Sarah Burke

Associate Professor

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Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2021)
Scanning tunnelling microscopy of topological materials (2021)

Topological materials have been at the forefront of condensed matter physics research over the past few decades. Characterised by electronic bands with non-trivial topological invariants, topological materials exhibit a number of interesting electronic properties, such as conducting chiral boundary states and linear electronic dispersions, and have been theorised for use in a variety of applications ranging from spintronic devices to quantum computing. Recently, topological semimetals were discovered, where the bulk electronic bands are understood in the framework of the high-energy relativistic Dirac equation and its conditional variations, the Weyl and Majorana equations. Furthermore, the vast permutations of material compounds available results in a nearly infinite sandbox for researchers to study, which has resulted in topological semimetals that have no high-energy analogue. One of such material classifications is the nodal-line semimetal, characterised by linear electronic band crossings that form lines or loops in momentum space. These nodal-line semimetals also exhibit exotic surface states, named drumhead states, which are an interesting and exciting new state with promises in high-temperature superconductivity and quantum computation. A large effort is being placed to find materials that can be used to study the fundamental properties of these materials and their resultant surface states. Scanning tunnelling microscopy (STM) provides a perfect tool to study the topological properties of materials, able to atomically resolve the surface structure and also provide insight into scattering selection rules, which are deeply dependent on the band topology. Two topological materials were studied using STM in this thesis: the topological nodal-line semimetal ZrSiTe and the topological insulator (Bi$_x$Sb$_{1-x}$)$_2$Te$_3$. ZrSiTe was studied with an emphasis on the quasiparticle scattering characteristics, measured using Fourier-transform scanning tunnelling spectroscopy. Two main scattering features are examined, one relating to the nodal line, and the other arising from the drumhead surface state. These studies mark the first time a drumhead state has been observed using a real space measurement. (Bi$_x$Sb$_{1-x}$)$_2$Te$_3$ was studied with an emphasis on the nano-scale transport characteristics, measured using 4-probe STM and scanning tunnelling potentiometry. Effects of step edges and domain boundaries on the local resistance are studied for a fractional substitution of $x = 0.19$.

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Investigations in Flatland: scanning tunnelling microscopy measurements of noble metal surface states and magnetic atoms on magnesium oxide (2018)

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.

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Single molecule perspectives of model organic semiconductors: Energy level mapping by high-resolution scanning probe microscopy (2017)

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.

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On-surface self-assembly and characterization of a macromolecular charge transfer complex by scanning tunneling microscopy and spectroscopy (2016)

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.

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Master's Student Supervision (2010 - 2020)
Scanning probe study of organic semiconducting molecules (2020)

When compared to conventional inorganic semiconductors, organic semiconductors are lightweight, flexible, and compatible with less expensive high-throughput manufacturing techniques. Applications of organic semiconductors in power generation and light emitting applications have been realized through the development of organic photovoltaic (OPV) and organic light emitting diode (OLED) devices. However, to optimize the performance and efficiency of these applications, the molecular orbital energy and the role of the exciton in charge generation and luminescence in organic materials need to be further explored. In this work, scanning probe microscopy (SPM) techniques including scanning tunnelling microscopy (STM), scanning tunnelling spectroscopy (STS), and scanning tunnelling microscopy luminescence (STML) were used to probe the electronic and optical properties of individual organic molecules deposited on insulating NaCl layers on a metallic substrate. Pixel-by-pixel STS energetically and spatially resolves molecular orbitals. Concurrent STML induces molecular luminescence through electron tunnelling, giving spectral information of the excitons and vibrational modes of the organic molecule on sub-nanometre length scales. The results presented here are the first signals of molecular emission obtained from our microscope, demonstrating the capability of our system in detecting single molecule luminescence. Conventional organic molecules 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) and zinc (II) phthalocyanine (ZnPc) were studied and the results compared to those presented in the literature. SPM was also performed on F₈ZnPc to explore the effects of fluorination. Our results revealed that electronic and structural changes due to the additional fluorine atoms and interactions with the substrate can affect the energy levels and luminescence of the molecule. Organic donor-acceptor molecules based on the hexamethylazatriangulene (HMAT) complex were also studied. The effects of various acceptor groups on the energetic gap and spatial distribution of molecular orbitals were explored for different HMAT derivatives using STS. We demonstrate the effects of gap engineering at the sub-molecular level for this promising class of optoelectronic organic materials.

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Organic photovoltaic films: the commissioning of and preliminary measurements on an organic molecular beam epitaxy system (2019)

Organic Photovoltaics (OPVs) may provide a means of achieving flexible and transparent solar cells, comprised of inexpensive materials and created through scalable processes. Compared to today’s dominant silicon-based solar cells, OPVs suffer from lower power conversion efficiency, and a principal barrier to efficient power conversion in OPVs lies in the separation of generated charges. In OPVs, photoabsorption results in a coulombically bound exciton; in order to generate free charges, we must engineer exciton dissociation. Thus, an understanding of the dynamics involved in exciton dissociation and the underlying electronic states that drive this separation is requisite to increasing the power conversion efficiency and developing commercially-viable OPV devices. In order to do this, we intend to map the energy landscapes of the system on a femto- to picosecond timescale, as well as an Angström length scale. To facilitate this mapping, we will use a combination of time- and angle-resolved photoemission spectroscopy (TR-ARPES) and scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) to analyze our films. Using a femtosecond pump-probe scheme, TR-ARPES measures the dynamic spectral properties of a system by monitoring a material’s electronic states after excitation. STM/S provides local information on the electronic structure, including both occupied and unoccupied states. Combined, these measurements will facilitate the understanding of energy level alignment, the band structure of the system, and the evolution of the excited states. Because of the quality and purity requirements for the samples, as well as the fragility of organic thin films, we must grow our films in-situ in a UHV environment. Over the past two years, we have designed and commissioned an organic molecular beam epitaxy (OMBE) growth chamber as well a home-built low energy electron diffraction (LEED) characterization chamber that is attached to an ARPES system. This thesis discusses the motivation and background information for this project in further detail, presents the experimental techniques required to understand and operate the OMBE and LEED chamber, describes the commissioning process of the OMBE, and touches on our preliminary growth recipes and data acquisition.

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Site Specific Heterogeneous Catalysis: CO and C2H4 Bonding with Fe-Terpyridine on a Ag(111) Surface Studied by Scanning Probe Microscopy (2019)

Heterogeneous catalysis is a key process in the manufacturing of chemicals, clean fuels, plastics, and pharmaceuticals, and often involves gaseous reactants catalyzed by solid surfaces producing gaseous products. At the solid-gas interface, a variety of potential nucleation sites and reaction pathways for chemical transformations exist. To track these different pathways and distinguish the main reaction from side reactions, it is essential to explore the surface site-by-site, at the atomic scale. Experimental techniques offering the ability to probe surfaces on a site- specific, atomic scale are limited, and rigorous knowledge of reaction mechanisms in many heterogeneous catalysis processes is lacking. In this work, Scanning Probe Microscopy (SPM) techniques including Scanning Tunneling Microscopy (STM), Scanning Tunneling Spectroscopy (STS), and non contact Atomic Force Microscopy (ncAFM) were utilized to examine the site-by-site surface transformation of small gaseous adsorbates at the single-molecule and submolecular scales.SPM techniques are able to access site-specific information from surfaces on the atomic scale, which is vital to elucidating reaction mechanisms in heterogeneous catalysis. For this work, an Fe-terpyridine coordination complex supported on a Ag(111) surface was tested as a catalyst for the transformation of gaseous carbon monoxide (CO) and ethylene (C₂H₄). Using SPM techniques, bonding was measured between CO and Fe-terpyridine active sites, and between C₂H₄ and the active sites at temperatures of T ≤ 30K. Physical structural changes in ncAFM and STM images as well as shifts in electronic structure obtained through STS measurements were used to characterize the bonds, and both Fe-CO and Fe-C₂H₄ bonds were preceded by metastable reaction intermediates. The measured bonding activity presented in this work demonstrates the potential of surface-supported Fe-terpyridine complexes as catalysts in the transformation of small hydrocarbons and oxides, and the fundamental mechanistic insight obtained through this SPM investigation opens the door to future optimization of these heterogeneous transformations.

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Coupling spatially and spectrally resolved optical measurements with a scanning probe system (2016)

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 [1]. 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 [2]. 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.

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