In-taek Han

Study of the field-screening effect of highly ordered carbon nanotube arrays

We have studied the field-screening effect provoked by the proximity of neighboring tubes by changing the tube height of highly ordered carbon nanotubes fabricated on porous anodic aluminum oxide templates. The field emission was critically affected by the tube height that protruded from the surface. The field emission was optimal when the tube height was similar to the intertube distance. The intertube distance to the tube height for maximum field emission is about one half the intertube distance predicted by Nilsson et al. [Appl. Phys. Lett. 76, 2071 (2000)].

Enhanced electron emission from carbon nanotubes through density control using in situ plasma treatment of catalyst metal

We controlled the density of carbon nanotubes (CNTs) through in situ NH3 plasma pretreatment and investigated field emission properties with the density variation. Ni catalytic layer was transformed into small nanoparticles with NH3 plasma pretreatment time and power. As NH3 plasma pretreatment time was increased, the growth rate of grown CNTs was gradually decreased. Also, the density of CNTs reduced from 2×109 to 8×106/cm2 with an increase in NH3 plasma pretreatment time from 10 to 30 min for the Ni layer of 10 A. With a decrease in the density of CNTs, the emission current density was increased and turn on electric field was decreased. We obtained large and uniform emission current (about 9 mA/emission area of 0.49 cm2) from CNTs film with the density of 8×106/cm2.

Multifunctional Supramolecular Hybrid Materials Constructed from Hierarchical Self‐Ordering of In Situ Generated Metal‐Organic Framework (MOF) Nanoparticles

A synergistic approach is described to engineer supramolecular hybrid materials based on metal-organic frameworks, encompassing HKUST-1 nanoparticles formed in situ, coexisting with an electrically conducting gel fiber network. The following findings are made: i) multistimuli-responsive structural transformation via reversible sol-gel switching and ii) radical conversion of a soft hybrid gel into a mechanically malleable, viscoelastic matter.

Well-ordered Co nanowire arrays for aligned carbon nanotube arrays

Abstract Well-ordered Co nanowire arrays formed on the porous anodic aluminum oxide (AAO) templates prepared by a two-step anodization technique have been used in the fabrication of well-aligned carbon nanotubes. Designed Co nanowire arrays can be made by controlling the pore arrays on AAO templates. By using them as a catalyst it is possible to fabricate the designed carbon nanotube arrays. Carbon nanotubes fabricated by disproportionation of CO were well graphitized, uniform in diameter and aligned vertically with respect to the plane of the template. It has been suggested that CO is an ideal precursor in fabrication of carbon nanotubes.

Integration and Electrical Properties of Carbon Nanotube Array for Interconnect Applications

Carbon nanotube (CNT) vertical integration and electrical properties are presented in full 6-inch wafer for interconnect applications. Series array of 1000 vias made of vertically grown CNTs is obtained with uniform electrical resistances within the wafer. Integration processes are implemented by following sequential steps: bottom electrode and via hole patterning, CNT growth and planarization, and top electrode patterning in a 6-inch wafer. Multiwall carbon nanotubes (MWNTs) are used for interconnection, titanium nitride for the bottom electrode, and aluminum with titanium adhesion layer for the top electrode. We have demonstrated well-defined CNT via series interconnection with 700 nm via diameters within the full wafer. Via resistance of 1.2 kΩ with CNT density of 2.7×1010/cm2is obtained with small resistance variation within the wafer, which also corresponds to 176 kΩ per one MWNT with 10 nm diameters. The possible approaches for further decrease of electrical resistance will be suggested.

Effects of growth parameters on the selective area growth of carbon nanotubes

Abstract Effects of growth parameters such as plasma intensity, flow rate, composition of reactant gases, growth temperature, and hole size on the selective area growth (SAG) of carbon nanotubes (CNTs) were investigated using the triode type field emission array structure in plasma enhanced chemical vapor deposition (PECVD) system . As the plasma intensity was increased, the diameter of CNTs was reduced from 180 to 90 nm, but the growth rate was promoted. With an increase in the NH 3 flow rate, the average diameter of CNTs was decreased due to the enhanced etching effect by NH 3 . An increase in the total flow rate of reactant gases reduced the growth rate of CNTs, but the average diameter of CNTs remained nearly constant. An increase in growth rate and diameter was observed at higher growth temperatures. As the hole size of the triode structure increased, the growth rate of CNTs gradually decreased because of the reduced lateral diffusion of reactant species into the hole.

Anti-frost coatings containing carbon nanotube composite with reliable thermal cyclic property

One of the most important applications for superhydrophobic coatings is anti-frosting for safety and energy conservation. Safety concerns are especially critical in cold-climate regions where the daily temperature fluctuation is large. However, superhydrophobic coatings have not been studied in terms of their thermomechanical reliability. In this study, wetting characteristics and stress relaxation behavior were quantitatively investigated with multi-walled carbon nanotube (MWNT)–silicone composite films under thermal cycling conditions. It is concluded that an open structure with numerous nanopores among the fillers, constituting air pockets described as the “Cassie structure,” is of great importance not only for developing a films superhydrophobic nature but also for accommodation of thermal stress that results from a difference in coefficient of thermal expansion between the coating and the substrate. The amount of stress relaxation for a 30 vol% MWNT–silicone composite film with open structure reaches ∼80% of the value for its counterpart with a closed structure and no pores. A superhydrophobic MWNT–silicone composite film that can endure over 4000 thermal cycles (−30 °C to room temperature) is fabricated by controlling the composition and microstructure of the composite. In addition, the importance of the size and shape of the nanofillers in delaying nucleation and growth of frost on superhydrophobic coatings is also discussed.

Anomalies in the Growth Temperature and Time Dependence of Carbon Nanotube Growth

CVD has been used to produce highly purified carbon nanotubes (CNTs) in large quantities, and also to obtain CNTs vertically aligned on substrates for use as electron emitters. Growth at low temperatures (due to the use of a large glass substrate with a low melting temperature), and well-aligned CNT growth are key factors for such applications. Compared to thermal CVD, plasma-enhanced (PE) CVD is better for lowering the growth temperature but not so good for large area synthesis. Two growth mechanisms for vertically aligned CNT growth using CVD have been proposed. Fan et al. proposed a bottom growth mechanism, where the carbonaceous gases could be supplied through the porous substrate. This theory has been questioned, since vertically aligned CNTs have also been successfully grown on a nonporous substrate. Several groups have proposed a cap growth mechanism, where the CNTs were grown from the front surface of the catalyst that exists on the top of the CNT in the form a cap. Such debates, and the lack of understanding of the growth mechanism, arise from an absence of experimental evidence. In general, vertically aligned CNTs are believed to be uniformly synthesized at relatively high temperatures, and their length is also believed to increase with growth time. In our study, however, we found unexpected behavior in the growth temperature and growth time during CNT growth using thermal CVD. With increasing temperatures, the degree of alignment became poorer and, in contrast to previously reported findings, [9±11] the length and density of the CNTs did not increase. The length of the CNTs increased in the initial stages of growth, but later became constant. This phenomenon, observed with highresolution transmission electron microscopy (HRTEM), led us to conclude that this unexpected catalytic growth behavior can only be explained by the cap growth mechanism. Figure 1 shows scanning electron microscopy (SEM) images (45 tilted) of multi-walled CNTs grown at 600 C, 650 C, 700 C, and 750 C for 20 min under a pressure of 5.5 torr. In our study, no pretreatment of the surface of the catalyst metal thin films, e.g., exposure to NH3 gas, [7] or a dipping in HF solution, was carried out prior to the CNT growth. In general, with increasing growth temperature, the CNTs become less defective and their length is expected to increase. As shown in Fig. 1a and 1b, the CNT density increased and vertical alignment improved at 650 C. However, on increasing the temperature to 750 C, the tube density decreased and the tube alignment became poorer. The tubes were randomly disordered on the substrate. This unusual observation is strongly correlated with the surface morphology of the Ni thin films. The right panel in Figure 1 shows the atomic force microscopy (AFM) images of Ni thin films that were prepared following exactly the same procedure as for the CNT growth. All the Ni thin films had a thickness of 120 nm and were prepared at 180 C for 4 h using radio-frequency (RF) magnetron sputtering. The asdeposited Ni thin film has a very uniform grain size of about 20 nm, as shown in the inset in Figure 1a. With increasing temperatures, the grain size and surface roughness increased, whereas the grain density decreased. From AFM image analysis, the average grain size of the films heattreated at 600 C, 650 C, 700 C, and 750 C were found to be about 30 nm, 30 nm, 50 nm, and 50 nm, respectively. The Ni films heat-treated at 600 C and 650 C still have a uniform grain size and similar surface roughness. The round grains, with a size of about 30 nm, are uniformly distributed over the entire substrate. However, AFM images of the Ni thin films formed at 700 C and 750 C showed less uniformly distributed grain sizes. With increasing temperature, some grains coalesced forming larger Ni grains, resulting in a wide distribution of grain sizes. The increase in diameter and coalescence of grains results from the migration of grain boundaries during heat treatment, which merges small grains into large ones. We note that, as previously reported, the average diameter of the CNTs does not exceed that of the Ni grains, suggesting that the grains act

Metallic conduction induced by direct anion site doping in layered SnSe2.

The emergence of metallic conduction in layered dichalcogenide semiconductor materials by chemical doping is one of key issues for two-dimensional (2D) materials engineering. At present, doping methods for layered dichalcogenide materials have been limited to an ion intercalation between layer units or electrostatic carrier doping by electrical bias owing to the absence of appropriate substitutional dopant for increasing the carrier concentration. Here, we report the occurrence of metallic conduction in the layered dichalcogenide of SnSe2 by the direct Se-site doping with Cl as a shallow electron donor. The total carrier concentration up to ~1020 cm−3 is achieved by Cl substitutional doping, resulting in the improved conductivity value of ~170 S·cm−1 from ~1.7 S·cm−1 for non-doped SnSe2. When the carrier concentration exceeds ~1019 cm−3, the conduction mechanism is changed from hopping to degenerate conduction, exhibiting metal-insulator transition behavior. Detailed band structure calculation reveals that the hybridized s-p orbital from Sn 5s and Se 4p states is responsible for the degenerate metallic conduction in electron-doped SnSe2.

A chlorinated barium titanate-filled polymer composite with a high dielectric constant and its application to electroluminescent devices

We have studied the effect of chlorination on the dielectric performance of a barium titanate (BTO)–polymer film. Functionalization of BTO powder particles with chlorine (Cl) atoms and/or Cl-containing functional groups was achieved by a simple treatment using chlorinated solvents. The chlorinated BTO (Cl-BTO) particles were incorporated into a cyanoethyl-based polymer and the mixture was spin-coated to produce a composite film. The dielectric constant of the composite film with Cl-BTO was as high as 208 at a frequency of 10 Hz, showing a 2.5-fold increase in dielectric constant compared to composites composed of the neat BTO and the cyanoethyl-based polymer at a frequency of 10 Hz. The observed dramatic increase in dielectric constant would be caused by the interfacial polarization due to the p-type doping effect resulting from the presence of strong electronegative Cl atoms. Given a relatively high dielectric constant, composite films with Cl-BTO were further exploited as a dielectric layer in an inorganic electroluminescence (EL) device. The luminance of the EL device with Cl-BTO was 4090 cd m−2 at a frequency of 1 kHz, showing a 2.03-fold increase in luminance compared to that with the untreated BTO. The improved performance of the EL device is attributed to a high dielectric constant of the composite films that allow for efficient charge carrier tunneling into the phosphor and therefore enhanced luminance and efficiency.