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6CCVD Research

Room-temperature hyperpolarization of polycrystalline samples with optically polarized triplet electrons - pentacene or nitrogen-vacancy center in diamond?

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2021-02-17
JournalMagnetic Resonance
AuthorsKoichiro Miyanishi, Takuya F. Segawa, Kazuyuki Takeda, Izuru Ohki, Shinobu Onoda
InstitutionsETH Zurich, The University of Osaka
Citations12
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”

This research demonstrates a method for achieving room-temperature 13C hyperpolarization in polycrystalline solids using optically polarized triplet electron spins, bypassing traditional low-temperature requirements.

  • Core Achievement: Successful room-temperature Dynamic Nuclear Polarization (DNP) of 13C nuclei via the Integrated Solid Effect (ISE) using direct electron-to-13C polarization transfer.
  • Systems Compared: Pentacene-doped [carboxyl-13C] benzoic acid (PBA) and Nitrogen-Vacancy (NV-) centers in microdiamonds.
  • Maximum Polarization: PBA achieved 0.12% 13C polarization (a 3600-fold enhancement over thermal equilibrium), significantly higher than the 0.01% (324-fold) achieved in microdiamonds.
  • Mechanism Insight (PBA Advantage): The transient nature of the pentacene excited triplet state (lifetime ~9 ”s) prevents the formation of persistent spin diffusion barriers, allowing for more efficient bulk polarization transport.
  • Mechanism Insight (NV- Limitation): The persistent paramagnetic nature of the NV- ground state and other defects (P1 centers) creates strong local fields, hindering 13C spin diffusion and limiting bulk polarization spread.
  • Engineering Guideline: The study characterizes key parameters (exchange probability, spin diffusion coefficient) necessary for selecting or engineering improved systems for room-temperature hyperpolarization of dilute nuclear spins.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Max 13C Polarization (PBA)0.12%Final polarization (3600x thermal)
Max 13C Polarization (Diamond)0.01%Microdiamonds (324x thermal)
Operating TemperatureRoom Temperature°CAll experiments
Static Magnetic Field (B0)0.35 to 0.5TField range used for DNP
ZFS Parameter D (NV-)2870MHzZero-Field Splitting (Ground Triplet State)
ZFS Parameter D (Pentacene)1350MHzZero-Field Splitting (Excited Triplet State)
13C T1 (PBA)474 ± 30sLongitudinal relaxation time
13C T1 (Diamond)99 ± 14sLongitudinal relaxation time
13C Spin Diffusion (PBA)9.75 x 10-20m2 s-1Estimated coefficient (13C enriched, 1H present)
13C Spin Diffusion (Diamond)7.28 x 10-18m2 s-1Calculated coefficient (Natural abundance 13C)
Pentacene Triplet Lifetime~9”sTime constant for lifetime decay
Optimal MW Pulse Width (PBA)30”stMW for ISE sequence
Optimal MW Pulse Width (Diamond)1250”stMW for ISE sequence
NV- Density (Microdiamond)8.9 x 1017cm-35 ppm concentration
P1 Center Density (Microdiamond)4.6 x 1018cm-326 ppm concentration (S=1/2 defects)

Key Methodologies

Section titled “Key Methodologies”

The dynamic nuclear polarization (DNP) was achieved using the Integrated Solid Effect (ISE) sequence, involving pulsed optical excitation, microwave irradiation, and magnetic field sweeping.

  1. Sample Preparation:

    • Microdiamonds: 500 ”m particles containing NV- (5 ppm) and P1 centers (26 ppm).
    • PBA: Microcrystals of [carboxyl-13C] benzoic acid doped with 0.04 mol% pentacene.
    • NV- Creation: Nanodiamonds were electron irradiated (1019 e-/cm2), annealed at 800 °C, and chemically cleaned (oxidation/acid boiling) to activate NV centers.

  2. Optical Excitation: Pulsed solid-state lasers were used to create hyperpolarized triplet electron spins (S=1).

    • NV- Excitation: 527 nm wavelength, 200 ns pulse length, 30 mJ pulse energy.
    • Pentacene Excitation: 594 nm wavelength, 200 ns pulse length, 6 mJ pulse energy.

  3. DNP Transfer (ISE): The ISE sequence was repeated (R up to 100 Hz) to accumulate 13C polarization.

    • Microwave irradiation and magnetic field sweeping were applied simultaneously to fulfill the Hartmann-Hahn condition (ωeff,e = ω0,C).
    • Optimal conditions were determined by varying the field sweep width (Bsweep), microwave duration (tMW), and microwave intensity (ω1,e).

  4. EPR Characterization: Pulse EPR (spin echo sequence) was used to measure the optically polarized triplet electron spectra and determine electron polarization (Pe).

    • EPR confirmed the partial crystalline pattern in microdiamonds and the dipolar powder pattern (Pake pattern) in nanodiamonds and PBA.

  5. NMR Detection: Enhanced 13C NMR spectra were measured at Larmor frequencies of 3.85 MHz (diamond) and 4.19 MHz (PBA).

    • 1H decoupling was applied during 13C signal acquisition for the PBA sample.

  6. Parameter Extraction: The exchange probability (Ο) and 13C spin diffusion coefficient (D) were determined by fitting the 13C polarization buildup curves under the rapid-diffusion limit assumption.

Commercial Applications

Section titled “Commercial Applications”

This technology is highly relevant to fields requiring enhanced magnetic resonance signals and room-temperature quantum control.

  • Biomedical Imaging (MRI/NMR): Provides a pathway for generating hyperpolarized agents (like 13C-labeled metabolites) at room temperature, simplifying hardware requirements compared to cryogenic DNP systems. This is crucial for metabolic imaging and clinical diagnostics.
  • Quantum Sensing and Metrology: The use of NV- centers reinforces their role as robust, room-temperature quantum defects for high-sensitivity magnetometers, electric field sensors, and thermometers.
  • Chemical Analysis and Spectroscopy: Enables high-sensitivity solid-state NMR for material science and chemistry, particularly for analyzing dilute or low-Îł nuclear spins (like 13C) in polycrystalline or amorphous samples.
  • Spintronics and Quantum Computing: The comparison between transient (Pentacene) and persistent (NV-) triplet states provides design principles for engineering new solid-state spin systems suitable for quantum information processing and storage.
  • Diamond Engineering: Drives demand for high-purity, isotopically controlled (low 14N, high 12C) Chemical Vapor Deposition (CVD) diamonds to minimize detrimental paramagnetic defects (P1 centers) that limit nuclear spin relaxation times.
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 (NV−) 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 1H−13C 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 13C−13C spin diffusion, examining various parameters which are beneficial or detrimental for successful bulk dynamic 13C polarization.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

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