I09-Surface and Interface Structural Analysis
|
Jens
Niederhausen
,
Antoni
Franco-cañellas
,
Simon
Erker
,
Thorsten
Schultz
,
Katharina
Broch
,
Alexander
Hinderhofer
,
Steffen
Duhm
,
Pardeep K.
Thakur
,
David A.
Duncan
,
Alexander
Gerlach
,
Tien-lin
Lee
,
Oliver T.
Hofmann
,
Frank
Schreiber
,
Norbert
Koch
Diamond Proposal Number(s):
[11415, 13740, 19033]
Open Access
Abstract: The vertical adsorption distances of the planar conjugated organic molecule 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) on hydroxylated ZnO(0001), determined with the x-ray standing wave technique (XSW), are at variance with adsorption geometries simulated with density functional theory for surface-structure models that consider terminating OH, whereas good agreement is found for PTCDI in direct contact with the topmost Zn layer. The consequential assignment of OH to subsurface sites is supported by additional, independent XSW and energy scanned photoelectron diffraction data and calls for a reconsideration of the prevalent surface models with important implications for the understanding of ZnO(0001) surfaces.
|
Feb 2020
|
|
I09-Surface and Interface Structural Analysis
|
Chiara
Bigi
,
Zhenkun
Tang
,
Gian Marco
Pierantozzi
,
Pasquale
Orgiani
,
Pranab Kumar
Das
,
Jun
Fujii
,
Ivana
Vobornik
,
Tommaso
Pincelli
,
Alessandro
Troglia
,
Tien-lin
Lee
,
Regina
Ciancio
,
Goran
Drazic
,
Alberto
Verdini
,
Anna
Regoutz
,
Phil D. C.
King
,
Deepnarayan
Biswas
,
Giorgio
Rossi
,
Giancarlo
Panaccione
,
Annabella
Selloni
Diamond Proposal Number(s):
[16041]
Abstract: Two-dimensional (2D) metallic states induced by oxygen vacancies (
V
O
s
) at oxide surfaces and interfaces provide opportunities for the development of advanced applications, but the ability to control the behavior of these states is still limited. We used angle resolved photoelectron spectroscopy combined with density-functional theory (DFT) to study the reactivity of
V
O
-induced states at the (001) surface of anatase
TiO
2
, where both 2D metallic and deeper lying in-gap states (IGs) are observed. The 2D and IG states exhibit remarkably different evolutions when the surface is exposed to molecular
O
2
: while IGs are almost completely quenched, the metallic states are only weakly affected. DFT calculations indeed show that the IGs originate from surface
V
O
s
and remain localized at the surface, where they can promptly react with
O
2
. In contrast, the metallic states originate from subsurface vacancies whose migration to the surface for recombination with
O
2
is kinetically hindered on anatase
TiO
2
(001), thus making them much less sensitive to oxygen dosing.
|
Feb 2020
|
|
I09-Surface and Interface Structural Analysis
|
G.
Vinai
,
C.
Bigi
,
A.
Rajan
,
M. D.
Watson
,
T.-l.
Lee
,
F.
Mazzola
,
S.
Modesti
,
S.
Barua
,
M.
Ciomaga Hatnean
,
G.
Balakrishnan
,
P. D. C.
King
,
P.
Torelli
,
G.
Rossi
,
G.
Panaccione
Diamond Proposal Number(s):
[21429]
Abstract: Among transition-metal dichalcogenides, mono and few-layers thick
VSe
2
has gained much recent attention following claims of intrinsic room-temperature ferromagnetism in this system, which have nonetheless proved controversial. Here, we address the magnetic and chemical properties of
Fe
/
VSe
2
heterostructure by combining element sensitive x-ray absorption spectroscopy and photoemission spectroscopy. Our x-ray magnetic circular dichroism results confirm recent findings that both native mono/few-layer and bulk
VSe
2
do not show intrinsic ferromagnetic ordering. Nonetheless, we find that ferromagnetism can be induced, even at room temperature, after coupling with a Fe thin film layer, with antiparallel alignment of the moment on the V with respect to Fe. We further consider the chemical reactivity at the
Fe
/
VSe
2
interface and its relation with interfacial magnetic coupling.
|
Jan 2020
|
|
I09-Surface and Interface Structural Analysis
|
David A.
Duncan
,
Nicolae
Atodiresei
,
Simone
Lisi
,
Phil J.
Blowey
,
Vasile
Caciuc
,
James
Lawrence
,
Tien-lin
Lee
,
Maria Grazia
Betti
,
Pardeep Kumar
Thakur
,
Ada
Della Pia
,
Stefan
Blügel
,
Giovanni
Costantini
,
D. Phil
Woodruff
Diamond Proposal Number(s):
[13625]
Open Access
Abstract: Theoretical formulations capable of modeling chemical interactions over 3–4 orders of magnitude of bond strength, from covalent to van der Waals (vdW) forces, are one of the primary goals in materials physics, and chemistry. Development of vdW corrections for density-functional theory has thus been a major research field for two decades. While many of these corrections are semiempirical, more theoretically rigorous ab initio functionals have been developed. The ab initio functional vdW-DF2, when coupled with the reoptimized B86 exchange function (vdW-DF2-rB86), has typically performed as well, if not better than most semiempirical formulations. Here we present a system, Co intercalation of graphene on Ir(111), for which a semiempirical correction predicts local corrugation maxima in locations at which the vdW-DF2-rB86 functional predicts global minima. Sub-angstrom precision quantitative structural measurements show better agreement with the semiempirical correction. We posit that it is balancing the weak vdW interaction with the stronger, even covalent, interactions that proves a challenge for the vdW-DF2-rB86 functional.
|
Dec 2019
|
|
I09-Surface and Interface Structural Analysis
|
Zachary W.
Lebens-higgins
,
David M.
Halat
,
Nicholas V.
Faenza
,
Matthew J.
Wahila
,
Manfred
Mascheck
,
Tomas
Wiell
,
Susanna K.
Eriksson
,
Paul
Palmgren
,
Jose
Rodriguez
,
Fadwa
Badway
,
Nathalie
Pereira
,
Glenn G.
Amatucci
,
Tien-lin
Lee
,
Clare P.
Grey
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[22250, 22148]
Open Access
Abstract: Aluminum is a common dopant across oxide cathodes for improving the bulk and cathode-electrolyte interface (CEI) stability. Aluminum in the bulk is known to enhance structural and thermal stability, yet the exact influence of aluminum at the CEI remains unclear. To address this, we utilized a combination of X-ray photoelectron and absorption spectroscopy to identify aluminum surface environments and extent of transition metal reduction for Ni-rich LiNi0.8Co0.2−yAlyO2 (0%, 5%, or 20% Al) layered oxide cathodes tested at 4.75 V under thermal stress (60 °C). For these tests, we compared the conventional LiPF6 salt with the more thermally stable LiBF4 salt. The CEI layers are inherently different between these two electrolyte salts, particularly for the highest level of Al-doping (20%) where a thicker (thinner) CEI layer is found for LiPF6 (LiBF4). Focusing on the aluminum environment, we reveal the type of surface aluminum species are dependent on the electrolyte salt, as Al-O-F- and Al-F-like species form when using LiPF6 and LiBF4, respectively. In both cases, we find cathode-electrolyte reactions drive the formation of a protective Al-F-like barrier at the CEI in Al-doped oxide cathodes.
|
Dec 2019
|
|
I09-Surface and Interface Structural Analysis
|
Abhinav
Prakash
,
Nicholas F.
Quackenbush
,
Hwanhui
Yun
,
Jacob T.
Held
,
Tianqi
Wang
,
Tristan
Truttmann
,
James M.
Ablett
,
Conan
Weiland
,
Tien-lin
Lee
,
Joseph C.
Woicik
,
K. Andre
Mkhoyan
,
Bharat
Jalan
Diamond Proposal Number(s):
[15845]
Abstract: Separating electrons from their source atoms in La-doped BaSnO3, the first perovskite oxide semiconductor to be discovered with high room-temperature electron mobility, remains a subject of great interest for achieving high-mobility electron gas in two dimensions. So far, the vast majority of work in perovskite oxides has focused on heterostructures involving SrTiO3 as an active layer. Here we report the demonstration of modulation doping in BaSnO3 as high room temperature mobility host without the use of SrTiO3. Significantly, we show the use of angle- resolved hard X-ray photoelectron spectroscopy (HAXPES) as a non-destructive approach to not only determine the location of electrons at the buried interface but also to quantify the width of electron distribution in BaSnO3. The transport results are in good agreement with the results of self-consistent solution to one-dimensional Poisson and Schrödinger equations. Finally, we discuss viable routes to engineer two-dimensional electron gas density through band-offset engineering.
|
Nov 2019
|
|
I09-Surface and Interface Structural Analysis
|
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.
|
Nov 2019
|
|
|
|
Arian
Arab
,
Xiaoran
Liu
,
Okan
Koksal
,
Weibing
Yang
,
Ravini U.
Chandrasena
,
Srimanta
Middey
,
Mikhail
Kareev
,
Siddharth
Kumar
,
Marius-adrian
Husanu
,
Zhenzhong
Yang
,
Lin
Gu
,
Vladimir N.
Strocov
,
Tien-lin
Lee
,
Jan
Minar
,
Rossitza
Pentcheva
,
Jak
Chakhalian
,
Alexander X.
Gray
Abstract: Artificial complex-oxide heterostructures containing ultrathin buried layers grown along the pseudocubic [111] direction have been predicted to host a plethora of exotic quantum states arising from the graphene-like lattice geometry and the interplay between strong electronic correlations and band topology. To date, however, electronic-structural investigations of such atomic layers remain an immense challenge due to the shortcomings of conventional surface-sensitive probes, with typical information depths of a few Ångstroms. Here, we use a combination of bulk-sensitive soft x-ray angle-resolved photoelectron spectroscopy (SX-ARPES), hard x-ray photoelectron spectroscopy (HAXPES) and state-of-the-art first-principles calculations to demonstrate a direct and robust method for extracting momentum-resolved and angle-integrated valence-band electronic structure of an ultrathin buckled graphene-like layer of NdNiO3 confined between two 4-unit cell-thick layers of insulating LaAlO3. The momentum-resolved dispersion of the buried Ni d states near the Fermi level obtained via SX-ARPES is in excellent agreement with the first-principles calculations and establishes the realization of an antiferro-orbital order in this artificial lattice. The HAXPES measurements reveal the presence of a valence-band (VB) bandgap of 265 meV. Our findings open a promising avenue for designing and investigating quantum states of matter with exotic order and topology in a few buried layers.
|
Oct 2019
|
|
I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[19524]
Abstract: Delafossite CuFeO2 photocathodes have recently attracted attention for water splitting due to their suitable band gap (∼1.5 eV) and high stability in aqueous media. The preparation of CuFeO2 usually requires long and energy‐intense treatments in an inert atmosphere for the full conversion of spinel CuFe2O4 to delafossite CuFeO2. Herein, we report the preparation and characterization of highly uniform and stable CuFeO2 thin films obtained via a combination of inexpensive ultrasonic spray pyrolysis followed by a short hybrid microwave treatment (∼4 min). The resulting films show good stability in alkaline media and produce a photocurrent of ∼650 μA/cm2 under 1.5 AM simulated sunlight and with oxygen bubbling. The effect of the rapid transformation from the spinel to the delafossite phase induced by hybrid microwave annealing was investigated with synchrotron‐based X‐ray absorption spectroscopy (XAS) and X‐ray photoelectron spectroscopy (XPS).
|
Sep 2019
|
|
I09-Surface and Interface Structural Analysis
|
X. C.
Huang
,
J. Y.
Zhang
,
M.
Wu
,
S.
Zhang
,
H. Y.
Xiao
,
W. Q.
Han
,
T.-l.
Lee
,
A.
Tadich
,
D.-c.
Qi
,
L.
Qiao
,
L.
Chen
,
K. H. L.
Zhang
Diamond Proposal Number(s):
[21432]
Abstract: This work reports a fundamental study on the electronic structure, optical properties, and defect chemistry of a series of Co-based spinel oxide (
Co
3
O
4
,
ZnCo
2
O
4
, and
CoAl
2
O
4
) epitaxial thin films using x-ray photoemission and absorption spectroscopies, optical spectroscopy, transport measurements, and density functional theory. We demonstrate that
ZnCo
2
O
4
has a fundamental bandgap of 1.3 eV, much smaller than the generally accepted values, which range from 2.26 to 2.8 eV. The valence band edge mainly consists of occupied
Co
3
d
t
6
2
g
with some hybridization with O
2
p
/Zn
3
d
, and the conduction band edge of unoccupied
e
∗
g
state. However, optical transition between the two band edges is dipole forbidden. Strong absorption occurs at photon energies above 2.6 eV, explaining the reasonable transparency of
ZnCo
2
O
4
. A detailed defect chemistry study indicates that Zn vacancies formed at high oxygen pressure are the origin of a high
p
-type conductivity of
ZnCo
2
O
4
, and the hole conduction mechanism is described by small-polaron hoping model. The high
p
-type conductivity, reasonable transparency, and large work function make
ZnCo
2
O
4
a desirable
p
-type transparent semiconductor for various optoelectronic applications. Using the same method, the bandgap of
Co
3
O
4
is further proved to be ∼0.8 eV arising from the tetrahedrally coordinated
Co
2
+
cations. Our work advances the fundamental understanding of these materials and provides significant guidance for their use in catalysis, electronic, and solar applications.
|
Sep 2019
|
|
