Near-field optical microscope observation of dye-containing nano-domains

Authors

  • N. Yamamoto,

    Corresponding author
    1. National Research Institute of Advanced Industrial Science and Technology, 1-8-31Midorigaoka, Ikeda, Osaka 563-8577, Japan
      Dr Noritaka Yamamoto. Tel.: +81 727 51 8613; fax: +81 727 51 9637; e-mail: noritaka.yamamoto@aist.go.jp
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  • T. Mizokuro,

    1. National Research Institute of Advanced Industrial Science and Technology, 1-8-31Midorigaoka, Ikeda, Osaka 563-8577, Japan
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  • H. Mochizuki,

    1. National Research Institute of Advanced Industrial Science and Technology, 1-8-31Midorigaoka, Ikeda, Osaka 563-8577, Japan
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  • S. Horiuchi,

    1. National Research Institute of Advanced Industrial Science and Technology, 1-8-31Midorigaoka, Ikeda, Osaka 563-8577, Japan
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  • T. Hayakawa,

    1. National Research Institute of Advanced Industrial Science and Technology, 1-8-31Midorigaoka, Ikeda, Osaka 563-8577, Japan
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  • T. Hiraga

    1. National Research Institute of Advanced Industrial Science and Technology, 1-8-31Midorigaoka, Ikeda, Osaka 563-8577, Japan
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Dr Noritaka Yamamoto. Tel.: +81 727 51 8613; fax: +81 727 51 9637; e-mail: noritaka.yamamoto@aist.go.jp

Summary

A novel method for forming dye-containing nano-domains in thin films using a polymer alloy system has been developed. The polymer alloy system (PS-b-PMMA), which consists of polystyrene (PS) and polymethyl methacrylate (PMMA), forms microphase separation in thin films. The film was treated using a previously reported technique under vacuum conditions, and an organic dye was selectively dispersed into the PS. Selective association of the dye (diarylethene; cis-1, 2-dicyano-1, 2-bis (2,4,5-trimethyl-3-thienyl) ethene) with the PS nano-domains was then observed, with both transmission electron microscopy and scanning near-field optical microscopy with an absorption spectrum.

Introduction

The need for higher density storage calls for development of new read/write techniques and storage materials. One candidate of read/write technique is near-field optical storage, which enables recording of marks smaller than the diffraction limit. Near-field evanescent light is not so high power, so this technique is not suitable for phase transition material. Nano-structured polymers, which self-organize into ordered structures, have been proposed as a higher-density storage material. However, most polymers are mutually immiscible: multicomponent polymer systems organize themselves into mesoscopic structures when they are cast into thin films. Block copolymer systems, with more than two different polymers linked at the ends of their chains, are more interesting because they form ordered nano-structures (Albrecht et al., 2000; Horiuchi et al., 2000; Yokoyama et al., 2000; Hayakawa, 2002). The distances between each domain are controllable as well as the domain size by choice of molecular weight and the polymer ratios. Attractive materials with novel optical properties have been derived from these nano-structures. Patterned media are preferable for high-density storage because the size and length of the mark that is employed can be reduced, and thus prevented from extending beyond nano-domain size. Polymer blending is useful for combining and improving the properties of commodity polymers used in the development of new materials. Because the properties of polymer blends are highly dependent upon their respective morphologies and interfaces, polymer blending is well suited to near-field surface storage. In this study, we demonstrate a selective dyeing technique for photochromic molecules used as storage material, and evaluate it using scanning near-field optical microscopy (SNOM). Photochromic molecules are amenable to photon-mode recording, which is considered to be a superior method of near-field optical recording owing to its high resolution and density, and its adaptability to re-writable media (Kaneuchi et al., 1997; Irie et al., 1999). We use the term ‘organic thin film’ not only in reference to the very small thickness of the film, but also to its particular properties and functions, which have not been achievable with conventional thin-film fabrication methods. We also discuss a vacuum technique, termed the ‘vapour transportation method’ (Hiraga et al., 2000), which shows promise in the preparation of a novel class of polymeric organic thin films. The present study seeks to provide an overview of the current status and future prospects for organic thin-film development. In addition, organic thin-film applications in industrial technology are considered.

The age of nano-technology dawned with the invention of scanning tunnelling microscopy (STM) in 1982 (Binning et al., 1982). Subsequently, several kinds of scanning probe microscopy (SPM) have also been developed, such as atomic force microscopy (AFM) (Binning & Quate, 1986). One of the latest – SNOM (Pohl et al., 1984) – is capable of versatile spectroscopy applications in a number of areas, including energy- and time-resolved spectroscopy, and polarization analysis. Whereas light transmission is relatively inefficient in conventional fibre probes, no such problem exists for photon-mode storage. Indeed, the high intensity of the light source can be problematic, as photochromic materials do not provide non-destructive readout. Feedback mechanisms coupled to shear force and fibre tip shape are not preferable for organic materials, because the tip cannot trace surface nano-structures and it cannot escape from the sample if an error occurs in tip–sample regulation, whereas a flexible fibre probe can avoid contact with a sample. The set-up discussed below for evaluating thin films with photochromic dye-containing nano-structure is available for both contact-mode and tapping-mode AFM (Muramatsu et al., 1995). In the present study, an AFM/SNOM system was employed.

Experiment

Materials and sample preparation

A diblock copolymer of polystyrene (PS) and polymethyl methacrylate (PMMA) (PS-b-PMMA) was prepared by conventional living anion polymerization (Horiuchi et al., 2000). (The average molecular weights of the PS and PMMA are 100 k and 600 k, respectively.) In addition, two thin films were prepared, one a block copolymer film, and the other a block copolymer/homopolymer blend, composed of PS and PMMA. In the copolymer/homopolymer film, rod-shaped PS domains (of less than 100 nm diameter) are dispersed in the PMMA matrix. After spin-coating a 0.5 wt% toluene solution on a glass substrate, the films were dried and annealed, and thermodynamically stable structures were kept under high vacuum at 170 °C for 1 day.

The films were then treated by a previously reported technique under vacuum conditions (Hiraga et al., 1990), in which an organic dye (diarylethene; cis-1, 2-dicyano-1, 2-bis (2,4,5-trimethyl-3-thienyl) ethane; Tokyo Kasei Co. Ltd) is selectively dispersed into the PS. Figure 1 shows the molecular structure of diarylethene together with the sample preparation scheme, referred to as the vacuum transportation method. The dye employed in the present experiment, diarylethene, has two different chemical forms, and is reversibly transformed from one (isomer A) into the other (isomer B) upon irradiation with light of appropriate wavelength. Photochromic reaction proceeded via irradiation with ultraviolet (UV) and visible light. In vacuum transportation, a sample substrate is first loaded in a Pyrex glass tube with a small amount of dye. After reaching a final pressure (∼10−6 Torr) via a vacuum pump, the glass tube is sealed by melting, to form an ampoule for the test piece. The ampoule is then set in an oven at a constant temperature of 115 °C for about 24 h (Hiraga et al., 1990; Mizokuro et al., 2002). Selectivity in dyeing is the primary advantage of this method, in which sublimation of the dye molecule is adjustable. Selectivity of the dye to the polymer depends on their mutual affinity. Molecular compound solubility can sometimes cause difficulties in creating an organic multilayer. In most cases, dip-coating and spin-coating can be employed to fabricate the organic thin layer. However, the solvents used in the second layer must not dissolve the first-layer compounds. When a compound is able to be sublimated, vapour transportation is a viable method for dyeing insoluble compounds.

Figure 1.

Representation of photochromic reaction of diarylethene (a), and schematic illustrations of the ‘vapour transport evaporation’ process. (b) Sample and dye are loaded into a glass tube and vacuumed to ultimate pressure, and (c) the glass tube is sealed and heated at the sublimation temperature of the dye molecules. GT, Pyrex glass tube; D, dye; S, sample substrate; CP, cutting point; HT, heater; TMP, turbo molecular pump; RT, rotary pump.

The sample is illuminated with a UV lamp (340 nm) just before the SNOM experiment to convert diarylethene to its B isomer, which has absorption at 532 nm.

Instruments

Topographic and SNOM images of the sample were taken with a commercial SNOM system (SPI-3700, Seiko Instruments). Details of the set-up are as described in Muramatsu et al. (1995). A bent aperture fibre probe with tip aperture diameter of 80–100 nm is used, in which the contact-mode AFM is safely applicable to observation of polymers. The feedback loop output signal provides a topographical AFM image of the sample surface. A 532-nm laser light generated by a CW Nd: YAG laser is coupled to the fibre end, illuminating the sample surface. Light transmitted through the sample is collected by an objective lens (40×, 0.5 NA, 0.3 mm: Sigma Koki Inc.). Because the working distance of this objective lens is short, samples are prepared on a cover glass (thickness of 0.15 mm). To avoid photo-isomerization of diarylethene compounds, we used a 780-nm diode laser for the optical lever. UV light of 340 nm is used to isomerize the photochromic molecules to their B isomer. Although 532-nm SNOM measurements are potentially destructive for this compound, we used a sufficiently weak power in the experiment to prevent destruction.

Simultaneous SNOM and AFM measurements were used to study the molecular orientation of the dispersed phase in the polymer alloy system. Images were obtained by scanning the sample in contact with the probe. Part of the light path is split towards a CCD camera, which allows simultaneous viewing of the tip and the sample. Lock-in detection provides the SNOM signal, and an optical image is obtained by scanning the sample surface.

Results and discussion

(PS-b-PMMA)/PS/PMMA system observed by TEM and AFM

Figure 2 shows two views of the nano-domain structure in a photochromic dye-containing PS/PMMA block copolymer (PS-b-PMMA) thin solid film, as observed using TEM and AFM. The sample was not treated with electron stains, such as osmium or ruthenium complexes. Mesoscopic phase separation is present in this polymer system. PS domains averaging 20 nm in size are dispersed in the PMMA matrix. By dispersing the dye into the polymer, dye-containing domains are visible by TEM. Figure 2(a) shows a spherical microphase structure (dark dots). In traditional TEM images, structures of polymer films are visualized with heavy metal staining. In this case, PS and PMMA phase structures are visualized without heavy metal staining, but by adding diarylethene, which contains sulphur atoms. A black circle corresponding to the PS domain with the dye is visible, where the dye has been selectively dispersed in the PS. The rod-shaped PS domain is perpendicular to the plane of observation. If the sample thickness is increased, thinner domains develop. The topographic image in Fig. 2(b) contains some small embossed areas, corresponding to the phase-separated PS domain in the TEM image. Because the evaporating rates of PS and PMMA differ in toluene solvent, an embossed structure is formed on the polymer surface. In the blended PS and PMMA system, PS occupies the higher domain, whereas the lower matrix consists of PMMA.

Figure 2.

TEM (a) and AFM (b) images of a microphase separation structure in thin solid film with PS/PMMA alloy system. This film forms microphase separation of PS and PMMA on the surface. The dark spot in the TEM image is from diarylethene molecules, which scatter electrons. A spot of about 20 nm in diameter is observed. The right-hand spot in the AFM image is higher within the whole area, and corresponds to the dark spot in the TEM image.

(PS-b-PMMA)/PS/PMMA ternary blend system

The PS nano-domain is densely stained when subjected to the diarylethene under high vacuum. However, this density of dye in the PS domain is not sufficient for SNOM absorption measurement because the PS domain is smaller than 20 nm, as shown in Fig. 2, and the thickness of the film is only 80 nm. A block copolymer/homopolymer blend of (PS-b-PMMA)/PMMA is therefore used. This blend is preferred to enlarge the PS domain. Figure 3 shows TEM, AFM topography and SNOM absorption images of the (PS-b-PMMA)/PMMA ternary blend system. The microphase separation structure of the PS/PMMA film is clearly visible, as shown in Fig. 3(a,b).

Figure 3.

(a) TEM, (b) AFM and (c) SNOM images of the (PS-b-PMMA)/PMMA ternary blend system. Adding PS with lower molecular weight than the alloy controls the domain size, whereas the addition of a similar low-molecular-weight PMMA controls distances between the domains. Laser light of 532 nm is used in the SNOM measurement, with the transmission light intensity of each pixel forming the image. The dark spot corresponds to the PS domain containing diarylethene molecules, which absorbs 532-nm light.

PS and PMMA homopolymers were each added to the (PS-b-PMMA) block copolymer, corresponding to the diameter of the PS domain and the space between the PS domains in the PMMA matrix, respectively. After spin-coating the 0.5 wt% toluene solution of the polymer mixture on a glass substrate, the film was treated under high vacuum at 170 °C for 24 h. In accordance with the procedure discussed in the previous section, diarylethene is dispersed in the PS domain by vapour transportation. In this case, the mean diameter of each PS domain is about 140 nm, and the mean length is about 180 nm, whereas the mean PS domain height is only about 8 nm. Some PS domains appear doughnut-shaped with an increase in domain size. When a solvent evaporates from copolymer/homopolymer at the substrate, homopolymers (PS and PMMA) with lower molecular weight are more easily moved than the copolymer molecule. Some of the PS homopolymer is left in the PMMA matrix and some of the PMMA homopolymer is left in the PS domain. The former forms smaller PS domains in PMMA matrix and the latter makes a hole in the PS domain. These phenomena are observed on Langmuir–Blodgett films: for example on l-α-sipalmitolphosphatidylethanolamine-nitrobenzoxadiazole (DPPE-NBD) monolayer (Horiuchi et al., 1988).

The PS domains seen here are visible as a result of the height differential between the PS and PMMA phases, caused by the different rates of solvent evaporation during the thin film drying process. In the SNOM optical image produced after UV light irradiation (Fig. 3c), a dark spot corresponding to the position in the topographic image is observed, similar to that seen in the TEM image. These results indicate that the dye-containing PS domain absorbs 532-nm light: it appears as a dark spot.

Once diarylethene molecules dye the PS domain, they will not dislodge from the PS polymer at atmospheric conditions. We tested a sample that did not originally contain dye, and found it did not show any contrast in the SNOM image. The thickness of the sample is about 80 nm as measured by AFM, and diarylethene dye is of low density, so the contrast in the SNOM image is not high. However, the dye-containing part is sufficiently enhanced visually to enable us to use recording media. This strongly suggests that the optical image contrast is obtained from the PS domain containing diarylethene molecules.

However, we unsure of the small change noted in the refractive index of domains that are dyed. The structure of the phase separation is small, and the film is extremely thin. Although the difference in refractive index between PS (n = 1.59) and PMMA (n = 1.49) is about 0.1, it cannot be directly observed in SNOM transmission imaging.

Conclusions

A dye-dispersed PS/PMMA block copolymer (PS-b-PMMA) was established by a combination vacuum and vapour transport method. This PS/PMMA block copolymer forms a nano-domain structure by microphase separation. Photochromic diarylethene molecules have variable sublimation characteristics and selective miscibility to PS polymers. The PS nano-domain is dyed densely for SNOM measurement when subjected to the diarylethene under high vacuum. We observed microphase separation containing photochromic dye using TEM and AFM. Diarylethene is converted to its B isomer, with absorption at 532 nm, and absorption imaging is demonstrated by SNOM. As expected from its bulk properties, the nano-domain also acts as a patterned mark for storage media.

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