A group of counterfeiters was recently caught producing fake luxury jewelry items by pouring molten metal into molds produced from the original jewelry. In a similar way, a template scaffold can produce an endless amount of molecular crystals of any shape or form. Researchers at Pohang University of Science and Technology (POSTECH) have used this principle to grow layered structures of two-dimensional molecular crystals and confirmed their intralayer and interlayer excitonic interactions.

Background

2D organic molecular solids are sought after as the next-generation semiconductor materials for applications such as flexible displays. However, past research on excitons has hit a roadblock due to organic molecules’ tendency to aggregate into formless clumps without a specific structure. Understanding the properties of excitons in crystals is crucial for future usage of 2D organic molecules in various applications.

The Research

The team at POSTECH created a layered structure by stacking up 2D organic molecular crystals using hexagonal boron nitride (hBN) as a mold. Boron nitride is an insulating material that does not conduct electricity. The team utilized it as a scaffold to successfully produce 2D organic molecular crystals and stacked these crystal layers on top of each other.

The researchers first determined the two-dimensional molecular crystals’ structure and arrangement by analyzing their degrees of optical absorption and patterns of electron diffraction. The team also confirmed intralayer and interlayer coupling of the molecular excitons after selectively separating the stacked 2D organic molecular crystals into the desired number of layers (or thicknesses).

Results

In a single layer, the molecular vibrations decreased as the temperature decreased, creating a more stable environment. As a result, excitons within the crystal were able to interact with relatively distant excitons, which was confirmed through changes in fluorescent energy. Moreover, as multiple layers were stacked, there was a continuous variation in the overall orientation of molecular transition dipoles due to the ‘mixing’ of interlayer excitons with intralayer excitons. However, once the thickness of the crystal reached a certain threshold, the direction of the interlayer molecular dipoles ceased to change and converged towards a single direction. It was observed that by manipulating the temperature and thickness of the layered structure, control over the interaction between excitons could be achieved.

Professor Sunmin Ryu, who led the study, explained that the research demonstrated the ability to control the physical properties of organic-molecule-based 2D materials and the movement of excitons. He expects the findings to lead to various applications in organic semiconductors and solar power generation. The method of producing 2D molecular crystals could be used to create a wide range of materials with tailored properties, opening up new possibilities in fields such as electronics and energy.

Chemistry

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