The IR and Raman spectra of the LaCoO3 perovskite (the formal occupancy on Co is d6, low spin, t2g6) are computed by imposing space groups (SG) of decreasing symmetry, from the cubic (SG Pm-3m, N. 221, and Fm-3m, N. 225), to the tetragonal (SG 140), rhombohedral (SG 167) and orthorhombic (SG 62) ones. The total energy differences between these structures, computed at the quantum mechanical level by using an all electron Gaussian type basis set and the full range hybrid B3LYP functional, is extremely small: 0.2 mEh between SG 167 and SG 62, 1.2 mEh between SG 140 and SG 62, and 4.5 mEh between the most stable structure and the cubic, ideal aristotype. These minor differences indicate that the experimentally proposed SG might be one of the (many) possible alternatives. The IR and Raman spectra are very rarely used for the identification of the symmetry of the perovskites at different temperatures. Here we investigate the evolution of the two spectra (IR and Raman) through the various competing space groups, exploring the possibility that they might be used for the identification of the low temperature structure (SG and position of the atoms) of the investigated compounds, and of perovskites in particular.

The treatment of the Hartree-Fock exchange (HFX) series in periodic systems is described when a Gaussian-type basis set is adopted, and its performance is documented with reference to the KCrF3 perovskite. The computational scheme for the exchange, implemented in the CRYSTAL code in its general lines more than 35 years ago (Causà et al., J. Chem. Phys. 92, 909 (1988)), is here documented for the first time with reference to the variables essential for the description of many properties of periodic systems: truncation of the exchange series, reduction of point symmetry, change of space group, and dimension of the unit cell: these features are involved in the calculation of the total energy, the equilibrium geometry, the relative stability of phases, the Jahn-Teller splitting and orbital ordering, the ferromagnetic versus antiferromagnetic energy difference, the vibrational frequencies, and the IR and Raman intensities, to quote the most important properties of interest. The same computational scheme is used for both pure HF and for full-range or range-separated hybrid functionals, in which a variable percentage X of HFX is used. To document the role of X, a very recently defined functional is employed, PBE(X), such that PBE(0) = PBE and PBE(25) = PBE0. The high efficiency of the algorithms implemented in CRYSTAL permits to perform calculations for supercells of KCrF3 containing, for example, 3430 atoms (7 × 7 × 7 supercell of the monoclinic cell, containing 10 atoms), when an all-electron basis set of triple-ζ quality is adopted.

The geometrical and electronic properties of LaFeO3 have been investigated by using an all electron Gaussian type basis set, the B3LYP full range hybrid functional and the CRYSTAL code, and compared with LaScO(3 )and LaCrO3 : the formal occupancy (according to the fully ionic model) of the transition metal d shell is d(0) (Sc), d(3) (t(2g)(3), Cr) and d(5) (t(2g)(3)e(g)(2), Fe), although the net d-charges resulting from a Mulliken analysis are closer to the occupancy in the neutral isolated atom (d(1), d(4), d(6)), with the additional d electron distributed over the 5 d orbitals, with both alpha and beta spin. The electronic structure for the ground state excludes the Jahn Teller deformation and the orbital ordering, so that the energy gain associated to the symmetry reduction from the ideal cubic structure is due to the rotation of the octahedra. This is here explored reducing the symmetry from the cubic (Pm3(-)m, N. 221), to the tetragonal (P4/m bm, N.127, and I 4/m cm, N. 140), and finally to the orthorhombic m (Pbnm, N.62) space group, with an overall energy gain of 28, 7 and 14 mE(h) for LaScO3, LaCrO3 , and LaFeO3 , respectively, corresponding to the rotations rather than the deformation of the octahedra, since the latter is extremely small. This study is complemented by the density of states, spin density maps, and ferro-antiferro energy differences.

The structural, electronic, and magnetic properties of the KCrF3 perovskite have been investigated using an all-electron Gaussian-type basis set and various functionals, including full-range and range-separated hybrids, as implemented in the crystal code. Structural optimizations were performed by imposing tetragonal (I4/mcm) and monoclinic (I112/m) space-group symmetries, corresponding to the experimentally observed phases above and below 250 K, respectively. Three AFM arrangements were considered and compared with the FM one: the AFMA phase (spin inversion between first-neighbor Cr ions along the c-axis) is more stable than the FM phase, in agreement with experimental evidence. The monoclinic I112/m structure is energetically favored over the tetragonal I4/mcm phase by 5.9 meV per transition-metal ion with the B3LYP functional, and by 3.4 meV when using PBE0 or HSE06. This small energy difference is associated with only minor changes in structural and electronic properties, including a slight volume contraction (-0.4%) and increase in the band gap (+2%). Mulliken population analysis indicates a Cr d-shell occupation of 3.907 |e|, which is very close to the formal d4 configuration. Nearly exactly three electrons populate the t2g manifold (0.974 |e| each), while the remaining electron is distributed between the dz2 (0.360 |e|) and dx2-y2 (0.624 |e|) orbitals. Spin-density maps clearly show the orbital ordering within the ab plane and provide insight into the stabilization mechanism of the AFMA phase, which is mediated by the polarization of the fluorine valence shell.

Wrinkles are out-of-plane deformations, commonly seen in chemical vapor deposition (CVD) grown graphene, that mostly arise from thermal expansion mismatches. These one-dimensional corrugations are believed to be centers of altered electrical and electronic properties for graphene. Herein, high-resolution electrical modes of Atomic Force Microscopy (AFM) were employed to measure the nanoscale current and work function distribution of graphene wrinkles. Tapping current measurements showcased an unforeseen increase up to two orders of magnitude compared to flat regions. As revealed by extensive first principles calculations (density functional theory employing CAM-B3LYP and PBE0 functionals) and experimental data (AFM advanced electrical modes), an interplay of mechanisms between polarization, separation from the substrate and strain gradients result in an impressive increase in vertical conductivity with possible implications for analogous surge also along wrinkle axes. Furthermore, characteristic variations in the work function of wrinkles, with respect to various substrates, could be clearly identified. The implications of our findings may pave the way for fine-regulation of conductivity via wrinkle engineering for the previously unexplored out-of-plane direction, as well as for the controlled formation of conductive channels.

The electronic and magnetic properties of diluted magnetic semiconductor (DMS) materials of the general formula NaY1-xCexSe2 (0.00 ≤ x ≤ 1.00) are studied using density functional theory (DFT) calculations. Alloys with fractional composition of the two rare earth elements (Y and Ce), which have not yet been synthesized, are predicted to exhibit magnetic transition and potential quantum spin liquid properties. As Ce concentration increases, these alloys transition from non-magnetic (NaYSe2) to antiferromagnetic (AFM), with NaCeSe2 forming magnetic layers of Ce3+ ions in a 2D triangular lattice. This structure suggests the potential for quantum spin liquid behavior. The Néel temperature (TN) of these compounds increases with Ce concentration, reaching 114 K at 100

This paper presents a multiscale approach to evaluate perovskite solar cell performance which determines material properties at the atomistic scale with first-principles calculations, and applies them in macro-scale device models. This work focuses on the MAPbI3 (MA = CH3NH3) perovskite and how its phase transitions impact on its optical, electronic, and structural properties which are investigated at the first-principles level. The obtained data are coupled to a numerical drift-diffusion device model enabling evaluation of the performance of corresponding single junction devices. The first-principles simulation applies a hybrid exchange-correlation functional adapted to the studied family of compounds. Validation by available experimental data is presented from materials properties to device performance, justifying the use of the approach for predictive evaluation of existing and novel perovskites. The coupling between atomistic and device models is described in terms of a framework for exchange of optical, vibrational, and electronic parameters between the two scales. The result of this theoretical investigation is a methodology for designing and optimising perovskite materials for both cell performance and stability, the key obstacle in the societal implementation of these record-breaking new materials.

Metal nanoclusters are atomically precise materials comprising metal core of few atoms exhibiting unique photoluminescence properties, unlike their bigger counterparts. Some metal nanocluster with ligand-to-metal charge transfer, long-lived excited state and excited triplet state contribute to inherent photosensitizing (PS) property. However, the therapeutic efficacy of PDT is hindered by the insufficient oxygen supply (O2) in tumor microenvironment. In the present work, cysteine-capped gold nanocluster (AuC) are studied for their unique molecular architecture for PS efficiency. The co-existence of monodispersed and self-assembled structures contribute to the photoluminescence from the quantum confinement of electronic states and aggregation-induced emission (AIE) based PS property, respectively. In-silico model was performed to study the interaction of cysteine to gold cluster, its ground and excited-state properties and the charge transfer mechanism. The AuC as PS generates cytotoxic radicals in both Type I and Type II photodynamic pathways and the dominant radical species involved were elucidated by EPR spectroscopy. In vitro analysis in HeLa cells showed excellent biocompatibility and bioimaging properties. The intracellular ROS production and Live/Dead assay confirmed the generation of ROS in HeLa cells upon laser irradiation. The image-guided photodynamic property with synergistic Type I and Type II PDT reactions of AuC promises its potential application in cancer therapy in both hypoxic and normoxic conditions

Abstract The crystal structure and ferroelectric properties of epitaxial ZrO2 films ranging from 7 to 42 nm thickness grown on La0.67Sr0.33MnO3 buffered (110)‐oriented SrTiO3 substrate are reported. By employing X‐ray diffraction, a tetragonal phase (t‐phase) at all investigated thicknesses, with slight in‐plane strain due to the substrate in the thinnest films, is confirmed. Further confirmation of the t‐phase is obtained through infrared absorption spectroscopy with synchrotron light, performed on ZrO2 membrane transferred onto a high resistive silicon substrate. Up to a thickness of 31 nm, the ZrO2 epitaxial films exhibit ferroelectric behavior, at variance with the antiferroelectric behavior reported previously for the t‐phase in polycrystalline films. However, the ferroelectricity is found here to diminish with increasing film thickness, with a polarization of 13 µC cm−2 and down to 1 µC cm−2 for 7 and 31 nm thick ZrO2 films, respectively. Given that the t‐phase is nonpolar, the observations emphasize the influence of external factors, in promoting polarization in t‐ZrO2 thin films. These findings provide new insights into the ferroelectric properties and structure of ZrO2 thin films, and open up new directions to investigate the origin of ferroelectricity in ZrO2 and to optimize this material for future applications.

The electronic and optical properties of a NaLaS2 compound, doped with different concentrations of Se and Te atoms, are explored using periodic density functional theory calculations. The primary objective is to identify light-absorbing materials for use in photovoltaic applications. It is found that the dopant concentration can induce a crystalline structure phase transition from cubic to trigonal, which is accompanied with drastic changes in both the nature (direct/indirect) and extent of the energy band gap. As the proportion of selenium atoms in the NaLaS2(1-x)Se2x alloys increases from 0 to 100%, the gap decreases from 2.93 to 2.41 eV, respectively. For the NaLaS2(1-x)Te2x alloys, the gap undergoes a (sharp) decrease reaching as low as 1.75 eV as the proportion of tellurium increases to 75%. The transport properties of the alloys reveal a significant drop in the effective mass and exciton binding energies, which is particularly marked for the NaLaS2(1-x)Te2x alloys with a Te concentration range of 25-100%. The exciton binding energy for NaLaS0.5Te1.5 is 4.37 meV, much less than the thermal energy at room temperature (25 meV). Therefore, the NaLaS0.5Te1.5 alloy shows potential as a light-absorbing material for photovoltaic applications that is worthy of further investigations.

An analysis of orbital magnetization in insulators is provided. It is shown that a previously proposed electronic orbital angular-momentum operator generalizes the "modern theory of orbital magnetization" to include nonlocal Hamiltonians. Expressions for magnetic transition dipole moments needed for the calculation of optical rotation and other properties are developed. A variety of issues that arise in this context are critically analyzed. These issues include periodicity of the operators, previously proposed band dispersion terms, and, if and where needed, evaluation of reciprocal space derivatives of orbital coefficients. Our treatment is used to determine the optical rotatory power of insulators employing a formulation that accounts for electric dipole-electric quadrupole (DQ), as well as electric dipole-magnetic dipole, contributions. An implementation in the public CRYSTAL program is validated against a model finite system and applied to the & alpha;-quartz mineral through linear-response timedependent density functional theory with a hybrid functional. The latter calculations confirmed the importance of DQ terms. Agreement against experiment was only possible with (i) use of a high-quality basis set, (ii) inclusion of a fraction of nonlocal Fock exchange, and (iii) account of orbital-relaxation terms in the calculation of response functions.

We report the Pockels electro-optic properties of ZrO2 and HfO2 orthorhombic Pbc21 and rhombohedral R3m ferroelectric phases, and we compare them to the well-known LiNbO3 Pockels material from density functional theory calculations using the CRYSTAL suite of programs. Specifically, three essential processes are explicitly investigated: The electronic, the ionic (or vibrational), and the piezoelectric contributions. Our calculations reveal that the ionic part coming from the low frequency phonon modes contributes the most to the electro-optic coefficients of rhombohedral LiNbO3 and of orthorhombic ZrO2 and HfO2. While these low-frequency modes show zero contribution to Pockels coefficients for the rhombohedral phase of the latter compounds.

We report on the crystal structure and ferroelectric properties of epitaxial ZrO$_2$ films ranging from 7 to 42 nm thickness grown on La$_{0.67}$Sr$_{0.33}$MnO$_3$-buffered (110)-oriented SrTiO$_3$ substrate. By employing X-ray diffraction, we confirm a tetragonal phase at all investigated thicknesses, with slight in-plane strain due to the substrate in the thinnest films. Further confirmation of the tetragonal phase was obtained through Infrared absorption spectroscopy with synchrotron light, performed on ZrO$_2$ membrane transferred onto a high resistive Silicon substrate. Up to a thickness of 31 nm, the ZrO$_2$ epitaxial films exhibit ferroelectric behavior, at variance with the antiferroelectric behavior reported previously for the tetragonal phase in polycrystalline films. However, the ferroelectricity is found here to diminish with increasing film thickness, with a polarization of about 13 $\mu$C.cm$^{-2}$ and down to 1 $\mu$C.cm$^{-2}$ for 7 nm and 31 nm-thick ZrO$_2$ films, respectively. This highlights the role of thickness reduction, substrate strain, and surface effects in promoting polarization in the tetragonal ZrO$_2$ thin films. These findings provide new insights into the ferroelectric properties and structure of ZrO$_2$ thin films, and open up new directions to investigate the origin of ferroelectricity in ZrO$_2$ and to optimize this material for future applications.

Structural, electronic, and thermoelectric properties of bridging OHb and terminal OHt groups adsorbed on stoichiometric SnO2 (110) surfaces have been investigated using density functional theory and semiclassical Boltzmann transport theory with effective core pseudopotential implemented in CRYSTAL17 program. Our results indicate that H and OH yield significant structural relaxation around the adsorption sites O-2c and Sn-5c. The results have shown that the absolute value of adsorption energy increases with decreasing the coverage from 1 to 1/4 monolayer. Mulliken charge analysis, band structures, and density of states were calculated and discussed. We found that H and OH adsorption increases the band gap energy from 2.81 eV for clean surfaces to 3.04, 2.95, and 2.89 eV with, respectively, 1, 1/2 and 1/4 monolayer surface coverages. Thermoelectric properties revealed that the presence of hydroxyl groups on the SnO2 (110) surfaces may enhance the Seebeck coefficient, electrical conductivity, and electronic thermal conductivity.