Modeling of Short-Pulse Laser Interactions with Monolithic and Porous Silicon Targets with an Atomistic-Continuum Approach.

The acquisition of reliable knowledge about the mechanism of short laser pulse interactions with semiconductor materials is an important step for high-tech technologies towards the development of new electronic devices, the functionalization of material surfaces with predesigned optical properties, and the manufacturing of nanorobots (such as nanoparticles) for bio-medical applications. The laser-induced nanostructuring of semiconductors, however, is a complex phenomenon with several interplaying processes occurring on a wide spatial and temporal scale. In this work, we apply the atomistic-continuum approach for modeling the interaction of an fs-laser pulse with a semiconductor target, using monolithic crystalline silicon (c-Si) and porous silicon (Si). This model addresses the kinetics of non-equilibrium laser-induced phase transitions with atomic resolution via molecular dynamics, whereas the effect of the laser-generated free carriers (electron-hole pairs) is accounted for via the dynamics of their density and temperature. The combined model was applied to study the microscopic mechanism of phase transitions during the laser-induced melting and ablation of monolithic crystalline (c-Si) and porous Si targets in a vacuum. The melting thresholds for the monolithic and porous targets were found to be 0.32 J/cm2 and 0.29 J/cm2, respectively. The limited heat conduction mechanism and the absence of internal stress accumulation were found to be involved in the processes responsible for the lowering of the melting threshold in the porous target. The results of this modeling were validated by comparing the melting thresholds obtained in the simulations to the experimental values. A difference in the mechanisms of ablation of the c-Si and porous Si targets was considered. Based on the simulation results, a prediction regarding the mechanism of the laser-assisted production of Si nanoparticles with the desired properties is drawn.

Molecular Dynamics Modeling of Pulsed Laser Fragmentation of Solid and Porous Si Nanoparticles in Liquid Media.

The production of non-toxic and homogeneous colloidal solutions of nanoparticles (NPs) for biomedical applications is of extreme importance nowadays. Among the various methods for generation of NPs, pulsed laser ablation in liquids (PLAL) has proven itself as a powerful and efficient tool in biomedical fields, allowing chemically pure silicon nanoparticles to be obtained. For example, laser-synthesized silicon nanoparticles (Si NPs) are widely used as contrast agents for bio visualization, as effective sensitizers of radiofrequency hyperthermia for cancer theranostics, in photodynamic therapy, as carriers of therapeutic radionuclides in nuclear nanomedicine, etc. Due to a number of complex and interrelated processes involved in the laser ablation phenomenon, however, the final characteristics of the resulting particles are difficult to control, and the obtained colloidal solutions frequently have broad and multimodal size distribution. Therefore, the subsequent fragmentation of the obtained NPs in the colloidal solutions due to pulsed laser irradiation can be utilized. The resulting NPs' characteristics, however, depend on the parameters of laser irradiation as well as on the irradiated material and surrounding media properties. Thus, reliable knowledge of the mechanism of NP fragmentation is necessary for generation of a colloidal solution with NPs of predesigned properties. To investigate the mechanism of a laser-assisted NP fragmentation process, in this work, we perform a large-scale molecular dynamics (MD) modeling of FS laser interaction with colloidal solution of Si NPs. The obtained NPs are then characterized by their shape and morphological properties. The corresponding conclusion about the relative input of the properties of different laser-induced processes and materials to the mechanism of NP generation is drawn.

Metabolomics reveals that PS-NPs promote lung injury by regulating prostaglandin B1 through the cGAS-STING pathway.

Nanoplastics have been widely studied as environmental pollutants, which can accumulate in the human body through the food chain or direct contact. Research has shown that nanoplastics can affect the immune system and mitochondrial function, but the underlying mechanisms are unclear. Lungs and macrophages have important immune and metabolic functions. This study explored the effects of 100 nm PS-NPs on innate immunity, mitochondrial function, and cellular metabolism-related pathways in lung (BEAS-2B) cells and macrophages (RAW264.7). The results had shown that PS-NPs exposure caused a decrease in mitochondrial membrane potential, intracellular ROS accumulation, and Ca2+ overload, and activated the cGAS-STING signaling pathway related to innate immunity. These changes had been observed at concentrations of PS-NPs as low as 60 µg/mL, which might have been comparable to environmental levels. Non-target metabolomics and Western Blotting results confirmed that PS-NPs regulated prostaglandin B1 and other metabolites to cause cell damage through the cGAS-STING pathway. Supplementation of prostaglandin B1 alleviated the immune activation and metabolic disturbance caused by PS-NPs exposure. This study identified PS-NPs-induced innate immune activation, mitochondrial dysfunction, and metabolic toxicity pathways, providing new insights into the potential for adverse outcomes of NPs in human life.

P53-response circRNA_0006420 aggravates lung cancer radiotherapy resistance by promoting formation of HUR/PTBP1 complex.

BACKGROUND: p53 wild-type lung cancer cells can develop radiation resistance. Circular RNA (circRNA) consists of a family of transcripts with exclusive structures. circRNA is critical in tumorigenesis and is a potential biomarker or therapeutic target. It is uncertain how circRNA expression and functions are regulated post-radiation in p53 wild-type cancer cells. METHODS: A549 or H1299 cells were divided into p53-wt and p53-KO groups by CRISPR/Cas9; both groups were subjected to 4 Gy ionizing radiation (IR: p53-wt-IR and p53-KO-IR). RNA-seq, CCK8, cell cycle, and other functional and mechanism experiments were performed in vivo. p53 gene knockout mice were generated to test the cell results in vitro. RESULTS: circRNAs were found in differential groups. circRNA_0006420 (IRSense) was upregulated in p53-wt cells but had the same expression level as p53-KO cells after radiation, indicating that p53 silencing prevents its upregulation after IR. In the presence of p53, upregulated IRSense post-radiation induces G2/M arrest by regulating DNA damage repair (DDR) pathway-related proteins. Meanwhile, upregulated IRSense post-radiation aggravates the radiation-induced epithelial-mesenchymal transition (EMT). Interestingly, in the presence of p53, it promotes IRSense/HUR/PTBP1 complex formation resulting in the promotion of the radiation-induced EMT. Moreover, c-Jun regulates the upregulation of p53 transcription after radiation treatment. For these lung cancer cells with p53, upregulated IRSense aggravates lung cancer cell proliferation and increases radiation resistance by interacting with HUR (ElAV-like protein 1) and PTBP1 (polypyrimidine tract-binding protein 1) in the nucleus. CONCLUSIONS: Lung cancer cells retaining p53 may upregulate circRNA_0006420 (IRSense) expression post radiation to form an IRSense/HUR/PTBP1 complex leading to radiotherapy resistance. This study furthers our understanding of the roles of circRNA in regulating the effect of radiotherapy and provides novel therapeutic avenues for effective clinical lung cancer therapies.

Steering Potential for Printing Highly Aligned Discontinuous Fibre Composite Filament.

DcAFF (discontinuous aligned fibre filament) is a novel material for fused filament fabrication (FFF) 3D printing made of highly aligned discontinuous fibres produced using high performance discontinuous fibre (HiPerDiF) technology. It reinforces a thermoplastic matrix to provide high mechanical performance and formability. Accurate printing of DcAFF poses a challenge, especially for complex geometries, because: (i) there is a discrepancy between the path where the filament experiences the adhering pressure from the filleted nozzle and the nozzle path; and (ii) the rasters display poor adhesion to the build platform immediately after deposition, which causes the filament to be dragged when the printing direction changes. This paper explains the implication of these phenomena on steering capabilities and examines the techniques for improving DcAFF printing accuracy. In the first approach, the machine parameters were adjusted to improve the quality of the sharp turning angle without changing the desired path, but this showed insignificant effects in terms of precision improvements. In the second approach, a printing path modification with a compensation algorithm was introduced. The nature of the inaccuracy of the printing at the turning point was studied with a first-order lag relationship. Then the equation to describe the deposition raster inaccuracy was determined. A proportional-integral (PI) controller was added to the equation to calculate the nozzle movement in order to bring the raster back to the desired path. The applied compensation path is shown to give an accuracy improvement in curvilinear printing paths. This is particularly beneficial when printing larger circular diameter curvilinear printed parts. The developed printing approach can be applied with other fibre reinforced filaments to achieve complex geometries.

Open Hole Tension of 3D Printed Aligned Discontinuous Composites.

This paper explores the use of Discontinuous Aligned Fibre Filament (DcAFF), a novel discontinuous fibre reinforced thermoplastic filament for 3D printing, to produce structural complex parts. Compared to conventional composite manufacturing, 3D printing has great potential in steering fibres around small structural features. In this current study, the initial thin carbon fibre (CF)-poly(L-lactic acid) (PLA) tape, produced with the High Performance Discontinuous Fibre (HiPerDiF) technology, is now reshaped into a circular cross-section filament, the DcAFF, using a bespoke machine designed to be scalable to high production rates rather than using a labour-intensive manual moulding method as in previous work. The filaments are then fed to a general-purpose 3D printer. Tensile and open-hole tensile tests were considered in this paper for mechanical and processability of DcAFF. The 3D printed specimens fabricated with the DcAFF show superior tensile properties compared to other PLA-based 3D printed composites, even those containing continuous fibres. Curvilinear open-hole tensile test samples were fabricated to explore the processability and performances of such material in complex shapes. The mechanical performance of the produced specimens was benchmarked against conventionally laid-up specimens with a cut hole. Although the steered specimens produced have lower strength than the fully consolidated samples, the raster generated by the printing path has turned the failure mechanism of the composite from brittle to ductile.

Nondestructive Femtosecond Laser Lithography of Ni Nanocavities by Controlled Thermo-Mechanical Spallation at the Nanoscale.

We present a new approach to femtosecond direct laser writing lithography to pattern nanocavities in ferromagnetic thin films. To demonstrate the concept, we irradiated 300 nm thin nickel films by single intense femtosecond laser pulses through glass substrate. Using a fluence above the ablation threshold, the process is destructive, leading to the formation of an ablation crater. By progressively lowering the laser fluence, the formation of closed spallation cavities below the ablation threshold is achieved. Systematic studies by the electron and optical interferometric microscopies, supported by molecular dynamics simulations, enabled us to gain an understanding of the thermo-mechanical spallation mechanism at the solid-molten interface. We achieved the fabrication of periodic arrangements of closed spallation nanocavities. Due to their topology, closed magnetic nanocavities can support unique couplings of multiple excitations (magnetic, optical, acoustic, spintronic). Thereby, they offer a unique physics playground for emerging fields in magnetism, magneto-photonic, and magneto-acoustic applications.

Formation of Periodic Nanoridge Patterns by Ultrashort Single Pulse UV Laser Irradiation of Gold.

A direct comparison of simulation and experimental results of UV laser-induced surface nanostructuring of gold is presented. Theoretical simulations and experiments are performed on an identical spatial scale. The experimental results have been obtained by using a laser wavelength of 248 nm and a pulse length of 1.6 ps. A mask projection setup is applied to generate a spatially periodic intensity profile on a gold surface with a sinusoidal shape and periods of 270 nm, 350 nm, and 500 nm. The formation of structures at the surface upon single pulse irradiation is analyzed by scanning and transmission electron microscopy (SEM and TEM). For the simulations, a hybrid atomistic-continuum model capable of capturing the essential mechanisms responsible for the nanostructuring process is used to model the interaction of the laser pulse with the gold target and the subsequent time evolution of the system. The formation of narrow ridges composed of two colliding side walls is found in the simulation as well as in the experiment and the structures generated as a result of the material processing are categorized depending on the range of applied fluencies and periodicities.

High-Order Harmonic Generation in Au Nanoparticle-Contained Plasmas.

Gold nanoparticles (NPs) have a wide range of applications in various fields. Here, we present high-order nonlinear optical studies of the plasmas produced from ablation of Au bulk targets and Au NP films deposited on paper and glass substrates. Experimentally, we analyze high-order harmonic generation (HHG) from gold NPs-containing plasmas. The HHG is produced by 35-fs pulses at 800 and 400 nm, while the plasmas are produced by femtosecond (35 fs, 800 nm), picosecond (200 ps, 800 nm), and nanosecond (5 ns, 1064 nm) pulses, respectively. High-order harmonics produced from ablated Au NPs on paper were 40 times stronger than the HHG from that ablated from the Au bulk targets. Through molecular dynamic simulations, we investigate the formation of gold NPs during laser ablation of a metal surface under different conditions.

Numerical Investigation of Ultrashort Laser-Ablative Synthesis of Metal Nanoparticles in Liquids Using the Atomistic-Continuum Model.

We present a framework based on the atomistic continuum model, combining the Molecular Dynamics (MD) and Two Temperature Model (TTM) approaches, to characterize the growth of metal nanoparticles (NPs) under ultrashort laser ablation from a solid target in water ambient. The model is capable of addressing the kinetics of fast non-equilibrium laser-induced phase transition processes at atomic resolution, while in continuum it accounts for the effect of free carriers, playing a determinant role during short laser pulse interaction processes with metals. The results of our simulations clarify possible mechanisms, which can be responsible for the observed experimental data, including the presence of two populations of NPs, having a small (5-15 nm) and larger (tens of nm) mean size. The formed NPs are of importance for a variety of applications in energy, catalysis and healthcare.

Pulse Duration and Wavelength Effects of Laser Ablation on the Oxidation, Hydrolysis, and Aging of Aluminum Nanoparticles in Water.

We analyzed the formation of the aluminum (Al) nanoparticles (NPs) with triangular shape obtained by ablating Al bulk in liquid using pulses with different durations (5 ns, 200 ps, and 30 fs) and wavelengths (355 nm, 800 nm, and 1064 nm). We report three stages of synthesis and aging of Al NPs: Formation, transformation, and stable stage. The NPs prepared by different pulses are almost identical at the initial stage. The effects of duration and wavelength of the ablation pulses on the aging of NPs are revealed. Pulse duration is determined to be essential for morphological transformation of NPs, while pulse wavelength strongly influences particle sizes. NPs produced by ultra-short pulses have smaller sizes and narrow size distribution. We demonstrate that oxidation and hydrolysis of Al in water are the results of ablation for all pulse durations and wavelengths, which also strongly modify the preferable reaction path of NPs in water, thus affecting the composition and morphology of triangle NPs. The results of modeling of the NPs generation in water due to a 50 ps laser pulse interacting with a thick Al target are presented. Water-based effects in the formation of NPs, their evolution, and solidification are considered from the mechanical and thermophysical points of view. The detailed analysis of the modeling results allowed for determination of the main mechanism responsible for the ablation process followed by the NPs formation.