Self-Powered Pyroelectric Sensing in Compositionally Graded Relaxor Thin Films

Pyroelectricity continues to be a significant area of research, driven in part by its promising applications including in thermal sensing and imaging and waste-heat energy conversion. Research efforts are toward improving the pyroelectric coefficient (π) and reducing the dielectric constant (ε) for energy-efficient thermal sensing. (1-x)PbMg_1/3Nb_2/3O₃-(x)PbTiO₃ [PMN-xPT], a canonical relaxor ferroelectric, exhibits large π (~ -550 μC/m²K under applied fields) as compared to ferroelectric materials such as LiNbO₃ (π~ -80 μC/m²K and PbZr_xTi_1-xO₃ (π~ -200 μC/m²K¹). Due to the lack of remanent polarization, PMN-xPT requires an external bias to induce polarization, increasing energy consumption in pyroelectric-sensing applications. Therefore, developing routes to "internally bias" such relaxor ferroelectric thin films is crucial for next-generation pyrosensors.

Here, we demonstrate large induced remanent polarization at zero bias by introducing built-in electric fields (~240 kV/cm) through compositional and strain gradients in relaxor ferroelectric thin films. This enables the operation of these materials for pyroelectric sensors without the need for an externally applied bias. Large strain gradients arise from engineered compositional gradients in PMN-xPT thin films and lattice mismatch with substrates. These combined effects contribute to the creation of large built-in potentials in the films, which enhance remanent polarization and suppress dielectric responses at zero bias, improving the figure of merit (π/ε) for pyrosensors. Specifically, we investigate the evolution of polarization, dielectric, and pyroelectric properties of 100-nm-thick compositionally graded PMN-xPT (x = 0.64 to 0.32) heterostructures grown via pulsed-laser deposition on DyScO₃, GdScO₃, and NdGaO₃ (110) substrates. Films grown on GdScO₃ substrates exhibited the highest built-in fields (~240 kV/cm) as compared to DyScO₃ (~217 kV/cm) and NdScO₃ (~53 kV/cm), as indicated from shifts in their polarization hysteresis loops. Such internal bias leads to a substantial reduction in the dielectric constant at zero bias, with the heterostructures grown on GdScO₃ (ε_r= 370) showing a 64% decrease as compared to the dielectric constant of heterostructures grown on NdScO₃ (ε_r=1014). Pyroelectric measurements were conducted using direct measurement techniques based on microfabricated electrothermal approaches. The maximum π for films grown on NdScO₃, DyScO₃, and GdScO₃ substrates were -228 μC/m²K, -147 μC/m²K, and -22 μC/m²K, respectively. We also employed pyrovoltage (open-circuit) detection, where the signal is proportional to the figure of merit (π/ε_r), showing that heterostructures grown on GdScO3 substrates enhanced the figure of merit by a factor of ~27 compared to NdScO3. To demonstrate their potential in thermal sensing, we fabricated thin-film based pyroelectric sensors from the graded thin films and evaluated their thermal sensitivity for detecting minimal temperature changes and broadband operation for frequency-dependent responsivity. Among the heterostructure variants, the graded films grown on GdScO₃ substrates showed the ability to detect temperature oscillation as small as ~17 mK in photovoltage detection mode. Compared to commercial LiNbO₃ pyrosensors, which are typically in bulk form with thicknesses ranging from 10s-100s micrometers, thin-film-based pyroelectric sensors provide several distinct advantages including smaller thermal mass (i.e., higher thermal sensitivity), and faster thermal equilibrium with its surroundings (allowing for broadband operation and higher sensing capabilities at heating frequencies up to 20 kHz). Ultimately the utilization of composition and strain gradients across the film thickness effectively induce remanent polarization in PMN-xPT relaxor ferroelectric thin films at zero bias, advancing the development of miniaturized pyrosensors with enhanced sensing sensitivity and energy efficiency.