Engineering bright fluorescent nitrogen-vacancy (NV) nano-diamonds: Role of low-energy ion-irradiation parameters

Bright emission from fluorescent nanodiamonds (FNDs) is highly desirable for optical bio-imaging applications. Here we report about optimized ion-irradiation and heat treatment conditions for efficient creation of Nitrogen-vacancy (NV) centers in high pressure high temperature (HPHT) grown type Ib Nanodiamond samples irradiated in energy range (20-50 KeV) and at varying fluence (1012-1016 ions/cm2). Different low energy ion irradiations are not detrimental for the crystallinity of nanodiamonds as confirmed by X-ray diffraction and HRTEM. The concentration of defects near the outer surface (non-diamond carbon) has been increased for highest ion dose (50 KeV, 1016 ions/cm2) as compared to lower ion dose (50 KeV, 1013 ions/cm2). The relative emission intensity of characteristic Zero-phonon lines of NV°, NV- centers as compared to broad emission in the range 550-750 nm was monitored with varying NV creation conditions. Sample irradiated at 50 KeV and with fluence of 1013 ions/cm2 show maximum emission in the phonon side band (550-750 nm) with maxima at 680 nm. These optimized ion irradiation conditions leads to the fabrication of 12.5±0.8 ppm concentration of NV- centers in nanodiamonds. This is desirable for their use as biomarker. This work provides a recipe for creating bright nanodiamonds for optical imaging applications.