Federation of Asian Chemical Societies (FACS)

  Last updated on 26 Apr., 2002      

Recent Advances in Chemistry Based Nanotechnology in Korea

Jinwoo Cheon and Sang-Min Lee
Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST) 
Taejon 305-701, Korea

Current chemical research activities related to nanotechnology are exploding in Korea. The research fields are getting wider and very active and some examples include topics on colloidal nanocrystals, inorganic/organic hybrid materials, nanoporous and catalysts, supramolecular chemistry, and molecular electronics. Synthesis, characterization, and applications of nanomaterials are important issues in materials chemistry because of their special characteristics different from bulk phases. For example, as changing of their size and shape in nanometer scale, their band-gap due to quantum confined band structure can be tuned and increasing surface/volume ratio of nanocrystals leads novel catalytic effects. The novel properties of nanomaterials promise novel science and technology in the near future. 

In this short review, some examples of recent advances in nanochemistry will be describes according to subjects.

1. Supramolecular materials:
Supramolecular chemistry is the chemistry of weak intermoleular interactions such as hydrogen bonding, electrostatic interaction, and van der Waals interaction, etc. Two keywords in supramolecular chemistry are recognition and self assembly. One goal in this field of study is to understand how biological molecules recognize each other and assemble into complex structures. Another important goal in supramolecular chemistry is the construction of functional (or 'smart') materials and devices using the principles that we learned from Mother Nature. A group led by K. Kim at Pohang University of Science and Technology has made significant progress in the development of chiral nanostrucutred materials. (Figure 1. Nature 2000, 404, 982)

In other hand, development of new supramolecular assemblies with desired structures, properties and functions by using the recognition and self-assembly principles done by Prof. M. H. P. Suh at Seoul National Univ. Her group is particularly interested in supramolecular systems that can provide the operating principles of molecular switches, molecular memories, molecular machines and sensors. One example is the self-assembly of a molecular floral lace with one-dimensional channels and inclusion of glucose (Angew. Chem. Int. Ed. 1999, 38, 1405) where a novel three-dimensional network containing one-dimensional channels, which is assembled by a nickel(II) macrocyclic complex as shown in the figure 2.

In the other hand, organic based supramolecular chemistry is nicely demonstrated. M. Lee at Yonsei University has explored a strategy to control the supramolecular structures self-assembled from molecular rods through attachment of flexible coil segments to their ends. In a given molecular rod, the relative lengths of the coil segments determine the resulting rod domain structures that include infinitely long cylinders and disk-like cylinders. These domains subsequently self-organized into 2-D hexagonal and 3-D body centered tetralgonal symmetries, respectively. 

2. Nanoporous Materials
Mesoporsous materials with well-defined structures have been widely explored. J. S. Suh at Seoul National Univ. has demonstrated well-ordered carbon nanotube structures which are of great importance for technical applications as cold-cathode flat panel displays. His group has reported for the first time highly ordered two-dimensional carbon nanotube arrays with perfect vertical alignment grown on AAO templates prepared by a two-step anodization process. In the two-step anodization process, after stripping away the thick aluminum oxide film obtained from the first long anodization, a porous thin alumina film with highly ordered pores are obtained by a subsequent re-anodization.

Solution based synthesis of another type of well-ordered mesoporous structures have been successfully studied by R. Ryoo at KAIST. One of the examples is the development of ordered nanoporous arrays of carbon supporting high dispersions of platinum nanoparticles (Figure 4. Nature 412, 169 (2001)) where the synthesis of highly ordered, rigid arrays of nanoporous carbon having uniform but tunable diameters. These structures are formed by using ordered mesoporous silicas as templates and the removal of matrix leaves a partially ordered graphitic framework. The resulting material provides high dispersion of these metal clusters and gives rise to promising electrocatalytic activity for oxygen reduction, which are relevant for fuel-cell technologies. 

3. Colloidal Nanoparticles and Inorganic Hybrid Materials
Synthesis and fabrication of inorganic nanocrystals of metals, semiconductors, and ceramics are of great interests and significant works carried out in Korea. Some examples include the size and shape controlled synthesis and applications of nanocrystals. K. S. Kim at POSTECH recently reported Ag nanowires which are formed via self-assembly organic templates. Obtained wires are one of the thinnest from the metallic materials and highly ordered with quantum properties.

J. Cheon group at KAIST has demonstrated novel chemical strategies for size and shape control over these quantum building blocks. Programmability of growth parameters and predictability of desired shape are successfully demonstrated in nm scale. Typical examples show rods, bi, tri, terapods and cubes. Controlled synthesis of multi-armed CdS nanorod architectures are shown in the figures. 

Hyeon and his co-workers at Seoul National University have reported the synthesis of highly crystalline and monodisperse maghemite γ-Fe2O3 nanocrystallites without a size-selection process (J. Am. Chem. Soc. 2001, 123, 12798) where particle size can be varied from 4 to 16 nm by controlling the experimental parameters. 

	Figure 7.

Also, a research group led by S. Lee at K-JIST utilize nanoparticles for emitting devices, and also to understand the evolution of the bulk properties from the molecular properties in this region. The optical properties of nanoparticles are determined by quantum confined effect, which drastically modifies the energy spectra of three-dimensionally confined mesoscopic particle. (for example, CdSe QDs and ZnSe QDs.) The overcoating process is performed to remove many of trap states at surface of QDs and to improve the quantum efficiencies. 

Inorganic hybrid layered materials are researched by J.-H. Choy at Seoul National University. The free modulation of interlayer distance in a layered high–transition temperature (high-Tc) superconductor is of crucial importance not only for the study of the superconducting mechanism but also for the practical application of high-Tc superconducting materials. Two-dimensional (2D) superconductors were achieved by intercalating a long chain organic compound into bismuth-based high-Tc cuprates. 

4. Nanoscale Assembly and Integration on 1, 2 Dimensional Structures
Molecular wire assisted assembles of zeolites to form novel superstructures are being conducted by Yoon K.B. at Sogang University. For example, the study of layer-by-layer assembly of zeolite crystals on glass with polyelectrolytes as ionic linkers clearly demonstrate the success of this concept. (Figure 9. J. Am. Chem. Soc. 2001, 123, 9769.). ZSM-5 crystals and glass plates tethered with trimethylpropylammonium iodide and sodium butyrate, respectively, (denoted as Z+, Z-, G+, and G-, respectively) were prepared. This report thus demonstrates the feasibility of layer-by-layer assembly of micrometer-sized zeolite crystals on glass through electrostatic interaction between surface-bound, fullfledged ionic centers, especially by use of polyelectrolyes as the linkers.

The study of magnetically coupled metal complexes with extended structures is conducted by Jung, D.-Y. at Sungkyunkwan University. A series of novel manganese α,ω-dicarboxylates, Mn2(H2O)[O2C(CH2)nCO2]2 (n = 3-12), were synthesized as single crystals or polycrystallines by the hydrothermal reaction of MnCl2 with the corresponding dicarboxylic acid in the presence of base. Their organic chains between two terminal carboxylates are close-packed and self-assembled between Mn-O layers. The relative conformations of two terminal carboxylate groups determine the spacing groups of crystal structures. All the prepared manganese dicarboxylate compounds involve the replication of the local crystal structure in the Mn-O monolayer to present an antiferromagnetic interaction. (Figure 10. Chem. Matter. 2001, 13, 2684)

Amine functionalized surface of silicon is studied by Park, J.W. at Pohang University of Science and Technology. The surface of a fused silica and oxidized silicon wafer (SiO2/Si(100)) was treated with (3-aminopropyl)- triethoxysilane, (3-aminopropyl)diethoxymethylsilane, and (3-aminopropyl)ethoxydimethylsilane in solution. The aminosilane layer was allowed to react with 4-nitrobenzaldehyde to form an imine, and absorbance (Asurf) of the imine was recorded by UV-vis spectroscopy and reported a determination of the absolute surface density of the amine group of aminosilylated layers, where a UV-vis spectrophotometer is a primary analytical tool. (Figure 11. Langmuir 1997, 13, 4305-4310)

Nanopatterning and self-assembled monolayers are researched by H. Lee at Hanyang university. Nanostructures down to a few tens of nanometers in size have been fabricated with both poly(3(2(5choloro-benzotriazolo)ethyl)thiophene Langmuir-Blodgett film and self-assembled mixed monolayers of 1,12-diaminododecane dihydrochloride and n-tridecylamine hydrochloride by the interaction with electrons emitted from a tip of AFM. The effect of functional groups of molecules on the AFM anodization has been studied in the optimized process conditions. (Figure 12. Thin Solid Film 2001, 393, 237)

Concluding Remarks
This short review shows some exciting recent results in the nanochemistry area of Korea, although not representing the whole due to the limitation of available spaces in this article. But I hope readers find at least the glimpse of current passion, enthusiasm, and the quality of researches in this field. The Ministry of Science and Technology of Korea realized the importance of this research field and promises substantial increase of research grants for the next 10 years and also started Nanotechnology Initiative Program in 2001. It is believed that more and more chemists will join this exacting and growing new research field in Korea. 

A Partial List of Institutions with Nanochemistry Researches

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