I09-Surface and Interface Structural Analysis
|
Jatinkumar
Rana
,
Joseph K.
Papp
,
Zachary
Lebens-higgins
,
Mateusz
Zuba
,
Lori A.
Kaufman
,
Anshika
Goel
,
Richard
Schmuch
,
Martin
Winter
,
M. Stanley
Whittingham
,
Wanli
Yang
,
Bryan D.
Mccloskey
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[22250]
Abstract: Though Li2MnO3 was originally considered to be electrochemically inert, its observed activation has spawned a new class of Li-rich layered compounds that deliver capacities beyond the traditional transition-metal redox limit. Despite progress in our understanding of oxygen redox in Li-rich compounds, the underlying origin of the initial charge capacity of Li2MnO3 remains hotly contested. To resolve this issue, we review all possible charge compensation mechanisms including bulk oxygen redox, oxidation of Mn4+, and surface degradation for Li2MnO3 cathodes displaying capacities exceeding 350 mAh g–1. Using elemental and orbital selective X-ray spectroscopy techniques, we rule out oxidation of Mn4+ and bulk oxygen redox during activation of Li2MnO3. Quantitative gas-evolution and titration studies reveal that O2 and CO2 release accounted for a large fraction of the observed capacity during activation with minor contributions from reduced Mn species on the surface. These studies reveal that, although Li2MnO3 is considered critical for promoting bulk anionic redox in Li-rich layered oxides, Li2MnO3 by itself does not exhibit bulk oxygen redox or manganese oxidation beyond its initial Mn4+ valence.
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Jan 2020
|
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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.
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Dec 2019
|
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I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[22148]
Abstract: We synthesized a new class of heterostructures by depositing CdS, CdSe, or CdTe quantum dots (QDs) onto α-V2O5 nanowires (NWs) via either successive ionic layer adsorption and reaction (SILAR) or linker-assisted assembly (LAA). SILAR yielded the highest loadings of QDs per NW, whereas LAA enabled better control over the size and properties of QDs. Soft and hard x-ray photoelectron spectroscopy in conjunction with density functional theory calculations revealed that all α-V2O5/QD heterostructures exhibited Type-II band offset energetics, with a staggered gap where the conduction- and valence-band edges of α-V2O5 NWs lie at lower energies (relative to the vacuum level) than their QD counterparts. Transient absorption spectroscopy measurements revealed that the Type-II energetic offsets promoted the ultrafast (10−12–10−11 s) separation of photogenerated electrons and holes across the NW/QD interface to yield long-lived (10−6 s) charge-separated states. Charge-transfer dynamics and charge-recombination time scales varied subtly with the composition of heterostructures and the nature of the NW/QD interface, with both charge separation and recombination occurring more rapidly within SILAR-derived heterostructures. LAA-derived α-V2O5/CdSe heterostructures promoted the photocatalytic reduction of aqueous protons to H2 with a 20-fold or greater enhancement relative to isolated colloidal CdSe QDs or dispersed α-V2O5 NWs. The separation of photoexcited electrons and holes across the NW/QD interface could thus be exploited in redox photocatalysis. In light of their programmable compositions and properties and their Type-II energetics that drive ultrafast charge separation, the α-V2O5/QD heterostructures are a promising new class of photocatalyst architectures ripe for continued exploration.
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Dec 2019
|
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I09-Surface and Interface Structural Analysis
|
Jack E. N.
Swallow
,
Benjamin A. D.
Williamson
,
Sanjayan
Sathasivam
,
Max
Birkett
,
Thomas J.
Featherstone
,
Philip A. E.
Murgatroyd
,
Holly J.
Edwards
,
Zachary W.
Lebens-higgins
,
David A.
Duncan
,
Mark
Farnworth
,
Paul
Warren
,
Nianhua
Peng
,
Tien-lin
Lee
,
Louis F. J.
Piper
,
Anna
Regoutz
,
Claire J.
Carmalt
,
Ivan P.
Parkin
,
Vin R.
Dhanak
,
David O.
Scanlon
,
Tim D.
Veal
Diamond Proposal Number(s):
[18428]
Open Access
Abstract: Transparent conductors are a vital component of smartphones, touch-enabled displays, low emissivity windows and thin film photovoltaics. Tin-doped In2O3 (ITO) dominates the transparent conductive films market, accounting for the majority of the current multi-billion dollar annual global sales. Due to the high cost of indium, however, alternatives to ITO have been sought but have inferior properties. Here we demonstrate that molybdenum-doped In2O3 (IMO) has higher mobility and therefore higher conductivity than ITO with the same carrier density. This also results in IMO having increased infrared transparency compared to ITO of the same conductivity. These properties enable current performance to be achieved using thinner films, reducing the amount of indium required and raw material costs by half. The enhanced doping behavior arises from Mo 4d donor states being resonant high in the conduction band and negligibly perturbing the host conduction band minimum, in contrast to the adverse perturbation caused by Sn 5s dopant states. This new understanding will enable better and cheaper TCOs based on both In2O3 and other metal oxides.
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Sep 2019
|
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I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[16005]
Abstract: Zn
M
I
I
I
2
O
4
(
M
I
I
I
=
Co
, Rh, Ir) spinels have been recently identified as promising
p
-type semiconductors for transparent electronics. However, discrepancies exist in the literature regarding their fundamental optoelectronic properties. In this paper, the electronic structures of these spinels are directly investigated using soft/hard x-ray photoelectron and x-ray absorption spectroscopies in conjunction with density functional theory calculations. In contrast to previous results,
ZnCo
2
O
4
is found to have a small electronic band gap with forbidden optical transitions between the true band edges, allowing for both bipolar doping and high optical transparency. Furthermore, increased
d
−
d
splitting combined with a concomitant lowering of Zn
s
/
p
conduction states is found to result in a
ZnCo
2
O
4
(
ZCO
)
<
ZnRh
2
O
4
(
ZRO
)
≈
ZnIr
2
O
4
(
ZIO
)
band gap trend, finally resolving long-standing discrepancies in the literature.
|
Aug 2019
|
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I09-Surface and Interface Structural Analysis
|
Sebastian A.
Howard
,
Christopher N.
Singh
,
Galo J.
Paez
,
Matthew
Wahila
,
Linda W.
Wangoh
,
Shawn
Sallis
,
Keith
Tirpak
,
Yufeng
Liang
,
David
Prendergast
,
Mateusz
Zuba
,
Jatinkumar
Rana
,
Alex
Weidenbach
,
Timothy M.
Mccrone
,
Wanli
Yang
,
Tien-lin
Lee
,
Fanny
Rodolakis
,
William
Doolittle
,
Wei-cheng
Lee
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[20647]
Open Access
Abstract: The discovery of analog LixNbO2 memristors revealed a promising new memristive mechanism wherein the diffusion of Li+ rather than O2− ions enables precise control of the resistive states. However, directly correlating lithium concentration with changes to the electronic structure in active layers remains a challenge and is required to truly understand the underlying physics. Chemically delithiated single crystals of LiNbO2 present a model system for correlating lithium variation with spectroscopic signatures from operando soft x-ray spectroscopy studies of device active layers. Using electronic structure modeling of the x-ray spectroscopy of LixNbO2 single crystals, we demonstrate that the intrinsic memristive behavior in LixNbO2 active layers results from field-induced degenerate p-type doping. We show that electrical operation of LixNbO2-based memristors is viable even at marginal Li deficiency and that the analog memristive switching occurs well before the system is fully metallic. This study serves as a benchmark for material synthesis and characterization of future LixNbO2-based memristor devices and suggests that valence change switching is a scalable alternative that circumvents the electroforming typically required for filamentary-based memristors.
|
Jul 2019
|
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I09-Surface and Interface Structural Analysis
|
Matthew J.
Wahila
,
Galo
Paez
,
Christopher N.
Singh
,
Anna
Regoutz
,
Shawn
Sallis
,
Mateusz J.
Zuba
,
Jatinkumar
Rana
,
M. Brooks
Tellekamp
,
Jos E.
Boschker
,
Toni
Markurt
,
Jack E. N.
Swallow
,
Leanne A. H.
Jones
,
Tim D.
Veal
,
Wanli
Yang
,
Tien-lin
Lee
,
Fanny
Rodolakis
,
Jerzy T.
Sadowski
,
David
Prendergast
,
Wei-cheng
Lee
,
W. Alan
Doolittle
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[20647, 21430]
Abstract: The metal-insulator transition of
NbO
2
is thought to be important for the functioning of recent niobium oxide-based memristor devices, and is often described as a Mott transition in these contexts. However, the actual transition mechanism remains unclear, as current devices actually employ electroformed
NbO
x
that may be inherently different to crystalline
NbO
2
. We report on our synchrotron x-ray spectroscopy and density-functional-theory study of crystalline, epitaxial
NbO
2
thin films grown by pulsed laser deposition and molecular beam epitaxy across the metal-insulator transition at
∼
810
∘
C
. The observed spectral changes reveal a second-order Peierls transition driven by a weakening of Nb dimerization without significant electron correlations, further supported by our density-functional-theory modeling. Our findings indicate that employing crystalline
NbO
2
as an active layer in memristor devices may facilitate analog control of the resistivity, whereby Joule-heating can modulate Nb-Nb dimer distance and consequently control the opening of a pseudogap.
|
Jul 2019
|
|
I09-Surface and Interface Structural Analysis
|
Zachary W.
Lebens-higgins
,
Nicholas V.
Faenza
,
Maxwell D.
Radin
,
Hao
Liu
,
Shawn
Sallis
,
Jatinkumar
Rana
,
Julija
Vinckeviciute
,
Philip J.
Reeves
,
Mateusz
Zuba
,
Fadwa
Badway
,
Nathalie
Pereira
,
Karena W.
Chapman
,
Tien-lin
Lee
,
Tianpin
Wu
,
Clare P.
Grey
,
Brent
Melot
,
Anton
Van Der Ven
,
Glenn G.
Amatucci
,
Wanli
Yang
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[19162]
Open Access
Abstract: Oxygen participation, arising from increased transition metal–oxygen covalency during delithiation, is considered essential for the description of charge compensation in conventional layered oxides. The advent of high-resolution mapping of the O K-edge resonant inelastic X-ray scattering (RIXS) provides an opportunity to revisit the onset and extent of oxygen participation. Combining RIXS with an array of structural and electronic probes for the family of Ni-rich LiNi0.8Co0.2−yAlyO2 cathodes, we identify common charge compensation regimes that are assigned to formal transition metal redox (<4.25 V) and oxygen participation through covalency (>4.25 V). From O K-edge RIXS maps, we find the emergence of a sharp RIXS feature in these systems when approaching full delithiation, which has previously been associated with lattice oxidized oxygen in alkali-rich systems. The lack of transition metal redox signatures and strong covalency at these high degrees of delithiation suggest this RIXS feature is similarly attributed to lattice oxygen charge compensation as in the alkali-rich systems. The RIXS feature's evolution with state of charge in conventional layered oxides is evidence that this feature reflects the depopulation of occupied O 2p states associated with oxygen participation.
|
Jul 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
|
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
|
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