Infrared detectors play an important role in research: many molecules absorb electromagnetic radiation in the infrared range in a very characteristic way. Just like people can be identified by their fingerprints, molecules can be identified by their infrared spectrum. Infrared spectroscopy is used in many different areas - from astrophysics to environmental analysis to the search for viruses.
For this method to work, it must be possible to detect even tiny doses of infrared radiation. Therefore, a material is needed that absorbs as much of the radiation as possible. Such a material has now been found at the TU Wien (Vienna). Using a special technical trick, it was possible to produce an ultra-thin gold layer measuring only 2 nanometers. In this way, about half of the radiation energy can be captured and converted into heat - in a completely uniform manner, over a wide range of the infrared spectrum.
Thinner layers - more resistance
"There are different ways to absorb an electromagnetic wave very well. You can come up with special antenna geometries or work with specially structured surfaces," says Niklas Luhmann, a doctoral student in Prof. Silvan Schmid's research group at the Institute for Sensor and Actuator Systems at TU Wien. "This allows us to produce almost perfect absorbers for a specific wavelength. But what you actually need is an absorber that absorbs radiation of very different wavelengths equally.”
The team therefore decided to take a different approach: "It has long been known that an electromagnetic wave is best absorbed when it hits a material with exactly the right surface resistance – which is 188 ohms," explains Niklas Luhmann. The problem is that this surface resistance is difficult to achieve.
The sheet resistance of a material depends on its thickness: The thicker the layer, the better it can conduct electrical current and the lower the resistance. With metals such as gold, for example, the desired surface resistance can only be achieved if the layer thickness is extremely thin - and until now, even the thinnest gold layers have been much too thick.
Copper oxide helps against gold lumps
"Normally gold has a tendency to clump together," says Luhmann. "When gold is evaporated onto silicon nitride, some of the gold atoms form a bond with silicon atoms, and that's exactly where other gold atoms are preferentially added. Small islands form on the surface, similar to drops of water on a window, forming due to surface tension.” A closed, continuous gold layer is only formed when a fairly large number of gold atoms have been used, and then the thickness of the layer is already so large that the surface resistance has become too small.
However, inspired by other experiments, for which different materials had been successfully combined, a solution to this problem was developed at TU Wien. First, copper was applied to silicon nitride. The copper reacts in the air to form copper oxide. A layer of gold can then be deposited on this copper oxide surface in a vacuum chamber. The copper oxide effectively prevents individual gold atoms from binding to the silicon nitride and thus reduces the formation of islands. In this way, an extremely thin gold layer can be created: while usually, only layers with a thickness of at least 7 nanometers can be produced, a 2-nanometer gold layer is created on copper oxide. This corresponds to about 7 gold atom layers.
Like a Guitar in the Sun
"This novel gold absorber can convert about half the energy of infrared rays directly into heat - a remarkably high proportion. And the most important thing is that it does this very evenly over an extremely wide range of wavelengths, from 2 µm to more than 20 µm," says Niklas Luhmann.
The heat is then dissipated, causing the detector to change its oscillation behavior. "Is is a bit like coating a guitar string with a thin layer of gold and placing the guitar in the sun," says Luhmann. "The string gets warmer, expands and changes its sound. And that is what we can measure." In this way, the Institute for Sensor and Actuator Systems is currently developing novel infrared sensors that can provide a variety of important research results.