The NV sample consisted of synthetic type Ib nanodiamond powders (MSY, ≤0.1 μm; Microdiamant) purified by nitration in concentrated sulfuric and nitric acid (H2SO4-HNO3), rinsed in deionized water, irradiated by a 3-MeV proton beam at a dose of 1 × 106 ions/cm2, and annealed in vacuum at 700°C for 2 hours to induce the formation of NV centers (Academia Sinica, Taipei, Taiwan) (38). The measured nanodiamond average size is 150.5 ± 23.3 nm.

The SiV sample consisted in nanodiamonds synthesized using a microwave plasma chemical vapor deposition (MPCVD) system from detonation nanodiamond seeds (size, 4 to 6 nm). The growth was carried out for 30 min in a gas mix of hydrogen/methane (100:1) at 900-W microwave power and 60-torr pressure. The synthesized nanodiamonds had sizes of ~0.3 to 1 μm.

For GeV centers, we looked at different samples. The first consisted of GeV centers synthesized using an MPCVD method, whereby the germanium was introduced externally as a solid or vapor source. The sample for the single GeV color centers is a high-purity single-crystal diamond from Element Six [N] <1 part per billion implanted with germanium ions at 35 keV using a nanoFIB system (ionLINE, Raith Nanofabrication) and an implantation dose of 100 Ge+ ions per spot. The sample was subsequently annealed at 1000°C for 30 min in high vacuum.

The second consisted in diamond nanoparticles hosting GeV color centers synthesized from mixtures of adamantane, C10H14 (purity, >99%; Sigma-Aldrich), with small amount of tetraphenylgermanium, C24H20Ge (purity, >95.5%; Sigma-Aldrich), at 9 GPa and 1500 to 1700 K, as described elsewhere (39). The concentration of Ge in the growth system was about 0.4% calculated relative to the carbon-germanium mixture, Ge/(Ge + C).

The third sample was a diamond membrane embedded with GeV color centers and was prepared as follows. The GeO2-covered membrane was placed in an MPCVD chamber, along with a ~1 mm × 1 mm piece of metallic germanium ~1 cm away. The conditions were as follows: hydrogen/methane ratio of 100:1 at 60 torr and microwave power of 900 W for 10 min to fabricate a ~400-nm intrinsic diamond layer that contains GeV color centers. The diamond membranes were then flipped 180° and thinned by an inductively coupled plasma reactive ion etching (ICP-RIE) with argon, oxygen, and SF6 etch (2:3:1) at a pressure of 45 mtorr, with a forward power of 500 and 100 W for the ICP and RIE, respectively.

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