5TH International Congress on Technology - Engineering & Science - Kuala Lumpur - Malaysia (2018-02-01)

Dft Electron Transport Study Of Ferrocene Molecule Along With Thiol And Amine Anchors

In general, the nature of the chemical linker group connecting the molecules to electrodes is a critically determinant factor influencing the transport properties through single molecular junctions [1], because they organize the energy gap alignment between the highest occupied molecular orbitals (HOMO)/lowest unoccupied molecular orbitals (LUMO) of molecule and electrode Fermi level. Therefore, comprehensive studies on the molecule-metal contact and appropriate choice of linkers would be essential for the development of molecular circuits. Ferrocene, belonging to the metallocene complexes with the formula FeC5H10, has been widely suggested in molecular electronic devices due to its exclusive features such as high degree of chemical and thermal stability and high structural flexibility [2,3]. Ferrocene-based nanoscale devices have been the subject of many theoretical and experimental investigations [4-6]. In this study, we demonstrate that how the nano-contacts affect the the electron transfer process of the metall-organic molecules, such as ferrocene. Our model system which is shown in Fig. 1, consists of a single ferrocene molecule attached to gold electrodes via linkers as Au(100)-S-Fe(Cp)2-X-Au(100) conformation, where X is S and NH2 linker. All calculations are performed based on density functional theory (DFT) plus a non-equilibrium Green’s function (NEGF) method. The generalized gradient approximation (GGA) is used for calculating the exchange correlation function. Geometry Optimization and electronic structure calculations have been performed in SIESTA package [7] and transport calculations are implemented in TRANSIESTA code [8]. For a deep insight to the transport processes at molecular states, we consider zero bias transmission coefficient as a function of energy for these devices around the Fermi level. As shown in Fig. 2, for the symmetric structure with S linker, the value of transmission at the Fermi level are significantly more than the values with the asymmetric device i.e. with S and NH2 anchors. The transmission spectra also imply that the transport mechanism may have been determined by the HOMO level in symmetric case while it is dominated by the LUMO level for the asymmetric case because of their adjacency to the Fermi energy. However, considering the width of the transmission peaks, it is clear that broadening of HOMO via thiol linker is higher than the LUMO through amine anchor, indicating a stronger coupling between the molecule and the electrodes. The increase in the transmission of S-ferrocene-S device can be attributed to the better charge transfer between the molecule and the electrodes that bring the HOMO level near to the Fermi level. The so-called tunnelling behaviour is also observed in the intermediate coupling regime. Note that, the significant role of linkers electronegativity is one of the major factors in tuning of transmission coefficient. Because N is one of the most electronegative elements, the electrons tend to hang out more toward the N atom when it is covalently bonded to Au and C atoms. Thus, it is predicted that the electron transfer process has not been accomplished well and transmission probability decrease in the structure while the valence electrons of S are less tightly held and form more delocalized bonds with metals such as Au resulted a more efficient transport mechanism. subsequently, the current-voltage (I-V) curves is considered in Fig. 3. As shown in the figure, the current of ferrocene device with symmetric structure (S linker) is remarkably higher than the asymmetric device over the entire bias voltage range. The I–V curves exhibit linear and nonlinear features. For the S-ferrocene-S device, the I–V characteristic displays two increasing nonlinear bias regimes. For a low bias (0-0.4 V), the I–V shape varies almost linearly, indicating metallic transport behaviours. In the second regime, there is a monotonic increase in the I–V curve from 0.4 to 0.6 V. By contrast, the current gradually enhance from 0 to 0.6 V in S-ferrocene-NH2 device because of a lower transmission peaks near the Fermi energy. The magnitudes of the currents depend on the distance between the two gold electrodes and the coupling strength between the electrodes and the molecule via linkers. The difference between the currents obtained for ferrocene linked with S and NH2 contacts appears to be the shift in the transmission peaks, which are due to the frontier molecular orbitals shift from negative to positive energies. Hence, the most appropriate anchoring group depends on the bias voltage range. Based on the I-V profile for these two junctions, highly nonlinear I–V curve is favoured because it can create switchable states. Hence, S atom is a better electron coupling group than NH2 linker. In summary our DFT study indicates that the nano-contacts influence significantly the electron transport of ferrocene molecular junctions so that S linker cause to augment the current which can be a promising candidate in developing nanodevices, while the NH2 anchoring group lead to lower the transport mechanism.
Roghayeh Farzadi, Hossain Milani Moghaddam, Davood Farmanzadeh