Spin-coated Thin Films Research Articles

EVALUATION ON THE EGFET PH SENSING PERFORMANCE OF SOL-GEL SPIN COATED TITANIUM DIOXIDE THIN FILM

For this study, TiO2 thin film was fabricated using the sol-gel spin coating method. The fabricated film was then applied as a sensing membrane in an extended gate field effect transistor (EGFET) pH sensor system. The pH sensing performance of the sol-gel spin-coated TiO2 was evaluated in terms of sensitivity, linearity, and hysteresis where the value obtained was 58.70 mV/pH, 0.9922, and 86.17 mV respectively. The drift rate of the sample when being measured for 12 consecutive hours was also determined where measurement in pH 4, pH 7, and pH 10 yield drift rate of 1.72 mV/h, 4.14 mV/h, and 6.05 mV/h respectively. Besides that, the TiO2 was characterized for its thickness (24.32 nm) and surface roughness (5.129 nm). From the results obtained, it was found that sol-gel spin-coated TiO2 thin film with thickness between 20 - 29 nm will have high pH sensitivity (more than 50 mV/pH).

Fabrication and characterization of InN-based metal-semiconductor-metal infrared photodetectors prepared using sol–gel spin coated technique

We report on the growth and characterization of undoped indium nitride (InN) thin films grown on a silicon substrate. The InN thin films were grown on aluminium nitride (AlN) template with gallium nitride (GaN) nucleation layer using a relatively simple and low-cost sol–gel spin coating method. The crystalline structure and optical properties of the deposited films were investigated. X-ray diffraction and Raman results revealed that InN thin films with wurtzite structure were successfully grown. For InN thin film grown on a substrate with the GaN nucleation layer, its strain and dislocation density are lower than that of the substrate with the AlN nucleation layer. From the ultra-violet-visible diffuse reflectance spectrum analysis, the energy bandgap of the InN thin films with the GaN layer was 1.70 eV. The potential application of the sol–gel spin-coated InN thin films was also explored. Metal–semiconductor–metal (MSM) infrared (IR) photodetectors were fabricated by depositing the platinum contacts using two interdigitated electrodes metal mask on the samples. The finding shows that the device demonstrates good sensitivity and repeatability towards IR excitation at a wavelength of 808 nm. The photodetector characteristics at dark and photocurrent conditions such as Schottky barrier height (SBH) and ideality factor are determined. Upon exposure to the IR source at 3V applied bias, InN/AlN/Si device configuration displays rapid rise time of 0.85 s and decay time of 0.78 s, while InN/GaN/AlNSi demonstrates slow rise time of 7.45 s and decay time of 13.75 s.

Flexible and Transparent Polymer-Based Optical Humidity Sensor

Thin spin-coated polymer films of amphiphilic copolymer obtained by partial acetalization of poly (vinyl alcohol) are used as humidity-sensitive media. They are deposited on polymer substrate (PET) in order to obtain a flexible humidity sensor. Pre-metallization of substrate is implemented for increasing the optical contrast of the sensor, thus improving the sensitivity. The morphology of the sensors is studied by surface profiling, while the transparency of the sensor is controlled by transmittance measurements. The sensing behavior is evaluated through monitoring of transmittance values at different levels of relative humidity gradually changing in the range 5–95% and the influence of up to 1000 bending deformations is estimated by determining the hysteresis and sensitivity of the flexible sensor after each set of deformations. The successful development of a flexible sensor for optical monitoring of humidity in a wide humidity range is demonstrated and discussed.

Photophysical and morphological properties of spin-coated and self-assembled carbazole derivative thin films

In this work, single-layered, bi-layered and tri-layered poly(vinyl-carbazole) (PVK) thin films were produced by spin-coating and self-assembling techniques and their morphological and photophysical properties were determined by time-resolved fluorescence spectroscopy, atomic force microscopy (AFM) and fluorescence microscopy. The effect of distinct aggregates formed in thin films on their photophysical and morphological properties was determined, evidencing that procedure control is essential to determine PVK thin film application. It was found that sandwich excimers are dominant in self-assembled films, evidenced by longer fluorescence lifetimes and red-shifted emissions, while in spin-coated films, shorter fluorescence lifetimes evidenced more efficient non-radiative energy transfer. AFM showed that consecutive deposited layers result in more uniform, thinner films when produced by spin-coating, but showing aggregation heterogeneity as evidenced by fluorescence spectroscopy. Yet, self-assembled films are rough, heterogeneous and thicker, showing prevalence of less variety of aggregates. These are crucial findings to address PVK application.

Touch sensor and photovoltaic characteristics of CuSbS2 thin films

Spin-coated chalcostibite CuSbS2 thin films (≈500 nm thick) were fabricated and the influence of the drying temperature on the structural, morphological, optical and thermoelectric properties of the films was investigated. Crystalline phase-pure chalcostibite has been obtained for the films dried at 180 °C and 210 °C, while below 180 °C these films are partially amorphous. Surprisingly, at drying temperature of 240 °C, a CuxS secondary phase appeared. The increase of the drying temperature leads to the increase of the particle size and the decrease of the optical band gap, which is interesting for optoelectronic applications. The highest power factor value was achieved for the film dried at 210 °C, due to the inexistence of secondary phases, which allowed realizing a stable thermoelectric touch sensor with a Vsignal/noise of 5. In addition, this film was tested as a photovoltaic (PV) device and a power conversion efficiency (PCE) of 0.030% with an open-circuit voltage (VOC) of 0.36 V, a short-circuit current density (JSC) of 0.278 mAcm−2 and a fill factor (FF) of 0.27 were obtained. Therefore, this work evidences a pathway toward developing bi-functional devices with simultaneously thermoelectric touch sensor and photovoltaic functions.

A study of thickness dependent microstructure of poly (3-hexylthiophene) thin films using grazing incidence x-ray diffraction

ABSTRACT Thin films of poly (3-hexylthiophene), abbreviated as P3HT, are widely used systems of semiconducting polymer for applications in organic electronic devices. We studied the microstructure of spin-coated thin films of P3HT using grazing incidence x-ray diffraction (GID) by combining high-resolution vertical scan with 2D intensity maps obtained using area detector. Microstructure of the films was quantitatively analyzed in terms of relative crystalline density, texture and coherence length. The structure of the films was relaxed by annealing at 110°C. The relaxed microstructure shows strong thickness dependence. Thick film (61 nm) was highly crystalline and textured. For thin film (32 nm), relative crystalline density was very weak but the coherence length of crystallites was larger and the orientation of crystallites was even more strongly textured. For thick film, the strong increase of crystalline density takes place through fresh nucleation and reorientation of crystallites already present in the films. For thin films, the substrate interaction induced very strong alignment of lamellar crystallites through degeneration of off-oriented crystallites and increased lamellar crystalline coherence. The drastic change of microstructure within a small thickness range is an important aspect to be considered while preparing thin film devices.

Effect of substituting the ionic radius in the structural and optoelectrical properties of spin-coated thin film synthesized by solvothermal method

High crystalline order, large grains and low defects are important parameters to enhance the structural and optoelectrical properties of a thin film. Here, it was found that these parameters affected by substituting ionic radius in the crystal structure of a thin film. In this report, three ionic radii of Cr2+, Cr3+ and Zn2+ ions were compared to get the optimum structural and optoelectrical properties represented in the CZTS, CCr2+TS and CCr3+TS thin films. Among the highest properties are the crystalline order, absorption coefficient, refractive index, dielectric constants and polarizability to the light. So, the authors believe that the CCr2+TS thin film can be used as an absorber layer instead of CZTS thin film for thin-film solar cell application.

Intercalation of Dyes in Graphene Oxide Thin Films and Membranes.

Intercalation of dyes into thin multilayered graphene oxide (GO) films was studied by neutron reflectivity and X-ray diffraction. Methylene blue (MB) penetrates the interlayer space of GO in ethanol solution and remains intercalated after the solvent evaporation, as revealed by the expansion of the interlayer lattice and change in chemical composition. The sorption of MB by thin GO films is found to be significantly stronger compared to the sorption of Crystal violet (CV) and Rose bengal (RB). This effect is attributed to the difference in the geometrical shape of planar MB and essentially nonflat CV and RB molecules. Graphite oxides and restacked GO films are found to exhibit different methylene blue (MB) sorptions. MB sorption by precursor graphite oxide and thin spin-coated films of GO is significantly stronger compared to freestanding micrometer-thick membranes prepared by vacuum filtration. Nevertheless, the sorption capacity of GO membranes is sufficient to remove a significant part of the MB from diluted solutions tested for permeation in several earlier studies. High sorption capacity results in strong modification of the GO structure, which is likely to affect permeation properties of GO membranes. Therefore, MB is not suitable for testing size exclusion effects in the permeation of GO membranes. It is not only hydration or solvation diameter but also the exact geometrical shape of molecules that needs to be taken into account considering size effects for penetration of molecules between GO layers in membrane applications.

Influence of concentration and annealing temperature on spin-coated metal oxide thin films for optoelectronic devices

The metal oxide thin films, such as cadmium oxide (CdO) and zinc oxide (ZnO) thin films, were deposited by sol–gel-derived spin coating on glass substrates. The precursor concentration and post-deposition annealing were varied as parameters in the preparation. The prepared films were analyzed from the characterization techniques, such as X-ray diffraction (XRD), UV–visible spectrophotometer, photoluminescence (PL), and Hall Effect measurements. The XRD patterns showed cubic and hexagonal phase for CdO and ZnO, respectively. The improvement of the crystallinity of the films was observed for the variation of precursor concentration and annealing. The refractive index, extinction coefficient, and optical conductivity were calculated from the optical analysis. The bandgap of the CdO and ZnO films have been changed marginally with concentration and annealing. Both the metal oxide films showed band edge emission from PL study and its intensity increased with annealing. The higher crystallite size of film showed the lower electrical resistivity of CdO and ZnO thin films. The CdO and ZnO films have found to be higher crystallite size, absorbance, and carrier concentration. The results obtained from the analysis was suitable for optoelectronic devices.

Structural, optical and Urbach energy properties of ITO/CdS and ITO/ZnO/CdS bi-layer thin films

The thin films of ITO/CdS and ITO/ZnO/CdS bi-layer have been fabricated and studied for structural, morphological, optical, and Urbach energy properties. ZnO thin films were coated onto ITO plated glass slides by a spin-coating method and CdS thin films by electron beam evaporation method. X-Ray Diffraction patterns of both ITO/CdS and ITO/ZnO/CdS thin films confirmed the hexagonal wurtzite structure of ZnO and hexagonal structure of CdS, respectively. AFM images show better uniformity and bigger grain size in the case of OTO/ZnO/CdS samples. Optical transmittance spectra show a slight redshift (~ 493 nm to ~ 505 nm) for the film ITO/ZnO/CdS compared to ITO/CdS. Urbach energy was calculated for all the samples to study the structural imperfections leading to the band tails. A decrease in Urbach energy is observed for ITO/ZnO/CdS compared to ITO/CdS. The obtained results indicate that the ITO/ZnO/CdS thin films are more suitable for optoelectronic devices compared to ITO/CdS films. The results obtained are discussed in detail and presented in this paper.

Detection of Gadolinium with an Impedimetric Platform Based on Gold Electrodes Functionalized by 2-Methylpyridine-Substituted Cyclam.

Gadolinium is extensively used in pharmaceuticals and is very toxic, so its sensitive detection is mandatory. This work presents the elaboration of a gadolinium chemical sensor based on 2-methylpyridine-substituted cyclam thin films, deposited on gold electrodes, using electrochemical impedance spectroscopy (EIS). The 2-methylpyridine-substituted cyclam (bis-N-MPyC) was synthesized in three steps, including the protection of cyclam by the formation of its CH2-bridged aminal derivative; the product was characterized by liquid 1H and 13C NMR spectroscopy. Spin-coated thin films of bis-N-MPyC on gold wafers were characterized by means of infrared spectroscopy in ATR (Attenuated Total Reflectance) mode, contact angle measurements and atomic force microscopy. The impedimetric chemical sensor was studied in the presence of increasing concentrations of lanthanides (Gd3+, Eu3+, Tb3+, Dy3+). Nyquist plots were fitted with an equivalent electrical circuit including two RC circuits in series corresponding to the bis-N-MPyC film and its interface with the electrolyte. The main parameter that varies with gadolinium concentration is the resistance of the film/electrolyte interface (Rp), correlated to the rate of exchange between the proton and the lanthanide ion. Based on this parameter, the detection limit obtained is 35 pM. The bis-N-MPyC modified gold electrode was tested for the detection of gadolinium in spiked diluted negative urine control samples.

Photoinduced Resist-free Imprinting (PRI) in fullerene thin films as revealed by Grazing Incidence Small-angle X-ray scattering

In this article we explore the potential of synchrotron Grazing Incidence Small Angle X-ray Scattering (GISAXS) to evaluate the extension of a Photoinduced Resist-free Imprinting (PRI) effect for grating-like Laser Induced Periodic Surface Structures (LIPSS) on spin-coated thin films of the fullerene derivative [6,6]-phenyl C71-butyric acid methyl ester (PC71BM). Valuable information about the nature of both the original fullerene LIPSS and of the remaining resist-free nanostructure in terms of geometry, size distribution, lattice periodicity and degree of order can be achieved by simulations of the GISAXS patterns. One of the most interesting features provided by the GISAXS analysis is the estimation of the paracrystalline distortion factor, which can be useful to evaluate the level of order in the nanostructure. Data revealed that LIPSS in PC71BM films can be described as one-dimensional paracrystalline lattices with levels of order in accordance with those typically observed in LIPSS of other materials. The PRI effect leaves a qualitatively similar nanostructure but about two times more disordered than the original LIPSS one. Our results show that GISAXS is a powerful tool to characterize Resist-free Imprinting procedures of potential relevance in nanotechnology.

Study on the effect of Ni co-doping on structural, micro-structural and optical properties of transparent AZO thin films

Abstract Optically transparent Zn0.97-xAl0.03NixO thin films were prepared using spin coating technique. The role of Ni concentration on the structural, microstructural, composition and optical properties were studied by means of XRD, SEM-EDS, RAMAN, UV–Visible spectroscopy. The prepared preferred crystalline orientation along the (002) plane (c-axis) with Ni concentration is seen in the prepared films. The findings of FESEM –EDX demonstrates the homogeneous, uniform and smooth texture of the prepared films with strong adherence to the glass substrate and the presence of nominal values of elements. In addition, the findings of Raman also confirm the absence of Ni related impurity phases. Furthermore, Raman peak shift and intensity variations of Raman spectra suggests that the prepared films have a high degree of crystallinity with lower residual stresses. Optical results suggested that spin coated thin films have optical transparency nearly 90% in the visible wavelength region. It was also observed that the energy bang-gap decreases with the concentration of Ni ions in the host ZnO matrix. Results examined indicate that spin-coated thin films are possible candidates for TCO applications.

Approaching Crystal Structure and High Electron Mobility in Conjugated Polymer Crystals.

Conjugated polymers usually form crystallized and amorphous regions in the solid state simultaneously, making it difficult to accurately determine their precise microstructures. The lack of multiscale microstructures of conjugated polymers limits the fundamental understanding of the structure-property relationships in polymer-based optoelectronic devices. Here, crystals of two typical conjugated polymers based on four-fluorinated benzodifurandione-based oligo(p-phenylene vinylene) (F4 BDOPV) and naphthalenediimide (NDI) motifs, respectively, are obtained by a controlled self-assembly process. The strong diffractivity of the polymer crystals brings an opportunity to determine the crystal structures by combining X-ray techniques and molecular simulations. The precise polymer packing structures are useful as initial models to evaluate the charge transport properties in the ordered and disordered phases. Compared to the spin-coated thin films, the highly oriented polymer chains in crystals endow higher mobilities with a lower hopping energy barrier. Microwire crystal transistors of F4 BDOPV- and NDI-based polymers exhibit high electron mobilities of up to 5.58 and 2.56cm2 V-1 s-1 , respectively, which are among the highest values in polymer crystals. This work presents a simple method to obtain polymer crystals and their precise microstructures, promoting a deep understanding of molecular packing and charge transport for conjugated polymers.

Bias Stress Stability of Solution-Processed Nano Indium Oxide Thin Film Transistor.

In this paper, the effects of annealing temperature and other process parameters on spin-coated indium oxide thin film transistors (In2O3-TFTs) were studied. The research shows that plasma pretreatment of glass substrate can improve the hydrophilicity of glass substrate and stability of the spin-coating process. With Fourier transform infrared (FT-IR) and X-ray diffraction (XRD) analysis, it is found that In2O3 thin films prepared by the spin coating method are amorphous, and have little organic residue when the annealing temperature ranges from 200 to 300 °C. After optimizing process conditions with the spin-coated rotating speed of 4000 rpm and the annealing temperature of 275 °C, the performance of In2O3-TFTs is best (average mobility of 1.288 cm2·V−1·s−1, Ion/Ioff of 5.93 × 106, and SS of 0.84 V·dec−1). Finally, the stability of In2O3-TFTs prepared at different annealing temperatures was analyzed by energy band theory, and we identified that the elimination of residual hydroxyl groups was the key influencing factor. Our results provide a useful reference for high-performance metal oxide semiconductor TFTs prepared by the solution method.

Crystallization of ZnO thin films via thermal dissipation annealing method for high-performance UV photodetector with ultrahigh response speed

ZnO-based ultraviolet (UV) photodetector can be easily fabricated by using sol–gel spin-coating method, however, the crystallization of amorphous state ZnO thin films is necessary to fabricate high performance UV photodetector. Thus, we devised a thermal dissipation annealing (TDA) method in which the heat transfer to the ZnO thin films can be synchronized with the heat release from the substrate. It was found that sol–gel spin-coated ZnO thin films can be crystallized through the mobility difference of ZnO molecules placed at the surface of ZnO thin films. Also, UV photodetector based on ZnO thin films annealed with the TDA method exhibited faster rise and decay time constant (τr = 35 ms and τd = 73 ms, respectively), a higher on/off current ratio, and reproducible photocurrent characteristics compared to that of the ZnO thin films annealed by using furnace and IR lamp. Therefore, these results indicated that the TDA method is a feasible alternative route for the fabrication of ZnO based high performance optoelectronic devices.

Mechanical Behavior of Spin-Coated Sb2S3 light Absorption Thin Films

To measure the mechanical properties of Sb2S3, a two-component compound semiconductor used in the light absorption layer of a solar cell, Sb2S3 thin films were formed on FTO glass using the spin coating method. The spin-coated Sb2S3 thin films were heat-treated at 200 <sup>o</sup>C in an Ar atmosphere for up to 1 hour to form a thin film with continuous crystalline structures. A nanoindentation system was used to measure the mechanical properties of the spin-coated Sb2S3 thin films, and the phenomena appearing during indentation were analyzed. We used the continuous stiffness measurement (CSM) technique, and Young's modulus and hardness measured with the indentation depth of 250 nm were about 53.1 GPa and 1.43 GPa, respectively. The results were analyzed and compared with literature values, which varied from 40 GPa for the nanowire forms of Sb2S3 to 117 GPa, based upon simulation results. Since there are few studies on the mechanical properties of spin-coated Sb2S3 thin films, the results of this study are worthwhile. Besides, we observed that the Sb2S3 thin film had a little brittleness in the indentation test at higher load, and the microstructure was pushed around the indenter depending on the degree of bonding to the FTO glass substrate. This is a matter to be considered when making flexible devices in the future.

Coherent optical interaction between plasmonic nanoparticles and small organic dye molecules in microcavities

We investigate the lasing performance of different composite gain materials consisting of small organic molecules, gold nanoparticles, and a polymer matrix mixed on a nanoscale within a spin-coated thin film. We experimentally demonstrate that the localized surface plasmon resonances of randomly distributed gold nanoparticles can oscillate in phase with the standing wave of the surrounding microcavity resonator and contribute to a lower lasing threshold.

In Situ Study of Order Formation in Mesoporous Titania Thin Films Templated by a Diblock Copolymer during Slot-Die Printing.

Slot-die printing, a large-scale deposition technique, is applied to fabricate mesoporous titania films. Printing is interesting, for example, for scaling up solar cells where titania films with an interconnected mesoporous network and a large surface-to-volume ratio are desired as photoanodes. A fundamental understanding of the structure evolution during printing is of high significance in tailoring these films. In this work, we provide important insights into the self-assembly of the slot-die-printed titania/polystyrene-block-poly(ethylene oxide) (PS-b-PEO) micelles into ordered hybrid structures in real time via in situ grazing-incidence small-angle X-ray scattering (GISAXS). GISAXS allows for tracking both vertical and lateral structure development of the film formation process. In the hybrid film, a face-centered cubic (FCC) structure is preferentially formed at the interfaces with air and with the substrate, while a defect-rich mixed FCC and body-centered cubic (BCC) structure forms in the bulk. After calcination, the surface and inner morphologies of the obtained nanostructured titania films are compared with the spin-coated analogues. In the printed films, the initially formed nanoscale structure of the hybrid film is preserved, and the resulting mesoporous titania film shows a superior order as compared with the spin-coated thin films which can be beneficial for future applications.

Major role of intrinsic defects in optical and electrical properties adopting a highly defect-controlled photoconductivity in spin-coated α-MoO3 thin films gained after UV illumination

Spin-coated MoO3 thin films were subsequently subjected to later thermal treatment (LTT) at different temperatures. The x-ray diffraction (XRD) results corroborated that the produced films crystallise in their α-phase with layer structure featured by preferential orientations along the (0k0) planes, and it was also revealed that the thermal energy gained by the later heat treatment plays a major role in enhancing crystallinity enlarging crystallite size. The optical spectroscopic analysis showed that in the visible and near-infrared regions, the average transmission of the film samples remarkably increases with increasing the LTT temperature, whereas the films prepared at 625 °C exhibits an average optical transmission of 79.92%. The optical bandgaps of the film samples were calculated to be of comparable values to the bulk one of α-MoO3 when increasing the LTT temperature from 375 to 525 °C, but it was found to be little greater than the bulk value by further increase in the LTT temperature. The DC electrical results revealed that raising the LTT temperature significantly enhances the electrical resistivity of the film samples, chiefly over the low working-temperatures. These results ascertained the realization of more than one conduction mechanism with different activation energies for the same film, and ramarkable upswings in activation energies were observed by increasing the LTT temperature. The photoconductivity (PC) analysis indicated the occurrence of various trapping processes associated with different photoexcitation energies. The PC analysis also corroborated that the highly resistive thin films exhibit much greater sensitivity to UV illumination compared with the remaining films, whereas the film prepared at the LTT temperature 625 °C presents the higher illumination current at the steady state condition exceeding the dark current value by a factor of 66.01.