Since the possibility to manipulate lattice impurity atoms using a focused electron beam in scanning transmission electron microscope was discovered in 2014 [1] and first experimentally demonstrated in graphene in 2017 [2,3], progress has been made to extend manipulation to different elements and even to other materials including single-walled carbon nanotubes and bulk silicon, as discussed in a recent review [4].However, despite advances in automation and machine-learning, the scaling up of the technique to build atomically precise patterns out of multiple impurity atoms has proven challenging.Electron-beam manipulation is challenging for two reasons: the same focused electron beam must be used to both manipulate and visualize the structures, and energy transfer from the beam electrons to the target atoms is stochastic.Together these mean that it is not possible to prevent unwanted interactions whereby an impurity moves in an undesired direction [2, 3], or worse yet, a carbon atom is knocked out instead of desired movement [1].However, although the interaction does not appear to be fully elastic [5], tuning the beam energy is effective in increasing the number of successful manipulations [6].The greatest initial challenge was the fabrication of samples with a high local concentration of impurities in an otherwise undamaged lattice, but techniques such as vacancy-mediated substitution of silicon adatoms [3,7] is able to produce highly suitable samples with more than a dozen potentially manipulable impurities in close proximity (Fig. 1a).However, attempts to manipulate these led to a surprising and disappointing result: most impurity atoms were quickly replaced by carbon (Fig. 1b), preventing the creation of multi-atom structures.This so-called carbon replacement process was first observed with germanium [8], and was particularly problematic for phosphorus where its probability was measured to be on par with other interactions [9].The most common silicon impurities were at first not thought to be as susceptible to the same problem, but it turned out that the cleaner the samples, the more likely it becameprecisely the issue mentioned above and shown in Fig. 1.We note that such data of failed experiments are usually not published.Although the atomistic details of this replacement mechanism are still murky, the process cannot be purely thermally activated.In our own experiments, phosphorus-and silicon-doped graphene samples have been heated to temperatures of up to 1000 °C for tens of cumulative hours, but lattice impurities can still be easily found, placing stringent constraints on possible related reaction barriers and suggesting that electron irradiation must play a pivotal role.Recent studies of the radiation stability of graphene at elevated temperatures have shed light on the source of the carbon atoms: these appear to be thermally diffusing adatoms whose migration barrier was estimated to be 0.33 eV [10].It does not seem possible to prevent this process by heating the samples, and although the source of the adatoms is unclear, they seem to be ubiquitous.However, considering the value of the migration barrier, experiments with ultra-stable cryogenic stages could be expected to provide a solution, as recently proposed [4].A second challenge to be tackled before the technique can be scaled up is sample drift.Although active drift compensation can be enabled by small subscan windows [3], we believe this is not ideal as it leads to a significant part of the irradiation dose missing the target atom and even triggering unwanted processes.Further, machine learning structure recognition we use for automation cannot easily keep track of drift.Active drift compensation with a parked electron beam can instead be enabled by real-time monitoring of the converged-beam electron diffraction pattern, whose center-of-mass displacement results from the real-space displacement of the beam.This can be compensated via opposing beam shifts, keeping the irradiation focused on the target atom, detecting atom jumps, and correcting the drift for the following scan frames.Image simulations and work on practical implementation will be discussed [11].

Understanding electron irradiation effects is vital not only for reliable transmission electron microscopy characterization, but increasingly also for the controlled manipulation of 2D materials. The displacement cross sections of monolayer hexagonal boron nitride (hBN) are measured using aberration-corrected scanning transmission electron microscopy in near ultra-high vacuum at primary beam energies between 50 and 90 keV. Damage rates below 80 keV are up to three orders of magnitude lower than previously measured at edges under poorer residual vacuum conditions, where chemical etching appears to dominate. Notably, it is possible to create single vacancies in hBN using electron irradiation, with boron almost twice as likely as nitrogen to be ejected below 80 keV. Moreover, any damage at such low energies cannot be explained by elastic knock-on, even when accounting for the vibrations of the atoms. A theoretical description is developed to account for the lowering of the displacement threshold due to valence ionization resulting from inelastic scattering of probe electrons, modeled using charge-constrained density functional theory molecular dynamics. Although significant reductions are found depending on the constrained charge, quantitative predictions for realistic ionization states are currently not possible. Nonetheless, there is potential for defect-engineering of hBN at the level of single vacancies using electron irradiation.

Although surface diffusion is critical for many physical and chemical processes, including the epitaxial growth of crystals and heterogeneous catalysis, it is particularly challenging to directly study. Here, we estimate the carbon adatom migration barrier on freestanding monolayer graphene by quantifying its temperature-dependent electron knock-on damage. Due to the fast healing of vacancies by diffusing adatoms, the damage rate decreases with increasing temperature. By analyzing the observed damage rates at 300–1073 K using a model describing our finite scanning probe, we find a barrier of (0.33 ± 0.03) eV.

Journal Article Adventures in Atomic Resolution in situ STEM Get access Andreas Postl, Andreas Postl University of Vienna, Faculty of Physics, Vienna, Austria Search for other works by this author on: Oxford Academic Google Scholar Thuy An Bui, Thuy An Bui University of Vienna, Faculty of Physics, Vienna, Austria Search for other works by this author on: Oxford Academic Google Scholar Fabian Kraft, Fabian Kraft University of Vienna, Faculty of Physics, Vienna, Austria Search for other works by this author on: Oxford Academic Google Scholar Alexandru Chirita, Alexandru Chirita University of Vienna, Faculty of Physics, Vienna, Austria Search for other works by this author on: Oxford Academic Google Scholar Gregor Leuthner, Gregor Leuthner University of Vienna, Faculty of Physics, Vienna, Austria Search for other works by this author on: Oxford Academic Google Scholar Heena Inani, Heena Inani University of Vienna, Faculty of Physics, Vienna, Austria Search for other works by this author on: Oxford Academic Google Scholar Clemens Mangler, Clemens Mangler University of Vienna, Faculty of Physics, Vienna, Austria Search for other works by this author on: Oxford Academic Google Scholar Kimmo Mustonen, Kimmo Mustonen University of Vienna, Faculty of Physics, Vienna, Austria Search for other works by this author on: Oxford Academic Google Scholar Jani Kotakoski, Jani Kotakoski University of Vienna, Faculty of Physics, Vienna, Austria Search for other works by this author on: Oxford Academic Google Scholar Toma Susi Toma Susi University of Vienna, Faculty of Physics, Vienna, Austria Corresponding author: toma.susi@univie.ac.at Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 28, Issue S1, 1 August 2022, Pages 2342–2343, https://doi.org/10.1017/S1431927622008996 Published: 01 August 2022

Substituting heteroatoms into graphene can tune its properties for applications ranging from catalysis to spintronics. The further recent discovery that covalent impurities in graphene can be manipulated at atomic precision using a focused electron beam may open avenues towards sub-nanometer device architectures. However, the preparation of clean samples with a high density of dopants is still very challenging. Here, we report vacancy-mediated substitution of aluminium into laser-cleaned graphene, and without removal from our ultra-high vacuum apparatus, study their dynamics under 60 keV electron irradiation using aberration-corrected scanning transmission electron microscopy and spectroscopy. Three- and four-coordinated Al sites are identified, showing excellent agreement with ab initio predictions including binding energies and electron energy-loss spectrum simulations. We show that the direct exchange of carbon and aluminium atoms predicted earlier occurs under electron irradiation, although unexpectedly it is less probable than the same process for silicon. We also observe a previously unknown nitrogen-aluminium exchange that occurs at Al─N double-dopant sites at graphene divacancies created by our plasma treatment.

Journal Article Automated Real-time Analysis of Atomic-resolution STEM Images Get access Jacob Madsen, Jacob Madsen Faculty of Physics, University of Vienna, Vienna, Austria Corresponding author: jacob.madsen@univie.ac.at Search for other works by this author on: Oxford Academic Google Scholar Andreas Postl, Andreas Postl Faculty of Physics, University of Vienna, Vienna, Austria Search for other works by this author on: Oxford Academic Google Scholar Toma Susi Toma Susi Faculty of Physics, University of Vienna, Vienna, Austria Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 25, Issue S2, 1 August 2019, Pages 166–167, https://doi.org/10.1017/S1431927619001569 Published: 01 August 2019

This work will show the coupling of a 6 kW el SOFC (Solid Oxide Fuel Cell) system developed by AVL using a stack module from Fraunhofer IKTS with a 5 kW cold absorption chiller developed by Graz University of Technology. The exhaust gas heat from the SOFC which is available at >200 °C will be used to operate the thermally driven absorption chiller. The SOFCs fuel flexibility was extended from gaseous fuels towards the utilization of liquid fuels such as conventional and synthetic diesel to address also transport applications. Besides a high electrical efficiency of 60 %, the option to generate cooling power in addition to heat is very attractive as it will enable new fields of applications in the transport sector such as conditioned freight transport and shipping containers or serving hotel loads of ships and trains which require simultaneous generation of electricity, heat and cooling capacity. An analysis of the system configuration and its dependency on various application scenarios is shown. Finally, the build-up of the prototype including test results will be shown in this work. % can be achieved. The SOFC system as well as the absorption chiller were built and tested. The SOFC shows a good steam reforming performance regarding the reformate composition required at the inlet of the SOFC. The integration of new balance of plant components such as the reformer, Diesel start-up burner and evaporator were successfully demonstrated.

This paper presents the current development status of AVLs stationary SOFC CCHP (combined cooling, heat and power) platform with special focus on the coupling of an SOFC with an absorption heat pump. Furthermore, the SOFCs fuel capability was extended towards the utilization of multiple renewable fuels such as biogas, biomass product gas and synthetic diesel. In the past AVL developed a natural gas operated SOFC CHP platform in the power range of 5 – 10 kWel with an electrical efficiency of >55 % using the Plansee/IKTS stack technology. For certain applications such as buildings, a heat driven operation mode leads to low operating hours per year for conventional CHP systems due to the low heat demand during summer season. The option to generate cooling power in addition to heat will increase the annual operating hours per year and thus economic efficiency significantly. This work will show the coupling of a stationary 6 kWel SOFC CHP system developed by AVL using an IKTS stack module with a 5 kWcooling absorption heat pump developed by TU Graz, Institute of Thermal Engineering. The exhaust gas heat from the SOFC which is available at >200 °C will be used to operate the thermally driven heat pump. The SOFC exhaust gas heat is transferred in the generator of the absorption heat pump to be converted to cooling power. To maximize the cooling power output of the SOFC CCHP system it is important to utilize as much SOFC exhaust gas heat as possible in the absorption heat pumping process. Furthermore, it is important to optimize the energy efficiency ratio of the absorption heat pump. In this context the influence of the cold and cooling water temperatures of the absorption heat pumping process on the system size and system total efficiency were investigated. It can be shown that for cold and cooling water temperatures as required in office buildings and hospitals a ratio between electrical power and cooling power of 4 - 6 can be achieved by the SOFC CCHP system. This analysis presents the capability of the SOFC CCHP system for a flexible and demand-oriented generation of electricity, heat and cooling power. As the electrical efficiency of the SOFC determines the exhaust gas heat output the system performance was further investigated towards the operation of renewable fuels. Biogas or synthetic diesel for instance may lead to lower electrical efficiencies compared to natural gas. Therefore, the type of fuel also influences the SOFC exhaust mass flow and temperature which is the main driving power for the absorption heat pump. In this work the fuel capability of the platform was extended from natural gas to renewable fuels. From economic point of view it is desired to benefit from volume effects for stacks and balance of plant components which is why AVLs system development approach is oriented towards a multi-fuel platform which requires only minor design adaptions when it comes to the utilization of different fuels including renewables. As a technology provider this approach is certainly a major advantage for customers who are used to a power generation technology that can be easily applied in various regions all over the world while facing varying fuel specifications. Therefore, this work will provide an overview of the most relevant renewable fuels and its composition for stationary SOFC CHP applications. Consequently, design synergies regarding the process design of the hot anode gas recirculation loop in terms of the recirculation ratio and reforming temperature which are influenced by the specific fuel, are shown. A comparison of system performance results based on process simulations will be discussed. The process simulation uses results from specific reforming tests for e.g. biogas and diesel which will be shown. Furthermore, compared to natural gas based systems a considerably higher effort has to be done in terms of the gas cleaning for biogas and biomass product gas. In case of biogas the main impurities are S, Cl, NH3 which need to be eliminated before sending the fuel to the steam reformer and stack respectively. A low cost gas cleaning concept for biogas based on several low temperature adsorption steps was developed by TU Graz. It can be shown that the absorption efficiency is highly depending on the fuel mixture. The influence of CH4, CO2 and H2O was identified to allow a proper design of the gas cleaning unit. Finally, the build-up of the prototype including first test results will be shown in this work.