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

Process Database

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 Material 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
Material Properties: 
High perpendicular magnetic anisotropy, coercivities as high as 2.7 kOe at 300K, CoO impurities
Step 1:

A gold-coated 5 MHz AT-cut crystal with a PS template acts as the electrodeposition template.

Step 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.

Step 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.

Step 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
Process 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

Equipment Requirements: 

Maxtek RQCM

References: 
A. Ursache, J.T. Goldbach, T.P. Russell, and M.T. Tuominen, J. Appl. Phys. 97, 10J322 (2005). doi:10.1063/1.1861393
DOI: 
10.1063/1.1861393
A. Ursache, M. Bal, J.T. Goldbach, R.L. Sandstrom, C.T. Black, T.P. Russell, and M.T. Tuominen, Mater. Res. Soc. Symp. Proc. 721, E3.e (2002)
T. Thurn-Albrecht, J. Schotter, G.A. Kästle, N. Emley, T. Shibauchi and L. Krusin-Elbaum et al., Science 290 p. 2126 (2000). doi:10.1126/science.290.5499.2126
DOI: 
10.1126/science.290.5499.2126
InterNano Taxonomy:
DOI: 
10.4053/pr006-071201