Microscopic and spectroscopic studies of CrCl3 functionalised two dimensional magnetic material

After the recent discovery of the persistent existence of ferromagnetism down to the mono- layer of CrI3, a significant amount of research has been conducted to explore and identify the novel properties of Cr-based magnetic materials. For our project, we have purposefully chosen to study the CrCl3 material due to its distinctive behavior of exhibiting weak van der Waals interaction between the layers as well as in-plane ferromagnetic order. In this thesis work, we reported the growth of monoclinic single crystal CrCl3 and confirmed its crystal phase and purity through X-ray diffraction (XRD) and Raman spectroscopy. Our observations confirmed the higher stability of exfoliated CrCl3 under ambient conditions compared to other chromium halides (CrX3). Its degradation rate is approximately four orders of magnitude slower than CrI3. Then we present a detailed step-by-step procedure for the exfoliation of CrCl3 on diverse substrates, with the objective of determining the optimal SiO2 thickness that enhances the visibility of single and few layers of CrCl3. To demonstrate the effectiveness of our approach, we conducted experiments on 270 nm and 285 nm SiO2/Si (100) substrates, exfoliating flakes of larger dimensions than graphene using the same methodological steps. The layer number identification was carried out using a large dataset, which helped to measure optical contrast accurately based on the number of layers, up to six layers. The optical contrast data were analyzed using the Fresnel’s equation formalism to determine the real and imaginary components of the wavelength-dependent refractive index of the material. Consequently, we calculate the layer and wavelength- dependent absorbance and reflectance, followed by the evaluation of optical contrast curves that illustrate significantly high contrast only within specific wavelength bands. This finding is subsequently validated through experimental UV-Vis absorbance data. Moreover, a micro-Raman study was performed at a wavelength of 532 nm to study the layers, down to two layers, and no significant spectral shifts were observed in relation to the layer number or bulk material. Here, We analyzed mechanically exfoliated CrCl3 flakes using various techniques to investi- gate the electronic structures of pure, oxidized, and defective monolayer (ML) CrCl3 phases. Surface oxidation of CrCl3 flakes occurs upon air exposure, resulting in the formation of oxidized O-CrCl3 structures and chromium oxide at the flake edges. Pure ML CrCl3 is an insulator with a band gap of 2.8 eV. Oxidized and Cl defective phases of ML CrCl3 in exfoliated CrCl3 flakes exhibit in-gap spin-polarized states and notable modifications of the electronic band structures. The Cl 2p core level data exhibits distinct signatures indicating the presence of Cl vacancies on the surface. Then we performed high resolution photo-emission (PE) spectroscopy on air and UHV cleaved CrCl3 and found a stable, but only partially ordered Cl-O-Cr phase on the surface. By studying electronic core levels (Cl 2p, Cr 2p, and 3p), we quantify the electron charge transfer to the Cr atom resulting from this modification and the increased exchange in- teraction between metal and ligand atoms. The analysis of multiplet components using the CMT4XPS code demonstrates the favourability of charge transfer and the reduced crystal field due to the established polarization field. Contrary to claims suggesting the presence of significant Cl and Cr atomic vacancies, we exclude this possibility based on the observed sign and magnitude of the shift in binding energy of core level electrons. Our methodological approach holds significant potential for accurately determining the structure of ordered sub-oxide phases in mono or bi-layer Cr trihalides. At first, we focused on measuring bulk or thick samples. Later, with the help of specific beam arrangements, we were able to expand our measurements to thin flakes and quantify their layers using Scanning Photoelectron Microscopy (SPEM). Our experiment successfully confirmed the existence of an ordered phase on the surface. However, we also discovered that the formation of Cl vacancies, which make room for oxygen, is limited in thinner flakes compared to thicker ones due to the restricted diffusion process. To support our findings, we compared them with other surface-sensitive techniques like Kelvin Probe Force Microscopy (KPFM). Our observations revealed that the work function increases with the thickness of flakes, which may be related to the increasing density of Cl vacancies acting as dissociation centers.