Nanoscale-NMR with Nitrogen Vacancy center spins in diamond
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-01-01 |
| Journal | Journal of the Korean Magnetic Resonance Society |
| Authors | JungâHyun Lee |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThe research details the development and application of high-spectral-resolution nanoscale Nuclear Magnetic Resonance (NMR) using Nitrogen-Vacancy (NV) centers in diamond.
- Problem Solved: Overcame the primary limitation of NV-NMRâpoor spectral resolutionâcaused by the short NV sensor spin lifetime (T1 ~ 3ms), which is orders of magnitude shorter than typical nuclear spin coherence times.
- Core Innovation (SR): The Synchronized Readout (SR) protocol was implemented. This technique actively detects the external AC signal, decoupling the NMR signal linewidth from the NV spin lifetime.
- Resolution Achievement: SR-NMR demonstrated spectral linewidths approaching 1 Hz for real liquid samples (water spin echo FWHM: 2.8 ± 0.3 Hz), sufficient for resolving molecular signatures like J-couplings and chemical shifts.
- SNR Maximization: Combining SR with Overhauser Dynamic Nuclear Polarization (DNP) resulted in a significant Signal-to-Noise Ratio (SNR) enhancement, achieving approximately x230 increase in signal magnitude.
- Sensitivity and Scale: The ensemble NV sensor achieved a magnetic field sensitivity of 30 pT Hz-1/2, enabling femtomole sensitivity within a picoliter detection volume.
- Versatility: The NV center system is a robust, solid-state spin sensor that operates under ambient conditions and is optically initialized (532 nm) and controlled via microwave signals.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| NV Spin System | Spin 1 | N/A | Electronic ground state (3A2) and excited state (3E). |
| Optical Initialization Wavelength | 532 | nm | Green laser used for spin polarization. |
| Fluorescence Emission Band | 600 to 800 | nm | Readout signal band. |
| NV Sensor Spin Lifetime (T1) | ~3 | ms | Traditional limit on interrogation duration. |
| Zero-Field Splitting (ZFS) | ~2.87 | GHz | Center frequency for Optically Detected Magnetic Resonance (ODMR). |
| Ensemble NV Concentration | â 3x1017 | cm-3 | Concentration within the 13 ”m NV-doped layer. |
| Ensemble Magnetic Field Sensitivity | 30 | pT Hz-1/2 | Measured performance using a 20 ”m diameter optical beam. |
| Spectral Resolution (Artificial AC Field) | 0.4 | mHz | Achieved using SR with T = 3000s averaging time. |
| Spectral Resolution (Water Spin Echo) | 2.8 ± 0.3 | Hz | Achieved using Coherently Averaged SR (CASR) on a real sample. |
| SNR Improvement (DNP) | x230 | N/A | Increase achieved by combining DNP with CASR. |
| Maximum Coherent Averaging Time | Up to 103 | s | Duration possible using the CASR protocol. |
Key Methodologies
Section titled âKey MethodologiesâThe high-resolution nanoscale NMR was achieved through a combination of advanced quantum control and signal processing techniques:
- Shallow NV Creation: NV sensor spins were created close to the diamond surface (few nm deep) using few keV nitrogen ion implantation energy to maximize the coupling to the external nuclear spin sample volume.
- AC Magnetometry Pulse Sequences: Conventional dynamical decoupling sequences, primarily CPMG (Carr-Purcell-Meiboom-Gill) or XY pulse sequences, were used to provide an AC signal filter, allowing the NV spin to accumulate phase only at the specific oscillating NMR signal frequency.
- Synchronized Readout (SR) Protocol: This technique involves periodic, synchronized readout of the NV sensorâs magnetometry response, mixing the external AC magnetic signal (Larmor oscillation) with the sensorâs spectral response.
- The effective frequency measured is Îf = |fac - fo|, where fac is the NMR frequency and fo is the center frequency of the magnetometry response.
- This active detection method ensures the NMR signal linewidth is independent of the short NV spin lifetime.
- Coherently Averaged Synchronized Readout (CASR): Used for real NMR samples where the initial phase of the oscillating field is locked to the SR sequence, allowing for coherent signal accumulation over extended periods (up to 1000s).
- Dynamic Nuclear Polarization (DNP): Overhauser DNP was employed to hyperpolarize the nuclear spins prior to CASR detection. This involved continuous pumping of polarization from the electronic spin to the nuclear spin (~300ms duration) to overcome the low SNR inherent to thermal polarization.
Commercial Applications
Section titled âCommercial ApplicationsâThe demonstrated capabilities of high-resolution, high-sensitivity NV-NMR open doors for applications in fields requiring analysis of extremely small or dilute samples.
- Pharmaceutical and Drug Discovery: High-resolution spectroscopy for identifying molecular NMR signatures (chemical shifts and J-couplings) in dilute solutions, enabling analysis with femtomole sensitivity.
- Catalysis Research: Monitoring chemical reactions and structural changes in catalytic processes using picoliter detection volumes.
- Single-Cell Biology: Performing NMR studies on single cells or small biological samples where volume constraints are critical.
- Quantum Sensing and Metrology: Utilizing the NV center as a leading platform for highly sensitive, atomic-scale magnetic field sensing under ambient conditions.
- Materials Science: Characterization of spin defects and nuclear environments in solid-state materials for quantum information science and device development.