Bong-Joong Kim

Highly Conductive PEDOT:PSS Nanofibrils Induced by Solution‐Processed Crystallization

The fabrication of electronic devices based on organic materials, known as ’printed electronics’, is an emerging technology due to its unprecedented advantages involving fl exibility, light weight, and portability, which will ultimately lead to future ubiquitous applications. [ 1 ] The solution processability of semiconducting and metallic polymers enables the cost-effective fabrication of optoelectronic devices via high-throughput printing techniques. [ 2 ] These techniques require high-performance fl exible and transparent electrodes (FTEs) fabricated on plastic substrates, but currently, they depend on indium tin oxide (ITO) coated on plastic substrates. However, its intrinsic mechanical brittleness and inferior physical properties arising from lowtemperature ( T ) processing below the melting T of the plastic substrates (i.e., typically below 150 °C) have increased the demand for alternative FTE materials. [ 3 ]

Kinetics of Individual Nucleation Events Observed in Nanoscale Vapor-Liquid-Solid Growth

We measured the nucleation and growth kinetics of solid silicon (Si) from liquid gold-silicon (AuSi) catalyst particles as the Si supersaturation increased, which is the first step of the vapor-liquid-solid growth of nanowires. Quantitative measurements agree well with a kinetic model, providing a unified picture of the growth process. Nucleation is heterogeneous, occurring consistently at the edge of the AuSi droplet, yet it is intrinsic and highly reproducible. We studied the critical supersaturation required for nucleation and found no observable size effects, even for systems down to 12 nanometers in diameter. For applications in nanoscale technology, the reproducibility is essential, heterogeneity promises greater control of nucleation, and the absence of strong size effects simplifies process design.

Realization of highly reproducible ZnO nanowire field effect transistors with n-channel depletion and enhancement modes

The authors demonstrate the highly reproducible fabrication of n-channel depletion-mode (D-mode) and enhancement-mode (E-mode) field effect transistors (FETs) created from ZnO nanowires (NWs). ZnO NWs were grown by the vapor transport method on two different types of substrates. It was determined that the FETs created from ZnO NWs grown on an Au-coated sapphire substrate exhibited an n-channel D mode, whereas the FETs of ZnO NWs grown on an Au-catalyst-free ZnO film exhibited an n-channel E mode. This controlled fabrication of the two operation modes of ZnO NW-FETs is important for the wide application of NW-FETs in logic circuits.

Hydrogen-induced morphotropic phase transformation of single-crystalline vanadium dioxide nanobeams.

We report a morphotropic phase transformation in vanadium dioxide (VO2) nanobeams annealed in a high-pressure hydrogen gas, which leads to the stabilization of metallic phases. Structural analyses show that the annealed VO2 nanobeams are hexagonal-close-packed structures with roughened surfaces at room temperature, unlike as-grown VO2 nanobeams with the monoclinic structure and with clean surfaces. Quantitative chemical examination reveals that the hydrogen significantly reduces oxygen in the nanobeams with characteristic nonlinear reduction kinetics which depend on the annealing time. Surprisingly, the work function and the electrical resistance of the reduced nanobeams follow a similar trend to the compositional variation due mainly to the oxygen-deficiency-related defects formed at the roughened surfaces. The electronic transport characteristics indicate that the reduced nanobeams are metallic over a large range of temperatures (room temperature to 383 K). Our results demonstrate the interplay between oxygen deficiency and structural/electronic phase transitions, with implications for engineering electronic properties in vanadium oxide systems.

Nucleation of Highly Dense Nanoscale Precipitates Based on Warm Laser Shock Peening

Warm laser shock peening (WLSP) is an innovative thermomechanical processing technique, which combines the advantages of laser shock peening (LSP) and dynamic aging (DA). It has been found that a unique microstructure with highly dense nanoscale precipitates surrounded by dense dislocation structures is generated by WLSP. In order to understand the nucleation mechanism of the highly dense precipitates during WLSP, aluminum alloy 6061 (AA6061) has been used by investigating the WLSP process with experiments and analytical modeling. An analytical model has been proposed to estimate the nucleation rate in metallic materials after WLSP. The effects of the processing temperature and high strain rate deformation on the activation energy of nucleation have been considered in this model. This model is based on the assumption that DA during WLSP can be assisted by the dense dislocation structures and warm temperature. The effects of the working temperature and dislocation density on the activation energy of precipitation have been investigated. This model is validated by a series of experiments and characterizations after WLSP. The relationships between the processing conditions, the nucleation density of precipitates and the defect density have been investigated.

Electrical and structural properties of antimony-doped p-type ZnO nanorods with self-corrugated surfaces

We report on p-type conductivity in antimony (Sb)-doped ZnO (ZnO:Sb) nanorods which have self-corrugated surfaces. The p-ZnO:Sb/n-ZnO nanorod diode shows good rectification characteristics, confirming that a p-n homojunction is formed in the ZnO nanorod diode. The low-temperature photoluminescence (PL) spectra of the ZnO:Sb nanorods reveal that the p-type conductivity in p-ZnO:Sb is related to the Sb(Zn)-2V(Zn) complex acceptors. Transmission electron microscopy (TEM) analysis of the ZnO:Sb nanorods also shows that the p-type conductivity is attributed to the Sb(Zn)-2V(Zn) complex acceptors which can be easily formed near the self-corrugated surface regions of ZnO:Sb nanorods. These results suggest that the Sb(Zn)-2V(Zn) complex acceptors are mainly responsible for the p-type conductivity in ZnO:Sb nanorods which have corrugated surfaces.

GaNAs as Strain Compensating Layer for 1.55 µm Light Emission from InAs Quantum Dots

GaNAs strain-compensating layers (SCLs) are applied to bury InAs quantum dots (QDs) grown on GaAs substrates. The main idea is the compensation of the compressive strain induced by InAs QDs with the tensile strain in the GaNAs SCLs to keep the total strain of the system minimum. The application of the GaNAs SCLs resulted in a systematic shift of photoluminescence (PL) peaks of the InAs QDs toward the longer wavelengths with the increase of the nitrogen (N) composition in GaNAs, and luminescence at a wavelength of 1.55 µm has been achieved from the InAs QDs for the N composition of 2.7% in the GaNAs SCL. This result is promising for the application of GaNAs SCL for InAs-QDs-based long-wavelength light sources for optical-fiber communication systems.

Growth Pathways in Ultralow Temperature Ge Nucleation from Au

Device integration on flexible or low-cost substrates has driven interest in the low-temperature growth of semiconductor nanostructures. Using in situ electron microscopy, we examine the Au-catalyzed growth of crystalline Ge at temperatures as low as 150 °C. For this materials system, the model for low temperature growth of nanowires, we find three distinct reaction pathways. The lowest temperature reactions are distinguished by the absence of any purely liquid state. From measurements of reaction rates and parameters such as supersaturation, we explain the sequence of pathways as arising from a kinetic competition between the imposed time scale for Ge addition and the inherent time scale for Ge nucleation. This enables an understanding of the conditions under which catalytic Ge growth can occur at very low temperatures, with implications for nanostructure formation on temperature-sensitive substrates.

Template-mediated nano-crystallite networks in semiconducting polymers

Unlike typical inorganic semiconductors with a crystal structure, the charge dynamics of π-conjugated polymers (π-CPs) are severely limited by the presence of amorphous portions between the ordered crystalline regions. Thus, the formation of interconnected pathways along crystallites of π-CPs is desired to ensure highly efficient charge transport in printable electronics. Here we report the formation of nano-crystallite networks in π-CP films by employing novel template-mediated crystallization (TMC) via polaron formation and electrostatic interaction. The lateral and vertical charge transport of TMC-treated films increased by two orders of magnitude compared with pristine π-CPs. In particular, because of the unprecedented room temperature and solution-processing advantages of our TMC method, we achieve a field-effect mobility of 0.25 cm(2) V(-1) s(-1) using a plastic substrate, which corresponds to the highest value reported thus far. Because our findings can be applied to various π-conjugated semiconductors, our approach is universal and is expected to yield high-performance printable electronics.

Growth and characterization of dilute nitride GaNxP1−x nanowires and GaNxP1−x/GaNyP1−y core/shell nanowires on Si (111) by gas source molecular beam epitaxy

We have demonstrated self-catalyzed GaN xP1−x and GaN xP1−x/GaNyP1−y core/shell nanowire growth by gas-source molecular beam epitaxy. The growth window for GaN xP1−x nanowires was observed to be comparable to that of GaP nanowires (∼585 °C to ∼615 °C). Transmission electron microscopy showed a mixture of cubic zincblende phase and hexagonal wurtzite phase along the [111] growth direction in GaN xP1−x nanowires. A temperature-dependent photoluminescence (PL) study performed on GaN xP1−x/GaNyP1−y core/shell nanowires exhibited an S-shape dependence of the PL peaks. This suggests that at low temperature, the emission stems from N-related localized states below the conduction band edge in the shell, while at high temperature, the emission stems from band-to-band transition in the shell as well as recombination in the GaN xP1−x core.