High-Precision and Rapid Direct Laser Writing Using a Liquid Two-Photon Polymerization Initiator

Two-photon polymerization based direct laser writing (DLW) is an emerging micronano 3D fabrication technology wherein two-photon initiators (TPIs) are a key component in photoresists. Upon exposure to a femtosecond laser, TPIs can trigger the polymerization reaction, leading to the solidification of photoresists. In other words, TPIs directly determine the rate of polymerization, physicochemical properties of polymers, and even the photolithography feature size. However, they generally exhibit extremely poor solubility in photoresist systems, severely inhibiting their application in DLW. To break through this bottleneck, we propose a strategy to prepare TPIs as liquids via molecular design. The maximum weight fraction of the as-prepared liquid TPI in photoresist significantly increases to 2.0 wt %, which is several times higher than that of commercial 7-diethylamino-3-thenoylcoumarin (DETC). Meanwhile, this liquid TPI also exhibits an excellent absorption cross section (64 GM), allowing it to absorb femtosecond laser efficiently and generate abundant active species to initiate polymerization. Remarkably, the respective minimum feature sizes of line arrays and suspended lines are 47 and 20 nm, which are comparable to that of the-state-of-the-art electron beam lithography. Besides, the liquid TPI can be utilized to fabricate various high-quality 3D microstructures and manufacture large-area 2D devices at a considerable writing speed (1.045 m s^-1). Therefore, the liquid TPI would be one of the promising initiators for micronano fabrication technology and pave the way for future development of DLW.

From Light to Structure: Photo Initiators for Radical Two-Photon Polymerization

Two-photon polymerization (2PP) represents a powerful technique for the fabrication of precise three-dimensional structures on a micro- and nanometer scale for various applications. While many review articles are focusing on the used polymeric materials and their application in 2PP, in this review the class of two-photon photo initiators (2PI) used for radical polymerization is discussed in detail. Because the demand for highly efficient 2PI has increased in the last decades, different approaches in designing new efficient 2PIs occurred. This review summarizes the 2PIs known in literature and discusses their absorption behavior under one- and two-photon absorption (2PA) conditions, their two-photon cross sections (σTPA ) as well as their efficiency under 2PP conditions. Here, the photo initiators are grouped depending on their chromophore system (D-π-A-π-D, D-π-D, etc.). Their polymerization efficiencies are evaluated by fabrication windows (FW) depending on different laser intensities and writing speeds.

D–π–A–π–D Initiators Based on Benzophenone Conjugate Extension for Two-Photon Polymerization Additive Manufacturing

A two-photon polymerization initiator is a kind of nonlinear optical material. With the demand for more efficient initiators in two-photon polymerization additive manufacturing, there are more and more related studies. In this paper, four conjugate-extended two-photon polymerization initiators with different alkane chain lengths were designed and synthesized, and single-photon, two-photon, and photodegradation experiments were carried out. Additive manufacturing experiments illustrated that the designed molecules can be used as two-photon initiators, and the writing speed can achieve 100,000 μm/s at a laser power of 25 mW. Through theoretical calculation and experimental comparison of the properties of molecules with different conjugation degrees, it was proven that a certain degree of conjugation extension can improve the initiation ability of molecules; however, this improvement cannot be extended infinitely. Solubility tests of different acrylates showed that molecules with different alkane chain lengths have varying solubility. Changing the molecular alkane chain length may be favorable to adapt to different monomers.