Templated Self‐Assembly of Colloidal Nanoparticles Controlled by Electrostatic Nanopatterning on a Si3N4/SiO2/Si Electret

Abstract

Metal and semiconductor nanoparticles display fascinating size-dependent structural, electronic, optical, magnetic, and chemical properties, which make them promising materials to be tailored and functionalized as fundamental building blocks for emerging nanotechnology applications. Because of the strong dependence of nanoparticle properties on their quantum-scale dimensions, the synthesis of nanoparticles with a small size and shape variation is of key importance. At present, size-, shape-, composition-, and surface-chemistry-controlled nanoparticles can be synthesized by colloidal-solution methods for a wide range of materials. Furthermore, strategies have been developed in which monodispersed nanoparticles can form self-assembled, long-range nanoparticle lattices (2D and 3D) under appropriate conditions. The next important challenge for emerging nanotechnological applications (chemical and biological sensing, electronics, optoelectronics) is to perform controlled and hierarchical self-assembly of monodispersed nanoparticles from the solution phase into ordered and specifically designed nanoparticle structures immobilized on solid-surface templates. In this paper, we report an approach for controlled assembly of metal (Au) and semiconducting (CdSe/ZnS core/shell) thiol-terminated nanoparticles onto electrically patterned Si3N4/SiO2/Si (NOS) electret films with an unprecedented resolution. In the past few years, several important breakthroughs in scanning-probe-based lithographic techniques using tip-induced local electrochemical reactions of self-assembled monolayers (SAMs) or tip-induced local-transport processes, such as dip-pen nanolithography, were developed for fabricating surface templates, which can be used to assemble nanoparticles on solid supports. In these techniques, the patterning process is realized by molecular reaction or transport through a water meniscus that naturally occurs between the tip and sample under ambient conditions. Therefore, the typical resolution and writing speed of such techniques is controlled by parameters such as probe scan speed, temperature, humidity, and molecule type. Furthermore, the reaction or transport rate is limited by the reaction or ink-transport process. As a result, these types of lithographic mechanism limit the line-writing speed to the range 0.1–10 lm s and the dotwriting time to the range of a few milliseconds to tens of seconds per dot (size-dependent) under ambient conditions. Recently, electrostatic-force-based assembly of nanoparticles has been proposed as a general, precise, and reliable methodology for such purposes. In the most direct type of electrostatic-force-based assembly, charge patterns are created by scanning-probe or microcontact charging techniques onto electret materials via electronor hole-tunneling processes. As the electret materials can retain electric charge or polarization for a long time, these charge patterns can be used as templates for assembling charged or polarizable nanoparticles. The major advantage of electrostatic lithography is that, in contrast to diffusion processes, the patterning speed can be enhanced over three orders of magnitude. However, because electrostatic forces scale with surface area and particle size, nanoscale electrostatic assembly of nanoparticles is a formidable task. Xerography (a form of electrophotography) is currently one of the prevailing methods for pattern generation or replication, with ∼ 100 lm resolution. In this process, charged toner particles (with diameters in the range of several micrometers) are attracted by electrostatic-charge patterns (latent electrostatic images) created on a photosensitive electret, and the images are developed with these toner particles. Very recently, nanoscale xerography (nanoxerography) using an electrical microcontact printing process or a scanningprobe-based process was proposed as a means for nanoscale pattern generation or replication. In this work, by using an optimized electret structure (NOS dielectric stack), improved local-charging conditions under high vacuum, and chemically modified nanoparticles (5 nm thiol-terminated gold colloids, see the literature and the Experimental section for the synthesis procedure), we are able to perform selective attachment of gold colloidal nanoparticles onto the patterned electret in a toluene solution at a resolution of ∼ 30 nm. Furthermore, in our process, only a C O M M U N IC A IO N S