11720
J. Phys. Chem. C 2009, 113, 11720–11724
Nanoparticle-Enhanced Photopolymerization† Kosei Ueno,‡,# Saulius Juodkazis,‡ Toshiyuki Shibuya,‡ Vygantas Mizeikis,§ Yukie Yokota,‡ and Hiroaki Misawa*,‡ Research Institute for Electronic Science, Hokkaido UniVersity, Sapporo 001-0021, Japan, DiVision of Global Research Leaders (Research Institute for Electronics), Shizuoka UniVersity, Hamamatsu 432-8561, Japan, and PRESTO, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan ReceiVed: February 26, 2009; ReVised Manuscript ReceiVed: May 4, 2009
Photopolymerization of commercial photoresist SU-8 assisted by plasmonic near-field enhancement in nanogaps separating gold nanoblocks was investigated. Photopolymerization rates enhanced by orders of magnitude were found in gold nanoblock structures with nanogaps narrower than 10 nm. The mechanisms responsible for local and nonlocal nonlinear photopolymerization of SU-8 are discussed. Introduction Nanoparticles of noble metals exhibit characteristic bands of optical attenuation at visible and infrared wavelengths due to localized surface plasmons.1-3 The localized surface plasmon bands are also associated with enhancement of the electromagnetic field due to its localization within a few nanometers’ distance from the surface of the nanoparticle.4-6 The locally enhanced near-field may also promote various nonlinear optical effects, like surface-enhanced Raman scattering (SERS), thus opening the possibility to use metal nanoparticles in ultrasensitive chemical sensors.7-10 New methods enabling photochemical reaction control are also typically realized using such metallic mesoscopic functionalized structures composed of features with sizes in the range from several nanometers to micrometers, in which the intensity of the incident electromagnetic field is enhanced due to the plasmonic Fro¨hlich condition and geometrical factor (also known as the “lightning rod effect”). Combination of both effects can help maximize the field intensity (I ) |E|2) enhancement. Field intensity enhancement by factors of up to ∼105 can be theoretically reached in nanocavities formed by closely spaced nanoparticles.11,12 The task of practical construction of such nanocavities requires fabrication techniques capable of structuring materials with resolution in the range of 1-10 nm in macroscopic areas with dimensions on the order of centimeters. Availability of such fabrication techniques would allow acceleration of photochemical reactions and its applications in various chemical sensors, photocatalysis, solar cells, and so forth.13 Interaction of the intense, locally enhanced electromagnetic field with matter is often a strongly nonlinear process. For example, photochemical processes can be additionally accelerated via secondary thermal, chemical, or mechanical effects. It is also helpful to remember that fluidic convection, flow, and surface tension mechanisms cannot be linearly extrapolated into the nanoscale.14 It is therefore important to study the role of these effects on the photochemical reactions in order to achieve practical results. In this work, we focus on peculiarities of †
Part of the “Hiroshi Masuhara Festschrift”. * To whom correspondence should be addressed. Phone: +81-11-7069358. Fax: +81-11-706-9359. E-mail:
[email protected]. ‡ Hokkaido University. § Shizuoka University. # Japan Science and Technology Agency.
photopolymerization assisted by nanoparticle plasmons in a commercial negative photoresist SU-8. The nanoscale system in which photopolymerization is induced consists of gold nanoparticles separated by nanometer-wide gaps between them. Such structures were demonstrated previously to exhibit large field enhancement factors due to predominant localization of the plasmonic near-field in the nanogaps.15 For example, photopolymerization of SU-8 by an enhanced electromagnetic field in the nanogaps using irradiation by a weak, incoherent source was achieved using similar structures.16 Here, we report on photopolymerization assisted by a nanostructured gold film, induced by femtosecond laser irradiation at irradiances per pulse as low as 50 MW/cm2, at which no photopolymerization occurs in unstructured gold films even after hours of laser irradiation. Spatial patterns of the photopolymerized material usually resemble the near-field patterns which induce photopolymerization.17 In this work, we demonstrate that although good qualitative resemblance between both patterns is obtained, various secondary effects, such as heating, heat diffusion, and cavity enhancement, should be also taken into account. Experimental Details To facilitate the field enhancement, gold nanoparticle structures were fabricated on glass substrates with nanometer-scale spatial resolution. The electron beam lithography (EBL) technique was used for the definition of masks of planar patterns of nanoparticles on the substrates, and the deposition of thin gold films over the mask was done by sputtering. To obtain the gold nanoparticles, liftoff of protected areas was subsequently performed.18 The geometry of the nanoblock patterns is described in detail in our previous reports15 and is also shown in Figure 1. The planar patterns consist of diagonally aligned squares with a side length of 120 nm, separated by narrow gaps of nanometer width between their nearest corners. Some of the fabricated structures consist of ensembles of nanoblock pairs, in which only two nearest nanoblocks interact strongly, producing surface plasmon modes localized in the nanogap of the pair, whereas a substantial separation between the neighboring pairs is maintained in order to limit the interpair electromagnetic coupling. Other structures consist of checkerboard patterns of nanoblocks, in which interactions occur between multiple nanoblocks, producing collective surface plasmon modes localized predominantly in the nanogaps. Series of samples having
10.1021/jp901773k CCC: $40.75 2009 American Chemical Society Published on Web 06/02/2009
Nanoparticle-Enhanced Photopolymerization
J. Phys. Chem. C, Vol. 113, No. 27, 2009 11721
Figure 1. (a) SEM images of gold nanoblock structures consisting of nanoblock pair and checkerboard patterns. Dimensions of the nanoblocks are (120 × 120 × 40) nm3. (b) Setup used for sample exposure and photopolymerization.
gap widths in the range from 1 to 40 nm were prepared. In all samples, gold structures occupy square regions of (10 × 10) µm2 on the substrate. These areas contain large ensembles (about 1000) of nanoblock pairs. High fidelity of the EBL allows preparation of nearly monodisperse ensembles of gold nanoparticles and has previously enabled fabrication of nanorod ensembles in which relative length variations by about 0.2% are detectable from the spectral shift of plasmonic peaks in the optical extinction spectra.19 Since an identical fabrication technique has been used for preparation of the samples in this study, its accuracy should be also similar, and statistical variations in the dimensions of nanoblocks and the nanogap width should not exceed 1.5 nm. The nanoblocks were formed by sputtering deposition and consist of 2 and 40 nm thick layers of chromium and gold, respectively, after which the photoresist mask was lifted off by washing in acetone. Scanning electron microscopy (SEM) images of the nanoblock pairs and checkerboards are shown in Figure 1a. As indicated above, the design size of nanoblocks is (120 × 120 × 40) nm3, which, according to previous experimental and theoretical studies, leads to plasmonic extinction peaks in the wavelength range of 700-800 nm, depending on the nanogap width.14 This spectral range overlaps well with the central wavelength of the laser; λ ) 800 nm used for optical exposure (see below). One can notice some structural imperfections, such as rough surface and rounded corners of the nanoblocks in the SEM images. However, it must be noted that the appearance of SEM images cannot be trusted entirely in evaluating the quality of the fabrication since imaging of such small features requires operation of the SEM apparatus close to its resolution limit (about 10 nm), at which various distortions and artifacts are likely to be present in the SEM images. Under these circumstances, the accuracy of the fabrication should be judged by carefully considering both the SEM images and the spectral response of the nanoparticles, as was done in our earlier study.19 After the fabrication, approximately a 1 µm thick film of SU-8 photoresist (MicroChem Co.) was deposited on the patterned areas of the substrate by spin-coating. Thus, nanoblocks became fully buried in the SU-8 film. Optical exposure experiments were carried out as shown schematically in Figure 1b. The samples were mounted on an Olympus optical microscope with an objective lens having a numerical aperture of NA ) 1.3. The beam of a femtosecond Ti:Sapphire laser (Tsunami, Spectra Physics) with a central wavelength of 800
Figure 2. SEM images of photopolymerized SU-8 regions in samples consisting of nanoblock pairs (a) and checkerboard patterns of nanoblocks (b-g) separated by 10 nm wide nanogaps and exposed by a linearly polarized laser beam along the directions indicated by arrows at a 516 MW/cm2 (per pulse) irradiance for different exposure times indicated in the figure. In (a), the nanogap area containing locally the photopolymerized region of SU-8 is emphasized by a dashed circle. The average height of the polymerized resist regions above the substrate reached 70 nm after 10 s and 150 nm after 100 s of exposure time.
( 6 nm, a pulse duration of 100 fs, and a repetition rate of 82 MHz was coupled into the microscope and focused on the sample by the lens. Both bulk gold and gold nanoparticles have negligible absorption at this wavelength (for nanoparticles, plasmonic extinction bands that can be observed at near-infrared and infrared wavelengths occur due to lossless elastic light scattering but not absorption), and direct heating by absorption was avoided. It is known that presurface melting of gold nanoparticles occurs already at 50 °C;20 gold nanoparticles 20 nm in diameter melt at 600 °C, a fraction of the bulk gold melting temperature of 1337 K.21 If heating and heating-induced melting were present, surface-tension-driven reshaping of the nanoblocks would modify their geometry, most likely increasing the nanogap width and suppressing field enhancement and optical extinction related to nanogap modes. This was not observed in our experiments, and we conclude that the influence of thermal effects on gold nanoparticles was negligible in the range of laser irradiances used. Results and Discussions Figure 2 shows typical examples of SU-8 photopolymerization patterns irradiated by laser pulses linearly polarized along
11722
J. Phys. Chem. C, Vol. 113, No. 27, 2009
the diagonals of the nanoblocks indicated by the arrows. In all samples, the nanogap width is 10 nm. Two nanoblock pairs shown on the left and right sides of Figure 2a, have different orientations of their long axis with respect to the polarization direction. For the left pair, so-called longitudinal plasmon (LP) modes of the pairs are predominantly excited, which are known to have the highest field intensity enhancement in the nanogaps. For the right pair, so-called transverse plasmon (TP) modes are excited. For these modes, spectral bands of optical extinction occur at somewhat shorter wavelengths, and field enhancement in the nanogap is weak. Two pairs of closely spaced nanoblocks shown in Figure 2a were irradiated under identical conditions, except for the orientation of the polarization with respect to the long axis of the pair. Thus, LP and TP modes were predominantly excited in the nanoblock pairs seen on the left and right sides of Figure 2a, respectively. The near-field of LP modes, strongly localized in the nanogap, produces substantial optical exposure of SU-8 in the nanogap, and the region of photopolymerized SU-8 bridging the nanogap can be clearly seen in the SEM image. In contrast, no photopolymerization in the nanogap can be induced by excitation of TP modes (earlier studies have indicated that the near-field of TP modes attains somewhat lower local intensity and is concentrated at the corners of nanoblocks along the polarization direction).16 Further examples of photopolymerization are given in Figure 2b-g for nanoblock ensembles arranged into checkerboard patterns. The symmetry of these structures obscures distinction between LP and TP modes, and their collective response is a mixture of these modes. However, the LP component is much more efficient in localizing the field in the nanogaps, and with increasing exposure dose, photopolymerization initially appears and remains stronger in the lines of nanogaps parallel to the polarization vector. This trend can be seen in Figure 2b and c. At higher exposure levels, photopolymerization gradually spreads over the entire pattern of gold nanoparticles. Such propagation of photopolymerization can be explained by diffusion of heat generated during exothermic cross-linking and its efficient redistribution assisted by gold nanoblocks. Thermal properties of gold nanostructures will be discussed later. It should be mentioned that polymerization around nanoblocks locally increases the refractive index of the photoresist, thus promoting polymerization along the direction perpendicular to polarization. The occurrence of a similar effect along the direction parallel to polarization vector has been reported earlier for the optical far-field region.22,23 In our case, the height of polymerized structures was limited to approximately 150 nm at maximum exposure, and the polymerization spread over the entire region of the nanostructures. This signifies the importance of the enhanced optical field, even for waveguide-mediated lateral growth of the polymerized film. Under the tight focusing used in the experiments, axial propagation of the photopolymerized volume was much smaller than both the Rayleigh length (approximately 3λ) and the thickness of the photoresist film (∼1 µm). Axial and lateral modifications of the polymerized structures after prolonged exposure at two different laser irradiances are compared in Figure 3. One can expect that by designing different patterns, propagation of polymerization can be guided beyond the illuminated area. The approximate size and total volume of photopolymerized SU-8 regions at different laser powers and exposure times were estimated from their footprint on the substrate observed by SEM and from their maximum height determined from atomic force microscopy (AFM), assuming a hemispherical shape of the top surface. These estimates are summarized in Figure 4. Expo-
Ueno et al.
Figure 3. Perspective-view SEM images of photopolymerized SU-8 on top of gold nanoblock pairs at different irradiances and exposure times. The nanoblock pairs are the same as those shown in Figure 2. The inset arrow shows incident polarization direction.
Figure 4. (a) Photopolymerized volume of SU-8 versus the irradiation time at different irradiance levels per pulse in structures with a nanogap width of 10 nm. (b) Dependence of the polymerization reaction rate on the laser power. Solid lines in the plots are guides to the eye only.
nential growth of the polymerized volume with exposure time is evident from Figure 4a. Although only two data points are available at the irradiance level of 0.6 MW/cm2, they exhibit the same general trend as those at higher irradiance levels. This trend reflects the behavior of the photopolymerization reaction
Nanoparticle-Enhanced Photopolymerization
J. Phys. Chem. C, Vol. 113, No. 27, 2009 11723
Figure 5. Photopolymerization of SU-8 in nanoblock pairs with different nanogap widths (indicated in the images) at a constant 52 MW/cm2 irradiance per pulse and 10 s exposure.
rate, shown in Figure 4b, which was estimated as a ratio between the total volume of the photopolymerized resist and the exposure time, and can be explained by self-propagation mechanisms discussed above. Photopolymerization in structures with different gap widths was compared visually using the SEM images shown in Figure 5. It is evident from the figure that nanogaps narrower than 10 nm are required in order to produce local field intensities sufficient for photopolymerization. On the other hand, no polymerization could be observed in structures with the narrowest nanogaps at exposures below 1 MW/cm2, even after prolonged exposures for 1000 s. Field enhancement in the nanogap and its penetration into the metallic gold of the nanoparticle may produce photopolymerization due to secondary exposure. One possible source of the secondary radiation is broadband continuum radiation and photoluminescence of gold, which is excited at the laser wavelength of 800 nm via two-photon absorption (TPA) and has a spectral peak near the 600 nm wavelength.24 Another mechanism may be related to hyper-Rayleigh scattering and second-harmonic generation at the gold surface.25,26 Finally, spontaneous and stimulated Raman scattering in regions of the photoresist subjected to a high-intensity field may also contribute to the secondary exposure. These effects are nonlinear and depend on the local field intensity due to the incident radiation, Eloc(ω0), and the re-emitted radiation, Eem ∼ Eloc(ω0 ( ∆ω); the overall enhancement factor becomes even larger. The contribution and importance of these phenomena are discussed below. Nonlinear optical interactions occurring in the spatial region of the enhanced optical field have profound influence on photopolymerization. The enhancement factor R of the electric field amplitude Ei can be described by treating metallic nanoparticle(s) as a cavity capable of trapping optical radiation.27 According to this picture, the two-particle cavity provided a field intensity enhancement by a factor of
√R )
|Eloc | 2 |Ei | 2
)
γradAc
Q2 4π2c2ηε0λ0 Veff
(1)
where Eloc is the local field, Q is the quality factor of the cavity, Veff is the effective mode volume, γrad is the radiative energy decay rate, Ac is the effective radiation cross section of the resonant cavity mode, c is the speed of light, ε0 is the permittivity of vacuum, λ0 is the wavelength of light in free space, and η is
the impedance of free space. The quantities Q and 1/Veff represent the spectral and spatial energy density of the resonant mode, respectively.27 Enhancement of the photoluminescence -2 , assuming and Raman scattering is expected to scale as ∝V eff that the reradiated wave may have a slight spectral shift with respect to the incident wave. It is helpful to remind here that photopolymerization in SU-8 occurs as chemically amplified cross-linking.28 The chemically amplified nature means that a single cross-linking event triggered via photoacid generation (PAG) leads to propagation of crosslinking and increased total cross-linked volume. Each molecule of epoxy-based photoresist SU-8 has eight cross-linking sites capable of PAG and polymerization promotion. The polymerization reaction is exothermic, and the released thermal energy leads to heating, which may in turn produce more cross-linking and also promote diffusion of the photo acids away from illuminated regions. In the nanoscale, thermal conductivity kc is reduced compared to the bulk value since the mean free path of the carriers, Λ, becomes reduced29
kc )
FcνΛ 3
(2)
where ν is the mean velocity of the thermal carriers (molecules, phonons, electrons), F is the density of the carriers, and c [J kg-1 K-1] is the specific heat. The mean free path of the thermal carriers can be expressed via the thermal velocity ν and average time interval between the scattering events, τ, tha tis, Λ ) ντ. When dimensions of objects become smaller than the macroscopic value of Λ (typically several tens of nanometers), scattering increases, and τ becomes reduced, causing a decrease of Λ and kc in eq 2. According to these considerations, nanoscale volume objects can localize heat more efficiently. This factor may provide an additional mechanism of positive feedback to exothermic reactions of cross-linking and polymerization in the nanometer-wide gap regions. In addition, chemical enhancement mechanisms should be also considered since chemical enhancement is usually present, and effective HOMO-LUMO electronic transitions in the photoinitiator including the photoresist are strongly affected by the presence of metals. As a rule, the effective band gap becomes narrower, and some midgap defect-type electronic levels appear upon adsorption to metal, leading to broadening of the electronic energy levels of the adsorbed species.30 This effect can be recognized by Raman line shifts of adsorbates. Such modifications of the electronic energy level structure increase the probability of nonlinear excitations; for example, TPA processes become transformed into a much more efficient two-step absorption. Conclusion We have observed and investigated photopolymerization of photoresist SU-8 facilitated by gold nanoblock structures capable of plasmonic near-field enhancement in the nanogaps between nanoblocks. At low laser irradiance levels of ∼50 MW/cm2, gap widths ranging from 1 to 10 nm were needed in order to produce photopolymerization. This finding reflects the circumstance that narrower gaps produce stronger field localization and higher intensity, which compensates the low average intensity of the incident laser beam. The volume of the photopolymerized resist was found to increase exponentially with the incident beam power. The volume increase is due to lateral spreading of the photopolymerized region on the
11724
J. Phys. Chem. C, Vol. 113, No. 27, 2009
substrate, while its height above the substrate saturates to the value of about 150 nm. For higher exposures, spreading of the photopolymerization extended to regions which were outside of the near-field localization regions, indicating the presence of secondary exposure mechanisms. Among such mechanisms, cavity-enhanced photoluminescence and second harmonic generation from the gold surface, as well as second harmonic generation and Raman scattering, are the most likely. In addition, reduced thermal conductivity and an increased refractive index of the exposed resist areas can provide a positive feedback mechanism which promotes spatial spreading of polymerization. These mechanisms can possibly be used in the future for controlled initiation and propagation of photopolymerization or other photochemical reactions using nanostructured metallic films. Acknowledgment. This work was supported by funding from the Ministry of Education, Culture, Sports, Science, and Technology of Japan: KAKENHI Grant-in-Aid for Scientific Research on the Priority Area “Strong Photon-Molecule Coupling Fields” (No. 470 (No. 19049001)) and 20710068, and Grants-in-Aid from Hokkaido Innovation through Nanotechnology Support (HINTS). References and Notes (1) Yu, Y.-Y.; Chang, S.-S.; Lee, C.-L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661–6664. (2) Link, S.; El-sayed, M. A. J. Phys. Chem. B 1999, 103, 8410–8426. (3) Kelly, K. L.; Coronado, Ed.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668–677. (4) Stockman, M. I.; Bergman, D. J.; Anceau, C.; Brasselet, S.; Zyss, J. Phys. ReV. Lett. 2004, 92, 057402-1-057402-4. (5) Bouhelier, A.; Beversluis, M. R.; Novotny, L. Appl. Phys. Lett. 2003, 83, 5041–5043. (6) Imura, K.; Nagahara, T.; Okamoto, H. J. Phys. Chem. B 2005, 109, 13214–13220.
Ueno et al. (7) Jeanmaire, D. L.; Van Duyne, R. P. J. Electroanal. Chem. 1977, 84, 1–20. (8) Nie, S.; Emory, S. R. Science 1997, 275, 1102–1106. (9) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Phys. ReV. Lett. 1997, 78, 1667–1670. (10) Sawai, Y.; Takimoto, B.; Nabika, H.; Ajito, K.; Murakoshi, K. J. Am. Chem. Soc. 2007, 129, 1658–1662. (11) Xu, H.; Baus, M. Phys. ReV. E 2000, 61, 3249–3251. (12) Hao, E.; Schatz, G. C. J. Chem. Phys. 2004, 120, 357–366. (13) Yu, K. F.; Tian, Y.; Tatsuma, T. Phys. Chem. Chem. Phys. 2006, 8, 5417–5420. (14) Tabeling, P. Introduction to Microfluidics; Oxford University Press: Oxford, U.K., 2005. (15) Ueno, K.; Juodkazis, S.; Mizeikis, V.; Sasaki, K.; Misawa, H. AdV. Mater. 2008, 20, 26–30. (16) Ueno, K.; Juodkazis, S.; Shibuya, T.; Yokota, Y.; Mizeikis, V.; Sasaki, K.; Misawa, H. J. Am. Chem. Soc. 2008, 130, 6928–6929. (17) Murazawa, N.; Ueno, K.; Mizeikis, V.; Juodkazis, S.; Misawa, H. J. Phys. Chem. C 2009, 113, 1147–1149. (18) Ueno, K.; Mizeikis, V.; Juodkazis, S.; Sasaki, K.; Misawa, H. Opt. Lett. 2005, 30, 2158–2160. (19) Ueno, K.; Juodkazis, S.; Mizeikis, V.; Sasaki, K.; Misawa, H. J. Am. Chem. Soc. 2006, 128, 14226–14227. (20) Plech, A.; Kotaidis, V.; Lorenc, M.; Boneberg, J. Nat. Phys. 2006, 2, 44–47. (21) Papon, P.; Leblon, J.; Meijer, P. H. E. The Physics of Phase Transitions: Concepts and Applications; Springer: Berlin, Germany, 2002. (22) Kewitsch, A. S.; Yariv, A. Opt. Lett. 1996, 21, 24–26. (23) Matsuo, S.; Miyamoto, T.; Tomita, T.; Hashimoto, S. Appl. Opt. 2007, 46, 8264–8267. (24) Beversluis, M. R.; Bouhelier, A.; Novotny, L. Phys. ReV. B 2003, 68, 115433-1115433-10. (25) Bouhelier, A.; Beversluis, M.; Hartschuh, A.; Novotny, L. Phys. ReV. Lett. 2003, 90, 013903-1013903-4. (26) Nappa, J.; Revillod, G.; Russier-Antoine, I.; Benichou, E.; Jonin, C.; Brevet, P. F. Phys. ReV. B 2005, 71, 165407-1165707-4. (27) Maier, S. A. Opt. Express 2006, 14, 1957–1964. (28) Liu, J.; Lu, Y. J. Fluoresc. 2004, 14, 343–354. (29) Rogers, B.; Pennathur, S.; Adams, J. Nanotechnology: Understanding Small Systems; CRC Press Taylor & Francis Group: Boca Raton, FL, 2008. (30) Gross, A. Theoretical Surface Science: A Microscopic PerspectiVe; Springer: Berlin, Germany, 2003.
JP901773K
