Proses Produksi Dan Pengemasan Yoghurtdicv. Cita Nasional Salatiga

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The Nature of Electron Mobility in Hybrid Perovskite CH₃NH₃PbI₃

CH₃NH₃PbI₃ is one of the most promising candidates for cheap and high-efficiency solar cells. One of its unique features is the long carrier diffusion length (>100 μm), but its carrier mobility is rather modest. The nature of the mobility is unclear. Here, using nonadiabatic wave function dynamics simulations, we show that the random rotations of the CH₃NH₃ cations play an important role in the carrier mobility. Our previous work showed that the electron and hole wave functions were localized and spatially separated due to the random orientations of the CH₃NH₃ cations in the tetragonal phase. We find that the localized carriers are able to conduct random walks due to the electrostatic potential fluctuation caused by the CH₃NH₃ random rotations. The calculated electron mobilities are in the experimentally measured range. We thus conclude that the carrier mobility of CH₃NH₃PbI₃ is likely driven by the dynamic disorder that causes the fluctuation of the electrostatic potential

Thermodynamic Oxidation and Reduction Potentials of Photocatalytic Semiconductors in Aqueous Solution

An approach is introduced to calculate the thermodynamic oxidation and reduction potentials of semiconductors in aqueous solution. By combining a newly developed ab initio calculation method for compound formation energy and band alignment with electrochemistry experimental data, this approach can be used to predict the stability of almost any compound semiconductor in aqueous solution. Thirty photocatalytic semiconductors have been studied, and a graph (a simplified Pourbaix diagram) showing their valence/conduction band edges and oxidation/reduction potentials relative to the water redox potentials is produced. On the basis of this graph, the thermodynamic stabilities and trends against the oxidative and reductive photocorrosion for compound semiconductors are analyzed, which shows the following: (i) some metal oxides can be resistant against the oxidation by the photogenerated holes when used as the n-type photoanodes; (ii) all the nonoxide semiconductors are susceptible to oxidation, but they are resistant to the reduction by the photogenerated electrons and thus can be used as the p-type photocathodes if protected from the oxidation; (iii) doping or alloying the metal oxide with less electronegative anions can decrease the band gap but also degrade the stability against oxidation

Dynamic Disorder and Potential Fluctuation in Two-Dimensional Perovskite

The structural and electronic properties of 2D perovskite (C₄H₉NH₃)₂PbBr₄ are investigated from first-principles calculations. It is found that despite the existence of carbon chain, the organic molecule C₄H₉NH₃ in 2D perovskite is able to rotate at room temperature, showing a highly dynamic behavior. The dynamic disorder of C₄H₉NH₃ introduces dynamic potential fluctuation, which is sufficient to localize the wave function and to separate the valence band maximum and conduction band minimum states. We further showed that, rather than a pure dipole rotation model, the disorder effect of the molecules can be better described by the motion of the net charge centers of the molecules, which contributes to the potential fluctuation. Hence, polar molecule is not the necessary condition to create potential fluctuation in perovskite, which is further demonstrated by the calculations of 2D inorganic perovskite Cs₂PbBr₄

Nanoscale Charge Localization Induced by Random Orientations of Organic Molecules in Hybrid Perovskite CH₃NH₃PbI₃

Perovskite-based solar cells have achieved high solar-energy conversion efficiencies and attracted wide attentions nowadays. Despite the rapid progress in solar-cell devices, many fundamental issues of the hybrid perovskites have not been fully understood. Experimentally, it is well-known that in CH₃NH₃PbI₃ the organic molecules CH₃NH₃ are randomly orientated at the room temperature, but the impact of the random molecular orientation has not been investigated. Because of the dipole moment of the organic molecule, the random orientation creates a novel system with long-range potential fluctuations unlike alloys or other conventional disordered systems. Using linear scaling ab initio methods, we find that the charge densities of the conduction band minimum and the valence band maximum are localized in nanoscales due to the potential fluctuations. The charge localization causes electron–hole separation and reduces carrier recombination rates, which may contribute to the long carrier lifetime observed in experiments

Electronic Properties of Electrical Vortices in Ferroelectric Nanocomposites from Large-Scale Ab Initio Computations

An original ab initio procedure is developed and applied to a ferroelectric nanocomposite, in order to reveal the effect of electrical vortices on electronic properties. Such procedure involves the combination of two large-scale numerical schemes, namely, the effective Hamiltonian (to incorporate ionic degrees of freedom) and the linear-scaling three-dimensional fragment method (to treat electronic degrees of freedom). The use of such procedure sheds some light into the origin of the recently observed current that is activated at rather low voltages in systems possessing electrical vortices. It also reveals a novel electronic phenomena that is a systematic control of the type of the band-alignment (i.e., type I versus type II) within the same material via the temperature-driven annihilation/formation of electrical topological defects

Tolerance of Intrinsic Defects in PbS Quantum Dots

Colloidal quantum dots exhibit various defects and deviations from ideal structures due to kinetic processes, although their band gap frequently remains open and clean. In this Letter, we computationally investigate intrinsic defects in a real-size PbS quantum dot passivated with realistic Cl-ligands. We show that the colloidal intrinsic defects are ionic in nature. Unlike previous computational results, we find that even nonideal, atomically nonstoichiometric quantum dots have a clean band gap without in-gap-states provided that quantum dots satisfy electronic stoichiometry

Electron Beam Manipulation of Nanoparticles

We report on electron beam manipulation and simultaneous transmission electron microscopy imaging of gold nanoparticle movements in an environmental cell. Nanoparticles are trapped with the beam and move dynamically toward the location with higher electron density. Their global movements follow the beam positions. Analysis on the trajectories of nanoparticle movements inside the beam reveals a trapping force in the piconewton range at the electron density gradient of 10³–10⁴ (e·nm^–2·s^–1)·nm^–1. Multiple nanoparticles can also be trapped with the beam. By rapidly converging the beam, we further can “collect” nanoparticles on the membrane surface and assemble them into a cluster

Electron Beam Manipulation of Nanoparticles

We report on electron beam manipulation and simultaneous transmission electron microscopy imaging of gold nanoparticle movements in an environmental cell. Nanoparticles are trapped with the beam and move dynamically toward the location with higher electron density. Their global movements follow the beam positions. Analysis on the trajectories of nanoparticle movements inside the beam reveals a trapping force in the piconewton range at the electron density gradient of 10³–10⁴ (e·nm^–2·s^–1)·nm^–1. Multiple nanoparticles can also be trapped with the beam. By rapidly converging the beam, we further can “collect” nanoparticles on the membrane surface and assemble them into a cluster

Electron Beam Manipulation of Nanoparticles

We report on electron beam manipulation and simultaneous transmission electron microscopy imaging of gold nanoparticle movements in an environmental cell. Nanoparticles are trapped with the beam and move dynamically toward the location with higher electron density. Their global movements follow the beam positions. Analysis on the trajectories of nanoparticle movements inside the beam reveals a trapping force in the piconewton range at the electron density gradient of 10³–10⁴ (e·nm^–2·s^–1)·nm^–1. Multiple nanoparticles can also be trapped with the beam. By rapidly converging the beam, we further can “collect” nanoparticles on the membrane surface and assemble them into a cluster