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news digest ♦ Novel Devices plane oriented emitters. (Credit: Zia lab/Brown University)


The research, published online on March 3rd in Nature Nanotechnology, was the collaborative effort of scientists from Brown University, Case Western Reserve University, Columbia University, and the University of California - Santa Barbara.


The new technique takes advantage of a fundamental property of thin films: interference.


Interference effects can be seen in the rainbow colours visible on the surface of soap bubbles or oil slicks. Scientists can analyse how light constructively and destructively interferes at different angles to draw conclusions about the film itself - how thick it is, for example. This new technique takes that kind of analysis one step further for light-emitting thin films.


“The key difference in our technique is we’re looking at the energy as well as the angle and polarisation at which light is emitted,” says Rashid Zia, assistant professor of engineering at Brown University and one of the study’s lead authors. “We can relate these different angles to distinct orientations of emitters in the film. At some angles and polarisations, we see only the light emission from in-plane emitters, while at other angles and polarisations we see only light originating from out-of-plane emitters.”


The researchers demonstrated their technique on two important thin-film materials, molybdenum disulphide (MoS2) and PTCDA. Each represents a class of materials that shows promise for optical applications. MoS2 is a two-dimensional material similar to graphene, and PTCDA is an organic semiconductor. The research showed that light emission from MoS2 occurs only from in-plane emitters. In PTCDA, light comes from two distinct species of emitters, one in-plane and one out-of-plane.


Rashid Zia continues, “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 of that light out and increase its overall efficiency.”


The same concept could apply to light-absorbing devices like solar cells. By understanding how the electronic excitations happen in the material, it could be possible to structure it in a way that converts more incoming light to electricity.


known, it may be possible to design structured devices that maximise those directional properties.


In most applications, thin-film materials are layered on top of each other. The orientations of emitters in each layer indicate whether electronic excitations are happening within each layer or across layers, and that has implications for how such a device should be configured.


“One of the exciting things about this research is how it brought together people with different expertise,” Zia notes. “Our group’s expertise at Brown is in developing new forms of spectroscopy and studying the electronic origin of light emission. The Kymissis group at Columbia has a great deal of expertise in organic semiconductors, and the Shan group at Case Western has a great deal of expertise in layered nanomaterials. Jon Schuller, the study’s first author, did a great job in bringing all this expertise together. Jon was a visiting scientist here at Brown, a postdoctoral fellow in the Energy Frontier Research Centre at Columbia, and is now a professor at UCSB.”


This work is further detailed in the paper, “ Orientation of luminescent excitons in layered nanomaterials,” by Jon A. Schuller et al in Nature Nanotechnology (2013), published online on 3rd March 2013. DOI: 10.1038/nnano.2013.20


Funding for the work was provided by the Air Force Office of Scientific Research, the Department of Energy, the National Science Foundation, and the Nanoelectronic Research Initiative of the Semiconductor Research Corporation.


Opel makes breakthrough with POET based n- and p-transistors


The result builds on the previous GaAs (gallium arsenide) based VCSEL milestone. It is a further verification that III-Vs can compete with silicon CMOS in WDM capable optoelectronic devices and functions, FETs and bipolar devices


Opel Technologies has achieved Milestone 4 in its Planar Optoelectronic Technology (POET), achieving radio frequency and microwave operation of both n-channel and p-channel transistors.


With this achievement, POET extends the capability of its unique monolithic platform to cover integration of a complete range of wavelength-division multiplexed (WDM) capable optoelectronic devices and functions.


This is in addition to complementary electronics based on n-channel and p-channel transistors as either field effect transistors (FETs) or bipolar devices.


Rashid Zia Zia also points out that once the orientation of the emitters is 134 www.compoundsemiconductor.net March 2013


For this milestone, 3inch POET wafers fabricated at BAE Systems (Nashua, NH) yielded submicron n-channel and micron-sized p-channel transistors operating at frequencies of 42 GHz and 3 GHz respectively. These operating frequencies are expected to be improved even further in the short term to


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