Research Seminar of Prof. Cai Weizhao from the University of Utah

Lecture Time: 14:30 on 2rd January, 2018

Lecture Room: C501 of Science and Innovation Building

Lecture Title: Aromatic hydrocarbons under pressure

Cai Weizhao

the University of Utah

Abstract

Pressure being analogous to temperature, is an appropriate approach to modify bond distances of materials and explore their exceptional physical properties. In this talk, I will present our recent results on two aromatic hydrocarbons under compression. First, I will show pressure-induced polymerization of benzene and its isotopologue C6D6. We found both isotopologues demonstrate negligible effect in the first-order phase transition and onset of chemical reaction. Raman spectra of polymeric products recovered from different pressures show a pronounced kinetic isotope effect: C6D6 polymerization reaction proceeds considerably slower than C6H6, leading to substantially lower fractions of sp 3 bonding in C6D6-polymers recovered from comparable pressures.

Then I will show another herringbone-type hydrocarbon benz[a]anthracene (BaA). The ambient-pressure phase of BaA was found to be stable up to ~15.0 GPa. And an exceptional piezochromic color change was observed simultaneously. DFT calculations suggest that the formation of crystalline polymeric phase can take place between 30-117 GPa. Optical absorption measurements reveal that the band gap of BaA decreases at high pressure, from 2.4 eV at 0.5 GPa to 1.0 eV at 50.6 GPa. The DFT calculations further suggest that the band gap of BaA in molecular phase could reduce to ~0.1 eV at 117 GPa. Photoconductivity measurements show a continuous increase of photocurrent in the molecular phase region, which is most likely originated from the increase of carrier mobility under pressure.