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Graphene enables the smallest, most sensitive sensors

Posted By Graphene Council, Saturday, September 19, 2020
We interview Peter Steeneken, from Graphene Flagship partner TU Delft, and Leader of the Graphene Flagship Sensors Work Package, on the advantages of graphene and related materials in the development of sensing devices – particularly NEMS. NEMS stands for nanoelectromechanical systems: a class of miniaturised devices that detect stimuli like air pressure, sound, light, acceleration or the presence of gases and chemical compounds.

NEMS production methods resemble those of the manufacture of classic transistors, so they can achieve similar production costs and widespread commercialisation. The Graphene Flagship is integrating graphene and related materials in NEMS. Keep reading to discover the future of miniaturised sensing!

"Graphene allows for ultimate force sensitivities in high-performance pressure sensors, microphones and accelerometers."- Peter Steeneken, Graphene Flagship 'Sensors' Leader

What exactly are NEMS sensors?

The NEMS acronym, meaning nanoelectromechanical systems, comprises a family of electric and electronic devices with nanometric dimensions that are mechanically movable. In the Graphene Flagship Sensors Work Package, we are mostly interested in NEMS sensors, which can measure air pressure, sound, light intensity, acceleration, or the presence of gases. To measure such forces you need motion, so movable parts are essential for NEMS.

Currently, MEMS (NEMS' micrometric 'big' cousins) have similar functionalities and are already produced in high volumes – up to billions of MEMS sensors per year – for devices like smartphones. Since they are produced using similar methods as CMOS electronics, they can be made small and with low production costs, which has accelerated their widespread commercialization.

NEMS are nanoscale devices – much smaller devices than classic MEMS. Their smaller size has several advantages: NEMS have higher sensitivity, and many of them can be placed on the same area that would be taken up by a single MEMS sensor. Moreover, NEMS are potentially cheaper, because they need less material to make, so more sensors can be produced from a single silicon wafer. The nanometric size of NEMS also enables new sensing functionalities. For instance, NEMS can even detect individual molecules and count them.

What innovative features does graphene bring to the NEMS field?

Since graphene is only one atom thick, it is the thinnest NEMS device-layer one can imagine. In terms of mechanical properties, graphene is stiff yet very flexible – suspended graphene can be deflected out-of-plane, allowing for ultimate force sensitivities in high-performance pressure sensors, microphones and accelerometers.

At the same time, graphene membranes are very robust. By tensioning graphene like a guitar string, its spring constant can be tuned and engineered to the desired value. The high electrical conductivity of graphene is also advantageous in electrical actuation, needed to provide the readout of sensors.

Although graphene is impermeable to gases in its pristine form – something that can be essential for pressure sensors – we can also tailor it with small pores and make it permeable or semi-permeable for gases and liquids, enabling completely new sensing functions. Compared to other types of thicker membranes, fluids can permeate at higher rates through graphene, which enables faster and lower power operation of sensing and separation devices. During the last years, the feasibility and potential of graphene for realizing novel and improved graphene NEMS sensors has become more apparent, as we describe in a recent review.1

"Graphene sensors could also increase our safety, [...] warn us in case of poor ventilation or remind us to wear a mask." - Peter Steeneken, Graphene Flagship 'Sensors' Leader

Graphene is one material in a huge family – can other layered materials be applied to NEMS devices as well?

Certainly. MEMS devices already use combinations of materials in the suspended layers: electrical conductors, semiconductors, insulators, optical and magnetic active layers, as well as piezoresistive and electric layers for sensing and actuation. We envisage that similar suspended heterostructures might be realised in NEMS by combining different types of layered materials.

We have already shown NEMS that use layered materials with high piezoresistive constants and others that showcase resistances that make them very sensitive to changes in gas compositions. Another approach for NEMS sensors would be to cover graphene with thin functionalisation layers, enabling new types of gas and biosensors as outlined in a recent focus issue edited by Arben Merkoci, from Graphene Flagship partner ICN2, Spain, and member of the Sensors Work Package.2

What are the applications of graphene-based NEMS sensors?

There is a wide range of applications that can be targeted. We could replace sensors in our mobile phones by smaller, more sensitive devices. These will allow better indoor navigation, thanks to acceleration and pressure sensors and directional low-noise microphones.

Graphene sensors could also increase our safety: our phone could warn us in case of poor ventilation, detecting increased CO2 levels in the environment – or remind us to wear a mask, if it senses that air pollution reaches dangerous thresholds. Beyond, high-end laboratory instruments, such as scanning probe microscopes, might also benefit from the flexibility of graphene.2

"With graphene, we could replace sensors in our smartphones by smaller, more sensitive devices." -  Peter Steeneken, Graphene Flagship 'Sensors' Leader

For you, which is the most exciting application of graphene for sensing?

I am excited about creating sensor platforms by combining multiple graphene sensors together. By making new combinations, sensors can become more selective and undesired crosstalk can be eliminated. Moreover, by combining the output of multiple sensors, we can extract more information about our environment.

For gas sensors, the combination of outputs provides a "fingerprint" of gas composition. Similarly, by combining outputs of accelerometers, pressure sensors, magnetometers, and microphones, we can deduce if someone is walking, biking, climbing stairs or driving a car.

I believe that some of the most exciting and impactful new applications of these graphene sensors will be in the medical domain: by developing graphene sensor platforms that can help us better detect and diagnose diseases. In fact, one of the latest Graphene Flagship spin-offs, INBRAIN Neuroelectronics, will design graphene-based sensors and implants to optimise the treatment of brain disorders, such as Parkinson's and epilepsy. Moreover recently, the production of graphene biosensors has advanced, and Graphene Flagship partner VTT, in Finland, already sells CMOS integrated multiplexed biosensor matrices for testing and development purposes.

Are graphene-enabled NEMS ready to jump onto the market?

During the last few years, we showed that graphene NEMS sensors can outperform current commercial MEMS sensors in several aspects. To get to the market, we need to show that graphene sensors can outperform current products in all aspects – including high-volume reliable production at a competitive cost.

To achieve this, more development is needed. The push of the Graphene Flagship towards industrialisation and large-scale manufacturing, will accelerate the NEMS sensors entry into the market. 

Just like MEMS, graphene NEMS have benefited from established CMOS fabrication methods, which facilitate high-volume low-cost production. Introducing a new material into a CMOS factory often takes between five and ten years of development.

These advances are achieved through international and multidisciplinary collaboration. In fact, the Graphene Flagship Sensors Work Package comprises a collaborative endeavour between industry and academia: Chalmers University of Technology (Sweden), ICN2, ICFO, Graphenea (Spain), RWTH Aachen, Bundeswehr University of Munich, Infineon Technologies (Germany), University of Tartu (Estonia), VTT (Finland) and TU Delft (Netherlands) - all Graphene Flagship partners.

With the support of the European Commission, the Graphene Flagship will soon start setting up set up an experimental pilot line to integrate graphene and related layered materials in a semiconductor platform. This will not only accelerate graphene device fabrication, but also accelerate the development of new graphene-enabled devices, providing an identical repeatable device fabrication flow.

Tags:  Arben Merkoci  Graphene  Graphene Flagship  ICN2  INBRAIN Neuroelectronics  Peter Steeneken  Sensors  TU Delft 

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Nanoelectromechanical sensors based on suspended 2D materials

Posted By Graphene Council, Wednesday, August 26, 2020
An international team of researchers have recently published a review article on nanoelectromechanical (NEMS) sensors based on suspended two-dimensional (2D) materials in the journal Research ("Nanoelectromechanical Sensors Based on Suspended 2D Materials"), an open-access multidisciplinary journal launched in 2018 as the first journal in the Science Partner Journal (SPJ) program.

The paper is an invited contribution to a special issue on “Progress and challenges in emerging 2D nanomaterials – preparation, processing, and device integration”, and has the purpose of contributing to the development of the field of 2D materials for sensor applications and to their integration with conventional semiconductor technology.

“I believe NEMS sensors based on 2D materials will be essential for satisfying the demand for integrated, high-performance sensors set by applications such as the Internet of Things (IoT) and autonomous mobility”, says Lemme, first author of the paper.

The review summarizes the many studies that have successfully shown the feasibility of using membranes of 2D materials in pressure sensors, microphones, mass and gas sensors – explaining the different sensor concepts and giving an overview of the relevant material properties, fabrication routes, and operation principles.

“Two-dimensional materials are ideally suited for sensors”, says Lemme, “as they allow realizing free-standing structures that are just one of a few atoms thick. This ultimate thinness can be a decisive advantage when it comes to nanoelectromechanical sensors, since the performance often depends critically on the thickness of the suspended part. Furthermore, many 2D materials have unique electrical, mechanical and optical properties that can be exploited for completely new concepts of sensor devices.”

The review – which includes contributions from RWTH Aachen University, AMO GmbH, Universität der Bundeswehr Munich, KTH Royal Institute of Technology, TU Delft, Infineon and the Kavli Institute of Nanoscience – discusses the different readout and integration methods of different sensors based on 2D materials, and provides comparisons against the state of the art devices to show both the challenges and the promises of 2D-materials based nanoelectromechanical sensing.

“Proof-of-concept sensor devices based on suspended 2D materials are almost always smaller than their conventional counterparts, show improved performances, and sometimes even completely novel functionalities”, says Peter G. Steeneken, leader of work-package 6 (Sensors) in the Graphene Flagship and co-author of the paper. “However, there are still enormous challenges to demonstrate that 2D material-based NEMS sensors can outperform conventional devices on all important aspects – for example, the establishment of high-yield manufacturing capabilities. The Graphene Flagship represents the ideal platform to address these challenges, as it fosters collaborations between world-leading groups to achieve a set of well-defined goals. This paper is an example of how, by bringing together complementary expertise, we can achieve more.”

Tags:  2D materials  AMO GmbH  Graphene  Graphene Flagship  Infineon  Kavli Institute of Nanoscience  KTH Royal Institute of Technology  Max Lemme  RWTH Aachen University  Sensors  TU Delft  Universität der Bundeswehr Munich 

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