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Interaction of the (2?3 × 3)rect. Adsorption-Site Basis and Alkyl-Chain Close Packing in Alkanethiol Self-Assembled Monolayers on Au(111): A Molecular Dynamics Study of Alkyl-Chain Conformation.

ABSTRACT: We show that the adsorption site basis of the (2?3 × 3)rect. phase of n-alkanethiol self-assembled monolayers plays a key role in determining the molecular conformation of the close-packed alkyl chains. Ten proposed reconstructed Au-S interfaces are used to explore the minimized energy alkyl-chain packing of n-decanethiol molecules using molecular dynamics with the all-atom description. In this comparative study, all models have the same alkyl-chain surface density of four molecules per unit cell; thus, differences are due to the headgroup spacing within the 4-molecule basis as opposed to the average surface density. We demonstrate for the first time the 4-molecule-basis twist structure driven by the packing of alkanethiol molecules in a large simulation box (100 molecules, 25 unit cells) using molecular dynamics. Our results validate the prediction put forward by Mar and Klein that to achieve the 4-molecule-basis twist symmetry observed by the experiment, the headgroups must deviate from the high-symmetry (?3 × ?3)R30° sites. The key structural parameters: tilt, twist, and end-group height, as well as their spatial order, are compared with experimental results, which we show is a highly sensitive approach that can be used to vet proposed Au-S interfacial models.

SUBMITTER: Bhattacharya S

PROVIDER: S-EPMC7301571 | biostudies-literature | 2020 Jun

REPOSITORIES: biostudies-literature

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Publications

Interaction of the (2√3 × 3)rect. Adsorption-Site Basis and Alkyl-Chain Close Packing in Alkanethiol Self-Assembled Monolayers on Au(111): A Molecular Dynamics Study of Alkyl-Chain Conformation.

Bhattacharya Soumya S Yothers Mitchell P MP Huang Liangliang L Bumm Lloyd A LA

ACS omega 20200602 23

We show that the adsorption site basis of the (2√3 × 3)rect. phase of <i>n</i>-alkanethiol self-assembled monolayers plays a key role in determining the molecular conformation of the close-packed alkyl chains. Ten proposed reconstructed Au-S interfaces are used to explore the minimized energy alkyl-chain packing of <i>n</i>-decanethiol molecules using molecular dynamics with the all-atom description. In this comparative study, all models have the same alkyl-chain surface density of four molecule ...[more]

PMID: 32566846

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Project description:Gold nanoparticles (AuNPs) with 14, 25 and 40nm diameters were functionalized with different chain length (C6, C8, C11 and C16) carboxylic acid terminated alkanethiol self-assembled monolayers (COOH-SAMs). X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to examine the changes in surface chemistry as both AuNP diameter and SAM chain length were varied. COOH-SAMs on flat gold surfaces were also examined and compared to the COOH-SAM on AuNP results. For a given surface, as the COOH-SAM chain length increased the XPS C/Au atomic ratio increased due to an increased number of carbon atoms per molecule in the overlayer and an increased attenuation of the Au substrate signal. For the C16 COOH-SAMs, as the size of AuNPs decreased the XPS C/Au atomic ratio and the apparent SAM thickness increased due to the increased curvature of the smaller AuNPs. The C16 COOH-SAMs on the flat Au had the lowest XPS C/Au atomic ratio and apparent SAM thickness of any C16 COOH-SAM covered Au surface. The effective take-off angles of the COOH-SAMs were also calculated by comparing the apparent thickness of COOH-SAMs with literature values. The effective take-off angle for C16 COOH-SAM on 14nm, 25nm and 40nm diameter AuNPs and flat Au were found to be 57°, 53°, 51° and 39°, respectively, for data acquired in a mode that collects a wide range of photoelectron take-off angles. The effective take-off angle for C16 COOH-SAM on 14nm AuNP and flat Au decreased to 52° and 0°, respectively, for data acquired in a mode that collects a narrow range of photoelectron take-off angles. The ToF-SIMS results showed similar changes in surface chemistry with COOH-SAM chain length and AuNP size. For example, the ratio of the sum of the C(1-4)H(x)O(y) positive ion intensities to the sum of the Au-containing positive ions intensities increased with decreasing AuNP size and increasing COOH-SAM chain length. Fourier transform IR spectroscopy in the attenuated total reflectance mode (FTIR-ATR) was used to characterize the crystallinity of the COOH-SAMs. The CH(2) stretching frequencies decreased with increasing COOH-SAM chain length on flat Au. The C16 COOH-SAM on the 14nm AuNPs exhibited a crystalline-like CH(2) stretching frequency. The size, size distribution, shapes and solution stability of AuNPs were investigated with transmission electron microscopy (TEM) and UV/VIS spectroscopy. As the average diameter of the AuNPs decreased the size distribution became narrower and the shape became more spherical.

Dataset Information

Interaction of the (2?3 × 3)rect. Adsorption-Site Basis and Alkyl-Chain Close Packing in Alkanethiol Self-Assembled Monolayers on Au(111): A Molecular Dynamics Study of Alkyl-Chain Conformation.

Publications

Interaction of the (2√3 × 3)rect. Adsorption-Site Basis and Alkyl-Chain Close Packing in Alkanethiol Self-Assembled Monolayers on Au(111): A Molecular Dynamics Study of Alkyl-Chain Conformation.

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