Scanning Tunneling Microscopy and Luminescence of Individual Nanostructures

Abstract

A detailed investigation and characterization of the local properties of individual nanoscopic structures is of great importance for the understanding of novel physical phenomena at the nanoscale as well as for the assessment of their possible use in future applications. A unique tool for the study of such structures are scanning probe methods. These methods do not only allow to obtain atomic scale information in both topographic and spectroscopic measurements at surfaces, but can also be employed for example to stimulate local photon emission. In this thesis the growth mechanism of inorganic and organic semiconductor nanostructures and the relation between their local structure and their electronic and optical properties is investigated by scanning probe techniques. In the first part of this thesis, the growth as well as the local electronic and morphologic properties of InAs/GaAs(001) quantum dots grown by molecular beam epitaxy are described. The recorded spectra show evidence of the discretization of the density of states that can be attributed to the zero dimensional confinement of charge carriers to the quantum dots. Moreover, the shape evolution of the quantum dots during the controlled removal of material through an in situ etching gas is studied. High resolution topographic images reveal that hereby an island shape transition takes place. This shape evolution is found to be the reverse of the shape evolution that typically takes place during the growth, indicating that thermodynamic factors play an important role during the growth process and the etching process. In addition, the mechanism of the material removal from the quantum dots is investigated in detail by the analysis of the size evolution of the islands as a function of the nominal amount of etched material. The second part of this thesis is dedicated to the study of individual CdSe nanowires grown by the solution-liquid-solid approach. These nanowires consist of small alternating sections of wurtzite and zincblende lattice structures. This structural arrangement also leads to an alternating electronic structure in the form of a type II band alignment along the long axis of the nanowires. The local electronic properties of the nanowires are analyzed by scanning tunneling spectroscopy. Tunneling current induced luminescence spectra from individual nanowires are acquired by the injection of holes and electrons and their subsequent radiative recombination. In contrast to previous studies on similar systems, the coupling of tip induced plasmons to the radiation process is not required. The shape and the position of the luminescence spectra is compared to photoluminescence spectra and Raman measurements. The photon energy is found to decrease with increasing wire diameter indicating quantum confinement of the charge carriers to the wire. The bulk bandgap, that can be extrapolated from the energy vs. diameter dependence of the emission peaks, indicates that the photons are emitted from zincblende type CdSe sections. Moreover, the diameter dependence reveals that the emission stems from carriers that are confined to quantum dot like parts of the wires. The lack of light emission from the wurtzite type segments is also re ected in the appearance of non-luminescent regions in the NW as shown by spatially resolved light intensity maps. Possible mechanisms for the light excitation are discussed. The investigation of single molecules of the luminescent organic emitter tris-(2-phenylpyridine)iridium(III) (Ir(ppy)3) is the topic of the last part of this thesis. To this end, Ir(ppy)3 molecules are deposited onto metal substrates and different types of thin insulating layers. Besides their structural and electronic characterization, the optical properties of single molecules are studied by tunneling current induced luminescence. No intrinsic light emission originating from the molecules can be observed neither on metal substrates nor on thin insulating layers. This indicates that in these systems, the molecules are still not sufficiently decoupled from the underlying metallic substrate. Preliminary results of tunneling current induced luminescence measurements of Ir(ppy)3 molecules on multilayers of C60 show great promise for exciting light emission on single molecules.