Article: NaxCoO2: A New Opportunity for rewritable Media?
Olivier Schneegans, Alec Moradpour, Oana Dragos, Sylvain Franger, Nita Dragoe, Loreynne Pinsard-Gaudart, Pascal Chrétien, Alexandre Revcolevschi, J. Am.Chem.Soc. (juin 2007)
DOI: 10.1021/ja069158v (http://pubs3.acs.org/acs/journals/d...)
Solid-state redox processes generally result in large, and consequently irreversible, structural modifications. Thus, for example, the generation of silicon oxide from silicon by an oxidative process involves a substantial disorganization of the initial silicon structure. Whenever reversible solid-state redox transformations are needed, a prerequisite towards this objective is the availability of solids exhibiting different stable redox stages associated with minor, namely reversible, structural changes.
We have investigated the Conducting-Probe Atomic Force Microscopy (CP-AFM) mediated surface-modifications that can be controllably reversed. Various conducting solids have previously been investigated by CP-AFM, but they generally involve irreversible surface modifications. Thus, for example, the oxidation of silicon and/or titanium by CP-AFM yields irreversible surface patterning. We decided to investigate mixed-valence cobalt oxide solids, such as NaxCoO2, as candidate materials.
We report in our preliminary results that the surface conductivity of NaxCoO2 can be reversibly modified by CP-AFM. More conducting (or more insulating) areas than the starting surface are obtained as a function of the probe–to-material bias. This electrochemical patterning reflects confined redox reactions of the cobalt oxide lattice, and involves sodium deintercalation (with oxidative bias yielding a more conducting surface) and sodium intercalation processes (with reductive bias yielding a less conducting surface).
These results open the way to reversible nanoscale modifications and might yield new scanning-probe-mediated high-density data storage. Moreover, this method enables three different surface-conductivity values: (1) more conducting, (2) less conducting, (3) the initial unmodified conductivity value. This special performance might allow using a ternary code, in place of the usual binary code for the data storage, and would therefore involve an additional increase in the potential data storage density.
These results involve a cooperative work of three different research groups: Laboratoire de Génie Electrique de Paris (LGEP, UMR 8507, Universités Paris VI and Paris-Sud, Supélec); Laboratoire de Physique des Solides (LPS, UMR 8502, Université Paris-Sud) and l’Institut de Chimie Moléculaire et des Matériaux (ICMMO, UMR 8182, Université Paris-Sud).
A C.N.R.S patent corresponding to this new high-density data storage opportunity (n° 07/03093) has been established on April 24th 2007.
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