Single-molecule Scale Nuclear Magnetic Resonance Spectroscopy using a Robust Near-Infrared Spin Sensor

Nuclear magnetic resonance (NMR) at the single-molecule level with atomic resolution holds transformative potential for structural biology and surface chemistry. Near-surface solid-state spin sensors with optical readout ability offer a promising pathway toward this goal. However, their extreme proximity to target molecules demands exceptional robustness against surface-induced perturbations. Furthermore, life science applications require these sensors to operate in biocompatible spectral ranges that minimize photodamage. In this work, we demonstrate that the PL6 quantum defect in 4H silicon carbide (4H-SiC) can serve as a robust near-infrared spin sensor. This sensor operates at tissue-transparent wavelengths and exhibits exceptional near-surface stability even at depth of 2 nm. Using shallow PL6 centers, we achieve nanoscale NMR detection of proton ($\mathrm{^{1}H}$) spins in immersion oil and fluorine ($\mathrm{^{19}F}$) spins in Fomblin, attaining a detection volume of $\mathrm{(3~nm)^3}$ and a sensitivity reaching the requirement for single-proton spin detection. This work establishes 4H-SiC quantum sensors as a compelling platform for nanoscale magnetic resonance, with promising applications in probing low-dimensional water phases, protein folding dynamics, and molecular interactions.

💡 Research Summary

This paper presents a breakthrough in nanoscale nuclear magnetic resonance (NMR) spectroscopy by exploiting the PL6 color center in 4H‑silicon carbide (4H‑SiC) as a robust, near‑infrared (NIR) spin sensor. The authors address two critical limitations of the widely used nitrogen‑vacancy (NV) center in diamond: (i) the need for visible‑light excitation, which can cause photodamage to biological samples, and (ii) charge‑state instability when the NV is placed within a few nanometers of the diamond surface. PL6 in 4H‑SiC emits in the 1038–1380 nm range and can be optically pumped with a 914 nm continuous‑wave laser, placing its operation in a tissue‑transparent window and dramatically reducing the risk of phototoxicity.

To create shallow sensors, the team implanted low‑energy (3 keV) ¹⁴N⁺ ions into an intrinsic 4H‑SiC epilayer, followed by high‑temperature annealing (1000–1050 °C). This process yields single PL6 defects located as shallow as 2 nm beneath the surface, as confirmed by NMR‑based depth calibration. Optical characterization shows single‑photon emission (g²(0) < 0.5) with a saturation count rate of ~460 kcps and a saturation power of ~558 kcps. Remarkably, a 2‑nm‑deep PL6 center retains its fluorescence intensity and spin readout contrast after 60 hours of continuous illumination at 2.2 × the saturation power, demonstrating unprecedented photostability for near‑surface quantum sensors.

NMR detection is performed using XY8‑k dynamical decoupling sequences, which act as narrow‑band filters for alternating magnetic fields. By sweeping the interpulse spacing τ, the filter peak aligns with the Larmor frequency of target nuclei, producing characteristic coherence dips. The authors demonstrate detection of ¹H spins in immersion oil and ¹⁹F spins in Fomblin. The measured proton gyromagnetic ratio (γ_H = 4.25 ± 0.08 kHz G⁻¹) matches the known value, confirming accurate magnetic field calibration. Fitting the proton spectra yields an RMS magnetic field of 3.7 µT, from which a sensor depth of 2.00 ± 0.04 nm is extracted, consistent with the NIR‑based depth measurement.

Sensitivity analysis across fourteen shallow PL6 centers shows a typical magnetic‑field sensitivity of ~350 nT Hz⁻¹ᐟ² for depths of 2–4 nm. The best performer (depth = 2.0 nm) reaches 307 ± 9 nT Hz⁻¹ᐟ², satisfying the threshold required for single‑proton electron‑nuclear double‑resonance (ENDOR) detection (≈300 nT Hz⁻¹ᐟ²). Numerical simulations indicate that a 2‑nm‑deep PL6 sensor samples magnetic fields from roughly 2500 protons, equivalent to an effective detection volume of (3 nm)³ and a statistical polarization of ~50 spins—precisely the regime of single‑molecule NMR.

Multi‑species spectroscopy is achieved with a correlation‑based pulse sequence that eliminates harmonic artifacts and provides T₁‑limited spectral resolution. In a Fomblin‑on‑SiC sample, the correlation signal reveals two distinct frequencies corresponding to ¹⁹F and a surface‑adsorbed ¹H layer. Fourier analysis yields linewidths of 10 kHz (¹⁹F) and 26 kHz (¹H). Depth analysis of this dataset indicates a PL6 depth of 3.6 ± 0.2 nm and a surface proton layer thickness of 0.8 ± 0.3 nm, consistent with previously reported water or hydrocarbon layers on SiC and diamond surfaces.

The discussion emphasizes that PL6 combines NIR optical addressability, sub‑2‑nm surface proximity, and exceptional photostability, delivering a sensitivity that meets the single‑spin detection frontier. The authors suggest further improvements via repetitive readout with nuclear ancillae, spin‑to‑charge conversion, and nanophotonic structures to boost photon collection. They also demonstrate that another SiC defect, PL5, can function as a shallow sensor at 1.8 nm depth, hinting at a broader family of robust SiC color centers for quantum sensing.

In summary, this work establishes PL6 centers in 4H‑SiC as a powerful platform for nanoscale NMR, capable of detecting proton and fluorine spins within a (3 nm)³ volume, operating in a biologically benign NIR window, and maintaining performance under prolonged illumination. The demonstrated sensitivity and robustness open pathways toward single‑molecule magnetic resonance imaging, real‑time monitoring of protein folding, and probing low‑dimensional water phases at the atomic scale.