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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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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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Symmetrical poly MUMPs-based piezoresistive microcantilever sensors with on-chip temperature compensation for microfluidics applications
(2008-05-01)
Tuantranont, Adisorn
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Lomas, Tanom
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Jaruwongrungsee, Kata
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Jomphoak, Apichai
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Wisitsoraat, Anurat
Microelectromechanical systems (MEMS)-based cantilever beam sensors for microfluidics applications with on-chip temperature sensors for temperature drift compensation were developed. The stress induced on gold surface with polysilicon piezoresistive sensing is demonstrated. In principle, adsorption of biochemical species on a functionalized surface of the microfabricated cantilever will cause surface stress and, consequently, cantilever bending. The sensing mechanism relies on the piezoresistive properties of the doped polysilicon wire encapsulated in the beam. The beam is constructed through multiusers MEMS Process (PolyMUMPs) foundry with postprocessing silicon etching. Bending analysis is performed so that the beam tip deflection can be predicted. The piezoresistor designs on the beams were varied, within certain constraints, so that the sensitivity of the sensing technique could be measured by external read-out circuit. The mass detection of 0.0058-0.0110 g is measured by the beam resistor series as a balanced Wheatstone bridge configuration. The voltage output of the bridge is directly proportional to the amount of bending in the MEMS cantilever. The temperature dependency and sensor performance have been characterized in experiments. Compensation by resisters on the substrate significantly reduces the temperature dependence. © 2008 IEEE.
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Simulation and experiment of microheater for a microfluidic polymerase chain reaction device
(2004-12-01)
Klaitabtim, Don
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Lomas, Tanom
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Wisitsora-At, Anurat
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Tuantranont, Adisorn
In this paper, we describe the simulation and experimental results of electroplated microheater for microfluidic-base polymerase chain reaction (PCR) device. This work used finite element analysis to simulate the temperature characteristics of a PCR device. The device consists of a reaction chamber using polydimethysiloxane (PDMS) with encapsulated micrhearter made of a nickel resistor wire. Design of the microheater mask layout using Tanner L-edit. Microheater can be made by low cost electroplating process. The temperature in the experiment was shown heating and cooling rates were 0.9°C/s and 0.8°C/s respectively. The thermal distribution of the device was also studied through finite elements modeling then compare to the experimental results. © 2004IEEE.
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Micromachined piezoresistive tactile sensor array fabricated by bulk-etched MUMPs process
(2003-07-14)
Lomas, Tanom
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Tuantranont, Adisorn
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Cheevasuvit, Fusak
This paper discusses the design, fabrication and testing of a 5×5 micromachined tactile sensor array for the detection of an extremely small force (micrometer-Newton range). Central contacting pads that are trampoline-shape suspended structures and sensor beams are formed using an anisotropic etching of silicon substrate of a MUMPs process chip. A piezoresistive layer of polysilicon embedded in sensor beams is used to detect the displacement of the suspended contacting pad. Each square tactile has dimension of 200 μm × 200 μm with 250 μm center-to-center spacing. The entire sensor area is 1.25 mm × 1.25 mm. The device was characterized under various normal force loads using weight microneedles. The individual sensor element shows the linear response to normal force with good repeatability.

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