Benchmark Assessment of Density Functional Methods on Group II–VI MX (M = Zn, Cd; X = S, Se, Te) Quantum Dots

In this work, we build a benchmark data set of geometrical parameters, vibrational normal modes, and low-lying excitation energies for MX quantum dots, with M = Cd, Zn, and X = S, Se, Te. The reference database has been constructed by <i>ab initio</i> resolution-of-identity second-order approximate coupled cluster RI-CC2/def2-TZVPP calculations on (MX)₆ model molecules in the wurtzite structure. We have tested 26 exchange-correlation density functionals, ranging from local generalized gradient approximation (GGA) and hybrid GGA to meta-GGA, meta-hybrid, and long-range corrected. The best overall functional is the hybrid PBE0 that outperforms all other functionals, especially for excited state energies, which are of particular relevance for the systems studied here. Among the DFT methodologies with no Hartree–Fock exchange, the M06-L is the best one. Local GGA functionals usually provide satisfactory results for geometrical structures and vibrational frequencies but perform rather poorly for excitation energies. Regarding the CdSe cluster, we also present a test of several basis sets that include relativistic effects via effective core potentials (ECPs) or via the ZORA approximation. The best basis sets in terms of computational efficiency and accuracy are the SBKJC and def2-SV­(P). The LANL2DZ basis set, commonly employed nowadays on these types of nanoclusters, performs very disappointingly. Finally, we also provide some suggestions on how to perform calculations on larger systems keeping a balance between computational load and accuracy

Modeling Surface Passivation of ZnS Quantum Dots

We report on the interaction between ZnS quantum dots and several surface ligands by means of pure Quantum Mechanical (QM) and hybrid Quantum Mechanics/Molecular Mechanics (QM/MM) methods. To shed light on the nature of the interaction, we focus our discussion on the structural and energetic aspects. The Zn₆S₆ cluster has been chosen to model the quantum dot core, while trimethylamine (NMe₃), trimethylphosphine (PMe₃), trymethylphosphine oxide (OPMe₃), methanol (MeOH), methanethiol (MeSH), and methaneselenol (MeSeH) have been employed to model the passivating ligands. Our results concerning the interaction between the cluster and one ligand of each type reveal that NMe₃, PMe₃, and OPMe₃ show a significantly greater affinity to Zn₆S₆ than MeOH, MeSH, and MeSeH. We noticed that the softer the heteroatom of the ligand bonded to the cluster, the greater the interaction energy. A comparative study of different amines shows that the interaction is strengthened with the number and the length of the alkyl substituents in the ligand. We demonstrated that the interaction is mainly electrostatic, even if an important polarization of the charge density is observed. Fully passivated complexes have also been investigated, and our calculations point out that the bond is weaker than in the complexes with a single bonded ligand, suggesting that the repulsive interactions between the ligands and the diminished charge acceptor capacity of the cluster come into play

Ligand adsorption energy and the postpurification surface chemistry of colloidal metal chalcogenide nanocrystals

The binding of ligands to nanometer-sized surfaces is a central aspect of colloidal nanocrystal (NC) research, for which CdSe NCs were mostly used as the model system to evaluate different surface chemistries. Here, we take the opposite approach and analyze the binding of a single ligand to two different materials. Using CdSe and CdS NCs of similar size and shape and purified with the same protocol, we show that both NCs are capped with tightly bound cadmium oleate (CdOA2). We systematically find that CdS NCs bind more CdOA2 per surface area and that a larger fraction of these ligands withstand displacement by butylamine (BuNH2) as compared to CdSe NCs. These findings concur with density functional theory simulations, which predict for CdS on average cadmium oleate displacement energy larger by 32 kJ/mol than for CdSe. Even so, the displacement isotherm indicates that for both NCs, the initially displaced ligands have the same displacement equilibrium constant. This result suggests that NC work-up codetermines the actual surface chemistry of NCs by effectively setting a displacement (free) energy threshold for ligands to remain bound throughout the purification process. As such, this work highlights that the actual surface chemistry of NCs post purification is the mixed result of intrinsic ligand-NC binding characteristics and concrete processing and purification methods used

Molecules with High Bond Orders and Ultrashort Bond Lengths: CrU, MoU, and WU

The structural and energetic parameters of MU heterobimetallic dimers (M = Cr, Mo, W) have been computed using the complete active space self-consistent-field method followed by second-order perturbation theory. Our results show that the effective bond order (EBO) of the MoU dimer (5.5) is higher than that for the tungsten dimer (5.2), known to date as the molecule with the highest EBO. These heterodimers present also ultrashort bond distances and remarkably large dissociation energies, which make these molecules suitable and interesting potential candidates in synthetic bimetallic organometallic chemistry

Efficient Hot Electron Transfer in Quantum Dot-Sensitized Mesoporous Oxides at Room Temperature

Hot carrier cooling processes represent one of the major efficiency losses in solar energy conversion. Losses associated with cooling can in principle be circumvented if hot carrier extraction toward selective contacts is faster than hot carrier cooling in the absorber (in so-called hot carrier solar cells). Previous work has demonstrated the possibility of hot electron extraction in quantum dot (QD)-sensitized systems, in particular, at low temperatures. Here we demonstrate a room-temperature hot electron transfer (HET) with up to unity quantum efficiency in strongly coupled PbS quantum dot-sensitized mesoporous SnO₂. We show that the HET efficiency is determined by a kinetic competition between HET rate (<i>K</i>_HET) and the thermalization rate (<i>K</i>_TH) in the dots. <i>K</i>_HET can be modulated by changing the excitation photon energy; <i>K</i>_TH can be modified through the lattice temperature. DFT calculations demonstrate that the HET rate and efficiency are primarily determined by the density of the state (DoS) of QD and oxide. Our results provide not only a new way to achieve efficient hot electron transfer at room temperature but also new insights on the mechanism of HET and the means to control it

Tuning Electron–Phonon Interactions in Nanocrystals through Surface Termination

We perform ab initio molecular dynamics on experimentally relevant-sized lead sulfide (PbS) nanocrystals (NCs) constructed with thiol or Cl, Br, and I anion surfaces to determine their vibrational and dynamic electronic structure. We show that electron–phonon interactions can explain the large thermal broadening and fast carrier cooling rates experimentally observed in Pb–chalcogenide NCs. Furthermore, our simulations reveal that electron–phonon interactions are suppressed in halide-terminated NCs due to reduction of both the thermal displacement of surface atoms and the spatial overlap of the charge carriers with these large atomic vibrations. This work shows how surface engineering, guided by simulations, can be used to systematically control carrier dynamics

Tuning Electron–Phonon Interactions in Nanocrystals through Surface Termination

We perform ab initio molecular dynamics on experimentally relevant-sized lead sulfide (PbS) nanocrystals (NCs) constructed with thiol or Cl, Br, and I anion surfaces to determine their vibrational and dynamic electronic structure. We show that electron–phonon interactions can explain the large thermal broadening and fast carrier cooling rates experimentally observed in Pb–chalcogenide NCs. Furthermore, our simulations reveal that electron–phonon interactions are suppressed in halide-terminated NCs due to reduction of both the thermal displacement of surface atoms and the spatial overlap of the charge carriers with these large atomic vibrations. This work shows how surface engineering, guided by simulations, can be used to systematically control carrier dynamics

Tuning Electron–Phonon Interactions in Nanocrystals through Surface Termination

We perform ab initio molecular dynamics on experimentally relevant-sized lead sulfide (PbS) nanocrystals (NCs) constructed with thiol or Cl, Br, and I anion surfaces to determine their vibrational and dynamic electronic structure. We show that electron–phonon interactions can explain the large thermal broadening and fast carrier cooling rates experimentally observed in Pb–chalcogenide NCs. Furthermore, our simulations reveal that electron–phonon interactions are suppressed in halide-terminated NCs due to reduction of both the thermal displacement of surface atoms and the spatial overlap of the charge carriers with these large atomic vibrations. This work shows how surface engineering, guided by simulations, can be used to systematically control carrier dynamics

An “Intermediate Spin” Nickel Hydride Complex Stemming from Delocalized Ni₂(μ-H)₂ Bonding

The nickel hydride complex [Cp′Ni­(μ-H)]₂ (<b>1</b>, Cp′ = 1,2,3,4-tetra­isopropyl­cyclo­penta­dienyl) is found to have a strikingly short Ni–Ni distance of 2.28638(3) Å. Variable temperature and field magnetic measurements indicate an unexpected triplet ground state for <b>1</b> with a large zero-field splitting of +90 K (63 cm^–1). Electronic structure calculations (DFT and CASSCF/CASPT2) explain this ground state as arising from half occupation of two nearly degenerate Ni–Ni π* orbitals

Electronic Structure of Ni₂E₂ Complexes (E = S, Se, Te) and a Global Analysis of M₂E₂ Compounds: A Case for Quantized E₂^<i>n</i>– Oxidation Levels with <i>n</i> = 2, 3, or 4

The diamagnetic compounds Cp′₂Ni₂E₂ (<b>1</b>: E = S, <b>2</b>: E = Se, <b>3</b>: E = Te; Cp′ = 1,2,3,4,-tetraisopropylcyclopentadienyl), first reported by Sitzmann and co-workers in 2001 [Sitzmann, H.; Saurenz, D.; Wolmershauser, G.; Klein, A.; Boese, R. <i>Organometallics</i> <b>2001</b>, 20, 700], have unusual E···E distances, leading to ambiguities in how to best describe their electronic structure. Three limiting possibilities are considered: case <b>A</b>, in which the compounds contain singly bonded E₂^2– units; case <b>B</b>, in which a three-electron E∴E half-bond exists in a formal E₂^3– unit; case <b>C</b>, in which two E^2– ions exist with no formal E–E bond. One-electron reduction of <b>1</b> and <b>2</b> yields the new compounds [Cp*₂Co]­[Cp′₂Ni₂E₂] (<b>1red</b>: E = S, <b>2red</b>: E = Se; Cp* = 1,2,3,4,5-pentamethylcyclopentadieyl). Evidence from X-ray crystallography, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy suggest that reduction of <b>1</b> and <b>2</b> is Ni-centered. Density functional theory (DFT) and ab initio multireference methods (CASSCF) have been used to investigate the electronic structures of <b>1</b>–<b>3</b> and indicate covalent bonding of an E₂^3– ligand with a mixed-valent Ni₂(II,III) species. Thus, reduction of <b>1</b> and <b>2</b> yields Ni₂(II,II) species <b>1red</b> and <b>2red</b> that bear unchanged E₂^3– ligands. We provide strong computational and experimental evidence, including results from a large survey of data from the Cambridge Structural Database, indicating that M₂E₂ compounds occur in quantized E₂ oxidation states of (2 × E^2–), E₂^3–, and E₂^2–, rather than displaying a continuum of variable E–E bonding interactions