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

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

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

Augmented Quantum Yield of a 2D Monolayer Photodetector by Surface Plasmon Coupling

Monolayer (1L) transition metal dichalcogenides (TMDCs) are promising materials for nanoscale optoelectronic devices because of their direct band gap and wide absorption range (ultraviolet to infrared). However, 1L-TMDCs cannot be easily utilized for practical optoelectronic device applications (e.g., photodetectors, solar cells, and light-emitting diodes) because of their extremely low optical quantum yields (QYs). In this investigation, a high-gain 1L-MoS₂ photodetector was successfully realized, based on the surface plasmon (SP) of the Ag nanowire (NW) network. Through systematic optical characterization of the hybrid structure consisting of a 1L-MoS₂ and the Ag NW network, it was determined that a strong SP and strain relaxation effect influenced a greatly enhanced optical QY. The photoluminescence (PL) emission was drastically increased by a factor of 560, and the main peak was shifted to the neutral exciton of 1L-MoS₂. Consequently, the overall photocurrent of the hybrid 1L-MoS₂ photodetector was observed to be 250 times better than that of the pristine 1L-MoS₂ photodetector. In addition, the photoresponsivity and photodetectivity of the hybrid photodetector were effectively improved by a factor of ∼1000. This study provides a new approach for realizing highly efficient optoelectronic devices based on TMDCs

Observing Grain Boundaries in CVD-Grown Monolayer Transition Metal Dichalcogenides

Two-dimensional monolayer transition metal dichalcogenides (TMdCs), driven by graphene science, revisit optical and electronic properties, which are markedly different from bulk characteristics. These properties are easily modified due to accessibility of all the atoms viable to ambient gases, and therefore, there is no guarantee that impurities and defects such as vacancies, grain boundaries, and wrinkles behave as those of ideal bulk. On the other hand, this could be advantageous in engineering such defects. Here, we report a method of observing grain boundary distribution of monolayer TMdCs by a selective oxidation. This was implemented by exposing directly the TMdC layer grown on sapphire without transfer to ultraviolet light irradiation under moisture-rich conditions. The generated oxygen and hydroxyl radicals selectively functionalized defective grain boundaries in TMdCs to provoke morphological changes at the boundary, where the grain boundary distribution was observed by atomic force microscopy and scanning electron microscopy. This paves the way toward the investigation of transport properties engineered by defects and grain boundaries

Synthesis of Centimeter-Scale Monolayer Tungsten Disulfide Film on Gold Foils

We report the synthesis of centimeter-scale monolayer WS₂ on gold foil by chemical vapor deposition. The limited tungsten and sulfur solubility in gold foil allows monolayer WS₂ film growth on gold surface. To ensure the coverage uniformity of monolayer WS₂ film, the tungsten source-coated substrate was placed in parallel with Au foil under hydrogen sulfide atmosphere. The high growth temperature near 935 °C helps to increase a domain size up to 420 μm. Gold foil is reused for the repeatable growth after bubbling transfer. The WS₂-based field effect transistor reveals an electron mobility of 20 cm² V^–1 s^–1 with high on–off ratio of ∼10⁸ at room temperature, which is the highest reported value from previous reports of CVD-grown WS₂ samples. The on–off ratio of integrated multiple FETs on the large area WS₂ film on SiO₂ (300 nm)/Si substrate shows within the same order, implying reasonable uniformity of WS₂ FET device characteristics over a large area of 3 × 1.5 cm²

Large Work Function Modulation of Monolayer MoS₂ by Ambient Gases

Although two-dimensional monolayer transition-metal dichalcogenides reveal numerous unique features that are inaccessible in bulk materials, their intrinsic properties are often obscured by environmental effects. Among them, work function, which is the energy required to extract an electron from a material to vacuum, is one critical parameter in electronic/optoelectronic devices. Here, we report a large work function modulation in MoS₂ via ambient gases. The work function was measured by an <i>in situ</i> Kelvin probe technique and further confirmed by ultraviolet photoemission spectroscopy and theoretical calculations. A measured work function of 4.04 eV in vacuum was converted to 4.47 eV with O₂ exposure, which is comparable with a large variation in graphene. The homojunction diode by partially passivating a transistor reveals an ideal junction with an ideality factor of almost one and perfect electrical reversibility. The estimated depletion width obtained from photocurrent mapping was ∼200 nm, which is much narrower than bulk semiconductors

Semiconductor–Insulator–Semiconductor Diode Consisting of Monolayer MoS₂, h‑BN, and GaN Heterostructure

We propose a semiconductor–insulator–semiconductor (SIS) heterojunction diode consisting of monolayer (1-L) MoS₂, hexagonal boron nitride (h-BN), and epitaxial p-GaN that can be applied to high-performance nanoscale optoelectronics. The layered materials of 1-L MoS₂ and h-BN, grown by chemical vapor deposition, were vertically stacked by a wet-transfer method on a p-GaN layer. The final structure was verified by confocal photoluminescence and Raman spectroscopy. Current–voltage (<i>I</i>–<i>V</i>) measurements were conducted to compare the device performance with that of a more classical p–n structure. In both structures (the p–n and SIS heterojunction diode), clear current-rectifying characteristics were observed. In particular, a current and threshold voltage were obtained for the SIS structure that was higher compared to that of the p–n structure. This indicated that tunneling is the predominant carrier transport mechanism. In addition, the photoresponse of the SIS structure induced by the illumination of visible light was observed by photocurrent measurements

Modulating Electronic Properties of Monolayer MoS₂ <i>via</i> Electron-Withdrawing Functional Groups of Graphene Oxide

Modulation of the carrier concentration and electronic type of monolayer (1L) MoS₂ is highly important for applications in logic circuits, solar cells, and light-emitting diodes. Here, we demonstrate the tuning of the electronic properties of large-area 1L-MoS₂ using graphene oxide (GO). GO sheets are well-known as hole injection layers since they contain electron-withdrawing groups such as carboxyl, hydroxyl, and epoxy. The optical and electronic properties of GO-treated 1L-MoS₂ are dramatically changed. The photoluminescence intensity of GO-treated 1L-MoS₂ is increases by more than 470% compared to the pristine sample because of the increase in neutral exciton contribution. In addition, the A_1g peak in Raman spectra shifts considerably, revealing that GO treatment led to the formation of p-type doped 1L-MoS₂. Moreover, the current <i>vs</i> voltage (<i>I–V</i>) curves of GO-coated 1L-MoS₂ field effect transistors show that the electron concentration of 1L-MoS₂ is significantly lower in comparison with pristine 1L-MoS₂. Current rectification is also observed from the <i>I–V</i> curve of the lateral diode structure with 1L-MoS₂ and 1L-MoS₂/GO, indicating that the electronic structure of MoS₂ is significantly modulated by the electron-withdrawing functional group of GO

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