Yoshiki Sakuma

Unidirectional nucleation followed by seamless stitching has emerged as a promising strategy for the scalable epitaxial growth of single-crystalline monolayer transition metal dichalcogenides on sapphire substrates, which holds potential for post-silicon electronics. In contrast, here we present a different growth mechanism for single-crystalline MoS ₂ on c-plane sapphire via metal-organic chemical vapor deposition (MOCVD). We show that the initial nucleation generates not only 0° and antiparallel 60° domains but also low-angle twisted domains, consistent with the coincidence site lattice framework. However, these rotationally misoriented domains are observed to deterministically self-align and merge into energetically preferred 0° domain during coalescence, yielding a continuous, unidirectional single-crystal. Additionally, by employing MoO ₂ Cl ₂ as a molybdenum precursor, we demonstrate that the growth of MoS ₂ occurs in a self-limiting manner. This epitaxial strategy is substantiated by a carrier mobility of 66 cm ² /Vs at room temperature and 749 cm ² /Vs at low temperatures. Our approach offers a practical and reproducible scheme for MOCVD-based van der Waals epitaxy for 2D electronics.

Highly oriented MoS2 monolayer (ML) on the sapphire(0001) substrate was grown by metal-organic chemical vapor deposition. The epitaxial configuration of ML-MoS2/sapphire has been studied using low-energy electron diffraction (LEED) combined with first-principles calculations. LEED analysis based on the dynamical diffraction theory revealed that the MoS2 ML is grown with the epitaxial relationship of MoS2 [112¯0] // Al2O3 [112¯0] and MoS2 [11¯00] // Al2O3 [11¯00] and that the formation of antiparallel (inversion) domains is effectively suppressed. The observed epitaxial relationship is insensitive to the direction of surface steps on the sapphire substrate.

We deposited a highly oriented MoS2 film on a 2-in. α-Al2O3 (0001) wafer by metalorganic chemical vapor deposition and determined the atomic configuration of the MoS2/α-Al2O3 (0001) stacking structure by performing atomic resolution electron microscopy observations along two orthogonal zone axis directions, i.e., the 112¯0 and 11¯00 directions of α-Al2O3. The results show that, first, the in-plane positions of Mo atoms coincide with those of the underlying Al and O atoms, and the 112¯0 direction of the monolayer 2H-MoS2 matches that of the α-Al2O3 substrate. Second, the α-Al2O3 surface was a reconstructed Al-I structure. Moreover, we performed the first-principles calculations using the observed in-plane atomic positions of the MoS2/α-Al2O3 structure as a starting configuration and found that the MoS2–Al2O3 distance is larger than the theoretical van der Waals distance. Because no ordered structures were observed between the MoS2 film and the Al2O3 substrate, the experimental and theoretical results strongly suggest that an amorphous interface layer exists between them. Such an amorphous interface layer is likely to weaken the MoS2–Al2O3 interaction that determines the stability of the MoS2/α-Al2O3 (0001) structure. We thus argue that controlling the interface layer is critical for fabricating highly oriented MoS2 films and is vital for improving the performance of field-effect transistors with MoS2 channels.

Toward future logic semiconductors at sub-1 nm nodes, unidirectional nucleation followed by seamless stitching has emerged as an exclusive strategy for scalable epitaxial growth of single-crystalline monolayer transition metal dichalcogenides on sapphire substrates. In contrast, here we present a novel growth mechanism for single-crystalline MoS₂ on c-plane sapphire via metal-organic chemical vapor deposition (MOCVD). The initial nucleation generates not only 0° and antiparallel 60° domains but also low-angle twisted domains, consistent with the coincidence site lattice framework. Nevertheless, these rotationally misoriented domains are deterministically self-aligned and merged into energetically preferred 0° domain during coalescence, yielding a continuous, unidirectional single-crystal. Additionally, by employing MoO₂Cl₂ as a molybdenum precursor, MoS₂ grows in a self-limiting manner. This epitaxial strategy is substantiated by phonon-limited mobility of 66 cm²/Vs at room temperature and 749 cm²/Vs at low temperatures. Our approach offers a practical and reproducible scheme for MOCVD-based van der Waals epitaxy in 2D electronics.

Dichalcogenides, such as molybdenum disulfide (MoS ₂ ), are being studied extensively due to their 2D feature and various material properties. Although crystal structures are critical for applications, conventional atomic structure analyses have a limited field of view. In this study, the crystal domains of monolayer MoS ₂ synthesized by metal–organic chemical vapor deposition (MOCVD) are analyzed using 4D scanning transmission electron microscopy (STEM) and unsupervised machine learning. Twist domains (±11°) are identified through the nonnegative matrix factorization (NMF) and hierarchical clustering of numerous (>22k) diffraction patterns from a wide field of view. Preprocessing for detecting noncentrosymmetry effectively visualizes the polarities of distinct MoS ₂ domains by highlighting the violation of Friedel's law in diffraction physics. Analyses reveal that the specimen deposited on Al ₂ O ₃ (0001) at 850 °C consists of domains measuring ≈100 nm in size and featuring many mirror‐twin boundaries. The findings provide valuable insights into optimizing the MOCVD process and elucidating crystal growth mechanisms.

Transition metal dichalcogenides (TMDC) are a family of atomic‐layer 2D materials (2dMs). Many researches have attempted to reinforce the unique properties by incorporating artificially designed nanostructures. Here, a highly photoluminescence (PL) enhancing platform is reported that is formed by combining high‐emittance plasmo–photonic metasurfaces with a large‐area 2dM, which is a continuous monolayer of TMDC of cm² dimension. It is experimentally demonstrated that the 2dM‐PL intensity at room temperature is enhanced by more than 1030‐fold in comparison with that observed on the flat gold film regions. The enhancement is associated with spectral transformation indicating a resonant effect of the metasurfaces. It is moreover revealed that the confocal PL image originates from a coherent single‐mode emission that forms a photon number distribution obeying the Poisson probability distribution. Thus, the 2dM‐fused metasurfaces function as efficient coherent quantum light sources. PL decay‐time analysis also reveals the coexistence of two components in the enhanced PL. The fast component exhibits a lifetime of 18 ps whereas the slow component decays at 4.4 ns. Both components are most likely to undergo radiative processes, which indicates that metal‐induced PL quenching is suppressed on these 2dM‐fused metasurfaces. The long lifetime component is ascribed to an excitonic intervalley transition.

2D materials such as transition metal dichalcogenides (TMDCs) are a new class of atomic‐layer materials possessing optical and electric properties that significantly depend on the number of layers. Electronic transitions can be manipulated in artificial resonant electromagnetic (EM) fields using metasurfaces and other designed nanostructures. Here, we demonstrate prominently resonant enhancement in the photoluminescence (PL) of atomic monolayer, WS ₂ , doped with a small quantity of Mo. The excitonic PL showed a strong enhancement effect on a higher‐order magnetic resonance of all‐dielectric metasurfaces consisting of periodic arrays of Si nanopellets. The PL intensity witnessed a 300‐fold enhancement compared to the reference PL intensity on a flat silicon dioxide (SiO ₂ ) layer, which suggests a drastic change in the dynamics of photoexcited states. Confocal PL microscopy and the analysis revealed that the single photons were coherently emitted from the TMDC monolayer on the metasurface. Furthermore, examining the PL lifetime in the ps and ns timescales clarified two exponential components at the prominent exciton PL: a short‐time component decaying in 22 ps and a long‐time component lasting over 10 ns. Therefore, we can infer that the radiative components were significantly activated in the TMDC monolayer on the metasurfaces in comparison to the reference monolayer on a flat SiO ₂ layer.

Two‐dimensional (2D) transition metal dichalcogenides (TMDCs) with unique electrical properties are fascinating materials used for future electronics. However, the strong Fermi level pinning effect at the interface of TMDCs and metal electrodes always leads to high contact resistance, which seriously hinders their application in 2D electronics. One effective way to overcome this is to use metallic TMDCs or transferred metal electrodes as van der Waals (vdW) contacts. Alternatively, using highly conductive doped TMDCs will have a profound impact on the contact engineering of 2D electronics. Here, a novel chemical vapor deposition (CVD) using mixed molten salts is established for vapor–liquid–solid growth of high‐quality rhenium (Re) and vanadium (V) doped TMDC monolayers with high controllability and reproducibility. A tunable semiconductor to metal transition is observed in the Re‐ and V‐doped TMDCs. Electrical conductivity increases up to a factor of 10⁸ in the degenerate V‐doped WS₂ and WSe₂. Using V‐doped WSe₂ as vdW contact, the on‐state current and on/off ratio of WSe₂‐based field‐effect transistors have been substantially improved (from ≈10^–8 to 10^–5 A; ≈10⁴ to 10⁸), compared to metal contacts. Future studies on lateral contacts and interconnects using doped TMDCs will pave the way for 2D integrated circuits and flexible electronics.

We investigate the photovoltaic properties of AlGaAs solar cells with embedded GaNAs quantum wells (QWs) with N concentrations in the range of 0-3.1%, for which the QW confinement energy can be tuned by adjusting the N concentration. We systematically study the dependence of open-circuit voltage V _OC in relation to the lowest band-to-band transition energy. In samples with low N concentrations (shallow QW confinement), V _OC degrades and is limited by the lowest transition energy in the solar cell, i.e., the QW transition. With increasing N concentration, N > 0.5% (deep QW confinement), V _OC does not degrade further and is no longer limited by the QW transition energy. The highest N sample exhibits a remarkably small offset between the lowest transition energy and the achieved V _OC of 0.23 V, which is beyond the detailed balance limit of standard solar cells. V _OC dependence is explained by analyzing the current-voltage (I-V) characteristics under different illumination conditions, from which information about the balance of escape and recombination rates of carriers from the QWs is extracted. In the deeply confined QWs, tunneling and thermal carrier escape is completely suppressed, allowing the recovery of V _OC .