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A Radiation-Tolerant g-C3N4 Dielectric Insulator: Local Structure Redistribution under γ-Irradiation
(2026-06-05)
Maluangnont, Tosapol
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Chaithaweep, Kanokwan
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Worathat, Supakarn
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Sangtawesin, Tanagorn
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Wasapinyokul, Kamol
Graphitic carbon nitride (g-C<inf>3</inf>N<inf>4</inf>) is a defect-rich polymeric material whose local structure is difficult to resolve using techniques dominated by average structural motifs. Here, pristine and γ-irradiated g-C<inf>3</inf>N<inf>4</inf> samples are examined using solid-state nuclear magnetic resonance (NMR) spectroscopy. Combined 13C, 15N, and 1H NMR reveal decreased terminal −NH<inf>2</inf> species and redistribution of interheptazine −NH– linkages with dose, while the heptazine framework remains intact. X-ray diffraction shows a non-monotonic evolution of stacking coherence, accompanied by IR evidence of C–N linkage modification. Despite these changes, the electrical resistivity remains extremely high at ∼109 Ω·cm and nearly independent of irradiation dose (10–400 kGy), temperature (RT–350 °C), and frequency (104–106 Hz). The dielectric permittivity decreases slightly from ∼7.2 to ∼4.7–5.2, while the dielectric loss tangent remains low (∼0.02). The refractive index similarly decreases from ∼2.6 in pristine g-C<inf>3</inf>N<inf>4</inf> to ∼2.2–2.3 after γ-irradiation. Analysis of the frequency dependence follows the universal dielectric response, consistent with correlated barrier hopping conduction with predominantly three-dimensional charge transport. Together, these observations show that γ-irradiation primarily reorganizes linkage environments without significantly perturbing the π-conjugated heptazine framework that governs charge transport. These characteristics make γ-irradiated g-C<inf>3</inf>N<inf>4</inf> promising for radiation-resistant dielectric and insulating components in nuclear and high-radiation electronic environments.
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Predictive Three-Dimensional CFD Modeling of Evaporation-Coupled PVDF/DMF Spin-Coating on Finite Square Substrates
(2026-01-01)
Vichachai, Ratiwat
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Thongsri, Jatuporn
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Wasapinyokul, Kamol
Predictive thickness control remains challenging for spin-coated polymer films on finite, noncircular substrates, particularly with solvent evaporation. This study presents a three-dimensional computational fluid dynamics framework to simulate poly(vinylidene fluoride) dissolved in dimethylformamide during spin-coating on a finite square substrate. The model resolved free-surface hydrodynamics, interfacial solvent evaporation, species transport, and concentration-dependent viscosity to capture evaporation-coupled thinning dynamics. The simulation results were corrected with a dry-film equation based on mass conservation to obtain the film profiles. Two simulated cases—nonevaporating and evaporating—were investigated for three parameters—spin-coating time, rotational speed, and solution concentration. The nonevaporating model was validated against the one-dimensional analytical solution, while the evaporating model was validated against experimental values. Both the simulated and experimental thicknesses exhibited good quantitative agreement, confirming that the model accurately captured the film formation mechanisms. Remaining discrepancies were attributed to liquid retention at the substrate edge, uncertainty in initial dispensed volume, and numerical resolution effects near the interfacial regions. The model revealed that evaporation-induced viscosity evolution significantly modified thinning behavior, particularly at low speeds and high concentrations, and amplified edge accumulation on finite substrates. This work establishes a predictive framework for thickness and profile control in spin-coated polymer films.
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Size-dependent and spatial variations in the structural properties of spin-coated poly(Vinylidene Fluoride) films
(2025-10-01)
Sukjit, Peemases
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Munpiriyakul, Pimpaporn
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Tuantranont, Adisorn
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Lomas, Tanom
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Borthai, Pawantree
The uniformity of structural properties in large spin-coated poly(vinylidene fluoride), or PVDF, films is crucial due to their widespread applications and thus requires careful investigations. In this study, variations in structural properties across PVDF films of different sizes and positions were examined. Thin PVDF films were fabricated by the spin-coating method onto rectangular substrates with lengths varying from 10 mm to 40 mm while maintaining a constant width of 10 mm. Three key characteristics – thickness, phase, and crystallinity – were characterized and analyzed. Two effects were investigated – the size-dependent effects, i.e., the property variations at a specific position due to the increasing film length, and the spatial effects, i.e., the property variations along distances from the film center within a film of a specific size. The average thickness of the fabricated film was in the range of 5.00 μm to 6.00 μm, while the crystalline size was in the range of 1.00 nm to 3.00 nm. For the size-dependent effects, at a specific position on the film, increasing the film length did not significantly affect the thickness; however, the phase of PVDF shifted toward a more chain-like β phase, while the crystalline size decreased. At the film center, the crystalline size decreased by 60.7% when the film size increased from the smallest to the largest. These changes resulted from four combined mechanisms: centripetal force, viscosity, evaporation rate, and shear force. For the spatial effects, in a film with a specific size, when the distance from the center increased, the thickness decreased, the phase remained β, and the crystalline size was smaller. For the largest 40 × 10 mm2 film, the thickness and crystalline size decreased by 16.7% and 3.6%, respectively, from the center to the edge of the film. These trends were attributed to the three combined mechanisms: centripetal force, viscosity, and evaporation rate. The findings of this study provide critical insights into a deeper understanding of property variations in spin-coated PVDF films among both different film sizes and different positions on a film, which is essential for optimizing their applications.
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Non-monotonic evolution of the responses of ZnO-nanoparticle UV-sensitive devices under ambient aging
(2023-08-01)
Sratongkham, Pittayathorn
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Chuenchom, Rattana
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Tuantranont, Adisorn
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Lomas, Tanom
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Wasapinyokul, Kamol
Ultraviolet (UV) sensitive devices based on spin-coated zinc-oxide nanoparticles (ZnO) were fabricated and characterised. They were subsequently stored in the ambient and dark conditions with low humidity for 56 days, during which time they were intermittently recharacterised to determine the aging effect on their sensing properties, including responsivity, sensitivity, and response and recovery times. Over the 56-day period, both the dark and illumination currents increased, causing the non-monotonic evolution of the performances of the devices – the responsivity improved by 9–18 folds, the sensitivity remained stable, and the response and recovery times deteriorated as they were 46 and 33 times longer, respectively. These changes were associated with an increase in the number of adsorbed oxygen molecules on the ZnO surface with time. This resulted in more photodesorbed oxygen molecules and thus more remaining charge carriers under illumination, which increased the photo-generated current and consequently responsivity. However, it also caused the current to rise and decay more slowly when the illumination appeared and disappeared, respectively, leading to the prolonged response and recovery times. The longer recovery times led in an increase in dark current, which, when combined with an increase in illumination current, resulted in a stable sensitivity. The trends of these sensing parameters were similar, but to varying degrees, regardless of the change in the radiation levels and ZnO layer thicknesses.
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Electrolytic exfoliation of few-layer graphene/sodium dodecylbenzenesulfonate for coin- and cylindrical-cell supercapacitor electrodes
(2023-06-01)
Lomas, Tanom
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Poochai, Chatwarin
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Sukjit, Peemases
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Wasapinyokul, Kamol
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Mensing, Johannes P.
Electrolytic exfoliation of graphite is a promising way to produce graphene quickly, inexpensively, and in an environmentally friendly manner. In this research, sodium dodecylbenzenesulfonate (SDBS), a commonly used anionic surfactant, was dissolved in 1 M H<inf>2</inf>SO<inf>4</inf> to produce SDBS-graphene via electrolytic graphite exfoliation. The AFM analysis validated the thickness of few-layer of SDBS-graphene between 5 and 15 nm. Then, a symmetric coin-cell (CR2032) supercapacitor (SC) comprised of SDBS-graphene and rGO (synthesized via Hummer's method) was assembled with 0.5 M H<inf>2</inf>SO<inf>4</inf> as an electrolyte. The highly exfoliated SDBS-graphene demonstrated a greater capacitive electrochemical response than rGO. The CV and GCD techniques revealed that the specific capacitance of SDBS-graphene was 150 F g−1 at 0.25 A g−1 with 20.4 Wh kg−1 of energy density and 494 W kg−1 of power density and that its percentage capacitive retention remained 95 % after 10,000 cycles at 3 A g−1, compared to the specific capacitance of reduced graphene. In addition, a cylindrical cell SC (CR32650) with SDBS-graphene demonstrated a capacitance of 220 F at 100 mA, along with an energy density of 33 kWh and a power density of 55 kW. This suggests that exfoliated SDBS-graphene may be utilized in SCs with high efficiency and long-term durability.
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Impressive Response of Spin-Coated ZnO Nanoparticle UV-Sensitive Devices with Various Thicknesses under Different UV Intensities
(2021-09-01)
Pawong, Whongsatorn
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Wasapinyokul, Kamol
We fabricated ultraviolet (UV) detectors based on spin-coated pure zinc oxide (ZnO) nanoparticles with a metal–semiconductor–metal configuration. Devices with various ZnO layer thicknesses were characterized under different UV intensities, and their responsivity, sensitivity, response time, and recovery time analyzed. The following performance was achieved: responsivity of 99.8 A W−1, sensitivity of 531.1, and response and recovery times of 0.01 s and 0.07 s, respectively. Increasing the thickness revealed monotonic effects on each property: the responsivity decreased, the sensitivity decreased, and the response and recovery times increased, mainly because of the thin penetration depth of ZnO and the lengthened cracks on the thicker layer. However, the effects of the UV intensity on the parameters were not monotonic. Indeed, as the intensity was increased, the responsivity decreased, the sensitivity first increased then decreased, the response time first increased before shortening, while the recovery time consistently shortened. Such trends resulted from the combination of several mechanisms: shrinkage of depletion layers, saturation of excitons, and saturation of trapping states. Increasing the radiation-on time shortened both the response and recovery times. This device performance is impressive compared with devices with more complicated material formats, device structure, or fabrication methods. Some complications in the work are also discussed.
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Accuracy of swanepoel method in calculation of polymer film thicknesses
(2021-08-01)
Kesornkhup, Sarunrit
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Tuantranont, Adisorn
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Lomas, Tanom
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Sriprachuabwong, Chakrit
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Wasapinyokul, Kamol
We studied the accuracy of the Swanepoel method in the calculation of thicknesses of spin-coated poly(methyl methacrylate) films with thicknesses up to 2500 nm. Their thicknesses were calculated by using the Swanepoel method and subsequently compared with the measured actual values. Results showed that both thicknesses followed identical trends where films with higher solution concentrations or slower spin-coating speeds were thicker. The relative difference between the Swanepoel and the actual thicknesses was explained through the thickness of the flat region where the interference of transmitted light occurred. For a film whose flat region was thinner or thicker than other features of the film, its Swanepoel thickness was lower or higher, respectively, than the actual average value. Errors of the Swanepoel thicknesses from the actual values were analysed to find their correlation with the film surface roughness and thickness. When the film roughness and thickness increased fivefold, the error increased threefold and 1.6 times, respectively, indicating that the effect of the film roughness was predominant. Mathematically, this effect was the result of the fact that when roughness increased, the interference pattern shrank, and hence the values deviated. For the effect of thickness, thicker films had higher roughness, and consequently higher errors. Errors of as low as 5% and 0.86% were observed for films with the roughness of less than 15 nm and those with the thickness of 1800 nm, respectively. This showed that the method can be used to calculate the thickness of μm-thick polymer films, with a good level of roughness, with satisfying accuracy.
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Mathematical model for thickness of off-center spin-coated polymer films
(2020-02-10)
Wasapinyokul, Kamol
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Panjasamanwong, Tanakrit
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Ponkasemsuk, Worathat
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Sriprachuabwong, Chakrit
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Lomas, Tanom
Off-center spin coating is a method to fabricate thin film on a substrate where the substrate is located at an off-center distance away from the rotating center of the spin coater. Here, a mathematical model to calculate the thickness of a film fabricated by an off-center spin-coating technique was developed and proposed. The model showed that the off-center film thickness was calculable by using four factors—the on-center film thickness, mass fraction of solid in the wet film, length of the substrate in the radial direction, and off-center distance. Simply, the off-center film thickness was inversely proportional to the off-center distance to the exponent of one-third, that is, the further the off-center distance, the thinner the film. The model was verified where the thicknesses of the films calculated by using the model were compared with the experimental values obtained from the off-center spin-coated films of poly(vinylidene fluoride) at various off-center distances. Both the modeled and the experimental data were of the same trend and in a good agreement with each other, indicating the validity of the model. The limitations of the model were also discussed. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48356.
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Optical transmittances of spin-coated polymer films with different spin step protocols
(2019-07-01)
Sukjit, Peemases
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Jaieau, Chatchai
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Wasapinyokul, Kamol
In this report, we studied the effects of different spin-coating step protocol on the optical transmittance of thin film polymer. Thin films of poly(methylmethacrylate), or PMMA, were spin-coated with two different spin step protocols - the speed-up and speed-down protocols. In each protocol, the spin-coating had two steps, the first and the second steps. In the speed-up protocol, the first step was slow, while the second faster step was faster. In the speed-down protocol, the first step was fast, while the second step was slower. All the films were subsequently characterised for their optical transmittance over the visible region. There were at least three points worth explained from the experimental results. Firstly, for the speedup protocol, when the second-step speed was slower than 4000 rpm, the transmittance of the film were varied and did not show any relation with the first-step speed. However, when the second-step speed was faster, all the films, regardless of the first-step speed, provided roughly equal transmittance. Secondly, for the speed-down protocol, when the second-step speed was slower than 3000 rpm, transmittances from all films were varied. But when the second-step speed was faster, all the films from all first-step speed gave roughly equal transmittance. As the transmittance is related to the film thickness, these results indicated that, regardless of what the protocol was, if the second-step speed was fast, the film thickness from any first-step speeds would be roughly equal to each other. Finally, for the same pair of speeds but with different protocols, a film with a speed-down protocol provided a relatively higher transmittance than one from a speed-up protocol, implying a thinner film. The results from this study could be further developed to understand how the spin-speed protocol could be chosen to provide a thin film with required thickness.
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A study of optical properties of thin film polymer spin-coated with different dispensing methods
(2019-07-01)
Phatcharawutikul, Ronakrit
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Tangtrakul, Kitbordee
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Wasapinyokul, Kamol
Thin films of poly(methyl methacrylate), PMMA, were fabricated by using a spin-coating process with two different dispensing methods: static and dynamic ones. In the former and latter methods, the solution was dropped onto the substrate when the substrate was stationary and spinning, respectively. The films were characterised for their optical transmittance. The results show that the films from the dynamic dispensing method would provide higher optical transmittances than those from the static ones, indicating the thinner films. This could be explained as, in the dynamic method, the film would always be in a dynamic state and therefore could not form the film easily, resulting in a thinner film. Another point worth noting is that, in the dynamic method, the dispensing height affected the film's transmittance. The higher the dispensing height, the higher the transmittance. This indicated a thinner film and could be explained that, in the dynamic method, a higher dispensing height would result in a larger proportion of the liquid drop to bounce off the substrate, hence less amount of liquid remained to form the film, leading to a thinner film. Such characteristic did not appear in the static method because in this method, the liquid was dropped carefully and covered all the substrate before the spinning started. The results from this study could help choosing the dispensing method to provide film with required optical properties.

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