In Situ Studies of Stress Environment in Amorphous Solids Using Negatively Charged Nitrogen Vacancy (NV–) Centers in Nanodiamond
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2020-12-08 |
| Journal | ACS Applied Polymer Materials |
| Authors | Kin On Ho, Man Yin Leung, Yiu Yung Pang, King Cho Wong, Ping Him Ng |
| Institutions | Chinese University of Hong Kong, Chinese University of Hong Kong, Shenzhen |
| Citations | 8 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”- Core Value Proposition: This research introduces a distinctive, in-situ quantum sensing protocol utilizing negatively charged Nitrogen Vacancy (NV-) centers in nanodiamond (ND) to microscopically map local pressure and shear strain within amorphous solids during chemical processing.
- Key Achievement (Stress Measurement): The methodology successfully measured local shear stress accumulation during the curing of polydimethylsiloxane (PDMS) and the polymerization of cyanoacrylate (AA glue). Calculated local shear stress values (29.6 MPa for PDMS; 104-148 MPa for AA glue) were found to be 3.3 to 5.8 times greater than the bulk Ultimate Tensile Strength (UTS).
- Strain vs. Pressure: Measurements confirmed that the solidification processes primarily induce shear strain (detected via transverse ZFS E), while local pressure changes (detected via longitudinal ZFS D) were negligible.
- Spatial Resolution and Sensitivity: The use of 100 nm NDs demonstrated higher sensitivity and spatial resolution compared to 1 µm NDs, revealing the spatial heterogeneity of strain accumulation and detecting residual reactions persisting long after macroscopic hardening.
- Methodological Breakthrough: The protocol performs measurements in situ without requiring an elastic substrate for stress analysis, enabling direct microscopic analysis of local properties at the nanoscale.
- Applicability: The technique is effective for transparent amorphous solids with internal stress in the order of MPa, with potential for pushing the limit to sub-MPa scale using better apparatus.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Sensor Type | Negatively Charged Nitrogen Vacancy (NV-) Center | N/A | Spin-1 quantum sensor in nanodiamond. |
| ND Sizes Used | 1 µm and 100 nm | µm, nm | Used to study size effect and spatial resolution. |
| ND Nitrogen Concentration | 3 | ppm | Concentration of NV centers in the nanodiamonds. |
| Longitudinal ZFS (D) (Ambient) | ≈ 2.87 | GHz | Zero-Field Splitting parameter related to pressure/temperature. |
| Temperature Dependence (dD/dT) | -74 | kHz/K | Calibration value used to eliminate temperature artifacts. |
| Gyromagnetic Ratio (γ) | 2.803 | MHz/G | Used for Zeeman splitting calculation under magnetic field (B). |
| PDMS Average ΔE (Strain Change) | 0.1 | MHz | Measured change in transverse ZFS during curing. |
| PDMS Local Shear Stress (Calculated) | ≈ 29.6 | MPa | Calculated from ΔE. 5.8x greater than bulk UTS. |
| PDMS Bulk UTS (Reference) | 5.13 ± 0.55 | MPa | Ultimate Tensile Strength (UTS) from literature. |
| AA Glue Average ΔE (1 µm ND) | 0.35 | MHz | Measured change in transverse ZFS during polymerization. |
| AA Glue Local Shear Stress (1 µm ND) | ≈ 104 | MPa | Calculated from ΔE. 3.3x greater than bulk UTS reference. |
| AA Glue Average ΔE (100 nm ND) | 0.5 | MHz | Measured change in transverse ZFS during polymerization. |
| AA Glue Local Shear Stress (100 nm ND) | ≈ 148 | MPa | Calculated from ΔE. 4.7x greater than bulk UTS reference. |
| Stress Susceptibility (Isotropic Assumption) | 1.75 | MHz/GPa | Parameter used for converting ΔE to shear stress (a = b = c). |
| PDMS Glass Transition Temp (Tg) | ≈ -100 | K | Indicates PDMS was in a viscous state during room temperature curing. |
| Excitation Wavelength | 520 | nm | Green laser used for NV- spin initialization and readout. |
| MW Transmission Wire Size | 50 | µm | Wire size used to deliver microwave signal for ODMR. |
Key Methodologies
Section titled “Key Methodologies”- Sensor and Substrate Preparation: Single-crystal nanodiamonds (NDs) of specified sizes (1 µm or 100 nm, 3 ppm N) were dispersed and drop-casted onto a plasma-cleaned glass slide. The slide was affixed to a custom Printed Circuit Board (PCB) containing a 50 µm microwave (MW) transmission wire.
- Sample Application: The pre-polymer (PDMS mixture or cyanoacrylate glue) was added onto the glass slide. NDs were strategically located at the interface between the glass and the amorphous solid to act as static, non-reactive probes.
- Confocal Microscopy Setup: A home-built confocal microscope was used for high-resolution spatial scanning (via a galvo mirror) and fluorescence collection. A 520 nm laser was focused onto the NDs for optical spin initialization and readout.
- Optically Detected Magnetic Resonance (ODMR): ODMR spectroscopy was performed repetitively over time (up to 48-60 hours) on selected individual NDs to measure the longitudinal ZFS (D) and transverse ZFS (E).
- Environmental Correction: Environment temperature was continuously monitored. Changes in D due to temperature fluctuations were calculated using the known derivative (dD/dT = -74 kHz/K) and subtracted from the raw data, isolating the spectral effects of pressure and local heating.
- Strain and Curing Time Analysis: The temporal dependence of the transverse ZFS (E) was fitted using an exponential function ($y = y_0 + a \cdot e^{-t/\tau}$). The curing time was defined as 2τ, and the total strain accumulation (ΔE) was defined as the difference between the final fitted E value and the initial E value (after adding the polymer).
- Local Stress Calculation: Local shear stress was approximated from the average measured ΔE using a simplified isotropic stress susceptibility parameter (1.75 MHz/GPa), based on the assumption that axial forces were negligible and shear forces were random in direction.
Commercial Applications
Section titled “Commercial Applications”| Industry/Field | Application Focus | Relevance to NV Sensing Protocol |
|---|---|---|
| Polymer Manufacturing | Optimization of Curing and Solidification Processes | Real-time monitoring of internal stress development (strain accumulation/relaxation) to prevent defects, cracks, and shape instability in polymeric products (e.g., coatings, films). |
| Adhesion Science | Microscopic Behavior of Adhesives (Glues) | Quantifying the high internal tension (shear stress) built up during polymerization, providing data crucial for selecting and utilizing adhesives in mechanical applications (e.g., strain gauge attachment). |
| Soft Condensed Matter Physics | Local Property Mapping and Dynamics | Studying spatial heterogeneity, viscoelasticity, and strain relaxation mechanisms in amorphous solids (glasses, gels, polymers) at sub-micron resolution. |
| Biomedical Engineering | Fabrication of Biocompatible Products | Refining manufacturing techniques for products in direct human contact, such as polydimethylsiloxane (PDMS) based contact lenses and medical devices, where internal stress affects long-term performance. |
| Quantum Sensing Technology | Development of Nanoscale Stress Sensors | Demonstrating the robustness and high sensitivity of ND-based NV- centers as versatile quantum sensors for mechanical quantities (pressure and strain) in complex chemical environments. |
| Advanced Materials Research | Investigation of Phase Transitions | Potential application to study glass transition processes driven by thermodynamic variables (temperature, pressure) and mechanical properties of novel organic materials (e.g., perovskite solar cells). |
View Original Abstract
Amorphous solids, which show characteristic differences from crystals, are common in daily usage. Glasses, gels, and polymers are familiar examples, and polymers are particularly important in terms of their role in construction and crafting. Previous studies have mainly focused on the bulk properties of polymeric products, and the local properties are less discussed. Here, we designed a distinctive protocol using the negatively charged nitrogen vacancy (NV-) center in nanodiamond to study properties inside polymeric products in situ. Choosing the curing of poly(dimethylsiloxane) (PDMS) and the polymerization of cyanoacrylate as subjects of investigation, we measured the time dependence of local pressure and strain in the materials during the chemical processes. From the measurements, we were able to probe the local shear stress inside the two polymeric substances in situ. By regarding the surprisingly large shear stress as the internal tension, we attempted to provide a microscopic explanation for the ultimate tensile strength (UTS) of a bulk solid. Our current methodology is applicable to any kind of transparent amorphous solids with the stress in the order of MPa and to the study of in situ properties in nanoscale. With better apparatus, we expect the limit can be pushed to sub-MPa scale.