[Paper Review] Graphene-based enhancement of near-field radiative-heat-transfer rectification

Landrieux, Simon, Ben-Abdallah, Philippe+1 more|arXiv (Cornell University)|Feb 1, 2022

Thermal Radiation and Cooling Technologies64 references18 citations

TL;DR

This paper proposes a graphene-enhanced near-field radiative thermal rectifier using VO2 and SiO2 substrates, where tunable graphene sheets at optimal chemical potentials enhance rectification efficiency by up to 14% at 100 nm separation. The enhancement arises from graphene plasmons mediating stronger coupling between substrate surface modes, inducing a transition from 1/d² to 1/d power-law dependence in heat flux, which maximizes asymmetry under temperature reversal.

ABSTRACT

International audience

Motivation & Objective

• To enhance near-field radiative heat transfer (RHT) rectification in a VO2/SiO2 heterostructure using graphene as a tunable mediator.
• To address the inefficiency caused by mismatched surface resonances between dissimilar materials in thermal rectifiers.
• To optimize graphene chemical potentials for maximum rectification efficiency across varying vacuum gaps.
• To investigate the role of graphene plasmons in modifying spectral and distance-dependent RHT characteristics.
• To identify the physical origin of non-monotonic efficiency behavior with respect to separation distance.

Proposed method

• Modeling a two-body system with VO2 and SiO2 substrates separated by a vacuum gap, each coated with a monolayer graphene sheet.
• Using Landauer-like decomposition of radiative heat flux: Φ = ∫₀^∞ dω/(2π) ∆Θ(ω) ∑ₚ ∫ d²k/(2π)² Tₚ(ω,k), where Tₚ is the transmission coefficient.
• Calculating reflection coefficients for graphene-covered surfaces using frequency- and wavevector-dependent surface conductivity σ(ω) and dielectric permittivity ϵm(ω).
• Optimizing chemical potentials µVO2 and µSiO2 for each gap distance d to maximize rectification coefficient η = (ΦF − ΦB)/ΦF.
• Analyzing spectral flux Φω and transmission TTM(ω,k) to identify mode coupling and resonance broadening effects.
• Comparing power-law scaling of ΦF and ΦB with d to identify transitions from 1/d² to 1/d behavior in the electrostatic regime.

Experimental results

Research questions

• RQ1 How does graphene coating affect the rectification coefficient of a VO2/SiO2-based near-field thermal diode?
• RQ2 What is the role of graphene plasmons in enhancing coupling between surface modes of VO2 and SiO2?
• RQ3 Why does the rectification efficiency exhibit a non-monotonic dependence on vacuum gap distance?
• RQ4 How does the power-law scaling of heat flux (1/d² vs. 1/d) relate to the efficiency enhancement?
• RQ5 Can optimal chemical potentials for graphene be tuned to maximize rectification across different separation distances?

Key findings

• The rectification coefficient η increases by up to 14% at a 100 nm gap when graphene chemical potentials are optimized, compared to the uncoated system.
• The enhancement is primarily due to increased forward heat flux ΦF, while backward flux ΦB remains largely unchanged, due to resonant coupling between graphene plasmons and substrate modes.
• Spectral analysis reveals broadened and more intense resonant modes in the forward configuration with graphene, especially in the 0.8–1.7×10¹⁴ rad/s range.
• A transition from 1/d² to 1/d power-law scaling of ΦF occurs below ~35 nm in the optimal graphene configuration, which enhances rectification efficiency at intermediate distances.
• The non-monotonic behavior of η with decreasing d arises because the 1/d transition in ΦF brings it closer to ΦB at very small gaps, reducing asymmetry despite higher absolute flux.
• The characteristic distance of the 1/d transition depends on the chemical potentials, with lower µ values shifting the transition to smaller d, but at the cost of mode detuning.