Surface Plasmon‐Enhanced Photodetection in Few Layer MoS2 Phototransistors with Au Nanostructure Arrays

2D Molybdenum disulfide (MoS₂) is a promising candidate material for high‐speed and flexible optoelectronic devices, but only with low photoresponsivity. Here, a large enhancement of photocurrent response is obtained by coupling few‐layer MoS₂ with Au plasmonic nanostructure arrays. Au nanoparticles or nanoplates placed onto few‐layer MoS₂ surface can enhance the local optical field in the MoS₂ layer, due to the localized surface plasmon (LSP) resonance. After depositing 4 nm thick Au nanoparticles sparsely onto few‐layer MoS₂ phototransistors, a doubled increase in the photocurrent response is observed. The photocurrent of few‐layer MoS₂ phototransistors exhibits a threefold enhancement with periodic Au nanoarrays. The simulated optical field distribution confirms that light can be trapped and enhanced near the Au nanoplates. These findings offer an avenue for practical applications of high performance MoS₂‐based optoelectronic devices or systems in the future.     

Au NPs@MoS2 Sub‐Micrometer Sphere‐ZnO Nanorod Hybrid Structures for Efficient Photocatalytic Hydrogen Evolution with Excellent Stability

MoS₂ shows promising applications in photocatalytic water splitting, owing to its uniquely optical and electric properties. However, the insufficient light absorption and lack of performance stability are two crucial issues for efficient application of MoS₂ nanomaterials. Here, Au nanoparticles (NPs)@MoS₂ sub‐micrometer sphere‐ZnO nanorod (Au NPs@MoS₂‐ZnO) hybrid photocatalysts have been successfully synthesized by a facile process combining the hydrothermal method and seed‐growth method. Such photocatalysts exhibit high efficiency and excellent stability for hydrogen production via multiple optical‐electrical effects. The introduction of Au NPs to MoS₂ sub‐micrometer spheres forming a core–shell structure demonstrates strong plasmonic absorption enhancement and facilitates exciton separation. The incorporation of ZnO nanorods to the Au NPs@MoS₂ hybrids further extends the light absorption to a broader wavelength region and enhances the exciton dissociation. In addition, mutual contacts between Au NPs (or ZnO nanorods) and the MoS₂ spheres effectively protect the MoS₂ nanosheets from peeling off from the spheres. More importantly, efficiently multiple exciton separations help to restrain the MoS₂ nanomaterials from photocorrosion. As a result, the Au@MoS₂‐ZnO hybrid structures exhibit an excellent hydrogen gas evolution (3737.4 μmol g^?1) with improved stability (91.9% of activity remaining) after a long‐time test (32 h), which is one of the highest photocatalytic activities to date among the MoS₂ based photocatalysts.     

Direct observation of epitaxial alignment of Au on MoS2 at atomic resolution

The morphology and structural stability of metal/2D semiconductor interfaces strongly affect the performance of 2D electronic devices and synergistic catalysis. However, the structural evolution at the interfaces has not been well explored particularly at atomic resolution. In this work, we study the structural evolution of Au nanoparticles (NPs) on few-layer MoS₂ by high resolution transmission electron microscope (HRTEM) and quantitative high-angle annular dark field scanning TEM. It is found that in the transition of Au from nanoparticles to dendrites, a dynamically epitaxial alignment between Au and MoS₂ lattices is formed, and Moiré patterns can be directly observed in HRTEM images due to the mismatch between Au and MoS₂ lattices. This epitaxial alignment can occur in ambient conditions, and can also be accelerated by the irradiation of high-energy electron beam. In situ observation clearly reveals the rotation of Au NPs, the atom migration inside Au NPs, and the transfer of Au atoms between neighboring Au NPs, finally leading to the formation of epitaxially aligned Au dendrites on MoS₂. The structural evolution of metal/2D semiconductor interfaces at atomic scale can provide valuable information for the design and fabrication of the metal/2D semiconductor nano-devices with desired physical and chemical performances.

     

Frictional and morphological properties of AuMoS2 films sputtered from a compact target

AuMoS₂ films 0.02–1.2 μm thick were sputtered from a target compacted from 5 wt.% Au plus 95 wt.% MoS₂ to investigate the frictional and morphological film growth characteristics. The gold dispersion effects in MoS₂ films are of interest to increase the densification and strengthening of the film structure. Three microstructural growth stages were identified on the nano-micro-macrostructural level. During sliding both sputtered AuMoS₂ and sputtered MoS₂ films have a tendency to break within the columnar region. The remaining or effective film, about 0.2 μm thick, performs the lubrication. The AuMoS₂ films displayed a lower friction coefficient with a high degree of frictional stability and less wear debris generation compared with pure MoS₂ films. The more favorable frictional characteristics of the AuMoS₂ films are attributed to the effective film thickness and the high density packed columnar zone which has a reduced effect on the fragmentation of the tapered crystallites during fracture.     

Au Dotted Magnetic Network Nanostructure and Its Application for On‐Site Monitoring Femtomolar Level Pesticide

A novel magnetically responsive and surface‐enhanced Raman spectroscopy (SERS) active nanocomposite is designed and prepared by direct grafting of Au nanoparticles onto the surface of magnetic network nanostructure (MNN) with the help of a nontoxic and environmentally friendly reagent of inositol hexakisphosphate shortly named as IP₆. The presence of IP₆ as a stabilizer and a bridging agent could weave Fe₃O₄ nanoparticles (NPs) into magnetic network nanostructure, which is easily dotted with Au nanoparticles (Au NPs). It has been shown firstly that the huge Raman enhancement of Au‐MNN is reached by an external magnetic collection. Au‐MNN presenting the large surface and high detection sensitivity enables it to exhibit multifunctional applications involving sufficient adsorption of dissolved chemical species for enrichment, separation, as well as a Raman amplifier for the analysis of trace pesticide residues at femtomolar level by a portable Raman spectrometer. Therefore, such multifunctional nanocomposites can be developed as a smart and promising nanosystem that integrates SERS approach with an easy assay for concentration by an external magnet for the effective on‐site assessments of agricultural and environmental safety.     

Cu2MoS4/Au Heterostructures with Enhanced Catalase‐Like Activity and Photoconversion Efficiency for Primary/Metastatic Tumors Eradication by Phototherapy‐Induced Immunotherapy

Photoimmunotherapy can not only effectively ablate the primary tumor but also trigger strong antitumor immune responses against metastatic tumors by inducing immunogenic cell death. Herein, Cu₂MoS₄ (CMS)/Au heterostructures are constructed by depositing plasmonic Au nanoparticles onto CMS nanosheets, which exhibit enhanced absorption in near‐infrared (NIR) region due to the newly formed mid‐gap state across the Fermi level based on the hybridization between Au 5d orbitals and S 3p orbitals, thus resulting in more excellent photothermal therapy and photodynamic therapy (PDT) effect than single CMS upon NIR laser irradiation. The CMS and CMS/Au can also serve as catalase to effectively relieve tumor hypoxia, which can enhance the therapeutic effect of O₂‐dependent PDT. Notably, the NIR laser‐irradiated CMS/Au can elicit strong immune responses via promoting dendritic cells maturation, cytokine secretion, and activating antitumor effector T‐cell responses for both primary and metastatic tumors eradication. Moreover, CMS/Au exhibits outstanding photoacoustic and computed tomography imaging performance owing to its excellent photothermal conversion and X‐ray attenuation ability. Overall, the work provides an imaging‐guided and phototherapy‐induced immunotherapy based on constructing CMS/Au heterostructures for effectively tumor ablation and cancer metastasis inhibition.     

Out‐of‐Plane Strain Induced in a Moiré Superstructure of Monolayer MoS2 and MoSe2 on Au(111)

Making contact of transition metal dichalcogenides (TMDCs) with a metal surface is essential for fabricating and designing electronic devices and catalytic systems. It also generates strain in the TMDCs that plays significant role in both electronic and phonon structures. Therefore, detailed understanding of mechanism of the strain generation is important to fully comprehend the modulation effect for the electronic and phonon properties. Here, MoS₂ and MoSe₂ monolayers are grown on Au surface by chemical vapor deposition and it is demonstrated that the contact with a crystalline Au(111) surface gives rise to only out‐of‐plane strain in both MoS₂ and MoSe₂ layers, whereas no strain generation is observed on polycrystalline Au or SiO₂/Si surfaces. Scanning tunneling microscopy analysis provides information regarding consequent specific adsorption sites between lower S (Se) atoms in the S? Mo? S (Se? Mo? Se) structure and Au atoms via unique moiré superstructure formation for MoS₂ and MoSe₂ layers on Au(111). This observation indicates that the specific adsorption sites give rise to out‐of‐plane strain in the TMDC layers. Furthermore, it also leads to effective modulation of the electronic structure of the MoS₂ or MoSe₂ layer.     

Solution‐Phase Epitaxial Growth of Perovskite Films on 2D Material Flakes for High‐Performance Solar Cells

The quality of perovskite films is critical to the performance of perovskite solar cells. However, it is challenging to control the crystallinity and orientation of solution‐processed perovskite films. Here, solution‐phase van der Waals epitaxy growth of MAPbI₃ perovskite films on MoS₂ flakes is reported. Under transmission electron microscopy, in‐plane coupling between the perovskite and the MoS₂ crystal lattices is observed, leading to perovskite films with larger grain size, lower trap density, and preferential growth orientation along (110) normal to the MoS₂ surface. In perovskite solar cells, when perovskite active layers are grown on MoS₂ flakes coated on hole‐transport layers, the power conversion efficiency is substantially enhanced for 15%, relatively, due to the increased crystallinity of the perovskite layer and the improved hole extraction and transfer rate at the interface. This work paves a way for preparing high‐performance perovskite solar cells and other optoelectronic devices by introducing 2D materials as interfacial layers.     

Morphological and frictional behavior of sputtered MoS2 films

From the texture and growth patterns of sputtered MoS₂ films deposited onto substrates, three regions can be distinguised: (1) a ridge formation region, (2) an equiaxed transition zone and (3) a columnar-fiber-like structure. The lubricating properties of sputtered MoS₂ films can be visually identified with respect to optical changes before and after rubbing. The orientation of the surface microcrystallites is identified, and the change in optical properties is explained. In sliding contact the sputtered film tends to break up at the base of the columnar region. Effective lubrication occurs with the film remaining on the substrate. This film is 0.18–0.22 microm thick.     

ZnS/C/MoS2 Nanocomposite Derived from Metal–Organic Framework for High‐Performance Photo‐Electrochemical Immunosensing of Carcinoembryonic Antigen

A hexafluorophosphate ionic liquid is used as a functional monomer to prepare a metal–organic framework (Zn‐MOF). Zn‐MOF is used as a template for MoS₂ nanosheets synthesis and further carbonized to yield light‐responsive ZnS/C/MoS₂ nanocomposites. Zn‐MOF, carbonized‐Zn‐MOF, and ZnS/C/MoS₂ nanocomposites are characterized by Fourier transform infrared spectroscopy, transmission electron microscopy, X‐ray diffraction pattern, scanning electron microscopy (SEM), element mapping, Raman spectroscopy, X‐ray photoelectron spectroscopy, fluorescence, and nitrogen‐adsorption analysis. Carcinoembryonic antigen (CEA) is selected as a model to construct an immunosensing platform to evaluate the photo‐electrochemical (PEC) performances of ZnS/C/MoS₂ nanocomposites. A sandwich‐type PEC immunosensor is fabricated by immobilizing CEA antibody (Ab₁) onto the ZnS/C/MoS₂/GCE surface, subsequently binding CEA and the alkaline phosphatase‐gold nanoparticle labeled CEA antibody (ALP‐Au‐Ab₂). The catalytic conversion of vitamin C magnesium phosphate produces ascorbic acid (AA). Upon being illuminated, AA can react with photogenerated holes from ZnS/C/MoS₂ nanocomposites to generate a photocurrent for quantitative assay. Under optimized experimental conditions, the PEC immunosensor exhibits excellent analytical characteristics with a linear range from 2.0 pg mL^?1 to 10.0 ng mL^?1 and a detection limit of 1.30 pg mL^?1 (S/N = 3). The outstanding practicability of this PEC immunosensor is demonstrated by accurate assaying of CEA in clinical serum samples.     

Enhanced photoelectrochemical properties and photocatalytic activity of porous TiO2 films with highly dispersed small Au nanoparticles

Small Au nanoparticles (NPs) with mean diameter of 4.1 nm were highly deposited on TiO₂ films via a simple electrostatic self-assembly method. The physically separated Au NPs, with a high surface density of 6.3 × 10¹¹ NPs/cm², were mainly distributed on the top layer of porous TiO₂ films. The deposition of Au NPs induced a negative shift (~ 100 mV) of the apparent flat band potential of Au-TiO₂ electrodes. The charge separation efficiency of the TiO₂ electrode increased from 72.1% to 88.5% by dispersing Au NPs. Whatever redox species were present in the electrolyte, the Au-TiO₂ electrode had higher photovoltage than the TiO₂ electrode. The photovoltage was very sensitive to added redox species such as O₂, O₃, and methanol, and the effect of adsorbed redox species on electron accumulation was discussed. The electrochemical impedance spectroscopic measurements revealed that the charge transfer resistance (R_ct) of Au-TiO₂ films was reduced to 16% of bare TiO₂ electrode, and the decreased R_ct corresponded to the increased photocatalytic activity of Au-TiO₂ films. The beneficial role of uniformly dispersed small Au NPs on the charge separation was discussed. By modifying TiO₂ films with small Au NPs, the photocatalytic activity of TiO₂ films for formaldehyde degradation increased about 2.5 times.     

From UV to Near‐Infrared Light‐Responsive Metal–Organic Framework Composites: Plasmon and Upconversion Enhanced Photocatalysis

The exploitation of photocatalysts that harvest solar spectrum as broad as possible remains a high‐priority target yet grand challenge. In this work, for the first time, metal–organic framework (MOF) composites are rationally fabricated to achieve broadband spectral response from UV to near‐infrared (NIR) region. In the core–shell structured upconversion nanoparticles (UCNPs)‐Pt@MOF/Au composites, the MOF is responsive to UV and a bit visible light, the plasmonic Au nanoparticles (NPs) accept visible light, whereas the UCNPs absorb NIR light to emit UV and visible light that are harvested by the MOF and Au once again. Moreover, the MOF not only facilitates the generation of “bare and clean” Au NPs on its surface and realizes the spatial separation for the Au and Pt NPs, but also provides necessary access for catalytic substrates/products to Pt active sites. As a result, the optimized composite exhibits excellent photocatalytic hydrogen production activity (280 µmol g^?1 h^?1) under simulated solar light, and the involved mechanism of photocatalytic H₂ production under UV, visible, and NIR irradiation is elucidated. Reportedly, this is an extremely rare study on photocatalytic H₂ production by light harvesting in all UV, visible, and NIR regions.     

CdSe quantum dot-sensitized Au/TiO2 hybrid mesoporous films and their enhanced photoelectrochemical performance

Novel CdSe quantum dot (QD)-sensitized Au/TiO₂ hybrid mesoporous films have been designed, fabricated, and evaluated for photoelectrochemical (PEC) applications. The Au/TiO₂ hybrid structures were made by assembly of Au and TiO₂ nanoparticles (NPs). A chemical bath deposition method was applied to deposit CdSe QDs on TiO₂ NP films with and without Au NPs embedded. We observed significant enhancements in photocurrent for the film with Au NPs, in the entire spectral region we studied (350–600 nm). Incident-photon-to-current efficiency (IPCE) data revealed an average enhancement of 50%, and the enhancement was more significant at short wavelength. This substantially improved PEC performance is tentatively attributed to the increased light absorption of CdSe QDs due to light scattering by Au NPs. Interestingly, without QD sensitization, the Au NPs quenched the photocurrent of TiO₂ films, due to the dominance of electron trapping over light scattering by Au NPs. The results suggest that metal NPs are potentially useful for improving the photoresponse in PEC cells and possibly in other devices such as solar cells based on QD-sensitized metal oxide nanostructured films. This work demonstrates that metal NPs can serve as light scattering centers, besides functioning as photo-sensitizers and electron traps. The function of metal NPs in a particular nanocomposite film is strongly dependent on their structure and morphology.      

Neue Aspekte bei der physikalischen Abscheidung kompakter Dünnschichten aus der Gasphase am Beispiel einer praxisrelevanten Festschmierstoffschicht

New aspects regarding sputter‐depositing dense coatings, in particular a solid lubricant of practical interest Using MoS₂‐based solid lubricant films as an example, this paper focuses on approaches to preventing sputter‐deposited films from attaining columnar microstructures. This is how it becomes possible to deposit tribological coatings that are characterised by a high durability. As a result of scanning electron microscopy on microstructures, texture measurements by means of XRD, and surface analysis, focusing on sputter‐deposited MoS₂ films, a concept of how to develop thick and dense MoS₂‐based solid lubricant films was generated. At an early stage of film growth a dense and flawless microstructure forms. In order to make the favourable properties of these underlayers available for tribological applications, metallic intralayers were introduced, by means of which the MoS₂ film growth was to be repeatedly interrupted.     

3D 1T‐MoS2/CoS2 Heterostructure via Interface Engineering for Ultrafast Hydrogen Evolution Reaction

Metallic phase (1T) MoS₂ has been regarded as an appealing material for hydrogen evolution reaction. In this work, a novel interface‐induced strategy is reported to achieve stable and high‐percentage 1T MoS₂ through highly active 1T‐MoS₂/CoS₂ hetero‐nanostructure. Herein, a large number of heterointerfaces can be obtained by interlinked 1T‐MoS₂ and CoS₂ nanosheets in situ grown from the molybdate cobalt oxide nanorod under moderate conditions. Owing to the strong interaction between MoS₂ and CoS₂, high‐percentage of metallic‐phase (1T) MoS₂ of 76.6% can be achieved, leading to high electroconductivity and abundant active sites compared to 2H MoS₂. Furthermore, the interlinked MoS₂ and CoS₂ nanosheets can effectively disperse the nanosheets so as to enlarge the exposed active surface area. The near zero free energy of hydrogen adsorption at the heterointerface can also be achieved, indicating the fast kinetics and excellent catalytic activity induced by heterojunction. Therefore, when applied in hydrogen evolution reaction (HER), 1T‐MoS₂/CoS₂ heterostructure delivers low overpotential of 71 and 26 mV at the current density of 10 mA cm^?2 with low Tafel slops of 60 and 43 mV dec^?1, respectively in alkaline and acidic conditions.     

Unveiling Active Sites for the Hydrogen Evolution Reaction on Monolayer MoS2

Here, the hydrogen evolution reaction (HER) activities at the edge and basal‐plane sites of monolayer molybdenum disulfide (MoS₂) synthesized by chemical vapor deposition (CVD) are studied using a local probe method enabled by selected‐area lithography. Reaction windows are opened by e‐beam lithography at sites of interest on poly(methyl methacrylate) (PMMA)‐covered monolayer MoS₂ triangles. The HER properties of MoS₂ edge sites are obtained by subtraction of the activity of the basal‐plane sites from results containing both basal‐plane and edge sites. The catalytic performances in terms of turnover frequencies (TOFs) are calculated based on the estimated number of active sites on the selected areas. The TOFs follow a descending order of 3.8 ± 1.6, 1.6 ± 1.2, 0.008 ± 0.002, and 1.9 ± 0.8 × 10^?4 s^?1, found for 1T′‐, 2H‐MoS₂ edges, and 1T′‐, 2H‐MoS₂ basal planes, respectively. Edge sites of both 2H‐ and 1T′‐MoS₂ are proved to have comparable activities to platinum (≈1–10 s^?1). When fitted into the HER volcano plot, the MoS₂ active sites follow a trend distinct from conventional metals, implying a possible difference in the reaction mechanism between transition‐metal dichalcogenides (TMDs) and metal catalysts.     

Generalized Scheme for High Performing Photodetectors with a p‐Type 2D Channel Layer and n‐Type Nanoparticles

A generalized scheme for the fabrication of high performance photodetectors consisting of a p‐type channel material and n‐type nanoparticles is proposed. The high performance of the proposed hybrid photodetector is achieved through enhanced photoabsorption and the photocurrent gain arising from its effective charge transfer mechanism. In this paper, the realization of this design is presented in a hybrid photodetector consisting of 2D p‐type black phosphorus (BP) and n‐type molybdenum disulfide nanoparticles (MoS₂ NPs), and it is demonstrated that it exhibits enhanced photoresponsivity and detectivity compared to pristine BP photodetectors. It is found that the performance of hybrid photodetector depends on the density of NPs on BP layer and that the response time can be reduced with increasing density of MoS₂ NPs. The rising and falling times of this photodetector are smaller than those of BP photodetectors without NPs. This proposed scheme is expected to work equally well for a photodetector with an n‐type channel material and p‐type nanoparticles.     

Annealing effect on the structural and optical properties of embedded Au nanoparticles in silicon suboxide films

Au/SiO_x nanocomposite films have been fabricated by co-sputtering Au wires and SiO₂ target using an RF magnetron co-sputtering system before the thermal annealing process at different temperatures. The structural and optical properties of the samples were characterized using X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), optical transmission, and reflection spectroscopy. XPS analysis confirms that the as-prepared SiO_x films are silicon-rich suboxide films. FESEM images reveal that with an increase in annealing temperature, the embedded Au NPs tend to diffuse toward the surface of the SiO_x films. In IR spectra, the intensity of the Si-O-Si absorption band increases with the annealing temperature. Optical spectra reveal that the position and intensity of the surface plasmon resonance (SPR) peak are dominated by the effect of the inter-particle distance and size of the Au NPs embedded in the SiO_x films, respectively. The SPR absorption peak shows the blue-shift from 672 to 600 nm with an increase in annealing temperature. The growth of silica nanowires (SiO_x NWs) is observed in the film prepared on a c-Si substrate instead of a quartz substrate and annealed at temperatures of 1000 °C.     

A Critical Review on Enhancement of Photocatalytic Hydrogen Production by Molybdenum Disulfide: From Growth to Interfacial Activities

Ultrathin 2D molybdenum disulfide (MoS₂), which is the flagship of 2D transition‐metal dichalcogenide nanomaterials, has drawn much attention in the last few years. 2D MoS₂ has been banked as an alternative to platinum for highly active hydrogen evolution reaction because of its low cost, high surface‐to‐volume ratio, and abundant active sites. However, when MoS₂ is used directly as a photocatalyst, contrary to public expectation, it still performs poorly due to lateral size, high recombination ratio of excitons, and low optical cross section. Besides, simply compositing MoS₂ as a cocatalyst with other semiconductors cannot satisfy the practical application, which stimulates the pursual of a comprehensive insight into recent advances in synthesis, properties, and enhanced hydrogen production of MoS₂. Therefore, in this Review, emphasis is given to synthetic methods, phase transitions, tunable optical properties, and interfacial engineering of 2D MoS₂. Abundant ways of band edge tuning, structural modification, and phase transition are addressed, which can generate the neoteric photocatalytic systems. Finally, the main challenges and opportunities with respect to MoS₂ being a cocatalyst and coherent light–matter interaction of MoS₂ in photocatalytic systems are proposed.     

Gap‐Mode Surface‐Plasmon‐Enhanced Photoluminescence and Photoresponse of MoS2

2D materials hold great potential for designing novel electronic and optoelectronic devices. However, 2D material can only absorb limited incident light. As a representative 2D semiconductor, monolayer MoS₂ can only absorb up to 10% of the incident light in the visible, which is not sufficient to achieve a high optical‐to‐electrical conversion efficiency. To overcome this shortcoming, a “gap‐mode” plasmon‐enhanced monolayer MoS₂ fluorescent emitter and photodetector is designed by squeezing the light‐field into Ag shell‐isolated nanoparticles–Au film gap, where the confined electromagnetic field can interact with monolayer MoS₂. With this gap‐mode plasmon‐enhanced configuration, a 110‐fold enhancement of photoluminescence intensity is achieved, exceeding values reached by other plasmon‐enhanced MoS₂ fluorescent emitters. In addition, a gap‐mode plasmon‐enhanced monolayer MoS₂ photodetector with an 880% enhancement in photocurrent and a responsivity of 287.5 A W^?1 is demonstrated, exceeding previously reported plasmon‐enhanced monolayer MoS₂ photodetectors.