How the Physicochemical Properties of the Bulk Material Affect the Ablation Crater Profile, Mass Balance, and Bubble Dynamics During Single-Pulse, Nanosecond Laser Ablation in Water.

Understanding the key steps that drive the laser-based synthesis of colloids is a prerequisite for learning how to optimize the ablation process in terms of nanoparticle output and functional design of the nanomaterials. Even though many studies focus on cavitation bubble formation using single-pulse ablation conditions, the ablation efficiency and nanoparticle properties are typically investigated under prolonged ablation conditions with repetition rate lasers. Linking single-pulse and multiple-pulse ablation is difficult due to limitations induced by gas formation cross-effects, which occur on longer timescales and depend on the target materials' oxidation-sensitivity. Therefore, this study investigates the ablation and cavitation bubble dynamics under nanosecond, single laser pulse conditions for six different bulk materials (Au, Ag, Cu, Fe, Ti, and Al). Also, the effective threshold fluences, ablation volumes, and penetration depths are quantified for these materials. The thermal and chemical properties of the corresponding bulk materials not only favor the formation of larger spot sizes but also lead to the highest molar ablation efficiencies for low melting materials such as aluminum. Furthermore, the concept of the cavitation bubble growth linked with the oxidation sensitivity of the ablated material is discussed. With this, evidence is provided that intensive chemical reactions occurring during the very early timescale of ablation are significantly enhanced by the bubble collapse.

Correction: Determining the role of redox-active materials during laser-induced water decomposition.

Correction for 'Determining the role of redox-active materials during laser-induced water decomposition' by Mark-Robert Kalus et al., Phys. Chem. Chem. Phys., 2019, 21, 18636-18651.

Determining the role of redox-active materials during laser-induced water decomposition.

Laser ablation in liquids (LAL) drives the decomposition of the liquid inducing the formation of a large number of different redox equivalents and gases. This not only leads to shielding effects and a decrease of the nanoparticle (NP) productivity but also can directly affect the NP properties such as the oxidation degree. In this study, we demonstrate that liquid decomposition during laser ablation in water is triggered by the redox activity of the 7 different bulk materials used; Au, Pt, Ag, Cu, Fe, Ti and Al, as well as by the reactivity of water with the plasma. Laser ablation of less-noble metals like aluminum leads to a massive gas evolution up to 390 cm3 per hour with molar hydrogen to oxygen ratios of 17.1. For more noble metals such as gold and platinum, water splitting induced by LAL is the dominant feature leading to gas volume formation rates of 10 up to 30 cm3 per hour and molar hydrogen to oxygen ratios of 1.2. We quantify the material-dependent ablation rate, shielding effects as well as the amount of hydrogen peroxide produced, directly affecting the yield and oxidation of the nanoparticles on the long-time scale.

Spontaneous Shape Alteration and Size Separation of Surfactant-Free Silver Particles Synthesized by Laser Ablation in Acetone during Long-Period Storage.

The technique of laser ablation in liquids (LAL) has already demonstrated its flexibility and capability for the synthesis of a large variety of surfactant-free nanomaterials with a high purity. However, high purity can cause trouble for nanomaterial synthesis, because active high-purity particles can spontaneously grow into different nanocrystals, which makes it difficult to accurately tailor the size and shape of the synthesized nanomaterials. Therefore, a series of questions arise with regards to whether particle growth occurs during colloid storage, how large the particle size increases to, and into which shape the particles evolve. To obtain answers to these questions, here, Ag particles that are synthesized by femtosecond (fs) laser ablation of Ag in acetone are used as precursors to witness the spontaneous growth behavior of the LAL-generated surfactant-free Ag dots (2⁻10 nm) into different polygonal particles (5⁻50 nm), and the spontaneous size separation phenomenon by the carbon-encapsulation induced precipitation of large particles, after six months of colloid storage. The colloids obtained by LAL at a higher power (600 mW) possess a greater ability and higher efficiency to yield colloids with sizes of <40 nm than the colloids obtained at lower power (300 mW), because of the generation of a larger amount of carbon 'captors' by the decomposition of acetone and the stronger particle fragmentation. Both the size increase and the shape alteration lead to a redshift of the surface plasmon resonance (SPR) band of the Ag colloid from 404 nm to 414 nm, after storage. The Fourier transform infrared spectroscopy (FTIR) analysis shows that the Ag particles are conjugated with COO⁻ and OH⁻ groups, both of which may lead to the growth of polygonal particles. The CO and CO2 molecules are adsorbed on the particle surfaces to form Ag(CO)x and Ag(CO2)x complexes. Complementary nanosecond LAL experiments confirmed that the particle growth was inherent to LAL in acetone, and independent of pulse duration, although some differences in the final particle sizes were observed. The nanosecond-LAL yields monomodal colloids, whereas the size-separated, initially bimodal colloids from the fs-LAL provide a higher fraction of very small particles that are <5 nm. The spontaneous growth of the LAL-generated metallic particles presented in this work should arouse the special attention of academia, especially regarding the detailed discussion on how long the colloids can be preserved for particle characterization and applications, without causing a mismatch between the colloid properties and their performance. The spontaneous size separation phenomenon may help researchers to realize a more reproducible synthesis for small metallic colloids, without concern for the generation of large particles.

How persistent microbubbles shield nanoparticle productivity in laser synthesis of colloids - quantification of their volume, dwell dynamics, and gas composition.

During laser synthesis of colloids, cavitation bubbles with lifetimes in the microsecond-scale form and shield the laser pulse leading to a decrease in nanoparticle output. A second type of productivity-limiting bubble that severely affects the productivity of the process is often neglected. With lifetimes from milliseconds to seconds, these persistent bubbles are systematically studied in this work by quantifying their composition, amount, size and dwell time in liquids with different viscosities and by relating the results to the nanoparticle productivities. It is found that during synthesis in water, water splitting occurs leading to persistent bubbles consisting of hydrogen and oxygen. In glycols, hydrogen and molecular carbon species containing microbubbles are formed. These persistent microbubbles shield up to 65% of the incoming laser beam depending on the liquid as well as the laser fluence and require attention by means of reducing their dwell time in the ablation zone and enhancing the nanoparticle output by liquid flow. The highest productivities and monodisperse quality are achieved in liquids with the lowest viscosities.

Substrate-mediated effects in photothermal patterning of alkanethiol self-assembled monolayers with microfocused continuous-wave lasers.

In recent years, self-assembled monolayers (SAMs) have been demonstrated to provide promising new approaches to nonlinear laser processing. Most notably, because of their ultrathin nature, indirect excitation mechanisms can be exploited in order to fabricate subwavelength structures. In photothermal processing, for example, microfocused lasers are used to locally heat the substrate surface and initiate desorption or decomposition of the coating. Because of the strongly temperature-dependent desorption kinetics, the overall process is highly nonlinear in the applied laser power. For this reason, subwavelength patterning is feasible employing ordinary continuous-wave lasers. The lateral resolution, generally, depends on both the type of the organic monolayer and the nature of the substrate. In previous studies we reported on photothermal patterning of distinct types of SAMs on Si supports. In this contribution, a systematic study on the impact of the substrate is presented. Alkanethiol SAMs on Au-coated glass and silicon substrates were patterned by using a microfocused laser beam at a wavelength of 532 nm. Temperature calculations and thermokinetic simulations were carried out in order to clarify the processes that determine the performance of the patterning technique. Because of the strongly temperature-dependent thermal conductivity of Si, surface-temperature profiles on Au/Si substrates are very narrow ensuring a particularly high lateral resolution. At a 1/e spot diameter of 2 µm, fabrication of subwavelength structures with diameters of 300-400 nm is feasible. Rapid heat dissipation, though, requires high laser powers. In contrast, patterning of SAMs on Au/glass substrates is strongly affected by the largely distinct heat conduction within the Au film and in the glass support. This results in broad surface temperature profiles. Hence, minimum structure sizes are larger when compared with respective values on Au/Si substrates. The required laser powers, though, are more than one order of magnitude lower. Also, the laser power needed for patterning decreases with decreasing Au layer thickness. These results demonstrate the impact of the substrate on the overall patterning process and provide new perspectives in photothermal laser patterning of ultrathin organic coatings.