Spontaneous electric fields in solid films: spontelectrics☆

Synthesis of 2,5,8-Tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzo[1,2-b:3,4-b':5,6-b″] Trithiophenes and Their Spontaneous Orientation Polarization in Thin Films

To investigate the relationship between molecular structures and spontaneous orientation polarization (SOP) in organic thin films, 2,5,8-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzo[1,2-b:3,4-b':5,6-b″] trithiophene (TPBTT) and its ethyl derivative (m-ethyl-TPBTT) were synthesized. Variable angle spectroscopic ellipsometry and two-dimensional grazing-incidence wide-angle X-ray scattering showed that the vacuum-deposited films of TPBTT and m-ethyl-TPBTT had a higher degree of molecular orientation parallel to the substrate compared with that of prototypical 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) due to the larger π-conjugated benzotrithiophene core. However, TPBTT films showed a lower SOP of +54.4 mV/nm than did the TPBi film (+77.3 mV/nm), indicating that the molecular orientation alone did not determine the SOP. In contrast, m-ethyl-TPBTT showed a larger SOP of +104.0 mV/nm in the film. Quantum chemical calculations based on density functional theory suggested that the differences in the stable molecular conformation and the permanent dipole moments between TPBTT and m-ethyl-TPBTT caused the differences in SOP. These results suggest that the simultaneous control of the orientational order and conformation of the molecules is important to achieving a large SOP in films.

Hidden singularities in spontaneously polarized molecular solids

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.

A theoretical study on spontaneous dipole orientation in ice structures

Spontaneous dipole orientation is studied for a set of simulated porous ASW ice films on a substrate held at temperatures ranging from 10 K to 140 K. It is found that the water dipoles in the films obtained at the lower temperatures are oriented such that a negative electric field with a magnitude of 10⁸-10⁹ V m^-1 is obtained. The magnitude of the field increases approximately linearly with height above the substrate, akin to experimental observations, although the magnitude of our field increases faster. A strong temperature dependence of the surface potential resulting from the spontelectric field is found, where the surface potential decreases when the substrate temperature increases. The surface potential finally becomes close to zero for temperatures around and above 110 K.

Comparative electron irradiations of amorphous and crystalline astrophysical ice analogues

Laboratory studies of the radiation chemistry occurring in astrophysical ices have demonstrated the dependence of this chemistry on a number of experimental parameters. One experimental parameter which has received significantly less attention is that of the phase of the solid ice under investigation. In this present study, we have performed systematic 2 keV electron irradiations of the amorphous and crystalline phases of pure CH₃OH and N₂O astrophysical ice analogues. Radiation-induced decay of these ices and the concomitant formation of products were monitored in situ using FT-IR spectroscopy. A direct comparison between the irradiated amorphous and crystalline CH₃OH ices revealed a more rapid decay of the former compared to the latter. Interestingly, a significantly lesser difference was observed when comparing the decay rates of the amorphous and crystalline N₂O ices. These observations have been rationalised in terms of the strength and extent of the intermolecular forces present in each ice. The strong and extensive hydrogen-bonding network that exists in crystalline CH₃OH (but not in the amorphous phase) is suggested to significantly stabilise this phase against radiation-induced decay. Conversely, although alignment of the dipole moment of N₂O is anticipated to be more extensive in the crystalline structure, its weak attractive potential does not significantly stabilise the crystalline phase against radiation-induced decay, hence explaining the smaller difference in decay rates between the amorphous and crystalline phases of N₂O compared to those of CH₃OH. Our results are relevant to the astrochemistry of interstellar ices and icy Solar System objects, which may experience phase changes due to thermally-induced crystallisation or space radiation-induced amorphisation.

Tuning Molecular Conformations to Enhance Spontaneous Orientation Polarization in Organic Thin Films

Three isomeric derivatives of 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) bearing ethyl groups on the N-phenyl moieties were synthesized to elucidate the effects of intramolecular interactions on spontaneous orientation polarization (SOP) in thin films. The films of the TPBi derivatives displayed enhanced SOP with a surface potential change of up to 1.8 times that for TPBi, and the p-substituted derivative exhibited the largest potential change reported to date (+141.0 mV/nm). Density functional theory calculations and single-crystal structure analysis suggest that the introduction of the ethyl groups switched the stable molecular conformation from C₁ to C₃ symmetry. Through analysis of the structural anisotropy in the films by spectral ellipsometry and two-dimensional (2D) grazing-incidence wide-angle X-ray scattering, we conclude that the conformational change of the molecules was the major factor underlying the SOP enhancement.

Sub-Hz Differential Rotational Spectroscopy of Enantiomers

We demonstrate for the first time high-precision differential microwave spectroscopy, achieving sub-Hz precision by coupling a cryogenic buffer gas cell with a tunable microwave Fabry–Perot cavity. We report statistically limited sub-Hz precision of (0.08 ± 0.72) Hz, observed between enantiopure samples of (R)-1,2-propanediol and (S)-1,2-propanediol at frequencies near 15 GHz. We confirm highly repeatable spectroscopic measurements compared to traditional pulsed-jet methods, opening up new capabilities in probing subtle molecular structural effects at the 10−10 level and providing a platform for exploring sources of systematic error in parity-violation searches. We discuss dominant systematic effects at this level and propose possible extensions of the technique for higher precision.

The rise of an exciton in solid ammonia

We trace a polymorphic phase change in solid ammonia films through the emergence of a Frenkel exciton at 194.4 nm, for deposition temperatures of 48 K, 50 K and 52 K. Observations on a timescale of hours give unparalleled access to the individual processes of nucleation and the phase change itself. The excitonic transition is forbidden in the low temperature phase, but greater flexing of the solid state structure in the higher temperature phase makes the transition allowed, as the nano-crystals approach ∼30 unit cells through nucleation. We find activation energies of 21.7 ± 0.6 kJ mol^-1 for nucleation and 22.8 ± 0.6 kJ mol^-1 for the phase change, corresponding to the breaking of two to three hydrogen bonds.

Low temperature aging in a molecular glass: the case of cis-methyl formate

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.

A mechanism for ageing in a deeply supercooled molecular glass

Measurements of the decay of electric fields, formed spontaneously within vapour-deposited films of cis-methyl formate, provide the first direct assessment of the energy barrier to secondary relaxation in a molecular glass. At temperatures far below the glass transition temperature, the mechanism of relaxation is shown to be through hindered molecular rotation. Magnetically-polarised neutron scattering experiments exclude diffusion, which is demonstrated to take place only close to the glass transition temperature.

Sign flipping of spontaneous polarization in vapour-deposited films of small polar organic molecules

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.

Crystallites and Electric Fields in Solid Ammonia

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.

Quantitative Anisotropic Analysis of Molecular Orientation in Amorphous N2O at 6 K by Infrared Multiple-Angle Incidence Resolution Spectrometry

The existence of molecular orientational order in nanometer-thick films of molecules has long been implied by surface potential measurements. However, direct quantitative determination of the molecular orientation is challenging, especially for metastable amorphous thin films at low temperatures. This study quantifies molecular orientation in amorphous N₂O at 6 K using infrared multiple-angle incidence resolution spectrometry (IR-MAIRS). The intensity ratio of the weak antisymmetric stretching vibration band of the ¹⁴N¹⁵NO isotopomer between the in-plane and out-of-plane IR-MAIRS spectra provides an average molecular orientation angle of 65° from the surface normal. No discernible change is observed in the orientation angle when a different substrate material is used (Si and Ar) at 6 K or the Si substrate temperature is changed in the range of 6-14 K. This suggests that the transient mobility of N₂O during physisorption is key in governing the molecular orientation in amorphous N₂O.

Self-Assembled Electret for Vibration-Based Power Generator

The vibration-based electret generators (EGs) for energy harvesting have been extensively studied because they can obtain electrical energy from ambient vibrations. EGs exhibit a sandwich structure of electrodes surrounding an air gap and an electret, which is a dielectric material with a quasi-permanent electrical charge or dipole polarisation. Various charging processes have been developed because the surface charge density (σ) of the electret determines the output power of the device. However, such processes are considered to constitute a key productivity-limiting factor from the mass production viewpoint, making their simplification or elimination a highly desired objective. Herein, a model EG that does not require any charging process by utilising the spontaneous orientation polarisation of 1,3,5-tris(1-phenyl-1H-benzimidazole-2-yl)benzene (TPBi) is demonstrated. The surface potential (V_sp) of an evaporated TPBi film has reached 30.2 V at a film thickness of 500 nm without using a charging process. The estimated σ of 1.7 mC m⁻² is comparable with that obtained using a conventional polymer-based electret after charging. Furthermore, V_sp is considerably stable in environmental conditions; thus, TPBi can be considered to be “self-assembled” electret (SAE). Application of SAE leads to developing an EG without requiring the charging process.

Librating dipoles as a probe of spontaneously electrical films and as a source of THz radiation

Pulsing a spontaneously electrical solid causes the molecular dipoles to oscillate and emit THz radiation.

The optical absorption spectra of spontaneously electrical solids: the case of nitrous oxide

Mud flats, Fejø, Denmark: a 2D representation of defect-free regions in a molecular crystal that can support Wannier–Mott excitons.

Assigning a structural motif using spontaneous molecular dipole orientation in thin films

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.

Non-linear and non-local behaviour in spontaneously electrical solids

We show that solids displaying spontaneous dipole orientation possess quite general non-local and non-linear characteristics expressed through their internal electric fields.

Wannier-Mott Excitons in Nanoscale Molecular Ices

The absorption of light to create Wannier-Mott excitons is a fundamental feature dictating the optical and photovoltaic properties of low band gap, high permittivity semiconductors. Such excitons, with an electron-hole separation an order of magnitude greater than lattice dimensions, are largely limited to these semiconductors but here we find evidence of Wannier-Mott exciton formation in solid carbon monoxide (CO) with a band gap of >8  eV and a low electrical permittivity. This is established through the observation that a change of a few degrees K in deposition temperature can shift the electronic absorption spectra of solid CO by several hundred wave numbers, coupled with the recent discovery that deposition of CO leads to the spontaneous formation of electric fields within the film. These so-called spontelectric fields, here approaching 4×10^{7}  V m^{-1}, are strongly temperature dependent. We find that a simple electrostatic model reproduces the observed temperature dependent spectral shifts based on the Stark effect on a hole and electron residing several nm apart, identifying the presence of Wannier-Mott excitons. The spontelectric effect in CO simultaneously explains the long-standing enigma of the sensitivity of vacuum ultraviolet spectra to the deposition temperature.

Spontaneous polarization of solid CO on water ices and some astrophysical implications

Reflection absorption infrared spectroscopy (RAIRS) is used to show that when 20 monolayer (ML) films of solid CO are laid down on solid water substrates at 20 to 24 K, the films polarize spontaneously.

Spontaneous electric fields in solid carbon monoxide

Reflection–absorption infrared spectroscopy (RAIRS) is shown to provide a means of observing the spontelectric phase of matter, the defining characteristic of which is the occurrence of a spontaneous and powerful static electric field within a film of material.

Spontaneously electrical solids in a new light

Reflection–absorption infrared spectroscopy (RAIRS) of nitrous oxide (N₂O) thin films is shown to provide an independent means of observing the spontelectric state; the first new structural phase of matter, with unique electrical properties, to have emerged in decades.

Investigations into the nature of spontelectrics: nitrous oxide diluted in xenon

The recent discovery of a new class of solids displaying bulk spontaneous electric fields as high as 10⁸ V m⁻¹, so-called ‘spontelectrics’, poses fundamental and unresolved problems in solid state physics.