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Vaporization-Condensation-Recrystallization Process-Mediated Synthesis of Helical m-Aminobenzoic Acid Nanobelts Seok Min Yoon,† In-Chul Hwang,† Namsoo Shin,‡ Docheon Ahn,‡ Sang Joo Lee,§ Jin Yong Lee,| and Hee Cheul Choi*,† Department of Chemistry, Pohang UniVersity of Science and Technology, San 31, Hyoja-Dong, Nam-Gu, Pohang 790-784, Korea, Pohang Accelerator Laboratory, Pohang UniVersity of Science and Technology (POSTECH), Pohang 790-784, Korea, Center for Computational Biology and Bioinformatics, Korea Institute of Science and Technology Information, 52 Eoeun-Dong, Yuseong-Gu, Daejeon 305-806, Korea, and Department of Chemistry, Sungkyunkwan UniVersity, 300 Chunchun-Dong, Jangan-Gu, Suwon 440-746, Korea ReceiVed July 11, 2007. In Final Form: August 23, 2007 One-dimensional (1D) helical organic nanostructures were synthesized by a modified vapor-solid (VS) process, called the vaporization-condensation-recrystallization (VCR) process. The conventional solution-phase synthetic methods generally mediate self-assemblies of repeating unit molecules. To provide enough intermolecular interaction forces among the unit molecules, such strategy requires specific designs and syntheses of complex unit molecules as they possess numerous functional groups including phenyl rings, hydroxyl groups, long aliphatic chains, etc. On the contrary, we found that small and simple organic molecules, for example, m-ABA, could be self-assembled by the VCR process, resulting in 1D helical organic nanostructures. When m-aminobenzoic acid (m-ABA) powders were vaporized and transported to be condensed on a cooler region, the condensates were recrystallized into 1D helical nanobelts. Each step of the VCR process was confirmed from control experiments performed by varying reaction times, substrate types, and reaction temperatures. Powder XRD data, SAED analysis, and theoretical calculations revealed that dimers of m-ABA molecules have repeating units, and the growth axis of m-ABA nanohelices is [100].

Introduction Organic nanostructures have drawn a great deal of attention due to their potential roles in organic nanoelectronics,1 as smart templates for inorganic materials,2,3 as organic-inorganic nanocomposites,4,5 and as soft and miniaturized cargo systems for drug delivery.6 Among the various types of organic nanostructures, such as spherical vesicles, fibers, ribbons, and tubules,7-11 helical belt-shaped one-dimensional (1D) nanostructures12 hold a special interest because of their unique optical activities as well as chirality-specific molecular interactions. * Corresponding author. E-mail: [email protected]. † Department of Chemistry, Pohang University of Science and Technology. ‡ Pohang Accelerator Laboratory, Pohang University of Science and Technology. § Korea Institute of Science and Technology Information. | Sungkyunkwan University. (1) Forrest, S. R. Nature 2004, 428, 911-918. (2) Raman, N. K.; Anderson, M. T.; Brinker, C. J. Chem. Mater. 1996, 8, 1682-1702. (3) Sone, E. D.; Zubarev, E. R.; Stupp, S. I. Angew. Chem., Int. Ed. 2002, 41, 1705-1709. (4) Sellinger, A.; Weiss, P. M.; Nguyen, A.; Lu, Y. R.; Assink, A.; Gong, W.; Brinker, C. J. Nature 1998, 394, 256-260. (5) Sayari, A.; Hamoudi, S. Chem. Mater. 2001, 13, 3151-3168. (6) Jeong, B. J.; Bae, Y. H.; Lee, D. S.; Kim, S. W. Nature 1997, 388, 860862. (7) Kimizuka, N.; Kawasaki, T.; Hirata, K.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6360-6361. (8) Pignataro, B.; Sardone, L.; Marletta, G. Langmuir 2003, 19, 5912-5917. (9) Hartgerink, J. D.; Beniash, E.; Stupp, S. I. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5133-5138. (10) (a) Fuhrhop, J.; Wang, T. Chem. ReV. 2004, 104, 2901-2937. (b) Lee, S. B.; Koepsel, R.; Warriner, H. E.; Russell, A. J. J. Am. Chem. Soc. 2004, 126, 13400-13405. (c) Gong, H.; Liu, M. J. Colloid Interface Sci. 2003, 258, 130134. (11) Kamiya, S.; Minamikawa, H.; Jung, J. H.; Yang, B.; Masuda, M.; Shimizu, T. Langmuir 2005, 21, 743-750. (12) (a) Engelkamp, H.; Middelbeek, S.; Nolte, R. J. M. Science 1999, 284, 785-788. (b) Ky Hirschberg, J. H. K.; Brunsveld, L.; Ramzi, A.; Vekemans, J. A. J. M.; Sijbesma, R. P.; Meijer, E. W. Nature 2000, 407, 167-170.

Conventionally, organic nanohelices have been synthesized by self-assembly reactions in solution phase. For example, 1D helical nanostructures have been made by self-assemblies of bolaamphiphiles,13 lipid bilayers,14,15 glucose,16 melamine,17 oligo(p-phenylene vinylene)s,18 aminoacid derivative,19 and diacetylenic alcoamides,20 etc. The main driving forces directing self-assembled 1D organic nanostructures are intermolecular interactions such as π-π interaction, hydrogen bonding, and van der Waals interaction, which lead the “unit” molecules to self-assemble. Accordingly, the unit molecules are required to be specifically designed as they hold long hydrophilic, hydrophobic chains, or numerous phenyl rings, which can provide sufficient clustering and stacking forces. Therefore, it is still challenging to develop a method by which organic nanohelices could be synthesized from “simple unit molecules”, allowing us to avoid complicated designing and synthesis steps. Here, we show unprecedented m-aminobenzoic acid (m-ABA) nanohelices formed via a modified vapor to solid (VS) phase transformation mechanism, called the vaporization-condensation-recrystallization (VCR) process. (13) (a) Shimizu, T. Macromol. Rapid Commun. 2002, 23, 311-331. (b) Song, J.; Cheng, Q.; Kopta, S.; Stevens, R. C. J. Am. Chem. Soc. 2001, 123, 3205-3231. (c) Shibakami, M.; Miyawaki, K.; Goto, R.; Shigeno, M. Jpn. J. Appl. Phys. 2004, 43, 4655-4658. (14) Fuhrhop, J.; Schnieder, P.; Boekema, E.; Helfrich, W. J. Am. Chem. Soc. 1988, 110, 2861-2867. (15) Fuhrhop, J.; Helfrich, W. Chem. ReV. 1993, 93, 1565-1582. (16) Jung, J. H.; John, G. J. Am. Chem. Soc. 2002, 124, 10674-10675. (17) Kimizuka, N.; Fujikawa, S.; Kuwahara, H.; Kunitake, T.; Marsh, A.; Lehn, J. M. J. Chem. Soc., Chem. Commun. 1995, 20, 2103-2104. (18) (a) Ajayaghosh, A.; Vijayakumar, C.; Varghese, R.; George, S. G. Angew. Chem., Int. Ed. 2006, 45, 456-460. (b) Ajayaghosh, A.; Varghese, R.; George, S. G.; Vijayakumar, C. Angew. Chem., Int. Ed. 2006, 45, 1141-1144. (19) Nakashima, N.; Asakuma, S.; Kunitake, T. J. Am. Chem. Soc. 1985, 107, 509-510. (20) Frankel, D. A.; O’Brien, D. F. J. Am. Chem. Soc. 1991, 113, 7436-7437.

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Figure 1. Photographs of the equipment and experimental details for the synthesis of m-ABA nanohelices. (a and b) m-ABA powder (left) and a Si(100) substrate (right) before and after VS reaction for 40 min, respectively. The as-grown m-ABA nanohelices are found as a chunk of white cotton on the Si(100) substrate in (b). (c and d) Photographs of a quartz tube in an electrical heating furnace, in which both m-ABA powder and a Si(100) substrate are located at the center of the furnace with a thermocouple and at the end of the heating region in the furnace, respectively. (e) Temperature gradient of electrical furnace when heating to 170 °C. Temperature is cooler and substrate position becomes more distant from the center.

The VS process has been widely utilized to produce various types of inorganic nanostructures including metal oxide helical nanobelts of ZnO,21 SnO2,22 and AlN nanocones.23 Such wide band gap metal oxide nanohelices are currently being studied for their applications in high performance resonator, inductor, and piezoelectric devices.24,25 However, there are only a few reported examples in which the VS process has been attempted to synthesize organic nanostructures.26-28 Experimental Section Synthesis of m-ABA Nanohelices. m-ABA powder (98%, SigmaAldrich) was used as received without further purification. 0.02 g of m-ABA powder was placed in the center of a quartz tube placed (21) Gao, P. X.; Ding, Y.; Mai, W.; Hughes, W. L.; Lao, C.; Wang, Z. L. Science 2005, 309, 1700-1704. (22) Yang, R.; Wang, Z. L. J. Am. Chem. Soc. 2006, 128, 1466-1467. (23) Liu, C.; Hu, Z.; Wu, Q.; Wang, X.; Chen, Y.; Sang, H.; Zhu, J.; Deng, S.; Xu, N. J. Am. Chem. Soc. 2005, 127, 1318-1322. (24) Bae, S. Y.; Lee, J.; Jung, H.; Park, J.; Ahn, J. P. J. Am. Chem. Soc. 2005, 127, 10802-10803. (25) Wang, Z. L.; Song, J. Science 2006, 312, 242-246. (26) Liu, H.; Li, Y.; Xiao, S.; Li, H.; Jiang, L.; Zhu, D.; Xiang, B.; Chen, Y.; Yu, D. J. Phys. Chem. B 2004, 108, 7744-7747. (27) Tong, W. Y.; Djurisic´, A. B.; Xie, M. H.; Ng, A. C. M.; Cheung, K. Y.; Chan, W. K.; Leung, Y. H.; Lin, H. W.; Gwo, S. J. Phys. Chem. B 2006, 110, 17406-17413. (28) Liu, H.; Zhao, Q.; Li, Y.; Liu, Y.; Lu, F.; Zhuang, J.; Wang, S.; Jiang, L.; Zhu, D.; Yu, D.; Chi, L. J. Am. Chem. Soc. 2005, 127, 1120-1121.

in a horizontal electrical heating furnace, and the quartz tube was flushed with Ar gas to remove ambient gases before any reaction. Under the steady flow of Ar, the system was heated from room temperature to 170 °C or designated temperatures (vaporization temperature) at a rate of 41 °C/min and maintained for 40 min. The m-ABA vapors transported by Ar were condensed at the end region of the furnace where the temperature was naturally dropped, resulting in the formation of semi-transparent wire-like crystal products in large quantity. The photographs of the reaction system are shown in Figure 1. For the substrate temperature control, a custom designed substrate holder connected to a proportional-integral-derivative controller (PID controller) was used. The heating lines and thermocouple were attached to the backside of a tungsten plate using a conducting aluminum tape. The samples were then located on top of the tungsten plate to control the substrate temperature. Characterizations. The morphologies, crystallinities, and populations of the products were characterized using scanning electron microscopy (SEM, XL30S, FEI), transmission electron microscopy (TEM, CM200, Philips), and an X-ray diffractometer (XRD, D/MAX 1400, RIGAKU).

Results and Discussion Mechanism of the Vaporization-Condensation-Recrystallization (VCR) Process. For the demonstration of VCR process-mediated formation of 1D organic nanostructures, we selected m-ABA as a precursor because it is one of the simplest

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Figure 2. SEM images of m-ABA nanohelices grown on (a) SiO2/Si and (b) Cu foil. (c) TEM image of a typical m-ABA nanohelix. (d) Selected area electron diffraction (SAED) pattern of a m-ABA nanohelix. The inset in (b) is a SEM image captured at an early stage of the growth of m-ABA nanohelices on a Si(100). Scheme 1. Schematic Representation of the Formation of m-ABA Nanohelices by the Vapor-Solid (VS) Process, During Which Vaporization, Condensation, and Recrystallization of m-ABA Occur Sequentially

molecules as it contains only one phenyl ring (-C6H4) with a carboxylic acid (-COOH) and an amine (-NH2) group at meta position, but it is difficult to grow into 1D nanostructures via self-assembly in solution phase due to lack of enough functional groups that are essential for intermolecular self-stacking. Despite the limited numbers of functional groups, high yields of m-ABA nanohelices were formed when m-ABA was evaporated at 170 °C under Ar flow and transported onto the solid substrates, which were placed in the cooler region at the end of the tube furnace (Figure 1). Figure 2a and b shows representative SEM images of m-ABA nanohelices grown on SiO2/Si (500 nm thermally grown SiO2 on a highly doped p-type Si wafer) and Cu foil (99%, Aldrich) substrates, respectively. Figure 2c depicts a representative TEM image of a m-ABA nanohelix. This particular nanohelix is 85 nm in width and twists 180° at every 920 nm in length. The thickness is estimated to be ca. 15 nm by measuring the width of the nanobelt at the thinnest point where it completely twists 90°, as indicated by an arrow. Although the crystal lattice information was not obtained from high-resolution TEM (HRTEM) mostly due to the instability of the sample under high energy of electron beam, m-ABA nanohelices turn out to be single crystalline as confirmed by powder XRD patterns and well-resolved diffracted electron spots from selected area electron diffraction (SAED) patterns taken by normal TEM (vide infra). In general, the mechanism of conventional VS process applied for the formation of 1D metal oxide semiconducting nanohelices

as well as organic-inorganic hybrid nanowires follows vaporization of precursors and random solidification of the precursor vapors in the cooler temperature region. The VS process involved in the growth of m-ABA nanohelices, however, seems to follow more specific vaporization and solidification pathways. During the experiments, we found that individual m-ABA nanohelices were rooted from single condensed precursor spots, like a blooming flower (Scheme 1 and inset of Figure 2b). This observation led us to propose that three-step phase transformation, that is, the vaporization-condensation-recrystallization (VCR) process, induces m-ABA nanohelices. The uniqueness of the VCR process was further studied by investigating the growth of m-ABA nanohelices as a function of reaction time and substrate type. As shown in Figure 3a, circular shape condensates were found on a solid substrate at the early stage of the reaction, which were generated by Brownian coagulation of m-ABA vapors under variable-temperature gradient.29 Upon prolonged reaction with other variables fixed, nanohelices were grown only from individual condensate spots (Figure 3b-d). The faster Ar flows, the smaller sizes of m-ABA condensates were obtained (Figure 4). We further found that the successful growth of m-ABA nanohelices through condensation-recrystallization steps does not necessarily require crystal parameter matching for the nucleation on the solid surface. Upon the growth of m-ABA (29) Flagan, R. C.; Lunden, M. M. Mater. Sci. Eng., A 1995, 204, 113-124.

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Figure 3. The effect of reaction time on the formation of m-ABA nanohelices. SEM images of m-ABA morphologies changing from condensates to helical nanobelts on Si(100) according to the reaction times of (a) 5, (b) 10, (c) 15, and (d) 20 min.

nanohelices on several different types of surfaces, such as Cu foil, SiO2, 12% hydrofluoric acid etched Si(100) and Si(111). m-ABA nanohelices with similar shapes and yields were obtained regardless of the substrate types, as long as the reaction time and flow rates were fixed (Figure 5). Morphology Control of m-ABA Nanostructures. During the VCR process of m-ABA, micro-millimeter length scale rods were also formed along with nanohelices. Hence, it is preferred to synthesize nanohelices preferentially. When we were conducting various control experiments to elucidate the key factors that would determine the helical morphology, we found that the substrate temperature critically affects the population of m-ABA nanohelices when carrier gas flow rate and vaporization temperature are constant. The substrate temperature was electronically controlled from 50 to 100 °C by using a proportional-integralderivative controller (PID controller, Figure S1 in Supporting Information). When the substrate temperature was 50 °C, only rod-shaped products were obtained (Figure 6a and b). However, helical nanobelts were dominantly produced as the substrate temperature was increased (Figure 6c and d). When the substrate temperature was higher than 100 °C, no nanobelt was found because most of the m-ABA vapors were not condensed on the substrates but preferred to be evacuated (Figure S2 in Supporting Information). Although it is still not clearly understood how chiral nanohelices could be grown from self-assemblies of achiral 3-ABA molecules, the preferential growth of helical nanobelts at high substrate temperature seems due to the degree of flatness of m-ABA condensates, which are dictated by their contact area onto a substrate. From metal droplet cases, it has been known that the contact area of a constant volume metal droplet is getting smaller as the substrate temperature increases. On such an occasion, most of the provided thermal energy is consumed for the

crystallization (or solidification) of the droplet.30,31 As a result, higher degree in solidification of a metal droplet is observed at higher temperature. Similarly, we observed that much smaller (less flatten) m-ABA condensates were formed at 80 °C (average diameter: 4.25 µm), while the condensates became more flatten at 50 °C (average diameter: 6.34 µm) (Figure S3 in Supporting Information). Consequently, helical nanobelts are grown from less flattened condensates, and rods from more flattened condensates. Structure of m-ABA Nanohelices. Because of the size and geometry limitations of m-ABA nanohelices, it was difficult to obtain their single-crystal structures. Instead, the structure of a m-ABA nanohelix was estimated from simulations by calculating intermolecular interactions enabling plausible packing of unit cells. First, a single-crystal structure of a m-ABA rod was obtained, and then the unit cell was used for the simulation. The single crystallographic data of a m-ABA rod revealed that it has monoclinic and catenated m-ABA (H2NC6H4COOH) instead of zwitterionic (+H3NC6H4COO-) structure, which is identical to the previously reported one.32 The usage of the unit cell of a single-crystal m-ABA rod for the simulation could be justified by comparing powder XRD patterns of rod-rich and nanohelixrich powder samples that were prepared by controlling substrate temperatures as explained in the previous section. As shown in Figure 7, both rod-rich and nanohelix-rich samples have identical crystal structures, which imply that the basic crystal structure of a single-crystal m-ABA rod is identical to that of a helical m-ABA nanobelt. Next, we carried out the semiempirical AM133 and ab initio calculations using a suite of Gaussian 03 programs.34 To acquire (30) FuKumoto, M.; Nishioka, E.; Matsubara, T. Surf. Coat. Technol. 1999, 120-121, 131-137. (31) Kang, B.; Waldvogel, J.; Poulikakos, D. J. Mater. Sci. 1995, 30, 49124925. (32) Voogd, J.; Verzijl, B. H. M.; Duisenberg, A. J. M. Acta Crystallogr., Sect. B 1980, 36, 2805-2806.

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Figure 4. The effect of carrier gas (Ar) flow rate on the condensate sizes and formation of m-ABA nanohelices: (a-d) SEM images of m-ABA nucleates formed under different Ar flow rates of 20, 50, 100, and 200 sccm, respectively. (e) Plot of the average diameter of m-ABA nucleates on a Si(100) surface as a function of Ar flow rate. The growth was performed at 170 °C (vaporization temperature) for 10 min.

Figure 5. The effect of substrate on the growth of m-ABA nanohelices. m-ABA nanohelices grown on (a) Si(100) including native oxide layers, (b) Si(100), (c) Si(111), (d) Cu, and (e) carbon-coated Si. The growth was performed at 170 °C (vaporization temperature) for 10 min under 100 sccm of Ar flow rate. Note that the native oxide layers of Si(100) and Si(111) are etched by 12% HF solution for (b) and (c), while native oxides on Cu are etched by 1 M HNO3 solution for (d).

some intimations for the formation of 1D nanobelts, especially helical nanostructures, the unit cell obtained from single crystallography was attempted to expand along the a-, b-, and c-axes. The expansion along the a-axis shows the formation of

dimers through the hydrogen bonding and π-π stacking (Figure 8a), while those along the b- and c-axes could not have enough intermolecular interactions to grow into stacked networks (Figure S4 in Supporting Information). Thus, the growing unit into a

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Figure 6. The effect of substrate temperature on the formation of m-ABA nanohelices. (a and b) SEM images of rod-type m-ABA structures obtained when the Si(100) substrate temperature is 50 °C. (c and d) SEM images of helical belt-type m-ABA structures when the substrate temperature is 80 °C.

Figure 7. Powder XRD patterns of m-ABA (a) rods and (b) helical nanobelts.

helical nanobelt should be m-ABA dimer and not the monomer. The dimer repeating unit was also evidenced by FT-IR spectra, which showed significantly red-shifted ν(CdO, 1633 cm-1) bands due to the formation of hydrogen bondings for both rod-rich and nanohelix-rich powder samples (Figure S5 in Supporting Information). There seem to be two stable m-ABA dimer conformers: one is interacting between two carboxylic acids and the other is interacting between one carboxylic acid and one amine group. The ab initio calculations at B3LYP/6-31G* level have revealed that the former conformer is more stable by ∼0 kcal/mol. Further, the CdO stretching vibrational frequencies for the former conformer were calculated to be 1719 and 1764 cm-1, while they are 1789 and 1796 cm-1 for the latter conformer. Considering that the CdO stretching vibrational frequency of the monomer m-ABA was calculated to be 1819 cm-1, the symmetric CdO stretching mode of the dimer shows red-shifts from that of the monomer by 100 and 30 cm-1 for the former and the latter, respectively. Referring to the recent study35 showing (33) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. Soc. 1985, 107, 3902-3909.

the 100 cm-1 red-shifted CdO stretching frequency for the aggregates between carboxylic acids from that of the monomer, we may conclude that the dimer conformer was used as the repeating unit for the nanohelix, and the rod must be the former conformer. Next, we tested if the expansion along the a-axis could generate a helical geometry as observed in our experiment. The AM1 optimized structures clearly show the helical structure for the aggregated m-ABA dimers as shown in Figure 8b and c. The close examination revealed that the terminal dimers at both ends represented more twisted angle than the inner ones. This may originate from the fact that the inner dimers interact with two neighbors (m-ABA dimers), while those at both ends interact with only one neighbor. Thus, the terminal dimers are twisted more to maximize the intermolecular interactions (called twisting force), which may cause additional m-ABA dimers to be helically positioned during the growth. Such a twisting force would become greater to yield high population of nanohelices if the dimer packing could expand laterally (along the b-axis). On the contrary, the twisting force might be minimized when the dimer packing prefers to expand along the c-axis, resulting in m-ABA rods. Accordingly, an expanded model for a m-ABA naonohelix was estimated from the higher level of ab initio calculations. The B3LYP/3-21G calculations for eight growing units show the 3.6 nm length and the twisted angle of 11.2° between the two terminal m-ABA dimers (Figure S6 in Supporting Information). Next, we investigated the inter-dimer interactions growing along the three different directions. As shown in Figure 9, there are four possible dimer-dimer interactions. Among these, type A was already shown to form the backbone by stacking of m-ABA dimers along the a-axis. Now, the expansion of the m-ABA dimer stacks (34) Frisch, M. J.; et al. Gaussian 03, revision A1; Gaussian, Inc.: Pittsburgh, PA, 2003. (35) Pawlukojc, A.; Leciejewicz, J. Chem. Phys. 2004, 299, 39-45.

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Figure 8. AM1 optimized geometry of the aggregated m-ABA dimers and their stacking along the a-axis via π-π interaction. (a) m-ABA dimer (gray balls, carbon; white balls, hydrogen; red balls, oxygen; blue balls, nitrogen; dashed line, hydrogen bonding). (b and c) AM1 optimized structures of 30 dimer-set along the a-axis seen at a different angle.

Figure 9. Inter-dimer interactions optimized by B3LYP/6-31G calculations. Types C and D are responsible for the successive expansions of type A along the c- and b-axes, which result in the increase in thickness and width of a m-ABA nanohelix, respectively.

(type A) toward b- and c-axes, which correspond to width and thickness, respectively, seems to occur as types B, C, and D. For the expansion of m-ABA dimer stacks along the axis responsible for the width increase, type D is more suitable rather than type B because the latter has a hydrogen bond between two amines from each dimer at large angle, so that the further expansion may distort the overall structure severely and may not result in the experimentally obtained helical structures. On the other hand, type D allows C-H‚‚‚N interactions between phenyl proton and the amine nitrogen, which provides more stable structures because it does not distort the building block. Next, the dimer stacks can grow into the axis responsible for the thickness increase by the interactions represented as type C. On the basis of these interpretation as well as experimental results, it is believed that the interaction types A (stacking interactions), C (hydrogen bonding), and D (C-H‚‚‚N interactions) are responsible for

the determination of the length, thickness, and the width, respectively. Furthermore, these results also explain the unsuccessful nanohelix formation but only straight rod-type structures from o- and p-ABA molecules, in which no driving force for twisting exists. On the basis of the calculated structural information, a SAED pattern of a m-ABA helix could be indexed. The SAED pattern in Figure 10b was obtained from [001] zone axis, and the direction matched with growth direction of helical nanobelts was [100]. Although the electron diffraction pattern is shown at a little tilted angle,36 the experimental result also agrees well with the simulated SAED pattern (Figure 10c).37 (36) Klug, A.; Crick, F. H. C.; Wyckoff, H. W. Acta Crystallogr. 1958, 11, 199-213. (37) Single Crystal 1.0.0, Crystal Maker Software Ltd.

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Figure 10. (a) TEM image of helical m-ABA nanobelt. (b) SAED pattern of helical m-ABA nanobelt from [001] zone axis. The growth axis of helical m-ABA nanobelt is [100]. (c) Simulated SAED pattern of m-ABA single crystal from [001] zone axis.

Conclusion We demonstrated that a simple and efficient modified VS process, that is, the VCR process, was successfully applied for the formation of 1D m-ABA nanohelices in high yield at relatively low temperature. Experimental observations as well as theoretical calculations support that m-ABA dimers grow into helical nanostructures through energetically stable intermolecular hydrogen bonding and π-π interactions. A successful application of VCR process to simple organic molecules is expected to provide great opportunities for the formation of diverse functional organic nanostructures. Among many possible applications, m-ABA nanohelices are expected to be applied for the construction of structure-mimicked metal helical nanobelts, which afford fundamental studies about couplings between surface plasmon and

electrons that are essential to designing optoelectronic devices and the generation of optically polarized plasmons. Acknowledgment. We gratefully thank the Basic Research Program of the KOSEF (R01-2004-000-10210-0), the Nano/Bio Science & Technology Program of MOST (2006-00955), the SRC/ERC Program (R11-2000-070-070020), and the Korean Research Foundation (MOEHRD, KRF-2005-005-J13103). Chul Ho Lee and Seung Ho Jung are thanked for the metal evaporation. Supporting Information Available: Ab initio calculation data, experimental setups for surface temperature control systems, and SEM images of m-ABA nanohelices grown at different substrate temperatures. This material is available free of charge via the Internet at http://pubs. acs.org. LA702071W