Structured Metal Surface Research Articles

Wettability Behaviour of Metal Surfaces after Sequential Nanosecond and Picosecond Laser Texturing.

This study examines the wettability behaviour of 304 stainless steel (304SS) and Ti-6Al-4V (Ti64) surfaces after sequential nanosecond (ns) and picosecond (ps) laser texturing; in particular, how the multi-scale surface structures created influence the lifecycle of surface hydrophobicity. The effect of different post-process treatments is also examined. Surfaces were analysed using Scanning Electron Microscopy (SEM), a white light interferometer optical profiler, and Energy Dispersive X-ray (EDX) spectroscopy. Wettability was assessed through sessile drop contact angle (CA) measurements, conducted at regular intervals over periods of up to 12 months, while EDX scans monitored elemental chemical changes. The results show that sequential (ns + ps) laser processing produced multi-scale surface texture with laser-induced periodic surface structures (LIPSS). Compared to the ns laser case, the (ns + ps) laser processed surfaces transitioned more rapidly to a hydrophobic state and maintained this property for much longer, especially when the single post-process treatment was ultrasonic cleaning. Some interesting features in CA development over these extended timescales are revealed. For 304SS, hydrophobicity was reached in 1-2 days, with the CA then remaining in the range of 120 to 140° for up to 180 days; whereas the ns laser-processed surfaces took longer to reach hydrophobicity and only maintained the condition for up to 30 days. Similar results were found for the case of Ti64. The findings show that such multi-scale structured metal surfaces can offer relatively stable hydrophobic properties, the lifetime of which can be extended significantly through the appropriate selection of laser process parameters and post-process treatment. The addition of LIPSS appears to help extend the longevity of the hydrophobic property. In seeking to identify other factors influencing wettability, from our EDX results, we observed a significant and steady rate of increase in the carbon content at the surface over the study period.

Comparative study of the anti-corrosive properties of thiols induced superhydrophobic surfaces

Fluoroalkyl thiols and alkyl thiols are widely used as low surface energy reagents for the preparation of superhydrophobic surfaces and corrosion resistant surfaces. Although the corrosion resistance of thiol-induced superhydrophobic surfaces has been well addressed previously, few studies have devoted to revealing the difference of fluoroalkyl thiol and alkyl thiol modified surfaces towards anti-corrosion. In this work, two types of superhydrophobic surfaces were prepared by respectively coating self-assembled monolayers (SAMs) of 1 H,1 H,2 H,2 H-perfluorodecanethiol (PFDT) and 1-dodecanethiol (DDT) on femtosecond laser engineered copper substrates. The formation mechanism of the SAMs was explained by analyzing the surface chemical composition. The water contact angle of the PFDT and DDT modified surfaces were measured to be 157.7° and 154.6°, respectively. Electrochemical tests showed that both SAMs significantly reduced the corrosion current density (Icorr) and corrosion rate of the copper surfaces, while simultaneously increased the charge transfer resistance (Rct) value by an order of magnitude compared to the pristine copper surface. The surface with PFDT SAM had the smallest Icorr and its Rct value was about 1.5 times larger than that with the DDT SAM. These results confirmed that both the SAMs enhanced the anti-corrosion properties of the copper surfaces, and the PFDT SAM was proved to be more effective. Furthermore, the anti-corrosion durability of the PFDT and DDT modified surfaces were compared after exposing them to salt, acid, and alkali solutions for 12 h, and the results showed that the surfaces maintained to be hydrophobic and anti-corrosive, and the PFDT induced superhydrophobic surface indicated better durability. Our results demonstrated that thiols SAMs could significantly improve the anti-corrosion property of structured metal surfaces, and that fluoroalkyl thiol was more desired for long-term anti-corrosive applications in comparison with alkyl thiol.

Mechanical Investigation of Recyclability for Sustainable Use of Laser-Based Metal–Polymer Joints

Metal–plastic hybrid components combine the strength of metal with the low density of plastic. Due to weight reduction, these components are becoming increasingly important. To reduce the need for raw materials, processes for the recyclability of hybrid compounds are being investigated to reuse the metal part. The aim of this research is to characterize the mechanical bond strength after laser-based cleaning and reuse of the metal component. For this purpose, laser radiation is used to introduce microstructures into the metal surface. Afterwards, the polymer is joined to the metal component with laser radiation. As a reference of the initial mechanical bond strength, the joined samples are examined in a tensile testing machine. The polymer residues remaining in the structured metal surface are removed with different laser-based cleaning strategies. The metal is used again to generate another hybrid joined sample with a new polymer component. The results of the subsequent tests in the tensile testing machine are used for a detailed analysis of the reusability. As a result of this investigation, the laser-cleaned specimens showed significant improvements in bond strength compared to the uncleaned specimens. The process of laser-based cleaning for the reuse of the metallic part of hybrid joined components provides a fundamental procedure for improving the circular economy. In the future, this study should be validated in subsequent investigations on realistic components with complex geometries.

Numerical research on supercavity sensing driven by guided resonance in a terahertz all-metal metamaterial absorber

Terahertz metamaterial sensing features high sensitivity and rich fingerprint spectrum, which shows an application potential of the great significance. However, limited by low-quality resonances and insufficient sensitivity, the sensing performance of terahertz metamaterials remains at a moderate level. In this paper, we numerical investigate a high-performance terahertz sensor based on plasmonic supercavity sensing. The proposed metamaterial sensor features an adjustable high quality absorption resonance ascribed to guided resonance excited by band-folding effect. Also, the terahertz plasmonic sensor inherently characterizes a high sensitivity owing to the all-metal design, which can limit the resonant field within the structured metal surface to fully couple with analyte. Based on these advantages, the proposed all-metal sensor demonstrates the maximum resonance frequency shift, sensitivity and sensing figure of merit are about 66%, 1.6 THz/RIU and 218, respectively, substantiating the sensing superiority. The proposed metamaterial absorber shows potential in spectroscopy, in addition to being a multifunctional absorber and sensor.

Crack-Initiated Durable Low-Adhesion Trilayer Icephobic Surfaces with Microcone-Array Anchored Porous Sponges and Polydimethylsiloxane Cover

Reducing unfavorable ice accretion on surfaces exposed in cold environment requires effective passive anti-icing/deicing techniques. Icephobic surfaces are widely applied on various infrastructures due to their low ice adhesion strength and flexibility, whereas their poor mechanical durability, common liquid infusion, weak resistance to contamination, and low bonding strength to substrates are the major remaining challenges. According to the fracture mechanics of ice layer, initiating cracks at the ice-solid interfaces via the proper design of internal structures of icephobic materials is a promising way to icephobicity. Herein, a crack initiating icephobic surface with porous PDMS sponges sandwiched between a protective, dense PDMS layer and a textured metal microstructure was proposed and fabricated. The combination of high- and low- stiffness PDMS layers anchored by the structured metal surface give the sandwich-like structure excellent icephobicity with both high durability and low ice adhesion (5.3 kPa in the icing-deicing cycles). The porosity and the elastic modulus of the PDMS sponges and the periodicity of the metal surface structures can both be tailored to realize enhanced icephobicity. The sandwich-like icephobic surface remained insignificantly changed under solid particle impacting and the durability characterized via linear abrasion tests was elevated compared with PDMS coating on flat metal surfaces. Additionally, the trilayer icephobic surface possesses durability, low ice adhesion strength, and improved resistance to contamination and is applicable on various surfaces.

Wetting behavior of gallium-based room temperature liquid metal (LM) on nanosecond-laser-structured metal surfaces

Gallium-based room-temperature liquid metal (LM) is a promising emerging functional material in e-skin, soft robots and thermal management systems for its fluidity, conductivity and nontoxicity. However, the intricate wettability of the LM with various surfaces impedes its further development. Knowledge on the gallium-based LM wetting properties especially with structured metal surfaces is relatively insufficient. Herein, wetting behavior of eutectic gallium-indium (EGaIn) on various micro-structured metal surfaces processed by nano-second laser ablation method is systematically studied. Increasing surface roughness, synergized with a high oxygen content, significantly strengthens the LM resistance. Micro-patterns composed of small, discontinuous facets exhibit better EGaIn repellence. Relative humidity (RH) is substantiated to influence the LM wetting behavior as well. Metal surfaces with excellent superlyophobicity (the contact angle> 160°, the adhesion force< 10 μN) are obtained. The LM repellence stability and anti-corrosion properties of the optimized treated metal surfaces are proved via the droplet impacting experiment and the corrosion test. These findings improve the understanding of gallium-based LM wetting behavior on structured metal surfaces, and the obtained LM-repellent metal surfaces will reduce undesired adhesion, blockage or LM corrosion during device preparation and long-term applications, which will facilitate its potential use.

General Background of SERS Sensing and Perspectives on Polymer‐Supported Plasmon‐Active Multiscale and Hierarchical Sensor Particles

AbstractSurface‐enhanced Raman scattering (SERS) is one of the most powerful analytical techniques for the identification of molecules. The substrate, on which SERS is dependent, contains regions of nanoscale gaps (hotspots) that hold the ability to concentrate incident electromagnetic fields and effectively amplify vibrational scattering signals of adsorbed analytes. While surface plasmon resonance from metal nanostructures is a central focus for the SERS effect, the support of polymers can be significantly advantageous to provide larger exposure of structured metal surfaces for efficient interactions with analytes. Characteristics of the polymer particles such as softness, flexibility, swellability, porosity, optical transparency, metal‐loading ability, and high surface area can allow diffusion of analytes and penetrating light deeply that can enormously amplify sensing outcomes. As polymer‐supported plasmon‐active sensor particles can emerge as versatile SERS substrates, the microfluidic platform is promising for the generation of sensor particles as well as for performing sequential SERS analysis of multiple analytes. Therefore, in this perspective article, the development of multifunctional polymer–metal composite particles, and their applications as potential sensors for SERS sensing through microfluidics are presented. A detailed background from the beginning of the SERS field and perspectives for the multifunctional sensor particles for efficient SERS sensing are provided.

SERS characterization of aggregated and isolated bacteria deposited on silver-based substrates.

Surface-enhanced Raman scattering (SERS), based on the enhancement of the Raman signal of molecules positioned within a few nanometres from a structured metal surface, is ideally suited to provide bacterial-specific molecular fingerprints which can be used for analytical purposes. However, for some complex structures such as bacteria, the generation of reproducible SERS spectra is still a challenging task. Among the various factors influencing the SERS variability (such as the nature of SERS-active substrate, Raman parameters and bacterial specificity), we demonstrate in this study that the environment of Gram-positive and Gram-negative bacteria deposited on ultra-thin silver films also impacts the origin of the SERS spectra. In the case of densely packed bacteria, the obtained SERS signatures were either characteristic of the secretion of adenosine triphosphate for Staphylococcus aureus (S. aureus) or the cell wall and the pili/flagella for Escherichia coli (E. coli), allowing for an easy discrimination between the various strains. In the case of isolated bacteria, SERS mapping together with principal component analysis revealed some variabilities of the spectra as a function of the bacteria environment and the bactericidal effect of the silver. However, the variability does not preclude the SERS signatures of various E. coli strains to be discriminated.

Gradient Index Devices for Terahertz Spoof Surface Plasmon Polaritons

Terahertz electromagnetic fields can be well confined and propagate at structured metal surfaces, known as spoof surface plasmon polaritons (SSPPs). Recent demonstrations of terahertz SSPP waveguid...

Lithium niobate and lithium tantalate based scalable terahertz pulse sources in reflection geometry.

A new type of THz source, working in reflection geometry, is proposed, where the pulse-front-tilt is introduced by a periodically micro-structured metal profile. For optical coupling, high refractive index nanocomposite fluid is used between the nonlinear optical material and the structured metal surface. Numerical simulations predict ∼87 and ∼85% optimized diffraction efficiencies for lithium niobate and lithium tantalate at 1030 and 800 nm pump wavelengths. The largest diffraction efficiencies can be achieved for a larger refractive index of the nanocomposite fluid than the index of the nonlinear material, for both cases. THz generation efficiencies of ∼3 and ∼1% are predicted for lithium niobate and lithium tantalate, respectively.

Terahertz Spoof Surface Plasmonic Logic Gates.

SummaryLogic gates are important components in integrated photonic circuitry. Here, a series of logic gates to achieve fundamental logic operations based on linear interference in spoof surface plasmon polariton waveguides are demonstrated at terahertz frequencies. A metasurface-based plasmonic source is adopted to couple free-space terahertz radiation into surface waves, followed by a funnel-shaped metasurface to efficiently couple the surface waves to the waveguides built on a domino structure. A single Mach-Zehnder waveguide interferometer can work as logic gates for four logic functions: AND, NOT, OR, and XOR. By cascading two such interferometers, NAND and NOR operations can also be achieved. Experimental investigations are supported by numerical simulations, and good agreement is obtained. The logic gates have compact sizes and high intensity contrasts for the output “1” and “0” states. More complicated functions can be envisioned and will be of great value for future terahertz integrated computing.

Excitation of odd-mode spoof surface plasmon polaritons and its application on low-pass filters

Dispersion properties of compact single- and double-side grooves SSPPs are analyzed and compared. The combination of two single-side grooves with opposite phases can sustain the odd-mode SSPPs, which is contrary to the dominantly even mode SSPPs existed on metal-dielectric interfaces in optical frequencies or on structured metal surfaces in the lower frequencies. Two low-pass filters based on odd-mode SSPPs working at 0–7 and 0–14.1 GHz are fabricated and measured. Both simulations and measurements validate that the proposed SSPPs structure can realize high-efficiency excitation of odd-mode SSPPs and low-loss transmission properties.

Structured metal double-blazed dispersion grating for broadband spectral efficiency achromatization.

The majority of spectrometers use reflective dispersion gratings with a metal-coated blazed grating profile for spectral decomposition. They achieve high diffraction efficiency at the design wavelength, which decays considerably in the adjacent longer and shorter wavelength ranges. We introduce a structured metal double-blazed grating with a high diffraction efficiency for a broadband spectral range, consisting of a sawtooth-like structured metal surface filled with a first dielectric transparent material. The planarized upper surface is covered with a second blazed profile of a different transparent material. We present a systematical theoretical analysis of the diffraction efficiency in reflection geometry, based on a scalar approach involving fundamental dispersion parameters such as Abbe numbers and relative partial dispersions of the materials. We find material combinations reducing the profile heights down to 1-2 µm.

Superhydrophobic Surfaces Enabled by Femtosecond Fiber Laser-Written Nanostructures

Inspired by the self-cleaning and water-repellent lotus leaf, we have developed an efficient process to create superhydrophobic metal surfaces using a femtosecond fiber laser and have investigated the mechanisms of the structured metal surfaces in relation to their hydrophobicity. The at will feature of femtosecond fiber lasers can write versatile patterns of hydrophobicity with nanoscale precision on any metal. The results show that the homogeneously distributed hierarchical structures exhibit multifunctional properties, including superhydrophobicity, self-cleaning, and light-trapping. By optimizing the fabrication conditions, we have achieved a contact angle as high as 171° and a rolling angle of less than 3°. The structure is also resistant to an extreme temperature range of −40 °C to 71 °C and temperature shocks from 20 °C to −40 °C. This research highlights the exciting potential applications of superhydrophobic metals in the aviation, biomedical, and solar energy industries and beyond.

Curved terahertz surface plasmonic waveguide devices.

Strongly confined surface waves can be achieved on periodically structured metal surfaces and are known as spoof surface plasmon polaritons (SPPs). In this work, several terahertz SPP devices based on curved waveguides are demonstrated. The transmittance and bending loss of 90-degree curved spoof SPP waveguides with a radius of curvature ranging from 200 to 2300 µm are investigated to identify the regime for high transmission. A commutator is designed and experimentally demonstrated. Furthermore, coupling equations are derived and verified for efficient coupling between bend-straight waveguides and between bend-bend waveguides. The results will be of great value for future integrated terahertz plasmonic systems.

Wettability transition modes of aluminum surfaces with various micro/nanostructures produced by a femtosecond laser

The wettability transition of hierarchical micro/nanostructure metal surfaces over time shows huge potential to fabricate the superhydrophilic and superhydrophobic surfaces without chemical coating. Although a large number of studies have reported the transition phenomenon on structured metal surfaces, the knowledge of which kind of micro/nanostructure contributes significantly to the acceleration or inhabitation of transition processes is still limited. In this article, the authors fabricated three kinds of typical micro/nanostructures with different topography features on aluminum surfaces by a femtosecond laser. The authors then systematically studied the relationship between the micro/nanostructures and the transition modes in ambient air. The results show that all structured surfaces show superhydrophilic nature initially but then become hydrophobic even superhydrophobic over time. The surfaces with regular microstructures hybrid with nanoripples and a few nanoparticles can achieve superhydrophobicity quickly, 27–30 days. Especially, these structures can realize superhydrophobicity within only 2 h if the low temperature heat treatment is performed on them in ambient air. In addition, the surfaces with regular deep microstructures hybrid with nanoripples and abundant nanoparticles can maintain superhydrophilicity for the longest time and show slow wettability transition mode due to the slower adsorption processes. These results may provide an aid to further understanding the wettability transition modes of different micro/nanostructures in ambient air and fabricating stable superhydrophobic and superhydrophilic surfaces without adding chemical coating.

All-metallic wide-angle metasurfaces for multifunctional polarization manipulation

Optical camouflage is a magical capability of animals as first noticed in 1794 by Erasmus Darwin in <em>Zoonomia</em>, but current biomimetic camouflage strategies cannot be readily applied in complex environments involving multispectral and in particular multi-polarization detection. Here we develop a plasmonic approach toward broadband infrared polarimetric crypsis, where the polarized thermal emission near the pseudo-Brewster angle is the main signal source and no existing polarizing camouflage technique has been discovered in nature. Based on all-metallic subwavelength structures, an electrodynamic resistance-reduction mechanism is proposed to avoid the significant polarization-dependent infrared absorption/radiation. It is also found that the structured metal surface presents giant extrinsic anisotropy regarding the phase shift between orthogonal polarization states, which helps to realize ultrahigh-efficiency and tunable polarization conversion in an unprecedented manner. Finally, we note that the catenary optical theory may provide a useful means to explain and predict these unusual performances.

Development of the Triboelectric Nanogenerator Using a Metal-to-Metal Imprinting Process for Improved Electrical Output.

Triboelectric nanogenerators (TENG), which utilize contact electrification of two different material surfaces accompanied by electrical induction has been proposed and is considered as a promising energy harvester. Researchers have attempted to form desired structures on TENG surfaces and successfully demonstrated the advantageous effect of surface topography on its electrical output performance. In this study, we first propose the structured Al (SA)-assisted TENG (SA-TENG), where one of the contact layers of the TENG is composed of a structured metal surface formed by a metal-to-metal (M2M) imprinting process. The fabricated SA-TENG generates more than 200 V of open-circuit voltage and 60 µA of short-circuit current through a simple finger tapping motion. Given that the utilization of the M2M imprinting process allows for the rapid, versatile and easily accessible structuring of various metal surfaces, which can be directly used as a contact layer of the TENG to substantially enhance its electrical output performance, the present study may considerably broaden the applicability of the TENG in terms of its fabrication standpoint.

Enhancing microwave absorption of metals by femtosecond laser induced micro/nano surface structure

It is well known that microwave radiation can be almost completely reflected by all the smooth metal surfaces. In this paper, we show, for the first time, that structured metal surface by femtosecond laser can largely reduce microwave reflectivity from the surface with absorption peaks in the range from 10 to 18 GHz. Our experimental results further show that blueshift of the microwave absorption peaks can be realized by different femtosecond laser scanning modes. The structured metal surface offers many advantages such as with thin thickness, high thermal stability and structural rigidity over the traditional microwave absorption technology based on nanoscale particles coatings. This technique provides a new approach for the reduction of microwave reflectance from metal surface.

Controlling FRET Enhancement Using Plasmon Modes on Gold Nanogratings

The optical properties of structured metal surfaces and nanoparticles can be engineered to influence the fluorescence properties of nearby quantum emitters through the manipulation of the local density of optical states (LDOS). Applying these techniques to Forster resonance energy transfer (FRET) is appealing but has proven to be a complicated and debated issue. In this paper, surface plasmons modes for a gold nanograting are found to enhance the FRET efficiency between nearby donor and acceptor molecules. Nanogratings support traveling surface plasmon waves with a broad range of wavelengths that follow a dispersion relationship, allowing for increases in the LDOS at targeted portions of the spectra of FRET paired molecules. Nearby excited fluorescent molecules may decay by launching a surface plasmon wave that couples into free-space light, which can be recovered. With this system, we measured the FRET efficiency for different plasmon wavelengths spanning both the donor and acceptor emission spectra. The...