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

Photoinduced Energy Shift in Quantum-Dot-Sensitized TiO₂: A First-Principles Analysis

We investigate the photoinduced dipole (PID) phenomenon, which holds enormous potential for the optimization of quantum dot-sensitized solar cells (QDSSCs), by means of first-principles electronic structure calculations. We demonstrate that the sensitization of the TiO₂ substrate with core/shell QDs produces almost no changes in the ground state but decisively improves the performance upon photoexcitation. In particular, the maximum attainable <i>V</i>_OC is predicted to increase by ∼25 meV due to two additive effects: (i) the displacement of the photoexcited hole away from the TiO₂ surface and (ii) the interfacial electrostatic interaction established between the TiO₂-injected electrons and the holes residing in the QD core. We believe that this work, explaining the mechanisms by which PID cells deliver better efficiencies, paves the way for the design of new QDSSCs with improved efficiencies

<i>Ab Initio</i> Molecular Dynamics Simulations of Methylammonium Lead Iodide Perovskite Degradation by Water

Protecting organohalide perovskite thin films from water and ambient humidity represents a paramount challenge for the commercial uptake of perovskite solar cells and, in general, of related optoelectronic devices. Therefore, understanding the perovskite/water interface is of crucial importance. As a step in this direction, here we present <i>ab initio</i> molecular dynamics simulations aimed at unraveling the atomistic details of the interaction between the methylammonium lead iodide (MAPbI₃) perovskite surfaces and a liquid water environment. According to our calculations, MAI-terminated surfaces undergo a rapid solvation process, driven by the interaction of water molecules with Pb atoms, which prompts the release of I atoms. PbI₂-terminated surfaces, instead, seem to be more robust to degradation, by virtue of the stronger (shorter) Pb–I bonds formed on these facets. We also observe the incorporation of a water molecule into the PbI₂-terminated slab, which could represent the first step in the formation of an intermediate hydrated phase. Interestingly, PbI₂ defects on the PbI₂-terminated surface promote the rapid dissolution of the exposed facet. Surface hydration, which is spontaneous for both MAI- and PbI₂-terminated slabs, does not modify the electronic landscape of the former, while the local band gap of the PbI₂-exposing model widens by ∼0.3 eV in the interfacial region. Finally, we show that water incorporation into bulk MAPbI₃ produces almost no changes in the tetragonal structure of the perovskite crystal (∼1% volume expansion) but slightly opens the band gap. We believe that this work, unraveling some of the atomistic details of the perovskite/water interface, may inspire new interfacial modifications and device architectures with increased stabilities, which could in turn assist the commercial uptake of perovskite solar cells and optoelectronic devices

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

Effect of Structural Dynamics on the Opto-Electronic Properties of Bare and Hydrated ZnS QDs

Quantum mechanical calculations on the structural and optoelectronic features of two realistic wurtzite-like ZnS quantum dot (QD) models, namely, (ZnS)₃₃ and (ZnS)₁₁₆, are presented both in vacuo and in an explicit water solution environment. Car–Parrinello molecular dynamics (CPMD) simulation and excited-state, Time-Dependent Density Functional Theory (DFT/TDDFT) calculations on extended models are combined to unravel hitherto inaccessible atomistic features of the investigated systems. Ultrasmall QDs are predicted to exhibit strong dynamical fluctuations. Accordingly, the bare (ZnS)₃₃ model undergoes a drastic structural rearrangement and evolves from the starting bulk-like structure to an amorphous phase. The geometrical changes occurring over the time are reflected on the opto-electronic properties. The band-edge states and the optical absorption onset both sizably vary along the CPMD trajectory. Eventually, the optical gap decreases due to the emergence of high-lying occupied orbitals. These midgap states are mainly localized in undercoordinated S sites and could act as trap states for the photogenerated holes. Water molecules are predicted to form strong Zn–OH₂ bonds with the surface Zn atoms. Hydration seems to lower the surface energy, stabilize the wurtzite polymorph, hinder the Zn–S bond breaking, and largely prevent the appearance of trap states. Besides, adsorbed water molecules produce a notable blue-shift of the optical gap. The electrostatic field induced by the solvent shell and the electron-donor properties of the water molecules are supposed to be responsible for the opening of the gap. Moreover, capping the QDs with water molecules increases the intensity of the lowest-lying electronic excitations. This study sheds light on the important opto-electronic modifications occurring for realistic QD in water solution and offers at the same time the methodological framework to investigate photocatalytic reactions mediated by ZnS

Quantum Dot Photoactivation of Pt(IV) Anticancer Agents: Evidence of an Electron Transfer Mechanism Driven by Electronic Coupling

Herein we elucidate the mechanism of photoreduction of the Pt­(IV) complex <i>cis,cis,trans-</i>[Pt­(NH₃)₂(Cl)₂(O₂CCH₂CH₂CO₂H)₂] (<b>1</b>) into Pt­(II) species (among which is cisplatin) by quantum dots (QDs), a process which holds potential for photodynamic therapy. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) methodologies, integrated by selected experiments, were employed to study the interaction and the light-induced electron transfer (ET) process occurring between two QD models and <b>1</b>. Direct adsorption of the complex on the nanomaterial surface results in large electronic coupling between the LUMO (lowest unoccupied molecular orbital) of the excited QD* and the LUMO+1 of <b>1</b>, providing the driving force to the light-induced release of the succinate ligands from the Pt derivative. As confirmed by photolysis experiments performed a posteriori, DFT highlights that QD photoactivation of <b>1</b> can favor the formation of preferred Pt­(II) photoproducts, paving the way for the design of novel hybrid Pt­(IV)–semiconductor systems where photochemical processes can be finely tuned

Carbo-Cages: A Computational Study

Inspired by their geometrical perfection, intrinsic beauty, and particular properties of polyhedranes, a series of carbo-cages is proposed in silico via density functional theory computations. The insertion of alkynyl units into the C–C bonds of polyhedranes results in a drastic lowering of the structural strain. The induced magnetic field shows a significant delocalization around the three-membered rings. For larger rings, the response is paratropic or close to zero, suggesting a nonaromatic behavior. In the carbo-counterparts, the values of the magnetic response are shifted with respect to their parent compounds, but the aromatic/nonaromatic character remains unaltered. Finally, Born–Oppenheimer molecular dynamics simulations at 900 K do not show any drastic structural changes up to 10 ps. In the particular case of a carbo-prismane, no structural change is perceived until 2400 K. Therefore, although carbo-cages have enthalpies of formation 1 order of magnitude higher than those of their parent compounds, their future preparation and isolation should not be discarded, because the systems are kinetically stable, explaining why the similar systems like carbo-cubane have already been synthesized