SYNTHESIS AND CHARACTERIZATION OF Au/CdSe & Ag/CdSe CORE/SHELL NANOPARTICLES

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1 International Journal of Metallurgical & Materials Science and Engineering (IJMMSE) ISSN Vol. 3, Issue 2, Jun 2013, TJPRC Pvt. Ltd. SYNTHESIS AND CHARACTERIZATION OF Au/CdSe & Ag/CdSe CORE/SHELL NANOPARTICLES A. GADALLA 1, M. MOHAMED 2 & D. HAMAD 3 1& 3 Physics Department, Faculty of Science, Assiut University, Assiut, Egypt 2 National Institute of Laser, Cairo University, Cairo, Egypt ABSTRACT Nanoparticles exhibit unique physical properties that are not found in their bulk counterpart. The synthesis structure which contains Au or Ag core metallic particles and a shell of CdSe semiconductor has the combined properties of quantum dots and the metallic particles. A new method has been developed to grow plasmonic semiconductor nanocomposites of Au/CdSe and Ag/CdSe nanostructure. The method based on preparing seed of metal nanoparticles which used as a seed using organometllic pyrolysis followed by adding the semiconductor precursors. Their chemical composition crystal structure is determined via X-Ray Diffraction. The collective optical properties of the plasmonic semiconductor nanohybrid has been measured using spectrophotometer techniques and compared to those individual components. The main features which observed in the hybrid nanostructure is broodning of Plasmonic band and decrease in its amplitude in addition to shifting excitonic energy band to higher wave length.the quenching of the emission of Au/CdSe has been observed due to photoinduced electron injection into metallic seed. The electron transfer processes from CdSe to the gold is more faster than that of the silver. For this reason, we can consider the CdSe/Au is strong plasmonicexitonic coupling but CdSe/Ag is week plasmonic-excitonic coupling. KEYWORDS: Au/CdSe & Ag/CdSe Nanoparticles, Organometllic Pyrolysis Meathod, X-Ray Diffraction, Optical Properties INTRODUCTION Recently many research efforts have been directed towards the engineering of the hybrid metal-semiconductor nanostructure 1-3, creating a new type of hybrid super lattices whose new properties offer great promise for applications in magnets, photovoltaics, electronic device, sensing and chemical biological application. Particular interest are a class of nanoparticles known as Quantum Dots QDs or semiconductor nanocrystals that have unique optical and electronic properties, tunable band gap, sharp emission band with broad excitation and strong resistance to photo bleaching. In the case of nanocrystals, the particle radius is smaller than Bohers radius exitons, both the carriers electrons are independently confined. This case is called the strong confiment regime.in this regime, the band gap decreases by increasing the size of the particles. Core/shell nanoparticles are nanostructures that have core made of a material coated with another material. Banin and co-workers 4 were able to synthesize hybrid nanostructure of semiconductor nanorods or tetrapods linked to metallic dots on the tips and they observe quenching of the emission due to electron transfer from the semiconductor rods to the Au dots. Surface plasmons SP, excited by the interaction between light and metal nanoparticles, can increase the density of states and the spontaneous emission rate of the semiconductor quantum-wells QW when the SP energy is inherently closed to the band-edge photoluminescence PL of semiconductor quantum dots QDs 5-8. Trials to prepare homogenous sample of Au-CdSe core shell particles have been made, but they failed to get control over the size and shape of the core-shell metallic semiconductor nanoparticles because of the lattice mismatch between the metal and the

2 60 A. Gadalla, M. Mohamed & D. Hamad semiconductor layer 9. Now it is possible to synthesize these nanocomposites in desired size and shape and with controlled improved properties such as increased stability, surface area, magnetic, optical and catalytic properties. Studying metallicsemiconductor nanoparticles with diameter less than 10 became a major disciphancy area of research during the past two decades. In this work, a method to prepare core/shell metallic-semiconductor nanodots of controlled size and shape via organometallic pyrolysis method has been developed to prepare Au/CdSe and Ag/CdSe nanocomposite. Our approach is based on heterogeneous nucleation and growth of the wurtzite CdSe nanodot on preexisting Au & Ag nanocrystals under mild experimental conditions. We also report the time-dependent optical properties of Au/CdSe & Ag/CdSe core shell during the growth process in organic media. EXPERIMENT Synthesis of Au/CdSe Core/Shell Nanoparticles The Au/CdSe nanoparticles have been synthesized with different size using organometallic pyrolysis in analogies to the procedure which developed by Murray et al 10. In three necked flask a mixture of 2ml of oleylamine and 1mM of AuHCl solution was heated up to 100oC, the color turned to be red which indicates gold nanoparticle formation. 2gm of TOPO was added to the previous mixture slowly and the temperate was increased up to 150oC gradually. This temperature was kept to be constant during the reaction. To form Shell of CdSe nanoparticles around the gold core, the CdSe precursor (CdO soluble in oleic acid and Se powder soluble in Trioctyl phosphine) was added drop wise into the reaction mixture. Once the orange color appears, a sample was taken out by withdrawing approximately 1ml from the reaction mixture and rapidly was injected in test tube containing 3ml toluene. This last step was repeated at different time intervals depending on color change observation. Seven samples with different sizes of Au/CdSe nanodoes could separate at different time interval to follow the nanoparticle growth. Synthesis of Ag/CdSe Core/Shell Nanoparticles In three necked flask, a mixture of 1:1 mole ratio of oleylamine and silver acetylacetonate (Ag acac) was heated up to o C. The color turned to be brownish yellow which indicates formation of silver nanoparticle. Excess of TOPO (2gm) was added and temperature increased up to 150 o C gradually. This temperature was kept to be constant during the reaction. Cadmium precursor (Cadmium oleate) and Selenium precursors (Se-TOP) were added to the previous mixture drop by drop with continuous stirring at the same constant temperature (150 o C). Samples were taken out by withdrawing approximately 1ml from the reaction mixture and rapidly injected in test tube contained 3ml toluene. All glassware was washed with aqua regain 3:1 ratio by volume of HCl, HNO 3 ), and rinsed many times with distilled water depending on color change observation. This last step was repeated at different time intervals depending on color change observation. Seven samples with different sizes of Ag/CdSe nanodoes could separated at different time interval. X-Ray Diffraction Pattern XRD the structural properties of nanoparticles were carried out with a Philips PW 1700 powder diffractmeter operating with CuKα anode(λ = ). Scans were done at 6 min 1 for 2θ values between 20 and 90. The samples were prepared by precipitating with methanol and washed with toluene solution. Ultraviolet and Visible UV-visible absorbance spectra of prepared Au/CdSe and Ag/CdSe core shell were measured with a spectrometer PerkinElmer Lambda 750 double beam spectrophotometer. Diluted solutions of target samples were placed in 1 cm UV quartz and the absorption was recorded within the appropriate scan range. The spectra were taken against the pure solvent

3 Synthesis and Characterization of Au/CdSe & Ag/CdSe Core/Shell Nanoparticles 61 reference of concern for each different sample. Photoluminescence PL properties of Au/CdSe and Ag/CdSe core/shell are characterized by PerkinElmer Lambda LS 55 Spectroflorometer to carry out the emission spectra for different sizes. RESULTS AND DISCUSSIONS Characterization of Au/CdSe Core/Shell Nanoparticles X-Ray Diffraction XRD peaks from atomic lattice of gold nanoparticles are seen in figure 1 and the peaks at 2θ = 38.23, 44.18, and are due to (111), (200), (220) and (311). These peaks are perfectly matched with the powder diffraction standard (JCPDS, ) which is in conformation of face centered cubic (fcc) structure of gold crystals. The peak corresponding to (111) plane is more intense than the other planes, suggesting that the (111) plane is in the predominant orientation. The average particle size of Au nanoparticle (~20) has been estimated by using Debye-Scherrer formula 11, 12. D = 0.9 λ / β cos θ. Where 'λ' is wave length of X-Ray ( ), β is FWHM (full width at half maximum), θ is the diffraction angle and D is particle diameter size. The calculated particle size details and d spacing () are shown in Table 1. 2Ѳ of the Intense Peak (deg) Figure 1: XRD Pattern of Gold Nanoparticles Table 1: The Particle Size of Gold Nanoparticles hkl FWHM of Intense Peak (β) Radians Size of the Partcle (D) d-spacing Figure 2: XRD Pattern of Au/CdSe Nanoparticles

4 62 A. Gadalla, M. Mohamed & D. Hamad XRD peaks of Au/CdSe nanoparticles, are seen in figure 2. The peak at 2θ = 24.01, 26.15, 35.74, 42.24, 46.19, and are due to (100), (002), (102), (110), (103), (112) and (202) planes of hexagonal CdSe crystal. Average particle size has been estimated by using Debye-Scherrer formula. The value of d spacing is calculated using Bragg s Law13, 2dsinθ = n λ. The calculated cell constant (a) is equal to (4.06 Aº) which is closed to reported data in literature, a= Aº (JCPPS file no ). Table 2 represents the calculated particle size details and 'd' spacing OPTICAL PROPERTIES 2θ of the Intense Peak (Deg) Table 2: The Particles Size of Au/CdSe Nanoparticles hkl FWHM of Intense Peak (Β) Radians Size of the Partcle (D) d-spacing The absorption spectra for Au/CdSe core/shell are shown in figure 3 for deferent shell thickness. The absorption spectra show one band at 520, sample a, due to surface Plasmon SP, absorption which is the spectral feature of gold spheres. Once CdSe precursors added, the surface Plasmon band shifted slightly to blue, samples b&c, due to of the dielectric constant of the surrounding. Different samples of the Au/CdSe hybrid nanostructure have been withdrawn after at different time interval to follow the nanoparticle growth. As the reaction time increase, CdSe nanocrystals start to form around gold seeds and the Plasmon band start to decay as show in the absorption spectra of sample d. Figure 3: Optical Absorption Spectra for Au /CdSe Core/ Shell Samples Increasing the reaction time more lead to complete formation of the CdSe clusters around gold and the exctonic band of CdSe became predominate, sample d. With increasing the CdSe loading, the broadening of the SPR peak increases and shift to the red direction comparing with the originals peak position, samples e,f,g. The calculated size of given prepared samples corresponding to the λ max of absorption spectra are shown in Table 3. This calculation was carried out by two methods, the polynomial fitting functions (Yu s et al 14 ) and effective mass approximation model (Brus's et al 15 ).). The sizing curves are shown in Figure 4.

5 Synthesis and Characterization of Au/CdSe & Ag/CdSe Core/Shell Nanoparticles 63 Table 3: Shell Sizes of the Prepared Au/ CdSe Nanocrystals Using Brus s et al and Yu et al Equations Samples no. λ max of UV/vis Peak () Shell Size Brus's et al, R() Shell Size Yu s et al, D() D E F G Figure 4: Sizing Curves for the Prepared Au/CdSe Nanocrystals The absorption and the emission spectra for sample of Au/CdSe are shown in Figure 5 for sample sizes 4.42 & The absorption spectra of these samples show the presence of the surface Plasmon of the gold spheres in addition to the band gap of the CdSe which is size dependent. The large particles showed the absorption at about 591 & 599 due to CdSe absorption.this confirms the formation of hybrid nanostructure of Au/CdSe. The emission of these samples is quenched completely due to the electron transfer to the metallic surface. Both absorption and emission spectra illustrate the strong coupling of Au nanoparticles and CdSe nanocrystals in the obtained system. Figure 5: Absorbance & Emission Spectra of Au/CdSe for Particles Size (a) 4.42 & (b) 4.74 CHARACTERIZATION OF Ag/CdSe CORE/SHELL NANOPARTICLES X-Ray Diffraction XRD diffraction of silver nanoparticales deposited on silicon slide is shown in Figure 6. The XRD shows that silver nanoparticles formed are of fcc crystalline. Four peaks at 2θ values of , , and deg corresponding to (111), (200), (220) and (311) planes of Silver is observed and compared with the standard powder diffraction card of (JCPPS, ); that was in conformation of face centered cubic (fcc) structure of silver. The average core size of pure silver Ag nanoparticle (~15) has been estimated by using Debye-Scherrer formula as shown previously. The calculated particle size details and 'd' spacing () are shown in Table 4.

6 64 A. Gadalla, M. Mohamed & D. Hamad Figure 6: XRD Pattern of Silver Nanoparticles Table 4: The Calculated Particle Size Details and 'd' Spacing () of Ag Nanoparticles 2Ө of the Intense Peak (deg) hkl FWHM of Intense Peak (β) Radians Size of the Partcle (D) d-spacing OPTICAL PROPERTIES Figure 7 showed the variation of the absorption spectra of Au/CdSe core/shell nanoparticles, during the course of the chemical growth. Controlling the shell thickness show a shift of the excitonic CdSe absorption peak to the red as the shell thickness increase. The Plazmon band of Ag nanodot appears at 400, involving the oscillations of the free electrons in the conduction band that occupy energy states immediately above the Fermi level. Once CdSe precursors have been added, the intensity of the Plasmon band of the silver core decreases, and small shoulder due to the formation of the CdSe was appear at higher wavelength (~512 ). The loading of theses oscillations with CdSe shell leads to shift towards the blue and a decrease in its amplitude in samples. That might be because of the change of the dielectric constant of the surrounding. Figure 7: Optical Absorption Spectra for Ag /CdSe Core/ Shell Samples of Shell Thickness as Indicated; (S 1, 2&3 ) for Ag Core and (S 4,5,6&7 ) for CdSe Shell With increasing the CdSe loading, the shell thickness of its layer increases and broadening of the surface plasmon (SP) peak, that start to appear at 553, and shift toward lower energy causing a red shift of the excitonic comparing with the original SP peak position. The plasmonic silver core enhances the optical abortively of the CdSe shell. The shell thickness calculation was carried out by two methods, the polynomial fitting functions Yu s et al 10 and effective mass

7 Synthesis and Characterization of Au/CdSe & Ag/CdSe Core/Shell Nanoparticles 65 approximation model Brus's et al 11. The calculated size of given prepared samples corresponding to the λ max of absorption spectra are shown in Table 5. And the sizing curve is depicted in Figure 8. Table 5: Calculated Shell Size of the Prepared Ag/CdSe Nanocrystals from the First Excitonic Absorption Band using Effective Brus s and Yu et al Equations Samples No. λ max of UV/vis Shell Thickness R () Shell Thickness D ( ) Peak () Brus's et al Yu s et al Figure 8: Sizing Curves (Shell Thickness) for the Prepared Ag/CdSe Nanocrystals Using Brus et al and Yu et al. al Equations The photoluminescence (PL) spectra of all the samples of Ag-CdSe core shell nanoparticles shown in Figure 9. The absorption maxima occur at higher energies than the emission maxima as expected, Stokes shift means the difference between the absorption and emission wavelengths. The large Stokes shift and narrow size distribution of these shell nanocrystals, along with the broad emission spectrum, would alleviate any efficiency loss attributed to self absorption. Figure 9: The Stoke Shift, where Absorption (Black Line) and Emission (Red Line) Spectra for all Samples of Ag-CdSe Nanocrystal of Different Sizes Equation

8 66 A. Gadalla, M. Mohamed & D. Hamad. Table 6: The Absorption Maxima λ Abs., the Emission Maxima λ PL, and the Stokes Shift Samples No. λ max of Abs. Spectra (Wavelength/) λ m ax of Emission Spectra (Wavelength / ) Stokes Shift () The presence of silver metallic core inside the CdSe nanoparticles affects the formed exciton and the associated phonon contribution. High localized electromagnetic field generated by the metal surface affect the coulomb interaction force and increases the intensity of the phonon peak. This reflected in the increasing the intensity of the core shell (stokes shift = 10 ), then reaches the maximum effect in (stokes shift = 16 ). This effect began to decrease again when the shell become thicker (stokes shift = 11&7). The large shell thickness of CdSe lead to decrease the excitonic band gap lowers than E-Fermi (E f ). This means that the electron transfers from metal to semiconductor. The coupling decrease again, and then start to increase more by increasing the shell thickness. This could be attributed to the fact that the plasmonic resonance of the silver nanoparticles at 420 is far away from the excitonic peaks of the CdSe nanoparticles which start to appear at 553 and shift towards lower energy by increasing the CdSe size. Table (6) shows the variation of emission maxima with the absorption maxima. This is linearly relationship of a slope ~ nearly one which clearly shown in Figure 10. Figure 10: The Linear Relation of the Absorption vs. Emission Maximum for Ag-CdSe Core s towards Lower Energy by Increasing the CdSe Size CdSe Size CONCLUSIONS The Au/CdSe and Ag/CdSe nanoparticles have been synthesized with different sizes using organometalic pyrolysis method with some modification. The Absorption spectra for the core/shell Au/CdSe & Ag/CdSe nanoparticles have shown a combined absorption of metal core SP band and the CdSe nanoshell. The absorption spectrum for CdSe nanoparticles exhibits remarkable red shift in the surface Plasmon (SP) energy band with size increase. Increasing the CdSe shelll in the Au/CdSe & Ag/CdSe nanoparticles gives the opportunity to tune the band gap of the nanoparticles to harvest a wide range of visible solar spectra. The emission spectra of Au/CdSe the exciton transition for CdSe leads to rapid quenching of the CdSe by photo induced electron injection into the Au nanoparticle. It has been observed that plasmonic gold has more influence on the optical properties and the quenching rate of the emission of CdSe nanoparticles much more than that of silver. This could be explained simpley because the plasmonic gold show its plasmon resonance at 520, which loacated in the same range as that of the optical absorption of CdSe nanocrystals (band gap absorption is

9 Synthesis and Characterization of Au/CdSe & Ag/CdSe Core/Shell Nanoparticles 67 ranged between ). In the mean while, silver nanoparticles absorb at 400, which is far in resonance of the optical absorption of the CdSe compared to that of gold particles. Thus, the qunching of the emission in case of Au/CdSe is much more effective, and the elctron transfer processes from CdSe to the gold is more faster than that of the silver. For this reason, we can consider that Au/CdSe is strong plasmonic-exciton coupling but Ag/CdSe is week plasmonic-excitonic coupling. The stoke shift values of core/shell Ag/CdSe samples were increased with decreasing the shell thickness. REFERENCES 1. Lin, H. Y.; Chen, Y. F.; Wu, J. G.; Wang, D. I.; Chen, C. C. (2006).Appl. Phys. Lett, 88, Lee, J.; Govorov, A. O.; Dulka, J.; Kotov, N. A. (2004).Nano Lett., 4, Markovich, G.; Collier, C. P.; Henrichs,S.E.; Remacle, F.; Levine, R. D. (1999).Heath, J. R. Acc. Chem. Res., 32, Banin, U. (2006). Journal of Physical Chemistry, B 110, Okamoto, K.; Nikl, I.; Narukawa, A.; Y.; Mukai, T.; Scherer, A. (2004). Nature Matter 3, Hecker, N. E.;Hopfel, R. A.; Sawaki, N.;Maier, T.; Strasser, G. (1999). Appl. Phys. Lett. 75, Gontijo, I.; Boroditsky, M.; Yablonovitch, E. S.; Mishra, U. K.; DenBaars, S. P, (1999). Phys. Rev. B, 60, Vuèkoviæ,J.; Lonèar, M.; Scherer, A.. (2000). IEEE J. Quant. Elec, 36, Lu, W. et al. (2005). Langmuir 21, Murray, C.B.; Norris,D.J and M Bawendi.G. (1993).Am. Chem. Soc, (115), Cullity, B.D. (2010). Addison-Wesley Company, USA. 12. Rita John, S; Florence S. (2009). Chalcogenide Lett., 6, T. Theivasanthi, M. (2010). Alagar, Archives of Physics Research, 1, Yu, W. W.; Qu, L.; Guo, W.; and Peng, X.(2003).Chem. Mater, 15, Brus, L. E. (1986). J. Phys Chem., 90, 2555.

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