Unveiling Fundamentals of Multi-Beam Pulsed Laser Ablation in Liquids toward Scaling up Nanoparticle Production.

Pulsed laser ablation in liquids (PLAL) is a versatile technique to produce high-purity colloidal nanoparticles. Despite considerable recent progress in increasing the productivity of the technique, there is still significant demand for a practical, cost-effective method for upscaling PLAL synthesis. Here we employ and unveil the fundamentals of multi-beam (MB) PLAL. The MB-PLAL upscaling approach can bypass the cavitation bubble, the main limiting factor of PLAL efficiency, by splitting the laser beam into several beams using static diffractive optical elements (DOEs). A multimetallic high-entropy alloy CrFeCoNiMn was used as a model material and the productivity of its nanoparticles in the MB-PLAL setup was investigated and compared with that in the standard single-beam PLAL. We demonstrate that the proposed multi-beam method helps to bypass the cavitation bubble both temporally (lower pulse repetition rates can be used while keeping the optimum processing fluence) and spatially (lower beam scanning speeds are needed) and thus dramatically increases the nanoparticle yield. Time-resolved imaging of the cavitation bubble was performed to correlate the observed production efficiencies with the bubble bypassing. The results suggest that nanoparticle PLAL productivity at the level of g/h can be achieved by the proposed multi-beam strategy using compact kW-class lasers and simple inexpensive scanning systems.

Green nanoparticle synthesis at scale: a perspective on overcoming the limits of pulsed laser ablation in liquids for high-throughput production.

Nanoparticles have become increasingly important for a variety of applications, including medical diagnosis and treatment, energy harvesting and storage, catalysis, and additive manufacturing. The development of nanoparticles with different compositions, sizes, and surface properties is essential to optimize their performance for specific applications. Pulsed laser ablation in liquid is a green chemistry approach that allows for the production of ligand-free nanoparticles with diverse shapes and phases. Despite these numerous advantages, the current production rate of this method remains limited, with typical rates in the milligram per hour range. To unlock the full potential of this technique for various applications, researchers have dedicated efforts to scaling up production rates to the gram-per-hour range. Achieving this goal necessitates a thorough understanding of the factors that limit pulsed laser ablation in liquid (PLAL) productivity, including laser, target, liquid, chamber, and scanner parameters. This perspective article explores these factors and provides a roadmap for increasing PLAL productivity that can be adapted to specific applications. By carefully controlling these parameters and developing new strategies for scaling up production, researchers can unlock the full potential of pulsed laser ablation in liquids.

Solvent Influence on the Magnetization and Phase of Fe-Ni Alloy Nanoparticles Generated by Laser Ablation in Liquids.

The synthesis of bimetallic iron-nickel nanoparticles with control over the synthesized phases, particle size, surface chemistry, and oxidation level remains a challenge that limits the application of these nanoparticles. Pulsed laser ablation in liquid allows the properties tuning of the generated nanoparticles by changing the ablation solvent. Organic solvents such as acetone can minimize nanoparticle oxidation. Yet, economical laboratory and technical grade solvents that allow cost-effective production of FeNi nanoparticles contain water impurities, which are a potential source of oxidation. Here, we investigated the influence of water impurities in acetone on the properties of FeNi nanoparticles generated by pulsed laser ablation in liquids. To remove water impurities and produce "dried acetone", cost-effective and reusable molecular sieves (3 Å) are employed. The results show that the Fe50Ni50 nanoparticles' properties are influenced by the water content of the solvent. The metastable HCP FeNi phase is found in NPs prepared in acetone, while only the FCC phase is observed in NPs formed in water. Mössbauer spectroscopy revealed that the FeNi nanoparticles oxidation in dried acetone is reduced by 8% compared to acetone. The high-field magnetization of Fe50Ni50 nanoparticles in water is the highest, 68 Am2/kg, followed by the nanoparticles obtained after ablation in acetone without water impurities, 59 Am2/kg, and acetone, 52 Am2/kg. The core-shell structures formed in these three liquids are also distinctive, demonstrating that a core-shell structure with an outer oxide layer is formed in water, while carbon external layers are obtained in acetone without water impurity. The results confirm that the size, structure, phase, and oxidation of FeNi nanoparticles produced by pulsed laser ablation in liquids can be modified by changing the solvent or just reducing the water impurities in the organic solvent.

Inactivation of SARS-CoV-2 by a chitosan/α-Ag2WO4 composite generated by femtosecond laser irradiation.

In the current COVID-19 pandemic, the next generation of innovative materials with enhanced anti-SARS-CoV-2 activity is urgently needed to prevent the spread of this virus within the community. Herein, we report the synthesis of chitosan/α-Ag2WO4 composites synthetized by femtosecond laser irradiation. The antimicrobial activity against Escherichia coli, Methicilin-susceptible Staphylococcus aureus (MSSA), and Candida albicans was determined by estimating the minimum inhibitory concentration (MIC) and minimal bactericidal/fungicidal concentration (MBC/MFC). To assess the biocompatibility of chitosan/α-Ag2WO4 composites in a range involving MIC and MBC/MFC on keratinocytes cells (NOK-si), an alamarBlue™ assay and an MTT assay were carried out. The SARS-CoV-2 virucidal effects was analyzed in Vero E6 cells through viral titer quantified in cell culture supernatant by PFU/mL assay. Our results showed a very similar antimicrobial activity of chitosan/α-Ag2WO4 3.3 and 6.6, with the last one demonstrating a slightly better action against MSSA. The chitosan/α-Ag2WO4 9.9 showed a wide range of antimicrobial activity (0.49-31.25 µg/mL). The cytotoxicity outcomes by alamarBlue™ revealed that the concentrations of interest (MIC and MBC/MFC) were considered non-cytotoxic to all composites after 72 h of exposure. The Chitosan/α-Ag2WO4 (CS6.6/α-Ag2WO4) composite reduced the SARS-CoV-2 viral titer quantification up to 80% of the controls. Then, our results suggest that these composites are highly efficient materials to kill bacteria (Escherichia coli, Methicillin-susceptible Staphylococcus aureus, and the yeast strain Candida albicans), in addition to inactivating SARS-CoV-2 by contact, through ROS production.

Comparison of ultrashort pulse ablation of gold in air and water by time-resolved experiments.

Laser ablation in liquids is a highly interdisciplinary method at the intersection of physics and chemistry that offers the unique opportunity to generate surfactant-free and stable nanoparticles from virtually any material. Over the last decades, numerous experimental and computational studies aimed to reveal the transient processes governing laser ablation in liquids. Most experimental studies investigated the involved processes on timescales ranging from nanoseconds to microseconds. However, the ablation dynamics occurring on a sub-nanosecond timescale are of fundamental importance, as the conditions under which nanoparticles are generated are established within this timeframe. Furthermore, experimental investigations of the early timescales are required to test computational predictions. We visualize the complete spatiotemporal picosecond laser-induced ablation dynamics of gold immersed in air and water using ultrafast pump-probe microscopy. Transient reflectivity measurements reveal that the water confinement layer significantly influences the ablation dynamics on the entire investigated timescale from picoseconds to microseconds. The influence of the water confinement layer includes the electron injection and subsequent formation of a dense plasma on a picosecond timescale, the confinement of ablation products within hundreds of picoseconds, and the generation of a cavitation bubble on a nanosecond timescale. Moreover, we are able to locate the temporal appearance of secondary nanoparticles at about 600 ps after pulse impact. The results support computational predictions and provide valuable insight into the early-stage ablation dynamics governing laser ablation in liquids.

Nanoparticle Additivation Effects on Laser Powder Bed Fusion of Metals and Polymers-A Theoretical Concept for an Inter-Laboratory Study Design All Along the Process Chain, Including Research Data Management.

In recent years, the application field of laser powder bed fusion of metals and polymers extends through an increasing variability of powder compositions in the market. New powder formulations such as nanoparticle (NP) additivated powder feedstocks are available today. Interestingly, they behave differently along with the entire laser powder bed fusion (PBF-LB) process chain, from flowability over absorbance and microstructure formation to processability and final part properties. Recent studies show that supporting NPs on metal and polymer powder feedstocks enhances processability, avoids crack formation, refines grain size, increases functionality, and improves as-built part properties. Although several inter-laboratory studies (ILSs) on metal and polymer PBF-LB exist, they mainly focus on mechanical properties and primarily ignore nano-additivated feedstocks or standardized assessment of powder feedstock properties. However, those studies must obtain reliable data to validate each property metric's repeatability and reproducibility limits related to the PBF-LB process chain. We herein propose the design of a large-scale ILS to quantify the effect of nanoparticle additivation on powder characteristics, process behavior, microstructure, and part properties in PBF-LB. Besides the work and sample flow to organize the ILS, the test methods to measure the NP-additivated metal and polymer powder feedstock properties and resulting part properties are defined. A research data management (RDM) plan is designed to extract scientific results from the vast amount of material, process, and part data. The RDM focuses not only on the repeatability and reproducibility of a metric but also on the FAIR principle to include findable, accessible, interoperable, and reusable data/meta-data in additive manufacturing. The proposed ILS design gives access to principal component analysis (PCA) to compute the correlations between the material-process-microstructure-part properties.

Towards laser printing of magnetocaloric structures by inducing a magnetic phase transition in iron-rhodium nanoparticles.

The development of magnetocaloric materials represents an approach to enable efficient and environmentally friendly refrigeration. It is envisioned as a key technology to reduce CO2 emissions of air conditioning and cooling systems. Fe-Rh has been shown to be one of the best-suited materials in terms of heat exchange per material volume. However, the Fe-Rh magnetocaloric response depends on its composition. Hence, the adaptation of material processing routes that preserve the Fe-Rh magnetocaloric response in the generated structures is a fundamental step towards the industrial development of this cooling technology. To address this challenge, the temperature-dependent properties of laser synthesized Fe-Rh nanoparticles and the laser printing of Fe-Rh nanoparticle inks are studied to generate 2D magnetocaloric structures that are potentially interesting for applications such as waste heat management of compact electrical appliances or thermal diodes, switches, and printable magnetocaloric media. The magnetization and temperature dependence of the ink's γ-FeRh to B2-FeRh magnetic transition is analyzed throughout the complete process, finding a linear increase of the magnetization M (0.8 T, 300 K) up to 96 Am2/kg with ca. 90% of the γ-FeRh being transformed permanently into the B2-phase. In 2D structures, magnetization values of M (0.8 T, 300 K) ≈ 11 Am2/kg could be reached by laser sintering, yielding partial conversion to the B2-phase equivalent to long-time heating temperature of app. 600 K, via this treatment. Thus, the proposed procedure constitutes a robust route to achieve the generation of magnetocaloric structures.

Nanoparticle Tracing during Laser Powder Bed Fusion of Oxide Dispersion Strengthened Steels.

The control of nanoparticle agglomeration during the fabrication of oxide dispersion strengthened steels is a key factor in maximizing their mechanical and high temperature reinforcement properties. However, the characterization of the nanoparticle evolution during processing represents a challenge due to the lack of experimental methodologies that allow in situ evaluation during laser powder bed fusion (LPBF) of nanoparticle-additivated steel powders. To address this problem, a simulation scheme is proposed to trace the drift and the interactions of the nanoparticles in the melt pool by joint heat-melt-microstructure-coupled phase-field simulation with nanoparticle kinematics. Van der Waals attraction and electrostatic repulsion with screened-Coulomb potential are explicitly employed to model the interactions with assumptions made based on reported experimental evidence. Numerical simulations have been conducted for LPBF of oxide nanoparticle-additivated PM2000 powder considering various factors, including the nanoparticle composition and size distribution. The obtained results provide a statistical and graphical demonstration of the temporal and spatial variations of the traced nanoparticles, showing ∼55% of the nanoparticles within the generated grains, and a smaller fraction of ∼30% in the pores, ∼13% on the surface, and ∼2% on the grain boundaries. To prove the methodology and compare it with experimental observations, the simulations are performed for LPBF of a 0.005 wt % yttrium oxide nanoparticle-additivated PM2000 powder and the final degree of nanoparticle agglomeration and distribution are analyzed with respect to a series of geometric and material parameters.

Laser Powder Bed Fusion of Polymers: Quantitative Research Direction Indices.

Research on Laser Powder Bed Fusion (L-PBF) of polymer powder feedstocks has raised over the last decade due to the increased utilization of the fabricated parts in aerospace, automotive, electronics, and healthcare applications. A total of 600 Science Citation Indexed articles were published on the topic of L-PBF of polymer powder feedstocks in the last decade, being cited more than 10,000 times leading to an h-index of 46. This study statistically evaluates the 100 most cited articles to extract reported material, process, and as-built part properties to analyze the research trends. PA12, PEEK, and TPU are the most employed polymer powder feedstocks, while size, flowability, and thermal behavior are the standardly reported material properties. Likewise, process properties such as laser power, scanning speed, hatch spacing, powder layer thickness, volumetric energy density, and areal energy density are extracted and evaluated. In addition, material and process properties of the as-built parts such as tensile test, flexural test, and volumetric porosity contents are analyzed. The incorporation of additives is found to be an effective route to enhance mechanical and functional properties. Carbon-based additives are typically employed in applications where mechanical properties are essential. Carbon fibers, Ca-phosphates, and SiO2 are the most reported additives in the evaluated SCI-expanded articles for L-PBF of polymer powder feedstocks. A comprehensive data matrix is extracted from the evaluated SCI-index publications, and a principal component analysis (PCA) is performed to explore correlations between reported material, process, and as-built parts.

Analysis of the Nanoparticle Dispersion and Its Effect on the Crystalline Microstructure in Carbon-Additivated PA12 Feedstock Material for Laser Powder Bed Fusion.

Driven by the rapid development of additive manufacturing technologies and the trend towards mass customization, the development of new feedstock materials has become a key aspect. Additivation of the feedstock with nanoparticles is a possible route for tailoring the feedstock material to the printing process and to modify the properties of the printed parts. This study demonstrates the colloidal additivation of PA12 powder with laser-synthesized carbon nanoparticles at >95% yield, focusing on the dispersion of the nanoparticles on the polymer microparticle surface at nanoparticle loadings below 0.05 vol%. In addition to the descriptors "wt%" and "vol%", the descriptor "surf%" is discussed for characterizing the quantity and quality of nanoparticle loading based on scanning electron microscopy. The functionalized powders are further characterized by confocal dark field scattering, differential scanning calorimetry, powder rheology measurements (avalanche angle and Hausner ratio), and regarding their processability in laser powder bed fusion (PBF-LB). We find that heterogeneous nucleation is induced even at a nanoparticle loading of just 0.005 vol%. Finally, analysis of the effect of low nanoparticle loadings on the final parts' microstructure by polarization microscopy shows a nanoparticle loading-dependent change of the dimensions of the lamellar microstructures within the printed part.

Fabrication by Laser Irradiation in a Continuous Flow Jet of Carbon Quantum Dots for Fluorescence Imaging.

Fluorescent carbon quantum dots (CQDs) are synthesized by laser irradiation of carbon glassy particles suspended in polyethylene glycol 200 by two methods, a batch and a flow jet configuration. The flow jet configuration is carried out by the simple combination of common laboratory objects to construct a home-made passage reactor of continuous flow. Despite the simplicity of the system, the laser energy is better harvested by the carbon microparticles, improving the fabrication efficiency a 15% and enhancing the fluorescence of CQDs by an order of magnitude in comparison with the conventional batch. The flow jet-synthesized CQDs have a mean size of 3 nm and are used for fluorescent imaging of transparent healthy and cancer epithelial human cells. Complete internalization is observed with a short incubation time of 10 min without using any extra additive or processing of the cell culture. The CQDs are well fixed in the organelles of the cell even after its death; hence, this is a simple manner to keep the cell information for prolonged periods of time. Moreover, the integrated photostability of the CQDs internalized in in vitro cells is measured and it remains almost constant during at least 2 h, revealing their outstanding performance as fluorescent labels.

From Complex Inorganic Oxides to Ag-Bi Nanoalloy: Synthesis by Femtosecond Laser Irradiation.

Bimetallic nanoalloys with a wide variety of structures and compositions have been fabricated through many diverse techniques. Generally, various steps and chemicals are involved in their fabrication. In this study, the synthesis of Ag-Bi nanoalloys by femtosecond laser irradiation of an inorganic oxide Ag2WO4/NaBiO3 target without any chemicals like reducing agents or solvent is presented. The interaction between these materials and the ultrashort pulse of light allows the migration of Ag and Bi atoms from the crystal lattice to the particles surfaces and then to the plasma plume, where the reduction of the positively charged Ag and Bi species in their respective metallic species takes place. Subsequently, the controlled nucleation and growth of the Ag-Bi alloyed nanoparticles occurs in situ during the irradiation process in air. Although at the bulk level, these elements are highly immiscible, it was experimentally demonstrated that at nanoscale, the Ag-Bi nanoalloy can assume a randomly mixed structure with up to 6 ± 1 atom % of Bi solubilized into the face-centered cubic structure of Ag. Furthermore, the Ag-Bi binary system possesses high antibacterial activity against Staphylococcus aureus (methicillin-resistant and methicilin-susceptible), which is interesting for potential antimicrobial applications, consequently increasing their range of applicability. The present results provide potential insights into the structures formed by the Ag-Bi systems at the nanoscale and reveal a new processing method where complex inorganic oxides can be used as precursors for the controlled synthesis of alloyed bimetallic nanoparticles.

Effects of femtosecond laser and other surface treatments on the bond strength of metallic and ceramic orthodontic brackets to zirconia.

Femtosecond laser has been proposed as a method for conditioning zirconia surfaces to boost bond strength. However, metallic or ceramic bracket bonding to femtosecond laser-treated zirconia surfaces has not been tested. This study compared the effects of four conditioning techniques, including femtosecond laser irradiation, on shear bond strength (SBS) of metallic and ceramic brackets to zirconia.Three hundred zirconia plates were divided into five groups: 1) control (C); 2) sandblasting (APA); 3) silica coating and silane (SC); 4) femtosecond laser (FS); 5) sandblasting followed by femtosecond laser (APA+SC). A thermal imaging camera measured temperature changes in the zirconia during irradiation. Each group was divided into 2 subgroups (metallic vs ceramic brackets). SBS was evaluated using a universal testing machine. The adhesive remnant index (ARI) was registered and surfaces were observed under SEM. Surface treatment and bracket type significantly affected the bracket-zirconia bond strength. SBS was significantly higher (p<0.001) for ceramic brackets in all groups (APA+FS > APA > FS > SC > control) than metallic brackets (APA+FS > FS > SC > APA > control). For metallic brackets, groups SC (5.99 ± 1.86 MPa), FS (6.72 ± 2.30 MPa) and APA+FS (7.22 ± 2.73 MPa) reported significantly higher bond strengths than other groups (p < 0.05). For ceramic brackets, the highest bond strength values were obtained in groups APA (25.01 ± 4.45 MPa), FS (23.18 ± 6.51 MPa) and APA+FS (29.22 ± 8.20 MPa).Femtosecond laser enhances bond strength of ceramic and metallic brackets to zirconia. Ceramic brackets provide significantly stronger adhesion than metallic brackets regardless of the surface treatment method.

Diffractive control of 3D multifilamentation in fused silica with micrometric resolution.

We show that a simple diffractive phase element (DPE) can be used to manipulate at will the positions and energy of multiple filaments generated in fused silica under femtosecond pulsed illumination. The method allows obtaining three-dimensional distributions of controlled filaments whose separations can be in the order of few micrometers. With such small distances we are able to study the mutual coherence among filaments from the resulted interference pattern, without needing a two-arm interferometer. The encoding of the DPE into a phase-only spatial light modulator (SLM) provides an extra degree of freedom to the optical set-up, giving more versatility for implementing different DPEs in real time. Our proposal might be particularly suited for applications at which an accurate manipulation of multiple filaments is required.