Abstract

We employed an in situ electrochemical cell in the transmission electron microscope (TEM) together with ex situ time-of-flight, secondary-ion mass spectrometry (TOF-SIMS) depth profiling, and FIB-helium ion scanning microscope (HIM) imaging to detail the structural and compositional changes associated with Na/Na(+) charging/discharging of 50 and 100 nm thin films of Sb. TOF-SIMS on a partially sodiated 100 nm Sb film gives a Na signal that progressively decreases toward the current collector, indicating that sodiation does not proceed uniformly. This heterogeneity will lead to local volumetric expansion gradients that would in turn serve as a major source of intrinsic stress in the microstructure. In situ TEM shows time-dependent buckling and localized separation of the sodiated films from their TiN-Ge nanowire support, which is a mechanism of stress-relaxation. Localized horizontal fracture does not occur directly at the interface, but rather at a short distance away within the bulk of the Sb. HIM images of FIB cross sections taken from sodiated half-cells, electrically disconnected, and aged at room temperature, demonstrate nonuniform film swelling and the onset of analogous through-bulk separation. TOF-SIMS highlights time-dependent segregation of Na within the structure, both to the film-current collector interface and to the film surface where a solid electrolyte interphase (SEI) exists, agreeing with the electrochemical impedance results that show time-dependent increase of the films' charge transfer resistance. We propose that Na segregation serves as a secondary source of stress relief, which occurs over somewhat longer time scales.

Lead halide perovskites are increasingly considered for applications beyond photovoltaics, for example, light emission and detection, where an ability to pattern and prototype microscale geometries can facilitate the incorporation of this class of materials into devices. Here we demonstrate laser direct write of lead halide perovskites, a remarkably simple procedure that takes advantage of the inverse dependence between perovskite solubility and temperature by using a laser to induce localized heating of an absorbing substrate. We demonstrate arbitrary pattern formation of crystalline CHNHPbBr on a range of substrates and fabricate and characterize a microscale photodetector using this approach. This direct write methodology provides a path forward for the prototyping and production of perovskite-based devices.

The lithiation of molybdenum disulfide (MoS 2 ) has been directly studied in situ in the TEM by observing specimens with the viewing direction parallel to the basal planes. The MoS 2 lamella was characterized by bright-field imaging during the lithiation, and both selected-area diffraction and high-resolution imaging before and after. An overall expansion of $5% along the c-direction was observed with concurrent local contraction. The contraction can be related to the expulsion of Mo as Li reduces it to form Li 2 S.Research into new materials for energy storage is at an exciting stage with both new materials and well-known materials showing new physical phenomena [1][2][3][4][5]. Layered materials such as the transition metal dichalcogenides (TMDCs) are attracting renewed interest for battery applications [6] as a special example of insertion electrode materials that can take advantage of the intercalation mechanism [7,8]. Work on the intercalation of alkali metals in TMDCs began in the 1970s [9-13], during which time many fundamental observations were made. For example, it was shown that the sodiated phase of NbSe 2 is orthorhombic with Na atoms occupying the octahedral sites in the van der Waals gap [9], and that the general stoichiometry of a fully-lithiated MX 2 compound (where M is a transition metal, usually in group 4B or 5B, and X is S, Se, or Te) is LiMX 2 except in the case of VSe 2 , which formed Li 2 VSe 2 [10].Although bulk molybdenum disulfide (MoS 2 ) can exist with different structures the 2H (hexagonal) phase is the most stable and is the predominant phase that occurs in nature [14]. The orthorhombic 1T structure is commonly observed after exfoliation, which is achieved by lithiating the MoS 2 and reacting the lithiated material with water [15][16][17]. Transition metal clustering in this phase can lead to 2 Â 1 (1T 0 ), 2 Â 2 (1T 00 ) and other supercell variations [18]. However, if these materials are to be used commercially in batteries it is likely that they will be in the form of sheets that are rather thicker than those formed by the exfoliation process. The present study addresses the lithiation of such practical materials.While considerable understanding of these materials can be achieved by examining specimens after lithiation or after delithiation (i.e. ex situ), observing the same specimens before, during and after lithiation has obvious advantages. In situ TEM is the only technique that allows specific sites on the specimen to be monitored in this way with atomic resolution. The usual geometry used for such studies is to examine the layer materials with the beam normal to the layers since this orientation greatly simplifies specimen preparation (see, e.g., [9]). In the present report, a novel experimental geometry was used such that the electron beam was parallel to the (0 0 0 1) planes. This geometry allowed direct observation of the Li intercalation process, in situ in the TEM. Lamellae of single crystal MoS 2 have been examined in this way; this approach can easily be extended to o...

No abstract