Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session B56: Focus Topic: Surface, Interface, and Thin Film Science of Organic Molecular Solids IFocus Live
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Sponsoring Units: DMP Chair: Emily Bittle, National Institute of Standards and Technology |
Monday, March 15, 2021 11:30AM - 11:42AM Live |
B56.00001: Controlling on-surface synthesis of 2D covalent organic networks using hydrogen Harshavardhan Murali, Zachery Enderson, Raghunath Dasari, Timothy C Parker, Seth R. Marder, Hong Li, Qingqing Dai, Jean-Luc Bredas, Phillip N First On-surface synthesis using dehalogenative Ullmann coupling on noble metals has been extensively used for creating metal-supported 2D covalent organic networks in ultrahigh vacuum. In principle, rational design of the precursor molecules determines the network and its electronic properties. In this work, we propose the introduction of atomic hydrogen into the reaction chamber as an additional route to engineer the on-surface reactions. Recent reports have demonstrated that atomic hydrogen assists in debromination and subsequent hydrogenation of adsorbed molecules[1]. We extend this technique to control the degree of hydrogenation of tribrominated heterotriangulene precursors[2] during the coupling process, which enables the creation of self-assembled islands of hydrogenated monomers, covalently-linked dimers, and macrocycle hexamers. We study these assemblies using STM and assess their structure using mass spectrometry. The sequence of n-mer assemblies enables a comparative study of electronic properties from single monomer to continuous 2D covalent network. |
Monday, March 15, 2021 11:42AM - 12:18PM Live |
B56.00002: 2D organic layers on surfaces: self-assembly and electronic structure Invited Speaker: Sylvie Rangan Understanding the basic mechanisms leading to the formation of 2D organic layers on surfaces, either via Van der Waals, ionic or covalent interactions, is a necessary step toward the development of controlled and ordered organic layers, for technological applications such as homogeneous doping of graphene or 2D organic topological insulators. Using a combination of scanning tunnel microscopy, various electron spectroscopies techniques and ab-initio calculations, we have studied several aspects of the self-assembly and reactivity of a particularly interesting class of molecules, tetraphenylporphyrins, on single crystal surfaces. |
Monday, March 15, 2021 12:18PM - 12:30PM Live |
B56.00003: Kondo Effect in a 2D Kagome Metal-organic Framework on a Metal Dhaneesh Kumar Gopalakrishnan, John Hellerstedt, Bernard Field, Benjamin Lowe, Yuefeng Yin, Nikhil V. Medhekar, Agustin E. Schiffrin The kagome lattice, hosting Dirac and flat electronic bands, offers a rich space to realise strongly correlated phases of matter in two-dimensional (2D) and layered materials. While strongly correlated electron phenomena have been observed in inorganic kagome systems, they remain unobserved in organic-based kagome ones. The latter offer versatile bottom-up synthesis protocols via self-assembly and metal-ligand coordination. Here, we report the synthesis of a 2D metal-organic framework (MOF) with honeycomb-kagome (HK) structure via coordination of Cu atoms with di-cyanoanthracene (DCA) molecules on an Ag(111) noble metal surface. Via low-temperature scanning tunneling spectroscopy (STS), we observe Kondo screening (by Ag(111) conduction electrons) of magnetic moments. From our temperature-dependent STS measurements, we determined a Kondo temperature of 124 (140) K associated with Cu (DCA, respectively). Our experiments are consistent with density functional theory and mean-field Hubbard modelling, which shows that the unpaired magnetic moments result directly from strong interactions between HK electrons. Our work paves the way for strongly correlated electronic phases of matter in - and strongly correlated electronics based on - atomically precise, self-assembled MOFs. |
Monday, March 15, 2021 12:30PM - 12:42PM Live |
B56.00004: Controlling energy level alignment at a chromophore/TiO2 interface using a helical peptide dipole Jonathan Viereck, Yuan Chen, Katherine Lloyd, Ryan Harmer, Sylvie Rangan, Elena Galoppini, Robert Allen Bartynski The sensitization of wide band gap transition metal oxide semiconductors by chromophores has become ubiquitous in the field of photovoltaics. Since performance hinges on the charge transfer across the interface, which in turn depends upon the alignment of the chromophore frontier orbitals with respect to the substrate band edges, it is crucial to find ways of controlling this energy alignment. |
Monday, March 15, 2021 12:42PM - 12:54PM Live |
B56.00005: Electronic and Magnetic Structure of Metal-Organic Lattices on Substrates Bernard Field, Dhaneesh Kumar Gopalakrishnan, John Hellerstedt, Yuefeng Yin, Agustin E. Schiffrin, Nikhil V. Medhekar Two-dimensional (2D) metal-organic frameworks (MOFs) have received substantial interest as potential new electronic materials, although their interactions with substrates have received little theoretical attention to date. We employ tools such as density functional theory (DFT), tight-binding, and mean-field Hubbard to investigate these systems. In particular, we focus on 2D MOFs with kagome geometry, where native electronic flat bands can give rise to strong electron-electron interactions. Owing to the latter, we find that a metal-organic dicyanoanthracene kagome lattice can exhibit frustrated antiferromagnetism for specific band filling factors. Different metallic substrates can support or suppress magnetism, as shown by both DFT and experimental measurements. We also investigate the effects of a decoupling atomically thin insulator (hexagonal boron nitride), and the consequences of MOF-surface interactions on previously predicted topological edge states. |
Monday, March 15, 2021 12:54PM - 1:06PM Live |
B56.00006: Is there charge transfer beyond the first layer? Alexander T Egger, Lukas Hörmann, Andreas Jeindl, Oliver Hofmann The adsorption of strong organic acceptors on metallic electrodes used to modify charge injection barriers in organic electronics. While for most adsorbates, the charge-transfer reaction is clearly limited to the molecules in direct contact with the substrate, for several systems experiments indicate charge-transfer to the second layer. Such long-ranged charge transfer is at variance with present level-alignment models. This raises the question if these experimental indications have been misinterpreted an a different effect, can explain the experimental findings. |
Monday, March 15, 2021 1:06PM - 1:18PM Live |
B56.00007: Identifying Surface Adsorbate Structures with Bayesian Inference and Atomic Force Microscopy Jari Järvi, Benjamin Alldritt, Ondrej Krejci, Milica Todorovic, Peter Liljeroth, Patrick Rinke Determining stable structures of organic molecular adsorbates requires both quantum mechanics and thorough exploration of the potential energy surface (PES). This is prohibitively expensive with density-functional theory (DFT). Bayesian Optimization Structure Search (BOSS) [1] is a new tool that combines DFT with Bayesian inference for accurate global structure search. BOSS applies strategic sampling to compute the complete PES with a small number of expensive DFT simulations. This allows a clear identification of stable structures and their energy barriers. |
Monday, March 15, 2021 1:18PM - 1:30PM Live |
B56.00008: Long-Range Surface-Assisted Molecule-Molecule Hybridization Marina Castelli, John Hellerstedt, Cornelius Krull, Spiro Gicev, Lloyd C. L. Hollenberg, Muhammad Usman, Agustin E. Schiffrin Metalated phthalocyanines are robust and versatile molecular complexes, whose properties can be tuned by changing its functional groups and central metal atom. Here, we studied the electronic structure of magnesium phthalocyanine (MgPc) - similar in structure and electronic properties to chlorophyll - on Ag(100) via low-temperature scanning probe microscopy (STM/STS/ncAFM) and density functional theory (DFT). Our ncAFM and STS data show that a single isolated MgPc molecule exhibits a flat four-fold symmetric morphology with doubly degenerate, partially populated lowest unoccupied molecular orbitals (LUMOs). Conversely, molecules with adjacent MgPc’s in proximity (within a distance of ~3 nm) show a lift of LUMO degeneracy and symmetry reduction, from four- to two-fold. We explain this interaction by a two-step hybridization process: MgPc orbitals first hybridize with Ag(100), forming molecule-surface orbitals with enhanced spatial extension that then hybridize in turn with neighboring molecules. This work highlights how orbital degeneracies and symmetries of molecular adsorbates can be significantly altered via surface-mediated intermolecular hybridization, over extended distances, having important implications for prospective molecule-based solid-state electronics. |
Monday, March 15, 2021 1:30PM - 1:42PM Live |
B56.00009: Similar Building Blocks, Diverse Structures: Understanding Self-Assembly of Acenequinones on Ag(111) Andreas Jeindl, Jari Domke, Lukas Hörmann, Falko Sojka, Roman Forker, Torsten Fritz, Oliver T. Hofmann The ongoing miniaturization in nanotechnology raises the need to understand (and control) the formation of surface polymorphs on a molecular level. Due to the intricate interaction mechanisms, complex physics and high configurational complexity at play, this understanding is still at the very beginning. |
Monday, March 15, 2021 1:42PM - 1:54PM Live |
B56.00010: Ogre: A Python Package for Molecular Crystal Surface Generation with Applications to Surface Energy and Crystal Habit Prediction Shuyang Yang, Imanuel Bier, Wen Wen, Jiawei Zhan, Saeed Moayedpour, Noa Marom We present Ogre, an open-source code for generating surface slab models from bulk molecular crystal structures. Ogre is written in Python and interfaces with the FHI-aims code to calculate surface energies at the level of density functional theory (DFT). The input of Ogre is the geometry of the bulk molecular crystal. The surface is cleaved from the bulk structure with the molecules on the surface kept intact. A slab model is constructed according to the user specifications for the number of molecular layers and the length of the vacuum region. Ogre automatically identifies all symmetrically unique surfaces for the user-specified Miller indices and detects all possible surface terminations. Ogre includes utilities to analyze the surface energy convergence and Wulff shape of the molecular crystal. Ogre is applied to three representative molecular crystals: the pharmaceutical aspirin, the organic semiconductor tetracene, and the energetic material HMX. The equilibrium crystal shapes predicted by Ogre are in agreement with experimentally grown crystals, demonstrating that DFT produces satisfactory predictions of the crystal habit for diverse classes of molecular crystals. Reference: The Journal of Chemical Physics 152, 244122 (2020). |
Monday, March 15, 2021 1:54PM - 2:06PM Live |
B56.00011: Probing the Interfacial Electronic Density of States of Perylenediimide Thin Films using Electronic Sum-Frequency Generation Daniel Cotton, Aaron Moon, Jon A Bender, Sean T Roberts Perylenediimide dyes (PDIs) are pigments known for crystallochromy, photophysical stability, and an ability to undergo exciton multiplication through singlet fission. The solid-state PDI band structure is intimately connected to local intermolecular coupling and is well described using a tight-binding Holstein Hamiltonian. Moreover, the intermolecular structure of PDI solids also modulates their ability to transport energy and engage in singlet fission. However, interfaces formed between PDI solids and other materials presents a different environment from the bulk, with potentially different properties. To use PDIs in novel organic-inorganic hybrid devices, the PDI-substrate interface must be understood. |
Monday, March 15, 2021 2:06PM - 2:18PM Live |
B56.00012: First principles prediction of two-dimensional metal-organic framworks: strong interplay between long-range dispersion and charge transfer Billal Sohail, Phil Blowey, Luke Rochford, Timothy LaFosse, David Duncan, Paul Ryan, David Warr, Tien Lee, Giovanni Constantini, Reinhard Maurer, Phil Woodruff When molecules strongly interact with interfaces, they exchange charge. Large organic moelcules interfaces are additionally stabilised by long-range dispersion forces. Correctly describing the interplay of charge-transfer and dispersion interactions at metal-organic interfaces is of paramount importance to correctly describe the structure, stability and electronic properties of such interfaces. In comparison with experiment, we will investigate the ability of dispersion-inclusive Density Functional Theory to describe the adsorption structure of strong donors such as alkali atoms and acceptor molecules such as TCNQ adsorbed at varying facets of Ag. In comparison to NIXSW measurment data on vertical adsorption heights, we find that all recently proposed dispersion-inclusive DFT methods fail to corrrectly capture the adsorption height of alkali ions at metal surfaces. Upon analysis, we propose a rescaling of dispersion parameters to correctly account for the change of atomic polarisability due to charge transfer in this system. Taking this into account gave rise to accurate structural models which enabled further analysis of the energetics and energy level alignment for adsorbed alkali-organic metal-organic frameworks. [1] |
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