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Alkali doping leads to charge-transfer salt formation in a two-dimensional metal–organic framework

DOI: 10.1021/acsnano.0c03133 DOI Help

Authors: Phil J. Blowey (University of Warwick) , Billal Sohail (University of Warwick) , Luke A. Rochford (University of Birmingham) , Timothy Lafosse (University of Warwick) , David A. Duncan (Diamond Light Source) , Paul Ryan (Diamond Light Source; Imperial College London) , Daniel Andrew Warr (University of Warwick) , Tien-lin Lee (Diamond Light Source) , Giovanni Costantini (University of Warwick) , Reinhard J. Maurer (University of Warwick) , David Phillip Woodruff (University of Warwick)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Acs Nano

State: Published (Approved)
Published: May 2020
Diamond Proposal Number(s): 15899 , 18191

Abstract: Efficient charge transfer across metal–organic interfaces is a key physical process in modern organic electronics devices, and characterization of the energy level alignment at the interface is crucial to enable a rational device design. We show that the insertion of alkali atoms can significantly change the structure and electronic properties of a metal–organic interface. Coadsorption of tetracyanoquinodimethane (TCNQ) and potassium on a Ag(111) surface leads to the formation of a two-dimensional charge transfer salt, with properties quite different from those of the two-dimensional Ag adatom TCNQ metal–organic framework formed in the absence of K doping. We establish a highly accurate structural model by combination of quantitative X-ray standing wave measurements, scanning tunnelling microscopy, and density-functional theory (DFT) calculations. Full agreement between the experimental data and the computational prediction of the structure is only achieved by inclusion of a charge-transfer-scaled dispersion correction in the DFT, which correctly accounts for the effects of strong charge transfer on the atomic polarizability of potassium. The commensurate surface layer formed by TCNQ and K is dominated by strong charge transfer and ionic bonding and is accompanied by a structural and electronic decoupling from the underlying metal substrate. The consequence is a significant change in energy level alignment and work function compared to TCNQ on Ag(111). Possible implications of charge-transfer salt formation at metal–organic interfaces for organic thin-film devices are discussed.

Journal Keywords: surface structure; charge transfer; two-dimensional; salt X-ray standing waves; density functional theory

Subject Areas: Chemistry, Materials

Instruments: I09-Surface and Interface Structural Analysis

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