Qun Zeng

The Formation Mechanism of Binary Semiconductor Nanomaterials: Shared by Single‐Source and Dual‐Source Precursor Approaches

One thing in common: The formation of binary colloidal semiconductor nanocrystals from single- (M(EEPPh2 )n ) and dual-source precursors (metal carboxylates M(OOCR)n and phosphine chalcogenides such as E=PHPh2 ) is found to proceed through a common mechanism. For CdSe as a model system (31) P NMR spectroscopy and DFT calculations support a reaction mechanism which includes numerous metathesis equilibriums and Se exchange reactions.

Mild and effective N-phthaloylation of amino acids

Summary.In the present work various free amino acids, including tryptophan and tyrosine, were effectively N-phthaloylated under reduced pressure and at lower temperature. Moreover, under these conditions, the presence of phthalic acid in phthalic anhydride did not hinder the N-phthaloylation of amino acids. This simple process is economic, environmentally friendly, and suitable for large-scale production.

Structures and optical absorptions of PbSe clusters from ab initio calculations.

Based on the low-lying structures of (PbSe)n (n = 1-10) clusters identified with a first-principles molecular dynamics approach, two growth patterns with distinct structure and energy evolutions were predicted for the even-n and odd-n clusters, respectively. Moreover, the clusters favor a simple cubic and bulk-like growth pattern, unlike the extensively studied II-VI clusters whose structural diversity has been well established. The overlap between 6p of Pb and 4p of Se makes not only the ordered and bulk-like structures but also a stable building block of (PbSe)4. The high stability of (PbSe)4 is recognized in terms of its binding energy, HOMO-LUMO gap, appearance in the structures of larger-size clusters, as well as its appearance in the fragmentation products of PbSe clusters. The geometrical and electronic structures of the PbSe clusters were further studied within the density functional theory framework including spin-orbital (SO) coupling. We found that SO coupling does not change the relative stability of the clusters but reduces their binding energy significantly. Particularly, the SO effect has a great impact on the UV-vis spectra of the clusters, which were simulated with time-dependent density functional theory at SO level of zeroth-order regular approximation.

Local and nonlocal contributions to molecular first-order hyperpolarizability: a Hirshfeld partitioning analysis.

Based on first-principles calculations, a decomposition scheme is proposed to investigate the molecular site-specific first-order hyperpolarizability (β) responses by means of Hirshfeld population analysis and finite field method. For a molecule, its β is decomposed into local and nonlocal contributions of individual atoms or groups. The former describes the response within the atomic sphere, while the latter describes the contributions from interatomic charge transfer. This scheme is then applied to six prototypical donor-acceptor (D-A) or D-π-A molecules for which the local and nonlocal hyperpolarizabilities are evaluated based on their MP2 density. Both the local and nonlocal parts exhibit site-specific characteristics, but vary differently with molecular structures. The local part depends mainly on the atomic attributes such as electronegativity and charge state, as well as its location in the molecule, while the nonlocal part relates to the ability and distance of charge delocalization within the molecule, increasing rapidly with molecular size. The proposed decomposition scheme provides a way to distinguish atomic or group contributions to molecular hyperpolarizabilities, which is useful in the molecular design for organic nonlinear optical materials.

First-principles study of ammonium ions and their hydration in montmorillonites

AbstractDensity functional theory calculations were performed to investigate the adsorption and hydration of an ammonium ion (NH4+) confined in the interlayer space of montmorillonites (MMT). NH4+ is trapped in the six-oxygen-ring on the internal surface and forms a strong binding with the surface O atoms. The hydration of NH4+ is affected significantly by the surface. Water molecules prefer the surface sites, and do not bind with the NH4+ unless enough water molecules are supplied. Moreover, the water molecules involved in NH4+ hydration tend to bind with the surface simultaneously. The hydration energy increases with the intercalated water molecules, in contrast to that in gas phase. In addition, the hydration leads to the extension of MMT basal spacing. FigureHydrated ammonium ion inside montmorillonite

Competition between monomer and dimer fragmentation pathways of cationic CuN clusters of N = 2–20

Density functional theory calculations are performed to study the structural evolution of cationic copper clusters of N = 2–20 size range. The first shape transition occurs at N = 4, from planar to three dimensional, while the second at N = 16, from layered to capped icosahedra. Distinct even–odd alternation and closing-shell effects in binding energy are noted when size varies. Two fragmentation pathways for each cation are investigated and compared to available experiments. The competition between the two pathways is rationalized from respective dissociation energies which are size and structure dependent.

Core–shell interaction and its impact on the optical absorption of pure and doped core-shell CdSe/ZnSe nanoclusters

The structural, electronic, and optical properties of core-shell nanoclusters, (CdSe)(x)@(CdSe)(y) and their Zn-substituted complexes of x = 2-4 and y = 16-28, were studied with density functional theory calculations. The substitution was applied in the cores, the shells, and/or the whole clusters. All these clusters are characterized by their core-shell structures in which the core-shell interaction was found different from those in core or in shell, as reflected by their bondlengths, volumes, and binding energies. Moreover, the core and shell combine together to compose a new cluster with electronic and optical properties different from those of separated individuals, as reflected by their HOMO-LUMO gaps and optical absorptions. With the substitution of Cd by Zn, the structural, electronic, and optical properties of clusters change regularly. The binding energy increases with Zn content, attributed to the strong Zn-Se bonding. For the same core/shell, the structure with a CdSe shell/core has a narrower gap than that with a ZnSe shell/core. The optical absorption spectra also change accordingly with Zn substitution. The peaks blueshift with increasing Zn concentration, accompanying with shape variations in case large number of Cd atoms are substituted. Our calculations reveal the core-shell interaction and its influence on the electronic and optical properties of the core-shell clusters, suggesting a composition-structure-property relationship for the design of core-shell CdSe and ZnSe nanoclusters.

First-principles study of O2 activation on ligand-protected Au32 clusters

Poly(N-vinyl-2-pyrrolidone) (PVP) is often used to protect active Au clusters from coalescence. The influences of PVP on the O2 adsorption on Au32 clusters were investigated using density functional theory calculations. Various low-lying structures of O2:Au32 and O2:Au32:PVP complexes, in which the Au32 is either neutral or anionic and the O2 is either molecular or dissociative, were identified. The PVP influences were evaluated in terms of the changes in geometry, adsorption energy, charge redistribution, spin density, and density of states upon PVP pre-adsorption. Our calculations reveal that PVP weakly adsorbs on the cluster surface, with rather small changes in the structural, geometrical and electronic properties that are relevant to the O2 activation. The activity of neutral or anionic Au32 towards O2 is kept or slightly enhanced by PVP because of the cooperative adsorption of PVP and O2. This is the structural basis of choosing PVP as the protective ligand for Au clusters.

A density functional theory study of the hydration of calcium ions confined in the interlayer space of montmorillonites

The structures of Ca2+ hydrates in the interlayer space of montmorillonites (MMT) were studied by periodic density functional theory (DFT) calculations under the GGA/PBE approximation. Affected by the internal surfaces, which are rich of negative charge, the Ca2+ hydration exhibits different behaviors from that in gas phase. The Ca2+ is located at the six-oxygen-ring (SOR) on the internal surface in dry MMT, while the incoming water molecules bind with the Ca2+, the O atoms on surface, and/or with each other. The water molecules have a tendency of forming a hydrogen bond (HB) network that connects the upper and lower surfaces. Attracted by surrounding water molecules, the Ca2+ gradually moves outward with increasing number of water molecules. Moreover, the hydration energy (EH) of Ca2+ is determined not only by the interaction between Ca2+ and H2O, but also by that among Ca2+, H2O and the surfaces. As a result, the EH has only small changes for additional incoming water molecules, in contrast to the great and monotonic decrease in gas phase.

Probing the surface structure of hydroxyapatite through its interaction with hydroxyl: a first-principles study

Understanding the interaction of the hydroxyapatite (HAp) surface with hydroxyl originating from either the alkalescent physiological environment or HAp itself is crucial for the development of HAp-based biomaterials. Periodical density functional theory calculations were carried out in this study to explore the interaction of the HAp (100), (010) and (001) facets with hydroxyl. Based on a comparison study of Ca-rich, PO4-rich and Ca–PO4–OH mixed surfaces, the interaction pattern, interaction energy and effect of an additional water molecule on the Ca–OH interaction were comprehensively studied. The formation of CaOH on the Ca-rich surface was energetically favored on (100) and (001), while Ca(OH)2 was energetically favored on (010). The Ca–water interaction was competitive, but had lower interaction energy than Ca–OH. Furthermore, Ca–O bonding and its influence on the OH stretching vibration were analyzed. Our calculations suggest that the hydroxyl-coated surface structure is more appropriate than the commonly used Ca-terminated surface model for studying HAp surface activity in its service environments.