Oxide electronics: Zinc oxide nanostructured LEDs and tantalum oxide nonvolatile memoriesPublished on July 18th, 2011 | By: Eileen De Guire
Just when I thought I could stroll back into the macro world, two new papers were published that complement our recent posts on PCMMs, supercapacitors, nanoporous materials and other nanostructured materials.
First, out of MIT comes “Face-selective electrostatic control of hydrothermal zinc oxide nanowire synthesis,” by Jaebum Joo, et. al. (see Nature Materials, doi:10.1038/nmat3069). Using hydrothermal synthesis, the group grew zinc oxide nanowires with controlled morphologies and functional properties. Morphologies synthesized ranged from platelets to needles with aspect ratios that spanned three orders of magnitude (~0.1-100 are reported).
The article abstract says a classical thermodynamic model was used to explain the growth inhibition mechanism “by means of the competitive and face-selective electrostatic adsorption on non-zinc complex ions at alkaline conditions.” An online story from MITnews clarifies that “the key turns out to be the electrostatic properties of the zinc oxide material as it grows from a solution.” When ions from other compounds are added to the solution (from which the ZnO is grown hydrothermally), they attach electrostatically and preferentially to the wire at only the sides or the ends, which inhibits growth in those directions (i.e., face-selective). The hydrothermal synthesis process temperature was less than 60°C, which opens up the possibility of manufacturing devices on or in polymers and plastics.
The team fabricated a functional LED array of ZnO nanowires, but ZnO also can be used in battery, sensor and other optical applications. In the MIT story Joo says this method and the ability to use it to control morpholgy could be applied to other materials, for example, titanium dioxide, a possible solar cell material. Joo also says the successful use of hydrothermal synthesis to manipulate nanostructure has “the potential for large-scale manufacturing.” (Perhaps a candidate technology for Obama’s recently announced Advanced Manufacturing Partnership?)
The second paper, published in the same issue of Nature Materials, is from a group at Samsung Electronics in Korea. In an interesting departure from the more common PCMM chalcogenide approach, they looked at tantalum oxide-based bilayer structures for nonvolatile memory devices. (See “A fast, high-endurance and scalable non-volatile memory device made from asymmetric Ta2O5-x/TaO2-x bilayer structures,” Nature Materials, doi:10.1038/nmat3070, by Myoung-Jae Lee, et. al.)
(Quick note: Based on the abstract and online images, this appears to be a material property- and performance-oriented paper, and it is not known whether the material is nanostructured.)
Like others researching nonvolatile memory materials, they are looking for “a material or device structure that satisfies high-density, switching-speed, endurance, retention and most importantly power — consumption criteria” (from the abstract). The paper describes an asymmetric passive switching device with an impressive cycling endurance of over 1012 and switching times of 10 ns. They were able to demonstrate a significant reduction of switching current and, therefore, power consumption.
The paper’s abstract postulates that there may be another benefit: “[B]y combining two such devices, each with an intrinsic Schottky barrier, we eliminate any need for a discrete transistor or diode in solving issues of stray leakage current paths in high-density crossbar arrays.”
Samsung’s published interest in nonvolatile memory materials combined with IBM’s recent proof-of-concept chalcogenide device seems to comprise pretty strong evidence that the leap from lab to prototype is underway, and Si-based flash memory may soon be just that, a memory.
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