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An unconventional “heteromorphic” superlattice (HSL) is realized, comprised of repeated layers of different materials with differing morphologies: semiconducting pc-In2O3 layers interleaved with insulating a-MoO3 layers. Originally proposed by Tsu in 1989, yet never fully realized, the high quality of the HSL heterostructure demonstrated here validates the intuition of Tsu, whereby the flexibility of the bond angle in the amorphous phase and the passivation effect of the oxide at interfacial bonds serve to create smooth, high-mobility interfaces. The alternating amorphous layers prevent strain accumulation in the polycrystalline layers while suppressing defect propagation across the HSL. For the HSL with 7:7 nm layer thickness, the observed electron mobility of 71 cm2/Vs, matches that of the highest quality In2O3 thin films. The atomic structure and electronic properties of crystalline In2O3 / amorphous MoO3 interfaces are verified using ab-initio molecular dynamics simulations and hybrid functional calculations. This work generalizes the superlattice concept to an entirely new paradigm of morphological combinations. 

Black phosphorus (BP) with unique 2D structure enables the intercalation of foreign elements or molecules, which makes BP directly relevant to high‐capacity rechargeable batteries and also opens a promising strategy for tunable electronic transport and superconductivity. However, the underlying intercalation mechanism is not fully understood. Here, a comparative investigation on the electrochemically driven intercalation of lithium and sodium using in situ transmission electron microscopy is presented. Despite the same preferable intercalation channels along [100] (zigzag) direction, distinct anisotropic intercalation behaviors are observed, i.e., Li ions activate lateral intercalation along [010] (armchair) direction to form an overall uniform propagation, whereas Na diffusion is limited in the zigzag channels to cause the columnar intercalation. First‐principles calculations indicate that the diffusion of both Li and Na ions along the zigzag direction is energetically favorable, while Li/Na diffusion long the armchair direction encounters an increased energy barrier, but that of Na is significantly larger and insurmountable, which accounts for the orientation‐dependent intercalation channels. The evolution of chemical states during phase transformations (from Li_xP/Na_xP to Li₃P/Na₃P) is identified by analytical electron diffraction and energy‐loss spectroscopy. The findings elucidate atomistic Li/Na intercalation mechanisms in BP and show potential implications for other similar 2D materials.

 

Using in situ electrical biasing transmission electron microscopy, structural and chemical modification to n–i–p‐type MAPbI₃solar cells are examined with a TiO₂electron‐transporting layer caused by bias in the absence of other stimuli known to affect the physical integrity of MAPbI₃such as moisture, oxygen, light, and thermal stress. Electron energy loss spectroscopy (EELS) measurements reveal that oxygen ions are released from the TiO₂and migrate into the MAPbI₃under a forward bias. The injection of oxygen is accompanied by significant structural transformation; a single‐crystalline MAPbI₃grain becomes amorphous with the appearance of PbI₂. Withdrawal of oxygen back to the TiO₂, and some restoration of the crystallinity of the MAPbI₃, is observed after the storage in dark under no bias. A subsequent application of a reverse bias further removes more oxygen ions from the MAPbI₃. Light current–voltage measurements of perovskite solar cells exhibit poorer performance after elongated forward biasing; recovery of the performance, though not complete, is achieved by subsequently applying a negative bias. The results indicate negative impacts on the device performance caused by the oxygen migration to the MAPbI₃under a forward bias. This study identifies a new degradation mechanism intrinsic to n–i–p MAPbI₃devices with TiO₂.

 

Although lithium‐ion batteries that run on the conversion reaction have high capacity, their cyclability remains problematic due to large volume changes and material pulverization. Dimensional confinement, such as 2D thin film or nanodots in a conductive matrix, is proposed as a way of improving the cyclic stability, but the lithiation mechanism of such dimensionally controlled materials remains largely unknown. Here, by in situ transmission electron microscopy, lithiation of thin RuO₂films with different thicknesses and directions of lithium‐ion diffusion are observed at atomic resolution to monitor the reactions. From the side‐wall diffusion in ≈4 nm RuO₂film, the ion‐diffusion and reaction are fast, called “interface‐dominant” mode. In contrast, in ≈12 nm film, the ion diffusion–reaction only occurs at the interface where there is a high density of defects due to misfits between the film and substrate, called the “interface‐to‐film” mode. Compared to the side‐wall diffusion, the reaction along the normal direction of the thin film are found to be sluggish (“layer‐to‐layer” mode). Once lithiation speed is higher, the volume expansion is larger and the intercalation stage becomes shorter. Such observation of preferential lithiation direction in 2D‐like RuO₂thin film provides useful insights to develop dimensionally confined electrodes for lithium‐ion batteries.