Multiphoton lithography (MPL) can use new organic semiconductor material (OS)


Development of innovative 3D micro and nanoscale technologies in nanoelectronics,[1] micro/nanoelectromechanical systems,[2] nanophotonics,[3] micro/nanofluidics,[4] and nano-biosciences[5] requires advancements in existing light induced-additive manufacturing techniques including electron beam lithography,[6] Digital Light Processing,[7] dip pen lithography,[8] stereolithography,[9] interference lithography,[10] and multiphoton lithography (MPL).[11] 

Among these techniques, MPL based on direct laser writing is state-of-the-art owing to its high spatial and temporal control as well as the versatility of photosensitive materials mostly composed of acrylate-based polymers/monomers or epoxy-based photoresists. One notable feature of MPL is its ability to fabricate 3D microstructures with subwavelength resolution typically 100–200 nm beyond the optical diffraction limit, achieved by combination of threshold effect and reduction of effective beam diameters due to multiphoton adsorption of pulsed femtosecond near-infrared laser beams (≈800 nm).[12, 13] Precise modulation of laser intensity, together with optimal writing speed and use of high numerical aperture objective lens have even enabled spatial resolution down to 15 nm.[14]

More importantly, MPL is an ideal tool for 3D printing of functional structures using photosensitive composite resins doped with stimuli-responsive nanomaterials. Semiconductive nanoparticles, magnetic nanoparticles, and metallic nanoparticles have been introduced into the resins to fabricate photoluminescent micro-nanostructures,[15] remotely controllable micro–nanomachines,[16] and conductive micro–nanoarchitectures,[17] respectively. However, these nanomaterials that usually exhibit inorganic properties are difficult to disperse homogenously in largely doped fashion within photopolymerizable resins without significant phase separation.[16, 18]

In recent years, there have been significant interests and efforts in fabrication of MPL-based electrically conductive structures due to their potential applications in emerging fields such as nanoelectronics, nanophotonics, plasmonics, and bioelectronics. Metal salts such as AgNO3[19, 20] and HAuCl4[17, 21, 22] with relatively high concentration (≈50 wt%) can be mixed with photoresist and MPL is employed for simultaneous photoreduction of metallic salts and photopolymerization of photoresist to fabricate highly conductive metallic nanoparticle-doped polymeric microstructures.

Although relatively high specific electrical conductivities up to ≈106 S m−1 wt%−1 (conductivity per concentration of metallic nanoparticles) can be achieved, metal-nanoparticle-loaded resins produce planar structures.[17] On the other hand, MPL-compatible composite resins containing carbon nanotubes (CNTs)[23-25] and graphene[26, 27] exhibit 3D microstructures. However, these nanomaterials demonstrate low specific electrical conductivities up to only ≈200 S m−1 wt%−1.[24]

To address these challenges, herein, we report a homogenous, transparent, and stable photosensitive resin doped with an organic semiconductor material (OS) to fabricate highly conductive 3D microstructures with high-quality structural features via MPL process. OSs are a broad family of π-conjugated molecules or polymers with alternating single and double bonds. The oxidized OSs (i.e., positively (p) doped) can reach conductivities up to several thousand S m−1.

To maintain electroneutrality, anions are intercalated within the bulk of p-doped OSs, providing both mobile electronic and ionic charge carriers.[28] Furthermore, OSs have many unique properties such as mixed ionic and electronic conduction, mechanical flexibility, large optical absorption and emission, and solution processability[29, 30] that are crucial for several applications including optoelectronics,[31, 32] printed electronics,[33, 34] and bioelectronics.[35-38] Poly(3,4-ethylenedioxythiophene) (PEDOT) is considered one of the most promising OSs due to its high electrical conductivity and chemical stability.[39, 40] 

Fabrication of organic bioelectronics has mostly relied on patterning and/or electrodeposition of OS on metal electrodes that are microfabricated using conventional multi-step lithography methods with limitations and challenges such as low resolution, 2D patterns, and/or complex and high-cost procedures.[41-43]

Here, for the first time we report applying MPL process to directly fabricate 3D organic semiconductor composite microstructures (OSCMs). We introduce a new MPL compatible resin composed of photopolymer poly(ethylene glycol) diacrylate (PEGA), organic semiconductor poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), photoinitiator 3-(trimethoxysilyl)propyl methacrylate (T-POL), and miscible agent dimethyl sulfoxide (DMSO) (Figure 1A, black boxes).

Microelectronic devices made of this OS composite resin were fabricated via MPL process and were characterized. Furthermore, proteins such as laminin and glucose oxidase were incorporated (Figure 1A, blue box) within MPL-based conductive microstructures and assessed for their biological activity and functionality. Compared to other 3D printing methods[44] such as ink-jet printing,[45] aerosol printing,[46] nozzle printing,[47] screen printing,[48] and lithography,[49] MPL offers facile fabrication of OS composite microstructures with high resolution and high aspect ratio that can be integrated with other MPL compatible materials such as insulating polymers to fabricate functional electronic circuits, biosensors and bioelectronics.

Details are in the caption following the image
Figure 1: Resin components and MPL fabrication process. A) Components of the OS composite resin: photopolymer poly(ethylene glycol) diacrylate (PEGA), organic semiconductor (OS) poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), photoinitiator 3-(trimethoxysilyl)propyl methacrylate (T-POL), miscible agent dimethyl sulfoxide (DMSO), and proteins such as laminin and glucose oxidase. B) Experimental setup for the MPL process, including resin, glass/PDMS substrates, 3D piezostage, controller, femtosecond (fs) laser, and the objective lens. C) The OS composite resin (yellow color) is crosslinked by a focused fs laser to create 3D OSCMs (green color). D) The sample is then rinsed in ethanol to remove any unsolidified resin, leaving the 3D OSCMs on the substrate.

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