The spin-dependent Seebeck effect and the charge and spin figure of merit in a hybrid structure of single-walled carbon nanotubes and zigzag-edge graphene nanoribbons

A hybrid structure of carbon nanotubes and graphene nanoribbons was predicted and synthesized (Y. Li et al., Nat. Nanotechnol., 2012, 7, 394–400; P. Lou, J. Phys. Chem. C, 2014, 118, 4475–4482). Herein, using the non-equilibrium Green's function (NEGF) combined with density functional theory (DFT), the thermal spin transport properties and the figure of merit (a material constant proportional to the efficiency of a thermoelectric couple made with the material) of a composite of single-walled carbon nanotubes and zigzag-edge graphene nanoribbons, labeled (6,6)SWCNT/n-ZGNR, are investigated for n = 1, 2, 3, and 8. The results manifest that spin-dependent currents with opposite flow directions were generated when a temperature gradient was applied between two electrodes, indicating the occurrence of the spin-dependent Seebeck effect (SDSE). Remarkably, when n = 3, the charge current is equal to zero, meaning that a perfect SDSE is observed. Moreover, a pure spin-dependent Seebeck diode (SDSD) effect can be observed. Finally, we notice that the device presents an n-type characteristic when n = 1, while the device has a p-type feature when n = 2. In particular, the spin-up thermopower is equal to the spin-down thermopower when n = 3; as a consequence, the charge thermopower is equal to zero, further demonstrating that a perfect SDSE is generated. These discoveries indicate that the (6,6)SWCNT/n-ZGNR is a promising candidate for spin caloritronics devices.