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. 2020 Feb 27;124(8):4511-4516.
doi: 10.1021/acs.jpcc.9b08768. Epub 2020 Jan 29.

Doping-Induced Electron Transfer at Organic/Oxide Interfaces: Direct Evidence From Infrared Spectroscopy

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Doping-Induced Electron Transfer at Organic/Oxide Interfaces: Direct Evidence From Infrared Spectroscopy

L Schöttner et al. J Phys Chem C Nanomater Interfaces. .
Free PMC article

Abstract

Charge transfer at organic/inorganic interfaces critically influences the properties of molecular adlayers. Although for metals such charge transfers are well documented by experimental and theoretical results, in the case of semiconductors, clear and direct evidence for a transfer of electrons or holes from oxides with their typically high ionization energy is missing. Here, we present data from infrared reflection-absorption spectroscopy demonstrating that despite a high ionization energy, electrons are transferred from ZnO into a prototype strong molecular electron acceptor, hexafluoro-tetracyano-naphthoquinodimethane (F6-TCNNQ). Because there are no previous studies of this type, the interpretation of the pronounced vibrational red shifts observed in the experiment was aided by a thorough theoretical analysis using density functional theory. The calculations reveal that two mechanisms govern the pronounced vibrational band shifts of the adsorbed molecules: electron transfer into unoccupied molecular levels of the organic acceptor and also the bonding between the surface Zn atoms and the peripheral cyano groups. These combined experimental data and the theoretical analysis provide the so-far missing evidence of interfacial electron transfer from high ionization energy inorganic semiconductors to molecular acceptors and indicates that n-doping of ZnO plays a crucial role.

Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Schematic energy level diagrams of electron acceptor molecules on (a) an undoped semiconductor and (b) an n-doped semiconductor.
Figure 2
Figure 2
Experimental IRRAS spectra of F6-TCNNQ on the nominally undoped ZnO(101̅0) measured with p-polarized light: (a) C=C/C–C region, the intensity of the multilayer data are scaled by a factor of 100 and (b) C≡N region.
Figure 3
Figure 3
Orientation of F6-TCNNQ in the monolayer (left) and multilayer (right) regimes. The polarization dependence of the IR-data indicates an orientation of the molecular plane parallel to the substrate.

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