Caroline Sunyong Lee

Transdermal transport efficiency during skin electroporation and iontophoresis

High-voltage pulsing has previously been shown to dramatically increase molecular transport across skin. The goal of this study was to examine the functional dependence of transdermal transport on pulse parameters and to make comparisons with iontophoresis. Transdermal transport of calcein, a model drug, was measured during low-voltage, constant electric fields (iontophoresis) and high-voltage pulsed electric fields (hypothesized to cause electroporation). In the first part of the study, the dependence of calcein flux caused by high-voltage pulses was determined as a function of pulse length, rate, polarity, waveform, and total pulsing time. In the second part, calcein transport numbers were calculated for both iontophoresis and high-voltage pulsing, and expressed as functions of pulse parameters. For both iontophoresis and high-voltage pulsing, transport numbers (or transport efficiency) ranged from 10−5 to 10−2 and were functions of voltage and current, but did not show dependence on pulse length, rate, energy, waveform, or total charge transferred. The resulting estimates of the area fraction of skin available to transport were larger during high-voltage pulsing ( 10−3 for small ions and 10−6 to 10−3 for calcein) than during iontophoresis (10−5 to 10−4 for small ions and 10−8 to 10−4 for calcein).

Gold nanoparticle modified graphitic carbon nitride/multi-walled carbon nanotube (g-C3N4/CNTs/Au) hybrid photocatalysts for effective water splitting and degradation

Gold nanoparticles (Au) used for stable plasmonic photocatalysts in hybrids of Au, graphitic carbon nitride (g-C3N4), and carbon nanotubes (CNTs), were evaluated for effective photodegradation of organic pollutants and photoelectrochemical (PEC) water splitting. These hybrids are formed at room temperature using sonication, and were shown to be effective for photodegradation of Rhodamine B (RhB) under irradiation with visible light. The hybrid samples resulted in a significant increase in photocatalytic activity compared with single-component samples of g-C3N4. In particular, the g-C3N4/CNTs/Au hybrids exhibited an exponential increase in the photocatalytic activity by a factor of almost 40. Structural and compositional analyses show the successful formation of ternary g-C3N4/CNTs/Au hybrids. The SPR due to the Au nanoparticles led to high optical absorbance, and the inclusion of the CNTs led to effective separation of photogenerated charge carriers, resulting in substantial improvement of the photocatalytic properties. PEC measurements indicate effective use of charge carriers, and open-circuit voltage decay measurements demonstrated increased lifetime of the photogenerated charge carriers in the hybrid samples. The ternary g-C3N4/CNTs/Au sample resulted in a large specific surface area, providing a large number of active sites for the adsorption of organic molecules. Therefore, a facile and room temperature fabrication method was shown to introduce Au and CNTs in the hybrid for substantial improvement of photocatalytic activities and effective water splitting.

Microcantilevers integrated with heaters and piezoelectric detectors for nano data-storage application

A thermomechanical writing system and a piezoelectric readback system have been demonstrated using silicon cantilevers integrated with heaters and piezoelectric sensors for a low-power scanning-probe-microscopy data-storage system. A thin polymethylmethacrylate film has been used as a media to record data bits of 50 nm in diameter and 25 nm in depth using the silicon cantilever. The sensitivity of 0.22 fC/nm was also obtained using the fabricated cantilever. Finally, to obtain readback signals using the piezoelectric cantilever, a patterned oxide wafer with 30 nm depth was scanned to show the distinctive charge signals.

Novel joining of dissimilar ceramics in the Si3N4–Al2O3 system using polytypoid functional gradients

A unique approach to crack-free joining of heterogeneous ceramics is demonstrated by the use of sialon polytypoids as Functionally Graded Materials (FGM) as defined by the phase diagram in the system, Si3N4-Al2O3. Polytypoids in the Al2O3-Si3N4 system offer a path to compatibility for heterogeneous ceramics. This paper describes successful hot press sintering of multilayered FGMs with 20 layers of thickness 500 mm each. Transmission Electron Microscopy was used to identify the polytypoids at the interfaces of different areas of the joint. It has been found that the 15R polytypoid was formed in the Al2O3-contained layers and the 12H polytypoid was formed in the Si3N4-contained layers.

Room-temperature synthesis of nanoporous 1D microrods of graphitic carbon nitride (g-C3N4) with highly enhanced photocatalytic activity and stability.

A one-dimensional (1D) nanostructure having a porous network is an exceptional photocatalytic material to generate hydrogen (H2) and decontaminate wastewater using solar energy. In this report, we synthesized nanoporous 1D microrods of graphitic carbon nitride (g-C3N4) via a facile and template-free chemical approach at room temperature. The use of concentrated acids induced etching and lift-off because of strong oxidation and protonation. Compared with the bulk g-C3N4, the porous 1D microrod structure showed five times higher photocatalytic degradation performance toward methylene blue dye (MB) under visible light irradiation. The photocatalytic H2 evolution of the 1D nanostructure (34 μmol g−1) was almost 26 times higher than that of the bulk g-C3N4 structure (1.26 μmol g−1). Additionally, the photocurrent stability of this nanoporous 1D morphology over 24 h indicated remarkable photocorrosion resistance. The improved photocatalytic activities were attributed to prolonged carrier lifetime because of its quantum confinement effect, effective separation and transport of charge carriers, and increased number of active sites from interconnected nanopores throughout the microrods. The present 1D nanostructure would be highly suited for photocatalytic water purification as well as water splitting devices. Finally, this facile and room temperature strategy to fabricate the nanostructures is very cost-effective.

Cellulose nano whiskers from grass of Korea

Polymer composites form a fascinating interdisciplinary area by bringing together biology and material science for wide verity of applications ranging from construction to biomedical technology. A deliberate interest in the development of eco-friendly material motivated the efforts toward research on cellulose composites due to its cheap, sustainable, recyclable, degradable nature and remarkable reinforcing properties at 167.5 GPa of Young’s modulus along the chain axis per theoretical estimations. The use of natural fiber for technical applications like automobile industry is restricted due to its incompatibility with generally hydrophobic host matrix and increase in weight of resulting products which provide a poor cost performance ratio. After resolving the incompatibility issues up to a satisfactory extent by adequate modification either in host or filler, it was assumed that dispersion and material properties may be enhanced with reduction in the size and increase in surface area by introducing nano fillers. Nano size (5-20 nm cross sections with length to several μm depending on source) rod like cellulose crystallites particles, known as cellulose nano whiskers (CNW), can be extracted from laterally stabilized fibrils bundle by removing amorphous region through controlled acid hydrolysis. These whiskers have been employed in reinforcing several polymers, which result in comparatively better mechanical properties. Nevertheless, such fibers have conquered many obstacles against industrial practices due to time consuming preparation procedure with very low yield, commercial unavailability, and most importantly, comparative higher cost through expensive source such as tunicate, bacterial, algal (valonia), brown algae (Oomycota) and commercially available microcrystalline cellulose. The low yield and availability of raw materials of these sources generally inhibit the penetration of this tremendous reinforcer for the development of daily use biodegradable products. The current attempt was made to obtain the cellulose nano whiskers from the cheapest source, grass of Koera for the first time, which may further broaden the use of these bio fillers ranging from commodity to constructive applications.

Nickel Line Patterning Using Silicon Supersonic Micronozzle Integrated with a Nanoparticle Deposition System

In this study, 3-µm- and 20-nm nickel powders were deposited on Si substrates to pattern a metal line using a nanoparticle deposition system (NPDS) at room temperature. The stand-off distance (SoD), the distance between the substrate and the end of nozzle, was varied from 300 to 1000 µm to determine its effect on deposition properties. A Ni line was successfully formed on the Si substrate. When 3-µm Ni powders were used, the thickness of the deposited layer on the Si substrate was measured to be 5.4 µm, and its width was 176.4 µm at a SoD of 300 µm. In contrast, the deposited average thickness at a SoD of 500 µm was 1.1 µm, with a width of 190.6 µm. Moreover, the deposited thickness was measured to be 6.4 µm using 20-nm Ni powders at a SoD of 300 µm. Thus, it was found that the deposited thickness decreased as SoD increased, indicating an inversely proportional relationship. For deposition behavior, depending on the size of powders, it was found that 20-nm Ni powders resulted in a thicker deposition than did 3-µm Ni powders, as momentum transfer between carrier gas and powders is inversely proportional to the powder size. Thus, as the powder size decreased, its spray velocity increased; hence, it is more effective to use nano powders for Ni line patterning. Surface resistivity of the deposited Ni line was 1.83 ×10-7 Ωm using 20-nm powders and 1.61 ×10-7 Ωm using 3-µm powders. These values are close to the standard resistivity value for bulk Ni, which is 6.9 ×10-8 Ωm, making NPDS a promising technique for metal line-fabrication equipment.

Mechanical properties of polytypoidally joined Si3N4-Al2O3

Crack-free joining of alumina and silicon nitride has been achieved by a unique approach introducing sialon polytypoids as a Functionally Graded Materials (FGM) bonding layer. The polytypoid compositions are identified in the phase diagram of the Si3N4-Al2O3system. The thermal stresses of this FGM junction were analyzed using a finite element analysis program (FEAP) taking into account both coefficient of thermal expansion (CTE) and modulus variations. From this analysis, the result showed a dramatic decrease in radial, axial and hoop stresses as the FGM changes from three layers to 20 graded layers. Scaling was considered, showing that the graded transition layer should constitute about 75% or more of the total sample thickness to reach a minimal residual stress. Oriented Vickers indentation testing was used to qualitatively characterize the strengths of the joint and the various interfaces. The indentation cracks were minimally or not deflected at the sialon layers, implying strong interfaces. Finally, flexural testing was conducted at room temperature and at high temperature. The average strength at room temperature was found to be 581 MPa and the average strength at high temperature (1200 degrees C) was found to be 262 MPa. Scanning electron microscope observation of fracture surfaces at a different loading rates indicated that the strength loss at higher temperatures was consistent with a softening of glassy materials present at grain junctions.

DEPOSITION OF Al2O3 POWDERS USING NANO-PARTICLE DEPOSITION SYSTEM

In this paper, alumina film was deposited using supersonic micronozzle in nano-particle deposition System (NPDS). Powder deposition at room temperature is important in the field of film deposition since high processing temperature can be a serious limitation for the deposition on flexible substrate. Previously, many studies have been reported on particle deposition, such as aerosol deposition method (ADM) or cold spray method. However, these deposition methods cannot be applied to various types of powders. Recently, NPDS using aluminum nozzle was designed to resolve these problems but it cannot deposit precise patterns less than 1 mm. In this study, alumina particles were deposited using Silicon-based micronozzle in NPDS. Three-dimensional silicon micronozzle was fabricated using semiconductor processing method, specifically deep reactive ion etching (DRIE) method. The silicon micronozzle fabricated by Bosch process is advantageous over the conventionally used nozzle, since the hardness of silicon is higher than that of aluminum and the lifetime can be increased. In this study, alumina nano-particles were accelerated to supersonic level at the neck of micronozzle and deposited on the substrate in a low vacuum condition. The film characteristics were evaluated using field-emission scanning electronic microscope (FE-SEM) and alpha step to measure its thickness of the deposited layer. The deposition result showed that alumina powders were successfully deposited using the fabricated micronozzle by means of NPDS.

Size-controlled BiOCl–RGO composites having enhanced photodegradative properties

Visible light-active bismuth oxychloride–reduced graphene oxide (BiOCl–RGO) composite photocatalysts were synthesised using a hydrothermal method at low temperature, and at a low cost. This approach reduced the recombination of electron–hole pairs and thereby provided more efficient photocatalysts. The size of BiOCl structure was controlled by polyvinylpyrrolidone (PVP) addition. Furthermore, formation of nanosized BiOCl sheets and BiOCl–RGO composites were confirmed by X-ray diffraction, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. Fabricated BiOCl–RGO composite with PVP exhibited better photocatalytic activity than pure BiOCl grown with and without PVP towards degradation of Rhodamine B (RhB). It was found that the composite photocatalyst degrades RhB completely within 310 min as compared with several hours for pure BiOCl. The improved photocatalytic performance of BiOCl–RGO composite was attributed to its high specific surface area (22.074 m2 g−1 and existence of polar surfaces, compared with 9.831 m2 g−1 for pure BiOCl). The analyses indicated that RGO helped to reduce recombination losses and improve electron transport. It also showed that presence of polar surfaces improved photocatalytic activity of BiOCl. Hence, BiOCl–RGO composite is a promising catalyst for the degradation of organic pollutants under visible light and could be used in applications such as water purification devices.