The scalable synthesis of high-quality single-crystal diamond films remains pivotal for next-generation extreme-performance devices. Iridium substrates offer exceptional promise for heteroepitaxy, yet early-stage growth mechanisms limiting crystal quality are poorly understood. An integrated multiscale investigation combining first-principles DFT calculations, molecular dynamics simulations, and experimental validation is presented to resolve the oriented attachment process governing diamond growth on Ir(100). Robust interfacial bonding at the interface and optimal carbon coverage are revealed to provide thermodynamic driving forces for primary nucleation. A critical angular tolerance enabling defect-free coalescence through crystallographic realignment is identified by molecular dynamics. Concurrent nucleation growth pathways are experimentally confirmed through SEM, AFM, and Raman spectroscopy, where nascent crystallites undergo spontaneous orientational registry to form continuous epitaxial domains. Grain boundary annihilation is observed upon lattice rotation aligning adjacent grains below the critical angle. Crucially, intrinsic atomic steps are generated on the resultant coalesced layer, eliminating conventional etching requirements for homoepitaxial thickening. This work advances fundamental understanding of single-crystal diamond growth mechanisms, facilitating enhanced quality control for semiconductor device manufacturing and quantum applications.

The precise control of thermal–kinetic parameters governs epitaxial perfection in functional oxide heterostructures. Herein, using Iridium/MgO(100) as a model system, the traditional “low-speed/high-temperature” paradigm is revolutionized through the combination of ab initio calculations, multiscale simulations, and subsequent deposition experiments. First-principles modeling reveals the mechanisms of Volmer–Weber (VW, island growth mode) nucleation at low coverage and Stranski–Krastanov (SK, layer-plus-island growth) transitions driven by interface metallization, stress release, and energy reduction, which facilitates coherent monolayer formation by lowering the energy barrier by ~34%. Molecular dynamics simulations demonstrate that the strategic co-optimization of substrate temperature (T_sub) and deposition rate (V_dep) induces an abrupt cliff-like drop in mosaic spread. Experimental validations confirm that this T-V synergy achieves unprecedented interfacial coherence, whereby AFM roughness reaches 0.34 nm (RMS) and the XRC-FWHM of 0.13° approaches single-crystal benchmarks. Notably, our novel “accelerated heteroepitaxy” protocol reduces growth time without compromising quality, addressing the efficiency–quality paradox in industrial-scale diamond substrate fabrication. These findings establish universal thermal–kinetic design principles applicable to refractory metal/oxide heterostructures for next-generation quantum sensors and high-power electronic devices.