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Self-assembled monolayers on metal oxides: Applications in nanotechnology

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  1. Tez No: 401362
  2. Yazar: OKTAY YILDIRIM
  3. Danışmanlar: PROF. DR. JURRIAAN HUSKENS, PROF. DR. GUUD RIJNDERS
  4. Tez Türü: Doktora
  5. Konular: Kimya Mühendisliği, Chemical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 2010
  8. Dil: İngilizce
  9. Üniversite: University of Twente
  10. Enstitü: Yurtdışı Enstitü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 121

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Özet (Çeviri)

This thesis describes the use of phosph(on)ate-based self-assembled monolayers (SAMs) to modify and pattern metal oxides. Metal oxides have interesting electronic and magnetic properties such as insulating, semiconducting, metallic, ferromagnetic etc. and SAMs can tailor the surface properties. FePt nanoparticles (NPs) are promising candidates for magnetic data storage applications due to their superior properties. In this thesis, the use of SAMs on conducting metal oxides for electrical applications and at the adsorption of magnetic NPs for data storage applications have been studied. By combining patterning techniques and selfassembly, functional inorganic-organic composite structures have been created. Chapter 1 provides a general introduction to this thesis. In Chapter 2, a literature overview of SAMs on metal oxides is given. SAM types were compared in terms of SAM formation (bond mechanism, interaction between the head-group and substrate surface, growth mechanism), quality (coverage, packing, order), structure (configuration) and stability. Techniques to pattern metal oxides with SAMs and several examples where SAMs were used in biomaterial or electronic applications, or as protective layers were covered. Phosp(on)ate-based molecules were found to form more densely packed, more ordered and more stable SAMs on metal oxides when compared to alkanoic acid-based molecules. At the same time, opportunies for further research have been identified. In Chapter 3, the assembly of phosph(on)ate-based SAMs with CH3, NH2, SH, COOH end groups on single crystalline-aluminum oxide (Al2O3) substrates is described. SAMs were bound to the substrate through the phosph(on)ate headgroups, had a homogeneous and high coverage, a tails-up orientation and a certain degree of order. This was shown by contact angle (CA), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The thickness of a TDP layer was smaller than the length of an extended TDP molecule. This indicated a tilt in the SAM layer. There was no indication of in-plane registry between the surface atoms of the substrate and the phosph(on)ate headgroup. To create chemically different regions, SAM patterns were prepared on Al2O3 by microcontact printing. In Chapter 4, the electrochemical properties of SAMs on conducting metal oxide Nb-STO are addressed. Unlike thiols on gold, the alkylphosphate SAMs on Nb- STO showed electrochemical stability over a wide voltage range of -2 to +2V as shown by cyclic voltammetry (CV). SAMs formed an insulating layer and inhibited the electrochemical activity of Nb-STO with an efficiency increasing with chain length. SAM-modified Nb-STO substrates had a higher resistance than bare substrates as shown by electrochemical impedance spectroscopy (EIS). Phosph(on)ate based- SAMs proved to be effective to electrically isolate the metal oxide surface and provided a stable system for electrochemical studies. This opens new possibilities to study electrochemical properties of semiconductors. Chapter 5 describes the low kinetic energy deposition of Pt top contacts on alkylphosphate SAMs by pulsed laser deposition (PLD) and electrical characterization of these SAMs on a conducting Nb-STO metal oxide susbtrate. Electrochemical Cu deposition showed that almost 100 % of the top contacts were insulated from the substrate by the SAM which shows the dense packing and robustness of the SAM. As a control experiment top contacts were prepared on a bare substrate without a SAM layer and Cu growth was seen on all of the Pt contacts. I-V measurements showed that SAM modification caused a dramatic decrease in the leakage current when compared to bare Nb-STO, which proved the efficiency of SAMs as dielectric organic thin films. The ability to prepare top electrodes with very high yield without crashing into the SAM layer opens new possibilities for the use of phosph(on)ate-based dielectric organic thin films on metal oxides for electronic device fabrication. Chapter 6 describes the controlled assembly of FePt NPs on phosph(on)atebased SAM-modified Al2O3 substrates. The NP coverage on Al2O3 substrates modified with organic monolayers could be controlled by varying the immersion time into a FePt NPs solution. NPs probably assembled on NH2 or COOH-terminated SAM-covered surfaces by ligand exchange since no assembly was observed when a CH3-terminated SAM was used. This provided the possibility to control the adhesion of NPs on surfaces by changing the surface chemistry. The morphology and coverage of the NP assemblies were observed by AFM and their ferromagnetic properties were studied before and after thermal annealing by a vibrating sample magnetometer (VSM). Alumina substrates were patterned by microcontact printing using NH2- terminated molecules as the ink, allowing local NP assembly. Thermal annealing under reducing conditions (96%N2/4%H2) led to a phase change of the FePt NPs from the disordered FCC phase to the ordered FCT phase. This resulted in ferromagnetic behavior at room temperature. Chapter 7 presents the preparation of high-resolution FePtAu NP patterns on an Al2O3 surface prepared by nanoimprint lithography (NIL) and nanomolding in capillaries (NAMIC). The polymer template generated by NIL behaved as a physical barrier and defined the patterned areas on the Al2O3 substrate. FePtAu nanoparticles (NPs) were assembled on phosph(on)ate SAM-modified polymer patterned substrates. After polymer removal, nano- and micro-scale, line and dot NP patterns were obtained on aluminum oxide substrates. Thick nanolines of NPs were obtained by NAMIC. FePtAu NPs are ferromagnetic as synthesized and therefore do not need to be annealed. Applying an external magnetic field during the assembly of FePtAu NPs on SAM-modified SrTiO3 (STO) resulted in angle-dependent magnetic properties which showed partial alignment of the NPs. The magnetic properties of the ferromagnetic NPs were addressed by VSM and those of the patterned NPs by magnetic force microscopy (MFM). The results showed that NIL combined with surface chemistry is a powerful method to create well-defined high resolution NP patterns over large areas. The process described in Chapters 6 and 7 can potentially be used in the fabrication of spintronic devices. The results described in this thesis show the versatility and efficiency of the use of phosph(on)ate-based SAMs to modify metal oxide surfaces. The use of SAMs on conducting metal metal oxides opens new possibilities for electrochemical studies. Metal top contact fabrication without causing shorts between the SAM and the substrate, combined with the insulating efficiency of the SAM, is promising for electrical device fabrication as well as for fundamental studies to understand the electrical properties of organic monolayers. Combining patterning techniques with chemical modification achieved by SAMs for controlled assembly and patterning magnetic nanoparticles on metal oxides can be used to prepare spintronic devices. By selecting the metal oxide properly, magnetic tunnel junctions can be fabricated. In principle, the surface modification and fabrication approach described throughout this thesis is applicable to a wide variety of metal oxides and NPs. This brings new possibilities for fabrication of functional hybrid organic-inorganic structure

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