Session R45: Surfaces, Interfaces, and Thin Films: Reactions and Dynamics

Photoemission Spectroscopy Experiments on Metalloporphyrin Architectures for Molecular Electronics

Metal Organic Frameworks (MOFs) based on metalloporphyrin architectures combine advantages of both organic and inorganic materials and therefore are interesting candidates for next generation molecular electronic devices and applications. They exhibit unique properties such as highly ordered structures, ultrahigh surface area, tunable nanopore size, and more interestingly tailorable electronic, and ionic properties.
In this work, we present Liquid Phase Epitaxy method to fabricate self-assembled monolayers on gold surfaces which enabled to host conductive MOFs with their ability to facilitate charge transfer and control the growth of the MOF thin films.
We used Photoemission Spectroscopy (PES) to reveal the electronic properties which is crucial for the construction of energy band diagrams of various metalloporphyrin derivatives coordinated to either copper or zinc based metal ions. The energy band diagrams revealed interesting details about the electronic properties when changing various metal ions or conductive ligands. The obtained results show the fabricated films exhibit semiconductor tunability of either p-type or n-type behavior as conducting medium of charge carrier transport which is a key prerequisite to build molecular electronic junctions and elaborated devices.   

Ultrafast electron dynamics in a thin C60 films observed with time and angle resolved photoemission

C60 derivatives are common acceptor materials for organic solar cells. Time- and angle-resolved photoelectron spectroscopy (tr-ARPES) is a powerful way to study the electron dynamics in these systems. However, experiments on perturbatively excited systems are very demanding. By combining an 88 MHz ultrafast XUV source [1] with time-of-flight momentum microscopy [2], we are able to perform experiments at high speed and low excitation fluence. Here we present first demonstration results on C60 films with this technique.
[1] C. Corder, et al, Structural Dynamics 5, 054301 (2018).
[2] K. Medjanik, et. al, Nature Materials 16, 615 (2017).   

Waves guided by the surface of a Lorentz model dielectric medium

The wavenumber, the phase velocity, the wave packet velocity, the energy transport velocity, and the attenuation coefficient are obtained for the wave guided by the surface of a Lorentz model dielectric medium.   

The TEM Studies of Tungsten Irradiated by Hellium

Tungsten (W) as one of the potential plasma-facing materials (PFMS) would suffer severe irradiation in nuclear fusion reactor which can results in deterioration of the related mechanical and physical properties. Understanding the irradiation effects on PFMS is then critical to improve their performances. Here, we studied the He ions irradiation effects by focusing on microstructures of the the W surfaces. As the irradiation time increased, the surface morphologies changed from smooth to full of nano-structures. TEM images show that the nano-structures contain lots of He voids, it is noted that the shapes of the he bubbles are faceted, which pointed out that the system prefers to develop low surface energy bubbles. And the nano-structures show good deformation feature although micro-carcks are developed inside.
  

Electronic Structure of Phase Separated Material Revealed by Nanospot Angle-Resolved Photoemission Spectroscopy

Phase separation frequently occurs in strong correlated materials. The electronic structures of different phases are very important but difficult to access due to very small grain size. As an electronic structure microscopy, nanospot angle-resolved photoemission spectroscopy (NanoARPES) can measure the electronic structure both in real and momentum spaces and provides the unique ability to measure the electronic structures of different phases in phase separated materials. Here I will present our recent experimental results on the electronic structure of phase separated material by using NanoARPES.   

Optimization and Characterization of Dip-Coated PEDOT:PSS on CdTe Solar Cells

Organic thin film solar cells have been identified as viable photovoltaic technology alternatives to conventional power production methods. In this work, we optimize the thin film deposition conditions for high-conductive grade poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) on pulsed laser deposited cadmium sulfide/cadmium telluride/indium tin oxide substrates. In our previous studies, we have determined that this particular substrate and polymer combination significantly improves the photovoltaic conversion efficiency due to a reduction of the Schottky barrier resistance. In this study, we focus on applying a uniform layer of the PEDOT:PSS back contact by optimizing thin film dip-coating parameters such as pulling rate, temperature, application duration, and drying procedures. The resultant back contacts are characterized by scanning electron microscopy, energy dispersive x-ray spectroscopy, and ellipsometry techniques. Electrical conductivity tests on full cells are also performed using a Keithley SourceMeter, and structure and efficiency results are discussed.   

Effect of illumination on the interplay between Dresselhaus and Rashba spin-orbit coupling in InAs quantum wells

Illumination of variable wavelength was used to study the dependence on the carrier concentration of the spin-orbit coupling and the semiconducting properties of a thin n-type InAs quantum well. Measurements of the sheet and Hall resistance were performed in variable magnetic field, field orientation, temperature, and under illumination with wavelengths of 400 nm up to 1300 nm. Beats in the Shubnikov-de Haas oscillations indicated the presence of strong spin-orbit coupling and the FFT of the oscillations points towards the presence of both Rashba and Dresselhaus spin-orbit interactions. Here we have used simulations to extract the dependence of the spin-orbit strengths on the carrier concentration. The simulations are based on the Hamiltonian of a 2D electron system with both types of spin-orbit interactions and subject to a perpendicular magnetic field. The results indicate that the Rashba coefficient decreases with increasing the carrier concentration, while the Dresselhaus coefficient remains mainly constant. These results are consistent with observations on similar materials where the carrier concentration was varied using applied gate voltage.   

Band Structure engineering in epitaxial LuSb thin films via dimensional confinement and bi-axial strain

Observation of extreme magnetoresistance (XMR) in rare-earth monopnictides has raised strong interest in understanding the role of its electronic structure. Here, the first demonstration of epitaxial synthesis of LuSb thin films on GaSb (001) substrates will be presented. Combining the techniques of molecular-beam epitaxy, low-temperature magnetotransport, angle-resolved photoemission spectroscopy, and hybrid density functional theory, we have unveiled the bandstructure of LuSb, where electron-hole compensation is identified as a mechanism responsible for XMR. However, by fabricating ultra-thin films, it is possible to controllably create an imbalance in the band fillings of electron and hole-like carriers in this otherwise compensated semimetal. Moreover, magnetoresistance behavior can also be tuned by application of bi-axial strain by synthesizing thin films of LuSb on lattice mis-matched substrates. Our work demonstrates the efficacy of epitaxial synthesis of rare-earth monopnictides to control its electronic structure, and thereby its physical properties   

High quality Bi2Se3 thin films grown on Van der Waals substrate BiInSe3

We grew high quality BiInSe₃ single crystals and developed an exquisite process to prepare these crystals as substrates. Well cleaved BiInSe₃ single crystals with subsequent in-situ thermal treatment can obtain atomically flat surface, which is evidenced by the sharp streaky reflection high energy electron diffraction (RHEED) patterns. By optimizing the growth recipe, we successfully grew high quality Bi₂Se₃ thin films on BiInSe₃ single crystal substrates. Owing to the weak Van der Waals bonding between the interlayers of BiInSe₃ substrates, we can easily transfer the Bi₂Se₃ thin films grown on these substrates. The high quality surface morphology and electrical transport characteristics, together with the transferrable feature all prove that BiInSe₃ could be a promising substrate for TI films growth. Our discovery could pave a new pathway for improving the sample quality of topological insulator thin films.   

Very different adsorption path ways of triatomic molecule H2O on isolated and domains of dangling bonds on the Si(100) surface

On the ideal clean Si(100) surface, dangling bond pairs (DBPs, denoted as -Si-Si-) cover the entire surface. Each dangling bond is an active adsorption site for adsorption of a gas molecule. In this study, isolated DBPs are prepared on a Si(100) surface as well-defined chemically-reactive sites for chemisorption of the prototypical H₂O triatomic molecules. The surrounding dangling bonds are passivated by chlorine-termination. Following saturation exposure to H₂O molecules at room temperature, the adsorbate configurations on these reactive sites as well as on the clean surface have been examined in atomic resolution using scanning tunneling microscopy.

The results show that, while the H-Si-Si-OH adsorbate configuration is common on a DBP, other configurations such as H-Si-O-Si-H and H-Si-Si-H are observed. These findings indicate that a seemingly simple chemisorption reaction on reactive sites involves not only the sites themselves but also the relevant surrounding bonds and adatoms.   

Reversible and Efficient Photo-Switching of Azobenzene Derivatives on an Insulator Surface Investigated by nc-AFM

Studying single molecular switches is of interest for a better understanding of fundamental physical and chemical processes, but also in view of their possible use in smart materials and molecular nanotechnology.
Azobenzene is a prototypical molecular switch, which changes between trans and cis isomers at the central N=N bond upon an external stimulus. Adsorption on solid substrates allows the study on the single molecule scale using scanning probe microscopy, but it has been shown that metallic substrates strongly influence the switching properties[1,2].
Here, we report non-contact atomic force microscopy (nc-AFM) results on the switching of individual Azobenzene derived molecules on an insulating calcite surface. It is demonstrated that unlike on metals [3] the molecules retain their efficient photoisomerization properties known from solution. Furthermore, the photoisomerization is shown to be reversible and selective, depending on the exciting wavelength.

[1] Choi et al., Phys. Rev. Lett. 96, 156106 (2006); [2] Dri et al., Nature Nanotechnology 3, 649 (2008); [3] Comstock et al., Phys. Rev. Lett 99, 038301 (2007)
  

Theory of coupled deposition and dissolution of solids, with focus on corrosion

Corrosion affects the durability of metals and structures and has important economic consequences, such as the continuous inspection for and replacement of damaged material. Corrosion often initiates locally and therefore, to increase the sustainable use of metals, a thorough knowledge of corrosion and its localized initiation is crucial. One technique to investigate this experimentally is the gold nanoplating technique on pure copper. This study investigates the pattern formation in this concurrent deposition-corrosion system. First we derive the general model for deposition-dissolution systems and then apply this to the gold-nanoplating example. This is the first model to implement non-linear reaction kinetics based on far-from equilibrium thermodynamics. Effects related to curvature induced diffusion and/or effective phase-separation upon deposition/dissolution are inherently included, phenomena which affect the formation dynamics of deposited film and the corroded bulk material-film interface. Other applications of the model are the passivation of stainless steels, the galvanization protection of Zn with respect to iron, and Li plating during continuous cycling of Li-Ion batteries.   

Morphological Instability of Grain Boundary under Lateral Applied Strain

The morphological instability of symmetrical high-angle tilt grain boundary induced by lateral applied strain is investigated using the phase field crystal model in two dimensions. By studying the growth rate of perturbations, we discover a threshold value of the applied strain below which the instability disappears. Furthermore, we construct the phase diagram of instability which is classified into two groups, namely the grain boundary structural transformation or the dislocation emission, based on the late stage behavior of the grain boundary. A theoretical model is proposed by taking into account a relatively soft solid-solid interface, the elasticity of solid grains, and the interfacial energy of grain boundary. The theoretical prediction of the growth rate and the phase diagram is in quantitatively good agreement with the simulation result. In addition, the theory also explains the inhomogeneous strain distribution observed in simulations.