ZnO nanomaterials have become appealing for next-generation micro/nanodevices owing to their unique properties. However, in-situ, one-step, patterned synthesis of ZnO nanomaterials with small grain sizes and high specific surface areas remains challenging. Conventional nanomaterial synthesis techniques require cumbersome and extremely time-consuming transfer and assembly processes or solution direct-patterning (SDP) techniques with limited precision and demanding rheological requirements. While breakthroughs in laser-based synthesis techniques have enabled simultaneous growth and patterning of these materials, device integration restrictions owing to pre-prepared laser-absorbing layers and high heat-affected zones remain a severe issue.
To address these challenges, prof. Xiong Wei's team from the Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology recently introduced a novel glycerol-assisted femtosecond laser direct writing (FsLDW) method to realize the single-step synthesis of high-specific surface areas patterned ZnO nanomaterials. Their findings were published in Light: Advanced Manufacturing, titled “Glycerol-assisted Grain Modulation in Femtosecond-Laser-induced Photochemical Synthesis of Patterned ZnO Nanomaterials”.
Figure 1. Schematic diagram of glycerol-assisted FsLDW method.
In this study, researchers prepared a liquid-phase precursor through the hydrolysis and condensation of zinc acetate, followed by spin coating to form a thin film on a substrate. Near-infrared femtosecond laser pulses were focused at the interface between the substrate and the precursor film. The laser focus can be considered as a miniature reaction chamber, where the multiphoton photochemical effects promote the cleavage and formation of chemical bonds, and initiate the synthesis of ZnO nanomaterials. Controlling laser beam scanning in a designed pathway allows patterned products be fabricated without masks. This method not only simplifies the patterned synthesis process of nanomaterials but also enables efficient device integration.
Figure 2. Glycerol-assisted FsLDW ZnO patterned nanomaterials.
Arbitrary ZnO micropatterns, including bow-shaped lines and logo patterns, were fabricated by controlling the laser beam scanning in a designed pathway. Moreover, by incorporating glycerol additives, they reduced the grain size, increased the specific surface area, and effectively enhanced the consistency and uniformity of the products, thereby overcoming bottlenecks encountered during the fabrication of integrated functional devices. This enhancement by glycerol is primarily attributed to its inhibitory effect on precursor hydrolysis and condensation processes, along with its spatial hindrance during the FsLDW process. This phenomenon provides a theoretical foundation for further optimization and improvement of material properties.
Figure 3. ZnO-based UV photodetector fabricated by glycerol-assisted FsLDW.
Researchers fabricated a ZnO photodetector based on this method to demonstrate the enhancement of device performance due to high specific surface area products. The high specific surface area achieved through the glycerol-assisted FsLDW increases the number of oxygen molecules adsorbed on the grain surface and expands the electron depletion region, thereby enabling an extremely low dark current and high switching ratio in the device.
In this method, the "cold processing" characteristic of femtosecond lasers greatly suppresses the heat-affected zone during the laser direct writing process, improving the precision of the products. Additionally, the extremely high peak power of the femtosecond laser induces nonlinear multiphoton absorption effects in the precursor, negating the necessity for a laser-absorbing layer. Similar glycerol-assisted FsLDW techniques can be employed to fabricate other metal oxide nanomaterials, such as tin, nickel, and titanium oxides, which is of great significance for improving the performance of functional devices based on surface effects, such as in gas sensing and catalysis.
Wang Yingchen and Xue Songyan, PhD students from the Wuhan National Laboratory for Optoelectronics, are the co-first authors of the paper, with Professor Xiong Wei as the corresponding author. Huazhong University of Science and Technology and Hubei Optics Valley Laboratory are the research institutions involved. This research work has received support from the National Natural Science Foundation of China.
Paper Link: https://www.light-am.com/article/pdf/preview/LAM2024060059.pdf
