Scientists from Europe’s Graphene Flagship have discovered the ability of graphene to provide effective microwave shielding, opening the door to use this two-dimensional material for reduction of microwave pollution and improvement of electromagnetic compatibility of upcoming nanoscale electronic devices.
Microwave communication is an ultra-high frequency wireless communication, enabling efficient data transmission with electromagnetic waves in the range of tens of gigahertz and establishing data links between the ground stations and Earth-orbiting satellites.
This kind of communication has now become very common in the modern world, thus facing problems such as interference and electromagnetic compatibility (EMC) due to crowding of the spectral bands assigned to different communications channels.
Regulations related to EMC demand the use of new instruments to address rigorous prerequisites regarding microwave shielding of both systems and components. As a result, new materials are being explored for use in future nanoelectronic devices as filters, shields, and coating layers.
The use of barriers that only reflect incident microwave radiation for shielding electronic devices are not adequate to address the electromagnetic pollution. Hence, EMC coatings capable of absorbing microwaves instead of reflecting are the subject of research, with a practical focus on layers having a thickness below a thousandth of a millmeter.
According to a research team headed by Philippe Lambin from the Université de Namur in Belgium, it is possible to achieve an efficient absorbent shield against microwaves with a graphene plane. Polina Kuzhir and Konstantin Batrakov from the Belarussian State University in Minska are the major contributors to the study.
The authors of the study are part of the Graphene Flagship, which is a consortium consisting of industrial and academic partners with a special focus on addressing the key scientific and technological challenges of Europe through long-term, multidisciplinary research initiatives.
The Lambin team showed the arithmetic increase of the conductivity of a number of graphene layers when they are separated by thin polymer spacers. By separating six graphene planes by PMMA (poly-methyl methacrylate) layers, the researchers were able to achieve optimum microwave absorption in the Ka communications band of 26.5-40GHz.
Scientists at the University of Eastern Finland in Joensuu have constructed multilayer microwave barriers by depositing a graphene layer at first on a copper foil substrate with chemical vapor deposition. A PMMA spacer of 600-800nm formed by spin coatings is then used to cover the graphene layer.
This is followed by etching away the copper with ferric chloride and transferring of the resulting graphene/PMMA heterostructure to a quartz substrate. The aforementioned steps are repeated until reaching the required number of graphene layers.
A single graphene layer is capable of absorbing up to 25% of incoming microwave radiation - a considerably higher performance for a one atom-thick material. It is possible to absorb up to 50% with a multilayer graphene/PMMA configuration.
This can be theoretically explained through the analysis of the reflection and transmission of a plane wave at the interface consisting of an ultra-thin conducting layer between two dielectric media. This approach helped the researchers to improve their graphene-PMMA structures for optimum absorption. The study results were also corroborated with stringent electromagnetic testing.
Lambin points out that the interface between air and the shielding material must also be considered. “We have found that the static conductivity of graphene is close to the value which relates the magnetic and electric fields in any electromagnetic radiation propagating in air. Thanks to this happy coincidence, graphene is an ideal material for absorbing radio waves, thus protecting sensitive electronic devices.”
Employing graphene/dielectric multilayers to absorb electromagnetic waves is not a new concept. For instance, a study proposed an ultra-broadband absorbing multilayer that operates in the terahertz region a few years ago. This operating range is much higher when compared to the Ka communications band described by the Graphene Flagship team.
Constructing a multilayer terahertz shield is a challenging task due to patterning of graphene planes at the micron scale for generation of surface plasmon resonances, which are oscillations of electrons propagating along the interfaces between various material layers. Conversely, the microwave barrier designed by the Lambin team is very simple with respect to fabrication and scalability.
Protection from external mechanical and chemical agents is essential for graphene/PMMA multilayers in practical applications. Hence, it is necessary to the quartz substrate to be faced outwards and coupled with a softer material. Material selection and thickness are the other aspects affecting microwave absorbance.
Deposition of stacks of few-layer graphene in a single step will increase process scalability significantly when compared to piling up of graphene monolayers with PMMA shuttles. Furthermore, any method increasing the conductivity of graphene will lower the count of atomic planes needed to optimize the microwave absorption level.
The study results have been featured in the Nature Scientific Reports journal.