Strain-induced piezoelectricity in heterostructures combining piezoelectric and non-piezoelectric layers is a phenomenon that arises from the mechanical coupling between dissimilar materials, leading to the generation of electric polarization under applied strain. This effect is particularly pronounced in van der Waals heterostructures, where atomically thin layers are stacked with precise control over their relative orientation and lattice mismatch. Among piezoelectric materials, transition metal dichalcogenides (TMDCs) such as MoS2 exhibit strong piezoelectric responses due to their non-centrosymmetric crystal structure in odd-numbered layers. When integrated with non-piezoelectric layers like graphene or hexagonal boron nitride (hBN), the heterostructure can exhibit enhanced or modulated piezoelectric properties, enabling novel functionalities in energy harvesting and electromechanical sensing.

The piezoelectric effect in TMDCs originates from the breaking of inversion symmetry in monolayer or odd-layered configurations. For instance, monolayer MoS2 possesses a hexagonal lattice with a lack of inversion symmetry, allowing for the generation of in-plane piezoelectric polarization when subjected to uniaxial strain. The piezoelectric coefficient of monolayer MoS2 has been experimentally measured to be approximately 3.73 pm/V, which is comparable to traditional piezoelectric materials like quartz. When combined with non-piezoelectric layers, the strain transfer across the heterostructure interface modifies the overall piezoelectric response. The non-piezoelectric layer acts as a strain modulator, redistributing mechanical stress and influencing the charge separation efficiency in the piezoelectric component.

The strain transfer mechanism in such heterostructures depends on the interfacial adhesion and the mechanical properties of the constituent layers. For example, graphene, with its high Young’s modulus (~1 TPa), can effectively transmit strain to an underlying MoS2 layer, amplifying the piezoelectric output. Conversely, softer non-piezoelectric layers may absorb strain, reducing the effective piezoelectric response. The interplay between these materials can be tuned by varying the stacking order, layer thickness, and interfacial quality. Studies have shown that heterostructures with alternating piezoelectric and non-piezoelectric layers can achieve a balance between mechanical flexibility and piezoelectric efficiency, making them suitable for applications requiring both durability and high energy conversion rates.

Energy harvesting applications benefit significantly from strain-induced piezoelectricity in heterostructures. The ability to convert mechanical energy from ambient vibrations, human motion, or acoustic waves into electrical energy offers a sustainable power source for low-power electronics. Heterostructures combining MoS2 and graphene, for instance, have demonstrated power densities in the range of 10–100 µW/cm² under moderate strain conditions. This performance is attributed to the synergistic effects of graphene’s high carrier mobility and MoS2’s strong piezoelectric coupling, which collectively enhance charge collection and reduce losses. Such systems are particularly promising for wearable electronics, where flexibility and lightweight properties are critical.

The scalability of these heterostructures further enhances their potential for large-area energy harvesting. Techniques like chemical vapor deposition (CVD) enable the growth of uniform TMDC films on flexible substrates, facilitating integration into practical devices. The non-piezoelectric layers often serve dual roles, acting as both strain modulators and conductive electrodes, simplifying device architecture. For instance, a heterostructure of MoS2 and hBN can leverage hBN’s insulating properties to prevent leakage currents while maintaining efficient strain transfer. This design minimizes parasitic losses and maximizes the net energy output.

Environmental stability is another advantage of these heterostructures. Unlike conventional piezoelectric polymers, which may degrade under humidity or high temperatures, TMDC-based heterostructures exhibit robust performance in harsh conditions. Encapsulation with non-piezoelectric layers like hBN further protects the active piezoelectric material from oxidation and mechanical wear, extending the operational lifetime of energy harvesting systems. This resilience is particularly valuable for applications in industrial monitoring or aerospace, where reliability under extreme conditions is paramount.

The frequency response of strain-induced piezoelectric heterostructures is another critical factor for energy harvesting. The inherent flexibility of these materials allows them to resonate at frequencies matching common environmental vibrations, such as those from machinery or human movement. By engineering the layer thickness and stacking sequence, the resonant frequency can be tailored to specific applications. For example, a heterostructure optimized for low-frequency vibrations (1–100 Hz) can efficiently harvest energy from footfalls or wind-induced vibrations, while higher-frequency designs may target acoustic energy harvesting.

In summary, strain-induced piezoelectricity in heterostructures combining piezoelectric and non-piezoelectric layers offers a versatile platform for mechanical energy conversion. The careful selection of materials and precise control over interfacial interactions enable enhanced piezoelectric responses, making these systems suitable for a wide range of energy harvesting applications. Their scalability, environmental stability, and tunable frequency response position them as promising candidates for powering next-generation wearable and autonomous devices. Future advancements in growth techniques and strain engineering will likely further improve their performance, paving the way for broader adoption in sustainable energy technologies.