Investigation of Terminal Group Effect on Electron Transport Through Open Molecular Structures

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1 Commun. Theor. Phys. 59 (2013) Vol. 59, No. 5, May 15, 2013 Investigation of Terminal Group Effect on Electron Transport Through Open Molecular Structures C. Preferencial Kala, 1 P. Aruna Priya, 2 and D. John Thiruvadigal 1, 1 Department of Physics and Nano Technology, Center for Materials Science and Nanodevices, SRM University, Kattankulathur , India 2 Department of Electronics and Communication Engineering, Center for Materials Science and Nanodevices, SRM University, Kattankulathur , India (Received November 2, 2012; revised manuscript received January 11, 2013) Abstract The effect of terminal groups on the electron transport through metal-molecule-metal system has been investigated using nonequilibrium Green s function (NEGF) formalism combined with extended Huckel theory (EHT). Au-molecule-Au junctions are constructed with borazine and BCN unit structure as core molecule and sulphur (S), oxygen (O), selenium (Se) and cyano-group (CN) as terminal groups. The electron transport characteristics of the borazine and BCN molecular systems are analyzed through the transmission spectra and the current-voltage curve. The results demonstrate that the terminal groups modifying the transport behaviors of these systems in a controlled way. Our result shows that, selenium is the best linker to couple borazine to Au electrode and oxygen is the best one to couple BCN to Au electrode. Furthermore, the results of borazine systems are compared with that of BCN molecular systems and are discussed. Simulation results show that the conductance through BCN molecular systems is four times larger than the borazine molecular systems. Negative differential resistance behavior is observed with borazine-cn system and the saturation feature appears in BCN systems. PACS numbers: b, h, b Key words: molecular electronics, extended Huckel theory (EHT), nonequilibrium Green s function (NEGF), quantum transport 1 Introduction Molecular electronics has received great attention because of the ultimate size limit of functional devices. The study of electron transport in metal-molecule-metal interfaces is the first step in the development of molecular electronics. At present, a number of theoretical models concentrating on the sandwich structure of an individual molecule between metallic electrodes using semi empirical [1 3] and first principle [4 6] theory are developed to provide quantitative description of molecular conduction. The semi-empirical method is computationally flexible compared to ab initio and it is able to capture several quantitative descriptions of molecules of moderate size. [7 9] Although current simulation methods can qualitatively describe some electronic properties of such simple systems, there are still a lot of issues waiting to be made understandable for the purpose of predicting, designing and controlling new molecular devices. Several experimental techniques have been developed, [10 11] including various scanning probes, break junctions, nano probes and crossed wires, and some reproducible measurements on single molecules have been made. There have also been intense theoretical investigations of transport properties of single molecules. Bai and co-workes [12 14] studied the electron transport of thiolated borazine and benzene using ab initio method and they demonstrated that the magnitude of benzene is much higher than borazine due to less delocalization. Also they observed Negative differential resistance (NDR) behavior for borazine in lower bias. Based on break junction experiment Xia et al. [15] recently analyzed the transport behavior of borazine molecule with different metal-molecule interface and reported that the enhancement of current is more for the bridge sited molecule. In this study, we apply nonequilibrium Green s function (NEGF) formalism [16 19] coupled with the extended Huckel theory (EHT) [20] to investigate the effect of terminal group on the electron transport through borazine and Au-BCN-Au systems. Borazine molecule (BN) has alternated arrangement of boron and nitrogen in benzene ring. If boron or nitrogen atoms were replaced by carbon atoms, BN will become BCN unit structure. [21 22] We investigated the density of states (DOS), transmission function (TF), the transmission spectra and the currentvoltage (I-V) characteristics to understand the influence of different terminal groups. 2 Theoretical Formulation In this section we present the formalism that is followed to investigate the transport properties of borazine and BCN molecular systems. The structural optimization and the transport properties of these two systems have been investigated using Atomistic Toolkit software (ATK). [23] Figures 1(a) and 1(b) show the optimized bo- Supported by DST-FIST Project. We gratefully acknowledge financial support from DST-FIST, Government of India Corresponding author, john.d@ktr.srmuniv.ac.in c 2013 Chinese Physical Society and IOP Publishing Ltd

2 650 Communications in Theoretical Physics Vol. 59 razine and BCN unit structures. Then the optimized molecules are sandwiched between gold (111) electrodes. The distance between left and right electrodes is adjusted and the whole system is optimized with gold atoms fixed by optimizing the total energy of the systems. Fig. 1 Schematic construction of optimal: (a) thiolated borazine, (b) thiolated BCN. ATK software can fully complete self-consistent process of the atomic-scale system and the semi-infinite electrodes coupling systems, the voltage across the conductor directly get involved in self-consistent calculation. In this calculation the open system can be divided into three parts: central scattering region, left electrode, and right electrode areas. For the left and right electrodes, we used 3 3 unit cell in the x and y directions to avoid the interaction between the molecules and the mirror image. The adsorption geometry is such that the molecules are located symmetrically at the top site of Au (111) surface. The general gradient approximation with double Zeta basis sets was used in the calculation. For convergence, the brillouin zone of the leads is sampled by K points in the direction of x, y, and z, where z is the electron transport direction. The NEGF-DFT self-consistency was controlled by a numerical tolerance of 10 5 ev. [24] The current through this system can be obtained from Landauer [25 26] formula according to the corresponding Green function. An electron incident from the source with energy E has a probability T(E) of being transmitted through the molecule to the drain. By calculating this transmission probability for a range of energies around the Fermi function E f of the lead, current is calculated using the Landauer formula. I = 2e T(E)[f(E µ 1 ) f(e µ 2 )]de, (1) h where T(E) is the transmission function through the device at energy E which is calculated using the non equilibrium Green s function formalism as given as T(E) = Tr(Γ 1 GΓ 2 G + ), where: broadening matrices Γ 1,2 = i(σ 1,2 Σ + 1,2 ) are defined as the anti - Hermitian parts of self energies Σ 1,2, and the molecular Green s function G is given by G(E) = (ES H + U SCF Σ 1 Σ 2 ) 1, (2) where S is the overlap matrix for non-orthogonal basis set of states, while H is the Hamiltonian of the neutral molecule. The self consistent potential U SCF is given by U SCF = U(N N eq ) and it is calculated by employing a simple self consistent field method. N eq is the equilibrium number of electrons in the molecule. µ 1, µ 2 are the electro chemical potential of left and right electrode respectively. (µ 1 µ 2 )/e = V is the electrical potential difference between left and right ends. Electrochemical Potential µ 1,2 is given as µ 1,2 = E f ± ev 2. (3) f(e µ 1,2 ) are the Fermi distribution functions of electrons in the left and right electrodes. The transmission coefficient and the current through the system have been obtained using ATK. 3 Result and Discussion Transmission functions (TF) are the main factor to decide the electron transport properties of the whole system. The TF magnitudes of borazine system with terminal group Se are larger in the lower and higher energy channel than the other three terminal groups as shown in Fig. 2(a). Fig. 2 Transmission of borazine (a) and BCN (b) systems with terminal groups, S, Se, CN, and O under zero bias.

3 No. 5 Communications in Theoretical Physics 651 For the borazine system with terminal group Se, the nearest TF peak to Fermi level is almost located at E f. This means that the conductance of borazine-se system is higher than borazine-s, borazine-o and borazine-cn because the transmission channels near the Fermi levels dominate the electron transport. Au-Se bond is more metallic in nature, and this gives rise to more charge mixing, smaller charge transfer, and stronger bonds, because of this the terminal group Se provides good conductance for borazine to Au electrode. For BCN molecular system, TF of the terminal group S, CN and Se are more broadened than O; this is due to coupling with the gold electrode. From Fig. 2(b), it is clear that the BCN-O system alone shows proper transmission peaks near the Fermi level leads more conduction. It can be concluded that the electron transmission will be greatly affected by different terminal groups as the transmission coefficients are changed which has been confirmed in our similar type of works. [27 28] Table 1 Isosurface MPSH-eigenstates for borazine molecular system using an isovalue of When the molecules interact with the Au (111) electrodes, the moleculars levels broaden into a continuum. The eigenstates of the whole metal-molecule-metal system consist of scattering states, which are molecular orbitallike in the molecules. [29] If an orbital is delocalized across the molecules, an electron that enters the molecules will have the higher probability of reaching the other end through that delocalized molecular orbital. This implies that the fully delocalized vacant molecular orbital served the channel of conduction in the electron transfer process. By calculating MPSH-eigenstates at the resonance energies (E HOMO, E LUMO ), the orbitals that are responsible for current flow through the molecule can be analyzed. The MPSH-eigenstates of borazine and BCN molecules with different terminal groups are represented in Table 1 and 2. From Tables 1 and 2 it is clear that the molecular orbitals for borazine and BCN systems are fully delocalized for HOMO resonance by providing a bath for better π-electron. But for LUMO resonance, most of the orbitals are localized across the molecules.

4 652 Communications in Theoretical Physics Vol. 59 Table 2 Isosurface MPSH-eigenstates for BCN molecular system using an isovalue of Fig. 3 Total density of states of the borazine (a) and BCN (b) with terminal group S, Se, O, CN. Figure 3 shows the total density of states for borazine and BCN molecular systems with terminal group S, O, CN and Se. The characteristics of borazine and BCN are changed when it is bonded to the electrodes through terminal groups. The states near E f are mainly contributed by π electrons of the molecule and have dominant effect on the electron transport. When different terminals groups are bonded to the molecule, the π electrons of the sys-

5 No. 5 Communications in Theoretical Physics 653 tem are different. We observed, the Se-borazine and O- BCN system has number of small DOS peaks, very close to the Fermi level, and this could lead to very good electron transport under small bias. the bias voltage because the energy of the p-orbitals is increased due to the influence of bias voltage. We observed that the transmission interval near the Fermi energy is increased at increasing bias, because incident electrons in these regions contribute most significantly to the transmission spectrum. Fig. 5 I-V characteristics for (a) borazine (b) BCN with terminal group S, Se, O, and CN. Fig. 4 Bias dependent transmission spectra for (a) borazine-s (b) BCN-S (c) borazine-cn. Figure 4 shows the transmission spectra of borazine- S, BCN-S and borazine-cn under different bias voltage. The magnitude of transmission function changes greatly with an increase in the bias voltage, which leads saturation behavior of BCN molecular systems. The same phenomena occur for the other borazine and BCN systems. But in borazine-cn system, by increasing the external bias from 0.6 V to 1 V the transmission regions are getting smaller and no additional transmission region will be included, this leads decrease in current and Negative differential resistance (NDR) appears in this voltage range. The peak of transmission function associated with HO- MOs in both the cases is shifted towards E f by increasing Figure 5 shows the I-V characteristics for four borazine and BCN molecular systems. The borazine-se system has the largest current and these results are more consistent with the experimental observations. [30 31] This is due the location of transmission peak is very close to Fermi level. In addition, the Selenium has two cross σ orbitals and one of them parallel to the σ orbital of borazine which will result efficient electron delocalization implies good electrical conductance. It is observed that the CN system also shows good conductance in the lower bias but for the biasing range 0.8 V to 1.5 V the transmission function will be affected which leads NDR. From Fig. 4(a), we observe that the two parts of the I-V curve with negative and positive bias exemplify NDR behavior, but we focus the positive bias of I-V curve, which can be understood by studying the coupling between the molecular orbitals in the borazine-cn system and the incident state in the electrode under various biases. In this system, when the bias increases above 0.8 V to 1.5 V, the magnitude of TF through HOMO and nearby orbitals continues to decrease and the LUMO will not be accessible, which leads negative slop of current and NDR observed. When the external bias increases from 1.6 V to 2 V the transmission at lower energy levels below HOMO increases and they are accessible, which leads to the increase of current. Compared to

6 654 Communications in Theoretical Physics Vol. 59 other systems, the S-Borazine and S-BCN molecular system exhibits poor conductance because its HOMO level is farther from the Fermi energy. Among the four BCN molecular systems we obtained large current for O-BCN due to better transmission. We conclude that Se is the best terminal group to couple borazine to Au electrode and O is the best one to couple BCN to Au electrode. From Fig. 5, the current magnitude of BCN molecular systems is four times larger than that of borazine. This is because; in BCN systems the presents of carbon atoms increase the delocalization of the electronic levels leads good conduction. [32 36] 4 Conclusion Au-molecule-Au system for borazine and BCN with terminal group S, Se, O and CN have been constructed, and the electron transmission function and I-V characteristics have been simulated. The transmission result shows that HOMO of all the systems is closer to Fermi level compared with their LUMO and hence HOMO dominates the electron transport. When the bias increases, the peaks of the transmission spectrum are shifted towards E f and there are considerable changes in their magnitudes. We found that the current is directly associated with the transmission spectrum, which is varied by the applied bias. The current magnitude of BCN is four times higher than the borazine, since borazine is less delocalized. The results demonstrated that terminal groups attached to the molecular devices offer the possibility of modifying their transport behavior in a controlled way. Among the four terminal groups, selenium provides better linkage between borazine and Au electrodes while oxygen is for BCN system. Negative differential resistance behavior has been observed for borazine CN system, this is due to the different coupling between the molecule and the electrodes. 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International Journal of ChemTech Research CODEN (USA): IJCRGG ISSN: 0974-4290 Vol.7, No.2, pp 695-699, 2014-2015 ICONN 2015 [4 th -6 th Feb 2015] International Conference on Nanoscience and Nanotechnology-2015