Dynamic Pyrophototronic Effect in 2D Devices

• Dynamic pyrophototronic effect is a phenomenon where time-varying temperature changes in materials with inherent or strain-induced polarization generate transient pyroelectric currents.
• It couples thermal transients with photoresponses, as demonstrated in strained WS2 phototransistors and ferroelectric α-In2Se3 devices, to enhance sensitivity and switching speed.
• Leveraging strain engineering and polarization control, device designs integrate pyroelectric and photovoltaic sensing for low-light detection and self-powered memory applications.

Searching arXiv for the cited papers and related pyrophototronic work. Dynamic pyrophototronic effect denotes a dynamic optoelectronic response in which a time-varying temperature perturbs polarization and launches a transient pyroelectric current while illumination simultaneously sustains a photoinduced current. In two-dimensional materials, this effect has been demonstrated in a strain-polarized 5-layer WS$_2$ phototransistor, where modulated light produces a four-stage current waveform through the coexistence of pyroelectric and photocurrent contributions, and in ferroelectric $\alpha$-In$_2$Se$_3$, where the instantaneous pyroelectric component $J_{pyro}=p\,dT/dt$ coexists with a temperature-dependent short-circuit photovoltaic current (Chakrabarty et al., 8 Jul 2025, Uzhansky et al., 2023). The defining feature is its dynamic character: it is expressed under optical or thermal modulation rather than only under steady-state illumination, and it becomes particularly prominent when the population of free carriers is low.

1. Physical basis and defining equations

The effect rests on the temperature dependence of polarization in materials that either possess spontaneous ferroelectric polarization or acquire strain-induced piezoelectric polarization. In odd-layer WS$_2$, which is non-centrosymmetric, biaxial strain $\varepsilon$ induces a piezoelectric polarization that can be written in simplified scalar form as $P_{piezo}=d\,\varepsilon$. The corresponding piezocharge at the WS$_2$/dielectric interface modifies the electrostatic potential and augments the internal field that separates free carriers (Chakrabarty et al., 8 Jul 2025).

In ferroelectric $\alpha$-In$\alpha$0Se$\alpha$1, the polarization $\alpha$2 depends sensitively on temperature. A time-varying temperature therefore induces a pyroelectric current density

$\alpha$3

where $\alpha$4 is the pyroelectric coefficient. Under steady illumination, the same in-plane polarization produces an internal in-plane electric field that drives a bulk photovoltaic current even at zero external bias. The dynamic pyrophototronic effect is thus the superposition of an instantaneous pyroelectric component proportional to $\alpha$5 and a more slowly varying photovoltaic component whose magnitude is modulated by temperature (Uzhansky et al., 2023).

In the strained WS$\alpha$6 phototransistor, the total dynamic responsivity is expressed as

$\alpha$7

with

$\alpha$8

This formulation emphasizes that the dynamic contribution is not simply an extension of steady photodetection; it depends on both thermal transients and the electrostatic operating point (Chakrabarty et al., 8 Jul 2025).

2. Material platforms and device realizations

Chakrabarty et al. implemented the effect in an ultra-thin 2D WS$\alpha$9 phototransistor fabricated on a p$_2$0 Si/300 nm SiO$_2$1 substrate, with a local bottom gate of Ti/Au (5 nm/20 nm), an exfoliated hBN gate dielectric, and a 5 L WS$_2$2 channel of approximately 3 nm thickness transferred by a PDMS stamp. The patterned device region, denoted Tp, used arrays of 450 nm-diameter holes with 15 nm depth in hBN, while the control region, denoted TNP, used non-patterned hBN. Source and drain contacts were Ti/Au (5 nm/100 nm), and the two transistors shared source and gate while retaining separate drains. Conformal deposition of WS$_2$3 over the patterned hBN induced a biaxial tensile strain of approximately 0.19% (Chakrabarty et al., 8 Jul 2025).

That strain state was verified by multiple probes. Raman spectroscopy showed a shift of the $_2$4 mode of approximately $_2$5 cm$_2$6, photoluminescence exhibited a red-shift of approximately 20 meV, piezo-force microscopy revealed butterfly loops and $_2$7 phase switching, and density functional theory calculations indicated a strain-induced dipole moment increase up to approximately 45% for 5 L material at $_2$8 strain. AFM confirmed conformal draping and uniform strain, Raman and PL mapping showed enhanced intensity and spectral shifts in Tp relative to TNP, piezoresponse force microscopy signals were absent in TNP, and cross-section HRTEM / HAADF-STEM showed a clean WS$_2$9/hBN interface without cracks (Chakrabarty et al., 8 Jul 2025).

The $_3$0-In$_3$1Se$_3$2 realization used a back-gated field-effect transistor based on an exfoliated flake of thickness in the few tens of nanometers on p$_3$3-Si/300 nm-SiO$_3$4. Source and drain electrodes were Cr (5 nm)/Au (50 nm), patterned by e-beam lithography, with channel length of approximately 2–5 $_3$5m. The degenerately doped p$_3$6-Si substrate served as back gate and was used both for pyroelectric measurements with the gate grounded and for non-volatile polarization switching using gate pulses up to $_3$7 V. Temperature control was provided by a Linkam heating stage with a K-type thermocouple, optical excitation by a broadband white LED spanning 420–720 nm at approximately 332 $_3$8W/cm$_3$9, and cryogenic measurements by a Lakeshore TTPX probe station with liquid-N$J_{pyro}=p\,dT/dt$0-based cooling rates up to 40 K/min (Uzhansky et al., 2023).

These two platforms are microscopically distinct: WS$J_{pyro}=p\,dT/dt$1 relies on strain-polarized non-centrosymmetric semiconducting layers, whereas $J_{pyro}=p\,dT/dt$2-In$J_{pyro}=p\,dT/dt$3Se$J_{pyro}=p\,dT/dt$4 relies on room-temperature stable ferroelectricity and the bulk photovoltaic effect. This suggests a broader materials class in which dynamic pyrophototronic behavior can arise whenever polarization, temperature transients, and photoinduced carrier generation are strongly coupled.

3. Dynamic response phenomenology

In the strained WS$J_{pyro}=p\,dT/dt$5 phototransistor, light modulated on and off at 1 Hz yields a four-stage dynamic response (Chakrabarty et al., 8 Jul 2025):

• Stage I (steady-dark): $J_{pyro}=p\,dT/dt$6, polarization is static, bound charges screen the piezo-dipoles, and only dark current $J_{pyro}=p\,dT/dt$7 flows.
• Stage II (light-on transient): $J_{pyro}=p\,dT/dt$8 warms the WS$J_{pyro}=p\,dT/dt$9, thermal vibrations disorder the strain-induced dipoles, spontaneous polarization decreases, bound carriers are released, and a sharp pyroelectric current $_2$0 flows in the same direction as the external drain bias.
• Stage III (steady-illumination): temperature stabilizes, $_2$1, $_2$2, and only photocurrent $_2$3 remains.
• Stage IV (light-off transient): $_2$4 cools the WS$_2$5, polarization recovers above its dark value, bound charge re-forms, and carriers flow in the reverse direction to produce an opposite $_2$6 pulse before returning to $_2$7.

The associated band-diagram sequence is described as a polarized dark band profile with bound carriers, a reduced band tilt during heating that releases those carriers, a steady illuminated state with photocarriers, and an enhanced band tilt during cooling that recaptures carriers (Chakrabarty et al., 8 Jul 2025). The unstrained TNP device lacks this built-in piezo-field and therefore lacks the dynamic pyropeak.

In $_2$8-In$_2$9Se$\varepsilon$0, the phenomenology is framed as coupled pyroelectric-photovoltaic transduction. Under dark heating from 30 $\varepsilon$1C to 40 $\varepsilon$2C over approximately 60 s, sharp pyroelectric current peaks of approximately 160 pA directly follow $\varepsilon$3. In a liquid-N$\varepsilon$4-cooled environment, both heating and cooling spikes are observed, and $\varepsilon$5 scales linearly with $\varepsilon$6 from $\varepsilon$7 K/s to $\varepsilon$8 K/s. Under continuous white-LED illumination, the short-circuit current is approximately 1 nA at room temperature and decreases to approximately 0.5 nA at 65 $\varepsilon$9C because the in-plane polarization weakens on heating (Uzhansky et al., 2023).

A central implication of both studies is that dynamic pyrophototronic response contains two observables at once: a transient, derivative-like signal associated with $P_{piezo}=d\,\varepsilon$0 and a steady or quasi-steady optical signal associated with illumination and polarization-controlled carrier separation. In the ferroelectric case this dual-mode behavior is explicit; in the WS$P_{piezo}=d\,\varepsilon$1 case it appears as the coexistence of transient pyrospikes and steady photocurrent.

4. Quantitative performance and operating regimes

The strained WS$P_{piezo}=d\,\varepsilon$2 device was developed for low optical power photodetection under 600 nm pulsed illumination at 1 Hz. At $P_{piezo}=d\,\varepsilon$3 V and $P_{piezo}=d\,\varepsilon$4 V, the steady-state responsivity of Tp was approximately 15.8 A/W at $P_{piezo}=d\,\varepsilon$5 nW, compared with approximately 0.35 A/W for TNP, corresponding to an approximately 45$P_{piezo}=d\,\varepsilon$6 boost attributed to enhanced light trapping. The dynamic responsivity including $P_{piezo}=d\,\varepsilon$7 peaked at approximately 0.7 A/W at $P_{piezo}=d\,\varepsilon$8 pW, which was approximately 8$P_{piezo}=d\,\varepsilon$9 larger than in the unstrained device. The report further states a specific detectivity of approximately $_2$0 Jones at 600 nm, rising to approximately $_2$1 Jones/cm under shot-noise-limited analysis, and rise/fall times reduced from approximately 70 ms in TNP to approximately 20 ms in Tp, corresponding to a 3$_2$2 speed improvement (Chakrabarty et al., 8 Jul 2025).

The same study established that the dynamic contribution is strongly regime-dependent. In wavelength sweeps from 450 to 900 nm at 1 nW, $_2$3 peaked near 660 nm and then fell in the sub-band-gap regime, whereas $_2$4 was nearly wavelength-agnostic in the visible range because it was thermally driven. In gate-voltage sweeps from $_2$5 V to $_2$6 V at 600 nm and 1 nW, $_2$7 maximized in the near-off regime, for example around $_2$8 V, while for $_2$9 threshold the large dark current and Joule heating masked the pyroelectric signal. In power sweeps from 50 pW to 3 nW, $\alpha$0 below approximately 1 nW, whereas above approximately 1 nW the photocurrent dominated and the pyroelectric contribution fell off (Chakrabarty et al., 8 Jul 2025).

In $\alpha$1-In$\alpha$2Se$\alpha$3, the directly measured pyroelectric coefficient was approximately 30.7 mC/m$\alpha$4K and the figure of merit approximately 135.9 m$\alpha$5/C. The short-circuit photocurrent had a temperature coefficient of approximately $\alpha$6 pA/K in the 30–40 $\alpha$7C range. Applying $\alpha$8 V, 30 s back-gate pulses non-volatility switched the in-plane polarization and flipped the sign of the short-circuit current on each light pulse, yielding an ON/OFF ratio of approximately $\alpha$9 with stable endurance over many cycles (Uzhansky et al., 2023).

Taken together, these metrics delineate two complementary operating windows. In the WS$\alpha$00 platform, the dynamic pyrophototronic contribution is strongest under weak optical excitation and near-off electrostatic bias; in the $\alpha$01-In$\alpha$02Se$\alpha$03 platform, the emphasis is on a large pyroelectric coefficient, zero-bias photovoltaic readout, and non-volatile reconfiguration of the current polarity.

5. Mechanistic interpretation and design rules

In strained WS$\alpha$04, the operative mechanism is a built-in piezo-field produced by biaxial tensile strain in a non-centrosymmetric channel. In the dark, carriers are bound to polarization charge and conduction is suppressed. Upon heating during illumination, the polarization decays, those carriers are released, and a transient pyroelectric current appears. The unstrained device lacks this polarization term and therefore lacks the corresponding dynamic pyropeak. The paper identifies several conditions that maximize the effect: biaxial tensile strain of approximately 0.1–0.3%, nanopatterned hBN with depth of approximately 15 nm and feature size of approximately 450 nm, gate bias near the off-state to minimize $\alpha$05, low incident power below 1 nW so that thermal transients dominate over steady photocarriers, and modulation frequency within the WS$\alpha$06 thermal time constant of approximately 20–70 ms (Chakrabarty et al., 8 Jul 2025).

The same work proposes device-level guidelines for low-light and spike-triggered sensing. These include use of a sub-wavelength patterned dielectric under a few-layer TMD channel to induce uniform biaxial strain, use of an ultra-thin odd-layer WS$\alpha$07 channel to exploit both piezoelectricity and high optical absorption, adjustment of gate bias so that the channel is just off in the dark, and a differential pair of identical Tp devices in which one is biased for pyro-sensitive operation and the other for purely photo-sensitive operation. A transimpedance plus subtractor circuit is then used to convert transient $\alpha$08 pulses into voltage spikes for real-time alert systems (Chakrabarty et al., 8 Jul 2025).

In $\alpha$09-In$\alpha$10Se$\alpha$11, the mechanism is framed differently but leads to an analogous coupling. Because the in-plane ferroelectric polarization weakens on heating, the short-circuit photovoltaic response decreases linearly with temperature in the 30–40 $\alpha$12C range according to $\alpha$13. At the same time, the temperature ramp itself produces the pyroelectric current. The result is simultaneous access to a static current proportional to the thermal state and a differential current proportional to its temporal derivative. The study further notes that no external amplifiers or biasing networks are needed for this dual-mode sensing mode (Uzhansky et al., 2023).

A plausible implication is that the phrase “dynamic pyrophototronic effect” encompasses at least two microscopic implementations: a strain-polarized semiconducting version in which transient heating releases carriers bound by piezoelectric charge, and a ferroelectric-photovoltaic version in which spontaneous polarization governs both pyroelectric displacement current and self-powered photocurrent.

6. Functional roles, interpretive nuances, and research directions

The most immediate application emphasized for the WS$\alpha$14 platform is event-based low-light detection. Gate tunability of the pyrophototronic current was leveraged to design an optical spike-triggered dynamic accident alert system with speed-specific control for self-driving applications under low light conditions. The principal device-level significance is the reported path to ameliorating the responsivity-speed trade-off in 2D photodetectors, since the same strained transistor exhibited enhanced dynamic responsivity and faster switching than the unstrained control (Chakrabarty et al., 8 Jul 2025).

In $\alpha$15-In$\alpha$16Se$\alpha$17, the coupled effect enables self-powered in-memory logic, integrated thermal and optical sensing, and a route to monolithic thermal-optical-energy devices. Because each pixel can store a digital or analog weight through non-volatile polarization reversal and be read out under illumination without external bias, the work identifies a photovoltaic memory mode in addition to sensing. The same current channel yields a static photocurrent for absolute-temperature photometric sensing and a transient pyroelectric pulse for differential thermal detection. The authors also note that, because the bulk photovoltaic effect does not rely on a p–n junction and can in principle generate photovoltages exceeding the bandgap, coupling it to a large pyroelectric coefficient opens a route to co-harvest solar and waste-heat energy in a single ultrathin monolithic device (Uzhansky et al., 2023).

Several interpretive nuances recur across the reported measurements. First, the transient signal should not be conflated with ordinary steady-state photocurrent: in strained WS$\alpha$18, the pyroelectric contribution is nearly wavelength-agnostic in the visible range, unlike the photocurrent which peaks near the band edge. Second, stronger conduction does not necessarily strengthen the dynamic effect: in the near-on regime, Joule heating and large dark current can mask $\alpha$19. Third, the absence of a visible opposite-sign current on ambient cooling in $\alpha$20-In$\alpha$21Se$\alpha$22 does not imply the absence of pyroelectricity, because the cooling rate is slower and a residual thermally generated photovoltaic base current screens the negative pyroelectric current; under faster cryogenic cooling, both heating and cooling spikes are recovered (Chakrabarty et al., 8 Jul 2025, Uzhansky et al., 2023).

The present demonstrations also delimit the current state of the field. In WS$\alpha$23, the effect has been shown under 1 Hz optical modulation and sub-nanowatt optical powers in a strain-engineered transistor geometry. In $\alpha$24-In$\alpha$25Se$\alpha$26, the proof-of-concept temperature steps were on the order of seconds because they were set by the Linkam stage, although the intrinsic thermal time constant of the atomically thin ferroelectric is expected to lie in the micro- to millisecond range once integrated with high-speed microheaters or laser heating (Uzhansky et al., 2023). This suggests that future work will likely focus on faster thermal actuation, tighter control of polarization landscapes, and architectures that preserve the differential pyroelectric signal while maintaining a stable optical baseline.