Highly Efficient, Ultrabroad PdSe2 Phototransistors from Visible to Terahertz Driven by Mutiphysical Mechanism

Two-dimensional topological semimetals: an emerging candidate for terahertz detectors and on-chip integration

The importance of terahertz (THz) detection lies in its ability to provide detailed information in a non-destructive manner, making it a valuable tool across various domains including spectroscopy, communication, and security. The ongoing development of THz detectors aims to enhance their sensitivity, resolution and integration into compact and portable devices such as handheld scanners or integrated communication chips. Generally, two-dimensional (2D) materials are considered potential candidates for device miniaturization but detecting THz radiation using 2D semiconductors is generally difficult due to the ultra-small photon energy. However, this challenge is being addressed by the advent of topological semimetals (TSM) with zero-bandgap characteristics. These semimetals offer low-energy excitations in proximity to the Dirac point, which is particularly important for applications requiring a broad detection range. Their distinctive band structures with linear energy-momentum dispersion near the Fermi level also lead to high electron mobility and low effective mass. The presence of topologically protected dissipationless conducting channels and self-powered response provides a basis for low-energy integration. In order to establish paradigms for semimetal-based THz detectors, this review initially offers an analytical summary of THz detection principles. Then, the review demonstrates the distinct design of devices, the excellent performance derived from the topological surface state and unique band structures in TSM. Finally, we outline the prospective avenues for on-chip integration of TSM-based THz detectors. We believe this review can promote further research on the new generation of THz detectors and facilitate advancements in THz imaging, spectroscopy, and communication systems.

Anisotropic Te/PdSe2 Van Der Waals Heterojunction for Self-Powered Broadband and Polarization-Sensitive Photodetection

Polarization-sensitive broadband optoelectronic detection is crucial for future sensing, imaging, and communication technologies. Narrow bandgap 2D materials, such as Te and PdSe2, show promise for these applications, yet their polarization performance is limited by inherent structural anisotropies. In this work, a self-powered, broadband photodetector utilizing a Te/PdSe2 van der Waals (vdWs) heterojunction, with orientations meticulously tailored is introduced through polarized Raman optical spectra and tensor calculations to enhance linear polarization sensitivity. The device exhibits anisotropy ratios of 1.48 at 405 nm, 3.56 at 1550 nm, and 1.62 at 4 µm, surpassing previously-reported photodetectors based on pristine Te and PdSe2. Additionally, it exhibits high responsivity (617 mA W-1 at 1550 nm), specific detectivity (5.27 × 1010 Jones), fast response (≈4.5 µs), and an extended spectral range beyond 4 µm. The findings highlight the significance of orientation-engineered heterostructures in enhancing polarization-sensitive photodetectors and advancing optoelectronic technology.

Alternating BiI3-BiI van der Waals Photodetector with Low Dark Current and High-Performance Photodetection

The emerging two-dimensional (2D) van der Waals (vdW) materials and their heterostructures hold great promise for optoelectronics and photonic applications beyond strictly lattice-matching constraints and grade interfaces. However, previous photodetectors and optoelectronic devices rely on relatively simple vdW heterostructures with one or two blocks. The realization of high-order heterostructures has been exponentially challenging due to conventional layer-by-layer arduous restacking or sequential synthesis. In this study, we present an approach involving the direct exfoliation of high-quality BiI3-BiI heterostructure nanosheets with alternating blocks, derived from solution-grown binary heterocrystals. These heterostructure-based photodetectors offer several notable advantages. Leveraging the "active layer energetics" of BiI layers and the establishment of a significant depletion region, our photodetector demonstrates a significant reduction in dark current compared with pure BiI3 devices. Specifically, the photodetector achieves an extraordinarily low dark current (<9.2 × 10-14 A at 5 V bias voltage), an impressive detectivity of 8.8 × 1012 Jones at 638 nm, and a rapid response time of 3.82 μs. These characteristics surpass the performance of other metal-semiconductor-metal (MSM) photodetectors based on various 2D materials and structures at visible wavelengths. Moreover, our heterostructure exhibits a broad-band photoresponse, covering the visible, near-infrared (NIR)-I, and NIR-II regions. In addition to these promising results, our heterostructure also demonstrated the potential for flexible and imaging applications. Overall, our study highlights the potential of alternating vdW heterostructures for future optoelectronics with low power consumption, fast response, and flexible requirements.

Uncooled Mid-Infrared Sensing Enabled by Chip-Integrated Low-Temperature-Grown 2D PdTe2 Dirac Semimetal

Next-generation mid-infrared (MIR) imaging chips demand free-cooling capability and high-level integration. The rising two-dimensional (2D) semimetals with excellent infrared (IR) photoresponses are compliant with these requirements. However, challenges remain in scalable growth and substrate-dependence for on-chip integration. Here, we demonstrate the inch-level 2D palladium ditelluride (PdTe2) Dirac semimetal using a low-temperature self-stitched epitaxy (SSE) approach. The low formation energy between two precursors facilitates low-temperature multiple-point nucleation (∼300 °C), growing up, and merging, resulting in self-stitching of PdTe2 domains into a continuous film, which is highly compatible with back-end-of-line (BEOL) technology. The uncooled on-chip PdTe2/Si Schottky junction-based photodetector exhibits an ultrabroadband photoresponse of up to 10.6 μm with a large specific detectivity. Furthermore, the highly integrated device array demonstrates high-resolution room-temperature imaging capability, and the device can serve as an optical data receiver for IR optical communication. This study paves the way toward low-temperature growth of 2D semimetals for uncooled MIR sensing.

Controllable Synthesis of 2H-1T' Mox Re(1- x ) S2 Lateral Heterostructures and Their Tunable Optoelectronic Properties

Constructing heterostructures and doping are valid ways to improve the optoelectronic properties of transition metal dichalcogenides (TMDs) and optimize the performance of TMDs-based photodetectors. Compared with transfer techniques, chemical vapor deposition (CVD) has higher efficiency in preparing heterostructures. As for the one-step CVD growth of heterostructures, cross-contamination between the two materials may occur during the growth process, which may provide the possibility of one-step simultaneous realization of controllable doping and formation of alloy-based heterostructures by finely tuning the growth dynamics. Here, 2H-1T' Mox Re(1- x ) S2 alloy-to-alloy lateral heterostructures are synthesized through this one-step CVD growth method, utilizing the cross-contamination and different growth temperatures of the two alloys. Due to the doping of a small amount of Re atoms in 2H MoS2 , 2H Mox Re(1- x ) S2 has a high response rejection ratio in the solar-blind ultraviolet (SBUV) region and exhibits a positive photoconductive (PPC) effect. While the 1T' Mox Re(1- x ) S2 formed by heavily doping Mo atoms into 1T' ReS2 will produce a negative photoconductivity (NPC) effect under UV laser irradiation. The optoelectronic property of 2H-1T' Mox Re(1- x ) S2 -based heterostructures can be modulated by gate voltage. These findings are expected to expand the functionality of traditional optoelectronic devices and have potential applications in optoelectronic logic devices.

Emerging Trends in 2D TMDs Photodetectors and Piezo-Phototronic Devices

The piezo-phototronic effect shows promise with regards to improving the performance of 2D semiconductor-based flexible optoelectronics, which will potentially open up new opportunities in the electronics field. Mechanical exfoliation and chemical vapor deposition (CVD) influence the piezo-phototronic effect on a transparent, ultrasensitive, and flexible van der Waals (vdW) heterostructure, which allows the use of intrinsic semiconductors, such as 2D transition metal dichalcogenides (TMD). The latest and most promising 2D TMD-based photodetectors and piezo-phototronic devices are discussed in this review article. As a result, it is possible to make flexible piezo-phototronic photodetectors, self-powered sensors, and higher strain tolerance wearable and implantable electronics for health monitoring and generation of piezoelectricity using just a single semiconductor or vdW heterostructures of various nanomaterials. A comparison is also made between the functionality and distinctive properties of 2D flexible electronic devices with a range of applications made from 2D TMDs materials. The current state of the research about 2D TMDs can be applied in a variety of ways in order to aid in the development of new types of nanoscale optoelectronic devices. Last, it summarizes the problems that are currently being faced, along with potential solutions and future prospects.

Ultrabroadband Imaging Based on Wafer-Scale Tellurene

High-resolution imaging is at the heart of the revolutionary breakthroughs of intelligent technologies, and it is established as an important approach toward high-sensitivity information extraction/storage. However, due to the incompatibility between non-silicon optoelectronic materials and traditional integrated circuits as well as the lack of competent photosensitive semiconductors in the infrared region, the development of ultrabroadband imaging is severely impeded. Herein, the monolithic integration of wafer-scale tellurene photoelectric functional units by exploiting room-temperature pulsed-laser deposition is realized. Taking advantage of the surface plasmon polaritons of tellurene, which results in the thermal perturbation promoted exciton separation, in situ formation of out-of-plane homojunction and negative expansion promoted carrier transport, as well as the band bending promoted electron-hole pair separation enabled by the unique interconnected nanostrip morphology, the tellurene photodetectors demonstrate wide-spectrum photoresponse from 370.6 to 2240 nm and unprecedented photosensitivity with the optimized responsivity, external quantum efficiency and detectivity of 2.7 × 10⁷  A W^-1 , 8.2 × 10⁹ % and 4.5 × 10¹⁵  Jones. An ultrabroadband imager is demonstrated and high-resolution photoelectric imaging is realized. The proof-of-concept wafer-scale tellurene-based ultrabroadband photoelectric imaging system depicts a fascinating paradigm for the development of an advanced 2D imaging platform toward next-generation intelligent equipment.

Ultralow-noise Terahertz Detection by p-n Junctions in Gapped Bilayer Graphene

Graphene shows strong promise for the detection of terahertz (THz) radiation due to its high carrier mobility, compatibility with on-chip waveguides and transistors, and small heat capacitance. At the same time, weak reaction of graphene's physical properties on the detected radiation can be traced down to the absence of a band gap. Here, we study the effect of electrically induced band gap on THz detection in graphene bilayer with split-gate p-n junction. We show that gap induction leads to a simultaneous increase in current and voltage responsivities. At operating temperatures of ∼25 K, the responsivity at a 20 meV band gap is from 3 to 20 times larger than that in the gapless state. The maximum voltage responsivity of our devices at 0.13 THz illumination exceeds 50 kV/W, while the noise equivalent power falls down to 36 fW/Hz^1/2.

Research Process on Photodetectors based on Group-10 Transition Metal Dichalcogenides

Rapidly evolving group-10 transition metal dichalcogenides (TMDCs) offer remarkable electronic, optical, and mechanical properties, making them promising candidates for advanced optoelectronic applications. Compared to most TMDCs semiconductors, group-10-TMDCs possess unique structures, narrow bandgap, and influential physical properties that motivate the development of broadband photodetectors, specifically infrared photodetectors. This review presents the latest developments in the fabrication of broadband photodetectors based on conventional 2D TMDCs. It mainly focuses on the recent developments in group-10 TMDCs from the perspective of the lattice structure and synthesis techniques. Recent progress in group-10 TMDCs and their heterostructures with different dimensionality of materials-based broadband photodetectors is provided. Moreover, this review accounts for the latest applications of group-10 TMDCs in the fields of nanoelectronics and optoelectronics. Finally, conclusions and outlooks are summarized to provide perspectives for next-generation broadband photodetectors based on group-10 TMDCs.

Sensitive Room-Temperature Graphene Photothermoelectric Terahertz Detector Based on Asymmetric Antenna Coupling Structure

A highly sensitive room-temperature graphene photothermoelectric terahertz detector, with an efficient optical coupling structure of asymmetric logarithmic antenna, was fabricated by planar micro-nano processing technology and two-dimensional material transfer techniques. The designed logarithmic antenna acts as an optical coupling structure to effectively localize the incident terahertz waves at the source end, thus forming a temperature gradient in the device channel and inducing the thermoelectric terahertz response. At zero bias, the device has a high photoresponsivity of 1.54 A/W, a noise equivalent power of 19.8 pW/Hz^1/2, and a response time of 900 ns at 105 GHz. Through qualitative analysis of the response mechanism of graphene PTE devices, we find that the electrode-induced doping of graphene channel near the metal-graphene contacts play a key role in the terahertz PTE response. This work provides an effective way to realize high sensitivity terahertz detectors at room temperature.

Investigating active area dependent high performing photoresponse through thin films of Weyl Semimetal WTe2

WTe₂ is one of the wonder layered materials, displays interesting overlapping of electron-hole pairs, opening of the surface bandgap, anisotropy in its crystal structure and very much sought appealing material for room temperature broadband photodection applications. Here we report the photoresponse of WTe₂ thin films and microchannel devices fabricated on silicon nitride substrates. A clear sharp rise in photocurrent observed under the illumination of visible (532 nm) and NIR wavelengths (1064 nm). The observed phoresponse is very convincing and repetitive for ON /OFF cycles of laser light illumination. The channel length dependence of photocurrent is noticed for few hundred nanometers to micrometers. The photocurrent, rise & decay times, responsivity and detectivity are studied using different channel lengths. Strikingly microchannel gives few orders of greater responsivity compared to larger active area investigated here. The responsivity and detectivity are observed as large as 29 A/W and 3.6 × 10⁸ Jones respectively. The high performing photodetection properties indicate that WTe₂ can be used as a broad band material for future optoelectronic applications.

Ultrasensitive Self-Driven Terahertz Photodetectors Based on Low-Energy Type-II Dirac Fermions and Related Van der Waals Heterojunctions

The exotic electronic properties of topological semimetals (TSs) have opened new pathways for innovative photonic and optoelectronic devices, especially in the highly pursuit terahertz (THz) band. However, in most cases Dirac fermions lay far above or below the Fermi level, thus hindering their successful exploitation for the low-energy photonics. Here, low-energy type-II Dirac fermions in kitkaite (NiTeSe) for ultrasensitive THz detection through metal-topological semimetal-metal heterostructures are exploited. Furthermore, a heterostructure combining two Dirac materials, namely, graphene and NiTeSe, is implemented for a novel photodetector exhibiting a responsivity as high as 1.22 A W^-1 , with a response time of 0.6 µs, a noise-equivalent power of 18 pW Hz^-0.5 , with outstanding stability in the ambient conditions. This work brings to fruition of Dirac fermiology in THz technology, enabling self-powered, low-power, room-temperature, and ultrafast THz detection.

Excitonic Insulator Enabled Ultrasensitive Terahertz Photodetection with Efficient Low-Energy Photon Harvesting

Despite the interest toward the terahertz (THz) rapidly increasing, the high-efficient detection of THz photon is not widely available due to the low photoelectric conversion efficiency at this low-energy photon regime. Excitonic insulator (EI) states in emerging materials with anomalous optical transitions and renormalized valence band dispersions render their nontrivial photoresponse, which offers the prospect of harnessing the novel EI properties for the THz detection. Here, an EI-based photodetector is developed for efficient photoelectric conversion in the THz band. High-quality EI material Ta₂ NiSe₅ is synthesized and the existence of the EI state at room temperature is confirmed. The THz scanning near-field optical microscopy experimentally reveals the strong light-matter interaction in the THz band of EI state in the Ta₂ NiSe₅ . Benefiting from the strong light-matter interaction, the Ta₂ NiSe₅ -based photodetectors exhibit superior THz detection performances with a detection sensitivity of ≈42 pW Hz^-1/2 and a response time of ≈1.1 µs at 0.1 THz at room temperature. This study provides a new avenue for realizing novel high-performance THz photodetectors by exploiting the emerging EI materials.

Rational Design of WSe2 /WS2 /WSe2 Dual Junction Phototransistor Incorporating High Responsivity and Detectivity

The excellent semiconducting properties and ultrathin morphological characteristics allow van der Waals (vdW) heterostructures based on 2D materials to be promising channel materials for the next-generation optoelectronic devices, especially in photodetectors. Although various 2D heterostructure-based photodetectors have been developed, the unavoidable trade-off between responsivity and detectivity remains a critical issue for these devices. Here, an ingenious phototransistor based on WSe₂ /WS₂ /WSe₂ dual-vdW heterostructures is constructed, performing both high responsivity and detectivity. In the charge neutrality point (gate voltage of -15 V and bias voltage of 1 V), this device demonstrates a pronounced photosensitivity, accompanying with high detectivity of 1.9 × 10¹⁴  Jones, high responsivity of 35.4 A W^-1 , and fast rise/fall time of 3.2/2.5 ms at 405 nm with power density of 60 µW cm^-2 . Density functional theory calculations, energy band profiles, and optoelectronic characteristics jointly verify that the high performance is ascribed to the distinctive device design, which not only facilitates the separation of photogenerated carriers but also produces a strong photogating effect. As a feasible application, an automotive radar system is demonstrated, proving that the device has considerable potential for application in vehicle intelligent assisted driving.

Waveguide-Integrated PdSe2 Photodetector over a Broad Infrared Wavelength Range

Hybrid integration of van der Waals materials on a photonic platform enables diverse exploration of novel active functions and significant improvement in device performance for next-generation integrated photonic circuits, but developing waveguide-integrated photodetectors based on conventionally investigated transition metal dichalcogenide materials at the full optical telecommunication bands and mid-infrared range is still a challenge. Here, we integrate PdSe₂ with silicon waveguide for on-chip photodetection with a high responsivity from 1260 to 1565 nm, a low noise-equivalent power of 4.0 pW·Hz^-0.5, a 3-dB bandwidth of 1.5 GHz, and a measured data rate of 2.5 Gbit·s^-1. The achieved PdSe₂ photodetectors provide new insights to explore the integration of novel van der Waals materials with integrated photonic platforms and exhibit great potential for diverse applications over a broad infrared range of wavelengths, such as on-chip sensing and spectroscopy.

Terahertz modulation characteristics of three nanosols under external field control based on microfluidic chip

Recently, with the widespread application of metamaterials in the terahertz (THz) modulation field, solid-state THz modulators have made breakthrough progress; however, there are still challenges in preparing flexible THz modulators with wide modulation bandwidths. In this study, a THz microfluidic chip was fabricated using cycloolefin copolymers with high transmission (90%) of THz waves. The THz modulation characteristics of TiO₂, Ag, and Fe₃O₄ nanosols under the control of an optical field, electric field, and magnetic field, respectively, were investigated. Under the action of photogenerated carrier migration, colloidal electrophoresis, and magneto-optical effect, all three nanosols exhibit broadband modulation performance in the frequency range of 0.3–2.4 THz, and the maximum modulation depth is 24%, 33%, and 54%, respectively. Contrary to previous studies based on traditional solid-state materials, this study innovatively explores the possibility of modulating THz waves with liquid materials, laying the foundation for the application of flexible liquid-film THz modulators.

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THz broadband amplitude modulation of liquid nanosols under external fields

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Using a microfluidic chip to reduce the absorption of THz waves by hydrogen bonds

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The experimental results lay a foundation for liquid-film THz modulators