ABSTRACT
As the electronics industry is integrating more and more new molecules to utilize them in logic circuits and memories to achieve ultra-high efficiency and device density, many organic structures emerged as promising candidates either in conjunction with or as an alternative to conventional semiconducting materials such as but not limited to silicon. Owing to rotaxane's mechanically interlocked molecular structure consisting of a dumbbell-shaped molecule threaded through a macrocycle, they could be excellent nanomachines in molecular switches and memory applications. As a nanomachine, the macrocycle of rotaxane can move reversibly between two stations along its axis under external stimuli, resulting in two stable molecular configurations known as "ON" and "OFF" states of the controllable switch with distinct resistance. There are excellent reports on rotaxane's structure, properties, and function relationship and its application to molecular electronics (Ogino, et al., 1984; Wu, et al., 1991; Bissell, et al., 1994; Collier, et al., 1999; Pease, et al., 2001; Chen, et al., 2003; Green, et al., 2007; Jia, et al., 2016). This comprehensive review summarizes [2]rotaxane and its application to molecular electronics. This review sorts the major research work into a multi-level pyramid structure and presents the challenges of [2]rotaxane's application to molecular electronics at three levels in developing molecular circuits and systems. First, we investigate [2]rotaxane's electrical characteristics with different driving methods and discuss the design considerations and roles based on voltage-driven [2]rotaxane switches that promise the best performance and compatibility with existing solid-state circuits. Second, we examine the solutions for integrating [2]rotaxane molecules into circuits and the limitations learned from these devices keep [2]rotaxane active as a molecular switch. Finally, applying a sandwiched crossbar structure and architecture to [2]rotaxane circuits reduces the fabrication difficulty and extends the possibility of reprogrammable [2]rotaxane arrays, especially at a system level, which eventually promotes the further realization of [2]rotaxane circuits.
ABSTRACT
As VLSI technology is shifting from microelectronics to nanoelectronics era, bi-stable [2]rotaxane emerges as a promising candidate for molecular electronics. A typical voltage-driven [2]rotaxane consists of a cyclobis-(paraquat-p-phenylene) macrocycle encircling a dumbbell shape molecular chain and moving between two stations on the chain: tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP). As a molecular switch, the macrocycle can move reversibly between two stations along its axis with appropriate driving voltage, resulting in two stable molecular conformational states with distinct high and low resistance. This makes it a well-suited candidate to represent binary states ("0" and "1") for digital electronics. In this work, we performed molecular simulation to investigate the switching mechanism of [2]rotaxane molecule. We used distance and angle variables to characterize the movement of the macrocycle along the chain, and compared the switching behavior of [2]rotaxane in water, ethanol, dimethyl ether and vacuum. The results show that the solvent environment plays an important role in the switching characteristics of [2]rotaxane molecule. The switching of [2]rotaxane is stable, controllable, reversible and repeatable. We also looked into potential failure mechanism of the [2]rotaxane, which could shed light on the fault model, testing and reliability enhancement of [2]rotaxane based molecular electronics. Our simulation results support that [2]rotaxane molecules possess potential to be used for molecular memory and logic applications.
