Hydrophilic‑Substrate‑Driven Self‑Polarization of PVDF for Ultra‑Low Noise Pyroelectric Sensors

Abstract

Polyvinylidene fluoride (PVDF) films are increasingly employed in flexible electronics for their piezo‑, pyroelectric, and ferroelectric capabilities. Conventional fabrication of poled films, however, typically requires multi‑step stretching and high‑temperature poling, limiting scalability and increasing defect density. We present a streamlined casting process that leverages hydrophilic surface modification of glass to induce spontaneous self‑polarization. The resulting films exhibit β‑phase content rising to 76.05 % after 8 h of piranha treatment, and deliver pyroelectric outputs comparable to thermally poled counterparts—all without an external poling field. To further suppress vibration‑induced piezoelectric noise, we integrate the films into a novel bilayer sensor architecture. Compared with a monolayer design, the bilayer sensor achieves a 38 dB signal‑to‑noise ratio (SNR) versus 18 dB, demonstrating a 93 % reduction in piezoelectric interference. These findings open a low‑cost, high‑performance route for wearable infrared and temperature sensing in noisy environments.

Introduction

PVDF and its copolymers have emerged as leading candidates for wearable devices and multifunctional sensors owing to their strong piezo‑ and pyroelectric responses, flexibility, and solution processability [1‑11]. Nonetheless, extracting robust pyroelectric performance remains challenging. Traditional routes involve two distinct steps: (i) mechanical or electro‑mechanical stretching to enrich the β‑phase, and (ii) thermal poling to align the dipole vectors perpendicular to the film surface [12‑20]. This two‑step sequence often yields films with limited active area, high defect density, and poor stability, and it demands meticulous handling to avoid dielectric breakdown [12‑20]. Moreover, the inherent piezoelectricity of PVDF makes monolayer infrared sensors highly susceptible to environmental vibrations, degrading the pyroelectric signal quality.

Recent studies have explored self‑polarization strategies that bypass external poling, such as casting on salt‑laden substrates [21‑25], spin coating [26‑27], Langmuir–Blodgett deposition [28], electrospinning [29‑35], and aqueous salt solutions [36]. While these methods can enhance piezoelectricity, they rarely address pyroelectric performance, and many are constrained to ultra‑thin films or require additional additives. Approaches that incorporate piezoelectric ceramics (e.g., PZT) to cancel vibrational noise are cumbersome and often degrade the dielectric performance of PVDF composites [37‑39]. Consequently, an efficient, scalable route to high‑quality pyroelectric PVDF films and low‑noise sensors is still needed.

In this work, we introduce a simple casting technique on hydrophilically treated glass that not only increases β‑phase content but also induces spontaneous self‑polarization. We further demonstrate a bilayer pyroelectric sensor that suppresses piezoelectric noise by exploiting the opposite signs of the pyroelectric and piezoelectric coefficients of PVDF. This integrated approach offers a promising pathway for robust, low‑cost infrared and temperature sensors suitable for harsh, noisy environments.

Methods

Preparation of the PVDF Film and the Bilayer Pyroelectric Sensor

Figure 1 outlines the fabrication workflow. A glass substrate was first immersed in a piranha solution (H₂SO₄ 98 % + H₂O₂ 30 % in a 7:3 volume ratio) and annealed at 60 °C for 2–8 h to generate a hydrophilic surface rich in Si–OH groups. PVDF powder (average Mw ≈ 5.34 × 10⁵ g mol⁻¹, Sigma‑Aldrich) was dispersed in N‑methyl‑pyrrolidone (NMP, 99 % purity) at 10 wt % and stirred at 50 °C for 4 h until a homogeneous solution was obtained. The solution was cast onto the treated substrate and dried at 80 °C for 10 h to remove NMP. To minimize edge effects, a 10 mm × 10 mm central area was cut from a 50 mm × 50 mm film, yielding a 50 µm‑thick sample. Aluminum electrodes (≈ 100 nm) were evaporated on both sides for electrical measurements.

Illustration of PVDF film and device preparation process. Step 1, the glass substrate was soaked in piranha solution for 2–8 h. Step 2, well‑stirred PVDF solution was cast on the substrate and dried at 80 °C for 10 h. Step 3, the PVDF film was peeled off from the substrate, and the edge was cut off to remove edge effect. Step 4, aluminum was evaporated onto both sides of the film as electrodes. Step 5, the bilayer device was fabricated by using PDMS pillars supported between the two layers as separators. Also indicated were schematics of the hydroxyl groups bonded on the surface of the glass substrate after treatment, hydrogen bonds formation after PVDF casting and orderly arrangement of the “ultra‑thin layer” at the bottom of PVDF film

Holes with diameters of 1 mm were created in a 1‑mm‑thick acrylic plate using a high‑power laser (type 4060, Ketai). Polydimethylsiloxane (PDMS) pillars were cast into the holes by mixing the base and curing agents at a 10:1 weight ratio and curing at 60 °C for 10 h. The bilayer sensor was assembled by gluing two polarized PVDF films with five pillars using an adhesive (type 810, LEAFTOP).

Physical Characterization and Testing Method

Contact angle (CA) was measured with a JC2000D1 meter to quantify substrate hydrophilicity. Fourier‑transform infrared (FTIR) spectroscopy (NICOLET 6700) assessed phase composition. Differential scanning calorimetry (DSC 7020) quantified crystallinity. Scanning electron microscopy (SEM Inspect F50) examined surface morphology. Electric displacement–electric field (D–E) curves were recorded with a HVI40904‑523 analyzer. Dielectric constants were obtained with a 4294A impedance analyzer. For pyroelectric measurement, a custom setup based on electrical modulation was used: a 980‑nm pulsed laser was driven by a square‑wave generator, and the generated current was amplified and recorded on a DSOX3012A oscilloscope. Piezoelectric responses were measured with a vibrator driven by a similar square‑wave generator.

Results and Discussion

The PVDF Film

Figure 2a shows that the CA of glass substrates decreases with immersion time, saturating after 8 h, indicating increased hydrophilicity due to more Si–OH groups. This trend correlates with a higher peeling force (inset). DSC data (Fig. 2b) reveal that the 8‑h treated film’s crystallinity rises by >50 % compared to untreated samples. FTIR spectra (Fig. 2c) display a progressive shift from the α‑phase peak at 764 cm⁻¹ to the β‑phase peak at 840 cm⁻¹; calculated β‑phase content reaches 76.05 % after 8 h—about 50 % higher than untreated films.

a CA of the glass substrates treated in piranha for different time, inset is the peeling force as a function of treatment time. b DSC pattern of PVDF samples. c FTIR spectra of PVDF samples, inset is β‑phase content as a function of treatment time calculated from the FTIR results. d Pyroelectric response of PVDF samples without undergoing thermal poling, inset is the simplified schematics of homemade signal readout circuit

Remarkably, the treated films exhibit a clear pyroelectric current—absent in untreated controls—indicating that the hydrophilic surface not only promotes β‑phase formation but also induces spontaneous dipole alignment. Comparative poling experiments (Fig. 3b) show that the treated film’s polarization is equivalent to a conventional thermal poling field of ~23 MV m⁻¹, while reverse poling can almost fully cancel the induced dipoles, confirming the bottom‑up relay mechanism driven by hydrogen bonding at the film/substrate interface (Fig. 3a). This bottom‑to‑top alignment relays through successive sub‑nanolayers, culminating in a uniformly polarized 50 µm film.

The Bilayer Pyroelectric Sensor

To mitigate piezoelectric noise, we devised a bilayer architecture where two identical PVDF films are separated by five PDMS pillars. The upper film acts as the temperature‑sensitive element, while the lower film serves as a vibration‑compensating reference. Finite‑element simulations (COMSOL Multiphysics) confirm that the pillars provide excellent thermal isolation (dT/dt in the lower layer remains flat) and mechanical coupling that ensures nearly identical piezoelectric responses in both layers. Experimental measurements (Fig. 4e,f) demonstrate that the bilayer sensor’s piezoelectric noise drops from ~0.5 V (monolayer) to ~0.05 V, while the pyroelectric signal remains ~4.1 V, yielding an SNR of 38 dB compared to 18 dB for the monolayer. This 93 % noise reduction validates the efficacy of the bilayer design for operation in acoustically noisy environments.

Simulation and measurement results of the bilayer‑structured pyroelectric sensor. a Explored schematic of the device structure. b Model and results of piezoelectric response simulation. c Model and results of thermal simulation. d Optical photo of the fabricated device. e Piezoelectric response at different frequencies. f Responses of the bilayer and conventional monolayer devices when simultaneously stimulated by mechanical vibration (5 Hz) and thermal irradiation (1 Hz)

Conclusions

We have demonstrated a facile, scalable casting method that employs hydrophilic glass treatment to self‑polarize PVDF films. The process elevates β‑phase content to 76 % and eliminates the need for external poling, while producing robust pyroelectric output. Coupling these films into a bilayer sensor architecture suppresses piezoelectric noise by ~93 % and maintains a high SNR of 38 dB, far surpassing conventional monolayer designs. This strategy paves the way for low‑cost, high‑performance infrared and temperature sensors suitable for wearable and harsh‑environment applications.

Abbreviations

CA

Contact angle

D‑E

Electric displacement‑electric field

DSC

Differential scanning calorimeter

FTIR

Fourier‑transform infrared

LB

Langmuir‑Blodgett

NMP

N‑methylpyrrolidone

PDMS

Polydimethylsiloxane

PVDF

Polyvinylidene fluoride

PZT

Lead zirconate titanate

SEM

Scanning electron microscope

SNR

Signal‑to‑noise ratio