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

Abstract

M any chemical and physical processes and properties, such as crystal growth, crystal morphology, and adsorption, are controlled by the atomic structure of the surface and its energy. This is even more important for nanostructured materials, in which the surface energy gives a considerable contribution to the total energy. Knowledge of the surface structures and energies allows predicting and optimizing the synthesis and, eventually, processing of nanocrystals into genuine nanomaterials. In recent years, PbSe nanomaterials have been intensively studied because of their attractive physical properties and promises for near-infrared opto-electronic applications. 1Ϫ9 Through fusion of smaller nanocrystal units, larger nanostructures can be fabricated with a wide variety of single crystal morphologies, such as nanorods, spheres, cubes, stars, and rings. 1,10Ϫ15 The experiments suggest that the efficient oriented attachmentOthe mutual alignment of nanocrystals during or prior to fusionOcan be attributed to dipole interactions caused by the electrically charged, Pb-or Se-terminated {111} facets. 10,12Ϫ15 However, this is currently just a hypothesis. Although it is experimentally proven that the nanocrystals carry dipole moments, 16,17 the experiments are not sufficiently sophisticated to prove the presence of nonreconstructed, fully polar {111} facets, and theoretical investigations are absent.There are many experimental and theoretical studies on the properties of bulk PbSe, 18Ϫ22 while the literature on the surface structure and energy is scarce. Kimmura et al. 23 grew a PbSe {111} surface on the {111} surface of BaF 2 . From the results of the high-resolution Rutherford backscattering spectroscopy, they found that the PbSe {111} surface is dominated by Pb atoms.Moreels and co-workers prepared colloidal PbSe nanocrystals of a uniform size. 24 They also investigated the interplay between the Pb-coordinating ligands and the structure of the PbSe surfaces in solution by nuclear magnetic resonance spectroscopy (NMR). It was concluded that the ligands have a strong impact on the nanocrystal surfaces and that capped PbSe nanocrystals are nonstoichiometric with a surface that is composed mainly of oleic acid capped lead atoms. 24 Ma et al. investigated the geometric and electronic properties of PbX (X ϭ S, Se, and Te) {100} surfaces using the density functional method. 25 However, there is no information about the surface energies.With the fast increasing interest in colloidal semiconductor nanocrystals, it is of key importance to investigate the surface structures and related energies of colloidal PbSe

A transferable force field for CdS-CdSe-PbS-PbSe solid systems

“…Similarity is also found between the surfaces of PbS 89 and PbSe. 38 The surface energies for CdX and PbX calculated by our force field are in reasonable agreement with our DFT calculations. The calculated surface energies of the {001} facet of PbX (∼0.29 J/m 2 ) are slightly higher than the DFT results (∼0.18 J/m 2 ).…”

“…30,[33][34][35][36][37][38] However, to simulate NCs containing more than a few thousands of atoms, DFT calculations are no longer possible because of the huge computational demands. Instead, classical molecular simulations techniques such as molecular dynamics (MD) or Monte Carlo (MC) using classical force fields are more commonly used to deal with large systems.…”

Depth dependence of vacancy formation energy at (100), (110), and (111) Al surfaces: A first-principles study

“…In general, the surface energies can inform about the equilibrium shape of nanocrystals [47] and reactivity of different surfaces for various surface phenomena [48]. The surface energy can be defined as the energy required per unit area, to create a given surface from the bulk crystal.…”

Revealing Interaction of Organic Adsorbates with Semiconductor Surfaces Using Chemically Enhanced Raman

“…Instead, surface reconstructions are very likely to occur, allowing (111) to remain stoichiometric, and therefore less polar and nonmetallic. 24 In our case, we let the surface be metallic, because we want to have a metallic system for comparison. Figure 2 addresses the issue of quantum confinement.…”