Changes In Film Structure Research Articles

Oxidant aggregate-induced porosity in vapour-deposited polymer films and correlated impact on electrochemical properties

ABSTRACT Persistently doped conjugated polymers are integral for energy storage, flexible electronics, and biosensors due to their unique ability to interact with both ionic and electronic currents. To maximise the performance of devices across these fields, research has focused on controlling material properties to optimise conductivities of both types of charge carriers. The challenge lies in improving ionic transport, which is typically the rate-limiting step in redox processes, without sacrificing electronic conductivity or desirable mechanical properties. Here we report on control of nanostructure in vapour deposited conducting polymer films and correlate changes in film structure with resulting electrochemical properties. Structural control is enabled by exploiting the growth of oxidant nanoaggregates during the reactive vapour deposition process. Relative to dense films, porous films exhibit faster response times in electrochemical testing. Scan rate analysis confirms a transition away from diffusion-limited charging kinetics and demonstrates the important role that porosity can play in ion transport through electroactive polymers. Advantageously, continuous polymer networks remain evident in nanostructured films, ensuring that high electronic conductivities are maintained along with high porosity. We find that such enhanced properties are retained even as polymer thickness increases ten-fold. The films reported herein may serve as robust electrodes in flexible electrochemical devices.

Structural Changes in Adsorbed Cytochrome c upon Applied Potential Characterized by Neutron Reflectometry.

The structural behavior of an electron-transfer protein, cytochrome c, at the 316L stainless steel electrode/aqueous interface was investigated over a range of applied potentials using neutron reflectometry supported by solution depletion isotherms, X-ray reflectometry, and quartz crystal microbalance measurements. A custom-made electrochemical cell allowed in situ observation of the adsorbed protein across a range of applied potentials; models fitted to the NR data showed a compact inner protein layer at the metal/electrolyte interface and a further thicker but highly diffuse layer that could be removed by rinsing. The overall amount adsorbed was found to be strongly dependent on the applied potential and buffer pH. Subtle but significant changes in the structure of the adsorbed protein layer were seen as the potential was swept between ±0.40 V, reflecting changing attractive/repulsive interactions between the protein's charged side groups and the surface. At greater applied potentials, irreversible changes in the stainless steel film structure were also observed and attributed to deuterium absorption into the metal.

Dielectric–Semiconductor Interface Limits Charge Carrier Motion at Elevated Temperatures and Large Carrier Densities in a High‐Mobility Organic Semiconductor

AbstractThe fundamental nature of charge transport in highly ordered organic semiconductors is under constant debate. At cryogenic temperatures, effects within the semiconductor such as traps or the interaction of charge carriers with the insulating substrate (dipolar disorder or Fröhlich polarons) are known to limit carrier motion. In comparison, at elevated temperatures, where charge carrier mobility often also decreases as function of temperature, phonon scattering or dynamic disorder are frequently discussed mechanisms, but the exact microscopic cause that limits carrier motion is debated. Here, the mobility in the temperature range between 200 and 420 K as function of carrier density is explored in highly ordered perylene‐diimide from 3 to 9 nm thin films. It is observed that above room temperature increasing the gate electric field or decreasing the semiconducting film thickness leads to a suppression of the charge carrier mobility. Via X‐ray diffraction measurements at various temperatures and electric fields, changes of the thin film structure are excluded as cause for the observed mobility decrease. The experimental findings point toward scattering sites or traps at the semiconductor–dielectric interface, or in the dielectric as limiting factor for carrier mobility, whose role is usually neglected at elevated temperatures.

On‐Site Surface Coordination Complexation via Mechanochemistry for Versatile Metal–Phenolic Networks Films

AbstractThe development of functional metal–phenolic networks (MPNs) films by a simple and green strategy on metallic plane substrates for surface modification is a big challenge. Herein, tribochemistry, as an effective and robust method to prepare MPNs films with versatile applications and controllable thickness (≈1.5 nm to 2.2 µm) by surface coordination complexation derived from tannic acid (TA) and metal oxides or ions (M) on plane substrates, is reported. The mechanism of the formation of TA‐M films is investigated in detail, showing that films are constructed by two‐layer structures. At the bottom of films, the chelation of TA active moieties and M is triggered by friction, facilitating the coating growth and reducing the friction coefficient. There is a downward trend in the concentration of M with the thickness of films increasing, which is attributed to the diffusion of metal ions. As a result, the dominant structure of films changes into the hydrogen bonding or π–π stacking interaction among oligomers derived from coupling of the TA active moieties. Such facile surface modification strategy can broaden in situ mechanochemical synthesis of functional layers and open a promising route for the design of patterning, antifouling, and controlled release coatings.

Control of Growth Mechanism of Electrodeposited Nanocrystalline NiFe Films

NiFe nanocrystalline films were formed onto Au film via pulsed electrolyte deposition with variable time of relaxation between the sequential pulses. Analysis of anomalous non-liner changes in the NiFe film structure showed the implementation of three mechanisms, resulting in “layer-by-layer”, “layer-plus-island”, or “island” growth. The change of the interpulse relaxation time provides the possibility to realize all three different mechanisms. The grown NiFe films may have different morphology. The ultrathin NiFe films have nanosized grains and exhibit high uniformity of structure, the films may be also combined of less that 10 nm nanoscale grains and their conglomerates, typically, about 50 nm in size, and, additionally, the film structure, consisting of isolated magnetic islands can be realized. The phenomenological explanation of the different mechanisms was obtained using atomic-force-microscopy (AFM) approach. It has been demonstrated that the growth mechanism can be controlled by nanocrystallites conglomeration, which is accelerated with increasing the relaxation time. The origin of the conglomeration process is mainly associated with high surface energy of nanosized grains.

The current-induced electroresistance in strain-modulated Pr0.5Sr0.5MnO3 film

Pr0.5Sr0.5MnO3 films were grown on ferroelectric substrates of 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 by pulsed-laser deposition method. The film structure and lattice change with electric field applied on the substrate are examined by X-ray diffraction. The electric field dependence of resistivity was compared with electric field dependence of lattice at room temperature, revealing a relation between resistance and strain. Current-induced electroresistance (CER) was studied by using different measuring current. With increasing electric filed a colossal decrease of CER at low temperature was achieved, indicating great strain effect. The piezoelectric strain effect on the magnetoelectric coupling at multiferroic interface was discussed.

Composition, Structure and Mechanical Properties of CrAlC Films Deposited by Arc Sputtering Technique

A focus point in this work was the research of the influence of deposition condition on properties of CrAlC films. CrAlC films were deposited by arc sputtering technique using arc sputtering of Cr-Al target and pulse arc sputtering of graphite target at different frequency. The multilayer [(CrAlC)+(a-C)]ncoatings were also obtained and investigated. Multilayer [(CrAlC)+(a-C)]nwere deposited with a systematic alternation of the pair [(CrAlC)+(a-C)], where a-C is the amorphous carbon layer. The structure, mechanical properties and fracture toughness of CrAlC films depend on pulse frequencies of the graphite sputtering. With increasing pulse frequency, the film structure changes from uniformly amorphous to amorphous with nanograins, the hardness of the films increases, the critical load of crack formation and film peeling tend to decrease. It is shown that a decrease in the thickness of the layers in [(CrAlC)+(a-C)]nleads to an increase in the crack resistance.

Thickness-dependent structural parameters of kesterite Cu2ZnSnSe4 thin films for solar cell absorbers

The influence of thickness on the structural parameters, off-stoichiometry and cation disorder in kesterite Cu2ZnSnSe4 films grown by flash deposition for solar cell absorbers is investigated employing X-ray diffraction, energy-dispersive X-ray and Raman spectroscopies. It is shown that the lattice parameters of Cu2ZnSnSe4 changed depending on film thickness: the 100 nm film turned out to be weakly stretched on the molybdenum-coated glass substrates, while, in the thicker films, the compressive deformation is defined. The causes of the changes in film structure are outlined. Raman spectra revealed secondary phases like Cu2SnSe3 detecting reduction of its fraction with an increment in thickness; also, the SnZn antisite defect fraction decreases. Simultaneously, the share of disordered kesterite phase associated with CuZn antisite defects rises with thickness. The obtained results can be useful for optimization of technological growth process.

Reactive Molecular Dynamics Modeling of Chloride-Induced Depassivation of Iron Passive Film in Alkaline Media

Chloride-induced depassivation of iron passive film in 13.5 pH NaOH solution was studied using Reactive Force Field Molecular Dynamics (ReaxFF-MD). The study involved the simulation of the passive oxide film formed in chloride-free 13.5 pH NaOH solution and the effect of incremental chloride addition to the solution on the oxide film. The interactions of chloride with the oxide film and the changes in the oxide film structure were investigated by monitoring different species, charge distributions, and the pair distribution functions over time. The relationship between different surface termination of the passive film (i.e., O-terminated surface vs. Fe-terminated surface) and chloride interactions was also studied. The results of the investigation were linked to existing depassivation models (e.g, the ion exchange model and the point defect model) and to a parallel Density Functional Theory (DFT) investigation of iron oxide and chloride interactions in water.

Low-temperature, plasma assisted, cyclic synthesis of MoS2

Thin film reaction based synthesis techniques are promising for large area, uniform two-dimensional transition metal dichalcogenide (TMD) layers such as MoS2. In this work, the impact of the initial molybdenum film composition (metallic versus oxidized) is explored. Alternating steps of Mo sputtering and H2S soaks are used in conjunction with plasma assisted synthesis techniques to synthesize films at low temperatures. Raman, photoluminescence, x-ray photoelectron spectroscopy, and atomic force microscopy are used to physically characterize the films' atomic structure, stoichiometry, and topography, while devices were fabricated to characterize their electronic properties. MoS2 synthesized from metallic Mo films were found to exhibit better atomic and electronic structure than MoS2 synthesized from MoOx films. Additionally, slowing the rate of synthesis by segmenting growth into repeating cycles resulted in much higher film quality. To understand the impact of atomic structure and stoichiometry on device performance, films synthesized at low temperature were exposed to various high temperature annealing conditions to induce changes in film structure and composition. Physical and electrical characterization reveal that stoichiometry has a significantly weaker influence on electronic performance than grain size and atomic structure. These results provide valuable information on the optimization of low temperature thin film reactions for TMD syntheses.

Electrical properties of thin films deposited from TMS/O2 in Microwave Multipolar Plasma reactor

Thin films have been deposited from pure Tetramethylsilane and a mixture of Tetramethylsilane and oxygen (TMS/O2). The addition of oxygen proportion to Tetramethylsilane vapors leads to the change in film structure which varies from organic to inorganic character close to SiOx-like film [1]. The electrical characterisation using Metal-Insulating-Metal structure permits the study of current-voltage (I (V)) curves behaviours. The results suggest that the carrier transport in the deposited films is limited by a space charge conduction mechanism.

Morphology of OLED Film Stacks Containing Solution-Processed Phosphorescent Dendrimers.

Organic light-emitting devices containing solution-processed emissive dendrimers can be highly efficient. The most efficient devices contain a blend of the light-emitting dendrimer in a host and one or more charge-transporting layers. Using neutron reflectometry measurements with in situ photoluminescence, we have investigated the structure of the as-formed film as well as the changes in film structure and dendrimer emission under thermal stress. It was found that the as-formed film stacks comprising poly(3,4-ethylenedioxythiophene):polystyrene sulfonate/host:dendrimer/1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (where the host was deuterated 4,4'-N,N'-di(carbazolyl)biphenyl or tris(4-carbazol-9-ylphenyl)amine, the host:dendrimer layer was solution-processed, and the 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene evaporated) had well-defined interfaces, indicating good wetting of each of the layers by the subsequently deposited layer. Upon thermal annealing, there was no change in the poly(3,4-ethylenedioxythiophene):polystyrene sulfonate/host:dendrimer interface, but once the temperature reached above the Tg of the host:dendrimer layer, it became a supercooled liquid into which 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene dissolved. When the film stacks were held at a temperature just above the onset of the diffusion process, they underwent an initial relatively fast diffusion process before reaching a quasi-stable state at that temperature.

Graphene-based humidity sensors: the origin of alternating resistance change

The response of a graphene-based humidity sensor is considered as a function of film structures. Analysis of the resistance changes due to water molecule adsorption on the graphene or multi-layer graphene (MLG) surface is performed for films with different structures and resistivities from hundreds of ohms/sq to hundreds of kilo-ohms/sq. The results revealed possible increase, decrease and non-monotonous behavior of resistance with changes in film structure. Adsorption of water molecules at grain boundary defects is assumed to lead to an increase in film resistivity due to the donor property of water and the p-type conductivity of graphene. Another type of conductive center with a higher capture cross-section is realized in the case of water molecule adsorption at edge defects in MLG films (the formation of conductive chains with ionic conductivity). If these chains form a continuous network the film resistivity decreases. The result of the competition between the opposite effects of the conductivity compensation and formation of the water-based conductive chains depends on the film structure and determines the response of humidity sensors. Sensor sensitivity is found to increase when only one type of defect determines water adsorption (edge defects or grain boundary defects).

Thermo-oxidation of laboratory-produced undoped and W-doped carbon films: A reaction product analysis

Thermo-oxidation is a technique where O2 gas is used to remove carbon co-deposits at elevated temperatures from plasma-facing materials of fusion devices. Following thermo-oxidation, the reaction products produced will need to be managed by a reactor's gas-handling system, including the tritium plant. Thus, it is important to know what molecular species are produced during oxidation. The reaction products of thermo-oxidation have previously been studied for undoped films indicating CO, CO2 and D2O as the reaction products. The present study directly compares the reaction product evolution of undoped films to that of films doped with tungsten.A glow discharge was used to produce sets of undoped and W-doped films on stainless steel (SS) foil substrates. X-ray photoelectron spectroscopy (XPS) found the W-doped films to contain an average 0.1 at.% W/(W+C). Laser thermal desorption spectroscopy (LTDS) was used to measure the areal D-concentration. Reaction products during oxidation were measured using a quadrupole mass spectrometer (QMS). Both the undoped and W-doped specimens were oxidized in 2Torr O2, at 350 °C for 4h, total. LTDS was then used to determine how much D had been removed during oxidation (93% and 32% of the D was removed after oxidation for the undoped and W-doped cases respectively). The reduced D loss for the W-doped specimens is attributed to a change in film structure due to the incorporation of W in the film. Oxidation reaction products for both undoped and W-doped films include CO, CO2, and D2O, with no finding of D2 or CD4 within experimental uncertainty. Particle accounting was performed for oxygen, deuterium and carbon atoms.

Thermal stability of ZIF-8 membranes for gas separations

The thermal stability of ZIF membranes is important for high temperature separation applications but has not been systematically studied. This work highlights the results of a thermal stability study of ZIF-8 membranes in terms of material structure, H2/CO2 gas permeation and separation characteristics. During binary and single gas temperature dependent permeance tests conducted from 25 to 250°C, both H2 and CO2 permeances decrease as a function of temperature. In the binary test, H2/CO2 selectivity increases between 25 and 225°C, and then decreases as temperature is further increased between 225–275°C. The results can be explained by the adsorption/diffusion mechanism. Beyond 275°C, H2/CO2 permeance and selectivity drastically increase with respect to temperature and is indicative of ZIF-8 membrane partial carbonization during the dynamic 30h temperature dependent test. The time/temperature dependency of the onset of ZIF-8 thin film structural change was deconvoluted in isothermal transient permeation experiments. Transient tests performed at 50, 100, 150 and 300°C for 24h indicate that ZIF-8 thin films maintain their crystallinity and structural integrity below 150°C. However, at temperatures of 150°C and greater the framework undergoes increased magnitudes of thermally induced carbonization as a function of temperature. Thermomechanically induced stresses between the ZIF-8 membrane thin film and α–alumina support may account for differences in static thermal stability observed when comparing ZIF-8 membranes and ZIF-8 crystalline powders.

Ageing mechanisms of polyurethane adhesive/steel interfaces

For the achievement of a complete understanding of the ageing and corrosion of adhesively bonded steel substrates, both the ageing of a polyurethane reference adhesive and the simultaneous corrosion of a mild steel substrate were studied by combined spectroscopic, electrochemical and microscopic techniques. A corundum-blasted mild steel was used as metal substrate in combination with a cross-linked reference polyurethane adhesive free from additional additives or fillers. The ageing was performed under full immersion in a chloride-free aerated electrolyte at elevated temperature. A strong influence of the water saturated polymer phase on the change in the interfacial oxide film structure of the metal substrate was observed. The thickening and hydroxylation of the metal oxide film during the corrosive delamination of the polyurethane film indicated that the de-bonding mechanism of the polyurethane adhesive is mainly based on interfacial water enrichment and a thereby induced growth of the interfacial oxide. The measured electrochemical properties of this oxide indicate that it is highly conductive and allows for oxygen reduction as the counter reaction to metal oxidation.

Thin Film Cathodes for Lithium and Beyond Lithium-Ion Batteries

The development of advanced batteries that have a higher energy density for current lithium-ion and beyond lithium-ion battery relies not only on the identification of new chemistries (e.g., a compatible combination of cathode, anode and non-aqueous electrolyte), but also mechanistic understanding of cathode performance during charge-discharge processes. For these reasons we focus on model thin film systems, which provide controllable geometry, orientation and surface roughness. Using pulsed laser deposition, we have grown lithium-based (e.g., LiMn2O4) and magnesium based (MgMn2O4) oxide thin films. Using these model systems, we have explored the electrochemical performance of these materials as new battery cathodes, as well as the intercalation mechanism of cations, namely Mg2+ and Li+, at the electrode/electrolyte interfaces. For example, the cubic phase of MgMn2O4 (MMOC) can only be found in powder form at high temperature (> 950 °C) or high pressure (> 15.6 GPa), but is stabilized as thin films, which exhibited reversible Mg2+ insertion and extraction in non-aqueous electrolyte such as Mg(TFSI)2 in diglyme with associated changes in bulk film structure and Mn-oxidation states. Using in situ X-ray reflectivity we have observed the film structural changes during charge-discharge processes. Our thin film model system studies provide a platform for material design and mechanistic understanding for lithium and beyond lithium-ion batteries. This work was supported by the Joint Center for Energy Storage Research (JCESR) through the Office of Basic Energy Sciences (BES), U.S. Department of Energy (DOE).

Swift heavy ion irradiation of metal containing tetrahedral amorphous carbon films

Thin carbon films were grown at room temperature on (001) n-Si substrate using dual cathode filtered vacuum arc deposition system. Graphite was used as a source of carbon atoms and separate metallic electrode was simultaneously utilized to introduce Ni or Cu atoms. Films were irradiated by 100MeV Ag7+ ions to fluences in the range 1×1010–3×1011cm−2. Rutherford backscattering spectroscopy, Raman scattering, scanning electron microscopy and atomic force microscopy in conductive mode were used to investigate film properties and structure change under irradiation. Some conductive channels having metallic conductivity type were found in the films. Number of such channels is less than number of impinged ions. Presence of Ni and Cu atoms increases conductivity of those conductive channels. Fluence dependence of all properties studied suggests different mechanisms of swift heavy ion irradiation-induced transformation of carbon matrix due to different chemical effect of nickel and copper atoms.

Structural and optical characterization of ZrO2 thin films grown on silicon and quartz substrates

Zirconium oxide thin films were grown successfully by thermal annealing of zirconium thin films deposited on quartz and silicon substrates by direct current magnetron sputtering technique. The structural and optical properties in relation to thermal annealing times were investigated. The X-ray diffraction patterns revealed that structure of films changes from amorphous to crystalline by increase of annealing times in range 60–240 min. The composition of films was determined by Rutherford back scattering spectroscopy. Atomic force microscopy results exhibited that surface morphology and roughness of films depend on the annealing time. The refractive index of the films was calculated using Swanepoel’s method. The optical band gap energy of annealed films decreased from 5.50 to 5.34 eV with increasing thermal annealing time.

Photoprecursor Approach Enables Preparation of Well-Performing Bulk-Heterojunction Layers Comprising a Highly Aggregating Molecular Semiconductor.

Active-layer morphology critically affects the performance of organic photovoltaic cells, and thus its optimization is a key toward the achievement of high-efficiency devices. However, the optimization of active-layer morphology is sometimes challenging because of the intrinsic properties of materials such as strong self-aggregating nature or low miscibility. This study postulates that the "photoprecursor approach" can serve as an effective means to prepare well-performing bulk-heterojunction (BHJ) layers containing highly aggregating molecular semiconductors. In the photoprecursor approach, a photoreactive precursor compound is solution-deposited and then converted in situ to a semiconducting material. This study employs 2,6-di(2-thienyl)anthracene (DTA) and [6,6]-phenyl-C71-butyric acid methyl ester as p- and n-type materials, respectively, in which DTA is generated by the photoprecursor approach from the corresponding α-diketone-type derivative DTADK. When only chloroform is used as a cast solvent, the photovoltaic performance of the resulting BHJ films is severely limited because of unfavorable film morphology. The addition of a high-boiling-point cosolvent, o-dichlorobenzene (o-DCB), to the cast solution leads to significant improvement such that the resulting active layers afford up to approximately 5 times higher power conversion efficiencies. The film structure is investigated by two-dimensional grazing-incident wide-angle X-ray diffraction, atomic force microscopy, and fluorescence microspectroscopy to demonstrate that the use of o-DCB leads to improvement in film crystallinity and increase in charge-carrier generation efficiency. The change in film structure is assumed to originate from dynamic molecular motion enabled by the existence of solvent during the in situ photoreaction. The unique features of the photoprecursor approach will be beneficial in extending the material and processing scopes for the development of organic thin-film devices.