Journal
ACS NANO
Volume 11, Issue 6, Pages 6252-6260Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acsnano.7b02449
Keywords
integer charge transfer; energy level alignment; ultrathin films; orbital tomography; scanning tunneling microscopy; pentacene; magnesium oxide
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Funding
- Austrian Science Fund (FWF) [P21330-N20, P27649-N20, P27427]
- Austrian Science Fund (FWF) [P27649, P27427] Funding Source: Austrian Science Fund (FWF)
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It is becoming accepted that ultrathin dielectric layers on metals are not merely passive decoupling layers, but can actively influence orbital energy level alignment and charge transfer at interfaces. As such, they can be important in applications ranging from catalysis to organic electronics. However, the details at the molecular level are still under debate. In this study, we present a comprehensive analysis of the phenomenon of charge transfer promoted by a dielectric interlayer with a comparative study of pentacene adsorbed on Ag(001) with and without an ultrathin MgO interlayer. Using scanning tunneling microscopy and photoemission tomography supported by density functional theory, we are able to identify the orbitals involved and quantify the degree of charge transfer in both cases. Fractional charge transfer occurs for pentacene adsorbed on Ag(001), while the presence of the ultrathin MgO interlayer promotes integer charge transfer with the lowest unoccupied molecular orbital transforming into a singly occupied and singly unoccupied state separated by a large gap around the Fermi energy. Our experimental approach allows a direct access to the individual factors governing the energy level alignment and charge-transfer processes for molecular adsorbates on inorganic substrates.
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