Phase transformation in emerging two-dimensional (2D) materials is crucial for understanding and controlling the interplay between structure and electronic properties. In this work, we investigate 2D In2Se3 synthesized via chemical vapor deposition, a recently discovered 2D ferroelectric material. We observed that In2Se3 layers with thickness ranging from a single layer to ∼20 layers stabilized at the β phase with a superstructure at room temperature. At around 180 K, the β phase converted to a more stable β′ phase that was distinct from previously reported phases in 2D In2Se3. The kinetics of the reversible thermally driven β-to-β′ phase transformation was investigated by temperature-dependent transmission electron microscopy and Raman spectroscopy, corroborated with the expected minimum-energy pathways obtained from our first-principles calculations. Furthermore, density functional theory calculations reveal in-plane ferroelectricity in the β′ phase. Scanning tunneling spectroscopy measurements show that the indirect bandgap of monolayer β′ In2Se3 is 2.50 eV, which is larger than that of the multilayer form with a measured value of 2.05 eV. Our results on the reversible thermally driven phase transformation in 2D In2Se3 with thickness down to the monolayer limit and the associated electronic properties will provide insights to tune the functionalities of 2D In2Se3 and other emerging 2D ferroelectric materials and shed light on their numerous potential applications (e.g., nonvolatile memory devices).

Tailoring the properties of two-dimensional (2D) crystals is important for both understanding the material behavior and exploring new functionality. Here we demonstrate the alteration of MoS 2 and metal-MoS 2 interfaces using a convergent ion beam. Different beam energies, from 60 eV to 600 eV, are shown to have distinct effects on the optical and electrical properties of MoS 2. Defects and deformations created across different layers were investigated, revealing an unanticipated improvement in the Raman peak intensity of multilayer MoS 2 when exposed to a 60 eV Ar + ion beam, and attenuation of the MoS 2 Raman peaks with a 200 eV ion beam. Using cross-sectional scanning transmission electron microscopy (STEM), alteration of the crystal structure after a 600 eV ion beam bombardment was observed, including generated defects and voids in the crystal. We show that the 60 eV ion beam yields improvement in the metal-MoS 2 interface by decreasing the contact resistance from 17.5 kΩ • µm to 6 kΩ • µm at a carrier concentration of n 2D = 5.4 × 10 12 cm −2. These results advance the use of low-energy ion beams to modify 2D materials and interfaces for tuning and improving performance in applications of sensors, transistors, optoelectronics, and so forth.

We report the first experimental characterization of isomerically pure and pristine C120 fullertubes, [5,5] C120-D5d(1) and [10,0] C120-D5h(10766). These new molecules represent the highest aspect ratio fullertubes isolated to date; for example, the prior largest empty cage fullertube was [5,5] C100-D5d(1). This increase of 20 carbon atoms represents a gigantic leap in comparison to three decades of C60–C90 fullerene research. Moreover, the [10,0] C120-D5d(10766) fullertube has an end-cap derived from C80-Ih and is a new fullertube whose C40 end-cap has not yet been isolated experimentally. Theoretical and experimental analyses of anisotropic polarizability and UV–vis assign C120 isomer I as a [5,5] C120-D5d(1) fullertube. C120 isomer II matches a [10,0] C120-D5h(10766) fullertube. These structural assignments are further supported by Raman data showing metallic character for [5,5] C120-D5d(1) and nonmetallic character for C120-D5h(10766). STM imaging reveals a tubular structure with an aspect ratio consistent with a [5,5] C120-D5d(1) fullertube. With microgram quantities not amenable to crystallography, we demonstrate that DFT anisotropic polarizability, augmented by long-accepted experimental analyses (HPLC retention time, UV–vis, Raman, and STM) can be synergistically used (with DFT) to down select, predict, and assign C120 fullertube candidate structures. From 10 774 mathematically possible IPR C120 structures, this anisotropic polarizability paradigm is quite favorable to distinguish tubular structures from carbon soot. Identification of isomers I and II was surprisingly facile, i.e., two purified isomers for two possible structures of widely distinguishing features. These metallic and nonmetallic C120 fullertube isomers open the door to both fundamental research and application development.

Van der Waals (vdW) heterostructures synthesized through the chemical vapor deposition (CVD) method allow creation and tuning of intriguing electronic and optical properties of twodimensional (2D) materials. Especially, local structures in the heterostructures, such as interfaces, edges and point defects, are critical for their wide range of potential application. However, up to now atomic scale measurements of local structures in as-grown 2D heterostructures on insulating substrates are still rare. Here we report our scanning tunneling microscopy (STM) and spectroscopy (STS) study of as-grown MoS2 monolayer and WS2/MoS2 heterobilayer on SiO2. The heterobilayer appears smoother than the MoS2 monolayer, with root mean square (RMS) roughness of 0.230 nm in the former and 0.329 nm in the latter. For the first time to our knowledge, we directly observed a novel type of continuous interfaces between the MoS2 monolayer and the top layer of the heterobilayer with atomic resolution. Our STS results and density functional theory (DFT) calculations revealed the band gaps of the heterobilayer and the 2 MoS2 monolayer. The finding of the continuous interfaces and the systematic characterizations could have significant impacts on optimizing and designing new 2D heterostructures.

Domain boundaries in ferroelectric materials exhibit rich and diverse physical properties distinct from their parent materials and have been proposed for broad applications in nanoelectronics and quantum information technology. Due to their complexity and diversity, the internal atomic and electronic structure of domain boundaries that governs the electronic properties remains far from being elucidated. By using scanning tunneling microscopy and spectroscopy (STM/S) combined with density functional theory (DFT) calculations, we directly visualize the atomic structure of polar domain boundaries in two-dimensional (2D) ferroelectric β′-In2Se3 down to the monolayer limit. We observe a double-barrier energy potential with a width of about 3 nm across the 60° tail-to-tail domain boundaries in monolayer β′-In2Se3. The results will deepen our understanding of domain boundaries in 2D ferroelectric materials and stimulate innovative applications of these materials.