Relevant Degree Programs
Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - Nov 2019)
At the atomic scale, surfaces exhibit a rich variety of physical phenomena that can be probed using a scanning tunnelling microscope (STM). The STM measures the quantum tunnelling of electrons between a metallic tip and conducting sample and can be used to characterize the nanoscale surface. This thesis presents STM measurements taken at low-temperature in ultra-high vacuum, which are used to characterize two different nanoscale environments: the two-dimensional surface states of Ag(111) and Cu(111) and the magnetic moments of iron and cobalt atoms deposited on a thin-film of magnesium oxide.Fourier-transform scanning tunnelling spectroscopy (FT-STS) analysis of quasiparticle interference, created by impurity scattering on the surfaces of the noble metals Ag(111) and Cu(111), is used to compare the most common modes of acquiring FT-STS data and shows, through both experiment and simulations, that artifact features can arise that depend on how the STM tip height is stabilized throughout the course of the measurement. Such artifact features are similar to those arising from physical processes in the sample and are susceptible to misinterpretation in the analysis of FT-STS in a wide range of important materials. A prescription for characterizing and avoiding these artifacts is proposed, which details how to check for artifacts using measurement acquisition modes that do not depend on tip height as a function of lateral position and careful selection of the tunnelling energy.In a separate set of experiments a spin resonance technique is coupled to an STM to probe the spin states of individual iron atoms on a magnesium oxide bilayer. The magnetic interaction between the iron atoms and surrounding spin centres shows an inverse-cubic distance dependence at distances greater than one nanometre. This distance-dependence demonstrates that the spins are coupled via a magnetic dipole-dipole interaction. By characterizing this interaction and combining it with atomic manipulation techniques a new form of nanoscale magnetometry is invented. This nanoscale magnetometer can be combined with trilateration to probe the spin structure of individual atoms and nanoscale structures. The information gained characterizing these new forms of magnetic sensing sets the stage for the study of complex magnetic systems like molecular magnets.
Organic semiconductors are a promising class of materials for many applications such as photovoltaics, light emitting diodes, and field-effect transistors. As these devices rely on the movement of charge at and near interfaces, understanding energy level alignment at these boundaries is essential to improve device performance. Differences in the local environment and surrounding molecular geometry have the potential to cause significant energy level shifts occurring on single molecule length scales, thus affecting device properties. Scanning Probe Microscopy is a family of techniques that allows investigation of materials on the molecular and submolecular level. Scanning Tunneling Spectroscopy (STS) allows for the mapping of electronic states with spatial and energetic resolution. Electrostatic Force Spectroscopic (EFS) mapping investigates the local charge distribution of surfaces even down to submolecular resolution. We utilize these techniques to investigate the prototypical semiconductors PTCDA and CuPc on NaCl(2ML)/Ag(111). Nanoislands of PTCDA were examined with STS, revealing strong electronic differences between molecules at the edges and those in the center, with energy level shifts of up to 400 meV. We attribute this to the change in electrostatic environment at the boundaries of clusters, namely via polarization of neighboring molecules. To further investigate the local electrostatics, we use EFS to probe the effect of adding charge to PTCDA molecules, both isolated and within clusters. We found that the charging energy depends on the initial local charge distribution by spatially resolving the charging events with sub-molecular resolution. In order to investigate the influence of interface geometry, we use pixel-by-pixel STS of the prototypical acceptor/donor system PTCDA/CuPc. We observe shifting of the donor and acceptor states in opposite directions, indicating an equilibrium charge transfer between the two. Further, we find that the spatial location of electronic states of both acceptor and donor is strongly dependent on the relative positioning of both molecules in larger clusters. The observation of these strong shifts illustrates a crucial issue: interfacial energy level alignment can differ substantially from the bulk electronic structure in organic materials. This has significant implications for device design, where energy level alignment strongly correlates to device performance.
Organic-based technologies have recently attracted significant interest. Characterization of their structure and properties at native length scales are essential for their implementation in devices. On-surface self-assembly of metal-organic frameworks is a simple way to fabricate molecular systems with specific functionalities. In this thesis work, the morphology and electronic structure of self-assembled linear nanochains, featuring a triiron linkage between two bisterpyridine-based ligands on an Ag(111) surface, have been investigated with scanning tunneling microscopy and spectroscopy. An in situ, clean and reliable on-surface preparation technique was developed for thermally-activated self-assembly of complexes based on the metal-organic motif of dyes used in photovoltaic and catalysis applications. Tunneling spectroscopy on the metal-organic nanostructures obtained suggests the formation of a coordination bond with charge transfer between metal and ligand. Furthermore, the electronic structure indicates the presence of the desired metal-to-ligand charge transfer optical transitions, characteristic of the related complexes. The unprecedented triiron coordination link has potential for being an efficient reaction center for catalysis applications, as well as for having interesting magneto, spin, and electronic properties. Each step and aspect of the chains formation process has been characterized via scanning tunneling microscopy measurements and growth studies, and the results are supported by density functional theory calculations. Additionally, the relevance and influence of the silver metal substrate on both bare ligands and chains has been investigated. Bare molecules show a strong interaction with the substrate, as demonstrated by their specific adsorption configurations and an electronic structure which is distinct from when they are electronically decoupled from the surface by an NaCl bilayer. When the molecules are in chains the silver plays a key role in the structure of the coordination link. This work shows the potential of using on-surface self-assembly and scanning tunneling microscopy and spectroscopy, not only to prepare with high-fidelity clean and controlled structures but also as a flexible platform to investigate and tailor functional properties of different systems for a large variety of applications where a solid support is essential.
Master's Student Supervision (2010 - 2018)
Developing a bottom-up understanding of the physics behind charge transfer processes on the nanometer scale will enable the focused design and synthesis of new materials which will revolutionize everything from solar cells to wearable electronics. Pushing our understanding of these processes to the nanometer scale is critical for next generation device development for two primary reasons. Firstly, modern electronic devices are fabricated ever smaller; to date IBM Research has already produced working chips using with gate widths only14 atoms (7 nm) wide . Secondly, for many devices which rely on charge transfer the important action is at the interface between materials; it is here that the energy level offset and other parameters can make or break a device. For modern organic devices, the interfacial region is in essence a nanometer-wide region: energy levels can differ by hundreds of meV only a few molecules awayfrom an interface . This thesis presents the design and execution of experiments which couple optical measurements with a scanning probe system. The marriage of optical and scanning probe systems enables simultaneous exploration of two complementary dimensions (optical and electronic) of the physics of the system under study, enabling the probing of parameters affecting charge transfer between single molecules. The custom-built system was used to explore optical and electronic properties of two prototypical organic molecules forming an acceptor-donor pair: 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) and copper (II) pthalocyanine. This proof-of-concept will allow future users to explore a wide variety of systems which may offer clues to how charge transfer processes occur at the nanometer scale.In the first part of this work I describe the motivation for our experiment as well as the experimental design and set-up. In the second part I detail how we used the enhanced optical-electrical scanning probe to observe real-space energy levels, luminescence (or lack thereof), and attempted optical excitations between two single organic molecules. Analysis of scanning tunnelling spectroscopy datacoupled with laser excitation as well as the results from experiments which in principle can measure sub-molecularly resolved luminescence show that the new optical system works as expected.