A multi-university research team has gained insight into how light is emitted from layered nanomaterials and other thin films through a new spectroscopic technique. The technique, called energy-momentum spectroscopy, allows researchers to tell whether or not the light emitted from a certain film is coming from emitters that are oriented along the plane of the film or perpendicular to it. By knowing the orientation of such emitters, engineers may be able to capitalize on the applications of thin films, such as in LEDs and solar cells. These results are due to a collaborative effort between Brown University, Case Western Reserve University, Columbia University, and the University of California—Santa Barbara.
Energy-momentum spectroscopy takes advantage of a well-known property of light – interference. Examples of such interference are the rainbows that you see on the surface of bubbles, or the surface of oil slicks. This property allows scientists to calculate properties of the material such as its thickness, by making use of the angle at which the light is hitting the thin film and the constructive/destructive properties of light which interference entails.
“The key difference in our technique is we’re looking at the energy as well as the angle and polarization at which light is emitted. We can relate these different angles to distinct orientations of emitters in the film. At some angles and polarizations, we see only the light emission from in-plane emitters, while at other angles and polarizations we see only light originating from out-of-plane emitters.” said Rashid Zia, assistant professor of engineering at Brown University and one of the study’s lead authors.
This technique has been used on two different films of different materials: molybdenum disulfide (MoS2) and PTCDA (acronym for a complex organic molecule, whose name is longer than this description of the long name). These two materials have potential optical applications. MoS2 is two-dimensional and similar to graphene, whilst PTCDA is essentially an organic semiconductor. The emissions from the MoS2 film are shown to come only from in-plane emitters, and PTCDA comes from both in-plane and out-of-plane emitters.
By knowing the orientation of the emitters, it is possible to construct devices with a structure that is able to maximize on this fact by layering the thin-films on top of one another. The orientation of the emitters tells us whether or not electrons will travel in the layer or across the layers.
“If you were making an LED using these layered materials and you knew that the electronic excitations were happening across an interface, then there’s a specific way you want to design the structure to get all the light out and increase its overall efficiency.” said Zia.
The same concept can be applied to any sort of light-absorbing device such as a solar cell. This study shows how with just the understanding of excitations in a material, we will be able to more efficiently convert incoming light to usable energy.
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