Dielectric function of hybrid perovskites at finite temperature investigated by classical molecular dynamics.

Ionic polarization and dielectric function play a fundamental role in the optoelectronic properties of hybrid perovskites, currently one of the most studied materials for next generation photovoltaics. The hybrid nature of the crystal, with molecular dipoles that can reorient within the inorganic lattice, gives rise to a complex dielectric response in the bulk material that has been largely studied and debated. Here, we investigate the nature and the relaxation properties of the dielectric polarization of hybrid perovskites at finite temperature by means of classical molecular dynamics. We provide evidence that a simple ionic model of classical interatomic forces is able to explain qualitatively the temperature and frequency dependence of the dielectric constant providing a picture that is fully consistent with experimental data. The constant dielectric function in the low-temperature phase is controlled by ionic displacements, while the temperature-dependent paraelectric behavior of the tetragonal phase is due to reorientation of dipoles that are responsible for the discontinuity at the orthorhombic-to-tetragonal transition. In the frequency domain, the molecular reorientations give rise to a broad band that is located in the 0.1 THz timescale at room temperature and that shifts down to the GHz timescale when cooling the system toward the tetragonal-to-orthorhombic phase transition. The relation between relaxation time and maximum absorption frequency is also clarified.

Low electron-polar optical phonon scattering as a fundamental aspect of carrier mobility in methylammonium lead halide CH3NH3PbI3 perovskites.

High carrier mobility is often invoked to justify the exceptionally long diffusion length in CH3NH3PbI3 perovskites. Using a combination of an ab initio band structure and scattering models, we present clear evidence that large electrical and Hall mobilities are crucially related to the low scattering rate of carriers with polar optical phonons, which represents the dominant mobility-limiting mechanism at room temperature. With a charge-injection regime at room temperature, we obtained carrier relaxation times (τrel) of ∼10 fs, which are typical of polar inorganic semiconductors, and electrical mobilities (µ) as high as ∼60 cm(2) V(-1) s(-1) and 40 cm(2) V(-1) s(-1) for electrons and holes, respectively, which were robustly independent on the injected carrier density in the range of n ∼ 10(14) cm(-3) to 10(20) cm(-3). In the absence of a significant concentration of trapping centers, these mobilities foster diffusion lengths of ∼10 µm for the low injection density regime (n ∼ 10(15) cm(-3)), which are in agreement with recent measurements for highly pure single-crystal perovskites.

Thermally Activated Point Defect Diffusion in Methylammonium Lead Trihalide: Anisotropic and Ultrahigh Mobility of Iodine.

We study the diffusion of point defects in crystalline methylammonium lead halide (MAPI) at finite temperatures by using all-atoms molecular dynamics. We find that, for what concerns intrinsic defects, iodine diffusion is by far the dominant mechanism of ionic transport in MAPI, with diffusivities as high as 7.4 × 10(-7) and 4.3 × 10(-6) cm(2) s(-1) at 300 K and single activation energies of 0.24 and 0.10 eV, for interstitials and vacancies, respectively. The comparison with common covalent and oxide crystals reveals the ultrahigh mobility of defects in MAPI. Though at room temperature the vacancies are about 1 order of magnitude more diffusive, the anisotropic interstitial dynamics increases more rapidly with temperature, and it can be dominant at high temperatures. Present results are fully consistent with the involvement of iodide ions in hysteresis and have implications for improvement of the material quality by better control of defect diffusion.

Temperature Evolution of Methylammonium Trihalide Vibrations at the Atomic Scale.

The temperature evolution of vibrations of CH3NH3PbI3 (MAPI) is studied by combining first principles and classical molecular dynamics and compared to available experimental data. The work has a fundamental character showing that it is possible to reproduce the key features of the vibrational spectrum by the simple physical quantities included in the classical model, namely the ionic-dispersive hybrid interactions and the mass difference between organic and inorganic components. The dynamics reveals a sizable temperature evolution of the MAPI spectrum along with the orthorhombic-to-tetragonal-to-cubic transformation and a strong dependence on molecular confinement and order. The thermally induced weakening of the H-I interactions and the anharmonic mixing of modes give two vibrational peaks at 200-250 cm(-1) that are not present at zero temperature and are expected to have detectable infrared activity. The infrared inactive vibrational peak at ∼140 cm(-1) due to molecular spinning disappears abruptly at the orthorhombic-to-tetragonal transition and forms a broad molecular band red-shifting progressively with temperature. This trend is correlated to the reduced confinement of the rotating cations due to thermal expansion of the lattice.

The study of polythiophene/water interfaces by sum-frequency generation spectroscopy and molecular dynamics simulations.

Semiconducting polymer/water interfaces are gaining increasing attention due to a variety of promising applications in the fields of biology and electrochemistry, such as electrochemically-gated transistors and photodetectors, which have been used for biosensing and neuroscience applications. However, a detailed characterization of the polymer surface in the presence of an aqueous environment is still lacking. In this work, we employed sum-frequency generation vibrational spectroscopy, a surface-specific technique compatible with electrochemical/biological conditions, to demonstrate that the surface of thin films of regio-regular poly-3-hexylthiophene (rr-P3HT) undergoes a molecular reorientation when exposed to aqueous electrolytes, with respect to their surface structure in air. Experimental results are corroborated by molecular dynamics simulations. Since surface molecular orientation is believed to play a fundamental role in electrochemical and environmental stability of conjugated polymers, the reported findings not only contribute to the fundamental understanding of conjugated polymer/water interfaces, but they may also have implications in the design of conjugated polymers for enhancing their performance in electrolytic environments.

Effects of TIPS-functionalization and perhalogenation on the electronic, optical, and transport properties of angular and compact dibenzochrysene.

We report a systematic comparative study on dibenzo[b,def]chrysene (angular) and dibenzo[def,mno] chrysene (compact) polyaromatic hydrocarbons and their bis-triisopropylsilylethynyl (TIPS)-functionalized and perhalogenated (F, Cl) counterparts. We used density functional theory (DFT) and time-dependent DFT to quantify the effects of morphology and chemical modifications on the electronic, optical, and transport properties. In particular, we compared electron affinity, ionization energy, fundamental gap, optical absorption, exciton binding energy, and reorganization energies for holes and electrons. For both TIPS-functionalization and halogen substitutions, we found larger electron affinities (nearly tripled with perchlorination). Ionization energies are found to be reduced for TIPS-functionalization (by ∼5%) and enhanced following halogen substitution (up to 17%). In both compact and angular dibenzochrysenes, the above trends reflect in a general reduction of the fundamental gap (up to 22%) following chemical modification. The effect of perhalogenation and TIPS-functionalization is always to increase molecular reorganization energies for both holes and electrons. Concerning the optical properties, we observe a redshift of the optical onset in all cases; for TIPS-functionalized molecules, in particular, we additionally found a remarkable enhancement of the absorption in the visible region.

Quantum confinement by an order-disorder boundary in nanocrystalline silicon.

We predict theoretically and show experimentally the occurrence of quantum confinement in hydrogenated nanocrystalline silicon. We prove that only valence states (positively charged carriers) are confined effectively within the nanograins. The emission associated to confined states is verified by photoluminescence experiments on nanocrystalline samples with controlled grain size. According to the present study, we propose nanocrystalline silicon as a promising material for oxygen-free optoelectronics, silicon-based memories and photovoltaics.

Atomic scale origin of crack resistance in brittle fracture.

We investigate the physical meaning of the intrinsic crack resistance in the Griffith theory of brittle fracture by means of atomic-scale simulations. By taking cubic SiC as a typical brittle material, we show that the widely accepted identification of intrinsic crack resistance with the free surface energy underestimates the energy-release rate. The strain dependence of the Young modulus and surface energy, as well as allowance for lattice trapping, improve the estimate of the crack resistance. In the smallest scale limit, crack resistance can be fitted by an empirical elastoplastic model.

DNA strand break rejoining in human lymphocytes during the S phase.

Induction and repair of DNA strand breaks in asynchronous and synchronized cultures of human lymphocytes was investigated by using the alkaline DNA-unwinding technique followed by chromatography on hydroxylapatite. Strand break rejoining in exponentially growing human PHA stimulated lymphocytes, irradiated with 20 Gy of X-rays, is temperature-dependent, being fast at 37 degrees C (half-time of a few minutes), and very slow at around 4 degrees C. In synchronized cells irradiated with the same X-ray dose, the repair capacity increases during S phase reaching its maximum when DNA is entirely duplicated.

[Effect of x-rays on the intracellular transport of thymidine in human lymphocytes stimulated by phytohemagglutinins]. / Effetto dei raggi X sul trasporto intracellulare della timidina in linfociti umani PHA stimolati.

The effect of X-irradiation on thymidine transport in human lymphocytes PHA stimulated was investigated. Mediated transport sistem, the predominant mechanism at low extracellular concentration of thymidine (less than 10(-7) M) in the medium is highly radiosensitive. The transport sistem was damaged considerably by high doses of X-rays (at least until 10 Krad); the decrease of thymidine uptake was a function of the time of incubation after irradiation. It suggest that repair mechanisms are not involved at high doses of X-rays within 120 minutes of incubation.