silica nanocomposites

Microelectronic Engineering 66 (2003) 171–179 www.elsevier.com / locate / mee

Nonlinear optical properties in CdS / silica nanocomposites S.G. Lu a , *, Y.J. Yu a , C.L. Mak a , K.H. Wong a , L.Y. Zhang b,c , X. Yao b,c a

Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong b FMRL, Tongji University, 71 Chifeng Road, Shanghai 200092, China c EMRL, Xi’ an Jiaotong University, Xi’ an 710049, China

Abstract Porous CdO / silica gel glasses were prepared through a sol–gel process. The nano-sized CdS was obtained by a gas reaction. For CdO / SiO 2 , the average pore radii were examined by mercury porosimetry. X-ray diffraction (XRD) was used to study the structural properties of the embedded CdS particles. Their crystallite sizes were estimated using Scherrer’s equation. The band gap energy of embedded CdS was evaluated by optical absorption measurement. A shift in the band gap energy, with respect to the bulk materials, was observed in CdS / SiO 2 nanocomposites. The amount of this shift increased with decreasing crystallite size. Based on the absorption spectra, the estimated effective mass of exciton was 0.315m 0 . The photoluminescence (PL) spectrum also showed a blue-shift characteristic near the energy band gap, but the shifted values were smaller than those of the absorption edge. Furthermore, a strong PL emission peak, observed in the range between 500 and 700 nm, also demonstrated a blue shift characteristic. The mechanism of this PL is regarded to be a re-combination process from surface states to the valence band, which resulted from the dangling bonds of the porous silica matrix, in the interband of the energy band gap of nanometre CdS particles. The 3rd nonlinear optical susceptibility was found to be 4.5310 211 esu (CW laser) and 2.3310 211 esu (pulse laser), respectively.  2003 Published by Elsevier Science B.V. Keywords: Nanometre; Sol–gel process; Photoluminescence; Nonlinear optical properties

1. Introduction Recently, nanometre II–VI semiconductor materials have attracted much attention because of their wide application as optical memories, and as fast response photoluminescent and electroluminescent devices [1–4]. Because these nanometre materials have a large percentage of surface atoms, they are very active under ambient conditions. In order to avoid the degradation of these active materials, they * Corresponding author. E-mail address: [email protected] (S.G. Lu). 0167-9317 / 03 / $ – see front matter  2003 Published by Elsevier Science B.V. doi:10.1016 / S0167-9317(03)00043-1

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are usually embedded into a host matrix to form composites, which are commonly called 0–3 nanocomposites for their zero dimensional active component. There are lots of studies reporting the quantum size effect and nonlinear optical effects in these nanocomposites [5–8]. These studies mainly focused on their optical absorption properties. However, to our knowledge, few reports [9–11] on the PL in these nanocomposites have been given. In this paper, crystallite nano-size CdS is successfully embedded into sol–gel silica. Blue-shift in the absorption edge, compared to bulk CdS, is observed in the absorption edge of the CdS / silica nanocomposite. A very strong PL peak related to the interaction between the nanometre CdS and the silica matrix, and a weaker PL peak relevant to the band-to-band emission are reported. These PL peaks can be described on the basis of the interaction between CdS nano-particles and the silica matrix.

2. Experimental procedures A two-step sol–gel process was used to prepare the gel glass. The CdO contents in our CdO / SiO 2 samples were 1, 2, 5, and 10 mass%. Firstly, tetraethyl orthosilicate (TEOS) was dropped into an acidic solution of deioned and ethanol alcohol with a pH value of 1.0 to 2.0, and stirred for 1 h. Secondly, cadmium acetate dissolved in an ethoxyethanol was added into the solution. The mixture was further stirred for another 30 min. After that, ammonia was added drop by drop into the solution to enhance its gelation speed. The final solution was stirred for another 30 min, then poured into glass utensils and placed in a closed environment for a couple of days to form a stiff gel. The gel was dried for 10 days at room temperature to form a dried gel and then annealed at 500 to 800 8C for several hours to get porous gel glasses. Finally the CdS / silica nanocomposite was obtained by passing H 2 S gas through the gel in a lower pressure atmosphere. By controlling the reaction time, different crystallite sizes of CdS can be obtained. The densities of samples after and before reaction with H 2 S gas are about 1.96 and 2.10 g / cm 3 respectively, measured by Archimedes method. The refractive indices are found to be 1.3 and 1.2, respectively, by use of an ellipsometer.

3. Results

3.1. Pore size distribution A mercury porosimeter (Carlo Erba) was used to study the effect of the annealing temperature on the pore size of CdO / SiO 2 gel glass. The results are presented in Fig. 1. With increased annealing temperature, both averaged pore radius and pore volume decrease, indicating that CdO / SiO 2 gel glass becomes denser. For samples annealed at 700 8C, pores with radius over 100 nm disappear. The most probable pore radius obtained at this temperature is about 5.6 nm.

3.2. XRD analysis The XRD patterns of the CdS / SiO 2 nanocomposites are shown in Fig. 2. The crystalline structure of CdS is of hexagonal phase in accordance with the JCPDS cards. By comparing Fig. 2a and b, we observed that the CdS / SiO 2 nanocomposites still contained a small portion of CdO in the SiO 2

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Fig. 1. Pore size distribution of CdO / SiO 2 gel glass by Hg-pressing method. (a) 400 8C32 h; (b) 500 8C32 h; (c) 700 8C32 h.

matrix. This indicates that the sulphidization of CdO was not complete. This may be due to the existence of CdO in the glass network, which cannot react with the in-flow H 2 S. After doing the peak separation and subtracting the instrumental broadening specified with a polycrystalline silicon powder, the crystallite sizes obtained by Scherrer’s formula for the k a 1 peak were between 3.5 and 4.5 nm. The relation between crystallite size and reaction time is shown in Table 1. This result consists of the porous size obtained by mercury porosimetry. The lattice constants a and c of the embedded CdS were found to be 0.4138 and 0.6758 nm, respectively, a little different from those of CdS bulk material (a50.4150 and c50.6738 nm). Our embedded nanoparticles have a larger c /a ratio than that of bulk materials. This larger value is, however, much closer to the theoretical value (c /a 5 1.6331) of

Fig. 2. XRD profiles of CdS / SiO 2 nanocomposites. (a) CdS / SiO 2 ; (b) CdO / SiO 2 ; (c) CdS.

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Table 1 Crystallite size of CdS versus reaction time with H 2 S Reaction time (h) Crystallite size (nm)

1 3.5

5 4.1

48 4.5

the hexagonal phase. The samples were ground into powder, dispersed in ethanol and observed under a TEM (JEOL 200 CX). The glass particles were found to be transparent, the electron diffraction patterns were attenuated, and only two diffraction rings were observed (Fig. 3). This is fewer than that

Fig. 3. Electronic diffraction pattern and rings of CdS crystallites in a CdS / SiO 2 nanocomposite.

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of BaTiO 3 particles (crystallite size about 50 nm) which have five rings [12], indicating that CdS crystallites are composed of two or more crystallites, whose size is even smaller.

3.3. Absorption and PL spectra The room temperature absorption spectra of nanocomposites, with different reaction times, were measured by a UV absorption spectrophotometer (Shimadzu UV-240). In general, the absorption edge due to the band transition of CdS is blue-shifted compared with bulk materials. The results have been published elsewhere [14]. The largest shift is about 0.254 eV with reaction time of 0.5 h. The PL spectra were measured by a UV–Vis–NIR spectrophotometer (Perkin-Elmer LS-50). The excitation (PLE) and PL spectra are shown in Figs. 4 and 5, respectively. For the PL spectrum, the excitation wavelength was 420 nm and a filter of 430 nm was used in order to prevent influence from the light source. The emission wavelength used for PLE spectra was 550 nm while the band gap energy of bulk CdS at room temperature is 2.38 eV (i.e. |520 nm).

Fig. 4. Excitation spectra of CdS / SiO 2 nanocomposites of different reaction times with H 2 S. (a) 5 h; (b) 7 h; (c) 48 h. Emission wavelength, 550 nm.

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Fig. 5. Emission spectra of CdS / SiO 2 nanocomposites of different reaction times with H 2 S. (a) 5 h; (b) 7 h; (c) 48 h. Excitation wavelength, 420 nm. Filter, 430 nm.

3.4. Third nonlinear optical susceptibility x (3) The degenerate four wave mixing (DFWM) method was used to measure the 3rd nonlinear optical susceptibility x (3) . The experimental set-up for the CW laser is shown in Fig. 6. The Ar 1 laser (Spectra Physics) has an output power as big as 4 W, and a wavelength of 514.5 nm. The power meter (HOP-1) may detect power as small as a pW. According to the theory [13], 3rd nonlinear optical susceptibility x (3) can be expressed as

x

(3)

] Al0 tg 21 (ŒR) 5 ]]]]] ] 3p cm0 mr LœI1 I2

(1)

where A is the area of the laser beam, l0 is the laser wavelength, R 5 I4 /I3 the conjugated reflectivity, c the light speed in a vacuum, m0 the magnetic susceptibility in the vacuum, mr the relative magnetic susceptibility of the sample, L the thickness of the sample, I1 and I2 are the pumped laser intensity and the transmitted laser intensity, respectively. The measured data are: I1 5 985 mW, I2 5 780 mW,

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Fig. 6. Third nonlinear optical susceptibility measurement set-up.

R 5 1.05 3 10 27 , l0 5 5.145 3 10 27 m, u 5 108, l 5 9.6 3 10 24 m, and A 5 3.6p 3 10 29 m 2 . In addition, the constants used are, m0 5 1, mr 5 4p 3 10 27 , L 5 l / cos 108 5 0.975 mm. So according to Eq. (1), we have x (3) 56.21310 219 (m / V)2 54.45310 211 (esu). We also did the DFWM measurement by use of a pulse laser (Nd-YAG, pulse width is 11 ns, and adding KDP SHG crystal), the result was 2.30310 211 (esu). The value approaches the above one obtained by a continuous laser.

4. Discussion For the absorption spectra, calculation [14] revealed that the effective mass of a confined exciton in CdS is 0.315m 0 , where m 0 is the mass of an electron. The value obtained here is less than that of the translational mass of an exciton (0.99m 0 ), but approaches the reduced mass of an exciton (0.154m 0 [15]). It seems that because the crystallite radius approximately equals the Bohr radius of an exciton (2.62 nm), the exciton becomes mainly a reduced one. Our result is different from that of Potter and Simmon’s work [5], where their effective mass was 0.92m 0 which is close to the translational mass. The discrepancy may be due to a smaller crystallite size in our work. For the PLE spectra (Fig. 4), three peaks at about 460, 410 and 390 nm were observed. The first peak (460 nm) shifted to a lower wavelength, i.e. larger energy with shorter reaction time in H 2 S. This peak is assigned as the jump of an electron directly from the valence band to the conduction band of the embedded CdS nanoparticles (band to band jump) [16]. Hence, an enlargement of the energy band gap caused by the quantum confinement of excitons in CdS particles is observed. However, the second and third one, probably due to some electronic transition of the SiO 2 matrix, have no obvious shift with crystallite size. Although both the PLE and absorption spectra showed a blue-shift characteristic of decreasing crystallite size, the amount of the shift in the absorption edge measured by absorption spectra is larger.

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For the PL spectra (Fig. 5), the emission peaks at |478 nm accounting for the combination of electrons in the conduction band with the holes in the valence band, show a blue-shift characteristic with decreasing CdS particle size and demonstrate the quantum size effect again. Another strong emission peak, ranging from 500 to 700 nm, also exhibits a blue-shift feature. Since the pure porous silica glass has no such strong emission in this wavelength region, this emission peak may be connected with the nanometre CdS. A possible explanation of this peak is that it originates from the nanosized CdS particles, whose electrons are excited to a higher energy state. Then, the electrons come down to an intermediate energy level through a non-radiation route and combine with the holes in the valence band to produce PL. The intermediate energy level may result from the surface states or the dangling bonds of the porous silica matrix [17]. Recently, the role of oxygen in the PL of porous silicon (P-Si) has been paid much attention [18]. In contrast to the situation in P-Si, the dangling bonds of Si–O in the sol–gel-derived glass network intrinsically exist. These dangling bonds can be further increased after the reaction of Cd 21 with S 22 through bond breaking, which has been verified by Raman spectroscopy [19]. Since there is a large number of dangling bonds in our nanocomposites, the concentration of inter-band levels is very high. Therefore the PL from the combination of inter-band to the valence band is much stronger than that directly from the conduction band to the valence band, which is demonstrated in Fig. 6. The 3rd nonlinear optical susceptibility was measured by using a degenerate four wave mixing method. Two kinds of methods had almost the same value. If considering that the mole concentration of CdS nanometre crystallites in the matrix SiO 2 is 5.45310 23 , and the dielectric constant of CdS approximately equals that of SiO 2 [20], so the x (3) value will be enlarged into an order of 4.22310 29 esu, which is slightly larger than that of bulk CdS (3.4310 29 esu) [21]. However, it is still a challenge to precisely measure the mole concentration through elemental chemical analysis, because one cannot distinguish the element Cd from the CdS crystallites or from the unreacted CdO?SiO 2 matrix. In addition, the measurement frequency used here is far from that of resonant frequency. Taking these factors into account, it is expected that the 3rd nonlinear optical susceptibility will be further enhanced. Nevertheless, the structural analysis indicates that there is a strong bond broken in the nanocomposite, which has a negative effect on the 3rd nonlinear optical properties.

5. Conclusion Porous CdS / silica nanocomposites were prepared through sol–gel and gas reaction processes. For CdO / SiO 2 , the average pore radii examined by Hg pressing method were |5.6 nm. XRD results indicate that the crystallite sizes of CdS were about 3.5 to 4.5 nm. A quantum size effect was evaluated by optical absorption measurement that had a blue-shift characteristic in the band gap energy with decreasing crystallite size. PL spectra also showed the blue-shift feature near the energy band gap, but the shift values were smaller than those of absorption edges. A strong PL peak was observed in the range from 500 to 700 nm. The mechanism of PL is regarded to be the interaction between the nanometre CdS particles and the porous silica matrix. The dangling bonds caused from the broken bonds were regarded to play an important role in the course of the PL emission phenomenon. The 3rd nonlinear optical susceptibility was obtained to be 4.5310 211 esu (CW laser) and 2.3310 211 esu (pulse laser), respectively.

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Acknowledgements This work was supported by the National ‘863’ New Materials Project of China. SGL would like to acknowledge financial support from the Centre for Smart Materials of the Hong Kong Polytechnic University. Thanks are also given to A.D. Tang and Y.E. Kong, Xi’an Institute of Optics and Precision Mechanics, CAS, for the measurement of the 3rd nonlinear optical coefficient.

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