Photoluminescence Segmentation within Individual Hexagonal Monolayer Tungsten Disulfide Domains Grown by Chemical Vapor Deposition

Terminal Atom-Controlled Etching of 2D-TMDs

The controlled etching of 2D transition metal dichalcogenides (2D-TMDs) is critical to understanding the growth mechanisms of 2D materials and patterning 2D materials but remains a major comprehensive challenge. Here, a rational strategy to control the terminal atoms of 2D-TMDs etched holes is reported. Using laser irradiation combined with an improved anisotropic thermal etching process under a determined atmosphere, terminal atom-controlled etched hole arrays are created on 2D-TMDs. By adjusting the gas atmosphere during the thermal etching stage, triangular etched hole arrays terminated by the tungsten zigzag (W-ZZ) edge (in an Ar/H₂ atmosphere), hexagonal etched hole arrays terminated alternately by the W-ZZ edge and sulfur (selenium) zigzag (S-ZZ or Se-ZZ) edge (in a pure Ar atmosphere), and triangular etched hole arrays terminated by the S-ZZ (Se-ZZ) edge (in an Ar/sulfur [selenium] vapor atmosphere) can be obtained. Density functional theory reveals the forming energy of different edges and the different activities of metal atoms and chalcogenide atoms under different atmospheres, which determine the terminal atoms of the holes. This work may enhance the understanding of the etching and growth of 2D-TMDs. The 2D-TMDs hole arrays constructed by this work may have important applications in catalysis, nonlinear optics, spintronics, and large-scale integrated circuits.

Spatial Control of Substitutional Dopants in Hexagonal Monolayer WS2 : The Effect of Edge Termination

The ability to control the density and spatial distribution of substitutional dopants in semiconductors is crucial for achieving desired physicochemical properties. Substitutional doping with adjustable doping levels has been previously demonstrated in 2D transition metal dichalcogenides (TMDs); however, the spatial control of dopant distribution remains an open field. In this work, edge termination is demonstrated as an important characteristic of 2D TMD monocrystals that affects the distribution of substitutional dopants. Particularly, in chemical vapor deposition (CVD)-grown monolayer WS₂ , it is found that a higher density of transition metal dopants is always incorporated in sulfur-terminated domains when compared to tungsten-terminated domains. Two representative examples demonstrate this spatial distribution control, including hexagonal iron- and vanadium-doped WS₂ monolayers. Density functional theory (DFT) calculations are further performed, indicating that the edge-dependent dopant distribution is due to a strong binding of tungsten atoms at tungsten-zigzag edges, resulting in the formation of open sites at sulfur-zigzag edges that enable preferential dopant incorporation. Based on these results, it is envisioned that edge termination in crystalline TMD monolayers can be utilized as a novel and effective knob for engineering the spatial distribution of substitutional dopants, leading to in-plane hetero-/multi-junctions that display fascinating electronic, optoelectronic, and magnetic properties.

Vapor-Liquid-Solid Growth of Morphology-Tailorable WS2 toward P-Type Monolayer Field-Effect Transistors

Although substantial efforts have been made, controllable synthesis of p-type WS₂ remains a challenge. In this work, we employ NaCl as a seeding promoter to realize vapor-liquid-solid (VLS) growth of p-type WS₂. Morphological evolution, including a one-dimensional (1D) nanowire to two-dimensional (2D) planar domain and 2D shape transition of WS₂ domains, can be well-controlled by the growth temperature and sulfur introduction time. A high growth temperature is required to enable planar growth of 2D WS₂, and a sulfur-rich environment is found to facilitate the growth of high-quality WS₂. Raman and photoluminescence (PL) mappings demonstrate uniform crystallinity and high quantum efficiency of VLS-grown WS₂. Moreover, monolayer WS₂-based field-effect transistors (FETs) are fabricated, showing p-type conducting behavior, which is different from previous reported n-type FETs from WS₂ grown by other methods. First-principles calculations show that the p-type behavior originates from the substitution of Na at the W site, which will form an additional acceptor level above the valence band maximum (VBM). This facile VLS growth method opens the avenue to realize the p-n WS₂ homojunctions and p/n-WS₂-based heterojunctions for monolayer wearable electronic, photonic, optoelectronic, and biosensing devices and should also be a great benefit to the development of 2D complementary metal-oxide-semiconductor (CMOS) circuit applications.

Symmetric domain segmentation in WS2flakes: correlating spatially resolved photoluminescence, conductance with valley polarization

The incidence of intra-flake heterogeneity of spectroscopic and electrical properties in chemical vapour deposited (CVD) WS₂flakes is explored in a multi-physics investigation via spatially resolved spectroscopic maps correlated with electrical, electronic and mechanical properties. The investigation demonstrates that the three-fold symmetric segregation of spectroscopic response, in topographically uniform WS₂flakes are accompanied by commensurate segmentation of electronic properties e.g. local carrier density and the differences in the mechanics of tip-sample interactions, evidenced via scanning probe microscopy phase maps. Overall, the differences are understood to originate from point defects, namely sulfur vacancies within the flake along with a dominant role played by the substrate. While evolution of the multi-physics maps upon sulfur annealing elucidates the role played by sulfur vacancy, substrate-induced effects are investigated by contrasting data from WS₂flake on Si and Au surfaces. Local charge depletion induced by the nature of the sample-substrate junction in case of WS₂on Au is seen to invert the electrical response with comprehensible effects on their spectroscopic properties. Finally, the role of these optoelectronic properties in preserving valley polarization that affects valleytronic applications in WS₂flakes, is investigated via circular polarization discriminated photoluminescence experiments. The study provides a thorough understanding of spatial heterogeneity in optoelectronic properties of WS₂and other transition metal chalcogenides, which are critical for device fabrication and potential applications.

The Hidden Flower in WS2 Flakes: A Combined Nanomechanical and Tip-Enhanced Raman Exploration

We report on tungsten disulfide (WS₂) flakes grown by chemical vapor deposition (CVD), which exhibit a flower-like surface structure above the primary few-layer flake with a triangular shape. The fine structure is only revealed in the mechanical, chemical, and electronic properties of the flake but not in the topography. The origin of this structure is the peculiar one-step growth during the CVD process that permits to control the sulfur concentration at any time. A high concentration of S at the onset of the deposition process leads to a rapid growth of the flake, resulting in tungsten vacancies. Reducing the sulfur concentration toward the end of the growth slows down the reaction and leads to sulfur vacancies. These microscale domains were studied by confocal- and tip-enhanced Raman spectroscopy revealing their chemical composition with high spatial resolution. A strong quenching of the photoluminescence in the tungsten-vacancy domains is observed. Atomic force microscope measurements, performed in intermittent contact mode, force modulation mode (including lateral force mode), and PeakForce quantitative nanomechanics mode, show that the mechanical properties of these domains differ. Within the tungsten-vacancy domains, the adhesion force is reduced, while the friction force increased. Kelvin probe force microscopy measurements show that the electronic properties of the flakes are modulated by these domains. The combined nanomechanical and nanospectroscopy measurements provide detailed insights on the inhomogeneous surface properties of the single WS₂ flake, further highlighting how its multidomain properties can be finely tuned using CVD.

Photoluminescence Lightening: Extraordinary Oxygen Modulated Dynamics in WS2 Monolayers

Transition metal dichalcogenide monolayers exhibit ultrahigh surface sensitivity since they expose all atoms to the surface and thereby influence their optoelectronic properties. Here, we report an intriguing lightening of the photoluminescence (PL) from the edge to the interior over time in the WS₂ monolayers grown by physical vapor deposition method, with the whole monolayer brightened eventually. Comprehensive optical studies reveal that the PL enhancement arises from the p doping induced by oxygen adsorption. First-principles calculations unveil that the dissociation of chemisorbed oxygen molecule plays a significant role; i.e., the dissociation at one site can largely promote the dissociation at a nearby site, facilitating the photoluminescence lightening. In addition, we further manipulate such PL brightening rate by controlling the oxygen concentration and the temperature. The presented results uncover the extraordinary surface chemistry and related mechanism in WS₂ monolayers, which deepens our insight into their unique PL evolution behavior.

Bandgap modulation in the two-dimensional core-shell-structured monolayers of WS2

Tungsten disulfide (WS2) has tunable bandgaps, which are required for diverse optoelectronic device applications. Here, we report the bandgap modulation in WS2 monolayers with two-dimensional core-shell structures formed by unique growth mode in chemical vapor deposition (CVD). The core-shell structures in our CVD-grown WS2 monolayers exhibit contrasts in optical images, Raman, and photoluminescence spectroscopy. The strain and doping effects in the WS2, introduced by two different growth processes, generate PL peaks at 1.83 eV (at the core domain) and 1.98 eV (at the shell domain), which is distinct from conventional WS2 with a primary PL peak at 2.02 eV. Our density functional theory (DFT) calculations explain the modulation of the optical bandgap in our core-shell-structured WS2 monolayers by the strain, accompanying a direct-to-indirect bandgap transition. Thus, the core-shell-structured WS2 monolayers provide a practical method to fabricate lateral heterostructures with different optical bandgaps, which are required for optoelectronic applications.

Application of two-dimensional layered materials in surface-enhanced Raman spectroscopy (SERS)

2D materials are promising SERS substrates. Seven feasible strategies to improve the SERS performance of 2D substrate materials are summarized. The prospect of future progress in SERS and possible challenges of 2D layered materials are put forwarded.

Strain-Induced Alternating Photoluminescence Segmentation in Hexagonal Monolayer Tungsten Disulfide Grown by Physical Vapor Deposition

Two-dimensional semiconductors exhibit strong light emission under optical or electrical pumping due to quantum confinement and large exciton binding energies. The regulation of the light emission shows great application potential in next-generation optoelectronic devices. Herein, by the physical vapor deposition strategy, we synthesize monolayer hexagonal-shaped WS₂, and its photoluminescence intensity mapping show three-fold symmetric patterns with alternating bright and dark regions. Regardless of the length of the edges, all domains with S-terminated edges show lower photoluminescence intensity, while all regions with W-terminated edges exhibit higher photoluminescence intensity. The photoluminescence segmentation mechanism is studied in detail by employing Raman spectroscopy, atomic force microscopy, high-resolution transmission electron microscopy, and Kelvin probe force microscopy, and it is found to originate from different strain distributions in the S-terminated region and the W-terminated region. The optical band gap determined by the photoluminescence in the dark region is ∼2 meV lower than that in the bright region, implying that more strain is stored in the S-terminated region than in the W-terminated region. The photoluminescence segmentation vanishes in transferred hexagonal-shaped WS₂ from the initial substrate to a fresh silicon substrate, further confirming the physical mechanism. Our results provide guidance for tuning the optical properties of two-dimensional semiconductors by controllable strain engineering.

Local Optical Properties in CVD-Grown Monolayer WS2 Flakes

Excitons dominate the light absorption and re-emission spectra of monolayer transition-metal dichalcogenides (TMD). Microscopic investigations of the excitonic response in TMD almost invariably extract information from the radiative recombination step, which only constitutes one part of the picture. Here, by exploiting imaging spectroscopic ellipsometry (ISE), we investigate the spatial dependence of the dielectric function of chemical vapor deposition (CVD)-grown WS2 flakes with a microscopic lateral resolution, thus providing information about the spatially varying, exciton-induced light absorption in the monolayer WS2. Comparing the ISE results with imaging photoluminescence spectroscopy data, the presence of several correlated features was observed, along with the unexpected existence of a few uncorrelated characteristics. The latter demonstrates that the exciton-induced absorption and emission features are not always proportional at the microscopic scale. Microstructural modulations across the flakes, having a different influence on the absorption and re-emission of light, are deemed responsible for the effect.

Valley manipulation in monolayer transition metal dichalcogenides and their hybrid systems: status and challenges

Recently, the emerging conceptual valley-related devices have attracted much attention due to the progress on generating, controlling, and detecting the valley degree of freedom in the transition metal dichalcogenide (TMD) monolayers. In general, it is known that achieving valley degree of freedom with long valley lifetime is crucial in the implementation of valleytronic devices. Here, we provide a brief introduction of the basic understandings of valley degree of freedom. We as well review the recent experimental advancement in the modulation of valley degree of freedom. The strategies include optical/magnetic/electric field tuning, moiré patterns, plasmonic metasurface, defects and strain engineering. In addition, we summarize the corresponding mechanisms, which can help to obtain large degree of polarization and long valley lifetimes in monolayer TMDs. Based on these methods, two-dimensional valley-optoelectronic systems based on TMD heterostructures can be constructed, providing opportunities for such as the new paradigm in data processing and transmission. Challenges and perspectives on the development of valleytronics are highlighted as well.

Atomically Asymmetric Inversion Scales up to Mesoscopic Single-Crystal Monolayer Flakes

Symmetry is highly relevant with various quantities and phenomena in physics. While the translational symmetry breaks at the edges of two-dimensional hexagonal crystalline flakes, it is usually associated with the breaking of central inversion symmetry that is yet to be observed in terms of physical properties. Here, we report an experiment-theory joint study on in-plane compressed single-crystal monolayer WS₂ flakes. Although the flakes show a hexagonal appearance with a C₆ symmetry, our density functional theory calculations predict that their in-plane strain, geometric structure, work-function, energy bandgap, and mechanical modulus are nonequivalent among the triangular regions with different edge terminations at the atomic scale, and the flakes exhibit self-patterns with a C₃ symmetry. Such nonequivalence of physical properties and concomitant self-patterns persist even in a 50 μm-sized monolayer WS₂, observed using atomic force microscopy. This indicates that the symmetry arising from the atomic geometry could preserve up to tens of microns for both geometric and properties of the flake, regardless of its mesoscopic geometry, i.e., C₆ here. Such a detectable mesoscopic scale and symmetric nano- to mesoscale patterns provide promising building blocks for 2D materials and devices and also allow edge terminations of 2D flakes to be directly distinguished.

Growth Mechanism of Alternating Defect Domains in Hexagonal WS2 via Inhomogeneous W-Precursor Accumulation

While a hexagonal WS₂ monolayer, grown by chemical vapor deposition, exhibits distinctive patterns in photoluminescence mapping, segmented with alternating S-vacancy (SV) and W-vacancy (WV) domains in a single crystal, the formation mechanism for native alternating defect domains remains unresolved to date. Here, the formation mechanism of alternating defect domains in hexagonal WS₂ via the precursor accumulation model is experimentally elucidated. A triangular WS₂ seed is initially formed, followed by a hexagonal flake. Alternating W-rich (SV) and W-deficient (WV) domains are constructed in hexagonal WS₂ flake, which is confirmed by confocal photoluminescence mapping and secondary ion mass spectroscopy. This is explained by the accumulation or scarcity of W-precursors at the edge of the WS₂ flake. The W-precursors accumulate near the edges of the initial triangular WS₂ seed over time, while they are deficient near the corners of the triangular WS₂ , eventually forming WV domains in the truncated hexagonal domains. The heterogeneous accumulation becomes more prominent in the presence of H₂ gas through desorption of the W-precursors.

Nearly 90% Circularly Polarized Emission in Monolayer WS2 Single Crystals by Chemical Vapor Deposition

Monolayer transition-metal dichalcogenides (TMDCs) in the 2H-phase are promising semiconductors for opto-valleytronic and opto-spintronic applications because of their strong spin-valley coupling. Here, we report detailed studies of opto-valleytronic properties of heterogeneous domains in CVD-grown monolayer WS₂ single crystals. By illuminating WS₂ with off-resonance circularly polarized light and measuring the resulting spatially resolved circularly polarized emission (P_circ), we find significantly large circular polarization (P_circ up to 60% and 45% for α- and β-domains, respectively) already at 300 K, which increases to nearly 90% in the α-domains at 80 K. Studies of spatially resolved photoluminescence (PL) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, Kelvin-probe force microscopy, and conductive atomic force microscopy reveal direct correlation among the PL intensity, defect densities, and chemical potential, with the α-domains showing lower defect densities and a smaller work function by 0.13 eV than the β-domains. This work function difference indicates the occurrence of type-two band alignments between the α- and β-domains. We adapt a classical model to explain how electronically active defects may serve as nonradiative recombination centers and find good agreement between experiments and the model. Scanning tunneling microscopic/spectroscopic (STM/STS) studies provide further evidence for tungsten vacancies (WVs) being the primary defects responsible for the suppressed PL and circular polarization in WS₂. These results therefore suggest a pathway to control the opto-valleytronic properties of TMDCs by means of defect engineering.

Ultrathin All-2D Lateral Graphene/GaS/Graphene UV Photodetectors by Direct CVD Growth

UV-sensitive lateral all-two-dimensional (2D) photodetecting devices are produced by growing the large band gap layered GaS between graphene electrode pairs directly using chemical vapor deposition methods. The use of prepatterned graphene electrode pairs on the Si wafer enables more than 200 devices to be fabricated simultaneously. We show that the surface chemistry of the substrate during GaS leads to selective growth in graphene gaps, forming the lateral heterostructures, rather than on the surface of graphene. The graphene/GaS/graphene lateral photodetecting devices are demonstrated to be sensitive to UV light only, with no measurable response to visible light. Furthermore, we demonstrate UV-band discrimination in photosensing, with measured photocurrents only produced for middle-UV and not for near-UV wavelength regions. The detection limit could reach down to 2.61 μW/cm² with a photoresponsivity as high as 11.7 A/W and a photo gain of 53.7 under 270 nm excitation. Gate-dependent modulation of the photocurrent is also demonstrated. The photodetectors exhibit long-term stability and reproducible ON-OFF switching behavior, with a response time lower than 60 ms. These results provide insights into how ultrathin UV sensing devices can be created using only 2D materials by exploiting large band gap 2D semiconductors such as GaS.

Inhomogeneous photocarrier dynamics and transport in monolayer MoS2 by ultrafast microscopy

Monolayer MoS₂ as a member of two-dimensional transition metal dichalcogenides (TMDs) has attracted considerable attention due to its superior optoelectronic properties. Understanding the photocarrier dynamics and transport in these two dimensional systems is beneficial for applications from photovoltaics to sensing. However, various structural defects strongly impact the dynamics and transport of photocarriers. Especially there lacks a precise measuring and understanding of photocarrier transport in TMDs. Here, femtosecond transient absorption spectroscopy and microscopy were employed to study the photocarrier dynamics and transport in monolayer MoS₂. Defect correlated photocarrier dynamics are observed across the monolayer MoS₂ where exciton formation and nonradiative recombination are the two dominant decay processes. To the best of our knowledge, we report two distinct photocarrier transport regimes in MoS₂ for the first time with diffusion coefficients of [Formula: see text] cm² s^-1 and [Formula: see text] cm² s^-1, by taking advantages of ultrafast microscopy with ∼20 nm spatial precision and ∼200 fs temporal resolution. These two regimes are ascribed to fast hot photocarrier diffusion and slow phonon-limited thermal diffusion, respectively. The results indicate that the initial fast photocarrier transport is less dependent on structural defects compared to photocarrier relaxation dynamics which may be useful for hot photocarrier extraction applications.

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.

Effect of layer and stacking sequence in simultaneously grown 2H and 3R WS2 atomic layers

In two-dimensional layered materials, layer number and stacking order have strong effects on the optical and electronic properties. Tungsten disulfide (WS₂) crystal, as one important member among transition metal dichalcogenides, has been usually prepared in a layered 2H prototype structure with space group P6₃/mmc ([Formula: see text]) in spite of many other expected ones such as 3R. Here, we report simultaneous growth of 2H and 3R stacked multilayer (ML) WS₂ crystals in large scale by chemical vapor deposition and effects of layer number and stacking order on optical and electronic properties. As revealed in Raman and photoluminescence (PL) measurements, with an increase in layer number, 2H and 3R stacked ML WS₂ crystals show similar variation of PL and Raman peaks in position and intensity. Compared to 2H stacked ML WS₂, however, 3R stacked one always exhibits the larger red (blue) shift of Raman [Formula: see text] (A_1g) peak and the appearance of PL A, B and I peaks at lower energies. Thereby, PL and Raman features depend on not only layer number but also stacking order. In addition, circularly polarized luminescence from two prototype WS₂ crystals under circularly polarized excitation has also been investigated, showing obvious spin or valley polarization of these CVD-grown multilayer WS₂ crystals.

Oxidation of Monolayer WS2 in Ambient Is a Photoinduced Process

We have studied the ambient air oxidation of chemical vapor deposition (CVD) grown monolayers of the semiconducting transition metal dichalcogenide (S-TMD) WS₂ using optical microscopy, laser scanning confocal microscopy (LSCM), photoluminescence (PL) spectroscopy, and atomic force microscopy (AFM). Monolayer WS₂ exposed to ambient conditions in the presence of light (typical laboratory ambient light for weeks or typical PL spectroscopy map) exhibits damage due to oxidation which can be detected with the LSCM and AFM, though may not be evident in conventional optical microscopy due to poorer contrast and resolution. Additionally, this oxidation was not random and was correlated with "high-symmetry" high intensity edges and red-shifted areas in the PL spectroscopy map, areas thought to contain a higher concentration of sulfur vacancies. In contrast, samples kept in ambient and darkness showed no signs of oxidation for up to 10 months. Low-irradiance/fluence experiments showed that samples subjected to excitation energies at or above the trion excitation energy (532 nm/2.33 eV and 660 nm/1.88 eV) oxidized in as little as 7 days, even for irradiances and fluences 8 and 4 orders of magnitude lower (respectively) than previously reported. No significant oxidation was observed for 760 nm/1.63 eV light exposure, which lies below the trion excitation energy in WS₂. The strong wavelength dependence and apparent lack of irradiance dependence suggests that ambient oxidation of WS₂ is initiated by photon-mediated electronic band transitions, that is, photo-oxidation. These findings have important implications for prior, present, and future studies concerning S-TMDs measured, stored, or manipulated in ambient conditions.

High-Performance WS2 Monolayer Light-Emitting Tunneling Devices Using 2D Materials Grown by Chemical Vapor Deposition

The solid progress in the study of a single two-dimensional (2D) material underpins the development for creating 2D material assemblies with various electronic and optoelectronic properties. We introduce an asymmetric structure by stacking monolayer semiconducting tungsten disulfide, metallic graphene, and insulating boron nitride to fabricate numerous red channel light-emitting devices (LEDs). All the 2D crystals were grown by chemical vapor deposition (CVD), which has great potential for future industrial scale-up. Our LEDs exhibit visibly observable electroluminescence (EL) at both 5.5 V forward and 7.0 V backward biasing, which correlates well with our asymmetric design. The red emission can last for at least several minutes, and the success rate of the working device that can emit detectable EL is up to 80%. In addition, we show that sample degradation is prone to happen when a continuing bias, much higher than the threshold voltage, is applied. Our success of using high-quality CVD-grown 2D materials for red light emitters is expected to provide the basis for flexible and transparent displays.

Microstructure and Elastic Constants of Transition Metal Dichalcogenide Monolayers from Friction and Shear Force Microscopy

Optical and electrical properties of 2D transition metal dichalcogenides (TMDCs) grown by chemical vapor deposition (CVD) are strongly determined by their microstructure. Consequently, the visualization of spatial structural variations is of paramount importance for future applications. This study demonstrates how grain boundaries, crystal orientation, and strain fields can unambiguously be identified with combined lateral force microscopy and transverse shear microscopy (TSM) for CVD-grown tungsten disulfide (WS₂ ) monolayers, on length scales that are relevant for optoelectronic applications. Further, angle-dependent TSM measurements enable the fourth-order elastic constants of monolayer WS₂ to be acquired experimentally. The results facilitate high-throughput and nondestructive microstructure visualization of monolayer TMDCs and insights into their elastic properties, thus providing an accessible tool to support the development of advanced optoelectronic devices based on such 2D semiconductors.

Scalable faceted voids with luminescent enhanced edges in WS2 monolayers

A scalable approach is needed in the formation of atomically flat edges with specific terminations to enhance local properties for optoelectronic, nanophotonic and energy applications. We demonstrate point defect clustering-driven faceted void formations with luminescent enhanced edges in WS2 monolayers during large-scale CVD growth and controlled annealing. With the aid of aberration-corrected scanning transmission electron microscopy (AC-STEM) high angle annular dark field (HAADF) imaging, we probed atomic terminations of S and W to explain observed luminescence enhancement in alternate edges. Faceted void formation in monolayer WS2 was found to be sensitive to annealing temperature, time, gas environment and precursor supply. Our observations of areal coverage evolution over time revealed competition between monolayer WS2 growth and void formation at 850 °C. While the initial stage was dominated by monolayer growth, defect generation and void growth dominated at later stages and provided an optimum processing window for monolayer WS2 as well as faceted void growth. Growth of faceted voids not only followed the geometry of monolayer facets but also showed similar atomic terminations at the edges and thus enabled local manipulation of photoluminescence enhancement with an order of magnitude increase in intensity. The developed CVD processing enabled multi-fold increase in the luminescent active edge length through the formation of faceted voids within the WS2 monolayer.

Chemical Vapor Deposition Growth of Two-Dimensional Monolayer Gallium Sulfide Crystals Using Hydrogen Reduction of Ga2S3

Two-dimensional gallium sulfide (GaS) crystals are synthesized by a simple and efficient ambient pressure chemical vapor deposition (CVD) method using a single-source precursor of Ga₂S₃. The synthesized GaS structures involve triangular monolayer domains and multilayer flakes with thickness of 1 and 15 nm, respectively. Regions of continuous films of GaS are also achieved with about 0.7 cm² uniform coverage. This is achieved by using hydrogen carrier gas and the horizontally placed SiO₂/Si substrates. Electron microscopy and spectroscopic measurements are used to characteristic the CVD-grown materials. This provides important insights into novel approaches for enlarging the domain size of GaS crystals and understanding of the growth mechanism using this precursor system.

Synthesis of Easily Transferred 2D Layered BiI3 Nanoplates for Flexible Visible-Light Photodetectors

Bismuth triiodide, BiI₃, is one of the promising 2D layered materials from the family of metal halides. The unique electronic structure and properties make it an attractive material for the room-temperature gamma/X-ray detectors, high-efficiency photovoltaic absorbers, and Bi-based organic-inorganic hybrid perovskites. Other possibilities including optoelectronic devices and optical circuits are envisioned but rarely experimentally confirmed yet. Here, we report the synthesis of vertical 2D BiI₃ nanoplates using the physical vapor deposition mechanism. The obtained products were found easy to be separated and transferred to other substrates. Photodetectors employing such 2D nanoplates on polyethylene terephthalate substrate are demonstrated to be quite sensitive to red light (635 nm) with good responsivity (2.8 A W^-1), fast stable photoresponse (3/9 ms for raise/decay times), and remarkable specific detectivity (1.2 × 10¹² jones), which attest to high comparability of the assembled components with many latest 2D nanostructured light sensors. In addition, such photodetectors exhibit outstanding mechanical stability and durability under different bending strains within the theoretically affordable levels, suggesting a variety of potential applications of 2D BiI₃ for flexible devices.

Edge optical scattering of two-dimensional materials

Rayleigh scattering has shown powerful abilities to study electron resonances of nanomaterials regardless of the specific shapes. In analogy to Rayleigh scattering, here we demonstrate that edge optical scattering from two-dimensional(2D) materials also has the similar advantage. Our result shows that, in visible spectral range, as long as the lateral size of a 2D sample is larger than 2 μm, the edge scattering intensity distribution of the high-angle scattering in k space is nearly independent of the lateral size and the shape of the 2D samples. The high-angle edge scattering spectra are purely determined by the intrinsic dielectric properties of the 2D materials. As an example, we experimentally verify this feature in single-layer MoS₂, in which A and B excitons are clearly detected in the edge scattering spectra, and the scattering images in k space and real space are consistent with our theoretical model. This study shows that the edge scattering is a highly practical and efficient method for optical studies of various 2D materials as well as thin films with clear edges.

Electrical Characterization of Discrete Defects and Impact of Defect Density on Photoluminescence in Monolayer WS2

Transition-metal dichalcogenides (TMDs) are an exciting class of 2D materials that exhibit many promising electronic and optoelectronic properties with potential for future device applications. The properties of TMDs are expected to be strongly influenced by a variety of defects which result from growth procedures and/or fabrication. Despite the importance of understanding defect-related phenomena, there remains a need for quantitative nanometer-scale characterization of defects over large areas in order to understand the relationship between defects and observed properties, such as photoluminescence (PL) and electrical conductivity. In this work, we present conductive atomic force microscopy measurements which reveal nanometer-scale electronically active defects in chemical vapor deposition-grown WS₂ monolayers with defect density varying from 2.3 × 10¹⁰ cm^-2 to 4.5 × 10¹¹ cm^-2. Comparing these defect density measurements with PL measurements across large areas (>20 μm distances) reveals a strong inverse relationship between WS₂ PL intensity and defect density. We propose a model in which the observed electronically active defects serve as nonradiative recombination centers and obtain good agreement between the experiments and model.

Phase engineering of seamless heterophase homojunctions with co-existing 3R and 2H phases in WS2 monolayers

Self-organized semiconductor-semiconductor heterostructures (3R-2H) that coexist in atomically thin 2D monolayers forming homojunctions are of great importance for next-generation nanoelectronics and optoelectronics applications. Herein, we investigated the defect controlled growth of heterogeneous electronic structure within a single domain of monolayer WS₂ to enable in-plane homojunctions consisting of alternate 2H semiconducting and 3R semiconducting phases of WS₂. X-ray photoelectron, Raman, and photoluminescence spectroscopy along with fluorescence and Kelvin probe force microscopy imaging confirm the formation of homojunctions, enabling a direct correlation between chemical heterogeneity and electronic heterostructure in the atomically thin WS₂ monolayer. Quantitative analysis of phase fractions shows 59% stable 2H phase and 41% metastable 3R phase estimated over WS₂ flakes of different sizes. Time-resolved fluorescence lifetime imaging confirms distinct contrast between 2H and 3R phases with two distinct lifetimes of 3.2 ns and 1.1 ns, respectively. Kelvin probe force microscopy imaging revealed an abrupt change in the contact potential difference with a depletion width of ∼2.5 μm, capturing a difference in work function of ∼40 meV across the homojunction. Further, the thermal stability of coexisting phases and their temperature dependent optical behavior show a distinct difference among 2H and 3R phases. The investigated aspects of the controlled in plane growth of coexisting phases with seamless homojunctions, their properties, and their thermal stability will enable the development of nanoscale devices that are free from issues of lattice mismatch and grain boundaries.

Atomic structure of defects and dopants in 2D layered transition metal dichalcogenides

Transmission electron microscopy can directly image the detailed atomic structure of layered transition metal dichalcogenides, revealing defects and dopants.

Two-dimensional light-emitting materials: preparation, properties and applications

We review the recent development in two-dimensional (2D) light-emitting materials and describe their preparation methods, optical/optoelectronic properties and applications.