The study of the reduction size effects on the material properties of nanostructured systems is fundamental for understanding physical phenomena otherwise not observed in bulk materials.
In particular, below a critical size, nanostructured materials can present phases that are metastable in macroscopic crystals or can even crystallize as new polimorphs, with properties different from the usual ones.
Pertanto, la scoperta, la caratterizzazione e l'ingegnerizzazione di tali fasi possono aprire la strada a nuove tipologie di dispositivi, con prospettiva di utilizzo dei materiali nanostrutturati a livello industriale.
Hence, the discover, charcterization and engineering of these new phases are promising to pave the way for the production of a new class of devices.
Our work [1] shows that the combined use of the HAADF-STEM (High Angle Annular dark Field- Scanning Transmission Electron Microscopy) technique and appropriate image contrast simulations is able to identify the metastable phases in the GeSbTe (GST) nanowires. GST is usually employed to fabricate phase change memory cells [2], candidate to replace FLASH memories in the near future.
Our study concerned the Ge1Sb2Te4 and Ge2Sb2Te5 compounds, the most widely employed alloys for memory devices.
According to the literature, these alloys have a rhomboedrical structure where Sb and Ge atoms occupy different sites. On the contrary, our work demonstrates that, in the nanowires, the Sb and Ge atoms indifferently fill these lattice sites resulting in a disorderd alloy while keeping unchanged its phase change properties [3].
This is a quite unexpected result as the disordered configuration is unstable, being energetically unfavourable, and it has never been observed in bulk GST.
The study of GST and IST (InSbTe) nanowires and their applications is done within SYNAPSE (Synthesis and functionality of chalcogenide Nanostructures for Phase change memories), FP7 Collaborative Project coordinated by M Longo, MDM Lab, Agrate Brianza
http://synapse.mdm.imm.cnr.it/
1- E. Rotunno, L. Lazzarini, M. Longo, V. Grillo. Crystal structure assessment of Ge-Sb-Te phase change nanowires, Nanoscale, 5, 1557, (2013)
2- P.H.S. Wong, S. Raoux, SB Kim, J. Liang, J.P. Reifenberg, B. Rajendran, M. Asheghi, K.E. Goodson. Phase Change Memory. IEEE proceedings, 98, 12 (2010)
3- M. Longo, R. Fallica, C. Wiemer, O. Salicio, M. Fanciulli, E. Rotunno, L. Lazzarini. Metal Organic Chemical Vapor Deposition of Phase Change Ge1Sb2Te4 Nanowires. Nanoletters, 12, 1509 (2012)
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