Spectroscopy and control of near-surface defects in conductive thin film ZnO

Abstract

Abstract:The electronic structure of inorganic semiconductor interfaces functionalized with extended π-conjugated organic molecules can be strongly influenced by localized gap states or point defects, often present at low concentrations and hard to identify spectroscopically. At the same time, in transparent conductive oxides such as ZnO, the presence of these gap states conveys the desirable high conductivity necessary for function as electron-selective interlayer or electron collection electrode in organic optoelectronic devices. Here, we report on the direct spectroscopic detection of a donor state within the band gap of highly conductive zinc oxide by two-photon photoemission spectroscopy. We show that adsorption of the prototypical organic acceptor C 60 quenches this state by ground-state charge transfer, with immediate consequences on the interfacial energy level alignment. Comparison with computational results suggests the identity of the gap state as a near-surface-confined oxygen vacancy.4

Decoupling the Impacts of Engineering Defects and Band Gap Alignment Mechanism on the Catalytic Performance of Holey 2D CeO2−x‐Based Heterojunctions

“…Further, the midgap Ru 4d band partially overlaps with Ce 4f band, which causes band tail state formation, which effectively lowers the E g . [68] The experimental E g of 2.73 eV is considerably less than the typical literature value of 3.00 eV [69] and less than the value of 3.2 eV calculated by DFT (Figure S10, Supporting Information). According to Xu et al, [22] the E g can be calculated on the basis of the [VO…”

Band Bending Engineering at Organic/Inorganic Interfaces Using Organic Self‐Assembled Monolayers

“…[8] In principle, for low doping concentrations and strong electron acceptors, band bending could be as large as the total band gap of the substrate (≈3.5 eV in ZnO). In practice, however, ΔΦ BB is almost always limited by the presence of defect states at or near the surface, such as oxygen vacancies, [11][12][13] provided they are present in sufficient concentrations.Since the type and concentration of these defects is difficult to control, band bending is typically hard to engineer. In this article, we use these defect states as an analogy to demonstrate a new concept that uses properly designed, covalently attached self-assembled monolayers (SAMs) to act like surface defects that limit band bending at desired values.…”

“…[8] In principle, for low doping concentrations and strong electron acceptors, band bending could be as large as the total band gap of the substrate (≈3.5 eV in ZnO). In practice, however, ΔΦ BB is almost always limited by the presence of defect states at or near the surface, such as oxygen vacancies, [11][12][13] provided they are present in sufficient concentrations.…”

Configuration-specific electronic structure of strongly interacting interfaces: TiOPc on Cu(110)

“…Figure 5(c) shows an intensity decrease in both the HOMO and f-LUMO following self-assembly. We studied this region in more detail by supplementing UPS with two-photon photoemission (2PPE) to take advantage of larger apparent cross sections for hybrid interface states in 2PPE [42,43]. Furthermore, the linearly polarized excitation source in 2PPE reduces background contributions by decreasing the number of symmetry-allowed transitions for photoemission from the Cu surface [31].…”