Temperature‑Dependent Raman Analysis of In‑Plane E₂g Phonons in Graphene and h‑BN Flakes

Abstract

Sp²‑bonded two‑dimensional materials such as graphene and hexagonal boron nitride (h‑BN) are celebrated for their exceptional thermal conductivities. We report micro‑Raman measurements of the in‑plane E₂g optical phonon peaks—≈1580 cm⁻¹ in graphene and ≈1362 cm⁻¹ in h‑BN—across a wide temperature span from –194 °C to +200 °C. Our data reveal that h‑BN flakes exhibit a markedly larger temperature‑induced frequency shift and linewidth broadening compared to graphene. Furthermore, the c‑axis thermal effect on the phonon frequency is more pronounced in h‑BN, whereas the broadening is comparable between the two materials. These findings deepen our understanding of the phonon‑mediated heat transport in layered sp² systems and inform the design of next‑generation thermal management devices.

Background

Graphene and h‑BN share a layered hexagonal lattice held together by weak van der Waals forces, yet each layer is bonded strongly via sp² hybridization. This structural similarity bestows them with remarkable in‑plane thermal conductivities, making them prime candidates for heat spreaders and thermoelectric applications. Heat transport in these crystals is dominated by lattice vibrations (phonons), and Raman spectroscopy offers a non‑destructive probe of phonon dynamics. The in‑plane E₂g mode—commonly called the G peak in graphene and the E₂g^high peak in h‑BN—shifts and broadens with temperature due to anharmonic phonon–phonon interactions and thermal expansion. While numerous studies have examined the temperature dependence of these modes in bulk and few‑layer samples, a direct, side‑by‑side comparison of graphene and h‑BN with matched thicknesses over a broad temperature range remains unexplored.

Experimental

High‑quality graphene and h‑BN flakes were produced by mechanical exfoliation onto 90 nm SiO₂/Si substrates. To minimize substrate‑induced perturbations and laser‑heating artifacts, we selected flakes containing dozens of layers. Thicknesses were verified by atomic force microscopy (AFM) in tapping mode, yielding values of 16–36 nm for both material families. Raman spectra were acquired using a Renishaw HR Evolution system equipped with a ×50 objective (NA = 0.45). A 532‑nm laser, <2 mW, illuminated the samples within a liquid‑nitrogen‑cooled Linkam stage, enabling temperature control from –194 °C to +200 °C. Spectral resolution of 0.5 cm⁻¹ was achieved with an 1800 lines mm⁻¹ grating, and each measurement was integrated for 20 s to ensure high signal‑to‑noise.

Results and Discussion

Room‑temperature Raman maps (Fig. 2) confirm sharp, defect‑free G and E₂g^high peaks, with minimal Si substrate interference. Upon heating, both peaks exhibit a clear red‑shift: ≈–18 cm⁻¹ for 16.2 nm h‑BN, ≈–12 cm⁻¹ for 36.2 nm h‑BN, and ≈–10 cm⁻¹ for the graphene counterparts. The temperature‑dependent phonon frequency, ω_ph, follows a second‑order polynomial, ω_ph = ω₀ + a T + b T², with coefficients listed in Table 1. The larger shifts in h‑BN (~1.5–2× those in graphene) are consistent with stronger multi‑phonon coupling predicted for bulk h‑BN (≈–10 cm⁻¹ from 100 K to 600 K) versus bulk graphite (≈–5 cm⁻¹).

The full width at half maximum (FWHM) also broadens linearly with temperature: Γ_ph = Γ₀ + c T. While h‑BN’s E₂g^high peak shows a broadening of ~1 cm⁻¹, graphene’s G peak remains almost unchanged, reflecting differences in anharmonic decay pathways. Table 2 summarizes the linewidth parameters. Thickness dependence reveals that both the frequency shift per °C·nm and linewidth slope increase modestly with layer number, highlighting the role of interlayer coupling and c‑axis thermal transport.

Conclusions

Our comparative Raman study demonstrates that h‑BN flakes are more sensitive to temperature than graphene of comparable thickness, both in phonon frequency shifts and linewidth broadening. The c‑axis thermal influence on phonon frequency is stronger in h‑BN, whereas broadening effects are similar. These insights into phonon thermodynamics are crucial for engineering efficient thermal interfaces and heat spreaders using layered sp² materials.

Abbreviations

AFM

Atomic force microscopy

FWHM

Full width at half maximum

h‑BN

Hexagonal boron nitride

vdW

Van der Waals