Cryogen-free low temperature high magnetic field ultra-high vacuum modular scanning tunneling microscope for the exploration of van der waals materials and nano-devices
Creators: name: Coe, Angela Mae
Publisher: name: No Publisher Supplied
Year: 2024

description: Scanning probe microscopy (SPM), particularly scanning tunneling microscopy (STM), has become a critical tool for locally probing material properties at the atomic level. STM continues to evolve to meet diverse and challenging research demands. Advancements have enabled high-resolution imaging and detailed analysis of complex materials and quantum states at low temperatures (LTs). However, challenges arise with LT-STM operation due to helium shortages, affecting the availability and cost of liquid helium which is essential for maintaining LTs. To address these challenges, cryogen-free closed-cycle cryostats (CCCs) have emerged, offering a sustainable alternative to liquid helium systems, since helium gas is recycled and liquid helium is not consumed. However, CCCs introduce high levels of mechanical vibration and acoustic noise, complicating integration with STM systems. Additionally, the need for ultra-high vacuum (UHV) environments and high magnetic fields further complicates STM design, particularly for experiments involving μm-sized 2D material sample devices. This thesis presents the design, development, and performance of an innovative STM head that integrates modularity, flexibility, stability, and compact size. The STM head is equipped with a quick-connect socket that is matched to a universal connector plug, enabling it to be transferred between systems. This head can be introduced into a vacuum system from room temperature and seamlessly transferred to various sites equipped with the connector plug, permitting multi-site STM operation is a variety of experimental conditions, including broad temperature ranges, ultra-high vacuum, high magnetic fields, and closed-cycle pulse-tube cooling. The STM’s compact size is achieved by a novel nested piezoelectric coarse walker design which allows for large orthogonal travel in the X, Y, and Z directions, ideal for studying both bulk and thin film samples ranging in size from mm to μm. Its stability and noise tolerance are demonstrated by achieving atomic resolution in ambient conditions on a laboratory desktop with no vibrational or acoustic isolation. This thesis introduces the design, construction, and performance of a cryogen free, UHV, LT, and high magnetic field platform for modular STM operation at variable temperatures, a base temperature of 4K, and magnetic fields up to 9T. This platform incorporates a novel internal vibration isolator to mitigate vibrations and a triple wall vacuum design to achieve low base temperatures. The CCC is incorporated into a UHV platform that supports in-situ sample and tip conditioning and alignment. Multi-site operation without breaking vacuum is facilitated by the quick-connect STM head and vacuum transfer methods, notably a low-profile vertical transfer mechanism.This thesis presents an internally mounted flexible mechanism that replaces large externally mounted vertical translation stages by utilizing collapsible components and gravity assisted tools to insert and extract the STM head into the LT region. This design substantially reduces the size of the vertical transfer mechanism which enables installation in standard laboratory spaces. The design eliminates also excess pumping volume and reduces susceptibility to vibrational noise. The combined innovations presented in this thesis significantly enhance STM capabilities, providing a platform for studying a wide range of samples under various experimental conditions. To demonstrate the system’s capabilities, and exploration of van der Waals heterostructures was conducted. This thesis presents the study of magic angle twisted-bilayer graphene (TBG) nearly aligned to hexagonal boron nitride (hBN). The stacking dependence and rotational misalignment of the layers is explored through STM topography and spectroscopy of the moiré superlattice to explore the formation of a moiré intercrystal.