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Nano-Scale Lithography and Microscopy of Organic Semiconductors

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Nano-Scale Lithography and Microscopy of Organic Semiconductors Nano-scale Lithography and Microscopy of Organic Semiconductors Dan Credgington University College London, 2010 A dissertation submitted for the degree of Doctor of Philosophy Acknowledgements Over the (long!) years of this degree, I‟ve been very fortunate to work with some wonderful and talented people; in particular I‟d like to thank Lisa, Gustaf, Mar, Laurent and all the various denizens of the LCN for providing not only a stimulating scientific environment, but also some light relief from the dark of the SNOM lab. I especially want to thank Olly, for spending so much time in it with me, and keeping me sane throughout. My biggest thanks go to Franco, for his experience, support, ideas and patience, not to mention the many excuses he found to fund my trips to conferences, workshops and schools. Finally I‟d like to thank my parents, and especially Claire, for supporting me unwaveringly for so long, and pushing me to finish the job. For this and more I am truly grateful. I, Dan Credgington confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. 2 Abstract The development of organic electronic and photonic devices increasingly requires the development of micro- and nano-structured morphologies, which in turn require the development of both prototyping and scalable patterning methods. This thesis presents investigations which explore and develop unconventional patterning techniques for a variety of conjugated polymers and organic molecules, using scanning near-field optical lithography (SNOL), scanning thermal lithography (SThL) and molecular self-assembly. Optimised formation of organic nanostructures is demonstrated, at resolutions which equal or better the current state of the art, with patterning resolution for isolated structures below 60nm for SNOL and 30nm for SThL. SThL in particular is demonstrated as a technique which can achieve serial write-speeds of over 100 µm/s, with significant potential for up-scaling. Furthermore, arbitrarily defined two-dimensional large-area nanostructures up to 20 × 20 µm are demonstrated using SNOL while maintaining both high resolution and the integrity of the probe. The nanostructures fabricated in the course of this work, and others, are characterised using both optical and topographic techniques, primarily atomic force microscopy and near- field microscopy. The detailed formation mechanisms for structures fabricated using SNOL via an in-situ conversion route are systematically investigated and contrasted with other formation routes, resulting in a comprehensive account of the factors affecting structure morphology. In addition, the optimised nanostructures achieved in this work are shown, within this context, to be very close to best achievable with an apertured scanning near- field system. 3 Table of contents ACKNOWLEDGEMENTS ........................................................................................................2 ABSTRACT ..................................................................................................................................3 1. INTRODUCTION .................................................................................................................6 1.1. OVERVIEW ..........................................................................................................................6 1.2. CONJUGATED POLYMERS ...................................................................................................7 1.3. CHEMISTRY .........................................................................................................................8 1.4. EXCITED STATES AND OPTOELECTRONIC PROPERTIES .................................................10 1.5. POLYMER BLENDS.............................................................................................................13 1.6. ORGANIC SEMICONDUCTOR DEVICES .............................................................................16 1.6.1. OLEDS ............................................................................................................................16 1.6.2. PHOTOVOLTAIC CELL ......................................................................................................18 1.7. LITHOGRAPHY TECHNIQUES ............................................................................................20 2. NEAR-FIELD MICROSCOPY AND LITHOGRAPHY: THEORY ............................22 2.1. FRESNEL-KIRCHHOFF INTEGRAL ....................................................................................22 2.2. THE FRESNEL APPROXIMATION ......................................................................................23 2.3. FRAUNHOFER DIFFRACTION ............................................................................................24 2.4. BREAKING THE DIFFRACTION LIMIT ...............................................................................26 2.5. THE NEAR-FIELD MICROSCOPE .......................................................................................29 2.6. NUMERICAL METHODS .....................................................................................................30 3. INSTRUMENTATION ......................................................................................................38 3.1. THE SCANNING NEAR-FIELD SYSTEM.............................................................................38 3.2. NEAR-FIELD PROBES .........................................................................................................42 3.3. IMPROVING RESONANCE ..................................................................................................43 4. SCANNING NEAR-FIELD PHOTOLITHOGRAPHY ..................................................46 4.1. LITHOGRAPHY OF PPV ....................................................................................................47 4.1.1. METHODOLOGY ..............................................................................................................47 4.1.2. MINIMUM FEATURE SIZE .................................................................................................49 4.1.3. COMPARISON WITH MODELLING .....................................................................................50 4.1.4. FEATURE FORMATION AND SHRINKAGE ..........................................................................52 4.1.5. MASS LOSS ......................................................................................................................55 4.1.6. ROLE OF SUBSTRATE REFLECTIONS ON FIELD PROFILE ...................................................64 4.1.7. COMPLEX PATTERNING AT HIGH RESOLUTION ................................................................70 4.2. LITHOGRAPHY OF F8OX ..................................................................................................71 4.3. LITHOGRAPHY OF BTOX .................................................................................................75 4.4. LITHOGRAPHY HBC-ACRYLATE .....................................................................................76 4.5. IN-SITU LITHOGRAPHY OF PMMA-AZOBENZENE ..........................................................81 4.6. CONCLUSIONS ...................................................................................................................90 5. SCANNING THERMAL LITHOGRAPHY ....................................................................92 4 5.1. LITHOGRAPHY USING A MICRO-THERMAL PROBE .........................................................93 5.1.1. INSTRUMENTATION .........................................................................................................93 5.1.2. LITHOGRAPHY .................................................................................................................94 5.1.3. ACHIEVABLE RESOLUTION ..............................................................................................95 5.2. LITHOGRAPHY USING A NANO-THERMAL PROBE ...........................................................96 5.2.1. INSTRUMENTATION .........................................................................................................96 5.2.2. LITHOGRAPHY .................................................................................................................97 5.3. CONCLUSIONS .................................................................................................................101 6. NEAR-FIELD MICROSCOPY .......................................................................................102 6.1. TRANSMISSION MODE SNOM ........................................................................................104 6.1.1. INSTRUMENTATION .......................................................................................................104 6.1.2. IMAGING OF HBC-PDI WHISKERS.................................................................................105 6.1.3. POLARISATION SNOM OF HBC-PDI FIBRES ................................................................115 6.2. REFLECTION MODE SNOM ............................................................................................116 6.2.1. INSTRUMENTATION .......................................................................................................116 6.2.2. PHOTOLUMINESCENCE IMAGING OF POLYROTAXANE FIBRES .......................................117 6.2.3. POLARISATION SNOM OF SUPRAMOLECULAR FIBRES..................................................121 6.3.
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