Chun Chan

Deposition and properties of tetrahedral amorphous carbon films prepared on magnetic hard disks

The areal density of information stored on the hard disk has doubled every two years. This substantial increase in disk storage has resulted from the application of giant magnetoresistance heads, new thin film media, and better electronic recording channels. However, such an increase cannot be easily attained without reducing the separation between the magnetic read-write head and magnetic recording medium surfaces. This can be achieved by using a thinner protective overcoat. In this study, ultrathin tetrahedral amorphous carbon (ta-C) films were deposited on magnetic hard disks (CoCrTa/Cr/NiP/Al–Mg) by a magnetic filtered cathodic arc deposition under different substrate bias voltages. The obtained films exhibited smoother surfaces than the uncoated disks as indicated by the atomic force microscopic measurement. The Raman spectra acquired showed a single asymmetric Lorentzian curve shape. Tetrahedral amorphous carbon coatings were subjected to an accelerated corrosion test in vapors of concentrated hydro...

Surface microstructure analysis of cubic boron nitride films by transmission electron microscopy

A simple coating technique was introduced to preserve the surface structure of samples for transmission electron microscopy (TEM) characterization, and used to study boron nitride (BN) films. A gold film precoated on the surface of BN films served to protect the BN surface against ion damages during sample preparation, and to separate and distinguish the film surface structure from the TEM glue. The technique enabled the observation of detailed surface microstructures of cubic BN (cBN) films, which provided direct evidences for understanding cBN growth mechanisms. The TEM sample technique is expected to be generally applicable to other film systems, particularly those with an amorphous topmost layer.

Effect of substrate surface on the structure and electronic properties of cubic boron nitride films

Cubic boron nitride (c-BN) films were prepared by mass-selected ion beam deposition (MSIBD) technique. The effects of substrate surface roughness were investigated by boron and nitrogen k-edge x-ray absorption near-edge structure, x-ray diffraction, and atomic force microscopy. All the films are a mixture of nanocrystalline sp3-bonded c-BN and sp2-bonded BN phases. The substrate with a rough surface causes a decrease of the c-BN phase content of the film on it. A significant large lattice contraction of the c-BN crystallites in the films, relative to the bulk, is observed. It is also found that the electronic structure of the nanocrystalline c-BN films by MSIBD technique is somewhat different from that of microcrystalline c-BN∕h-BN references. We attribute the effect of the nature of the substrate on the morphology and structure of the c-BN films to the orientation of sp2-bonded graphitic BN basal plane on the top surface of the films during their growth, and the lattice contraction and energy band structure modification of c-BN films to the large compressive stress, respectively.Cubic boron nitride (c-BN) films were prepared by mass-selected ion beam deposition (MSIBD) technique. The effects of substrate surface roughness were investigated by boron and nitrogen k-edge x-ray absorption near-edge structure, x-ray diffraction, and atomic force microscopy. All the films are a mixture of nanocrystalline sp3-bonded c-BN and sp2-bonded BN phases. The substrate with a rough surface causes a decrease of the c-BN phase content of the film on it. A significant large lattice contraction of the c-BN crystallites in the films, relative to the bulk, is observed. It is also found that the electronic structure of the nanocrystalline c-BN films by MSIBD technique is somewhat different from that of microcrystalline c-BN∕h-BN references. We attribute the effect of the nature of the substrate on the morphology and structure of the c-BN films to the orientation of sp2-bonded graphitic BN basal plane on the top surface of the films during their growth, and the lattice contraction and energy band struct...

X-ray absorption studies on cubic boron nitride thin films

Cubic boron nitride (c-BN) films synthesized by various energetic species assisted physical vapor deposition and chemical vapor deposition techniques on Si and diamond-coated Si substrates have been investigated by boron and nitrogen K-edge angle-resolved x-ray absorption near-edge structure in both total electron yield and fluorescence yield modes. X-ray absorption spectrum has been developed to study the film structure, the quantity and distribution of the partially ordered turbostratic (t-BN) and amorphous (a-BN) sp2-hybridized BN phases, and the t-BN∕a-BN ratios. The preferred direction of the t-BN basal planes at the interface between c-BN and substrate is found to be normal or nearly normal to the substrate. The content of the sp2-bonded BN in the c-BN films deposited on diamond-coated Si substrates reduces remarkably. The modifications of the electronic structure of the c-BN films with respect to bulk hexagonal BN and c-BN have been investigated and the crystallinity of c-BN films has also been eva...

Molecular Details of the PH Domain of ACAP1BAR-PH Protein Binding to PIP-Containing Membrane

ACAP1 proteins were previously reported to specifically bind PIP2-containing cell membranes and form well-structured protein lattices in order to conduct membrane tubulation. We carried out molecular dynamics simulations to characterize orientation of the PH domains with respect to the BAR domains inside the protein dimer. Followed by molecular dynamics simulations, we present a comprehensive orientation analysis of PH domain under different states including unbound and bound with lipids. We have examined two binding pockets on the PH domain and present PMF profiles of the two pockets to account for their preference to PIP2 lipids. Combining orientation analysis and studies of binding pockets, our simulations results reveal valuable molecular basis for protein-lipid interactions of ACAP1 proteins during membrane remodeling process.

Diamond and cubic boron nitride: synthesis and electronic applications

This paper reviews recent progress in the deposition of diamond and cubic boron nitride (cBN) films. It aims at the effort of preparing smooth surfaces, improving crystallinity and enhancing adhesion of diamond and cBN films. The properties of these materials, multi-stage growth processes and difference in synthesis of diamond and cBN films are discussed. Resolving the nucleation and growth stages, deposition at different temperatures, post deposition treatment, possible chemistry, and the mechanisms behind are introduced as well. We report the most striking results achieved in our laboratories including the deposition of thin smooth oriented diamond film with coalescent diamond crystals and the deposition of thick cubic boron nitride films yielding well-resolved Raman spectra. The difference between the syntheses of polycrystalline and nano-crystalline diamond films are shown to be in gas phase environments. While the CH/sub 3/ radicals are responsible for the growth of polycrystalline films the CVD environment with abundant C/sub 2/ dimers results in the deposition of nanocrystalline diamond. Special attention deserves manufacturing single crystal diamond nanotips and their arrays. These single crystal diamond nano-lips have a very high aspect ratio, an apical angle of 28/spl deg/ and a radius of 5 nm. Finally, we present a review of potential and current applications of diamond in electronic areas.

DIAMOND GROWN ON STEEL VIA IN-SITU FORMED INTERLAYERS

Diamond films were grown on steel substrates by a hot filament chemical vapor deposition method without any special substrate pretreatment. When the deposition time was extended from 15 to 83 hrs and above, non-graphitic interfacial layers were formed below diamond films. The interfacial layers were solid and mediated good adherence of diamond films on steel substrates. The interfacial layers were found to be iron and chromium carbides with traces of oxygen as determined by chemical analysis using a scanning Auger microscopy (SAM). The carbide interfacial layers probably resulted from graphitic conversion processes. It is assumed that the graphitic conversion was established on chemical reaction pathways driven by thermal diffusion processes between the earlier formed graphitic layers and atomic constituents of the steel substrates.

Investigation of diamond film deposition on steel without and with ion beam nitriding pretreatment

The surface morphology, film quality, interface structure, growth rate and adhesion of diamond films prepared by hot filament chemical vapor deposition on commercial steel substrates with and without ion beam nitriding pretreatment were investigated by scanning electron microscopy, x-ray diffraction, scanning Auger spectroscopy, a Vickers microindenter, and microRaman spectroscopy. Apart from quite different surface morphologies, both the quality and growth rate of diamond films deposited on the ion beam nitrided steel substrates were higher than those on the unnitrided steel substrates. Similar diamond/steel interface structure were observed on both the nitrided and unnitrided steel. Diamond films were grown on steel substrates via carbide interlayers formed in situ during the diamond deposition rather than via a graphitic soot interlayer. Ion beam nitriding appeared to enhance the diamond growth through thickening of the carbide interlayer. The adhesion of diamond films on the nitrided steel was tested ...

Temperature-Dependent Lipid Extraction from Membranes by Boron Nitride Nanosheets

Two-dimensional (2D) materials can mechanically insert into cell membranes and extract lipids out, thus leading to the destruction of cell integrity. On the one hand, the cytotoxicity of 2D materials can be harnessed in surface engineering to resist biofouling, and on the other hand, it causes great concern with in vivo biomedical applications ranging from drug delivery to nanoimaging. To understand the nature of this cytotoxic behavior and find strategies to control it, we performed molecular dynamics (MD) simulations on the lipid extraction of hexagonal boron nitride (BN) nanosheets from lipid membranes. Interestingly, we observed that the lipid extraction behavior suddenly disappears as temperature decreases. Structural analyses revealed that this temperature dependence is related to the lipid membrane phase transition, which was confirmed by an additional membrane model with phase state regulated by cholesterol. The potential of mean force calculation was adopted to clarify the thermodynamic origin of these results, which also indicates directions to adjust the lipid extraction behavior of nanomaterials. Overall, this work suggests that the cytotoxic mechanical interactions between 2D materials and cell membranes can be controlled by temperature and other factors which can induce phase transitions of lipid membranes and that the thermodynamic threshold of the lipid extraction varies for surfaces with different curvature. This work clarifies the thermodynamics in the lipid extraction phenomenon of 2D materials and indicates possible strategies to adjust the antibacterial performance or cytotoxicity of 2D materials.

Myopathy-inducing mutation H40Y in ACTA1 hampers actin filament structure and function.

In humans, more than 200 missense mutations have been identified in the ACTA1 gene. The exact molecular mechanisms by which, these particular mutations become toxic and lead to muscle weakness and myopathies remain obscure. To address this, here, we performed a molecular dynamics simulation, and we used a broad range of biophysical assays to determine how the lethal and myopathy-related H40Y amino acid substitution in actin affects the structure, stability, and function of this protein. Interestingly, our results showed that H40Y severely disrupts the DNase I-binding-loop structure and actin filaments. In addition, we observed that normal and mutant actin monomers are likely to form distinctive homopolymers, with mutant filaments being very stiff, and not supporting proper myosin binding. These phenomena underlie the toxicity of H40Y and may be considered as important triggering factors for the contractile dysfunction, muscle weakness and disease phenotype seen in patients.