Semiconducting Cross-Linked Polymer Nanowires Prepared by High

Oct 1, 2012 - High-energy charged particle irradiation of cross-linking polymers gives nanowires formed by cross-linking reactions along the ion track...
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Semiconducting Cross-Linked Polymer Nanowires Prepared by HighEnergy Single-Particle Track Reactions Shu Seki,*,† Akinori Saeki,† Wookjin Choi,† Yuta Maeyoshi,† Masaki Omichi,*,† Atsushi Asano,† Kazuyuki Enomoto,† Chakkooth Vijayakumar,† Masaki Sugimoto,‡ Satoshi Tsukuda,§ and Shun-ichiro Tanaka§ †

Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Japan Atomic Energy Agency, Takasaki Advanced Radiation Research Institute, 1233 Watanuki-machi, Takasaki, Gunma 370-1292, Japan § Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ‡

S Supporting Information *

ABSTRACT: High-energy charged particle irradiation of cross-linking polymers gives nanowires formed by cross-linking reactions along the ion track trajectories. Here, the direct formation of nanowires consisting of a conjugated polymer by single-particle nanofabrication technique (SPNT) is investigated. Poly(9,9′-di-n-octylfluorene) (PFO), regioregular poly(3-hexylthiophene) (rrP3HT), and poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) underwent an efficient cross-linking reaction upon irradiation, resulting in the formation of 1-dimensional nanostructures with high and desired aspect ratio reaching up to ∼200. The size of nanowires was perfectly interpreted by well-sophisticated theoretical aspects based on the statistical theory of polymer backbone configurations, suggesting that simple cross-linking reactions of the polymers determine the size and structure of nanowires. PFO based nanostructures exhibited sharp and intense emission with high fluorescence quantum yield indicating the absence of any significant inter/intra polymer chromophore interactions in the nanowires assemblies.

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INTRODUCTION The applications of high-energy charged particles, which can induce high-density energy deposition along the particle path, have extended to many fields in recent years. The interactions of MeV order high-energy charged particle with the matters have been used in cancer radiotherapy,1 breed improvement,2,3 etc., and their feasibility in materials science is also of considerable interest, where it has been known as nuclear track fabrication.4−6 The inhomogeneous energy deposition along a single-particle trajectory (ion track) plays a crucial role, allowing for high spatial selectively in the distribution of the radiation dose.7−9 The inhomogeneous energy deposition along a single particle trajectory (ion track) plays a crucial role, allowing for high spatial selectively in the distribution of the radiation dose. High-energy charged particles have often been locally characterized by their hard interaction with organic materials, giving cylinder-like nanospace along the ion tracks where reactive intermediates (ion radicals, neutral radicals, etc.) are produced nonhomogeneously with extremely high density.6,10,11 The chemical reactions within the limited nanospace are feasible to produce one-dimensional (1-D) nanomaterials, and we have successfully fabricated nanowires based on the cross-linking reactions in the polymer thin films as target materials for MeV order high-energy charged particles.10−16 The present negative tone imaging technique along the single particle (single-particle nanofabrication technique, SPNT) is © 2012 American Chemical Society

applicable to a variety of polymeric materials and their hybrids with equally controlled sizes (length, thickness, number density, etc.). Contrary to the conventional electron-beam or laser lithographic technique,17,18 the SPNT is essentially free from the engineering factors originated from beam spot size, beam diffraction, depth of focus, optical alignment, etc., because a 1-D nanowire is produced by a corresponding high-energy single particle. Conjugated polymers are a class of materials that combine the electrical and optical properties of traditional semiconductors with the mechanical and processing advantages of polymers. Poly(9,9′-di-n-octylfluorene) (PFO), regioregular poly(3-hexylthiophene) (rrP3HT), and poly[2-methoxy-5-(2′ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) are an important class of conducting polymers that can be used in flexible optoelectronic devices such as light-emitting diodes,19 photovoltaic cells,20 and field-effect transistors.21 Nanostructures based on conjugated polymer are expected to be applicable to nanosized optoelectronic devices. Not only the conjugated polymer materials but also small π-conjugated molecules has also successfully fabricated into 1-D nanostructures by the diverse methods of template-assisted synthesis, Received: July 12, 2012 Revised: September 14, 2012 Published: October 1, 2012 12857

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Figure 1. AFM topographic images of nanowires based on poly(9,9′-di-n-octylfluorene) (PFO) produced by SPNT. Images (a)−(c) were observed in the thin films of PFO (thickness = 400 nm) with a variety of scales ((a) 4 × 4 μm; (b) 2 × 2 μm; and (c) 1 × 1 μm, respectively, with the colorheight scale of 18 nm) after irradiation of 450 MeV Xe particles at the fluence of 1.0 × 109 ions cm−2.

Figure 2. AFM topographic images of nanowires based on regioregularpoly(3-hexylthiophene) (rrP3HT) produced by SPNT. Images (a)−(c) were observed in the thin films of rrP3HT (thickness = 450 nm) with a variety of scales ((a) in plane, 4 × 4 μm, color-height, nm; (b) 2 × 2 μm; (c) 1 × 1 μm) after irradiation of 450 MeV Xe particles at the fluence of 3.0 × 109 ions cm−2.

Fabrication of Nanowires. PFO and rrP3HT were dissolved in toluene and o-dichlorobenzene (3 wt %), respectively, and spin-coated on Si substrates. MEH-PPV was dissolved in toluene (1 wt %) and spin-coated on Si substrate. The thickness of the films was confirmed by a stylus surface profiler (Veeco, Dektak 150). The films of PFO, rrP3HT, and MEH-PPV were irradiated by 450 MeV 129Xe25+ or 490 MeV 192 Os30+ particles from cyclotron accelerator at Japan Atomic Energy Agency, Takasaki Advanced Radiation Research Institute. The number of incident particles was controlled from 5.0 × 108 to 1.0 × 1010 ions cm−2 to prevent overlapping of the particle trajectories. The irradiated films were developed directly in toluene, chlorobenzene, or chloroform for 1−30 min. Characterization of Nanowires. The sizes and shapes of the nanostructure formed along particle trajectories were observed using an AFM (SEIKO Instruments Inc., Nanocute OP and Nanonavi II). The observation of PFO nanowires on a quartz substrate was carried out using a fluorescence microscope (Olympus, DSU-IX81-SET). The ultraviolet−visible (UV−vis) absorption spectrum of PFO nanowires on a quartz substrate was recorded using a UV−vis spectrophotometer (JASCO, V-570). The fluorescence spectra of PFO nanowires on a quartz substrate were measured using a fluorescence spectrophotometer (Hitachi High-Technologies Co., F-2700). The loss of kinetic energy of particles due to penetration through the polymer films was estimated using the SRIM 2010 calculation code.24 Photoconductivity of the nanowire with a variety of aspect ratio was examined by a noncontact conductivity measurement system (flash-photolysis timeresolved conductivity measurement, FP-TRMC25,26) on a quartz substrate with the higher number density of the nanowires as 1010 wires cm−2.

electrospinning, nanolithography, self-assembly, etc., and revealing unique opto-electronic features.22 These highly sophisticated approaches are giving finely defined 1-D structures of π-conjugated molecules with the respective molecular design to control intermolecular weak interactions and/or chemical reactions; however, no versatile method has been reported to be applicable for a wide range of conjugated materials. In this article, the nanowires based on conjugated polymers were prepared by applying SPNT to their thin films. The isolated nanowires based on conjugated polymers were observed by atomic force microscope (AFM), and the sizes (length and radius of the cross-section of nanowire) were also estimated quantitatively. The stability of nanowires was discussed in terms of cross-linking efficiency in detail. Finally, the optical properties of nanowires based on PFO were compared with that of the solution and thin film.

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EXPERIMENTAL SECTION Synthesis. All reagents and chemicals were purchased from Nacalai Tesque, Inc., and Aldrich Chemical Co. Ltd. unless otherwise stated. Poly(9,9′-di-n-octylfluorene)s (PFO) were purchased from Aldrich Chemical Co. Ltd. or synthesized by the Suzuki−Miyaura coupling of 9,9′-dioctyl-2,7-dibromofluorene and 9,9′-dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester with (tetrakis(triphenylphosphine)palladium(0) (3 mol %) as the catalyst in toluene and aqueous 2 M K2CO3.23 The number-average molecular weights (Mn) of PFO were 6.0 × 103, 1.0 × 104, and 1.2 × 104 g mol−1. Regioregular poly(3hexylthiophene) (rrP3HT, Mn = 5.0 × 104 g mol−1) and poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV, Mn = 3.7 × 104 g mol−1) were purchased from Aldrich Chemical Co. Ltd. and used without further purification. 12858

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Figure 3. AFM topographic images of nanowires based on poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) produced by SPNT. Images (a)−(c) were observed in the thin films of MEH-PPV (thickness = 400 nm) with a variety of scales ((a) 4 × 4 μm; (b) 2 × 2 μm; and (c): 1 × 1 μm, respectively, with the color-height scale of 22 nm) after irradiation of 450 MeV Xe particles at the fluence of 1.0 × 109 ions cm−2.

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RESULTS AND DISCUSSION The thin films of PFO, rrP3HT, and MEH-PPV were exposed to 450 MeV Xe or 490 MeV Os particles at low fluences (5.0 × 108−1.0 × 1010 ions cm−2). The low fluence irradiation without overlapping between ion tracks produced single ion events in the films. The incident single ion penetrates the film, and then, the ion bombardment releases active intermediates at high density within a limited area along the single ion track. These intermediates form an inhomogeneous spatial distribution in the ion track due to the various chemical reactions involved. The cross-linking reactions along the ion track were mainly caused in PFO, rrP3HT, and MEH-PPV, resulting in the formation of a cross-linked cylinder-like nanogel (nanowire). The noncross-linked area can be removed by development with organic solvents, utilizing the difference in solubility of the cross-linked and noncross-linked polymers. The nanowires formed by ion bombardment can therefore be completely isolated. However, the nanowires lie down on the substrates during development procedures, thus the nanowires based on PFO, rrP3HT, and MEH-PPV were visualized by AFM as shown in Figures 1, 2, and 3. It has been reported that the sizes and number density of nanowires can be easily controlled by some parameters of both ion beam and polymers, especially the length of nanowires reflects the thickness of the original film. The length of PFO nanowires were in good agreement with the initial film thickness. However, the lengths of nanowires based on rrP3HT and MEH-PPV were not uniform because these nanowires fragmented to several parts during developmental procedures. It is considered that the stability of nanowires was strongly influenced by the number of crosslinking points induced into inner polymer chains. Thus, it is expected that the polymer with high cross-linking efficiency for ionizing radiation gives stabilized nanowires. The stabilization of nanowires with high density cross-links induced in PFO provides high and uniform aspect ratio of 1-D nanowires reaching up to ∼200. Recently, the development of a bulk heterojunction (BHJ) solar cell was reported successfully using 1-D conjugated polymer nanowires self-assembled with poly(3-alkylthiophene)s.27 The performance of the BHJ solar cell devices increased considerably both in power conversion and incident photon-to-electron efficiencies with an increase in the aspect ratio or nanowires ranging from 50−260. The present technique provides a versatile technique to produce conjugated polymer nanowires with the aspect ratio in the similar range, and fabricate the nanowires directly onto a variety of substrates including electrodes (Supporting Information, Figure S1). Conjugated polymer nanowires have been expected to give effective pathways for the charge carriers in the

electronic devises, and charge carrier mobility in the nanowires was also examined by Field-Effect Transistor device characteristics and/or Space Charge Limited Current measurement.28,29 The mobility of charge carriers along the nanowire axis was also examined by the noncontact conductivity measurement; flashphotolysis time-resolved microwave conductivity (FP-TRMC) measurement technique for the PFO nanowires (Supporting Information, Figure S2). The nanowires showed clear conductivity transients upon injection of charge carriers via photoexcitation (Supporting Information, Figure S3), giving the minimum mobility value of positive charges as 2 × 10−4 cm2 V−1 s−1. In case of self-assembled conjugated polymer nanowires, a considerable increase in charge carrier mobility was reported with an increase in the aspect ratio of nanowires; however, small dependence of mobility was observed for the PFO nanowires. FP-TRMC measurement probes predominantly the local motion of charge carriers with alternating electric field of probing microwaves, and this is the case giving small dependence on the local mobility of charge carriers in the present PFO nanowires. It should also be noted that the observed minimum value of mobility was almost consistent to the value observed for noncross-linked PFO, suggesting that the PFO nanowires preserves semiconducting nature with enough high hole mobility even after inducement of cross-links by SPNT. The radiation sensitivity (reaction efficiency) of polymers, such as cross-linking and scission G values (number of reactions per 100 eV of absorbed dose) have been studied extensively for several types of radiation. The G values are strongly dependent on the molecular structures. The specific properties of ion beams have been characterized by the linear energy transfer (LET), given by the energy deposition of an incident particle per unit length. The ion track radius is also an important parameter in such reactions, reflecting the local spatial distribution of energy deposited by an incident ion and influencing the character of subsequent chemical reactions. Thus, deposited energy density and cross-linking efficiency are very important factors for formation of the nanowire by SPNT. The size of the cross-section of the nanowire was dependent on LET, cross-linking G-values (G(x)), and molecular weight of polymers. The average radial sizes (r) of the nanowires were well interpreted by the theoretical model as follows:11 ⎡ ⎛ e1/2r ⎞⎤−1 LETG ( x ) mN p ⎥ ⎢ln⎜ ⎟ r2 = ⎜ ⎢ 400πρA ⎣ ⎝ rc ⎟⎠⎥⎦

(1)

where A is Avogadro’s number, m is the mass of a monomer unit, ρ (g nm−3) is the density of the polymer, N is the degree 12859

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of polymerization, LET (eV nm−1) is the averaged energy deposition of the incident particle per unit length along the trajectory, and rc and rp are the radii of the core and penumbra areas, defined from the equation theorem of collisions of charged particles with matter.30 All nanowires are no longer standing; they collapsed onto the substrate observed as the 2-D images. Assuming that the crosssection of nanowires was transformed into a simple ellipse, the real value of the radius of the cross-section (r) is defined using the following equation.15 Here, the values of r1 and r2 were defined as the half-width and half-height at half-maximum of the AFM trace of the cross-sections, respectively.

r=

r1r2

high local motion of the side chain is allowed, giving highly stabilized states of neutral radicals and hence the lower contribution to the cross-linking reactions.31 This is the case giving the lower cross-linking efficiency in MEH-PPV in comparison with the other polymers with long n-alkyl chains. The G(x) values in PFO cross-linking exhibit small dependence on the LET values of the incident charged particle of 490 MeV Os (LET = 13 000 eV/nm) and 450 MeV Xe (LET = 10 000 eV/nm) (Table 1). This implies that the equivalent spatial density of reactive radicals contribution cross-linking reactions were introduced at the surfaces of the nanowires based on the second-order manner of radical−radical coupling reactions in PFO. In contrast, the irradiation to the thin film of rrP3HT and MEH-PPV gave fragmented nanowires on the substrate as shown in Figure 3 because the sufficient number of cross-links to form nanogel structures was not introduced within each ion track along the projectile ions. These results indicate that the cross-linking efficiency (G(x)) is a very important factor to stabilize the nanowires. Figure 4 shows the AFM images of nanowires produced in PFOs having different molecular weights. The correlation between size (radius of the nanowire cross-section) and molecular weight was investigated precisely, and a clear increase in radius was observed as the molecular weight of PFOs increased. The radii of the PFO-based nanowires were 6.1, 7.0, and 7.5 nm for PFOs with molecular weights of 6.0, 10, and 12 × 103 g mol−1, respectively. On the basis of eq 1, the primary parameter determining the radii is the molecular weight (mN) of the target polymers in case of the identical LET value of incident particle because the dependence of G(x) on LET value can be negligible as summarized in Table 1. This gives striking interpretation to dependence of the radii on the molecular weight of PFO. Figure 5 shows radius distribution of the nanowires based on PFOs with molecular weight of 6.0, 10, and 12 × 103 g mol−1, respectively. The distribution of r increases with an increase in molecular weight. The value of radius distribution was defined as the standard derivation (σ) of r. Figure 6 shows the dependence of σ on the degree of polymerization (N, number of monomer units in polymer chains) of PFO. According to the statistical theory on the conformation of polymer chains, the gyration radius (Rg) of polymer molecules is determined by the following formula, using an impermeable rigid-body approximation and the Mark−Houwink equation:

(2)

The radii of the PFO (Os, Mn = 12000), rrP3HT, and MEHPPV nanowires were determined to be 7.5, 8.2, and 4.0 nm by AFM measurement and calculation on the ellipse model (eq 2), respectively. The radii, LET, and G(x) of nanowires based on a variety of targets are summarized in Table 1. Table 1. Radii, LET, and Cross-Linking Efficiencies of Nanowires Based on a Variety of Targets mN (g mol−1)

ρ (g cm−3)

r (nm)

LETa (eV nm−1)

G(x) ( 100 eV−1)

PFO

1.2 × 104

1.0

rrP3HT MEHPPV

5.0 × 104 3.7 × 104

1.1 0.99

6.9a 7.5b 8.2 4.0

10000a 13000b 10000a 10000a

2.3a 2.5b 0.77a 0.22a

targets

a The values of r and G(x) were observed for 450 MeV Xe irradiation with the calculated LET values for the identical particles by the SRIM 2010 calculation code.24 bThe values of r, G(x), and LET for 490 MeV Os particles.

The values of G(x) for PFO, rrP3HT, and MEH-PPV were 1.8, 0.77, and 0.22 by calculation using eq 1, with empirical parameters. The G(x) values of PFO and rrP3HT were much larger than that of MEH-PPV. The differences of G(x) among these polymers were clearly reflected in the shape of nanowires, especially the length. High-energy particles penetrating into thin films of the PFO with high cross-linking efficiency promote effective cross-linking reactions between the long n-alkyl side chains (radical−radical reactions) along the trajectories. However in MEH-PPV, bearing branched alkyl chains, the neutral radicals produced on the alkyl side chains transformed rapidly into tertiary radials via initially formed primary and penultimate radials at the room temperature where enough

⎛ N ⎞ν aσ = R g = κ ⎜ ⎟ ⎝ Mm ⎠

(3)

Figure 4. AFM images of nanowires based on poly(9,9-di-n-octylfluorene) (PFO) produced by SPNT. Images (a)−(c) were observed in the thin films of PFO (Mn = (a) 6.0 × 103, (b) 1.0 × 104, and (c) 1.2 × 104 g mol−1) after irradiation of 490 MeV Os particles at the fluence of 5.0−10 × 108 ions cm−2. 12860

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Figure 5. Radii of nanowires based on poly(9,9′-di-n-octylfluorene) (PFO) (Mn = (a) 6.0 × 103, (b) 1.0 × 104, and (c) 1.2 × 104 g mol−1) produced by SPNT. The nanowires were formed by 490 MeV Os particles irradiation on PFO film at the fluence of 5.0−10 × 108 ions cm−2.

Figure 7a shows the UV−vis absorption spectra of the thin solid film and nanowires prepared from PFO (Mn = 1.2 × 104 g mol−1) on quartz substrates. The absorption spectrum of nanowires showed nearly identical features as that of the thin film, except a few nanometers of red-shifting in the absorption maximum. The absorption maximum of the film was found to be at 378 nm, whereas that of the nanowires was at 387 nm. This indicates a possible difference in the chromophore interactions in the nanowires when compared to that of the film. The other two samples of PFO with lower molecular weights also showed similar absorption characteristics. Figure 7b−d shows the fluorescence spectra of the nanowires based on PFO (Mn = 1.2 × 104, 1.0 × 104, and 6.0 × 103 g mol−1 respectively) along with the corresponding spectra of thin solid film and toluene solution for reference. All three samples in toluene, where they exist in a molecularly dispersed state, exhibited sharp and narrow emission with vibronic features. However, the thin film emission was relatively broad and red-shifted with minimum vibronic features. This could be attributed to the aggregation of fluorene chromophores through inter/intra polymer interactions in the film state. The emission maximum obtained for the nanowire samples prepared from lower molecular weight samples (Figures 7c,d) was found to be more red-shifted than that of the corresponding spectrum in solution as well as in film state. However, the emission maximum of the higher molecular weight sample (Figure 7b) was red-shifted when compared to the solution and blue-shifted to that of the film. The emission from the lower molecular weight sample (Figure 7d) showed marked difference from both solution and film state, whereas the emission became more and more comparable with that of the solution state with

Figure 6. Correlation between the degree of polymerization (N) and the observed standard deviation of radius (σ) of poly(9,9′-ndioctylfluorene) (PFO) nanowires. The values of standard distribution were recorded after development with chlorobenzene.

where a is a scaling factor, Mm denotes the mass of a polymer chain unit, and κ is a constant.32,33 The correlation represented in Figure 6 is well described by eq 3, giving a value for the index ν of 0.62 in the case of development by chlorobenzene. The observed value of ν suggests that the polymer chains on the surface of nanowires was held in a relatively expanded state. This is due to contact with a good solvent (chlorobenzene for PFO) for the polymer during the development procedure. It should be noted that AFM observation of PFO nanowires has been performed in the air after development procedure. Because of the interaction of the nanowires and the substrate, the actual gyration radius (Rg) of the polymer molecules during development in the solvent is memorized on the substrate even after the drying process.

Figure 7. (a) Optical absorption spectra and (b−d) fluorescence spectra of nanowires (purple), thin films (blue), and toluene solutions of poly(9,9′di-n-octylfluorene) (PFO) (Mn = (a,b) 1.2 × 104, (c) 1.0 × 104, and (d) 6.0 × 103 g mol−1). The nanowires were formed by 450 MeV Xe particles irradiation on PFO film at the fluence of 1.0 × 1010 ions cm−2. The irradiated specimens were developed by toluene. The fluorescence spectra were recorded after excitation of the samples with 350 nm light. 12861

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fluorescence. The fluorescence of the nanowires based on PFO were observed and visualized by a fluorescence microscope (Supporting Information, Figure S4). The number of bright blue fluorescent dots (∼80 μm−2) observed in the image was almost identical to the number of nanowires in that particular area (100 μm−2). This suggests the role of PFO nanowires as emissive components in the case of hybridization with the other nanowires by the present SPNT technique.

increasing molecular weights (Figures 7b,c). Significant shift of emission maximum to the red-region and a considerable difference in the emission characteristics in lower molecular weight samples indicates the damage of the polymers on highenergy particle irradiation. However, the possibility for such damages diminishes with an increase in the molecular weight. Surprisingly, the emission obtained for all three nanowire samples were very sharp with strong vibrational features, indicating that the inter/intra polymer chromophore interactions were negligible in the nanowires. In addition to that, they exhibited a considerable emission in the range of 500−550 nm, which is particularly visible in Figures 7b,d. It has been reported that polyfluorenes easily oxidize to form keto-defect sites resulting in green emission due to energy transfer from fluorene to the keto-defects. In the present case, such oxidation processes are possible during the irradiation of high energy ion beam and subsequent exposure to the air, and hence, the emission in the 500−550 nm range must be originated from the keto-defect sites.34,35,37−39 The value of fluorescence quantum yield of PFO nanowires (40%) was about 2-fold higher than that of the thin solid film (20%). The extremely high quantum efficiency of fluorescence reiterates that the fluorene chromophores do not exhibit stacking interactions with neighboring units in the nanowires assembly. Fluorescence intensity of nanowires increases sublinearly with an increase in fluence, hence the number density of nanowires on the substrate as shown in Figure 8a. The volume

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CONCLUSIONS We revealed that the SPNT is universally applicable to fabricate one-dimensional nanostructures of various conducting polymers with finely controlled sizes. The stability of nanowires was strongly dependent on the cross-linking G-value of the polymers. Fluorescence emission analysis revealed that the polymers undergo some structural damage on irradiation, which significantly diminished with an increase in the molecular weights. The irradiation process also results in the formation of chemical defects in the case of polyfluorenes leading to the observation of emission between 500−550 nm. However, the irradiated samples exhibited sharp and intense emission with high fluorescence quantum yield indicating the absence of any significant inter/intra polymer interactions in the nanowire assemblies. This work illustrates the use of SPNT for the fabrication of nanostructures of functional polymers with precise control over the dimensions, useful for advanced device applications.

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing AFM topographic images of PFO nanowires on an Au substrate and on a quartz substrate for noncontact conductivity measurement, the conductivity transient observed for the nanowires by the noncontact conductivity measurement, and a fluorescence microscope image of PFO nanowires. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 8. Relationship between fluorescence intensity of poly(9,9′-din-octylfluorene) (PFO) nanowires and fluence for 490 MeV Os particles irradiation to PFO.

AUTHOR INFORMATION

Corresponding Author

*Tel/Fax:+81-6-6879-4586. E-mail: [email protected]. jp. Notes

fraction of nanowires (g) to the total volume of target polymers has been quantitatively derived as following simple expression of a form

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS Funding Program for NextGeneration World-Leading Researches (NEXT Programs) and KAKENHI from MEXT Japan (No. 22226017). This work was performed with the facility shearing program provided by JAEA.

g = 1 − exp( −nπr 2)

where n (m−2) is fluence of incident particles.11,12 On the basis of the value of r = 7.5 nm, fluence of 4.6 × 1011 cm−2 is predicted to give track−track overlapping at 50%; however, the sublinear increase was observed even in far lower range of fluence. As observed in the AFM images in Figures 2 and 3, nanowires show in-plane distribution of upside lines, which reflect trajectories of incident particle and are orthogonal initially to the substrate surface. The length of upside lines was 60 times longer than the radius of cross-section, and an entanglement of nanowires occurs in the development procedure. This is the case giving the fluorescence quenching via nanowire−nanowire interaction on the surface even in the 2 orders of magnitude lower range of fluence. Without the entanglement in the lower range of fluence, the nanowires exhibit extremely high quantum efficiency of

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REFERENCES

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dx.doi.org/10.1021/jp3069249 | J. Phys. Chem. B 2012, 116, 12857−12863