Reduction of the Work Function of Gold by N-Heterocyclic Carbenes

Abstract
N-Heterocyclic carbenes (NHCs) bind strongly to gold and other metals. This work experimentally probes the effect of NHCs on the work function (WF) of gold for the first time, theoretically analyzes the origin of this effect, and examines the effectiveness of NHC-modified gold as an electron-injecting electrode. UV photoelectron spectroscopy shows the WF of planar gold is reduced by nearly 2 eV to values of 3.3–3.5 eV. This effect is seen for NHCs with various heterocyclic cores, and with either small or large N,N′-substituents. DFT calculations indicate the WF reduction results from both the interface dipole formed between the NHC and the gold and from the NHC molecular dipole. For N,N′-diisopropyl-NHCs, an important contributor to the former is charge transfer associated with coordination of the carbene carbon atom to gold. In contrast, the carbene carbon of N,N′-2,6-diisopropylphenyl-NHCs is not covalently bound to gold, resulting in a lower interface dipole; however, a larger molecular dipole partially compensates for this. Single-layer C60 diodes with NHC-modified gold as the bottom electrode demonstrate high rectification ratios and show that these electrodes can act as effective electron-injecting contacts, suggesting they may be useful for a variety of materials applications.

Citation
Kim HK, Hyla AS, Winget P, Li H, Wyss CM, et al. (2017) Reduction of the Work Function of Gold by N-Heterocyclic Carbenes. Chemistry of Materials 29: 3403–3411. Available: http://dx.doi.org/10.1021/acs.chemmater.6b04213.

Acknowledgements
The research was partly supported by the National Science Foundation (NSF) under the CCI Center for Selective C–H Functionalization, Grant CHE-1205646, the IGERT: Nanostructured Materials for Energy Storage and Conversion, Grant DGE-1069138, and the CRIF Program, Grant CHE-0946869. We also acknowledge generous support from KAUST; we thank the IT Research Computing Team and Supercomputing Laboratory at KAUST for providing computational and storage resources. B.K., F.A.L., and C.F.-H. acknowledge support from the Office of Naval Research Award N00014-04-1-0313.

Publisher
American Chemical Society (ACS)

Journal
Chemistry of Materials

DOI
10.1021/acs.chemmater.6b04213

Additional Links
http://pubs.acs.org/doi/full/10.1021/acs.chemmater.6b04213

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