Langmuir Nanoarchitectonics: One-Touch Fabrication of Regularly

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Article pubs.acs.org/Langmuir

Langmuir Nanoarchitectonics: One-Touch Fabrication of Regularly Sized Nanodisks at the Air−Water Interface Taizo Mori,†,‡ Keita Sakakibara,†,‡ Hiroshi Endo,† Misaho Akada,†,‡ Ken Okamoto,† Atsuomi Shundo,† Michael V. Lee,† Qingmin Ji,† Takuya Fujisawa,†,§ Kenichiro Oka,∥ Mutsuyoshi Matsumoto,∥ Hideki Sakai,§ Masahiko Abe,§ Jonathan P. Hill,†,‡ and Katsuhiko Ariga*,†,‡ †

World Premier International (WPI) Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ JST, CREST, Gobancho, Chiyoda-ku, Tokyo 102-0076, Japan § Department of Pure and Applied Chemistry, Faculty of Science and Technology and ∥Department of Materials Science and Technology, Faculty of Industrial Science, Tokyo University of Science, 2641 Yamazaki, Noda 278-8510, Japan ABSTRACT: In this article, we propose a novel methodology for the formation of monodisperse regularly sized disks of several nanometer thickness and with diameters of less than 100 nm using Langmuir monolayers as fabrication media. An amphiphilic triimide, tri-n-dodecylmellitic triimide (1), was spread as a monolayer at the air−water interface with a water-soluble macrocyclic oligoamine, 1,4,7,10-tetraazacyclododecane (cyclen), in the subphase. The imide moieties of 1 act as hydrogen bond acceptors and can interact weakly with the secondary amine moieties of cyclen as hydrogen bond donors. The monolayer behavior of 1 was investigated through π−A isotherm measurements and Brewster angle microscopy (BAM). The presence of cyclen in the subphase significantly shifted isotherms and induced the formation of starfish-like microstructures. Transferred monolayers on solid supports were analyzed by reflection absorption FT-IR (FT-IR-RAS) spectroscopy and atomic force microscopy (AFM). The Langmuir monolayer transferred onto freshly cleaved mica by a surface touching (i.e., Langmuir−Schaefer) method contained disk-shaped objects with a defined height of ca. 3 nm and tunable diameter in the tens of nanometers range. Several structural parameters such as the disk height, molecular aggregation numbers in disk units, and 2D disk density per unit surface area are further discussed on the basis of AFM observations together with aggregate structure estimation and thermodynamic calculations. It should be emphasized that these well-defined structures are produced through simple routine procedures such as solution spreading, mechanical compression, and touching a substrate at the surface. The controlled formation of defined nanostructures through easy macroscopic processes should lead to unique approaches for economical, energy-efficient nanofabrication.

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INTRODUCTION

processes for constructing nanosized and quantum-sized structures from molecules and atoms have recently been paid much attention. An important target in material nanoarchitectonics is the preparation of well-defined quantum-sized objects,4 which are categorized as quantum dots, wires, or wells usually with a selected dimension being less than 10 nm. In particular, 2D materials such as graphene have recently been intensively investigated.5 Additionally, the restriction of structure to a 2D plane converts extended quantum wells to size-controlled quantum disk objects whose thickness is a few nanometers, perhaps leading to quantum effects orthogonal to the object plane and confined in-plane effects when widths are between 10

Practical and theoretical studies on nanoscience and nanotechnology are dominated by nanomaterials with controlled dimensions and morphologies. Fabrication techniques have been developed to provide nanomaterials with defined structures that are created by top-down methods such as sophisticated microfabrication techniques. However, top-down techniques are not practical for all materials, especially in cases of thermally and mechanically fragile soft matter. Therefore, the so-called bottom-up approaches are expected to contribute more to the nanostructuring of such materials. Bottom-up approaches rely on self-assembly processes to form selected structures through the spontaneous association of atoms, molecules, clusters, and particles.1 Such strategies can be included in the wider concept of nanoarchitectonics2,3 in which nanostructured materials and/or materials with internal nanostructures are constructed using various building blocks through appropriate interactions between them. In particular, © XXXX American Chemical Society

Special Issue: Interfacial Nanoarchitectonics Received: October 30, 2012 Revised: December 25, 2012

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Figure 1. Methodology for the formation of regularly sized nanodisk-containing Langmuir monolayers as fabrication media.

molecular interactions at the air−water interface create selected molecular pairs that in turn lead to regular patterns with molecular-level unit structures.19 However, strong local interactions cannot be used to control large-scale domain sizes, and the use of weak interactions should provide greater control over the domain size in 2D compared to the stronger and more specific molecular recognition events. Dewetting processes are widely known to be useful for the formation of regular patterns.20 In one excellent example, Chi and coworkers proposed a method to create regularly aligned and onedimensionally extended nanoscopic channels through anisotropic dewetting using the Langmuir−Blodgett (LB) process.21 Continuous shifts of 1D interfacial lines during the vertical lifting of a solid substrate through a Langmuir monolayer resulted in regular 1D nanostructures. If the process is similarly conducted in two dimensions by horizontal-touching transfer of the Langmuir monolayer, then regularly shaped 2D nanostructures such as quantum disks could be formed. In fact, horizontal dewetting processes on the macroscopic scale are known to yield regular 2D patterns.22 To establish a novel methodology for the formation of regularly sized organic nanodisks through a nonlithographic interfacial process based on the strategy described above (Figure 1), an amphiphilic triimide, tri-n-dodecylmellitic triimide (1),18,23 and a water-soluble macrocyclic oligoamine, 1,4,7,10-tetraazacyclododecane (cyclen), were investigated. The imide moieties of 1 act as hydrogen bond acceptors and can interact weakly with the secondary amine moieties of cyclen as hydrogen bond donors. The Langmuir monolayer that formed was transferred onto freshly cleaved mica by the surface touching (i.e., Langmuir−Schaefer) method. We have already demonstrated the validity of this methodology (i.e., Langmuir nanoarchitectonics) in a preliminary communication.18 In this article, we report detailed analyses of this method as well as new experimental results including the formation of mesoscale

and 100 nm. For example, a phase transition from half-metal to semiconductor for ErP quantum disks can be induced by reducing their thickness to less than 3 nm.6 Also, the regulation of vector magnetism in two dimensions within magnetic disks of diameters of ca. 100 nm is useful for information propagation in cellular automata.7 Anticipated potential uses for nanosized and quantum-sized disk objects include artificial atoms,8 applications involving surface plasmon resonance,9 logic and memory operations,10 and the direct observation of individual biological species.11 Because lithographic processes for preparing defined size or shape nanosized and quantum-sized disk objects, especially those composed of soft materials, remain limited by high costs and low energy efficiency, alternative approaches based on nanoarchitectonic-type self-assembly into 2D structures and the subsequent transcription of those into the desired materials should be applied.12 However, most self-assembly processes using block copolymers,13 oxidized organogelators,14 or saltfree surfactant mixtures15 provide disklike objects with excessive thicknesses (several tens of nanometers) and polydispersities. In contrast, molecular assemblies confined at interfaces such as Langmuir monolayers, lipid bilayers, and bicelles have thicknesses of only several nanometers, although they usually possess extended lateral dimensions (i.e., micrometer scale or more) even after phase separation16 or other processes.17 In this article, we propose a novel methodology for the creation of monodisperse size-controlled disk objects of several nanometers thickness and with diameters of less than 100 nm using Langmuir monolayers as fabrication media (Figure 1). This kind of methodology can be referred to as Langmuir nanoarchitectonics. The proposed technique can be realized by combining two processes: (i) relatively weak intermolecular interactions within an interfacial medium and (ii) 2D quick dewetting.18 We have successfully demonstrated that specific B

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Figure 2. (A) π−A isotherms for 1 on aqueous solutions of cyclen of various concentrations at 20.0 ± 0.2 °C. (B) Molecular area values at 10 mN m−1 in plot A as a function of cyclen concentration ([G]). (C) Temperature dependence of π−A isotherms for 1 on an aqueous solution of 1.0 mM cyclen. (D) Molecular area values at 10 mN m−1 in plot C as a function of the subphase temperature. (BAM) was performed on a custom-built setup equipped with a CCD camera and a video recorder. A He−Ne laser (632.8 nm) was used as the monitoring light.

objects and the transcription of soft disk objects into hard metallic structures.

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EXPERIMENTAL SECTION

RESULTS As shown in Figure 2A, the surface pressure (π)−molecular area (A) isotherm of 1 on pure water contains a plateau region at a surface pressure of about 17 mN m−1 characteristic of a phase transition. This plateau region extends down to less than 0.3 nm2, which corresponds to only half of the expected area for an ideal close-packed state of three alkyl chains (ca. 0.6 nm2). Therefore, this transition is not a typical transition from the expanded phase to the condensed phase of monomolecular layers. The molecular area for the observed transition (1.15 nm2) corresponds to an intermolecular distance of 1.18 nm for a 2D hexagonal model structure (see below). This transition might be regarded as a conversion from a 2D to a 3D structure when pure water is used as the subphase. The presence of cyclen at various concentrations (up to 2.0 mM) in the subphase led to a decrease in the molecular area in the low-pressure region and an increase in the transition-point pressure. These tendencies were promoted when the cyclen concentration in the subphase was increased. As summarized in Figure 2B, the decrease in the molecular area of the monolayer state below the transition point appears to reach saturation. This behavior may reflect conformational and orientational changes of 1 upon binding of cyclen from the subphase. Similar variations in molecular areas have often been observed to occur during molecular recognition at the air−water interface.19 The addition of cyclen induces increases in both the transition pressure and pressure in the plateau region. Although the detailed mechanism of the latter isotherm behaviors may be complex, it is reasonable to suggest that the binding of cyclen to 1 tends to suppress the transition from the 2D to 3D phase.

Materials. Solvents and materials were purchased from Tokyo Chemical Industry Co. Ltd. (Tokyo, Japan) and Wako Pure Chemical Co. (Osaka, Japan) and were used without further purification. 1H NMR spectra were recorded on a JEOL AL300BX spectrometer (300 MHz) in chloroform-d1 with tetramethylsilane as an internal standard. Chemical shifts (δ) and coupling constants (J) are given in ppm and Hz, respectively. MALDI-TOF-MS spectra were acquired using a Shimadzu-Kratos AXIMA+ mass spectrometer with dithranol as the matrix. Water used for the subphase was distilled using an Autostill WG220 (Yamato) and deionized using a Milli-Q Lab (Millipore). Its specific resistance was greater than 18 MΩ cm. Spectroscopic-grade chloroform (Kanto Chemical Industries, Ltd.) was used as the spreading solvent. Details of the synthesis of triimide derivatives (1) have been reported previously.18,23 Measurements. π−A isotherms were measured at 20.0 °C (except a few examples) using a computer-controlled film balance (USI System FSD-300, USI, Fukuoka, Japan). Fluctuation of the subphase temperature was within ±0.2 °C. Following spreading of the sample, the solvent was allowed to evaporate for 15 min. Compression was commenced at a rate of 0.2 mm s−1. The horizontal lifting method was used to deposit the surface monolayer onto a substrate. Monolayers were transferred onto freshly cleaved mica substrates (for AFM observation) or gold-deposited glass substrates (for FT-IR-RAS measurement) at different surface pressures. The cast film was prepared from a mixed solution of 1 in chloroform (1.4 mM) and cyclen in methanol (1.4 mM). Topographic images of films were taken with an atomic force microscope (AFM, SPA400, Seiko Instruments, Japan). Measurements were taken in constant force mode in air at 293 K using a 20 μm × 20 μm scan head and a silicon nitride tip on a cantilever with a spring constant of 0.022 N m−1. Reflection absorption FT-IR (FT-IR-RAS) spectra were recorded on a Nicolet 670SX FT-IR spectrophotometer equipped with an MCT detector. All data were collected at a spectral resolution of 4 cm−1. Brewster angle microscopy C

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without cyclen in the subphase, corresponding to disordered alkyl chains that include a gauche conformation (Figure 3B). These observations imply that the packing of molecules of 1 is not predominantly determined by the crystalline packing of the alkyl chains. The morphologies of the films at the air−water interface and on a solid substrate (mica) were investigated by using Brewster angle microscopy (BAM) and atomic force microscopy (AFM), respectively. BAM observation was carried out at various pressures with and without cyclen in the subphase (1.0 mM). The formation of any objects could not be detected in monolayers of 1 on pure water (Figure 4a−c) at the transition pressures stated. These observations indicate that significant aggregation does not occur and that molecules of 1 are homogeneously dispersed on pure water. In sharp contrast, the presence of cyclen induced the formation of microscale objects upon compression of a monolayer of 1 on 1.0 mM aqueous cyclen above the transition pressure (Figure 4d−f). AFM observations of the transferred monolayer reveal that these objects could be assigned as “microstarfish” structures. These structures were similarly found for samples transferred to mica from the subphase (dewetting process) in either vertical dipping mode (Figure 5a) or horizontal drawing-up mode (Figure 5b). The microstarfish objects have diameters of 1 to 3 μm and heights of 30 to 60 nm. The formation of the regular microstarfish objects could not be achieved by the simple solution mixing of 1 with cyclen so that only interfacial interactions between the two components lead to the formation of these regularly shaped objects. There are only a few examples reported of starlike objects prepared using different synthesis strategies.24 The corresponding objects are less prevalent in the sample transfer from air (quick dewetting) in surface touching mode. Although the microstarfish objects (Figure 5c) and their collapsed forms (Figure 5d) could be detected in selected areas on mica, most of the substrate was covered homogeneously with small dotlike objects (Figure 5e,f). Interestingly, the latter dotlike objects could not be detected in the samples prepared by vertical dipping and horizontal drawing-up modes. Structures of 1 with cyclen obtained by the surface touching method on mica surfaces were further analyzed by AFM observation.18 The objects take dot or disk shapes (Figure 6). Height and diameter data of the objects on a mica surface were collected and converted into a distribution correlation between height and diameter (Figure 6a). Strikingly, these plots reveal the formation of structures with narrowly distributed heights and diameters that possess some dependency on the applied surface pressure. Objects prepared at 15 mN m−1 (red plots) have diameters of 46 ± 7 nm and thicknesses of 2.6 ± 0.3 nm. When the surface pressure was increased to 30 mN m−1 (blue plots), the diameter also increased to 73 ± 6 nm but the thickness was maintained in the range of 2.9 ± 0.4 nm. These observations demonstrate that the formation of monodisperse regularly sized nanodisks of controlled size occurs simply by touching a solid substrate to a Langmuir monolayer of 1 with cyclen in the subphase Such monodispersely sized regular disk objects could be obtained only in the presence of cyclen by the appropriate surface touching method. The necessity of the interaction of cyclen with triimide 1 is suggested by comparing AFM images of a film of 1 transferred from a pure water surface because this contains only irregular and continuous morphologies. However, the copresence of these two molecular species is not sufficient for the formation of regularly sized disks as indicated by AFM

Higher energy (i.e., surface pressure) is required for this transition at higher concentrations of cyclen. The binding of cyclen to 1 most likely stabilizes the monolayer state of 1. The variation of the subphase temperature also shifts the isotherms of 1 with 1.0 mM cyclen in the subphase (Figure 2C). At higher surface pressures (more than ca. 20 mN m−1), the isotherms apparently expand as the temperature is increased. This can be explained by simple thermal expansion. The temperature dependency of the isotherms below the transition pressure is not as simple as that seen for shifts in molecular areas at 10 mN m−1 as a function of the subphase temperature (Figure 2D). Increasing the temperature from 10 to 20 °C induced a decrease in the molecular area of 1, and the molecular area increased during the temperature increase from 20 to 30 °C. This rather complicated behavior may originate from two mechanisms such as the rearrangement of molecular conformation to a more compact structure upon the binding of cyclen together with the thermal molecular expansion. To investigate the binding of cyclen to 1 at the air−water interface, FT-IR spectra of the transferred monolayer on a goldcoated glass slide were taken in the reflection−absorption spectral (RAS) mode.18 In spectra in the 1700 to 1800 cm−1 region, the ν(CO) band at 1735 cm−1 for the monolayer of 1 on pure water shifted to 1730 cm−1 for monolayer 1 transferred from 1.0 mM aqueous cyclen (Figure 3A). Broad and split

Figure 3. (A) FT-IR-RAS spectra (1300−1800 cm−1) of (a) cyclen on a KBr plate, (b) an LB monolayer film of 1 prepared on pure water at 30 mN m−1 and an LB monolayer film of 1 prepared on an aqueous solution of cyclen (1.0 mM) at surface pressures of (c) 15 and (d) 30 mN m−1. Gold-coated glass was used as a substrate for samples b−d. (B) FT-IR-RAS spectra (2750−3050 cm−1) of LB films of 1: a monolayer transferred onto a gold-coated glass substrate from (a) pure water at a surface pressure of 30 mN m−1 and an aqueous solution of cyclen (1.0 mM) at surface pressures of (b) 15 and (c) 30 mN m−1.

peaks of δ(N−H) bands appear in the 1600−1700 cm−1 region for monolayer 1 transferred from the cyclen subphase, which were not observed in the monolayer of 1 transferred from pure water, and these are significantly shifted relative to standard δ(N−H) bands. These features indicated that 1 and cyclen interact through hydrogen bonding and that cyclen is included within the transferred monolayer of 1. Unexpectedly, alkyl chains in the film appear to be less well packed as inferred from the CH2 asymmetric band in the IR spectrum. The peak remains in a range between 2924 and 2927 cm−1 with and D

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Figure 4. BAM images of monolayer 1 (a−c) on pure water and (d−f) on an aqueous solution of cyclen (1.0 mM).

Figure 5. AFM images of LB films of 1 transferred from an aqueous solution of cyclen (1.0 mM) at a surface pressure of 30 mN m−1 on mica by (a) the vertical dipping method, (b) the horizontal drawing-up method, and (c−f) the surface touching method.

images of cast films of the two components and a solution mixture. The green plots in Figure 6 represent the heights and diameters of objects obtained by casting an equimolar mixed solution of 1 and cyclen. The plots of these objects’ dimensions indicate wide distributions. Therefore, the addition of cyclen to

1 facilitates the formation of certain types of assembly, but these are of poor structural regularity with large distributions of both diameter and height. To demonstrate the benefits of the nanodisks as templating architectures, platinum was used to coat the LB film. A E

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Figure 6. (a) Distribution correlation between height and diameter of disklike objects on a mica surface (red, LB films of 1/cyclen at 15 mN m−1; blue, LB films of 1/cyclen at 30 mN m−1; green, cast films of 1/cyclen). (b, c) Inserted AFM image and (d) cross section representing typical morphologies of 1 transferred from an aqueous solution of cyclen (1.0 mM) at a surface pressure of 30 mN m−1.

metallic nanodisks by vacuum deposition on an LB film as a versatile and stable template.

scanning electron microscope (SEM) image of the platinumcoated monolayer film prepared on an aqueous solution of cyclen at a surface pressure of 30 mN m−1 (Figure 7) reveals

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DISCUSSION In this section, the sizes of the disklike objects obtained are analyzed with respect to several structural parameters (height of the disks, number of molecules per disk (molecular aggregation number), and number of disks per unit area (2D disk density)) to assess the formation mechanism of the regularly sized nanodisks. The heights of disks were experimentally determined as being approximately 3 nm (2.6 ± 0.3 and 2.9 ± 0.4 nm for 15 and 30 mN m−1, respectively), which can be compared to the size of molecule 1. That is, if dodecyl chains of 1 stand perpendicular to the triimide core while retaining an all-trans conformation, then the thickness of a monolayer should be approximately 1.5 nm (C−C distance = 0.154 nm, C−C−C angle = 109.28°, thickness of graphene = 0.34 nm). The estimated molecular size is almost half of the disk height. Therefore, the nanodisks’ height is consistent with a bilayer structure (i.e., reverse bicelle) where cyclen is likely trapped between two triimide molecules with their planar aromatic rings positioned parallel. Although it is difficult to elucidate the

Figure 7. SEM image of an LB film of 1 coated with platinum. The original LB monolayer film of 1 was prepared on an aqueous solution of cyclen (1.0 mM) at a surface pressure of 30 mN m−1. The histogram indicates the disk diameter estimated from the SEM image.

bright areas corresponding to platinum disks with a uniform diameter of 76 ± 9 nm indicating that the platinum coating yielded a structure nearly identical to that of the original LB film. This finding demonstrates the feasibility of preparing

Figure 8. Plots for aggregation number analysis on a monolayer of 1 formed on the water surface (A) in the absence of cyclen and (B) in the presence of 1.0 mM cyclen. F

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Figure 9. Formation mechanism of the microstarfish object. (a) 1 and cyclen self-assembled into 1D columnar stacks. (b) The microstarfish object is formed from the aggregation of 1D columnar stacks. (c) AFM of LB films of 1 transferred from an aqueous solution of cyclen (1.0 mM) at a surface pressure of 30 mN m−1 on mica by the horizontal drawing-up method.

Aggregation numbers of grown nanodisks at higher surface pressures were estimated from the molecular area at the transition points where molecules of 1 are expected to have their most densely packed state in two dimensions. Data of the π−A isotherms indicate that molecular areas at the transition point are ca. 1.15 and 0.95 nm2 for pure water and the 1.0 mM cyclen subphase, respectively. These values agree reasonably well with our molecular model estimations of 2D molecular packing. Applying the latter molecular area (0.95 nm2) to actual averaged disk sizes (in diameter, 46 ± 7 nm and 73 ± 6 nm at 15 and 30 mN m−1, respectively) and assuming bilayer formation within the disk provides average molecular aggregation numbers of 3600 and 8900 within each nanodisk prepared at 15 and 30 mN m−1, respectively. Interestingly, the average aggregation number increases as the applied surface pressure increases. This observation is contrary to that observed for conventional micelles in solution where the average aggregation number decreases as the applied surface pressure increases.27 To determine whether the number of disks increases or the disks themselves increase in size upon compression, the number of disks per unit area (2D disk density) was estimated. The number of nanodisks per 1.0 μm2 was counted and averaged using numerous AFM images to provide 61 ± 6 and 72 ± 8 for 15 and 30 mN m−1, respectively. These values of the number of disks per unit area should be corrected in consideration of the compression ratio of the surface pressure from 15 to 30 mN m−1. The compression ratio was calculated on the basis of the results of π−A measurement by dividing the surface area at 30 mN m−1 by that at 15 mN m−1 to be 0.78. Therefore, the actual number of disks (61 ± 6) in the range of 1.0 × 1.0 μm2, which was prepared at 15 mN m−1, could be corrected to be 78 ± 8 through division by the compression ratio. The corrected number of disks (78 ± 8) is thus similar to the actual number of disks prepared at 30 mN m−1 (72 ± 8), suggesting that the number of disks per unit area does not depend on the surface pressure. These considerations lead to a plausible conclusion for the effect of pressure on disk

bilayer formation mechanism, the following mechanism is plausible. Disk heights are similar at 15 and 30 mN m−1 (below and above the phase-transition point) so that the transition on the aqueous phase is probably not related to bilayer formation.25 The surface touching deposition technique is sometimes known to yield bilayer structures through layer overturn, and the presence of cyclen should strongly enhance the association of the head groups of 1 in forming a bilayer structure. The numbers of molecules per disk (molecular aggregation number) were estimated. Prior to calculation of the aggregation numbers in nanodisks, aggregation numbers of 1 during the initial stages of assembly are estimated from the π−A isotherm at lower pressures (