Atomic and Electronic Structure of Quantum Dots Measured with Scanning Probe Techniques

Abstract

This thesis deals with low temperature scanning tunneling microscopy/spectroscopy and atomic force microscopy (LT-STM/STS and AFM) studies on colloidal semiconductor and graphene quantum dots (g-QDs). These nanostructures are interesting because they show tunable electrical and optical properties due to the quantum confinement effect. The high spatial resolution of scanning probe microscopy makes it possible to measure the topography and the electronic properties of individual QDs. In addition to the experiments on the different types of QDs, this thesis also includes our experiments on the moiré pattern of an epitaxial graphene monolayer on Ir(111) and the measurement of the intermolecular interaction between two carbon monoxide molecules probed with the frequency modulation AFM (FM-AFM). In the first chapter, we give a general introduction to the working principles of STM/STS and FM-AFM. In addition, we introduce the basic concepts of double barrier tunneling junction, electron-phonon coupling in semiconductor nanocrystals and the use of a tuning fork force sensor in AFM. In chapter 2, we discuss electron-phonon coupling in CdSe quantum dots (spheres and rods) probed by LT-STM. We resolve the polaronic states of the QDs, which are induced by the coupling between the tunneling electrons and longitudinal optical phonons. Our data show how the electron-phonon coupling strength depends on the orbital envelope function in these QDs and how the conductance spectrum evolves from shell-tunneling into the shell-filling regime once the tip is placed closer to the sample. Numerical calculations based on a dielectric continuum model can reproduce the experimentally measured electron-phonon coupling strength. In chapter 3, we discuss the quantum transport properties of PbSe-CdSe core-shell QDs measured by LT-STM. We observed a series of equally spaced double resonances in the conductance spectra of these QDs. The phenomenon is explained by hole induced electron transport, the hole occupation in the PbSe core enabled by the strong and relatively symmetric potential barrier of the CdSe shell. In chapter 4, the moiré pattern of an epitaxial graphene monolayer on the Ir(111) surface investigated with tuning fork based FM-AFM is presented and discussed. We applied FM-AFM to measure the geometric corrugation of the moiré pattern more directly. In chapter 5, we present our investigation on the confined electronic states in g-QDs with atomically well defined structure on Ir(111). We map the spatially resolved tunneling conductance that allows us to visualize how the confined states evolve with the energy in real space. The low energy carriers still have a linear dispersion down to the smallest g-QDs we have measured (about 2nm in diameter). Chapter 6 is devoted to the FM-AFM experiments on probing the intermolecular interaction between two carbon monoxide molecules. With one CO molecule attached to the AFM tip apex, we measure how the intermolecular interaction depends on the distance between the molecule on the tip and another molecule adsorbed on the Cu(111) surface. We found that at a close distance (few Å), there is a repulsive interaction induced by the Pauli repulsion between the electrons in the outer orbitals in these molecules.