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Institute
Microscopic and spectroscopic characterization of interfaces and dielectric layers for OFET devices
(2009)
Copper oxide (CuO) is an attractive p-type oxide semiconductor with narrow band gap that has intriguing properties such as non-toxicity, chemical stability, electrocatalytic activity, abundant availability and low production cost. Polymeric materials have attracted great interest owing to their high flexibility, light weight, low cost production and high resistance against corrosion. Poly(vinylidene fluoride) (PVDF) is a commonly used piezoelectric polymer due to its excellent flexibility and piezoelectric properties. In this work, we adopted a one-pot synthesis of CuO nanowires by facile and selective chemical etching process of Cu substrate. XPS, XRD, SEM data ascribed the formation of polycrystalline CuO nanowires over the entire Cu surface. It has been observed that the uniformity, shape and size of the nanowires could be conveniently controlled by etching time and etchant concentration. We have also prepared piezoelectric nanogenerator based on the flexible PVDF/CuO composite film, where the traditional electrical poling treatment was avoided. In addition, dielectric properties have been studied to demonstrate the role of interfacial polarization.
A self-poled piezoelectric nanogenerator (NG) based on bismuth oxide (Bi2O3) nanostructures (BNS)-doped poly(vinylidene fluoride) (PVDF) is demonstrated. The in-situ formation of BNS incorporated in the PVDF matrix is realized through a simple solution casting technique that favors the nucleation and stabilization of 99.6% of the electroactive phase in PVDF (i.e., beta/gamma-phases). The enhancement of the output voltage (3.6 V) and current (2.4mA) of the NG based on a nonelectrically poled BNS containing PVDF composite film is achieved and demonstrated by simple repeated human finger imparting. The improvement of the output performance is influenced by the generation of the electroactive polar beta-phase in PVDF, due to the electrostatic interactions between the –CH2–/–CF2– dipoles of PVDF and the surface charge of the BNS.
The NGs are able to charge up capacitors demonstrating that the power generation from such a hybrid device structure can be utilized for powering various portable devices.
The design and development of intrinsically stretchable all-organic self-powered sensors concurrently perceiving temperature and pressure remain a challenge but deliver an exciting platform to realize environmentally friendly wearable electronics. In this approach, a biomimetic all-organic stretchable energy harvester is designed by a xylitol-added poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS/Xyl) film as a compatible overlay electrode with polyaniline-reinforced one-dimensional aligned poly(vinylidene fluoride) hybrid electroactive soft nanowires. The gradient of elastic modulus between the electrode and the active nanowire component enables the all-organic device to manifest excellent power-generating performance under external temperature fluctuation (∼3 μW/m2 under ΔT ∼ 92 K) and mechanical force (∼31 μW/cm2 at 30 N). Importantly, the device renders simultaneous energy scavenging of temperature and pressure changes under pressing and stretching conditions (∼20%). The excellent mechanosensitivity (∼100 mV/N), fast response time (∼1 ms), outstanding mechanical and thermal stability, and good temperature resolution <10 K enable the harvester to act as an epidermal sensor, which simultaneously detects and discriminates both subtle pressure and thermal deviations exposed to an epidermis surface. The real-time recording and wireless transferring of physiological signals to a smartphone indicate an effective way to realize remote healthcare monitoring for early intervention.
Flexible and wearable e-skin sensors are attracting a great interest for their smart sensing applications in next-generation electronics. However, implant ability, sensitivity, and biosignal detection capability in a self-powered manner are the prime concerns in embedded devices. In particular, electrode compatibility and imperishability have become challenging issues in wearable sensors due to the poor compatibility and fragileness of metal electrodes. In this context, we report on a skin-interactive metal-free spongy electrode in a piezoelectric sensor where highly aligned poly(vinylidenefluoride) (PVDF) nanofibers (NFs) arrays are introduced as the piezoelectric active component and conducting polyaniline- (PANI-) coated PVDF (PANI−PVDF) NFs mats served as flexible electrodes. Notably, a 99% yield of piezoelectric phases of the aligned PVDF arrays is the key factor to exhibit promising mechano-sensitivity (0.8 V/kPa) performance that in turn helps in human-health monitoring. The sensor shows excellent mechanical to electrical energy conversion that enable to sense human finger touch (10 V under 10 kPa) with energy conversion efficiency of 53%. Most importantly, due to the compatible electrodes excellent mechanical stability has been found showing negligible degradation over 12,000 periodic cycles. Furthermore, under mechanical stimuli, it is also possible to charge up a capacitor (1μF) to 4 V within 60 s confirming the possibility to use the device as a self-powered piezo-organic-e-skin sensor (POESS). This type of structural design enables to trace elusive movement of muscles and the operation in several conditions such as bending, compression and stretching. We demonstrated various human gestures monitoring, such as wrist bending, neck stretching, and arm compressions, throat movements during drinking water, coughing actions, and swallowing. In addition, diverse specific phonation recognition, heart-pulse measurement and its respective short-time Fourier transform (STFT) analysis indicate an efficient and convenient way of monitoring human-health status particularly in hospital-free mode.
This paper reports a self-powered,flexible, piezo- and pyro-electric hybrid nanogenerator (NG) device that can be fixed on different locations of human skin for detecting static and dynamic pressure variations and can also monitor temperature fluctuations during the respiration process. An efficient and cost-effective fabrication strategy has been developed to create electrospun poly(vinylidene fluoride) (PVDF)/graphene oxide (GO) nanofibers, which are used to create a highly sensitive wearable pressure sensor and pyroelectric breathing sensor. The sensor can accurately and rapidly
detect pressures as low as 10 Pa with a high sensitivity (4.3 V/kPa), a key performance indicator for wearable sensors. Importantly, the sensor exhibits a high sensitivity to bending and stretching by finger, wrist, and elbow. The pressure sensor is also highly sensitive to vocal vibrations when attached to the human throat. The device can generate a maximum output power density of∼6.2 mW/m2 when subjected to a compressive stress, which enhances its range of applications. Moreover, it is demonstrated that doping with GO improves the pyroelectric energy harvesting and sensing performance of the device under repeated temperature fluctuations. The PVDF/GO-based nanogenerator has a maximum pyroelectric output power density of∼1.2 nW/m2 and can sense temperature changes during respiration, which makes it promising as a pyroelectric breathing sensor. It is demonstrated that processing of the PVDF-GO self-powered multifunctional pressure and pyroelectric breathing sensor can be up-scaled for fabricating compact and high-performance electronic skins for application in health monitoring, motion detection, and portable electronics.
In this work we report on the preparation of poly(vinylidene fluoride) (PVDF) films with various mass fractions (wt%) of hydrated rare earth metal salt [(NH4)4Ce(SO4)4.2H2O] (CAS) and its electroactive phase (β- and γ-phase) formation. It was found that CAS can yield high amount of electroactive phases in PVDF. Compared to the neat PVDF film, a significantly increment of dielectric constant (ε ∼ 41) and a low loss factor (tan δ ∼ 0.8) were observed in the PVDF-CAS composite films. Thus the PVDF-CAS composite films with high electroactive phase and superior dielectric properties may meet the requirements of next generation electronic components.
Natural piezoelectric materials are of increasing interest, particularly for applications in biocompatible, implantable, and flexible electronic devices. In this paper, we introduce a cost-effective, easily available natural piezoelectric material, that is, sugar in the field of wearable piezoelectric nanogenerators (PNGs) where low electrical output, biocompatibility, and performance durability are still critical issues. We report on a high-performance piezoorganic nanogenerator (PONG) based on the hybridization of sugar-encapsulated polyvinylidenefluoride (PVDF) nanofiber webs (SGNFW). We explore the crucial role of single-crystal sugar having a fascinating structure along with the synergistic enhancement of piezoelectricity during nanoconfinement of sugar-interfaced macromolecular PVDF chains. As a consequence, the SGNFW-based PONG exhibits outstanding electricity generation capability (e.g.,∼100 V under 10 kPa human finger impact and maximum power density of 33 mW/m2) in combination with sensitivity to abundantly available different mechanical sources (such as wind flow, vibration, personal electronics, and acoustic vibration). Consequently, it opens up suitability in multifunctional self-powered wearable sensor designs for realistic implementation. In addition, commercially available capacitors are charged up effectively by the PONG because of its rapid energy storage capability. The high performance of the PONG not only offers“battery-free”energy generation (several portable units of light-emitting diodes and a liquid crystal display screen are powered up without using external storage) but also promises its use in wireless signal transmitting systems, which widens the potential in personal health care monitoring. Furthermore, owing to the geometrical stress confinement effect, the PONG is proven to be a highly durable power-generating device validated by stability test over 10 weeks. Therefore, the organic nanogenerator would be a convenient solution for portable personal electronic devices that are expected to operate in a self-powered manner.
The electroactive β phase of poly(vinylidenefluoride) (PVDF) is induced due to the aging time of PVDF solutions. The feasibility of the combination of the three crystalline polymorphs (α, β and γ) is demonstrated where
their relative proportion within the PVDF film can be tailored by the simple monitoring of the preparation conditions. To identify all these phases, Fourier transform infrared (FT-IR) spectroscopy is carried out and it is spotlighted that the vibrational bands at 510 and 841 cm-1 are not sufficient to state the formation of the β phase. The main aim of this work is devoted to develop a better understanding on the thermal stability of these several phases of PVDF, which has a longstanding ambiguity
persisting in this area. It has been found that the in situ thermal FT-IR spectroscopy is one of the best alternatives to understand this important
issue. It is ascertained that the β phase is the least thermally stable phase
among α, β and γ phases, whereas the γ phase is the most thermally stable phase.
Exploring efficient and inexpensive electrocatalysts for the oxidation of water is of great importance for various electrochemical energy
storage and conversion technologies. In the present study, a new water-soluble [Ce
III(DMF) (HSO4)3] complex was synthesized and characterized
by UV−vis, photoluminescence, and high-resolution X-ray photoelectron spectroscopy techniques. Owing to classic 5d →4f transitions, an intense
photoluminescence in the UV region was observed from the water-soluble [CeIII(DMF)(HSO4)3] complex. A stacking electrode was designed where
self-assembled L-cysteine monolayer modified gold was immobilized with the synthesized cerium complex and was characterized by scanning electron microscopy, electrochemical impedance spectroscopy, and cyclic voltam-metry. The resulting electrode, i.e., [CeIII(DMF)(HSO4)3]−L-cysteine-Au
stacks shows high electrocatalytic water oxidation behavior at an
overpotential of η ≈0.34 V under neutral pH conditions. We also demonstrated a way where the overpotential is possible to decrease upon irradiation of UV light.
Ytterbium (Yb3+) assisted porous poly(vinylidene fluoride) (PVDF) composite film comprising flexible ferroelectretic nanogenerator (FTNG) is highlighted where traditional poling treatment is completely avoided. The piezoelectric output of FTNG is realized by the co-operative activity of self-polarized -CH2/-CF2 dipoles with porous electret-like structure in the composite film. Owing to extraordinary ferroelectric and dielectric properties, FTNG is acting as a highly efficient mechanical energy harvester. It is capable to capture several forms of abundant mechanical energy arising from humanfinger movements, machine vibrations and sound waves. As a proof of concept, under compressive deformation, FTNG is enable to instantly powers up several consumer electronics and thus provides a promising strategy for achieving self-powered electronic devices.
Cerium(III)-,N-dimethylformamide-bisulfate
[Ce(DMF)(HSO4)3] complex is doped into poly(vinylidene
fluoride) (PVDF) to induce a higher yield (99%) of the
electroactive phases (β- and γ-phases) of PVDF. A remarkable enhancement of the output voltage (∼32 V) of a nano-generator (NG) based on a nonelectrically poled cerium(III) complex containing PVDF composite film is achieved by
simple repeated human finger imparting, whereas neat PVDF
does not show this kind of behavior. This high electrical output resembles the generation of self-poled electroactiveβ-phase in
PVDF due to the electrostatic interactions between the fluoride of PVDF and the surface-active positive charge cloud of the
cerium complex via H-bonding and/or bipolar interaction among the opposite poles of cerium complex and PVDF, respectively.
The capacitor charging capability of the flexible NG promises its applicability as piezoelectric-based energy harvester. The cerium(III) complex doped PVDF composite film exhibit an intense photoluminescence in the UV region, which might be due to
a participation of electron cloud from negative pole of bipolarized PVDF. This fact may open a new area for prospective development of high-performance energy-saving flexible solid-state UV light emitters.
A flexible nanogenerator (NG) is fabricated with a poly(vinylidene fluoride) (PVDF)film, where deoxyribonu-cleic acid (DNA) is the agent for the electroactive β-phase
nucleation. Denatured DNA is co-operating to align the
molecular−CH2/−CF2 dipoles of PVDF causing piezo- electricity without electrical poling. The NG is capable of harvesting energy from a variety of easily accessible mechanical
stress such as human touch, machine vibration, football
juggling, and walking. The NG exhibits high piezoelectric
energy conversion efficiency facilitating the instant turn-on of several green or blue light-emitting diodes. The generated energy can be used to charge capacitors providing a wide scope
for the design of self-powered portable devices.
A polymer nanogenerator based on silver nanoparticles doped electrospun P(VDF-HFP) nanofibers
(2015)
The rising energy demand within the increasing mobility of human society has driven the development of new alternative power sources for portable devices with ultra-low electric power consumption.
Based on its piezoelectric properties poly-vinylidene fluoride (PVDF) is a good candidate for mechanical energy harvesting for such devices. Moreover the copolymer poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)] offers better film flexibility and cost-effectiveness than pure PVDF.
We report on a polymer nanogenerator (PNG) based on electrospun P(VDF-HFP) nanofibers doped with silver nanoparticles (Ag-NPs). The electrospun fibers were characterized by FTIR, XPS, SEM and pressure imparting probe. It has been found that the yield of the piezoelectric phase is increased by the addition of Ag-NPs due to an interaction between surface charges of Ag-NPs and the molecular dipoles. Furthermore, defects in the P(VDF-HFP) electrospun fibers are removed. Accordingly, a significant enhancement in the output power of the PNG was found. Similar trends were also observed with Pd-NPs.
We report on the electrospinning of poly(vinylidene fluoride–hexafluoropropylene) [P(VDF–HFP)] nanofibers doped with silver nanoparticles for the preparation of a polymer based nanogenerator (PNG). It has been found that the yield of the piezoelectric phase is increased by the addition of silver nanoparticles. Furthermore, defects in the P(VDF–HFP) electrospun fibers are removed resulting in a significant enhancement in the output power of the PNG. A maximum generated PNG output voltage of 3 V with a current density of 0.9 μA cm−2 is achieved.
The electroactive Beta-phase formation in Poly(vinylidene fluoride) by gold nanoparticles doping
(2012)
Polyvinylidene fluoride (PVDF) is a typical fluoro-polymer with complicated polymorphisms due to the semi-crystalline nature. Generally, four crystalline phases, α, β, γ and δ may exist in PVDF crystals. The nonpolar α- and δ-phase has a monoclinic lattice with trans-gauche conformation (TGTG⁄), which is thermodynamically most stable for PVDF. The polar β phase with all-trans (TTTT) planar zigzag conformation is attractive for its piezo-, pyro-, and ferroelectric properties. The polar β-Phase and also semi-polar γ-phase (TTTGTTTG⁄) have obtained wide device applications such as acoustic transducers, piezoelectric sensors, energy harvesting generators, pyroelectric sensors, non-volatile memories and energy storage capacitors. In this work, we explore the possibility to nucleate β-phase in PVDF hydrated salt composite films by addition of the hydrated Rare earth metal salt, (NH4)4 Ce (SO4)4, 2H2O) for the piezo- ferro and high dielectric based device applications. It has been found that the water molecules in (NH4)4 Ce (SO4)4, 2H2O could form hydrogen bonds with PVDF during the crystallization process and the hydrogen bonds promoted the electro-active crystalline β- phase in PVDF with all -trans conformations. In addition, the DMF (solvent used for films preparation) can also assist the rare-earth based nanoparticle preparation, as a result the electrostatic interaction of the surface charge of the nanoparticles and molecular dipoles in PVDF can resist the electroactive β- phase in PVDF.
The effect of X-ray photoelectron spectroscopy measurement on P(VDF-TrFE) copolymer thin films
(2012)
The impact of prolonged X-ray irradiation during X-ray photoelectron spectroscopy (XPS) measurement was investigated on poly(vinylidene-trifluoroethylene) (P(VDF-TrFE)) thin films. It was observed that
prolonged X-ray irradiation can accelerate the crosslinking of P(VDF-TrFE) and diminish the ferroelectric phase. Fourier transform infrared spectroscopy (FT-IR) data indicate that the ferroelectric phase diminishes completely after 360 kJ of X-ray irradiation dose and it induces the paraelectric phase. In this work,
the main emphasis was given to the optimization of the X-ray irradiation dose during XPS measurements
to maintain the ferroelectric phase within the copolymer films.
Control of β- and γ-Phase formation in electroactive P(VDF-HFP) films by silver nano-particle doping
(2012)
Microscopic and Spectroscopic Characterization of Interfaces and Dielectric Layers for OFET Devices
(2008)
Ferroelectric properties of copolymers of vinylidene fluoride with trifluoroethylene (P[VDF-TrFE]) has become great deal of interest due to potential use in non volatile memory technology. Permanent dipoles (formed between the alternating hydrogen and fluorine atom within the copolymer) ordering of orientation are crucial for device efficiency. Here we focus on thickness dependence dipole orientation in these co-polymer films. In our systematic studies, we found there is prominent dipole re-orientation effect also for the intrinsic films (~10 nm thickness) before applying the electric field. It is observed after the application of the field, the dipoles re-oriented in opposite direction. Therefore, we conclude that the lowest thickness of the film should have well grown crystalline lamellae structure. As a result, c-axis of crystallites acts as well define manner in plane of the surface and for that reason the b-axis is more free. Because the b-axis is parallel to the polarization direction, it is easy to get a preferred orientation of the dipoles. For thicker films (here up to 100 nm), the average orientation effect is hindered, so a very low re-orientation effect has been observed. For this investigation, we used Near Edge X-ray Absorption Spectroscopy, at the U 49/2-PGM beam line of BESSY-II, Berlin. This work is supported by DFG within SPP1157 (SCHM 745/11-2).
The ferroelectric copolymer poly(vinylidene fluoride trifluoroethylene) (P(VDF-TrFE)) is a possible material for low cost applications as organic nonvolatile memory element. We present an XPS interface investigation of the copolymer/electrode interface. Below 100nm film thickness, interface phenomena becomes important. Possible interface reactions could lead to a reduction of the ferroelectric functionality.We present a comparative study of the two interfaces Al/P(VDF-TrFE) and PEDOT:PSS/P(VDF-TrFE). The result is a clear indication of a surface reaction in between the Aluminum-electrodes. In contrast, for PEDOT:PSS the measurements shows a layer by layer formation of the interface PEDOT:PSS/PVDF without any reaction. Second, we present a study of remanent polarization of ultrathin spincoated films of the copolymer, in a thickness range from 5 to 100mn, with organic electrodes like PEDOT:PSS. The measurements are carried out by using the flatband shift of a capacitance-voltage (CV) characteristic. This shift of flatband voltage is used as value for the electric polarization of the polymer.
Due to potential use in non volatile memory application, Poly(vinylidene fluoride/trifluoroethylene) (P(VDF-TrFE)) is a great deal of interest, especially to produce organic field effect transistors (OFET). Right now it was found that Aluminum is one of the cost-effective inorganic electrode candidate. On the other hand, poly(3,4 ethylenedioxythiophene):poly(4-styrenesulfonate)(PEDOT:PSS) shows promising behavior for the application as organic electrode. In part of our investigations we have systematically studied surface interactions of organic ferroelectric thin films (P(VDF-TrFE))with inorganic (Al) and organic layers (PEDOT:PSS). By X-ray photoelectron spectroscopy (XPS) is has been found that there is a interfacial reaction with Al and the copolymer. In contrast, for electrodes, made of PEDOT: PSS, we found not such kind of behavior. In this case, we found no evidence for any surface reaction. Furthermore, we present our results concerning the orientation of permanent dipoles in the co-polymer (formed between hydrogen and fluorine atom). Here, we use Near-edge X-ray absorption fine structure (NEXAFS) analysis at the U 49/2 - PGM2 beam line of the BESSY-II Synchrotron. This work is supported by Deutsche Forschungsgemeinschaft (DFG) within priority program 1157 (DSCH 745/11-1).