Room-temperature hyperpolarization of polycrystalline samples with optically polarized triplet electrons - pentacene or nitrogen-vacancy center in diamond?
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-02-17 |
| Journal | Magnetic Resonance |
| Authors | Koichiro Miyanishi, Takuya F. Segawa, Kazuyuki Takeda, Izuru Ohki, Shinobu Onoda |
| Institutions | ETH Zurich, Kyoto University Institute for Chemical Research |
| Citations | 12 |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: Room-Temperature Hyperpolarization in Diamond and Pentacene
Section titled âTechnical Documentation & Analysis: Room-Temperature Hyperpolarization in Diamond and PentaceneâExecutive Summary
Section titled âExecutive SummaryâThis document analyzes the research demonstrating room-temperature 13C hyperpolarization using optically polarized triplet electrons in diamond NV- centers and pentacene-doped benzoic acid (PBA). The findings highlight the critical role of material purity and defect control, areas where 6CCVDâs advanced MPCVD diamond solutions provide a distinct advantage.
- Core Achievement: Successful room-temperature 13C Dynamic Nuclear Polarization (DNP) achieved in polycrystalline samples (microdiamonds and PBA) using the Integrated Solid Effect (ISE).
- Performance Metrics: Achieved 13C polarization enhancement of 324-fold (0.01% polarization) in NV- microdiamonds and 3600-fold (0.12% polarization) in PBA.
- Material Challenge Identified: The study explicitly notes that paramagnetic defects (e.g., P1 centers in diamond) create spin-diffusion barriers, severely limiting the nuclear spin-lattice relaxation time (T1,C) and overall bulk polarization efficiency.
- Solution Requirement: High-purity, low-defect CVD diamond is identified as the optimal material to extend T1,C and improve DNP performance, directly aligning with 6CCVDâs core Single Crystal Diamond (SCD) offering.
- Mechanism Comparison: The persistent nature of the NV- ground triplet state allows for longer microwave contact times (tMW), but the permanent local fields create diffusion barriers. PBAâs transient triplet state avoids diffusion barriers but is limited by short triplet lifetimes.
- 6CCVD Value Proposition: We supply high-purity MPCVD SCD wafers with engineered defect concentrations, enabling researchers to overcome the T1,C limitations observed in the microdiamond samples.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Maximum 13C Polarization (Diamond) | 0.01 | % | Microdiamonds (NV-) |
| Maximum 13C Polarization (PBA) | 0.12 | % | Pentacene-doped Benzoic Acid |
| Polarization Enhancement (Diamond) | 324 | fold | Compared to thermal equilibrium |
| Polarization Enhancement (PBA) | 3600 | fold | Compared to thermal equilibrium |
| 13C Longitudinal Relaxation Time (Diamond) | 99 ± 14 | s | T1,C (Microdiamonds) |
| 13C Longitudinal Relaxation Time (PBA) | 474 ± 30 | s | T1,C (PBA) |
| NV- Zero-Field Splitting (D) | 2870 | MHz | Ground Triplet State |
| Pentacene Zero-Field Splitting (D) | 1350 | MHz | Excited Triplet State |
| Magnetic Field Range (DNP) | 0.3 to 0.5 | T | Room Temperature Experiments |
| Microdiamond Particle Size | ~500 | ”m | Used for DNP |
| NV- Concentration (Microdiamonds) | 8.9 x 1017 (5) | cm-3 (ppm) | Required for DNP |
| P1 Center Concentration (Microdiamonds) | 4.6 ± 0.1 x 1018 (26) | cm-3 (ppm) | Detrimental paramagnetic defect |
| Diamond Spin Diffusion Coefficient (D) | 7.28 x 10-18 | m2 s-1 | Calculated for naturally abundant 13C |
| PBA Spin Diffusion Coefficient (D) | 9.75 x 10-20 | m2 s-1 | Estimated for 13C-enriched sample |
Key Methodologies
Section titled âKey MethodologiesâThe hyperpolarization was achieved using the Integrated Solid Effect (ISE) sequence, combining optical excitation, microwave irradiation, and magnetic field sweeping.
- Sample Preparation (NV- Diamond): Microdiamonds were electron irradiated (fluence of 1019 e-/cm2) at room temperature to form vacancies, followed by high-temperature annealing (800 °C under vacuum) to create NV centers.
- Surface Cleaning: âDarkâ nanodiamonds were chemically cleaned via oxidation (550 °C in air) and boiling acid cleaning (H2SO4/HNO3) to remove amorphous sp2 carbon surface defects.
- Optical Excitation: Pulsed lasers were used: 527 nm (30 mJ) for NV centers and 594 nm (6 mJ) for pentacene, both with a 200 ns pulse length.
- ISE Sequence Application: The ISE pulse sequence was repeated (R, repetition rate) to accumulate 13C polarization until a steady state was reached, balancing DNP buildup and nuclear spin-lattice relaxation.
- Optimization: Experimental parameters (magnetic-field sweep width Bsweep, microwave pulse width tMW, and microwave intensity $\omega_{1,e}$) were optimized to maximize 13C magnetization.
- NV- Optimal Conditions: Bsweep ~ 5 mT, tMW ~ 1250 ”s, $\omega_{1,e}$ ~ 3.74 MHz.
- PBA Optimal Conditions: Bsweep ~ 20 mT, tMW ~ 30 ”s, $\omega_{1,e}$ ~ 3.44 MHz.
- NMR Detection: Enhanced 13C magnetization was detected using a radiofrequency $\pi$/2 pulse, with 1H decoupling applied for the benzoic acid sample.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThe research highlights a critical material limitation: the presence of high concentrations of native paramagnetic defects (P1 centers) in the microdiamonds, which act as spin-diffusion barriers and severely reduce the nuclear spin-lattice relaxation time (T1,C). 6CCVDâs expertise in high-purity MPCVD diamond growth directly addresses this challenge, enabling superior DNP performance.
Applicable Materials for Hyperpolarization Research
Section titled âApplicable Materials for Hyperpolarization ResearchâTo replicate and extend this research, 6CCVD recommends the following materials, engineered for optimal spin properties:
| Material Grade | Description | Application Relevance | 6CCVD Capability |
|---|---|---|---|
| High-Purity SCD | Single Crystal Diamond (SCD) with nitrogen concentration < 1 ppm (P1 centers below detection limits). | Critical for T1,C Extension. Eliminates the P1 center spin-diffusion barriers, maximizing nuclear spin relaxation times and bulk polarization efficiency, as suggested by the authors. | SCD wafers up to 10x10 mm; thickness 0.1 ”m - 500 ”m. |
| Engineered NV SCD | Single Crystal Diamond with controlled NV- concentration (via nitrogen doping during growth or post-processing). | Ideal for maximizing the density of optically polarized triplet electrons while maintaining high crystal quality and low background defects. | Precise control over NV density and depth for ensemble DNP. |
| Optical Grade PCD | Polycrystalline Diamond (PCD) plates with high thermal conductivity and low birefringence. | Suitable for large-area DNP applications requiring high power handling and large sample volumes (up to 125 mm diameter). | Plates up to 125 mm; thickness 0.1 ”m - 500 ”m. |
| Boron-Doped Diamond (BDD) | SCD or PCD doped with Boron. | Potential for use as highly conductive electrodes or for exploring alternative DNP mechanisms in conductive environments. | Custom BDD doping levels available. |
Customization Potential
Section titled âCustomization Potentialâ6CCVD provides comprehensive engineering services to tailor diamond materials precisely to complex DNP and NMR experimental requirements:
- Custom Dimensions: We supply large-area PCD plates (up to 125 mm diameter) or high-quality SCD wafers, offering superior homogeneity compared to the microdiamond powders used in the study.
- Ultra-Smooth Polishing: To minimize surface defects (like the sp2 carbon noted in the nanodiamonds) that absorb laser light and degrade spin coherence, 6CCVD offers polishing services:
- SCD: Surface roughness (Ra) < 1 nm.
- PCD: Surface roughness (Ra) < 5 nm (for inch-size plates).
- Integrated Metalization: For researchers developing integrated DNP/NMR devices requiring microwave delivery structures, 6CCVD offers in-house metalization services, including Au, Pt, Pd, Ti, W, and Cu deposition.
- Precision Fabrication: Custom laser cutting and shaping services are available to create specific geometries required for microwave cavities or magnetic field alignment.
Engineering Support
Section titled âEngineering Supportâ6CCVDâs in-house PhD team specializes in the physics and chemistry of MPCVD diamond defects and spin properties. We can assist researchers in material selection for similar Room-Temperature Hyperpolarization projects, focusing on optimizing the trade-off between NV concentration (source of polarization) and P1 concentration (source of relaxation/diffusion barriers). Our expertise ensures that the supplied diamond maximizes the nuclear spin-lattice relaxation time (T1,C) necessary for high bulk polarization.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
Abstract. We demonstrate room-temperature 13C hyperpolarization by dynamic nuclear polarization (DNP) using optically polarized triplet electron spins in two polycrystalline systems: pentacene-doped [carboxyl-13C] benzoic acid and microdiamonds containing nitrogen-vacancy (NVâ) centers. For both samples, the integrated solid effect (ISE) is used to polarize the 13C spin system in magnetic fields of 350-400 mT. In the benzoic acid sample, the 13C spin polarization is enhanced by up to 0.12 % through direct electron-to-13C polarization transfer without performing dynamic 1H polarization followed by 1Hâ13C cross-polarization. In addition, the ISE has been successfully applied to polarize naturally abundant 13C spins in a microdiamond sample to 0.01 %. To characterize the buildup of the 13C polarization, we discuss the efficiencies of direct polarization transfer between the electron and 13C spins as well as that of 13Câ13C spin diffusion, examining various parameters which are beneficial or detrimental for successful bulk dynamic 13C polarization.