title: Etching of Crystalline ZnO Surfaces upon Phosphonic Acid Adsorption: Guidelines for the Realization of Well-Engineered Functional Self-Assembled Monolayers.
abstract: Functionalization of metal oxides by means of covalently bound self-assembled monolayers (SAMs) offers a tailoring of surface electronic properties such as their work function and, in combination with its large charge carrier mobility, renders ZnO a promising conductive oxide for use as transparent electrode material in optoelectronic devices. In this study, we show that the formation of phosphonic acid-anchored SAMs on ZnO competes with an unwanted chemical side reaction, leading to the formation of surface precipitates and severe surface damage at prolonged immersion times of several days. Combining atomic force microscopy (AFM), X-ray diffraction (XRD), and thermal desorption spectroscopy (TDS), the stability and structure of the aggregates formed upon immersion of ZnO single crystal surfaces of different orientations [(0001̅), (0001), and (101̅0)] in phenylphosphonic acid (PPA) solution were studied. By intentionally increasing the immersion time to more than 1 week, large crystalline precipitates are formed, which are identified as zinc phosphonate. Moreover, the energetics and the reaction pathway of this transformation have been evaluated using density functional theory (DFT), showing that zinc phosphonate is thermodynamically more favorable than phosphonic acid SAMs on ZnO. Precipitation is also found for phosphonic acids with fluorinated aromatic backbones, while less precipitation occurs upon formation of SAMs with phenylphosphinic anchoring units. By contrast, no precipitates are formed when PPA monolayer films are prepared by sublimation under vacuum conditions, yielding smooth surfaces without noticeable etching.
authors.affiliation: Fachbereich Physik, Molekulare Festkörperphysik and Wissenschaftliches Zentrum für Materialwissenschaften (WZMW), Philipps-Universität Marburg , Renthof 7, 35032 Marburg, Germany.
authors.forename: Alexandra
authors.initials: A
authors.lastname: Ostapenko
authors.affiliation: Interdisciplinary Center for Molecular Materials (ICMM) and Computer-Chemistry-Center (CCC), Department of Chemistry and Pharmacy, Friedrich-Alexander-Universität Erlangen-Nürnberg , Nägelsbachstrasse 25, 91052 Erlangen, Germany.
authors.forename: Tobias
authors.initials: T
authors.lastname: Klöffel
authors.affiliation: Fachbereich Chemie and Wissenschaftliches Zentrum für Materialwissenschaften (WZMW), Philipps-Universität Marburg , Hans-Meerwein-Strasse 4, 35043 Marburg, Germany.
authors.forename: Jens
authors.initials: J
authors.lastname: Eußner
authors.affiliation: Fachbereich Chemie and Wissenschaftliches Zentrum für Materialwissenschaften (WZMW), Philipps-Universität Marburg , Hans-Meerwein-Strasse 4, 35043 Marburg, Germany.
authors.forename: Klaus
authors.initials: K
authors.lastname: Harms
authors.affiliation: Fachbereich Chemie and Wissenschaftliches Zentrum für Materialwissenschaften (WZMW), Philipps-Universität Marburg , Hans-Meerwein-Strasse 4, 35043 Marburg, Germany.
authors.forename: Stefanie
authors.initials: S
authors.lastname: Dehnen
authors.affiliation: Interdisciplinary Center for Molecular Materials (ICMM) and Computer-Chemistry-Center (CCC), Department of Chemistry and Pharmacy, Friedrich-Alexander-Universität Erlangen-Nürnberg , Nägelsbachstrasse 25, 91052 Erlangen, Germany.
authors.forename: Bernd
authors.initials: B
authors.lastname: Meyer
authors.affiliation: Fachbereich Physik, Molekulare Festkörperphysik and Wissenschaftliches Zentrum für Materialwissenschaften (WZMW), Philipps-Universität Marburg , Renthof 7, 35032 Marburg, Germany.
authors.forename: Gregor
authors.initials: G
authors.lastname: Witte
keywords: ZnO; etching; phosphonate; phosphonic acid; precipitation; self-assembled monolayer