2nd February 2023

8:30 - 9:00: Positronium ELECTROchemistry: In situ study of electrochemical processes by positronium annihilation

Philipp Brunner

This talk expands the field of application of positron and positronium annihilation towards an in situ tool for monitoring electrochemical processes in aqueous electrolytes [1,2]. In water, a high amount of positronium, a hydrogen-like bound state of a positron and an electron, is found. By adding different solutes to water, the amount of formed positronium, but also its lifetime can be significantly influenced.

Two interesting species in this context are the Fe(CN)63-/4- and the Fe2+/3+ redox couple. Fe(CN)63- ions inhibit the positronium formation and oxidize the formed positronium, whereas Fe(CN)64- ions only inhibit the positronium formation and Fe2+ ions only interact with positronium via spin conversion reactions. Based on these quite different properties of the positronium chemistry of the used ions, highly reversible variations of the mean positron lifetime could be observed in an aqueous K3[Fe(CN)6 as well as a Fe2+/3+ based electrolyte upon electrochemical switching between the oxidation states Fe(CN)<sub6>3- and Fe(CN)64-, and Fe3+ and Fe2+, respectively. The dynamic in situ measurements of the positron lifetime were conducted by using cyclic voltammetry. While continuously changing the applied potential of the electrochemical cell, both the resulting current and the variation of the positron lifetime could be monitored. The observed hysteresis like behavior of the mean positron lifetime perfectly correlates with the shift between the reduction and oxidation peaks in the cyclic voltammogram. Furthermore, the variation of the mean positron lifetime with the Fe2+/3+ concentration ratio could be quantitatively described by a reaction rate model for positronium formation and annihilation. The highly reversible galvanostatic charging behavior of the Fe2+/3+ electrolyte monitored by positron lifetime underlines the attractive application potentials of positronium electrochemistry for in situ studies of iron-based redox-flow battery electrolytes.</sub6>

[1] P. Brunner et al., Phys. Chem. Chem. Phys. 23, 25278 (2021)
[2] P. Brunner et al., J. Chem. Phys. 157, 234202 (2022)

9:00 - 9:30: Correlated Mott insulators in strong electric fields: Role of phonons in heat dissipation

Tommaso Mazzocchi

Mott-insulating models can undergo an insulator-to-metal transition when subject to a constant bias voltage [1], which makes them suitable to describe the resistive switch observed in correlated insulators [2]. State-of-the-art techniques in nonequilibrium physics rely on the coupling to fermion baths to dissipate the field-induced excess energy [1,3]. However, a realistic description of heat-exchange processes cannot do without the inclusion of phonons. In [4] we study a single-band Hubbard model in a static electric field coupled to electron and acoustic phonon baths. The nonequilibrium steady-state is addressed via the dynamical mean-field theory using the auxiliary master equation approach as impurity solver. Phonons are included via the Migdal approximation. Using both the electron and phonon baths the steady-state current is slightly enhanced by phonons for field strengths close to half of the gap and suppressed at the gap resonance. With phonons alone, dissipation can occur only at the resonances and the current at the metallic phase is suppressed by almost one order of magnitude.

[1] C. Aron, 10.1103/PhysRevB.86.085127
[2] E. Janod et al., 10.1002/adfm.201500823
[3] Y. Murakami et al., 10.1103/PhysRevB.98.075102
[4] T. M. Mazzocchi et al., 10.1103/PhysRevB.106.125123


9:30 - 10:00: Influence of phononic dissipation on impact ionization processes in a photodriven Mott insulator

Paolo Gazzaneo

It has been suggested that in strongly correlated materials, highly photoexcited charge carriers could use their extra energy to excite additional carriers across the Mott gap via impact ionization [1,2]. However, the influence of electron-phonon scattering on photocurrent and impact ionization in Mott photovoltaic setups is still an open question. We address this issue in a nonequilibrium steady state study on the occurrence of impact ionization in a simplified model of a Mott photovoltaic device in presence of acoustic phonons [3], consisting of a Mott-insulating layer coupled to two wide-band fermion leads. For a small hybridization to the leads, we obtain a peak in the photocurrent as a function of the driving frequency which can be associated with impact ionization processes, while for larger hybridizations we find a suppression of impact ionization with respect to direct photovoltaic excitations. The effect of acoustic phonons produces a slight enhancement of the photocurrent for small driving frequencies and a suppression at frequencies around the main peak at all considered hybridization strengths.

[1] E. Manousakis, Phys. Rev. B 82, 125109 (2010)
[2] J. E. Coulter et al., Phys. Rev. B 90, 165142 (2014)
[3] Gazzaneo et al, Phys. Rev. B 106, 195140 (2022)

10:30 - 11:00: Laser-Induced Graphene for Soft and Thin Electronics

Alexander Dallinger

The conversion of commercial polymers into conductive porous graphene by direct laser scribing with a CO2 infrared laser, creating the Laser Induced Graphene (LIG), is an easy and highly scalable production method for conductive track patterning [1]. LIG is a 3D porous material, exhibiting very high surface area, excellent conductivity and high thermal stability. Which makes it a desired material with applications in many fields such as sensing, actuation and energy. Many properties of LIG can be tuned by the processing parameters (laser) or the processing environment. For example, by using an inert atmosphere or certain processing parameters the wetting behavior of LIG can be tuned from hydrophilic to superhydrophobic without any additional chemical treatment.

While polyimide and other rigid commercially available polymer sheets represent excellent LIG precursors, their unfitting mechanical properties and poor gas permeability are limiting their use in fields such as epidermal electronics or soft robotics. Strategies such as a transfer or embedding onto soft and flexible materials allows to extend the field of usage.

By transferring the LIG onto a substrate which provides excellent conformal adhesion on skin, stretchability, high breathability and waterproof stability, epidermal sensors were demonstrated. The sensors include electromyography (EMG), tactile and strain/respiration sensing [2]. Self sensing soft robotic actuators were made from LIG/PDMS composites and a thermoresponsive hydrogel deposited by initiated chemical vapor deposition. The embedded LIG was used as a joule heating element to trigger a reversible actuation induced by the collapse of the hydrogel [3].

A wearable electrochemical sensor based on LIG was used to analyze sweat. LIG electrodes were chemically modified to sense a change in pH and a change in concentration of uric acid and tyrosine in sweat [4].

[1] Lin, J.; Peng, Z.; Liu, Y.; Ruiz-Zepeda, F.; Ye, R.; Samuel, E. L. G.; Yacaman, M. J.; Yakobson, B. I.; Tour, J. M. Laser-Induced Porous Graphene Films from Commercial Polymers. Nature Communications 2014, 5, 5714. doi.org/10.1038/ncomms6714.
[2] Dallinger, A.; Keller, K.; Fitzek, H.; Greco, F. Stretchable and Skin-Conformable Conductors Based on Polyurethane/Laser-Induced Graphene. ACS Appl. Mater. Interfaces 2020. doi.org/10.1021/acsami.0c03148.
[3] Dallinger, A.; Kindlhofer, P.; Greco, F.; Coclite, A. M. Multiresponsive Soft Actuators Based on a Thermoresponsive Hydrogel and Embedded Laser-Induced Graphene. ACS Appl. Polym. Mater. 2021. doi.org/10.1021/acsapm.0c01385.
[4] Vivaldi, F.; Dallinger, A.; Poma, N.; Bonini, A.; Biagini, D.; Salvo, P.; Borghi, F.; Tavanti, A.; Greco, F.; Di Francesco, F. Sweat Analysis with a Wearable Sensing Platform Based on Laser-Induced Graphene. APL Bioengineering 2022, 6 (3), 036104. https://doi.org/10.1063/5.0093301.


11:00 - 11:30: Interlayer orbital overlap governing thin-film geometry: the role of interfacial charge transfer

Fabio Calcinelli

Organic thin films can assume a wide variety of structures,and their polymorphism is influenced by several factors, including the substrates on which they grow. Predicting which structure a thin film will assume on a substrate is impossible through conventional first-principle modeling alone, because the number of possible polymorphs quickly exceeds computational limitations. However, recent developments in machine-learning assisted structure search have made structure-to-property investigations accessible.

Employing smart-data machine learning, we demonstrate the impact that different substrates can have on the geometry of the first two layers of a thin film. We identify the energetically most favourable geometries for benzoquinone on silver and on graphene, and compare their electronic properties. While the polymorphs formed in the first layer of benzoquinone are very similar, for the second layer we find two significantly different structures. We explain this difference as an effect of the interplay between different charge transfer on the two substrates, and different interlayer orbital overlap for the two structures. This also signifies that the two structures will exhibit relevantly different vertical charge carrier mobilities, according to the hopping model. Furthermore, the importance of calculating overlaps directly from the eigenstate vectors, rather than from cubefiles, is discussed.


11:30 - 12:00: Application of atomistic force field potentials for the prediction of heat transport and phonon properties in metal-organic frameworks

Sandro Wieser

Metal-organic frameworks (MOFs) are a class of highly porous hybrid materials with many possible applications, for example, in catalysis, in gas storage and separation, or as electronic components. Many of these application involve processes generating heat. For optimal large-scale implementation this heat needs to be dissipated effectively, which is why it is our objective to understand structurally dependent heat transfer processes in MOFs.

In this work, non-equilibrium molecular dynamics (NEMD) simulations were employed for an accurate prediction of the thermal conductivity and to analyze the flow of thermal energy in real space. Due to the associated high computational cost, it was necessary to develop computationally efficient force field potentials (FFPs) based on ab-initio reference data. The focus was on an accurate description of phonon properties in the respective materials, as those play a crucial role for thermal transport. As an initial approach, MOF-FF [1] type force fields were parametrized to model the atomistic interactions in a group of isoreticular metal-organic frameworks (IRMOFs). This allowed to identify the interface between node and linker as the primary heat transfer bottleneck in MOFs [2]. To develop structure-to-property relations for such systems, the structure of the base MOF was systematically varied, changing the nature of the metal ions, and changing the length and nature of the organic linkers [3].

A drawback of this approach is that developing MOF-FF type potentials is extremely costly regarding the use of human resources and for more complex MOFs also turned out to be not sufficiently accurate. To overcome these problems, recently developed machine-learning methodologies were explored to provide an easier approach for generating force-field potentials. This led to the creation of astonishingly accurate moment tensor potentials (MTPs) [4], which can be generated with minimal human input and only slightly increased computational costs. The MTPs were extensively tested based on the elastic and phonon properties of a set of the most commonly studied MOFs, yielding highly promising results. This will greatly increase the multitude of MOFs for which structurally-dependent heat transport properties can be estimated at close to ab-initio accuracy.

[1] S. Bureekaew, S. Amirjalayer, M. Tafipolsky, C. Spickermann, T. K. Roy, R. Schmid, Phys. Status Solidi Basic Res. 2013, 250, 1128.

[2] S. Wieser, T. Kamencek, J. P. Dürholt, R. Schmid, N. Bedoya-Martínez, E. Zojer, Adv. Theory Simulations 2021, 4, 2000211.

[3] S. Wieser, T. Kamencek, R. Schmid, N. Bedoya-Martínez, E. Zojer, Nanomaterials 2022, 12, 2142.

[4] I. S. Novikov, K. Gubaev, E. V Podryabinkin, A. V Shapeev, Mach. Learn. Sci. Technol. 2021, 2, 025002.


Poster session in connection with the FoE Advanced Materials Science Poster Day

Sven Kiefer: Clouds in 3D Exoplanet Atmospheres

Helena Lecoq Molinos: A study of vanadium oxides as nucleation candidates

Simon Fernbach: Reliable force field potential for thermal transport in AlN

Alexander Dallinger: Soft Actuators Based on Liquid Crystal Elastomer and Laser Induced Graphene

Nanna Bach-Møller: Aggregation and charging of mineral cloud particles under high-energy irradiation

Jayatee Kanwar: Hydrocarbon chemistry in inner regions of planet forming disks

Theodoros Dimitriadis: Capillary-driven water transport by contrast wettability-based durable surfaces

Adrian Kirchner: VUV Dual Comb Spectroscopy

Marcel Simhofer: Kinetics of the Hydrogenation Processes in Palladium studied by in situ Dilatometry

Lara Novak: Electrochemical detection of fluoride ions in water using nanoporous gold modified by a boronic acid terminated self-assembled monolayer