Optical spectrum of bottom-up graphene nanoribbons: towards efficient atom-thick excitonic solar cells.

Recently, atomically well-defined cove-shaped graphene nanoribbons have been obtained using bottom-up synthesis. These nanoribbons have an optical gap in the visible range of the spectrum which make them candidates for donor materials in photovoltaic devices. From the atomistic point of view, their electronic and optical properties are not clearly understood. Therefore, in this work we carry out ab-initio density functional theory calculations combine with many-body perturbation formalism to study their electronic and optical properties. Through the comparison with experimental measurements, we show that an accurate description of the nanoribbon's optical properties requires the inclusion of electron-hole correlation effects. The energy, binding energy and the corresponding excitonic transitions involved are analyzed. We found that in contrast to zigzag graphene nanoribbons, the excitonic peaks in the absorption spectrum are a consequence of a group of transitions involving the first and second conduction and valence bands. Finally, we estimate some relevant optical properties that strengthen the potential of these nanoribbons for acting as a donor materials in photovoltaic.

Effect of lateralized epileptic discharges on the thought flow.

PURPOSE: Pauses in verbal expression of thought flow in patients with partial epilepsy was studied in order to achieve a best comprehension of brain interhemispheric influences. METHOD: We studied thirty nine patients with partial epileptic seizures and twenty four volunteers (control group). Free association of ideas was used as a method to establish the thought flow, starting with a stimulus word (subject name) that must be followed by spontaneous and consecutive word-phrase. Patients were instructed to say any thought that came to their minds without any kind of censoring. Subject responses were recorded through a PC keyboard and time intervals (pauses) between each word-phrase were processed by a software developed for this purpose. RESULTS: Time intervals reduction among associations in right lesional group were compared to left lesional, right non-lesional and control groups. An increase of associative time intervals was observed in the right non-lesional group compared to left non-lesional group. CONCLUSIONS: We suggest a possible inhibitory inter-hemispheric function of the right hemisphere modulating verbal expression of the thought flow. Considering the hypothesis of the inter-hemispheric inhibition by the right hemisphere on left hemisphere, we admit that in epileptic patients with right hemispheric lesion there is an attenuation of the inter-hemispheric inhibition and an increase in thought flow, whereas in patients without a lesion but a paroxysmal epileptic activity in right temporal lobe there is an exacerbation of this inhibitory function and consequent reduction in the thought flow.