Nanoscale electro-optics of metals and molecules using UHV-STM

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Imagine a light source so small that it is able to generate information from just a single molecule. That's the idea behind this project. But why should we want to do this? The background blurb runs something like this. Electronic device sizes are continually being reduced to yield greater processing power in a smaller space at greater speed; the same goes for the size of the basic memory element in data storage devices. However, the drive to sub-100 nm sizes (1 nm is one millionth of 1 mm) is based on pushing conventional patterning technology to new limits; it's basically a glossed-up version of the same old technology that was used to produce micron scale stuff a decade or more ago. Instead of chipping away bit by bit at the size thing, could it not be treated in a more revolutionary fashion? Is it possible, for example, to make processing elements based on single molecules? That would knock the processing element size down to 1 nm or so. But how do we go about assessing this possibility? In real terms that is, not nice thought experiments. To do this we need to address single molecules and read information from them.The instrument that will be used to do this is called a scanning tunnelling microscope or STM for short. It can hold an electrically conducting tip an incredibly small distance (1 nm or less) above a conducting surface. When a bias is applied between tip and surface some electrons cross the gap - a small current flows (pA to nA scale). This happens by a quantum mechanical process called tunnelling - in terms of 'classical' physics, no current should flow because the gap is electrically insulating! As you might guess, there is some pretty nifty feedback involved in keeping the tip a fixed distance of just 1 nm above the sample. The STM can then move the tip in a controlled way to build up an image of the sample surface with atomic resolution - you can see the atoms poking right out there! Obviously, it will 'see' any molecules that are placed on the surface.That takes care of the molecular addressing. The next and really neat aspect of this approach is that the information output can be optical. When electrons flow between the tip and the sample (either direction is OK), some light is generated. If a molecule is sitting in there, the light output can have more to do with the molecule than the properties of either the tip or the underlying substrate. There are then two further steps that the project will try to address. First, we would like a 'smart' molecule on the surface, preferably one that can change shape and properties between two different, stable states. Such molecules actually exist and we intend using a remarkably simple one called azobenzene. The idea is to change the state of the molecule by an electrical (voltage) pulse applied to the tip and see if the light output changes. (Or, conversely, address the molecule with an external light pulse and monitor a change in its conductivity.) The second thing is to improve the optical output and here we have some ideas on really tiny antenna. To get a feel for this, think about the receiving antenna or ariel on your TV. It's size is in the 0.1 to 1 m range and that's because it's designed to match the wavelength of the TV carrier signal - about 0.4m for a signal of frequency 750 MHz. Scaling to the optical/near-infrared region (wavelength 400-1000 nm) implies an antenna length of a few 100 nm. Such antenna will be realized on the basis of fascinating entities known as carbon nanotubes. These are a whole story in themselves, but the important point in relation to this work is that they can be grown perpendicularly out of a surface. The nanotubes we will use will be 25-50 nm in diameter and several 100 nm long. The idea is to custom manufacture the STM tip as an optical antenna so that it efficiency transmits optical information from individually addressed molecules.