Abstract:
This thesis aims to create a chemical approach for fabricating controlled Fe-Se system
nanoparticles and comprehending the formation process. Chapter 1 of this thesis gives an
overview of the history and progress of essential formation processes for better understanding and
controlling the Fe-Se system properties. Chapter 2 investigates the theoretical crystal habit of all
the phases of the Fe-Se system using BFDH (pure crystallographic approach) and HP (periodic
bond chain vector approach) models. Chapter 3 deals with the Fe-Se system's complete phase
transformation sequence using a wet-chemical method, from plotting the phase diagram to
optimizing the appropriate conditions required to synthesize individual phases. Finally, the cause
for the similarities and dissimilarities in theoretical crystal habits and real-life experimental
morphologies is examined. Chapter 4 explains the correlation between the phases and the
magnetic properties of the Fe-Se system. The magnetic properties vary enormously with change
in Fe:Se ratio due to the change in Fe2+/Fe3+ ratio, crystal field environment around Fe-ions,
magnetocrystalline anisotropy, Fe-vacancies, and so forth. It is observed that Fe3Se4 has the most
unique and interesting magnetic properties. Chapter 5 deals with the growth directions of Fe3Se4
in different reaction conditions. The entire roadmap is laid-out―starting from the formation of
the unit cell to the diffusion and attachment of monomers and the fate of various facet growth.
Finally, we have shown how the distinct growth in various facets influences the magnetic
properties. In chapter 6, a conscious effort has been made to fabricate the monoclinic M3Se4
compounds (where M can be Fe, Co, or Ni). The influence of transition metal (M) on the magnetic
properties is investigated for monoclinic M3Se4 NPs. The Fe3Se4 is observed to be well-known
ferrimagnetic with a Curie temperature of nearly 322 K. However, the other two compounds,
Co3Se4 and Ni3Se4, are examined to be paramagnetic over the whole measurement temperature
range (5 K to 300 K). Chapter 7 summarizes the work done in this thesis and makes
recommendations for future study in many domains.
