Tuning electron transport in metal films and graphene with organic monolayers
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- Tez No: 402043
- Danışmanlar: PROF. DR. W. G. VAN DER WIEL
- Tez Türü: Doktora
- Konular: Biyomühendislik, Elektrik ve Elektronik Mühendisliği, Bioengineering, Electrical and Electronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2014
- Dil: İngilizce
- Üniversite: University of Twente
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 138
Özet
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Özet (Çeviri)
Introducing organic molecules into electronic devices has attracted significant research interest due to its promise in both technological development and fundamental research. Organic molecular materials offer advantages such as low costs, easy fabrication, mechanical flexibility, light weight and chemical tunability, which can be engineered to obtain various functionalities. Organic molecules are used for many purposes as is summarized in Chapter 1. In this thesis work, they were used for magnetic doping of a thin metal film and for modifying electrical properties of graphene. Theoretical concepts used throughout the thesis are introduced in Chapter 2. The main theoretical concepts are (1) the Kondo effect, which is a many-body phenomenon originating from the interaction of the spin of an isolated magnetic impurity with the spins of surrounding conduction electrons, (2) weak(anti-)localization, a correction to the resistivity due to the electron interference in disordered metals, and (3) crystal field theory, explaining the electronic structure and the origin of the spin state of the cenral metal ion in the molecular complexes used in this thesis work. In Chapter 3, the experimental methods that were used throughout the thesis work are explained. The preparation of SAMs on thin Au films and SiO2/Si substrates, and the deposition techniques that were used for Au capping of the SAMs are described. Low-temperature electric and magnetic characterization systems, synchrotron radiation techniques and surface-enhanced Raman spectroscopy are introduced. A novel molecular fabrication method for magnetic doping of thin metal films with isolated impurities up to high concentrations is discussed in Chapter 4. A monolayer of paramagnetic molecular complexes, with a spin 1/2 core ion (Co complex), were inserted into thin disordered Au films as magnetic dopants. It was possible to dilute the magnetic impurity concentration by mixing the magnetic molecular complexes with their non-magnetic counterpart molecules (Zn complex: same molecules, but with spin 0 core ion) in solution. The concentration of magnetic impurities in the metal film was directly proportional to the concentration of magnetic complexes in solution. Kondo and weak localization measurements demonstrated that the magnetic impurity concentration can be systematically varied up to ~800 ppm concentration without any sign of interimpurity interaction, or undesired clustering often suffered from in alternative methods. Our fabrication method proved to be easy to apply and reproducible to a high degree for more than 5 independent fabrication runs. The results showed that magnetic impurity concentrations as high as 800 ppm are not high enough to cause impurities to interact with each other. Our molecular spin doping technique can be used for the investigation of very important physical phenomena such as the Kondo effect, RKKY interaction which can lead to further understanding of impurity-electron, impurity-impurity interactions. It is also discussed in Chapter 4 that the Kondo effect is strong when a thin layer of Au is sputtered onto the monolayers on Au. Related to this, in Chapter 5, the effect of Au deposition over the monolayers (Au capping) on the magnetotransport properties is discussed. Au deposition was applied by two different techniques; magnetron sputtering and e-beam deposition. The kinetic energy per deposited atom in sputtering is about 2 orders of magnitude higher than for e-beam deposition. It was observed that the Au capping deposited by ebeam deposition increased the Kondo upturn and Kondo temperature (TK) slightly compared to the uncapped case, while capping by magnetron sputtering had a more pronounced effect. The higher Kondo upturn and TK, accompanied with smaller phase coherence lengths, can be attributed to enhanced interaction between the Co-ions and the Au conduction electrons. XAS and Raman spectroscopy showed that the molecular structure remained intact after e-beam evaporation. However upon sputtering, molecular bonds were broken during deposition. The change in the electronic environment of the Co2+ ions turned out to be the predominant reason for the higher TK for the sputtered samples. These results imply that it is possible to change the strength of impurity-host interactions in such systems, by tuning the deposition parameters of Au top layers. This allows for tuning of the magnetic impurity states, e.g. binding energy with respect to the host Fermi energy, and orbital overlap via the ligand structure around the ions.Graphene has attracted significant attention due to its exotic band structure and properties such as light weight, flexibility, optical transparency and very high charge carrier mobility. It is known that the electronic properties of graphene are severely affected by molecular species/adsorbates on the surface, or at the interface between the substrate and the graphene. In Chapter 6, molecular complexes (which involved the same metal ion-terpyridine groups that were discussed in Chapter 4 and Chapter 5) were inserted between graphene sheets and SiO2/Si substrates The original aim was to observe the effect of the spin of the Co complex on the graphene conduction. However, temperature dependent measurements did not show any significant effect, possibly due to the much larger influence of defects or grain edges in graphene. Instead, it was observed that the molecular complexes induced p-type doping of the graphene, which resulted in a hysteresis effect due to charge transfer and trapping. The sample-to-sample variations were larger than any differences for molecular layers containing different molecules (Co complexes, Zn complexes or a mixture of the two complexes). However, large hysteresis effects were only observed for samples containing molecular complexes. It was shown that our molecularlayer/ graphene system can be used to make memory devices exhibiting up to 4 different resistance states.
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