Conductivity Of Thin Films Research Articles

Carbon nanofiber-reinforced Pt thin film-based airflow sensor for respiratory monitoring

Recognizing abnormal respiratory patterns is vital for detecting emergency signs or symptoms for diagnosing disease and dysfunction. This paper suggests design and fabrication of a novel airflow sensor based on platinum (Pt) thin films reinforced by carbon nanofibers (CNFs). The conductive Pt thin films were supported by polydimethylsiloxane (PDMS), which endows the sensors with great bendability and ultrahigh sensitivity. CNFs bridge and deflect the microcracks formed in Pt thin film when subjected to external stress, resulting in increased piezoresistive properties. The Pi-shaped air flow sensor was subjected to various airflow rates to study the sensor performance including sensitivity, response time, and recovery time. The results indicate the sensor possesses high sensitivity (27.6 mV (m/s)-1) and low response time (> 0.6 s) with a low-velocity threshold of 15 L min-1 or 0.83 m/s. A finite-element model was developed in COMSOL Multiphysics package to study fluid-solid interactions and piezoresistive effects of the Pt-CNFs/PDMS nanocomposite. Direct measurements of sensor tip displacement were also quantified using single-point laser Doppler vibrometry, which was compared against the numerical simulations. The calibration plot and the numerical results are in good agreement. When compared with previous studies, our airflow sensor showed superior sensing performance in terms of their sensor length, sensitivity, and velocity threshold under various experimental conditions. As a proof of concept, we tested the airflow sensor for monitoring a human respiratory pattern for two extreme conditions, sitting and running.

Scalable areal capacity of SbSxCy+z micro-thin-film cathodes for lithium-metal polysulfide batteries

The wearable electronics and nanorobotics demand high energy density devices such as thin-film solid-state lithium-sulfur (Li-S) or lithium-metal polysulfide (Li-MSx) batteries on a small footprint area, which requires the adoption of modern thin-film fabrication technologies. The fabrication of efficient sulfur based thin film cathodes with a stable cycling performance for Li-MSx batteries is a serious challenge because of the insulating nature and low meting point of sulfur. Herein, we have developed highly conductive thin film cathodes of amorphous SbSxCy+z with chemically bonded sulfur by adopting magnetron sputtering, and the cycling stability was enhanced by fluorinated ethylene carbonate (FEC) as an electrolyte additive. The thin film metal polysulfide cathode is used directly to build Li-MSx battery with FEC modified electrolyte, which exhibits an excellent cyclic stability, commendable rate capability, and high areal capacity of 250 μAh/cm2 for at least 150 cycles. This study offers an opportunity to fabricate shuttle-effect free robust Li-MSx batteries with stable cycling performance.

Multipurpose nanocomposite resist for free-standing transparent conductive thin films

Nanocomposites formed by silver nanowires (AgNWs) embedded in a polymer matrix are a convenient way to deposit thin films with electrical conductance and high transparency on different substrates. Nanocomposite resists containing AgNWs in a poly(methyl methacrylate) solution can be effectively used to produce conductive coatings in a straightforward manner. Here, we show that by adding a sacrificial layer of polyvinylpyrrolidone on a glass substrate, prior to the nanocomposite resist, it is possible to obtain large-area free-standing films of about 450 nm with electrical conductance and high transparency. The films can be transferred to different surfaces and materials including non-flat substrates. The formation of conductive stacks by piling two layers was also demonstrated. The optical, electrical, and structural properties of these free-standing films were studied obtaining films with transmittance T(%) = 78% at 550 nm, sheet resistance Rs = (670 ± 40) Ω sq−1 and surface roughness Ra = (50 ± 10) nm. We studied the strain resistance behavior of films transferred to polyethylene terephthalate sheets under bending tests finding a sensitivity of (0.51 ± 0.01) Ω deg−1 and a gradual increase in the resistance during cycling. In addition, thin flexible supports can be added by covering the nanocomposite film with polydimethylsiloxane (PDMS) prior to its release, enhancing the mechanical robustness and improving the manipulation of the films.

Fabrication of graphene, graphite and multi wall carbon nano tube based thin films and their potential application as strain sensor

Carbon based nanomaterials are extensively used in the manufacturing of flexible electronic devices due to their numerous favorable properties. Such devices have shown remarkable sensitivity with excellent stability when implemented in various flexible environments. But the formulation of conductive inks using carbon based nanomaterials presented some challenges in the optimization of various ink parameters like rheology, surface tension, particle size etc. In this paper, we present graphite, graphene and multi-wall carbon nano tube (MWCNT) based flexible resistive strain sensors on polymer substrate using a simple and solvent free fabrication process. A controllable conductive thin film is obtained and designed into two different patterns viz. ‘Straight Line’ (line) and ‘Serpentine’ (serp) to enhance the sensing performance of the film. The electrical performance has been evaluated in terms of current-voltage characteristics. The result indicates the resistive behavior of all the devices under investigation. The bending measurements have been conducted; the resistive strain sensor exhibits significant resistance variation during bending applications. There has been a significant change in the electrical resistance of each sample for different strain level i.e. ∼ 20% for MWCNT(line) and MWCNT (serp), ∼30% for Graphite(line) and Graphene (line) and ∼50% to 60% for Graphite (serp) and Graphene (serp). High sensitivity of the ‘Serpentine’ patterns in comparison to ‘Straight Line’ patterns indicates the importance of various patterns in the manufacturing of flexible sensors. Surface morphology and structural information of the fabricated devices have also been analyzed and investigated.

PLD of transparent and conductive AZO thin films

The pulsed laser deposition (PLD) of aluminium (2 wt %) doped with zinc oxide (98 wt %) (AZO) on glass substrates showed structures and morphology in the nanoscale. The deposition of the AZO thin films was completed at chosen substrate temperatures and low partial pressure of 20 mTorr. The laser energy density fixed at 4 J/cm2 appeared suitable for the ablation of bulk AZO and reduced the probable arrival of droplets onto the substrates. Moreover, the AZO films were deposited at a target-to-substrate distance of 5.78 cm, and the calculated crystallite size ranged from 31.1 to 45.4 nm. The lowest film resistivity of 6.8 × 10−5 Ω cm at an average optical transmittance of 83 % in the UV–vis–NIR region confirms the effect and relevance of the preparation procedure. These observations confirm the AZO thin film crystallinity with an average bandgap energy of 3.36 eV extrapolated relative to the photon energy. Thus, the PLD technique ensures a more significant avenue toward the growth of crack-free and reliable thin films with improved electrical and optical properties.

Eutectic CsHSO4-Coordination Polymer Glasses with Superprotonic Conductivity.

Superprotonic phase transition in CsHSO4 allows fast protonic conduction, but only at temperatures above the transition temperature of 141 °C (Tc). Here, we preserve the superprotonic conductivity of CsHSO4 by forming a binary CsHSO4-coordination polymer glass system, showing eutectic melting. Their anhydrous proton conductivities below Tc are at least 3 orders of magnitude higher than CsHSO4 without compromising conductivity at higher temperatures or the need for humidification, reaching 6.3 mS cm-1 at 180 °C. The glass also introduces processability to the conductor, as its viscosity below 103 Pa·s can be achieved at 65 °C. Solid-state NMR and X-ray pair distribution functions reveal the oxyanion exchanges and the origin of the preserved conductivity. Finally, we demonstrate the preparation of a micrometer-scale thin-film proton conductor showing low resistivity with high transparency (transmittance >85% between 380-800 nm).

The Growth Mechanism of a Conductive MOF Thin Film in Spray‐based Layer‐by‐layer Liquid Phase Epitaxy

AbstractThe layer‐by‐layer liquid‐phase epitaxy (LBL‐LPE) method is widely used in preparing metal–organic framework (MOF) thin films with the merits of controlling thickness and out‐of‐plane orientation for superior performances in applications. The LBL‐LPE growth mechanism related to the grain boundary, structure defect, and orientation is critical but very challenging to study. In this work, a novel “in‐plane self‐limiting and self‐repairing” thin‐film growth mechanism is demonstrated by the combination study of the grain boundary, structure defect, and orientation of Cu3(HHTP)2‐xC thin film via microscopic analysis techniques and electrical measurements. This mechanism results a desired high‐quality MOF thin film with preferred in‐plane orientations at its bottom for the first time and is very helpful for optimizing the LBL‐LPE method, understanding the growth cycle‐dependent properties of MOF thin film, and inspiring the investigations of the biomimetic self‐repairing materials.

The Growth Mechanism of a Conductive MOF Thin Film in Spray-based Layer-by-layer Liquid Phase Epitaxy.

The layer-by-layer liquid-phase epitaxy (LBL-LPE) method is widely used in preparing metal-organic framework (MOF) thin films with the merits of controlling thickness and out-of-plane orientation for superior performances in applications. The LBL-LPE growth mechanism related to the grain boundary, structure defect, and orientation is critical but very challenging to study. In this work, a novel "in-plane self-limiting and self-repairing" thin-film growth mechanism is demonstrated by the combination study of the grain boundary, structure defect, and orientation of Cu3 (HHTP)2 -xC thin film via microscopic analysis techniques and electrical measurements. This mechanism results a desired high-quality MOF thin film with preferred in-plane orientations at its bottom for the first time and is very helpful for optimizing the LBL-LPE method, understanding the growth cycle-dependent properties of MOF thin film, and inspiring the investigations of the biomimetic self-repairing materials.

Design, fabrication and characterization of nanostructured SiO2/TiO2/ITO conductive Bragg reflectors for optoelectronic applications

Conductive Bragg Reflectors SiO2/TiO2/ITO thin films have been grown on glass substrate at room temperature by thermal evaporation and then were annealed in annealing pressure 600 mbar at different temperatures ranging from 250 to 550 °C for 30 min. X ray diffraction results suggested that all films have polycrystalline structure and the peak intensities are increased by increasing annealing temperature. Scanning electron microscopy images showed that crystallinity and homogeneity of films depend on the annealing temperature. Residual Stress, Strain and Dislocation density are decreased by increasing annealing temperature. Conductive DBR films prepared at 550 °C have a very smooth surface with an RMS roughness of 0.25 nm. For films prepared at 550 °C, high reflectance (94.4%) over the visible wavelength region of spectrum (at wavelength 591 nm) was obtained. Increasing annealing temperature (Ta) from 250 to 450 °C reduced resistivity of conductive DBR thin films from (17.8 ×10−4 Ω cm) to (3.8 ×10−4 Ω cm). Further increase of temperature up to 550 °C had no effect on the resistivity.

Chemical Vapor Deposition of Edge-on Oriented 2D Conductive Metal–Organic Framework Thin Films

We demonstrated the synthesis of a conductive two-dimensional metal-organic framework (MOF) thin film by single-step all-vapor-phase chemical vapor deposition (CVD). The synthesized large-area thin film of Cu3(C6O6)2 has an edge-on-orientation with high crystallinity. Cu3(C6O6)2 thin film-based microdevices were fabricated by e-beam lithography and had an electrical conductivity of 92.95 S/cm. Synthesis of conductive MOF thin films by the all-vapor-phase CVD will enable fundamental studies of physical properties and may help to accomplish practical applications of conductive MOFs.

Recent Developments and Implementations of Conductive Polymer-Based Flexible Devices in Sensing Applications.

Flexible sensing devices have attracted significant attention for various applications, such as medical devices, environmental monitoring, and healthcare. Numerous materials have been used to fabricate flexible sensing devices and improve their sensing performance in terms of their electrical and mechanical properties. Among the studied materials, conductive polymers are promising candidates for next-generation flexible, stretchable, and wearable electronic devices because of their outstanding characteristics, such as flexibility, light weight, and non-toxicity. Understanding the interesting properties of conductive polymers and the solution-based deposition processes and patterning technologies used for conductive polymer device fabrication is necessary to develop appropriate and highly effective flexible sensors. The present review provides scientific evidence for promising strategies for fabricating conductive polymer-based flexible sensors. Specifically, the outstanding nature of the structures, conductivity, and synthesis methods of some of the main conductive polymers are discussed. Furthermore, conventional and innovative technologies for preparing conductive polymer thin films in flexible sensors are identified and evaluated, as are the potential applications of these sensors in environmental and human health monitoring.

Liquid-metal micro-networks with strain-induced conductivity for soft electronics and robotic skin

Thin-film devices made of room-temperature liquid metals (LMs) have contributed to the development of electronic skin for human-robot/machine interfaces but still have limitations, including degradations of performance and robustness under repeated deformations. In this paper, we describe an interesting phenomenon of the formation of LM microscale networks (LMMNs) and propose to use the LMMNs for fabricating thin-film conductors. A simple layer-by-layer (LBL) deposition process enables the growth of a hierarchical structure of LM microdroplets that forms a conductive network (i.e., LMMN) when stretched. The strain-history behavior of LMMNs allows conductivity enhancement up to 2.37 × 106 S m−1 in response to increased tensile strains. By adjusting the number of LM layers in LBL deposition, the gauge factor (0.2 ≤ GF ≤ 1), the linearity, and the sheet resistance of LMMN films can be easily controlled, providing high potentials in various applications, including skin-mountable circuits, energy harvesters, and soft artificial skin.

On the Effect of the Co-Introduction of Al and Ga Impurities on the Electrical Performance of Transparent Conductive ZnO-Based Thin Films.

In this study, a set of ZnO-based thin films were prepared on glass substrates at various substrate temperatures via the direct current magnetron sputtering of ceramic targets with the following compositions: pure ZnO, Al-doped ZnO with doping levels of 1 and 2 at.%, Ga-doped ZnO with doping levels of 1 and 2 at.%, and (Al, Ga)-co-doped ZnO with doping levels of 1 and 2 at.% for each impurity metal. The dependencies of sheet resistance, carrier concentration, and Hall mobility on the substrate temperature were studied for the deposited films. The results of evaluating the electrical performances of the films were compared with the data of their XRD study. According to the XRD data, among all the deposited ZnO films, the maximum crystallinity was found in the co-doped thin film with doping levels of 2 at.% for each impurity metal, deposited at a substrate temperature of 300 °C. It was revealed that the observed increase in the Hall mobility and carrier concentration for the co-doped films may, in particular, be due to the difference in the preferred localization of Ga and Al impurities in the ZnO film: the Ga ions were mainly incorporated into the crystal lattice of ZnO nanocrystallites, while the Al impurity was mostly localized in the intercrystalline space at the grain boundaries.

Thermally stable and conductive nickel-incorporated gallium oxide thin-film electrode for efficient GaN microscale light-emitting diode arrays

Microscale light-emitting diodes (µLEDs) have attracted considerable attention as next-generation solid-state lighting sources owing to their reliable performance and attractive properties, such as easy miniaturization and stable operation in various movements. However, the quantum efficiency of μLEDs is lower than that of larger-size LEDs, which hinders their use in high-performance μLED display applications. Herein, a thermally stable and highly conductive GaOx thin-film as a p-type contact electrode is demonstrated by using electric-field-induced doping treatment (EDT) to achieve high-performance GaN µLEDs. The proposed GaOx electrode exhibits high transmittance (92%) and low specific contact resistance (3.5 × 10-3 Ωcm2), along with high thermal stability (over 10 years at 77 °C). Transmission electron microscopy analyses show that conductive channels are formed in the GaOx electrode based on the diffusion of metallic Ni species from the top metal because of EDT, thereby facilitating efficient hole injection into the μLED pixels with little spreading to the passivation layers. Consequently, the µLED array with the GaOx electrode exhibited a 12% higher light output power and 57% higher current level than those of a µLED array with conventional ITO electrodes. The results of this study can guide efforts dedicated to further improving the performance of ITO-based optoelectronic devices, including µLEDs.

Van der Waals Epitaxy of Thin Gold Films on 2D Material Surfaces for Transparent Electrodes: All-Solution-Processed Quantum Dot Light-Emitting Diodes on Flexible Substrates.

With the assistance of van der Waals (vdW) epitaxy, nanometer-thick and highly conductive gold films are deposited onto MoS2 surfaces for use as transparent anode electrodes in quantum dot light-emitting diodes (QLEDs) on poly(ethylene terephthalate) (PET) substrates. After transferring wafer-scale and monolayer MoS2 to PET substrates, 10 nm thick gold (Au) films are deposited onto the two-dimensional (2D) material surfaces as anode electrodes. Bounded only by weak vdW forces on 2D material surfaces, the diffusive Au adatoms tend to facilitate lateral growth and lead to the formation of continuous and highly conductive thin metal films in the nanometer regime. The Au film exhibits excellent tensile bending stability for its sheet resistance, which is superior to that of rigid indium-tin oxide (ITO) films on PET substrates. Thermally stable CdSe@CdZnS/ZnS QLEDs are fabricated on the PET substrate. Compared with devices fabricated on sapphire substrates, the phenomenon of sub-bandgap turn-on is observed for the flexible device. Based on our demonstrations, the high conductivity and robust durability toward substrate bending make the nanometer-thick Au film grown on 2D material surfaces a promising candidate to replace current ITO anode electrodes for flexible device applications.

A combinatorial study on ZnO-In2O3-SnO2 system: The effects of different postgrowth annealing conditions on optical and electrical properties

ZnO-In2O3-SnO2 (ZITO) thin film library was produced via the combinatorial approach. The films were deposited using a magnetron sputtering system. Varying ZITO compositions were obtained in a single deposition run by employing a custom-made triangular type of substrate carrier magazine. The effect of various postgrowth annealing atmospheres on the electrical and optical properties of the films were examined. Air, Ar, forming gas (Ar+4 vol% H2), and successive annealing under forming gas + argon atmospheres were studied. Room temperature (RT) deposited films were identified either crystalline or amorphous depending on the location of the substrate on the holder. All these samples exhibited average visible transmittance (Tvis) below 75 % and sheet resistance (RS) higher than 50 Ω/□. Annealing under air atmosphere improved the optical and electrical properties of the films significantly, but not simultaneously for the same composition. Although better optical improvement was achieved by annealing under Ar and increase in electrical conductivity after annealing under forming gas, optimum properties have been obtained with the latter condition. Tvis values above 85 % and RS values below 50 Ω/□ were attained. On the other hand, successive annealing did not provide any advantage in reaching optimal samples when compared to single gas atmosphere annealings. Further, increasing the annealing temperature were found to be beneficial particularly for the conductivity of many compositions. Finally, this study has introduced a systematic approach to produce transparent conductive oxide thin films with reduced indium content that can be suitable for many optoelectronic applications.

Single-beam ion source enhanced growth of transparent conductive thin films

A single-beam ion source was developed and used in combination with magnetron sputtering to modulate the film microstructure. The ion source emits a single beam of ions that interact with the deposited film and simultaneously enhances the magnetron discharge. The magnetron voltage can be adjusted over a wide range, from approximately 240 to 130 V, as the voltage of the ion source varies from 0 to 150 V, while the magnetron current increases accordingly. The low-voltage high-current magnetron discharge enables a ‘soft sputtering mode’, which is beneficial for thin-film growth. Indium tin oxide (ITO) thin films were deposited at room temperature using a combined single-beam ion source and magnetron sputtering. The ion beam resulted in the formation of polycrystalline ITO thin films with significantly reduced resistivity and surface roughness. Single-beam ion-source-enhanced magnetron sputtering has many potential applications in which low-temperature growth of thin films is required, such as coatings for organic solar cells.

Measurement of apparent diffusion constant on the cross‐section of thin PVA films under a free swelling condition with a facile electronic device

AbstractThe importance of predicting the diffusion coefficient to describe the diffusion process in glassy polymers is obvious. However, most of the direct measurements of diffusion coefficients are time‐consuming, expensive and system intrusive. Here, we report a facile method to measure the apparent diffusion constant on the cross‐section of thin PVA film with a device that is structured with a thin conductive film. The results can employ diffusion and the solution conductivity theory well. The diffusion time and the conductivity of the device are determined by the process of solution passing through the cross‐section of thin PVA film and reaching the conductive substrate. The data detected the apparent diffusion constant in diffusion and the concentration gradient of the solution on the interface between the PVA film and conductive film. This device provides a simple, rapid, and effective way to study the apparent diffusion constant in PVA film. It holds great potential in predicting the diffusion process of a thin glassy polymer film.

Surface Morphology of Textured Transparent Conductive Oxide Thin Film Seen by Various Probes: Visible Light, X-rays, Electron Scattering and Contact Probe.

Fluorine-doped tin oxide thin films (SnO2:F) are widely used as transparent conductive oxide electrodes in thin-film solar cells because of their appropriate electrical and optical properties. The surface morphology of these films influences their optical properties and therefore plays an important role in the overall efficiencies of the solar cells in which they are implemented. At rough surfaces light is diffusely scattered, extending the optical path of light inside the active layer of the solar cell, which in term improves light absorption and solar cell conversion efficiency. In this work, we investigated the surface morphology of undoped and doped SnO2 thin films and their influence on the optical properties of the films. We have compared and analysed the results obtained by several complementary methods for thin-film surface morphology investigation: atomic force microscopy (AFM), transmission electron microscopy (TEM), and grazing-incidence small-angle X-ray scattering (GISAXS). Based on the AFM and TEM results we propose a theoretical model that reproduces well the GISAXS scattering patterns.

OH- Conductivity and Water Uptake of Anion Exchange Thin Films Under Humidity Control

Anion exchange membrane fuel cells (AEMFCs) have attracted widespread attention because of the lower cost of using non-precious metal catalysts and high oxygen reduction reaction (ORR) kinetics in alkaline conditions.[1] The previous research focused on the water uptake and OH- conduction properties of anion exchange membranes (AEMs) as thick forms.[2–5] The fuel cell reaction occurs at the triple-phase interface where the junction of ion-conductive ionomer, catalyst, and fuel/oxidant. The ionomer plays an important role to deliver OH- ion between the thick membrane andto electrochemical catalysts in fuel cells. However, the investigation of OH- ion conduction and hydration properties of thin ionomers is important but not sufficient.This work demonstrated the relations between OH- conductivity and water uptake of anion exchange thin films for the first time.[6] The reported poly[(9,9-bis(6′-(N,N,N-trimethylammonium)-hexyl)-9H-fluorene)-alt-(1,4-benzene)] (PFB+)[6] was synthesized as a model ionomer. We established in situ methods for measuring OH- conductivity and water uptake of anion exchange thin films[7] because OH- ion easily exchanges for carbon dioxide in the air. The OH- conductivity of 273 nm-thick PFB+ thin film form at 25 °C under 95 % relative humidity (RH) is comparable to the reported OH- conductivity value of PFB+ bulk membrane. Reduced OH- conductivity and water uptake were observed in 30 nm-thick PFB+ film compared to thicker 273 nm-thick PFB+ film. This reduced OH- conductivity was caused by the decreased number of water molecules contained in thinner PFB+ films. Under the same number of water molecules contained, similar OH- conductivity results can be obtained for both 273 and 30 nm-thick films as shown in Figure. Results show a different trend compared to the case of the proton conductive thin films.[8] References [1] G. Merle, M. Wessling, K. Nijmeijer, J. Memb. Sci. 2011, 377, 1–35.[2] J. Y. Jeon, S. Park, J. Han, S. Maurya, A. D. Mohanty, D. Tian, N. Saikia, M. A. Hickner, C. Y. Ryu, M. E. Tuckerman, S. J. Paddison, Y. S. Kim, C. Bae, Macromolecules 2019, 52, 2139–2147.[3] J. Chen, M. Guan, K. Li, S. Tang, ACS Appl. Mater. Interfaces 2020, 12, 15138–15144.[4] U. Salma, Y. Nagao, Polym. Degrad. Stab. 2020, 179, 109299.[5] C. G. Arges, L. Zhang, ACS Appl. Energy Mater. 2018, 1, 2991–3012.[6] W. H. Lee, A. D. Mohanty, C. Bae, ACS Macro Lett. 2015, 4, 453–457.[7] F. Wang, D. Wang, Y. Nagao, ChemSusChem 2021, 14, 2694–2697.[8] Y. Nagao, Sci. Tech. Adv. Mater. 2020, 21, 79–91. Figure 1