The Effect of Intercalation on the Structure and Properties of Layered MSe2 (M = Nb, Ti)

Abstract

Layered (2D) materials are an intriguing class of materials which has fascinated the researchers around the globe due to their excellent mechanical flexibility, reduced dimensionality and interlayer van der Waals interaction. The tunable dimensionality, availability of larger surface area and weak interlayer interaction of these 2D materials provides manifold applications in the field of electronics, optoelectronics, catalysis, energy generation and storage, spintronics, chemical and biological sensors, solar cells, supercapacitors, etc. 2D materials covers various classes of layered materials starting from organic layered materials, layered oxides, layered halides and layered chalcogenides. Particularly, layered transition metal dichalcogenides (LTMDs) have attracted enormous research attention because of their broad range of electronic properties from insulator to superconductors. Further, the structural and physical properties of LTMDs, specifically group IV, group V, and group VI metal chalcogenide, can be tuned by intercalating with various intercalants. The thesis highlights the effect of various type of intercalant on the structure and properties of 2H–NbSe2 and 1T–TiSe2. The thesis can be divided majorly into seven chapters, where the first two chapters include introduction to LTMDs, synthetic process and characterization tools used to study the various effects of intercalation. An introduction to the LTMDs, the polytypic behavior, charge density wave (CDW), superconductivity, intercalation process is briefly presented in chapter one. Chapter two details a comprehensive synthetic process i.e., high temperature solid-state method and the associated processes for synthesis of pure phase intercalated LTMDs. Further to identify and analyze the phases, various characterization techniques that applied along with the working theories, instrument details and data collection conditions are entailed in chapter two. The third, fourth and fifth chapters detail the effect of p-block, s-block and d-block element intercalant, respectively on the structural and physical properties of 2H–NbSe2. The detrimental effect of Sn (p-block) intercalation on the superconductivity of 2H–NbSe2 is studied in chapter three on the basis of data obtained from powder XRD, Raman spectroscopy, magnetic and resistivity properties measurements. The intercalation process suppresses the superconductivity of 2H–NbSe2, which is related to the lattice expansion due to the insertion of Sn in the vdW gap and a plausible explanation on the valence factor of intercalant on the properties is included. On the other hand, Mg (s-block element) have a minimal effect on the superconductivity and also on the structure. The said effect has been explained through theoretical and experimental studies and attributed to the unfavorable interaction of Mg with NbSe2 layers in MgxNbSe2. Furthermore, Fe (d-block element) has an interesting effect on the structural property of 2H–NbSe2. At 900 ℃, a mixture phase (2H+4H) is obtained for pristine NbSe2. Intercalation of Fe in the octahedral void of vdW gap results in three distinct type of phase transition in FexNbSe2. In composition range 0 ≤ x < 0.2 the mixture phase transforms to 2H phase where Fe is randomly distributed throughout the vdW gap. Further increase in Fe content in FexNbSe2 (0.2 < x ≤ 0.25), the 2H phase transform to an ordered 2𝑎𝑜 x 2𝑎𝑜 x 1𝑐𝑜 superstructure denoted as 2H(I) phase, which crystallizes in the same space group (P63/mmc) as that of the parent compound and Fe atoms occupy alternate octahedral (Oh) void in an ordered manner. Finally in the range of 0.25 < x ≤ 0.5, the 2𝑎𝑜 x 2𝑎𝑜 x 1𝑐𝑜 superstructure transforms to √3𝑎𝑜 x √3𝑎𝑜 x 1𝑐𝑜 superstructure with a polar space group P6322. Intercalation of Fe induces antiferromagnetism. The disordered phase shows weak short range antiferromagnetism and spin glass like behavior below 25 K. The ordered phase, Fe0.25NbSe2, show a long range antiferromagnetic ordering at a much higher temperature as compared to the disordered phase with TN = 144 K. Further increase in Fe content, the antiferromagnetic ordering temperature decreases to 60 K for Fe0.40NbSe2. The variation of the magnetic ordering temperature strongly correlated with the structure and position of magnetic ions. The sixth chapter describes the effect of Sn, Pb and Dy intercalants on the structure and properties of 1T–TiSe2. Particularly, a detailed study on the effect of Dy intercalation on the structure and magnetic properties has been carried out. Dy has a minimal effect on the structure of 1T–TiSe2, whereas intercalation of Dy induces paramagnetism till 4% and further intercalation results in antiferromagnetic ordering below 4.5 K. The last chapter includes the summary, future works and bibliography. In summary, the various factors which has been found to affect the extent of intercalation and the effect of intercalation on 2H–NbSe2 and 1T–TiSe2, has been correlated and categorized on the basis of type of intercalant.