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1. Electrically Insulating Flexible Films with Quasi‐1D van der Waals Fillers as Efficient Electromagnetic Shields in the GHz and Sub‐THz Frequency Bands

   https://doi.org/10.1002/adma.202007286  

   Barani, Zahra; Kargar, Fariborz; Ghafouri, Yassamin; Ghosh, Subhajit; Godziszewski, Konrad; Baraghani, Saba; Yashchyshyn, Yevhen; Cywiński, Grzegorz; Rumyantsev, Sergey; Salguero, Tina_T; et al (February 2021, Advanced Materials)

   Abstract Polymer composite films containing fillers comprising quasi‐1D van der Waals materials, specifically transition metal trichalcogenides with 1D structural motifs that enable their exfoliation into bundles of atomic threads, are reported. These nanostructures are characterized by extremely large aspect ratios of up to≈10⁶. The polymer composites with low loadings of quasi‐1D TaSe₃fillers (<3 vol%) reveal excellent electromagnetic interference shielding in the X‐band GHz and extremely high frequency sub‐THz frequency ranges, while remaining DC electrically insulating. The unique electromagnetic shielding characteristics of these films are attributed to effective coupling of the electromagnetic waves to the high‐aspect‐ratio electrically conductive TaSe₃atomic‐thread bundles even when the filler concentration is below the electrical percolation threshold. These novel films are promising for high‐frequency communication technologies, which require electromagnetic shielding films that are flexible, lightweight, corrosion resistant, inexpensive, and electrically insulating. 

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2. Highly Stretchable Polymer Composite with Strain‐Enhanced Electromagnetic Interference Shielding Effectiveness

   https://doi.org/10.1002/adma.201907499  

   Yao, Bin; Hong, Wei; Chen, Tianwu; Han, Zhubing; Xu, Xinwei; Hu, Renchao; Hao, Jianyu; Li, Changhao; Li, He; Perini, Steven_E; et al (February 2020, Advanced Materials)

   Abstract Polymer composites with electrically conductive fillers have been developed as mechanically flexible, easily processable electromagnetic interference (EMI) shielding materials. Although there are a few elastomeric composites with nanostructured silvers and carbon nanotubes showing moderate stretchability, their EMI shielding effectiveness (SE) deteriorates consistently with stretching. Here, a highly stretchable polymer composite embedded with a three‐dimensional (3D) liquid‐metal (LM) network exhibiting substantial increases of EMI SE when stretched is reported, which matches the EMI SE of metallic plates over an exceptionally broad frequency range of 2.65–40 GHz. The electrical conductivities achieved in the 3D LM composite are among the state‐of‐the‐art in stretchable conductors under large mechanical deformations. With skin‐like elastic compliance and toughness, the material provides a route to meet the demands for emerging soft and human‐friendly electronics. 

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3. Role of Surface-Grafted Polymers on Mechanical Reinforcement of Metal–Organic Framework–Polymer Composites

   https://doi.org/10.1021/acsapm.3c01195  

   Yang, Xiaozhou; Hu, Yongtao; Bonnett, Brittany L; Blosch, Sarah E; Cornell, Hannah D; Gibbons, Bradley; Santos, Claudio Amaya; Ilic, Stefan; Morris, Amanda J (October 2023, ACS Applied Polymer Materials)

   Utilizing metal–organic frameworks (MOFs) as reinforcing fillers for polymer composites is a promising strategy because of the low density, high specific modulus, and tunable aspect ratio (AR). However, it has not been demonstrated for the MOF-reinforced polymer composite using MOFs with high AR and polymer-grafted surface, both of which are extremely important factors for efficient load transfer and favorable particle–matrix interaction. To this end, we designed an MOF–polymer composite system using high AR MOF PCN-222 as the mechanical reinforcer. Moreover, we developed a synthetic route to graft poly(methyl methacrylate) (PMMA) from the surface of PCN-222 through surface-initiated atomic transfer radical polymerization (SI-ATRP). The successful growth of PMMA on the surface of PCN-222 was confirmed via proton nuclear magnetic resonance and infrared spectroscopy. Through thermogravimetric analysis, the grafting density was found to be 0.18 chains/nm2. The grafted polymer molecular weight was controlled ranging from 50.3 to 158 kDa as suggested by size exclusion chromatography. Finally, we fabricated MOF–polymer composite films by the doctor-blading technique and measured the mechanical properties through the tension mode of dynamic mechanical analysis. We found that the mechanical properties of the composites were improved with increasing grafted PMMA molecular weight. The maximum reinforcement, a 114% increase in Young’s modulus at 0.5 wt % MOF loading in comparison to pristine PMMA films, was achieved when the grafted molecular weight was higher than the matrix molecular weight, which was in good agreement with previous literature. Moreover, our composite presents the highest reinforcement measured via Young’s modulus at low weight loading among MOF-reinforced polymer composites due to the high MOF AR and enhanced interface. Our approach offers great potential for lightweight mechanical reinforcement with high AR MOFs and a generalizable grafting-from strategy for porphyrin-based MOFs. 

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4. Ultrathin MXene film interaction with electromagnetic radiation in the microwave range

   https://doi.org/10.1063/5.0176575  

   Rakhmanov, Roman; Shuck, Christopher Eugene; Al_Hourani, Jamal; Ippolito, Stefano; Gogotsi, Yury; Friedman, Gary (November 2023, Applied Physics Letters)

   The quick progress in communication technologies demands superior electromagnetic interference (EMI) shielding materials. However, achieving a high shielding effectiveness (SE) with thin films, which is needed for microscale, flexible, and wearable devices, through absorption of EM radiation remains a challenge. 2D titanium carbide MXene, Ti3C2Tx, has been shown to efficiently reflect electromagnetic waves. In this paper, we investigated the electromagnetic shielding of ultrathin printed Ti3C2Tx films and recorded absorption up to 50% for 4 nm-thick films. This behavior is explained by impedance matching. Analysis of the sheet impedance in the X-band frequency range allows us to correlate the EMI shielding mechanism with the electrical conductivity measured within the same range. The average bulk in-plane conductivity for 4 to 40 nm-thick films reaches 106 S/m, while the average relaxation time is estimated at around 2.3 ps. Our figures of merit are similar to those reported for ultrathin metal films, such as gold, showing that an abundant MXene material can replace noble metals. We demonstrate that the MXene conductivity mechanism does not change from direct current to THz. The conventional method of reporting EMI SE is correlated with absolute values of transmitted, reflected, and absorbed power, which allows us to interpret previous results on MXene EMI shielding. Considering the easy deposition of thin MXenes films from solution onto a variety of surfaces, our findings offer an attractive alternative for shielding microscale devices and personal electronics. 

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5. Screening for electrically conductive defects in thin functional films using electrochemiluminescence

   https://doi.org/10.1039/D3AY00687E  

   Quinn, Harley; Wang, Wenlu; Werner, Jörg G; Brown, Keith A (July 2023, Analytical Methods)

   Multifunctional thin films in energy-related devices often must be electrically insulating where a single nanoscale defect can result in complete device-scale failure. Locating and characterizing such defects presents a fundamental problem where high-resolution imaging methods are needed to find defects, but imaging with high spatial resolution limits the field of view and thus the measurement throughput. Here, we present a novel high-throughput method for detecting sub-micron defects in insulating thin films by leveraging the electrochemiluminescence (ECL) of luminol. Through a systematic study of reagent concentrations, buffers, voltage, and excitation time, we identify optimized conditions at which it is possible to detect sub-micron defects at high-throughput. Extrapolating from the signal to background observed for detecting 440 nm wide lines and 620 nm diameter circles, we estimate the minimum detectable features to be lines as narrow as 2.5 nm in width and pinholes as small as 70 nm in radius. We further explore this method by using it to characterize a nominally insulating phenol film and find conductive defects that are cross-correlated with high-resolution atomic force microscopy to provide feedback to synthesis. Given this assay’s inherent parallelizability and scalability, it is expected to have a major impact on the automated discovery of multifunctional films. 

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