Quantum Tunnelling and the Scanning Tunnelling Microscope

In the last post, we learnt that the uncertainty principle means that for microscopic systems, we cannot say for certain what exactly will happen when one thing affects another: all we can say is that there is a certain probability that an outcome will occur. Although this may seem like a backward step from classical physics, where we can say with absolute certainty what will happen, quantum physics is the only accurate model we have for predicting the outcomes of quantum systems. However, the probabilities we do calculate do translate into real life, and thus the technologies built on them can perform in ways that would be impossible under the laws of classical physics, giving these new technologies huge advantages over older technologies.

Consider throwing a ball straight up into the air: it rises, reaches a definite apex where it's vertical velocity is zero, and then falls back down. Now, imagine that instead of a macroscopic ball, we're dealing with a microscopic particle. As we've already discussed, it's impossible to tell the exact position of our particle at any time: so what does this mean for the "flight" of our particle? It means that it's impossible to tell when the particle reaches any sort of "apex", be it the natural limit to its movement through space, some sort of barrier, or a gap between materials. More importantly, the uncertainty principle means that there's is even some probability that the particle will pass on even further!  This probability decreases the further the particle moves past the point it should, and quickly drops toward zero, meaning that it is unlikely to see particles passing through metres of concrete, for example. This phenomena is is known as "quantum tunnelling" and is one of the most useful practical applications of the Uncertainty Principle. 

The scanning tunnelling microscope (STM) is the most direct application of quantum tunnelling in technology. It uses the ability of an electron current to tunnel to image surfaces of materials right down to the individual atoms that make it up. It was first invented in 1981 by IBM Zurich, and has since become absolutely essential to anyone studying the atomic structure of solid materials, physicists, biologists and nanotechnologists included.

The STM works by passing a very small current round a metal needle which is then moved over the surface of a sample.  The metal needle at its tip is only one atom thick. The metal tip is held at around 5 nanometres, 0.000000005m, from the surface, meaning that mathematically, the wavefunctions of the electrons in the current flowing around the metal needle and the electrons in the surface overlap and because the metal tip is held at a different voltage to the surface of the material, the electrons in the current begin to tunnel across at a rate proportional to the distance between the metal tip and the surface of the material. This flow of electrons across the gap creates a tiny current which can be amplified and then measured. The smaller the distance between the metal tip and the surface, the more electrons will tunnel across the gap and the larger the current will be. The tunnelling current is incredibly sensitive to even tiny changes in distance, so even the difference between one atom and the next can be detected. Creating an image with an STM is analogous to taking a rubbing of a tree bark: the places where there are ridges in the bark give you darker lines, like how places where the atoms are closer to the metal tip will give you higher currents. Using a computer and a current amplifier, it’s possible to interpret the values of current measured and build up an image of the surface of the material.

STMs have helped to revolutionise the way scientists in a variety of fields work when studying microscopic structures. For example, before the invention of the STM, if a chemist wanted to learn about what chemicals work with what catalysts, they would have to go through a trial and error type experiment, simply trying to fit “keys in locks”, as it were. However, by using an STM, chemists can now image and study the actual active sites of the catalysts and other chemicals, and fit them together without having to go through the trial and error process.  Furthermore, by taking advantage of the fact that an STM is not limited to just vacuums and can function in any atmosphere at a large range of temperatures, it is possible to watch these chemical reactions occur in real time at a molecular level. This allows us to build a much greater understanding of the processes involved. The STM has also been used to study strands of DNA to help us understand the behaviour of genetic material in more detail, which could lead to the development of new medicines to treat genetic diseases. 

However, the STM isn’t perfect. It can only provide a view of the first layer or so of a material, as “the experimental ‘‘image’’ is relatively insensitive to the positions of atoms beyond the first atomic layer”. It also requires the material to be an electrical conductor, which limits the numbers of materials it can be used to scan. It also requires the material to be extremely stable and clean, the tip of metal point to be very sharp, the sample to be isolated from any external vibrations, and sophisticated electronics. These requirements can make using an STM a challenging and expensive process.   

Another month, another blog post. I should really do these more regularly. This one was going to be part of my EP, but I had to scrap it as my question wasn't really suitable for the qualification, as there's not enough debate in the discussion of -how- an effect is used. I didn't want to waste all the research I've done, so I'm going to slowly adapt my dissertation into a number of posts. 

Thanks for reading as always, 

GM ^^