Formation Of Surface Structures Research Articles

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.

GLOSSY CARBON ELECTRODES WITH LASER-INDUCED PERIODIC SURFACE STRUCTURES FOR ELECTROCHEMILUMINESCENT ANALYSIS

In this paper, the possibility of using glassy carbon electrodes modified with laser-induced periodic surface structures for conducting electrochemical and electrochemiluminescent studies and creating appropriate analytical systems is investigated. Modification of the surface of the electrodes is carried out with the help of a femtosecond laser. The surface morphology of the modified electrodes by atomic force microscopy shows the stable formation of periodic surface structures. Electrochemiluminescence studies indicate the possibility of effective use of such modified electrodes for the creation of electrochemiluminescent analytical systems.

Ultra-short pulsed laser ablation of decagonal AlCoNi and AlCoCuNi quasicrystals

Al70Co20Ni10 and Al70Co15Cu10Ni5 (at%) decagonal quasicrystals (DQCs) were subjected to femtosecond laser ablation at a laser wavelength of 800 nm, in deionized water (DI). The generated nanoparticles (NPs) confined within the liquid medium were studied for optical and structural properties. The structural analysis revealed the retention of the targets phase in the NPs. In case of Al70Co20Ni10, formation of Al3Ni2 phase was confirmed through the SAED patterns and HRTEM micrographs. Contrary to this, the formation of Al-Ni phase was not observed in Al70Co15Cu10Ni5. This is attributed to the presence of Cu in the latter sample, which restricts the mixing of Ni with Al. The HRTEM micrographs in both cases showed that Al2O3 was present at the edges of the NPs. The optical analysis confirmed the formation of Al2O3 in both cases. However, in the case of Al70Co15Cu10Ni5, a very weak signal corresponding to CuO was detected. This is attributed to the diffusion of Cu along with Al at the edge. The ablated region of the target’s surface was studied for the formation of laser-induced periodic surface structures (LIPSS) also known as ripples, and the type of ripples formed was identified.

Highly regular nanogratings on amorphous Ge films via laser-induced periodic surface sublimation

Despite the fact that monocrystalline germanium (Ge) was historically the first material where formation of laser-induced periodic surface structures (LIPSSs) was observed in 1965 and then extensively studied, practically relevant thin films of amorphous Ge (a-Ge) were occasionally ignored by these studies so far. Here, highly regular LIPSSs were observed on the surface of magnetron-sputtered a-Ge films under near-IR femtosecond laser exposure. Formation of these regular structures was explained by excitation and interference of the surface waves at the interface of photoexcited Ge that was confirmed by performed calculations and full-wave simulations. At the same time, remarkable structural regularity of these LIPSSs unveiled their ablation-free formation scenario involving laser-driven oxidation of the Ge followed by ultra-clean sublimation of the oxide. Laser patterning in vacuum was found to regulate oxidation/sublimation rates of the a-Ge and its oxide improving the structure quality and opening pathways for practical applications in optoelectronics, solar light harvesting and near-IR photonics.

Laser machining of free-standing silicon nitride membranes

Patterning free-standing Silicon Nitride membranes has enabled the development of various SiN-based devices such as quantum resonators, biosensors, temperature detectors, and plasmonic devices. Patterning SiN membranes usually involves expensive and time-consuming cleanroom techniques that limit flexibility in the development of new SiN membrane designs. While ultrafast laser machining has been proposed to create micron to submicron features in these membranes, laser machining of large area features has not been reported as it often results in membrane failure due to stress concentration during laser machining. We show that continuous laser machining can be used to create a crack and to extract the fracture toughness of the membrane (1.9 ± 0.3 MPa.m1/2). To successfully patterned SiN membranes without failure, a low stress laser machining method is developed, where stress concentration can be minimized by adjusting the laser drilling sequence, a result confirmed by finite element simulations. Complex shapes can thus be machined over a large area with a very high success rate. Laser parameters are optimized to improve the quality of the laser-cut edges without the formation of laser-induced periodic surface structure. Transmission Electron Microscopy and Electron Energy Loss Spectroscopy indicate a modification of the Si/N ratio over a small region next to the laser machined features. These results demonstrate that low stress ultrafast laser machining can be used to create patterns in SiN membranes quickly and economically, enabling the rapid development of new membrane designs.

Formation and modulation of multi-pulse femtosecond laser-induced periodic surface structures by electromagnetic and partial differential equation simulations.

In order to demonstrate the formation of laser-induced periodic surface structures (LIPSS), simulations were performed to investigate the effect of multiple femtosecond laser pulses with different laser energy densities on a Ti6Al4V surface. In this work, a set of partial differential equations calculating the electron and lattice temperature variations, followed by coupling with an electric field, is used to analyze the evolution of the periodic surface structure induced by the interaction of the femtosecond laser with the material. As the number of pulses increases, the surface structure of the material changes from none to produce LIPSS structure and from low spatial frequency LIPSS (LSFL) structure to high spatial frequency LIPSS (HSFL) structure. In order to compare the results, single-point laser scanning ablation experiments were carried out at femtosecond laser energy. The experimental results are consistent with the simulation results.

Formation of laser-induced periodic surface structures on Zr-based bulk metallic glasses with different chemical composition

Bulk metallic glasses (BMG) are amorphous metal alloys known for their unique physical and mechanical properties. In the present study, the formation of femtosecond (fs) laser-induced periodic surface structures (LIPSS) on the Zr-based BMGs Zr46Cu46Al8, Zr61Cu25Al12Ti2, Zr52.5Cu17.9Al10Ni14.6Ti5 (Vit105) and Zr57Cu15.4Al10Ni12.6Nb5 (Vit106) was investigated as a function of their different chemical composition. For this purpose, LIPSS were generated on the sample surfaces in an air environment by fs-laser irradiation (λ = 1025 nm, τ = 300 fs, frep = 100 kHz). The surface topography was characterized by scanning electron microscopy and atomic force microscopy. Moreover, the impact of LIPSS formation on the structure and chemical surface composition was analyzed before and after fs-laser irradiation by X-ray diffraction and X-ray photoelectron spectroscopy as well as by transmission electron microscopy in combination with energy dispersive X-ray spectroscopy. Despite the different chemical composition of the investigated BMGs, the fs-laser irradiation resulted in almost similar properties of the generated LIPSS patterns. In the case of Zr61Cu25Al12Ti2, Vit105 and Vit106, the surface analysis revealed the preservation of the amorphous state of the materials during fs-laser irradiation. The study demonstrated the presence of a native oxide layer on all pristine BMGs. In addition, fs-laser irradiation results in the formation of laser-induced oxide layers of larger thickness consisting of an amorphous ZrAlCu-oxide. The precise laser-structuring of BMG surfaces on the nanoscale provides a versatile alternative to thermoplastic forming of BMG surfaces and is of particular interest for the engineering of functional material surfaces.

Materials Space-Tectonics: Atomic-level Compositional and Spatial Control Methodologies for Synthesis of Future Materials.

This review proposes a novel methodology, materials space-tectonics, as a game-changing concept of materials synthesis. Many reactions occur at surfaces and interfaces, making the formation of well-designed surface and interfacial structures a crucial matter. To establish both a bulk material body (i.e., framework) and an empty void space, careful consideration is required to "nano-architect" the space. Traditional porous materials, such as mesoporous silicas, zeolites, and metal-organic frameworks (MOFs), lack advanced cooperative functionalities due to their largely monotonous pore geometries and limited conductivities. To overcome these limitations and create functional structures with surface-specific functions, "Materials space-tectonics" is proposed as a game-changing methodology for future materials synthesis. This review summarizes recent examples of materials synthesis according to the synthesis of building blocks (i.e., tectons) and their hybridization. It also provides practical tips for successfully implementing materials syntheses and showcase various state-of-the-art performances from the viewpoint of practical applications. Finally, the potential integration of materials space-tectonics with emerging technologies such as materials informatics is discussed.

Effect of film thickness on laser induced surface structures formation on optically transparent polymer films

Laser-induced periodic surface structures can be prepared on polymer thin films, despite the absence of light absorption at the wavelength of the irradiating laser, by using a bilayer approach. Nanostructuring of a poly(3-hexylthiophene-2,5-diyl) (P3HT) layer, by laser irradiation at 532 nm, covered by a thin layer of a non-absorbing polymer, in this case, the ferroelectric copolymer poly(vinylidene fluoride - trifluoro ethylene),P(VDF-TrFE), allows the development of a grating-like structure on the surface. The range of thicknesses of the upper layer, which leads to the formation of LIPSS, has been determined. It is verified that for a thickness larger than 40 nm, no periodic structures are formed, neither in the upper layer nor in the bottom P3HT one. This can be explained considering a mechanical restriction which prevents P3HT rearrangement. The upper nanostructured surface layer exhibits ferroelectric properties as proven by piezoresponse force microscopy measurements.

Influence of zirconium and copper sub-layer in cell integrations on femtosecond laser-processed Ti thin films

The creation of novel biocompatible Ti-based thin films with a Zr or Cu sub-layer modified by ultrafast laser processing is studied. To prepare bioactive surfaces, ultrafast laser processing is focused on the formation of laser-induced periodic surface structures (LIPSS) with the production of oxide phases at the surfaces. Two differently designed multilayer thin films Ti/Cu/Ti and Ti/Zr/Ti were deposited on the silicon using the ion sputtering method. The Ti thin film contains Cu or Zr sub-layer (thickness of 10 nm) at the 10 nm below the surface. The composition and surface morphology variations for these systems, deposited and laser-processed under the same experimental conditions, were caused only by different thermo-physical properties of the sub-layer (Cu or Zr). The surface morphology in the form of LIPSS, led to improved cell adhesion and stable cells/thin films interface compared to as-deposited samples. Field-emission scanning electron microscopy and MTT analysis revealed that laser processing of both systems increased cell adhesion, proliferation, and metabolical activity of L929 mouse fibroblast cells compared to non-modified flat surfaces. Overall, the biocompatibility of Zr-containing thin films is better than Ti/Cu/Ti system. Further, laser processing and formation of LIPSS makes Ti/Zr/Ti thin films excellent candidate for biomedical .

Femtosecond Laser‐Induced Periodic Surface Structuring of the Topological Insulator Bismuth Telluride

AbstractSurface structuring of topological insulator Bi2Te3 single crystals with femtosecond laser by varying pulses N and energy E is reported. Interesting effects related to laser‐induced periodic surface structures formation in this class of materials are evidenced. At low pulse energy, a clear formation of periodic, subwavelength ripples oriented orthogonally to the laser polarization is observed; those are restricted to an annular region surrounding a featureless central disk as the laser energy progressively increases. The structural analysis shows that some degree of crystallinity is preserved in the rippled area, but the central disk is amorphous resembling what is observed for germanium (Ge) and is associated with the hindering of surface structure formation due to a thick melted surface layer. Interestingly, at larger fluence or number of pulses, a transition to suprawavelength grooves occurs within the annular region covered by surface structures. The findings demonstrate a clear incubation behavior, suggesting that the formation of laser‐induced periodic surface structures is coherent with the general features of the process already reported for other materials. However, the disappearance of these structures in the central area, possibly resulting from the influence of the depth of the melt layer, indicates a mixed behavior for Bi2Te3.

Formation of low- and high-spatial frequency laser-induced periodic surface structures (LIPSSs) in ALD-deposited MoS2

The formation of laser-induced periodic surface structures (LIPSSs) on the atomic layer-deposited (ALD) molybdenum disulfide (MoS2) upon femtosecond laser processing is studied experimentally. Laser-processing parameters such as average laser power and the scan speed at which the formation of the periodic nanostructures takes place are identified. Optical and scanning electron microscopy are applied to identify the parameter regions for the different LIPSS formations and transitions between them. High- and low-spatial frequency LIPSS (HSFL and LSFL) with two distinct periods λLSFL ≈ 1.1 μm and λHSFL ≈ 83 nm can be observed. The HSFL are dominating at higher and the LSFL at lower laser average powers. Formation of LIPSS is found to inhibit laser ablation at lower scan speeds.

Ergosterol distribution controls surface structure formation and fungal pathogenicity.

Ergosterol, the major sterol in fungal membranes, is critical for defining membrane fluidity and regulating cellular processes. Although ergosterol synthesis has been well defined in model yeast, little is known about sterol organization in the context of fungal pathogenesis. We identified a retrograde sterol transporter, Ysp2, in the opportunistic fungal pathogen Cryptococcus neoformans. We found that the lack of Ysp2 under host-mimicking conditions leads to abnormal accumulation of ergosterol at the plasma membrane, invagination of the plasma membrane, and malformation of the cell wall, which can be functionally rescued by inhibiting ergosterol synthesis with the antifungal drug fluconazole. We also observed that cells lacking Ysp2 mislocalize the cell surface protein Pma1 and have abnormally thin and permeable capsules. As a result of perturbed ergosterol distribution and its consequences, ysp2∆ cells cannot survive in physiologically relevant environments such as host phagocytes and are dramatically attenuated in virulence. These findings expand our knowledge of cryptococcal biology and underscore the importance of sterol homeostasis in fungal pathogenesis. IMPORTANCE Cryptococcus neoformans is an opportunistic fungal pathogen that kills over 100,000 people worldwide each year. Only three drugs are available to treat cryptococcosis, and these are variously limited by toxicity, availability, cost, and resistance. Ergosterol is the most abundant sterol in fungi and a key component in modulating membrane behavior. Two of the drugs used for cryptococcal infection, amphotericin B and fluconazole, target this lipid and its synthesis, highlighting its importance as a therapeutic target. We discovered a cryptococcal ergosterol transporter, Ysp2, and demonstrated its key roles in multiple aspects of cryptococcal biology and pathogenesis. These studies demonstrate the role of ergosterol homeostasis in C. neoformans virulence, deepen our understanding of a pathway with proven therapeutic importance, and open a new area of study.

Fermented corn bran: A by-product with improved total phenolic content and antioxidant activity

Corn bran is a milling by-product of the corn industry; however, it has been considered a potential source of phenolic compounds. This study was conducted to evaluate the potential of solid-state fermentation with Lactobacillus reuteri and Lactobacillus plantarum for improving the phenolic compounds and antioxidant activity of corn bran. The fermentation caused a significant increase in the total phenolic content (TPC) of fermented corn brans compared to the control sample. Highest TPC was obtained after 48 h of fermentation. The antioxidant activity, measured through different methodologies (DPPH, ABTS, ORAC assays), was higher in the fermented corn bran samples than in the control one. Fermentation increased the concentration of free phenolic acids, especially ferulic acid and p-coumaric acid, in the fermented samples. Fourier-transform infrared spectroscopy indicated that fermentation promoted the degradation of some ester bonds in the substrate structure, which could lead to the release of free phenolic acids. Meanwhile, scanning electron microscopy showed that the fermentation led to the formation of a loose surface structure in the fermented samples. The results revealed that solid-state fermentation was an effective method for the enhancement of phenolic compounds, suggesting that this bioprocessing technology may help improve the antioxidant activity of fermented corn bran.

Azeotropic Binary Solvent System Containing Nonfluorinated Polymer-Grafted Silica Nanoparticles for the Fabrication of a Superhydrophobic Surface.

This study describes a method for fabricating a superhydrophobic surface on glass via a colloidal deposition technique based on solvent evaporation-induced aggregation. Silica nanoparticles with a low grafting density of long-chain poly(cyclohexyl methacrylate) (PCH) were dispersed in a binary solvent system consisting of tetrahydrofuran (THF) and methanol (MeOH) with an azeotropic point and the nonfluorinated and hydrophobic PCHMA having a solubility parameter similar to that of THF. In the early stages of evaporation, the binary mixtures tend to induce the aggregation of PCH-NP due to the azeotropic point of the solvent components, leading to the formation of surface structures ranging from smooth to rough on the substrate. By adjusting the initial ratio of the binary solvents, a superhydrophobic coating with a water contact angle of 154 ± 2° and a sliding angle of less than 10° was achieved at a THF content of 60 wt %. This facile approach using azeotropes successfully shows that changes in the solvent composition of the binary solvent system during evaporation can be used to prepare superhydrophobic coatings with well-controlled surface structures.

Cell Response on Laser-Patterned Ti/Zr/Ti and Ti/Cu/Ti Multilayer Systems

Arranged patterns obtained via ultrafast laser processing on the surface of Ti/Cu/Ti/Si and Ti/Zr/Ti/Si thin-film systems are reported. Two differently designed multilayer thin films Ti/Cu/Ti/Si and Ti/Zr/Ti/Si were deposited on silicon using the ion sputtering method. The bioactive surfaces of these systems involve the formation of laser-induced periodic surface structures (LIPSS) in each of the laser-written lines of mesh patterns on 5 × 5 mm areas. The formation of nano- and micro-patterns with an ultra-thin oxide film on the surfaces was used to observe the effects of morphology and proliferation of the MRC-5 cell culture line. To determine whether Ti-based thin films have a toxic effect on living cells, an MTT assay was performed. The relative cytotoxic effect, as a percentage of surviving cells, showed that there was no difference in cell number between the Ti-based thin films and the control cells. There was also no difference in the viability of the MRC-5 cells, except for the Ti/Cu/Ti/Si system, where there was a slight 10% decrease in cell viability.

The Synergistic Effects of Aminosilane Coupling Agent on the Adhesion Performance of Silane Primer for Silicone Resin Thermal Protection Coating.

As a bridge between the coating and the substrate, the primer has a direct impact on the adhesion performance of silicone resin thermal protection coating. In this paper, the synergistic effects of an aminosilane coupling agent on the adhesion performance of silane primer were investigated. The results show that silane primer containing N-aminoethyl-3-aminopropylmethyl-dimethoxysilane (HD-103) formed a continuous and uniform film on the surface of the substrate. Two amino groups of HD-103 were conducive to moderate and uniform hydrolysis of the silane primer system, and the introduction of dimethoxy groups was more conducive to the improvement of interfacial layer density and the formation of the planar surface structure, thus enhancing the bond strength at the interface. When the content was 13 wt%, it exhibited excellent synergistic effects on adhesive properties, and the adhesive strength reached 1.53 MPa. The possible morphology and composition of the silane primer layer were investigated by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). A thermogravimetric infrared spectrometer (TGA-IR) was used to analyze the thermal decomposition of the silane primer layer. The results showed that the alkoxy groups in the silane primer were first hydrolyzed to form Si-OH, and then the dehydration and condensation reactions between Si-OH and the substrate formed a firm network structure.

In situ observation of phase-locking growth of LIPSS promoted by random deposits or pre-formed nano/microstructures on a SiC single crystal

In recent years, real-time microscopic imaging of laser-induced periodic surface structure (LIPSS) formation during laser micromachining was found to be favorable for investigating the physical origin of the LIPSS on various materials. In this paper, an ultraviolet-oblique-illumination high-resolution microscopic imaging setup, combined with a focal spot enlarging technique, was used for observing the initiation, development, and maturation of the LIPSS. The research focuses on the influence of random surface deposits, laser-etched microtrench, and pre-existing ripples on laser-induced LIPSS under pulse-by-pulse irradiation. Compared to the flat region on the SiC surface, both random surface deposits and the microtrench promote the phase-locking growth of the LIPSS, but in different ways. The finite-difference time-domain method shows that random surface deposits may promote the LIPSS growth through the action of the interference field of incident light with its scattered light on deposits. A microtrench covered with high-spatial-frequency LIPSS may assist LIPSS growth by absorption enhancement-induced easier transferring of the incident periodic field. Finally, phase-locking growth of the LIPSS was found to be the result of the light-trapping effect of high-refractive-index sidewalls of the LIPSS valley. Our results showed that local morphologies were very important in LIPSS formation.