Second Harmonic Generation Imaging Reveals Existence of Second Bandgap in a 2D Structure

Using time-resolved second harmonic generation (TSHG) microscopy and density functional theory (DFT) calculations, researchers at Daegu Gyeongbuk Institute of Science and Technology (DGIST) have demonstrated the existence of upper bandgap of atomic rhenium disulfide (ReS₂) layers in the conductive atomic structure of ionization energy. The researchers observed the ionization energy area of 2D atomic structures using a TSHG imaging system that they developed.

The team’s TSHG imaging system can generate images of the sounds of atomic-layered structure in 300 nm of high resolution, increasing the sensitivity of measurement of the dispersion effects of layer noise and observed electron movements inside transition bands — visible rays and near-UV rays — using the probing energy of IR bandwidth.

Professor Hyunmin Kim said, “Through this research, we will be able to clarify the structure of multilayer bandgaps existing in various atopic structures besides the rhenium disulfide that was observed this time. It provided important elements to analyze the unidentified causes of electronic activities that contribute to driving the optical sensors and photocatalysts of various 2D structures.” In the future, Kim said, he hopes to develop a device that will operate both optically and electrically by using a new bandgap.

Professor Hyunmin Kim (left) and professor Jaedong Lee (right) at DGIST demonstrated the possible existence of a 0.5-eV bandgap at a distance of 4 eV from the ground state of the bilayer ReS₂ crystal using time-resolved SHG microscopy. Courtesy of Daegu Gyeongbuk Institute of Science and Technology (DGIST).

Professor Jaedong Lee, who calculated the theories for the research, said, “We could observe multilayer bandgaps in this research, which will greatly help with related research such as observing bandgaps of junction structures and improving device agglomeration in the future.”

The research was published in Light: Science and Application (https://doi.org/10.1038/s41377-018-0100-3).