Plasma and nanoparticle shielding during pulsed laser ablation in liquids cause ablation efficiency decrease

Understanding shielding cross-effects is a prerequisite for maximal power-specific nanosecond laser ablation in liquids(LAL).However,discrimination between cavitation bubble(CB),nanoparticle(NP),and shielding,e.g.,by the plasma or a transient vapor layer,is challenging.Therefore,CB imaging by shadowgraphy is performed to better understand the plasma and laser beam-NP interaction during LAL.By comparing the fluence-dependent CB volume for ablations performed with 1 ns pulses with reports from the literature,we find larger energy-specific CB volumes for 7 ns-ablation.The increased CB for laser ablation with higher ns pulse durations could be a first explanation of the efficiency decrease reported for these laser systems having higher pulse durations.Consequently,1 ns-LAL shows superior ablation efficiency.Moreover,a CB cascade occurs when the focal plane is shifted into the liquid.This effect is enhanced when NPs are present in the fluid.Even minute amounts of NPs trapped in a stationary layer decrease the laser energy significantly,even under liquid flow.However,this local concentration in the sticking film has so far not been considered.It presents an essential obstacle in high-yield LAL,shielding already the second laser pulse that arrives and presenting a source of satellite bubbles.Hence,measures to lower the NP concentration on the target must be investigated in the future.

Time resolved studies reveal the origin of the unparalleled high efficiency of one nanosecond laser ablation in liquids

Laser ablation in liquid is a scalable nanoparticle production method with applications in areas like catalysis and biomedicine.Due to laser-liquid interactions,different energy dissipation channels such as absorption by the liquid and scattering at the ablation plume and cavitation bubble lead to reduced laser energy available for nanoparticle production.Ultrashort pulse durations cause unwanted nonlinear effects in the liquid,and for ns pulses,intra-pulse energy deposition attenuation effects are to be expected.However,intermediate pulse durations ranging from hundreds of picoseconds up to one nanosecond have rarely been studied in particular in single-pulse settings.In this study,we explore the pico-to nanosecond pulse duration regimes to find the pulse duration with the highest ablation efficiency.We find that pulse durations around 1-2 ns enable the most efficient laser ablation in liquid since the laser beam shielding by the ablation plume and cavitation bubble sets in only at longer pulse durations.Furthermore,pump-probe microscopy imaging reveals that the plume dynamics in liquids start to differ from plume dynamics in air at about 2 ns after pulse impact.

Characterizing bubble interaction effects in synchronous-double-pulse laser ablation for enhanced nanoparticle synthesis

To further advance nanomaterial applications and reduce waste production during synthesis,greener and sustainable production methods are necessary.Pulsed laser ablation in liquid(PLAL)is a green technique that enables the synthesis of nanoparticles.This study uses synchronous-double-pulse PLAL to understand bubble interaction effects on the nanoparticle size.By adjusting the lateral separation of the pulses relative to the maximum bubble size,an inter-pulse separation is identified where the nanoparticle size is fourfold.The cavitation bubble pair interaction is recorded using a unique coaxial diffuse shadowgraphy system.This system allows us to record the bubble pair interaction from the top and side,enabling the identification of the bubble’s morphology,lifetime,volumetric,and displacement velocity.It is found that the collision and collapse of the bubbles generated at a certain inter-pulse separation results in a larger nanoparticle size.These results mark a significant advancement by controlling the abundance of larger nanoparticles in PLAL,where previous efforts were primarily focused on reducing the average nanoparticle size.The experimentally observed trends are confirmed by numerical simulations with high spatial and temporal resolution.This study serves as a starting point to bridge the gap between upscaled multi-bubble practices and fundamental knowledge concerning the determinants that define the final nanoparticle size.

Laser-generated high entropy metallic glass nanoparticles as bifunctional electrocatalysts

High entropy metallic glass nanoparticles(HEMG NPs)are very promising materials for energy conversion due to the wide tuning possibilities of electrochemical potentials offered by their multimetallic character combined with an amorphous structure.Up until now,the generation of these HEMG NPs involved tedious synthesis procedures where the generated particles were only available on highly specialized supports,which limited their widespread use.Hence,more flexible synthetic approaches to obtain colloidal HEMG NPs for applications in energy conversion and storage are highly desirable.We utilized pulsed laser ablation of bulk high entropy alloy targets in acetonitrile to generate colloidal carbon-coated CrCoFeNiMn and CrCoFeNiMnMo HEMG NPs.An in-depth analysis of the structure and elemental distribution of the obtained nanoparticles down to single-particle levels using advanced transmission electron microscopy(TEM),energy-dispersive X-ray spectroscopy(EDX),X-ray diffraction(XRD),and Xray photoelectron spectroscopy(XPS)methods revealed amorphous quinary and senary alloy phases with slight manganese oxide/hydroxide surface segregation,which were stabilized within graphitic shells.Studies on the catalytic activity of the corresponding carbon-HEMG NPs during oxygen evolution and oxygen reduction reactions revealed an elevated activity upon the incorporation of moderate amounts of Mo into the amorphous alloy,probably due to the defect generation by atomic size mismatch.Furthermore,we demonstrate the superiority of these carbon-HEMG NPs over their crystalline analogies and highlight the suitability of these amorphous multi-elemental NPs in electrocatalytic energy conversion.

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 surfaaant-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.

Multidimensional thermally-induced transformation of nest-structured complex Au-Fe nanoalloys towards equilibrium

Bimetallic nanoparticles are often superior candidates for a wide range of technological and biomedical applications owing to their enhanced catalytic,optical,and magnetic properties,which are often better than their monometallic counterparts.Most of their properties strongly depend on their chemical composition,crystallographic structure,and phase distribution.However,little is known of how their crystal structure,on the nanoscale,transforms over time at elevated temperatures,even though this knowledge is highly relevant in case nanoparticles are used in,e.g.,high-temperature catalysis.Au-Fe is a promising bimetallic system where the low-cost and magnetic Fe is combined with catalytically active and plasmonic Au.Here,we report on the in s/fi;temporal evolution of the crystalline ordering in Au-Fe nanoparticles,obtained from a modern laser ablation in liquids synthesis.Our in-depth analysis,complemented by dedicated atomistic simulations,includes a detailed structural characterization by X-ray diffraction and transmission electron microscopy as well as atom probe tomography to reveal elemental distributions down to a single atom resolution.We show that the Au-Fe nanoparticles initially exhibit highly complex internal nested nanostructures with a wide range of compositions,phase distributions,and size-depended microstrains.The elevated temperature induces a diffusion-controlled recrystallization and phase merging,resulting in the formation of a single face-centered-cubic ultrastructure in contact with a body-centered cubic phase,which demonstrates the metastability of these structures.Uncovering these unique nanostructures with nested features could be highly attractive from a fundamental viewpoint as they could give further insights into the nanoparticle formation mechanism under non-equilibrium conditions.Furthermore,the in situ evaluation of the crystal structure changes upon heating is potentially relevant for high-temperature process utilization of bimetallic nanoparticles,e.g.,during catalysis.

Laser-based in situ embedding of metal nanoparticles into bioextruded alginate hydrogel tubes enhances human endothelial cell adhesion

Alginate is a widely used hydrogel in tissue engineering owing to its simple and non-cytotoxic gelation process, ease of use, and abundance. However, unlike hydrogels derived from mammalian sources such as collagen, alginate does not contain cell adhesion Iigands. Here, we present a novel laser ablation technique for the in situ embedding of gold and iron nanoparticles into hydrogels. We hypothesized that integration of metal nanoparticles in alginate could serve as an alternative material because of its chemical biofunctionalization ability （coupling of RGD ligands） to favor cell adhesion. Cytocompatibility and biofunctionality of the gels were assessed by cell culture experiments using fibroblasts and endothelial cells. Nanoparticles with an average particle size of 3 nm （gold） and 6 nm （iron） were generated and stably maintained in alginate for up to 6 months. Using an extrusion system, several centimeter-long alginate tubes with an outer diameter of approximately 3 mm and a wall thickness of approximately 150 μm were manufactured. Confocal microscopy revealed homogeneously distributed nanoparticle agglomerates over the entire tube volume. Endothelial cells seeded on iron-loaded gels showed significantly higher viability and an increased degree of spreading, and the number of attached cells was also elevated in comparison to the control and gold-loaded alginates. We conclude that laser-based in situ integration of iron nanoparticles （40.01 wt.%） in alginate is a straightforward method to generate composite materials that favor the adhesion of endothelial cells. In addition, we show that nanoparticle integration does not impair the alginate＇s gelation and 3D biofabrication properties.