image: Figure | Principe of LOTB. a. Schematic diagram of the bubble flattening process with LOTB. In stage (i), liquid-filled nanobubbles are generated during wet transfer; in stage (ii), a high-temp field region and a low-temp field region are generated under laser irradiation, which sublimates the TMD film to generate a sacrifice region and enables the phase transition of the inclusions, respectively; in stage (iii), the inclusions inside the nanobubbles flow out of the sacrifice region under the pressure difference inside and outside nanobubbles, which flattens the film together with a stress-pulling effect. b. Simulated temperature distribution (black line) under the irradiation of a Gaussian laser beam (green line, normalized) with an intensity of 106.1 mW/µm2 and beam diameter of 0.3 µm. The red and blue regions indicate the high-temp field and low-temp field regions, respectively. c. AFM topography of a transferred 1L-MoS2 sample with nanobubbles. The inset shows the optical image of the 1L-MoS2 film, where the dashed rectangle indicates the region of the AFM topography. d. Aspect ratio change of a nanobubble under different irradiation laser power , where the orange area indicates the phase transition region leading to an abrupt change of aspect ratio and the black dashed line indicates the power threshold. The insets show the corresponding AFM topography images of the same nanobubble under different irradiation laser powers. e Calculated pressure difference inside and outside the nanobubble versus the aspect ratio. The radius of the nanobubble is taken as 200 nm.
Credit: Benfeng Bai et al.
Two-dimensional (2D) van der Waals materials, such as transition metal dichalcogenides, have emerged as important candidate materials for next-generation chip-scale optoelectronic devices. However, during their growth and transfer processes, various nanodefects such as nanobubbles are inevitably generated. These nanobubbles, though tiny with dimensions typically from 10 nm to 1 μm, may significantly alter the local dielectric environment and tensile strain of 2D materials and therefore are rather detrimental to the constructed devices. To date, there is no post-processing method that can effectively eliminate nanobubbles in 2D materials after fabrication and transfer, which has been a major obstacle in the development of 2D material-based devices.
In a new paper published in Light: Advanced Manufacturing, a team of scientists, led by Professor Benfeng Bai and Hong-Bo Sun from State Key Laboratory of Precision Measurement Technology and Instruments, Department of Precision Instrument, Tsinghua University, China, have developed a novel all-optical method called laser optothermal nanobomb (LOTB) for efficient flattening of nanobubbles in 2D materials. This method leverages an optothermally induced phase transition and stress-pulling effect. Under focused continuous-wave laser irradiation, a high-temperature region sublimates the 2D film to create a tiny sacrificial breach, while a surrounding low-temperature region vaporizes the liquid inclusions inside the bubbles. The resulting pressure difference drives the gas out through the breach, flattening the film in approximately 50 milliseconds without damaging the material's intrinsic optoelectronic properties.
The scientists demonstrate that LOTB reduces surface roughness by more than 70% in 1L-MoS2 films, as validated by nano-photoluminescence and Raman spectroscopy. To address scalability, they further developed dual-beam and multi-shot strategies, which significantly expand the flattened area and improve processing effectiveness. These strategies enable precise, large-area defect repair with high throughput.
“Our study not only reveals the formation dynamics of nanobubbles and explores how they change under laser irradiation, but also provides a novel principle and technique for nanodefect repair in two-dimensional materials,” the scientists note. “This method achieves high-efficiency flattening, non-destructive optoelectronic properties, precise control, and excellent long-term stability. With its high process compatibility under ambient conditions, it offers a reliable solution for defect repair in 2D material-based devices.”
“These features give this method outstanding advantages in the mass production of two-dimensional materials and devices. It is expected to play an important role in expanding the application prospects of high-performance 2D material devices in the next-generation semiconductor industry.”the scientists forecast.
Journal
Light: Advanced Manufacturing
Article Title
Laser optothermal nanobomb for efficient flattening of nanobubbles in van der waals materials