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Laser-induced periodic surface structures (LIPSS) and their combination with self-ordered microstructures forming hierarchical Spikes enable a large variety of different surface functionalizations for applications in the fields of optics, fluidics, tribology, or medicine. Moreover, high spatial frequency LIPSS (HSFL) provide an appealing and straightforward way for surface nanostructuring featuring spatial periods even below 100 nm – far beyond the optical diffraction limit. However, the imposed surface functionalities are usually caused by both, topographic and chemical surface alterations. For exploring these effects in detail, multi-method characterizations were performed here for HSFL and hierarchical Spikes processed on Ti-6Al-4V alloy upon irradiation with near-infrared ps-laser pulses (1030 nm wavelength, ~1 ps pulse duration, 1 – 400 kHz pulse repetition rate) under different laser and scan processing conditions. The following sample characterization involved morphological and topographical investigations by scanning electron microscopy (SEM), atomic force microscopy (AFM), stylus profilometry (SP), and white light interference microscopy (WLIM), as well as near-surface chemical analyses by X-ray photoelectron spectroscopy (XPS), hard X-ray photoelectron spectroscopy (HAXPES) and depth-profiling time-of-flight secondary ion mass spectrometry (TOF-SIMS). The results allow to qualify the laser ablation depth, the geometrical HSFL/Spikes characteristics and provide detailed insights into the depth extent and the nature of the ps-laser-induced near-surface oxidation arising from the laser-processing in ambient air and into the relevance of heat-accumulation effects at high pulse repetition rates. Moreover, the direct comparison of the HAXPES and XPS data reveals the role of surface-covering organic contaminants adsorbed from the ambient atmosphere without the uncertainties and potential sputter reduction potentially caused by ion-sputter depth profiling.
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.
Laser-induced periodic surface structures (LIPSS) have gained remarkable attention as they represent a universal phenomenon that is often accompanying laser-processing. Such LIPSS enable a large variety of different surface functionalizations for applications in the fields of optics, fluidics, tribology, or medicine. Moreover, so-called “high spatial frequency LIPSS” (HSFL) provide an appealing and straightforward way for surface nanostructuring featuring spatial periods even below 100 nm – far beyond the optical diffraction limit. However, the imposed surface functionalities are usually caused by both, topographic and chemical surface alterations. For exploring these effects in detail, multi-method characterizations were performed here for HSFL on processed Ti- 6Al- 4V alloy upon irradiation with near-infrared ps-laser pulses (1030 nm wavelength, ~1 ps pulse duration, 1 – 400 kHz pulse repetition rate) under different laser and scan processing conditions. The sample characterization involved morphological and topographical investigations by scanning electron microscopy (SEM), atomic force microscopy (AFM), stylus profilometry (SP), and white light interference microscopy (WLIM), as well as near-surface chemical analyses by X-ray photoelectron spectroscopy (XPS), hard X-ray photoelectron spectroscopy (HAXPES) and depth-profiling time-of-flight secondary ion mass spectrometry (TOF-SIMS). The results allow to qualify the laser ablation depth, the geometrical HSFL characteristics and provide detailed insights into the depth extent and the nature of the ps-laser-induced near-surface oxidation. Significance for medical applications will be outlined.
Surface nanostructures provide the possibility to create and tailor surface functionalities mainly via controlling their topography along with other chemical and physical material properties. One of the most appealing technologies for surface functionalization via micro- and nanostructuring is based on laser processing. This can be done either via direct contour-shaping of the irradiated material using a tightly focused laser beam or in a self-ordered way that allows employing larger laser beam diameters along with areal scanning to create a variety of laser-induced periodic surface structures (LIPSS). For the latter approach, particularly ultrashort pulsed lasers have recently pushed the borders across long-lasting limitations regarding the minimum achievable feature sizes and additionally boosted up the production times. This chapter reviews the plethora of recently investigated applications of LIPSS—for example, via imposing diffractive or plasmonic structural colors, the management of liquids and surface wetting properties, biomedical and bioinspired functionalities, beneficial effects in tribology for reducing friction and wear, the manipulation of optical scattering and absorption in photovoltaics, or the modification of magnetic or superconducting surface properties in other energy applications. The footprint of the LIPSS-based technology is explored in detail regarding the current state of industrialization, including an analysis of the market and associated LIPSS production costs.
Laser-induced periodic surface structures (LIPSS) have gained remarkable attention as they represent a universal phenomenon that is often accompanying laser-processing. Such LIPSS enable a plethora of different surface functionalizations for applications in the fields of optics, fluidics, tribology, or medicine. Due to the inherent multiscale nature of processes involved in LIPSS formation, their in-situ observation requires experimental techniques that combine high temporal resolution with the appropriate spatial resolution. In this respect fourth generation light sources, namely short wavelength, short pulse free electron lasers (FELs) are offering new and fascinating possibilities. This work contribution will discuss the results of scattering experiments carried at the FLASH free electron laser at DESY (Hamburg, Germany), which allowed us to resolve laser-induced structure formation at surfaces on the nm to sub-µm length scale and in temporal regimes ranging from ps to several ns with sub-ps resolution. A ps-optical pump / fs-XUV scattering probe scheme was employed to 100 nm thick laser-excited silicon films, while recording snapshots of the transmitted XUV scattering patterns at various delay times after the laser pulse impact. On timescales ranging from hundred ps until several ns almost quantitative agreement was observed between certain features of the recorded scattering patterns and predictions of the first-principles theory of J.E. Sipe and coworkers. Other scattering features appearing with a delay of ~100 ps and lasting for ~1 ns are attributed to capillary surface waves being excited at the laser-melted film material while ablation proceeds. Our superior spatio-temporal resolution experiments allow to uniquely reveal and distinguish early signatures of coherent/plasmonic electromagnetic scattering effects, separately followed by hydrodynamic matter reorganization.
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.
Laser-induced periodic surface structures (LIPSS) represent a unique route for functionalizing materials through the fabrication of surface nanostructures. Commercial AISI 316L stainless steel (SS316L) surfaces are laser treated by ultraviolet 300 ps laser pulses in a laser line scanning (LLS) approach. Processing parameters are optimized (pulse energy of 2.08 µJ, pulse repetition frequency of 300 kHz, and suitable laser scan and sample displacement rates) for the generation of low spatial frequency LIPSS over a large 25 × 25 mm2 area. Different angles of incidence of the laser radiation (0°, 30°, and 45°) and different linear laser beam polarizations (s and p) produce a plethora of rippled surface morphologies at distinct grains. Scanning electron microscopy and 2D Fourier transforms, together with calculations of the optical energy deposited at the treated surfaces using Sipe's first-principles electromagnetic scattering theory, are used to study and analyze in detail these surface morphologies. Combined with electron backscattering diffraction, analyses allow associating site-selectively various laser-induced-surface morphologies with the underlying crystalline grain orientation. Resulting grain orientation maps reveal a strong impact of the grain crystallographic orientation on LIPSS formation and point toward possible strategies, like multi-step processes, for improving the manufacturing of LIPSS and their areal coverage of polycrystalline technical materials.
Surfaces of metallic copper and copper alloys effectively inactivate microorganisms and viruses. However, the exact inactivation mode is still under debate. Main factors are assumed to include direct contact with the metallic surface, influx of Cu(I)/Cu(II) ions and the generation of reactive oxygen species (ROS). Laser-induced periodic surface structures (LIPSS) are frequently reported to act antibacterial, mainly by prevention of bacterial adhesion due to a limited number of possible adhesion points or by increasing the overall surface of intrinsically antibacterial materials. In time-kill experiments with E. coli and S. aureus we analyzed the impact of LIPSS on the toxicity of metallic copper and brass. We also conducted ROS accumulation assays and conclude that the application of LIPSS is not generally straight forward to obtain or improve antibacterial surfaces. Thus, the antibacterial effects of LIPPS.