Jin Joo

Large-scale soft colloidal template synthesis of 1.4 nm thick CdSe nanosheets.

Two-dimensional (2D) nanocrystals have attracted tremendous attention from many researchers in various disciplines because of their unique properties. Since ways of making graphene were devised, there have been significant research efforts to synthesize free-standing 2D nanocrystals of various materials, including metals, oxides, and chalcogenides. Many of these 2D nanocrystals have been generated from exfoliation of materials with layered structures, and tiny amounts of products are generally produced. CdSe nanocrystals are among the most intensively studied nanostructured materials, owing to their many size-dependent optical and electrical characteristics and resulting exciting applications. Herein, we report on the large-scale synthesis of single-layered and lamellar-structured 2D CdSe nanocrystals with wurtzite crystal structure as thin as 1.4 nm by a soft colloidal template method. These free-standing 2D nanocrystals with insulating organic layers at the interface could find many interesting electronic and optoelectronic applications, including in quantum cascade lasers and resonant tunneling diodes utilizing their multiple quantum well structures. Compared to materials with layered crystal structures such as graphite, the synthesis of free-standing 2D nanocrystals of nonlayered materials such as CdSe is extremely challenging, because selective growth along one specific facet among several with similar energies is required. For example, in CdSe with a hexagonal wurtzite crystal structure, a (0001) facet has significantly higher surface energy than other facets, which leads to the formation of many one-dimensional nanostructures. Although there is a slight difference in the surface energies of (1120) and (1100) facets, quantum-confined thin CdSe 2D nanocrystals could not be synthesized using a conventional colloidal chemical route that employs thermal decomposition of precursors at high temperature, because the small difference in the surface energies of these two facets is negated by the high reaction temperature. Consequently, there have been only a few reports on the successful chemical synthesis of 2D CdSe nanocrystals. For example, CdSe inorganic–organic hybrid lamellar structures and CdSe nanoplatelets with zinc-blende structure were synthesized using colloidal chemical routes. However, their 2D growth mechanism has not been clearly elucidated. Furthermore, nanostructural control to form single-layered or multiple-layered nanosheets has not been demonstrated. In the current approach to creating 2D CdSe nanocrystals, we employed a soft template method, and we were able to synthesize not only free-standing single-layered CdSe nanosheets but also lamellar-structured nanosheets by controlling the interaction between organic layers in 2D templates of cadmium chloride alkyl amine complexes. It has been reported that the complex of cadmium halide and diamine can form a cadmium halide /diamine alternating layered structure through diamine bridging and hydrogen bonding between hydrogen atoms of the amine and halogen atoms. Likewise, a [CdCl2(RNH2)2] lamellar complex, which is used herein as a soft template, is expected to form lamellar structures composed of 2D arrays of CdCl2 and alkyl amine by van der Waals attraction between hydrocarbon sidechains of the alkyl amine. The small-angle X-ray scattering (SAXS) patterns of [CdCl2(RNH2)2] complexes with butylamine (BA), octylamine (OA), and dodecylamine (DA) show 00l orders of reflection, which confirms that the complexes formed typical lamellar structures (Supporting Information, Figure S1). A [CdCl2(OA)2] lamellar complex was chosen as the soft template for the synthesis of lamellarstructured CdSe nanosheets because of its optimum reactivity. [*] J. S. Son, Dr. J. Joo, K. Park, Dr. J. H. Kim, Dr. K. An, J. H. Yu, S. G. Kwon, Dr. S.-H. Choi, Prof. T. Hyeon National Creative Research Initiative Center for Oxide Nanocrystalline Materials and School of Chemical and Biological Engineering Seoul National University Seoul 151-744 (Korea) Fax: (+82)2-886-8457 E-mail: thyeon@snu.ac.kr

Giant Zeeman splitting in nucleation-controlled doped CdSe:Mn2+ quantum nanoribbons

Doping of semiconductor nanocrystals by transition-metal ions has attracted tremendous attention owing to their nanoscale spintronic applications. Such doping is, however, difficult to achieve in low-dimensional strongly quantum confined nanostructures by conventional growth procedures. Here we demonstrate that the incorporation of manganese ions up to 10% into CdSe quantum nanoribbons can be readily achieved by a nucleation-controlled doping process. The cation-exchange reaction of (CdSe)(13) clusters with Mn(2+) ions governs the Mn(2+) incorporation during the nucleation stage. This highly efficient Mn(2+) doping of the CdSe quantum nanoribbons results in giant exciton Zeeman splitting with an effective g-factor of approximately 600, the largest value seen so far in diluted magnetic semiconductor nanocrystals. Furthermore, the sign of the s-d exchange is inverted to negative owing to the exceptionally strong quantum confinement in our nanoribbons. The nucleation-controlled doping strategy demonstrated here thus opens the possibility of doping various strongly quantum confined nanocrystals for diverse applications.

Dimension-Controlled Synthesis of CdS Nanocrystals: From 0D Quantum Dots to 2D Nanoplates

The dimension-controlled synthesis of CdS nanocrystals in the strong quantum confinement regime is reported. Zero-, one-, and two-dimensional CdS nanocrystals are selectively synthesized via low-temperature reactions using alkylamines as surface-capping ligands. The shape of the nanocrystals is controlled systematically by using different amines and reaction conditions. The 2D nanoplates have a uniform thickness as low as 1.2 nm. Furthermore, their optical absorption and emission spectra show very narrow peaks indicating extremely uniform thickness. It is demonstrated that 2D nanoplates are generated by 2D assembly of CdS magic-sized clusters formed at the nucleation stage, and subsequent attachment of the clusters. The stability of magic-sized clusters in amine solvent strongly influences the final shapes of the nanocrystals. The thickness of the nanoplates increases in a stepwise manner while retaining their uniformity, similar to the growth behavior of inorganic clusters. The 2D CdS nanoplates are a new type of quantum well with novel nanoscale properties in the strong quantum confinement regime.

Magnetic mesoporous materials for removal of environmental wastes

We have synthesized two different magnetic mesoporous materials that can be easily separated from aqueous solutions by applying a magnetic field. Synthesized magnetic mesoporous materials, Mag-SBA-15 (magnetic ordered mesoporous silica) and Mag-OMC (magnetic ordered mesoporous carbon), have a high loading capacity of contaminants due to high surface area of the supports and high magnetic activity due to the embedded iron oxide particles. Application of surface-modified Mag-SBA-15 was investigated for the collection of mercury from water. The mercury adsorption using Mag-SBA-15 was rapid during the initial contact time and reached a steady-state condition, with an uptake of approximately 97% after 7h. Application of Mag-OMC for collection of organics from water, using fluorescein as an easily trackable model analyte, was explored. The fluorescein was absorbed into Mag-OMC within minutes and the fluorescent intensity of solution was completely disappeared after an hour. In another application, Mag-SBA-15 was used as a host of tyrosinase, and employed as recyclable catalytic scaffolds for tyrosinase-catalyzed biodegradation of catechol. Crosslinked tyrosinase in Mag-SBA-15, prepared in a two step process of tyrosinase adsorption and crosslinking, was stable enough for catechol degradation with no serious loss of enzyme activity. Considering these results of cleaning up water from toxic inorganic and organic contaminants, magnetic mesoporous materials have a great potential to be employed for the removal of environmental contaminants and potentially for the application in large-scale wastewater treatment plants.

Fabrication of novel mesoporous dimethylsiloxane-incorporated silicas

Mesoporous dimethylsiloxane-incorporated silica materials are synthesized which exhibit high surface areas, regular mesopores, thick walls, high hydrophobicity and high mechanical strength.

Recombinant production and biochemical characterization of a hypothetical acidic shell matrix protein in Escherichia coli for the preparation of protein-based CaCO3 biominerals

The biological structure of shells consists of highly organized calcium carbonate (CaCO3) crystals, which have remarkable mechanical and biological properties compared to the pure mineral form of CaCO3. It has been known that the organization of these biominerals is controlled by a relatively tiny amount of organic components such as shell matrix proteins. Here, we successfully produced a recombinant hypothetical acidic shell matrix protein in Escherichia coli, although the protein is composed of highly repetitive and biased amino acid sequences. About 15mg/L purified protein with greater than 95% purity was obtained in the 400 mL lab scale flask culture. The protein was able to efficiently form a complex with calcium ions, and spherulitic calcite crystals were synthesized in the presence of the recombinant protein. We expect that biomineralization using the recombinant protein could not only overcome the limited amount of protein available for biomineralization studies and biomineral preparation in practical aspects, but also provide opportunities to enable biomimetic synthesis of notable bio-composites based on organic-inorganic complexation.

Synthesis and catalytic applications of uniform-sized nanocrystals

We developed a new generalized synthetic procedure to produce monodisperse nanocrystals of many transition metals, metal oxides, and metal sulfides without a size selection process. Highly-crystalline and monodisperse nanocrystals were synthesized from the thermal decomposition of metal-surfactant complexes. We synthesized monodisperse spherical nanocrystals of metals (Fe, Cr, Cu, Ni, and Pd), metal oxides (γ-Fe 2 O 3 , Fe 3 O 4 , CoFe 2 O 4 , MnFe 2 O 4 , NiO, and MnO), and metal sulfides (CdS, ZnS, PbS, and MnS). We reported the ultra-large-scale synthesis of monodisperse nanocrystals by the thermolysis of metal-oleate complexes. We synthesized as much as 40 grams of monodisperse magnetite nanocrystals using 1 L reactor. By controlling the nucleation and growth processes, we were able to synthesize monodisperse magnetite nanoparticles with particle sizes of 4, 6, 7, 8, 9, 10, 11, 12, 13, 14, and 16 nm. Multi-gram scale synthesis of CdS, ZnS, PbS, and MnS were achieved from the thermolysis of metal-surfactant complexes in the presence of sulfur. We synthesized uniform Cu 2 O coated Cu nanoparticles from the thermal decomposition of copper acetylacetonate, followed by air oxidation. We successfully used these nanoparticles for the catalysts for Ullmann type amination coupling reactions of aryl chlorides. We synthesized core/shell-like Ni/Pd bimetallic nanoparticles from the consecutive thermal decomposition of metal-surfactant complexes. The nanoparticle catalyst was atom-economically applied for various Sonogashira coupling reactions.

Preparation of CaCO3 and CaO Nanoparticles via Solid-State Conversion of Calcium Oleate Precursor

Calcium carbonate (CaCO3) and monodisperse calcium oxide nanoparticles (CaO NPs) are prepared by the calcination of solid-state calcium oleate precursor in air condition. The effect of calcination temperature on the synthesis of CaCO3 and CaO NPs is examined. The polymorphism is confirmed by X-ray diffraction spectroscopy (XRD), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). The sample morphologies including their size and size distribution are investigated by field emission scanning electron microscopy (FESEM). Calcination of calcium oleate between 400 and 550 °C results in CaCO3 NPs with mean sizes from 82 to 98 nm, whereas monodisperse spherical CaO NPs are obtained at 650 °C and an average size is estimated to be 40 nm. Beyond 650 °C, the size of CaO NPs increases with broad size distribution. The results of this study provide a novel approach to monodisperse CaCO3 and CaO NPs that can be applied in a variety of fundamental and industrial fields.

Effects of Calcification Inhibitors on the Viability of the Coralline Algae Lithophyllum yessoense and Corallina pilulifera

Coralline algae, the algal whitening phenomenon-causing seaweeds, are characterized by calcareous deposits in the cell wall. The viability of the coralline algae Lithophyllum yessoense and Corallina pilulifera was quantitated using a triphenyltetrazolium chloride assay and eight calcification inhibitors. Among these inhibitors, ferric citrate showed the strongest inhibition of coralline algae viability. The concentrations of ferric citrate conferring 50% inhibition were 1.7 and 3.8 mM for L. yessoense and C. pilulifera, respectively. Thus, at a specific concentration and in a localized area, ferric citrate may be used to prevent the blooming of coralline algae.

Preparation of urushiol-containing poly(methyl methacrylate) copolymers for antibacterial and antifouling coatings

Synthesis of an eco-friendly and efficient antibacterial and antifouling coatings is presented by exploiting urushiol, a natural varnishing material. Since urushiol has inherent outstanding surface-protecting and antimicrobial properties, a series of poly (methyl methacrylate)-urushiol polymer compositions were prepared and fabricated into films. The prepared films were subjected to antimicrobial and antifouling studies. The polymer systems were characterized by various physico-spectroscopic techniques such as 1H NMR, Fourier transform infrared spectroscopy, differential scanning calorimetry, atomic force microscopy, and thermal gravimetric analysis. The confocal laser scanning micrographs, obtained for Pseudomonas aeruginosa biofilm formation, demonstrated an excellent antimicrobial response of the urushiol-incorporated polymers against this bacterial strain. We also demonstrated an inhibitory attachment effect against Navicula incerta, a fouling microalgal strain.