# Novel Molecular Non-Volatile Memory: Application of Redox-Active Molecules
## 2 Molecular Logic Switching Devices  
Two fundamentally different approaches to molecular electronics are graphically termed \"top-down\" and \"bottom-up\". Instead of top-down, which includes making nano-scale structures by machining and etching techniques, molecular electronics rely on the \"bottom-up\" approach, by taking advantage of molecule self-assembly. Bottom-up refers to building organic or inorganic structures by atom-by-atom or molecule-by-molecule techniques. In the past few decades, research on molecular electronics has been focusing more and more on the combination of top-down device fabrication (mainly lithography) with bottom-up molecule self-assembly. The crossbar molecular circuit is one of the earliest logic forms of molecular memory arrays. The switching element is a metal/molecule/metal sandwich junction, wherein the molecules are located at the cross-section of two nanoscale metal wires (Figure 1). The early demonstration of such junction molecular switching devices utilized molecules consisting of two mechanically-interlocked rings [12; 13; 14; 15]. The molecular monolayers were deposited as a Langmuir-Blodgett film. The mechanical motion of these molecules is an activated redox process, and two stable and electrochemically-switchable states of the molecules form the logic on and off states. As shown in Figure 1, the molecular switching devices can exhibit a \(\sim\)10\({}^{2}\) on/off current ratio, but with limited endurance cycles. Based on such switching devices, an 8 \(\times\) 8 crossbar has been built by the Hewlett-Packard research group [16]. This approach has the advantage of architectural simplicity and the potential of high density via fabrication of highly dense nanowires. However, this approach has two major disadvantages, including a high rate of defective switching elements and the difficulty in controlling the metal/molecule interface. Even though a defect-tolerant architecture had been established earlier, the metal/molecule problems in such junction switching devices and crossbar circuits are still to be addressed [17]. In later publications, it was reported that the electron transport phenomena in the metal/molecule/metal junction were molecule independent; instead, they were dominated by electrode reactions with molecules, and the intrinsic spacing of the energy levels might be modified [18; 19; 20]. Another architecture utilizing metal/molecule/metal junction is the so-called nanocell molecular circuit, proposed by researchers at Rice University and Yale University [21; 22; 23; 24]. A nanocell is a two-dimensional network of self-assembled metallic particles connected by molecules that show reprogrammable negative differential resistance (illustrated in Figure 2) [21; 22]. The nanocell is surrounded by a small number of lithographically-defined metal lines that provide input and output leads. The active component for a nanocell is also a metal/molecule/metal switch, and one pathway may include more than one molecular switch [25; 26; 27]. The molecule pathways can be learned first by developing appropriate computation algorithms; then, the whole cell can be programmed to perform a particular function. Such a nanocell molecular circuit uses similar molecular switches as the crossbar molecular circuit, but the circuit architecture is quite different. Compared to the crossbar approach, the nanocell approach has the advantages of large-scale circuit fabrication and integration. However, the disadvantage is that it relies on complex programming algorithms. Meanwhile, both the nanocell and crossbar approaches may have the same difficulty in controlling the metal/molecule interface. In addition to the metal/molecule/metal switching devices, molecules have also been tested as the channel material in a standard MOSFET structure [28, 29, 30, 31].