Raman thermometry for 2D materials: promise and pitfalls

Graphene's introduction posed challenges for traditional thermal measurement techniques due to its unique structure as an atom-thick membrane. Optothermal Raman thermometry addressed these challenges by using a laser to both heat and probe the crystal, allowing temperature readings from the vibrational spectrum of the material. This method was pivotal in early measurements of suspended graphene, revealing ultrahigh room-temperature conductivities and sparking extensive debate over the accuracy of absorbed laser power measurements.

Raman scattering provides insights into a crystal's phonons, with temperature changes affecting these phonons and shifting characteristic peaks. In graphene, the G and 2D bands are well-studied examples. By increasing laser power and observing peak shifts, researchers can measure temperature changes without physical contact. When graphene is suspended, heat flows radially through the layer, and the ratio of absorbed power to temperature rise determines thermal conductivity.

The method's advantages include the absence of fabricated heaters or thermometers on the crystal, micrometer spatial selectivity, and chemical specificity. It applies across the 2D material family, particularly in suspended-membrane geometries that other methods cannot address. Initial measurements of suspended single-layer graphene reported thermal conductivities exceeding those of natural diamond, establishing a new field of study.

Subsequent research refined these measurements by directly assessing absorption through transmitted and reflected beams, and by comparing supported and suspended graphene to quantify substrate effects on conductivity. Independent electrothermal studies on large CVD sheets supported these findings, though strict same-sample corroboration remains challenging.

Absorbed power remains a critical factor, as conductivity scales with the fraction of the incident beam absorbed by the thin crystal. Early studies often relied on theoretical absorbance values, contributing to discrepancies in reported conductivities. Modern practices measure transmission and reflection on the same membrane to address this.

Temperature-rise uncertainty in Raman thermometry involves multiple factors, including calibration of the Raman temperature coefficient and peak-position estimation. The radial-conduction model assumes known boundary conditions and spot profiles, with interfacial conductance at the rim being a separate consideration.

Raman thermometry and electrothermal methods serve as complementary approaches, providing independent cross-checks for supported and free-standing 2D layers. While both methods have unique error structures, they share some assumptions that can bias results. Modern electrothermal techniques even incorporate Raman's laser heating while using resistive readout, trading absorbance challenges for calibration ones.

Early discrepancies in graphene conductivity values were largely due to differences in assumed versus measured absorbance, with additional contributions from thermometry calibration and geometry. As absorption measurements improved, the spread of values narrowed, reflecting the method's maturation.

Design considerations should focus on supported-state measurements unless the application involves suspended graphene. Raman thermometry is applicable to other materials, such as transition-metal dichalcogenides, provided certain conditions are met.

This article is part of the ACS thermal metrology knowledge hub, providing insights into optothermal Raman thermometry and electrothermal approaches. For specific feasibility and quotes, contact our thermal testing team.