A material may appear flawless on the surface, yet fail to function properly. The cause lies in structural defects hidden within two-dimensional thin films, which are considered key materials for next-generation semiconductor devices. Recently, a Korean research team developed an optical analysis method that can identify these invisible defects using light.
A research team led by Professor Sunmin Ryu and Ph.D. candidate Yeri Lee (Department of Chemistry) at Pohang University of Science and Technology (POSTECH) has developed an interferometric second-harmonic generation (SHG) imaging approach capable of optically identifying hidden structural defects in thin films of hexagonal boron nitride (hBN), a promising material as a key component for next-generation semiconductor technologies. The study was published in Advanced Materials , a leading journal in materials science.
From smartphones and artificial intelligence (AI) to quantum computers, two-dimensional materials are emerging as key building blocks for next-generation electronic technologies. Among them, hBN is often referred to as a “protective layer for 2D materials” due to its excellent insulating properties, which help prevent current leakage. However, when hBN is synthesized over large areas, regions known as “antiparallel domains” can form within the film, where the crystal orientations are reversed. Much like sailors rowing in opposite directions aboard the same boat, these internal signals can interfere with one another, potentially degrading its electrical and optical performance despite its seemingly intact appearance. Conventional analytical techniques such as transmission electron microscopy(TEM) and scanning tunneling microscopy (STM) offer highly precise observations, but they are not well suited for rapid, large-area analysis. Raman spectroscopy, while nondestructive, also has limitations in directly distinguishing antiparallel domains.
To overcome these limitations, the researchers focused on second-harmonic generation (SHG) imaging. SHG is a nonlinear optical phenomenon in which light at twice the frequency of the incident light is generated when light interacts with certain materials. By introducing an external reference signal and precisely analyzing the phase difference between the two signals, the team confirmed that antiparallel domains with SHG phases differing by exactly 180 degrees are widely present, even in regions that appear to have the same orientation.
In particular, by comparing ten hBN thin films grown under different conditions, the team found that variations in SHG intensity are closely associated not only to differences in crystal orientation but also to destructive interference between antiparallel domains. In other words, the attenuation of light caused by signals generated in opposite directions enables quantitative evaluation of structural inhomogeneity in the crystal.
The team also established optical criteria for evaluating the crystallinity and structural uniformity of hBN by correlating SHG intensity with Raman spectroscopy data and crystal orientation dispersion. This achievement goes beyond the detection of specific defect, opening the way for rapid, systematic quality assessment of large-area two-dimensional materials.
“This study demonstrates that antiparallel domains within hBN, which have long been difficult to identify directly, can be optically distinguished,” said Professor Sunmin Ryu of POSTECH. “We expect this approach to serve as an important analytical tool not only for optimizing the growth conditions of two-dimensional materials, but also for advancing next-generation electronic, optical, and quantum devices.”
This work was supported by the Mid-Career Researcher Program of the National Research Foundation of Korea and the Global Research Center for Systems Chemistry.
Advanced Materials
Ubiquitous Antiparallel Domains in 2D Hexagonal Boron Nitride Uncovered by Interferometric Nonlinear Optical Imaging
4-May-2026