A Simple Way to Achieve Legible and Local Controllable Patterning for Polymers Based on a Near-Infrared Pulsed Laser

Photothermal Nanomaterials: A Powerful Light-to-Heat Converter

All forms of energy follow the law of conservation of energy, by which they can be neither created nor destroyed. Light-to-heat conversion as a traditional yet constantly evolving means of converting light into thermal energy has been of enduring appeal to researchers and the public. With the continuous development of advanced nanotechnologies, a variety of photothermal nanomaterials have been endowed with excellent light harvesting and photothermal conversion capabilities for exploring fascinating and prospective applications. Herein we review the latest progresses on photothermal nanomaterials, with a focus on their underlying mechanisms as powerful light-to-heat converters. We present an extensive catalogue of nanostructured photothermal materials, including metallic/semiconductor structures, carbon materials, organic polymers, and two-dimensional materials. The proper material selection and rational structural design for improving the photothermal performance are then discussed. We also provide a representative overview of the latest techniques for probing photothermally generated heat at the nanoscale. We finally review the recent significant developments of photothermal applications and give a brief outlook on the current challenges and future directions of photothermal nanomaterials.

Modification of Polymeric Surfaces with Ultrashort Laser Pulses for the Selective Deposition of Homogeneous Metallic Conductive Layers

In recent years, the demand for highly integrated and lightweight components has been rising sharply, especially in plastics processing. One strategy for weight-saving solutions is the development of conductive tracks and layouts directly on the polymer housing parts in order to be able to dispense with the system integration of additional printed circuit boards (PCB). This can be conducted very advantageously and flexibly with laser-based processes for functionalizing polymer surfaces. In this work, a three-step laser-based process for subsequent selective metallization is presented. Conventional injection molded components without special additives serve as the initial substrate. The Laser-Based Selective Activation (LSA) uses picosecond laser pulses to activate the plastic surface to subsequently deposit palladium. The focus is on determining the amount of deposited palladium in correlation to the laser and scan parameters. For the first time, the dependence of the metallization result on the accumulated laser fluence (F_acc) is described. The treated polymer parts are characterized using optical and scanning electron microscopy as well as a contact-type profilometer.

Aramid textile with near-infrared laser-induced graphene for efficient adsorption materials

Porous carbon materials show great application potential in the field of adsorption. However, the preparation process of carbon adsorption materials relies on high temperature, high energy consumption, many steps, and long time. Most of them exist in the form of powder or block, and the practical application scenarios are limited and difficult to recycle. In this study, based on in-situ carbonization of polymer precursor, we directly generated laser-induced graphene (LIG) on the surface of commercial aramid textile using a low-energy near-infrared laser in air, and prospected the application prospect of the prepared aramid/graphene textile in the field of adsorption. Under a certain laser energy, the photothermal reaction promotes the breaking of the CO and CN bonds in the surface layer of the aramid fiber, and reorganizes into a graphene structure at an instantaneous high temperature, while the overall flexible structure of the textile was not destroyed. Further, adsorption materials based on the as-prepared aramid/graphene textiles were also designed, including VOC-adsorbing textile in air and dye-adsorbing textile in water. Using low-energy near-infrared laser to directly achieve LIG writing in commercial textiles under air condition will provide an efficient, environmentally friendly, and designable direction for the large-scale fabrication of textile adsorption products.

The Impact of Selected Laser-Marking Parameters and Surface Conditions on White Polypropylene Moldings

Laser marking of polymer materials is a technology that is increasingly used in industry. Polypropylene (PP) shows a low ability to absorb electromagnetic radiation in the near-infrared range (λ = 1064 nm). The paper presents the influence of the surface condition of white-colored polypropylene moldings on the efficiency of their marking with a laser beam. In addition, the operation of the commercial laser marking additive (LMA) Lifolas M 117009 UN, intended to support the process of laser marking of polyolefin surfaces, was verified. The study is an attempt to combine laser operating parameters, material, and geometric properties of PP moldings to obtain the expected quality of graphic symbols. The test samples were made by injection molding method with the use of a specially designed modular injection mold. The molding cavities were prepared with various methods of metal processing, thanks to which obtained moldings differed in surface condition. The marking effects were assessed based on colorimetric tests and digital image analysis. The 0.5 wt% LMA content resulted in obtaining a graphic sign with high contrast in comparison to the background. The gradual increase in the modifier content resulted in a further increase in contrast. These values depended on the degree of surface finish of the samples, and therefore on the roughness parameters. Samples with a rough surface finish showed higher contrast compared to surfaces with a high surface finish. It was also found that for the analyzed moldings, the laser-marking process should be performed with the use of a low head velocity (450–750 mm/s) and a high concentration of the laser beam (0.03–0.05 mm).

Pitaya-Structured Microspheres with Dual Laser Wavelength Responses for Polymer Laser Direct Writing

A unique pitaya-structured graphene/TiO₂@PS microsphere with dual laser wavelength responses is designed and prepared via a facile approach of polymer melt blending. The graphene/TiO₂ particles ("pitaya seeds") are homogeneously distributed in the polystyrene ("pitaya pulp") of the microspheres with an average size of 1.5 μm. The graphene in microspheres serves not only as a laser absorber that has responses to both 355 nm UV and 1064 nm NIR lasers but also as a reducing agent of TiO₂ during laser direct writing (LDW). As expected, benefiting from the unique pitaya-structured structure, the graphene/TiO₂@PS microsphere can remarkably improve the performance of both NIR and UV LDW of polymers. The results of characterizations reveal that the black color caused by NIR LDW is due to the generation of the amorphous carbon and the color change after UV LDW is owing to the formation of black sp/sp² carbon compounds. Meanwhile, some TiO₂ in microspheres is reduced into the black/gray titanium oxides of Ti²⁺ and Ti³⁺ after NIR and UV LDW, respectively. The above co-contribution endows the graphene/TiO₂@PS microspheres with an outstanding color-changing ability. This pitaya-structured microsphere will have a profound effect on polymers' laser direct writing.

Preparation and Laser Marking Properties of Poly(propylene)/Molybdenum Sulfide Composite Materials

In this study, using molybdenum sulfide (MoS₂) as laser-sensitive particles and poly(propylene) (PP) as the matrix resin, laser-markable PP/MoS₂ composite materials with different MoS₂ contents ranging from 0.005 to 0.2% were prepared by melt-blending. A comprehensive analysis of the laser marking performance of PP/MoS₂ composites was carried out by controlling the content of laser additives, laser current intensity, and the scanning speed of laser marking. The color difference test shows that the best laser marking performance of the composite can be obtained at the MoS₂ content of 0.02 wt %. The surface morphology of the PP/MoS₂ composite material was observed after laser marking using a metallographic microscope, an optical microscope, and a scanning electron microscope (SEM). During the laser marking process, the laser energy was absorbed and converted into heat energy to cause high-temperature melting, pyrolysis, and carbonization of PP on the surface of the PP/MoS₂ composite material. The black marking from carbonized materials was formed in contrast to the white matrix. Using X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and Raman spectroscopy, the composite materials before and after laser marking were tested and characterized. The PP/MoS₂ composite material was pyrolyzed to form amorphous carbonized materials. The effect of the laser-sensitive MoS₂ additive on the mechanical properties of composite materials was investigated. The results show that the PP/MoS₂ composite has the best laser marking property when the MoS₂ loading content is 0.02 wt %, the laser marking current intensity is 11 A, and the laser marking speed is 800 mm/s, leading to a clear and high-contrast marking pattern.

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.

Laser induced porous electrospun fibers for enhanced filtration of xylene gas

With the development of industry, the harm caused by volatile organic compound (VOC) gases to the human body has received much attention. This study reveals as the first attempt to apply laser irradiation technique to the preparation of porous electrospun fibers with excellent low-concentration VOC gases adsorption properties. The laser-sensitive polycarbonate (PC) fibers prepared from electrospinning was treated in air by scanning with a neodymium-doped yttrium aluminum garnet (Nd: YAG) pulsed laser beam to achieve porous structure. During the laser irradiation process, a series of changes such as melting, thermal degradation, and carbonization of the polymer fibers can change the surface structure. The morphology of the porous structure is related to the degree of laser-induced carbonization, and the laser current is an important parameter for determining the degree of laser-induced carbonization of a particular polymer. The results indicate that porous carbon structures can be created on the surface of the fiber membrane by controlling the degree of laser-induced carbonization, and a highly xylene gas adsorption efficiency is exhibited. This study may provide useful insights for developing electrospun porous fibers with VOC adsorption by simple, effective and environmentally friendly laser post-processing process.

Facile Fabrication of High-Contrast and Light-Colored Marking on Dark Thermoplastic Polyurethane Materials

In this work, using ferroferric oxide (Fe₃O₄) and zirconium oxide (ZrO₂) as laser-sensitive particles and thermoplastic polyurethane (TPU) as the matrix resin, a series of TPU/Fe₃O₄/ZrO₂ composites were prepared by melt blending, and the effect of the laser marking additive content, composition, and laser marking parameters on the laser marking properties of composites was investigated. The laser marking mechanism of Fe₃O₄/ZrO₂ additives and the role of each component in TPU laser marking were studied by metallographic microscopy, color difference test, scanning electron microscopy, and Raman spectroscopy. Fe₃O₄ nanoparticles as a laser sensitizer component, on the one hand, can act as a pigment to make the TPU substrate black and, on the other hand, can absorb laser energy to contribute to the formation of laser markings on TPU composite surfaces. In addition, the introduction of ZrO₂ nanoparticles can help absorb the laser energy, while the contrast can be improved to enhance the laser marking performance of the TPU composite. Through thermogravimetric analysis, the changes in the thermally stable properties of TPU composites before and after laser marking were investigated, and the results indicated that Fe₃O₄/ZrO₂ nanoparticles can absorb the laser energy, causing melting and pyrolysis of the TPU backbone at a high temperature, to produce a gaseous product resulting in foaming. Finally, the high-contrast and light-colored markings were formed on the black TPU composite surface. This work provides a facile method for producing high-contrast and light-colored markings on the dark TPU composite surface.

New Strategy to Achieve Laser Direct Writing of Polymers: Fabrication of the Color-Changing Microcapsule with a Core-Shell Structure

This paper proposed an efficient and environmentally friendly strategy to prepare a new color-changing microcapsule with a core-shell structure for laser direct writing of polymers, and only the physical melt blending of polymers was employed. The laser absorber (SnO₂) and the easily carbonized polymer (PC) were designed as the "core" and the "shell" of the microcapsule, respectively. The microcapsules were in situ formed during melt blending. Scanning electron microscopy, transmission electron microscopy, and energy-dispersive spectrometry confirmed the successful preparation of SnO₂/PC microcapsules with a core-shell structure. Their average diameter was 2.2 μm, and the "shell" thickness was 0.21-0.24 μm. As expected, these SnO₂/PC microcapsules endowed polymers with an outstanding performance of near-infrared (NIR) laser direct writing. Raman spectroscopy and X-ray photoelectron spectroscopy indicated that the color change was ascribed to the polymer carbonization because of the instantaneous high temperature caused by the SnO₂ absorption of NIR laser energy. Optical microscopy observed a thick carbonization layer of 234 μm. Moreover, Raman depth imaging revealed the carbonization distribution, confirming that the amorphous carbon produced by the carbonization of the PC "shell" is the key factor of SnO₂/PC microcapsules to provide polymers an outstanding performance of laser direct writing. This color-changing microcapsule has no selectivity to polymers because of providing a black color source (the carbonization of PC) itself, ensuring the high contrast and precision of patterns or texts after laser direct writing for all general-purpose polymers. We believe that this novel strategy to achieve laser direct writing of polymers will have broad application prospects.

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

Our work proposed a facile strategy for selective fabrication of the precise metalized patterns onto polymer substrates through the laser direct structuring (LDS) technology using organocopper compounds. Copper oxalate (CuC₂O₄) and copper acetylacetonate [Cu(acac)₂] which can be used as laser sensitizers were first introduced into an acrylonitrile-butadiene-styrene (ABS) matrix for preparing LDS materials. After the activation with 1064 nm pulsed near-infrared laser, the Cu⁰ (metal copper) was generated from CuC₂O₄ and Cu(acac)₂ and then served as catalyst species for the electroless copper plating (ECP). A series of characterizations were conducted to investigate the morphology and analyze the surface chemistry of ABS/CuC₂O₄ and ABS/Cu(acac)₂ composites. Specially, the X-ray photoelectron spectroscopy analysis indicated that 58.3% Cu²⁺ in ABS/CuC₂O₄ was reduced to Cu⁰, while this value was 63.9% for ABS/Cu(acac)₂. After 30 min ECP, the conductivities of copper circuit on ABS/CuC₂O₄ and ABS/Cu(acac)₂ composites were 1.22 × 10⁷ and 1.58 × 10⁷ Ω^-1·m^-1, respectively. Moreover, the decorated patterns and near-field communication circuit were demonstrated by this LDS technology. We believe that this study paves the way for developing organocopper-based LDS materials, which have the potential for industrial applications.

Flash Light Millisecond Self-Assembly of High χ Block Copolymers for Wafer-Scale Sub-10 nm Nanopatterning

One of the fundamental challenges encountered in successful incorporation of directed self-assembly in sub-10 nm scale practical nanolithography is the process compatibility of block copolymers with a high Flory-Huggins interaction parameter (χ). Herein, reliable, fab-compatible, and ultrafast directed self-assembly of high-χ block copolymers is achieved with intense flash light. The instantaneous heating/quenching process over an extremely high temperature (over 600 °C) by flash light irradiation enables large grain growth of sub-10 nm scale self-assembled nanopatterns without thermal degradation or dewetting in a millisecond time scale. A rapid self-assembly mechanism for a highly ordered morphology is identified based on the kinetics and thermodynamics of the block copolymers with strong segregation. Furthermore, this novel self-assembly mechanism is combined with graphoepitaxy to demonstrate the feasibility of ultrafast directed self-assembly of sub-10 nm nanopatterns over a large area. A chemically modified graphene film is used as a flexible and conformal light-absorbing layer. Subsequently, transparent and mechanically flexible nanolithography with a millisecond photothermal process is achieved leading the way for roll-to-roll processability.

Selective Metallization Induced by Laser Activation: Fabricating Metallized Patterns on Polymer via Metal Oxide Composite

Recently, metallization on polymer substrates has been given more attention due to its outstanding properties of both plastics and metals. In this study, the metal oxide composite of copper-chromium oxide (CuO·Cr₂O₃) was incorporated into the polymer matrix to design a good laser direct structuring (LDS) material, and the well-defined copper pattern (thickness =10 μm) was successfully fabricated through selective metallization based on 1064 nm near-infrared pulsed laser activation and electroless copper plating. We also prepared polymer composites incorporated with CuO and Cr₂O₃; however, these two polymer composites both had very poor capacity of selective metallization, which has no practical value for LDS technology. In our work, the key reasons causing the above results were systematically studied and elucidated using XPS, UV-vis-IR, optical microscopy, SEM, contact angle, ATR FTIR, and so on. The results showed that 54.0% Cu²⁺ in the polymer composite of CuO·Cr₂O₃ (the amount =5 wt %) is reduced to Cu⁰ (elemental copper) after laser activation (irradiation); however, this value is only 26.8% for the polymer composite of CuO (the amount =5 wt %). It was confirmed that to achieve a successful selective metallization after laser activation, not only was the new formed Cu⁰ (the catalytic seeds) the crucial factor, but the number of generated Cu⁰ catalytic seeds was also important. These two factors codetermined the final results of the selective metallization. The CuO·Cr₂O₃ is very suitable for applications of fabricating metallic patterns (e.g., metal decoration, circuit) on the inherent pure black or bright black polymer materials via LDS technology, which has a prospect of large-scale industrial applications.

A Dual-Responsive Nanocomposite toward Climate-Adaptable Solar Modulation for Energy-Saving Smart Windows

In this work, a novel fully autonomous photothermotropic material made by hybridization of the poly(N-isopropylacrylamide) (PNIPAM) hydrogel and antimony-tin oxide (ATO) is presented. In this photothermotropic system, the near-infrared (NIR)-absorbing ATO acts as nanoheater to induce the optical switching of the hydrogel. Such a new passive smart window is characterized by excellent NIR shielding, a photothermally activated switching mechanism, enhanced response speed, and solar modulation ability. Systems with 0, 5, 10, and 15 atom % Sb-doped ATO in PNIPAM were investigated, and it was found that a PNIPAM/ATO nanocomposite is able to be photothermally activated. The 10 atom % Sb-doped PNIPAM/ATO exhibits the best response speed and solar modulation ability. Different film thicknesses and ATO contents will affect the response rate and solar modulation ability. Structural stability tests at 15 cycles under continuous exposure to solar irradiation at 1 sun intensity demonstrated the performance stability of such a photothermotropic system. We conclude that such a novel photothermotropic hybrid can be used as a new generation of autonomous passive smart windows for climate-adaptable solar modulation.

Fabricating Metallic Circuit Patterns on Polymer Substrates through Laser and Selective Metallization

Nowadays, with the rapid development of portable electronics, wearable electronics, LEDs, microelectronics, and bioelectronics, the fabrication of metallic circuits onto polymer substrates with strong adhesion property is an ever-increasing challenge. In this study, the high-resolution and well-defined metallic circuits were successfully prepared on the polymer surface via laser direct structuring (LDS) based on copper hydroxyl phosphate [Cu₂(OH)PO₄], and the key mechanism of the selective metallization was systematically investigated. XPS confirmed that Cu⁰ (elemental copper) was formed through photochemical reduction reaction of Cu₂(OH)PO₄, after 1064 nm NIR pulsed laser irradiation. During the electroless plating, because it is the important active catalytic center, this newly formed Cu⁰ was the key factor to achieve the successful selective metallization. SEM revealed that after the electroless plating, the copper layer actually physically anchored into the polymer substrate, giving an excellent mechanical adhesion property of the obtained metallic patterns. In addition, the micro-Raman surface imaging approved the generation of the amorphous carbon on the polymer composites' surface after NIR laser irradiation, and the chemical reaction region caused by the pulsed laser spot was found at approximately 40 μm. This environmentally friendly and effective strategy for fabricating circuit patterns on the polymer surface has a possible application in the printed circuit plate (PCB) industry.

Local Controllable Laser Patterning of Polymers Induced by Graphene Material

Graphene has been successfully applied to the field of polymer laser patterning. As an efficient 1064 nm near-infrared (NIR) pulsed laser absorber, only 0.005 wt % (50 ppm) of graphene prepared by mechanical exfoliation endowed polymer materials with very good NIR pulsed laser patterning. Optical microscopy observed that the generated black patterns came from the local discoloration of the polymer surface subjected to the laser irradiation, and the depth of the discolored layer was determined to be within 221-348 μm. The X-ray photoelectron spectroscopy confirmed that the polymer surface discoloration was contributed by the local carbonization of polymers caused by graphene due to its high photothermal conversion capacity. Raman depth imaging successfully detected that the generated carbon in the discolored layer was composed of amorphous carbon and complex sp/sp²-carbon compounds containing C≡C or conjugated C═C/C≡C structures. This study also provides a simple guideline to fabricate laser-patterning polymer materials based on graphene. We believe that graphene has broad application prospects in the field of polymer laser patterning. Importantly, this work opens up a valuable, feasible direction for the practical application of this new carbon material.