Amal Al Ghaferi

Microscopic Droplet Formation and Energy Transport Analysis of Condensation on Scalable Superhydrophobic Nanostructured Copper Oxide Surfaces

Utilization of nanotechnologies in condensation has been recognized as one opportunity to improve the efficiency of large-scale thermal power and desalination systems. High-performance and stable dropwise condensation in widely-used copper heat exchangers is appealing for energy and water industries. In this work, a scalable and low-cost nanofabrication approach was developed to fabricate superhydrophobic copper oxide (CuO) nanoneedle surfaces to promote dropwise condensation and even jumping-droplet condensation. By conducting systematic surface characterization and in situ environmental scanning electron microscope (ESEM) condensation experiments, we were able to probe the microscopic formation physics of droplets on irregular nanostructured surfaces. At the early stages of condensation process, the interfacial surface tensions at the edge of CuO nanoneedles were found to influence both the local energy barriers for microdroplet growth and the advancing contact angles when droplets undergo depinning. Local surface roughness also has a significant impact on the volume of the condensate within the nanostructures and overall heat transfer from the vapor to substrate. Both our theoretical analysis and in situ ESEM experiments have revealed that the liquid condensate within the nanostructures determines the amount of the work of adhesion and kinetic energy associated with droplet coalescence and jumping. Local and global droplet growth models were also proposed to predict how the microdroplet morphology within nanostructures affects the heat transfer performance of early-stage condensation. Our quantitative analysis of microdroplet formation and growth within irregular nanostructures provides the insight to guide the anodization-based nanofabrication for enhancing dropwise and jumping-droplet condensation performance.

Enhanced electrical properties of vertically aligned carbon nanotube-epoxy nanocomposites with high packing density

During their synthesis, multi-walled carbon nanotubes can be aligned and impregnated in a polymer matrix to form an electrically conductive and flexible nanocomposite with high backing density. The material exhibits the highest reported electrical conductivity of CNT-epoxy composites (350 S/m). Here, we show how conductive atomic force microscopy can be used to study the electrical transport mechanism in order to explain the enhanced electrical properties of the composite. The high spatial resolution and versatility of the technique allows us to further decouple the two main contributions to the electrical transport: (1) the intrinsic resistance of the tube and (2) the tunneling resistance due to nanoscale gaps occurring between the epoxy-coated tubes along the composite. The results show that the material behaves as a conductive polymer, and the electrical transport is governed by electron tunneling at interconnecting CNT-polymer junctions. We also point out the theoretical formulation of the nanoscale electrical transport between the AFM tip and the sample in order to derive both the composite conductivity and the CNT intrinsic properties. The enhanced electrical properties of the composite are attributed to high degree of alignment, the CNT purity, and the large tube diameter which lead to low junction resistance. By controlling the tube diameter and using other polymers, the nanocomposite electrical conductivity can be improved.

Enhanced organic solar cells efficiency through electronic and electro-optic effects resulting from charge transfers in polymer hole transport blends

We demonstrate that blending fluorinated molecules in PEDOT:PSS hole transport layers (HTL) induces charge transfers which impact on both charge extraction and photogeneration within organic photovoltaic (OPV) devices. OPVs fabricated with modified HTL and two photoactive polymer blends led systematically to power conversion efficiencies (PCE) increases, with PTB7:PC70BM blend exhibiting PCE of ∼8.3%, i.e. ∼15% increase compared to pristine HTL devices. A reduced device-to-device characteristics variations was also noticed when fluorinated additives were used to modify the PEDOT:PSS. Shading lights onto the effect of HTL fluorination, we show that the morphology of the polymer:PCBM blends remains surprisingly unaffected by the fluorinated HTL surface energy but that, instead, the OPVs are impacted not only by the HTL electronic properties (work function, dipole layer, open circuit voltage, charge transfer dynamic) but also by alteration of the complex refractive indices (photogeneration, short circuit current density, external quantum efficiencies, electro-optic modelling). Both mechanisms find their origin in fluorination induced charge transfers. This work points towards fluorination as a promising strategy toward combining both external quantum efficiency modulation and power conversion efficiency enhancement in OPVs. Charge transfers could also be used more broadly to tune the optical constants and electric field distribution, as well as to reduce interfacial charge recombinations within OPVs.

Optimizing the dispersion conditions of SWCNTs in aqueous solution of surfactants and organic solvents

The optimum concentration of surfactants in aqueous medium and the optimum sonication time for organic medium to achieve the best dispersion of SWCNTs are investigated in this study. Scanning electron microscope (SEM), atomic force microscope (AFM), and UV-vis-NIR study show that the optimum surfactant concentrations to achieve best dispersion for 0.25mg/mL of SWCNTs are 9-10mg/mL for (sodium dodecylbenzenesulfonate (SDBS)) and 8-9mg/mL for (sodium deoxycholate (DOC)). The diameter of the SWCNT lies between 0.7 and 2.5nm in the case of aqueous solvent, which is in good agreement with the chirality. Moreover, SEM analysis reveals the presence of well-dispersed nanotubes along with some amount of surfactant particles; that is, some nanotubes might be covered with surfactants. Best dispersion of SWCNTs in organic solvents (1,2-Dichloroethane (DCE) and N,N-Dimethylformamide (DMF)) is achieved by four hours of tip sonication. It realized that organic solvents provide pure SWCNTs after drying, which may possibly have SWCNTs with intrinsic properties.

An amino acid-based swift synthesis of zinc oxide nanostructures

Zinc oxide (ZnO) nanostructures (rods, ellipsoids and flowers) were synthesized using a zinc precursor and L-lysine in aqueous medium at room temperature and/or at elevated temperature. The average thicknesses of different zinc oxide nanorods samples synthesized by this eco-friendly route are in the range of 109 to 291 nm. The synthesized ZnO samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effects of concentration, concentration ratio of reactants, heat, zinc precursor or precipitating agent on the resulting ZnO have been studied. XRD analysis of nanorods proved the formation of hexagonal ZnO. Results revealed that lysine is responsible for the selective formation of 1-D morphologies even at room temperature. Acetate precursor was found to yield thinner nanorods than nitrate precursor. Based on the results, a plausible mechanism of formation of nanostructures has been proposed.

Plasmonic nanofluids enhanced solar thermal transfer liquid

Plasmonic nanostructures suspended in a liquid are known to offer enhanced absorption of light and improved photo-thermal efficiency comparing with conventional solar absorbers. This approach localizes high temperatures to the interior of the liquid through efficient trapping of incoming light via scattering and absorption mechanisms. Theoretical studies show that Ag exhibits the highest efficiency of plasmonic excitations, and the plasmonic absorption band can be shifted to cover the visible wavelength ranges by loading the Ag NPs onto silica core. In this work, silica-core decorated with Ag NPs are synthesized through the chemical reduction method and their morphological and optical properties are characterized using transmission electron microscope (TEM) and UV-Vis-NIR spectrophotometer. The characterization results show the potentials of light absorbing plasmonic metal-dielectric nanoparticles suspended in water for producing steam at high efficiencies upon solar illumination. The experimental work su...

Carbon nanomaterials based TSVs for dual sensing and vertical interconnect application

We discuss fabrication and characterization of TSVs filled with carbon nano-materials (CNM) for dual function of sensing and vertical interconnect for hostile environment applications (Corrosive High Temperature and Pressure). Nano-composites, made by functionalization of CNTs were integrated using dispersion in epoxy resin and inkjet techniques to fill up the TSVs and provide sensing surface. The results reveal ability for the nano-composite to fill vias with electrical conductivity path and sensing established through the wafer backside.

Multi-wall carbon nanostructured paper: characterization and potential applications definition

In the present work, a free-standing paper-like sample made of a three-dimensional web of multiwall carbon nanotubes that exhibit cross-linking and wall sharing is characterized with respect to its electrical, electrochemical, surface and wetting properties. All these parameters are studied simultaneously to assess the potential of the sample for demanding applications with multiple material requirements. It is observed that many characteristics of the one-dimensional counterpart are retained by the three-dimensional web. In addition, the properties of this nanostructured material are compared against zero-, one- two- and three-dimensional carbon-based materials, while potential applications that require high electrical conductivity and energy storage capabilities and related properties, are discussed.

Carbon Nanostructure‐Based Scale Sensors Using Inkjet Printing and Casting Techniques

In this chapter, the fabrication and characterization of scale sensor using carbon nanotubes (CNTs) are discussed. Two different methods are used to prepare the carbon nanomaterials for the sensor fabrication: CNT casting and the CNT inkjet printing. In addition, the sensors are integrated into Kelvin architectures. The electrical resistance of the carbon nanomaterial films is measured with and without adding a drop of brine to the surface of the film. The films are characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), and energy dispersive X‐ray spectroscopy (EDS). Electrical resistance of the casted CNT films and five layers of CNT inkjet printing are found to be close to 40.0 kΩ and 1.00 kΩ, respectively. Adding one drop of brine solution on the surface of the casted CNT film and five layers of CNT inkjet printing changed the resistance by 50% and 75%, respectively. The resetting process is done for all sensors by soaking in deionized water (DI water) for some time, and the electrical resistance is measured and found to be close to the initial electrical resistance.

Divergent surface properties of multidimensional sp (2) carbon allotropes: the effect of aging phenomena.

Despite the current interest in the scientific community in exploiting divergent surface properties of graphitic carbon allotropes, conclusive differentiation remains elusive even when dealing with parameters as fundamental as adhesion. Here, we set out to provide conclusive experimental evidence on the time evolution of the surface properties of highly oriented pyrolytic graphite (HOPG), graphene monolayer (GML) and multiwalled carbon nanotubes (MWCNTs) as we expose these materials to airborne contaminants, by providing (1) statistically significant results based on large datasets consisting of thousands of force measurements, and (2) errors sufficiently self-consistent to treat the comparison between datasets in atomic force microscopy (AFM) measurements. We first consider HOPG as a model system and then employ our results to draw conclusions from the GML and MWCNT samples. We find that the surface properties of aged HOPG are indistinguishable from those of aged GML and MWCNT, while being distinct from those of cleaved HOPG. Herein, we provide a sufficient body of evidence to disregard any divergence in surface properties for multidimensional sp (2) carbon allotropes that undergo similar aging processes.