Atomic-scale observation of localized phonons at FeSe/SrTiO3 interface.

In single unit-cell FeSe grown on SrTiO3, the superconductivity transition temperature features a significant enhancement. Local phonon modes at the interface associated with electron-phonon coupling may play an important role in the interface-induced enhancement. However, such phonon modes have eluded direct experimental observations. The complicated atomic structure of the interface brings challenges to obtain the accurate structure-phonon relation knowledge. Here, we achieve direct characterizations of atomic structure and phonon modes at the FeSe/SrTiO3 interface with atomically resolved imaging and electron energy loss spectroscopy in an electron microscope. We find several phonon modes highly localized (~1.3 nm) at the unique double layer Ti-O terminated interface, one of which (~ 83 meV) engages in strong interactions with the electrons in FeSe based on ab initio calculations. This finding of the localized interfacial phonon associated with strong electron-phonon coupling provides new insights into understanding the origin of superconductivity enhancement at the FeSe/SrTiO3 interface.

Highly Reproducible Epitaxial Growth of Wafer-Scale Single-Crystal Monolayer MoS2 on Sapphire.

2D semiconducting transition-metal dichalcogenides (TMDs) have attracted considerable attention as channel materials for next-generation transistors. To meet the industry needs, large-scale production of single-crystal monolayer TMDs in highly reproducible and energy-efficient manner is critically significant. Herein, it is reported that the high-reproducible, high-efficient epitaxial growth of wafer-scale monolayer MoS2 single crystals on the industry-compatible sapphire substrates, by virtue of a deliberately designed "face-to-face" metal-foil-based precursor supply route, carbon-cloth-filter based precursor concentration decay strategy, and the precise optimization of the chalcogenides and metal precursor ratio (i.e., S/Mo ratio). This unique growth design can concurrently guarantee the uniform release, short-distance transport, and moderate deposition of metal precursor on a wafer-scale substrate, affording high-efficient and high-reproducible growth of wafer-scale single crystals (over two inches, six times faster than usual). Moreover, the S/Mo precursor ratio is found as a key factor for the epitaxial growth of MoS2 single crystals with rather high crystal quality, as convinced by the relatively high electronic performances of related devices. This work demonstrates a reliable route for the batch production of wafer-scale single-crystal 2D materials, thus propelling their practical applications in highly integrated high-performance nanoelectronics and optoelectronics.

Epitaxial Growth of High-Quality Monolayer MoS2 Single Crystals on Low-Symmetry Vicinal Au(101) Facets with Different Miller Indices.

Epitaxial growth of wafer-scale monolayer semiconducting transition metal dichalcogenide single crystals is essential for advancing their applications in next-generation transistors and highly integrated circuits. Several efforts have been made for the growth of monolayer MoS2 single crystals on high-symmetry Au(111) and sapphire substrates, while more prototype growth systems still need to be discovered for clarifying the internal mechanisms. Herein, we report the epitaxial growth of unidirectionally aligned monolayer MoS2 domains and single-crystal films on low-symmetry Au(101) vicinal facets via a facile chemical vapor deposition method. On-site scanning tunneling microscopy observations reveal the formation of a specific rectangular Moiré pattern along the [101̅] step edge of Au(101) and along its perpendicular direction. The perfect lattice constant matching of MoS2/Au(101) along the substrate high-symmetry directions (i.e., Au[101̅], Au [010]) as well as the step-edge-guiding effect are proposed to facilitate the robust epitaxy. Multiscale characterizations further confirm the domain-boundary-free feature of the monolayer MoS2 films merged by unidirectionally aligned monolayer domains. This work hereby puts forward a symmetry mismatched epitaxial system for the direct synthesis of monolayer MoS2 single crystals, which should deepen our understanding about the epitaxy of 2D layered materials and propel their applications in various fields.

Phonon transition across an isotopic interface.

Isotopic mixtures result in distinct properties of materials such as thermal conductivity and nuclear process. However, the knowledge of isotopic interface remains largely unexplored mainly due to the challenges in atomic-scale isotopic identification. Here, using electron energy-loss spectroscopy in a scanning transmission electron microscope, we reveal momentum-transfer-dependent phonon behavior at the h-10BN/h-11BN isotope heterostructure with sub-unit-cell resolution. We find the phonons' energy changes gradually across the interface, featuring a wide transition regime. Phonons near the Brillouin zone center have a transition regime of ~3.34 nm, whereas phonons at the Brillouin zone boundary have a transition regime of ~1.66 nm. We propose that the isotope-induced charge effect at the interface accounts for the distinct delocalization behavior. Moreover, the variation of phonon energy between atom layers near the interface depends on both of momentum transfer and mass change. This study provides new insights into the isotopic effects in natural materials.