The researchers used high-throughput calculations to reveal van der Waals interactions between layers of two-dimensional materials

Recently, Liu Weimin, academician of the Chinese Academy of Sciences, researcher of the Lanzhou Institute of Chemical Physics of the Chinese Academy of Sciences, and Professor Qi Weihong of Northwestern Polytechnical University have revealed the relationship between the binding energy and the binding force between the layers of two-dimensional materials through high-throughput calculations and experiments, and proposed that only the interlayer binding force can be used as a criterion for the difficulty of stripping two-dimensional materials (rather than the binding energy). 

The van der Waals interaction between the layers of the two-dimensional material is a non-bond interaction, relative to the chemical bond, this weaker interaction makes the two-dimensional materials can be arbitrarily stacked without the need for interlayer matching lattice, easy to form two-dimensional material van der Waals heterojunction and moiré and other superstructures, while bringing many novel physical, chemical, mechanical and other properties. At present, academics are biased to use interlayer binding energy to characterize the difficulty of stripping two-dimensional materials. 

The researchers believe that the use of interlayer binding can characterize the difficulty of peeling two-dimensional materials is not accurate enough, and only interlayer bonding forces can more accurately characterize the difficulty of two-dimensional material peeling. On the potential energy-layer spacing curve of the two-dimensional material, the binding energy corresponds to the potential well depth of the potential energy curve, and the binding force corresponds to the inflection point of the potential energy curve change (equivalent to the second derivative of the potential energy curve is zero), the interlayer binding force and the binding energy change trend are not the same, some two-dimensional materials have large interlayer binding force but small binding energy, while other two-dimensional materials have small binding force but large binding energy. The interlayer adhesion force of a two-dimensional material directly corresponds to the peeling force of the material, rather than the binding energy. 

Based on this idea, the researchers performed high-throughput calculations by invoking the First Principles Calculation Program (VASP) through their own descent algorithm and obtained interlayer binding energy and binding force data for 230 common two-dimensional materials (Figure 1).

Figure 1: High-throughput calculation of binding energy and binding force of 230 two-dimensional materials

The calculation results show that the trend of interlayer binding energy and binding force of two-dimensional materials is indeed not exactly consistent (that is, high binding energy does not mean large binding force). The researchers also designed experiments to test the accuracy of theoretical predictions. For three two-dimensional materials: graphene, hexagonal boron nitride and indium selenide, the two-dimensional material is attached to a homemade silicon ball probe through a unique assembly method, and these probes are used to perform high-precision interlayer adhesion tests under the atomic force microscope. The results of the study are exactly consistent with the calculation predictions, namely: F graphene> F boron nitride> F indium selenide. This proves theoretically and experimentally that only interlayer-to-layer binding forces can more accurately characterize the difficulty of stripping of two-dimensional materials. The relevant experimental results are shown in Figure 2, where Fig. 2(a) is the two-dimensional material clad probe preparation method; Figure 2(b) is the graphene probe SEM photo; Figure 2(c) is the graphene typical 8×8 mining point determination result; Figure 2(d) is the typical force-displacement curve of graphene, hexagonal boron nitride and indium selenide; Figure 2(e) is the graphene determination result statistics; Figure 2 (f) is the hexagonal boron nitride determination result statistics; Figure 2 (g) is the indium selenide determination result statistics.

Figure 2: AFM interlayer binding force measurement results

Prediction of the readibility of new two-dimensional materials or the design of two-dimensional van der Waals materials must be determined by studying the size of their interlayer binding forces; the interlayer bond stability of two-dimensional material devices also needs to be accurately characterized by binding forces. The interlayer adhesion data of 230 common two-dimensional materials presented in this work is reliable and complete. The results of this study contribute to a deep understanding of the van der Waals interaction between layers of two-dimensional materials and the establishment of physical models of two-dimensional materials, and provide a theoretical basis for the application of two-dimensional materials. 

The research was published under the title “High-Throughput Calculation of Interlayer van der Waals Forces Validated with Experimental Measurements.”Researchabove. Northwestern Polytechnical University is the first completion unit, and Lanzhou Institute of Chemicals is the second completion unit. 

The above work has been supported by the National Natural Science Foundation of China, the Basic Scientific Research Fund of the Central Universities and the Analysis and Testing Center of Northwestern Polytechnical University. (Source: Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences)

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