Packing density and structure effects on energy-transfer dynamics in argon collisions with organic monolayers

Abstract

A combined experimental and molecular-dynamics simulation study has been used to investigate energy-transfer dynamics of argon atoms when they collide with n-alkanethiols adsorbed to gold and silver substrates. These surfaces provide the opportunity to explore how surface structure and packing density of alkane chains affect energy transfer in gas-surface collisions while maintaining the chemical nature of the surface. The chains pack standing up with 12 degrees and 30 degrees tilt angles relative to the surface normal and number densities of 18.9 and 21.5 A(2)molecule on the silver and gold substrates, respectively. For 7-kJmol argon scattering, the two surfaces behave equivalently, fully thermalizing all impinging argon atoms. In contrast, these self-assembled monolayers (SAMs) are not equally efficient at absorbing the excess translational energy from high-energy, 35 and 80 kJmol, argon collisions. When high-energy argon atoms are scattered from a SAM on silver, the fraction of atoms that reach thermal equilibrium with the surface and the average energy transferred to the surface are lower than for analogous SAMs on gold. In the case of argon atoms with 80 kJmol of translational energy scattering from long-chain SAMs, 60% and 45% of the atoms detected have reached thermal equilibrium with the monolayers on gold and silver surfaces, respectively. The differences in the scattering characteristics are attributed to excitation efficiencies of different types of surface modes. The high packing density of alkyl chains on silver restricts certain low-energy degrees of freedom from absorbing energy as efficiently as the lower-density monolayers. In addition, molecular-dynamics simulations reveal that the extent to which argon penetrates into the monolayer is related to packing density. For argon atoms with 80-kJmol incident energy, we find 16% and 7% of the atoms penetrate below the terminal methyl groups of C(10) SAMs on gold and silver, respectively.

Improvement of heat transfer efficiency at solid-gas interfaces by self-assembled monolayers

“…Furthermore, molecular beams with energies much higher than thermal energies have been used in most detailed studies of gassurface scattering. [3][4][5] Nevertheless, such experiments have shown that a highly efficient solid-gas energy exchange occurs when the solid surface is modified with selfassembled monolayers (SAMs). 4,6,7 This suggests that even in the case of the scattering of the gas characterized by thermal energies, highly efficient thermal energy exchange can be achieved with SAM modified surfaces.…”

“…The longer collision time and much higher TAC on the SAM surface indicate the Ar-SAM thermal energy exchange is mainly through the trappingdesorption channel in which the gas atoms lose memory of their incident parameters and thermalize with the surface upon desorption. 4 The longer collision time and better energy exchange might be caused by two factors associated with SAMs. One is the terminal pseudo-atoms in the SAM are much lighter than the Au atoms in the bare surface such that the gas atoms have better mass matching with CH 3 (m Ar /m CH3 % 2.6) compared to that with Au (m Ar /m Au % 0.2).…”

Lanthanide complexes on Ag nanoparticles: Designing contrast agents for magnetic resonance imaging

“…From this analysis we found 2610 complexes per Ag particle ( Table 2). The corresponding footprint of complexes on the 10 nm particle surface would be approximately 12 Å 2 /complex, which is relatively small and suggests that some complexes remain unbound forming a looped oligomeric structure above the metal surface [54,55].…”

Collision-induced annealing of octanethiol self-assembled monolayers by high-kinetic-energy xenon atoms

“…62 For alkanethiol films with larger amounts of corrugation, more energy is deposited compared to less-corrugated samples. The low-frequency vibrational modes that are primarily responsible for energy accommodation are more accessible in the highly corrugated films, giving rise to a 7% increase in energy transferred.…”