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<span style="font-size:10.5pt; font-family:"Arial","sans-serif"; color:#1F497D">***** Seminar *****</span><span style="color:#1F497D"></span></p>
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<b><span style="font-size:16.0pt; color:#1F497D">Department of Electrical and Computer Engineering</span></b><span style="font-size:16.0pt; color:#1F497D"></span></p>
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<b><span style="font-size:16.0pt; color:#1F497D">Materials Engineering Program</span></b></p>
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<b><span style="font-size:14.0pt; color:#1F497D">Center for Integrated Bio and Nano Systems</span></b></p>
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<b><span style="font-size:14.0pt; color:#1F497D"> March 22, 2019</span></b><span style="font-size:14.0pt; color:#1F497D"></span></p>
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<b><span style="font-size:14.0pt; color:#1F497D">10:30 a.m., Room: CBB 108 </span>
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<b><span style="font-size:18.0pt; color:#1F497D">Two-dimensional Semiconductors for Chiral Phononics and Ferroelectric Switches </span></b></p>
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<span style="font-size:18.0pt; color:#1F497D">Hanyu Zhu</span></p>
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<span style="font-size:16.0pt; color:#1F497D">Rice University</span></p>
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<b><span style="color:#1F497D">Abstract</span></b><span style="font-size:11.0pt; font-family:"Calibri","sans-serif"; color:#1F497D">:</span><span style="color:#1F497D"> Two dimensional (2D) semiconductors, such as monolayer transition metal dichalcogenides
(TMDC) are promising channel materials for transistors beyond 5 nm nodes. On the other hand, the rich physics in a sub-nanometer-confined, freestanding covalent lattice extend to more than just gate-tunable conductivity. For example, the family of compound
MX2 (M=Mo, W and X=S, Se, Te) exhibit multiple crystalline structures, each having unique electronic properties according to their lattice symmetry. In this seminar, I will first explain the consequence of inversion symmetry breaking in the hexagonal monolayer,
such as an optical band gap, second-order nonlinearity, and particularly the valley degree of freedom. Then I will introduce the concept of chiral phonons as a result of in-plan inversion asymmetry, in which the atomic vibration takes the form of unidirectional
rotation. We experimentally observed chiral phonons using ultrafast optical spectroscopy through their chiral coupling to valley-polarized electrons. The spin-momentum-locking property allows these phonons to not only encode information themselves but also
potentially act as spin and valley switches. Finally, I will demonstrate the control of out-of-plane asymmetry in the quintuple-layered M’2X3 compounds (M’=Al, Ga, In). We found non-volatile and switchable out-of-plane polarization, i.e. ferroelectricity,
in In2Se3 by electromechanical and nonlinear optical imaging. In contrast to conventional ferroelectric materials that becomes unstable below a critical thickness, In2Se3 shows locking between the out-of-plane dipoles and in-plane lattice asymmetry that leads
to a very high transition temperature without the assistance of extrinsic screening. Together, these results indicate the exciting possibilities of manipulating the crystal structure for ultrafast electronic devices.
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<b><span style="color:#1F497D">Bio</span></b><span style="color:#1F497D">: Hanyu is an assistant professor of Materials Science and NanoEngineering at Rice University. He earned his B.S. in Mathematics and Physics in Tsinghua University in China, when he got
into the field of nanomaterials. He obtained his Ph.D. in Applied Science and Technology in the University of California at Berkeley for studying electromechanics of atomically thin crystals. After that, he conducted post-doctoral research developing novel
optical spectroscopy for phonons, the quantized lattice vibration. He started the Emerging Quantum and Ultrafast materials Lab in 2018 with a focus on structural engineering of low dimensional and correlated materials.
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<span style="color:#1F497D">Contact Prof. Jiming Bao (<a href="mailto:jbao@uh.edu">jbao@uh.edu</a>) if you would like to meet with Dr. Zhu.</span></p>
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