Molecular Thin Films Research Articles

Biomimetic macroscopic hierarchical moire gratings.

Large-scale hierarchical macroscopic moire gratings resembling the surface structure of Peruvian lily flower petals are fabricated on azobenzene molecular glass thin films using a Lloyd's mirror interferometer. It is shown that nanostructured linear and crossed moire gratings can be made with pitch values reaching a few millimeters. Also, using atomic force microscopy, scanning electron microscopy, optical microscopy, and surface profilometry techniques, it is shown that the obtained moire gratings have two-fold or three-fold hierarchical structures fabricated using a simple all optical technique.

Intermolecular interaction and cooperativity in an Fe(II) spin crossover molecular thin film system

Compact domain features have been observed in spin crossover [Fe{H2B(pz)2}2(bipy)] molecular thin film systems via soft x-ray absorption spectroscopy and photoemission electron microscopy. The domains are in a mixed spin state that on average corresponds to roughly 2/3 the high spin occupation of the pure high spin state. Monte Carlo simulations support the presence of intermolecular interactions that can be described in terms of an Ising model in which interactions beyond nearest-neighbors cannot be neglected. This suggests the presence of short-range order to permit interactions between molecules beyond nearest neighbor that contribute to the formation of largely high spin state domains structure. The formation of a spin state domain structure appears to be the result of extensive cooperative effects.

Perylene bisimides-based molecular dyads with different alkyl linkers for single-component organic solar cells

Two kinds of small molecular dyads (SMDs) based on trithiophene-benzodithiophene-rhodanine (DR3TBDT) main backbone and PBI termini, named SMD2-C6 and SMD2-C12, were designed with different lengths of alkyl linkers (C6H12 and C12H24). The length effect of alkyl linkers on the photovoltaic performance of the two SMDs have been systematically studied. The results show that the length of alkyl linkers can simutaneously tune the energy levels and phase separation in molecular dyad thin films. As a result, a power conversion efficiency of 1.33% was obtained for SMD2-C12 based single-component organic solar cells (SCOSCs), showing higher open-circuit voltage, short-circuit current density, and fill factor than those of SMD2-C6 based devices. Our findings provide a strategy for designing new SMDs and reveal the importance of alkyl linkers' length on the morphology of SMDs toward improved SCOSC performance.

Microporous Cyclodextrin Film with Funnel‐type Channel Polymerized on Electrospun Cellulose Acetate Membrane as Separators for Strong Trapping Polysulfides and Boosting Charging in Lithium–Sulfur Batteries

The “shuttle effect” of polysulfides hampers the commercialization of lithium–sulfur (Li‐S) batteries. Here, a thin molecular sieve film was decorated on the surface of an electrospun cellulose acetate (CA) membrane derived from recycled cigarette filters, where the truncated cone structure β‐cyclodextrin (β‐CD) was selected as the building block to physically block and chemically trap polysulfides while simultaneously dramatically speeding up ion transport. Furthermore, on the β‐CD free side of the separator facing the cathode, graphite carbon (C) was sputtered as an upper current collector, which barely increases the thickness. These benefits result in an initial discharge performance of 1378.24 mAh g−1 and long‐term cycling stability of 863.78 mAh g−1 after 1000 cycles at 0.2 C for the battery with the β‐CD/CA/C separator, which is more than three times that of the PP separator after 500 cycles. Surprisingly, the funnel‐type channel of β‐CD generates a differential ionic fluid pressure on both sides, speeding up ion transport by up to 69%, and a 65.3% faster charging rate of 9484 mA g−1 was achieved. The “funnel effect” of a separator is regarded as a novel and high‐efficiency solution for fast charging of Li‐S and other lithium secondary batteries.

Freestanding non-covalent thin films of the propeller-shaped polycyclic aromatic hydrocarbon decacyclene

Molecularly thin, nanoporous thin films are of paramount importance in material sciences. Their use in a wide range of applications requires control over their chemical functionalities, which is difficult to achieve using current production methods. Here, the small polycyclic aromatic hydrocarbon decacyclene is used to form molecular thin films, without requiring covalent crosslinking of any kind. The 2.5 nm thin films are mechanically stable, able to be free-standing over micrometer distances, held together solely by supramolecular interactions. Using a combination of computational chemistry and microscopic imaging techniques, thin films are studied on both a molecular and microscopic scale. Their mechanical strength is quantified using AFM nanoindentation, showing their capability of withstanding a point load of 26 ± 9 nN, when freely spanning over a 1 μm aperture, with a corresponding Young’s modulus of 6 ± 4 GPa. Our thin films constitute free-standing, non-covalent thin films based on a small PAH.

Thermoelectric properties of organic thin films enhanced by π–π stacking

Thin films comprising synthetically robust, scalable molecules have been shown to have major potential for thermoelectric energy harvesting. Previous studies of molecular thin-films have tended to focus on massively parallel arrays of discrete but identical conjugated molecular wires assembled as a monolayer perpendicular to the electrode surface and anchored via a covalent bond, know as self-assembled monolayers. In these studies, to optimise the thermoelectric properties of the thin-film there has been a trade-off between synthetic complexity of the molecular components and the film performance, limiting the opportunities for materials integration into practical thermoelectric devices. In this work, we demonstrate an alternative strategy for enhancing the thermoelectric performance of molecular thin-films. We have built up a series of films, of controlled thickness, where the basic units—here zinc tetraphenylporphyrin—lie parallel to the electrodes and are linked via π–π stacking. We have compared three commonly used fabrications routes and characterised the resulting films with scanning probe and computational techniques. Using a Langmuir-Blodgett fabrication technique, we successfully enhanced the thermopower perpendicular to the plane of the ZnTPP multilayer film by a factor of 10, relative to the monolayer, achieving a Seebeck coefficient of −65 μV K−1. Furthermore, the electronic transport of the system, perpendicular to the plane of the films, was observed to follow the tunnelling regime for multi-layered films, and the transport efficiency was comparable with most conjugated systems. Furthermore, scanning thermal microscopy characterisation shows a factor of 7 decrease in thermal conductance with increasing film thickness from monolayer to multilayer, indicating enhanced thermoelectric performance in a π–π stacked junction.

Plasmon-Enhanced Exciton Delocalization in Squaraine-Type Molecular Aggregates.

Enlarging exciton coherence lengths in molecular aggregates is critical for enhancing the collective optical and transport properties of molecular thin film nanostructures or devices. We demonstrate that the exciton coherence length of squaraine aggregates can be increased from 10 to 24 molecular units at room temperature when preparing the aggregated thin film on a metallic rather than a dielectric substrate. Two-dimensional electronic spectroscopy measurements reveal a much lower degree of inhomogeneous line broadening for aggregates on a gold film, pointing to a reduced disorder. The result is corroborated by simulations based on a Frenkel exciton model including exciton-plasmon coupling effects. The simulation shows that localized, energetically nearly resonant excitons on spatially well separated segments can be radiatively coupled via delocalized surface plasmon polariton modes at a planar molecule-gold interface. Such plasmon-enhanced delocalization of the exciton wave function is of high importance for improving the coherent transport properties of molecular aggregates on the nanoscale. Additionally, it may help tailor the collective optical response of organic materials for quantum optical applications.

Electrochemically deposited molecular thin films on transparent conductive oxide substrate: combined DC and AC approaches for characterization

Transparent conductive oxides such as indium tin oxide (ITO) substrates are commonly employed as prime materials for optoelectronic applications. Enhancement in functions of such devices often compels stable and robust modification of the ITO substrate to improve its interfacial charge transfer characteristics. Thereby, in this work, naphthyl modifier multilayer films are fabricated on ITO substrate using conventional electrochemical reduction of 1-naphthyl diazonium salts (NAPH-D) via altering its concentration ranging from 2 mM to 12 mM with a step size of 2. Surface coverage was significantly tuned by varying NAPH-D concentration, keeping other parameters such as the number of scans and scan rate constant. For lower concentrations (2 mM), the molecular thickness ∼6 nm was obtained, whereas higher concentrations (12 mM) produced around 15–18 nm thickness. Atomic force microscopy (AFM), cyclic voltammetry, and electrochemical impedance spectroscopy (EIS) in the presence of a ferrocene redox probe also supports the formation of well packed molecular film grown on the ITO surface. Further, the wettability property of the grafted naphthyl film was investigated at different surface coverages and correlated with charge transfer resistance (RCt) obtained from EIS studies.

Coordination-driven opto-electroactive molecular thin films in electronic circuits

An electrochemical method is utilized to prepare terpyridine layers on ITO, taking advantage of layer-by-layer deposition, growing opto-electro-active coordination compounds for the fabrication of electronic devices and circuit elements.

Ultrafast charge dynamics in molecular thin films

Conjugated molecular thin films have potential for novel optoelectronic devices such as flexible solar cells and display devices. However, a detailed understanding of their excited state dynamics is crucial for their technological applications. Tetrapyrrole complexes are an important class of conjugated organic molecules for optoelectronic applications due to their high light gathering ability and thermal stability. In this review, we summarize excited state dynamics of porphyrin and phthalocyanine molecular thin films investigated by pump–probe techniques. The relaxation dynamics for metallo-phthalocyanine molecules are observed to be ultrafast in nature and are in femtosecond time scale while that for metal free phthalocyanine are moderately fast. In case of metallo-phthalocyanines exciton lifetime is much longer than for metal free phthalocyanines. The central metal atom plays an important role in the excited state dynamics of porphyrin molecular thin films thus providing additional excitation channels. Time-resolved photoemission spectroscopy reveals about the evolution of charge transfer excitons at donor–acceptor interfaces and their dissociation mechanism. In case of metallo-phthalocyanines the exciton transport is dominated by hopping mechanism rather than spatial coherent length of excitons. The pump–probe studies provide a basis for appropriate selection of molecules for technological applications.

Simulation of Optical Properties of Inhomogeneous Molecular Contamination Thin Film Outgassed from Non-Metallic Materials

Simulation of Optical Properties of Inhomogeneous Molecular Contamination Thin Film Outgassed from Non-Metallic Materials

Magnetic Field Perturbations to a Soft X-ray-Activated Fe (II) Molecular Spin State Transition

The X-ray-induced spin crossover transition of an Fe (II) molecular thin film in the presence and absence of a magnetic field has been investigated. The thermal activation energy barrier in the soft X-ray activation of the spin crossover transition for [Fe{H2B(pz)2}2(bipy)] molecular thin films is reduced in the presence of an applied magnetic field, as measured through X-ray absorption spectroscopy at various temperatures. The influence of a 1.8 T magnetic field is sufficient to cause deviations from the expected exponential spin state transition behavior which is measured in the field free case. We find that orbital moment diminishes with increasing temperature, relative to the spin moment in the vicinity of room temperature.

Novel optoelectronic technique for direct tracking of ultrafast triplet excitons in polymeric semiconductor

The exciton physics of organic semiconductors is exotic. It is a domain in which singlet and triplet kinetics both play an important role in determining the performance of various optoelectronic devices. Since triplet excitons are non-emissive, it brings further challenges in the understanding of triplet kinetics. In this work, we have studied the effect of polymer chain packing on triplet diffusion in the polyfluorene based polymeric system, which is known to give efficient organic light emitting diode (OLED) efficiency for display devices. Furthermore, this polyfluorene system exhibits an efficient triplet–triplet fusion process, which provides singlet excitons as delayed fluorescence and becomes a tool to study triplet exciton kinetics. We have developed a unique method to trace the position of the triplet exciton in the emissive layer of OLEDs by analyzing angle-resolved delayed electroluminescence emission patterns as a function of time. This study could provide exciton transport kinetics in the transverse direction from the substrate plane. Furthermore, direct visualization of the delayed photoluminescence imaging technique could provide lateral transport kinetics of triplet excitons. Results suggest that the diffusion is significantly anisotropic in thinner films. As the thickness of the film increases, anisotropy reduces in triplet transport. Moreover, we noticed that in thicker polymeric semiconductor films, diffusivity approaches close to ultrahigh 10−3 cm2 s−1, which is similar to the values that are reported for acene-based molecular crystalline thin films. Our results also provide important insight into efficient electroluminescence in unusually thick (1.2 μm) polyfluorene-based emissive layers of OLEDs.

Electrochemical Potential-Driven High-Throughput Molecular Electronic and Spintronic Devices: From Molecules to Applications.

Molecules are fascinating candidates for constructing tunable and electrically conducting devices by the assembly of either a single molecule or an ensemble of molecules between two electrical contacts followed by current-voltage (I-V) analysis, which is often termed "molecular electronics". Recently, there has been also an upsurge of interest in spin-based electronics or spintronics across the molecules, which offer additional scope to create ultrafast responsive devices with less power consumption and lower heat generation using the intrinsic spin property rather than electronic charge. Researchers have been exploring this idea of utilizing organic molecules, organometallics, coordination complexes, polymers, and biomolecules (proteins, enzymes, oligopeptides, DNA) in integrating molecular electronics and spintronics devices. Although several methods exist to prepare molecular thin-films on suitable electrodes, the electrochemical potential-driven technique has emerged as highly efficient. In this Review we describe recent advances in the electrochemical potential driven growth of nanometric various molecular films on technologically relevant substrates, including non-magnetic and magnetic electrodes to investigate the stimuli-responsive charge and spin transport phenomena.

Ordering of Small Molecules on Hydrophobic Self-Assembled n-Alkanethiols: Delicate Balance of Interfacial and Intermolecular Interactions

Growth of molecular thin films with desired orders and orientations has become technologically relevant as the electronic industries seek new opportunities and applications. However, the delicate balance of interfacial and intermolecular forces and their complex influence on thin-film growths still require more understanding. Here, the effects of a hydrophobic self-assembled monolayer (SAM) surface on the crystallization of four common solvents—acetonitrile, ethanol, methanol, and water—are investigated. Despite the absence of significant substrate–molecule forces, unexpected oriented growth is observed for these molecules except water. Acetonitrile and ethanol form a sustaining vertical assembly order with long-range crystalline structures. Coincident epitaxy with small lattice mismatches is found to be essential to these orderings, which are energetically favored but without a dominant azimuthal orientation. In contrast, a preferred in-plane registry of methanol overlayers is observed for an ultrathin nominal thickness and becomes lost in slightly thicker films. Such thickness-dependent ordering of methanol assemblies can be explained with semi-commensurate epitaxy with a tensile strain of ∼6.6% along hydrogen-bonded chains, whose quick relief results in the loss of order and even the phase. These rich observations suggest that SAM surfaces offer good opportunities for selective crystallization of molecular films worthy of further investigations.

Dynamic resonant waveguide crossed gratings for wavelength-selective polarization conversion and optical modulation

Two-dimensional Resonant Waveguide Crossed Gratings (RWCG) were fabricated on azobenzene molecular glass thin films and their resonance behavior was studied once placed in between orthogonally aligned polarizers. Normally-incident polychromatic light was transmitted and/or reflected from these RWCGs only in narrow positive peaks. In addition, the central wavelength and transmitted intensity of these positive peaks were actively modulated by an external light source. Furthermore, a dynamic volume birefringence behavior related to the photomechanical effect of the azobenzene chromophores was observed. A mechanism to explain the polarization conversion of the resonant light using RWCGs at normal incidence was also proposed.

Universal Scalings in Two-Dimensional Anisotropic Dipolar Excitonic Systems.

Low-dimensional excitonic materials have inspired much interest owing to their novel physical and technological prospects. In particular, those with strong in-plane anisotropy are among the most intriguing but short of general analyses. We establish the universal functional form of the anisotropic dispersion in the small k limit for 2D dipolar excitonic systems. While the energy is linearly dispersed in the direction parallel to the dipole in plane, the perpendicular direction is dispersionless up to linear order, which can be explained by the quantum interference effect of the interaction among the constituents of 1D subsystems. The anisotropic dispersion results in a E^{∼0.5} scaling of the system density of states and predicts unique spectroscopic signatures including: (1)disorder-induced absorption linewidth, W(σ)∼σ^{2.8}, with σ the disorder strength, (2)temperature dependent absorption linewidth, W(T)∼T^{s+1.5}, with s the exponent of the environment spectral density, and (3)the out-of-plane angular θ dependence of the peak splittings in absorption spectra, ΔE(θ)∝sin^{2}θ. These predictions are confirmed quantitatively with numerical simulations of molecular thin films and tubules.

Avoiding the Center-Symmetry Trap: Programmed Assembly of Dipolar Precursors into Porous, Crystalline Molecular Thin Films.

Liquid-phase, quasi-epitaxial growth is used to stack asymmetric, dipolar organic compounds on inorganic substrates, permitting porous, crystalline molecular materials that lack inversion symmetry. This allows material fabrication with built-in electric fields. A new programmed assembly strategy based on metal-organic frameworks (MOFs) is described that facilitates crystalline, noncentrosymmetric space groups for achiral compounds. Electric fields are integrated into crystalline, porous thin films with an orientation normal to the substrate. Changes in electrostatic potential are detected via core-level shifts of marker atoms on the MOF thin films and agree with theoretical results. The integration of built-in electric fields into organic, crystalline, and porous materials creates possibilities for band structure engineering to control the alignment of electronic levels in organic molecules. Built-in electric fields may also be used to tune the transfer of charges from donors loaded via programmed assembly into MOF pores. Applications include organic electronics, photonics, and nonlinear optics, since the absence of inversion symmetry results in a clear second-harmonic generation signal.

Recent Progress in the Consistent Interpretation of Complementary Spectroscopic Results Obtained on Molecular Systems

AbstractResearch on organic thin films is largely driven by potential (opto‐)electronic applications and turns out to be no less intriguing from a fundamental point of view. Numerous studies make it clear that the understanding of device‐relevant molecular thin film architectures is quite challenging—often hampered by insufficient spectroscopic data and the lack of a consistent interpretation of the available datasets. Consequently, speculative aspects prevail in the discussion of energy levels in conjunction with the optical properties of organic thin films. Adequate spectroscopic techniques applicable to thin films of organic molecules (typical thicknesses required for devices are in the nanometer range) with the necessary sensitivities are rather demanding. Some of those methods were developed or significantly improved in the recent past. Here, the now available complementary spectroscopies are briefly surveyed with particular emphasis on some techniques that have not yet become widespread standards, and a non‐exhaustive set of examples of acquired experimental results are provided. For a consistent interpretation of the latter, the concepts brought forward in the literature considering the role of initial and final states of spectroscopic processes are outlined, with important consequences for quantitatively correct energy diagrams.

Inline holographic inscription of diffractive lenses in azobenzene molecular glass thin films.

A simple inline holographic setup is used to fabricate holographic diffractive lenses using off-the-shelf components. The resulting surface relief gratings are inscribed directly in azobenzene-functionalized thin films with pitches that agree well with a theoretical Fresnel zone plate. The annular gratings have an outer radius of approximately 9 mm and an inner radius of less than 4 mm. Interfering laser beams, circularly polarized in the same direction, generally produce poor-quality gratings in azo-films, but the addition of a reference beam lens greatly improved their consistency and produced quality gratings with depths up to 400 nm. Multiple exposures produce multifocal diffractive lenses, while angling the sample resulted in focal lines, instead of focal points.