Semiconducting metal sulphide (MS) nanoparticles (NPs) and their superlattices are in the focus of numerous researcher groups since years. These NPs are relatively easy to prepare and exhibit interesting size-dependent electrical, optical, and chemical properties which make them attractive for various applications [1]. They are used in sensor, biomedicine, and optoelectronic applications. Particulary, MS NPs can be used to enhance the energy conversion in organic solar cells [2]. Control over the NPs size uniformity, their shape, and decoration by organic ligand shell is important for their effective employment.
Further enhancement and scaling up of the NPs properties can be achieved by their assembly into superlattices. Of particular importance is the question whether NPs assemble randomly with respect to their atomic planes or along a certain direction of the atomic lattice, and what facilitates such iso-oriented attachment. Superlattices consisting of iso-oriented, single-crystalline NPs, so called mesocrystals [3], have potential for applications as photonic crystals, magnetic storage media, and in technical devices where large stiffness in combination with high elasticity is desired.
In the talk, we report on two X-ray small angle scattering (SAXS) studies of NPs formation [4] and NPs ordering into mesocrystals [5], respectively. In the first study, we applied transmission SAXS in real-time during growth of ZnS NPs. The NPs were formed by reaction of zinc chloride with elemental sulphur dissolved in oleylamine at temperatures 170 °C and 215 °C, respectively. We have simulated the time-dependent SAXS profiles by a model considering spherical ZnS NPs core decorated by organized shell of oleylamine molecules, i.e. core shell particles, which interact via the sticky hard-sphere potential. The SAXS analysis allowed for characterizing influence of temperature, reaction time, and sulphur concentration on NPs structure. In the second study, we use grazing incidence SAXS (GISAXS) to investigate ordering of PbS nano-crystals functionalized with oleic acid in mesocrystals. The mesocrystals were prepared by drying from hexane dispersion of NPs (0.5 mg/mL) on electron microscopy grids and Si 001 substrates. In both studies, the SAXS analysis is combined with X-ray diffraction characterization, to study the metal sulfide lattice of NPs core, and electron microscopy techniques to obtain microscopic images of NPs and their ordering. Additionally, for PbS mesocrystals, selected area electron diffraction (SAED) is used to get insight into alignment of lattice of PbS nano-crystals with the NPs supper-lattice orientation.
For the ZnS NPs, the real-time in situ SAXS measurements reveal that the initial nucleation of the NPs occurs in reversed micelles, which disappear when the NPs start to grow. After a rapid initial growth phase, the NPs' growth slows down after about 30 min of the reaction. Higher reaction temperature or higher sulphur concentration leads to larger nanoparticles and stronger attractive potential between them. By changing the molar ratio of sulphur to zinc salt from one to five in the initial solution, the particle size increases from 2.4 to 3.9 nm. An organized oleylamine ligand shell appears around NPs after their size exceeds certain critical diameter which is around 2.8 nm. The thickness of the shell is about 2.6 nm which matches well with the length of an oleylamine molecule. The polydispersity of NPs is around 30 % and it is independent on the employed reaction conditions.
For the mesocrystals formed of PbS nano-crystals functionalized with oleic acid, we observe body-centered tetragonal (BCT) superlattice of cuboctahedral NPs. The latice paratmers of the BCT NPs superalttice are a=b=10.7 nm and c=12.8 nm for NPs average diameter of 6.3 nm. The NPs superlattices are oriented with [011]SL and [001]SL zone axes parallel to the sample surface normal for thinner (i.e., few NPs layers) and thicker NPs films. By comparing scanning transmission electron microscopy images of the superalttice and the SAED of the nano-crystals' PbS lattices we conclude that the PbS lattice planes {100}PbS are parallel to the {100}SL planes of the NPs superlattice. At the same time, the PbS nano-crystals are oriented with [011]PbS zone axis parallel to the sample normal, i.e. parallel to the [011]SL superlattice zone axis for thinner films. We explain formation of the BCT lattice and the alignment of the nano-crystal lattice with the superlattice of the NPs by favourable mutual interaction of nano-crystals' facets covered with ligand where forces between neighbouring {100}PbS −{100}PbS and {111}PbS −{111}PbS facets are balanced.
In both studies, SAXS has crucial role for characterization of NPs and their superlattices and the employment of the scattering technique for these kind of material systems will be highlighted in the talk.