Acoustic sensors leveraging piezoelectric two-dimensional materials such as molybdenum disulfide (MoS2) and hexagonal boron nitride (hBN) represent a significant advancement in microelectromechanical systems (MEMS). These sensors exploit the intrinsic piezoelectric properties of atomically thin layers to detect mechanical vibrations, translating them into measurable electrical signals. Their high sensitivity, miniaturization potential, and compatibility with semiconductor fabrication processes make them ideal for applications ranging from ultrasonic imaging to industrial nondestructive testing.

The operational principle of these sensors relies on the direct piezoelectric effect, where mechanical strain induces a polarization charge in the material. In 2D piezoelectric materials, this effect is highly pronounced due to their reduced dimensionality and lack of inversion symmetry in certain crystal phases. For instance, monolayer MoS2 in the 1H phase exhibits strong in-plane piezoelectricity, with coefficients measured at approximately 3.73 pm/V for single-layer configurations. Similarly, hBN, though less studied, demonstrates out-of-plane piezoelectric responses under specific stacking arrangements. These properties enable the design of resonant acoustic sensors with tunable frequency responses, dictated by material thickness, geometry, and clamping conditions.

Resonant frequency response is a critical parameter for piezoelectric 2D material-based acoustic sensors. The resonant frequency (f) of a clamped circular diaphragm, for example, can be approximated by the relation f = (2.4048 / (2πa)) * sqrt(D / ρh), where a is the radius, D is the flexural rigidity, ρ is the mass density, and h is the thickness. For a monolayer MoS2 membrane with a radius of 10 µm, the fundamental resonance typically falls in the range of 10–100 MHz, making it suitable for high-frequency ultrasonic applications. The quality factor (Q) of these resonators is influenced by material damping and environmental conditions, with values exceeding 1000 in vacuum for optimized structures. The integration of such membranes into MEMS platforms enhances their performance by providing precise mechanical support and electrical interfacing.

MEMS integration of 2D piezoelectric materials involves several fabrication challenges and solutions. One common approach is the transfer of pre-synthesized monolayers onto prefabricated MEMS cavities using polymer-assisted techniques. Alternatively, direct growth via chemical vapor deposition (CVD) on sacrificial substrates allows for better alignment and strain control. Electrode patterning, typically using gold or graphene contacts, ensures efficient charge collection while minimizing parasitic capacitances. The use of silicon or silicon nitride substrates provides mechanical stability and thermal management, critical for maintaining sensor performance under operational stresses. Advanced encapsulation methods, such as atomic layer deposition (ALD) of Al2O3, protect the sensitive 2D layers from environmental degradation without significantly affecting their piezoelectric response.

In ultrasonic imaging, these sensors offer distinct advantages over conventional piezoelectric ceramics like lead zirconate titanate (PZT). Their sub-nanometer thickness enables ultra-wide bandwidth detection, capable of resolving features smaller than the wavelength of the incident sound. For instance, a MoS2-based transducer with a center frequency of 50 MHz can achieve a bandwidth exceeding 80%, facilitating high-resolution imaging of biological tissues at micrometer scales. Additionally, their biocompatibility and flexibility allow for conformal attachment to curved surfaces, enabling endoscopic and catheter-based imaging probes. Experimental studies have demonstrated the feasibility of detecting ultrasound waves with pressures as low as 1 mPa/√Hz, rivaling the sensitivity of bulk piezoelectric detectors.

Beyond medical imaging, these sensors find utility in industrial and environmental monitoring. Their high resonant frequencies and low power consumption make them suitable for gas sensing, where acoustic wave propagation changes due to mass loading by adsorbed molecules. In underwater acoustics, arrays of 2D piezoelectric sensors can achieve directional sensitivity with minimal crosstalk, enhancing sonar systems' resolution. The absence of lead in these materials also addresses growing regulatory restrictions on hazardous substances in electronic devices.

Future developments in this field are likely to focus on improving material synthesis to achieve larger-area, defect-free monolayers with consistent piezoelectric properties. Heterostructures combining multiple 2D materials, such as MoS2/graphene hybrids, could further enhance sensitivity and multifunctionality. Advances in nanofabrication techniques will enable more complex MEMS architectures, including multi-resonator arrays for parallel signal processing. The integration of machine learning algorithms for real-time signal analysis could unlock new applications in predictive maintenance and structural health monitoring.

In summary, acoustic sensors based on piezoelectric 2D materials represent a transformative technology with broad applicability in high-frequency sensing. Their unique combination of mechanical and electrical properties, coupled with MEMS compatibility, positions them as a promising alternative to traditional piezoelectric systems. Continued research into material optimization and device integration will further expand their capabilities, paving the way for next-generation ultrasonic and vibrational sensing platforms.