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.
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Nov 2018
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I09-Surface and Interface Structural Analysis
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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.
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Jun 2019
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I09-Surface and Interface Structural Analysis
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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.
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Dec 2018
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I09-Surface and Interface Structural Analysis
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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.
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Nov 2018
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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.
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Nov 2018
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I09-Surface and Interface Structural Analysis
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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.
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Jul 2019
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I09-Surface and Interface Structural Analysis
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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.
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Jul 2019
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I09-Surface and Interface Structural Analysis
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Z. h.
Lim
,
N. F.
Quackenbush
,
A. n.
Penn
,
M.
Chrysler
,
M.
Bowden
,
Z.
Zhu
,
J. M.
Ablett
,
T.-L.
Lee
,
J. m.
Lebeau
,
J. C.
Woicik
,
P. v.
Sushko
,
S. a.
Chambers
,
J. h.
Ngai
Diamond Proposal Number(s):
[17449]
Abstract: We report charge transfer and built-in electric fields across the epitaxial
SrNb
x
Ti
1
−
x
O
3
−
δ
/
Si
(
001
)
interface. Electrical transport measurements indicate the formation of a hole gas in the Si and the presence of built-in fields. Hard x-ray photoelectron measurements reveal pronounced asymmetries in core-level spectra that arise from these built-in fields. Theoretical analysis of core-level spectra enables built-in fields and the resulting band bending to be spatially mapped across the heterojunction. The demonstration of tunable charge transfer, built-in fields, and the spatial mapping of the latter, lays the groundwork for the development of electrically coupled, functional heterojunctions.
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Jul 2019
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I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[21432]
Abstract: This work reports a systematical study on the relationship of electronic structure to oxygen evolution reaction (OER) activity of NixCo3-xO4 (x=0-1) mixed oxides. The specific OER activity is substantially increased by 16 times from 0.02 mA cm-2BET for pure Co3O4 to 0.32 mA cm-2BET for x=1 at an overpotential of 0.4 V and exhibits a strong correlation with the amount of Ni ions in +3 oxidation state. X-ray spectroscopic study reveals that inclusion of Ni3+ ions upshifts the occupied valence band maximum (VBM) by 0.27 eV toward the Fermi level (EF), and creates a new hole (unoccupied) state located ~1 eV above the EF. Such electronic features favour the adsorption of OH surface intermediates on NixCo3-xO4, resulting in enhanced OER. Furthermore, the emerging hole state effectively reduces the energy barrier for electron transfer from 1.19 eV to 0.39 eV, and thereby improves the kinetics for OER. The electronic structure features that lead to a higher OER in NixCo3-xO4 can be extended to other transition metal oxides for rational design of highly active catalysts.
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Aug 2019
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I09-Surface and Interface Structural Analysis
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Slavomir
Nemsak
,
Mathias
Gehlmann
,
Cheng-Tai
Kuo
,
Shih-Chieh
Lin
,
Christoph
Schlueter
,
Ewa
Mlynczak
,
Tien-Lin
Lee
,
Lukasz
Plucinski
,
Hubert
Ebert
,
Igor
Di Marco
,
Ján
Minár
,
Claus M.
Schneider
,
Charles S.
Fadley
Diamond Proposal Number(s):
[11516, 12032]
Open Access
Abstract: The dilute magnetic semiconductors have promise in spin-based electronics applications due to their potential for ferromagnetic order at room temperature, and various unique switching and spin-dependent conductivity properties. However, the precise mechanism by which the transition-metal doping produces ferromagnetism has been controversial. Here we have studied a dilute magnetic semiconductor (5% manganese-doped gallium arsenide) with Bragg-reflection standing-wave hard X-ray angle-resolved photoemission spectroscopy, and resolved its electronic structure into element- and momentum- resolved components. The measured valence band intensities have been projected into element-resolved components using analogous energy scans of Ga 3d, Mn 2p, and As 3d core levels, with results in excellent agreement with element-projected Bloch spectral functions and clarification of the electronic structure of this prototypical material. This technique should be broadly applicable to other multi-element materials.
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Aug 2018
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