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Bottom-up synthesis of vertically oriented two-dimensional materials
Abstract
Understanding nucleation and growth of two-dimensional (2D) and layered materials is a challenging topic due to the complex van der Waals interactions between layers and substrate. The morphology of 2D materials is known vary depending on experimental conditions. For the case of MoS2, the morphology has been shown to vary from rounded (molybdenum rich) domains to equilateral triangular (sulfur rich) domains. These different morphologies can result in drastically different properties, which can be exploited for applications in catalytic reactions, digital electronics, optoelectronics, and energy storage. Powder vaporization (PV) synthesis of molybdenum disulfide (MoS2) can yield vertical domains, however, these domains are often ignored when the morphology evolution of MoS2 is discussed, thereby completely omitting a major part of the impact of the Mo:S ratio to the growth mode of MoS2 during PV. Combining experimental and numerical simulation methods, we reveal a vertical-to-horizontal growth mode transition for MoS2 that occurs in the presence of a molybdenum oxide partial pressure gradient. Transmission electron microscopy reveals that the growth of vertical MoS2 results from initial seeding of single crystalline molybdenum dioxide, followed by sulfurization from the substrate upward to form vertically oriented MoS2 domains.
... Precise control of the growth morphology and coverage of atomically thin structures requires understanding the underlying physical mechanisms and realizing the correlation and significance of experimentally controllable parameters 34,35 . Several attempts have been made to understand these mechanisms and guide the synthesis process for the controlled growth of 2D materials. ...
... presence of precursors' concentration gradient normal to the substrate will also promote the formation of out-of-plane growth via the Mullins-Sekerka mechanism 34,35 . Thus, the highest uniformity of 2D films can be achieved when the precursors' concentration is homogeneously distributed over the substrate with the minimum gradient in the direction normal to the substrate. ...
Reproducible wafer-scale growth of two-dimensional (2D) materials using the Chemical Vapor Deposition (CVD) process with precise control over their properties is challenging due to a lack of understanding of the growth mechanisms spanning over several length scales and sensitivity of the synthesis to subtle changes in growth conditions. A multiscale computational framework coupling Computational Fluid Dynamics (CFD), Phase-Field (PF), and reactive Molecular Dynamics (MD) was developed – called the CPM model – and experimentally verified. Correlation between theoretical predictions and thorough experimental measurements for a Metal-Organic CVD (MOCVD)-grown WSe2 model material revealed the full power of this computational approach. Large-area uniform 2D materials are synthesized via MOCVD, guided by computational analyses. The developed computational framework provides the foundation for guiding the synthesis of wafer-scale 2D materials with precise control over the coverage, morphology, and properties, a critical capability for fabricating electronic, optoelectronic, and quantum computing devices.
... For example, a slight variation of the Mo:S ratio along the CVD reactor leads to different growth morphologies [3]. Also, a change in the growth direction from in-plane to normal to the substrate's plane upon changing the substrate's location has been reported triggered by the gradient of the precursor concentration [4,5]. Different 2D materials have already been synthesized using the CVD technique by trial-and-error experimentation. ...
... Our results revealed a more uniform precursor distribution over the substrate when the substrate is rotating and a higher concentration of precursors at the center that potentially leads to multilayer growth. In contrast, most of the precursor concentration in the case of the stationary substrate is concentrated at its front edge, leading to large concentration gradients, which promotes the formation of out-of-plane growth and agglomeration by Mullins-Sekerka instability [4,5]. Substrate's rotation also enhances the intermixing of precursors, providing a more uniform W: Se ratio and a more consistent final synthesis morphology over the substrate, as shown in our PF simulations in Fig. 2c-d. ...
The exotic properties of 2D materials made them ideal candidates for applications in quantum computing, flexible electronics, and energy technologies. A major barrier to their adaptation for industrial applications is their controllable and reproducible growth at a large scale. A significant effort has been devoted to the chemical vapor deposition (CVD) growth of wafer-scale highly crystalline monolayer materials through exhaustive trial-and-error experimentations. However, major challenges remain as the final morphology and growth quality of the 2D materials may significantly change upon subtle variation in growth conditions. Here, we introduced a multiscale/multiphysics model based on coupling continuum fluid mechanics and phase-field models for CVD growth of 2D materials. It connects the macroscale experimentally controllable parameters, such as inlet velocity and temperature, and mesoscale growth parameters such as surface diffusion and deposition rates, to morphology of the as-grown 2D materials. We considered WSe 2 as our model material and established a relationship between the macroscale growth parameters and the growth coverage. Our model can guide the CVD growth of monolayer materials and paves the way to their synthesis-by-design.
Graphic abstract
... Theoretical calculations and computer simulations at different length and temporal scales have been used to explain the experimentally observed growth morphologies of h-BN. These tools, ranging from atomistic-scale first-principles calculations and molecular dynamics (MD) simulations to mesoscale kinetic Monte Carlo and phase-field approaches to macroscopic heat transfer and fluid dynamics, could shed light on the mechanisms governing the growth of 2D materials and guide the experimental growth [27][28][29]. An extensive review of these methods is presented in [30]. ...
... The current macroscale multiphysics model predicts the spatial distribution of precursor concentration, carrier gas velocity, pressure, and temperature within the CVD growth chamber by coupling the mass flow (Navier-Stokes), heat transfer (conduction and convection), and flow-assisted mass transport [28,29,43] equations, respectively, i.e. ...
There is a lack of knowledge on the fundamental growth mechanisms governing the characteristics of 2D materials synthesized by the chemical vapor deposition (CVD) technique and their correlation with experimentally controllable parameters, which hindered their wafer-scale synthesis. Here, we pursued an analytical and computational approach to access the system states that are not experimentally viable to address these critical needs. We developed a multiscale computational framework correlating the macroscale heat and mass flow with the mesoscale morphology of the as-grown 2D materials by solving the coupled system of heat/mass transfer and phase-field equations. We used hexagonal boron nitride (h-BN) as our model material and investigated the effect of substrate enclosure on its growth kinetics and final morphology. We revealed a lower concentration with a more uniform distribution on the substrate in an enclosed-growth than open-growth. It leads to a more uniform size distribution of the h-BN islands, consistent with existing experimental investigations.
... Thus, having a thorough understanding of the macroscale physics and processes is essential for controlling the growth of 2D materials and their synthesis by design. We can classify the macroscale models of the growth chamber into four groups: (i) experiment-based models, where rate equations and their constants used to describe the growth 170 are determined from experiments; (ii) analytical models, where the governing equations are simplified and solved analytically 171 ; (iii) adaptive models, where a set of experiments are used to train the model 172 ; and (iv) multiphysics models, where the coupled system of governing equations at different length and temporal scales are solved numerically 173 . Among these methods, the last group of models have key advantages, providing a profound understanding to the growth process, flexibility to apply to different growth conditions, and the ability to optimize the process. ...
... For example, it was revealed that the concentration gradient of the precursor is a critical factor in determining the growth morphology of 2D materials, where a low concentration gradient along the substrate leads to the formation of multigrain 2D films. In contrast, moderate planar concentration gradients lead to isolated islands of varying morphologies, and an out-ofplane gradient leads to the formation of standing 2D materials 173,177 . Furthermore, macroscale models can be combined with lower scale models to form a multiscale platform with high fidelity, e.g., ref. 8 . ...
The successful discovery and isolation of graphene in 2004, and the subsequent synthesis of layered semiconductors and heterostructures beyond graphene have led to the exploding field of two-dimensional (2D) materials that explore their growth, new atomic-scale physics, and potential device applications. This review aims to provide an overview of theoretical, computational, and machine learning methods and tools at multiple length and time scales, and discuss how they can be utilized to assist/guide the design and synthesis of 2D materials beyond graphene. We focus on three methods at different length and time scales as follows: (i) nanoscale atomistic simulations including density functional theory (DFT) calculations and molecular dynamics simulations employing empirical and reactive interatomic potentials; (ii) mesoscale methods such as phase-field method; and (iii) macroscale continuum approaches by coupling thermal and chemical transport equations. We discuss how machine learning can be combined with computation and experiments to understand the correlations between structures and properties of 2D materials, and to guide the discovery of new 2D materials. We will also provide an outlook for the applications of computational approaches to 2D materials synthesis and growth in general.
... Current growth chamber models adopt four major approaches: (1) the rate equations are parameterized experimentally [119], (2) simplified analytical governing equations are used [120], (3) an adaptive model is experimentally trained to determine the optimum growth parameters [121][122][123], and (4) the coupled system of equations governing the growth processes at different spatial and time scales is numerically solved [124][125][126][127][128][129]. Models using the first approach require several trial experiments, where extrapolation to different experimental setups and beyond the specific experimental conditions is difficult. ...
... Additionally, controlling the surface energy of the substrate via thermal treatments can enable epitaxial growth of TMDs like MoS 2 [2,163] and WSe 2 [167], provided growth temperatures are high enough to enable sufficient adatom mobility and to limit nucleation density [62]. In addition to the use of epitaxial substrates, relative metal and chalcogen ratios present during growth have also been shown to play a role in 2D material domain orientation [62,82,127,129,168]. Beyond the use of epitaxial substrates, substrate functionalization and seeding methods have been developed to achieve selectivearea growth of 2D materials, however, precise control over domain orientation through these approaches has not yet been demonstrated. ...
Since their modern debut in 2004, 2-dimensional (2D) materials continue to exhibit scientific and industrial promise, providing a broad materials platform for scientific investigation, and development of nano- A nd atomic-scale devices. A significant focus of the last decade's research in this field has been 2D semiconductors, whose electronic properties can be tuned through manipulation of dimensionality, substrate engineering, strain, and doping (Mak et al 2010 Phys. Rev. Lett. 105 136805; Zhang et al 2017 Sci. Rep. 7 16938; Conley et al 2013 Nano Lett. 13 3626-30; Li et al 2016 Adv. Mater. 28 8240-7; Rhodes et al 2017 Nano Lett. 17 1616-22; Gong et al 2014 Nano Lett. 14 442-9; Suh et al 2014 Nano Lett. 14 6976-82; Yoshida et al 2015 Sci. Rep. 5 14808). Molybdenum disulfide (MoS 2 ) and tungsten diselenide (WSe 2 ) have dominated recent interest for potential integration in electronic technologies, due to their intrinsic and tunable properties, atomic-scale thicknesses, and relative ease of stacking to create new and custom structures. However, to go 'beyond the bench', advances in large-scale, 2D layer synthesis and engineering must lead to 'exfoliation-quality' 2D layers at the wafer scale. This roadmap aims to address this grand challenge by identifying key technology drivers where 2D layers can have an impact, and to discuss synthesis and layer engineering for the realization of electronic-grade, 2D materials. We focus on three fundamental areas of research that must be heavily pursued in both experiment and computation to achieve high-quality materials for electronic and optoelectronic applications.
... However, studying the crystallization kin-etics of 2D materials grown by CVD is challenging due to the variability inherent to the operation of a CVD furnace. For example, considerably different morphologies of MoS 2 (vertically oriented, triangular, hexagonal, rounded, and rods) have been reported for CVD experiments performed under similar conditions [25][26][27][28]. In order to elucidate the crystallization kinetics of MoS 2 and mitigate the challenges presented by CVD, we crystallize an amorphous MoS 2 film and use in situ Raman spectroscopy to monitor the evolution of the 2H-MoS 2 Raman peaks. ...
... However, for both low and high crystallization temperatures we do not observe any vertical layers and instead only detect layers which have formed parallel to the substrate. In addition to our results, a survey of literature contrasting vertical and horizontal growth of MoS 2 [26,[48][49][50], suggests that the precursor, sulfurization mechanism and substrate interactions play a larger role over temperature in determining the growth direction of MoS 2 layers. ...
Developing effective strategies to synthesize 2D materials such as molybdenum disulfide (MoS2) necessitates a fundamental understanding of the thermodynamics and kinetics controlling the nucleation and growth processes. Studying crystallization kinetics of MoS2 with conventional synthesis methods, such as chemical vapor deposition, is challenging because there is a complex set of thermally-activated events happening simultaneously. By combining high-throughput experimentation with in situ Raman spectroscopy we show that the migration-limited crystallization kinetics of MoS2 can be directly observed. During isothermal heating we find that nucleation of MoS2 happens rapidly and that the crystallization rate follows an Arrhenius temperature relationship, yielding an energy barrier of 1.03 eV/atom. The relationship between temperature, crystal quality, and layer orientation is determined with transmission electron microscopy and Raman spectroscopy, revealing that elevated crystallization temperatures improve crystal quality and reduce defect formation.
... The cross-section TEM specimens were prepared by focused ion beam (FIB), following the typical TEM specimen preparation method for 2D materials using a Quanta 3D FEG. 40,41 A carbon and Pt layer were deposited to protect the Co(OH) 2 sample during the FIB sampling and milling process. The damage caused by the Ga ion beam of 30 kV was removed with a low voltage of 5 kV. ...
The irreversible phase transition from a layered to a spinel structure, typically observed in an intercalation-type electrode, has been recognized as one of the main causes of capacitance fading, structural instability, and thermal instability in Li-ion batteries. However, observing the phase transition in a real environment in real-time is still challenging. Here, cobalt hydroxide, Co(OH) 2 was irradiated with an electron beam in a transmission electron microscope, and the phase transition was intensively investigated using in-situ high resolution transition electron microscopy. Both the alpha and beta-Co(OH) 2 phases changed into a spinel Co 3 O 4 phase, and formed nanograins. However, the alpha Co(OH) 2 showed a faster phase transition and dramatic volume shrinkage during the phase transition, which led to layer bending/discontinuity and the development of cracks. Our results provide a detailed explanation of the mechanism behind the deterioration of the layered structure and the emergence of defects during the phase transition, providing crucial information for designing the next generation battery.
... Multi-layered materials show strong chemical bonds within the layer but weak van der Waals forces in between 1 , which allows these materials to be physically or chemically thinned to a single atomic two-dimensional (2D) layer 2 . The synthesis of 2D materials has become critical in modern materials research [3][4][5][6][7][8][9][10][11][12][13] because of their unique physicochemical properties that differ from their bulk counterparts. Specifically, these materials with defined geometries exhibit unique shape-dependent properties and have been successfully used in nanoelectronics devices 14 . ...
Transition metal dichalcogenides (TMDCs) are potential materials for future optoelectronic devices. Grain boundaries (GBs) can significantly influence the optoelectronic properties of TMDC materials. Here, we have investigated the mechanical characteristics of tungsten diselenide (WSe2) monolayers and failure process with symmetric tilt GBs using ReaxFF molecular dynamics simulations. In particular, the effects of topological defects, loading rates, and temperatures are investigated. We considered nine different grain boundary structures of monolayer WSe2, of which six are armchair (AC) tilt structures, and the remaining three are zigzag (ZZ) tilt structures. Our results indicate that both tensile strength and fracture strain of WSe2 with symmetric tilt GBs decrease as the temperature increases. We revealed an interfacial phase transition for high-angle GBs reduces the elastic strain energy within the interface at finite temperatures. Furthermore, brittle cracking is the dominant failure mode in the WSe2 monolayer with tilted GBs. WSe2 GB structures showed more strain rate sensitivity at high temperatures than at low temperatures.
... 52 Recently, 2D forms of Ga, In, Sn, Cu, Bi, Pb, and Ag confined between EG and SiC were made possible by CHet. 4,54 Figure 5a illustrates the CHet method for 2D metal synthesis at the EG/SiC interface. First, SiC (0001) is annealed at high temperature to sublime Si atoms and leave C atoms on the surface to recrystallize into a carbon-rich layer (the buffer layer) partially bound to SiC. ...
Since the isolation of graphene in 2004, two-dimensional (2D) materials research has rapidly evolved into an entire subdiscipline in the physical sciences with a wide range of emergent applications. The unique 2D structure offers an open canvas to tailor and functionalize 2D materials through layer number, defects, morphology, moiré pattern, strain, and other control knobs. Through this review, we aim to highlight the most recent discoveries in the following topics: theory-guided synthesis for enhanced control of 2D morphologies, quality, yield, as well as insights toward novel 2D materials; defect engineering to control and understand the role of various defects, including in situ and ex situ methods; and properties and applications that are related to moiré engineering, strain engineering, and artificial intelligence. Finally, we also provide our perspective on the challenges and opportunities in this fascinating field.
... The second method is the bottom-up approach, where a monolayer is directly grown on a substrate. 13 In this case, 2D materials, which do not have 3D analogs or a layered bulk structure, can be synthesized. The experimental search for such novel 2D materials is time-consuming and expensive; therefore, theoretical and computational methods become important in the material discovery field. ...
... The second method is the bottom-up approach, where a monolayer is directly grown on a substrate. 13 In this case, 2D materials, which do not have 3D analogs or a layered bulk structure, can be synthesized. The experimental search for such novel 2D materials is time-consuming and expensive; therefore, theoretical and computational methods become important in the material discovery field. ...
... The subsequent sulphurization for the period of 20 minutes transformed into MoS2 nanosheets on CNF by reducing MoO(3-x). 7, 30,31 Stage II, we deposited the NbS2 on MoS2-CNF to fabricate the NbS2/MoS2-CNF heterostructure which could increase the Sulphur sites as it is vital for effective HER. To initiate the NbS2 nanosheets deposition, we changed the reactor tube from position I to position II by moving the reactor tube towards zone-II until NbCl5 reaches Zone-I (figure S1). ...
There is an urgent need to develop an efficient and non-precious electrode materials for practical electrocatalyst hydro-gen evolution reaction (HER) application to restrain the increasing energy demand. In the present work, we report an efficient and low-cost electrode with high stability for binder-free water electrolysis under all ranges of pH from 0 to 14. Herein, two-dimensional (2D) heterostructure of NbS2/MoS2 ultra-thin vertical nanosheets were grown on carbon nano-fiber with high aspect ratio by one-step chemical vapor deposition approach. The resultant hybrid catalyst demonstrates superlative HER performance with small onset potential (41 mV @ ~0pH, 22 mV @ ~7pH and 32 mV @ ~14pH) and very low overpotential (0.23 V @ ~0pH, 0.21 V @ ~7pH and 0.33 V @ ~14pH to reach 50 mA/cm2) vs. RHE. Besides, the fabricated NbS2/MoS2-CNF displays excellent chronoamperometry stability more than 50 hours in all pH ranges. The proposed heterostructure holds the vital prerequisites for being a significant electrode material owing to multiple HER active edge and planar sulphur sites, excellent barrier free charge transfer ability towards the electrolyte and impressive en-durance. Overall, the 2D/1D hybrid heterostructure appeared to be a precious metal free flexible electrode for excellent HER performance under wide range of pH for water splitting applications.
... Different methods have been adopted, e.g., application of shear among the graphene layers, [8] the saturation of the graphene layers by radical groups, [4] and metallic catalyst. [9,10] Various theoretical and computational methods are utilized to study synthesis of 2D materials [11][12][13][14], specifically the diamondization process, e.g., Molecular Dynamics [7], DFT [4], and ab initio. [11,12] Among these methods, Molecular Dynamics has been widely used to study the phase transformation and surface effects in materials [17][18][19][20][21][22][23][24]. ...
Diamond is the hardest superhard material with excellent optoelectronic, thermomechanical, and electronic properties. Here, we have investigated the possibility of a new synthesis technique for diamane and diamond thin films from multilayer graphene at pressures far below the graphite → diamond transformation pressure. We have used the Molecular Dynamics technique with reactive force fields. Our results demonstrate a significant reduction (by a factor of two) in the multilayer graphene → diamond transformation stress upon using a combined shear and axial compression. The shear deformation in the multilayer graphene lowers the phase transformation energy barrier and plays the role of thermal fluctuations, which itself promotes the formation of diamond. We revealed a relatively weak temperature dependence of the transformation strain and stresses. The transformation stress vs. strain curve for the bulk graphite drops exponentially for finite temperatures.
... Ruzmetov Additional substrates have been explored for TMD epitaxy. For example, the formation of well-aligned vertical MoS 2 fins on 4H-and 6H-SiC substrates was demonstrated by PVT for catalytic applications (68,69). In this case, the epitaxial relationship is MoS 2 [0001]//SiC < 1120 > and arises due to the high MoO 3 :S ratio used for growth; a lower ratio resulted in planar domains. ...
Transition metal dichalcogenide (TMD) monolayers and heterostructures have emerged as a compelling class of materials with transformative properties that may be harnessed for novel device technologies. These materials are commonly fabricated by exfoliation of flakes from bulk crystals, but wafer-scale epitaxy of single-crystal films is required to advance the field. This article reviews the fundamental aspects of epitaxial growth of van der Waals bonded crystals specific to TMD films. The structural and electronic properties of TMD crystals are initially described along with sources and methods used for vapor phase deposition. Issues specific to TMD epitaxy are critically reviewed, including substrate properties and film-substrate orientation and bonding. The current status of TMD epitaxy on different substrate types is discussed along with characterization techniques for large-area epitaxial films. Future directions are proposed, including developments in substrates, in situ and full-wafer characterization techniques, and heterostructure growth.
Expected final online publication date for the Annual Review of Materials Research, Volume 50 is July 1, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
... The second method is the bottom-up approach, where a monolayer is directly grown on a substrate. 13 In this case, 2D materials, which do not have 3D analogs or a layered bulk structure, can be synthesized. The experimental search for such novel 2D materials is time-consuming and expensive; therefore, theoretical and computational methods become important in the material discovery field. ...
Defects in the tin dioxide determine its main characteristics, which are widely used in gas sensors. In the tin dioxide, the native and impurity defects were investigated using the density functional theory. It was shown that the oxygen vacancies dominate between the uncharged defects and the cobalt atoms occupy the places of tin atoms in the case of the cobalt doping. The found defects structures explain the contradictory experimental results obtained previously.
... Monolayer MoS 2 has excellent photoelectric properties due to its high electron mobility and unique band gap structure. The bulk MoS 2 is an indirect bandgap semiconductor with band gap energy about 1.29 eV, band gap energy increases with the decrease of MoS 2 layers number, thus monolayer MoS 2 becomes the direct bandgap semiconductor with 1.90 eV [7][8][9]. Due to the existence of a direct band gap, field effect transistors based on monolayer MoS 2 have higher current switching ratio and electron mobility at room temperature [10,11]. In ...
Two-dimensional transition metal dichalcogenides (TMDs) have attracted attention from researchers in recent years. Monolayer molybdenum disulfide (MoS2) is the direct band gap two-dimensional crystal with excellent physical and electrical properties. Monolayer MoS2 can effectively compensate for the lack of band gap of graphene in the field of nano-electronic devices, which is widely used in catalysis, transistors, optoelectronic devices, and integrated circuits. Therefore, it is critical to obtain high-quality, large size monolayer MoS2. The large-area uniform high-quality monolayer MoS2 is successfully grown on an SiO2/Si substrate with oxygen plasma treatment and graphene quantum dot solution by atmospheric pressure chemical vapor deposition (APCVD) in this paper. In addition, the effects of substrate processing conditions, such as oxygen plasma treatment time, power, and dosage of graphene quantum dot solution on growth quality and the area of the monolayer of MoS2, are studied systematically, which would contribute to the preparation of large-area high-quality monolayer MoS2. Analysis and characterization of monolayer MoS2 are carried out by Optical Microscopy, AFM, XPS, Raman, and Photoluminescence Spectroscopy. The results show that monolayer MoS2 is a large-area, uniform, and triangular with a side length of 200 μm, and it is very effective to treat the SiO2/Si substrate by oxygen plasma and graphene quantum dot solution, which would help the fabrication of optoelectronic devices.
... [27][28][29][30][33][34][35][36][37][38][39][40][41][42][43][44][45] However, only a few theoretical studies have been performed to understand the growth behavior of TMDs due to their complex compound structures. [46][47][48][49][50][51][52] Recently, kinetic Monte Carlo (kMC) simulations have also been employed to examine the growth processes of TMDs. [50][51][52] However, the growth rates that were obtained for compact triangular domains at 1273 K (~10 −6 μm/s, 50~1 0 −5 μm/s, 51~1 0 −4 μm/s 52 ) were much lower than that obtained by Gao et al. 10 (26 μm/s). ...
The ultrafast growth of large-area, high-quality WSe2 domains with a compact triangular morphology has recently been achieved on a gold substrate via chemical vapor deposition. However, the underlying mechanism responsible for ultrafast growth remains elusive. Here, we first analyze growth processes and identify two possible pathways that might achieve ultrafast growth: Path 1, fast edge attachment and ultrafast edge diffusion; Path 2, fast kink nucleation and ultrafast kink propagation. We perform kinetic Monte Carlo simulations and first-principles calculations to assess the viability of these two paths, finding that Path 1 is not viable due to the high edge diffusion barrier calculated from first-principles calculations. Remarkably, Path 2 reproduces all the experimental growth features (domain morphology, domain orientation, and growth rate), and the associated energetic data are consistent with first-principles calculations. The present work unveils the underlying mechanism for the ultrafast growth of WSe2, and may provide a new route for the ultrafast growth of other two-dimensional materials.
... Bottom-up techniques, e.g., chemical vapor deposition (CVD), tend to aggregate material at the most fundamental level (atoms) in a layer-wise process to produce the desired stoichiometry and morphology [90,91]. Several bottom-up techniques have been employed in the fabrication of nanosheets; CVD and atomic layer deposition (ALD) are among the more important techniques [92,93]. ...
Intensive research effort is currently focused on the development of efficient, reliable, and environmentally safe electrochemical energy storage systems due to the ever-increasing global energy storage demand. Li ion battery systems have been used as the primary energy storage device over the last three decades. However, low abundance and uneven distribution of lithium and cobalt in the earth crust and the associated cost of these materials, have resulted in a concerted effort to develop beyond lithium electrochemical storage systems. In the case of non-Li ion rechargeable systems, the development of electrode materials is a significant challenge, considering the larger ionic size of the metal-ions and slower kinetics. Two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides, MXenes and phosphorene, have garnered significant attention recently due to their multi-faceted advantageous properties: large surface areas, high electrical and thermal conductivity, mechanical strength, etc. Consequently, the study of 2D materials as negative electrodes is of notable importance as emerging non-Li battery systems continue to generate increasing attention. Among these interesting materials, graphene has already been extensively studied and reviewed, hence this report focuses on 2D materials beyond graphene for emerging non-Li systems. We provide a comparative analysis of 2D material chemistry, structure, and performance parameters as anode materials in rechargeable batteries and supercapacitors. Open image in new window
... 8,16,17 At the mesoscale, the phase-field approach [18][19][20][21] has been applied to investigating the effect of deposition flux and diffusivity on the island morphologies 20,22,23 as well as the interactions among different islands. 24 At the macroscale, finite element methods (FEM) focused on transport phenomena have been employed to predict the distribution of velocities, concentrations and concentration gradients of precursors during synthesis, 9,25 which have provided useful guidance for experimental synthesis by correlating the concentration gradient distribution with the growth modes of 2D materials. However, since most of these existing investigations are focused on different individual aspects of the entire synthesis process, i.e., atomistic calculations on thermodynamic properties, reaction mechanisms and pathways, mesoscale models on crystal growth kinetics, and FEM on transport phenomena, the coupling and interactions among these different aspects, which are required for developing the direct connection between the CVD control parameters and 2D island growth morphologies, are highly demanding. ...
2D growth: multiscale simulations capture MoS2 CVD growth dynamics A computational multiscale framework is capable of modelling the growth morphology during chemical vapour deposition (CVD) synthesis of MoS2. A team led by Kasra Momeni at Louisiana Tech University and Long-Qing Chen at Pennsylvania State University developed a hierarchical model capable of predicting the size and distribution of atomically thin materials synthetized by CVD in relation to several control parameters of the reactor chamber. The equations governing heat and mass transport in the reactor were solved numerically to determine the thermodynamic state of the precursor and its concentration within the chamber. The resulting concentration distributions were then coupled to the mesoscale phase-field equations describing the growth morphology. This framework can be used to explore the growth diagrams resulting in the formation of 2D materials under specific parameters, or, conversely, to find optimal growth conditions as a function of the desired material morphology.
... Current growth chamber models adopt four major approaches: (1) the rate equations are parameterized experimentally, 117 (2) simplified analytical governing equations are used, 118 (3) an adaptive model is experimentally trained to determine the optimum growth parameters, [119][120][121] and (4) the coupled system of equations governing the growth processes at different spatial and time scales is numerically solved. [122][123][124][125][126][127] Models using the first approach require several trial experiments, where extrapolation to different experimental setups and beyond the specific experimental conditions is difficult. Although the second class of models provides the flexibility for adjusted experimental conditions, it is commonly oversimplified. ...
Two dimensional (2D) materials continue to hold great promise for future electronics, due to their atomic-scale thicknesses and wide range of tunable properties. However, commercial efforts in this field are relatively recent, and much progress is required to fully realize 2D materials for commercial success. Here, we present a roadmap for the realization of electronic-grade 2D materials. We discuss technology drivers, along with key aspects of synthesis and materials engineering required for development of these materials. Additionally, we highlight several fundamental milestones required for realization of electronic-grade 2D materials, and intend this article to serve as a guide for researchers in the field.
... Figure 6d shows the optical image of the uniform MoS 2 monolayers with several centimeters in size, which are grown on a SiO 2 /Si substrate [42]. By using varying growth parameters, the transition between vertical and horizontal growth was achieved [151,152]. ...
Low-dimensional layered transition metal dichalcogenides (TMDs) have recently emerged as an important fundamental research material because of their unique structural, physical and chemical properties. These novel properties make these TMDs a suitable candidate in numerous potential applications. In this review, we briefly summarize the properties of low-dimensional TMDs, and then focus on the various methods used in their preparation. The use of TMDs in electronic devices, optoelectronic devices, electrocatalysts, biosystems, and hydrogen storage is also explored. The cutting-edge future development probabilities of these materials and numerous research challenges are also outlined in this review.
... 19 Wu et al. synthesized high quality MoS 2 ,WS 2 , and multilayer films by the sequential sulfurization of Mo and W metal for vertical heterostructures. 20 Using chemical vapor deposition (CVD), Vil a et al. controlled the orientation of the MoS 2 growth by varying the MoO x :S 2 ratio while sulfurizing MoO 3 at 750 C. 21 Feng et al. controlled the grain size of single layer MoS 2 by controlling the H 2 with MoO 3 and elemental sulfur. 22 In addition to the oxide, MoCl 5 and elemental sulfur have been used to grow monolayer MoS 2 at high temperatures. ...
Molybdenum sulfide films were grown by atomic layer deposition on silicon and fused silica substrates using molybdenum hexafluoride (MoF6) and hydrogen sulfide at 200 °C. In situ quartz crystal microbalance (QCM) measurements confirmed linear growth at 0.46 Å/cycle and self-limiting chemistry for both precursors. Analysis of the QCM step shapes indicated that MoS2 is the reaction product, and this finding is supported by x-ray photoelectron spectroscopy measurements showing that Mo is predominantly in the Mo(IV) state. However, Raman spectroscopy and x-ray diffraction measurements failed to identify crystalline MoS2 in the as-deposited films, and this might result from unreacted MoFx residues in the films. Annealing the films at 350 °C in a hydrogen rich environment yielded crystalline MoS2 and reduced the F concentration in the films. Optical transmission measurements yielded a bandgap of 1.3 eV. Finally, the authors observed that the MoS2 growth per cycle was accelerated when a fraction of the MoF6 pulses were substituted with diethyl zinc.
... The 2D numerical model was developed using the commercial finite element software package COMSOL [31], where the coupled system of heat transfer, equation (1), diffusion, equation (2), and fluid flow, equation (3), are considered [32]. The partial pressure of Mo precursor, i.e. concentration of Mo, and its distribution is the key element in determining the growth mode: vertical versus lateral. ...
Among post-graphene two dimensional (2D) materials, transition metal dichalcogenides (TMDs, such as MoS2) have attracted significant attention due to their superior properties for potential electronic, optoelectronic and energy applications. Scalable and controllable powder vapor transport (PVT) methods have been developed to synthesize 2D MoS2 with controllable morphologies (i.e. horizontal and vertical), yet the growth mechanism for the transition from horizontal to vertical orientation is not clearly understood. Here, we combined experimental and numerical modeling studies to investigate the key growth parameters that govern the morphology of 2D materials. The transition from vertical to horizontal growth is achieved by controlling the magnitude and distribution of the precursor concentration by placing the substrate at different orientations and locations relative to the source. We have also shown that the density of as-grown nanostructures can be controlled by the local precursor-containing gas flow rate. This study demonstrates the possibility for engineering the morphology of 2D materials by controlling the concentration of precursors and flow profiles, and provides a new path for controllable growth of 2D TMDs for various applications.
The low evaporation temperature and carcinogen classification of commonly used molybdenum trioxide (MoO3) precursor render it unsuitable for the safe and practical synthesis of molybdenum disulfide (MoS2). Furthermore, as evidenced by several experimental findings, the associated reaction constitutes a multistep process prone to the formation of uncontrolled amounts of intermediate MoS2−yOy phase mixed with the MoS2 crystals. Here, molybdenum dioxide (MoO2), a chemically more stable and safer oxide than MoO3, was utilized to successfully grow cm-scale continuous films of monolayer MoS2. A high-resolution optical image stitching approach and Raman line mapping were used to confirm the composition and homogeneity of the material grown across the substrate. A detailed examination of the surface morphology of the continuous film revealed that, as the gas flow rate increased by an order of magnitude, the grain-boundary separation dramatically reduced, implying a transition from a kinetically to thermodynamically controlled growth. Importantly, the single-step vapor-phase sulfurization (VPS) reaction of MoO2 was shown to suppress intermediate state formations for a wide range of experimental parameters investigated and is completely absent, provided that the global S:Mo loading ratio is set higher than the stoichiometric ratio of 3:1 required by the VPS reaction.
The layer- and morphology-dependent properties of two-dimensional molybdenum disulfide (MoS2) have established its relevance across broad applications in electronics, optoelectronics, sensing, and catalysis. Understanding how to manipulate the material growth to achieve the desired properties is the key to tailoring the material towards a specific application. In this work, we investigate the growth of vertically standing MoS2 nanosheets by chemical vapor deposition on vicinal and on-axis 4H-SiC (0001) substrates. In both cases the MoS2 flakes exhibit three preferred orientations, aligning with the 〈112¯0〉substrate directions due to strain minimization of a MoO2 intermediate phase. Whereas MoS2 grown on vicinal SiC substrates exhibits strict near-vertical alignment, scanning electron microscopy and near-edge X-ray absorption fine structure (NEXAFS) measurements indicate a near-random vertical orientation when MoS2 is grown on on-axis SiC. Photoemission spectroscopy and NEXAFS measurements indicate the presence of defects and disordered edges which establish the suitability of the material for applications in sensing and catalysis.
Two-dimensional (2D) porous coordination polymers (PCPs) is a kind of porous crystalline material formed by metal nodes and organic ligands through coordination bonds. The PCPs exhibit unique features such as flexible structures, abundant accessible active sites, and high surface areas. The greatest challenge on the commercialization of 2D PCPs and their nano-composites is to achieve a controllable and facile synthesis of 2D PCPs with low-cost and high-quality. Recently, significant progress has been made in the structure and morphological regulation of 2D PCPs. It is still of considerable significance to further explore the synthetic mechanism and unique properties of 2D PCP and their nano-composites. In this review, recent research progress in the synthesis of 2D PCP materials and their applications are summarized. Firstly, the development process of 2D PCP is briefly introduced, then the synthesis strategy of 2D PCP and their nano-composites are discussed. Subsequently, the potential applications of 2D PCP and their nano-composites in electro-catalysis and electronic transport are introduced. Finally, the challenges and prospects for the synthesis and applications of the 2D PCPs and their nano-composites are addressed for future developments.
Two-dimensional materials, including transition-metal dichalcogenides (TMDs), have aroused wide interest due to their various applications in gas sensors and photoelectric devices. However, achieving a uniform distribution of materials on the substrate is still a challenging and urgent problem. In this article, the factors influencing the material distribution during the chemical vapor deposition (CVD) process are investigated by a simulation method, and the results show that the unequal velocity distributions caused by the carrier gas flow on the growing surface play an important role in influencing the crystal distribution. Boundary layer theory is used to explain the mechanism of the effect of the carrier gas flow on the growth uniformity. An improved method is proposed to improve the material uniformity by changing the placement of the Si/SiO2 substrate from face down to face up. The simulation and experimental results show that uniformly distributed MoS2 with a large size and good quality can be obtained with the improved method. This work provides a method for preparing uniformly distributed TMDs on a substrate, which should facilitate subsequent research of two-dimensional materials and promote the application of two-dimensional materials.
The two-dimensional (2D) metal-organic frameworks (MOFs) and their derivatives with excellent dimension-related properties, e.g. high surface areas, abundantly accessible metal nodes, and tailorable structures, have attracted intensive attention as energy storage materials and electrocatalysts. A major challenge on the road toward the commercialization of the 2D MOFs and their derivatives is to achieve facile and controllable synthesis of the 2D MOFs with a high quality and at a low cost. Significant developments have been made in the synthesis and applications of the 2D MOFs and their derivatives in recent years. In this review, we first discuss the state-of-the-art synthetic strategies (including both the top-down and bottom-up approaches) for the 2D MOFs. Subsequently, we review the most recent application progresses of the 2D MOFs and their derivatives in the fields of electrochemical energy storage (e.g., batteries and supercapacitors) and electrocatalysis (of classical reactions such as HER, OER, ORR, and CO2RR). Finally, the challenges and promising strategies for the synthesis and applications of the 2D MOFs and their derivatives are addressed for the future development.
The continuous scaling of transistors has led to unprecedented challenges for interconnect technologies. Conventional barriers fail when thinned below 4 nm; therefore, novel materials and back‐end‐of‐line (BEOL) compatible synthesis are urgently needed. 2D transition metal dichalcogenides present a unique opportunity for addressing the scaling of interconnects. Here, nanometer thick Nb‐incorporated MoS2 is successfully synthesized at BEOL compatible temperatures and their abilities of blocking Cu atom diffusion are investigated. Nb incorporation of MoS2 is systematically studied at 450 °C and its growth dynamics is compared with those carried out at high temperatures. The addition of a few percent Nb in MoS2 enhances breakdown time by more than 100×, reaching a failure time >12 500 s under the electric field of 7 MV cm−1. These results suggest that integration of Nb‐incorporated MoS2 in electronic technologies is a promising route for the sub‐5 nm technology node. In this work, MoS2 thin film synthesized at 450 °C by metal organic chemical vapor deposition is consistently studied. Complementary characterizations are utilized to understand MoS2 thin film microstructure and its impact on copper diffusion barrier performance. A powder‐based doping method is demonstrated with the purpose of incorporating niobium atoms throughout the MoS2 film to better qualify for being copper diffusion barrier.
Transition metal dichalcogenide (TMD) monolayers and heterostructures have emerged as a compelling class of materials with transformative new science that may be harnessed for novel device technologies. These materials are commonly fabricated by exfoliation of flakes from bulk crystals, but wafer-scale epitaxy of single crystal films is required to advance the field. This article reviews the fundamental aspects of epitaxial growth of van der Waals bonded crystals specific to TMD films. The structural and electronic properties of TMD crystals are initially described along with sources and methods used for vapor phase deposition. Issues specific to TMD epitaxy are critically reviewed including substrate properties and film-substrate orientation and bonding. The current status of TMD epitaxy on different substrate types is discussed along with characterization techniques for large area epitaxial films. Future directions are proposed including developments in substrates, in situ and full wafer characterization techniques and heterostructure growth.
Non-Li metal ion rechargeable battery systems e.g. Na, K, Mg, Ca ion systems are at the brink of playing a major role for sustainable energy and grid storage, in part owing to their significant availability as compared to Li-ion rechargeable systems. However, non-Li-based systems pose their own unique set of challenges; the large ionic size of the respective ions especially for Na and K systems, weak kinetics and low voltage window of Mg ion systems etc., which prevents efficient reversibility. Developing efficient electrode materials with novel morphologies is one of the main ways to harness the potential on the non-Li ion systems. It is here that two-dimensional (2D) layered materials which have excellent structural, electrochemical, and mechanical properties can be considered to be prime candidates for negative electrode materials in non-Li-based energy storage systems. Therefore, research in the various aspects of 2D materials encompassing their fabrication techniques, tailoring their morphology, and application as anodes in non-Li systems have significantly increased in recent years, with more expected increase in the future. With this perspective in mind, here we provide an exhaustive review of the structure, properties of various 2D materials (graphene, phosphorene, and transition metal dichalcogenides), their performances as anode materials in emerging non-Li-based energy storage systems, and the obstacles that must be overcome at each stage.
Efficient hydrogen evolution reaction (HER) using two-dimensional layered materials as electrocatalyst with high-permormance remains a challenging task due to the insufficient edge active sites. In this regard, herein, molybdenum disulphide nanosheets with rich active sulphur sites are vertically grown on graphene surface by chemical vapour deposition process. The direct integration of vertically aligned MoS2 nanosheets on graphene forms van der Waals (vdW) heterojunction which facilitates a barrier free charge transport towards the electrolyte as a result of unique and well-matched energy band alignment at the interface. The prospective combination of Ohmic graphene/MoS2 heterostructure and high electrocatalytic edge activity of sulphur deliver an incredible and small turn on potential of 0.14 V vs. RHE in acid electrolyte solution. Most importantly, the use of vertical vdW device architexture exhibits nearly 8X improvement in HER than that of layered counterpart. Besides, the HER reaction is highly stable over 50 hours of continuous run even after 150 days. The combined analysis of our study make certain that the graphene/MoS2 heterostructure will be an efficient alternative electrode for the low-cost and large scale electrochemical applications.
Reduced molybdenum oxide, generally MoO3-x, is a family of conductive metal oxides. During the reduction from MoO3 to various MoO3-x phases, the dielectric material becomes increasingly metallic all the way to MoO2. The interesting property through oxygen control has recently intrigued research of the MoO3-x family in multiple research areas including electrical and electrochemical application. One of the research difficulties in reduced molybdenum oxide is the lack of effective tools to control the oxygen level. Herein, we report a facile synthesis method by temperature-controlled synthesis of triangular MoO2 and square Mo4O11. The triangular and square flakes show a metallic behavior in our DC measurement with a conductivity as high as ~940 S/cm and ~28 S/cm respectively. Further Mott-Schottky analysis of samples regrown on carbon fiber paper (CFP) reveals the hole mobility as high as 105 – 108 cm2 V-1 s-1. The decrease in oxygen level from Mo4O11 to MoO2 affects the density of states mapped in Mo 4d orbitals, leading to a higher conductivity for triangular MoO2. The excessive hole carriers and p-type behavior in Mott-Schottky measurement at high frequencies could be attributed to potential oxygen acceptors and molybdenum vacancies resulted from low reduction power of hydrogen.
In the previous chapter, brief history of the development and fundamentals of 2D materials and vdW heterostructures and the very first methods to isolate them are provided. Graphene can be considered as the funding layer for the field of 2D materials. And we are able to continuously branch out from graphene to other kinds of 2D layers, which sometimes is so called “beyond graphene” 2D layers, and also the sciences and engineering behind them. A heterostructure made of 2D semiconducting materials is an important remark toward flexible and low-power optoelectronics in the future. Analogously, 2D TMDCs represent a new class of building blocks. By combining certain of them, interesting physical sciences and practical applications can be created out of our hands. However, current methods for making a vdW heterostructure may not always provide good material interfaces. This challenge inspired my graduate research on synthetic 2D layers and their heterostructures and discovery of their properties. This chapter covers some practical aspects of thin-film deposition and also methods used for depositing 2D TMDC domains and films. The transport mechanism for 2D material devices is dominated by a few scattering events, which a lot of time are related to the interface of 2D materials and their substrates. This chapter, therefore, provides all necessary knowledges that are not all included in the later chapter which focused on the properties, devices of synthetic 2D layers, 2D/2D vdW heterostructures, and 2D/3D heterostructures.
MoS2-based electrocatalysts are promising cost-effective replacements for Pt-based catalysts for hydrogen evolution by water splitting, yet achieving high current density at low over-potential remains a challenge. Herein, a binder-free electrode of MoS2/CNF (carbon nanofiber) is prepared by electrospinning and subsequent thermal treatment. The growth of MoS2 nanoplates contained within or protruding out from the CNF can be controlled by adding urea or ammonium bicarbonate to the electrospinning precursors, due to the crosslinking effects of urea and the increased porosity caused by pyrolysis of ammonium bicarbonate allowing growth through pores in the CNF. By virtue of the abundant exposed edges in this microstructure and strong bonding between the catalyst and the conductive carbon network, the composite material exhibits ultra-high electrocatalytic hydrogen evolution activity in acidic solutions, with current densities of 500 and 1000 mA/cm2 at overpotentials of 380 and 450 mV, respectively, exceeding the performance of many reported MoS2-based catalysts and even commercial Pt/C catalysts. Thus MoS2/CNF membranes show potential as efficient and flexible binder-free electrodes for electrocatalytic hydrogen production.
This work describes the wafer-scale standing growth of (002)-plane-oriented layers of WS2 and their suitability for use in self-biased broadband high-speed photodetection. The WS2 layers are grown using large-scale sputtering, and the effects of the processing parameters such as the deposition temperature, time, and sputtering power are studied. The structural, physical, chemical, optical, and electrical properties of the WS2 samples are also investigated. Based on the broadband light absorption and high-speed in-plane carrier transport characteristics of the WS2 layers, a self-biased broadband high-speed photodetector is fabricated by forming a type-II heterojunction. This WS2/Si heterojunction is sensitive to ultraviolet, visible, and near-infrared photons and shows an ultrafast photoresponse (1.1 s) along with excellent responsivity (3 mA/W) and specific detectivity (1010 Jones). A comprehensive Mott-Schottky analysis is performed to evaluate the device parameters of the device, such as the frequency-dependent flat-band potential and carrier concentration. Further, the photodetection parameters of the device, such as its linear dynamic range, rising time, and falling time, are evaluated in order to elucidate its spectral and transient characteristics. The device exhibits remarkably improved transient and spectral photodetection performances as compared to those of photodetectors based on atomically thin WS2 and two-dimensional materials. These results suggest that the proposed method is feasible for the manipulation of vertically standing WS2 layers that exhibit high in-plane carrier mobility and allow for high-performance broadband photodetection.
In this data article, vertically grown SnS layers were investigated. The growth processes of vertical SnS layers were discussed in our article [1]. This data article provides the chemical analysis using the XPS measurements for the SnS sample grown on a Si wafer. Deposition time varying SnS morphology changes were observed by FESEM. The cross-sectional images were achieved to monitor the SnS layer thickness. Refractive index of the grown SnS film was calculated using the reflectance data. A self-operating photoelectric was realized with structuring of SnS layers on the n-type Si wafer. Transient photoresponses were achieved by tuning the switching frequencies.
This study achieved wafer-scale, high quality tin monosulfide (SnS) layers. By using a solid-state reaction, the vertically aligned SnS layers spontaneously grew with sulphur reduction from the sputtered SnS2 particles without any post processes. The quality of the SnS vertical layers was observed by a high resolution transmission electron microscopy, which confirmed an interlayer space of 0.56 nm for a perfect matching to the theoretical value. The phase purity of the SnS was confirmed by Raman spectroscopy. The intrinsic energy bandgap value (1.6 eV) of SnS is attractive for photoelectric devices. To form a heterojunction, the vertical SnS layers were grown on a p-type Si substrate. Due to the nanoscale size and vertical standing features of the SnS layers, a significantly low reflection (<5%) was realized for the SnS/n-Si heterojunction device. As a photovoltaic cell, the device provides a higher open circuit voltage (> 300 mV). For photodetection, the response speed is faster than 15 s for near infrared wavelength photons, which is a 1000 times improvement over the horizontal shape device. The vertically standing SnS layers show high photoreactive performance, which confirms that the functional design of 2D materials is an effective route to achieve enhanced photoelectric devices, such as photodetectors and solar cells.
Heterostructures unconstrained by epitaxy have generated considerable excitement due to the discovery of emergent properties - properties not found in either constituent. Heterostructures enable the surfaces on either side of two-dimensional (2D) layers to be used to systematically investigate phenomena such as superconductivity and magnetism in the 2D limit. The ability to choose constituents facilitates the prediction of emergent properties created by the unusual coordination environments at incommensurate interfaces. There have already been many reviews on heterostructures, focusing on a variety of topics that reflect the diverse interest in this area as well as the potential for new technologies. Hence this review focuses mainly on the synthesis and structural characterization of heterostructures containing transition metal dichalcogenides (TMD). This review only briefly discusses 2D materials and TMD/TMD heterostructure devices and the performances that have been achieved. This review provides a historical context for the rapid development of this field and discusses proposed mechanisms for emergent properties. Up to now, the materials used in heterostructures have mainly been materials with 2D structures, as these compounds can be easily cleaved into ultrathin layers. This review discusses the expansion of heterostructure constituents to include materials that do not have 2D structures. Structural changes and charge redistribution between adjacent (or even more distant) layers are likely to be larger for 3D constituents than with 2D constituents based on known misfit layer compounds. Systematic changes in properties with layer thickness, layer sequences, and the identity of constituents will increase our understanding of emergent properties and how they can be optimized.
Nanomaterials have been widely investigated as high performance catalysts due to their extremely enhanced surface-to-volume ratio compared with bulk materials. Among nanomaterials, transition metal dichalcogenides (TMDs) have attracted many interests and been studied as a good candidate for the electrocatalysts due to the capability in tuning their catalytically active sites. One of the primary methods for the synthesis of TMDs is chemical vapour deposition (CVD) which enables the growth of high quality, large-area TMDs with a uniform, atomically thin feature and a layer number controllability. In addition, high degree of freedoms of CVD methods can provide effective routes for realizing various morphologies and structures of synthesized TMD films with versatile catalytic functionalities. This review is intended to deliver a focused overview of CVD growth routes of functional TMDs with catalytic functionality that enables the enhancement of the HER performances. Two growth strategies for the generation of catalytically active sites in functional TMDs are introduced. The first strategy is the activation of TMD basal plane by creating the additional defects such as S vacancies in the post-growth stage. The second is the effective production of catalytic edges by engineering morphologies and structures of CVD-grown TMDs in the mid-growth stage. The resulting HER performances for each CVD growth route are also discussed. This review will provide insights for the design of synthetic schemes and catalytic systems of CVD-grown functional TMDs for high performance HER applications.
Monolayer molybdenum disulphide (MoS$_2$) is a promising two-dimensional (2D) material for nanoelectronic and optoelectronic applications. The large-area growth of MoS$_2$ has been demonstrated using chemical vapor deposition (CVD) in a wide range of deposition temperatures from 600 {\deg}C to 1000 {\deg}C. However, a direct comparison of growth parameters and resulting material properties has not been made so far. Here, we present a systematic experimental and theoretical investigation of optical properties of monolayer MoS$_2$ grown at different temperatures. Micro-Raman and photoluminescence (PL) studies reveal observable inhomogeneities in optical properties of the as-grown single crystalline grains of MoS$_2$. Close examination of the Raman and PL features clearly indicate that growth-induced strain is the main source of distinct optical properties. We carry out density functional theory calculations to describe the interaction of growing MoS$_2$ layers with the growth substrate as the origin of strain. Our work explains the variation of band gap energies of CVD-grown monolayer MoS$_2$, extracted using PL spectroscopy, as a function of deposition temperature. The methodology has general applicability to model and predict the influence of growth conditions on strain in 2D materials.
Edge-oriented MoS2 nanopetals complexed with basal-oriented MoS2 thin films have been mildly grown through a simple atmospheric pressure chemical vapor deposition (APCVD) process with the reaction of MoO3 and S. Dense nanopetals with hexagonal structure exposed numerous chemically reactive edge sites. The roles of growth temperature, time and S/MoO3 mass ratio have been carefully investigated to tune the morphology and density of the as-grown products. Importantly, the carbon nanotube (CNT) films were used as the substrates for growing MoS2 nanopetals. The MoS2/CNT composites, directly as working electrodes, showed remarkable and stable electrocatalytic activity in hydrogen evolution reaction (HER), as manifested with a low oneset overpotential of ~100 mV and a small Tafel slope of 49.5 mV/decade. The development of the MoS2/CNT electrode provides a promising way to fabricate other multifunctional electrodes.
We report on the growth of molybdenum disulphide (MoS2) using H2S as a gas-phase sulfur precursor that allows controlling the domain growth direction of domains in both vertical (perpendicular to the substrate plane) and horizontal (within the substrate plane), depending on the H2S:H2 ratio in the reaction gas mixture and temperature at which they are introduced during growth. Optical and atomic force microscopy measurements on horizontal MoS2 demonstrate the formation of monolayer triangular-shape domains that merge into a continuous film. Scanning transmission electron microscopy of monolayer MoS2 shows a regular atomic structure with a hexagonal symmetry. Raman and photoluminescence spectra confirm the monolayer thickness of the material. Field-effect transistors fabricated on MoS2 domains that are transferred onto Si/SiO2 substrates show a mobility similar to previously reported exfoliated and chemical vapor deposition-grown materials.
Vertically aligned nanosheet heterostructures with partly reduced MoO3 cores and adjustable MoS2 shells were fabricated via two-step chemical vapor deposition (CVD). The as-synthesized MoS2/MoOx heterostructures exhibit enhanced visible-light photocatalytic activity and good compatibility in a wide range of PH values (e.g. 2–12) for the degradation of organic dyes, both of which are of significance for practical applications. The vertically grown nanosheets form a three-dimensional (3D) mesh structure, creating a large specific surface area for the optical absorption and the catalytic redox reaction. In particular, the sulfidation-produced MoS2 coating layer provides an effective protective against photocorrosion in a wide PH window, and meanwhile modulates the energy band structure to promote the absorption of visible-light photons and the separation of photo-generated electron–hole pairs.
Scientists increasingly witness the applications of MoS2 and MoO2 in the field of energy conversion and energy storage. On the one hand, MoS2 and MoO2 have been widely utilized as promising catalysts for electrocatalytic or photocatalytic hydrogen evolution in aqueous solution. On the other hand, MoS2 and MoO2 have also been verified as efficient electrode material for lithium ion batteries. In this review, the synthesis, structure and properties of MoS2 and MoO2 are briefly summarized according to their applications for H2 generation and lithium ion batteries. Firstly, we overview the recent advancements in the morphology control of MoS2 and MoO2 and their applications as electrocatalysts for hydrogen evolution reactions. Secondly, we focus on the photo-induced water splitting for H2 generation, in which MoS2 acts as an important co-catalyst when combined with other semiconductor catalysts. The newly reported research results of the significant functions of MoS2 nanocomposites in photo-induced water splitting are presented. Thirdly, we introduce the advantages of MoS2 and MoO2 for their enhanced cyclic performance and high capacity as electrode materials of lithium ion batteries. Recent key achievements in MoS2- and MoO2-based lithium ion batteries are highlighted. Finally, we discuss the future scope and the important challenges emerging from these fascinating materials.
Two-dimensional materials are an emerging class of new materials with a wide range of electrical properties and potential practical applications. Although graphene is the most well-studied two-dimensional material, single layers of other materials, such as insulating BN (ref. 2) and semiconducting MoS2 (refs 3, 4) or WSe2 (refs 5, 6), are gaining increasing attention as promising gate insulators and channel materials for field-effect transistors. Because monolayer MoS2 is a direct-bandgap semiconductor due to quantum-mechanical confinement, it could be suitable for applications in optoelectronic devices where the direct bandgap would allow a high absorption coefficient and efficient electron-hole pair generation under photoexcitation. Here, we demonstrate ultrasensitive monolayer MoS2 phototransistors with improved device mobility and ON current. Our devices show a maximum external photoresponsivity of 880 A W(-1) at a wavelength of 561 nm and a photoresponse in the 400-680 nm range. With recent developments in large-scale production techniques such as liquid-scale exfoliation and chemical vapour deposition-like growth, MoS2 shows important potential for applications in MoS2-based integrated optoelectronic circuits, light sensing, biomedical imaging, video recording and spectroscopy.
The controlled synthesis of highly crystalline MoS2 atomic layers remains a challenge for the practical applications of this emerging material. Here, we developed an approach for synthesizing MoS2 flakes in rhomboid shape with controlled number of layers by the layer-by-layer sulfurization of MoO2 microcrystals. The obtained MoS2 flakes showed high crystallinity with crystal domain size of ~10 μm, significantly larger than the grain size of MoS2 grown by other methods. As a result of the high crystallinity, the performance of back-gated field effect transistors (FETs) made on these MoS2 flakes was comparable to FETs based on mechanically exfoliated flakes. This simple approach opens up a new avenue for controlled synthesis of MoS2 atomic layers, and will make this highly crystalline material easily accessible for fundamental aspects and various applications.
The remarkable properties of graphene have renewed interest in inorganic, two-dimensional materials with unique electronic and optical attributes. Transition metal dichalcogenides (TMDCs) are layered materials with strong in-plane bonding and weak out-of-plane interactions enabling exfoliation into two-dimensional layers of single unit cell thickness. Although TMDCs have been studied for decades, recent advances in nanoscale materials characterization and device fabrication have opened up new opportunities for two-dimensional layers of thin TMDCs in nanoelectronics and optoelectronics. TMDCs such as MoS(2), MoSe(2), WS(2) and WSe(2) have sizable bandgaps that change from indirect to direct in single layers, allowing applications such as transistors, photodetectors and electroluminescent devices. We review the historical development of TMDCs, methods for preparing atomically thin layers, their electronic and optical properties, and prospects for future advances in electronics and optoelectronics.
Controlling surface structure at the atomic scale is paramount to developing effective catalysts. For example, the edge sites of MoS(2) are highly catalytically active and are thus preferred at the catalyst surface over MoS(2) basal planes, which are inert. However, thermodynamics favours the presence of the basal plane, limiting the number of active sites at the surface. Herein, we engineer the surface structure of MoS(2) to preferentially expose edge sites to effect improved catalysis by successfully synthesizing contiguous large-area thin films of a highly ordered double-gyroid MoS(2) bicontinuous network with nanoscaled pores. The high surface curvature of this catalyst mesostructure exposes a large fraction of edge sites, which, along with its high surface area, leads to excellent activity for electrocatalytic hydrogen evolution. This work elucidates how morphological control of materials at the nanoscale can significantly impact the surface structure at the atomic scale, enabling new opportunities for enhancing surface properties for catalysis and other important technological applications.
The isolation of graphene in 2004 from graphite was a defining moment for the "birth" of a field: Two-dimensional (2D) materials. In recent years, there has been a rapidly increasing number of papers focusing on non-graphene layered materials, including transition-metal dichalcogenides (TMDs), because of the new properties and applications that emerge upon 2D confinement. Here we review significant recent advances and important new developments in 2D materials "beyond graphene". We provide insight into the theoretical modeling and understanding of the van der Waals (vdW) forces that hold together the 2D layers in bulk solids, as well as their excitonic properties and growth morphologies. Additionally, we highlight recent breakthroughs in TMD synthesis and characterization and discuss the newest families of 2D materials, including monoelement 2D materials (i.e., silicene, phosphorene, etc.) and transition metal carbide- and carbon nitride-based MXenes. We then discuss the doping and functionalization of 2D materials beyond graphene that enable device applications, followed by advances in electronic, optoelectronic, and magnetic devices and theory. Finally, we provide perspectives on the future of 2D materials beyond graphene.
The precise control of edge geometry and crystal shape of monolayer MoS2 is particular of importance for their applications in nanoelectronics and photo-electro catalysts. Here we reveal a crucial role of chemical potential in the determination of equilibrium shape (ES) and edge structure of monolayer MoS2 by using density-functional theory calculations. Applying Wulff construction rule, our results demonstrate the shape evolution of monolayer MoS2 flake from the dodecagonal shape, then to the hexagonal shape, to the triangular shape with the variation of chemical potential from the Mo-rich to the S-rich condition, and the edge structure of ES changes correspondingly from mixed zigzag/armchair edges to pure zigzag edges. This finding can be applied to explain extensive experimental observations about the morphology of MoS2 domains. Meanwhile, the edge magnetism and electronic structures of monolayer MoS2 domains are found to be dependent on their edge structure, which provides specific guidance for the magnetic modulation of monolayer MoS2 and designing more effective MoS2-based catalysts.
Conspectus In the wake of the discovery of the remarkable electronic and physical properties of graphene, a vibrant research area on two-dimensional (2D) layered materials has emerged during the past decade. Transition metal dichalcogenides (TMDs) represent an alternative group of 2D layered materials that differ from the semimetallic character of graphene. They exhibit diverse properties that depend on their composition and can be semiconductors (e.g., MoS2, WS2), semimetals (e.g., WTe2, TiSe2), true metals (e.g., NbS2, VSe2), and superconductors (e.g., NbSe2, TaS2). The properties of TMDs can also be tailored according to the crystalline structure and the number and stacking sequence of layers in their crystals and thin films. For example, 2H-MoS2 is semiconducting, whereas 1T-MoS2 is metallic. Bulk 2H-MoS2 possesses an indirect band gap, but when 2H-MoS2 is exfoliated into monolayers, it exhibits direct electronic and optical band gaps, which leads to enhanced photoluminescence. Therefore, it is important to learn to control the growth of 2D TMD structures in order to exploit their properties in energy conversion and storage, catalysis, sensing, memory devices, and other applications. In this Account, we first introduce the history and structural basics of TMDs. We then briefly introduce the Raman fingerprints of TMDs of different layer numbers. Then, we summarize our progress on the controlled synthesis of 2D layered materials using wet chemical approaches, chemical exfoliation, and chemical vapor deposition (CVD). It is now possible to control the number of layers when synthesizing these materials, and novel van der Waals heterostructures (e.g., MoS2/graphene, WSe2/graphene, hBN/graphene) have recently been successfully assembled. Finally, the unique optical, electrical, photovoltaic, and catalytic properties of few-layered TMDs are summarized and discussed. In particular, their enhanced photoluminescence (PL), photosensing, photovoltaic conversion, and hydrogen evolution reaction (HER) catalysis are discussed in detail. Finally, challenges along each direction are described. For instance, how to grow perfect single crystalline monolayer TMDs without the presence of grain boundaries and dislocations is still an open question. Moreover, the morphology and crystal structure control of few-layered TMDs still requires further research. For wet chemical approaches and chemical exfoliation methods, it is still a significant challenge to control the lateral growth of TMDs without expansion in the c-axis direction. In fact, there is plenty of room in the 2D world beyond graphene. We envisage that with increasing progress in the controlled synthesis of these systems the unusual properties of mono- and few-layered TMDs and TMD heterostructures will be unveiled.
Two-dimensional (2D), layered transition metal dichalcogenides (TMDCs) can grow in two different growth directions, that is, horizontal and vertical. In the horizontal growth, 2D TMDC layers grow in planar direction with their basal planes parallel to growth substrates. In the vertical growth, 2D TMDC layers grow standing upright on growth substrates exposing their edge sites rather than their basal planes. The two distinct morphologies present unique materials properties suitable for specific applications, such as horizontal TMDCs for optoelectronics and vertical TMDCs for electrochemical reactions. Precise control of the growth orientation is essential for realizing the true potential of these 2D materials for large-scale, practical applications. In this Letter, we investigate the transition of vertical-to-horizontal growth directions in 2D molybdenum (or tungsten) disulfide and study the underlying growth mechanisms and parameters that dictate such transition. We reveal that the thickness of metal seed layers plays a critical role in determining their growth directions. With thick (>∼3 nm) seed layers, the vertical growth is dominant, while the horizontal growth occurs with thinner seed layers. This finding enables the synthesis of novel 2D TMDC heterostructures with anisotropic layer orientations and transport properties. The present study paves a way for developing a new class of 2D TMDCs with unconventional materials properties.
Atmospheric-pressure chemical vapor deposi-tion (CVD) is used to grow monolayer MoS 2 two-dimensional crystals at elevated temperatures on silicon substrates with a 300 nm oxide layer. Our CVD reaction is hydrogen free, with the sulfur precursor placed in a furnace separate from the MoO 3 precursor to individually control their heating profiles and provide greater flexibility in the growth recipe. We intentionally establish a sharp gradient of MoO 3 precursor concentration on the growth substrate to explore its sensitivity to the resultant MoS 2 domain growth within a relatively uniform temperature range. We find that the shape of MoS 2 domains is highly dependent upon the spatial location on the silicon substrate, with variation from triangular to hexagonal geometries. The shape change of domains is attributed to local changes in the Mo:S ratio of precursors (1:>2, 1:2, and 1:<2) and its influence on the kinetic growth dynamics of edges. These results improve our understanding of the factors that influence the growth of MoS 2 domains and their shape evolution. I n recent years, two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs), including MX 2 (M = Mo, W; X = S, Se), have attracted a great deal of attention because of their unique structure as well as remarkable physical and chemical properties. 1−3 As a member of the TMD family, molybdenum disulfide (MoS 2) with a direct band gap of 1.8 eV for a single molecular layer is complementary to the zero-bandgap graphene, with potential in catalysis, valleytronic applications, nanoelectronics, and optoelectonic devices. 2−7
With advances in exfoliation and synthetic techniques, atomically thin films of semiconducting transition metal dichalcogenides have recently been isolated and characterized. Their two-dimensional structure, coupled with a direct band gap in the visible portion of the electromagnetic spectrum, suggests suitability for digital electronics and optoelectronics. Towards that end, several classes of high-performance devices have been reported along with significant progress in understanding their physical properties. Here, we present a review of the architecture, operating principles, and physics of electronic and optoelectronic devices based on ultrathin transition metal dichalcogenide semiconductors. By critically assessing and comparing the performance of these devices with competing technologies, the merits and shortcomings of this emerging class of electronic materials are identified, thereby providing a roadmap for future development.
Well-defined ultrathin MoS2 nanoplates are developed by a facile solvent-dependent control route from single source precursor for the first time. The obtained ultrathin nanoplate with a thickness of ~5nm features high density of basal edges and abundant unsaturated active S atoms. The multistage growth process is investigated and the formation mechanism is proposed. Ultrathin MoS2 nanoplates exhibit an excellent activity for hydrogen evolution reaction (HER) with a small onset potential of 0.09 V, a low Tafel slope of 53 mV dec-1 and remarkable stability. This work successfully demonstrates that the introduction of unsaturated active S atoms into ultrathin MoS2 nanoplates for enhanced electrocatalytic properties is feasible through a facial one-step solvent control method, and that this may open up a potential way for designing more efficient MoS2-based catalysts for HER.
Homogeneous large-area graphene monolayers were successfully prepared ex situ on 6H-SiC(0001). The samples have been studied systematically and the results are compared with those from a sample cut from the same wafer and prepared by in situ heating. The formation of smaller graphene flakes was found on the in situ prepared sample, which is in line with earlier observations. Distinctly different results are observed from the ex situ graphene layers of different thicknesses, which are proposed as a guideline for determining graphene growth. Recorded C 1s spectra consisted of three components: bulk SiC, graphene (G), and interface (I), the latter being a 6√3 layer. Extracted intensity ratios of G/I were found to give a good estimate of the thickness of graphene. Differences are also revealed in micro low energy electron diffraction images and electron reflectivity curves. The diffraction patterns were distinctly different from a monolayer thickness up to three layers. At a larger thickness only the graphitelike spot was visible. The electron reflectivity curve showed a nice oscillation behavior with kinetic energy and as a function of the number of graphene layers. The graphene sheets prepared were found to be very inert and the interface between the substrate and the layer(s) was found to be quite abrupt. No free Si could be detected in or on the graphene layers or at the interface.
Layered materials consist of molecular layers stacked together by weak interlayer interactions. They often crystallize to form smooth thin films, nanotubes, and platelet or fullerene-like nanoparticles due to the anisotropic bonding. Structures that predominately expose edges of the layers exhibit high surface energy and are often considered unstable. In this communication, we present a synthesis process to grow MoS2 and MoSe2 thin films with vertically aligned layers, thereby maximally exposing the edges on the film surface. Such edge-terminated films are metastable structures of MoS2 and MoSe2, which may find applications in diverse catalytic reactions. We have confirmed their catalytic activity in hydrogen evolution reaction (HER), in which the exchange current density correlates directly with the density of the exposed edge sites.
MoS(2) nanosheet-coated TiO(2) nanobelt heterostructures-referred to as TiO(2) @MoS(2) -with a 3D hierarchical configuration are prepared via a hydrothermal reaction. The TiO(2) nanobelts used as a synthetic template inhibit the growth of MoS(2) crystals along the c-axis, resulting in a few-layer MoS(2) nanosheet coating on the TiO(2) nanobelts. The as-prepared TiO(2) @MoS(2) heterostructure shows a high photocatalytic hydrogen production even without the Pt co-catalyst. Importantly, the TiO(2) @MoS(2) heterostructure with 50 wt% of MoS(2) exhibits the highest hydrogen production rate of 1.6 mmol h(-1) g(-1) . Moreover, such a heterostructure possesses a strong adsorption ability towards organic dyes and shows high performance in photocatalytic degradation of the dye molecules.
X-ray photoelectron spectroscopy (XPS) and laser Raman spectroscopy (LRS) have been used to examine the thermal reduction products of thin-film molybdenum trioxide under flowing hydrogen or nitrogen gases. Some of the LRS measurements were made in situ during the reduction reaction using a custom quartz cell reactor. Thermal reduction of MoO{sub 3} was studied over the range 350-730 {degrees}C. At most temperatures, MoO{sub 2} and MoO{sub 3} were the only species measured by LRS. Two intermediate molybdenum species, believed to be Mo(IV) and Mo(V), were detected by XPS, along with MoO{sub 2} and MoO{sub 3}. In one set of sample reduced in the 450 {degrees}C range, localized reduction could be attributed to the presence of impurities. From these impurities, a chemical {open_quotes}front{close_quotes} is propagated, and at the front, a metastable species intermediate between MoO{sub 3} and MoO{sub 2} is identified. 42 refs., 8 figs., 2 tabs.
The stability of the shape of a spherical particle undergoing diffusion‐controlled growth into an initially uniformly supersaturated matrix is studied by supposing an expansion, into spherical harmonics, of an infinitesimal deviation of the particle from sphericity and then calculating the time dependence of the coefficients of the expansion. It is assumed that the pertinent concentration field obeys Laplace's equation, an assumption whose conditions of validity are discussed in detail and are often satisfied in practice. A dispersion law is found for the rate of change of the amplitude of the various harmonics. It is shown that the sphere is stable below and unstable above a certain radius R c , which is just seven times the critical radius of nucleation theory; analogous conclusions are obtained for the solidification problem. The results for the sphere are used to discuss the stability of nonspherical growth forms.
Large-area MoS(2) atomic layers are synthesized on SiO(2) substrates by chemical vapor deposition using MoO(3) and S powders as the reactants. Optical, microscopic and electrical measurements suggest that the synthetic process leads to the growth of MoS(2) monolayer. The TEM images verify that the synthesized MoS(2) sheets are highly crystalline.
Inorganic solids are an important class of catalysts that often derive their activity from sparse active sites that are structurally
distinct from the inactive bulk. Rationally optimizing activity is therefore beholden to the challenges in studying these
active sites in molecular detail. Here, we report a molecule that mimics the structure of the proposed triangular active edge
site fragments of molybdenum disulfide (MoS2), a widely used industrial catalyst that has shown promise as a low-cost alternative to platinum for electrocatalytic hydrogen
production. By leveraging the robust coordination environment of a pentapyridyl ligand, we synthesized and structurally characterized
a well-defined MoIV-disulfide complex that, upon electrochemical reduction, can catalytically generate hydrogen from acidic organic media as
well as from acidic water.
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