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Semiconductor Nanoparticles on Solid Substrates: Film Structure, Intermolecular Interactions, and Polyelectrolyte Effects Zhiyong Tang, Ying Wang, and Nicholas A. Kotov* Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 Received February 3, 2002. In Final Form: June 7, 2002 Citrate-stabilized CdSe or CdSe/CdS core-shell nanoparticles (NPs) were adsorbed on the standard silicon wafers bearing either a short-chain covalently bound adsorption promoter (3-aminopropyl)triethoxylsilane (APTES) or macromolecular adsorption promotersspolyethylenimine (PEI) or poly(diallydimethylammonium) chloride (PDDA). The aim of this study is 2-fold: (1) to compare different methods of NP processing into thin films and (2) to elucidate the effect of the long-chain dynamics on the NP film structure. Systematic atomic force microscopy study of the films revealed that both types of NPs produced densely packed films on PDDA, while rarified films with significant clustering formed on PEI and APTES. The difference in NP layer morphologies was rationalized on the basis of intermolecular NP-polyelectrolyte interactions. Importantly, we observed that the adsorption layer of the weak polyelectrolyte PEI could alter its chain distribution by partial wrapping around the NPs, while no disturbance in APTES and PDDA monolayer by NP was observed. This was attributed to more labile binding of PEI to the solid substrate than for other adsorption promoters.
Introduction Semiconductor nanoparticles (NPs) are being vigorously investigated as new materials for traditional electronic and optical applications, as chemical and biological sensors, and as building blocks for conceptually novel molecule-based devices.1-3 Most of these practical implementations of nanotechnology will require the immobilization of NPs on various substrates in the form of thin films. One of the new techniques that can be used for NP processing in thin films is the layer-by-layer (LBL) assembly on polyelectrolytes (PEs).4,5-7 It is both potent and simple. It also affords a high degree of structural control and quality of the coatings.8-11 Given the fact that there are several approaches to the preparation of NP thin films, it is important to understand the differences in the film structure obtained by this and other comparable techniques. Therefore, we studied the morphology of three types of NP layers adsorbed on (1) a strong PE, (2) a weak PE, and (3) a self-assembled monolayer of (3-aminopropyl)triethoxylsilane (APTES). The NP films on covalently bound adsorption promoters such as APTES are often used in the fundamental research and model devices based on single NPs.12-16 NP/PE thin * To whom correspondence may be addressed. E-mail: kotov@ okstate.edu. Tel: 405-744-3991. Fax: 405-744-6007. (1) Alivisatos, A. P. Science (Washington, D.C.) 1996, 271, 933-937. (2) Alivisatos, A. P. NATO Sci. Ser., Ser. C 1999, 519, 405-416. (3) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. Rev. Mater. Sci. 2000, 30, 545-610. (4) Decher, G. Science 1997, 277, 1232-1237. (5) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 9, 13065-13069. (6) Kotov, N. A. MRS Bull. 2001, 26, 992-997. (7) Gao, M. Y.; Richter, B.; Kirstein, S.; Mohwald, H. J. Phys. Chem. B 1998, 102, 4096-4103. (8) Fendler, J. H. Chem. Mater. 2001, 13, 3196-3210. (9) Mamedov, A. A.; Belov, A.; Giersig, M.; Mamedova, N. N.; Kotov, N. A. J. Am. Chem. Soc. 2001, 123, 7738-7739. (10) Murzina, T. V.; Nikulin, A. A.; Aktsipetrov, O. A.; Ostrander, J. W.; Mamedov, A. A.; Kotov, N. A.; Devillers, M. A. C.; Roark, J. Appl. Phys. Lett. 2001, 79, 1309-1311. (11) Ostrander, J. W.; Mamedov, A. A.; Kotov, N. A. J. Am. Chem. Soc. 2001, 123, 1101-1110.
films represent a convenient method for the preparation of organized nanostructured composites. Both tasks require better structural control over the NP packing, which is essential for various nanotechnological applications. We found that the morphology of the films differed significantly in NP clustering and the overall particle density. The observed differences in the structure of the thin films are rationalized on the basis of intermolecular interactions between NPs and the partner organic layer. In comparison with the self-assembled films of short molecules and strong PEs, the NP films on substrates bearing weak PE layers are distinguished by their ability to adjust their conformation accommodating attracted NPs. Experimental Section Poly(diallydimethylammonium) chloride (PDDA) of average molecular weight 200 000 was purchased from Aldrich. Polyethylenimine (PEI) of average molecular weight 750 000 was purchased from Sigma. (3-Aminopropyl)triethoxylsilane (APTES) was obtained from Fluka. Milli-Q-deionized water was used for all the experiments. Aqueous colloidal solutions of citrate-stabilized CdSe and coreshell CdSe/CdS NPs were prepared as described previously.17 Both colloidal solutions were used as prepared, with the concentration of CdSe and CdSe/CdS particles of 4 × 10-4 M referring to Se2-. Method 1. CdSe or CdSe/CdS NPs on PDDA-modified substrate: (1) dipping of silicon wafers into a solution of PDDA (10 mM) for 10 min, (2) rinsing with water for 2 min, (3) dipping into the dispersion of CdSe or CdSe/CdS NPs (pH ) 9) for 10 min, (12) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221-5230. (13) Colvin, V. L.; Schlamp, M. C.; Allvisatos, A. P. Nature (London) 1994, 370, 354-357. (14) Ulman, A. Adv. Mater. (Weinheim, Fed. Repub. Ger.) 1993, 5, 55-57. (15) Grabar, K. C.; Brown, K. R.; Keating, C. D.; Stranick, S. J.; Tang, S. L.; Natan, M. J. Anal. Chem. 1997, 69, 471-477. (16) Klein, D. L.; McEuen, P. L.; Katari, J. E. B.; Alivisatos, A. P. Nanotechnology 1996, 7, 397-400. (17) Rogach, A. L.; Nagesha, D.; Ostrander, J. W.; Giersig, M.; Kotov, N. A. Chem. Mater. 2000, 12, 2676-2685.
10.1021/la025601d CCC: $22.00 © 2002 American Chemical Society Published on Web 07/30/2002
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Figure 1. Absorption (A) and luminescence (B) spectra of “bare” CdSe NPs (solid lines) and CdSe/CdS NPs (dashed lines).
Figure 2. 1 µm × 1 µm TM-AFM images of on PDDA (A), PEI (B), and APTES (C) modified silicon wafers. (4) rinsing with water again for 2 min. The preparation of CdSe or CdSe/CdS NPs on PEI-modified substrate was the same as above, using 10 mM of PEI in the first step. Method 2. CdSe or CdSe/CdS NPs on APTES self-assembled monolayer (SAM) modified substrate: (1) the functionalization of clean silicon wafers with an NH2-terminated SAM by immersion in a 1 mM toluene solution of APTES for 1 h, followed by heating at 110 °C for 10 min, (2) immersion of the APTES-derived substrate in a solution of CdSe or CdSe/CdS NPs (pH ) 9) for 10 min and then rinsing with ultrapure water for 2 min. Atomic force microscopy (AFM) measurements were performed, in air, by using a Nanoscope IIIa system (Digital Instruments Inc., Santa Barbara, CA) operating in the tapping mode (TM). TESP silicon tips with a cantilever length of 125 µm and a characteristic frequency of 300-330 kHz were employed for image acquisition. All height measurements were obtained by using the box cursor in the bearing analysis of image analysis software of Nanoscope IIIA.18 The height distribution curves were obtained from the analysis software package supplied with the instrument. Silicon wafers used as substrates were purchased from Virginia Semiconductors. Oxidation in air results in the formation of a stable layer of silica on their surface, to which all surface modifiers are chemically or physically bound.
Results and Discussion 1. Nanoparticles. Citrate-capped CdSe and CdSe/CdS NPs are stable in weak base solution, pH 9, because of the reciprocal repellency of surface negative charges. Both solutions of CdSe and CdSe/CdS NPs show a peak corresponding to the first excitonic transition in the absorption spectra (Figure 1A). The corresponding light emission transition can be observed as a pronounced photoluminescence peak (full width at half-maximum ) 35-50 nm), which is significantly enhanced by capping (18) Ramirez-Aguilar, K. A.; Rowlen, K. L. Langmuir 1998, 14, 25622566.
the CdSe surface with a higher band gap inorganic layer of CdS (Figure 1B).19-21 The average size of CdSe is evaluated to be 4 nm from UV absorption maxima, while corresponding sizes of CdSe and CdSe/CdS were measured to be 4 and 5 nm from transmission electron microscopy (TEM) data, respectively.17 Although these NP dispersions have useful optical properties, they are utilized in this study as model colloids, which differ in size while being very similar in surface chemistry. 2. Morphology of the Supporting Organic Layer. The choice of the organic substances for NP adsorption was governed by the typical PEs used in LBL assembly, such as PDDA and PEI. For comparative study, APTES was chosen because of its ability to form dense selfassembled monolayers on silica always present on silicon wafers and for the similarity of its amino group responsible for the NP attraction to that in PEI. While some other headgroups, such as thiols, could probably result in stronger bonding to NP surface, this choice of the adsorption promoters should reveal the differences in adsorption layer structure introduced by PEs as compared with the low molecular weight compounds. Figure 2 represents the TM-AFM images of APTES-, PEI-, and PDDA-modified silicon wafers. All three modified surfaces are fairly featureless and flat. For PDDAmodified substrate, the root-mean-square value of surface roughness is 0.10 nm and the largest surface height difference is 0.4 nm. The corresponding values are 0.14 and 0.6 nm for PEI-modified one and 0.29 and 1.0 nm for (19) Dabbousi, B. O.; Rodriguez-Viejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101, 9463-9475. (20) Peng, X.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019-7029. (21) Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. 1996, 100, 468471.
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Figure 3. 1 µm × 1 µm TM-AFM height images of citrate-capped CdSe NPs on PDDA (A), PEI (B), and APTES (C) modified silicon wafers. 1 µm × 1 µm TM-AFM height images of CdS/CdSe core-shell on PDDA (D), PEI (E), and APTES (F) modified silicon wafers. 1 µm × 1 µm phase images of CdS/CdSe core-shell on PDDA (G), PEI (H), and APTES (I) modified silicon wafers.
APTES-modified one. Obviously, the silicon wafers with PDDA are the smoothest, while those with APTES are the coarsest surface among the three types of substrates. The area of topographically identifiable domains is several tens of nanometers for PEI- or PDDA-modified substrates and several hundreds of nanometers for APTES-modified one. No change in surface morphology can be observed for any of the adsorption promoters after immersing both modified substrates in the solution with pH 7-9 for 10 min. 3. Nanoparticle Density and Distribution. Figure 3 shows tapping mode AFM images of CdSe or CdSe/CdS NPs on the modified silicon wafers. On the PDDA-coated surface, both CdSe (Figure 3A) and CdSe/CdS colloids (Figure 3D and Figure 3G) are adsorbed in a homogeneous close-packed monolayer with little aggregation or multilayer formation. The scratch test (Supporting Information) reveals that indeed the observed topography originates from nanoparticle adsorption and not from etching or other substrate-related chemistry. The difference in size of “naked” CdSe and core-shell CdSe/CdS NPs can be clearly
seen in the AFM images. The height of individual NPs was found to be 4.3 and 5.8 nm for CdSe and CdSe/CdS, respectively, which coincides well with the TEM and optical spectroscopy data.17 Unlike PDDA, both CdSe (Figure 3B and Figure 3C) and CdSe/CdS (Figure 3E and Figure 3F) colloids adsorb on PEI- and APTES-bearing substrates as well-separated particles. Additionally, the corresponding phase images reveal that AFM dots on PEIor APTES-modified surface are actually the clusters of single semiconductor NPs (Figure 3H and Figure 3I). Previously, we hypothesized that the dense NP monolayers formed when both attractive forces to the PE films and repulsive forces between the NP were strong.11,22 When one of these components significantly exceeds the other one, then the organization of the film suffers either from chaotic clustering and high roughness (NP-PE attraction is too strong) or low particle density (NP-NP repulsion is too strong). The variations in the NP layer morphologies (22) Pastoriza-Santos, I.; Koktysh, D. S.; Mamedov, A. A.; Giersig, M.; Kotov, N. A.; Liz-Marzan, L. M. Langmuir 2000, 16, 2731-2735.
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Figure 4. Height distribution of citrate-capped CdSe NPs on PDDA (A), PEI (B), and APTES (C) modified silicon wafers. CdS/CdSe core-shell on PDDA (D), PEI (E), and APTES (F) modified silicon wafers. CdSe/CdS core-shell on PDDA.
seen in Figure 3 can be qualitatively rationalized by considering the interactions between NP and adsorption promoters. We should note that a quantitative picture of the intermolecular interactions in such a multibody system as NPs + PE is difficult to obtain because a large variety of forces is involved in both attractive and repulsive interactions of the NP ensemble.23-25 The discussion presented here should be considered as an effort to clarify the most essential interactions among them, which, in turn, can simplify the quantitative treatment as well as the structural control of the NP films.11 For PDDA adsorbed on silicon wafers, there are numerous quaternary ammonium cations, which remain positively charged in a wide range of pH. So at pH 9.0, i.e., the conditions in NP dispersions used here, there is a strong electrostatic attraction of PDDA and highly negatively charged NPs.22,26 When PEI or APTES molecules are adsorbed on a silica layer coating the wafers, adsorption of NPs is promoted by primary or secondary amino groups. While pKa of the amino groups in solution is 10.6,27,28a it becomes several pH units lower in monolayers on solid substrates being from 7.428b to 3.9.28c Since there are only a few positively charged amino groups at pH 9.0, there is little, if any, attractive long-range electrostatic interaction between the NPs and PEI- or APTES-modified Si wafers. Adsorption of the nanoparticles should be primary credited to different short-range forces such as hydrogen bonding, coordinative forces with Cd2+-rich semiconductor surface, hydrophobic forces, and other van der Waals interactions. As compared to PDDA, the overall balance of forces in PEI/NP and APTES/NP pairs shifts toward repulsion. Consequently, the particle density on PDDA is ∼1500 and ∼720 units/µm2 for CdSe and CdSe/ CdS films.29 The films of the two other adsorption promoters reveal particle densities virtually identical to (23) Kotov, N. A. Nanostruct. Mater. 1999, 12, 789-796. (24) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 81538160. (25) Hammond, P. T. Curr. Opin. Colloid Interface Sci. 2000, 4, 430442. (26) Liu, Y. J.; Wang, A. B.; Claus, R. J. Phys. Chem. B 1997, 101, 1385-1388. (27) Zhu, T.; Fu, X.; Mu, T.; Wang, J.; Liu, Z. Langmuir 1999, 15, 5197-5199. (28) (a) Zhu, T.; Zhang, X.; Wang, J.; Fu, X.; Liu, Z. Thin Solid Films 1998, 327-329, 595-598. (b) Zhang, H.; He, H.-X.; Wang, J.; Mu, T.; Liu, Z.-F. Appl. Phys. A 1998, 66, S269-S271. (c) Vezenov, D.; Noy, A.; Rozsnyai, L. F.; Lieber, C. J. Am. Chem. Soc. 1997, 119, 2006-2015.
each other: ∼280 units/µm2 for CdSe, ∼480 units/µm2 for CdSe/CdS. All these values are obviously smaller than that of closely packed monolayer of NPs, i.e., ∼40000 units/ µm2, which means that even for quite densely packed films on PDDA, the in-plane gaps between the NPs are wide.30 This should be specifically referred to the citrate-stabilized CdSe and CdSe/CdS. Other nanoparticles with different capping agents may produce thicker films with much denser packings.31 4. Formation of NP Aggregates. It is also essential to understand the origin of different NP clustering patterns when the organic supporting layer is varied. The NP aggregation mode of NPs on PEI- and APTES-modified silicon wafers can be analyzed in a quantitative manner by taking the advantage of height distribution diagrams of AFM images reflecting the diameter of adsorbed species.18,32,33 Figure 4 represents the height analysis data for CdSe and CdSe/CdS NPs on the modified substrates obtained from AFM analysis of images presented in Figure 3 and similar to them by Digital Instruments/Veeco software. On PDDA-modified substrate, the peaks are centered around 4 and 6 nm for CdSe (Figure 4a) and CdSe/CdS (Figure 4d) NPs, respectively, which confirms that NPs on PDDA exist preferentially in the form of a submonolayer. On PEI- or APTES-modified wafers, the height diagram peaks shift to larger values indicating the presence of aggregates. Both CdSe and CdSe/CdS colloids show stronger agglomeration on PEI-modified substrate than on APTES. The maxima of the CdSe diagrams are observed at 11 and 7 nm for PEI and APTES, respectively, while those of CdSe/CdS distributions are located at 20 and 17 nm for PEI and APTES, respectively. These data translate into an average number of particles per aggregate as 2.8 for CdSe-PEI, 1.8 for CdSe-APTES, 4.0 for CdSe/CdS-PEI, and 3.4 for CdSe/CdS-APTES. (29) These values need to be considered as the lower limit of particle density because of the broadening of lateral features by the AFM tip. (30) In LBL multilayer films, the average distance between the NPs becomes much smaller because of the particle stacking on top of each other. The width of interparticle gaps in the composite should be comparable to the PE chain diameter, i.e., a few nanometers. (31) Rogach, A.; Koktysh, D.; Harrison, M.; Kotov, N. A. Chem. Mater. 2000, 12 (6), 1526-1528. (32) Sato, H.; Ohtsu, T.; Komasawa, I. J. Colloid Interface Sci. 2000, 230, 200-204. (33) Gerion, D.; Pinaud, F.; Williams, S. C.; Parak, W. J.; Zanchet, D.; Weiss, S.; Alivisatos, A. P. J. Phys. Chem. B 2001, 105, 8861-8871.
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Whether the clustering behavior originates from aggregation of NP in solution or on the surface, it can be understood considering the balance between NP-PE attractive and repulsive forces. First, let us address the difference between PDDA-supported layers and those afforded by PEI and APTES. Despite the stronger attraction to the substrate resulting in denser films, the NP layers on PDDA are also smoother, which correlates well with the “strong attraction-strong repulsion” premise mentioned above. A simple model of an aggregate is “extra” NPs topping the existing monomolecular 2D NP island. The electrostatic repulsion between these additional NPs and the underlying sample plane in rarified NP layers produced by APTES and PEI adsorption promoters is much weaker than that in the densely packed layers formed on PDDA. For a well-packed monolayer, the approaching NPs experience the repulsion from the entire negatively charged surface, while for films made of isolated NP islands this repulsion is significantly reduced due to the absence of the repulsive interaction with uncovered surface, which, in fact, is weakly attractive. Analogously, the attraction to highly positively charged surface of PDDA can break apart NP clusters existing in solution. Therefore, the probability of the presence of NP clusters, no matter whether they are formed in situ or adsorbed as a single unit from the solution, is higher in a dispersed submonolayer of NPs than in a densely packed layer (Figure 3 PDDA vs Figure 3 PEI & APTES). In the case of PEI and APTES, stronger clustering is also aided by powerful van der Waals interactions with nonionized NH2 groups abundant in both compounds. Once the aggregate is formed, short-range dispersion forces come into play, which prevents the removal of NPs during the rinsing stage unlike those weakly adsorbed on the substrate surface. High molecular mass and polarizability of NPs make them very significant for the adsorption layer. Additionally, the dipole forces between the NPs exceptionally strong.37 Second, it is necessary to rationalize the greater size of aggregates formed on PEI than on APTES. It cannot be attributed to the intrinsic roughness of the organic supporting layers because the APTES-modified substrate exhibits actually slightly greater roughness than those bearing PEI (Figure 2). Instead, it should be associated with the existence of partially detached segments of the polyelectrolytes protruding away from the interface into the NP dispersion. They can serve as catalysts for NP aggregation as was demonstrated previously for magnetite nanocrystals.34 Therefore, these segments promote aggregation in 3D, while the agglomeration on the substrates modified with short APTES molecules is limited mainly to the flat surface. 5. Polyelectrolyte Wrapping. An interesting structural feature related to the macromolecular nature of the supporting layer can be seen in Figure 3. There are distinct “collars” around NPs that appear substantially higher than the polyelectrolyte film away from them on the PEImodified wafers (shown as the arrow in Figure 3B and Figure 3E). They have not been observed on PDDA- or APTES-bearing substrates. Detailed morphological information can be further obtained through imaging the rarified NP films assembled on the PE-modified silicon wafers under different pH conditions. The highly dispersed NPs films even on PDDA can be prepared by immersing the substrate in dilute CdSe solution (5 × 10-5 M) for a short time (30 s), while the pH values of the CdSe solution are adjusted by the adding of acid, instead of buffer solution, to avoid possible salt effect (34) Correa-Duarte, M. A.; Giersig, M.; Kotov, N. A.; Liz-Marzan, L. M. Langmuir 1998, 14, 6430-6435.
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on the adsorption promoter layer.35,36 Figure 5A shows the AFM image of CdSe NPs on the PDDA-modified substrates at pH 9.0. No collar structure is observed on it, and similar results are also observed after CdSe NPs are assembled on PDDA-modified silicon wafers at pH 7.0. On the contrary, the wider collars appear and their numbers are increasing with pH value increase between 7.0 and 9.0, when CdSe NPs are adsorbed on the PEImodified surface (Figure 5B-D). The similar features cannot be seen in AFM images when the PEI-modified wafer was immersed in solution without NPs regardless of pH value. Therefore, the collars in panels B and E of Figure 3 and cloudlike disturbances in the PE monolayer seen all over the images in panels B-D of Figure 5 come from the interaction between NPs and PEI layer. From the AFM and corresponding cross-sectional image (Figure 5E), one can clearly see that single CdSe NP is wrapped by the rising PEI chains. To understand the genesis of these structural features, one needs to include another fundamental component of the NP + PE force map that is the interactions of the PE and the substrate. When a NP lands on the substrate, it prompts the structure of the initially adsorbed PEI layers to change. The PE may or may not have the ability to reorganize itself in response to this stimulus. Provided that the PE-substrate interactions are fairly weak, the labile segments of the PE chain surrounding the NP become attracted to it and eventually wrap around it. Additionally, adsorbed parts of the macromolecules adjust their conformation to best accommodate the NP and to maximize the work done by attractive forces. This restructuring is facilitated by the weakness of the electrostatic attraction of PEI to the silica layer coating the wafers. When pH value of the NPs solution increases from 7.0 to 9.0, the attraction between the NPs and PEI becomes stronger, too, as indicated by the greater number of particles adsorbed (Figure 5). Naturally, the width of NP collars and the number of cloudlike disturbances in PEI monolayer also increase. The mobility of the PEI chains escalates with pH because the electrostatic adsorption to a negatively charged SiO2 surface becomes weaker as PEI becomes less positive at higher pH, while for NP, the similar trend is apparently compensated by the more diverse van der Waals forces with organic stabilizer shells around NPs. This model corroborates well with the fact that the NP wrapping by organic chains is not observed on APTES. These molecules are short and covalently bound to silica film on a silicon wafer, while the electrostatic interaction of PDDA macromolecules retaining high positive charge even in basic media with the wafer is too strong to allow for its dynamic restructuring. Conclusion Thus, NP submonolayers formed on three different adsorption promoters, i.e., PDDA, PEI, and APTES, have been considered here, and their structure can be represented by the diagrams in Figure 6. The marked difference in the structural characteristics of the NP layers such as overall particle density, distribution, clustering, and roughness can be understood on the basis of electrostatic forces combined with short-range van der Waals forces. The simple qualitative considerations worked well explaining the modes of NP adsorption facilitated by the (35) Tedeschi, C.; Moehwald, H.; Kirstein, S. J. Am. Chem. Soc. 2001, 123, 954-960. (36) McAloney, R. A.; Sinyor, M.; Dudnik, V.; Goh, M. C. Langmuir 2001, 17, 6655-6663. (37) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297 (5579), 237-240.
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Figure 5. 3 µm × 3 µm TM-AFM height images of citrate-capped CdSe NPs on PDDA-modified silicon wafer at pH 9.0 (A). CdSe NPs on PEI-modified silicon wafer at pH 7.0 (B), 8.0 (C), and 9.0 (D). AFM height and cross-section images of single CdSe NP on PEI-modified silicon wafer at pH 9.0 (E).
Figure 6. Structural sketch of NPs adsorbed on PDDA (A), PEI (B), and APTES (C) modified silicon wafers.
selected organic molecules. However, the accurate assessment of the contribution of specific interactions remains a challenge because of the problems with their experimental isolation. Previously we were able to demonstrate the role of hydrophobic interactions in the assembly of NPs.11 In this work, the choice of adsorption promoters allowed us to reveal the effect of the macromolecular nature of the polyelectrolyte as opposed to the short-chain adsorption promoters. It should be emphasized that the collected data indicate the necessity to take into account the dynamic restructuring of the macromolecular chains especially for weak PEs. The comparison of the PEI and APTES shows the presence of the detached PE segments resulting in the greater roughness of NP films. We also observed the ability of the PE film to adjust its chain distribution and accumulate around the adsorbate, thereby increasing the attraction forces. The high quality of the NP films obtained on strong PEs, such as PDDA, should be attributed to the high charge and low mobility of the adsorbed PDDA chain
unless extreme ionic conditions are in place.34 It can be expected that some structural reorganization of the PDDA monolayer can occur at the point of immediate contact with NPs to increase the work of attractive long-distance electrostatic forces; however other than AFM, experimental methods such as grazing angle FTIR are needed for its assessment. Acknowledgment. N.A.K. thanks NSF CAREER (CHE-9876265), NSF Biophotonics Initiative (BES0119483), AFOSR (F49620-99-C-0072), and OCAST (AR99(2)-026) for the financial support of this research. Supporting Information Available: AFM image of the area with a scratch in CdSe nanoparticle film adsorbed on PDDA. This material is available free of charge via the Internet at http://pubs.acs.org. LA025601D
