Observation of Novel Charge Ordering and Study of Unique Interlayer Oxidation in VdW Itinerant Ferromagnet Fe3GeTe2

Abstract

Low dimensional materials show unique properties as compared to their 3D counterparts which make them a fascinating system to study. The paradigm shift in the study of 2D materials happened when Andre Geim and Konstantin Novoselov isolated a single atomic layer of graphite known as graphene in 2004. The field of 2D materials is an active area of research that encompasses a wide range of materials such as metals, insulators, semiconductors, and superconductors. One such material is the vdW itinerant ferromagnet Fe3GeTe2 (FGT) with a Curie temperature of 220 K, which retains its ferromagnetic properties up to monolayer. In the first part of the thesis, various vdW materials like graphite, NbSe2, MnPS3, hBN and Fe3GeTe2 have been studied using techniques such as AFM, Raman spectroscopy, HR-TEM and transport measurements. Successful isolation and characterization of monolayer graphene was achieved followed by the preparation of TBG (Twisted Bilayer Graphene) with a twist angle of 8.7°. Band gap of MnPS3 was calculated to be 2.75 eV by studying its heterostructures with graphene. Exfoliation of FGT was performed with the thinnest flake obtained to be a trilayer. Intensity of Raman modes at 120 cm-1 and 154 cm-1 were found to be enhanced with decreasing thickness of the exfoliated FGT flakes. A few FGT crystals were found to have excess Fe concentration which are accommodated either in-between the layers or inside the layer. FGT is known to be an air-sensitive material that oxidizes in ambient conditions. Oxidation was studied on exfoliated FGT flakes where surface oxidation starts at around 4 hours of exposure with rate of oxidation increasing exponentially with temperature. Raman modes are shifted to 124 cm-1 and 142 cm-1 for oxidized flakes hinting at structural change in the flakes. Thickness of the FGT flakes was found to increase in a sigmoidal fashion which is explained by oxygen intercalation in between the FGT layers. Interlayer oxidation in FGT is hypothesized considering the oxidation of the protected side exfoliated on PDMS and confirmed through transport measurements.

In the second part of the thesis UHV-STM at 4.2 K has been used to study the surface of FGT at the atomic scale. STM images revealed regions of FGT that were pristine and Fe-intercalated. Also various kinds of intrinsic atomic vacancies namely - Te, Fe I and Fe II defects were imaged and identified unambiguously. Charge ordering is observed in Fe-intercalated regions in FGT samples as (√3a0 x √3a0)R30° modulation as well as 2a0 x a0. Spectroscopy measurements showed a decrease in Kondo temperature for Fe-intercalated regions.