High Vacuum Sublimation Research Articles

Design and Processing as Ultrathin Films of a Sublimable Iron(II) Spin Crossover Material Exhibiting Efficient and Fast Light-Induced Spin Transition.

Materials based on spin crossover (SCO) molecules have centered the attention in molecular magnetism for more than 40 years as they provide unique examples of multifunctional and stimuli-responsive materials, which can be then integrated into electronic devices to exploit their molecular bistability. This process often requires the preparation of thermally stable SCO molecules that can sublime and remain intact in contact with surfaces. However, the number of robust sublimable SCO molecules is still very scarce. Here, we report a novel example of this kind. It is based on a neutral iron(II) coordination complex formulated as [Fe(neoim)2], where neoimH is the ionogenic ligand 2-(1H-imidazol-2-yl)-9-methyl-1,10-phenanthroline. In the first part, a comprehensive study, which covers the synthesis and magnetostructural characterization of the [Fe(neoim)2] complex as a bulk microcrystalline material, is reported. Then, in the second part, we investigate the suitability of this material to form thin films through high-vacuum sublimation. Finally, the retainment of all present SCO capabilities in the bulk when the material is processed is thoroughly studied by means of X-ray absorption spectroscopy. In particular, a very efficient and fast light-induced spin transition (LIESST effect) has been observed, even for ultrathin films of 15 nm.

Nanoscale Quantized Oscillations in Thin‐Film Growth Greatly Enhance Transconductance in Organic Transistors

AbstractA growth mode of pentacene thin films deposited by high vacuum sublimation where the morphology versus thickness h “rings” back and forth between rough 3D films with pyramid islands and smooth 2D films with ziqqurat islands is discovered. The roughness σ versus h exhibits seamless coherent oscillations whose amplitude and wavelength increase as integer multiples of 1.5 ML thickness. The quantized oscillations are reconducted to dynamic wetting/dewetting transitions involving the upper layers of pentacene film. Importantly, the transconductance of organic field effect transistors, either in solid state or electrolyte‐gated, exhibits antiphase oscillations with one‐decade swing. Charge mobilities in the wetting regime reach 0.1 cm2 V−1 s−1, in line with high‐end values reported for thin‐film pentacene transistors. Controlling this growth mode enables the limitations of charge transport imposed by the roughening transition to be overcome, a universal feature of high vacuum growth to date.

Calculation of the nonstoichiometry area of nanocrystalline palladium (II) oxide films

Nanocrystalline palladium (II) oxide films were synthesised using thermal oxidation in the oxygen atmosphere of the initial ultradispersed metal palladium layers with a thickness of ~ 35 nanometres that were obtained on SiO2/Si (100) substrates using the method of thermal sublimation in high vacuum. Using X-ray analysis, it was established that during thermal oxidation in the oxygen atmosphere within the temperature range T = 670–970 K the values of the a and c parameters of the tetragonal lattice as well as the unit cell volume of nanocrystalline PdO films increased monotonously with the rise of the temperature reaching the maximum values at T = 950–970 K. It was found that the parameters of the tetragonal lattice and the unit cell volume of nanocrystalline PdO films decreased as the oxidation temperature increased up to T > 970 KBased on the ratio of the c/a parameters, it was shown that the main contribution to the deformation phenomena of the tetragonal lattice were mostly due to the increase in the elementary translations along the coordination axes OX and OY. Based on an assumption that the ionic component of the chemical bond is essential to the palladium (II) oxide structure, we suggested a method for the calculation of the range of the nonstoichiometry area for nanocrystalline PdO films, using the reported data on the radii of cation Pd2+ and anion O2- taking into account their coordination environment. The results of the calculations showed that nanocrystalline PdO films synthesised with an oxygen pressure of ~ 105 kPa are characterisedby the two-sided homogeneity region in relation to the stoichiometric ratio of the components. The homogeneity region of nanocrystalline PdO films is characterised by the retrograde solidus line in the range of the temperatures T = 770–1070 K.

Properties and degradation of manganese(III) porphyrin thin films formed by high vacuum sublimation

Manganese porphyrins are of interest due to the optical, electronic and magnetic properties of the central metal ion, coupled to the low bandgap of the polyaromatic ring. These attractive characteristics are harnessed in solutions or in ultra-thin films, such as, for example, self-assembled monolayers. However, for devices, thicker films deposited using a controlled and reproducible method are required. Here we present the morphological, structural, chemical and optical properties of manganese(III) tetraphenylporphyrin chloride (MnTPPCl) thin films deposited using organic molecular beam deposition, typically employed to process analogue molecules for applications such as organic photovoltaics. We find, using a combination of UV-vis and X-ray photoelectron spectroscopies, that the sublimation process leads to the scission of the Mn–Cl bond. The resultant film is a Mn(II)TPP:Mn(III)TPPCl blend where approximately half the molecules have been reduced. Following growth, exposure to air oxidizes the Mn(II)TPP molecule. Through quantitative analysis of the time-dependent optical properties, the oxygen diffusion coefficient (D) [Formula: see text] is obtained, corresponding to a slow bulk oxidation following fast oxidation of a 8-nm-thick surface layer. The bulk diffusion D is lower than for analogous polycrystalline films, suggestion that grain boundaries, rather than molecular packing, are the rate-limiting steps in oxidation of molecular films. Our results highlight that the stability of the axial ligands should be considered when depositing metal porphyrins from the vapor phase, and offer a solvent-free route to obtain reproducible and smooth thin films of complex materials for engineering film functionalities.

Characterization of vacuum-deposited films of hexachloro-substituted subphthalocyanines for photovoltaic applications

Abstract The thin film properties of the new hexachloro-pyrazinosubporphyrazine compound, Cl6-pSubPz, were studied in comparison with those of an effective molecular acceptor, hexachloro-substituted subphthalocyanine, Cl6-SubPc. The latter compound is widely used in organic photovoltaics as electron acceptor (n-type layer). It was shown that with high-vacuum sublimation Cl6-pSubPz forms stable, smooth and highly textured films on many functional inorganic surfaces. The position of the main electronic absorption band (Q-band) of this compound in the visible range coincides with the maximum of the solar spectrum. Specific conductivity of the sublimed films of both studied compounds lies in the same range, but its thermal activation energy for Cl6-pSubPz is two times higher than for Cl6-SubPc.

Vertical Tunnel Junction Embedding a Spin Crossover Molecular Film

AbstractThin films of a molecular spin crossover (SCO) Iron(II) complex featuring a high transition temperature are grown by sublimation in high vacuum on TSAu and investigated by X‐ray and UV photoelectron spectroscopies. Temperature‐dependent studies demonstrate that the thermally induced spin crossover behavior is preserved in thin films. A large‐area ultrathin switchable spin crossover molecular vertical tunnel junction with top electrodes of the liquid eutectic of gallium and indium, for which the spin‐state switching of the films induces a two orders of magnitude change in the tunneling current density flowing through the junction, is reported here. The results on large‐area junctions, rationalized by density functional theory calculations, demonstrate the high potential of SCO‐based switchable molecular junctions as functional devices.

Synthesis and optical properties of poly[4-methacryloxy-(4′-carboxy)-azobenzene

In the present work, the homopolymer built from the free radical polymerization of methacrylic monomer incorporating an azobenzene side-group has been synthesized and structurally characterized. The optical properties such as refractive index, extinction coefficient, absorption coefficient and optical energy band gap of poly[4-methacryloxy-(4′-carboxy)-azobenzene] thin film prepared by high vacuum sublimation method were determined using spectroscopic ellipsometry combined with transmittance measurements.We found that in the spectrum of extinction coefficient there are absorption bands, which are assigned as the n-π* and π-π* electronic transitions of the azo compound.

Chemical tailoring of Single Molecule Magnet behavior in films of Dy(III) dimers

The insertion of Single Molecule Magnets (SMMs) in spintronic devices has drawn much attention thanks to their fascinating quantum and classical magnetic properties. Although many efforts have been done in this direction, we are still in search of optimal molecules, which preserve intact the magnetic bistability at the nanoscale. In this frame, we have synthesized a new dysprosium-based dimer characterized by SMM behavior and a double-butterfly-shape hysteresis. The complex has been successfully deposited by sublimation in high vacuum and the peculiar SMM behavior of the complex is fully retained at the nanoscale demonstrating that a modification of the dimer structure aimed for enhancing the SMM character is compatible with nanostructuration purposes.

Organization of Pentacene Molecules on Anisotropic Ultrathin HfO[sub 2]/Al[sub 2]O[sub 3] Templates for Organic Thin-Film Transistors Using an Ion-Beam Treatment

This article investigates improving organic thin-film transistor (OTFT) characteristics by using an alignable high dielectric constant film to control the self-organization of pentacene molecules. The process, based on the growth of a pentacene film with high vacuum sublimation, is a method of self-organization that uses ion-beam (IB) bombardment of the surface that serves as the gate dielectric layer. X-ray photoelectron spectroscopy indicates that the IB increases the structural anisotropy of the film, and X-ray diffraction patterns show that increasing the anisotropy may lead to the self-organization of pentacene molecules in the first polarized monolayer. An effective mobility of was achieved, which is significantly different from the mobility in pentacene films that are not aligned. The proposed OTFT devices with an ultrathin structure as the gate dielectric layer were operated at a gate voltage lower than 5 V.

Light emitting nanofibers from functionalised para-quaterphenylenes: theory, synthesis and application

Growth of mutually aligned organic nanofibers from functionalised p-quaterphenylene has been demonstrated on a mica substrate via high vacuum sublimation. The fibers bridge the gap between microscopic and macroscopic dimensions according to their typical width in nanometer scale and length up to several hundred micrometers. They are an important step forward towards integrated nanoscopic optoelectronics since they proof that it is possible to generate ordered organic nanoaggregates from functionalised building blocks. These p-quaterphenylenes show well-organised growth with determinded morphology and emit intense blue light similar to the para-hexaphenylene. Prior to their synthesis, more complex functionalised p-oligophenylenes have been designed using theoretical calculations to focus on the most promising molecular building blocks with respect to their functionality and NLO properties, leading to increased optical activity of the resulting nanofibers.

Field Effect Transistors with Organic Semiconductor Layers Assembled from Aqueous Colloidal Nanocomposites

We demonstrate field effect transistors based on organic semiconductor molecules dispersed in a self-organized polystyrene (PS) latex bead matrix. An aqueous colloidal composite made of PS and tetrahexylsexithiophene (H4T6) is deposited with a micropipet into the channel of a bottom-contact field effect transistor. The beads self-organize into a network whose characteristic distances are governed by their packing. The semiconductor molecules crystallize in the interstitial voids, leading to the growth of large interconnected domains. Depending on the bead size and the ratio between H4T6 and PS, the fraction of the different phases in the polymorph can be controlled. In the transistors where the H4T6 metastable "red phase" is the largest, the device response and the charge mobility are comparable to those of sexithienyl thin films grown by high-vacuum sublimation.

Scanning electron microscopy of impurity structures in snow

We examined well-sintered, mid-winter snow from Mammoth Mountain, California using an environmental scanning electron microscope equipped with a cold stage. Ring-like filaments were observed at all grain boundaries in a highly sublimated, sputter-coated sample 2 days after collection, though filaments were much harder to find in uncoated samples. Filaments appeared in uncoated specimens from the same snow sample five months later, though X-ray microanalysis failed to detect soluble impurities in the filaments, grain boundary grooves or on ice grain surfaces above background noise levels. After 8 months in storage and under forced sublimation in high vacuum, the same sample produced not only grain boundary filaments but complex, web-like structures, nodules and films that, in extreme cases, formed castings of completely sublimated snow grains. Energy dispersive X-ray spectrometry reveals that the filaments and webs are composed of the same set of light elements typical of the soluble impurities in Sierra snow. The spectra also reveal significant spatial variability in element ratios. Impurities probably concentrate in a quasi-liquid layer at the air/ice interface and in the disordered grain boundary region where crystal lattices are misoriented. If concentrated at grain boundaries these light elements could act as dopants, reducing grain boundary energy, reducing melting temperatures and promoting the sintering process.

Formation of fullerenes by pyrolysis of perchlorofulvalene and its derivatives

In a retro-synthetic approach, [60]fullerene might be accessible by condensing six fulvalene fragments. In order to explore the potential of such a route for direct synthesis of [60]fullerene we have investigated the pyrolysis of perchlorofulvalene (PCF). Low temperature pyrolysis of PCF at 250 °C resulted mainly in the formation of dimers, trimers, tetramers and products of subsequent intramolecular condensation of these oligomers. Increasing the temperature to 300–350 °C leads to the formation of perchlorinated polynuclear aromatic hydrocarbons. Pyrolysis at 400–450 °C gives a cross-linked polymer structure which is the result of intermolecular condensation of the polynuclear aromatic intermediates. Pyrolysis at higher temperatures (>500 °C) mainly leads to graphite. It was found that the two-step pyrolysis of PCF (heating first at 450 °C, after that at 750 °C) yielded a fullerene containing soot via an intermediate polynuclear aromatic net. High temperature rearrangement of the latter gave fullerenes C 60 and C 70. The best results were obtained when a PCF oligomer obtained by Ullmann condensation was used as a precursor. By two-step pyrolysis and further high vacuum sublimation of the soot the fullerenes C 60 and C 70 were obtained in extractable amounts.

In situ X-ray synchrotron study of organic semiconductor ultra-thin films growth

In this work we present an X-ray diffraction study of the early stages of growth of an organic semiconductor (sexithiophene, T6) thin film prepared by high vacuum sublimation. Specular reflectometry and grazing incidence X-ray diffraction were used to monitor the formation of T6 films on silicon oxide. Our results show that T6 grows as a crystalline layer from the beginning of the evaporation. The reflectometry analysis suggests that, in the range of rates and temperatures studied, the growth is never layer by layer but rather 3D in nature. In-plane GIXD has allowed us to observe for the first time a thin film phase of T6 formed of molecules standing normal to the substrate and arranged in a compressed unit cell with respect to the bulk, i.e. the unit cell parameters b and c are relatively smaller. We have followed the dynamics of formation of this new phase and identified the threshold of appearance of the bulk phase, which occurs above ≈5–6 monolayers. These results are relevant to the problem of organic thin film transistors, for which we have previously demonstrated experimentally that only the first two monolayers of T6 films are involved in the electrical transport. The layers above the second one do not effectively contribute to charge mobility, either because they are more “disordered” or because of a screening of the gate field.

Freezing of Binary Colloidal Systems for the Formation of Hierarchy Assemblies

Cryo-etch scanning electron microscopy (cryo-etch SEM) of aqueous gels composed of colloidal silica nanoparticles in the 1−40 nm range and liposomes of ∼200 nm gave unique morphologies. The aqueous gels are frozen at subcooled liquid nitrogen and fractured to obtain a fresh surface. High-vacuum sublimation of ice from the freshly exposed surface (etching) results in the formation of a hierarchy assembly, characterized by granular fences composed of colloidal silica and liposomes surrounded by empty areas in which amorphous ice originally resided. The biocompatible character of this ice segregation induced self-assembly (ISISA) process that allows for the preservation of the structural integrity of liposomes within the assembly is demonstrated by fluorescence anisotropy performed at the binary colloidal aqueous gels and differential scanning calorimetry and electron microscopy at the hierarchy assembly. The resulting assembly shows an interesting dual character, with one colloidal entity supporting the structure (e.g., silica) and the other providing functionality (e.g., liposomes).

Aroma Active Compounds of Bulgogi

ABSTRACT: Volatile components in bulgogi were extracted by high vacuum sublimation (HVS) and solid phase microextraction (SPME). The extracts were then analyzed by gas chromatography‐olfactometry (GCO) conducted on 2 columns with different polarities. Aroma dilution methods were used with a serial dilution of the extract in HVS‐GCO and varying the GC injector split ratios in SPME‐GCO to determine aroma active compounds of bulgogi. In HVS‐GCO, methional (described as cooked potato/soy sauce) exhibited the highest flavor dilution (FD) factor, followed by 2‐acetyl‐2‐thiazoline (described as nutty/popcorn/peanut), furaneol (described as caramel/sweet), and 2‐ethyl‐3,5‐dimethylpyrazine (described as nutty/coffee/chocolate). In SPME‐GCO, allyl mercaptan (described as garlic/bulgogi) showed the highest FD factor, followed by methional (described as cooked potato/soy sauce), 2‐ethyl‐3,5‐dimethylpyrazine (described as nutty/coffee/chocolate), and 2‐acetylpyrazine (described as nutty/peanut/cooked rice). These sulfur‐containing compounds and heterocyclic compounds, which had high FD factors and characteristic odor notes, could be important to bulgogi flavor.

Yttrium triiodide, YI3

Single crystals of YI3 were obtained by high-vacuum sublimation of the crude product at 1213 K. The crystal structure is that of the BiI3 structure type. Two-thirds of the octa­hedral voids between every second hexa­gonally closest-packed layer of I− ions are occupied by Y3+ ions. Thereby, the YI6 octa­hedra share common edges within Bγ2/3A slabs in the stacking sequence ⋯γ2/3A□Bγ2/3A□Bγ2/3A□Bγ2/3⋯.

Ultrastructure in frozen/etched saline solutions: on the internal cleansing of ice.

Seawater, with its 3.5% salt content, freezes into hexagonal ice (Ih) that encloses concentrated brine within its matrix. When unsubmerged sea ice reaches a certain height and temperature, the brine drains downward through narrow channels. This mechanism was now modeled by frozen 2-3.5% saline as investigated by cryo-etch high-resolution secondary electron microscopy. Thus, saline was either plunge-frozen in liquid ethane at -183 degrees C or else high-pressure frozen to -105 degrees C in 5-6 ms. Ice from a freshly exposed surface was then subjected to a high-vacuum sublimation ("etching"), a procedure that removes pure bulk ice in preference to ice from frozen hydrated salt. After chromium-coating the etched surface with a 2-nm film, the sample was examined by cryo-HRSEM. Granular icy "fences" were seen surrounding empty areas where amorphous ice had originally resided. Since the fences, about 1-2 mum high, survived the etching, it is likely that they consist of frozen brine. The presence of such fences suggests that, during freezing, saline can purge itself of salt with remarkable speed (5-6 ms). Alternatively, channels (perhaps routed around submicroscopic crystallites of cubic ice (Ic) embedded in the amorphous ice at -105 degrees C) can guide the migration of salt to the periphery of ice patches. Macromolecules fail to form fences because they diffuse too slowly or because they are too large to pass through the channels.

New SYNTHETIC METHOD OF 13H-DIBENZO[A,DE]ANTHRACENE-13-ONE

The polycyclic aromatic ketone, 13H-dibenzo[a,de]anthracene-13-one (5,6-BBz) is useful as a starting material for synthesis of undecacyclic aromatic compounds by means of condensation. To synthesize 5,6-BBz, glycerol condensation of benzo[a]anthraquinone was conducted. The condensation, however, gave two structural isomers besides 5,6-BBz and the isolation was very difficult; separation of the crude products by column chromatography or high-vacuum sublimation was unsuccessful because of their similarity in structure and vapor pressure. Only a little 5,6-BBz was obtained by repeated recrystallization, but the amount was insufficient for condensation. Thus, we developed a new synthetic method which affords 5,6-BBz selectively. 9-(o-Chlorobenzoyl)anthracene was synthesized by the Friedel-Crafts' reaction of anthracene with o-chlorobenzoyl chloride and aluminum chloride anhydride. In order to suppress the production of 9,10-di(o-chlorobenzoyl) anthracene, the reaction was performed at low temperature. The crude products obtained were purified by column chromatography on activated alumina and then recrystallized with benzene, which yielded yellow crystals of 9-(o-chlorobenzoyl)anthracene. The structure of 9-(o-chlorobenzoyl)anthracene was determined by X-ray diffraction analysis for the first time. Cyclo-dehydrohalogenation of 9-(o-chlorobenzoyl) anthracene gave 5,6-BBz selectively, the amount of which was sufficient for synthesizing undecacyclic aromatic hydrocarbons.

Orienting Tetracene and Pentacene Thin Films onto Friction-Transferred Poly(tetrafluoroethylene) Substrate

Oriented films of tetracene and pentacene have been obtained by high vacuum sublimation onto oriented poly(tetrafluoroethylene) (PTFE) substrates. Polymorphism, orientation, and morphology of the pentacene and tetracene films are studied as a function of deposition parameters [substrate temperature (Ts), deposition time (t), and deposition rate (τ)] using X-ray and electron diffraction, transmission and scanning electron microscopies and atomic force microscopy (AFM). Oriented films of the triclinic structures reported by Campbell et al. (Acta Crystallogr. 1961, 14, 705; 1962, 15, 289) have been obtained in well-defined Ts-ranges. Oriented acene films of 50 nm consist of a continuous texture of flat-lying and terraced domains with a dense (0 0 1) contact plane and a high degree of in-plane orientation. Specific nucleation sites of pentacene on the PTFE substrate have been identified by AFM in the early stage of deposition. The similarity found in the growth phenomenology of tetracene and pentacene allows ...