Structural and magnetic studies of core-shell and hollow nanoparticles

Quy K Ong, Purdue University

Abstract

Nanoparticles can self-assemble or be directed to assemble into ordered structures, as determined from balance of forces acting upon them. In Chapter I, the concept of self-assembly and directed assembly is introduced followed by a discussion on surface and interparticle forces. Externally applied fields are particularly useful in directed assembly, and can be magnetic, electric, or optical forces in origin. In this chapter, we investigate methods leading to the directed assembly of cobalt nanoparticle rings around gold nanowires (nano-rotaxanes), and show that both dielectrophoretic and electrophoretic forces can be used to create such nanoscale heterostructures. Surfaces and material interfaces often play a significant role in governing the magnetic properties of nanoparticles. In Chapter II, we study the magnetic behaviors of core-shell Fe@Fe3O4 and hollow Fe 3O4 nanoparticles and reveal an unusual exchange-bias effect related to interfacial frozen spins. Hysteresis measurements of core–shell particles at 5 K after field cooling exhibit a large loop shift associated with unidirectional anisotropy, whereas Fe3O4 hollow nanoparticles support much smaller shifts. Both core–shell and hollow particles exhibit sharp demagnetization jumps at low fields associated with a sudden switching of shell moments. Temperature-dependent magnetizations of core–shell particles at high field show a deviation between field cooling and zero field cooling curves below 30 K, suggesting the presence of frozen spins at the interface. These frozen interfacial spins play an important role in mediating the exchange coupling between the ferromagnetic core and ferrimagnetic shell. We have also explored several experimental conditions that affect the relative intensity of the interfacial frozen spins such as temperature, repeated measuring field cycling, and age-dependent oxidation. With respect to nanoparticle synthesis, transitional-metal nanoparticles are often prepared by solvothermal routes or by the reduction of ionic salts. In chapter III, we show that TOPO has the potential to mediate the solvothermal synthesis of Co nanoparticles via a redox coupling process. The viability of the redox coupling is established by using TOP to convert Co-oleate into Co nanoparticles. Co nanoparticles synthesized from this route can then be coated with a thin shell of iron oxide, and further transformed into hollow cobalt ferrite nanoparticles upon heating or irradiation by a TEM electron beam. The transformation is accompanied by an enlargement of particle size, and is affected by environmental factors. Magnetic studies revealed that hollow cobalt ferrite nanoparticles possess higher blocking temperatures than their parent core–shell Co@FexOy nanoparticles. In Chapter IV, ultrathin Au nanowires (d<2 >nm) with extremely high aspect ratio were synthesized by reduction of Au(III) in oleylamine (OAm), and carefully analyzed by a combination of HRTEM and STM studies. While the nanowires are highly crystalline with apparent growth along the <111> direction, some segments are marked with amorphous defects, which we postulate to be an ionic Au(I)-OAm complex. The growth mechanism is proposed as template nucleation and anisotropic growth, in conjunction with oriented attachment.

Degree

Ph.D.

Advisors

Wei, Purdue University.

Subject Area

Physical chemistry

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