Laser synthesis of colloidal Si@Au and Si@Ag nanoparticles in water via plasma-assisted reduction

Laser synthesis of nanoparticles in organic solvents - products, reactions, and perspectives

Laser synthesis and processing of colloids (LSPC) is an established method for producing functional and durable nanomaterials and catalysts in virtually any liquid of choice. While the redox reactions during laser synthesis in water are fairly well understood, the corresponding reactions in organic liquids remain elusive, particularly because of the much greater complexity of carbon chemistry. To this end, this article first reviews the knowledge base of chemical reactions during LSPC and then deduces identifiable reaction pathways and mechanisms. This review also includes findings that are specific to the LSPC method variants laser ablation (LAL), fragmentation (LFL), melting (LML), and reduction (LRL) in organic liquids. A particular focus will be set on permanent gases, liquid hydrocarbons, and solid, carbonaceous species generated, including the formation of doped, compounded, and encapsulated nanoparticles. It will be shown how the choice of solvent, synthesis method, and laser parameters influence the nanostructure formation as well as the amount and chain length of the generated polyyne by-products. Finally, theoretical approaches to address the mechanisms of organic liquid decomposition and carbon shell formation are highlighted and discussed regarding current challenges and future perspectives of LSPC using organic liquids instead of water.

Merging of Bi-Modality of Ultrafast Laser Processing: Heating of Si/Au Nanocomposite Solutions with Controlled Chemical Content

Ultrafast laser processing possesses unique outlooks for the synthesis of novel nanoarchitectures and their further applications in the field of life science. It allows not only the formation of multi-element nanostructures with tuneable performance but also provides various non-invasive laser-stimulated modalities. In this work, we employed ultrafast laser processing for the manufacturing of silicon-gold nanocomposites (Si/Au NCs) with the Au mass fraction variable from 15% (0.5 min ablation time) to 79% (10 min) which increased their plasmonic efficiency by six times and narrowed the bandgap from 1.55 eV to 1.23 eV. These nanostructures demonstrated a considerable fs laser-stimulated hyperthermia with a Au-dependent heating efficiency (~10-20 °C). The prepared surfactant-free colloidal solutions showed good chemical stability with a decrease (i) of zeta (ξ) potential (from -46 mV to -30 mV) and (ii) of the hydrodynamic size of the nanoparticles (from 104 nm to 52 nm) due to the increase in the laser ablation time from 0.5 min to 10 min. The electrical conductivity of NCs revealed a minimum value (~1.53 µS/cm) at 2 min ablation time while their increasing concentration was saturated (~1012 NPs/mL) at 7 min ablation duration. The formed NCs demonstrated a polycrystalline Au nature regardless of the laser ablation time accompanied with the coexistence of oxidized Au and oxidized Si as well as gold silicide phases at a shorter laser ablation time (<1 min) and the formation of a pristine Au at a longer irradiation. Our findings demonstrate the merged employment of ultrafast laser processing for the design of multi-element NCs with tuneable properties reveal efficient composition-sensitive photo-thermal therapy modality.

Enhancement of Metal Nanostructure Deposition on Silicon Laser-Induced Periodic Surface Structures by Galvanic Replacement

We report a chemically motivated, single-step method to enhance metal deposition onto silicon laser-induced periodic surface structures (LIPSSs) using reactive laser ablation in liquid (RLAL). Galvanic replacement (GR) reactions were used in conjunction with RLAL (GR-RLAL) to promote the deposition of Au and Cu nanostructures onto a Si LIPSS. To increase the deposition of Au, sacrificial metals Cu, Fe, and Zn were used; Fe and Zn also enhanced the deposition of Cu. We show that the deposited metal content, surface morphology, and metal crystallite size can be tuned based on the difference in electrochemical potentials of the deposited and sacrificial metal. Compared to the Au and Cu reference samples, GR more than doubled the metal content on the LIPSS and reduced metal crystallite sizes by up to 20%. The ability to tune the metal content and crystalline domain size simultaneously makes GR-RLAL a potentially useful approach in the manufacturing of functional metal-LIPSS materials such as surface-enhanced Raman spectroscopy substrates.

Environmentally Friendly Improvement of Plasmonic Nanostructure Functionality towards Magnetic Resonance Applications

Plasmonic nanostructures have attracted a broad research interest due to their application perspectives in various fields such as biosensing, catalysis, photovoltaics, and biomedicine. Their synthesis by pulsed laser ablation in pure water enables eliminating various side effects originating from chemical contamination. Another advantage of pulsed laser ablation in liquids (PLAL) is the possibility to controllably produce plasmonic nanoparticles (NPs) in combination with other plasmonic or magnetic materials, thus enhancing their functionality. However, the PLAL technique is still challenging in respect of merging metallic and semiconductor specific features in nanosized objects that could significantly broaden application areas of plasmonic nanostructures. In this work, we performed synthesis of hybrid AuSi NPs with novel modalities by ultrashort laser ablation of bulk gold in water containing silicon NPs. The Au/Si atomic ratio in the nanohybrids was finely varied from 0.5 to 3.5 when changing the initial Si NPs concentration in water from 70 µg/mL to 10 µg/mL, respectively, without requiring any complex chemical procedures. It has been found that the laser-fluence-insensitive silicon content depends on the mass of nanohybrids. A high concentration of paramagnetic defects (2.2·× 10¹⁸ spin/g) in polycrystalline plasmonic NPs has been achieved. Our findings can open further prospects for plasmonic nanostructures as contrast agents in optical and magnetic resonance imaging techniques, biosensing, and cancer theranostics.

Multigram-Scale Production of Hybrid Au-Si Nanomaterial by Laser Ablation in Liquid (LAL) for Temperature-Feedback Optical Nanosensing, Light-to-Heat Conversion, and Anticounterfeit Labeling

Recent progress in hybrid optical nanomaterials composed of dissimilar constituents permitted an improvement in the performance and functionality of novel devices developed for optoelectronics, catalysis, medical diagnostics, and sensing. However, the rational combination of contrasting materials such as noble metals and semiconductors within individual hybrid nanostructures via a ready-to-use and lithography-free fabrication approach is still a challenge. Here, we report on a two-step synthesis of hybrid Au-Si microspheres generated by laser ablation of silicon in isopropanol followed by laser irradiation of the produced Si nanoparticles in the presence of HAuCl₄. Thermal reduction of [AuCl₄]^- species to a metallic gold phase, along with its subsequent mixing with silicon under laser irradiation, creates a nanostructured material with a unique composition and morphology, as revealed by electron microscopy, tomography, and elemental analysis. A combination of basic plasmonic and nanophotonic materials such as gold and silicon within a single microsphere allows for efficient light-to-heat conversion, as well as single-particle SERS sensing with temperature-feedback modality and expanded functionality. Moreover, the characteristic Raman signal and hot-electron-induced nonlinear photoluminescence coexisting within the novel Au-Si hybrids, as well as the commonly criticized randomness of the nanomaterials prepared by laser ablation in liquid, were proved to be useful for the realization of anticounterfeiting labels based on a physically unclonable function approach.

Ag-Decorated Si Microspheres Produced by Laser Ablation in Liquid: All-in-One Temperature-Feedback SERS-Based Platform for Nanosensing

Combination of dissimilar materials such as noble metals and common semiconductors within unified nanomaterials holds promise for optoelectronics, catalysis and optical sensing. Meanwhile, difficulty of obtaining such hybrid nanomaterials using common lithography-based techniques stimulates an active search for advanced, inexpensive, and straightforward fabrication methods. Here, we report one-pot one-step synthesis of Ag-decorated Si microspheres via nanosecond laser ablation of monocrystalline silicon in isopropanol containing AgNO₃. Laser ablation of bulk silicon creates the suspension of the Si microspheres that host further preferential growth of Ag nanoclusters on their surface upon thermal-induced decomposition of AgNO₃ species by subsequently incident laser pulses. The amount of the AgNO₃ in the working solution controls the density, morphology, and arrangement of the Ag nanoclusters allowing them to achieve strong and uniform decoration of the Si microsphere surface. Such unique morphology makes Ag-decorated Si microspheres promising for molecular identification based on the surface-enhanced Raman scattering (SERS) effect. In particular, the designed single-particles sensing platform was shown to offer temperature-feedback modality as well as SERS signal enhancement up to 10⁶, allowing reliable detection of the adsorbed molecules and tracing their plasmon-driven catalytic transformations. Considering the ability to control the decoration degree of Si microspheres by Ag nanoclusters via amount of the AgNO₃, the developed one-pot easy-to-implement PLAL synthesis holds promise for gram-scale production of high-quality hybrid nanomaterial for various nanophotonics and sensing applications.

Black Au-Decorated TiO2 Produced via Laser Ablation in Liquid

The rational combination of plasmonic and all-dielectric concepts within hybrid nanomaterials provides a promising route toward devices with ultimate performance and extended modalities. Spectral matching of plasmonic and Mie-type resonances for such nanostructures can only be achieved for their dissimilar characteristic sizes, thus making the resulting hybrid nanostructure geometry complex for practical realization and large-scale replication. Here, we produced amorphous TiO₂ nanospheres decorated and doped with Au nanoclusters via single-step nanosecond-laser irradiation of commercially available TiO₂ nanopowders dispersed in aqueous HAuCl₄. Fabricated hybrids demonstrate remarkable light-absorbing properties (averaged value ≈96%) in the visible and near-IR spectral range mediated by bandgap reduction of the laser-processed amorphous TiO₂ as well as plasmon resonances of the decorating Au nanoclusters. The findings are supported by optical spectroscopy, electron energy loss spectroscopy, transmission electron microscopy, and electromagnetic modeling. Light-absorbing and plasmonic properties of the produced hybrids were implemented to demonstrate catalytically passive SERS biosensor for identification of analytes at trace concentrations and solar steam generator that permitted to increase water evaporation rate by 2.5 times compared with that of pure water under identical 1 sun irradiation conditions.

Nanosecond-Laser Generation of Nanoparticles in Liquids: From Ablation through Bubble Dynamics to Nanoparticle Yield

A comprehensive picture of the nanosecond-laser generation of colloidal nanoparticles in liquids is nowadays the demand of their high-throughput industrial fabrication for diverse perspective biomedical, material science, and optoelectronic applications. In this study, using silicon as an example, we present a self-consistent experimental visualization and theoretical description of key transient stages during nanosecond-laser generation of colloidal nanoparticles in liquids: plasma-mediated injection of ablated mass into the liquid and driving the vapor bubble, finalized by the colloid appearance in the liquid. The explored fundamental transient stages envision the basic temporal and spatial scales, as well as laser parameter windows, for the demanded high-throughput nanosecond-laser generation of colloidal nanoparticles in liquids.

Nucleation and growth of gold nanoparticles initiated by nanosecond and femtosecond laser irradiation of aqueous [AuCl4]. Phys Chem Chem Phys 2018;20:28465-28475. [PMID: 30411753 PMCID: PMC6310131 DOI: 10.1039/c8cp05774e]

Irradiation of aqueous [AuCl4]- with 532 nm nanosecond (ns) laser pulses produces monodisperse (PDI = 0.04) 5 nm Au nanoparticles (AuNPs) without any additives or capping agents via a plasmon-enhanced photothermal autocatalytic mechanism. Compared with 800 nm femtosecond (fs) laser pulses, the AuNP growth kinetics under ns laser irradiation follow the same autocatalytic rate law, but with a significantly lower sensitivity to laser pulse energy. The results are explained using a simple model for simulating heat transfer in liquid water and at the interface with AuNPs. While the extent of water superheating with the ns laser is smaller compared to the fs laser, its significantly longer duration can provide sufficient energy to dissociate a small fraction of the [AuCl4]- present, resulting in the formation of AuNPs by coalescence of the resulting Au atoms. Irradiation of initially formed AuNPs at 532 nm results in plasmon-enhanced superheating of water, which greatly accelerates the rate of thermal dissociation of [AuCl4]- and accounts for the observed autocatalytic kinetics. The plasmon-enhanced heating under ns laser irradiation fragments the AuNPs and results in nearly uniform 5 nm particles, while the lack of particles' heating under fs laser irradiation results in the growth of the particles as large as 40 nm.