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Periodic and aperiodic two-dimensional nanostructures with hierarchical order have been prepared by a combined top–down/bottom–up approach. This method allows 7 nm nanoparticles to be positioned with a accuracy of 10 nm or less, with a separation distance of several micrometers. The Figure is an optical dark field microscopy image of a square arrangement of Au dots on a Si wafer.

the yield of functional circuits was limited to about 80%.More robust constructions should help to circumvent thisproblem in the future.This demonstration of fabrication of a complex, electri-cally and electrochemically functional system uses self-assembly in four ways: i) in recognition of the “lock” and“key” components, by exploiting hydrophobic interactionsbetween surfaceswith complementary shapes; ii)in tailoringthe surface properties of the subunits, by forming a hydro-philic SAM on the exposed surface of gold to prevent coat-ingwith the liquid adhesive;iii) injoining the subunits,byse-lectively forming a thin film of adhesive on the hydrophobicsurfaces by interaction of these surfaces with droplets of thisadhesive suspended in water; iv) in containing the mercurydrop as a coherent structure in the copper cup. The successof this process depends on forming the surfaces of the sub-units designed to interact into precise, complementaryshapes, and on patterning these surfaces into hydrophobicand hydrophilic regions. To achieve electrical contact repro-ducibly, some degree of mechanical freedom was required.The liquid mercury provided the system with the requiredflexibility: as long as the cathode wire of the LED touchedthe mercury,electrical contact was maintained.Simple shape complementarity, in combination with flu-idic shear and gravitational forces, has been used by Smithand co-workers in an elegant demonstration of the fillingof shaped depressions in a planar substrate by small micro-electronic devices.