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The chemical selectivity and faradaic efficiency of high‐index Cu facets for the CO2 reduction reaction (CO2RR) is investigated. More specifically, shape‐controlled nanoparticles enclosed by Cu {hk0} facets are fabricated using Cu multilayer deposition at three distinct layer thicknesses on the surface facets of Au truncated ditetragonal nanoprisms (Au DTPs). Au DTPs are shapes enclosed by 12 high‐index {310} facets. Facet angle analysis confirms DTP geometry. Elemental mapping analysis shows Cu surface layers are uniformly distributed on the Au {310} facets of the DTPs. The 7 nm Au@Cu DTPs high‐index {hk0} facets exhibit a CH4 : CO product ratio of almost 10 : 1 compared to a 1 : 1 ratio for the reference 7 nm Au@Cu nanoparticles (NPs). Operando Fourier transform infrared spectroscopy spectra disclose reactive adsorbed *CO as the main intermediate, whereas CO stripping experiments reveal the high‐index facets enhance the *CO formation followed by rapid desorption or hydrogenation.

We set out a model of the stock market in which investors with heterogeneous beliefs update their type based on the past performance of neighbours in an arbitrary social network.We study how the network structure and the degree of agents’ attention to performance affect the coupled price-type dynamics. Types converge to a group-consensus characterised by network centrality if updating is purely social and to either the group’s most fundamental or most chartist type if updating is purely performance-based, with the time to convergence being finite and proportional to the network diameter. For intermediate cases, we identify two key mechanisms which can make group-consensus non-monotonic with respect to investors’attention to performance. These results shed light on when performance-based updating from social networks is stabilising – or destabilising – for asset prices. As an application, ourmodel can explain price bubbles and price oscillations by network-performance effects.

Solar cells incorporating organic inorganic metal halide perovskites as the absorber material have achieved power conversion efficiencies of more than 25% after only a decade of research. The extremely rapid improvement in efficiency of perovskite solar cells compared to pre-established absorber materials, such as silicon, cadmium telluride, or gallium arsenide, is mainly due to their cheap and easy low-temperature, solution-processing preparation techniques such as spin coating. To further improve the power conversion efficiency of perovskite solar cells, it is still necessary to develop a fundamental understanding of the device physics. The focus of this work lies, therefore, on investigating the energy band diagram of perovskite solar cells, both in the dark and under illumination at open circuit conditions, predominantly using photoelectron spectroscopy (PES). Two different architectures are investigated and compared: i) the classical architecture where the perovskite absorber is deposited onto the electron extraction layer and ii) the inverted architecture where the perovskite is deposited onto the hole extraction layer. Initial experiments showed that perovskite absorbers are extremely light-sensitive, meaning that even small intensities of background light, like the visible light emitted from the X ray source, can induce a photovoltage which will significantly affect the PES measurements by shifting all spectra to higher or lower binding energies. To shield the sample from the visible light emitted by the X ray source, the setup of the X ray photoelectron spectroscopy (XPS) system used in this work was improved by installing an aluminum window in between the sample and the X ray source. In the next step, a comparative study of several different perovskite absorbers on n type SnO2 (classical) and p type NiOx substrates (inverted architecture) was performed. It was proven that the underlying substrate has no effect on the doping level of the perovskite absorbers, as it has previously been proposed in the literature. The perovskite absorbers are always measured to be n doped. It is suggested that the literature reported substrate effect originates from background light during the measurement. This leads to an unnoticed photovoltage resulting in a binding energy shift of all spectra, which leads to the determination of incorrect doping levels. For both architectures, the majority of the photovoltage and hence the open-circuit voltage of the full device is identified at the n-type perovskite | p-type hole extraction layer interface. The interfaces between the perovskite and the respective hole extraction layer (classical: spiro MeOTAD and inverted: NiOx) were then investigated in detail. For the perovskite | spiro MeOTAD interface a classical step-by-step interface experiment was performed. Since spiro MeOTAD films used in perovskite solar cell devices are usually doped with LiTFSI, at first a vacuum deposition process of LiTFSI doped spiro MeOTAD through co-evaporation of both materials was developed. The interface characterization proved that a band bending occurs in the dark, which changes to a flat band situation under illumination, corresponding to a surface photovoltage. For the inverted architecture, the perovskite | NiOx interface was investigated using the tapered cross-section PES method, which demonstrated the presence of a band bending in the dark as well. Finally, the results from the photovoltage measurements and the detailed interface characterizations were combined to derive complete energy band diagrams for both architectures under dark and illuminated open-circuit conditions.

Galley proofs for the paper Carbon-Assisted Stable Silver Nanostructures

We study general properties of pairwise stable networks in homogeneous societies, i.e. when agents' utilities differ only with respect to their network position while their names do not matter. Rather than assuming a particular functional form of utility, we impose general link externality conditions on utility such as ordinal convexity and ordinal strategic complements. Depending on these rather weak notions of link externalities, we show that pairwise stable networks of various structure exist. For stronger versions of the convexity and strategic complements conditions, we are even able to characterize all pairwise stable networks: they are nested split graphs (NSG). We illustrate these results with many examples from the literature, including utility funtions that arise from games with strategic complements played on the network and utility funtions that depend on centrality measures such as Bonacich centrality.

Habilitationsschrift der Fakultät für Wirtschaftswissenschaften

Increasing concerns about climate change have given rise to the formation of International Environmental Agreements (IEAs) as a possible solution to limit global pollution effects. In this paper, we study the stability of IEAs in a repeated game framework where we restrict to strategies which are simple and invariant to renegotiation. Our main contribution to the literature on IEAs is that we allow for heterogeneous patterns of pollution such that additional to a global effect of pollution there are local pollution effects represented by a network structure. We show that stable IEAs exist if the network structure is balanced. Too large asymmetries in the degree of local spillovers may however lead to non-existence of stable structures. The generality of our approach allows for several applications to general problems in the provision of public goods.

We present a model of opinion formation where individuals repeatedly engage in discussion and update their opinion in a social network similarly to the DeGroot model. Abstracting from the standard assumption that individuals always report their opinion truthfully, agents in our model interact strategically in the discussion such that their stated opinion can di er from their true opinion. The incentive to do so is induced by agents' preferences for conformity. Highly conforming agents will state an opinion which is close to their neighbors' while agents with low level of conformity may be honest or even overstate their opinion. We model opinion formation as a dynamic process and identify conditions for convergence to consensus. Studying the consensus in detail, we show that an agent's social in uence on the consensus opinion is increasing in network centrality and decreasing in the level of conformity. Thus, lower conformity fosters opinion leadership. Moreover, assuming that the initial opinion is a noisy signal about some true state of the world, we consider the mean squared error of the consensus as an estimator for the true state of the world. We show that a society is \wise, i.e. the mean squared error is smaller, if players who are well informed are less conform, while uninformed players conform more with their neighbors.

This paper studies the properties of convexity (concavity) and strategic complements (substitutes) in network formation and the implications for the structure of pairwise stable networks. First, different definitions of convexity (concavity) in own links from the literature are put into the context of diminishing marginal utility of own links. Second, it is shown that there always exists a pairwise stable network as long as the utility function of each player satisfies convexity in own links and strategic complements. For network societies with a profile of utility functions satisfying concavity in own links and strategic complements, a local uniqueness property of pairwise stable networks is derived. The results do neither require any specification on the utility function nor any other additional assumptions such as homogeneity.

Since the seminal contribution of Jackson & Wolinsky 1996 [A Strategic Model of Social and Economic Networks, JET 71, 44-74] it has been widely acknowledged that the formation of social networks exhibits a general conflict between individual strategic behavior and collective outcome. What has not been studied systematically are the sources of inefficiency. We approach this omission by analyzing the role of positive and negative externalities of link formation. This yields general results that relate situations of positive externalities with stable networks that cannot be “too dense” in a well-defined sense, while situations with negative externalities tend to induce “too dense” networks.