Electrochemically Modulated Interaction of MXenes with Microwaves

Dynamic control of electromagnetic wave jamming is needed for protecting electronic devices working at gigahertz frequencies. We describe a method for modulating reflection and absorption of electromagnetic waves using micron-thick films of 2D carbides (MXenes).
Published in Materials
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We frequently tune the sound volume and screen brightness of our TVs and personal electronic devices. We can deem light in the room and adjust brightness of our desk lamps to achieve comfortable illumination. Dynamic control of interactions between infrared light and materials is being explored for adaptive thermal management in functional textiles. We can change the transparency and tint of electrochromic windows when flying a Boeing Dreamliner with a push of a button. Can we achieve the same level of active control with radio waves? Why can’t we modulate the reflection and absorption of microwaves the same way we do it for light or sound waves? This would allow one to turn on and off communication ability and protection of electronic devices.

Electromagnetic interference (EMI) shielding is essential for protecting electronic components against electromagnetic jamming, as the number of wireless communication devices, particularly operating at gigahertz frequencies, in industrial and private space has increased by orders of magnitude in the past decades. Dynamic control of electromagnetic wave jamming is a significant technological challenge for protecting these electronic devices and a variety of other communication technologies. It’s potentially possible to vary permeation of radio waves through a compressible foam material. Changing its density by compression and stretching can change the reflection and absorption of microwaves, enabling tunable EMI shielding capability. However, the required foam thickness of several millimeters hinders their application in integrated electronics. Mechanical compression or stretching is also impractical for controlling material properties and functions in majority of applications.

Tuning the response of electromagnetic waves with thin films of nanomaterials or nanostructures, aimed at developing highly integrated and adaptive shielding, is of particular interest. This tunability, however, is challenging when the wavelength is much longer than the size scale of nanomaterials, i.e., microwaves have the wavelength in the millimeter range. To date, thin EMI shielding coatings only offer ‘static’ protection. Alternatively, electrical control of the interactions between electromagnetic waves and materials has been proven effective in optoelectronic and infrared devices. Yet, a key feature of current EMI shielding materials is high conductivity, like in conventional metals, such as aluminum, nickel and copper, and carbon materials (graphene, nanotubes, carbon fibers, etc.), which leads to reflection of incident waves. Generally, the electrical conductivity of metals is not affected by external stimuli. Thus, no active control is possible. The charge density can be adjusted in graphene through the insertion of ions into the interlayer space but is limited in the microwave range for EMI shielding due to the graphene’s inert and hydrophobic surfaces, as well as electrical double-layer (EDL) charge storage mechanism. Also, the shielding ability of reduced graphene oxide, which can be processed into films required for shielding applications, is far from optimal. Hence, active manipulation of the interaction between millimeter- and centimeter-length waves and thin films remains elusive due to these fundamental challenges.

MXenes, a large family of two-dimensional (2D) transition metal carbides and nitrides, have emerged as promising EMI shielding materials with higher shielding effectiveness than metals and carbons and the ability to produce micron-thin freestanding films. While MXenes have high density of states at the Fermi level, their conductivity and other optoelectronic properties are tunable, like in semiconductors. Transition metal layers, such as titanium in Ti3C2, facilitate electron transport and a large redox-active oxide/hydroxide-like surface of MXenes terminated by oxygen and hydroxyl groups {Ti3C2O2 or Ti3C2(OH)2} enable high-rate redox charge storage. The surface redox reaction accompanied by ion intercalation leads to electron transfer and a change of the oxidation state of the transition metal in the surface layer of MXene accompanied by the change of interlayer spacing, which further affects the electrical conductivity of MXene electrodes. Hence, the combination of metallic conductivity and redox charge storage renders MXenes unique in their response to incident EM waves and offers a possibility to regulate EMI shielding behavior.

In our article Electrochemically Modulated Interaction of MXenes with Microwaves in Nature Nanotechnology (https://www.nature.com/articles/s41565-022-01308-9), we introduce electrochemically modulated EMI shields made from MXene films with aqueous gel electrolytes (Fig. 1a), which enable reversible modulation of EMI shielding capability. We selected five common MXenes (V2CTx, Ti2CTx, Ti3C2Tx, V4C3Tx, and Nb4C3Tx; Tx represents the surface groups) for this study (Fig. 1b). They represent various MXene structures, have different colors and a wide range of conductivities. The intercalation of ions into MXene layers was achieved by applying a small potential (under 1 V) to the MXene electrodes (Fig. 1c). Conversely, the ions deintercalated during discharge, making the operation reversible and cyclic. A cross-sectional scanning electron microscopy image of a thin (about 30 times thinner than a human hair) MXene film shows the well-aligned 2D flakes (Fig. 1d). The films were produced by a simple and scalable spray coating process, just like painting a wall with a spray gun, but using a dispersion of MXene flakes in water. 15×15 cm2 flexible MXene-based shields were fabricated, but there is no size limit for this technology.

Fig. 1. Electrochemically modulated EMI shielding behaviors of MXene films. a, Schematic of the shield, which consists of MXene electrodes on a PET substrate and an electrolyte-containing polymer membrane. b, Digital photos of MXene electrodes. c, Illustration of ions intercalation between MXene layers for tuning EMI shielding. d, Scanning electron microscope image of the cross-section of a MXene film, showing the aligned layers.

Fig. 2 shows how the reversible tunability of EMI shielding is realized by electrochemically driven ion intercalation and de-intercalation; this results in charge transfer accompanied by expansion and shrinkage of the MXene layer spacing. The effect was stable over more than 500 cycles. Bidirectional tunability of shielding efficiency is achieved with a continuous transition from EDL to redox charge storage in MXene electrode films with aqueous electrolytes within a 1 V window. Particularly, the surface redox processes in MXene electrodes, accompanied by a change in the oxidation state of the transition metal, ion intercalation, and interlayer spacing expansion/contraction, change the ratio of absorption to reflection and total EMI shielding efficiency. We systematically performed in situ measurements with several MXenes and electrolytes to verify the effects of electric double layer and redox behaviors on the reflection and absorption of electromagnetic waves, offering an insight into the fundamental understanding of the interactions of microwaves with thin MXene films and building a platform for developing adaptive EMI shields with dynamic regulation. Different from static EMI shielding, our method offers opportunities to achieve active modulation that can adapt to demanding environments. The demonstrated control of reflection and absorption of centimeter-scale electromagnetic waves in the X-band (8.2-12.4 GHz, the most common radar frequency range) by micrometer-thin MXene films provides a realistic approach toward adaptive electromagnetic protection.

Fig. 2. Illustration of the mechanisms of electrochemically modulated EMI shielding: the charge transfer, oxidation state change, and layer spacing change.

Furthermore, we demonstrate an EMI shielding alertor through electrochemical oxidation of MXene films. This is a single-use electrical ‘switch’ with a transition from EMI shielding film to electromagnetic wave-transparent film, which is achieved through irreversible anodic oxidation of the MXene electrode.

The developed method for modulating the reflection and absorption of incident electromagnetic waves using various submicron-thick MXene thin films is powerful and versatile, but the control of electromagnetic wave response at gigahertz frequencies can be further optimized by modification of MXenes and confined electrolytes. The combination of varied electrical conductivities and redox-active transition metal atoms makes electrochemically controlled MXene shields promising for fundamental studies aimed at understanding the interaction of nanometer-scaled MXene sheets with centimeter-scale electromagnetic waves. Also, the observed effect of tunable shielding can be used for developing new microwave technologies. For example, object identification can be achieved with the use of active MXene shields having varied electromagnetic signals. Furthermore, we envision that our concept can be combined with metasurfaces by patterning MXene electrodes to achieve precise local control of reflection, absorption, and transmission of microwaves in thin films, which can be an exciting direction to explore.

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