Adhesion mechanisms of the contact interface of TiO2 nanoparticles in films and aggregates

Fundamental knowledge about the mechanisms of adhesion between oxide particles with diameters of few nanometers is impeded by the difficulties associated with direct measurements of contact forces at such a small size scale. Here we develop a strategy based on AFM force spectroscopy combined with all-atom molecular dynamics simulations to quantify and explain the nature of the contact forces between 10 nm small TiO2 nanoparticles. The method is based on the statistical analysis of the force peaks measured in repeated approaching/retracting loops of an AFM cantilever into a film of nanoparticle agglomerates and relies on the in-situ imaging of the film stretching behavior in an AFM/TEM setup. Sliding and rolling events first lead to local rearrangements in the film structure when subjected to tensile load, prior to its final rupture caused by the reversible detaching of individual nanoparticles. The associated contact force of about 2.5 nN is in quantitative agreement with the results of molecular dynamics simulations of the particle–particle detachment. We reveal that the contact forces are dominated by the structure of water layers adsorbed on the particles’ surfaces at ambient conditions. This leads to nonmonotonous force–displacement curves that can be explained only in part by classical capillary effects and highlights the importance of considering explicitly the molecular nature of the adsorbates

Cyberspace, Blockchain, Governance:How Technology Implies Normative Power and Regulation

Technologies and their inherent design choices create normative structures that affect governance. This chapter aims to illustrate how blockchain technology in particular introduces new norms into a legal framework. We first analyze the different forms of governance by distinguishing between old and new governance. With a view to code that functions as legal norms, Blockchain technology is particularly suited to create governance structures and mechanisms. However, one needs to be aware of the norms that are implicitly introduced into the legal system by a specific blockchain technology. We look at the blockchain technology that underlies cryptocurrencies such as Bitcoin. This blockchain introduces a decentralized, transparent, cryptographically locked and thus immutable shared ledger. In summary, these design choices have normative powers over the user and over user interaction. If this is indeed the case, then regulators have to actively assess newly introduced digital ledger technology and other technologies for their effect on the normative and legal system.</p

Dissociation of O2 molecules on strained Pb(111) surfaces

By performing first-principles molecular dynamics calculations, we systematically simulate the adsorption behavior of oxygen molecules on the clean and strained Pb(111) surfaces. The obtained molecular adsorption precursor state, and the activated dissociation process for oxygen molecules on the clean Pb surface are in good agreements with our previous static calculations, and perfectly explains previous experimental observations [Proc. Natl. Acad. Sci. U.S.A. 104, 9204 (2007)]. In addition, we also study the influences of surface strain on the dissociation behaviors of O2 molecules. It is found that on the compressed Pb(111) surfaces with a strain value of larger than 0.02, O2 molecules will not dissociate at all. And on the stretched Pb(111) surfaces, O2 molecules become easier to approach, and the adsorption energy of the dissociated oxygen atoms is larger than that on the clean Pb surface

Growth of platinum clusters via addition of Pt (II) complexes: a first principles investigation

The growth of platinum clusters by the addition of Pt(II) complexes to clusters with an average oxidation state higher than zero is investigated by means of first principles molecular dynamics. In the simulations, PtCl2(H2O)(2) complexes react with Pt-12, Pt12Cl4, and Pt13Cl6 clusters with no need for reducing electrons. The analysis of the electronic density of states of the growing clusters shows that immediately after the adsorption of the Pt(II) complex on the cluster surface the electronic states of the complex extend over the whole cluster. In a process that gradually increases the mean coordination number of the atoms within the cluster, the adsorbed Pt atom is progressively incorporated into the cluster structure and quickly becomes indistinguishable from the other Pt atoms. Considerable rearrangement of the cluster structure and redistribution of the chlorine ligands on the cluster surface characterize the observed reactions. These results are fully consistent with the autoaccelerating kinetics observed in cluster growth experiments and support a face-capping growth mechanism that can account for shape-controlled cluster fabrication

Piezo-spectroscopic stress measurement near PZT-microstructures on silicon

Domain formation and polarization in ferroelectric films probably are strongly influenced by intrinsic stress. Raman microscopy offers the possibility to get informations about the stress state even in film structures with lateral dimensions of several ten microns. PZT (Pb[Zr,Ti]O3) microstructures with Pt bottom electrodes sputtered on silicon wafers were investigated using the Raman peak of the single crystalline silicon. Near edges of Pt and PZT films Raman shifts |\u394\u3bd| 641,5 cm-1 were measured corresponding to stresses in the silicon substrate near the interface in the order of several hundred MPa. The Raman shift profiles are dependent on the particular geometry of the investigated structures and specific micro structure defects near the edge. An estimate of the stress in the PZT film was obtained by modelling the stress in the silicon as a function of the distance from one edge using a finite element code

Molecular Dynamics Simulations of the Silica-Cell Membrane Interaction: Insights on Biomineralization and Nanotoxicity

The interaction of silica (SiO2) with biological systems is complex and contradictory. On the one hand, silica is at the basis of several biomineralization processes (e.g., in sponges). On the other hand, silica nanoparticles and dust may lead to silicosis and, at the cellular level, hemolysis. These toxic responses are strongly dependent on the silica polymorph and their root causes are still under debate. Both silica biomineralization and silica-induced nanotoxicity could be related to similar mechanisms of molecular recognition between the cellular membranes and the surface of the SiO2 particles. On the basis of this hypothesis, we employed classical molecular dynamics simulations, coupled to advanced sampling techniques, to achieve an atomistic picture of the interactions between different types of silica nanoparticles and the membrane of erythrocytes. Our predicted free-energy profiles associated with membrane crossing give no evidence for segregation of nanoparticles at the membrane/water interface, irrespective of their Si nuclearity, structure, and charge. The associated molecular trajectories, however, are suggestive of a possible direct translocation mechanism, in which silica nanoclusters elicit both local and large-scale effects on the membrane dynamics and stability. This gives hints on possible pathways for silica nanotoxicity based on nanoparticle-induced membrane perforation

Nucleation of platinum clusters on biopolymers: a first principles study of the molecular mechanism

5nonenoneL.C.CIACCHI; M.MERTIG; R.SEIDEL; W.POMPE; A. DE VITAL. C., Ciacchi; M., Mertig; R., Seidel; W., Pompe; DE VITA, Alessandr

Tidying up the conformational ensemble of a disordered peptide by computational prediction of spectroscopic fingerprints

The most advanced structure prediction methods are powerless in exploring the conformational ensemble of disordered peptides and proteins and for this reason the "protein folding problem" remains unsolved. We present a novel methodology that enables the accurate prediction of spectroscopic fingerprints (circular dichroism, infrared, Raman, and Raman optical activity), and by this allows for "tidying up" the conformational ensembles of disordered peptides and disordered regions in proteins. This concept is elaborated for and applied to a dodecapeptide, whose spectroscopic fingerprint is measured and theoretically predicted by means of enhanced-sampling molecular dynamics coupled with quantum mechanical calculations. Following this approach, we demonstrate that peptides lacking a clear propensity for ordered secondary-structure motifs are not randomly, but only conditionally disordered. This means that their conformational landscape, or phase-space, can be well represented by a basis-set of conformers including about 10 to 100 structures. The implications of this finding have profound consequences both for the interpretation of experimental electronic and vibrational spectral features of peptides in solution and for the theoretical prediction of these features using accurate and computationally expensive techniques. The here-derived methods and conclusions are expected to fundamentally impact the rationalization of so-far elusive structure-spectra relationships for disordered peptides and proteins, towards improved and versatile structure prediction methods