In every decade for the past 30 years, the transmission electron microscope (TEM) has developed new topics that dominated the decade. In the 1990s, environmental electron microscopy allowed samples to be examined under more natural conditions. From 2000 to 2010, aberration correction brought samples into sharper focus, like a double pair of eyeglass lenses. In the current decade, looking at reactions taking place inside the TEM, in situ, is the major topic for microscopy.
Why is in situ microscopy so important and why now? To find out, we spoke with Bernd Kabius, senior scientist at Penn State’s Materials Research Institute and acknowledged expert in TEM.
BK: In situ microscopy is an overarching, long-term topic. It is one of our two major focus areas in TEM at MRI. The other is soft matter TEM; that includes both polymers and incorporating TEM with the life sciences. In the past, most microscopy has been post mortem. It is a forensic examination where you have to guess what happened. There are ways to make sure your results are meaningful, but nevertheless, you don’t really see what happens.
An example of this is an experiment my colleagues at Lawrence Livermore National Lab did a while ago. Using an ultrafast microscope, they watched the reaction front moving through a binary alloy. They detected that there is a liquid phase at the reaction front that they had never suspected. That is something that happens far below the melting point. If they hadn’t seen it, they would have had no idea there was an intermediary stage.
Why are we only starting to use in situ TEM now?
BK: The reason is that all the technological components for in situ microscopy are now here. One problem has been that the space inside the microscope has been constrained. In order to get low aberration and high resolution, the pole pieces of the objective lens have to be very close together. That restricted the space to 2mm. There was not much room for anything but a sample there.
But then came aberration correction in the last decade, and that opened up the gap for the objective lens. Now you have 5mm of space to work with, and in the future 1cm or more will be possible.
Another important technology advance is MEMS. Five millimeters is still not a very large space, so we have to make things very small to get experiments done. Our Nanofab at Penn State, which is located one floor above our microscopy facility, gives us the ability to do customized stages.
What capabilities do we have at Penn State?
BK: We recently acquired two stages, a heating/nano biasing stage – that is a stage that can heat up to 1200 °C very fast and very stably – and a liquid stage, a much rarer device. I believe there are only a few in the world. The nice thing is we can do these experiments in our high-level Titan microscope, which is capable of chemical and elemental analysis at very high resolution (0.07nm). These analytical tools are available while you do the experiment.
The potential applications are really wide. The combination of heating and biasing opens up a lot of research: microelectronics, soft materials like polymers that have electrical properties, up to basic research such as what happens to electron shells under an electric field.
The liquid stage allows us to observe processes you normally could not observe in a microscope when you have to work in a vacuum. This stage allows us to encapsulate a liquid between two silicon nitride windows. These windows can be made really thin, 50nm or less. These are electron transparent and you can see what happens between the windows.
This stage opens up another new area of research, for instance, in batteries. Because we have four electrodes in this stage, it can also be used to perform electrochemical experiments, and batteries are electrochemical devices. The world needs better batteries for cars and for energy storage in general. And we have a big battery program at Penn State already. This stage also can heat up 100 °C, not more, because of course, the liquid would evaporate and the gas pressure would destroy silicon nitride. We have already begun experiments in both stages.
Are these stages expensive?
BK: Yes, you could buy a very nice car for the price of one of these – $100,000-$200,000.
Wow, nice car.