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Functional diversification at the single-device level has become essential for emerging optical neural network (ONN) development. Stable ferroelectricity harnessed with strong light sensitivity in α-In 2 Se 3 holds great potential for developing ultrathin neuromorphic devices. Herein, we demonstrated an all-2D van der Waals heterostructure-based programmable synaptic field effect transistor (FET) utilizing a ferroelectric α-In 2 Se 3 nanosheet and monolayer graphene. The devices exhibited reconfigurable, multilevel nonvolatile memory (NVM) states, which can be successively modulated by multiple dual-mode (optical and electrical) stimuli and thereby used to realize energy-efficient, heterosynaptic functionalities in a biorealistic fashion. Furthermore, under light illumination, the prototypical device can toggle between volatile (photodetector) and nonvolatile optical random-access memory (ORAM) logic operation, depending upon the ferroelectric-dipole induced band adjustment. Finally, plasticity modulation from short-term to prominent long-term characteristics over a wide dynamic range was demonstrated. The inherent operation mechanism owing to the switchable polarization-induced electronic band alignment and bidirectional barrier height modulation at the heterointerface was revealed by conjugated electronic transport and Kelvin-probe force microscopy (KPFM) measurements. Overall, robust (opto)electronic weight controllability for integrated in-sensor and in-memory logic processors and multibit ORAM systems was readily accomplished by the synergistic ferrophotonic heterostructure properties. Our presented results facilitate the technological implementation of versatile all-2D heterosynapses for next-generation perception, optoelectronic logic systems, and Internet-of-Things (IoT) entities.

Heterostructures based on two-dimensional materials offer the possibility to achieve synergistic functionalities, which otherwise remain secluded by their individual counterparts. Herein, ferroelectric polarization switching in α-In 2 Se 3 has been utilized to engineer multilevel nonvolatile conduction states in a partially overlapping α-In 2 Se 3 -MoS 2 -based ferroelectric semiconducting field effect device. In particular, we demonstrate how the intercoupled ferroelectric nature of α-In 2 Se 3 allows to nonvolatilely switch between n - i and n - i - n type junction configurations based on a novel edge state actuation mechanism, paving the way for subnanometric scale nonvolatile device miniaturization. Furthermore, the induced asymmetric polarization enables enhanced photogenerated carriers' separation, resulting in an extremely high photoresponse of ∼1275 A/W in the visible range and strong nonvolatile modulation of the bright A- and B- excitonic emission channels in the overlaying MoS 2 monolayer. Our results show significant potential to harness the switchable polarization in partially overlapping α-In 2 Se 3 -MoS 2 based FeFETs to engineer multimodal, nonvolatile nanoscale electronic and optoelectronic devices.

Two dimensional (2D) van der Waals heterostructures (vdWHs) have unique potential in facilitating the stacking of layers of different 2D materials for optoelectronic devices with superior characteristics. However, the fabrication of large area all-2D heterostructures is still challenging towards realizing practical devices at a reduced cost. In the present work, we have demonstrated a rapid yet simple, impurity-free and efficient sonication-assisted chemical exfoliation approach to synthesize hybrid vdWHs based on 2D molybdenum disulphide (MoS 2 ) and tungsten disulphide (WS 2 ), with high yield. Microscopic and spectroscopic studies have confirmed the successful exfoliation of layered 2D materials and formation of their hybrid heterostructures. The co-existence of 2D MoS 2 and WS 2 in the vdWH hybrids is established by optical absorption and Raman shift measurements along with their chemical stiochiometry determined by x-ray photoelectron spectroscopy. The spectral response of the vdWH/Si (2D/3D) heterojunction photodetector fabricated using the as-synthesized material is found to exhibit broadband photoresponse compared to that of the individual 2D MoS 2 and WS 2 devices. The peak responsivity and detectivity are found to be as high as ∼2.15 A W -1 and 2.05 × 10 11 Jones, respectively for an applied bias of -5 V. The ease of fabrication with appreciable performance of the chemically synthesized vdWH-based devices have revealed their potential use for large area optoelectronic applications on Si-compatible CMOS platforms., (© 2023 IOP Publishing Ltd.)

We present a novel maskless device fabrication technique for rapid prototyping of two-dimensional (2D)-based electronic materials. The technique is based on a thermally activated and self-developed cyclic polyphthalaldehyde (c-PPA) resist using a commercial Raman system and 532 nm laser illumination. Following the successful customization of electrodes to form field effect transistors based on MoS 2 monolayers, the laser-induced electronic doping of areas beneath the metal contacts that were exposed during lithography was investigated using both surface potential mapping and device characterization. An effective change in the doping level was introduced depending on the laser intensity, i.e., low laser powers resulted in p-doping, while high laser powers resulted in n-doping. Fabricated devices present a low contact resistance down to 10 kΩ·μm at a back-gate voltage of V G = 80 V, which is attributed to the laser-induced n-type doping at the metal contact regions.

Transition metal dichalcogenides (TMDs) and their heterojunctions are drawing immense research interest for various applications including infrared detection. They are being studied with different semiconductor materials to explore their heterojunction properties. In this regard, we report a MoSe 2 /Si heterojunction broadband photodiode which is highly sensitive for a wide spectral range from 405 nm to 2500 nm wavelength with the maximum responsivity of ∼522 mA W -1 for 1100 nm of incident light. The hydrothermal synthesis approach leads to the imperfect growth of the MoSe 2 , creating defects in the lattice, which was confirmed by x-ray photo-spectroscopy. These sub-bandgap defects caused high optical absorption of the SWIR light as observed in the absorption spectra. The speed of the device ranges to 18/10 μs for 10 kHz modulated light. Furthermore, the photodetector has been fully operational even at zero bias voltage, making it a potential contender for self-powered photodetection.

The interface of transition-metal dichalcogenides (TMDCs) and high- k dielectric transition-metal oxides (TMOs) had triggered umpteen discourses because of the indubitable impact of TMOs in reducing the contact resistances and restraining the Fermi-level pinning for the metal-TMDC contacts. In the present work, we focus on the unresolved tumults of large-area TMDC/TMO interfaces, grown by adopting different techniques. Here, on a pulsed laser-deposited MoS 2 thin film, a layer of TiO 2 is grown by atomic layer deposition (ALD) and pulsed laser deposition (PLD). These two different techniques emanate the layer of TiO 2 with different crystallinities, thicknesses, and interfacial morphologies, subsequently influencing the electronic and optical properties of the interfaces. Contrasting the earlier reports of n-type doping at the exfoliated MoS 2 /TiO 2 interfaces, the large-area MoS 2 /anatase-TiO 2 films had realized a p-type doping of the underneath MoS 2 , manifesting a boost in the extent of p-type doping with increasing thickness of TiO 2 , as emerged from the X-ray photoelectron spectra. Density functional analysis of the MoS 2 /anatase-TiO 2 interfaces, with pristine and interfacial defect configurations, could correlate the interdependence of doping and the terminating atomic surface of TiO 2 on MoS 2 . The optical properties of the interface, encompassing photoluminescence, transient absorption and z -scan two-photon absorption, indicate the presence of defect-induced localized midgap levels in MoS 2 /TiO 2 (PLD) and a relatively defect-free interface in MoS 2 /TiO 2 (ALD), corroborating nicely with the corresponding theoretical analysis. From the investigation of optical properties, we indicate that the MoS 2 /TiO 2 (PLD) interface may act as a promising saturable absorber, having a significant nonlinear response for the sub-band-gap excitations. Moreover, the MoS 2 /TiO 2 (PLD) interface had exemplified better phototransport properties. A potential application of MoS 2 /TiO 2 (PLD) is demonstrated by the fabrication of a p-type phototransistor with the ionic-gel top gate. This endeavor to analyze and perceive the MoS 2 /TiO 2 interface establishes the prospectives of large-area interfaces in the field of optics and optoelectronics.

While two-dimensional (2D) layered MoS 2 nanosheets have been extensively studied owing to their fascinating optoelectronic properties, less attention has been paid to the corresponding zero-dimensional nano-crystals. In this contribution, we report the efficacy of MoS 2 nanocrystals for their size tunable properties for optical and photocatalytic applications. We have synthesized differently sized (10-70 nm) crystalline, hexagonal 2H-MoS 2 nanoparticles (NPs) dispersed in DMF solvent using a simple exfoliation technique. Synthesized NPs are found to exhibit size-dependent optical properties and excitation-dependent fluorescence characteristics in the visible region, which are otherwise not observed in bulk or 2D MoS 2 layers. Size tunable bandgap and broad absorbance and emission spectrum covering the visible range could be exploited in the fabrication of various opto-electronic devices. Charge carrier emission dynamics of differently sized MoS 2 NPs are investigated using time correlated single photon counting (TCSPC) spectroscopic technique. We found two time components, one in the order of several hundreds of ps, which arises due to the radiative recombination of charge carriers, while the other one is of the order of a few ns, which emanates from the defect states of MoS 2 NPs. The average time constants are found to decrease with increase in particle size. A noticeable photocatalytic activity of the synthesized MoS 2 NPs under visible light illumination for the degradation of brilliant green dye is also demonstrated for the first time and the effect of size variation of NPs in the dye degradation process is reported.

The characteristics of a novel 0D/3D heterojunction photodetector fabricated using solution-processed colloidal MoS 2 quantum dots (QDs) on GaAs is presented. MoS 2 QDs with a dimension of ∼2 nm, synthesized by a standard sono-chemical exfoliation process with 2D layers have been used for the purpose. The microscopic and spectroscopic studies confirmed the formation of semiconducting (2H phase) MoS 2 QDs. The photodetectors were fabricated using n-GaAs substrates with two different doping concentrations resulting in n-n heterojunctions between n-type 0D MoS 2 QDs and bulk n-GaAs. The devices fabricated using GaAs with a higher doping concentration, showed an increase in the reverse current of the order of ∼10 2 upon illumination, while the same with a lower doping concentration showed an increase of the order of ∼10 3 . All the heterojunction photodetector devices show a broadband operation over the visible wavelength range of 400-950 nm, with a peak responsivity of the devices being observed at 500 nm. The peak responsivity and detectivity are found to be ∼400 mA W -1 and ∼4 × 10 12 Jones, respectively, even without any external applied bias, which are useful for self-powered photodetection. The results indicate that colloidal MoS 2 /GaAs based hybrid heterostructures provide a platform for fabricating broadband photodetectors by using highly absorbing MoS 2 QDs, which may show the pathway towards next-generation optoelectronic devices with superior detection properties.

A unique light trapping mechanism associated with nano-conical textured black Si templates has been utilized to achieve improved photoresponse in MoS 2 QDs/Si heterojunctions over a wide wavelength range from visible to near infrared. Black Si templates have been fabricated by a simple and cost effective metal assisted chemical etching technique followed by spin-coating of colloidal MoS 2 quantum dots (QDs) to form the heterojunction. A peak responsivity of as high as ∼1.39 A W -1 at ∼665 nm for a bias of 5 V has been achieved. The responsivity value is higher as compared to recently published results having similar device structure. The combination of MoS 2 QDs and black Si has resulted in a broader spectral response with enhanced optical absorption in the nano-conical heterojunction devices. Finite element based optical simulation results revealed the superiority of MoS 2 QDs/Si nano-conical heterojunctions due to improved light trapping. The results appear attractive for next generation Si CMOS compatible broad band photodetectors using two dimensional semiconductors.