Electrostatic properties of maghemite (γ- Fe₂O₃) nanocrystalline quantum dots determined by electrophoretic deposition

Abstract

We applied a DC electric field between two flat electrodes to attract thermally charged maghemite (γ-Fe(2)O(3)) nanocrystalline quantum dots dissolved in hexane to form smooth, robust, large area and apparently identical films of equal thickness on both electrodes. Visible microscopy, scanning electron microscopy, atomic force microscopy and profilometry showed that the electrophoretically deposited dot films were very smooth with an rms roughness of ∼10 nm for ∼0.2 µm thick films. The films were of high quality. They did not re-dissolve in hexane (as do those formed by dry casting), which is a good solvent for these dots, or in common cleaning solvents such as water, alcohols and acetone. The deposition on both electrodes implies there are both positively and negatively thermally charged dots, unlike conventional electrophoretic deposition. We used simple thermodynamics to explain the results of electrophoretic deposition macroscopically. To connect the macroscopic nature of the deposition to the microscopic nature of the dots we performed electrophoretic mobility measurements of the dots and the results seem to complement the thermodynamic treatment.

Thermal Charging of Colloidal Quantum Dots in Apolar Solvents: A Current Transient Analysis

“…Hence, a proper, thermodynamic understanding of this preferential charging requires a view on the countercharge. Third, this work shows that charges are present in QD dispersions in apolar solvents, yet it does not address possible mechanisms of charge generation, which is clearly of importance for applications where rapid charging or discharging is important, such as electrophoretic deposition , or (avoiding of) photoionization.…”

“…An aspect of these colloidal NP dispersions in apolar solvents that systematically returned in the literature is that of dipoles − or charges on the NPs. ,,− Using an apolar solvent (2,2,4,4,6,6,8-heptamethylnonane) and excess free ligand to stabilize the dispersion, Shim et al . showed that dispersions of CdSe QDs in apolar solvents have a nonzero conductivity, a result they related to charges on the QDs.…”

“…showed that dispersions of CdSe QDs in apolar solvents have a nonzero conductivity, a result they related to charges on the QDs. Further confirmation of the fact that QDs can become charged in apolar media came from electrophoretic deposition, which was used to form thin films of CdSe and Fe 2 O 3 QDs. , Alternatively, the idea that colloidal QDs can become charged upon illumination was demonstrated more directly using electrostatic force microscopy. , In addition, Shevchenko et al . showed that additives like oleic acid and tri- n -octylphosphine oxide can change the average charge on colloidal NPs and they proposed a link between charges on colloidal NPs and the structure of binary superlattices formed upon self-assembly of dispersed NPs .…”

“…[10][11][12] An aspect of these colloidal NP dispersions in apolar solvents that systematically returned in the literature is that of dipoles [13][14][15][16][17] or charges on the NPs. 13,14,[18][19][20][21][22] Using an apolar solvent (2,2,4,4,6,6,8-heptamethylnonane) and excess free ligand to stabilize the dispersion, Shim et al showed that dispersions of CdSe QDs in apolar solvents have a nonzero conductivity, 14 a result they related to charges on the QDs. Further confirmation of the fact that QDs can become charged in apolar media came from electrophoretic deposition, which was used to form thin films of CdSe and Fe 2 O 3 QDs.…”

Toward Dynamic Control over TiO2 Nanocrystal Monolayer-by-Monolayer Film Formation by Electrophoretic Deposition in Nonpolar Solvents

“…Recently, EPD conducted on nanocrystals that were suspended in nonpolar solvents has increased in popularity, in contrast to the more traditional approach of using polar solvents (methanol, acetone, water, etc.) to produce films, coatings, and casts. − Two of the driving factors for using nonpolar solvents are: 1) to suppress the van der Waals interactions between nanocrystals while in suspension, which frequently results in particle agglomeration or aggregation before the nanocrystals reach the electrode; and 2) to minimize electrolysis of the suspension solvent, which leads to difficult-to-control hydrodynamics and electrohydrodyamics of the suspension.…”