# Novel Molecular Non-Volatile Memory: Application of Redox-Active Molecules
## 3 Redox-Active Molecule-Based Non-Volatile Memory  
Appropriate modifications of the molecular structures and switching elements have been designed to change the switching kinetics and to enhance the performance for different memory applications. Different device platforms have been engineered to interface with the molecules, such as dielectrics, oxides, nanowires, carbon nanotubes, and so forth. A well-known method to achieve a memory effect is by employing redox-active elements. The redox mechanism involves electrons being transferred from one element to another, such that a donor element gives up an electron (oxidation) to an accepting element (reduction). This process usually requires an external stimulus for redox reactions, such as an electric field or change in temperature. Such redox-active molecule-based memory devices can show fast speed, low operation voltage and high reliability, due to the intrinsic redox behavior of the molecules. Therefore, it is very attractive for DRAM and flash memory device applications. The research group at North Carolina State University proposed hybrid CMOS/molecular memory devices, in which the redox-active charge-storage molecules were incorporated into Si structures to generate a new class of electronic memory devices [32; 33; 34; 35; 36]. These redox-active molecules, which can be designed to self-assemble on surfaces as monolayers, exhibit charge-storage states at discrete voltage levels. This approach can provide a smooth transition from CMOS technology to  
Figure 3: Structure and electrical characteristics of a molecular FET. (**a**) Structure of (18-phenoxyocytadecyll)trichlorosilane (PhO-OTS); (**b**) structure of the organic semiconductor pentacene; (**c**) cross-section of a pentacene organic field-effect transistor (OFET); (**d**) output and transfer characteristics, with subthreshold region showing a subthreshold swing of 100 mV per decade [31]. Reproduced with permission from [31], © Nature Publishing Group, 2004.  
molecular electronics technology by integrating the vast infrastructure of developed CMOS technology with inherent molecular properties. This approach focuses on developing hybrid molecular non-volatile memory with redox-active molecules as the active charge-storage medium. The redox-active molecule used in such molecular memory usually consists of a redox-active component, a linkage component and surface attachment groups. Figure 4 shows two examples of such molecules with different redox-active components. Physically, the redox-active component acts as the charge-storage center, with both the linkage and the surface group acting as the insulator. From this standpoint, different redox-active component can be designed or synthesized for multiple redox states, thus for complex and high memory density. The linkage component can be engineered to tune the memory retention properties, while the surface attachment groups can be specifically designed for attachment on different surfaces via covalent bonds. The redox component, such as the Fe in 4-ferrocenylbenzyl alcohol (Fc-BzOH) and Zn in 5-(4-hydroxymethylphenyl)-10,15,20-trimesitylporphinatozinc(II) (Por-BzOH) (shown in Figure 4), is the charge-storage medium, which can be in neutral and positively-charged states through losing electrons. The molecular components surrounding the redox center act as the barrier against charge loss. The electrons tunnel through the barrier during oxidation and reduction processes. The electrolyte/molecule interface acts as another barrier between the molecule and top gate electrode, which blocks the electron tunnel into the molecule from the electrolyte to reduce an oxidized molecule. The positive charges in the molecule can be reduced at a lower gate potential, wherein an electron tunnels back to the molecule from the Si substrate. The SAM attachment of redox molecules to Si, SiO\({}_{2}\) or other surfaces was typically carried out by using chemical solution deposition [35; 36; 37].