# Novel Molecular Non-Volatile Memory: Application of Redox-Active Molecules
## 5 Nanowire/Nanotube-Based Molecular Memory
Figure 10: Comparison of designs and operations between (**a**) conventional silicon flash memory and (**b**) molecular memory for multilevel nonvolatile data storage; (**c**) comparison of multilevels in a two-bit silicon flash memory and molecular memory; (**d**) molecular structure of Fe\({}^{2+}\)-terpyridine compound. Reproduced with permission from [53; 54], © AIP Publishing LLC, 2004,2007. Recently, the NIST researchers have extended their work on the capacitor memory cell to a novel top-gated nanowire molecular flash memory based on Si nanowire FETs [47; 55]. The memory cell consists of a self-aligned top-gated Si nanowire FET with \(\alpha\)-ferrocenylethanol redox-active molecules attached to a Si nanowire surface (Figure 11). The molecules are effectively protected by engineered top gate dielectrics. The injected charges are mainly located in the redox centers of molecules, with trace amount stored in the molecule/Al\({}_{2}\)O\({}_{3}\) interface traps. The molecular flash memory cell exhibited fast P/E speed and excellent P/E endurance (>10\({}^{9}\) P/E cycles). These characteristics are attributed to both the intrinsic charging properties of redox-active molecules and the protection of the molecules obtained by the self-alignment fabrication enabling clean solid-molecule and dielectric interfaces. However, similar to their previous capacitor molecular memory, only \(\sim\)20% of the molecules were effectively involved in the redox process. Further research efforts are required to increase the redox efficiency so as to lower the operation voltage and improve the operation speed. The memory density can be further enhanced by using redox molecules with multiple redox centers. Such a multi-bit memory concept is more reasonable and feasible than that by just modulating the voltage level, as the physically discrete charged states of the molecule can enable precise controlling of the charged states. In addition to the traditional redox-active molecules, novel molecules and clusters have been synthesized and implemented in molecular memory. In 2014, [PERSON] _et al._ reported a study on molecular nanowire flash memory using core-shell polyoxometalate (POM) molecules for charge storage (Figure 12) [56]. The POM molecule functions as the switching component in the memory associated with the oxidation of selenite template at the cluster core and the reduction of the metal oxide cluster cage. The unique oxidation process of the selenite template at the cluster core was observed only after the initial excitation and cannot be recreated with consecutive operations, leading to a new type of \"write-once-erase\" memory. The flash memory with a 4-nm Si nanowire channel and POM for charge storage has shown excellent retention properties with no charge store decay after 336 h. With the increasing oxidation state of POM molecules, the on/off ratio can reach as high as 10\({}^{11}\)[56]. Even though the operation voltage and the switching time of such molecular memory are still quite large, further engineering to shorten the distance from the control gate to the nanowire channel  
Figure 11: **(Left)** TEM image of the cross-section of a molecular flash memory device; **(Right)** discrete charging behavior of the Ru\({}_{2}\) molecular flash memory and the endurance properties. Reproduced with permission from [55], © American Chemical Society, 2015.  
and to reduce the number of POMs can be expected to effectively lower the voltage and the write time into the sub-picosecond region. As illustrated by the above experimental work, the performance of molecular flash memory depends on not only the redox behavior of the molecules, but also the preparation of the transistor platform and the integration of the molecules in the memory structure. While the memory endurance is mainly determined by the mechanism of the charge storage, further improving the memory performance, such as the memory density, speed and power, requires optimized engineering on the transistor platform structure and an enhanced molecule/semiconductor interface.