Synthesis of Nanostructures by Electrochemical Processing

Transcription

1 Synthesis of Nanostructures by Electrochemical Processing Giovanni Zangari, Robert M. Metzger, Bill Butler University of Alabama at Tuscaloosa The work presented was partly sponsored through DOD grant no. DAAH MINT Fall Review November 7, 2001

2 Porous Al oxide: a versatile nanostructured template Synthesis of ordered Al oxide porous structures Growth of magnetic materials in Al oxide: particles and wires Perspectives and opportunities in manotechnology: magnetism electronics hybrid systems Outline

3 Anodization of Aluminum Electrochemical oxidation Morphology controlled by selection of the electrolyte film forming conditions high efficiency, neutral electrolytes: barrier films acid or alkaline electrolytes: porous films Main electrolytes: di- and tribasic acids H 3 PO 4, (COOH) 2, H 2 SO 4 A Aluminum POWER SUPPLY + - Graphite 2Al + (3O 2- ) ox = Al 2 O 3 + 6e - H 2 O = 2H + + (O 2- ) aq (O 2- ) aq = (O 2- ) ox C Al = Al e -

4 Morphology of Porous Al Oxide Cell diameter D c : D c (nm) = V (V< 20 V) D c (nm) = V (V> 20 V) Pore diameter D p : D p (nm) = V increases with ph increases with T Pore size: 5 nm 1 µm Pore length up to 100 µm

5 Pore Ordering Process: n-step anodization Electropolishing to flatten the Al surface and form a mound pattern. Anodization of Al in (COOH) 2 for 0.5 to 12 hours Dissolution oxide film in HgCl 2. Further anodization of Al. H. Masuda, K. Fukuda; Science 268, 1466 (1995) 30 min 12 h

6 Long-range Order Ordering process slowed down at defects, impurities

7 Ordered Area, S ord / µm Limits in Long-range Order Polycrystalline Al, % n * 10 min 2 µm 2 /h n * 1 hour one-cycle Ordering rate = 0.25 µm 2 /h Total anodization time, t = n t c / hours V. Konovalov et al; Proc. ECS, Vol Metzger et al; IEEE MAG 36, 30 (2000) Area of ordered regions increases linearly with time Intrinsic limits due to the chemical corrosion of Al in the anodizing electrolyte

8 True Long-range Order in Al-oxide Nano-molding using a SiC stamper Masuda et al. APL 71, 2770 (1997) Formation of anodization windows by optical lithography Li et al. Electrochem. Sol. St. Lett. 3 (2000) Induced ordering is feasible

9 Normalized Magnetization 1 0 AC-ECD of Co in Al-oxide Out of plane In-plane 120 nm Field (Oe) Counts Mean 1203 nm STD 197 nm ratio = 16% Particle length (nm) 10 nm STD/<L> = 16 % no reorientation of the anisotropy

10 Pulse-Reverse Electrodeposition 500 Hz asymmetric pulse-reverse electrodeposition of Co, 10% cathodic, 90% anodic Impose high nucleation density, increase relaxation time Cathodic Current (A) Cathodic Potential (V) Deposition time (s)

11 Normalized Magnetization Normilized Magnitization Field (Oe) Out of plane 120 nm PR-ECD of Co in Al oxide In-plane In-plane Out of plane 5 nm Field (Oe) Counts Co particle length (nm) STD/<L> = 5 % reorientation of the anisotropy Mean 615 nm STD 32 nm Ratio = 5% (b)

12 PR-ECD Co: Nanoparticles M. Sun et al; APL 78, 2964 (2001) Co deposited by pulse-reverse electrodeposition. <L> = 74 nm, diameter 60 nm. Polycrystalline, random orientation. Standard deviation of particle length ~ 0

13 PR-ECD Co: Nanowires [00.2] Single crystalline nanowires Layered structure probably due to the PR process HCP c-axis perpendicular to the layer structure

14 Nanoholes in Al Oxide Bottom Topography of Al Oxide After Ar + Ion Milling Controllable hole dimension can provide nano-sized constrictions T. Xu et al; Nanoletters, in press

15 Summary of Current Status Regular and uniform templates Uniform magnetic nanostructures Polycrystalline nanoparticles, transforming to single crystalline nanowires Versatile template for nanofabrication nm nm nm Al 2 O 3 membrane Al 2 O 3 membrane Al 2 O 3 membrane conductive support

16 Atom Probe Analysis of Multilayer Structures Atomic Probe Field Ion Microscopy has the capability to characterize local structure and composition of multilayers with quasi-atomic resolution ideal for the study of interfaces Specimen preparation is challenging, requiring patterning of the film in a needle with radius ~ 50 nm

17 Nanoscale Templates for Atom Probe Studies nm Cu Al 2 O 3 After Cu ECD After Al 2 O 3 etching After Al 2 O 3 etching, metal columns of variable size would provide versatile templates for the growth of samples suitable to atom probe investigations

18 Comparative Atom Probe Studies of Multilayers GMR multilayers can be grown by sputtering or ECD ECD in Al oxide templates easily provides for CPP- GMR systems Comparison of interface quality in state-of-the-art sputtered MLs with ECD MLs P.R. Evans et al. APL 76, 481 (2000) CoNiCu/Cu MLs

19 Opportunities in Nanoscale Magnetism nm Spatially confined spin valves for current-induced switching studies Magnetic constrictions: domain wall propagation spin-dependent transport Magnetization switching nm Cu Co Ni Constriction

20 Miscellaneous Opportunities Detection of conductance quantization through nanowires Li et al; APL 72, 894 (1998) Molecular, gas sensors Li et al; APL 76, 1333 (2000) Protein bioassaying Pena et al; Science 294, 137 (2001) Derivatization of organic molecules to preferentially bind to predefined metals: organic/inorganic devices, nanoconnectors, low contact resistance

21 Outlook Availability of simple, reliable methods for the synthesis of micro- to nanoscale templates Growth of nanoparticles and nanowires is possible: not confined to metals Opportunities in (just a few examples): atom probe studies of interfaces nanoscale magnetism coupling molecular/metallic systems at the nanoscale