Effective production of fluorescent nanodiamonds containing negatively-charged nitrogen-vacancy centers by ion irradiation
Graphical abstract
Schematic diagram of ion irradiation for producing NV−s and evaluation by optically detected magnetic resonance (ODMR).
Introduction
Fluorescent nanodiamonds (FNDs) have been attracting much attention as new bio-imaging probes due to their unique optical properties. The negatively-charged nitrogen-vacancy color center (NV−) in FNDs, which is formed by a substitutional nitrogen atom combining with an adjacent vacancy, is responsible for fluorescence that is emitted in the near-infrared region (600–800 nm) with a high quantum yield of ~ 0.7 [1], [2]. Its well-known photo-stability of displaying neither photo-blinking nor photo-bleaching enables long-term in vivo and in vitro imaging, which is not readily achieved by commonly used fluorescent agents such as organic dyes and fluorescent proteins [3], [4], [5]. It has also been reported that FNDs are highly bio-compatible and non-toxic to various kinds of cell types, and are thus superior to quantum dots which often show cyto-toxicity [6], [7]. These advantages of FNDs over conventional fluorescent dyes could make them a potential candidate for optical bio-imaging probes [5], [8], [9].
Optically detected magnetic resonance (ODMR) has been demonstrated for detection of a single NV− at ambient conditions [10], and significant attention has been paid in the field of quantum information [11], magnetic sensing [12], [13] and bio-application [14], [15]. In particular, advanced applications in the nano-scale sensing of magnetic [16], [17] and electric fields [18] and temperature [19] require a long-time coherence of spin state, which strongly depends on the quality of the diamond matrix. For this reason, methods for synthesizing high quality nanodiamonds and the efficient production of NV−s are highly desired. Here, we report the effects of ion irradiation on the production of NV−s in nanodiamonds.
There are two types of nitrogen-vacancy centers (NVCs), each with a different charged state [20], [21]. One is a neutral NVC (NV0, electron spin: S = 1/2), while the other is a negatively-charged NVC (NV−, electron spin: S = 1), represented by zero-phonon lines at 575 nm and 637 nm respectively [22], [23]. Both NVCs emit fluorescence, but only the NV− allows for measurement by ODMR. The ground state of NV− is a spin-triplet, and the spin sublevels, ms = 0 and ms = ± 1 are split by 2.87 GHz. The spin sublevels ms = ± 1 are degenerated in the absence of an external magnetic field. Microwave (MW) irradiation at this frequency induces electron spin magnetic resonance between ms = 0 and degenerated ms = ± 1 spin levels, which results in a reduction of the fluorescence intensity from NV− due to spin-dependent intersystem crossing [10]. On the other hand, NV0 is insensitive to MW irradiation. By comparing the fluorescence intensity upon MW irradiation, we can distinguish the two types of NVCs, as shown later.
In general, the concentration of NVCs in diamond is on the order of sub ppm [16]. The existence probability of NVCs contained in nanodiamonds is lower than that in bulk diamonds, because vacancies are annihilated at the surface [24]. In order to increase the concentration of NVCs in nanodiamonds, the method of ion irradiation is commonly used for creating carbon vacancies [3], [25]. In this technique, ionized atoms are accelerated and are allowed to penetrate into nanodiamonds, where collisions of the ions with atoms force the carbon atoms to be expelled, resulting in the creation of vacancies. Subsequent thermal annealing at 800 °C leads to trapping of moving vacancies by nitrogen atoms which pre-exist in the diamond lattice, thereby forming an NVC [26]. Although this method is quite effective, optimal conditions for producing a high quantity of NV−s in nanodiamonds are still unknown. In this study, we also investigated how ion irradiation affects the production of nanodiamonds containing NV−s, and their ODMR spectra when irradiated by H+, He+, Li+ and N+ ions. Furthermore, we elucidated the optimal irradiation condition.
Section snippets
Sample preparation
In our experiments, we used synthetic type Ib nanodiamonds which contain typically 100 ppm nitrogen atoms (Micron + MDA Element Six). A suspension of nanodiamonds in Milli-Q water was centrifuged at 15,000 rpm for 20 min, and the supernatant was freeze-dried. The median size of the nanodiamond particles was determined by dynamic light scattering (DLS) to be 25.9 nm (Fig. 1). 2 μl of a nanodiamond suspension in Milli-Q water at a concentration of 2 mg/ml was dropped on the surface of a silicon wafer
Results and discussion
The numbers of nanodiamonds containing NV−s produced per mm2 as a function of energy and ion dose for H+, He+, Li+ and N+ irradiation are listed in Table 2. Fig. 3 shows a bar chart representation of the values given in Table 2. The results of H+ irradiation show no or very few instances of nanodiamonds containing NV−s after irradiation with less than or equal to 1 × 1012 ions per cm2. Production of nanodiamonds containing NV−s increased as ion dose increased, but doses higher than 1 × 1014 ions per
Conclusion
We explored the optimal conditions of ion irradiation for creating nanodiamonds containing NV−s with a median particle size of 26 nm. The obtained best conditions were He+ ion irradiation at 60 keV or 80 keV with a dose of 1 × 1013 ions per cm2 for the production of high-quantity and high-quality nanodiamonds containing NV−s. The consequent NV−s exhibited strong intensities and narrow widths in ODMR spectra, which is generally an indication of good crystalline. We also demonstrated that irradiation
Author contributions
S.S. and Y.Y. contributed equally.
Prime novelty statement
ODMR for NV−s has attracted a lot of attention for its variety of applications; however, appropriate irradiation condition for creating NV−s still remains unknown. This is the first work reporting the ion irradiation conditions for producing high-quantity and high-quantity nanodiamonds containing NV−s.
Acknowledgment
This research was supported by World Premier International Research Center Initiative (WPI-iCeMS) from MEXT Japan, Core Research for Evolutional Science (CREST) (Grant 116517 to M.S.) from Japan Science and Technology Agency, Open Advanced Research Facilities Initiative of Japan Atomic Energy Agency (Grant 11B-C04 to Y.H.), Funding Program for Next Generation World-Leading Researchers (NEXT Program) (Grant LS072 to Y.H.) and Grant-in-Aid for Scientific Research on Innovative Areas (Grant
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Present address: Tandem Accelerator Complex, Research Facility Center for Science and Technology, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan.