Diamond-based carbon composite films have long been recognized as promising candidates for high-performance field emission cathodes owing to their exceptional chemical inertness, high mechanical strength, and tunable electronic properties. Among them, diamond-graphite composite (GDC) films, which integrate the superior structural stability of the diamond phase and the high electrical conductivity of the graphitic phase, have garnered increasing attention from the field emission research community. Nevertheless, the insufficient long-term electron emission stability under continuous operating conditions has long been the critical bottleneck hindering the practical application of GDC films.
To address this bottleneck issue, a simple and efficient in-situ hydrogen/argon (H2/Ar) plasma treatment modification strategy is proposed to prepare nanocrystalline diamond@diamond graphite composite (D@GDC) films with surface rich nanocrystalline diamond phase gradient structure, so as to achieve synergistic optimization of electron emission performance and long-term emission stability, and reveal the underlying mechanism of plasma film surface interaction. In the experiment, GDC films were first deposited on silicon substrates using liquid n-butylamine as the sole carbon source. Subsequently, in-situ H2/Ar plasma treatment was performed on the as-prepared GDC films, and a series of D@GDC films were fabricated by adjusting the flow ratio of H2 to Ar. The microstructure, phase composition, carbon chemical bonding states of the as-obtained films, along with the plasma reaction process, were systematically characterized by FESEM, GIXRD, Raman, XPS, and in-situ OES. The field emission performance of the films was tested through a direct-current (DC) field emission test platform.
The experimental results demonstrate that H2/Ar plasma exerts a dual effect of selective etching and in-situ regrowth on GDC films. Specifically, the plasma preferentially etches the sp2-hybridized graphitic phase, meanwhile, the etching of the graphitic phase generates abundant carbon-containing active species, which induce the in-situ growth of a nanocrystalline diamond layer on the film surface. With the increase of the Ar flow ratio, the relative content of sp3-bonded carbon (sp3-C) on the film surface increases continuously, reaching a maximum of 56.03% at an Ar content of 75%. The pristine GDC film exhibits a low turn-on electric field of 1.76 V/μm and a current density up to 9.15 mA/cm2 at an applied electric field of 4 V/μm, yet suffered a severe current density attenuation of as high as 60.64% after a 120-minute continuous emission test. Among all H2/Ar plasma-treated samples, the D@GDC-3 sample treated at an H2/Ar flow ratio of 50∶50 achieves the optimal balance between emission performance and stability, with a turn-on electric field as low as 1.93 V/μm, a current density of 4.0 mA/cm2 at an applied electric field of 5.82 V/μm, and a current density attenuation of only 12.54% after the 120-minute continuous emission test. Mechanism analysis reveals that the nanocrystalline diamond protective layer with high sp3-C content constructed on the film surface can effectively mitigate the field-induced ion bombardment damage during the field emission process, while the dual-layer structure simultaneously takes into account the high electron transport efficiency of the graphitic phase in the base layer and the excellent structural stability of the diamond phase in the surface layer.
In summary, the in-situ H2/Ar plasma treatment strategy proposed in this work successfully constructs a hierarchical "nanocrystalline diamond surface layer @diamond-graphite composite base layer" structure on GDC films, effectively breaking through the long-standing bottleneck of balancing the emission performance and stability of GDC films. This work provides a facile and efficient novel strategy for the structural design of high-performance diamond-based field emission cathodes, and lays a solid experimental foundation for their practical application in advanced vacuum electronic devices.
Key words
diamond-graphite /
field emission /
plasma post-treatment /
MPCVD /
stability
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Funding
National Natural Science Foundation of China (62374140); Sichuan Science and Technology Program (2025NSFSC2063); Natural Science Foundation of Sichuan Province (24zs9103)
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