# Novel Molecular Non-Volatile Memory: Application of Redox-Active Molecules
## 5 Nanowire/Nanotube-Based Molecular Memory  
Semiconductor nanowire and nanotube MOSFETs have been regarded as the building blocks for nanoelectronics devices and circuits [48; 49; 50]. Compared to planar devices based on bulk materials, the nanowires have smaller channels and a larger surface-to-volume ratio. Thus, they require less stored charges to induce the same memory window for the memory application. Moreover, the nanowire or nanotube channel can enable a gate-surrounding structure, allowing excellent electrostatic gate control. Integrating redox-active molecules with semiconductor nanowire or nanotube FETs for solid-state flash memory will not only improve the memory performance by utilizing the advantageous properties of redox-active molecules, but also will provide an efficient platform to study the redox-active molecules with microelectronics characterization metrologies. Figure 8: (a) Molecule structure of \(\alpha\)-ferrocenylethanol and a schematic of the Metal-Al\({}_{2}\)O\({}_{3}\)-Ferrocene-Oxide-Silicon (MAFOS) capacitor memory structure; (b) \(\Delta\)V\({}_{\text{FB}}\) of the molecular memory and control samples as a function of program/erase (P/-E) voltage; (c) charge retention at room temperature; (d) device endurance with different P/Epulse widths. Reproduced with permission from [47], © AIP Publishing LLC, 2013. Figure 9: Nanowire-based non-volatile memory, with the nanowire surface functionalized with redox-active molecules. Logic on and off states are represented by different charged states. Reproduced with permission from [51], © American Chemical Society, 2002. In 2002, the researchers from Harvard University reported a molecular non-volatile memory based on a back-gated nanowire FET with the nanowire surface functionalized with cobalt phthalocyanine (CoPc) redox-active molecules (Figure 9) [51]. By modulating the channel conductance with back gate voltage, reversible switching between on and off states has been achieved, with the on/off ratio exceeding 10\({}^{4}\). The retention (>20 min) and endurance (>100 cycles) of the molecular memory device have outperformed the earlier junction molecular switching devices. Integrated arrays of independently-addressable molecular nanowire memory have also been characterized, which demonstrated another advantage of implementing nanowire structure memory in programmable logic arrays. The memory performance has been further improved with molecule and device engineering. In 2004, the University of Southern California research groups reported a back-gate nanowire FET-based molecular memory, which utilized In\({}_{2}\)O\({}_{3}\) nanowires and a family of bis(terpyridine)-Fe(II) molecules for charge storage (Figure 10) [52; 53]. It was demonstrated that bis(terpyridine)-Fe(II) with a larger or more complex linkage component showed better retention, due to the increase in the tunneling barrier. In addition, by applying gate voltage pulses with different amplitudes, multilevels in memory cells can be illustrated by altering the population of the reduced/oxidized molecules [53]. However, such multilevel memory realization depends on the gate voltage applied to oxidize/reduce a portion of molecules in SAM. Further cell scaling or increasing the number of memory levels will be extremely difficult, as the complicated top-down device fabrication and device structure will inevitably lead to significant device variation and a blurring of the multiple storage states. In addition, the demonstrated device performance is still far below the standards for on-chip memory applications. This is partly because of the back gate device structure, unclean steps in the nanowire FET device fabrication process and, most importantly, the absence of protection for redox-active molecules.