Electrophoretic deposition of multifunctional nanocrystal films

An alternative to the conventional dry cast and spin cast methods of forming films of nanocrystals is electrophoretic deposition. A pair of Au-on-Si electrodes was submerged in a nanocrystal solution (∼3 nm diameter CdSe/TOPO nanocrystals in hexane) and high dc voltage was applied across them in a dark room. Uniform, identical, ∼μm-thick films deposited on both the positive and negative electrodes after a typical 30 min run. Photoluminescence (PL) of the films showed that they were indeed made of dense arrays of the nanocrystals. The film thickness depended on the initial particle density in the solution, the applied dc voltage, and deposition time. Optical micrographs, scanning electron microscope (SEM) and atomic force microscope (AFM) images showed that the films had very few defects, and that the rms roughness of the films was <2.5 nm. With patterned electrodes patterned films with features as small as 1 μm were formed. Treatment of the electrode surface with an adhesion promoter before deposition and of the electrophoretically deposited films afterwards by linker molecules makes the films much more stable in solvents such as hexane, chloroform, methanol, acetone and chlorobenzene.; The electrophoretic deposition method was found to work for other electrodes (e.g., ITO) and some other nanocrystals (e.g., maghemite) also.; In addition to these single component nanocrystal films, films were also deposited from mixtures of nanocrystals. Electrophoretic deposition from mixtures of CdSe and maghemite nanocrystals in hexane led to the formation of identical films on the positive and negative electrodes composed of both types of nanocrystals. In contrast, electrophoretic deposition from mixtures of CdSe and Au nanocrystals in hexane led to the formation of films of nanocrystals on only the positive electrode and these films were composed of only CdSe nanocrystals; the formation of films on the negative electrode was suppressed. As the density of charged Au nanocrystals was decreased below that of charged CdSe nanocrystals, the suppression of CdSe nanocrystal film formation on the negative electrode decreased, and films composed of CdSe nanocrystals formed on both electrodes.; Preliminary studies on the electrical, thermal, and mechanical properties of these films are reported. Interdot coupling in dry cast films of small and large CdSe nanocrystals is also reported.