Synthesis and characterizations of silver-fullerene C70 nanocomposite

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Films of C70 fullerene containing silver nanoparticles were synthesized by thermal ... detection of low intensity vibrational modes of C70 in Raman scattering is ...
APPLIED PHYSICS LETTERS 93, 103114 共2008兲

Synthesis and characterizations of silver-fullerene C70 nanocomposite R. Singhal,1,a兲 D. C. Agarwal,1 S. Mohapatra,1 Y. K. Mishra,1 D. Kabiraj,1 F. Singh,1 D. K. Avasthi,1 A. K. Chawla,2 R. Chandra,2 G. Mattei,3 and J. C. Pivin4 1

Inter University Accelerator Centre, P.O. Box No. 10502, New Delhi 110067, India Institute Instrumentation Centre, Indian Institute of Technology Roorkee, Roorkee 247667, India 3 Department of Physics, University of Padova, via Marzolo 8, 35131 Padova, Italy 4 CSNSM, IN2P3-CNRS, Batiment 108, F-91405 Orsay Campus, France 2

共Received 5 June 2008; accepted 4 August 2008; published online 12 September 2008兲 Films of C70 fullerene containing silver nanoparticles were synthesized by thermal codeposition. Optical absorption studies revealed that surface plasmon resonance of Ag nanoparticles occurs at unusually large wavelength, which showed a regular redshift from 521 to 581 nm with increase in metal content from 4.5% to 28%. It is explained by the Maxwell–Garnett theory considering the absorbing nature of fullerene matrix. Rutherford backscattering and transmission electron microscopy were performed to quantify metal content and the particle size, respectively. A better detection of low intensity vibrational modes of C70 in Raman scattering is observed due to surface enhanced Raman scattering. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2976674兴 The synthesis of fullerene, an allotrope of carbon, in spherical 共C60兲 and ellipsoidal 共C70兲 form and embedding of metal into fullerene matrices have attracted attention for technological applications.1–4 Noble metal nanoparticles 共NPs兲 are of interest due to their localized surface plasmon resonance 共LSPR兲 resulting in a strong absorption of light at a particular wavelength in visible region.5–7 The transparency of C60 and C70 in the visible region and their reduced reactivity with noble metals make nanocomposites of fullerenes and noble metals particularly interesting for plasmonic integrated devices. The LSPR wavelength depends on various factors such as the size, shape, interparticle separation, and dielectric function of the matrix. The instability of Ag NPs at ambient conditions 共due to oxidation兲 especially in silica matrix has been a major concern in synthesis of Ag based nanocomposites.8,9 Carbon-based matrices are interesting for protecting these particles against oxidation. Fullerene is a functional material having applications in various fields such as electronic devices, memory devices, drug delivery, catalysis, coatings, and especially in biology due to its biocompatibility.2–4 By embedding the Ag NPs in fullerenes, the optical properties of noble metal NPs as well as the properties of fullerene coexist and, therefore, the dual properties can be effectively used. In this letter, we report the synthesis of a bifunctional Ag– C70 nanocomposite and the tuning of its LSPR wavelength 共from 521 to 581 nm兲 simply by varying the metal volume fraction. Unusually large LSPR wavelength in Ag-fullerene nanocomposite thin films as compared to that in Ag-silica or Ag-polymer is explained by Maxwell– Garnett theory. We further demonstrate that weak Raman modes of C70 could be observed in these Ag– C70 nanocomposites due to surface enhanced Raman scattering 共SERS兲. Ag– C70 nanocomposites with increasing metal concentration denoted as a, b, and c were synthesized by evaporating Ag and C70 simultaneously from two crucibles. The actual metal concentration and thickness of the films on Si substrates were measured by Rutherford backscattering specAuthor to whom correspondence should be addressed. Tel.: ⫹91-1126893955. FAX: ⫹91-11-26893666. Electronic addresses: rahuliuac@ gmail.com and [email protected].

troscopy. Ag atomic fractions were found to be of 4.5%, 15.5%, and 28% in the films a, b, and c, respectively, and their thicknesses of comparable value. UV-Vis absorption spectra of films on quartz substrates were recorded using a dual beam U-3300 Hitachi spectrometer. X-ray diffraction 共XRD兲 spectra on quartz substrates were recorded at a glancing angle of 1°, using Cu K␣ radiation 共1.5406 Å兲. Transmission electron microscopy 共TEM兲 of films deposited on carbon coated Cu grids was performed using a FEI TECNAI 20 microscope operated at 200 kV. Micro-Raman measurements on quartz substrates were performed using Renishaw inVia Raman microscope with Ar ion laser excitation at 514 nm with very low power 共⬍1 mW and 20⫻ objective兲 to avoid any heating effect. The TEM micrograph of Ag– C70 nanocomposite thin film c is shown in Fig. 1. Uniformly distributed and almost

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FIG. 1. TEM micrograph corresponding to Ag– C70 nanocomposite films c. The SAED pattern for the nanocomposite film c is shown in the inset.

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FIG. 2. 共Color online兲 XRD spectra for the Ag– C70 nanocomposite films a, b, and c having different metal fraction. The reflections by 共111兲 and 共200兲 planes reveal the fcc phase of Ag NPs.

spherical particles can clearly be seen in the bright field images. Selected area diffraction pattern 共SAED兲 for the nanocomposite film c is shown in the inset of Fig. 1. The average particle size was found to be 2.8, 3.6, and 5.4 nm for the films a, b, and c respectively. It increases almost linearly with the Ag concentration. Here it is interesting to note that formation of NPs is possible in as-deposited film without any annealing whereas thermal codeposition of Ag and silica requires annealing for the formation of metal NPs.10–13 This is attributed to the high diffusivity of Ag in C70 as compared to that of Ag in silica. For the higher concentration of metal, average interatomic distance of initially diffusing atoms is smaller and therefore a larger number of Ag atoms participate in the nucleation and growth process leading to a bigger size particle. Glancing angle XRD patterns of the Ag– C70 nanocomposites 共films a, b and c兲 are shown in Fig. 2. The C70 film has poor crystallinity on quartz14 and therefore does not appear in XRD. With increase in metal concentration, XRD peaks become sharper. We estimated the average diameter of Ag NPs by Scherrer formula using the full width at half maximum of peaks corresponding to 共111兲 reflection and it comes out to be 2.8, 3.4, and 4.2 nm for the films a, b, and c respectively. It may be noted that the 2␪ value for Ag 共111兲 decreased by about 0.3° with increase in Ag fraction, which implies increase in d value for bigger size NPs. Such lattice expansion occurs due to the decreasing surface tension with increasing size.15,16 The absorption spectra of Ag– C70 nanocomposite films 共a-4.5%Ag, b-15.5%Ag, and c-28.0%Ag兲 along with that of a pure C70 film are shown in Fig. 3共a兲. These spectra show broad LSPR bands indicating that Ag NPs are formed. The LSPR wavelength is redshifted from 521 to 581 nm with the increase in Ag concentration from 4.5% to 28%. The broad absorption is mainly due to the overriding of absorption due to LSPR on the absorption of C70. The shift of LSPR peak position is almost linear with the Ag concentration as shown in the inset of the Fig. 3共a兲. Takele et al.17 reported a small red shift 共⬃30 nm兲 in the LSPR peak position of Ag NPs in various carbon-based matrices such as Nylon, Teflon AF, and polymethyl methacrylate by varying metal concentration from 4% to 25%. Mishra et al.18 reported a redshift

FIG. 3. 共Color online兲 The UV-visible absorption spectra for the Ag– C70 nanocomposites a, b, and c showing a clear redshift in LSPR position are shown in 共a兲. The almost linear shift in LSPR position with metal fraction is shown in inset 共a兲. Theoretically simulated absorption spectra 共using Maxwell–Garnett theory兲 for Ag– C70 nanocomposites are shown in 共b兲.

共⬃24 nm兲 in the position of LSPR peak of Ag NPs in silica by varying the metal concentration from 4% to 46%. Such shifts are mainly due to the mutual polarizations of the particles. But in the case of Ag– C70 nanocomposite, the redshift in LSPR position is quite large 共⬃60 nm兲. Considering the large metallic fraction in our samples and that the fullerene film is an absorbent matrix, we used the Maxwell–Garnett theory to properly simulate the extinction cross section and the LSPR position of the nanocomposite. For metal nanoclusters, the dielectric function of Ag, corrected for the size of nanoclusters according to TEM results was used, whereas the dielectric function of C70 film was modeled according to Ref 19. These results are shown in Fig. 3共b兲 and were found to closely match with experimental results. Thus the unusual large SPR and its large redshift are explained without the considering the static charge transfer effect.20–22 For pure C70 at room temperature, 53 Raman active modes are predicted 共12 A1⬘ + 22 E2⬘ + 19 E1⬙兲 from the D5h point group symmetry according to group theory.23,24 Many groups reported large number of vibrational modes of C70 共Refs. 25 and 26兲 but all 53 vibrational modes of C70 molecule have not been observed. Figure 4 compares the Raman spectrum of the Ag– C70 nanocomposite film with that of pure C70 film having same thickness as of nanocomposite film. A significant enhancement of weak Raman modes of C70 molecules is clearly observed. Insets 共a兲 and 共b兲 of Fig. 4 show some theoretically predicted vibrational modes27 at wavenumbers of 474 共not shown in inset兲, 533, 547, and 787 cm−1, which were not visible due to low intensity in

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FIG. 4. 共Color online兲 Comparison of Raman spectra of Ag– C70 nanocomposite film with pure C70 fullerene film having the same thickness as the nanocomposite film. Insets 共a兲 and 共b兲 clearly show some vibrational modes, which were hidden due to low intensity.

pure C70 film. This is attributed to the surface plasmons of the Ag NPs, which enhance the polarizibility of the C70 molecule, because of the increase in electric field. There is a possibility of using Ag– C70 nanocomposite for SERS applications. In conclusion, we report the synthesis of Ag NPs embedded in C70 matrix with the tunability of LSPR wavelength from 521 to 581 nm by varying the metal fraction from 4.5% to 28%. Unusually large LSPR wavelength and its large change with metal content was explained by Maxwell– Garnett theory considering the absorbing nature of the fullerene matrix. TEM and XRD results confirm the formation of Ag NPs in fcc phase in C70 matrix. The appearance of hidden vibrational modes of C70 in Raman spectrum of this nanocomposite film indicates the possibility of application of this nanocomposite in surface enhanced Raman spectroscopy. We are grateful to Professor A. Gupta, Centre Director, UGC-DAE CSR, Indore, India for providing the GAXRD facility. The author 共R.S.兲 is thankful to Ms. Ritu, R/S, IIT Rookee for providing necessary literature for this study.

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