Kyujin Choi

Reflection terahertz time-domain spectroscopy of RDX and HMX explosives

We report on our study of RDX and HMX, two of the most commonly used explosive materials, in bulk pellets with reflection terahertz time-domain spectroscopy in the frequency range of 0.3–3 THz. The maximum entropy method was utilized to correct our raw reflection data against the phase error due to the relative displacement between the sample and the reference. Both the refractive index n and the extinction coefficient k in the terahertz region were acquired for these two explosives without a Kramers-Kronig analysis. Both RDX and HMX exhibit a series of distinct peaks not quite detectable in the more conventional transmission-type measurements due to their high terahertz absorptivity. Our results are compared with the literature data on powder samples.

CoFe2O4 nanostructures with high coercivity

Nanometer-sized ferrite magnetic materials are the subject of intense research interest due to their potential applications in high-density magnetic information storage. One of the most explored ferrite materials is the cobalt ferrite (CoFe2O4). We have synthesized cobalt ferrite nanowires using cobalt ferrite nanoparticles in a porous anodic alumina template (AAT). The process of embedding ferrimagnetic particles into the pores was assisted by the magnetic field of a permanent magnet placed in vacuum directly under the substrate. Particles synthesized in the template were subsequently annealed at 600°C for 2h in Ar gas forming arrays of cobalt ferrite nanowires inside the AAT. The morphology of the ferrite before and after annealing was observed using a field-emission scanning electron microscope. The crystallographic structure of the nanowires was analyzed using x-ray diffraction and transmission electron microscopy. The magnetization was measured by a superconducting quantum interference device. The co...

Energy Band Gap and Optical Transition of Metal Ion Modified Double Crossover DNA Lattices

We report on the energy band gap and optical transition of a series of divalent metal ion (Cu(2+), Ni(2+), Zn(2+), and Co(2+)) modified DNA (M-DNA) double crossover (DX) lattices fabricated on fused silica by the substrate-assisted growth (SAG) method. We demonstrate how the degree of coverage of the DX lattices is influenced by the DX monomer concentration and also analyze the band gaps of the M-DNA lattices. The energy band gap of the M-DNA, between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), ranges from 4.67 to 4.98 eV as judged by optical transitions. Relative to the band gap of a pristine DNA molecule (4.69 eV), the band gap of the M-DNA lattices increases with metal ion doping up to a critical concentration and then decreases with further doping. Interestingly, except for the case of Ni(2+), the onset of the second absorption band shifts to a lower energy until a critical concentration and then shifts to a higher energy with further increasing the metal ion concentration, which is consistent with the evolution of electrical transport characteristics. Our results show that controllable metal ion doping is an effective method to tune the band gap energy of DNA-based nanostructures.

Terahertz and optical study of monolayer graphene processed by plasma oxidation

We report on our terahertz and optical study of monolayer graphene grown by chemical vapor deposition and processed by plasma oxidation. The plasma oxidation induces oxygen-related defects, and the resulting disorder increases the sheet resistance of graphene as measured via terahertz spectroscopy. The excitonic absorption peak weakens considerably and blue shifts upon plasma oxidation, resulting in higher transmittance in both the visible and ultraviolet regions. Our oxygen plasma-treated graphene also exhibits a free-carrier doping effect as confirmed by the blue shift of the Raman G band.

Terahertz, optical, and Raman signatures of monolayer graphene behavior in thermally reduced graphene oxide films

We report on our joint spectroscopic study of the thermal reduction process of quasi-monolayer graphene oxide films grown on fused silica substrates by spin-coating. We estimate that about 65% of our film area consists of monolayer platelets of reduced graphene oxide, based on our quantitative analysis of the local atomic force microscopy topography. With thermal annealing under suitable conditions, clear signatures of monolayer graphene behavior were identified in the resonant excitonic absorption at 4.55 eV, the overall decrease in the visible-range transmission, the re-emergence of the Raman 2D band, the red-shift of the Raman G band toward the monolayer position, and the decrease in the optical sheet resistance in the terahertz range.

Terahertz electrodynamics and superconducting energy gap of NbTiN

Terahertz electrodynamics of superconducting NbTiN has been studied in the spectral range of 8-70 cm–1 above and below the critical temperature of Tc = 14.1 K. Our transmission terahertz time-domain spectroscopy technique allows for independent and accurate determination of both the real part σ1 and the imaginary part σ2 of the optical conductivity σ as a function of frequency ω and temperature T without a Kramers-Kronig analysis. A clear signature of the superconducting energy gap Δ(T) is observed in the real part σ1 of the optical conductivity below Tc, with 2Δ(0) = 36.5 cm−1 corresponding to 2Δ(0)/kBTc = 3.72. No indication of strong-coupling effects was observed in the imaginary part σ2 toward zero frequency. Our results, including the temperature dependence of the penetration depth λ(T), are in excellent quantitative agreement with weak coupling BCS theory and Mattis-Bardeen formula.

Optical and magnetic properties induced by structural confinement of ternary chalcogenide in SBA-15 nanotube

The amorphous ternary material, Ni3(SbTe3)2 was prepared using a rapid precipitation reaction between the Zintl phase K3SbTe3 and NiBr2 in solution. This material induced by the incorporation of ternary chalcogenide into long range ordered mesoporous SBA-15 was transformed into ternary nanoparticles. Structural confinement of the amorphous ternary chalcogenide in the mesoporous silica tube causes the suppression of phonon coupling to photoexcited carriers in the confined ternary chalcogenide, resulting in narrower photoluminescence (PL) band than those of the bulk ternary chalcogenide. It also gives rise to ferromagnetic behavior of the nanoparticles made from the material, which is a spin glass with the freezing temperature of 4.0 K in a bulk form. The physical and chemical properties of obtained ternary bulk and nanoparticles have been investigated by x-ray diffraction, superconducting quantum interference device, transmission electron microscopy, Brunauer–Emmett–Teller, and PL (steady-state photolumine...

Optical deposition of Molybdenum disulfide on a fiber facet

We could successfully deposit Molybdenum disulfide on a single mode fiber facet. The deposition was done by a 1552nm fiber laser. The results obtained by SEM shows that the core area of the fiber was completely covered by MoS2.

Terahertz time-domain spectroscopy of NiO x thin films

We have measured the terahertz transmittance of NiOx thin films grown on Si by thermal evaporation. The frequency-dependent conductivities were determined without resorting to a Kramers-Kronig analysis. Large changes in these spectral functions occurred due to varying deposition rate and annealing temperature. We observed a direct correlation between these parameters with the electronic and optical properties of NiOx thin films.

Fabrication and characteristics of thin-film waveguides based on DNA biomaterials

In the development of optoelectronic devices, deoxyribonucleic acid has been a representative material which has received much recent attention as a promising research area. We constructed waveguide based on optical fiber components and polydimethylsiloxane (PDMS) channel with discrete two dimensional nano-scale DNA monolayers grown by a self-assembly process and DNA complexes with cetyltrimethylammonium (DNA-CTMA) based on different kinds of solution; butanol, ethanol, and hexafluoroisopropanol. We measured the spectral profile of waveguides using a near-infrared laser source. We expect that biomaterials such as DNA-lipid complexes will have a unique application in optical devices compared with traditional fossil fuel-based polymers.