I06-Nanoscience (XPEEM)
|
Tuhin
Maity
,
Manisha
Bansal
,
Nives
Strkalj
,
Kapildeb
Dolui
,
Di
Zhang
,
Zihao
He
,
Guillaume F
Nataf
,
Adam
Lovett
,
Massimo
Ghidini
,
Sarnjeet S.
Dhesi
,
Ping
Lu
,
Haiyan
Wang
,
Weiwei
Li
,
Judith L.
Macmanus-Driscoll
Diamond Proposal Number(s):
[22427]
Open Access
Abstract: We investigate the emergence and optimization of conventional exchange bias (EB) in ultrathin (<10 nm) ferroelectric (FE) BaTiO3 (BTO)/ferromagnetic (FM) La0.67Sr0.33MnO3 (LSMO) epitaxial bilayers without an antiferromagnetic (AFM) material. The EB originates from the electronic orbital reconstruction at the FE-FM interface due to the ferroelectric polarization. We achieve maximum EB of approximately 42 Oe with single-domain polarization in nine-unit-cell-thick BTO, setting the BTO thickness above the critical threshold for ferroelectricity yet below the thickness of strain relaxation and multidomain breakdown. Furthermore, the LSMO layer needs to be thick enough to sustain both the FM layer and polarization-induced AFM spin configuration at the LSMO/BTO interface, yet as thin as possible to enable the EB loop shift. The temperature, training, field, and thickness dependence of the EB confirm that the LSMO/BTO interface exhibits conventional EB despite its unconventional origin. Using x-ray magnetic circular dichroism, scanning transmission electron microscopy, and density-functional-theory calculations, we confirm that the macroscopic EB effect originates from the interfacial AFM spin configuration in LSMO driven by FE-induced d-orbital modifications in interfacial Mn ions. Thus, we engineer strong interfacial EB coupling in artificial multiferroics without a conventional AFM material by controlling FE polarization, highlighting the potential for advanced spintronic applications.
|
Nov 2024
|
|
I06-Nanoscience (XPEEM)
I09-Surface and Interface Structural Analysis
|
Dong
Li
,
Hongguang
Wang
,
Kaifeng
Li
,
Bonan
Zhu
,
Kai
Jiang
,
Dirk
Backes
,
Larissa S. I.
Veiga
,
Jueli
Shi
,
Pinku
Roy
,
Ming
Xiao
,
Aiping
Chen
,
Quanxi
Jia
,
Tien-Lin
Lee
,
Sarnjeet S.
Dhesi
,
David O.
Scanlon
,
Judith L.
Macmanus-Driscoll
,
Peter A.
Van Aken
,
Kelvin H. L.
Zhang
,
Weiwei
Li
Diamond Proposal Number(s):
[25425, 26901, 29616, 31069]
Open Access
Abstract: Transition metal oxides are promising candidates for the next generation of spintronic devices due to their fascinating properties that can be effectively engineered by strain, defects, and microstructure. An excellent example can be found in ferroelastic LaCoO3 with paramagnetism in bulk. In contrast, unexpected ferromagnetism is observed in tensile-strained LaCoO3 films, however, its origin remains controversial. Here we simultaneously reveal the formation of ordered oxygen vacancies and previously unreported long-range suppression of CoO6 octahedral rotations throughout LaCoO3 films. Supported by density functional theory calculations, we find that the strong modification of Co 3d-O 2p hybridization associated with the increase of both Co-O-Co bond angle and Co-O bond length weakens the crystal-field splitting and facilitates an ordered high-spin state of Co ions, inducing an emergent ferromagnetic-insulating state. Our work provides unique insights into underlying mechanisms driving the ferromagnetic-insulating state in tensile-strained ferroelastic LaCoO3 films while suggesting potential applications toward low-power spintronic devices.
|
Jun 2023
|
|
I06-Nanoscience (XPEEM)
|
Dong
Li
,
Bonan
Zhu
,
Dirk
Backes
,
Larissa S. I.
Veiga
,
Tien-Lin
Lee
,
Hongguang
Wang
,
Qian
He
,
Pinku
Roy
,
Jiaye
Zhang
,
Jueli
Shi
,
Aiping
Chen
,
Peter A.
Van Aken
,
Quanxi
Jia
,
Sarnjeet S.
Dhesi
,
David O.
Scanlon
,
Kelvin H. L.
Zhang
,
Weiwei
Li
Diamond Proposal Number(s):
[25425, 26901, 29616]
Abstract: Strain engineering of epitaxial transition metal oxide heterostructures offers an intriguing opportunity to control electronic structures by modifying the interplay between spin, charge, orbital, and lattice degrees of freedom. Here, we demonstrate that the electronic structure, magnetic and transport properties of
La
0.9
Ba
0.1
MnO
3
thin films can be effectively controlled by epitaxial strain. Spectroscopic studies and first-principles calculations reveal that the orbital occupancy in Mn
e
g
orbitals can be switched from the
d
3
z
2
−
r
2
orbital to the
d
x
2
−
y
2
orbital by varying the strain from compressive to tensile. The change of orbital occupancy associated with Mn
3
d
-O
2
p
hybridization leads to dramatic modulation of the magnetic and electronic properties of strained
La
0.9
Ba
0.1
MnO
3
thin films. Under moderate tensile strain, an emergent ferromagnetic insulating state with an enhanced ferromagnetic Curie temperature of 215 K is achieved. These findings not only deepen our understanding of electronic structures, magnetic and transport properties in the
La
0.9
Ba
0.1
MnO
3
system, but also demonstrate the use of epitaxial strain as an effective knob to tune the electronic structures and related physical properties for potential spintronic device applications.
|
Apr 2022
|
|
I09-Surface and Interface Structural Analysis
|
Jueli
Shi
,
Ethan A.
Rubinstein
,
Weiwei
Li
,
Jiaye
Zhang
,
Ye
Yang
,
Tien-Lin
Lee
,
Changdong
Qin
,
Pengfei
Yan
,
Judith L.
Macmanus-Driscoll
,
David O.
Scanlon
,
Kelvin H.l.
Zhang
Diamond Proposal Number(s):
[24219]
Open Access
Abstract: Oxide semiconductors are key materials in many technologies from flat-panel displays，solar cells to transparent electronics. However, many potential applications are hindered by the lack of high mobility p-type oxide semiconductors due to the localized O-2p derived valence band (VB) structure. In this work, the VB structure modulation is reported for perovskite Ba2BiMO6 (M = Bi, Nb, Ta) via the Bi 6s2 lone pair state to achieve p-type oxide semiconductors with high hole mobility up to 21 cm2 V−1 s−1, and optical bandgaps widely varying from 1.5 to 3.2 eV. Pulsed laser deposition is used to grow high quality epitaxial thin films. Synergistic combination of hard x-ray photoemission, x-ray absorption spectroscopies, and density functional theory calculations are used to gain insight into the electronic structure of Ba2BiMO6. The high mobility is attributed to the highly dispersive VB edges contributed from the strong coupling of Bi 6s with O 2p at the top of VB that lead to low hole effective masses (0.4–0.7 me). Large variation in bandgaps results from the change in the energy positions of unoccupied Bi 6s orbital or Nb/Ta d orbitals that form the bottom of conduction band. P–N junction diode constructed with p-type Ba2BiTaO6 and n-type Nb doped SrTiO3 exhibits high rectifying ratio of 1.3 × 104 at ±3 V, showing great potential in fabricating high-quality devices. This work provides deep insight into the electronic structure of Bi3+ based perovskites and guides the development of new p-type oxide semiconductors.
|
Jan 2022
|
|
I09-Surface and Interface Structural Analysis
|
X. C.
Huang
,
W.-W.
Li
,
S.
Zhang
,
F. E.
Oropeza
,
G.
Gorni
,
V. A.
De La Pena-O'Shea
,
T.-L.
Lee
,
M.
Wu
,
L.-S.
Wang
,
D.-C.
Qi
,
L.
Qiao
,
J.
Cheng
,
K. H. L.
Zhang
Diamond Proposal Number(s):
[24219]
Abstract: In this paper, we report insights into the local atomic and electronic structure of
NiCo
2
O
4
epitaxial thin films and its correlation with electrical, optical, and magnetic properties. We grew structurally well-defined
NiCo
2
O
4
epitaxial thin films with controlled properties on
Mg
Al
2
O
4
(
001
)
substrates using pulsed laser deposition. Films grown at low temperatures (
<
400
∘
C
) exhibit a ferrimagnetic and metallic behavior, while those grown at high temperatures are nonmagnetic semiconductors. The electronic structure and cation local atomic coordination of the respective films were investigated using a combination of resonant photoemission spectroscopy, x-ray absorption spectroscopy, and ab initio calculations. Our results unambiguously reveal that the
Ni
3
+
valence state promoted at low growth temperature introduces delocalized
Ni
3
d
-derived states at the Fermi level (
E
F
), responsible for the metallic state in
NiCo
2
O
4
, while the
Co
3
d
-related state is more localized at higher binding energy. In the semiconducting films, the valence state of Ni is lowered and
∼
+
2
. Further structural and defect chemistry studies indicate that the formation of oxygen vacancies and secondary CoO phases at high growth temperature are responsible for the
Ni
2
+
valence state in
NiCo
2
O
4
. The
Ni
3
d
-related state becomes localized away from
E
F
, opening a band gap for a semiconducting state. The band gap of the semiconducting
NiCo
2
O
4
is estimated to be
<
0.8
eV
, which is much smaller than the quoted values in the literature ranging from 1.1 to 2.58 eV. Despite the small band gap, its optical transition is
d
−
d
dipole forbidden, and therefore, the semiconducting
NiCo
2
O
4
still shows reasonable transparency in the infrared-visible light region. The present insights into the role of
Ni
3
+
in determining the electronic structure and defect chemistry of
NiCo
2
O
4
provide important guidance for use of
NiCo
2
O
4
in electrocatalysis and opto-electronics.
|
Sep 2021
|
|
|
|
Anna
Abfalterer
,
Javad
Shamsi
,
Dominik J.
Kubicki
,
Christopher N.
Savory
,
James
Xiao
,
Giorgio
Divitini
,
Weiwei
Li
,
Stuart
Macpherson
,
Krzysztof
Gałkowski
,
Judith L.
Macmanus-Driscoll
,
David O.
Scanlon
,
Samuel D.
Stranks
Open Access
Abstract: Optoelectronic devices based on lead halide perovskites are processed in facile ways, yet are remarkably efficient. There are extensive research efforts investigating lead-free perovskite and perovskite-related compounds, yet there are challenges to synthesize these materials in forms that can be directly integrated into thin film devices rather than as bulk powders. Here, we report on the colloidal synthesis and characterization of lead-free, antifluorite Cs2ZrX6 (X = Cl, Br) nanocrystals that are readily processed into thin films. We use transmission electron microscopy and powder X-ray diffraction measurements to determine their size and structural properties, and solid-state nuclear magnetic resonance measurements reveal the presence of oleate ligand, together with a disordered distribution of Cs surface sites. Density functional theory calculations reveal the band structure and fundamental band gaps of 5.06 and 3.91 eV for Cs2ZrCl6 and Cs2ZrBr6, respectively, consistent with experimental values. Finally, we demonstrate that the Cs2ZrCl6 and Cs2ZrBr6 nanocrystal thin films exhibit tunable, broad white photoluminescence with quantum yields of 45% for the latter, with respective peaks in the blue and green spectral regions and mixed systems exhibiting properties between them. Our work represents a critical step toward the application of lead-free Cs2ZrX6 nanocrystal thin films into next-generation light-emitting applications.
|
Nov 2020
|
|
I07-Surface & interface diffraction
|
Baodan
Zhao
,
Yaxiao
Lian
,
Linsong
Cui
,
Giorgio
Divitini
,
Gunnar
Kusch
,
Edoardo
Ruggeri
,
Florian
Auras
,
Weiwei
Li
,
Dexin
Yang
,
Bonan
Zhu
,
Rachel A.
Oliver
,
Judith L.
Macmanus-Driscoll
,
Samuel D.
Stranks
,
Dawei
Di
,
Richard H.
Friend
Diamond Proposal Number(s):
[17223]
Abstract: Light-emitting diodes based on halide perovskites have recently reached external quantum efficiencies of over 20%. However, the performance of visible perovskite light-emitting diodes has been hindered by non-radiative recombination losses and limited options for charge-transport materials that are compatible with perovskite deposition. Here, we report efficient, green electroluminescence from mixed-dimensional perovskites deposited on a thin (~1 nm) lithium fluoride layer on an organic semiconductor hole-transport layer. The highly polar dielectric interface acts as an effective template for forming high-quality bromide perovskites on otherwise incompatible hydrophobic charge-transport layers. The control of crystallinity and dimensionality of the perovskite layer is achieved by using tetraphenylphosphonium chloride as an additive, leading to external photoluminescence quantum efficiencies of around 65%. With this approach, we obtain light-emitting diodes with external quantum efficiencies of up to 19.1% at high brightness (>1,500 cd m−2).
|
Oct 2020
|
|
I06-Nanoscience (XPEEM)
|
Weiwei
Li
,
Bonan
Zhu
,
Ruixue
Zhu
,
Qiang
Wang
,
Ping
Lu
,
Yuanwei
Sun
,
Clodomiro
Cafolla
,
Zhimin
Qi
,
Aiping
Chen
,
Peng
Gao
,
Haiyan
Wang
,
Qing
He
,
Kelvin H. L.
Zhang
,
Judith L.
Macmanus‐driscoll
Diamond Proposal Number(s):
[17284]
Open Access
Abstract: Control of BO6 octahedral rotations at the heterointerfaces of dissimilar ABO3 perovskites has emerged as a powerful route for engineering novel physical properties. However, its impact length scale is constrained at 2–6 unit cells close to the interface and the octahedral rotations relax quickly into bulk tilt angles away from interface. Here, a long‐range (up to 12 unit cells) suppression of MnO6 octahedral rotations in La0.9Ba0.1MnO3 through the formation of superlattices with SrTiO3 can be achieved. The suppressed MnO6 octahedral rotations strongly modify the magnetic and electronic properties of La0.9Ba0.1MnO3 and hence create a new ferromagnetic insulating state with enhanced Curie temperature of 235 K. The emergent properties in La0.9Ba0.1MnO3 arise from a preferential occupation of the out‐of‐plane Mn d 3z 2−r 2 orbital and a reduced Mn eg bandwidth, induced by the suppressed octahedral rotations. The realization of long‐range tuning of BO6 octahedra via superlattices can be applicable to other strongly correlated perovskites for exploring new emergent quantum phenomena.
|
Aug 2020
|
|
I06-Nanoscience (XPEEM)
|
Weiwei
Li
,
Bonan
Zhu
,
Qian
He
,
Albina Y.
Borisevich
,
Chao
Yun
,
Rui
Wu
,
Ping
Lu
,
Zhimin
Qi
,
Qiang
Wang
,
Aiping
Chen
,
Haiyan
Wang
,
Stuart A.
Cavill
,
Kelvin H. L.
Zhang
,
Judith L.
Macmanus‐driscoll
Diamond Proposal Number(s):
[17284]
Open Access
Abstract: Ultrathin epitaxial films of ferromagnetic insulators (FMIs) with Curie temperatures near room temperature are critically needed for use in dissipationless quantum computation and spintronic devices. However, such materials are extremely rare. Here, a room‐temperature FMI is achieved in ultrathin La0.9Ba0.1MnO3 films grown on SrTiO3 substrates via an interface proximity effect. Detailed scanning transmission electron microscopy images clearly demonstrate that MnO6 octahedral rotations in La0.9Ba0.1MnO3 close to the interface are strongly suppressed. As determined from in situ X‐ray photoemission spectroscopy, O K‐edge X‐ray absorption spectroscopy, and density functional theory, the realization of the FMI state arises from a reduction of Mn eg bandwidth caused by the quenched MnO6 octahedral rotations. The emerging FMI state in La0.9Ba0.1MnO3 together with necessary coherent interface achieved with the perovskite substrate gives very high potential for future high performance electronic devices.
|
Nov 2019
|
|
I09-Surface and Interface Structural Analysis
|
Jia-Ye
Zhang
,
Weiwei
Li
,
Robert L. Z.
Hoye
,
Judith
Macmanus-Driscoll
,
Melanie
Budde
,
Oliver
Bierwagen
,
Le
Wang
,
Yingge
Du
,
Matthew
Wahila
,
Louis F. J.
Piper
,
Tien-Lin
Lee
,
Holly
Edwards
,
Vinod R.
Dhanak
,
Hongliang
Zhang
Diamond Proposal Number(s):
[16005]
Abstract: NiO is a p-type wide bandgap semiconductor of use in various electronic devices ranging from solar cells to transparent transistors. Understanding and improving its optical and transport properties have been of considerable interest. In this work, we have investigated the effect of Li doping on the electronic, optical and transport properties of NiO epitaxial thin films grown by pulsed laser deposition. We show that Li doping significantly increases the p-type conductivity of NiO, but all the films have relatively low room-temperature mobilities (< 0.05 cm2 V−1s−1). The conduction mechanism is better described by small-polaron hoping model in the temperature range of 200 K < T <330 K, and variable range hopping at T <200 K. A combination of x-ray photoemission and O K-edge x-ray absorption spectroscopic investigations reveal that the Fermi level gradually shifts toward the valence band maximum (VBM) and a new hole state develops with Li doping. Both the VBM and hole states are composed of primarily Zhang-Rice bound states, which accounts for the small polaron character (low mobility) of hole conduction. Our work provides guidelines for the search for p-type oxide materials and device optimization.
|
Jan 2018
|
|
