Accurate Electronic Structure Calculations of Critical Materials supported on Gold(111) Surface using Large-Scale DFT Computations

PI Anh Ngo, Physical Sciences and Engineering
DIagram showing the origin of molecule friction

Figure 1. Origin of molecule friction. (a) DFT result of a total density plot of 6P on Ag(111) surface. (b) CMD results for distance (traveled by the 6P molecule over 1 ns) as a function of force plots, which reveals the minimum lateral force required to move along the parallel and sideways directions. (c) The adsorption geometry of the 6P onAg(111). Molecular dynamic simulations are performed along the parallel (P) and sideways (S) directions. The green and red colored regions depict the surface areas for the molecule to move one atom site along parallel and sideways, respectively, during lateral displacement. [ Nano Letters 21 (15), 6391-6397 ]

Project Summary

The lateral force needed to move a molecule on a surface is important for understanding dynamics as well as related tribological phenomena at the atomic and molecular scales. Using density functional theory and classical molecular dynamics simulations, we demonstrated that it is easier to move the para-sexiphenyl (6P) molecule on the surface of Ag(111) in the direction parallel to their molecular axis rather than sideways. This observation was well in agreement with the experimental findings that 1D molecules required almost double the lateral force to move them on the surface sideways than along the parallel direction.

Project Description

With the advancement of surface science techniques, such as atomic force microscopy (AFM) and scanning tunneling microscopy (STM), it has become possible to gain deeper insight into the lateral forces and corresponding tribological phenomena of the atoms, molecules, and nanoscale systems on surfaces. Although a myriad of studies have been conducted to elucidate the friction at the nano- and atomic scale, many tribological phenomena are yet to be understood. At the atomic scale, the potential energy landscape of surfaces plays a critical role in friction. For this experimental study, we chose para-sexiphenyl (6P) molecules because they have a representative 1D shape. 6P is composed of sixπ-rings connected as a linear chain (Figure 1b), and it is widely investigated due to potential applications in optoelectronic devices. An atomically flat Ag(111) surface, to study 6P molecules moving along the direction parallel to its molecular axis and sideways to the axis, was chosen. 

To unravel the experimentally observed lateral force phenomenon, we have performed density functional theory (DFT) and classical molecular dynamics (CMD) calculations. Density Functional Theory (DFT) calculations were carried out with the Vienna ab initio simulation package (VASP) code. Geometrically relaxed DFT calculations including vdW interactions of the 6P adsorbed on a Ag(111) surface confirm the adsorption of the molecule along the [110] surface directions, in agreement with the experimental finding (Figure 1a below). The calculations also reveal twisting of alternateπ-rings (indicated with arrows in Figure 1a) with a torsional angle of∼18°.

Next, using DFT computed adsorption structure of 6P on Ag(111) as input, we performed classical molecular dynamics (CMD) simulations were performed within the canonical ensemble (NVT) to (a) identify the threshold force required to displace 6P on Ag (111) along parallel and sideways directions, as well as (b) elucidate the conformational dynamics associated with the surface sliding of 6P along these directions (under applied force). The calculations provide the threshold lateral force to displace the molecule along the parallel direction, FLP, as ∼125 pN while that along the sideways direction, FLS, as ∼200 pN. Thus, threshold lateral force along the sideways direction is larger than that of the parallel direction, and the ratio of the lateral forces, FLS/FLP, is 1.6. This ratio relatively agrees with the experiments.