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.
[Show abstract][Hide abstract] ABSTRACT: In molecular and organic electronic systems, the electrode material has considerable influence on the performance of the resulting device. The advent of scanning tunneling microscopy (STM) and its various spectroscopic extensions has allowed for exploration of the polymer-electrode interface with atomic-scale resolution. Here I present the use of STM to analyze such systems. Specifically, STM and microwave-frequency alternating current STM (ACSTM) of conducting polymers on different substrates can shed new light on how the electrode material electronically affects the adhered polymer structure. The polymers used in this work are polydiacetylene nanowires and poly(3-hexylthiophene) (P3HT) monolayer, bilayer, and thin films. For both polymers, we can use the convolution of electronic and topographic information inherent in STM topography images to extrapolate information about the electronic structure. It is also possible to acquire information about the work function, the density of states (DOS), relative energy level positions, and the differential capacitance via spectroscopic measurements. In particular, capacitance imaging requires a novel technique known as ACSTM that can be used to probe relative carrier concentration. This thesis presents analyses of PDA and P3HT on graphite and molybdenum disulfide. For P3HT thin films, gold and platinum substrates are also studied. The results indicate a strong substrate-dependent charge transfer that is further illuminated through ACSTM and other spectroscopic investigations. In this work, preliminary investigations of photovoltaic P3HT:fullerene films are also discussed.
[Show abstract][Hide abstract] ABSTRACT: A molecular wire candidate, the polydiacetylene chain, fabricated in a substantial support layer of monomers self-assembled on a highly ordered pyrolytic graphite surface, was studied using scanning tunneling microscopy and spectroscopy. The density of states of individual polymers and constituent monomers were observed on the same surface, and then compared with the calculated results. The spectrum delineating the density of states of the polydiacetylene wire clearly reveals the theoretically predicted pi-band and band edge singularities of the one-dimensional polymer.
[Show abstract][Hide abstract] ABSTRACT: Scanning-tunneling spectroscopy experiments performed on conjugated polymer films are compared with three-dimensional numerical model calculations for charge injection and transport. It is found that if a sufficiently sharp tip is used, the field enhancement near the tip apex leads to a significant increase in the injected current, which can amount to more than an order of magnitude and can even change the polarity of the predominant charge carrier. We show that when charge injection from the tip into the organic material predominates, it is possible to probe the electronic properties of the interface between the organic material and a metallic electrode directly by means of tip height versus bias voltage measurements. Thus, one can determine the alignment of the molecular orbital energy levels at the buried interface, as well as the single-particle band gap of the organic material. By comparing the single-particle energy gap and the optical absorption threshold, it is possible to obtain an estimate of the exciton binding energy. In addition, our calculations show that by using a one-dimensional model, reasonable parameters can only be extracted from z-V and I-V curves if the tip apex radius is much larger than the tip height. In all other cases, the full three-dimensional problem needs to be considered.
Physical Review B 07/2004; 70(4). DOI:10.1103/PhysRevB.70.045202 · 3.74 Impact Factor
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