First-principles study on transport property of molecular} device with non-collinear electrodes

Abstract

Molecular device is the ultimate electronic devices in the view point sense of scale size.Electron transport in molecular device shows obvious quantum effect,and the transport property of molecular device will be strongly affected by the chemical and structural details,including the contact position and method between the molecule and electrodes,the angle between two electrodes connecting to the molecule.However,we notice that in the existing reports on device simulations from first principles the two electrodes are always in a collinear case.Even for multi-electrode simulations,one usually used to adopt orthogonal electrodes,namely,each pair of the electrodes is in a collinear case.As the electrode configuration will clearly affect the transport property of a device on a nanometer scale,the first principles quantum transport studies with non-collinear electrodes are of great importance,but have not been reported yet.In this paper,we demonstrate the calculations of a transport system with non-collinear electrodes based on the state-of-the-art theoretical approach where the density functional theory (DFT) is combined with the Keldysh non-equilibrium Green's function (NEGF) formalism. Technically,to model a quantum transport system with non-collinear electrodes,the center scattering region of the transport system is placed into an orthogonal simulation box in all the other quantum transport simulations,while one or two electrodes are simulated within a non-orthogonal box.This small change in the shape of the simulation box of the electrode provides flexibility to calculate transport system with non-collinear electrodes,but also increases the complexity of the background coding.To date,the simulation of transport system with non-collinear electrodes has been realized only in the Nanodcal software package. Here,we take the Au-benzene (mercaptan)-Au molecular devices for example,and systematically calculate the quantum transport properties of the molecular devices with various contact positions and methods,and specifically,we first demonstrate the effect of the angle between the two electrodes on the transport property of molecular device from first principles.In our NEGF-DFT calculations performed by Nanodcal software package,the double- polarized atomic orbital basis is used to expand the physical quantities,and the exchange-correlation is treated in the local density approximation,and atomic core is determined by the standard norm conserving nonlocal pseudo-potential.Simulation results show that the chemical and structural details not only quantitatively affect the current value of the molecular device,but also bring new transport features to a device,such as negative differential resistance.From these results,we can conclude that the physics of a transport system having been investigated in more detail and a larger parameter space such as the effect of the contact model having been assessed by a comparison with ideal contacts,further understanding of the transport system can be made and more interesting physical property of the device can be obtained,which will be useful in designing of emerging electronics.