Multimethod System with XPS, LEEM and SPM

Progress in the development of plasmon-enabled light-harvesting technologies requires a
better understanding of their fundamental operating principles and current limitations. Here, we employ
picosecond time-resolved X-ray photoemission spectroscopy to investigate photoinduced electron transfer in
a plasmonic model system composed of 20 nm sized gold nanoparticles (NP_s) attached to a nanoporous film
of TiO₂. The measurement provides direct, quantitative access to transient local charge distributions from
the perspectives of the electron donor (AuNP) and the electron acceptor (TiO₂). On average, approximately
two electrons are injected per NP, corresponding to an electron injection yield per absorbed photon of 0.1%.
Back electron transfer from the perspective of the electron donor is dominated by a fast recombination
channel proceeding on a time scale of 60 ± 10 ps and a minor contribution that is completed after ∼1 ns.
The findings provide a detailed picture of photoinduced charge carrier generation in this NP−semiconductor
junction, with important implications for understanding achievable overall photon-to-charge conversion
efficiencies.

M. Borgwardt, J. Mahl, F.Roth, L. Wenthaus, F. Brauße, M.Blum,
K. Schwarzburg, G. Liu, F.M. Toma, and O.Gessner
The Journal of Physical Chemistry Letters. 2020, 11, 5476−5481

Methanol is a promising chemical for the safe and efficient storage of hydrogen, where methanol conversion reactions can
generate a hydrogen‐containing gas mixture. Understanding the chemical state of the catalyst over which these reactions
occur and the interplay with the adsorbed species present is key to the design of improved catalysts and process conditions.
Here we study polycrystalline Cu foils using ambient pressure X‐ray spectroscopies to reveal the Cu oxidation state and
identify the adsorbed species during partial oxidation (CH₃OH + O₂), steam reforming (CH₃OH + H₂O), and autothermal
reforming (CH₃OH + O₂ + H₂O) of methanol at 200 °C surface temperature and in the mbar pressure range. We find that the
Cu surface remains highly metallic throughout partial oxidation and steam reforming reactions, even for oxygen‐rich
conditions. However, for autothermal reforming the Cu surface shows significant oxidation towards Cu₂O. We rationalise
this behaviour on the basis of the shift in equilibrium of the CH₃OH^* + O^* ⇌ CH₃O^* + OH^* caused by the addition of H₂O.

B. Eren, C. G. Sole, J. S. Lacasa, D. Grinter, F. Venturini, G. Held,
C. S. Esconjauregui, and R. S. Weatherup
Physical Chemistry Chemical Physics, 2020, 00, 1‐8

The development of high performance gas sensors that operate at room temperature
has attracted considerable attention. Unfortunately, the conventional
mechanism of chemiresistive sensors is restricted at room temperature by
insufficient reaction energy with target molecules. Herein, novel strategy for
room temperature gas sensors is reported using an ionic-activated sensing
mechanism. The investigation reveals that a hydroxide layer is developed by
the applied voltages on the SnO₂ surface in the presence of humidity, leading
to increased electrical conductivity. Surprisingly, the experimental results
indicate ideal sensing behavior at room temperature for NO₂ detection with
sub-parts-per-trillion (132.3 ppt) detection and fast recovery (25.7 s) to 5 ppm
NO₂ under humid conditions. The ionic-activated sensing mechanism is
proposed as a cascade process involving the formation of ionic conduction,
reaction with a target gas, and demonstrates the novelty of the approach. It
is believed that the results presented will open new pathways as a promising
method for room temperature gas sensors.

Y. G. Song, Y.-S. Shim, J. M. Suh, M.-S. Noh, G. S. Kim,
K. S. Choi, B. Jeong, S. Kim, H. W. Jang, B.-K. Ju,
and C.-Y. Kang
Small 2019, 15, 1902065

Bulk ZrO₂ is a highly active and selective catalyst for dehydrogenation of propane (PDH), in which coordinatively
unsaturated Zr cations (Zr_cus⁴⁺) serve as active sites. Substitution of dopant ions into Zr lattice can improve its catalytic activity by generating more Zr_cus⁴⁺ sites. In this work, a series of vanadium-doped ZrO₂ metal oxides (VZrO-x) have been prepared and the influences of vanadium content on their properties have been systematically investigated. Various characterization techniques showed that an appropriate amount of vanadium dopant helps more Zr_cus⁴⁺ sites to be created by a structural transformation and H₂ pretreatment. However, excess vanadium dopant led to a negative effect on the catalytic activity owing to the formation of
bulk-like V₂O₅ crystallites. The catalytic activity of VZrO-x is well correlated with the amount of Lewis acid sites
because Zr_cus⁴⁺ cations correspond to Lewis acid sites. The VZrO-8 catalyst exhibited two times higher activity
than pure ZrO₂. Moreover, for repeated cycles the activity was totally recovered by oxidative regeneration
followed by reductive pretreatment. Finally, the performance test results showed that H₂ co-feeding can further
enhance the activity by suppressing coke deposition during PDH.

N. Jeon, H. Choe, B.Jeong, and Y. Yun
Catalysis Today

Bulk ZrO₂ has shown high activity and selectivity for propane dehydrogenation (PDH). Its catalytic performance
can be enhanced by substitution of the Zr ions with suitable dopant ions. In this study, a series of Cu-doped ZrO₂
metal oxides (CuZrO-x) have been prepared using the co-precipitation method and the effects of the amount of
dopant on their properties have been investigated. The appropriate amount of Cu dopant promotes the creation
of oxygen vacancies and coordinatively unsaturated Zr_cus⁴⁺ sites, active sites for PDH; however, excess Cu forms
bulk CuO in the CuZrO-x catalysts resulting in decreased activity. The catalytic activity for PDH is closely
correlated to the amount of weak Lewis acid sites in CuZrO-x. Moreover, a comparison of the CuZrO-x properties
with those of Cu-impregnated ZrO₂ has shown that the amount of acid sites and therefore, the catalytic performance,
depends on the doping method.

N. Jeon, H. Choe, B.Jeong, and Y. Yun
Applied Catalysis A, General 586 (2019) 117211

The success of many emerging molecular electronics concepts hinges on an atomistic understanding of
the underlying electronic dynamics. We employ picosecond time-resolved x-ray photoemission spectroscopy
(tr-XPS) to elucidate the roles of singlet and triplet excitons for photoinduced charge generation at a copperphthalocyanine–
C₆₀ heterojunction. Contrary to common belief, fast intersystem crossing to triplet excitons
after photoexcitation is not a loss channel but contributes to a significantly larger extent to the time-integrated
interfacial charge generation than the initially excited singlet excitons. The tr-XPS data provide direct access to
the diffusivity of the triplet excitons D_CuPc = (1.8 ± 1.2) × 10⁻⁵ cm²/s (where CuPc is copper-phthalocyanine)
and their diffusion length L_diff = (8 ± 3) nm.

F. Roth, S.Neppl, A. Shavorskiy, T. Arion, J.Mahl, H. O. Seo,
H. Bluhm, Z.Hussain, O. Gessner, and W. Eberhardt.
PHYSICAL REVIEW B 99, 020303(R) (2019)

Charge transport measurements form an essential tool in condensed matter physics. The usual approach is to contact a sample by two or four probes, measure the resistance and derive the resistivity, assuming homogeneity within the sample. A more thorough understanding, however, requires knowledge of local resistivity variations. Spatially resolved information is particularly important when studying novel materials like topological insulators, where the current is localized at the edges, or quasi-two-dimensional (2D) systems, where small-scale variations can determine global properties. Here, we demonstrate a new method to determine spatially-resolved voltage maps of current-carrying samples. This technique is based on low-energy electron microscopy (LEEM) and is therefore quick and non-invasive. It makes use of resonance-induced contrast, which strongly depends on the local potential. We demonstrate our method using single to triple layer graphene. However, it is straightforwardly extendable to other quasi-2D systems, most prominently to the upcoming class of layered van der Waals materials.

J. Kautz, J. Jobst, C. Sorger, R. M. Tromp, H. B. Weber und S. J. van der Molen
Scientific Reports volume 5, Article number: 13604 (2015)

X-ray photoelectron spectroscopy (XPS) is one of the most powerful techniques to quantitatively ana-lyze the chemical composition and electronic structure of surfaces and interfaces in a non-destructivefashion. Extending this technique into the time domain has the exciting potential to shed new lighton electronic and chemical dynamics at surfaces by revealing transient charge configurations withelement- and site-specificity. Here, we describe prospects and challenges that are associated with theimplementation of picosecond and femtosecond time-resolved X-ray photoelectron spectroscopy atthird-generation synchrotrons and X-ray free-electron lasers, respectively. In particular, we discuss aseries of laser-pump/X-ray-probe photoemission experiments performed on semiconductor surfaces,molecule-semiconductor interfaces, and films of semiconductor nanoparticles that demonstrate the highsensitivity of time-resolved XPS to light-induced charge carrier generation, diffusion and recombinationwithin the space charge layers of these materials. Employing the showcase example of photo-inducedelectronic dynamics in a dye-sensitized semiconductor system, we highlight the unique possibility toprobe heterogeneous charge transfer dynamics from both sides of an interface, i.e., from the perspectiveof the molecular electron donor and the semiconductor acceptor, simultaneously. Such capabilities willbe crucial to improve our microscopic understanding of interfacial charge redistribution and associatedchemical dynamics, which are at the heart of emerging energy conversion, solar fuel generation, andenergy storage technologies.

S. Neppl, O. Gessner
Journal of Electron Spectroscopy and Related Phenomena 200 (2015) 64–77

An apparatus for sub-nanosecond time-resolved ambient-pressure X-ray photoelectron spectroscopy
studies with pulsed and constant wave X-ray light sources is presented. A differentially pumped
hemispherical electron analyzer is equipped with a delay-line detector that simultaneously records
the position and arrival time of every single electron at the exit aperture of the hemisphere with
∼0.1 mm spatial resolution and ∼150 ps temporal accuracy. The kinetic energies of the photoelectrons
are encoded in the hit positions along the dispersive axis of the two-dimensional detector. Pumpprobe
time-delays are provided by the electron arrival times relative to the pump pulse timing. An
average time-resolution of (780 ± 20) ps (FWHM) is demonstrated for a hemisphere pass energy Ep
= 150 eV and an electron kinetic energy range KE = 503–508 eV. The time-resolution of the setup
is limited by the electron time-of-flight (TOF) spread related to the electron trajectory distribution
within the analyzer hemisphere and within the electrostatic lens system that images the interaction
volume onto the hemisphere entrance slit. The TOF spread for electrons with KE = 430 eV varies
between ∼9 ns at a pass energy of 50 eV and ∼1 ns at pass energies between 200 eV and 400 eV. The
correlation between the retarding ratio and the TOF spread is evaluated by means of both analytical
descriptions of the electron trajectories within the analyzer hemisphere and computer simulations of
the entire trajectories including the electrostatic lens system. In agreement with previous studies, we
find that the by far dominant contribution to the TOF spread is acquired within the hemisphere. However,
both experiment and computer simulations show that the lens system indirectly affects the time
resolution of the setup to a significant extent by inducing a strong dependence of the angular spread
of electron trajectories entering the hemisphere on the retarding ratio. The scaling of the angular
spread with the retarding ratio can be well approximated by applying Liouville’s theorem of constant
emittance to the electron trajectories inside the lens system. The performance of the setup is demonstrated
by characterizing the laser fluence-dependent transient surface photovoltage response of a
laser-excited Si(100) sample. © 2014 AIP Publishing LLC.

A. Shavorskiy, S. Neppl, D. S. Slaughter, J. P. Cryan, K. R. Siefermann, F. Weise,
Mf Lin, C. Bacellar, M. P. Ziemkiewicz, I. Zegkinoglou, M. W. Fraund, C.
Khurmi, M. P. Hertlein, T. W. Wright, N. Huse, R. W. Schoenlein, T. Tyliszczak, G.
Coslovich, J.
AIP Review of Scientific Instruments 85, 093102 (2014)

Understanding interfacial charge-transfer processes on the atomic level is
crucial to support the rational design of energy-challenge relevant systems such as solar cells,
batteries, and photocatalysts. A femtosecond time-resolved core-level photoelectron spectroscopy
study is performed that probes the electronic structure of the interface between
ruthenium-based N3 dye molecules and ZnO nanocrystals within the first picosecond after
photoexcitation and from the unique perspective of the Ru reporter atom at the center of the
dye. A transient chemical shift of the Ru 3d inner-shell photolines by (2.3 ± 0.2) eV to higher
binding energies is observed 500 fs after photoexcitation of the dye. The experimental results
are interpreted with the aid of ab initio calculations using constrained density functional
theory. Strong indications for the formation of an interfacial charge-transfer state are presented,
providing direct insight into a transient electronic configuration that may limit the
efficiency of photoinduced free charge-carrier generation.

K.R. Siefermann, C. D. Pemmaraju, S. Neppl, A. Shavorskiy,
A. A. Cordones, J. Vura-Weis, D. S. Slaughter, F. P. Sturm, F. Weise,
H. Bluhm, M. L. Strader, H. Cho, MF Lin, C. Bacellar,
C. Khurmi, J. Guo, G. Coslovich, J. S. Robinson, R. A. Kaindl,
R. W.
The Journal of Physical Chemistry Letters. 2014, 5, 2753−2759