Growth Of Organic Thin Films Research Articles

Single-molecular diffusivity and long jumps of large organic molecules: CoPc on Ag(100).

Energy dissipation and the transfer rate of adsorbed molecules do not only determine the rates of chemical reactions but are also a key factor that often dictates the growth of organic thin films. Here, we present a study of the surface dynamical motion of cobalt phthalocyanine (CoPc) on Ag(100) in reciprocal space based on the helium spin-echo technique in comparison with previous scanning tunnelling microscopy studies. It is found that the activation energy for lateral diffusion changes from 150meV at 45-50K to ≈100meV at 250-350K, and that the process goes from exclusively single jumps at low temperatures to predominantly long jumps at high temperatures. We thus illustrate that while the general diffusion mechanism remains similar, upon comparing the diffusion process over widely divergent time scales, indeed different jump distributions and a decrease of the effective diffusion barrier are found. Hence a precise molecular-level understanding of dynamical processes and thin film formation requires following the dynamics over the entire temperature scale relevant to the process. Furthermore, we determine the diffusion coefficient and the atomic-scale friction of CoPc and establish that the molecular motion on Ag(100) corresponds to a low friction scenario as a consequence of the additional molecular degrees of freedom.

Closing the loop: autonomous experiments enabled by machine-learning-based online data analysis in synchrotron beamline environments.

Recently, there has been significant interest in applying machine-learning (ML) techniques to the automated analysis of X-ray scattering experiments, due to the increasing speed and size at which datasets are generated. ML-based analysis presents an important opportunity to establish a closed-loop feedback system, enabling monitoring and real-time decision-making based on online data analysis. In this study, the incorporation of a combined one-dimensional convolutional neural network (CNN) and multilayer perceptron that is trained to extract physical thin-film parameters (thickness, density, roughness) and capable of taking into account prior knowledge is described. ML-based online analysis results are processed in a closed-loop workflow for X-ray reflectometry (XRR), using the growth of organic thin films as an example. Our focus lies on the beamline integration of ML-based online data analysis and closed-loop feedback. Our data demonstrate the accuracy and robustness of ML methods for analyzing XRR curves and Bragg reflections and its autonomous control over a vacuum deposition setup.

Crystalline organic thin films for crystalline OLEDs (II): weak epitaxy growth of phenanthroimidazole derivatives.

The ordered molecular arrangement of crystalline organic semiconductors facilitates high carrier mobility and light emission in organic light-emitting diode (OLED) devices. It has been demonstrated that the weak epitaxy growth (WEG) process is a valuable crystallization route for fabricating crystalline thin-film OLEDs (C-OLEDs). Recently, C-OLEDs based on crystalline thin films of phenanthroimidazole derivatives have exhibited excellent luminescent properties such as high photon output at low driving voltage and high power efficiency. Achieving effective control of organic crystalline thin film growth is crucial for the development of new C-OLEDs. Herein, we report the studies on morphology structure and growth behavior of the phenanthroimidazole derivative WEG thin films. The oriented growth of WEG crystalline thin films is determined by channeling and lattice matching between the inducing layer and active layer. Large-size and continuous WEG crystalline thin films can be obtained by controlling the growth conditions.

Toward Ideal Metal–Organic Framework Thin-Film Growth via Automated Layer-by-Layer Deposition: Examples Based on Perylene Diimide Linkers

Photoactive metal–organic framework (MOF) thin films offer an opportunity for translating the advantages of periodic, crystalline, and tailorable light-harvesting materials directly into devices such as those for photoelectrochemical solar energy conversion. In this study, we report the fabrication of light-absorbing perylene-diimide-containing pillared-paddlewheel MOF thin films using an automated layer-by-layer (LbL) deposition technique. Our focus here is on optimizing the growth of representative chromophoric MOFs as oriented films of uniform and predefined thickness. Growth was examined as a function of metal identity, pillaring ligand composition, and supporting-surface chemical functionality. Application of atomic force microscopy (AFM) and complementary techniques revealed that the surface-supported MOFs initially display island-type film growth (Volmer–Weber growth), resulting in comparatively rough films. Further growth is accompanied by the merging of islands, resulting in films that, depending on experimental details, can be remarkably smooth (i.e., roughness on the order of ±1 nm (one structural repeat unit in the pillaring direction)). These details include the use of 1,4-diazabicyclo[2.2.2]octane (DABCO) as a MOF pillar and ALD-grown zinc oxide as a film support (ALD = atomic layer deposition). Also helpful for mitigating island-type growth, at least in part, is the replacement of Zn2+ by Cu2+ as the metal component of the MOF. Notably, each of these adjustments entails replacing weaker chemical bonds with stronger ones.

Simultaneous measurement of X-ray scattering and photoluminescence during molecular deposition

We present a simultaneous in situ measurement of X-ray scattering and photoluminescence (PL) spectroscopy during organic thin-film growth in a vacuum. We have designed a modified deposition chamber that broadens its utilization and enables complementary information on the structure and optoelectronic properties. In a pilot experiment, we investigated the growth of diindenoperylene (DIP) thin films in real time. We found a direct correlation between PL and grazing-incidence wide-angle X-ray scattering (GIWAXS), showing the formation of a new DIP polymorph starting at a layer thickness of ∼14 nm. Furthermore, we show that the PL signal is more sensitive to the arrangement of the molecules than GIWAXS in the limit of small layer thicknesses. The presented experimental setup paves the way for performing simultaneous PL and X-ray scattering measurements during the growth of various materials suitable for optoelectronic applications.

Simulating the growth of amorphous organic thin films

Physical vapor deposition is used widely to produce organic electronics devices, in particular for OLEDs in smartphones, TVs and other display applications. Despite its long history, recent years have seen a number of surprising observations, such as deposition-induced emitter orientation and built-in electrostatic potentials in thin organic films. Modeling the details of these effects is complicated by the long time scales involved in the underlying processes. In this paper, we compare two different modeling approaches, which both aim to simulate the physical vapor deposition (PVD) process for small organic molecules. We compare a molecular dynamics approach, based on a classical bead-spring force field and time integration, with a Metropolis Monte Carlo approach, where the intramolecular degrees are limited to the torsional rotations. To analyze the resulting structures, we calculate the density and radial distribution functions (RDF) of all films. We observe a good agreement for the RDFs, but an approximately 10% higher density for the films generated by the molecular dynamics approach. Additionally, we investigate the anisotropic nature of such morphologies by calculating the ordinary and extra-ordinary refractive index for each material. Finally, we calculate electron and hole mobilities with an Kinetic Monte Carlo protocol.

Epitaxy of an Organic Semiconductor Templated by Molecular Monolayer Crystals

Molecular template growth is an efficient approach to enhance the crystallinity of organic semiconductor materials deposited by physical vapor deposition methods. Lattice matching, large grain size, and large monolayer coverage are critical figures of merits to evaluate effective templates. Here, solution-processed molecular monolayer crystals (MMCs) with 100% monolayer coverage and large single-crystalline domain size present the optimum form of molecular templates, which are successfully utilized for the growth of organic semiconductor thin films. Commensurate epitaxy growth over a large area was observed, and the reason for widely observed lamella structures in thin films was disclosed. In addition, the MMCs are proven effective in templating the growth of various high-performance organic semiconductor materials into orientated thin films. The MMC template provides an efficient approach to enhance the crystallinity and regulate the molecular orientation of organic semiconductor thin films.

Collective orientation barrier in growth of organic thin films revealed by pMAIRS

We used $p$-polarized multiple-angle incidence resolution spectrometry (pMAIRS) to investigate a collective orientation barrier (COB) in the growth of organic semiconductor (OSC) films. We demonstrate a temperature-dependent variation in the growth of pentacene (PEN) on $\mathrm{Si}{\mathrm{O}}_{2}$ films as a model system. The molecular orientation varied from lying to standing as the growth temperature increased. This change suggests that the formation of a standing orientation is thermally activated compared with the lying state. The nucleation of standing-oriented islands occurs by molecular self-assembly at sufficiently high temperatures. Conversely, molecules deposit in a flat-lying state at low temperatures owing to hindrance of the kinetic barrier to reorientation from lying to standing states. The COB is defined as a collective energy barrier for the reorientation processes throughout the growth. The COB is quantitatively estimated from an Arrhenius plot of the probability to form a standing orientation, derived from the dichroic ratio measured by pMAIRS. We found that the COB in the growth of PEN on the $\mathrm{Si}{\mathrm{O}}_{2}$ system was approximately 0.02 eV, which is a key parameter for determining the molecular orientation. A quantitative evaluation of the COB will be applicable to other systems and enable effective control over the molecular orientation of OSC films.

Tailoring the Optical Response of Pentacene Thin Films via Templated Growth on Hexagonal Boron Nitride.

The optoelectronic properties of organic thin films are strongly dependent on their molecular orientation and packing, which in turn is sensitive to the underlying substrate. Hexagonal boron nitride (hBN) and other van der Waals (vdW) materials are known to template different organic thin film growth modalities from conventional inorganic substrates such as SiO2. Here, the morphology and temperature-dependent optical properties of pentacene films grown on hBN are reported. Pentacene deposited on hBN forms large-grain films with a molecular π-face-on orientation unlike the dendritic edge-on thin-film phase on SiO2. Pentacene/hBN films exhibit a 40 meV lower free exciton emission than pentacene/SiO2 and an unconventional emission energy temperature dependence. Time-resolved photoluminescence (PL) decay measurements show a long-lived signal in the π-face-on phase related to delayed emission from triplet-triplet fusion. This work demonstrates that growth on vdW materials provides a pathway for controlling optoelectronic functionality in molecular thin films.

Morphological Transitions in Organic Ultrathin Film Growth Imaged by In Situ Step-by-Step Atomic Force Microscopy

In situ atomic force microscopy (AFM) allowed us to investigate the evolution at the early stages of the growth of organic thin films. An ultrahigh-vacuum atomic force microscope, integrated with a...

Surface modification of PLA nets intended for agricultural applications

Exclusion nets create a physical barrier between pests and crops, serving as a practical tool to reduce pesticide use in a wide variety of crops, and their negative impacts on the environment. While the efficiency of this method has been recently assessed, protection from pests is incomplete, as microorganisms such as fungi are not hindered by netting. In order to avoid favorable conditions for microbial development (e.g. wet leaf surfaces), the present study discusses two hydrophobic surface treatment approaches, each applied to two PLA substrates that can be used as exclusion nets to reduce water ingress on protected fruit trees. Both a commercial exclusion net and a fused deposition modelled (or 3D printed) mesh were treated via solvent-free and solvent-induced modifications. Photo-initiated chemical vapor deposition (PICVD) allowed organic thin film growth on the substrate at near-ambient conditions. Dip-dip-dry (DDD) exploited solvent-induced topology modification to yield hydrophobic PLA samples, showing various microstructures (crazes or hierarchical topology) depending on the substrate properties. Both substrates were efficiently rendered hydrophobic after surface treatments, with water contact angles climbing from 73° to 101° (PICVD) or 143° (DDD) for printed PLA samples, and to 131° (PICVD) or 135° (DDD) for commercial exclusion nets. The hierarchical features present on DDD-treated printed PLA substrates showed potential for molecular adsorption, paving the way for a method to increase the phytosanitary effect of agricultural textiles by active ingredient incorporation (limonene is used here as a test case). Surface properties were characterized with SEM imaging, contact angle measurements, tensile tests and drop penetration assessments (rain simulation).

From flat to tilted: gradual interfaces in organic thin film growth.

We investigate domain formation and local morphology of thin films of α-sexithiophene (α-6T) on Au(100) beyond monolayer coverage by combining high resolution scanning tunneling microscopy (STM) experiments with electronic structure theory calculations and computational structure search. We report a layerwise growth of highly-ordered enantiopure domains. For the second and third layer, we show that the molecular orbitals of individual α-6T molecules can be well resolved by STM, providing access to detailed information on the molecular orientation. We find that already in the second layer the molecules abandon the flat adsorption structure of the monolayer and adopt a tilted conformation. Although the observed tilted arrangement resembles the orientation of α-6T in the bulk, the observed morphology does not yet correspond to a well-defined surface of the α-6T bulk structure. A similar behavior is found for the third layer indicating a growth mechanism where the bulk structure is gradually adopted over several layers.

Electric-Field-Tunable Growth of Organic Semiconductor Crystals on Graphene.

Growth of organic semiconductor thin films on a two-dimensional template is affected by its properties and is not well understood. This growth process dictates a thin film's final morphology and crystal structure and is controlled by the interactions between ad-molecules and the template. Here, we report that the template's charge density determines the tuning of such interactions. We observe the dependence of pentacene nucleation on charge carrier density ng in graphene under an applied electric field and contact-doping and then deduce that the interaction energy EA between the ad-molecule and the graphene is related linearly to ng. This tunability of EA allows control of the pentacene crystals growth. We exploit these findings to demonstrate that graphene, in which ng is controlled, can be used to template pentacene thin films for improved optoelectronic properties, such as electrical conductivity and exciton diffusion length.

Ultrahigh-vacuum organic molecular-beam deposition system for in situ growth and characterization.

A compact ultrahigh-vacuum molecular-beam deposition system has been developed for the in situ synthesis of organic thin films and multilayers. The system incorporates all the features (heater, thickness monitor, evaporators) necessary for controlled organic thin-film growth. It can be used independently, or it can be docked to the in situ growth system and transferred to other instruments of the PGM beamline, thus allowing extensive film preparation and characterization. A manipulator dedicated to specimen preparation and organic-film deposition with temperature control between 200 K and ∼800 K has been developed. The design and performance of the system are reported with emphasis on a novel solution of masks developed to achieve position-dependent film deposition. To demonstrate the enhanced capabilities of the PGM beamline in the growth and in the characterization of electronic-structure studies of organic molecular films and their heterostructures through synchrotron-based spectroscopies, this paper presents some preliminary results of a study of Fe-phthalocyanine growth on Si substrates and on in situ prepared La0.67Sr0.33MnO3 buffer layers on SrTiO3 single crystal.

Storage Life Modeling and Analysis for Contacting Slip Ring Based on Physics of Failure

The storage life of the contacting slip ring in the inertial navigating platform cannot be easily estimated because of the difficulty of obtaining field data. This paper proposes a new approach about the storage life modeling and analysis for the contacting slip ring based on the combination of failure mechanism and statistical data. The analysis of failure mechanism shows that the increase of contact resistance caused by the stress relaxation of brushes and organic thin-film growth leads to the storage failure of the contacting slip ring in most cases. Based on the microscopic contact and thin-film electrical conduction mechanism, the relaxation resistance and thin-film resistance growth models are developed, which can transform the failure data of contact resistance into the storage life data. Some measures are presented to extend the storage life of the contacting slip ring through the life prediction and analysis of critical factors.

Orientation transitions during the growth of imine covalent organic framework thin films

The first experimental observation of a rare re-entrant transition during COF thin film growth reveals independent nucleation and growth kinetic processes.

Characterization of effective conductive layer of nano organic thin film using reflectance spectroscopy

We propose an optical approach for analyzing the formation of the conductive layer during organic thin film growth. The relationship between the properties of multi-layer film, such as thickness and optical coefficients, and the corresponding differential reflectance spectrum (DRS) is derived as math formula based on the effective medium approximation. With the deduced formula, the thickness of the deposited film, for example, electron transport layer in this paper, can be estimated according to the measured DRS data. But, in fact, the fitting error always exists. It is, on the other hand, a useful evidence to indicate the actual situation of the thin film. A concept of the normalized fitting error (NFE) is offered here to equivalently assess the fitting results of all DRS data in the growth process. The curve of NFE versus time is proposed to analyze the growth revolution of the thin film and reveal the inner physical mechanism. In order to demonstrate the performance of the proposed method, an organic field effect transistor (OFET) with a bottom-gate structure is fabricated and pentacene organic thin film is deposited by vacuum thermal evaporation, as an electron transport layer, on the top of the transistor, i. e., an insulator substrate of Si/SiO2. The reflected optical spectrum and the current between the drain and the source of the OFET device are investigated in real time in the growth process. It has been reported that pentacene has three kinds of crystal structures and their optical properties differ from each other. The actual phase of the pentacene film in our experiment is discussed at first. The fitting results show that the pentacene layer exists mainly in thin film phase here. Then, the thickness of SiO2 layer is determined to be 296 nm, which is close to the design value of 300 nm. With those parameters, a four-layer model is used to calculate the thickness of the organic film. The thickness data indicate that the film appears to be linearly growing and the growth rate is 0.2 nm/min. Next, the NFE is plot as a function of time. In this plot, the curve of the NFE increases quickly at the beginning of the growth and reaches to a positive peak at 70 min. After that, the NFE decreases and then keeps constant for a while. When the measured current-time curve is added into this plot, one finds that the increase of the current happens at the same time with the peak of the NFE. It implies that the NFE is related to the structure change of the organic film and thus linked indirectly to the electronic property. The peak of the NFE, to a certain extent, reveals the completeness of the organic conductive layer. As a result, the presented optical approach is valuable for analyzing the electronic status of the organic thin film, especially if the electronic test cannot be performed.

Spiral growth mode in DMDPC organic thin film transistors by physical vapor deposition

We report the observation of a screw-dislocation-driven spiral growth of DMDPC organic thin films. The existence of screw dislocations was clearly confirmed by the observations of outcropped stepsand spiral fringes.

Growth and application of organic thin films by co-evaporant induced crystallization

Growth and application of organic thin films by co-evaporant induced crystallization

Polarization-dependent differential reflectance spectroscopy for real-time monitoring of organic thin film growth

By monitoring the reflectance of a sample surface during deposition of a thin organic film, one can obtain information with submonolayer resolution in real-time. A special kind of optical spectroscopy is Differential Reflectance Spectroscopy (DRS), which compares the reflectance before and during deposition of a thin film or any other change of the surface optical properties. In this work, we present an extended DRS setup that allows monitoring simultaneously both linear polarization states (s and p) of the reflected light. We implement polarization-dependent DRS to monitor the growth of perflouropentacene thin films on a Ag(110) single crystal. The setup allows us to deduce the optical anisotropy of the sample and, in particular, the preferred orientation of the molecules on the surface.