Development, fabrication and characterization of graphene and bismuth hall sensors for scanning hall probe microscopy

Scanning Hall Probe Microscopy (SHPM) is a powerful magnetic imaging technique which provides high magnetic field and spatial resolution, simultaneously with the topography of magnetic and superconducting materials. It is a quantative method which can be operated under high magnetic fields and a wide temperature range. The Hall sensor which is sensitive to the perpendicular component of the magnetic field emanating from the specimen determines resolution. In this work, single layer Graphene Hall Probes (GHP) were fabricated using Chemical Vapor Deposition (CVD) grown graphene on copper foils, transferred to Silicon wafers. The Hall coefficient and field sensitivity of GPHs were measured to be 0.18 Ω/G and 0.20 G/√Hz, respectively, for a 3 μA drive current at room temperature. For the first time, GHP is successfully used for magnetic imaging in 3–300 K range in SHPM with quartz crystal Atomic Force Microscopy (AFM) feedback. This study has demonstrated that graphene is an alternative material to be used for magnetic imaging in SHPM. Alternatively, Bismuth based Hall probes are also fabricated by advanced lithographic techniques. Since Bismuth is a semimetal with low carrier concentration, it has been considered as a promising Hall sensor material. For the first time, SHPM images of NdFeB demagnetized magnet were acquired using Bi hall sensor fabricated by Electron Beam Lithography (EBL) with quartz crystal AFM feedback

Coalescence of few layer graphene grains grown by chemical vapor deposition and their stacking sequence

Few layer graphene is attractive due to its extraordinary electronic and optical properties, which are strongly influenced by the orientation between the layers called as stacking sequence. It is challenging to synthesize high quality large size single or multi layer graphene crystals on the metal catalyst using chemical vapor deposition technique. The present work is about synthesis of few layer graphene grains on platinum foil using ambient pressure chemical together vapor deposition technique. The main focus is how the different grains coalesced and maintain the stacking sequence. Different characterization techniques are used to analyze the grains when they are in the process of merging to make a bigger grain. Scanning electron microscopy clearly shows different stacking sequences and merging of different nucleation sites of different grains. Interestingly, different stacking sequences are observed during the process of coalescence of grains. Raman spectroscopy gives accurate information about the number of layers and their stacking sequence. We observed Bernal AB and twisted layer stacking in the grains when they were combining together to grow into a bigger size. The full width at half maximum (FWHM) value of 2D Raman peaks appeared in the range of 52-69 cm(-1) which shows an increase from the value of single layer graphene (30.18 cm(-1)) and identifies Bernal stacking in grains. For twisted stacking FWHM values lie in the range of 19-32 cm(-1).Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) ; European Union (EU