Molecular crossbar circuit

Fig. 1.16 (a) The demonstration of point addressablity within a 64-bit molecular switch crossbar circuit utilized as a molecular RAM. This... 

Fig. 1.18 A SEM image composed of a nanometer-scale molecular switch crossbar circuit.

FIGURE 1 A series of 100-element crossbar circuits, the prominent circuits of molecular electronics. The molecule of interest is typically sandwiched between the intersection of two crossed wires. This very simple circuit can be used even in the presence of manufacturing defects and can be fabricated at dimensions that far exceed the best lithographic methods. The device densities in these circuits approach lO /cm for the smallest crossbars. 

Single-walled carbon nanotubes (SWNTs) had been considered for the crossbar components of the defect-tolerant molecular computers but they have been found to be too difficult to handle due to their insolubility and their tendency to form bundles or ropes. Instead, metallic nanowires have become the materials of choice used in the construction of the crossbar devices, with ultrahigh-density lattices and circuits being built, having groups of nanowires 8 nm in diameter and 16 nm apart in layers perpendicular to each other to create nanowire junction densities of 1011 per cm2.52 The process does not depend on self-assembly but rather on molecular beam epitaxy. 

A crossbar array based scheme is at the moment perceived as the most realistic strategy to combine nano-scale components with CMOS circuits [67], where a concept for neuromorphic networks based on molecular singleelectron transistors [68] has also been proposed. 

DNA self-assembly could be used in a variety of ways to solve this problem molecular components (e.g., AND, OR, and NOT gates, crossbars, routing elements) could be chemically attached to DNA tiles at specific chemical moieties, and subsequent self-assembly would proceed to place the tiles (and hence circuit elements) into the appropriate locations. Alternatively, DNA tiles with attachment moieties could self-assemble into the desired pattern, and subsequent chemical processing would create functional devices at the positions specified by the DNA tiles. None of these approaches has yet been convincingly demonstrated, but it is plausible that any of them could eventually succeed to produce two- or three-dimensional circuits with nanometer resolution and precise control of chemical structure. 

Additionally, if SWNTs are to be used as the crossbars, connection of molecular switches via covalent bonds introduces sp -hybridized carbon atom linkages at each junction, disturbing the electronic nature of the SWNT and possibly rendering useless the SWNTs in the first place. Non-covalent bonding of the device molecule to the SWNT will probably not provide the conductance necessary for the circuit to operate. Therefore, continued work is being done to devise and construct crossbar architectures that address these challenges. 

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