Exploring Favorable Supramolecular Interactions of Multifluorinated Aromatics in Dendronized Push-Pull Chromophores for Electro-Optics.

Multifluorinated aromatics serve as supramolecular synthons in the research of organic electro-optic (EO) materials by exploiting π-π stacking interaction between the aromatic hydrocarbon and multifluorinated aromatic groups for performance improvement. However, non-classical hydrogen bonding remains largely unexplored in fluorinated EO dendrimers. In this study, three Fréchet-type generation 1 benzyl ether co-dendrons were synthesized by replacing one benzyl group with 2,3,5,6-tetrafluorobenzyl (p-HF4Bz), pentafluorobenzyl (C6F5Bz), and 2,3,4,5-tetrafluorobenzyl (o-HF4Bz) groups, to afford the benzoic acid derivatives D1, D2, and D3, which were further bonded to the donor and π-bridge moieties to afford three co-dendronized push-pull phenyltetraene chromophores EOD1, EOD2, and EOD3, respectively. The weak C-H⋅⋅⋅X (X = O, F) interactions in the crystal structure of D1 cumulatively add to the benzoic acid dimers to form an extended hydrogen-bonded network, while D2 is crystallized into a centric one-dimensional chain with strong intermolecular interactions. The poled films of EOD1 with PMMA exhibited the largest and most stable EO activity with optical homogeneity among the series. The results identify the effectiveness of weak but favorable hydrogen bonds enabled by the enhanced carbon acidity of p-HF4Bz synthon in D1, over the interactions in D2 and D3, for the rational design of supramolecular EO dendrimers.

Ultrahigh Lubricity between Two-Dimensional Ice and Two-Dimensional Atomic Layers.

Low temperature and high humidity conditions significantly degrade the performance of solid-state lubricants consisting of van der Waals (vdW) atomic layers, owing to the liquid water layer attached/intercalated to the vdW layers, which greatly enhances the interlayer friction. However, using low temperature in situ atomic force microscopy (AFM) and friction force microscopy (FFM), we unveil the unexpected ultralow friction between two-dimensional (2D) ice, a solid phase of water confined to the 2D space, and the 2D molybdenum disulfides (MoS2). The friction of MoS2 and 2D ice is reduced by more than 30% as compared to bare MoS2 and the rigid surface. The phase transition of liquid water into 2D ice under mechanical compression has also been observed. These new findings can be applied as novel frictionless water/ice transport technology in nanofluidic systems and promising high performance lubricants for operating in low temperature and high humidity environments.

Controlled Adhesion of Ice-Toward Ultraclean 2D Materials.

The scalable 2D device fabrication and integration demand either the large-area synthesis or the post-synthesis transfer of 2D layers. While the direct synthesis of 2D materials on most targeted surfaces remains challenging, the transfer approach from the growth substrate onto the targeted surfaces offers an alternative pathway for applications and integrations. However, the current transfer techniques for 2D materials predominantly involve polymers and organic solvents, which are liable to contaminate or deform the ultrasensitive atomic layers. Here, novel ice-aided transfer and ice-stamp transfer methods are developed, in which water (ice) is the only medium in the entire process. In practice, the adhesion between various 2D materials and ice can be well controlled by temperature. Through such controlled adhesion of ice, it is shown that the new transfer methods can yield ultrahigh quality and exceptional cleanliness in transferred 2D flakes and continuous 2D films, and are applicable for a wide range of substrates. Furthermore, beyond transfer, ice can also be used for cleaning the surfaces of 2D materials at higher temperatures. These novel techniques can enable unprecedented ultraclean 2D materials surfaces and performances, and will contribute to the upcoming technological revolutions associated with 2D materials.

Anomalous fracture in two-dimensional rhenium disulfide.

Low-dimensional materials usually exhibit mechanical properties from those of their bulk counterparts. Here, we show in two-dimensional (2D) rhenium disulfide (ReS2) that the fracture processes are dominated by a variety of previously unidentified phenomena, which are not present in bulk materials. Through direct transmission electron microscopy observations at the atomic scale, the structures close to the brittle crack tip zones are clearly revealed. Notably, the lattice reconstructions initiated at the postcrack edges can impose additional strain on the crack tips, modifying the fracture toughness of this material. Moreover, the monatomic thickness allows the restacking of postcrack edges in the shear strain-dominated cracks, which is potentially useful for the rational design of 2D stacking contacts in atomic width. Our studies provide critical insights into the atomistic processes of fracture and unveil the origin of the brittleness in the 2D materials.

Spontaneously Ordered Hierarchical Two-Dimensional Wrinkle Patterns in Two-Dimensional Materials.

Achieving two-dimensionally (2D) ordered surface wrinkle patterns is still challenging not only for the atomic-thick 2D materials but also in general for all soft surfaces. Normally disordered 2D wrinkle patterns on isotropic surfaces can be rendered via biaxial straining. Here, we report that the 1D and 2D ordered wrinkle patterns in 2D materials can be produced by sequential wrinkling controlled by thermal straining and vertical spatial confinement. The various hierarchical patterns in 2D materials generated by our method are highly periodic, and the hexagonal crystal symmetry is obeyed. More interestingly, these patterns can be maintained in suspended monolayers after delamination from the underlying surfaces which shows the great application potentials. Our new approach can simplify the patterning processes on 2D layered materials and reduce the risk of damage compared to conventional lithography methods, and numerous engineering applications that require nanoscale ordered surface texturing could be empowered.

Site-specific electrical contacts with the two-dimensional materials.

Electrical contact is an essential issue for all devices. Although the contacts of the emergent two-dimensional materials have been extensively investigated, it is still challenging to produce excellent contacts. The face and edge type contacts have been applied previously, however a comparative study on the site-specific contact performances is lacking. Here we report an in situ transmission electron microscopy study on the contact properties with a series of 2D materials. By manipulating the contact configurations in real time, it is confirmed that, for 2D semiconductors the vdW type face contacts exhibit superior conductivity compared with the non-vdW type contacts. The direct quantum tunneling across the vdW bonded interfaces are virtually more favorable than the Fowler-Nordheim tunneling across chemically bonded interfaces for contacts. Meanwhile, remarkable area, thickness, geometry, and defect site dependences are revealed. Our work sheds light on the significance of contact engineering for 2D materials in future applications.

Catalyzed Kinetic Growth in Two-Dimensional MoS2.

It remains difficult to control the morphology of two-dimensional (2D) materials via direct chemical vapor deposition (CVD) growth. In particular, off-equilibrium (kinetic) growth may produce flakes with non-Wulff shapes (e.g., high-index edges, symmetrical shapes, etc.), which are potentially useful; however, a general controllable approach for the kinetic growth of 2D materials is currently lacking. In this work, we pushed the CVD growth of 2D MoS2 into deep kinetic regime, by using potassium chloride (KCl) as catalyst and plasma pretreatment on growth substrates. The unprecedented nonequilibrium high-index faceting and unusual high-symmetry shapes in 2D materials have been realized. The growth mechanism of high-index facets is rationalized based on the theory of kinetic instability on crystal surfaces. This new vapor-liquid-adatom-solid (VLAS) growth mechanism-synergistic capture of multiple vapor phase molecules by the catalyst particles on corners and the oversaturated adatom diffusion along adjacent edges can offer great opportunities for shape engineering on 2D materials. The high-quality, rapid, and controllable synthesis of high-index facets (edges) and other non-Wulff shapes of 2D transition metal dichalcogenides will benefit the developments in 2D materials.

Critical Stable Length in Wrinkles of Two-Dimensional Materials.

The emergent two-dimensional (2D) materials are atomically thin and ultraflexible, promising for a variety of miniaturized, high-performance, and flexible devices in applications. On one hand, the ultrahigh flexibility causes problems: the prevalent wrinkles in 2D materials may undermine the ideal properties and create barriers in fabrication, processing, and quality control of materials. On the other hand, in some cases the wrinkles are used for the architecturing of surface texture and the modulation of physical/chemical properties. Therefore, a thorough understanding of the mechanism and stability of wrinkles is highly needed. Herein, we report a critical length for stabilizing the wrinkles in 2D materials, observed in the wrinkling and wrinkle elimination processes upon thermal annealing as well as by our in situ TEM manipulations on individual wrinkles, which directly capture the evolving wrinkles with variable lengths. The experiments, mechanical modeling, and self-consistent charge density functional tight binding (SCC-DFTB) simulations reveal that a minimum critical length is required for stabilizing the wrinkles in 2D materials. Wrinkles with lengths below a critical value are unstable and removable by thermal annealing, while wrinkles with lengths above a critical value are self-stabilized by van der Waals interactions. It additionally confirms the pronounced frictional effects in wrinkles with lengths above critical value during dynamical movement or sliding.

In Situ Scanning Transmission Electron Microscopy Observations of Fracture at the Atomic Scale.

The formation, propagation, and structure of nanoscale cracks determine the failure mechanics of engineered materials. Herein, we have captured, with atomic resolution and in real time, unit cell-by-unit cell lattice-trapped cracking in two-dimensional (2D) rhenium disulfide (ReS_{2}) using in situ aberration corrected scanning transmission electron microscopy (STEM). Our real time observations of atomic configurations and corresponding strain fields in propagating cracks directly reveal the atomistic fracture mechanisms. The entirely brittle fracture with non-blunted crack tips as well as perfect healing of cracks have been observed. The mode I fracture toughness of 2D ReS_{2} is measured. Our experiments have bridged the linear elastic deformation zone and the ultimate nm-sized nonlinear deformation zone inside the crack tip. The dynamics of fracture has been explained by the atomic lattice trapping model. The direct visualization on the strain field in the ongoing crack tips and the gained insights of discrete bond breaking or healing in cracks will facilitate deeper insights into how atoms are able to withstand exceptionally large strains at the crack tips.

Superior Dielectric Screening in Two-Dimensional MoS2 Spirals.

Metals have the best dielectric screening capability among all materials; however, it is usually difficult to fabricate continuous and uniform ultrathin (few-atomic-layer thickness) metal films. Conversely, high-quality atomic-thick semiconductor or semimetal materials (so called two-dimensional materials) such as graphene or MoS2 can be readily obtained and robust in ambient conditions; however, their dielectric screening capabilities are greatly reduced by their reduced dimensionality. Particularly, in the vertical direction, the dielectric screening of two-dimensional materials is insufficient; thus, the performances of devices by two-dimensional materials were easily affected by the coulomb-scattering or other kind of sources. Herein, we propose that with a screw dislocation connecting the van der Waals layers in two-dimensional MoS2 spiral structures, excellent dielectric screening in the vertical direction can be achieved. Our Kelvin force microscopy directly demonstrates that the external impurity charges can be perfectly screened by a theoretically minimum number of layers (two layers) in the MoS2 spirals. This spiral structure-assisted screening approach paves new way to the design of high-performance ultrathin electrical and optical devices.

Edge Delamination of Monolayer Transition Metal Dichalcogenides.

Delamination of thin films from the supportive substrates is a critical issue within the thin film industry. The emergent two-dimensional, atomic layered materials, including transition metal dichalcogenides, are highly flexible; thus buckles and wrinkles can be easily generated and play vital roles in the corresponding physical properties. Here we introduce one kind of patterned buckling behavior caused by the delamination from a substrate initiated at the edges of the chemical vapor deposition synthesized monolayer transition metal dichalcogenides, led by thermal expansion mismatch. The atomic force microscopy and optical characterizations clearly showed the puckered structures associated with the strain, whereas the transmission electron microscopy revealed the special sawtooth-shaped edges, which break the geometrical symmetry for the buckling behavior of hexagonal samples. The condition of the edge delamination is in accordance with the fracture behavior of thin film interfaces. This edge delamination and buckling process is universal for most ultrathin two-dimensional materials, which requires more attention in various future applications.