Maeng-Je Seong

Direct vapor phase growth process and robust photoluminescence properties of large area MoS2 layers

There has been growing research interest in the use of molybdenum disulfide in the fields of optoelectronics and energy harvesting devices, by virtue of its indirect-to-direct band gap tunability. However, obtaining large area thin films of MoS2 for future device applications still remains a challenge. In the present study, the amounts of the precursors (S and MoO3) were varied systematically in order to optimize the growth of highly crystalline and large area MoS2 layers by the chemical vapor deposition method. Careful control of the amounts of precursors was found to the key factor in the synthesis of large area highly crystalline flakes. The thickness of the layers was confirmed by Raman spectroscopy and atomic force microscopy. The optical properties and chemical composition were studied by photoluminescence (PL) and X-ray photoelectron spectroscopy. The emergence of strong direct excitonic emissions at 1.82 eV (A-exciton, with a normalized PL intensity of ∼55 × 103) and 1.98 eV (B-exciton, with a normalized PL intensity of ∼5 × 103) of the sample at room temperature clearly indicates the high luminescence quantum efficiency. The mobility of the films was found to be 0.09 cm2/(V·s) at room temperature. This study provides a method for the controlled synthesis of high-quality two-dimensional (2D) transition metal dichalcogenide materials, useful for applications in nanodevices, optoelectronics and solar energy conversion.

Gold nanoparticle–DNA aptamer composites as a universal carrier for in vivo delivery of biologically functional proteins

Although the delivery of biologically functional protein(s) into mammalian cells could be of tremendous value to biomedical research, the development of such technology has been hindered by the lack of a safe and effective delivery method. Here, we present a simple, efficient, and versatile gold nanoparticle-DNA aptamer conjugate (AuNP-Apt)-based system, with nanoblock-like properties, that allows any recombinant protein to be loaded without additional modifications and delivered into mammalian living systems. AuNP-Apt-based protein delivery system was able to deliver various proteins into variety of cell types in vitro without showing cytotoxicity. This AuNP-Apt system was also effective for the local and systemic targeted delivery of proteins in vivo. A local injection of the AuNP-Apt loaded with the apoptosis-inducing BIM protein efficiently inhibited the growth of xenograft tumors in mice. Furthermore, an intravenous injection of AuNP-Apt loaded with both epidermal growth factor (EGF) and BIM resulted in the targeted delivery of BIM into a xenograft tumor derived from EGF receptor-overexpressing cancer cells with no detectable systemic toxicity. Our findings show that this system can serve as an innovative platform for the development of protein-based biomedical applications.

Coexistence of bi-stable memory and mono-stable threshold resistance switching phenomena in amorphous NbOx films

Both bi-stable memory and mono-stable threshold switching are observed in amorphous NbOx films. In addition, the transition between memory and threshold switching can be induced by changing external electrical stress. Raman spectroscopy and transmission electron microscope data show that the NbOx film is self-assembled into a layered structure consisting of a top metal-rich region and a bottom oxygen-rich region. The volume ratio of the two regions depends on the film thickness. Our experimental results suggest that different characteristics of conducting filaments in the two regions result in thickness dependence of switching types and the transition between memory and threshold switching.

“Textured” Network Devices: Overcoming Fundamental Limitations of Nanotube/Nanowire Network-Based Devices†

Single-walled carbon nanotubes (swCNTs) and nanowires are strong candidate materials for next-generation devices such as high-mobility field-effect transistors (FETs), ultrasensitive sensors, and so on. One approach for practical device applications can be thin-film devices based on nanotube/ nanowire networks. However, such network-based devices have been suffering from various fundamental limitations. For example, transistors based on swCNTnetworks usually have a poor on–off ratio due to metallic swCNTs in the network channels. Furthermore, nanotube/nanowire network-based devices in general exhibit low mobility and conductivity with nanoscale channel width due to the poor scaling behavior of percolated network channels. Herein,

Large-scale assembly of carbon nanotube-based flexible circuits for DNA sensors

We present a simple but efficient method to prepare carbon nanotube (CNT)-based flexible devices embedded in polymer substrates. In this strategy, a methyl-terminated self-assembled monolayer is first coated on a solid substrate as a release layer, and CNT-network devices fabricated on it are directly transferred into a poly(dimethylsiloxane) (PDMS) mold, resulting in flexible CNT-network devices embedded in PDMS. The embedded circuits exhibit stable operation even after significant bending. We also propose Raman spectroscopy as a powerful tool to remotely characterize the CNT-network device structures covered by a polymer layer. As a proof of concept, we demonstrate DNA sensors utilizing the fabricated CNT-network devices.

Near bandgap second-order nonlinear optical characteristics of MoS2 monolayer transferred on transparent substrates

We have investigated the second-order nonlinear optical (NLO) properties of CVD-grown MoS2 monolayer (ML) transferred onto transparent substrates such as fused silica and polyethylene terephthalate. The physical properties of the transferred MLs were characterized by optical and NLO methods. We measured the second-order susceptibility χ(2) in the spectral range of λ= 1064–1600 nm in which the corresponding second harmonic radiation resonates with the exciton levels. It was found that χ(2) is strongly enhanced by up to a factor of 5 near the A- and B-exciton levels due to two-photon resonance. The absolute χ(2) values of our samples determined by both reflection and transmission geometry are on par with that of as-grown MLs. Our results imply that the cavity-confinement scheme can be employed for maximizing the nonlinear optical efficiency of atomically thin transition metal dichalcogenides for transparent/flexible optoelectronics applications, especially when oriented stacking of transferred MLs are contr...

High‐Performance Photoconductive Channels Based on (Carbon Nanotube)–(CdS Nanowire) Hybrid Nanostructures

A photoconductive channel based on hybrid nanostructures comprising carbon nanotubes (CNTs) and CdS nanowires is fabricated by a directed assembly strategy and catalyst-assisted chemical vapor deposition (CVD). The photoconductive channels simultaneously exhibit large photocurrent and fast response speed. Furthermore, it can be easily applied to surfaces that are not flat, such as a glass tube. This is a simple but efficient strategy for various optoelectronic applications.

Enhanced protein-mediated binding between oligonucleotide–gold nanoparticle composites and cell surfaces: co-transport of proteins and composites

The measurement of the binding force between a cell membrane and an oligonucleotide-functionalized, gold-coated tip using atomic force microscope force spectroscopy showed enhanced binding forces due to the presence of proteins in the buffer. The cellular uptake of oligonucleotide–gold nanoparticle composites was also enhanced when the composites were coated with serum proteins. Confocal microscopy image analysis of fluorescently labeled serum proteins and the oligonucleotides of the composites revealed co-transport of proteins and the composites.

Functional Role of bdm During Flagella Biogenesis in Escherichia coli

The biofilm-dependent modulation gene (bdm) has recently been shown to play a role in osmotic-induced formation of biofilm in Escherichia coli. In this study, we demonstrated that deletion of bdm results in down-regulation of flagella biosynthesis genes and, consequently, a defect in E. coli motility. In addition, we employed atomic force microscopy to confirm the absence of flagella-like structures on the surface of bdm-null cells. These findings indicate that bdm plays a key role in regulatory pathway for the formation of flagella.

Effect of graphene oxide ratio on the cell adhesion and growth behavior on a graphene oxide-coated silicon substrate.

Control of living cells on biocompatible materials or on modified substrates is important for the development of bio-applications, including biosensors and implant biomaterials. The topography and hydrophobicity of substrates highly affect cell adhesion, growth, and cell growth kinetics, which is of great importance in bio-applications. Herein, we investigate the adhesion, growth, and morphology of cultured breast cancer cells on a silicon substrate, on which graphene oxides (GO) was partially formed. By minimizing the size and amount of the GO-containing solution and the further annealing process, GO-coated Si samples were prepared which partially covered the Si substrates. The coverage of GO on Si samples decreases upon annealing. The behaviors of cells cultured on two samples have been observed, i.e. partially GO-coated Si (P-GO) and annealed partially GO-coated Si (Annealed p-GO), with a different coverage of GO. Indeed, the spreading area covered by the cells and the number of cells for a given culture period in the incubator were highly dependent on the hydrophobicity and the presence of oxygenated groups on GO and Si substrates, suggesting hydrophobicity-driven cell growth. Thus, the presented method can be used to control the cell growth via an appropriate surface modification.