Two-dimensional Transition Metal Dichalcogenides Research Articles

Impact of monolayer WS2 surface properties on the gate dielectrics formation by atomic layer deposition

Two-dimensional transition metal dichalcogenides (2D TMDs), such as MoS2 and WS2, have emerged as promising channel materials for future generation transistors. However, carbon-based surface contaminants pose a significant challenge in the formation of high-quality metal–oxide–semiconductor gate stacks for 2D TMDs. Carbon-based surface contaminants are known to be present even on directly grown 2D TMDs that have not been in contact with polymers. These organic contaminants affect precursor adsorption during atomic layer deposition (ALD) of gate dielectrics on 2D TMDs and as such the 2D-dielectric interface. This study examines the effectiveness of predeposition annealing in mitigating carbon-based contaminants while maintaining the integrity of a directly grown WS2 monolayer on a SiO2 substrate. We show that a WS2 monolayer on a SiO2/Si substrate remains stable during vacuum annealing at temperatures up to 400 °C. Water contact angle measurements and x-ray photoelectron spectroscopy confirm that the surface concentration of carbon starts to decrease at 150 °C. Thermal anneal improves the surface coverage of Al2O3 for both conventional chemisorption-based ALD and physisorbed-precursor-assisted ALD processes by facilitating more effective Al2O3 nucleation on the WS2 monolayer. The impact of predeposition anneal on the Al2O3 growth behavior in both processes can be explained by changes in surface contaminant levels. Our results underscore the importance of surface pretreatment in dielectric deposition on 2D TMDs and demonstrate that predeposition anneal is an effective method to enhance ALD-based dielectric deposition on directly grown 2D TMDs.

Two-Dimensional ZrS2 and HfS2 for Making Sub-10 nm High-Performance P-Type Transistors.

Two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors have been recognized as reliable candidates for future sub-10 nm physical gate length field-effect transistors (FETs). However, the device performance of 2D P-type devices is far inferior to that of N-type devices, which seriously hinders the development of complementary metal-oxide-semiconductor (CMOS) integrated circuits. Herein, we presented that two new 2D TMDC channel materials, ZrS2 and HfS2, can realize high-performance P-type MOSFETs through first-principles quantum transport simulations. Different from the 2D MoS2 and WSe2, the continuous in-plane p-orbitals at the valence band edge of 2D ZrS2 and HfS2 lead to a small hole effective mass of 0.24 m0. As a result, 2D ZrS2 and HfS2 P-type MOSFETs with 10 nm gate length possess an on-state current (Ion) as high as 2000 μA/μm. Moreover, even when the gate length shrinks to 5 nm, the Ion can also reach ∼1500 μA/μm with the energy delay product ranging from 3 × 10-30 to 1 × 10-29 Js/μm, which are better than many other 2D P-type MOSFETs like MoS2 and WSe2. Our work demonstrates that 2D ZrS2 and HfS2 are competitive channel materials for constructing future sub-10 nm P-type high-performance FETs.

Scalable fabrication of vertically arranged Bi2Se3 crossbar arrays for memristive device applications

Despite the unique advantages of the memristive switching devices based on two-dimensional (2D) transition metal dichalcogenides, scalable growth technologies of such 2D materials and wafer-level fabrication remain challenging. In this work, we present the gold-assisted large-area physical vapor deposition (PVD) growth of Bi2Se3 features for the scalable fabrication of 2D-material-based crossbar arrays of memristor devices. This work indicates that gold layers, prepatterned by photolithography processes, can catalyze PVD growth of few-layer Bi2Se3 with 100-folds larger crystal grain size in comparison with that grown on bare Si/SiO2 substrates. We also present a fluid-guided growth strategy to improve growth selectivity of Bi2Se3 on Au layers. Through the experimental and computational analyses, we identify two key processing parameters, i.e., the distance between Bi2Se3 powder and the target substrate and the distance between the leading edges of the substrate and the substrate holder with a hollow interior, which plays a critical role in realizing large-scale growth. By optimizing these growth parameters, we have successfully demonstrated cm-scale highly-selective Bi2Se3 growth on crossbar-arrayed structures with an in-lab yield of 86%. The whole process is etch- and plasma-free, substantially minimizing the damage to the crystal structure and also preventing the formation of rough 2D-material surfaces. Furthermore, we also preliminarily demonstrated memristive devices, which exhibit reproducible resistance switching characteristics (over 50 cycles) and a retention time of up to 106 s. This work provides a useful guideline for the scalable fabrication of vertically arranged crossbar arrays of 2D-material-based memristive devices, which is critical to the implementation of such devices for practical neuromorphic applications.

Device simulation study of multilayer MoS2 Schottky barrier field-effect transistors

Molybdenum disulfide (MoS2) is a representative two-dimensional layered transition-metal dichalcogenide semiconductor. Layer-number-dependent electronic properties are attractive in the development of nanomaterial-based electronics for a wide range of applications including sensors, switches, and amplifiers. MoS2field-effect transistors (FETs) have been studied as promising future nanoelectronic devices with desirable features of atomic-level thickness and high electrical properties. When a naturallyn-doped MoS2is contacted with metals, a strong Fermi-level pinning effect adjusts a Schottky barrier and influences its electronic characteristics significantly. In this study, we investigate multilayer MoS2Schottky barrier FETs (SBFETs), emphasizing the metal-contact impact on device performance via computational device modeling. We find thatp-type MoS2SBFETs may be built with appropriate metals and gate voltage control. Furthermore, we propose ambipolar multilayer MoS2SBFETs with asymmetric metal electrodes, which exhibit gate-voltage dependent ambipolar transport behavior through optimizing metal contacts in MoS2device. Introducing a dual-split gate geometry, the MoS2SBFETs can further operate in four distinct configurations:p - p,n - n,p - n, andn - p. Electrical characteristics are calculated, and improved performance of a high rectification ratio can be feasible as an attractive feature for efficient electrical and photonic devices.

Multiple Conformal-Contact Transfer of Large-Area Crack-Free Transition Metal Dichalcogenide Stacks

Abstract Atomically-thin two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as an ideal platform for both physics investigation and device applications. By stacking different layers into homo- or hetero-structures, an extra degree of freedom is involved in further tuning their properties, thereby boosting scenarios in twistronics, moiré photonics and optoelectronics. However, interfacial imperfections such as contaminations and cracks, frequently occur during the layer stacking sequence and accumulate layer by layer, greatly degenerating the interface quality. In this study, we developed a multiple conformal-contact transfer method to construct TMD stacks with crack-free intrinsic interfaces. The design of a deformable buffer layer is crucial to guarantee the conformal contact and intact transfer of each layer, contributing to the successful construction of centimetre-scale TMD stacks up to 8 layers. Precise control over spatial location and interlayer twist angle is also feasibly achieved, evidenced by the stacking-dependent interlayer exciton effects in WS2-WSe2 heterostructures. This work provides a facile and precise approach for architecting 2D stacks with perfect interfaces, which will further accelerate the customized design for their device functionalization.

Quantifying relaxation time constants in MoS2xSe2(1-x) alloys: Impact of Stoichiometry and Si/SiO2 interference

Despite a decade of extensive research on two-dimensional transition metal dichalcogenides, there are still some gaps in our understanding of their remarkable properties, limiting their potential applications. One such gap relates to the properties of these alloys, which offer tunable band gaps and can serve as the basis for various optoelectronic and nonlinear optical devices. A comprehensive understanding of the ultrafast charge carrier excitation and relaxation dynamics as well as their characteristic relaxation time constants, is crucial for the further use of these materials in specific applications. In this work, we experimentally investigate the dynamics of ultrafast relaxation processes in monolayer MoS2xSe2(1-x) alloys on a Si/SiO2 substrate using time-resolved spectroscopy. We determine the characteristic relaxation time constants for structures with different stoichiometric compositions and refine these constants by accounting for interference effects arising from the substrate.

Layer Number and Stacking Engineering of MoS2 Crystals for High-Performance Polarization-Sensitive Photodetector.

The layer and stacking engineering of two-dimensional (2D) transition-metal dichalcogenides (TMDs) gives rise to novel phenomena and multiapplications; thus, TMDs have garnered considerable attention. However, the precisely customized fabrication of stacked 2D materials to date is largely limited to the lack of effective and controllable growth strategies, prone to the unpredictable stacking orders and randomly distributed nucleation sites. Here, we devise an optimized chemical vapor deposition approach for modulating the MoS2 single crystals from monolayer to multilayer with diverse stacking configurations. Significantly, the phototransistor based on monolayer MoS2 single crystal exhibits an ultrasensitive performance with a high photoresponsivity (R) of 3.3 × 104 A W-1 and a remarkable detectivity (D*) of above 1.7 × 1014 Jones at 405 nm light illumination. Ultralow-frequency and angle-resolved polarized Raman spectroscopy is used to systematically uncover the delicate interlayer interactions and crystallographic anisotropy. Moreover, the polarization-sensitive photodetectors using 1-3L MoS2 show a layer number-dependent anisotropic performance, with dichroism ratios of 1.36, 1.44, and 1.52. This work offers a promising method to not only enable the fabrication of new customized layer-, stacking-, and twist-2D materials but also provides the foundation for the development of advanced polarization-sensitive and optoelectronic devices based on stacking transitions.

Exchange-enhanced spin-orbit splitting and its density dependence for electrons in monolayer transition metal dichalcogenides

We show that spin-orbit splitting (SOS) in monolayers of semiconducting transition metal dichalcogenides (TMDs) is substantially enhanced by an electron-electron interaction. This effect, similar to the exchange enhancement of the electron g factor, is most pronounced for conduction band electrons (in particular, in MoS2), and it has a nonmonotonic dependence on the carrier sheet density n. That is, the SOS enhancement is peaked at the onset of filling of the higher-energy spin-split band by electrons, n*, which also separates the regimes of slow (at n<n*) and fast (for n>n*) spin and valley relaxation of charge carriers. Moreover, this density itself is determined by the enhanced SOS value, making the account of exchange renormalization important for the analysis of the spintronic performance of field-effect transistors based on two-dimensional TMDs. Published by the American Physical Society 2024

Quasi-Zero-Dimensional Source/Drain Contact for Fermi-Level Unpinning in a Tungsten Diselenide (WSe2) Transistor: Approaching Schottky-Mott Limit.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) known for their exceptional electrical and optical properties have emerged as promising channel materials for next-generation electronics. However, as strong Fermi-level pinning (FLP) between the metal and the 2D TMDC material at the source/drain (S/D) contact decides the Schottky barrier height (SBH), the transistor polarity is fixed to a certain type, which remains a challenge for the 2D TMDC field-effect transistors (FETs). Here, a S/D contact structure with a quasi-zero-dimensional (quasi-0D) contact interface, in which the dimensionality reduction effect alleviates FLP, was developed to gain controllability over the polarity of the 2D TMDC FET. As a result, conventional metal contacts on the WSe2 FET showed n-type characteristics due to strong FLP (pinning factor of 0.06) near the conduction band, and the proposed quasi-0D contact enabled by the Ag conductive filament on the WSe2 FET exhibited p-type characteristics with a SBH very close to the Schottky-Mott rule (pinning factor of 0.95). Furthermore, modeling of Schottky barriers of conventional contacts, one-dimensional (1D) contacts, and quasi-0D contacts revealed that the SBH of the quasi-0D contact is relatively less subject to interface dipoles that induce FLP, owing to more rapid decaying of dipole energy. The proposed contact in this study provided a method that progressed beyond the alleviation of FLP to achieve controllable polarity. Moreover, reducing the contact dimensionality to quasi-0D will enable high compatibility with the further scaled-down nanoscale device contact structure.

Comparative analysis of thermoelectric properties in bulk 2H and monolayer MoS2: a first-principles study at high temperatures

The pursuit of high-efficiency heat-to-electricity conversion is one of the indispensable driving forces toward future renewable energy production. The two-dimensional (2D) transition metal dichalcogenide, such as molybdenum disulfide (MoS2), is at the forefront of research due to its outstanding heat propagation features and potential applications as a thermoelectric material. Using the first-principles density functional theory coupled with the semi-classical Boltzmann transport equation within the constant relaxation time approximation, we present the thermoelectric and energy transport in the bulk 2H and monolayer MoS2 material system. In order to advance the underlying physics, we calculate several crucial transport parameters such as electrical conductivity, electronic thermal conductivity, Seebeck coefficient, and power factor as a function of the reduced chemical potential for different doping types and temperatures, in addition to the electron energy dispersion relation of the material system. Our comprehensive study employs the Shankland interpolation algorithm and the rigid band approximation to attain a high degree of accuracy. This thorough investigation reveals the high Seebeck coefficient of 1534 and 1550 μ V/K at 500 K for the bulk 2H and monolayer MoS2, respectively. Furthermore, the ultrahigh power factor values of 9.21 × 1011 and 3.69 × 1011 Wm −1 K −2 s −1 are shown at 800 K in the bulk 2H and monolayer MoS2, respectively. Based on the power factor results, our in-depth analysis demonstrates that the bulk 2H MoS2, when compared to monolayer MoS2, exhibits great potential as a promising semiconducting thermoelectric material for advanced high-performance energy device applications.

Exciton dissociation in two-dimensional transition metal dichalcogenides: Excited states and substrate effects

Exciton dissociation in two-dimensional transition metal dichalcogenides: Excited states and substrate effects

Liquid Nitrogen Exfoliation and Nanodispersion of WS2 for Thermocatalysts.

The efficient and clean exfoliation of single and/or few-layer nanosheets of WS2, a two-dimensional transition metal dichalcogenides, remains a significant challenge. In this study, a simple exfoliation method was proposed to produce ultrathin WS2 nanosheets by combining the liquid nitrogen exfoliation and nanodispersion techniques. This approach efficiently exfoliated WS2 into several layers of nanosheets via rapid temperature changes and mechanical stress without inducing defects or contamination. After five cycles of heating/liquid nitrogen and nanodispersion, the resulting WS2 nanosheets (WS2-5N ND) were confirmed to have been successfully exfoliated into 1-4 layers. When applied as a promoter in a thermocatalyst for the selective catalytic reduction of NOX using NH3, 2V3WS2/Ti (WS2-5N ND) exhibited excellent NOX conversion and N2 selectivity, along with excellent durability even in the presence of SO2. This result was greater than 2V3WS2/Ti (WS2-5N) subjected to only liquid nitrogen exfoliation, proving the importance of the simultaneous action of both methods. This method is expected to be an important contribution to ongoing research on high-performance WS2-based catalysts, thereby opening up potential opportunities for a wide range of applications.

Broadband self-powered MoS2/PdSe2/WSe2 PSN heterojunction photodetector for high-performance optical imaging and communication

Two-dimensional (2D) semi-metal transition metal dichalcogenides (TMDs) have drawn significant attention for their distinctive physical properties. However, the inherent high dark current of these materials and the single structure of detectors hinder the further development of photodetectors with high performance. Here, we construct a PSN (p-type semiconductor/semi-metal/n-type semiconductor) architecture by sandwiching 2D semi-metal between two semiconductor layers. In this architecture, the top and bottom layers generate an internal built-in electric field, while the middle layer serves as an absorption layer for low-energy photons and facilitates the dissociation of photo-generated carriers. As a result, the heterojunction device demonstrates a wide spectrum optical response from visible to infrared light (405 nm to 1550 nm) without requiring an external voltage. Working in self-powered mode at room temperature, the device achieves a responsivity of 0.56 A/W, a detectivity of 5.63 × 1011 Jones, and a rapid response speed of 190/74 µs. Additionally, the device shows potential for applications in fast optical communication and multi-wavelength optical imaging. This work presents a novel approach for developing a new type of broadband, self-powered, high-performance miniaturized semi-metal-based photodetector.

Understanding Iron-Doping Modulating Domain Orientation and Improving the Device Performance of Monolayer Molybdenum Disulfide.

Domain orientation modulation and controlled doping of two-dimensional (2D) transition-metal dichalcogenides (TMDCs) are two pivotal tasks for synthesizing wafer-scale single crystals and boosting device performances. However, realizing two such targets and uncovering internal physical mechanisms remain daunting challenges. We develop an accurate Fe doping strategy, which enables domain orientation control and electron mobility improvement of monolayer MoS2. By tuning of the Fe dopant dosages, parallel steps with different heights are formed, which induce edge-nucleation of unidirectionally aligned monolayer MoS2. In parallel, Fe doping induces the down shift of the conduction band minimum of monolayer MoS2 and matches well with the work function of an electrode, which reduces Schottky barrier height and delivers ultralow contact resistance (561 Ω μm) and excellent electron mobility (37.5 cm2 V-1 s-1). The modulation mechanism is clarified by combining theory calculations and electronic structure characterizations. This work hereby provides a new paradigm for synthesizing wafer-scale 2D TMDC single crystals and constructing high-performance devices.

Prediction of a novel two-dimensional superatomic Cd6S2 monolayer for photocatalytic water splitting.

Two-dimensional transition metal dichalcogenides possess a significant specific surface area, adjustable bandgaps, and excellent optical absorption properties, rendering them highly conducive to photocatalytic applications. Herein, a MoS2-like 2D superatomic Cd6S2 monolayer is predicted, wherein the octahedral Cd6 superatom unit connects with S atoms via six vertices. Chemical bonding analysis reveals that the remarkable dynamic, thermal, and mechanical stability of the Cd6S2 monolayer results from the covalent Cd-S bonds and the 6-center 8-electron (6c-8e) delocalized bond within the Cd6 core, which ensures the chemical octet rule for both the S atom and the Cd6 superatom. Demonstrating notable optical absorption coefficients and a strain-tuned energy band structure, the Cd6S2 monolayer emerges as a viable candidate for catalyzing the solar-powered splitting of water. This work offers an alternative avenue to modify or improve the properties of 2D materials for photocatalytic applications through superatomic assembly.

Transition Metal Dichalcogenides in Electrocatalytic Water Splitting

Two-dimensional transition metal dichalcogenides (TMDs), also known as MX2, have attracted considerable attention due to their structure analogous to graphene and unique properties. With superior electronic characteristics, tunable bandgaps, and an ultra-thin two-dimensional structure, they are positioned as significant contenders in advancing electrocatalytic technologies. This article provides a comprehensive review of the research progress of two-dimensional TMDs in the field of electrocatalytic water splitting. Based on their fundamental properties and the principles of electrocatalysis, strategies to enhance their electrocatalytic performance through layer control, doping, and interface engineering are discussed in detail. Specifically, this review delves into the basic structure, properties, reaction mechanisms, and measures to improve the catalytic performance of TMDs in electrocatalytic water splitting, including the creation of more active sites, doping, phase engineering, and the construction of heterojunctions. Research in these areas can provide a deeper understanding and guidance for the application of TMDs in the field of electrocatalytic water splitting, thereby promoting the development of related technologies and contributing to the solution of energy and environmental problems. TMDs hold great potential in electrocatalytic water splitting, and future research needs to further explore their catalytic mechanisms, develop new TMD materials, and optimize the performance of catalysts to achieve more efficient and sustainable energy conversion. Additionally, it is crucial to investigate the stability and durability of TMD catalysts during long-term reactions and to develop strategies to improve their longevity. Interdisciplinary cooperation will also bring new opportunities for TMD research, integrating the advantages of different fields to achieve the transition from basic research to practical application.

An in-depth study of the synthesis of ReSe2 for anisotropic Raman characteristics

Abstract Two-dimensional transition metal dichalcogenides (TMDs) have received more interest for their potential role in future electronic and optoelectronic applications. Unlike other TMDs, Rhenium diselenide (ReSe2) stands out for its distinctive anisotropic growth characteristics. These unique features arise from its low lattice symmetry and interlayer decoupling, this has sparked significant interest among researchers. Previous research has indicated the presence of various growth patterns, including dendritic formations and structures resembling flowers. In this study, we effectively produced ReSe2 using the ‘Tilting Boat’ method to achieve growth on a 21 μm scale. Through precise manipulation of the growth conditions, we successfully attained flakes of 21 μm scale in comparison to prior findings. Moreover, we successfully produced a variety of shapes, including triangles, diamonds, and hexagons, on 1 × 1 cm2 Si/SiO2 substrates. Furthermore, we achieved the successful production of a continuous ReSe2 film on a 1 × 3 cm2 Al2O3 substrate. We verified the distinct anisotropic properties of ReSe2 via Raman Spectroscopy. Furthermore, we acquired three-dimensional visual representations of ReSe2 flakes and continuous films via SEM measurements. By employing EDS data and analysing x-ray photoelectron spectroscopy spectra, we have established a compositional ratio of 1:2 for Re and Se, which aligns with the MX2 structure. This confirmation indicates the successful synthesis of high-quality ReSe2 flakes.

Synergistic effects of advanced MoSe2/MnO2 nanocomposite for direct yellow dye degradation for enhanced photocatalytic applications

MoSe2, a significant component within the class of two-dimensional transition metal dichalcogenides (TMDCs), boasts diverse photocatalysis characteristics. Herein, this research focuses on the hydrothermal approach to fabricate and thoroughly characterize Molybdenum diselenide (MoSe2) nanoparticles and Manganese dioxide (MnO2) nanoflakes as their co-catalyst with controllable morphologies size and enhanced photocatalytic activity. The samples were advanced characterizations such as SEM and HR-TEM to examine morphology, Powder X-ray diffraction (PXRD) to validate phase and crystal structure, Fourier transform infrared (FTIR) spectroscopy for analysing functional groups and bonds, and XPS for insights into elemental composition and chemical state of the nanocomposite. Due to the efficient separation of photogenerated electron-hole pairs facilitated by the rapid transfer of electrons by the addition of MnO2 and MoSe2/MnO2 nanocomposite demonstrates significantly enhanced photocatalytic performance and excellent stability in Direct Yellow (DY), a common pollutant found in industrial effluents. The MoSe2/MnO2 nanocomposite photocatalytic activity was 1.2 to 1.6 times higher than that of its individual components of MoSe2 and MnO2, indicating synergistic effects leading to enhanced performance. Consequently, this study illustrates that the MoSe2/MnO2 nanocomposite effectively degrades direct yellow dye under base conditions through photocatalysis.

Long-Lived Topological Flatband Excitons in Semiconductor Moiré Heterostructures: A Bosonic Kane-Mele Model Platform.

Moiré superlattices based on two-dimensional transition metal dichalcogenides (TMDs) have emerged as a highly versatile and fruitful platform for exploring correlated topological electronic phases. One of the most remarkable examples is the recently discovered fractional quantum anomalous Hall effect (FQAHE) under zero magnetic field. Here, we propose a minimal structure that hosts long-lived excitons-a ubiquitous bosonic excitation in TMD semiconductors-with narrow topological bosonic bands. The nontrivial exciton topology originates from hybridization of moiré interlayer excitons and is tunable by controlling twist angle and electric field. At small twist angle, the lowest exciton bands are isolated from higher energy bands and provide a solid-state realization of the bosonic Kane-Mele model with topological flatbands, which could potentially support the bosonic version of FQAHE.

Diverse Excitonic Phenomena in Asymmetric Trilayer Transition Metal Dichalcogenide Heterostructures.

Interlayer excitons formed in two-dimensional transition metal dichalcogenide (TMD) heterostructures can be easily tuned due to the large spatial separation. In this work, we discuss the electronic and excitonic optical properties of trilayer heterostructures MoS2/MoSSe/WSe2 and MoS2/MoSSe/MoSe2 using state-of-the-art GW+BSE calculations. In both trilayer geometries, we discover a variety of exciton states, including interlayer excitons, every-other-layer excitons, and their hybridized states, h-IX. Importantly, the h-IXs are optically bright through hybridizing with the intralayer excitons, and the radiative lifetimes of h-IXs range from subnanoseconds to tens of microseconds at 77 K, depending on their compositions. We also reveal that the diversity of the low-lying IXs in MoS2/MoSSe/MoSe2 is higher than that of MoS2/MoSSe/WSe2, because more energy levels participate in transitions in MoS2/MoSSe/MoSe2. Our findings demonstrate that the appropriate energy alignment via manipulating the Janus layer is crucial for realizing rich excitonic states in trilayer TMD heterostructures.