Formation Of Surface Structures Research Articles

Efficient ablation, further GHz burst polishing, and surface texturing by ultrafast laser

A large variety of ultrafast laser‐matter interaction regimes and different processed surface finishing qualities of stainless steel can be achieved by varying the laser processing parameters. The optimization of the laser fluence to the most efficient ablation also leads to the lowest surface roughness of the ablated material. High laser fluence together with a high pulse repetition rate leads to heat accumulation, which results in the formation of microstructures with various morphology and scales. The use of GHz bursts allows a rough stainless‐steel surface to be smoothed and even polished. In this work, the fast and high‐quality milling of 2.5D cavities was demonstrated. Laser beam spot optimization was used to increase the throughput of the high‐power laser (67.8 W), resulting in an ablation rate of 13 mm3/min. The complex cavities were produced by laser milling and cutting in layers. The formation of surface structures on stainless steel because of laser irradiation has been demonstrated. The possibilities of further GHz burst polishing were examined on previously laser‐milled stainless steel using with lowest possible surface roughness. The ability to polish the surface with microstructures below the original roughness was demonstrated by bursts of ultrashort pulses with a repetition rate in the GHz range. It has been demonstrated that mold milling in stainless steel designed for LED diffusers can be fabricated using a modern femtosecond laser source together with beam size optimization technique for efficient ablation with low surface roughness, layer‐by‐layer milling techniques, and GHz burst polishing techniques.This article is protected by copyright. All rights reserved.

Polarization and kinetics of optical emission during laser induced periodic surface structure formation on crystalline silicon

Polarization and kinetics of secondary optical emission, which occurs during the formation of laser-induced periodic surface structures (LIPSSs) on crystalline silicon by a femtosecond laser beam in air, were analyzed. It was found that the emission lines of ablated Si atoms and formed in plasma SiN molecules are unpolarized, and they decay on the nanosecond scale. The second harmonic generation (SHG) is enhanced by laser-induced surface plasmons, and the SHG polarization basically repeats the polarization of the laser. The results can be used for optical monitoring of LIPSS formation in real time, which is especially important for laser processing of large non-planar surfaces.

Electrochemical and semiconducting properties of multifunctional smart conductive fabrics based on polypyrrole

AbstractWe have fabricated multifunctional conductive fabrics composed of polypyrrole/BaTiO3/poly(acrylonitrile‐co‐methylacrylate) (PPy/BaTiO3/P(AN‐co‐MA)(PBA) utilizing a simple process including dip coating and in situ chemical polymerization of pyrrole, which oxidized both ammonium persulfate (APS) and ammonium persulfate + iron (III) chloride (APS + FeCl3) for flexible and wearable textile applications. The influence of the oxidant nature, PPy content, and BaTiO3 on the surface structures, electrochemical, and semiconducting characteristics of the fabrics was examined using scanning electrochemical microscopy (SEM), electrochemical impedance spectroscopy (EIS), and the Mott–Schottky (M–S) studies. The study revealed that the size of the polymer nanostructures on fabric has an impact on the electrochemical impedance properties. Specifically, a decrease in diameter (when FeCl3 + APS is used) or the formation of more compact swelling surface structures (when only APS is used) is associated with changes in the conductivity of the coated fabric. Based on the EIS tests, the composite coating (S0.1, S0.2, and S0.3) with FeCl3 has a higher electrical conductivity compared to the coating with APS (PBA). The M–S tests indicate that the semiconducting characteristics of coated fabrics are dependent upon the kind of oxidant, the amount of PPy, and the presence of BaTiO3.

Investigating the transition from high to low spatial frequency periodic nanoripples on TiO2 (0 0 1) oriented surface upon irradiation with UV-femtosecond pulses

The formation of laser-induced periodic surface structures (LIPSS) on a TiO2 (001) oriented surface upon irradiation with UV-fs laser pulses was analyzed. Two systems of low spatial frequency LIPSS (LSFL) perpendicular to the direction of laser polarization are observed after N = 5 laser pulses. One was developed on the edges of the crater created by the first laser pulse, while the other appeared in the center of crater and spread towards the edges. When N was greater than 10, the two LIPSS systems merge. Both LSFL systems have a spatial period of approximately 200 nm and are due to the coherent superposition of surface waves scattered on inhomogeneities of a transient metallic surface. However they differ by the type of defects involved in the excitation process of surface waves. While a pronounced and localized rim triggered LSFL on the edge of the laser spot, LSFL in the central part nucleated at the shallow high spatial frequency LIPSS (HSFL), which are parallel to the laser polarization. A mechanism is proposed to explain the role of the HSFL in the formation of LSFL. Numerical simulations of the absorbed laser power density support the evolution from HSFL to LSFL as the number of laser pulses increased.

Impact of helium and hydrogen plasma exposure on surface damage and erosion of tungsten

The impact of helium plasma exposure on the tungsten surface damage structure development and erosion has been investigated by comparing the impact of hydrogen plasma exposure. Crystal orientation dependence of the undulating surface structure formation and erosion rate is observed on the plasma-exposed tungsten surface independently from the plasma species. The top surface of the plasma exposed tungsten has a tendency to {100} plane independently from the initial surface orientation. Although hydrogen and/or helium cause no erosion in tungsten under incident ion energy exposure conditions below the sputtering threshold, inevitable minute impurities, like oxygen, play an essential role in erosion, and significant erosion can be observed even at 30 eV.

Cost-effective superhydrophobic ZnO films with adjustable wetting behaviors deposited via solution precursor plasma spray process

In this work, ZnO films featuring hierarchical micron−/nanometer-sized structures were deposited for adjusting wettability by a simple solution precursor plasma spray (SPPS) process using cost-effective zinc acetate solution. The effects of solution flow rate on the surface microstructures, surface roughness, crystalline orientation, and wetting behaviors of the films were investigated comprehensively. The results showed that high solution flow rate facilitated the formation of apparent hierarchical surface structures featuring more uniformly-distributed micron-sized protrusions and higher surface roughness. The increase of solution flow rate also promoted the crystalline growth along the non-polar (100) plane. With the solution flow rate increasing from 10 ml/min to 40 ml/min, the synergy of the surface structures and crystalline growth orientation resulted in a sharp decrease of the surface energy of the films from ~74.4 mJ/m2 to ~12.6 mJ/m2 and a dramatic elevation of water contact angles from ~130° to ~155°. The wetting states of different films were elucidated by correlating the wetting behaviors with the surface structures. The SPPS-deposited ZnO films showed high adhesion to water droplets, similar to the wetting behaviors of the petal effect, which could be used as a “mechanical hand” for non-loss microdroplet transfer. The work shows that SPPS represents a promising method to deposit cost-effective superhydrophobic films with adjustable wetting behaviors.

Surface blackening of titanium alloy by laser nanosecond pulses

The possibility of developing and blackening the surface of titanium alloy by nanosecond pulses of an ArF laser, a Nd:YAG laser, and a copper vapor laser for use in space engineering is studied in this work. For various irradiation regimes, the optimal parameters for the formation of the necessary surface structures were determined. The possibility of applying a reliable color layer without the use of any paints on the surface of a titanium alloy as a result of treatment with an Nd:YAG laser beam is shown. In particular, this method of laser processing will make it possible to obtain black color on the surface of devices used in outer space to absorb and dissipate solar energy, and it is expected to increase the radiation resistance of such coatings and their service life.

Oxidation-driven 2D LIPSS on amorphous Si films atop metal layers: The interplay of near-field and far-field effects

We investigate the formation of Laser-Induced Periodic Surface Structures (LIPSS) on amorphous Si films coated on Al using circularly polarized femtosecond lasers. By controlling laser fluence, we induce oxidation-based surface patterns, with their regularity governed by the interplay between near-field and far-field contributions. Achieving the right balance between these effects results in a consistent hexagonal pattern. Varying the underlying metal type further revealed its impact on pattern quality: strong far-field contributions led to irregular patterns, while dominant near-field effects resulted in extensive oxidation. Our study reveals the critical balance between near-field and far-field effects in optimizing 2D LIPSS patterns, emphasizing the role of the underlying metal.

The effect of molten pool evolution on the formation of surface structures during laser polishing

As a novel surface treatment technology, in-situ laser polishing aims to improve surface finish of 3D printed parts, while laying the foundation for the integrated processing of 3D printing and laser polishing. However, the polishing process will introduce a few new surface structures (undercuts, step structures, ripples, etc.), which will affect the surface roughness. In this work, the surface roughness and surface structure after high power and low power polishing were compared respectively through experiments. In addition, a 2D transient numerical model was developed to explore the forming mechanism of different structures. The evolution of temperature field, velocity field and freedom surface of melting pool under different parameters were analyzed by the model. The results showed that the solidification induced surface structure with large roughness was generated after high power polishing. During its formation, the thermocapillary force was dominant and drove the tangential flow of fluid. While, low power polishing resulted in ripple structure with small roughness. The emergence of this structure was primarily due to the capillary force as the leading force in the melting pool flow process, which drove the normal flow of the melt, thus eliminating the large surface curvature. Furthermore, the width of the melting pool in the simulation was compared with the width of the single scanning track, and they were in good agreement. Finally, by adjusting the test strategy, it was found that the surface roughness could reach the lowest (0.88 ± 0.06 μm) only when the two structures coexisted.

Porosity Engineering of Dried Smart Poly(N-isopropylacrylamide) Hydrogels for Gas Sensing.

A recent study unveiled the potential of acrylamide-based stimulus-responsive hydrogels for volatile organic compound detection in gaseous environments. However, for gas sensing, a large surface area, that is, a highly porous material, offering many adsorption sites is crucial. The large humidity variation in the gaseous environment constitutes a significant challenge for preserving an initially porous structure, as the pores tend to be unstable and irreversibly collapse. Therefore, the present investigation focuses on enhancing the porosity of smart PNiPAAm hydrogels under the conditions of a gaseous environment and the preservation of the structural integrity for long-term use. We have studied the influence of polyethylene glycol (PEG) as a porogen and the application of different drying methods and posttreatment. The investigations lead to the conclusion that only the combination of PEG addition, freeze-drying, and subsequent conditioning in high relative humidity enables a long-term stable formation of a porous surface and inner structure of the material. The significantly enhanced swelling response in a gaseous environment and in the test gas acetone is confirmed by gravimetric experiments of bulk samples and continuous measurements of thin films on piezoresistive pressure sensor chips. These measurements are furthermore complemented by an in-depth analysis of the morphology and microstructure. While the study was conducted for PNiPAAm, the insights and developed processes are general in nature and can be applied for porosity engineering of other smart hydrogel materials for VOC detection in gaseous environments.

Chemical and Topographical Changes upon Sub‐100‐nm Laser‐Induced Periodic Surface Structure Formation on Titanium Alloy: The Influence of Laser Pulse Repetition Rate and Number of Over‐Scans

Titanium and its alloys are known to allow the straightforward laser‐based manufacturing of ordered surface nanostructures, so‐called high spatial frequency laser‐induced periodic surface structures (HSFL). These structures exhibit sub‐100 nm spatial periods – far below the optical diffraction limit. The resulting surface functionalities are usually enabled by both, topographic and chemical alterations of the nanostructured surfaces. For exploring these effects, multi‐method characterizations were performed here for HSFL processed on Ti–6Al–4V alloy upon irradiation with near‐infrared ps‐laser pulses (1030 nm, ≈1 ps pulse duration, 1–400 kHz) under different laser scan processing conditions, i.e., by systematically varying the pulse repetition frequency and the number of laser irradiation passes. The sample characterization involved morphological and topographical investigations by scanning electron microscopy (SEM), atomic force microscopy (AFM), tactile stylus profilometry, as well as near‐surface chemical analyses hard X‐ray photoelectron spectroscopy (HAXPES) and depth‐profiling time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS). This provides a quantification of the laser ablation depth, the geometrical HSFL characteristics and enables new insights into the depth extent and the nature of the non‐ablative laser‐induced near‐surface oxidation accompanying these nanostructures. This allows to answer the questions how the processing of HSFL can be industrially scaled up, and whether the latter is limited by heat‐accumulation effects.

Formation of two-dimensional laser-induced periodic surface structures on titanium by GHz burst mode femtosecond laser pulses

GHz burst mode femtosecond (fs) laser pulses, which consist of a series of pulse trains with ultra-fast intervals of several hundred picoseconds, have offered distinct features for material processing compared to conventional irradiation of laser pulses (single-pulse mode). We apply GHz burst mode processing to fabricate laser-induced periodic surface structures (LIPSS) on the material surfaces. In our previous work for silicon (Si), we have found that GHz burst mode can create unique two-dimensional (2D) LIPSS composed of both parallel and perpendicular to the laser polarization direction. We proposed that the formation of 2D-LIPSS is attributed to the synergetic contributions of electromagnetic and hydrodynamic mechanisms. To further investigate more detailed formation mechanisms and explore practical applications, we employ titanium (Ti), whose properties are significantly different from Si. We demonstrate that GHz burst mode fs laser pulses (central wavelength: 1,030 nm, intra-pulse width: 230 fs, intra-pulse repetition rate (an intra-pulse interval): 4.88 GHz (205 ps) and burst pulse repetition rate: 10 kHz) can also fabricate 2D-LIPSS on Ti surfaces. We attribute the dominant formation mechanism of 2D-LIPSS to the generation of hot spots with highly enhanced electric fields due to transient change of material properties during GHz burst pulse irradiation. Based on this speculation, properly tailoring the shapes of the burst pulse with an optimum intra-pulse number enables the creation of well-defined 2D-LIPSS. Furthermore, essentially homogeneous 2D-LIPSS can be formed in a large area by laser scanning of a focused fs laser beam with a stage scanning speed of 5 mm/s.

Numerical study of ultrafast optical instability causing structured light absorption in metal

We present the first-principle numerical study of nonlinear decay of a femtosecond laser pulse into a pair of surface plasmon polaritons (SPP) during reflection from a rough metallic surface. The ultrafast dynamics of the decay was studied at damaging laser fluences of about 1 J/cm2, and the principal role of the electronic collision rate growth was proved. The resulting strongly inhomogeneous heating of metal is an important stage of laser-induced phenomena like ablation, terahertz radiation generation, and periodic surface structures formation.

Tributyrin microcapsule prepared by ultrahigh methoxylated pectin combination with maltodextrin: The characterization, gastrointestinal digestion, and fecal fermentation behavior

A modified ultrahigh methoxylated pectin (UHMP, DM>90%) combined with maltodextrin was used to prepare tributyrin microcapsules. The encapsulation performance of microcapsules and their potential application in gut health were investigated in this study. Results showed that tributyrin was successfully encapsulated into the matrix of microcapsules, and there is no extensive chemical reaction occurred during encapsulation. The UHMP is conducive to the dispersion of tributyrin, while the maltodextrin as wall material promoted the formation of smooth surface structure of microcapsules. Microencapsulation is able to provide thermal protection for tributyrin, as reflected by the increased decomposition temperature and the decreased weight loss rate. In addition, microencapsulation effectively masks the unpleasant odor of tributyrin, resulting in a significantly reduced off-odor retention rate. During small intestinal digestion, the UHMP-stabilized tributyrin emulsions partly demulsified and led to the release of tributyrin, however, a significant higher tributyrin retention rate (88.1 ± 1.3%) was observed in UHMP-stabilized tributyrin emulsions when compared with the blank group (60.1 ± 4.6%). Through in vitro fecal fermentation, it was found that UHMP-stabilized tributyrin emulsions can regulate bacterial community by stimulating the growth of probiotics (e.g. Lachnospira, Phascolarctobacterium, and Parabacteroides) and decreasing the relative abundances of harmful microbes (e.g. Klebsiella and Fusobacterium). Furthermore, UHMP-stabilized tributyrin emulsions promote the production of SCFAs, especially, the content of butyric acid increased from 1.59 ± 0.32 mmol/L to 10.58 ± 1.89 mmol/L. The present study provides a feasible approach for the preparation of tributyrin microcapsule, which improves its application performance in gut health.

Dynamic surfaces of latex films and their antifouling applications

Latex polymer particles have been widely used in industry and everyday life. For decades the fabrication of “smart” latex film from latex particles has been a great challenge due to the difficulty in the synthesis of the functional latex particles by traditional emulsion polymerization using small molecular surfactants. In this manuscript, a simple and environmentally-friendly approach to the fabrication of “smart” latex films with dynamic surfaces is reported. Latex particles with poly(n-butyl methacrylate) (PnBMA) in the cores and zwitterionic poly-3-[dimethyl-[2-(2-methylprop-2-enoyloxy) ethyl]azaniumyl]propane-1-sulfonate (PDMAPS) in the shells are synthesized by reversible addition-fragmentation chain transfer (RAFT) mediated surfactant-free emulsion polymerization. The kinetics for the emulsion polymerization is studied, and the latex particles are analyzed by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and dynamic light scattering (DLS). Latex films are prepared by casting aqueous solutions of the latex particles at temperatures above the glass transition temperature (Tg) of PnBMA. On the dried latex film, the hydrophobic PnBMA blocks occupy the top surface; after water treatment, the hydrophilic PDMAPS blocks migrate to the surface. A change in the surface hydrophilicity results in a change in the water contact angle of the latex film. A mechanism for the formation of the dynamic surface structure is proposed in this research. Antifouling applications of the latex films are investigated. Experimental results indicate that the water-treated latex film is able to efficiently inhibit protein adsorption and resist bacterial adhesion.

Investigating Laser-Induced Periodic Surface Structures (LIPSS) Formation in Silicon and Their Impact on Surface-Enhanced Raman Spectroscopy (SERS)

Laser-induced periodic surface structures (LIPSS) have gained significant attention due to their ability to modify the surface morphology of materials at the micro-nanoscale and show great promise for surface functionalization applications. In this study, we specifically investigate the formation of LIPSS in silicon substrates and explore their impact on surface-enhanced Raman spectroscopy (SERS) applications. This study reveals a stepwise progression of LIPSS formation in silicon, involving three distinct stages of LIPSS: (1) integrated low-spatial-frequency LIPSS (LSFL) and high-spatial-frequency LIPSS (HSFL), (2) principally LSFL and, (3) LSFL at the edge of the irradiated spot, elucidating the complex interplay between laser fluence, pulse number, and resulting surface morphology. Furthermore, from an application standpoint, these high-quality multi-scale periodic patterns lead to the next step of texturing the entire silicon surface with homogeneous LIPSS for SERS application. The potential of LIPSS-fabricated silicon substrates for enhancing SERS performance is investigated using thiophenol as a test molecule. The results indicate that the Au-coated combination of LSFL and HSFL substrates showcased the highest enhancement factor (EF) of 1.38×106. This pronounced enhancement is attributed to the synergistic effects of localized surface plasmon resonance (LSPR) and surface plasmon polaritons (SPPs), intricately linked to HSFL and LSFL characteristics. These findings contribute to our understanding of LIPSS formation in silicon and their applications in surface functionalization and SERS, paving the way for sensing platforms.

A theoretical and experimental study of surface micro/nanostructure formation in ultraprecision diamond milling

Ultraprecision diamond milling (UPDM) is a very promising machining method, producing high-quality surface structures with nanometric surface roughness Sa and sub-micrometric form accuracy PV. Due to its high flexibility, it has been widely employed to fabricate complex freeform/structured surfaces. However, there exists little comprehensive understanding of its surface structure formation. Therefore, this study focuses on discussing surface structure formation in the UPDM process. Firstly, a geometric model was proposed for the relationship among the diamond tool with eccentricity, machining motion with tool tilt, and surface formation in the UPDM process. Secondly, the simulation for surface structure formation of UPDM was carried out for the proposed geometric model, considering different tool tilt angles. Finally, a series of UPDM experiments were conducted at the different tool tilt angles. By comparing with the simulation results, it was found that the simulation results were in good consistence with the experimental results. The simulation and experimental results indicated that in the UPDM process the diamond tool eccentricity would prominently induce the droplet-like microstructures on the machined surface, and the tilt angle would dramatically produce the hexagon-like nanostructures. Further, with the increase of the tool tilt angle, the micro/nanostructures’ PV first increased, then decreased, and finally remained stable. Interestingly, UPDM could be directly modulated for the fabrication of micro/nanostructures. Significantly, the study gives a deep insight into surface micro/nanostructure formation of UPDM.

Laser-Induced Periodic Surface Structures on Layered GaSe Crystals: Structural Coloring and Infrared Antireflection.

We study structural and morphological transformations caused by multipulse femtosecond-laser exposure of Bridgman-grown ϵ-phase GaSe crystals, a van der Waals semiconductor promising for nonlinear optics and optoelectronics. We unveil, for the first time, the laser-driven self-organization regimes in GaSe allowing the formation of regular laser-induced periodic surface structures (LIPSSs) that originate from interference of the incident radiation and interface surface plasmon waves. LIPSSs formation causes transformation of the near-surface layer to amorphous Ga2Se3 at negligible oxidation levels, evidenced from comprehensive structural characterization. LIPSSs imprinted on both output crystal facets provide a 1.2-fold increase of the near-IR transmittance, while the ability to control local periodicity by processing parameters enables multilevel structural color marking of the crystal surface. Our studies highlight direct fs-laser patterning as a multipurpose application-ready technology for precise nanostructuring of promising van der Waals semiconductors, whose layered structure restricts application of common nanofabrication approaches.

Formation of Periodic Surface Structures on Semiconductors under Mid-Infrared Free-Electron Laser Irradiation

Formation of Periodic Surface Structures on Semiconductors under Mid-Infrared Free-Electron Laser Irradiation

2D compound structures with deep subwavelength period on silicon fabricated by double time delayed femtosecond laser beams

In this work, two orthogonally polarized femtosecond laser beams are employed to irradiate a p-doped silicon wafer with an electrical resistivity of 0.008 Ω·cm. It is interesting to find that 2D compound structures composed of sub-wavelength periodic ripples and deep sub-wavelength nanodot array can be produced when proper laser fluence and time delay between the dual laser beams are used. The formation of the periodic ripples can be explained by the interference between the preceding incident laser and it induced surface plasmon polaritons (SPPs). The periodic nanodot array has a period down to ∼200 nm and the radius of the nanodot is ∼30 nm, most of which appear at the boundary between the ditch and ridge of the ripple. During the ripples’ formation, the residual melting silicon is most probably located at the boundary between the ditch and ridge of the ripple. Furthermore, the period of the nanodot array is roughly equal to the perimeter of the nanodot. Therefore, it is considered that the dot array may be generated due to the Rayleigh-Taylor instability of the melting silicon. It is also noted that these nanodots are all uniformly arranged along vertical lines, indicating that the subsequent incident laser may break the stochastic characteristic of the Rayleigh-Taylor instability and produce the 2D periodic dot array. The thermo-hydrodynamical process combined with the interference effect between SPPs and the incident laser can benefit the formation of complex surface structures with versatile functions.