Plasmonic hot electron induced structural phase transition in a MoS2 monolayer

Hot Electron-Based Near-Infrared Photodetection Using Bilayer MoS2

Recently, there has been much interest in the extraction of hot electrons generated from surface plasmon decay, as this process can be used to achieve additional bandwidth for both photodetectors and photovoltaics. Hot electrons are typically injected into semiconductors over a Schottky barrier between the metal and semiconductor, enabling generation of photocurrent with below bandgap photon illumination. As a two-dimensional semiconductor single and few layer molybdenum disulfide (MoS2) has been demonstrated to exhibit internal photogain and therefore becomes an attractive hot electron acceptor. Here, we investigate hot electron-based photodetection in a device consisting of bilayer MoS2 integrated with a plasmonic antenna array. We demonstrate sub-bandgap photocurrent originating from the injection of hot electrons into MoS2 as well as photoamplification that yields a photogain of 10(5). The large photogain results in a photoresponsivity of 5.2 A/W at 1070 nm, which is far above similar silicon-based hot electron photodetectors in which no photoamplification is present. This technique is expected to have potential use in future ultracompact near-infrared photodetection and optical memory devices.

Gram-Scale Aqueous Synthesis of Stable Few-Layered 1T-MoS2 : Applications for Visible-Light-Driven Photocatalytic Hydrogen Evolution

Most recently, much attention has been devoted to 1T phase MoS2 because of its distinctive phase-engineering nature and promising applications in catalysts, electronics, and energy storage devices. While alkali metal intercalation and exfoliation methods have been well developed to realize unstable 1T-MoS2 , but the aqueous synthesis for producing stable metallic phase remains big challenging. Herein, a new synthetic protocol is developed to mass-produce colloidal metallic 1T-MoS2 layers highly stabilized by intercalated ammonium ions (abbreviated as N-MoS2). In combination with density functional calculations, the X-ray diffraction pattern and Raman spectra elucidate the excellent stability of metallic phase. As clearly depicted by high-angle annular dark-field imaging in an aberration-corrected scanning transmission electron microscope and extended X-ray absorption fine structure, the N-MoS2 exhibits a distorted octahedral structure with a 2a0 × a0 basal plane superlattice and 2.72 Å Mo-Mo bond length. In a proof-of-concept demonstration for the obtained material's applications, highly efficient photocatalytic activity is achieved by simply hybridizing metallic N-MoS2 with semiconducting CdS nanorods due to the synergistic effect. As a direct outcome, this CdS:N-MoS2 hybrid shows giant enhancement of hydrogen evolution rate, which is almost 21-fold higher than pure CdS and threefold higher than corresponding annealed CdS:2H-MoS2.

Active Light Control of the MoS2 Monolayer Exciton Binding Energy

Plasmonic excitation of Au nanoparticles deposited on a MoS2 monolayer changes the absorption and photoluminescence characteristics of the material. Hot electrons generated from the Au nanoparticles are transferred into the MoS2 monolayers, resulting in n-doping. The doping effect of plasmonic hot electrons modulates the dielectric permittivity of materials, resulting in a red shift of both the absorption and the photoluminescence spectrum. This spectroscopic tuning was further investigated experimentally by using different Au nanoparticle concentrations, excitation laser wavelengths, and intensities. An analytical model for the photoinduced modulation of the MoS2 dielectric function and its exciton binding energy change is developed and used to estimate the doping density of plasmonic hot electrons. Our approach is important for the development of photonic devices for active control of light by light.

Graphene Quantum Dots Doping of MoS2 Monolayers

Graphene quantum dots (GQDs) interacting with molybdenum disulfide (MoS2 ) monolayers induce an effective photoexcited charge transfer at the interface. Both the photoluminescence (PL) and valley polarization of this GQDs/MoS2 heterostructure can be modulated under various doping charge densities. The photon-exciton interaction is used to explain and calculate the heterostructure PL control, and is further applied to the valley-polarization tuning.

Stabilization of 1T-MoS₂ Sheets by Imidazolium Molecules in Self-Assembling Hetero-layered Nanocrystals

We report a facile, room-temperature assembly of MoS2-based hetero-layered nanocrystals (NCs) containing embedded monolayers of imidazolium (Im), 1-butyl-3-methylimidazolium (BuMeIm), 2-phenylimidazolium, and 2-methylbenzimidazolium molecules. The NCs are readily formed in water solutions by self-organization of the negatively charged, chemically exfoliated 0.6 nm thick MoS2 sheets and corresponding cationic imidazole moieties. As evidenced by transmission electron microscopy, the obtained NCs are anisotropic in shape, with thickness varying in the range 5-20 nm and lateral dimensions of hundreds of nanometers. The NCs exhibit almost turbostratic stacking of the MoS2 sheets, though the local order is preserved in the orientation of the imidazolium molecules with respect to the sulfide sheets. The atomic structure of NCs with BuMeIm molecules was solved from powder X-ray diffraction data assisted by density functional theory calculations. The performed studies evidenced that the MoS2 sheets of the NCs are of the nonconventional 1T-MoS2 (metallically conducting) structure. The sheets' puckered outer surface is formed by the S atoms and the positioning of the BuMeIm molecules follows the sheet nanorelief. According to thermal analysis data, the presence of the BuMeIm cations significantly increases the stability of the 1T-MoS2 modification and raises the temperature for its transition to the conventional 2H-MoS2 (semiconductive) counterpart by ∼70 °C as compared to pure 1T-MoS2 (∼100 °C). The stabilizing interaction energy between inorganic and organic layers was estimated as 21.7 kcal/mol from the calculated electron density distribution. The results suggest a potential for the design of few-layer electronic devices exploiting the charge transport properties of monolayer thin MoS2.

Characterization of the structural defects in CVD-grown monolayered MoS2 using near-field photoluminescence imaging

Structural defects can critically influence the electrical and optical properties of monolayered molybdenum disulfide (MoS2) grown by chemical vapor deposition (CVD); thus, convenient optical methods that can visualize grain boundaries (GBs) and other structural defects are in great demand. Although photoluminescence (PL) imaging can identify the presence of relatively large defects, the limited spatial resolution of PL imaging prevents the identification of nanosized structural defects in the monolayered MoS2. Additionally, the origin of the PL signal contrast observed at certain types of structural defects, such as GBs, is not yet understood. Here, we present near-field PL images of CVD-grown monolayered MoS2, collected to identify nanosized line defects and adlayer defects in the monolayered MoS2. Our results of correlated scanning electron microscopy imaging and the inspection of near-field PL profiles of line defects and GBs suggest that decreased PL on GBs is due to the local physical damage of the MoS2 film rather than due to the presence of localized states.

Dispersive growth and laser-induced rippling of large-area singlelayer MoS2 nanosheets by CVD on c-plane sapphire substrate

Vapor-phase growth of large-area two-dimensional (2D) MoS₂ nanosheets via reactions of sulfur with MoO₃ precursors vaporized and transferred from powder sources onto a target substrate has been rapidly progressing. Recent studies revealed that the growth yield of high quality singlelayer (SL) MoS₂ is essentially controlled by quite a few parameters including the temperature, the pressure, the amount/weight of loaded source precursors, and the cleanup of old precursors. Here, we report a dispersive growth method where a shadow mask is encapsulated on the substrate to ‘indirectly’ supply the source precursors onto the laterally advancing growth front at elevated temperatures. With this method, we have grown large-area (up to millimeters) SL-MoS₂ nanosheets with a collective in-plane orientation on c-plane sapphire substrates. Regular ripples (~1 nm in height and ~50 nm in period) have been induced by laser scanning into the SL-MoS₂ nanosheets. The MoS₂ ripples easily initiate at the grain boundaries and extend along the atomic steps of the substrate. Such laser-induced ripple structures can be fundamental materials for studying their effects, which have been predicted to be significant but hitherto not evidenced, on the electronic, mechanical, and transport properties of SL-MoS₂.

Plasmonic Hot Electron Induced Photocurrent Response at MoS2-Metal Junctions

We investigate the wavelength- and polarization-dependence of photocurrent signals generated at few-layer MoS2-metal junctions through spatially resolved photocurrent measurements. When incident photon energy is above the direct bandgap of few-layer MoS2, the maximum photocurrent response occurs for the light polarization direction parallel to the metal electrode edge, which can be attributed to photovoltaic effects. In contrast, if incident photon energy is below the direct bandgap of MoS2, the photocurrent response is maximized when the incident light is polarized in the direction perpendicular to the electrode edge, indicating different photocurrent generation mechanisms. Further studies show that this polarized photocurrent response can be interpreted in terms of the polarized absorption of light by the plasmonic metal electrode, its conversion into hot electron-hole pairs, and subsequent injection into MoS2. These fundamental studies shed light on the knowledge of photocurrent generation mechanisms in metal-semiconductor junctions, opening the door for engineering future two-dimensional materials based optoelectronics through surface plasmon resonances.

Plasmonic hot electron enhanced MoS2 photocatalysis in hydrogen evolution

With plasmonic hot electron doping, the molybdenum disulfide (MoS2) monolayer with deposited Au@Ag nanorattles effectively enhanced the hydrogen evolution reaction (HER) efficiency. The maximum photocatalysis is achieved under plasmon resonance excitation, and is actively controlled by the incident laser wavelength and power intensity. The localized phase transition of MoS2 is achieved and characterized to explicate this plasmon-enhanced hydrogen evolution. The proposed MoS2-nanoparticle composite combines surface plasmons and planar 2D materials, and pioneers a frontier field of plasmonic MoS2 photocatalysis.

Wavelength tunable microdisk cavity light source with a chemically enhanced MoS2 emitter

In this work, we report an integrated narrowband light source based on thin MoS2 emissive material coupled to the high quality factor whispering gallery modes of a microdisk cavity with a spatial notch that enables easy out-coupling of emission while it yields high spatial coherence and a Gaussian intensity distribution. The active light emitting material consists of chemically enhanced bilayer MoS2 flakes with a thin atomic layer deposited SiO2 protective coating that yields 20-times brighter chemically enhanced photoluminescence compared to as-exfoliated monolayers on the microdisk. Quality factors ≈ 1000 are observed as well as a high degree of spatial coherence. We also experimentally achieve effective index tuning of cavity coupled emission over a full free spectral range. The thermal response of this system is also studied. This work provides new insights for nanophotonic light sources with atomically thin active media.

Two-dimensional transition metal dichalcogenide nanosheet-based composites

This review summarizes and discusses the synthetic strategies, properties and applications of two-dimensional transition metal dichalcogenide nanosheet-based composites, with emphasis on those new appealing structures, properties and functions.

Phase engineering of transition metal dichalcogenides

The co-existence of 2H, 1T and 1T′ phases in monolayered TMDs.

Phase stability and Raman vibration of the molybdenum ditelluride (MoTe2) monolayer

A DFT study on the phase stability of the different phases of the MoTe₂ monolayer in their free standing state.

Plasmon-induced hot carrier science and technology

The discovery of the photoelectric effect by Heinrich Hertz in 1887 set the foundation for over 125 years of hot carrier science and technology. In the early 1900s it played a critical role in the development of quantum mechanics, but even today the unique properties of these energetic, hot carriers offer new and exciting opportunities for fundamental research and applications. Measurement of the kinetic energy and momentum of photoejected hot electrons can provide valuable information on the electronic structure of materials. The heat generated by hot carriers can be harvested to drive a wide range of physical and chemical processes. Their kinetic energy can be used to harvest solar energy or create sensitive photodetectors and spectrometers. Photoejected charges can also be used to electrically dope two-dimensional materials. Plasmon excitations in metallic nanostructures can be engineered to enhance and provide valuable control over the emission of hot carriers. This Review discusses recent advances in the understanding and application of plasmon-induced hot carrier generation and highlights some of the exciting new directions for the field.

Synthesis of porous gold nanoparticle/MoS2 nanocomposites based on redox reactions

We develop a facile method for preparing the porous gold nanoparticles (Au-NPs)/2H-form MoS₂ nanocomposite that forms a unique 3-dimensional structure and shows a high surface enhanced Raman spectroscopy effect.

Plasmonic pumping of excitonic photoluminescence in hybrid MoS2-Au nanostructures

We report on the fabrication of monolayer MoS2-coated gold nanoantennas combining chemical vapor deposition, e-beam lithography surface patterning, and a soft lift-off/transfer technique. The optical properties of these hybrid plasmonic-excitonic nanostructures are investigated using spatially resolved photoluminescence spectroscopy. Off- and in-resonance plasmonic pumping of the MoS2 excitonic luminescence showed distinct behaviors. For plasmonically mediated pumping, we found a significant enhancement (∼65%) of the photoluminescence intensity, clear evidence that the optical properties of the MoS2 monolayer are strongly influenced by the nanoantenna surface plasmons. In addition, a systematic photoluminescence broadening and red-shift in nanoantenna locations is observed which is interpreted in terms of plasmonic enhanced optical absorption and subsequent heating of the MoS2 monolayers. Using a temperature calibration procedure based on photoluminescence spectral characteristics, we were able to estimate the local temperature changes. We found that the plasmonically induced MoS2 temperature increase is nearly four times larger than in the MoS2 reference temperatures. This study shines light on the plasmonic-excitonic interaction in these hybrid metal/semiconductor nanostructures and provides a unique approach for the engineering of optoelectronic devices based on the light-to-current conversion.

Theoretical predictions for hot-carrier generation from surface plasmon decay

Decay of surface plasmons to hot carriers finds a wide variety of applications in energy conversion, photocatalysis and photodetection. However, a detailed theoretical description of plasmonic hot-carrier generation in real materials has remained incomplete. Here we report predictions for the prompt distributions of excited 'hot' electrons and holes generated by plasmon decay, before inelastic relaxation, using a quantized plasmon model with detailed electronic structure. We find that carrier energy distributions are sensitive to the electronic band structure of the metal: gold and copper produce holes hotter than electrons by 1-2 eV, while silver and aluminium distribute energies more equitably between electrons and holes. Momentum-direction distributions for hot carriers are anisotropic, dominated by the plasmon polarization for aluminium and by the crystal orientation for noble metals. We show that in thin metallic films intraband transitions can alter the carrier distributions, producing hotter electrons in gold, but interband transitions remain dominant.