Using non equilibrium Green's function formalism coupled with density functional theory, we carry out electronic transport calculation in two types of molecular devices, one constructed by linear monoatomic carbon chain (center dot center dot center dot C-C-C center dot center dot center dot) and the other by two carbon chains capped with a phenyl ring (center dot center dot center dot C-C-Ph-C-C center dot center dot center dot), sandwiched between two z-shape electrodes, constructed by zigzag-armchair-zigzag (zzac-zz) graphene nanoribbons (GNRs). The potential difference between the z-shape contacts can be varied by employing an external d.c. voltage source. Thus, one may observe the variation of conductivity through the channels. The current voltage (I-V) characteristics of the proposed resistors show N-type negative differential resistance (NDR), within a particular voltage region. The figure of merit or PVR (peak to valley) ratio (I-peak/I-valley) gets significantly increased, on capping the chains with phenyl ring. A higher value of PVR in I-V characteristics enhances the possibility of applications utilizing NDR. The calculated I-V characteristic is asymmetric and the rectification ratio is found to be 7, in case of the linear carbon chain. The rectification ratio R(V) = I(V)/I(V) is an important parameter which determines, its suitability as rectifying device. It has been demonstrated that on varying the conformation of the phenyl ring with respect to the plane of electrodes, the transport properties of the system can be modulated. Interestingly, I-V characteristics are asymmetric and show dual NDR peaks in perpendicular conformation of the phenyl ring, with respect to the electrodes in the (center dot center dot center dot C-C-Ph-C-C center dot center dot center dot) system. The figure of merit is found to be respectively 8 and 51 for the first and second NDR regions. The later value is extremely high, making it an excellent candidate for potential applications. Moreover, the multi peak NDR device may be widely used in multiple-valued logics. Only a limited number of multiple NDR peak molecular-based nano systems have so far been reported, which are quite complex; by contrast the present system seems to be quite simple. The physical phenomenon of NDR was explained in the light of molecular projected self-consistent Hamiltonian (MPSH) and also the evolution of the frontier molecular orbitals (HOMO-LUMO) as well as transmission under various external bias voltages. (C) 2015 Elsevier B.V. All rights reserved.
