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Suppressing Plasmonic Heating in Aqueous Environments with Hexagonal Boron Nitride

This study demonstrates that integrating hexagonal boron nitride (hBN) thin flakes as heat spreaders can reduce plasmonic heating in aqueous environments by up to 60% compared to glass, with cooling efficiency governed by flake thickness and interfacial thermal conductance, as validated through finite-element simulations and cross-grating wavefront microscopy experiments.

Original authors: Martina Russo, Roland van der Vegt, Bohai Liu, Sam Beijers, Sara Salera, Guillaume Baffou, Klaas-Jan Tielrooij, Peter Zijlstra

Published 2026-05-04

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CC BY 4.0

Original authors: Martina Russo, Roland van der Vegt, Bohai Liu, Sam Beijers, Sara Salera, Guillaume Baffou, Klaas-Jan Tielrooij, Peter Zijlstra

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you have a tiny, glowing speck of gold sitting in a drop of water. When you shine a laser on it, this speck gets incredibly hot—so hot that it could damage delicate things nearby, like biological sensors or tiny computer chips. This is called "plasmonic heating." It's like trying to cool down a hot cup of coffee by placing it on a thin piece of paper; the paper just gets hot too, and the heat doesn't go anywhere.

This paper is about finding a better "cooling mat" for these tiny hot spots. The researchers discovered that a special, ultra-thin material called hexagonal boron nitride (hBN) acts like a super-efficient heat spreader.

Here is the story of how they solved the problem, explained simply:

The Problem: The "Hot Spot" Trap

When gold nanoparticles are hit with light, they absorb energy and turn it into heat. If you put them on a standard glass slide (like a microscope slide), the heat gets stuck. Glass is a poor conductor of heat, so the temperature spikes right where the particle is, potentially ruining sensitive experiments.

The Solution: The "Heat Highway"

The researchers tried placing the gold particles on a flake of hBN instead of bare glass. Think of hBN as a super-highway for heat.

Glass is like a dirt road; heat moves slowly and gets stuck.
hBN is like a high-speed train track. It allows heat to zoom away sideways (laterally) very quickly, spreading the energy out so the specific spot doesn't get as hot.

The Experiment: Measuring the Heat

To prove this worked, the team used a clever tool called Cross-Grating Wavefront Microscopy (CGM).

How it works: Imagine looking at a hot object through a special lens that can see how the air (or water) bends light because of the heat. The hotter the water gets, the more it bends the light.
The Magic: This tool allowed them to "see" the temperature map around the gold particle without touching it or using any dyes. It was like having a thermal camera that could see heat on a scale smaller than a virus.

They also used this same tool to measure how thick the hBN flakes were. Usually, measuring the thickness of something this thin requires heavy, clunky machines or slow chemical tests. But CGM acted like a "magic ruler," measuring the thickness instantly just by looking at how the light passed through the flake.

The Big Discovery: Thickness Matters

The researchers found that the thickness of the hBN flake changes how well it cools the gold particle:

Too Thin (The "Paper Towel" Effect): If the hBN flake is very thin (just a few layers), it doesn't have enough "mass" to soak up the heat. It's like trying to cool a hot pan with a single sheet of paper towel; the paper gets hot immediately and can't help much.
Just Right (The "Cooling Pad" Effect): As the hBN flake gets thicker, it becomes a better heat sink. It has enough capacity to absorb the heat and spread it out efficiently.
The Result: By using the right thickness of hBN, they were able to reduce the temperature rise by about 60% compared to using plain glass.

Two Ways Heat Escapes

The study also revealed two ways the heat leaves the gold particle:

The Direct Path: Heat jumps straight from the gold into the hBN (like stepping off a hot stove onto a cool floor).
The Indirect Path: Heat goes from the gold into the surrounding water, and then the water passes the heat to the hBN.

Even if the gold doesn't touch the hBN perfectly, the hBN can still cool things down by stealing the heat from the water around the particle.

Why This Matters

This research gives scientists a new rulebook for building tiny, heat-sensitive devices. If you are building a biosensor (to detect viruses) or a microchip, you don't want your device to overheat and break. By sandwiching your tiny components between a layer of hBN and glass, you can keep them cool and running smoothly, just like putting a high-tech cooling pad under a gaming laptop.

In short: They found a way to use a special, transparent, atom-thin material to act as a super-cooler for tiny hot spots, and they proved it works by "seeing" the heat with a special light-based camera.

1. Problem Statement

Plasmonic nanostructures, particularly gold and silver nanoparticles, are widely used for their ability to confine and enhance electromagnetic fields. However, when illuminated at their resonance frequency, a significant portion of absorbed optical energy is converted into heat via non-radiative decay (Landau damping, electron-electron scattering, and electron-phonon coupling). This phenomenon, known as plasmonic heating, causes localized temperature rises that can:

Degrade performance in heat-sensitive applications (biosensing, nanophotonics, microelectronics).
Induce thermal drift, modify refractive indices, and damage molecular probes or nearby structures.
Cause thermal accumulation in densely integrated architectures where heat dissipation pathways are restricted.

Conventional cooling strategies often fail at the nanoscale due to limited heat transport and high interfacial thermal resistance. While high-thermal-conductivity substrates like diamond or graphene have been explored, they suffer from drawbacks such as optical absorption, electrical conductivity interference, or complex fabrication. There is a critical need for a material that offers high thermal conductivity, optical transparency, chemical stability, and electrical insulation to manage heat in aqueous environments.

2. Methodology

The authors employed a combined simulation-experiment framework to investigate hexagonal boron nitride (hBN) as a nanoscale heat spreader.

A. Numerical Simulations (Finite Element Modeling)

Model: Simulated 100 nm gold nanospheres (GNPs) immobilized on glass or on hBN flakes of varying thickness ( $t_{hBN}$ ), immersed in water.
Parameters: Systematically varied hBN thickness (1–100 nm), in-plane thermal conductivity ( $k_{\parallel}$ ), and interfacial thermal conductance ( $G$ ) at all interfaces (gold-water, gold-hBN, water-hBN, etc.).
Geometry: Modeled nanoparticles as truncated spheres to account for realistic contact areas ( $f$ ) with the substrate, acknowledging the faceted nature of gold crystals.
Metric: Defined a "cooling factor" as the ratio of temperature rise with hBN ( $\Delta T_{hBN}$ ) to temperature rise on bare glass ( $\Delta T_{SiO2}$ ).

B. Experimental Setup

Sample Preparation:
- hBN flakes were mechanically exfoliated from powder onto SiO2/Si and transferred to glass coverslips using a dry-transfer method (PDMS/PC stamp).
- 100 nm citrate-capped gold nanospheres were drop-casted onto the samples, immobilizing them on both bare glass and hBN flakes.
- Samples were placed in a fluidic chamber filled with water.
Characterization Technique: Cross-Grating Wavefront Microscopy (CGM)
- Nanothermometry: Used CGM (a quadriwave lateral shearing interferometry technique) to measure optical path differences (OPD) induced by the thermo-optic effect ($dn/dT$) in the water surrounding the heated particles. This allowed for label-free, non-invasive mapping of temperature distributions with sub-nanometer sensitivity.
- Thickness Characterization: Leveraged the same CGM setup to measure hBN thickness non-invasively by correlating OPD with the effective refractive index, validated against Atomic Force Microscopy (AFM).
Excitation: A 532 nm laser was used to heat the GNPs, while a white LED provided the probe beam for CGM imaging.

3. Key Contributions

Demonstration of hBN as a Nanoscale Heat Spreader: Proved that hBN flakes can significantly mitigate plasmonic heating in aqueous environments, reducing steady-state temperature rises by up to ~60% compared to glass.
Identification of Heat Dissipation Pathways: Revealed two distinct mechanisms:
- Direct Pathway: Heat conduction from the nanoparticle directly to the hBN (dominant with larger contact areas).
- Indirect Pathway: Heat transfer from the nanoparticle to the surrounding water, then to the hBN (dominant even with point contacts).
Thickness-Dependent Regimes: Established that cooling efficiency is governed by different limiting factors depending on flake thickness:
- Thin Flakes (<10 nm): Limited by the heat capacity of the hBN layer itself.
- Thick Flakes (>20 nm): Limited by interfacial thermal conductance between the nanoparticle and the hBN.
Novel Application of CGM: Generalized the use of CGM for rapid, non-invasive, all-optical thickness characterization of non-absorbing 2D materials, eliminating the need for destructive AFM or complex Raman fitting.

4. Key Results

Cooling Efficiency: Experimental results showed a monotonic improvement in cooling with increasing hBN thickness. Flakes around 100 nm thick achieved the maximum reduction (~60%) in temperature rise.
Simulation-Experiment Agreement: Finite-element simulations closely matched experimental data, confirming that high in-plane thermal conductivity ( $k_{\parallel} \approx 250-650$ W m $^{-1}$ K $^{-1}$ ) allows hBN to laterally spread heat effectively before it reaches the glass substrate.
Interfacial Impact: The study highlighted that even with minimal contact area (point contact), hBN provides significant cooling due to the indirect water-mediated pathway. However, maximizing cooling requires optimizing both the contact area and the interfacial thermal conductance ( $G_{GNP-hBN}$ ).
Thickness Characterization: The CGM-based thickness measurement showed a strong correlation with AFM (accuracy within 3 nm) across a range of 3–110 nm, validating it as a high-throughput alternative for 2D material characterization.

5. Significance and Impact

Design Guidelines: The paper provides concrete design principles for integrating 2D materials into thermally sensitive platforms. It emphasizes that for optimal thermal management, one must balance hBN thickness (to ensure sufficient heat capacity) and interface engineering (to ensure efficient heat transfer).
Broad Applicability: The findings are directly applicable to improving the stability and reliability of:
- Biosensors: Preventing thermal denaturation of biomolecules.
- Nanophotonics & Integrated Circuits: Mitigating thermal drift and signal degradation.
- Energy Conversion: Enhancing efficiency in solar harvesting and catalysis.
Methodological Advance: The dual use of CGM for both temperature mapping and material characterization offers a powerful, non-invasive tool for the high-throughput analysis of 2D material-based architectures.

In conclusion, this work establishes hexagonal boron nitride as a superior candidate for nanoscale thermal management in plasmonic systems, offering a solution that combines high thermal conductivity with the optical and electrical transparency required for advanced photonic and biological applications.