Orthogonal Functionalization of Oxide/Metal Nanostructures: Carboxylate and Thiol Self-Assembled Monolayers on 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 (SAMs) of 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 denser and slightly more upright adlayer upon heating to 400 K. On the Au(111) surface partially covered by CoO nanoislands, BPDCA preferentially binds to CoO. BPDCA adopts a tilted or upright orientation on CoO with one carboxylate group bound to the surface and a free carboxylic acid group pointing towards the vacuum. When DT is dosed onto BPDCA-precovered CoO/Au(111), the BPDCA SAM is preserved while a DT SAM grows on the Au regions between the CoO islands. This observation demonstrates that the oxide-bound carboxylate layer is sufficiently robust to confine 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.