Microfibrous Thin Films Research Articles

Luminescent solar concentrator containing Parylene-C microfibrous thin film infiltrated with Lumogen F Red 305

Abstract Luminescent solar concentrators (LSCs) were made by the infiltration of microfibrous thin films (µFTFs) of Parylene C by Lumogen F Red 305 (LFR305), in order to maximize the concentration of light made available to a photovoltaic solar cell (PVSC). The Parylene-C µFTFs with either tilted columnar or chevronic morphology were fabricated using physicochemical vapor deposition and infiltrated with LFR305 using thermal evaporation. Application of a voltage across a photoresistor illuminated by a solar simulator (AM1.5) through an LFR305-infiltrated Parylene-C LSC resulted in an enhancement of the ON-OFF minimum current ratio by a factor of 1.5 ± 0.29 compared to the ratio before LFR305 infiltration/deposition, regardless of morphology; furthermore, LFR305 infiltration enhanced the ON-OFF minimum current ratio by 56.5 ± 21.5 %, depending on the morphology. The LSC concentration factor was determined to be 8.7 ± 3.4 after integration with a monocrystalline-silicon solar cell, depending on the morphology.

Long‐wavelength infrared characteristics of multifunctional microfibrous thin films of Parylene C

AbstractCalculations show that structurally chiral, three‐dimensionally periodic, microfibrous thin films (μFTFs) of Parylene‐C can function as both polarization‐insensitive and circular‐polarization‐selective photonic crystals in the long‐wavelength infrared regime. Careful engineering is required for bandstop filters in order to avoid the numerous absorption peaks of bulk Parylene‐C. The photonic functionality of the Parylene‐C μFTFs is in addition to their phononic, hydrophobic, and biomedical functionalities.

Effects of constant-voltage stress on the stability of Parylene-C columnar microfibrous thin films

A systematic study of the degradation caused by electrical stress in Parylene-C columnar microfibrous thin films (μFTFs) was carried out to explore their suitability for applications in flexible electronics. The columnar μFTFs were fabricated by directing a collimated flux of Parylene-C monomers normally toward a planar substrate in a low-vacuum chamber. Metal-insulator-metal (MIM) capacitors incorporating Parylene-C columnar μFTFs were fabricated and electrically characterized to examine reliability and electrical-stress resistance. The measured leakage current in and capacitance of MIMs were studied as functions of constant-voltage stress (CVS) applied to the MIMs. Room-temperature leakage current in the MIMs before and after CVS was found to be space-charge limited. The space charge comprises defects introduced during fabrication, no new defects being induced by the CVS. Kohlrausch-Williams-Watts relaxation was exploited to understand transient leakage-current behavior, and characteristic times in the 3.5–3.9 s range and stretch factors in the 4.2–5.2 range were determined. These parameters suggest that carrier trapping at defects and their polarization orientation are related to space-charge formation. Furthermore, capacitance dependences on time and frequency were found to be good indicators of the CVS-induced degradation and stability. Charge buildup in the Parylene-C columnar μ-FTFs is accompanied by capacitance decrease with CVS duration. Extrapolation of the capacitance-decrease dependence on CVS duration enables predictions of reliability over extended periods.

Optical oxygen sensors based on microfibers formed from fluorinated copolymers

Polymeric microfibers, particularly the fluorinated copolymers’ fibers, are promising functional materials for photoelectric devices, sensing, and energy storage due to their various surface characteristics. However, to obtain the high fluorine content copolymer-based fibers with considerable uniformity is likely to be especially challenging, which seriously debilitates their applications in sensor devices. Herein, for the first time, we presented a high fluorine content platinum porphyrin-grafted poly(isobutyl methacrylate-co-dodecafluoroheptyl methacrylate) copolymers (PtTFPP-p(IBM-co-DFHMA)) microfibrous thin-films used as optical oxygen sensors. The porous thin-film frameworks were formed of uniform microfibers, which afforded an exceptional improvement in sensitivity and exhibited 584% higher sensitivity than the solid sensing film owing to the large specific surface area, porous structures and oxygen diffusion enhancement by fluorine elements. Additionally, the remarkable emission intensity-changing characteristic of the microfiber sensing film under various air pressures facilitates convenient visualization of pressure distributions on film surface. The characteristics are particularly important for the computational fluid dynamics simulations in various sensing fields such as unsteady flow visualization and unsteady pressure measurements, etc. Owing to its attractive advantages and versatile performance, fluorine-containing copolymers fibers are expected to provide a new strategy for the rational design of high performance gas sensor devices.

Selectablity of mechanical and dielectric properties of Parylene-C columnar microfibrous thin films by varying deposition angle

Columnar microfibrous thin films (μFTFs) were fabricated with a collimated flux of Parylene-C monomers directed at an angle with respect to the substrate plane in a vacuum chamber. The porosity of the columnar μFTFs was found to decrease with the deposition angle and lie between 0.38 and Both the static Young’s modulus and the yield strength of the columnar μFTFs were found to be higher in the morphologically significant plane than in the plane normal to it. In both loading directions, static Young’s moduli and yield strengths are about two orders of magnitude lower for the columnar μFTFs than the corresponding parameters of the bulk material, making the columnar μFTFs softer. The lowest relative permittivity of the fabricated columnar μFTFs in the 1–1000 kHz range was found to be about 70% of that of the bulk material. The static Young’s moduli, yield strengths, and the relative permittivity were correlated to the porosity, crystallinity, and the deposition angle. Both mechanical and dielectric properties of the columnar μFTFs can be controlled by selecting appropriately for application in flexible electronics.

Dielectric Properties of and Charge Transport in Columnar Microfibrous Thin Films of Parylene C

Parylene-C microfibrous thin films ( $\mu $ FTFs), grown using oblique-angle physicochemical vapor deposition, were examined for potential use as low-relative-permittivity (i.e., low- $\kappa $ ) interlayer dielectrics in integrated circuits and, more importantly, flexible electronics. These films were characterized using capacitance–voltage–temperature (CVT) and current–voltage–temperature experiments at different temperatures (ranging from 298 to 420 K) and frequencies (ranging from 1 kHz to 1 MHz). Field emission scanning electron microscopy revealed the Parylene C $\mu $ FTFs to be highly porous. Consequently, their $\kappa $ values are at least 20% lower than those of bulk Parylene C. The dependences of $\kappa $ on frequency and temperature suggest that molecular dipole oscillations are responsible for charge polarization in the $\mu $ FTFs. The dc leakage current in the $\mu $ FTFs at temperatures not exceeding 100 °C (~373 K) was found to arise from Poole–Frenkel (PF) emissionmechanism with a barrier energy of about 0.77 eV. Moreover, when fitted to the PF model, the experimental data yielded high-frequency values $\kappa _{\infty }$ of $\kappa $ in agreement with those obtained from CVT experiments, thus confirming our identification of PF as themajor responsiblemechanism and confirming the low- $\kappa $ characteristic of microfibrous Parylene C. The ac current transport in the $\mu $ FTFs was found attributable to small-polaron-tunneling hopping conduction and characterized by the power law $ \omega ^{s}$ , with $s\in [{0.082,0.85}]$ increasing with temperature. AC conduction in the $\mu $ FTFs is temperature-activated with an activation energy that decreases from 0.020 to 0.012 eV as frequency increases from 1 kHz to 1 MHz.

Parylene-C microfibrous thin films as phononic crystals

Phononic bandgaps of Parylene-C microfibrous thin films (s) were computationally determined by treating them as phononic crystals comprising identical microfibers arranged either on a square or a hexagonal lattice. The microfibers could be columnar, chevronic, or helical in shape, and the host medium could be either water or air. All bandgaps were observed to lie in the 0.01–162.9-MHz regime, for microfibers of realistically chosen dimensions. The upper limit of the frequency of bandgaps was the highest for the columnar and the lowest for the chiral . More bandgaps exist when the host medium is water than air. Complete bandgaps were observed for the columnar with microfibers arranged on a hexagonal lattice in air, the chevronic with microfibers arranged on a square lattice in water, and the chiral with microfibers arranged on a hexagonal lattice in either air or water. The softness of the Parylene-C s makes them mechanically tunable, and their bandgaps can be exploited in multiband ultrasonic filters.

Studies of Parylene C Microfibrous Thin Films Electrical Properties

Parylene C is known for its flexibility, transparency, biocompatibility, and dielectric reliability. Parylene C is, hence, researched for use as a gate dielectric in transistors and capacitors for flexible electronics as well as for nano/microelectromechanical system applications [1-3]. In this study, we use metal–insulator–metal (MIM) capacitor structures to examine the electrical characteristics of columnar microfibrous thin films (µFTFs) of Parylene C prepared using oblique-angle physicochemical deposition [4,5], whereby a collimated vapor Parylene C is directed at an angle χv with respect to the exposed face of a planar substrate. The values of χv used were 30°, 60°, and 90°. We found that the low-frequency relative permittivity of the columnar µFTF increases with χv. This is explained in terms of the porosity as a function of χv, the porosity being higher for lower χv. Leakage current and charge deep-level transient spectroscopy (Q-DLTS) measurements at temperatures in the 230–470 K range and at frequencies in the 1–1000 kHz range were used to evaluate the dielectric integrity of the columnar µFTFs. Observed capacitance transients and Q-DLTS peaks in temperature were analyzed to uncover carrier traps located in the Parylene C bulk and at the metal/Parylene C interfaces in the MIM structures. The variations of DC and AC conductivities can be explained in terms of charge transport mechanisms involving thermionic emission and electron hopping. W. T. Wondmagegn, N. T. Satyala, I. Mejia-Silva, D. Mao, S. Gowrisanker, H. N. Alshareef, H. J. Stiegler, M. A. Quevedo–Lopez, and B. E. Gnade, Thin Solid Films, 519(13), 4313–4318 (2011).T. Trantidou, D. J. Payne, V. Tsiligkiridis, Y.-C., Chang, C. Toumazou, and T. Prodromakis, Sensors and Actuators B: Chemical, 186(1), 1–8 (2013).B. Park B., K-J. Im, K. Cho, and S. Kim, Organic Electronics, 9(5), 878–882(2008).C. Chindam, N. M. Wonderling, A. Lakhtakia, O. O. Awadelkarim, and W. Orfali, Applied Surface Science, 345(1), 145–155 (2015).Khawaji, I. H., Chindam, C., Orfali, W., Awadelkarim, O. O., Lakhtakia, A. ECS Transactions, 69(5), 113-119 (2015).

Studies of Parylene C Microfibrous Thin Films Electrical Properties

We have fabricated and measured the room-temperature capacitance in metal-insulator-metal (MIM) structures utilizing the Parylene C films as the insulators in the frequency range 1 kHz to 1 MHz. The Parylene-C films were of variable thicknesses and were either spin-on dense films or columnar microfibrous thin films (μFTFs) grown using oblique angle physicochemical vapor deposition. The values of the vapor tilt angle, χv, used were 30°, 60°, or 90°. Expectedly, columnar microfibrous thin film of Parylene C is observed to have lower values of k than the dense one. However, it is observed that the dielectric constant of the μFTFs is increased with increasing deposition tilt angle χv. This is explained in terms of film porosity as a function of χv, which is enhanced in films deposited at lower tilt angles. The dependence of k on measurement frequency suggests that molecular dipole effects dominate charge polarization in the films.

Temperature-dependent dynamic moduli of Parylene-C columnar microfibrous thin films

Columnar microfibrous thin films (μFTFs) of the polymer Parylene C with different microfiber inclination angles were fabricated using a physicochemical vapor deposition process whereby a collimated flux of hot monomers is obliquely directed towards a planar substrate. Each μFTF was then subjected to cyclic elastic loading in one of two orthogonal directions lying wholly in the substrate plane: either (i) normal or (ii) parallel to the morphologically significant plane of the μFTF. Dynamic storage and loss moduli were determined in the 1–80 Hz frequency range for temperatures between −40 °C and 125 °C. For all columnar μFTFs, the storage and loss moduli for normal loading did not exceed their counterparts for parallel loading. All columnar μFTFs were found to be softer than a bulk film of Parylene C. In both bulk and columnar forms, Parylene C was found to be rheologically not simple.

Electrical Studies on Parylene-C Columnar Microfibrous Thin Films

Columnar microfibrous thin films (μFTFs) of Parylene C are deposited on top of p-type Si substrate using physicochemical vapor deposition to form metal-insulator-semiconductor and metal-insulator-metal structures utilizing the Parylene-C μFTFs as the gate dielectric. The columnar μFTFs were characterized for their electrical properties and dielectric integrity using leakage-current and capacitance measurements at temperatures in the 200K - 500 K range and at frequencies in the range 1 kHz to 1 MHz. Observed variations in leakage current and capacitance were found to be consistent with transport mechanisms involving the Poole–Frenkel and Schottky emission mechanisms as well as carrier trapping/detrapping at interfacial traps.

Electrical Studies on Parylene-C Columnar Microfibrous Thin Films

Future advances in solid-state device technology are dependent on innovations in multifunctional materials and novel deposition processes and, hence, a wide spectrum of new material compositions and morphologies are being explored. For this study, microfibrous thin films (µFTFs) of Parylene C [1,2] were deposited on top of p-type Si substrates using physicochemical vapor deposition in a PDS2010 Labcoater to form metal-insulator–semiconductor (MIS) and metal–insulator–metal (MIM) capacitor structures utilizing the Parylene-C µFTF as the gate dielectric. Each µFTF was deposited with a collimated vapor flux of Parylene-C monomers inclined at 60° to the substrate plane and had a thickness of about 100 µm. The Parylene-C µFTFs were characterized for electrical properties and dielectric integrity using leakage current, equivalent series resistance and reactance, capacitance measurements at temperatures ranging from –40 °C to 125 °C and frequencies ranging from 1 kHz to 1 MHz, as well as field-emission scanning electron microscopy. The observed variations in leakage current can be explained in terms of transport mechanisms involving thermionic emission and electron hopping. Capacitance and dielectric loss data were analyzed. We found that higher leakage current does not allow for a reliable estimation of the dielectric constant. [1] Pursel, S., Horn, M. W., Demirel, M. C., & Lakhtakia, A. (2005). Growth of sculptured polymer submicronwire assemblies by vapor deposition, Polymer, 46(23), 9544–9548. [2] Chindam, C., Wonderling, N. M., Lakhtakia, A., Awadelkarim, O. O., & Orfali, W. (2015). Microfiber inclination, crystallinity, and water wettability of microfibrous thin-film substrates of Parylene C in relation to the direction of the monomer vapor during fabrication, Applied Surface Science, 345(1), 145–155.

The Effects of Morphology on the Dielectric and Mechanical Properties of Parylene-C Microfibrous Thin Films

Silicon-based CMOS material set is rather restrictive for the realization of the full potential of NEMS/MEMS and, hence, a wide spectrum of new material compositions and morphologies are being explored nowadays. Because of its extensive applications in electronic and medical devices as well as because of its chemical inertness and thermal stability, a polymer called Parylene C was used to grow microfibrous thin films using physicochemical vapor deposition in a PDS2010 Parylene-C labcoater on p-type Si or brass substrates. The thin films are assemblies of parallel microfibers tilted at an angle χv with respect to the substrate plane. During deposition, a collimated vapor flux of Parylene C was directed at an angle χv with respect to the same plane. Values of χv used were 10°, 25°, 45°, 60°, and 90°. Both χ and the mass density increase with increasing χv. Using Parylene-C microfibrous thin films as the dielectric layers in metal-insulator–semiconductor (MIS) and metal–insulator–metal (MIM) capacitor structures, we were able to study their dielectric properties. The dependence of the “effective” relative permittivity eeff on χv was observed to be controlled by the amplitude of the applied AC signal, and eeff was found to decrease with increasing signal frequency. The MIS and MIM leakage current was used to study the dependence of hopping conduction on both the frequency and the temperature. These observations were interpreted relative to series/parallel networks of miniature resistors and capacitors. Also, we undertook a dynamic mechanical analysis, whereby a Parylene-C microfibrous thin film was clamped at two ends and subjected to cyclic loading. The storage and the loss moduli were measured as functions of frequency, in the frequency range 5 to 200 Hz, and temperatures up to 125 °C. The glass transition temperature was identified as 65 °C from the temperature maximum of the loss modulus.

Frequency- and temperature-dependent storage and loss moduli of microfibrous thin films of Parylene C

Elastic cyclic loading of ~110µm-thick Parylene C thin films comprising parallel slanted microfibers was performed in the frequency range 5–200Hz, to determine the dependences of their storage and loss moduli on the temperature and the frequency. The glass-transition temperature was identified as about 65°C from the temperature-maximum of the loss modulus. A low-frequency peak (at 115–135Hz) in the variation of the loss modulus is due to the resonance of an individual microfiber loaded by the remaining microfibers in the film.