Single Color Centers in Nanodiamonds

• Single color centers in nanodiamonds are optically active point defects with narrow spectral lines and robust photostability, ideal for room-temperature quantum applications.
• Advanced synthesis methods—including CVD, HPHT, and ion implantation—achieve controlled defect incorporation, precise dipole alignment, and high conversion efficiencies.
• Integration with photonic platforms enhances single-photon emission and spin control, driving progress in quantum sensing, communication, and scalable quantum devices.

Single color centers in nanodiamonds are optically active point defects embedded within nanocrystalline diamond, functioning as solid-state quantum emitters with room-temperature stability, narrow spectral lines, and robust photostability. These systems, particularly the nitrogen-vacancy (NV), silicon-vacancy (SiV), and group-IV vacancy centers (GeV, SnV), have become central to quantum photonics, nanoscale sensing, and bioimaging due to their unique optical and spin properties. Their nanometer-scale hosts enable integration into hybrid platforms, direct interfacing with photonic structures, and scalable deployment as indistinguishable single-photon sources.

1. Synthesis and Structural Control

Nanodiamond synthesis methods for single color centers comprise both top-down and bottom-up approaches, each with distinct implications for defect incorporation, crystal quality, and yield of desired color centers.

• CVD and Heteroepitaxial Growth: Subwavelength “nanoisland” crystals (e.g., 160 nm × 75 nm) can be heteroepitaxially grown on oriented templates such as Ir(001)/YSZ/Si(001) via microwave plasma CVD. Bias-enhanced nucleation (BEN) and controlled methane concentration (e.g., CH₄/H₂ = 0.5–3 % at 40 mbar) yield cubo-octahedral particles with sharp (001) facets and defined crystal orientation (Neu et al., 2011, Arend et al., 2015). Defined orientation determines in-plane dipole alignment of incorporated centers and enables controlled coupling to photonic modes.
• Seeded High-Pressure Growth: Ultrasmall (<5 nm) nanodiamonds with engineered color centers can be synthesized via high-pressure, low-temperature routes employing organic diamondoid seed molecules (e.g., adamantane, azaadamantane) that survive HPHT conditions (5–8 GPa, 550–650 °C) and template specific atomic substitutions (e.g., N for NV centers) (Zapata et al., 2017).
• Ion Implantation: Group-IV centers (GeV⁻, SnV⁻) are created in 180 nm nanodiamonds via targeted 57–60 keV ion implantation, followed by annealing at ~1200 °C to mobilize vacancies and enable complex formation. High conversion efficiency (~90 % for GeV⁻) can be achieved, though at the expense of collateral creation of other defects (e.g., additional NVs) (Sachero et al., 25 Mar 2025).
• Nanodiamond Size, Strain, and Quality: The probability of single-center incorporation is size-dependent for NV centers, peaking at 15–20 % in 60–70 nm nanocrystals and rapidly decreasing below 50 nm (0706.2518). For SiV in ultrasmall crystals (e.g., 10 nm), sub-GPa strain levels and high crystalline quality (as determined by Raman analysis and narrow ZPL distributions) are achievable by optimized HPHT synthesis and post-growth purification (Bolshedvorskii et al., 2018).

2. Optical and Spectroscopic Properties

Color centers in nanodiamonds exhibit distinct emission spectra characterized by narrow zero-phonon lines (ZPL), vibrational sidebands, and, in some cases, fine-structure splitting dependent on strain and defect identity.

• NV Centers:
  • Emission: ZPL at 637 nm (NV⁻) and 575 nm (NV⁰) with broad phonon sidebands extending to 800 nm.
  • Debye–Waller factor: low (~4–5 % in NDs).
  • Room-temperature single-photon emission confirmed via g^{(2)}(0)<0.5; photostable with high quantum efficiency in neutral charge state (Berthel et al., 2015, 0706.2518).
• SiV Centers:
  • Emission: ZPL in 730–750 nm range, typically means at 738–742.6 nm (strain dependent), linewidths FWHM 0.7–2.5 nm (mean 1.3 nm in heteroepitaxial NDs; as narrow as 1 nm in site-selected NIs), and up to 5.9 nm (mean) in <10 nm NDs (Neu et al., 2011, Bolshedvorskii et al., 2018, Arend et al., 2015).
  • High Debye–Waller factor: ~68–70 % (direct measurement and from S ≈ 0.38).
  • Inhomogeneous broadening is dominated by local microstresses and strain fields; precise ZPL position is highly sensitive to the local lattice environment (Lindner et al., 2018).
  • In rare cases, additional narrow lines >825 nm have been observed, tentatively assigned to purely electronic transitions (Neu et al., 2011).
• Group-IV Centers (GeV⁻, SnV⁻):
  • ZPL: GeV⁻ at 602 nm, SnV⁻ at 620 nm.
  • Fine structure: ground-state splittings of 428 GHz (GeV⁻) and 854 GHz (SnV⁻).
  • Linewidth: GeV⁻ 440 ± 40 MHz, SnV⁻ 300 ± 30 MHz (at 5–12 K) (Sachero et al., 25 Mar 2025).
  • High purity single-photon emission: g^{(2)}(0) = 0.15 (GeV⁻), 0.03 (SnV⁻).
• Unidentified NIR Defects:
  • Single-photon emitter with ZPL at ~780 nm, sub-GHz linewidth (660 MHz @ 10 K), high ZPL emission (Debye–Waller factor ≈0.87), and robust emission stability under low-power resonant excitation (Tran et al., 2017).

Table: Representative ZPL and Linewidth Values for Single Centers in Nanodiamonds

Center	ZPL (nm)	FWHM (nm, RT unless specified)	Debye–Waller Factor
NV⁻	637	>10	~0.04
SiV⁻	737–750	0.7–2.5 (het-epi, 1.0–1.8 site-select, ~5.9 in 10 nm NDs)	~0.7
GeV⁻	602	0.0015 (440 MHz @ 5–12K)	~0.8
SnV⁻	620	0.0010 (300 MHz @ 5–12K)	~0.7
NIR Center	780	0.0013 (660 MHz @ 10K)	0.87

3. Spin and Photophysical Characteristics

Single color centers in nanodiamonds retain spin and photon emission features key to quantum applications:

• Single-Photon Emission: All families of centers display antibunching behavior (g^{(2)}(0)<0.5). Saturated count rates can reach several hundred kilocounts per second (e.g., 0.5–1 Mcps for HPHT-grown NV in <5 nm NDs; 75–143 kcps for SiV in 10 nm NDs; 1.2×10⁵ cps for NIR centers) (Zapata et al., 2017, Bolshedvorskii et al., 2018, Tran et al., 2017).
• Lifetime and Brightness: Excited-state lifetimes are short for SiV (<1.5 ns) and group-IV centers (1.8–3 ns), enabling high repetition rates. NV centers in NDs commonly show 12–22 ns decay times (Berthel et al., 2015, Bolshedvorskii et al., 2018, Sachero et al., 25 Mar 2025).
• Photon Statistics and Dynamics: NV⁻ centers display photon bunching and a quantum efficiency that drops to ~0.5 at high excitation (role of dark shelving state), while NV⁰ centers remain nearly ideal two-level systems (Q ≈ 1). Photochromism and blinking are observed in some charge states and center types, especially in strain-modified or surface-influenced SiV centers (Berthel et al., 2015, Lindner et al., 2018).
• Polarization: Both absorption and emission are highly polarized (visibility >90–100 % for SiV), with fixed dipole orientation set by the host crystal (Neu et al., 2011, Neu et al., 2013).

4. Environmental Effects: Strain, Inhomogeneity, and Surface Treatments

• Strain Effects: In SiV nanodiamonds, bimodal clustering of ZPLs arises from strain. A tightly clustered group (730–741 nm) exhibits broad linewidths (5–17 nm), attributable to local stresses; a second “modified” group shows ZPLs from 715–835 nm with narrower widths (1–4 nm) and possible chemical or charge-state modification (Lindner et al., 2018).
• Surface Chemistry and Annealing: Air oxidation and other treatments can remove damaged surface layers, narrow both the diamond Raman and optical emission lines, reduce stress-induced inhomogeneity, and enhance photon purity for single emitters (Neu et al., 2013). Post-growth annealing is necessary in seeded-synthesis approaches to activate NV fluorescence after introduction of vacancies (Zapata et al., 2017).
• Host-Dependent Coherence: Spin coherence times (T₂) for NV in NDs are typically 15 µs (vs. up to 350 µs in ultrapure bulk), while ultrasmall NDs display increased spectral diffusion and phonon-induced broadening. For SiV, electronic spin T₂* in bulk is ~35 ns; in 10 nm NDs, room-T coherence is unmeasured but cryogenic T₂* may be preserved under low strain (0706.2518, Bolshedvorskii et al., 2018, Lindner et al., 2018).

5. Engineering and Device Integration

• Positioning and Control: Site-selective heteroepitaxial growth on patterned substrates (e.g., via electron-beam lithography, ICP-RIE) enables deterministic placement of diamond nanoislands with single SiV incorporation yields of ~1 % and <20 nm lithographic accuracy (Arend et al., 2015). Seeded growth potentially offers atomically deterministic color center placement, pending further isotopic and spectral verification (Zapata et al., 2017).
• Coupling to Photonic Platforms: Defined orientation and geometry (e.g., subwavelength nanoislands) minimize internal reflection losses and set transition dipole alignment, enhancing coupling to waveguides and cavities, simplifying integration into nanophotonic circuits, and maximizing photon out-coupling (Neu et al., 2011).
• Spontaneous Emission Engineering: Placement of nanodiamonds on photonic structures (e.g., opal photonic crystals) increases spontaneous emission rates by ~1.5× through modification of the local density of optical states, predominantly due to orientation-dependent field effects (Inam et al., 2011).
• Scalability and Yield Constraints: While deterministic approaches show promise for site control, yields for single emitter incorporation remain sub-unity (e.g., ~0.15 % for SiV in 10 nm HPHT NDs, ~0.01 for SiV in site-selected islands), with ongoing work to improve through seed, process, and catalyst optimization (Bolshedvorskii et al., 2018, Arend et al., 2015, Zapata et al., 2017).

6. Applications and Limitations

Single color centers in nanodiamonds are advancing quantum sensing, integrated photonics, and quantum information science:

• Quantum Sensing: Single NVs in nanodiamonds facilitate nanoscale magnetometry and electrometry in biological and fluidic contexts, enabled by robust spin-state readout and operation at ambient conditions (0706.2518).
• Quantum Photonics and Communication: Bright, photostable, and spectrally narrow SiV, GeV, and SnV centers serve as single-photon and entangled-photon sources. Sub-GHz homogeneous linewidths and high Debye–Waller factors (e.g., 0.87 for NIR centers in NDs) enable indistinguishable photon emission required for two-photon interference and quantum protocols (Tran et al., 2017, Sachero et al., 25 Mar 2025).
• Spin Registers and Multi-Atom Centers: Atomically-designed seeds permit encoding of multi-atom quantum registers (e.g., NV–^13C) in ultrasmall diamonds, with prospects for deterministic two-qubit systems. NE8 centers and more complex architectures become accessible with further seed-chemistry advances (Zapata et al., 2017).

Limitations include spectral inhomogeneity due to strain or surface effects (especially for SiV), quantum yield reduction at high excitation for NV⁻, yield limitations in ultrasmall NDs, unmeasured room-T spin coherence for group-IV centers in small hosts, and incomplete elimination of modified (non-canonical) centers in many processing routes.

7. Outlook and Open Challenges

Key challenges in the field encompass:

• Achieving high-yield, atomically precise incorporation of single or multi-atom color centers in ultrasmall hosts via designed precursor molecules.
• Minimizing strain and inhomogeneous broadening in <100 nm NDs to reach Fourier-transform-limited optical linewidths at room temperature.
• Determining spin coherence times of single group-IV centers in ultrasmall NDs, including phonon-confinement suppression of decoherence.
• Surface passivation strategies to suppress blinking and charge noise, thereby stabilizing emission and spin properties.
• Expansion to other defect families, with controlled site, charge, and chemical state (e.g., GeV, SnV, NE8) for scalable quantum devices (Sachero et al., 25 Mar 2025, Lindner et al., 2018, Zapata et al., 2017).

Sustained advances in material synthesis, atomic defect engineering, and surface/interface control are required to translate the unique photonic and spintronic capabilities of single color centers in nanodiamonds into robust, scalable technologies for quantum computation, communications, and nanoscale sensing.