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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Tomáš
Rauch
,
Victor A.
Rogalev
,
Maximilian
Bauernfeind
,
Julian
Maklar
,
Felix
Reis
,
Florian
Adler
,
Simon
Moser
,
Johannes
Weis
,
Tien-lin
Lee
,
Pardeep K.
Thakur
,
Jörg
Schäfer
,
Ralph
Claessen
,
Jürgen
Henk
,
Ingrid
Mertig
Diamond Proposal Number(s):
[19512]
Abstract: The diamond and zinc-blende semiconductors are well-known and have been widely studied for decades. Yet, their electronic structure still surprises with unexpected topological properties of the valence bands. In this joint theoretical and experimental investigation, we demonstrate for the benchmark compounds InSb and GaAs that the electronic structure features topological surface states below the Fermi energy. Our parity analysis shows that the spin-orbit split-off band near the valence band maximum exhibits a strong topologically nontrivial behavior characterized by the
Z
2
invariants
(
1
;
000
)
. The nontrivial character is a consequence of the nonzero spin-orbit coupling and is imposed by the chosen constituents, in contrast to the conventional topological phase transition mechanism which relies on tuning parameters in the system Hamiltonian. Ab initio-based tight-binding calculations resolve topological surface states in the occupied electronic structure of InSb and GaAs, further confirmed experimentally by soft x-ray angle-resolved photoemission from both materials. Our findings are valid for all other materials whose valence bands are adiabatically linked to those of InSb, i.e., many diamond and zinc-blende semiconductors, as well as other related materials, such as half-Heusler compounds.
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Jun 2019
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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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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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Diamond Proposal Number(s):
[9118]
Abstract: Variable-period x-ray standing wave (VPXSW) studies have been carried out using 3 keV x rays and
photoelectron detection. Two model surfaces have been used, a native SiO2 layer (20 Å thick) on bulk silicon, and
a purpose-built multilayer surface comprising a chloroform/water marker layer (12 Å thick) on an ionic liquid
spacer layer (211 Å thick) deposited on a SiO2/Si substrate at 90 K. By using photoelectron detection, both
chemical and elemental sensitivity were achieved. The surfaces were modeled using dynamic x-ray scattering
for x-ray intensity, and attenuation of photoelectrons transmitted through the layers, to produce simulations
which accurately reproduced the experimental VPXSW measurements. VPXSW measurements made using the
substrate, spacer layer, and marker layer photoelectron signatures produced consistent structural values. This
work demonstrates that VPXSW can be used to determine chemically specific layer thicknesses within thick
(300 Å) surface structures composed of the light elements B, C, N, O, F, and Cl with an accuracy of 10 to
15 Å, perpendicular to the surface.
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Oct 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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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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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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Martin
Schwarz
,
Manuela
Garnica
,
David A.
Duncan
,
Alejandro
Pérez Paz
,
Jacob
Ducke
,
Peter S.
Deimel
,
Pardeep K.
Thakur
,
Tien-lin
Lee
,
Angel
Rubio
,
Johannes V.
Barth
,
Francesco
Allegretti
,
Willi
Auwärter
Diamond Proposal Number(s):
[14624]
Abstract: The tetrapyrrole macrocycle of porphine is the common core of all porphyrin molecules, an interesting class of π-conjugated molecules with relevance in natural and artificial systems. The functionality of porphines on a solid surface can be tailored by the central metal atom and its interaction with the substrate. In this study, we present a local adsorption geometry determination for cobalt porphine on Cu(111) by means of complementary scanning tunneling microscopy, high-resolution X-ray photoelectron spectroscopy and X-ray standing wave measurements, and density functional theory calculations. Specifically, the Co center was determined to be at an adsorption height of 2.25 ± 0.04 Å occupying a bridge site. The macrocycle adopts a moderate asymmetric saddle-shape conformation, with the two pyrrole groups that are aligned perpendicular to the densely packed direction of the Cu(111) surface tilted away from the surface plane.
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Feb 2018
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I09-Surface and Interface Structural Analysis
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Matthias
Meier
,
Zdeněk
Jakub
,
Jan
Balajka
,
Jan
Hulva
,
Roland
Bliem
,
Pardeep K.
Thakur
,
Tien-lin
Lee
,
Cesare
Franchini
,
Michael
Schmid
,
Ulrike
Diebold
,
Francesco
Allegretti
,
David
Duncan
,
Gareth S.
Parkinson
Diamond Proposal Number(s):
[13817]
Open Access
Abstract: Accurately modelling the structure of a catalyst is a fundamental prerequisite for correctly predicting reaction pathways, but a lack of clear experimental benchmarks makes it difficult to determine the optimal theoretical approach. Here, we utilize the normal incidence X-ray standing wave (NIXSW) technique to precisely determine the three dimensional geometry of Ag1 and Cu1 adatoms on Fe3O4(001). Both adatoms occupy bulk-continuation cation sites, but with a markedly different height above the surface (0.43 ± 0.03 Å (Cu1) and 0.96 ± 0.03 Å (Ag1)). HSE-based calculations accurately predict the experimental geometry, but the more common PBE + U and PBEsol + U approaches perform poorly.
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Jan 2018
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