Abstract
The ability of nanostructures to confine electrons at discrete energy levels makes them a promising platform for novel applications in chemistry, optics and information processing. However, these new exciting possibilities come with huge challenges associated with accessing and characterizing objects at the nanometer scale. The general objective of this thesis is to extend the use of atomic force microscopy (AFM), a technique known for resolving the force of individual atoms on a surface, to investigate single-electron charging of various nanostructures. My specific goal was to develop a technique to measure the electronics properties of nanostructure through tunnelling rate measurements, starting with (i) large metallic grains, where the charge becomes discrete although the electron energy level spacing is negligible, to (ii) quantum dots (QDs), where the reduced dimensionality leads to clear energy level spacing, and finally to (iii) organometallic molecules, where the system reorganizes to a different configuration upon the addition of an electron.
In the first chapters, I review the theoretical background of single-electron charging of a quantum dot electrostatically coupled with a mechanical oscillator and provide an efficient algorithm to calculate the response of the coupled electro-mechanical system using linear response theory. I then show that such system can be realized experimentally by using a low-temperature AFM operating at a temperature of 4 K. A simple design using a single-fiber to both excite a cantilever beam and detect its motion by interferometry is proposed. I also show that a calibration of the transfer function of the excitation system is necessary in order to accurately measure forces from measured spectra. Moreover, I demonstrate that nanostructures need to be embedded in a single tunnel junction to adequately examine them with AFM. The description of a low cost, wet lab approach developed to electrically insulate gold nanoparticles from a gold back-electrode by chemisorbing them on a self-assembled monolayer of alkanethiols is also included in this thesis.
One of the key contributions of the present work resides in chapter 5. In this chapter, theoretical and experimental studies of the effect of the density of states of a QD on the rate of single-electron tunneling that can be directly measured by electrostatic force microscopy (e-EFM) experiments are then exposed. In e-EFM, the motion of a biased atomic force microscope cantilever tip modulates the charge state of a QD in the Coulomb blockade regime. The charge dynamics of the dot, which is detected through its back-action on the capacitively coupled cantilever, depends on the tunneling rate of the QD to a back-electrode. The density of states of the QD can therefore be measured through its effect on the energy dependence of tunneling rate. Experimental data on individual 5 nm colloidal gold nanoparticles that exhibit a near continuous density of state at 77 K are presented. In contrast, analysis of already published data on self-assembled InAs QDs at 4 K clearly reveals discrete degenerate energy levels.
In the last chapter of this thesis, I exhibit early results showing for the first time that the reorganization energy of nanostructures upon charging can be measured through its unique effect on the response of the AFM at different oscillation amplitudes of the cantilever. First experimental accounts of that effect are provided by performing single-electron charging of ferrocene (Fe(C5H5)2) molecules.