Self-assembly and polarizability of capped CdS nanocrystals on silicon surfaces

The zinc-blende CdS nanocrystals with particle size less than 10nm were prepared in solution with a molecular surfactant. The particle surface was modified with capping molecules, bis(2-ethylhexyl) sulfosuccinate disodium salt (AOT) or hexametaphosphate (HMP). The colloid solution was deposited on silicon wafer and the assembly of these nanocrystals was studied by atomic force microscopy. It was found that the CdS/AOT formed a monolayer by simply dropping the colloid solution on silicon surface, while CdS/HMP exhibited 3D structure with prefered orientation in the similar deposition process. A dynamic electric force microscopy was performed and the polarizability was measured from the resonance of cantilever under applied alternating voltage between tip and substrate. The dielectric constant of CdS nanocrystal was estimated by comparing the different electric capacitance of Au and CdS nanocrystals.