I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[17635]
Open Access
Abstract: Using the chemically specific techniques of normal incidence X-ray standing waves and photoelectron diffraction, we have investigated the dissociative adsorption of formic acid on the Fe3O4(001) surface, specifically probing the local structures of both the adsorbed formate and resulting surface hydroxyl. Using model independent direct methods, we reinforce the observations of a previous surface X-ray diffraction study that the formate molecule adsorbs with both oxygens atop octahedrally coordinated surface Fe cations and that ∼60% of the formate is adsorbed in the so called tet site. We additionally determine, for the first time, that the surface hydroxyl species are found at the so called int site. This confirms previous DFT predictions and reinforces the pivotal role the surface hydroxyl plays in lifting the subsurface cation vacancy termination of the Fe3O4(001) surface.
|
Dec 2021
|
|
I09-Surface and Interface Structural Analysis
|
F.
Offi
,
K.
Yamauchi
,
S.
Picozzi
,
V.
Lollobrigida
,
A.
Verna
,
C.
Schlueter
,
T.-L.
Lee
,
A.
Regoutz
,
D. J.
Payne
,
A.
Petrov
,
G.
Vinai
,
G. M.
Pierantozzi
,
T.
Pincelli
,
G.
Panaccione
,
F.
Borgatti
Diamond Proposal Number(s):
[11322]
Abstract: Hybridization of electronic states and orbital symmetry in transition metal oxides are generally considered key ingredients in the description of both their electronic and magnetic properties. In the prototypical case of
La
0.65
Sr
0.35
MnO
3
(LSMO), a landmark system for spintronics applications, a description based solely on Mn
3
d
and O
2
p
electronic states is reductive. We thus analyzed elemental and orbital distributions in the LSMO valence band through a comparison between density functional theory calculations and experimental photoelectron spectra in a photon energy range from soft to hard x rays. We reveal a number of hidden contributions, arising specifically from La
5
p
, Mn
4
s
, and O
2
s
orbitals, considered negligible in previous analyses; our results demonstrate that all these contributions are significant for a correct description of the valence band of LSMO and of transition metal oxides in general.
|
Oct 2021
|
|
I09-Surface and Interface Structural Analysis
|
P. T. P.
Ryan
,
P. L.
Lalaguna
,
F.
Haag
,
M. M.
Braim
,
P.
Ding
,
D. J.
Payne
,
J. V.
Barth
,
Tien-Lin
Lee
,
D. P.
Woodruff
,
F.
Allegretti
,
D. A.
Duncan
Diamond Proposal Number(s):
[24113]
Open Access
Abstract: Utilising normal incidence X-ray standing waves we rigourously scrutinise the “inverted model” as the adsorption structure of free-base tetraphenyl porphyrin on Cu(111). We demonstrate that the iminic N atoms are anchored at near-bridge adsorption sites on the surface displaced laterally by 1.1 ± 0.2 Å in excellent agreement with previously published calculations.
|
Mar 2020
|
|
I09-Surface and Interface Structural Analysis
|
P. T. P.
Ryan
,
M.
Meier
,
Z.
Jakub
,
J.
Balajka
,
J.
Hulva
,
D. J.
Payne
,
T.-L.
Lee
,
C.
Franchini
,
F.
Allegretti
,
G. S.
Parkinson
,
D. A.
Duncan
Diamond Proposal Number(s):
[13817]
Open Access
Abstract: In this work, the adsorption height of Ag adatoms on the Fe3O4(001) surface after exposure to CO was determined using normal incidence x-ray standing waves. The Ag adatoms bound to CO (
Ag
CO
1
Ag1CO
) are found to be pulled out of the surface to an adsorption height of 1.15 Å ± 0.08 Å, compared to the previously measured height of 0.96 Å ± 0.03 Å for bare Ag adatoms and clusters. Utilizing DFT+vdW+U calculations with the substrate unit cell dimension fixed to the experimental value, the predicted adsorption height for
Ag
CO
1
Ag1CO
was 1.16 Å, in remarkably good agreement with the experimental results.
|
Feb 2020
|
|
|
|
Zdenek
Jakub
,
Jan
Hulva
,
Paul T. P.
Ryan
,
David A.
Duncan
,
David J.
Payne
,
Roland
Bliem
,
Manuel
Ulreich
,
Patrick
Hofegger
,
Florian
Kraushofer
,
Matthias
Meier
,
Michael
Schmid
,
Ulrike
Diebold
,
Gareth S.
Parkinson
Open Access
Abstract: The structure of a catalyst often changes in reactive environments, and following the structural evolution is crucial for the identification of the catalyst's active phase and reaction mechanism. Here we present an atomic-scale study of CO oxidation on a model Rh/Fe3O4(001) “single-atom” catalyst, which has a very different evolution depending on which of the two reactants, O2 or CO, is adsorbed first. Using temperature-programmed desorption (TPD) combined with scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS), we show that O2 destabilizes Rh atoms, leading to the formation of RhxOy clusters; these catalyze CO oxidation via a Langmuir–Hinshelwood mechanism at temperatures as low as 200 K. If CO adsorbs first, the system is poisoned for direct interaction with O2, and CO oxidation is dominated by a Mars-van-Krevelen pathway at 480 K.
|
Feb 2020
|
|
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
|
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
|
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.
|
Jun 2018
|
|
|
|
Pranab Kumar
Das
,
Jagoda
Sławińska
,
Ivana
Vobornik
,
Jun
Fujii
,
Anna
Regoutz
,
Juhan M.
Kahk
,
David O.
Scanlon
,
Benjamin J.
Morgan
,
Cormac
Mcguinness
,
Evgeny
Plekhanov
,
Domenico
Di Sante
,
Ying-Sheng
Huang
,
Ruei-San
Chen
,
Giorgio
Rossi
,
Silvia
Picozzi
,
William R.
Branford
,
Giancarlo
Panaccione
,
David J.
Payne
Abstract: The delicate interplay of electronic charge, spin, and orbital degrees of freedom is in the heart of many novel phenomena across the transition metal oxide family. Here, by combining high-resolution angle-resolved photoemission spectroscopy and first principles calculations (with and without spin-orbit coupling), the electronic structure of the rutile binary iridate, IrO2, is investigated. The detailed study of electronic bands measured on a high-quality single crystalline sample and use of a wide range of photon energy provide a huge improvement over the previous studies. The excellent agreement between theory and experimental results shows that the single-particle DFT description of IrO2 band structure is adequate, without the need of invoking any treatment of correlation effects. Although many observed features point to a 3D nature of the electronic structure, clear surface effects are revealed. The discussion of the orbital character of the relevant bands crossing the Fermi level sheds light on spin-orbit-coupling-driven phenomena in this material, unveiling a spin-orbit-induced avoided crossing, a property likely to play a key role in its large spin Hall effect.
|
Jun 2018
|
|
I09-Surface and Interface Structural Analysis
|
T.
Pincelli
,
V.
Lollobrigida
,
F.
Borgatti
,
A.
Regoutz
,
B.
Gobaut
,
C.
Schlueter
,
T.-L.
Lee
,
D. J.
Payne
,
M.
Oura
,
K.
Tamasaku
,
A. Y.
Petrov
,
P.
Graziosi
,
F. Miletto
Granozio
,
M.
Cavallini
,
G.
Vinai
,
R.
Ciprian
,
C.
Back
,
G.
Rossi
,
M.
Taguchi
,
H.
Daimon
,
G.
Van Der Laan
,
G.
Panaccione
Diamond Proposal Number(s):
[11322]
Abstract: In the rapidly growing field of spintronics, simultaneous control of electronic and magneticproperties is essential, and the perspective of building novel phases is directly linked to the control of tuning parameters, for example, thickness and doping. Looking at the relevanteffects in interface-driven spintronics, the reduced symmetry at a surface and interface corresponds to a severe modification of the overlap of electron orbitals, that is, to a change of electron hybridization. Here we report a chemically and magnetically sensitive depth-dependent analysis of two paradigmatic systems, namely La1xSrxMnO3 and (Ga,Mn)As. Supported by cluster calculations, we find a crossover between surface and bulk in the electron hybridization/correlation and we identify a spectroscopic fingerprint of bulk metallic character and ferromagnetism versus depth. The critical thickness and the gradient of hybridization are measured, setting an intrinsic limit of 3 and 10 unit cells from the surface,respectively, for (Ga,Mn)As and La1xSrxMnO3, for fully restoring bulk properties.
|
Jul 2017
|
|
