Finer detail

For years the wild frontier of particle physics was mapped remotely: a picture of its laws sketched and revised through theory and experimentation. The notion that one day we would be able to see individual atoms and, crucially, observe their relationships, was the stuff dreams were made of.

Advances in electron microscopy, however, have made this possible and we can now see into some of the furthest, or smallest, parts of inner space.

The ability to explore the world on the nanoscale has enabled science to advance dramatically, and innovations are promised in biological and materials technologies that will have applications across many fields.

Last year one of the most advanced, commercially available scanning/ transmission electron microscopes (STEMs) was installed at the London Centre for Nanotechnology (LCN) — a joint venture between Imperial College and University College London. The Titan 80/300, unveiled by FEI in 2005, is the only one of its kind in the UK.

So far FEI has sold around 50 Titans to various research organisations around the world where they are investigating atomic structures in semiconductors, steel, coatings, catalysts, solar cells, LEDs, fuel cells as well as protein structures in cells and the structure of viruses.

Imperial and UCL bought their Titan with a £2.4m EPSRC grant and Imperial spent a further £500,000 creating an electromagnetically and acoustically shielded room to house the machine. For that price, said Imperial’s Dr David McComb, the centre has acquired a capability that is unique in the UK.

While other similar microscopes might be more powerful in terms of imaging, the Titan will allow the different teams at LCN to see, for the first time, how one atom interacts with the others around it.

According to McComb this ability is a leap forward in microscopy and materials analysis. ‘It has had a major impact on the things that users like me can actually do,’ he said.

The system was funded on the basis of eight specific projects that the centre felt would yield significant advances in certain given fields of research.

One such project is to examine the structure of quantum dots to optimise their use in optoelectronics and fibreoptic communications. Quantum dots are used to emit light for the purposes of fibreoptic communications. By ‘tuning’ those emissions it should be possible to gain better control of the bandwidths available in fibreoptics and improve their efficiency.

Titan is also being used to investigate areas of interface in the structure of bone and other mineralised tissues. ‘If we think of bone as a composite material with an organic and inorganic phase, what we are particularly interested in is looking at the interface between those two phases. We want to understand what that interface should look like in healthy tissue, and then we can look at pathological states eventually and try to understand what has changed about that interface,’ said McComb. He added that the project will ultimately give scientists some insight into the characteristics associated with wasting diseases such as osteoporosis and others that attack mineralised tissue.

With so many projects and only one microscope, McComb said it runs ‘pretty much 24/7’, with the day divided into morning, afternoon and evening sessions. The Titan has won a string of design awards since its launch, and although McComb said his teams had experienced some software problems, the technology itself is in a class of its own. ‘In terms of the mechanical aspects — the design of the electronics and the lens systems — it really is a well-designed microscope,’ he said.

It is the novel lens design in particular that makes the Titan superior to its competitors, said McComb. The great advantage of an electron microscope over an optical one is that you can vary the strength of the lens by changing the amount of current through the lens coil to alter the magnetic field. However, until now it has not been possible to switch precisely back and forth between magnetic fields, due to the legacy effects of one field on the next — a phenomenon known as hysteresis. This creates a degree of error in the lens.

‘The real strength, and one of the things that is underplayed with this microscope is what FEI calls the constant power lenses. I’ve been doing microscopy for more than 20 years and one of the problems you would have on every other transmission electron microscope (TEM) is that when you switch between operating modes, the electromagnetic lens goes from a state with plenty of current flowing through the coil to a small amount — or even no current at all.’

A microscopist will change modes hundreds of times a day, and so the magnetic field in the lens will be constantly changing, as will its temperature. McComb said that for switching modes it is ideal to determine the amount of current necessary for each mode, store those values and then switch between them on the computer.

‘What you would find on every other microscope, however, is that you are close but not exactly there. The reason is that the current you thought you needed assumed the lens was at a certain temperature. As soon as you drop the lens down to a lower value of current it starts to cool down. then you go back to the value you were at which is no longer the value required for that magnetic field.’

To combat this error FEI has put two coils into each lens, which makes it twice as heavy and almost twice as expensive. The current is passed down both coils in opposite directions, but the total current at any time is constant. ‘By changing the proportion of the current flowing in opposite directions we can give the lens any value we want, but the total current never changes. This means the temperature of the lens never changes, so hysteresis does not occur.

‘I don’t think people have really appreciated how useful that is. What it means is that a user would perhaps set up a microscope in normal mode and align to do some diffraction, then would switch to high resolution TEM imaging mode, and then perhaps change to an analytical, STEM, mode. If you have gone to the trouble to do the alignment for STEM, and then you think, I should have recorded an image from that area in TEM mode, at that point in a normal microscope you wouldn’t want to go backwards because you might need to redo all the alignments that you just spent an hour doing. With the Titan, you don’t hesitate. There is no hysteresis, so you know you can switch between modes and you won’t lose your alignments. And that makes a huge difference to the way a user operates the system.’ The constant power lens, said McComb, gives the Titan a flexibility and stability above any other microscope in the UK, but he said, the technology that really stands out in terms of what it can do is the monochromator. This is the piece of equipment that allows the researchers to learn something about the atom and its relationship to those immediately around it, and focus for the first time on these vitally important areas of interface between the materials of our world.

‘We use a technique called electron energy loss spectroscopy, which allows us to interrogate the environment of the atom. So we can not only ask what the identity of the atom is, we can start to ask questions like how many other atoms are there around it, what type they are and how are they bonded to the atom in the middle?’

To put it more simply McComb said: ‘To understand any interface, you need to understand what atoms in that interface are actually doing. If the paint starts peeling off your car, its not necessarily a problem with the metal the car is made of or an inherent problem with the paint, its a problem associated with interface between the paint and the metal, and to understand that you need to understand how the atoms at the surface of the metal interact with the atoms at the surface of the paint.’

Titan is giving UK scientists the ability to probe the atomic interface at the heart of a host of materials