Sooyoung Park

Polymorphic and mechanochromic luminescence modulation in the highly emissive dicyanodistyrylbenzene crystal: secondary bonding interaction in molecular stacking assembly

Understanding the role of self-assembly and electronic interactions of constituent molecules in determining optoelectronic properties of a molecular solid is a fundamental and essential issue in material science. Particularly, the correlation between the molecular stacking mode and modulated optical properties has barely been established in spite of its scientific and technological importance. Herein, we present dicyanodistyrylbenzene-based highly luminescent crystals which uniquely exhibit polymorphism and mechanochromism. In these materials, various secondary bonding interactions (local dipole interaction, C–H⋯π interaction, and C–H⋯N hydrogen bond) play key roles in molecular stacking assembly, as well as in polymorphic and mechanochromic behaviors. The two different polymorphic phases of dicyanodistyrylbenzene crystal were correlated to the different modes of local dipole coupling, which caused a substantial alternation of π–π overlap and excited state delocalization to give differently colored fluorescence emission. Most uniquely, the phase transformation between those crystalline phases was effected through thermal and mechanical processes. We have comprehensively carried out in-depth and systematic optical, structural, and photophysical investigations to establish unambiguous structure–property relationships.

A Proposal on an Optimized Device Structure With Experimental Studies on Recent Devices for the DRAM Cell Transistor

We have experimentally analyzed the leakage mechanism and device degradations caused by the Fowler-Nordheim (F-N) and hot carrier stresses for the recently developed dynamic random-access memory cell transistors with deeply recessed channels. We have identified the important differences in the leakage mechanism between saddle fin (S-Fin) and recess channel array transistor (RCAT). These devices have their own respective structural benefits with regard to leakage current. Therefore, we suggest guidelines with respect to the optimal device structures such that they have the advantages of both S-Fin and RCAT structures. With these guidelines, we propose a new recess-FinFET structure that can be realized by feasible manufacturing process steps. The structure has the side-gate form only in the bottom channel region. This enhances the characteristics of the threshold voltage (VTH), ON/OFF currents, and the retention time distributions compared with the S-Fin structure introduced recently.

Characterization of Channel-Diameter-Dependent Low-Frequency Noise in Silicon Nanowire Field-Effect Transistors

The low-frequency noise in the silicon nanowire field-effect transistor (SNWFET) is characterized using SNWFETs with different channel diameters <i>d</i>_NW. The current density and the simulation result indicate that the volume inversion as manifested by the spatial charge distribution is enhanced in smaller <i>d</i>_NW. The measured noise data are discussed based on the number and correlated mobility fluctuation model. It is shown that the low-frequency noise decreases in smaller <i>d</i>_NW. This <i>d</i>_NW-dependent noise behavior is clarified in terms of the effective oxide trap density and the fraction of inversion charges near the Si-SiO₂ interface.

Gene expression profiling of human hepatoblastoma using archived formalin-fixed and paraffin-embedded tissues

We elucidated the genetic profile of hepatoblastomas (HBLs) to identify diagnostic and prognostic markers. RNA was extracted from 32 formalin-fixed, paraffin-embedded HBLs and corresponding nonneoplastic liver (NNL) tissues, and cDNA-mediated annealing, selection, extension, and ligation (DASL) chip assays were performed. Immunohistochemistry was performed to confirm the expression of Yin Yang 1 (YY1) protein in HBL. Twenty-four genes that were associated with signal transduction, cell–cell adhesion, cell cycle regulation, and apoptosis were differentially expressed in HBL and NNL tissues. Two apoptosis-associated genes, MYCN and BIRC5, were highly upregulated in HBL. Eight genes, including YY1 and IGF1, were upregulated in HBL cases that had a poor prognosis. Thirty-eight genes, including YY1, were differentially expressed according to histologic differentiation of HBL, and the immunohistochemical expression of YY1 was correlated with poor HBL differentiation. Thus, using DASL chip assays, we report the gene expression profiles of HBL, which suggest new candidate prognostic and diagnostic genetic markers and putative therapeutic targets for HBL.

Characteristics of gate-all-around silicon nanowire field effect transistors with asymmetric channel width and source/drain doping concentration

We performed 3D simulations to demonstrate structural effects in sub-20 nm gate-all-around silicon nanowire field effect transistors having asymmetric channel width along the channel direction. We analyzed the differences in the electrical and physical properties for various slopes of the channel width in asymmetric silicon nanowire field effect transistors (SNWFETs) and compared them to symmetrical SNWFETs with uniform channel width. In the same manner, the effects of the individual doping concentration at the source and drain also have been investigated. For various structural conditions, the current and switching characteristics are seriously affected. The differences attributed to the doping levels and geometric conditions are due to the electric field and electron density profile.

A high-performance ambipolar organic field-effect transistor based on a bidirectional π-extended diketopyrrolopyrrole under ambient conditions

A novel bidirectional π-extended 2,5-dihydro-1,4-dioxo-3,6-di-2-thienyl-1,4-diketopyrrolo[3,4-c]pyrrole derivative (DPP-2T) with the 4-(2,2-dicyanovinyl)phenyl group, (DPP-2T2P–2DCV), has been synthesized and characterized in order to achieve a high-performance organic semiconductor. The HOMO/LUMO energies of DPP-2T2P–2DCV were estimated to be −5.36 and −3.81 eV, respectively, based on their redox potentials, which were very similar to the other bidirectional π-extended DPP-2T analogue DPP-4T–2DCV. The calculated HOMO/LUMO values (HOMO: −5.43 eV, LUMO: −3.56 eV) based on the optimized geometry agreed well with the experimental values. DPP-2T2P–2DCV exhibits ambipolar TFT response with reasonably balanced electron and hole mobilities of 0.168 cm2 V−1 s−1 and 0.015 cm2 V−1 s−1 by solution process, respectively, which is the best result for solution processable DPP-based ambipolar small molecule semiconductors measured under ambient atmosphere.

Optical and electrical characteristics of asymmetric nanowire solar cells

We propose an asymmetric radial structure developed via simulation that improves the optical and electrical characteristics of silicon nanowire (SiNW) solar cells. This nanostructure is designed by shrinking the bottom core diameter and holding the top core diameter fixed in the SiNW, which results in a total reflection of the incident light in the outer wall of the shell due to the difference in the refractive index. The reflection enhances light trapping and concentration, which results in a 10 times higher optical generation rate and greater optical absorption in the high energy regime as compared with the fundamental symmetric radial structure. Further, we found that the efficiency is increased by over 10% when the bottom core diameter is decreased. The proposed structure has great potential to effectively improve the efficiency in concert with optimizing the design parameters.

Novel Blue Organic Light Emitting Materials

The new blue light emitting materials containing anthracene were designed, synthesized and characterized. The materials were prepared in high yields by well-known reactions such as Grignard reaction, synthesis of boronic acid, and Suzuki coupling reaction. The non-coplanar structures of materials were confirmed by using the PM3 parameterization in the Hyper Chem 5.0 program (Hypercube) in order to fully optimize the structure of molecule. The obtained materials showed a thermal stability and a good film forming ability. It is found that the energy transfer from substituents to anthracene took place effectively.

The exciton transition in extremely shallow quantum well structures: Strong coupling between the distributions in the quantized and in‐plane directions

The planar radius and the binding energy of excitons in extremely shallow quantum well (ESQW) structures are calculated theoretically. As the strength of an applied electric field increases, the exciton planar radius increases and the binding energy decreases rapidly compared with a conventional quantum well, which explains the rapid peak quench in ESQW structures. We also calculate the energy difference between heavy hole and light hole exciton transitions in quantum wells with various barrier heights. Due to the small energy difference in ESQW structures, the light hole peak is not clearly resolved from heavy hole peak by room‐temperature photocurrent experiments.

An Analysis of the Field Dependence of Interface Trap Generation under Negative Bias Temperature Instability Stress using Wentzel–Kramers–Brillouin with Density Gradient Method

In this paper, the Si–H bond dissociation rate is calculated under a negative bias temperature instability (NBTI) condition that considers the quantum effect on the hole density in the inversion layer of a metal–oxide–semiconductor field-effect transistor (MOSFET). The physical model used in this study is composed of two terms: the number of holes in that Si–H bond, and the polarization of the Si–H bond under an external electric field. By adopting a density-gradient (DG) method with a penetration boundary condition and the Wentzel–Kramers–Brillouin (WKB) approximation, the penetrated hole density profile in the gate oxide and the tendency towards the hole amount in the Si–H bond according to the electric field have been identified and compared with other works. The results show that the NBTI field dependence and the lifetime of the devices under NBTI stress correlate to the power-law dependency.