In the present investigation, a nitrogen-doped multilayer homoepitaxial single crystal diamond is synthesized on a high-pressure high temperature (HPHT) Ib-type diamond substrate using the microwave plasma chemical vapor deposition (MPCVD) method. When 0.15 sccm of nitrogen was added in the gas phase, the growth rate of the doped layer was about 1.7 times that of the buffer layer, and large conical and pyramidal features are formed on the surface of the sample. Raman mapping and photoluminescence imaging of the polished cross sectional slice shows a broadband emission, with a characteristic zero phonon line (ZPL) at 575 nm in the doped layers, and large compressive stress was formed in the nitrogen-doped layers. X-ray topography shows that the defects at the interface can induce dislocation. The pyramid feature is formed at the defect, and more nitrogen-related defects are formed in the pyramid region. Thin nitrogen-doped multilayers were successfully prepared, and the thickness of the nitrogen-doped and buffer layers was about 650 nm each. The indentation measurements reveal that the thin nitrogen-doped multilayers are ultra-tough (at least ~22 MPa m^1/2), compared to the Ib-type HPHT seed substrate (~8 MPa m^1/2) and the unintentionally doped chemical vapor deposition (CVD) single crystal diamond (~14 MPa m^1/2).

Producing inch-scale, binder-free ultrahard diamond presents a formidable challenge due to the limitations of the conventional high-pressure and high-temperature method. Here, we report a customized microwave plasma chemical vapor deposition technique with high-frequency gas-switching control. By periodically introducing nitrogen, a transient local non-equilibrium growth mode is established, enabling the synthesis of free-standing ultrahard diamond wafers up to 3 mm thick and 5 inches in diameter. The wafers exhibit a Vickers hardness of ~208.3 GPa, comparable to the hardest nano-twinned diamonds, and show exceptional wear resistance—abrasive ratio ~7 times higher than polycrystalline diamond substrate. High-resolution transmission electron microscopy reveals an ultra-dense three-dimensional interlocked stacking fault network (density up to 4.3×10¹² cm⁻²), contributing to superior mechanical properties. This process also allows deposition on commonly used three-dimensional tool surfaces. This work provides a scalable strategy for producing ultrahard, inch-scale diamond suitable for demanding applications in precision machining, semiconductor and aerospace industries.