Nanostructured Metal Surfaces Research Articles

Ag-Nanostars for the Sensitive SERS Detection of Dyes in Artistic Cross-Sections—Madonna della Misericordia of the National Gallery of Parma: A Case Study

In historical paintings, the detection of low amounts of pigments and dyes by Raman spectroscopy can sometimes be challenging, in particular for fluorescent dyes. This issue can be overcome by using SERS (surface-enhanced Raman spectroscopy) which takes advantage of the properties of nanostructured metal surfaces to quench fluorescence and enhance Raman signals. In this work, silver nanostars (AgNSs) are applied for the first time to real art samples, in particular to painting cross-sections, exploiting their effective SERS properties for pigment identification. The case study is the Madonna della Misericordia of the National Gallery of Parma (Italy). Cross-sections were analyzed at first by optical microscopy, SEM-EDS, and micro-Raman spectroscopy. Unfortunately, in some cross-sections, the application of conventional Raman spectroscopy was hindered by an intense background fluorescence. Therefore, AgNSs were deposited and used as SERS-active agent. The experimentation was successful, allowing us to identify a modern dye, namely copper phthalocyanine. This result, together with the detection of other modern pigments (titanium white) and expert visual examination, allowed to reconstruct the painting history, postdating its realization from the 15th century (according to the Gallery inventory) to 19th century with a heavy role of recent (middle 20th century) restoration interventions.

Temperature-dependent wetting characteristics of micro–nano-structured metal surface formed by femtosecond laser

The hydrophilicity induced by micro–nano-structured surface has aroused great attention for promising applications. In this work, the temperature-dependent surface wettability with different micro–nano-structures prepared by femtosecond lasers is studied systematically. It is found that the contact angle of both the micro- and nano-structural hydrophilic surfaces reduces as the ambient temperature decreases from 60 to 20 °C, but the contact angles of the typical microscale surfaces increase when the surface temperature reduces from 20 to 5 °C when the surface temperature is lower than ambient temperature. Furthermore, it is found that some microscale structural samples show a much weaker temperature dependence than others, which is experimentally proved to be due to the strong capillary effects in those specific microscale structures. By contrast, hydrophobic micro–nano-structural samples with the same structure obtained by silanization process always show that the hydrophobicity of both nano- and micro-structured surfaces decreases when temperature decreases from 20 to 5 °C. To the best of our knowledge, we are the first to report the temperature-dependent wettability of hydrophilic micro–nano-structured surfaces with surface temperature below ambient temperature. The findings in this work can be applied to the micro- and nanoscale structural surface formed by other methods, and it will be beneficial to the engineering design and manufacture of micro–nano-structured surfaces to achieve the desired performance of the wetting function, which is of great significance to the application of hydrophilic micro–nano-structured surfaces in varied temperature surroundings.

Green manufacturing of nanostructured Al-Based sustainable self-cleaning metallic surfaces

Atmospheric degradation of the metallic components initiated by the accumulation of contaminants and humidity significantly limits the performance and sustainability of many engineering systems. Aluminium and its alloys are widely used in different industries such as food processing, marine, aerospace, automobiles and household appliances. The leaching of aluminium can cause significant environmental and health-related issues. In the present study, a facile and environment-friendly (hot water treatment) processing route was utilized to fabricate sustainable Al surfaces without the use of any chemical reagents and toxic solvents. The developed nanostructured Al surface showed immense mechanical, chemical and thermal durability restricting corrosion and leaching of aluminium. The processed Al composed of self-assembled 2D nanostructures showed extreme wettability ranging from superhydrophilic (θ < 5°) to superhydrophobicity (SHS) (θ > 160°), depending upon post-processing conditions. The developed SHS has showed high water (droplet impacts) and dust repellence under static and dynamic conditions. This is related with effective entrapment of air, prompted by the presence of nanostructured morphology, leading to stable Cassie state. Results showed that SHS possess better self-cleaning ability compared to the superhydrophilic counterpart. The SHS showed better sustainability in terms of high thermal stability in a temperature range of −80 °C to 350 °C and corrosion resistance under prolonged immersion for 48 h in salt media. Compared to unprocessed counterpart, SHS showed 40 times reduction in the corrosion rate, which is significantly high when compared with several other processing routes. The present study showed an eco-friendly and straightforward approach which can be effectively used for fabricating multifunctional metallic surfaces with widespread applications.

Bright upconverted emission from light-induced inelastic tunneling.

Upconverted light from nanostructured metal surfaces can be produced by harmonic generation and multi-photon luminescence; however, these are very weak processes and require extremely high field intensities to produce a measurable signal. Here we report on bright emission, 5 orders of magnitude greater than harmonic generation, that can be seen from metal tunnel junctions that we believe is due to light-induced inelastic tunneling emission. Like inelastic tunneling light emission, which was recently reported to have 2% conversion efficiency per tunneling event, the emission wavelength recorded varies with the local electric field applied; however, here the field is from a 1560 nm femtosecond pulsed laser source. Finite-difference time-domain simulations of the experimental conditions show the local field is sufficient to generate tunneling-based inelastic light emission in the visible regime. This phenomenon is promising for producing ultrafast upconverted light emission with higher efficiency than conventional nonlinear processes.

(Invited) Isolating an Individual Ion Emitter and Light-Induced Tunneling Upconversion

This talk will review two recent discoveries made in my group. Single Photon Emitters: Intrigued by the promise of single-photon quantum technologies, many have studied the properties of single ion defect emitters, particularly lanthanides in a crystal matrix (including praseodymium, cerium, and erbium). I will report on a way of reliably isolating individual erbium ion emitters in nanocrystals with the potential for 100% individual emitter yield in post-selection. We used a nanoplasmonic (metal nanostructure) aperture optical tweezer to reliably isolate individual erbium ion emitters in individually trapped NaYF4 nanocrystals. The nanoplasmonic aperture solved two major problems: (1) it enhanced the emission rate so that single ions could be detected reliably in a short time, and (2) it isolated the single Er containing nanocrystal from the other nanocrystals in solution. This new demonstration follows our recently published work showing 400× increased emission using the nanoapertures. Light-Induced Upconversion Emission: Upconverted light from nanostructured metal surfaces can be produced by harmonic generation and multi-photon luminescence; however, these are very weak processes and require extremely high field intensities to produce a measurable signal. Here we report on bright emission, 5 orders of magnitude greater than harmonic generation, that can be seen from metal tunnel junctions due to light-induced inelastic tunneling. Like inelastic tunneling light emission, which was recently reported to have 2% conversion efficiency per tunneling event, the emission wavelength recorded varies with the local electric field applied; however, here the field is from a 1560 nm femtosecond pulsed laser source. Finite-difference time-domain simulations of the experimental conditions show the local field is sufficient to generate tunneling-based inelastic light emission in the visible regime. This phenomenon is promising for producing ultrafast upconverted light emission with higher efficiency than conventional nonlinear processes.

Silver-chitosan nanocomposite as a plasmonic platform for SERS sensing of polyaromatic sulfur heterocycles in oil fuel

Herein, a silver-chitosan nanocomposite for application in surface enhanced Raman spectroscopy (SERS) sensing was proposed. It was shown that optically transparent chitosan coatings with 0.8 μm thickness allow penetration of target analytes to silver nanoparticles and the analysis in both polar and nonpolar solvents. Under the chosen conditions, chitosan formed continuously smooth films and coatings stabilizing rough nanostructured metallic surfaces and served as a suitable matrix for immobilization, uniform spreading, and preconcentration of the analytes. Polycyclic aromatic sulfur heterocycles were chosen as target analytes being one of the most important fuel quality markers, hazardous components, and the hardest-to-remove impurities. For the most effective immobilization and even distribution of the analytes onto a nanostructured metallic surface, an additional polymer layer of chitosan was found to be needed. The presence of thin films of chitosan resulted in higher reproducibility of SERS spectra as compared to bare nanostructured silver substrates. Additionally, the developed nanocomposite SERS sensors provided the rapid determination of dibenzothiophene and its derivatives in isooctane with the threshold of detection better than 0.1 μM. This approach was successfully applied in the analysis of real fuel samples and the results agreed well with independently measured FTIR and GC-MS data.

Coarse-Grained Molecular Dynamics Simulations of Boundary Lubrication on Nanostructured Metal Surfaces

Coarse-grained molecular dynamics simulations were carried out to investigate the frictional properties of lubricant molecules on nanostructured metal surfaces. The simulation cell consists of a lubricant film enclosed between two metal surfaces. The attractive potential of specific iron atoms on the surface was modified such that the lubricant molecule adsorb preferentially on these atoms. These particular iron atoms were arranged to reproduce a grain boundary surface nanostructure. It is found that below the critical normal stress, the strength of the interaction between Fe and the lubricant molecule has little effect on the friction coefficient. However, the behavior of the lubricant film on the metal surface is sensitive to the interaction strength. Large attraction forces increase the adherence of the lubricant film.

6th International conference “Nanobiophysics: fundamental and applied aspects”

6th International conference “NANOBIOPHYSICS: Fundamental and Applied Aspects” (NBP-2019) took place on October 1-4, 2019 at the Institute of Physics of the National Academy of Sciences of Ukraine (Kyiv, Ukraine). Previous five conferences, starting from 2009, were organized due to joint efforts of B. Verkin Institute for Low Temperature Physics and Engineering of the NAS of Ukraine and Institute of Physics of the NAS of Ukraine on biennial basis in Kyiv and Kharkiv alternatively.63 leading scientists from 14 countries took part in the conference, Ukraine, Belarus, Italy, France, Latvia, Lithuania, Estonia, Germany, Poland, Check Republics, Canada, Denmark, USA and ARE were represented. 36 scientific lectures, including 17 invited, and 27 poster presentations were made. The goal of the conference was in exchange of information concerning recent achievements in the field of nanobiophysics, aimed at the inspiration of further advancement of basic investigations as well as innovations in the related areas of nanotechnology. Friendly format of the scientific meeting provided the exchange of ideas in such urgent topics as physical properties of biomolecular nanosystems, interactions of biomolecules with nanoparticles and nanostructured surfaces, enhancement of optical processes in biomolecules by nanostructured metal surfaces, formation of nanohybrids of biomolecules with carbon nanostructures, theory and computer modeling of nanoscopic biological systems, applications of bionanomaterials and bionanocomposites. In the framework of the conference special sessions for communications between the participants of the current international projects HORIZON 2020 and NATO Workshop were organized. Book of abstract based on NBP-2019 materials was published.

Mechanism of superhydrophilic to superhydrophobic transition of femtosecond laser-induced periodic surface structures on titanium

The wettability is an imperative property of a solid surface. For decades, superhydrophobic and superhydrophilic surfaces have been focus of extensive research owing to their importance for both fundamental research and in many biomedical, aeronautical, and industrial applications. There have been many studies reported on wettability transition of micro- and nanostructured metal surfaces over time, but there seems to be no well-accepted mechanism to address this phenomenon. Therefore, a detailed study of the surface chemical composition of the laser ablated surfaces at different time points is necessary to develop an understanding of the factors resulting in change in wettability over time. The aim of the present work was to investigate the mechanism underlying the wettability transition of femtosecond laser generated periodic surface structures on titanium between the superhydrophilic and superhydrophobic wetting states under ambient conditions. The time-dependent wettability of the laser treated surfaces assessed by the sessile drop method. The samples exhibited superhydrophilic behavior immediately after laser texturing and became superhydrophobic over time. Detailed surface chemical analyses by X-ray photoelectron spectroscopy revealed that the unique electronic structures of Ti2O3 and TiO2 and resulting in hydrophilic and hydrophobic hydration structures, respectively, played a crucial role in the observed wettability transition.

Present and Future of Surface-Enhanced Raman Scattering.

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

(Keynote) Tuning the Activity and Selectivity of CO2 Electroreduction Catalysts through Nanostructuring and Controlled Oxidation

CO2 electroreduction (CO2RR) into useful chemicals and fuels is a promising technology that might be utilized to minimize the impact that the increasing industrial CO2 emissions are having on the environment. In this talk I will discuss the use of O2-plasma treatments and pulsed potential CO2 electrolysis for the activation of nanostructured metallic surfaces and the re-generation of surface oxides. Although plasma-oxidized silver surfaces were found to display a considerably decreased overpotential for the production of CO, the hydrogen evolution reaction (HER), a competing reaction against CO2 reduction, was found to increase over time. More stable and C1-product selective SnOx/AgOx catalysts will be presented. Additionally, a direct correlation between the presence of Cu2O on well-defined single crystal and nanostructured Cu surfaces and the enhanced production of C2+ products will be illustrated. Electrochemical tests combined with ex situ SEM/TEM-EDS, quasi in situ XPS, operando EC-AFM and XAFS data will be used to extract correlations between the activity, selectivity and stability of CO2RR catalysts and their morphology, structure, composition, and chemical state.

Planar Aperiodic Arrays as Metasurfaces for Optical Near-Field Patterning

Plasmonic metasurfaces have spawned the field of flat optics using nanostructured planar metallic or dielectric surfaces that can replace bulky optical elements and enhance the capabilities of traditional far-field optics. Furthermore, the potential of flat optics can go far beyond far-field modulation and can be exploited for functionality in the near-field itself. Here, we design metasurfaces based on aperiodic arrays of plasmonic Au nanostructures for tailoring the optical near-field in the visible and near-infrared spectral range. The basic element of the arrays is a rhomboid that is modulated in size, orientation, and position to achieve the desired functionality of the micron-size metasurface structure. Using two-photon-photoluminescence as a tool to probe the near-field profiles in the plane of the metasurfaces, we demonstrate the molding of light into different near-field intensity patterns and active pattern control via the far-field illumination. Finite element method simulations reveal that the near-field modulation occurs via a combination of the plasmonic resonances of the rhomboids and field enhancement in the nanoscale gaps in between the elements. This approach enables optical elements that can switch the near-field distribution across the metasurface via wavelength and polarization of the incident far-field light and provides pathways for light matter interaction in integrated devices.

Electrodeposited Manganese Oxide on Tailored 3D Bimetallic Nanofoams for Energy Storage Applications

Three‐dimensional (3D) electrode design has great advantages over its two‐dimensional (2D) counterparts, including higher mass loading of active material, enhanced ion diffusion, and electron charge transfer. Commercial 3D porous structures (i.e., Ni foams) do not fit the purpose of the ideal 3D electrode for supercapacitors, in which surface area (per cm2) is more important than large pore volume. These characteristics, however, can be tuned by the dynamic hydrogen bubble template (DHBT) electrodeposition, a route that is used to tailor 3D nanostructured (multi‐) metallic porous surfaces. In addition to the higher surface area and tailored porosity, these 3D nanostructures can be subsequently functionalized with different species such as metal oxides or other compounds. Therefore, a facile two‐step electrochemical fabrication of 3D composite electrode composed of a bimetallic foam functionalized with manganese (Mn) oxide is proposed. The effect of applied current densities on the distribution and structure of Mn oxide (MnOx) electrodeposited over the bare foam is discussed. The results demonstrate that this route paves the way to design high‐surface‐area architectures for charge storage electrodes with enhanced electrochemical performance (194 F g−1 mg−1 of electrodeposited MnOx at 0.5 A g−1) and high charge–discharge rate capabilities (91% capacitance retention at 20 A g−1) for supercapacitor applications.

Coarse-grained molecular dynamics simulation of boundary lubrication of nanostructured metal surfaces

Coarse-grained molecular dynamics simulation of boundary lubrication of nanostructured metal surfaces

Efficient fabrication of gradient nanostructure layer on surface of commercial pure copper by coupling electric pulse and ultrasonics treatment

Severe plastic deformation can be easily produced on metal surfaces by coupling the micro thermal shock from high peak pulse current and the micro mechanical shock from ultrasonics. Moreover, an efficient method for preparing a gradient nanostructured metal surface by coupling electric pulse and ultrasonics treatment (CEPUT) is developed in this study. The variation in microstructure and hardness of the specimen are investigated by electron backscatter diffraction, transmission electron microscope, X-ray diffraction, and nano-indentation measurement. Results showed that on the treated copper surface with CEPUT, the original grain boundaries are no longer recognized, the average grain size decreases from 48.77 μm to 39.22 nm, and the thickness of severe plastic deformation layer reaches to approximately 500 μm. Moreover, the hardness reaches to 2.105 GPa, and CEPUT also reduces the texture in the sample surface. A computational model is developed and the grain refinement mechanism is proposed to describe the electrical-thermal-mechanical phenomena during CEPUT. The proposed simple and cost-effective method of grain refinement and to produce the graded materials is effective, especially in the materials of high thermal and electrical conductivity.

Nature Inspired Plasmonic Structures: Influence of Structural Characteristics on Sensing Capability

Surface enhanced Raman scattering (SERS) is a powerful analytical technique that allows the enhancement of a Raman signal in a molecule or molecular assemblies placed in the proximity of nanostructured metallic surfaces, due to plasmonic effects. However, laboratory methods to obtain of these prototypes are time-consuming, expensive and they do not always lead to the desired result. In this work, we analyse structures existing in nature that show, on a nanoscale, characteristic conformations of photonic crystals. We demonstrate that these structures, if covered with gold, change into plasmonic nanostructures and are able to sustain the SERS effect. We study three different structures with this property: opal, a hydrated amorphous form of silica (SiO2·nH2O); diatoms, a kind of unicellular alga; and peacock tail feather. Rhodamine 6G (down to 10−12 M) is used to evaluate their capability to increase the Raman signal. These results allow us to define an alternative way to obtain a high sensitivity in Raman spectroscopy, currently achieved by a long and expensive technique, and to fabricate inexpensive nanoplasmonic structures which could be integrated into optical sensors.

Nanostructured gold films exhibiting almost complete absorption of light at visible wavelengths

Nanostructured metal surfaces have been known to exhibit properties that deviate from that of the bulk material. By simply modifying the texture of a metal surface, various unique optical properties can be observed. In this paper, we present a simple two step electrochemical process combining electrodeposition and anodization to generate black gold surfaces. This process is simple, versatile and up-scalable for the production of large surfaces. The black gold films have remarkable optical behavior as they absorb more than 93% of incident light over the entire visible spectrum and also exhibit no specular reflectance. A careful analysis by scanning electron microscopy reveals that these unique optical properties are due to their randomly rough surface, as they consist in a forest of dendritic microstructures with a nanoscale roughness. This new type of black films can be fabricated to a large variety of substrates, turning them to super absorbers with potential applications in photovoltaic solar cells or highly sensitive detectors and so on.

Spatial resolution versus contrast trade-off enhancement in high-resolution surface plasmon resonance imaging (SPRI) by metal surface nanostructure design.

Surface plasmon resonance imaging (SPRI) is an optical near-field method used for mapping the spatial distribution of chemical/physical perturbations above a metal surface without exogenous labeling. Currently, the majority of SPRI systems are used in microarray biosensing, requiring only modest spatial resolution. There is increasing interest in applying SPRI for label-free near-field imaging of biological cells to study cell/surface interactions. However, the required resolution (sub-µm) greatly exceeds what current systems can deliver. Indeed, the attenuation length of surface plasmon polaritons (SPP) severely limits resolution along one axis, typically to tens of µm. Strategies to date for improving spatial resolution result in a commensurate deterioration in other imaging parameters. Unlike the smooth metal surfaces used in SPRI that support purely propagating surface modes, nanostructured metal surfaces support "hybrid" SPP modes that share attributes from both propagating and localized modes. We show that these hybrid modes are especially well-suited to high-resolution imaging and demonstrate how the nanostructure geometry can be designed to achieve sub-µm resolution while mitigating the imaging parameter trade-off according to an application-specific optimum.

“Nickel Nanoflowers” with Surface-Attached Fluoropolymer Networks by C,H Insertion for the Generation of Metallic Superhydrophobic Surfaces

Metallic superhydrophobic surfaces (SHSs) combine the attractive properties of metals, such as ductility, hardness, and conductivity, with the favorable wetting properties of nanostructured surfaces. Moreover, they promise additional benefits with respect to corrosion protection. For the modification of the intrinsically polar and hydrophilic surfaces of metals, a new method has been developed to deposit a long-term stable, highly hydrophobic coating, using nanostructured Ni surfaces as an example. Such substrates were chosen because the deposition of a thin Ni layer is a common choice for enhancing corrosion resistance of other metals. As the hydrophobic coating, we propose a thin film of an extremely hydrophobic fluoropolymer network. To form this network, a thin layer of a fluoropolymer precursor is deposited on the Ni substrate which includes a comonomer that is capable of C,H insertion cross-linking (CHic). Upon UV irradiation or heating, the cross-linker units become activated and the thin glassy film of the precursor is transformed into a polymer network that coats the surface conformally and permanently, as shown by extensive extraction experiments. To achieve an even higher stability, the same precursor film can also be transformed into a chemically surface-attached network by depositing a self-assembled monolayer of an alkane phosphonic acid on the Ni before coating with the precursor. During cross-linking, by the same chemical process, the growing polymer network will simultaneously attach to the alkane phosphonic acid layer at the surface of the metal. This strategy has been used to turn fractal Ni "nanoflower" surfaces grown by anisotropic electroplating into SHSs. The wetting characteristics of the obtained nanostructured metallic surfaces are studied. Additionally, the corrosion protection effect and the significant mechanical durability are demonstrated.

Surface enhanced Raman spectroscopy in microchip electrophoresis

Coupling microchip capillary electrophoresis to surface enhanced Raman spectroscopy (MCE-SERS) combines the high separation power of capillary electrophoresis with the capability to obtain vibrational fingerprint spectra for compound identification. Raman spectroscopy is a structurally descriptive and label-free detection method which is particularly suited for chemical analysis because it is non-destructive and allows the identification of analytes. However, it suffers from poor sensitivity and sometimes even requires acquisition times far longer than the typical peak width of electrophoretic separations. The Raman intensity can be drastically improved if the analyte is brought into close proximity to nanostructured metal surfaces or colloids due to the surface enhancement effect. This paper presents a novel approach in the field of MCE-SERS on-line coupling. The key element of the developed glass microfluidic device is a dosing structure which consists of two side channels joining the MCE channel symmetrically after the electrophoretic separation of the analytes. The dosing channel supplies silver nanoparticles (Ag-NPs), to the separated electrophoretic zones which facilitates an on-the-fly recording of SERS-spectra of the separated compounds. The functionality of the MCE-SERS chip was evaluated by the analysis of a rhodamine model mixture within 90 s achieving RSD of migration times below 1.5%. The approach was successfully applied for the analysis of the food additive riboflavin in a barbecue sauce.