Pronounced polarization-induced energy level shifts at boundaries of organic semiconductor nanostructures.

Organic semiconductor devices rely on the movement of charge at and near interfaces, making an understanding of energy level alignment at these boundaries an essential element of optimizing materials for electronic and optoelectronic applications. Here we employ low temperature scanning tunneling microscopy and spectroscopy to investigate a model system: two-dimensional nanostructures of the prototypical organic semiconductor, PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) adsorbed on NaCl (2 ML)/Ag(111). Pixel-by-pixel scanning tunneling spectroscopy allows mapping of occupied and unoccupied electronic states across these nanoislands with sub-molecular spatial resolution, revealing strong electronic differences between molecules at the edges and those in the centre, with energy level shifts of up to 400 meV. We attribute this to the change in electrostatic environment at the boundaries of clusters, namely via polarization of neighbouring molecules. The observation of these strong shifts illustrates a crucial issue: interfacial energy level alignment can differ substantially from the bulk electronic structure in organic materials.

Collision-like interactions between acoustic and electrical signals that produce startle reflexes in reticular formation sites.

A startle-like response can be evoked at low currents by one-pulse electrical stimulation of reticular formation sites from the rostrolateral pons to the caudomedial medulla. To test whether this response is mediated by the same reticular formation neurons as those that mediate the acoustic startle, we delivered a brief, subthreshold acoustic stimulus followed by an 0.1-ms electrical pulse to one side of the reticular formation of rats. The current thresholds for electrical startle were usually powerfully reduced (50-80%) whenever the acoustic stimulation was presented within 5 ms of the electrical pulse. This summation was, however, interrupted by brief (0.2-1.0 ms) spike-like increases in threshold when the electrical pulse was delivered 4.0-4.6 ms after the offset of the acoustic stimulus. The timing of the spike-like increase in threshold shifted to longer intervals in more caudal sites, consistent with the conduction of action potentials in the startle pathway. For example, the increase occurred at an interval of 4.1 ms near the ventral lateral lemniscus (VLL) and at intervals of 4.4-4.6 ms for sites in the pontine or medullary reticular formation. The increases in startle threshold are attributed to collisions between antidromic action potentials evoked by the electrical pulses and orthodromic action potentials evoked by the acoustic stimuli. These results suggest that the neurons in reticular formation that produce the acoustic startle reflex overlap greatly with the neurons that mediate electrically evoked startle-like responses. Also, the acoustic signals mediating the startle reflex must be, in large part, a synchronous volley of action potentials conducted by longitudinal bundles of reticular formation axons.