Lead halide perovskites constitute a very promising class of materials for a broad range of solar and opto-electronic applications. Perovskites exhibit many unusual properties, and recent experiments demonstrate an unusual temperature dependence of charge carrier lifetimes. Focusing on the all-inorganic CsPbBr3, and using a combination of ab initio nonadiabatic molecular dynamics and time-domain density functional theory, we demonstrate that the unconventional behavior arises due to a highly anharmonic nature of atomic motions in perovskites. As temperature increases, perovskite structure undergoes a notable deformation, reflected in tilting of octahedral units, and experiences large scale anharmonic movements away from the equilibrium geometry. As a result, the electronic energy gap increases, and phonon-induced loss of coherence within the electronic subsystem accelerates. These two factors slow down nonradiative electron-hole recombination, which constitutes the main limitation on efficiencies of perovskite solar, optical and electronic devices. The increase of charge carrier lifetimes with temperature is particularly beneficial in applications, because materials heat up, for instance, from sunlight during solar energy harvesting. The behavior of the all-inorganic halide perovskite investigated here is different from that of hybrid organic-inorganic perovskites, which exhibit additional disorder associated with reorientations of the asymmetric organic cations. The reported simulations generate an in-depth understanding of the unusual properties of inorganic perovskites, relevant for photocatalytic, photovoltaic, electronic and optical applications.
The distribution of charge carriers in metal halide perovskites draws strong interest of the solar cell community, with experiments demonstrating that edges of various microstructures can improve material performance. This is rather surprising since edges and grain boundaries are often viewed as the main source of charge traps. We demonstrate by ab initio quantum dynamics simulations that edges of the CH3NH3PbBr3 perovskite create shallow trap states that mix well with the valence and conduction bands of the bulk, and therefore, support mobile charge carriers. Charges are steered to the edges energetically, facilitating dissociation of photo−generated excitons into free carriers. The edge−driven charge separation extends carrier lifetimes due to decreased overlap of the electron and hole wave functions, which leads to reduction of the nonadiabatic coupling responsible for nonradiative electron−hole recombination. Reduction of spatial symmetry near the edges activates additional vibrational modes that accelerate coherence loss within the electronic subsystem, further extending carrier lifetimes. Enhanced atomic motions at edges increase fluctuations of edge energy levels, enhancing mixing with band states and improving charge mobility. The simulations contribute to the atomistic understanding of the unusual properties of metal halide perovskites, generating the fundamental knowledge needed to design high−performance optoelectronic devices.
In pnictide RbEuFe4As4, superconductivity sets in at 36 K and coexists, below 15−19 K, with the long-range magnetic ordering of Eu 4f spins. Here we report scanning tunneling experiments performed on cold-cleaved single crystals of the compound. The data revealed the coexistence of large Rb-terminated and small Eu-terminated terraces, both manifesting 1 × 2 and \sqrt 2 × \sqrt 2 reconstructions. On \sqrt 2 × \sqrt 2 surfaces, a hidden electronic order with a period ∼5 nm was discovered. A superconducting gap of ∼7 meV was seen to be strongly filled with quasiparticle states. The tunneling spectra compared with density functional theory calculations confirmed that flat electronic bands due to Eu 4f orbitals are situated ∼1.8eV below the Fermi level and thus do not contribute directly to Cooper pair formation.
NaCl is one of the simplest compounds and was thought to be well-understood, and yet, unexpected complexities related to it were uncovered at high pressure and in low-dimensional states. Here, exotic hexagonal NaCl thin films on the (110) diamond surface were crystallized in the experiment following a theoretical prediction based on ab initio evolutionary algorithm USPEX. State-of-the-art calculations and experiments showed the existence of a hexagonal NaCl thin film, which is due to the strong chemical interaction of the NaCl film with the diamond substrate.
We demonstrate that halide content strongly affects nonradiative electron–hole recombination in all-inorganic perovskite quantum dots (QDs). Using time domain density functional theory and nonadiabatic molecular dynamics, we show that replacing half of the bromines with iodines in a CsPbBr3 QD extends the charge carrier lifetime by a factor of 5, while complete replacement extends the lifetime by a factor of 8. Doping with iodines decreases the nonadiabatic charge–phonon coupling because iodines are heavier and slower than bromines and because the overlap between the electron and hole wave functions is reduced. In general, the nonradiative electron–hole recombination proceeds slowly, on a nanosecond time scale, due to small sub-1 meV nonadiabatic coupling and short sub-10 fs coherence times. The obtained recombination times and their dependence on the halogen content show excellent agreement with experiments. Our study suggests that the power conversion efficiencies of solar cells can be controlled by changing the halide composition in all-inorganic perovskite QDs.
An attractive two-dimensional semiconductor with tunable direct bandgap and high carrier mobility, black phosphorus (BP) is used in batteries, solar cells, photocatalysis, plasmonics and optoelectronics. BP is sensitive to ambient conditions, with oxygen playing a critical role in structure degradation. Our simulations show that BP oxidation slows down charge recombination. This is unexpected, since typically charges are trapped and lost on defects. First, BP has no ionic character. It interacts with oxygen and water weakly, experiencing little perturbation to electronic structure. Second, phosphorus supports different oxidation states and binds extraneous atoms avoiding deep defect levels. Third, soft BP structure can accommodate foreign species without disrupting periodic geometry. Finally, BP phonon scattering on defects shortens quantum coherence and suppresses recombination. Thus, oxidation can be regarded as production of a self-protective layer that improves BP properties. These BP features should be common to other mono-elemental 2D materials, stimulating energy and electronics applications.
Quantum dot (QD) solids represent a new type of condensed matter drawing high fundamental and applied interest. Quantum confinement in individual QDs, combined with macroscopic scale whole materials, leads to novel exciton and charge transfer features that are particularly relevant to optoelectronic applications. This Perspective discusses the structure of semiconductor QD solids, optical and spectral properties, charge carrier transport, and photovoltaic applications. The distance between adjacent nanoparticles and surface ligands influences greatly electrostatic interactions between QDs and, hence, charge and energy transfer. It is almost inevitable that QD solids exhibit energetic disorder that bears many similarities to disordered organic semiconductors, with charge and exciton transport described by the multiple trapping model. QD solids are synthesized at low cost from colloidal solutions by casting, spraying, and printing. A judicious selection of a layer sequence involving QDs with different size, composition, and ligands can be used to harvest sunlight over a wide spectral range, leading to inexpensive and efficient photovoltaic devices.
Supercapacitors based on carbon nanomaterials are attracting much attention because of their high capacitance enabled by large specific surface area. The introduction of heteroatoms such as N or O enhances the specific capacitance of these materials. However, the mechanisms that lead to the increase in the specific capacitance are not yet well-studied. In this Letter, we demonstrate an effective method for modification of the surface of carbon nanowalls (CNWs) using DC plasma in atmospheres of O2, N2, and their mixture. Processing in the plasma leads to the incorporation of ∼4 atom % nitrogen and ∼10 atom % oxygen atoms. Electrochemical measurements reveal that CNWs functionalized with oxygen groups are characterized by higher capacitance. The specific capacitance for samples with oxygen reaches 8.9 F cm−3 at a scan rate of 20 mV s−1. In contrast, the nitrogen-doped samples demonstrate a specific capacitance of 4.4 F cm−3 at the same scan rate. The mechanism of heteroatom incorporation into the carbon lattice is explained using density functional theory calculations.
Recently, a new phase of hydrogen hydrates has been observed at ∼5−7 kbar and ∼170−250 K. X-ray diffraction patterns do not allow determination of its structure unambiguously. In this work, we perform classical molecular dynamics simulation of hydrogen hydrates and select two possible structures. One of these structures is not a typical clathrate and has never been observed for hydrates. In this study, we pay special attention to the choice of the model parameters in order to reveal the corresponding sensitivity of the results.