Pulse sequence design for high field NMR with NV centers in dipolarly coupled samples

At a Glance

Metadata	Details
Publication Date	2025-08-22
Journal	Scientific Reports
Authors	Carlos Munuera-Javaloy, Ander Tobalina, J. Casanova
Institutions	University of the Basque Country
Citations	1
Analysis	Full AI Review Included

Technical Documentation & Analysis: High-Field NV-NMR in Dipolarly Coupled Samples

Source Paper: Pulse sequence design for high field NMR with NV centers in dipolarly coupled samples (Scientific Reports, 2025)

Executive Summary

This research introduces a novel quantum sensing protocol utilizing Nitrogen Vacancy (NV) ensembles in diamond to achieve high-resolution Nuclear Magnetic Resonance (NMR) spectroscopy in solid-state, dipolarly coupled samples. This breakthrough directly addresses a major limitation in microscale NMR, opening new avenues for materials science and pharmaceutical research.

Core Achievement: Successful simulation of chemical shift identification in solid-state ethanol (C₂H₆O) at high external magnetic fields ($B_{0} = 2.1$ T).
Methodology: Synchronization of two radiation channels: Radio Frequency (RF) for nuclear spin decoupling (using an advanced LG4 sequence) and Microwave (MW) for NV ensemble sensing (using a tailored CPMG-like sequence).
Decoupling Efficacy: The LG4 sequence effectively mitigates strong homonuclear dipole-dipole interactions, which typically distort spectra in solid materials.
High-Field Advantage: Operating at high magnetic fields increases sample thermal polarization, leading to a significantly enhanced NMR signal.
Precision Limit: The method’s precision is ultimately limited only by the intrinsic coherence time of the nuclear sample ($T_{2}$), surpassing limitations imposed by NV dephasing.
Application Potential: Enables microscale NMR for low-diffusion, solid-state materials, including Active Pharmaceutical Ingredients (APIs), energy storage materials, and biological structures (e.g., Alzheimer-related molecules).

Technical Specifications

The following hard data points were extracted from the simulation and experimental requirements outlined in the paper:

Parameter	Value	Unit	Context
External Magnetic Field ($B_{0}$)	2.1	T	High-field operation for enhanced polarization.
RF Rabi Frequency (Decoupling)	2π × 100 to 2π × 200	kHz	Required range for effective LG4 decoupling.
MW Rabi Frequency (Sensing)	20	MHz	Used for NV control pulses (attainable by state-of-the-art antennas).
Simulated Dipolar Coupling Strength	Up to 17	kHz	Homonuclear coupling strength in solid ethanol.
Detected Chemical Shift (High)	327 (3.66 ppm)	Hz	Frequency shift for three protons in ethanol.
Detected Chemical Shift (Low)	106 (1.19 ppm)	Hz	Frequency shift for two protons in ethanol.
Sample Coherence Time (T₂)	0.2	s	Intrinsic limit of the method’s precision.
Operating Temperature	300	K	Room temperature operation capability.
Total Evolution Time	0.5	s	Total time simulated for sample evolution.

Key Methodologies

The protocol relies on precise, synchronized control over both the nuclear spins in the sample (via RF) and the NV ensemble sensor (via MW).

Nuclear Spin Decoupling (RF Channel):
- Application of the advanced LG4 (Lee-Goldburg 4-block) sequence using an off-resonant continuous RF field.
- The LG4 sequence is designed to cancel homonuclear dipole-dipole interactions up to higher orders, resulting in narrower spectral lines.
- High RF Rabi frequencies (up to 2π × 200 kHz) are required to exceed the dipolar coupling strength (17 kHz) by an order of magnitude for optimal decoupling.
Signal Generation and Encoding:
- The RF field simultaneously generates a slow-frequency NMR signal trackable by the NV sensor.
- The effective nuclear energy shifts ($\delta_{i}$) are encoded in the amplitude variation of the sample’s longitudinal magnetization.
NV Ensemble Sensing (MW Channel):
- A tailored MW pulse sequence (a modified CPMG-like structure) is applied to the NV ensemble.
- The timing of the π pulses is adjusted to maximize the contrast of the recorded spectra by aligning the projecting axis with the major axis of the magnetization ellipse.
- The MW sequence enables the detection of the magnetic field emitted by the driven sample in a heterodyne frame.
Data Extraction:
- The measurement outcomes are Fourier transformed to extract the sinusoidal components corresponding to the effective frequencies ($\delta_{i}^{*}$).
- Target nuclear shifts ($\delta_{i}$) are then obtained via a direct analytical mapping (Eq. 5).

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the high-quality MPCVD diamond materials and custom engineering required to implement and advance this cutting-edge NV-NMR protocol, particularly for solid-state applications.

Applicable Materials

To replicate or extend this high-field, high-resolution NV-NMR research, the following 6CCVD materials are essential:

Optical Grade Single Crystal Diamond (SCD):
- Requirement: NV centers require extremely low strain and high purity to maximize coherence time ($T_{2}$). Optical grade SCD provides the necessary crystalline quality for creating stable, high-density NV ensembles with superior performance compared to lower-grade materials.
- 6CCVD Capability: We supply SCD plates with Ra < 1nm polishing, minimizing surface defects that can degrade NV coherence, which is critical for achieving the precision demonstrated in this protocol.
Custom Thickness SCD:
- Requirement: Precise control over the NV layer depth relative to the sample surface is crucial for optimizing signal coupling.
- 6CCVD Capability: We offer SCD wafers in thicknesses ranging from 0.1µm up to 500µm, allowing researchers to select the optimal depth for surface-sensitive or bulk measurements.

Customization Potential

The complexity of this protocol—requiring synchronized RF and MW delivery at high frequencies—necessitates highly integrated diamond devices.

Integrated Antenna Fabrication: The paper relies on state-of-the-art RF antennas to achieve Rabi frequencies up to 2π × 200 kHz.
- 6CCVD Capability: We offer in-house metalization services (including Au, Pt, Ti, W, and Cu) for fabricating micro-antennas and transmission lines directly onto the diamond substrate. This integration minimizes signal loss and ensures the homogeneity of the driving field, which is particularly effective in the small volume regime studied.
Custom Dimensions and Substrates:
- 6CCVD Capability: We provide custom diamond plates and wafers up to 125mm (PCD) or precise SCD dimensions, mounted on robust substrates (up to 10mm thick) for easy handling and integration into existing high-field NMR setups.

Engineering Support

This research highlights the transition of NV-NMR from liquid-state to challenging solid-state systems, applicable to fields like materials science and pharmaceutical analysis (e.g., characterizing APIs and excipients).

Expert Consultation: 6CCVD’s in-house PhD team specializes in MPCVD growth parameters, NV creation, and surface engineering. We can assist researchers in optimizing material selection, NV density, and surface termination for similar Solid-State NMR Spectroscopy projects.
Global Supply Chain: We ensure reliable, global shipping (DDU default, DDP available) of highly specialized diamond components, supporting international research efforts.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Abstract Diamond-based quantum sensors have enabled high-resolution NMR spectroscopy at the microscale in scenarios where fast molecular motion averages out dipolar interactions among target nuclei. However, in samples with low-diffusion, ubiquitous dipolar couplings challenge the extraction of relevant spectroscopic information. In this work we present a protocol that enables the scanning of nuclear spins in dipolarly-coupled samples at high magnetic fields with a sensor based on nitrogen vacancy (NV) ensembles. Our protocol is based on the synchronized delivery of radio frequency (RF) and microwave (MW) radiation to eliminate couplings among nuclei in the scanned sample and to efficiently extract target energy-shifts from the sample’s magnetization dynamics. In addition, the method is designed to operate at high magnetic fields leading to a larger sample thermal polarization, thus to an increased NMR signal. The precision of our method is ultimately limited by the coherence time of the sample, allowing for accurate identification of relevant energy shifts in solid-state systems.