Transition metal oxide (TMO) has been intensively utilized in catalysis, energy and nanotechnology due to its wide band gap, high carrier mobility and high stability. The hybridization of TMO with organic materials supplies an efficient route for constructions of novel organic electric and optoelectronic devices. It requires not only a stable interface, but also a matched energy level alignment of the frontier molecular orbitals with the energy gap of the oxide semiconductor allowing for an efficient charge transfer or separation at the interface. Therefore the investigation of the electronic structures at organic/TMO interfaces is of great significance for further understanding the behaviors of organic devices as well as for their functional optimizations.
Prof. Faqiang Xu’s group has comprehensively studied the chemistry and electronic structures of the terephthalic acid (TPA)/TiO2(110) interface at the Surface Physics Endstation of NSRL by virtue of the synchrotron radiation based photoemission techniques. The adsorption of TPA on TiO2(110)-(1×1) at room temperature leads to the formation of dicarboxylate species at lower coverage (∼0.2 ML) followed by the formation of monocarboxylate species below 1 ML, indicating a geometry variation from flat-lying to upright adsorption and conforming well the previous adsorption model. Additionally, the interfacial electronic structures of TPA/TiO2(110) demonstrate a straddling lineup of the energy levels and no gap states present at the TPA/TiO2(110) interface. It turns out that a large energy difference with a value of ~1.1 eV is present between the lowest unoccupied molecular orbital (LUMO) of TPA and the conduction band (CB) of TiO2. This type of interface is energetically favorable for electron transfer rather than charge separation therein. The present research is suggested to provide a deep understanding on the charge transport properties at the heterogeneous organic-oxide interface, which is highly desirable for exploring new generation of hybrid energy devices. (J. Phys. Chem. C 2013, 117, 21351)

图1 (left) A representative fitting of O 1s spectrum of 0.8 ML TPA. The inset is a schematic assignment of the various surface oxygen species upon the adsorption of one TPA molecule on TiO2 (110)-(1 ×1) surface. (right) Schematic of the energy level alignment at the TPA/TiO2(110) interface.

Moreover Xu’s group also researched the FePc/MoO3 interface. MoO3 is an insulator with a high electric affinity value (~6.5 eV) and a deep valence band structure which serves it as an excellent hole transport buffer in the fabrication of organic optical and electronic devices. It was found that some Mo 4d band gap states (BGS) were introduced due to the partial reduction of MoO3. The energy level alignment studies of the FePc/MoO3 interface demonstrate that the BGS acts as “energy ladders” in MoO3 buffer layer and facilitate the hole injection from the anode into the FePc layer. (Appl. Surf. Sci. 2013, 271, 352)

图2 Energy level alignment scheme of (a) FePc/MoO3 (3nm)/ITO and (b) FePc/MoOx (3nm)/ITO interfaces. The blue solid circles represent electrons and the red hollow circles represent holes.