Scanning tunneling spectroscopy on organic semiconductors: Experiment and model

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.

Substrate-Mediated Intermolecular Hybridization in Binary Phthalocyanine Superstructures

“…Scanning tunneling spectroscopy was performed in constant-current distance-voltage mode in order to minimize the likelihood of tip-induced molecular motion or chemical reaction. [14][15][16] Theory. First-principles electronic structure calculations, based on density functional theory (DFT) approach, were performed with the Quantum ESPRESSO package, 17 using plane waves and ultrasoft pseudopotential 18 implementation.…”

Electrical Scanning Probe Microscopy on Active Organic Electronic Devices

“…[26,28] Although again attention must be paid to tip-sample contact forces, most of this disagreement can be resolved by accounting for the 3D tip-substrate geometry present during c-AFM measurements using numerical simulations, [30] as suggested by Kemerink and coworkers for STM experiments. [31] Using a combination of numerical calculation and experiment, we found that for a range of tip diameters and sample thicknesses, the current density could be expressed through a simple modification to the field-dependent form of the Mott-Gurney Law. [30] Using this formula to fit a range of c-AFM data with different tip diameters, sample thicknesses, and polymers, we were able to obtain reasonable agreement between mobilities measured with c-AFM and those measured in macroscopic diodes.…”

Electronic transport properties and orientation of individualMn12single-molecule magnets

“…This finding is discussed with respect to calculations on electronic transport through Mn 12 molecules. Furthermore, distancevoltage ͑z-U͒ spectra 28 obtained from individual Mn 12 molecules indicate a preferential orientation of the molecular easy axis approximately perpendicular to the surface. The results are further corroborated by macroassisted I-U spectroscopy and represent an important step toward future magnetic-field-dependent STS studies.…”

“…The step height can be used as an estimation of the real height of the molecules. 28 A small uncertainty arises from the fact that the tunneling distance between tip and Mn 12 -th as well as 4-MOBCA is slightly different due to the different electronic structure. 25 Nonetheless, the small spread of the measured height values indicates a preferential orientation of the molecules either with the easy axis perpendicular or parallel to the surface.…”

“…Although additional tunable parameters as well as details of the measurement geometry and the electronic structure of the molecules have to be included in future experiments and more advanced simulations, the order of magnitude of the E F -HOMO difference required to adjust the simulated to the experimental spectra is in good agreement with calculations on the electronic structure of Mn 12 molecules. 10,11,36 As demonstrated for different molecules, 28 the presence of a conductance gap in molecules can be used for distancevoltage spectroscopy. Here, we show that this is also valid for Mn 12 -th and that there is a signature of a preferred orientation of the molecular easy axis with respect to the surface.…”