Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session G52: Focus Topic: Surface, Interface, and Thin Film Science of Organic Molecular Solids IFocus Session
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Sponsoring Units: DMP Chair: Daniel Dougherty, North Carolina State University Room: Mile High Ballroom 1E |
Tuesday, March 3, 2020 11:15AM - 11:27AM |
G52.00001: Using Deposition Rate to Control Liquid Crystal-like Order in Vapor Deposited Organic Glasses Camille Bishop, Jacob Thelen, Yuhui Li, Eliot H Gann, Michael Toney, Lian Yu, Dean DeLongchamp, Mark Ediger Physical vapor deposition can prepare glassy materials with high degrees of structural anisotropy compared to liquid-cooled glasses. In this work, Posaconazole, a molecule with no known liquid crystal phases, is vapor-deposited to create smectic-like glasses with a high degree of orientational and positional order [Bishop et. al. PNAS 116, 21421 (2019)]. Deposition parameters are varied to prepare glasses with a range of liquid crystal-like order. We show that increasing the substrate temperature and decreasing the deposition rate have the same effect upon vapor-deposited glass structure, and describe the relationship using a “Deposition Rate-Substrate Temperature Superposition”. We expect that this superposition principle can be used to understand surface dynamics during vapor deposition, in order to optimize structural order in glasses for organic electronic applications. |
Tuesday, March 3, 2020 11:27AM - 12:03PM |
G52.00002: 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 particularly interesting model systems: Zinc tetraphenylporphyrins (ZnTPP) on single crystal surfaces. |
Tuesday, March 3, 2020 12:03PM - 12:15PM |
G52.00003: Electric field manipulation of the molecular spin state of a Fe(II) spin crossover complex Guanhua Hao, Aaron Mosey, Xuanyuan Jiang, Alpha T. N'Diaye, Xin Zhang, Jian Zhang, Ruihua Cheng, Xiaoshan Xu, Peter A Dowben The spin crossover (SCO) phenomenon, in 3d transition metal compounds, has potential applicability in molecular spintronic devices for low-cost flexible memory. Isothermal changes of the electronic structure has now been achieved for the Fe(II) spin crossover complex [Fe{H2B(pz)2}2(bipy)], where pz = tris(pyrazol-1-yl)- borohydride and bipy = 2,2’-bipyridine, by external electric fields. This isothermal voltage-controlled switching is evident in thin film bilayer structures where the molecular spin crossover film is adjacent to a molecular ferroelectric thin film (the tested examples being polyvinylidene fluoride hexafluoropropylene or croconic acid (C5H2O5)). These organic ferroelectric substrates appear to lock the spin crossover molecular complex largely in the low or high spin state depending on the direction of ferroelectric polarization, in both a planar two terminal diode structure and for a transistor structure. Moreover, the spin state change is observed to be accompanied by a conductance change, where higher conductance occurs at a high spin state while lower conductance is observed for a low spin state, and is seen to be nonvolatile, i.e. the spins state is retained in the absence of an applied electric field. |
Tuesday, March 3, 2020 12:15PM - 12:27PM |
G52.00004: Expanding the surface chemistry synthetic toolkit: facile carbon (sp2)-nitrogen(sp2) bond formation on Au[111] and Ag[111] Raymond Blackwell, Ilya Piskun, Joaquim Somoza, Fangzhou Zhao, Steven Louie, Angel Rubio, Felix Fischer Surface assisted synthesis offers a unique pathway towards the creation of atomically precise nanostructures that cannot be achieved in solution. The Ulmann coupling—where surface stabilized radicals form covalent bonds prior to polymerization or cyclization—has emerged as a promising route for accessing novel architectures, in particular graphene nanoribbons where the electronic structure is intimately linked with the atomic structure. This reaction, however, has been limited to the formation of C-C bonds, restricting its overall scope. The ability to create novel linkages on the surface would allow for a deeper understanding of the interplay between atomic and electronic structure. Here we demonstrate facile C-N bond formation on different coinage metal surfaces at substantially lower temperatures than polymerization and subsequent C-C bond formation. Scanning tunneling microscopy and spectroscopy (STM/STS) performed conjointly with first-principles calculations confirm the desired product. The mechanism for C-N formation will be discussed along with strategies for the synthesis of extended 1-dimensional structures. |
Tuesday, March 3, 2020 12:27PM - 12:39PM |
G52.00005: Scanning Tunneling Microscopy and Spectroscopy of Heterotriangulene-based 2D Polymers Zachery Enderson, Harshavardhan Murali, Raghunath Dasari, Timothy C Parker, Seth R. Marder, Hong Li, Qingqing Dai, Simil Thomas, Jean-Luc E Bredas, Phillip N First Bottom-up synthesized covalent organic frameworks (COFs) provide a means to customize the properties of a 2D polymer from its monomer precursors. The 2D-crystalline DTPA (dimethylmethylene-bridged triphenylamine) COF is synthesized on Au(111) or Ag(111) through Ullman-type coupling1 . Theory predicts that the as-grown closed-shell electronic structure is semiconducting. Heating in vacuum selectively cleaves methyl groups from the monomer bridge sites, but leaves the framework intact, creating a COF resembling an ultra-flat covalent network of triangulene molecules. Enticing electronic properties are predicted, depending on the specific termination of the bridge sites. Calculations for a H-terminated case indicate that the “radical” COF will be a half-metal (fully spin-polarized density of states at the Fermi energy)2. Using an LT-STM, we present new information on the bridge configuration of the demethylated structure, on the electronic structure of the DTPA COF in its closed-shell (methylated) and “radical” (demethylated) forms, and on the interaction between the COF and substrate. |
Tuesday, March 3, 2020 12:39PM - 12:51PM |
G52.00006: Enhancement of Magnetoelectric Coupling in Ferroelectric Vinylidene Fluoride/Ferroelectric Cobalt by insertion of a dielectric layer of Magnesium Oxide. Aashish Subedi, Keith D Foreman, Shireen Adenwalla Poly (vinylidene fluoride), or PVDF combines the advantages of organic based electronics with the useful bi-stable polarization characteristic of ferroelectricity. Magnetoelectric coupling between PVDF, and ferromagnetic cobalt thin films has been well established. Because VDF thin films may be deposited in vacuum, the interface between Co and VDF is clean and sharp, with little to no oxidation. We studied magnetoelectric coupling in these heterostructures, with VDF deposited directly on Co without breaking vacuum. This pristine interface resulted in very weak magnetoelectric coupling. The introduction of oxides of cobalt between these layers from exposure in air resulted in an increase in the surface coupling but presented challenges of surface purity and irreproducibility. Thus, we investigated the effect of controlled growth of a dielectric with high K, MgO. We focus on the growth and heat treatment of MgO on ferromagnetic cobalt. Consequently, we examine the interfacial interaction between the Cobalt, MgO and VDF layers and analyze how this affects the out of plane magnetic anisotropy of Cobalt. |
Tuesday, March 3, 2020 12:51PM - 1:03PM |
G52.00007: Electric Field Control of Molecular Charge State in a Single-Component 2D Organic Nanoarray Agustin Schiffrin, Dhaneesh Kumar, Yuefeng Yin, Nikhil Medhekar, Cornelius Krull Quantum dots (QD) with an electric-field-addressable charge state are promising for digital information storage, single-electron transistors and quantum computing. Semiconductor QDs often offer limited control on size and composition, and low potential for scalability or miniaturization. Owing to their tunability and self-assembly capability, organic molecular building blocks can be used for the synthesis of two-dimensional (2D) QD arrays. Here, we report the self-assembly of 9, 10-dicyanoanthracene (DCA) molecules on Ag(111) into periodic 2D arrays, where the molecular charge state (neutral or negative) can be altered – individually and depending on the adsorption site – by the local electric field of a scanning tunneling microscope tip. Charging is enabled by an effective DCA/Ag(111) tunneling barrier and electric-field-driven electron population of the lowest unoccupied molecular orbital (LUMO). Subtle site-dependent variations of the DCA adsorption height result in a spatial modulation of the molecular polarizability, dielectric constant and LUMO energy level alignment, giving rise to a spatially dependent likelihood of charging. Our work offers potential for high-density 2D self-assembled arrays of QDs whose charge state can be addressed individually by an electric field. |
Tuesday, March 3, 2020 1:03PM - 1:15PM |
G52.00008: Low Energy Electron Interactions with Aliphatic and Aromatic Self-assembled Monolayers on Au(111) Carl Ventrice, Jodi Grzeskowiak Electron scattering experiments have been performed on SAMs of the aliphatic molecule decanethiol and the aromatic molecule biphenylthiol grown on Au(111) via vapor phase deposition. The molecules take on either a lying down (LD) or standing up (SU) geometry, depending on coverage. From LEED, the crystal structure of both the LD phase and SU phase SAMs grown from both types of molecules is easily disrupted upon electron irradiation. Changes in peak intensities and positions of the TPD and XPS spectra indicate that the C-S bonds are cleaved for both molecules upon electron irradiation. In the SU phase of the aliphatic molecule, a large chemical shift in the C-1s and large reduction in the hydrocarbon fragments of the TPD spectra are observed. The LD phase of the aliphatic molecule and both phases of the aromatic molecule show only subtle changes to the spectra. The relative insensitivity of these SAMs to electron beam damage is attributed to charge delocalization effects. For the LD phase of the aliphatic molecule, excess charge is easily quenched since it is in direct contact with the metallic substrate. For the aromatic molecule, conjugated double bonds within the aromatic rings allow for excess charge delocalization, resulting in a lower probability for C-C bond breakage. |
Tuesday, March 3, 2020 1:15PM - 1:27PM |
G52.00009: Interface properties of FePc adsorbed on Ag (100) with and without NaCl monolayer
M. Jabrane 1,2, M.Y. El Hafidi1 , M. El Hafidi1 , A. Kara2
1 Laboratoire de la physique la matière condensée, Faculty of Sciences Ben M’Sik, Hassan II University Meysoun Jabrane For spintronic based technologies, current research focuses on developing novel nanoscale materials that combines molecular magnetism with organic spintronic devices. In this context, we studied magnetic properties of Iron-Phthalocyanine (FePc) adsorbed on various metallic surfaces. We found that FePc loses its magnetization when deposited on Cu(111) and Ag(111) with a charge transfer of 0.7e and 0.8e towards the molecule, respectively. Therefore, the interaction between FePc and the substrates affects the properties of the molecule. To circumvent the loss of FePc magnetic state, we suggest adding a monolayer of an insulator. |
Tuesday, March 3, 2020 1:27PM - 1:39PM |
G52.00010: Electric-field assisted nucleation processes of croconic acid films Yifan Yuan, Xuanyuan Jiang, Shashi Poddar, Xiaoshan Xu Growth of organic thin films using physical vapor deposition typically follows a three-dimensional mode, resulting in a rough surface, which undermines their application potentials. To address this issue, we have studied the effect of electric field and temperature on the growth dynamics, especially the heterogeneous nucleation process, of croconic acid (CA) films, taking advantage of the large dipole of the molecules and the ferroelectric polarization of the molecular crystals. We found that the nucleation rate has a maximum at an intermediate temperature, and the electric field shifts the maximum nucleation rate towards the lower temperature. An analysis using classical nucleation theory suggests that the electric field decreases sublimation temperature, increases the wetting angle, and decreases the surface diffusion barrier. These results provide important insight into the growth of molecular crystal films under electric fields and pave a way to fabricate films with better surface characteristics for molecular ferroelectric films. |
Tuesday, March 3, 2020 1:39PM - 1:51PM |
G52.00011: Ultra-low switching Energy Organic Memories Thirumalai Venkatesan, Sreetosh Goswami, Sreebrata Goswami
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Tuesday, March 3, 2020 1:51PM - 2:03PM |
G52.00012: Computational Raman spectroscopy for organic adsorbates on graphene Alexey Zayak, Sajjad Afroosheh How could we “see” the processes that occur in complex atomic-scale heterogeneous interfaces? It was suggested that Surface-enhanced Raman spectroscopy presents a keen approach for studying such systems, implying utilization of the Chemical Enhancement (CE). The latter is expected to report all information about the interfacial electronic coupling, however, a substantial learning curve is ahead before that information can be fully understood. The case of Graphene Enhanced Raman Spectroscopy (GERS) appears to be particularly intriguing. A delocalized electron system of graphene coupled with localized electronic states of organic adsorbates makes a significant challenge for understanding their Raman spectroscopy. We present a systematic computational study of Tetracyanoquinodimethane (TCNQ) adsorbed on graphene. This talk will report about simulations of surface-enhanced Raman spectra along with the supporting electronic structure analysis, focusing on the interfacial charge transfer and electron-phonon coupling. The application of a wide range of external electric biases allows us to modulate the charge transfer across the interface and reveal its role in corresponding changes of Raman spectra. |
Tuesday, March 3, 2020 2:03PM - 2:15PM |
G52.00013: Comparitive STM and LEED study of the self-assembly of theobromine on Au(111) and few layer graphene on SiC(0001) Ismail Baltaci, Malte Schulte, Carsten Westphal Graphene has unique electronic and structural properties and is of interest in a broad range of applications. The functionalization of graphene by molecular adsorbates enables applications like organic sensors and solar cells. As the weak interaction between substrate and adsorbate is important, graphene and Au(111) are suitable substrates due to their low reactivity. Compounds of the xanthine group have a great effect on the human central nervous system resulting in energy arousal and increased cognitive function1,2. Hence they are of great significance for applications in pharmacology, toxicology, and biochemistry3,4. |
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