Despite the encouraging properties and research of 2D MoS 2 , an ongoing issue associated with the oxidative instability remains elusive for practical optoelectronic applications. Thus, in‐depth understanding of the oxidation behavior of large‐scale and homogeneous 2D MoS 2 is imperative. Here the structural and chemical transformations of large‐area MoS 2 multilayers by air‐annealing with altered temperature and time via combinatorial spectro‐microscopic analyses (Raman spectroscopy, X‐ray photoelectron spectroscopy, and atomic force microscopy) are surveyed. The results gave indications pertaining to temperature‐ and time‐dependent oxidation effects: i) heat‐driven elimination of redundant residues, ii) internal strain stimulated by the formation of MoO bonds, iii) deterioration of the MoS 2 crystallinity, iv) layer thinning, and v) morphological transformation from 2D MoS 2 layers to particles. Photoelectrical characterization of the air‐annealed MoS 2 is implemented to capture the link between the oxidation behavior of MoS 2 multilayers and their photoelectrical properties. The photocurrent based on MoS 2 air‐annealed at 200 °C is assessed to be 4.92 µA, which is 1.73 times higher than that of pristine MoS 2 (2.84 µA). The diminution in the photocurrent of the photodetector based on MoS 2 air‐annealed above 300 °C in terms of the structural, chemical, and electrical conversions induced by the oxidation process is further discussed.

A synthetic platform for industrially applicable two-dimensional (2D) semiconductors that addresses the paramount issues associated with large-scale production, wide-range photosensitive materials, and oxidative stability has not yet been developed. In this study, we attained the 6 in. scale production of 2D SnSe semiconductors with spatial homogeneity using a rational synthetic platform based on the thermal decomposition of solution-processed single-source precursors. The long-range structural and chemical homogeneities of the 2D SnSe layers are manifested using comprehensive spectroscopic analyses. Furthermore, the capability of the SnSe-based photodetectors for broadband photodetection is distinctly verified. The photoresponsivity and detectivity of the SnSe-based photodetectors are 5.89 A W-1 and 1.8 × 1011 Jones at 532 nm, 1.2 A W-1 and 3.7 × 1010 Jones at 1064 nm, and 0.14 A W-1 and 4.3 × 109 Jones at 1550 nm, respectively. The minimum rise times for the 532 and 1064 nm lasers are 62 and 374 μs, respectively. The photoelectrical analysis of the 5 × 5 SnSe-based photodetector array reveals 100% active devices with 95.06% photocurrent uniformity. We unequivocally validated that the air and thermal stabilities of the photocurrent yielded from the SnSe-based photodetector are determined to be >30 d in air and 160 °C, respectively, which are suitable for optoelectronic applications.

Despite the encouraging inherent properties of and research progress on two-dimensional (2D) MoS2, an ongoing issue associated with oxidative instability remains unsolved for practical optoelectronic applications. Thus, in-depth understanding of the oxidation behavior of large-scale and homogeneous 2D MoS2 is imperative. In this study, the structural and chemical transformations of large-area MoS2 multilayers were investigated by air-annealing at various temperatures and times by performing Raman spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. The results explicitly indicate the temperature- and time-dependent oxidation effects: heat-driven elimination of redundant residues, internal strain stimulated by the formation of Mo–O bonds, deterioration of the MoS2 crystallinity, layer thinning, and morphological transformation from 2D MoS2 layers into particles. Photoelectrical characterization of the air-annealed MoS2 was implemented to capture the link between the oxidation behavior of large-scale MoS2 multilayers and their photoelectrical properties.

Abstract 2D van der Waals (vdW) hetero integration, which features exotic interplanar interactions derived from mixed‐dimensional heterostructures, is an emergent platform for implementing high‐performance electronics and broadband/wavelength‐tunable photodetectors. However, the production of large‐area 2D spatially homogeneous transition‐metal dichalcogenides (TMDs) and elucidation of the electrostatic dynamics governing the interlayer interactions are two paramount prerequisites for realizing practical 2D‐TMD‐heterostructure‐based photodetectors. Here, a wafer‐scale synthesis of mixed‐dimensional Pt–MoS2‐based vdW heterostructures is unprecedentedly demonstrated by manipulating the platinization conditions. The rationally designed platinization yields dimensionality‐tailored Pt, including Pt nanofilm, Pt nanoparticles, and Pt atoms, with MoS2 as host platform. From density functional theory calculations, this study insights that Mo vacancy sites on the MoS2 surface are thermo‐dynamically favorable sites for Pt with an adsorption energy of −2.25 eV, then Pt clusters are sequentially formed neighboring the specific Pt‐substituted position with a formation energy of 1.30 eV. Intensive microscopic and spectroscopic analyses reveal the structural, chemical, and electrical features, validating the proposed dynamics‐related mechanism. The dimensionality‐tailored vdW heterostructures exhibit outstanding optoelectrical properties with excellent photoresponsivity (2.04 mA W−1) and highly sensitive detectivity (9.82 × 106 cm Hz1/2 W−1).

Even though molybdenum disulfide (MoS 2 ) possesses superb features, its practical application in optoelectronics is hindered by the lack of a reliable synthetic route for producing large‐scale two‐dimensional (2D) MoS 2 with spatial homogeneity and a pertinent patterning technique that can be used to realize 2D MoS 2 ‐based photodetector arrays. To resolve these issues, the effectiveness of an unprecedented combination of a solution‐based synthetic route with resist‐ and etching‐free patterning of 2D MoS 2 mediated by pre‐defined photosensitive polyimide (PSPI) for realizing 2D MoS 2 ‐based photodetector arrays is demonstrated. A solution‐based large‐area compatible approach is adopted for the synthesis of MoS 2 . Comprehensive structural and chemical analysis of the MoS 2 synthesized by altering the concentration of ammonium tetrathiomolybdate is performed to determine the optimum conditions for enhancing the photoelectrical responses of MoS 2 ‐based photodetectors. It is ascertained that the photocurrent of MoS 2 ‐based photodetectors fabricated by PSPI‐assisted patterning is unequivocally greater than that of photodetectors produced via the conventional photolithography process owing to the absence of photoresist residues associated with impeding photon absorption and providing scattering centers of photogenerated carriers.

To maximize the synergistic effects of hybridized heteromaterials in terms of device performance of piezoelectric nanogenerators (PNGs), we demonstrate an unprecedented strategy for the direct growth of perovskite piezoelectric barium titanate (BTO) nanoparticles (NPs) on two-dimensional graphene oxide (GO) via a simple hydrothermal method. The mutual interactions between the strongly coupled heteromaterials in terms of their structural, chemical, and electrical variations are systematically explored. From these comprehensive spectroscopic and microscopic examinations, we ascertain that the hybridization of BTO NPs with GO enables to enhance the piezoelectric response of the PNGs compared with those of pristine BTO NPs-based PNGs and simply mixed BTO NPs/GO-based PNGs. This can be understood by the synergistic interplay of the heteromaterials associated with (i) the formation of homogeneous size distribution of the BTO NPs after the hybridization, (ii) healing of oxygen vacancies in the BTO crystals and a simultaneous improvement in the crystallinity of GO through chemical reduction.

A new type of coaxial hetero‐structured MoS2/SnO2 nanotube (MS‐NT) array is rationally designed for flexible visible light photodetector. Herein, a suitable solution for fabricating MS‐NT comprising SnO2 with controlled thickness and strain is proposed. The thickness of SnO2 deposited on MoS2 nanotubes are delicately manipulated by adjusting atomic layer deposition (ALD) cycles, which cause the internal tensile strain of the hybrid nanotubes. The photocurrent of the MS‐NT‐based photodetector exhibits high photoresponsivity values of 198.4 mA W‐1 and external quantum efficiency of 0.78%. In the analysis, a linear correlation between the excitation power and the photocurrent extracted from the MNT‐based devices is discerned. At the same time, a superlinearity of photocurrent in the MS‐NT arrays‐based device is found. Density functional theory calculations are adopted to unveil the strong correlation between the strain, bandgap, and photocurrent of the MS‐NT, which is a decisive factor for understanding the experimental evidences. The detailed mechanism for enhance photoresponsivity is further suggested for the strain‐tailored MS‐NT arrays‐based device. Such peculiar properties are expected to open a new chapter in the era of smart optical devices that require low driving voltage and high power efficiency.

To gain the target functionality of graphene for gas detection, nonfocused and large-scale compatible MeV electron beam irradiation on graphene with Ag patterns is innovatively adopted in air for chemical patterning of graphene. This strategy allows the metal-assisted site-specific oxidation of graphene to realize monolithically integrated graphene-chemically patterned graphene (CPG)-graphene homojunction-based gas sensors. The size-tunable CPG patterns can be mediated by regulating the size of Ag prepatterns. The impacts of highly energetic electron irradiation (HEEI) on graphene are summarized as follows: (i) the selective p-type doping and the defect generation of graphene by the HEEI-induced oxidation, (ii) the resistance of the homojunction devices manipulated by the HEEI dose, (iii) the band gap opening of graphene as well as the lowering of the Fermi level, (iv) the work function values for pristine graphene and CPG corresponding to 4.14 and 4.88 eV, respectively, and (v) graphene-CPG-graphene homojunction for NO2 gas, revealing an 839% enhanced gas response compared with that of the pristine graphene-based gas sensor.

Piezoelectric two-dimensional (2D) transition-metal dichalcogenides such as molybdenum disulfide (MoS2) have recently attracted significant attention owing to their applicability for fabrication of flexible power generators. In this study, novel piezoelectric nanogenerators (PNGs) consisting of 2D piezoelectric MoS2 shells are fabricated where an Al2O3 thin layer deposited on the surface of polystyrene (PS) beads is used to avoid collapse of the spherical MoS2 shells under the high growth temperature. In addition, the MoS2 shell size is controlled by adjusting the PS bead size and the effects of the MoS2 shell size on power generation characteristics are investigated. Our PNG based on the piezoelectric MoS2 shells produces a peak output voltage of approximately 1.2 V at a pressure of 4.2 kPa. The minimum pressure for power generation by tapping is 0.3 kPa. This novel method is very promising for development of the next-generation PNGs based on 2D semiconductor piezoelectric materials.

Single source precursors for coating and subsequent thermal decomposition processes enable a large-scale, low-cost synthesis of two-dimensional transition metal dichalcogenides (TMDs). However, practical applications based on two-dimensional TMDs have been limited by the lack of applicable single source precursors for the synthesis of p-type TMDs including layered tungsten diselenide (WSe2). We firstly demonstrate the simple and facile synthesis of WSe2 layers using a newly developed precursor that allows improved dispersibility and lower decomposition temperature. We study the thermal decomposition mechanism of three types of (Cat+)2[WSe4] precursors to assess the most suitable precursor for the synthesis of WSe2 layers. The resulting chemical and structural exploration of solution-processed WSe2 layers suggests that the (CTA)2[WSe4] may be a promising precursor because it resulted in the formation of high-crystalline WSe2. In addition, this study verifies the capability of WSe2 layers for multifunctional applications in optoelectronic and electronic devices. The photocurrent of WSe2-based photodetectors shows an abrupt switching behavior under periodic illumination of visible or IR light. The extracted photoresponsivity values for WSe2-based photodetectors recorded at 0.5 V correspond to 26.3 mA W-1 for visible light and 5.4 mA W-1 for IR light. The WSe2-based field effect transistors exhibit unipolar p-channel transistor behavior with a carrier mobility of 0.45 cm2 V-1 s-1 and an on-off ratio of ∼10.

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.

Functional van der Waals heterojunctions of transition metal dichalcogenides are emerging as a potential candidate for the basis of next‐generation logic devices and optoelectronics. However, the complexity of synthesis processes so far has delayed the successful integration of the heterostructure device array within a large scale, which is necessary for practical applications. Here, a direct synthesis method is introduced to fabricate an array of self‐assembled WSe2/MoS2 heterostructures through facile solution‐based directional precipitation. By manipulating the internal convection flow (i.e., Marangoni flow) of the solution, the WSe2 wires are selectively stacked over the MoS2 wires at a specific angle, which enables the formation of parallel‐ and cross‐aligned heterostructures. The realized WSe2/MoS2‐based p–n heterojunction shows not only high rectification (ideality factor: 1.18) but also promising optoelectrical properties with a high responsivity of 5.39 A W−1 and response speed of 16 µs. As a feasible application, a WSe2/MoS2‐based photodiode array (10 × 10) is demonstrated, which proves that the photosensing system can detect the position and intensity of an external light source. The solution‐based growth of hierarchical structures with various alignments could offer a method for the further development of large‐area electronic and optoelectronic applications.

Arrays of van der Waals gaps were manufactured by synthesizing the vertically aligned graphene layer stacked between two copper (Cu) catalytic films. The Cu-graphene-Cu laminated structure was obtained by directly synthesizing graphene on a patterned Cu film followed by depositing a second copper layer for optical measurements. The synthesis of graphene on the Cu surface was optimized by adjusting the synthesis temperatures and pre-annealing time using plasma enhanced chemical vapor deposition (PECVD). Resonant Raman spectroscopy measurements reveal that graphene can be synthesized on both bulk Cu foil and relatively thin Cu film under the same growth mechanism using PECVD. Structural and optical characterizations of the array of graphene van der Waals gaps were implemented by the transmission electron microscope and terahertz-time domain spectroscopy (THz-TDS). In THz-TDS, the measured THz amplitude transmitted through the graphene van der Waals gap slit array was constant regardless of the gap width determined by the number of graphene layers between the Cu thin films in a single slit. These results imply that the optical dielectric constant of graphene at THz frequencies in the out-of-plane direction is linearly proportional to the gap width. Our results of the manufacturing method can be adopted to investigate mechanical, electrical, and optical properties of other 2D materials such as h-BN, MoS2, and others. Furthermore, metal-graphene-metal structures with vertical orientations can be used in many electronic, optic, and optoelectronic applications.

Despite many encouraging properties of transition metal dichalcogenides (TMDs), a central challenge in the realm of industrial applications based on TMD materials is to connect the large-scale synthesis and reproducible production of highly crystalline TMD materials. Here, the primary aim is to resolve simultaneously the two inversely related issues through the synthesis of MoS2(1- x ) Se2 x ternary alloys with customizable bichalcogen atomic (S and Se) ratio via atomic-level substitution combined with a solution-based large-area compatible approach. The relative concentration of bichalcogen atoms in the 2D alloy can be effectively modulated by altering the selenization temperature, resulting in 4 in. scale production of MoS1.62 Se0.38 , MoS1.37 Se0.63 , MoS1.15 Se0.85 , and MoS0.46 Se1.54 alloys, as well as MoS2 and MoSe2 . Comprehensive spectroscopic evaluations for vertical and lateral homogeneity in terms of heteroatom distribution in the large-scale 2D TMD alloys are implemented. Se-stimulated strain effects and a detailed mechanism for the Se substitution in the MoS2 crystal are further explored. Finally, the capability of the 2D alloy for industrial application in nanophotonic devices and hydrogen evolution reaction (HER) catalysts is validated. Substantial enhancements in the optoelectronic and HER performances of the 2D ternary alloy compared with those of its binary counterparts, including pure-phase MoS2 and MoSe2 , are unambiguously achieved.

In article number 1904194, Seoung-Ki Lee, Jong-Hyun Ahn, and co-workers demonstrate a heterostructure array based on transition-metal dichalcogenides, via a solution-based self-assembly method. The heterostructure is composed of p-WSe2 wires and n-MoS2 wires in parallel- or cross-aligned structure. This WSe2/MoS2 p–n junction array exhibits outstanding electrical and optoelectrical properties, including high rectifying behavior and photoresponsivity with fast response time.

Low-dimensional nanostructures and their complementary hybridization techniques are in the vanguard of technological advances for applications in transparent and flexible nanoelectronics due to the intriguing electrical properties related to their atomic structure. In this study, we demonstrated that welding of Ag nanowires (NWs) encapsulated in graphene was stimulated by flux-optimized, high-energy electron beam irradiation (HEBI) under ambient conditions. This methodology can inhibit the oxidation of Ag NWs which is induced by the inevitably generated reactive ozone as well as improve of their electrical conductivity. We have systematically explored the effects of HEBI on Ag NWs and graphene. The optimized flux for HEBI welding of the Ag NWs with graphene was 150 kGy, which decreased the sheet resistance of the graphene/Ag NWs to 12 Ohm/sq. Following encapsulation with graphene, the initial chemical states of the Ag NWs were well-preserved after flux-tuned HEBI, whereas graphene underwent local HEBI-induced defect generation near the junction area. We further employed resonant Raman spectroscopy to follow the structural evolution of the sacrificial graphene in the hybrid film after HEBI. Notably, the sheet resistance of the welded Ag NWs encapsulated with graphene after HEBI was well-maintained even after 85 days.

We developed a paper-type electrode for the next-generation wearable devices by complementary hybridization of methyl cellulose and carbon nanotubes (CNTs). The large-scale hybrid paper was fabricated using an incomparably simple and low-cost solution process. Structural features, surface hydrophobicity, and chemical interaction between CNTs and cellulose of the hybrid papers were systematically explored by adjusting the concentration of CNTs. The hybrid papers demonstrated superb mechanical durability even under highly strained conditions such as folding, crumpling and re-flattening regardless of the concentration of CNTs. The electrical properties of the CNTs/methyl cellulose were well-preserved even when folded as origami or submerged in water.

Complementary hybridization of ZnO thin films with aluminum tetraphenylporphyrin (Al(III)TPP), zinc tetraphenylporphyrin (Zn(II)TPP) and H2TPP (tetraphenylporphyrin) was adopted for tuning the hybrid thin film transistor (TFT) performance and improving their flexibility. After the hybridization with the organic layers, the chemical and structural features of ZnO thin films were well-preserved as compared with those of solely ZnO thin films. The existence of organic layers was monitored by X-ray photoelectron spectroscopy depth profiling. We fabricated the TFT based on ZnO/organic layers, resulting in the on-off ratio and threshold voltage of the devices manipulated by selecting the organic layers. These results can be understood by the performance tuning mechanisms related with the electron charge transfer induced by a work function difference. Remarkably, a significant improvement of the flexibility in the hybrid films was achieved without any significant loss in optical transmittance, which will be high demand in transparent and flexible electronics.

We rationally designed a new type of hybrid materials, molybdenum disulfide (MoS2) synthesized by Mo pre-deposition followed by subsequent sulfurization process directly on thermal chemical vapor deposition (TCVD)-grown graphene, for applications in a multifunctional device. The synthesis of stoichiometric and uniform multilayer MoS2 and high-crystalline monolayer graphene was evaluated by X-ray photoelectron spectroscopy and Raman spectroscopy. To examine the electrical transport and photoelectrical properties of MoS2-graphene hybrid films, field effect transistors (FETs) and visible-light photodetectors based on MoS2-graphene were both fabricated. As a result, the extracted mobility for MoS2-graphene hybrid FETs was two times higher than that of MoS2 FETs. In addition, the MoS2-graphene photodetectors revealed a significant photocurrent with abrupt switching behavior under periodic illumination. (C) 2016 Elsevier B.V. All rights reserved.