AVS2012 Session TF+EM+SE+NS-ThM: Nanostructuring Thin Films

In this presentation, several examples of uniquely plasma-enabled nanostructuring of thin film materials for applications in energy conversion and storage, environmental monitoring, and bio-sensing. Strong emphasis is made on atom-, energy-efficiency, and environment-friendliness of plasma-based nanotechnologies.

1. Introduction: Atom- and energy-efficient nanotechnology is the ultimate Grand Challenge for basic energy sciences as has recently been road-mapped by the US Department of Energy. This ability will lead to the energy- and matter-efficient production of functional nanomaterials and devices for a vast range of applications in energy, environmental and health sectors that are critical for a sustainable future. Here we present examples related to atom- and energy-efficient nanoscale synthesis of advanced nanomaterials for energy conversion and storage, environmental sensing, and also discuss effective cancer cell treatment using low-temperature plasmas.

2. Atom- and energy-efficient nanostructure production for energy storage: Here we show an example of a recent achievement of a very low amount of energy per atom (~100 eV/atom) in the synthesis of MoO₃ nanostructures for energy storage (e.g., Li-ion battery) applications. This was achieved by using time-programmed nanosecond repetitive spark in open air between Mo electrodes. Highly-controlled dosing of Mo and O atoms was achieved through the controlled evaporation and dissociation reactions and maintaining reactive chemistry in air. These nanomaterials show excellent electrochemical and energy storage performance.

3. Environment-friendly, single-step solar cell production: Highly-efficient (conversion efficiency 11.9%, fill factor 70 %) solar cells based on the vertically-aligned single-crystalline nanostructures have been produced without any pre-fabricated p-n junctions in a very simple, single-step process of Si nanoarray formation by etching p-type Si wafers in low-temperature environment-friendly plasmas of argon and hydrogen mixtures. The details of this process and the role of the plasma are discussed.

4. Metal-nanotube/graphene environmental and bio-sensors: Plasma processing was successfully applied for the fabrication of hybrid nanomaterials based on metal-decorated carbon nanotubes and vertically aligned graphenes. The applications of these structures in environmental (gas) and bio-sensing (SERC/plasmonic) platforms are presented. The vertically-aligned graphene structures have been grown without catalyst and any external substrate heating, owing to the unique plasma properties.

Nanometer-scale, vapor-deposited multilayers are an ideal class of materials for systematic, detailed investigations of reactive properties. Created in a pristine vacuum environment by sputter deposition, these high-purity materials have well-defined reactant layer thicknesses between 1 and 1000 nm, minimal void density and intimate contact between layers. If designed appropriately, these energetic materials can be ignited at a single point and exhibit a subsequent, high-temperature, self-propagating formation reaction. The nanometer-scale periodicity set through design tailors the effective diffusion length of the subsequent self-propagating reaction.

With this presentation, we describe effects of the nanometer-scale, multilayer periodicity on i) the reactivity of multilayers in different surrounding gaseous environments and ii) the reaction front morphology as viewed in the plane of the multilayer. We show that nickel/titanium and titanium/boron multilayers are affected by the surrounding gaseous environment, and describe how the magnitude of average propagation speed depends on multilayer periodicity. Fine multilayer designs are characterized by fast reaction waves, and there is no difference in average propagation speed when reacted in air (atm. pressure) versus vacuum (1 mTorr). Coarse multilayer designs are generally slower and are affected by secondary oxidation reactions when conducted in air. These thick multilayer designs are affected by the pressure of the surrounding gaseous environment with enhanced propagation speeds owing to the highly exothermic reaction of Ti with O. Regarding the effects of nanometer-scale multilayer periodicity on reaction front morphology, we show that reactive multilayers often have a smooth reaction front when layer periodicity is small. However, multilayers having larger periodicity (and hence larger effective diffusion lengths) exhibit reaction front instabilities and complex reaction front morphologies.

In this talk, we also stress how the propensity to oxidize and the propensity to form reaction front instabilities (as affected through nanometer-scale design) impact final properties of the multilayers for applications such as localized joining.

Sandia is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.