🔬 mesoscale physics

Large Nernst effect in chemically derived multilayer graphene at millitesla magnetic fields

This paper demonstrates that chemically derived multilayer graphene films with thickness and carrier density fluctuations exhibit a large Nernst effect at millitesla magnetic fields, achieving transverse thermopower values up to 250 $\mu$ V/K and highlighting their potential for scalable thermoelectric energy generation.

Original authors: Valentin Semkin, Denis Borisenko, Yana Litun, Oleg Kononenko, Dmitry Mylnikov, Alexey Bocharov, Dmitry Svintsov

Published 2026-09-09

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CC BY 4.0

Original authors: Valentin Semkin, Denis Borisenko, Yana Litun, Oleg Kononenko, Dmitry Mylnikov, Alexey Bocharov, Dmitry Svintsov

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Heat is often seen as waste, a byproduct of machines running or electricity flowing that simply dissipates into the air. Yet, for decades, scientists have searched for materials that can turn this wasted warmth directly back into useful electricity, a process known as thermoelectricity. The challenge lies in finding a substance that conducts electricity well but blocks heat, a difficult combination to achieve. Graphite, the material found in pencil lead, and its thinner cousin, graphene, are abundant and excellent conductors, making them attractive candidates. However, they possess a fundamental flaw for this task: they are "compensated" materials, meaning they contain nearly equal numbers of positive and negative charge carriers. These opposing charges tend to cancel each other out, resulting in almost no voltage when heated, rendering them useless for generating power under normal conditions.

A team of researchers at the Moscow Institute of Physics and Technology and Skanda Rus has discovered a way to bypass this limitation using a phenomenon called the Nernst effect. This effect occurs when a material is heated while sitting in a magnetic field. In most materials, heating creates a voltage that pushes charges in a straight line from hot to cold. But in a magnetic field, the positive and negative charges are deflected in opposite directions sideways, rather than just moving forward. Because they move in the same sideways direction despite having opposite charges, they add up instead of canceling out, creating a strong transverse voltage. Historically, observing this effect required extremely cold temperatures or massive, powerful magnets, making it a curiosity for laboratory physics rather than a practical tool. The researchers set out to see if this effect could be coaxed out of ordinary, chemically produced layers of graphene at room temperature using only a small, permanent magnet.

The team worked with a macroscopic film of multilayer graphene, a material grown through a chemical process that results in a sheet with varying thickness and a complex internal structure. They shaped this film into a long, narrow strip and placed it on a stage where they could control the magnetic field. Instead of heating the entire strip, they used a focused beam of infrared light to heat a tiny spot on the film, creating a sharp temperature difference in a very specific location. By moving this laser spot across the film and measuring the voltage that appeared between different contacts, they mapped how the material responded to heat under different magnetic conditions.

At first, with no magnetic field, the results were chaotic. The voltage measured at different points fluctuated wildly, changing from positive to negative as the laser moved. This randomness was caused by the uneven nature of the film, where tiny regions acted like separate islands of positive or negative charge. However, the moment the researchers introduced a very weak magnetic field, the chaos vanished. Even at a field strength of just 4 millitesla—a tiny fraction of the force used in a standard MRI machine—the random fluctuations smoothed out. The voltage became consistent and strong, appearing clearly at the edges of the film. The transverse voltage, which flows sideways across the material, grew to match and then exceed the voltage that flowed straight along the length of the film.

The researchers calculated that the material's ability to generate this sideways voltage was remarkably high. At the weakest field they tested, the effect was already comparable to the best results seen in other high-performance materials. As they increased the magnetic field to 315 millitesla, the voltage generation capability rose to 250 microvolts per kelvin. This is a significant finding because it demonstrates that the Nernst effect can be large and useful in a material that is easy to manufacture and does not require extreme cooling. The study also revealed that the effect is strongest when the magnetic field lines align with the direction of the heat flow and the resulting current, a condition they confirmed by measuring the voltage at specific triangular contacts on the film.

The team noted that the success of this experiment relied on the high mobility of the electrons and holes within the graphene film, which was likely enhanced by the way the layers were twisted relative to one another during growth. This internal structure allowed the charges to move freely enough to respond strongly to the magnetic field. While the researchers did not claim to have built a working power generator, their results suggest that large-scale sheets of this material could be used to harvest energy from heat sources using only small, permanent magnets. The work challenges the long-held view that graphite-based materials are too "compensated" to be useful for thermoelectric applications, opening a new path for turning waste heat into electricity using materials that are abundant and easy to synthesize.

Technical Summary: Large Nernst Effect in Chemically Derived Multilayer Graphene

Problem Statement
Thermoelectric generators based on graphite and graphene face a fundamental limitation: their compensated semimetallic nature leads to nearly zero thermopower (Seebeck coefficient, $S$ ) due to electron-hole compensation. While methods such as metal-induced doping, intercalation, and electric field gating exist to shift the Fermi level, they often lack controllability in multilayer systems or are difficult to scale. Furthermore, the Nernst effect (transverse thermopower, $S_{xy}$ ), which can generate large currents in compensated semimetals under a magnetic field, has historically been restricted to extreme conditions (very low temperatures, very high magnetic fields, or ultra-high mobility structures). It remains unclear whether scalable, chemically derived multilayer graphene (CVD) can exhibit a significant Nernst effect under ambient conditions, given that CVD synthesis typically introduces grain boundaries and contaminants that reduce carrier mobility.

Methodology
The authors investigated a macroscopic Hall-bar device fabricated from a chemically derived multilayer graphene film (dimensions $1660 \times 200$ $\mu$ m). The film exhibited significant thickness variations (20–100 nm) and random interlayer twists.

Characterization: Magnetoresistance (MR) and Hall effect measurements were performed at room temperature up to 1 T to determine carrier mobility and film uniformity.
Photo-Thermoelectric (PTE) Mapping: A spatially resolved technique was employed using a focused mid-infrared quantum cascade laser ( $\lambda_0 = 8.1$ $\mu$ m, spot size $\sigma \approx 20$ $\mu$ m) to induce localized temperature gradients.
Magnetic Field Range: Measurements were conducted under ambient conditions using a permanent magnet providing fields up to $B \approx 315$ mT, as well as an electromagnet up to 1 T.
Voltage Measurement: PTE voltages were measured between source-drain contacts (longitudinal) and Hall contacts (transverse) while scanning the laser spot across the sample. Theoretical estimates of thermopower were derived by solving the heat conduction equation and linking the measured voltages to the laser-induced temperature rise.

Key Results

High Mobility and Non-Uniformity: The film demonstrated a large magnetoresistance of 100% at 1 T. Analysis of the low-field MR yielded intrinsic carrier mobilities of $(3\text{--}5) \times 10^4$ cm $^2$ /(V s), attributed to twist-induced interlayer decoupling. However, the apparent Hall mobility was much lower ( $\sim 1.5 \times 10^3$ cm $^2$ /(V s)) due to strong spatial fluctuations in carrier density and thickness.
Emergence of the Nernst Effect: At zero magnetic field, the PTE voltage showed strong spatial fluctuations in sign and magnitude, indicative of random electron-hole puddles. Upon applying a weak magnetic field ( $B \approx 4$ mT), the transverse (Nernst) voltage became comparable to the longitudinal Seebeck voltage at the metal contacts.
Magnitude of Transverse Thermopower:
- At $B^* \approx 4$ mT, the transverse thermopower $S_{xy}$ was estimated to be $\sim 10$ $\mu$ V/K.
- As the field increased to $B \approx 315$ mT, $S_{xy}$ rose sub-linearly to approximately $250\text{--}270$ $\mu$ V/K.
- The Nernst coefficient $N = S_{xy}/B$ was estimated at 2500 $\mu$ V/(K T) in weak fields, significantly exceeding values reported for compensated InSb and predicted for trilayer graphene.
Spatial Coherence: The application of a magnetic field caused the random photovoltage puddles to coalesce into a fixed-sign signal at the sample edges (positive at one edge, negative at the other), confirming the additive nature of electron and hole currents under the Nernst effect.
Contact Geometry Enhancement: By utilizing triangular Hall probes and positioning the laser spot adjacent to the probe apex, the authors achieved a further enhancement of the transverse signal. This geometry maximized the overlap between the virtual current field and the Nernst current density, yielding a maximum voltage of $\sim 200$ $\mu$ V at 41 mT, compared to 45 $\mu$ V for standard source-drain measurements.

Significance and Claims
The paper demonstrates that large-scale, chemically derived multilayer graphene can exhibit a substantial Nernst effect under ambient conditions using only millitesla magnetic fields. The authors claim that the high intrinsic mobility of the twisted CVD film, combined with the electron-hole compensation mechanism, allows for thermoelectric voltages that rival or exceed longitudinal Seebeck voltages at very low fields.

The study suggests that the Nernst effect in such materials is a promising avenue for thermoelectricity generation, potentially offering an alternative to traditional doping strategies. The authors note that the effect is robust enough to be observed with permanent magnets, making it relevant for practical applications. They conclude that while the exact mechanism of mobility enhancement in non-twisted CVD graphene remains an open question, the observed performance in twisted films highlights the potential of graphitic compounds for thermoelectric harvesting, particularly in composite media involving ferromagnetic nanoparticles. The work does not claim immediate commercial viability but establishes the physical feasibility of large Nernst coefficients in scalable, room-temperature graphene systems.

Technical Summary: Large Nernst Effect in Chemically Derived Multilayer Graphene

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