A POSTECH research team led by Prof. Moon-Ho Jo in the department of materials science and engineering (also in the division of advanced materials science) discovered a new optical feature by grafting different two-dimensional materials and increased the possibility of developing advanced two-dimensional optical devices.
Prof. Jo’s research team adjusted the relative orientation between MoS2 and WS2 and materialized light absorption and emission in MoS2/WS2 monolayer stacks. By having succeeded in controlling optical features of two-dimensional materials, the research team opened up the possibility of developing advanced optical devices including two-dimensional illuminators, laser machines, and photo detectors.
Atomic two-dimensional materials have multiple atom layers, and when they are mono-layered, different physical properties are observed. The best example is graphene that is a single layer of a carbon atom.
Such two-dimensional materials are expected to become next-generation semiconductors thanks to their excellent physical and chemical properties. MoS2 and WS2 are also layered compounds that could be torn off into single layers, and prospective candidates for the next-generation semiconductor that can replace graphene.
Two-dimensional materials have their own orientation. When two atom layers are stacked and their orientations are tuned to each other, their interactive reponses strengthen and increase the applicability of the materials as electronic or optical devices. Especially, such materials despite nano-sized thickness can absorb over 10 percent of light and considerably increase the performance of optical devices. The researchers succeeded in creating a new process and having the rotation angles in MoS2/WS2 monolayer stacks be tuned to each other.
The monolayer stacks formed by the team produce a high level of electron momentum and the efficiency of electron transfer. To bring about this result, the researchers adopted an unconventional method to make the structure of the stacks.
Conventionally, mono-layered materials used to be stripped through the mechanical method and physically transcribed to make a stacked structure. However, relative orientation between monolayers were misfit and interlayer reponses were too weak and limited.
The POSTECH team unconventionally adopted the large-area chemical vapor deposition*. It made sure that the rotational angle between atom layers was zero and orientations were matched.
*Chemical Vapor Deposition is a method to synthesize a material onto a chosen substrate using chemical reactions between vapors produced when materials are heated at high temperatures.
Prof. Jo said, “We demonstrated for the first time that tuning orientations between two atom layers can control the properties of light absorption and emission at the new wavelengths. Therefore, a variety of photophysical phenomena are expected to be better researched.” He added, “Since such materials absorb and emit visible light that can be detected by human eyes, the performance of two-dimensional optical devices could be drastically improved.”
The research outcome was published in the June 23rd online issue of Nature Communications (IF 11.470). This research was jointly conducted with Prof. Choi Hyungyong’s research team in the school of electrical and electronic engineering at Yonsei University and Prof. Ji Hoon Shim’s team of POSTECH department of chemistry.