Formation and Structure of Highly Ordered Self-Assembled Monolayers on Au(111) via Vapor Deposition of Dioctyl Diselenides

The formation and growth processes of octaneselenolate (C8Se) self-assembled monolayers (SAMs) on Au(111) from dioctyl diselenides were investigated by scanning tunneling microscopy (STM) as a function of vapor deposition time at 363 K. STM observations revealed unique surface features of the C8Se SAMs on Au(111) prepared at 363 K for 1 h, consisting of various types of narrow, bright molecular rows and broad, bright molecular rows with three-directional orientations. After increasing the deposition time from 1 and 6 h, interestingly, the structural quality of C8Se SAMs is greatly enhanced, showing the formation of tri-directional highly ordered domains with distinct domain boundaries on an entire Au(111) surface. The observed ordered phase can be described by a (2 × 2√7)rect packing structure with an arial density of 28.9 Å2/molecule. C8Se SAMs at 363 K for 24 h consisted of long-range, highly ordered domains and large, disordered domains. The ordered phase can be described by a (√3 × √23)rect packing structure with an arial density of 23.3 Å2/molecule, which is denser than that of C8Se SAMs formed by vapor deposition at 363 K and for 6 h. This study clearly demonstrates that vapor deposition is a highly effective method for preparing highly ordered alkyl selenolate SAMs on Au(111). Furthermore, molecular-scale STM results provide new insights into the formation and growth processes of alkyl selenolate SAMs on Au(111) formed by vapor deposition of dialkyl diselenides at high temperatures.