Fabrication and characterization of graphene-superconductor devices

Graphene is the first single-atom-thick two-dimensional material and exhibits a large set of interesting properties. This thesis consists of two parts. The first regards the growth of large-area graphene using chemical vapor deposition (CVD). Graphene is grown using CVD on copper catalyst showing high quality with charge carrier mobility exceeding 3000 cm2/Vs.Wet chemical etching is used to transfer graphene to insulating substrates. Cu is removed using either diluted HNO3 or diluted HCl with a small amount of added H2O2. To allow for faster transfer and avoid consuming copper, a hydrogen-bubbling method is developed to delaminate graphene from Cu. Graphene transferred this way shows properties similar to those of graphene transferred using wet etching.To avoid transfer-related issues, graphene is grown non-catalytically directly on insulating substrates such as SiO2, Al2O3, and Si3N4. The grain size is only ~10 nm due to the lack of catalytic activity during growth. Such graphene shows inferior electronic properties with mobility in the order of ~tens of cm2/Vs. Despite that, sheet resistance around kΩ, the possibility to grow several layer thick films, and optical properties similar to those of pristine graphene make it an interesting material.A method for cleaning graphene mechanically using atomic force microscopy (AFM) is developed. By appropriate choice of the applied force, atomically smooth (roughnes

Fabrication and characterization of graphene-superconductor devices

Graphene is the first single-atom-thick two-dimensional material and exhibits a large set of interesting properties. This thesis consists of two parts. The first regards the growth of large-area graphene using chemical vapor deposition (CVD). Graphene is grown using CVD on copper catalyst showing high quality with charge carrier mobility exceeding 3000 cm2/Vs.Wet chemical etching is used to transfer graphene to insulating substrates. Cu is removed using either diluted HNO3 or diluted HCl with a small amount of added H2O2. To allow for faster transfer and avoid consuming copper, a hydrogen-bubbling method is developed to delaminate graphene from Cu. Graphene transferred this way shows properties similar to those of graphene transferred using wet etching.To avoid transfer-related issues, graphene is grown non-catalytically directly on insulating substrates such as SiO2, Al2O3, and Si3N4. The grain size is only ~10 nm due to the lack of catalytic activity during growth. Such graphene shows inferior electronic properties with mobility in the order of ~tens of cm2/Vs. Despite that, sheet resistance around kΩ, the possibility to grow several layer thick films, and optical properties similar to those of pristine graphene make it an interesting material.A method for cleaning graphene mechanically using atomic force microscopy (AFM) is developed. By appropriate choice of the applied force, atomically smooth (roughnes

Cleaning graphene using atomic force microscope

We mechanically clean graphene devices using an atomic force microscope (AFM). By scanning an AFM tip in contact mode in a broom-like way over the sample, resist residues are pushed away from the desired area. We obtain atomically flat graphene with a root mean square (rms) roughness as low as 0.12 nm after this procedure. The cleaning also results in a shift of the charge-neutrality point toward zero gate voltage, as well as an increase in charge carrier mobility

Detection of graphene microelectromechanical system resonance

We present an experimental setup for fast detection of resonances of graphene microelectromechanical structures of different quality. The relatively simple technique used to read-out of the resonance frequency is the main advantage of the proposed system. The resolution is good enough to detect vibrations of the graphene resonator with the quality factor of ∼24 and resonance frequency of 104 MHz

Family of graphene-based superconducting devices

A family of highly sensitive devices based on a graphene nanobridge and superconducting electrodes has been developed, manufactured, and examined. These devices can be used to create a graphene-based integral receiver. A cold-electron bolometer prototype with superconductor-insulator-normal metal tunnel junctions has been studied. Its response to a change in the temperature and external microwave radiation has been measured. A superconducting quantum interferometer with a graphene strip as a weak coupling between superconducting electrodes has been examined. The corresponding modulation of the voltage by a magnetic field at a given current has been measured. The effect of the gate voltage on the resistance of graphene has been analyzed for these samples. To confirm that graphene is single-layer, measurements with the reference samples were performed in high magnetic fields, displaying the half-integer quantum Hall effect

Unusual thermopower of inhomogeneous graphene grown by chemical vapor deposition

We report on thermopower (TEP) and resistance measurements of inhomogeneous graphene grown by chemical vapor deposition (CVD). Unlike the conventional resistance of pristine graphene, the gate-dependent TEP shows a large electron-hole asymmetry. This can be accounted for by inhomogeneity of the CVD-graphene where individual graphene regions contribute with different TEPs. At the high magnetic field and low temperature, the TEP has large fluctuations near the Dirac point associated with the disorder in the CVD-graphene. TEP measurements reveal additional characteristics of CVD-graphene, which are difficult to obtain from the measurement of resistance alone

Quantum Hall effect in graphene decorated with disordered multilayer patches

Quantum Hall effect (QHE) is observed in graphene grown by chemical vapour deposition using platinum catalyst. The QHE is even seen in samples which are irregularly decorated with disordered multilayer graphene patches and have very low mobility (<500 cm2V−1s−1). The effect does not seem to depend on electronic mobility and uniformity of the resulting material, which indicates the robustness of QHE in graphene

Templated Growth of Covalently Bonded Three-Dimensional Carbon Nanotube Networks Originated from Graphene

A template-assisted method that enables the growth of covalently bonded three-dimensional carbon nanotubes (CNTs) originating from graphene at a large scale is demonstrated. Atomic force microscopy-based mechanical tests show that the covalently bonded CNT structure can effectively distribute external loading throughout the network to improve the mechanical strength of the material

Determination of the Bending Rigidity of Graphene via Electrostatic Actuation of Buckled Membranes

The small mass and atomic-scale thickness of graphene membranes make them highly suitable for nanoelectromechanical devices such as e.g. mass sensors, high frequency resonators or memory elements. Although only atomically thick, many of the mechanical properties of graphene membranes can be described by classical continuum mechanics. An important parameter for predicting the performance and linearity of graphene nanoelectromechanical devices as well as for describing ripple formation and other properties such as electron scattering mechanisms, is the bending rigidity, {\kappa}. In spite of the importance of this parameter it has so far only been estimated indirectly for monolayer graphene from the phonon spectrum of graphite, estimated from AFM measurements or predicted from ab initio calculations or bond-order potential models. Here, we employ a new approach to the experimental determination of {\kappa} by exploiting the snap-through instability in pre-buckled graphene membranes. We demonstrate the reproducible fabrication of convex buckled graphene membranes by controlling the thermal stress during the fabrication procedure and show the abrupt switching from convex to concave geometry that occurs when electrostatic pressure is applied via an underlying gate electrode. The bending rigidity of bilayer graphene membranes under ambient conditions was determined to be $35.5^{+20}_{-15}$ eV. Monolayers have significantly lower {\kappa} than bilayers