Spin-Coating Electrostatic Self-Assembly:Fabrication Method for CdSe Nanoparticle Monolayer

Abstract

The precise positioning of functionally distinct nanoparticles (NPs) in integration of organic and inorganic materials into hybrid optoelectronic structures appears to be an essential prerequisite for the realization of the high-performance electronic and optical devices. Numerous attempts have been made to fabricate uniform 2D NP monolayers with using diverse techniques such as self-assembly technique, electrophoretic deposition, Langmuir-Blodgett technique, electrostatic interaction, and DNA hybridization. In general, immobilization by means of solution-dipped self-assembly is accomplished via surface modification of substrate with functional groups that provide an attractive interaction with deposited nanoparticles. Functional groups such as thiol, pyridil, amino, and carboxy can all be used to immobilize metal NPs on various oxide surfaces. The electrostatic attraction between oppositely charged entities has also been exploited for the immobilization of negatively charged gold NPs on poly(ethyleneimine)-modified substrate. To date, however, there has been no recognizable breakthrough in the preparation of large laterally extended NP monolayers for the development of applications such as metal nanoparticle memory and nanoparticle-based LED device. The spin coating electrostatic self-assembly (SCESA) method was successfully shown to allow rapid fabrication of a well-ordered structure of multilayer thin films, and to realize a variety of multilayer heterostructures on a solid substrate. Here, this method is adopted to immobilize ~6 nm diameter carboxylic acid-derivatized CdSe nanoparticle in a single layer on an amino-terminated self-assembled monolayer. The morphology of the spin-assembled NP monolayer (i.e., surface roughness and domain feature), as revealed by atomic force microscope (AFM), was compared with that of a sample prepared by solution-dip electrostatic self-assembly (ESA) method. CdSe NPs were synthesized based on the Peng’s preparation method. CdSe NPs are synthesized with hydrophobic organic capping agents such as TOPO/TDPA, which in general limits their interaction with specialized materials particularly in an aqueous environment. For the charged CdSe NPs, the hydrophophic capping ligands were exchanged with a thiol-containing organic acid, i.e. mercaptoacetic acid (MAA), as described elsewhere. The MAA-modified NPs at a concentration of (10 mg/mL) do not aggregate and remain stable in an aqueous 0.1 M TRIS buffer solution (pH = 9) over several months, indicating that the MAA layer coats the entire surface of each individual particle. The size of the modified NPs was investigated using transmission electron microscopy (TEM). The sample was prepared by placing a drop of the dispersion onto a carboncoated copper grid and allowing it to dry in air. The TEM measurements were performed on a FEI (TECNAI G F20) microscope at an accelerating voltage of 200 KV. From the TEM image analysis, the size of MAA-derivatized CdSe NPs (MAA-CdSe) was determined to 6.0 ± 0.4 nm (Figure 1). The electrophoretic mobility of the MAA-CdSe NPs was obtained from an average of five measurements in 0.1 M TRIS buffer solution (pH = 9), performed at the stationary level using an electrophoretic light scattering method (Otsuka Electronics, Photal ELS-8000). The mobilities (μ) were converted to the zeta (ζ )-potential using the Smoluchowski relation ζ = μη/e, where η and e are the viscosity and permittivity of the solution, respectively. The zeta potential of the negatively charged particles was determined to be −48.2 mV when dispersed in 0.1 M TRIS buffer. Among the several deposition methods available, we were particularly interested in SCESA, because it is expected to provide a unique condition for immobilizing NPs in a single layer on a solid substrate. Figure 2 is a simplified illustration for the immobilization of the MAA-CdSe NPs in a single layer on an amino-terminated monolayer, self-assembled during the aminopropylsilanization reaction on a silicon substrate, as described elsewhere. The preparation of a MAA-CdSe monolayer based on the SCESA method was carried out in accordance with the well-established procedures described previously. In this case, ca. 0.5 mL MAACdSe in TRIS buffer solution (~1 mg/mL, pH = 9) was deposited onto a silicon substrate, which was then spun at a speed of 5000 rpm for 20 seconds. Subsequently, 1 mL of