This thesis describes a part of the first steps towards DNA sequencing via tunneling current measurement though graphene electrodes. The focus lies in particular on the optimization of the...Show moreThis thesis describes a part of the first steps towards DNA sequencing via tunneling current measurement though graphene electrodes. The focus lies in particular on the optimization of the fabrication and the characterization of the graphene electrodes. Progress has been made in the fabrication of atomically sharp graphene edges supported by a Si/SiO2 substrate with prefabricated contact pads. The optimization of reactive ion etching parameters was analyzed using SEM. However some problems were uncovered with the prefabricated wafers, since the contact pads seem to cause to large a step for the graphene to bridge. This leads to the tearing of graphene after some time when unsupported by a protective top layer. The creation of a tunneling junction with two graphene electrodes using a modified STM device confirmed the previously found results. The same hysteresis in the I-Z curve was observed which is likely due to carbon bonds forming between the two graphene electrodes, however more research is necessary to fully analyze the edge composition of the graphene. Knowing fully what the edge of the graphene looks like on the atomic scale is also important for future steps since this is needed to chemically functionalize the graphene edge and to tunneling current measurements more reproducible. The prefabricated wafers in combination with new sample holders with integrated wiring made to fit the STM also made the use of a backgate possible. However this caused some instability in the tunneling current which needs to be resolved before any valuable measurements can be taken. These measurements should also give us information about the composition of the tunneling junction that is formed.Show less
The work presented in this thesis is part of a larger project towards the goal of reading DNA via the tunneling current in a graphene-graphene nanogap. This work focuses on preparing the transition...Show moreThe work presented in this thesis is part of a larger project towards the goal of reading DNA via the tunneling current in a graphene-graphene nanogap. This work focuses on preparing the transition of a graphene twisted edge junction from ambient conditions to liquid environments. Improvements to the fabrication of the electrodes are made by means of silanizing the Si/SiO2 substrate. The use of OTS seems promising, but more measurements are required for valid conclusions. The current contributions of the electrodes in Milli-Q water are investigated component-wise by cyclic voltammetry. Thin pour reprorubber epoxy is demonstrated to be sufficient in insulating the contact and wiring. A double-layer approach to insulating the graphene with PMMA is investigated, but the proposed method does not yield satisfactory results. In order to continue towards sufficiently insulated graphene electrodes, it is suggested to move on to different sample preparation methods or different insulating materials.Show less
Various experiments have indicated that electron transfer through nonmagnetic chiral hydrocarbon molecules is spin dependent. This property is called the chiral-induced spin selectivity (CISS)...Show moreVarious experiments have indicated that electron transfer through nonmagnetic chiral hydrocarbon molecules is spin dependent. This property is called the chiral-induced spin selectivity (CISS) effect and its presence cannot be explained with current theoretical models. To detect the CISS effect experimentally, chiral molecules have to be isolated. Therefore, transport through a single chiral molecule must be measured with STM. A fabrication procedure for sharp and stable nickel STM tips has been developed using electrochemical etching. The produced tips are very sharp and have an average opening angle of 18.9° and an apex size below 200 nm. The magnetic polarisation of the tip can be reversed in situ by a magnetic field, induced by a coil around the tip, which has been demonstrated. The quality and the magnetisation of the tips have been tested on HOPG, gold and nickel with STM. Clear large-scale images were obtained on all materials. Atomic resolution was obtained on HOPG. With these challenges resolved, STM measurements using a nickel tip with both magnetic polarisations can be performed in the study of single chiral molecules as spin filter and the investigation of the CISS effect.Show less
The chiral-induced spin selectivity effect (the CISS effect) is the effect where the electron transmission yield through chiral molecules depends on the spin orientation of the electron. The CISS...Show moreThe chiral-induced spin selectivity effect (the CISS effect) is the effect where the electron transmission yield through chiral molecules depends on the spin orientation of the electron. The CISS effect has been demonstrated. However, to formulate a model describing the effect, quantitative testing is required. Therefore, we want to make samples where single chiral molecules are embedded in a host matrix, a self-assembling monolayer of insulating thiols. Consequently, we want to measure the tunneling current through these single chiral molecules by using STM with a ferromagnetic tip. With a coil around the ferromagnetic tip, we can change the magnetization and thus the spin polarization in situ. In the framework of the project, we aim to measure chiral-induced spin selectivity in single chiral molecules. In this thesis specifically methods of sample and STM tip fabrication are discussed. Our work proposes the first steps towards quantitative testing of chiral-induced spin selectivity.Show less
In this project, magneto-transport measurements are performed on exfoliated trigonal tantalum disulfide (1T-TaS2) flakes with a top-contact Hall-bar geometry. Transport measurements reveal the...Show moreIn this project, magneto-transport measurements are performed on exfoliated trigonal tantalum disulfide (1T-TaS2) flakes with a top-contact Hall-bar geometry. Transport measurements reveal the presence of charge density waves and therelatednearly-commensuratetocommensuratephasetransition. Thephase transition is shown to depend on both the thickness of the crystal and the cooling rate. The observed critical thickness is approximately 80 nm, relatively large compared to literature. A clear deviation from literature is observed in the resistive behavior during the transition. The increase in resistivity due to the transition is considerably smaller and less abrupt. This deviation may be attributed to partial switching of the crystal, i.e. only some of the layers switching to the commensurate phase. Hall effect measurements in the supercooled phase, i.e. below the suppressed phase transition, reveal large charge carrier density and extremely low mobility, both in agreement with literature. A down-turn is observed in the supercooled phase at low temperatures. This down-turn is also observed in literature, however its origin is not discussed. We propose that this down-turn is caused by weak anti-localization. The phase coherence length, extracted from fitting of the Hikami-Larkin-Nagaoka model (HLN)modeltotheweakanti-localizationpeaks,followsapowerlawwithexponent γ = −0.341±0.03. This exponent suggests that the electron transport is one-dimensional, substantiating the notion that in the nearly-commensurate phase, the electron transport is dominated by the domain boundaries.Show less
In this thesis research is presented into multiple unexplained phase transitions of the two antiferromagnets FePS3 and Fe1-xS (pyrrhotite). X-ray diffraction and resistivity measurements of...Show moreIn this thesis research is presented into multiple unexplained phase transitions of the two antiferromagnets FePS3 and Fe1-xS (pyrrhotite). X-ray diffraction and resistivity measurements of pressurised FePS3 show that this material undergoes at least two crystallographic phase transitions, strongly related to the appearance of an exotic metallic state when pressures over approximately 10GPa are applied. Magnetisation, specific heat and resistivity measurements of Pyrrhotite highlight for the first time the dual nature of the Besnus transition. Compelling evidence is presented that excludes the proposition of extrinsic magnetic coupling between differently ordered vacancy superstructures as the underlying mechanism. Instead the presence of local magnetic domain formation is highlighted.Show less
During this project we have tested and characterized a new cryogenic STM (Scanning Tunneling Microscope) with a uniquely designed coarse XY stage. We have used this STM the perform STM break...Show moreDuring this project we have tested and characterized a new cryogenic STM (Scanning Tunneling Microscope) with a uniquely designed coarse XY stage. We have used this STM the perform STM break junction experiments on both clean gold samples and samples covered by OPE-3 molecules. In the results for the clean gold samples we retrieve the characteristic and well known features of single atom gold wires. For the results of the samples covered by molecules we see faint traces of the molecules in the conductance histogram, although they are not as clear as compared to results in literature. Cryogenic experiments using this STM have proven to be difficult and only partially successful.Show less
Using the notched wire mechanically controllable break junction technique charge transport of a single-molecule junction, containing the OPE3 molecule, is studied at room temperature. By deposition...Show moreUsing the notched wire mechanically controllable break junction technique charge transport of a single-molecule junction, containing the OPE3 molecule, is studied at room temperature. By deposition of molecules from solution between two gold leads we were able to determine the conductance of the OPE3 at (1.6±0.3)·10−4G0. The measurement speed of the LabVIEW control program for this experiment was improved by more than a factor of 7 compared to the previous version.Show less
The structure and conductive behaviour of graphene was studied under breaking. We were able to break and reconnect graphene using the mechanical break junction and observed a change in conductive...Show moreThe structure and conductive behaviour of graphene was studied under breaking. We were able to break and reconnect graphene using the mechanical break junction and observed a change in conductive behaviour from ohmic to non-linear; a possible explanation for the effect was offered. SEM, AFM and Raman spectroscopy determined that no large structural damage was caused by the breaking, nor was a decrease in graphene quality towards the break edge observed up until 1 micron near the edge; graphene was established to be present to within at least 100 nm of the edge. Furthermore, a modified mechanical break junction design for a graphene nanojunction was tested and deemed complicated: it was advised to loosen the requirements for future experiments. Lastly, an approach to electrochemically etching a graphene nanojunction was tested and approached theoretically; it was deemed not feasible using current graphene transfer techniques.Show less
In recent years Electronic Double Layer (EDL) gating using ionic liquids or organic electrolytes has been successful in tuning carrier densities in materials such as ZnO [1], SrTiO3 [2–4], La2...Show moreIn recent years Electronic Double Layer (EDL) gating using ionic liquids or organic electrolytes has been successful in tuning carrier densities in materials such as ZnO [1], SrTiO3 [2–4], La2-xSrxCuO4 [5] and YBa2Cu3O7- [6]. The potential for studying High-Tc superconductors and Metal-Insulator transition with great flexibility has been shown. The carriers are pulled into a 1nm layer by the accumulation of ions on the surface (EDL). Here the formation the EDL in time is studied on atomically flat SrTiO3 (100) single crystals and electron doped High-Tc superconductor Nd2-xCexCuO4 thin films. At temperatures close to the melting point of the ionic liquid (DEMETFSI) the ability to separate the EDL formation on the contacts from the formation on STO is demonstrated by measuring resistance and gate current as a function of time. This despite the presence of a significant Faradaic current most likely due to oxidation/reduction at the gold surface. It is argued that the EDL formation on the STO originates from the contacts and its growth largely depends on the mobility of the ions. Measurements on (undoped) Nd2CuO4 show a resistance decrease up to 90% at Vg = 3V and thereby showing the potential of EDL gating on electron doped cuprates.Show less
