Field emission regime of the scanning tunnelling microscope

This thesis presents the development and application of the scanning field emission microscopy (SFEM) as a surface-sensitive imaging and spectroscopy tool capable of probing nanoscale variations in local electronic structure. Operating in the field emission regime of a dual-mode STM system, SFEM enables high-resolution mapping of absorbed and secondary electron signals, offering complementary contrast mechanisms to conventional STM at higher electron energies. The transition from STM to SFEM is achieved by retracting the tip by several nanometres and applying a high negative bias, generating a focused primary electron beam that interacts with the sample surface. Particular emphasis is placed on understanding the origins of absorbed current contrast and correlating it with underlying material properties.
The thesis begins with an introduction including a detailed project motivation and chapter overview. Chapter 2 outlines the theoretical background of STM and SFEM, including tunnelling and field emission theory as well as electron–solid interactions. Chapter 3 describes the instrumentation, tip preparation and acquisition protocols for dual-mode STM and SFEM operation, along with secondary electron detection and complementary techniques. Chapters 4, 5, 6 present the results of SFEM across both metallic and semiconducting surfaces. Chapter 4 demonstrates elemental contrast with Fe and Er thin films on W(110) using absorbed current imaging and STM, supported by field emission resonance spectroscopy to quantify local work function variations. Chapter 5 reveals chemically phase-separated domains within co-deposited Fe–Er alloy nanoislands, resolved through SFEM. Chapter 6 applies SFEM to Er silicide growth on Si(111), showing contrast mechanisms linked to geometry and electronic structure and introduces energy-resolved spectroscopy to detect electron energy loss features.