Fabrication of Highly Ordered Al 2 O 3 Nanohole Arrays As a Nanostructured Template

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1 Fabrication of Highly Ordered Al 2 O 3 Nanohole Arrays As a Nanostructured Template Jie Gong, Bill Butler and Giovanni Zangari Materials Science Program University of Alabama at Tuscaloosa This work was funded by DOD grant no. DAAH MINT Fall Review, November, 2001

2 Introduction Porous Al oxide: a versatile nanostructured template Synthesis of ordered Al 2 O 3 porous structures Periodic nanoholes produced by ion milling of barrier layer of anodic porous alumina Sputtering of Cu on the nanohole side to produce a conductive support for further nanofabrication by electrodeposition Perspectives and opportunities in nanotechnology: magnetism electronics hybrid systems

3 Anodization of Aluminum Electrochemical oxidation: n- step anodization Morphology controlled by selection of the electrolyte film forming conditions Acid or alkaline electrolytes: porous films Electrolytes used: (COOH) 2 Pore size: 5 nm 1 mm, increases with ph and T Pore length up to 100 µm A Aluminum POWER SUPPLY + - Graphite 2Al + (3O 2- ) ox = Al 2 O 3 + 6e - H (O 2- ) aq = (O 2-2 O = 2H + + (O 2- ) aq ) ox Al = Al e - C

4 Pore Ordering Process (1) Electropolishing: to flatten the Al surface Anodization Stripping Al 2 O 3 : in : in (COOH) 2 H 2 CrO 4 + H 3 PO 4 RMS roughness: 0.36 nm Imprinted pattern on Al after stripping

5 Pore Ordering Process (2) Anodization (2 nd ): in (COOH) 2 for 12 hours Stripping Al 2 O 3 : in H 2 CrO 4 + H 3 PO 4, 30 min Roughness still large Imprinted pattern on Al after stripping

6 Pore Ordering Process (3) Anodization (3 nd ): in (COOH) 2 for 5.5 hours Highly ordered porous alumina

7 Long-range Order (SEM observation) Planar view crosssection Ordering process slowed down at defects, impurities

8 Pore Ordering Process (4) Separation of alumite nanostructure and Al substrate: in HgCl 2 solution Periodic barrier U-shape caps can be discerned

9 Nanoholes on barrier side Ion milling on barrier side can open nano-holes Controllable hole dimension can provide nano-sized constrictions Upper: (left) ion milling 15 min; (right) 10 min Lower: 5 min

10 Conductive support on barrier side Fully blocked nanoholes 200 nm copper was sputtered on the nanohole barrier side Cu provides a full covered conductive support for further nanofabrication Upper: (left) ion milling 15 min; (right) 10 min Lower: 5 min

11 Attempts For Copper Electrodeposition Current Density (ma/cm 2 ) time (s) Current transient: Cu in holes Left: Current transient of DC potentiostatic electrodeposition of Cu into nanoholes with a conductive Cu layer. Right: cross-sectional SEM image of nanowires formed in nanoholes by electrodepsotion

12 Summary of Current Status Regular and uniform templates can be obtained Controllable hole dimension (15 ~ 40 nm) to form nanocontacts at the barrier hole position Versatile template for nanofabrication: spatially confined spin valves and magnetic constrictions nm nm Al 2 O 3 membrane Cu Al 2 O 3 membrane conductive support Co constrictions

13 Future Work Growth of isolated Cu nanowires by electrodeposition Growth of isolated Cu/FeCoNiCu/Cu multilayered nanostructure by electrodeposition Growth of Ni nanocontact Opportunities in (just a few examples): atom probe studies of interfaces nanoscale magnetism coupling molecular/metallic systems at the nanoscale