I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[8450]
Abstract: We present a systematic X-ray standing wave (XSW) study of the π-conjugated organic compound 2H-tetraphenylporphyrin (2HTPP) and copper(II)-tetraphenylporphyrin (CuTPP) on Cu(111) at room and low temperatures. We exploit the feature of thermally activated self-metalation of 2HTPP to CuTPP to study the influence of the central metal atom on the bonding distance of the molecule to the substrate surface. Comparison between the average adsorption distances of the carbon and nitrogen atoms of 2HTPP reveals a distorted molecule with the nitrogen atoms being closer to the surface than the carbon ones on average. Additionally, the measured positions of the two chemically inequivalent types of nitrogen atoms (iminic and aminic) of 2HTPP indicate a distorted porphyrin ring. After the chemical reaction from 2HTPP to CuTPP at 500 K, no change of the adsorption distance of the carbon skeleton is seen, but the entire molecule becomes flattened. Despite the changes upon metalation, adsorption distances for both molecules show a strong interaction with the substrate in comparison to similar π-conjugated molecules.
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May 2014
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I09-Surface and Interface Structural Analysis
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Nan
Yang
,
Alex
Belianinov
,
Evgheni
Strelcov
,
Antonello
Tebano
,
Vittorio
Foglietti
,
Daniele
Di Castro
,
Christoph
Schlueter
,
Tien-lin
Lee
,
Arthur P.
Baddorf
,
Nina
Balke
,
Stephen
Jesse
,
Sergei V.
Kalinin
,
Giuseppe
Balestrino
,
Carmela
Aruta
Diamond Proposal Number(s):
[9221]
Abstract: A systematic study by reversible and hysteretic electrochemical strain microscopy (ESM) in samples of cerium oxide with different Sm content and in several working conditions allows disclosing the microscopic mechanism underlying the difference in electrical conduction mechanism and related surface activity, such as water adsorption and dissociation with subsequent proton liberation. We have measured the behavior of the reversible hysteresis loops by changing temperature and humidity, both in standard ESM configuration and using the first-order reversal curve method. The measurements have been performed in much smaller temperature ranges with respect to alternative measuring techniques. Complementing our study with hard X-ray photoemission spectroscopy and irreversible scanning probe measurements, we find that water incorporation is favored until the doping with Sm is too high to allow the presence of Ce3+. The influence of doping on the surface reactivity clearly emerges from all of our experimental results. We find that at lower Sm concentration, proton conduction is prevalent, featured by lower activation energy and higher electrical conductivity. Defect concentrations determine the type of the prevalent charge carrier in a doping dependent manner.
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Dec 2014
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I09-Surface and Interface Structural Analysis
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J.
Sforzini
,
L.
Nemec
,
T.
Denig
,
B.
Stadtmüller
,
T. L
Lee
,
C.
Kumpf
,
S.
Soubatch
,
U.
Starke
,
P.
Rinke
,
V.
Blum
,
Francois
Bocquet
,
Stefan
Tautz
Diamond Proposal Number(s):
[8449, 10271]
Open Access
Abstract: We measure the adsorption height of hydrogen-intercalated quasifreestanding monolayer graphene on the
(0001) face of 6H silicon carbide by the normal incidence x-ray standing wave technique. A density
functional calculation for the full ð6
ffiffiffi
3 p × 6
ffiffiffi
3 p Þ-R30° unit cell, based on a van derWaals corrected exchange
correlation functional, finds a purely physisorptive adsorption height in excellent agreement with experiments,
a very low buckling of the graphene layer, a very homogeneous electron density at the interface, and
the lowest known adsorption energy per atom for graphene on any substrate.Astructural comparison to other
graphenes suggests that hydrogen-intercalated graphene on 6H-SiCð0001Þ approaches ideal graphene.
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Mar 2015
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I09-Surface and Interface Structural Analysis
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N.
Yang
,
E.
Strelcov
,
A.
Belianinov
,
A.
Tebano
,
V.
Foglietti
,
C.
Schlueter
,
T.-L.
Lee
,
S.
Jesse
,
S. V.
Kalinin
,
G.
Balestrino
,
C.
Aruta
Abstract: Water adsorption, splitting, and proton liberation were investigated on Sm0.1Ce0.9O2-δ thin films by scanning probe microscopy. An irreversible volume expansion was observed by applying a positive bias with increased temperature. The volume expansion is also linearly dependent on the relative humidity. A reversible water adsorption process and its effect on the conductivity were also investigated by electrochemical strain microscopy and first order reversal curve under a number of experiment conditions. The presence of a Ce3+ along with OH groups, detected by hard x-ray photoemission spectroscopy established a clear correlation between the water incorporation and the valence state of Ce.
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Oct 2015
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I09-Surface and Interface Structural Analysis
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Abstract: Organic–inorganic halide perovskite solar cells have rapidly evolved over the last 3 years. There are still a number of issues and open questions related to the perovskite material, such as the phenomenon of anomalous hysteresis in current–voltage characteristics and long-term stability of the devices. In this work, we focus on the electron selective contact in the perovskite solar cells and physical processes occurring at that heterojunction. We developed efficient devices by replacing the commonly employed TiO2 compact layer with fullerene C60 in a regular n–i–p architecture. Detailed spectroscopic characterization allows us to present further insight into the nature of photocurrent hysteresis and charge extraction limitations arising at the n-type contact in a standard device. Furthermore, we show preliminary stability data of perovskite solar cells under working conditions, suggesting that an n-type organic charge collection layer can increase the long-term performance.
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Jun 2015
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I09-Surface and Interface Structural Analysis
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Anna
Regoutz
,
Isha
Gupta
,
Alexantrou
Serb
,
Ali
Khiat
,
Francesco
Borgatti
,
Tien-Lin
Lee
,
Christoph
Schlueter
,
Piero
Torelli
,
Benoit
Gobaut
,
Mark
Light
,
Daniela
Carta
,
Stuart
Pearce
,
Giancarlo
Panaccione
,
Themistoklis
Prodromakis
Diamond Proposal Number(s):
[0240]
Open Access
Abstract: TiO2 is commonly used as the active switching layer in resistive random access memory. The electrical characteristics of these devices are directly related to the fundamental conditions inside the TiO2 layer and at the interfaces between it and the surrounding electrodes. However, it is complex to disentangle the effects of film “bulk” properties and interface phenomena. The present work uses hard X-ray photoemission spectroscopy (HAXPES) at different excitation energies to distinguish between these regimes. Changes are found to affect the entire thin film, but the most dramatic effects are confined to an interface. These changes are connected to oxygen ions moving and redistributing within the film. Based on the HAXPES results, post-deposition annealing of the TiO2 thin film was investigated as an optimisation pathway in order to reach an ideal compromise between device resistivity and lifetime. The structural and chemical changes upon annealing are investigated using X-ray absorption spectroscopy and are further supported by a range of bulk and surface sensitive characterisation methods. In summary, it is shown that the management of oxygen content and interface quality is intrinsically important to device behavior and that careful annealing procedures are a powerful device optimisation technique.
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Jan 2016
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I09-Surface and Interface Structural Analysis
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J.
Sforzini
,
P.
Hapala
,
M.
Franke
,
G.
Van Straaten
,
A.
Stöhr
,
S.
Link
,
S.
Soubatch
,
P.
Jelínek
,
T.-L.
Lee
,
U.
Starke
,
M.
Švec
,
F.
Bocquet
,
S.
Tautz
Diamond Proposal Number(s):
[10271]
Open Access
Abstract: We investigate the structural and electronic properties of nitrogen-doped epitaxial monolayer graphene
and quasifreestanding monolayer graphene on 6H-SiCð0001Þ by the normal incidence x-ray standing wave
technique and by angle-resolved photoelectron spectroscopy supported by density functional theory
simulations. With the location of various nitrogen species uniquely identified, we observe that for the same
doping procedure, the graphene support, consisting of substrate and interface, strongly influences the
structural as well as the electronic properties of the resulting doped graphene layer. Compared to epitaxial
graphene, quasifreestanding graphene is found to contain fewer nitrogen dopants. However, this lack of
dopants is compensated by the proximity of nitrogen atoms at the interface that yield a similar number of
charge carriers in graphene.
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Mar 2016
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I09-Surface and Interface Structural Analysis
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Abstract: Proton-conducting perovskite oxides form a class of solid electrolytes for novel electrochemical devices operating at moderate temperatures. Here, we use hard X-ray photoelectron spectroscopy, scanning transmission electron microscopy, and density functional theory calculations to investigate the structure and elucidate the origin of the fast proton transport properties of strained ultrathin films of Y-doped BaZrO3 grown by pulsed lased deposition on NdGaO3. Our study shows that our BaZr0.8Y0.2O3 films incorporate a significant amount of Y dopants, and to a lesser extent also Zr ions, substituting for Ba2+, and that these substitutional defects agglomerate forming columnar regions crossing vertically from the surface to the interface the entire film. Our calculations also show that, in regions rich in Y substitutions for both Zr and Ba, the proton transfer process involves nearly zero-energy barriers, indicating that A-site cation substitutions by Y lead to fast transport pathways and hence are responsible for the previously observed enhanced values of the proton conductivity of these perovskite oxide films.
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Mar 2016
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I09-Surface and Interface Structural Analysis
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N.
Yang
,
E.
Di Bartolomeo
,
V.
Foglietti
,
C.
Cantoni
,
A.
Belianinov
,
A.
Tebano
,
S.
Licoccia
,
T- L
Lee
,
C.
Schlueter
,
S. V.
Kalinin
,
G.
Balestrino
,
C.
Aruta
Abstract: In this paper, we report on BaZr0.8Y0.2O3-x (BZY) thin films grown on highly mismatched NdGaO3 (110) substrates by RHEED assisted pulsed laser deposition. The conduction and electrochemical performances are studied by Electrochemical Impedance Spectroscopy and Electrochemical Strain Microscopy respectively. Conductivity, as well as electrochemical response improves while decreasing the thickness, indicating that the proton movement is prompt at the defective interface region. Structural defects at the interface between film and substrate are clearly displayed by x-ray diffraction, RHEED patterns and transmission electron microscopy. The role of chemical defects on BZY film properties is elucidated by Hard X-ray Photoelectron Spectroscopy. Our results demonstrate that both structural dislocations and chemical defects influence the proton conduction and reaction process in BZY thin films.
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May 2016
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I09-Surface and Interface Structural Analysis
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Peter
Deimel
,
Reda M.
Bababrik
,
Bin
Wang
,
Philip
Blowey
,
Luke
Rochford
,
Pardeep K.
Thakur
,
Tien-Lin
Lee
,
Marie-Laure
Bocquet
,
Johannes V.
Barth
,
Phil
Woodruff
,
David
Duncan
,
Francesco
Allegretti
Diamond Proposal Number(s):
[8940]
Open Access
Abstract: The strong parallels between coordination chemistry and adsorption on metal surfaces, with molecules and ligands forming local bonds to individual atoms within a metal surface, have been established over many years of study. The recently proposed “surface trans-effect” (STE) appears to be a further manifestation of this analogous behaviour, but so far the true nature of the modified molecule–metal surface bonding has been unclear. The STE could play an important role in determining the reactivities of surface-supported metal–organic complexes, influencing the design of systems for future applications. However, the current understanding of this effect is incomplete and lacks reliable structural parameters with which to benchmark theoretical calculations. Using X-ray standing waves, we demonstrate that ligation of ammonia and water to iron phthalocyanine (FePc) on Ag(111) increases the adsorption height of the central Fe atom; dispersion corrected density functional theory calculations accurately model this structural effect. The calculated charge redistribution in the FePc/H2O electronic structure induced by adsorption shows an accumulation of charge along the σ-bonding direction between the surface, the Fe atom and the water molecule, similar to the redistribution caused by ammonia. This apparent σ-donor nature of the observed STE on Ag(111) is shown to involve bonding to the delocalised metal surface electrons rather than local bonding to one or more surface atoms, thus indicating that this is a true surface trans-effect.
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Jun 2016
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