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Ordered Deposition of Inorganic Clusters from Micellar Block Copolymer Films Joachim P. Spatz, Stefan Mo¨ssmer, Christoph Hartmann, and Martin Mo¨ller* Universita¨ t Ulm, Organische Chemie III/Makromolekulare Chemie, D-89081 Ulm, Germany
Thomas Herzog, Michael Krieger, Hans-Gerd Boyen, and Paul Ziemann Universita¨ t Ulm, Abteilung Festko¨ rperphysik, D-89081 Ulm, Germany
Bernd Kabius Forschungszentrum Ju¨ lich, Institut fu¨ r Festko¨ rperforschung, D-52425 Juelich, Germany Received January 21, 1999. In Final Form: September 1, 1999 A method is presented for generating quasiregular arrays of nanometer-sized noble metal and metal oxide clusters on flat substrates by the use of a polymer template. The approach is of general applicability to other metals and various oxides. In the first step, polymeric micelles with a polar core were generated by dissolution of poly(styrene)-block-poly(2-vinylpyridine) in toluene. These micelles were used as nanocompartments that were loaded with a defined amount of a metal precursor. The metal ions can be reduced in such a way that exactly one elemental or oxidic particle is formed in each micelle, where each particle is of equal size. By dipping a flat substrate into a dilute solution, a monolayer of the micelles was obtained whereby the embedded equally large particles became arranged in a mesoscopic quasihexagonal two-dimensional (2-D) lattice. Exposure to an oxygen plasma allowed removal of the polymer completely, leaving the naked metal particles firmly attached to the substrate in the same quasihexagonal order as in the monomicellar film. A modified procedure in which the precursor salt was not reduced before the plasma treatment yielded clusters of identical size and in the same 2-D order. The size (height) of the clusters could be varied between 1 and 15 nm depending on the concentration of the metal salt. The interparticle distance could be varied between 30 and 140 nm by using block copolymers with different lengths of the blocks. Such lattices of Au particles have been used to bind streptavidin proteins in an ordered array.
Introduction Recent developments in nanoparticles and nanostructured materials have opened new opportunities for building up functional nanostructures. Complementary to lithographic patterning techniques, formation of nanostructures from nanoscale building blocks that themselves are synthesized from atoms and molecules has become increasingly important. This so-called bottom-up approach is based on the controlled assembly of molecules and nanoparticles.1 The aim is to produce materials and devices that take advantage of physical, chemical, and biological principles whose causes are found on the nanometer scale.2 New properties are due to surface effects and size reductions to the point where a characteristic length scale of a physical phenomenon becomes comparable with at least one length of the structure. Engineered nanostructures include quantum dots, nanowires, nanotubes, nanolayers, and two- or three-dimensional structures thereof (see articles and references in ref 1). An important task within this context is the production of heterogeneous * To whom correspondence should be addressed. E-mail:
[email protected]. (1) (a) Whitesides, G. M.; Mahias, J. P.; Seto, C. T. Science 1990, 254, 1312. (b) Nanophase Materials: Synthesis, Properties and Applications; Hodjipanayis, G. C., Siegel, R. W., Eds.; Kluwer: Dordrecht, 1994. (c) Nanofabrication and Biosystems; Hoch, H. C., Jelinski, L. W., Craighead, H. G., Eds.; Cambridge University Press: Cambridge, 1996. (2) (a) Goldhaber-Gordon, M. S.; Montemerlo, J. Ch.; Love, G. J.; Ellenbogen, J. C. Proc. IEEE 1997, 85, 521. (b) Collins, P. G.; Zettl, A.; Bando, H.; Thess, A.; Smalley, R. E. Science 1997, 278, 100.
surfaces combining areas with different chemical and physical properties on the scale of nanometers. Such nanopatterned surfaces can lead to peculiar physical and biological phenomena,3,4 for example, if used as substrate for crystal growth or wetting experiments as well as in studies on cell immobilization. Furthermore, nanopatterned surfaces can provide novel platforms for the assembly of more complicated functional nanostructures and nanodevices. Efficient parallel generation of a surface nanopattern represents a major challenge that has been tackled by very different concepts lately. One approach is soft lithography, which has been shown to be viable at dimensions down to several 10-nm units.5 An alternative concept uses self-assembled templates as a substrate for inorganic materials.6 Yet another approach is based on etching procedures of glass nanocomposites of aluminum or silica with HF.7 Pattern formation is also possible by the ordered deposition of surfactant-encapsulated nanoparticles. In this case nanofilms of densely packed spheres have been (3) Dogterom, M.; Fe´lix, M.-A.; Guet, C. C.; Leibler, S. J. Cell Biol. 1993, 121, 1357. (4) Chen, Ch. S.; Mrksich, M.; Hang, S.; Whitesides, G. M.; Ingber, D. E. Science 1997, 276, 1425. (5) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Science 1996, 272, 85. (6) (a) Aksay, A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273, 892. (b) Lee, T.; Yao, N.; Aksay, I. A. Langmuir 1997, 13, 3866. (7) Pearson, D. H.; Tonucci, R. J. Adv. Mater. (Weinheim, Ger.) 1996, 8, 1031.
10.1021/la990070n CCC: $19.00 © 2000 American Chemical Society Published on Web 11/10/1999
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reported where the distance between the particles is limited to the small spacer volume of the low-molecularweight surfactants.8 The approach described here is based on the selfassembly of a diblock copolymer to micelles, which serve as a compartment for the formation of nanoparticles.9 The diblock copolymer micelles are loaded with a suitable transition metal salt and deposited as a micellar monofilm onto a substrate. In a subsequent step the diblock copolymer is removed by means of an oxygen plasma, leaving behind regularly arranged uniform nanoparticles of the noble metal or a metal oxide. In this way a nanoscopic chemically heterogeneous surface pattern can be generated by direct deposition of a noble metal or a metal oxide cluster on various substrates. The size of the inorganic dots is controlled by the amount of inorganic precursor compound that has been added per micelle; the distance depends on the molecular weight of the block copolymer. In comparison with the low-molecular-weight surfactant approach, the macromolecular approach allows the separation of 1-nm-sized particles by up to 140 nm. Several reports have been published by different groups in which an inorganic functional component has been incorporated selectively into microdomains of a block copolymer and in which they even have been transferred to monocrystals arranged in a regular array with periodicities varying between 10 and 200 nm (for reviews see ref 10 and papers cited therein).9 Correspondingly, it has been demonstrated that surfactant-stabilized nanocrystals as well as nanocrystals that have been formed in microgel particles can be used to prepare rather regularly ordered nanoparticulate films.11 The novel aspect of this work is the fact that the regularly arranged inorganic dots are freely accessible to chemical and physical interaction and are remarkably stable although they are no longer supported or protected by an organic coating. Experimental Section Materials. Tetrachloroauric acid (HAuCl4) (Fluka, purum), palladium acetate [Pd(OAc)2] (Degussa), and potassium trichloroethylene platinate (II) (Aldrich) were used as received. Toluene (Merck, p.A.) was freshly distilled over sodium/benzophenone, tetrahydrofuran (THF) (Merck, p.A.) from potassium. Methanol (puris.) and petrol ether (puris.) have been purified by distilling them on a vacuum evaporator. The monomers, styrene (Merck, 99%) and 2-vinylpyridine (2VP) (Merck, 99%), were twice distilled at reduced pressure from KOH and CaH2 respectively. Biotin-HPDP were obtained from Pierce, streptavidin from Sigma, tributylphosphine and octadecyltrimethoxysilane from Aldrich. Anhydrous hydrazine was prepared by thermolysis of hydrazine cyanurate12 on a high-vacuum line, as shown below (Figure 1). The temperature was raised to 200 °C and the evolving hydrazine was distilled in an ampule, equipped with a poly(tetrafluoroethylene) (PTFE) valve, and stored in the nitrogenfilled glove box before use. Substrates. Mica plates were received from BAL-TEC (P ) 10, BU 006 027-T) and freshly cleaved before use. Thin glass plates (Berliner Glas KG, Robax-Glas AF 45) were cut to 10 × 10 × 1.2 mm3 pieces, immersed into H2SO4/H2O2 (1/1 vol) for 2 (8) Abe, K.; Hanada, T.; Yoshida, Y.; Tanigaki, N.; Takiguchi, H.; Nagasawa, H.; Nakamoto, M.; Yamaguchi, T.; Yase, K. Thin Solid Films 1998, 327-329, 524. (9) (a) Spatz, J. P.; Mo¨ssmer, S.; Mo¨ller, M. Chem. Eur. J. 1996, 2, 1552. (b) Spatz, J. P.; Roescher, A.; Mo¨ller, M. Adv. Mater. (Weinheim, Ger.) 1996, 8, 337. (c) Antonietti, M.; Fo¨rster, S.; Hartmann, J.; Oestreich, S. Macromolecules 1996, 29, 3800. (10) (a) Mo¨ller, M.; Spatz, J. P. Curr. Opin. Colloid Interface Sci. 1997, 2, 177. (b) Fo¨rster, S.; Antonietti, M. Adv. Mater. (Weinheim, Ger.) 1998, 10, 195. (11) Whetten, R. L.; Khoury, J. T.; Landman, U. Adv. Mater. (Weinheim, Ger.) 1996, 8, 428. (12) Nachbaur, E.; Leiseder, G. Monatsh. Chem. 1971, 102, 1718.
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Figure 1. Schematic drawing of the experimental setup, used for the preparation of anhydrous hydrazine by thermal decomposition of hydrazinium cyanurate on a high-vaccum line. h, rinsed three times with double distilled water, and rinsed three times with ethanol (p.A.) before use. n+-Doped Si wafers (CrysTec, Berlin) were cleaned in an ultrasonic acetone (p.A.) bath for 2 h. Carbon grids (mesh size: 200 × 200 µm2) for electron microscopy were coated with a Formvar film (approximately 20 nm) and a layer of carbon (approximately 5 nm). Polymerization. Block copolymers of poly(styrene)-blockpoly(2-vinylpyridine) (PS-b-P2VP) with different molecular weights were synthesized by means of living, anionic polymerization. The prepurified monomers were dried on a high-vacuum line by three subsequential freezing-evacuation-melting cycles and distilled under high vacuum in ampules equipped with PTFE valves, following standard techniques.13 For purification of styrene and 2VP, LiAlH4 and CaH2, respectively, have been used. The ampules were transferred into a nitrogen-filled glove box and stored in the refrigerator at -30 °C. THF was given over LiAlH4 and degassed by freezing-evacuation-melting cycles and distilled under high vacuum into a flask which stored in a glovebox prior to use. The polymerization was carried out in this glove box at -78 °C. After putting the solvent inside the polymerization flask, a few drops of styrene were added and the solution was subsequently titrated by the initiatior for receiving the characteristic orange-yellow color of the living polystyryl anions. Afterward, the calculated amount of s-BuLi was added, followed by styrene by means of a syringe. One hour later, LiCl was added to decrease the high nucleophilicity of the polystyryl anions (5 mol % excess in relation to the living polymeric chain ends). An aliquot of the polymer solution was separated, terminated with methanol, and analyzed with size-exclusion chromatography (SEC) to determine the molecular weight and the molecular weight distribution of the first block. Afterward, the second monomer, 2VP, was slowly added by means of a syringe at -78 °C and the polymerization mixture was stirred for an additional 30 min. After termination with methanol and precipitation in petrol ether, the polymers were characterized by means of SEC, proton-nuclear magnetic resonance (1H NMR), and elemental analysis (Table 1). Analysis. SEC. Molecular weights and molecular weight distribution were obtained from SEC with N,N-dimethylacetamide (for PS-b-P2VP) as eluent. The setup consisted of Waters µ-Styragel columns with pore sizes of 105, 104, and 103 Å and a guard column. Sample detection was performed by a Waters 410 differential refractometer. The setup was calibrated with narrow polystyrene samples from PSS (Mainz). For the calculation of molecular weights PSS scientific 3.0 b-61 software was used. 1H NMR. Spectra of polymer solutions in deuterated chloroform were recorded on a Bruker AC-200 (200 MHz). Chemical shifts (ppm) are relative to TMS. (13) Ku¨nstle, H. Block Copolymer Composites With Semiconductor Nanocrystals. Ph.D. Thesis, Holland 1993.
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Table 1. Molecular Weights and Block Lengths of the Synthesized Block Copolymers polymera PS(x)-b-P2VP(y)
PS Mnb (g/mol)
PS Mw/Mnb
block Mnc (g/mol)
block Mw/Mnc
P2VP DPd 1H-NMR
P2VP DPe EA
325-75 (ref 13) 190-190 800-860 1700-450
19.900 83.800 180.200
1.09 1.08 1.09
52.400 40.900 175.700 225.400
1.09 1.09 1.12 1.13
80 196 876 472
84 187 857 466
a x and y give the numbers of repeating units according to the monomer/initiator ratio. b Molecular weight of the first block, obtained from SEC in DMA with narrow distributed PS standards. c Molecular weight of the block copolymer, obtained from SEC in DMA with narrow distributed PS standards. d Block length of P2VP, calculated from the styrene/2VP composition obtained by 1H NMR in relation to the PS block. e Block length of P2VP, derived from elemental analysis.
Loading of Micelles with Metal Precursors. A 0.5 wt % solution (c ) 5 mg/mL) of the diblock copolymer PS(x)-b-P2VP(y) (x ) 190, 325, 800, 1700; y ) 190, 75, 860, 450, respectively) in dry toluene (V ) 5 mL) was stirred for 5 h. Liter equivalents of the metal precursor [HAuCl4, Pd(OAc)2, K(PtCl3C2H4)] per pyridine unit (L ) 0.1-0.5) were added and stirred for at least 24 h. Under strong stirring, 2 mL of the micellar solution were quickly added to 5 mL of a 0.02 vol % solution of nonaqueous hydrazine in toluene. The excess hydrazine (15%) was neutralized with hydrochloric acid directly after reduction. The hydrazinium chloride precipitated. Preparation of Mono Films. Substrates (glass, n+-doped Si wafer, mica, carbon-coated copper grid) were coated with a micellar monolayer using a KSV Sigma 70 system from KSV Instruments. The substrates were fixed on a substrate holder, dipped into the micellar solution (c ) 5 mg/mL) with a velocity of v ) 40 mm/min, and pulled out of the solution with a velocity of 10 mm/min. The covered substrates were dried by exposure to air. Plasma Etching. The substrates coated with a monolayer of micelles were treated with a gas plasma using a plasma etcher P300 (Plasma Electronics) with a high-frequency generator (Dressler rf-generator lpg 133c). In the case of oxygen plasma (5.0, MTI) the substrates were treated for 10 min at 120 W. The gas pressure was 0.45 mbar. The temperature close to the substrates did not exceed 100 °C. Scanning Force Microscopy (SFM). SFM investigations were performed with a Nanoscope III (Digital Instruments) operating in tapping mode or contact mode. The oscillation frequency for tapping mode was set to approximately 360 kHz depending on the Si cantilever (k = 50 N/m, Nanosensors). Si3N4 cantilevers (k = 0.06 N/m, Nanosensors) were used for contact mode. High-Resolution Transmission Electron Microscopy (HRTEM). HRTEM images were recorded with a CM 200 FEG (CS correcture).14 A Si wafer with Au particles was fixed with Araltit and microtomed with a diamond knife perpendicular to the Si wafer surface. Thin slides (80-100 nm thickness) were transferred onto a carbon-coated copper grid for investigations. X-ray Photoelectron Spectroscopy (XPS). The excitation of the valence electrons of small Au clusters on an n+-doped Si wafer was carried out under ultrahigh vacuum using an X-ray tube (AlKR radiation) at an energy of 1486.6 eV. A multichannel detector (five channeltrons) has been applied. Attenuated Total Reflection (ATR). The ATR experiment was realized in the Kretschmann configuration.15 Glass substrates consisting of BK-7 (Spindler & Hoyer) were mounted on a BK-7 glass prism using an index matching oil (Pfeiffer + Balzers). The glass plates were covered by thermal evaporation with about 45 nm of silver and 5-10 nm of SiO2. The size of a glass plate was 10 mm in height and 20 mm in length. One part of the substrate was used as reference area, the other was covered with metal salt-loaded micelles. Two different light sources were used for the excitation of the plasma oscillations: A semiconductor laser (DS 670 OEM, Spindler & Hoyer), working with a wavelength of 670 nm; and a highpressure xenon short-arc lamp (Osram), running with a power (14) Heiner, M.; Rose, H.; Uhlemann, S.; Schwan, E.; Kabius, B.; Urban, K. Ultramicroscopy 1998, 74, 53. (15) Kretschmann, E. Z. Phys. 1971, 241, 313.
of 75 W. The monochromatization was done by a PTI model 101, with a wavelength of 400-800 nm. The angle scan measurements were realized by rotating a mirror, reflecting the light beam into a telescope configuration. Binding of Biotin and Streptavidin to Au Dot Arrays. The Au dot arrays on glass were treated directly after the O2 plasma process with a 1wt % solution of octadecyltrimethoxysilane (Aldrich) in toluene (c ) 0.01 mg/mL). The octadecyltrimethoxysilane-modified surfaces were dried by exposure to air. Biotin-HPDP (Pierce) was dissolved to 0.1 wt % in dimethylformamide (DMF) (p.a., Aldrich). The Biotin-HPDP was reduced by adding 5 vol % of tributylphosphine (Fluka).3 After 10 min the solution was diluted four times in water/methanol (1/1) and the glass with the Au dot arrays was immersed into this solution for 30 min. The sample was rinsed three times with methanol (p.a., Aldrich) and immersed in an 80-mL buffer solution of piperazineN,N′-bis[2-ethanesulfonic acid] (PIPES) (Sigma, 99%) brought to pKa ) 6.8 with KOH. Streptavidin (Sigma) was dissolved to 0.02 wt % in the buffer solution. The sample was transferred from the pure buffer to the streptavidin solution for at least 15 min. SFM images were investigated under the buffer solutions in contact mode.
Results and Discussion In a first step, PS-b-P2VP diblock copolymers differing in molecular weight were dissolved in toluene. Toluene dissolves preferentially the PS block, whereas the P2VP is almost insoluble. As a consequence, the diblock copolymers associated to micelles at a rather low concentration and the amount of molecularly dissolved block copolymer was vanishing small.16,17 When such a solution was treated with a suitable amount of solid HAuCl4, AuCl4- ions were bound as counterions in the polar core of the micelles by protonating the pyridine units. The micelles were formed in equilibrium and the amount of gold per micelle varied only in narrow limits.8 Typically up to one equivalent of HAuCl4 per 2VP unit can be taken up by such a micellar solution.16 Afterward the toluene solution of the HAuCl4-loaded micelles was mixed with a solution of anhydrous hydrazine in dry toluene and the Au3+ ions were reduced to form one elemental gold particle in each micelle whose size was controlled by the amount of aurate ions originally loaded to the core of the respective micelles (Figure 2).9 To prepare a thin film, a suitable flat substrate, that is, a mica or a glass plate or also a carbon-coated Cu grid, was dipped into the solution and pulled out at a controlled velocity of approximately 10 mm/min. The films do not necessarily represent an equilibrium structure, for example, nonequilibrium morphologies are formed in cases where the core/corona volume ratio is larger than 0.150.2. However, already in the case of the pure diblock (16) Spatz, J. P.; Sheiko, S. S.; Mo¨ller, M. Macromolecules 1996, 29, 3220. (17) (a) Cheong Chan, Y.; Schrock, R. R.; Cohen, R. E. J. Am. Chem. Soc. 1992, 114, 7295. (b) Moffit, M.; Eisenberg, A. Chem. Mater. 1995, 7, 1178.
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Figure 2. Schematic drawing of the micelle formation of poly(styrene)-block-poly(2-vinylpyridine) (PS-b-P2VP) block copolymers in toluene. After complexation of HAuCl4 to the pyridine units in the micellar core, the metal compound can be reduced to the zero-valent state by chemical conversion, leading to exactly one gold particle in each block copolymer micelle. Figure 4. Schematic drawing of the formation of naked gold clusters via oxygen plasma treatment. Irrespective of whether metal salt-loaded block copolymer micelles or gold nanoparticles containing micelles are used, the same pattern is obtained after degradation of the polymer.
Figure 3. Transmission electron micrographs of thin films cast from micellar solutions of PS(190)-b-P[2VP(HAuCl4)0.5(190)]. (a) Micelles where the half-number of the pyridine units are neutralized by adding tetrachloroauric acid. The black dots mark small Au clusters within each micelle upon electron beam irradiation. (b) Gold particles with a diameter of 9 nm in each block copolymer micelle after treating a 2-mL aliquot of a c ) 5 mg/mL concentrated PS(190)-b-P[2VP(HAuCl4)0.5(190)] solution with anhydrous hydrazine.
copolymer solutions, the fast evaporation of the solvent in combination with vitrification of the polymer (Tglass of PS and P2VP is approximately 104 °C) did not allow major structural transformations during formation of the dry film, and the micelles originally formed in dilute solution remain intact. This kinetic stability is significantly enhanced by the neutralization of the pyridine units by the gold acid, which yields an ionic core block.16 As a consequence, the films can even be annealed without endangering the structural integrity of the micelles. The formation of a closed monofilm of densely packed micelles is effected by long-range van der Waals interactions and capillary forces acting between the micelles during evaporation of the solvent in the adherent film, and enabled by the intrinsic stability of the relatively thick monomicellar films as investigated intensively by Nagayama and colleagues.18,19 Higher rates of pulling result in a thicker coverage of the substrate. Thus the coverage of the substrate can be adjusted by varying the concentration and the velocity by which the substrate is retracted from the solution.20 By empirical choice of the appropriate conditions we have been able to prepare micellar monofilms extending over an area of up to 3 × 3 cm2. Two types of micellar monofilms were prepared as schematically shown below: (a) Films from micelles whose core was loaded by polymer bound AuCl4- ions and (b) films in which each micelle contained a single gold particle. Figure 3 depicts transmission electron micrographs of a type a and a type b film. In both cases a thin film was cast on a carboncoated copper grid. The bright-field image in Figure 3b shows the rather regular arrangement of isolated gold particles, each generated within the core of one micelle. (18) (a) Denkov, N. D.; Yoshimura, H.; Nagayama, K.; Kouyama, T. Phys. Rev. Lett. 1996, 76, 2354. (b) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303. (19) (a) Dushkin, C. D.; Kralchevsky, P. A.; Paunov, V. N.; Yoshimura, H.; Nagayama, K. Langmuir 1996, 12, 641. (b) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303. (c) Denkov, N. D.; Yoshimura, H.; Nagayama, K.; Kouyama, T. Phys. Rev. Lett. 1996, 76, 2354. (d) Adachi, E.; Nagayama, K. Langmuir 1996, 12, 1836. (20) Meiners, J. C.; Quintel-Ritzi, A.; Mlynek, J.; Elbs, H.; Krausch, G. Macromolecules 1997, 30, 4945.
Figure 5. (a) SFM topography image of a monomicellar film cast from a PS(1700)-b-P[2VP(HAuCl4)0.3(450)] solution onto a glass substrate. The height profile indicates the lateral periodicity of 90 nm and the height variations of the micelles mainly contributing from the mismatch of the horizontal line indicated in the image with the top of the micelles. (b) Same sample as in (a) but after the oxygen plasma treatment, resulting in naked Au particles on the glass substrate. The height profile of the horizontal line indicated in the image demonstrates the unchanged lateral periodicity of 90 nm after the plasma process and the 8-nm height of the naked Au particles. The length of one image side corresponds to 1.1 µm in both cases.
In Figure 3a the core of each micelle is marked by a large number of tiny spots. These are ultrasmall gold particles that were generated by irradiation with the electron beam during observation.9 Both types of micellar films were exposed to a gas plasma to remove the polymer. In most cases, an oxygen plasma was used. Alternatively, the polymer could also be removed by means of a CF4 or a hydrogen plasma. The plasma treatment was chosen because of the relatively moderate thermal conditions to avoid extensive heating of the sample. Under the appropriate conditions, the temperature during the plasma treatment did not exceed 100 °C. Irrespective of whether the micelles were loaded with HAuCl4 or already contained a single gold particle each, the plasma treatment resulted in the deposition of small metal particles on the bare substrate. The location and order of the particles corresponded to the pattern of the closely packed micelles in the original film (Figure 4). The SFM images in Figure 5 (contact mode) demonstrate the transformation for a micellar monofilm of a PS(1700)b-P[2VP(HAuCl4)0.3(450)] diblock copolymer on glass. Figure 5a depicts a topographic image of the film as cast from a micellar toluene solution. The micrograph displays the outer contour of the polymer micelles. The bright spots mark elevations where the gold salt-loaded cores are located. The profile at the bottom of the image gives the height variations along the black marking. The average
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lateral periodicity is about 90 nm. The height of the micelles was 35 nm. Height variations in the profile are mainly caused by the mismatch of the horizontal line and the top of the micelles. Figure 5b shows the SFM topography of the same sample after it was treated with an oxygen plasma. The image became sharper as the soft polymer coat was removed. The height of the elevations was reduced to 8 nm and in between them the SFM needle probed the bare substrate. The height profile along a line running over several naked Au clusters is indicated in the SFM image. To evaluate the lateral diameter of the particles we had to deconvolute the scan profile over the particle with respect to the tip radius (corrected for the tip radius).21 The particle size was 8 nm in height and about 10 nm in the horizontal width, again variations of the height corrugations in the profile origin mainly from the mismatch of the horizontal line with the top of the Au clusters. The lateral periodicity of the pattern remained unchanged compared with that of the micellar film in Figure 5a, that is, 90 nm. The pattern, size, and shape of gold clusters as observed by SFM was the same irrespective of whether the pyridine units in the core were partially neutralized by HAuCl4 or whether each micelle contained a preformed gold particle. This indicates that the reduction and crystallization of the gold upon the plasma treatment of the salt-filled micelles must be faster than full degradation of the polymer. In this case, the reduction can be attributed to intermediate oxidation products of the polymer such as CO and electrons from the plasma. To evaluate whether the polymer coating was removed completely a sample was investigated by XPS. As the sensitivity of XPS is based on the conductivity of the substrate, a micellar monofilm of PS(1700)-b-P[2VP(HAuCl4)0.3(450)] was deposited on a strongly n+-doped Si wafer. In this case we could not remove the polymer by an oxygen plasma as the inevitable oxidation of the substrate would result in an insulating interlayer. This can be avoided by using a CF4 plasma even though this is not as effective as an O2 plasma for the removal of organic materials.22 Figure 6 depicts a topographic SFM image of the sample after treatment with the CF4 plasma. Although the plasma did not oxidize the silicon substrate it eroded it substantially, as can be seen at the increased roughness compared with Figure 5b. Only very small amounts of carbon were indicated. The integrated C-1s signal (Figure 7a) was as small as that from a pure reference Si wafer and can be attributed to contaminations known to be present even under ultrahigh vacuum conditions. The question arises as to whether the observed particles are elemental gold or Au2O3. Recently it has been reported that the plasma treatment of thin gold films resulted in the formation of gold oxide.23 However, in the presence of air, Au2O3 is metastable. Annealing will result in decomposition to elemental gold. To evaluate the state of oxidation of the metal, a freshly prepared sample on a Si wafer treated with an oxygen plasma was measured after the treatment with an oxygen plasma (Figure 7b) and after thermal treatment at 960 °C (Figure 7c). The Au-4f signal decreased in the latter case but the peak position remained unchanged. The decreased signal intensity can be attributed to loss of Au by evaporation and could be (21) Sheiko, S.; Mo¨ller, M.; Reuvekamp, E. M. C. M.; Zandbergen, H. W. Phys. Rev. B: Condens. Matter 1992, 48, 5675. (22) Brodie, I.; Muray, J. J. The Physics of Micro/Nano-Fabrication; Plenum Press: New York, 1992. (23) Maya, L.; Paranthaman, M.; Thundat, T.; Bauer, M. L. J. Vac. Sci. Technol., B 1996, 14, 15-21.
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Figure 6. SFM topography image of a monomicellar film cast from a PS(1700)-b-P[2VP(HAuCl4)0.3(450)] solution onto a strongly n+-doped Si wafer after CF4 plasma treatment. In comparison with Figure 2, the height profile in the image below shows a higher surface roughness derived from the CF4 plasma eroding the Si wafer.
correlated to a decrease in the cluster height (see below for the discussion of the remarkable thermal stability of the Au clusters). Transformation of Au2O3 to elemental gold upon thermal treatment would have been indicated by a shift in the peak position by about 2 eV for the unaged and the annealed sample. Removal of the polymer matrix by the plasma procedure can be monitored by ATR spectroscopy, the principle of which is sketched in the inset of Figure 8a.24 A glass prism was covered by a thin (46 nm) Ag film and the reflected intensity of a laser beam (λ ) 670 nm) was determined as a function of the angle of incidence Θ. At a well-defined angle, a surface plasmon polariton is exited, leading to a resonance-like depression of the reflected intensity (open squares) as shown in Figure 8a. Fitting to theory (solid line through the data points) delivers the real (′) and imaginary (′′) part of the Ag dielectric constant. To protect the silver film against oxidation by the O2 plasma it was coated by a thin layer of SiO2. This resulted in a shift of the resonance by about +2° (open circles). When a monolayer of the PS(1700)-b-P2VP(450) diblock copolymer micelles was deposited on the Ag-SiOx sandwich (open diamonds) the resonance was shifted by another 1.5° toward higher angles. Exposure to an oxygen plasma removed the polymer and the resonance of the SiO2-coated silver film was recovered (closed triangles, Figure 8a). Because the ATR technique has a sensitivity of well below a monolayer of any adsorbed molecules,25 this result proved that there was no residual polymer left after the plasma treatment. Figure 8b shows the corresponding ATR data for a mono film of Au-loaded diblock copolymer micelles, PS(1700)-b-P[2VP(450)‚Au]. In this case, a significant shift (cf. enlarged view in the inset) of the resonance relative (24) Krieger, M. Diplomarbeit, Universita¨t Ulm, Ulm, Deutschland, 1998. (25) Raether, H. Surface Plasmons; Springer-Verlag: Berlin, 1986.
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Figure 7. XPS spectrum of Au particles on a strongly n+-doped Si wafer (a) directly after the CF4 plasma treatment, (b) directly after the O2 plasma treatment, and (c) after the O2 plasma treatment plus additional heating to 960 °C.
Figure 9. High-resolution transmission electron micrograph of an O2 plasma-treated monomicellar film of PS(1700)-bP[2VP(HAuCl4)0.3(450)] on a strongly n+-doped Si wafer microtomed perpendicular to the substrate surface and imaged along the wafer plane. The image shows a crystalline Au particle of 6 nm in diameter and the crystalline Si wafer. The oxidized and for that reason amorphous Si of the substrate is not visible. The pyramidal shape of the substrate underneath the particle results from the mask effect of the Au particle during the plasma procedure.
Figure 8. (a) ATR investigations of a plasma glass plate covered by Ag (open squares), Ag + SiOx after plasma treatment (open circles), Ag + SiOx + polymer after plasma treatment (closed triangles), and Ag + SiOx + polymer before plasma treatment (open diamonds). The inset shows the experimental setup. (b) ATR investigations of a plasma glass plate covered by Ag (open squares), Ag + SiOx after plasma treatment (open circles), and Ag + SiOx + Au nanoparticles after plasma treatment (closed triangles). The inset shows the enlargement of the shift resulting from the deposition of the Au nanoparticles. The solid line represents the fit to theory as described in the text.
to the polymer-free Ag-SiOx bilayer was observed, indicating the presence of the Au nanoparticles.
In addition, the Au nanoparticles were observed directly by SFM and found to have a similar size, size distribution, and lateral periodicity to the clusters shown in Figure 2b, although the thin layer of SiOx showed a surface roughness slightly higher than in the case of glass. The theoretical fit of the experimental data (solid line through the closed triangles) delivered a real part for the dielectric constant of ′ ) -9, which convincingly demonstrated the metallic character of the nanoparticles and corresponded closely to the bulk value of gold, ′ ) -13 (ref). Assuming an additional thin polymer film just under the particles did not allow us to obtain such a high-quality fit as given in Figure 8b. Figure 9 shows an HRTEM of an ultramicrotomed crosssection of an O2 plasma-treated monomicellar film of PS(1700)-b-P[2VP(HAuCl4)0.3(450)] on an n+-doped Si wafer. The HRTEM image shows a side-on view of a crystalline Au particle approximately 6 nm in diameter.
Ordered Deposition of Inorganic Clusters
The particle sits on a pyramid growing out of the crystalline Si wafer. The image can be explained if one considers that the O2 plasma has oxidized the top layer of the Si wafer and that the resulting layer of SiO2 is not visible because of its amorphous character. The Au particle must have acted as a kind of mask protecting the silicon within the pyramid below the gold particle against oxidation. It has been mentioned already without further discussions that the gold particles were remarkably stable against thermal treatment. In all cases the gold particles were strongly bound to substrates such as glass and SiO2 and could not be removed either by washing with different solvents or treatment in an ultrasonic bath. Even careful rubbing with a soft tissue has been possible. When a sample like the one in Figure 5b was annealed at 800 °C, the structure of the Au nanodot array did not change in the presence of an electrolyte either. For example, when we annealed 10-nm (height) Au clusters on SiO2, which were prepared from a monofilm of PS(1700)-b-P[2VP(HAuCl4)0.5(450)] by O2 plasma treatment for 4, 8, and 12 h at 800 °C, SFM images showed no evidence of particle movement or particle coagulation on the substrate. Also, no particle broadening due to wetting of the substrate was observed. By the same procedure Au particles of 4 nm in height were deposited onto a SrTiO3 wafer. When the sample was heated for 20 min to 800 °C in an argon/ oxygen atmosphere the SFM images remained the same. Also in this case, no indication for cluster movement or cluster coagulation was observed, demonstrating the remarkable stability of the Au particles. The cause of the stability is assumed to be the even size distribution impeding Ostwald ripening and the effective binding of the clusters to the substrate. It must be mentioned that the melting temperature of Au nanoparticles is expected to decrease drastically if their size is reduced further and that the reported stability can most likely not be extrapolated to smaller particles.26 Previous preparation techniques of Au particles that do not involve the in situ formation, for example, on glass, do show coagulation of the particles and a weak adhesion of Au particles.27 The SF micrographs in Figure 10 demonstrate how the periodicity and the size of the Au particles can be varied by using diblock copolymers of different molecular weight. Figure 10a depicts a sample that was prepared from a toluene solution of PS(800)-b-P[2VP(HAuCl4)0.5(860)] on freshly cleaved mica and subsequently treated with an oxygen plasma. Figure 10b and c show corresponding preparations from PS(325)-b-P[2VP(HAuCl4)0.5(75)] and PS(1700)-b-P[2VP(HAuCl4)0.1(450)]. The gold particles exhibit a uniform height of 12 nm, 2 nm, and 1 nm, respectively. The apparent diameter of the particles is exaggerated because of to the finite radius of the tip apex, which was determined to be approximately 15 nm.21 The respective periodicities were 80 nm, 30 nm, and 140 nm. The periodicity of the sample in Figure 10b differs from that in Figure 5b, although the same polymer was used. The remarkable variation is explained by the different substrate. For the sample in Figure 5, glass, which has a smaller surface energy than mica, has been used as the substrate. The coating procedure effects the absolute coverage of the substrate; the corona block can become more or less stretched, which affects the distance between the particles. This will be subject of a detailed discussion in a forthcoming report. (26) Lai, S. L.; Guo, I. Y.; Petrova, V.; Remeneth, G.; Allen, L. H. Phys. Rev. Lett. 1996, 77, 99. (27) Kreibig, U.; Vollmer, M. In Optical Properties of Metal Clusters; Toennies, J. P., Ed.; Springer Series in Materials Science; Springer Verlag: Heidelberg, 1995.
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Figure 10. SFM topography images of Au cluster arrays on mica substrates after the oxygen plasma treatment. The different interparticle distances are obtained by varying the lengths of the polymer blocks in the following way: (a) PS(800)b-P[2VP(HAuCl4)0.5(860)], (b) PS(325)-b-P[2VP(HAuCl4)0.5(75)], and (c) PS(1700)-b-P[2VP(HAuCl4)0.1(450)]. The length of each image corresponds to 3 µm.
To demonstrate the general applicability of our technique, we prepared corresponding cluster arrays of naked Pt and Pd particles. Figure 11 shows topographic SF micrographs of Pd particles of Ø ) 2 nm (a) and naked Pt particles of Ø ) 3 nm (b) that were deposited on a glass substrate from micellar solutions of PS(1700)-b-P{2VP[Pd(Ac)2]0.2(450)} and PS(1700)-b-P{2VP[K(PtCl3C2H4)]0.2(450)}, respectively. Again, the plasma treatment allowed removal of the organic component fully. The lateral distance between the nanoclusters resulted as in the case of Au clusters on the same substrate to approximately 90 nm.
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Figure 11. SFM topography images of (a) Pd and (b) Pt cluster arrays. The length of each image corresponds to 1 µm. The profiles show the height variations of the scan line indicated in the micrographs.
The fact that the nanodots are not stabilized by an organic coating makes them candidates for locally selective binding of macromolecular compounds and biomolecules. To demonstrate this, we used an array of 2-3-nm-high Au particles to arrange streptavidin molecules in a defined pattern. In this case, the Au particles were deposited on a glass slide. The decorated glass slide was then treated with a solution of octadecyltrimethoxysilane. Because of the reaction with the SiOH groups of the glass surface, the alkane chains were grafted onto the glass, leaving the inert Au nanoparticles uncovered. According to the molecular weight of the octadecyltrimethoxysilane chains, the thickness of the layer was 2-3 nm, resembling the height of the Au nanoparticles. Figure 12a depicts a contact mode SF micrograph that was recorded from such an octadecyltrimethoxysilane-modified Au nanodot array on glass under water. Only a very weak unclear topographical contrast is observed, as the thickness of the octadecyltrimethoxysilane monolayer is comparable with the height of the Au nanoparticles. When such a substrate was treated with a water/ methanol/DMF solution containing approximately 0.2 wt
Figure 12. SFM micrograph of a glass plate where Au nanoparticles with 2-3 nm in height and 80 nm in lateral periodicity were deposited and (a) the glass was coated by polyethylene glycol (1.3 × 1.3 µm2); (b) after wetting the substrate from (a) with a reduced solution of biotin-HPDP and (after cleaning with a pure buffer) with a solution of streptavidin (1.3 × 1.3 µm2), and (c) the same procedure as described under (b) but without a protecting layer of octadecyltrimethoxysilane (5 × 5 µm2). The latter results in the formation of twodimensional lamellas of streptavidin as reported by others.28
% of reduced biotin-HPDP, the thiol groups bound selectively to the Au particles. Because of their small size, the biotin molecules could not be detected by SFM and micrographs such as the one shown in Figure 12a were obtained. Figure 12b shows a contact-mode SF micrograph of the same substrate as in Figure 9a but after the covering with streptavidin. Biotin is known to bind specifically to one of the four binding sides of a streptavidin molecule with a binding constant of Kdiss ) 10-15 mol/L.28 The
Ordered Deposition of Inorganic Clusters
aqueous solution was replaced for at least 15 min by a buffered 5 mmol solution of streptavidin. The streptavindin molecules had sufficient time to bind to the biotin binding sides, which were fixed to the Au nanoparticles.3 Clearly, the topographic contrast in the SF image recovered was due to the binding of the rather large streptavidin molecules (5 × 4.5 × 4.5 nm3). The elevations in Figure 12b correspond to the distance of the biotin-modified Au particles. When the same experiments were repeated with a gold-cluster-decorated glass slide, which was not modified by octadecyltrimethoxysilane coating, unselective binding of streptavidin was observed. This is shown in Figure 12c, where large areas of the glass substrate got covered by a closed layer of streptavidin. Uncontrolled adsorption of streptavidin on glass has been reported by others.28 Eventually, such arrays of streptavidin might be applied to bind other proteins such as kinesin, dynein, RNA, and DNA, which, in turn, can be used as patterned substrates for well-defined motility assays or expression experiments in the future.29 In summary, the approach described here represents a simple procedure for the preparation of nanometer-sized dots in a rather regular pattern with distances of a few nanometers up to 140 nm so far. The versatility of the (28) (a) Schmitt, F.-J.; Knoll, W. Biophys. J. 1991, 60, 716. (b) Weisenhorn, A. L.; Schmitt, F.-J.; Knoll, W.; Hansma, P. K. Ultramicroscopy 1992, 42-44, 1125. (29) Turner, D. C.; Chang, C.; Fang, K.; Brandow, S. L.; Murphy, D. B. Biophys. J. 1995, 69, 2782.
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method includes also a broad choice of particle sizes, which particle sizes could be varied between 1 and 15 nm. The regular decoration requires only a minimum roughness of the substrate and wetting by the micellar solution, two conditions that are easily fulfilled by most substrates. Without further effort we have prepared samples of >3 × 3 cm2. Besides the diblock copolymers used here, various other amphiphilic diblock copolymers form reverse micelles in a nonpolar solvent and can be used to bind a large variety of metal compounds such as H2PtCl6, Pd(Ac)2, TiCl4, etc. Chemical transformation of the inorganic compound can be performed before deposition of the micellar film or upon removal/oxidation of the film and even after removal. So far, less noble compounds can only be deposited in their oxidic form, that is, TiO2, Fe2O3, InOx, whereas noble metals are deposited in the elementary state. Besides flatness, the only requirement for the substrate is that it must be resistant to the plasma gas applied. Thus a large number of substrates can be used, for example, gold, glass, sapphire, SrTiO3, mica, or also materials with surfaces that are made passive by an oxygen layer like silicon, GaAs, InP, or SixNy. Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft (SFB 239, Teilprojekt F5) and the BMBF (03D0054) is gratefully acknowledged. LA990070N
