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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Jack E. N.
Swallow
,
Benjamin A. D.
Williamson
,
Sanjayan
Sathasivam
,
Max
Birkett
,
Thomas J.
Featherstone
,
Philip A. E.
Murgatroyd
,
Holly J.
Edwards
,
Zachary W.
Lebens-higgins
,
David A.
Duncan
,
Mark
Farnworth
,
Paul
Warren
,
Nianhua
Peng
,
Tien-lin
Lee
,
Louis F. J.
Piper
,
Anna
Regoutz
,
Claire J.
Carmalt
,
Ivan P.
Parkin
,
Vin R.
Dhanak
,
David O.
Scanlon
,
Tim D.
Veal
Diamond Proposal Number(s):
[18428]
Open Access
Abstract: Transparent conductors are a vital component of smartphones, touch-enabled displays, low emissivity windows and thin film photovoltaics. Tin-doped In2O3 (ITO) dominates the transparent conductive films market, accounting for the majority of the current multi-billion dollar annual global sales. Due to the high cost of indium, however, alternatives to ITO have been sought but have inferior properties. Here we demonstrate that molybdenum-doped In2O3 (IMO) has higher mobility and therefore higher conductivity than ITO with the same carrier density. This also results in IMO having increased infrared transparency compared to ITO of the same conductivity. These properties enable current performance to be achieved using thinner films, reducing the amount of indium required and raw material costs by half. The enhanced doping behavior arises from Mo 4d donor states being resonant high in the conduction band and negligibly perturbing the host conduction band minimum, in contrast to the adverse perturbation caused by Sn 5s dopant states. This new understanding will enable better and cheaper TCOs based on both In2O3 and other metal oxides.
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Sep 2019
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[15132]
Open Access
Abstract: Combining X-ray photoelectron spectroscopy with the standing wave technique we investigated adsorption of a monolayer of water on Ti-oxide terminated SrTiO3(001) in ultra-high vacuum (UHV). At room temperature, the surface is water free but hydroxylated. A quarter monolayer of hydroxyl is tightly bound (1.85 ± 0.06) Å above the TiO2 surface. Deposited at low temperature, a monolayer of water adsorbs with the oxygen located (2.55 ± 0.2) Å above the surface, apparently close to atop Ti, but H2O is unstable at 200K. A fraction desorbs, in part under the X-ray beam, but a major fraction of H2O dissociates immediately, with the liberated hydrogen most likely attaching to a surface oxygen. The produced hydroxyls bind only loosely to the surface, are unstable at 200K and rapidly desorb once the surface is water-free. Although our study was conducted in UHV, the presented results suggests that Ti-oxide terminated SrTiO3(001) may possess a high catalytic activity toward hydrolysis under realistic conditions.
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Jun 2019
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I09-Surface and Interface Structural Analysis
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Anders L.
Jørgensen
,
David A.
Duncan
,
Claus F. P.
Kastorp
,
Line
Kyhl
,
Zeyuan
Tang
,
Albert
Bruix
,
Mie
Andersen
,
Bjork
Hammer
,
Tien-lin
Lee
,
Liv
Hornekaer
,
Richard
Balog
Diamond Proposal Number(s):
[16243]
Abstract: Functionalization of graphene on Ir(111) is a promising route to modify graphene by chemical means in a controlled fashion at the nanoscale. Yet, the nature of such functionalized sp3 nanodots remains unknown. Density functional theory (DFT) calculations alone cannot differentiate between two plausible structures, namely true graphane and substrate stabilized graphane-like nanodots. These two structures, however, interact dramatically differently with the underlying substrate. Discriminating which type of nanodots forms on the surface is thus of paramount importance for the applications of such prepared nanostructures. By comparing X-ray standing wave measurements against theoretical model structures obtained by DFT calculations we are able to exclude the formation of true graphane nanodots and clearly show the formation graphane-like nanodots.
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Jun 2019
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Abstract: Surface-assisted Ullmann coupling is the workhorse of on-surface synthesis. Despite its obvious relevance, many fundamental and mechanistic aspects remain elusive. In order to shed light on individual reaction steps and their progression with temperature, Temperature-Programmed X-ray Photoelectron Spectroscopy (TP-XPS) experiments are performed for a prototypical model system. The activation of the coupling by initial dehalogenation is tracked by monitoring Br 3d core levels, whereas the C 1s signature is used to follow the emergence of metastable organometallic intermediates and their conversion to the final covalent products upon heating in real time. The employed 1,3,5 tris(4 bromophenyl)benzene precursor is comparatively studied on Ag(111) vs. Au(111), whereby intermolecular bonds and network topologies are additionally characterized by Scanning Tunneling Microscopy (STM). Besides the well-comprehended differences in activation temperatures for debromination, the thermal progression shows marked differences between the two surfaces. Debromination proceeds rapidly on Ag(111), but is relatively gradual on Au(111). While on Ag(111) debromination is well explained by first-order reaction kinetics, thermodynamics prevail on Au(111), underpinned by a close agreement between experimentally deduced and Density Functional Theory (DFT) calculated reaction enthalpies. Thermodynamically controlled debromination on Au(111) over a large temperature range implies an unexpectedly long life-time of surface-stabilized radicals prior to covalent coupling, as corroborated by TP-XPS of C 1s core levels. These insights are anticipated to play an important role regarding our ability to rationally synthesize atomically precise low-dimensional covalent nanostructures on surfaces.
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Feb 2019
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[12800]
Abstract: The adsorption of organic molecules on solid substrates is important to applications in fields such as catalysis, photovoltaics, corrosion inhibition, adhesion, and sensors. The molecular level description of the surface-molecule interaction and of the adsorption structures in these complex systems is crucial to understand their properties and function. Here we present the investigation of one such system, benzotriazole (BTAH) on single crystal Cu(111) in vacuum conditions. BTAH is the most widely used corrosion inhibitor for copper and thus a molecule of great industrial relevance. We show that the co-application of a wide range of spectroscopic techniques with theoretical methods provides unique insight in the description of the atomistic details of the adsorbed structures. Specifically, spectroscopic photoemission, absorption and standing wave experiments combined with ab initio computational modeling allowed us to identify that benzotriazole forms overlayers of intact BTAH when deposited at low temperature and it dissociates into BTA and H at room temperature and above. The dissociated molecule then forms complex structures of mixed chains and dimers of BTA bound to copper adatoms. Our work also reveals that copper adatoms at low concentrations, such as the theoretically predicted superstructures cannot be be resolved by means of current X-ray photoelectron spectroscopy (XPS) as the modelled Cu 2p spectra are practically indistinguishable from those for a Cu surface without adatoms. Overall this study significantly deepens understanding of BTAH on Cu – a system studied for more than 50 years – and it highlights the benefits of combining spectroscopic and computational methods in order to obtain a complete picture of a complex adsorption system.
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Jan 2019
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[14624]
Open Access
Abstract: By combining X-ray photoelectron spectroscopy, X-ray standing waves and scanning tunneling microscopy, we investigate the geometric and electronic structure of a prototypical organic/insulator/metal interface, namely cobalt porphine on monolayer hexagonal boron nitride (h-BN) on Cu(111). Specifically, we determine the adsorption height of the organic molecule and show that the original planar molecular conformation is preserved in contrast to the adsorption on Cu(111). In addition, we highlight the electronic decoupling provided by the h-BN spacer layer and find that the h-BN–metal separation is not significantly modified by the molecular adsorption. Finally, we find indication of a temperature dependence of the adsorption height, which might be a signature of strongly-anisotropic thermal vibrations of the weakly bonded molecules.
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Nov 2018
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I09-Surface and Interface Structural Analysis
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Jack E. N.
Swallow
,
Benjamin A. D.
Williamson
,
Max
Birkett
,
Alex
Abbott
,
Mark
Farnworth
,
Thomas J.
Featherstone
,
Nianhua
Peng
,
J.
Kieran Cheetham
,
Paul
Warren
,
Anna
Regoutz
,
David A.
Duncan
,
Tien-lin
Lee
,
David O.
Scanlon
,
R.
Vin Dhanak
,
Tim D.
Veal
Abstract: Fluorine-doped tin oxide (FTO) is a commercially successful transparent conducting oxide with very good electrical (resistivities < 1 × 10 3 Ω⋅ cm) and optical properties (transmittance >85%). These properties coupled with cheap and large-scale deposition on float-glass lines means FTO has found commercial use in, for example, low emissivity windows and solar cells. However, despite its widespread application, a detailed understanding is lacking of the doping and defects in FTO. Recent work [1] has suggested that the fluorine interstitial plays a major role in limiting the conductivity of FTO. Here we present synchrotron radiation high energy x-ray photoemission spectroscopy (XPS) of the fluorine 1s core level of FTO films without in situ surface preparation. This probes deeper than standard XPS and shows that the fluorine interstitial is present not just at the surface of the films and is not an artefact of argon ion sputtering for surface preparation.
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Nov 2018
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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]
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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Caio C.
Silva
,
Marcella
Iannuzzi
,
David A.
Duncan
,
Paul
Ryan
,
Katherine T.
Clarke
,
Johannes T.
Kuchle
,
Jiaqi
Cai
,
Wouter
Jolie
,
Christoph
Schlueter
,
Tien-lin
Lee
,
Carsten
Busse
Diamond Proposal Number(s):
[12558, 16796]
Abstract: The structure of graphene on Ru(0001) has been extensively studied over the last decade, yet with no general agreement. Here we analyse graphene's valleys and hills using a combination of x-ray standing waves (XSW) and density functional theory (DFT). The chemical specicity of XSW allows an independent analysis of valleys and hills which, together with DFT model, results in the precise determination of the distance between the at, strongly bonded valleys of graphene and the substrate, as well the corrugation presented in the weakly bounded hills. From the theoretical viewpoint, the good agreement with experiment validates the choices regarding the unit cell size and the non-local correlation functional.
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Jul 2018
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