Fig. 1: Synthesis and characterization of the inch-scale ultrahard diamond wafers using customized microwave plasma chemical vapor deposition (MP-CVD) method.

a Schematic of the polycrystalline diamond growth process with cyclic nitrogen doping, illustrating changes of gas compositions and the corresponding plasma groups with the rapid flow switching. MFC: Mass Flow Controller. b Comparation of CN group signal (inset showing the magnified spectrum in the blue-shaded region) and C₂ group signal during the nitrogen valve open and close periods within a single cycle detected by optical emission spectrometer. c Intensity variation of optical emission spectrometer characteristic peaks for C₂ and CN groups over 3000 growth cycles. d Temperature monitoring of the MP-CVD chamber over 3000 growth cycles, with temperature fluctuations corresponding to the cyclic doping process. e Photograph of a typical 3-mm-thick, 5-inch free-standing polycrystalline diamond wafer grown by the cyclic nitrogen doping process. f XRD pattern of the growth wafer, with characteristic crystal plane peaks consistent with the standard diamond structure (red bars), with the (111) plane showing the strongest reflection. g Raman spectrum of the diamond wafer, indicating the coexistence of sp³, sp², and Trans-polyacetylene (TPA, refer to ultrananocrystalline diamond grain⁶⁷) signals. h SEM image of the diamond wafer, showing a pyramidal surface morphology (indicated by orange dashed lines); representative SEM image were obtained from five independent experiments showing similar results. Source data are provided as a Source Data file.