Dielectric microbead-decorated conductive polymer composites towards ultrathin high-performance terahertz absorbing coatings

Abstract

The rapid development of terahertz (THz) technology necessitates efficient absorbers to mitigate electromagnetic interference (EMI). Here, we demonstrate a simple strategy of incorporating dielectric microbeads into conductive polymer composites to enhance THz absorption. Polystyrene (PS) microbeads embedded in poly(3,4-ethylenedioxythiophene) (PEDOT) improve impedance matching, introduce dielectric scattering centers, and reshape PEDOT sheets to induce multiple scattering. Optimized composites achieve EMI shielding effectiveness exceeding 40 dB, and reflection loss below −38 dB at a thickness of 0.7 mm. The coatings exhibit strong adhesion, environmental stability, and conformability, offering a versatile route towards ultrathin, lightweight and durable THz absorbers for future electromagnetic protection systems.

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Shangzhi Chen

Prof. Shangzhi Chen works at the University of Electronic Science and Technology of China (UESTC). He obtained his bachelor's degree in Materials Science and Engineering from Shanghai Jiao Tong University, China, in 2012, his master's degree in Nanoelectronics from KU Leuven, Belgium, in 2016, and his PhD in Applied Physics from Linköping University, Sweden, in 2021. After that, he conducted his postdoctoral research at the Cavendish Laboratory, University of Cambridge, UK, until 2024, when he joined UESTC. His research focuses on the development of (semi-)conductive polymers and molecules for advanced nanophotonic, electronic, and electrochemical applications.

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1. Introduction

The terahertz (THz) spectral range (0.1–10 THz) is of strategic significance across diverse domains, including wireless communications,¹ atmospheric remote sensing,² security inspection,³ and biomedical imaging.⁴ In these applications, THz absorbers are indispensable components, serving to mitigate electromagnetic interference (EMI), suppress background noise, and ensure reliable target calibration.^5–7 Conventional absorbing materials such as ferromagnetic composites, though effective in the millimeter-wave regime, suffer from reduced efficiency at higher frequencies.^8–10 Metamaterial-based structures can achieve high absorption, but their effective bandwidth is generally too narrow for practical use.^11–13 Emerging two-dimensional (2D) materials, including graphene and MXene, also face an intrinsic absorption limit of 50% for electromagnetic waves for a single layer.^14–17 A widely adopted strategy to circumvent this limitation involves the design of porous architectures, where multiple scattering occurring at absorber/air interfaces (e.g., graphene/air^18,19 or MXene/air²⁰) significantly suppresses surface reflection and enhances absorption. Using this approach, ultrahigh THz absorption exceeding 99.99% has been demonstrated in the 0.3–1.65 THz range.²⁰ Recently, we further introduced lightweight and sustainable conductive polymers and cellulose to construct aerogels,⁶ achieving broadband THz absorption with reflection loss (RL) below −30 dB and EMI shielding effectiveness (EMI SE) above 50 dB over 0.25–1.20 THz.

Nevertheless, as THz technology advances toward miniaturization and system-level integration, practical absorber designs must not only provide high RL, EMI SE, and broad effective bandwidth, but also satisfy additional requirements such as reduced weight, sub-millimeter thickness, and high mechanical conformity for integration onto arbitrary surfaces. Strategies based on porous architectures, such as aerogels^{6,18,19,21,22} and (organo-)hydrogels,^23,24 often struggle to achieve thicknesses below 1 mm, since their inherent discontinuity and inhomogeneity significantly compromise performance. Attempts to fabricate absorbers by dispersing MXene into polymer matrices have yielded pastes or paints with excellent EMI SE (>50 dB); however, the shielding effect in such systems is predominantly reflection-driven rather than absorption-dominated in the THz regime.^25,26 Chemical modification of graphene nanosheets to adjust interlayer spacing can induce multiple scattering of THz waves.²⁷ Utilizing this approach, we achieved absorption-dominated EMI SE, with single-layer coatings exhibiting average absorption approaching 70% across 0.25–1.20 THz – representing notable progress in the field.²⁷ Nonetheless, achieving average absorption above 95% typically requires micro- and macro-structured surfaces, such as periodic wedges or inverted pyramids.²⁷ To date, it remains a challenge to develop THz absorbing coatings that simultaneously deliver high absorption (>90%), and small thickness (<1 mm) across a broad effective bandwidth (e.g., >0.1 THz).

In this study, we introduced insulating polystyrene (PS) microbeads into a conductive polymer matrix for THz absorption enhancement. The PS microbeads were uniformly dispersed and functioned as air-like pores, inducing multiple scattering that could suppress surface reflection and improve absorption. By incorporating the waterborne polyurethane (WPU) as a binder, the absorber can be formulated into paint, enabling facile deposition onto arbitrary substrates for EMI protection. Through further optimization, the coating achieved an EMI SE exceeding 40 dB and an average absorption above 99.0% at a thickness of ∼0.7 mm – among the highest reported for THz absorbing coatings. This work demonstrates a novel strategy for designing high-performance THz absorbers, and the developed coatings provide a promising route towards lightweight, thin, flexible, and conformable broadband THz absorbers that satisfy current practical requirements.

2. Results and discussion

As illustrated in Fig. 1a, multiple scattering of electromagnetic waves in porous absorbers (i.e., aerogels^6,18 or conductive foams^20,28–30) originates from repeated reflections and transmissions at the interfaces between air and the absorbing medium. Replacing air with a dielectric of suitable refractive index can produce a similar impedance-matching effect to porous absorbers. To examine this, we first analyze the optical response of a single interface. For normal incidence, the reflection and transmission at the interface between two media are governed by the Fresnel equations:³¹where n₁ and n₂ are the complex refractive indices of the incident and transmitted media, respectively, and Real() denotes the real part. To realize high-performance absorbers, interface reflection should be minimized while transmission into the lossy phase should be enhanced to increase the probability of absorption; this is achieved when the refractive index contrast |n₁−n₂| is small. In the THz range, conductive materials (e.g., conductive polymers or graphene) are effective absorbers whose optical parameters are well described by the classical Drude model. Their complex refractive index follows from the angular frequency-dependent conductivity and permittivity:^32–34where ω is the angular frequency, τ is the momentum-averaged scattering time, ε_∞ is the high-frequency permittivity, and ε₀ is the vacuum permittivity. The DC electrical conductivity (σ_DC) of the material can be calculated via σ_DC = neμ, with n being the carrier density, μ being the mobility, and e being the elementary charge. As a representative conductive polymer, we consider poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as a model system and, for the following calculations, assume n of 1 × 10²⁰ cm⁻³ and μ of 1 cm²·V⁻¹·s⁻¹ (see Fig. S1 for resulted optical parameter dispersions).

Fig. 1 Concept of introducing dielectric microbeads for THz absorbers. (a) Schematic comparison of aerogel and composite paint (PS microbeads can improve impedance matching, introduce dielectric scattering centers, and reshape PEDOT:PSS sheets to induce multiple scattering). (b) Calculated THz reflection at the interface between a conductive polymer and a dielectric material with varying refractive index. (c) Calculated THz transmission at the same interface.

With the above framework, we calculated the influence of the dielectric refractive index on the THz reflection and transmission of the interfaces (Fig. 1b and c) in the spectral range of 0.25–1.20 THz. As the refractive index increases, the reflection decreases significantly, particularly at higher frequencies, while the transmission correspondingly increases. Specifically, increasing refractive index from 1.0 to 2.0 results in a remarkable enhancement of the average THz transmission from 0.36 to 0.58, and a reduction of the average THz reflection from 0.65 to 0.42 (see Fig. S2). These results are consistent with the expectation that dielectric incorporation improves impedance matching between air and conductive polymers, effectively acting as an anti-reflection layer. This suggests that introducing pores composed of dielectrics other than air can also induce multiple scattering within the system. Compared to aerogels, this strategy is more practical, as embedding micro-sized dielectric particles into conductive polymers provides a straightforward route to form such “pores”. Based on this insight, we propose that incorporating dielectric particles into conductive polymers can serve as an effective scattering medium, simultaneously enhancing transmission and suppressing reflection.

To demonstrate this concept, we selected commercially available polystyrene (PS) microbeads as the model system, which exhibit a refractive index of ∼1.6 in the THz range.³⁵ As illustrated in Fig. 2a, the proposed THz absorbing coating can be fabricated via a straightforward process (more details can be found in the Method section). Specifically, PEDOT:PSS and PS microbeads were blended in aqueous solution, and water-borne polyurethane (WPU) was introduced as the binder to form a viscous paint suitable for coating applications. In this demonstration, PS microbeads with a diameter of 3 μm were employed, allowing a large number of microbeads to be densely incorporated into the composite. Pure PEDOT:PSS samples exhibit a morphology of closely stacked sheets with sub-micrometer thickness and lateral dimensions ranging from several micrometers to over 100 μm (Fig. 2b and Fig. S3). Upon incorporation of PS microbeads, the morphology changes markedly, as shown in Fig. 2c–g and Fig. S3, with PEDOT:PSS-to-PS volume ratios varying from 5 : 1 to 1 : 1. At a low ratio (5 : 1), PS microbeads are sparsely distributed and partially covered by PEDOT:PSS sheets. Increasing the PS content (4 : 1 and 3 : 1) induces curvatures in the PEDOT:PSS sheets, with the microbeads embedded in the sheets and in some cases appearing to bisect the microbeads through their centers. Energy-dispersive X-ray spectroscopy (EDS) of the 3 : 1 composite (Fig. 2h) confirms that sulfur from PEDOT:PSS is uniformly distributed, forming a continuous conductive network. Notably, the induced curvatures disrupt the initial close stacking of pure PEDOT:PSS sheets and introduce inter-sheet gaps, which are highly favorable for multiple scattering, consistent with our previous study.²⁷ At higher PS content (2 : 1 and 1 : 1), PS microbeads dominate the morphology, shadowing the PEDOT:PSS sheets and significantly influencing their shape and size. In this case, the continuity of the conductive network formed by PEDOT:PSS sheets may be compromised, less favorable for effective THz wave absorption.³⁶

Fig. 2 Fabrication and microstructure of PS microbeads-incorporated conductive polymer coatings. (a) Schematic illustration of the fabrication process. (b) Scanning electron microscopy (SEM) image of pure PEDOT samples. (c)–(g) SEM images of composites with PEDOT:PSS to PS volume ratio varying from 5 : 1 to 1 : 1. (h) EDS elemental mapping of carbon (left) and sulfur (right) for the composite with a PEDOT:PSS to PS volume ratio of 3 : 1.

With this microstructural understanding, we performed THz-time domain spectroscopy (THz-TDS) to investigate the THz absorption performance of these composites in the spectral range of 0.25–1.20 THz (Fig. 3a–c and Fig. S4). The pure PS microbead sample exhibits a relatively high average transmission of 45.68% (approaching 80% at 0.3 THz), as expected from its low conductivity, which provides little conductive loss for THz absorption. In contrast, pure PEDOT:PSS shows a lower average transmission of 7.88%, yet this value remains higher than that of the composites. This is likely due to its relatively high reflection (3.36%) compared with the composites (below 1%), consistent with the morphology of large-sized closely stacked sheets (Fig. 2b), which act as effective reflectors of incident THz waves. Incorporating PS microbeads into the PEDOT:PSS sheets effectively suppresses both reflection and transmission. At a PEDOT:PSS to PS volume ratio of 4 : 1, the average reflection and transmission were reduced to 1.07% and 0.27%, respectively, corresponding to an average THz absorption exceeding 98% at a thickness of merely 0.7 mm. Fig. 3c further reveals the non-monotonic variation of THz reflection and transmission with composition: both initially decrease, reaching a minimum at 4 : 1, and then increase at higher PS contents. This trend is consistent with the morphological evolution. At low PS contents, the microbeads intercalate between PEDOT:PSS sheets or are embedded into these sheets, increasing interlayer spacing and promoting multiple scattering. At higher PS loadings, however, the microbeads dominate the morphology, altering the size and shape of the PEDOT:PSS sheets and disrupting their continuous conductive network, leading to a deterioration of THz absorption performance.

Fig. 3 Optimization of the volume ratio between PEDOT:PSS and PS. (a) THz reflection curves. (b) THz transmission curves. (c) Average reflection and transmission values in the THz range. (d) RL (reflection loss) curves. (e) EMI SE (electromagnetic interference shielding effectiveness) curves. (f) Average-RL and SE values in the THz range. The PEDOT:PSS to PS microbeads volume ratio was varied from 5 : 1 to 1 : 1. For comparison, pure PEDOT:PSS (1 : 0) and pure PS (0 : 1) samples were also characterized.

For practical applications, reflection loss (RL) and electromagnetic interference (EMI) shielding effectiveness (SE) in the THz range were also evaluated (Fig. 3d–f). Both parameters show similar trends to reflection and transmission. In practice, –RL and SE values above 10 dB are generally considered sufficient for THz applications. Pure PEDOT:PSS and PS samples, however, fail to meet this requirement, with both –RL and SE around or below 10 dB, confirming their limited functionality in this frequency range. By contrast, composites with PEDOT:PSS to PS volume ratios of 5 : 1 (RL = −20.1 dB, SE = 21.1 dB), 4 : 1 (RL = −20.9 dB, SE = 30.1 dB), and 3 : 1 (RL = −20.2 dB, SE = 29.1 dB) readily surpass the 20 dB threshold. These excellent performances can be attributed to three synergistic mechanisms, corroborated by morphological characterization: (1) PS microbeads act as anti-reflection media, improving impedance matching between PEDOT:PSS and air, thereby enhancing transmission and suppressing surface reflection; (2) PS microbeads behave as dielectric pores, providing efficient scattering centers that increase the probability of THz absorption; and (3) incorporation of PS microbeads modifies the morphology of PEDOT:PSS sheets, introducing curvatures that serve as additional sources of multiple scattering. Other factors may also play a role, such as secondary doping effects of insulating PS, which may enhance the conductivity of the composite.^37,38 At a volume ratio of 2 : 1, the composite achieves an SE of 18.9 dB and an RL of −17.9 dB, slightly below the required level. These results indicate that there exists a relatively broad range of PEDOT:PSS to PS volume ratios that yield –RL and SE values above 20 dB, which is advantageous for large-scale production where compositional homogeneity cannot always be precisely controlled. This finding highlights the robustness of the composite design for producing high-performance THz coatings with high tolerance to variations in recipe ratios.

We further investigated the effect of the WPU binder concentration on the overall THz performance of composite coatings (Fig. 4 and Fig. S5, S6). To determine the optimal binder content, the PEDOT : PSS to PS volume ratio was fixed at 4 : 1, while the WPU concentration was systematically varied from 99.5 wt% to 96.0 wt%. As shown in Fig. 4a–c, increasing the WPU content leads to a monotonic decrease in transmission down to below 1%, accompanied by a monotonic increase in reflection from <0.1% to ∼0.75%. This behavior can be attributed to the extend propagation path length of THz waves caused by the denser packing of PS spheres and the associated curvatures of PEDOT micro-sized sheets, as well as enhanced conductivity from the increased PEDOT fraction in the composites, collectively resulting in strong THz absorption (Fig. S6). Notably, this trend is more pronounced at higher frequencies than at lower frequencies, likely because the 3 μm PS microbeads are less effective in scattering longer-wavelength THz radiation. At a WPU binder content of 96.0 wt%, the coating achieves ∼99% absorption, an average SE of 40.5 dB, and an average RL of −38.0 dB (Fig. 4d–f). Although further reduction of the WPU concentration could potentially enhance the performance, it would compromise the viscosity of the composite paste, making it unsuitable for coating spreading. Importantly, the performance obtained at 96.0 wt% WPU is already comparable with state-of-the-art THz absorbing coatings,³⁹ considering the small thickness of only 0.7 mm. Table S1 summarizes representative high-performance THz absorbers reported in the literature for comparison with our work, further demonstrating that our samples exhibit clear advantages in both effective bandwidth and overall terahertz absorption performance.

Fig. 4 Optimization of the WPU weight concentration. (a) THz reflection curves. (b) THz transmission curves. (c) Average reflection and transmission values in the THz range. (d) RL (reflection loss) curves. (e) EMI SE (electromagnetic interference shielding effectiveness) curves. (f) Average –RL and SE values in the THz range. The weight concentration of WPU was varied from 99.5 wt% to 96.0 wt%.

For practical applications such as THz absorption coatings, achieving high THz absorption is a key requirement. We here explored the potential of these composites in diverse application scenarios. As displayed in Fig. 5a, a free-standing paper fabricated from the composite paste has mechanical flexibility and can be repeatedly folded into arbitrary shapes without damage, indicating its suitability as an effective EMI shielding layer in wearable and flexible electronics. The film's outstanding mechanical robustness and ultralight weight were further highlighted in a load-bearing test: despite a total mass of only 0.2 g, the paper supported a 200 g weight – 1000 times its own mass – without rupturing or undergoing significant deformation (Fig. 5b and Fig. S7). This exceptional strength-to-weight ratio underscores its promise for weight-sensitive applications such as aerospace stealth coatings and lightweight electromagnetic shielding armor. Environmental stability under harsh conditions represents another critical advantage. The composite coating was immersed in 30% sodium chloride solution, simulating aggressive marine environments, for 40 days. As shown in Fig. 5c and d, the SE remained nearly unchanged before and after immersion. This remarkable durability is attributed to the film's intrinsic hydrophobicity, as confirmed by a water contact angle of 102° (Fig. 5e). The strong hydrophobic surface acts as a non-wetting barrier, preventing electrolyte penetration and thereby protecting the material from oxidation degradation and corrosion. In addition to the above, the composite paste can be readily deposited onto arbitrary substrates, independent of surface roughness or geometry. To demonstrate this, the coating was applied to a ship model with complex surface features, forming a uniform, conformal matte film that preserved its hydrophobicity and corrosion resistance (Fig. 5f). This demonstration highlights the material's strong potential for military stealth technologies and protective coating in marine environments, where resistance to mechanical stress, salt spray, and harsh environmental conditions is essential. Furthermore, electrical characterization reveals the insulating nature of the composites (Fig. S8), which provides an additional advantage for their application in electronic device encapsulation and system integration, minimizing the potential risk of electric interference and ensuring stable system operation. Overall, the integration of high THz absorption, mechanical flexibility, environmental robustness, and conformal coating capability positions this composite as a promising candidate towards next-generation electromagnetic systems.

Fig. 5 Demonstration of THz absorption coatings in different application scenarios. (a) Foldability test showing repeated folding and unfolding without rupture. (b) Stress–strain curve of a free-standing composite wire (inset: a 0.2 g wire supporting a 200 g weight without rupture). (c) Salt corrosion resistance after immersion in 30% NaCl solution for 40 days. (d) Photographs of samples immersed in 30% NaCl solution on Day 1 and Day 40. (e) Water contact angle measurement of the composite coating. (f) Conformal application of the coating on a ship model with complex surface geometry.

3. Conclusions

In this study, we demonstrate a strategy of incorporating dielectric microbeads into conductive polymers to achieve high-performance THz absorbing coatings. The microbeads not only improve surface impedance matching and act as effective scatters that enhance THz absorption, but also reshape conductive polymer micro-sheets and induce curvatures that promote multiple scattering. By optimizing the composition, the coating achieves >99% absorption, EMI SE above 40 dB, and RL of below −38 dB, at a thickness of only 0.7 mm. Moreover, the composites can be applied to a wide range of substrates, exhibiting excellent adhesion, environmental stability, and conformability. These attributes make the coatings highly suitable for applications requiring thin, robust, and conformal films under harsh conditions. Beyond PEDOT:PSS, the strategy can be extended to other material systems to further advance the development of next-generation electromagnetic protection technologies.

4. Method

4.1. Materials and coating preparation

Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS, PH1000, with a solid content of 1.3 wt%) was purchased from Heraeus (Germany). Polystyrene micro-beads (PS, diameter of 3 μm, aqueous solution with a solid content of 1.3 wt%) were purchased from Rego Biotechnology (China). Waterborne polyurethane (WPU, with a solid content of 30 wt%) was purchased from Shenzhen Jitian Chemical Co., Ltd (China). DI water was used throughout the study to avoid unintended ionic effects.

The electromagnetic absorbing agent was prepared by blending the PEDOT:PSS and PS microbeads in different ratios, namely 5 : 1, 4 : 1, 3 : 1, 2 : 1, and 1 : 1. The mixture was then transferred into a glass beaker and subjected to ultrasonic treatment (40 kHz, 300 W) for 30 minutes to ensure homogeneity. The well-mixed suspension was then rapidly frozen at −18 °C for 12 hours and subsequently lyophilized under vacuum (< 0.5 Pa, Labconco FreeZone) for 24 hours to completely remove the water, giving rise to fluffy powders. For comparison, control samples of pure PEDOT (without PS) and pure PS (without PEDOT) were also prepared under identical conditions.

The electromagnetic coating and films were prepared by adding WPU binders in the above agents with different weight ratios, including 0.5 wt%, 1.0 wt%, 1.5 wt%, 2.5 wt%, and 4.0 wt%. The obtained mixture was thoroughly ground using an agate mortar for 30 minutes to form uniform slurries without agglomeration. For coating, the slurry can be drop-cast onto clean substrates (e.g., glass) and spread into a film using a doctor's blade with a gap of 0.7 mm. The film was then dried on a hotplate at 80 °C in air for 2 hours and cooled gradually to room temperature to prevent cracking and ensure the uniformity of the film thickness.

4.2. Microstructural characterization

Microstructural characterization was performed using a field emission scanning electron microscopy (FE-SEM, Nova Nano SEM 450, FEI Company) operated at 10 kV. Elemental mapping was conducted via an energy dispersive spectrometer (EDS, Oxford Instruments) attached to the SEM. To enhance the conductivity and minimize the charging artifacts, all samples were sputter-coated (Quorum Q150T ES) with a 5 nm thick gold layer.

4.3. Terahertz absorption performance measurement

The THz optical properties of the composite coatings were characterized using a THz-TDS system (Fico, Zomega Terahertz Corporation) equipped with both reflection and transmission modules. The THz beam spot size was approximately 2 mm in diameter. All measurements were performed under controlled environmental conditions at 23 ± 1 °C and relative humidity below 8% RH, to minimize atmospheric absorption. The reflectance (R) and transmittance (T) were then derived from these signals through a Fourier transform process. The absorption (A) equal to 1-R-T. The reflection loss (RL) and shielding effectiveness (SE) were extracted from the time-domain signals according towhere E_s is the amplitude of the THz wave reflected from the sample, indicates that the sample is measured with placement on a metal plate, simulating conditions consistent with practical applications. E_ref is the amplitude of the reference signal reflected from an aluminum mirror (assumed to be a perfect reflector), and E_air is the amplitude of the transmitted wave through the sample normalized to air.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

The data supporting this article have been included in the main text and as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5tc03391h.

Acknowledgements

The authors acknowledge support from the National Natural Science Foundation of China (62405044, 6251101062, 62235004, 62311530115, and 62505042), the Natural Science Foundation of Sichuan Province (2025ZNSFSC0337), and the Swedish Research Council (VR, 2022-06214).

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