I09-Surface and Interface Structural Analysis
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Phil J.
Blowey
,
Billal
Sohail
,
Luke A.
Rochford
,
Timothy
Lafosse
,
David A.
Duncan
,
Paul
Ryan
,
Daniel Andrew
Warr
,
Tien-Lin
Lee
,
Giovanni
Costantini
,
Reinhard J.
Maurer
,
David Phillip
Woodruff
Diamond Proposal Number(s):
[15899, 18191]
Open Access
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.
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May 2020
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Open Access
Abstract: X-ray photoemission and X-ray absorption spectroscopy are important techniques to characterize chemical bonding at surfaces and are often used to identify the strength and nature of adsorbate–substrate interactions. In this study, we judge the ability of X-ray spectroscopic techniques to identify different regimes of chemical bonding at metal–organic interfaces. To achieve this, we sample different interaction strength regimes in a comprehensive and systematic way by comparing two topological isomers, azulene and naphthalene, adsorbed on three metal substrates with varying reactivity, namely the (111) facets of Ag, Cu, and Pt. Using density functional theory, we simulate core-level binding energies and X-ray absorption spectra of the molecular carbon species. The simulated spectra reveal three distinct characteristics based on the molecule-specific spectral features which we attribute to types of surface chemical bonding with varying strength. We find that weak physisorption only leads to minor changes compared to the gas-phase spectra, weak chemisorption leads to charge transfer and significant spectral changes, and strong chemisorption leads to a loss of the molecule-specific features in the spectra. The classification we provide is aimed at assisting interpretation of experimental X-ray spectra for complex metal–organic interfaces.
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Jan 2023
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Open Access
Abstract: Diamond-based materials have unique properties that are exploited in many electrochemical, optical, thermal, and quantum applications. When grown via chemical vapor deposition (CVD), the growth rate of the (110) face is typically much faster than the other two dominant crystallographic orientations, (111) and (100). As such, achieving sufficiently large-area and high-quality (110)-oriented crystals is challenging and typically requires post-growth processing of the surface. Whilst CVD growth confers hydrogen terminations on the diamond surface, the majority of post-growth processing procedures render the surface oxygen-terminated, which in turn impacts the surface properties of the material. Here, we determine the oxygenation state of the (110) surface using a combination of density functional theory calculations and X-ray photoelectron spectroscopy experiments. We show that in the 0–1000 K temperature range, the phase diagram of the (110) surface is dominated by a highly stable phase of coexisting and adjacent carbonyl and ether groups, while the stability of peroxide groups increases at low temperatures and high pressures. We propose a mechanism for the formation of the hybrid carbonyl-ether phase and rationalize its high stability. We further corroborate our findings by comparing simulated core-level binding energies with experimental X-ray photoelectron spectroscopy data on the highest-quality (110)-oriented diamond crystal surface reported to date.
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Jan 2022
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I07-Surface & interface diffraction
I09-Surface and Interface Structural Analysis
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Philip J.
Mousley
,
Luke A.
Rochford
,
Paul T. P.
Ryan
,
Philip
Blowey
,
James
Lawrence
,
David A.
Duncan
,
Hadeel
Hussain
,
Billal
Sohail
,
Tien-Lin
Lee
,
Gavin R.
Bell
,
Giovanni
Costantini
,
Reinhard J.
Maurer
,
Christopher
Nicklin
,
D. Phil
Woodruff
Diamond Proposal Number(s):
[14884, 4884]
Open Access
Abstract: 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.
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Apr 2022
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[17261, 20785]
Open Access
Abstract: The results are presented of a detailed combined experimental and theoretical investigation of the influence of coadsorbed electron-donating alkali atoms and the prototypical electron acceptor molecule 7,7,8,8-tetracyanoquinodimethane (TCNQ) on the Ag(100) surface. Several coadsorption phases were characterized by scanning tunneling microscopy, low-energy electron diffraction, and soft X-ray photoelectron spectroscopy. Quantitative structural data were obtained using normal-incidence X-ray standing wave (NIXSW) measurements and compared with the results of density functional theory (DFT) calculations using several different methods of dispersion correction. Generally, good agreement between theory and experiment was achieved for the quantitative structures, albeit with the prediction of the alkali atom heights being challenging for some methods. The adsorption structures depend sensitively on the interplay of molecule–metal charge transfer and long-range dispersion forces, which are controlled by the composition ratio between alkali atoms and TCNQ. The large difference in atomic size between K and Cs has negligible effects on stability, whereas increasing the ratio of K/TCNQ from 1:4 to 1:1 leads to a weakening of molecule–metal interaction strength in favor of stronger ionic bonds within the two-dimensional alkali–organic network. A strong dependence of the work function on the alkali donor–TCNQ acceptor coadsorption ratio is predicted.
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Jan 2023
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I09-Surface and Interface Structural Analysis
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Benedikt P.
Klein
,
Nadine J.
Van Der Heijden
,
Stefan R.
Kachel
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Markus
Franke
,
Claudio K.
Krug
,
Katharina K.
Greulich
,
Lukas
Ruppenthal
,
Philipp
Müller
,
Phil
Rosenow
,
Shayan
Parhizkar
,
Francois C.
Bocquet
,
Martin
Schmid
,
Wolfgang
Hieringer
,
Reinhard J.
Maurer
,
Ralf
Tonner
,
Christian
Kumpf
,
Ingmar
Swart
,
J. Michael
Gottfried
Diamond Proposal Number(s):
[16259]
Open Access
Abstract: The interaction of carbon-based aromatic molecules and nanostructures with metals can strongly depend on the topology of their π-electron systems. This is shown with a model system using the isomers azulene, which has a nonalternant π system with a 5-7 ring structure, and naphthalene, which has an alternant π system with a 6-6 ring structure. We found that azulene can interact much more strongly with metal surfaces. On copper (111), its zero-coverage desorption energy is 1.86 eV, compared to 1.07 eV for naphthalene. The different bond strengths are reflected in the adsorption heights, which are 2.30 Å for azulene and 3.04 Å for naphthalene, as measured by the normal incidence x-ray standing wave technique. These differences in the surface chemical bond are related to the electronic structure of the molecular π systems. Azulene has a lowlying LUMO that is close to the Fermi energy of Cu and strongly hybridizes with electronic states of the surface, as is shown by photoemission, near-edge x-ray absorption fine-structure, and scanning tunneling microscopy data in combination with theoretical analysis. According to density functional theory calculations, electron donation from the surface into the molecular LUMO leads to negative charging and deformation of the adsorbed azulene. Noncontact atomic force microscopy confirms the deformation, while Kelvin probe force microscopy maps show that adsorbed azulene partially retains its in-plane dipole. In contrast, naphthalene experiences only minor adsorption-induced changes of its electronic and geometric structure. Our results indicate that the electronic properties of metal-organic interfaces, as they occur in organic (opto)electronic devices, can be tuned through modifications of the π topology of the molecular organic semiconductor, especially by introducing 5-7 ring pairs as functional structural elements.
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Feb 2019
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I09-Surface and Interface Structural Analysis
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Benedikt P.
Klein
,
Juliana M.
Morbec
,
Markus
Franke
,
Katharina K.
Greulich
,
Malte
Sachs
,
Shayan
Parhizkar
,
Francois C.
Bocquet
,
Martin
Schmid
,
Samuel J.
Hall
,
Reinhard J.
Maurer
,
Bernd
Meyer
,
Ralf
Tonner
,
Christian
Kumpf
,
Peter
Kratzer
,
J. Michael
Gottfried
Diamond Proposal Number(s):
[16259]
Abstract: Interfaces between polycyclic π-electron systems and metals play prominent roles in organic or graphene-based (opto)electronic devices, in which performance-related parameters depend critically on the properties of metal/semiconductor contacts. Here, we explore how the topology of the π-electron system influences the bonding and the electronic properties of the interface. We use azulene as a model for nonalternant pentagon-heptagon (5-7) ring pairs and compare it to its isomer naphthalene, which represents the alternant 6-6 ring pair. Their coverage-dependent interaction with Ag(111) and Cu(111) surfaces was studied with the normal-incidence X-ray standing wave (NIXSW) technique, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, UV and X-ray photoelectron spectroscopy (UPS, XPS), and density functional theory (DFT). Coverage-dependent adsorption heights and spectroscopic data reveal that azulene forms shorter interfacial bonds than naphthalene and engages in stronger electronic interactions with both surfaces. These differences are more pronounced on Cu. Increasing coverages lead to larger adsorption heights, indicating bond weakening by intermolecular repulsion. The extensive DFT calculations include dispersive interactions using: (1) the DFT-D3 scheme, (2) the vdWsurf correction based on DFT-TS, (3) a Many-Body Dispersion (MBD) correction scheme, and (4) the D3surf scheme. All methods predict the adsorption heights reasonably well with an average error below 0.1 Å. The stronger bond of azulene is attributed to its nonalternant topology, which results in a reduced HOMO-LUMO gap and brings the LUMO energetically close to the Fermi energy of the metal, causing stronger hybridization with electronic states of the metal surfaces.
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Nov 2019
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Open Access
Abstract: X-ray photoemission (XPS) and near edge x-ray absorption fine structure (NEXAFS) spectroscopy play an important role in investigating the structure and electronic structure of materials and surfaces. Ab initio simulations provide crucial support for the interpretation of complex spectra containing overlapping signatures. Approximate core-hole simulation methods based on density functional theory (DFT) such as the delta-self-consistent-field (ΔSCF) method or the transition potential (TP) method are widely used to predict K-shell XPS and NEXAFS signatures of organic molecules, inorganic materials and metal–organic interfaces at reliable accuracy and affordable computational cost. We present the numerical and technical details of our variants of the ΔSCF and TP method (coined ΔIP-TP) to simulate XPS and NEXAFS transitions. Using exemplary molecules in gas-phase, in bulk crystals, and at metal–organic interfaces, we systematically assess how practical simulation choices affect the stability and accuracy of simulations. These include the choice of exchange–correlation functional, basis set, the method of core-hole localization, and the use of periodic boundary conditions (PBC). We particularly focus on the choice of aperiodic or periodic description of systems and how spurious charge effects in periodic calculations affect the simulation outcomes. For the benefit of practitioners in the field, we discuss sensible default choices, limitations of the methods, and future prospects.
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Feb 2021
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I09-Surface and Interface Structural Analysis
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P. J.
Blowey
,
R. J.
Maurer
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L. A.
Rochford
,
D. A.
Duncan
,
Jie Hun
Kang
,
D. A.
Warr
,
A. J.
Ramadan
,
T.-L.
Lee
,
P. K.
Thakur
,
G.
Costantini
,
K.
Reuter
,
D. P.
Woodruff
Diamond Proposal Number(s):
[9459, 14524, 15899]
Open Access
Abstract: The local structure of the non-planar phthalocyanine, vanadyl phthalocyanine (VOPc), adsorbed on Cu(111) at a coverage of approximately one half of a saturated molecular layer, has been investigated by a combination of normal-incidence X-ray standing waves (NIXSW), scanned-energy mode photoelectron diffraction (PhD) and density-functional theory (DFT), complemented by scanning tunnelling microscopy (STM). Qualitative assessment of the NIXSW data clearly shows that both ‘up’ and ‘down’ orientations of the molecule (with V=O pointing out of, and into, the surface) must coexist on the surface. O 1s PhD proves to be inconclusive regarding the molecular orientation. DFT calculations, using two different dispersion correction schemes, show good quantitative agreement with the NIXSW structural results for equal co-occupation of the two different molecular orientations and clearly favour the Many Body Dispersion (MBD) method to deal with long-range dispersion forces. The calculated relative adsorption energies of the differently-oriented molecules at the lowest coverage show a strong preference for the ‘up’ orientation, but at higher local coverages, this energetic difference decreases and mixed orientation phases are almost energetically equivalent to pure ‘up’ oriented phases. DFT-based Tersoff-Hamann simulations of STM topographs for the two orientations cast some light on the extent to which such images provide a reliable guide to molecular orientation.
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Oct 2018
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[17261, 20785]
Abstract: A quantitative structural investigation is reported, aimed at resolving the issue of whether substrate adatoms are incorporated into the monolayers formed by strong molecular electron acceptors deposited onto metallic electrodes. A combination of normal-incidence X-ray standing waves, low-energy electron diffraction, scanning tunnelling microscopy, and X-ray photoelectron spectroscopy measurements demonstrate that the systems TCNQ and F4TCNQ on Ag(100) lie at the boundary between these two possibilities and thus represent ideal model systems with which to study this effect. A room-temperature commensurate phase of adsorbed TCNQ is found not to involve Ag adatoms, but to adopt an inverted bowl configuration, long predicted but not previously identified experimentally. By contrast, a similar phase of adsorbed F4TCNQ does lead to Ag adatom incorporation in the overlayer, the cyano end groups of the molecule being twisted relative to the planar quinoid ring. Density functional theory (DFT) calculations show that this behavior is consistent with the adsorption energetics. Annealing of the commensurate TCNQ overlayer phase leads to an incommensurate phase that does appear to incorporate Ag adatoms. Our results indicate that the inclusion (or exclusion) of metal atoms into the organic monolayers is the result of both thermodynamic and kinetic factors.
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Mar 2022
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