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I am a PhD candidate looking into the Switching of Molecular Quantum Dot Cellular Automata for Mixed Valence Molecules, under the supervision of Dr Gerard Edwards and Dr Graham Spink.

Transistors have been continually getting smaller since their invention. This is in part due to the desire to keep up with Moore’s Law, but also because the smaller the transistors, the more can fit on a computer chip. However, we are now approaching an impasse with conventional transistors due to issues like heat dissipation melting the tiny components and leakage current (through quantum mechanical tunnelling) disrupting the binary information. This impasse is where Molecular Quantum Dot Cellular Automata (QCA) can move in and replace the traditional transistor computing paradigm.

Molecular QCA is a nano-computing paradigm first posited by Craig Lent, University of Notre Dame, USA. The idea is that, rather than being held in the current flow through a transistor, the binary information would be held within the charge configuration of a molecule. For example, one of the proposed molecules for this device has three “dots” (charge occupation sites) and a single free charge to occupy them. This means that the molecule has three possible states that it can take, labelled “1”, “0” and “null” (the three states required for computing). The charge in the molecule uses tunnelling to travel between each of the possible dots. The beauty of this idea is that it completely avoids any of the issues with small transistors; the lack of a flowing current in the QCA paradigm means that heat dissipation is significantly decreased, and leakage current is not a problem.


States of a QCA cell


Above: The three states of a QCA cell described in the text: "1" (left), "0" (middle) and "null" (right).

A QCA cell can consist of two molecules, each with three dots and one charge (six dots and two charges total). The idea is to utilise Coulomb repulsion effects between the cells to carry information. When two of these cells are placed next to each other the charges within them will repel each other causing them to align themselves into the same state (“0” or ”1”). This means these cells could replace not just the transistors in a computer but also the wires. QCA can also be used to make the standard logic gates (“AND”, “OR”, “NOT” etc…) as well as a “majority” gate which has three inputs and outputs the majority. The third “null” state of the QCA cell is necessary to allow clocking control. This makes the QCA approach incredibly useful for computing.

The aim of my research is to calculate the switching of QCA to provide further insight into how a device like this would work in a real-life setting. My work builds on previous research by looking into the effect of heat dissipation on the tunnelling effects of the QCA cell in more detail. So far, I have simulated two dot and three dot molecules using different protocols for bit writing and erasure. The next step is to incorporate heat dissipation effects into these simulations showing how this would affect the tunnelling of the charges within the molecules.