Vacuum-deposited Films Research Articles

Surface thermal fluctuation spectroscopy study of ultra-thin ionic liquid films on quartz

We apply a surface thermal fluctuation spectroscopy technique to investigate the liquid nature of ionic liquid (IL) thin films and micro-droplets. While the spectrum for bulk IL was well explained by a simple Newtonian fluid model, a significant contribution of viscoelasticity was suggested for the vacuum-deposited thin films and micro-droplets. For a sample of IL micro-droplets with statistical distributions of their size and thickness, the scanning surface fluctuation spectroscopy was demonstrated to spatially map out the thickness and viscoelasticity values for effectively investigating the correlation between them, successfully reproducing a typical surface morphology of the sample as well.

Comparative experiments on hydrogenation of titanium films and foils

Comparative experiments have been performed for the gas-diffusion hydrogenation of vacuum-deposited titanium films and commercially produced titanium foils. The sorption and thermodesorption characteristics of obtained hydrogenated materials and the values of hydrogen mass content in these materials have been defined. Differences in the resulting structure of deposited films and foils after hydrogenation are shown.

Graphene Metapixels for Dynamically Switchable Structural Color.

Structural coloration providing vibrant and tailored colors enables broad applications. Existing strategies of structural coloration either use resonances or diffraction induced by arrayed nanostructures with element sizes at a wavelength scale or are based on interference from vacuum-deposited large-area thin films. It is extremely challenging to achieve full color pixels with diffraction-limited resolution without sophisticated multiple-step nanostructure fabrication or externally applied field control. Realization of dynamically switchable full color displays with diffraction-limited resolution is even harder. This work demonstrates a structural color strategy with developed anisotropic graphene metapixels. The anisotropic optical property is given by the intrinsic birefringence of the layered structure of graphene metamaterials, and each metapixel is spatially encoded by direct laser printing with diffraction-limited resolution (250 nm). The colors can be dynamically and instantly switched by controlling the scattering of the light source to excite different modes based on the strong anisotropic optical properties of the graphene metapixels. The low-cost large-scale fabrication method allows experimental demonstration of a large-area (4 in.) flexible full color optical switchable display. Such a simple, effective and flexible method promises broad practical applications in color display and color image sensing related fields.

Optimizing the plasma oxidation of aluminum gate electrodes for ultrathin gate oxides in organic transistors

A critical requirement for the application of organic thin-film transistors (TFTs) in mobile or wearable applications is low-voltage operation, which can be achieved by employing ultrathin, high-capacitance gate dielectrics. One option is a hybrid dielectric composed of a thin film of aluminum oxide and a molecular self-assembled monolayer in which the aluminum oxide is formed by exposure of the surface of the aluminum gate electrode to a radio-frequency-generated oxygen plasma. This work investigates how the properties of such dielectrics are affected by the plasma power and the duration of the plasma exposure. For various combinations of plasma power and duration, the thickness and the capacitance of the dielectrics, the leakage-current density through the dielectrics, and the current–voltage characteristics of organic TFTs in which these dielectrics serve as the gate insulator have been evaluated. The influence of the plasma parameters on the surface properties of the dielectrics, the thin-film morphology of the vacuum-deposited organic-semiconductor films, and the resulting TFT characteristics has also been investigated.

Dielectric constant of thin film graphitic carbon nitride (g-C3N4) and double dielectric Al2O3/g-C3N4

Graphitic carbon nitride (g-C3N4), a highly stable wide bandgap material, is yet to be evaluated for thin-film applications in devices since its basic electrical properties are not understood. We study its vacuum-deposited thin film form, and in combination with Al2O3, in sandwiched devices using capacitance–voltage (C-V) and current–voltage (I-V) characteristics over a wide range of temperatures and frequencies. The dielectric constant of g-C3N4 alone is between 7 and 8 for frequencies 100 Hz to 100 kHz, but it is almost double at 14–16 when used in conjunction with a thin layer of Al2O3. The increased dielectric constant is attributed to additional polarization at the interface of the two dielectrics. The leakage current density is of the order of 10–7 A/cm2 and shows slight asymmetry. The mechanisms of current transport are mainly space charge limited at fields higher than 5 × 105 V/cm. We attribute the small but significant difference between forward and reverse bias to the presence of the negative sheet charge at the interface between the two dielectrics. The interfacial charge density is estimated to be 1011 cm−2.

Polymorphism and structure formation in copper phthalocyanine thin films.

Many polymorphic crystal structures of copper phthalocyanine (CuPc) have been reported over the past few decades, but despite its manifold applicability, the structure of the frequently mentioned α polymorph remained unclear. The base-centered unit cell (space group C2/c) suggested in 1966 was ruled out in 2003 and was replaced by a primitive triclinic unit cell (space group P 1). This study proves unequivocally that both α structures coexist in vacuum-deposited CuPc thin films on native silicon oxide by reciprocal space mapping using synchrotron radiation in grazing incidence. The unit-cell parameters and the space group were determined by kinematic scattering theory and provide possible molecular arrangements within the unit cell of the C2/c structure by excluded-volume considerations. In situ X-ray diffraction experiments and ex situ atomic force microscopy complement the experimental data further and provide insight into the formation of a smooth thin film by a temperature-driven downward diffusion of CuPc molecules during growth.

Direct observation and evolution of electronic coupling between organic semiconductors

The electronic wave functions of an atom or molecule are affected by its interactions with its environment. These interactions dictate electronic and optical processes at interfaces, and is especially relevant in the case of thin film optoelectronic devices such as organic solar cells. In these devices, charge transport and interfaces between multiple layers occur along the thickness or vertical direction, and thus such electronic interactions between different molecules---same or different---are crucial in determining the device properties. Here, we introduce an in situ spectroscopic ellipsometry data analysis method called differential analysis in real time (DART) with the ability to directly probe electronic coupling due to intermolecular interactions along the thickness direction using vacuum-deposited organic semiconductor thin films as a model system. The analysis, which does not require any model fitting, reveals direct observations of electronic coupling between frontier orbitals under optical excitations leading to delocalization of the corresponding electronic wave functions with thickness or, equivalently, number of molecules away from the interface in C60 and MeO-TPD deposited on an insulating substrate $(\mathrm{Si}{\mathrm{O}}_{2})$. Applying the same methodology for C60 deposited on phthalocyanine thin films, the analyses shows strong, anomalous features---in comparison to C60 deposited on $\mathrm{Si}{\mathrm{O}}_{2}$---of the electronic wave functions corresponding to specific excitation energies in C60 and phthalocyanines. Translation of such interactions in terms of dielectric constants reveals plasmonic type resonance absorptions resulting from oscillations of the excited state wave functions between the two materials across the interface. Finally, reproducibility, angstrom-level sensitivity, and simplicity of the method are highlighted showcasing its applicability for studying electronic coupling between any vapor-deposited material systems where real-time measurements during thin film growth are possible.

Effect of the conformer distribution on the properties of amorphous organic semiconductor films for organic light-emitting diodes.

With the remarkable improvement in the electrical and optical properties of organic light-emitting diodes (OLEDs) in recent years, the details of the higher-order structure of vacuum-deposited amorphous organic films and its formation mechanism need to be understood. In particular, to clarify the effect of the higher-order structure on the film properties, it is necessary to analyze the molecular aggregation states in the vacuum-deposited amorphous films. Toward their deep understanding, the higher-order structure and film properties have often been discussed with relation to the surface diffusion and structural relaxation of the molecules immediately after deposition on the film surface. However, the effect of the variety of conformers, which is specific to amorphous organic materials, on the thermal and electrical properties of the films has not been deeply discussed. In this study, we focused on three structural isomers of OLED materials and discuss the effect of the conformer distribution on the molecular aggregation states and thermal and electrical properties of the vacuum-deposited films. From their comparison, we found that the properties of the film composed of a relatively small number of stable conformers are superior to those of the other two films composed of relatively large numbers of stable conformers. This superiority originates from formation of aggregates of the same conformer, which become the starting points for crystallization when the film is heated. Our detailed comparison and discussion focusing on the variety of conformers will lead to a deeper understanding of the molecular aggregation states and physical properties of amorphous organic films.

Optoelectronic property comparison for isostructural Cu2BaGeSe4 and Cu2BaSnS4 solar absorbers

This study identifies key underlying differences in electronic properties of vacuum-deposited Cu2BaGeSe4 and Cu2BaSnS4 thin films.

The effect of molecular structure on the efficiency of 1,4-diazine–based D–(π)–A push-pull systems for non-doped OLED applications

A series of novel D–A and D–π–A push-pull systems based on a pyrazine and quinoxaline acceptor, bearing various electron-donating triphenylamine and carbazole moieties, are compared. A significant difference in electrochemical and photophysical properties was found depending on molecular structure. The compounds have strong solvatochromic properties. Quinoxaline-containing systems exhibit delayed fluorescence (DF) in thermal vacuum deposition films. Despite the low quantum yield of fluorescence in the solid state (less than 10%), organic light-emitting diodes with sufficiently high efficiency (4.2 cd/A) have been fabricated on the basis of this push-pull systems. The best results were obtained for compounds exhibiting DF. The possible channel for increasing the efficiency of OLED can be associated with the “hot excitons” mechanism.

Investigation of 4,6-di(hetero)aryl-substituted pyrimidines as emitters for non-doped OLED and laser dyes

Two novel V-shaped push-pull systems based on a pyrimidine acceptor have been designed and investigated. Low-temperature measurements of the fluorescence and delayed luminescence spectra demonstrated that the emission bands of the compounds have a charge-transfer character.Despite the fact that compounds in thermal vacuum deposition films have a low fluorescence quantum yield, OLED devices based on them show high efficiency, which can be associated with the emission mechanism through delayed fluorescence. It is found that photoproducts, obtaining upon exposure to UV-irradiation of fluorophores in chloroform solution, exhibit laser activity in the red region of the spectrum.

Transparent Hole-Transporting Materials Containing Partially Oxygen-Bridged Triphenylamine Skeletons for Efficient Perovskite Solar Cells

Transparent organic semiconductors are desirable for organic optoelectronic devices. While electrical performance is often improved by increasing the p-conjugation length, the resulting red-shifted absorption profile typically leads to loss of transparency. Therefore, it is a great challenge to design and develop organic semiconductor materials that possess both high electrical performance and transparency in visible light region. We have previously developed promising transparent hole-transporting materials (HTMs) based on the quasiplanar skeletons of partially oxygen-bridged triphenylamines.In the present study, we have designed new transparent HTMs by introducing diarylamines into the 7-position of a quasiplanar partially oxygen-bridged triphenylamine. The combination of an oxygen-bridged triphenylamine strong donor and a diarylamine weak donor makes the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) localize on the oxygen-bridged triphenylamine and diarylamine, respectively, leading to minimal orbital overlap between the HOMO and the LUMO, therefore, weak absorption. Three HTMs, namely, HND-Cbz, HND-DTP, and HND-NOMe, were synthesized by connecting a partially oxygen-bridged triphenylamine structure to carbazole, dithieno[3,2-b:2′,3′-d]pyrrole, and bis(4-methoxyphenyl)amine through Buchwald-Hartwig amination with yields of 91%, 70%, and 88%, respectively. All of three materials are amorphous and thermally stable with decomposition temperatures higher than 350 °C. The vacuum-deposited thin films are transparent, with absorption edges from 405 nm to 419 nm. The space-charge limited current (SCLC) hole mobilities of HND-Cbz, HND-DTP, and HND-NOMe are 1.3 x 10- 7, 3.5 x 10- 5 and 1.4 x 10- 4 cm2 V- 1 s- 1, respectively while the HOMO energy levels are –5.34, –5.30 and –5.08 eV. The HOMO levels lie above the corresponding valence band of typical perovskites (e.g. –5.45 eV for MAPbI3 or –5.56 for Cs0.05FA0.15MA0.80PbI2.75Br0.25), making these molecules potentially good candidates for hole-transporting materials in perovskite solar cells. To verify this, perovskite solar cells having the structures ITO/SnO2/Cs0.05FA0.15MA0.80PbI2.75Br0.25/HTM/Au for the solution processed HTM and ITO/SnO2/Cs0.05FA0.15MA0.80PbI2.75Br0.25/HTM/MoO3/Au/MoO3 for the vacuum processed HTM layer, were fabricated. With HND-Cbz, HND-DTP and HND-NOMe as the HTM, the best power conversion efficiency reached 13.7%, 15.0% and 17.2%, respectively. Figure 1

Unprecedented switching endurance affords for high-resolution surface temperature mapping using a spin-crossover film

Temperature measurement at the nanoscale is of paramount importance in the fields of nanoscience and nanotechnology, and calls for the development of versatile, high-resolution thermometry techniques. Here, the working principle and quantitative performance of a cost-effective nanothermometer are experimentally demonstrated, using a molecular spin-crossover thin film as a surface temperature sensor, probed optically. We evidence highly reliable thermometric performance (diffraction-limited sub-µm spatial, µs temporal and 1 °C thermal resolution), which stems to a large extent from the unprecedented quality of the vacuum-deposited thin films of the molecular complex [Fe(HB(1,2,4-triazol-1-yl)3)2] used in this work, in terms of fabrication and switching endurance (>107 thermal cycles in ambient air). As such, our results not only afford for a fully-fledged nanothermometry method, but set also a forthcoming stage in spin-crossover research, which has awaited, since the visionary ideas of Olivier Kahn in the 90’s, a real-world, technological application.

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.

In situ vacuum ellipsometry approach to investigation of glass transition behavior in ionic liquid thin films

Glass transition behavior of vacuum-deposited ionic liquid thin films was investigated by in situ ellipsometry. The glass transition temperatures of [omim][TFSA] thin films (≥50 nm) were determined from the temperature-dependent light ellipticity and found to be almost identical to the bulk value regardless of the thickness. Moreover, it was suggested that the sequentially deposited [omim][PF6]/[omim][TFSA] bi-layer film became homogeneous and had one glass transition temperature close to the value of the corresponding bulk mixture for each weight fraction, which could be explained by the theoretical fast mutual diffusion of ILs within the film relative to the total deposition time.

Synthesis and Characterization of Fluorenone-Based Donor-Acceptor Small Molecule Organic Semiconductors for Organic Field-Effect Transistors

In this study, a series of new fluorenone-based small molecules were synthesized and characterized as organic semiconductors for organic field-effect transistors. Thermal, optical, and electrochemical properties of the new compounds were characterized. Furthermore, thin films of the developed compounds were employed as organic semiconductors, and vacuum-deposited film of fluorenone derivative with alkylated double thiophene exhibited p-channel device characteristics with hole mobility as high as 0.02 cm2/Vs and current on/off ratio of 107. In addition, surface morphology and microstructure of vacuum deposited films were analyzed and correlated with the electrical characteristics.

Methanol Oxidation Catalyzed by Copper Nanoclusters Incorporated in Vacuum-Deposited HKUST-1 Thin Films

Metal-organic framework (MOF) thin films allow the unique properties of MOFs to be incorporated into membranes and micro-electronic devices and also permit studies of catalysis employing powerful u...

Spin Transition of an Iron(II) Organoborate Complex in Different Polymorphs and in Vacuum-Deposited Thin Films: Influence of Cooperativity.

Two polymorphic modifications (1-I and 1-II) of the new spin crossover (SCO) complex [Fe{H2B(pz)(pypz)}2] (pz = pyrazole, pypz = pyridylpyrazole; 1) were prepared and investigated by differential scanning calorimetry (DSC), magnetic measurements, Mößbauer, vibrational, and absorption spectroscopy as well as single-crystal and X-ray powder diffraction. DSC measurements reveal that upon heating the thermodynamically metastable form 1-II to ∼178 °C it transforms into 1-I in an exothermic reaction, which proves that these modifications are related by monotropism. Both forms show thermal SCO with T1/2 values of 390 K (1-II) and 270 K (1-I). An analysis of the crystal structures of 1-II and the corresponding Zn(II) (2) and Co(II) (3) complexes that are isotypic with 1-I reveals that form II consists of dimers coupled by strong intramolecular π···π interactions, which is not the case for 1-I. In agreement with these findings, investigations of thin films of 1, where significant π···π interactions should be absent, reveal SCO behavior similar to that of 1-I. These results underscore the importance of cooperativity for the spin-transition behavior of this class of complexes.

Increasing efficiency of hybrid p-CuI/n-Cl6SubPc heterojunction through the interface engineering

Microscopic ordering of molecules in thin vacuum-deposited films of hexachlorinated subphthalocyanine (Cl6SubPc) is achieved through selection of a deposition surface and temperature. While being amorphous on many practical substrates, Cl6SubPc crystallizes when deposited on the surface of cuprous iodide (CuI), a well-known orientation and hole-transporting material in organic electronic devices. This “disorder-order” transition is triggered by annealing and eventually affects the macroscopic properties of Cl6SubPc films, such as conductivity and optical spectra. The halide semiconductor CuI acts here not only as a growth surface, but also as a donor component of the hybrid p-CuI/n-Cl6SubPc heterojunction, whose morphology is also influenced by annealing. The archetypal thin-film solar cells employing thus structured junction deliver efficiency approaching 2%, in contrast with the photovoltaically less active control devices incorporating amorphous Cl6SubPc layer.

Low Temperature (

To achieve fully-solution processed n-i-p perovskite solar cells (PSCs), jet-sprayed silver nanowires (AgNWs) are used to replace vacuum-deposited Ag film as the top electrode of n-i-p structure PSCs. AgNWs are covered with a layer of polyvinylpyrrolidone (PVP) because of the addition of PVP during the growth process of AgNWs. This PVP will deteriorate the conductivity of AgNWs and become a charge extraction barrier as an electrode of a PSC. A low temperature (<40 °C) atmospheric-pressure dielectric-barrier-discharge-jet (DBDjet) is used to post-treat AgNWs after completing the fabrication of n-i-p PSCs. The average cell efficiency improved from 9.069% (without DBDjet treatment) to 12.09% (DBDjet with scan rate of 0.5 cm s−1), and the best performing cell achieves efficiency of 14.037% (DBDjet with scan rate of 1 cm s−1). DBDjet plasma treatment removes PVP, improving the interfacial contacts among AgNWs and between AgNWs layer and hole transport layer; this in turn increases the cell efficiency. Sheet resistance measurement, X-ray photoelectron spectroscopy, water contact angle measurement, and electrochemical impedance spectroscopy all show evidence of the removal of the PVP layer by DBDjet plasma treatment.