Conformations and charge transport characteristics of biphenyldithiol self-assembled-monolayer molecular electronic devices: A multiscale computational study

Abstract

We report a computational study of conformations and charge transport characteristics of biphenyldithiol ͑BPDT͒ monolayers in the ͑ ͱ 3 ϫ ͱ 3͒R30°packing ratio sandwiched between Au͑111͒ electrodes. From force-field molecular-dynamics and annealing simulations of BPDT self-assembled monolayers ͑SAMs͒ with up to 100 molecules on a Au͑111͒ substrate, we identify an energetically favorable herringbone-type SAM packing configuration and a less-stable parallel packing configuration. Both SAMs are described by the ͑2 ͱ 3 ϫ ͱ 3͒R30°unit cell including two molecules. With subsequent density-functional theory calculations of one unit cell of the ͑i͒ herringbone SAM with the molecular tilt angle Ϸ 15°, ͑ii͒ herringbone SAM with Ϸ 30°, and ͑iii͒ parallel SAM with Ϸ 30°, we confirm that the herringbone packing configuration is more stable than the parallel one but find that the energy variation with respect to the molecule tilting within the herringbone packing is very small. Next, by capping these SAMs with the top Au͑111͒ electrode, we prepare three molecular electronic device models and calculate their coherent charge transport properties within the matrix Green's function approach. Current-voltage ͑I -V͒ curves are then obtained via the Landauer-Büttiker formula. We find that at low-bias voltages ͉͑V͉ Շ 0.2 V͒ the I -V characteristics of models ͑ii͒ and ͑iii͒ are similar and the current in model ͑i͒ is smaller than that in ͑ii͒ and ͑iii͒. On the other hand, at higher-bias voltages ͉͑V͉ տ 0.5 V͒, the I -V characteristics of the three models show noticeable differences due to different phenyl band structures. We thus conclude that the BPDT SAM I -V characteristics in the low-bias voltage region are mainly determined by the Si-Au interaction within the individual molecule-electrode contact, while both intramolecular conformation and intermolecular interaction can affect the BPDT SAM I -V characteristics in the high-bias voltage region.

Odd−Even Effects in Charge Transport across Self-Assembled Monolayers

“…Charge transport through insulating organic matter almost certainly occurs (at least over distances of less than a few nanometers) via non-resonant coherent tunneling. 39,[45][46][47][48][49][50][51][52][53][54][55] Understanding the relationship between molecular structure and electronic properties in organic systems would improve our ability i) to model electron transport in relevant biological systems (e.g. redox proteins) and ii) to evaluate the potential of, and perhaps to design, electronic devices based on organic components.…”

First-principles approach to the charge-transport characteristics of monolayer molecular-electronics devices: Application to hexanedithiolate devices

“…2͑b͔͒ as the energetically optimal position. 10 Next, with the S atoms bridging the fcc hollow site of the Au͑111͒ electrode, we allowed the two Au layers of the electrodes on the molecule ͑S͒ side to relax and checked whether any significant reorganization of the molecule or Au atoms occurs. We repeated this process for progressively increasing electrode-electrode gap distances d = 13.7, 13.9, 14.1, 14.3, and 14.5 Å ͑Fig.…”

“…This is because we have observed a rather noticeable deviation in the projected density of states ͑PDOS͒ of the Au surface layer from its bulk counterpart in our previous study of the BPDT devices. 10 We used the next three layers of Au atoms ͑surface+ 1-surface+ 3 Au layers͒ as the "electrode" part in the MGF calculations. This setup is schematically outlined in Fig.…”

“…We contrast this with the observation in -conjugated BPDT molecules. 10 Unlike the case of C6DT molecules where the transmission channel is essentially determined by the DT bridges and the C6 core plays a secondary role, in the case of BPDT molecules, BP core levels are significantly hybridized with DT bridge levels which results in a large variation of conductance with the molecule tilting and monolayer packing. We note that in both cases DT bridge levels play an important role in characterizing the device transmission properties and conclude that the characteristics of the devices based on small molecules strongly depend on the nature of metal-electrode contacts.…”

“…Currently, the code is coupled with a Gaussian basis set code, 9 but it can be easily generalized to other LCAO DFT or tight-binding codes. This method has been successfully applied to the investigation of molecular electronic devices based on monolayers of simple biphenyldithiolate 10 ͑BPDT͒ and complex catenane molecules. 11 The MGF approach has been previously implemented on the basis of zero-dimensional ͑molecular͒ quantum chemistry codes [12][13][14][15] and periodic solid state codes, [16][17][18][19][20] and applied to the study of molecular electronic devices.…”