Non-annealed Films Research Articles

Interplay between Exciton and Free Carriers in Organolead Perovskite Films

For highly interested organolead perovskite based solar cells, the exciton and free carriers are the photoproducts in the working layers. In this study, we revealed their two forms of relations depending on heat-annealing condition. In non-annealed films and single crystal, they are in density-dependent dynamical balance (co-existing). For the sufficiently heat-annealed films, they present a significant emissive exciton-carrier collision (ECC). The two relations indicate the emergence of a subgrain morphology within the tetragonal phase of crystal grain, induced by heat annealing process. Such subgrain structure could be assigned to a ferroelastic twinning structure recently found inside the crystal grain of the films. Since the heat annealing is a general procedure in preparing perovskite working layers, we propose that the ECC and subgrain morphology widely exist in real devices. We suggest that the subgrain structure provides another level of morphological basis for in depth understanding high performance of organolead perovskite working layers.

Reversible p-type conductivity in H passivated nitrogen and phosphorous codoped ZnO thin films using rapid thermal annealing

We demonstrate reversible p-type nature of pulsed laser deposited (P, N) codoped ZnO thin films using rapid thermal annealing process. As grown thin films exhibited change in conductivity from p to n-type over a span of 120days. Non-annealed n-type thin films contain unintentional donor impurities such as hydrogen and carbon. X-ray photoelectron spectroscopy and Raman measurements conclusively show that hydrogen passivates nitrogen acceptors by forming NH complex. Carbon can be annealed out at 600°C, whereas, the dissociation of NH complex takes place at 800°C. The films revert its p-type nature at an annealing temperature of 800°C.

Reconstructing the energy band electronic structure of pulsed laser deposited CZTS thin films intended for solar cell absorber applications

We report here on the use of pulsed KrF-laser deposition (PLD) technique for the growth of high-quality Cu2ZnSnS4 (CZTS) thin films onto Si, and glass substrates without resorting to any post sulfurization process. The PLD-CZTS films were deposited at room temperature (RT) and then subjected to post annealing at different temperatures ranging from 200 to 500°C in Argon atmosphere. The X-ray diffraction and Raman spectroscopy confirmed that the PLD films crystallize in the characteristic kesterite CZTS structure regardless of their annealing temperature (Ta), but their crystallinity is much improved for Ta≥400°C. The PLD-CZTS films were found to exhibit a relatively dense morphology with a surface roughness (RMS) that increases with Ta (from ∼14nm at RT to 70nm at Ta=500°C with a value around 40nm for Ta=300–400°C). The optical bandgap of the PLD-CZTS films, was derived from UV–vis transmission spectra analysis, and found to decrease from 1.73eV for non-annealed films to ∼1.58eV for those annealed at Ta=300°C. These band gap values are very close to the optimum value needed for an ideal solar cell absorber. In order to achieve a complete reconstruction of the one-dimensional energy band structure of these PLD-CZTS absorbers, we have combined both XPS and UPS spectroscopies to determine their chemical bondings, the position of their valence band maximum (relative to Fermi level), and their work function values. This enabled us to sketch out, as accurately as possible, the band alignment of the heterojunction interface formed between CZTS and both CdS and ZnS buffer layer materials.

Excited-State Proton Transfer and Conformational Relaxation of 2-(4'-Pyridyl)benzimidazole in Nafion Films.

The effect of annealing on the acidity and water uptake of Nafion films has been studied by using the acidity sensing fluorophore 2-(4'-pyridyl)benzimidazole (4PBI). The difference in acidity and the microenvironment of the fluorophore in annealed and nonannealed films is brought out in this study. The annealed film is found to have less water uptake than nonannealed films. The amount of water uptake increases upon acid treatment of the films, as all the steady-state and time-resolved behaviour of the molecule in nonannealed films is restored. These observations are rationalised by the formation of anhydrides upon annealing and their hydrolysis to sulfonic acid groups upon acid treatment. Interestingly, the acidity of annealed films is found to be even less than that of Na+ exchanged films, indicating that annealing removes more protons from the Nafion films than cation exchange can.

Thickness Dependence of Acidity and Microstructure in Nafion Films

AbstractThin films of Nafion of various thickness viz. 6 and 1 µm, 280, 80 and 33 nm have been probed using the fluorescent molecule 2‐(2′‐pyridyl)benzimidazole (2PBI), at low as well as high hydration levels. The acidity of these films has been found to be less than that of Nafion 117 membrane at hydrated conditions. This has been ascribed to the high water uptake of the films compared to the membrane and the consequent decrease in the local concentration of protons. The fluorophore reports a significant decrease in the acidity of the film upon annealing them, for all values of thickness. Surprisingly, the acidity in annealed film is lesser than in nonannealed films in which H3O+ ions are exchanged with Na+ ions. Acid treatment of the annealed films causes a reversal of this effect, except for the film with thickness less than 80 nm. Thus, pockets inaccessible to proton appear to form in the ultrathin films. The effective proton concentration depends on the thickness of the films and decreases with decrease in thickness of the films. This may be due to the association of protons to sulfonate ions at lower thickness.

Effects of temperature on electromechanical properties and ultrasonic performance of piezoelectric cellular PP films

Ferroelectrets are a recent addition to the family of piezoelectric polymers. Ferroelectrets are space-charge electrets with a heterogeneous and usually cellular foam structure. They are flexible, available in large areas and show strong piezoelectric response. Due to their low acoustic impedance they have strong application potential in air-borne ultrasonic transducers. Here, the influence of temperature on the electromechanical and ultrasonic properties of polypropylene (PP) ferroelectrets was addressed. PP ferroelectrets were subjected to repeated temperature variations. The characterization was performed by means of dielectric resonance spectroscopy (DRS) and Laser-Doppler Vibrometry (LDV). Profound variations in the piezoelectric properties during thermal cycling were observed and correlated to the changes in the elastic properties. Exposure to elevated temperatures resulted in a reduction in the piezoelectric response and an increase in the elastic stiffness constant which also shifted the thickness-extension resonance peaks towards higher values. Large differences in piezoelectric properties were discovered between the first and the subsequent cycles. To decrease the influence of temperature cycling on the piezoelectric properties of PP ferroelectrets, annealing was used to advantage. Annealing treatments with different durations and different annealing temperatures were investigated. A particular procedure yielding foam stiffening and a sufficiently high piezoelectric activity was determined and was employed for further investigations. The piezoelectric properties of the annealed ferroelectrets varied much less when compared to non-annealed ferroelectrets. Characterization of thermally cycled annealed and non-annealed films revealed relatively homogeneous phase and amplitude distributions of the surface-vibration below the resonance, which resembles a piston-like response. Piston-like response was not significantly deteriorated by thermal cycling below the resonance frequency. This was corroborated for all samples by measuring the acoustic directivity of them in the farfield, at frequencies below the resonance. At the resonance, much higher distortions of the response were observed as revealed from the phase images.

Strain status of epitaxial Ge film on a Si (001) substrate

An epitaxial Ge film was grown on a Si (001) substrate via a two-step process through the molecular beam epitaxy technique. The strain status of non-annealed and annealed epitaxial Ge films was determined by X-ray diffraction, Raman spectroscopy, and a combination of high-resolution transmission electron microscopy and geometric phase analysis. Results showed that the strain in non-annealed and annealed epitaxial Ge films is nonhomogeneous from the Ge/Si interface to the Ge film surface. The strain parallel to the interface in the non-annealed epitaxial Ge film is compressive; this strain reaches a minimum near the surface and a maximum at the interface. By contrast, the strain parallel to the interface in the annealed epitaxial Ge film is tensile; this strain reaches a minimum at the interface and a maximum near the surface.

MgO magnetic tunnel junctions of enduring F-type upon annealing

The authors performed magnetotransport experiments to determine whether annealing alters the oxygen vacancy-mediated tunnelling potential landscape of the central portion of a MgO ultrathin film within sputtered CoFeB/MgO/CoFeB magnetic tunnel junctions. Using the Îrel method reveals a temperature-dependent tunnelling barrier height for a non-annealed barrier that arises from single oxygen vacancies (F centres) and is qualitatively identical to that found for its partly and fully annealed counterparts. Thus these MTJs with F centres remain of F-type upon annealing. This explicitly confirms that the large tunnel-magnetoresistance (TMR) increase upon annealing results mainly from structural modifications of MgO and CoFeB and not from vacancy pairing within the barrier. Photoluminescence spectra performed on both annealed and non-annealed thin MgO films grown on CoFeB electrodes support this conclusion. This work should promote renewed scrutiny over the precise impact of annealing on tunnelling magnetotransport across MgO.

Imaging the effects of annealing on the polymorphic phases of poly(vinylidene fluoride).

The effect of annealing on the phase transformation and the dielectric properties of poly(vinylidene fluoride) (PVDF) is explored using quasi-single molecule (quasi-SM) microscopy. The solvatochromic properties of nile red (NR) are employed to measure the spatial distribution of the local dielectric constant (ε) in ∼30 μm thick PVDF films before and after annealing at 90 °C. The results presented here demonstrate that nonannealed films exhibit much larger ε distributions, both in terms of magnitude and distribution, when compared to annealed films. The polymorphic phase of PVDF before and after annealing is also confirmed using X-ray diffraction. Nonannealed films are found to be in the γ-phase with annealing promoting the transition to primarily β-phase. Combining these results, we conclude that the decrease in ε with annealing time is due to the phase transformation from γ- to β-phase. Using quasi-SM imaging techniques one can readily visualize the ε environments present within different polymorphic phases of PVDF.

Study the structural, optical and electrical properties of sprayed Zinc oxide (ZnO) thin films before and after annealing temperature

Zinc oxide (ZnO) thin films were deposited on glass substrates by spray pyrolysis technique by decomposition of zinc acetate dihydrate in ethanol. The ZnO thin films were deposited at 350 ◦ C and annealed at 500 ◦ C for 2 h. The substrate was R217102 glass in a size of 30 cm × 7.5 cm × 0.1 cm. The films exhibited a hexagonal wurtzite structure with a strong (002) preferred orientation. The higher value of crystallite size (20.29 nm) was attained with films annealed at 500 ◦ C, probably due to improvement of the crystallinity of the films with heating. The average transmittance of the films, by UV-Vis, was over of 95 %. The band gap energy varied from 3.279 to 3.288 eV for the non-annealed and annealed films, respectively. The electrical resistivity was measured to be 0.4 � .cm.

Reversible Tuning of Chemical Structure of Nafion Cast Film by Heat and Acid Treatment

Effects of annealing have been studied on the chemical structure, water uptake, and acidity of the cast Nafion thin film of thickness ∼ 6 μm using a fluorescent probe, 2-(3'-pyridyl)benzimidazole (3PBI), and attenuated total reflactance infrared (ATR-IR) spectroscopy. Nonannealed films and thick Nafion 117 membrane have been used as reference materials, in order to develop a complete understanding of the effect. Annealing has been found to cause a decrease in acidity of otherwise highly acidic ionomer, as sensed by the fluorescent probe and loss of water, as reflected in the ATR-IR spectrum. This observation is surprising and cannot be explained in the light of previous reports of physical changes. Our ATR-IR study has revealed changes in the chemical structure of the hydrophilic part of the ionomer, leading to the formation of sulfonic acid anhydrides. This phenomenon can rationalize the decreased acidity reported in our fluorescence study. Interestingly, acid treatment of the annealed film restores the acidity of the unannealed films. This cannot be rationalized simply by a greater proton uptake from the solution, as the film has to be electroneutral. It appears that the anhydrides formed during the annealing process undergo acid hydrolysis, leading to an increase in the number of SO3(-) groups and, consequently, an increase in the number of H3O(+) ions in the water channels. Besides, the films can be hydrated to an extent that is much greater than Nafion membranes, but the water uptake of acid treated annealed film is slightly less than that of nonannealed films. Hence, we conclude that, along with annealing, acid treatment, a procedure that is generally not performed on the cast films, is an important pretreatment procedure to improve the acidity and hence the transport properties of the cast film.

PH-stability and pH-annealing of H-bonded multilayer films prepared by layer-by-layer spin-assembly

Spin-assisted assembly and pH-induced disintegration of H-bonded multilayers of poly(N-vinylpyrrolidone) and tannic acid (PVPON/TA) have been investigated. Multilayers showed significantly different pH-induced disintegration profiles depending on how long the multilayers were kept in the dry state between exposures to buffer solutions indicating relaxations of the molecules. To understand the effect of kinetic trapping and the consequent relaxations towards equilibrium on the pH-stability of H-bonded PVPON/TA spin-LbL multilayers, spin-coated multilayers were prepared at different nonequilibrium states by varying the spinning rate and the solution-film static contact time. The pH-stability of multilayers increased with increasing spinning rate and decreasing static contact time. The gradual decrease of bilayer thickness with increasing spinning rate indicated the flattening of the molecules. The enhancement of the pH-stability of multilayers was attributed to both the flattening and the relaxations of molecules having nonequilibrium conformations. A transition from linear to exponential growth profiles was observed with increasing static contact times indicating in and out diffusion in PVPON/TA system. pH-treatment of multilayers just below the critical disintegration pH, a process we call pH-annealing, enhanced their pH-stability significantly. Multilayers annealed longer at pH 7.0 showed better pH-stability (up to 1h) at pH 9.0 whereas nonannealed films disintegrated within 10min. The introduced pH-annealing process is promising for practical applications of H-bonded systems such as controlled drug delivery.

Study of the optical properties of etched alpha tracks in annealed and non-annealed CR-39 polymeric detectors

The UV–visible absorption spectra of virgin and alpha particle-irradiated, annealed and non-annealed CR-39 polymeric track detectors were investigated using a UV–visible spectrometer (Shimadzu mini 1240). Isothermal annealing experiments were carried out on poly allyl diglycol carbonate (PADC) films based nuclear track detectors (NTDs) exposed to a 241Am source. A shifting and broadening of the UV–visible peaks was observed as a result of the etched alpha tracks in the non-annealed and annealed PADC films. The UV–visible spectra of the virgin and non-annealed α-irradiated PADC polymer films displayed a decreasing trend in their optical energy band gaps, both direct and indirect, whereas those measured for the annealed α-irradiated ones showed no significant change. This drop in the energy band gap with increasing fluence is discussed on the basis of the alpha particle- and thermal annealing-induced modifications in the PADC polymeric detector. The results clearly showed that the values for the indirect energy band gap were lower than the corresponding values for the direct band gap. In addition, the Urbach energy was estimated from the Urbach edge, and exhibited roughly the same trend as the optical band gap. Finally, this study presents new results showing that the annealed PADC films were highly insensitive to alpha particles.

Nanoscale imaging of dense fiber morphology and local electrical response in conductive regioregular poly(3-hexylthiophene)

We present a thermal annealing method for producing micrometer long and dense semi-crystalline fiber structures in poly(3-hexylthiophene) (P3HT) films. The thin film topology and local electrical properties are investigated by conductive atomic force microscopy (c-AFM), and three-dimensional device models are used to illustrate the effects of current spreading, anisotropic mobility and traps. In-plane current flow along high mobility fibers provides a mechanism for c-AFM contrast in fibrous films. The comparison of c-AFM results between the annealed and non-annealed P3HT films suggests that trap effects are pronounced in disordered P3HT thin films prior to thermal annealing. The methodologies we demonstrate here on the archetypal P3HT film can be generalized for understanding the correlation between structure and local electrical properties in a variety of polymer and polymer nanocomposite systems.

Two-step annealed CdS/CdSe co-sensitizers for quantum dot-sensitized solar cells

CdS/CdSe co-sensitizers on TiO2 films were annealed using a two-step procedure; high temperature (300 °C) annealing of TiO2/CdS quantum dots (QDs), followed by low temperature (150 °C) annealing after the deposition of CdSe QDs on the TiO2/CdS. For comparison, two types of films were prepared; CdS/CdSe-assembled TiO2 films conventionally annealed at a single temperature (150 or 300 °C) and non-annealed films. The 300 °C-annealed TiO2/CdS/CdSe showed severe coalescence of CdSe QDs, leading to the blocked pores and hindered ion transport. The QD-sensitized solar cell (QD-SSC) with the 150 °C-annealed TiO2/CdS/CdSe exhibited better overall energy conversion efficiency than that with the non-annealed TiO2/CdS/CdSe because the CdSe QDs annealed at a suitable temperature (150 °C) provided better light absorption over long wavelengths without the hindered ion transport. The QD-SSC using the two-step annealed TiO2/CdS/CdSe increased the cell efficiency further, compared to the QD-SSC with the 150 °C-annealed TiO2/CdS/CdSe. This is because the 300 °C-annealed, highly crystalline CdS in the two-step annealed TiO2/CdS/CdSe improved electron transport through CdS, leading to a significantly hindered recombination rate.

Antibacterial and Water Purification Activities of Self-Assembled Honeycomb Structure of Aerosol Deposited Titania Film

A simple and rapid room-temperature aerosol deposition method was used to fabricate TiO(2) films for photokilling/photdegradation applications. TiO(2) particles were accelerated to supersonic speeds and fractured upon impacting a glass substrate to form a functional thin film, a process known as aerosol deposition. After deposition, the films were annealed at various temperatures, and their photokilling/photodegradation performances following ultraviolet (UV) exposure were evaluated by counting the number of surviving bacterial colonies, and by a methylene blue decolorization test. The photocatalytic performances of all TiO(2) films were obtained under weak UV exposure (0.6 mW/cm(2)). The film density, crystalline phase, and surface roughness (morphology) were measured by scanning electron microscopy, X-ray diffraction, UV-visible spectroscopy, and atomic force microscopy. The unique, self-assembled honeycomb structure of the aerosol deposited films contributed to the increase in surface area because of extreme roughness, which enhances the photokilling and photodegradation performance. Nonannealed films yielded the best photocatalytic performance due to their small crystalline sizes and large surface areas due to increased surface roughness.

New Columnar Zn-Phthalocyanine Designed for Electronic Applications

Columnar liquid crystals are composed of disk-shaped aromatic molecules surrounded by flexible side chains, where molecules self-assemble in columns and thereby form large surface-oriented domains. These systems are known for their good charge and exciton transport along the columns, with mobilities approaching those of aromatic single crystals. Such semiconducting materials are promising for devices applications, since the output efficiency can be tuned by properly aligning columns. In the work presented here, the synthesis and characterization of a new Zn-phthalocyanine (ZnPc) is described which exhibits remarkable properties, such as hexagonal columnar order, achieved by cooling down from the isotropic phase to room temperature. Such order was confirmed by optical microscopy and X-ray diffraction experiments. Diodes were constructed using spin-coated films, and the conductive properties were investigated by current versus voltage analysis, where mobilities of 10(-3) and 10(-2) cm(2)/(V s) were obtained for the nonannealed and annealed films, respectively.

Preparation of Non-annealed Anatase TiO2 Films on ITO substrates by Anodizing in Hot Phosphate/glycerol

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Quasistatic and dynamic mechanical properties of Al–Si–Cu structural films in uniaxial tension

In this article, the quasistatic and dynamic tensile properties of aluminum–silicon–copper (Al–Si–Cu) alloy films are described. The films were deposited by sputtering onto thermally oxidized Si wafers, and then half of the wafers were heat treated at 623 K in nitrogen gas for 1 h. Specially developed environment-controlled uniaxial tensile test equipment was used to carry out the quasistatic tensile test, stress relaxation test, and cyclic loading test at temperatures ranging from room temperature (RT) to 573 K in high vacuum, and the influence of annealing on the mechanical characteristics was investigated. The Young’s modulus did not show annealing dependency. The mean value was 65 GPa at RT, and gradually decreased with increasing test temperature. The yield stresses of nonannealed and annealed films were 168.5 and 129.6 MPa, respectively, which also decreased with temperature rise. In stress relaxation test results, creep exponents in respective films were obtained from curve fitting using the Norton law, which indicated that creep deformation was restricted by annealing. The cyclic loading test was performed under stress- and displacement-amplitude-constant modes. The stress-amplitude-constant mode test provided creep deformation acceleration to failure, whereas the displacement-amplitude-constant mode test showed a gradual drop of stress amplitude. The stress-amplitude change was compared with stress relaxation curves; consequently, the creep deformation was dominant to the degradation of Al–Si–Cu films subjected to cycling loading with constant displacement amplitude.

Instability of nanostructures patterned in polystyrene under high electric field gradients

A nanolithography technique based on the lateral displacement of electrically biased AFM tip was developed for nanostructures formation of 30–100nm in width and 1–10nm in height in the polystyrene (PS) films. It was demonstrated that the nanostructures patterned in annealed PS films (90K Mw) show slow exponential relaxation between 55 and 265h depending on their size. Relaxation of the nanostructures in non-annealed films usually occurred in minutes. It was observed that in the annealed samples a negative electric charge accumulated in the areas where the nanostructures formed while in the non-annealed samples only the positive charge in exposed areas was detected using the electric force microscopy. After 320h of monitoring under the humidity maintained between 25 and 27% it was suggested that slow dynamical changes of the nanostructures can be attributed to the negative electric charge dissipation in the annealed samples.