Orthogonal Functionalization of Oxide/Metal Nanostructures: Carboxylate and Thiol Self-Assembled Monolayers of CoO/Au(111)

The orthogonal self-assembly of anchored molecular monolayers on chemically heterogeneous substrates offers a route for bottom-up chemical nanopatterning and area-selective deposition. Herein, we investigated the selective growth of self-assembled monolayers n-decanethiol (DT) and 4,4'-biphenyldicarboxylic acid (BPDCA) on a pristine Au(111) surface and on CoO nanoislands on Au(111) by physical vapor deposition (PVD) under ultrahigh vacuum (UHV) conditions. We combined in-situ infrared reflection-absorption spectroscopy (IRAS) applied during PVD and scanning tunneling microscopy (STM).

At 300 K, DT formed a lying-down phase on Au(111), which evolved into a more dense and slightly more upright adlayer upon heating to 400 K. On the Au(111) surface partially covered by CoO nanoislands, the BPDCA preferentially binds to CoO. BPDCA adopts a tilted or upright orientation on CoO with one carboxylate group bound to surface and a free carboxylic acid group pointing towards the vacuum. When DT is dosed onto the BPDCA-precovered CoO/Au(111), the BPDCA SAM remains unchanged while a DT SAM grows on the Au regions in between. This observation demonstrates that the oxide-bound carboxylate layer is sufficiently robust to confine the DT to the Au regions. Our results provide a vacuum-based procedure for orthogonal functionalization of a metal/oxide nanostructure to synthesize a nanopatterned SAM with different terminating functional groups.