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Langmuir 2006, 22, 3028-3034
Structure and Mechanism of the Deposition of Multilayers of Polyelectrolytes and Nanoparticles Basel Abu-Sharkh* Department of Chemical Engineering, KFUPM, Dhahran 31261, Saudi Arabia ReceiVed July 23, 2005. In Final Form: February 2, 2006
MD simulation of the layer-by-layer assembly of polyelectrolytes (PEs) and nanoparticles (NPs) revealed that the assembly process is electrostatically driven with alternating charge reversal and an overcompensation mechanism. Layers were observed to grow in the lateral direction as well as in a direction normal to the surface. Weakly adsorbed PE molecules were observed to desorb from the flat and NP surfaces. Those molecules are attracted by suspended NPs in solution. PE molecules do not only pull NPs toward the surface but bridge NPs both in solution and on the surface, forming agglomerates and islands. The first double layer differs in structure from the second double layer as a result of strong adsorption of the PE molecules to the rigid surface.
1. Introduction Layer-by-layer assembly (LBL) is one of the most important methods of thin-film deposition.1 This technique has been successfully used for the deposition of oppositely charged polyelectrolytes (PEs), nanoparticles (NPs), and various other materials. The method uses very simple equipment and produces high-quality films with thickness that can be controlled on the nanometer scale. Unlike other techniques of NP-polymer composite formation, LBL produces highly homogeneous composite films of PEs and NPs. Experimental studies of the multilayers formed by PEs and NPs show that the thickness of the layers follows a linear trend and that the thickness of each monolayer corresponds to the diameter of the nanoparticle or the thickness of a monolayer of the PE chain.2-4 In some cases, two to three monolayers are formed in each deposition cycle, indicating the looping/ entanglement of charged polymer chains with charged nanoparticles.5-7 In the case of large NPs, for example, yittrium iron garnet and other large hydrophilic NPs made from oxides, the thickness obtained was noted to be significantly lower than the average value expected for densely packed layers of the same diameter, and very low surface coverage was observed. However, stable growth of the NPs with poly(diallyl dimethylammonium) chloride (PDDA) was observed for 50 depositions. A study of the effect of NP size on the structure of the multilayer film indicated that the density and thickness of the films as well as the adsorption kinetics appear to be strongly dependent on the size of the particles, with smaller particles favoring the formation of smooth, dense films with a higher content of NPs.8 The growth of YIG films prepared by layer-by-layer assembly was found to * E-mail:
[email protected]. (1) Kotov, N. A. In Multilayer Thin Films; Decher, G., Schlenoff, J. B., Eds.; Wiley-VCH: Weinheim, Germany, 2003. (2) Lvov, Y. M.; Rusling, J. F.; Thomsen, D. L.; Papadimitrakopoulos, F.; Kawkami, T.; Kunitake, T. Chem. Commun. 1998, 1229-1230. (3) Teldeshi, C.; Mohwald, H.; Kirstein, S. J. Am. Chem. Soc. 2001, 123, 954-960. (4) Fang, M.; Kim, C. H.; Saupe, G. B.; Kim, H. N.; Waraksa, C. C.; Miwa, T.; Fujishima, A.; Mallouk, T. E. Chem. Mater. 1999, 11, 1526-1532. (5) Rogach, A. L.; Koktysh, D. S.; Harrison, M.; Kotov, N. A. Chem. Mater. 2000, 12, 1526-1528. (6) Serizawa, T.; Takeshita, H.; Ahashi, M. Langmuir 1998, 14, 4088-4094. (7) Hicks, J. F.; Shon, Y. S.; Murray, R. W. Langmuir 2002, 18, 2288-2294.
occur via two deposition modes: sequential adsorption of densely packed layers (normal growth mode) and in-plane growth of isolated particle domains (lateral expansion mode). Microscopy results indicate that lateral growth is based on the interplay of particle/particle and particle/polyelectrolyte interactions rather than on a substrate effect. The lateral expansion mode is a general attribute of layer-by-layer deposition and can be observed for various aqueous colloids. The switch from lateral to normal growth mode was found to be effected by grafting charged organic hydrophobic groups to YIG nanoparticles. Hydrophobic interactions between the hydrocarbon groups of the modified YIG and polyelectrolyte significantly increase the attractive component of the particle/polyelectrolyte and particle/particle interactions. The effect of ionic strength on the adsorption behavior and structure formation of PE/NP films was also investigated.9 Low ionic strength solutions gave stable adsorbed films with a reproducible stratified multilayer structure. The films formed in high ionic strength solutions were initially much thicker but also less stable. A significant desorption was observed to take place in conjunction with the second exposure to NPs.10 Despite numerous experimental investigations of the structure of PE/NP multilayer assemblies, the theoretical models of electrostatic self-assembly are still very limited. All of the theoretical studies considered only oppositely charged PE multilayer assembly.11-14 None of these models describes the assembly of PE/NP multilayers. Computer simulation is a valuable tool that can give insight into the molecular phenomena and mechanism of PE/NP multilayer formation. It can be used to elucidate factors influencing the self-assembly, explain some experimental observations, and verify theoretical models. Very few MC and MD simulation studies have been devoted to investigating multilayer formation (8) Bogdanovic, G.; Sennerfors, T.; Zhmud, B.; Tiberg, F. J. Colloid Interface Sci. 2002, 255, 44-51. (9) Sennerfors, T.; Bogdanovic, G.; Tiberg, F. Langmuir 2002, 18, 64106415. (10) Ostrander, J. W.; Mamedov, A A.; Kotov, N. A. J. Am. Chem. Soc. 2001, 123, 1101-1110. (11) Netz, R. R., Joanny, J. F. Macromolecules 1999, 32, 9013-9025. (12) Park, S. Y.; Rubner, M. F.; Mayes, A. M. Langmuir 2002, 18, 96009604. (13) Castlenov, M.; Joanny, J. F. Langmuir 2000, 16, 7524-7532, (14) Solis, F. J.; del la Cruz M. O. J. Chem. Phys. 1999, 110, 11517-11522.
10.1021/la052004t CCC: $33.50 © 2006 American Chemical Society Published on Web 03/01/2006
Polyelectrolyte/Nanoparticle Multilayer Deposition
on spherical, planar, and cylindrical surfaces.15-20 However, no molecular simulation studies have been reported for the multilayer assembly of PEs and NPs. The objective of this article is to investigate the mechanism of formation of PE/NP multilayers and visualize the interaction between the NP and PE molecules during the deposition process. A second objective of this study is to investigate the structure of the multilayers deposited from salt-free solutions on a flat surface.
Langmuir, Vol. 22, No. 7, 2006 3029
term 1:
∑i K2(bi - b0)2 + K3(bi - b0)3 + K4(bi - b0)4 +
term 2:
∑i H2(θi - θ0)2 + H3(θi - θ0)3 + H4(θi - θ0)4+
term 3:
∑i V1[1 - cos(φi - φ01)] + V2[1 - cos(2φi - φ02)] + V3[1 - cos(3φi - φ03)] + qiqj
2. Simulation Details
term 4:
∑ i>j r
+
ij 2 ∑ i>j
σij
r ij
We conducted a coarse-grained MD simulation of the LBL assembly of polyelectrolyte molecules and nanoparticles from dilute solutions. The polyelectrolyte chains consist of Np ) 64 monomer beads. The absolute value of the charge on each charged monomer bead is equal to 1. A coarse-grained system was used to reduce the overall number of particles in the system and subsequently to reduce the number of computations per time step. In addition, the overall dynamics of the system is accelerated because the free-energy profile in the system is less bumpy.21-23 The diameter of a bead σ in the chain is 4 Å, which corresponds to 1.6 monomers of NaPSS with a monomer size of 2.5 Å. The nanoparticle was modeled using a C60 fullerene sphere. Two surface charge densities were used for nanoparticles: a high charge of -30 corresponding to a single-charge monovalent charge on every second carbon and a low charge of -6 corresponding to six monovalent charged beads of C60 distributed uniformly over the surface. All charges in the system are monovalent, discrete, and fixed on the beads. The diameter of the particle is 7.114 Å. Each bead of the C60 NP or PE has a mass of 12 atomic mass units. Increasing the mass of each bead to 72 was not found to influence the equilibrium configuration of the system. Multilayers were deposited from dilute solutions with a volume fraction of 0.05 on a hexagonal packed surface composed of 289 spherical particles that are constrained in place. The dimensions of the surface are 42.8 Å × 42.8 Å. Each bead has a mass of 12 atomic mass units. The surface is located at Z ) 0. The total surface charge is -144, corresponding to a negative monovalent unit charge on every other bead. A neutral soft repulsive wall was placed at the top of the simulation box to avoid the escape of counterions and chains to the lower side of the charged surface. The upper wall is identical to the lower surface with the exception that it interacts with other particles by a force field that corresponds to the repulsive term of eq 5. A 100-Å-thick layer of vacuum was placed on top of the neutral wall to eliminate interaction between the lower side of the charged wall and particles inside the simulation box. The force field used to model the chains is a simplified form of the polymer-consistent force field (PCFF) described by an equation of the form24,25 (15) Messina, R. Macromolecules 2004, 37, 621. (16) Messina, R.; Holm, C.; Kremer, K. Langmuir 2003, 19, 4473. (17) Messina, R. J. Chem. Phys. 2003, 119, 8133. (18) Panchagnula, V.; Jeon J.; Dobrynin, A. V. Phys. ReV. Lett. 2004, 93, 037801. (19) Panchagnula, V.; Jeon, J.; Rusling, J. F.; Dobrynin, A. V. Langmuir 2005, 21, 1118-1125. (20) Abu-Sharkh, B. J. Chem. Phys. 2005, 123, 114907/1-114907/6. (21) Marrink, S. J.; Mark, A. E. J. Am. Chem. Soc. 2003, 125, 15233-15242. (22) Stevens, M.; Hoh, J. H.; Woolf, T. B. Phys. ReV. Lett. 2003, 91, 188102188104. (23) Aksimentiev, A.; Schulten, K. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 4337-4338. (24) Sun, H.; Mumby, S. J.; Maple, J. R.; Hagler, A. T. J. Am. Chem. Soc. 1994, 116, 2978. (25) Sun, H. J. Comput. Chem. 1994, 15, 752.
term 5:
[ ( ) ( )] rij
9
-3
σij
6
rij
(1)
The force field employs a quartic polynomial for bond stretching (term 1) and angle bending (term 2) and a three-term Fourier expansion for torsions (term 3). Term 4 is the Coulombic interaction between the atomic charges, and the attractive part of term 5 represents the van der Waals interactions. The force field parameters of the chain are given in Table 1.24,25 Electrostatic interactions between charged beads are calculated using the Ewald sum method.23 A relative dielectric constant r of 80 was used to account for the screening of charges by an implicit solvent (water).24 van der Waals cross-interaction parameters are calculated using24,25
(σi3σj3) ij ) 2xij 6 (σi + σj6) σij )
(σi6 + σj6)1/6 2
Nonbonded van der Waals interactions were calculated with a cutoff distance of 2.5 σ, where σ is the diameter of a chain bead. Standard long-range corrections were applied.25 Simulations were carried out in the NVT ensemble with periodic boundary conditions in three dimensions. A constant temperature was accomplished by linking the system to the Andersen thermostat.26 A simulation time step of 3 fs was used. Simulations were performed using a procedure that is similar to the procedures used for the simulation of polyelectrolyte multilayers.15-20 The procedure resembles the experimental deposition of multilayers that proceeds by the immersion of a charged substrate into a dilute polyelectrolyte solution followed by a rinsing step to remove excess, unadsorbed molecules and finally immersion in a dilute suspension containing the nanoparticles followed by a second rinsing step. The charged surface was constructed, and its counterions were dispersed throughout the simulation box. Ten positively charged polyelectrolyte molecules were then inserted into the box along with their counterions. The concentration of chains was kept at a volume fraction of 0.05. The simulation box was subsequently equilibrated for 20 ns, during which time equilibration was confirmed by monitoring the total energy and concentration profiles of the various species in the system. Unadsorbed polyelectrolyte molecules were then removed along with their counterions, representing a rinsing step. These molecules were deleted from the simulation box. Equilibrating the system for 200 ps following deletion of the unadsorbed molecules was not found to cause any desorption or major reorganization of adsorbed molecules. Twenty (26) Karasawa, N.; Goddard, W. A. Macromolecules, 1992, 25, 7268.
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Abu-Sharkh Table 1. Force Field Parameters
T ) 300 K mass C Fe
12.011 55.847
bond potential C-C
b0 1.53
K2 299.67
K3 -501.77
K4 679.81
angle potential C-C-C
φ0 112.67
H2 39.516
H3 -7.443
H4 -9.5583
torsion potential C-C-C-C
V1 0
φ01 0
V2 0.0514
φ02 0
nonbond interactions Fe C
σ 2.6595 4.0100
13.889 0.0540
ij ) 2xij σij )
V3 -0.143
φ03 0
(σi3σj3) (σi6 + σj6)
(σi6 + σj6)1/6 2
(high charge) or 80 (low charge) negatively charged nanoparticles were subsequently added to the box along with their oppositely charged counterions at a volume fraction of 0.05. The system was again equilibrated for 20 ns. Unadsorbed NPs were then removed along with their counterions, representing a rinsing step. After a molecule or a nanoparticle was adsorbed on the surface, the volume of the simulation cell was adjusted so that the concentration of molecules in the solution phase remained constant. In addition, it was observed that the molecules that are close to the surface interact with the surface whereas molecules that are far away do not feel the presence of the surface. As a result, after adjusting the volume of the simulation cell, the molecules and nanoparticles in the solution phase were redistributed uniformly throughout the liquid phase to simulate a real solution in which molecules are uniformly distributed. This process was repeated three or four times until no more adsorption was observed, indicating equilibrium between the surface and the solution. Another layer of the PE and nanoparticles was subsequently added using the same procedure described above. Overall charge neutrality was always maintained in the system using counterions. The four depositions represent two complete dipping cycles. After completing the depositions, the system was further annealed for a total of 20 ns. Equilibration was confirmed again by monitoring the energy and concentration profiles in the system. Deciding which molecules are adsorbed was based on the following criterion: If a bead or more of a PE molecule or an NP is less than a distance of 1.1σ from a surface particle or an already adsorbed PE molecules, then that molecule is considered to be adsorbed on the surface. Otherwise, the molecule or NP is considered not to be adsorbed and is removed in the rinsing step.
3. Results and Discussion The first deposition step of the cationic PE on the negatively charged substrate resulted in the formation of a layer of adsorbed molecules that assume three different conformations: train, loop, and tail as shown in Figure 1. The tails and loops extend into the solution phase in the absence of added salt because of strong intramolecular electrostatic repulsion. However, the train sections of the chains adhere to the surface. These structures have been experimentally observed and reported in the literature for PE molecules adsorbed on charged surfaces.27
During the initial stages of the highly charged NP deposition step, three processes were observed to occur (Figure 2). Initially, the extended tail segments reach for an NP and start adsorbing and folding at its surface (Figure 2a). Once this adsorption takes place, the NP either pulls the PE from the surface or the PE pulls the NP toward the surface (Figure 2b and c). The mechanism that dominates depends on the fraction of the PE molecule that is in direct contact with the surface in the train conformation. Molecules with a small train fraction were easily pulled away from the surface. It is expected that this process depends on many factors, for example, charges on the surface, PE, and NP in addition to the sizes of the PE molecule and NP. Molecules that are strongly anchored to the surface start folding around the NP, bringing it gradually closer to the surface. It was also observed that PE molecules pulled from the surface bridge NPs and bring them together in small agglomerates composed of two to three NPs. Those globules are removed from the simulation box during the rinsing step (Figure 2d). The desorption of PE molecules or NPs from previously deposited layers has been experimentally observed.1 Upon adsorption of the nanoparticles, the net charge of the system changes from +112 to -132, corresponding to the adsorption of six NPs and the desorption of one cationic PE molecule. Deposition of the particles with low charge proceeded following a similar mechanism; however, much less desorption of PE chains by NPs was observed. Deposition mostly proceeded by the adsorption of a PE end on an NP surface, followed by folding and gradual pulling of the NP toward the surface. The second cationic PE layer was deposited using a procedure similar to the one described earlier. The mechanism of deposition in this step proceeded by the initial adsorption of one or both ends of the PE chain on the surface of a nanoparticle(s) or an uncovered surface, followed by gradual folding and adsorption of the chain on the surface of the NP. In this case, the NP appeared to be very stable on the surface, and no desorbtion of NPs was observed. Similar to the first PE layer, some tail segments were also extended to the solution phase. A second nanoparticle layer was subsequently deposited using the same procedure described earlier. The three processes that were observed during the first deposition cycle are also observed during the second layer deposition. In addition, some NPs were deposited in the uncovered areas of the surface. Furthermore, the NPs were stacked not only parallel but (27) Klitzing, R. V.; Steitz, R. In Handbook of Polyelectrolytes and Their Applications; Tripathy, S. K., Kumar, J., Nalwa, H. S., Eds.; American Scientific Publishers: Stevenson Ranch, CA, 2002; Vol. 1.
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Figure 1. Structure of the first cationic PE layer.
Figure 3. Structure of the PE/NP system with (a) high-charge NPs and (b) low-charge NPs.
Figure 2. Mechanism of deposition of the first layer of nanoparticles.
also perpendicular to the surface. Because many nanoparticles are bridged together by the polyelectrolyte, both in solution and on the surface, the topolgy of the deposited group depends on the topology of the bridged particles. In general, the particles assemble together in the form of islands that are deposited horizontally or vertically on the surface. Islands containing up to three NPs bridged by one or more chains were observed. A similar formation of islands has been observed experimentally.15 After a very long equilibration time of 100 ns, the NP and PE system forms a highly ordered system in which particles are assembled in well-defined layers. It is also observed that one chain from the first polymer layer can extend to encapsulate nanoparticles from the first and second layers. The system with low-charge NPs provided higher surface coverage, and many more NP were deposited in the first cycle compared to the number of strongly charged NPs. Figure 3 shows the conformation of the simulation box after two deposition cycles for the strongly charged NP system (a) and after four cycles for the system with low NP charge (b). The different chain colors in Figure 3b indicate PE chains deposited at different cycles. It can be observed that more than one layer can be deposited in one deposition cycle. A similar phenomenon was observed during the deposition of HgTe- and citrate-stabilized gold NPs in addition to large latex colloids
deposited from solutions in the presence of salt.5,6 It is also observed that well-defined layers are formed in both cases. We also observed that the strongly charged NPs do not completely cover the substrate in the first deposition and that the NPs are not densely packed as shown in Figure 4a. A similar phenomenon was observed for 32 nm yittrium iron garnet (YIG) NPs as well as for some other large hydrophilic NPs made of oxides. However, the density of the NPs was high enough to reverse the surface charge as shown in Figure 5. It is also observed in Figure 4 that the NPs form island. A similar phenomenon was observed using scanning electron microscopy of PDDA/barium ferrate NPs and PDDA/latex films. Our simulations show that such islands probably start to form in the solvent phase before the NPs are deposited onto the substrate. However, some of the surface area that was not covered in the first deposition cycle was covered by PE and NP after the second deposition cycle (Figure 4b). Figure 5 shows the net charge of the system with highly charged NPs (excluding counterions) after each deposition step. It can be observed that the level of charge reversal is nearly uniform for the PE and NP steps. The charge reversal effected by the NP layer is slightly higher than that caused by the PE layer. The concentration profiles in the Z direction of the NP and PE beads are shown in Figure 6, part a for the system with highly charged NPs and in part b for the system with low-charge NPs. Concentration profiles of beads as a function of the distance from the plate (Z) are calculated by taking volumetric slices of thickness 0.2 Å in the Z dimension. A histogram of Z positions was then constructed as a trajectory-averaged quantity. Number densities are therefore in units of beads/Å3. The sharp peaks of the first PE and NP layers result from the adsorption of PE tail segments on the rigid surface and the subsequent adsorption of
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Abu-Sharkh
Figure 4. Top view of (a) the first double layer and (b) two double layers of the system with high-charge NPs.
Figure 6. Concentration profiles of the cationic PE and NP normal to the surface for the system with (a) high-charge NPs and (b) lowcharge NPs.
Figure 5. Net charge after the first, second, and third depositions in the system with high-charge NPs.
NPs onto those PE segments. The subsequent layers are more flat and have a more uniform distribution of concentration. In addition, significant overlap and penetration of the two components are observed. The lateral expansion of the NP layer in the system containing high-charge NPs is illustrated in Figure
7, which shows the concentration profiles of the PE molecules and NPs after the depositions of the first and second layers. It can be clearly seen that the concentration of both PE molecules and NPs in the first layer (first peaks in Figure 7a and b) increases after the deposition of the second layer, indicating that more PE molecules and NPs were deposited in the first layer during the second cycle of depositions. Figure 8 shows the intermolecular radial distribution function of the PE segments and NP beads. Comparison between the low-charge and high-charge NP systems indicates that the NPs are more strongly correlated in the low-charge system. NPs are more closely packed and can approach one another more easily than in the high-charge NP system because of less-repulsive interactions. Furthermore, the PE molecules are more strongly correlated at short distances in the high-charge NP system because many of them need to assemble on the highly charged NP surface in order to neutralize it, resulting in close proximity of the PE molecules. In addition, the need for more PE segments to
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Figure 7. Concentration profiles of (a) the PE and (b) the NPs after deposition of one layer and two layers.
neutralize the highly charged NP causes the NP-PE correlation to be stronger in the high-charge NP system. Figure 9 shows the normalized distribution of the radius of gyration of PE molecules in the system with low charge. A bimodal curve can be observed. The second peak corresponds to the molecules that are adsorbed or are near the surface. These molecules are more expanded as a result of the adsorption of a large part of them on the flat surface in a stretched conformation, resulting in a large radius of gyration. The first peak corresponds to molecules in the second layer and subsequent layers. Molecules in these layers are less expanded because they fold around the NPs, resulting in a smaller radius of gyration. The local orientation of chains relative to the surface may be measured using the orientation correlation function. To this end, we define unit vectors between adjacent monomers:
u)
ri - r i - 1 |ri - ri - 1|
(2)
Figure 8. Intermolecular radial distribution function of the system with (a) high-charge NPs and (b) low-charge NPs.
The scalar product between two such unit vectors describes the angle between the chain tangent vector and the surface vector:
cos R(r) ) uchain‚usurface
(3)
The distance r denotes the distance between the centers of mass of the chain segment and the surface vector. The orientation correlation is defined by using the second Lengendre polynomial:
1 P2(r) ) [3 cos2 R(r) - 1] 2
(4)
Figure 10 shows the orientation correlation function of the three first PE layers in the low-charge NP system. For the first layer, it can be observed that a near-perfect parallel orientation is observed at short distances (P2 ≈ 1). This results from the strong adsorption of PE segments on the surface. As the distance between the surface and chain segments increases, the parallel orientation is maintained to a lower extent as indicated by the positive values
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Figure 9. Normalized distribution of the radius of gyration of PE molecules in the system with low-charge NPs. The first peak corresponds to molecules in the second layer and subsequent layers. The second peak corresponds to molecules in the first layer.
Figure 10. Orientation correlation function of the first, second, and third layers with the surface in the system with low-charge NPs.
of P2 even as far as 20 Å from the surface. The second layer also shows a high level of orientation (P2 ≈ 0.8) with the surface at a short distance of 11 Å, but a perpendicular orientation is observed at a distance of 17 Å. This parallel orientation is associated with segments that are adsorbed on the sides of the NPs. The third layer also shows a high level of parallel orientation (P2 ≈ 0.8) with the surface at short distances (21 Å). Figure 11 shows the orientation correlation function between PE molecules deposited in subsequent layers. The negative P2 at closest contact indicates near-parallel orientation. This parallel orientation is the only possible orientation of segments at closest contact. The value of P2 increases to 0.6 for the first- to secondlayer correlation and to 0.4 for the second- to third- and thirdto fourth-layers correlations. The surface effect is apparent in the first- to second-layer orientation correlation.
Abu-Sharkh
Figure 11. Orientation correlation function of the first with second, second with third, and third with fourth layers in the system with low-charge NPs.
4. Conclusions In conclusion, multilyers of PE and NPs were found to be electrostatically driven with alternating charge reversal and an overcompensation mechanism. Layers were observed to grow in the lateral direction as well as normal to the surface. Lateral growth is facilitated by the adsorption of more PE molecules on the uncovered areas of the surface. The newly adsorbed PE molecules attract more oppositely charged NP particles to the surface, resulting in lateral growth. Growth normal to the surface is caused by PE chains adsorbed onto NP surfaces. Weakly adsorbed PE molecules were observed to desorb form the flat and NP surfaces. Those molecules are attracted by suspended NPs in solution. PE molecules do not only pull NPs toward the surface but also bridge NPs both in solution and on the surface, forming agglomerates and island. The first double layer differs in structure from the second double layer as a result of a strong surface effect. The charge of the NP has a strong influence on the structure of the multilayers. High charge leads to the formation of island and incomplete coverage of the surface whereas the system with low NP charge tends to provide more surface coverage. This might be a result of strong repulsive interactions between different NPs and NPs with similarly charged surfaces. The lateral repulsion of NPs prevents the packing of NPs on the surface, thus islands are formed. This strong repulsion is not sufficiently neutralized by the PE, resulting in the adsorption of only a small number of NPs in each step. This problem is not encountered in the low-NP-charge system, and as a result, more surface coverage is feasible. There are similarities between the NP-PE and PEPE multilayer systems. For example, the mechanism of deposition is very similar, in which a particle or part of the PE chain is initially adsorbed, followed by reorientation and packing on the surface. Rearrangement and desorption of weakly adsorbed chains is also observed in both cases, although NPs demonstrate a stronger ability to desorbing previously adsorbed chains than oppositely charged PEs. In both cases, the first two layers have a structure that is different from that of subsequent layers. The first layers are well structured, and their structure is strongly influenced by the highly ordered wall structure. LA052004T
