Femtosecond Laser Structuring Research Articles

Hierarchical micro/nanostructured TiO2/Ag substrates based on femtosecond laser structuring: A facile route for enhanced SERS performance and location predictability

Hierarchical micro/nanostructured TiO2/Ag substrates for surface-enhanced Raman scattering (SERS) application are fabricated by depositing Ag nanoparticles on the surface of TiO2 hierarchical micro/nanostructures, which are formed by combining femtosecond laser structuring with scalable hydrothermal treatment. Our results show that the morphology of hierarchical micro/nano TiO2/Ag architectures can significantly influence SERS performance. Compare with one-dimensional TiO2/Ag nanostructures hydrothermally fabricated on laser untreated Ti surface, a four-fold enhancement is shown in the SERS signal with excellent reproducibility. Most importantly, enhancement is significantly stronger in the valleys of our microstructures than on the hills due to geometrical orientation of nanostructures on the surface of the microstructure. The site predictability for SERS enhancement is one of the major roadblock for SERS applications. Present study provides an effective way to enhance the site predictability for optimal SERS in addition to a significant enhancement in signal strength.

Spiky Nickel Electrodes for Electrochemical Oxygen Evolution Catalysis by Femtosecond Laser Structuring

Micro- and nanostructured Ni/NiO surfaces were generated by femtosecond laser structuring for oxygen evolution reaction in alkaline water electrolysis cells. For two different angles between the laser beam and the nickel surface, two different types of laser-structured electrodes were prepared, characterized, and compared with a plane tempered nickel electrode. Their electrochemical activities for the oxygen evolution reaction were tested by using cyclic and linear sweep voltammetry. The chemical surface composition was investigated by X-ray photoelectron spectroscopy. Laser structuring increased the overall electrochemical performance by more than one order of magnitude. The overpotential of the laser-structured electrodes for the oxygen evolution reaction was decreased by more than 100 mV due to high defect densities of the structures created by the laser ablation process.

Femtosecond laser structuring of Janus foam: Water spontaneous antigravity unidirectional penetration and pumping

In this letter, we report a Janus foam that is fabricated by the femtosecond laser structuring of copper foam. Each side of the Janus foam possesses different wettabilities: one is superhydrophilic while the other is hydrophobic. On the Janus foam, water can spontaneously antigravity transport from the lower hydrophobic side to the upper superhydrophilic side without an input of external energy, but cannot pass through in the opposite direction. The Janus foam can be considered to have “water diode” characteristics, and it enables the continuous antigravity transport of water without requiring any external forces. The Janus foam is then used to design a “water pump”. These findings will aid the design and development of fluid delivery technologies.

Porous Nickel Nano‐Foam by Femtosecond Laser Structuring for Supercapacitor Application

AbstractA focused femtosecond laser beam was scanned across a nickel electrode in a line pattern with different line distances to generate a large electrochemical surface area for charge storage. During the laser structuring process, small metal particles were generated and sintered to a porous foam‐like structure, the so‐called laser‐induced nano‐foam (LINF), which strongly adheres to the substrate surface. The structuring was carried out in argon atmosphere, in order to prevent oxidation of the LINF structure during the structuring process. The topography of the LINF was investigated by scanning electron microscopy and laser scanning microscopy. The electrochemical surface area of the electrodes was determined by cyclic voltammetry based on the charging of the double‐layer. The total capacity of the nickel LINF electrodes was measured by galvanostatic charge‐discharge to test their capability for supercapacitor applications. The surface area enlargement and therefore the total capacity increases with decreasing line distance. The LINF structure provides a surface area enlargement up to a factor of 1600 and a total capacity up to 2 C cm−2.

Long-term wettability of titanium surfaces by combined femtosecond laser micro/nano structuring and chemical treatments

Two types of titanium surfaces covering with TiO2 micro/nanostructures were fabricated by femtosecond laser structuring followed by NaOH hydrothermal or H2O2 oxidation treatments. After exposure to air, the former surface maintained hydrophilic for 10 days and had a minimum contact angle less than 20° even after 50 days, while the latter surface changed to superhydrophobic with a contact angle more than 150° after 50 days. Detailed analysis in terms of morphology, phase transformation and surface chemistry indicated that the changes of wettability behavior were associated with the adsorption of organic compounds from the surrounding atmosphere. The surface prepared by laser and H2O2 oxidation not only had a stronger adsorption ability than the surface prepared by laser and NaOH treatment, but also exhibited a relative higher hydrophobic hydrocarbon tail in the absorbed compounds, which resulted different wettability evolution trend for the two types of titanium surfaces.

Femtosecond laser structuring of graphite anodes for improved lithium-ion batteries: Ablation characteristics and process design

Lithium-ion batteries are widely used as energy storage devices due to their high energy density and versatile applicability. Key components of lithium-ion batteries are electrically isolated electrodes and a liquid electrolyte solution which enables ion transport between the electrodes. Laser structuring of electrodes is a promising approach to enhance the high-current capability of lithium-ion batteries by reducing cell internal resistances, as a larger contact area of the active material with the electrolyte solution is created. In the work described here, lithium-ion battery anodes were structured by locally ablating small fractions of the coating using femtosecond laser pulses with infrared wavelengths. A study on ablation characteristics depending on different process parameters such as laser fluence and repetition rate was performed. Special focus was on the ablation efficiency, enabling an optimized process design. The influence of the electrode composition was taken into account by studying the ablation behavior at a varying binder content. Evenly distributed micro holes were chosen in order to keep active material removal at a minimum. To evaluate the effect of structured graphite anodes on the electrochemical properties of lithium-ion batteries, test cells were manufactured and galvanostatically cycled at different current rates. Results show improvements in high-current performance which is expressed by an increased discharge capacity yield.

Laser‐Textured Metal Substrates as Photoanodes for Enhanced PEC Water Splitting Reactions

We demonstrate the effect of femtosecond laser structuring of titanium substrates to increase the absorption, photoconversion, and overall photoelectrochemical water splitting (PEC) performance compared to pristine metal substrates, independent of any additional top coat layers. The influence of ultra short laser pulse patterning on PEC efficiency is investigated toward spectroscopic (UV‐Vis), microscopic (SEM), crystallographic (XRD), and compositional (XPS) properties. The beneficial effect of a periodically patterned substrate is attributed to enhanced specific surface area and improved in‐plane light trapping when compared to flat surfaces. Photoanodes for water splitting experiments fabricated by titanium and iron oxide films on laser pre‐patterned Ti substrates are also found to show enhanced PEC efficiency (0.057 mA cm−2) when compared to unpatterened substrates (0.028 mA cm−2). The lower absolute PEC efficiencies are due to extreme thin films.

Fabrication of Biomimetic Fog-Collecting Superhydrophilic-Superhydrophobic Surface Micropatterns Using Femtosecond Lasers.

The exciting functionalities of natural superhydrophilic and superhydrophobic surfaces served as inspiration for a variety of biomimetic designs. In particular, the combination of both extreme wetting states to micropatterns opens up interesting applications, as the example of the fog-collecting Namib Desert beetle shows. In this paper, the beetle's elytra were mimicked by a novel three-step fabrication method to increase the fog-collection efficiency of glasses. In the first step, a double-hierarchical surface structure was generated on Pyrex wafers using femtosecond laser structuring, which amplified the intrinsic wetting property of the surface and made it superhydrophilic (water contact angle < 10°). In the second step, a Teflon-like polymer (CF2) n was deposited by a plasma process that turned the laser-structured surface superhydrophobic (water contact angle > 150°). In the last step, the Teflon-like coating was selectively removed by fs-laser ablation to uncover superhydrophilic spots below the superhydrophobic surface, following the example of the Namib Desert beetle's fog-collecting elytra. To investigate the influence on the fog-collection behavior, (super)hydrophilic, (super)hydrophobic, and low and high contrast wetting patterns were fabricated on glass wafers using selected combinations of these three processing steps and were exposed to fog in an artificial nebulizer setup. This experiment revealed that high-contrast wetting patterns collected the highest amount of fog and enhanced the fog-collection efficiency by nearly 60% compared to pristine Pyrex glass. The comparison of the fog-collection behavior of the six samples showed that the superior fog-collection efficiency of surface patterns with extreme wetting contrast is due to the combination of water attraction and water repellency: the superhydrophilic spots act as drop accumulation areas, whereas the surrounding superhydrophobic areas allow a fast water transportation caused by gravity. The presented method enables a fast and flexible surface functionalization of a broad range of materials including transparent substrates, which offers exciting possibilities for the design of biomedical and microfluidic devices.

OLED Luminaires: Device Arrays with 99.6% Geometric Fill Factor Structured by Femtosecond Laser Ablation.

The plethora of design opportunities renders organic light emitting diodes (OLEDs) ideal luminaires for general lighting applications. Progressing from lab-scale device concepts to large-area applications calls for smart device designs that are scalable and, at the same time, unsusceptible to resistive losses within the electrodes. By employing direct pulsed femtosecond laser structuring, we fabricate OLED luminaires comprising monolithically interconnected OLED arrays. We determine the laser ablation thresholds and the optimized process parameters for all functional layers. The clean laser cuts with precise ablation depths show no detectable damage to adjacent layers or any ridges, hence avoiding unwanted short-cuts or device isolation. All processes are scalable. The 3-fold structuring of the OLED luminaire is confined within 45 μm and hence below the resolution limit of the human eye, yielding a geometric fill factor beyond 99% and therefore a very homogeneous device perception.

Transparent superhydrophobic surfaces with high adhesion generated by the combination of femtosecond laser structuring and wet oxidation

In this work we demonstrate a fabrication process for the generation of quartz surfaces showing both high transparency and superhydrophobic behaviour. A thin a-Si layer on a quartz wafer was selectively structured by a femtosecond laser. After laser structuring the sample was thermally (wet) oxidized at 1100° until the a-Si layer was fully converted to SiO2. Since SiO2 is hydrophilic, an assistant layer was used to shift the contact angle of the material to a value higher than 90°. A thin layer of Teflon-like polymer was deposited by a plasma process onto the oxidized SiO2 surface. With this process chain it is possible to generate a functional surface with a water contact angle (CA) of up to 163° combined with high transparency close to 80% and a high adhesion with a CA hysteresis of up to 151°.

Enhanced photocatalytic properties of hierarchical three-dimensional TiO2 grown on femtosecond laser structured titanium substrate

Three-dimensional micro-/nanostructured TiO2 (3D-TiO2) fabricated on titanium substrate effectively improves its performance in photocatalysis, dye-sensitized solar cell and lithium-ion battery applications. In this study, the hierarchical 3D-TiO2 with anatase phase directly grown on femtosecond laser structured titanium substrate is reported. First, the primary columnar arrays were fabricated on the surface of titanium substrate by femtosecond laser structuring. Next, the secondary nano-sheet substructures were grown on the primary columnar arrays by NaOH hydrothermal treatment. Followed by ion-exchange process in HCl and annealing in the air, the hierarchical anatase 3D-TiO2 was achieved. The hierarchical anatase 3D-TiO2 exhibited enhanced performances in light harvesting and absorption ability compared to that of nano-sheet TiO2 grown on flat titanium surface without femtosecond laser structuring. The photocatalytic degradation of methyl orange reveals that photocatalytic efficiency of the hierarchical anatase 3D-TiO2 was improved by a maximum of 57% compared to that of nano-sheet TiO2 (55% vs 35%). Meanwhile, the hierarchical anatase 3D-TiO2 remained mechanically stable and constant in consecutive degradation cycles, which promises significance in practical application.

Selective femtosecond laser structuring of dielectric thin films with different band gaps: a time-resolved study of ablation mechanisms

Ultrashort pulse lasers have been increasingly gaining importance for the selective structuring of dielectric thin films in industrial applications. In a variety of works the ablation of thin \(\hbox {SiO}_{2}\) and \({\hbox {SiN}}_{x}\) films from Si substrates has been investigated with near infrared laser wavelengths with photon energies of about 1.2 eV where both dielectrics are transparent (\(E_{{\mathrm{gap,SiO}_{2}}}\approx 8\,\hbox {eV}; E_{{\mathrm{gap,SiN}}_{x}}\approx 2.5\,\hbox {eV}\)). In these works it was found that few 100 nm thick \(\hbox {SiO}_{2}\) films are selectively ablated with a “lift-off” initiated by confined laser ablation whereas the \(\hbox {SiN}_{{x}}\) films are ablated by a combination of confined and direct laser ablation. In the work at hand, ultrafast pump-probe imaging was applied to compare the laser ablation dynamics of the two thin film systems directly with the uncoated Si substrate—on the same setup and under identical parameters. On the \(\hbox {SiO}_{2}\) sample, results show the pulse absorption in the Si substrate, leading to the confined ablation of the \(\hbox {SiO}_{2}\) layer by the expansion of the substrate. On the \(\hbox {SiN}_{{x}}\) sample, direct absorption in the layer is observed leading to its removal by evaporation. The pump-probe measurements combined with reflectivity corrected threshold fluence investigations suggest that melting of the Si substrate is sufficient to initiate the lift-off of an overlaying transparent film—evaporation of the substrate seems not to be necessary.

Ridge waveguide laser in Nd:LiNbO3 by Zn-diffusion and femtosecond-laser structuring

Ridge waveguide lasers have been fabricated on Nd3+ doped LiNbO3 crystals. The fs-laser writing technique was used to define ridge structures on a gradient-index planar waveguide fabricated by Zn-diffusion. This planar waveguide was formed in a z-cut LiNbO3 substrate homogeneously doped with a 0.23% of Nd3+ ions. To obtain lateral light confinement, the surface was then micromachined using a multiplexed femtosecond laser writing beam, forming the ridge structures. By butting two mirrors at the channel waveguide end-facets, forming a waveguide laser cavity, TM-polarized laser action at 1085 nm was achieved by end-fire TM-pumping at 815 nm. The waveguide laser shows a threshold of 31 mW, with a 7% of slope efficiency.

Surface structure formation in WE54 Mg alloy subjected to ultrafast laser texturing

Pico- and femtosecond laser surface structuring of WE54 Mg alloy was investigated in this paper. In particular, the effective response of rare earth elements during laser structuring and the surface structure formation mechanisms were studied. A 10 W picosecond laser, having a wavelength of 1064 nm, and a 1.5 W femtosecond laser, having a wavelength of 795 nm, were used to irradiate WE54 surface to change the surface morphology. A number of pulses varying from 100 to 1500 were applied to create a crater at single and multiple points on the WE54 alloy surface in an air medium. Taylor Hobson profilometer and energy dispersive x-ray spectroscopy (EDS) techniques were employed in order to analyze the modification in surface morphology and chemical composition, respectively. A crater depth around 1.1 μm was measured by the profilometer. It was observed that a compound of yttrium and neodymium was formed during the process of rapid thermalization, which is investigated by x-ray diffraction. The surface portion containing yttrium was converted into a white crystal-like structure as observed by optical and scanning electron microscope (SEM). The white color compound of rare earth element is turned into gray, then dark, and is finally ablated from the surface with increasing scanning number and number of laser pulses. The change in color represents the change in optical properties of WE54. SEM analysis also revealed that no cracks were present on the laser structured surface; hence, stress effect on the surface can be neglected. Explosive vaporization of the substrate was observed in pico- and femtosecond laser structuring. The investigation on ultrafast laser structuring suggests that the surface properties created by laser structuring of WE54 can be controlled if proper laser processing is applied.

Femtosecond laser structuring of silver-containing glass: Silver redistribution, selective etching, and surface topology engineering

Femtosecond direct laser writing in silver-containing phosphate glasses allows for the three-dimensional (3D) implementation of complex photonic structures. Sample translation along or perpendicular to the direction of the beam propagation has been performed, which led to the permanent formation of fluorescent structures, either corresponding to a tubular shape or to two parallel planes at the vicinity of the interaction voxel, respectively. These optical features are related to significant modifications of the local material chemistry. Indeed, silver depletion areas with a diameter below 200 nm were evidenced at the center of the photo-produced structures while photo-produced luminescence properties are attributed to the formation of silver clusters around the multiphoton interaction voxel. The laser-triggered oxidation-reduction processes and the associated photo-induced silver redistribution are proposed to be at the origin of the observed original 3D luminescent structures. Thanks to such material structuring, surface engineering has been also demonstrated. Selective surface chemical etching of the glass has been obtained subsequently to laser writing at the location of the photo-produced structures, revealing features with nanometric depth profiles and radial dimensions strongly related to the spatial distributions of the silver clusters.

Impact of evolving surface nanoscale topologies in femtosecond laser structuring of Ni-based superalloy CMSX-4

We investigate the role of surface nanoscale topographies in the inhomogeneous coupling of laser radiation to Ni-based superalloy CMSX-4 surfaces and in the formation of laser-induced periodic surface structures. The initial surface arbitrary roughness is already able, upon laser exposure, to induce low-spatial-frequency and high-spatial-frequency structures, actively determining an interference of incoming and scattered fields resulting in spatial energy modulation. A topology variation with the incoming dose via the number of pulses determines a correlated evolution in the regular ripple arrangements. The scattering pattern is severely influenced by the scattering source geometries. Therefore, we equally study experimentally the role of one-dimensional nanoscale grooves in determining polarization-dependent structuring patterns at increasing irradiation dose. Finally, the role of surface nanostructures in generating the surface modulation of deposited energy is analyzed by finite-difference-time-domain simulation. Ripple formation in multi-pulses is a result of the feedback process between light and nanostructures.

Femtosecond laser structuring of novel electrodes for 3D fuel cell design with increased reaction surface

ABSTRACTThe scalable storage of renewable energy by means of converting water to hydrogen fuels electrochemically hinges on fundamental improvements in catalytic materials. However, many applications exist where an extended lifetime is virtually crucial for their functionality and success, e.g. in case of limited accessibility such as tire pressure sensors or biomedical implants. For these kinds of applications, the ultimate power supply should be a self-renewing energy source. This strategy is pursued by the concept of Micro Energy Harvesting (MEH). Within a MEH system a micro generator converts ambient energy to electrical energy for driving an application. Unfortunately, it is not ensured that the ambient energy level will maintain always high enough to provide sufficient power to the system as harvested energy usually manifests itself in rather irregular, random and low-energy bursts. One appealing form of integrated energy storage is the use of H2/air, a so called fuel cell type (FC) battery. Such devices promise very high volumetric energy densities of more than 2000 Wh/l. Consequently, this type of battery has recently attracted more and more attention and primary as well as secondary cells have been realized. Alkaline polymer electrolyte fuel cells have been recognized as the most promising solution in order to overcome the dependency on noble metal catalysts. Nevertheless, further improvements for these kinds of fuel cells have to be reached with respect to high power. Therefore, one promising approach is to increase the skin surface of porous chromium decorated nickel electrodes for enhancement of exchange current density by forming three-dimensional (3D) microstructures directly into the electrode. Therefore, a novel laser structuring process was applied using ultrashort laser pulses. Ultrashort laser processing of complex multimaterial systems for energy storage allow for precise material removal without changing the material properties. By applying this novel laser-based structuring technique, 3D microstructures could be formed permitting shortened diffusion lengths between the electrolyte and the electrode surface being necessary for increased exchange current densities.

Femtosecond Laser Structuring of Polymer Optical Fibers for Backscatter Sensing

Focused femtosecond laser pulses are used to create scattering damage in the core of polymer optical fibers (POF). The resulting structures are investigated regarding morphology by light microscopy and backscattered intensity using optical time domain reflectometry (OTDR). Quasi-distributed optical fiber sensing is proposed by evaluating backscatter changes at the inscribed scatter centers. Application examples for quasi-distributed measurement of fiber bends and temperature are demonstrated.

Tailor-made three-dimensional hybrid scaffolds for cell cultures

The construction of the ideal three-dimensional scaffold for cell culture is one of the most intriguing topics in tissue engineering. It has been shown that cells can be cultured on most organic biomimetic materials, which now are losing popularity in favour of novel, hybrid systems. In this study, a series of photosensitive sol–gel hybrid materials, based on silicon–zirconium and silicon–titanium oxides, have been investigated for their suitability in three-dimensional scaffold fabrication. These materials can be structured by two-photon polymerization, a laser-based technique allowing the fabrication of micrometre-size structures with submicron resolution. The work presented here examined the effect of the organic/inorganic composition of the materials on cell behaviour and the establishment of a ‘cell-culture friendly’ environment. This is vital for cell adhesion, growth and differentiation, as the organic part of the material provides the soft matrix for cell growth, whereas the inorganic component gives the mechanical stability and rigidity of the three-dimensional structures. In addition, the use of femtosecond laser structuring permits the fabrication of a wide range of mechanically stable scaffolds of different sizes and shapes to be tested in terms of cell viability, proliferation and orientation.

Water sprints uphill on glass

As a material, glass has been widely used in traditional fluidic devices and more recently in micro- and nanofluidic devices, and its wetting properties are crucially important in these applications. In this work, by using a high-intensity femtosecond laser structuring technique, we create a unique surface pattern on glass that transfer a regular glass surface to superwicking. As a result, water defies the gravity and sprints vertically uphill along the structured glass surface at an unprecedented velocity of 3.8 cm/sec. Our study shows that the dynamics of this fast self-propelled water flow follows a square root of time dependence.