Papers by Author: Chwee Teck Lim

Abstract: In this paper, the effects of Na+ concentration on the overstretching transition of B-DNA molecule at physiological temperature are studied by both experimental and numerical methods. Using optical tweezers, the relationship of external force and relative extension is obtained by stretching single B-DNA molecule at 37°C. As the concentration increases from 0.909mM to 909mM, the overstretching transition force increases from 65.65 ± 1.2pN to 43.07 ± 1.2pN. An analytical expression is derived, which shows that overstretching transition force is linear with the natural logarithm of salt concentration. Based on a previous model, a three-dimensional model is proposed herein and solved by Metropolis Monte Carlo method. The bending deformation of DNA backbones, cooperativity of base-stacking interactions, electrostatic interactions, and spatial effects of DNA double helix structure are taken into account. Our key contribution is that the electrostatic energy is explicitly given as a function of folding angle and Na+ concentration. A new parameter is also introduced to account for the cooperativity of base-stacking interactions. The numerical results of this model are in good agreement with our experimental results.

Abstract: The effects of oxygen (O2) reactive ion etching (RIE) on the field emission (FE) properties of aligned CuO nanowire films are investigated systematically. It is found that the FE performance of the films is largely enhanced after initial exposure to reactive oxygen ions but degrades after extended treatment. As comparison, Ar RIE is also used to treat CuO nanowires, which, however, results in the deterioration of FE properties. The enhanced FE after O2 RIE is attributed to the shaper morphology, cleaner surface and better conductivity. On the other hand, increased work function and non-crystallized surface structure cause the deterioration of FE of CuO nanowires after Ar RIE treatments.

Abstract: Flake-shaped hematite (α-Fe2O3) nanostructure has been successfully fabricated by using a hot-plate to directly heat Fe foil or Fe-coated substrates in air at 300oC. After heating, the surface of the samples was found to be populated with α-Fe2O3 nanoflakes. Such growth of α-Fe2O3 nanoflakes was very substrate-friendly. They can be formed on blank Si wafer, patterened Si, AFM tips, silica sphere, quartz, glass slide, Al foil and electrochemically etched W tip. The formation process and the final products were investigated by glancing angle x-ray diffraction (GAXRD), micro-Raman, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate the final products are single crystalline α-Fe2O3 nanoflakes vertically standing on the Fe3O4 film that acts as the precursor for growth of α-Fe2O3. The α-Fe2O3 nanoflakes formed by this method show very sharp tip with the tip radii as small as several nanometers and large surface to volume ratio. Such nanoflakes may be potentially useful as novel candidates for future electron field emission and gas senor devices. Furthermore, it is believed that this simple and substrates-friendly method is useful in extending the applications of α-Fe2O3 nanostructures.

Abstract: A simple technique to synthesis crystalline Tungsten Oxide nanowires is presented. Using a standard thermal hotplate, a pure 99.9% tungsten foil is annealed to 484 ± 5 oC under ambient condition to generate vapor deposition of the heated materials on a piece of 150μm thick glass cover slide pressing on the tungsten foil. Tungsten oxide nanowires are found to deposit on the cover slide facing the heated tungsten foil. These tungsten oxide nanowires were characterized with SEM, TEM, EDX, micro-Raman and XRD. The crystalline nanowires were found to be straight and clean with a diameter of 10-300nm and a length of a few tens of micrometers.

Abstract: Nanoindentation is a useful technique to measure hardness as well as elastic and timedependent plastic properties of materials with nanometer resolution. The measurement of elastic modulus of polymeric materials remains challenging due to their viscoelastic behavior. Clay reinforced nylon6 nanocomposites are found to have great improvement in the elastic modulus and tensile strength due to exfoliated hybrid structure. However, its mechanical properties have not been well investigated. In the present study, hardness and elastic modulus of nylon6-5wt%clay nanocomposites were investigated using nanoindentation. Creep effects of the nanocomposites on the unloading stiffness, which directly relates to the elastic modulus, were studied under various unloading rates and holding periods. It was found that the elastic modulus and hardness of nylon6-5wt%clay nanocomposites increased by 58% and 80%, respectively, as compared to pure nylon6. Experimental results for both polycarbonate and nylon6-5wt%clay nanocomposites showed that loading rate had no significant effects on the unloading stiffness. However, stiffness decreased to more consistent values after longer holding periods (more than 30 sec) and at faster unloading rates. The results indicated that creep behavior of the polymers affects the measurement of the unloading stiffness and may possibly overestimate the elastic modulus. Errors in the stiffness measurements from nanoindentation could be minimized with appropriate loading, unloading and holding conditions.