Synthesis, Surface Modification, and Multilayer Construction of Mixed

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Langmuir 2004, 20, 11169-11174

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Synthesis, Surface Modification, and Multilayer Construction of Mixed-Monolayer-Protected CdS Nanoparticles Takaaki Tsuruoka,† Kensuke Akamatsu,*,‡ and Hidemi Nawafune*,‡ Graduate School of Science and Faculty of Science and Engineering, Konan University, 8-9-1 Okamoto, Higashinada, Kobe 658-8501, Japan Received May 21, 2004. In Final Form: September 16, 2004 Herein, we describe a study aimed at synthesizing mixed-monolayer-protected CdS nanoparticles and investigating the reactivity of surface-bound functional groups in order to facilitate the immobilization of nanoparticles on a solid substrate as well as the construction of a three-dimensional nanocomposite. CdS nanoparticles initially prepared by the reverse micelle method were used to modify nanoparticle surfaces with 1-decanethiol molecules by ligand exchange. Subsequently, 11-mercapto-1-undecanol was partially incorporated by a place exchange reaction, thereby providing stable, mixed-monolayer-protected CdS nanoparticles. The nanoparticles obtained at each step were characterized by FT-IR and UV-vis spectroscopy, transmission electron microscopy, and elemental analysis. The reactivity of surface hydroxyl groups was verified by a reaction with isocyanate-bearing molecules that provide carbamate bonds in high yields at ambient temperature. The obtained mixed-monolayer-protected nanoparticles were also successfully immobilized on a glass substrate through a carbamate-bond-forming reaction that could be further utilized for multilayer construction in a layer-by-layer fashion.

Introduction Ligand-protected nanoparticles consisting of semiconductor cores surrounded by organic monolayers have attracted considerable interest for applications in materials science and nanotechnology. The interest in these nanomaterials is motivated by the unique optical and electrical properties of the semiconductor cores induced by the quantum size effect. Additionally, the organic surroundings can provide stability and additional functionality to the nanoparticles. These unique properties of ligand-protected nanoparticles have made them promising nanomaterials for various potential applications including electronics,1,2 optics,3-6 and biosensors.7-9 Thin films are of particular interest for the development of novel devices that utilize the specific characteristics of semiconductor nanoparticles. Moreover, the nanoparticles could be either immobilized onto solid substrates or incorporated into solid matrixes in order to construct composite thin films.3,4,6,10-13 * To whom correspondence should be addressed. E-mail: [email protected]. † Graduate School of Science. ‡ Faculty of Science and Engineering. (1) Klein, D. L.; Roth, R.; Lim, A. K. L.; Alivisatos, A. P.; McEuen, P. L. Nature 1997, 389, 699. (2) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (3) Gao, M.; Richter, B.; Kirstein, S.; Mohwald, H. J. Phys. Chem. B 1998, 102, 4096. (4) Lee, J.; Sundar, V. C.; Heine, J.; Bawendi, M. G.; Jemsen, K. F. Adv. Mater. 2000, 12, 1102. (5) Coe, S.; Woo, W.; Bawendi, M.; Bulovic, V. Nature 2002, 420, 800. (6) Tessler, N.; Medvedev, V.; Kazes, M.; Kan, S.; Banin, U. Science 2002, 295, 1506. (7) Bruchez, M.; Moronne, M.; Gin, P.; Shimon, W.; Alivisatos, A. P. Science 1998, 281, 2013. (8) Chan, W. C. W.; Nie, S. Science 1998, 281, 2016. (9) Gerion, D.; Parak, W. J.; Williams, S. C.; Zanchet, D.; Micheel, C. M.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 7070. (10) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. (11) Hao, E.; Lian, T. Langmuir 2000, 17, 7879. (12) Tang, Z.; Wang, Y.; Kotov, N. A. Langmuir 2002, 18, 7035. (13) Lowman, G. M.; Nelson, S. L.; Graves, S. M.; Strouse, G. F.; Buratto, S. K. Langmuir 2004, 20, 2057.

It is preferable to construct these films in ordered structures without mutual aggregation. These immobilized semiconductor nanoparticles are expected to be used for potential applications such as single electron transistors (SETs),1 light-emitting diodes (LEDs),3,5,6 and biosensing devices.7-9 Recently, there has been an intense effort to synthesize a wide variety of semiconductor nanoparticles in the liquid phase. These methods enable the preparation of relatively monodisperse semiconductor nanoparticles with control over the crystallinity, the size, and the composition within a wide range. Reverse micelle methods have been successful in terms of preparing metal oxide and other compound semiconductor nanoparticles stabilized by a monolayer of surfactants.14,15 Hot injection methods have also been widely used in order to prepare highly monodisperse, highly luminescent semiconductor nanoparticles.16-18 These methods produce crystalline nanoparticles with fewer defects using high temperature solvents and appropriate surface-bound ligands on the resulting semiconductor nanoparticles. Although these studies enable the synthesis of stable ligand-protected semiconductor nanoparticles with various sizes, shapes, and compositions, very few studies have been conducted on the surface structures and properties of ligand molecules, which are determined by the initial steps of synthesis. The surface properties of ligand-stabilized nanoparticles such as solubility and reactivity depend on the molecular structure and composition of the ligands. Diverse modification procedures have been used to improve the properties and the functionalities of semiconductor nanoparticles. Control of surface properties has been recently (14) Ogawa, S.; Hu, K.; Fan, F. F.; Bard, A. J. J. Phys. Chem. B 1997, 101, 5707. (15) Nakanishi, T.; Ohtani, B.; Uosaki, K. J. Phys. Chem. B 1998, 102, 1571. (16) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (17) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 183. (18) Reiss, P.; Bleuse, J.; Pron, A. Nano Lett. 2002, 2, 781.

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demonstrated by employing facile methods such as ligand exchange and place exchange modifications.19 Although these surface modification methods have been relatively successful for the preparation of gold nanoparticles protected by single molecules with specific binding affinities, such as alkanethiols19,20 and single-stranded DNAs,21,22 thus far, few have been reported for semiconductor nanoparticles.14,23,24 The development of a method that allows effective surface modification of semiconductor nanoparticles with diverse surface functionalities would be extremely beneficial for the development of electrical and optical devices using these semiconductor nanomaterials. In this study, we report an experimental investigation of surface chemical modifications of CdS nanoparticles in order to form mixed-monolayer-protected nanoparticles, along with a covalent-bonding-based layerby-layer approach for the fabrication of nanocomposite films on glass substrates. The process involves the preparation of CdS nanoparticles by the reverse micelle method, surface modification with alkanethiol molecules, and partial introduction of hydroxyl groups onto the CdS nanoparticles. Attachment of CdS nanoparticles on glass substrates was achieved through a carbamate-bondforming reaction between hydroxyl groups on the nanoparticles and monolayers bearing isocyanate groups on a glass substrate. Multilayer nanocomposite thin films with a high volume fraction of CdS nanoparticles can also be fabricated using a stepwise surface-confined reaction of CdS nanoparticle units and isocyanate-bearing monomer units, which serve as linker molecules, in a layer-by-layer fashion. Experimental Section Materials. Na2S‚9H2O (98.0%), heptane (99.0%), and sodium dioctyl sulfosuccinate (AOT, 75.0%) were purchased from Wako Chemical Co. and used as received. Toluene (99.5%) and tetrahydrofuran (THF, 99.5%) were purchased from Wako Chemical Co. and dehydrated by distillation prior to use. Cd(ClO4)2‚6H2O, 1-decanethiol (96%), and 11-mercapto-1-undecanol (97%) were purchased from Aldrich and used as received. Glass substrate (Matsunami Micro cover glass; thickness, 0.12-0.17 mm) was used for immobilization of the nanoparticles. Preparation of CdS Nanoparticles Using the Reverse Micelle Method. CdS nanoparticles were prepared according to previous literature.15 Typically, two heptane solutions of AOT (0.2 M, 50 mL) were prepared. An aqueous solution of Cd(ClO4)2 (0.4 M) was added to one heptane solution, while an aqueous solution of Na2S (0.4 M) was added to the other solution in order to achieve a [H2O]/[AOT] ratio of 6 for both solutions. The solutions were stirred for 3 h. The micellar solution containing Cd(ClO4)2 was then added slowly to the micellar solution containing Na2S at room temperature. CdS nanoparticles were obtained after the solution was stirred for 3 h. Surface Modification of CdS Nanoparticles. 1-Decanethiol (DT) molecules (4.3 mmol) were added to a heptane solution of CdS nanoparticles (1.5 µM). This solution was stirred for 5 h, and methanol was subsequently added in order to remove the AOT molecules. After the methanol phase was removed, the heptane phase was evaporated. The residual solution was then dropped into a large volume of methanol, and the resultant yellow (19) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (20) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782. (21) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (22) Demers, L. M.; Mirkin, C. A.; Mucic, R. C.; Reynolds, R. A.; Letsinger, R. L.; Elghanian, R.; Viswanadham, G. Anal. Chem. 2000, 72, 5535. (23) Mitchell, G. P.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1999, 121, 8122. (24) Pardo-Yissar, V.; Katz, E.; Wasserman, J.; Willner, I. J. Am. Chem. Soc. 2003, 125, 622.

Tsuruoka et al. Scheme 1. Schematic Illustration of Exchange of Protective Molecules from AOT to DT by Ligand Exchange Reaction on the Surface of CdS Nanoparticles and Partial Introduction of MUO by Place Exchange Reaction

precipitate was filtered off using a 0.2-µm membrane filter, yielding purified DT-protected CdS nanoparticles. THF containing 11-mercapto-1-undecanol (MUO) molecules (17 µmol, 5 mL) was then added into a THF solution of the purified nanoparticles (0.13 µmol, 20 mL). After stirring for 3 h, the solution was dropped into a large amount of toluene and the resultant yellow precipitate was filtered off, yielding purified DT/MUO-protected CdS nanoparticles. Construction of Nanoparticle-Based Nanocomposite Films. The glass substrate (1 cm × 2 cm) was washed with 5 M aqueous NaOH solution at 80 °C for 5 min and then immersed in toluene containing hexamethylene diisocyanate (1 M, 5 mL) at room temperature for 4 h under an argon atmosphere, followed by rinsing with toluene under ultrasonication. The isocyanatebearing glass substrate was then immersed in THF containing CdS nanoparticles (10 mg, 5 mL) for 4 h. After the glass substrate was rinsed with THF under ultrasonication, the glass substrate with the first CdS monolayer was again immersed in toluene containing hexamethylene diisocyanate for 4 h. This process was repeated in order to form three-dimensional multilayered structures. Characterization. Absorption spectra were recorded using a Hitachi U-2001 double beam spectrophotometer. The Fourier transform infrared (FT-IR) spectra were obtained using a Nicolet Magna-IR 550 spectrometer. The transmitted signal was averaged for 64 scans at 4 cm-1 resolution. Elemental analysis of the particles was performed using a JSM-6340F field emission scanning electron microscope (FE-SEM) equipped with an energydispersive X-ray (EDX) microanalyzer operating at 15 kV. A JEOL JEM-2010 transmission electron microscope (TEM) operating at 200 kV was used to investigate the size and the size distribution of the nanoparticles. The samples for TEM observation were prepared by dropping THF solution containing CdS nanoparticles onto a carbon-coated TEM grid. The surface morphology of the CdS nanoparticles immobilized on the glass substrate was observed by atomic force microscope (AFM, contact mode) using a JEOL JSPM-4210. A standard silicon nitride tip was used for image acquisition.

Results and Discussion Synthesis of DT-Protected CdS Nanoparticles. Scheme 1 shows a schematic of the procedure used in the present work to prepare the mixed-monolayer-protected CdS nanoparticles. Two different surface modification processes were used to obtain CdS nanoparticles protected by both DT and MUO molecules. The surface organic monolayers at each modification step were characterized

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Figure 1. FT-IR spectra of initial AOT-protected CdS nanoparticles (a) and the nanoparticles obtained after ligand exchange with DT (b) followed by place exchange with MUO (c). Table 1. Results of EDX Elemental Analysis for CdS Nanoparticles Protected by Different Molecules protective molecules noa DT DT/MUO

S (at. %)

Cd (at. %)

51.5 54.5 55.3

48.5 45.5 44.7

a The precipitates were obtained by adding methanol into the solution containing original AOT-protected CdS nanoparticles.

by FT-IR, as shown in Figure 1. First CdS nanoparticles were prepared by the reverse micelle method in heptane solution. TEM observation was used to determine the mean diameter of the AOT-protected CdS nanoparticles as 5.9 nm (standard deviation ) 0.9 nm), and the resulting nanoparticles were used as the starting materials for following surface modification procedures. The FT-IR spectrum for AOT-protected nanoparticles shows the asymmetric and symmetric C-H stretching band around 2900 cm-1 and the CdO stretching band at 1736 cm-1 (Figure 1a). Both bands are assigned to AOT molecules.15 Surface ligand exchange of AOT with DT can be achieved by adding excess DT, relative to AOT, in heptane solution. After reaction for 5 h and purification with methanol, the successful ligand exchange is verified as shown in Figure 1b. The only peaks observed are the characteristic peaks assigned to the asymmetric and symmetric CH2 stretching bands at 2920 and 2850 cm-1, respectively. The AOTrelated peak (CdO stretching, 1736 cm-1) disappears. The CdS nanoparticles could be obtained as solid powders with a waxlike texture and were observed to be soluble in nonpolar solvents such as hexane, toluene, chloroform, and THF but were insoluble in polar solvents. These results indicate that AOT molecules initially bound to CdS nanoparticle surfaces were exchanged with DT molecules due to the stronger binding affinity of mercapto groups on DT to the Cd atoms on the particle surface, resulting in the formation of Cd-S bonds. Introduction of DT molecules is further supported by EDX elemental analysis (Table 1); the sulfur content in solid powders obtained after the purification is observed to increase as compared with the CdS nanoparticles with no protective molecules obtained by addition of methanol into the initial solution containing AOT-protected nanoparticles. Forma-

Figure 2. Optical absorption spectra of CdS nanoparticles protected by AOT (dashed line), DT (solid line), and DT/MUO (dotted line). The spectra were obtained by dissolving the CdS nanoparticles in heptane for AOT- and DT-protected nanoparticles and in THF for DT/MUO-protected nanoparticles.

tion of a Cd-S bond may also be confirmed by the absence of the S-H stretching band around 2600 cm-1 in the FTIR spectrum (Figure 1b). In addition, the change in the intensity of the C-H stretching region may be due to the structural difference in the alkyl chain between the AOT (nonlinear) and DT (linear) molecules. In Figure 2, the band onset in the UV-vis absorption spectra is observed to shift to slightly longer wavelengths after the reaction. The red shift in the band onset may be due to the change in the surface structure of the CdS nanoparticles, the details remaining to be investigated. Partial Introduction of MUO Molecules to DTProtected CdS. To further initiate the surface modification process, a portion of THF solution containing MUO molecules was added to THF solution containing DTprotected CdS nanoparticles. After reaction for 3 h, the addition of a large amount of toluene, a good solvent for the initial DT-protected nanoparticles, induced precipitation of the nanoparticles. The products obtained from repeated washings with toluene were soluble in THF but were found to be insoluble in hexane, toluene, and chloroform. Figure 1c shows the FT-IR spectrum of the products, in which the C-H stretching bands remain unchanged, whereas new bands appear around 3398 cm-1 and at 1100 cm-1. These signals are attributed to the O-H stretching and C-O stretching vibrations, respectively.25 A S-H stretching band does not arise. These results indicate that the MUO molecules are partially introduced onto the DT-protected CdS nanoparticles by simply adding the MUO molecules to the solution, yielding mixedmonolayer-protected nanoparticles (DT/MUO-protected CdS nanoparticles). The polar hydroxyl groups thus introduced were thought to be responsible for the changes in the solubility of the resulting nanoparticles: the insoluble nature of the nanoparticles in nonpolar solvents. The sulfur content in the nanoparticles obtained is comparable with that in the DT-protected nanoparticles (Table 1); therefore partial introduction of MUO molecules onto the nanoparticle surface can be achieved by place exchange reaction between surface-bound DT and free MUO molecules in an almost stoichiometric manner (1:1 ratio). The UV-vis spectrum of the product has an almost identical onset wavelength to that obtained for DT(25) Persson, H. H. J.; Caseri, W. R.; Suter, U. W. Langmuir 2001, 17, 3643.

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Figure 3. TEM image and electron diffraction pattern of DT/ MUO-protected CdS nanoparticles on a carbon-coated TEM grid.

protected CdS nanoparticles (Figure 2), indicating that size and surface structures of CdS nanoparticles are preserved through the additional surface modification. Although analogous stoichiometric exchange has been observed and studied in detail for gold nanoparticles with a strong binding affinity to mercapto groups,19 the present results are considered to be the first successful surface modification to a semiconductor surface with thiolderivatives, providing mixed-monolayer-protected particles. Since the place exchange reaction can be mediated by a shift of the dynamic equilibrium between the fixed DT molecules bound to the CdS surface and free MUO molecules in the solution phase, the degree of surface modification could be controlled by varying the reaction conditions. Although the precise percentage of protective molecules remains to be determined, a longer reaction (mixing) time and a higher concentration of MUO molecules in the solution resulted in greater surface modifications and an increase in the MUO content of surface protective layers. This increase paralleled the corresponding decrease in DT content. Thus, greater surface modifications resulted in THF-insoluble nanoparticles with increased solubility in polar solvents such as methanol, ethanol, and dimethylformamide. A TEM image of DT/MUO-protected CdS nanoparticles is shown in Figure 3. The mean size of the resulting CdS nanoparticles was 5.9 nm (standard deviation, 0.9 nm). The high-resolution image and electron diffraction pattern confirm that the nanoparticles were highly crystalline and well isolated on the carbon-coated grids. The plane distances were determined to be 0.361, 0.206, and 0.175 nm, which corresponded well with the (111), (220), and (311) planes of CdS with cubic zinc blende structure.26 The size and size distribution of the DT/MUO-protected CdS nanoparticles were in good agreement with those in the initial AOT-protected CdS nanoparticles, indicating that the present surface modification process provided mixed-monolayer-protected particles without any structural transition, size evolution, or mutual aggregation of CdS cores. Carbamate-Bond-Forming Reaction at the CdS Nanoparticle Surface. To clearly demonstrate the functionalization of the CdS nanoparticles, the reactivity of the particles was investigated by a covalent-bondforming reaction of the surface-bound hydroxyl groups. The carbamate-bond-forming reaction was used in the present work to confirm the reactivity. This reaction has (26) The file for X-ray powder diffraction standards by JCPDS: CdS, 10-454.

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Figure 4. FT-IR spectra of DT/MUO-protected CdS nanoparticles obtained after reaction with isocyanic acid n-octadecyl ester (a) and octadecylcarbamic acid ethyl ester (b).

been reported to proceed cleanly in ambient conditions for the coupling of molecules with hydroxyl and isocyanate groups.25 Mixing of the DT/MUO-protected CdS nanoparticles with isocyanic acid n-octadecyl ester in THF yielded solid powders after reaction for 4 h followed by precipitation using ethanol. The FT-IR spectrum of the resulting powders indicates that almost complete formation of a carbamate bond is achieved (Figure 4a). The characteristic peaks appear as new features in the spectrum at 3314 cm-1 (N-H stretching), 1686 cm-1 (Cd O stretching), 1539 cm-1 (C-N-H stretching), 1274 cm-1 (C-N stretching), and 1048 cm-1 (C-O stretching), all of which are attributable to a carbamate bond. The results are in good agreement with the IR spectrum for hydroxylgroup-terminated self-assembled monolayers with phenylene diisocyanate on gold substrates.25 Additionally, the spectrum is quite similar in shape to that of octadecylcarbamic acid ethyl ester (Figure 4b) prepared by reaction of isocyanic acid n-octadecyl ester with ethanol.27 The broad O-H stretching band observed before the reaction (in Figure 1c) disappears, which further confirms the clean coupling reaction used to form carbamate bonds. The resulting products were soluble in toluene (insoluble before reaction) due to introduction of long hydrophobic alkyl chains to the nanoparticle surface. Covalent-Bond-Directed Immobilization of CdS Nanoparticles on the Glass Substrate. The ability of surface hydroxyl groups to form carbamate bonds was further used for the immobilization of CdS nanoparticles on glass substrates. The isocyanate group functionality should be introduced onto the glass substrate due to the coupling scheme employed here. To introduce the isocyanate, the glass substrate was first treated with concentrated alkaline solution, which resulted in exposure of surface hydroxyl groups on the substrate. The resulting hydrophilic surface was then modified by reaction with hexamethylene diisocyanate (Scheme 2). The motivation lies in the assumption that one of the two isocyanate groups reacts with a surface hydroxyl group, whereas the other would be directed away from the surface and consequently available for further reactions. Immobilization of CdS nanoparticles was characterized by AFM in order to assess the surface topography of the layer. The DT/MUO-protected CdS nanoparticles were (27) Octadecylcarbamic acid ethyl ester was prepared as follows: isocyanic acid n-octadecyl ester was dissolved in ethanol (final concentration, 50 mM) and stirred for 1 h. The excess ethanol was then evaporated to give octaecylcarbamic acid ethyl ester powder.

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Figure 5. AFM images of bare glass substrate (a), 1 layer of CdS nanoparticles (b), and 10 layers of CdS nanoparticles (c). Scheme 2. Schematic Illustration for Functionalization of the Glass Substrate Using Hexamathylene Diisocyanate and Immobilization of DT/MUO-Protected CdS Nanoparticles on the Isocyanate-Bearing Substrate through Carbamate-Forming Reaction

Figure 6. TEM image for 1 layer of CdS nanoparticles immobilized on a glass substrate. The image was taken after removal of the layer using HF vapor.

dissolved in THF, and the isocyanate-bearing glass substrate was then immersed in the solution for 4 h. As shown in Figure 5b, although individual CdS nanoparticles are not distinguished after 1-layer deposition, some domains with dimensions smaller than a few tens of nanometers, which cannot be observed on the bare glass surface (Figure 5a), are clearly observed. The surface roughness of the deposited layer is relatively small even after immobilization (RMS ) 3.7 nm); therefore the observed morphological features are thought to be due to the slightly different packing density of the immobilized CdS nanoparticles. This is presumably due, in part, to the inhomogeneous distribution of the first isocyanate functionality on the substrate on a molecular scale. To confirm that the observed morphology resulted from the immobilized CdS nanoparticles, TEM observation was performed for a sample with 1-layer deposition of CdS nanoparticles. This was achieved by removing the layer from the glass substrate using HF vapor.28 A typical TEM image is shown in Figure 6. Although the arrangement of immobilized CdS nanoparticles in the image could not be truly reflected as those on the glass substrate due to sample processing, the nanoparticles clearly have a mean diameter of 5.9 nm. This diameter is consistent with the starting DT/MUO-protected nanoparticles. The image further demonstrates that only the monolayer of CdS nanoparticles was immobilized. When the isocyanatebearing substrate was immersed in ethanol for 10 min (28) The TEM sample for CdS nanoparticles immobilized on glass substrates was prepared as follows. A thin carbon layer was vacuumdeposited on a sample with 1-layer deposition, and the sample was then exposed to hydrogen fluoride (HF) vapor for 30 s using aqueous HF solution (10%). The film was then separated from the substrate by floating in a water bath and placed on a carbon-coated TEM grid.

before immersing in the solution containing CdS nanoparticles or when the hydroxyl-group-bearing substrate (without hexamethylene diisocyanate treatment) was used for the experiment, no layers of CdS nanoparticles were observed in both AFM and TEM images (results not shown). This indicates that the nanoparticles only attached to a glass surface with isocyanate-group-bearing layers via a carbamate-bond-forming reaction. It is important to note that the long duration of the immobilization reaction was conservatively employed in order to ensure the immobilization came to completeness and was not optimized; shorter reaction times (less than 1 h) have also been successfully employed to immobilize the nanoparticles. One of the advantages of the present immobilization method is that it is a universal method applicable to fabrication of multilayer structures of these monolayerprotected nanoparticles. The layers thus deposited can possess functional groups that originate from the top layer of each cycle. These functional groups can then be used to immobilize the next target species, providing multilayers of both CdS nanoparticles and organic spacer linkers in a layer-by-layer fashion. To demonstrate this capability, a carbamate-bond-forming reaction was repeated using hexamethylene diisocyanate as the linker molecules. Successive immobilization was performed by simple alternating immersion in toluene solution, and the immersion cycle was continued at least 20 times. UV-vis spectroscopy was used to monitor the multilayer construction as a function of the number of layers of CdS nanoparticles, and the representative results are shown in Figure 7. Immobilization of CdS nanoparticles resulted in an increase in absorbance, and the onset wavelength is observed to remain constant as the layers grew. The spectra show a uniform increase in absorbance at 450 nm with the number of layers (inset, Figure 7), indicating a regular immobilization of the nanoparticles on the sub-

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Figure 7. Absorption spectra of CdS-nanoparticle-based multilayer films on a glass substrate. Layers 1-10 of the CdS nanoparticles are shown from the lower to the upper spectrum. Inset: Plots of absorbance at 450 nm for films as a function of the number of layers.

strates.29 In the AFM image shown in Figure 5c, a surface morphology similar to that in Figure 5b is observed after 10-layer deposition, with the exception of a slight increase in the domain size, suggesting that this immobilization process is highly reproducible. A height profile for the film yielded only a 4 nm difference between the highest and lowest points in the area investigated, comparable to the film obtained after 1-layer deposition (Figure 5b). These results further suggest that each immersion cycle immobilizes approximately the same quantity of CdS nanoparticles. The carbamate bond formed between each CdS nanoparticle and the substrate prevents the CdS nanoparticles from being removed by, for example, repeated washing with several organic solvents and water at a wide range of pH under ultrasonication. Using an experimentally determined molar absorption coefficient of CdS nanoparticles in Figure 7,30 an average increase of 0.037 in absorbance over the 20 reaction cycle (corresponding to 10 layers of CdS nanoparticles) indicates that 1.2 × 10-12 mol cm-2 of CdS nanoparticles are immobilized on the underlying layers in one immobilization step, the value of which is higher than that of other semiconductor (29) The CdS nanoparticles immobilized on glass substrates showed no detectable emission peak because of the small amount of nanoparticles (few monolayers) and low quantum efficiency of the present CdS nanoparticles. The CdS nanoparticles protected by AOT, DT, and DT/ MUO molecules did not show exciton-related emission but showed a very weak emission peak at about 600 nm due to trap-state emission, similar to the case of AOT-protected CdS nanoparticles previously reported by another group (Harruff, B. A.; Bunker, C. E. Langmuir 2003, 19, 893). (30) The molar absorption coefficient of the CdS nanoparticles at 450 nm in dilute THF solution was determined as 1.6 × 106 M-1 cm-1.

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nanoparticles with comparable size immobilized via electrostatic interaction.12 Assuming that the 5.9 nm CdS nanoparticles are surrounded by a mixed monolayer of 1.5 nm in thickness,25 about 9 nm particles were immobilized on the glass substrate and underlying layers during each deposition cycle. If 9 nm particles were immobilized on the surface in a hexagonal close packed arrangement, a numerical calculation gives a surface coverage of 2.3 × 10-12 mol cm-2. Compared with the estimated surface coverage described above, this value suggests that the immobilized CdS nanoparticles are not arranged in a close packing arrangement; however, relatively high surface coverage can be achieved. The coverage of the immobilized nanoparticles may be controlled by varying the percentage of protective molecules as well as the number of covalent bonds formed during immobilization. Optimization of these variables is currently in progress. Conclusions The present study describes the synthesis and surface modification of monolayer-protected CdS nanoparticles. Decanethiol-protected CdS nanoparticles were easily synthesized by ligand exchange using AOT-protected nanoparticles. The obtained nanoparticles could be treated as solid powders, have a high stability, and are soluble in a number of organic solvents. Further modification of the surface protective layers of CdS nanoparticles is achievable simply by a place exchange reaction with a hydrophobicgroup-bearing molecule, mercaptoundecanol, thus providing better control over surface polarity. This increased control is thought to be associated with solubility and reactivity of the resulting mixed-monolayer-protected CdS nanoparticles. Single-step modulation of the surface hydroxyl group was demonstrated by a carbamate-bondforming reaction with isocyanate functional groups, which proceeded cleanly in ambient reaction conditions. This surface chemistry is versatile and offers an approach for the use of mixed-monolayer-protected CdS nanoparticles as building blocks for the construction of multilayer nanocomposite thin films in a layer-by-layer fashion. The immobilization process is driven by the formation of a carbamate bond after reaction between surface-bound hydroxyl groups and isocyanate groups introduced onto a glass substrate. This provided a controlled fabrication of a thin nanocomposite layer with relatively smooth surface morphology. This methodology may be applicable to other types of semiconductor nanoparticles and nanorods, as well as useful for general applications in the fabrication of electronic and photonic devices with diverse functionalities. The potential utilities of this technology are currently under investigation. LA048729Z