Molecular Logic Gates Electronic Conductance

The molecular logic gates listed so far were investigated in solutions. The major challenge is the identification of methods to incorporate these functional molecules into solid-state devices while maintaining their signal transduction abilities. SAMs are the most promising for the ordering of wires and devices on the Au surfaces, with rigid rod systems standing nearly perpendicular to the surface, the thiol groups dominating the adsorption sites on the Au (Tour 2000).

Electronically configurable molecular-based logic gates have been fabricated from an array of configurable switches, each consisting of a monolayer of redox-active rotaxanes sandwiched between metal electrodes. The redox properties of these molecular compounds, as measured in solution, do translate well into solidstate device properties. A single monolayer of the catenane anchored with amphiphilic phospholipids counterions and sandwiched between an n-type polycrystalline Si (poly-Si) electrode and a Ti/Al top electrode can act as a switching device. This device exhibits hysteretic (bistable) current/voltage characteristics. The switch can be opened at 12 V, closed at 22 V, and read between 0.1 and 0.3 V and may be recycled many times. The switches were read by monitoring current flow at reducing voltages in air and at room temperature. The switches were irreversibly opened by applying an oxidizing voltage across the device. Once the conditions for addressing and configuring the individual devices were determined, linear arrays of devices into AND and OR wired logic gates were configured. The truth table of any AND gate is such that a high response is only recorded when both inputs are high. The high and low current levels of those gates were separated by factors of 15 and 30, respectively, which is a significant enhancement over that expected for wired logic gates (Collier 1999).

The switching mechanism is illustrated in Fig. 12.14. The ground state "co-conformer" [Ao] of this catenane has the TTF unit located inside the cyclophane. Upon oxidation, the TTF unit becomes positively charged, and the Coulombic repulsion between TTF1 and the tetracationic cyclophane causes the crown ether to circumrotate to give co-conformer [B1], which will reduce back to [Bo] when the bias is returned to 0 V. This bistability is the basis of this device. The energy gap between the highest occupied and lowest unoccupied molecular orbitals for co-conformer [Bo] must be narrower in energy than the corresponding gap in co-conformer [Ao], implying that, in a solid state device, tunneling current through the junction containing [Bo] will be greater.

Thus, this co-conformer represents the "switch closed" state, and the ground state co-conformer [Ao] represents the "switch open" state (Collier 2000). A molecule in two-dimensional molecular electronics circuits is shown in the Fig. 12.15. Addressing an array of bistable [2]rotaxanes through a two dimensional crossbar arrangement provides the device element of a current-driven molecular electronic circuit. The development of the [2]rotaxane switches through an iterative, evolutionary process is described in (Luo 2002). The arrangement reported here allows both memory and logic functions to use the same elements (Luo 2002).

Fig. 12.14. Switching mechanism of a rotaxane molecule. A system that was made even more complicated by integrating the switchable molecules in two-dimensional molecular electronics circuits shown in the Fig. 12.15

Fig. 12.15. A simple yet versatile circuit architecture, known as a crossbar, is shown at increasing levels of complexity in terms of both fabrication and function. Each junction is a switching device, with black arrows corresponding to open switches and red arrows to closed switches. Wires that are utilised to address or modify the switches are highlighted in red, except for the case of the 2D Logic Circuit in which the configuration of the circuit (where the junctions are both switches and diodes) has been indicated.

Fig. 12.15. A simple yet versatile circuit architecture, known as a crossbar, is shown at increasing levels of complexity in terms of both fabrication and function. Each junction is a switching device, with black arrows corresponding to open switches and red arrows to closed switches. Wires that are utilised to address or modify the switches are highlighted in red, except for the case of the 2D Logic Circuit in which the configuration of the circuit (where the junctions are both switches and diodes) has been indicated.

Another electron-conductance based logical gates are based on polyacetylene that consists of a chain of carbon atoms held together by alternating double and single bonds through which electrons are thought to travel in small packets called solitons. Hence, different structures of polyacetylene can be used as soliton switches. Soliton switches can be used much like relays and combined to perform the simple logic functions required to form Boolean gates. (Groves 1995).

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