Session G31: Instrumentation 1: Gas, NMR, Materials

Pd80Co20 Nanohole Arrays Coated with Poly(methyl methacrylate) for High-Speed Electrical Hydrogen Sensing with a Part-per-Billion Detection Limit

As hydrogen gas increasingly becomes critical as a carbon-free energy carrier, the demand for robust hydrogen sensors for leak detection and concentration monitor will continue to rise. However, to date, there are no lightweight sensors capable of meeting the required performance metrics for the safe handling of hydrogen. Here, we report an electrical hydrogen gas sensor platform based on a resistance nanonetwork derived from Pd-Co composite hole arrays (CHAs) on a glass substrate, which meets or exceeds these metrics. In optimal nanofabrication conditions, a single poly(methyl methacrylate)(PMMA)-coated CHA nanosensor exhibits a response time (t₈₀) of 1.0 s at the lower flammability limit of H₂ (40 mbar), incredible sensor accuracy (<1% across 5 decades of H₂ pressure), and an extremely low limit of detection (LOD) of <10 ppb at room temperature. Remarkably, these nanosensors are extremely inert against CO and O₂ gas interference, display robust long-term stability in air over 2 months with diminutive power requirement (∼2 nW) and applied magnetic field (up to 3 kOe), a crucial metric for leak detection and concentration control. Further, a 50-nm thin film of fullerene C₆₀ is sandwiched between the glass substrate and Pd-Co layer for the purpose of maximizing the interacting surface area of the sensor. Remarkably, C₆₀/PdCo/PMMA sensor exhibits a superior response time t₉₀ of <0.5 s at 40 mbar while the accuracy, LOD, inertness and stability are conserved. 

Sub-second and ppm-level Optical Sensing of Hydrogen Using Templated Control of Nano-hydride Geometry and Composition

The use of hydrogen as a clean and renewable alternative to fossil fuels requires a suite of flammability mitigating technologies, particularly robust sensors for hydrogen leak detection and concentration monitoring. To this end, we have developed a class of lightweight optical hydrogen sensors based on a metasurface of Pd nano-patchy particle arrays, which fulfills the increasing requirements of a safe hydrogen fuel sensing system with no risk of sparking. The structure of the optical sensor is readily nano-engineered to yield extraordinarily rapid response to hydrogen gas (<3 s at 1 mbar H₂) with a high degree of accuracy (<5%). By incorporating 20% Ag, Au or Co, the sensing performances of the Pd-alloy sensor are significantly enhanced, especially for the Pd₈₀Co₂₀ sensor whose optical response time at 1 mbar of H₂ is just ~0.85 s, while preserving the excellent accuracy (<2.5%), limit of detection (2.5 ppm), and robustness against aging, temperature, and interfering gases. The superior performance of our sensor places it among the fastest and most sensitive optical hydrogen sensors.   

Building a Faraday Force Magnetometer for Magnetization Measurements of Quantum Materials at 50mk Under High Magnetic Fields

The purpose of this presentation is to discuss the fabrication and experimental setup of a previously designed Faraday Force Magnetometer used for magnetization measurements of Quantum Materials. New design updates have been implememtned on this probe, measuring 20mm x 20mm x 15mm, and measurements have been completed validating previously claimed sensitivity resolutions. This probe, placed in a dilution refrigerator and cooled to 50mk, can test magnetization of materials up to fields of 14T with a resolution better than 10^−4 emu. By the time of this presentation we hope to present new measurements of CeCl3 and other samples with a nominal sensitivity of 10^-6 emu. We will discuss limitation of sensitivity and challenges in practical implications at ultra low temperatures.   

Probing quasi two-dimensional materials using Nuclear Magnetic Resonance

Our ability to design and produce a variety of thin films and two-dimensional materials has greatly evolved during the last two decades. This technical breakthrough allows for the study of physical properties that do not transpose to bulk materials. The fragility and complexity of such materials require technological improvements to characterize samples without causing damage to them. Nuclear Magnetic Resonance (NMR) is a well-known technique used to probe local magnetism in a great variety of materials and requires no direct contact to the studied samples. However, conventionally, this technique requires a large number of spins (10^20) in order to be efficient, which can be partially solved by decreasing the temperature and increasing the applied magnetic field. This limits the accessibility of the phase diagram and the study of magnetic phase transitions of materials. Here, we demonstrate that planar superconducting NbN resonators in proximity to the surface of a thin sample can be used has an alternative to standard coils for NMR measurements. These devices allow for improved sensitivity NMR measurements of thin films at low temperatures and pave the way towards probing atomically thin samples. This technique is also easily implemented within a conventional solid-state NMR set-up.    

Magnetic Resonance Imaging with a Multichannel Optically Pumped Magnetometer

Ultra-low field (ULF) magnetic resonance imaging (MRI) is a promising method for anatomical imaging with various advantages over conventional MRI such as low cost and low weight. Previously, multichannel parallel ULF MRI was realized only with multichannel superconducting quantum interference devices (SQUIDs), but the cryogenic operation is a serious drawback. We aim to develop a more practical parallel ULF MRI based on a multichannel optically pumped magnetometer (OPM) and multiple flux transformers. The OPM is the most sensitive cryogen-free magnetic sensor in a broad range of frequencies from kHz to MHz. In this talk, we describe the basic principle and the design of our ULF MRI system. We also present our recent progress on the reduction of crosstalk between sensing channels and the development of a single-module multichannel OPM.   

Characterization of Micro-Magnetic Thin Films on Oscillators and Effects of Varying Angle of Applied Field

We report the further development of a fiber-optic interferometer system with variable applied DC and AC magnetic fields for the characterizations of micromagnets & micromagnetic films on small AFM oscillators. The system has measured and calibrated displacements to determine the resonant frequencies(~0.8-800 kHz), quality factors(~30-2000), amplitudes (0.001-10 nm) and spring constants (~0.1 N/m) of resonances. The driven response to AC magnetic field gradients (~1x10-4 - 1x10-3 T/m) has provided direct measurement of the dependence of the magnetic force on the angle of the applied field. Improvements in precision for angles of applied fields has let us measure a resonant frequency shift of δω/ω = ~10^{-3} for thin films with a 90 degree shift in angle of a polarizing field. The difference between a direct force Anti-Helmholtz geometry and a more strongly torsionally driven Helmholtz geometry has also been explored. This work supports our studies in nuclear magnetic resonance force microscopy (NMRFM).   

Multi-harmonic ElectroThermal Spectroscopy (METS) for a non-invasive and spatially resolved determination of electrochemical processes in an electrochemical cell

Lock-in detection techniques like the 3-omega method have been used for determining spatially resolved thermal properties. In electrochemistry, Electrochemical Impedance Spectroscopy (EIS), another lock-in technique, is ubiquitously used for studying transport resistance and charge transfer resistance. However, EIS lacks spatial information and cannot differentiate mechanisms of the charge transfer resistance at the electrode. In this work, we present a lock-in detection technique based on thermal signatures of electrochemical processes. The processes are differentiated from the harmonics and the phase of the heat generated when an Alternating Current (AC) is passed through the cell. The spatial resolution is achieved from the frequency dependent thermal penetration depth. We name this method Multi-harmonic ElectroThermal Spectroscopy (METS). A four-point probe RTD sensor acting as a METS sensor can be employed on the exterior of the cell to non-invasively probe spatially resolved electrochemical information. In summary, METS provides a unique capability to directly probe and differentiate electrochemical processes with spatial information.     

New Technique for Microwave Spectroscopy on Micron-Scale Materials

In recent years, interesting materials have emerged which are only available as micron-scale flakes whose GHz conductivity spectra are predicted to yield insights into their novel physics. These materials include twisted bilayer graphene, bilayer dichalcogenides, and artificial lattices for analog quantum simulations. For most current broadband GHz spectroscopy techniques, micron-scale flakes effect a signal that is too weak to be useful. We have developed a new broadband GHz spectroscopy technique that allows one to deduce both the excitation spectra and free carrier response of micron-scale flakes without requiring sophisticated sample preparation or measurement techniques.

 

We present results from proof-of-principle experiments conducted for micron-scale P⁺ monolayer regions encapsulated in Si, whose conductivity spectrum is expected to be constant in frequency for microwave frequencies. The results show excellent i) qualitative agreement with those expectations, and ii) excellent quantitative agreement with DC resistance measurements, indicating the ability to perform sensitive contact-free conductivity spectroscopy on µm-scale flakes. These theoretical results also show that P⁺ monolayers can support microwave applications, such as ESR in QIS.