[Paper Review] Phonon Transport in Suspended Single Layer Graphene

Xiangfan Xu, Yu Wang+11 more|arXiv (Cornell University)|Dec 14, 2010

Thermal properties of materialsMaterials Science19 citations

TL;DR

This study presents the first temperature-dependent phonon transport measurements in suspended copper-based chemical vapor deposition (Cu-CVD) single-layer graphene (SLG) from 15 K to 380 K. Using microfabricated suspended devices, it demonstrates that thermal conductance per unit area scales as ∼1.7 × 10⁵ T¹.⁵³ W/m²K, indicating dominant heat transport by flexural acoustic (ZA) phonons, and approaches 30% of the ballistic limit at low temperatures, confirming high thermal quality in CVD-grown graphene.

ABSTRACT

We report the first temperature dependent phonon transport measurements in suspended Cu-CVD single layer graphene (SLG) from 15K to 380K using microfabricated suspended devices. The thermal conductance per unit cross section $σ$/A increases with temperature and exhibits a peak near T~280K ($\pm$10K) due to the Umklapp process. At low temperatures (T<140K), the temperature dependent thermal conductivity scales as ~T^{1.5}, suggesting that the main contribution to thermal conductance arises from flexural acoustic (ZA) phonons in suspended SLG. The $σ$/A reaches a high value of 1.7$ imes10^5 T^{1.5}$ W/m^2K, which is approaching the expected ballistic phonon thermal conductance for two-dimensional graphene sheets. Our results not only clarify the ambiguity in the thermal conductance, but also demonstrate the potential of Cu-CVD graphene for heat related applications.

Motivation & Objective

• To measure temperature-dependent thermal conductance in suspended single-layer graphene (SLG) to resolve ambiguities in intrinsic thermal transport properties.
• To determine the dominant phonon modes responsible for heat conduction in free-standing SLG, especially at low temperatures.
• To evaluate the thermal performance of Cu-CVD-grown graphene relative to exfoliated graphene and theoretical ballistic limits.
• To investigate the role of phonon-phonon Umklapp scattering and defect-related scattering in limiting thermal conductivity.

Proposed method

• Fabricated microfabricated suspended devices on SiNₓ/Si wafers with pre-patterned Pt heater and sensor resistors.
• Transferred large-area, high-quality Cu-CVD-grown SLG onto suspended membranes using a dry transfer process.
• Employed a suspended microbridge configuration with Cr/Au contacts to ensure good thermal and electrical contact.
• Measured thermal conductance per unit area (σ/A) via Joule heating and resistance change in the heater and sensor, using a four-terminal configuration.
• Analyzed temperature dependence of σ/A and fitted data to power laws (T¹.⁵) to identify phonon transport mechanisms.
• Used thermopower measurements to probe carrier type and detect phonon drag effects, confirming absence of phonon drag in suspended SLG.

Experimental results

Research questions

• RQ1 What is the temperature dependence of thermal conductance in suspended Cu-CVD single-layer graphene?
• RQ2 Which phonon branch dominates heat transport in suspended SLG, particularly at low temperatures?
• RQ3 To what extent does phonon transport in Cu-CVD graphene approach the ballistic limit?
• RQ4 How do Umklapp scattering and defect-related scattering influence thermal conductivity at different temperatures?
• RQ5 Is there evidence of phonon drag in suspended CVD graphene, and what does this imply about carrier-phonon coupling?

Key findings

• Thermal conductance per unit area (σ/A) in suspended Cu-CVD SLG scales as ∼1.7 × 10⁵ T¹.⁵³ W/m²K from 15 K to 140 K, indicating dominant contribution from flexural acoustic (ZA) phonons.
• At room temperature (300 K), σ/A reaches 3.8 × 10⁸ W/m²K, comparable to values reported for exfoliated SLG.
• The low-temperature σ/A scaling (T¹.⁵) indicates that phonon transport in the 500 nm channel length sample approaches 30% of the theoretical ballistic limit for graphene.
• A peak in thermal conductivity near 280 K (±10 K) is observed due to increasing Umklapp scattering at higher temperatures, followed by a decrease.
• Thermopower measurements show a linear voltage response with Joule heating, with a zero extrapolation at T = 0 K, confirming the absence of phonon drag in suspended CVD SLG.
• Deviation from ballistic transport is attributed to residual scattering from PMMA residues, CVD-specific defects, ripples, and isotopic ¹³C impurities.