Characterization of a Ferroelectric-Gated Graphene Memory Device Fabricated on a Flexible Substrate by Transfer Process.

The mechanical flexibility of both ferroelectric polymer and graphene provides the possibility for the memory device based on ferroelectric polymer and grapheme to operate on flexible substrate. Here, a memory device was fabricated on flexible substrate through the continuous transfer process of the two units with the ferroelectric polymer and the graphene hybrid film as one unit, and characterized. In particular, characteristics were maintained even with repetitive bending. The transfer process demonstrated in this paper is useful for implementing a memory device on a large-area substrate by consuming a very small amount of graphene.

Highly Sensitive and Stable Humidity Sensor Based on the Bi-Layered PVA/Graphene Flower Composite Film.

Two-dimensional (2D) materials and their composites have gained significant importance as the functional layer of various environmental sensors and nanoelectronics owing to their unique properties. This work reports for the first time a highly sensitive, fast, and stable humidity sensor based on the bi-layered active sensing area composed of graphene flower (GF) and poly (vinyl alcohol) PVA thin films for multifunctional applications. The GF/PVA humidity sensor exhibited stable impedance response over 15 days, for a relative humidity (RH) range of (40-90% RH) under ambient operating conditions. The proposed bi-layered humidity sensor also exhibited an ultra-high capacitive sensitivity response of the 29 nF/%RH at 10 kHz and fast transient response of 2 s and 3.5 s, respectively. Furthermore, the reported sensor also showed a good response towards multi-functional applications such as non-contact skin humidity and mouth breathing detection.

Hydrophilicity Control of Ag-Paste Electrodes by UV/Ozone Treatment.

Ag-paste is used as an electrode material in various fields as a manufacturing advantage that enables solution processing. However, when a subsequent thin film is formed on the solidified Ag-paste electrode, there is a fear that the bonding force between the Ag-paste electrode and the subsequent thin film is weakened and peeled off due to the low surface energy of the Agpaste electrode. It is necessary to increase the surface energy of the Ag-paste electrode surface since it ultimately directly affects the yield of the device or product. In this study, the UV/ozone treatment process was introduced to increase the Ag-paste surface energy, thereby making the surface hydrophilic. Additionally, it was confirmed that the UV/ozone treatment process affected only the surface of the Ag-paste electrode by extracting the contact resistance.

Polarization Reversal Characteristics in Capacitors with a Metal/Insulator/Ferroelectric/Metal Structure.

A metal/insulator/ferroelectric/metal structure was fabricated using a covering of approximately 10 nm thick of an insulating polystyrene film on a ferroelectric poly(vinylidene fluoride-trifluoroethylene) film. To fabricate several samples, the thickness of the ferroelectric film was held constant while the thickness of the insulating film was varied from 8 to 24 nm. The polarization- voltage relationships were measured to extract the main parameters, in this case the remanent polarization, depolarization, coercive voltage and biased voltage values. As the insulating film becomes thicker, the remanent polarization and coercive voltage values tended to increase. On the other hand, depolarization and biased voltage values decreased. By analyzing the above mentioned parameters, a certain optimum insulator thickness could be predicted. This work shows that metal/insulator/ferroelectric/metal devices are more useful than metal/ferroelectric/metal capacitors.

Highly Efficient and Wide Range Humidity Response of Biocompatible Egg White Thin Film.

Biopolymers are a solution to solve the increasing problems caused by the advances and revolution in the electronic industry owing to the use of hazardous chemicals. In this work, we have used egg white (EW) as the low-cost functional layer of a biocompatible humidity sensor and deposited it on gold (Au) interdigitated electrodes (IDEs) patterned through the state-of-the-art fabrication technology of thermal vacuum evaporation. The presence of hydrophilic proteins inside the thin film of EW makes it an attractive candidate for sensing humidity. Usually, the dependence of the percentage of relative humidity (%RH) on the reliability of measurement setup is overlooked for impedimetric humidity sensors but we have used a modified experimental setup to enhance the uniformity of the obtained results. The characteristics of our device include almost linear response with a quick response time (1.2 s) and fast recovery time (1.7 s). High sensitivity of 50 kΩ/%RH was achieved in the desirable detection range of 10-85%RH. The device size was intentionally kept small for its potential integration in a marketable chip. Results for the response of our fabricated sensor for dry and wet fingertips, along with determining the rate of breathing through the mouth, are part of this study, making it a potential device for health monitoring.

A Full-Range Flexible and Printed Humidity Sensor Based on a Solution-Processed P(VDF-TrFE)/Graphene-Flower Composite.

A novel composite based on a polymer (P(VDF-TrFE)) and a two-dimensional material (graphene flower) was proposed as the active layer of an interdigitated electrode (IDEs) based humidity sensor. Silver (Ag) IDEs were screen printed on a flexible polyethylene terephthalate (PET) substrate followed by spin coating the active layer of P(VDF-TrFE)/graphene flower on its surface. It was observed that this sensor responds to a wide relative humidity range (RH%) of 8-98% with a fast response and recovery time of 0.8 s and 2.5 s for the capacitance, respectively. The fabricated sensor displayed an inversely proportional response between capacitance and RH%, while a directly proportional relationship was observed between its impedance and RH%. P(VDF-TrFE)/graphene flower-based flexible humidity sensor exhibited high sensitivity with an average change of capacitance as 0.0558 pF/RH%. Stability of obtained results was monitored for two weeks without any considerable change in the original values, signifying its high reliability. Various chemical, morphological, and electrical characterizations were performed to comprehensively study the humidity-sensing behavior of this advanced composite. The fabricated sensor was successfully used for the applications of health monitoring and measuring the water content in the environment.

Polarization Reversal Characteristics of Ferroelectric-Schottky Diode Hybrid Structure.

The polarization reversal characteristics were measured by fabricating a device with top electrode/n-type semiconductor/ferroelectric/bottom electrode structure. It was observed that the hysteresis curves were changed according to the polarity of the applied voltage. When a positive voltage was applied, depolarization was hardly observed. However, a significant depolarization occurred when a negative voltage was applied. An attempt was made to set up an equivalent circuit using a Schottky diode to model the semiconductor-ferroelectric structure. This study is expected to be useful for setting protocol of electronic device composed of semiconductor-ferroelectric hybrid structure.

Imaging Conductivity Changes in Monolayer Graphene Using Electrical Impedance Tomography.

Recently, graphene has gained a lot of attention in the electronic industry due to its unique properties and has paved the way for realizing novel devices in the field of electronics. For the development of new device applications, it is necessary to grow large wafer-sized monolayer graphene samples. Among the methods to synthesize large graphene films, chemical vapor deposition (CVD) is one of the promising and common techniques. However, during the growth and transfer of the CVD graphene monolayer, defects such as wrinkles, cracks, and holes appear on the graphene surface. These defects can influence the electrical properties and it is of interest to know the quality of graphene samples non-destructively. Electrical impedance tomography (EIT) can be applied as an alternate method to determine conductivity distribution non-destructively. The EIT inverse problem of reconstructing conductivity is highly non-linear and is heavily dependent on measurement accuracy and modeling errors related to an accurate knowledge of electrode location, contact resistances, the exact outer boundary of the graphene wafer, etc. In practical situations, it is difficult to eliminate these modeling errors as complete knowledge of the electrode contact impedance and outer domain boundary is not fully available, and this leads to an undesirable solution. In this paper, a difference imaging approach is proposed to estimate the conductivity change of graphene with respect to the reference distribution from the data sets collected before and after the change. The estimated conductivity change can be used to locate the defects on the graphene surface caused due to the CVD transfer process or environment interaction. Numerical and experimental results with graphene sample of size 2.5 × 2.5 cm are performed to determine the change in conductivity distribution and the results show that the proposed difference imaging approach handles the modeling errors and estimates the conductivity distribution with good accuracy.

Effect of Randomly Grown Morphology of ZnO Nanorods in Inverted Organic Solar Cells.

Here, we analyzed the photovoltaic properties of the inverted organic solar cells (IOSCs) by using randomly oriented medium density ZnO nanorods (ZnO-NR) synthesized hydrothermally at low temperature conditions to avoid morphological defects. The IOSC with ZnO-NR (length < 150 nm) of medium density and random orientation showed an improvement of 83% in power conversion efficiency compared to the cell with (length < 20 nm) hydrothermally grown ZnO-NR. The optimized hydrothermal growth conditions for ZnO-NR enhanced the photovoltaic performance indicators by reducing recombination rate evidenced by the photovoltaic data. The qualitative elemental analysis of the ZnO-NR based interface was performed by the EDX, which confirmed that the as-grown ZnO-NR contain the Zn and O elements.