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Cassidy A, Pijpers FP, Field D. Hidden singularities in spontaneously polarized molecular solids. J Chem Phys 2023; 158:144501. [PMID: 37061481 DOI: 10.1063/5.0138642] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 04/17/2023] Open
Abstract
Films of dipolar molecules formed by physical vapor deposition are, in general, spontaneously polarized, with implications ranging from electron transfer in molecular optoelectronic devices to the properties of astrochemical ices in the interstellar medium. Polarization arises from dipole orientation, which should intuitively decrease with increasing deposition temperature, T. However, it is experimentally found that minimum or maximum values in polarization vs T may be observed for cis-methyl formate, 1-propanol, and ammonia. A continuous analytic form of polarization vs T is developed, which has the property that it is not differentiable at all T. The minima and maxima in polarization vs T are marked by singularities in the differential of this analytic form. This exotic behavior is presently unique to films of dipolar species and has not been reported, for example, in the related magnetic phases of spin glasses.
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Affiliation(s)
- Andrew Cassidy
- Center for Interstellar Catalysis, Department of Physics and Astronomy, Aarhus University, Aarhus, Denmark
| | - Frank P Pijpers
- Korteweg - de Vries Institute for Mathematics, University of Amsterdam, Amsterdam, Netherlands
| | - David Field
- Center for Interstellar Catalysis, Department of Physics and Astronomy, Aarhus University, Aarhus, Denmark
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Cassidy A, Jørgensen MRV, Glavic A, Lauter V, Plekan O, Field D. Low temperature aging in a molecular glass: the case of cis-methyl formate. Phys Chem Chem Phys 2021; 23:15719-15726. [PMID: 34279530 DOI: 10.1039/d1cp01332g] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Glassy films of cis-methyl formate show spontaneous dipole orientation on deposition from the vacuum, the so-called 'spontelectric effect', creating surface potentials and electric fields within the films. We follow the decay of these fields, and their accompanying dipole orientation, on the hours timescale at deposition temperatures between 40 K and 55 K. Our data trace the low temperature 'secondary decay' mechanism, at tens of degrees below the glass transition temperature of 90 K. We show that secondary decay is due to molecular rotation, with associated activation energies lying between 0.1 and 0.2 eV. Diffusion is absent, as established from published neutron reflectivity data. Using an analytical model for the spontelectric effect, data are placed on a quantitative footing, showing that angular motion in excess of 50° reproduces the observed values of activation energies. Exploitation of the spontelectric effect is new in the study of glass aging and is shown here to give insight into the elusive processes which take place far from the molecular glass transition temperature.
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Affiliation(s)
- Andrew Cassidy
- Center for Interstellar Catalysis and Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, Aarhus C, Denmark.
| | - Mads R V Jørgensen
- Center of Materials Crystallography, iNano & Department of Chemistry, Aarhus University, Langelandsgade 140, Aarhus C, Denmark and MAX IV Laboratory, Lund University, Fotongatan 2, Lund, Sweden
| | - Artur Glavic
- Laboratory for Neutron and Muon Instrumentation, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland
| | - Valeria Lauter
- Neutron Scattering Division, Oak Ridge National Lab, Oak Ridge, TN 37831, USA
| | - Oksana Plekan
- Sincrotrone Trieste S.C.P.A., Area Science Park, Strada Statale 14, km 163.5, I-34149 Basovizza, Trieste, Italy
| | - David Field
- Center for Interstellar Catalysis and Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, Aarhus C, Denmark.
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Field D, Cassidy A. Comment on "Spontaneous Polarization of Cryo-Deposited Films for Five Normal Saturated Monohydroxy Alcohols, C nH 2n+1OH, n = 1-5". J Phys Chem B 2021; 125:7568-7569. [PMID: 34185522 DOI: 10.1021/acs.jpcb.1c02361] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- David Field
- Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, Aarhus 8000 C, Denmark
| | - Andrew Cassidy
- Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, Aarhus 8000 C, Denmark
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Tourlakis GM, Adamopoulos SAT, Gavra IK, Milpanis AA, Tsagri LF, Pachygianni ASG, Chatzikokolis SS, Tsekouras AA. Sign flipping of spontaneous polarization in vapour-deposited films of small polar organic molecules. Phys Chem Chem Phys 2021; 23:14352-14362. [PMID: 34169950 DOI: 10.1039/d1cp01584b] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Films of polar molecules vapour-deposited on sufficiently cold substrates are not only amorphous, but also exhibit charge polarization across their thickness. This is an effect known for 50 years, but it is very poorly understood and no mechanism exists in the literature that can explain and predict it. We investigated this bulk effect for 18 small organic molecules as a function of substrate temperature (30-130 K). We found that, as a rule, alcohol films have the negative end on the vacuum side at all temperatures. Alkyl acetates and toluene showed positive voltages which reached a maximum around the middle of the temperature range investigated. Tetrahydrofuran showed positive voltages which dropped with increasing deposition temperature. Diethyl ether, acetone, propanal, and butanal showed positive film voltages at low temperatures, negative at intermediate temperatures and again positive voltages at higher temperatures. In all cases, film voltages were monitored during heating leading to film evaporation. Film voltages were irreversibly eliminated before film elimination, but voltage profiles during temperature ramps differed vastly depending on compound and deposition temperature. In general, there was a gradual voltage reduction, but propanal, butanal, and diethyl ether showed a change in voltage sign during temperature ramp in films deposited at low temperatures. All these data expand substantially the experimental information regarding spontaneous polarization in vapour-deposited films, but still require complementary measurements as well as numerical simulations for a detailed explanation of the phenomenon.
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Affiliation(s)
- Georgios M Tourlakis
- Physical Chemistry Laboratory, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis, Zografos, GR-15784, Greece.
| | - Sotirios Alexandros T Adamopoulos
- Physical Chemistry Laboratory, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis, Zografos, GR-15784, Greece.
| | - Irini K Gavra
- Physical Chemistry Laboratory, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis, Zografos, GR-15784, Greece.
| | - Alexandros A Milpanis
- Physical Chemistry Laboratory, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis, Zografos, GR-15784, Greece.
| | - Liveria F Tsagri
- Physical Chemistry Laboratory, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis, Zografos, GR-15784, Greece.
| | - Aikaterini Sofia G Pachygianni
- Physical Chemistry Laboratory, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis, Zografos, GR-15784, Greece.
| | - Stylianos S Chatzikokolis
- Physical Chemistry Laboratory, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis, Zografos, GR-15784, Greece.
| | - Athanassios A Tsekouras
- Physical Chemistry Laboratory, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis, Zografos, GR-15784, Greece.
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Stubbing JW, McCoustra MRS, Brown WA. A new technique for determining the refractive index of ices at cryogenic temperatures. Phys Chem Chem Phys 2020; 22:25353-25365. [PMID: 33140768 DOI: 10.1039/d0cp02373f] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
A reflection-absorption optical (RAO) spectrometer, operating across the ultra-violet/visible (UV/visible) wavelength region, has been developed that allows simultaneous measurements of optical properties and thickness of thin solid films at cryogenic temperatures in ultrahigh vacuum. The RAO spectrometer enables such measurements to be made after ice deposition, as opposed to most current approaches which make measurements during deposition. This allows changes in the optical properties and in the thickness of the film to be determined subsequent to thermal, photon or charged particle processing. This is not possible with current techniques. A data analysis method is presented that allows the wavelength dependent n and k values for ices to be extracted from the reflection-absorption spectra. The validity of this analysis method is shown using model data from the literature. New data are presented for the reflection UV/visible spectra of amorphous and crystalline single component ices of benzene, methyl formate and water adsorbed on a graphite surface. These data show that, for benzene and methyl formate, the crystalline ice has a larger refractive index than amorphous ice, reflecting changes in the electronic environment occurring in the ice during crystallisation. For water, the refractive index does not vary with ice phase.
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Affiliation(s)
- James W Stubbing
- Department of Chemistry, School of Life Sciences, University of Sussex, Falmer, Brighton, BN1 9QJ, UK.
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Cassidy A, James RL, Dawes A, Field D. Crystallites and Electric Fields in Solid Ammonia. ChemistryOpen 2020; 9:983-990. [PMID: 33024652 PMCID: PMC7528761 DOI: 10.1002/open.202000118] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/30/2020] [Revised: 06/03/2020] [Indexed: 11/12/2022] Open
Abstract
Absorption spectra of vacuum-deposited films of ammonia have been obtained in the range 115 nm to 310 nm for a set of 15 deposition temperatures, Td, between 20 K and 80 K. Results focus upon the region 115 nm to 130 nm in overlapping D, E, F and G←X Rydberg transitions involving Wannier-Mott excitons. We identify two phases of ammonia, showing the solid to be polymorphic. Peak absorption wavelengths in the region of interest are found to shift to the red by 299 cm-1, for Td between 20 K to 50 K, and 1380 cm-1 for Td between 55 K to 80 K. Shifts provide evidence for the presence of spontaneously generated electric fields in these films, of values in excess of 108 V m-1 for Td of 20 K to 50 K to a few times 107 V m-1 for 55 K to 80 K. Results enable us to place a lower limit of 1.58 nm on the size of crystallites in the low temperature regime. This dimension represents 16 unit cells or 64 species, giving a more quantitative description than the nebulous term amorphous, as applied to solid ammonia. We also determine that crystallites formed in the high temperature regime contain, within ±20 %, 1688, 756 and 236 molecules of ammonia, respectively at Td of 65 K, 60 K and 55 K.
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Affiliation(s)
- Andrew Cassidy
- Department of Physics and AstronomyAarhus UniversityNy Munkegade 1208000AarhusDenmark
| | - Rachel L. James
- School of Physical SciencesThe Open UniversityWalton HallMilton KeynesMK7 6AAUK
| | - Anita Dawes
- School of Physical SciencesThe Open UniversityWalton HallMilton KeynesMK7 6AAUK
| | - David Field
- Department of Physics and AstronomyAarhus UniversityNy Munkegade 1208000AarhusDenmark
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Field D. Librating dipoles as a probe of spontaneously electrical films and as a source of THz radiation. Phys Chem Chem Phys 2019; 21:26606-26614. [DOI: 10.1039/c9cp05746c] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Pulsing a spontaneously electrical solid causes the molecular dipoles to oscillate and emit THz radiation.
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Affiliation(s)
- D. Field
- Department of Physics and Astronomy
- University of Aarhus
- Aarhus
- Denmark
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Roman M, Dunn A, Taj S, Keolopile ZG, Rosu-Finsen A, Gutowski M, McCoustra MRS, Cassidy AM, Field D. Assigning a structural motif using spontaneous molecular dipole orientation in thin films. Phys Chem Chem Phys 2018; 20:29038-29044. [DOI: 10.1039/c8cp06010j] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023]
Abstract
Guided by the spontelectric behaviour of thin films of cis-methyl formate, infrared observations and computational investigations reveal the dimer structural motif of the crystalline solid.
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Affiliation(s)
- M. Roman
- Institute of Chemical Sciences
- Heriot-Watt University
- Edinburgh
- UK
| | - A. Dunn
- Institute of Chemical Sciences
- Heriot-Watt University
- Edinburgh
- UK
| | - S. Taj
- Institute of Chemical Sciences
- Heriot-Watt University
- Edinburgh
- UK
| | - Z. G. Keolopile
- Institute of Chemical Sciences
- Heriot-Watt University
- Edinburgh
- UK
- Department of Physics
| | - A. Rosu-Finsen
- Institute of Chemical Sciences
- Heriot-Watt University
- Edinburgh
- UK
| | - M. Gutowski
- Institute of Chemical Sciences
- Heriot-Watt University
- Edinburgh
- UK
| | | | - A. M. Cassidy
- Department of Physics and Astronomy
- University of Aarhus
- Aarhus
- Denmark
| | - D. Field
- Department of Physics and Astronomy
- University of Aarhus
- Aarhus
- Denmark
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