Elsevier

Applied Surface Science

Volume 418, Part A, 1 October 2017, Pages 393-400
Applied Surface Science

Full Length Article
The effect of electrodes on 11 acene molecular spin valve: Semi-empirical study

https://doi.org/10.1016/j.apsusc.2017.02.175Get rights and content

Highlights

  • A semi-empirical approach to analyze the electron transport characteristics of 11 acene molecular spin valve.

  • The effect of electrodes modifying the spin-dependence behaviours of these systems in a controlled way.

  • Iron electrode is an efficient one to construct 11-acene single molecular spin valve.

Abstract

A new revolution in electronics is molecular spintronics, with the contemporary evolution of the two novel disciplines of spintronics and molecular electronics. The key point is the creation of molecular spin valve which consists of a diamagnetic molecule in between two magnetic leads. In this paper, non-equilibrium Green’s function (NEGF) combined with Extended Huckel Theory (EHT); a semi-empirical approach is used to analyse the electron transport characteristics of 11 acene molecular spin valve. We examine the spin-dependence transport on 11 acene molecular junction with various semi-infinite electrodes as Iron, Cobalt and Nickel. To analyse the spin-dependence transport properties the left and right electrodes are joined to the central region in parallel and anti-parallel configurations. We computed spin polarised device density of states, projected device density of states of carbon and the electrode element, and transmission of these devices. The results demonstrate that the effect of electrodes modifying the spin-dependence behaviours of these systems in a controlled way. In Parallel and anti-parallel configuration the separation of spin up and spin down is lager in the case of iron electrode than nickel and cobalt electrodes. It shows that iron is the best electrode for 11 acene spin valve device. Our theoretical results are reasonably impressive and trigger our motivation for comprehending the transport properties of these molecular-sized contacts.

Introduction

Spintronics enable us to utilize spin degree of freedom of the electron. Comparing to conventional electronics, spintronics devices provide power efficient, fast responsive devices such as MRAM, spin LED etc. By using nanomaterials we can increase the efficiency of these devices. Using various theoretical methods new materials for spintronics applications are being studied [1], [2], [3].

During the last decades, the molecular spintronic devices have gained magnetism coupled with the long spin-diffusion length which produces a valuable excitement in the next generation of organic spintronic devices [4], [5], [6], [7]. This is achieved by both the metal-molecule contact and molecule itself.

The engineering of spin transport in small conjugated organic molecules [8], [9], [10], graphene [11], [12], [13] and magnetic molecules [14], [15], has achieved a tremendous analysis because of the significant applications in the molecular spintronic devices. Interestingly, 11-acenes molecule belongs to the organic molecular candidates and forms one of the most attractive materials due to their typical structural geometries and excellent electronic features [16], [17].

Several theoretical studies were carried out to understand the different electronic properties of these molecules. Bendiko and his co-workers [18] analysed the characteristics of shortest acene rings and they suggested that, 6-acene molecule have a magnetic ground state. Jiang & Dai [19] studied the electronic ground state of higher acene molecular junctions. They reported that, the magnetic ground state of higher acene molecules is stabilized. [20]

Recently S. Caliskan & A. Laref [21] analysed the spin transport properties of n-polyacene molecules with Ni electrodes. They suggested that, in these molecular devices, structural configuration and molecular electronics are playing an essential role in spin-polarized transport properties. Previous studies have inferred that, in short acene molecular junctions the majority spin states are more localized and they cannot hold any magnetic moment and representing paramagnetic states [6], [10], [21], [22]. So that we focused on 11-acene molecular junction to explore the influence of electrodes on spin dependent transport characteristics.

In general, the electrical properties of molecular devices are understood by measuring the transport through a molecule sandwiched between metallic electrodes [5]. The transport characteristics of molecular devices are determined by both the metal-molecule contact and molecule itself. The local electronic structure of molecular sites controls the mechanism of a molecular device. For this occasion, the metal-molecule contact should capture a valuable attention.

In this study, we investigate the transport properties of 11-acene molecular junctions by varying the electrodes such as Iron (Fe), Cobalt (Co) and Nickel (Ni). In these molecular devices the geometric, electronic and magnetic properties are effectively modulated by varying the electrodes. These properties are analysed by using non-equilibrium Green’s function (NEGF) [23], [24], [25] combined with Extended Huckel Theory (EHT) [26], a semi-empirical [27], [28], [29], [30] approach to model the spin dependent transport of 11-acene molecular system. We investigated the spin polarised device density of states, projected device density of states of carbon and the electrode element, transmission spectrum, transmission pathways, total energy and charge transfer of these devices to understand the influence of different electrodes over the 11-acene molecular junction.

Section snippets

Model and computational method

For the 11-acene molecular junctions the spin-resolved transport properties are determined by implementing a fully spin-polarized DFT with a NEGF scheme in an Atomistic Tool Kit (ATK) package [31]. The structural optimization of 11-acene junctions are computed using DFT, where the wave are represented using a linear combination of atomic orbitals (LCAO) as a basis set. The atomic cores are described by nonlocal, norm-conserving scalar-relativistic Troullier-Martins pseudopotentials [32], [33].

Results and discussion

In our work, the electronic structure and spin transport properties of the 11- acene molecular junction is analysed with various electrodes. Here the left and right electrodes are joined in parallel (PL) and anti-parallel (APL) magnetic configuration. At first we compute the spin polarized total density of states (DOS) of the three molecular systems for both spin up and spin down carriers, which is represented Fig. 2. It is observed that, for the molecular system with cobalt electrode shows

Conclusion

Our theoretical investigation demonstrated the effect of electrode for tuning the spin transport properties of 11-acene molecular system. By changing the electrodes for the 11-acene molecular junction the electronic structure, magnetic and transport characteristics are manipulated in a controlled manner.

From DOS and Transmission spectra of these systems shows that all the three system can act as a molecular spin valve. Out of these 11-acene with Fe system shows more stability than Co and Ni,

Acknowledgment

We gratefully acknowledge financial support for this project from DST-FIST, Government of India (Ref. No SR/FST/PSI-010/2010).

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