Development and challenges of polarization-sensitive photodetectors based on 2D materials

Abstract

Polarization-sensitive photodetectors based on two-dimensional (2D) materials have garnered significant research attention owing to their distinctive architectures and exceptional photophysical properties. Specifically, anisotropic 2D materials, including semiconductors such as black phosphorus (BP), tellurium (Te), transition metal dichalcogenides (TMDs), and tantalum nickel pentaselenide (Ta₂NiSe₅), as well as semimetals like 1T′-MoTe₂ and PdSe₂, and their diverse van der Waals (vdW) heterojunctions, exhibit broad detection spectral ranges and possess inherent functional advantages. To date, numerous polarization-sensitive photodetectors based on 2D materials have been documented. This review initially provides a concise overview of the detection mechanisms and performance metrics of 2D polarization-sensitive photodetectors, which are pivotal for assessing their photodetection capabilities. It then examines the latest advancements in polarization-sensitive photodetectors based on individual 2D materials, 2D vdW heterojunctions, nanoantenna/electrode engineering, and structural strain integrated with 2D structures, encompassing a range of devices from the ultraviolet to infrared bands. However, several challenges persist in developing more comprehensive and functional 2D polarization-sensitive photodetectors. Further research in this area is essential. Ultimately, this review offers insights into the current limitations and challenges in the field and presents general recommendations to propel advancements and guide the progress of 2D polarization-sensitive photodetectors.

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1. Introduction

Photodetectors are fundamental components in light sensing systems, playing a vital role across diverse domains including remote sensing, environmental monitoring, and medical diagnostics.^1–4 The advancement of photodetectors is primarily steered by emerging technologies such as photonic integrated circuits, the Internet of Things (IoT), and automation.^5–7 These technologies continually propel improvements in photodetector performance and capabilities, enabling them to achieve advanced functionalities such as wide wavelength detection and polarization-sensitive photoresponse.

Light is an electromagnetic wave encapsulating essential elements like intensity, wavelength, phase, and polarization. While the human eye can perceive light intensity within a constrained wavelength range (approximately 390–760 nm),^8,9 a deeper understanding of light is essential for comprehensive exploration of our surroundings. Human ingenuity allows us to effectively alter existing conditions through the judicious application of tools. Among these tools, photodetectors are indispensable instruments for unraveling the mysteries of light. They possess the remarkable ability to convert the intricate properties of light into electric currents, rendering them perceivable to human senses. Notably, the polarization of light, characterized by its vector properties, poses a challenge for intuitive human observation. However, the detection of polarized light holds critical applications across diverse fields. In the realm of biology and medicine, polarized light detection has become a valuable tool for studying biological structures and physiological processes. Polarization-sensitive imaging techniques, such as polarization microscopy, can provide deep insights into the dynamics of cells, tissues, and biomolecules, thereby facilitating the diagnosis and monitoring of various medical conditions.^10,11 Furthermore, polarization-sensitive photodetectors can distinguish between different polarization states of light, enabling the acquisition of vast amounts of information about the properties and interactions of light with matter. This knowledge has been instrumental in driving advancements across a wide range of disciplines, from quantum computing to materials science.^12,13 Therefore, the development of detectors tailored for polarized light detection is of paramount importance.

The concept of designing polarization-sensitive photodetectors dates back half a century,^14,15 yet early iterations often sacrificed compactness to fulfill their objectives,^16–18 hindering widespread adoption. Recent years have witnessed the emergence of micro-nano photodetectors,^19–21 marking a significant leap in technology. Since the inception of the first graphene (Gr) nanosheet in 2004, two-dimensional (2D) materials have stepped into the forefront of photodetector technology. Leveraging their distinctive properties, these materials have emerged as the premier option for micro-nano photodetectors.

After years of advancement, micro-nano photodetectors boasting enhanced photoresponsivity and rapid response attributes have found widespread utility in realms such as communications, biomedicine, and remote sensing.^22–24 Yet, the current landscape reveals a limitation: although traditional photodetectors crafted from materials like silicon (Si), germanium (Ge), and III–V semiconductors exhibit superior sensitivity, it is difficult to distinguish polarized light. 2D materials are an effective solution to solving this issue. They exhibit strong light–matter interactions, adjustable band gaps based on thickness, and high carrier mobility that greatly enhance the performance of photodetectors. Additionally, their high extinction ratio allows them to overcome problems caused by signal crosstalk in traditional detectors.^25,26 Moreover, they also possess significant linear/nonlinear photoresponse and an ultra-fast energy transfer rate.^27–29 Most crucially, due to their rich band structures and naturally passivated surfaces, 2D materials with different capabilities can be artificially stacked to form homogeneous/heterogeneous junctions to maximize functionality.^30–34 Additionally, 2D materials can easily integrate with 1D/3D structures to enhance their inherent capabilities.^25,35 These abilities grant 2D materials with significant flexibility in the field of photodetectors.

In recent years, numerous polarization-sensitive photodetectors based on 2D materials have emerged, capable of detecting polarized light. These include devices utilizing semiconductor materials such as ReSe₂,³⁶ ReS₂,³⁷ GeSe,³⁸ GeS₂,³⁹ as well as semi-metallic materials like1T′-MoTe₂³⁴ and PdSe₂.⁴⁰ Previous studies have demonstrated the flexible application of these 2D materials in various photodetector designs, leveraging their unique properties. Additionally, there have been reports of combining nanoantennas with 2D materials for polarized light detection.^25,41 Furthermore, some studies have explored alternative methods, such as strain engineering, to achieve polarized light detection (Fig. 1).^42,43

Fig. 1 Development of 2D polarization-sensitive photodetectors in the past five years. (a) Reproduced with permission.⁴⁴ Copyright 2020, The Royal Society of Chemistry. (b) Reproduced with permission.⁴⁵ Copyright 2020, Tsinghua University Press. (c) Reproduced with permission.⁴⁶ Copyright 2021, American Chemical Society. (d) Reproduced with permission.⁴⁷ Copyright 2021, The Royal Society of Chemistry. (e) Reproduced with permission.⁴⁸ Copyright 2022, American Chemical Society. (f) Reproduced with permission.⁴⁹ Copyright 2022, The Chinese Ceramic Society. (g) Reproduced with permission.⁵⁰ Copyright 2023, Wiley-VCH GmbH. (h) Reproduced with permission.⁵¹ Copyright 2023, American Chemical Society. (i) Reproduced with permission.³⁶ Copyright 2024, Wiley-VCH GmbH. (j) Reproduced with permission.³⁰ Copyright 2023, Wiley-VCH GmbH. (k) Reproduced with permission.³⁸ Copyright 2024, Wiley-VCH GmbH. (l) Reproduced with permission.⁵² Copyright 2024, Wiley-VCH GmbH. (m) Reproduced with permission.⁵³ Copyright 2024, American Chemical Society.

Although there have been some discussions on 2D polarization-sensitive photodetection, a detailed and up-to-date systematic description remains lacking. Therefore, it is imperative to provide a comprehensive classification and organization of existing 2D polarization-sensitive photodetectors. This review examines the key applications of 2D materials in polarized light detection over the past five years, with a focus on the working principles and latest research advancements of 2D material-based polarization-sensitive photodetectors. This review categorizes these devices into three main types: single anisotropic material devices, anisotropic material heterostructure devices, and nanoantenna/electrode engineering and structural strain devices. This classification effectively encompasses the majority of 2D material-based polarization-sensitive photodetectors. Subsequently, the latest research findings are introduced and compared. Finally, the ongoing challenges and opportunities in this field are summarized, and potential strategies to address them are envisioned, aiming to guide explorations and fully leverage the pivotal role of 2D materials in the future optoelectronic industry.

2. Polarization-sensitive photodetection mechanism

Before we delve into our discussion, it is essential to introduce the working mechanism of 2D polarization-sensitive photodetectors, explaining how they convert polarized light into polarization-dependent currents. Additionally, we need to discuss the parameters used to evaluate the performance of photodetectors and their definitions. This chapter is divided into two main sections: the first section will focus on the general working principles of photodetectors, explaining how they operate to convert light into an electrical signal. The second section will provide details on the working principles of 2D polarization-sensitive photodetectors specifically, including how they are able to detect the polarization state of incident light.

2.1 Working mechanism of photodetectors

Converting light signals into electrical signals is crucial for photodetectors. This process primarily involves the conversion of incident photons into charge carriers or current within the device. The primary working mechanisms include the photoconductive effect, photogating effect, photo-thermoelectric effect, photovoltaic effect, and pyro-phototronic effect.^26,54,55 In the following sections, we will introduce each of these mechanisms individually.

The processes of photoconductive effect and photogating effect require the application of bias voltage to generate current in the device. In the case of the photoconductive effect, the increase in conductivity is primarily attributed to the increase in photo-generated charge carriers in the semiconductor under illumination.^56,57 The applied external voltage causes charge carriers to move in opposite directions, resulting in the photocurrent. The photogating effect is considered a special type of photoconductive effect, where electrons or holes generated by incident light are captured by carrier traps within the semiconductor. The captured carriers have a longer lifetime compared to the transit time of other carriers in the channel, thus modulating the semiconductor's conductivity.^58,59 Simultaneously, charged traps in the material can act as local gates, introducing a photo-induced gate voltage to effectively control the conductivity of the channel. For 2D devices, introducing the photogating effect is a crucial method for enhancing their electrical performance.

The photo-thermoelectric effect generally exists in photo-thermoelectric devices. In these devices, local heating caused by laser irradiation leads to temperature differences (ΔT) between the two ends of the channel,⁶⁰ resulting in the generation of photo-thermoelectric voltage difference (ΔV). The ratio of the ΔV to the ΔT is the Seebeck coefficient (S):⁶¹

Under illumination, photo-generated electron–hole pairs are separated by the built-in electric field at the heterojunction interface and collected by the electrodes, a phenomenon known as the photovoltaic effect.⁶² This effect is commonly utilized in photodiodes, as they can generate current without the need for an external bias voltage, endowing these devices with self-powering capabilities. The appropriate built-in electric field is crucial for this effect, making it widely applicable in Schottky junctions (metal–semiconductor).^55,63,64 It is worth mentioning that the open-circuit voltage and the short-circuit current are two essential parameters of photovoltaic devices and key indicators for evaluating the self-powering capabilities of the devices. The pyro-phototronic effect is a working mechanism that has emerged in recent years based on the pyroelectric effect. It refers to the transient temperature changes affecting the photocurrent of a detector at the moment a laser is turned on or off. These temperature fluctuations induce a pyro-electric polarization field in the pyroelectric material, which, combined with the existing built-in electric field, collectively governs the separation of photo-generated electron–hole pairs.^65–67 When utilized appropriately, this effect can also serve as an effective method for enhancing the performance of photodetectors.

There are many parameters for evaluating the performance of photodetectors, among which the most concerned are responsivity (R), specific detectivity (D*), on/off ratio (I_on/I_off) and response time (τ_rise/τ_fall).^68–70 Specifically, R indicates the photoresponse intensity of the device, D* and I_on/I_off indicate the sensitivity of the device to the environment, τ_rise/τ_fall, indicate the response speed of the device. Another two parameters that can directly affect the performance of the device, internal quantum efficiency (IQE) and external quantum efficiency (EQE) are also concern. In addition, noise equivalent power (NEP) represents the minimum optical signal power that the device can detect from the total noise. Photoconductive gain (G) is also valued in photoconductive devices. In order to gain a more detailed understanding of these parameters, Table 1 presents their definitions and formulas in detail.^71–74 Among them, there are generally two most commonly used formulas for calculating D*, both of which are listed in the table. However, D* calculated using NEP can often more persuasively represent the sensitivity of the device. Therefore, the D* performance mentioned later in this review are all derived from the NEP formula.

Table 1 Figures-of-merit of photodetectors

Parameter	Expression	Description
Photocurrent (Iph, A)	Iph = Ilight − Idark	Ilight and Idark respectively represent the photodetector current under laser illumination and in the dark state.
Responsivity (R, A W^−1)		R is defined as the ratio of photocurrent to incident optical power density, where P represents the incident optical power density, and S denotes the effective photosensitive area of the device.
Noise equivalent power (NEP, W Hz^−1/2)		NEP refers to the minimum optical signal power that can be detected or distinguished from the total noise. The inoise is the noise current. photodetector.
Specific detectivity (D*, Jones)		B is the bandwidth.
		q is the electronic charge.
Photoconductive gain (G)		τ is the photocarrier lifetime, tl is the carrier transit time. The transit time is dependent on the applied bias voltage (V), carrier mobility (μ) and the length of the channel (L).
External quantum efficiency (EQE)		EQE is defined as the photon utilization ratio, which is the ratio of the number of incident photons to the number of generated excitons. The h is the Planck constant (6.626 × 10^−34 J s^−1), c is the light velocity, λ is the wavelength of incident. light,
Internal quantum efficiency (IQE)		Where A is the absorption coefficient of the material. The IQE formula mainly considers the number of photons absorbed.
Response time (τrise/τfall, s)	τrise and τfall	τrise and τfall are defined as the time measured from 10%/90% to 90%/10% of the net photocurrent.
On/off ratio (Ion/Ioff)		Ion/Ioff refers to the ratio of photodetector current under laser illumination to dark current.

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2.2 Working mechanism of polarization-sensitive photodetectors

To systematically understanding the photoresponse mechanism of the polarization-sensitive photodetectors, the constitutive relation containing nonlinear terms is firstly discussed. The electric field E(t) can be written as:^26,75where E_dc is the static electric field corresponding to direct current (dc), E₁(ω) is the optical field at the circular frequency ω. The static current density Jⁱ_dc can be written as:

Jⁱ_dc = σ^(1);ij_dc(0)E^j_dc + 2σ^(2);ikm(ω, −ω)E^k₁(E^m₁)* + 6σ^(3);ikm(0, ω, −ω)E^j_dc(E^m₁)* + … (3)

where σ^(1);ij_dc(0), σ^(2);ikm(ω, −ω) and σ^(3);ikm(0, ω, −ω) represent the dc conductivity, second-order conductivity tensor and third-order conductivity tensor, respectively, with subscripts i, j, k, m = {x, y, z} indicating Cartesian coordinates. Furthermore, we can write eqn (3) as:

Jⁱ_dc = σ⁽¹⁾E⁽¹⁾ + σ⁽²⁾EE +… (4)

Under light illumination, the current change is completely determined by the nonlinear conductivity tensors σ⁽²⁾ and σ⁽³⁾. When these components exhibit anisotropy depending on the different directions of the dc field and optical field. The change in photocurrent density under different polarized light excitations can be expressed as:

ΔJⁱ_dc = ΔΔσ⁽¹⁾·E^j_dc + σ⁽²⁾·EE +… = ΔσΔE·E^j_dc + σ⁽²⁾·EE +… (5)

At this point, the photodetector can achieve a polarization-sensitive response to polarized light.

The polarization performance of the photodetector can be affected by varying the parameters in eqn (5). In addition, a crucial metric for evaluating the polarization performance of photodetectors is the polarization ratio (PR).^76–78 Although various definitions and expressions of this parameter exist in different articles, the fundamental definition remains consistent. Essentially, there are two primary expression methods: one using (I_max − I_min)/(I_max + I_min),³³ while the other using I_max/I_min.^34,77 In this review, we uniformly express the PR as the ratio of the maximum to minimum photocurrent (PR = I_max/I_min). A higher value of PR indicates a stronger ability of the device to discriminate between polarization angle differences.

3. Polarization-sensitive photodetectors based on individual 2D materials

The study of polarization-sensitive photodetectors based on low-dimensional materials dates back to 2001.⁷⁶ Initially, one-dimensional (1D) nanowires, characterized by their unique structure, garnered significant attention in the research of micro/nano polarization-sensitive photodetectors. Subsequently, 2D materials with asymmetric structures have been reported to enable the detection of polarized light. Notably, over the past five years, 2D materials have garnered considerable interest, prompting researchers to explore their potential for detecting polarized light and assess their practical applications. In this section, we review several representative polarization-sensitive photodetectors based on individual 2D materials from recent years. For ease of comparison and reference, the performance parameters of these devices are summarized in Table 2.

Table 2 Performance comparison of polarization-sensitive photodetectors based on individual 2D materials

Material	Wavelength (nm)	R (A W^−1)	D* (Jones)	τrise/τfall (ms)	PR	Ref.
GeSe	532	1.6 × 10^5	2.9 × 10^13	—	1.3	79
GeSe	830	400	—	5.0/2.0	2.16	80
GeSe	633	7.5	3.04 × 10^8	—	1.55	81
GeSe	808	30	—	—	1.1	35
ReSe2	635	0.379	6.8 × 10^11	18.8/21.6	3.1	82
ReS2	665	959	1.61 × 10^12	130/160	1.72	83
ReS2	650	0.28	4.22 × 10^9	2.63/2.11	2.79	84
1T′-MoTe2	850	1.2	7.68 × 10^12	0.016/0.007	1.2	85
PdSe2	532	0.0013	2.55 × 10^7	0.004/0.014	1.3	86
PdSe2	532	0.035	—	11/6	1.9	45
PdSe2	7400	—	6.7 × 10^6	0.047/0.051	2.06	87
PdSe2	1064	1.1	1.3 × 10^11	0.003/0.006	1.6	88
SiP	365	17	2.5 × 10^11	0.06/0.03	—	89
	532	—	—	—	2.9
Te	1550	327	6.08 × 10^7	0.022/0.023	2.05	90
t-InTe	365	12.05	3.85 × 10^11	100/200		91
	808	8.37	2.68 × 10^11	—	1.81
VP	520	341	—	—	3.9	92
In2Te5	445	0.171	5.08 × 10^9	—	1.26	93
	638	0.139	4.13 × 10^9	420/530	1.34
β-Bi2O3	365	71.91	6.09 × 10^13	0.4/0.2	1.91	94
Ta2NiSe5	638	13	3.5 × 10^10	0.012/0.010	1.47	95
Ta2NiSe5	1064	44	1.2 × 10^12	98/82	3.24	96
Ta2NiSe5	1350	198.1	3.3 × 10^9	27.4/28.3	1.46	97
PbSnS2	520	21.9	—	72/8	1.25	98
In2SnS4	532	0.0934	8.49 × 10^7	0.02/0.02	1.7	99

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Germanium selenide (GeSe) is a very classical anisotropic 2D material. It has a similar structure to black phosphorus (BP), holding strong in-plane anisotropy, making it very suitable for application in polarization-sensitive photodetectors. However, unlike BP, GeSe is very stable in air. As early as 2012, Xue et al.¹⁰⁰ first reported the liquid phase synthesis of GeSe nanosheets and characterized the anisotropy of photocurrent of the devices. They respectively fabricated GeSe bottom–top contact device and bottom–bottom contact device (Fig. 2a) to characterize the polarization-sensitive photoresponse properties along the a-axis and b-axis, respectively. This was an early exploration of polarization-sensitive photodetector with breakthrough significance. In 2018, Zhai's team⁷⁹ reported a polarization-sensitive photodetector made of a mechanically exfoliated GeSe nanoplate. It utilized gate-voltage modulation to achieve new breakthroughs in the photodetection performance of GeSe-based photodetectors. At V_g = −80 V, the R reaches 1.6 × 10⁵ A W⁻¹, and D* reaches 2.9 × 10¹³ Jones. Additionally, the PR under 532 nm laser irradiation reaches 1.3. Fig. 2b shows the schematic diagram of the device structure and the normalized photocurrent as a function of polarization angle. It has provided a good reference for constructing high-performance polarization-sensitive photodetectors based on 2D GeSe.

Fig. 2 (a) Schematic illustration of two kinds of single micrometer-sized GeSe nanosheet-based device with top-contact and bottom-top-contact. Reproduced with permission.¹⁰⁰ Copyright 2012, WILEY-VCH Verlag GmbH & Co. KGaA. (b) Schematic structure of GeSe-based photodetector and the normalized photocurrent as a function of polarization angle. Reproduced with permission.⁷⁹ Copyright 2018, WILEY-VCH Verlag GmbH & Co. KGaA. (c) Schematic diagram of a ReSe₂ device structure. Reproduced with permission.¹⁰¹ Copyright 2016, American Chemical Society. (d) 3D schematic image of the 2D ReSe₂ Schottky photodetector with geometry-asymmetric contacts on symmetric Au electrodes, time-resolved self-powered photocurrent curves under different polarization angles and polar plot of the photocurrent at different polarization angles. Reproduced with permission.⁵¹ Copyright 2024, American Chemical Society. (e) Schematic diagram of a balanced ReS₂ device structure and Polar plot of the photocurrent response measured at different gate voltages. Reproduced with permission.⁸⁴ Copyright 2024, Wiley-VCH GmbH. (f) Schematic diagram of a 1T′-MoTe₂ device and polar plot of the photocurrent at different polarization angles. Reproduced with permission.⁸⁵ Copyright 2023, American Chemical Society.

While some polarization-sensitive photodetectors based on 2D GeSe have been reported, in-plane optical anisotropy of GeSe was not the focus of research at that time. In 2019, Yang et al.¹⁰² systematically studied the in-plane optical anisotropy of GeSe, including its anisotropic light absorption, reflection, extinction and refraction. By using angle-resolved photoemission spectroscopy for the first time, they experimentally observed the anisotropic band structure of GeSe and interpreted the origin of its optical anisotropy. This research effectively stimulated the exploration of GeSe in polarization-sensitive photodetectors and provided basic information on the optical anisotropy of GeSe. In recent years, people are no longer satisfied with just relying on the intrinsic in-plane anisotropy of GeSe to detect polarized light. He et al.⁸⁰ reported a polarization-sensitive photodetector based on GeSe with photoinduced short-wave infrared detection. By inducing Ge vacancies in GeSe through illumination, the photoresponse range of GeSe photodetector has been widened from the visible light to 1600 nm. For the first time, polarization-sensitive photoresponse was achieved at 1310 nm and 1550 nm (sub-waveband radiation), realizing infrared polarization-sensitive photoresponse in GeSe. The validity of the experimental method was also verified using GeS. This study not only provided guidance for the exploration of GeSe, but also offered great reference value for the research of other 2D materials in polarization-sensitive photodetectors.

Additionally, group VII transition metal dichalcogenides (TMDs) like 2D ReSe₂ and ReS₂ have also garnered significant research attention. Their photoresponse ranges span the visible light and near-infrared ranges. Due to their distorted octahedral symmetrical structure, they present good linear polarization sensitivity, making them suitable for application in polarization-sensitive photodetectors. In 2016, Zhang et al.¹⁰¹ first reported a photodetector based on 2D ReSe₂ nanosheet (Fig. 2c). They synthesized ReSe₂ nanosheets using chemical vapor deposition (CVD) method. The top-gated field effect transistor (FET) device based on ReSe₂ achieved a maximum I_on/I_off ratio of 10⁷. They then performed gate modulation control on the devices using SiO₂ and h-BN as gate dielectrics, respectively. It was found that the carrier mobility could be increased by more than 500 times when using an h-BN dielectric layer. They applied photocurrent mapping to investigate the polarized characteristics of ReSe₂ devices under gate voltage modulation, providing guidance for subsequent research on ReSe₂ polarization-sensitive photodetectors. Mathew et al.¹⁰³ in 2022 also synthesized ReSe₂ nanosheets using the CVD method. They then fabricated a light-emitting transistor (LET) using asymmetric platinum (Pt)/chromium (Cr) electrodes. Due to the asymmetric electrodes, it achieved balanced electron–hole densities and field-effect mobility, thereby realizing a low turn-on voltage device. The device stably emitted polarization-related near-infrared spectra at room temperature. It provided a very effective reference for future realization of infrared polarization light detection based on ReSe₂. Just this year, Liu and Zheng et al.⁸² constructed a series of 2D ReSe₂ Schottky photodetectors with geometrically asymmetric contacts. Due to differences in Schottky barrier heights caused by the asymmetry in contact area, interface states, and thickness variations, these devices exhibited impressive optoresponse performance and polarization sensitivity in self-power mode. Fig. 2d illustrates the schematic diagram of the device structure and the polarization-sensitive photocurrent under 635 nm laser illumination in self-power mode. The device achieved PR of 3.1 and 3.6 under 635 nm and 808 nm lasers, respectively. Its outstanding imaging capabilities were also demonstrated. This work reveals the enormous potential of ReSe₂ Schottky photodetectors with geometrically asymmetric contacts in high-performance, self-powered, and polarization-sensitive photodetection.

Also in 2016, Liu et al.¹⁰⁴ first reported a broadband polarization-sensitive photodetector based on few-layer ReS₂. Since then, people began to realize the huge potential of ReS₂ in the field of polarized light detection. Lin et al.⁸³ vertically stacked two independent ReS₂ nanobelt devices at an angle of 60° to construct a linear polarization polarimeter that can distinguish the linear polarization state of incident light in the 590 to 800 nm range. The single ReS₂ device also achieved a PR of up to 1.72 at 665 nm and had an ultra-high responsivity of 959 A W⁻¹. This provided reference for subsequent fabrication of high-performance ReS₂ devices. Hu et al.⁸⁴ proposed a balanced photodetector based on 2D ReS₂ this year, which exhibited good polarization anisotropy (Fig. 2e). The device consists of two photodetectors of ReS₂ that are perpendicular to each other in crystal orientation. A single device can show a PR of 2.79 under 650 nm laser irradiation. When the two devices are combined together due to their vertical orientation in crystal, their sensitivity to polarized light can be enhanced by more than two orders of magnitude. This work provides a new approach for further developing 2D polarization-sensitive photodetectors.

The response range of early introduced 2D materials mainly focused on the visible light band. Although some methods can realize the detection of ultraviolet or infrared polarized light, the effect is not satisfactory. Researchers started to explore low bandgap 2D materials to pursue polarization light detection in the infrared band. As is well known, MoTe₂ is an excellent TMD material. But when it transforms from the 2H phase to the 1T′ phase similar to ReSe₂ and ReS₂, it exhibits stronger spin–orbit coupling and ferromagnetism, and has semimetal properties (with a small bandgap when single-layer or few-layer, and the bandgap gradually disappears as the thickness increases). In 2016, Beams et al.¹⁰⁵ first systematically studied the structural characteristics of 1T′-MoTe₂ with different layer numbers, revealing its strong optical anisotropy caused by structural asymmetry. Subsequently, in 2019, Zhu et al.¹⁰⁶ experimentally obtained the birefringent refractive indices of few-layer 1T′-MoTe₂. These studies provided indispensable fundamentals for the subsequent exploration of polarization-sensitive photodetectors based on 1T′-MoTe₂. In 2023, Mao et al.⁸⁵ synthesized wafer-scale (1 inch) few-layer 1T′-MoTe₂ using an improved chemical vapor deposition method with precursor predeposition. They fabricated asymmetric Pd/Au electrodes photodetectors. Fig. 2f shows the schematic diagram of the device structure and the polarization plot of the polarization-sensitive photocurrent. Even when expanded to 1200 nm, the device still maintains high performance, with a PR of 1.2. This study provides inspiration for the future large-scale application of 1T′-MoTe₂ in polarization-sensitive photodetectors.

In addition, the other semimetal 2D material PdSe₂ also presents potential in polarization-sensitive photodetectors. PdSe₂ shows a unique pentagonal structure, which endows it with some unique optical, electrical and thermoelectric properties. Excellent carrier mobility and strong in-plane anisotropy have made it a darling in polarization-sensitive photodetector research in recent years. Liang et al.¹⁰⁷ reported a polarization-sensitive photodetector based on 2D PdSe₂, which had an ultra-high responsivity of 708 A W⁻¹ in 1064 nm and experimentally determined the optical polarization anisotropy of PdSe₂. The device schematic diagram and polarization-sensitive photocurrent plot are shown in Fig. 3a. Zhong et al.⁴⁵ fabricated polarization-sensitive photodetectors using mechanically exfoliated PdSe₂ and verified its strong anisotropy through polarization-resolved absorption spectra measurements. Jiang et al.⁸⁸ constructed a lateral PdSe₂ p–i–n homojunction photodetector through electrostatic doping, and verified its polarization distinction capability (PR = 1.6) in the near-infrared range (1064 nm). Fig. 3b shows the schematic structure diagram of the device and the polarization-sensitive photocurrent plot. The device demonstrated excellent performance (R = 1.1 A W⁻¹, D* = 1.3 × 10¹¹ Jones) at 1064 nm, providing potential guidance for fabricating next-generation high-performance polarization-sensitive photodetectors. In addition, the photo-thermoelectric effect of PdSe₂ was also revealed. Li et al.⁸⁶ demonstrated a PdSe₂ photodetector grown by CVD, verifying that the current was from the temperature difference caused by the laser irradiation, confirming the existence of the photo-thermoelectric effect in PdSe₂. It also demonstrated the polarization light detection capability (PR = 1.3) of the device under the photo-thermoelectric effect. Dai et al.⁸⁷ made full use of the photo-thermoelectric effect of PdSe₂, successfully extending the detection band of the2D PdSe₂-based photodetector to 10.5 μm. They fabricated a PdSe₂ device using asymmetrical electrodes of multilayer graphene and Au, with a large difference in thermal conductivity between the two electrodes, which could effectively increase the temperature difference in the channel caused by the laser irradiation. This helped it achieve PR values of 2.06 and 1.21 at 4.6 μm and 10.5 μm, respectively. This research provides powerful guidance for the development of 2D PdSe₂ in the field of infrared polarization-sensitive photodetection based on the photo-thermoelectric effect.

Fig. 3 (a) Schematic diagram of the PdSe₂-based photodetector and polar plot of photocurrent variation with polarization angle. Reproduced with permission.¹⁰⁷ Copyright 2019, WILEY-VCH Verlag GmbH & Co. KGaA. (b) Schematic illustration of the device, schematic cross-sectional illustration of the doping distribution and polar plots of the photocurrent as a function of polarization angle. Reproduced with permission.⁸⁸ Copyright 2023, American Chemical Society. (c) Schematic illustration of the polarization dependent photoresponse measurements of Te nanowire device arrays and time-resolved photoresponse recorded with light polarization direction changing from 0 to 360°. Reproduced with permission.⁹⁰ Copyright 2023, Wiley-VCH GmbH. (d) Schematic representation of the GeAs₂ polarization-sensitive photodetector and polar plot of the normalized polarization-dependent photocurrent (red square) and reflectance contrast (green circle). Reproduced with permission.¹⁰⁸ Copyright 2018, WILEY-VCH Verlag GmbH & Co. KGaA. (e) The schematic of polarization-sensitive 2D t-InTe-based photodetector and polarization angle-dependent time-resolved photoresponse curves. Reproduced with permission.⁹¹ Copyright 2024, Wiley-VCH GmbH. (f) Schematic diagram of GeS₂ photodetector and polar plot of photocurrents under different polarization angle of incident light. Reproduced with permission.³⁹ Copyright 2019, WILEY-VCH Verlag GmbH & Co. KGaA.

In a creative spirit, researchers have consciously synthesized 2D materials with asymmetric structures to advance the development of 2D polarization-sensitive photodetectors. This approach leverages their unique structural characteristics for enhanced polarized light detection. Tellurium (Te), an elemental 2D semiconductor featuring a quasi-1D crystal structure, exhibits anisotropic physical properties, making it a particularly popular 2D material in recent years. Wei et al.⁹⁰ induced the directional epitaxial growth of quasi-1D Te nanowires through physical vapor deposition in nano-grooves on annealed m-plane sapphire. The resulting Te-based photodetector demonstrated a broadband response ranging from 532 nm to 2530 nm and achieved polarization-sensitive infrared light detection (PR = 2.05 at 1550 nm). The device structure and time-resolved photocurrent at different polarization angles are shown in Fig. 3c. Li et al.¹⁰⁸ synthesized single crystal GeAs₂ through chemical vapor transport (CVT) and conducted a comprehensive study of its structure, vibration, electrical properties, and in-plane optical anisotropy. The few-layer GeAs₂-based polarization-sensitive photodetector exhibited anisotropic photodetection capacity (Fig. 3d), with a PR can reach 2, sparking interest in the development of IV–V compounds. Zhou et al.⁹¹ first prepared large-area tetragonal (t-) InTe nanosheets using CVD. Due to its inherent narrow bandgap (1.28 eV) and low symmetry, the 2D t-InTe photodetector demonstrated a PR of 1.81 at near-infrared band (808 nm), surpassing most 2D anisotropic materials. The schematic diagram of the device structure and time-resolved photocurrent at different polarization angles are shown in Fig. 3e. Yang et al.³⁹ reported an ultraviolet polarization-sensitive photodetector based on 2D GeS₂ (Fig. 3f), addressing the gap in 2D polarization-sensitive photodetectors within the ultraviolet range. With a bandgap greater than 3 eV, GeS₂ can effectively detect light in the ultraviolet range. Additionally, its intrinsic structural asymmetry enables a PR of 2.1 in the ultraviolet range. This work provides valuable guidance for exploring polarized light detection in the ultraviolet spectrum. Zhao et al.⁸⁹ demonstrated a polarization-sensitive photodetector based on 2D SiP, as shown in Fig. 4a. Due to the intrinsic structural properties of SiP, this device achieves a fast response time of 30 microseconds and a PR up to 2.9. Recently, Liu et al.⁹² conducted a study on a polarization-sensitive photodetector utilizing 2D violet phosphorene (VP), which possesses an asymmetric structure similar to BP. Fig. 4b illustrates the schematic diagram of the device structure, along with its 2D colormap depicting the anisotropic photocurrent. The device exhibited a remarkable responsivity of up to 341 A W⁻¹ and a PR of 3.9. These findings suggest a promising future for VP in the realm of 2D polarization-sensitive photodetection. Furthermore, Cheng et al.⁹³ utilized density functional theory-based electronic density calculations to demonstrate the strong in-plane anisotropy of In₂Te₅. They subsequently developed a polarization-sensitive photodetector based on this material, which exhibited PR of 1.26 and 1.34 at 445 nm and 638 nm, respectively. This research has the potential to stimulate further exploration of III–IV group 2D materials (Pentatellurides M₂Te₅, where M = Al, Ga, In, etc.) for polarized light detection applications. Additionally, Guo et al.⁹⁴ employed Bi₂O₃ powder and Bi₂Se₃ fragments as precursors and additives, respectively, and adopted a surface-assisted passivation growth strategy to synthesize 2D, ultra-thin β-Bi₂O₃ nanosheets. They subsequently developed a polarization-sensitive photodetector based on this material, which demonstrated a remarkable PR of 1.91 in the ultraviolet 365 nm wavelength range, and exhibited excellent ultraviolet polarization imaging capabilities. This work opens up new avenues for the synthesis of 2D materials with polarization-sensitive performance in the ultraviolet spectral region.

Fig. 4 (a) Schematic representation of the polarization-sensitive photodetector based on SiP flake and time-resolved photoresponse with light polarization direction along 0–180° recorded under 532 nm laser sources. Reproduced with permission.⁸⁹ Copyright 2021, Wiley-VCH GmbH. (b) Schematic representation of the polarization-sensitive photodetector based on the 2D VP nanosheet and 2D colormap of the anisotropic photocurrent of the photodetector. Reproduced with permission.⁹² Copyright 2023, Wiley-VCH GmbH. (c) A diagrammatic illustration of nanoflake-based polarization-sensitive photodetectors utilizing Ta₂NiSe₅ material and photocurrent mapping obtained by scanning photodetectors under the 638 nm laser with the polarized angle varying from 0° to 180°. Reproduced with permission.⁹⁵ Copyright 2023, Wiley-VCH GmbH. (d) Schematic diagram of the Ta₂NiSe₅ photodetector device and anisotropic photocurrent under 1550 nm laser illumination. Reproduced with permission.⁹⁷ Copyright 2021, Elsevier Ltd. (e) Schematic diagram of the angle-resolved polarization-sensitive photoresponse measurement setup and anisotropic photocurrent for the linear polarized lasers at 400 nm. Reproduced with permission.⁹⁶ Copyright 2021, American Chemical Society. (f) Schematic diagram of synergistic additive of salt and molecular sieve-assisted CVD growth process, the transfer characteristics of the device along the x and y directions at V_ds = 3 V and normalized I_ph as a function of polarization angle. Reproduced with permission.⁹⁹ Copyright 2021, Wiley-VCH GmbH.

In recent years, ternary materials have garnered significant attention from researchers due to their asymmetric structures and unique properties. Ta₂NiSe₅ is a representative example. Both experimental and theoretical investigations have established that it possesses a bandgap of 0.36 eV at room temperature, which exhibits minimal dependence on thickness.¹⁰⁹ This narrow bandgap enables it to interact with light in the far infrared range. Furthermore, as a ternary compound, Ta₂NiSe₅ offers the advantage of tuning its electronic and optoelectronic properties through stoichiometric variations, thereby providing greater flexibility. Its inherent in-plane anisotropy also renders it an ideal candidate for broadband polarization photodetection. Zhang's team⁹⁵ demonstrated an h-BN-encapsulated Ta₂NiSe₅ polarization-sensitive photodetector with a working wavelength spanning from 520 nm to 4.6 μm. Their work systematically evaluated the polarization-sensitive photoresponse at 638 nm (Fig. 4c). Due to the intrinsic structural properties of Ta₂NiSe₅, the photodetector exhibited considerable polarized light detection capability. However, the study did not explore polarization-sensitive photodetection in the infrared range. Zhang et al.⁹⁷ utilized the CVT method to prepare Ta₂NiSe₅ nanosheets. The resultant photodetector exhibited a broadband photoresponse ranging from 405 nm to 4.6 μm. Fig. 4d illustrates the device schematic and photocurrent polarization plot under 1550 nm light, achieving a PR of 1.46, realizing effective exploration of infrared polarized light detection capability of Ta₂NiSe₅. Qiao et al.⁹⁶ also studied Ta₂NiSe₅ grown via the CVT method, systematically examining its anisotropic electronic transport and optoelectronic properties. The Ta₂NiSe₅-based photodetector achieved a maximum PR of 3.24 at 1064 nm (Fig. 4e). This research contributes to the understanding and development of applications involving in-plane anisotropic 2D materials in photodetectors. Additionally, Zhai's group⁹⁸ synthesized 2.4 nm thick PbSnS₂ nanosheets using the CVD method and systematically investigated its anisotropic optical and electrical properties. Zuo et al.⁹⁹ employed the same method to synthesize 2D In₂SnS₄ nanosheets and achieved control over its anisotropy through gate voltage modulation. Fig. 4f depicts the schematic diagram of material growth, transfer curve characteristics of the device along different directions, and normalized photocurrent as a function of polarization angle. The In₂SnS₄ device exhibited a maximum PR of 1.70 at a gate voltage of −20 V. This research not only offers insights into the synthesis of ternary materials but also provides an effective method for modulating the polarization-sensitive photoresponse.

4. Polarization-sensitive photodetectors based on 2D material heterostructures

While polarization-sensitive photodetectors based on single 2D materials have demonstrated impressive performance, various methods have been attempted to further enhance their polarization sensitivity. Among these, heterogeneous structure engineering stands out as the most promising strategy. This approach leverages the exceptional properties of different materials and may uncover novel mechanisms. Consequently, it offers an effective pathway to achieving high-performance polarization-sensitive photodetectors. In this section, we review the recent advancements in polarization-sensitive photodetectors based on 2D vdWs heterojunctions. Table 3 provides a comparison of the performance of polarization-sensitive photodetectors utilizing 2D vdWs heterojunctions. To facilitate a comprehensive understanding of their working principles, we classify these photodetectors into three categories: type-I band alignment, type-II band alignment, type-III band alignment and Schottky band alignment, as depicted in Fig. 5.

Table 3 Performance comparison of polarization-sensitive photodetectors based on 2D heterostructures

Materials	Wavelength (nm)	R (A W^−1)	D* (Jones)	τrise/τfall (ms)	PR	Ref.
GeS2/PbSe	1300	320	10^12	—	3.36	110
Gr/2H-MoTe2/BP	1310	0.17	2 × 10^9	—	6	111
Te/MoSe2	405	2.106	2.91 × 10^13	22/25	16.39	47
WSe2/Ta2NiSe5	780	22 000	2 × 10^12	0.0018	1.35	33
WSe2/Ta2NiSe5	532	0.034	1.08 × 10^10	0.094/0.091	3.84	112
WSe2/Te	405	402	9.28 × 10^13	1.7/3.2	2.5	30
2H-MoTe2/Ta2NiSe5/Gr	2200	5.79	10^10	—	1.44	52
2H-MoTe2/As0.4P0.6	405	1590	1.2 × 10^12	0.025/0.03	10	50
GeAs/InSe	635	0.357	2 × 10^11	25/25	18	113
GeAs/WS2	635	0.509	1.08 × 10^12	1.5/1.3	—	114
	1310	—	—	—	4.5
	1550	—	—	—	3.1
MoSe2/GeSe/MoSe2	635	0.22	6.6 × 10^10	—	12.5	38
β-In2Se3/Te	1310	2	2.14 × 10^9	0.49/0.51	4.95	115
MoS2/GaAs	780	0.0352	1.96 × 10^13	0.0034/0.0156	4.8	116
2H-MoTe2/Ta2NiSe5/2H-MoTe2	635	0.06	4 × 10^12	0.0035/0.0042	16.3	117
	1550	—	—	—	8.1
ReSe2/ReS2	350	126.56	3.16 × 10^11	0.006/0.0089	—	118
	638	16.24	4.06 × 10^10	—	1.2
	980	0.66	1.66 × 10^9	—	2.0
VP/InSe	1064	182.8	7.86 × 10^12	—	—	53
VP/Gr/InSe	1064	230	4.64 × 10^13	0.017/0.018	2.59
ReSe2/WSe2	638	0.57	10^10	0.002/0.0025	2	46
WSe2/Ta2NiSe5/WSe2	635	0.436	1.0 × 10^12	0.42/0.64	13.9	51
ReSe2/SnSe2	635	0.096	1 × 10^10	0.996/1.42	12.26	36
ReSe2/MoS2	635	0.011	1 × 10^9	270/320	5.1
Gr/ReSe2/SnSe2	635	0.144	2.4 × 10^10	0.752/0.928	13.27
Rubrene/WS2	405	227	4 × 10^11	0.18/0.21	1.5	119
2H-MoTe2/GeSe/MoS2	635	0.732	2.3 × 10^12	17.06/21.41	11.2	120
ReSe2/MoS2	638	3.52	10^11	5.0/9.1	1.35	49
2H-MoTe2/1T′-MoTe2/MoSe2	1310	0.76	3 × 10^9	13/10	55	121
1T′-MoTe2/MoS2	915	0.079	1.2 × 10^10	0.18/0.202	1.98	122
1T′-MoTe2/WS2	635	2.04	1.2 × 10^12	0.045/0.049	5	123
1T′-MoTe2/WS2/1T′ -MoTe2	635	1.06	5.6 × 10^11	0.048/0.048	13
Gr/PdSe2/Ge	980	0.691	1.73 × 10^13	0.006/0.092	91.2	40
PdSe2/CdTe	780	0.324	3.3 × 10^12	0.0049/0.0083	4.4	124
PdSe2/GaN	360	0.249	7.9 × 10^12	0.028/0.122	4.5	48
PdSe2/Si	980	0.726	3.19 × 10^14	0.025/0.034	75	44
PdSe2/Te	1550	0.617	5.27 × 10^10	0.005/0.004	3.56	125

---

Fig. 5 Schematic diagram of four heterojunction band alignments.

Band alignment in 2D vdWs heterojunctions has been a subject of extensive discussion. When 2D semiconductors form vdWs heterojunctions, there are three possible types of band alignments: type-I, type-II, and type-III. However, the separation of bands in type-III structures hinders the recycling and utilization of photo-generated carriers at the interface, significantly impacting photoelectric conversion efficiency. Additionally, photodetectors with type-III band alignment are prone to uncontrollable internal electric field effects during operation, potentially influencing photoresponse. From a material integration perspective, the separated energy bands make it challenging to effectively combine the optical or electrical properties of the constituent materials. Consequently, research on type-III band alignment remains relatively sparse, except for specific applications such as band-to-band transitions. Due to the rarity of type-III band heterojunction structures, this reiew focuses solely on analyzing type-I and type-II band alignments in 2D semiconductor heterojunctions.

4.1 Polarization-sensitive photodetectors based on 2D heterostructures with type-I band alignment

Type-I band alignment typically occurs in heterojunctions where one component possesses a narrower bandgap. Well-designed 2D heterostructures featuring type-I band alignment are more stable and exhibit superior integration of optical and electrical properties compared to type-II band alignment.¹²⁶ Recently, our team reported a polarization-sensitive photodetector based on a WS₂/Te type-I band heterojunction.³⁰ This device demonstrated exceptional performance, including a responsivity of 405 A W⁻¹, a detectivity of 9.28 × 10¹³ Jones, rapid rising and falling times of 1.7 ms and 3.2 ms, respectively, and a PR of 2.5. Furthermore, it was successfully integrated into a polarization-sensitive imaging system. Fig. 6a illustrates the schematic diagram of the device and the polarization-sensitive imaging results. These remarkable performances, particularly the anisotropic properties, were significantly enhanced compared to previously reported single Te-based devices, highlighting the superiority of type-I band heterojunctions in polarization-sensitive photodetection. Additionally, Zhao et al.⁴⁷ reported an outstanding MoSe₂/Te type-I band heterojunction polarization-sensitive photodetector. Fig. 6b displays the device schematic and polarization-sensitive photocurrent plot. This device exhibited a PR of 16.39 in self-driving mode, approximately one order of magnitude higher than that of single Te-based devices. This substantial enhancement can be attributed to the high quality of Te nanoflakes prepared by the solution synthesis method and the effective utilization of electrons under type-I band alignment. Apart from Te, BP, another narrow-bandgap 2D material, has been extensively studied for its anisotropy. Recently, there have been novel approaches to utilizing BP. Shen et al.¹¹¹ described a gate-tunable polarization-sensitive photodetector based on BP. This device employed a vertical Gr/2H-MoTe₂/BP heterostructure as the light-active layer on a sapphire (Al₂O₃) substrate, with an h-BN dielectric layer. The device effectively detected infrared (1550 nm) polarized light and achieved polarization reversal in both visible light (638 nm) and infrared light (1550 nm) by tuning the gate voltage. This research offers new insights into narrow-bandgap 2D materials, significantly advancing their development in the field of polarization-sensitive photodetectors.

Fig. 6 (a) Schematic diagram of the polarization-sensitive photodetection measurement and polarization imaging diagram. Reproduced with permission.³⁰ Copyright 2023, Wiley-VCH GmbH. (b) Optical microscope image and schematic diagram of a MoSe₂/Te device structure and polarization-sensitive photocurrent for the incident wavelengths of 405 nm. Reproduced with permission.⁴⁷ Copyright 2021. The Royal Society of Chemistry. (c) Schematic diagram of a WSe₂/Ta₂NiSe₅ device structure and testing of polarization-sensitive photocurrent of the device at different voltages under 1310 nm light. Reproduced with permission.³³ Copyright 2024, Nature Publishing Group. (d) Schematic diagram of a Gr/Ta₂NiSe₅/2H-MoTe₂ device structure and polarization-sensitive photocurrent polarization diagram. Reproduced with permission.⁵² Copyright 2024, Wiley-VCH GmbH. (e) Schematic diagram of the heterostructure device and the polar plots of the relationship between photocurrent and polarization angle under 1100 nm and 1500 nm light, respectively. Reproduced with permission.¹¹⁰ Copyright 2024, Wiley-VCH GmbH. (f) Schematic diagram of a 2H-MoTe₂/As_0.4P_0.6 device structure, wavelength-dependent photocurrent testing graph of the device and polarization-sensitive light absorption testing diagrams at 405 nm and 1310 nm respectively. Reproduced with permission.⁵⁰ Copyright 2023, Wiley-VCH GmbH.

In the preceding section, Ta₂NiSe₅, a narrow-bandgap 2D material, was highlighted, having gained significant popularity in recent years. Its distinctive structural attributes render it highly favored for use in single-material polarization-sensitive photodetectors and equally appealing for constructing vdW heterostructures. Liu et al.³³ presented a WSe₂/Ta₂NiSe₅ type-I band heterojunction photodetector (depicted in Fig. 6c), where tunneling facilitated by photogating enhanced both the responsivity and response speed of the device concurrently. Additionally, they revealed that under an electric field, photogating-assisted tunneling enabled carrier multiplication and acceleration. This device also exhibited tunable PR that was wavelength-dependent, offering valuable insights for wavelength identification. The operational principle of this device hinges on the photogating effect, primarily attributed to the photo-induced gate voltage generated by trap states resulting from confined electrons or holes.⁵⁹ The contribution of the type-I band structure is indispensable here, as it facilitates the trapping of carriers under light illumination, leading to the formation of trap states and subsequently photo-induced gating to assist tunneling. Simultaneously, Zhu et al.¹¹² demonstrated another promising application of the WSe₂/Ta₂NiSe₅ vdW heterojunction as a polarization-sensitive photodetector. In self-driving mode, the device achieved a high detectivity of 1.08 × 10¹⁰ Jones and an ultra-fast response time of merely 91 μs. Moreover, owing to the intrinsic structural anisotropy of Ta₂NiSe₅, the device possessed robust polarization-sensitive photodetection and high-resolution polarization-sensitive imaging capabilities. Leveraging these features, the device facilitated the realization of multimode optoelectronic logic gates through the co-modulation of light′s on/off state, polarization angle, gate voltage, and bias voltage. This research offers significant theoretical and technical support for the advancement of multifunctional optoelectronic devices. Recently, our team reported a high-performance polarization-sensitive photodetector (Fig. 6d) based on Ta₂NiSe₅.⁵² We constructed a three-layer vdW heterostructure comprising Ta₂NiSe₅, 2H-MoTe₂, and Gr. Under light illumination, a substantial number of polarization-sensitive carriers were generated in the top Ta₂NiSe₅ layer, swiftly separated by the middle 2H-MoTe₂ layer, and efficiently collected by the bottom Gr layer. This device successfully detected polarized light ranging from 520 nm to 2200 nm and demonstrated high-resolution polarization-sensitive imaging capabilities. The type-I band alignment between Ta₂NiSe₅ and 2H-MoTe₂ facilitated rapid carrier transfer. This work provides invaluable insights for exploring infrared polarization-sensitive imaging technologies.

Thus far, the exploration of application perspectives for 2D polarization-sensitive photodetectors has been constrained, and the number of reported 2D semiconductor materials exhibiting anisotropy remains relatively limited. Consequently, it is particularly crucial to explore novel applications for polarization-sensitive photodetectors and to identify new 2D anisotropic semiconductor materials. Liu et al.¹¹⁰ described an optoelectronic transistor based on a GeS₂/PbSe vdWs heterojunction, which exhibits precise narrowband selective spectral detection (accuracy of 96.7%) in the short-wave infrared (SWIR) range and has high in-plane anisotropy and electrical reconfigurability. Fig. 6e shows the schematic diagram of the heterostructure device and the polar plots of the relationship between photocurrent and polarization angle under 1100 nm and 1500 nm light, respectively. The device also demonstrated deep learning analysis in combination with artificial neural networks. This paves the way for exploring new applications of 2D devices in this field in the future. Li et al.⁵⁰ constructed a vdWs heterojunction using As_0.4P_0.6 with in-plane anisotropic structure and 2H-MoTe₂, realizing a reconfigurable wide spectral band polarization-sensitive photodetector. As_0.4P_0.6 is a very promising 2D anisotropic material, but has been mentioned very little in previous studies. This device can achieve wide spectral band polarized light detection from 405 nm to 1550 nm, and has a high responsivity of 1590 A W⁻¹ at 405 nm. Due to the wavelength-dependent polarization reversal characteristics of the As_0.4P_0.6 nanosheets, polarization reversal can be observed in both light absorption and photocurrent of the device (Fig. 6e). The detection mode can also be reconfigured from polarization-independence to polarization-susceptibility by changing the gate voltage. Many of these characteristics originate from the intrinsic structural properties of As_0.4P_0.6. This work introduces another very promising material for exploring 2D polarization-sensitive photodetectors and paves the way for realizing multifunctional polarization-sensitive photodetectors.

4.2 Polarization-sensitive photodetectors based on 2D heterostructures with type-II band alignment

Compared with type-I band alignment, type-II band alignment is more common in 2D optoelectronic detectors. The type-II band structure can more effectively utilize the photon energy. Electrons and holes can be separated in different materials, which can effectively reduce the recombination probability and achieve more complete separation. Moreover, the type-II band structure is easier to design and can provide more transition paths for carriers. Compared with type-I band alignment, it can more effectively realize performance enhancement of devices. In this section, we focus on some representative anisotropic materials and newly reported ones.

Rhenium diselenide (ReSe₂) and rhenium disulfide (ReS₂), both group VII transition metal dichalcogenides introduced previously, are two typical examples. As early as 2018, Liu et al.¹²⁷ employed a two-step epitaxial growth to synthesize large-area monolayer ReS₂/ReSe₂ lateral heterojunction. They observed polarization-sensitive photocurrent characteristics, demonstrating the linear polarization behavior in a 2D lateral heterostructure (Fig. 7a). However, they did not consider the lattice matching between the two anisotropic materials nor explore how to maximize the PR following their combination. Four years later, Li et al.¹¹⁸ prepared another ReSe₂/ReS₂ vdWs heterojunction polarization-sensitive photodetector by an alternating stacking method. This device successfully achieved high responsivity, wavelength-dependent bipolar photoresponse, ultrafast and polarization-sensitive photodetection capabilities. The device exhibited a positive response in the ultraviolet to visible spectral range and a negative response above 760 nm. This research revealed the enormous potential of ReX₂ (X = Se, S) vdWs heterostructures in multifunctional polarization-sensitive photodetection. However, the study found that the stacking method weakened the linear polarization sensitivity of the ReSe₂/ReS₂ device, but did not alter the stacking method to improve the polarization sensitivity of the device. In the same year, the research group constructed a polarization-sensitive photodetector based on a ReSe₂/MoS₂ vdWs heterojunction using the same device structure.⁴⁹ Its detection spectrum was no longer limited by the carrier mobility and bandgap of ReX₂ that are independent of the number of layers. This device successfully realized polarized light detection up to the near-infrared band (1250 nm) using the same device structure. By replacing the material system from ReX₂ to ReSe₂/MoS₂, the detector's spectral range was expanded without those limitations of intrinsic properties of ReX₂.

Fig. 7 (a) Schematic diagram of a horizontal ReSe₂/ReS₂ heterojunction device structure, optical microscope image and polarization-sensitive photocurrent testing graph. Reproduced with permission.¹²⁷ Copyright 2018, WILEY-VCH Verlag GmbH & Co. KGaA. (b) Schematic diagram of a Gr/ReSe₂/SnSe₂ device structure, optical microscope image and polarization-sensitive photocurrent polarization diagram. Reproduced with permission.³⁶ Copyright 2024, Wiley-VCH GmbH. (c) Schematic diagram of a semi-vertical WSe₂/ReSe₂ heterojunction device structure, optical microscope image, band alignment diagram and polarization-sensitive photocurrent mapping diagram. Reproduced with permission.⁴⁶ Copyright 2021, American Chemical Society. (d) Schematic diagram of a MoSe₂/GeSe/MoSe₂ device structure, polarization-sensitive photocurrent polarization diagram and polarization-sensitive photocurrent time-resolved curve. Reproduced with permission.³⁸ Copyright 2024, Wiley-VCH GmbH. (e) Schematic diagram of a MoS₂/GeSe/2H-MoTe₂ device structure, polarization-sensitive photocurrent time-resolved curve and polarization diagram. Reproduced with permission.¹²⁰ Copyright 2024, WILEY-VCH GmbH.

Recently, Pan et al.³⁶ demonstrated three polarization-sensitive photodetectors based on ReSe₂ vdWs heterojunctions at once using a vertical stacking approach. They respectively fabricated ReSe₂/SnSe₂, ReSe₂/MoS₂, and Gr/ReSe₂/SnSe₂ vdWs heterojunction photodetectors and systematically studied and compared their optoelectronic properties. It was found that combining Gr with ReSe₂/SnSe₂ can increase the PR of the device by more than four times. The schematic and polarization-sensitive photocurrent at polar plot of the Gr/ReSe₂/SnSe₂ device are shown in Fig. 7b, which can be attributed to the synergetic effect of unilateral depletion and photoinduced tunneling mechanism after adding a few layers of Gr. In addition, the application capabilities demonstrated, such as clear and stable quaternary logic state polarization imaging without any power supply and excellent polarization modulation for optical communication, inject vitality into the development of 2D polarization-sensitive photodetectors. Fig. 7c shows a WSe₂/ReSe₂ heterojunction photodetector with a semi-vertical geometry and its mapping graph of polarization-sensitive photocurrent.⁴⁶ It has a standard type-II band alignment with its main working wavelength at 980 nm in the near infrared. Additionally, the device demonstrated digital incoherent holographic 3D imaging in a self-driven mode, providing new reference for future applications of 2D polarization-sensitive photodetectors. Ahn et al.¹²⁸ also demonstrated a polarization-sensitive photodetector based on a 2D ReS₂/2H-MoTe₂ vdWs heterojunction. It can respond to near-infrared light up to 1310 nm and realize polarized light detection. Moreover, using Pt and Ti/Au as electrodes at the two ends respectively, this device achieves a rectification ratio of four orders of magnitude.

This year, two different research groups consecutively reported on high-performance polarization-sensitive photodetectors utilizing vdWs heterostructures based on 2D GeSe. In their work, An et al.³⁸ demonstrated polarization-sensitive photodetectors utilizing MoSe₂/GeSe bilayer and MoSe₂/GeSe/MoSe₂ trilayer vdWs heterostructures. They primarily focused on the polarization-sensitive photoresponse of these devices under 635 nm laser illumination in self-driving mode. As illustrated in Fig. 7d, the MoSe₂/GeSe/MoSe₂ device exhibited a significant PR of 12.5, which is 3.5 times higher than that of the MoSe₂/GeSe device and five times higher than that of the single GeSe device. Another trilayer heterostructure device employing the 2H-MoTe₂/GeSe/MoS₂ structure demonstrated ultra-strong antibipolar transfer characteristics through the incorporation of GeSe.¹²⁰ This structure holds significant potential for applications such as frequency doubling and binary phase shift keying units in the communication field. Under self-driving mode operation at 635 nm, the device exhibited a PR of 11.2, as shown in Fig. 7e. Comparative testing further verified that the anisotropy of its photocurrent originates from the structural characteristics of GeSe. In addition to GeSe, Huo's team demonstrated two polarization-sensitive photodetectors based on 2D vdWs heterostructures of GeAs/WS₂ and GeAs/InSe.^113,114 Their polarization performance was attributed to GeAs, which exhibits classic type-II band alignment, enabling high responsivities of 509 mA W⁻¹ and 357 mA W⁻¹ for the GeAs/WS₂ and GeAs/InSe devices, respectively, at 635 nm. Furthermore, due to the narrow bandgap (0.83 eV) of GeAs, the GeAs/WS₂ device achieved PR values of 4.5 and 3.1 at short-wave infrared wavelengths of 1310 nm and 1550 nm, respectively (Fig. 8a). Leveraging the high performance of InSe, the GeAs/InSe device fully exploited their performance advantages, achieving a PR of 18 at 635 nm in self-driving mode (Fig. 8b), surpassing most 2D polarization-sensitive photodetectors. This year, Huo's team also demonstrated a polarization-sensitive photodetector based on a 2D β-In₂Se₃/Te vertical vdWs heterostructure. Benefiting from the proven high performance of In₂Se₃ and the strong anisotropy of Te, this device exhibited high performance in the short-wave infrared region, with responsivities of 2 A W⁻¹ at 1310 nm and 0.71 A W⁻¹ at 1550 nm, and detectivities of 2.14 × 10⁹ Jones and 7.3 × 10⁸ Jones, respectively.¹²⁹ Additionally, it demonstrated potential for polarization-sensitive imaging sensors. Fig. 8c shows the device schematic and polarization-sensitive photocurrent curve as well as polarization imaging at 1310 nm, and this device can also be used for infrared ASCII code communication transmission. These studies by Huo's team provide new opportunities to further explore 2D polarization-sensitive photodetectors.

Fig. 8 (a) The polarization sensitive photocurrent of the GeAs/WS₂ device under 1310 nm and 1550 nm lasers with varying polarization angles. Reproduced with permission.¹¹⁴ Copyright 2022, American Chemical Society. (b) The time resolved photocurrent of GeAs/InSe heterojunction under polarized light with varying polarization angle from 0° to 330° under 635 nm and polarization sensitive photocurrent as a function of the polarization angle in the polar coordinates. Reproduced with permission.¹¹³ Copyright 2021, Wiley-VCH GmbH. (c) The schematic structure of the β-In₂Se₃/Te polarization-sensitive photodetector, the polarization-resolved photoresponse under 1310 nm and imaging patterns of the letters “Te” at 0° and 90°, respectively. Reproduced with permission.¹¹⁵ Copyright 2024, Wiley-VCH GmbH. (d) The schematic structure of the InSe/Gr/VP polarization-sensitive photodetector and angle-resolved photocurrent data. Reproduced with permission.⁵³ Copyright 2024, American Chemical Society. (e) Schematic diagram of the Nb₂GeTe₄/MoS₂ photodetector under the MIR light irradiation, atomic structure of Nb₂GeTe₄ and MoS₂, optical image of the heterojunction and the linear polarization photoresponse of the heterostructure for the 3.7, 4.6, and 11.0 μm MIR light. Reproduced with permission.¹³⁰ Copyright 2023, Wiley-VCH GmbH.

Based on the preceding mentioned 2D anisotropic material VP, Ahmad et al.⁵³ proposed a type-II VP/InSe vdWs heterostructure recently. Under 1064 nm light excitation, this device exhibited an R of 182.8 A W⁻¹ and a D* of 7.86 × 10¹² Jones. Moreover, they found that inserting a few layers of Gr between VP and InSe can effectively enhance the photodetection performance (Fig. 8d). Due to the intrinsic in-plane anisotropy of VP, this device also showed a PR of 2.7. Zheng et al.¹¹⁹ proposed an organic/inorganic 2D vdWs heterostructure polarization-sensitive photodetector. By combining the organic single crystal rubrene with the 2D WS₂, and leveraging the better carrier mobility, longer exciton diffusion length, anisotropic charge transport and unique linear dichroism of organic single crystals, the photodetection performance was extended, realizing an R of 1000 A W⁻¹ and a response time of 180 μs. Additionally, a PR of 1.7 was achieved in the visible light region by utilizing the anisotropy of rubrene, providing very valuable reference for exploring new types of 2D polarization-sensitive photodetectors.

In recent years, ternary 2D tellurides materials including TaIrTe₄, NbIrTe₄, Nb₂SiTe₄ and Nb₂GeTe₄ have attracted attention due to their optical anisotropy, narrow bandgap and topological insulator properties, and they are considered potential candidates for far-infrared polarization-sensitive photodetection. Han et al.¹³⁰ described a polarization-sensitive photodetector based on an Nb₂GeTe₄/MoS₂ vdWs heterostructure, which covers a wide spectral range from 3.7 to 11 μm. Combining the clear photovoltaic effect in the heterojunction region and the bolometric effect of MoS₂, this device achieved PR values of 48 and 34 under 4.6 μm and 11 μm light irradiation, respectively. Interestingly, under 3.7 μm light, the device exhibited a bipolar photoresponse, with PR showing −3.38 (Fig. 8e), which can be attributed to the bolometric effect of MoS₂ leading to a negative dominant response at specific wavelengths. Based on these characteristics, this device has been successfully applied to bipolar infrared polarization coding communication and polarization imaging.

4.3 Polarization-sensitive photodetectors based on 2D Schottky heterostructures

The Schottky band alignment discussed in this section pertains specifically to vdWs heterojunctions comprising one or more 2D semimetal materials. Such heterojunctions typically exhibit pronounced photovoltaic effects, rendering them highly suitable for self-driving photodetectors characterized by ultra-fast response speeds. Over the past five years, 2D anisotropic semimetal materials, particularly 1T′-MoTe₂ and PdSe₂, have garnered significant attention. This section primarily reviews and summarizes the strengths and weaknesses of 1T′-MoTe₂ and PdSe₂ in Schottky vdWs heterojunction polarization-sensitive photodetectors reported in recent years. It is anticipated that this analysis will offer valuable insights for the future development of Schottky band alignment heterojunction photodetectors. Recently, Pan et al.³⁴ demonstrated a polarization-sensitive photodetector based on a 2H-MoTe₂/1T′-MoTe₂/MoSe₂ trilayer vdWs heterostructure. This photodetector exhibits a strong photovoltaic effect, enabling highly self-driven current without an external applied voltage for polarized light detection. Fig. 9a shows the device structural diagram and band alignment diagram. The type-II band alignment between the bottom MoSe₂ layer and the upper 2H-MoTe₂ layer effectively promotes the separation of electrons and holes, while the intermediate 1T′-MoTe₂ layer enhances exciton dissociation, significantly improving device performance. The incorporation of 1T′-MoTe₂ enables the device to achieve a PR of 55 at the near-infrared wavelength of 1310 nm, surpassing most 2D polarized photodetectors. Fig. 9b illustrates a wide-spectral-range polarization-sensitive photodetector based on an MoS₂/1T′-MoTe₂ vdW heterostructure.¹²² Due to the structural characteristics that induce a photovoltaic effect, this photodetector can respond to polarized light from 520 nm to 1550 nm without an applied bias voltage. Most notably, it achieves polarization imaging applications in the near-infrared band under scattering media, which holds significant promise for the development of next-generation polarization imaging devices. Additionally, there have been advancements in the study of how changes in crystal orientation caused by stacking 2D polarization-sensitive materials affect polarization sensitivity. Wu et al.¹²³ systematically investigated the stacking configurations of their 1T′-MoTe₂/WS₂ 2D vdW heterostructure polarization-sensitive photodetector (Fig. 9c). They employed a 1T′-MoTe₂/WS₂/1T′-MoTe₂ structure and compared the changes in polarization sensitivity by altering the stacking directions of the longitudinal nanoflakes of 1T′-MoTe₂. The results revealed that by optimizing the crystal orientation, the PR could be increased by nearly five times. This provides new insights and degrees of freedom for future exploration of 2D polarization-sensitive photodetectors.

Fig. 9 (a) Schematic diagram of a 2H-MoTe₂/1T′-MoTe₂/MoSe₂ device structure and band alignment diagram Reproduced with permission.³⁴ Copyright 2024, Wiley-VCH GmbH. (b) Schematic diagram of a MoS₂/1T′-MoTe₂ device structure and its polarization-sensitive photocurrent polarization diagrams under different wavelength light. Reproduced with permission.¹²² Copyright 2023, American Chemical Society. (c) Optical microscope images of 1T′-MoTe₂/WS₂ vdWs heterojunction devices with three different stacking ways. Reproduced with permission.¹²³ Copyright 2022, Wiley-VCH GmbH. (d) Schematic diagram and polarization-sensitive photocurrent testing graph of a PdSe₂/Te device structure. Reproduced with permission.¹²⁵ Copyright 2024, Wiley-VCH GmbH. (e) Schematic diagram of a Gr/PdSe₂/Ge device structure, band alignment diagram and polarization-sensitive photocurrent polarization diagrams under different wavelengths. Reproduced with permission.⁴⁰ Copyright 2019, American Chemical Society. (f) Schematic diagram and polarization-sensitive photocurrent testing graph of a PdSe₂/GaN device structure. Reproduced with permission.⁴⁸ Copyright 2022, American Chemical Society.

PdSe₂'s excellent intrinsic structural characteristics render it highly suitable for detecting polarized light across a broad spectral band through the construction of vdW heterojunctions with other 2D materials. Recently, Wang et al.¹²⁵ meticulously oriented the crystals of two anisotropic materials through their stacking method and fabricated a Te/PdSe₂ vdW heterojunction polarization-sensitive photodetector. This device achieved a detection range exceeding 4 μm and exhibited PR of 3.56 and 1.62 at 1550 nm and 4 μm, respectively (Fig. 9d). However, despite the careful selection of the stacking direction, the observed breakthrough in PR did not fully meet expectations for the combination of these two highly anisotropic materials. Professor Wu from Zhengzhou University has made significant contributions to the research of 2D PdSe₂. Over the past few years, he has consistently demonstrated his expertise in vdW heterojunction polarization-sensitive photodetectors based on 2D PdSe₂.^40,44,48,124 As early as 2019, he introduced a polarization-sensitive photodetector utilizing a Gr/PdSe₂/Ge vdW heterojunction. Fig. 9e illustrates the device schematic and polarization-sensitive photocurrent curve, demonstrating polarized light detection from ultraviolet (365 nm) to near-infrared (1550 nm). The highest PR achieved was 112.2, confirming that the polarization originates from the intrinsic structural anisotropy of PdSe₂. This work injected considerable confidence into the field of polarization-sensitive photodetectors based on PdSe₂. Professor Wu's other work is also groundbreaking. His team proposed a 2D PdSe₂/3D CdTe hybrid-dimensional vdW heterojunction, leveraging the structural characteristics of PdSe₂ and the unique hybrid-dimensional geometric structure. This device can detect polarized light in the long-wavelength infrared spectrum (10.6 μm) at room temperature and achieve stable imaging within this band. This detection spectral range surpasses most polarization-sensitive photodetectors based on PdSe₂. Additionally, by combining wide bandgap GaN and PdSe₂ (Fig. 9f), they successfully realized ultraviolet light (360 nm) polarization-sensitive photodetection in the self-driving mode. This combination exhibited excellent performance in ultraviolet polarization-sensitive imaging and secure ultraviolet light communication. Professor Wu's research has provided invaluable insights for the development of high-performance polarization-sensitive ultraviolet (infrared) photodetectors based on 2D PdSe₂. Furthermore, Zeng et al.¹³¹ demonstrated a polarization-sensitive photodetector utilizing a PdSe₂/perovskite Schottky heterojunction, achieving polarized light detection across the deep ultraviolet to near-infrared range with a PR of 6.04 and rapid response times of 3.5/4 μs. The high-quality imaging capability at 808 nm underscores the promising potential of this device in future optoelectronic systems.

5. Polarization-sensitive photodetectors based on nanoantenna/electrode engineering and structural strain

The polarization-sensitive photodetectors introduced earlier rely on the intrinsic structural anisotropy of active materials for polarized light detection. Enhancement of this anisotropy is typically achieved by constructing two- or three-layer vdW heterojunctions with other materials to improve the PR. However, this method, which relies on the inherent structural anisotropy of materials, is easily constrained by other material characteristics, thereby limiting the detection spectral range and performance. In recent years, researchers have explored alternative methods for achieving polarized light detection, including the incorporation of nanoantenna structures, unique electrode engineering designs, and the application of artificial strain in 2D materials. These approaches enable non-anisotropic materials to detect polarized light or combine in-plane and out-of-plane anisotropy to further enhance the PR. This section reviews some representative works in this area from recent years, aiming to provide guidance for the future development of 2D polarization-sensitive photodetectors.

In 2020, Abbasi et al.¹³² developed a single-metal polarization-sensitive photodetector utilizing asymmetric gold nanostructures. Through the optical resonance and localized light absorption generated by plasmons, they achieved polarized light detection in the near-infrared band via the photothermal effect. However, the authors did not attempt to integrate 2D materials with their nanostructures, limiting the potential for further references. Typically, such optical structures exploit optical anisotropy through asymmetrical light absorption or directional optical resonances. Fig. 10a depicts a polarization-sensitive photodetector incorporating a plasmonic nanocavity and a monolayer of MoS₂.²⁵ The plasmonic structure was crafted by milling nanogrooves onto an Au substrate using focused ion beam milling, followed by the transfer of 2D MoS₂ onto it. MoS₂, a well-known isotropic material, exhibited a more than 24-fold enhancement in photoluminescence due to the nanostructure. Polarized light detection was also achieved at 633 nm, yielding a PR of approximately 4, despite using an isotropic material. This enhancement can be attributed to the improved light absorption in the plasmonic nanocavities and the out-of-plane anisotropy resulting from varying refractive indices in different directions. This study presents a viable approach for exploring 2D polarized photodetectors. Enhancing the polarization sensitivity of anisotropic materials is more challenging compared to enabling isotropic materials to detect polarized light using nanoantennas. To effectively enhance polarization sensitivity, a more nuanced investigation of the optical effects of plasmonic structures and the selection of structures that align with the crystal orientation of 2D materials is required. BP, a classic anisotropic material, serves as an ideal candidate for such experiments. Fig. 10b illustrates a polarization-sensitive photodetector combining plasmonic nanocavities with BP and its light absorption under various conditions.⁴¹ The device enhanced BP's absorption of linearly polarized light by 185 times and its infrared emission by 18 times. This enhancement is attributed to the near-field enhancement of the plasmonic structure and polarization conversion that aligns well with BP's anisotropy. However, this study did not explore the electrical manifestation of such strong light absorption in terms of polarization-sensitive photodetection, nor did it provide insights into polarization-sensitive photodetection conversion.

Fig. 10 (a) Schematic of the hybrid structure of MoS₂ polarized photodetector with Au NG array. Reproduced with permission.²⁵ Copyright 2023, Tsinghua University Press. (b) Schematic diagram of a BP/plasma structural device and absorption testing graphs of BP under different conditions. Reproduced with permission.⁴¹ Copyright 2021, American Chemical Society. (c) Perspective view of the graphene device and enlarged views of the asymmetric nanostructure-based and symmetric architecture-based device. Reproduced with permission.¹³³ Copyright 2024, American Chemical Society. (d) Images of GeSe SWA at different angles. Reproduced with permission.³⁵ Copyright 2023, Elsevier B.V. and Science China Press. (e) The photocurrent of the Ga₂O₃ device under 0° and 90° polarized light and time-resolved photoresponse under illumination with polarization angle from 0° to 180°. Reproduced with permission.⁴² Copyright 2024, Science China Press. (f) Schematic diagram of a curled WSe₂ device and optical microscope images during the curling process. Reproduced with permission.⁴³ Copyright 2024, American Chemical Society.

Electrode engineering represents another strategy for fabricating high-performance 2D polarization-sensitive photodetectors. Zhang et al.¹³³ recently introduced a series of asymmetric plasma nanostructures fabricated on graphene planes, encompassing stripes and triangular tip arrays. These nanostructures, serving as electrodes for graphene photodetectors, are illustrated in Fig. 10c, which depicts a typical schematic of the structure. Upon light excitation, the non-centrosymmetric metallic nanostructures exhibit robust light–matter interactions, with localized fields near the tips, resulting in an asymmetric electric field. This field can generate a PR of nearly 10. In a related study, Zhang et al.¹³⁴ proposed a plasmonic cavity-integrated graphene phototransistor for polarization detection. Unlike the previously mentioned stripe/triangle electrodes, they employed multiple stripe/single stripe electrodes. In their composite structure, for polarization perpendicular to the metal stripes, the local field near the graphene was enhanced, significantly boosting graphene's absorption. Conversely, for polarization along the stripe direction, most of the light was reflected, suppressing graphene's light absorption. Through this method, they achieved a PR of up to 30 in the near-infrared range. These two studies offer new perspectives for the development of multifunctional graphene photodetectors.

Due to the uncertainty regarding how nanoantenna enhance the anisotropy of materials, it is deemed more appropriate to directly modify the materials themselves by integrating nanostructures onto 2D nanosheets. In 2023, Zhou et al.³⁵ reported a subwavelength array (SWA) structure based on GeSe, where the zigzag and armchair axes of GeSe were determined using angle-resolved Raman spectroscopy. Subsequently, nanoarrays were designed along the armchair axis and directly etched onto GeSe nanosheets. Fig. 10d illustrates the details of these nanoarrays. By aligning the nanoarrays with the atomic lattice arrangement along the low-symmetry direction and suppressing the high-symmetry direction, the PR of the device was enhanced from 1.6 to 18. Additionally, the device exhibited a wide power range of 40 dB in the near-infrared band at 808 nm, demonstrating exceptional weak light detection capabilities.

Another approach to enhancing polarization sensitivity in 2D materials involves applying suitable strain. Zhang et al.⁴² investigated the effects of various degrees of compressive and tensile strain on β-Ga₂O₃ and its photoresponse (Fig. 10e). They discovered that applying a 0.7% compressive strain significantly increased the PR of the device by approximately 100 times, representing a substantial enhancement. Density functional theory (DFT) calculations attributed this enhancement to an increase in the absorption coefficient ratio. Experimentally, compressive or tensile strain, induced by inward or outward bending, disrupts the crystal's original periodicity and symmetry by altering the atomic arrangement, which in turn affects the electronic structure and polarized light absorption. The experimental demonstration that tensile and compressive strain respectively weaken and enhance the polarization sensitivity of β-Ga₂O₃ further supports this explanation. The atomic reorganization that occurs after introducing mechanical strain can change the anisotropy of the crystal. This phenomenon was also observed in another study, where a MoS₂ photodetector achieved polarized light detection after applying mechanical strain.¹³⁵ However, excessive mechanical strain can lead to current instability and device damage,^136,137 which is undesirable. To avoid strain-induced damage to the internal structure of materials, Zhang et al.⁴³ proposed an alternative method. They utilized solution-induced curling of thin-layer 2D materials to achieve controlled curling of specific areas and degrees by adjusting the solution amount. This method was initially proposed by Professor Duan from Hunan University.¹³⁸ By curling WSe₂, they created 1D/2D homogeneous junction structures of WSe₂, an isotropic material, and introduced in-plane anisotropy, enabling polarized light detection. Fig. 10f illustrates the schematic diagram of the curling process and optical microscope images of the curled structures. These studies present innovative approaches for achieving polarized light detection or enhancement, providing valuable insights and references for the exploration and development of 2D polarization-sensitive photodetectors.

6. Methods for improving the polarization sensitivity of polarization-sensitive photodetectors

With the advent of 2D polarization-sensitive photodetectors, researchers have increasingly focused on enhancing their polarization sensitivity. However, this field still appears to be under-explored. In this section, we will review and summarize some representative methods aimed at improving the polarization sensitivity of these devices, offering valuable insights and references.

In aforementioned section, we mentioned a subwavelength array (SWA) structure based on GeSe, reported by Zhou et al.,³⁵ which is a highly effective method. After determining the orientation of the crystal axes, this method fabricates nanoarrays directly on GeSe nanoplates through etching. This results in enhanced light absorption in the low-symmetry direction and suppressed light absorption in the high-symmetry direction, thereby improving the PR of the device from 1.6 to 18. This provides an excellent demonstration of the effectiveness of the method. Furthermore, Wu et al.⁷⁷ demonstrated an ultra-sensitive polarization-sensitive photodetector with a homojunction in-plane structure defined by ferroelectric domains in BP. The ferroelectric domains of p-doped and n-doped regions in the up-polarized and down-polarized states form an in-plane PN junction within the BP plane. They analyzed and observed the enhancement effect of ferroelectric fields on electron–phonon interaction in a specific direction of the device, which in turn enhances the photothermoelectric effect in that direction. In their structure, the combined effects of the photovoltaic effect and enhanced photothermoelectric effect promote anisotropic transport of photocurrent, resulting in a PR as high as 288. The design concept of modulating and enhancing 2D devices with ferroelectric fields is also applicable to other materials, providing an important approach for designing high-performance polarization-sensitive photodetectors. Recently, Xue et al.¹³⁹ developed a controllable growth strategy for synthesizing WS₂ stripe arrays with defined chirality and coherent polarity directly from a materials synthesis perspective. The synthesized WS₂ stripe arrays exhibit clear chirality and can readily achieve ultra-sensitive polarization light detection. Furthermore, their synthesis method enables systematic study of chirality-dependent 2D photodetectors and provides a general approach for customizing and large-scale integrating chiral materials.

Among these methods, SWA improves the asymmetry of charge distribution, suppresses matrix elements along the zigzag direction and local surface plasmons, and increases light absorption and photoexcitation probability along the armchair direction. This provides a valuable reference for designing optical structures to enhance the polarization sensitivity of devices. On the other hand, ferroelectric field modulation utilizes external factors to control the current along a specific crystallographic direction of the material, allowing it to be increased or decreased, thereby modifying the polarization sensitivity of the device. This can inspire researchers to explore the use of various external factors (electric field, magnetic field, etc.) to manipulate the charge carrier transport in materials, thereby enhancing the polarization sensitivity of devices.

7. Summary and outlook

After many years of development, 2D polarization-sensitive photodetectors have achieved significant breakthroughs. Their numerous excellent performance characteristics render them promising for a wide range of polarized light detection applications, including broadband detection, weak light detection, ultra-fast detection, and high-sensitivity detection. This review discusses the latest research advancements in 2D materials, as well as their heterojunction and hybrid structures, within the context of polarization-sensitive photodetectors. The first part introduces polarization-sensitive photodetectors based on single 2D materials. These devices’ ability to detect polarized light primarily stems from the intrinsic in-plane anisotropy of 2D materials. However, they often suffer from limitations such as a narrow response range, poor responsivity, and low PR. In contrast, the second part presents 2D vdW heterojunction polarization-sensitive photodetectors, which offer greater flexibility. By stacking and combining different materials, their properties can be integrated to significantly enhance the photoresponse and PR. However, their polarized light detection capability still relies on the intrinsic in-plane anisotropy of 2D materials. The third part introduces works that achieve polarized light detection or enhancement by combining nanoantennas with 2D materials or modifying the properties of the 2D materials themselves. These approaches can more effectively regulate the polarization sensitivity of the devices. However, they often face limitations in detection bands. To realize high-performance and wide-band 2D polarization-sensitive photodetectors, further efforts are still required.

In summary, there remain numerous avenues worth exploring in the realm of polarization-sensitive photodetectors based on 2D materials. Thus far, the exploration of polarization-sensitive photodetectors in deep ultraviolet, far infrared, and terahertz bands has been scarce, constrained by the inherent band structural characteristics of anisotropic 2D materials. This necessitates ongoing efforts to develop novel anisotropic 2D materials or incorporate nanoantennas to facilitate photodetection in these bands. However, research in this area is relatively limited and may necessitate extensive investigation. Moreover, the pursuit of more comprehensive 2D polarization-sensitive photodetectors is crucial. Polarized light detection has undergone significant development over the years, driving the advancement of numerous emerging applications. These include polarization-based encrypted optical communications, polarization-sensitive bioimaging, polarization-based chemical sensing, polarization-assisted medical diagnostics, as well as polarization-aided techniques for the manipulation of quantum states. The current lack of comprehensiveness in 2D polarization-sensitive photodetectors hinders their participation in numerous emerging applications, posing a significant impediment to their development. Additionally, the exploration and modification of a broader range of 2D material structures merit attention, as exemplified by the fabrication of nanoarrays in GeSe nanosheets. Enhancing the inherent performance of 2D materials to achieve our objectives may be feasible, albeit in-depth research in this domain remains limited due to technical constraints and may emerge as a focal point in future endeavors. Recently, research on 2D vdWs heterojunction polarization-sensitive photodetectors has predominantly centered on single transistors or diodes, despite frequent mention of this topic. The large-scale development of 2D material growth, array fabrication for integrated devices, and chip-based applications remains underdeveloped due to associated difficulties. These areas are particularly pivotal for advancing 2D vdWs heterojunctions.

Lastly, the rapid progression of ultrafast laser imaging over the past two years has garnered considerable attention. Utilizing ultrafast lasers in transient absorption spectroscopy can effectively characterize carrier states in 2D materials, thereby aiding in the comprehension of charge transfer mechanisms in 2D devices. However, pertinent research in 2D polarization-sensitive photodetectors is nearly absent, presenting a potential breakthrough for developing novel 2D polarization-sensitive photodetectors.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (62175040, 61805044), Guangdong Basic and Applied Basic Research Foundation (2024A1515012688), and Guangdong Provincial Key Laboratory of Information Photonics Technology (2020B121201011).

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