Jiuxiang Dai

The paper examines the constraints of interlayer coupling in synthetically stacked MoS₂ flakes, assessing the impact of stacking variables on the vertical architectures of two-dimensional (2D) structures with varying thicknesses. A clear difference between 2D stacked metamaterials and interface-driven stackings is established as a new design criterion for 2D material-based devices for optoelectronics.

Abstract

Interlayer interactions are one of the crucial parameters of two-dimensional (2D) layered materials-based junctions. Understanding the limits of interlayer coupling and defining the “maximum building block thickness” in artificially stacked 2D layered materials are key tasks that hold significant importance, not only in fundamental physics, but also in practical applications such as electronics, photonics, and optoelectronics. Here, the interlayer coupling limits are optically investigated of a model 2D layered semiconductor, MoS₂, revealing the evolution of distinct interaction mechanisms between layers via artificial stacking. As the total thickness increases, a reduction in the stacking angle influence on the properties of the homojunctions is reflected in the photoluminescence and second harmonic generation responses. The results show that the effective coupling limit for vertically stacked 2D metamaterials resides in three-layer flakes. The findings pave the way to advanced and complex devices of 2D superlattices for photonics and optoelectronics.

Characterizing single layer graphene (SLG) films directly on their growth substrates is highly beneficial to study their intrinsic properties. Charge doping and strain are among the prominent properties of interest to assess SLG quality. In this work, the use of selected area electron diffraction and Raman spectroscopy as means to quantify intrinsic strain in chemical vapor deposition grown SLG films is demonstrated.

Abstract

Quantifying the intrinsic properties of 2D materials is of paramount importance for advancing their applications. Large-scale production of 2D materials merits the need for approaches that provide direct information about the role of growth substrate on 2D material properties. Transferring the 2D material from its growth substrates can modify the intrinsic properties of the asgrown 2D material. In this study, suspended chemical vapor deposition (CVD) graphene films are prepared directly on their growth substrates in a high-density grid array. The approach facilitates the quantification of intrinsic strain and doping in suspended CVD graphene films. To achieve this, transmission electron microscopy and large-area Raman mapping are employed. Remarkably, the analysis reveals consistent patterns of compressive strain (≈−0.2%) both in the diffraction patterns and Raman maps obtained from these suspended graphene films. By conducting investigations directly on the growth substrates, the potential influences introduced during the transfer process are circumvented effectively. Consequently, the methodology offers a robust and reliable means of studying the intrinsic properties of 2D materials in their authentic form, uninfluenced by the transfer-induced alterations that may skew the interpretation of their properties.

Observing the gain and loss of PDMS and LiNbO₃ surface molecules through an in situ monitoring strategy for oxygen plasma. Saturation of oxygen-ion and ─OH on the surfaces is achieved, leading to the optimal modification effect. The formation of Si─O─Nb occurred between PDMS and LiNbO₃. The PDMS- LiNbO₃ microfluidic chip exhibited excellent performance, with the highest pressure exceeding 60 psi.

Abstract

Acoustic microfluidic chips, fabricated by combining lithium niobate (LiNbO₃) with polydimethylsiloxane (PDMS), practically find applications in biomedicine. However, high-strength direct bonding of LiNbO₃ substrate with PDMS microchannel remains a challenge due to the large mismatching of thermal expansion coefficient at the interface and the lack of bonding theory. This paper elaborately reveals the bonding mechanisms of PDMS and LiNbO₃, demonstrating an irreversible bonding method for PDMS-LiNbO₃ heterostructures using oxygen plasma modification. An in-situ monitoring strategy by using resonant devices is proposed for oxygen plasma, including quartz crystal microbalance (QCM) covered with PDMS and surface acoustic wave (SAW) fabricated by LiNbO₃. When oxygen plasma exposure occurs, surfaces are cleaned, oxygen ions are implanted, and hydroxyl groups (-OH) are formed. Upon interfaces bonding, the interface will form niobium-oxygen-silicon covalent bonds to realize an irreversible connection. A champion bonding strength is obtained of 1.1 MPa, and the PDMS-LiNbO₃ acoustic microfluidic chip excels in leakage tests, withstanding pressures exceeding 60 psi, outperforming many previously reported devices. This work addresses the gap in PDMS-LiNbO₃ bonding theory and advances its practical application in the acoustic microfluidic field.

Thermomechanical nanomolding is extended to fabricate 2D metal nanostructures with widths < 50 nm, depths ≈ 0.5—1 µm and lengths ≈7 mm at wafer scale at back end of line (BEOL) compatible conditions, where the microstructure of the feedstock is transferred to the molded nanostructure.

Abstract

With shrinking dimensions in integrated circuits, sensors, and functional devices, there is a pressing need to develop nanofabrication techniques with simultaneous control of morphology, microstructure, and material composition over wafer length scales. Current techniques are largely unable to meet all these conditions, suffering from poor control of morphology and defect structure or requiring extensive optimization or post-processing to achieve desired nanostructures. Recently, thermomechanical nanomolding (TMNM) has been shown to yield single-crystalline, high aspect ratio nanowires of metals, alloys, and intermetallics over wafer-scale distances. Here, TMNM is extended for wafer-scale fabrication of 2D nanostructures. Using In, Al, and Cu, nanomold nanoribbons with widths < 50 nm, depths ≈0.5–1 µm and lengths ≈7 mm into Si trenches at conditions compatible is successfully with back end of line processing . Through SEM cross-section imaging and 4D-STEM grain orientation maps, it is shown that the grain size of the bulk feedstock is transferred to the nanomolded structures up to and including single crystal Cu. Based on the retained microstructures of molded 2D Cu, the deformation mechanism during molding for 2D TMNM is discussed.

An ultrathin layer of samarium (Sm) with a thickness equivalent to ≈2 monolayers is remarkably effective at passivating polycrystalline copper (Cu) films toward oxidation in ambient air. The ultrathin Sm layer lowers the work function of the Cu film down to ≈3.8 eV, making this approach attractive for applications requiring a low-work-function electrode with high stability in air.

Herein, it is reported that how a layer of samarium (Sm) with a thickness equivalent to ≈2 atoms (0.8 nm) deposited by thermal evaporation is remarkably effective at passivating polycrystalline copper (Cu) toward oxidation in ambient air. To monitor the rate of Cu oxidation in real time, slablike Cu films with a thickness of 9 nm are fabricated on glass modified with a layer of 3-mercaptopropyl silatrane, which immobilizes condensing Cu atoms by reaction with the thiol moiety, promoting slablike film formation at very low thickness. Upon exposure to ambient air the rate of increase in electrical resistance due to reaction with oxygen and water is slowed by more than an order of magnitude when the Cu film is capped with the ultrathin Sm layer. After 1 year, the resistance increases by ≈30% as compared to ≈190% for Cu films without an ultrathin Sm layer. Photoelectron spectroscopy, atomic force microscopy, and Kelvin probe measurements shed light on the underlying mechanism of passivation. Additionally, the ultrathin Sm layer is greatly lowering the work function of polycrystalline Cu films making this approach attractive for applications requiring a low-work-function electrode with high stability in air.

In this study, the impact of thermal annealing on the interaction between monolayer MoS₂ and Au using Raman spectroscopy is investigated. Two predominant interaction modes are identified: strongly coupled regions where MoS₂ is hybridized to Au with electron doping and minimal strain and weakly coupled regions characterized by slight hole doping and 1.0% tensile strain.

Herein, the impact of thermal annealing on the interaction between monolayer MoS₂ and Au using Raman spectroscopy is investigated. It is found that MoS₂ has two main modes of interactions with the underlying Au being either weakly coupled or strongly coupled. The regions strongly coupled to Au are hybridized to Au, minimally strained, and electron doped. The weakly coupled regions are found to be slightly hole doped with tensile strain of 1.0%. The overall areal coverage of the strongly coupled regions is not increased by thermal annealing, and the variability in the degree of hybridization increases at annealing temperatures above 100 °C. The data also show that monolayer MoS₂ starts to decouple from Au around 100 °C, becoming fully decoupled above 200–250 °C, suggesting that monolayer MoS₂ produced by Au-assisted mechanical exfoliation may be more easily transferred off Au at elevated temperatures.

The local space-confined chemical vapor deposition growth technique for single-crystal TBG with a wide range of twist angles on liquid copper substrates is reported. The clean surface, high crystallinity, thermal stability, stacking structure, growth mechanism, and electrical transport properties of as-grown TBG are investigated. This work provides a convenient avenue for investigating the physical properties of TBG.

Abstract

Twisted bilayer graphene (TBG) generates significant attention in the fundamental research of 2D materials due to its distinct twist-angle-dependent properties. Exploring the efficient production of TBG with a wide range of twist angles stands as one of the major frontiers in moiré materials. Here, the local space-confined chemical vapor deposition growth technique for high-quality single-crystal TBG with twist angles ranging from 0° to 30° on liquid copper substrates is reported. The clean surface, pristine interface, high crystallinity, and thermal stability of TBG are verified by using comprehensive characterization techniques including optical microscopy, electron microscopy, and secondary-ion mass spectrometry. The proportion of TBG in bilayer graphene reaches as high as 89%. In addition, the stacking structure and growth mechanism of TBG are investigated, revealing that the second graphene layer develops beneath the first one. A series of comparative experiments illustrates that the liquid copper surface, with its excellent fluidity, promotes the growth of TBG. Electrical measurements show the twist-angle-dependent electronic properties of as-grown TBG, achieving a room-temperature carrier mobility of 26640 cm² V⁻¹ s⁻¹. This work provides an approach for the in situ preparation of 2D twisted materials and facilitates the application of TBG in the fields of electronics.

Theoretical calculations predict that the self-diffusion coefficients (D) of H atom between layers of transition-metal dichalcogenides (TMDCs) is affected by the layer composition, but also by layer stacking. The highest diffusivity is expected for materials with Mo and Se elements in the R-type of stacking, which is present in moiré structures with small twist angles approaching 0°.

Abstract

Hydrogen is a crucial source of green energy and is extensively studied for its potential usage in fuel cells. The advent of 2D crystals (2DCs) has taken hydrogen research to new heights, enabling it to tunnel through layers of 2DCs or be transported within voids between the layers, as demonstrated in recent experiments by Geim's group. In this study, it investigates how the composition and stacking of transition-metal dichalcogenide (TMDC) layers influence the transport and self-diffusion coefficients (D) of hydrogen atoms using well-tempered metadynamics (WTMetaD) simulations. The findings show that modifying either the transition metal or the chalcogen atoms significantly affects the free energy barriers (ΔF) and, consequently, the self-diffusion of hydrogen atoms between the 2DC layers. In the Hhh$H^h_h$ polytype (2H stacking), MoSe₂ exhibits the lowest ΔF, while WS₂ has the highest, resulting in the largest D for the former system. Additionally, hydrogen atoms inside the RhM$R_h^M$ (or 3R) polytype encounter more than twice lower energy barriers and, thus, much higher diffusivity compared to those within the most stable Hhh$H^h_h$ stacking. These findings are particularly significant when investigating twisted layers or homo- or heterostructures, as different stacking areas may dominate over others, potentially leading to directional transport and interesting materials for ion or atom sieving.

Transition metal phosphides (TMPs) have excellent electrochemical and catalytic properties due to charge rearrangement in the hybridization behavior of the metal-atom d-orbitals and P-atom p-orbitals. This review systematically introduces the relevant studies on TMPs as cathode hosts of lithium-sulfur batteries (LSBs), while analyzing TMPs′ catalytic characteristics for the intermediate product polysulfides. Finally, the challenges and outlook of TMPs in LSBs are elaborated.

Abstract

Lithium–sulfur batteries (LSBs) with ultra-high energy density (2600 W h kg⁻¹) and readily available raw materials are emerging as a potential alternative device with low cost for lithium-ion batteries. However, the insulation of sulfur and the unavoidable shuttle effect leads to slow reaction kinetics of LSBs, which in turn cause various roadblocks including poor rate capability, inferior cycling stability, and low coulombic efficiency. The most effective way to solve the issues mentioned above is to rationally design and control the synthesis of the cathode host for LSBs. Transition metal phosphides (TMPs) with good electrical conductivity and dual adsorption-conversion capabilities for polysulfide (PS) are regarded as promising cathode hosts for new-generation LSBs. In this review, the main obstacles to commercializing the LSBs and the development processes of their cathode host are first elaborated. Then, the sulfur fixation principles, and synthesis methods of the TMPs are briefly summarized and the recent progress of TMPs in LSBs is reviewed in detail. Finally, a perspective on the future research directions of LSBs is provided.

The perspective highlights recent advancements in wet and dry transfer techniques for two-dimensional (2D) materials and devices, and reviews multi-scale cleanliness assessment methodologies and passive/active cleaning strategies. The interfacial wetting role played in these methods is emphasized, suggesting that a thorough understanding could lead to the development of effective cleaning strategies to fully harness the potential of 2D materials in the fields of electronics and optoelectronics.

Abstract

Two-dimensional (2D) materials have tremendous potential to revolutionize the field of electronics and photonics. Unlocking such potential, however, is hampered by the presence of contaminants that usually impede the performance of 2D materials in devices. This perspective provides an overview of recent efforts to develop clean 2D materials and devices. It begins by discussing conventional and recently developed wet and dry transfer techniques and their effectiveness in maintaining material “cleanliness”. Multi-scale methodologies for assessing the cleanliness of 2D material surfaces and interfaces are then reviewed. Finally, recent advances in passive and active cleaning strategies are presented, including the unique self-cleaning mechanism, thermal annealing, and mechanical treatment that rely on self-cleaning in essence. The crucial role of interface wetting in these methods is emphasized, and it is hoped that this understanding can inspire further extension and innovation of efficient transfer and cleaning of 2D materials for practical applications.

Nature Materials, Published online: 05 December 2023; doi:10.1038/s41563-023-01745-4

Nature Electronics, Published online: 05 December 2023; doi:10.1038/s41928-023-01080-1

Nature Electronics, Published online: 05 December 2023; doi:10.1038/s41928-023-01075-y

Publication date: April 2024

Source: Progress in Materials Science, Volume 142

Author(s): Kalainathan Sivaperuman, Anju Thomas, Ravikumar Thangavel, Logu Thirumalaisamy, Soundarrajan Palanivel, Sudhagar Pitchaimuthu, Nazmul Ahsan, Yoshitaka Okada