Ultrafast electron dynamics and enhanced optical ...

JOURNAL OF APPLIED PHYSICS 98, 033528 共2005兲

Ultrafast electron dynamics and enhanced optical nonlinearities of CdScapped Au/ BaTiO3 composite film Yong Yanga兲 and Masayuki Nogami Department of Materials Science and Engineering, Nagoya Institute of Technology, Showa, Nagoya 466-8555, Japan

Jianlin Shi and Hangrong Chen State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China

Ye Liu and Shixiong Qian Physics Department, National Key Laboratory of Applied Surface Physics, Fudan University, Shanghai 200433, People’s Republic of China

共Received 20 January 2005; accepted 23 June 2005; published online 11 August 2005兲 The third-order nonlinear optical responses of gold nanoparticles capped by CdS shells of different thicknesses embedded in BaTiO3 films were investigated by off-resonance femtosecond optical Kerr effect technology at 800 nm. The nonlinearities of the Au–CdS core-shell composite nanoparticles exhibit a significant enhancement, compared with their single components, and the ␹共3兲 value apparently increases with the increase in the CdS shell thickness. The electric-field distribution in the composite nanoparticles was calculated using Neeve’s theory 关A. E. Neeves and M. H. Birnboim, J. Opt. Soc. Am. B 6, 787 共1989兲兴 to explain the nonlinear enhancement mechanism, which was mainly attributed to the enhancement of the localized electric field in the CdS shell. The electron relaxation dynamics in the Au–CdS nanoparticles with a core-shell structure were found to be the same as that in pure gold nanoparticle systems using the time-resolved pump-probe technique measured at 800 nm. © 2005 American Institute of Physics. 关DOI: 10.1063/1.2001727兴 I. INTRODUCTION

Materials with a large third-order optical nonlinearity and ultrafast response time are essential for light-controlled phase and refractive index modulation for future applications in various fields such as optical telecommunications, optical data storage, optical computation, and information processing.1 Nanometer-sized metal-particle-based composite materials exhibit a large third-order nonlinear response due to the enhancement of the local-field factor near the surface-plasmon resonance.2 Recently, Chen and co-workers prepared Ag/ BaTiO3 共Ref. 3兲 and Au/ BaTiO3 films4 using pulse laser deposition and observed large optical nonlinear responses using the Z-scan method. Another class of composite materials, semiconductor nanoparticles embedded in a dielectric matrix,5 also exhibit a large third-order nonlinearity enhancement at both resonant and nonresonant frequencies, originating from the quantum size effect and dielectric confinement effect, respectively. However, the nonlinear properties of semiconductor-metal composite nanoparticles were seldom reported due to the difficulties in the preparation by physical methods. The composites of nanoparticles with a metallic core and semiconductor shell, or with a semiconductor core and metallic shell, suspended in a nonlinear medium should exhibit a strongly enhanced nonlinear response by several orders of magnitude higher than their single component based on the theoretical calculations by Neeves and Birnboim.6,7 The advances in self-assembly a兲

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chemistry offer an exciting pathway for the construction of materials with designer-specified optical, electrical, and catalytic properties. In this Letter, we applied the self-assembling technique to synthesize the gold nanoparticle core capped with CdS shells of different thicknesses. The nonresonant third-order optical nonlinear responses were investigated using the femtosecond optical Kerr effect 共OKE兲 technique. To investigate the nonlinear enhancement mechanism, the electric-field distribution in composite nanoparticles was calculated according to the electrostatic approximation by Laplace’s equation under the boundary conditions appropriate to the model of core-shell nanoparticles. The ultrafast electron relaxation dynamics was also studied by the timeresolved pump-probe technique. II. EXPERIMENT

Gold nanoparticles capped with CdS shells of different thicknesses were prepared and fabricated into BaTiO3 films.8,9 Briefly, 4 ml of a 24.3-mM HAuCl4-ethanol solution was added to a 100-ml boiling solution of trisodium citrate 共1.0 mM兲 with rapid stirring. After the solution had turned purple red within 30 s, it was quickly cooled in an ice bath and concentrated to 20 ml in a vacuum. CdS colloids were prepared with Cd共CH3COO兲2 and CS共NH2兲2, Cd共CH3COO兲2 · 2H2O 共1.0 mmol兲 was dissolved in acetic acid 共20 ml兲, and CS共NH2兲2 共3.0 mmol兲 was dissolved in 2-methoxyethanol 共20 ml兲. The two solutions were then mixed and stirred for 60 min at 40 ° C. The gold colloid was injected into the CdS sol at different molar ratios in order to control the shell thickness. The gold colloid soon turned

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brown from purple red. After 30 min, the mixed solution turned transparent from brown, which indicated that the gold colloids had been capped by the Cd2+-thiourea complexes. The Au–CdS core-shell colloid solution was slowly injected into the BaTiO3 precursor solution.8 Thin films were dip coated on glass slides at a withdrawal speed of 75 mm/ min. Each film was dried at 60 ° C for 10 min and successively heated to 500 ° C for 20 min in a N2 atmosphere. The nonresonance third-order nonlinear susceptibilities ␹共3兲 of the composite films were measured in accordance with the OKE at room temperature. The details of the experimental arrangement for the OKE have been described elsewhere.10 The laser source was a mode-locked Ti:sapphire femtosecond laser 共Spectra-Physics Tsunami兲 operating at a repetition of 82 MHz. The output pulse centered at 800 nm with a duration of 80 fs was split into pump and probe pulses and then focused tightly on the sample with the spot size of about 50 ␮m. The OKE signal was detected by a photodiode connected to a lock-in amplifier and stored in a computer. CS2 was used as the standard sample, with its model of ␹共3兲 being 1.0⫻ 10−13 esu at 800 nm. III. RESULTS AND DISCUSSION

The size distribution and shape of the Au–CdS composite nanoparticles were characterized by transmission electron microscopy 共TEM兲, as shown in Fig. 1共a兲. The typical coreshell structure of a composite nanoparticle is clearly observed. The composite particles are spherical, their average diameter is around 10 nm and the diameter of the Au core is around 4 nm. The other two samples have the same structure and the same diameter as the Au cores, but the diameters of the composite particles are 6 and 8 nm, respectively 共Table I兲. Figure 1共b兲 shows the UV-VIS absorption optical spectra of composite films with different CdS shell thicknesses. These curves have onsets at 480– 500 nm, which are attributed to the absorption edge of the CdS nanocrystals. These absorption peaks at about 590 nm are attributed to the surface plasma resonance 共SPR兲 of the gold nanoparticles. The transition between the 5d10 level and unoccupied conduction bands leads to the SPR absorption spectra for gold nanoparticles.11 It is interesting to note that the SPR peak is redshifted with the increase in the CdS shell thickness. This redshift is also consistent with the theoretical analysis result by Neeves and Birnboim6 that the SPR peak would be redshifted as the shell thickness is increased for the dielectric constants ␧shell ⬎ ␧matrix 共␧shell = 5.4 and ␧matrix = 3.6, measured by ellipsometry兲. The OKE signals of the Au–CdS core-shell nanoparticles 共CSNs兲 embedded in BaTiO3 films are shown in Fig. 2. The third-order nonlinear susceptibility ␹共3兲共model兲 of the thin film can be estimated using the following equation:10

␹共3兲 =

冉冊冉冊冉冊 Is Iref

1/2

ns nref

2

Lref 1 共3兲 ␹ , Ls R ref

共1兲

where n is the linear refractive index, I is the OKE signal intensity, L is the interaction distance of the pump light and probe light in the sample, R is the correction coefficient, and

FIG. 1. 共a兲 The TEM micrograph of the Au-CdS core-shell composite nanoparticles incorporated in the BaTiO3 film. 共b兲 The UV-VIS absorption spectra of the Au– CdS / BaTiO3 films with different CdS shell thicknesses. The inset shows the changes in the SPR position with the CdS shell thickness.

the subscripts s and ref stand for the sample and reference, respectively. The relative parameters and calculated ␹共3兲 values of the films are shown in Table I. The calculated ␹共3兲 value of the sample with the total diameter d2 = 10 nm is 共1.58± 0.20兲 ⫻ 10−10 esu, which is about one order of magnitude larger than the summation of equal molar single gold and CdS nanoparticles embedded in the BaTiO3 films prepared under the same conditions 共they are 0.66⫻ 10−11 and 1.58⫻ 10−11 esu, respectively兲.9 The ␹共3兲 value increases with the increase in the CdS shell thickness. The enhancement of the third-order nonlinear susceptibilities ␹共3兲 of the films should be attributed to the internanoparticle interactions or the coherent coupling between the CdS and Au nanoparticles during laser excitation.12 The local field can be concentrated in both the interior and the exterior neighborhoods of the CSNs at the surface-mediated plasmon


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TABLE I. Nonlinear optical properties and decay times of Au– CdS / BaTiO3 films with different CdS shell thicknesses.

Sample Au/BTO CdS/BTO Au-CdS/BTO

Diameter of core 共nm兲 8 5 4 4 4

Diameter of total particle 共nm兲

␹共3兲共10−11 esu兲

␶e-e 共fs兲

␶e-ph 共ps兲

␶ph-ph 共ps兲

6 8 10

0.66± 0.12 1.58± 0.30 5.83± 1.20 7.24± 1.40 15.8± 2.00

95± 5 90± 5 90± 5

1.7± 0.03 1.8± 0.04 1.6± 0.04

⬎160 147± 6 125± 4

resonance.6 Therefore, the increased field is then utilized for the self-enhancement of the optical nonlinearity from each component of the composite films including the Au core, CdS shell, and BaTiO3 medium. Figure 3 is the calculated spatial dependence of the electric-field ratio in Au–CdS composite nanoparticles based on the electrostatic approximation by the solution of Laplace’s equation under the boundary conditions appropriate to the CSN model.6 It is evident from the inset of Fig. 3 that the electric field E1 in the core and E2 in the shell region are significantly enhanced at the resonance wavelength 共598 nm兲. However, when excited at 800 nm, the amplitude of the electric field E1 in the core region is less than that of E0, while the electric field E2 in the CdS shell region gets substantially enhanced compared to the far field from the particle E0; then the enhanced electric field E2 in the CdS shell region is dominant for the enhancement of the effective optical nonlinearity in our samples. Furthermore, with the increase of CdS shell thickness, both amplitudes of the electric fields in the core E1 and shell E2 regions increase, which results in the further enhanced third-order nonlinearity. The femtosecond transient dynamics under excitation at 800 nm for our samples were examined by a standard pump and probe technique. The incident 80-fs laser pulse was split into a pump pulse and a probe pulse by a beam splitter, and the pump power was 60 mW. The normalized transmittances of the probe beam from these samples were recorded as a function of the delay time as shown in Fig. 4.

FIG. 2. Femtosecond OKE responses of Au– CdS / BaTiO3 films with different CdS shell thicknesses.

As is well known, the electron relaxation dynamic process in a metal nanoparticle system with ultrafast light excitation occurs in several steps.13,14 First, the electrons are excited to energy states higher than the Fermi level and the electron distribution is a nonthermal distribution according to Fermi-Dirac statistics. The relaxation processes start to thermalize the electrons via electron-electron 共e-e兲 scattering and electron scattering with the surface. The corresponding time scale is of the order of a few hundreds of femtoseconds, depending on the metal and conditions of the excitation. The next step involves the electron-phonon 共e-ph兲 coupling,

FIG. 3. 共a兲 Calculated magnitudes of the electric-field ratio in the Au-core and CdS-shell regions with different CdS shell thicknesses at 800 nm. For the CdS shell and BaTiO3 matrix, ␧2 / ␧0 = 5.4 and ␧3 / ␧0 = 3.6 共measured result兲, respectively. 共b兲 The electric-field ratio of E1 / E0 and E2 / E0 in a sample with a total particle diameter d2 = 10 nm as a function of wavelength.


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冋冉冊册冋冉冊冉冊册

⌬T/T = 1 − exp + A2 exp

−t ␶e-e

−t

␶ph-ph

A1 exp

+ ␥off .

−t ␶e-ph

共3兲

According to this formula, the fitted results for the sample with a total diameter d2 = 8 nm are ␶e-e = 90 fs, ␶e-ph = 1.8 ps, and ␶ph-ph = 147 ps, respectively. The decay times of 90 fs and 1.8 ps for the e-e and e-ph interactions are consistent with those 共70 fs and 1.55 ps兲 in the 3.3-nm gold nanoshell measured at 805 nm.15 These transient bleach data for other samples are also shown in Table I. These decay times for e-e and e-ph in the three samples change very slightly due to the same gold sizes, the same neighboring CdS medium, and the same measurement conditions. However, the time scale for the ph-ph interactions decreases as the CdS shell thickness increases. The differences should result from the different conduction rates of the heat inside the surrounding medium. In our case, the thermal dissipation inside the media includes the heat conductivity in the CdS shell and BaTiO3 matrix. The thermal conductivity of CdS 共20 W m−1 K−1兲 is apparently higher than that of the BaTiO3 matrix 共6 W m−1 K−1兲. With the increase in the CdS shell thickness, the heat dissipation inside the media would occur more easily. IV. CONCLUSIONS

FIG. 4. 共a兲 Transient bleach recovery data of a sample with a total particle diameter of d2 = 8 nm. 共b兲 The experimental 共open circles兲 and fitted 共solid line兲 results.

which leads to an equilibrium of the electron and lattice temperatures. This relaxation process takes place within a few picoseconds. The last step is the heat transfer from the metal nanoparticles to the environment in a long time scale of over 100 ps. This decay also depends on two contributions: the heat exchange between gold particles and the surrounding medium and the conduction of the heat inside the surrounding medium. Averitt et al.15 investigated the ultrafast electron dynamics in Au2S – Au composite nanoparticles using a femtosecond laser with a pulse width of 50 fs and used the following expression derived from an extension of the two-temperature model 共TTM兲 to include an expression related to the energy stored in the nonthermal electron distribution:

冋冉冊册冉冊

⌬T/T = 1 − exp

−t ␶e-e

exp

−t + ␥off , ␶e-ph

共2兲

where ␥off is due to the temperature-dependent effects that persist on a time scale longer than the experiment. Because the measured time scale in our case is much longer than that in Averitt’s experiment 共6 ps兲, it clearly shows the twocomponent recovery processes. Therefore, the above formula can be modified based on this case,

In summary, we have performed off-resonance femtosecond OKE experiments and calculations based on Neeve’s theory for gold nanoparticles capped by CdS shells of different thicknesses. The results establish the following conclusions: 共a兲 the nonlinear optical susceptibility in the Au–CdS composite nanoparticle was enhanced due to the core-shell structure; 共b兲 with an increase in the CdS shell thickness, both amplitudes of the electric fields in the core E1 and in the shell E2 regions increase in the BaTiO3 matrix, which results in the further enhanced third-order nonlinearities; and 共c兲 the relaxation dynamics of composite nanoparticles was similar to the electron relaxation dynamic process in a pure gold nanoparticle system at 800 nm. ACKNOWLEDGMENTS

Financial support and granting of the postdoctoral fellowship 共P04416兲 of this work from the Japan Society for Promotion of Science is gratefully acknowledged. This work is also supported by the NITECH 21st Century COE Program “World Ceramics Center for Environmental Harmony,” the Fundamental Research Program of China 共2002 CB613300兲 and Shanghai Nanomaterial Special Fund 共No. 0252nm083兲. 1

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