ARTICLE pubs.acs.org/JPCC
Nanoscale Two-Photon Induced Polymerization of Diacetylene LangmuirBlodgett Film by Near-Field Photoirradiation Akira Sakamoto,*,† Kazuhiko Mori,† Kohei Imura,‡,§,||,^ and Hiromi Okamoto‡,§ †
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Materials Science Division, Graduate School of Science and Engineering, Saitama University, Shimo-okubo 255, Sakura-ku, Saitama 338-8570, Japan ‡ Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan § The Graduate University for Advanced Studies, Okazaki, Aichi 444-8585, Japan PRESTO, Japan Science and Technology Agency, Honcho Kawaguchi, Saitama 332-0012, Japan
bS Supporting Information ABSTRACT: We have succeeded in nanoscale polymerization of LangmuirBlodgett (LB) films of cadmium 10,12-pentacosadiynoate (CdPA) by near-field two-photon excitation with femtosecond pulses. The LB films of CdPA were excited locally through an aperture of a near-field optical fiber probe by femtosecond-pulsed near-infrared radiation. The formation of polydiacetylene (PDA) films has been confirmed by two-photon induced and ordinary (linear) luminescence and Raman measurements by using a scanning near-field optical microscope. The polymerization of CdPA to PDA was induced by the femtosecond-pulsed near-infrared radiation but not by continuous wave radiation. This result confirms that the polymerization was not induced by heat arising from the near-field irradiation. The number of photons involved in the polymerization reaction was estimated to be 2 from the dependence of photoluminescence intensities of PDA on the power of incident light for polymerization.
1. INTRODUCTION Scanning near-field optical microscopy overcomes the restriction of spatial resolution of the conventional optical microscope determined by the diffraction limit of light1 and allows us to investigate optical properties on nanometric scales. Simultaneous topographic imaging is also practicable by this method, which is also occasionally advantageous for the studies of nanomaterials. In addition to such well-known advantages in nanoscale spectroscopy and imaging, the near-field optical method has a potential for fabricating various nanoscale structures on the basis of photochemical reactions. Near-field-based nanofabrication techniques have been developed for (1) material modification such as photopolymerization, (2) material deposition under gaseous materials such as chemical vapor deposition, and (3) material removal such as photoablation and photochemical etching.26 The apertureless near-field method has also been exploited for nanofabrication.6 Many types of diacetylene derivatives (R—CtC—CtC— R0 ) polymerize under heat, ultraviolet (UV), and γ-ray irradiation, forming blue- or red-colored polydiacetylene (PDA) with π-electron conjugated systems (Scheme 1).7 The polymerization reaction proceeds in various states such as single crystals,8 LangmuirBlodgett (LB) films,9 self-assembled thin films,10 and evaporated films.11 While UV irradiation is usually used in photoinduced polymerization of diacetylene to PDA, twophoton induced polymerization using visible light (633 nm) r 2011 American Chemical Society
has been demonstrated for spin-coated films on glass substrates.12 Because of unique properties of PDA in electronic conductivities and nonlinear optical susceptibilities, their applications to organic electronic devices and nonlinear optical and electro-optical devices were proposed.13,14 If we can photoinduce and control polymerization of PDA in nanometer scale, we may apply this technique to manufacturing various electronic and optical nanoscale devices made of PDA such as electronic circuits and waveguides. LB films are self-organized thin films with molecular-level flatness and are suitable to demonstrate fabrication of nanostructures by photoinduced polymerization with near-field irradiation of light, as well as to verify polymerization by near-field optical measurements. We have tried in this study nanoscale two-photon induced polymerization of LB films of a diacetylene derivative to PDA by near-field irradiation using a scanning near-field optical microscope (SNOM). Photoinduced polymerization of diacetylenes to PDA by near-infrared multiphoton excitation has several advantages over the usual UV-induced polymerization. For example, photodegradation of PDA is suppressed under near-infrared excitation as compared with UV excitation, because PDA has no electronic absorption in the near-infrared region. Then it may Received: October 20, 2010 Revised: March 6, 2011 Published: March 23, 2011 6190
dx.doi.org/10.1021/jp1100522 | J. Phys. Chem. C 2011, 115, 6190–6194
The Journal of Physical Chemistry C
ARTICLE
Scheme 1. Polymerization of CdPA LB Film (R1 = (CH2)11CH3, R2 = (CH2)8COO)
also suppress side reactions, and consequently the extent of polymerization might be much more easily controllable as compared with the UV polymerization. From the experimental aspect, the visible to near-infrared radiation is much easier to handle in a SNOM with an aperture-type optical fiber probe. In the present study, we have found that two-photon-induced polymerization of diacetylenes to PDA occurs locally in cadmium 10,12-alkyldiynoate LB films (Scheme 1), by irradiating femtosecond near-infrared pulses through a near-field probe aperture under the SNOM. Optical properties of the locally photopolymerized PDA films investigated by the SNOM are also discussed.
2. EXPERIMENTAL SECTION 10,12-Pentacosadiynoic acid (CH3(CH2)11—CtC—CtC —(CH2)8COOH, abbreviated as PA) was purchased from Tokyo Chemical Industry Co., Ltd., and purified by recrystallization from a n-hexane/chloroform (10:1, v/v) mixed solvent. The monolayers of cadmium salts of PA (cadmium 10,12-pentacosadiynoate, abbreviated as CdPA) were prepared by spreading a chloroform solution of PA (∼104 mol dm3) on an aqueous buffer solution (pH 6.8) containing CdCl2 (3 104 mol dm3) and KHCO3 (3 104 mol dm3). The surface pressurearea (πA) isotherms were measured for the monolayers using a Lauda film balance at 15 C. The monolayer of cadmium salts was transferred to a quartz substrate (0.1 mm thick) by the LangmuirBlodgett (LB) method15 at a surface pressure of 25 mN m1 and a temperature of 15 C. The transferred LB films of cadmium salts were annealed at 50 C for 72 h in a thermostated water tank, in order to form the highly oriented molecular alignment.16 A home-built apertured-type SNOM17,18 was operated under ambient conditions. The annealed LB films of CdPA were illuminated locally through an aperture of a gold-coated nearfield optical fiber probe (JASCO). The typical diameter of the aperture was 50100 nm. A shear-force feedback method was utilized to maintain the near-field probe in the vicinity of the sample surface, and its feedback signal was used to construct a topographic image. A mode-locked Ti:sapphire laser (SpectraPhysics MaiTai, 780 nm,
