Songhui Kim

Distance control for a near-field scanning microwave microscope in liquid using a quartz tuning fork

We demonstrate a scanning near-field microwave microscope (NSMM) in the liquid environment using a tuning fork shear-force feedback method to control the distance between tip and sample. Only the probe tip for the NSMM is immersed in water. The dry part of the probe is attached to one prong of a quartz tuning fork and directly coupled to a high-quality dielectric resonator at an operating frequency f=4.5–5.5GHz. This distance control method is independent of the local microwave characteristics. The amplitude of the tuning fork was used as a set point of the distance control parameter in the liquid. To demonstrate the distance regulation system, we present the NSMM images of a copper film in air and liquid without and with readjustment of the distance set point, as well as an image of a DNA film in buffer solution. Imaging under buffer environments is of particular interest for future studies of biomolecular association reactions on solid supports.

Sodium chloride sensing by using a near-field microwave microprobe

The authors observed the NaCl concentration of solutions using a near-field microwave microprobe (NFMM). Instead of the usual technique, they take advantage of the noncontact evaluation capabilities of a NFMM. A NFMM with a high Q dielectric resonator allows observation of small variations of the permittivity due to changes in the NaCl concentration. By measuring the reflection coefficient S11, they could observe the concentration of NaCl. The measured signal-to-noise was about 53dB and the minimum detectible signal was about 0.005dB∕(mg∕ml). In order to determine the probe selectivity, they measured a mixture solution of NaCl and glucose.

Tip–sample distance control for near-field scanning microwave microscopes

We demonstrate a near-field scanning microwave microscope (NSMM) which uses a tuning fork shear-force feedback method to control the distance between tip and sample. This distance control method is independent of local microwave characteristics. The probe tip for the NSMM is attached to one prong of a quartz tuning fork and directly coupled to a high-quality microstrip resonator with a dielectric resonator at an operating frequency of f=4.5–5.5 GHz. The amplitude of the tuning fork was used as a distance control parameter in the feedback system. To demonstrate the ability of the distance regulation system, we present topographic images of an uneven conducting metal sample and compare the height response and the NSMM image.

Nondestructive high spatial resolution imaging with a 60 GHz near-field scanning millimeter-wave microscope

We demonstrate a nondestructive millimeter-wave surface imaging technique using a near-field scanning millimeter-wave microscope (NSMM) with a resonant standard waveguide probe at an operation frequency f=60 GHz. A chemically etched metallic probe tip was coupled to the resonant rectangular waveguide. By properly tuning the tunable resonator and the probe tip we could improve sensitivity and spatial resolution of the NSMM. By measuring the change of the quality factor in the near-field zone, near-field scanning millimeter-wave images of patterned metal films and YBa2Cu3Oy thin films were obtained with a spatial resolution better than 1 μm. We observed the dependence of the current density NSMM images of patterned indium tin oxide films on the bias currents.

Antiferromagnetic order induced by gadolinium substitution in Bi2Se3 single crystals

Magnetic topological insulators can serve as a fundamental platform for various spin-based device applications. We report the antiferromagnetic order induced by the magnetic impurity dopants of Gd in GdxBi2−xSe3 and the systematic results with varying the Gd concentration x ( = 0.14, 0.20, 0.30, and 0.40). The antiferromagnetic order is demonstrated by the magnetic susceptibility, electrical resistivity, and specific heat measurements. The anomaly observed at TN = 6 K for x ≥ 0.30 shifts towards lower temperature with increasing the magnetic field, indicative of antiferromagnetic ground state. The Gd substitution into Bi2Se3 enables not only tuning the magnetism from paramagnetic to antiferromagnetic for high x (≥ 0.30) but also giving a promising candidate for antiferromagnetic topological insulators.

Optical Characterization of Sensory Rhodopsin II Thin Films using a Near-field Scanning Microwave Microscope

We report the electro-optical properties of the sensory rhodopsin II using a near-field scanning microwave microscope(NSMM). Rhodopsin was known as a photoreceptor pigment with a retinal as a chromophore via a protonated Schiff base and consists of seven transmembrane segments. The sensory rhodopsin II, expressing E. coli UT5600 with endogenous retinal biosynthesis system and purified with affinity chromatography in the presence of 0.02 % DM (Dodecyl Maltoside) from Natronomonas pharaonis. We measured the absorption spectra and the transients difference of sensory rhodopsin II from Natronomonas pharaonis using a UV/VIS spectrophotometer with Nd-Yag Laser (532 nm). The absorption spectra of NpSR II showed a typical rhodopsin spectrum with a left shoulder region and the photointermediates spectra of NpSR II-ground state (

A Study on Space Charge of Organic Pentacene/metal Interface

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Nano resolution imaging technique with a near-field scanning microwave microscope

Surface potential properties at the interface of pentacene thin films on gold (Au) and aluminum (Al) surfaces were investigated by using a near-field scanning microwave microprobe (NSMM). The surface potential formed across the pentacene film was observed by measuring the microwave reflection coefficient and compared with the result of a Kelvin-probe method. The obtained reflection coefficient of the pentacene thin films on Al was decreased as the pentacene film thickness increased due to the increased accumulation of negative space charges, while for Au was essentially constant.

Magneto-optical imaging using a garnet indicator film prepared on glass substrates

We demonstrated a near-field scanning microwave microscope (NSMM) with a nano spatial resolution using a hybrid tip. In order to understand the function of the probe, we fabricated the hybrid tip with a flat shaped shoulder and a reduced length of the tapered part using a conventional chemical etching technique. Two different NSMM images were observed for patterned Cr films on glass substrates and analyzed as a function of the apex and the cone angle of the tapered part. The hybrid tips were coupled to a high quality dielectric resonator at an operating frequency f = 4.46 GHz. By using the hybrid tip, we demonstrated an improved high contrast NSMM image of lambda phage DNA on a glass substrate.