**2. Fabrication of suspended 2D materials**

Generally, to measure the mechanical properties of 2D materials experimentally, a suspended structure needs to be fabricated. There are mainly two ways of fabricating a structure sus‐ pended with 2D materials. One approach is to transfer the 2D materials directly onto the prepatterned substrates [3, 7, 18–23]. The other approach is to transfer the 2D materials on the substrate first and then remove the sacrificial layer beneath the transferred 2D materials [24– 31]. **Figure 1(a)** and **(b)** shows the two schematics of the first fabrication approach, while **Figure 1(c)** shows the second approach.

In the first approach, taking SiO2/Si substrate for example, holes or cavities are patterned in the SiO2 layer with lithography and wet/dry etching techniques, as shown in the first step of **Figure 1(a)**. Then, 2D materials are transferred onto the prepatterned SiO2/Si substrate by the exfoliation method, forming suspended 2D materials structure (second step of **Figure 1(a)**). With this method, the suspended structure can be fabricated theoretically on various kinds of substrates. However, with the conventional mechanical exfoliation method, since the 2D Mechanical Properties and Applications of Two-Dimensional Materials http://dx.doi.org/10.5772/104209 221

**1. Introduction**

advanced nanoelectromechanical systems (NEMS).

220 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

**2. Fabrication of suspended 2D materials**

**1(c)** shows the second approach.

mechanical as well as mechanoelectric transduction properties.

Since the first successful preparation of graphene by mechanical exfoliation from graphite crystals in 2004 [1], two-dimensional (2D) materials have attracted dramatic attention due to their extraordinary physical properties (ultralow weight, high Young's modulus, and high strength) [2–7] and outstanding electrical properties [1] compared with conventional bulk materials. In the past few years, graphene, with the highest measured Young's modulus (~1 TPa) [3], is the most widely studied 2D material. Studies have shown that graphene filled into the polymer matrices can reinforce the mechanical properties of the composites significantly [8]. However, pristine graphene does not have a bandgap [9], which limits its applications in certain fields requiring a semiconducting material. As a potential substitute material of graphene, the transition metal dichalcogenides (TMDCs, e.g., MoS2 and WSe2) and black phosphorus (BP) with an intrinsic bandgap [10, 11] possess the potential for electronics and optoelectronics applications [12–15] and open a new field for 2D materials study. Moreover, the existence of piezoelectricity and the more sensitive piezoresistive effect in TMDCs compared with graphene under mechanical deformation make them more interesting for innovative applications including tactile strain sensors [16], nanogenerators [17], and

In this review, first, we introduce the common approaches used for fabricating suspended 2D material structures. Then, characterization methods for extracting the in-plane and out-ofplane mechanical properties of 2D materials are presented. A summary of the experimental results is given. In the last section, we introduce the electrical output change of 2D materials induced by mechanical deformation—piezoresistive and piezoelectric effects. In addition, we provide some example applications of 2D materials that make use of their extraordinary

Generally, to measure the mechanical properties of 2D materials experimentally, a suspended structure needs to be fabricated. There are mainly two ways of fabricating a structure sus‐ pended with 2D materials. One approach is to transfer the 2D materials directly onto the prepatterned substrates [3, 7, 18–23]. The other approach is to transfer the 2D materials on the substrate first and then remove the sacrificial layer beneath the transferred 2D materials [24– 31]. **Figure 1(a)** and **(b)** shows the two schematics of the first fabrication approach, while **Figure**

In the first approach, taking SiO2/Si substrate for example, holes or cavities are patterned in the SiO2 layer with lithography and wet/dry etching techniques, as shown in the first step of **Figure 1(a)**. Then, 2D materials are transferred onto the prepatterned SiO2/Si substrate by the exfoliation method, forming suspended 2D materials structure (second step of **Figure 1(a)**). With this method, the suspended structure can be fabricated theoretically on various kinds of substrates. However, with the conventional mechanical exfoliation method, since the 2D

**Figure 1.** Schematic of two representative approaches of suspended structures fabrication. (a, b) Transfer of the 2D ma‐ terials directly onto the prepatterned substrates; (c) Suspension of the 2D materials by etching the sacrificial layer un‐ derneath.

material sheets distribute randomly onto the substrates, the 2D material sheets may not cover necessarily the specific hole in the SiO2 layers, bringing the challenge of improving the production rate. Normally, two methods can be employed to address this problem. One method is to fabricate repeatable patterns (e.g., hole arrays) in the substrate [3, 7, 22] to enhance the probability of producing the suspended structures, as shown in **Figure 2(a)**. Another method is to employ a modified exfoliation method (transfer printing/stamping [20, 32]), with a transparent viscoelastic material as the carrier for the 2D materials, which enables a precise transfer of 2D materials to the desirable location [19, 21].

**Figure 2.** (a) Optical image of graphene suspended over hole arrays [35]. (b) SEM images of a MoS2 bridge supported on Au electrodes [23]. (c) SEM image of suspended graphene stripe under Au electrodes [25].

In most cases, electrical signal needs to be applied to the suspended 2D materials; therefore, metal contacts need to be made to contact the suspended 2D materials, as shown in the third step of **Figure 1(a)**. In order to avoid the common wet process inducing the collapse of 2D membranes, the shadow mask method [27] instead of lithography should be used. In addition, by combining conventional lithography, lift-off of deposited metal and transfer printing/ stamping of 2D materials, one can realize the fabrication of suspended 2D materials supported on patterned metal contacts, as depicted in **Figures 1(b)** and **2(b)**.

The schematic of the second approach of fabricating a suspended 2D material structure is presented in **Figure 1(c)**. After the transfer of 2D material onto substrate and metal contacts deposition, the 2D material is suspended by etching the underlying sacrificial layer with the predeposited metal contacts acting as the etching mask and clamping of the 2D materials. In this method, SiO2 has been used widely as the sacrificial layer, which is removed commonly by anisotropic wet etching with buffered hydrofluoric acid (BOE). In order to prevent the 2D materials from collapsing due to the surface tension between the 2D materials and BOE, the drying process is operated normally in a critical point dryer (CPD) [25, 29, 30]. In addition, the etching time needs to be adjusted carefully to control the undercut of SiO2 beneath the metal contacts to prevent the metal from collapsing. Although a more complex structure can be fabricated with this approach, as shown in **Figure 2(b)**, the wet etching involved in the fabrication process may introduce some contamination in the 2D materials, which may degrade the performance of the devices. Moreover, the acids used in this process are not suitable for some 2D materials, such as Bi2Se3 [33] and metals (e.g., Ti, Al). Thus, to avoid acid etching in the fabrication process, photoresist can be used as the sacrificial layer instead of SiO2 which can be removed with photoresist developers [33, 34].
