Template Assisted Electrostatic Assembly of Colloidal Au Nanoparticles

Abstract

Selective assembly of nanoparticles onto patterned surfaces using template-assisted convection driven self-assembly methods have recently gained much attention since it may make possible integration of nanoparticles to active device structures [1,2]. Convection driven assembly utilizes the increased capillary forces at the vapor-liquid-surface contact line during controlled solvent evaporation to assemble nanoparticles onto nanostructrued surfaces which normally takes several tens of minutes. Here, we present a method where convection driven nanoparticle assembly on structural templates was enhanced by the application of an electrostatic field between the substrate and counter electrode, allowing nanoparticle assembly at far reduced processing time. The potential difference applied between the substrate and the counter electrode creates an electric field, which allows negatively charged nanoparticles to be guided to the substrate surface.

We use an n-type Si substrate coated with thermally evaporated Al. The 20 nm thickness Al is oxidized to act as an insulating layer. The insulator acts to prevent Au nanoparticle agglomeration by allowing charges to remain on the nanoparticle. An array of 40 x 40 nm2 holes were patterned in 150 nm thick PMMA using electron beam lithography, which acted as the structural template for nanoparticle assembly. A drop of solution containing high density 20 nm diameter colloidal Au nanoparticles was placed on the patterned surface. Then the counter electrode was placed on the liquid surface and 5~10 V was applied between the counter electrode and substrate to initiate electrophoresis. As shown in figure 2, nanoparticles were assembled mostly inside the resist hole structural template and not on the resist surface. The nanoparticle diameter dnc / linewidth lW = 0.5, which means that the maximum number of nanoparticle density per hole for monolayer coverage is 2. However, we observed an average site density 1.3. This was attributed to electrostatic repulsion of existing nanparticles, preventing additional nanoparticles assembling into the hole structures. We will present template assisted electrostatic assembly results on patterns with differing dnc/lW ratios to investigate the relationship between the capillary forces and applied electric fields. Also, capacitive interaction between the nanoparticles and substrate will be investigated for possible application of the assembly process to the fabrication of nanoscale charge storage devices.