Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session X10: Surface Science of Organic Molecular Solids, Films, and Nanostructures - Analysis, Imaging, Spectroscopy, and Simulation of Electronic States and ProcessesFocus
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Sponsoring Units: DMP DCMP Chair: Christoph Boehme, University of Utah Room: BCEC 151B |
Friday, March 8, 2019 8:00AM - 8:36AM |
X10.00001: Quintet multiexciton dynamics in singlet fission Invited Speaker: Dane McCamey, Selected by Focus Topic organizer (Boehme) |
Friday, March 8, 2019 8:36AM - 8:48AM |
X10.00002: Charge-state controlled imaging of electronic transitions in single molecules Laerte Patera, Fabian Queck, Philipp Scheuerer, Jascha Repp Electron transfer plays a crucial role in many chemical reactions, from photosynthesis to combustion and corrosion. However, the way in which redox reactions affect individual molecules and, in particular, their electronic structure, remains largely unclear. Unveiling these fundamental aspects requires the development of experimental tools allowing the observation of electron transfer down to the single molecule level. Here, we demonstrate the capability of performing tunnelling experiments on non-conductive substrates to map the orbital structure of isolated molecules upon electron transfer. By driving a change in the redox state of a molecule synchronized with the oscillating tip of an Atomic Force Microscope (AFM), previously inaccessible electronic transitions are resolved in space and energy. Our results unveil the effects of electron transfer and polaron formation on the single-orbital scale, thus opening the door to the investigation of complex redox reactions and charging-related phenomena with sub-ångström resolution. |
Friday, March 8, 2019 8:48AM - 9:00AM |
X10.00003: Direct visualization of hydrogen-transfer intermediate states by scanning tunneling microscopy Deliang Bao, Rongting Wu, Linghao Yan, YELIANG WANG, Junhai Ren, Yanfang Zhang, Qing Huan, Yuyang Zhang, Shixuan Du, Sokrates T Pantelides, H.-J. Gao Hydrogen atoms bonded within molecular cavities often undergo tunneling or thermal-transfer processes that play major roles in diverse physical phenomena. Such transfers may or may not entail intermediate states. The existence of such fleeting states is typically determined by indirect means, while their direct visualization has not been achieved. Here we use density-functional-theory (DFT) calculations and scanning-tunneling-microscopy (STM) simulations to predict a unique STM-image signature of thermal-transfer double-C morphology for the cis-intermediate of the two-hydrogen transfer process that occurs in metal-free naphthalocyanine (NPc) molecules adsorbed on Ag(111) surfaces. As guided by the theoretical predictions, at elaborately adjusted scanning temperature and bias, STM experiments achieve a direct visualization of the cis-intermediate. This work demonstrates a practical way to directly visualize elusive intermediates, which enhances understanding of the quantum dynamics of hydrogen atoms. |
Friday, March 8, 2019 9:00AM - 9:12AM |
X10.00004: Imaging Binding Structures of Small Molecules to Metallo-porphyrin Using Scanning Tunneling Microscopy MinHui Chang, Yun Hee Chang, Na Young Kim, Un Seung Jeon, Howon Kim, Kyung Min Kim, Yong-Hyum Kim, Se-Jong Kahng Binding reaction between small gas molecules and metallo-porphyrins play crucial roles in functional processes of biological systems such as oxygen delivery, muscle contraction, and synaptic transmission. Their geometrical structures such as tilted binding of NO to metallo-porphyrin have been recently confirmed by high-resolution scanning tunneling microscopy (STM) images at the single molecule level. Here, we present STM images of further systems, di, tri, and quadra-atomic small molecules, binding to metallo-porphyrin on Au(111). We observed square ring, rectangular ring, and center-bright structures for three different small molecules. With the help of density functional theory (DFT) calculations, we reproduce the experimental STM images in the simulated images. Thus, our study shows that geometric structures of small molecules binding to metallo-porphyrins can be probed with STM combined with DFT methods. |
Friday, March 8, 2019 9:12AM - 9:24AM |
X10.00005: Sub-Monolayer Annealed CuPc on Cu(111): Defect Hindered Dynamic Clusters William Huxter, Jun Nogami, Chandra Veer Singh Annealed sub-monolayer copper phthalocyanine on Cu(111) was studied with room temperature scanning tunneling microscopy (STM) and density functional theory (DFT). At coverages of 0.25 ML we observed that annealing produces a disordered cluster network of CuPc that attach together by forming C-C bonds across isoindole lobes. At lower coverages we find that smaller CuPc clusters (less than ~20 CuPc in size) are mobile on the Cu(111) surface. This dynamic motion was tracked by repeated STM image scans. Subsequent deposition of CuPc (without annealing) produced a Cu(111) surface covered with highly mobile single CuPc molecules and less mobile CuPc clusters. This enabled resolution of surface defects that were found to immobilize the CuPc clusters through a defect pinning mechanism. |
Friday, March 8, 2019 9:24AM - 9:36AM |
X10.00006: Structure and electronic properties of end states of self-assembled 1D covalent molecular chains on Au(111) Ales Cahlik, John Hellerstedt, Martin Svec, V M Santhini, Simon Pascal, Sigurour Ingi Erlingsson, Karel Vyborny, Olivier Siri, Pavel Jelinek One-dimensional structures offer a rich ecosystem for realizing quantum states with potential application for advanced information technologies. Surface confined molecular self-assembly is one avenue for creating 1d systems, where the extant structure is controlled by the precursor shape, and functional group interlinking chemistry. Here we study self-assembled 1d chains of zwitterionic molecule bis-bidentate ditopic (DABQDI) on Au(111) in ultrahigh vacuum, measured at 5K using combined scanning tunneling and non-contact atomic force microscopies (STM/ nc-AFM). Submolecular resolution achieved with a CO- functionalized tip offers detailed structural information, specifically regarding the unusual hydrogen bonds linking the precursor units. In-gap electronic states near the Fermi energy are observed via scanning tunneling spectroscopy (STS), strongly localized to the chain ends. We present our latest efforts to understand and simulate the observed structures via density functional theory (DFT) and nc-AFM simulations, and rationalize the observed electronic properties via modelling inspired by the Su, Schrieffer, Heeger (SSH) one-dimensional tight binding model. |
Friday, March 8, 2019 9:36AM - 9:48AM |
X10.00007: Long-Range Energy Level Shifts Induced by Single Impurity Molecules in C60 Thin Films Erik Mårsell, Bingkai Yuan, Katherine Cochrane, Miriam D DeJong, David J Jones, Moritz Riede, Sarah Burke Organic photovoltaics (OPV) is a promising technology for low-cost, flexible solar cells with low embodied energy. However, the efficiency remains low due to high exciton binding energies. The main driving force behind exciton dissociation is the energy landscape around the donor-acceptor interface. To optimize device efficiency, we therefore need to better understand the pathways of exciton dissociation at interfaces in OPV materials and how they correlate with the energy landscape. We use scanning tunneling microscopy and spectroscopy to study model systems consisting of thin films of C60 with single molecules of pure and fluorinated zinc phthalocyanine (ZnPc, F4ZnPc, and F8ZnPc) added. We measure how a molecule affects the energy levels in the surrounding C60 matrix; they shift by up to 150 meV depending on the degree of fluorination of the impurity molecule. This shift prevails over at least several C60 molecules from the impurity. This large and long-range shift induced by the phthalocyanine, and heavily influenced by the fluorine atoms, opens up new possibilities for controlled design of the energy landscape in the OPV heterojunction to optimize charge transfer efficiency. |
Friday, March 8, 2019 9:48AM - 10:00AM |
X10.00008: Anharmonic Vibrational Raman Spectra in Polymorphic Molecular Crystals: Density-Functional-Perturbation Theory and Machine Learning Nathaniel Raimbault, Mariana Rossi Anharmonic contributions to vibrational Raman spectra of molecular crystals can be decisive to identify the structure of different polymorphs. In this work we characterise the low-frequency Raman spectral region of different polymorphs of the flexible aspirin and paracetamol crystals. We include anharmonicities through the time-correlation formalism, combining ab initio molecular dynamics and density-functional perturbation theory (DFPT) implemented in a full-potential, all-electron framework [1]. Lattice expansion and anharmonic thermal nuclear motion strongly affect the collective vibrations of the low-frequency region. This effect is much less pronounced at higher frequencies. We obtain excellent agreement with experimental lineshapes, highlighting the necessity of going beyond the harmonic approximation. In order to bypass the cost of DFPT evaluations of the polarizability tensor, we employ different forms of Kernel Ridge Regression (KRR) and discuss their efficiency. Training our models on several hundreds of points, we reproduce Raman spectra that would otherwise require the calculation of tens of thousands of points. This technique is extended to surface-sensitive vibrational sum-frequency generation. |
Friday, March 8, 2019 10:00AM - 10:12AM |
X10.00009: Evaluation of Pulay forces due to change in overlap of subspace-projection orbitals in constrained DFT: Application in calculation of reorganization energy of adsorbed molecule Subhayan Roychoudhury, David D. O'Regan, Stefano Sanvito Total energy of a constrained DFT calculation depends on population of subspace(s). When such subspace is defined in terms of orbitals localized on host atoms, ionic translations change the overlap of corresponding projection orbitals generating unconventional Pulay terms. We derive an exact expression for such Pulay terms maintaining non-orthogonality of projection orbitals. The corrected forces are implemented in the linear scaling DFT code ONETEP and, in conjunction with constrained DFT, are used to calculate the reorganization energy of a pentacene molecule adsorbed on a graphene flake. We show that subspace population depends on the choice of projection orbitals centred on a given set of host atoms and that non-orthogonal Wannier functions offer more precise results compared to pseudo-atomic orbitals. Our calculations of reorganization energy, performed by including ensemble DFT, correction for periodic boundary conditions and dispersion corrections show that, in general, reorganization energy of an adsorbed pentacene is lower than that of one in gas phase. This is consistent with effect of steric hindrance. |
Friday, March 8, 2019 10:12AM - 10:24AM |
X10.00010: Impacts of Electrostatic Screening and Interfacial Charge Transfer on Molecular Donor-Acceptor Heterojunctions Studied by Scanning Probe Microscopy/Spectroscopy Andrew Tan, Pengpeng Zhang A common need in organic electronics is the thorough understanding and control of the electronic structures and interfacial properties of molecular thin films on inorganic substrates. However, when donor (D) and acceptor (A) molecular heterojunctions are concerned, the interfacial charge transfer effects can compete with intermolecular charge transfer, which, along with the electrostatic effects from the substrate, exerts a significant substrate perturbation on the heterojunction. Employing archetypal D and A organic molecules and a variety of different supporting substrates, we show that the presence of the substrate, even a weakly interacting one, can still significantly perturb the intrinsic properties of the D-A heterojunction via interfacial charge transfer. Furthermore, substrate electrostatic screening can be modulated by interfacial charge transfer. This study highlights the impacts of the substrate electrostatic environment and the interfacial coupling on molecular electronic structures, an essential aspect in the applications of organic and molecular electronic devices. |
Friday, March 8, 2019 10:24AM - 10:36AM |
X10.00011: Spectroscopic evidence of two-impurity Kondo effect in potassium doped p-sexiphenyl films on Au (111) Yajun Yan, Wei Chen, Mingqiang Ren, Yilin Zhang, Tong Zhang, Donglai Feng Molecular magnetism is usually observed in organic coordination compounds consisting of transition metals or rare-earth metals, while it is rare in transition metal free or full-shell molecules. In this work, we grow high-quality p-sexiphenyl (PSP) monolayer film on Au (111) substrate, and then doped it with potassium (K) atoms. Various ordered phases are observed, with different structural and electronic properties. For a particular ordered phase, we observe obvious Kondo resonance peaks in isolated/misarranged PSP molecules, but clear splitting of such peaks is observed in PSP dimers. These phenomena share strong similarities to the two-impurity Kondo effect. Combined with theoretical calculation, we find that local spin is induced in PSP molecules by charge transfer from K atoms, leading to the appearance of Kondo resonance and the magnetic interaction of two Kondo impurities. Such two-impurity Kondo effect has only been reported in quantum dot system or dilute adatoms system, but the splitting of the Kondo resonance peak is much smaller than in our PSP system, which is as large as 5 meV. Our study not only provides a new and simple platform for investigating the two-impurity Kondo effect, but also provides a new route to construct a Kondo lattice on metal substrate. |
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