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involve electrostatic interactions,[2] sedimentation,[3] and solvent evaporation.[4] Here we present two approaches that we have successfully demonstrated for fabricating 2D and 3D ordered arrays of nanoparticles.[5] The first technique utilizes a combination of IgCP, selective dewetting, confined crystallization, and thermal reactions to form 2D ordered arrays of magnetic nanoparticles on Si substrates. The second technique utilizes confined self-assembly under continuous sonication to form 3D crystalline assemblies from aqueous dispersions of colloidal particles. EXPERIMENT 2D Ordered Arrays by Microcontact Printing Figure 1A shows the precedure for making 2D arrays of nanoparticles on Si substrates. After being cleaned and rinsed with deionized water, the surface of a Si wafer was patterned by gCP with a 0.2% solution (in toluene) of octadecyltrichlorosilane (OTS). The patterned wafer 115 Mat. Res. Soc. Symp. Proc. Vol. 571 © 2000 Materials Research Society

was then immersed in a 0.01 M nitrate solution [for example, Co(N0 3)2 or Ni(N0 3)2] in isopropanol. Upon removal from the solution, the liquid remaining on the Si surface selectively dewetted the hydrophobic regions and formed very small droplets on the patterned grids of hydrophilic regions. After the alcohol evaporated, a small particle of the nitrate salt was formed in each hydrophilic region. These nitrate particles were converted into metal oxides by heating the substrate in air at 600 'C. Further heating at 400 'C in a flow of H 2 reduced the oxide to the magnetic metal.

A

B F Native oxide

I

Microcontact printing

with CH3 (CH2 )17 SiCI3

f

'

SAM of -Si(CH2 )17 CH3

Dipping into an isopropanol

nitrate solution and withdraw Nitrate solution 'Npfiotoresist frame (-12 g±mthick) 1) Assemble 480-nm particles 2) Assemble nanoparticles f3) Dry and remove top substrate

1) Drying at room temperature 0

2) Heating at 600 C in air MI oxide Metal

I

Reduction by H2 at 400"C

[1111•1111Si

F

Metal particle

"111 Ii

Figure 1. (A) Schematic procedure for the formation of 2D arrays of magnetic particles. (B) Schematic outline for the self-assembly of monodispersed particles into ordered 3D arrays. 3D Crystalline Arrays by Confined Self-Assembly Confined self-assembly was used to obtain 3D crystalline arrays of nanoparticles (Figure IB). A packing cell was formed from a frame of photoresist (Microposit 1075, Shipley, MA)

116

between two glass slides forming a hollow rectangular area. The formation of this packing cell has been described in detail elsewhere. [6] A hole was drilled or chemically etched into the top substrate over which a glass tube was attached, allowing for injection of aqueous dispersions of nanoparticles into the cell. Monodispersed polystyrene (PS) beads and silica colloids (50 and 100 nm in diameter) were diluted to -0.05% (wt) with deionized water prior to use. The aqueous solvent drained from the cell through channels selectively patterned in one side of the frame. A slight N2 pressure assisted the flow of solvent through the chann