I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[10370, 11915, 13837]
Abstract: Molecular monolayer films containing two different types of molecules (so called heteromolecular films) are promising candidates for the controlled functionalization of metal-organic hybrid interfaces. This is particularly true for blends formed by charge donor and acceptor molecules. Here we study heteromolecular monolayer systems containing 3,4,9,10-perylene-tetra-carboxylic-dianhydride (PTCDA) as charge acceptor, and either copper-II- or tin-II-phthalocyanine (CuPc or SnPc) as charge donor, adsorbed on Ag(111). We find that both systems exhibit structural phases with identical lateral ordering (iso-structural phases), which is an important prerequisite for studying the role of the central metal atom without competing effects caused by different lateral structures. Using normal incidence x-ray standing waves and photoemission tomography we find distinct differences in the (vertical) geometric and electronic structure for the heteromolecular systems under study: While the vertical structure of CuPc is essentially unaffected by mixing with PTCDA, the SnPc clearly reacts to the formation of a blend by reducing its adsorption height by approx. 0.2 Å. Also, the vertical structure of the PTCDA anhydride groups changes strongly: While the anhydride oxygen atoms are located below the perylene core for most mixed phases, for one of the PTCDA+CuPc phases it is lying above the perylene core. Regarding the electronic structure we find that while mixing with PTCDA causes a complete depletion of the CuPc former lowest unoccupied molecular orbital (FLUMO), the SnPc FLUMO is pinned to the Fermi level instead, and thus it remains partially filled. We demonstrate that all these differences are driven by the rearrangement of the substrate electron density in the vicinity of the PTCDA molecules, which are caused by the interaction with the metal phthalocyanine molecules.
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Mar 2018
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I09-Surface and Interface Structural Analysis
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Philipp
Scheiderer
,
Matthias
Schmitt
,
Judith
Gabel
,
Michael
Zapf
,
Martin
Stuebinger
,
Philipp
Schütz
,
Lenart
Dudy
,
Christoph
Schlueter
,
Tien-lin
Lee
,
Michael
Sing
,
Ralph
Claessen
Diamond Proposal Number(s):
[14106, 15200, 15856, 18372]
Abstract: The Mott transistor is a paradigm for a new class of electronic devices—often referred to by the term Mottronics—which are based on charge correlations between the electrons. Since correlation‐induced insulating phases of most oxide compounds are usually very robust, new methods have to be developed to push such materials right to the boundary to the metallic phase in order to enable the metal–insulator transition to be switched by electric gating. Here, it is demonstrated that thin films of the prototypical Mott insulator LaTiO3 grown by pulsed laser deposition under oxygen atmosphere are readily tuned by excess oxygen doping across the line of the band‐filling controlled Mott transition in the electronic phase diagram. The detected insulator to metal transition is characterized by a strong change in resistivity of several orders of magnitude. The use of suitable substrates and capping layers to inhibit oxygen diffusion facilitates full control of the oxygen content and renders the films stable against exposure to ambient conditions. These achievements represent a significant advancement in control and tuning of the electronic properties of LaTiO3+x thin films making it a promising channel material in future Mottronic devices.
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May 2018
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[15748]
Abstract: Atomically thin two-dimensional (2D) materials exhibit superlative properties dictated by their intralayer atomic structure, which is typically derived from a limited number of thermodynamically stable bulk layered crystals (e.g., graphene from graphite). The growth of entirely synthetic 2D crystals - those with no corresponding bulk allotrope - would circumvent this dependence upon bulk thermodynamics and substantially expand the phase space available for structure-property engineering of 2D materials. However, it remains unclear if synthetic 2D materials can exist as structurally and chemically distinct layers anchored by van der Waals (vdW) forces, as opposed to strongly bound adlayers. Here, we show that atomically thin sheets of boron (i.e., borophene) grown on the Ag(111) surface exhibit a vdW-like structure without a corresponding bulk allotrope. Using X-ray standing wave (XSW)-excited X-ray photoelectron spectroscopy (XPS), the positions of boron in multiple chemical states are resolved with sub-Ångström spatial resolution, revealing that the borophene forms a single planar layer that is 2.4 Å above the unreconstructed Ag surface. Moreover, our results reveal that multiple borophene phases exhibit these characteristics, denoting a unique form of polymorphism consistent with recent predictions. This observation of synthetic borophene as chemically discrete from the growth substrate suggests that it is possible to engineer a much wider variety of 2D materials than those accessible through bulk layered crystal structures.
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Apr 2018
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I09-Surface and Interface Structural Analysis
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Qi
Wang
,
Antoni
Franco-cañellas
,
Penghui
Ji
,
Christoph
Buerker
,
Rong-bin
Wang
,
Katharina
Broch
,
Pardeep Kumar
Thakur
,
Tien-lin
Lee
,
Haiming
Zhang
,
Alexander
Gerlach
,
Lifeng
Chi
,
Steffen
Duhm
,
Frank
Schreiber
Diamond Proposal Number(s):
[10443]
Abstract: Organic heterostructures are a central part of a manifold of (opto)electronic devices and serve a variety of functions. Particularly, molecular monolayers on metal electrodes are of paramount importance for device performance as they allow tuning energy levels in a versatile way. However, this can be hampered by molecular exchange, i.e., by interlayer diffusion of molecules toward the metal surface. We show that the organic–metal interaction strength is the decisive factor for the arrangement in bilayers, which is the most fundamental version of organic–organic heterostructures. The subtle differences in molecular structure of 6,13-pentacenequinone (P2O) and 5,7,12,14-pentacenetetrone (P4O) lead to antithetic adsorption behavior on Ag(111): physisorption of P2O but chemisorption of P4O. This allows providing general indicators for organic–metal coupling based on shifts in photoelectron spectroscopy data and to show that the coupling strength of copper-phthalocyanine (CuPc) with Ag(111) is in between that of P2O and P4O. We find that, indeed, CuPc forms a bilayer when deposited on a monolayer P4O/Ag(111) but molecular exchange takes place with P2O, as shown by a combination of scanning tunneling microscopy and X-ray standing wave experiments.
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Apr 2018
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I09-Surface and Interface Structural Analysis
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P.
Schütz
,
D. V.
Christensen
,
V.
Borisov
,
F.
Pfaff
,
P.
Scheiderer
,
L.
Dudy
,
M.
Zapf
,
J.
Gabel
,
Y. Z.
Chen
,
N.
Pryds
,
Victor
Rogalev
,
V. N.
Strocov
,
C.
Schlueter
,
T.-l.
Lee
,
H. O.
Jeschke
,
R.
Valentí
,
M.
Sing
,
R.
Claessen
Abstract: The spinel/perovskite heterointerface γ−Al2O3/SrTiO3 hosts a two-dimensional electron system (2DES) with electron mobilities exceeding those in its all-perovskite counterpart LaAlO3/SrTiO3 by more than an order of magnitude, despite the abundance of oxygen vacancies which act as electron donors as well as scattering sites. By means of resonant soft x-ray photoemission spectroscopy and ab initio calculations, we reveal the presence of a sharply localized type of oxygen vacancies at the very interface due to the local breaking of the perovskite symmetry. We explain the extraordinarily high mobilities by reduced scattering resulting from the preferential formation of interfacial oxygen vacancies and spatial separation of the resulting 2DES in deeper SrTiO3 layers. Our findings comply with transport studies and pave the way towards defect engineering at interfaces of oxides with different crystal structures.
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Oct 2017
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[8436]
Abstract: The interaction of oxalic acid with the Cu(110) surface has been investigated by a combination of scanning tunnelling microscopy (STM), low energy electron diffraction (LEED), soft X-ray photoelectron spectroscopy (SXPS), near-edge X-ray absorption fine structure (NEXAFS) and scanned-energy mode photoelectron diffraction (PhD), and density functional theory (DFT). O 1s SXPS and O K-edge NEXAFS show that at high coverages a singly deprotonated monooxalate is formed with its molecular plane perpendicular to the surface and lying in the
[11¯0]
azimuth, while at low coverage a doubly-deprotonated dioxalate is formed with its molecular plane parallel to the surface. STM, LEED and SXPS show the dioxalate to form a (3 × 2) ordered phase with a coverage of 1/6 ML. O 1s PhD modulation spectra for the monooxalate phase are found to be simulated by a geometry in which the carboxylate O atoms occupy near-atop sites on nearest-neighbour surface Cu atoms in
[11¯0]
rows, with a Cu-O bondlength of 2.00±0.04 Å. STM images of the (3 × 2) phase show some centred molecules attributed to adsorption on second-layer Cu atoms below missing [001] rows of surface Cu atoms, while DFT calculations show adsorption on a (3 × 2) missing row surface (with every third [001] Cu surface row removed) is favoured over adsorption on the unreconstructed surface. O 1s PhD data from dioxalate is best fitted by a structure similar to that found by DFT to have the lowest energy, although there are some significant differences in intramolecular bondlengths.
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Oct 2017
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I09-Surface and Interface Structural Analysis
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P. T. P.
Ryan
,
Z.
Jakub
,
J.
Balajka
,
J.
Hulva
,
M.
Meier
,
J. T.
Kuchle
,
P. J.
Blowey
,
P. K.
Thakur
,
C.
Franchini
,
D. J.
Payne
,
D. P.
Woodruff
,
L. A.
Rochford
,
F.
Allegretti
,
T.-l.
Lee
,
G. S.
Parkinson
,
D. A.
Duncan
Diamond Proposal Number(s):
[16403, 18191, 13817]
Open Access
Abstract: The normal incidence X-ray standing wave (NIXSW) technique has been used to follow the evolution of the adsorption geometry of Ni adatoms on the Fe3O4(001)-(√2 × √2)R45° surface as a function of temperature. Two primary surface region sites are identified: a bulk-continuation tetrahedral site and a sub-surface octahedral site, the latter site being preferred at higher annealing temperatures. The ease of incorporation is linked to the presence of subsurface cation vacancies in the (√2 × √2)R45° reconstruction and is consistent with the preference for octahedral coordination observed in the spinel compound NiFe2O4.
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Jun 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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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[8436]
Open Access
Abstract: The archetypal electron acceptor molecule, TCNQ, is generally believed to become bent into an inverted bowl shape upon adsorption on the coinage metal surfaces on which it becomes negatively charged. New quantitative experimental structural measurements show that this is not the case for TCNQ on Ag(111). DFT calculations show that the inclusion of dispersion force corrections reduces not only the molecule-substrate layer spacing but also the degree of predicted molecular bonding. However, complete agreement between experimentally-determined and theoretically-predicted structural parameters is only achieved with the inclusion of Ag adatoms into the molecular layer, which is also the energetically favoured configuration. The results highlight the need for both experimental and theoretical quantitative structural methods to reliably understand similar metal–organic interfaces and highlight the need to re-evaluate some previously-investigated systems.
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Jul 2018
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I09-Surface and Interface Structural Analysis
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Justin L.
Andrews
,
Junsang
Cho
,
Linda
Wangoh
,
Nuwanthi
Suwandaratne
,
Aaron
Sheng
,
Saurabh
Chauhan
,
Kelly
Nieto
,
Alec
Mohr
,
Karthika J.
Kadassery
,
Melissa R.
Popeil
,
Pardeep K.
Thakur
,
Matthew
Sfeir
,
David C.
Lacy
,
Tien-lin
Lee
,
Peihong
Zhang
,
David F.
Watson
,
Louis F. J.
Piper
,
Sarbajit
Banerjee
Diamond Proposal Number(s):
[12764, 16005]
Abstract: Addressing the complex challenge of harvesting solar energy to generate energy-dense fuels such as hydrogen requires the design of photocatalytic nanoarchitectures interfacing components that synergistically mediate a closely interlinked sequence of light-harvesting, charge separation, charge/mass transport, and catalytic processes. The design of such architectures requires careful consideration of both thermodynamic offsets and interfacial charge-transfer kinetics to ensure long-lived charge carriers that can be delivered at low overpotentials to the appropriate catalytic sites while mitigating parasitic reactions such as photocorrosion. Here we detail the theory-guided design and synthesis of nanowire/quantum dot hetero-structures with interfacial electronic structure specifically tailored to promote light-induced charge separation and photo-catalytic proton reduction. Topochemical synthesis yields a metastable β-Sn0.23V2O5 compound exhibiting Sn 5s-derived midgap states ideally positioned to extract photogenerated holes from interfaced CdSe quantum dots. The existence of these midgap states near the upper edge of the valence band (VB) has been confirmed and β-Sn0.23V2O5/CdSe heterostruc-tures have been shown to exhibit a 0 eV midgap state-VB offset, which underpins ultrafast sub-picosecond hole transfer. The β-Sn0.23V2O5/CdSe heterostructures are further shown to be viable photocatalytic architectures capable of efficacious hydrogen evolution. The results of this study underscore the criticality of precisely tailoring the electronic structure of semi-conductor components through the design and synthesis of novel compounds and demonstrates the remarkable utility of p-block lone-pair states to effect rapid charge separation necessary for photocatalysis.
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Nov 2018
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