Hyungdong Lee

Direct exfoliation and dispersion of two-dimensional materials in pure water via temperature control

The high-volume synthesis of two-dimensional (2D) materials in the form of platelets is desirable for various applications. While water is considered an ideal dispersion medium, due to its abundance and low cost, the hydrophobicity of platelet surfaces has prohibited its widespread use. Here we exfoliate 2D materials directly in pure water without using any chemicals or surfactants. In order to exfoliate and disperse the materials in water, we elevate the temperature of the sonication bath, and introduce energy via the dissipation of sonic waves. Storage stability greater than one month is achieved through the maintenance of high temperatures, and through atomic and molecular level simulations, we further discover that good solubility in water is maintained due to the presence of platelet surface charges as a result of edge functionalization or intrinsic polarity. Finally, we demonstrate inkjet printing on hard and flexible substrates as a potential application of water-dispersed 2D materials.

Direct Alignment and Patterning of Silver Nanowires by Electrohydrodynamic Jet Printing

Highly aligned and patterned silver nanowires (Ag NWs) are investigated by using electrohydrodynamic (EHD) jet printing. Interaction between the flow field and the electric field as well as the mechanical stretching of the fiber jet can successfully align the Ag NWs inside the jet fiber. This technique can be applied in fabricating 1D nanostructures-based printed micro/nanoscale devices.

Directly printed stretchable strain sensor based on ring and diamond shaped silver nanowire electrodes

In this study, we fabricated a stretchable Silver nanowires (Ag NWs)/PDMS composite strain sensor with arbitrary micro-pattern electrodes using dispensing nozzle printing. In order to ensure a mechanically stable design, we proposed two types of electrodes: patterns of overlapped rings and diamonds. We also demonstrated that the electrical resistance could be modified according to the printing speed because the number of conductive fillers was proportional to the liquid ejection time. We also conducted static simulation for the two geometries to study the effect of the patterns when the strain sensor is stretched. We achieved highly stretchable strain sensor (up to 60% strain) with a suitable electrode design. Based on experimental results, it is expected that directly drawn electronic skin (E-skin) via the printing method can be fabricated with multifunctional sensing abilities in the near future.

Low angle forward reflected neutral beam source and its applications

As one of the many nano-device fabrication techniques employed in the semiconductor industry, neutral beams are being examined using various methods to solve possible charge-related problems that occur during device processing. This review introduces a neutral beam generated by surface neutralization of an ion beam using a low angle forward reflection technique and explains its application to various areas such as surface treatments and etching. The neutralization efficiency of an ion beam using a low angle forward reflection technique was approximately 99.7%. When a metal-oxide-semiconductor device was etched using a reactive neutral beam, it was confirmed that charge-related problems such as aspect-ratio-dependent etching and gate oxide charging could be removed using reactive neutral beam etching instead of conventional reactive ion etching. Neutral beams can be beneficial to other devices such as the III-V device and field emission device.

Directly Drawn Poly(3-hexylthiophene) Field-Effect Transistors by Electrohydrodynamic Jet Printing: Improving Performance with Surface Modification

In this study, direct micropatterning lines of poly(3-hexylthiophene) (P3HT) without any polymer binder were prepared by electrohydrodynamic jet printing to form organic field-effect transistors (OFETs). We controlled the dielectric surface by introducing self-assembled monolayers and polymer thin films to investigate the effect of surface modifications on the characteristics of printed P3HT lines and electrical performances of the OFETs. The morphology of the printed P3HT lines depended on the surface energy and type of substrate. The resulting OFETs exhibited high performance on octadecyltrichlorosilane-modified substrates, which was comparable to that of other printed P3HT OFETs. In order to realize the commercialization of the OFETs, we also fabricated a large-area transistor array, including 100 OFETs and low-operating-voltage flexible OFETs.

Surface Analysis of Atomic-Layer-Etched Silicon by Chlorine

Silicon atomic-layer etching (ALET) was carried out by the adsorption of Cl 2 to form silicon chlorides followed by the desorption of the silicon chlorides formed on the surface by irradiating an Ar neutral beam of a low energy. The silicon surface during the silicon ALET was investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy, and its etch mechanism was studied. XPS data showed that during the Cl 2 adsorption stage, silicon chloride bonding peaks related to SiCl and SiCl 3 were formed. The peak intensity related to SiCl bonding is slowly decreased during the desorption of silicon chlorides, and when the SiCl bondings are removed completely by irradiating enough Ar neutral-beam dose to the surface, the saturated silicon etch rate of 1.36 A/cycle which is related to one Si monolayer per cycle could be obtained. Also, at this condition, the surface roughness is close to the roughness of the silicon substrate itself. The SiCl bondings formed on the silicon surface during the adsorption stage are related to the desorption species during the ALET, while the SiCl 3 bondings are related to the species formed with the surface silicon damaged during the desorption stage.

Precise Depth Control and Low-Damage Atomic-Layer Etching of HfO2 using BCl3 and Ar Neutral Beam

The etch characteristics of HfO 2 by atomic-layer etching (ALET) were investigated using a BCl 3 /Ar neutral beam. The effect of ALET on surface modification and etch-depth control was also examined. Self-limited etching of HfO 2 could be obtained using BCl 3 ALET. This was attributed to the absorption of BCl 3 by the Langmuir isotherm during the absorption stage and the vaporization of hafnium-chlorides/boron oxychlorides formed on the surface during the desorption stage. In addition, the surface composition of HfO 2 was not altered by etching during ALET.

Growth of carbon nanotubes: effect of Fe diffusion and oxidation

The diffusion and surface oxidation rates of Fe deposited on Si and barrier layers of Al/SiO2 and Al2O3/SiO2 have been comparatively studied and correlated with the growth of carbon nanotubes (CNTs). Initially, Fe/Si, Fe/Al/SiO2/Si and Fe/Al2O3/SiO2/Si samples were subjected to thermal chemical vapour deposition (CVD) at ∼650°C for ∼30 min to grow the CNTs. Scanning electron microscopy analysis showed that the height of the CNTs on the Fe/Al2O3/SiO2/Si samples was relatively high (∼9.5–11 µm), as compared with the other samples. To investigate this, a few as-prepared samples were thermally annealed at ∼650°C for ∼30 min and characterized by dynamic secondary ion mass spectroscopy (D-SIMS) and X-ray photoelectron spectroscopy (XPS). The D-SIMS results showed that the diffusion depth, x Fe, and magnitude of the diffusivity, D Fe, of the Fe atoms are highest for the Fe/Si sample. This is attributed to vacancy-mediated migration, which leads to the formation of unstable, non-stoichiometric Fe–Si and Fe–O–Si phases. However, for the Fe/Al2O3/SiO2/Si samples, the magnitudes of x Fe and D Fe are found to be the lowest, which indicates steric hindrance to Fe by the Al2O3 layers. The XPS analysis revealed that the surface metallic state, after annealing, is almost unaffected for the Fe/Al2O3/SiO2/Si samples, whereas the majority of the Fe precipitate was observed to be oxidized in the case of the other samples.

Effect of neutral beam etching of p-GaN on the GaN device characteristics

GaN materials were etched using a CF4-based neutral beam, and its etch damage characteristics were compared with those etched with a CF4-based inductively coupled plasma (ICP). Photoluminescence data showed that the neutral beam etched GaN materials show fewer defects on the surface compared to the GaN materials etched by ICP. Also, the current–voltage characteristics of GaN light emitting diodes fabricated with p-GaN etched by the neutral beam showed less damage compared to those fabricated with p-GaN by the ICP. When a photonic crystal-like structure having 2-μm-diameter microlens array was formed using the neutral beam etching on the p-GaN of the GaN device, an increase of 20% in the optical emission intensity could be observed without significantly increasing the forward voltage (0.7V).

Self-Assembly of Silver Nanowire Ring Structures Driven by the Compressive Force of a Liquid Droplet

In a nanowire dispersed in liquid droplets, the interplay between the surface tension of the liquid and the elasticity of the nanowire determines the final morphology of the bent or buckled nanowire. Here, we investigate the fabrication of a silver nanowire ring generated as the nanowire encapsulated inside of fine droplets. We used a hybrid aerodynamic and electrostatic atomization method to ensure the generation of droplets with scalable size in the necessary regime for ring formation. We analytically calculate the compressive force of the droplet driven by surface tension as the key mechanism for the self-assembly of ring structures. Thus, for potential large-scale manufacturing, the droplet size provides a convenient parameter to control the realization of ring structures from nanowires.