Temperature sensing at the nanoscale has become increasingly critical in modern microelectronics and biomedical applications, where precise thermal monitoring is essential for performance optimization and safety. Two-dimensional materials such as graphene and transition metal dichalcogenides (TMDCs) have emerged as promising candidates due to their exceptional thermal and electrical properties. These materials enable high-sensitivity temperature detection through mechanisms like the thermal coefficient of resistance (TCR), while their atomic thickness allows for minimal invasiveness in thermal mapping. This article explores the principles, advancements, and applications of 2D material-based temperature sensors, focusing on TCR behavior and nanoscale thermal imaging without overlapping with thermoelectric energy conversion or macroscopic thermal management.
The thermal coefficient of resistance is a fundamental parameter defining how a material's electrical resistance changes with temperature. In graphene, TCR arises primarily from electron-phonon interactions and defect scattering. Monolayer graphene exhibits a TCR in the range of 0.0005 to 0.002 per Kelvin at room temperature, depending on doping levels and substrate interactions. The linearity of TCR in graphene across a broad temperature range makes it suitable for high-resolution sensing. TMDCs like MoS2 and WS2 demonstrate higher TCR values, typically between 0.003 and 0.01 per Kelvin, due to their semiconducting nature and stronger temperature-dependent carrier mobility. The layer-dependent bandgap in TMDCs allows tuning TCR by varying thickness, offering flexibility in sensor design.
Nanoscale thermal mapping leverages the high spatial resolution enabled by 2D materials. Graphene's high thermal conductivity, exceeding 2000 W/mK, ensures rapid thermal equilibration, while its atomic thickness permits minimal disruption to local heat flow. Techniques such as scanning thermal microscopy (SThM) integrate graphene probes to achieve sub-100 nm resolution in temperature mapping. TMDCs, with lower thermal conductivity (20-100 W/mK for monolayers), provide complementary advantages in thermal isolation, enhancing sensitivity in localized measurements. By functionalizing these materials with thermally responsive coatings, researchers have achieved temperature resolution below 10 mK in microscale regions.
In microelectronics, 2D material temperature sensors address critical challenges in device reliability and thermal management. Integrated graphene sensors monitor hotspot formation in high-performance processors, where localized heating can exceed 150°C. The ultrafast response time of graphene, on the order of nanoseconds, allows real-time detection of transient thermal fluctuations in transistors. TMDC-based arrays are deployed in 3D integrated circuits to map interlayer heat dissipation, preventing thermal crosstalk. These sensors operate without introducing parasitic capacitance, a key advantage over conventional metal-based thermistors. The compatibility of 2D materials with silicon processing enables direct fabrication on wafers, simplifying integration.
Biomedical applications exploit the mechanical flexibility and biocompatibility of 2D material sensors. Graphene patches with multiplexed TCR detectors provide continuous skin temperature monitoring with minimal discomfort, achieving accuracy comparable to clinical thermometers. Functionalized MoS2 sensors enable intracellular temperature measurements, resolving metabolic heat variations within single cells. In minimally invasive surgery, TMDC-coated catheters perform real-time thermal imaging of tissues, detecting inflammation or tumor margins with sub-millimeter precision. The absence of toxic elements in these materials ensures safety for long-term implantation, unlike traditional metal-oxide sensors.
Fabrication techniques for these sensors emphasize precision and scalability. Chemical vapor deposition (CVD) produces large-area graphene films transferred onto target substrates, while mechanical exfoliation yields high-quality TMDC flakes for research prototypes. Lithographic patterning defines sensor geometries down to 100 nm, crucial for minimizing thermal mass. Encapsulation layers of hexagonal boron nitride (hBN) protect the active materials from environmental degradation while preserving thermal contact. Recent advances in roll-to-roll processing suggest potential for cost-effective mass production of flexible sensor arrays.
Performance metrics highlight the superiority of 2D material sensors. Graphene-based devices demonstrate temperature resolution below 0.1 K with response times under 1 ms, outperforming platinum RTDs in dynamic measurements. MoS2 sensors achieve noise-equivalent temperature differences of 50 mK at microscale dimensions, surpassing thin-film thermocouples. The mechanical robustness of these materials allows operation under repeated bending cycles, with less than 5% TCR variation after 10,000 flexes in polymer-embedded designs.
Challenges remain in standardizing TCR calibration across fabrication batches and mitigating substrate-induced variability. Strain coupling in flexible applications can introduce resistance changes unrelated to temperature, requiring compensation algorithms. Ongoing research explores heterostructures combining graphene and TMDCs to optimize TCR linearity while suppressing interference effects. The development of readout integrated circuits tailored for high-impedance 2D material sensors further enhances system-level performance.
Future directions include quantum-enhanced sensing exploiting graphene's Landau levels for ultra-high precision below 1 mK, and bioresorbable TMDC sensors for temporary medical implants. The integration of machine learning for real-time thermal anomaly detection in complex electronic systems represents another growing application. As fabrication maturity improves, 2D material temperature sensors are poised to become ubiquitous in applications demanding miniaturization, speed, and accuracy unattainable with conventional technologies. Their unique combination of properties continues to inspire innovations across engineering and life sciences, from preventing microprocessor failures to unlocking new diagnostic capabilities in medicine.