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. 2026 Mar 26;8(7):2714–2724. doi: 10.1021/acsaelm.6c00115

Zero-Bias Photodetection and Opto-Synaptic Plasticity in BP/MoS2 and WS2/PdSe2 van der Waals Heterostructures

Ofelia Durante †,*, Loredana Viscardi †, Adolfo Mazzotti †, Sebastiano De Stefano †, Andres Castellanos-Gomez ‡, Marika Schleberger §, Antonio Di Bartolomeo †,*
PMCID: PMC13085519  PMID: 42004737

Abstract

van der Waals (vdW) heterostructures assemble atomically thin crystals into defect-free interfaces, where band alignment, interlayer coupling, and built-in electric fields can be engineered for multifunctional, low-power optoelectronics. This Spotlight provides a concise, critical overview of two complementary platforms - BP/MoS2 and WS2/PdSe2 - highlighting how their type-II or type-I band alignment enables optoelectronic operations. In back-gate transistors, both BP/MoS2 and WS2/PdSe2 heterojunctions exhibit transfer curves with high ON/OFF ratios that can reach 107–108 and hysteresis, which is reduced in a vacuum. In BP/MoS2, deterministic stacking and contact/work function engineering stabilize a type-II band offset that supports zero (V ds = 0 V) or low (|V ds| ≤ 5 V) bias operation with fast, dual-time scale photoresponse (from hundreds of ms to ∼1 s) and linear behavior up to 50 μW optical power. Pressure tunes transport mechanisms (from thermionic to band-to-band tunnelling and ohmic), and four-probe data provide a band offset of 68 meV and zero-bias operation at 600 nm, consistent with MoS2-driven visible selectivity. Similarly, WS2/PdSe2 stacks enable visible-band photodetection at low bias, with pressure-tunable transport and photoresponse; the pressure-dependent photocurrent makes them promising for optoelectronic pressure sensing. In a gate-assisted regime (negative V gs in high vacuum), persistent photoconductivity enables opto-synaptic plasticity in WS2/PdSe2 heterojunctions with paired-pulse facilitation (PPF) of 137% and post-tetanic potentiation (PTP) of 300% under 1 s pulse trains. Overall, this Spotlight examines synthesis pathways, interfacial structure, and device-level behavior, and discusses stability, uniformity, and readout compatibility as practical levers for translating single-device demonstrations into reproducible arrays.

Keywords: van der Waals heterostructures, black phosphorus, MoS2 , WS2 , PdSe2 , mid-infrared photodetection, in-sensor memory, neuromorphic vision

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1. Introduction

Over the past five years, van der Waals (vdW) optoelectronics has progressed from proof-of-concept demonstrations to application-oriented device platforms. − By assembling atomically thin crystals across clean interfaces, vdW heterostructures provide a modular route to optoelectronic functionalities that are difficult to achieve in bulk or epitaxial systems. Because adjacent layers couple through vdW forces rather than covalent bonds, lattice matching requirements are relaxed and interface disorder can be reduced. In this regime, device behavior is governed less by bulk composition and more by interface physics, specifically band alignment, interlayer coupling, and integrated fields, which can be designed to control carrier generation, separation, transport, and recombination with limited energy budgets. These ingredients have enabled the realization of pixels that operate at low (|V ds| ≤ 5 V) or zero (V ds = 0 V) bias and begin to combine detection with elementary preprocessing or memory at the front end. ,

This Spotlight focuses on two complementary vdW heterostructure families as case studies: black phosphorus/molybdenum disulfide (BP/MoS2) and tungsten disulfide/palladium diselenide (WS2/PdSe2). In BP/MoS2, the anisotropy and thickness-tunable bandgap of BP naturally pair with the robust semiconductor properties of MoS2, often producing type II offsets that favor charge separation and rapid photoresponsivity. In WS2/PdSe2, the strong optical transitions of WS2 combine with the electronic versatility of PdSe2 enabling alignment windows that include type I scenarios, paving the way for light-efficient recombination control and device functions beyond intensity sensing, such as multimodal operation and optoelectronic plasticity. Considered together, these platforms cover different areas of the design space and illustrate how interface control directly affects the behavior at the application level. The joint discussion of BP/MoS2 and WS2/PdSe2 is particularly instructive because the two systems cover complementary alignment regimes and functionalities. BP/MoS2 exemplifies type II, short-transit, low/zero-bias photodetection, while WS2/PdSe2 enables broadband operation and electrical reconfigurability. Together, they provide a design grammar that links synthesis and interfacial structure to device-level properties and functions ready for application in low-power multifunctional pixels. Against this backdrop, recent reports on BP/MoS2 and WS2/PdSe2 address complementary levers and, within this framework, establish practical design rules. As part of the “Spotlight on Applications” series, this article provides a concise and critical report on the current state of the art and summarizes representative findings into applicable design rules and trade-offs for low-power photodetection and sensor functionality. We use BP/MoS2 and WS2/PdSe2 as complementary case studies to link synthesis, assembly choices, and interfacial structure to operational regimes to practical constraints relevant to scalability.

In BP/MoS2, pressure-dependent transport in vertical junctions has been reported, with applied pressure acting as a post-fabrication knob to tune barrier heights and interlayer coupling while preserving low-power operation; self-powered photoconductivity with (near) zero bias and a stable, integrated, field-driven response suitable for programmable responsivity, and dominant n-type conduction with fast photoresponsivity, establishing contact/channel design rules for short-transition devices. In WS2/PdSe2, a visible-light photodetector with pressure-responsive behavior has been reported, highlighting the mechano-optical coupling in a single vdW junction; optoelectronic synaptic characteristics under optical/electrical pulse protocols have also been observed, indicating learning pathways in the sensor at the pixel level. To guide the reader, Table summarizes the design space in a visual map linking platforms (BP/MoS2, WS2/PdSe2), controllable levers (device configuration, contacts/work function, pressure, gate), transport mechanisms (thermionic emission, band-to-band tunneling, persistent photoconductivity), and application metrics (V oc at 0 V, ON/OFF, response time, PPF/PTP). This map is used as an organizing thread for the sections that follow.

1. BP/MoS2 and WS2/PdSe2 Heterostructures: Key Parameters Taken from Selected Studies.

ref device configuration spectra band operating regime key parameters
BP/MoS2 heterostructure; ultrafast pump–probe study Visible (ultrafast excitation) No device bias Electron transfer ∼54 fs (1L-BP → MoS2); thickness-dependent charge transfer
BP/MoS2 phototransistor Visible–NIR Biased; wavelength/gate controlled NPC ↔ PPC polarity switching; wavelength-tunable crossover (thickness/gate dependent)
MoS2/BP/MoS2 JFET (npn) photodetector NIR → mid-IR (≈1550–3600 nm) Low-bias JFET R ≈ 9.04 A W−1 @ 1550 nm; D* ≈ 5.36 × 109 Jones
Vertical 4-probe BP/MoS2 (self-powered) Visible (∼600 nm) 0 V (photovoltaic) ΦB ≈ 68 meV (T-dependent I–V); zero-bias operation (mV-scale Voc)
Vertical BP/MoS2 heterojunction (back-gate) Visible Pressure-dependent transport Thermionic → BTBT → ohmic regimes (kink at low V); dark current increases in high vacuum
BP/MoS2 junction with Cr asymmetric contacts 450–2400 nm (white-laser) Biased (gate-tunable) Dominant n-type conduction; fast photoresponse (≈sub-200 ms scale)
PdSe2/WS2 heterostructure photodetector (CVD WS2 + selenized Pd) 532–1550 nm V ds = 2 V (zero gate) R = 3.91 mA W−1@ 635 nm; τ_rise/τ_decay = 49/90 ms
PdSe2/WS2 polarization-sensitive photosynapse 488–1550 nm Synaptic operation; polarization readout Polarization sensitivity ≈ 10.55 @ 980 nm; dark current ≈ 0.31 nA
WS2/PdSe2 photosynapse (opto-synaptic FET) Visible–NIR High vacuum; V gs = –50 V Persistent photoconductivity enabling synaptic plasticity under optical/electrical pulses; opto-synaptic plasticity with PPF ≈ 137% and PTP ≈ 300% under 1 s pulse trains. In-sensor (pixel-level) memory
WS2/PdSe2 heterostructure; back-gated visible photodetector and pressure sensor Visible (peak @ 620 nm) Low-bias (0–5 V); ambient ↔ 10–4 mbar Responsivity up to 1.2 A W–1 @ 620 nm; pressure-tunable photocurrent (vacuum ↔ air)
WS2/PdSe2 heterostructure; three-terminal photoelectric synapse transistor Visible Gate-assisted synaptic operation; low-bias readout (V ds = 1 V) with Vbg pulsed modulation; vacuum measurements ON/OFF ≈ 106; energy ≈ 2.4 pJ per event; multimodal analog synaptic plasticity and weight updateability; CIFAR-10 accuracy 92.8%
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2. Results

In this section, this Spotlight further discusses the two selected platforms, BP/MoS2 and WS2/PdSe2, to illustrate how controlled synthesis and stacking determine the interfacial structure, which in turn governs device-level properties and, ultimately, application-ready functionalities. Together, these families occupy complementary alignment regimes (type II for BP/MoS2; tunable type I/II for WS2/PdSe2) and feature distinct knobs (contact/work function engineering, deterministic transfer, and atmosphere/pressure control) that translate interface physics into low-power pixels capable of sensing, storing, and performing elementary preprocessing on the sensor. The section is organized as follows: Section discusses BP/MoS2 case studies spanning contacts/band alignment, pressure-tunable transport, and self-powered four-probe operation; Section addresses WS2/PdSe2 junctions under pressure/adsorbate control and synaptic functionality enabled by persistent photoconductivity (PPC).

2.1. BP/MoS2 Heterostructures: Literature Survey

Recent literature underscores interface engineering as the primary lever governing carrier separation, transport, and functionality in BP/MoS2 heterostructures.

At the fundamental limit, pump–probe experiments report femtosecond interlayer electron transfer from single-layer BP to MoS2 within ∼ 54 fs - directly linking stacking geometry and layer thickness to exciton dissociation efficiency. Moving to device-level operation, both lateral and vertical BP/MoS2 junctions provide programmable polarity and band-edge selectivity as application levers that go beyond simple responsivity scaling.

In a representative BP-MoS2 phototransistor, photoconductance reversibly switches from negative to positive when excitation exceeds the MoS2 absorption limit; the crossover wavelength is set by MoS2 thickness and can be tuned electrostatically, enabling multistate logic and synaptic-like behaviors encoded in the photonic input. Switching occurs on practical time scales (seconds), illustrating how band alignment and trapping can be harnessed as functional resources rather than treated solely as limitations.

Recent device reports further highlight how interfacial design extends performance into the near- and mid-infrared. A MoS2/BP/MoS2 junction FET photodetector achieves high sensitivity at telecommunications and mid-IR wavelengths (∼9.0 A W–1, D* ≈ 5.4 × 109 Jones at 1550 nm; ∼7.3 A W–1, D* ≈ 4.3 × 109 Jones at 3.6 μm), emphasizing efficient interlayer collection in a compact stack. BP/MoS2 functionalized heterojunctions also demonstrate how chemical engineering of the interface can switch between ultrafast sensing and nonvolatile optoelectronic memory: ON/OFF ratios ∼ 3.5 × 107, dark current ∼0.13 pA, responsivity up to ∼22 A W–1, with rise/fall times ≈130/260 μs and ∼90 stable conductance states for in-sensor memory. Since the following figures of merit are extracted from different device architectures and operating conditions (e.g., illumination wavelength/irradiance, biasing, pressure/vacuum, and device area), they are reported here only to illustrate the overall qualitative performance and trade-offs relevant to the application, and are not intended for rigorous comparison between different studies.

Thermal transport across BP/MoS2 interfaces - critical for pixel stability under continuous-wave illumination - also benefits from vdW engineering: simulations indicate that interfacial thermal conductance can increase by ∼167% as temperature rises from 100 to 350 K, reflecting the sensitivity of phonon coupling to temperature and defect concentration. Collectively, complementary studies over the last five years further map a broad BP/MoS2 design space, including twist-/stacking-dependent electronic structure, gating-enabled nonlinear transport (e.g., Negative Differential Resistance), metasurface-assisted mid-IR photodiodes, and multifunctional heterostructure concepts, reinforcing interface control as a central lever for performance and versatility. − ,−

Building on the BP/MoS2 literature context above, the following subsections highlight three representative vertical-stack case studies that operationalize complementary interface-control levers and enable practical take-home design rules. Section addresses contact/band-alignment landscapes leading to dominant n-type conduction and fast photoresponse; Section discusses pressure-dependent transport and the low-bias crossover from thermionic emission to band-to-band tunnelling; and Section examines self-powered photoconduction in four-probe configurations to isolate junction-limited behavior and quantify the effective interfacial barrier.

2.1.1. Dominant n-Type Conduction with Fast Photoresponse

Recent work reports a BP/MoS2 heterostructure in which interface-controlled transport yields a fast photoconductive response characterized by two distinct time constants, linear power dependence, and pronounced bias-polarity asymmetry in the current arising from band alignment and contact effects. Atomic Force Microscopy (AFM) and Raman characterization (Figure a,b) support a clean heterogeneous interface with BP in the multilayer regime and MoS2 close to the monolayer limit, while time-resolved photocurrent traces (Figure c-d) reveal a fast component (τ1, hundreds of ms) and a slower component (τ2, hundreds of ms to ∼1 s), commonly associated with the presence of BP in heterostructures, characterized by higher mobility than MoS2, and slower trapping/detrapping at the interface and flake edges.

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(a) AFM composite of the BP/MoS2 heterojunction with Cr/Au electrodes. (b) Raman spectra of pristine MoS2 and BP and of the overlap region (BP/MoS2). (c) Time-resolved photocurrent traces under white-laser illumination for increasing incident power (10–50 μW) at V ds= −5 V (295 K, 0.7 mbar). (d) Rise/decay constants versus incident power extracted from double-exponential fits. (e) Photocurrent versus incident power at V ds= ± 5V (linear regime over the tested range). (f) Output characteristics in dark and under illumination (drain connected to BP; 0.7 mbar, 295 K). (g) Band-diagram sketches of BP/MoS2 with Cr contacts at equilibrium and under bias/illumination, highlighting a type-II offset, Schottky contact at Cr/MoS2 (electron barrier ≈ 0.4–0.5 eV) and ohmic contact at Cr/BP, which together rationalize the dominant n-type transport and the larger photocurrent for negative V ds. Adapted from ref . Copyright 2024 The Author(s). Published by Elsevier B.V. under the CC BY 4.0 license.

Power-dependent measurements (Figure e) show a linear scaling of I ph with incident power at V ds = ± 5 V, supporting a photoconductive mechanism where free-carrier density increases with photon flux. The polarity asymmetry of the current with higher current at negative V ds (Figure f) can be rationalized by a staggered type-II junction predicted by Anderson’s rule, together with asymmetric Cr contacts (ohmic type at interface with BP and Schottky type with 0.4–0.5 eV barrier on MoS2).

Using representative band parameters (MoS2: Φ ≈ 4.5 eV, χ ≈ 4.2 eV, E g ≈ 1.8 eV; ,,, BP: Φ ≈ 4.2 eV, χ ≈ 4.0 eV, E g ≈ 0.3 eV ,, ), the BP/MoS2 interface is consistent with a modest electron barrier (∼0.15 eV) that only weakly impedes transport. Indeed, as demonstrated in the following, four-probe measurements indicate a slightly lower barrier of 0.068 eV. The band bending suppresses hole transport from BP to MoS2 while enabling electron transport, although electron transfer from MoS2 to BP remains limited by the Cr/MoS2 Schottky barrier. The band alignment enables efficient photocarrier separation, particularly under reverse bias conditions (V ds < 0 V), with electrons drifting toward the MoS2 layer and holes toward the BP layer (Figure g).

In BP/MoS2 vertical stacks, a type-II offset combined with asymmetric contacts can yield polarity-selective, linear photoconductive gain; however, maintaining fast, reproducible response requires suppressing interfacial trapping.

2.1.2. Pressure-Dependent Transport in Vertical Stacks

Pressure provides a practical knob to modulate transport in vertical BP/MoS2 heterojunctions. Despite identical Cr/Au contacts, the polarity asymmetry shown in Figure a,b differs from the electrical measurements reported in Figure f, indicating that junction electrostatics dominate over metal choice alone. In particular, differences in BP thickness can modify gate screening and series resistance, shifting where voltage drops and how band bending develops across the overlap region.

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Dark output characteristics at (a) 1.6 mbar and at (b) 9.5 × 10–5 mbar for different V gs. Panel (b) shows bias-polarity asymmetry and “kink” in the current at small bias. (c) Dark output curve at V gs = +10 V measured at 9.5 × 10–5 mbar. Higher vacuum yields larger dark current and a pronounced kink. The fitting lines indicate dominant transport mechanisms: thermionic emission, band-to-band tunnelling (BTBT), and ohmic transport. (d-f) Type-II BP/MoS2band diagrams illustrating three conduction regimes: I (0 < V ds < 0.2 V) thermionic emission from BP to MoS2; II (0.2 < V ds < 1 V) BTBT across the junction; III (V ds < 0) current limited by the MoS2/metal Schottky barrier and the BP/MoS2 interfacial barrier, consistent with n-type behavior. Reproduced with permission from ref . Copyright 2025 The Author(s). Published by Elsevier Ltd. Licensed under CC BY-NC-ND 4.0.

Under reduced pressure, suppression of adsorbate-induced trapping increases the dark current in BP/MoS2 heterojunctions with Schottky contacts and reveals a characteristic low-bias “kink” in forward I–V curves, marking a crossover from thermionic emission to band-to-band tunnelling (BTBT). As shown in Figure a, the dark output characteristics at 1.6 mbar are lower than those at 9.5 × 10–5 mbar (Figure b), consistent with reduced adsorption and a decreased trap/scattering density under high vacuum conditions. The “kink”, highlighted in Figure b by an orange dotted circle, appears around V ds ≈ 0.1 V for positive V gs and can be interpreted in terms of a small set of transport regimes (Figure c): (i) at low forward bias (0–0.2 V), Figure d, Schottky-like thermionic emission dominates; (ii) at intermediate bias (0.2–1 V), Figure e, BTBT becomes accessible and produces the kink/changed slope; and (iii) at higher bias (V ds > 1 V), series resistance in the BP and MoS2 channels drives an effectively ohmic response. For V ds < 0, Figure f, transport is strongly suppressed and limited by the MoS2/metal Schottky barrier together with the BP/MoS2 interfacial barrier, consistent with the overall n-type character of the stack. Pressure - via adsorbate/trap control - remodels the effective barrier landscape and can change the dominant transport pathway (thermionic → BTBT), enabling behavior similar to that of a low-voltage, high-gain tunnel diode under high vacuum conditions or low-noise regimes at higher pressures relevant to detection modes.

2.1.3. Self-Powered Photoconduction

Self-powered operation in BP/MoS2 junctions is better assessed using a four-point contact (4PP) geometry (two electrodes per flake), which minimizes contact-resistance artifacts and highlights junction-related transport. In the configuration described in ref , photovoltaic parameters (short circuit current, I sc, and open circuit voltage, V oc) are evaluated versus temperature (100–400 K) and wavelength (450–700 nm), clarifying how the effective interfacial barrier and spectral absorption govern the photoresponse. Device architecture and optical characterization are shown in Figure a,b, while dark I–V curves acquired from 100 to 400 K under reduced pressure (Figure c) show increasing conductance and evolving rectification with temperature, consistent with thermally activated release from traps/defects and progressive desorption of O2/H2O that reduces scattering and increases electron density. A standard thermionic-emission diode analysis yields a small effective interfacial barrier for electrons, ΦB ∼ 68 meV at V ds = −1 V (Richardson plot inset of Figure c), indicating a weakly electron-blocking interface compatible with what was reported before. Under monochromatic illumination (450–700 nm), 4PP I–V curves retain rectification and exhibit a clear photovoltaic response at zero (Vds = 0 V) bias (Figure d), with I sc and V oc varying with wavelength. V oc reaches ∼2.5 mV at λ ≈ 600 nm (inset of Figure d), and time-resolved measurements (Figure e,f) show that photoconductance peaks for λ ≲ 600 nm and decreases for longer wavelengths, consistent with efficient excitation when photon energy exceeds the MoS2 bandgap (∼1.8 eV).

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(a) Schematic of the BP/MoS2 heterostructure with Cr/Au leads in a four-probe configuration. (b) Processed 100× optical image highlighting the overlap region. (c) Four-probe I–V characteristics of the junction in the dark at different temperatures; inset: Richardson plot at V ds = −1 V, and linear fit (dashed curves) with a dashed linear fit used to extract the barrier height, ΦB. (d) Four-probe I–V curves under monochromatic illumination at selected wavelengths; inset: Open circuit voltage as a function of the wavelength. (e) Conductance versus time at fixed drain current I d = 7 nA, during exposure to a single laser pulse with wavelength stepped from 400 to 700 nm. (f) Photoconductance as a function of the wavelength. Adapted from ref . Copyright 2025 The Author(s). Published by Wiley-VCH GmbH under the CC BY 4.0 license.

Light-dark transitions again display two characteristic time constants, attributable to fast channel transport and slower trapping/detrapping at the interface. Four-probe measurements show that a small effective interfacial barrier enables self-powered MoS2/BP operation with visible-band selectivity (peak near ∼600 nm). Engineering ΦB and trap dynamics is therefore a direct route to stable, ultralow-power front-end pixels and spectrally selective sensing.

2.2. WS2/PdSe2 heterostructures: Literature survey

The WS2/PdSe2 material pair offers a complementary vdW platform in which broadband optical absorption combines with electrical reconfigurability driven by band alignment, defect chemistry, and environmental coupling. WS2 supplies robust excitonic transitions and controllable doping, whereas PdSe2 contributes a narrow-gap electronic channel with strong sensitivity to adsorbates and gate fields. At the device level, WS2/PdSe2 vdW photodiodes allow control on junction polarity and rectification: gate-tunable switching between p-n and n-p modes and rectification ratios continuously adjustable from ∼10–4 to ∼104 via band-to-band tunnelling control have been reported, which is attractive for polarity-programmable front ends.

The key properties of WS2 - carrier mobility, excitonic optical constants, and defect chemistry - are now well mapped, providing a mature toolbox for doping/defect engineering and synthesis routes that translate directly into heterostructure performance.

Beyond visible-band operation, WS2 -based broadband stacks highlight strategies for zero-bias and infrared response through defect-assisted gap reduction and optical field management. A pyramidal WS2/Si mixed junction achieves R ≈ 0.29 A W–1, D* ≈ 2.6 × 1014 Jones, and ultrabroad response spectrum ranging from 265 nm to 3.0 μm at 0 V, while WS2/Si type-II junctions report responsivity of ∼23 A W–1and specific detectivity ∼1.6 × 1012 Jones with mid-IR response, indicating viable building blocks for low-power arrays. Directional/polarization-sensitive functionality can also be introduced by coupling WS2 with anisotropic layers: WS2/BP stacks retain robust anisotropy in optical response (e.g.,∼1.8× polarization ratio for neutral excitons), pointing to direction-sensitive pixels without external polarizers. The same materials set further supports cross-domain sensing modalities through complementary surface chemistry.

PdSe2 films anchored to WS2 yield a reported ∼67% response to 50 ppm of H2 at 100 °C, illustrating cointegrated environmental sensing based on chemo resistive/catalytic effects. Additional photodiode implementations reinforce broadband detection capabilities in WS2/PdSe2: a device fabricated via direct selenization of Pd onto WS2 monolayers followed by transfer is reported to exhibit type-I band alignment and a broadband response from 532 to 1550 nm. Under 635 nm excitation, rise/fall times of ∼49/90 ms at V ds = 2 V and a sublinear photocurrent-intensity scaling (α ≈ 0.76–0.85) are consistent with trap-assisted recombination and lifetime limitations; ON/OFF reaches ∼103 in ambient conditions, and responsivity peaks at low irradiance (e.g., ∼3.9 mA W–1) before decreasing at higher power.

Synaptic-type operation has been demonstrated in a two-terminal WS2/PdSe2 photosynapse spanning the visible-to-infrared (≈488–1550 nm), combining very low dark current (∼0.31 nA) with strong bias sensitivity (ratio ≈10.55 at 980 nm) and device-level learning functions (e.g., paired pulse facilitation PPF ≈138% and bias-encoded optical programming/reading of synaptic weights). Moreover, three-terminal 2D WS2/PdSe2 can reach an ON/OFF ratio of ∼106 operating at an ultralow energy cost of ∼2.4 pJ per event. Under electrical gating, the device supports multimodal analog synaptic functions, including robust plasticity and reliable, updateable synaptic weights.

Building on the broader WS2/PdSe2 landscape outlined above, the following subsections highlight two representative case studies that leverage the strong environmental coupling of this interface in complementary ways. Section focuses on pressure/adsorbate-controlled transport and photoresponse to realize an optoelectronic pressure sensor, while Section discusses persistent-photoconductivity regimes under vacuum and gate bias that enable synapse-like optoelectronic plasticity.

2.2.1. WS2/PdSe2 Heterostructures as a Pressure Optoelectronic Sensor

WS2/PdSe2 stacks provide an optoelectronic pressure-sensing modality in which ambient conditions tune both transport and photoresponse. Varying pressure from 10–4 mbar to ambient under controlled illumination produces reversible shifts in transfer characteristics and a photocurrent that is strongly pressure dependent. Devices assembled by mechanical exfoliation and deterministic dry transfer, (Figure a) offer clean interfaces suitable for isolating adsorbate-driven effects; AFM thickness estimates (Figure b) place WS2 close to the monolayer regime. Figure c illustrates the WS2/PdSe2 band alignment before (top) and after (bottom) contact. In the top panel, isolated materials are referenced to the vacuum level using ϕ, χ, and E gap, yielding the positions of E C, E V, and E F; WS2 shows a wide gap (∼2 eV) and n-type behavior, whereas PdSe2 has a small gap (∼0.1 eV) and p-type character with E F near E V. The other parameters for WS2 are ϕ = 4.6 eV, χ = 3.9 eV, and for PdSe2 they are ϕ = 5.1 eV, χ = 4.6 eV. In the bottom panel, when contact is established, Fermi-level equilibration drives electron transfer from WS2 to PdSe2 (ϕWS2 < ϕPdSe2), depleting WS2 and partially compensating holes in PdSe2 near the interface. The resulting built-in field sets a type-I alignment that favors visible-band absorption in WS2 and charge storage in PdSe2. Figures d,e confirm that WS2 is n-type with mobility that increases markedly in high vacuum, while PdSe2 evolves from ambipolar behavior at ambient pressure to predominantly n-type after removal of H2O/O2 adsorbates. In the heterostructure, these adsorbate-controlled polarity and mobility shifts translate into pressure-tunable transfer curves (Figure f): hysteresis narrows from ∼70 V at ambient to ∼15 V in high vacuum, consistent with reduced charge trapping upon desorption, and strong gate control is retained with ON/OFF ∼107–108 across the pressure cycle.

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(a) Optical micrograph of a WS2/PdSe2 heterostructure. (b) AFM height line profiles for the flakes: WS2 (left) and PdSe2 (right). (c) WS2/PdSe2 band alignment before (top) and after (bottom) contact. Work function and electron affinity values are representative and may vary with thickness, doping, and adsorbates (environment). , Transfer curves at ambient pressure and in high vacuum at a pressure of 10–4 mbar for a single (d) WS2 and (e) PdSe2 flake-based devices. (f) Transfer curves of the WS2/PdSe2 heterostructure at a fixed V ds = 5 V in different states. Transfer curves of the WS2/PdSe2 heterostructure under supercontinuum laser at (g) ambient pressure and in (h) high vacuum at a pressure of 10–4 mbar. (i) Photocurrent of the WS2/PdSe2 heterostructure at λ = 620 nm under pressures from high vacuum at 10–4 mbar to ambient pressure. Adapted from ref . Copyright 2025 The Author(s). Published by IOP Publishing Ltd., under the CC BY 4.0 license.

Under white-light illumination (Figures g,h), current increases at all gate biases and the transfer curve shifts more strongly in high vacuum, preventing complete turn-off within the measured V gs window. Spectral measurements further show a pressure-dependent photocurrent at λ = 620 nm (Figure i), directly linking adsorbate state to optoelectronic output. Overall, the combined pressure-tunable transfer characteristics and visible-band photoresponse support WS2/PdSe2 heterostructures as candidates for low-power on-chip phototransistors and for chemical/environmental sensing modalities where adsorbates act as functional knobs. In WS2/PdSe2, adsorbate-controlled polarity/mobility and trap-mediated hysteresis provide an intrinsic route to pressure-tunable electronics and photoresponse, enabling multimodal (light + pressure) sensing at low bias.

2.2.2. WS2/PdSe2 Heterostructures as an Optoelectronic Synaptic Device

WS2/PdSe2 junctions can exhibit persistent photoconductivity (PPC) under high vacuum and negative gate bias, enabling synapse-like optoelectronic behavior under optical/electrical pulse stimuli. Devices assembled by deterministic stacking on a back-gated Si/SiO2 platform (Figure a) show PPC-driven conductance states consistent with a history-dependent “synaptic weight”. Power-dependent measurements (Figure b) show that illumination increases the drain current and that successive pulses incrementally raise the conductance, while the conductance remains elevated after the light is switched off. Under negative gate bias (e.g., V gs = −50 V), trapped holes at interfacial sites suppress recombination and prolong electron lifetime in the conduction channel, providing a physical basis for PPC and pulse-programmable conductance.

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(a) Schematic of the device and electrical setup; inset: AFM line profiles of both flakes. (b) Response to individual 1-s light pulses at V ds = 5 V and V gs = −50 V as the incident laser power increases. (c) Train of 15 1-s pulses separated by 1 s at V ds = 5 V and V gs = −50 V at different incident laser power. (d) PPF (left axis) and PTP (right axis) versus incident laser power. (e) Conceptual diagram of brain operation. (f) Demonstration of learning → forgetting → relearning in the WS2/PdSe2 synaptic device. Adapted from ref . Copyright 2025 The Author(s). Published by IOP Publishing Ltd., under the CC BY 4.0 license.

In this regime, pulse trains produce stepwise current increments (Figure c). Paired-pulse facilitation (PPF) is quantified as A 2/A 1 (in %), where A 1and A 2 are the peak current responses to two consecutive light pulses, yielding PPF ≈ 137%. , Post-tetanic potentiation (PTP) (in %) is quantified as A n/A 1 (in %), where A n is the peak response after a pulse train; PTP remains around ≈ 300% (Figure d). Learning-forgetting-relearning cycles are also reproduced (Figures e,f): a training sequence increases conductance (learning), followed by gradual decay (forgetting), and subsequent stimuli recover the trained state with fewer pulses (relearning), consistent with accelerated recall relative to initial training. PPC under high vacuum and negative gate bias provides a programmable, history-dependent conductance state in WS2/PdSe2 that can encode synaptic weight (PPF/PTP and learning cycles); array-level stability and reproducibility will hinge on controlling traps and environmental coupling.

3. Conclusions and Outlook

This Spotlight connects deterministic assembly and interfacial design to device-level metrics and application-oriented behavior in two complementary vdW platforms: BP/MoS2 and WS2/PdSe2. In this article, we provide a concise and critical status report and extract actionable design rules linking synthesis and assembly choices and interfacial structure to operational regimes and integration constraints relevant to scalable low-power pixels. Clean, completely dry transfers, controlled alignment of thickness/twist bands, and trap landscapes that regulate generation, separation, transport, and recombination with limited energy budgets emerge as recurring enablers for front-end pixels capable of beginning to detect, store, and preprocess information. In vertical layouts, BP/MoS2 acts as a short-passage type II archetype. Recent work reports n-type conduction with fast and reproducible photoresponsivity, showing an almost linear scaling with incident power and a clear polarity asymmetry that is consistent with interfacial band offsets and asymmetric Cr contacts. Complementary measurements across ambient-to-high-vacuum conditions reveal a low-bias “kink” associated with a crossover from thermionic emission to band-to-band tunnelling, indicating pressure as a practical post-fabrication lever for adjusting barriers and coupling between layers. Four-probe measurements further support self-powered photoconduction with a small interfacial barrier (∼68 meV), visible-band selectivity with a peak near ∼600 nm, and two characteristic time scales related to fast channel transport and slower trapping/detrapping dynamics. Collectively, these studies point to practical design rules for engineering contacts and interfaces in vertical, short-transit photodetectors operating at zero (V ds = 0 V) and low bias (|V ds| ≤ 5 V).

The WS2/PdSe2 platform complements the BP/MoS2 heterojunction and enables broadband absorption together with reconfigurability. Single flake controls showed pressure-induced mobility/polarity changes which, when combined in a stack, produce tunable gate phototransistors with large ON/OFF ratios (107–108), reduced hysteresis under high vacuum, and a pressure-coupled photoresponse pathway toward multimodal pixels. In a gate-assisted regime (negative Vgs in high vacuum), the junction exhibits persistent photoconductivity enabling optoelectronic synaptic behavior: short-term plasticity with PPF ≈ 137% and PTP ≈ 300% and learning-forgetting-relearning similar to those in the brain, where fewer pulses are needed to recover a trained state. Taken together, these observations support the existence of internal memory within the sensor and allow for preprocessing directly at the pixel level, enabling novel applications in unconventional computing.

The translation of the performance of a single device into reproducible arrays appears to hinge on a small set of immediate measures:

  • (i)

    Stability and passivation, particularly for BP (e.g., h-BN/ALD encapsulation) to curb oxidation and drift;

  • (ii)

    Uniformity/variability control via deterministic stacking, twist/thickness metrology (±1–2°; ±1 layer) and gentle modeling of the trap landscape;

  • (iii)

    Short-transit vertical architectures that support zero/low bias operation without sacrificing bandwidth;

  • (iv)

    Readout compatibility, CDS/noise budgeting, and per-pixel programmability of bias response and synaptic weight within a CMOS back-end. As an illustrative intermediate reference, a modestly sized array (e.g., 32 × 32) that achieves high functional efficiency (on the order of ≳95%) and controlled pixel-to-pixel sensitivity dispersion (on the order of ∼10–15%) would represent a useful step forward compared to individual devices; however, the required array size and acceptable dispersion depend on the application and architecture and can be mitigated by calibration and readout/processing schemes.

More broadly, the convergence of interface control and programmable junction physics enables pixels that detect, store, and preprocess information at extremely low power: from zero-bias BP/MoS2 photodiodes delivering stable front-end signals, to WS2/PdSe2 elements that embed synaptic weights and multimodal (light + pressure) signaling in hardware. Looking ahead, heterogeneous focal planes combining BP/MoS2 for NIR/MWIR detection with WS2/PdSe2 for visible/polarimetry/learning could shift computation toward the sensor, reduce data movement, and support efficient neuromorphic vision. With continued advances in passivation, uniformity, and vertical transport, vdW heterostructures are poised to move from interesting device concepts to scalable, ready-to-use arrays for robust imaging and low-power on-pixel intelligence.

Acknowledgments

A.D.B. thanks the University of Salerno, Italy, for funding this project through grant ORSA243813. ICMM-CSIC authors acknowledge support from the Severo Ochoa Centres of Excellence program through Grant CEX2024-001445-S, funded by MICIU/AEI/10.13039/501100011033. A.C.-G. acknowledges support from Grants PDC2023-145920-I00 and PID2023-151946OB-I00, funded by MICIU/AEI/10.13039/501100011033 and, respectively, by the European Union NextGenerationEU/PRTR (PDC2023-145920-I00) and by ERDF/EU (PID2023-151946OB-I00). A.C.-G. also acknowledges funding from the European Research Council (ERC) through the ERC-PoC 2024 StEnSo project (grant agreement 101185235) and the ERC-2024 SyG SKIN2DTRONICS project (grant agreement 101167218). M.S. acknowledges support from the Deutsche Forschungsgemeinschaft within the IRTG 2803:2D Mature, project No. 461605777, as well as project No. 429784087. M.S., L.V., and O.D. acknowledge support by the clean room staff of A. Lorke, especially G. Prinz.

The authors declare no competing financial interest.

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