Stability of cubic tin sulphide nanocrystals: role of ammonium chloride surfactant headgroups.

New semiconducting metastable cubic phases have been recently discovered in the tin monosulfide and monoselenide systems. Surface energy calculations and experimental studies indicate that this cubic π-phase is stabilized by specific ligand adsorption on the surface. In this work, it is shown experimentally that the synthesis carried out using mixtures of oleylamine and oleylammonium chloride (OACl) surfactants results in the cubic phase, transforming the growth from orthorhombic to cubic nanoparticles with increasing OACl concentration up to a limiting point. Complementary ab initio calculations find that adsorbed ligands lower the surface energies for both the cubic phase and the orthorhombic phase, relative to the pristine surfaces. The decrease in the surface energy increases with ligand coverage. Stronger binding energies to the cubic phase suggest a higher coverage, and therefore preferential stabilization of this phase. Upon further increasing the coverage, the surface energy becomes negative, effectively destabilizing the particles in agreement with experimental observations. Bonding analysis shows that Cl bonds to Sn and replaces missing Sn-S bonds at the surface of the cubic structure. In the competing orthorhombic layered phase, Cl also bonds to a Sn atom but at the expense of one of the Sn-S bonds of this Sn atom. This observation can explain the trends of the surface energies. This combined experimental and computational analysis sheds light on the stabilization processes of these nano-materials and indicates the path to improve synthetic routes.

Beneficial Impurities and Phase Control in Colloidal Synthesis of Tin Monoselenide.

The effect of impurities in colloidal synthesis of SnSe in oleylamine surfactant was investigated. Specifically, it was found that impurities such as water, hydrochloric acid, and carbon dioxide stabilize the recently discovered cubic phase of tin monoselenide, π-SnSe. We describe the reaction that releases HCl to the reaction medium through reaction of SnCl2 with moisture and its subsequent reaction with oleylamine, transforming it from neutral to charged surfactant. A similar path occurs in the case of CO2, which reacts with oleylamine to give charged oleylammonium-oleylcarbamate molecular pairs. By exposing the reaction to controlled concentrations of "beneficial contaminants", hitherto a major source of irreproducibility in this synthesis, we demonstrate phase and shape control from π-SnSe cubes to α-SnSe rods.

Mapping Charge Distribution in Single PbS Core - CdS Arm Nano-Multipod Heterostructures by Off-Axis Electron Holography.

We synthesized PbS core-CdS arm nanomultipod heterostructures (NMHs) that exhibit PbS{111}/CdS{0002} epitaxial relations. The PbS-CdS interface is chemically sharp as determined by aberration corrected transmission electron microscopy (TEM) and compared to density functional theory (DFT) calculations. Ensemble fluorescence measurements show quenching of the optical signal from the CdS arms indicating charge separation due to the heterojunction with PbS. A finite-element three-dimensional (3D) calculation of the Poisson equation shows a type-I heterojunction, which would prevent recombination in the CdS arm after optical excitation. To examine charge redistribution, we used off-axis electron holography (OAEH) in the TEM to map the electrostatic potential across an individual heterojunction. Indeed, a built-in potential of 500 mV is estimated across the junction, though as opposed to the thermal equilibrium calculations significant accumulation of positive charge at the CdS side of the interface is detected. We conclude that the NMH multipod geometry prevents efficient removal of generated charge carriers by the high energy electrons of the TEM. Simulations of generated electron-hole pairs in the insulated CdS arm of the NMH indeed show charge accumulation in agreement with the experimental measurements. Thus, we show that OAEH can be used as a complementary methodology to ensemble measurements by mapping the charge distribution in single NMHs with complex geometries.

New nanocrystalline materials: a previously unknown simple cubic phase in the SnS binary system.

We report a new phase in the binary SnS system, obtained as highly symmetric nanotetrahedra. Due to the nanoscale size and minute amounts of these particles in the synthesis yield, the structure was exclusively solved using electron diffraction methods. The atomic model of the new phase (a = 11.7 Å, P2(1)3) was deduced and found to be associated with the rocksalt-type structure. Kramers-Kronig analysis predicted different optical and electronic properties for the new phase, as compared to α-SnS.