Graduation Year

2023

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Manh-Huong Phan, Ph.D.

Committee Member

Hariharan Srikanth, Ph.D.

Committee Member

Sarath Witanachchi, Ph.D.

Committee Member

David Mandrus, Ph.D.

Committee Member

Minh Tuan Trinh, Ph.D.

Keywords

transition metal dichalcogenides, low-dimensional, ferromagnetism, van der Waals magnets

Abstract

Atomically thin transition metal dichalcogenide (TMD) semiconductors hold enormous potential for modern optoelectronic devices, magnetic sensing, and quantum communications. Long-range ferromagnetism can be induced in TMD monolayers by introducing a small amount of magnetic dopants to form diluted magnetic semiconductors (DMSs), making them promising candidates for applications in ultralow-power and ultracompact spintronic nanodevices. This dissertation research aims to address two contemporary challenges in the fields of nano-magnetism and spintronics: controllability of magnetic dopants in TMD monolayers in the form of MX2 (M = W, Mo; X = S, Se) and understanding the fundamental physical mechanisms that give rise to magnetism in these two-dimensional (2D) systems. Magnetic transition metal (V, Fe) -doped TMD monolayers were synthesized by an advanced liquid-phase precursor-assisted chemical vapor deposition (CVD) method. The samples were then characterized utilizing high-resolution scanning transmission electron microscopy (HR-STEM), vibrating sample magnetometry (VSM), magnetic force microscopy (MFM), the anomalous Hall effect (AHE), and optical (photoluminescence and Raman) measurements. Our breakthrough discoveries have included the first demonstration of the tunable and robust ferromagnetism in vanadium-doped tungsten dichalcogenide monolayers (e.g., V-doped WS2 and V-doped WSe2) at room temperature. The saturation magnetization is optimized by varying the vanadium concentration and this approach achieved the highest magnetic moments at an optimal doping level ever attained for atomically thin V-doped TMDs. In addition, we discovered thermally induced spin flipping effect in the V-doped WSe2 monolayers, which can be achieved at low external magnetic fields and manipulated by modifying the V concentration. As the underlying origin of ferromagnetism in such 2D systems has remained elusive due to the combined intrinsic (from magnetic dopants) and defective (from vacancies) magnetic contributions, our current efforts are to decouple these contributions to enhance the magnetic moment through a combined experimental and theoretical study on MX2 monolayers with tunable V and Fe dopants. Combined with HR-STEM, VSM, and MFM studies, and density functional theory calculations we are able to shed light on how dopant and dopant-induced spins couple to induce ferromagnetic (FM) or antiferromagnetic (AFM) order when their distances are tuned by doping, as well as their coupling to spins induced by defects or vacancies. By interfacing these 2D-DMSs with graphene or two different TMDs to form 2D van der Waals heterostructures such as V-MoS2/graphene and MoS2/WS2, we show the new ways of harnessing the excellent magnetic and magneto-optical properties in these 2D systems. This study paves a new pathway for the development of novel 2D-TMD magnetic semiconductors to fulfill the increasing requirements of ultrafast spintronic and valleytronic devices and forms a new foundation for the fundamental understanding of 2D van der Waals heterostructures for the future of quantum communications and technologies.

Download

Share

COinS