Nitrogen-doped carbon nanotubes under electron irradiation simulated with a tight-binding model

Experiments show that nitrogen-doped carbon nanotubes subjected to the electron beam in a transmission electron microscope can easily lose dopant atoms and that overall they are less stable under electron irradiation than the pristine tubes. To understand the lower stability of nitrogen-doped nanotubes we use a density-functional-theory-based tight-binding model and simulate impacts of energetic electrons onto the nanotubes. We show that the dopant atom displacement energy and thus the electron threshold energy is lower for nanotubes with smaller diameter and that, independent of the nanotube diameter, the dopant nitrogen atoms can be displaced more easily than the host carbon atoms. Our results set a limit on the threshold electron energy for damage production in N-doped tubes and indicate that spatially localized electron irradiation of doped nanotubes can be used for local atomic and band structure engineering.Peer reviewe

Measurements of the time-dependent specific heat of amorphous materials

The time dependence of the specific heat of several amorphous materials, such as a-SiO2, a-As2S3, and a-As, has been studied in the temperature range 0.1-1 K. It is found that below 0.3 K in all the materials studied the short-time specific heat at about 10 μsec is considerably smaller than the long-time specific heat, but larger than the Debye value. Above 0.3 K most of the specific heat is coupled to the phonons already at 10 μsec. However, measurements at very long time scales reveal that the specific heat has a component with a logarithmic time dependence, as proposed by the tunneling model, although only part of the total specific heat can be ascribed to it

Enhanced Volatile Organic Compounds emissions and organic aerosol mass increase the oligomer content of atmospheric aerosols

Secondary organic aerosol (SOA) accounts for a dominant fraction of the submicron atmospheric particle mass, but knowledge of the formation, composition and climate effects of SOA is incomplete and limits our understanding of overall aerosol effects in the atmosphere. Organic oligomers were discovered as dominant components in SOA over a decade ago in laboratory experiments and have since been proposed to play a dominant role in many aerosol processes. However, it remains unclear whether oligomers are relevant under ambient atmospheric conditions because they are often not clearly observed in field samples. Here we resolve this long-standing discrepancy by showing that elevated SOA mass is one of the key drivers of oligomer formation in the ambient atmosphere and laboratory experiments. We show for the first time that a specific organic compound class in aerosols, oligomers, is strongly correlated with cloud condensation nuclei (CCN) activities of SOA particles. These findings might have important implications for future climate scenarios where increased temperatures cause higher biogenic volatile organic compound (VOC) emissions, which in turn lead to higher SOA mass formation and significant changes in SOA composition. Such processes would need to be considered in climate models for a realistic representation of future aerosol-climate-biosphere feedbacks.Research at the University of Cambridge was supported by a Marie Curie Intra-European fellowship (project no. 254319) and the ERC grant no. 279405. We thank the SAPHIR and TNA2012 team in Jülich for supporting our measurements and the support by EUROCHAMP2 contract no. 228335. The field-work was funded by ERC grant 227463 and the Academy of Finland Centre of Excellence (grants 1118615 and 272041) and by the Office of Science (BER), US Department of Energy via Biogenic Aerosols - Effects on Clouds and Climate (BAECC). European Union’s Horizon 2020 research and innovation programme under grant agreement no. 654109 and previously from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 262254. We thank the Met Office for use of the NAME model. S.C. thanks the UK Natural Environment Research Council for her studentship

The time dependent specific heat of dielectric glasses

Measurements on the time dependence of the specific heat exist now both at short and long timescales. Below 0.3 K the short time (~10 µs) specific heat of all the materials studied is smaller than the long time specific heat, but larger than the Debye value. Above 0.2 K most of the specific heat is coupled to the phonons already at 10 µs. However, measurements at very long timescales reveal that the specific heat has a component with a logarithmic time dependence, as proposed by the tunneling model, although only part of the total specific heat can be ascribed to it