Direct experimental evidence for substrate adatom incorporation into a molecular overlayer

While the phenomenon of metal substrate adatom incorporation into molecular overlayers is generally believed to occur in several systems, the experimental evidence for this relies on the interpretation of scanning tunneling microscopy (STM) images, which can be ambiguous and provides no quantitative structural information. We show that surface X-ray diffraction (SXRD) uniquely provides unambiguous identification of these metal adatoms. We present the results of a detailed structural study of the Au(111)-F4TCNQ system, combining surface characterization by STM, low-energy electron diffraction, and soft X-ray photoelectron spectroscopy with quantitative experimental structural information from normal incidence X-ray standing wave (NIXSW) and SXRD, together with dispersion-corrected density functional theory (DFT) calculations. Excellent agreement is found between the NIXSW data and the DFT calculations regarding the height and conformation of the adsorbed molecule, which has a twisted geometry rather than the previously supposed inverted bowl shape. SXRD measurements provide unequivocal evidence for the presence and location of Au adatoms, while the DFT calculations show this reconstruction to be strongly energetically favored

Direct experimental evidence for substrate adatom incorporation into a molecular overlayer

While the phenomenon of metal substrate adatom incorporation into molecular overlayers is generally believed to occur in several systems, the experimental evidence for this relies on the interpretation of scanning tunneling microscopy (STM) images, which can be ambiguous and provides no quantitative structural information. We show that surface X-ray diffraction (SXRD) uniquely provides unambiguous identification of these metal adatoms. We present the results of a detailed structural study of the Au(111)-F4TCNQ system, combining surface characterization by STM, low-energy electron diffraction, and soft X-ray photoelectron spectroscopy with quantitative experimental structural information from normal incidence X-ray standing wave (NIXSW) and SXRD, together with dispersion-corrected density functional theory (DFT) calculations. Excellent agreement is found between the NIXSW data and the DFT calculations regarding the height and conformation of the adsorbed molecule, which has a twisted geometry rather than the previously supposed inverted bowl shape. SXRD measurements provide unequivocal evidence for the presence and location of Au adatoms, while the DFT calculations show this reconstruction to be strongly energetically favored

Computational characterisation of donor-acceptor molecules at metal-organic interfaces

In the field of organic electronics, the interface between metal substrate and (sub)monolayer organic material is of crucial interest as it determines the functionality and efficiency of organic-electronic technologies such as organic light emitting diodes (OLED), organic field effect transistors (OFET) and organic photovoltaics (OPV). In an effort to support this burgeoning multi-disciplinary field, this work aims to identify and understand structure-to-property relationships of a variety of hybrid interfaces. Utilising state-of-the-art computation, theoretical calculations at the Density Functional Theory level are presented and compared to both qualitative and quantitative experimental benchmarks - where possible. The systems of interest are comprised of either strong electron donor species such as alkali atoms, or organic electron acceptor molecules such as 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its derivatives, adsorbed on coinage metal substrates. Information gathered by studying these systems provide general structure-to-property relationships which can be used in the context of co-adsorbed phases at metal surfaces. In particular, the influence of long range dispersion interactions and correct description of such forces which determine the adsorption geometry between substrate and adsorbate. In addition, there are many concomitant processes which arise due to adsorption such as, the “push back” effect, intrinsic dipoles and electrostatic interactions. The implications of such effects can influence structural rearrangement which ultimately impact the energy level alignment across the metal-organic interface. To this end, the improvement of our understanding of energy level alignment and how to exert control over it poses the greatest challenge within the field. Improvement of quantum efficiencies of the aforementioned organic electronic technologies is highly dependent upon elucidating the nature of the non-trivial metal-organic interface. Probing the electronic structure is of upmost importance as will be shown in this work utilising well-established methods