Growth Of Transition Metal Dichalcogenides Research Articles

Synthesis of Few-Layered Transition-Metal Dichalcogenides by Ion Implantation of Chalcogen and Metal Species into Sapphire.

The growth of transition-metal dichalcogenides (TMDCs) has been performed so far using most established thin-film growth techniques (e.g., vapor phase transport, chemical vapor deposition, molecular beam epitaxy, etc.). However, because there exists no self-limiting mechanism for the growth of TMDCs, none of these techniques allows precise control of the number of TMDC layers over large substrate areas. Here, we explore the ion implantation of the parent TMDC atoms into a chemically neutral substrate for the synthesis of TMDC films. The idea is that once all of the ion-implanted species have reacted together, the synthesis reaction stops, thereby effectively stopping growth. In other words, even if there is no self-limiting mechanism, growth stops when the nutrients are exhausted. We have co-implanted Mo and S ions into c-oriented sapphire substrates using various doses corresponding to 1- to 5-layer atom counts. We find that the subsurface region of the sapphire substrates is amorphized by the ion implantation process, at least for implanted doses of 2-layer atom counts and over. For all doses, we have observed the formation of MoS2 material inside the sapphire after postimplantation annealing between 800 and 850 °C. We report that the order of implantation (i.e., whether S or Mo is implanted first) is an important parameter. More precisely, samples for which S is implanted first tend to yield thin crystals with a large lateral extension (more than 200 nm for 5-layer doses) and mainly located at the interface between the amorphized and crystalline sapphire. When Mo is first implanted, the MoS2 crystals still predominantly appear at the amorphous-crystalline interface (which is much rougher), but they are much thicker, suggesting a different nucleation mechanism.

Controllable and Reproducible Growth of Transition Metal Dichalcogenides by Design of Experiments

AbstractControllable and reproducible synthesis of 2D materials is crucial for their future applications. Chemical vapor deposition (CVD) promises scalable and high‐quality growth of 2D materials. However, to optimize CVD growth, multiple parameters have to be carefully selected. Design of experiments (DoE) is a consistent and versatile tool to optimize all parameters simultaneously in a controlled way. This study exploits DoE statistical approaches to show how the CVD growth of transition metal dichalcogenides (TMDs) can be optimized, using tungsten disulfide as an example. A designed set of 29 different processes is used to cover the entire parameter space. The resulting growth output is characterized in terms of material morphology for factors such as single crystal size and continuous film size. The nonlinear model used to fit the output as a function of input parameters provides crucial insights into the nontrivial CVD process ensuring easy and systematic growth optimization. The predicted processes show successful optimization with respect to both the resulting material and the process stability. This powerful technique can be adapted for different setups and other TMD materials.

Y‐Stabilized ZrO2 as a Promising Wafer Material for the Epitaxial Growth of Transition Metal Dichalcogenides

Y‐stabilized ZrO2 (YSZ) as a promising single‐crystal wafer material for the epitaxial growth of transition metal dichalcogenides applicable for both physical (PVD) and chemical vapor deposition (CVD) processes is used. MoS2 layers grown on YSZ (111) exhibit sixfold symmetry and in‐plane epitaxial relationship with the wafer of () MoS2 || () YSZ. The PVD‐grown submonolayer thin films show nucleation of MoS2 islands with a lateral size of up to 100 nm and a preferential alignment along the substrate step edges. The layers exhibit a strong photoluminescence yield as expected for the 2H‐phase of MoS2 in a single monolayer limit. The CVD‐grown samples are composed of triangular islands of several micrometers in size in the presence of antiparallel domains. The results represent a promising route toward fabrication of wafer‐scale single‐crystalline transition metal dichalcogenide layers with a tunable layer thickness on commercially available wafers.

In situ strain-induced phase transition and defect engineering in CVD-synthesized atomically thin MoS2

Alkali metal halides have recently received great attention as additives in the chemical vapor deposition (CVD) process to promote the growth of transition metal dichalcogenides (TMDs). However, the multi-faceted role of these halide salts in modulating the properties and quality of TMD monolayers remains mechanistically unclear. In this study, by introducing excessive gaseous sodium chloride (NaCl) into the CVD system, we demonstrate that preferential NaCl deposition along the monolayer edges causes large in situ strain that can invoke localized domains of high defect density and 2H to 1T phase transition. High-resolution scanning transmission electron microscopy, Raman mapping and molecular dynamics simulations revealed that higher NaCl concentrations can promote the coalescence of independent local strain domains, further increasing the 1T/2H phase ratio and defect density. Furthermore, excessive NaCl was also proven by density functional theory calculations to convert thermodynamic growth to kinetic growth, accounting for the unique cloud-shaped MoS2 crystals acquired. Compared with post-growth strain processing methods, this one-step approach for phase and defect engineering not only represents a deeper understanding of the role that NaCl plays in the CVD process, but also provides a convenient means to controllably synthesize conductive/defect-rich materials for further electrocatalysis and optoelectronic applications.

Tungsten Oxide Mediated Quasi-van der Waals Epitaxy of WS2 on Sapphire.

Conventional epitaxy plays a crucial role in current state-of-the art semiconductor technology, as it provides a path for accurate control at the atomic scale of thin films and nanostructures, to be used as the building blocks in nanoelectronics, optoelectronics, sensors, etc. Four decades ago, the terms "van der Waals" (vdW) and "quasi-vdW (Q-vdW) epitaxy" were coined to explain the oriented growth of vdW layers on 2D and 3D substrates, respectively. The major difference with conventional epitaxy is the weaker interaction between the epi-layer and the epi-substrates. Indeed, research on Q-vdW epitaxial growth of transition metal dichalcogenides (TMDCs) has been intense, with oriented growth of atomically thin semiconductors on sapphire being one of the most studied systems. Nonetheless, there are some striking and not yet understood differences in the literature regarding the orientation registry between the epi-layers and epi-substrate and the interface chemistry. Here we study the growth of WS2 via a sequential exposure of the metal and the chalcogen precursors in a metal-organic chemical vapor deposition (MOCVD) system, introducing a metal-seeding step prior to the growth. The ability to control the delivery of the precursor made it possible to study the formation of a continuous and apparently ordered WO3 mono- or few-layer at the surface of a c-plane sapphire. Such an interfacial layer is shown to strongly influence the subsequent quasi-vdW epitaxial growth of the atomically thin semiconductor layers on sapphire. Hence, here we elucidate an epitaxial growth mechanism and demonstrate the robustness of the metal-seeding approach for the oriented formation of other TMDC layers. This work may enable the rational design of vdW and quasi-vdW epitaxial growth on different material systems.

Crystal violet as CMOS-compatible alkali-free promoter for CVD growth of MoSe2 monolayers: Comparative surface analysis with alkali-based promoter

The wafer-level growth of transition metal dichalcogenides (TMDs) using chemical vapor deposition (CVD) is essential for industrial applications involving complementary metal-oxide-semiconductor (CMOS) processes. In the CVD processes of TMDs, promoters containing alkali elements such as Na and K are typically used to obtain high-quality crystals. However, the use of alkali elements is rigorously restricted in CMOS processes because of contamination problems. In this study, we investigated crystal violet (CV) as a promising alkali-free promoter in the CVD growth of MoSe2. Particularly, the surface properties were intensively examined by optical, chemical, and topographical analyses and compared to those of samples prepared with NaOH as a representative alkali-based promoter. Na residues causing a doping effect were observed in the NaOH process, whereas the doping effect was negligible in the CV process. After the transfer process from the as-grown state, the strain effect in the NaOH process was prominently changed, resulting in wrinkles on MoSe2, whereas the wrinkles were negligible in the CV process, suggesting relatively smaller strain accumulation in the CV process during crystal growth. Our results showed the feasibility of CV as an alkali-free promoter for utilizing CVD-grown TMDs in industrial CMOS processes.

Non-epitaxial single-crystal 2D material growth by geometric confinement.

Two-dimensional (2D) materials and their heterostructures show a promising path for next-generation electronics1-3. Nevertheless, 2D-based electronics have not been commercialized, owing mainly to three critical challenges: i) precise kinetic control of layer-by-layer 2D material growth, ii) maintaining a single domain during the growth, and iii) wafer-scale controllability of layer numbers and crystallinity. Here we introduce a deterministic, confined-growth technique that can tackle these three issues simultaneously, thus obtaining wafer-scale single-domain 2D monolayer arrays and their heterostructures on arbitrary substrates. We geometrically confine the growth of the first set of nuclei by defining a selective growth area via patterning SiO2 masks on two-inch substrates. Owing to substantial reduction of the growth duration at the micrometre-scale SiO2 trenches, we obtain wafer-scale single-domain monolayer WSe2 arrays on the arbitrary substrates by filling the trenches via short growth of the first set of nuclei, before the second set of nuclei is introduced, thus without requiring epitaxial seeding. Further growth of transition metal dichalcogenides with the same principle yields the formation of single-domain MoS2/WSe2 heterostructures. Our achievement will lay a strong foundation for 2D materials to fit into industrial settings.

Phase-Field Modeling of Chemical Vapor-Deposited 2D MoSe2 Domains with Varying Morphology for Electronic Devices and Catalytic Applications

Two-dimensional (2D) crystalline growth of transition metal dichalcogenides (TMDs) by atmospheric pressure chemical vapor deposition (APCVD) is highly sensitive to any changes in growth parameters. At an optimized growth temperature and transition metal flux, 2D compact domains grow primarily triangular/hexagonal in shape that transform into dendritic structures at higher transition metal flux. With changes in the local flux (or, local chalcogen-to-transition metal vapor ratio), domain morphologies on the substrate vary with distance for locations away from the center. In this work, phase field simulations are carried out to emulate experimentally observed morphology evolution as a function of transition metal flux. Our model demonstrates the critical roles of precursor flux and attachment time in controlling the domain morphologies which is further established by fractal dimension analysis. Evolution of patterns simulated as a function of flux and attachment time can help to identify more precise combination of growth parameters leading to a specific growth mode. Consistent with the experimental observations, the model also reproduces the interaction among multiple domains. Our findings could be useful for achieving controlled growth of 2D domains with desired shape (compact or dendritic) as required for the next-generation electronic and optoelectronic devices, and efficient catalytic applications.

Patterned Growth of Transition Metal Dichalcogenide Monolayers and Multilayers for Electronic and Optoelectronic Device Applications.

A simple, large area, and cost-effective soft lithographic method is presented for the patterned growth of high-quality 2D transition metal dichalcogenides (TMDs). Initially, a liquid precursor (Na2 MoO4 in an aqueous solution) is patterned on the growth substrate using the micromolding in capillaries technique. Subsequently, a chemical vapor deposition step is employed to convert the precursor patterns to monolayer, few layers, or bulk TMDs, depending on the precursor concentration. The grown patterns are characterized using optical microscopy, atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and photoluminescence spectroscopy to reveal their morphological, chemical, and optical characteristics. Additionally, electronic and optoelectronic devices are realized using the patterned TMDs and tested for their applicability in field effect transistors and photodetectors. The photodetectors made of MoS2 line patterns show a very high responsivity of 7674 A W-1 and external quantum efficiency of 1.49 × 106 %. Furthermore, the multiple grain boundaries present in patterned TMDs enable the fabrication of memtransistor devices. The patterning technique presented here may be applied to many other TMDs and related heterostructures, potentially advancing the fabrication of TMDs-based device arrays.

Ultrahigh Sulfur Loading Tolerant Cathode Architecture with Extended Cycle Life for High Energy Density Lithium–Sulfur Batteries

AbstractLithium–sulfur batteries are regarded as the imminent energy storage device for high energy density applications. However, at practical sulfur loadings >5 mg cm−2, the cell suffers from severe capacity fade and durability. In the present work, a hybrid MoS2–WS2 heterodimensional structure is reported. The strain induced growth of transition metal dichalcogenides preferentially exposes edge sites and maximizes the geometric coverage for anchoring‐diffusion‐conversion of polysulfides to restrain the shuttle effect at practical S‐loadings. The systematic analysis (5–50 mg cm−2 of S‐loadings) reveals that the unique cathode architecture exhibits reversible S‐loading tolerance up to 28 mg cm−2. A high initial areal capacity of 32 mAh cm−2 with an area specific energy density of 67 mWh cm−2 is achieved with a low electrolyte volume/S‐loading ratio of 5 mL g−1. The strategy presented here can unlock high S‐loading Li–S cells with extended cyclability and high energy density.

Strategies for Controlled Growth of Transition Metal Dichalcogenides by Chemical Vapor Deposition for Integrated Electronics.

In recent years, transition metal dichalcogenide (TMD)-based electronics have experienced a prosperous stage of development, and some considerable applications include field-effect transistors, photodetectors, and light-emitting diodes. Chemical vapor deposition (CVD), a typical bottom-up approach for preparing 2D materials, is widely used to synthesize large-area 2D TMD films and is a promising method for mass production to implement them for practical applications. In this review, we investigate recent progress in controlled CVD growth of 2D TMDs, aiming for controlled nucleation and orientation, using various CVD strategies such as choice of precursors or substrates, process optimization, and system engineering. We then survey different patterning methods, such as surface patterning, metal precursor patterning, and postgrowth sulfurization/selenization/tellurization, to mass produce heterostructures for device applications. With these strategies, various well-designed architectures, such as wafer-scale single crystals, vertical and lateral heterostructures, patterned structures, and arrays, are achieved. In addition, we further discuss various electronics made from CVD-grown TMDs to demonstrate the diverse application scenarios. Finally, perspectives regarding the current challenges of controlled CVD growth of 2D TMDs are also suggested.

Monolayer MoS2 of high mobility grown on SiO2 substrate by two-step chemical vapor deposition

We report a novel two-step ambient pressure chemical vapor deposition (CVD) pathway to grow high-quality MoS2 monolayer on the SiO2 substrate with large crystal size up to 110 μm. The large specific surface area of the pre-synthesized MoO3 flakes on the mica substrate compared to MoO3 powder could dramatically reduce the consumption of the Mo source. The electronic information inferred from the four-probe scanning tunneling microscope (4P-STM) image explains the threshold voltage variations and the n-type behavior observed in the two-terminal transport measurements. Furthermore, the direct van der Pauw transport also confirms its relatively high carrier mobility. Our study provides a reliable method to synthesize high-quality MoS2 monolayer, which is confirmed by the direct 4P-STM measurement results. Such methodology is a key step toward the large-scale growth of transition metal dichalcogenides (TMDs) on the SiO2 substrate and is essential to further development of the TMDs-related integrated devices.

Wafer-scale monolithic integration of full-colour micro-LED display using MoS2 transistor.

Large-scale growth of transition metal dichalcogenides and their subsequent integration with compound semiconductors is one of the major obstacles for two-dimensional materials implementation in optoelectronics applications such as active matrix displays or optical sensors. Here we present a novel transition metal dichalcogenide-on-compound-semiconductor fabrication method that is compatible with a batch microfabrication process. We show how a thin film of molybdenum disulfide (MoS2) can be directly synthesized on a gallium-nitride-based epitaxial wafer to form a thin film transistor array. Subsequently, the MoS2 thin film transistor was monolithically integrated with micro-light-emitting-diode (micro-LED) devices to produce an active matrix micro-LED display. In addition, we demonstrate a simple approach to obtain red and green colours through the printing of quantum dots on a blue micro-LED, which allows for the scalable fabrication of full-colour micro-LED displays. This strategy represents a promising route to attain heterogeneous integration, which is essential for high-performance optoelectronic systems that can incorporate the established semiconductor technology and emerging two-dimensional materials.

In-depth characterization of stacking faults forming during the growth of Transition-Metal Di-Chalcogenides (TMDCs) by ambient pressure-CVD

Transition-metal di-chalcogenides (TMDCs) are van der Waals (vdW) layered materials with unique properties, important for the semiconductor industry. Generally, MX2 (where M = metal and X = chalcogen atoms, such as S, Se, or Te) type TMDCs contain easily exfoliable layers, turning to essentially 2D materials. Planar structural defects, such as stacking faults (SFs), change the atomic arrangement, further improving electronic properties, making these materials useful for various applications (optoelectronic devices, spintronics, gas sensing, catalysis, energy storage, thermoelectrics, etc.) Previous reports on TMDC synthesis with specific SFs highlight the challenge in obtaining them. Even more, for a successful application, these SFs should be stable and consistent under experimental conditions. Our previous work on facile preparation of 2H-WSe2 on W foil in an ambient-pressure chemical vapor deposition (APCVD) exhibited inherent selenium vacancies forming during the synthesis. In current research, we present detailed characterization of SFs forming during this synthesis in various 2H structures: (tungsten di-selenide) WSe2, (molybdenum di-selenide) MoSe2 and (tungsten di-sulfide) WS2. The stability of these defects as a function of post-annealing treatments is also discussed.

Imaging and analysis of multidimensional atomically thin transition metal dichalcogenide layers on diatom frustules

Diatom frustules can exhibit multiple unique properties owing to their multidimensional and hierarchical pore structures. Herein, we report on the growth of transition metal dichalcogenides (TMDCs) on diatom frustules using metal–organic chemical vapor deposition. Structural analysis results revealed the conformal growth of primarily monolayer TMDCs not only on the outer surface but also inside the pore structures. Characterization and imaging using photoluminescence and micro-Raman analyses revealed that the resulting structures exhibit high-quality and uniform monolayer MoS2 films on the diatom frustules throughout the complex morphologies. Furthermore, these atomically thin layers provided a uniform conducting path for electrons from the sample surface to ground during scanning electron microscopy operations. This feature can minimize the reduction and/or deformation of the signals obtained with a conventional conducting layer coating, thereby enabling imaging and analysis of the diatom frustules during energy-dispersive X-ray spectroscopy measurements and focused ion-beam processing.

Precise Layer Control and Electronic State Modulation of a Transition Metal Dichalcogenide via Phase-Transition-Induced Growth.

Wafer-scale growth of transition metal dichalcogenides with precise control over the number of layers, and hence the electronic state is an essential technology for expanding the practical application of 2D materials. Herein, a new growth method, phase-transition-induced growth (PTG), is proposed for the precisely controlled growth of molybdenum disulfide (MoS2 ) films consisting of one to eleven layers with spatial uniformity on a 2 in. wafer. In this method, an energetically unstable amorphous MoSx Oy (a-MoSx Oy ) phase is effectively converted to a thermodynamically stable crystalline MoS2 film. The number of MoS2 layers is readily controlled layer-by-layer by controlling the amount of Mo atoms in a-MoSx Oy , which is also applicable for the growth of heteroatom-inserted MoS2 . The electronic states of intrinsic and Nb-inserted MoS2 with one and four layers grown by PTGare are analyzed based on their work functions. The work function of monolayer MoS2 effectively increases with the substitution of Nb for Mo. As the number of layers increases to four, charge screening becomes weaker, dopant ionization becomes easier, and ultimately the work function increases further. Thus, better electronic state modulation is achieved in a thicker layer, and in this respect, PTG has the advantage of enabling precise control over the film thickness.

Two-Step Growth of Uniform Monolayer MoS2 Nanosheets by Metal-Organic Chemical Vapor Deposition.

To achieve large area growth of transition metal dichalcogenides of uniform monolayer thickness, we demonstrate metal–organic chemical vapor deposition (MOCVD) growth under low pressure followed by a high-temperature sulfurization process under atmospheric pressure (AP). Following sulfurization, the MOCVD-grown continuous MoS2 film transforms into compact triangular crystals of uniform monolayer thickness as confirmed from the sharp distinct photoluminescence peak at 1.8 eV. Raman and X-ray photoelectron spectroscopies confirm that the structural disorders and chalcogen vacancies inherent to the as-grown MOCVD film are substantially healed and carbon/oxygen contaminations are heavily suppressed. The as-grown MOCVD film has a Mo/S ratio of 1:1.6 and an average defect length of ∼1.56 nm, which improve to 1:1.97 and ∼21 nm, respectively, upon sulfurization. The effect of temperature and duration of the sulfurization process on the morphology and stoichiometry of the grown film is investigated in detail. Compared to the APCVD growth, this two-step growth process shows more homogenous distribution of the triangular monolayer MoS2 domains across the entire substrate, while demonstrating comparable electrical performance.

Large-area growth of MoS2 at temperatures compatible with integrating back-end-of-line functionality

Direct growth of transition metal dichalcogenides over large areas within the back-end-of-line (BEOL) thermal budget limit of silicon integrated circuits is a significant challenge for 3D heterogeneous integration. In this work, we report on the growth of MoS2 films (∼1–10 nm) on SiO2, amorphous-Al2O3, c-plane sapphire, and glass substrates achieved at low temperatures (350 °C–550 °C) by chemical vapor deposition in a manufacturing-compatible 300 mm atomic layer deposition reactor. We investigate the MoS2 films as a potential material solution for BEOL logic, memory and sensing applications. Hall-effect/4-point measurements indicate that the ∼10 nm MoS2 films exhibit very low carrier concentrations (1014–1015 cm−3), high resistivity, and Hall mobility values of ∼0.5–17 cm2 V−1 s−1, confirmed by transistor and resistor test device results. MoS2 grain boundaries and stoichiometric defects resulting from the low thermal budget growth, while detrimental to lateral transport, can be leveraged for the integration of memory and sensing functions. Vertical transport memristor structures (Au/MoS2/Au) incorporating ∼3 nm thick MoS2 films grown at 550 °C (∼0.75 h) show memristive switching and a stable memory window of 105 with a retention time >104 s, between the high-low resistive states. The switching set and reset voltages in these memristors demonstrate a significant reduction compared to memristors fabricated from pristine, single-crystalline MoS2 at higher temperatures, thereby reducing the energy needed for operation. Furthermore, interdigitated electrode-based gas sensors fabricated on ∼5 nm thick 550 °C-grown (∼1.25 h) MoS2 films show excellent selectivity and sub-ppm sensitivity to NO2 gas, with a notable self-recovery at room temperature. The demonstration of large-area MoS2 direct growth at and below the BEOL thermal budget limit, alongside memristive and gas sensing functionality, advances a key enabling technology objective in emerging materials and devices for 3D heterogeneous integration.

Initial stage of MBE growth of MoSe2 monolayer

An atomically thin MoSe2 layer has been synthesized on mica using molecular beam epitaxy (MBE). The polymorphous of the MoSe2 layer depends on the coverage and the growth temperature. At low coverages and low growth temperature, 1T-MoSe2 forms in addition to a comparable quantity of 2H-MoSe2. The metastable 1T-MoSe2 transfers gradually to the stable 2H-MoSe2 before the completion of the first monolayer. The current result sheds some light on the complexity of the nucleation and growth of transition metal dichalcogenide (TMDC) monolayers and implies a possible route for a phase selective synthesis using MBE.

Vertical Chemical Vapor Deposition Growth of Highly Uniform 2D Transition Metal Dichalcogenides.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted great attention due to their physical and chemical properties that make them promising in electronics and optoelectronics. Because of the difficulties in controlling concentrations of solid precursors and spatially nonuniform growth dynamics, it is challenging to grow 2D TMDCs over large areas with good uniformity and reproducibility so far, which significantly hinders their practical use. Here we report a vertical chemical vapor deposition (VCVD) design with gaseous precursors to grow monolayer TMDCs with a uniform density and high quality over the whole substrate and with excellent reproducibility. Such a gaseous VCVD design can well control the three key parameters in TMDC growth, including precursor concentration, gas flow, and temperature, which cannot be done in a currently widely used horizontal CVD system with solid precursors. Statistical results show that VCVD-grown monolayer TMDCs including MoS2 and WS2 are of high uniformity and quality on substrates over centimeter size. We also fabricated multiple van der Waals heterostructures by one-step transfer of VCVD-grown TMDCs, owning to their good uniformity. This work sheds light on the growth of 2D materials with high uniformity on a large-area substrate, which can be used for the wafer-scale fabrication of 2D materials and their heterostructures.