The width of the G-band (ГG) can be used to get an idea of the number of defects (even if there are some differences depending on the sample, as seen in Part 3 with doped graphene, for example). The D4 band has also recently been understood in the context of the double resonance mechanism [61].
Raman Spectroscopy of Different Aromatic Carbons
The G band is due to the E2gphonon at the center of the Brillouin zone (and is called the Г point). The presence of hydrogen systematically decreases the position of the G band (for ta-C/ta-C:H and a-C/ta-C:H).
Discussion
IG(ω,λL) =Ig(ω,λL) +Ir(ω,λL) +Ic(ω,λL) (23) where ω is the Raman frequency, λL is the laser wavelength and Ig,Ir is the intensities of nc-G, fused aromatic rings and olefinic chain respectively. It has been one of the best advances made in recent years to overtake the old Tuinstra and Ferrari. Regarding what already exists in the literature about curvature and/or fingerprints of phonon confinement in Raman spectra, analysis of pyrocarbons, which can consist of winding planes of different lengths, has revealed that the D bandwidth is sensitive to the effects of graphene curvature [123,23.
Among other changes, the increase of the D-bandwidth with the pressure increase is more pronounced than that of the G-band, leading to a rapid evolution of the D-bandwidth compared to the G-bandwidth. Part of the answer was obtained by observing a band shift of the G band [8], but none was obtained for defective graphene regarding the D bandwidths. The arrow in Figure 25b starts from the line in the confinement model and suggests that curvature provides an additional broadening of the G band.
Recently, an additional source of broadening for the G band was discovered by studying graphene membranes under pressure and attributed to strain (if the strain is higher than 1%) [294].
Conclusions
Ix (expressed in arbitrary units related to the number of counts on the detector): height of the band marked x. Ax (expressed in arbitrary units related to the number of counts on the detector): integrated area of the band labeledx. Evolution of the Raman spectra from single, few and multilayer graphene with increasing disorder.Phys.
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Signature of the two-dimensional phonon scattering in graphene probed by double-resonant Raman scattering.Phys.
Corrosion Resistance and Durability of
Superhydrophobic Copper Surface in Corrosive NaCl Aqueous Solution
- Introduction
- Materials and Methods
- Results and Discussion
- Conclusions
The main aim of the present study is to investigate the corrosion resistance and durability of superhydrophobic copper surfaces. The chemical stability of the superhydrophobic copper surfaces in the corrosive aqueous NaCl solution (3.5 wt%) over a short period of time was assessed. Mild abrasion tests and ultrasonic vibration were performed to test the durability of the superhydrophobic copper surfaces.
Figure 5 shows the variation of water contact angles of superhydrophobic copper surfaces immersed in a 3.5% NaCl aqueous solution. Effect of immersion time on the static water contact angle of a superhydrophobic copper surface. The potentiodynamic polarization curve of the superhydrophobic copper surface was compared with the curves of the pristine copper surface and the trichlorosilane-coated copper surface (reference surface), as shown in Figure 6 .
In addition, the corrosion current density of the super hydrophobic surface was less than 1% of that of the pristine copper surface.
On the Durability and Wear Resistance of Transparent Superhydrophobic Coatings
Wetting Theories, Surface Roughness and Robust Metastable Superhydrophobicity
The most hydrophobic component is the epicuticular wax, and the apparent contact angle of water on wax is only slightly over 90◦ , which is certainly not sufficient to explain the extreme non-wettability of certain plant surfaces. In addition, also due to the specific shape of the Lotus leaf protrusions, this strategy lowers the sensitivity of the superhydrophobic state to the protrusion distance. From the tribological point of view, energy- or stress-dissipating ceramic nanoparticles or organic substances should be incorporated into the coatings, such as rubbery domains [17].
Mie scattering occurs when the diameter of the surface features is close to the wavelength of the incident photon, and Rayleigh scattering occurs when the size of the surface features is much smaller than the wavelength of the incident photon. Note that here we define "contact angle" as the apparent contact angle that is stable over time on the surface. In other words, the apparent contact angle on the superhydrophobic surface can still be greater than 150°, but the droplet will not roll or slide on the coating when tilted.
On such surfaces, the actual contact angle is established at submicron scales along the contact line, known as tacky superhydrophobicity.
Wear Damage Resistance Characterization Methods
In other words, metastable states (minimum liquid-solid contact), where minimum droplet-surface contact points are maintained, must have a long lifetime. This strategy avoids the need for high-steepness projections that may be more prone to erosion, wear, and breakage. A self-similar hierarchical texture throughout the non-wettable surface, film or coating mass should be engineered to resist material removal due to wear [9];
Once transparency is also introduced into the equation, resistance to durability becomes more problematic since transparent surface structure roughness should not increase Mie and Rayleigh scattering [18]. In this review, we will focus on coating resistance to wear-induced abrasion of transparent non-wetting coatings. It is believed that abrasion is the most relevant durability parameter that can be convincing for potential industrial applications as well as for outdoor anti-graffiti installations.
Wear of transparent non-wettable coatings has not been adequately addressed so far and further R&D efforts are needed to enable commercialization.
Transparent Non-Wettable Coatings from Nanoparticle Assembly
The stability of the Cassie state (droplets rolling away freely) of the coatings against the external pressure was investigated. The extreme stability of the Cassie-Baxter state in a micro/nanoporous coating was attributed to the bilayer porous structure. After depressurization, the contact line partially recovered in that dehumidification of the nanoporous substrate took place.
The optical transmittance of the resulting coating was greater than 80% over a broad spectrum of ultraviolet and visible wavelengths. To improve the mechanical stability of the nanoparticle films, chemical vapor deposition (CVD) of tetraethoxysilane in the presence of ammonia was performed. Bursting led to an increase in the slip angle and ultimately to loss of superhydrophobicity.
Dotted zone shows loss of nanoparticles from the fiber surface; (c) Photograph of blue-stained water on the coated polyester mesh surface (left) and a photograph of an uncoated surface on white paper for comparison; (d) Contact angle and sliding angle measurements of the superhydrophobic masks using aqueous solutions with a pH range of 2–14 [37].
Transparent Non-Wettable Coatings from Pattern Transfer to Polymers
It was found that the fabricated mesh retained its superhydrophobicity and low water slip angle due to the PFDTS surface treatment, although the SiO2 nanoparticles modified with PFOTS are removed by the abrasion (Figure 11b). They claimed that it was not necessary to control the refractive index of the materials to improve transparency and to contain a strong chemical or physical bond between the particles or the particles and the substrate to improve abrasion resistance. Therefore, compared to the traditional technology, the combination of the transparent mesh and the hierarchical structure of SiO2 nanoparticles seems to be an effective and simple method to improve the abrasion resistance and transparency of these superhydrophobic films (Figure 11c, d). a) Schematic of the manufacturing procedure for the superhydrophobic polyester mesh.
Thirdly, the mesh is treated with SiO2 nanoparticles modified with PFOTS using a spray method; (b) FE-SEM images of the superhydrophobic polyester mesh after 100 cycles of abrasion with a pressure of ~10 kPa. On a large-sized template of the transparent polydimethylsiloxane (PDMS) elastomer surface, perfectly ordered microstructures with an inverted trapezoidal cross-section were fabricated with two consecutive PDMS replication processes and a three-dimensional diffuser lithography technique. They also applied a PDMS-based coating to achieve both a highly transparent superhydrophobic surface and an anti-adhesion layer coating for high-resolution nanoimprint lithography through intrinsic low surface energy and easy release of PDMS.
Thus, the superomniphobic property of PDMS microstructures was attributed to their ability to prevent local Cassie Baxter-Wenzel transition as lateral diffusion occurred immediately after the composite interface disappeared.
Transparent Non-Wettable Coatings from Ceramic-Based Nanostructures
Their approach not only enabled the design of surface microstructures with graded-index anti-reflection characteristics, suppressing surface reflection by optical impedance matching between interfaces, but also enabled self-cleaning capability through surface chemistry modification. In addition, the average penetration depth (calculated from the lateral and vertical displacements) of the indenter is almost the same for both the reference substrate and the coated sample for loads at half and full load. This result suggests that the scratch resistance behavior of the applied films is comparable to that of the underlying substrate material.
Metal oxides are generally known to exhibit significant wetting towards water molecules due to the high possibility of synergistic hydrogen bonding interactions at the solid–water interface. By applying an antifouling treatment, reduction in the performance of the solar devices was inhibited. For example, the stripe-coated solar cells maintained a high conversion efficiency of 92% of the uncoated solar cell even in the presence of oil contamination.
Note that the sputtered borosilicate films are generally somewhat rougher than the underlying substrates; (b) Comparison of coefficient of friction profiles as a function of scratch distance for fused silica substrates with and without a dense borosilicate layer.
Recommendations for Future Directions
Conclusions and Outlook
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