In-Depth Investigation of Copper Surface Chemistry Modification by Ultrashort Pulsed Direct Laser Interference Patterning

Abstract

Surface patterning in the micro- and nanometer-range by means of pulsed laser interference has repeatedly proven to be a versatile tool for surface functionalization. With these techniques, however, the surface is often changed not only in terms of morphology but also in terms of surface chemistry. In this study, we present an in-depth investigation of the chemical surface modification occurring during surface patterning of copper by ultrashort pulsed direct laser interference patterning (USP-DLIP). A multimethod approach of parallel analysis using visualizing, topography-sensitive, and spectroscopic techniques allowed a detailed quantification of surface morphology as well as composition and distribution of surface chemistry related to both processing and atmospheric aging. The investigations revealed a heterogeneous surface composition separated in peak and valley regions predominantly consisting of Cu2O, as well as superficial agglomerations of CuO and carbon species. The evaluation was supported by a modeling approach for the quantification of XPS results in relation to heterogeneous surface composition, which was observed by means of a combination of different spectroscopic techniques. The overall results provide a detailed understanding of the chemical and topographical surface modification during USP-DLIP, which allows a more targeted use of this technology for surface functionalization.

Increasing Antibacterial Efficiency of Cu Surfaces by targeted Surface Functionalization via Ultrashort Pulsed Direct Laser Interference Patterning

“…While on polished Cu, viable cell count reduction reaches a critical amount of 99.9% [13] after ≈85 min, the same level is reached on the USP-DLIP surfaces already between 60 and 70 min. Surprisingly, the antibacterial properties of as-processed surfaces seem to exceed those of immersion etched samples after 60 min despite a significantly higher amount of surface oxidation, [14,45] whereas the post-treated surfaces dominate in bacterial killing in the first 30 min. Comparison of the colony forming units (CFUs) reduction rate in relation to the smooth Cu reference reveal a 4.5-fold enhancement on immersion etched surfaces in this early phase, which decreases to ≈3.6-fold during further exposure.…”

“…was also recently verified for the surfaces investigated in this study. [45] However, the rapid increase in CA has to be emphasized especially for as-processed USP-DLIP surfaces where elevated values around 90° could be detected already 24 h after laser processing (see Figure 2a,b). The quick attainment of high CA values in the first aging phase indicates a prominent effect of sticking on the droplet spreading by the rough surface topography alongside the chemical modification during aging, which also leads to higher drop shape anisotropy as spreading is less suppressed parallel to the line pattern orientation (compare Figures 2a and 2b).…”

“…In a former study, these sub-structures could be identified as agglomerations of Cu 2 O formed by re-deposition of ablated matter during USP-DLIP processing. [45] Post-processing by immersion etching in 3% citric acid was shown to fully remove surface oxide revealing both the metallic substrate material as well as a surface morphology mainly induced by mobilized and rapidly cooled melt fronts resulting from the phase explosion ablation mechanism (see Figure 1b). Consequently, the surface roughness Rq is highest in the peak area of the as-processed surfaces, which can be attributed to the flake-like sub-structure, since in case of the immersion etched samples peak roughness almost equals the roughness measured in the valleys, as presented in Table 1.…”

“…It has to be noted that the ratio of agglomeration between hydrophilic carbo-oxygen and hydrophobic C-C/C-H bonding species was shown to vary in favor of the latter type on as-processed surfaces during aging in a recent study, while a similar ratio was already stabilized after 24 h of aging on immersion etched samples. [45] Hence, wetting of as-processed surfaces would be expected to show a more pronounced hydrophilic behavior in comparison to etched surfaces slightly after processing from a chemical aspect. However, CA measurements show the opposite, which highlights the significant influence of the difference in surface roughness.…”

Laser Thermochemical High-Contrast Recording on Thin Metal Films

“…However, the diffraction limit restricts the direct laser writing technique. On the other hand, researchers have developed cutting-edge ways to overcome the limit, for instance, direct interference patterning by registration of an interference pattern from two or more laser beams [9][10][11][12][13][14][15][16]. Other most recent experiments tackle this problem by the laser-induced formation of oxide layers on metals of titanium (Ti) [17] and vanadium groups (V, Zr, Ta, et al) [18][19][20].…”