Indirect Band Gap Emission by Hot Electron Injection in Metal/MoS₂ and Metal/WSe₂ Heterojunctions

Optical spectroscopic detection of Schottky barrier height at a two-dimensional transition-metal dichalcogenide/metal interface

Atomically thin two-dimensional transition-metal dichalcogenides (2D-TMDs) have emerged as semiconductors for next-generation nanoelectronics. As 2D-TMD-based devices typically utilize metals as the contacts, it is crucial to understand the properties of the 2D-TMD/metal interface, including the characteristics of the Schottky barriers formed at the semiconductor-metal junction. Conventional methods for investigating the Schottky barrier height (SBH) at these interfaces predominantly rely on contact-based electrical measurements with complex gating structures. In this study, we introduce an all-optical approach for non-contact measurement of the SBH, utilizing high-quality WS2/Au heterostructures as a model system. Our approach employs a below-bandgap pump to excite hot carriers from the gold into WS2 with varying thicknesses. By monitoring the resultant carrier density changes within the WS2 layers with a broadband probe, we traced the dynamics and magnitude of charge transfer across the interface. A systematic sweep of the pump wavelength enables us to determine the SBH values and unveil an inverse relationship between the SBH and the thickness of the WS2 layers. First-principles calculations reveal the correlation between the probability of injection and the density of states near the conduction band minimum of WS2. The versatile optical methodology for probing TMD/metal interfaces can shed light on the intricate charge transfer characteristics within various 2D heterostructures, facilitating the development of more efficient and scalable nano-electronic and optoelectronic technologies.

Plasma Processing and Treatment of 2D Transition Metal Dichalcogenides: Tuning Properties and Defect Engineering

Two-dimensional transition metal dichalcogenides (TMDs) offer fascinating opportunities for fundamental nanoscale science and various technological applications. They are a promising platform for next generation optoelectronics and energy harvesting devices due to their exceptional characteristics at the nanoscale, such as tunable bandgap and strong light-matter interactions. The performance of TMD-based devices is mainly governed by the structure, composition, size, defects, and the state of their interfaces. Many properties of TMDs are influenced by the method of synthesis so numerous studies have focused on processing high-quality TMDs with controlled physicochemical properties. Plasma-based methods are cost-effective, well controllable, and scalable techniques that have recently attracted researchers' interest in the synthesis and modification of 2D TMDs. TMDs' reactivity toward plasma offers numerous opportunities to modify the surface of TMDs, including functionalization, defect engineering, doping, oxidation, phase engineering, etching, healing, morphological changes, and altering the surface energy. Here we comprehensively review all roles of plasma in the realm of TMDs. The fundamental science behind plasma processing and modification of TMDs and their applications in different fields are presented and discussed. Future perspectives and challenges are highlighted to demonstrate the prominence of TMDs and the importance of surface engineering in next-generation optoelectronic applications.

Piezo-Activated Atomic-Thin Molybdenum Disulfide/MXene Nanoenzyme for Integrated and Efficient Tumor Therapy via Ultrasound-Triggered Schottky Electric Field

Monolayer molybdenum disulfide (MoS₂ ) nanoenzymes exhibit a piezoelectric polarization, which generates reactive oxygen species to inactivate tumors under ultrasonic strain. However, its therapeutic efficiency is far away from satisfactory, due to stackable MoS₂ , quenching of piezo-generated charges, and monotherapy. Herein, chitosan-exfoliated monolayer MoS₂ (Ch-MS) is composited with atomic-thin MXene, Ti₃ C₂ (TC), to self-assemble a multimodal nanoplatform, Ti₃ C₂ -Chitosan-MoS₂ (TC@Ch-MS), for tumor inactivation. TC@Ch-MS not only inherits piezoelectricity from monolayer MoS₂ , but also maintains remarkable stability. Intrinsic metallic MXene combines with MoS₂ to construct an interfacial Schottky heterojunction, facilitating the separation of electron-hole pairs and endowing TC@Ch-MS increase-sensitivity magnetic resonance imaging responding. Schottky interface also leads to peroxidase mimetics with excellent catalytic performance toward H₂ O₂ in the tumor microenvironment under mechanical vibration. TC@Ch-MS possesses the superior photothermal conversion efficiency than pristine TC under near-infrared ray illumination, attributed to its enhanced interlaminar conductivity. Meanwhile, TC@Ch-MS realizes optimized efficiency on tumor apoptosis with immunotherapy. Therefore, TC@Ch-MS achieves an integrated diagnosis and multimodal treatment nanoplatform, whereas the toxicity to normal tissue cells is negligible. This work may shed fresh light on optimizing the piezoelectric materials in biological applications, and also give prominence to the significance of intrinsic metallicity in MXene.

Ultrafast Thermionic Electron Injection Effects on Exciton Formation Dynamics at a van der Waals Semiconductor/Metal Interface

Inorganic van der Waals bonded semiconductors such as transition metal dichalcogenides are the subject of intense research due to their electronic and optical properties which are promising for next-generation optoelectronic devices. In this context, understanding the carrier dynamics, as well as charge and energy transfer at the interface between metallic contacts and semiconductors, is crucial and yet quite unexplored. Here, we present an experimental study to measure the effect of mutual interaction between thermionically injected and directly excited carriers on the exciton formation dynamics in bulk WS₂. By employing a pump-push-probe scheme, where a pump pulse induces thermionic injection of electrons from a gold substrate into the conduction band of the semiconductor, and another delayed push pulse that excites direct transitions in the WS₂, we can isolate the two processes experimentally and thus correlate the mutual interaction with its effect on the ultrafast dynamics in WS₂. The fast decay time constants extracted from the experiments show a decrease with an increasing ratio between the injected and directly excited charge carriers, thus disclosing the impact of thermionic electron injection on the exciton formation dynamics. Our findings might offer a new vibrant direction for the integration of photonics and electronics, especially in active and photodetection devices, and, more in general, in upcoming all-optical nanotechnologies.

Engineering Plasmonic Environments for 2D Materials and 2D-Based Photodetectors

Two-dimensional layered materials are considered ideal platforms to study novel small-scale optoelectronic devices due to their unique electronic structures and fantastic physical properties. However, it is urgent to further improve the light–matter interaction in these materials because their light absorption efficiency is limited by the atomically thin thickness. One of the promising approaches is to engineer the plasmonic environment around 2D materials for modulating light–matter interaction in 2D materials. This method greatly benefits from the advances in the development of nanofabrication and out-plane van der Waals interaction of 2D materials. In this paper, we review a series of recent works on 2D materials integrated with plasmonic environments, including the plasmonic-enhanced photoluminescence quantum yield, strong coupling between plasmons and excitons, nonlinear optics in plasmonic nanocavities, manipulation of chiral optical signals in hybrid nanostructures, and the improvement of the performance of optoelectronic devices based on composite systems.

Optical and Material Characteristics of MoS2/Cu2O Sensor for Detection of Lung Cancer Cell Types in Hydroplegia

In this study, n-type MoS₂ monolayer flakes are grown through chemical vapor deposition (CVD), and a p-type Cu₂O thin film is grown via electrochemical deposition. The crystal structure of the grown MoS₂ flakes is analyzed through transmission electron microscopy. The monolayer structure of the MoS₂ flakes is verified with Raman spectroscopy, multiphoton excitation microscopy, atomic force microscopy, and photoluminescence (PL) measurements. After the preliminary processing of the grown MoS₂ flakes, the sample is then transferred onto a Cu₂O thin film to complete a p-n heterogeneous structure. Data are confirmed via scanning electron microscopy, SHG, and Raman mapping measurements. The luminous energy gap between the two materials is examined through PL measurements. Results reveal that the thickness of the single-layer MoS₂ film is 0.7 nm. PL mapping shows a micro signal generated at the 627 nm wavelength, which belongs to the B2 excitons of MoS₂ and tends to increase gradually when it approaches 670 nm. Finally, the biosensor is used to detect lung cancer cell types in hydroplegia significantly reducing the current busy procedures and longer waiting time for detection. The results suggest that the fabricated sensor is highly sensitive to the change in the photocurrent with the number of each cell, the linear regression of the three cell types is as high as 99%. By measuring the slope of the photocurrent, we can identify the type of cells and the number of cells.

Theoretical design of SnS2-graphene heterojunctions with vacancy and impurity defects for multi-purpose photoelectric devices

SnS₂ with atomic thickness has attracted extensive attention in the field of photocatalysis due to its special physicochemical properties, suitable band gap, low cost and low environmental toxicity. However, the application of SnS₂ in the field of optoelectronics is restricted by its low photocatalytic efficiency and carrier mobility. In this study, vacancies and transition metal atoms are introduced into a SnS₂ monolayer to modulate its physicochemical properties. Meanwhile, the SnS₂ monolayer modified by vacancies and transition metal atoms is combined with graphene to form a heterostructure, which promotes the separation of photogenerated electron-hole pairs. The results of theoretical calculations show that the SnS₂/graphene heterojunction can promote the separation of photogenerated carriers in intrinsic monolayer SnS₂, and improve the photocatalytic efficiency and carrier mobility. The modification of Sn vacancies and Fe, Co atoms not only expands the visible light response range of the SnS₂/graphene heterojunction, but also introduces magnetism, which is expected to be applied in spin optoelectronic materials. In this work, defects, doping and heterojunction assembly are rationally integrated, which provides a new idea for the design and development of spin optoelectronic devices based on monolayer SnS₂.

Anomalously polarised emission from a MoS2/WS2 heterostructure

We report circularly polarised emission, with helicity opposite to the optical excitation, from a van der Waals heterostructure (HS) consisting of a monolayer MoS₂ and three-layer WS₂. Selective excitation of the MoS₂ layer confirms that this cross-polarized emission is due to the charge transfer from the WS₂ layers to the MoS₂ layer. We propose that the high levels of n-doping in the constituent layers due to sulphur vacancies and defects give rise to an enhanced transition rate of electrons from the k valley of WS₂ to the k' valley of MoS₂, which leads to the emission, counter polarized to the excitation. Simulations based on the rate equation model support this conclusion.

Nanostructure-based photoelectrochemical sensing platforms for biomedical applications

As a newly developed and powerful analytical method, the use of photoelectrochemical (PEC) biosensors opens up new opportunities to provide wide applications in the early diagnosis of diseases, environmental monitoring and food safety detection. The properties of diverse photoactive materials are one of the essential factors, which can greatly impact the PEC performance. The continuous development of nanotechnology has injected new vitality into the field of PEC biosensors. In many studies, much effort on PEC sensing with semiconductor materials is highlighted. Thus, we propose a systematic introduction to the recent progress in nanostructure-based PEC biosensors to exploit more promising materials and advanced PEC technologies. This review briefly evaluates the several advanced photoactive nanomaterials in the PEC field with an emphasis on the charge separation and transfer mechanism over the past few years. In addition, we introduce the application and research progress of PEC sensors from the perspective of basic principles, and give a brief overview of the main advances in the versatile sensing pattern of nanostructure-based PEC platforms. This last section covers the aspects of future prospects and challenges in the nanostructure-based PEC analysis field.

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.

A two-step chemical vapor deposition process for the growth of continuous vertical heterostructure WSe2/h-BN and its optical properties

Direct growth of WSe₂ on hexagonal boron nitride via a two step CVD process.

Non-Carbon 2D Materials-Based Field-Effect Transistor Biosensors: Recent Advances, Challenges, and Future Perspectives

In recent years, field-effect transistors (FETs) have been very promising for biosensor applications due to their high sensitivity, real-time applicability, scalability, and prospect of integrating measurement system on a chip. Non-carbon 2D materials, such as transition metal dichalcogenides (TMDCs), hexagonal boron nitride (h-BN), black phosphorus (BP), and metal oxides, are a group of new materials that have a huge potential in FET biosensor applications. In this work, we review the recent advances and remarkable studies of non-carbon 2D materials, in terms of their structures, preparations, properties and FET biosensor applications. We will also discuss the challenges facing non-carbon 2D materials-FET biosensors and their future perspectives.

Cobalt-doped MoS2 enhances the evolution of hydrogen by piezo-electric catalysis under the 850 nm near-infrared light irradiation

Hydrogen is a clean shuttle of energy storage that can naturally reserve solar and wind energy, and it can be released.

Enhanced excitation and emission from 2D transition metal dichalcogenides with all-dielectric nanoantennas

The recently emerged concept of all-dielectric nanophotonics based on optical Mie resonances in high-index dielectric nanoparticles has proven to be a promising pathway to boost light-matter interactions at the nanoscale. In this work, we discuss the opportunities enabled by the interaction of dielectric nanoresonators with 2D transition metal dichalcogenides (2D TMDCs), leading to weak and strong coupling regimes. We perform a comprehensive analysis of bright exciton photoluminescence (PL) enhancement from various 2D TMDCs, including WS₂, MoS₂, WSe₂, and MoSe₂ via their coupling to Mie resonances of a silicon nanoparticle. For each case, we find the system parameters corresponding to maximal PL enhancement taking into account excitation rate, Purcell factor and radiation efficiency. We demonstrate numerically that all-dielectric Si nanoantennas can significantly enhance the PL intensity from 2D TMDC by a factor of hundred through precise optimization of the geometrical and material parameters. Our results may be useful for high-efficiency 2D TMDC-based optoelectronic, nanophotonic, and quantum optical devices.

Physics of excitons and their transport in two dimensional transition metal dichalcogenide semiconductors

Two-dimensional (2D) group-VI transition metal dichalcogenide (TMD) semiconductors, such as MoS₂, MoSe₂, WS₂ and others manifest strong light matter coupling and exhibit direct band gaps which lie in the visible and infrared spectral regimes. These properties make them potentially interesting candidates for applications in optics and optoelectronics. The excitons found in these materials are tightly bound and dominate the optical response, even at room temperatures. Large binding energies and unique exciton fine structure make these materials an ideal platform to study exciton behaviors in two-dimensional systems. This review article mainly focuses on studies of mechanisms that control dynamics of excitons in 2D systems – an area where there remains a lack of consensus in spite of extensive research. Firstly, we focus on the kinetics of dark and bright excitons based on a rate equation model and discuss on the role of previous ‘unsuspected’ dark excitons in controlling valley polarization. Intrinsically, dark and bright exciton energy splitting plays a key role in modulating the dynamics. In the second part, we review the excitation energy-dependent possible characteristic relaxation pathways of photoexcited carriers in monolayer and bilayer systems. In the third part, we review the extrinsic factors, in particular the defects that are so prevalent in single layer TMDs, affecting exciton dynamics, transport and non-radiative recombination such as exciton–exciton annihilation. Lastly, the optical response due to pump-induced changes in TMD monolayers have been reviewed using femtosecond pump–probe spectroscopy which facilitates the analysis of underlying physical process just after the excitation.

Two-dimensional (2D) group-VI transition metal dichalcogenide (TMD) semiconductors, such as MoS₂, MoSe₂, WS₂ and others manifest strong light matter coupling and exhibit direct band gaps which lie in the visible and infrared spectral regimes.

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.

Direct and indirect light emissions from layered ReS2-x Se x (0 ≤ x ≤ 2)

ReS₂ and ReSe₂ have recently been enthusiastically studied owing to the specific in-plane electrical, optical and structural anisotropy caused by their distorted one-layer trigonal (1 T) phase, whereas other traditional transition-metal dichalcogenides (TMDCs, e.g. MoS₂ and WSe₂) have a hexagonal structure. Because of this special property, more and versatile nano-electronics and nano-optoelectronics devices can be developed. In this work, 2D materials in the series ReS_2-x Se _x (0 ≤ x ≤ 2) have been successfully grown by the method of chemical vapor transport. The direct and indirect resonant emissions of the complete series of layers can be simultaneously detected by polarized micro-photoluminescence (μPL) spectroscopy when the thickness of the ReS_2-x Se _x is greater than ∼70 nm. When it is less than 70 nm, only three direct excitonic emissions-E ₁^ex, E ₂^ex and E _S^ex-are detected. For the thick (bulk) ReS_2-x Se _x , more stacking of the ReX₂ monolayers even flattens and shifts the valence-band maximum from Γ to the other K- or M-related points, thus leading to the coexistence of direct and indirect resonant light emissions from the c-plane ReX₂. The transmittance absorption edge of each bulk ReX₂ (a few microns thick) usually has a lower energy than those of the direct E ₁^ex and E ₂^ex excitonic emissions to form indirect absorption. The coexistence of direct and indirect emissions in ReX₂ is a unique characteristic of a 2D layered semiconductor possessing triclinic low symmetry.

Broad-Band Photocurrent Enhancement in MoS2 Layers Directly Grown on Light-Trapping Si Nanocone Arrays

There has been growing research interest in realizing optoelectronic devices based on the two-dimensional atomically thin semiconductor MoS₂ owing to its distinct physical properties that set it apart from conventional semiconductors. However, there is little optical absorption in these extremely thin MoS₂ layers, which presents an obstacle toward applying them for use in high-efficiency light-absorbing devices. We synthesized trilayers of MoS₂ directly on SiO₂/Si nanocone (NC) arrays using chemical vapor deposition and investigated their photodetection characteristics. The photoresponsivity of the MoS₂/NC structure was much higher than that of the flat counterpart across the whole visible wavelength range (for example, it was almost an order of magnitude higher at λ = 532 nm). Strongly concentrated light near the surface that originated from a Fabry-Perot interference in the SiO₂ thin layers and a Mie-like resonance caused by the Si NCs boosted the optical absorption in MoS₂. Our work demonstrates that MoS₂/NC structures could provide a useful means to realize high-performance optoelectronic devices.

Ultrathin gold film modified optical properties of excitons in monolayer MoS2

The incident angle for maximum C excitonic absorption deviates from the SPR angle due to the ultrathin-gold-film-induced optical scattering.

Engineering the hot spots in squared arrays of gold nanoparticles on a silver film

Density of nanoparticle (NP) arrays affects the hot spots distribution and strength in NP-metal film (NP-MF) geometry. In-depth understanding of the variation of electromagnetic (EM) field enhancement with NPs density is essential for wide applications of the NP-MF geometry such as surface-enhanced spectroscopies and enhanced efficiency of optoelectronic devices. Here, we show that the field distribution in the NP array on the metal film is greatly enhanced and confined at the NP-NP junctions for very small horizontal gap (g) between neighboring NPs, whereas the fields at the NP-MF junction are extremely small. When gradually increasing g, the field enhancement at the NP-NP junction decreases, along with the gradually enhanced fields at the NP-MF junction. We show that there is an optimal value of horizontal gap (∼75 nm for 80 nm Au NP array on Ag film with 532 nm normal incidence), indicating that the average field enhancement in NP-MF geometry can be optimized by adjusting the horizontal gap. More importantly, it is found that the EM field enhancement is greatly decreased when g fulfills the requirement to couple the 532 nm incident light into SPPs, because of the interference between the LSPR and the SPPs, which leads to a Fano dip at the incident wavelength of 532 nm.

Layer Control of WSe2 via Selective Surface Layer Oxidation

We report Raman and photoluminescence spectra of mono- and few-layer WSe2 and MoSe2 taken before and after exposure to a remote oxygen plasma. For bilayer and trilayer WSe2, we observe an increase in the photoluminescence intensity and a blue shift of the photoluminescence peak positions after oxygen plasma treatment. The photoluminescence spectra of trilayer WSe2 exhibit features of a bilayer after oxygen plasma treatment. Bilayer WSe2 exhibits features of a monolayer, and the photoluminescence of monolayer WSe2 is completely absent after the oxygen plasma treatment. These changes are observed consistently in more than 20 flakes. The mechanism of the changes observed in the photoluminescence spectra of WSe2 is due to the selective oxidation of the topmost layer. As a result, N-layer WSe2 is reduced to N-1 layers. Raman spectra and AFM images taken from the WSe2 flakes before and after the oxygen treatment corroborate these findings. Because of the low kinetic energy of the oxygen radicals in the remote oxygen plasma, the oxidation is self-limiting. By varying the process duration from 1 to 10 min, we confirmed that the oxidation will only affect the topmost layer of the WSe2 flakes. X-ray photoelectron spectroscopy shows that the surface layer WOx of the sample can be removed by a quick dip in KOH solution. Therefore, this technique provides a promising way of controlling the thickness of WSe2 layer by layer.

Moving graphene devices from lab to market: advanced graphene-coated nanoprobes

After more than a decade working with graphene there is still a preoccupying lack of commercial devices based on this wonder material. Here we report the use of high-quality solution-processed graphene sheets to fabricate ultra-sharp probes with superior performance. Nanoprobes are versatile tools used in many fields of science, but they can wear fast after some experiments, reducing the quality and increasing the cost of the research. As the market of nanoprobes is huge, providing a solution for this problem should be a priority for the nanotechnology industry. Our graphene-coated nanoprobes not only show enhanced lifetime, but also additional unique properties of graphene, such as hydrophobicity. Moreover, we have functionalized the surface of graphene to provide piezoelectric capability, and have fabricated a nano relay. The simplicity and low cost of this method, which can be used to coat any kind of sharp tip, make it suitable for the industry, allowing production on demand.

An oxygen-vacancy rich 3D novel hierarchical MoS2/BiOI/AgI ternary nanocomposite: enhanced photocatalytic activity through photogenerated electron shuttling in a Z-scheme manner

Herein, we propose the photocatalytic mechanism, involving a Z-scheme and oxygen vacancy states, for the MoS₂/BiOI/AgI nanocomposite.

Direct Plasmon-Driven Photoelectrocatalysis

Harnessing the energy from hot charge carriers is an emerging research area with the potential to improve energy conversion technologies.1-3 Here we present a novel plasmonic photoelectrode architecture carefully designed to drive photocatalytic reactions by efficient, nonradiative plasmon decay into hot carriers. In contrast to past work, our architecture does not utilize a Schottky junction, the commonly used building block to collect hot carriers. Instead, we observed large photocurrents from a Schottky-free junction due to direct hot electron injection from plasmonic gold nanoparticles into the reactant species upon plasmon decay. The key ingredients of our approach are (i) an architecture for increased light absorption inspired by optical impedance matching concepts,4 (ii) carrier separation by a selective transport layer, and (iii) efficient hot-carrier generation and injection from small plasmonic Au nanoparticles to adsorbed water molecules. We also investigated the quantum efficiency of hot electron injection for different particle diameters to elucidate potential quantum effects while keeping the plasmon resonance frequency unchanged. Interestingly, our studies did not reveal differences in the hot-electron generation and injection efficiencies for the investigated particle dimensions and plasmon resonances.