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Despite the low thermoelectric (TE) efficiency of graphene, its flexibility features are attractive for flexible and wearable next‐generation thermoelectric applications. So, it will be highly desirable to synthesize graphene‐based high TE material. Hence, the possibility of significant enhancement of the TE performance in ZnO/graphene heterostructure is investigated. The ZnO monolayer has a direct band gap of 3.3 eV, while a band gap of 5 meV in the ZnO/graphene heterostructure is found. The highest ZT ≈ 2.4 in the n‐doped ZnO/graphene heterostructure at 500 K is obtained, whereas the ZnO monolayer shows ZT ≈ 1.3 at 700 K. Particularly, this giant ZT in the ZnO/graphene heterostructure is found even at a low carrier concentration (≈1011 carrier cm−2). Besides, the ZnO/graphene heterostructure also displays a ZT ≈ 0.8 even at 300 K with a very low carrier concentration (≈1010 carrier cm−2). This outstanding TE performance originates from the TE coefficient advantages of each layer; high electrical conductivity from graphene and high Seebeck coefficient from ZnO incorporate with reduced thermal conductivity in the heterostructure. The findings will stimulate further studies to confirm the results as well as the development of flexible TE generators based on graphene for Internet of things thermoelectric applications. [ABSTRACT FROM AUTHOR]

We investigated the electronic structure and magnetic properties of hydrogenated bilayer graphene and graphene/h-BN heterostructure as a function of applied uniaxial compression normal to the interface plane using van der Waals density functional theory. The H atoms preferred a cluster formation was found in the hydrogenated bilayer graphene and graphene/h-BN systems with a non-magnetic ground state. With increasing external pressure, the equivalence of two sublattices in graphene was broken because of the interaction between graphene and the underlying layer and the site dependent adsorption energy difference was greatly enhanced with the applied pressure. This resulted in a transition from an NM ground state to a ferromagnetic in hydrogenated bilayer graphene and graphene/h-BN heterostructure. Thus, we propose that the magnetic property can be manipulated by external pressure and that hydrogenated systems can be utilized for potential spintronics applications at low H concentrations. [ABSTRACT FROM AUTHOR]

[Display omitted] • The origin of the ferromagnetism in the 2D Co doped ZnO system was investigated. • The graphene/Co doped ZnO/graphene multilayer structure was also considered. • The hydrogen effect was found to be the origin of the ferromagnetism in both systems. • High Curie temperature (∼500 K) was found in both systems using the Heisenberg model. • Both systems showed optical transparency in the visible spectrum. Despite extensive efforts to find room temperature ferromagnetic (FM) two-dimensional (2D) materials, only a few 2D FM materials have been experimentally synthesized. Particularly, it is a still challenging task to find optically transparent room temperature 2D FM systems. Recently, room temperature ferromagnetism was confirmed in 2D graphene/Co doped ZnO/graphene multilayer structure. However, in our calculations, the Co doped ZnO (ZCO) monolayer showed an antiferromagnetic (AFM) ground state. Moreover, both ZCO/graphene bilayer and graphene/ZCO/graphene trilayer systems also exhibited an AFM state. Thus, we investigated the possibility of hydrogen-induced ferromagnetism. After the introduction of the H atom (H-ZCO), we found a ferromagnetic (FM) ground state in both H-ZCO monolayer and graphene/H-ZCO/graphene trilayer systems. We obtained that the FM state appeared only when the H atom stayed on the same ZCO layer. Thus, we propose that the ferromagnetism in the graphene/ZCO/graphene originates from the H effect. Also, we obtained a Curie temperature (T C) of 500 K in H-ZCO and graphene/H-ZCO/graphene trilayer systems. Moreover, we find that the graphene/H-ZCO/graphene shows optically transparent behavior in the visible frequencies. [ABSTRACT FROM AUTHOR]

Abstract: Using the Vienna ab initio simulation package (VASP) we have explored structural, adsorption, and magnetic properties of graphene/BN/Co(111) with the help of semi empirical and non-local van der Waals functional methods such as DFT-D2, vdW-DF2, optPBE-vdW, and optB88-vdW. In the most stable optimized structure, one carbon is on top of B atom and the other carbon is on top of hollow site of BN layer. No buckling geometry is found in both graphene and BN layers with vdW-DF2 method. In contrast, all three other approaches show that a buckling feature takes place in BN layer adjacent to Co substrate and due to this buckling an adsorption energy of graphene is significantly enhanced. Very interestingly, the graphene has an energy gap only in one spin band due to the influence of Co and an insulating state in BN layer adjacent to graphene is observed. In addition, the Co layer displays half metallic states as well. This feature suggests that the graphene/BN/Co(111) can be utilized for ideal tunneling magnetoresistance (TMR) structure because a high TMR ratio is expected. [Copyright &y& Elsevier]

We have investigated the magnetization reversal induced by carrier doped graphene (Be, B, N, O, or Cl doping) in Ni/Graphene/Co. In undoped case, the magnetic layers have an antiferromagnetic (AFM) coupling and this is still preserved from Be to O doping. We find magnetization reversal from AFM to ferromagnetic interaction induced by Cl doped graphene. In addition, the Ni and the Co layers show the opposite spin asymmetry near the Fermi level and this implies that each layer will generate completely different in-plane spin current in the same direction if an external electric field is applied. [ABSTRACT FROM AUTHOR]

We investigated the electric field dependent band structures and optical properties of graphene/phosphorene bilayer. Remarkably, the effective electron Fermi velocity in graphene/phosphorene was comparable to that of pure graphene monolayer. Moreover, this feature was not disrupted even in the presence of a finite external electric field. The electronic band structure changed from a semiconductor to a metallic state with an external electric field strength of 0.5 V/Å. In addition, the electric field enhanced the structural stability of a bilayer system because the binding energy was increased approximately 20–30% as compared with that at zero electric field. We found that the optical anisotropy regarding the electric polarization along armchair and zigzag directions disappeared, but the anisotropic optical properties still survived regarding the electric polarization for in-plane and perpendicular directions. The small effective mass, high speed of electrons, and optical property in graphene/phosphorene bilayer may provide a promising way for graphene based device applications. [ABSTRACT FROM AUTHOR]

Using the DFT-D2 van der Waals density functional theory method, we have investigated thickness dependent optical properties for polarized light in multilayer BN/graphene/BN structures by changing the stacking order. It has been found that the optical properties of multilayers are strongly subjective to the polarization of the light, whereas no meaningful dependence of the thickness and stacking order is observed. At infrared frequency, a high refractive index is found in parallel polarization and it is suppressed by increasing the frequency, while it is almost constant for perpendicular polarization. A huge reflectivity is also achieved at infrared frequency in 3 and 4 monolayer (ML) films for parallel polarization, whereas it becomes almost constant in perpendicular polarization. In addition, the frequency dependent optical conductivity is found in parallel polarization, but it vanishes for perpendicular polarization. [ABSTRACT FROM AUTHOR]

Abstract: Using the full potential based WIEN 2K method, we have explored thickness dependent energy band gaps and effective masses of BN layer sandwiched by graphene (G/BN/G) and graphene sandwiched by BN (BN/G/BN) systems. Here, capping and substrate layer coverages are changed from 1 monolayer (ML) to 2ML and 1 to 4ML, respectively. In G/BN/G systems, we find rather small energy gaps and the energy dispersions become quadratic near the K-point. Unlike in G/BN/G, a trilayer BN/G/BN shows large gap of 117meV and it decreases as the number of BN layer increases, but we still find a gap about 90meV in BN (2ML)/G/BN (4ML) system. The thickness dependent suppression of band gap in BN/G/BN can be nicely interpreted in terms of interlayer distance from BN substrate to graphene. Furthermore, very interestingly, the energy dispersion is nearly linear near the K-point and this linearity is still preserved even in all BN/G/BN systems. Surprisingly, the effective mass decreases as the number of BN layer increases. For instance, the smallest effective mass of 0.00235 me is estimated in conduction band along K − Γ direction. Overall, our calculations may suggest that the BN/G/BN system has potential application for fast radio frequency (RF) device or on-off switching transistor because the linearity and small effective mass is kept without any external factors such as electric field, strain, and doping. [Copyright &y& Elsevier]