Surface NMR using quantum sensors in diamond

NMR is a noninvasive, molecular-level spectroscopic technique widely used for chemical characterization. However, it lacks the sensitivity to probe the small number of spins at surfaces and interfaces. Here, we use nitrogen vacancy (NV) centers in diamond as quantum sensors to optically detect NMR signals from chemically modified thin films. To demonstrate the method’s capabilities, aluminum oxide layers, common supports in catalysis and materials science, are prepared by atomic layer deposition and are subsequently functionalized by phosphonate chemistry to form self-assembled monolayers. The surface NV-NMR technique detects spatially resolved NMR signals from the monolayer, indicates chemical binding, and quantifies molecular coverage. In addition, it can monitor in real time the formation kinetics at the solid–liquid interface. With our approach, we show that NV quantum sensors are a surface-sensitive NMR tool with femtomole sensitivity for in situ analysis in catalysis, materials, and biological research.

The characterization of surface processes at the molecular level is essential for understanding fundamental processes in industrial catalysis, energy conversion, electronic circuits, targeted drug delivery, and biosensing (1). However, many analytical techniques used in surface science are inaccessible under ambient or chemically relevant conditions. Therefore, it remains challenging to perform chemical analysis under the conditions in which these processes occur (2, 3). Commonly used surface sensitive methods, such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy, and secondary ion mass spectroscopy can perform chemical analysis but require ultra-high vacuum and expensive equipment (4). Great efforts have been devoted to extending XPS analysis to near ambient conditions (2). Indeed, both near-ambient pressure XPS and extended X-ray absorption fine structure have significantly expanded the applicability of these X-ray–based techniques for understanding reaction mechanisms at chemically active interfaces (2, 5). However, both methods require intense synchrotron radiation to achieve high sensitivity and resolution, which limits their practical accessibility and increases their cost. State-of-the-art surface-sensitive spectroscopy techniques, such as sum frequency generation and second harmonic generation, can perform analysis under ambient conditions but require technically complex equipment such as femtosecond lasers (6). Even with all these techniques available, molecular dynamics or chemical reaction kinetics at surfaces are still challenging to probe experimentally (7) (SI Appendix, Supplementary Note 1).NMR spectroscopy is one of the major tools for chemical and structural analysis in chemistry, biology, and materials science. Solid-state NMR in particular (8) has advanced understanding of a range of systems, including metal organic frameworks (9), batteries (10), and catalysts (11). However, sensitivity remains a challenge for traditional NMR spectroscopy, making studies at surfaces difficult because of the limited numbers of nuclear spins. Recently, surface-enhanced NMR spectroscopy (DNP-SENS) relying on hyperpolarization such as dynamic nuclear polarization (12, 13) or xenon-based techniques (14) gained research momentum and enabled probing spins located at surfaces. However, even in highly porous materials with greater than 1,000 m²/g surface area, the concentration of NMR-active nuclei of interest often remains low (e.g., 1 mmol of surface atoms/g), which requires long averaging times to obtain solid-state NMR spectra with reasonable signal-to-noise ratios (SNR) (12) (SI Appendix, Supplementary Note 2).Here, we demonstrate the use of quantum sensors in diamond as a surface-sensitive spectroscopy technique that works at ambient conditions and can probe planar interfaces on the microscopic length scale with far greater sensitivity (femtomoles, see Materials and Methods) than conventional NMR. The spectroscopic technique relies on the nitrogen vacancy (NV) point defect, consisting of a nitrogen impurity (N) and an adjacent vacancy (V) in the carbon lattice of diamond. These spin-1 defects allow for optical detection of magnetic resonance and have been established as highly sensitive nanoscale magnetic field sensors (15, 16). Near-surface NV centers are sensitive to magnetic fields from the Larmor precession of nuclei from samples positioned outside of the diamond. This enables nanoscale NMR detection—even down to a single molecule (17) or spin (18, 19). The measurement volume of such NV sensors (20, 21) corresponds to a hemisphere whose radius is roughly their depth below the surface in the diamond lattice (e.g., 5 to 10 nm). At this small length scale, the thermal polarization of the nuclear spins can be neglected since spin noise dominates for a small number of spins (22, 23). For that reason, the NMR signal strength is independent of the applied magnetic field B₀, reducing experimental complexity and costs, which makes the technique accessible to a broader community. Previously published nanoscale NV-NMR experiments detected NMR signals from either bulk samples [such as viscous oils (21, 22, 24)] or samples tethered to (17) or placed directly on the diamond surface (25). In this work, we propose the use of NV centers in diamond combined with state-of-the-art thin film deposition techniques as a general platform to detect NMR signals with high sensitivity and spatial resolution even from nondiamond surfaces. This approach is general and allows for the probing of a variety of surfaces and interfaces with NMR, thereby enabling their chemistry to be explored. Here, we use atomic layer deposition (ALD), a technology that can be applied to synthesize films of a wide variety of materials with high thickness precision to coat the diamond with amorphous aluminum oxide (Al₂O₃). Al₂O₃ provides an exemplary surface of high technical relevance in optoelectronic applications and acts as structural support in various catalytic processes (26). In a proof-of-concept study for this surface-sensitive spectroscopic technique, we probe the chemical modification of the Al₂O₃ surface with phosphonate anchoring during the formation of a self-assembling monolayer (SAM) (27).