Graphene electrodes revolutionize the scaling of piezoelectric NEMS resonators

(Nanowerk Spotlight) Tiny sensors are becoming ubiquitous in an increasing number of applications for smart mobile devices, automotive, healthcare and environmental monitoring. Recent advancements in sensor miniaturization, low power consumption and low cost can be seen as a harbinger for a new era of sensing in which the data collected from multiple individual smart sensor systems are combined to get information about the environment that is more accurate and reliable than the individual sensor data.

By leveraging such sensor 'fusion' it will be possible to acquire accurate, real-time and almost complete information about the environments in which humans live, work and play. This technology provides a huge potential for the development of the Internet of Things in which physical and virtual objects are linked through the application of sensing and communication capabilities.

In this context, micro and nano-electromechanical systems (NEMS) are important for several advanced applications for instance in radio frequency (RF) wireless communications, sensors and switches. The two principal components common to most electromechanical systems irrespective of scale are a mechanical element and transducers. The mechanical element either deflects or vibrates in response to an applied force. Depending on their type, the mechanical elements can be used to sense static or time-varying forces. The transducers in MEMS and NEMS convert the mechanical energy into electrical signals and vice versa.

Due to the intrinsically high electromechanical transduction efficiency in piezoelectric materials, many MEMS and NEMS applications are driven by on-chip piezoelectric actuation and sensing of high frequency vibration in miniaturized free-standing micro- and nanomechanical structures.

In conventional piezoelectric MEMS and NEMS resonators, a significant amount of energy is lost due to the damping effect induced by the metal electrodes attached to the resonant body of the device: the metal electrodes, commonly used today, dampen the vibration of the resonators through their mass, as an acoustic guitar mute dampens the vibration (sound) of the guitar strings.

Therefore, designing ideal electrodes that simultaneously guarantee low mechanical damping and electrical loss as well as high electromechanical coupling in ultra-low-volume piezoelectric nanomechanical structures has become an important research goal in the NEMS field.

A team of researchers at Northeastern University, led by Matteo Rinaldi, demonstrated that this fundamental challenge can be addressed by using an atomically thin graphene layer as an electrode.

"The key challenges associated with the development of high performance MEMS and NEMS resonators for RF wireless communication and sensing applications are the isolation of energy-dissipating mechanisms and scaling of the device volume in the nanoscale size-range," Rinaldi, an assistant professor in the Department of Electrical and Computer Engineering at Northeastern University, tells Nanowerk. "We show that a virtually massless and strainless single-atom-thick sheet of carbon, closely mimics an 'ideal electrode' for piezoelectric nanoelectromechanical resonators. This allows graphene-electrode based piezoelectric NEMS resonators to operate at their theoretical 'unloaded' frequency-limits, with significantly improved electromechanical performance compared to metal-electrode counterparts, despite their reduced volumes."

Rinaldi and his colleagues reported their findings in Nano Letters ("Graphene as a Massless Electrode for Ultrahigh-Frequency Piezoelectric Nanoelectromechanical Systems").

A false-color SEM image of the NEMS resonator. A freestanding aluminum nitride (AlN) nanoplate (the central rectangular green plate), released from the underlying silicon substrate, is supported mechanically at two ends. A bottom metal interdigitated electrode (shown in dark gray underneath the AlN nanoplate), connected to the two electrical terminals of the device, and a top electrically floating graphene electrode (the gray rectangular area on top of AlN nanoplate), are employed for the piezoelectric actuation and sensing of a high frequency lateral-extensional mode of vibration in the nanoplate. The zoomed-in views highlight the boundary of the patterned graphene electrode on the anchor region of the device and the honeycomb lattice of the atomically thin and virtually massless graphene electrode. (Image: Zhenyun Qian)

"While a metal electrode cannot be arbitrarily thinned to reduce its mass and damping, graphene is electrically conductive even as an atomically thin and virtually massless electrode," says graduate student Zhenyun Qian, the lead author on the paper.

"The new electrodes also benefit from the fact that – unlike conventional metal electrodes that form chemical bonds with underlying substrates – the mechanically transferred graphene remains attached through weak van der Waals interactions, and virtually 'floats' over the piezoelectric layer with minimal mechanical interactions," says Swastik Kar, an assistant professor in the Department of Physics at Northeastern University, and a collaborator on the project.

This, along with the two-dimensional (2D) massless nature of graphene, creates an effective mechanical isolation of the electrode from the resonator.

"In fact" says Rinaldi, "our experiments indicate that the graphene electrode does not induce any mechanical perturbation to the vibrating piezoelectric aluminum nitride (AIN) nano-plate and in this sense it behaves as a mechanically 'isolated' electrode: the graphene electrode does not change the mass of the resonator and it dissipates less acoustic energy than a conventional metal electrode attached to the resonator."

He points out that, despite this effective mechanical 'isolation' of the graphene electrode, the team's experiments show that the electromechanical coupling of graphene-electrode devices remains similar to that of conventional metal-electrode devices, highlighting the extraordinary result that a single atom sheet of carbon is as effective in confining the radio frequency electric field within the AlN membrane as a 100 nm thick metal film.

The researchers also demonstrate the remarkable way in which the mechanical resonance of these graphene-electrode piezoelectric NEMS devices can be completely quenched by chemical modification of the graphene electrode conductivity – i.e. fluorination – which has great potential for ultra-sensitive molecular detection.

Furthermore, using CVD-grown graphene, they demonstrate scalability at the wafer-level making the technology suitable for mass production.

"Thanks to the unique features demonstrated in our paper, we expect this technology to lead to a new paradigm for high-performance, miniaturized, power efficient sensing and radio frequency wireless communication systems," says Rinaldi.

He gives two potential application examples:

1) Radio frequency filters used in mobile communication devices

Micro acoustic piezoelectric resonators are the dominant technology in the market for RF filters used in mobile communication devices. Multiple wireless communication standards such as WiFi, LTE, GSM, are nowadays used in mobile communication and each of them uses a small section (band) of the spectrum of radio communication frequencies. Therefore, filtering of frequencies is crucial for the functionality of modern multi-band radios.

A micro-acoustic piezoelectric filter uses the mechanical resonance of a vibrating piezoelectric micro-structure to pass a range of signal frequencies, but to block others: thanks to the piezoelectric effect this mechanical vibration is converted into electrical signal and vice versa. Therefore, only the electrical signal matching the mechanical resonance frequency of the micro-structure will pass.

The selectivity of the filter (precision of signal separation) depends on the quality factor, Q, of the resonator which is a measure of the damping acting on the vibrating micro-mechanical structure. High Q means low damping, hence good selectivity.

"We demonstrate that by replacing a conventional metal electrode with a virtually massless graphene electrode, the mechanical quality factor of the vibrating structure is improved without any loss of electromechanical coupling. Therefore, the use of a massless graphene electrode could potentially lead to RF filters with improved signal separation precision," notes Rinaldi.

"Advanced micro-acoustic piezoelectric filter designs could be implemented by staking few layers of heavily doped graphene to form virtually massless electrodes with sheet resistance values reaching ∼1 Ω per square or below," adds Kar.

2) High resolution chemical, physical and biological sensors

The physical and electrical properties of conventional metal electrodes fundamentally limits the volume scaling of piezoelectric resonators which is instead highly desired for multiple sensing applications: reducing the volume of the resonator makes it extremely sensitive to targeted external perturbations (molecules, infrared radiation, magnetic field, depending on the particular design) but typically volume scaling is associated with a deterioration of Q, the electrode damping effect is even more significant at nano scale due to the fact metal electrode cannot be arbitrarily thinned.

"We demonstrate that graphene can address this fundamental issue," notes Rinaldi. "In fact we show that graphene-electrode based piezoelectric nanoelectromechanical resonators can operate at their theoretically 'unloaded' frequency-limits with significantly improved electromechanical performance compared to metal-electrode counterparts, despite their reduced volumes."

He adds that this represents a spectacular trend inversion in the scaling of piezoelectric electromechanical resonators, opening up new possibilities for the implementation of nanoelectromechanical resonant sensors with unprecedented performance.

Going forward, the team's vision is to develop a new class of 2D-materials-coupled NEMS that can potentially have an impact in a variety of advanced sensing, detection, and communication applications.