Jiuxiang Dai

MX₂-based devices for high-performance circuits are expected to be introduced in production after the Si-sheet-based CFET. A stacked-sheet MX₂ device will look like that prepared by process simulation. The status of the process development needed to produce such a device in a fab environment, and the gaps to be overcome are described.

Abstract

Large-area 2D-material-based devices may find applications as sensor or photonics devices or can be incorporated in the back end of line (BEOL) to provide additional functionality. The introduction of highly scaled 2D-based circuits for high-performance logic applications in production is projected to be implemented after the Si-sheet-based CFET devices. Here, a view on the requirements needed for full wafer integration of aggressively scaled 2D-based logic circuits, the status of developments, and the definition of the gaps to be bridged is provided. Today, typical test vehicles for 2D devices are single-sheet devices fully integrated in a lab environment, but transfer to a more scaled device in a fab environment has been demonstrated. This work reviews the status of the module development, including considerations for setting up fab-compatible process routes for single-sheet devices. While further development on key modules is still required, substantial progress is made for MX₂ channel growth, high-k dielectric deposition, and contact engineering. Finally, the process requirements for building ultra-scaled stacked nanosheets are also reflected on.

A novel and feasible gradient orbital coupling strategy is proposed for tuning the oxygen reduction reaction performance through the construction of Co 3d-O 2p-Eu 4f unit sites on Eu₂O₃–Co model. This strategy optimizes the e_g occupancy of Co sites, and weakens the OO bond, and ultimately breaks the scaling relation between *OOH and *OH at Co–O–Eu unit sites.

Abstract

The development of highly efficient and economical materials for the oxygen reduction reaction (ORR) plays a key role in practical energy conversion technologies. However, the intrinsic scaling relations exert thermodynamic inhibition on realizing highly active ORR electrocatalysts. Herein, a novel and feasible gradient orbital coupling strategy for tuning the ORR performance through the construction of Co 3d-O 2p-Eu 4f unit sites on the Eu₂O₃–Co model is proposed. Through the gradient orbital coupling, the pristine ionic property between Eu and O atoms is assigned with increased covalency, which optimizes the e_g occupancy of Co sites, and weakens the OO bond, thus ultimately breaking the scaling relation between *OOH and *OH at Co–O–Eu unit sites. The optimized model catalyst displays onset and half-wave potential of 1.007 and 0.887 V versus reversible hydrogen electrode, respectively, which are higher than those of commercial Pt/C and most Co-based catalysts ever reported. In addition, the catalyst is found to possess superior selectivity and durability. It also reveals better cell performance than commercial noble-metal catalysts in Zn–air batteries in terms of high power/energy densities and long cycle life. This study provides a new perspective for electronic modulation strategy by the construction of gradient 3d–2p–4f orbital coupling.

Abstract

Transition metal dichalcogenides (TMD) heterostructure is widely applied for second harmonic generation (SHG) and holds great promises for laser source, nonlinear switch, and optical logic gate. However, for atomically thin TMD heterostructures, low SHG conversion efficiency would occur due to reduction of light—matter interaction length and lack of phase matching. Herein, we demonstrated a facile directional SHG amplifier formed by MoS₂/WS₂ monolayer heterostructures suspended on a holey SiO₂/Si substrate. The SHG enhancement factor reaches more than two orders of magnitude in a wide spectral range from 355 to 470 nm, and the radiation angle is reduced from 38° to 19° indicating higher coherence and better emission directionality. The giant SHG enhancement and directional emission are attributed to the great excitation and emission field concentration induced by a self-formed vertical Fabry—Pérot microcavity. Our discovery gives helpful insights for the development of two-dimensional (2D) nonlinear optical devices.

2D In₄Te₃ single crystals are synthesized via self-modulation-guided growth strategy, which exhibits high crystallinity, excellent second harmonic generation, and ultralow thermal conductivity. This work also provides opportunities to synthesize a family of metal tellurides with ultrathin thickness and high crystallinity, which will greatly boost the potential for thermoelectric applications.

Abstract

Ultralow thermal conductivity materials have triggered much interest due to diverse applications in thermal insulation, thermal barrier coating, and especially thermoelectrics. Two dimensional (2D) indium tellurides with ultralow thermal conductivity provide a versatile platform for tailoring the heat transfer, exploring new candidates for thermoelectrics, and achieving miniature, lightweight, and highly integrated devices. Unfortunately, their nanostructure and structure-related heat transfer properties at a 2D scale are much less studied due to difficulties in material fabrication. The ionic character between interlayers and strong covalent bonds in 3D directions impede the anisotropic growth of indium telluride flakes; meanwhile, the low environmental stability and chemical reactivity of tellurium also limit the fabrication of high-quality tellurides, thus hindering the exploration of thermal transport properties. Here, a self-modulation-guided growth strategy to synthesize high-quality 2D In₄Te₃ single crystals with ultralow thermal conductivity (0.47 W m⁻¹ K⁻¹) is developed. This strategy can also be extended to synthesize a series of highly crystallized metal tellurides, providing excellent candidates for further application in thermoelectrics.

In this study, a four-inch wafer-scale c-Si tapered microwire (TMW) solar cell with 21.1%-efficiency by utilizing the conventional fabrication processes is successfully demonstrated. Our device shows the highest power conversion efficiency among the previously reported MW-based radial junction solar cells. It is believe that our solar cell will accelerate the commercialization of MW-based radial junction solar cells.

Abstract

Microwire (MW)-based radial junction crystalline silicon (c-Si) solar cells have great potential as an emerging energy device with an efficiency of over 20%. However, the competitive efficiency of MW-based c-Si solar cells in realizing a wafer-scale device is limiting its commercialization. In this study, the aim is to demonstrate that conventional fabrication techniques can be applied to MW-based solar cells while not only increasing the size from the lab-scale to the wafer-scale but also retaining an efficiency of >20%. Surprisingly, an improvement in open-circuit voltage and fill factor is observed with an increase in device size, due to the reduction of recombination loss at the device edge. Finally, a successful demonstration of 21.1% efficiency at 4-inch wafer-scale (25 cm²) in c-Si MW solar cell is observed, while an efficiency of 20.6% at a lab-scale size (1 cm²) is observed.

Nature Electronics, Published online: 15 September 2022; doi:10.1038/s41928-022-00824-9

Lattice matched Mo₂C−Mo₂N was formed by a gradient heating epitaxial growth method. The unique heterointerface promotes hydrogen evolution kinetics in both acid and alkaline media by generating near-zero hydrogen-adsorption free energy and facilitating water dissociation. As a result, electrocatalytic hydrogen evolution performance in pH-universal media is achieved that is comparable to commercial Pt/C.

Abstract

An optimized approach to producing lattice-matched heterointerfaces for electrocatalytic hydrogen evolution has not yet been reported. Herein, we present the synthesis of lattice-matched Mo₂C−Mo₂N heterostructures using a gradient heating epitaxial growth method. The well lattice-matched heterointerface of Mo₂C−Mo₂N generates near-zero hydrogen-adsorption free energy and facilitates water dissociation in acid and alkaline media. The lattice-matched Mo₂C−Mo₂N heterostructures have low overpotentials of 73 mV and 80 mV at 10 mA cm⁻² in acid and alkaline solutions, respectively, comparable to commercial Pt/C. A novel photothermal-electrocatalytic water vapor splitting device using the lattice-matched Mo₂C−Mo₂N heterostructure as a hydrogen evolution electrocatalyst displays a competitive cell voltage for electrocatalytic water splitting.

Nature Communications, Published online: 12 September 2022; doi:10.1038/s41467-022-33082-6

Nature Electronics, Published online: 12 September 2022; doi:10.1038/s41928-022-00820-z

Centimeter-scaled van der Waals passivation of transition metal dichalcogenides by stacking thermally grown hydrocarbon (HC) dielectrics is demonstrated to enhance the electric performance and stability of the field-effect transistors (FETs). HC encapsulation onto MoSe₂ FETs fundamentally suppresses the electric degradation caused by an interfacial defect by screening the molecular adsorption.

Abstract

Development of efficient surface passivation methods for semiconductor devices is crucial to counter the degradation in their electrical performance owing to scattering or trapping of carriers in the channels induced by molecular adsorption from the ambient environment. However, conventional dielectric deposition involves the formation of additional interfacial defects associated with broken covalent bonds, resulting in accidental electrostatic doping or enhanced hysteretic behavior. In this study, centimeter-scaled van der Waals passivation of transition metal dichalcogenides (TMDCs) is demonstrated by stacking hydrocarbon (HC) dielectrics onto MoSe₂ field-effect transistors (FETs), thereby enhancing the electric performance and stability of the device, accompanied with the suppression of chemical disorder at the HC/TMDCs interface. The stacking of HC onto MoSe₂ FETs enhances the carrier mobility of MoSe₂ FET by over 50% at the n-branch, and a significant decrease in hysteresis, owing to the screening of molecular adsorption. The electron mobility and hysteresis of the HC/MoSe₂ FETs are verified to be nearly intact compared to those of the fabricated HC/MoSe₂ FETs after exposure to ambient environment for 3 months. Consequently, the proposed design can act as a model for developing advanced nanoelectronics applications based on layered materials for mass production.

Well-controlled van der Waals trilayer MoS₂ is used as the semiconductor channel, which can successfully address the long-standing issue of performance degradation from physical-vapor deposited metal contact, leading to the demonstration of high-speed MoS₂ transistors with a high drain current (589 µA µm⁻¹ at V _ds = 1 V) as well as record-high saturation velocity (4.2 × 106 cm s⁻¹) at room temperature.

Abstract

Two-dimensional MoS₂ field-effect transistors (FETs) have great potential for next-generation electronics due to their excellent electronic properties with an atomic thin channel. However, multiple challenges exist for the monolayer MoS₂ channel, including interface scattering and ohmic contact. In this work, well-controlled trilayer MoS₂ with high mobility and large single crystals is successfully grown on soda-lime glass substrates using chemical vapor deposition, with a lateral size of up to 148 µm, which is the largest reported size to date. A record high on/off ratio of ≈10¹² and a high carrier mobility of 62 cm² V⁻¹ s⁻¹ of trilayer MoS₂ FETs are demonstrated, showing notable advantages compared with the monolayer counterpart. The long-standing issue of monolayer MoS₂ performance degradation from physical vapor deposited metal contact can be mitigated by the trilayer MoS₂ channel, achieving the lowest contact resistance of 350 Ω µm using the common method of e-beam evaporated Ni. Moreover, 40-nm channel-length trilayer MoS₂ FETs using ultrathin HfLaO dielectrics exhibit a high current of 589 µA µm⁻¹ at a supply voltage of 1 V at room temperature, which increases to 1162 µA µm⁻¹ at 4.3 K, the highest among those using commonly evaporated metal. Record high electron saturation velocity of 4.2 × 10⁶ cm s⁻¹ can be achieved at room temperature.

Light: Science & Applications, Published online: 02 September 2022; doi:10.1038/s41377-022-00956-9

Nature Photonics, Published online: 01 September 2022; doi:10.1038/s41566-022-01067-y

npj 2D Materials and Applications, Published online: 08 September 2022; doi:10.1038/s41699-022-00338-0

npj 2D Materials and Applications, Published online: 08 September 2022; doi:10.1038/s41699-022-00339-z

Nature, Published online: 07 September 2022; doi:10.1038/s41586-022-05024-1

Abstract

Graphene nanoribbons (GNRs) are regarded as an ideal candidate for beyond-silicon electronics. However, synthesis of aligned GNR arrays on insulating substrates with high efficiency is challenging. In this work, we develop a facile strategy, involving KOH pre-treatment and high-temperature annealing, to construct parallel steps on the two-fold symmetry a-plane sapphire substrate. Horizontal GNRs as narrow as 15.1 nm with global alignment across a region of 20 mm² are then grown on the step edge-enriched substrate through plasma enhanced chemical vapor deposition (PECVD) method. GNRs align well along the atomic steps on sapphire ( \([1\bar 100]\) direction) with their widths and densities swiftly adjustable by step morphology modification on substrate surface. A step-edge confined growth mechanism is proposed, attributing the constraint on the nanoribbon broadening to a relatively low growth temperature in PECVD, which restrains the activation energy to suppress GNRs across step edges on sapphire and prevents detrimental nanoribbon widening. The results provide a new perspective for scalable synthesizing well aligned nanoribbons of other two-dimensional materials.