wileyonlinelibrary.comThird, monolayer TMDs show piezoelectricity due to the broken inversion symmetry. [5,6] As a result, monolayer TMDs indicate significant potential for flexible optoelectronics, [7,8] piezotronics, [5,6] mechanically enhanced nanocomposites, [9] and smart materials for strain sensing. [10] In these applications, strain is inevitably a critical object requiring comprehensive understanding.So far, substantial efforts have been spent on the study of strain engineering to the band structure of TMDs by using bending, [11][12][13][14][15] high-pressure compression, [16][17][18][19][20][21][22][23][24] and tensile elongation. [25] Their results indicate strain is an effective way to tune the bandgap from direct to indirect, generate the redshifts of trion and exciton peaks, and change the material conductivity from semiconducting to metallic. [26] However, an important question of how strain relaxes inside TMDs is still open.Herein, we investigate the strain relaxation of monolayer WS 2 triangular crystals deposited on polydimethylsiloxane (PDMS) substrate. The uniaxial tensile strain applied to the WS 2 crystals is transferred from PDMS substrate when the substrate is elongated by a loading frame. We observe that the trion and exciton PL peaks undergo a redshift when the substrate strain is increased from 0 to 0.16. However, the redshifts stop when the substrate strain is further increased from 0.16 to 0.32. This is caused by the strain relaxation in WS 2 through Strain-dependent electrical and optical properties of atomically thin transition metal dichalcogenides may be useful in sensing applications. However, the question of how strain relaxes in atomically thin materials remains not well understood. Herein, the strain relaxation of triangular WS 2 deposited on polydimethylsiloxane substrate is investigated. The photoluminescence of trions (X -) and excitons (X 0 ) undergoes linear redshifts of ≈20 meV when the substrate tensile strain increases from 0 to 0.16. However, when the substrate strain further increases from 0.16 to 0.32, the redshifts cease due to strain relaxation in WS 2 . The strain relaxation occurs through formation of wrinkles in the WS 2 crystal. The pattern of wrinkles is found to be dependent on the relative angle between an edge of the triangular WS 2 crystal and tensile strain direction. Finite element simulations of the strain distribution inside the WS 2 crystals explain the experimental observations.

Some highly ordered compounds of graphene oxide (GO), e.g., the so-called clamped and unzipped GO, are shown to have piezoelectric responses via first-principles density functional calculations. By applying an electric field perpendicular to the GO basal plane, the largest value of in-plane strain and strain piezoelectric coefficient, d 31 are found to be 0.12% and 0.24 pm/V, respectively, which are comparable with those of some advanced piezoelectric materials. An in-depth molecular structural analysis reveals that deformation of the oxygen doping regions in the clamped GO dominates its overall strain output, whereas deformation of the regions without oxygen dopant in the unzipped GO determines its overall piezoelectric strain. This understanding explains the observed dependence of d 31 on oxygen doping rate, i.e., higher oxygen concentration giving rise to a larger d 31 in the clamped GO whereas leading to a reduced d 31 in the unzipped GO. As the thinnest two-dimensional piezoelectric materials, GO has a great potential for a wide range of MEMS/NEMS actuators and sensors. *

Driven by the increasing demand for micro-/nano-technologies, stimuli-responsive shape memory materials at nanoscale have recently attracted great research interests. However, by reducing the size of conventional shape memory materials down to approximately nanometre range, the shape memory effect diminishes. Here, using density functional theory calculations, we report the discovery of a shape memory effect in a two-dimensional atomically thin graphene oxide crystal with ordered epoxy groups, namely C8O. A maximum recoverable strain of 14.5% is achieved as a result of reversible phase transition between two intrinsically stable phases. Our calculations conclude co-existence of the two stable phases in a coherent crystal lattice, giving rise to the possibility of constructing multiple temporary shapes in a single material, thus, enabling highly desirable programmability. With an atomic thickness, excellent shape memory mechanical properties and electric field stimulus, the discovery of a two-dimensional shape memory graphene oxide opens a path for the development of exceptional micro-/nano-electromechanical devices.

Phosphorene, the single-layer form of black phosphorus, as a new member of atomically thin material family, has unique puckered atomistic structure and remarkable physical and chemical properties. In this paper, we report a discovery of an unexpected electromechanical energy conversion phenomenon-shape memory effect-in Li doped phosphorene P4Li2, using ab initio density functional theory simulations. Two stable phases are found for the two-dimensional (2D) P4Li2 crystal. Applying an external electric field can turn on or off the unique adatom switches in P4Li2 crystals, leading to a reversible structural phase transition and thereby the shape memory effect with an tunable strain output as high as 2.06%. Our results demonstrate that multiple temporary shapes are attainable in one piece of P4Li2 material, offering programmability that is particularly useful for device designs. Additionally, the P4Li2 displays superelasticity that can generate a pseudoelastic tensile strain up to 6.2%. The atomic thickness, superior flexibility, excellent electromechanical strain output, the special shape memory phenomenon, and the programmability feature endow P4Li2 with great application potential in high-efficient energy conversion at nanoscale and flexible nanoelectromechanical systems.

Hydroxylation as a technique is mainly used to alter the chemical characteristics of hexagonal boron nitride (h-BN), affecting physical features as well as mechanical and electromechanical properties in the process, the extent of which remains unknown. In this study, effects of functionalization on the physical, mechanical, and electromechanical properties of h-BN, including the interlayer distance, Young's modulus, intrinsic strength, and bandgaps were investigated based on density functional theory. It was found that functionalized layers of h-BN have an average distance of about 5.48 Å. Analyzing mechanical properties of h-BN revealed great dependence on the degree of functionalization. For the amorphous hydroxylated hexagonal boron nitride nanosheets (OH-BNNS), the Young's modulus moves from 436 to 284 GPa as the coverage of -OH increases. The corresponding variations in the Young's modulus of the ordered OH-BNNS with analogous coverage are bigger at 460-290 GPa. The observed intrinsic strength suggested that mechanical properties are promising even after functionalization. Moreover, the resulted bandgap reduction drastically enhanced the electrical conductivity of this structure under imposed strains. The results from this work pave the way for future endeavors in h-BN nanocomposites research.