Molecular ligands guide individual nanocrystals to a soft-landing alignment on surfaces

Elongated CdSe nanocrystals (nanorods) coated with long phosphonate ligands are deposited by pulse valve injection under ultrahigh vacuum on atomically clean surfaces. The alignment of individual CdSe nanorods is observed on the Si(100)-2x1:H surface by scanning tunneling microscopy, and on the basal plane of graphite by scanning electron microscopy. From calculations, the minimum van der Waals interaction energies between isolated molecular ligands and the surfaces occurs along specific directions. These tiny interactions guiding the orientated soft-landing of the CdSe nanorods show evidence of collective ligand effects.