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Porous anodic oxide (PAO) films are grown by electrochemical oxidation of metals in a solution that dissolve the oxide. During the film formation, voltage increases linearly until the peak, then declines to steady-state value. PAO finds extensive usage as templates and substrates in many applications, such as solar cells and optical devices. However, the mechanism underlying the observed initiation and evolution of self-organized PAO structure is not understood. Recent studies on pore formation points out the role of plastic flow on pore initiation process [1–3]. In this study, we characterized stress profiles and its evolution during the film formation prior to the pore initiation. Phase-shifting curvature interferometry was used to monitor sample curvature change during film growth and subsequent dissolution. Oxide films were grown at constant current densities in 0.4 M H3PO4 until different thicknesses and subsequently current was turned off to dissolve grown oxide film. The stress profile of oxide film was revealed by in-situ monitoring the curvature change during dissolution of oxide film period after anodizing [4] . In addition, morphological evolution of oxide film during film growth was characterized using SEM. Oxide films growth until different thicknesses values up to onset of pore initiation instability. The measured stress change during film growth was in excellent agreement with prior measurements [5]. Measured stress profiles showed that for thin films <20 nm, compressive stress was evenly dispersed through the thickness. However, for thicker films, the stress is concentrated within 20-nm thick layer near the solution interface. Transition in the stress profile coincides with oxide film thickness associated with initial roughening instability at the solution interface [6]. SEM images also showed that the first instability initiated at an oxide thickness of 20 nm with stable surface roughness pattern with a length scale of 20 nm. After the initial instability, the stress level near the solution interface became increasingly compressive as oxide film thickens. This behavior continued until the moment of self-ordered pore initiation when the both oxide thickness and the integrated oxide stress rapidly decreased to steady-state values. Morphological change during anodizing coincides with the stress transient, which could be attributed to the relaxation of elastic stress due to onset of plastic flow. Thus, plastic yielding in the oxide may induce a second instability mechanism involving pore initiation, leading to final self-ordered pore pattern. ACKNOWLEDGMENTS Support was provided by the National Science Foundation (CMMI-100748). REFERENCES [1] Garcia-Vergara, S.J., et al. Electrochim. Acta. 2006, 52, 681. [2] Houser, J.E., Hebert, K.R. Nature Mater. 2009, 8, 415. [3] Oh, J., Thompson, C.V. Electrochim. Acta. 2011, 56, 4044. [4] Çapraz, Ö.Ö., Shotriya, P., Hebert, K.R. J. Electrochem. Soc. 2014, 161, D256. [5] Çapraz, Ö.Ö., Hebert, K.R., Shotriya, P. J. Electrochem. Soc. 2013, 160, D501. [6] Hebert, K.R., et al. Nature Mater. 2012, 11, 162.

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Stress evolution of anodic alumina films prior to the pore formation

Porous anodic oxide (PAO) films are grown by electrochemical oxidation of metals in a solution that dissolve the oxide. During the film formation, voltage increases linearly until the peak, then declines to steady-state value. PAO finds extensive usage as templates and substrates in many applications, such as solar cells and optical devices. However, the mechanism underlying the observed initiation and evolution of self-organized PAO structure is not understood. Recent studies on pore formation points out the role of plastic flow on pore initiation process [1–3]. In this study, we characterized stress profiles and its evolution during the film formation prior to the pore initiation. Phase-shifting curvature interferometry was used to monitor sample curvature change during film growth and subsequent dissolution. Oxide films were grown at constant current densities in 0.4 M H3PO4 until different thicknesses and subsequently current was turned off to dissolve grown oxide film. The stress profile of oxide film was revealed by in-situ monitoring the curvature change during dissolution of oxide film period after anodizing [4] . In addition, morphological evolution of oxide film during film growth was characterized using SEM. Oxide films growth until different thicknesses values up to onset of pore initiation instability. The measured stress change during film growth was in excellent agreement with prior measurements [5]. Measured stress profiles showed that for thin films <20 >nm, compressive stress was evenly dispersed through the thickness. However, for thicker films, the stress is concentrated within 20-nm thick layer near the solution interface. Transition in the stress profile coincides with oxide film thickness associated with initial roughening instability at the solution interface [6]. SEM images also showed that the first instability initiated at an oxide thickness of 20 nm with stable surface roughness pattern with a length scale of 20 nm. After the initial instability, the stress level near the solution interface became increasingly compressive as oxide film thickens. This behavior continued until the moment of self-ordered pore initiation when the both oxide thickness and the integrated oxide stress rapidly decreased to steady-state values. Morphological change during anodizing coincides with the stress transient, which could be attributed to the relaxation of elastic stress due to onset of plastic flow. Thus, plastic yielding in the oxide may induce a second instability mechanism involving pore initiation, leading to final self-ordered pore pattern. ACKNOWLEDGMENTS Support was provided by the National Science Foundation (CMMI-100748). REFERENCES [1] Garcia-Vergara, S.J., et al. Electrochim. Acta. 2006, 52, 681. [2] Houser, J.E., Hebert, K.R. Nature Mater. 2009, 8, 415. [3] Oh, J., Thompson, C.V. Electrochim. Acta. 2011, 56, 4044. [4] Çapraz, Ö.Ö., Shotriya, P., Hebert, K.R. J. Electrochem. Soc. 2014, 161, D256. [5] Çapraz, Ö.Ö., Hebert, K.R., Shotriya, P. J. Electrochem. Soc. 2013, 160, D501. [6] Hebert, K.R., et al. Nature Mater. 2012, 11, 162.