Intact sublimation of silicon nanocrystals evidenced via HREM imaging and EELS in a dedicated STEM

Silicon nano crystals (NCs) have attracted considerable interest for possible uses in optoelectronics1 As the particle size decreases the properties of NCs become increasingly sensitive to the surface termination.2, 3 Monolayer chemistries4-10 have been exploited to control the physicochemical properties. NCs are often prepared by vapour-phase deposition techniques; using these they can be conveniently analysed via gas phase analysis techniques, such as mass spectrometry. This cannot be employed, however, if NCs are not synthesized in the gas phase. Here we present a STEM study of undecyl-capped SiNCs, evaporated intact upon heating in ultrahigh vacuum at 200 °C and collected on a variety of solid substrates, including carbon-coated TEM grids. The BF- and HAADF lattice images confirm that the particles have a crystalline core with Si-lattice spacings. The presence of Si in the core is also confirmed by Si-L edge EELS, which reveals furthermore the presence of a surface oxide.