We present a proof-of-concept magnetic resonance detection of organic radical 2,2-diphenyl-1-picrylhydrazyl via a nanostructured on-chip graphene quantum dot bolometer. In a common electron paramagnetic resonance setup, the microwave propagates from a source into a sample and back to a detector. Yet, by using on-chip detection, it is possible to skip the whole detection arm and, thus, mitigate lab expenses on instrument and cooling costs. The bolometric detection was demonstrated at a frequency of 151 GHz and a temperature of 15 K.

Topics
Quantum confinement, Magnetic resonance, Phonons, Amplifiers, Printed circuit board, Electromagnetic radiation detectors, Terahertz radiation, Thermodynamic properties, Electron paramagnetic resonance spectroscopy, Graphene

Bolometers are very sensitive detection devices for measurements of electromagnetic radiation power in a broad frequency range, including terahertz (300 GHz–30 THz) and infrared bands (300 GHz–430 THz). The principle is based on heating a material with highly temperature-dependent electrical resistance caused by incident electromagnetic radiation.1 They are widely used in astrophysics due to their high sensitivity and ability to detect a tiny amount of incident electromagnetic radiation.2 These are commonly used in electron paramagnetic resonance (EPR) spectroscopy, infrared (IR) spectroscopy, and in the microwave/terahertz technology.3–8 

There are two important parameters determining the performance of a bolometer. The first is electrical responsivity R (V/W), which defines the efficiency of the bolometer, that is, the ability to convert incoming electromagnetic radiation to a measurable output (higher is better). The second parameter is the noise-equivalent power NEP (W/ Hz⁠), which represents the sensitivity (lower is better).9 The working principle of bolometers can be described in the following way. The incident electromagnetic radiation with power P ( t ) = P 0 + Δ P   cos ( ω t )⁠, where ω is the angular frequency modulating the power. Such radiation initiates the heat transfer as follows:
(1)
where C is the heat capacity, G is the heat conductivity, and T0 is the substrate lattice temperature. The temperature change is then
(2)
The responsivity R ( ω ) is then
(3)
where Δ V DC is the voltage change caused by electromagnetic radiation, in the case of ω ≪ 1 τ⁠; then, R ( ω ) ∝ 1 G and for ω ≫ 1 τ⁠, the R ( ω ) ∝ 1 ω C⁠, where τ = C G is the bolometer thermal time constant.10 

Nowadays, commercial bolometric detectors are based on (III–V) InSb semiconductors (e.g., QMC Instruments Ltd.) with the use of hot electrons that occur only at very low temperatures, usually units of kelvin. At these temperatures, the electron gas in a metal is weakly bound to phonons. Electrons will get out of equilibrium with phonons after they absorb electromagnetic radiation and create high-energy electron gas.11 This can also be achieved by the use of a hot-electron graphene-based bolometer, as shown in Fig. 1. The detailed description of electron temperature T e can be found in the supplementary material of our previous work on graphene quantum dot (GQD) bolometers.12 There are three separate constants regarding our GQD bolometers: (i) intrinsic GQD thermal constant in the order of nanoseconds,12 (ii) bolometer response time constant dependent on detection via electrical circuitry and this was estimated to be < 100 ms from the response decay, (iii) and the last is the lock-in time constant, which was set to 1 s.

FIG. 1.
Left: the incident power P in from THz radiation hits the graphene with electrons temperature T e on the substrate with temperature T 0. Right: the scheme illustrates the thermal conductance G th from a graphene to the substrate lattice via phonon emission G ep and electron diffusion G diff.
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Left: the incident power P in from THz radiation hits the graphene with electrons temperature T e on the substrate with temperature T 0⁠. Right: the scheme illustrates the thermal conductance G th from a graphene to the substrate lattice via phonon emission G ep and electron diffusion G diff⁠.

FIG. 1.
Left: the incident power P in from THz radiation hits the graphene with electrons temperature T e on the substrate with temperature T 0. Right: the scheme illustrates the thermal conductance G th from a graphene to the substrate lattice via phonon emission G ep and electron diffusion G diff.
View largeDownload slide

Left: the incident power P in from THz radiation hits the graphene with electrons temperature T e on the substrate with temperature T 0⁠. Right: the scheme illustrates the thermal conductance G th from a graphene to the substrate lattice via phonon emission G ep and electron diffusion G diff⁠.

Close modal

Graphene is a pure two-dimensional crystal with a hexagonal lattice composed of carbon atoms with distinctive electric, thermal, mechanical properties,18 extraordinary stiffness,19 and high charge carrier mobility, which stems from a weak interaction between electrons and phonons.20 These properties, along with small heat capacity, make graphene a promising material for very sensitive and responsive bolometers by using hot electrons.

However, a monolayer graphene as such is not the best option for functional bolometric devices due to its small dependence on the temperature of the electrical resistance, which changes less than 30% from 30 mK to room temperature.21 This property is caused by the above-mentioned weak electron–phonon scattering.22 This would make pristine graphene not suitable for bolometric devices. This issue was overcome by different designs that are mentioned in Table I, that is, by using a dual gate structure on the bilayer graphene in such a way to alter the graphene bandgap13 or by creating defects in graphene that cause a strong electron localization.12,14,23

TABLE I.

Comparison of different graphene-based designs for bolometric detection with corresponding responsivity R, NEP, and operating temperature.

DesignRef R (V/W) NEP ( W / Hz ) Temperature
Commercial InSb bolometerQMC  5 × 10 3  5.0 × 10 − 13  4.2 K 
Dual gate on bilayer graphene13   2 × 10 5  3.3 × 10 − 14  5 K 
Defects in graphene14   6 × 10 6  1.2 × 10 − 15  1.5 K 
Graphene quantum dots12   1 × 10 10  2.0 × 10 − 16  2.5 K 
Magnetic tunnel junction15   4 × 10 6  2.4 × 10 − 12  Uncooled 
Graphene Josephson junction16   ⋯  7.0 × 10 − 19  19 mK 
Superconductor graphene junction17   ⋯  3.0 × 10 − 20  50 mK 
DesignRef R (V/W) NEP ( W / Hz ) Temperature
Commercial InSb bolometerQMC  5 × 10 3  5.0 × 10 − 13  4.2 K 
Dual gate on bilayer graphene13   2 × 10 5  3.3 × 10 − 14  5 K 
Defects in graphene14   6 × 10 6  1.2 × 10 − 15  1.5 K 
Graphene quantum dots12   1 × 10 10  2.0 × 10 − 16  2.5 K 
Magnetic tunnel junction15   4 × 10 6  2.4 × 10 − 12  Uncooled 
Graphene Josephson junction16   ⋯  7.0 × 10 − 19  19 mK 
Superconductor graphene junction17   ⋯  3.0 × 10 − 20  50 mK 
View Large

In our previous work,24 we tested EPR detection with transmission as a function of frequency, at zero magnetic field. This response was measured by varying the frequency in steps and did not provide a smooth signal. Here, we demonstrate the standard EPR technique by modulating the magnetic field and providing a response with better resolution, as a smooth signal that can be compared with numerical spin Hamiltonian simulation.

In our study, we used GQD, described in detail previously,12,23 utilizing the quantum confinement. The quantum confinement in GQDs is a patterned nanoconstriction that limits electron flow through the graphene bow tie. The nanoconstriction introduces quantum confinement gap and the current through the bow tie occurs via thermal activation due to this gap, resulting in very strong temperature dependent resistance. This, in turn, allows us to detect drops in microwave radiation intensity due to resonance absorption from the unpaired electron. Therefore, this sensitive detector enables us to perform EPR spectroscopy. Such GQD served as a bolometer and a substrate for an on-chip detector of organic radical 2,2-diphenyl-1-picrylhydrazyl (DPPH), which is a standard stable radical marker for electron paramagnetic resonance testing25 with applications, such as radical scavenger, in antioxidant assays.26,27

This approach completely omits the need for a detection arm of a spectrometer setup, and thus, decreases the power loss of the signal along the way to the detector. In a conventional high field EPR spectrometer, the detector (superconducting bolometer, Schottky diode detector, frequency mixer, etc.) is usually located several meters away from the sample outside of the variable temperature insert. Depending on the spectrometer's design and operating frequency, losses in the sample–detector path can reach up to 10 dB. Accordingly, having the detector just beneath the sample is advantageous. External bolometers need to be cooled down to cryogenic temperatures, whereas the on-chip bolometer is cooled together with a studied sample. This significantly reduces the measurement costs and eliminates the need for an externally cooled bolometer device. The overall measurement scheme for bolometric detection of magnetic resonance is shown in Fig. 2. 2,2-diphenyl-1-picrylhydrazyl was obtained (Sigma-Aldrich) as powder and dissolved in dichloromethane (Penta, 99%). The 50 μl of 10 mM solution was drop-cast in ambient conditions onto the already wire-bonded GQD chip.

FIG. 2.
Scheme of the measurement for the bolometer detection of magnetic resonance. The chip with the graphene quantum dot was wire bonded to the chip expander and placed into our sample holder with a modulation coil around the sample. The bias voltage of 1.1 V from a current preamplifier drove the current through the device. The signal from the preamplifier was coupled to the lock-in amplifier which provided alternating current modulation of 27 Hz to the sample. The DPPH sample was measured while being irradiated by continuous microwave with a frequency 151 GHz and held at 15 K.
View largeDownload slide

Scheme of the measurement for the bolometer detection of magnetic resonance. The chip with the graphene quantum dot was wire bonded to the chip expander and placed into our sample holder with a modulation coil around the sample. The bias voltage of 1.1 V from a current preamplifier drove the current through the device. The signal from the preamplifier was coupled to the lock-in amplifier which provided alternating current modulation of 27 Hz to the sample. The DPPH sample was measured while being irradiated by continuous microwave with a frequency 151 GHz and held at 15 K.

FIG. 2.
Scheme of the measurement for the bolometer detection of magnetic resonance. The chip with the graphene quantum dot was wire bonded to the chip expander and placed into our sample holder with a modulation coil around the sample. The bias voltage of 1.1 V from a current preamplifier drove the current through the device. The signal from the preamplifier was coupled to the lock-in amplifier which provided alternating current modulation of 27 Hz to the sample. The DPPH sample was measured while being irradiated by continuous microwave with a frequency 151 GHz and held at 15 K.
View largeDownload slide

Scheme of the measurement for the bolometer detection of magnetic resonance. The chip with the graphene quantum dot was wire bonded to the chip expander and placed into our sample holder with a modulation coil around the sample. The bias voltage of 1.1 V from a current preamplifier drove the current through the device. The signal from the preamplifier was coupled to the lock-in amplifier which provided alternating current modulation of 27 Hz to the sample. The DPPH sample was measured while being irradiated by continuous microwave with a frequency 151 GHz and held at 15 K.

Close modal

The GQD bolometers were fabricated by a multi-step lithography process from large-area epitaxial graphene grown on the SiC substrate. The defects in graphene were produced by Pd sputtering and the resist was cleaned by aqua regia treatment. The detailed fabrication procedure was reported previously.12,24,28,29

The measurements on bolometers were enabled by a chip sample holder for EPR. The design was reported in detail recently.30 The holder features a custom printed circuit board (PCB) chip expander (SEANT Technology, CZE) placed in the center of the magnetic field. The measured chip was placed on 1 mm thick single crystal Al2O3—sapphire (Crytur, CZE), which works as a heat sink. This design offers up to 16 electric contacts for the sample, temperature sensor, and heater. The 8 × 8 mm2 graphene bolometer on SiC was placed onto the custom PCB chip expander and devices on chip were wire bonded by a TPT HB 16 wire bonder in ISO 8 CEITEC Nano cleanrooms.

EPR spectra were acquired on a home-built spectrometer with custom software31 featuring a signal generator, an amplifier–multiplier chain (both Virginia Diodes, Charlottesville, VA, USA), a quasi-optical microwave bridge (Thomas Keating, Billingshurst, UK), and a 16 T cryogen free magnet system with a variable temperature insert, allowing for temperatures down to 1.8 K (Cryogenic, London, UK).

According to our estimations, changes in the device's current due to EPR transitions in the sample are within the pA range. To detect such low currents, we used phase-sensitive detection consisting of a low-noise current preamplifier (Stanford Research Systems, Model SR570) followed by a lock-in amplifier (Zurich instruments MFLI 500 kHz). The preamplifier's sensitivity was set to 20 μA/V, and it also provided the bias voltage of 1.1 V for the device. The lock-in amplifier generated AC to drive a modulation coil built in the chip sample holder. We used magnetic field modulation with 27 Hz and 3.5 mT amplitude. As a result, the recorded spectrum is the first derivative of EPR absorption with respect to the magnetic field. EPR spectrum was simulated using EasySpin (version 6.0.0-dev.51), a toolbox for Matlab.32 

We measured the bolometer's current–voltage (I/V) characteristics shown in Fig. 3, which gave us information about responsiveness of a bolometer to incident microwave irradiation. The I/V curve displays the non-linear behavior and does not follow the simple Ohm's law approximation. Instead, apparent Joule's heating was observed as we increased the voltage from 0 to 1 V. The increase in the current was detected upon incidence of 151 GHz microwave irradiation (red line) compared to dark without a microwave (black line) sweep. Both measurements were done at 15 K and in an applied external magnetic field of 5.5 T. The absorbed power Δ P was calculated by finding the differential resistance/conductance out of the microwave ON (red line) at zero bias voltage (dotted black line), because at zero bias the only contribution to heating is from the microwave radiation. We did the same for the microwave OFF (black line) at zero bias voltage and marked a gray dot where the green dotted line has the same slope as the black dotted line. At this point, the electrons are heated by the Joule power to the same temperature as they were heated by the microwave irradiation at zero bias voltage. The absorbed power Δ P is then as follows:
(4)
where the current and voltage are taken from derivatives at zero bias voltage for microwave ON and OFF. This yields the detected absorbed power Δ P of 47 pW. The responsivity R is then as follows:
(5)
which is comparable with previously reported responsivity for GQDs12,23 and orders of magnitude higher than other approaches listed in Table I, especially taking into account operating temperature of 15 K.
FIG. 3.
Current–voltage characteristics comparing microwave OFF (black) and ON (red) in an applied static magnetic field of 5.5 T, a temperature of 15 K, and a frequency of 151 GHz. The inset shows the absorbed power of 47 pW at zero bias voltage. The responsivity is then calculated from Δ V DC at arbitrarily chosen current of 1 nA.
View largeDownload slide

Current–voltage characteristics comparing microwave OFF (black) and ON (red) in an applied static magnetic field of 5.5 T, a temperature of 15 K, and a frequency of 151 GHz. The inset shows the absorbed power of 47 pW at zero bias voltage. The responsivity is then calculated from Δ V DC at arbitrarily chosen current of 1 nA.

FIG. 3.
Current–voltage characteristics comparing microwave OFF (black) and ON (red) in an applied static magnetic field of 5.5 T, a temperature of 15 K, and a frequency of 151 GHz. The inset shows the absorbed power of 47 pW at zero bias voltage. The responsivity is then calculated from Δ V DC at arbitrarily chosen current of 1 nA.
View largeDownload slide

Current–voltage characteristics comparing microwave OFF (black) and ON (red) in an applied static magnetic field of 5.5 T, a temperature of 15 K, and a frequency of 151 GHz. The inset shows the absorbed power of 47 pW at zero bias voltage. The responsivity is then calculated from Δ V DC at arbitrarily chosen current of 1 nA.

Close modal
Once we tested the bolometer responsiveness, we have drop-cast DPPH onto a bolometer chip covered with active GQDs. Our rough estimate of spins involved for detection is as follows. The number of spins N deposited on the whole SiC substrate is approximately N = c · V · N A ≈ 3 × 10 17⁠, where c is the concentration (10 mM), V is the volume (50 μl), and N A is the Avogadro constant. The covered active area of graphene, i.e., the GQD + graphene bow ties, was roughly 20 × 10 = 200 μm2. This leads to a conservative estimate of 1012 spins detectable by our device, which is comparable to the limit of detection for high-frequency systems with dedicated external bolometer detection5,33 and Schottky-diode mixers.34,35 The primary aim was to display proof-of-concept; therefore, the spin sensitivity is a mere estimate and was not primarily sought after in our study contrary to EPR works with voltage-controlled detection.36–38, Figure 4 shows EPR spectrum of DPPH. One unpaired electron in radical gives rise to a total spin quantum number S = 1 2⁠. Only the electron Zeeman contribution to the overall Hamiltonian was considered. The spin Hamiltonian used for simulation is as follows:
(6)
where μ B is the Bohr magneton, B → is the externally applied magnetic field, g e ↔ is the Landé g-factor, and S ̂ is the spin angular momentum operator. The best simulation gave an isotropic g value of 2.0036, which is in good agreement with previously reported value for DPPH.39 As we measure the powder signal, the g-factor anisotropy can introduce small inhomogeneous broadening of the line. The magnetic field inhomogeneity (100 ppm in our magnet) should not matter as we sense samples from the very small volume. There is a very small intrinsic anisotropy of DPPH recently reported.40 There is also dependency on the DPPH preparation routes discussed41 resulting in different line shapes. Nevertheless, DPPH serves as a good standard for high field EPR measurements.42 In our study, the linewidth peak-to-peak for simulation was 2.2 mT (50% Gaussian + 50% Lorentzian contribution). It is possible to fit the spectrum with only Lorentzian peak line shape and 3.5 mT modulation amplitude. However, addition of Gaussian line shape introduces inherent inhomogeneous line broadening and yields the best result.
FIG. 4.
EPR spectra of DPPH on the bolometric chip for S = 1 2 measured at 151 GHz and 15 K. The g value obtained from spin Hamiltonian simulation gave 2.0036.
View largeDownload slide

EPR spectra of DPPH on the bolometric chip for S = 1 2 measured at 151 GHz and 15 K. The g value obtained from spin Hamiltonian simulation gave 2.0036.

FIG. 4.
EPR spectra of DPPH on the bolometric chip for S = 1 2 measured at 151 GHz and 15 K. The g value obtained from spin Hamiltonian simulation gave 2.0036.
View largeDownload slide

EPR spectra of DPPH on the bolometric chip for S = 1 2 measured at 151 GHz and 15 K. The g value obtained from spin Hamiltonian simulation gave 2.0036.

Close modal

We have reported proof-of-concept bolometric detection of organic radical DPPH by using a nanostructured GQD. This approach eliminates the detection arm in the spectrometer by the direct measurement of samples on the bolometric chip. The current limitations lie in the temperature stability to prevent excessive heating during the measurement, and this can be solved by design adjustment where additional heater and heat sink are used to keep the sample at constant temperature. The current signal-to-noise (SNR) presents a drawback albeit the reported EPR spectrum is for simple one shot measurement. Multiple scans would improve the SNR by a factor of n⁠, where n is the number of scans. Although the bolometer responsivity is high, the net current noise contributes significantly to lower the SNR. The read-out speed from the bolometer is also limited by the design, and further improvements are necessary. The advantage of a GQD bolometer is that it operates even in high magnetic fields, and it is possible to perform EPR spectroscopy measurement with frequency modulation. It is also possible to reuse the chip up to a few times depending on the molecule of interest and its solubility. While considering these current technology limitations, this work presents a novel approach of utilizing graphene to provide nanostructured bolometric detection in EPR spectroscopy. Future work will focus on the suppression of current noise and measurability of other, more complex, samples.

This work was supported by the U.S. Office of Naval Research (Nos. N00014-16-1-2674 and DURIP N00014-17-1-2436) and the NSF (No. ECCS-1610953). Research at NRL was supported by the Office of Naval Research. Research at CEITEC was supported by the Ministry of Education, Youth and Sports of the Czech Republic under the project CEITEC 2020 (No. LQ1601), by MŠMT Inter-Excellence (No. LTAUSA19060, 2020–2022), and by the ERC, under the European Union's Horizon 2020 research and innovation programme (GA No. 714850), and Electric-field control of spin-qubits in quantum paraelectrics with funding source: GAĆR Standard (No. 23-05578S). We acknowledge the CzechNanoLab Research Infrastructure supported by MEYS CR (No. LM2023051).

The authors have no conflicts to disclose.

J. Hrubý: Conceptualization (equal); Investigation (equal); Writing – original draft (equal). O. Laguta: Data curation (equal); Validation (equal); Writing – original draft (equal). A. Sojka: Writing – original draft (equal). L. St. Marie: Investigation (equal); Visualization (equal). R. Myers-Ward: Conceptualization (equal). D. K. Gaskill: Conceptualization (equal). A. El fatimy: Conceptualization (equal); Methodology (equal). P. Barbara: Conceptualization (equal); Writing – original draft (equal). P. Neugebauer: Supervision (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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