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Because of this problem, Chan


says “freeing the electrons” has been a focus in developing organic semi- conductors for solar cells, light sen- sors and many other optoelectronic applications. Now, two physics research


groups at KU, led by Chan and Hui Zhao, professor of physics and astron- omy, have effectively generated free electrons from organic semiconductors when combined with a single atomic


layer of molybdenum disulfide (MoS2, a recently discovered two-dimensional (2D) semiconductor. The introduced 2D layer allows the electrons to escape from “holes” and move freely. “One of the prevailing assump-


tions is free electrons can be generat- ed from the interface as long as elec- trons can be transferred from one material to another in a relatively short period of time — less than one- trillionth of a second,” Chan says. “However, my graduate students Ti- ka Kafle and Bhupal Kattel and I have found the presence of the ultra- fast electron transfer in itself is not sufficient to guarantee the genera- tion of free electrons from the light absorption. That’s because the ‘holes’ can prevent the electrons from mov- ing away from the interface. Wheth - er the electron can be free from this binding force depends on the local en- ergy landscape near the interface.” Chan also says that the energy land- scape of the electrons could be seen as a topographic map of a mountain. “A hiker chooses his path based


on the height contour map,” he says. “Similarly, the motion of the electron at the interface between the two ma- terials is controlled by the electron energy landscape near the interface.” Chan and Zhao’s findings will


help develop general principles of how to design the “landscape” to free the electrons in such hybrid materi- als. The discovery was made by com- bining two highly complementary ex- perimental tools based on ultrafast


Breakthrough Material Could


lasers, time-resolved photoemission spectroscopy in Chan’s lab and tran- sient optical absorption in Zhao’s lab. Both experimental setups are located in the basement of the Integrated Science Building. In the time-resolved photoemis-


sion spectroscopy experiment, Kafle used an ultrashort laser pulse that only exists for 10 quadrillionths of a second to trigger the motion of elec- trons. The advantage of using such a short pulse is the researcher knows precisely the starting time of the electron’s journey. Kafle then used another ultrashort laser pulse to hit the sample again at an accurately controlled time relative to the first pulse. This second pulse is energetic enough to kick out these electrons from the sample. By measuring the energy of these


electrons (now in a vacuum) and using the principle of energy conservation, the researchers were able to figure out the energy of electrons before they were kicked out and thus reveal the journey of these electrons since they were hit by the first pulse. Because on- ly electrons near the front surface of the sample can be released by the sec- ond pulse, the position of the electron relative to the interface is also re- vealed with atomic precision. In the transient optical absorp-


tion measurements, Peng Yao and KU graduate Peymon Zereshki, both su- pervised by Zhao, also used a two- pulse technique, with the first pulse initiating the electron motion in the same way. However, in their measure- ments, the second pulse does the trick of monitoring electrons by detecting the fraction of the second pulse that is reflected from the sample, instead of kicking out the electrons. “Because light can penetrate a


longer distance, the measurement can probe electrons in the entire depth of the sample and therefore provide complementary information to the first techniques that are more ‘surface sensitive,’” Zhao says. “These measurements enabled us to reconstruct the trajectory of the elec- tron and determine conditions that enable the effective generation of free electrons.” Web: www.news.ku.edu r


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Special Focus: PCB and Assembly..................... 52


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