Sub-femtomolar Isothermal Desoprtion and Reaction Kinetics on Microhotplate Sensor Platforms

The population of adsorbates on a semiconductor surface directly influences the physical and chemical properties of the semiconductor. In the case of a metal-oxide semiconductor, the adsorbing species can change its electrical conductivity, a phenomenon which forms the operating principle of gas sensors. The interaction of adsorbed oxygen species on a metaloxide surface with reducing or oxidizing gases leads to an increase or decrease in electrical conductivity respectively. Miniature gas sensors called microhotplates (developed at the National Institute of Standards and Technology) are excellent surface science tools to explore surface reactions on semiconducting metal-oxide films. This thesis outlines how the desorption kinetics may be modeled in situations where the effects of finite heating rate, and system pumping rate are intertwined with the desorption rate, and how it is possible to estimate these time constants from isothermal desorption of sub-femtomolar coverages. Benzoic acid on reduced SnO2 was used as a model system to demonstrate the technique. It was observed to adsorb at coverages below 0.005 monolayers with an activation energy for desorption of 97 kJ/mol. The uptake, reaction pathways, and desorption kinetics of 2-propanol on TiO2 and SnO2 films were studied to demonstrate new microhotplate-based techniques to probe the fundamental surface processes that lead to electrical conductivity changes in chemiresistive gas sensors. Uptake and pulsed desorption measurements showed that reproducible coverages of 2-propanol could be prepared during low temperature adsorption, while interlaced, mass-resolved desorption pulses quantified indications of conversion to propene on oxidized TiO2 and SnO2 that correlate with conductivity changes. Fractional isothermal desorption data for 2-propanol on the oxidized TiO2 film suggest that the surface is energetically heterogeneous. A Monte Carlo model gives an average binding energy of 102 kJ/mol with a standard deviation of 15.7 kJ/mol, assuming diffusion is negligible on the timescale of the microhotplate’s heating pulse. The technique can thus show how a microsensor platform can provide a better understanding of the principles of sensor operation by determining, from sub-femtomolar quantities of adsorbates on a single microsensor, coverage, pumping speed, desorption rate, and reactivity of surface interactions and their effect on the sensing film conductivity

Gloss Dynamics of Inkjet Printers

Inkjet printing is a popular non-impact technology with widespread use in home and office applications. The basic principle involves propelling ink drops of different colors on a substrate. High quality images, near photographic quality, are now possible. The gloss of the printed substrate is an important quality attribute. Printed gloss depends on a number of characteristics of the media and the ink, but a good fundamental understanding is not available in the literature. The dependence of the gloss on the media and ink characteristics is reported in this work. The experimental results are compared with values predicted by a mathematical model. The dynamic post-printing gloss was studied with a specially constructed apparatus, which measured the laser reflectance of the printed surface within 40 ms after drop impact. Both pigmented and dye-based inks are used with rapidly absorbing porous media and swelling polymer-coated media. Various properties of the media such as surface roughness, ink absorption rates, pore size distribution, oil absorption capacity, wettability, and gloss were characterized along with ink properties like surface tension, viscosity, and filtercake resistance or the filtercake forming ability of the pigmented inks. The model and experimental results show that the gloss of dye-based inks on porous media depends on the media roughness. Gloss on swellable media depends on the roughness of the wet swollen polymer coating. The gloss of pigmented inks on porous media is determined by the ink pigment size and the dry media gloss. The gloss on swellable media is determined by ink pigment size and the wet roughness. The model predictions compare well to experiments for a wide range of parameters

Multi-Dimensional Sensors and Sensing Systems

A universal microelectromechanical (MEMS) nano-sensor platform having a substrate and conductive layer deposited in a pattern on the surface to make several devices at the same time, a patterned insulation layer, wherein the insulation layer is configured to expose one or more portions of the conductive layer, and one or more functionalization layers deposited on the exposed portions of the conductive layer to make multiple sensing capability on a single MEMS fabricated device. The functionalization layers are adapted to provide one or more transducer sensor classes selected from the group consisting of: radiant, electrochemical, electronic, mechanical, magnetic, and thermal sensors for chemical and physical variables and producing more than one type of sensor for one or more significant parameters that need to be monitored