Francesca Nunzi

Nearly Monodisperse Insulator Cs4PbX6 (X = Cl, Br, I) Nanocrystals, Their Mixed Halide Compositions, and Their Transformation into CsPbX3 Nanocrystals

We have developed a colloidal synthesis of nearly monodisperse nanocrystals of pure Cs4PbX6 (X = Cl, Br, I) and their mixed halide compositions with sizes ranging from 9 to 37 nm. The optical absorption spectra of these nanocrystals display a sharp, high energy peak due to transitions between states localized in individual PbX64– octahedra. These spectral features are insensitive to the size of the particles and in agreement with the features of the corresponding bulk materials. Samples with mixed halide composition exhibit absorption bands that are intermediate in spectral position between those of the pure halide compounds. Furthermore, the absorption bands of intermediate compositions broaden due to the different possible combinations of halide coordination around the Pb2+ ions. Both observations are supportive of the fact that the [PbX6]4– octahedra are electronically decoupled in these systems. Because of the large band gap of Cs4PbX6 (>3.2 eV), no excitonic emission in the visible range was observed. The Cs4PbBr6 nanocrystals can be converted into green fluorescent CsPbBr3 nanocrystals by their reaction with an excess of PbBr2 with preservation of size and size distributions. The insertion of PbX2 into Cs4PbX6 provides a means of accessing CsPbX3 nanocrystals in a wide variety of sizes, shapes, and compositions, an important aspect for the development of precisely tuned perovskite nanocrystal inks.

Inherent electronic trap states in TiO2 nanocrystals: effect of saturation and sintering

We report a quantum mechanical investigation on the nature of electronic trap states in realistic models of individual and sintered anatase TiO2 nanocrystals (NCs) of ca. 3 nm diameter. We find unoccupied electronic states of lowest energy to be localized within the central part of the NCs, and to originate from under-coordinated surface Ti atoms lying mainly at the edges between the (100) and (101) facets. These localized states are found at about 0.3–0.4 eV below the fully delocalized conduction band states, in good agreement with both electrochemical and spectro-electrochemical results. The overall Density-Of-States (DOS) below the conduction band (CB) can be accurately fitted to an exponential distribution of states, in agreement with capacitance data. Water molecules adsorbed on the NC surface raise the energy and reduce the number of localized states, thus modifying the DOS. As a possible origin of additional trap states, we further investigated the oriented attachment of two TiO2 NCs at various possible interfaces. For the considered models, we found only minor differences between the DOS of two interacting NCs and those of the individual constituent NCs. Our results point at the presence of inherent trap states even in perfectly stoichiometric and crystalline TiO2 NCs due to the unavoidable presence of under-coordinated surface Ti(IV) ions at the (100) facets.

Structural and Electronic Properties of Photoexcited TiO2 Nanoparticles from First Principles

The structure and energetics of excitons and individual electron and hole polarons in a model anatase TiO2 nanoparticle (NP) are investigated by means of Density Functional Theory (DFT) and Time Dependent (TD)-DFT calculations. The effect of the Hartree-Fock exchange (HF-exc) contribution in the description of TiO2 NPs with unpaired electrons is examined by comparing the results from semilocal and hybrid DFT functionals with different HF-exc percentages, including a long-range corrected hybrid functional. The performances of TD-DFT and ground state (SCF) DFT approaches in the description of the photoexcited polaron states in TiO2 NPs are also analyzed. Our results confirm that the HF-exc contribution is essential to properly describe the self-trapping of the charge carriers. They also suggest that long-range corrected functionals are needed to properly describe excited state relaxation in TiO2 NPs. TD-DFT geometry optimization of the lowest excited singlet and triplet states deliver photoluminescence values in close agreement with the experimental data.

Shape and Morphology Effects on the Electronic Structure of TiO2 Nanostructures: From Nanocrystals to Nanorods

We carry out an accurate computational analysis on the nature and distribution of electronic trap states in shape-tailored anatase TiO2 structures, investigating the effect of the morphology on the electronic structure. Linear nanocrystal models up to 6 nm in length with various morphologies, reproducing both flattened and elongated rod-shaped TiO2 nanocrystals, have been investigated by DFT calculations, to clarify the effect of the crystal facet percentage on the nanocrystal electronic structure, with particular reference to the energetics and distribution of trap states. The calculated densities of states below the conduction band edge have been very well fitted assuming an exponential distribution of energies and have been correlated with experimental capacitance data. In good agreement with the experimental phenomenology our calculations show that elongated rod-shaped nanocrystals with higher values of the ratio between (100) and (101) facets exhibit a relatively deeper distribution of trap states. Our results point at the crucial role of the nanocrystal morphology on the trap state density, highlighting the importance of a balance between the low-energy (101) and high-energy (100)/(001) surface facets in individual TiO2 nanocrystals.

Cr(CO)3‐Activated Diels–Alder Reaction on Single‐Wall Carbon Nanotubes: A DFT Investigation

Breaking barriers: In agreement with experimental evidence, it was found by means of high-level DFT calculations that the Cr(CO)(3) metal fragment considerably reduces the reaction energy barrier-for both the concerted and stepwise reaction mechanisms (see graphic)-of the Diels-Alder reaction of butadiene on (5,5) carbon nanotubes.The reaction mechanism and the effect of Cr(CO)(3) on the Diels-Alder reaction of butadiene on (5,5) carbon nanotube sidewalls have been investigated by high-level DFT calculations. We investigated both concerted and stepwise reaction pathways on closed-shell and open-shell potential-energy surfaces. In agreement with recent experimental evidence, we found the Cr(CO)(3) metal fragment to considerably reduce the reaction energy barrier, both for the concerted and stepwise reaction mechanisms. The latter mechanism, previously found to be higher in energy with traditional substrates, appears to compete with the concerted mechanism on the carbon nanotube sidewalls. An analysis of the frontier molecular orbitals on the pristine and Cr(CO)(3)-complexed carbon nanotubes allowed us to identify the reason for this inverted reactivity pattern and the role of the transition metal on the addition of the diene.

Ab Initio Simulation of the Absorption Spectra of Photoexcited Carriers in TiO2 Nanoparticles

We investigate the absorption spectra of photoexcited carriers in a prototypical anatase TiO2 nanoparticle using hybrid time dependent density functional theory calculations in water solution. Our results agree well with experimental transient absorption spectroscopy data and shed light on the character of the transitions. The trapped state is always involved, so that the SOMO/SUMO is the initial/final state for the photoexcited electron/hole absorption. For a trapped electron, final states in the low energy tail of the conduction band correspond to optical transitions in the IR, while final states at higher energy correspond to optical transitions in the visible. For a trapped hole, the absorption band is slightly blue-shifted and narrower in comparison to that of the electron, consistent with its deeper energy level in the band gap. Our calculations also show that electrons in shallow traps exhibit a broad absorption in the IR, resembling the feature attributed to conductive electrons in experimental spectra.

The interaction of Cr(CO)3 on the (n, 0) nanotube side-walls: a density functional study through a cluster model approach

Density functional calculations have been performed to investigate the functionalization of single-wall carbon nanotubes (SWNTs) with the Cr(CO)3 metal fragment, employing extended molecular models. A circumcoronene molecule (C54H18), made by the fusion of 19 hexagonal carbon rings, can be regarded as a fragment of a graphene sheet. To reproduce the curvature of the SWNT surface, suitable geometric constraints have been imposed on the C54H18 model, freezing the positions of the outer hydrogens along the directions of the nanotube C-C bonds. Geometry optimizations have then been performed under this constraint on the Cr(CO)3-C54H18 complexes, pointing out the most favourable coordination sites on the hexagonal rings of the carbon atom surface and the electronic properties of the resulting system. The effect of the curvature on the metal coordination to nanotubes has been analysed by investigating the interaction of the Cr(CO)3 metal complex with the C54H18 molecules, modelling (n, 0) nanotubes with different degrees of curvature, i.e. with various values of the chiral vector (n, 0).

Stabilization through p-dimethylaminobenzaldehyde of a new NLO-active phase of [E-4-(4-dimethylaminostyryl)-1-methylpyridinium] iodide: synthesis, structural characterization and theoretical investigation of its electronic properties

Co-crystallization of p-N(CH3)2C6H4CHO and E-4-(4-dimethylaminostyryl)-1-methylpyridinium iodide ([DAMS]I) gives a new solid state form (A), characterized by high non-linear-optical (NLO) activity and quite different from the well known [DAMS]I salt (centrosymmetric and therefore inactive). The X-ray structural characterization, although affected by the extended disorder of p-N(CH3)2C6H4CHO, addresses a new kind of aggregation for the [DAMS+] chromophore molecules. In fact, together with the often encountered J-type aggregation, an unprecedented “fishbone” coupling is observed. Calculations using time-dependent density functional theory (TDDFT) prove that the absence of a J-aggregation band in the electronic absorption spectrum is due to the additional intermolecular interaction that quenches the expected J-type signal. Correlation between supramolecular arrangements of the chromophores and NLO properties is also discussed.

A pyracylene model for the interaction of transition metals with fullerenes: a density functional study

Pyracylene, a polynuclear aromatic hydrocarbon which is made up of two benzene and two cyclopentadienyl rings, has been employed as a model to study the interaction of transition metal complexes with fullerenes. To reproduce adequately the geometric and electronic structure of fullerene with a pyracylene model, we had to impose suitable geometric constraints forcing the pyramidalisation angle on the two central carbon atoms to assume a value similar to that observed in C60. Density functional calculations were then performed on (PH3)2M(C14H8) (M = Ni, Pd, Pt) molecules. The results have been analysed in terms of the Chatt–Dewar–Duncanson model and show that the constrained pyracylene is a fairly good model to study the interaction of transition metals with fullerene: geometries are reproduced within 0.02 A and the bond dissociation energies are slightly underestimated by only 10–40 kJ mol−1.