzoldi,P.andHaglund Jr, R.F., Applied Physics Letters, 19!96,68,3820. Sangregorio, C., Gale&, M., Bardi, U. and Baglioni, P., L.ungtnuir, 1996,12 5800. See, for example, Nonlinear Optics of Organic Molecules and Polymers, 4s. H.S. Nalwa and S. Miyata, CRC Press, Boca Raton, 1997. Sakagucbi, T., Sbimizu, Y., Miya, M., Fukuxni,T., Ohta, K., and Nagata, A., Chemistry Letters, 1992,281.

7. 8. 9.

Kreibig, U. and Fragstein, C.V.,2. Physik, 1969,224,307. Kreibig, U. and Volmer, U., Optical Properties of Metal Clusters, Springer, Berlin, 1995. Cbae, L., Lee, M., Kim, H.K. and Moon, D.W.,Bulletin of the Korean Chemical Society, 1997,18, 886.