Fabrication and charge transport measurements on graphene-based nanostructures in the quantum hall regime

Quantum Hall effect(QHE) is not only important from fundamental physics point of view but also it provides the international resistance standard. Therefore, it has a direct impact on the whole electronics industry in terms of reaching the ultimate precision in any application. Achieving QHE at higher currents near the breakdown regime is crucial for improving the resistance standard. Graphene seems to be a good candidate for the resistance metrology towards better precision and wider application under less strict conditions due to its unique electronic properties. In this thesis, we first investigated the breakdown of the QHE in mechanically exfoliated single layer graphene samples on SiOx substrates. We found that the breakdown emerges as a gradual increase in the longitudinal resistivity rather than an abrupt jump. We have also observed that the deviation of the Hall resistance with current remains very small until an abrupt increase around jx = 5A=m. The exponential dependence of the conductivity on the current is attributed to impurity mediated inter-Landau level tunnelling of carriers. As a second study, graphene samples were suspended and electrically characterized at temperatures ranging from room temperature to 20 mK at magnetic fields between 0-12 Tesla. Various techniques were developed to fabricate suspended devices and treated them to reach ultra-high cleanliness. These techniques lead us to produce devices with charge mobility values in excess of 10⁶ cm²V⁻¹s⁻¹. We observed that in these devices, the minimum conductivity around the Dirac point can exceed the theoretically predicted value of 4e²/πh. In such monolayer graphene devices, quantum Hall filling factors v= 0, ∓1 can also emerge in the magneto-transport measurements in addition to the expected 2(2n+1) plateaus. The presence of these plateaus in these ultra high quality suspended samples indicate the lifting of the valley and spin degeneracy

Growth of thin graphene layers on stacked SiC surface in ultra high vacuum

We demonstrate a technique to produce thin graphene layers on C-face of SiC under ultra high vacuum conditions. A stack of two SiC substrates comprising a half open cavity at the interface is used to partially confine the depleted Si atoms from the sample surface during the growth. We observe that this configuration significantly slows the graphene growth to easily controllable rates on C-face SiC in UHV environment. Results of low-energy electron diffractometry and Raman spectroscopy measurements on the samples grown with stacking configuration are compared to those of the samples grown by using bare UHV sublimation process

Tuning of 2D rod-type photonic crystal cavity for optical modulation and impact sensing

We propose a novel way of mechanical perturbation of photonic crystal cavities for on-chip applications. We utilize the equivalence of the 2D photonic crystals with perfect electric conductor (PEC) boundary conditions to the infinite height 3D counterparts for rod type photonic crystals. Designed structures are sandwiched with PEC boundaries above and below and the perturbation of the cavity structures is demonstrated by changing the height of PEC boundary. Once a defect filled with air is introduced, the metallic boundary conditions is disturbed and the effective mode permittivity changes leading to a tuned optical properties of the structures. Devices utilizing this perturbation are designed for telecom wavelengths and PEC boundaries are replaced by gold plates during implementation. For 10 nm gold plate displacement, two different cavity structures showed a 21.5 nm and 26 nm shift in the resonant wavelength. Optical modulation with a 1.3 MHz maximum modulation frequency with a maximum power consumption of 36.81 nW and impact sensing with 20 μs response time (much faster compared to the commercially available ones) are shown to be possible

Intermodal coupling as a probe for detecting nanomechanical modes

Nanoelectromechanical systems provide ultrahigh performance in sensing applications. The sensing performance and functionality can be enhanced by utilizing more than one resonance mode of a nanoelectromechanical-systems device. However, it is often challenging to measure mechanical modes at high frequencies or modes that couple weakly to output transducers. In this paper, we propose the use of intermodal coupling as a mechanism to enable the detection of such modes. To implement this method, a probe mode is continuously driven and monitored using a phase-locked loop, while an auxiliary drive signal scans for other modes. Each time the auxiliary drive signal excites the corresponding mode by matching the mechanical frequency, the effective tension within the structure increases, which in turn causes a frequency shift in the probe mode. The location and width of these frequency shifts can be used to determine the frequency and quality factor of mechanical modes indirectly. Intermodal coupling can be used as a tool to obtain the spectrum of a mechanical structure even if some of these modes cannot be detected conventionally

Genetic algorithm-driven surface-enhanced raman spectroscopy substrate optimization

Surface-enhanced Raman spectroscopy (SERS) is a highly sensitive and molecule-specific detection technique that uses surface plasmon resonances to enhance Raman scattering from analytes. In SERS system design, the substrates must have minimal or no background at the incident laser wavelength and large Raman signal enhancement via plasmonic confinement and grating modes over large areas (i.e., squared millimeters). These requirements impose many competing design constraints that make exhaustive parametric computational optimization of SERS substrates pro-hibitively time consuming. Here, we demonstrate a genetic-algorithm (GA)-based optimization method for SERS substrates to achieve strong electric field localization over wide areas for recon-figurable and programmable photonic SERS sensors. We analyzed the GA parameters and tuned them for SERS substrate optimization in detail. We experimentally validated the model results by fabricating the predicted nanostructures using electron beam lithography. The experimental Raman spectrum signal enhancements of the optimized SERS substrates validated the model predictions and enabled the generation of a detailed Raman profile of methylene blue fluorescence dye. The GA and its optimization shown here could pave the way for photonic chips and components with arbitrary design constraints, wavelength bands, and performance targets

Control of the graphene growth rate on capped SiC surface under strong Si confinement

The effect of the degree of Si confinement on the thickness and morphology of UHV grown epitaxial graphene on (0 0 0 −1) SiC is investigated by using atomic force microscopy and Raman spectroscopy measurements. Prior to the graphene growth process, the C-face surface of a SiC substrate is capped by another SiC comprising three cavities on its Si-rich surface with depths varying from 0.5 to 2 microns. The Si atoms, thermally decomposed from the sample surface during high temperature annealing of the SiCcap/SiCsample stack, are separately trapped inside these individual cavities at the sample/cap interface. Our analyses show that the growth rate linearly increases with the cavity height. It was also found that stronger Si confinement yields more uniform graphene layers