Session J2: Thin Film Growth

Unique PLD Production of Two Pure Titania Phases on Sapphire Substrates

Two pure tetragonal phases of titanium dioxide – anatase and rutile – were grown on c-cut sapphire substrates via pulsed-laser deposition by changing only the growth and annealing conditions such as ambient gas pressure, substrate temperature and laser pulse repetition rate, and without changing the substrate, target, or working gas. The production of multiple phases of a material using a single target and a single substrate is critical since changing either the target or the substrate can obfuscate correlations between growth conditions and the performance of the resulting film. Further, there is no prior report of the production of anatase titania on sapphire; in fact, the review presented by Janisch et al. predicted that anatase-on-sapphire was "impossible." For applications in which titania is used for its optical properties, sapphire substrates may be preferential, as they may significantly decrease the cost of production and increase the survivability of titania films – assuming the induced strain does not affect the final performance of the system.   

Polymer-assisted deposition epitaxial Li(Ni,Co,Mn)O2 thin films

Rechargeable lithium-ion batteries are widely used in mobile devices, hybrid, plug-in hybrid, and electric vehicles. The performance of batteries strongly depends on the structure, morphology, and properties of electrode materials. A great effort has been made to synthesize a variety of electrode materials and to understand the role of the electronic structure of redox active materials to improve the energy density, rate capability, and cycling stability. It is generally considered that positive electrode determines the specific capacity and the energy density of batteries. Li(Ni,Co,Mn)O$_{\mathrm{2}}$, a layered material, has gained considerable attention as the positive electrode due to its high specific capacity and thermal stability. To understand the nature of the electrochemical reaction, it is expected that single crystal-like electrode materials may offer better understanding of its effects on crystallographic orientation on the electrochemical properties. To this end, epitaxial Li(Ni,Co,Mn)O$_{\mathrm{2}}$ thin films of different orientations have been successfully grown on SrTiO$_{\mathrm{3}}$ substrates from a solution method, called polymer-assisted deposition. The films have been characterized by x-ray diffraction, atomic force microscope, and cross-section high resolution transmission electron microscope.   

Stability of Block Copolymers Thin Films During Direct Immersion Annealing

Nano architectures in ordered block copolymers (BCP) thin films have inspired a variety of new applications. For example, the uniformly sized and shaped nano domains formed in the films have been used for nanolithography and high-density information storage media. Direct Immersion Annealing (DIA) is a novel and robust annealing technique, that has been shown to enhance the quality and rate of ordering in BCP thin films overcoming the major drawbacks of thermal annealing. As for now, DIA results have been reported for substrates with relatively high surface energy and there have been no studies performed concerning the effects of surface energy on the stability of BCP thin films during DIA. In this study we use a gradient surface energy substrate coat it with a thin film (100 nm) of PS-PMMA and anneal using DIA for varying amounts of time. This allowed us to investigate the role that surface energy plays in the stability of the film during the annealing process. Our results show that the stability of the film during DIA is highly dependent of the surface energy of the substrate. We were also able to determine a critical surface energy transition point which marks the separation from stable to unstable region and which can be correlated to the surface energy of the solvent mixture.   

Bottom-up preparation of nanopore array with self-aligned nanogap electrodes for single biomolecule characterization

Nanopore sensors, an emerging third-generation DNA sequencing technique, exemplify a new strategy in the characterization of biomolecules at single molecule level. In such designs, the dimension of the sensor matches precisely with a single target molecule, so that the presence and/or motion of the molecule inside the sensor can generate measurable time-dependent electrical read-out signals containing significant local structural information. However, existing fabrication processes face great challenges such as the scalability and reproducibility of fabrication, lack of control of translocation, and low specificity in read-out signals. Here we propose a new framework of preparing nanopore device arrays with the additional integration of a pair of embedded nanogap electrodes in a self-aligned manner. Specifically, we will introduce our impedance-based feedback control system for the electrochemical deposition of metal on pre-defined nanoscale electrodes within a confined space to construct sub-10 nm nanopores with gate electrodes. The precise control of the critical dimensions of the gap and its mechanism will be demonstrated and discussed. Our design can provide a promising platform for the scalable preparation of single-molecule characterization devices with active translocation control and recognition tunneling readout signals.   

Metallic substrates for improved electroplating of carbon nanotube structures

Metal electroplating of carbon nanotube (CNT) forests is a possible method for metal MEMS fabrication. Recent work has demonstrated that electroplating an interconnected carbon nanotube structure is possible through direct electrical contact to the CNT structure. However, this is not ideal as in many MEMS designs, features need to be both electrically and mechanically isolated from each other. Our current efforts have been directed toward electroplating CNT patterns through indirect electrical contact. Where indirect contact means electrical contacting to the CNT's through a patterned metallic lead that sits between the substrate and the grown CNT forests. As the CNT forest catalysts layer involves an alumina diffusion barrier and the iron catalyst layer, this is not as simple as it sounds. We will report on the current status and recent results of establishing an electric signal from the metallic leads through to the carbon nanotubes.   

Conformal Coating of Carbon Nanotube Forests

Carbon nanotube forests have a unique vertical growth that has been coupled with materials infiltration techniques including LPCVD, CVD, and electroplating to make a range of structures. Examples include MEMS, chromatography media, and batteries where the novel features include high aspect ratios and infiltration using a wide variety of materials. The CNT template with nanotube spacings on the order of 100 nm and heights up to 250 $\mu $mcreates difficult challenges to uniform coating of the infiltration materials. Atomic Layer Deposition, where diffusion into the forest and the deposition reaction are uncoupled may be the ideal infiltration process in these structures. We will discuss experiments where ALD was used to deposit oxides into CNT forests. Various chemical priming and surface functionalization were explored. This is a work in progress and ideal coating processes have not yet been determined.