We found that there are 38 S atoms and 8 Se atoms at the interface, indicating that during the two-step epitaxial growth, the first grown WSe 2 layer had W-terminals and favored W─S bonds. From the experimental 3D atomic model, we combined multislice simulations with sAET to estimate the 3D precision of the method to be 4, 15, 6, and 15 pm for the Mo, S, W, and Se atoms, respectively (see Materials and Methods). Using scanning AET (sAET) ( 19), we determined the 3D atomic coordinates and atomic species of an interface region, containing 488 Mo, 991 S, 150 W, and 257 Se atoms and 16 S/Se vacancies ( Fig.
PHONON DISPERSIO DAHED VERTICAL SERIES
A tilt series of 12 images was acquired from an interface region of an epitaxial MoS 2-WSe 2 lateral heterojunction (see Materials and Methods, Fig. The experiment was conducted with an aberration-corrected scanning transmission electron microscope (STEM), operated at 60 kV in annular dark-field (ADF) mode. In contrast, the phonon dispersion derived from the minimum energy state of the heterojunction is absent of the local interface phonon modes, indicating the importance of using experimental 3D atomic coordinates as direct input to better predict the properties of heterointerfaces. The experimentally measured 3D atomic coordinates, representing a metastable state of the heterojunction, were used as direct input to first-principles calculations to reveal new phonon modes localized at the heterointerface, which were corroborated by the measurements of spatially resolved electron energy-loss spectroscopy (EELS).
PHONON DISPERSIO DAHED VERTICAL FULL
We observed various crystal defects-including vacancies, substitutional defects, bond distortion, and atomic-scale ripples-and quantitatively characterized the 3D atomic displacements and full strain tensor across the heterointerface. Here, using a MoS 2-WSe 2 lateral heterojunction as a model, we applied atomic electron tomography (AET) ( 16– 20) to determine the 3D atomic coordinates and crystal defects at the heterointerface with picometer precision. However, real heterointerfaces neither have perfect crystal lattices nor are in the minimum energy states. Because of the difficulty in directly measuring these 3D coordinates, such studies often use perfect crystal lattices ( 12– 14), statistically incorporate crystal defects into the interface ( 15), and relax the atomic configurations to the minimum energy states. On the computational side, density functional theory (DFT) can be used to predict the properties of heterostructures ( 12– 15) but requires knowledge of the 3D local atomic coordinates. Although 2D lateral and vertical heterostructures have been actively studied for fundamental interest and practical applications ( 1– 9), our current understanding of the atomic structure at the heterointerface has primarily relied on aberration-corrected electron microscopy and scanning probe microscopy ( 6– 11), which provide either 2D projection images or surface structure. We expect that this work will pave the way for correlating structure-property relationships of a wide range of heterostructure interfaces at the single-atom level.Ī major challenge in materials design and engineering is to tailor the 3D atomic structures at the interface to achieve the desired properties. By using the experimental 3D atomic coordinates as direct input to first-principles calculations, we reveal new phonon modes localized at the interface, which are corroborated by spatially resolved electron energy-loss spectroscopy. We observe point defects, bond distortion, and atomic-scale ripples and measure the full 3D strain tensor at the heterointerface. Here, we use atomic electron tomography to determine the 3D local atomic positions at the interface of a MoS 2-WSe 2 heterojunction with picometer precision and correlate 3D atomic defects with localized vibrational properties at the epitaxial interface. The three-dimensional (3D) local atomic structures and crystal defects at the interfaces of heterostructures control their electronic, magnetic, optical, catalytic, and topological quantum properties but have thus far eluded any direct experimental determination.