Laser-Induced Selective Metallization on Polymer Substrates Using Organocopper for Portable Electronics

Direct Laser Writing: From Materials Synthesis and Conversion to Electronic Device Processing

Direct Laser Writing (DLW) has been increasingly selected as a microfabrication route for efficient, cost-effective, high-resolution material synthesis and conversion. Concurrently, lasers participate in the patterning and assembly of functional geometries in several fields of application, of which electronics stand out. In this review, recent advances and strategies based on DLW for electronics microfabrication are surveyed and outlined, based on laser material growth strategies. First, the main DLW parameters influencing material synthesis and transformation mechanisms are summarized, aimed at selective, tailored writing of conductive and semiconducting materials. Additive and transformative DLW processing mechanisms are discussed, to open space to explore several categories of materials directly synthesized or transformed for electronics microfabrication. These include metallic conductors, metal oxides, transition metal chalcogenides and carbides, laser-induced graphene, and their mixtures. By accessing a wide range of material types, DLW-based electronic applications are explored, including processing components, energy harvesting and storage, sensing, and bioelectronics. The expanded capability of lasers to participate in multiple fabrication steps at different implementation levels, from material engineering to device processing, indicates their future applicability to next-generation electronics, where more accessible, green microfabrication approaches integrate lasers as comprehensive tools.

Splashing-Assisted Femtosecond Laser-Activated Metal Deposition for Mold- and Mask-Free Fabrication of Robust Microstructured Electrodes for Flexible Pressure Sensors

Flexible pressure sensors play an indispensable role in flexible electronics. Microstructures on flexible electrodes have been proven to be effective in improving the sensitivity of pressure sensors. However, it remains a challenge to develop such microstructured flexible electrodes in a convenient way. Inspired by splashed particles from laser processing, herein, a method for customizing microstructured flexible electrodes by femtosecond laser-activated metal deposition is proposed. It takes advantage of the catalyzing particles scattered during femtosecond laser ablation and is particularly suitable for moldless, maskless, and low-cost fabrication of microstructured metal layers on polydimethylsiloxane (PDMS). Robust bonding at the PDMS/Cu interface is evidenced by the scotch tape test and the duration test over 10 000 bending cycles. Benefiting from the firm interface, the developed flexible capacitive pressure sensor with microstructured electrodes presents several conspicuous features, including a sensitivity (0.22 kPa^-1 ) 73 times higher than the one using flat Cu electrodes, ultralow detection limit (<1 Pa), rapid response/recovery time (4.2/5.3 ms), and excellent stability. Moreover, the proposed method, inheriting the merits of laser direct writing, is capable of fabricating a pressure sensor array in a maskless manner for spatial pressure mapping.

Low-power laser manufacturing of copper tracks on 3D printed geometry using liquid polyimide coating

Silver nanoparticle photoreduction synthesis by direct laser writing is a process that enables copper micro-track production on very specific polymers. However, some important 3D printing polymers, such as acrylonitrile butadiene styrene (ABS) and acrylates, do not accept this treatment on their surface. This work presents an approach to produce copper microcircuitry on 3D substrates from these materials by using direct laser writing at low power (32 mW CW diode laser). We show that by coating a thin layer of polyimide (PI) on a 3D-printed geometry, followed by a sequence of chemical treatments and low-power laser-induced photoreduction, copper tracks can be produced using silver as catalyst. The surface chemistry of the layer through the different stages of the process is monitored by FTIR and X-ray photoelectron spectroscopy. The copper tracks are selectively grown on the laser-patterned areas by electroless copper deposition, with conductivity (1.2 ± 0.7) × 10⁷ S m^-1 and a width as small as 28 μm. The patterns can be written on 3D structures and even inside cavities. The technique is demonstrated by integrating different circuits, including a LED circuit on 3D printed photopolymer acrylate and a perovskite solar cell on an ABS 3D curved geometry.

Enhanced Performance of Triboelectric Nanogenerators and Sensors via Cold Spray Particle Deposition

In this study, a high-performance triboelectric nanogenerator (TENG) is developed based on cold spray (CS) deposition of composite material layers. Composite layers were fabricated by cold spraying of micron-scale tin (Sn) particles on aluminum (Al) and polytetrafluoroethylene (PTFE) films, which led to improved TENG performance owing to functionalized composite layers as friction layers and electrodes, respectively. As-sprayed tin composite layers not only enhanced the flow of charges by strong adhesion to the target layer but also formed a nano-microstructure on the surface of the layers, thereby increasing the surface area during friction. More importantly, the electricity generation performance was improved more than 6 times as compared to the TENG without CS deposition on it. From parametric studies, the TENG using the cold-sprayed composite layer produced an electrical potential of 1140 V for a simple structure with a 25.4 × 25.4 mm² contact area. We also optimize the geometry and fabrication process of the TENG to increase the manufacturing efficiency while reducing the processing cost. The resultant sprayed layers and structures exhibited sustainable robustness by showing consistent electrical performance after the mechanical adhesion test. The proposed manufacturing approach is also applicable for processing three-dimensional (3D) complex layers owing to the technological convergence of a cold spray gun attached to a robotic arm, which makes possible to fabricate the 3D TENG. To elaborate, a composite layer having the shape of a 3D ball is produced, and the exercise status of the ball is monitored in real-time. The fabricated 3D ball using the TENG transmitted a distinguishable signal in real-time according to the state of the ball. The proposed TENG sensing system can be utilized as a self-powered sensor without the need of a battery, amplifier, and rectifier. The results of this study can potentially provide insights for the practical material design and fabrication of self-powered TENG systems.

Cluster-enabled patterning of copper nanostructures from aqueous solution using a femtosecond laser

A one-step method for patterning low-resistivity nanoscale copper wire is proposed herein to solve the challenging issues of using common metals rather than noble metal nanostructures fabricated by direct laser writing in solution. A complexing and a reducing agent were introduced for the single-photon absorption of copper solution in the visible range and to enable two-photon absorption with a femtosecond laser. Copper clusters were generated prior to direct laser writing to decrease induced laser energy during two-photon absorption and accelerate copper nanowire patterning to avoid the boiling of copper solution. A surfactant was used to restrain the overgrowth of copper clusters to obtain written nanowires with high uniformity. By controlling the laser writing parameters, the obtained copper wire had a minimum width of 230 nm and a resistivity of 1.22 × 10^-5Ω·m. Our method paves the way for the fabrication of common metal nanodevices by direct laser writing.

The Effect of Antimony (III) Oxide on the Necessary Amount of Precursors Used in Laser-Activated Coatings Intended for Electroless Metallization

The article presents research on the potential use of organometallic compounds with the addition of antimony (III) oxide Sb₂O₃ as a coating additive that will make coatings susceptible to electroless metallization after prior surface irradiation with 193 nm wavelength laser radiation and a different number of laser pulses. The surface modification and activation effects were assessed by optical-imagining as well as by scanning electron microscopy (SEM) with energy dispersive analysis (EDX). It was found that the presence of Sb₂O₃ in the coating made it possible to reduce the content of the copper complex, causing an intensive surface ablation, resulting in the formation of a conical structure with a higher content of metallic copper nuclei.

Autocatalytic Laser Activator for Both UV and NIR Lasers: Preparation of Circuits on Polymer Substrates by Selective Metallization

In laser-induced selective metallization (LISM), conventional laser activators only work at a single laser wavelength. This study reported a new laser activator (MoO₃) very suitable for both 355 nm UV and 1064 nm near-infrared (NIR) lasers for the first time. When applying MoO₃ to polymers, the prepared Cu layer on laser-activated polymers showed a good conductivity (2.63 × 10⁶ Ω^-1·m^-1) and excellent adhesion. Scanning electron microscopy, optical microscopy, and resistance analysis revealed the excellent LISM performance of the polymer/MoO₃ composites, and the quality of the Cu layer prepared using the UV laser is much better than that using the NIR laser. The limit width of the copper wire prepared by the UV laser is as narrow as 30.1 μm. We also confirmed the mechanism of MoO₃ initiating electroless copper plating after laser activation to be the autocatalytic mechanism, which is very different from the conventional reduction mechanism. The effect of laser activation was only to expose the MoO₃ active species to the polymer surface. X-ray diffraction and tube experiments revealed that the activity of α·h-MoO₃ was higher than that of α-MoO₃. X-ray photoelectron spectroscopy indicated that a part of Mo⁶⁺ was reduced to Mo⁵⁺ during laser activations, leading to the increase of the oxygen vacancies in MoO₃ and possibly further enhancing the activity of MoO₃. Besides, the micro-rough structures caused by the laser on the polymer surface provided riveting points for successfully depositing the copper layer. The Ni-Cu, Ag-Cu, and Au-Ni-Cu layers were obtained via the continued deposit of other metals on the Cu layer. The resistances of these metal layers had much better stability than that of the neat Cu layer. Furthermore, the Au layer further enhanced the conductivity of the circuit. The proposed strategy is easy for large-scale industrial applications, which will greatly expand the application scenarios of the LISM field.

Scalable Strategy to Directly Prepare 2D and 3D Liquid Metal Circuits Based on Laser-Induced Selective Metallization

Selective wetting of a gallium-based liquid metal on copper circuits is one of the ways to prepare liquid metal circuits. However, the complex fabrication processes of an adhesion layer between copper circuits (or patterns) and substrates were still inevitable, limiting scalable applications. Our work developed a facile way to directly prepare 2D and 3D liquid metal circuits by combining laser-induced selective metallization and selective wetting for the first time. The copper template was obtained on elastomers using laser-induced selective metallization, and high-resolution liquid metal circuits were fabricated by brushing Galinstan on the copper template in the alkali solution. The distribution of Cu element not only was on the top surface but also extended to the interior of the elastomer substrate. This revealed that the Cu layer prepared by laser-induced selective metallization is born to firmly embed into the substrate, which endowed the circuits with strong adhesion, reaching the highest 5B level. Moreover, the prepared liquid metal circuits (or patterns) had a typical layered structure. The liquid metal circuits exhibit good flexibility, stretchability, self-healing ability, and acid-alkaline resistance. Compared with the traditional methods of patterning liquid metals, fabricating liquid metal circuits based on laser-induced selective metallization has irreplaceable advantages, such as strong adhesion between circuits and substrate, fabricating 3D circuits, good acid-alkaline resistance, cost-effectiveness, maskless use, time savings, arbitrary design of patterns, and convenient operation, which endow this method with great application prospect.

Direct Laser Writing of Copper Micropatterns from Deep Eutectic Solvents Using Pulsed near-IR Radiation

In this study, we developed a method for the fabrication of electrically conductive copper patterns of arbitrary topology and films on dielectric substrates, by improved laser-induced synthesis from deep eutectic solvents. A significant increase in the processing efficiency was achieved by acceptor substrate pretreatment, with the laser-induced microplasma technique, using auxiliary glass substrates and optional laser post-processing of the recorded structures; thus, the proposed approach offers a complete manufacturing cycle, utilizing a single, commercially available, pulsed Yb fiber laser system. The potential implications of the presented research are amplified by the observation of laser-induced periodic surface structures (LIPSSs) that may be useful for the further tuning of tracks' functional properties.

Flexible Metal Electrodes by Femtosecond Laser-Activated Deposition for Human-Machine Interfaces

Flexible metal electrodes are essential for flexible electronics, where the main challenge is to obtain mask-free patterned metals directly on substrates such as poly(dimethylsiloxane) (PDMS) at low cost. This work highlights a feasible strategy named femtosecond laser-activated metal deposition for electroless deposition of metals (Cu, Ni, Ag, and Au) on PDMS, which is suitable for maskless and low-cost fabrication of metal layers on PDMS and even on other materials of different natures including polyethylene terephthalate, paper, Si, and glass. The electrical conductivity of the PDMS/Cu electrode is comparable to that of bulk Cu. Moreover, robust bonding at the PDMS/Cu interface is evidenced by a scotch tape test and bending test of more than 20,000 cycles. Compared with previous studies using a nanosecond laser, the restriction on absorbing sensitizers could be alleviated, and catalysts could originate from precursors without polymer substrates under a femtosecond laser, which may be attributed to nonlinear absorption and ultrashort heating time with the femtosecond laser. Implementing a human-machine interface task is demonstrated by recognizing hand gestures via a multichannel electrode array with high fidelity to control a robot hand.

A Fullerene-Platinum Complex for Direct Functional Patterning of Single Metal Atom-Embedded Carbon Nanostructures

The development of patterning materials ("resists") at the nanoscale involves two distinct trends: one is toward high sensitivity and resolution for miniaturization, the other aims at functionalization of the resists to realize bottom-up construction of distinct nanoarchitectures. Patterning of carbon nanostructures, a seemingly ideal application for organic functional resists, has been highly reliant on complicated pattern transfer processes because of a lack of patternable precursors. Herein, we present a fullerene-metal coordination complex as a fabrication material for direct functional patterning of sub-10 nm metal-containing carbon structures. The attachment of one platinum atom per fullerene molecule not only leads to significant improvement of sensitivity and resolution but also enables stable atomic dispersion of the platinum ions within the carbon matrix, which may gain fundamentally new interest in functional patterning of hierarchical carbon nanostructures.

Comparative Evaluation of Cu(acac)2 and {[Cu(μ-O,O'-NO3) (L-arg) (2,2'-bpy)]·NO3}n as Potential Precursors of Electroless Metallization of Laser-Activated Polymer Materials

This paper presents a comparative assessment of Cu(acac)2 and {[Cu(μ-O,O'-NO3) (L-arg)(2,2'-bpy)]·NO3}n as potential precursors for the electroless metallization of laser activated polymer materials. Coatings consisting of polyurethane resin, one of the two mentioned precursor compounds, and antimony oxide (Sb2O3), as a compound strongly absorbing infrared radiation, were applied on the polycarbonate substrate. The coatings were activated with infrared Nd: YAG laser radiation (λ = 1064 nm) and electroless metallized. It was found that after laser irradiation, a micro-rough surface structure of the coatings was formed, on which copper was present in various oxidation states, as well as in its metallic form. For selected parameters of laser irradiation, it was possible to deposit a copper layer on the coating containing Cu(acac)2 and Sb2O3, which is characterized by high adhesion strength. It was also found that the {[Cu(μ-O,O'-NO3) (L-arg)(2,2'-bpy)]·NO3}n complex was not an effective precursor for the electroless metallization of Nd:YAG laser activated coatings. An attempt was made to determine the influence of the precursor chemical structure on the obtained metallization effects.

Laser Synthesis and Microfabrication of Micro/Nanostructured Materials Toward Energy Conversion and Storage

Nanomaterials are known to exhibit a number of interesting physical and chemical properties for various applications, including energy conversion and storage, nanoscale electronics, sensors and actuators, photonics devices and even for biomedical purposes. In the past decade, laser as a synthetic technique and laser as a microfabrication technique facilitated nanomaterial preparation and nanostructure construction, including the laser processing-induced carbon and non-carbon nanomaterials, hierarchical structure construction, patterning, heteroatom doping, sputtering etching, and so on. The laser-induced nanomaterials and nanostructures have extended broad applications in electronic devices, such as light-thermal conversion, batteries, supercapacitors, sensor devices, actuators and electrocatalytic electrodes. Here, the recent developments in the laser synthesis of carbon-based and non-carbon-based nanomaterials are comprehensively summarized. An extensive overview on laser-enabled electronic devices for various applications is depicted. With the rapid progress made in the research on nanomaterial preparation through laser synthesis and laser microfabrication technologies, laser synthesis and microfabrication toward energy conversion and storage will undergo fast development.

Laser-induced deposition of copper from deep eutectic solvents: optimization of chemical and physical parameters

Fabrication of conductive copper structures on the surface of various dielectric materials is quite important in many fields of science.

Top-Down Direct Preparation of Orange-Yellow Dye Similar to Psittacofulvins from Commercial Polymer by Laser Writing

Laser manufacturing is a promising method for the design and preparation of high value-added materials. When the laser acts on the polymer precursors, some wonderful phenomena will always occur and accompanied by the generation of new substances. Herein, we report a top-down approach for the direct preparation of orange-yellow dye that is similar to psittacofulvins from commercial polymer resins by laser writing. Conjugated double bonds and micro-rough structures are formed simultaneously on laser-irradiated polymer substrate surfaces. The typical polyconjugated structures of psittacofulvin dyes were confirmed by micro-Raman and Raman imaging results. Temperature-dependent Fourier transform infrared and X-ray photoelectron spectroscopy further demonstrated the formation mechanism of laser-induced psittacofulvins dyes based on the chemical composition. Further, optical microscopy, laser confocal microscopy, and scanning electron microscopy were carried out to characterize the physical morphologies of laser-irradiated polymer substrates. A unique advantage of preparing psittacofulvins dye using laser writing is its simple steps, and the dye can be converted directly from the appropriate precursor substrate. Interestingly, the laser-irradiated polymer substrate surface undergoes color change. This laser-induced color patterning is attractive due to the characteristics of high precision, flexibility, and maskless; any patterns can be easily designed and produced on the polymer at desired positions.