I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[24284]
Open Access
Abstract: A previous investigation of the structure of TCNQ adsorbed on Ag(111) using normal-incidence X-ray standing waves (NIXSW) and density functional theory (DFT) provided indirect evidence that Ag adatoms must be incorporated into the molecular overlayer. New surface X-ray diffraction (SXRD) results, presented here, provide direct evidence for the presence and location of these Ag adatoms and clearly distinguishes between two alternative models of the adatom registry favored by two different DFT studies.
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Feb 2023
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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
,
Matthew A.
Stoodley
,
Matthew
Edmondson
,
Luke A.
Rochford
,
Marc
Walker
,
Lars
Sattler
,
Sebastian
Weber
,
Gerhard
Hilt
,
Leon B. S.
Williams
,
Tien-Lin
Lee
,
Alex
Saywell
,
Reinhard J.
Maurer
,
David A.
Duncan
Diamond Proposal Number(s):
[25379]
Open Access
Abstract: Ultra-high vacuum deposition of the polycyclic aromatic hydrocarbons azupyrene and pyrene onto a Cu(111) surface held at a temperature of 1000 K is herein shown to result in the formation of graphene. The presence of graphene was proven using scanning tunneling microscopy, x-ray photoelectron spectroscopy, angle-resolved photoemission spectroscopy, Raman spectroscopy, and low energy electron diffraction. The precursors, azupyrene and pyrene, are comparatively large aromatic molecules in contrast to more commonly employed precursors like methane or ethylene. While the formation of the hexagonal graphene lattice could naively be expected when pyrene is used as a precursor, the situation is more complex for azupyrene. In this case, the non-alternant topology of azupyrene with only 5- and 7-membered rings must be altered to form the observed hexagonal graphene lattice. Such a rearrangement, converting a non-alternant topology into an alternant one, is in line with previous reports describing similar topological alterations, including the isomerization of molecular azupyrene to pyrene. The thermal synthesis route to graphene, presented here, is achievable at comparatively low temperatures and under ultra-high vacuum conditions, which may enable further investigations of the growth process in a strictly controlled and clean environment that is not accessible with traditional precursors.
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Nov 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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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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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[13947]
Open Access
Abstract: The adsorption structure of truxenone on Cu(111) was determined quantitatively using normal-incidence X-ray standing waves. The truxenone molecule was found to chemisorb on the surface, with all adsorption heights of the dominant species on the surface less than ∼2.5 Å. The phenyl backbone of the molecule adsorbs mostly parallel to the underlying surface, with an adsorption height of 2.32 ± 0.08 Å. The C atoms bound to the carbonyl groups are located closer to the surface at 2.15 ± 0.10 Å, a similar adsorption height to that of the chemisorbed O species; however, these O species were found to adsorb at two different adsorption heights, 1.96 ± 0.08 and 2.15 ± 0.06 Å, at a ratio of 1:2, suggesting that on average, one O atom per adsorbed truxenone molecule interacts more strongly with the surface. The adsorption geometry determined herein is an important benchmark for future theoretical calculations concerning both the interaction with solid surfaces and the electronic properties of a molecule with electron-accepting properties for applications in organic electronic devices.
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Dec 2021
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[15899, 18191]
Abstract: The structure of coadsorption phases formed on Ag(111) by TCNQ (7,7,8,8-tetracyanoquinodimethane) with Cs are compared with previously reported coadsorption phases formed with K, following investigation by scanning tunnelling microscopy (STM), low energy electron diffraction, soft X-ray photoelectrons spectroscopy and normal incidence X-ray standing waves (NIXSW). For each alkali we identify two ordered phases, one with an alkali: TCNQ stoichiometry of 1:1 and the other 2:1. STM images show the molecular organisation is the same for Cs and K, although only the K2TCNQ phase is commensurate with the substrate. A previously-published detailed structure determination of the K2TCNQ phase, complemented by density function theory calculations that identify bonding strengths, showed that the binding within the layer is much stronger than that of the layer to the substrate. Insensitivity to commensuration is thus to be expected. The situation for KTCNQ and CsTCNQ is less clear; these ordered incommensurate overlayers clearly have strong intralayer bonding, but the relative strength of the average overlayer-substrate bonding is unknown. NIXSW data show that the alkalis in these phases occupy adsorption sites far more distant from the substrate than the TCNQ molecules when compared to the near coplanar alkali-TCNQ geometry of K2TCNQ and Cs2TCNQ. Ultraviolet photoelectron spectra show increasing bonding shifts of TCNQ orbital states with alkali coverage.
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Nov 2020
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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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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[12975]
Abstract: A combination of scanning tunneling microscopy, low-energy electron diffraction,and low-energy electron microscopy (LEEM) has been used to identify the structural phases formed by 7,7,8,8-tetracyanoquinodimethane (TCNQ) on Ag(111). These comprise a two-dimensional gas phase, a low-density commensurate (LDC) phase, and a higher-density incommensurate (HDI) phase. LEEM also shows the presence of an additional “precursor-HDI” phase with a surface unit mesh area only ≈3% less than the HDI phase. Normal incidence x-ray standing-wave measurements of the HDI phase yield almost identical structural parameters to the LDC phase for which a full structure determination has been previously reported. The results show TCNQ does not adopt the inverted bowl distortion favored in earlier density functional theory calculations of TCNQ on coinage metal surfaces, but the N atoms are twisted out of the molecular plane, an effect found for the LDC phase to be due to incorporation of Ag adatoms. The possible role of Ag adatoms in the HDI phase, and in the transition from the precursor-HDI phase, is discussed.
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Nov 2019
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I09-Surface and Interface Structural Analysis
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P. J.
Blowey
,
R. J.
Maurer
,
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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