Towards molecular bits: molecules that can represent binary information

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When the first commercially successful microprocessor was launched in 1974 it contained roughly 6000 transistors on one silicon chip, each transistor representing one bit of binary information. Thirty-two years later, the Pentium 4 has 42 million transistors on a chip, giving an idea of how miniaturisation has progressed in that time. However, transistors are not going to become very much smaller because of two problems. Firstly, as the size of a transistor decreases, it becomes more difficult to make it turn on and off cleanly (a transistor is just a switch that uses on and off states to represent 0 and 1). Secondly, transistors generate heat, and transistor densities are becoming so high that removing that heat and stopping the chips from melting is a real challenge.The aim of this proposed research is to synthesise some chemical compounds that could be used to replace transistors in electronic devices, using a new way of representing and manipulating binary information called the Quantum-dot Cellular Automata (QCA) paradigm. A QCA cell consists of four metallic dots arranged in a square, each dot being able to accommodate an electrical charge. Electrically charging the dots at two of the opposite corners of a cell can be used to encode 0, while charging the other two dots represents 1. Where more than one QCA cell is present, the arrangement of the charged dots within one cell affects that in neighbouring cells. Because like charges repel each other, dots that are next to each other tend to have opposite charges, which means that where two cells are side-by-side the second will have the same value (0 or 1) as the first. Conversely, two cells sharing a corner will adopt opposite values. This second kind of arrangement, where a value of the input cell of 1 is converted to an output cell value of 0, is equivalent to a NOT gate in conventional electronics. Other kinds of logic gate can be mimicked by other arrangements of cells, and so the correct arrangement of cells could function as a computer.It has been shown that an arrangement of QCA cells with metallic nanoparticles about 50 nm in size at the corners can work as a logic gate at milliKelvin temperatures, and whilst this is too cold for general purpose computing it is predicted that if the metallic components can be shrunk to about 2 nm then they will work at room temperature. Two nanometres is about the same size as a single molecule, so if we can build a molecule that contains an array of four centres, each of which can be switched 'on' or 'off' as described above, then we might be able to use them in computing.In chemical terms, switching each 'dot' on or off is simply a redox process - adding or removing an electron - so each corner needs to be made from something that is stable in more than one oxidation state. What is proposed is to join four such entities together with some chemical scaffolding to form molecular QCA cells, and then investigate their behaviour to see whether they would work as desired.