Currently, the direct synthesis of inch-scale single-crystal graphene on insulating substrates is limited by the lack of metal catalysis, suitable crystallization conditions, and self-limiting growth mechanisms. In this study, we investigated the direct growth of adlayer-free ultra-flat wafer-scale single-crystal monolayer graphene on insulating substrates by the multi-cycle plasma-etching-assisted chemical vapor deposition (MPE-CVD) method. Firstly, an angstrom-scale growth nanochamber was created by fabricating single-crystal Cu(111) foils on Al2O3(0001) substrates. Graphene was then directly synthesized at the interface between Cu(111) and Al2O3(0001) by MPE-CVD. After growth, the Cu(111) foil was detached using a liquid-nitrogen-assisted separation method, and the ultra-high-quality single-crystal graphene film was experimentally achieved on Al2O3(0001). This work breaks the bottleneck in the direct synthesis of single-crystal monolayer graphene on insulating substrates and paves the way for next-generation carbon-based atomic electronics and semiconductor nanodevices.

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The electronic structure of double perovskite PrMnNiO was studied using core x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. The 2p x-ray absorption spectra show that Mn and Ni are in 4+  and 2+  states respectively. Based on charge transfer multiplet analysis of the 2p XPS spectra of both ions, we find charge transfer energies [Formula: see text] of 3.5 and 2.5 eV for Ni and Mn respectively. The ground state of Ni and Mn ions reveal a higher d electron count of 8.21 and 3.38 respectively as compared to the ionic values. The partial density of states clearly show a charge transfer character of the system for U  -  J [Formula: see text] 2 eV. The O 1s edge absorption spectra reveal a band gap of 0.9 eV, which is close to the value estimated from analysis of Ni and Mn 2p photoemission and absorption spectra. The combined analysis of nature of spectroscopic data and first principles calculations reveal that the material is a p  -  d type charge transfer insulator with an intermediate covalent character according to the Zannen-Sawatzy-Allen phase diagram.

First-principles calculations are performed for the recently synthesized monolayer MoSi2N4 [Science 369, 670–674 (2020)]. We show that N vacancies are energetically favorable over Si vacancies, except for Fermi energies close to the conduction band edge in the N-rich environment, and induce half-metallicity. N and Si vacancies generate magnetic moments of 1.0 and 2.0 μB, respectively, with potential applications in spintronics. We also demonstrate that N and Si vacancies can be used to effectively engineer the work function.