Structures of Annealed Decanethiol Self-Assembled Monolayers on Au(111):  an Ultrahigh Vacuum Scanning Tunneling Microscopy Study

Abstract

While the structures of self-assembled monolayers (SAMs) of alkanethiols on Au(111) are extensively studied and well-known, new structures and complex phase behavior have been progressively discovered when coverage of these layers falls below saturation. Structures and phase transitions of annealed decanethiol monolayers on Au(111) surfaces were systematically investigated using scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. Rich structures were revealed as a result of annealing in UHV. At temperatures below 345 K, no significant changes in coverage were observed, although the size of two-dimensional crystalline c(4√3 × 2√3)R30° domains increases as annealing progresses. A two-dimensional melting occurs at 345 ± 5 K and was captured in situ from time-dependent STM studies. Above 400 K, significant desorption takes place. In the temperature range of 345−400 K, within which desorption progresses to gradually decrease the surface coverage, a variety of striped phases have been observed, each having distinct molecular-level packing and unit cells. Well-known striped phases have been confirmed: (p × √3), with p values (integer or half-integer multiples of the Au(111) periodicity) of 7.5, 9, and 11. In addition, new structures such as mixed striped phases and mesh-like structures are revealed, which are often found to coexist with the regions of pure striped phases. The systematic investigations of the structural and phase evolution shed light on the SAM desorption process at the molecular level.

Coexistence of Closely Packed c(4 × 2) and Striped Phases in Self-Assembled Monolayers of Decylthiocyanates on Au(111)

“…The surface structure of DT SAMs observed here is quite consistent with a number of previous STM studies for alkanethiol SAMs. 5,11,24,26,27 This result strongly supports the idea that DT SAMs are able to reach saturated monolayer coverage with these preparation conditions.…”

“…The ordered domains appeared to be striped phases with an inter-row distance of 2.3 -2.6 nm; such striped phases, in which the alkyl chain was lying flat on the gold surface, often appear at the initial stage of SAM growth 28,29 or from SAMs formed after thermal annealing. 26 We found the surface structure of DTC SAMs to be quite similar to those of OTC SAMs, 24 which implies that OTC and DTC molecules have a nearly identical adsorption behavior in solution. The inferior quality of thiocyanate SAMs compared to thiol SAMs may be due to the lower sticking coefficient of thiocyanates, the high activation barrier for S-CN bond cleavage, or the lower adsorption activity of a sulfur joined to an electron withdrawing CN group.…”

Exchange Reactions between Alkanethiolates and Alkaneselenols on Au{111}

“…There are numerous phases of even simple alkanethiolates and alkaneselenolates on Au{111}. 26,86,111 We observed a monolayer transition from a full-coverage sulfur-bound C10 SAM to a full-coverage selenium-bound C12Se, taking note of how the gold substrate directs the progression of the reaction and the structures of the products.…”

Stability of large-area molecular junctions

“…Phase transitions and desorption of alkanethiol monolayers on Au(1 1 1) have been observed at elevated temperatures with STM [20,21,[24][25][26][27]. The two dimensional melting temperature of alkanethiol SAMs increases with increasing chain length of the alkane backbone [22,23,27].…”

“…Thermal stability of SAMs on Au surfaces has been studied extensively [17][18][19][20][21][22][23]. Phase transitions and desorption of alkanethiol monolayers on Au(1 1 1) have been observed at elevated temperatures with STM [20,21,[24][25][26][27].…”