(Invited) The Effects of Laser Ablation Texturing and Nanoparticles on Anodic Nanotube and Porous Film Formation

Pits and pores in oxidized metals and silicon can be produce by a variety of self-limiting and, in some cases, self-organizing electrochemical processes. Here we discuss both high voltage anodization – which is well known for making oxidized metal nanostructures – and stain etching to produce porous silicon. High-voltage anodization – denoted Matsuda-Schmuki chemistry with reference to the work, e.g., of Masuda et al. for Al1-4 and Schmuki et al. for Ti or other metals,5,6 – involves an intricate balance between oxide growth and dissolution in which pore initiation and propagation is convoluted with the need for strain relief because of the expansion inherent to oxide formation. We use laser ablation with a nanosecond pulsed Nd:YAG laser operating at 532 nm to introduce texture (pillars, pits and nanoparticles) prior to anodization.7 Texturing of the substrate prior to anodization introduces features that can modify either the initiation of pores or strain relief during oxidation. Thereby it can affect the development of the resulting structures. Texturization also increases reaction rates by the introduction of lattice defects as well as increased surface area. Results for a number of metals including Al, Ti and Zn will be discussed. The SEM image shown in Figure (a) is of a porous F-doped ZnO layer grown on a laser ablated Zn substrate when anodized in an NH4F/ethylene glycol solution at 60 V. As shown in Fig. (b), TiO2 nanotube formation by anodization of pillar and nanoparticle Ti substrates is extremely facile. Stain etching of Si powders can produce high surface area and highly photoluminescent porous powders. PL bands from blue to red have been observed, as shown in Figure (c). PL in the red to near IR is extremely long lived, exhibiting multi-exponential decay with lifetime components in excess of 100 µs. Biological tissue autofluorescence exhibits lifetimes in the nanosecond range. Thus, the long-lived PL from Si nanoparticles is promising for bioimaging applications in which image acquisition is delayed after the initial photoexcitation.8 Furthermore, porous powders are readily terminated and chemically passivated with alkyl groups by hydrosilylation with, e.g., 1-alkenes. These alkyl-terminated powders not only retain their photoluminescence, they also retain long-lived PL. The characteristics of the porous powder respond to changes in the starting material (wafer reclaim chunks, reactor powder, metallurgical grade powder), etching conditions and chemical passivation of the powder after etching. We will discuss factors that influence the specific surface area, PL and structure of the porous particles. 1 H. Masuda, H. Yamada, M. Satoh, H. Asoh, M. Nakao, and T. Tamamura, Appl. Phys. Lett. 71, 2770 (1997). 2 H. Masuda, F. Hasegwa, and S. Ono, J. Electrochem. Soc. 144, L127 (1997). 3 H. Masuda and M. Satoh, Jpn. J. Appl. Phys. 35, L126 (1996). 4 H. Masuda and K. Fukuda, Science 268, 1466 (1995). 5 K. Lee, A. Mazare, and P. Schmuki, Chem. Rev. 114, 9385 (2014). 6 J. M. Macak, H. Tsuchiya, A. Ghicov, K. Yasuda, R. Hahn, S. Bauer, and P. Schmuki, Curr. Opin. Solid State Mater. Sci. 11, 3 (2007). 7 A. S. Ganas, D. A. Znamensky, N. M. Alba, J. L. Hernández-Pozos, and Kurt W. Kolasinski, ECS Trans. 69, 155 (2015). 8 L. Gu, D. J. Hall, Z. T. Qin, E. Anglin, J. Joo, D. J. Mooney, S. B. Howell, and M. J. Sailor, Nature Communications 4, 2326 (2013). Figure 1