Active Light Control of the MoS2 Monolayer Exciton Binding Energy

Surface Doping and Dual Nature of the Band Gap in Excitonic Insulator Ta2NiSe5

Excitons in semiconductors and molecules are widely utilized in photovoltaics and optoelectronics, and high-temperature coherent quantum states of excitons can be realized in artificial electron-hole bilayers and an exotic material of an excitonic insulator (EI). Here, we investigate the band gap evolution of a putative high-temperature EI Ta2NiSe5 upon surface doing by alkali adsorbates with angle-resolved photoemission and density functional theory (DFT) calculations. The conduction band of Ta2NiSe5 is filled by the charge transfer from alkali adsorbates, and the band gap decreases drastically upon the increase of metallic electron density. Our DFT calculation, however, reveals that there exist both structural and excitonic contributions to the band gap tuned. While electron doping reduces the band gap substantially, it alone is not enough to close the band gap. In contrast, the structural distortion induced by the alkali adsorbate plays a critical role in the gap closure. This work indicates a combined electronic and structural nature for the EI phase of the present system and the complexity of surface doping beyond charge transfer.

Plasmon-Induced Hot Electrons in Nanostructured Materials: Generation, Collection, and Application to Photochemistry

Plasmon refers to the coherent oscillation of all conduction-band electrons in a nanostructure made of a metal or a heavily doped semiconductor. Upon excitation, the plasmon can decay through different channels, including nonradiative Landau damping for the generation of plasmon-induced energetic carriers, the so-called hot electrons and holes. The energetic carriers can be collected by transferring to a functional material situated next to the plasmonic component in a hybrid configuration to facilitate a range of photochemical processes for energy or chemical conversion. This article centers on the recent advancement in generating and utilizing plasmon-induced hot electrons in a rich variety of hybrid nanostructures. After a brief introduction to the fundamentals of hot-electron generation and decay in plasmonic nanocrystals, we extensively discuss how to collect the hot electrons with various types of functional materials. With a focus on plasmonic nanocrystals made of metals, we also briefly examine those based upon heavily doped semiconductors. Finally, we illustrate how site-selected growth can be leveraged for the rational fabrication of different types of hybrid nanostructures, with an emphasis on the parameters that can be experimentally controlled to tailor the properties for various applications.

Ultrafast Coherent Exciton Couplings and Many-Body Interactions in Monolayer WS2

Transition metal dichalcogenides (TMDs) are quantum confined systems with interesting optoelectronic properties, governed by Coulomb interactions in the monolayer (1L) limit, where strongly bound excitons provide a sensitive probe for many-body interactions. Here, we use two-dimensional electronic spectroscopy (2DES) to investigate many-body interactions and their dynamics in 1L-WS2 at room temperature and with sub-10 fs time resolution. Our data reveal coherent interactions between the strongly detuned A and B exciton states in 1L-WS2. Pronounced ultrafast oscillations of the transient optical response of the B exciton are the signature of a coherent 50 meV coupling and coherent population oscillations between the two exciton states. Supported by microscopic semiconductor Bloch equation simulations, these coherent dynamics are rationalized in terms of Dexter-like interactions. Our work sheds light on the role of coherent exciton couplings and many-body interactions in the ultrafast temporal evolution of spin and valley states in TMDs.

Anomalous layer-dependent photoluminescence spectra of supertwisted spiral WS2

Twisted stacking of two-dimensional materials with broken inversion symmetry, such as spiral MoTe2 nanopyramids and supertwisted spiral WS2, emerge extremely strong second- and third-harmonic generation. Unlike well-studied nonlinear optical effects in these newly synthesized layered materials, photoluminescence (PL) spectra and exciton information involving their optoelectronic applications remain unknown. Here, we report layer- and power-dependent PL spectra of the supertwisted spiral WS2. The anomalous layer-dependent PL evolutions that PL intensity almost linearly increases with the rise of layer thickness have been determined. Furthermore, from the power-dependent spectra, we find the power exponents of the supertwisted spiral WS2 are smaller than 1, while those of the conventional multilayer WS2 are bigger than 1. These two abnormal phenomena indicate the enlarged interlayer spacing and the decoupling interlayer interaction in the supertwisted spiral WS2. These observations provide insight into PL features in the supertwisted spiral materials and may pave the way for further optoelectronic devices based on the twisted stacking materials.

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

van der Waals 2D transition metal dichalcogenide/organic hybridized heterostructures: recent breakthroughs and emerging prospects of the device

The near-atomic thickness and organic molecular systems, including organic semiconductors and polymer-enabled hybrid heterostructures, of two-dimensional transition metal dichalcogenides (2D-TMDs) can modulate their optoelectronic and transport properties outstandingly. In this review, the current understanding and mechanism of the most recent and significant breakthrough of novel interlayer exciton emission and its modulation by harnessing the band energy alignment between TMDs and organic semiconductors in a TMD/organic (TMDO) hybrid heterostructure are demonstrated. The review encompasses up-to-date device demonstrations, including field-effect transistors, detectors, phototransistors, and photo-switchable superlattices. An exploration of distinct traits in 2D-TMDs and organic semiconductors delves into the applications of TMDO hybrid heterostructures. This review provides insights into the synthesis of 2D-TMDs and organic layers, covering fabrication techniques and challenges. Band bending and charge transfer via band energy alignment are explored from both structural and molecular orbital perspectives. The progress in emission modulation, including charge transfer, energy transfer, doping, defect healing, and phase engineering, is presented. The recent advancements in 2D-TMDO-based optoelectronic synaptic devices, including various 2D-TMDs and organic materials for neuromorphic applications are discussed. The section assesses their compatibility for synaptic devices, revisits the operating principles, and highlights the recent device demonstrations. Existing challenges and potential solutions are discussed. Finally, the review concludes by outlining the current challenges that span from synthesis intricacies to device applications, and by offering an outlook on the evolving field of emerging TMDO heterostructures.

Photoluminescence Enhancement of Monolayer WS2 by n-Doping with an Optically Excited Gold Disk

In nanophotonics and quantum optics, we aim to control and manipulate light with tailored nanoscale structures. Hybrid systems of nanostructures and atomically thin materials are of interest here, as they offer rich physics and versatility due to the interaction between photons, plasmons, phonons, and excitons. In this study, we explore the optical and electronic properties of a hybrid system, a naturally n-doped monolayer WS2 covering a gold disk. We demonstrate that the nonresonant excitation of the gold disk in the high absorption regime efficiently generates hot carriers via localized surface plasmon excitation, which n-dope the monolayer WS2 and enhance the photoluminescence emission by regulating the multiexciton population and stabilizing the neutral exciton emission. The results are relevant to the further development of nanotransistors in photonic circuits and optoelectronic applications.

Plasmon photocatalytic CO2 reduction reactions over Au particles on various substrates

Surface plasmonic effects have been widely used in photocatalytic reactions like CO₂ conversion in the past decades. However, owing to the significant controversy in the physical processes of plasmon photocatalytic reactions and difficulty in realizing CO₂ reduction, the influence mechanism of the plasmon effect on the CO₂ photoreduction is still under debate. In this study, Au particles deposited on various substrates were employed to acquire insights into the plasmon photocatalytic CO₂ reduction, including SiO₂, n-Si, p-Si, TiO₂-SiO₂, TiO₂-n-Si, and TiO₂-p-Si. It was found that the plasmon resonant enhancement (PRE) effect of Au-SiO₂ caused by the Au plasmon was stronger than that of Au-TiO₂-SiO₂ and Au-n-Si (Au-p-Si) in the visible-light range, while it was weaker for Au-n-Si (Au-p-Si) samples than Au-TiO₂-n-Si (Au-TiO₂-p-Si). The simulation results agree with the experimental conclusions. The photocatalytic results indicated that the catalytic activity of Au-n-Si (Au-p-Si) samples was lower than that of Au-TiO₂-n-Si (Au-TiO₂-p-Si), and Au-SiO₂ was lower than Au-TiO₂-SiO₂ and Au-n-Si (Au-p-Si) samples, suggesting that the direct electron transfer (DET) mechanism was dominant here compared with the PRE mechanism.

Achieving near-perfect light absorption in atomically thin transition metal dichalcogenides through band nesting

Near-perfect light absorbers (NPLAs), with absorbance, [Formula: see text], of at least 99%, have a wide range of applications ranging from energy and sensing devices to stealth technologies and secure communications. Previous work on NPLAs has mainly relied upon plasmonic structures or patterned metasurfaces, which require complex nanolithography, limiting their practical applications, particularly for large-area platforms. Here, we use the exceptional band nesting effect in TMDs, combined with a Salisbury screen geometry, to demonstrate NPLAs using only two or three uniform atomic layers of transition metal dichalcogenides (TMDs). The key innovation in our design, verified using theoretical calculations, is to stack monolayer TMDs in such a way as to minimize their interlayer coupling, thus preserving their strong band nesting properties. We experimentally demonstrate two feasible routes to controlling the interlayer coupling: twisted TMD bi-layers and TMD/buffer layer/TMD tri-layer heterostructures. Using these approaches, we demonstrate room-temperature values of [Formula: see text]=95% at λ=2.8 eV with theoretically predicted values as high as 99%. Moreover, the chemical variety of TMDs allows us to design NPLAs covering the entire visible range, paving the way for efficient atomically-thin optoelectronics.

Role of Bilayer Graphene Microstructure on the Nucleation of WSe2 Overlayers

Over the past few years, graphene grown by chemical vapor deposition (CVD) has gained prominence as a template to grow transition metal dichalcogenide (TMD) overlayers. The resulting two-dimensional (2D) TMD/graphene vertical heterostructures are attractive for optoelectronic and energy applications. However, the effects of the microstructural heterogeneities of graphene grown by CVD on the growth of the TMD overlayers are relatively unknown. Here, we present a detailed investigation of how the stacking order and twist angle of CVD graphene influence the nucleation of WSe₂ triangular crystals. Through the combination of experiments and theory, we correlate the presence of interlayer dislocations in bilayer graphene with how WSe₂ nucleates, in agreement with the observation of a higher nucleation density of WSe₂ on top of Bernal-stacked bilayer graphene versus twisted bilayer graphene. Scanning/transmission electron microscopy (S/TEM) data show that interlayer dislocations are present only in Bernal-stacked bilayer graphene but not in twisted bilayer graphene. Atomistic ReaxFF reactive force field molecular dynamics simulations reveal that strain relaxation promotes the formation of these interlayer dislocations with localized buckling in Bernal-stacked bilayer graphene, whereas the strain becomes distributed in twisted bilayer graphene. Furthermore, these localized buckles in graphene are predicted to serve as thermodynamically favorable sites for binding WSe_x molecules, leading to the higher nucleation density of WSe₂ on Bernal-stacked graphene. Overall, this study explores synthesis-structure correlations in the WSe₂/graphene vertical heterostructure system toward the site-selective synthesis of TMDs by controlling the structural attributes of the graphene substrate.

Energy transfer and charge transfer between semiconducting nanocrystals and transition metal dichalcogenide monolayers

Nowadays, as a result of the emergence of low-dimensional hybrid structures, the scientific community is interested in their interfacial carrier dynamics, including charge transfer and energy transfer. By combining the potential of transition metal dichalcogenides (TMDs) and nanocrystals (NCs) with low-dimensional extension, hybrid structures of semiconducting nanoscale matter can lead to fascinating new technological scenarios. Their characteristics make them intriguing candidates for electronic and optoelectronic devices, like transistors or photodetectors, bringing with them challenges but also opportunities. Here, we will review recent research on the combined TMD/NC hybrid system with an emphasis on two major interaction mechanisms: energy transfer and charge transfer. With a focus on the quantum well nature in these hybrid semiconductors, we will briefly highlight state-of-the-art protocols for their structure formation and discuss the interaction mechanisms of energy versus charge transfer, before concluding with a perspective section that highlights novel types of interactions between NCs and TMDs.

Impact of synthesis temperature and precursor ratio on the crystal quality of MOCVD WSe2monolayers

Structural defects in transition metal dichalcogenide (TMDC) monolayers (ML) play a significant role in determining their (opto)electronic properties, triggering numerous efforts to control defect densities during material growth or by post-growth treatments. Various types of TMDC have been successfully deposited by MOCVD (metal-organic chemical vapor deposition), which is a wafer-scale deposition technique with excellent uniformity and controllability. However, so far there are no findings on the extent to which the incorporation of defects can be controlled by growth parameters during MOCVD processes of TMDC. In this work, we investigate the effect of growth temperature and precursor ratio during MOCVD of tungsten diselenide (WSe₂) on the growth of ML domains and their impact on the density of defects. The aim is to find parameter windows that enable the deposition of WSe₂ML with high crystal quality, i.e. a low density of defects. Our findings confirm that the growth temperature has a large influence on the crystal quality of TMDC, significantly stronger than found for the W to Se precursor ratio. Raising the growth temperatures in the range of 688 °C to 791 °C leads to an increase of the number of defects, dominating photoluminescence (PL) at low temperatures (5.6 K). In contrast, an increase of the molar precursor ratio (DiPSe/WCO) from 1000 up to 100 000 leads to less defect-related PL at low temperatures.

Catalytic Activity of Defect-Engineered Transition Me tal Dichalcogenides Mapped with Atomic-Scale Precision by Electrochemical Scanning Tunneling Microscopy

Unraveling structure-activity relationships is a key objective of catalysis. Unfortunately, the intrinsic complexity and structural heterogeneity of materials stand in the way of this goal, mainly because the activity measurements are area-averaged and therefore contain information coming from different surface sites. This limitation can be surpassed by the analysis of the noise in the current of electrochemical scanning tunneling microscopy (EC-STM). Herein, we apply this strategy to investigate the catalytic activity toward the hydrogen evolution reaction of monolayer films of MoSe₂. Thanks to atomically resolved potentiodynamic experiments, we can evaluate individually the catalytic activity of the MoSe₂ basal plane, selenium vacancies, and different point defects produced by the intersections of metallic twin boundaries. The activity trend deduced by EC-STM is independently confirmed by density functional theory calculations, which also indicate that, on the metallic twin boundary crossings, the hydrogen adsorption energy is almost thermoneutral. The micro- and macroscopic measurements are combined to extract the turnover frequency of different sites, obtaining for the most active ones a value of 30 s^-1 at -136 mV vs RHE.

Plasmon-exciton couplings in the MoS2/AuNP plasmonic hybrid structure

The understanding and engineering of the plasmon-exciton coupling are necessary to control the innovative optoelectronic device platform. In this study, we investigated the intertwined mechanism of each plasmon-exciton couplings in monolayer molybdenum disulfide (MoS₂) and plasmonic hybrid structure. The results of absorption, simulation, electrostatics, and emission spectra show that interaction between photoexcited carrier and exciton modes are successfully coupled by energy transfer and exciton recombination processes. Especially, neutral exciton, trion, and biexciton can be selectively enhanced by designing the plasmonic hybrid platform. All of these results imply that there is another degree of freedom to control the individual enhancement of each exciton mode in the development of nano optoelectronic devices.

Exciton tuning in monolayer WSe2 via substrate induced electron doping

We report large exciton tuning in WSe2 monolayers via substrate induced non-degenerate doping. We observe a redshift of ∼62 meV for the A exciton together with a 1-2 orders of magnitude photoluminescence (PL) quenching when the monolayer WSe2 is brought in contact with highly oriented pyrolytic graphite (HOPG) compared to dielectric substrates such as hBN and SiO2. As the evidence of doping from HOPG to WSe2, a drastic increase of the intensity ratio of trions to neutral excitons was observed. Using a systematic PL and Kelvin probe force microscopy (KPFM) investigation on WSe2/HOPG, WSe2/hBN, and WSe2/graphene, we conclude that this unique excitonic behavior is induced by electron doping from the substrate. Our results propose a simple yet efficient way for exciton tuning in monolayer WSe2, which plays a central role in the fundamental understanding and further device development.

Plasmon-Induced Enhanced Light Emission and Ultrafast Carrier Dynamics in a Tunable Molybdenum Disulfide-Gallium Nitride Heterostructure

The effect of localized plasmon on the photoemission and absorption in hybrid molybdenum disulfide-Gallium nitride (MoS₂-GaN) heterostructure has been studied. Localized plasmon induced by platinum nanoparticles was resonantly coupled to the bandedge states of GaN to enhance the UV emission from the hybrid semiconductor system. The presence of the platinum nanoparticles also increases the effective absorption and the transient gain of the excitonic absorption in MoS₂. Localized plasmons were also resonantly coupled to the defect states of GaN and the exciton states using gold nanoparticles. The transfer of hot carriers from Au plasmons to the conduction band of MoS₂ and the trapping of excited carriers in MoS₂ within GaN defects results in transient plasmon-induced transparency at ~1.28 ps. Selective optical excitation of the specific resonances in the presence of the localized plasmons can be used to tune the absorption or emission properties of this layered 2D-3D semiconductor material system.

Optical Response of CVD-Grown ML-WS2 Flakes on an Ultra-Dense Au NP Plasmonic Array

The combination of metallic nanostructures with two-dimensional transition metal dichalcogenides is an efficient way to make the optical properties of the latter more appealing for opto-electronic applications. In this work, we investigate the optical properties of monolayer WS2 flakes grown by chemical vapour deposition and transferred onto a densely-packed array of plasmonic Au nanoparticles (NPs). The optical response was measured as a function of the thickness of a dielectric spacer intercalated between the two materials and of the system temperature, in the 75–350 K range. We show that a weak interaction is established between WS2 and Au NPs, leading to temperature- and spacer-thickness-dependent coupling between the localized surface plasmon resonance of Au NPs and the WS2 exciton. We suggest that the closely-packed morphology of the plasmonic array promotes a high confinement of the electromagnetic field in regions inaccessible by the WS2 deposited on top. This allows the achievement of direct contact between WS2 and Au while preserving a strong connotation of the properties of the two materials also in the hybrid system.

Enhanced light-matter interaction in two-dimensional transition metal dichalcogenides

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials, such as MoS₂, WS₂, MoSe₂, and WSe₂, have received extensive attention in the past decade due to their extraordinary electronic, optical and thermal properties. They evolve from indirect bandgap semiconductors to direct bandgap semiconductors while their layer number is reduced from a few layers to a monolayer limit. Consequently, there is strong photoluminescence in a monolayer (1L) TMDC due to the large quantum yield. Moreover, such monolayer semiconductors have two other exciting properties: large binding energy of excitons and valley polarization. These properties make them become ideal materials for various electronic, photonic and optoelectronic devices. However, their performance is limited by the relatively weak light-matter interactions due to their atomically thin form factor. Resonant nanophotonic structures provide a viable way to address this issue and enhance light-matter interactions in 2D TMDCs. Here, we provide an overview of this research area, showcasing relevant applications, including exotic light emission, absorption and scattering features. We start by overviewing the concept of excitons in 1L-TMDC and the fundamental theory of cavity-enhanced emission, followed by a discussion on the recent progress of enhanced light emission, strong coupling and valleytronics. The atomically thin nature of 1L-TMDC enables a broad range of ways to tune its electric and optical properties. Thus, we continue by reviewing advances in TMDC-based tunable photonic devices. Next, we survey the recent progress in enhanced light absorption over narrow and broad bandwidths using 1L or few-layer TMDCs, and their applications for photovoltaics and photodetectors. We also review recent efforts of engineering light scattering, e.g., inducing Fano resonances, wavefront engineering in 1L or few-layer TMDCs by either integrating resonant structures, such as plasmonic/Mie resonant metasurfaces, or directly patterning monolayer/few layers TMDCs. We then overview the intriguing physical properties of different van der Waals heterostructures, and their applications in optoelectronic and photonic devices. Finally, we draw our opinion on potential opportunities and challenges in this rapidly developing field of research.

Surface plasmon enhancement in the different spatial distributions of nanowire and two-dimensional material

Surface plasmon (SP) nanostructures have been widely researched to improve the low light absorption of two-dimensional transition metal dichalcogenides (TMDCs). However, the impact of the different coupling forms of them,...

Water dissociation and association on mirror twin boundaries in two-dimensional MoSe2: insights from density functional theory calculations

The adsorption and dissociation of water molecules on two-dimensional transition metal dichalcogenides (TMDs) is expected to be dominated by point defects, such as vacancies, and edges. At the same time, the role of grain boundaries, and particularly, mirror twinboundaries (MTBs), whose concentration in TMDs can be quite high, is not fully understood. Using density functional theory calculations, we investigate the interaction of water, hydroxyl groups, as well as oxygen and hydrogen molecules with MoSe2 monolayers when MTBs of various types are present. We show that the adsorption of all species on MTBs is energetically favorable as compared to that on the basal plane of pristine MoSe2, but the interaction with Se vacancies is stronger. We further assess the energetics of various surface chemical reactions involving oxygen and hydrogen atoms. Our results indicate that water dissociation on the basal plane should be dominated by vacancies even when MTBs are present, but they facilitate water clustering through hydroxyl groups at MTBs, which can anchor water molecules and give rise to the decoration of MTBs with water clusters. Also, the presence of MTBs affects oxygen reduction reaction (ORR) on the MoSe2 monolayer. Unlike Se vacancies which inhibit ORR due to a high overpotential, it is found that the ORR process on MTBs is more efficient, indicating their important role in the catalytic activity of MoSe2 monolayer and likely other TMDs.

High-Yield Exfoliation of Monolayer 1T'-MoTe2 as Saturable Absorber for Ultrafast Photonics

Liquid-phase exfoliation can be developed for the large-scale production of two-dimensional materials for photonic applications. Although atomically thin 2D transition metal dichalcogenides (TMDs) show enhanced nonlinear optical properties or photoluminescence quantum yield relative to the bulk phase, these properties are weak in the absolute sense due to the ultrashort optical path, and they are also sensitive to layer-dependent symmetry properties. Another practical issue is that the chemical stability of some TMDs (e.g., Weyl semimetals) decreases dramatically as the thickness scales down to monolayer, precluding application as optical components in air. To address these issues, a way of exfoliating TMDs that ensures instantaneous passivation needs to be developed. Here, we employed a polymer-assisted electrochemical exfoliation strategy to synthesize PVP-passivated TMDs monolayers that could be spin coated and restacked into organic-inorganic superlattices with well-defined X-ray diffraction patterns. The segregation of restacked TMDs (e.g., MoS₂) by PVP allows the inversion asymmetry of individual layers to be maintained in these superlattices, which allows second harmonic generation and photoluminescence to be linearly scaled with thickness. PVP-passivated monolayer 1T'-MoTe₂ saturable absorber fabricated from these flakes exhibits fast response and recovery time (<150 fs) and pulse stability. Continuous-wave mode-locking based on 1T'-MoTe₂ saturable absorber in a fiber ring laser cavity has been realized, attaining a fundamental repetition rate of 3.15 MHz and pulse duration as short as 867 fs at 1563 nm.

Purcell-enhanced photoluminescence of few-layer MoS2 transferred on gold nanostructure arrays with plasmonic resonance at the conduction band edge

Plasmonic coupling of metallic nanostructures with two-dimensional molybdenum disulfide (MoS2) atomic layers is an important topic because it provides a pathway to manipulate the optoelectronic properties and to overcome the limited optical cross-section of the materials. Plasmonic enhanced light-matter interaction of a MoS2 layer is known to be mainly governed by optical field enhancement and the Purcell effect, while the discrimination of the contribution from each mechanism to the plasmonic enhancement is challenging. Here, we investigate photoluminescence (PL) enhancement from few-layer MoS2 transferred on Au nanostructure arrays with controlled localized surface plasmon resonance (LSPR) spectral positions that were detuned from the excitation wavelengths. Two distinctive regimes in LSPR mode-dependent PL enhancement were revealed showing a maximum enhancement (∼40-fold) with zero detuning and a modest enhancement (∼10-fold) with the red-shift detuned LSPR from the excitation wavelength, which were attributed to LSPR-induced optical field enhancement and the Purcell effect, respectively. By applying the experimental parameters into the Purcell effect formalism, an effective mode volume of ∼0.016λ03 was estimated. Our work provides an insight into how to utilize few-layer MoS2 as a base material for optoelectronics by harnessing Purcell-enhanced optical responsivity.

Room-Temperature Exciton-Based Optoelectronic Switch

Excitons, bound pairs of electrons and holes, could act as an intermediary between electronic signal processing and optical transmission, thus speeding up the interconnection of photoelectric communication. However, up to date, exciton-based logic devices such as switches that work at room temperature are still lacking. This work presents a prototype of a room-temperature optoelectronic switch based on excitons in WSe₂ monolayer. The emission intensity of WSe₂ stacked on Au and SiO₂ substrates exhibits completely opposite behaviors upon applying gate voltages. Such observation can be ascribed to different doping behaviors of WSe₂ caused by charge-transfer and chemical-doping effect at WSe₂ /Au and WSe₂ /SiO₂ interfaces, respectively, together with the charge-drift effect. These interesting features can be utilized for optoelectronic switching, confirmed by the cyclic PL switching test for a long time exceeding 4000 s. This study offers a universal and reliable approach for the fabrication of exciton-based optoelectronic switches, which would be essential in integrated nanophotonics.

Highly efficient and controllable photoluminescence emission on a suspended MoS2-based plasmonic grating

Being a new class of materials, transition metal dichalcogenides are paving the way for applications in atomically thin optoelectronics. However, the intrinsically weak light-matter interaction and the lack of manipulation ability has lead to poor light emission and tunable behavior. Here, we investigate the fluorescence characteristic of monolayer molybdenum disulfide on a metal narrow-slit grating, where a highly efficient, 471 times photoluminescence enhancement are realized, based on the hybrid surface plasmon polaritons resonances and the decreased influence of substrate. Moreover, the emitted intensity and polarization are controllable due to the polarization-dependent characteristic and anisotropy of grating. The manipulations of light-matter interactions in this special system provide a new insight into the fluorescent emission process and open a new avenue for high-performance low dimensional materials devices designs.

Depletion-Mediated Uniform Deposition of Nanorods with Patterned, Multiplexed Assembly

Device-scale, uniform, and controllable deposition of nanoparticles on various substrates is fundamentally important not only for the fabrication of thin-film devices but also for the large sample statistics of single-particle performances. However, it is challenging to obtain such predefined depositions using a simple and efficient method. Here, we present a novel strategy for obtaining the uniform and particle density/spacing-tunable deposition of nanorods on a linker-free substrate. The deposition is driven by the tailored particle-substrate depletion attraction owing to the size-matched design of the substrate roughness and the nanorod diameter. Both gold nanorods and upconversion nanorods were applied to demonstrate the generality of the method. The high particle density of more than 21 per μm² and correspondingly the small particle spacing of fewer than 0.3 μm were achieved on a scalable substrate template. On this basis, orientational ordering and pattern-selective deposition of nanorods were realized by controlling the liquid flow rate and employing the substrate with patterned roughness areas, respectively. With the roughness-directed density-tunable depositions of nanorods integrated onto a single platform, multiplexed gold nanorod assembly and programmable surface-enhanced Raman mapping were achieved, with a promising prospect in information encoding by using the Raman signals as the translation units. The thermal stability and related transition temperature of about 160 °C of gold nanorods were also revealed as an application of single-particle statistics. This practical method could be extended to wide ranges of potential applications in plasmonic coupling devices, cryptography, or single-particle performance statistics with the feature of the high-throughput, low-cost, and scalable fabrication.

Dielectric Nanowire Hybrids for Plasmon-Enhanced Light-Matter Interaction in 2D Semiconductors

Monolayer transition metal dichalcogenides (TMDs) with a direct band gap are suitable for various optoelectronic applications such as ultrathin light emitters and absorbers. However, their weak light absorption caused by the atomically thin layer hinders more versatile applications for high optical gains. Although plasmonic hybridization with metal nanostructures significantly enhances light-matter interactions, the corrosion, instability of the metal nanostructures, and the undesired effects of direct metal-semiconductor contact act as obstacles to its practical application. Herein, we propose a dielectric nanostructure for plasmon-enhanced light-matter interaction of TMDs. TiO₂ nanowires (NWs), as an example, are hybridized with a MoS₂ monolayer on various substrates. The structure is implemented by placing a monolayer MoS₂ between a TiO₂ NW for a photonic scattering effect and metallic substrates with a spacer for the plasmonic Purcell effect. Here, the thin dielectric spacer is aimed at minimizing emission quenching from direct metal contact, while maximizing optical field localization in ultrathin MoS₂ near the TiO₂ NW. An effective emission enhancement factor of ∼22 is attained for MoS₂ near the NW of the hybrid structure compared to the one without NWs. Our work is expected to facilitate a hybridized platform based on 2D semiconductors for high-performance and robust optoelectronics via engineering dielectric nanostructures with plasmonic materials.

Light-Controlled Near-Field Energy Transfer in Plasmonic Metasurface Coupled MoS2 Monolayer

The energy transfer from plasmonic nanostructures to semiconductors has been extensively studied to enhance light-harvesting and tailor light-matter interactions. In this study, the efficient energy transfer from an Au metasurface to monolayered MoS2 within a near-field coupling regime is reported. The metasurface is designed and fabricated to demonstrate strong photoluminescence (PL) and cathodoluminescence (CL) emission spectra. In the coupled heterostructure of MoS2 with a metasurface, both the Raman shift and absorption spectral intensities of monolayered MoS2 are affected. The spectral profile and PL peak position can be tailored owing to the energy transfer between plasmonic nanostructures and semiconductors. This is confirmed by ultrafast lifetime measurement. A theoretical model of two coupled oscillators is proposed, where the expanded general solutions (EGS) of such a model result in a series of eigenvalues that correspond to the renormalization of energy levels in modulated MoS2. The model can predict the peak shift up to tens of nanometers in hybrid structures and hence provides an alternative method to describe energy transfer between metallic structures and two-dimensional (2D) semiconductors. A viable approach for studying light-matter interactions in 2D semiconductors via near-field energy transfer is presented, which may stimulate the applications of functional nanophotonic devices.

Gate-tunable trion binding energy in monolayer MoS2 with plasmonic superlattice

Two-dimensional transition metal dichalcogenides exhibit promising potential and attract the attention of the world in the application of optoelectronic devices owing to their distinctive physical and chemical properties. The real-time control of light-matter interactions in semiconductor devices through an external optical resonant cavity is crucial for designing next-generation optoelectronic devices. Here, we report the spectroscopic identification of trion binding energy in monolayer MoS₂ field-effect transistors with plasmonic nanoresonators. In consequence, the binding energy could be regulated dynamically through an external electric field. In addition, after increasing the carrier injection, the evidence of the enhanced trion binding energy can also be observed, which can be utilized for researching magneto-plasmons. The ability to dynamically control the optical properties by electrostatic doping opens a platform for designing next-generation optoelectronic and valleytronic applications in two-dimensional crystals with accurate and precise tailored responses.

Two Switchable Plasmonically Induced Transparency Effects in a System with Distinct Graphene Resonators

General plasmonic systems to realize plasmonically induced transparency (PIT) effect only exist one single PIT mainly because they only allow one single coupling pathway. In this study, we propose a distinct graphene resonator-based system, which is composed of graphene nanoribbons (GNRs) coupled with dielectric grating-loaded graphene layer resonators, to achieve two switchable PIT effects. By designing crossed directions of the resonators, the proposed system exists two different PIT effects characterized by different resonant positions and linewidths. These two PIT effects result from two separate and polarization-selective coupling pathways, allowing us to switch the PIT from one to the other by simply changing the polarization direction. Parametric studies are carried to demonstrate the coupling effects whereas the two-particle model is applied to explain the physical mechanism, finding excellent agreements between the numerical and theoretical results. Our proposal can be used to design switchable PIT-based plasmonic devices, such as tunable dual-band sensors and perfect absorbers.

Near-Unity Light Absorption in a Monolayer WS2 Van der Waals Heterostructure Cavity

Excitons in monolayer transition-metal-dichalcogenides (TMDs) dominate their optical response and exhibit strong light-matter interactions with lifetime-limited emission. While various approaches have been applied to enhance light-exciton interactions in TMDs, the achieved strength have been far below unity, and a complete picture of its underlying physical mechanisms and fundamental limits has not been provided. Here, we introduce a TMD-based van der Waals heterostructure cavity that provides near-unity excitonic absorption, and emission of excitonic complexes that are observed at ultralow excitation powers. Our results are in full agreement with a quantum theoretical framework introduced to describe the light-exciton-cavity interaction. We find that the subtle interplay between the radiative, nonradiative and dephasing decay rates plays a crucial role, and unveil a universal absorption law for excitons in 2D systems. This enhanced light-exciton interaction provides a platform for studying excitonic phase-transitions and quantum nonlinearities and enables new possibilities for 2D semiconductor-based optoelectronic devices.

Hybridizing Plasmonic Materials with 2D-Transition Metal Dichalcogenides toward Functional Applications

Recently, 2D transition metal dichalcogenides (TMDs) have become intriguing materials in the versatile field of photonics and optoelectronics because of their strong light-matter interaction that stems from the atomic layer thickness, broadband optical response, controllable optoelectronic properties, and high nonlinearity, as well as compatibility. Nevertheless, the low optical cross-section of 2D-TMDs inhibits the light-matter interaction, resulting in lower quantum yield. Therefore, hybridizing the 2D-TMDs with plasmonic nanomaterials has become one of the promising strategies to boost the optical absorption of thin 2D-TMDs. The appeal of plasmonics is based on their capability to localize and enhance the electromagnetic field and increase the optical path length of light by scattering and injecting hot electrons to TMDs. In this regard, recent achievements with respect to hybridization of the plasmonic effect in 2D-TMDs systems and its augmented optical and optoelectronic properties are reviewed. The phenomenon of plasmon-enhanced interaction in 2D-TMDs is briefly described and state-of-the-art hybrid device applications are comprehensively discussed. Finally, an outlook on future applications of these hybrid devices is provided.

Plasmonically induced transparency in in-plane isotropic and anisotropic 2D materials

General two-dimensional (2D) material-based systems that achieve plasmonically induced transparency (PIT) are limited to isotropic graphene only through unidirectional bright-dark mode interaction. Moreover, it is challenging to extend these devices to anisotropic 2D films. In this study, we exploit surface plasmons excited at two crossed grating layers, which can be formed either by dielectric gratings or by the 2D sheet itself, to achieve dynamically tunable PIT in both isotropic and anisotropic 2D materials. Here, each grating simultaneously acts as both bright and dark modes. By taking isotropic graphene and anisotropic black phosphorus (BP) as proofs of concept, we reveal that this PIT can result from either unidirectional bright-dark or bidirectional bright-bright and bright-dark mode hybridized couplings when the incident light is parallelly/perpendicularly or obliquely polarized to the gratings, respectively. Identical grating parameters in isotropic (crossed lattice directions in anisotropic) layers produce polarization-independent single-window PIT, whereas different grating parameters (coincident lattice directions) yield polarization-sensitive double-window PIT. The proposed technique is examined by a two-particle model, showing excellent agreement between the theoretical and numerical results. This study provides insight into the physical mechanisms of PIT and advances the applicability and versatility of 2D material-based PIT devices.

Sensitive Terahertz Detection and Imaging Driven by the Photothermoelectric Effect in Ultrashort-Channel Black Phosphorus Devices

Terahertz (THz) photon detection is of particular appealing for myriad applications, but it still lags behind efficient manipulation with electronics and photonics due to the lack of a suitable principle satisfying both high sensitivity and fast response at room temperature. Here, a new strategy is proposed to overcome these limitations by exploring the photothermoelectric (PTE) effect in an ultrashort (down to 30 nm) channel with black phosphorus as a photoactive material. The preferential flow of hot carriers is enabled by the asymmetric Cr/Au and Ti/Au metallization with the titled-angle evaporation technique. Most intriguingly, orders of magnitude field-enhancement beyond the skin-depth limit and photon absorption across a broadband frequency can be achieved. The PTE detector has excellent sensitivity of 297 V W^-1, noise equivalent power less than 58 pW/Hz^0.5, and response time below 0.8 ms, which is superior to other thermal-based detectors at room temperature. A rigorous comparison with existing THz detectors, together with verification by further optical-pumping and imaging experiments, substantiates the importance of the localized field effect in the skin-depth limit. The results allow solid understanding on the role of PTE effect played in the THz photoresponse, opening up new opportunities for developing highly sensitive THz detectors for addressing targeted applications.

Charge Transfer-Mediated Blue Luminescence in Plasmonic Ag-Cu2O Quantum Nanoheterostructures

Metal-semiconductor hybrid nanoheterostructures have the possibility to exhibit new synergic properties other than the combination of properties from discrete components due to the interaction of metal and semiconductor components at the interfaces. Here, we have synthesized Ag-Cu₂O eyeball-shaped quantum nanoheterostructures with diameter ranging between 8 and 12 nm using a single-step low-cost solvothermal process. It is observed that the presence of a minimum 3% of Ag is required for the formation of Ag-Cu₂O quantum nanoheterostructures. The formation of nanoheterostructures has introduced new synergic properties like intense blue luminescence and surface-enhanced Raman scattering due to the interactions between Ag and Cu₂O atoms at the interfaces. The significant presence of charge transfer through the interfaces is identified from the peak shift of Raman modes. The increase in the electron density at the metal surface due to the charge transfer and the recombination of these electrons with sp- or d-band holes of Ag could be the effective mechanism of the observed blue luminescence. The blue luminescence of Ag-Cu₂O quantum nanoheterostructures together with its low band gap value (≈2.3 eV) is believed to have important applications in optoelectronic devices.

Polarized resonant emission of monolayer WS2 coupled with plasmonic sawtooth nanoslit array

Transition metal dichalcogenide (TMDC) monolayers have enabled important applications in light emitting devices and integrated nanophotonics because of the direct bandgap, spin-valley locking and highly tunable excitonic properties. Nevertheless, the photoluminescence polarization is almost random at room temperature due to the valley decoherence. Here, we show the room temperature control of the polarization states of the excitonic emission by integrating WS₂ monolayers with a delicately designed metasurface, i.e. a silver sawtooth nanoslit array. The random polarization is transformed to linear when WS₂ excitons couple with the anisotropic resonant transmission modes that arise from the surface plasmon resonance in the metallic nanostructure. The coupling is found to enhance the valley coherence that contributes to ~30% of the total linear dichroism. Further modulating the transmission modes by optimizing metasurfaces, the total linear dichroism of the plasmon-exciton hybrid system can approach 80%, which prompts the development of photonic devices based on TMDCs.

Here the authors show that WS₂ coupled with a plasmonic sawtooth nanoslit array is an efficient exciton-plasmon hybrid system which enables polarization modulation of the excitonic emission at the nanoscale up to 80% and observation of valley coherence at room temperature.

Seeded-growth of WS2 atomic layers: the effect on chemical and optical properties

The growth of high quality materials from well-defined seeds is a well-established and beneficial procedure, as it enables the control of the crystal orientation, domain size, phase and chemical composition of nanocrystals, thin films and 3D crystals. The seeded-growth approach for 2D transition metal dichalcogenides (TMDs) is investigated, envisaging that seeds have a great impact on the chemical composition of the grown layers and thus, on their chemical and optical properties. The controlled nucleation and narrow domain size distribution of single crystalline WS2 atomic layers are demonstrated by employing the seeded-growth approach. The growth of single layer WS2 domains from well-defined Au seeds leads to nanoparticle (NP) decoration over the domain in a very peculiar way that might be related to the growth mechanism of such atomic-layers. The segmentation in Raman enhancement and photoluminescence maps of exciton and trion emissions well correlate with the presence of Au NPs observed in electron microscopy and chemical maps obtained by time-of-flight secondary ion mass spectroscopic imaging. This work emphasizes the importance of the seed material and its effect on the grown 2D material and may lead to novel methodologies for controlled growth, doping and the formation of hybrid materials to be used in catalysis, sensors and optoelectronics.

Highly Transparent and Surface-Plasmon-Enhanced Visible-Photodetector Based on Zinc Oxide Thin-Film Transistors with Heterojunction Structure

Highly transparent zinc oxide (ZnO)-based thin-film transistors (TFTs) with gold nanoparticles (AuNPs) capable of detecting visible light were fabricated through spray pyrolysis on a fluorine-doped tin oxide substrate. The spray-deposited channel layer of ZnO had a thickness of approximately 15 nm, and the thickness exhibited a linear increase with an increasing number of sprays. Furthermore, the ZnO thin-film exhibited a markedly smoother channel layer with a significantly lower surface roughness of 1.84 nm when the substrate was 20 cm from the spray nozzle compared with when it was 10 cm away. Finally, a ZnO and Au-NP heterojunction nanohybrid structure using plasmonic energy detection as an electrical signal, constitutes an ideal combination for a visible-light photodetector. The ZnO-based TFTs convert localized surface plasmon energy into an electrical signal, thereby extending the wide band-gap of materials used for photodetectors to achieve visible-light wavelength detection. The photo-transistors demonstrate an elevated on-current with an increase of the AuNP density in the concentration of 1.26, 12.6, and 126 pM and reach values of 3.75, 5.18, and 9.79 × 10⁻⁷ A with applied gate and drain voltages. Moreover, the threshold voltage (Vth) also drifts to negative values as the AuNP density increases.

Molybdenum Disulfide Nanosheet/Quantum Dot Dynamic Memristive Structure Driven by Photoinduced Phase Transition

MoS₂ 2D nanosheets (NS) with intercalated 0D quantum dots (QDs) represent promising structures for creating low-dimensional (LD) resistive memory devices. Nonvolatile memristors based 2D materials demonstrate low power consumption and ultrahigh density. Here, the observation of a photoinduced phase transition in the 2D NS/0D QDs MoS₂ structure providing dynamic resistive memory is reported. The resistive switching of the MoS₂ NS/QD structure is observed in an electric field and can be controlled through local QD excitations. Photoexcitation of the LD structure at different laser power densities leads to a reversible MoS₂ 2H-1T phase transition and demonstrates the potential of the LD structure for implementing a new dynamic ultrafast photoresistive memory. The dynamic LD photomemristive structure is attractive for real-time pattern recognition and photoconfiguration of artificial neural networks in a wide spectral range of sensitivity provided by QDs.

Tunable Chemical Coupling in Two-Dimensional van der Waals Electrostatic Heterostructures

Heterostructures of two-dimensional (2D) atomic crystals provide fascinating molecular-scale design elements for emergent physical phenomena and functional materials, as integrating distinct monolayers into vertical heterostructures can afford coupling between disparate properties. However, the available examples have been limited to either van der Waals (vdW) or electrostatic (ES) heterostructures that are solely composed of noncharged and charged monolayers, respectively. Here, we propose a "vdW-ES heterostructure" chemical design in which charge-neutral and charged monolayer-building blocks with highly disparate chemical and physical properties are conjugated vertically through asymmetrically charged interfaces. We demonstrate vdW-ES heteroassembly of semiconducting MoS₂ and dielectric Ca₂Nb₃O₁₀^- (CNO) monolayers using an amphipathic molecular starch, resulting in the emergence of trion luminescence observed at the lowest energy among MoS₂-related materials, probably due to interfacial confinement effects given by vdW-ES dual interactions. In addition, interface engineering leads to tailored exciton of the vdW/ES heterostructures owing to the pronounced dielectric proximity effects, bringing an intriguing interlayer chemistry to modify 2D materials. Furthermore, the current approach was successfully extended to create a graphene/CNO heterostructure, which verifies the versatility of the preparative method.

All-silicon reconfigurable metasurfaces for multifunction and tunable performance at optical frequencies based on glide symmetry

Dielectric metasurfaces have opened promising possibilities to enable a versatile platform in the miniaturization of optical elements at visible and infrared frequencies. Due to high efficiency and compatibility with CMOS fabrication technology, silicon-based metasurfaces have a remarkable potential for a wide variety of optical devices. Adding tunability mechanisms to metasurfaces could be beneficial for their application in areas such as communications, imaging and sensing. In this paper, we propose an all-silicon reconfigurable metasurface based on the concept of glide symmetry. The reconfigurability is achieved by a phase modulation of the transmitted wave activated by a lateral displacement of the layers. The misalignment between the layers creates a new inner periodicity which leads to the formation of a metamolecule with a new sort of near-field interaction. The proposed approach is highly versatile for developing multifunctional and tunable metadevices at optical frequencies. As a proof of concept, in this paper, we design a bifunctional metadevice, as well as a tunable lens and a controllable beam deflector operating at 1.55 μm.

Ultrafast nonlinear absorption enhancement of monolayer MoS2 with plasmonic Au nanoantennas

In this work, we experimentally study the nonlinear absorption enhancement of saturable absorption and two-photon absorption on a hybrid structure comprising a monolayer MoS₂ and Au nanoantennas via femtosecond I-scan measurement. Specifically, a 13-fold increment in the linear absorption coefficient is attained at 1.85 eV, along with an 8-fold enhancement of the two-photon absorption coefficient at 1.65 eV, which is attributed to exciton-plasmon coupling resonance and plasmonic hot electron transfer. The exciton-plasmon coupling effect is characterized by stable photoluminescence experiments. Furthermore, the exciton recombination time is extracted from the pump-probe measurement, whose value in the hybrid structure is shortened from 18.5 ps (pure MoS₂) to 1.84 ps. Our findings facilitate a new perspective to modulate the nonlinear optical response and to promote the performance of nonlinear photonic devices.

Cu2O-Au nanowire field-effect phototransistor for hot carrier transfer enhanced photodetection

In metal-semiconductor hybrid nanostructures, metal absorbs incident photons and generates hot carriers. The hot carriers are injected into the adjacent semiconductor and subsequently contribute to photocurrent. This process increases the conversion efficiency of optoelectronic devices and provides a new path of photodetectors. In this work, we report an enhanced photodetector by hot holes transfer, which is based on Au nanoparticles decorated p-type Cu₂O nanowires. The photodetector achieves an enhanced photo-responsivity up to 0.314 A W^-1, a higher detectivity of 3.7 × 10¹⁰ Jones. The response time and external quantum efficiency of the Cu₂O-Au nanowires photodetector are 3.7 times faster and 18.2 times higher than that of the Cu₂O nanowires, respectively. The findings indicate that Cu₂O-Au nanowires would be a promising candidate in developing novel plasmonic hot carrier devices.

Ultrafast Carrier Dynamics of the Exciton and Trion in MoS2 Monolayers Followed by Dissociation Dynamics in Au@MoS2 2D Heterointerfaces

Many-body states like excitons, biexcitons, and trions play an important role in optoelectronic and photovoltaic applications in 2D materials. Herein, we studied carrier dynamics of excitons and trions in monolayer MoS₂ deposited on a SiO₂/Si substrate, before and after Au NP deposition, using femtosecond transient absorption spectroscopy. Luminescence measurements confirm the presence of both an exciton and trion in MoS₂, which are drastically quenched after deposition of Au NPs, indicating electron transfer from photoexcited MoS₂ to Au. Ultrafast study reveals that photogenerated free carriers form excitons with a time scale of ∼500 fs and eventually turn into trions within ∼1.2 ps. Dissociation of excitons and trions has been observed in the presence of Au, with time scales of ∼600 fs and ∼3.7 ps, respectively. Understanding the formation and dissociation dynamics of the exciton and trion in monolayer MoS₂ is going to help immensely to design and develop many new 2D devices.

Defect-Controlled Nucleation and Orientation of WSe2 on hBN: A Route to Single-Crystal Epitaxial Monolayers

A defect-controlled approach for the nucleation and epitaxial growth of WSe₂ on hBN is demonstrated. The WSe₂ domains exhibit a preferred orientation of over 95%, leading to a reduced density of inversion domain boundaries (IDBs) upon coalescence. First-principles calculations and experimental studies as a function of growth conditions and substrate pretreatment confirm that WSe₂ nucleation density and orientation are controlled by the hBN surface defect density rather than thermodynamic factors. Detailed transmission electron microscopy analysis provides support for the role of single-atom vacancies on the hBN surface that trap W atoms and break surface symmetry leading to a reduced formation energy for one orientation of WSe₂ domains. Through careful control of nucleation and extended lateral growth time, fully coalesced WSe₂ monolayer films on hBN were achieved. Low-temperature photoluminescence (PL) measurements and transport measurements of back-gated field-effect transistor devices fabricated on WSe₂/hBN films show improved optical and electrical properties compared to films grown on sapphire under similar conditions. Our results reveal an important nucleation mechanism for the epitaxial growth of van der Waals heterostructures and demonstrate hBN as a superior substrate for single-crystal transition-metal dichalcogenide (TMD) films, resulting in a reduced density of IDBs and improved properties. The results motivate further efforts focused on the development of single crystal hBN substrates and epilayers for synthesis of wafer-scale single crystal TMD films.

Plasmonically-powered hot carrier induced modulation of light emission in a two-dimensional GaAs semiconductor quantum well

A hot-electron-enabled route to controlling light with dissipative loss compensation in semiconductor quantum light emitters has been realized for tunable quantum optoelectronic devices via a two-species plasmon system. The dual species nano-plasmonic system is achieved by combining UV-plasmonic gallium metal nanoparticles (GaNPs) with visible-plasmonic gold metal nanoparticles (AuNPs) on a near-infrared two-dimensional GaAs/AlGaAs quantum well emitter. It has been demonstrated that while hot carrier-powered charge-transfer processes can result in semiconductor doping and increased optical absorption, photo-generated carrier density in the quantum well can also be modulated by off-resonant plasmonic interaction without thermal dissipation. Merging these essential emitter-friendly optical characteristics in the two-species plasmon system, we effectively modulate the frequency of the emitted light. The wavelength of the emitted light is tuned by the plasmonically powered hot electron process induced by the AuNPs with a 10-fold emission enhancement induced by the GaNPs. The additional plasmonic element provides functionality to achieving an active plasmonic light emitter that is otherwise far from reach with conventional single plasmonic material-based semiconductors.

Probing Evolution of Local Strain at MoS2-Metal Boundaries by Surface-Enhanced Raman Scattering

Strain usually exists in two-dimensional (2D) materials and devices, and its presence drastically modulates their properties. When 2D materials interface with noble metals, local strain and surface plasmon can couple at the metal-2D material boundaries, delivering a lot of intriguing phenomena. Current studies are mostly focused on the explanations of these strain-related phenomena based on a static point of view. Although strain can typically be relaxed in many environments, the time evolution of strain at metal-2D material interfaces remains largely unknown. In this work, we investigate the evolution of local strain at Ag-MoS₂ boundaries by surface-enhanced Raman scattering. With the split of MoS₂ Raman peaks as an indicator of local strain, it is found that the originally localized strain at Ag-MoS₂ boundaries evolves and relaxes with time into a delocalized strain in MoS₂ plane. The time to start the strain relaxation depends on the number of layers of MoS₂ flakes, suggesting that the relaxation may result from the mechanical instability of the interface between the topmost MoS₂ layer and the underlying materials. The relaxation occurs in a certain period of time, i.e., ∼70 days for 1L and ∼30 days for 3L. Accompanying the strain relaxation, surface sulfurization of Ag also occurs, a process that reduces the strength of locally enhanced electric field. Our results not only provide a deep understanding of strain evolution at metal-MoS₂ interfaces but also shed light on the optimization of MoS₂-based device fabrications.

Enhanced Light-Matter Interactions in Self-Assembled Plasmonic Nanoparticles on 2D Semiconductors

Two-dimensional (2D) transition-metal dichalcogenide (TMD) monolayers of versatile material library are spotlighted for numerous unexplored research fields. While monolayer TMDs exhibit an efficient excitonic emission, the weak light absorption arising from their low dimensionality limits potential applications. To enhance the light-matter interactions of TMDs, while various plasmonic hybridization methods have been intensively studied, controlling plasmonic nanostructures via self-assembly processes remains challenging. Herein, strong light-matter interactions are reported in plasmonic Ag nanoparticles (NPs) hybridized on TMDs via an aging-based self-assembly process at room temperature. This hybridization is implemented by transferring MoS₂ monolayers grown via chemical vapor deposition onto thin-spacer-covered Ag films. After a few weeks of aging in a vacuum desiccator, the Ag atoms in the heterolayered film diffuse to the MoS₂ layers through a SiO₂ spacer and self-cluster onto MoS₂ point defects, resulting in the formation of Ag-NPs with an estimated diameter of ≈50 nm. The photoluminescence intensities for the Ag-NP/MoS₂ hybrids are enhanced up to 35-fold compared with bare MoS₂ owing to the local field enhancement near the plasmonic Ag-NPs. The localized surface plasmon resonances modes of this hybrid are systematically investigated via numerical simulations and dark-field scattering microscopy.

Near Infrared Random Lasing in Multilayer MoS2

We demonstrated room temperature near infrared (NIR) region random lasing (RL) (800-950 nm), with a threshold of nearly 500 μW, in ∼200 nm thick MoS2/Au nanoparticles (NPs)/ZnO heterostructures using photoluminescence spectroscopy. The RL in the above system arises mainly due to the following three reasons: (1) enhanced multiple scattering because of Au/ZnO disordered structure, (2) exciton-plasmon coupling because of Au NPs, and (3) enhanced charge transfer from ZnO to thick MoS2 flakes. RL has recently attracted tremendous interest because of its wide applications in the field of telecommunication, spectroscopy, and specifically in biomedical tissue imaging. This work provides new dimensions toward realization of low power on-chip NIR random lasers made up of biocompatible materials.

Revealing the microscopic CVD growth mechanism of MoSe2 and the role of hydrogen gas during the growth procedure

Understanding the microscopic mechanisms for the nucleation and growth of two-dimensional molybdenum diselenide (2D MoSe₂) via chemical vapor deposition (CVD) is crucial towards the precisely controlled growth of the 2D material. In this work, we employed a joint use of transmission electron microscopy and CVD, in which the 2D MoSe₂ were directly grown on a graphene membrane based on grids, that enables the microstructural characterization of as-grown MoSe₂ flakes. We further explore the role of hydrogen gas and find: in an argon ambient, the primary products are few-layer MoSe₂ flakes, along with MoO _x nanoparticles; while with the introduction of H₂, single-layer MoSe₂ became the dominant product during the CVD growth. Quantitative analysis of the effects of H₂ flow rate on the flake sizes, and areal coverage was also given. Nevertheless, we further illuminated the evolution of shape morphology and edge structures of single-layer MoSe₂, and proposed the associated growth routes during a typical CVD process.

Ultrafast, asymmetric charge transfer and slow charge recombination in porphyrin/CNT composites demonstrated by time-domain atomistic simulation

The versatile photochemical properties of porphyrin molecules make them excellent candidates for solar energy applications. Carbon nanotubes (CNTs) exhibit superior charge conductivity and have been combined with porphyrins to achieve efficient and ultrafast charge separation. Experiments show that the charge separated state lives less than 10 ps, which is too short for applications. Using real-time time-dependent tight binding density functional theory (DFTB) combined with non-adiabatic molecular dynamics (NAMD), we model photo-induced charge separation and recombination in two porphyrin/CNT composites. Having achieved excellent agreement with the experiment for the electron transfer from the porphyrins to the CNT, we demonstrate that hole transfer can be achieved upon CNT excitation, although in a less efficient way. By exciting the CNT one can extend light harvesting into lower energies of the solar spectrum and increase solar light conversion efficiency. We also show that the charge separated state can live over 1 ns. The two orders of magnitude difference from the experimental lifetime could arise due to the presence of defects or metallic tubes in the samples. The charge separated state is long-lived because the non-adiabatic electron-phonon coupling is very small, less than 1 meV, and the quantum coherence is short, 15-20 fs. The excited states in the isolated porphyrins and CNT live around 100 ps, in agreement with experiments as well. The porphyrin/CNT interaction occurs through the π-electron systems of the two species. The non-radiative relaxation is promoted by both high and low frequency phonons, with higher frequency phonons playing more important roles in electron relaxation than in hole relaxation. Low frequency phonons contribute significantly to the decay of the charge separated state, because they modulate the relative positions of the porphyrins and the CNT. The time-domain atomistic simulations provide a detailed understanding of the charge separation and recombination mechanisms, and generate valuable guidelines for the optimization of photovoltaic efficiency in modern nanoscale materials.