InterNano - a project of the National Nanomanufacturing Network - Pulse-reversed electrodeposition with QCM monitoring to make magnetic nanowires in a nanotemplate

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Process Description #6

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6. Pulse-reversed electrodeposition with QCM monitoring to make magnetic nanowires in a nanotemplate

Cobalt nanowires with high perpendicular magnetic anisotropy are formed in a diblock copolymer film template using a pulse-reversed voltage with QCM monitoring. This in situ monitoring system along with the pulse-reversed field enables new control over the magnetic crystal growth.

Contributors: Andrei Ursache, Mark Tuominen, T. P. Russell
Lab: NSF Center for Hierarchical Manufacturing

Manufactured Nanomaterial or Structure: Cobalt nanowires (15 nm diameter, 500 nm long, in a hexagonally packed formation with 24 nm spacing.)
Chemical composition: Cobalt, CoO impurities
Physical Form: High aspect ratio nanowires
Properties: High perpendicular magnetic anisotropy, coercivities as high as 2.7 kOe at 300K, CoO impurities

Process:

1.	A gold-coated 5 MHz AT-cut crystal with a PS template acts as the electrodeposition template.
2.	An electrolyte solution consisting of deionized water, methanol, cobalt sulfate, and boric acid is formed. To maintain a pH of 6.0, sodium tetraborate is added.
3.	With the sample in solution, two pulses are applied (-1.0 and -0.35 V) for 10 ms each. The intention is to remove atoms with low binding energies from the surface to maintain better crystal formation.
4.	By monitoring the resonant frequency of the (Maxtek RQCM) QCM and electrodeposition current simultaneously one can control the length of the nanowires accurately and have a full knowledge of current efficiency. Dotted line: electrode potential, solid line: current, dashed line: EQCM frequency change Dotted line: electrode potential, solid line: current, dashed line: EQCM frequency change XRD data for pulse-reversed Co nanowire array XRD data for pulse-reversed Co nanowire array

Notes: A pH of 6.0 is important to maintain is needed to get the preferred crystalline orientation of the cobalt nanowires. This of course would differ for other materials.

Raw Materials

P(S-b-MMA) block copolymer as described in this process (link to BCP process)
Quartz crystal microbalance
Saturated calomel electrode (SCE), reference electrode
Platinum foil, counter electrode
Electrolyte consisting of deionized water, methanol (surfactant, CH3OH, 20% by volume), cobalt sulfate (CoSO4·7H2O, 1 M), and boric acid (H3BO3, 0.6 M)
Sodium tetraborate for pH adjustment (Na2B4O7·10H2O, 0.1 M)

Environmental Variables: Clean room environment

Tags: Block copolymers, Electrochemical quartz crystal microbalance (QCM), Electrodeposition, Electrodeposition, Magnetic

DOI: 10.4053/nnn071201pr/1

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