Sung-Min Choi

Thermally Switchable One- and Two-Dimensional Arrays of Single-Walled Carbon Nanotubes in a Polymeric System

Fabrication of highly ordered arrays of single-walled carbon nanotubes (SWNTs) has been of great interest for a wide range of potential applications. Here, we report thermally switchable one- and two-dimensional arrays of individually isolated SWNTs formed by cooperative self-assembly of functionalized SWNTs and a block copolymer/water system. Small-angle X-ray scattering measurements reveal that when the block copolymer/water system is in an isotropic phase, two-dimensional hexagonal arrays of SWNTs are formed by depletion attraction, and when the block copolymer/water system is in a lamellar phase, one-dimensional lattices of SWNTs intercalated in the polar regions of the polymeric lamellar structure are formed by entropically driven segregation and two-dimensional depletion attraction. These two SWNT arrays are thermally interchangeable, following the temperature-dependent phase behavior of the block copolymer/water system.

Structural phase transitions of Ge2Sb2Te5 cells with TiN electrodes using a homemade W heater tip

The phase transitions of a Ge2Sb2Te5 cell with a volume of 20×20×0.1μm3 were carried out by applying a reset pulse (10V and 50ns) and a subsequent set pulse (5V and 300ns) using a homemade W heater tip fabricated by focused ion beam lithography. The phase transformation from a crystalline state to an amorphous state was confirmed by measuring the I-V curves and observation with a cross-sectional transmission electron microscope both before and after applying the reset pulse. The electron diffraction pattern obtained from the transformed area clearly showed the amorphous state. The resistance value of the transformed amorphous area was two orders higher than that of the original crystalline phase. This difference in the resistance value between the reset and set states was maintained for 20 reset/set pulse cycles. It is expected that this experimental setup can be used to evaluate the fatigue behavior of Ge2Sb2Te5 cells with reset/set pulse cycles.

Phase behavior of hexa-peri-hexabenzocoronene derivative in organic solvent.

The phase behavior of HBC-C12 in p-xylene at various concentrations has been investigated by differential scanning calorimetry, polarized optical microscopy, small angle neutron scattering and wide-angle X-ray scattering techniques, and a phase diagram depending on concentration and temperature has been determined with detailed structural information. At room temperature, HBC-C12 in p-xylene forms needle-like crystalline aggregates with a rectangular columnar packing which is essentially the same as the crystalline phase of HBC-C12 in bulk. Upon heating, it undergoes two transitions, a transition from the rectangular columnar packing to another rectangular columnar packing with a small step change in the lattice parameter c, and then a transition to isotropic phase. In the isotropic phase, HBC-C12 molecules form stacked cylindrical particles which consist of ∼11-13 molecules depending on temperature. Upon cooling, HBC-C12 in p-xylene makes an abrupt transition from the isotropic to the rectangular columnar packing, which is in contrast with the gradual columnar to isotropic transition upon heating. These results provide important information for optimizing the conditions of discotic liquid crystal solution for the fabrication of DLC thin films using solution-processing techniques.

Single-walled carbon nanotube induced re-entrant hexagonal phases in a Pluronic block copolymer system

We report a new single-walled carbon nanotube (SWNT)-induced re-entrant lyotropic phase behavior of Pluronic F127 block copolymers in water which show two hexagonal phase regions. The small angle X-ray scattering measurements show that as the concentration is increased from 5 to 100 wt% by evaporation, the F127–SWNT/water system exhibits isotropic–hexagonal–(FCC/hex)–BCC–hexagonal–lamellar phase transitions, which is in contrast to the F127/water system showing isotropic–FCC–BCC–hexagonal-lamellar phase transitions. This clearly indicates that, in the F127–SWNT/water system, the 1sthexagonal phase is induced by the presence of long one-dimensional SWNTs and the 2ndhexagonal phase originates from the phase behavior of F127 itself. The presence of SWNTs also induced the shifts of phase transition concentrations and the change of domain sizes. The SWNTs in the hexagonal phases are located in the hydrophobic core (PPO domain) of F127 cylinders, making their orientation parallel to the [001] direction. The possible orientations of SWNTs (which maintain their individuality or very small bundle state) in the FCC and BCC phases were identified based on the epitaxial transitions between corresponding phases (hexagonal to FCC, FCC to BCC, and BCC to hexagonal): [110] in the FCC and 〈111〉 in the BCC.

Highly ordered self-assembly of 1D nanoparticles in phospholipids driven by curvature and electrostatic interaction.

Self-assembly of 1D nanoparticles such as carbon nanotubes or nanorods into highly ordered superstructures using various interactions has been of great interest as a route toward materials with new functionalities. However, the phase behavior of 1D nanoparticles interacting with surrounding materials, which is the key information to design self-assembled superstructures, has not been fully exploited yet. Here, we report for the first time a new phase diagram of negatively charged 1D nanoparticle and cationic liposome (CLs) complexes in water that exhibit three different highly ordered phases, intercalated lamellar, doubly intercalated lamellar, and centered rectangular phases, depending on particle curvature and electrostatic interactions. The new phase diagram can be used to understand and design new highly ordered self-assemblies of 1D nanoparticles in soft matter, which provide new functionalities.

Construction of an optimal witness for unknown two-qubit entanglement.

Whether entanglement in a state can be detected, distilled, and quantified without full state reconstruction is a fundamental open problem. We demonstrate a new scheme encompassing these three tasks for arbitrary two-qubit entanglement, by constructing the optimal entanglement witness for polarization-entangled mixed-state photon pairs without full state reconstruction. With better efficiency than quantum state tomography, the entanglement is maximally distilled by newly developed tunable polarization filters and quantified by the expectation value of the witness, which equals the concurrence. This scheme is extendible to multiqubit Greenberger-Horne-Zeilinger entanglement.

Green Synthesis of High-Purity Mesoporous Gold Sponges Using Self-Assembly of Gold Nanoparticles Induced by Thiolated Poly(ethylene glycol)

A facile and green synthesis method for mesoporous gold sponges has been developed, which involves a simple mixing of a very small amount of thiolated-poly(ethylene glycol) (SH-PEG) and citrate-covered gold nanoparticles (Au NPs) in aqueous solution at room temperature. While SH-PEG molecules have been widely used as biocompatible hydrophilic capping agents for Au NPs for stable dispersion in aqueous solution, here they are used as destabilizing agents. When SH-PEG molecules are mixed with citrate-covered Au NPs at the molar ratio ranging from 3 to 20 (SH-PEG/Au NP), mesoporous gold sponges with randomly interconnected 3D network structures are formed within 2 to 3 h. This is driven by the destabilization of negatively charged citrate molecules on Au NPs by a small number of SH-PEG molecules bonded on the particle surface, which results in the decrease in zeta potential and thus the assembly of Au NPs into porous sponges. The use of very low concentration of SH-PEG (ca. 20-200 nM) in aqueous solution at room temperature makes the method highly eco-friendly as well as results in high-purity as-synthesized gold sponges (98.7 wt %). The mesoporous gold sponges fabricated with the present method exhibit a high SERS activity, making them highly applicable for sensitive SERS detection of molecules.

Hydration forces between surfaces of surfactant coated single-walled carbon nanotubes

The interaction force between functionalized single-walled carbon nanotubes (SWNTs) plays an important role in the fabrication of self-assembled and highly ordered SWNT arrays for a wide range of potential applications. Here, we measured interaction force between SWNTs encapsulated with polymerized surfactant monolayer (p-SWNTs). The balance between the repulsion between p-SWNTs and the osmotic pressure exerted by poly(ethylene glycol) in aqueous solution results in two-dimensional hexagonal arrays of p-SWNTs with very small surface to surface distances (<1 nm). The interaction force measured by the osmotic pressure technique shows characteristic decay length of hydration force in its origin.

Scalable thermal synthesis of a highly crumpled, highly exfoliated and N-doped graphene/Mn-oxide nanoparticle hybrid for high-performance supercapacitors

A scalable thermal method to synthesize a highly crumpled, highly exfoliated and N-doped graphene/Mn-oxide nanoparticle hybrid for high performance supercapacitors has been demonstrated. Reduction of graphene-oxide (GO), nanometer scale crumpling, high level of exfoliation, N-doping of graphene and decoration with Mn-oxide nanoparticles, each of which significantly contributes to a high specific capacitance (958 F g−1 at 5 mV s−1) in a synergetic way, are achieved in a single thermal process, thermal annealing of GO–Gly–Mn(NO3)2·4H2O mixture at 500 °C followed by a rapid quenching with liquid nitrogen. N-doping of graphene is predominantly done in the form of pyrrolic-like and pyridine-like nitrogens, and Mn-oxide nanoparticles are formed on the surface of graphene as MnO2 and Mn3O4. The nanometer scale crumpling of graphene sheets, which is achieved by rapid quenching of graphene in the presence of Mn-oxide nanoparticles on its surface, induces an exceptionally high degree of exfoliation of graphenes and prevents restacking of graphene sheets during a repeated charge–discharge process, providing a high specific surface area (1006 m2 g−1) and high cycle stability (94.1% retention after 1000 cycles), respectively. The simplicity of the synthesis process and the high performance of supercapacitors make it an easily scalable and industrially applicable method.

Highly Ordered and Highly Aligned Two‐Dimensional Binary Superlattice of a SWNT/Cylindrical‐Micellar System

We report a highly ordered intercalated hexagonal binary superlattice of hydrophilically functionalized single-walled carbon nanotubes (p-SWNTs) and surfactant (C12 E5 ) cylindrical micelles. When p-SWNTs (with a diameter slightly larger than that of the C12 E5 cylinders) were added to the hexagonally packed C12 E5 cylindrical-micellar system, p-SWNTs positioned themselves in such a way that the free-volume entropies for both p-SWNTs and C12 E5 cylinders were maximized, thus resulting in the intercalated hexagonal binary superlattice. In this binary superlattice, a hexagonal array of p-SWNTs is embedded in a honeycomb lattice of C12 E5 cylinders. The intercalated hexagonal binary superlattice can be highly aligned in one direction by an oscillatory shear field and remains aligned after the shear is removed.