Liwei Huang

Intercalating Oleylamines in Graphite Oxide

Graphite oxide has been synthesized from raw graphite particles and been treated with various mass amounts of oleylamine as intercalants to form intercalation compounds. X-ray diffraction patterns reveal that the inter-sheet distances strongly depend on the graphite oxide to oleylamine mass ratios. The equilibrium-like behavior implies diffusion-dominated oleylamine adsorption on graphite oxide in solution and excluded volume intercalations among oleylamine-adsorbed graphite oxide during restacking. The intercalation compounds are soluble in organic solvents, and their applications in the fabrication of transparent and conductive coatings have been demonstrated.

Hybrid nanostructure heterojunction solar cells fabricated using vertically aligned ZnO nanotubes grown on reduced graphene oxide

Using reduced graphene oxide (rGO) films as the transparent conductive coating, inorganic/organic hybrid nanostructure heterojunction photovoltaic devices have been fabricated through hydrothermal synthesis of vertically aligned ZnO nanorods (ZnO-NRs) and nanotubes (ZnO-NTs) on rGO films followed by the spin casting of a poly(3-hexylthiophene) (P3HT) film. The data show that larger interfacial area in ZnO-NT/P3HT composites improves the exciton dissociation and the higher electrode conductance of rGO films helps the power output. This study offers an alternative to manufacturing nanostructure heterojunction solar cells at low temperatures using potentially low cost materials.

Hydrogenation of Mg film and Mg nanoblade array on Ti coated Si substrates

The hydrogenation of Mg film and Mg nanoblade array fabricated on Ti coated Si substrates has been studied and compared. The nanoblades start to absorb hydrogen at a temperature between 250 and 300°C, which is much lower than 350°C for Mg film. However, the saturated total hydrogen uptake in nanoblades is less than half of that in the film, resulting from MgO formation by air exposure. The nanoblade morphology with large surface area and small hydrogen diffusion length, and the catalytic effect of Ti layer, are two main reasons for the nanoblade hydrogenation behavior.

Design optimization of custom engineered silver-nanoparticle thermal interface materials

Thermal interface material (TIM) is a major hurdle in heat flow for typical chip/heat sink assemblies. In many electronic devices, hot spots occur in areas of high activity during the device operation. These hot spots can lead to high thermal gradients, which in turn result in performance and reliability hindrances. The elevated, non-uniform power density confronted with conventional TIMs that contain a uniform layer of high thermal conductivity material for the entire chip can be extremely insufficient in many applications. In this paper, a custom engineered, Ag-nanoparticle (Ag-NP) TIM that targets directly to the high power density region is introduced for achieving better thermal-mechanical-electrical performance at low cost. These nanoparticles can be inkjet printed on hot spots and sintered at a relative low temperature (~120°C) to create a continuous metallic layer that is in good contact with both the chip and heat sink, whereas the conventional particle-laden TIM covers the lower power density area. A computational model is developed to examine the overall thermal performance and reliability of the hybrid Ag-NP/conventional TIM as a function of the bondline thickness, applied pressure, deposition pattern, and surface roughness. The results show great improvements compared with a high-performance indium solder.

Sintering Metal Nanoparticle Films

We have carried out several measurements in order to understand the process of metal nanoparticle (MNP) film sintering. Small angle neutron scattering has been used to reveal the average diameters of silver and gold nanoparticles (Ag-NPs and Au-NPs) used in this study to be 4.6 and 3.8 nm, respectively, with a size distribution of ca. 20%. Spun- cast Ag-NP and Au-NP films have been sintered at temperature ranges of 80-160degC and 180-210 degC, respectively, for various times. The resulting film composition, morphology and electric resistance have been revealed. Upon sintering, the organic content in MNP films reduces to less than 10% while the overall film thickness reduces to about the half of the as-cast film thickness. The resistance of sintered Ag-NP films can vary over more than 7 decades depending on the sintering temperature. The conductivity of Ag-NP films sintered at 150degC is 2.4 times 10-8 Omegam. The transport properties are affected by both the composition and morphology of sintered films.

Inkjet printing of palladium source and drain electrodes on individual single-wall carbon nanotubes to fabricate field effect transistors

Field effect transistors (FETs) have been fabricated on individual single-wall carbon nanotubes by inkjet printing of source and drain electrodes using palladium nanoparticles. The FETs display p-type semiconducting characteristics, whereas subsequent thermal annealing shows that both hole-doping and Schottky contacts have an influence on the electrical transport. The data show that inkjet printing is a viable tool for the facile fabrication of single-tube nanoelectronics.

Low-Temperature Solution Synthesis of Zinc Oxide Nanotubes

Zinc oxide nanotubes (ZnO-NTs) have been successfully synthesized at ∼60°C using wet chemical method. ZnO-NTs have diameters of 100–200 nm and lengths of 1–2 μm. Transmission electron microscopy shows that as-synthesized NTs are single-crystal hexagonal structure and grown along [0001] direction. X-ray diffraction, ultravisible absorption, and photoluminescence results reveal that thermal treatments at low temperature of <500°C in air cannot apparently improve the crystal properties of as-synthesized ZnO.