Patterned silver nanoparticle films by an ion complexation process in thermally evaporated fatty acid films

Patterned silver nanoparticle films by an ion complexation process in thermally evaporated fatty acid films

Materials Research Bulletin 37 (2002) 1613±1621 Patterned silver nanoparticle ®lms by an ion complexation process in thermally evaporated fatty acid ...

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Materials Research Bulletin 37 (2002) 1613±1621

Patterned silver nanoparticle ®lms by an ion complexation process in thermally evaporated fatty acid ®lms Saikat Mandal, S.R. Sainkar, Murali Sastry* Materials Chemistry Division, National Chemical Laboratory, Pune 411008, India (Refereed) Received 13 June 2001; accepted 22 May 2002

Abstract The formation of silver nanoparticle ®lms in a patterned manner on suitable substrates is described. The protocol for realising such structures comprises of the following steps. In the ®rst step, patterned ®lms of a fatty acid are thermally evaporated onto solid supports using suitable masks (e.g. a TEM grid). Thereafter, the fatty acid ®lm is immersed in silver nitrate solution and Ag‡ ions entrapped in the lipid matrix by electrostatic complexation with the carboxylate ions of the fatty acid molecules. The ®nal step involves the reduction of the Ag‡ ions in situ thus leading to the formation of silver nanoparticles within the patterned lipid matrix. The process of metal ion incorporation and reduction may be repeated a number of times to increase the nanoparticle density in the lipid matrix. The silver nanoparticle density may also be increased by dissolution of the fatty acid molecules in suitable solvents. The process of Ag‡ ion entrapment and formation of silver nanoparticles within the patterned lipid matrix has been followed by quartz crystal microgravimetry, UV±VIS spectroscopy, FTIR, SEM and EDX. The process described shows immense potential for extension to assemblies of nanoparticles in more intricate patterns as well as to the growth of semiconductor quantum dots in such patterns. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Composites; A. Nanoparticles; A. Thin ®lms; B. Chemical synthesis; B. Intercalation

*

Corresponding author. Tel.: ‡91-20-5893044; fax: ‡91-20-5893952/044. E-mail address: [email protected] (M. Sastry). 0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 2 ) 0 0 8 1 8 - 8

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1. Introduction Nanotechnology is being hailed as the technology of the new millennium. The different optoelectronic and physicochemical properties of nanoscale matter vis-aÁ-vis bulk materials has long been realised and attempts are being made to harness these properties for commercial application [1]. One important goal in nanotechnology, and an important one in the context of commercial application of nanoparticles, is the organisation of nanoparticles in thin ®lm form. The bottom-up method for the formation of nanoscale structures is currently receiving considerable attention [2]. This approach is based on the synthesis of nanoparticles (often by the colloidal route) followed by organisation on suitable surfaces using principles of self-assembly. A number of protocols have been developed for the self-assembly of nanoparticles as monolayers [3±10] and as superlattice structures [11±20]. While these protocols enable formation of planar thin ®lms of nanoparticles, there are very few reports in the literature on in-plane spatial control over the nanoparticle structures formed. In this direction, He et al. [21] and Aizenberg et al. [22] have very recently shown that selfassembled monolayers (SAMs) deposited by microcontact printing (mCP) on gold

Ê thick StA ®lm during immersion in a Fig. 1. (A) QCM mass uptake recorded with time for a 500 A 4 10 M AgNO3 solution. Each cycle of immersion (indicated next to the mass uptake curve) is followed by reduction of the Ag‡ ions with hydrazine (see text for details). The inset is a schematic of the process for obtaining patterned silver nanoparticle assemblies from fatty acid ®lms. It consists of deposition of StA using a TEM grid as a mask on a suitable substrate (step 1). This is followed by immersion in AgNO3 solution and electrostatic entrapment of the Ag‡ ions in the fatty acid matrix (step 2). The ®nal step is the reduction of the Ag‡ ions in situ to yield silver nanoparticles in the patterned lipid matrix (step 3). The reduction process may be followed by further ion corporation and reduction (steps 2 and 3) in a cyclic manner (details in the text). (B) UV±VIS spectra recorded from a Ê thick ®lm after four successive cycles of Ag‡ incorporation and reduction with hydrazine. The 500 A ion incorporation cycle is indicated next to the respective spectra.

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thin ®lms enables deposition of the monolayers with good spatial control over differing chemical functionality. Using such patterned SAMs, they were able to assemble nanoparticles using electrostatic interaction between the terminal functional groups in the SAM and the charges on the colloidal particles in a spatially programmed manner [21,22]. For completeness, we would like to mention the work of Resch et al. who demonstrated the use of scanning force microscopy in the manipulation of nanoparticles to form spatially ordered nanoparticle assemblies [23]. This technique would not classify as a self-assembly method and would be more in the nature of a nanorobotics approach to organised nanoparticle assemblies. It is clear from the above that there is a substantial gap in currently available experimental methods for realising patterned nanoparticle assemblies. In this communication, we attempt to address this lacuna and demonstrate the synthesis of patterned silver nanoparticle assemblies. The entire process is illustrated in the schematic in the inset of Fig. 1. The ®rst step consists of the deposition of patterned thin ®lms of stearic acid (StA) by vacuum evaporation using a suitable mask on solid substrates. The patterned fatty acid ®lm is then immersed in aqueous AgNO3 solution and Ag‡ ions incorporated in the lipid matrix by attractive electrostatic interaction with the negatively charged carboxylate ions of the fatty acid molecules. The silver stearate ®lm is treated with hydrazine vapour to reduce the metal ions in situ thus leading to the formation of silver nanoparticles within the patterned lipid host. The technique described herein draws inspiration from the earlier work from this laboratory on the spontaneous self-organisation of thermally evaporated fatty acid ®lms during immersion in electrolyte solutions [24] and considerably enhances the capability of the protocol. 2. Experimental details Ê Stearic acid (CH3(CH2)16COOH, Aldrich, used as-received) ®lms of 500 A thickness were deposited by thermal evaporation in an Edwards E306A vacuum coating unit at a pressure of better that 1  10 7 Torr. The ®lms were deposited on a Si(1 1 1) wafer (for Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction measurements), on a quartz substrate (for UV±VIS spectroscopy studies) and a gold-coated 6 MHz quartz crystal for quartz crystal microbalance (QCM) Ê thick StA ®lm was also deposited on a Si(1 1 1) wafer using a measurements. A 500 A transmission electron microscope grid as a mask to yield a patterned StA ®lm for scanning electron microscopy (SEM) and energy dispersive analysis of X-ray (EDX) measurements. After deposition of the StA ®lms, the StA-coated QCM crystal was immersed in 10 4 M AgNO3 solution (pH ˆ 5:2) and the change in frequency of the quartz crystal was measured ex situ after thorough washing and drying of the crystal for various times of immersion of the crystal in the electrolyte solution. The frequency of the quartz crystal resonator was measured using an Edwards FTM5 frequency counter that had a frequency resolution and stability of 1 Hz. For the 6 MHz crystal used in

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this study, this translates into a mass resolution of 12 ng/cm2. The frequency changes were converted to mass loading using the standard Sauerbrey formula [25]. On equilibration of the Ag‡ ion concentration in the fatty acid ®lm, the silver stearate ®lm was placed in a closed container along with a petridish containing hydrazine. The reduction of the Ag‡ ions in situ by the hydrazine vapour was carried out for 1 h. A rapid colour change was observed in the initially colourless silver stearate ®lm to a light brown colour clearly indicating the formation of silver nanoparticles in the fatty acid matrix. This process was followed by three additional cycles of Ag‡ incorporation and reduction, the QCM mass uptake being recorded for each additional cycle of Ê thick Ag Ag‡ incorporation as well. UV±VIS spectroscopy measurements of a 500 A nano-StA ®lm on quartz during the different stages of nanoparticle formation were carried out on a Hewlett-Packard HP 8542A diode array spectrophotometer operated at a resolution of 2 nm while XRD measurements were performed in the transmission mode on a Philips PW 1830 instrument operating at 40 kV voltage and a current of 30 mA with Cu Ka radiation. FTIR measurements of the silver stearate/Ag nano ®lms were carried out on a Shimadzu FTIR-8201 PC instrument operated in the diffuse re¯ectance mode at a resolution of 4 cm 1. SEM measurements on the patterned StA/ Ag nano ®lms were carried out on a Leica Stereoscan-440 scanning electron microscope equipped with a Phoenix EDX attachment. EDX spectra were recorded in the spot-pro®le mode by focusing the electron beam onto speci®c regions of the Ag nano-StA patterned ®lm. 3. Results and discussion Ê thick StA ®lm Fig. 1A shows the QCM mass uptake data recorded for the 500 A ‡ during four successive cycles of Ag incorporation, the reduction of the silver ions being accomplished by hydrazine treatment as mentioned above. The cycle of ion incorporation is indicated in the ®gure. At the electrolyte solution pH ˆ 5:2, the StA molecules in the fatty acid ®lm are expected to be completely ionised thus leading to maximum electrostatic interaction between the Ag‡ ions and the carboxylate ions of the lipid ®lm. The QCM mass uptake recorded during the ®rst immersion cycle seen in Fig. 1A is found to be ca. 18,000 ng/cm2 and is seen to be complete within ca. 100 min of immersion. Comparing the silver ion mass uptake with the mass uptake Ê thick StA of 4600 ng/cm2 results in a Ag‡:StA recorded for the as-deposited 500 A molar ratio of ca. 10:1 while from purely electrostatic considerations, a 1:1 Ag‡:StA molar ratio is expected. This result clearly indicates overcompensation of the negative charge of the fatty acid matrix by the positively charged silver ions and is similar to the charge overcompensation that occurs in the layer-by-layer assembly protocol of oppositely charge entities such as polyelectrolytes, biomacromolecules, etc. [18]. Another important source of uncertainty in estimating the molar ratio of the Ag‡ ions to the StA molecules arises from the method of measurement, viz. QCM. In the QCM measurements, application of the Sauerbrey equation implicitly assumes that the lipid ®lm possesses acoustic properties nearly identical to that of the underlying quartz

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support [25]. While this is a fair approximation where ®lms of covalently bonded solids are concerned, it is not clear that this approximation is valid for molecular solids such as StA which are held together by weak, van der Waals interactions. Indeed, the acoustic `stiffness' of the lipid ®lm would be expected to change drastically before and after incorporation of the silver ions making the use of such an approximation even more indeterminate. We believe this aspect is responsible for the large overcompensation of negative charge in the fatty acid matrix by the silver cations observed in this study and future studies based on independent acoustic measurements will hopefully resolve this issue. After the ®rst cycle of ion incorporation, the silver stearate ®lm was treated with hydrazine to form silver nanoparticles in the fatty acid matrix. This results in re-generation of free acid molecules and the possibility of carrying out additional ion incorporation and reduction thus leading to an increase in the nanoparticle density/increase in the size of existing nanoparticles in the lipid matrix. Three additional cycles of ion exchange and reduction were carried out and the QCM mass uptakes recorded for these cycles is shown in Fig. 1A. It is observed that the additional mass uptake due to incorporation of Ag‡ ions is much less for these cycles (ca. 4000 ng/cm2) than for the ®rst cycle (18,000 ng/cm2). This may be due to blockage of diffusion pathways for the Ag‡ ions due to the presence of the silver nanoparticles in the lipid matrix or due to co-ordination of a percentage of the StA molecules to the surface of the nanoparticles, which would render them unavailable for ion incorporation. A combination of both factors may also contribute to the reduced ion uptake observed. Treatment of the silver stearate ®lms with hydrazine resulted in the ®lm turning to a light brown colour indicative of silver nanoparticle formation as brie¯y mentioned earlier. The process of silver nanoparticle formation by hydrazine reduction of the Ê thick StA ®lm on quartz during four successive cycles of ion Ag‡ ions in a 500 A entrapment and reduction was followed by UV±VIS spectroscopy and the spectra recorded are shown in Fig. 1B (the ion incorporation cycle is indicated next to the respective spectrum). A strong resonance at ca. 440 nm is clearly seen in all the ®lms and arises due to excitation of surface plasmon vibrations in the silver nanoparticles [26,27]. Furthermore, the surface plasmon resonance intensity increases with number of cycles of silver ion incorporation indicating an increase in the concentration of the silver nanoparticles in the ®lm. This could be seen as a visible increase in the darkness of the brown colouration in the Ag nano-StA ®lm. On establishing the formation of silver nanoparticles by hydrazine treatment of silver stearate ®lms, we proceeded to repeat the procedure with the TEM grid Ê thick StA ®lm on a Si(1 1 1) substrate. The SEM image recorded patterned 500 A after one cycle of Ag‡ ion incorporation and reduction by hydrazine is shown in Fig. 2A. The silver nanoparticle assemblies are clearly observed within the squares of the StA matrix. Spot-pro®le EDX measurements were carried out at points within one of the gaps between the StA segments and on the Ag nano-StA element (indicated by arrows in Fig. 2A) and are shown in Fig. 2B and C, respectively. It is seen that while there is strong Ag signature from within the StA elements (Fig. 2C), there is no

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Ê thick Ag nano-StA ®lm deposited on a Si(1 1 1) Fig. 2. (A) SEM picture of a patterned 500 A substrate. The accompanying EDX pro®les (B and C) have been recorded from the regions indicated by crosses in the SEM picture (see text for details).

evidence of the silver signals from within the gaps (Fig. 2B). This is an important result and clearly shows that the Ag nanoparticles formed by Ag‡ ion incorporation and reduction by hydrazine are faithful to the underlying patterned StA template. It is interesting to note that there is very little spill-over of the nanoparticles into the gaps of the patterned ®lm indicating a high degree of ®delity to the StA template. While the use of such a masking procedure in the generation of patterned metal ®lms is not novel, the use of a thermally evaporated patterned lipid ®lm to entrap metal ions and grow nanoparticles by reduction has, to the best of our knowledge, not been reported on so far. It is conceivable that this process could be extended to more intricate structures using more sophisticated masks. The process of Ag‡ incorporation in the thermally evaporated StA ®lm is readily studied by FTIR spectroscopy. Fig. 3A shows the FTIR spectra recorded from the asÊ thick StA ®lm on Si(1 1 1) wafer (curve 1); the StA ®lm after the ®rst deposited 500 A ‡ cycle of Ag ion incorporation (curve 2) and the StA ®lm after four cycles of Ag‡ incorporation and hydrazine reduction (curve 3). Three bands labelled a±c at 1695, 1520 and 1470 cm 1, respectively have been identi®ed in the ®gure. The band at 1695 cm 1 (feature a, curve 1) is assigned to the carbonyl stretch vibration of the carboxylic acid groups in the StA ®lms (curve 1) and is clearly missing from both the silver stearate (curve 2) and Ag nano-StA ®lms (curve 3). On formation of silver stearate, the carboxylate stretch frequency appears at 1520 cm 1 (feature b, curve 2)

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Ê Fig. 3. (A) FTIR spectra in the spectral range 1350±1800 cm 1 recorded from an as-deposited 500 A thick StA ®lm on a Si(1 1 1) substrate (curve 1); the StA ®lm after one cycle of immersion in AgNO3 solution (curve 2) and the StA ®lm after four cycles of ion incorporation and reduction with hydrazine (curve 3). Three prominent FTIR bands labelled a±c are identi®ed in the ®gure and discussed in the Ê thick StA ®lm on Si(1 1 1) text. (B) The XRD (1 1 1) Bragg re¯ections recorded from a 500 A ‡ substrate after each of four cycles of Ag ion incorporation and reduction with hydrazine. The cycle of ion incorporation and reduction is indicated next to the respective curve. A Lorentzian ®t to the XRD pattern recorded from the StA ®lm after one cycle of Ag‡ entrapment and reduction is shown in the ®gure.

and is a clear indication of formation of the metal salt of StA [28]. After formation of the silver nanoparticles (curve 3), the band at 1520 cm 1 splits into two bands and may be attributed to un-reacted silver stearate ®lms and StA molecules bound to the surface of the silver nanoparticles. A third feature at 1470 cm 1 is seen in all the spectra and arises from the methylene scissoring vibrations of the StA hydrocarbon chains in the nanocomposite ®lm. An estimate of the silver nanoparticle size grown in the StA matrix by the method outlined above was made from the broadening of the (1 1 1) Bragg re¯ection using the Debye±Scherrer formula [29]. Fig. 3B shows the XRD patterns recorded from the Ag nano-StA ®lms on Si(1 1 1) wafer after four successive cycles of Ag‡ incorporation and reduction. The Lorentzian ®t to the XRD pattern from the ®rst cycle ®lm is shown in Fig. 3B. Similar ®ts (data not shown) were done for the other ®lms as well and the Ê for silver nanoparticle sizes estimated from the analysis was 250, 245, 260 and 255 A cycles one to four, respectively. This clearly shows that successive cycles of silver ion incorporation and reduction does not result in growth of already nucleated nanoparticles in the lipid matrix. The increase in the plasmon resonance intensity (Fig. 1B) mentioned earlier must therefore be attributed to a large extent to nucleation of new silver nanoparticles during each cycle of ion entrapment.

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4. Conclusions In conclusion it has been shown that silver nanoparticle ®lms may be synthesised in patterned fatty acid ®lms by an ion complexation and reduction process, thus addressing an important gap in available protocols for realising spatially programmed nanoparticle assemblies. The versatility of the technique lies in the possibility of doing interesting chemistry via entrapment of different ions in the lipid matrix either simultaneously or sequentially and carrying out different reactions to yield, for example, nanoparticles of semiconductors and oxides. More intricate patterns of the de®ning lipid matrix may also be envisaged and these aspects are being pursued vigorously. Acknowledgments Saikat Mandal would like to thank the University Grants Commission (UGC), Government of India for a research fellowship. This work was partially funded by a grant from the Indo-French Centre for the Promotion of Advanced Scienti®c Research (IFCPAR), New Delhi, and is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

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