Laser-induced Periodic Nanostructures Research Articles

Change of Adhesion Properties of Bioinspired Laser-Induced Periodic Nanostructures towards Cribellate Spider Nanofiber Threads by Means of Thin Coatings

We investigated the effect of additional continuous functional coatings, which changed the hydrophilic–hydrophobic properties of the surface without heavily influencing the surface topography at the nanoscale, on the antiadhesive properties of bioinspired laser-induced periodic nanostructures. These nanostructures mimic the antiadhesive structures on the silk-combing area on the legs of cribellate spiders, the calamistrum. The thin films were deposited by matrix-assisted laser deposition and characterized by infrared spectroscopy, X-ray photoelectron spectroscopy, water contact angle measurements, and adhesion tests using capture threads from the cribellate spider webs. In all cases, the nanoripples were preserved and these structured surfaces showed lower adhesion forces compared to flat controls, although not significant. However, this effect was totally overwhelmed by the difference between the adhesion forces on surfaces with different chemical compositions. The largest adhesion forces were observed on hydrophilic surfaces and the lowest ones on hydrophobic surfaces. The fact that the antiadhesion between nanofibers and the nano-structured areas depends strongly on the chemical composition of the surface can be explained by the specific adhesion between individual chemical groups due to frequency dependencies in the theory of van der Waals forces. However, explaining these adhesion properties just by the categories “hydrophilic” or “hydrophobic” is oversimplified.

Effects of laser pulse duration on the formation dynamics of laser-induced periodic nanostructures.

Formation dynamics of laser-induced periodic surface structures (LIPSSs) on the SiC substrates were described with varying pulse numbers and pulse duration. As the number of laser pulses increases, two significant transformations become evident in the progression of structural formations. First from surface roughening with nanoparticles to LIPSS with the period that is slightly shorter than the laser wavelength. Second it turns to LIPSS with a period less than half the laser wavelength. It is found that maintaining the crystallinity is the key to changing the structures. In the cases of longer pulse width than sub-nanoseconds, no LIPSS formations are observed or LSFL does not change to HSFL because the irradiated area is poly-crystallized.

Surface micromorphology and nanostructures evolution in hybrid laser processes of slicing and polishing single crystal 4H-SiC

Slicing and post-treatment of SiC crystals have been a significant challenge in the integrated circuit and microelectronics industry. To compete with wire-sawing and mechanical grinding technology, a promising approach combining laser slicing and laser polishing technologies has been innovatively applied to increase utilization and decrease damage defects for single crystal 4H-SiC. Significant material utilization has been achieved in the hybrid laser processes, where material loss is reduced by 75% compared to that of conventional machining technologies. Without any special process control or additional treatment, an internally modified layer formed by laser slicing can easily separate the 4H-SiC crystals using an external force of about ∼3.6 MPa. The modified layer has been characterized using a micro-Raman method to determine residual stress. The sliced surface exhibits a combination of smooth and coarse appearances around the fluvial morphology, with an average surface roughness of over Sa 0.89 µm. An amorphous phase surrounds the SiC substrate, with two dimensions of lattice spacing, d = 0.261 nm and d = 0.265 nm, confirmed by high-resolution transmission electron microscopy (HRTEM). The creation of laser-induced periodic surface nanostructures in the laser-polished surface results in a flatter surface with an average roughness of less than Sa 0.22 µm. Due to the extreme cooling rates and multiple thermal cycles, dissociation of Si-C bonding, and phase separation are identified on the laser-polished surface, which is much better than that of the machining surface. We anticipate that this approach will be applicable to other high-value crystals and will have promising viability in the aerospace and semiconductor industries.

Laser-induced deep-subwavelength periodic nanostructures with large-scale uniformity

Femtosecond lasers are capable of fabricating uniform periodic nanostructures with a near-wavelength periodicity; however, it is challenging to produce subwavelength nanostructures with large-scale uniformity. Here, we investigate femtosecond laser-induced self-assembly of periodic nanostructures on Si-on-Pt hybrid ultrathin films via photothermal-induced oxidation. The coexistence of scattering light and surface plasmon polaritons on the hybrid films gives rise to a diversity of surface morphologies. Depending on the laser power and sample scanning velocity, beyond the traditional one-dimensional nanogratings that exhibit a near-wavelength periodicity, two types of nanostructures with subdiffraction-limit periodicity while large-scale uniformity are also observed. The first type, occurring at high laser energy and low scanning velocity, is generated by the spatial frequency doubling of the traditional laser-plasmon-interfering nanogratings. It exhibits a periodicity of <λ2. The second type, deep-subwavelength nanostructures, takes place at low pulse energy or low scanning velocity. It is in the form of two-dimensional nanoparticles and has a periodicity of <λ4. The far-field laser-plasmon interference associated with near-field scattering is attributed to the formation of such deep-subwavelength nanostructures, as confirmed by finite-difference time-domain numerical simulations. Our work provides a route toward high-throughput laser fabrication of large-scale deep-subwavelength periodic nanostructures.

Generation mechanisms of laser-induced periodic nanostructures on surfaces of microgrooves

Functional surfaces with hierarchical micro-/nanostructures play a vital role in improving performances in emerging fields over the past decades. In this work, we propose a hybrid manufacturing method for efficiently fabricating hierarchical nanotextured microgrooves by combining fly cutting and femtosecond laser nanopatterning. Nanostructures, including nanogratings and nanoholes, can be uniformly generated in a V-shaped groove by controlling laser polarization. We experimentally study the influence of the laser parameters (laser fluence and effective pulse number) to investigate the morphological evolution of nanostructures in the V-shaped groove under laser irradiation. By theoretical calculations and analyses, we find that the interference between the incident and reflected laser is helpful for ablating the nanogratings. In addition, this interference can also couple with the laser-SPPs (surface plasmon polaritons) interference and form nanoholes. The results enrich the diversity of achievable laser-induced periodic surface structures on complex surfaces and provide a new way to efficiently fabricate large-area hierarchical micro-/nanostructures at low cost.

Threshold effect on the femtosecond laser-induced periodic subwavelength structure: An analytical approach

For linearly polarized femtosecond laser-induced periodic nanostructures on a metal surface, the underlying physics of self-formation patterns is yet to be fully explained. We demonstrated here that the stabilized periodic interface electrostatic field (Coulomb field) arising from the surface phase-locking resonant dominates the self-formation of ripple structures. The kinetics of ultrafast laser ablation on solid surface were discussed. The threshold fluence and upper limit fluence for self-formation of ripple structures were calculated. The underlying mechanisms were clarified with both simulation and experimental results. Analytical formula for the interspace between ripples (i.e., wavelength of surface Langmuir wave) was developed. As illustrated by the experimental results, the competition of the two ablation mechanisms, namely surface Coulomb ablation and direct ultrafast laser ablations, was shown to result in distinct structuring patterns for a typical Gaussian pulse laser irradiation on the target surface at the appropriate laser energy fluence.

A hybrid machine learning approach to determine the optimal processing window in femtosecond laser-induced periodic nanostructures

Surface nanostructuring could enhance surface properties such as strength, self-cleaning, anti-fog and anti-bacterial properties. Femtosecond laser-induced periodic surface structures (LIPSS) is a nanoscale structure created with laser technique. However, its quality is significantly influenced by the complicated interrelationship between the various laser processing and material parameters. Hitherto the selection of the appropriate laser parameters mainly depends on personal experience in conjunction with many time-consuming experimental trials. To have a simple, fast, and intelligent process, a hybrid machine learning method is proposed to determine the optimized processing window for femtosecond laser-induced nanostructures. Firstly, k-means clustering method was applied to automatically classify the laser-induced nanostructures into good and bad quality classes. Before clustering, dimensionality reduction methods were applied to reduce the high dimension of image data and to extract features. Different dimensionality reduction methods including principal component analysis (PCA), local linear embedding (LLE), t-random adjacent embedding (t-SNE) and transfer learning were explored. Transfer learning showed a much better result compared with other dimensionality reduction methods. Transfer learning VGG19 model achieved the highest accuracy of 90.6 %. After clustering, the image was labelled as good and bad clusters accordingly. The labeled image was trained using artificial neural network (ANN), random forest (RF), decision tree (DT), support vector machine (SVM), k-nearest neighbors (KNN) and Naive Bayesian Classifier (NBC) algorithms for the prediction of laser processing results. The results show that DT gives the best accuracy of 96.7 %. Finally, an optimal laser processing window for femtosecond laser-induced nanostructures was determined.

Enhanced hydrogen production through alkaline electrolysis using laser-nanostructured nickel electrodes

This study describes the fabrication of ultrafast laser-induced periodic nanostructures on Nickel sheets and their use as cathodes in alkaline electrolysis. For the first time, to the best of our knowledge, laser-nanostructured Ni sheets were used as cathode electrodes in a custom-made electrolysis cell at actual, Hydrogen producing conditions, and their efficiency has been compared to the untreated Nickel sheets. The electrochemical evaluation showed higher Jpeaks, lower overpotential, and enhanced double-layer capacitance for the nanostructured electrode. A decrease in the Tafel slope was also found for the nanostructured electrode. The hydrogen production efficiency was found to be 3.7 times larger for the laser-nanostructured Nickel electrode, which was also confirmed by current-time measurements during electrolysis. Also, a novel approach is proposed to improve the stability of the current density during electrolysis and, therefore, the hydrogen production process by about 10%.

Surface Superconductivity Changes of Niobium Sheets by Femtosecond Laser-Induced Periodic Nanostructures

Irradiation with ultra-short (femtosecond) laser beams enables the generation of sub-wavelength laser-induced periodic surface structures (LIPSS) over large areas with controlled spatial periodicity, orientation, and depths affecting only a material layer on the sub-micrometer scale. This study reports on how fs-laser irradiation of commercially available Nb foil samples affects their superconducting behavior. DC magnetization and AC susceptibility measurements at cryogenic temperatures and with magnetic fields of different amplitude and orientation are thus analyzed and reported. This study pays special attention to the surface superconducting layer that persists above the upper critical magnetic field strength Hc2, and disappears at a higher nucleation field strength Hc3. Characteristic changes were distinguished between the surface properties of the laser-irradiated samples, as compared to the corresponding reference samples (non-irradiated). Clear correlations have been observed between the surface nanostructures and the nucleation field Hc3, which depends on the relative orientation of the magnetic field and the surface patterns developed by the laser irradiation.

A combined model for formation mechanism of ripples induced by femtosecond laser on silicon carbide

In this article, a comparative study is performed to explore the formation mechanism of laser-induced periodic surface structures (LIPSS) using femtosecond laser on silicon carbide (SiC). The optical properties of SiC transformed to a metal-like state when the excited carrier density exceeds 2e22 cm−3 under laser irradiation and significantly altered the laser energy deposition. The laser-induced carrier density coupled with the optical properties and the incident laser intensity profile were calculated. The two-temperature model was used to describe the energy transformation from electron to lattice and guided to select appropriate laser conditions for producing ripples. The spatial period of laser-induced periodic nanostructures was calculated in frequency domain based on the Drude-Sipe model that comprised the estimation of free carrier density. The calculated range of the spatial period of LIPSS was about from 776 to 544 nm. Additionally, the experimental value was about 719–483 nm, which contained a good consistency with the theoretical results. This work provides a new approach for the prediction and fabrication of the LIPSS on SiC.

Periodicity control of laser-induced periodic nanostructures by thin deposition layer on sapphire substrate

This study concludes the possibility of controlling the period of the laser-induced periodic surface structures (LIPSSs) by deposition of a layer of different kinds of material on the processing sample. When the Pt-Pd or ZnO layers were deposited on the sapphire substrate, the period of the LIPSS formed at the sapphire surface was significantly shortened. For deposition layers thicker than 40 nm, LIPSS with periods much shorter than 330 nm, which is the period of LIPSS without a deposition layer, were formed. Some case periods of around 50 nm were obtained, which were one twentieth of the applied wavelength.

Laser Formation of Periodic Micro- and Nanostructures on the Surface of Monocrystalline Silicon

Experimental studies of the features of the formation of laser-induced periodic nanostructures on the surface of silicon wafers in the zones of action of second, millisecond and nanosecond laser pulses are conducted in the work. The results of microscopic investigations by optical and electron microscopes of periodic structures formed on surfaces with crystallographic orientation (111), (100) are presented. The obtained results can be used to optimize the laser pulse mode for controlled micro- nanostructuring of the semiconductor surface.

Generalized Plasmonic Modelling of the Effect of Refractive Index on Laser-Induced Periodic Nanostructures

Laser-induced periodic surface structures (LIPSS) have been studied theoretically employing generalized plasmonic modelling on several dielectric materials such as SiO2, Al2O3, ZnO, AlAs and diamond exposed to 800 nm wavelength multi-pulse femtosecond laser irradiation. The study of the optical properties of the materials during laser irradiation reveals a formation of a metallic like pseudo-material on the irradiated layer during excitation. A study of the grating periodicity of the nanostructures shows that the materials having a high refraction index allow LIPSS formation with a wide range of grating periodicities. Results also show High Spatial Frequency LIPSS formation with periodicities 3 to 8 times lower than the laser wavelength.

Laser-induced periodic surface nanostructures formation and modification in polyethylene terephthalate (PET)

Polyethylene terephthalate (PET) is a thermoplastic polymer used in a large variety of industries as clothing, packaging and aeronautics among many others. Laser-inducted periodic surface nanostructures presents great potential as long as the modifications of materials are delivered in a controlled manner. However the formation of nanostructures having different morphological properties presents itself a difficult errand. The purpose of this paper is to present how the surface structure of Polyethylene terephthalate (PET) changes at nanoscale levels after irradiation with laser beams of spatially variant polarization. Utilizations of such nanostructures can be seen in a multitude of applications varying from electronics and textiles to aeronautical and military implementations. For the creation of the new nanostructures has been used an YAG solid state laser (Spectra Physics, Quanta Ray) with a pulse duration of 4 ns and a repetition rate of 10 Hz for different spatial periods and at different inclination angles, an Atomic Force Microscope and a Scanning Electron Microscope in order to analyze the newly created nanostructures. Such Polyethylene terephthalate (PET) modified structures have been used as well in combination with fiber glass compounds in order to produce highly resistant engineering resins to heat and impact. The formation of specific controllable surface nanostructures given different functionalities are of major interest as a technique to improve or even produce completely new specific oriented properties. This study reveals how the structure of polyethylene terephthalate surface changes after laser-induced treatment.

Formation of femtosecond laser-induced periodic nanostructures on GaN

The femtosecond laser-induced periodic surface structure (LIPSS) method has attracted considerable interest as a way to form fine structures with dimensions below the laser wavelength. These structures are highly reproducible, demonstrating that this method is suitable for applications such as quantum devices. In this study, we investigated the shape and crystalline state of a LIPSS to clarify the material properties and laser conditions that define its structure. While a LIPSS composed of GaN maintained its crystallinity, disturbances in its local crystalline orientation were generated. The direction and periodicity of the LIPSS did not depend on the in-plane crystal direction of GaN or the scanning direction, but the aspect ratio of each LIPSS depended on the in-plane crystal direction. The LIPSS with exposed {1–100}GaN faces showed high aspect ratios, with narrower widths and deeper depths, than LIPSS with different exposed faces.

Friction reduction of steel by laser-induced periodic surface nanostructures with atomic layer deposited TiO2 coating

We report on the reduction of friction of steel treated by femtosecond laser pulses and subsequent atomic layer deposition. Uniform laser-induced periodic surface structures (LIPSS) are produced on the surfaces of stainless steel samples by an 800-nm femtosecond laser. These nano-textured surfaces are then deposited with TiO2 coating by means of atomic layer deposition (ALD). Friction tests show that the average friction coefficients of the treated samples have been greatly reduced. The improvement of tribological properties is attributed to the intact ripple structures reduce the contact area between friction pairs and act as reservoirs for wear particles. Moreover, the TiO2 coating further reduces adhesion and plastic deformation, which is beneficial to improving tribological properties. It is suggested that the synergistic effect of LIPSS and ALD TiO2 coating had a more marked potential for improving tribological properties.

In-situ high-resolution visualization of laser-induced periodic nanostructures driven by optical feedback

Optical feedback is often evoked in laser-induced periodic nanostructures. Visualizing the coupling between surfaces and light requires highly-resolved imaging methods. We propose in-situ structured-illumination-microscopy to observe ultrafast-laser-induced nanostructures during fabrication on metallic glass surfaces. This resolves the pulse-to-pulse development of periodic structures on a single irradiation site and indicates the optical feedback on surface topographies. Firstly, the quasi-constancy of the ripples pattern and the reinforcement of the surface relief with the same spatial positioning indicates a phase-locking mechanism that stabilizes and amplifies the ordered corrugation. Secondly, on sites with uncorrelated initial corrugation, we observe ripple patterns spatially in-phase. These feedback aspects rely on the electromagnetic interplay between the laser pulse and the surface relief, stabilizing the pattern in period and position. They are critically dependent on the space-time coherence of the exciting pulse. This suggests a modulation of energy according to the topography of the surface with a pattern phase imposed by the driving pulse. A scattering and interference model for ripple formation on surfaces supports the experimental observations. This relies on self-phase-stabilized far-field interaction between surface scattered wavelets and the incoming pulse front.

The Role of Coupled Nanoplasmon Excitation in Growth Mechanism of Laser-Induced Self-Organized Nanostructures in AgCl-Ag Waveguide Thin Films

Formation mechanism of laser-induced spontaneous periodic nanostructures in thin light-sensitive AgCl waveguide films, doped by silver nanoparticles, is studied. It is found that the initial size, geometry, and surface coverage parameters of Ag nanoparticles instate preconditions for the nanostructure formation. These parameters play essential roles in coupled nanoplasmon excitation in silver nanoclusters, which in turn influences scattering of the incident light from the silver clusters. Some parts of the scattered light propagate as waveguide modes in the film and interference with the incident light. Afterward, migration of the Ag nanoparticles into the minima of the interference pattern forms the nanostructure. Simultaneously, excitation of coupled nanoplasmons in the neighboring clusters enhances the scattered light intensity. It is observed that longer exposure results in destruction of the formed nanostructures, because of creation of electrical joint between agglomerated clusters in the interference pattern’s minima, which leads to weakening of the TE mode excitation and, consequently, domination of the Gaussian profile of the incident light. This leads to the deceleration of the self-organized nanostructure development, growth rate, and quality. We have found that competition between the Gaussian profile of the incident laser field and the interference field causes a finally saturating oscillatory behavior of the self-organizing process.

Highly periodic laser-induced nanostructures on thin Ti and Cu foils for potential application in laser ion acceleration

The feasibility of femtosecond laser-induced periodic nanostructures on thin Ti and Cu foils (thickness down to 1 μm) is demonstrated. At pulse durations of 120 fs and a wavelength of 400 nm, periods of 61 nm to 320 nm were obtained. Particle-in-cell simulations of laser ion acceleration processes with such nanostructured targets indicate their potential for high energy particle physics applications. In particular, a measurable enhancement of the proton cut-off energy and a significant enhancement of the number of accelerated particles compared to non- or weakly structured targets of same thickness and material are expected.

Reduction of Friction of Metals Using Laser-Induced Periodic Surface Nanostructures

We report on the effect of femtosecond-laser-induced periodic surface structures (LIPSS) on the tribological properties of stainless steel. Uniform periodic nanostructures were produced on AISI 304L (American Iron and Steel Institute steel grade) steel surfaces using an 800-nm femtosecond laser. The spatial periods of LIPSS measured by field emission scanning electron microscopy ranged from 530 to 570 nm. The tribological properties of smooth and textured surfaces with periodic nanostructures were investigated using reciprocating ball-on-flat tests against AISI 440C balls under both dry and starved oil lubricated conditions. The friction coefficient of LIPSS covered surfaces has shown a lower value than that of the smooth surface. The induced periodic nanostructures demonstrated marked potential for reducing the friction coefficient compared with the smooth surface.