Han-Kyu Choi

Metal-Catalyzed Chemical Reaction of Single Molecules Directly Probed by Vibrational Spectroscopy

The study of heterogeneous catalytic reactions remains a major challenge because it involves a complex network of reaction steps with various intermediates. If the vibrational spectra of individual molecules could be monitored in real time, one could characterize the structures of the intermediates and the time scales of reaction steps without ensemble averaging. Surface-enhanced Raman scattering (SERS) spectroscopy does provide vibrational spectra with single-molecule sensitivity, but typical single-molecule SERS signals exhibit spatial heterogeneities and temporal fluctuations, making them difficult to be used in single-molecule kinetics studies. Here we show that SERS can monitor the single-molecule catalytic reactions in real time. The surface-immobilized reactants placed at the junctions of well-defined nanoparticle-thin film structures produce time-resolved SERS spectra with discrete, step-transitions of photoproducts. We interpret that such SERS-steps correspond to the reaction events of individual molecules occurring at the SERS hotspot. The analyses of the yield, dynamics, and the magnitude of such SERS steps, along with the associated spectral characteristics, fully support our claim. In addition, a model that is based on plasmonic field enhancement and surface photochemistry reproduces the key features of experimental observation. Overall, the result demonstrates that it is possible, under well-controlled conditions, to differentiate the chemical and physical processes contributing to the single-molecule SERS signals, and thus shows the use of single-molecule SERS as a tool for studying the metal-catalyzed organic reactions.

Stem-piped light activates phytochrome B to trigger light responses in Arabidopsis thaliana roots

Light conducted through plant tissues activates a photoreceptor in the roots of Arabidopsis thaliana. Plant stems pipe light to roots Light affects not only the development and physiology of the shoots (stems, leaves, and flowers) of plants but also the underground root system. Light triggers shoot cells to release signals that travel to the root and affect the development and physiology of the root system. Like shoot cells, root cells also have photoreceptors that can be activated by light, leading Lee et al. to investigate if light actually reaches these underground parts of the plant. Exposing Arabidopsis thaliana shoots to light while protecting the roots from light activated the photoreceptor phyB in the roots. In the root, phyB activated Hy5, a transcription factor that mediates cellular responses to light and was important for growth of the primary root and for root gravitropism, the proper downward orientation of roots. Arabidopsis stems efficiently conducted only certain wavelengths of light to the root tissues, and these conducted wavelengths activated phyB directly in the roots. These findings demonstrate that roots not only receive information about light conditions through signaling molecules that travel from the shoot to the root in response to light but also directly perceive light that is conducted through the plant tissues. The roles of photoreceptors and their associated signaling mechanisms have been extensively studied in plant photomorphogenesis with a major focus on the photoresponses of the shoot system. Accumulating evidence indicates that light also influences root growth and development through the light-induced release of signaling molecules that travel from the shoot to the root. We explored whether aboveground light directly influences the root system of Arabidopsis thaliana. Light was efficiently conducted through the stems to the roots, where photoactivated phytochrome B (phyB) triggered expression of ELONGATED HYPOCOTYL 5 (HY5) and accumulation of HY5 protein, a transcription factor that promotes root growth in response to light. Stimulation of HY5 in response to illumination of only the shoot was reduced when root tissues carried a loss-of-function mutation in PHYB, and HY5 mutant roots exhibited alterations in root growth and gravitropism in response to shoot illumination. These findings demonstrate that the underground roots directly sense stem-piped light to monitor the aboveground light environment during plant environmental adaptation.

PERFORMANCE OF THE SRAO 6-METER RADIO TELESCOPE

We introduce and describe performance of the 6-meter telescope of Seoul Radio Astronomy Observatory (SRAO). All the softwares and instruments except the antenna structure and its driving system are developed for ourselves. The SIS mixer type receiver resulted in the receiver noise temperature less than 50 K (DSB) over the whole 3-mm radio window. An autocorrelation spectrometer, developed first in Korea, provides maximum 50 MHz band width over 1024 channels. Antenna surface is measured and adjusted using template method and radio holography which resulted in a superb surface accuracy bet-ter than 30. Accordingly, the aperture and beam efficiences amount to and , respectively, largely independent of frequency in the 85 - 115 GHz range. It is also found that telescope pointing errors are less than 10 in both azimuth and elevation and that antenna gain is almost constant against elevation greater than , without adjusting sub-reflector position. The SRAO 6-meter telescope is now fully operational and all these characteristics verify that observations are carried out with high precision and fidelity.

Identification of the First Elementary Step in the Photocatalytic Reduction of Nitrobenzenethiols on a Metallic Surface

Reduction of nitrobenzene is widely used for the assessment of the catalytic activities of nanoparticles, yet its mechanism is still largely unverified. Here, using the surface-enhanced Raman scattering (SERS), we have identified an intermediate of the first step in the photocatalytic reduction of nitrobenzenethiols (NBTs) on a metallic surface. The formation of the intermediate is identified by a fast red-shift of the NO2 symmetric-stretching peak of the SERS spectra of reacting NBTs, prior to the slow intensity decay. On the basis of the laser power dependences of the rates of spectral changes, electrochemical SERS, and quantum chemical calculations, we conclude that the intermediate is the anion radical of nitrobenzenethiol that is formed by the metal-to-molecule single-electron transfer reaction. The subsequent intensity decay of the peak, which is the rate-determining step of the whole reduction reaction, corresponds to another single-electron reduction of the anion radical into dihydroxyaminobenzenethiol or dianion of NBT.

Low-Power Optical Trapping of Nanoparticles and Proteins with Resonant Coaxial Nanoaperture Using 10 nm Gap

We present optical trapping with a 10 nm gap resonant coaxial nanoaperture in a gold film. Large arrays of 600 resonant plasmonic coaxial nanoaperture traps are produced on a single chip via atomic layer lithography with each aperture tuned to match a 785 nm laser source. We show that these single coaxial apertures can act as efficient nanotweezers with a sharp potential well, capable of trapping 30 nm polystyrene nanoparticles and streptavidin molecules with a laser power as low as 4.7 mW. Furthermore, the resonant coaxial nanoaperture enables real-time label-free detection of the trapping events via simple transmission measurements. Our fabrication technique is scalable and reproducible, since the critical nanogap dimension is defined by atomic layer deposition. Thus our platform shows significant potential to push the limit of optical trapping technologies.

Development of the SRAO 1024-Channel Digital Auto-Correlator

We developed a 1024-channel digital auto-correlation spectrometer for the Seoul Radio Astronomy Observatory (hereafter the SRAO-1KACS). The SRAO-1KACS has two main modules: the IF-to-baseband converter (IFBC) module and the 1024-channel auto-correlator (1KACR) module. The input frequency range of the IFBC module is from 1.5 to 1.55 GHz with a dynamic range of −4 ∼ +3 dBm. The 1KACR module performs calculations of auto-correlation coefficients by the ‘accumulation and modulo-2-counting method’ in 3-level configuration. The system is controlled by a Linux-based personal computer. The SRAO-1KACS provides 3 different observational modes: 50, 25, and 12.5 MHz bandwidth modes. The channel losses are 20%, 12%, and 8% for each bandwidth mode, respectively. Various tests were executed including lab tests and astronomical tests. Lab tests were performed for a 1.5625MHz sinusoidal wave input and for a white noise source. We also executed astronomical tests in 12CO J=1−0 emission line at 115.2712 GHz, which showed that SRAO-1KACS can be used at astronomical observatories.

Frequency-Domain Proof of the Existence of Atomic-Scale SERS Hot-Spots

The existence of sub-nanometer plasmonic hot-spots and their relevance in spectroscopy and microscopy applications remain elusive despite a few recent theoretical and experimental evidence supporting this possibility. In this Letter, we present new spectroscopic evidence suggesting that Angstrom-sized hot-spots exist on the surfaces of plasmon-excited nanostructures. Surface-enhanced Raman scattering (SERS) spectra of 4,4-biphenyl dithiols placed in metallic junctions show simultaneously blinking Stokes and anti-Stokes spectra, some of which exhibit only one prominent vibrational peak. The activated vibrational modes were found to vary widely between junction sites. Such site-specific, single-peak spectra could be successfully modeled using single-molecule SERS induced by a hot-spot with a diameter no larger than 3.5 Å, located at the specific molecular sites. Furthermore, the model, which assumes the stochastic creation of hot-spots on locally flat metallic surfaces, consistently reproduces the intensity distributions and occurrence statistics of the blinking SERS peaks, further confirming that the sources of the hot-spots are located on the metallic surfaces. This result not only provides compelling evidence for the existence of Angstrom-sized hot-spots but also opens up the new possibilities for the vibrational and electronic control of single-molecule photochemistry and real-space visualization of molecular vibration modes.

Research and development of digital autocorrelation spectrometer

This paper describes the system configuration and the some results of the prototype wide-band digital autocorrelation spectrometer for radio astronomy observation, which will be used as back-end spectrometer for the 2/3mm dual channels SIS receiver at Taeduk Radio Astronomy Observatory. This spectrometer consists of high-speed sampler module, circular memory buffer module, and correlator module based on QUAINT correlator chip. The developed digital autocorrelator shows good performance results at 50MHz and 100MHz mode with the 10MHz CW source input. After some calibration procedure, this spectrometer can be use as back-end system at TRAO

Waveguide-Integrated Compact Plasmonic Resonators for On-Chip Mid-Infrared Laser Spectroscopy

The integration of nanoplasmonic devices with a silicon photonic platform affords a new approach for efficient light delivery by combining the high field enhancement of plasmonics and the ultralow propagation loss of dielectric waveguides. Such a hybrid integration obviates the need for a bulky free-space optics setup and can lead to fully integrated, on-chip optical sensing systems. Here, we demonstrate ultracompact plasmonic resonators directly patterned atop a silicon waveguide for mid-infrared spectroscopic chemical sensing. The footprint of the plasmonic nanorod resonators is as small as 2 μm2, yet they can couple with the mid-infrared waveguide mode efficiently. The plasmonic resonance is directly measured through the transmission spectrum of the waveguide with a coupling efficiency greater than 70% and a field intensity enhancement factor of over 3600 relative to the evanescent waveguide field intensity. Using this hybrid device and a tunable mid-infrared laser source, surface-enhanced infrared absorption spectroscopy of both a thin poly(methyl methacrylate) film and an octadecanethiol monolayer is successfully demonstrated.

A DESIGN DEVELOPMENT OF 400MHz BAND AUTOCORRELATOR FOR RADIO ASTRONOMY OBSERVATION

This paper is the research and development including the system design and the prototype system building of the 400MHz wide-band digital autocorrelation spectrometer system for radio astronomy observation, which will be used as back-end signal processing unit of the Dual channel SIS receiver at Taeduk Radio Astronomy Observatory. So in this paper, we performed development of the high speed digitizing sampler, the circular memory buffer, and the correlator module for the 400MHz wide-band digital autocorrelator. This developed system will be use at TRAO after the housing and some calibration.