Flexible 2D Materials beyond Graphene: Synthesis, Properties, and Applications

Abstract

properties and are proved to be potential in the fields of optoelectronics, photonics, biosensing as well as energy conversion and storage. Although centimeter-scale graphene single crystal has been obtained, it is not suitable for electronic switches in traditional transistors due to its semimetallic nature and large dark-current. Even many technologies have been proposed to create the energy bandgap in graphene, the value is remaining too small to realize practical applications. [2] Therefore, scientists are gradually changing the direction of research focusing on 2D materials beyond graphene, such as transition metal dichalcogenides (TMDs), graphitic carbon nitride, MXenes, hexagonal boron nitride (h-BN), hexagonal metal oxide (h-MO), and transition metal oxides (TMOs). As expected, 2D materials can provide a completely suspended surface without dangling bonds, so that the corresponding devices can minimize the influence of various surface states (Figure 1). [3] Benefitting from these excellent features, the 2D transistor can obtain a very low subthreshold swing (SS≈70 mV decade −1 ), which is comparable to the most advanced silicon (Si) transistor. [4] Due to excellent mechanical flexibility and compatibility with low-temperature manufacturing processes, 2D materials are expected to be used in flexible electronic products with plastic substrates. [5][6][7] The use of 2D materials in flexible electronic products can overcome several technical challenges in systems based on traditional materials, such as low mobility (typically below 10 cm 2 v −1 s −1 ), high driving voltage of organic materials, [8,9] complex technological processes, and large leakage current of polycrystalline Si. [10,11] Beyond their excellent electrical properties, 2D materials also exhibit a high degree of flexibility and transparency. Strong covalent bonds in the plane (Figure 1) make the 2D material mechanically robust, resulting in high fracture strains. [12][13][14] Mechanical robustness makes 2D materials suitable for flexible electronic products (Figure 2 reveals the roadmap of 2D materials used in flexible devices) that are subject to mechanical strain and cyclic stress. In addition, their atomic thickness makes them also suitable for transparent electronic products due to their high transparency in the visible range.In this paper, we first introduce the properties and preparation methods of 2D materials beyond graphene. Then, the applications of 2D materials in flexible devices are reviewed.2D materials are now at the forefront of state-of-the-art nanotechnologies due to their fascinating properties and unique structures. As expected, low-cost, high-volume, and high-quality 2D materials play an important role in the applications of flexible devices. Although considerable progress has been achieved in the integration of a series of novel 2D materials beyond graphene into flexible devices, a lot remains to be known. At this stage of their development, the key issues concern how to make further improvements to highperformance and scal...

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