pubs.acs.org/Langmuir © 2010 American Chemical Society
Structural Characterization of a Spin-Assisted Colloid-Polyelectrolyte Assembly: Stratified Multilayer Thin Films )
M. Kiel,† S. Mitzscherling,† W. Leitenberger,† S. Santer,† B. Tiersch,‡ T. K. Sievers,§ H. M€ohwald,§ and M. Bargheer*,†, Institute of Physics and Astronomy, and ‡Institute of Chemistry, University of Potsdam, Potsdam, Germany, § Max-Planck Institute of Colloids and Interfaces, Potsdam, Germany, and Helmholtz Centre Berlin, Berlin, Germany )
†
Received September 8, 2010. Revised Manuscript Received October 12, 2010 The assembly of polyelectrolytes and gold nanoparticles yields stratified multilayers with very low roughness and high structural perfection. The films are prepared by spin-assisted layer-by-layer self-assembly (LbL) and are characterized by X-ray reflectivity (XRR), UV-vis spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy (TEM). Typical structures have four repeat units, each of which consists of eight double layers (DL) of poly(sodium 4-styrenesulfonate)/poly(allylamine hydrochloride), one monolayer of gold nanoparticles (10 nm diameter), and another layer of poly(allylamine hydrochloride). XRR scans show small-angle Bragg peaks up to seventh order, evidencing the highly stratified structure. Pronounced Kiessig fringes indicate a low global roughness, which is confirmed by local AFM measurements. TEM images corroborate the layered structure in the growth direction and nicely show the distinct separation of the individual particle layers. An AFM study reveals the lateral gold particle distribution within one individual particle layer. Interestingly, the spin-assisted deposition of polyelectrolytes reduces the roughness induced by the particle layers, leading to self-healing of roughness defects and a rather perfect stratification.
Introduction Ultrathin polymer films with inorganic nanoparticles have attracted much attention during the last few decades because of their unique electronic, optical, and catalytic properties.1-3 The versatility of these films has been demonstrated in numerous applications (e.g., photonic structures,4,5 surface processing,6,7 electronic and electrochemical devices,8-12 and medical applications13). Often, these applications require the nanoparticles to be arranged in stratified 2D monolayers. Spin-assisted layer-by-layer deposition (LbL) allows for the preparation of highly stratified multilayered films14 in a time- and effort-efficient way at low cost. Very robust structures can be produced by the deposition of oppositely charged molecules (polyelectrolytes). Early on, the incorporation of nanoscale objects with surface charges into polyelectrolyte films15 and the preparation of a multilayered structure with three alternating layers of gold nanoparticles and polyelectrolytes that showed a
few Bragg peaks originating from the layer spacing of the particles in the XRR characterization were reported.16,17 The idea to use polymers to separate particle layers by well-defined spacers with nanometer thickness has been transferred to semiconducting18,19 and magnetic20,21 particles. Only very few cases have demonstrated perfect assemblies indicated by the presence of high-order Bragg reflections for CdS in polyelectrolytes14 or CdTe in phospholipids.22 Situations with very small particle diameter and very thick polymer spacers naturally led to perfect stratification as evidenced by TEM.23 Here, we present the preparation and characterization of a multilayered structure composed of gold nanoparticles and ultrathin polyelectrolyte spacer layers that still ensure the clear separation of the gold particles. The well-defined separation of gold-containing layers (i.e., the stratification of the sample) was verified by XRR and TEM experiments. Polyelectrolyte layers with a thickness comparable to the particle diameter are sufficient to separate particles of adjacent layers.
*Corresponding author. E-mail:
[email protected].
(1) Hammond, P. T. Adv. Mater. 2004, 16, 1271–1293. (2) Srivastava, S.; Kotov, N. A. Acc. Chem. Res. 2008, 41, 1831–1841. (3) Kinge, S.; Crego-Calama, M.; Reinhoudt, D. N. ChemPhysChem 2008, 9, 20–42. (4) Wang, T. C.; Cohen, R. E.; Rubner, M. F. Adv. Mater. 2002, 14, 1534–1537. (5) Nolte, A. J.; Rubner, M. F.; Cohen, R. E. Langmuir 2004, 20, 3304–3310. (6) Crisp, M. T.; Kotov, N. A. Nano Lett. 2003, 3, 173–177. (7) Hattori, H. Adv. Mater. 2001, 13, 51–54. (8) Yu, A.; Liang, Z.; Cho, J.; Caruso, F. Nano Lett. 2003, 3, 1203–1207. (9) Wohltjen, H.; Snow, A. W. Anal. Chem. 1998, 70, 2856–2859. (10) He, J.-A.; Mosurkal, R.; Samuelson, L. A.; Li, L.; Kumar, J. Langmuir 2003, 19, 2169–2174. (11) Rogach, A. L.; Koktysh, D. S.; Harrison, M.; Kotov, N. A. Chem. Mater. 2000, 12, 1526–1528. (12) Liang, Z. Q.; Dzienis, K. L.; Xu, J.; Wang, Q. Adv. Funct. Mater. 2006, 16, 542–548. (13) Sinani, V. A.; Koktysh, D. S.; Yun, B.-G.; Matts, R. L.; Pappas, T. C.; Motamedi, M.; Thomas, S. N.; Kotov, N. A. Nano Lett. 2003, 3, 1177–1182. (14) Cho, J.; Char, K.; Hong, J.-D.; Lee, K.-B. Adv. Mater. 2001, 13, 1076–1078. (15) Feldheim, D. L.; Grabar, K. C.; Natan, M. J.; Mallouk, T. E. J. Am. Chem. Soc. 1996, 118, 7640–7641.
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Preparation The samples were fabricated by the layer-by-layer technique (LbL). We embedded gold nanoparticles (10 nm in diameter) into a matrix of anionic poly(sodium 4-styrenesulfonate) (PSS, (16) Decher, G. Science 1997, 277, 1232–1237. (17) Schmitt, J.; Decher, G.; Dressick, W. J.; Brandow, S. L.; Geer, R. E.; Shashidhar, R.; Calvert, J. M. Adv. Mater. 1997, 9, 61–65. (18) Joly, S.; Kane, R.; Radzilowski, L.; Wang, T.; Wu, A.; Cohen, R. E.; Thomas, E. L.; Rubner, M. F. Langmuir 2000, 16, 1354–1359. (19) Lvov, Y.; Ariga, K.; Onda, M.; Ichinose, I.; Kunitake, T. Langmuir 1997, 13, 6195–6203. (20) Grigoriev, D.; Gorin, D.; Sukhorukov, G. B.; Yashchenok, A.; Maltseva, E.; Mohwald, H. Langmuir 2007, 23, 12388–12396. (21) Mamedov, A. A.; Kotov, N. A. Langmuir 2000, 16, 5530–5533. (22) Yuan, B.; Xing, L.-L.; Zhang, Y.-D.; Lu, Y.; Mai, Z.-H.; Li, M. J. Am. Chem. Soc. 2007, 129, 11332–11333. (23) Krishnan, R. S.; Mackay, M. E.; Duxbury, P. M.; Pastor, A.; Hawker, C. J.; Van Horn, B.; Asokan, S.; Wong, M. S. Nano Lett. 2007, 7, 484–489.
Published on Web 11/08/2010
DOI: 10.1021/la103609f
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Figure 1. Preparation steps for spin-assisted layer-by-layer assembly. Mw = 70 000 g/mol) and cationic poly(allylamine hydrochloride) (PAH, Mw = 56 000 g/mol). The polyelectrolytes and the gold nanoparticle solution (cw = 0.01%) were purchased from SigmaAldrich. PSS was dialyzed before use. PAH was used as purchased. Fused silica glass substrates were cleaned in an ultrasonic bath of acetone for 1 min, rinsed with purified water (Milli-Q), and dried with N2. Mica sheets were cleaned by pulling off the top layer with tape. The area of the substrates was typically 20 20 mm2. Spin-assisted multilayer assembly was carried out as follows. Unless otherwise noted, a constant spin speed of 3000 rpm was used. Each substrate was first coated with a poly(ethyleneimine) solution (PEI, cw = 1%, no salt) to compensate for small surface inhomogeneities in charge density. While spinning, 0.02 mL of the polyelectrolyte solution was dropped onto the sample. After 10 s, purified water was dropped onto the sample three times to wash off the residual solution and excess polyelectrolyte. In between each adsorption step, the samples were dried by waiting another 30 s while spinning. Multilayers with the composition substrate/PEI[(PSS/PAH)nAuNP/PAH]m were prepared by alternatingly spin coating with negatively charged PSS and positively charged PAH solutions (cw = 0.1%, 1 M NaCl) (Figure 1). Because of their negative surface charge, the gold nanoparticles were always adsorbed after PAH layer deposition. For this purpose, the spin coater was stopped and the sample was wetted with approximately 0.5 mL of the gold suspension. After 30 min, the suspension was spun off at 1000 rpm and the sample was rinsed with purified water (Milli-Q). The low rotational speed required N2 gas flow to support the drying process.
Results and Discussion Layering and Distribution along the z Growth Direction. The sample to be analyzed by XRR has the composition PEI[(PSS/PAH)8Au-NP/PAH]4 and was prepared as described in the
Preparation section. The X-ray reflectivity measurements were made at the BESSY II synchrotron radiation facility in Berlin. X-ray reflectivity θ-2θ scans were performed using monochromatic X-rays with a photon energy of 8774 eV derived from a single silicon crystal monochromator at the EDR beamline. XRR measurements were interpreted by simulation of XRR spectra from electron density profiles using the Paratt formalism.24 The measured XRR spectrum shows Bragg peaks up to seventh order (Figure 2a), which proves a well-stratified layering. In an ideal structure, the electron density of the sample along the z growth direction would essentially be given by the sum of the homogeneous polymer electron density distribution and a parabolic electron density profile originating from the gold spheres with identical size (10 nm) and position along the z direction. The electron density of this ideal structure is shown as a green dashed line in Figure 2c, and the resulting XRR curve is given with the same color code in Figure 2a. The correct Bragg peak positions verify the assumed multilayer thickness; however, the lower measured intensities reflect imperfections in the positioning and size of the particles in the film. We account for a slight disorder of the nanoparticles by convoluting the perfect electron density profile with a Gaussian function. Using a fully statistical displacement of particles out of the perfect layering plane according to a Gaussian would allow the first three Bragg peaks in the experimental data to be well fitted, but the higher orders would vanish (not shown). Figure 2c shows the electron density profile (solid black line) that was used to fit the data in Figure 2a (solid black line). This profile represents a compromise between the two extremes. It is the sum of an ideally ordered structure (30%) and a structure in which the gold particles are smeared out by a Gaussian function with a fwhm of Δz = 8.4 nm (70%). This Δz value is a measure of the vertical positioning error and the size variation of the particles. It can be seen in Figure 2c that between two gold layers the electron density still decays down to the pure polymer value indicated by the vertical dotted line. Note that in this fitting procedure we still assume that all gold layers are equal distances from each other. Despite this restriction, the simulation and experiment for the XRR curve match for this sample very
Figure 2. (a) X-ray reflectivity (XRR) scan (red circles). The first of the seven Bragg peaks is hidden by the total reflection, and the second peak has an intensity of 80%. The green dashed line is a simulation according to an electron-density profile, where an ideal layering of particles is assumed (inset). The solid black line shows a simulation of the real structure, taking into account a statistical distribution of particles in the z growth direction. The pronounced Kiessig fringes prove a small surface roughness and a total sample thickness of 120 nm. The average distance between two adjacent gold layers is 19.4 nm. Gold particles have an average diameter of 10 nm, and their positioning error along z is Δz = 8.5 nm. (b) Schematic cross section of the sample. (c) Electron density along the z direction. The vertical dotted line indicates the average electron density of PAH/PSS, and the green dashed line indicates that of an ideally stratified film, with the electron densities of the individual gold films being purely parabolic. The solid black line shows the electron density of the sample, where the particle density is smeared out by a Gaussian function with a fwhm of 8.4 nm as a result of the imperfect positioning of gold particles in the z direction. The measured film has the composition substrate/PEI[(PSS/PAH)8Au-NP/PAH]4. 18500 DOI: 10.1021/la103609f
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Figure 3. TEM cross section of a stratified film. This film was prepared by dip coating and illustrates over eight polyelectrolyte/ particle layers the influence of polyelectrolyte spacer layers on the roughness. The yellow box highlights the self-smoothing of an intermediate rough surface within two spacer layers of polyelectrolytes. The white bar corresponds to 200 nm.
well. Figure 3 shows the TEM image of a different sample with well-stratified layers; however, here the gold layer spacing is not equidistant. Accordingly, the XRR curve (not shown) cannot consist of sharp Bragg peaks evidencing the artificial periodicity. Particle Distribution in the Plane. The average electron density derived from XRR (Figure 2c) allows us to calculate the particle volume filling factor of 3.4% (VAu/Vtotal). This value is equivalent to a lateral packing density within each gold layer of 19.5 particles per (100 nm)2 or 17% with respect to the closest packed layer (Figure 4c). The calculated packing density is confirmed by the AFM topography shown in Figure 4a. Here, the number of particles is 17.8 per (100 nm)2, which corresponds to a 15.5% packing density. The value of the particle packing density derived from XRR and AFM can be verified by UV-vis transmission/absorption spectroscopy (Figure 5). The measured absorption spectrum (black solid line) shows a maximum at 540 nm due to the gold particle plasmon resonance and a gold interband absorption at λ < 480 nm.25 We also calculated the UV-vis absorption spectrum (red dashed line) by applying the Maxwell-Garnett effective medium theory using the packing density (volume filling factor) derived from XRR and the dielectric constants of gold and of the polyelectrolytes.26 The calculated and recorded spectra are in good agreement, especially with respect to the spectrally integrated absorption, which supports the packing density calculated from our XRR and AFM data. Layering Fluctuations and Surface Roughness. For the calculation of the XRR curve, we assumed an electron density profile, which naturally has a gradient at the film/air interface. This gradient is determined exclusively by the distribution of gold particles that form the topmost layer24 and contains no further assumption on surface roughness from other origins. The simulation using this density profile matches the decay of the XRR curve and the modulation amplitude of the Kiessig fringes. Because the XRR signal is averaged over a large surface area of 10 1 mm2, the electron density profile reflects this area average. Thus, local (24) McMorrow, D.; Als-Nielsen, J. Elements of Modern X-ray Physics; John Wiley & Sons: New York, 2001; p 336. (25) Rechberger, W.; Hohenau, A.; Leitner, A.; Krenn, J. R.; Lamprecht, B.; Aussenegg, F. R. Opt. Commun. 2003, 220, 137–141. (26) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; John Wiley & Sons: New York, 1995.
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Figure 4. (a) Topography of gold nanoparticles on 10 DL of PSS/ PAH as measured with AFM (tapping mode). The area is 410 410 nm2. The white bar represents 20 nm. Notably, the particle diameter is 10 nm but is imaged larger because of the AFM tip effects. (b) Height of the surface along the line in panel a. (c) Schematic of the packing density (17%) of gold particles at the surface according to XRR measurements (where every sixth place is occupied). This is in agreement with the AFM image in panel a.
Figure 5. UV-vis absorption spectrum (solid black line) of a sample with the following composition: substrate/PEI[(PSS/PAH)8Au-NP/ PAH]4(PSS/PAH)8. The red dashed line shows the calculated absorption spectrum of gold particles embedded in a polyelectrolyte matrix. This spectrum was determined with the Maxwell-Garnett effective medium theory.
layering perfection as measured by AFM and TEM can be expected to be even better. Figure 3 shows a TEM cross section of a dip-coated, highly stratified film with eight gold layers and polyelectrolyte spacer thicknesses grown on a glass substrate.27 The spacer-layer thickness varies between 18 and 29 nm (bottom to top). The local layering perfection persists from the substrate up to the surface. The TEM image indicates that the roughness induced by particles can even be reduced by the deposition of several polyelectrolyte layers (yellow box in Figure 3). This self-healing capability for roughness defects greatly supports the formation of stratified films. A higher degree of stratification, (i.e., the clear separation of gold layers) is facilitated by larger spacer layers. Locally, as evidenced by TEM cross sections, the fluctuations in the gold layer thickness can be much smaller than the global average of 8.4 nm as measured by (27) The dip-coated film was chosen for presentation here because it allows for good visualization of the film composition. For spin coating, we used mica substrates, which strongly splintered upon preparation of the TEM samples and thus led to a random distribution of mica splitters in the picture, preventing nice imaging. For the dip-coated film, the same polyelectrolytes were used but at lower concentrations in solution: PEI (10-3 M, 1 M NaCl), PAH (10-3 M, 0.5 M NaCl), and PSS (10-3 M, 0.5 M NaCl). For both assembly methods, we used the same gold particle solution and applied the same layering sequence. For the adsorption of the polyelectrolytes and the nanoparticles, the substrate was immersed in the solutions for 10 and 60 min, respectively. After each adsorption step, the sample was dipped into freshly purified water twice, first for 30 s and then for 10 min.
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Figure 6. Decrease in the surface roughness due to additional layers of PSS/PAH. 0 double layers (DL) correspond to the PAH-capped gold layer, the negative value to the pure gold layer.
XRR. Thus, the spin-coated sample with a spacing of eight double layers, for which the electron density map has been derived from the XRR data (Figure 2), corresponds to the minimum spacer thickness that leads to perfect stratification throughout a large sample area of several millimeters in the sense that between each particle layer the electron density decays to the polyelectrolyte value. The fact that the low local roughness persists up to the sample surface is verified by determining the surface roughness rms of 2.0 nm locally over an area of 410 410 nm2 by AFM (Figure 4) before covering the last gold layer with eight double layers of polyelectrolytes. This rms value corresponds to peak-to-valley modulations that are somewhat larger as illustrated by the outline in Figure 4b. The outline suggests that gold particles with a diameter of 10 nm sink into the polymer surface approximately 5 nm. For the ideal case, where the half spheres stick out of a perfectly flat surface with an in-plane density of 15.4% (as determined by the AFM study above), we calculated an rms value of 0.76 nm. Thus, the observed surface roughness of 2.0 nm is only partially caused by the fact that the gold half-spheres stick out of the surface. As observed in the TEM images, AFM as a local probe also yields a value that is somewhat smaller than the global XRR measurement. Note that the 8.4 nm fwhm of the Gaussian function, which was used to smear out the electron density profile of an ideally layered structure used to simulate the XRR curve, corresponds to a global rms value for positioning fluctuation of 4.2 nm, which is approximately a factor of 2 larger
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than the local value derived by AFM. This indicates a high degree of lateral homogeneity and the flatness of the sample. Smoothing. To investigate the self-healing of roughness defects in detail, we conducted an AFM roughness study. We monitored the change in roughness after the adsorption of a particle layer followed by the deposition of up to eight polyelectrolyte double layers (DL) as capping. The roughness was determined after every PAH adsorption step by evaluating areas of 390 390 nm2 that show no surface defects and no occasional dust particles. Figure 6 depicts the measured surface roughness as a function of the number of double layers as capping. The initial roughness of about 2 nm of the gold layer decreases by approximately a factor of 2. We note here that the XRR (AFM) measurements reported in Figure 2 (Figure 4) have a single (no) PAH capping layer covering the top gold particle layer.
Conclusions Multilayer samples of gold nanoparticles in a polyelectrolyte matrix with an excellent degree of stratification and low roughness were prepared by spin-assisted layer-by-layer self-assembly. All conclusions drawn from the characterization by X-ray reflectivity measurements on a typical sample are verified by additional techniques: The layering is verified by TEM, the surface roughness is verified by AFM, and the filling factor (i.e., the packing density of gold particles) is verified by counting particles in the AFM picture and by the magnitude of the surface plasmon absorption seen in the optical absorption spectra. During the adsorption of gold particles in the spin-assisted layer-by-layer deposition process, the particles penetrate halfway into the polyelectrolyte matrix. In particular, we demonstrate that the global structure integrated over several millimeters shows position fluctuations that are only approximately twice as large as on a few hundred nanometer length scale. Such information is crucial for future experiments where the interparticle-interaction is fine tuned by carefully adjusting the average separation of particles in the plane and the minimum separation between planes. Because the optical properties depend on clustering in the plane and on the layer separation, this provides a facile way to obtain films with defined optical properties. Acknowledgment. We thank DFG for funding this project via BA 2281/3-1
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