Christian G Carvajal

ZnO/Al:ZnO Transparent Resistive Switching Devices Grown by Atomic Layer Deposition for Memristor Applications

ZnO has intrinsic semiconductor conductivity because of an unintentional doping mechanism resulting from the growth process that is mainly attributable to oxygen vacancies (VO) positioned in the bandgap. ZnO has multiple electronic states that depend on the number of vacancies and the charge state of each vacancy. In addition to the individual electron states, the vacancies have different vibrational states. We developed a high-temperature precursor vapor mask technique using Al2O3 to pattern the atomic layer deposition of ZnO and Al:ZnO layers on ZnO-based substrates. This technique was used to create a memristor device based on Al:ZnO thin films having metallic and semiconducting and insulating transport properties ZnO. We demonstrated that adding combination of Al2O3 and TiO2 barrier layers improved the resistive switching behavior. The change in the resistance between the high- and low-resistivity states of the memristor with a combination of Al2O3 and TiO2 was approximately 157%. The devices were exposed to laser light from three different laser diodes. The 450 nm laser diode noticeably affected the combined Al2O3 and TiO2 barrier, creating a high-resistivity state with a 2.9% shift under illumination. The high-resistivity state shift under laser illumination indicates defect shifts and the thermodynamic transition of ZnO defects.

Multilayered Approach for TiO2 Hollow-Shell-Protected SnO2 Nanorod Arrays for Superior Lithium Storage

The ability to control the growth of materials with nanosized precision as well as a complex hollow morphology provides rationale for the study of systems comprising both characteristics. This study explores the design of TiO2 hollow nanotube shells deposited by atomic layer deposition (ALD) on vertically aligned SnO2 nanorods grown using the vapor-liquid-solid technique. The sacrificial template approach in combination with highly conformal coating advantages of ALD resulted in a highly reproducible method to create a large surface area covered by TiO2-protected SnO2 nanorods, which are about 60-100 nm in diameter and approximately 1 μm in length. ZnO was used as a sacrificial layer to create a 30 nm gap between SnO2 nanorods and 10 nm of TiO2 shells. Chemical etching of the sacrificial layer was used to create the desired hollow nanocomposite. A coin half-cell battery has been assembled using the TiO2-protected SnO2 nanorods as an anode electrode and lithium foil as a counter electrode and tested for lithium storage during 70 cycles of charge/discharge in a range of 0.5-2.5 V. The TiO2 hollow shell functioned as a good and robust enhancer for both absolute capacity and current rate capabilities of vertically aligned SnO2 nanorods; an improvement in cyclic stability was also observed. This advanced self-standing hollow configuration provides several unique advantages for energy storage device applications including enhanced lithiation for superior energy storage performance.