Ultrafast Photoresponse Research Articles

Large-scale m-GeS2 grown on GaN for self-powered ultrafast UV photodetection

Ultraviolet (UV) photodetection has been of great importance in the fields of aerospace, space communications, and remotely sensed images. However, conventional UV photodetectors (PDs) have been facing intrinsic drawbacks of complicate structural issues and interference from ambient visible light. Therefore, wide bandgap semiconductors have attracted significant attention. Herein, self-powered PDs based on the monolithic germanium disulfide (m-GeS2)/gallium nitride (GaN) pn heterojunction have been proposed with a large-size m-GeS2 over 2 cm2. The electronic and optical properties of the m-GeS2/GaN heterojunction are investigated via experiments and first-principles methods. The PDs reveal an impressive performance in self-powered response and high responsivity and detectivity of 16.8 mA W−1 and 1.03 × 1011 Jones, respectively. Further analyses determined that the PDs show an ultrafast photoresponse with a rise and fall time of < 0.30 and 1.10 ms at 365 nm. Consequently, this study provides a feasible method for the synthesis of large-sized m-GeS2 and demonstrates its enormous potential in high-performance, self-powered UV photodetection.

Synthesis of interface modified MoS2/ZnO heterostructure via simple hydrothermal method and their enhanced UV photodetection characteristics with ultrafast photoresponse speed

Ultraviolet (UV) photodetectors employing ZnO nanorods (NRs) exhibit high photocurrent stability; however, UV photodetectors fabricated using ZnO NRs have several disadvantages. In this study, we fabricated high-performance UV photodetector using hydrothermally grown MoS2/ZnO heterostructure. The surface morphology of MoS2/ZnO heterostructure exhibited sea-urchin structure with a greater length and diameter than ZnO NRs. In addition, the formation of MoS2/ZnO heterostructure can form a type-II band alignment at interface between MoS2 and ZnO, which facilitates the regulation of electron movement with large surface area. With respect to UV photoresponse, MoS2/ZnO heterostructure exhibited a significantly improved photocurrent value compared to ZnO NRs and MoS2/ZnO heterostructure had extremely fast UV photoresponse speed with highly improved photoresponsivity, photosensitivity, and photoselectivity. Therefore, it was concluded that hydrothermally grown MoS2/ZnO heterostructure is a suitable solution to drawbacks of ZnO-based UV photodetectors; consequently, the proposed method can be used to fabricate ZnO-based high-performance optoelectronics applied in various fields.

Synergistic‐engineered van der Waals photodiodes with high efficiency

AbstractVan der Waals (vdW) heterostructures based on two‐dimensional transition‐metal dichalcogenides have provided unprecedented opportunities for photovoltaic detectors owing to their strong light‐matter interaction and ultrafast interfacial charge transfer. Despite continued advancement, insufficient control of photocarrier behaviors still limits the external quantum efficiency (EQE) and operation speed of such detectors. Here, we propose a synergistic strategy of contact‐configuration design and thickness‐modulation to construct high‐performance vdW photodiodes based on the typical type II heterostructure (MoS2/WSe2). Through integrating three contact architectures into one device to exclude other factors, we solid the superiority of designed 1L‐MoS2/WSe2/graphene heterostructures incorporating efficient photocarrier collection and gate modulation. Together with leveraging the layer‐number‐dependent properties of WSe2, we observe the critical thickness of WSe2 (11 layers) for the highest EQE, which verifies the thickness‐dependent competition between photocarrier generation, dissociation, and collection. Finally, we demonstrate the synergistic‐engineered vdW heterostructure can trigger record‐high EQE (61%) and manifest ultrafast photoresponse (4.1 μs) at the atomically thin limit (8 nm). The proposed strategy enables architecture‐design and thickness‐engineering to unlock the potential to realize high‐performance optoelectronic devices.

Van der Waals MoS2/Two-Dimensional Perovskite Heterostructure for Sensitive and Ultrafast Sub-Band-Gap Photodetection.

Two-dimensional (2D) hybrid perovskites have been extensively studied as the promising light-sensitive materials in the photodetectors owing to their improved structural stability over that of their three-dimensional counterparts. However, the application of the 2D perovskite-based photodetector in the near-infrared (NIR) region is obstructed by the large intrinsic optical band gap. Herein, we develop a novel van der Waals heterostructure composed of few-layer 2D perovskite/MoS2 nanoflakes, which exhibits high-sensitivity detection performance over a broad spectral region, from the visible region to the telecommunication wavelength (i.e., 1550 nm). In particular, the photoresponsivity and specific detectivity under an 860 nm laser reach 121 A W-1 and 4.3 × 1014 Jones, respectively, whereas the individual nanoflakes show no response under the same wavelength. Meanwhile, the response time at the microsecond (μs) level is obtained, shortened by around 3 orders of magnitude compared to that of the constituting layers. The sensitive and ultrafast photoresponse at the NIR wavelength stems from the strong interlayer transition of sub-band-gap photons and the rapid separation of the photogenerated carriers by the built-in field within the heterojunction area. Our results not only provide an effective approach to achieve sub-band-gap photodetection in 2D perovskite-based structures but also suggest a universal strategy to fabricate high-performance optoelectronic devices.

A visible to near-infrared nanocrystalline organic photodetector with ultrafast photoresponse

A high performance organic photodetector with an ultrafast photoresponse of 1.8 μs, a high responsivity of 680 mA W−1 and a detectivity of 6.35 × 1013 Jones was fabricated. The small Frenkel exciton binding energy ensures that excitons can be efficiently dissociated.

Ultrafast photoresponse of vertically oriented TMD films probed in a vertical electrode configuration on Si chips.

Integrated photodetectors based on transition metal dichalcogenides (TMDs) face the challenge of growing their high-quality crystals directly on chips or transferring them to the desired locations of components by applying multi-step processes. Herein, we show that vertically oriented polycrystalline thin films of MoS2 and WS2 grown by sulfurization of Mo and W sputtered on highly doped Si are robust solutions to achieve on-chip photodetectors with a sensitivity of up to 1 mA W−1 and an ultrafast response time in the sub-μs regime by simply probing the device in a vertical arrangement, i.e., parallel to the basal planes of TMDs. These results are two orders of magnitude better than those measured earlier in lateral probing setups having both electrodes on top of vertically aligned polycrystalline TMD films. Accordingly, our study suggests that easy-to-grow vertically oriented polycrystalline thin film structures may be viable components in fast photodetectors as well as in imaging, sensing and telecommunication devices.

Wafer‐Scale InN/In2S3 Core–Shell Nanorod Array for Ultrafast Self‐Powered Photodetection

AbstractSelf‐powered photodetectors have paved the way for electronic applications in fields such as civilian communication, infrared mapping, and industrial automatic control. However, most self‐powered photodetectors have faced photoresponse‐speed and device‐scale bottlenecks. Herein, a novel, self‐powered detector with an ultrafast response speed based on a core–shell InN/In2S3 nanorod array is proposed. A wafer‐scale InN/In2S3 nanorod array with good homogeneity is synthesized on Si substrates via a simple two‐step method involving molecular beam epitaxy and chemical vapor deposition. The photodetector device exhibits excellent self‐powered properties and a high current on/off ratio of 5 × 103. Further analyses determined that the device have an excellent photovoltaic responsivity and detectivity of 140 mA·W−1 and 4.0 × 1010 Jones, respectively (0 V). Impressively, the device exhibits an ultrafast photoresponse with a rise/fall time of 22/32 µs. The self‐powered InN/In2S3 photodetector with an ultrafast response speed shows superior potential for electronic applications. The core–shell nanostructure hybrid heterojunction introduces a novel idea for wafer‐scale nano‐photodetectors.

Wide-bandgap semiconductor microtubular homojunction photodiode for high-performance UV detection

Here we report a novel ZnO microtubular homojunction photodiode (PD) fabricated by optical vapor supersaturated precipitation (OVSP) and sputtering methods. The acceptor-rich ZnO (A-ZnO) microtube grown by OVSP possesses a great UV–visible rejection ratio of ~3.77 and a high external quantum yield of 1.4 × 103%. The formed A-ZnO/n-ZnO homojunction UV-PD realizes an ultrafast photoresponse down to 6 ms (three orders of magnitude faster) and boosts the sensitivity by 2.6-folds with respect to the bare A-ZnO microtube. It is beneficial for weak-signal UV detection down to the threshold of 0.4 mW/cm2 under random excitation conditions, by which the detectivity up to 6.67 × 1012 cmHz1/2/W is achieved. The ZnO microtubular homojunction paves a new way for design of high-performance wide-bandgap semiconductor UV-PDs, promoting their practical applications in future.

Ultrafast Nanoscale Gradient Junction Self‐Powered Schottky Photodetectors for Vision‐Like Object Classification

AbstractEmulating human vision for pattern classification is essential in intelligent machines, including pilotless vehicles and humanoid robots. Traditionally, analog visual information is captured by photosensors, which are sequentially classified using a physically separated algorithm‐based unit, such as a computer. Among such units, a self‐powered photodetector with a quick information sensing feature can be utilized for an unprecedented pattern classification; however, designing an ultrafast photodetector while maintaining a high sensitivity is critical. Herein, an alternative photovoltaic‐effect‐based, highly sensitive, self‐powered, broadband, ultrafast gradient junction nanoscale Schottky photodetector is introduced, that can sense input optical information without latency. Importantly, the proposed device can sense optical input within a ≈40 ns duration and demonstrate a 3‐dB bandwidth wider than 0.5 MHz, providing a throughput of 10 million bits per second. Photoconductive atomic force microscopy and Kelvin probe force microscopy independently revealed the photodynamic characteristic at a nanometer (≈35 nm) scale. Further, an array that can classify nontrivial patterns even with noisy inputs is developed. A unique solution to achieve an ultrafast photo response in self‐powered mode and classify the input patterns is provided. The proposed approach can be extended to several other artificial neural sensors, such as tactile, audio, and thermal sensors.

Critical Strain-Induced Photoresponse in Folded Graphene Superlattices.

Strain engineering is the most effective method to break the symmetry of the graphene lattice and achieve graphene band gap tunability. However, a critical strain (>20%) is required to open the graphene band gap, and it is very difficult to achieve such a large strain. This limits the development of experimental research and optoelectronic devices based on graphene strain. In this work, we report a method for preparing large-strain graphene superlattices via surface energy engineering. The maximum strain of the curved lattice could reach 50%. In particular, our pioneering work reports the behavior of an ultrafast (as short as 6 ps) photoresponse in a strained folded graphene superlattice. The photocurrent map shows a large increase (up to 102) of the photoresponsivity in the tensile graphene lattice, which is generated by the interaction between the strained and pristine graphene. Through Raman spectroscopy, Kelvin probe force microscopy, and high-resolution transmission electron microscopy, we demonstrate that the ultrathreshold strain in the graphene bends triggers the opening of the graphene band gap and results in a unique photovoltaic effect. This work deepens the understanding of the strain-induced change of the photoelectrical properties of graphene and proves the potential of strained graphene as a platform for the generation of novel high-speed, miniaturized graphene-based photodetectors.

Novel Authentic and Ultrafast Organic Photorecorders Enhanced by AIE‐Active Polymer Electrets via Interlayer Charge Recombination

AbstractOrganic photonic memory, featuring a variety of glamorously light‐driven characteristics, is rapidly growing into an indispensable building block for next‐generation optical communication systems. However, the ambiguity of their operating mechanism associated with the limitation of photoadaptive materials as an electronics promoter results in the slow development of photonic transistor‐based devices. In this study, the conjugated polymers composed of donor–acceptor motifs with typical aggregation‐induced emission (AIE) behaviors are designed and successfully discover high‐performance photoprogrammable memory. Moreover, the mechanism of photoboosted recording behavior, attributed to the recombination of the formed interlayer excitons right after simultaneous excitation without applying vertical and parallel electric‐field at the interface in‐between active semiconductor and AIE polymers, is cautiously corroborated by steady‐state PL and pulse PL measurements. The AIE‐polymer memory devices perform ultrafast photoresponse time of 0.1 ms, an outstanding current switch ratio up to 106, and retention stability over 40 000 s without significant dissipation. Furthermore, photoresponsive AIE‐polymer electrets not only modulate the memory performance through the emission wavelength but easily switch storage behavior of nonvolatile memory from flash to WORM by adjusting the torsion‐angle through the motif of the donor and acceptor moieties. These findings open an avenue for designing conjugated polymer electret for ultrafast optical storage devices.

Ultrafast Dynamics and Photoresponse of a Fungi-Derived Pigment Xylindein from Solution to Thin Films.

Organic semiconductor materials have recently gained momentum due to their non-toxicity, low cost, and sustainability. Xylindein is a remarkably photostable pigment secreted by fungi that grow on decaying wood, and its relatively strong electronic performance is enabled by π-π stacking and hydrogen-bonding network that promote charge transport. Herein, femtosecond transient absorption spectroscopy with a near-IR probe was used to unveil a rapid excited-state intramolecular proton transfer reaction. Conformational motions potentially lead to a conical intersection that quenches fluorescence in the monomeric state. In concentrated solutions, nascent aggregates exhibit a faster excited state lifetime due to excimer formation, confirmed by the excimer→charge-transfer excited-state absorption band of the xylindein thin film, thus limiting its optoelectronic performance. Therefore, extending the xylindein sidechains with branched alkyl groups may hinder the excimer formation and improve optoelectronic properties of naturally derived materials.

Fabrication of 1D Te/2D ReS2 Mixed-Dimensional van der Waals p-n Heterojunction for High-Performance Phototransistor.

The superior optical and electronic properties of the two-dimensional (2D) rhenium disulfide (ReS2) makes it suitable for nanoelectronic and optoelectronic applications. However, the internal defects coupled with with the low mobility and light-absorbing capability of ReS2 impede its utilization in high-performance photodetectors. Fabrication of mixed-dimensional heterojunctions is an alternative method for designing high-performance hybrid photodetectors. This study proposes a mixed-dimensional van der Waals (vdW) heterojunction photodetector, containing high-performance one-dimensional (1D) p-type tellurium (Te) and 2D n-type ReS2, developed by depositing Te nanowires on ReS2 nanoflake using the dry transfer method. It can improve the injection and separation efficiency of photoexcited electron-hole pairs due to the type II p-n heterojunction formed at the ReS2 and Te interface. The proposed heterojunction device is sensitive to visible-light sensitivity (632 nm) with an ultrafast photoresponse (5 ms), high responsivity (180 A/W), and specific detectivity (109), which is superior to the pristine Te and ReS2 photodetectors. As compared to the ReS2 device, the responsivity and response speed is better by an order of magnitude. These results demonstrate the fabrication and application potential of Te/ReS2 mixed-dimensional heterojunction for high-performance optoelectronic devices and sensors.

Scalable fabrication of long-wave infrared PtSe2-G heterostructure array photodetectors

Two-dimensional (2D) materials with excellent optoelectronic properties have attracted tremendous research interest in recent years. The promising performances of photodetectors based on 2D materials, such as ultrafast photoresponse and ultrahigh photoresponsivity, have been demonstrated in the visible to short-wavelength infrared spectrum range (0.8–2 μm). However, high performance, room temperature operation long-wavelength infrared (LWIR) photodetection is challenging. The detectors based on graphene usually exhibit low photoresponsivity due to the low optical absorption and short carrier lifetime. In addition, the relatively large bandgap of transition metal dichalcogenides limited the photoresponse bandwidth. Here, we report a way to fabricate a scalable device array of room-temperature operation LWIR PtSe2-G heterostructure detectors. The photoresponsivity at 10.6 μm up to ∼300 mA/W is obtained. The long-wave infrared light in the pico-watt range could be detected at room temperature by the PtSe2-G heterostructure detector. This result indicates that the PtSe2-G heterostructure device could be a highly competitive candidate for an uncooled LWIR detector. It also opens a way for a scalable array infrared focus plane device for the LWIR image.

Ultrafast Photocurrent Response and High Detectivity in Two-Dimensional MoSe2-based Heterojunctions.

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials have garnered great attention on account of their novel properties and potential to advance modern technology. Recent studies have demonstrated that TMDCs can be utilized to create high-performing heterostructures with combined functionality of the individual layers and new phenomena at these interfaces. Here, we report an ultrafast photoresponse within MoSe2-based heterostructures in which heavily p-doped WSe2 and MoS2 flakes share an undoped MoSe2 channel, allowing us to directly compare the optoelectronic properties of MoSe2-based heterojunctions with different 2D materials. Strong photocurrent signals have been observed in both MoSe2-WSe2 and MoSe2-MoS2 heterojunctions with a photoresponse time constant of ∼16 μs, surmounting previous MoSe2-based devices by three orders of magnitude. Further studies have shown that the fast response is independent of the integrated 2D materials (WSe2 or MoS2) but is likely attributed to the high carrier mobility of 260 cm2 V-1 s-1 in the undoped MoSe2 channel as well as the greatly reduced Schottky barriers and near absence of interface states at MoSe2-WSe2/MoS2 heterojunctions, which lead to reduced carrier transit time and thus short photocurrent response time. Lastly, a high detectivity on the order of ∼1014 Jones has been achieved in MoSe2-based heterojunctions, which supersedes current industry standards. These fundamental studies not only shed light on photocurrent generation mechanisms in MoSe2-based heterojunctions but also open up new avenues for engineering future high-performance 2D optoelectronic devices.

Photoresponse properties of CdSe thin film photodetector

In the present investigation, the performance of highly sensitive photodetector based on CdSe thin film has been demonstrated. The cadmium selenide (CdSe) films were deposited successfully by spray pyrolysis method on glass substrate. The as-deposited films were used as visible light detector. Further, the structural, morphological, and optical properties of CdSe films were investigated through X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy, and optical absorption spectrophotometer. The XRD studies show that the produced samples are polycrystalline in nature. A UV–Visible absorbance spectrum indicates direct allowed transition with bandgap about 1.75 eV. The SEM micrographs depict uniformly distributed well defined uneven hexagonal grains. The CdSe film-based photodetector exhibits high responsivity, detectivity and shows an ultrafast photoresponse. The excellent photoresponse and high photosensitivity of CdSe film makes it a strong candidate for high-performance photodetectors.

Lead-Free Cs4CuSb2Cl12 Layered Double Perovskite Nanocrystals.

Concerns about the toxicity of lead-based perovskites have aroused great interest for the development of alternative lead-free perovskite-type materials. Recently, theoretical calculations predict that Pb2+ cations can be substituted by a combination of Cu2+ and Sb3+ cations to form a vacancy-ordered layered double perovskite structure with superior optoelectronic properties. However, accessibilities to this class of perovskite-type materials remain inadequate, hindering their practical implementations in various applications. Here, we report the first colloidal synthesis of Cs4CuSb2Cl12 perovskite-type nanocrystals (NCs). The resulting NCs exhibit a layered double perovskite structure with ordered vacancies and a direct band gap of 1.79 eV. A composition-structure-property relationship has been established by investigating a series of Cs4CuxAg2-2xSb2Cl12 perovskite-type NCs (0 ≤ x ≤ 1). The composition induced crystal structure transformation, and thus, the electronic band gap evolution has been explored by experimental observations and further confirmed by theoretical calculations. Taking advantage of both the unique electronic structure and solution processability, we demonstrate that the Cs4CuSb2Cl12 NCs can be solution-processed as high-speed photodetectors with ultrafast photoresponse and narrow bandwidth. We anticipate that our study will prompt future research to design and fabricate novel and high-performance lead-free perovskite-type NCs for a range of applications.

Defect-Mediated Transport in Self-Powered, Broadband, and Ultrafast Photoresponse of a MoS2/AlN/Si-Based Photodetector

By combining unique properties of ultrathin 2D materials with conventional 3D semiconductors, devices with enhanced functionalities can be realized. Here, we report a self-powered and ultrafast pho...

In Situ Conformal Coating of Polyaniline on GaN Microwires for Ultrafast, Self-Driven Heterojunction Ultraviolet Photodetectors.

Independent and zero-maintenance systems would be in urgent need in the near future internet of things. Here, we present high-performance, self-driven organic/inorganic heterojunction ultraviolet (UV) photodetectors (PDs) by in situ polymerization of polyaniline (PANI) on Gallium nitride microwires. The GaN microwires with a high crystalline quality are grown on patterned Si substrates by metal organic chemical vapor deposition. Using a facile in situ chemical polymerization method, PANI is conformally coated on the surface of GaN microwires. The constructed GaN/PANI hybrid microwire PD exhibits a high responsivity of 178 mA/W, a remarkable detectivity of 4.67 × 1014 jones, and an ultrafast UV photoresponse speed (rise time of 0.2 ms and fall time of 0.3 ms) under zero bias. The intimate heterojunction in the form of N-Ga-N bonds between GaN and PANI may account for the observed high performances. The presented self-driven microwire UV PDs featuring ultrahigh-speed (sub-millisecond) response to UV light may find applications in future nano/micro-photosensor networks.

Quantitative Understanding of Charge-Transfer-Mediated Fe3+ Sensing and Fast Photoresponse by N-Doped Graphene Quantum Dots Decorated on Plasmonic Au Nanoparticles.

The formation of a heterostructure with plasmonic nanoparticles drastically alters the optoelectronic properties of graphene quantum dots (GQDs), resulting in exceptional properties. In the present work, we prepare nitrogen-doped GQDs decorated on gold nanoparticles (Au@N-GQDs) by a one-step green reduction method and study its extraordinary fluorescence and photoresponse characteristics. The as-prepared Au@N-GQDs show more than one order of magnitude enhancement in the fluorescence intensity as compared to the bare N-GQDs, which is attributed to hot electron generation and improved absorption in N-GQDs by local field enhancement and the modification of the edge functional groups. Because of the selective coordination to Fe3+ ions, the Au@N-GQDs exhibit extraordinary quenching of fluorescence, with ultrahigh sensitivity for the detection of Fe3+ (<1 nM). A new model for the charge-transfer dynamics is developed involving the Langmuir's law of adsorption to explain the unusual quenching, which strongly deviates from the known models of static/dynamic quenching. The proposed sensor is successfully implemented for the ultrasensitive detection of Fe3+ ions in human serum and Brahmaputra river water samples, representing its high potential applications in clinical as well as environmental diagnosis. Additionally, because of its high absorption in the UV-vis-NIR region and high charge density with long life excitons, the Au@N-GQDs are utilized as photodetectors with ∼104 times faster response than that of bare N-GQDs. The Au@N-GQD-based photodetector possesses a high responsivity of ∼1.36 A/W and a remarkably high external quantum efficiency of ∼292.2%, which is much superior to the GQD-based photodetectors reported till date. The underlying mechanism of ultrafast photoresponse is ascribed to the transfer of hot electrons along with the tunneling of the electrons from Au NPs to N-GQDs as well as the defect reduction of N-GQDs by the incorporation of Au NPs. Without the use of any charge transporting layer, the outstanding performance of N-GQD-based plasmonic photodetector opens up unique opportunities for future high-speed optoelectronic devices.