At Pohang University of Science and Technology, researchers led by Professor Sunmin Ryu and Ph.D. candidate Yeri Lee have cracked an invisible problem that was sabotaging one of tomorrow's most promising materials. Hexagonal boron nitride—hBN—looks perfect under the microscope, yet hidden defects lurking within its atomic structure were degrading its ability to protect the delicate electronics of next-generation devices. Now, using nothing but light itself, the team can finally see what was always there.
The discovery matters because hBN is emerging as essential infrastructure for everything from AI chips to quantum computers. Its exceptional insulating properties make it a natural shield against current leakage in two-dimensional materials, earning it the nickname "protective layer for 2D materials." But when hBN is grown across large areas—as it must be for practical use—something unexpected happens. Regions called antiparallel domains form within the film, places where crystal orientations reverse direction, like sailors rowing in opposite directions on the same boat. The material's surface appears flawless, yet these internal conflicts create destructive interference patterns that degrade electrical and optical performance.
Detecting these hidden defects has been frustratingly difficult. Conventional tools like transmission electron microscopy and scanning tunneling microscopy can see them with atomic precision, but they're too slow and too limited in scope for analyzing large areas. Raman spectroscopy avoids damaging the sample, but it cannot directly distinguish antiparallel domains from the normal material around them. The team needed a new approach.
They turned to second-harmonic generation, or SHG, an optical phenomenon that occurs when light at twice the frequency of incoming light is generated within certain materials. By introducing an external reference signal and precisely measuring the phase difference between the two signals, the researchers could identify antiparallel domains that differ by exactly 180 degrees—opposites that reveal themselves through their mathematical fingerprints. When they studied 10 hBN thin films grown under different conditions, a pattern emerged: variations in SHG intensity correlated directly with both crystal orientation and the destructive interference caused by antiparallel domains working against each other.
This breakthrough does more than spot a specific flaw. By correlating SHG intensity with Raman spectroscopy data, the team established optical criteria for evaluating crystallinity and structural uniformity across large areas. The result is a rapid, systematic quality assessment tool that could transform how two-dimensional materials are manufactured and verified.
"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," Professor Ryu said in the announcement of their findings, published in Advanced Materials.
The implications ripple outward. As semiconductor manufacturers scale up production of 2D materials, they gain a nondestructive way to ensure quality in real time. Materials that were once invisible defects can now be detected before they reach the next stage of device fabrication. For an industry racing to build the quantum computers and AI chips of the future, seeing the invisible could make all the difference.