08 septembre 2009

Model of a ferrocene molecule anchored on a silicon surface via a (a) short or (b) long spacer

The immobilization of redox compounds – which are able to gain or lose electrons – on silicon paves the way for the development of new hybrid memories. The SCIB, in collaboration with the LETI, showed that by increasing the length of the link between the redox centre and the silicon, the kinetic transfer charge is lowered. This effect could lead to devices with higher charge retention time.


In molecular hybrid memories, information is stored as a charge trapped on the redox compound that is grafted to the silicon device. As with other memories, the writing speed and charge retention are two critical parameters. For molecular memories relying on the redox states of the compound, these parameters are conflicting. Writing the information -- by transferring charges from the silicon to the redox centre – requires good communication between the surface and the molecule. In order to increase the retention time, the redox center must be as isolated as possible from the surface. This specificity of molecules to communicate with silicon can be modulated by the nature of the molecule itself, and also its environment.


In order to act upon these two criteria, different spacers – alkyl chains with different lengths -- have been used to establish functions on the silicon with different molecules: metalloporphyrins (molecules found in nature and also synthesized in the laboratory) and ferrocene. The charge transfers between the surface and the molecules have been identified by electrochemistry. These studies showed that the length of the spacer has a profound effect on electronic communication.  A short spacer favours the overlap of electronic orbitals of the molecule and the silicon, thereby leading to a lower charge retention time. On the contrary, a long spacer isolates the redox centre from the surface and slows the charge migration. These results will help us to identify the spacer length which will allow reaching equilibrium between writing speed and charge retention, and that is therefore best suited for the targeted memory application.


Further reading: K. Huang, et al., Chemphyschem 10 (2009) 963


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