I09-Surface and Interface Structural Analysis
|
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.
|
Jun 2019
|
|
I09-Surface and Interface Structural Analysis
|
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.
|
Jun 2019
|
|
I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[9012, 12558]
Abstract: We study the effect of alkali metal intercalation (Cs and Li) on the geometry of graphene on Ir(111) using the x-ray standing waves technique. For both alkali metals, the increase in the mean height of the carbon layer does not depend on the lateral structure or the density of the intercalated layer. For Li, full delamination of graphene from the metal substrate is found already for a small amount of intercalant. Even though Lithium lifts graphene to a smaller height, it is much more efficient in ironing out the corrugation of pristine graphene on Ir(111).
|
May 2019
|
|
I09-Surface and Interface Structural Analysis
|
Anna
Regoutz
,
Alex M.
Ganose
,
Lars
Blumenthal
,
Christoph
Schlueter
,
Tien-lin
Lee
,
Gregor
Kieslich
,
Anthony K.
Cheetham
,
Gwilherm
Kerherve
,
Ying-sheng
Huang
,
Ruei-san
Chen
,
Giovanni
Vinai
,
Tommaso
Pincelli
,
Giancarlo
Panaccione
,
Kelvin H. L.
Zhang
,
Russell G.
Egdell
,
Johannes
Lischner
,
David O.
Scanlon
,
David J.
Payne
Diamond Proposal Number(s):
[12673]
Abstract: Theory and experiment are combined to gain an understanding of the electronic properties of OsO2, a poorly studied metallic oxide that crystallizes in the rutile structure. Hard and soft valence-band x-ray photoemission spectra of OsO2 single crystals are in broad agreement with the results of density-functional-theory calculations, aside from a feature shifted to high binding energy of the conduction band. The energy shift corresponds to the conduction electron plasmon energy measured by reflection electron energy loss spectroscopy. The plasmon satellite is reproduced by many-body perturbation theory.
|
Feb 2019
|
|
I09-Surface and Interface Structural Analysis
|
Wei-cheng
Lee
,
Matthew
Wahila
,
Shantanu
Mukherjee
,
Christopher N.
Singh
,
Tyler
Eustance
,
Anna
Regoutz
,
Hanjong
Paik
,
Jos E.
Boschker
,
Fanny
Rodolakis
,
Tien-lin
Lee
,
Darrell G.
Schlom
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[20647, 21430]
Open Access
Abstract: We investigate the electronic structure of epitaxial VO
2
2
films in the rutile phase using density functional theory combined with the slave-spin method (DFT + SS). In DFT + SS, multi-orbital Hubbard interactions are added to a DFT-fit tight-binding model, and slave spins are used to treat electron correlations. We find that while stretching the system along the rutile
c
c
-axis results in a band structure favoring anisotropic orbital fillings, electron correlations favor equal filling of the
t
2g
t2g
orbitals. These two distinct effects cooperatively induce an orbital-dependent redistribution of the electron occupations and spectral weights, driving strained VO
2
2
toward an orbital selective Mott transition (OSMT). The simulated single-particle spectral functions are directly compared to V L-edge resonant X-ray photoemission spectroscopy of epitaxial 10 nm VO
2
2
/TiO
2
2
(001) and (100) strain orientations. Excellent agreement is observed between the simulations and experimental data regarding the strain-induced evolution of the lower Hubbard band. Simulations of rutile NbO
2
2
under similar strain conditions are performed, and we predict that an OSMT will not occur in rutile NbO
2
2
. Our prediction is supported by the high-temperature hard x-ray photoelectron spectroscopy measurement on relaxed NbO
2
2
(110) thin films with no trace of the lower Hubbard band. Our results indicate that electron correlations in VO
2
2
are important and can be modulated even in the rutile phase before the Peierls instability sets in.
|
Feb 2019
|
|
I09-Surface and Interface Structural Analysis
|
Daniel W.
Davies
,
Aron
Walsh
,
James J.
Mudd
,
Chris F.
Mcconville
,
Anna
Regoutz
,
J. Matthias
Kahk
,
David J.
Payne
,
Vin R.
Dhanak
,
David
Hesp
,
Katariina
Pussi
,
Tien-lin
Lee
,
Russell G.
Egdell
,
Kelvin H. L.
Zhang
Diamond Proposal Number(s):
[8441]
Abstract: Indium oxide is widely used as transparent electrode in optoelectronic devices and as a photocatalyst with activity for reduction of CO2. However, very little is known about the structural and electronic properties of its surfaces, particularly those prepared under reducing conditions. In this report, directional ‘lone-pair’ surface states associated with filled 5s2 orbitals have been identified on vacuum-annealed In2O3(111) through a combination of hard and soft X-ray photoemission spectroscopy and density functional theory calculations. The lone pairs reside on indium ad-atoms in a formal +1 oxidation state, each of which traps two electrons into a localised hybrid orbital protruding away from the surface and lying just above the valence band maximum in photoemission spectra. The third electron associated with the ad-atoms is delocalised into the conduction band, thus producing the surface electron accumulation layer identified previously on vacuum-annealed In2O3(111) (1×1) surfaces. The surface structure is further supported by low energy electron diffraction, but there is no chemical shift in indium core level x-ray photoelectron spectra between surface In(I) ad-atoms and bulk In(III). The 5s2 lone pairs confer Lewis basicity on the surface In sites and may have a pronounced impact on the catalytic or photo-catalytic activity of reduced In2O3.
|
Dec 2018
|
|
I09-Surface and Interface Structural Analysis
|
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.
|
Nov 2018
|
|
I09-Surface and Interface Structural Analysis
|
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.
|
Nov 2018
|
|
I09-Surface and Interface Structural Analysis
|
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.
|
Nov 2018
|
|
I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[12558]
Abstract: Epitaxial hexagonal boron nitride on Ir(111) is significantly modified by adsorption and intercalation of alkali-metal atoms. Regarding geometry, intercalation lifts the two-dimensional layer from its substrate and reduces the characteristic corrugation imprinted by direct contact with the metal substrate. Moreover, the presence of charged species in close proximity to the hexagonal boron nitride (hBN) layer strongly shifts the electronic structure (valence bands and core levels). We used scanning tunneling microscopy, low-energy electron diffraction, x-ray photoelectron spectroscopy (XPS), and the x-ray standing wave technique to study changes in the atomic structure induced by Cs adsorption and intercalation. Depending on the preparation, the alkali-metal atoms can be found on top and underneath the hexagonal boron nitride in ordered and disordered arrangements. Adsorbed Cs does not change the morphology of hBN/Ir(111) significantly, whereas an intercalated layer of Cs decouples the two-dimensional sheet and irons out its corrugation. XPS and angle-resolved photoelectron spectroscopy reveal a shift of the electronic states to higher binding energies, which increases with increasing density of the adsorbed and intercalated Cs. In the densest phase, Cs both intercalates and adsorbs on hBN and shifts the electronic states of hexagonal boron nitride by 3.56 eV. As this shift is not sufficient to move the conduction band below the Fermi energy, the electronic band gap must be larger than 5.85 eV.
|
Nov 2018
|
|
