Graduation Year

2022

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Humberto R. Gutierrez, Ph.D.

Committee Member

Manh-Huong Phan, Ph.D.

Committee Member

Garrett Matthews, Ph.D.

Committee Member

Stephen Saddow, Ph.D.

Keywords

Transition Metal Dichalcogenides, Two Dimensional Magnetism

Abstract

In recent years, spintronics has gained increasing interest due to the possibility of storing and processing information through the manipulation of both the charge and spin of an electron. Dilute magnetic semiconductors are ideal for the fabrication of such devices as they display carrier-mediated ferromagnetism which allows the electronic control of magnetism. Transferring these properties into the two-dimensional (2D) realm is very attractive for both fundamental research and novel applications. The recent discovery of long-range magnetic order in 2D materials has attracted a growing effort in the search for new functional 2D materials that can display ferromagnetic properties at room-temperature. Although intrinsic layered transition metal dichalcogenides (TMDs) show little to no magnetic properties, different approaches that include elemental substitutional doping, nanoparticle decoration, passivation, and defect engineering have shown promising outcomes in enhancing their transport properties as well as magneticproperties.

In this dissertation, we explored alternative post-growth techniques with the aim of inducing room- temperature magnetic order into 2D semiconductor TMDs, such as MoSe2. The MoSe2 monolayers were synthesized using a water assisted chemical vapor deposition (CVD) method developed in our lab. In the first project, the pre-grown MoSe2 films were introduced in a quartz tube reactor for a post- growth treatment at high temperature while MnCl2 powders were evaporated up stream at lower temperature. The resulting materials, 2D MoSe2 films decorated with Mn or MnOx nanoparticles (NPs), exhibit sizable room-temperature ferromagnetism (FM). However, this approach introduces a variety of elements, such as NPs containing Mn atoms, chlorine doping, and a high density of defects created at high temperatures in the reactive environment, that could be magnetically active on the MoSe2 monolayer. To separate these potential sources of magnetism, three distinct approaches were explored in the second and third projects of this dissertation. For the second project, hybrid materials with mixed dimensionality (0D/2D) were prepared by covering the MoSe2 monolayer with sub- nanometer overlayers of V2O5, MnO2 or FeCl3, thermally deposited in high vacuum. The TMD film was kept at room temperature to minimize defects formation. The resulting overlayer has NPs-like morphology, and the hybrid material presents room-temperature FM. Since this technique is compatible with the use of shadow masks, the overlayers were also deposited to form periodic 2D patterns of magnetic and non-magnetic domains that are seamlessly connected laterally. These novel structures, presented here for the first time, pave the way for further development of truly 2D magnetic metamaterials, such as 2D magnonic crystals. In the third project, we explored halogen doping as a potential source of magnetism in MoSe2. For chlorine doping, the CVD-grown MoSe2 was dipped in dilute HCl solution. The doping level was controlled by varying the HCl concentration, as well as the dipping time. After this acid treatment, the MoSe2 shows enhanced room temperature FM that depends on the reaction parameters. Other halogen elements, Florine and Bromine, were also explored, and a similar FM behavior was observed. In parallel, different methodologies to create defects in a controlled fashion were tested. For instance, plasma etching or exposure to UV radiation was conducted in controlled atmospheres. The UV treatment was combined with in situ passivation, which gave rise to the enhanced magnetism in MoSe2. Using shadow masks, once again, a site-selective UV exposure & passivation protocol was developed to create 2D patterns of engineered defects.

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