Formation Of Surface Nanostructures Research Articles

Defect-Rich CuZn Nanoparticles for Model Catalysis Produced by Femtosecond Laser Ablation.

Femtosecond laser ablation of Cu0.70Zn0.30 targets in ethanol led to the formation of periodic surface nanostructures and crystalline CuZn alloy nanoparticles with defects, low-coordinated surface sites, and, controlled by the applied laser fluence, different sizes and elemental composition. The Cu/Zn ratio of the nanoparticles was determined by energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and selected area electron diffraction. The CuZn nanoparticles were about 2-3 nm in size, and Cu-rich, varying between 70 and 95%. Increasing the laser fluence from 1.6 to 3.2 J cm-2 yielded larger particles, more stacking fault defects, and repeated nanotwinning, as evident from high-resolution transmission electron microscopy, aided by (inverse) fast Fourier transform analysis. This is due to the higher plasma temperature, leading to increased random collisions/diffusion of primary nanoparticles and their incomplete ordering due to immediate solidification typical of ultrashort pulses. The femtosecond laser-synthesized often nanotwinned CuZn nanoparticles were supported on highly oriented pyrolytic graphite and applied for ethylene hydrogenation, demonstrating their promising potential as model catalysts. Nanoparticles produced at 3.2 J cm-2 exhibited lower catalytic activity than those made at 2.7 J cm-2. Presumably, agglomeration/aggregation of especially 2-3 nm sized nanoparticles, as observed by postreaction analysis, resulted in a decrease in the surface area to volume ratio and thus in the number of low-coordinated active sites.

From groove to hillocks – Atomic-scale simulations of swift heavy ion grazing impacts on CaF2

Surface nanopatterning of CaF2 by swift heavy ions irradiation under oblique angles is studied with a combination of the event-by-event Monte Carlo particle transport model and molecular dynamics simulations. The model describes the electronic system excitation and energy transfer to the lattice followed by the atomic response. The approach allowed us to simulate the kinetics of the electronic ensemble excited by a grazing ion demonstrating that the presence of the surface does not reduce the energy of the lattice as expected. On the contrary, the track core temperature near the surface is slightly higher than in the bulk, because electrons reflected from the surface bring a part of the energy back to the core.The formation kinetics of entire grazing ion tracks is studied revealing the mechanisms of various surface nanostructures formation. Depending on the penetration depth, the ion produces a groove bordered by hillocks, a single chain of nanohillocks, a huge hillock at the end of the rift, and a single continuous structure afterward. The critical depth of the material expulsion from the surface equals approximately to the transient molten zone radius (∼4–4.5 nm). The simulated structures are in reasonable agreement with the available experimental data.

Nanoscale wear evolution on a polystyrene/poly (n-butyl methacrylate) blend

We have investigated the formation of surface nanostructures caused by early-stage wear of polystyrene (PS) and poly(n-butyl methacrylate) (PnBMA) thin films as pure materials, or mixed in the form of a 1:1 blend. To this end, atomic force microscopy (AFM) was used to repeatedly scratch the sample surfaces, measure the accompanying friction forces, and image the resulting features. In the very first stage ordered ripples are formed in all cases. As the process goes on, hillocks are nucleated on the crests of the ripples, and progressively released in the form of nanoplastics while the ripples become wider and less regular. On the blend surface the more compliant PnBMA presents more corrugated ripple structures and larger friction oscillations than PS in the beginning, but the scenario becomes again more complex as the wear test is repeated and the original ‘rim and hole’ geometry of the blend is disrupted. Quite noticeably, the wear damage is reduced if the surfaces are scraped forth and back and not only in one direction. The influence of the scan pattern (distance between scan lines) and of the normal force (well below and above the force threshold of about 50 nN leading to surface ripples in the first stage) have been also addressed.

Helium bubble size effects on the surface morphological response of plasma-facing tungsten

Abstract We report a simulation study on the effects of helium (He) bubble size on the morphological evolution and pattern formation on the surface of tungsten used as a plasma-facing component (PFC) in nuclear fusion devices. We have carried out a systematic investigation based on self-consistent dynamical simulations of surface morphological evolution according to an atomistically-informed, 3D continuum-scale model that captures well the relevant length and time scales of surface nanostructure formation in PFC tungsten. The model accounts for PFC surface diffusion, driven by the biaxial compressive stress originating from the over-pressurized He bubbles in the near-surface region of PFC tungsten as a result of He plasma exposure, combined with the formation of self-interstitial atoms in tungsten that diffuse toward the PFC surface and the flux of surface adatoms generated as a result of surface vacancy-adatom pair formation upon He implantation; this transport of surface adatoms contributes to the anisotropic growth of surface nanostructural features due to the different rates of adatom diffusion along and across step edges of islands on the tungsten surface. Our detailed analysis reveals that varying the average He bubble size plays an important role in the PFC surface growth kinetics as well as the resulting surface topography. Specifically, we find that an increase in the He bubble size leads to a deceleration in the growth rate of the tungsten nanotendrils that emanate from the PFC surface. We also find that the separation distance between the resulting surface features increases with increasing He bubble size, as well as over time. This coarsening effect is a thermally activated process resulting in an accurate description of the temperature dependence of the average surface feature separation by an Arrhenius relation.

Initial growth of tungsten fuzz induced by bubble-driven surface stress layer under helium irradiation

Low-energy, high-flux He exposure to tungsten (W) surface leads to the formation of surface nanostructures and severe morphological changes, which may eventually erode the W divertor and threaten the operation of the reactor. In this study, the response of polycrystalline W under low-energy He+ irradiation at different temperatures has been investigated in order to analyze the early stage of nanostructure formation. It is found that the interactions of high-density over-pressured He bubbles result in the formation of surface stress layers. The significant effect of temperature on the surface stress leads to differences in the incubation dose of W fuzz growth. The interaction between the planar network and the underlying W matrix is weak under surface stress, and the W fuzz grows on the surface once the stress reaches a threshold. Thereafter, tensile stress-driven cracking causes the 3D growth of W nanofibers.

Influence of surface modification on droplet mobility during dropwise condensation

AbstractA two‐step surface fabrication technique is used to modify the wetting characteristics of copper sheets for heat transfer application. It consists of oxygen plasma treatment that results in the formation of surface nanostructures, and a post‐functionalization process using self‐assembled monolayers. The copper samples are characterized by contact angle measurements, scanning electron microscopy, and atomic force microscopy. Results show that surface roughness, described using hybrid and functional roughness parameters, varies with irradiation power. The plasma‐treated copper samples become superhydrophobic with water contact angles ranging from 150° to 160° after the post‐functionalization. Condensation experiments are conducted to explore the influence of surface wettability on condensate formation. Results show that droplets formed on the superhydrophobic copper sheets are smaller and more mobile with sweep events that are 9x more frequent compared to those on untreated copper sheets. Sweep events are shown to be influenced by surface porosity, which is defined using standard roughness parameters. It is demonstrated that higher surface porosity results to higher sweep interval.

Surface nanostructures formation induced by highly charged ions: Kinetic and potential energy dependence

We applied the thermal spike model to interpret the interplay of the kinetic energy with the potential energy in the nanostructures formation of various materials, i.e., the two dimensional freestanding monolayer MoS2, mica, KCl and CaF2, induced by xenon ions for the wide range of the charge state (15+ to 50+) and the kinetic energy from several keV up to 10 MeV. The deposition of the potential energy in the solid is extracted from the non-equilibrium charge state distributions when the highly charged ions are transporting in the solids. Also, the experiments of the nanostructure formation on the surfaces of the mica samples induced by Xeq+ (28+ to 35+) at the kinetic energies of 66.7 keV and 288 keV have been performed and the potential energy thresholds for the hillock formation have been measured to support our calculations and explanations. The increase of the damage pore radius as a function of the potential energy has been interpreted for the freestanding monolayer MoS2. For the bulk materials (mica, KCl and CaF2), the interplay of the kinetic energy and the potential energy has been calculated and discussed and the general agreements have been reached between the model calculations and the experimental results.

Onset of fuzz formation in plasma-facing tungsten as a surface morphological instability

Based on simulation results and theoretical analysis according to an atomistically informed continuum-scale model for the surface morphological response of tungsten plasma-facing component (PFC) in nuclear fusion devices, we demonstrate that the onset of the well-known surface nanostructure (known as ``fuzz'') formation in PFC tungsten is the outcome of a stress-induced surface morphological instability. Specifically, we show that formation and growth of nanotendrils emanating from the exposed surface of PFC tungsten, a precursor to fuzz formation, is caused by a long-wavelength surface morphological instability triggered by compressive stress in the PFC near-surface layer due to over-pressurized helium (He) bubbles forming in this near-surface region through implantation of low-energy He ions from the plasma. Using linear stability theory (LST), we predict the onset of surface growth in response to low-amplitude perturbations from a planar PFC surface morphology and calculate the average spacing between nanotendrils growing from the PFC surface, in excellent agreement with self-consistent dynamical simulations of surface morphological response according to the fully nonlinear surface evolution model starting with random fluctuations from the planar surface morphology that result in nanoscale surface roughness. In addition, we examine the morphological response of the PFC surface to low-amplitude perturbations of very long wavelengths from its planar morphology and interpret fully the simulation results on the basis of a weakly nonlinear tip-splitting instability theory, which predicts a post-instability nanotendril pattern formation with nanotendril separation consistent with the LST predictions regardless of the initial surface perturbation. Finally, we compare our simulation predictions for surface nanotendril growth with experimental measurements of fuzz layer growth on the exposed surface of PFC tungsten. Our simulation results are in very good agreement with the surface growth measurements at the early stages of fuzz growth, further establishing the onset of fuzz formation in PFC tungsten as the outcome of a stress-driven PFC surface morphological instability.

Effect of High-Energy Shot Peening on Surface Nanocrystallization and Integrity of AISI 4140 Steel: A Numerical and Experimental Investigation

High-energy shot peening (HESP) can introduce not only compressive residual stresses but also nanocrystalline layer close to the surface. Surface nanocrystallization leads to great improvement of surface hardness and fatigue property of treated metallic components. In this paper, a finite element method (FEM) was proposed to investigate the effects of HESP parameters, including surface coverage, shot diameter and shot velocity, on the grain refinement phenomenon. The quantitative relationship between the grain size and the equivalent plastic strain (PEEQ) was obtained by comparing the data from FEM simulation and x-ray diffraction in experiments. The surface topography and the depth profiles of microhardness and full width at half maximum were analyzed with the aim of assessing and verifying the surface integrity and nanostructure formation after HESP treatment. The results showed that appropriate increment of surface coverage, shot diameter and shot velocity was beneficial to grain refinement and nanostructure formation on a treated surface after HESP treatment. Through reasonable process control of HESP, microcracks did not appear on a treated surface even when the shot peening surface coverage reached up to 1000%.

Nanomorphology of eco-friendly colloidal inks, relating non-fullerene acceptor surface energy to structure formation

This work unravels the intricate relationship between non-fullerene acceptor material surface energy and nanostructure formation in organic nanoparticle colloids.

Response of NIH 3T3 Fibroblast Cells on Laser-Induced Periodic Surface Structures on a 15×(Ti/Zr)/Si Multilayer System.

Ultrafast laser processing with the formation of periodic surface nanostructures on the 15×(Ti/Zr)/Si multilayers is studied in order to the improve cell response. A novel nanocomposite structure in the form of 15×(Ti/Zr)/Si multilayer thin films, with satisfying mechanical properties and moderate biocompatibility, was deposited by ion sputtering on an Si substrate. The multilayer 15×(Ti/Zr)/Si thin films were modified by femtosecond laser pulses in air to induce the following modifications: (i) mixing of components inside of the multilayer structures, (ii) the formation of an ultrathin oxide layer at the surfaces, and (iii) surface nano-texturing with the creation of laser-induced periodic surface structure (LIPSS). The focus of this study was an examination of the novel Ti/Zr multilayer thin films in order to create a surface texture with suitable composition and structure for cell integration. Using the SEM and confocal microscopies of the laser-modified Ti/Zr surfaces with seeded cell culture (NIH 3T3 fibroblasts), it was found that cell adhesion and growth depend on the surface composition and morphological patterns. These results indicated a good proliferation of cells after two and four days with some tendency of the cell orientation along the LIPSSs.

Simulations of Formation of Nanostructure in Silicon Surface by Single Slow Highly Charged Ion

To understand the mechanisms of surface erosion induced by slow highly-charged ion (SHCI) bombardment, the surface nanostructure formation in Si (111) surface by single Xe44+ ion was studied by using molecular-dynamics (MD) simulations, based on analyzing the multiple electron emission of the substrate. The time evolutions of the temperature, energy, pressure and density of the substrate have been systematically studied. The results show the bombardment of the incident SHCI resulting in an explosive event in the surface. A shock wave propagating at ~104 m/s is formed in the system during the initial 175 fs. After this initial shock, many particles are ejected from the surface since the extreme non-equilibrium of the system. And at t=370 fs, a crater-like nanostructure with diameter of ~40 Å and depth of ~18 Å is formed at the incident site.

Creation of surface nanometer-scale plasma region by irradiation with slow highly charged ions

Slow highly charged ions (HCIs) were used to create a plasma region of nanometer scale on the surface of different materials. The creation of the plasma in a localized region is accompanied by the formation of surface nanostructures. The plasma expansion approach is used to explain the creation mechanism of HCI-induced nanostructures in lithium niobate single crystals, considering the Maxwellian and non-Maxwellian electron distributions. The numerical solutions of the applied hydrodynamic equations show that the profile of the early stage expanded plasma is independent of the ratio of ion-to-electron temperatures. However, an increase in the temperature ratio leads to a wider expansion domain. In addition, the effect of material constituents ions are studied in terms of the modifications induced in the plasma-expanded region.

Exposures of bulk W and nanostructured W coatings to medium flux D plasmas

• Exposures of nanostructured W coatings and bulk W was performed to high energy D plasmas in GyM. • Formation of nanostructures was observed after exposures. • Nanostructures strongly depend on the structure of W. • Development of blisters was observed on compact coatings. In this work, we discuss the exposures of polycrystalline tungsten (W) and nanostructured W coatings to high energy ( ~ 30 ÷ 300 eV ) medium-flux D ( ~ 10 20 m - 2 s - 1 ) deuterium plasmas of the linear machine GyM at an overall fluence of 10 24 m - 2 . The energy and particle flux are tailored to that of charge-exchange neutrals at the vessel main chamber of present-day tokamaks and expected in ITER. Scanning Electron Microscopy measurements show the formation of surface nanostructures that are found to strongly depend on the W morphology and crystallinity, as well as on the energy of the ion species. The obtained nanostructures are few nanometres deep and similar to the one obtained after exposure of bulk W to divertor-like conditions. Dedicated annealing experiments are performed on selected samples, showing that the nanostructures developed on W coatings are thermally stable up to their recrystallisation temperature. The development of micrometric-sized blisters is also observed and found to strongly depend on the features of the W coatings, the substrate and the applied bias voltage.

Features of the cadmium chalcogenide substrates with surface nanostructure

The effect of single crystalline CdTe, CdSe, CdS substrates normalizing annealing on their optical properties was investigated, and the surface nanostructure formation was determined by the study of optical transmission spectra, photoluminescence and atomic force microscopy (AFM) topograms. Possibility obtaining of intense luminescence in the border region, formed by interband recombination and transitions due to size quantization of the charge carrier energy, is determined.

Stoichiometric influences on ion beam nanopatterning of CoSi binary compound

Impact of initial stoichiometry of binary compound on surface nanostructure formation with low energy ion irradiation has been studied. Different stoichiometric CoSi surfaces are irradiated by Ar+ ion beam with energy of 700 eV, fluence of 7.5×1018 ions cm−2 and angle of incidence 67∘. Within a narrow window of stoichiometric variation, self-organized nanoripples have been observed. The ripple structures are well formed for stoichiometric ratios of 40:60 for Co:Si. Nanoscale ripples start growing for a concentration of about Co22Si78. The root mean square (rms) roughness initially decreases and then increases slightly as Co increases from low to medium concentrations. The evolution of different morphologies has been corroborated from the behavior of power spectral densities (PSD). Correlation lengths are extracted from atomic force microscopy (AFM) images to corroborate the ripple formation region only within a specific stoichiometric range. Differential sputtering yields provide a rationale for the observed pattern evolution.

Enhanced Activity of Pr6O11 and CuO Infiltrated Ce0.9Gd0.1O2 Based Composite Oxygen Electrodes

Operation of solid oxide fuel/electrolysis cells (SOFC/SOEC) at high temperatures (T > 850 °C) is accompanied by degradation phenomena, which severely affect the operational lifetime of the cell. Degradation processes are expected to occur slower at low temperatures. However, significant reduction in electrocatalytic activity of the oxygen electrode, is one of the major challenges in decreasing the operating temperature down to 500 °C–650 °C. Recently, Pr6O11 infiltrated Ce0.9Gd0.1O2 (CGO) based electrodes have been proposed to realize high electrochemical performance at intermediate temperature. In this study, Pr-oxide has been infiltrated into a well performing sub-micro La0.6Sr0.4Co0.2Fe0.8O3/Ce0.9Gd0.1O2 (LSCF/CGO) composite electrode as well as a poorly performing LaNi0.6Fe0.4O3/Ce0.9Gd0.1O2 (LNF/CGO) electrode. The results are compared with Cu-oxide infiltration. Formation of Pr rich oxide surface nanostructures, resulted in a pronounced reduction of polarization resistances by a factor of 3 in state of the art LSCF/CGO electrodes. Even better performances were achieved in Pr6O11 infiltrated LNF/CGO, demonstrating low polarization resistances of 0.074 ∓ 0.002 and 0.146 ∓ 0.002 Ω.cm2 at 600 °C and 550 °C, respectively. Low degradation rates were observed over a 200 h durability test at 650 °C, illustrating the potential of Pr6O11 infiltrated CGO oxygen electrodes for intermediate temperature SOFC/SOEC application.

Design and application of gas cluster accelerator for surface smoothing and nanostructures formation

A custom-built gas cluster ion source with energy up to 50 keV is constructed, and Ar, CO<sub>2</sub>, N<sub>2</sub>, and O<sub>2</sub> are used as the working gases. The clusters are formed by a metal supersonic conical nozzle with critical diameter in a range of 65–135 μm and a cone angle of 14°. The nozzle is powered in the pulsed mode, which improves the pumping conditions, and also makes it possible to increase the gas pressure in the stagnation zone to 15 atm and thereby obtain larger clusters. Based on the principle of ultrasonic expansion, gas cluster ions with an average size of 3000 atoms are obtained. The cluster beam current of 50 μA is obtained. The Ar cluster beam, which is less reactive, is used for treating surface, namely, surface smoothing and formation of self-assembled nanostructures. The Ar cluster bombardment perpendicular to the surface of the substrate is used to demonstrate the smoothing of the surface of Si wafers, Ti coating, and Au film. For the initial Si wafer, its root-mean-square (RMS) roughness of 1.92 nm decreases down to 0.5 nm after cluster beam treatment. The cleaning effect of the cluster beam is also observed very well. The one-dimensional (1D) isotropic power spectral density of the Si surface topography before and after smoothing are also discussed. The off-normal irradiation Ar cluster beam is also used to form self-assembled surface nanoripple arrays on the surface of flat ZnO single crystal substrates. The ripple formation is observed when the incident angle of the cluster beam is in a range of 30°–60°. The process of nanoripple fabrication is significantly governed by the cluster beam incident angle, energy and dose. The nano-ripples formed on the flat substrates remain eolian sand ripples and their formation starts at the incident angle of 30°. The most developed nanoripples are observed at the incident angle within a range of 45°–60°. The surface morphology and characteristic distribution of the nano-structures on the flat ZnO substrate are also analyzed by the two-dimensional (2D) power spectral density function. Next, Ar cluster beam is used for irradiating the ZnO nanorod arrays grown on the Si substrate. Due to various angles between the nanorod’s axis and the substrate normal, the conditions of the ripple formation on the nanorod facets are also realized. The dependence of wavelength on the accelerating voltage of the cluster ions and the dose are studied. Similar dependence of wavelength on accelerating voltage and dose are found for nanorods. Comparing with the flat ZnO surface, nanoripples on the ZnO nanorod faces at high irradiation doses demonstrate an ordering effect, and morphology of the ripples resembles that of more parallel steps rather than eolian ripples.

Механизм формирования наноструктур гексагональной симметрии на поверхности металлов последовательностью сдвоенных ультракоротких импульсов излучения ортогональной поляризации

An experimental results causing the ordered hexagonal symmetry surface nanostructures formation on metals under the sequence of scanned doubled collinear femtosecond pulses of orthogonal polarization and variable time-delay have been analized. It was shown that the structures production caused by the dynamic grating formation by the first pulse, excitation of two paires of surface plasmon polaritons in symmetric to the polarization vector directions of second pulse directions with 60 degrees angle between formed respective dynamic gratings and forth-fold first grating degeneration. So three gratings of three-flold symmetry about surface normal creates experimentally observable resonant surface relief of hexagonal symmetry.

Effects of deep subsurface damages on surface nanostructure formation in laser recovery of grinded single-crystal silicon wafers

A nanosecond pulsed Nd:YVO4 laser was irradiated on a boron-doped single-crystal silicon wafer after rough and fine grinding processes to recover the grinding-induced subsurface damages. The surface topography of samples was investigated by using a white-light interferometer, a scanning electron microscope, and an atomic force microscope; while the crystallinity was analyzed by a laser micro-Raman spectrometer. It was found that surface nanostructures were generated by the Mullins-Sekerka instability, which remained on the surface under recoil pressure and surface tension. The rough grinding-induced deep subsurface damages influenced the interface instability between liquid and solid silicon during recrystallization process. By increasing pulse width and decreasing laser peak irradiance, the subsurface damage was recovered and a flat surface with surface roughness of ~1 nm Sa was obtained. This study reveals important correlations among grinding-induced latent subsurface defects, laser peak irradiance and nanoscale surface topography formation in laser recovery, which contributes to high quality silicon wafer manufacturing.