Chi-Man Lawrence Wu

N-P transition sensing behaviors of ZnO nanotubes exposed to NO2 gas.

Hydrothermally synthesized ZnO nanotube arrays were used to fabricate the gas sensor for detection of the toxic gas NO2. The ZnO nanotube gas sensor exhibited sensitive response to NO2 down to 500 ppb at low temperature 30 degrees C. Abnormal temperature-dependent and concentration-dependent N-P transition behaviors were observed. The excellent sensing ability of the sensor and their anomalous conductivity behaviors may be attributed to the unique surface conductivity related to the hollow nanostructure feature. These sensing behaviors may help to comprehensively understand the sensing mechanism of 1D ZnO nanostructures and improve the selectivity of the ZnO gas sensor.

Improvements of microstructure, wettability, tensile and creep strength of eutectic Sn–Ag alloy by doping with rare-earth elements

To improve the properties of the eutectic Sn-Ag lead-free solder alloy, various amounts of mixed rare-earth (RE) elements, mainly Ce and La, were added. The microstructure, wetting properties, melting behavior, mechanicalproperties, and creep behavior were studied. It was revealed that RE elements can refine the intermetallics, and with 0.5% RE addition, the RE-bearing phase can be detected in the microstructure of the slow-cooled alloy. The results of differential scanning calorimetry indicate that the melting points of the RE-doped alloys are slightly lower than that of the Sn-3.5Ag and have a eutectic peak. The wetting property and creep resistance of the Sn-3.5Ag-0.25RE alloy are better than those of the Sn-3.5Ag alloy. The creep properties were studied at the temperatures of 303, 348, and 393 K, at various stress levels between 8 and 34 MPa. The stress exponents of the Sn-3.5Ag and Sn-3.5Ag-0.25RE were obtained at these temperatures. Tensile, creep, and wetting properties were found to improve with the addition of RE elements. The improvement of creep resistance is due to the fine dispersion of intermetallics and the decrease in interface energy between matrix and intermetallics. The wettability improvement is mainly due to the accumulation of RE elements at the solder/flux interface, leading to the reduction of the interfacial tension between solder and flux.

White-light spectral interferometry for surface plasmon resonance sensing applications.

A novel differential phase detecting surface plasmon resonance (SPR) sensor based on white-light spectral interferometry is presented. Our proposed scheme employs a white-light source for SPR excitation and measures the corresponding SPR phase change at the optimized coupling wavelength with fixed angle of incidence across the visible spectrum. Compared to existing laser based phase detecting schemes, this system offers optimal sensitivity and extended dynamic range of measurement without any compromise in phase detection resolution. Results obtained from sodium chloride solutions indicate that the detection limit is 
2.6×10⁻⁷ RIU over a refractive index range of 10⁻² RIU, which is considerably wider than that achievable by existing laser based approach, thus making our scheme very attractive for practical SPR sensing applications.

Adsorption of nucleobase pairs on hexagonal boron nitride sheet: hydrogen bonding versus stacking

The adsorption of hydrogen-bonded and stacked nucleobase pairs on the hexagonal boron nitride (h-BN) surface was studied by density functional theory and molecular dynamics methods. Eight types of nucleobase pairs (i.e., GG, AA, TT, CC, UU, AT, GC, and AU) were chosen as the adsorbates. The adsorption configurations, interaction energies, and electronic properties of the nucleobase pair on the h-BN surface were obtained and compared. The density of states analysis result shows that both the hydrogen-bonded and stacked nucleobase pairs were physisorbed on h-BN with minimal charge transfer. The hydrogen-bonded base pairs lying on the h-BN surface are significantly more stable than the stacked forms in both the gas and water phase. The molecular dynamics simulation result indicates that h-BN possessed high sensitivity for the nucleobases and the h-BN surface adsorption could revert the base pair interaction from stacking back to hydrogen bonding in aqueous environment. The h-BN surface could immobilize the nucleobases on its surface, which suggests the use of h-BN has good potential in DNA/RNA detection biosensors and self-assembly nanodevices.

Differential spectral phase interferometry for wide dynamic range surface plasmon resonance biosensing

We introduce a novel wide dynamic range phase-sensitive surface plasmon resonance (SPR) biosensor based on differential spectral interferometry. Superseding conventional spectroscopic approach where only the SPR dip is monitored, our system acquires the spectral phase information of the entire electromagnetic field that undergoes SPR transformation. Since the SPR-induced phase change is highly wavelength specific with fixed incident angle, ultra-high sensitivity achievable through phase-sensitive detection, as reported herein, is maintained continuously across the spectral domain in response to refractive index changes. Our system has demonstrated a detection limit of 2.2×10(-7) in terms of refractive index unit (RIU) using standard single-layer gold surface. In terms of biosensing performance, the estimated detection sensitivity obtained from bovine serum albumin (BSA) antibody-antigen binding experiments is 0.5 ng ml(-1).

Effects of Cu Contents in Sn–Cu Solder on the Composition and Morphology of Intermetallic Compounds at a Solder/Ni Interface

The reaction between Sn– x Cu ( x = 0.1, 0.3, 0.7, 0.9, and 1.5 wt%) solder alloys and Ni at 260, 280, and 290 °C for 60 s was studied to reveal the effect of Cu content on the composition and morphology of intermetallic compounds (IMCs) formed at the interface between solder and substrate. The results indicated that Cu concentration greatly affects both the composition and morphology of the IMC between the solder and Ni substrate. In particular, when the Cu concentration was less than or equal to0.3 wt%, (Cu x Ni 1− x ) 3 Sn 4 IMCs were formed at the interface. When the Cu concentration was 0.7 wt%, large facets type of (Cu x Ni 1− x ) 6 Sn 5 were mixed with (Cu x Ni 1− x ) 3 Sn 4 in the IMC layer. At Cu concentrations higher than the eutectic one, e.g., 0.9 and 1.5 wt%, stick-shaped (Cu x Ni 1− x ) 6 Sn 5 compounds were detected, but the (Cu x Ni 1− x ) 3 Sn 4 IMCs disappeared. The formation and growth mechanism of the (Cu x Ni 1− x ) 6 Sn 5 compound were analyzed. The evolution tendency of the composition and morphology of the IMCs at the three testing temperatures was found to be the same.

Interfacial microstructure and strength of lead-free Sn-Zn-RE BGA solder bumps

Rare earth (RE) elements, primarily La and Ce, were doped in Sn-Zn solder to improve its properties such as wettability. The interfacial microstructure evolution and shear strength of the Sn-9Zn and Sn-9Zn-0.5RE (in wt%) solder bumps on Au/Ni/Cu under bump metallization (UBM) in a ball grid array (BGA) were investigated after thermal aging at 150 /spl deg/C for up to 1000 h. In the as-reflowed Sn-9Zn solder bump, AuSn/sub 4/ intermetallic compounds (IMCs) and Au-Zn circular IMCs formed close to the solder/UBM interface, together with the formation of a Ni-Zn-Sn ternary IMC layer of about 1 /spl mu/m in thickness. In contrast, in the as-reflowed Sn-9Zn-0.5RE solder bump, a spalled layer of Au-Zn was formed above the Ni layer. Sn-Ce-La and Sn-Zn-Ce-La phases were found near the interface at positions near the surface of the solder ball. Upon thermal aging at 150 /spl deg/C, the concentration of Zn in the Ni-Zn-Sn ternary layer of Sn-9Zn increased with aging time. For Sn-9Zn-0.5RE, the Au-Zn layer began to dissolve after 500 h of thermal aging. The shear strength of the Sn-9Zn ball was decreased after the addition of RE elements, although it was still higher than that of the Sn-37Pb and Sn-36Pb-2Ag Pb-bearing solders. The fracture mode of the Sn-9Zn system was changed from ductile to partly brittle after adding the RE elements. This is mainly due to the presence of the brittle Au-Zn layer.

Lead free solder alloys Sn-Zn and Sn-Sb prepared by mechanical alloying

The mechanical alloying (MA) processes of tin-9zinc (Sn-9Zn) and tin-5antimony (Sn-5Sb) were analyzed using an X-ray diffractometer, a differential scanning calorimeter (DSC) and a scanning electron microscope (SEM). The results showed that supersaturated solid solution and intermetallic compound were produced during the MA process. The size of powder particles could be controlled under the proper MA conditions and was reduced to 5 to 20 μm. As a surfactant, proper addition of rosin allows MA to occur smoothly. However, it may also be a source of contamination of the metallic powder. It is considered that MA may be a technique analogous to casting to prepare solder alloys.

Common-path spectral interferometry with temporal carrier for highly sensitive surface plasmon resonance sensing

Incorporating the temporal carrier technique with common-path spectral interferometry, we have successfully demonstrated an advanced surface plasmon resonance (SPR) biosensing system which achieves refractive index resolution (RIR) up to 2 × 10(-8) refractive index unit (RIU) over a wide dynamic range of 3 × 10(-2) RIU. While this is accomplished by optimizing the SPR differential phase sensing conditions with just a layer of gold, we managed to address the spectral phase discontinuity with a novel spectral-temporal phase measurement scheme. As the new optical setup supersedes its Michelson counterpart in term of simplicity, we believe that it is a significant contribution for practical SPR sensing applications.

Methanol Oxidation on Pt3Sn(111) for Direct Methanol Fuel Cells: Methanol Decomposition

PtSn alloy, which is a potential material for use in direct methanol fuel cells, can efficiently promote methanol oxidation and alleviate the CO poisoning problem. Herein, methanol decomposition on Pt3Sn(111) was systematically investigated using periodic density functional theory and microkinetic modeling. The geometries and energies of all of the involved species were analyzed, and the decomposition network was mapped out to elaborate the reaction mechanisms. Our results indicated that methanol and formaldehyde were weakly adsorbed, and the other derivatives (CHxOHy, x = 1-3, y = 0-1) were strongly adsorbed and preferred decomposition rather than desorption on Pt3Sn(111). The competitive methanol decomposition started with the initial O-H bond scission followed by successive C-H bond scissions, (i.e., CH3OH → CH3O → CH2O → CHO → CO). The Brønsted-Evans-Polanyi relations and energy barrier decomposition analyses identified the C-H and O-H bond scissions as being more competitive than the C-O bond scission. Microkinetic modeling confirmed that the vast majority of the intermediates and products from methanol decomposition would escape from the Pt3Sn(111) surface at a relatively low temperature, and the coverage of the CO residue decreased with an increase in the temperature and decrease in partial methanol pressure.