Unveiling transient current response in bilayer oxide-based physical reservoirs for time-series data analysis

Physical reservoirs employed to map time-series data and analyze extracted features have attracted interest owing to their low training cost and mitigated interconnection complexity. This study reports a physical reservoir based on a bilayer oxide-based dynamic memristor. The proposed device exhibits a nonlinear current response and short-term memory (STM), satisfying the requirements of reservoir computing (RC). These characteristics are validated using a compact model to account for resistive switching (RS) via the dynamic evolution of the internal state variable and the relocation of oxygen vacancies. Mathematically, the transient current response can be quantitatively described according to a simple set of equations to correlate the theoretical framework with experimental results. Furthermore, the device shows significant reliability and ability to distinguish 4-bit inputs and four diverse neural firing patterns. Therefore, this work shows the feasibility of implementing physical reservoirs in hardware and advances the understanding of the dynamic response.

Nanostructured doping of WSe2via block copolymer patterns and its self-powered photodetector application

Transition metal dichalcogenides (TMDs), e.g., MoS₂, MoSe₂, ReS₂, and WSe₂, are effective materials for advanced optoelectronics owing to their intriguing optical, structural, and electrical properties. Various approaches for manipulating the surface of the TMDs have been suggested to unleash the optoelectronic potential of the TMDs. Herein, we employed the self-assembly of the poly(styrene-b-methyl methacrylate) (PS-b-PMMA) block copolymer (BCP) to prepare a nanoporous pattern and generate nanostructured charge-transfer p-doping on the WSe₂ surface, maximizing the depletion region in the absorber layer. After the spin coating and thermal annealing of PS-b-PMMA, followed by the selective etching of PMMA cylindrical microdomains using oxygen reactive-ion plasma, nanopatterned WO_x with high electron affinity was grown on the WSe₂ surface, forming a three-dimensional homojunction. The nanopatterned WO_x significantly expanded the depletion region in the WSe₂ layer, thus enhancing optoelectronic performance and self-powered photodetection. The proposed approach based on the nanostructured doping of the TMDs via BCP nanolithography can help create a promising platform for highly functional optoelectrical devices.

Bias-Modified Schottky Barrier Height-Dependent Graphene/ReSe2 van der Waals Heterostructures for Excellent Photodetector and NO2 Gas Sensing Applications

Herein, we reported a unique photo device consisting of monolayer graphene and a few-layer rhenium diselenide (ReSe₂) heterojunction. The prepared Gr/ReSe₂-HS demonstrated an excellent mobility of 380 cm²/Vs, current on/off ratio ~ 10⁴, photoresponsivity (R ~ 74 AW^-1 @ 82 mW cm^-2), detectivity (D^* ~ 1.25 × 10¹¹ Jones), external quantum efficiency (EQE ~ 173%) and rapid photoresponse (rise/fall time ~ 75/3 µs) significantly higher to an individual ReSe₂ device (mobility = 36 cm² V^-1s^-1, Ion/Ioff ratio = 1.4 × 10⁵-1.8 × 10⁵, R = 11.2 AW^-1, D* = 1.02 × 10¹⁰, EQE ~ 26.1%, rise/fall time = 2.37/5.03 s). Additionally, gate-bias dependent Schottky barrier height (SBH) estimation for individual ReSe₂ (45 meV at V_bg = 40 V) and Gr/ReSe₂-HS (9.02 meV at V_bg = 40 V) revealed a low value for the heterostructure, confirming dry transfer technique to be successful in fabricating an interfacial defects-free junction. In addition, HS is fully capable to demonstrate an excellent gas sensing response with rapid response/recovery time (39/126 s for NO₂ at 200 ppb) and is operational at room temperature (26.85 °C). The proposed Gr/ReSe₂-HS is capable of demonstrating excellent electro-optical, as well as gas sensing, performance simultaneously and, therefore, can be used as a building block to fabricate next-generation photodetectors and gas sensors.

Tuning the sensing responses towards room-temperature hypersensitive methanol gas sensor using exfoliated graphene-enhanced ZnO quantum dot nanostructures

A suitable and non-invasive methanol sensor workable in ambient temperature conditions with a high response has gained wide interest to prevent detrimental consequences for industrial workers from its low-level intoxication. In this work, we present a tunable and highly responsive ppb-level methanol gas sensor device working at room temperature via a bottom-up synthetic approach using exfoliated graphene sheet (EGs) and ZnO quantum dots (QDs) on an aluminum anodic oxide (AAO) template. It is verified that EGs-supported AAO with a vertical electrode configuration enabled high and fast-responsive methanol sensing. Moreover, the hydroxyl and carboxyl groups of the high surface area EGs and ZnO QDs with a 3.37 eV bandgap efficiently absorbing UV light led to 56 times high response due to the enhanced polarization on the sensor surface compared to non-UV-radiated EGs/AAO at 800 ppb of methanol. The optimal resonance frequency of methanol is determined to be 100 kHz, which could detect methanol with high response of 2.65% at 100 ppm. The limit of detection (LOD) concentration is obtained at 2 ppb level. This study demonstrates the potential of UV-assisted ZnO, EGs, and AAO-based capacitance sensor material for rapidly detecting hazardous gaseous light organic molecules at ambient conditions, and the overall approach can be easily expanded to a novel non-invasive monitoring strategy for light and hazardous volatile organic exposures.

Inducing Strong Light-Matter Coupling and Optical Anisotropy in Monolayer MoS2 with High Refractive Index Nanowire

Mixed-dimensional heterostructures combine the merits of materials of different dimensions; therefore, they represent an advantageous scenario for numerous technological advances. Such an approach can be exploited to tune the physical properties of two-dimensional (2D) layered materials to create unprecedented possibilities for anisotropic and high-performance photonic and optoelectronic devices. Here, we report a new strategy to engineer the light-matter interaction and symmetry of monolayer MoS₂ by integrating it with one-dimensional (1D) AlGaAs nanowire (NW). Our results show that the photoluminescence (PL) intensity of MoS₂ increases strongly in the mixed-dimensional structure because of electromagnetic field confinement in the 1D high refractive index semiconducting NW. Interestingly, the 1D NW breaks the 3-fold rotational symmetry of MoS₂, which leads to a strong optical anisotropy of up to ∼60%. Our mixed-dimensional heterostructure-based phototransistors benefit from this and exhibit an improved optoelectronic device performance with marked anisotropic photoresponse behavior. Compared with bare MoS₂ devices, our MoS₂/NW devices show ∼5 times enhanced detectivity and ∼3 times higher photoresponsivity. Our results of engineering light-matter interaction and symmetry breaking provide a simple route to induce enhanced and anisotropic functionalities in 2D materials.

Broadband, Ultra-High-Responsive Monolayer MoS2/SnS2 Quantum-Dot-Based Mixed-Dimensional Photodetector

Atomically thin two-dimensional (2D) materials have gained significant attention from the research community in the fabrication of high-performance optoelectronic devices. Even though there are various techniques to improve the responsivity of the photodetector, the key factor limiting the performance of the photodetectors is constrained photodetection spectral range in the electromagnetic spectrum. In this work, a mixed-dimensional 0D/2D SnS₂-QDs/monolayer MoS₂ hybrid is fabricated for high-performance and broadband (UV-visible-near-infrared (NIR)) photodetector. Monolayer MoS₂ is deposited on SiO₂/Si using chemical vapor deposition (CVD), and SnS₂-QDs are prepared using a low-cost solution-processing method. The high performance of the fabricated 0D/2D photodetector is ascribed to the band bending and built-in potential created at the junction of SnS₂-QDs and MoS₂, which enhances the injection and separation efficiency of the photoexcited charge carriers. The mixed-dimensional structure also suppresses the dark current of the photodetector. The decorated SnS₂-QDs on monolayer MoS₂ not only improve the performance of the device but also extends the spectral range to the UV region. Photoresponsivity of the device for UV, visible, and NIR region is found to be ∼278, ∼ 435, and ∼189 A/W, respectively. Fabricated devices showed maximum responsivity under the visible region attributed to the high absorbance of monolayer MoS₂. The response time of the fabricated device is measured as ∼100 ms. These results reveal that the development of a mixed-dimensional (0D/2D) SnS₂-QDs/MoS₂-based high-performance and broadband photodetector is technologically promising for next-generation optoelectronic applications.

Photophysical Dynamics in Semiconducting Graphene Quantum Dots Integrated with 2D MoS2 for Optical Enhancement in the Near UV

The hybrid structure of zero-dimensional (0D) graphene quantum dots (GQDs) and semiconducting two-dimensional (2D) MoS₂ has been investigated, which exhibit outstanding properties for optoelectronic devices surpassing the limitations of MoS₂ photodetectors where the GQDs extend the optical absorption into the near-UV regime. The GQDs and MoS₂ films are characterized by Raman and photoluminescence (PL) spectroscopies, along with atomic force microscopy. After outlining the fabrication of our 0D-2D heterostructure photodetectors comprising GQDs with bulk MoS₂ sheets, their photoresponse to the incoming radiation was measured. The hybrid GQD/MoS₂ heterostructure photodetector exhibits a high photoresponsivity R of more than 1200 A W^-1 at 0.64 mW/cm² at room temperature T. The T-dependent optoelectronic measurements revealed a peak R of ∼544 A W^-1 at 245 K, examined from 5.4 K up to 305 K with an incoming white light power density of 3.2 mW/cm². A tunable laser revealed the photocurrent to be maximal at lower wavelengths in the near ultraviolet (UV) over the 400-1100 nm spectral range, where the R of the hybrid GQDs/MoS₂ was ∼775 A W^-1, while a value of 2.33 × 10¹² Jones was computed for the detectivity D* at 400 nm. The external quantum efficiency was measured to be ∼99.8% at 650 nm, which increased to 241% when the wavelength of the incoming laser was reduced to 400 nm. Time-resolved measurements of the photocurrent for the hybrid devices resulted in a rise time τ_rise and a fall time τ_fall of ∼7 and ∼25 ms, respectively, at room T, which are 10× lower compared to previous reports. From our promising results, we conclude that the GQDs exhibit a sizable band gap upon optical excitation, where photocarriers are injected into the MoS₂ films, endowing the hybrids with long carrier lifetimes to enable efficient light absorption beyond the visible and into the near-UV regime. The GQD-MoS₂ structure is thus an enabling platform for high-performance photodetectors, optoelectronic circuits, and quantum devices.

Directed Energy Transfer from Monolayer WS2 to Near-Infrared Emitting PbS-CdS Quantum Dots

Heterostructures of two-dimensional (2D) transition metal dichalcogenides (TMDs) and inorganic semiconducting zero-dimensional (0D) quantum dots (QDs) offer useful charge and energy transfer pathways, which could form the basis of future optoelectronic devices. To date, most have focused on charge transfer and energy transfer from QDs to TMDs, that is, from 0D to 2D. Here, we present a study of the energy transfer process from a 2D to 0D material, specifically exploring energy transfer from monolayer tungsten disulfide (WS₂) to near-infrared emitting lead sulfide-cadmium sulfide (PbS-CdS) QDs. The high absorption cross section of WS₂ in the visible region combined with the potentially high photoluminescence (PL) efficiency of PbS QD systems makes this an interesting donor-acceptor system that can effectively use the WS₂ as an antenna and the QD as a tunable emitter, in this case, downshifting the emission energy over hundreds of millielectron volts. We study the energy transfer process using photoluminescence excitation and PL microscopy and show that 58% of the QD PL arises due to energy transfer from the WS₂. Time-resolved photoluminescence microscopy studies show that the energy transfer process is faster than the intrinsic PL quenching by trap states in the WS₂, thus allowing for efficient energy transfer. Our results establish that QDs could be used as tunable and high PL efficiency emitters to modify the emission properties of TMDs. Such TMD-QD heterostructures could have applications in light-emitting technologies or artificial light-harvesting systems or be used to read out the state of TMD devices optically in various logic and computing applications.

A review of molybdenum disulfide (MoS2) based photodetectors: from ultra-broadband, self-powered to flexible devices

Two-dimensional transition metal dichalcogenides (2D TMDs) have attracted much attention in the field of optoelectronics due to their tunable bandgaps, strong interaction with light and tremendous capability for developing diverse van der Waals heterostructures (vdWHs) with other materials. Molybdenum disulfide (MoS2) atomic layers which exhibit high carrier mobility and optical transparency are very suitable for developing ultra-broadband photodetectors to be used from surveillance and healthcare to optical communication. This review provides a brief introduction to TMD-based photodetectors, exclusively focused on MoS2-based photodetectors. The current research advances show that the photoresponse of atomic layered MoS2 can be significantly improved by boosting its charge carrier mobility and incident light absorption via forming MoS2 based plasmonic nanostructures, halide perovskites-MoS2 heterostructures, 2D-0D MoS2/quantum dots (QDs) and 2D-2D MoS2 hybrid vdWHs, chemical doping, and surface functionalization of MoS2 atomic layers. By utilizing these different integration strategies, MoS2 hybrid heterostructure-based photodetectors exhibited remarkably high photoresponsivity raging from mA W-1 up to 1010 A W-1, detectivity from 107 to 1015 Jones and a photoresponse time from seconds (s) to nanoseconds (10-9 s), varying by several orders of magnitude from deep-ultraviolet (DUV) to the long-wavelength infrared (LWIR) region. The flexible photodetectors developed from MoS2-based hybrid heterostructures with graphene, carbon nanotubes (CNTs), TMDs, and ZnO are also discussed. In addition, strain-induced and self-powered MoS2 based photodetectors have also been summarized. The factors affecting the figure of merit of a very wide range of MoS2-based photodetectors have been analyzed in terms of their photoresponsivity, detectivity, response speed, and quantum efficiency along with their measurement wavelengths and incident laser power densities. Conclusions and the future direction are also outlined on the development of MoS2 and other 2D TMD-based photodetectors.

Enhanced electrical and broad spectral (UV-Vis-NIR) photodetection in a Gr/ReSe2/Gr heterojunction

Excellent electrical and photoelectrical study of vertical integration by layered two-dimensional materials having gate tunable broad spectral (UV-Vis-NIR) light detection response.

UV-Visible Photodetector Based on I-type Heterostructure of ZnO-QDs/Monolayer MoS2

Monolayer MoS₂ has shown excellent photoresponse properties, but its promising applications in high-sensitivity photodetection suffer from the atomic-thickness-limited adsorption and band gap-limited spectral selectivity. Here we have carried out investigations on MoS₂ monolayer-based photodetectors with and without decoration of ZnO quantum dots (ZnO-QDs) for comparison. Compared with monolayer MoS₂ photodetectors, the monolayer ZnO-QDs/MoS₂ hybrid device exhibits faster response speed (1.5 s and 1.1 s, respectively), extended broadband photoresponse range (deep UV-visible), and enhanced photoresponse in visible spectrum, such as higher responsivity over 0.084 A/W and larger detectivity of 1.05 × 10¹¹ Jones, which results from considerable injection of carries from ZnO-QDs to MoS₂ due to the formation of I-type heterostructure existing in the contact interface of them.

Highly efficient and flexible photodetector based on MoS2–ZnO heterostructures

Two-dimensional (2D) transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS₂) and tungsten diselenide (WSe₂), have recently attracted attention for their applicability as building blocks for fabricating advanced functional materials. In this study, a high quality hybrid material based on 2D TMD nanosheets and ZnO nanopatches was demonstrated. An organic promoter layer was employed for the large-scale growth of the TMD sheet, and atomic layer deposition (ALD) was utilized for the growth of ZnO nanopatches. Photodetectors based on 2D TMD nanosheets and ZnO nanopatches were successfully fabricated and investigated, which showed a high photoresponsivity of 2.7 A/W. Our novel approach is a promising and effective method for the fabrication of photodetectors with a new structure for application in TMD-based transparent and flexible optoelectronic devices.

Two-dimensional transition metal dichalcogenides (TMDs) such as molybdenum disulfide, have recently attracted attention for their applicability as building blocks for fabricating advanced functional materials.

Ultrahigh Photoresponsive Device Based on ReS2 /Graphene Heterostructure

Heterostructures that combine graphene and transition metal dichalcogenides, such as MoS₂ , MoTe₂ , and WS₂ , have attracted attention due to their high performances in optoelectronic devices compared to homogeneous systems. Here, a photodevice based on a hybrid van der Waals heterostructure of rhenium disulfide (ReS₂ ) and graphene is fabricated using the stacking method. The device presents a remarkable ultrahigh photoresponsivity of 7 × 10⁵ A W^-1 and a detectivity of 1.9 × 10¹³ Jones, along with a fast response time of less than 30 ms. Tremendous photocurrents are generated in the heterostructure due to the direct bandgap, high quantum efficiency, and strong light absorption by the multilayer ReS₂ and the high carrier mobility of graphene. The ReS₂ /graphene heterostructured device displays a high photocurrent under the applied gate voltages due to the photogating effect induced by the junction between graphene and ReS₂ . The ReS₂ /graphene heterostructure may find promising applications in future optoelectronic devices, providing a high sensitivity, flexibility, and transparency.

Near Infrared Random Lasing in Multilayer MoS2

We demonstrated room temperature near infrared (NIR) region random lasing (RL) (800-950 nm), with a threshold of nearly 500 μW, in ∼200 nm thick MoS2/Au nanoparticles (NPs)/ZnO heterostructures using photoluminescence spectroscopy. The RL in the above system arises mainly due to the following three reasons: (1) enhanced multiple scattering because of Au/ZnO disordered structure, (2) exciton-plasmon coupling because of Au NPs, and (3) enhanced charge transfer from ZnO to thick MoS2 flakes. RL has recently attracted tremendous interest because of its wide applications in the field of telecommunication, spectroscopy, and specifically in biomedical tissue imaging. This work provides new dimensions toward realization of low power on-chip NIR random lasers made up of biocompatible materials.

Comparison of Electrical and Photoelectrical Properties of ReS2 Field-Effect Transistors on Different Dielectric Substrates

As one of the newly discovered transition-metal dichalcogenides (TMDs), rhenium disulfide (ReS₂) has been investigated mostly because of its unique characteristics such as the direct band gap nature even in bulk form, which is not prominent in other TMDs (e.g., MoS₂, WSe₂, etc.). However, this material possesses a low mobility and an on/off ratio, which restrict its usage in high-speed and fast switching applications. Low mobilities or on/off ratios can also be caused by substrate scattering as well as environmental effects. In this study, we used few-layer ReS₂ (FL-ReS₂) as a channel material to investigate the substrate-dependent mobility, current on/off ratio, Schottky barrier height (SBH), and trap density of states of different dielectric substrates. The hexagonal boron nitride (h-BN)/FL-ReS₂/h-BN structure was observed to exhibit a high mobility of 45 cm² V^-1 s^-1, current on/off ratio of about 10⁷, the lowest SBH of about 12 mV at a zero back-gate voltage ( V_bg), and a low trap density of states of about 5 × 10¹³ cm^-3. These quantities are reasonably superior compared to the FL-ReS₂ devices on SiO₂ substrates. We also observed a nearly 5-fold improvement in the photoresponsivity and external quantum efficiency values for the FL-ReS₂ devices on h-BN substrates. We believe that the photonic characteristics of TMDs can be improved by using h-BN as the substrate and capping layer.

Ultraviolet Wavelength-Dependent Optoelectronic Properties in Two-Dimensional NbSe2-WSe2 van der Waals Heterojunction-Based Field-Effect Transistors

Atomically thin two-dimensional (2D) van der Waals (vdW) heterostructures are one of the very important research issues for stacked optoelectronic device applications. In this study, using the transferred and stacked NbSe₂-WSe₂ films as electrodes and a channel, we fabricated the field-effect transistor (FET) devices based on 2D-2D vdW metal-semiconductor heterojunctions (HJs) and systematically studied their ultraviolet (UV) wavelength-dependent electrical and photoresponse properties. Upon the exposure to UV light with a wavelength of 365 nm, the NbSe₂-WSe₂ vdW HJFET devices exhibited threshold voltage shift toward positive gate bias direction, a current increase, and a nonlinear photocurrent increase upon applying a gate bias due to the contribution of the photogenerated hole current. In contrast, for the 254 nm UV-irradiated FET devices, the drain current was decreased dramatically and the threshold voltage was negatively shifted. The time-resolved photoresponse properties showed that the device current after turning off the 254 nm UV light was completely and much more rapidly recovered compared with the case of the persistent photocurrent after turning off the 365 nm UV light. Interestingly, we found that the wettability of the WSe₂ surface was changed with increasing irradiation time only after 254 nm UV irradiation. The measured wetting behavior on the WSe₂ surface provided direct evidence that the experimentally observed UV-wavelength-dependent phenomena was attributed to the UV-induced dissociative adsorption of oxygen and water molecules, leading to the modulation of charge trap states on the photogenerated and intrinsic carriers in the p-type WSe₂ channel. This study will help provide an understanding of the influence of environmental and electrical measurement conditions on the electrical and optical properties of 2D-2D vdW HJ devices for a variety of device applications through the stacking of 2D heterostructures.