Search Results (5)

Controlling the balance between piezoelectric and flexoelectric effects is crucial for tailoring the electromechanical responses of a material. In twisted graphene, it is found that the electromechanical response near the domain walls (DWs) is dominated by either the flexoelectric effect as in twisted bilayer graphene (tBLG) or the piezoelectric effect as in twisted monolayer–bilayer graphene (tMBG). The codominance of both effects in a single system is rare. Here, utilizing lateral piezoresponse force microscopy (LPFM), we show that piezoelectric and flexoelectric effects can coexist and are equally important in twisted double bilayer graphene (tDBG), termed as the piezo-flexoelectric effect. Unlike tBLG and tMBG, distinctive two-step LPFM spatial profiles are captured across the moiré DWs of tDBG. By decomposing the LPFM signal into axisymmetric and antisymmetric components, we find that the angular dependence of both components satisfies sinusoidal relations. Quantitatively, the in-plane piezoelectric coefficient of DWs in tDBG is determined to be 0.15 pm/V by dual AC resonance tracking (DART) LPFM measurement. The conclusion is further supported by continuum mechanics simulations. Our results demonstrate that the stacking configuration serves as a powerful tuning knob for modulating the electromechanical responses of twisted van der Waals materials. Full article

Graphene is undoubtedly the carbon allotrope that has attracted the attention of a myriad of researchers in the last decades more than any other. The interaction of external or intercalated Li and Li⁺ with graphene layers has been the subject of particular attention for its importance in the applications of graphene layers in Lithium Batteries (LiBs). It is well known that lithium atoms and Li⁺ can be found inside and/or outside the double layer of graphene, and the graphene layers are often twisted around its parallel plane to obtain twisted graphene with tuneable properties. Thus, in this research, the interactions between Li and Li⁺ with bilayer graphene and twisted bilayer graphene were investigated by a first-principles density functional theory method, considering the lithium atom and the cation at different symmetry positions and with two different adsorption configurations. Binding energies and equilibrium interlayer distances of filled graphene layers were obtained from the computed potential energy profiles. This work shows that the twisting can regulate the interaction of bilayer graphene with Li and Li⁺. The binding energies of Li⁺ systematically increase from bilayer graphene to twisted graphene regardless of twisted angles, while for lithium atoms, the binding energies decrease or remain substantially unchanged depending on the twist angles. This suggests a higher adsorption capacity of twisted graphene towards Li⁺, which is important for designing twisted graphene-based material for LiB anode coating. Furthermore, when the Li or Li⁺ is intercalated between two graphene layers, the equilibrium interlayer distances in the twisted layers increase compared to the unrotated bilayer, and the relaxation is more significant for Li⁺ with respect to Li. This suggests that the twisted graphene can better accommodate the cation in agreement with the above result. The outcomes of this research pave the way for the study of the selective properties of twisted graphene. Full article

Diamanes are unique 2D carbon materials that can be obtained by the adsorption of light atoms or molecular groups onto the surfaces of bilayer graphene. Modification of the parent bilayers, such as through twisting of the layers and the substitution of one of the layers with BN, leads to drastic changes in the structure and properties of diamane-like materials. Here, we present the results of the DFT modelling of new stable diamane-like films based on twisted Moiré G/BN bilayers. The set of angles at which this structure becomes commensurate was found. We used two commensurate structures with twisted angles of θ = 10.9° and θ = 25.3° with the smallest period as the base for the formation of the diamane-like material. Previous theoretical investigations did not take into account the incommensurability of graphene and boron nitride monolayers when considering diamane-like films. The double-sided hydrogenation or fluorination of Moiré G/BN bilayers and the following interlayer covalent bonding led to the opening of a gap up to 3.1 eV, which was lower than the corresponding values of h-BN and c-BN. The considered G/BN diamane-like films offer great potential in the future for a variety of engineering applications. Full article

Twisted van der Waals heterostructures were recently found to possess unique physical properties, such as superconductivity in magic angle bilayer graphene. Owing to the nonhomogeneous stacking, the energy of twisted van der Waals heterostructures are often higher than their AA or AB stacking counterpart, therefore, fabricating such structures remains a great challenge in experiments. On the other hand, one dimensional (1D) coaxial van der Waals structures has less freedom to undergo phase transition, thus offer opportunity for fabricating the 1D cousin of twisted bilayer graphene. In this work, we show by molecular dynamic simulations that graphene nanoribbons can self-assemble onto the surface of carbon nanotubes driven by van der Waals interactions. By modifying the size of the carbon nanotubes or graphene nanoribbons, the resultant configurations can be controlled. Of particular interest is the formation of twisted double walled carbon nanotubes whose chiral angle difference can be tuned, including the 1.1° magic angle. Upon the longitudinal unzipping of such structures, twisted bilayer graphene nanoribbons can be obtained. As the longitudinal unzipping of carbon nanotubes is a mature technique, we expect the strategy proposed in this study to stimulate experimental efforts and promote the fast growing research in twistronics. Full article

While it is known that the double-layer Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+y}$ (BSCCO) cuprate superconductor exhibits a one-dimensional (1D) incommensurate superlattice (IS), the effect of IS on the electronic structure remains elusive. Following the recent shift of interest from an underdoped phase to optimum and overdoped phases in BSCCO by increasing the hole doping x, controlled by the oxygen interstitials concentration y, here we focus on the multiple splitting of the density of states (DOS) peaks and emergence of higher order van Hove singularities (VHS) due to the 1D incommensurate superlattice. It is known that the 1D incommensurate wave vector $\mathbf{q}=ϵ\mathbf{b}$ (where $\mathbf{b}$ is the reciprocal lattice vector of the orthorhombic lattice) is controlled by the misfit strain between different atomic layers in the range $0.209$–$0.215$ in BSCCO and in the range $0.209$–$0.25$ in Bi${}_2$Sr${}_2$Ca${}_{1-x}$Y${}_x$Cu${}_2$O${}_{8+y}$ (BSCYCO). This work reports the theoretical calculation of a complex pattern of VHS due to the 1D incommensurate superlattice with large 1D quasi-commensurate supercells with the wave vector $ϵ=9/\eta$ in the range $36>\eta >43$. The similarity of the complex VHS splitting and appearing of higher order VHS in a mismatched CuO${}_2$ bilayer with VHS due to the moiré lattice in strained twisted bilayer graphene is discussed. This makes a mismatched CuO${}_2$ bilayer quite promising for constructing quantum devices with tuned physical characteristics. Full article