Real-space measurement of the potential distribution inside organic semiconductors.
ABSTRACT We demonstrate that the soft nature of organic semiconductors can be exploited to directly measure the potential distribution inside such an organic layer by scanning-tunneling microscope (STM) based spectroscopy. Keeping the STM feedback system active while reducing the tip-sample bias forces the tip to penetrate the organic layer. From an analysis of the injection and bulk transport processes it follows that the tip height versus bias trace obtained in this way directly reflects the potential distribution in the organic layer.
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ABSTRACT: Scanning tunneling microscopy (STM) has been used to study polydiacetylene (PDA) nanowires and their electronic coupling with the surface. PDA nanowires exhibit intriguing substrate-dependent electronic effects when probed at varying sample bias voltage conditions on different substrate electrode materials, in this case, highly ordered pyrolytic graphite (HOPG) and molybdenum disulfide (MoS(2)). An analysis of nanowire heights over a wide range of bias voltages shows strong polymer-substrate contact effects, the strength of which is reflected in the asymmetry of the height-voltage data on each substrate. On HOPG, PDA nanowires exhibit a decrease in height as the bias voltage magnitude is reduced, and the height is substantially greater at negative voltages than at positive voltages. On MoS(2), PDA nanowires appear with much higher contrast than on HOPG when imaged at the same negative bias conditions. At positive bias voltages on MoS(2), the nanowires are invisible in all STM images, yet the unpolymerized molecules can still be imaged. These effects are necessarily electronic in origin. Surprisingly, only the polymer nanowires exhibit any bias-dependent change; the unpolymerized molecules are imaged at all bias voltages on both substrates. Additionally, the substrate affects how the unpolymerized molecules are ordered. In some areas, the molecules are arranged such that part of the monolayer is offset from the correct threefold symmetry direction by a slight misfit angle. On HOPG, this misfit is approximately 6 degrees, while on MoS(2), it is approximately 11 degrees. Interactions with the substrate thus play a role both in electronic structure and in molecular alignment.ACS Nano 09/2008; 2(8):1571-80. · 12.03 Impact Factor
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ABSTRACT: The temperature dependence of the built-in voltage of organic semiconductor devices is studied. The results are interpreted using a simple analytical model for the band bending at the electrodes. It is based on the notion that, even at zero current, diffusion may cause a significant charge density in the entire device, and hence a temperature dependent band bending. Both magnitude and temperature dependence of the built-in potential of various devices are consistently described by the model, as the effects of a thin LiF layer between cathode and active layer.Applied Physics Letters 06/2006; · 3.79 Impact Factor
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ABSTRACT: A diodelike device has been fabricated by organic molecular beam deposition of pentacene on a surface-modified, single-crystalline Au substrate. Using the substrate as the first and the tip of a scanning tunneling microscope (STM) as the second electrode, transport characteristics of this organic semiconductor (OSC) device were investigated. The probed pentacene islands are single crystalline and defect-free and consist of few molecular layers only. The current-voltage characteristics of this device reveal a pronounced asymmetry. For negative polarity, the current characteristics is almost independent of the layer thickness. For positive polarity, the current onset is shifted significantly to larger voltages with increasing layer thickness. Numerical simulations for a two-dimensional model system allow us to identify the injection properties of the STM tip as reasons for this pronounced asymmetry. For negative substrate bias the creation of holes in the valence band occurs by tunneling of electrons to the tip whereas in the opposite case holes have to be transported through the OSC layer from the substrate. Thus, for low positive voltage the hole current limits the device current. Once the resulting voltage drop between layer and tip becomes larger than the barrier for electron injection, direct tunneling of electrons into the pentacene conduction band becomes possible and n conduction begins to dominate.Journal of Applied Physics 01/2007; 102:033708. · 2.21 Impact Factor