Growth of Wafer-Scale Standing Layers of WS₂ for Self-Biased High-Speed UV–Visible–NIR Optoelectronic Devices

This work describes the wafer-scale standing growth of (002)-plane-oriented layers of WS₂ and their suitability for use in self-biased broad-band high-speed photodetection. The WS₂ layers are grown using large-scale sputtering, and the effects of the processing parameters such as the deposition temperature, deposition time, and sputtering power are studied. The structural, physical, chemical, optical, and electrical properties of the WS₂ samples are also investigated. On the basis of the broad-band light absorption and high-speed in-plane carrier transport characteristics of the WS₂ layers, a self-biased broad-band high-speed photodetector is fabricated by forming a type-II heterojunction. This WS₂/Si heterojunction is sensitive to ultraviolet, visible, and near-infrared photons and shows an ultrafast photoresponse (1.1 μs) along with an excellent responsivity (4 mA/W) and a specific detectivity (∼1.5 × 10¹⁰ Jones). A comprehensive Mott–Schottky analysis is performed to evaluate the parameters of the device, such as the frequency-dependent flat-band potential and carrier concentration. Further, the photodetection parameters of the device, such as its linear dynamic range, rising time, and falling time, are evaluated to elucidate its spectral and transient characteristics. The device exhibits remarkably improved transient and spectral photodetection performances as compared to those of photodetectors based on atomically thin WS₂ and two-dimensional materials. These results suggest that the proposed method is feasible for the manipulation of vertically standing WS₂ layers that exhibit high in-plane carrier mobility and allow for high-performance broad-band photodetection and energy device applications

Compliance-Free Multileveled Resistive Switching in a Transparent 2D Perovskite for Neuromorphic Computing

We demonstrate the pulsed voltage tunable multileveled resistive switching (RS) across a promising transparent energy material of (C₄H₉NH₃)₂PbBr₄. The X-ray diffraction and scanning electron microscopy results confirm the growth of (001) plane-orientated nanostructures of (C₄H₉NH₃)₂PbBr₄ with an average size of ∼360 nm. The device depicts optical transmittance higher than 70% in the visible region and efficient absorbance in the ultraviolet region. The current–voltage measurement shows the bipolar RS. In addition, depending on the magnitude of applied electric pulse, the current across the device can be flipped in four different levels, which remain stable for long time, indicating multimode RS. Further, the current across the device increases gradually by applying continuous pulses, similar to the biological synapses. The observed results are attributed to the electric field-induced ionic migration across the (C₄H₉NH₃)₂PbBr₄. The existing study should open a new avenue to apply this promising energy material of perovskite for multifunctional advanced devices

Highly Enhanced Photoresponsivity of a Monolayer WSe₂ Photodetector with Nitrogen-Doped Graphene Quantum Dots

Hybrid structures of two-dimensional (2D) materials and quantum dots (QDs) are particularly interesting in the field of nanoscale optoelectronic devices because QDs are efficient light absorbers and can inject photocarriers into thin layers of 2D transition-metal dichalcogenides, which have high carrier mobility. In this study, we present a heterostructure that consists of a monolayer of tungsten diselenide (ML WSe₂) covered by nitrogen-doped graphene QDs (N-GQDs). The improved photoluminescence of ML WSe₂ is attributed to the dominant neutral exciton emission caused by the n-doping effect. Owing to strong light absorption and charge transfer from N-GQDs to ML WSe₂, N-GQD-covered ML WSe₂ showed up to 480% higher photoresponsivity than that of a pristine ML WSe₂ photodetector. The hybrid photodetector exhibits good environmental stability, with 46% performance retention after 30 days under ambient conditions. The photogating effect also plays a key role in the improvement of hybrid photodetector performance. On applying the back-gate voltage modulation, the hybrid photodetector shows a responsivity of 2578 A W^–1, which is much higher than that of the ML WSe₂-based device

Light Soaking Phenomena in Organic–Inorganic Mixed Halide Perovskite Single Crystals

Recently, organic–inorganic mixed halide perovskite (MAPbX₃; MA = CH₃NH₃⁺, X = Cl^–, Br^–, or I^–) single crystals with low defect densities have been highlighted as candidate materials for high-efficiency photovoltaics and optoelectronics. Here we report the optical and structural investigations of mixed halide perovskite (MAPbBr_3–<i>x</i>I_<i>x</i>) single crystals. Mixed halide perovskite single crystals showed strong light soaking phenomena with light illumination conditions that were correlated to the trapping and detrapping events from defect sites. By systematic investigation with optical analysis, we found that the pseudocubic phase of mixed halide perovskites generates light soaking phenomena. These results indicate that photoinduced changes are related to the existence of multiple phases or halide migrations

Multiphoton Absorption Coefficients of Organic–Inorganic Lead Halide Perovskites CH₃NH₃PbX₃ (X = Cl, Br, I) Single Crystals

Hybrid organic–inorganic lead halide perovskites have recorded unprecedented improvement in efficiency as fourth-generation photovoltaic materials. Recently, they have attracted enormous interest in nonlinear optics stemming basically from their excellent optoelectronic properties. Here, we investigate multiphoton absorption (MPA) in high-quality MAPbX₃ (MA = CH₃NH₃ and X = Cl, Br, I) bulk single crystals synthesized by an inverse-temperature crystallization (ITC) method. The two-photon absorption (2PA) coefficients under picosecond pulse excitation are determined to be β = 23 ± 2 cm/GW and 9 ± 1 cm/GW for MAPbI₃ and MAPbBr₃ at λ = 1064 nm, and 13 ± 2 cm/GW for MAPbCl₃ at λ = 532 nm. The 2PA coefficients are comparable to those of conventional semiconductors having similar bandgaps and can be explained by a two-band model. Furthermore, we characterize the three-photon absorption behavior of MAPbCl₃ at λ = 1064 nm, yielding γ = 0.05 ± 0.01 cm³/GW². The polarization dependence of MPA is also probed to experimentally estimate the degree of anisotropy. The hybrid perovskites are promising materials for nonlinear optical applications due to polarization-dependent MPA response and subsequent strong photoluminescence emission, especially for the Br- and I-containing compounds

Effects of TiO₂ Interfacial Atomic Layers on Device Performances and Exciton Dynamics in ZnO Nanorod Polymer Solar Cells

The performances of organic electronic and/or photonic devices rely heavily on the nature of the inorganic/organic interface. Control over such hybrid interface properties has been an important issue for optimizing the performances of polymer solar cells bearing metal-oxide conducting channels. In this work, we studied the effects of an interfacial atomic layer in an inverted polymer solar cell based on a ZnO nanorod array on the device performance as well as the dynamics of the photoexcited carriers. We adopted highly conformal TiO₂ interfacial layer using plasma enhanced atomic layer deposition (PEALD) to improve the compatibility between the solution-prepared active layer and the ZnO nanorod array. The TiO₂ interfacial layer facilitated exciton separation and subsequent charge transfer into the nanorod channel, and it suppressed recombination of photogenerated carriers at the interface. The presence of even 1 PEALD cycle of TiO₂ coating substantially improved the short-circuit current density (<i>J</i>_sc), open circuit voltage (<i>V</i>_oc), and fill factor (FF), leading to more than 2-fold enhancement in the power conversion efficiency (PCE). The dynamics of the photoexcited carriers in our devices were studied using transient absorption (TA) spectroscopy. The TA results clearly revealed that the TiO₂ coating played a key role as an efficient quencher of photogenerated excitons, thereby reducing the exciton lifetime. The electrochemical impedance spectra (EIS) provided further evidence that the TiO₂ atomic interfacial layer promoted the charge transfer at the interface by suppressing recombination loss

Photochemical Reaction in Monolayer MoS₂ <i>via</i> Correlated Photoluminescence, Raman Spectroscopy, and Atomic Force Microscopy

Photoluminescence (PL) from monolayer MoS₂ has been modulated using plasma treatment or thermal annealing. However, a systematic way of understanding the underlying PL modulation mechanism has not yet been achieved. By introducing PL and Raman spectroscopy, we analyze that the PL modulation by laser irradiation is associated with structural damage and associated oxygen adsorption on the sample in ambient conditions. Three distinct behaviors were observed according to the laser irradiation time: (i) slow photo-oxidation at the initial stage, where the physisorption of ambient gases gradually increases the PL intensity; (ii) fast photo-oxidation at a later stage, where chemisorption increases the PL intensity abruptly; and (iii) photoquenching, with complete reduction of PL intensity. The correlated confocal Raman spectroscopy confirms that no structural deformation is involved in slow photo-oxidation stage; however, the structural disorder is invoked during the fast photo-oxidation stage, and severe structural degradation is generated during the photoquenching stage. The effect of oxidation is further verified by repeating experiments in vacuum, where the PL intensity is simply degraded with laser irradiation in a vacuum due to a simple structural degradation without involving oxygen functional groups. The charge scattering by oxidation is further explained by the emergence/disappearance of neutral excitons and multiexcitons during each stage

Negative and Positive Persistent Photoconductance in Graphene

Persistent photoconductance, a prolonged light-induced conducting behavior that lasts several hundred seconds, has been observed in semiconductors. Here we report persistent negative photoconductance and consecutive prominent persistent positive photoconductance in graphene. Unusually large yields of negative PC (34%) and positive PC (1652%) and remarkably long negative transient response time (several hours) were observed. Such high yields were reduced in multilayer graphene and were quenched under vacuum conditions. Two-dimensional metallic graphene strongly interacts with environment and/or substrate, causing this phenomenon, which is markedly different from that in three-dimensional semiconductors and nanoparticles

Optical and Facet-Dependent Carrier Recombination Properties of Hendecafacet InGaN/GaN Microsized Light Emitters

A hendecafacet (HF) microsized light emitter based on an InGaN/GaN multiple quantum well (MQW) is grown via selective area metal–organic chemical vapor deposition. The HF microsized light emitter is found to possess four crystallographic facets, (0001), {11̅01}, {112̅2}, and {11–20}. Distinct facet-dependent emission properties, investigated by confocal scanning photoluminescence (PL) and cathodoluminescence (CL) measurements, are found to originate from differences in indium composition and InGaN quantum well thickness of the MQW. Facet-dependent recombination properties, examined by temperature-dependent micro-PL and PL streak images, suggest that the localization energy and nonradiative recombination of carriers at MQW on each facet are varied with the polarization fields and threading dislocations. Besides, scanning time-resolved PL measurements reveal that the recombination lifetime around the edge where different facets meet is shorter than that in the facet regions, implying such nonradiative recombination can be a significant obstacle for achieving high quantum efficiency microstructured light-emitting diodes

Metal–Insulator–Semiconductor Diode Consisting of Two-Dimensional Nanomaterials

We present a novel metal–insulator–semiconductor (MIS) diode consisting of graphene, hexagonal BN, and monolayer MoS₂ for application in ultrathin nanoelectronics. The MIS heterojunction structure was fabricated by vertically stacking layered materials using a simple wet chemical transfer method. The stacking of each layer was confirmed by confocal scanning Raman spectroscopy and device performance was evaluated using current versus voltage (<i>I</i>–<i>V</i>) and photocurrent measurements. We clearly observed better current rectification and much higher current flow in the MIS diode than in the p–n junction and the metal–semiconductor diodes made of layered materials. The <i>I</i>–<i>V</i> characteristic curve of the MIS diode indicates that current flows mainly across interfaces as a result of carrier tunneling. Moreover, we observed considerably high photocurrent from the MIS diode under visible light illumination