Detailing the molecular makeup of materials – from solar cells to organic LEDs and transistors to medically important proteins – is not always a crystal-clear process. To understand how materials work at these microscopic scales and to better design materials to improve their function, it's necessary to know not only about their composition but also their molecular arrangement and microscopic imperfections.
A research team working at the Department of Energy's Lawrence Berkeley National Laboratory has demonstrated infrared imaging of an organic semiconductor known for its electronics capabilities, revealing key nanoscale details about the nature of its crystal shapes and orientations as well as defects that affect its performance.
To achieve this imaging breakthrough, researchers from Berkeley Lab's Advanced Light Source (ALS) and the University of Colorado-Boulder combined the power of infrared light from the ALS and infrared light from a laser with a tool known as an atomic force microscope. The ALS, a synchrotron, produces light in a range of wavelengths or "colors" – from infrared to X-rays – by accelerating electron beams near the speed of light around bends.
The researchers focused both sources of infrared light onto the tip of the atomic force microscope, which works a bit like a record-player needle; it moves across the surface of a material and measures the subtlest of surface features as it lifts and dips. The technique allows researchers to tune the infrared light in on specific chemical bonds and their arrangement in a sample, show detailed crystal features, and explore the nanoscale chemical environment in samples.
"Our technique is broadly applicable," said Hans Bechtel, an ALS scientist. "You could use this for many types of material – the only limitation is that it has to be relatively flat" so that the tip of the atomic force microscope can move across its peaks and valleys.