Discovery of two-dimensional (2D) materials has catalyzed a paradigm shift in materials science and device physics engineering [1,2]. Due to their outstanding semiconducting properties, such as high charge carrier mobility, large optical absorption, and high ON/OFF ratio, they offer extraordinary opportunities for developing compact, flexible, and high-performance devices across various domains such as optoelectronics, sensing, and photodetection [3]. Early carrier investigation reveals that 2D graphene, with its unique electronic properties, initially paved the way for this revolution. Subsequently, conventional 2D materials such as transition metal dichalcogenides (TMDCs; such as MoS₂, WS₂, and InSe) garnered significant attention due to their semiconducting nature making them suitable for a broad spectrum and many other device applications [4,5]. Despite the research progress, the pursuit of these 2D materials with more specialized characteristics has become increasingly critical, particularly for emerging applications such as polarized photodetection and advanced imaging technologies [6,7]. One such material, pentagonal 2D palladium diselenide (PdSe₂) which belongs to the group-10 TMDCs, has emerged as an interesting material for developing the next-generation photoelectronoic device due to its outstanding semiconducting characteristics [8]. PdSe₂ exhibits unique pentagonal crystal structure which is distinguished from the other 2D materials with hexagonal or trigonal structures [9]. This distinctive arrangement of crystal structure realizes strong in-plane anisotropy, which directly influences its electronic, optical, and mechanical properties. Such anisotropic behavior is very important for photoelectronic applications such as broadband photodetection and polarization-sensitive applications, where the ability of a material to selectively interact with light of different polarizations can enhance device performance [10]. In addition, PdSe₂ exhibits several optoelectronic properties such as a layer-dependent tunable bandgap, excellent air stability, and a broad spectral response, making it a potential candidate for developing the polarized photodetectors based on the 2D materials.

The definition of polarized photodetection is a process wherein a detector senses the orientation of an incident light’s electric field [11]. Polarized photodetectors have many applications in advanced technological sectors such as atmospheric monitoring, biomedical diagnostics, quantum information processing, and so forth [12]. Anisotropic materials like black phosphorus (BP), violet phosphorus, indium selenide, and rhenium disulfide have shown great promises for improving polarized photodetection due to their inherent ability to interact differently with light depending on its polarization state [13]. Rather than anisotropic materials, a combination of more than 2D materials or low-dimensional materials in the form of 2D van der Waals (vdWs) also shows exciting features to develop polarized photodetectors with reasonable polarization sensitivity [2,14]. These 2D materials and their vdWs heterostructures have shown considerable potential for developing the polarized photodetectors with considerable anisotropic sensitivity. However their performance is often constrained by limitations in their anisotropic properties or difficulties in tuning their electronic structures for specific applications [15,16]. Among 2D anisotropic materials, pentagonal structural PdSe₂ offers a solution to these challenges. Its anisotropic electronic and optical behavior, combined with strong light-matter interactions, positions it as an ideal material for detecting polarized light across a wide range of wavelengths, from visible to infrared [17]. Furthermore, the tunable bandgap of PdSe₂ allows for the design of devices with tailored optoelectronic characteristics, optimizing performance for various polarization detection applications [18].

This review aims to provide a comprehensive overview of the current state of research on PdSe₂ with a particular focus on its potential for polarized photodetection and imaging technologies. We highlight the key figures of merit for detection of polarization. After that, we focus on the structural and electronic properties of PdSe₂, emphasizing its strong polarization sensitivity, tunable bandgap, and environmental stability, which make it ideal for developing optoelectronic devices such as broadband photodetectors and their applications in polarization detection-based imaging systems. Next, we provide the recent research progress of the polarized photodetectors based on PdSe₂ and their vdWs heterostructures with other low-dimensional materials. We also address the challenges in scalable synthesis, material stability, and integration with other low-dimensional materials, offering future research directions to optimize PdSe₂ for commercial applications. Thanks to its outstanding optoelectronic properties, PdSe₂ is at the forefront of optoelectronic materials, poised to drive innovations in polarization photodetection.

Synthesis techniques of the PdSe₂ are very important to tune the semiconductor properties of the devices. In this section, we will shed light on the advanced synthesis techniques to prepare PdSe₂ including their advantages or disadvantages.

Chemical vapor deposition (CVD) is one of the most used methods for synthesizing PdSe₂ due to its ability to produce high-quality thin films at a relatively large scale. A group of researchers prepared 2D PdSe₂ flakes by employing space-confined CVD under ambient pressure as depicted in Fig. 1(a), by utilizing PdCl₂ as the Pd source and Se powder as the Se source. According to the schematic diagram, one end of the crucible containing the Se powder is left open, while the polished SiO₂ side faces downward over the PdCl₂ crucible, which has a 3 mm opening at the other end. By using this technique, they achieved a large single crystal with domain sizes exceeding 1 mm, exhibiting an anisotropic structure. This anisotropic growth pattern, with a length-to-width ratio of approximately 100:1 (see Fig. 1(a)) could significantly influence the material’s physical properties, such as its in-plane anisotropic optical characteristics. PdSe₂ adopts a pentagonal lattice structure (Pbca space group, No. 61), where four Se atoms pass through the Pd layer, breaking the symmetry, as demonstrated in Fig. 1(b). In addition, they employed Raman spectra to confirm the arrangements of crystal structure and quality of the PdSe₂ nanoflakes. According to the measurements, the Raman spectra reveal four distinct peaks at approximately 143.6 cm⁻¹ (${\mathrm{A}}_{\mathrm{g}}^1 $), 206 cm⁻¹ (${\mathrm{A}}_{\mathrm{g}}^2 $), 221.4 cm⁻¹ (${\mathrm{B}}_{\mathrm{lg}}^2 $), and 256 cm⁻¹ (${\mathrm{A}}_{\mathrm{g}}^3 $), as shown in Fig. 1(c) [19]. By optimizing the CVD system, 2D material’s preparation can be realized in high quality nanoflakes with optical and dielectric properties which can be effectively used for development of the promising optoelectronic devices. Keeping the concept in mind, a team of researchers achieved the preparation of few-layer PdSe₂ on sapphire using a novel three-zone CVD system, as shown in Fig. 1(d). In this process, continuous few-layer PdSe₂, with a controllable thickness on a centimeter scale, was produced in a tube furnace with PdCl₂ and Se serving as the precursors. The furnace is divided into three zones, each maintained at different temperatures. The Se powder was placed in zone 1 and heated to its melting point of 250 °C. PdCl₂ was in zone 2, while a centimeter-scale sapphire substrate was positioned in zone 3, which was heated to either 500 °C or 600 °C, respectively. The evaporated Se and Pd precursors were transported using an Ar/H₂ gas mixture at flow rates of 300/30 sccm. By optimizing the growth time, large-scale PdSe₂ nanoflakes with layer counts varying from 3L to 15L were achieved where L is the layers. They observed that the growth times for the 3L, 5L, 8L, 12L, and 15L PdSe₂ films were 2 min, 6 min, 12 min, 20 min, and 26 min, respectively (see Fig. 1(e)) [20].

Mechanical exfoliation, also known as the “Scotch tape” method, and electrochemical exfoliation are two simple techniques for producing thin flakes of PdSe₂ from bulk crystals. Usually, mechanical exfoliation techniques are widely used to tear the 2D materials from bulk crystals to few layers or monolayers. Recently, a group of researchers presented electrochemical exfoliation techniques to transform bulk PdSe₂ materials into few-layer PdSe₂. In this process, bulk PdSe₂ single crystals undergo an electrochemical molecular intercalation technique, as shown in Fig. 1(f). Tetrabutylammonium (TBA), a quaternary alkylammonium cation, was used as the intercalant, which remained stably attached to the nanosheet surface. This resulted in the formation of 2D PdSe₂ superlattices (TBA-PdSe₂), even after repeated cycles of sonication and washing with N,N-dimethylformamide (DMF). In addition, Raman spectra reveal that the high crystalline quality of a pristine five-layer PdSe₂ nanosheet (approximately 3 nm thick), exfoliated from bulk crystals, displayed prominent Raman modes, including A_g and B_1g, observed at 145 cm⁻¹, 210 cm⁻¹, 225 cm⁻¹, and 258 cm⁻¹ (see Fig. 1(g)) [21].

PdSe₂ emerged as a particularly interesting candidate due to its unique pentagonal crystal structure, which imparts a range of anisotropic electronic and optical characteristics [22].

One of the most compelling features of PdSe₂ is its layer-dependent electronic structure [23]. Like many other 2D materials [24,25], PdSe₂ exhibits a tunable bandgap that can be controlled by varying the number of layers. In its monolayer form, PdSe₂ is a semiconductor with a direct bandgap of approximately 1.3 eV. This bandgap is suitable for a wide range of optoelectronic applications, particularly in the visible and near-infrared (NIR) spectral regions, which are of great interest for photodetection. As the number of layers increases, the bandgap decreases, eventually leading to a semi-metallic behavior in the bulk form of PdSe₂ [26]. For further investigations of the bandgap of PdSe₂, a group of researchers investigated the bandgap tunability in the layers of PdSe₂. They observed that the bandgap of PdSe₂ changes according to the number of the layers. According to their investigations, the bandgap of PdSe₂ changes with the thickness. Fig. 2(a) shows that as the thickness increases, the optical band edge shifts toward lower energy levels. Using this data, the bandgap values were calculated as a function of the number of layers. As shown in Fig. 2(b), the bandgap for a thickness of 1.2 nm is 1.08 eV. As the thickness increases from 3 to 40 layers, the bandgap progressively decreases to 0.29 eV. When the layer count reaches 50, the bandgap approaches 0.00 eV, indicating a shift from a semiconductor to a semimetal. To provide further insights into how thickness affects the energy band structure of PdSe₂ films, ultraviolet photoemission spectroscopy was also performed. According to the UPS investigations, work function values were 4.32 eV, 4.45 eV, 4.70 eV, 4.83 eV, 5.07 eV, 5.16 eV, and 5.27 eV for thicknesses of 1.2 nm, 2.0 nm, 4.0 nm, 8.0 nm, 12.0 nm, 16.0 nm, and 20.0 nm, respectively. Additionally, the difference between the Fermi level and the valence band maximum (VBM) was found to be 0.50 eV, 0.45 eV, 0.35 eV, 0.26 eV, 0.10 eV, 0.06 eV, and 0.00 eV for the corresponding thicknesses [27]. A team of researchers conducted detailed electrical transport measurements to determine the actual bandgap of PdSe₂. By employing techniques such as Hall measurements and dual-gate field-effect transistors (FETs), they estimated the bandgap to be approximately 0.3 eV as shown in Fig. 2(c). This result helps resolve previous discrepancies in reported bandgap values and provides valuable insights into the potential of PdSe₂ for infrared detection applications. Furthermore, the study highlights the material’s in-plane anisotropy, which may significantly impact its application in optoelectronic devices [28].

Experiments have demonstrated that PdSe₂ exhibits a significant difference in optical absorption when the light is polarized parallel to or perpendicular to the crystallographic axes of the material [29,30]. This anisotropic absorption is a direct consequence of the pentagonal lattice symmetry and is a key factor in PdSe₂ suitability for polarized photodetection. This ability to differentiate between polarization states allows for the development of photodetectors with high polarization sensitivity, which is essential for advanced imaging technologies, environmental sensing, and even biomedical diagnostics [31]. To confirm this anisotropy property of PdSe₂, a group led by Liang et al. [18] conducted angle-resolved polarized Raman spectroscopy. The Raman spectra were obtained using a 532 nm laser in a backscattering configuration. As depicted in Fig. 2(d), the Raman spectra of the A_g mode at 151 cm⁻¹ show a periodic variation as the rotation angle increases in 15° increments, ranging from 0° to 360°. The maximum intensities occur at 0°, 90°, 180°, and 360°, while the minimum values appear at 45°, 135°, 225°, and 315°. Furthermore, as a photodetection mode, the linear polarization dependence was analyzed by mechanically adjusting the linear polarization angle of the excitation laser. Fig. 2(e) illustrates the variation in photocurrent as a function of the polarizer’s angle. The photocurrent shows a periodic change with the polarization angle, peaking at 45° and 135°, and reaching a minimum at 90° and 180°. These findings highlight the potential of PdSe₂ as a medium for linear dichroism, which could be beneficial in polarization based applications [18].

A group of researchers revealed their anisotropy properties. In their investigations, they employed a state-of-the-art technique such as angle resolved transport measurements on few-layer PdSe₂ as shown in Fig. 2(f). In this process, a set of twelve electrodes are distributed in a circular pattern with equal angular spacing, where the pair of electrodes aligned with the b-axis is designated as the 0° reference point. The current-voltage curve (I_ds-V_ds) at various angles (Fig. 2(g)) indicates good Ohmic contact and a clear dependence on angle. To further investigate the anisotropic behavior, the angle-dependent conductivity (Fig. 2(h)) was derived from the I_ds-V_ds data along six different directions, based on the following equation: σ(θ) = σ_acos²θ + σ_bsin²θ, where σ_a and σ_b represent the conductance along the a-axis, b-axis, respectively and θ is the fitting parameter. The conductance ratio (σ_b/σ_a) they achieved was approximately 2.25, which closely aligned with the theoretically predicted value of about 2.5 [32], and exceeded other anisotropic 2D materials such as BP [33].

The FET configuration is among the most widely employed architecture for PdSe₂-based photodetectors. In this mechanism, a thin layer of PdSe₂ is used as the channel material, with source and drain electrodes on either side to control the flow of current. A gate electrode modulates the carrier density in the channel, enabling control over the device’s electrical properties, including its photocurrent response. Based on the amazing optoelectronic properties of the PdSe₂, a group of researchers demonstrated a polarized photodetector based on few-layer PdSe₂ as shown in Fig. 3(a). Remarkably, polarization-sensitive photodetectors based on this material demonstrate anisotropic photodetection behavior, with dichroic ratios reaching approximately 1.8 at 532 nm and 2.2 at 369 nm, with polarization orientations shifted by 90° along the a-axis and b-axis, respectively (see Fig. 3(b). This distinctive polarization selection in photodetection is linked to intrinsic linear dichroism conversion [32].

Recent studies have demonstrated that vdWs heterostructures incorporating PdSe₂ can significantly improve device responsivity and polarization sensitivity [34]. For instance, devices composed of PdSe₂ and MoS₂ have shown enhanced performance in terms of photocurrent generation, particularly when the incident light is polarized along the optimal direction for each material [35,36]. This ability to combine and optimize materials at the atomic level is a key reason why vdWs heterostructures represent a major frontier in the development of 2D material-based photodetectors. Based on this motivation, a group of researchers demonstrated a MoS₂/PdSe₂-based vdWs heterostructure for polarization photodetection as shown in the optical image of Fig. 3(c). As proposed, the structure provides a strong built-in electric potential at the surface of the device which is responsible for highly sensitive polarization detection. The performance of the device can be tuned by adjusting the built-in electric field via the gate voltage. They observed that, as the gate voltage is increased from −30 V to +30 V, the responsivity (R) is increased from 7.5 A/W to 13 A/W, while the detectivity (D*) is improved from 1.53×10⁹ Jones to 2.63×10⁹ Jones. Fig. 3(d) presents the anisotropic ratio (dichroism ratio) of 3.09 which is greater than several reported polarization photodetectors based on 2D materials. The results demonstrate that MoS₂/PdSe₂ polarization photodetectors outperform many other polarization detectors [37]. Combination of 2D materials with narrow-bandgap 2D materials such as Te (0.3 eV) is realized as an effective strategy to improve the polarization sensitivity at the broadened spectrum. A group of researchers introduced a polarization-sensitive photodetector based on the Te/PdSe₂ vdWs heterostructure which exhibits an exceptionally wide spectral response. By integrating experimental studies with the density functional theory (DFT) calculations, they establish the optimal stacking orientation between single-crystalline Te and PdSe₂, which maximizes the optical in-plane anisotropy of the heterojunction with the self-power mode mechanism. As a photodetector, the device shows R of 15.5 A/W, D* of 2.17×10¹¹ Jones, rapid response time of 4.3 μs, and wide broadband spectrum range from 405 nm to 4000 nm. Importantly, the Te/PdSe₂ heterojunction photodetector shows a high dichroic ratio of 1.62 at 4000 nm (Figs. 3(e) and (f)), indicating strong potential for polarization-sensitive short-wavelength infrared (SWIR) photodetectors. They claimed that their study could realize self-driven broadband polarization optoelectronic probing spanning the visible to infrared spectrum [38].

Hybrid structures, which incorporate PdSe₂ with low-dimensional materials, for instance, quantum dots (QDs) metal nanoparticles or organic semiconductors, can be realized as another innovative approach to enhancing the photodetector performance [39]. These hybrid structures offer the ability to fine-tune the optical and electronic properties of PdSe₂ based-devices for specific applications, particularly by enhancing light absorption and polarization sensitivity.

By keeping the concept, a group of researchers fabricated the self-driven, polarization-sensitive, broadband photovoltaic detector by transferring 2D PdSe₂ onto vertically aligned Si nanowire arrays (SiNWAs). The strong light confinement effect in the PdSe₂/SiNWA mixed-dimensional vdWs heterostructure results in noticeable photovoltaic behavior under laser light illumination (200 nm to 4600 nm), enabling the device to function as a self-powered photodetector without an external power source. As a photodetector, it demonstrated high R of 726 mA/W, D* of 3.19×10¹⁴ Jones, and fast response time of 25.1/34 ms. Additionally, the large surface-to-volume ratio of the PdSe₂ film and SiNWAs enhances the device’s sensitivity to relative humidity, and its light-enhanced humidity sensing capabilities were also explored. Regarding polarization detection, it shows impressive photoresponse capabilities, such as high sensitivity to polarized light, which arises from the asymmetric pentagonal crystal structure of PdSe₂ with its puckered form. Fig. 3(g) provides a schematic of the experimental setup using the PdSe₂/SiNWA device. In this setup, a polarizer is used to convert incoming light into polarized light, while the polarization angle is adjusted with a half-wave plate. Fig. 3(h) exhibits the polarization angle-dependent photocurrent, normalized to the minimum value. The results, measured at V_ds = 0 V, show that the photocurrent reaches a maximum at 0° (and 180°) and a minimum at 90° (and 270°), providing a high polarization sensitivity of 75 [40]. This outstanding polarization sensitivity is much larger than other anisotropic materials such as GeS₂ [42] and BP [43]. These impressive findings suggest that the hybrid structures such as PdSe₂/SiNWA mixed-dimensional heterostructure device have significant potential for use in high-performance, polarization-sensitive broadband photodetection applications. In addition, Schottky junction-based device is also suitable to enhance the polarization sensitivity. Schottky junction-based device plays a crucial role in enhancing the sensitivity of photodetectors, reducing the dark current and also achieving a quick photoresponse by leveraging the unique properties of the metal-semiconductor interface [44,45]. Based on this motivation, a team of researchers reported a vertical PdSe₂/GaN vdWs Schottky junction, created through an in-situ growth process, for developing the highly sensitive UV-polarized photodetection. Regarding fabrication of the device, the 2D PdSe₂ layer is directly grown on a GaN substrate using a low-temperature post-chalcogenation technique, ensuring a pristine and contamination-free junction interface. The fabricated device shows high R of 249.9 mA/W and a quick photoresponse of 28.5 µs at the wavelength of 360 nm. In terms of polarization detection, the device shows polarization sensitivity of 4.5 at the wavelength of 360 nm. Such high polarization sensitivity is attributed to the Schottky junction-based structure and this value is also comparable to that with the other 2D materials and their heterostructures based-polarized photodetectors as shown in Fig. 3(i) [41]. The Schottky junction is further shown to be highly effective for high-resolution polarized UV imaging and secure UV optical communication. This study provides a practical approach for creating high-performance, polarized UV photodetectors based on 2D materials. Despite the significant advantages of 2D PdSe₂ over conventional 2D materials like MoS₂, WSe₂, and InSe, there are also some disadvantages, as listed below in Table 1.

PdSe2, a promising member of the group-10 TMDCs family, offers exceptional potential for advanced polarized photodetectors due to its tunable electronic properties, unique pentagonal crystal structure, and high polarization sensitivity. Despite progress in PdSe₂-based photodetectors, several challenges must be addressed to unlock its full potential. This section discusses these challenges and outlines future research directions to foster innovation and commercialization.

Scalable and high-quality synthesis of PdSe₂ remains a significant hurdle. Current methods, such as mechanical and liquid exfoliation, are limited in scale and can result in inconsistent semiconducting properties. Large-scale applications demand reliable, uniform production of PdSe₂ films. Future research should focus on improving scalable methods like CVD, molecular beam epitaxy, and atomic layer depositition to produce defect-free, environmentally stable materials with a controlled layer thickness for integration into commercial devices.

Integrating PdSe₂ with conventional semiconducting technologies presents compatibility challenges with Si-based electronics, flexible substrates, and optical systems. Combining PdSe₂ with complementary materials such as graphene or MoS₂ in vdWs heterostructures can enhance device performance but poses difficulties in achieving seamless interfaces due to lattice mismatches and thermal expansion differences. Advanced interface engineering and hybrid device designs are necessary to address these issues.

Although PdSe₂ is more stable than materials like BP and violet phosphorus, environmental factors such as humidity and temperature can degrade its performance over time, especially in harsh conditions. Protective coatings, such as hexagonal boron nitride (h-BN) or polymer encapsulation, are critical for maintaining long-term device reliability. Future research should evaluate PdSe₂-based devices under real-world conditions to ensure their durability and performance.

Strain engineering and doping hold promise for tailoring PdSe₂’s photoelectronic properties, enhancing its sensitivity across broadband wavelengths. These techniques could enable advanced applications such as polarization-sensitive imaging and wearable devices, paving the way for next-generation photoelectronics.

Current dichroic ratios in PdSe₂-based polarized photodetectors are below the practical threshold of 10–20 required for real-world applications. Enhancing material properties and novel device engineering are crucial to bridge this performance gap.

Beyond polarized photodetectors, PdSe₂ could benefit emerging fields like quantum computing, telecommunications, and energy harvesting due to its tunable bandgap, polarization sensitivity, and high electron mobility. Expanding research into these areas could unlock new opportunities for this versatile material [46,47].

The authors declare no conflicts of interest.