Wonsuk Jung

Ultraconformal Contact Transfer of Monolayer Graphene on Metal to Various Substrates

The direct transfer method of large area monolayer CVD graphene from Cu foil to various substrates such as PET, PDMS, and glass is developed using mechano-electro-thermal forces based on ultraconformal contact without any metal etching process or additional carrier layers in a solid-state process. Transferred graphene presents both excellent quality (with no residues, few defects, or no folding) and remarkable mechanical and electrical stability.

Domain structure in a micron-sized PbZr1−xTixO3 single crystal on a Ti substrate fabricated by hydrothermal synthesis

Hydrothermal synthesis is an attractive method for the deposition of ferroelectric thin films because it can realize a low processing temperature of 200 °C or less. In this study, micron-sized PbZr1−xTixO3 (PZT) single crystals and thin films were fabricated on a Ti substrate at 160 °C by hydrothermal synthesis. The ferroelectricity of the crystals was investigated using local piezoresponse hysteresis, while their domain structure was investigated using high-resolution transmission electron microscopy (HRTEM) measurement. The isolated PZT single crystal exhibited hysteresis behavior of the piezoresponse, which represents ferroelectricity. From the HRTEM observations of a crystallite in polycrystalline PZT thin film, we found a twinned domain structure in the form of a … c/a/c/a … polydomain pattern. On the basis of the HRTEM observations, we proposed a three-dimensional domain structure for the isolated PZT single crystal; the c/a domain walls form along coherent {101} planes in the tetragonal cell. This ...

Switchable single c-domain formation in a heteroepitaxial PbTiO3 thin film on a (001) Nb-SrTiO3 substrate fabricated by means of hydrothermal epitaxy

We used hydrothermal epitaxy to fabricate a heteroepitaxial PbTiO3 (PTO) thin film on a Nb-doped (001) cubic SrTiO3 (NSTO) substrate while avoiding the phase transition. By means of transmission electron microscopy and piezoresponse force microscopy analyses, it was confirmed that no a domain formed in the heteroepitaxial PTO film; the film had a single +c-domain structure at an as-synthesized state. From the measurement of the polarization-voltage hysteresis curve, large remanent polarization (2Pr:144μC∕cm2) resulting from the single +c-domain structure and an imprint induced by an asymmetric electrode configuration were observed. It is suggested that the single +c-domain structure was caused by the interface of the PTO film and solution rather than the interface of the PTO film and NSTO electrode under the hydrothermal conditions. This hydrothermally synthesized PTO film is anticipated to be very suitable for high-performance engineering applications.

Ferroelectric properties and fatigue behavior of heteroepitaxial PbZr1−xTixO3 thin film fabricated by hydrothermal epitaxy below Curie temperature

A heteroepitaxial PbZr1−xTixO3 (PZT) thin film was fabricated by means of hydrothermal epitaxy at 210 °C below Curie temperature without undergoing the paraelectric to ferroelectric phase transition. From transmission electron microscope and piezoresponse force microscope observations, it was confirmed that the fabricated PZT thin films had only a −c monodomain without an a domain in the as-synthesized state. The polarization-electric-field hysteresis curve and the fatigue behavior of the heteroepitaxial PZT capacitor with a Pt top and n-type semiconductor bottom electrode was observed. The remanent polarization 2Pr of the PZT capacitor was about 63μC∕cm2. This value was much lower compared to that of the PbTiO3 capacitor, which was also fabricated by means of hydrothermal epitaxy at 160 °C below Curie temperature. It was suggested that a lower polarization of the PZT capacitor was due to the nonswitchable interfacial layer grown in the initial growth stage. However, this layer did not exert an influence ...

Size dependence of initial polarization direction in nanosized epitaxial PbTiO3 islands fabricated by hydrothermal epitaxy below Curie temperature

Nanosized epitaxial PbTiO3 (PTO) islands with a mono c domain were fabricated via self-assembly on a Nb-SrTiO3 (NSTO) substrate at 160 °C by hydrothermal epitaxy without undergoing the paraelectric to ferroelectric phase transition. The direction of the polarization at the as-fabricated state was observed with an island size up to a lateral area of 104nm2 and thickness of 12 nm (determined by atomic force microscope) using a piezoresponse force microscope. We found a critical size below which the negative polarization is stable at the as-fabricated state and above which the positive polarization is stable. This phenomenon was discussed under two assumptions: that the PTO islands are grown initially with positive polarization during fabrication, and the energy barrier for the transition from the positive polarization state to the negative polarization state is dependent on the ferroelectric island size.

Retention loss phenomena in hydrothermally fabricated heteroepitaxial PbTiO3 films studied by scanning probe microscopy

We observed the retention loss phenomena of the nanodomains with an average diameter of 36nm and that of the square domains with a size of 1 and 25μm2 that were reversed by an applying electric field at an atomic force microscopy conductive tip in a heteroepitaxial PbTiO3 thin film, which was fabricated via hydrothermal epitaxy below Curie temperature, TC. While the nanodomains did not undergo significant retention loss until 5.3×106s, the square domains revealed some retention loss for a fixed period after long latent periods. The observed phenomena were explained in terms of the instability of the curved c∕c domain wall and the compressive strain energy. Analyses showed that the nanodomains composed a cylinder extending to the bottom electrode; however, the square domains had a curved c∕c domain wall, including the compressive strain energy, and these factors caused the retention loss.

Flow-induced voltage generation over monolayer graphene in the presence of herringbone grooves

While flow-induced voltage over a graphene layer has been reported, its origin remains unclear. In our previous study, we suggested different mechanisms for different experimental configurations: phonon dragging effect for the parallel alignment and an enhanced out-of-plane phonon mode for the perpendicular alignment (Appl. Phys. Lett. 102:063116, 2011). In order to further examine the origin of flow-induced voltage, we introduced a transverse flow component by integrating staggered herringbone grooves in the microchannel. We found that the flow-induced voltage decreased significantly in the presence of herringbone grooves in both parallel and perpendicular alignments. These results support our previous interpretation.

Superstrong encapsulated monolayer graphene by the modified anodic bonding

We report a superstrong adhesive of monolayer graphene by modified anodic bonding. In this bonding, graphene plays the role of a superstrong and ultra-thin adhesive between SiO2 and glass substrates. As a result, monolayer graphene presented a strong adhesion energy of 1.4 J m(-2) about 310% that of van der Waals bonding (0.45 J m(-2)) to SiO2 and glass substrates. This flexible solid state graphene adhesive can tremendously decrease the adhesive thickness from about several tens of μm to 0.34 nm for epoxy or glue at the desired bonding area. As plausible causes of this superstrong adhesion, we suggest conformal contact with the rough surface of substrates and generation of C-O chemical bonding between graphene and the substrate due to the bonding process, and characterized these properties using optical microscopy, atomic force microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy.

Observation of [110] surface band within {101} a-domain of heteroepitaxial PbTiO3 thin film fabricated by hydrothermal epitaxy

Heteroepitaxial PbTiO3 film on an Nb-doped (001) cubic SrTiO3 substrate was fabricated by hydrothermal epitaxy at 200°C. Piezoresponse force microscopy and x-ray Θ rocking curves confirmed that the film showed a c∕a∕c∕a multi-domain structure even though it did not undergo a cubic paraelectric (PE) to tetragonal ferroelectric (FE) phase transition. After heat treatment of this film at 600°C, we observed the [110] surface band within the a-domain, which was formed through the PE to FE phase transition. We also found that a [110] surface band existed along the (11¯1) plane within the a-domain. We predicted that the [110] surface band would be monoclinic phase due to the interaction of two different variants of a-domains in the presence of a c-domain in the heteroepitaxial PbTiO3 film.

Differences between LTE and 3G service customers

This paper assesses the differences between 3G and LTE subscribers.The validity of the traditional determinants of customer satisfaction is tested.The effects of customer satisfaction and switching costs on loyalty are analyzed.This study finds that LTE users care more about data service quality than 3G users. South Korea has now become one of the worlds leading mobile service markets with the launch of its advanced LTE services; the LTE service subscription rate in Korea has exceeded 50% as of November 2013. This development raises the question of how LTE service subscribers may differ from 3G service subscribers. Within the context of this rapidly changing environment of mobile service markets, this study examines the validity of the traditional determinants of customer satisfaction in data-service oriented mobile communications markets, analyzes the effects of customer satisfaction and switching costs on customer loyalty, and assesses the difference between 3G and LTE subscribers with respect to their determinants of customer loyalty. The results show that there are significant differences between 3G and LTE subscribers while traditional factors such as call quality, data quality, customer support, and price perception are still significant determinants of customer satisfaction.