Modeling of Carbon based Nanostructures
Understanding and ultimately designing materials with nanostructured components by means of modelling and simulations, often requires a combination of established techniques at various scales to be linked into well defined scale-bridging models. Here, some of our recent work focussing on simulations of carbon-based nanostructures is presented along with accompanying experiments, if present. Areas of current interest are: Tunnel formation in graphite, for further nano-functionalization of graphitic materials; understanding of the polymer based Single Wall Carbon Nanotubes (SWCNTs) purification mechanisms; transition metal based decoration of graphite nano-flakes; alkali metal based intercalation of graphite bilayers; Boltzmann transport simulations of graphene based field emitting transistors (GFETs); etc.
Spin-Crossover and Massive Anisotropy Switching of 5d Transition Metal Atoms on Graphene Nanoflakes
In spin crossover phenomena, the magnetic moment of a molecule is switched by external means. Here we theoretically predict that several 5d-transition metals (TMs) adsorbed on finite graphene flakes undergo a spin crossover, resulting from multiple adsorption minima, that are absent in the zero-dimensional limit of benzene and the two-dimensional limit of graphene. The different spin states are stable at finite temperature and can be reversibly switched with an electric field. The system undergoes a change in magnetic anisotropy upon spin crossover, which facilitates read-out of the spin state. The TM-decorated nanoflakes thus act as fully controlled single-ion magnetic switches.
Beljakov, I.; Meded, V.; Symalla, F.; Fink, K.; Shallcross, S.; Ruben, M.; Wenzel W
Catalytic subsurface etching of nanoscale channels in graphite
Catalytic hydrogenation of graphite has recently attracted renewed attention as a route for nanopatterning of graphene and to produce graphene nanoribbons. These reports show that metallic nanoparticles etch the surface layers of graphite or graphene anisotropically along the crystallographic zig-zag ‹11–20› or armchair ‹10–10› directions. The etching direction can be influenced by external magnetic fields or the supporting substrate. Here we report the subsurface etching of highly oriented pyrolytic graphite by Ni nanoparticles, to form a network of tunnels, as seen by scanning electron microscopy and scanning tunnelling microscopy. In this new nanoporous form of graphite, the top layers bend inward on top of the tunnels, whereas their local density of states remains fundamentally unchanged. Engineered nanoporous tunnel networks in graphite allow for further chemical modification and may find applications in various fields and in fundamental science research.
Lukas M., Meded V. , Vijayaraghavan A., Song L., Ajayan P.M., Fink K. , Wenzel W., Krupke R.
Selective Dispersion of Single-Walled Carbon Nanotubes with Specific Chiral Indices by Poly(N- decyl-2,7-carbazole)
Physico-chemical methods to sort single-walled carbon nanotubes (SWNTs) by chiral index are presently lacking but are required for in-depth experimental analysis and also for potential future applications of specific species. Here we report the unexpected selectivity of poly(N-decyl-2,7-carbazole) to almost exclusively disperse semiconducting SWNTs with differences of their chiral indices (n − m) ≥ 2 in toluene. The observed selectivity complements perfectly the dispersing features of the fluorene analogue poly(9,9-dialkyl-2,7-fluorene), which disperses semiconducting SWNTs with (n − m) ≤ 2 in toluene. The dispersed samples are further purified by density gradient centrifugation and analyzed by photoluminescence excitation spectroscopy. All-atom molecular modeling with decamer model compounds of the polymers and (10,2) and (7,6) SWNTs suggests differences in the π−π stacking interaction as origin of the selectivity. We observe energetically favored complexes between the (10,2) SWNT and the carbazole decamer and between the (7,6) SWNT and the fluorene decamer, respectively. These findings demonstrate that subtle structural changes of polymers lead to selective solvation of different families of carbon nanotubes. Furthermore, chemical screening of closely related polymers may pave the way toward simple, low-cost, and index-specific isolation of SWNTs.
Lemasson F. A., Strunk T., Gerstel P., Hennrich F., Lebedkin S., Barner-Kowollik C., Wenzel W., Kappes M. M., Mayor M.