Semester and Master theses

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As of February 2016 our group is no longer active, as Prof. Batlogg has been transitioned into emeritus status. Therefore we can no longer accept master or semester students.

Examples of Semester and Master theses in our lab

(Summaries written by the students)

OFETs with [EMIM][TFSI] as gate dielectric

In this semester thesis the behavior of [EMIM][TFSI] as gate dielectric for organic field effect transistors is tested. For this purpose various devices with rubrene and TCNQ as semiconductors are considered. Linear mobilities up to 3 cm^2/Vs and saturated mobilities even up to 13 cm^2/Vs are measured for rubrene devices with bottom contact and top gate.

Furthermore, temperature’s influence on the transistor’s performance and on the capacitance of [EMIM][TFSI] is investigated.

Thermal expansion near phase transitions in NaxCoO2

NaxCoO2 has emerged to be a promising material for thermoelectric energy conversion. The discovery of superconductivity in hydrated Na0.3CoO2 makes the study of the NaxCoO2 system to be an active area of research. A new capacitive dilatometer makes it possible to investigate the thermal expansion and the magnetostriction of crystals at high accuracy.

This work is focused at the completion and calibration of this new dilatometer and its application to the study of NaxCoO2 with xnominal = 0.85. Phase diagrams below 30 K were measured for both crystal directions, and an investigation of the Na rearrangements near 200 K and at 285 K was performed. The data are in agreement with previous observations of phase transitions at 8 K and 22 K and spin- op near 8 Tesla using various other methods. The recently discovered Na rearrangement near 200 K may have shown up as a strange behavior of the magnetostriction at 230 K - 240 K. It was also observed that the temperature of the Na rearrangement at 285 K depends on the magnetic field if the expansion measurement is started at 150 K initial temperature, while no dependence was seen when it started at 270 K.

Pentacene thin film transistors with different gate dielectrics on a  flexible substrate

Flexible organic thin film transistors in bottom-gate geometry were fabricated with two different gate dielectrics: the inorganic SiNx and the organic parylene N. Kapton-based substrates with pre-deposited unstructured Cr gate and SiNx coating were provided by the ETH Institute of Electronics. Pentacene active layer and Au (Cr) top contacts were deposited via physical vapor deposition.

The characteristics of the produced transistors were recorded and found to be widely spread: Mobilities ranged from 0.01 to 0.2 cm^2/Vs , threshold voltages from -60 to -10 V and on/off ratios from 10^5 to 10^8. Some of the spread was linked to the different dielectrics, but other factors may have played significant roles, in particular, the uneven surface of the substrate. In a last step, the transistors were bent to well-defined radii and device performance was recorded as a function of bending.

Difference in conductivity due to SAM formation of two types of organosilane at rubrene single crystal's surface

In this semester work the growth of two different organosilane selfassembled monolayers (SAM), C8H4Cl3F13Si resp. C10H4Cl3F17Si, at the surface of rubrene crystals has been investigated. During the SAM growth process a drastic increase of the rubrene crystal's surface conductivity (approximately two orders of magnitude higher than the rubrene's initial conductivity) has been observed. Furthermore, the two SAMs have been tested against each other whereas a considerable difference in conductivity (9.5 ± 1.3) · 10−8 Ohm−1 could be found.

Modelling and Parameter Extraction on Pentacene Organic Thin Film Transistors (PEN-OTFTs)

The focus of this thesis are Organic Thin Film Transistors (OTFTs). Devices in an inverted top contact staggered geometry with Pentacene (PEN) as organic active material with and without additional charge transfer layers at the gate and contact interfaces have been fabricated using thermal evaporation on Si-wavers with thermally grown oxide as gate dielectric and their electronic charactersitics have been measured. Knowledge about the conduction mechanism(s) in PEN in particular and disordered organic and anorganic semiconductors in general has been compiled to achieve a good overview. Furthermore extraction methods have been compiled, adapted and developed to extract relevant parameters of the electrical behavior and a modeling framework has been developed focusing on band-like transport of charge carriers allowing for the input of an arbitrary density of states (DOS).

Widerstandsmessungen an NaxCoO2

Die Semesterarbeit fand im Rahmen der Erforschung des Materialverbundes NaxCoO2 statt. Im Vordergrund standen Widerstandsmessungen, die an diesem Material bei unterschiedlichen Na Konzentrationen x und teilweiser Zuführung kleiner Konzentrationen des vierten Elements Ca durchgeführt wurden. Die dabei entstandenen Unterschiede waren deutlich. Dies ist insofern verblüffend, da doch die Konzentration des Kalziums nur 1/15 Teil der Natriumkonzentration betrug. Mit dem Kalzium stellten wir eine Energielücke im Kristall fest, die wir aber leider nicht genau zu verstehen wussten. In den anderen Messungen, mit einer Natriumkonzentration von x = 0.85, konnten wir in Vereinbarung mit anderen gemachten Beobachtungen eine Phasenübergang in der Umgebung von 22 K und 338 K sehen. Um verbesserte Resultate bei ganz kleinen Widerständen (kleiner als 10-3 Ohm) zu erhalten, versuchten wir die Messungen an den Proben mit einem Lock-In durchzuführen.

Organic charge-transfer interfaces between TTF and TCNQ

Goal of my first master project was to fabricate and investigate the properties of TTF-TCNQ organic charge-transfer interfaces. First, different methods for growing single crystals of both compounds are explored. It is shown that high quality single crystals can be grown from different solutions, by physical vapor transport method as well as by sublimation on the hot plate. Furthermore, techniques to confine the crystals shape are explored. It is shown, that the shape of TTF crystals can be changed from needle like to thin lamella platelets. In a second part, different techniques to fabricate crystal interfaces are investigated ranging form mechanical lamination to direct growth of crystals on top of each other.

The possibility to grow TTF single crystals directly on top of TCNQ single crystals by physical vapor transport method as well as in hexane solution is demonstrated. Finally, interfaces are characterized by temperature and growth dependent C-V measurements. The metallicity of the TTF-TCNQ interface as reported in literature could not been supported so far. However, conductivity increases of several orders of magnitudes were observed for compound interfaces compared to the properties of the single compounds.

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Sun Jul 23 08:52:08 CEST 2017
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