Direct Piezoresistive Self-Sensing of Electromechanical Motion in Suspended Nanostructures

Nanoelectromechanical systems (NEMS) offer superior mechanical performance. They are better-sensing devices than their forerunner, microelectromechanical systems (MEMS), which have penetrated our everyday lives as various sensors in our mobile devices and gadgets. Because of their extreme sensitivity and high mechanical compliance, NEMS shows the potential roadmap towards futuristic sensing technologies.

One of the main challenges in NEMS resonators is the efficient read-out of nanoscale mechanical motion. In this regard, purely electrical transduction is a promising avenue to explore due to its potential for ON-chip implementation. However, as device dimensions shrink, the signal associated with the device operation becomes progressively weaker. Hence, in NEMS resonator read-out, it becomes challenging to distinguish the target signal from the noisy background originating from a plethora of sources.

Therefore, to improve the signal-to-noise ratio (SNR), frequency-mixing and lock-in detection, technically known as the "1-omega" method, is widely used in NEMS resonators, especially those incorporating low-dimensional materials like graphene and carbon nanotubes. Implementing these mixing measurements requires coherent control over circuit parameters, necessitating challenging configurations, which can be a hassle.

This study introduces a mixing technique that is much simpler to implement compared to existing techniques, reducing the requirements of complex circuit setups. The intrinsic piezoresistivity of the material for the vibrating structural layer in the NEMS resonators is leveraged here to generate a signal that quadratically scales with the vibration. Consequently, reading out this specific signal through a lock-in amplifier requires minimal circuit load. This technique has been called the direct piezoresistive method.

The possibility of implementing this technique can be traced to modeling the modulation of the conductance of a vibrating membrane, which has a noticeable piezoresistive character. A nanofabricated reduced graphene oxide-based NEMS resonator was used for the experimental validation. Through a synergy of theory, COMSOL-based simulations, and experiments, a specific piezoresistive component could be utilized that reduces circuit load and improves the SNR considerably compared to existing methods like the 1-omega method.

This self-sensing method maintains a stable electrical background, meaning that the motion of the structure does not interfere with the baseline signal, unlike the 1-omega method, leading to more precise measurements. It also shows that the direct piezoresistive method doesn’t introduce any effect of phase mismatch between the mixing signals at the output. It allows for easy discernment of the resonance parameters and characteristics, a significant advantage over the 1-omega technique. This method is especially beneficial for material systems where capacitive transconductance (essential in the 1-omega method) is weak, making the technique more holistic and allowing for a wide-ranging material selection for NEMS.

The direct piezoresistive method offers a versatile solution to detecting nanoscale vibrations in NEMS resonators. It introduces the key advantage of not requiring complex signal processing or specialized materials, making it widely applicable across different nanostructures. With fewer components, precise measurements of vibrational displacement in nanoscale resonators can be accomplished with relatively facile control. This simple yet effective technique contributes to the ongoing effort to develop more efficient sensing technologies for nanoscale applications.