The response of charge to externally applied electric fields is an important basic property of any material system, as well as one critical for many applications. Here, we examine the behaviour and dynamics of charges fully confined on the nanometre length scale. This is accomplished using CdSe nanocrystals of controlled radius (1-2.5 nm) as prototype quantum systems. Individual electron-hole pairs are created at room temperature within these structures by photoexcitation and are probed by terahertz (THz) electromagnetic pulses. The electronic response is found to be instantaneous even for THz frequencies, in contrast to the behaviour reported in related measurements for larger nanocrystals and nanocrystal assemblies. The measured polarizability of an electron-hole pair (exciton) amounts to approximately 10(4) A(3) and scales approximately as the fourth power of the nanocrystal radius. This size dependence and the instantaneous response reflect the presence of well-separated electronic energy levels induced in the system by strong quantum-confinement effects.

In this work, we have developed a new fabrication method for nanoparticle (NP) assemblies for Li-ion battery electrodes that require no additional support or conductive materials such as polymeric binders or carbon black. By eliminating these additives, we are able to improve the battery capacity/weight ratio. The NP film is formed by using electrophoretic deposition (EPD) of colloidally synthesized, monodisperse cobalt NPs that are transformed through the nanoscale Kirkendall effect into hollow Co(3)O(4). EPD forms a network of NPs that are mechanically very robust and electrically connected, enabling them to act as the Li-ion battery anode. The morphology change through cycles indicates stable 5-10 nm NPs form after the first lithiation remained throughout the cycling process. This NP-film battery made without binders and conductive additives shows high gravimetric (>830 mAh/g) and volumetric capacities (>2100 mAh/cm(3)) even after 50 cycles. Because similar films made from drop-casting do not perform well under equal conditions, EPD is seen as the critical step to create good contacts between the particles and electrodes resulting in this significant improvement in battery electrode assembly. This is a promising system for colloidal nanoparticles and a template for investigating the mechanism of lithiation and delithiation of NPs.

A dc electric field is used to attract charged CdSe nanocrystals in hexane to rapidly form very smooth, robust, large-area, several micron-thick films of equal thickness on both electrodes. This deposition on both electrodes implies there are both positively and negatively thermally charged dots, unlike conventional electrophoretic deposition. With patterned electrodes, controllable and locally selective assembly is achieved.

Controlled purification of the CdSe nanocrystal primitive solution critically affects the electrophoretic deposition of these nanocrystals. Film morphology is optimized after two precipitation steps with no TOPO/TOP added in either step. This suggests the need to minimize impurities in the solution or to remove a controlled, small fraction of the ligands on the nanocrystal surface. The smoothness of electrophoretically deposited nanocrystal films is excellent (<3 nm rms roughness) and the appearance of particulates of nanocrystals due to homogeneous nucleation is minimized, even during patterned deposition. Treating the films with cross-linker molecules makes the electrophoretically deposited films very resistant to dissolution and flaking in many solvents.