We performed studies of the self-organization processes in nanoporous alumina membranes at initial and late stages of aluminium anodization by using scanning electron microscopy (SEM) and small-angle neutron scattering (SANS). SEM observations indicated three stages in the self-organization of nanopores in alumina: (1) nucleation of random nanopores with a broad radius distribution, (2) narrowing the radius distribution and (3) slow evolution of the nanoporous structure toward ordering of

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... res into large domains. SANS studies revealed orientational correlation between ordered domains of nanopores, which is characterized by a small misorientation angle. For the samples with high aspect ratio of nanopores, the SANS patterns showed azimuthal smearing, which was attributed to the redistribution of nanopores between the domains during their growth. PACS: 61.46.-w, 82.45.Yz Introduction Aluminium anodization [1] is one of the most controllable self-organized processes yielding vertically aligned uniform cylindrical nanopores. These nanopores can be used as nano-beakers for either self-assembly or synthesis of nanomaterials. Nanoporous alumina (NPA) membranes have been used to fabricate a wide variety of diverse nanostructures such as quantum dots [2], nanowires [3, 4], carbon nanotubes [5], nanostructured films [6] and photonic structures [7]. The geometry of the nanoporous structure, such as interpore distance, pore diameter and length, can be varied in a wide range by adjusting conditions of aluminium anodization and postanodizing treatment [8]. Besides vertical alignment, nanopores tend to form ordered hexagonal arrays under specific conditions [9]. Thus, laterally disordered as well as highly-ordered nanoporous alumina membranes can be fabricated. Although experimental SANS studies on nanoporous alumina have been already reported [17][18][19][20][21], most of them were performed on samples with disordered porous structure. Also, to the best of our knowledge, there are no SANS studies that reveal dynamics of the self-organization processes in nanoporous alumina. In this paper small-angle neutron scattering (SANS) and scanning electron microscopy (SEM) studies have been used in a complementary manner, which enabled us to study self-organization of the nanoporous structure on different stages of the anodization process. Experimental The nanoporous alumina membranes were fabricated by anodization of 99.99% pure aluminium sheets. The aluminium substrates were first annealed at 500 °C in argon atmosphere and then electropolished in a mixture of perchloric acid and ethanol 1:4 to obtain a smooth mirror-like surface. The anodization of the electropolished aluminium was performed in a two-electrode cell with the aluminium sheet as anode and a platinum mesh as the counter electrode under the constant potential of 40V in 0.3M oxalic acid electrolyte at 5 °C. The electrolyte solution was vigorously stirred to improve the uniformity of the temperature distribution and to minimize diffusion limitations in long nanopores. The ordering of the nanopores during initial stages of their growth was studied by interruption of the anodization process, selective dissolution of the anodic oxide layer in a mixture of chromic (20 g/l) and phosphoric (66 g/l) acids at 60 °C and SEM observation of structural features remained on the surface of aluminium, which replicates the morphology of the Al/Al 2 O 3 interface (figure 1).