X-ray induced ultrafast charge transfer in thiophene-based conjugated polymers controlled by core-hole clock spectroscopy

We explore ultrafast charge transfer (CT) resonantly induced by hard X-ray radiation in organic thiophene-based polymers at the sulfur K-edge. A combination of core-hole clock spectroscopy with real-time propagation time-dependent density functional theory simulations gives an insight into the electron dynamics underlying the CT process. Our method provides control over CT by a selective excitation of a specific resonance in the sulfur atom with monochromatic X-ray radiation. Our combined experimental and theoretical investigation establishes that the dominant mechanism of CT in polymer powders and films consists of electron delocalisation along the polymer chain occurring on the low-femtosecond time scale.

Electronic Structure and Chemical Bonding in Methylammonium Lead Triiodide and Its Precursor Methylammonium Iodide

A detailed examination of the electronic structures of methylammonium lead triiodide (MAPI) and methylammonium iodide (MAI) is performed with ab initio molecular dynamics (AIMD) simulations based on density functional theory, and the theoretical results are compared to experimental probes. The occupied valence bands of a MAPI single crystal and MAI powder are probed with X-ray photoelectron spectroscopy, and the conduction bands are probed from the perspective of nitrogen K-edge X-ray absorption spectroscopy. Combined, the theoretical simulations and the two experimental techniques allow for a dissection of the electronic structure unveiling the nature of chemical bonding in MAPI and MAI. Here, we show that the difference in band gap between MAPI and MAI is caused chiefly by interactions between iodine and lead but also weaker interactions with the MA⁺ counterions. Spatial decomposition of the iodine p levels allows for analysis of Pb-I σ bonds and π interactions, which contribute to this effect with the involvement of the Pb 6p levels. Differences in hydrogen bonding between the two materials, seen in the AIMD simulations, are reflected in nitrogen valence orbital composition and in nitrogen K-edge X-ray absorption spectra.

Experimental and Theoretical Core Level and Valence Band Analysis of Clean Perovskite Single Crystal Surfaces

A detailed understanding of the surface and interface properties of lead halide perovskites is of interest for several applications, in which these materials may be used. To develop this understanding, the study of clean crystalline surfaces can be an important stepping stone. In this work, the surface properties and electronic structure of two different perovskite single crystal compositions (MAPbI₃ and Cs_x FA_{1- x} PbI₃ ) are investigated using synchrotron-based soft X-ray photoelectron spectroscopy (PES), molecular dynamics simulations, and density functional theory. The use of synchrotron-based soft X-ray PES enables high surface sensitivity and nondestructive depth-profiling. Core level and valence band spectra of the single crystals are presented. The authors find two carbon 1s contributions at the surface of MAPbI₃ and assign these to MA⁺ ions in an MAI-terminated surface and to MA⁺ ions below the surface. It is estimated that the surface is predominantly MAI-terminated but up to 30% of the surface can be PbI₂ -terminated. The results presented here can serve as reference spectra for photoelectron spectroscopy investigations of technologically relevant polycrystalline thin films, and the findings can be utilized to further optimize the design of device interfaces.

Coupling Methylammonium and Formamidinium Cations with Halide Anions: Hybrid Orbitals, Hydrogen Bonding, and the Role of Dynamics

The electronic structures of four precursors for organic-inorganic hybrid perovskites, namely, methylammonium chloride and iodide, as well as formamidinium bromide and iodide, are investigated by X-ray emission (XE) spectroscopy at the carbon and nitrogen K-edges. The XE spectra are analyzed based on density functional theory calculations. We simulate the XE spectra at the Kohn-Sham level for ground-state geometries and carry out detailed analyses of the molecular orbitals and the electronic density of states to give a thorough understanding of the spectra. Major parts of the spectra can be described by the model of the corresponding isolated organic cation, whereas high-emission energy peaks in the nitrogen K-edge XE spectra arise from electronic transitions involving hybrids of the molecular and atomic orbitals of the cations and halides, respectively. We find that the interaction of the methylammonium cation is stronger with the chlorine than with the iodine anion. Furthermore, our detailed theoretical analysis highlights the strong influence of ultrafast proton dynamics in the core-excited states, which is an intrinsic effect of the XE process. The inclusion of this effect is necessary for an accurate description of the experimental nitrogen K-edge X-ray emission spectra and gives information on the hydrogen-bonding strengths in the different precursor materials.

Mixed-Halide Double Perovskite Cs2 AgBiX6 (X=Br, I) with Tunable Optical Properties via Anion Exchange

Lead-free double perovskites, A2 M+ M'3+ X6 , are considered as promising alternatives to lead-halide perovskites, in optoelectronics applications. Although iodide (I) and bromide (Br) mixing is a versatile tool for bandgap tuning in lead perovskites, similar mixed I/Br double perovskite films have not been reported in double perovskites, which may be due to the large activation energy for ion migration. In this work, mixed Br/I double perovskites were realized utilizing an anion exchange method starting from Cs2 AgBiBr6 solid thin-films with large grain-size. The optical and structural properties were studied experimentally and theoretically. Importantly, the halide exchange mechanism was investigated. Hydroiodic acid was the key factor to facilitate the halide exchange reaction, through a dissolution-recrystallization process. In addition, the common organic iodide salts could successfully perform halide-exchange while retaining high mixed-halide phase stability and strong light absorption capability.

Sensitivity of Nitrogen K-Edge X-ray Absorption to Halide Substitution and Thermal Fluctuations in Methylammonium Lead-Halide Perovskites

The performance of hybrid perovskite materials in solar cells crucially depends on their electronic properties, and it is important to investigate contributions to the total electronic structure from specific components in the material. In a combined theoretical and experimental study of CH₃NH₃PbI₃-methylammonium lead triiodide (MAPI)-and its bromide cousin CH₃NH₃PbBr₃ (MAPB), we analyze nitrogen K-edge (N 1s-to-2p*) X-ray absorption (XA) spectra measured in MAPI and MAPB single crystals. This permits comparison of spectral features to the local character of unoccupied molecular orbitals on the CH₃NH₃ ⁺ (MA⁺) counterions and allows us to investigate how thermal fluctuations, hydrogen bonding, and halide-ion substitution influence the XA spectra as a measure of the local electronic structure. In agreement with the experiment, the simulated spectra for MAPI and MAPB show close similarity, except that the MAPB spectral features are blue-shifted by +0.31 eV. The shift is shown to arise from the intrinsic difference in the electronic structure of the two halide atoms rather than from structural differences between the materials. In addition, from the spectral sampling analysis of molecular dynamics simulations, clear correlations between geometric descriptors (N-C, N-H, and H···I/Br distances) and spectral features are identified and used to explain the spectral shapes.

Core-Level Binding Energy Reveals Hydrogen Bonding Configurations of Water Adsorbed on TiO_{2}(110) Surface

Using x-ray photoelectron spectroscopy of the oxygen 1s core level, the ratio between intact (D_{2}O) and dissociated (OD) water in the hydrated stoichiometric TiO_{2}(110) surface is determined at varying coverage and temperature. In the submonolayer regime, both the D_{2}O∶OD ratio and the core-level binding energy of D_{2}O (ΔBE) decrease with temperature. The observed variations in ΔBE are shown with density functional theory to be governed crucially and solely by the local hydrogen bonding environment, revealing a generally applicable classification and details about adsorption motifs.

Intercalation of transition metals in aluminene bi-layers: An ab initio study

Using first principles calculations based on density functional theory (DFT), we probe various possible stacking arrangements of bilayer aluminene and intercalate six transition metal (TM) atoms (Ti, Cr, Mn, Fe, Co, and Ni) in unique bilayer aluminene systems. Further, we calculate valence charge density and electron localization function to ascertain the nature of bonding present in both the pristine and TM-intercalated composite systems. Intercalation of Cr, Mn, and Fe is found to result in the magnetic ground state. For Ti, Co, and Ni-intercalated systems, the starting trigonal symmetry has changed to a tetragonal symmetry. Co and Ni intercalated systems exhibit much higher (negative) formation energies compared to the other composite systems. In addition, nesting of the Fermi surface has been probed for the Co and Ni intercalated systems and observations indicate the possibility of the presence of charge density wave in the systems. A dispersion-corrected DFT study suggests that the van der Waals interaction is not likely to play a crucial role in determining the properties of both the pristine and TM-intercalated systems.

Remarkable Structural Effect on the Gold-Hydrogen Analogy in Hydrogen-Doped Gold Cluster

In accordance with the well established gold-hydrogen analogy, a hydrogen atom mimics the properties of a gold atom in gold clusters. In a recent study it has been demonstrated that the properties of a hydrogen atom doped small gold cluster (Au₇H) are not in conformity with the aforementioned analogy. In this paper we study the properties of the Au₇H cluster exhaustively to re-examine the validity of the gold-hydrogen analogy in the context of adsorption of CO and O₂ molecules on pristine gold and hydrogen atom doped gold clusters. For this purpose we first determine the most stable structure of the Au₇H cluster by using an ab initio density functional theory based method with generalized gradient approximation (GGA) and Meta-GGA exchange-correlation functionals. We carry out geometry optimization by considering various planar and three-dimensional isomers of the Au₇H cluster as initial geometries. We find that the lowest energy structure of Au₇H is a planar one with C_{2 v} symmetry, and it is very close to the structure of the Au₈ cluster with D_{4 h} symmetry. Furthermore, to examine the validity of the gold-hydrogen analogy we carry out a detailed investigation of the adsorption of CO and O₂ molecules on the most stable as well as various other low energy isomers of the Au₇H cluster. We find that the adsorption energies and the extent of activation of CO and O₂ molecules on the most stable planar isomer of Au₇H are almost the same as those on the parent Au₈ cluster with D_{4 h} symmetry proving the validity of the gold-hydrogen analogy. On the other hand, for the high energy three-dimensional isomers of the Au₇H cluster obtained from the pristine Au₈ cluster with T _d symmetry, we find a significant enhancement in adsorption energy as well as the extent of activation of CO and O₂ molecules as compared to those for the corresponding pristine cluster. Therefore, the high reactivity of the 3D isomer of the Au₇H cluster may be attributed to its existence in a state which is higher in energy than its most stable planar isomer.

Structural, electronic and optical properties of 2,5-dichloro-p-xylene: experimental and theoretical calculations using DFT method

The vibrational spectra including FT-IR and FT-Raman for 2,5-dichloro-p-xylene (DCPX) have been recorded.

The benefits of a comprehensive rehabilitation program in patients diagnosed with spastic quadriplegia

Spastic quadriplegia has as an etiopathogenic substrate, a non-progressive brain lesion; however, the clinical manifestations of the disease evolve over time. Children diagnosed with spastic quadriplegia show a variety of symptoms in different areas: sensorimotor, emotional, cognitive, and social. The purpose of this study was to assess the functional status in patients diagnosed with spastic quadriplegia, who followed a complex medical rehabilitation program, during a year, and highlight the importance of using physical and kinetic techniques in improving their status. A total of 10 children diagnosed with spastic quadriplegia were included in the study and the Gross Motor Function Classification System (GMFCS) and manual ability classification system (MACS) were used to evaluate the functionality status of each patient. Every patient was evaluated initially (T1), after six months of program (T2), and after they completed the study. All the children were originally monitored daily, for 5 days per week for a period of one month, then two times a week for a year. A statistically significant difference regarding the modification of the GMFCS and MACS stage was found, which occurred between the first and the third evaluation. The inverse correlation of the statistical significance between the ages of patients and the decrease in GMFCS or MACS stage was highlighted; the younger the patient, the more the scale decreased. A direct link between the gross motor function and the manual ability was noticed. Applying a complex rehabilitation program has proven efficient by improving both the gross motor functionality and the manual ability.

Molecular structure, vibrational spectral assignments (FT-IR and FT-RAMAN), NMR, NBO, HOMO–LUMO and NLO properties of 2-nitroacetophenone based on DFT calculations

The FT-IR and FT-Raman analyses of 2-nitro acetophenone (2NAP) have been carried out by density functional theory (DFT) calculations based on B3LYP level with 6-31G*/6-311[Formula: see text]G** basis set. The gauge-independent atomic orbital (GIAO) method has been used to get 1H NMR and [Formula: see text]C NMR chemical shifts. From DFT calculations, various parameters such as atomic charges, HOMO–LUMO energies and Dipole moment have been obtained. The molecular electronic potential (MEP) has also been derived for 2NAP. In order to find the electronic excitation energies, oscillator strength and nature of the respective excited states, the closed-shell singlet calculation has been utilized. MOLVIB program has been employed to calculate total energy distribution (TED) and normal coordinate analysis. Natural bond orbital (NBO) analysis has also been carried out by DFT calculations with B3LYP/6-311[Formula: see text]G** basis set.

Successful autotransplantation of a mature mesiodens to replace a traumatized maxillary central incisor

AIM

This case describes the successful transplantation of a mature mesiodens tooth to replace a traumatized maxillary central incisor.

SUMMARY

A 17-year-old male attended 1 week after a traumatic injury to his left maxillary central incisor (tooth 21). Radiographs revealed a horizontal root fracture and a poor prognosis. The tooth was atraumatically removed and replaced with a mesiodens lying in the same region. After stabilization, root canal treatment was performed and aesthetics were restored with a tooth coloured restoration. A 2-year follow-up revealed the tooth had good aesthetics and function.

KEY LEARNING POINTS

A supernumerary nonfunctional tooth such as a mesiodens can be successfully used to replace a missing permanent tooth by autotransplantation. Autotransplantation has a high success rate if case selection is good, appropriate surgery is carried out and excellent hygiene is maintained. Autotransplantation should be considered as one of the most biologic techniques for replacing a missing tooth with minimal cost. Autotransplantation can be carried out even after complete root formation in the donor tooth.

Silicene beyond mono-layers--different stacking configurations and their properties

We carry out a computational study on the geometric and electronic properties of multi-layers of silicene in different stacking configurations using state-of-the-art ab initio density functional theory based calculations. In this work we investigate the evolution of these properties with increasing number of layers (n) ranging from 1 to 10. Although a mono-layer of silicene possesses properties similar to those of graphene, our results show that the geometric and electronic properties of multi-layers of silicene are strikingly different from those of multi-layers of graphene. We observe that strong inter-layer covalent bonding exists between the layers in multi-layers of silicene as opposed to weak van der Waals bonding which exists between the graphene layers. The inter-layer bonding strongly influences the geometric and electronic structures of these multi-layers. Like bi-layers of graphene, silicene with two different stacking configurations AA and AB exhibits linear and parabolic dispersions around the Fermi level, respectively. However, unlike graphene, for bi-layers of silicene, these dispersion curves are shifted in the band diagram; this is due to the strong inter-layer bonding present in the latter. For n > 3, we study the geometric and electronic properties of multi-layers with four different stacking configurations, namely AAAA, AABB, ABAB and ABC. Our results on cohesive energy show that all the multi-layers considered are energetically stable. Furthermore, we find that the three stacking configurations (AAAA, AABB and ABC) containing tetrahedral coordination have much higher cohesive energy than the Bernal (ABAB) stacking configuration. This is in contrast to the case of multi-layers of graphene, where ABAB is reported to be the lowest energy configuration. We also observe that bands near the Fermi level in lower energy stacking configurations AAAA, AABB and ABC correspond to the surface atoms and these surface states are responsible for the semi-metallic character of these multi-layers.