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

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

doi:10.1021/acsnano.0c03133

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

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

^*Corresponding author for this work

Research output: Contribution to journal › Article › peer-review

14 Citations (Scopus)

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.

Original language	English
Pages (from-to)	7475-7483
Number of pages	9
Journal	ACS Nano
Volume	14
Issue number	6
DOIs	https://doi.org/10.1021/acsnano.0c03133
Publication status	Published - 23 Jun 2020

Bibliographical note

Funding Information:
The authors thank Diamond Light Source for allocations SI15899 and NT18191 of beam time at beamline I09 that contributed to the results presented here. P.J.B. acknowledges financial support from Diamond Light Source and EPSRC. G.C. acknowledges financial support from the EU through the ERC Grant “VISUAL-MS” (Project ID: 308115). B.S. and R.J.M. acknowledge doctoral studentship funding from the EPSRC and the National Productivity Investment Fund (NPIF). R.J.M. acknowledges financial support via a UKRI Future Leaders Fellowship (MR/S016023/1). We acknowledge computing resources provided by the EPSRC-funded HPC Midlands+ Computing Centre (MR/S016023/1) and the EPSRC-funded Materials Chemistry Consortium for the ARCHER U.K. National Supercomputing Service (EP/R029431/1). R.J.M. acknowledges fruitful discussions with O. Hofmann (TU Graz).

Publisher Copyright:
© 2020 American Chemical Society.

Keywords

charge transfer
density functional theory
surface structure
two-dimensional salt
X-ray standing waves

ASJC Scopus subject areas

General Materials Science
General Engineering
General Physics and Astronomy

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10.1021/acsnano.0c03133

Cite this

@article{e21c20114a1c46dd9211746651cf377a,

title = "Alkali Doping Leads to Charge-Transfer Salt Formation in a Two-Dimensional Metal-Organic Framework",

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.",

keywords = "charge transfer, density functional theory, surface structure, two-dimensional salt, X-ray standing waves",

author = "Blowey, {Phil J.} and Blowey, {Phil J.} and Billal Sohail and Rochford, {Luke A.} and Timothy Lafosse and Duncan, {David A.} and Ryan, {Paul T.P.} and Ryan, {Paul T.P.} and Warr, {Daniel Andrew} and Lee, {Tien Lin} and Giovanni Costantini and Maurer, {Reinhard J.} and Woodruff, {David Phillip}",

note = "Funding Information: The authors thank Diamond Light Source for allocations SI15899 and NT18191 of beam time at beamline I09 that contributed to the results presented here. P.J.B. acknowledges financial support from Diamond Light Source and EPSRC. G.C. acknowledges financial support from the EU through the ERC Grant “VISUAL-MS” (Project ID: 308115). B.S. and R.J.M. acknowledge doctoral studentship funding from the EPSRC and the National Productivity Investment Fund (NPIF). R.J.M. acknowledges financial support via a UKRI Future Leaders Fellowship (MR/S016023/1). We acknowledge computing resources provided by the EPSRC-funded HPC Midlands+ Computing Centre (MR/S016023/1) and the EPSRC-funded Materials Chemistry Consortium for the ARCHER U.K. National Supercomputing Service (EP/R029431/1). R.J.M. acknowledges fruitful discussions with O. Hofmann (TU Graz). Publisher Copyright: {\textcopyright} 2020 American Chemical Society.",

year = "2020",

month = jun,

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doi = "10.1021/acsnano.0c03133",

language = "English",

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journal = "ACS Nano",

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AU - Blowey, Phil J.

AU - Sohail, Billal

AU - Rochford, Luke A.

AU - Lafosse, Timothy

AU - Duncan, David A.

AU - Ryan, Paul T.P.

AU - Warr, Daniel Andrew

AU - Lee, Tien Lin

AU - Costantini, Giovanni

AU - Maurer, Reinhard J.

AU - Woodruff, David Phillip

N1 - Funding Information: The authors thank Diamond Light Source for allocations SI15899 and NT18191 of beam time at beamline I09 that contributed to the results presented here. P.J.B. acknowledges financial support from Diamond Light Source and EPSRC. G.C. acknowledges financial support from the EU through the ERC Grant “VISUAL-MS” (Project ID: 308115). B.S. and R.J.M. acknowledge doctoral studentship funding from the EPSRC and the National Productivity Investment Fund (NPIF). R.J.M. acknowledges financial support via a UKRI Future Leaders Fellowship (MR/S016023/1). We acknowledge computing resources provided by the EPSRC-funded HPC Midlands+ Computing Centre (MR/S016023/1) and the EPSRC-funded Materials Chemistry Consortium for the ARCHER U.K. National Supercomputing Service (EP/R029431/1). R.J.M. acknowledges fruitful discussions with O. Hofmann (TU Graz). Publisher Copyright: © 2020 American Chemical Society.

PY - 2020/6/23

Y1 - 2020/6/23

N2 - 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.

AB - 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.

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

Abstract

Bibliographical note

Keywords

ASJC Scopus subject areas

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