Emerging Methods and Applications in van der Waals Materials and Heterostructures

 Two-dimensional materials are advancing rapidly in condensed matter physics,  holding the potential to revolutionize nanotechnology through atomically thin conducting sheets for atomic-scale circuits. Since the discovery of the electric-field effect  in mechanically exfoliated graphene nanocircuits, various compounds with electrical properties ranging from metals and insulators to superconductors and magnets have  been identified. These materials, known as van der Waals (vdW) materials, exhibit  unique structural anisotropy and weak van der Waals bonds in one dimension, offering  numerous experimental avenues for exploration.  

This thesis explores innovative methods and applications of vdW materials and  heterostructures. We begin with a comprehensive introduction to vdW materials  and heterostructures, discussing their unique properties, fabrication techniques, and  potential. Next, we investigate the direct measurement of ferroelectric polarization in  bilayer WT_e2, demonstrating ferroelectric behavior in metallic systems and examining  the influence of carrier density on polarization.  

We introduce a novel tunneling spectroscopy approach using “via” contacts in  hexagonal boron nitride (hBN), enabling precise characterization of the density of  states in 2D materials such as NbS_e2 and graphene. This method enhances understanding of interface effects and barrier properties at the nanoscale.  

Further, we explore the integration of vdW heterostructures with programmable  substrates. First, we use ultra-low-voltage electron-beam lithography (ULV-EBL)  to pattern buried ferroelectric B-substituted AlN (Al_1−xB_xN) thin films, enabling  tailored electrostatic control over vdW layers and the creation of engineered superlat tices. Second, we focus on graphene interactions with the ferroelectric layer, detailing  synthesis, electronic properties, and transport phenomena in graphene/Al_1−xB_xN devices.  

Finally, we summarize the key findings and suggest future research directions to  advance and refine this work. Collectively, this thesis advances the understanding of  vdW materials and their heterostructures, showcasing innovative methodologies for  their characterization and integration, and laying the groundwork for future applications in nanoelectronics and quantum simulation.