Harmonizing Material Quantity and Terahertz Wave Interference Shielding Efficiency with Metallic Borophene Nanosheets

doi:10.21203/rs.3.rs-4763119/v1

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Harmonizing Material Quantity and Terahertz Wave Interference Shielding Efficiency with Metallic Borophene Nanosheets

https://doi.org/10.21203/rs.3.rs-4763119/v1

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Materials with electromagnetic interference (EMI) shielding in the terahertz (THz) regime, while minimizing the quantity used, are highly demanded for future information communication, healthcare and mineral resource exploration applications. Currently, there is often a trade-off between the amount of material used and the absolute EMI shielding effectiveness (EESt) for the EMI shielding materials. Here, we address this trade-off by harnessing the unique properties of two-dimensional (2D) β12-borophene (β12-Br) nanosheets. Leveraging β12-Br’s light weight and exceptional electron mobility characteristics, which represent among the highest reported values to date, we simultaneously achieve a THz EMI shield effectiveness (SE) of 70 dB and an EESt of 4.8 × 105 dB·cm2/g (@0.87 THz) using a β12-Br polymer composite. This surpasses the values of previously reported THz shielding materials with an EESt less than 3 × 105 dB·cm2/g and a SE smaller than 60 dB, while only needs 0.1 wt.% of these materials to realize the same SE value. Furthermore, by capitalizing on the composite’s superior mechanical properties, with 158% tensile strain at a Young’s modulus of 33 MPa, we demonstrate the high-efficiency shielding performances of conformably coated surfaces based on β12-Br nanosheets, suggesting their great potential in EMI shielding area.

Physical sciences/Nanoscience and technology/Nanoscale materials/Two-dimensional materials

Physical sciences/Materials science/Materials for optics/Nanophotonics and plasmonics

THz radiation, spanning from 0.1 to 10 THz in frequency, represents a frontier in technology with vast potential across multiple applications including information communications, radar imaging, noninvasive inspection, and biomedical sensing¹. Its appeal lies in distinctive characteristics, such as low photon energy, robust penetration capabilities and the ability to discern characteristic molecular signatures within this spectral range. However, despite its promise, THz electronic and optoelectronic devices are not immune to electromagnetic interference (EMI) pollution, a challenge well-known in the radio-frequency realm^2–4. Such interference can markedly degrade device performance and do harm to the surrounding environment, underscoring the need for effective EMI shielding materials.

An ideal EMI shielding material should effectively block electromagnetic waves (EMW) while simultaneously ensure that the isolated electromagnetic field does not emit into the surrounding environment. Meanwhile, such a material is also anticipated to be low-cost and easily producible, lightweight and possess good stretchability, enabling it to conform to arbitrary surfaces. This flexibility is particularly crucial for effective EMI shielding in wearable devices and portable equipment for consumer electronics.

Currently, research on THz EMW shielding materials follows two main approaches. On one hand, traditional materials originally designed for the microwave frequency range are still utilized, such as metal thin films^5–7 and carbon-based composite films, including conductive carbon inks^{8, 9}, carbon fibers^{9, 10}, carbon nanotubes^{9, 11}, and graphene^{9, 12}. These materials are highly conductive and inhibit EMW through reflection, resulting in a significant portion of the waves being radiated into the surrounding environment, thus producing secondary pollution^{5, 13}. This inherent characteristic renders them less suitable for serving as EMI shielding materials in miniaturized devices with a demand of high integration. Alternatively, EMW can be shielded through absorption by the charged carriers within the EMI shielding materials. This has led to a recent shift in research focus towards exploring new-type materials with high electromagnetic absorption properties. These materials include polymer composite films^{14, 15}, nanocomposite films^{16, 17}, multilayer assemblies^{5, 15}, porous foams/aerogel composites^{17, 18}, and so on. Typically, these materials consist of segregated conductive fillers with a high density of free electrons with large mobility, facilitating the absorption of EMW through scattering at defects and interfaces presenting on each filler surface, followed by re-absorption by the free electrons. In particular two–dimensional (2D) conductive materials, such as MXene and graphene, have recently been employed as the fillers due to their outstanding electrically conductive characteristics. For example, a very recent study demonstrates that MXene assemblies can approach the intrinsic absorption limit in the 0.5–10 THz frequency range, with a small thickness of 10.2 nm¹⁹. A thin graphene/PMMA nanolaminate composite is displayed with a high conductivity to exhibit a shielding effectiveness (SE) reaching 60 dB for a small thickness of 33 µm¹⁵. Our recent study indicates that a transparent MXene film can have an SE of 21 dB over a wide frequency range of 0.1–10 THz²⁰.

While these exciting results provide a pathway for developing high-performance THz EMI shielding materials, there usually needs a trade-off between the mass amount of material used and the shielding effectiveness. This typically leads to an absolute EMI shielding effectiveness (EES_t) of less than 3 × 10⁴ dB·cm²/g and an SE smaller than 60 dB (Table S1, Supporting Information)¹⁵. To further enhance the shielding performance of these materials, an increased filler content or increment of the material’s thickness is usually necessary. However, this inevitably increases the overall mass and thickness of EMI shielding material, which could compromise their mechanical stability and limit their developments in portable and miniaturized devices requiring lightweight, flexible, and conformable coatings. All these requirements highlight the necessity for a flexible shielding material with both high EES_t and shielding SE values. Moreover, for practical applications, an environmentally favorable and scalable manufacturing process for the EMI shielding materials is urgently demanded, yet very challenging^{2, 21}.

Borophene, often termed the boron analogue of graphene, represents a unique class of 2D material characterized by its presence in multiple phases, stemming from the arrangement and deficiency of boron atoms^{22, 23}. These phases encompass triangular, honeycomb, stripe and rectangular configurations, among others²⁴. The diverse structural landscape of borophene, composed of the lightest solid element, underpins its ultralow density and distinguished flexibility, characterized by a high Young’s modulus²². Furthermore, borophene is predicted to have outstanding electronic mobility alongside a large carrier density, suggesting it is one of the very few metallic or semi–metallic 2D materials with high conductivity^{25, 26}. These properties render borophene an attractive candidate for embedding into polymer matrices to develop advanced flexible EMI shielding materials. However, the absence of a synthesis method for growing freestanding metallic borophene with high yield has hindered their developments on THz EMI shielding area, causing the low EES_t smaller than 0.125 dB·cm²/g and SE lower than 42 dB^{27, 28}, far from fully utilizing its advantages.

Here, we establish a scalable synthesis method for creating a wafer-scale composite of polydimethylsiloxane (PDMS) embedded with few-layer β₁₂-Br single crystalline nanosheets²⁹ as conductive fillers. The resulting composite film can reach a maximum size of up to 5 inches and has a thickness ranging from 0.2 to 4 mm. The single-crystalline β₁₂-Br fillers display superhigh-density free electrons (~ 3.4×10¹⁹ m^− 2) and large conductivity. These free electrons are excited by THz illumination, dissipating through scattering at the boundaries of the β₁₂-Br nanosheets. This phenomenon significantly contributes to the exceptional performance of the composite film in THz EMI shielding. The film achieves a mean SE as high as over 70 dB and an ultrahigh EES_t of 4.8 × 10⁵ dB·cm³/g, the highest value to date as we know, across the 0.5 to 2 THz frequency range, with potential extension up to 10 THz. These results overwhelm those of previously reported EMI shielding materials designed for the THz spectral regime, which require at least 10⁴ times more mass than the 2D borophene utilized in our current study to realize the comparable SE value. Additionally, the flexible composite film exhibits an ultrahigh tensile strain of over 158% at a Young’s modulus of 33 MPa, enabling effective shielding of conformably coated surfaces. The results undoubtedly underscore the potential of few-layer metallic β₁₂-Br nanosheets as extremely efficient, low-density, and highly elastic materials for advanced THz shielding applications.

Fabrication and characterization of β₁₂-Br/PDMS composites

The β₁₂-Br comprises five atoms per unit cell, characterized by alternating rows of empty and filled hexagons along the x-direction. This arrangement usually produces stripes of vacancies along this axis. Along the y-direction, the structure consists of columns featuring a continuous line of atoms interspersed with incomplete hexagons, as shown in Fig. 1a. The unique crystalline structure of β₁₂-Br results in a high electron density (3.4 × 10⁹/m²), accompanied by a large electron mobility (2.84 × 10⁶ cm²/(V·s)) and a high Young’s modulus (382 GPa) ^24,25. Furthermore, it is the most stable among the various allotropes of borophene in its freestanding state^{23, 29–32}. In this study, the borophene nanosheets were synthesized by our developed low-temperature liquid phase exfoliation (LTLE) technique (see Methods for details)³³, as depicted in Fig. 1b. The crystalline structure and stoichiometric ratio of the products are respectively confirmed by X-ray diffraction (XRD) and confocal Raman spectroscopy, revealing the nanosheets are the β₁₂ phase with high crystallinity (Supplementary Figs. 1 and 2). The β₁₂-Br nanosheets are a few atomic layers in thickness and well-dispersed into an aqueous solution, with an average thickness about 4 nm (Fig. 1c, Supplementary Fig. 3) and a six-fold symmetry (Fig. 1d, Supplementary Fig. 4). Additionally, over 94 at.% of the boron atoms remain unoxidized (Supplementary Figs. 5 and 6).

It should be noted that fabricating robust, stretchable, and continuous thin film based on pure borophene nanosheets alone has significant challenges, especially when attempting to scale up to larger area²⁷. Instead, we employ the β₁₂-Br nanosheets as the fillers embedded into PDMS film to form a flexible composite. Specifically, the composite film was produced using a simple sol-gel method (Fig. 1b), where the β₁₂-Br nanosheets were intricately linked together via robust intermolecular bonding interactions involving hydrogen atoms from PDMS molecules and boron atoms located on the surface of the β₁₂-Br (Fig. 1a). This approach has been previously employed, where α-Br flakes were used as fillers²⁷. However, due to the inefficient EMI shielding of α-Br reported in those studies, elevated concentration of borophene, up to 100 wt.%, was necessary. This undoubtedly causes the difficulties in achieving uniform and continuous thin films owe to potential cracking of the PDMS matrix. For the β₁₂-Br, as discussed below, leveraging the ultrahigh EES_t of the β₁₂-Br allows for exceptionally low borophene content of less than 0.5 wt.% in the matrix and minimizing the breakage probability of PDMS intermolecular bond. This obvious decrease of the β₁₂-Br content facilitates their homogeneous dispersion within the PDMS matrix (Fig. 1f, inset), enabling the formation of flexible large-area thin films up to 5 inches (Fig. 1e, f).

In-situ electrical measurements are conducted on individual β₁₂-Br nanosheet to explore its native electrical transport behaviors, as shown in Fig. 2a. Based on the I-V characteristics exhibit a clear linear relationship (inset), the average electrical conductivity ($\:\stackrel{-}{{\sigma\:}_{B}}$) of individual nanosheets can be calculated to be about 3.0 × 10⁴ S/m, suggesting their metallic conduction behaviors. It is noted that the electrical conductivity of a single β₁₂-Br nanosheet with a 4-nm thickness is two orders of magnitude higher than that of individual MoS₂ (303.03 S/m) and WS₂ (416.67 S/m)³⁴ and close to that of graphene (~ 10⁶ S/m)³⁵. Figure 2c gives the morphology and measurement circuit of bare β₁₂-Br nanosheet film with a 400-nm thickness. As presented in Fig. 2d, the mean sheet resistance ($\:\stackrel{-}{{R}_{S}}$) of the β₁₂-Br nanosheet film is determined to be 1.7 × 10⁴ Ω/sq according to the I-V curves (inset), which is approximately five orders of magnitude lower than that (6.0 × 10⁹ Ω/sq) of the nanostructure film consisted of the mixed β₁₂-Br and χ₃-Br phases^{36, 37}. The above transport results prove the superior crystallinity and purity of β₁₂-Br sheets synthesized by our method, suggesting the β₁₂-Br nanosheets with ultrahigh electrical conductivity should have promising future in THz-wave EMI shielding.

THz shielding performance and mechanism of β₁₂-Br/PDMS composite film

The THz-wave shielding performance of the β₁₂-Br/PDMS composite film was evaluated using a THz time-domain spectroscope (THz-TDS) system, as given in the inset of Fig. 3a. From Fig. 3a, the excellent EMW shielding performances of 1-mm-thickness composite film can extend across a very wide frequency range from 0.1 to 7 THz, highlighting that the β₁₂-Br/PDMS composite film serves as an ultra-broadband EMW shielding material. To better comprehend the contribution of different components to the THz shielding performance of the composite film, both the reflectance and transmittance spectra of the composite film are measured together to obtain the EMI absorption effectiveness (SE_A) and reflection effectiveness (SE_R), respectively. It is obviously seen that the average SE_A reaches up to 60 dB while the SE_R is only ~ 5 dB when the illumination frequency is larger than 0.2 THz, revealing that the absorption loss should dominate over the THz-wave shielding behaviors of β₁₂-Br/PDMS composite film rather than the reflection or transmission loss. With a thickness of 2 mm and containing 0.13 wt.% of β₁₂-Br, the EMI SE of the composite film can reach as high as 70 dB (Fig. 3c). Furthermore, the EMI EES_t, which offers a more comprehensive assessment of the shielding performance of the material under idealized conditions by considering the mass of the shielding material and the spot size of incident EMW (see Methods), is obtained to be in the range of 2.5 × 10⁵ – 4.8 × 10⁵ dB·cm²/g across the frequency range of 0.8 THz to 2 THz. Notably, several films, comprising of graphene or MXene composites among others, demonstrate the EMI SSE_t maxima capped at 3 × 10⁵ dB·cm²/g. However, these composite materials, even at concentrations of at least 5 wt.%, fell short of achieving the benchmark of 80 dB in terms of EMI SE (Fig. 3d, Table S1, Supporting Information). In stark contrast, the β₁₂-Br/PDMS film, comprising a mere 0.13 wt.% weight fraction and measuring 2 mm in thickness, showcased an impressive average EMI SSE_t of 4.8 × 10⁵ dB·cm²/g. Furthermore, with an increase in the weight fraction to 0.5 wt.% and the thickness to 4 mm, the film attained an average EMI SE as high as 85 dB. Such comparison evidently underscores that despite of the evidently improved performance of the β₁₂-Br/PDMS film, the mass of the shielding materials is four orders of magnitude smaller than that of other excellent shielding materials previously reported.

A series of β₁₂-Br/PDMS composite films were prepared to investigate the influence of the weight ratio (wt.%) of β₁₂-Br nanosheets and the film thickness on the THz EMI shielding performance (Supplementary Fig. 7). The power of THz EMW is effectively dissipated by the composite film, specifically by the β₁₂-Br nanosheets, which is elucidated by monitoring the SE spectra measured from composite films with varying concentrations of β₁₂-Br nanosheet but identical thickness (2 mm). By progressively increasing the borophene content from 0.13 to 2.1 wt.%, the EMI SE initially experiences rapid augmentation, eventually reaching a plateau for mass ratios over 0.5 wt.%. Notably, the highest SE achieved can arrive at 76 dB at 0.87 THz for a mass ratio of 2.1 wt.%, unveiling that the EMW shielding of the composite film is primarily attributed to the addition of β₁₂-Br nanosheets.

The aforementioned results clearly show that even a small (2.1 wt.%) addition of β₁₂-Br nanosheet into the polymer matrix can yield a superhigh EMI SE of 76 dB. This obviously surpasses previous findings, where an SE of only 42 dB was achieved with 100 wt.% of α-Br filled into the same matrix²⁷. Most of all, for a weight ratio of 0.13 wt.%, the EMI SSE_t can reach up to 4.8 × 10⁵ dB·cm³/g, superior to all reported composite films with conductive fillers such as graphene and MXene (Fig. 3d). Simultaneously, the EMI SE value of β₁₂-Br/PDMS composite film can still maintain as high as 68 dB, which achieves the best value reported up to date.

The addition of β₁₂-Br nanosheet fillers is found to be very essential for evidently improving the THz shielding performance of composite film. If the filler is changed from bulk boron powers to the β₁₂-Br nanosheets, the THz shielding performances of the composite film were significantly improved (Fig. 4a). For instance, in a composite film with 0.13 wt.%, the SE already reaches as high as 68 dB at 0.87 THz. To probe the decisive factors of the EMW shielding performance of the β₁₂-Br/PDMS composite film, the dependence of EMI SSE_t on the mass ratio of borophene nanosheets is characterized, revealing a nonlinear increase with the reduction of the weight ratio (Fig. 4b). The THz-wave shielding performance of the composite film is strongly dependent on the film thickness, as manifested from the EMI SE spectra against the film thickness (Fig. 4c). If the weight ratio of borophene nanosheets was kept at 0.5 wt.%, both of the EMI SE and SSE_t values monotonically increase with the film thickness (Fig. 4d). Particularly, for a film thickness of 4 mm, the maximum EMI SE and SSE_t can reach as high as 85 dB and 2.5 × 10⁵ dB·cm²·g^− 1 at 0.87 THz, respectively. These results evidently demonstrate the exceptional EMW shielding performance of the β₁₂-Br/PDMS composite film.

Afterwards, the mechanism governing the strong THz EMW absorption of the β₁₂-Br/PDMS composite film are further explored. The preceding discussion unambiguously suggests that the incident EMW are efficiently absorbed and dissipated by various β₁₂-Br nanosheets within the composite film. Each single crystalline β₁₂-Br nanosheet supports a high concentration of free electrons with large mobilities. Upon excitation by the THz wave, these free electrons become excited and accelerated by the electric field of the EMW, leading to collective oscillations. However, due to the small planar size of the borophene nanosheets (approximately 3 µm, Fig. 1c), the electrons encounter boundaries and suffer from multiple reflections, ultimately losing their kinetic energies into the lattice of the β₁₂-Br nanosheets, as seen in Fig. 4e. In this manner, nearly all of the absorbed EMW are dissipated as heat, resulting in the ultrahigh EMI absorption effectiveness observed.

This mechanism can be further supported by simulating the dynamics of electrons and the corresponding absorption of EMW energy within a β₁₂-Br nanosheet (Methods and Supplementary Section 5). The behavior of free electrons in β₁₂-Br nanosheet in response to the EMW is governed by the alternating-current conductivity, which can be characterized using the Drude model (Supplementary Figs. 8 and 9). For a disk-shaped sheet with a monolayer thickness, multiple electromagnetic resonances, characterized by strong absorption peaks, can be excited upon irradiation by the THz wave (Supplementary Fig. 10). As shown in Fig. 4f, these resonances induce strong EMW localizations in β₁₂-Br nanosheets, where the associated free electrons become localized and subsequently reflected by the disk boundary (Fig. 4g). Accordingly, lots of Joule loss will occur within the borophene nanosheets (Fig. 4h), which can be further confirmed by THz near-field optical measurements (Method) conducted on an individual β₁₂-Br nanosheet (Fig. 4i). As a result, the Joule loss leads to a very strong absorption efficiency of up to 50% at the resonance, even for a nanosheet with a monolayer thickness (Supplementary Fig. 10).

It is noteworthy that, according to the simulation results, the resonance frequency of the β₁₂-Br nanosheet is strongly dependent on its size and shape, covering a broad spectral range, for example, from 0.2 to 2 THz (Supplementary Fig. 10). However, the maximum absorption efficiency nearly remains unvaried at 50%, irrespective of the geometrical parameters of the borophene nanosheets (Supplementary Fig. 10). Consequently, the broadband yet relatively steady THz EMI absorption shielding performance of the β₁₂-Br/PDMS composite film observed in our study should originate from the containing β₁₂-Br nanosheet with varied thickness and shapes in the composite film.

Interestingly, even after being exposed to air over 15 days, the EMI shielding performances of the β₁₂-Br/PDMS composite film showed no obvious degradation and retained 92.94% of the original performance (Fig. 4j), revealing the high stability of the β₁₂-Br/PDMS film under ambient conditions.

Mechanical properties of the β₁₂-Br/PDMS composite film

Good mechanical property is another critical requirement for THz EMW shielding materials in portable and wearable electronic devices^{5, 38}. By taking advantage of the exceptional Young’s modulus of β₁₂-Br (theoretical value of 163 to 382 GPa)³⁹, the composite film can be easily bent up to a high angle of 180° without any breakage or cracks observed through 500 bending experiments, revealing its excellent flexibility. Further uniaxial tensile experiments were conducted to quantitatively assess the elastic behavior of the composite film with various weight fractions of borophene nanosheets. It is noted that the 0.13 wt.% β₁₂-Br/PDMS composite film exhibits a tensile strain, σ_s, of 50% at a tensile stress, ε_s, of 13 MPa, while that with 2.10 wt.% achieves a σ_s of 158% at ε_s = 33 MPa (Fig. 5a). The corresponding Young’s moduli, E_c = σ_s/ε_s × 100,⁴⁰ were approximately 26 MPa and 21 MPa, respectively. One can obviously see that the mechanical properties of the composite film are gradually enhanced with increasing the weight fraction of borophene, overwhelming those of many other high-modulus 2D materials (Table S2, Supporting Information). Also, this observed trend corroborates that the ultrahigh theoretical Young’s modulus of borophene nanosheets should be responsible for the excellent mechanical properties of the composite film.

THz shielding measurements were then performed on the 2-mm-thickness composite film with 0.5 wt.% β₁₂-Br nanosheets. After 500 bending cycles, the mean EMI SE value of the film slightly decreased from 81 dB to 75 dB, with an average fading rate as low as 0.015% per bending cycle (Fig. 5b). Similarly, the mean THz EMI SE value of the composite film mildly reduced from 78 dB to 71 dB after 500 stretching cycles, with an average fading rate of less than 0.018% per stretching cycle (Fig. 5c). Despite of these tiny attenuation, the sample can recover over 80% of its initial EMI SE efficiency for THz waves after both bending and stretching tests. These findings validate the great potential of β₁₂-Br/PDMS composite film to combine high flexibility with superior mechanical strength, making them promising candidates for advanced flexible electronic applications.

Application demonstration of the β₁₂-Br/PDMS composite film

The discussion above clearly exhibits the excellent THz shielding performance and flexibility of the β₁₂-Br/PDMS composite film, which ensure them highly suitable for applications in EMW shielding of objects with irregular surfaces. To this end, the β₁₂-Br/PDMS composite film was employed to conformably coated onto practical objects to show its excellent EMI SE upon THz EMW illumination. A 1-mm-thickness β₁₂-Br/PDMS composite film was placed on the surface of a dry leaf for THz shielding imaging (Fig. 5d). As observed in Fig. 5e, the THz signal is almost completely absorbed where the composite film covers the leaf, causing the emergence of the shadow for imaging leaf. Additionally, the same composite film was wrapped around a human finger, effectively blocking off the THz signal at the wrapped region. Moreover, the composite film can also be fabricated into different patterns to match the outlines of target objects for demonstrating different characters, as presented in Fig. 5f. It is obviously seen that the contour profile of “Sun Yat-sen University” is very clear and sharp, further showcasing the versatility of the composite film in being processed into EMW shielding materials with arbitrary shapes to meet diverse application requirements.

Borophene, an important member of 2D material family, has attracted much attention since the birth due to its unique electrical and mechanical properties, as well as low mass density. While their applications in energy conversion and storage have been widely studied, their photonic and optoelectronic applications remain unexplored. The main obstacle lies in that it is still a challenging issue to produce freestanding borophene with high yield and high purity out of a variety of different phases. In this study, we demonstrate the successful applications of borophene in THz EMW shielding by developing a facile approach for creating composite film consisted of PDMS filled with single-phase and high-crystallinity few-layer β₁₂-Br nanosheets of high conductivity. Such composite film, which can be scaled up to 5 inches and with tailorable thickness, exhibits an ultrahigh SE and EESt over 70 dB and 4.8 × 10⁵ dB·cm³/g, respectively, with only 0.13 wt.% filling ratio of the β₁₂-Br nanosheets. This successfully circumvents the trade-off between the amount of filling material used and the shielding effectiveness, which usually present in the previous reported EMI shielding materials designed for the THz spectral regime.

Moreover, the remarkable mechanical property of single crystalline β₁₂-Br nanosheets guarantees the composite film not only handily conforms to different object surfaces with various three-dimensional curvatures, but also produces efficient shielding on these objects. One possible application for our flexible composite film is to prevent the EMW pollution in electrical and optoelectrical circuits from the surrounding environment or adjacent neighborhoods in future 6G communication networks or large-scale integrated circuits. Besides, with its low-weight, high-efficiency, good-flexibility, and high-stability, and broadband shielding performances, the composite film based on β₁₂-Br nanosheets could be successfully utilized in a wearable device for daily consumer electronics in the future.

Materials. Boron powders (99.8%, 325 mesh) were purchased from ZhongNuo (China) Co. Ltd. Polydimethylsiloxane (PDMS, SYLGARD 184) and N-methyl pyrrolidone (NMP, 99.8%) was bought from Dow–Corning (USA) Co. Ltd. and Innochem (China) Co. Ltd, respectively.

Sample Fabrication. β₁₂-Br nanosheets can be synthesized using our previously developed LTEP method³³. And PDMS gel was prepared by mixing the base component with the hardener in a weight ratio of 10:1.

The procedures to fabricate β₁₂-Br/PDMS composite film are as follows. Firstly, the as-grown β₁₂-Br nanosheet powders were dissolved into 1 mL deionized water via 30 min of bath ultrasonic dispersion, and subsequently added into the PDMS solvent to form the uniform sol by a 1 hour of continuous stirring. Secondly, the sol was coated on the surface of laboratory dish and sat for 2 days in air to realize the solidification of the gel. Finally, they were handed into the vacuum chamber and treated at 60 ^oC for about 12 hours to remove the residual moisture. Through the above procedures, the fabrication of large-area β₁₂-Br/PDMS composite film was accomplished, as shown in Fig. 1b.

Characterization. The thickness of β₁₂-Br nanosheets was measured by an atom force microscope (AFM, Bruker Dimension Icon). The morphology and crystalline structure of the nanosheets were investigated by a scanning electron microscope (SEM, Zeis Supra 60) and transmission electron microscope (TEM, FEI Titan 80–300). The scanning transmission electron microscope (STEM), energy disperse X-ray (EDX) mapping and electron energy loss spectroscopy (EELS) techniques were carried out in a JEM ARM200F thermal-field emission microscope with a Cs corrector probe working at 300 kV. For the high-angle annular dark field (HAADF) measurement, a convergence angle of about 21 mrad and collection angle range of 65–172 mrad were adopted for the incoherent atomic number imaging. The chemical compositions were analyzed by XRD patterns recorded on a D-MAX 2200 VPC system. The Raman spectrum of the β₁₂-Br nanosheets was obtained by inVia Reflex (532 nm laser) made by Renishaw. And the current-voltage characteristics of the borophene nanosheet were tested in an ultra-high vacuum (UHV) probe made by Wavetest.

Computational model of β₁₂-Br nanosheet. The theoretically model of few-layer β₁₂-Br nanosheet was obtained by the density functional theory (DFT), and more details can be found in our previous report³³.

THz Shielding Measurements. The THz shielding effectiveness of the samples was studied using a THz-TDS (Toptica) system at room temperature under N₂. The samples were attached onto a hollow iron plate for test, and THz wave focused on the sample with a spot radius of 2.5 mm. The EMI SE of the material can be described by decibels (dB) and derived using the following equation:⁵

EMI SE (dB) = $\:\text{-}\text{20}{\text{log}}_{\text{10}}\left(\frac{{\text{E}}_{\text{in}}}{{\text{E}}_{\text{out}}}\right)$ (1)

, where $\:{\text{E}}_{\text{in}}$ and $\:{\text{E}}_{\text{out}}$ denote the incident field strength and transmitted field intensity of THz waves, respectively. The EMI SSE_t of the material can be calculated based on the following equation:¹⁵

EMI SSE_t (dB·cm²·g^− 1) = $\:\frac{\text{EMI SE · }{\text{S}}^{\text{2}}}{\text{m}}$ (2)

, where S and m respectively represent the area of the focal spot of THz wave and the mass of β₁₂-Br nanosheets.

THz Optical Nanoimaging. Optical nanoimaging was conducted on the samples using a scattering-type THz optical microscope (THz-NeaSNOM, Neaspec GmbH). To image the β₁₂-Br nanosheet in real space, a THz laser with tunable frequency from 0.1 to 3 THz was focused onto both the sample and a metal-coated AFM tip (25PtIr200B-H, Rocky Mountain Nanotechnology) with a radius below 20 nm. The back-scattered light from the tip was demodulated and detected at a harmonic higher than that of the tip.

Numerical Simulation. Because β₁₂-Br nanosheet belongs to a metallic 2D material, its conductivity ($\:{\sigma\:}_{jj}$) can be obtained by the Drude model and written as: ^{41, 42}

$$\:{\sigma\:}_{jj}=\frac{i{D}_{j}}{\pi\:\left(\omega\:+\frac{i}{\tau\:}\right)}\:,\:{D}_{j}=\frac{\pi\:{e}^{2}n}{{m}_{j}}$$

, where j represents the x or y direction of the optical axis of β₁₂-Br nanosheet in our paper. In Eq. (3), $\:e,\:n,\:\omega\:,\:\tau\:,\:{D}_{j}$ and $\:{m}_{j}$ stand for electron charge, density of electrons, frequency of excitation, carrier lifetime, Drude weight along x or y direction, and effective electron mass in x and y directions, respectively. Therefore, the real ($\:{\epsilon\:}_{r,jj}$) and imaginary ($\:{\epsilon\:}_{i,jj}$) parts of the complex permittivity along each direction can be derived based on the following equation:

$$\:{\epsilon\:}_{r,jj}={\epsilon\:}_{r}-\frac{{e}^{2}n}{{m}_{j}{\epsilon\:}_{0}h\left({\omega\:}^{2}+\frac{1}{{\tau\:}^{2}}\right)}\:,\:{\epsilon\:}_{i,jj}=\frac{\raisebox{1ex}{${e}^{2}n$}\!\left/\:\!\raisebox{-1ex}{$\tau\:$}\right.}{{m}_{j}{\epsilon\:}_{0}h\omega\:\left({\omega\:}^{2}+\frac{1}{\tau\:}\right)}$$

, where $\:{\epsilon\:}_{r}=\text{11}$ is the relative permittivity, $\:{\epsilon\:}_{0}=\text{8.854}\times\:{\text{10}}^{\text{\--}\text{12}}$ F/m is the vacuum permittivity, and h represents the thickness of β₁₂-Br nanosheet. The effective electron mass (m_x/m_y) of β₁₂-borophene nanosheet along x or y directions are respectively 3.5 m₀ and 3.7 m₀, where $\:{m}_{0}=\text{9.11}\times\:{\text{10}}^{\text{\--}\text{31}}$ kg is the mass of electron²⁵.

Using Matlab R2021b software, the permittivity spectra in the frequency range of 0.1 ̶ 2 THz of β₁₂-Br nanosheets can be calculated according to Eqs. (3) and (4), where the carrier density ranges from 1.0 $\:\times\:$ 10¹⁹ to 5.0 $\:\times\:$ 10¹⁹ m^− 2 and the mean free time of electrons varies from 3 $\:\times\:$ 10^–14 to 12 $\:\times\:$ 10^–14 s. And then, the permittivity spectra were imported into CST software to construct a β₁₂-Br nanosheet. Finally, we carried out the finite-difference time-domain simulations (FDTD, Lumerical Inc.) to simulate the THz absorbance of β₁₂-Br nanosheets.

Data availability

The authors declare that the main data supporting the findings of this study are available within the paper. Extra data are available from the corresponding authors upon reasonable request. Source data are provided with this paper.

Online content

Any methods, additional references, Nature Research reporting summaries, source data, extended data, supplementary information, acknowledgements, details of author contributions and competing interests; and statements of data and code availability are available at https://doi.org/xxxx.

Competing interests

The authors declare no competing interests.

Author contributions

S. D., H. C. and F. L. proposed and supervised the projects. H.L. synthesized the materials, characterized their surface morphology and chemical compositions, and carried out the THz shielding performances measurements. Z.C. and H.Z. simulated the absorption spectra of borophene in THz band by FTDT software. X.W. was responsible for the terahertz imaging of the β₁₂-Br/PDMS composite film. J.W. created a flowchart illustrating the experimental process. All the authors involved in the analysis and discussion of the experimental results. And all authors approve to submit the final version of the manuscript.

Acknowledgements

The authors are very thankful for the support of the National Key Research and Development Program of China (2022YFA1203503), the National Science Foundation of China (Grant No. 51872337), Guangdong Basic and Applied Basic Research Foundation (Grant Nos. 2021A1515012592, 2020B1515020009), the Science and Technology Department of Guangdong Province (Grant No. 2020B1212060030), and Guangzhou Science and Technology Program (Grant 2024A04J6359).

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Harmonizing Material Quantity and Terahertz Wave Interference Shielding Efficiency with Metallic Borophene Nanosheets

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