Energetics of Polar and Nonpolar Facets of PbSe Nanocrystals from Theory and Experiment

Surface energies of the distinct facets of nanocrystals are an important factor in the free energy and hence determine the nanocrystal morphology, chemical and physical properties, and even interparticle dipole interactions. Here we investigate the stability and atomic structure of polar and nonpolar PbSe surfaces by combining first-principles calculations with high-resolution transmission electron microscopy (TEM). For uncapped surfaces, the calculations predict that the nonpolar {100} surface is the most stable with a surface energy of 0.184 J m−2, while the nonpolar {110} and reconstructed {111}-Pb surfaces have surface energies of 0.318 J m−2 and 0.328 J m−2, respectively. Fully polar {111} surfaces are structurally unstable upon relaxation. These findings are in good agreement with TEM observations showing that capped nanocrystals have a nearly spherical, multifaceted morphology, while cubical shapes with predominantly {100} facets are obtained when the capping molecules are removed through heating in vacuum. During this process, however, also multipolar surfaces can temporarily exist just after the removal of the surfactants. These metastable {111} surfaces consist of ribbon-like nanodomains, whereby the ribbons are alternating in polarity. The calculations confirm that these multipolar surfaces are energetically more favorable than fully polar surfaces. The consequences for capped nanocrystals (a dominant Pb-oleate termination) and nanocrystal fusion (a shorter interaction range of dipole interactions) are discussed

Morphological Transformations and Fusion of PbSe Nanocrystals Studied Using Atomistic Simulations

Molecular dynamics simulations are performed on capped and uncapped PbSe nanocrystals, employing newly developed classical interaction potentials. Here, we show that two uncapped nanocrystals fuse efficiently via direct surface attachment, even if they are initially misaligned. In sharp contrast to the general belief, interparticle dipole interactions do not play a significant role in this “oriented attachment” process. Furthermore, it is shown that presumably polar, capped PbSe{111} facets are never fully Pb- or Se-terminated

Chemical Transformation of Au-Tipped CdS Nanorods into AuS/Cd Core/Shell Particles by Electron Beam Irradiation

We demonstrate that electron irradiation of colloidal CdS nanorods carrying Au domains causes their evolution into AuS/Cd core/shell nanoparticles as a result of a concurrent chemical and morphological transformation. The shrinkage of the CdS nanorods and the growth of the Cd shell around the Au tips are imaged in real time, while the displacement of S atoms from the CdS nanorod to the Au domains is evidenced by high-sensitivity energy-dispersive X-ray (EDX) spectroscopy. The various nanodomains display different susceptibility to the irradiation, which results in nanoconfigurations that are very different from those obtained after thermal annealing. Such physical manipulations of colloidal nanocrystals can be exploited as a tool to access novel nanocrystal heterostructures

Morphological Transformations and Fusion of PbSe Nanocrystals Studied Using Atomistic Simulations

Molecular dynamics simulations are performed on capped and uncapped PbSe nanocrystals, employing newly developed classical interaction potentials. Here, we show that two uncapped nanocrystals fuse efficiently via direct surface attachment, even if they are initially misaligned. In sharp contrast to the general belief, interparticle dipole interactions do not play a significant role in this “oriented attachment” process. Furthermore, it is shown that presumably polar, capped PbSe{111} facets are never fully Pb- or Se-terminated

Chemical Transformation of Au-Tipped CdS Nanorods into AuS/Cd Core/Shell Particles by Electron Beam Irradiation

We demonstrate that electron irradiation of colloidal CdS nanorods carrying Au domains causes their evolution into AuS/Cd core/shell nanoparticles as a result of a concurrent chemical and morphological transformation. The shrinkage of the CdS nanorods and the growth of the Cd shell around the Au tips are imaged in real time, while the displacement of S atoms from the CdS nanorod to the Au domains is evidenced by high-sensitivity energy-dispersive X-ray (EDX) spectroscopy. The various nanodomains display different susceptibility to the irradiation, which results in nanoconfigurations that are very different from those obtained after thermal annealing. Such physical manipulations of colloidal nanocrystals can be exploited as a tool to access novel nanocrystal heterostructures

Nanogold: A Quantitative Phase Map

The development of the next generation of nanotechnologies requires precise control of the size, shape, and structure of individual components in a variety of chemical and engineering environments. This includes synthesis, storage, operational environments and, since these products will ultimately be discarded, their interaction with natural ecosystems. Much of the important information that determines these properties is contained within nanoscale phase diagrams, but quantitative phase maps that include surface effects and critical diameter (along with temperature and pressure) remain elusive. Here we present the first quantitative equilibrium phase map for gold nanoparticles together with experimental verification, based on relativistic ab initio thermodynamics and in situ high-resolution electron microscopy at elevated temperatures

Morphological Transformations and Fusion of PbSe Nanocrystals Studied Using Atomistic Simulations

Molecular dynamics simulations are performed on capped and uncapped PbSe nanocrystals, employing newly developed classical interaction potentials. Here, we show that two uncapped nanocrystals fuse efficiently via direct surface attachment, even if they are initially misaligned. In sharp contrast to the general belief, interparticle dipole interactions do not play a significant role in this “oriented attachment” process. Furthermore, it is shown that presumably polar, capped PbSe{111} facets are never fully Pb- or Se-terminated

Anisotropic Cation Exchange in PbSe/CdSe Core/Shell Nanocrystals of Different Geometry

We present a study of Cd²⁺-for-Pb²⁺ exchange in PbSe nanocrystals (NCs) with cube, star, and rod shapes. Prolonged temperature-activated cation exchange results in PbSe/CdSe heterostructured nanocrystals (HNCs) that preserve their specific overall shape, whereas the PbSe core is strongly faceted with dominance of {111} facets. Hence, cation exchange proceeds while the Se anion lattice is preserved, and well-defined {111}/{111} PbSe/CdSe interfaces develop. Interestingly, by quenching the reaction at different stages of the cation exchange new structures have been isolated, such as core–shell nanorods, CdSe rods that contain one or two separated PbSe dots and fully zinc blende CdSe nanorods. The crystallographically anisotropic cation exchange has been characterized by a combined HRTEM/HAADF-STEM study of heterointerface evolution over reaction time and temperature. Strikingly, Pb and Cd are only intermixed at the PbSe/CdSe interface. We propose a plausible model for the cation exchange based on a layer-by-layer replacement of Pb²⁺ by Cd²⁺ enabled by a vacancy-assisted cation migration mechanism

Stabilization of Rock Salt ZnO Nanocrystals by Low-Energy Surfaces and Mg Additions: A First-Principles Study

Whereas bulk zinc oxide (ZnO) exhibits the wurtzite crystal structure, nanoscale ZnO was recently synthesized in the rock salt structure by addition of Mg. Using first-principles methods, we investigated two stabilization routes for accessing rock salt ZnO. The first route is stabilization by Mg addition, which was investigated by considering ZnO–MgO mixed phases. The second route is through size effects, as surface energies become dominant for small nanocrystal sizes. We discovered that the surface energy of rock salt ZnO is surprisingly low at 0.63 J m^–2, which is lower than those of wurtzite and zinc blende ZnO and lower than that of rock salt MgO. We predict that pure rock salt ZnO is stable for nanocrystals smaller than 1.6 nm, and that Mg additions can greatly extend the size range in which the rock salt phase is stable. Both mixed-phase and core–shell models were considered in the calculations. The present approach could be applied to predict the stabilization of many other nanocrystal phases in deviating crystal structures