Juyeong Oh

Controlling the luminescence emission from palladium grafted graphene oxide thin films via reduction

The role of palladium (Pd) in the reduction of graphene oxide (GO) thin films was investigated using a Pd assisted grafting technique. The structural and optical characteristics of these thin films were obtained from various spectroscopic analyses, which confirmed increased C[double bond, length as m-dash]C-C aromatic ring vibration and oxidation of Pd with Ar annealing. In Pd free GO, annealing of films resulted in restoration of sp(2) clusters; however, Pd grafting with non-annealed film enhanced the possibility of restoration and further annealing dramatically increased the restoration rate with enhanced blue photoluminescence (PL) emission. The blue PL emission originates from sp(2) cluster sites and the yellow-green PL from defect trapped states. As reduction of GO increased, yellow-green emission decreased and blue PL became the prominent emission. These experimental findings open up a new feasible pathway for controlling the luminescence emission from graphene oxide that furthers the technological advancement of graphene based optoelectronic devices.

Substrate and buffer layer effect on the structural and optical properties of graphene oxide thin films

Graphene oxide (GO) thin films on various substrates show surprising variations of their structural and optical properties. These variations were also studied by depositing GO via introducing a gold nanoparticles buffer layer on quartz substrate. The effect of the substrate as well as buffer layer results in short range order crystallization in deposited GO films with an increase in inter-planar spacing. XPS analysis shows that GO undergoes reduction when spin coated on ITO/glass substrate. The deposited GO films exhibit luminescence emission, and the introduction of gold buffer layer results in a blue shift of the photoluminescent emission spectra. The GO on gold buffer layer shows almost constant optical absorption in the whole visible spectral region like graphene. The present study indicates that buffer layer effects and the interaction between different substrates and GO is strong enough to affect the oxygen linkages in GO which in turn changes its structural and optical properties, which may find potential application in graphene based optoelectronic device fabrication.

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.

Origin of variation in switching voltages in threshold-switching phenomena of VO2 thin films

We investigated the origin of the variation in switching voltages in threshold-switching of VO2 thin films. When a triangular-waveform voltage signal was applied, the current changed abruptly at two switching voltages, i.e., VON (insulator-to-metal) and VOFF (metal-to-insulator). VON and VOFF were measured by changing the period of the voltage signal, the temperature of the environment, and the load resistance. We observed that either VON or VOFF varied significantly and had different dependences with respect to the external parameters. Based on the mechanism of the metal–insulator transition induced by Joule heating, numerical simulations were performed, which quantitatively reproduced all of the experimental results. From the simulation analysis, the variation in the switching voltages for threshold-switching was determined to be thermal in origin.

Unconventional Terahertz Carrier Relaxation in Graphene Oxide: Observation of Enhanced Auger Recombination Due to Defect Saturation

Photoexcited carrier relaxation is a recurring topic in understanding the transient conductivity dynamics of graphene-based devices. For atomically thin graphene oxide (GO), a simple free-carrier Drude response is expected to govern the terahertz (THz) conductivity dynamics--same dynamics observed in conventional CVD-grown graphene. However, to date, no experimental testimony has been provided on the origin of photoinduced conductivity increase in GO. Here, using ultrafast THz spectroscopy, we show that the photoexcited carrier relaxation in GO exhibits a peculiar non-Drude behavior. Unlike graphene, the THz dynamics of GO show percolation behaviors: as the annealing temperature increases, transient THz conductivity rapidly increases and the associated carrier relaxation changes from mono- to biexponential decay. After saturating the recombination decay through defect trapping, a new ultrafast decay channel characterized by multiparticle Auger scattering is observed whose threshold pump fluence is found to be 50 μJ/cm2. The increased conductivity is rapidly suppressed within 1 ps due to the Auger recombination, and non-Drude THz absorptions are subsequently emerged as a result of the defect-trapped high-frequency oscillators.

Broadband supercontinuum generation using a hollow optical fiber filled with copper-ion-modified DNA

We experimentally demonstrated supercontinuum generation through a hollow core photonic bandgap fiber (HC-PBGF) filled with DNA nanocrystals modified by copper ions in a solution. Both double-crossover nano DNA structure and copper-ion-modified structure provided a sufficiently high optical nonlinearity within a short length of hollow optical fiber. Adding a higher concentration of copper ion into the DNA nanocrystals, the bandwidth of supercontinuum output was monotonically increased. Finally, we achieved the bandwidth expansion of about 1000 nm to be sufficient for broadband multi-spectrum applications.

Artificial Rod and Cone Photoreceptors with Human-Like Spectral Sensitivities

Photosensitive materials contain biologically engineered elements and are constructed using delicate techniques, with special attention devoted to efficiency, stability, and biocompatibility. However, to date, no photosensitive material has been developed to replace damaged visual-systems to detect light and transmit the signal to a neuron in the human body. In the current study, artificial nanovesicle-based photosensitive materials are observed to possess the characteristics of photoreceptors similar to the human eye. The materials exhibit considerably effective spectral characteristics according to each pigment. Four photoreceptors originating from the human eye with color-distinguishability are produced in human embryonic kidney (HEK)-293 cells and partially purified in the form of nanovesicles. Under various wavelengths of visible light, electrochemical measurements are performed to analyze the physiological behavior and kinetics of the photoreceptors, with graphene, performing as an electrode, playing an important role in the lipid bilayer deposition and oxygen reduction processes. Four nanovesicles with different photoreceptors, namely, rhodopsin (Rho), short-, medium-, and longwave sensitive opsin 1 (1SW, 1MW, 1LW), show remarkable color-dependent characteristics, consistent with those of natural human retina. With four different light-emitting diodes for functional verification, the photoreceptors embedded in nanovesicles show remarkably specific color sensitivity. This study demonstrates the potential applications of light-activated platforms in biological optoelectronic industries.

Imaging of single retinal ganglion cell with differential interference contrast microscopy (Conference Presentation)

Glaucoma is a progressive optic neuropathy, characterized by the selective loss of retinal ganglion cells (RGCs). Therefore, monitoring the change of number or morphology of RGC is essential for the early detection as well as investigation of pathophysiology of glaucoma. Since RGC layer is transparent and hyporeflective, the direct optical visualization of RGCs has not been successful so far. Therefore, glaucoma evaluation mostly depends on indirect diagnostic methods such as the evaluation of optic disc morphology or retinal nerve fiber layer thickness measurement by optical coherence tomography. We have previously demonstrated single photoreceptor cell imaging with differential interference contrast (DIC) microscopy. Herein, we successfully visualized single RGC using DIC microscopy. Since RGC layer is much less reflective than photoreceptor layer, various techniques including the control of light wavelength and bandwidth using a tunable band pass filter were adopted to reduce the chromatic aberration in z-axis for higher and clearer resolution. To verify that the imaged cells were the RGCs, the flat-mounted retina of Sprague-Dawley rat, in which the RGCs were retrogradely labeled with fluorescence, was observed by both fluorescence and DIC microscopies for direct comparison. We have confirmed that the cell images obtained by fluorescence microscopy were perfectly matched with cell images by DIC microscopy. As conclusion, we have visualized single RGC with DIC microscopy, and confirmed with fluorescence microscopy.

Generation of Broadband Supercontinuum through Nonlinear Interaction in DNA Materials

We demonstrated supercontinuum generation through DNA materials. The DNA material, which is modified copper ion, provided optical nonlinearity within a hollow optical fiber. The supercontinuum spectrum is optimized by fiber length and copper ion concentration for the bandwidth of more than 800nm.

Metal ion sensing solution containing double crossover DNA

The current study describes metal ion sensing with double crossover DNAs (DX1 and DX2), artificially designed as a platform of doping. The sample for sensing is prepared by a facile annealing method to grow the DXs lattice on a silicon/silicon oxide. Adding and incubating metal ion solution with the sensor substrate into the micro-tube lead the optical property change. Photoluminescence (PL) is employed for detecting the concentration of metal ion in the specimen. We investigated PL emission for sensor application with the divalent copper. In the range from 400 to 650 nm, the PL features of samples provide significantly different peak positions with excitation and emission detection. Metal ions contribute to modify the optical characteristics of DX with structural and functional change, which results from the intercalation of them into hydrogen bonding positioned at the center of double helix. The PL intensity is decreased gradually after doping copper ion in the DX tile on the substrate.