Alkali Doping Leads to Charge-Transfer Salt Formation in a Two-Dimensional Metal–Organic Framework

Phil J Blowey,Paul T P Ryan,Daniel Andrew Warr,Luke A Rochford,Giovanni Costantini,Timothy Lafosse,Billal Sohail,Tien-Lin Lee,Reinhard J Maurer,David A Duncan,David Phillip Woodruff

doi:10.1021/acsnano.0c03133

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.

Highlights

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
By including atoms or molecules that are strong donors of electrons into the TCNQ-metal interface, a competition between donor, acceptor, and metal can be created, which has the potential to modify structure, stability, and the ensuing electronic properties of the interface. These ideas have been explored by a density functional theory (DFT) investigation of TCNQ coadsorbed with alkali atoms on a silver surface, which has highlighted the potential to control electrostatic properties such as the surface work function and the energy barrier at the metal−organic interface.[14]
As realistic simulations using density-functional theory (DFT) can only be performed for commensurate phases, we present here only the results for this phase, for which scanning tunneling microscope (STM) images and a LEED

Summary

Introduction

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. By including atoms or molecules that are strong donors of electrons into the TCNQ-metal interface, a competition between donor, acceptor, and metal can be created, which has the potential to modify structure, stability, and the ensuing electronic properties of the interface These ideas have been explored by a density functional theory (DFT) investigation of TCNQ coadsorbed with alkali atoms on a silver surface, which has highlighted the potential to control electrostatic properties such as the surface work function and the energy barrier at the metal−organic interface.[14]. Detailed analysis of the electronic structure of the 2D metal−organic-framework (2DMOF) formed by this coadsorption reveals it to be a de facto quasi-free-standing two-dimensional organic salt, formed at the expense of a strongly reduced interaction between the metal surface and the molecular adlayer This interaction competition between donor, acceptor, and metal directly translates into changes in molecular energy level alignment and work function, both of which can potentially be further tuned by various means. This behavior contrasts with the more typical 2D-MOF formed by TCNQ and Ag adatoms in the absence of the coadsorbed K

Methods

Results

Conclusion