Seungwoo Lee

The Effect of Na2CO3 on the Catalytic Gasification of Rice Straw over Nickel Catalysts Supported on Kieselguhr

Rice straw was catalytically gasified over nickel catalysts supported on kieselguhr. Ni catalyst activated the gasification step from the formed oil. The effect of sodium carbonate on the formation of carbonized solid, oil and gas composition was also investigated. With the addition of sodium carbonate, the gas yield was largely increased and the formation of oil through liquefaction also increased. A reaction pathway on the gasification of rice straw was discussed.

Multibit MoS2 Photoelectronic Memory with Ultrahigh Sensitivity

A novel multibit MoS2 photoelectronic nonvolatile memory device is developed by synergistically combining rational device designs and the efficient transfer of large-area MoS2 flakes. The MoS2 photoelectronic memory exhibits excellent memory characteristics, including a large programming/erasing current ratio that exceeds 107 , multilevel data storage of 3 bits (corresponding to eight levels), performance stability over 200 cycles, and stable data retention over 104 s.

Vertically Oriented, Three-Dimensionally Tapered Deep-Subwavelength Metallic Nanohole Arrays Developed by Photofluidization Lithography

The field-gradient, superficial photo fluidization of azomaterials allows a specific 3D nano-silhouette to be shaped over a large area, so as to get easy access to a 3D-tapered, deep sub-wavelength Au nanohole (20 nm spatial size) array. The squeezing of visible light into the deep sub-wavelength point and the relevant extraordinary optical transmission (EOT) are achieved using this 3D-tapered, 20 nm Au nanohole.

Deterministic assembly of metamolecules by atomic force microscope-enabled manipulation of ultra-smooth, super-spherical gold nanoparticles

Atomic force microscope (AFM)-enabled manipulation of individual metallic nanoparticles (NPs) has proven useful for assembling diverse structural motifs of metamolecules. However, for the reliable verifications of their electric/magnetic behaviors and translations into practical applications (e.g., metasurfaces), currently available assembly of polygonal shaped metallic NPs with size and shape distributions should be further advanced. Here, we discover conditions for AFM-enabled, deterministic assembly of highly uniform, super-spherical gold NPs (AuNPs) into the metamolecules, which can show the designed electric/magnetic resonance behaviors in a highly reliable fashion. The use of super-spherical AuNPs together with the controlled adhesive properties of an AFM tip allows us to linearly and continuously push AuNPs toward the pre-programed directions and positions with minimized slipping away effect. Thus, a versatile and fast (as little as few minutes per each metamolecule) assembly of metamolecules with unprecedented structural fidelity becomes possible via AFM-enabled manipulation; enabling a high precision engineering of electromagnetic properties with metamolecules.

Microfluidic Generation of Monodisperse and Photoreconfigurable Microspheres for Floral Iridescence-Inspired Structural Colorization

Flowering plants have evolved their structural coloration strategies exquisitely to split various vivid colors into a spatially distributed sequence by using the 2D diffraction elements, confined onto a curved surface. Herein, A new strategy of artificially mimicking floral-inspired grating diffractive motifs onto the monodisperse and smooth microspheres is reported, developed by microfluidics; thus, the accessible design space of structural colorization can be greatly expanded.

Heterogeneously Assembled Metamaterials and Metadevices via 3D Modular Transfer Printing.

Metamaterials have made the exotic control of the flow of electromagnetic waves possible, which is difficult to achieve with natural materials. In recent years, the emergence of functional metadevices has shown immense potential for the practical realization of highly efficient photonic devices. However, complex and heterogeneous architectures that enable diverse functionalities of metamaterials and metadevices have been challenging to realize because of the limited manufacturing capabilities of conventional fabrication methods. Here, we show that three-dimensional (3D) modular transfer printing can be used to construct diverse metamaterials in complex 3D architectures on universal substrates, which is attractive for achieving on-demand photonic properties. Few repetitive processing steps and rapid constructions are additional advantages of 3D modular transfer printing. Thus, this method provides a fascinating route to generate flexible and stretchable 2D/3D metamaterials and metadevices with heterogeneous material components, complex device architectures, and diverse functionalities.

Colloidal superlattices for unnaturally high-index metamaterials at broadband optical frequencies.

The recent advance in the assembly of metallic nanoparticles (NPs) has enabled sophisticated engineering of unprecedented light-matter interaction at the optical domain. In this work, I expand the design flexibility of NP optical metamaterial to push the upper limit of accessible refractive index to the unnaturally high regime. The precise control over the geometrical parameters of NP superlattice monolayer conferred the dramatic increase in electric resonance and related effective permittivity far beyond the naturally accessible regime. Simultaneously, effective permeability change, another key factor to achieving high refractive index, was effectively suppressed by reducing the thickness of NPs. By establishing this design rule, I have achieved unnaturally high refractive index (15.7 at the electric resonance and 7.3 at the quasi-static limit) at broadband optical frequencies (100 THz ~300 THz). I also combined this NP metamaterial with graphene to electrically control the high refractive index over the broad optical frequencies.

Security analysis and applications of standard key agreement protocols

The use of cryptographic system to provide the privacy of transmitted message over network is increasing gradually. A key agreement protocol is the most important part to establish a secure cryptographic system and the effort to standardize the key agreement protocols is in rapid progress. In this paper, we study properties and operation of standard key agreement protocols and analyze the security of their protocols under several active attacker models such as active impersonation, key-compromised impersonation, (full/half) forward secrecy, known-key passive attack, known-key impersonation attack and so on. Then, we suggest the standard for selecting key agreement protocols for each application, and find the most suitable key agreement protocol for each application.

Design of optical metamaterial mirror with metallic nanoparticles for floating-gate graphene optoelectronic devices

The purpose of this work is to conceive the idea for using the gate dielectrics of floating-gate memory device (i.e., Au nanoparticle (AuNP) monolayer embedded within polymeric matrix) as a magnetic mirror, so as to harness the broadband light absorption of thin film optoelectronics. In particular, we systematically examined whether the versatile assembly of spherical AuNP monolayer can be indeed treated as the effective magnetic mirror for floating-gate graphene optoelectronic device. High amenability of the AuNP assembly with the large-area device fabrication procedures may make this strategy widely applicable to various thin film optoelectronic devices. Our study thereby advances the design of mirror for thin film optoelectronics.

Assembly of “3D” plasmonic clusters by “2D” AFM nanomanipulation of highly uniform and smooth gold nanospheres

Atomic force microscopy (AFM) nanomanipulation has been viewed as a deterministic method for the assembly of plasmonic metamolecules because it enables unprecedented engineering of clusters with exquisite control over particle number and geometry. Nevertheless, the dimensionality of plasmonic metamolecules via AFM nanomanipulation is limited to 2D, so as to restrict the design space of available artificial electromagnetisms. Here, we show that “2D” nanomanipulation of the AFM tip can be used to assemble “3D” plasmonic metamolecules in a versatile and deterministic way by dribbling highly spherical and smooth gold nanospheres (NSs) on a nanohole template rather than on a flat surface. Various 3D plasmonic clusters with controlled symmetry were successfully assembled with nanometer precision; the relevant 3D plasmonic modes (i.e., artificial magnetism and magnetic-based Fano resonance) were fully rationalized by both numerical calculation and dark-field spectroscopy. This templating strategy for advancing AFM nanomanipulation can be generalized to exploit the fundamental understanding of various electromagnetic 3D couplings and can serve as the basis for the design of metamolecules, metafluids, and metamaterials.