Synthesis of Metal Nanoparticles and Patterning in Polymeric Films

Feb 16, 2017 - The Institute of Scientific and Industrial Research, Osaka University, 8-1 ... Department of Chemistry, Konan University, 8-9-1 Okamoto...
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Synthesis of Metal Nanoparticles and Patterning in Polymeric Films Induced by Electron Nanobeam Hiroki Yamamoto,*,† Takahiro Kozawa,† Seiichi Tagawa,† Muneyuki Naito,§ Jean-Louis Marignier,‡ Mehran Mostafavi,‡ and Jacqueline Belloni*,‡ †

The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Osaka, 567-0047 Japan Laboratoire de Chimie Physique, ELYSE, Université Paris-Sud, Bât. 349, 91405 Orsay, France § Department of Chemistry, Konan University, 8-9-1 Okamoto Higashinada, Kobe, Hyogo 658-8501, Japan ‡

ABSTRACT: Using a scanning electron nanobeam, thin polymeric films loaded with metal nanoparticles of silver or gold are prepared by a one-step irradiation-induced reduction of the solid solutions of the metal ions embedded in the polymer. In the first part, the feasibility of the film synthesis is demonstrated and the preparation conditions are optimized for the choice of the matrix, the irradiation dose, the heating to develop the particles, and the dissolution of unexposed areas to reveal the film loaded with metal nanoparticles. The metal nanoparticles are observed by either optical absorption or microscopy. The mechanism of the reduction of metal ions and of the polymer cross-linking is deduced from the average absorbance measurements. In the second part, in view of realizing specific patterns of high resolution using the electron nanobeam, groups of nanobeam scans may produce 200 nm wide lines that can be separated by unexposed spaces of adjustable width, where precursors were dissolved. The resolution of the very narrow electron nanobeam has been exploited to demonstrate the achievement of nanopatterning on polymer films using a direct-writing process. This method supplies interesting applications such as masks, replicas, or imprint molds of improved density and contrast.

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INTRODUCTION

Metal nanoparticles embedded in mineral glasses are known to have been prepared since antiquity by fusing metal salt precursors with the glass matrix that acquires, even at low loadings, the intense and very stable colors of the specific surface plasmon bands of ultradivided metal nanoparticles. The nucleation mechanism of the metal atoms into clusters and nanoparticles inside sodalime silicate glasses has been recently investigated at room temperature, in particular, by inducing the reduction of ions, without any additive, by light absorption from a laser beam5 or by ion6 or γ irradiation.7 Ionizing radiations, able to penetrate into zeolites exchanged by silver ions at the surface of the cavities, generate in situ small silver oligomers observed by optical absorption or electron spin resonance (ESR).8,9 The nucleation mechanism in zeolites has been observed by pulse radiolysis coupled to time-resolved optical absorption.10,11 After exchanging metal ions at the surface of Nafion pores and channels, the formation and reactions of metal clusters were also studied by pulse radiolysis.12−14 The dose-rate effects could be accounted for by the nuclearity-dependent properties of the silver oligomers, namely, by their possible reoxidation by the protons.13 Silver, palladium, or nickel clusters were also produced as embedded

One of the present challenges in preparing stimulus-responsive polymer films,1 such as hardmasks for lithography or templates such as nanoimprints,2,3 is to enhance their density. Because conventional resists such as most polymer materials are easily attacked by atomic oxygen in the etching process, one of the possible ways to increase the etching durability is by incorporating inorganic species such as metal particles into resists. With the feature size becoming smaller (< 30 nm), important problems such as pattern collapse and poor pattern transfer occur in conventional polymer-based photoresist materials. Normally, to overcome this problem, a spin-on-glass (SOG) is used as an intermediate hard mask.4 However, this trilayer system of conventional approach is complicated and costly because it needs two times of reactive ion etching. Therefore, the replacement of the conventional trilayer system by less complex processes is needed. This can be overcome by using hybrid nanomaterials such as hafnium hydroxide oxide material.2 The difficulty of making polymer films loaded with metal nanoparticles is to incorporate them inside the polymer film, and, in view of application for hard masks and templates, to control their localization in an accurate geometry with a high resolution. © 2017 American Chemical Society

Received: December 13, 2016 Revised: February 10, 2017 Published: February 16, 2017 5335

DOI: 10.1021/acs.jpcc.6b12543 J. Phys. Chem. C 2017, 121, 5335−5340

Article

The Journal of Physical Chemistry C

irradiating the film is only 2 nm; however, a controlled magnetic scanning system is used at 8.3 μm s−1. Actually, the irradiation may be more or less uniform throughout the film. Two different experimental irradiation protocols have been used. (i) In view of observing after irradiation the optical absorbance of the film that is supported on quartz, the nanobeam delivers, over an area of 6 × 6 mm2 (in some cases 4.2 × 3.6 mm2), a series of irradiation dots, spaced by 30 nm from axis to axis and centered on the impact of the 2 nm nanobeam where the dose is the highest. The successive scans of the beam are also spaced by 30 nm one from each other. However, because of the scattering of the electrons, the irradiation spots are much more extended than the nanobeam section and are almost overlapping. Nevertheless, the dose between the dots is lower than the average value. As a result, the energy of the nanobeam is deposited almost randomly over the irradiated area. The mean dose is 2.6 to 6.6 J cm−2 (80 to 200 μC cm−2). Thus the beam irradiates a large exposure range, up to 6 × 6 = 36 mm2 within 16 to 39 h (or 4.2 × 3.6 mm2 within 7 to 16 h). After irradiation, the film seems to be almost unchanged and is still colorless (Ag+) or yellow (AuIII), but, after heating (Figure 1, step 4), the exposed part acquires the specific surface plasmon yellow color for silver/nanoparticles films and the specific deep-purple color for gold nanoparticles/films. In contrast with the unexposed parts of the film, the exposed parts are now insoluble in the casting solvent PGMEA, and this difference provides a way to reveal the nanoparticle-loaded film from the ion-loaded film. (ii) In the second irradiation process, to create more contrast on the 100 nm thin film supported on silicon, the dots are much closer (2.5 nm). Patterns are created, on an area of 150 × 150 μm2, with groups of parallel irradiated lines of variable width, each constituted of several nanobeam scans also in close vicinity (2.5 nm). The lines are separated by unirradiated spaces of adjustable width. The film is then treated by heating and revealed by dissolution in PGMEA as above. The dosimetry for the film irradiation using the 75 keV electron nanobeam was done by the collection of charges by using a Faraday cup on the basis that a charge of 100 μC cm−2 of 75 kV electrons corresponds to a dose of 330 kGy.19 The penetration of 75 kV electrons is 100 μm in water or 94 μm in PS films and 85 μm in PMMA films, which is most part of the thickness of the films (120 μm). The dose absorbed in the film is more homogeneous and higher than when dots are distant by 30 nm and is 28.8 mC cm−2. The irradiated films are then ready for characterization by optical spectrophotometry (UV−vis spectrophotometer, JASCO V-570), optical microscopy (Keyence VHX), electron microscopy (HRTEM, JEM-3000F (300 kV)) equipped with EDS, and atomic force microscopy (AFM, SPI 3800/SPA 300 or N-3900/SPA-300HV) (Figure 1, step 5).

in poly(vinyl alcohol) polymer by radiation-induced reduction in water of the ionic precursors simultaneously with the crosslinking of the polymer into a solid film.15,16 In the case of nickel, the dried film containing the nanoparticles was ferromagnetic.14 Besides, extensive works have been devoted to the irradiation of polymer films for lithographic applications.1,17 The aim of the present work is to take advantage of the radiolytic method to reduce metal ions into particles embedded in a polymer and to cross-link the polymer to make it insoluble and fixed on the support. Both nanoparticles and cross-linked polymer are generated without additives via a one-step exposure. Moreover, by using an electron nanobeam, as in nanolithographic studies, the process will permit, with a high spatial resolution, the production of metal nanoparticles and of the cross-linked polymer exclusively in the irradiated parts of the polymer films. The mechanisms of this direct-writing process, the properties of the loaded films, and the possible patterning were explored.

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EXPERIMENTAL METHODS All of the reagents were analytically pure. The metal salts AgClO4 and KAuCl4, propylene glycol monomethyl ether acetate (PGMEA), poly(styrene) (PS 4000), and poly(methyl methacrylate) (PMMA 15000) were purchased from Aldrich. For the synthesis of the films, the salts AgClO4 or KAuCl4 and the polymers PS (in the case of AgClO4) or PMMA (in the case of KAuCl4) are first dissolved in the casting solvent PGMEA. The preparation of the films is described in the scheme of Figure 1. A volume (4 mL) of the casting solutions

Figure 1. Scheme of the radiolytic synthesis steps of the polymer films loaded by metal nanoparticles.

of PGMEA containing silver ions and PS or gold ions and PMMA (step 1) is deposited drop by drop on a quartz plate at the temperature of 80−90 °C (step 2) so that the solvent is rapidly evaporated and the metal ions remain intimately dispersed in the thin PS or PMMA polymer matrix. Then, the ion-loaded film is still baked at 80 °C for 24 h. In this stage, the evaporation process is reversible and the film may be dissolved back into PGMEA. The obtained PS or PMMA films containing the metal ions are 120 μm thick and 3 × 3 cm2 wide. Note that the mild prebake stage at 80 °C is unable to reduce the ions. The film is then exposed, perpendicularly to the surface (Figure 1, step 3), to the electron nanobeam of 75 kV of ISIR, Osaka University.18 The cross section of the electron beam

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RESULTS AND DISCUSSION Part 1: Randomly Irradiated Films. Silver Nanoparticles in PS Films. First, AgClO4 at the concentration of 1.9 × 10−2 mol L−1 and the polymer polystyrene (PS) at the monomer concentration of 0.38 mol L−1 were dissolved in PGMEA used as the casting solvent (Figure 1). After evaporation of the casting solvent, the thin and transparent film of PS containing silver ions, with selected silver loading and thickness, is ready for electron beam irradiation. The Ag+ concentration in the PS 5336

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The Journal of Physical Chemistry C film (dielectric constant = 2.4, specific mass = 1.05 g cm−3) is now 0.51 mol L−1, corresponding to a loading in PS of 5.2 wt % Ag. After irradiation following protocol (i), the film is still colorless. The structure of the film is such that the diffusion of ions and atoms inside the polymer during the irradiation is blocked. However, after heating, only the exposed part acquires the specific yellow color of silver nanoparticles (Figure 2).

Figure 3. Optical absorption spectra of PS films containing AgClO, before and after increasing irradiation doses delivered by the 75 kV electron nanobeam, then heating to 120 °C for 5 min. The film thickness is 120 μm.

whatever was the initial irradiation dose absorbed (80 or 200 μC cm−2). In AFM images of the irradiated part of the film, the individual silver nanoparticles have a mean diameter of 6 nm and are randomly distributed (Figure 4a). The electron beam

Figure 2. Films of polystyrene (3 × 3 cm2, thickness 120 μm) containing silver ions partly unexposed (transparent) and partly exposed to irradiation (yellow) by using the nanobeam of 75 kV electrons (average dose: 6.6 J cm−2) on the restricted area of 6 × 6 mm2 and after heating to 120 °C during 5 min.

Clearly, the radiation-induced cluster formation is strictly restricted to the exposed area, but its absorbance intensity is too small to be detected before the heating step. Note that heating alone (120 °C, 5 min) without irradiation is unable to start the reduction in the unexposed part of the film. Developable nuclei are provided exclusively by the electron beam irradiation. Indeed, single silver ions in the unexposed part are not reduced by heating because their reduction potential is much more negative than that of ions adsorbed on multiatoms nuclei of the exposed part.12,13 This discrimination and amplification beyond a critical nuclearity is comparable to that of the photographic development.1 The optical absorption spectra at increasing doses after heating are shown in Figure 3. The absorbance maximum is at 410 nm as for the surface plasmon band of silver nanoparticles. The absorbance does not increase much between 80 and 200 μC cm−2. As described above, the dose absorbed is not strictly uniform over the sample. It is locally higher under the nanometric beam spots and lower in the areas irradiated by only the scattered electrons, but the heating finally induces the same growth. The excess dose induces a modification of the polymer by cross-linking (PS), which makes the film insoluble in the casting solvent PGMEA. Because these changes are due to chain reactions, they probably extend somewhat further than the strict impact of the nanobeams. However, because of the nonhomogeneous irradiation of the film, only part of the sample area is sufficiently irradiated to reduce all of the ions into nanoparticles. During heating and diffusion, many more supplementary ions, initially present in the weakly irradiated part of the film, are thermally reduced at the surface of the silver clusters produced during the irradiation step and acting as catalysts.20 So the measured average absorbance of the 100 μm film increases to 0.37, almost

Figure 4. Silver nanoparticles synthesized in PS films irradiated using the 75 keV electron nanobeam (dose 200 μC cm−2), then heated to 120 °C for 5 min and revealed in PGMEA. (a) AFM image. (b) TEM image. (c) Pentagonal silver nanoparticle. (d) Electron diffraction pattern of the fcc structure of a pentagonal nanoparticle.

cross section is very small, but forward and back scattering is induced at the interface between the polymer and the substrate. Therefore, no memory remains of the initial distribution in dots. TEM images also show nanoparticles of 5 nm embedded in the film (Figure 4b). In bright-field TEM images, some clusters are larger and display a pentagonal shape (Figure 4c) that we assign to five multitwinned clusters with an fcc structure, as shown by the electron diffraction patterns (Figure 4d). This particular crystal shape is often obtained in the 5337

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The Journal of Physical Chemistry C radiolytic synthesis of silver clusters in solution and is assigned to the growth from an initial single icosahedral nucleus.14 It is noteworthy that the unirradiated parts of the film containing the metal salt are reversibly soluble back into PGMEA. Gold Nanoparticles in PMMA Film. The film is prepared by dissolving KAuCl4 (6.45 × 10−3 mol L−1) and the polymer PMMA (0.38 mol L−1) in PGMEA. After evaporation of the solvent (Figure 1), the thin film of PMMA (dielectric constant = 2.7, specific mass = 1.18 g cm−3) of selected thickness (120 μm) containing the AuIII gold ions that absorb in the UV− visible is yellow before irradiation (Figure 5, left). The

Figure 6. Optical absorption spectra at increasing doses delivered by the 75 keV electron beam and heating at 120 or 150 °C for 5 min of the KAuIIICl4−PMMA film (0.17 mol L−1). The spectrum before irradiation is due to AuIII ions. After irradiation but without heating the spectrum is almost the same.

The size of gold nanoparticles in the film observed by AFM and TEM is from 5 to 15 nm (Figure 7a−c). The EDS spectrum with the Au peak at 10 keV is shown in Figure 7d.

Figure 5. Gold nanoparticles embedded in PMMA films (120 μm). Left: before irradiation, the film of 3 × 3 cm2 is colored by AuIII ions at 0.17 mol L−1. Right: after irradiation using the 75 keV electron beam (200 μC cm−2) and heating at 150 °C during 5 min, the irradiated area (4.2 × 3.6 mm2) is deep purple due to gold nanoparticles.

concentration of AuIII in the PMMA film is 0.17 mol L−1, corresponding to a loading in Au/PMMA of 3.2 wt %. The unirradiated film can be reversibly dissolved into the casting solvent. After irradiation of the thin film at an average dose of 200 μC cm−2, the film is still yellow, meaning that it contains mostly gold ions, with only a very small part being reduced into Au0 (Figure 5, left). However, after a short heating to 150 °C for 5 min, the exposed area becomes deep purple (Figure 5, right). That means that, as for silver-PS films, gold ions are locally reduced in the series of spots, but the irradiated area constitutes a small part of the whole sample and the cluster absorbance is too low to be detected. However, as in the silver−PS film, the heating lets surrounding ions diffuse and achieve their reduction onto the radiation-induced nuclei that grow, but no reduction induced by heating is observed in unexposed parts. Hence, the grown gold nanoparticles with the typical purple color are observed exclusively in exposed areas. The spectra at increasing doses absorbed by the AuIII− PMMA film and after heating at two different temperatures are shown in Figure 6. The absorbance for the surface plasmon band of gold nanoparticles is A = 0.13 for 200 μC cm−2. The maximum is at 560 nm, which is at a somewhat higher wavelength than in polar liquids but close to the maximum in THF.21 The reduction of AuIII ions into Au nanoparticles requires, for the same radiolytic yield, a dose three times larger than that for silver. This explains that under irradiation only and without heating the spectrum is predominantly that of gold ions absorbing in the UV−visible with a maximum at 320 nm. However, the development of the purple color after heating with A = 0.13 indicates that the ions diffuse and are thermally reduced at the surface of the clusters radiolytically produced. The UV−vis absorbance of ions is less intense (Figure 6).

Figure 7. Gold nanoparticles formed in 3.2 wt % Au/PMMA films supported on quartz after irradiation and heating. Mean dose 200 μC cm−2 or 6.6 J cm−2. (a) AFM image of gold nanoparticles in PMMA films. (b) Bright-field TEM image. (c) Same sample at higher magnification. (d) EDS spectrum.

Mechanism of the Formation of Metal Nanoparticles in Polymer Films. The main component of the film is the polymer (PS or PMMA for silver or gold clusters, respectively) that indeed absorbs the major part of the dose. Likewise for other molecules, the primary effect of irradiation is to excite or ionize the most abundant molecules, that is, the polymer, into the radical cation and an electron PS vvv → PS*, PS+•, e−

(1a) +•

PMMA vvv → PMMA*, PMMA , e

−

(1b)

The electron is trapped readily in the polymeric film 5338

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The Journal of Physical Chemistry C PS + e− → ePS−•

PMMA + e− → ePMMA −

of Ag/PS films, the phenomena induced by e-beam account for a negative tone resist because the exposed areas undergo a cross-linking reaction. Figure 8 presents optical micrographs of

(2a) (2b) +•

In competition, part of the electrons and cations of PS or PMMA+• undergo a geminate recombination that is efficient in these films of low dielectric constant PS+• + e− → PS

(3a)

PMMA+• + e− → PMMA

(3b)

A part of the escaped electrons are transferred to the metal ions, reducing them efficiently into atoms and eventually into clusters Ag + + ePS− → Ag 0 → ... → Ag n

(4a)

Au III + 3ePMMA − → Au 0 → ... → Au n

(4b)

When the other part of the electrons is scavenged by the polymer, the negative anions PS−• or PMMA−• induce the cross-linking or scission of the polymer, respectively. Simultaneously, the radical cations PS+• (or PMMA+•), not involved in the geminate recombination, may also initiate the cross-linking (or the degradation) of the polymer, as observed in solutions.21 This process makes the irradiated part of the film insoluble in PGMEA and permits the separation, by differential solubility, of the irradiated and unexposed parts of the film. PS+• + nPS → cross‐linked PS+•

(5a)

PMMA+• + PMMA → degraded PMMA+•

(5b)

Figure 8. Patterns obtained after irradiation by the nanoelectron beam. They are constituted by silver nanoparticles embedded in cross-linked PS supported on silicon. Loading Ag/PS = 5.1 wt %. Chip size: (150 × 150) μm2. The width of the lines depends on the number of scans of irradiation printed by the electron nanobeam in close vicinity (2.5 nm between two successive scans). The space without irradiation between two lines is also adjustable. (a) Group of five lines of 1 μm, each resulting from 400 scans of the nanobeam. The lines are separated by 1 μm of unirradiated area. (b) Group of five lines of 500 nm, each resulting from 200 scans of the nanobeam. The lines are separated by 1.5 μm of unexposed area. (c) Group of five lines of 500 nm, each resulting from 200 scans of the nanobeam. The lines are separated by an unexposed area 500 nm wide. (d) Group of 10 lines of 200 nm, each resulting from 80 scans of the nanobeam. The lines are separated by 800 nm of unirradiated area.

It is also not excluded that the radical cations reoxidize part of the metal atoms or oligomers, resulting in a lower radiolytic yield. Eventually, heating favors the short diffusion of surrounding ions in the vicinity of the exposed area of the film to the surface of radiation-induced nuclei and their growth by thermal reduction. As a result, the silver or gold nanoparticles are thus directly embedded during the irradiation in the depth of the cross-linked or chain-scissioned polymeric film. Part 2: Nanopatterning in Thin Ag Nanoparticles/PS Films. The above results have demonstrated the possibility to take advantage of the electron beam irradiation to induce simultaneously metal nanoparticle and cross-linking of the polymer to synthesize an unsoluble metal-loaded film. On the same basis of the choice of the components, of the conditions of preparation and irradiation of the composite film (Ag 5.2 wt %/PS), the high spatial resolution of the electron nanobeam may be exploited to create nanopatterns by “direct writing”. In this study of e-beam nanopatterning, the thin films of silver/PS are spin-coated at 3000 rpm for 30 s. The typical thickness is 100 nm. The very narrow electron nanobeam (2 nm cross section) is used to generate the nanostructure such as lines pattern. The distance between two successive irradiation dots or two successive scans is now 2.5 nm. The corresponding homogeneous dose is 29 mC cm−2 (950 J cm−2), that is, 145 times higher than in the above experiments. The ions are thus reduced at a higher fraction than above. The number of scans per line and the distance of unirradiated areas between lines are variable. After e-beam irradiation, the films are heated to 150 °C for 5 min to enhance the metal ion reduction. The thin films are then treated for 30 s in PGMEA to remove the unexposed area. Nanostructure dimensions are characterized by optical microscopy. In the case

line and space patterns of PS films of improved density and contrast containing Ag nanoparticles and deposited on Si. Although the exposure dose is not optimized, the most highly defined line and space widths on Si are 200 and 500 nm, respectively. The SEM image of the 500 nm patterns of cross-linked PS containing Ag nanoparticles is shown in Figure 9. The most important part of the silver NPs is within the polymeric lines. Actually, we also observe a very low number of silver nanoparticles in the area unexposed to the electron nanobeam, but these nanoparticles are found mostly near the lines. Further work is in progress to optimize the process to get Ag nanoparticles exclusively in exposed areas. The potential application of thin polymeric films containing metal nanoparticles as a mask of high resolution for lithography or as a template such as an imprint is thus demonstrated.

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CONCLUSIONS Conditions have been found for radiolytically synthesizing stable metal nanoparticles of silver or gold embedded in polymeric films by using an electron nanobeam. Simultaneously, the interaction of the electron beam with the polymer induces a cross-linking (PS) or a degradation (PMMA) that renders it insoluble in the initial casting solvent and fixed to the support. It permits a final differential solubilization and removal 5339

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(3) Oleksak, R. P.; Ruther, R. E.; Luo, F.; Fairley, K. C.; Decker, S. R.; Stickle, W. F.; Johnson, D. W.; Garfunkel, E. L.; Herman, G. S.; Keszler, D. A. Chemical and Structural Investigation of HighResolution Patterning with Hafsox. ACS Appl. Mater. Interfaces 2014, 6, 2917−2921. (4) Introduction to Microlithography, 2nd ed.; Thompson, L. F., Willson, C. G., Bowden, M. J., Eds.; ACS Professional Reference Book, American Chemical Society: Washington, DC, 1994; p 237. (5) Valentin, E.; Bernas, H.; Ricolleau, C.; Creuzet, F. Ion Beam ″Photography″: Decoupling Nucleation and Growth of Metal Clusters in Glass. Phys. Rev. Lett. 2001, 86, 99−102. (6) Bernas, H.; Chaumont, J.; Cottereau, E.; Meunier, R.; Traverse, A.; Clerc, C.; Kaitasov, O.; Lalu, F.; Ledu, D.; Moroy, G.; et al. Progress Report on Aramis, The 2 MV Tandem at Orsay. Nucl. Instrum. Methods Phys. Res., Sect. B 1992, 62, 416−420. (7) Espiau de Lamaestre, R.; Béa, H.; Bernas, H.; Marignier, J.-L.; Belloni, J. Irradiation-Induced Ag Nanocluster Nucleation in Silicate Glasses: Analogy with Photography. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 777−794. (8) Michalik, J.; Azuma, N.; Sadlo, J.; Kevan, L. Silver Agglomeration in Sapo-5 and Sapo-11 Molecular-Sieves. J. Phys. Chem. 1995, 99, 4679−4686. (9) Gachard, E.; Belloni, J.; Subramanian, M. A. J. Mater. Sci. Optical and ESR Spectroscopic Studies of Silver Clusters in Ag, Na-Y Zeolite by γ-Irradiation. J. Mater. Chem. 1996, 6, 867−870. (10) Yordanov, I.; Knoerr, R.; De Waele, V.; Bazin, P.; Thomas, S.; Rivallan, M.; Lakiss, L.; Metzger, T. H.; Mintova, S. Elucidation of Pt Clusters in the Micropores of Zeolite Nanoparticles Assembled in Thin Films. J. Phys. Chem. C 2010, 114, 20974−20982. (11) Knoerr, R.; Yordanov, I.; De Waele, V.; Mintova, S.; Mostafavi, M. Preparation of Colloidal BEA Zeolite Functionalized with Pd Aggregates as a Precursor for Low Dimensionality Sensing Layer. Sens. Lett. 2010, 8, 497−501. (12) Platzer, O.; Amblard, J.; Marignier, J. L.; Belloni, J. Nanosecond Pulse Radiolysis Study of Metal Aggregation in Polymeric Membranes. J. Phys. Chem. 1992, 96, 2334−2340. (13) Amblard, J.; Platzer, O.; Ridard, J.; Belloni, J. Computerized Simulation of Silver Aggregation and Corrosion in Polymeric Membranes. J. Phys. Chem. 1992, 96, 2341−2344. (14) Amblard, J.; Belloni, J.; Platzer, O. Synthèse Radiolytique et Réactivité Catalytique de Nanoagrégats Bimétalliques Greffés sur Diverses Electrodes. J. Chim. Phys. 1991, 88, 835−843. (15) Belloni, J.; Mostafavi, M.; Remita, H.; Marignier, J. L.; Delcourt, M.-O. Radiation-Induced Synthesis of Mono- and Multi-Metallic Nanocolloids and Clusters. New J. Chem. 1998, 22, 1239−1255. (16) Korchev, A. S.; Shulyak, T. S.; Slaten, B. L.; Gale, W. F.; Mills, G. Sulfonated Poly(Ether Ether Ketone)/Poly(Vinyl Alcohol) Sensitizing System for Solution Photogeneration of Ag, Au and Cu Crystallites. J. Phys. Chem. B 2005, 109, 7733. (17) Ito, H.; Willson, C. G. Chemical Amplification in the Design of Dry Developing Resist Materials. Polym. Eng. Sci. 1983, 23, 1012. (18) Yamamoto, H.; Nakano, A.; Okamoto, K.; Kozawa, T.; Tagawa, S. Polymer Screening Method for Chemically Amplified Electron Beam and X-Ray Resists. Jpn. J. Appl. Phys. 2004, 43, 3971−3973. (19) Yamamoto, H.; Kozawa, T.; Nakano, A.; Okamoto, K.; Yamamoto, Y.; Ando, T.; Sato, M.; Komano, H.; Tagawa, S. Proton Dynamics in Chemically Amplified Electron Beam Resists. Jpn. J. Appl. Phys. 2004, 43, L848−L850. (20) Mostafavi, M.; Marignier, J.-L.; Amblard, J.; Belloni, J. Nucleation Dynamics of Silver Aggregates. Simulation of the Photographic Development Process. Radiat. Phys. Chem. 1989, 34, 605−617. (21) Yamamoto, H.; Kozawa; Tagawa, S.; Naito, M.; Marignier, J.-L.; Mostafavi, M.; Belloni, J. Radiation-Induced Synthesis of Metal Nanoparticles in Ethers THF and PGMEA. Radiat. Phys. Chem. 2013, 91, 148−155.

Figure 9. SEM image of the Ag/PS patterns 500 nm wide (same conditions of irradiation as in Figure 8c). The Ag nanoparticles appear as white dots in gray polymer lines.

of the initial unexposed parts of the films containing the metal ion precursors Because the diffusion of the particles may be blocked in this material, the resolution of the very narrow electron nanobeam has been exploited to demonstrate the feasibility and the achievement of nanopatterning on these films. Therefore, this method supplies interesting applications such as masks, replica, or imprint molds of improved density and contrast.

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AUTHOR INFORMATION

Corresponding Authors

*H.Y.: E-mail: [email protected]. *J.B.: E-mail: [email protected]. ORCID

Jacqueline Belloni: 0000-0001-6623-393X Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 21st Century Center of Excellence (COE) program ‘Towards Creating New Industries Based on Inter-Nanoscience’ and the Collaboration contract between the Centre National de la Recherche Scientifique and the Institute for Scientific and Industrial Research. This work was supported in part by JSPS KAKENHI Grant Numbers (Nos. 24656447, 26706027, and 16K14439) from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). A part of this work was supported by “Nanotechnology Platform Project (Nanotechnology Open Facilities in Osaka University)” of Ministry of Education, Culture, Sports, Science and Technology, Japan. We thank Professor Shu Seki for his help for part of AFM measurements.

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REFERENCES

(1) Chen, J.-K.; Chang, C.-J. Fabrications and Applications of Stimulus-Reponsive Polymer Films and Patterns on Surfaces ; a Review. Materials 2014, 7, 805−875. (2) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Imprint of sub-25 nm vias and Trenches in Polymers. Appl. Phys. Lett. 1995, 67, 3114−3116. 5340

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