Reactivity of Gas-Phase Radicals with Organic Surfaces

Abstract

In chemical reactions at the gas–surface interface, the heterogeneity in structure of reaction sites plays a critical role in determining surface reactivity. This Perspective describes reaction mechanisms in such systems and details the use of in situ scanning probe microscopy to investigate reactions of gas-phase radicals with self-assembled alkanethiolate monolayers on gold surfaces. For both atomic hydrogen and atomic chlorine reagents, the presence of defects in the alkanethiolate surface order has a substantial influence on what reactions can occur and the speed at which they do so. Data acquired from a series of images were modeled using kinetic Monte Carlo simulations, and a surface radical reaction model was developed to explain the observed evolution of surface structure as the reactions proceed.

Influence of Structural Dynamics on the Kinetics of Atomic Hydrogen Reactivity with Low-Temperature Alkanethiolate Self-Assembled Monolayers

“…It is well-documented in previous studies that the primary reaction mechanism of atomic H with short-chain alkanethiolate SAMs (C ≤ 12) is through the cleavage of the sulfur–gold bonds, thereby removing entire thiols from the surface. This reactivity, as shown in our reaction of an 8C SAM in Figure , initially appears around the thiolate domain grain boundaries and subsequently expands outward, in agreement with previous results. ,− The removal of thiolate molecules promotes the formation of various low-density phases, including both ordered lying-down, or striped, phases and disordered 2D fluids. − These low-density regions continue to grow as the reaction proceeds, consuming the hexagonal close-packed (φ-phase) domains until only low-density alkanethiol and gold adatom islands ,, remain. We consider the reaction to be complete when no φ-phase SAM is left on the Au(111) surface .…”

Chain-Length-Dependent Reactivity of Alkanethiolate Self-Assembled Monolayers with Atomic Hydrogen

“…Several groups have studied thiolate SAM reactivity with energetic gas species such as H, ,− O, − and O 3 , − as adsorption energetics and reaction dynamics are of great interest to the surface science community. For example, techniques such as X-ray photoelectron spectroscopy , and reflection absorption infrared spectroscopy have been used to explore the effect of chain length on the reactivity of alkanethiolate SAMs with atomic gases.…”

“…The direct-imaging and nondestructive nature of scanning probe microscopy typically allows better investigation of the mechanistic details of such reactions. Furthermore, results from previous STM studies of thiolate SAMs already provide us with important information on how variables such as temperature, ,− local surface environment, and exposure to reactive gases − ,, will influence a SAM’s film structure. For example, earlier work by Kandel’s group shows how 1-octanethiolate SAMs react with atomic hydrogen and how the resultant monolayer damage is characterizable with STM.…”

Temporally and Spatially Resolved Oxidation of Si(111)-(7 × 7) Using Kinetic Energy Controlled Supersonic Beams in Combination with Scanning Tunneling Microscopy

“…Supersonic molecular beams (SMBs) present an incisive tool for studying energetic site specific reactivity − on surfaces including oxygen. − Nolan and co-workers , report two types of molecular oxygen adsorption processes on Pt(111) depending on the incident translational energy ( E i ). They utilized in situ high-resolution electron energy loss spectroscopy (HREELS) with SMBs to determine whether molecular oxygen experiences superoxo-like or peroxo-like precursor states prior to dissociating on the surface.…”