Seung Hyun

Bioinspired Dual Stimuli-Responsive Membranous System with Multiple On–Off Gates

Stimuli-responsive polymers have been widely used for controlled release of several biomolecules. In general, a single stimulus among various stimuli, for instance, temperature, pH, or light, has been used for these polymers. Although some stimuli are applied together, one cannot control each stimulus independently at a given stimulus-responsive polymer. However, to mimic biological system like cell membrane, multiple on-off gates utilizing independent control of dual (or multiple) stimuli should be used. Here, we introduce a stimuli-responsive membrane controlled by two orthogonal stimuli. For this purpose, the top and the bottom parts of anodized aluminum oxide membrane walls are independently grafted by thermoresponsive poly(N-isopropylacrylamide) and pH-responsive poly(acrylic acid), respectively, by using surface-initiated atom transfer radical polymerization. The membrane clearly showed two independent on-off gates depending on temperature and pH. Furthermore, through light irradiation of two different wavelengths (near-infrared and ultraviolet), temperature and pH were also controlled independently and promptly. Thus, this membrane shows two independent on-off gating of the transport of a model biomolecule of fluorescein isothiocyanate-labeled bovine serum albumin. This strategy suggests the potential of independently modified membrane in layers as stimuli-responsive on-off gates for the application of artificial cell membrane.

High density array of multiferroic nanoislands in a large area

We report the innovative fabrication of a high density array of multiferroic BiFeO3 (BFO) nanoislands on a conductive substrate in a large area. The anodic aluminum oxide (AAO) template has widely been used to fabricate highly arranged nanostructures, because of easy control of pore size and perfect hexagonal pore packing. The existing AAO mask-assisted pulsed laser deposition (PLD) method is limited to fabricating a nanoisland array in a small area. To supplement the shortcoming of this method, a thick AAO membrane in a large area was electrochemically detached and floated on polystyrene (PS) film without crack. Then, a nanoporous polystyrene (PS) template was prepared by dry etching with the thick AAO membrane mask, followed by spin coating of the BFO precursor on the PS template. After removing the PS template at high temperature, we prepared a high density array of multiferroic BFO nanoislands in a large area epitaxially grown on an STO:Nb (100) substrate. A high density array of BFO nanoislands in a large area showed both ferroelectricity of individual nanoislands obtained by piezoresponse force microscopy (PFM) and macroscopic magnetism measured by a superconducting quantum interference device (SQUID) based magnetic property measurement system (MPMS). A high density array of BFO nanoislands could be employed as a next-generation memory device capable of electric writing and magnetic reading (or vice versa).

Towards the limit of ferroelectric nanostructures: switchable sub-10 nm nanoisland arrays

Ultrahigh density arrays of ferroelectric PbTiO3 (PTO) nanoislands with various feature sizes were epitaxially grown by utilizing block copolymer micelles. Piezoresponse and ferroelectric properties were clearly observed in the PTO nanoislands by band excitation piezoresponse force microscopy. In particular, PTO nanoislands were fully switchable even with a volume as small as 79 nm3. Accordingly, it is expected that the volume of the switchable ferroelectric nanoislands can be further decreased. The obtained results show that an areal density of over 10 Tb in−2 for the ferroelectric memory devices would be possible.

A Facile Route for Patterned Growth of Metal-Insulator Carbon Lateral Junction through One-Pot Synthesis

Precise graphene patterning is of critical importance for tailor-made and sophisticated two-dimensional nanoelectronic and optical devices. However, graphene-based heterostructures have been grown by delicate multistep chemical vapor deposition methods, limiting preparation of versatile heterostructures. Here, we report one-pot synthesis of graphene/amorphous carbon (a-C) heterostructures from a solid source of polystyrene via selective photo-cross-linking process. Graphene is successfully grown from neat polystyrene regions, while patterned cross-linked polystyrene regions turn into a-C because of a large difference in their thermal stability. Since the electrical resistance of a-C is at least 2 orders of magnitude higher than that for graphene, the charge transport in graphene/a-C heterostructure occurs through the graphene region. Measurement of the quantum Hall effect in graphene/a-C lateral heterostructures clearly confirms the reliable quality of graphene and well-defined graphene/a-C interface. The direct synthesis of patterned graphene from polymer pattern could be further exploited to prepare versatile heterostructures.

Origin of f-orbital-bonding insensitivity to spin-orbit coupling in UO2

The origin of negligible spin-orbit coupling effects on the ground-state properties of UO2 is examined using the LDA+DMFT and the LSDA+U methods. Although the previous theoretical results neglecting the spin-orbit coupling (SOC) are in reasonable agreement with experiments, this insensitivity has not been investigated. Via the charge density distribution, we show that the SOC does not affect the U-O bonding states which are directly related to the bonding properties (e.g., lattice constant, bulk modulus, and oxidation/reduction energies).