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Solid-state Li–S batteries are attractive due to their high energy density and safety. However, it is unclear whether the concepts from liquid electrolytes are applicable in the solid state to improve battery performance. Here, we demonstrate that the nanoscale encapsulation concept based on Li2S@TiS2 core–shell particles, originally developed in liquid electrolytes, is effective in solid polymer electrolytes. Using in situ optical cell and sulfur K-edge X-ray absorption, we find that polysulfides form and are well-trapped inside individual particles by the nanoscale TiS2 encapsulation. This TiS2 encapsulation layer also functions to catalyze the oxidation reaction of Li2S to sulfur, even in solid-state electrolytes, proven by both experiments and density functional theory calculations. A high cell-level specific energy of 427 W·h·kg–1 is achieved by integrating the Li2S@TiS2 cathode with a poly­(ethylene oxide)-based electrolyte and a lithium metal anode. This study points to the fruitful direction of borrowing concepts from liquid electrolytes into solid-state batteries.

Selective adsorption of ligands on nanocrystal surfaces can affect oxidative etching. Here, we report the etching of palladium nanocrystals imaged using liquid cell transmission electron microscopy. The adsorption of surface ligands (i.e., iron acetylacetonate and its derivatives) and their role as inhibitor molecules on the etching process were investigated. Our observations revealed that the etching was dominated by the interplay between palladium facets and ligands and that the etching exhibited different pathways at different concentrations of ligands. At a low concentration of iron acetylacetonate (0.1 mM), rapid etching primarily at {100} facets led to a concave structure. At a high concentration (1.0 mM), the etch rate was decreased owing to a protective film of iron acetylacetonate on the {100} facets and a round nanoparticle was achieved. Ab initio calculations showed that the differences in adsorption energy of inhibitor molecules on palladium facets were responsible for the etching behavior.

An understanding of nanocrystal shape evolution is significant for the design, synthesis, and applications of nanocrystals with surface-enhanced properties such as catalysis or plasmonics. Surface adsorbates that are selectively attached to certain facets may strongly affect the atomic pathways of nanocrystal shape development. However, it is a great challenge to directly observe such dynamic processes in situ with a high spatial resolution. Here, we report the anomalous shape evolution of Ag2O2 nanocrystals modulated by the surface adsorbates of Ag clusters during electron beam etching, which is revealed through in situ transmission electron microscopy (TEM). In contrast to the Ag2O2 nanocrystals without adsorbates, which display the near-equilibrium shape throughout the etching process, Ag2O2 nanocrystals with Ag surface adsorbates show distinct facet development during etching by electron beam irradiation. Three stages of shape changes are observed: a sphere-to-a cube transformation, side etching of a cuboid, and bottom etching underneath the surface adsorbates. We find that the Ag adsorbates modify the Ag2O2 nanocrystal surface configuration by selectively capping the junction between two neighboring facets. They prevent the edge atoms from being etched away and block the diffusion path of surface atoms. Our findings provide critical insights into the modulatory function of surface adsorbates on the shape control of nanocrystals.

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.

Colloidal cesium lead halide perovskite nanocrystals exhibit unique photophysical properties including high quantum yields, tunable emission colors, and narrow photoluminescence spectra that have marked them as promising light emitters for applications in diverse photonic devices. Randomly oriented transition dipole moments have limited the light outcoupling efficiency of all isotropic light sources, including perovskites. In this report we design and synthesize deep blue emitting, quantum confined, perovskite nanoplates and analyze their optical properties by combining angular emission measurements with back focal plane imaging and correlating the results with physical characterization. By reducing the dimensions of the nanocrystals and depositing them face down onto a substrate by spin coating, we orient the average transition dipole moment of films into the plane of the substrate and improve the emission properties for light emitting applications. We then exploit the sensitivity of the perovskite electronic transitions to the dielectric environment at the interface between the crystal and their surroundings to reduce the angle between the average transition dipole moment and the surface to only 14° and maximize potential light emission efficiency. This tunability of the electronic transition that governs light emission in perovskites is unique and, coupled with their excellent photophysical properties, introduces a valuable method to extend the efficiencies and applications of perovskite based photonic devices beyond those based on current materials.

As the lightest and cheapest transition metal dichalcogenide, TiS

As a typical transition metal dichalcogenide, MoS

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.

Because of their enhanced kinetic properties, nanocrystallites have received much attention as potential electrode materials for energy storage. However, because of the large specific surface areas of nanocrystallites, they usually suffer from decreased energy density, cycling stability, and effective electrode capacity. In this work, we report a size-dependent excess capacity beyond theoretical value (170 mA h g–1) by introducing extra lithium storage at the reconstructed surface in nanosized LiFePO4 (LFP) cathode materials (186 and 207 mA h g–1 in samples with mean particle sizes of 83 and 42 nm, respectively). Moreover, this LFP composite also shows excellent cycling stability and high rate performance. Our multimodal experimental characterizations and ab initio calculations reveal that the surface extra lithium storage is mainly attributed to the charge passivation of Fe by the surface C–O–Fe bonds, which can enhance binding energy for surface lithium by compensating surface Fe truncated symmetry to c...

van der Waals (vdW) forces, despite being relatively weak, hold the layers together in transition metal dichalcogenides (TMDs) and play a key role in their band structure evolution, hence profoundly affecting their physical properties. In this work, we experimentally probe the vdW interactions in MoS2 and other TMDs by measuring the valence band maximum (VBM) splitting (Δ) at K point as a function of pressure in a diamond anvil cell. As high pressure increases interlayer wave function coupling, the VBM splitting is enhanced in 2H-stacked MoS2 multilayers but, due to its specific geometry, not in 3R-stacked multilayers, hence allowing the interlayer contribution to be separated out of the total VBM splitting, as well as predicting a negative pressure (2.4 GPa) where the interlayer contribution vanishes. This negative pressure represents the threshold vdW interaction beyond which neighboring layers are electronically decoupled. This approach is compared to first-principles calculations and found to be widel...

We report controllable anisotropic light emission of photons originating from vertically aligned transition dipole moments in spun-cast films of CsPbBr

A self-consistent linear scaling three-dimensional fragment (LS3DF) method is used to study the dipole moments and internal electric fields of large ZnO nanorods. Our ab initio calculations reveal that the ZnO nanorod with unpassivated (1010) side surface has a side surface contribution per Zn-O dimer 10 times larger than the bulk contribution per Zn-O bond. Detailed analysis is used to decompose the total dipole moment into different contributions. It is found that the total dipole moment of a nanorod can be calculated from the different parts with their dielectric screening described by a continuous model. We also show the effect of the dipole moment on the interior electronic structure of the nanorod.

The atomic-scale restructuring of hex-Pt(100) induced by carbon monoxide with a wide pressure range was studied with a newly designed chamber-in-chamber high-pressure STM and theoretical calculations. Both experimental and DFT calculation results show that CO molecules are bound to Pt nanoclusters through a tilted on-top configuration with a separation of approximately 3.7-4.1 A. The phenomenon of restructuring of metal catalyst surfaces induced by adsorption and, in particular, the formation of small metallic clusters suggests the importance of studying structures of catalyst surfaces under high-pressure conditions for understanding catalytic mechanisms.

We report detailed ab initio calculations of poly(3-hexylthiophene) (P3HT) on top of a ZnO (1010) surface. We studied different absorption sites and orientations. We found that the P3HT chain prefers to lay along the dimer row direction of the ZnO surface. We also found strong coupling between the P3HT molecule and the ZnO substrate in the conduction band states, while minimum coupling in the valence band states.

The size-dependent electronic structure of CdTe quantum wires is determined by density functional theory using the local density approximation with band-corrected pseudopotential method. The results of the calculations are then used to assign the size-dependent absorption spectrum of colloidal CdTe quantum wires synthesized by the solution-liquid-solid mechanism. Quantitative agreement between experiment and theory is achieved. The absorption features comprise transitions involving the highest 25-30 valence-band states and lowest 15 conduction-band states. Individual transitions are not resolved; rather, the absorption features consist of clusters of transitions that are determined by the conduction-band energy-level spacings. The sequence, character, and spacing of the conduction-band states are strikingly consistent with the predictions of the simple effective-mass-approximation, particle-in-a-cylinder model. The model is used to calculate the size dependence of the electron effective mass in CdTe quantum wires.

Type II core-shell nanowires based on III-V and II-VI semiconductors are designed to provide the highly desirable but not readily available feature--efficient charge separation--and concurrently address the different material challenges specific for a few key renewable energy applications: including hydrogen generation via photoelectrochemical water splitting, dye-sensitized solar cells, and conventional solar cells. They also open up new avenues for studying novel physics and material sciences in reduced dimensionality of very unusual quasi-one-dimensional systems. A first-principles density function theory within the local density approximation (LDA) is used for the electronic structure calculation and a valence-force-field method for the structural relaxation, and empirical corrections to the LDA errors are applied.

LiFePO4 has long been held as one of the most promising battery cathode for its high energy storage capacity. Meanwhile, although extensive studies have been conducted on the interfacial chemistries in Li-ion batteries,1-3 little is known on the atomic level about the solid-liquid interface of LiFePO4/electrolyte. Here, we report battery cathode consisted with nanosized LiFePO4 particles in aqueous electrolyte with an high charging and discharging rate of 600 C (3600/600 = 6 s charge time, 1 C = 170 mAh g(-1)) reaching 72 mAh g(-1) energy storage (42% of the theoretical capacity). By contrast, the accessible capacity sharply decreases to 20 mAh g(-1) at 200 C in organic electrolyte. After a comprehensive electrochemistry tests and ab initio calculations of the LiFePO4-H2O and LiFePO4-EC (ethylene carbonate) systems, we identified the transient formation of a Janus hydrated interface in the LiFePO4-H2O system, where the truncated symmetry of solid LiFePO4 surface is compensated by the chemisorbed H2O molecules, forming a half-solid (LiFePO4) and half-liquid (H2O) amphiphilic coordination environment that eases the Li desolvation process near the surface, which makes a fast Li-ion transport across the solid/liquid interfaces possible.

Charge hopping and percolation in quantum dot (QD) solids has been widely studied, but the microscopic nature of the percolation process is not understood or determined. Here we present the first imaging of the charge percolation pathways in two-dimensional PbS QD arrays using Kelvin probe force microscopy (KPFM). We show that under dark conditions electrons percolate via in-gap states (IGS) instead of the conduction band, while holes percolate via valence band states. This novel transport behavior is explained by the electronic structure and energy level alignment of the individual QDs, which was measured by scanning tunneling spectroscopy (STS). Chemical treatments with hydrazine can remove the IGS, resulting in an intrinsic defect-free semiconductor, as revealed by STS and surface potential spectroscopy. The control over IGS can guide the design of novel electronic devices with impurity conduction, and photodiodes with controlled doping.

Single-particle plus many-particle calculations of the electronic states of semiconductor nano dumbbells show how geometrical features (e.g., the width of the dumbbell wire) determine, through quantum confinement and electron−electron correlation effects, the spatial localization and the degree of entanglement of the electronic wave functions. Remarkably, we find that correlation effects can alter carrier localization and that the degree of wave function entanglement can be tuned by changing the diameter of the dumbbell wire. We further show how the exciton binding energy depends on the nano dumbbell geometry.

The electronic structures of zinc blende InP quantum rods are calculated using an atomistic pseudopotential method. The quantum rods in ref [001] and [111] growth directions are studied. We found dramatic differences in the valence bands between the wurtzite CdSe, zinc blende InP [111], and InP [001] quantum rods. We also found an unexpected Γ-X coupling in the conduction bands of the [111] direction quantum wires. Our calculated results await experimental confirmations.

The structural and electronic properties of MoS2/MoSe2 bilayers are calculated using first-principles methods. It is found that the interlayer van der Waals interaction is not strong enough to form a lattice-matched coherent heterostructure. Instead, a nanometer-scale Moiré pattern structure will be formed. By analyzing the electronic structures of different stacking configurations, we predict that the valence-band maximum (VBM) state will come from the Γ point due to interlayer electronic coupling. This is confirmed by a direct calculation of a Moiré pattern supercell containing 6630 atoms using the linear scaling three-dimensional fragment method. The VBM state is found to be strongly localized, while the conduction band minimum (CBM) state is only weakly localized, and it comes from the MoS2 layer at the K point. We predict such wave function localization can be a general feature for many two-dimensional (2D) van der Waals heterostructures and can have major impacts on the carrier mobility and other electronic and optical properties.

We developed an ab-initio multiscale method for simulation of carrier transport in large disordered systems, based on direct calculation of electronic states and electron-phonon coupling constants. It enabled us to obtain the never seen before rich microscopic details of carrier motion in conjugated polymers, which led us to question several assumptions of phenomenological models, widely used in such systems. The macroscopic mobility of disordered poly(3- hexylthiophene) (P3HT) polymer, extracted from our simulation, is in agreement with experimental results from the literature.

We report a 100,000-fold increase in the conductance of individual CdSe nanorods when they are electrically contacted via direct solution phase growth of Au tips on the nanorod ends. Ensemble UV-Vis and X-Ray photoelectron spectroscopy indicate this enhancement does not result from alloying of the nanorod. Rather, low temperature tunneling and high temperature (250-400 K) thermionic emission across the junction at the Au contact reveal a 75percent lower interface barrier to conduction compared to a control sample. We correlate this barrier lowering with the electronic structure at the Au-CdSe interface. Our results emphasize the importance of nanocrystal surface structure for robust device performance and the advantage of this contact method.

The band-edge exciton fine structure of wurtzite CdSe nanocrystals is investigated by a plane-wave pseudopotential method that includes spin-orbit coupling, screened electron-hole Coulomb interactions, and exchange interactions. Large-scale, systematic simulations have been carried out on quantum dots, nanorods, nanowires, and nanodisks. The size and shape dependence of the exciton fine structure is explored over the whole diameter-length configuration space and is explained by the interplay of quantum confinement, intrinsic crystal-field splitting, and electron-hole exchange interactions. Our results show that the band-edge exciton fine structure of CdSe nanocrystals is determined by the origin of their valence-band single-particle wave functions. Nanocrystals where the valence-band maximum originates from the bulk A band have a "dark" ground-state exciton. Nanocrystals where the valence-band maximum is derived from the bulk B band have a "quasi-bright" ground-state exciton. Thus, the diameter-length configuration map can be divided into two regions, corresponding to dark and quasi-bright ground-state excitons. We find that the dark/quasi-bright ground-state exciton crossover is not only diameter-dependent but also length-dependent, and it is characterized by a curve in the two-parameter space of diameter and length.