2D Materials and their Application to Near Infrared Detection

1. Introduction

In the modern era of the electronics industry, photodetectors are essential parts. It is essentially an optoelectronic device that generates electricity from light. They are also called photosensors, which work primarily as the optical receivers to convert information from the light into electrical signals. They are crucial for capture, identification, and visualization of optical information [1]. The first photodetector emerged during World War II (1933) by Edgar Kutzscher, who found that lead sulfide (PbS) can be used as a photoconducting detector to detect hot emissions (infrared radiation) from bombers and ships. Our exploration of photo detectors began relatively recently, in the early 1990s [2]. In stark contrast, humanity’s fascination with the nature of light stretches back to as early as 300 BC. This timeline shows our long-standing effort to understand light, laying the groundwork for the development of advanced photodetector technologies. The research on light started at 300 BC with Aristotle, followed by Huygens, who found the wave nature of light in the seventeenth century. Later in the eighteenth century, Maxwell predicted that those waves were electromagnetic waves with both electric and magnetic components. After many remarkable findings and theories in the nineteenth century, Einstein proposed that “light is composed of small packets of electromagnetic energy called photons.” He even proved it with the help of the photoelectric effect, which is the basic principle behind the working of photodetectors [3]. Photodetectors work based on the principle of creating electron-hole pairs when exposed to light. When photons with energies equal to or greater than the material’s bandgap hit the material, they excite electrons from the valence band to the conduction band, leaving behind positively charged holes. These conduction band electrons and valence band holes can move due to concentration gradients or electric fields, whether intrinsic or externally applied. These photogenerated electron-hole pairs may recombine and release light in the absence of an electric field. They can, however, be separated as free charge carriers by an electric field, and some of these free charges will gather at the electrode of the photodetector to form a photocurrent. This phenomenon of photocurrent generation is depicted in Figure 1. The strength of the incoming light has a direct correlation with the photocurrent’s magnitude at a particular wavelength [4]. In 1800, Sir Frederick William Herschel paved the way to let us know about the wide spectrum of light. He was the first man to find that there is light beyond the red portion of the spectrum, and he named it “Calorific rays” which is now called “Infrared radiation.” In 1801, followed by his finding, Johann Wilhelm Ritter found that there is also light beyond the violet portion, and he named it “Chemical rays” which is now called “Ultraviolet.” Ultraviolet to infrared light is all the electromagnetic radiation with different wavelengths that constitutes the electromagnetic spectrum. The ability to detect light in different sub-bands has many uses in industries, the military, agriculture, and the biological sciences. Displays, video imaging, electronic eyes, optoelectronic storage, industrial safety, and other applications use visible (Vis) light detection, while ultraviolet (UV) light detection is used in military surveillance, space exploration, border security, environmental monitoring, and sterilization procedures, among other applications. Fire alarms, precision strikes, satellite remote sensing, optical communication, night vision technologies, remote control systems, analytical science, and navigational aids are just a few of the applications for infrared (IR) light detection [5]. Figure 2 illustrates the broad electromagnetic spectrum, highlighting the wavelength ranges and the materials typically employed for detecting radiation in each spectral region. An ideal photodetector should efficiently convert light into electrical signals, detect weak optical signals, respond quickly, and utilize photons for charge carrier generation. These qualities are evaluated through figures of merit like responsivity, response time, noise equivalent power, detectivity, quantum efficiency, and gain, which are influenced by material properties such as bandgap, absorption coefficient, and charge carrier dynamics. Understanding these metrics helps assess material performance and optimization in photodetector applications. The formulas for each figure of merit and how they relate to material behavior and performance have been discussed in the following sections.

2. Fabrication techniques of MoS₂ heterostructure photodetectors

The concept of assembling and fabricating 2D materials, including MoS₂, into heterostructures was first proposed by Wang et al. in their work on high-quality graphene electronics [22], leading to the discovery of numerous novel properties in these engineered systems. These heterostructures can be broadly classified into two categories: vertically stacked heterostructures, where layers are assembled via van der Waals interactions, and epitaxially grown planar heterostructures, where materials are seamlessly integrated through direct growth, ensuring strong in-plane bonding. Among the various methods of fabrication of these heterostructures, the Chemical Vapor Deposition (CVD) and deterministic transfer methods are the powerful, efficient way to attain the better devices with remarkable properties. These methods were briefly discussed here. A high-precision method for creating MoS₂ heterostructures, the deterministic transfer method guarantees regulated monolayer stacking free from contamination or misalignment. The schematic experimental setup and the deterministic transfer process have been illustrated in the Figure 3(a) and (b) respectively. The development of high-performance van der Waals (vdW) heterostructures, where exact interlayer coupling directly affects electronic and optical properties, requires this technique. First, bulk crystals of monolayer or few-layer MoS₂ are usually exfoliated by applying liquid exfoliation, Nitto tape, or scotch tape on SiO₂/Si, sapphire, or other substrates. A viscoelastic polymer, like polydimethylsiloxane (PDMS) or poly(methyl methacrylate) (PMMA), is applied to the monolayer as a support layer during the transfer process. With the aid of micromanipulators and an optical microscope, the MoS₂ flake is precisely positioned over the target substrate or another two-dimensional material (e.g., WS₂, graphene). A temperature-controlled stamp-assisted technique (≈ 40–100°C) can be used to achieve the clean adhesion while reducing interfacial contamination. Ultimately, the pristine interface is restored by annealing (approximately 300–400°C in Ar/H₂ atmosphere) or acetone removal of the polymer layer [23].

Chemical Vapor Deposition (CVD) is a widely used bottom-up approach for the fabrication of vertical 2D heterostructures, enabling the growth of planar multi-junction heterostructures. Several strategies have been explored to achieve scalable and controllable fabrication of both vertical and planar heterostructures with atomically sharp heterojunctions and clean interfaces. The two-step CVD method, commonly used for this purpose, involves growing layered crystals as a substrate for the second layer. The rate of gas flow and synthesis time play crucial roles in the formation of vertically stacked and lateral heterostructures. For instance, the growth of a 2D GaSe/MoSe2 heterostructure was achieved using this method, though large lattice misfits between the layers can result in incommensurate superstructures [24]. Additionally, a stacked TMD/hBN heterostructure was synthesized using a Ni-Ga alloy and Mo foil as substrates, where the alloy promoted the formation of the hBN honeycomb lattice while Mo served as a source of Mo [25]. The multi-step CVD method offers greater flexibility and control by switching the direction and modulating the components of the gas flow. This approach allows for sharper heterojunction boundaries and enables sequential growth of multi-junction heterostructures. For example, a WS₂ − WS₂(1 − x) Se2x monolayer planar heterostructure with tunable band alignment was successfully grown by modulating the gas flow direction and temperature. Similarly, a one-pot synthesis strategy was developed by placing two precursor powders (MoX₂ and WX₂) in the same boat, with different gas flows promoting the selective growth of MoX₂ and WX₂, respectively [26]. This multi-step approach, compared to one-step or two-step CVD methods, offers more precise control, enabling the creation of spatially distinct optoelectronic devices by separating electrons and holes into different materials. Dong Hee Shin et al. fabricated MoS₂/SQD:SiO₂ photodiodes by direct chemical vapor deposition (CVD) of MoS₂ onto SiO₂-embedded silicon quantum dot (SQD) multilayers. The SQD:SiO₂ multilayers were first prepared by reactive ion beam sputtering, followed by high-temperature annealing at 1100°C in a nitrogen atmosphere to form well-embedded quantum dots. High-resolution transmission electron microscopy (HRTEM) and photoluminescence (PL) confirmed the presence of SQDs. For MoS₂ deposition, SQD: SiO₂ substrates were placed downstream of MoO₃ powder, and hydrogen sulfide (H₂S) was introduced to synthesize single- and multilayer MoS₂ films at 600°C. The MoO₃ density was controlled to regulate the film thickness, which was analyzed using Raman spectroscopy. Atomic bonding states of MoS₂ were characterized using X-ray photoelectron spectroscopy (XPS), while its optical absorption was evaluated using a UV-Vis-NIR spectrophotometer. To enhance photodetector (PD) performance, a bathocuproine (BCP) layer was spin-coated onto the Si surface, serving as an electron-selective contact to minimize leakage current. Finally, Au and InGa electrodes were deposited using photolithography and a lift-off process. This fabrication method ensures high-quality MoS₂ growth with minimized defects at the MoS₂/SQD:SiO₂ interface, leading to enhanced carrier transport. The resulting photodetector achieves exceptional performance, including a high external quantum efficiency (EQE) of 92%, a detectivity (D*) of 6.1 × 10¹³ Jones, and an ultrafast response time (60 ns rise/756 ns fall), making it one of the fastest MoS₂-based PDs reported. The integration of SQDs further boosts light absorption and carrier extraction, making this fabrication approach highly effective for high-speed and broadband photodetection Figure 4 [27].

The MoS₂/GaN/Si heterostructure photodetector (PD) was fabricated by Deependra Kumar Singh et al. to achieve self-powered, broadband, and spectrally distinctive photodetection. A 250 nm GaN thin film was first grown on Si(111) substrates using plasma-assisted molecular beam epitaxy (PAMBE). The GaN layer plays a crucial role in forming a heterojunction with MoS₂, enabling efficient carrier transport and built-in potential formation. Following GaN deposition, MoS₂ thin films were grown using pulsed laser deposition (PLD). An MoS₂ pellet (2 cm diameter) served as the target, positioned 4.5 cm from the substrate. The deposition was performed at a base pressure of 5.0 × 10⁻⁶ mbar, using a pulsed laser source with a repetition rate of 3 Hz, 1000 laser pulses, and a laser energy of 200 mJ, resulting in a laser fluence of 1 J/cm². During deposition, the chamber pressure was maintained at 1.5 × 10⁻⁵ mbar. The sample underwent in situ annealing at 700°C for 20 minutes, followed by slow cooling at a ramp rate of 5°C/min to enhance film crystallinity. This fabrication approach enables the realization of a high-performance photodetector with a broadband response (300–1100 nm) and an ultrahigh responsivity of 23.81 A/W at 995 nm under low-intensity illumination (0.075 mW/cm²) in a self-powered mode. The device exhibits a unique feature of photocurrent polarity inversion in the NIR region due to a competitive mechanism between the photothermal electric (PTE) effect in MoS₂ and the built-in potential at the MoS₂/GaN/Si heterojunction. The demonstrated wavelength-dependent polarity switching opens new possibilities for low-power optoelectronic applications, making this PD one of the most advanced MoS₂-based devices for future filter-less and energy-efficient photodetection technologies [28].

Minkyun Son et al. fabricated MoS₂/HfS₂ heterojunction photodetector. Firstly, MoS₂ was synthesized via a two-step CVD process in a 2-inch tubular furnace. A SiO₂/p-Si substrate was pre-deposited with MoO₃ and NaCl using thermal evaporation, then placed in a crucible at the center of the furnace, while 4 g of sulfur was positioned 20 cm upstream. The system was heated at 30°C/min under 200 sccm Ar flow at 760 Torr, held at 150°C for 15 min, and then heated at 25°C/min to 500°C. At this point, sulfur vaporization began, and when the chamber temperature reached 750°C, the sulfur heating zone reached 200°C, ensuring uniform MoS₂ growth. The temperature was maintained for 20 min before natural cooling. The grown MoS₂ exhibited large flake sizes (∼200 μm) with 86% coverage, providing a high-quality platform for heterostructure fabrication. The HfS₂/MoS₂ vertical heterojunction was synthesized in a 1-inch tubular furnace with MoS₂/SiO₂ placed at the center and a mixture of HfCl₄ (0.02 g) and NaCl (0.01 g) positioned further away in a crucible. Upstream, 4 g of sulfur was kept 15 cm from MoS₂, with H₂ (15 sccm) and Ar (20 sccm) as carrier gases. The system was heated at 30°C/min to 150°C (held for 15 min), followed by a faster ramp at 40°C/min to 500°C, after which the sulfur heating zone was activated. When the main heater reached 950°C, sulfur reached 150°C, maintaining these conditions for 30 min before natural cooling. This controlled synthesis enabled the selective growth of HfS₂ on MoS₂ due to strong interfacial interactions, forming a vertical heterojunction with a large interface area, enhancing interlayer exciton generation. The conduction band edge of HfS₂ (5.2 eV) resulted in a narrow interlayer bandgap, facilitating IR detection. The built-in potential at the interface suppressed dark current and created gradient band bending, ensuring efficient carrier drift. The resulting HfS₂/MoS₂ photodetector demonstrated exceptional performance, achieving a photoresponsivity of ≈600 A/W and detectivity (D*) of ≈7 × 10¹³ Jones at 1550 nm and ≈1500 A/W with D* ≈ 2 × 10¹⁴ Jones at 980 nm. The device exhibited rapid response times (rise/decay: 60/71 μs), surpassing conventional MoS₂-based detectors. This interlayer exciton-driven IR photodetector, fabricated via a scalable two-step CVD process, marks a significant advancement in high-performance 2D optoelectronic devices Figure 5 [30].

Subhrajit Mukherjee et al. synthesized MoS₂ quantum dots (QDs) via solvent-assisted sonochemical exfoliation, modified to ensure uniform film deposition on Si for vertical Si/MoS₂ p-n heterojunction fabrication. Since high-boiling DMF caused QD aggregation, prolonged sonication was followed by 2 days of vigorous stirring for size uniformity. Gradual centrifugation yielded QD precipitates of different sizes, which were dried under Ar at ∼60°C and redispersed in ethanol for spin-coating onto H-passivated p-type Si (0.77 Ω-cm). Device fabrication included Au (∼70 nm) as the top electrode and Al (∼80 nm) on the Si backside via thermal evaporation (base pressure ∼ 1 × 10⁻⁶ Torr). For LED fabrication, a ∼ 20 nm Al-doped ZnO (AZO) film was deposited via pulsed laser deposition (PLD) at ∼300°C (base pressure ∼ 5 × 10⁻⁶ Torr, energy density ∼ 2 J/cm², repetition rate ∼ 5 Hz), exhibiting a resistivity of ∼10⁻³ Ω·cm and ∼ 92% transmittance. MoS₂ QDs exhibited size-dependent emission (2–26 nm) with nearly stoichiometric 2H-phase and n-type doping due to Cl, confirmed via XPS. Time-resolved PL showed superior carrier lifetimes (1.5–2.5 ns) compared to MoS₂ flakes (∼10 ps). The 0D/3D heterostructures demonstrated rectifying characteristics (ideality factor ∼ 9.0) and bias-dependent electroluminescence (450–800 nm), indicating potential for LED applications. Spectral responsivity increased with decreasing QD size, reaching 0.85 A/W and a peak detectivity of ∼8 × 10¹¹ Jones at −2 V for ∼2 nm QDs, outperforming commercial Si homojunctions and graphene/Si devices. The study highlights the potential of colloidal n-MoS₂ QDs for Si-compatible, large-area multifunctional optoelectronic devices using 0D/3D heterojunctions [29].

3. MoS₂ heterostructure for near IR detection

Our recent work has employed metal passivation by Zn and Fe on MoS₂ to achieve high-performance in the NIR regime and low dark current. As seen in Figure 6d, the synthesized pure MoS₂ (MoS), Zn-MoS, and Fe-MoS are tested in both light and dark conditions. The thermally persuaded charge carriers, also known as leakage current, are typically responsible for the dark current (I_dark), which is measured at 1.5x10⁻⁶ A, 3.98x10⁻⁶ A, and 2.81x10⁻⁶ A for MoS, Zn-MoS, and Fe-MoS devices, respectively. Ag (metal electrode)-MoS₂ (semiconductor) photodetectors with leakage current (I_leak) exhibit Schottky behavior in dark conditions, as demonstrated by the obtained I-V curve. The carrier injection, which regulates the flow of carriers into semiconductors, is directly impacted by the metal-semiconductor interface. Surface passivation of the (Zn and Fe)-MoS₂ surface, which reduces the surface states, could counteract the fermi level pinning (FLP) effect that would deteriorate the carrier injection through the metal-semiconductor (M-S) interface. Band bending (uplift/downshift) is facilitated by the gap states formed by the metal contact between the semiconductor energy bands or by crystal defects or distortion that induce the surface states. Additionally, in order to prevent leakage, the FLP effect is suppressed by altering the surface of MoS₂ with Fe and Zn, which passivates the dangling bonds on the surface and allows for the establishment of a downward shift in fermi energy. Therefore, as illustrated in Figure 6d, the enhanced photoresponse of the devices is indicated by the increased current (I_illum) of 1.89x10⁻⁶ A, 9.77x10⁻⁶ A, and 6.25x10⁻⁶ A for MoS, Zn-MoS, and Fe-MoS under the light illumination. I_ph = I_dark - I_illum is used to calculate the photocurrent (I_ph) at a bias voltage of 3 V. For MoS, Zn-MoS, and Fe-MoS, the enhanced I_ph values are 0.3x10⁻⁶ A, 5.79x10⁻⁶ A, and 3.44x10⁻⁶ A, respectively. The trapping mechanism can be clearly understood from the temporal photoresponse measurement. Figure 6a shows non-exponential behavior in the light ON–OFF condition in the I_llum -t curve, indicating. The trapping of carriers induced photocurrent. For 10 seconds, the MoS first cycle maintains a dark steady-state current of 1.59x10⁻⁶ A. Light is illuminated in such a situation, and an increase in current (1.89x10⁻⁶ A) is observed with a similar steady state for 10 s. The behavior returns with a similar trend as seen in the previous cycle when the dark and light conditions are switched. Comparably, the dark current for (Zn and Fe)-MoS is 3.98x10⁻⁶ and 2.81x10⁻⁶ A, respectively, and it abruptly rises to 9.77x10⁻⁶ and 6.25x10⁻⁶ A when illuminated. Figure 6 illustrates the response’s improved stability across various cycles (a, b, and c). For MoS, Zn-MoS, and Fe-MoS, the computed rise time (τ_r) and decay time (τ_d) are 0.8, 1.3, and 1 s, and 1, 1.2, and 1.5 s, respectively [31]. This is what causes the photocurrent’s abrupt increase and rapid decay. In another work by Vabbina et al. designed and fabricated a shortwave infrared (SWIR) photodiode by integrating a few layers of p-type molybdenum disulfide (MoS₂) onto a graphene layer, forming a van der Waals (vdW) heterojunction. This device exhibited an extensive spectral response covering the SWIR region, as shown in Figure 7b. The photodetector achieved a peak responsivity of 1.26 A/W at a wavelength of 1.44 μm, with a corresponding specific detectivity of 4 × 10¹⁰ Jones. The broadband spectral response of this photodetector was attributed to the coexistence of two distinct operational mechanisms: the optical bandgap excitation mode and the internal photoemission mode. In optical bandgap excitation mode, when the energy of incident photons is greater than the intrinsic bandgap of MoS₂, electron-hole pairs are generated, resulting in a strong photocurrent. This mechanism is responsible for the device’s response at shorter wavelengths, where MoS₂ can efficiently absorb and convert incident photons into charge carriers. In the internal photoemission mode, in addition to direct bandgap excitation, the photodetector can also operate through an internal photoemission process. In this mode, electron-hole pairs are excited even by photons with energy lower than the bandgap of MoS₂. This phenomenon occurs due to an internal transition mechanism between the graphene and MoS₂ layers. When photons with sub-bandgap energy strike the junction, they induce electronic transitions that facilitate photoemission across the potential barrier formed at the MoS₂/graphene interface. This process enables the detection of longer wavelengths beyond the MoS₂ absorption edge, significantly extending the device’s spectral response. The combination of these two modes allows the photodetector to operate efficiently over a wide spectral range, making it highly suitable for SWIR applications. Figure 7a illustrates the internal transition mechanism and the resulting photoemission across the heterojunction, further explaining the broadband response characteristics of the device [32].

Long et al. introduced an advanced shortwave infrared (SWIR) photodiode based on a MoS₂/graphene/WSe₂ van der Waals (vdW) heterojunction, in which a monolayer of graphene was sandwiched between a bottom p-type MoS₂ layer and a top n-type WSe₂ layer, all deposited onto a SiO₂/Si substrate. This unique layered architecture enabled the photodiode to leverage the distinct optical and electronic properties of each material, leading to enhanced performance in terms of broadband absorption and carrier transport. One of the primary advantages of this device configuration was the inclusion of a graphene interlayer, which contributed significantly to broadband light absorption [33]. Unlike conventional semiconductors with a fixed bandgap, graphene, being a zero-bandgap material, possesses a continuous density of electronic states that allows it to absorb a wide spectrum of incident light, from visible to infrared wavelengths. This characteristic was instrumental in extending the photodetection range of the device far beyond that of MoS₂ or WSe₂ alone. The incorporation of MoS₂ (p-type) and WSe₂ (n-type) formed a well-defined p–n junction, which played a crucial role in the efficient separation and transport of photogenerated charge carriers. Upon illumination, electron-hole pairs were generated at different regions of the heterostructure. The built-in electric field at the MoS₂/WSe₂ junction facilitated the rapid separation of these charge carriers, reducing recombination losses and improving the overall quantum efficiency of the photodetector.

The combination of broadband absorption in graphene and the efficient charge separation mechanism in the p–n heterojunction allowed the device to achieve a remarkably wide spectral response. The photodetector exhibited sensitivity across a broad wavelength range, spanning from 0.4 μm (visible) to 2.4 μm (SWIR), as illustrated in Figure 7c. Furthermore, the synergistic effects of the vdW heterojunction led to an improvement in specific detectivity, enabling the device to operate with high sensitivity even in low-light conditions. The MoS₂/graphene/WSe₂ vdW heterojunction photodiode demonstrated a significant advancement in optoelectronic device engineering by integrating the unique properties of 2D materials to achieve high-performance broadband photodetection. Its ability to detect wavelengths from the visible to the SWIR region, coupled with high carrier mobility and efficient charge separation, makes it highly suitable for applications in optical sensing, imaging, and telecommunications. More notably, the MoS₂/graphene/WSe₂ vdW heterojunction photodiode exhibited an impressive responsivity of approximately 2.64 × 10³ A/W at a forward bias of 1 V when illuminated with 1.064 μm wavelength light. This high responsivity was accompanied by a remarkable specific detectivity of 1.1 × 10¹³ Jones and an external quantum efficiency (EQE) reaching nearly 10⁴. An EQE exceeding unity indicates the presence of a photoconductive gain in the device, which can be attributed to several key mechanisms.

First, the short transit distance in the Vertical Junction: The vertical charge transport between MoS₂, graphene, and WSe₂ allowed photogenerated carriers to traverse the junction efficiently, reducing recombination losses. Then, the interlayer inelastic tunneling effect is the presence of graphene as an intermediate layer that facilitated charge transfer across the heterojunction through inelastic tunneling, further enhancing carrier collection efficiency, and finally, the rapid carrier transport, which is the high carrier mobility within the vdW heterostructure, enabled efficient charge separation and extraction, contributing to the significant photoconductive gain. Additionally, the photodetector demonstrated a fast response time across various wavelengths. For instance, at 1.064 μm, the device exhibited a rise time of 24.1 ms, as shown in Figure 7d. This rapid response, combined with its high responsivity and detectivity, underscores the potential of the MoS₂/graphene/WSe₂ heterojunction photodiode for high-performance optoelectronic applications, including infrared imaging and optical communication [36]. Long et al. demonstrated the fabrication of a mid-wave infrared (MWIR) photodiode by utilizing the narrow optical bandgap and the ease of heterojunction formation in two-dimensional black arsenic-phosphorus (b-AsP), specifically in the form of As_xP_1−x. The resulting b-AsP/MoS₂ stacked van der Waals (vdW) heterojunction photodiode exhibited outstanding MWIR photodetection capabilities. Under broadband illumination in the 3–5 μm range, the device achieved an exceptional specific detectivity exceeding 4.9 × 10⁹ Jones at zero bias and room temperature. This performance significantly surpassed that of certain commercially available infrared photodetectors, including PbSe-based and thermistor-based IR photodetectors, as illustrated in Figure 8a. In conclusion, the b-AsP/MoS₂ vdW heterojunction photodiode demonstrated superior MWIR photodetection performance, achieving a high specific detectivity of 4.9 × 10⁹ Jones at zero bias and room temperature. Its broadband response in the 3–5 μm range outperformed conventional IR photodetectors, highlighting the potential of 2D materials for next-generation infrared sensing applications. The exceptional specific detectivity of the b-AsP/MoS₂ vdW heterojunction photodiode was primarily attributed to its suppressed dark current, resulting from the potential barrier at the p-type b-As_xP_1−x and n-type MoS₂ heterojunction, as illustrated in Figure 8(b) and (c). This barrier effectively reduced noise, leading to a two-order magnitude improvement in detectivity compared to a bare b-As_xP_1−x photoconductor. In addition to its high detectivity, the photodiode exhibited an ultrafast response time of approximately 0.5 ms at 4 μm, as shown in Figure 8b [37]. Several black phosphorus (bP)-based vdW heterojunction photodiodes, such as the MoS₂/bP heterojunction for 1.55 μm detection, have also shown excellent performance in the shortwave infrared (SWIR) region in addition to MWIR detection [26], the WSe₂/bP/MoS₂ sandwich-like heterojunction for 1.55 μm [38], and the PdSe₂/bP heterojunction for 1.31 μm detection [35]. These results highlight the versatility of bP-based vdW heterojunctions for broadband infrared photodetection. 2D layered materials-based heterojunctions and CNT-based heterojunctions were fabricated, and both of their properties were compared, from which it is proven that the problems of organic NIR detectors and CNT-based NIR detectors, such as environmental instability and low sensitivity, are said to be overcome with the help of layered materials-based heterojunctions. The graphene/MoS₂ heterojunction is fabricated using a two-step process in which a single layer of graphene is grown on the Cu foil by the chemical vapor deposition method, and then it is transferred to the SiO₂/Si substrate by patterning it as an 800 nm nanoribbon using electron beam lithography, and then MoS₂ flakes are exfoliated and transferred onto the graphene channel by means of PMMA-mediated transfer technique. Finally, source and drain electrodes were also constructed using electron beam lithography. Further, the WeSe₂/MoS₂ heterojunction is fabricated by means of mechanical exfoliation and micromanipulation methods. WSe₂ was transferred to the SiO₂/Si substrate after getting exfoliated from the crystal. MoS₂ flake was placed on WSe₂ by means of a micromanipulation method with the aid of polydimethylsiloxane film and a 3-axis manipulator. Finally, Au/Cr and Pd electrodes were constructed on MoS₂ and WSe₂ flakes, respectively, using electron beam lithography and lift-off techniques (Figure 9a and b). Both the heterojunctions show the diode rectifying behavior (forward bias) in the dark, and the photocurrent was increased when the heterojunctions were illuminated with NIR light (785 cw laser), which shows its response in the NIR region. (Figure 9c and d) The inset figure represents the on/off current ratio at V = ±1 V, which was about 10 and 200, respectively. In particular, WSe₂/MoS₂ heterojunction exhibited photovoltaic effect. The photocurrent density is measured at V = - 0.5 V for both the heterojunctions by using the laser diode of wavelength 808 nm which is turned on and off repeatedly (Figure 9e and f). The photocurrent rise and decay times were shorter than 1 s, which makes it better than CNT/ZnO heterojunction with the rise and fall times of 90 and 70 s, respectively [35].

4. Summary and outlook

2D material-based infrared (IR) photodetectors have emerged as promising candidates for next-generation optoelectronic devices due to their exceptional properties, including tunable bandgaps, high carrier mobility, and strong light-matter interactions. These photodetectors can be classified into three main categories based on device configurations: photoconductors, photodiodes, and nanophotonic-integrated photodetectors. Photoconductors, particularly graphene-based ones, offer high-speed performance, while photodiodes exhibit improved photoresponsivity and noise reduction due to p–n junctions. Additionally, integrating optical nanostructures enhances light absorption, leading to higher responsivity and detectivity. Despite significant advancements, several challenges include limited spectral coverage: most 2D material-based IR photodetectors operate in the SWIR and MWIR ranges, with only a few materials extending to LWIR/FIR, crucial for thermal imaging and biosensing. The Incomplete performance characterization in various photodetector evaluations focus on single-wavelength illumination, lacking full spectral response data needed for optimization. In addition to this, the real-world applicability concerns rely on laser illumination, making practical deployment uncertain. Using blackbody illumination would provide a more realistic assessment. Finally, the scalability issues in current fabrication methods, primarily mechanical exfoliation, hinder large-area and uniform production, limiting commercial viability. To advance 2D material-based IR photodetectors, the area that requires further exploration is bandgap engineering through 2D material alloys—adjusting material composition (e.g., InxGa1 − xP alloys) could enable LWIR to FIR detection. Further, developing CVD growth methods for high-quality, large-area 2D materials tailored for IR photodetectors and comprehensive performance evaluation, i.e., systematic characterization across multiple wavelengths to optimize spectral response. Remarkably, integration with optical nanostructures unambiguously enhances light absorption via waveguides, cavities, plasmonic structures, and large-scale detector arrays, which address integration challenges for high-resolution imaging applications requiring CMOS-compatible processing. By overcoming these challenges, 2D material-based IR photodetectors could revolutionize imaging, sensing, and optoelectronic technologies, paving the way for high-performance, scalable, and broadband photodetection systems.