Abstract
For the realization of molecular electronics, one essential goal is the ability to systematically fabricate molecular functional components in a well-controlled manner. Experimental techniques have been developed such that π-stacked ethylbenzene molecules can now be routinely induced to self-assemble on an H-terminated Si(100) surface at precise locations and along precise directions. Electron transport calculations predict that such molecular wires could indeed carry an electrical current, but the Si substrate may play a considerable role as a competing pathway for conducting electrons. In this work, we investigate the effect of placing substituent groups of varying electron donating or withdrawing strengths on the ethylbenzene molecules to determine how they would affect the transport properties of such molecular wires. The systems consist of a line of π-stacked ethylbenzene molecules covalently bonded to a Si substrate. The ethylbenzene line is bridging two Al electrodes to model current through the molecular stack. For our transport calculations, we employ a first-principles technique where density functional theory (DFT) is used within the non-equilibrium Green’s function formalism (NEGF). The calculated density of states suggest that substituent groups are an effective way to shift molecular states relative to the electronic states associated with the Si substrate. The electron transmission spectra obtained from the NEGF–DFT calculations reveal that the transport properties could also be extensively modulated by changing substituent groups. For certain molecules, it is possible to have a transmission peak at the Fermi level of the electrodes, corresponding to high conduction through the molecular wire with essentially no leakage into the Si substrate.
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Notes
The van der Waals radius cutoff was set to 15.0 Å, so that the terms corresponding to interactions over distances greater than this value are assumed to be zero.
The reason these levels have finite broadening is because of the π-stacking interaction with molecules in neighboring images of the periodic system.
Note that the effect of including the Si substrate is to shift the positions of the transmission peaks by ca. 0.5 eV and they become split due to hybridization with the Si states.
Comparing the transmission peak positions in Fig. 5b to the PDOS positions in Fig. 4e, we see that they differ. This is because in position of the DOS peaks are relative to the Si substrate, while in the transmission spectrum they are relative to the E F of the Al electrodes. In other words, the peaks in the transmission spectra are shifted (by ca. −1.5 eV) relative to the PDOS plots.
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Acknowledgments
We thank Dr. Gino A. DiLabio for numerous helpful discussions. We are grateful to Sharcnet for access to computational resources and NSERC for financial support.
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Published as part of the special collection of articles celebrating the 50th anniversary of Theoretical Chemistry Accounts/Theoretica Chimica Acta.
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Smeu, M., Wolkow, R.A. & Guo, H. Conduction modulation of π-stacked ethylbenzene wires on Si(100) with substituent groups. Theor Chem Acc 131, 1085 (2012). https://doi.org/10.1007/s00214-011-1085-7
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DOI: https://doi.org/10.1007/s00214-011-1085-7