**2.2. Actuation**

**Figure 6.** Schematic of device structure and fundamental characteristics of the all-elastomeric transparent stretchable

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

**Figure 7.** (a) Optical image of the single-atomic layer MoS2 flake. Blue and yellow spheres represent Mo and S atoms, respectively.(b) Polar plot of the second harmonic intensity from single-layer MoS2 as a function of the crystal's azimu‐ thal angle θ.(c) A typical flexible device with single-layer MoS2 flake and electrodes at its zigzag edges.(d) Operation

scheme of the single-layer MoS2 piezoelectric device [43].

gated sensor [42].

The crumpling of materials is widely observed in various objects as small as biological membranes, in objects as thin as a piece of paper, and in systems as large as the Earth's crust. 2D actuation systems responsive to electrochemical, light, and other external stimuli can convert different energy forms (electric, chemical, photonic, thermal, etc.) to mechanical energy that is potentially profitable for diverse applications ranging from robots, sensors to memory chips [15, 44]. Theoretically, Roger et al. [45] have studied the electrochemical actuation of monolayer graphene upon charge injection and ionic liquid (IL) electrolyte immersion. They have concluded that the electrostatic double layer could induce strains of more than 1% and its contribution to the overall strain was always equal to or higher than that of the quantum-mechanical strain (~0.2%) from charge injection of −0.1 e per C atom. Based on these theoretical predictions, GO is in principle an excellent material for actuators and artificial muscles.

Zhang, et al. [46] performed molecular dynamics simulation to create a nanosized graphene origami box. By warping the top graphene layer downward and the bottom graphene layer upward, the cross-shaped cubic graphene nanocage could encapsulate nano objects such as biomolecules (**Figure 8**). This paradigm opens up a new avenue to control the 3D architecture of 2D-layered materials and provides a feasible way to exploit and fabricate the 2D nanosized actuators.

**Figure 8.** Programmable graphene folding with designed morphology.(a) and (b) represent the initial and final config‐ urations of a graphene nanocage, respectively.(c) Schematic illustrating rapid loss of H2O in GO and subsequent crum‐ pling of GO nanosheets [46, 47].

Ma et al. [47] experimentally proved that GO in aqueous solution could be aerosolized and dried to produce crumpled nanopaper-like sheets, which are similar to the graphene nanocage. They used online size selection and aerosol mass analysis to determine the fractal dimension (*D*) of crumpled GO nanosheets. *D* is able to be tuned by altering solvent conditions. Typically, a 10% acetone mixture increases *D* to 2.68 ± 0.02 from 2.54 ± 0.04. Calculations of the confine‐ ment force indicate that crumpling of GO nanosheets is driven by the capillary force associated with rapid loss of the solvent.

Similarly, a fluidic motion of alcohol molecules which across the interlayer gap in layered double hydroxide (LDH) could enable rapid and reversible tuning of interlayer spacing of the LDH at sub-Å precision, reported by Ishihara et al. [48]. This so-called hydrogen bond–driven "homogeneous intercalation" mechanism could be used in rapid, reversible, and ultraprecise actuation of LDH materials.

Similar approaches could also be applied to magadiite [49] and layered potassium hexaniobate [50]. Novel photoactivated artificial muscle model units could be obtained as they reported. For example, as seen in **Figure 9**, it is clearly observed on a cross-cut section of the layered hybrid film that upon photoirradiation of a layered hexaniobate intercalated with a polyfluor‐ oalkyl azobenzene derivative, a very large magnitude lateral movement (sliding) of the nanosheets was reversibly induced. By applying this strategy, organic/inorganic hybrid nanosheets reversibly and horizontally slide on a macroscale upon on/off photoirradiation, which results in vertically shrinking and expansion of the interlayer spaces in the layered hybrid structure. The sliding movement of the structure on such a giant scale is the first example of an artificial muscle model unit having remarkable similarity to that in natural muscle fibrils.

**Figure 9.** Schematic diagram of the niobate nanosheet sliding movement and interlayer distance change induced by photochemical trans–cis isomerization of the azobenzene molecules [50].

#### **2.3. Communication**

**Figure 8.** Programmable graphene folding with designed morphology.(a) and (b) represent the initial and final config‐ urations of a graphene nanocage, respectively.(c) Schematic illustrating rapid loss of H2O in GO and subsequent crum‐

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

Ma et al. [47] experimentally proved that GO in aqueous solution could be aerosolized and dried to produce crumpled nanopaper-like sheets, which are similar to the graphene nanocage. They used online size selection and aerosol mass analysis to determine the fractal dimension (*D*) of crumpled GO nanosheets. *D* is able to be tuned by altering solvent conditions. Typically, a 10% acetone mixture increases *D* to 2.68 ± 0.02 from 2.54 ± 0.04. Calculations of the confine‐ ment force indicate that crumpling of GO nanosheets is driven by the capillary force associated

Similarly, a fluidic motion of alcohol molecules which across the interlayer gap in layered double hydroxide (LDH) could enable rapid and reversible tuning of interlayer spacing of the LDH at sub-Å precision, reported by Ishihara et al. [48]. This so-called hydrogen bond–driven "homogeneous intercalation" mechanism could be used in rapid, reversible, and ultraprecise

Similar approaches could also be applied to magadiite [49] and layered potassium hexaniobate [50]. Novel photoactivated artificial muscle model units could be obtained as they reported. For example, as seen in **Figure 9**, it is clearly observed on a cross-cut section of the layered hybrid film that upon photoirradiation of a layered hexaniobate intercalated with a polyfluor‐ oalkyl azobenzene derivative, a very large magnitude lateral movement (sliding) of the nanosheets was reversibly induced. By applying this strategy, organic/inorganic hybrid nanosheets reversibly and horizontally slide on a macroscale upon on/off photoirradiation, which results in vertically shrinking and expansion of the interlayer spaces in the layered

pling of GO nanosheets [46, 47].

with rapid loss of the solvent.

actuation of LDH materials.

Emotional (or feeling) communication skills are natural behavior in biological systems. However, similar communication between humans and autonomous robots is a tremendous challenge to be achieved.

Appearance and texture identification in an artificial skin allows creating and broadcasting emotional cues, which may facilitate the acquisition of the robot's emotional behavior. The fabrication of a network consisting of mechanically flexible sensors is the key to achieve artificial intelligence that comes into direct contact with humans for biomedical applications such as prosthetic skin. To mimic the interaction behavior such as tactile sensing properties of natural skin, large arrays of pixel sensors on a flexible and stretchable substrate are usually required [51]. The integration of 2D materials in FET arrays as the dielectric layer leads to a new type of active sensing devices which not only have high sensitivity but also enable to initiate responsive interactive behavior. In this context, there are several cases reported. For example, Wang et al. [52] integrated various electronic, sensor, and light-emitting components (involving both organic and inorganic materials) on a thin plastic substrate (**Figure 10**). This work demonstrated a possibly practical technology platform serving as a flexible userinteractive system that could not only detect and spatially map external stimuli such as pressure, but also respond with a seamlessly integrated display. The responsive pressure profile is instantaneously visible without the need of sophisticated data acquisition circuits and electronic boards on such systems. Such approach based on integration of 2D functional materials into a flexible thin film device could lead to an emerging and hot research topic, i.e., electronic skin, or e-skin.

**Figure 10.** (a) Cross-sectional schematic showing one pixel of the interactive e-skin device, consisting of various com‐ ponents.(b) Schematic of the e-skin circuit matrix.(c) Photograph of an integrated device showing that light is locally emitted when the device surface is touched. (d) PDMS slabs with C, A, and L shapes are prepared and used to apply pressure onto the sensor array.(e) Green, blue, and red color interactive e-skins are used to spatially map and display the pressure [52].
