Vacancy tuned thermoelectric properties and high spin filtering performance in graphene/silicene heterostructures

Flexible thermoelectrics in crossed graphene/hBN composites

Nanostructures exhibit unusual properties due to the dominance of quantum mechanical effects. In addition, the geometry of a nanostructure can have a strong influence on its physical properties. Using the tight-binding and force-constant approaches with the help of the non-equilibrium Green's function method, the transport and thermoelectric properties of cross-shaped (X-shaped) composite heterostructures are studied in two cases: Mixed graphene and h-BN (HETX-CBN) and all graphene (HETX-C) cross-shaped structures. Our numerical results show that an X-shaped structure helps to manipulate its electronic and phononic properties. The transport energy gap can be tuned in the range of ~ 0.8 eV by changing one arm width. Due to the drastic decrease in the electronic conductance of HETX-CBN and the dominance of the phononic thermal conductance, the ZT performance is degraded despite the high Seebeck coefficient value (in the order of meV). However, HETX-C has better ZT performance due to better electronic conductance and lower phononic/electronic thermal ratio, it can enhance the ZT ~ 2.5 times compared to that of zigzag graphene nanoribbon. The thermoelectric properties of the system can be tuned by controlling the size of the arms of the device and the type of its atoms.

Tuning phononic and electronic contributions of thermoelectric in defected S-shape graphene nanoribbons

Thermoelectrics as a way to use waste heat, is essential in electronic industries, but its low performance at operational temperatures makes it inappropriate in practical applications. Tailoring graphene can change its properties. In this work, we are interested in studying the transport properties of S-shape graphene structures with the single vacancy (SV) and double vacancy (DV) models. The structures are composed of a chiral part, which is an armchair graphene nanoribbon, and two zigzag graphene ribbons. We investigate the changes in the figure of merit by means of the Seebeck coefficient, electronic conductance, and electronic and phononic conductances with the vacancies in different device sizes. The transport properties of the system are studied by using the non-equilibrium Green’s function method, so that the related Hamiltonians (dynamical matrices) are obtained from the tight-binding (force constant) model. The maximum figure of merit (ZT) obtains for the DVs in all lengths. Physical properties of such a system can be tuned by controlling various parameters such as the location and the type of the defects, and the device size. Our findings show that lengthening the structure can reduce phononic contribution, and single vacancies than double vacancies can better distinguish between electronic thermal conductance behavior and electronic conductance one. Namely, vacancy engineering can significantly increase thermoelectric performance. In the large devices, the SVs can increase the ZT up to 2.5 times.