Nam Joo Lee

The interlayer screening effect of graphene sheets investigated by Kelvin probe force microscopy

We report on the interlayer screening effect of graphene using Kelvin probe force microscopy (KPFM). By using a gate device configuration that enables the supply of electronic carriers in graphene sheets, the vertical screening properties were studied from measuring the surface potential gradient. The results show layer-dependence of graphene sheets, as the number of graphene layers increases, the surface potential decreases exponentially. In addition, we calculate the work function-related information of the graphene layers using KPFM.

Scanning probe microscopy study of microcells from the organ surface Bonghan corpuscle

Microcells from organ surface Bonghan corpuscles [B. H. Kim, J. Acad. Med. Sci. DPR Kor. 90, 1 (1963)] of mammals have been studied by using optical microscopy, transmission electron microscopy and immunohistochemistry. In order to further investigate their physical and electrical properties at better resolution, many different modes of scanning probe microscopy were used in this research. Their surface morphology was studied by topography imaging and error-signal imaging of atomic force microscopy and their mechanical properties were investigated by force modulation microscopy. Electrostatic force microscopy was also used for their electrical characterization.

A study on the resistance switching of Ag2Se and Ta2O5 heterojunctions using structural engineering

The resistive random access memory (RRAM) devices with heterostuctures have been investigated due to cycling stability, nonlinear switching, complementary resistive switching and self-compliance. The heterostructured devices can modulate the resistive switching (RS) behavior appropriately by bilayer structure with a variety of materials. In this study, the bipolar resistive switching characteristics of the bilayer structures composed of Ta2O5 and Ag2Se, which are transition-metal oxide (TMO) and silver chalcogenide, were investigated. The bilayer devices of Ta2O5 deposited on Ag2Se (Ta2O5/Ag2Se) and Ag2Se deposited on Ta2O5 (Ag2Se/Ta2O5) were fabricated for investigation of the RS characteristics by stacking sequence of Ta2O5 and Ag2Se. All operating voltages were applied to the Ag top electrode with the Pt bottom electrode grounded. The Ta2O5/Ag2Se device showed that a negative voltage sweep switched the device from high resistance state (HRS) to low resistance state (LRS) and a positive voltage sweep switched the device from LRS to HRS. On the contrary, for the Ag2Se/Ta2O5 device a positive voltage sweep switched the device from HRS to LRS, and a negative voltage sweep switched it from LRS to HRS. The polarity dependence of RS was attributed to the stacking sequence of Ta2O5 and Ag2Se. In addition, the combined heterostructured device of both bilayer stacks, Ta2O5/Ag2Se and Ag2Se/Ta2O5, exhibited the complementary switching characteristics. By using threshold switching devices, sneak path leakage can be reduced without additional selectors. The bilayer heterostructures of Ta2O5 and Ag2Se have various advantages such as self-compliance, reproducibility and forming-free stable RS. It confirms the possible applications of TMO and silver chalcogenide heterostructures in RRAM.

Resistive switching characteristics of Ag 2 Se thin film

Electrical properties of silver selenide (Ag2Se) thin film, which is a thickness of 500 nm, were studied using conducting atomic force microscopy (c-AFM). A resistive switching was observed in the sweep voltage range from -7 to +7 V at room temperature. In addition, the Ag/Ag2Se/Au structure was compared with the Ag2Se/Au structure. It was considered that the resistive switching behavior may cause Ag filaments. The typical resistive characteristics of the Ag2Se thin film were presented, and the mechanism was discussed.

Electrostatic Force Microscopy of Metallic Ion-Intercalated DNA

The local charge of plasmid DNA has been studied using electrostatic force microscopy (EFM). The sample was prepared by immersing plasmid DNA in ZnCl2. The interaction of the tip with the positive local charge was stronger for the DNA immersed in a solution of higher concentration, and this phenomenon was evidence of the positive metallic ion intercalation into DNA.

Electrical characteristics of organic molecular wires by scanning probe microscopy

Reportedly, molecular wires which have intrinsically different characteristics from semiconductor nano wires show some unique transport properties. However, it seems that there is not so much understanding about their local properties. So, in this work, we characterized electrical properties of carotenoid which had differently substituted end-groups by using Scanning Probe Microscopy. To confirm the difference of the electrochemistry with substituted end-groups, we performed cyclic voltammetry in au aqueous solution. Electrochemical reaction was detected in the potential region between 0.5 and 0.75 V vs. Ag rceil AgCI rceil KCl (sat), whereas the conductance values were showed difference of differently substituted end-groups. Physisorptional effect between molecule and Au substrate was removed by introducing thiol[SH] group at each end of molecule resulting in covalent bond. After inserting these molecules into a 1-methylsulfanyl-octadecane monolayer pre-deposited on Au substrate, an Au nanoparticle is attached to the other end of each molecule via the protruding thiol group. To confirm the chemical conjunction between au Au nanoparticle and carotenedithiol molecule, we performed Electrostatic Force. These measurements allow us to simulate the transport property of carotenoid molecule. Followed IN measurement reconfirmed the results.A PtIr coated AFM probe is used as an electrode to contact the molecule through the Au nanoparticle. They reveal that methoxy-phenyl substituted carotenedithiol molecules showed better conducting properties and bromo-phenyl substituted carotenedithiol molecules showed worse than phenyl-substituted one did. I also obtained that current-voltage (I-V) curves were quantized as integer multiples of one fundamental curve. These allow us to simulate the transport property of single carotenoid molecule.