Non-van der Waals layered materials are mechanically stripped into two-dimensional crystals

Recently, Geng Fengxia’s team at Soochow University published a new study entitled “Mechanical cleavage of non-van der Waals structures towards two-dimensional crystals” in the journal Nature Synthesis.

In this study, aiming at the challenge that non-van der Waals layered materials have electron interaction between layers and are difficult to directly peel off with tape, this study transforms the non-van der Waals layered structure into a metastable form by mechanical calendering to the relative slippage of adjacent sheets, effectively weakens the interlayer interaction, and realizes the mechanical peeling of a series of materials, including metals (Sb, Bi), semiconductors (SnO, V2O5, Bi2O2Se) and superconducting materials KV3Sb5. It is also found that the corresponding thin-layer two-dimensional structure has strong thickness dependence characteristics.

The corresponding author of the study is Professor Geng Fengxia of Soochow University, and the co-first authors are Jiang Kun, Ji Jinpeng, Gong Wenbin and Ding Ling.

Two-dimensional materials are limited in thickness to the nanometer level, between one and a few atoms, and therefore often have unique physical properties and applications that differ from those of the block. The mechanical peeling method, also known as the scotch tape method, is considered one of the most effective methods to obtain high-quality two-dimensional materials because it does not involve any chemical reactions, is able to maintain its original structure and intrinsic properties. However, this method is usually only suitable for block structures where the layer-to-layer interaction is dominated by weak van der Waals forces, such as graphene, hexagonal boron nitride (h-BN), transition metal disulfides (TMDs), metal-organic frameworks (MOFs), and black phosphorus. Many functional materials have electronic interactions between adjacent layers or faces, making it difficult to strip these materials directly to a single or few-layer structure via tape.

Recently, Geng Fengxia’s team of Soochow University developed a peeling method for non-van der Waals layered structural materials, which used mechanical calendering to induce relative sliding of adjacent layers, detached the material from the stable state, effectively weakened the interlayer bonding effect, and then successfully obtained the two-dimensional structure of a series of materials through mechanical peeling, including metal materials Bi, Sb, semiconductor materials SnO, V2O5, Bi2O2Se and superconducting compound KV3Sb5.

Figure 1: Strategy schematic; Differential electron density map representing the bulk structure of the material and AFM image of the corresponding two-dimensional material.

Figure 2: Theoretical prediction results using SnO materials as an example.

Figure 3: Characterization of SnO crystal structure before and after interlayer mutual slip.

The morphological characterization of SEM can clearly show that the crystal plane has undergone domino-like slip. From the XRD crystal structure characterization, it can be seen that the layer spacing increases slightly, and the in-plane crystal plane (100) and (110) spacing also increases, and the crystal plane edge can be inferred by comparing the spacing change relationship[110]Direction slippage. Cross-sectional sectional characterization of crystals before and after slip also gives consistent results. The critical stress required for sliding measured by in-situ AFM was 2.8 MPa, which was basically consistent with the theoretical calculation of 3.17 MPa, which was much smaller than the critical stress required to peel off the monolayer. The force required for a 50×50 μm2 microcrystallite is only 7.9 mN, which can be achieved by experimental mechanical calendering. In the Raman characterization, it was found that the intensity of the Eg peak in the plane brought by slip was significantly reduced, which may be the result of local coordination change. At the same time, the bandwidth increases, and the sample color changes from blue-black to dark brown. These results suggest that small disturbances in the interlayer structure of non-van der Waals materials can lead to changes in material structure and properties.

Figure 4: Morphology, structure, and stability characterization of mechanically stripped two-dimensional SnO nanosheets.

Figure 5: Comparison of physical properties of two-dimensional materials and blocks and known van der Waals materials.

The bandgap characterization and theoretical results of the obtained two-dimensional materials show that Sb changes from the metal of the bulk to the wide-bandgap semiconductor of the two-dimensional material (2.01 eV). SnO is regulated from infrared 0.60 eV for bulk samples to 3.65 eV for monolayer samples, which has a wider regulation interval than known van der Waals materials. The thin-layer KV3Sb5 (2-5 nm) is an ideal material for studying unconventional superconductivity in two-dimensional Kagome lattices and with charge density waves. (Source: Science Network)

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