Molecular Adsorption on ZnO(101̅0) Single-Crystal Surfaces: Morphology and Charge Transfer

Abstract

While ZnO has excellent electrical properties, it has not been widely used for dye-sensitized solar cells, in part because ZnO is chemically less stable than widely used TiO(2). The functional groups typically used for surface passivation and for attaching dye molecules either bind weakly or etch the ZnO surface. We have compared the formation of molecular layers from alkane molecules with terminal carboxylic acid, alcohol, amine, phosphonic acid, or thiol functional groups on single-crystal zinc oxide (1010) surfaces. Atomic force microscopy (AFM) images show that alkyl carboxylic acids etch the surface whereas alkyl amine and alkyl alcohols bind only weakly on the ZnO(1010) surface. Phosphonic acid-terminated molecules were found to bind to the surface in a heterogeneous manner, forming clusters of molecules. Alkanethiols were found to bind to the surface, forming highly uniform monolayers with some etching detected after long immersion times in an alkanethiol solution. Monolayers of hexadecylphosphonic acid and octadecanethiol were further analyzed by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and electrochemical measurements. AFM scratching shows that thiols were bound strongly to the ZnO surface, suggesting the formation of strong Zn-S covalent bonds. Surprisingly, the tridentate phosphonic acids adhered much more weakly than the monodentate thiol. The influence of organic grafting on the charge transfer to ZnO was studied by time-resolved surface photovoltage measurements and electrochemical impedance measurements. Our results show that the grafting of thiols to ZnO leads to robust surfaces and reduces the surface band bending due to midgap surface states.

Etching of Crystalline ZnO Surfaces upon Phosphonic Acid Adsorption: Guidelines for the Realization of Well-Engineered Functional Self-Assembled Monolayers

“…When providing sufficient thermal energy and time during preparation, this yields exceptionally well ordered SAMs of one phase only. 37 By contrast, on oxidic substrates undesirable side reactions, such as partial dissolution of substrate surfaces, have been reported, 13,20,25,26,38,39 which are made responsible for the failure and improper behavior of electronic devices. 40,41 Therefore, a closer analysis of the immersion chemistry of such hybrid systems is required.…”

First principles investigations on the electronic structure of anchor groups on ZnO nanowires and surfaces

“…16,26 Alkanethiols were found to bind to the surface, forming highly uniform monolayers with some etching detected after long immersion times in an alkanethiol solution. 25 However, earlier investigations reported non-uniform coverages or even physisorbed layers of these functional groups. 24 Several chemisorbed [27][28][29][30][31] and physisorbed molecules [32][33][34][35][36][37] on ZnO have been theoretically investigated.…”

“…24 On the other hand, recent atomic force microscopy (AFM) measurements have refuted the rightness of carboxylic acids for the modification of ZnO, showing that alkyl carboxylic acids etch the surface. 25 Thiols have also been suggested to form strong bonds on ZnO. 16,26 Alkanethiols were found to bind to the surface, forming highly uniform monolayers with some etching detected after long immersion times in an alkanethiol solution.…”

A review of in situ surface functionalization of gallium nitride via beaker wet chemistry

“…The use of Kelvin force probe microscopy (KFPM), a technique similar to AFM, provides information for identifying differences, both qualitatively as well as quantitatively, between the surface potential of various adsorbates. Previously reported findings of the attachment of phosphonic acids to indium tin oxide 25,45 and zinc oxide 43 show a change in the charge distribution on the surfaces based on a modulation of the work functions of the surfaces. For electronic devices, this information is crucial in determining multi-material compatibility for electrical interface interactions.…”

“…The presence of certain groups provides the capability of altering numerous surface properties. 25,43,44 In the case of DSSCs, the use of phosphonic acids limits the amount of surface charge recombination and promotes work function matching. 45 The various proposed modes of phosphonic group attachment to the surface of GaN are shown in Fig.…”