Controllable Modulation of the Electronic Structure of ZnO(1010) Surface by Carboxylic Acids

A systematic investigation of the correlation between bonding geometries and electronic structures of mercapto-acetic acid molecule on the ZnO(101̅0) nonpolar surface is reported. The geometric structure calculation results are consistent with the recent Fourier transform infrared attenuated total reflectance (FT-IR-ATR) findings. The mercapto-acetic acid molecule can contribute an abundance of band gap states to ZnO. Monolayer functionalized ZnO(101̅0) is on the verge of a metal to insulator transition, which is consistent with the experimental findings of the conductivity increase by 6 orders of magnitude. The electrostatic net charge transfer from the molecule to ZnO is around 0.3 electrons for all configurations, but the electronic structure and adsorption energy of carboxylic molecules on ZnO(101̅0) show strong configuration dependence. This is also the magic of the organic molecule-oxide interface. The mercapto-acetic acid molecule functionalized ZnO also shows facet-dependent characteristics while the monolayer functionalized ZnO(21̅1̅0) does not show metal to insulator transition. Acetic acid does not contribute to the band gap states of ZnO(101̅0), whereas benzoic acid and 9-anthracenecarboxylic acid do contribute an abundance of band gap states to ZnO(101̅0). 9-Anthracenecarboxylic acid functionalized ZnO(101̅0) shows a smaller energy difference between the conduction band minimum (CBM) and highest occupied molecular orbital (HOMO), compared to mercapto-acetic acid under the same situation. Our findings are useful to understand the effect of surface functionaliztion on ZnO-based solar cells, biosensor applications, oxide surface nanofabrications, and molecular electronics.