Kwang-Ryeol Lee

Growth of carbon nanotubes by chemical vapor deposition

Abstract The growth behavior of carbon nanotubes (CNT) deposited from C2H2 by thermal CVD method was investigated. Nickel particles of diameter ranging from 15 to 90 nm were used as the catalyst. CNTs were deposited in various environments of N2, H2, Ar, NH3 and their mixtures to investigate the effect of the environment on the CNT growth behavior. The deposition was performed at 850°C in atmospheric pressure. In pure N2 environment, thick carbon layer deposition occurred on the substrate without CNT growth. The Ni particles encapsulated by the carbon deposition could not work as the catalyst in this condition. However, the growth of CNT was enhanced as the H2 concentration increased in the mixture of N2 and H2 environment. In pure H2 environment, randomly tangled CNTs could be obtained. The growth of CNT was much enhanced when using NH3 as the environment gas. Vertically aligned CNTs could be deposited in NH3 environment, whereas the CNT growth could not be obtained in the mixture of N2 and H2 environment of the same ratio of N/H. These results were discussed in terms of the passivation of the catalyst caused by the excessive deposition of carbon on the catalyst surface. For the deposition of the CNT, the decomposition rate of C2H2 should be controlled to supply carbon for nanotube growth without passivation of the catalyst surface by excessive carbon deposition. The present work showed that the composition of environment gas significantly affects the reaction kinetics in the CNT growth. It is also noted that nitride surface layer formation on Ni catalyst in NH3 environment can affect the CNT growth behavior.

Effect of residual stress on the Raman-spectrum analysis of tetrahedral amorphous carbon films

Tetrahedral amorphous carbon (ta-C) films deposited by the filtered vacuum arc process have large compressive residual growth stresses that depend on the atomic-bond structure. We observed that the G peak of the Raman spectrum shifts to higher frequency by 4.1±0.5 cm−1/GPa due to the residual compressive stress. This value agrees well with the calculated Raman-peak shift of the graphite plane due to applied stress. By considering the effect of residual stress on the G-peak position, we also observe a similar dependence between the G-peak position and the atomic-bond structure in both ta-C and hydrogenated amorphous carbon (a-C:H) films; namely, that a higher sp2 bond content shifts the G-peak position to higher frequency.

Wrinkled, Dual-Scale Structures of Diamond-Like Carbon (DLC) for Superhydrophobicity

We present a simple two-step method to fabricate dual-scale superhydrophobic surfaces by using replica molding of poly(dimethylsiloxane) (PDMS) micropillars, followed by deposition of a thin, hard coating layer of a SiO(x)-incorporated diamond-like carbon (DLC). The resulting surface consists of microscale PDMS pillars covered by nanoscale wrinkles that are induced by residual compressive stress of the DLC coating and a difference in elastic moduli between DLC and PDMS without any external stretching or thermal contraction on the PDMS substrate. We show that the surface exhibits superhydrophobic properties with a static contact angle over 160 degrees for micropillar spacing ratios (interpillar gap divided by diameter) less than 4. A transition of the wetting angle to approximately 130 degrees occurs for larger spacing ratios, changing the wetting from a Cassie-Cassie state (C(m)-C(n)) to a Wenzel-Cassie state (W(m)-C(n)), where m and n denote micro- and nanoscale roughness, respectively. The robust superhydrophobicity of the Cassie-Cassie state is attributed to stability of the Cassie state on the nanoscale wrinkle structures of the hydrophobic DLC coating, which is further explained by a simple mathematical theory on wetting states with decoupling of nano- and microscale roughness in dual scale structures.

Nanoscale patterning of microtextured surfaces to control superhydrophobic robustness.

Most naturally existing superhydrophobic surfaces have a dual roughness structure where the entire microtextured area is covered with nanoscale roughness. Despite numerous studies aiming to mimic the biological surfaces, there is a lack of understanding of the role of the nanostructure covering the entire surface. Here we measure and compare the nonwetting behavior of microscopically rough surfaces by changing the coverage of nanoroughness imposed on them. We test the surfaces covered with micropillars, with nanopillars, with partially dual roughness (where micropillar tops are decorated with nanopillars), and with entirely dual roughness and a real lotus leaf surface. It is found that the superhydrophobic robustness of the surface with entirely dual roughness, with respect to the increased liquid pressure caused by the drop evaporation and with respect to the sagging of the liquid meniscus due to increased micropillar spacing, is greatly enhanced compared to that of other surfaces. This is attributed to the nanoroughness on the pillar bases that keeps the bottom surface highly water-repellent. In particular, when a drop sits on the entirely dual surface with a very low micropillar density, the dramatic loss of hydrophobicity is prevented because a novel wetting state is achieved where the drop wets the micropillars while supported by the tips of the basal nanopillars.

Tribological behavior of silicon-incorporated diamond-like carbon films

The tribological behavior between silicon-incorporated diamond-like carbon (Si-DLC) films and a steel ball was investigated from the viewpoint of tribochemical reaction. The films were deposited on Si(100) wafers from radio-frequency glow discharge of mixtures of benzene and dilute silane gases. The tribological behavior was investigated by using a ball-on-disk type wear rig in ambient atmosphere. The variation of the friction coefficient with the number of contact cycles was compared among films having of various silicon concentrations from 0 to 9.5 at%. It was observed that the friction coefficient decreased with increasing silicon concentration in the films. Furthermore, the friction behavior became more stable even at a small amount of silicon less than 0.5 at% incorporated. By analyzing the composition of the debris formed, we could conclude that the low and stabilized friction coefficient is intimately related to the formation of the silicon-rich oxide debris. These results are consistent with a previously suggested mechanism that the hydrated silica debris results in the low friction coefficient in a humid environment.

An experimental study of the influence of imperfections on the buckling of compressed thin films

The role of imperfections on the initiation and propagation of buckle driven delaminations in compressed thin films has been demonstrated using experiments performed with diamond-like carbon (DLC) films deposited onto glass substrates. The surface topologies and interface separations have been characterized by using the Atomic Force Microscope (AFM) and the Focused Ion Beam (FIB) imaging system. The wavelengths and amplitudes of numerous imperfections have been measured by AFM and the interface separations characterized on cross sections made with the FIB. Chemical analysis of several sites, performed using Auger Electron Spectroscopy (AES), has revealed the origin of the imperfections. The incidence of buckles has been correlated with the imperfection wavelength. The findings have been rationalized in terms of theoretical results for the effect of imperfections on the energy release rate.

Structural dependence of mechanical properties of Si incorporated diamond-like carbon films deposited by RF plasma-assisted chemical vapour deposition

Abstract Mechanical properties and atomic bond structure of Si incorporated diamond-like carbon (Si-DLC) films were investigated. The films were deposited by 13.56 MHz r.f.- plasma-assisted chemical vapour deposition (r.f.-PACVD), using mixtures of benzene and diluted silane (SiH4/H2 10:90) as the reaction gases. Si concentration in the film was varied from 0 to 17 atomic (at.)% by increasing the diluted silane fraction from 0 to 95%. It was observed that the mechanical properties of the film changed significantly when the Si concentration was less than 5 at.%. In this concentration range, hardness, residual stress and elastic constants increased with increasing Si concentration. For higher concentrations of Si, the mechanical properties showed saturated behaviour. The changes of the mechanical properties are discussed in terms of the content of three-dimensional inter-links of the atomic bond network.

Determination of elastic modulus and Poisson's ratio of diamond-like carbon films

A simple technique to measure the elastic modulus and Poisson’s ratio of diamond-1ike carbon (DLC) films deposited on Si substrate was suggested. This technique involved etching a side of Si substrate using the DLC film as an etching mask. The edge of the DLC overhang, which is free from constraint of the Si substrate, exhibits periodic sinusoidal shape. By measuring the amplitude and the wavelength of the sinusoidal edge, we can determine the strain of the film required to adhere to the substrate. Combined with an independent stress measurement by laser reflection method this technique allows calculation of the biaxial elastic modulus, E=O1 2 nU where E is the elastic modulus and n Poisson’s ratio of the DLC films. By comparing the biaxial elastic modulus with plane-strain modulus E=O1 2 n 2 U measured by nanoindentation, we could further determine the elastic modulus and Poisson’s ratio, independently. The mechanical properties of DLC films deposited by r.f. PACVD were characterized using this technique. The films were prepared by using C6H6 r.f. glow discharge at a self bias voltage of 400 V and a deposition pressure of 1.33 Pa. The elastic modulus and Poisson’s ratio were 87 ^ 18 GPa and 0:22 ^ 0:33, respectively. The effects of the etching depth and the film thickness were also discussed. q 1999 Elsevier Science S.A. All rights reserved.

Effect of environment on the tribological behavior of Si-incorporated diamond-like carbon films

An experimental study was performed to discover the effect of environment on the tribological behavior of Si-incorporated diamond-like carbon (Si-DLC) film slid against a steel ball. The films were deposited on Si (1 0 0) wafers by a radio frequency glow discharge of mixtures of benzene and dilute silane gases. Experiments using a ball-on-disk test-rig were performed in vacuum, dry air and ambient air. It was observed that coefficient of friction decreased as the environment changed from vacuum to dry air. Results also showed that low and stable friction related closely to the smoothening of track surfaces and the formation of silicon-rich oxide debris.

Tribological behavior of sliding diamond-like carbon films under various environments

Abstract Tribological behaviors of amorphous diamond-like carbon (DLC) films were experimentally evaluated under various environments using a steel ball-on-disk wear-rig at dry sliding surfaces. The DLC films were prepared on Si wafer by r.f. PACVD method using benzene (C 6 H 6 ). Every test was performed under the normal load 4.9 N and sliding velocity of 0.05 m/s, and the coefficients of friction of DLC films were measured with the contact cycle. When the test was performed under a vacuum, the coefficient of friction of DLC films showed a significant fluctuation with the contact cycles, remarkably accompanied with roll-shaped polymeric wear debris. Another test using thermally-baked DLC films showed that the friction was lower and more stable. Under a humidity controlled air, friction of DLC films was also stable. Tests with dry O 2 and N 2 gas were extended to find any other environmental factors which could affect the tribological behavior of DLC films. It was found that tribological behavior of DLC films was dependent on the formation of friction layers which were mainly affected by the environment. In the final discussion of this work, effects of environment on the tribological behavior of DLC films were combined and discussed in terms of tribo-chemistry of DLC films.