Tuning the Band Bending and Controlling the Surface Reactivity at Polar and Nonpolar Surfaces of ZnO through Phosphonic Acid Binding

Abstract

ZnO is a prime candidate for future use in transparent electronics; however, development of practical materials requires attention to factors including control of its unusual surface band bending and surface reactivity. In this work, we have modified the O-polar (0001̅), Zn-polar (0001), and m-plane (101̅0) surfaces of ZnO with phosphonic acid (PA) derivatives and measured the effect on the surface band bending and surface sensitivity to atmospheric oxygen. Core level and valence band synchrotron X-ray photoemission spectroscopy was used to measure the surface band bending introduced by PA modifiers with substituents of opposite polarity dipole moment: octadecylphosphonic acid (ODPA) and 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctylphosphonic acid (FOPA). Both PAs act as surface electron donors, increasing the downward band bending and the strength of the two-dimensional surface electron accumulation layer on all of the ZnO surfaces investigated. On the O-polar (0001̅) and m-plane (101̅0) surfaces, the ODPA modifier produced the largest increase in downward band bending relative to the hydroxyl-terminated unmodified surface of 0.55 and 0.35 eV, respectively. On the Zn-polar (0001) face, the FOPA modifier gave the largest increase (by 0.50 eV) producing a total downward band bending of 1.00 eV, representing ∼30% of the ZnO band gap. Ultraviolet (UV) photoinduced surface wettability and photoconductivity measurements demonstrated that the PA modifiers are effective at decreasing the sensitivity of the surface toward atmospheric oxygen. Modification with PA derivatives produced a large increase in the persistence of UV-induced photoconductivity and a large reduction in UV-induced changes in surface wettability.

Simultaneous Effect of Ultraviolet Radiation and Surface Modification on the Work Function and Hole Injection Properties of ZnO Thin Films

“…Noteworthy, the recovery is smaller for the SAM‐treated samples. This is consistent with the results from previous studies on the stability of persistent photoconductivity of neat and SAM‐treated ZnO layers in air, where the presence of a dense SAM was proposed to slow down the readsorption of oxygen and other air‐derived acceptor molecules by the ZnO surface …”

“…Storing the UV soaked sample in the dark goes along with a partial recovery of the WF, which attains a saturated value of 4.3 eV after ≈48 h. We noticed that the WF never regains the original value of the initial preillumination state, demonstrating the persistent effect of UV soaking. Earlier work reported persistent WF shifts upon UV light exposure of TiO 2 and ZnO if the sample is kept in vacuum or in an inert gas atmosphere …”

“…Meanwhile, for modified samples with high initial WF (here: (FBPA:BPA) 1:1, 3:1, and 1:0) UV soaking does not produce a major effect, as the ZnO Fermi level remains pinned. For these samples, UV exposure causes a slight increase in the current at high reverse bias, which may be caused by an increase in the electrical conductivity of the ZnO due to photodoping …”

The Impact of Dipolar Layers on the Electronic Properties of Organic/Inorganic Hybrid Interfaces

“…Moreover, the adsorption of physisorbed or covalently bonded molecules causes only very local charge rearrangements, which do not notably affect the surface band bending, [85,154] unless the surface chemistry is substantially altered (e.g., by the removal of hydroxyl groups). [155] For the adsorption of organic electron acceptors on n-type or electron donors on p-type materials, however, the semiconducting nature of the substrate can have a tremendous impact. Provided that the adsorbate depletes charge carriers (i.e., electrons or holes) at the surface and in the bulk, this results in an additional band bending, ΔBB.…”

Influence of dielectric barrier plasma treatment of ZnMgAl alloy‐coated steel on the adsorption of organophosphonic acid monolayers

“…A 1‐mM solution of ODPA in ethanol was used in order to form a self‐assembled monolayer on the surface. The concentration of 1mM has been established as a standard concentration for obtaining self‐assembling monolayers on various substrates 18,19 . The samples were immersed in the ODPA solution for 24 h immediately after the plasma treatments, subsequently rinsed with ethanol and dried with a stream of air.…”