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.

We present a first-principles approach to calculate the phonon thermal conductivity based on the Green-Kubo formalism. In this approach, the density functional theory energy is distributed to each atom, and a two-step method in the molecular dynamics is introduced to avoid the atomic position R wrapping problem in a periodic system when the heat current is calculated. We show that this first-principles Green-Kubo approach is particularly suitable for disordered systems like amorphous and liquid, where the thermal conductivities are small due to strong phonon scattering but difficult to be calculated using anharmonic interaction energy. We have applied our method to liquid Ar, liquid Si, and amorphous Si. The calculated thermal conductivities agree well with previous theoretical and experimental results. We have also compared our method to previous works combining first-principles simulations with the Green-Kubo formalism.

The mixed quantum-classical nonadiabatic molecular dynamics (NAMD) is a powerful tool to study many phenomena, especially ultrafast carrier transport and cooling. Carrier decoherence and detailed balance are two major issues in NAMD. So far, there is no computationally inexpensive approach to incorporate both effects. While the decoherence effect can be easily included in the state density matrix formalism and the detailed balance can be included in surface hopping or the wave function collapse approach, it is difficult to include both of them in a unified formalism. In this work we introduce a state density matrix formalism (referred to as P-matrix) including both the decoherence and detailed balance effects for NAMD. This method is able to explicitly treat the decoherence between different pairs of adiabatic states. Moreover, the off-diagonal density matrix elements are divided into two parts, corresponding to energy-increasing and energy-decreasing transitions. The detailed balance is then enforced by a Boltzmann factor applied to the energy-increasing transition part. The P-matrix formalism is applied to study hot-hole cooling and transfer processes in Si quantum dot (QD) systems. The calculated hot-carrier relaxation time is consistent with experiments. In a QD-pair system, the hot-hole cooling time shows weak dependence on the QD spacing. However, the hot-carrier transfer rate from one QD to another is found to decrease exponentially with the QD-QD distance. When the QD spacing is small (∼1 nm), the hot-carrier transfer can be very efficient. It is also shown that the explicit treatment of decoherence time is important in order to treat this hot-carrier transfer correctly.

The formation energies and charge-transition levels of intrinsic point defects in lead halide perovskite CsPbBr₃ are studied from first-principles calculations. It is shown that the formation energy of dominant defect under Br-rich growth condition is much lower than that under moderate or Br-poor conditions. Thus avoiding the Br-rich condition can help to reduce the defect concentration. Interestingly, CsPbBr₃ is found to be highly defect-tolerant in terms of its electronic structure. Most of the intrinsic defects induce shallow transition levels. Only a few defects with high formation energies can create deep transition levels. Therefore, CsPbBr₃ can maintain its good electronic quality despite the presence of defects. Such defect tolerance feature can be attributed to the lacking of bonding-antibonding interaction between the conduction bands and valence bands.

First-principles calculations have been performed on the band structure of mono- and few-layer TiS3 nanofilms. It is found that the band gap character of the TiS3 films is quite robust, almost independent of layer thickness, vertical strain and stacking order, which is in sharp contrast to most other two-dimensional materials, such as MoS2. The robustness of the band gap originates from the location of the CBM and VBM states, which are at the center atoms of TiS3, and are thus unaffected by the layer-layer coupling. Such a property of TiS3 nanofilms promises good application potential in nanoelectronics and optoelectronics, and also makes TiS3 a good platform to study the electronic properties of a material in the two-dimensional limit.

As a typical transition metal dichalcogenide, MoS₂ offers numerous advantages for nanoelectronics and electrochemical energy storage due to its unique layered structure and tunable electronic properties. When used as the anode in lithium-ion cells, MoS₂ undergoes intercalation and conversion reactions in sequence upon lithiation, and the reversibility of the conversion reaction is an important but still controversial topic. Here, we clarify unambiguously that the conversion reaction of MoS₂ is not reversible, and the formed Li₂S is converted to sulfur in the first charge process. Li₂S/sulfur becomes the main redox couple in the subsequent cycles and the main contributor to the reversible capacity. In addition, due to the insulating nature of both Li₂S and sulfur, a strong relaxation effect is observed during the cycling process. This study clearly reveals the electrochemical lithiation-delithiation mechanism of MoS₂, which can facilitate further developments of high-performance MoS₂-based electrodes.

While a large variety of organic and molecular materials have been found to exhibit charge memory effects, the underlying mechanism is not well-understood, which hinders rational device design. Here, we study the charge retention mechanism of a nanoscale memory system, an organic monolayer on a silicon substrate, with Au nanoparticles on top serving as the electrical contact. Combining scanning probe imaging/manipulation and density functional simulations, we observe stable charge retention effects in the system and attributed it to polaron effects at the amine functional groups. Our findings can pave the way for applications in nonvolatile memory, neuromorphic computing, and high dielectric breakdown devices.

Phase transformations have been a prominent topic of study for both fundamental and applied science. Solid-liquid reaction-induced phase transformations can be hard to characterize, and the transformation mechanisms are often not fully understood. Here, we report reversible phase transformations between a metal (Pb) nanocrystal and a viscous liquid-like phase unveiled by in situ liquid cell transmission electron microscopy. The reversible phase transformations are obtained by modulating the electron current density (between 1000 and 3000 electrons Å^-2 s^-1). The metal-organic viscous liquid-like phase exhibits short-range ordering with a preferred Pb-Pb distance of 0.5 nm. Assisted by density functional theory and molecular dynamics calculations, we show that the viscous liquid-like phase results from the reactions of Pb with the CH₃O fragments from the triethylene glycol solution under electron beam irradiation. Such reversible phase transformations may find broad implementations.

Halide perovskites are of great interest because of their large potential for a broad range of applications, such as photovoltaics, light-emitting diodes, and photodetectors. However, currently the state-of-the-art highly emissive halide perovskite materials contain toxic lead ions, and the synthesis of color-stable efficient blue-emissive perovskite analogs remains challenging. The discovery of lead-free, non-toxic halide materials with efficient blue light emission is promising for practical light-emitting applications. Here, we have discovered a series of highly blue-emissive all-inorganic Cu(I)-containing rare-earth halide compounds in the solid state at room temperature. The Cu(I) ions in the compounds show a 3-coordination geometry with the halide ions and connect Sc-Cl octahedra to form clusters in alternating layers, which play a crucial role in the regulation of the electronic structure and emission properties of this series of rare-earth halide cluster compounds.

We report controllable anisotropic light emission of photons originating from vertically aligned transition dipole moments in spun-cast films of CsPbBr₃ nanocubes. By depositing films of nanocrystals on precoated substrates we can control the packing density and resultant radiation pattern of the emitted photons. We develop a technical framework to calculate the average orientation of light emitters, i.e., the angle between the transition dipole moment vector (TDM) and the substrate. This model is applicable to any emissive material with a known refractive index. Theoretical modeling indicates that oriented emission originates from an anisotropic alignment of the valence band and conduction band edge states on the ionic crystal lattice and demonstrates a general path to model the experimentally less accessible internal electric field of a nanosystem from the photoluminescent anisotropy. The uniquely accessible surface of the perovskite nanoparticles allows for perturbation of the normally isotropic emissive transition. The reported sensitive and tunable TDM orientation and control of emitted light will allow for applications of perovskite nanocrystals in a wide range of photonic technologies inaccessible to traditional light emitters.