Graphite and diamond are made of a substance called carbon (C). Graphene is the separation of a single layer of carbon atom from graphite. Even though they are made of the same atoms, they are classified as completely different materials depending on the number of atoms and their arrangement. Recently, a research team at POSTECH has demonstrated a single-layer molecular crystal sandwiched between two-dimensional molecular crystals (2DMCs) for the first time.
A research team led by Professor Sunmin Ryu and Ph.D. candidate Seonghyun Koo in collaboration with Professor Ji Hoon Shim of POSTECH's Department of Chemistry and the National Institute for Materials Science in Japan have fabricated single-layer tetracene molecular crystals (MC) and reported on their optical properties. This was the first such case in the field of 2D molecular crystals so far. The findings from the study were recently published in Nano Letters, a journal with global influence in the field of nanotechnology.
Since Andre Geim's research team at University of Manchester in 2004 separated graphene with a thickness of one carbon atom from graphite through mechanical exfoliation, research on 2D materials has exploded. But it has advanced in favor of inorganic crystals that bond chemically. Molecular crystals such as tetracene are made up of weak van der Waals bonds, so it is quite difficult to bring them to the molecular level are very unstable in atmospheric conditions.
To this, the research team succeeded in constructing single-layer tetracene crystals using two-dimensional inorganic crystals such as graphene or hexagonal boron nitride (hBN) as substrates. In addition, covering the hBN using the dry transfer greatly improved its stability against heat and light.
The atomic force microscopy (AFM) and wide-field photoluminescence imaging were introduced to obtain the 2D shape of single-layer tetracene crystals the size of several tens of micrometers. The research team investigated the electronic structure of the sample with polarized absorption and emission spectromicroscopy combined with SHG spectroscopy.
The research team obtained the absorption and emission spectra of the tetracene crystal according to the polarization of the incident light, and found that both the absorption and emission were maximized at a specific angle. This meant that the material has crystallinity and the orientation of tetracene crystals can be determined experimentally through absorption and emission. In addition, it was confirmed that the Davydov splitting, which is significantly increased compared to the 3D crystal, is an index indicating the intrinsic properties of the 2D crystal.
In this study, the structure of the material was preserved and the photostability was greatly improved by confining the 2D tetracene crystals between hexagonal boron nitride or graphene. The wide-area photoluminescence imaging technique of crystals and optical second-harmonic generation allowed the growth of monolayer films of tetracene crystals the size of several tens of micrometers, and it was found that a specific orientation was preferred when grown on a boron nitride substrate. The research team secured the theoretical validity of the layered structure through first-principles calculations.
As it was confirmed that the thickness of the same organic crystals can actively modulate physical properties, it is expected to be widely applicable in OLED that uses organic crystals and in organic photoelectric energy conversion.
"The nonlinear proliferation of excitons in tetracene crystals is garnering attention as an eco-friendly research that can overcome the thermodynamic efficiency limits of solar cells," explained Professor Sunmin Ryu. He added, "The 2D tetracene crystals can be used as the principal material for forming the light absorption layer of solar cells and will be a starting point for research on the structure and properties of low-dimensional molecular crystals."