Energy harvesting systems based on piezoelectric and triboelectric nanomaterials are in great demand, as they can provide routes for the development of self-powered devices which are highly flexible, stretchable, mechanically durable, and can be used in a wide range of applications. Our recent research interest mainly focuses on the fabrication of nanogenerators (NGs) based on 1D/2D nanomaterials such as zinc oxide nanowire/nanosheets and graphene. Furthermore, the efforts towards developing triboelectric NGs using those materials are in progress.

Nanogenerator (NG) is a promising new concept for harvesting mechanical energy from ambient sources and developing self-powered electronic systems. Here, we report a development of flexible NGs from sponge-like nanoporous piezoelectric polyvinylidene fluoride (PVDF) thin films that were fabricated by a casting-etching method in wafer-scale. The nanoporous PVDF NGs can be directly attached to the surface of an electronic device (e.g. a cell phone), and effectively convert mechanical energy from ambient surface oscillations to electricity using the device&’s own weight to enhance the amplitude. An output voltage of 11.0 V and current of 9.8 mu;A was demonstrated using a 2 cm2 sized PVDF NG. Multiple PVDF NGs were integrated in series and operate synchronically to raise the output power for the operation of electronic devices. In conclusion, a novel integratable NG design based on sponge-like nanoporous piezoelectric PVDF thin films was successfully demonstrated. This type of NG can generate significantly electrical energy by harvesting mechanical energy from surface oscillations. This technique is considerably scalable and integratable, providing a promising solution for developing practical self-powered electronic devices.

Piezoelectric nanodevices can offer crucial advantages for energy harvesting, mechanical sensing/actuation, biomedical devices, optoelectronics, and more. In fact, in comparison with conventional piezoelectric materials, nanostructures can be highly deformed by tiny forces, exhibit outstanding mechanical properties, and can have substantially higher piezoelectric coefficients.
However, many issues must be addressed, both at the theoretical and at the technological level, in order to design and fabricate optimal piezo-nano-devices on flexible substrates.
In the first part of this presentation, I show how shape, electrical boundary conditions, position of contacts, type of mechanical input, and charge transport properties can all crucially affect the piezopotential, the mechanical-to-electrical conversion efficiency, and the on-off ratios of piezoelectric nanodevices. As an example, for a given mechanical input, laterally deflected cylindrical nanowires with total bottom contacts result in lower piezopotential and mechanical-to-electrical conversion efficiency than vertically compressed nanowires; as another example, the mechanical-to-electrical static conversion efficiency of vertically compressed nanowires is almost independent of strain at small strains and increases at higher strains, thus confirming that the ability of nanostructures to withstand extreme deformations without fracture may be a key for increasing the efficiency of nanogenerators. As another example, conical nanowires may prove advantageous for nanogenerators, piezotronics, and piezophototronics.
In the second part I will highlight the challenges to overcome for taking full advantage of piezoelectric nanodevices on flexible substrates, with special reference to both modeling (e.g. parasitic impedances, transient phenomena, and scaling of the electro-mechanical properties, including the Young modulus and piezoelectric coefficients) and fabrication (e.g. low-temperature processing and complexity of in-liquids real-time monitoring).
Finally, I will illustrate some piezoelectric nanodevices on flexible substrates.

High flexibility and easy tunability have been important advantages for one-dimensional nanowires (NWs) in wide applications of nanoscale integrated circuits and high sensitive sensor etc. We present our work on NW lasers from fabrication, tuning to applications. First of all, we fabricated various NW lasers: semiconductor NW-microfiber hybrid lasers [1], semiconductor NW-microdisk cavity hybrid lasers [2], and semiconductor NW-microfiber ultraviolet-green-red three color lasers [3]. The hybrid structure laser combine the high gain of semiconductor and high quality factor of micro cavity, offering low threshold and easily manipulation properties. Secondly, we focus on achieving tuning/modulating/enhancing the afore-mentioned NW lasers. We demonstrate an efficient way to achieve wide wavelength tunable lasers on single non-doped CdSe NWs [4]. The laser wavelengths are tuned from 746 nm to 706 nm by changing the length of a single NW (402-nm-diameter), spanning a range as large as 40 nm. The length dependent wavelength tuning is attributed to absorption-emission-absorption (AEA) of the propagating light in the NWs. We also utilize the bending effects to modulate the polarization of NW lasers [5]. Furthermore, we work on the modulation of polarization and modes of NW lasers by electrical tuning. Finally, we use the tunable laser to achieve super-resolution microscope and investigate the behind new physical mechanism. Our work pioneered in the NW lasers may provide a way of fabricating ultra-compact nanophotonic devices and may find its applications in single molecule detection, integrated optical circuits, and optical communications etc.
References:
[1]Q. Yang, X. Jiang, X. Guo, Y. Chen, L. Tong, “Hybrid structure laser based on semiconductor nanowires and a silica microfiber knot cavity”, Appl. Phys. Lett. 94 (2009) 101108.
[2]G. Wang, X. Jiang, M. Zhao, Y. Ma, H. Fan, Q. Yang, L. Tong, M. Xiao, “Microlaser based on a hybrid structure of a semiconductor nanowire and a silica microdisk cavity”, Opt. Express 20 (2012) 29472-29478.
[3]Y. Ding, Q.Yang, X.Guo, S. Wang, F. Gu, J. Fu, Q. Wan, J. Cheng, and L. Tong, “Nanowires/microfiber hybrid structure multicolor laser”, Opt. Express 17 (2009) 21813.
[4]J. Li , C. Meng , Y. Liu , X. Wu , Y. Lu , Y. Ye , L. Dai , L. Tong, X. Liu, Q. Yang “Wavelength Tunable CdSe Nanowire Lasers Based on the Absorption-Emission-Absorption Process” Adv. Mater. DOI: 10.1002/adma.201203692 (2012)
[5]W. Yang, Y. Ma, Y. Wang, C. Meng, X.Wu, Y. Ye, L. Dai, L. Tong, X. Liu, Q. Yang, “Bending effects on lasing action of semiconductor nanowires”, Opt. Express 21 (2013) 2025-2031

Large interconnected collections of small, ultrathin inorganic solar cells and piezoelectric elements provide routes to high performance energy production systems in flexible, and even stretchable, designs. This talk describes materials, techniques for assembly and concepts in mechanics for this type of technology. Device examples built using silicon, compound semiconductors and piezoelectric materials derived from wafer-scale sources of material illustrate an ability to achieve, simultaneously, high efficiency operation in energy conversion and extreme levels of flexibility and stretchability.

Piezoelectric materials can convert mechanical energy into electrical energy, and piezoelectric devices made of various inorganic materials and organic polymers have been demonstrated. However, synthesizing such materials often requires toxic materials, harsh conditions and/or complex procedures. Recently, it was shown that hierarchically organized natural materials, such as bones, collagen fibrils and peptide nanotubes, can display piezoelectric properties. In this paper, we will demonstrate our innovative approach to produce biomaterial-based piezoelectric energy generation. Recently, we establish that the piezoelectric and liquid crystalline properties of M13 bacteriophage (phage) can be used to generate electrical energy. Using piezoresponse force microscopy, we characterized the structure-dependent piezoelectric properties of phage at the molecular level. We then showed that self-assembled thin films of phage could exhibit piezoelectric strengths of up to 7.8 pm/V. It is also possible to modulate the dipole strength of phage, and hence tune their piezoelectric response by genetically engineering the phage&’s major coat proteins. Finally, we developed a phage-based piezoelectric generator that produces up to 100 nA of current and 900 mV of potential, and use it to operate a liquid crystal display. Because biotechnology techniques enable large-scale production of genetically modified phages, phage-based piezoelectric materials potentially offer a simple and environment-friendly approach to piezoelectricity generation.

A number of unique and interesting energy harvesting devices utilizing materials with extremely large magnetostrictive constants (lambda;) such as TbFe (Terfenol) have been demonstrated in literature. When compared to piezoelectric devices though the number paper dealing with these materials is only a small fraction. The reason for this may be due to a lack of materials with suitable properties. Magnetostrictive materials can change the direction of their magnetization with the application of a stress which is referred to as the Villari effect. This action is analogous to rotating a magnet. By using Maxwells equations it can be seen that this rotation of magnetic moment can be used to harvest energy. GaFe (Galfenol) overcomes many limitations of typical magnetostrictive materials in that it has a large lambda;, is rare-earth-free, and ductile. In addition to that, our group has pioneered electrodeposition of the material which allows us to deposit thin films directly onto various substrates and structures. To date we have demonstrated deposition on planar substrates of glass, metals, Si, GaAs and into high aspect ratio nanoporous films using a rotating disc electrode (RDE). Recently, a technique for depositing directly onto a cylindrical substrate using a rotating cylinder electrode (RCE) has been developed with allows for direct integration of energy harvesting materials onto a shaft or tubular structure. EDS results of our cylindrical films show that we can control the composition of our alloy (10%-40% at.% Ga ) by adjusting the deposition potential 1.4 to 1.8V, the rotation rate 100 to 1000rpm, and by adjusting the ratio of Ga:Fe in solution. As deposited films have been measured using a capacitance bridge to have a lambda; between 50 and 120ppm, while our nanowires have been measured via atomic force microscopy to have a lambda; of 40ppm. With this unique ability to electrodeposit magnetostrictive material onto a wide range of substrates, new opportunity exist for developing integrated energy harvesting micro and nano-structured devices.

Among several piezoelectric materials, ZnO has been considered as one of the most promising candidates for the realization of nanogenerator due to their nontoxicity and cost effectiveness by comparison to other materials. However, the performance of ZnO piezoelectric nanogenerator (NG) has been limited largely due to excessive electrons existing in ZnO, which screens piezoelectric potential generated by mechanical deformation of the crystal structure. Nominally undoped intrinsic ZnO exhibits n-type behavior due to defect levels induced by interstitials and oxygen vacancies, leading to Fermi level close to the conduction band edge. Thus, large excessive electrons can be easily found even at room temperature. To alleviate this issue, several attempts have been made up to date. In our report, as an alternate solution, we investigated CuO, p-type metal oxide semiconductor with a bandgap of 1.35 eV, to form a metal oxide only CuO-ZnO p-n junction, where CuO naturally demonstrates p-type behavior due to cation-deficiency, acting as a hole conductor. In this approach, we demonstrated greatly enhanced piezoelectric energy conversion performance of ZnO flexible piezoelectric NG, exhibiting 7-fold higher output voltages and one order of magnitude higher output currents by comparison to the devices without CuO layer.

ZnO nanogenerators(NGs) have gained great attentions as a material system to fabricate NGs due to nontoxicity and cost effectiveness of ZnO by comparison to other materials. However, relatively low piezoelectric dipole moment coupled with potential screening effect by excess electrons in ZnO (wurtzite crystal structure) necessitates additional process steps to realize high performance NGs. In this respect, several attempts have been made to fabricate NGs with perovskite ceramic materials such as PZT and BaTiO3. Inherent piezoelectric properties of these materials are superior to ZnO, providing a more advantageous platform to realize high performance NGs. Several reports have been made based on these material systems. In our work, we investigated NGs based on BaTiO3 nanoparticle composite, which is more environmental friendly material compared with PZT. The NGs were fabricated by forming a multilayer structure with BaTiO3-PVDF composite. Compared with previously demonstrated BaTiO3 composite based NG, our approach provides relatively simple fabrication process, allowing multiple layer stacking, which results in enhanced output power per unit area. After polling process at high electric field, the NGs consistently exhibits over 8 V peak-to-peak output voltage with a current density fo ~1.3 uA/cm^2 at normal bending conditions on a bending stage. Upon the applied pressure with a finger tip normal to the device surface, the devices show the output voltage and current density of ~50 V and ~10 uA/cm^2, respectively. Our simple approach allows to realize large scale high performance NGs with high flexibility and reliability.

The emergence of electronic devices/systems with unprecedented functionalities mandatorily requires portable, flexible and sustainable power sources. Energy harvesting technology that can efficiently generate electricity from ambient environmental energy is the prerequisite for the realization of such new power sources. But due to their intrinsic limitation of unstable and uncontrollable electrical output, it is desirable to hybridize them with energy storage devices into single units, which could be capable of sustainably providing a stable output through utilizing the energy in the environment.
For the electricity generation, mechanical energy is one of the best choices as energy source owing to its universally-existence in our living environment and human bodies. A new type of devices—triboelectric nanogenerators (TENGs)—based on contact electrification has been recently invented as an effective and adaptive technology to generate electricity from motions. However, because the device structure and material properties have not been optimized, the output is still insufficient for sustainably driving electronic devices/systems. Here, we demonstrated a rationally designed arch-shaped TENG as a flexible mechanical energy harvester with extremely-high power output. Through purposely introducing thermal stress on the surface, a flexible substrate was made into a naturally-bent shape, so that a steady gap formed between two triboelectric layers at strain-free conditions. This design facilitates the separation of the opposite tribo-charges, thus maximizes the electrical driving force. This unique structure, together with the surface modification of tribo-layers, helps to largely enhance the output voltage, and power density to 230 V and 128 kW/m3, respectively, with an efficiency as high as 10~39%. For the first time, it realized the instantaneous driving of tens of regular electronic devices (LEDs), and also the fully charging of a Li-ion battery. [1]
For the development of sustainable power sources, we further hybridized the arch-shaped TENG with a flexible Li-ion-battery into as a single device—a flexible self-charging power unit (SCPU), which allows a battery to be charged directly by ambient mechanical motion. This physical hybridization enables a new operation mode: the “sustainable mode”, in which the environmental mechanical energy is scavenged to charge the battery while the battery keeps driving an external load. In this mode, the demonstrated SCPU can provide a continuous and sustainable DC current of 2 µA at a stable voltage of 1.55 V for as long as there is mechanical motion/agitation. It can be used to continuously drive a UV sensor for extended period of time. Thus, the SCPU can serve as an independent and sustainable power unit, which will meet the general requirement of almost any electronic device. [2]
[1] Wang, S. H.; Wang, Z. L. et al. Nano Lett. 2013, 13, 2226-2233.
[2] Wang, S. H.; Wang, Z. L. et al. ACS Nano Under review.

One-dimensional (1D) piezoelectric and ferroelectric nanomaterials, such as PZT, ZnO nanowires, have attracted great attention due to their fascinating properties and potential applications ranging from high-density ferroelectric random access memory (FeRAM) and high performance nanogenerators [1,2]. In particular, hierarchical complex structure on the basis of ZnO nanowires has been designed and prepared to realize novel nanodevices in the recent work [3,4]. In this study, we apply a two-step hydrothermal process to fabricate hierarchical PbTiO3/ZnO (PTOZ) nanostructures. Unique brush-like three-dimensional (3D) nanostructures of the samples was achieved, where secondary ZnO nanorods grew on the surface of the single-crystal nanowires of ferroelectric tetragonal PbTiO3. The number density and the length/diameter ratio of the secondary ZnO can be easily tailored by changing the reaction time. Photoluminescence (PL) properties of the as-prepared and post-annealing treated samples were studied at room temperature. Compared with pure ZnO nanorods, such hybrid nanostructures show a stronger broad visible emission centered at 632nm. Moreover, after annealing treatment in air, the visible emission intensity of the samples was significantly enhancemed, accompanied with a red shift in the peak center.
The orange-red emission is commonly attributed to the presence of excess oxygen in the ZnO [5]. For the PTOZ nanostructures, we proposed that the polarization surface of PbTiO3 nanowires could make additional contribution to the orange-red emission in the hierarchical PTOZ nanostructures by changing absorption of oxygen on their surfaces. These hybrid 3D nanostructures could be ideal candidates for optoelectronic and piezoelectronics nanodevice applications.
Reference:
1. Xiao, Z.; Ren, Z. H.; Liu, Z. Y.; Wei, X.; Xu, G.; Liu, Y.; Li, X.; Shen, G. and Han, G. R. J.Mater.Chem.,2011, 21, 3562.
2. Wang, Z. L.; Zhu, G.; Yang, Y.; Wang, S. H. and Pan, C. F. Materials Today 2012, 15, 532-543.
3. Cheng, C. W.; Liu, B.; Yang, H. Y.; Zhou, W. W.; Sun, L.; Chen, R.; Yu, S. F.; Zhang, J. X.; Gong, H.; Sun, H. D. and Fan, J. H. ACS Nano. 2009, 10, 3069.
4. Qin, Y.; Wang, X. D. and Wang, Z. L. Nature. 2008, 451, 809.
5. Djuriscaron;icacute;, A. B.; Leung, Y. H.; Tam, K. H.; Hsu, Y. F.; Ding, L.; Ge, W. K.; Zhong, Y. C.; Wong, K. S.; Chan, W. K.; Tam, H. L.; Cheah, K. W.; Kwok, W. M. and Phillips, D. L. Nanotechnology 2007, 18, 095702.

In the field of organic electronics a major research focus lies on the solution processabilty of organic semiconductor materials. Although some materials are designed for solution processing, i.e. Phenyl-C61-butyric acid methyl ester (PCBM), it is repeatedly reported in literature that deposition of PCBM via thermal evaporation in vacuum is performed to achieve either ultrathin or very clean films suitable for e.g. photoelectron spectroscopy characterization.
We have investigated the effects of thermal evaporation on PCBM in a very interdisciplinary approach, applying physical characterization techniques such as photoelectron (PES) and infrared (IR) spectroscopy in combination with device engineering and chemical analysis using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), ultra performance liquid chromatography-coupled mass spectrometry (UPLC-MS), and nuclear magnetic resonance spectroscopy (NMR). The methods were applied to thin films prepared by both solution based deposition techniques and thermal evaporation. It is found, that evaporation proves to be an inefficient deposition route for thin layers of chemically pure PCBM. Changes in the IR spectrum of the PCBM films already indicate a change in the molecular structure of PCBM. The UPLC chromatogram of the redissolved organic film proves the formation of several molecular species, including bare C60. Especially the crucible residue after evaporation contains a large fraction of disintegration products. 1H-NMR of the crucible residues shows the formation various new functional groups in the side-chain of PCBM. The effect of degradation on the electronic properties was however found to be limited, as an almost unchanged ionization potential of 6.1 eV was determined with UPS for both the solution processed as well as the evaporated films. By simulating the evaporation conditions by TGA, PCBM sample purity was reduced to roughly 90 %. Bulk-heterojunction solar cells were fabricated with pure and thermally treated PCBM samples to investigate their impact on device performance.

The development of a method for wirelessly “radiating” power generated by biomechanical motion could enable new fundamental insights and applications in soft robotics, sensor networks and remote medical monitors. Biomechanical power harvesting is gaining interest due to decreasing power requirements in portable electronic devices. Various approaches have been investigated to harvest this biomechanical power, including piezoelectricity and triboelectricity. Significantly less explored is the use of magnetic materials for wireless harvesting of biomechanical power. For example, if a flexible magnetostrictive sample is interfaced with a biomechanical source, the persistent biomechanical motion will continuously deform the sample, which will radiate electromagnetic power.
Here we demonstrate the feasibility of wirelessly harvesting power from biomechanical motion using flexible magnetostrictive Terfenol-D nanoribbons, which were deposited, fabricated and transfer printed onto biocompatible silicone. A Terfenol-D film was sputterer-deposited onto an oxidized Si wafer substrate containing photolithographically patterned resist lines. Terfenol-D nanoribbons were obtained following lift-off. The transfer involved complete removal of the Si substrate, rendering the sample flexible. The flexible nanoribbons exhibited excellent magnetic characteristics; indeed, the observed saturation magnetization value of ~1.3 Tesla is, to our knowledge, the highest value reported to date for any sputtered Terfenol-D film. As a proof of principle for demonstrating the potential of the magnetostrictive nanoribbons to wirelessly harvest biomechanical power, flexible nanoribbon generators were attached to a wind-up toy capable of generating a flapping motion. The power wirelessly radiated from this system was then measured. The result indicates that magnetostrictive nanoribbons which are deformed periodically can transmit power to a receiver without any physical contact, enabling new advances in wirelessly harvesting power from a biomechanical source.

Interfaces between polymers and inorganic materials can be found at all levels of flexible organic electronics and photovoltaics, including in the active electronic or photovoltaic layers, at electrical contacts or interconnection lines, and within the protective encapsulation and transparent barrier layers. In the case of flexible protective barriers, the integrity of such polymer-inorganic interfaces are of critical importance to the lifetime of devices and their modules, since interfacial damage in the form of loss of adhesion or delamination can degrade device performance and create fast diffusion pathways for potentially harmful environmental species such as moisture and oxygen.
We present research methodologies for quantifying the effect of interfacial chemistry together with the role of environment, temperature and mechanical stress on the adhesion of model polymer-oxide interfaces. Measurements of adhesive strength are reported as a function of pre-exposure to accelerated aging conditions, including elevated temperature, ambient humidity, and irradiation with ultraviolet light. We demonstrate that adhesive strength is degraded after exposure to ultraviolet light, and use XPS measurements to correlate this degradation to photo-induced chemical changes at the interface. Importantly, we also show that the wavelength of the UV irradiation (UVA vs. UVB) affects many properties of the polymer-oxide system, including delamination behavior, adhesive strength, and the polymer yellowing rate. The importance of understanding such mechanisms of environmentally-assisted degradation of polymer-inorganic interfaces is underscored by the need for improved accelerated life testing procedures, which form the basis for more accurate lifetime predictions.

The development of a cheap, flexible, and reliable barrier film technology is critical for the integration and commercialization of flexible photovoltaics and organic electronics devices. A promising emerging barrier technology utilizes multiple thin films of alternating organic/inorganic layers that is amenable to roll-to-roll processing while maintaining the desired ultra-low diffusion barrier properties. In operation, the barrier films are subject to weathering effects such as diurnal temperature cycling, moisture and chemical erosion, and UV degradation. The organic/inorganic interfaces are highly susceptible to damage from these degradation sources and limit the operational lifetime of the barrier film. We demonstrate a novel method to increase the organic/inorganic interfacial adhesion strength in a model system of poly (methyl methacrylate) (PMMA) and silicon oxide through interfacial patterning. An array of nanoscale patterns is etched into the PMMA through the use of nanosphere lithography. A thin layer of silicon oxide is conformally deposited onto the patterned PMMA through plasma enhanced chemical vapor deposition. The patterned interface exhibit an order of magnitude increase in adhesion strength over that of a non-patterned interface. X-ray photoelectron spectroscopy (XPS) of the delamination surface shows that the fracture pathway changes from the oxide/PMMA interface in the unpatterned system to within the PMMA in the patterned system. Absorption spectroscopy was used to determine the effect of the nanostructured interface on the optical transmission of the samples. Applied to barrier films, interfacial patterning can be used to increase adhesion between the organic/inorganic layers and resistance of delamination under environment degradation.

Molybdenum (Mo) thin films were prepared on flexible polyimide (PI) substrates by direct current (DC) magnetron sputtering method. Mo thin films prepared at different sputtering gas pressures and powers have been characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), four-probe resistivity meter and adhesion test, respectively. The influence of different deposition factors on the crystal structures, surface morphology, electrical and adhesion properties of Mo thin films have been discussed. The experiment results indicate that increasing sputtering gas pressures lead to worse crystal structures, worse electrical properties and better adhesion; the design of a bilayer Mo thin films can achieve better electrical and adhesion properties; increasing sputtering powers result in better electrical and adhesion properties, but it will lead to worse crystal structures and electrical and adhesion properties after beyond a certain power value. Therefore, a bilayer Mo thin film deposited at a higher sputtering power is more suitable for the back contact electrode of CIGS thin film solar cells.

Range anxiety is considered as one of the major hurdles in the acceptance of a pure electric vehicle (EV) by the mass market. In order to alleviate this concern, auto makers are researching batteries which can store more energy per unit weight. However it might take decades of development to reach the energy density of gasoline even with the advanced formulations under consideration. The objective of this research is to consider an alternative approach which refreshes the batteries of an EV by delivering energy to the vehicle while in motion. Methods of directing intense energy beams to a moving target have been developed by Directed Energy Weapons Systems Program of the military and some of those can be adopted to EVs [1].
A typical EV has a large cross-sectional area (~10 m2) and a roof area which is about a third of the total cross-section (~3 m2). Three recharging schemes which take advantage of these aspects of the vehicle are proposed:
(a) By AC magnetic induction while the EV is moving at very slow speed, for example through a toll gate or temporarily stationary, for example waiting at a red light. AC current is provided to coils placed on the ground to create a time varying magnetic field perpendicular to the road surface. This magnetic field induces current in receiver coils wrapped around the EV. Frequency of the AC input needs to be set high (600 Hz or more) in order to provide a significant amount power during the brief presence of the vehicle over the coils.
(b) By permanent magnets placed on the road surface along highways. A static magnetic field perpendicular to the road surface is generated by sheets of magnets placed on the road surface. High speed motion of the vehicle over the magnetic field induces a current at receiver coils wrapped around the EV. Since this is a passive method which does not provide power, magnetic sheets are placed at downhill locations along the highway where the EV can maintain its motion with minimal battery usage.
(c) By a pulsed laser directed at photovoltaic (PV) panels placed on the roof of the EV. Diode lasers which are small and relatively inexpensive are installed on overhead structures such as traffic signs. The laser sends high power (kW) pulses to the roof of the EV as it approaches and passes below the laser unit. A conversion efficiency of 50% can be attained with a PV panel matched to the nearly monochromatic light of the laser.
Technical requirements necessary to refresh 1-3% of the energy stored in a typical EV battery will be provided during the presentation.
[1] R.J. Katt, "Selected Directed Energy Research and Development for U.S. Air Force Aircraft Applications," National Academies Press, Publication 18497, Washington, DC 2013

Encapsulation of electronic devices based on organic materials that are prone to degradation even under normal atmospheric conditions with hermetic barriers is crucial for increasing their lifetime. A challenge is to develop “ultrabarriers” that are impermeable, flexible, and preferably transparent. Another important requirement is that they must be compatible with organic electronics fabrication schemes (i.e. must be solution processable, deposited at room temperature and be chemically inert). Here, we report lifetime increase of 1,300 hours for poly(3-hexylthiophene) (P3HT) films encapsulated by uniform and continuous thin (~10 nm) films of reduced graphene oxide (rGO). This level of protection against oxygen diffusion is substantially better than conventional polymeric barriers such as CytopTM, which degrades after only 350 hours despite being 400 nm thick. Analysis using atomic force microscopy, x-ray photoelectron spectroscopy and high resolution transmission electron microscopy suggest that the superior oxygen gas barrier property of rGO is due to the close interlayer distance packing and absence of pinholes within the impermeable sheets. These material properties can be correlated to the enhanced lag time of 500 hours. Our results provide new insight for the design of high performance and solution processable transparent “ultrabarriers” for a wide range of encapsulation applications.
H.Yamaguchi et al. "Reduced Graphene Oxide Thin Films as Ultrabarriers for Organic Electronics" Adv. Energy Mater. (2013), Article first published online: 10 OCT 2013, doi: 10.1002/aenm.201300986

Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) has been considered as one of the potential and useful conducting polymers for various optoelectronic applications because of its solubility, flexibility, low cost and conductivity. It is accepted that quantum yield of films are less than their solution counterparts and has a “temporal tail” in the decay dynamics. We try to understand if a correlation exists between the “temporal tail” and quantum yield in films. We investigate the origin of a “temporal tail” by a temperature dependent study of the spectra. We also study the electric-field effect on the “tail” and describe the results on the basis of a model that implies that the long-lived `tail&’ luminescence in film is a result of energy transfer from “ H-like aggregates” present in the film.

Piezo-phototronic effect is demonstrated in the ZnO/P3HT solar cell system and detailed study is conducted regarding the influence of crystallization and doping level of ZnO on the strength of piezo-phototronic effect as well as the overall solar cell performance. By testing and comparing five groups of samples prepared under different conditions, optimized parameters are determined for the most efficient piezo-phototronic enhancement of solar cell performance. The general principles and regularities provided in this study are universal and applicable to all solar cell systems involving piezoelectric semiconductor materials and could provide substantial guidance on further increasing performances of commercial solar cells based on CdTe, GaAs etc. and also spur the development of flexible solar cells for smart applications in various situations.

Failures in electronic devices can be classified as i) catastrophic failures which destroy the device or ii) degradation failures which causes a reduction in device performance. Device bias, resultant channel temperature, passivation, and material interactions may all cause or contribute to failure. For the analysis of such types of defects, it is very important to access the ‘heart&’ of the device and to observe the front side of the die. This is done by removing the epoxy resin which plays an important part in insulation between the electrical parts of the device as well as to provide protection against atmospheric corrosion. The most widely used solution is to use the laser decapsulation process along with chemical depackaging technique to obtain a good surface of the die for further analysis. The use of silica for reinforcement of polymeric insulation like epoxy resin in microelectronic packaging industry is essential for its dielectric and thermal properties since, addition of silica improves the thermal dissipation . However, decapsulation of chips, tend to leave behind minute traces of this epoxy which remain bound to crevices of the semiconductor device.
In this paper, decapsulation studies were carried out on Gallium Nitride High Electron Mobility Transistor (GaN HEMT) devices and the physics of degradation attributed to epoxy was analyzed. Although the sealant materials are supposedly electrically inactive, a contribution to the device performance is observed. These electrically active traces are believed to contribute to the degradation in the performance of the device as evident by the IV measurements for different gate voltages. The use of photoemission microscopy for the analysis of this particular type of packaging related failure has been put forward in this work. The main aim of this paper is to study the effects of the remnant material on the performance of the component and to analyze its contribution towards electrical degradation by using photoemission microscopy.

Single wall carbon nanotubes (CNT) were functionalized to p- or n-type materials, and a thermoelectric power generating device made of CNT films was designed and fabricated to operate a glucose sensor. The electronic transport properties were optimized by adjusting the amount of surfactants on CNTs for p-type films as well as reducing CNTs with multiple dopants for n-type. The thermopower and electrical conductivity of optimized n-type nanotubes were measured to be -86 µV/K and 5,200 S/m, respectively. The p-type nanotube samples have a thermopower and electrical conductivity value of +100 µV/K and 11,000 S/m, respectively. Electronic band structures with the Fermi level and carrier mobilities of the CNTs were experimentally investigated, which have elucidated the carrier type and relatively large thermopower values.
The thermoelectric device was made of 72 p-type and 72 n-type CNT films electrically connected in series and thermally in parallel so as to maximally utilize temperature gradients. This device produced 465 mV at a temperature gradient of ΔT = 49 K, which is much larger than those of typical polymer based composites. The thermoelectric device was integrated with a glucose sensor and the self-powered sensor system was operated by using 1.8 µW from the thermoelectric device at ΔT = 32 K. We demonstrated that flexible organic thermoelectric devices are promising for portable and self-powered sensors.

At present considerable research interest exists for magnetic structure driven ferroelectricity in double perovskite oxides. We report an investigation of such properties in Y2XMnO6 (X = Co, Ni). On the basis of magnetic and crystal environment, we expect multiferroicity in these compounds that crystallize in P21/n space group. Under this symmetry, Co2+/Ni2+ are arranged alternately with respect to Mn4+ (along c-axis) in an one dimensional Ising chain. This leads to the development of up-up-down-down spin ordering below the magnetic ordering temperature. We have also synthesized phase pure Y2(Cu/Zn)MnO6 that do not possess P21/n symmetry and therefore no features related to magneto-electric coupling are expected.
In Y2CoMnO6, both DC and AC susceptibility as a function of temperature shows anti ferromagnetic transition at ~ 80K. By measuring pyroelectric current as a function of temperature, we confirm emergence of spontaneous ferroelectric polarization below the magnetic ordering temperature. We note that such high temperature improper ferroelectricity due to spin ordering is extremely rare. We identify that the up-up-down-down magnetic symmetry with the alternate magnetic ions along c-axis could give rise to such phenomena due to magneto-striction. The behaviour of induced polarization with varying poling field and temperature cycling confirm the development of genuine ferroelectricity at such high temperatures. This development of ferroelectricity with up-up-down-down ordering along c-axis is also confirmed by neutron diffraction measurement at T = 2.7 K. The saturation polarization is estimated to be ~ 65µC/m2 and saturation magnetization at 5T corresponds to 6.2 µB/ f.u. Preliminary results on Y2NiMnO6 confirm anti- ferromagnetic transition at TN ~ 86 K but the associated ferroelectricity is yet to be unambiguously established due to leaky characteristics.

Energy harvesting with piezoelectric material provides a compact and reliable method to convert the irregular mechanical energy in the environment into electric energy. Compared with traditional polycrystalline ceramics, single crystal piezoelectric nanostructures are more flexible and robust and they can accommodate larger strain input, which leads to a higher power density. Because of the non-toxicity and various existed synthesis methods, piezoelectric zinc oxide (ZnO) nanowire is widely studied for this application. ZnO is also a semiconductor and the internal free electron can partially screen the generated potential, while a Schottky contact helps block the charge from flowing through the nanowire. The effect of the free charge and the Schottky contact adds on the length dependence of the output from a nanowire, while such study is still lacking. Here by depositing a series of electrodes on a single ZnO nanowire sited on a flexible substrate, we characterized the energy conversion from a single nanowire with different length and different types of contact. This study shows the influence of the dimension and the semiconducting property of a nanowire on its mechanical-electric energy conversion, and it provides a critical design guideline for a high output ZnO nanogenerator.

Piezotronics based on piezoelectric fine wires have show diverse of potential applications [1-3]. A piezotronic device is normally characterized by its capable of output pulsed piezoelectrical signals upon a load of strain. Strain sensor or tactile sensor could be a promising application for piezotronic devices [1, 2]. This field is pioneered by Zhong Lin Wang&’s group. Zhong Lin Wang&’s group has just demonstrated a tactile imaging device based on an array of piezotronic transistors [1]. This work opened a broad range of potential applications such as human-electronic interfacing, smart skin and micro- and nanoelectromechanical systems. The unique thing of this type of tactile sensor is that its function mechanism is similar with that of tactile sensing of human beings, which is essentially a bio-piezo effect.
In this report, we demonstrate a horizontally aligned piezotronic component based on a ZnO piezoelectric fine wire bridging between two copper electrodes. We integrate a fiber across the ZnO piezoelectric fine wire and so it forms a ridge across the piezotronic component after PDMS packaging. The ridge over the piezotronic component serves as a strain introducing component mimic human being finger prints. The devices are used to touch (press and slide on) a series of objects with period textures on the surface. The output piezoelectric pulses are recorded and correlated to the period textures of the objects been touched.
This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 61172041, 91123018, 61172040), and the Fundamental Research Funds for the Central Universities. The authors also thank Prof. Yong Qin for his valuable comments on the device fabrications.
Reference:
[1] Wenzhuo Wu, Xiaonan Wen, Zhong Lin Wang, Taxel-Addressable matrix of vertical-nanowire piezotronic transistors for active and adaptive tactile imaging, Science 340, 952 (2013).
[2] Rusen Yang, Yong Qin, Liming Dai and Zhong Lin Wang, Power generation with laterally packaged piezoelectric fine wires, Nature Nanotechnology, 4, 34-39 (2009).
[3] Jun Zhou, Yudong Gu, Peng Fei, Wenjie Mai, et al., Flexible Piezotronic Strain Sensor, Nano Lett. 8, 3035-3040 (2008).

I will present methods to fabricate, test and characterize ZnO nanorod-based energy harvesters and the impact of surface passivation of the ZnO on power output for a vibrational harvesting system and a photovoltaic. Previous studies have focused on improving the performance of these devices. In this regard, suppression of semiconductor carrier screening and improvement in device architecture had been reported. However, the impact of ZnO nanorod passivation on p-n junction devices has not been studied.
The first section of the presentation will discuss the passivation of ZnO nanorods using CuSCN in ZnO/PEDOT:PSS (poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)) p-n junction-type piezoelectric energy harvesting devices. The maximum power density across its optimal load is determined by testing the device across variable resistors showing higher output from passivated devices. The increase in voltage and voltage-driven current is discussed with relation to changes in the carrier lifetime.
A comparison between passivated and non-passivated ZnO devices shows that the passivated device generated an open-circuit voltage of 212 mV and short circuit current of 1 mA/cm2 which was around twice the value of non-passivated device (90 mV and 0.66 mA/cm2). When tested across a range of load resistances, the maximum power density was also found to have almost doubled from 36.00 µW/cm2 across a 1.38 k#8486; load for the non-passivated device to 64.40 µW/cm2 (1.67 k#8486;) for the passivated device. We deduce that the voltage and voltage-driven current density of the passivated ZnO device improved due to three-fold increase in carrier life time.
The second part of the presentation will focus on the photovoltaic (PV) effect of perovskite materials and methods of passivating the surface of ZnO to enable thin film growth. I will report on the design, fabrication and testing of a solid-state perovskite enhanced ZnO solar cell. The p-type perovskite material used was bismuth ferrite (BFO), so far not implemented in a ZnO solar cells.
The cell uses FTO and Au electrodes sandwiching semiconductor layers and a CuSCN hole conductor, the ZnO was sensitized with N719. XRD confirmed the presence of BFO with the presence of a single [110] BFO peak. The results show a significant increase in the performance with efficiencies increased by almost 3 times with the BFO coating with an efficiency of 0.272%. The BFO acts as a buffer from electron recombination. However, the relative increase in performance of the solar cell when BFO was added shows promise of BFO as a PV material.
In summary I will show that there are a range of energy harvesting devices that can be produced using a surface passivation layer on a ZnO nanorod architecture and that by careful control of the process parameters enhanced energy harvesting is possible.

In this work, we developed a new thin-film-based flexible technology for both energy generation and contact sensing. The device is rationally structured through stacking layers of flexible substrates, transparent electrodes, and electrification layers. By a unique operating principle that couples contact electrification and electrostatic induction, the device is able to produce electricity between electrodes as a result of contact with other objects. On one hand, it can be effectively used for power generation. Triggered by human foot steps, a device with dimensions of 15 cm by 15 cm could deliver a open-circuit voltage of 800 V and a maximum short-circuit current of 3 mA, corresponding to a maximum power density of 4.2 W/m2 that was delivered to an external load. As a direct power supply, it successfully lighted up over 1,000 LED bulbs simultaneously. On the other hand, it can also be applied for sensing applications as a self-powered sensor that does not require external power supplies. For a contact object that has an area of 6.5 cm2, the device exhibits an ultra-high sensitivity of 0.9 V/Pa. The produced electric signal from a device that was embedded under a carpet has been demonstrated to trigger a wireless alarm system when people walked by. Such a demonstration was also applicable for direct contact from a human finger and hand grabbing, indicating immediate applications of the technology in security, surveillance, tactile imaging, and touch pad technology, among others.

It would be of great interest to develop energy harvesters that can convert ambient mechanical energies such as vibrations or air flows to power wireless devices such as sensors, controllers, transmitters and receivers. Over the past decade, piezoelectric cantilever (PC) energy harvesters have been increasingly investigated for this application. Traditional PCs consist of equal length of piezoelectric and nonpiezoelectric layers. Here we investigated the energy harvesting performance of PCs with unequal piezoelectric and nonpiezoelectric lengths (two-section PCs) both theoretically and experimentally. Under the application of step-wise tip forces it was shown that a longer nonpiezoelectric layer is preferred for generating a higher induced voltage while a longer piezoelectric layer reduces the induced voltage due to charge cancellation.^1 On the other hand under harmonic base vibrations, there exists an optimal nonpiezoelectric-to-piezoelectric length ratio at which output voltage, current, and power can be maximized.^2 Theoretically, a two-section PC was considered within the framework of a beam theory for step-wise tip forces and within the framework of Euler-Bernoulli beam theory with electromechanical coupling for harmonic base vibrations. The results of the theoretical analysis were in good agreement with the experiments.
To utilize air flow, we present a piezoelectric flow energy harvester (PFEH) by adding a cylindrical extension to a piezoelectric cantilever. The flow induced vibration of the cylindrical extension causes the piezoelectric cantilever to vibrate at the natural frequency of the PFEH. The PFEH provides a low-cost, compact and scalable power source for small electronics by harvesting energy from ambient flows such as wind and water streams.^3 Prototypes were tested in both laminar and turbulent air flows demonstrating the feasibility of the design. Turbulence excitation was found to be the dominant driving mechanism of the PFEH with additional vortex shedding excitation contribution in the lock-in region. The voltage and power output of the PFEH increased with increasing size of the extension. We also showed that a rectangular extension helped generate a higher voltage and power than a circular one of the same dimensions. Finally, the PFEH was successfully used to power a green LED intermittently with an instant power output of more than 12 mW by using a charge storage capacitor and a low-power temperature sensor continually with an average power output of 18 mu;W at 4.7 m/s wind velocity.
References
1. X. Gao, W. Y. Shih, and W.-H. Shih, Smart Mater. Struct., 18, 125018 (2009)
2. X. Gao, W.-H. Shih, and W. Y. Shih, Appl. Phys. Lett. 97, 233503 (2010)
3. X. Gao, W.-H. Shih and W. Y. Shih, IEEE Transactions on Industrial Electronics, 60, 1116-1118 (2013)

Triboelectrification is an effect that is known to each and every one probably ever since the ancient Greek time, but it is usually taken as a negative effect and is avoided in many technologies. We have recently invented a triboelectric nanogenerator (TENG) that is used to convert mechanical energy into electricity by a conjunction of triboelectrification and electrostatic induction. As for this power generation unit, in the inner circuit, a potential is created by the triboelectric effect due to the charge transfer between two thin organic/inorganic films that exhibit opposite tribo-polarity; in the outer circuit, electrons are driven to flow between two electrodes attached on the back sides of the films in order to balance the potential. Ever since the first report of the TENG in January 2012, the output power density of TENG has been improved for five orders of magnitude within 12 months. The area power density reaches 313 W/m2, volume density reaches 490 kW/m3, and a conversion efficiency of ~50% has been demonstrated. The TENG can be applied to harvest all kind mechanical energy that is available but wasted in our daily life, such as human motion, walking, vibration, mechanical triggering, rotating tire, wind, flowing water and more. Alternatively, TENG can also be used as a self-powered sensor for actively detecting the static and dynamic processes arising from mechanical agitation using the voltage and current output signals of the TENG, respectively, with potential applications for touch pad and smart skin technologies. The TENG is possible not only for self-powered portable electronics, but also as a new energy technology with a potential of contributing to the world energy in the near future.
[1] Z.L. Wang “Triboelectric Nanogenerators as New Energy Technology for Self-Powered Systems and as Active Mechanical and Chemical Sensors”, ACS Nano, DOI 10.1021/nn404614z.

Nano electromechanical systems (NEMS) developed using piezoelectric nanowires have gained an immense interest in energy harvesting applications as they are able to convert several different forms of mechanical energy sources into electric power and thereby function as reliable power sources for ultra-low power wireless electronics. In this work, a piezoelectric nanowire vibrational energy harvester is fabricated through the development of a synthesis process for vertically aligned barium titanate (BaTiO3) nanowire arrays directly on a conductive substrate. These poled ferroelectric NW arrays are characterized through direct vibration excitation and demonstrated to provide efficient harvesting of mechanical vibrational energy producing an average power density of ~6.27 mu;W/cc from 1g acceleration. In order to substantiate the superior energy harvesting performance of the newly developed BaTiO3 NW arrays, a direct comparison is made with conventional ZnO NW arrays. It will be shown that the ferroelectric BaTiO3 NW energy harvester has ~16 times greater power density than the ZnO NW NEMS energy harvester from the same acceleration input. In addition to energy harvesting applications nanowire arrays can also find use in dynamics sensors, however, to date, no results have demonstrated the capability to use aligned piezoelectric nanowire arrays as a highly accurate nano-electromechanical system-based dynamic sensor with a wide operating bandwidth and high coherence. It will also be shown that the ferroelectric nanowires can provide excellent sensitivity to dynamic excitation and function as an accurate accelerometer. A high sensitivity of up to 50mVg-1 is observed from the sensor composed of vertically aligned barium titanate nanowire arrays, thus providing performance comparable to many commercial accelerometer systems.

More than 50% of total input energy is wasted as heat in various industrial processes. If we could harness a small fraction of the waste heat while satisfying the economic demands of cost versus performance, then thermoelectric (TE) power generation could bring substantial positive impacts. To meet these demands single-crystal semiconductor nanowire networks have been investigated as a method to achieve advanced TE devices because of their predicted large reduction in thermal conductivity and increase in power factor.
To further our goal of developing practical and economical TE devices, we designed and developed a material platform that combines a semiconductor nanowire network and a semiconductor thin film integrated directly on a mechanically flexible metallic substrate. We assessed the potential of this platform by using indium phosphide (InP) nanowire networks and a doped poly-silicon (poly-Si) thin film combined on copper sheets. InP nanowires were grown by metal organic chemical vapor deposition (MOCVD). In the nanowire network, InP nanowires were grown in three-dimensional networks in which electrical charges and heat travel under the influence of their characteristic scattering mechanisms over a distance much longer than the mean length of the constituent nanowires. Subsequently, plasma-assisted CVD was utilized to form a poly-Si thin film to prevent electrical shorting when an ohmic copper top contact was made. An additional facet to this design is the utilization of multiple materials to address the various temperature ranges at which each material is most efficient at heat to energy conversion. The utilization of multiple materials could enable the enhancement of total power generation for a given temperature gradient. We will investigate the use of poly-Si thin films combined with InP nanowires to enhance TE properties. TE parameters, power production, and challenges of a large area nanowire device on a flexible metallic substrate will be presented. We will discuss our design and testing of a new large area, scalable, mechanically flexible TE devices.

A way to response the energy consumption and to produce sustainable nonpolluting power source is energy harvesting from our environment, as a renewable energy source. This energy source are always available everywhere, such as a vibration, wind, fluid, etc. Recently, there have been attempts to fabricate energy generators using piezoelectric, pyroelectric, triboelectric materials, and so on. Here, we concurrently use piezoelectric and triboelectic effects to produce a high output power. Contact electrification between the PVDF film and a PTFE sheet generate electrical signals by triboelectric and electrostatic effects, and the PVDF film also produce piezoelectricity when applied a mechanical force. An arch-shape PI film was attached to the PVDF film, which plays role in driving force for bending and releasing. An Al and two Au electrodes were deposited on the PTFE sheet and the both sides of the PVDF film, respectively. In the experimental results, the PVDF film produced higher output current than the triboelectricity, in contrary the output voltage from triboelectric effect was much higher than the PVDF film when being pressed the generator. Finally, the two effects were connected parallelly, resulting in the output voltage of ~ 100 V and the output current of ~220 uA in dimension of 7 X 4 cm2. The generating power was successfully tested with 60 LEDs lighting. Thus, we expect that this hybrid harvesting technique using triboelectric and piezoelectric effects can be used in various applications as a power source and apply to electronics directly.

Polymer solar cell (PSC) technology is an efficient and cost-effective solution for harvesting of solar energy. Transparent PSCs, in particular, have great potential for building-integrated photovoltaics, where the photovoltaic elements can simultaneously serve as building materials and power generators while maintaining an attractive building exterior.
In this talk, I will talk about high-performance, visibly transparent or semi-transparent polymer solar cells fabricated via solution processing. The photoactive layer of the single-junction PSCs harvest solar energy largely from the near-infrared region while being less sensitive to visible photons. In a conventional single junction device consisting of a highly transparent silver nanowire metal oxide composite top electrode, a 4% power-conversion efficiency (PCE) was achieved with a maximum transparency of 66% at 550 nm. To further enhance the performance and tune the external appearance, tandem device architectures were employed. New low bandgap polymers based on benzodithiophene and diketopyrrolopyrrole units were designed and synthesized. A novel interconnecting layer between the two sub-cells was also designed to make efficient tandem devices. As a result tandem devices recorded a high PCE of 7% with a maximum transparency of 30% at 550 nm while a PCE of 6% was achieved with a maximum transparency of 40% at 550 nm (device area = 0.1 cm2). Finally, we demonstrate non-transparent flexible devices with PCEs over 4% and large area (device area = 10 cm2) devices with PCEs over 3.5%. By incorporating transparent silver nanowire electrode, we believe semi-transparent, large area devices with PCE over 2% is in reach shortly.

Graphene is a promising electrode material for various flexible energy applications, but its conductivity and work-function need to be optimized to enhance the performance of graphene-based energy devices. This can be performed by various doping methods including atomic substitution, molecular adsorption, covalent functionalization, substrate induced doping, and the use of metallic thin films or nanoparticles. The substitution of carbon atoms with B or N as well as the covalent functionalization are advantageous in terms of long-term stability, which is, however, unfavorable due to significant decrease in carrier mobility and conductivity. Thus, we report a simple method to tune the electrical properties of CVD graphene by molecular vapors, where the dopants in vapor phase are mildly adsorbed on graphene surface without direct contact with solution. The vapor-phase doping provides not only very high carrier concentration but also good long-term stability in air, which is particularly important for practical energy and display applications.
On the other hand, graphene quantum dots (GQDs) exhibit great potential for various optoelectronic applications due to their size-dependent and edge-sensitive photoluminescence properties. Previously, GQDs were mainly synthesized from graphene oxides by chemical exfoliation under strongly acidic environment or by multi-step lithographic methods including masking, patterning and lift-off, which hindered the efficient preparation of high-quality GQDs for practical applications. In addition, nitrogen-functionalization or doping is known to be very helpful to tailor the intrinsic properties of GQDs, but it needs further complicated wet-chemical reactions. Here we present a very simple solvent-free method to prepare nitrogen-doped GQDs (N-GQDs) by directly applying nitrogen plasma to as-grown graphene on Cu. The resulting N-GQDs can be transferred as a film-like layer or easily dispersed in an organic solvent. We confirm that the N-GQDs whose sizes are narrowly distributed around ~5 nm show strong photoluminescence when excited at 365 nm, which would be useful for a wide range of optoelectronic, electrochemical, and energy storage applications.

The fragility of many modern organic semiconductors might limit their applicability in applications demanding mechanical compliance. For example, many pure polymers, polymer-fullerene blends, and the transparent conductor indium tin oxide all fracture at tensile strains of two percent or less on compliant substrates. The design of organic semiconductors that can be deformed significantly would facilitate roll-to-roll production, mechanical robustness for potable applications, conformal bonding to curved surfaces other than cylinders, and would enable large-scale solar farms based on ultra thin organic modules that can survive forces of the outdoor environment. This paper describes our efforts to understand and control the structural parameters that influence the mechanical properties of modern conjugated polymers. Our conclusions include the effect of the side chain in determining the elasticity, ductility, and adhesion of polymers and their blends with fullerenes, and how this effect can be predicted by theory. Ultra-compliant materials are used for the first time in solar cell that can be stretched and conformed to hemispherical surfaces without damage. We also describe the synthesis of all-conjugated block copolymers whose goal is to maximize both electronic properties and mechanical compliance. A new polymerization based on cross-coupling of conjugated “macromonomers” enables block-copolymer-like materials from step-growth polymerizations. Mechanical and photovoltaic properties of these segmented copolymers are also reported. Our results should inform the engineering of new semiconducting polymers for flexible and stretchable applications.

Recent advancements in the processing of electronically monodisperse carbon nanotubes have enabled the exploration of a wide range of functional devices based on random networks of nanotubes. In particular, thin film transistors (TFTs) using solution-processed semiconductor-enriched nanotubes have been demonstrated to exhibit excellent electrical properties with high uniformity on both rigid and flexible substrates. High hole mobilites of up to 50 cm2/Vs with high Ion/Ioff of up to 106 have been reported for nanotube TFTs. Here, we monolithically integrate organic photodetectors (OPDs) made of regioregular poly(3-hexylthiophene) (P3HT) and [6, 6]-phenyl C61-butyric acid methyl ester (PCBM) on top of a nanotube backplane consisting of an 18×18 pixel array (physical size of 2 cm by 1.5 cm). The absorption spectrum of P3HT:PCBM bulk-heterojunction photodetectors is optimal for visible light imaging applications. Furthermore, by integrating a Gd2O2S:Tb (GOS) scintillator film on top of the imager, a mechanically flexible X-ray imager is demonstrated.

In this talk, I will discuss new polymers we have been developing for high performance solar cells. We also developed a variety of functional polymers for Si-based lithium ion battery electrodes.

The broad application of solar energy harvesting in everyday products, e. g. by integration in clothing, requires mechanically flexible photovoltaic (PV) cells to be available in large volumes and at low costs. PV devices with indium tin oxide (ITO) free transparent electrodes printed on plastic foils are promising candidates for this approach, because they can be produced on large scale in an industrial roll-to-roll fabrication process. In order to compensate for the rather high resistive losses in ITO-free transparent conductive materials, current collecting grids comprising high electrical conductivities need to be included to reduce the sheet resistances. Printing and subsequent sintering of conductive inks or pastes based on metal nanoparticle dispersions is a convenient deposition method for these structures. The traditional approach of sintering by prolonged heating has the decisive disadvantage of being rather slow, thereby requiring long processing times and being difficult for application in R2R manufacturing. This is especially true if inexpensive commodity polymers are used as the substrate materials, which cannot withstand high temperatures. In addition, inks containing nanoparticles based on non-noble metals are prone to oxidation during thermal sintering, which demands working under protective atmosphere. Sintering methods which are fast enough to allow R2R processing at reasonable speeds and avoid oxidation even when operating in air are therefore in high demand.
In this contribution, photonic flash sintering is presented as a fast and highly efficient processing technology for improving the conductivities of printed metal nanoparticle structures on plastic foils. Due to the selective heating of the metal deposits by the absorption of visible light flashes, for which the substrate itself is transparent, high temperatures can be achieved locally for very short times. Using this approach, resistivity drops of more than six orders of magnitude are achieved within fractions of a second. For silver based inks, both integration in printed organic solar cells with improved power conversion efficiencies and R2R application have been demonstrated. Because the photonic sintering process occurs much faster than the oxidation of copper nanoparticles, the technology can also be applied to copper inks without the need of a protective atmosphere.

In this work, we presented the fabrication and testing of four types of stretchable electrodes design for piezoresistive sensor base on silver-nanowires (AgNWs) and polydimethylsiloxane (PDMS). The effect of oxygen plasma treatment and electron beam evaporation (e-beam evaporation) on the stretchability and contact resistance between the electrodes and piezoresistive material were investigated with the assistance of SEM images and EDS spectra. The oxygen plasma treatment could degrade the stretchability and contact quality, but improved the metal layer evaporated on the AgNWs/PDMS conductor. The e-beam evaporation could optimize the contact quality and insulate the AgNWs network from effect of oxygen plasma treatment. However, the untreated PDMS usually had a poor adhesion with the metal layer. So for the performance improvement of AgNWs/PDMS electrode, efforts should be made on design of the fabricating process or introduction of new technology that could keep the density of the AgNWs network, meanwhile enhance the adhesion strength of metal layer to the AgNWs/PDMS conductor. The results could be used to guide the fabrication of qualified electrodes that comprise high stretchability and low contact resistance for skin-like piezoresistive sensor.

Two anthracene-based star-shaped conjugated small molecules, 5',5''-(9,10-bis((4-hexylphenyl)ethynyl)anthracene-2,6-diyl)bis(5-hexyl-2,2'-bithiophene), HBantHBT, and 5',5''-(9,10-bis(phenylethynyl)anthracene-2,6-diyl)bis(5-hexyl-2,2'-bithiophene), BantHBT, were used as electron-cascade donor materials by incorporating them into organic photovoltaic cells prepared using a poly((5,5-E-alpha-((2-thienyl)methylene)-2-thiopheneacetonitrile)-alt-2,6-[(1,5-didecyloxy)naphthalene])) (PBTADN):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) blend. The small molecules penetrated the PBTADN:PC71BM blend layer to yield complementary absorption spectra through appropriate energy level alignment and optimal domain sizes for charge carrier transfer. A high short-circuit current (JSC) and fill factor (FF) were obtained using solar cells prepared with the ternary blend. The highest photovoltaic performance of the PBTADN:BantHBT:PC71BM blend solar cells was characterized by a JSC of 11.0 mAcm-2, an open circuit voltage (VOC) of 0.91 V, a FF of 56.4%, and a power conversion efficiency (PCE) of 5.6% under AM1.5G illumination (with a high intensity of 100 mW-2). The effects of the small molecules on the ternary blend were investigated by comparison with the traditional poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) system.

There is an inevitable need to increase our dependence on renewable and sustainable energy resources. In addition to harvesting energy from the solar, wind, and biomass resources, there is a decent amount of work being done on harvesting energy from smaller resources such as the human body for smaller mobile energy needs such as charging or powering gadgets. Cotton, composed of cellulose and hemicellulose and used for apparels, is well known for both its thermal and electrical insulating properties. However, it needs to be electrically conductive to enable transfer of small amounts of electrical charges. In this study we present a facile technique of coating graphene layers on cotton fibers, making them electrically conductive without compromising its mechanical properties. The electrically conductive cotton as apparel can be potentially used to monitor and analyze one&’s heart rate and breathing and cool down one&’s body on a hot summer's day. Scanning electron microscopy (SEM) was used to carry out morphological studies of the surface and interface of graphene layers and cotton fibers. An insulation resistance meter and a digital voltmeter were used to measure the electrical conductivity of pristine and the graphene coated cotton fibers respectively.

PNNL is developing a new generation of micro-batteries based on lithium/ carbon monofluoride (Li/CFx with x = 1) chemistry. This talk will review the fundamental chemistry, engineering design and the applications of such micro-batteries. The project was motivated by the Juvenile Salmon Acoustic Telemetry System (JSATS) project supported by the U.S. Army Corps of Engineers, Portland District, to develop the smallest micro-acoustic transmitter commercially available to date. To study smaller fish and permit implantation by injection using a needle, the JSATS micro-acoustic transmitter was reduced in weight and volume. The micro-battery achieves a steady high-rate pulse current with required lifetime while significantly reduces the weight and volume. The newly designed micro-batteries have intrinsically lower impedance than the batteries currently used in JSATS transmitters, leading to significantly improved electrochemical performances within a wide operating temperature range from -5°C to 25°C. The high power, long life micro-batteries can have potential for many other applications and have been used as in-situ probe to understand the chemical transformations in advanced lithium batteries.

Photovoltaic materials convert photon energy into stored electrical energy that can be used to generate mechanical work. Less attention has been paid to the direct conversion of light into mechanical work. Such photomechanical materials can be useful not only for solar energy conversion, but also as power sources for nanomachines. In order to harness photomechanical materials as power sources for nanomachines design, their photomechanical response needs to be (i) reversible, (ii) stable and (iii) fast. We have used molecular crystalline nanometer and micron-scale structures that absorb near ultraviolet and visible light and then execute well-defined motions such as twisting, bending and coiling. Examples include microribbons composed of 9-anthracenecarboxylic acid (9AC) that twist and untwist repeatedly under uniform illumination condition; and nanowires of dimethyl 2-(3-(anthracen-9-yl)allylidene)-malonate (DMAAM) that curl into tightly coiled bundles upon irradiation.
We recently began investigating fluorinated anthracene derivatives, which show significantly improved photomechanical response. Microneedles composed of 1-fluoro-9-anthracenecarboxylic acid (1F-9AC) respond 3~4 times faster than those composed of unsubstituted 9-anthracenecarboxylic acid (9AC) and have better photostability. The photo-induced motion can be repeated more than 100 times. These needles can do useful work, for example by moving heavy cubic NaCl crystals in viscous solution. Preliminary attempts to incorporate 1F-9AC crystalline microribbons into lipid bilayers in order to control their placement on surfaces will also be described. This work will aid in advancing the design of flexible energy conversion devices.

We have developed two types of optothermal field effect transistors (FET) based on the pyroelectric effect of the piezoelectric substrates. The first example is a graphene-lead zirconate titanate (PZT) system.1 Under the incidence of an infrared (IR) laser beam, the drain current can be increased or decreased depending on the direction of the polarization of the PZT substrate. The drain current sensitivity of the optothermal FET can reach up to 360 nA/mW at a drain field of 6.7 kV/m more than 5 orders of magnitude higher than that of the photogating transistors based on carbon nanotube on SiO2/Si substrate. Graphene is an excellent component for pyroelectric FET due to its high optical transparency and conductance.
The second example is a composite consisting of single zinc oxide nanowire and lead zirconate titanate (ZnO NW-PZT).2 Under IR laser illumination, the transconductance of the ZnO NW can be modulated by optothermal gating. The drain current can be increased or decreased by IR illumination depending on the polarization orientation of the PbZr0:3Ti0:7O3 (PZT) substrate. Furthermore, by combining the photocurrent behavior in the UV range and the optothermal gating effect in the IR range, the wide spectrum of response of current to light offers a variety of opportunities for nanoscale optoelectronic devices.
References
1. C.-Y. Hsieh, Y.-T. Chen, W.-J. Tan, Y.-F. Chen, W. Y. Shih, and W.-H. Shih, “Graphene-PZT Optothermal Field Effect Transistors,” Appl. Phys. Lett., 100, 113507 (2012)
2. C.-Y. Hsieh, M.-L. Lu, J.-Y. Chen, Y.-T. Chen, Y.-F. Chen, W. Shih, and W.-H. Shih, "Single ZnO nanowire-PZT Optothermal Field Effect Transistors," Nanotechnology, 23, 355201 (2012)

We have fabricated stretchable GaAs solar cells using transfer printing technique. First, GaAs solar cell layers were grown on a GaAs wafer by metal organic vapor deposition(MOCVD) with AlAs sacrificial layer. For the separation of solar cell layers from GaAs wafer, AlAs sacrificial layer was selectively etched by diluted hydrogen fluoride solution. Because of high etching selectivity between GaAs solar cell layer and AlAs sacrificial layer, GaAs solar cells were separated from GaAs wafer without any damage. The GaAs solar cell layers separated by this epitaxial lift off technique were transfer printed onto the pre-strained substrate for the formation of noncoplanar interconnects. The microcell array with deformable interconnects can isolate the solar cell layers from strains that could cause fracture because the interconnects accommodate nearly all of the deformation. We observed photovoltaic characteristics of GaAs solar cells before and after transfer printing. Also, the mechanical properties of the stretchable solar cells were investigated using systematic stretching tests.

Supercapacitors have been considered as one of the most promising energy storage devices because of their many advantages including high power density, faster charge/discharge process and longer lifespan Unfortunately, the practical applications of supercapacitors are largely hindered due to the lack of high-performance electrode materials at a reasonable cost. The development of nanostructured materials, especially metal oxides, will undoubtedly provide a promising solution to enhance the capacitive performance because of their high surface area, short electron and ion transport pathways. At the same time, it is also noted that the poor intrinsic conductivity of these oxide materials, extra contact resistance and increased “dead surface” due to the introduction of conductive agent and polymer binder during the conventional thin film electrode preparation still seriously limit their performance. Recently, an emerging new concept is to grow electroactive nanostructures on conductive substrates to be directly used as integrated electrodes for supercapacitors, which could solve the above mentioned problems.In this work, different NiCo2O4 nanostructures including nanowires, hybrid nanowires and mesoporous nanosheets can be successfully grown on various conductive substrates including Ni foam, Ti foil, stainless steel foil and flexible graphic papers, with simple low temperature and environmentally friendly solution methods. The supercapacitive performance of these structures is investigated in details.
From the comparison of their supercapacitive performance, it is notable that the NiCo2O4 mesoporous nanosheets-Ni foam architecture exhibits the best, wich both higher areal capacitance and better cycling stability. The possible reasons could be related to the structural features of the architecture: First, the mesoporous feature of the NiCo2O4 nanosheets largely increases the amount of electroactive sites. In addition, the highly porous film structure formed by the interconnected nanosheets greatly facilitates transport of the electrolyte. Furthermore, the direct growth of interconnected two-dimensional nanosheets on conductive substrate could ensure good mechanical adhesion, and more importantly good electrical contact with the conductive substrate that also serves as the current collector in such highly integrated electrodes. The FESEM images of the NiCo2O4 mesoporous nanosheets after charging/discharging for 3000 cycles at a high current density of 25 mA/cm2 indicate that the morphology and structure of the nanosheets are well retained except for the increased roughness on the nanosheet surface, caused by the high rate redox reactions during the repeated charge/discharge processes.

Energy harvesting nanotechniques have been remarkably developed for last five years for micro/nano-power devices and self-powered sensor systems. The miniaturized micro/nano devices can generate electrical power in various mechanism based on piezoelectric, pyroelectric, electromagnetic and triboelectric effects. On the other hand, the energy storage system for the created electricity from the nanounits is significant issue for continuous and practical energy generation devices. Accordingly, energy units, which simultaneous convert and storage electrical power will be significant prospects for the future energy harvesters. Alternately, ionic liquids associated electrochemical systems for energy scavenging and storage are ideal for this purpose.
Ionic liquids are defined as liquids composed entirely of ions in fluid form. The ionic liquids are utilized in wide range of applications such as lithium batteries, fuel cells, supercapacitors and electromechanical actuators as electron-conducting materials. Among those applications, the electromechanical actuators show a strain deformation in the device under stimulus of an applied voltage. However, the reverse effect of the electrical unit under the mechanical strain then may leads to energy creation using the ion liquid based electrochemical transducers.
Flexibility and stretchability of energy harvesters are also important features for body-movement-driven-devices such as artificial muscles, wearable human patches and electronic cloth. Recently, several researches on flexible nanocomposite based generators have been demonstrated for energy generation with high flexibility.
In this work, the ionic polymer gel was adopted as an electron-conducting polymer to fabricate a nanocomposite generator which convert mechanical energy into electrochemical energy. The nanocomposite consists of the ionic liquid 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide, [EMI]-[TFSI], in polymer matrix for flexibility and transformability to any distortionally bent changes. As-fabricated nanocomposite based energy harvesting cell acts as a transducer which can convert and store electrical power based on electrochemical potential difference and electrostatical energy strorage system. This approach suggests new operation mechanism in energy harvesting technique with a strong advantage to the energy charging system.
This highly flexible nanocomposite generator is versatile to acquire irregular or random mechanical energy sources from variable environmental conditions for self-powering biomechanical applications such as artificial muscles and wearable human patches and electronic cloth.

Harvesting energy from the ambient mechanical energy sources are highly desirable for powering portable electronics, biomedical, and healthcare applications. Recently, a new type of power generating device based on triboelectric effects coupled with electrostatic effects have been demonstrated to be a powerful means of harvesting the mechanical energy. However, most of such triboelectric generators required sophisticated design and a high degree of integration. Nanoparticles and nanowires-based generators is also likely to deform and degrade under large applied force. Here, we present a new type of triboelectric generator structure based on an inverse opal structured film, named as a sponge film. The sponge film becomes more flexible than the flat film, increasing the contact area and the capacitance by the increase in effective ε/d value under the same force. Not only triboelectric charges on both the contacting surfaces but also additional charges on inner pore surfaces by the electrostatic induction may help to enhance the electrical output performance. Thus, the sponge-structured generators show large output power of approximately 4.8 mW /cm², giving over 10-fold enhancement compared with the flat film. In addition, by coating more hydrophobic Teflon layer on the surface of the sponge-structured film as well as the surface of inner pores, we will show that the generator is less sensitive to the humidity.

The research for the achievement of efficient organic photovoltaics (OPVs) has become one of the most popular topics in recent years caused by their potential to be analternative source of green energy. Polymer solar cells have many desirable features including large-scale fabrication utilizing low cost solution processability. The development of novel materials is necessary to enhance the coverage of the solar spectrum and the absorption coefficients, which can improve the lower power conversion efficiency and smaller photo current as compared to the case of inorganic solar cells. The HOMO energy level of the donor, such as low band gap conjugated polymer, and the LUMO energy level of the acceptor, PCBM, needs to be optimized to maximize the attainable open circuit voltage (Voc)
Main chain donorminus;acceptor (Dminus;A) alternating design is a very efficient way to tune the absorption and energy level of conjugated polymers through the intramolecular charge transfer from donor to acceptor unit. In this work Dioctyloxy substituted dimethyl-2H-benzimidazole (DOMBI) was prepared and copolymerized with thiophene (or bithiophene) and carbazole using both Stille and Suzuki polymerization. In DOMBI, the sulfur at 2-position of BT unit was replaced with dialkyl substituted carbon and dioctyloxy groups were introduced at 5,6-position of MBI unit. In addition to this, dioctyloxy groups on 5,6-position will improve the absorption at the longer wavelength region. The devices with PTDOMBI with PC71BM showed VOC value of 0.58 V, JSC value of 4.03 mA/cm2, and FF of 0.32 giving power conversion efficiency of 0.76%.

There is currently a strong need for development of inexpensive, flexible, light-weight and environmentally friendly energy storage devices [1]. As a result of this, research is carried out to develop versatile and flexible electrode materials as a complement to the materials used in contemporary batteries and supercapacitors. This has resulted in an increased interest in electronically conducting polymers as it is well-known [2, 3] that these materials can be used to manufacture all-polymer-based batteries and supercapacitors. The latter devices, however, generally suffer from problems due to low capacities, low charging rates, poor cycling stability and rapid self-discharge [2]. Possible approaches to circumvent, at least some of these problems, involve the use of composites comprising conducting polymers and carbon nanotubes [4] or nanocellulose [5].
We have shown [5] that flexible cellulose and polypyrrole composites, obtained by chemical polymerization of pyrrole on a cellulose substrate derived from the Cladophora sp. algae, can be used as paper-based electrode materials for environmental friendly charge storage devices. As these types of devices exhibit good cycling stability even at high charge and discharge rates, this type of devices provide exciting possibilities for the development of green and foldable devices as well as for a range of new applications which are incompatible with conventional batteries and supercapacitors.
This presentation will focus on the results of our recent research concerning the cell resistance of polypyrrole and cellulose composite based charge storage devices. It will be shown that this cell resistance is influenced by both the thickness of the polypyrrole/cellulose composite and that of the separator. The influence of the porosity of the separator, and the contact resistances between the current collectors and the composite, on the cell resistance, as well as the possibilities of designing inexpensive all-organic energy storage devices with promising performance regarding cycling stability and rate capability will also be discussed.
References
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[3] J. Heinze, in Organic Electrochemistry, Eds. Ole Hammerich and H. Lund, 4th Ed, Marcel Dekker,
New York, 2001.
[4] C. Meng, C. Liu, L. Chen., C. Hu, S. Fan, Nano Letters, 2010, 10, 4025.
[5] G. Nyström, A. Razaq, M. Stroslash;mme, L. Nyholm, A. Mihranyan, Nano Letters 2009, 9, 3635.

Organic photovoltaic (OPV) devices provide a offer great opportunities as renewable energy sources. The design and synthesis of low-bandgap polymers for use as electron donor materials for bulk heterojunction (BHJ) have attracted remarkable attention. A series of new donor-acceptor π-conjugated small molecules incorporating benzodithiophene(BDT) as an electron donating unit have designed and synthesized in solar cells with phenanthrothiadiazole(PT) and dimethyl-2H-benzimidazole(MBI) as the electron accepting unit. The DBDTPT and DBDTMBI, were synthesized by stille reaction and evaluated in OPVs. The DBDTPT shows absorption band with maximum peak at about 361 nm in solution. The HOMO and LUMO levels of DBDTPT were exhibited at -5.46 and -3.65 eV, respectively. The small molecule has good thermal stability. The HOMO-LUMO energy bandgaps of this material shows 1.81 eV for DBDTPT.

Indium tin oxide (ITO) and Al-doped zinc oxide are generally used for transparent conductive electrodes, but they have a number of weakness. Sputtering to produce such metal oxide thin films is not only too expensive in cost, but also fragility of the film cracking when the substrate is bent, leading to device malfunction. Silver nanowires (Ag NWs) have emerged as one of the most promising alternative for a traditional metal-oxide transparent electrode such as ITO, because they suggest optical transmittance and surface resistivity identical to their contenders. However, previous work does not fully exploit the superior properties to that of control devices on ITO substrates.
In this work, we present a novel way of preparing highly transparent and conductive Ag NWs electrodes using mayer rod method, which shows enhanced device performance compared to device using ITO electrodes. The sheet resistance of Ag NW film on a substrate is 20-50 Omega;/sq. Within the spectrum range from 400 to 800 nm, the optical transmittance is over 86% for lambda; >480 nm. The device can be fabricated by the direct deposition of small molecular weight organic materials and a metal cathode on the Ag NWs anodes. The devices using Ag NWs results in an enhancement of the luminance value at least 10% compared to that from a conventional OLED on ITO substrates.

Herein we describe the stretching and conformal bonding (i.e., decal-transfer printing) of organic solar cells in both the “conventional” and “inverted” configurations to hemispherical glass surfaces with radii of 8 mm. This action produces equivalent biaxial tensile strains of 24%, which many materials used in organic electronic devices cannot accommodate without fracture. Consideration of the mechanical properties of conjugated polymers reveals a surprising effect of a single structural parameter—the length of the alkyl side chain—on the elasticity and ductility of regioregular polythiophene. This analysis enables selection of materials that can accommodate sufficient tensile strain for non-planar applications. For polymer-fullerene solar cells, devices based on the elastic and ductile poly(3-octylthiophene) (P3OT) exhibit typical photovoltaic properties when bonded to hemispherical glass substrates, while those based on the relatively brittle poly(3-hexylthiophene) (P3HT) exhibit extensive cracking, which degrades the photovoltaic effect significantly. The results suggest that mechanical properties should be taken into account when designing and selecting organic semiconductors for applications that demand significant deformation.

Unlike piezoelectric polarization change, which is caused by the stress, the flexoelectric polarization change is caused by the strain gradient. The crystal non-centrosymmetry requirement essential for piezoelectric response is replaced by the non-centrosymmetry of the sample geometry as long as the sizes involved are small enough to make the flexoelectric response appreciable. Tagantsev estimated the flexoelectric coefficient µ, the ratio of the induced polarization to the strain gradient in ionic crystals, to be of order ke/a, where k is the dielectric constant, and e/a asymp; 0.4 nC/m, the ratio of the ionic charge to the lattice constant.
In our studies, we investigated the flexoelectric response in ferroelectric copolymers of vinylidene fluoride with trifluoroethylene (VDF-TrFE) and relaxor terpolymers of VDF-TrFE with chlorotrifluoroethylene. By using a simple cantilever measurement technique, while monitoring remanent polarization through the pyroelectric response, we are able to measure the flexoelectric response in thin films and isolate and correct for piezoelectric contributions, which would otherwise dominate the flexoelectric measurements. The upper limit for the flexoelectric coefficient of the copolymer P(VDF-TrFE) was mu; le; 191 ± 17 nC/m in the ferroelectric phase and µ = 18 ± 1 nC/m in the paraelectric phase. The relaxor terpolymer P(CVDF-TrFE-CTFE) had µ = 30 ± 1.5 nC/m at room temperature. Temperature-dependent studies were also carried out to better understand the origin of the flexoelectric response. In the case of the relaxor terpolymer, the flexoelectric signal as a function of temperature scaled very closely with the dielectric constant, as has been proposed in the case of oxide ferroelectrics. To further reduce the piezoelectric response in the copolymer, we cooled samples at zero field to reduce the remnant polarization to less than 2% of its maximum value. Even with this there is still a residual piezoelectric contribution to the overall signal. These studies are aimed to harvest the flexoelectric energy as an alternative to piezoelectric analogues.

Most applications of non-grid-connected solar modules require battery storage and power electronics for maximum power point tracking and battery charge control. Although flexible solar modules, flexible batteries, and flexible electronic circuits have each been extensively studied and optimized, they are typically fabricated separately, and power conversion circuits used for solar energy harvesting usually consist of conventional electronic components mounted on a rigid printed circuit board. Compared to subtractive processes such as printed circuit board etching and photolithography, additive printing processes for electronic device fabrication can reduce materials waste and process complexity. Additive processes also allow a diversity of devices to be fabricated on the same substrate in a single production line. Furthermore, since multiple components of a solar energy harvesting system require similar materials functionalities, such as metal contacts and interconnects, each printing pass could be used to deposit layers of multiple components at the same time. Thus, printing offers great potential for high-throughput and low-cost production of integrated solar energy harvesting and storage systems. We have fabricated flexible photovoltaics, batteries, capacitors, resistors and inductors using a variety of scalable printing techniques compatible with roll-to-roll or sheet-by-sheet fabrication processes. We present a strategy for production of multiple types of passive components simultaneously on the same substrate, and a design for a solar sheet in which a printed solar module, battery and power electronics are integrated together into a single flexible piece.

In this paper, we propose a novel privacy filter to prevent disclosure of personal information, while maintaining the high transmittance. Transparent vertical array structure was introduced given that it distorts the image by refraction and total reflection [1]. In contrast to the conventional display privacy filter which significantly decreases normal transmittance [2], the proposed privacy filter rarely affects the normal transmittance as it does not include the opaque material. To confirm the feasibility of our model, transparent cubic structure with refractive index of 1.5 was analyzed using lay tracing method [3]. It was found that the refraction and reflection of obliquely incident light in the transparent cubic structure result in the copy, displacement, and loss of the image, yielding distortion of image. Further, this simulation results were confirmed with the actual test results conducted with PDMS (1.4 cm x 1.4 cm x 1.8 cm) vertical array structure.
Finally, the privacy filter film which is the array of transparent PDMS vertical structure (100 mu;m x 100 mu;m x 200 mu;m) was fabricated by MEMS technology [4]. The privacy filter film on the smart phone rarely influenced the display in the normal direction whereas clear recognition of the texts was unachievable when looking from the side. Thus, the suggested privacy filter is well-suited for the mobile display device given that it provides high transmittance and it requires simple fabrication process which can lead to mass production.
[1] R. Dumke, M. Volk, T. Müther, F. B. J. Buchkremer, G. Birkl, and W. Ertmer, Phys. Rev. Lett. 89, 9 (2002).
[2] H. Yoon, S.G. Oh, D.S. Kang, J.M. Park, S.J. Choi, K.Y. Suh, K. Char, and H.H. Lee, Nat. Commun. 2, 455 (2011).
[3] A. C Hamilton and J. Courtial, J. Opt. A: Pure Appl. Opt. 10 (2008).
[4] S.S. Oha, C.G. Choi, Y.S. Kim, Microelectronic Engineering 87 (2010).

Supercapacitors as electrical energy storage devices have exhibited a high power density but a low energy density compared to batteries. In order to improve rather poor energy density of supercapacitors, various pseudocapacitve metal oxides have been actively investigated. Among those metal oxides, manganese oxide has attracted extensive interest owing to its advantages such as cost-efficiency, high energy capacity, natural abundance, and environmentally friendly property. However, the low electrical conductivity of manganese oxide limited its use as electrode materials to be combined with highly conductive carbon-based materials including carbon nanotubes
In this study, we report on the fabrication of planar-type flexible micro-supercapacitors (MSCs) using Au electrodes coated with MWNT film and the top-most layer of MWNT/Mn3O4 nanoparticle (NP) composite. MWNT thin film was formed via layer-by-layer (LBL) assembly of MWNTs functionalized with amine group and MWNTs with carboxyl group in water, and the hydrothermally synthesized composite of MWNT/Mn3O4 NP was coated on top of the MWNT film. In the planar-type MSC, the ion-channel length was 150 mu;m. By using PVA-H3PO4 gel electrolyte, the fabricated MSCs worked in the potential between 0 and 0.8 V. The addition of the top layer MWNT/Mn3O4 NP composite enhanced the performance of the MSCs dramatically to give a volumetric capacitance of 38.8 F/cm3 at scan rate of 10 mV/s and the Coulomb efficiency of ~100%: the volumetric capacitance of 2.8 F/cm3 was obtained in case of using just MWNT film. Even after the repetitive operation by 10^4 times, the capacitance remained ~88.3% of the original value.
MSC arrays fabricated on a PET film showed stable electrochemical properties upon 1,000 times bending with both compressive and tensile stress. Furthermore, we could increase the output voltage and the capacitance by deliberately designing the circuit of MSC arrays, which could operate various active nano-devices.

A novel sol-gel condition that produces a highly compressed organic/inorganic hybrid glass has been discovered. The compressed sol-gel glass was based on alkylene-bridged polysilsesquioxane doped with chromium. The sol-gel thin film prepared under a novel acidic condition has shown a low thermal conductivity and a high compressibility. Interestingly, the highly compressed hybrid glass also showed a strong ‘acoustic response&’ as strong as liquid. Usually, when the laser beam goes through a solid medium, the density wave is linear; because, in solid media, heat doesn&’t decay through the solid medium effectively. The acoustic response generated from the novel glass was as compressive as liquid. In laser experiments, we calculated ‘coefficient of phonon diffraction (D),&’ which was proportional to the coefficient of thermal conductivity. The number (D) of the novel glass was FIVE times smaller than that of a normal glass; the thermal conductivity of the novel glass was FIVE times less than that of a normal glass. Due to its low thermal conductivity, the sol-gel film serves like a ‘HEAT GENERATOR (ENERGY-STORAGE MATERIAL)&’; the heat gets transferred into expansion or compression wave effectively. This is a new concept that could bring novel energy storage materials with a good optical clarity. It can also be fabricated on a variety of substrates and thus can be used for flexible electronics, including flexible energy-storage, energy conversion, and optically transparent energy devices.

Flexible energy storage devices have received tremendous interest recently due to the increasing demand for sustainable and renewable energy in modern electronic industry. In this study, TiO2 nanorod arrays grown directly on Ti foil were prepared by a facile one-step wet chemical synthesis process. The morphology and microstructures of the as-prepared samples were characterized by field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), respectively. By using a three-electrode system, the electrochemical properties of the TiO2 nanorod arrays are systematically investigated by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy tests. The TiO2 nanorod arrays showed a good specific capacitance and exhibited excellent cycle life by retaining more than 98% of the initial specific capacitance after 1000 cycles. The growth mechanism of these TiO2 nanorod arrays will be discussed in the presentation.

The interfacial properties of semiconducting materials are important to establish the working mechanisms of optoelectronic devices as well as to improve their performance. The electronic structure at an interface could strongly depend on how the materials make contact with each other, which in turn is governed by many factors such as interfacial dipole moments induced by the molecular orientation and electronic coupling of the materials. Accordingly, it is necessary to investigate that how the properties of p/n interfaces influence device performance under conditions that more closely resemble those of an actual device. Well-defined bilayer structures could be more suitable for the fundamental investigation of p/n (or donor/acceptor) interfaces in organic optoelectronics. However, the bilayer devices composed of organic donor/acceptor materials often induce diffusion and intermixing of the donor and acceptor molecules at the interface by application of heat and pressure during transfer the films. Therefore, to investigate the relation between their structure and device performance, novel architectures with well-defined interfaces are needed.
Here we fabricated the novel architecture of self-assembled P(VDF-TrFE):P3HT-composited photoactive layer that consists of P3HT domain in P(VDF-TrFE) matrix for hybrid photovoltaic devices (HPDs) and demonstrated the ferroelectric effect on photovoltaic behavior and energy band tuning at donor-acceptor interface. The P(VDF-TrFE):P3HT/ZnO-device exhibits the enhanced diode behavior and photovoltaic characteristics as compared to the P3HT:ZnO HPD. Further, VOC has been tuned by control application of forward and reversed external electric fields in HPD. These dramatic changes in photovoltaic behavior depending on the poling process indicate that photovoltaic performance can be enhanced by the artificial control of dipole moments in ferroelectric materials, leading to additional driving force for charge extraction and the change in the energy band at donor-acceptor interface of HPDs.

Over the past 1-2 years there has been a growing interest in developing flexible/stretchable batteries that can used as a power source for wearable monitoring and sensing devices. Commercially available batteries are bulky, non-flexible and they negate the advantages offered by these new class of devices. Early demonstrations in flexible batteries were based on reducing the thickness of the active layers and using flexible current collectors. These batteries were flexible but they have a limit on their bending limit. Reports on stretchable batteries have been based on supporting the anode and cathode on stretchable current collectors or using rigid islands of active mass on stretchable substrate. In our previous work, we demonstrated highly flexible batteries with a mesh-supported design. The mesh substrate enabled electrodes with capacity as high as 6mAh/cm^2 and bend limiting of 9mm. Similar concept was used to fabricate stretchable electrodes by embedding the active materials inside a stretchable conductive fabric. These batteries showed no loss in capacity even when stretched by 100%. Compliant batteries in their current form as still orders of magnitude thicker than the flexible/stretchable sensors and circuits. In this talk, we propose a battery design with electrode thickness less than 10 micron. The thin electrode design will reduce the strain generated on the electrode during flexing and decrease the minimum bending radius of the battery. Due to the thin form-factor of the electrodes, the batteries can be made stretchable by supporting them on a pre-strained elastomer. The capacity of the battery can be easily increased by folding the battery multiple time without increase the thickness of the inactive component.

We developed a hybrid-nanostructured Si electrode for flexible Li ion batteries. A uniformly distributed nano-hairy Si anode was fabricated on a polyimide substrate. Through the in-situ lithiation test, we could elucidate the characteristic lithiation behavior of the nano-hairy Si anodes: the isotropic volume expansion accommodated by compliant movement. This unique nano-hairy Si anode structure dramatically improve capacity, cycle life and rate capabilities during the coin-cell test by effectively suppressing mechanical failures such as buckles and delamination. Subsequently, we successfully demonstrated the flexible Li-ion Batteries by implementing the nano-hairy Si anode.

We have developed new biopolymer based composite materials incorporating electroactive and conjugated polymers. The biopolymer lignin is found in green plants, and makes up 20-30 % of the weight of wood. Derivatized lignin is available as a waste product from paper pulp processing. Lignin derivatives are incorporated as an anion and dopant in the conjugated polymer during polymerization from the monomer, and act as an electronic wire accessing the biopolymer. Quinones can be generated in the lignin derivative during initial redox cycling, causing the oxidation of monolignols. The density of quinone groups is high enough to double the charge storage capacity of the polymer, compared to more standard forms, and much improve the capacitance. Inclusion of more redox species is possible, to increase the charge density, and to modify the redox potential where charge is stored. This is demonstrated with three alternative smaller quinones, with redox potential spanning from the anodic to the cathodic range. Inclusion of these smaller quinones does not severely compromise the inclusion of the lignin derivative, and all quinones contribute to the capacitance. Different redox species may be incorporated in these biopolymer composites, and a striking example is the polyoxymetalates. These materials, with three waves from the redox of molybdenum, and with quinone redox in addition, add several redox steps. Such electrodes exchange mainly protons with aqueous electrolytes during redox, making these electron/proton exchange materials. They also show high capacitance and improved charge storage compared to the polymer/lignin derivative host material.

To meet the requirements of the next-generation electronic industry, there is a strong need for the development of lightweight, flexible, inexpensive and environmentally-friendly high-performance supercapacitor and battery electrodes [1]. Contemporary functional electrodes for supercapacitors and batteries generally contain carbon black and polymeric binders to increase the electrical conductivity and to obtain better physical contact between the active materials and the current collector [2]. As these additive materials do not contribute significantly to the electrochemical charge storage capacity, the development of free-standing, high active mass electrodes is becoming one of the key challenges.
Conducting polymers, such as polypyrrole (PPy) and polyaniline are particularly interesting electrode materials in this quest since these polymers exhibit fast redox switching, high conductivity, mechanical flexibility, and low weight [3]. Recent work has also demonstrated that nanocellulose fibers can serve as a promising substrate material for conducting polymer based electrodes due to its high abundance in nature and its well-established industrial use [4]. We have thus shown that electrodes composed of PPy coated nanocellulose composite combine high conductivity with good mechanical properties and that this be can used to obtain free-standing, additive-free high-performance supercapacitors electrodes [5, 6].
The present communication will focus on the results of our recent research on the possibility of using polypyrrole coated nanocellulose fibers in the manufacturing of free-standing, high active mass paper electrodes for sustainable energy-storage devices. It will be shown that this type of devices exhibit high cell capacitances at high current densities during thousands of cycles in aqueous solutions and that this approach holds great promise for the fabrication of high performance electrodes for advanced energy-storage devices.
References
[1] M. S. Whittingham, Chem. Rev. 2004, 104, 4271.
[2] D.-H. Ha, M. A. Islam, R. D. Robinson, Nano Lett. 2012, 12, 5122.
[3] P. Novak, K. Muller, K. S. V. Santhanam, O. Haas, Chem. Rev. 1997, 97, 207.
[4] G. Zheng, Y. Cui, E. Karabulut, L. Waring;gberg, H. Zhu, L. Hu, MRS Bull. 2013, 38, 320.
[5] G. Nyström, A. Razaq, M. Stroslash;mme, L. Nyholm, A. Mihranyan, Nano Lett. 2009, 9, 3635.
[6] L. Nyholm, G. Nyström, A. Mihranyan, M. Stroslash;mme, Adv. Mater. 2011, 23, 3751.

High-performance flexible energy-storage devices have great potential as power sources for wearable devices, artificial electronic skins, distributed sensors and other portable electronics. However, one major limitation to realize these applications is the development of flexible electrodes with reliable mechanical property and excellent electrochemical performance. Currently, employed batteries and supercapacitors are mainly based on relatively rigid and fragile electrodes which are not suitable for flexible device fabrication due to their poor mechanical properties. Here we report an aqueous-based solution process towards efficient fabrication of high performance flexible lithium electrodes. This was accomplished by using a synergistic combination of conducting polymer, CNTs and nanoparticle building blocks. Our afforded interpenetrating network of CNT-conducting polymer hydrogel are observed to possess good mechanical property, high conductivity and facile ion transport, thus leading to facile electrode kinetics and high strain tolerance during electrode volume change. Due to these structural merits, we are able to achieve flexible electrodes high-rate capability and high cycling stability. For example, in a charge/discharge time of 40 s, flexible TiO2 electrodes achieved a capacity of 76 mAh/g at mass loading of 2.6 mg/cm2. Furthermore, an areal capacity of 2.2 mAh/cm2 can be obtained for flexible SiNP-based electrodes at 0.1C rate, which have yet been achieved using previous electrode structures with the same active materials. This simple yet efficient solution process provides great promise for fabrication of a variety of high performance flexible electrodes.

There is a growing demand for flexible electrodes and electrolytes to form the basis of conformable energy storage devices. Such structures would find use in wearable systems and other niche applications where the seamless integration with a host is desirable.
We have been involved in the development of flexible and stretchable electrodes based on organic conductors : organic conducting polymers and nanostructured carbons. Here we will present our latest developments in these areas.
We have also been involved in the development of highly stretchable electrolytes based on ionic liquids imbibed within a host polymer.
The combination of these electromaterial systems presents truly flexible and stretchable devices. Our studies also highlight the challenges that must be overcome to fully optimise the performance of the individual components. These challenges will ultimately be overcome using innovative approaches to device fabrication.

Flexible energy storage can enable novel applications but requires new consideration beyond traditional energy devices. Here I will present our research in the past several years on developing energy storage devices with paper, textile and sponge substrates. These substrates offer interesting three-dimensional porous scaffolds for loading conducting nanomaterials and active energy storage materials. We have demonstrated superior performance on supercapacitors and batteries. We have also shown excellent flexibility and stretch ability of these devices.

Recently many batteries have been demonstrated to have flexible and stretchable qualities, but these batteries currently lack the aesthetic qualities of fine fabrics or jewelry that might command attention on the runway or a high price at the department store. In this presentation we discuss work towards the integration of energy storage materials with fine fabrics and jewelry. Beyond the traditional qualities one measures in a battery (energy density, capacity and cycle life), the haute couture battery must be considerate of aesthetic features such as drape and visual consistency. The battery is not “snapped” into a dress, rather it may have to withstand being sewn or hewn. For all applications the battery must be safe enough to wear, and for “pret-a-porter” applications, washed readily. If the battery is to be integrated into jewelry, it must take a shape and texture consistent with the artist&’s vision, not vice versa.
We will demonstrate a number of implementations of batteries that are capable of flexible, stretchable and high potential operation (through design integrated series cell stacking). We will demonstrate both primary and secondary chemistries that can be processed and blended in a manner consistent with the standard fabrication of clothing and jewelry, rather than requiring a clean room or chemistry lab. We will discuss how these new features may be quantified, and how traditional battery processes can be reworked to add new functionality to clothing while meeting the subjective requirements of beautiful wearable objects.

Applications for flexible and wearable electronic devices are finding their way in products for sports and healthcare, military technology, as well as safety and construction equipment. Some notable examples include, Nike Fit, Addidas MiCoach, and the UnderArmour heart monitor. It should be noted that these devices still use conventional pouch cell or hard batteries and electrochemical capacitors. Our previous work focused on screen printing carbon materials into cotton, polyester and carbon fiber textiles to create an energy storage device. However, that design is hindered by delamination of carbon from the fiber surface during use. In this work we describe a knitted, all-solid supercapacitor based on carbon materials (e.g., activated carbon and graphene), which is embedded into cellulose based yarns.
Carbon materials are introduced to cotton yarns through a process of ‘Natural Fiber Welding&’ to create capacitive yarns This process utilizes ionic liquid-based solvents to partially embed carbon materials into the cellulose fiber surface with minimal modification to the cellulose substrate. A small amount (0.5 wt.%) of solubilized cellulose binder increases the mechanical integrity of coatings to limit delamination from the cotton substrate.
These capacitive yarns are plied with stainless steel yarn and act as the electrode and current collector, respectively. These yarns are knitted into fabric on a Shima Seiki 3D knitting machine into 2-electrode symmetric supercapacitors as a single sheet of fabric and are coated in electrolyte. Rather than a liquid electrolyte, a PVA based gel electrolyte is used to eliminate the possibility of leaking. The knitting equipment is industrial grade, meaning any devices we design can be sent to a factory and mass-produced. At present our yarns have the highest reported capacitance per length for a carbon yarn capacitor, up to 45 mF/cm. Here we also report on the first all-knitted supercapacitor, its performance and future challenges.

The piezotronic effect describes the coupling of piezoelectric polarization and the intrinsic electric field in a space charge region for the purpose of tuning charge transport behaviors of semiconductor materials and devices. The piezotronic effect has a significant influence on the heterostructure&’s electronic properties by precise modification of the interfacial energetics via mechanical strain. The merit of this approach is that it allows for a device to be composed of materials which are still individually optimized for their specific bulk electronic properties while allowing the independent optimization of their heterointerface. We first discuss barrier-height engineering of a heterogeneous semiconductor interface manifested by a PEC half-cell, where the influence of the remnant piezoelectric polarization on the photocurrent of water splitting was studied as a function of strain applied to the anode. The direct interaction between piezoelectric polarization and electrochemical processes is denoted as piezocatalysis effect. This provides a direct pathway for mechanical to chemical energy conversion. By straining a piezoelectric ferroelectric PMN-PT beam in water, we experimentally demonstrated that piezoelectric potential can raise the energy of electrons at the surface of piezoelectric material (or electrode) to such a level that is sufficient to drive proton reduction reactions within its immediate vicinity. The piezocatalytic efficiency (~0.2% - ~2.4%) was found to depend sensitively upon the length of straining state, consistent with the limitations imposed by electrochemical reaction kinetics in the DI water environment. The piezotronic effect has also been applied to the ZnO/PbS quantum dot (QD) heterojunction for engineering the interfacial band structure and depletion region. This method escalated the solar energy efficiency by 30% when a relatively small strain -0.25% was applied to the QDSC under low-intensity illumination. The enhancement of short circuit current and efficiency was mostly due to the expansion of depletion region in PbS, as a result of piezoelectric polarization-induced charge redistribution at the ZnO/PbS interface. These initial investigations of the piezotronic effect open a new route toward efficient and effective energy harvesting and conversion, particular in the flexible systems where strain could be significant.

Photovoltaic technology converting solar energy into electricity is playing more and more important roles in providing the society with clean energy and reducing the impact of burning fossil fuels on environment. How to increase the photovoltaic conversion efficiency has been studied very intensively during the past couple of decades. In this talk, I will present our effort using nanostructures to improve the conversion efficiency by both designing new structures and inventing new transparent electrode, specifically I will discuss our development on nano coaxial cable solar cells to separate the photon and charge path ways, ultra thin film solar cells for efficient light absorption, and super flexible transparent electrode for efficient collection of charges.

GaAs is a desirable material for a variety of applications including high-efficiency photovoltaics, THz emitters, and neutron detectors due to its wide band gap and high electron mobility. Large area and flexible form-factors for these devices require thin GaAs, which has previously been achieved by chemically etching sacrificial layers in hydrofluoric acid, or by release of GaAs grown on silicon or germanium substrates. In this talk, we demonstrate the ability to create thin, flexible sheets of GaAs (<2-100 mu;m thick) from bulk GaAs using a controlled spalling-based process that requires only minutes of processing time and much less hazardous materials. A metal layer is deposited in tension onto the GaAs surface to cleave a sheet from the wafer substrate. Crystallographic fracture behavior and experimental factors that influence the spall depth, and hence the thickness of the GaAs sheet, are discussed. The integrity of the spalled material is probed by characterizing the performance of a spalled single-junction GaAs photovoltaic cell, demonstrating that the spalled material shows no degradation compared to a conventionally fabricated device.

Transparent flexible electrodes are necessary for roll-to-roll processing fabrication, which is a cost-efficient way to make organic electronics such as organic solar cells (OSCs). In order to meet the requirements (e.g. transparency, conductivity, work function, gas-barrier, flexibility, etc.) multi-layer electrodes on a substrate have become active.
“Flextrode” is a flexible transparent electrode consisting of zinc oxide (ZnO)/PEDOT:PSS/silver grid/polyethylene terephthalate fabricated by the roll-to-roll process. The Flextrode provides a ZnO surface on a high-conductivity electrode while maintaining reasonable transparency and good flexibility. It is optimized to be used as the cathode (i.e. low work function) in inverted OSCs. The as-received Flextrode samples have a surface contamination layer that is insulating. Prior to being used in OSCs, this contamination layer need be removed. In this study, we applied two surface cleaning methods, i.e., UV-ozone and oxygen plasma, with various treatment times. The samples after cleaning are characterized by x-ray/ultraviolet photoelectron spectroscopies, conductive atomic force microscopy, 4-point probe conductivity measurement, UV-visible transmittance measurement, and water contact angle measurement. Based on these experimental results, we found the optimal conditions and were able to recover the work functions without damaging the Flextrode.

The development of a method for integrating highly efficient energy conversion materials onto soft tissue could yield breakthroughs in energy harvesting systems for implantable biomedical devices. Further, the scaling of such materials down to nanometer levels may yield novel probes for studying fundamental mechanical responses of cells. Of particular interest are materials and devices which can conform to soft, curved surfaces such as skin, and operate in vital environments that may involve both flexing and stretching modes. Piezoelectric crystals are a particularly interesting category of energy conversion materials whose properties have been extensively characterized in the bulk. They are brittle inorganic crystals which are processed at high temperatures, and thus are thermally and mechanically incompatible with soft biological matter. Our group has shown advances in the fabrication and integration of highly efficient nanopiezoelectrics on flexible and stretchable substrates. Yet, questions remain about how to engineer these interfaces to be compatible with biological systems. Here we propose new strategies for addressing these questions. First, we have investigated the fabrication, characterization, and device integration of new classes of nanopiezoelectrics. Next, we have interfaced these materials with cells to act as fundamental probes of mechanical deformations of cells in response to electrical excitations. Finally, we have scaled these nanopiezoelectrics to macroscopic dimensions and biointerfaced them with tissue. This research suggests exciting implications for the direct biointerfacing of nanomaterials with cells and tissue, both as fundamental probes and for bioelectromechanical energy harvesting.

In this presentation, we will discuss out work on the fabrication of flexible fiber-based thermoelectric modules and motion sensors. The flexible fibers coated with semiconductor nanocrystals show significantly reduced manufacture cost (up to 4 orders) while maintaining similar efficiency to convert waste heat into electricity. Similar fibers can also be manufactured into motion sensors to detect the air fluctuations caused by moving objects as small as fruit flies. We will also present our latest work with Air Force Research Lab on the experimental studies and theoretical modeling of heat transfer in these fiber-based devices using 3 Omega method.

Organic electronic materials have already found commercial success as an alternative to conventional inorganics in devices such as photovoltaics, transistors, and LEDs. Only recently have organic electronic materials been seriously considered for thermoelectric applications. Organic materials have a few key advantages over conventional thermoelectric materials: the general disorder of organic materials presents an inherently low thermal conductivity, and they can be solution deposited at ambient temperatures and pressures, which allows them to be deposited on flexible substrates for novel, conformal form factor devices. The first challenge for organic thermoelectric materials is increasing the charge carrier concentration and mobility for improved conductivity, without greatly reducing the thermopower. Charge carrier transport is still not well understood in these materials, so there is no obvious path for optimizing these materials. Here we report on the challenges of optimizing the thermoelectric performance of high-mobility semiconducting polymers including P3HT and PBTTT-C14. We will report results using a variety of doping mechanisms that demonstrate striking structure-property relationships between the power factor and electrical conductivity. Our results will be placed in the context of organic semiconductors, thin film inorganic, and hybrid thermoelectrics suggesting the most promising optimization methods for different classes of organic materials. Recent results on flexible thermoelectric generators based on organic materials will be presented.

The Seebeck coefficient (S) and electrical conductivity (σ) are measured and systematically investigated for two n-channel conjugated polymers: a polymer recently synthesized in our lab, poly(PyDI-ethynylene), and commercially available Polyera N2200. N2200 is a blue polymer and shows ambipolar conductivity, whereas poly(PyDI-ethynylene) is yellow and observed to be unipolar. The purpose of this study is to characterize the terms in the thermoelectric power factor (SSσ) for these materials in order to understand their potential for the n-channel leg of thermoelectric generators, and to determine fundamental behavior regarding the enhancement of power factor in these systems, including hybrids with inorganic particles.
We characterized two chemically similar versions of poly(PyDI-ethynylene), but which have drastically different molecular weights. Unlike the low-MW polymer (4 kDa, 1.76 PDI), the polymer with higher MW (20 kDa, 3.47 PDI) also contains a flourinated phenyl end-cap. The poly(PyDI-ethynylene) with increased MW shows slightly improved conductivity in its undoped form (0.0001 to 0.001 S/cm), and a much greater S (-40 to -220 mu;V/K). N2200 (100 kDa, 3-6 PDI) shows still greater conductivity (0.01 S/cm), as expected due to higher intermolecular overlaps of diimide cores, but only slightly greater power factor due to lower intrinsic S (-130 mu;V/K). Power factors are 0.0012 µW/mK² for low-MW poly(PyDI-ethynylene), 0.014 µW/mK² for higher MW poly(PyDI-ethynylene), and 0.029 µW/mK² for N2200. To our knowledge, these are the first reports of intrinsic n-type thermoelectric behavior in organic polymers.
We also investigated hybrid thin-films utilizing inorganic additives. Here we focus on two additives, tin (II) chloride or sodium niobate, utilizing 20 and 80 wt% in polymers. Power factors of pristine polymers are enhanced by additives in all cases. We observe that tin (II) chloride is a weak dopant for both poly(PyDI-ethynylene) and N2200. S increases greatly for both polymers with 20 wt% tin, but more for N2200 (-1000 to -1500 mu;V/K), and increases further for higher tin concentration (-2000 to -2500 mu;V/K). For sodium niobate blends, S increases to around -500 mu;V/K for both polymers with lower concentration and rises above -5000 mu;V/K for 80 wt% blends, while conductivity is unaffected. The greatest power factor was obtained using the ionic ceramic at high concentration, and was similar for both n-channel polymers, around 10 mu;W/mK², followed by 80 wt% tin (II) chloride blends, with around 2-7 mu;W/mK², within one order of magnitude of high-performing p-type polymer composites. Leveraging further recent advances in n-polymer doping, we anticipate equivalent power factors from both composite polarities to be achievable in the near future, and thus application of flexible thermoelectric generators for remote light-power applications.

I will present progress by our research group, in an overview talk, on graphene-based and graphene-derived ultracapacitor systems. Several recent papers include:
Kim, TaeYoung; Jung, Gyujin; Yoo, Seonmi; Suh, Dwang S.; Ruoff, Rodney S.; Activated Graphene-Based Carbons as Supercapacitor Electrodes with Macro- and Mesopores. ACS Nano (2013), 7 (8), Aug. 2013 , 6899-6905
Tsai, Wan-Yu; Lin, Rongying; Murali, Shanthi; Zhang, LiLi; McDonough, John K.; Ruoff, Rodney S.; Taberna, Pierre-Louis; Gogotsi, Yury; Simon, Patrice Outstanding performance of activated graphene based supercapacitors in ionic liquid electrolyte from -50 to 80 oC. Nano Energy (2013), DOI: 10.1016/j.nanoen.2012.11.006.
Zhang, Li Li; Zhao, Xin; Stoller, Meryl D.; Zhu, Yanwu; Ji, Hengxing; Murali, Shanthi; Wu, Yaping; Perales, Stephen; Clevenger, Brandon; Ruoff, Rodney S. Highly Conductive and Porous Activated Reduced Graphene Oxide Films for High-Power Supercapacitors. Nano Letters (2012), 12, 1806-1812.
Zhu, Yanwu; Murali, Shanthi; Stoller, Meryl D.; Ganesh, K. J.; Cai, Weiwei; Ferreira, Paulo J.; Pirkle, Adam; Wallance, Robert M.; Cychosz, Katie A.; Thommes, Matthias; Su, Dong; Stach, Eric A.; Ruoff, Rodney S. Carbon-Based Supercapacitors Produced by Activation of Graphene. Science (2011), 332, 1537-1541.

Recently, paper is getting great deal of attention as a flat substrate for flexible energy storage because it has a strong adhesion to solvent-dispersed electrochemically active materials such as ink[1,2]. Owing to the feasibility of using printable solution processes, paper could be used as a substrate in batteries and electrochemical capacitors. For coating or printing of well-dispersed carbon nanomaterials, they are usually first encapulated by surfactant molecules or polymers and then suspended in the solvent as separated particles. It is well known that surfactants act as an insulator by resisting the charge transfer between the carbon nanomaterials[3]. This causes an undesirable decrease in the conductivity. In the case of paper, high conductivity can be achieved even with surfactants because paper is believed to remove the surfactants from carbon nanomaterials. However, the electrical conductivity and electroactive performance of the combination system of a paper and carbon nanomaterials still need to be enhanced.
Hence, we have reported the development of flexible graphene-paper electrode (GPE) with a flat surface, whose internal structure has been formed with gradient porous build-up by 2-hydroxyethyl cellulose (HC). The HC solution was employed as filler for the 3D hierarchical porous fiber structures of filter paper to provide a flat surface of graphene layer on the paper electrode. In addition, HC solution served as a binder to form the gradient porous graphene layer, enabling it to create an anchoring force between the porous graphene layer and the filter paper. The water soluble HC in GPE enhanced the absorption of electrolyte and acted as electrolyte reservoir to maintain high ion flux. It was found that the GPE showed not only a good capacitance as a result of the low surface resistance offered by the flat surface but also a high electrochemical activity due to the internal porous structure. The morphology of GPE was investigated using a scanning electron microscope, and the surface resistance of the GPE as a function of graphene content was determined using four-probe method. The electrochemical performance of the GPE was evaluated by cyclic voltammetry. The gravimetric capacitance of GPE was found to be 120 F/g of graphene, and the capacitance retention was within ca. 96% for over 500 cycles. This could be attributed to both the low surface resistance resulting from the flat surface and the high electrochemical activity caused by the gradient porous structure. This unique structure not only offers an enhanced conductivity and good electrical contact between the electrode and electrolyte but also helps GPE to maintain good cyclic stability, proving its potential for use in various rechargeable and portable energy-storage devices.
[1] L. B. Hu at. all, Proc. Natl. Acad. Sci. USA 106, 21490 (2009).
[2] V. L. Pushparaj at. all, Proc. Natl. Acad. Sci. USA 104, 13574 (2007).
[3] Z. Weng at. all, Adv. Energy Mater. 1, 917 (2011)

The requirement of flexible and stretchable all-solid-state thin film lithium batteries have been steady increased as increasing the development of wearable, flexible and soft electronic devices such as smart cards, bio sensors, and roll-up displays. In order to satisfy the requirements, using the conventional polymer substrate is necessary for low price and light weight battery. However, the cathode thin film, which is a critical component of thin film battery, certainly needs to be crystallized by annealing process. Ultimately, the polymer substrate goes through sudden changes because of its poor resistance to high temperature, so a new direction for crystallization at low temperature should be considered.
Here, we fabricated flexible all-solid-state thin film lithium battery using Sn-LiMn2O4 cathode, LIPON electrolyte, and Al-Si anode thin film on polyimide (PI) substrate. The deposited cathode thin films were crystallized by the excimer laser annealing. Structural properties of the thin films were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The electrochemical properties were measured using WBC3000 battery cycler. The results of this study shows the applicability of well-crystallized cathode film at low temperature, and it will be an important method in flexible devices.

Cadmium sulfide (CdS)/Cadmium telluride (CdTe) thin film solar cells have been considered to be low cost and high efficient photovoltaic devices due to the high light absorption and the simple fabrication process. Also, it has been previously reported that flexible photovoltaic devices can be demonstrated on flexible substrate such as glass and polymer substrates.
CdS/CdTe thin films are generally deposited on the indium tin oxide (ITO) layer as the transparent conductive layer (TCL). However, the ITO films are not suitable to the flexible devices because of the poor mechanical properties. Therefore it has been expected that the graphene, which has high optical transmittance, thermal/electrical conductivity and good mechanical properties, can be an alternative material. Also, the polymer substrate used for fabricating the flexible photovoltaic devices cannot offer the thermal stability at high temperature growth condition.
In this study, we demonstrated CdS/CdTe thin film structures grown on the flexible thin glass substrate using the trilayer graphene as the TCL. When the CdS thin films are deposited on the pristine graphene layer, the CdS thin flims with high quality cannot be obtained. Therefore the ultraviolet (UV) or electron-beam (e-beam) were irradiated on the surface of graphene transferred on the top of thin glass, then the CdS thin films were grown on the defective graphene by chemical bath deposition (CBD) method. After that, the CdTe thin films were deposited on the CdS thin films by close-spaced sublimation (CSS) process. The properties of the CdS/CdTe thin films grown on the flexible thin glass using the trilayer graphene as the TCL will be presented.

The piezoelectric semiconductor materials has emerged as the most attractive material for nanogenerator (NG) based prototype applications such as piezotronics, piezophotonics and energy harvesting due to the coupling of piezoelectric and semiconducting dual properties. Understanding the mechanism for high power generation, charge transport behavior, energy band modulations and role of depletion width in piezoelectric semiconducting p-n junction through piezoelectric charges, developed by external mechanical strains are essential for various NG. Here, we demonstrated enhancement of output power of one dimensional zinc oxide (ZnO) based NG using p-type semiconductor polymer and by controlling their energy band at depletion width in piezoelectric semiconducting p-n junction interface and native defects presented in as grown ZnO NWs. The piezoelectric output performance from the P3HT coated ZnO based NG was several times higher than that from the pristine ZnO based NG under application of same vertical compressive force. Holes from p-type P3HT polymer significantly reduced the piezoelectric potential screening effect caused by free electrons in ZnO. Theoretical investigations by COMSOL were also carried out in order to understand the improvement in the performance of surface passivated ZnO based NG in the terms of free carriers concentration and holes diffusion due to formation p-n junction at interface ZnO and P3HT and formed depletion width.

We present that the electroluminescence (EL) properties and the output voltage of flexible N-face p-type GaN thin films can be tuned by strain-induced piezo-potential generated across the metal-semiconductor-metal structures. Under different staining conditions (convex and concave bending modes), the transport properties of the GaN films can be changed due to the spontaneous polarization of the films. The I-V characteristics with the bending modes show that the convex bending can increase the current across the films by the decrease in the barrier height at the metal-semiconductor contact, increasing the EL intensity of the P-N junction device. At convex bending, it is also shown that the flexible p-type GaN films can generate an output voltage of up to 1.0 V, while at concave bending, 0.4 V. The change of the band bending with the crystal polarity of GaN films was investigated using high-resolution photoemission spectroscopy. This study has great significance on the practical applications of GaN in optoelectronic devices and nanogenerators under a working environment.

Bulk heterojunction polymer solar cells have the potential to provide a low-cost, easily scaled up and lightweight using wet-processes such as ink-jet printing and roll-to-roll printing techniques. We have been developing inverted polymer solar cells with metal oxide electron collection layer such as amorphous titanium oxide (TiOx), zinc oxide (ZnO), and zinc sulphide. We have previously reported the fabrication and the characterization of flexible polymer solar cells with a sol-gel TiOx.1) However, because the sol-gel TiOx precursor solution was unstable with respect to moisture, it was essential to handle the precursor solution in a N2 filled glove box. Herein we report that a novel ZnO precursor solution, which contains zinc acetylacetate (Zn(acac)2) as the zinc source and acetylacetone (AA) as the complexing agent, makes it possible to prepare the ZnO film on flexible PET-ITO substrates at low temperatures and in air.2)
A ZnO solid film with thickness of about 100 nm was prepared on the substrate using the sol-gel method. The ZnO precursor solutions were prepared in 2-methoxyethanol, which contained Zn(acac)2 and AA. The solutions were spin-coated onto the ITO, and the precursor films were heated at 100 °C for 1 h. A mixed solution of P3HT and PCBM (w/w = 5:4) was spin-coated onto the ITO/ZnO substrate. A PEDOT:PSS dispersion in water containing 0.5 wt% Triton-X 100 was spin-coated onto the PCBM:P3HT layer. The Au back electrode was vacuum-deposited at 2×10-5 Torr onto the PEDOT:PSS layer. The effective area of the solar cell was restricted to 1 cm2, by depositing the Au electrode with a shadow mask.
Reproducible cell performances were obtained despite the ZnO cells being fabricated in air and at low temperature, using a novel ZnO precursor solution containing Zn(acac)2 and AA. The power conversion efficiency (PCE) of the flexible ZnO cells without sealing was ca. 2.4% under irradiating AM1.5G simulated sunlight. To improve the performance of the flexible cell, a small amount of Nitrobenzene (NB) was added to the CB solution of P3HT:PCBM, being denoted the CB:NB solvent. Though NB was a poor solvent for P3HT and PCBM, it was known that its addition improves the crystallinity and the thermal stability of P3HT and PCBM in a solid film. When the blend film was prepared using CB containing 2 vol% NB, the improved performance of PCE = 3.5% was obtained. However, its durability was relatively low, for example, the PCE dropped to 2.1% after light irradiation for 100 h in air. On the other hand, when the PCBM:P3HT film was prepared using only CB solvent, the PCE of the non-sealed flexible cell maintained 96% of the maximum value (PCE = 2.4%) ever after light irradiation for 100 h in air.
References
1) I. Sasajima, S. Uesaka, T. Kuwabara, T. Yamaguchi, and K. Takahashi: Org. Electron., 12 (2011) 113.
2) T. Kuwabara, T. Nakashima, T. Yamaguchi, and K. Takahashi: Org. Electron., 13 (2012) 1136.

Energy harvesting systems which are flexible, stretchable, transformable to any surface topology, and mechanically durable may be used in a wide range of applications such as robotic sensory skins, wearable communication devices, electronic textile, and especially biomedical devices due to their high sustainability of all movements and body deformations. Human body has promising harvesting energy sources like thermal energy and mechanical energy. It would be highly desirable to harvest both types of energy from a single cell of nanogenerator at the same time not only to miniaturize an nanogenerator device but also to obtain high output performance from the device. In this work, we fabricated a highly stretchable, hybrid energy-scavenging nanogenerator based on the micro-patterned piezoelectric P(VDF-TrFE) polymer, micro-patterned polydimethylsiloxane (PDMS)-carbon nanotubes (CNTs) composite and graphene nanosheets.

Hydrogen is becoming increasingly important in green energy solutions around the
globe, yet it is still challenging to produce, store and release the hydrogen gas in a
controlled manner. We demonstrate a simple approach for the preparation of
thermoresponsive porous polymer-ruthenium nanoparticle composite materials that
catalytically produce and store hydrogen gas at room temperature, and its inherent
thermoresponsiveness, which allows for “on-demand” release of the stored gas.
The catalytically active ruthenium nanoparticles are embedded into the polymer in a
dynamic fashion by exploiting a versatile host-guest system based on the macrocycle
cucurbit[7]uril. The materials demonstrate reproducible behavior over many cycles
and the catalytic activity and release temperature are easily modulated by the
formulation.
The ruthenium-polymer nanocomposites matrix swell when introduced to ammonia borane solution, hold the catalytically produced H2 gas at room temperature. Upon heating to above its LCST, the matrix deswell and expels the gas out. This process is reproducible when the material filtered out and a new aliquot of ammonia borane is introduced again below the LCST.

Traditionally, researchers have observed that there is an inherent competition between the electronic properties and mechanical compliance of conjugated polymer thin films. This inverse relationship is undesirable for applications in flexible and stretchable electronics, which require high mechanical compliance while retaining high charge-carrier mobilities. We have found that minor structural variations in a conjugated polymer can have a large influence on both the mechanical and photovoltaic properties of the materials. Poly(3-hexylthiophene) (P3HT) blended with the fullerene derivative, PCBM, is quite stiff and brittle. Its tensile modulus is an order of magnitude greater (>1 GPa) than that of a blend in which the polymer has two additional methylene units in its alkyl chain, poly(3-octylthiophene) (P3OT). The power conversion efficiency of devices that use P3HT:PCBM is, however, substantially better than those of P3OT:PCBM. This paper describes our efforts to achieve the “best of both worlds”—that is, the design of a mechanically robust and stretchable polymer:fullerene system with photovoltaic properties that are close to state-of-the-art. We correlated the mechanical and photovoltaic properties of four polymers designed to exhibit the properties of both P3HT and P3OT: (1) a physical blend (P3HT:P3OT); (2) a block copolymer (P3HT-b-P3HT); (3) a random copolymer (P3HT-co-P3OT); and (4) poly(3-heptylthiophene) (P3HpT), whose side chain contains seven carbon atoms. Measured values of power conversion efficiency (PCE) plotted as a function of tensile modulus revealed that the four “hybrid” materials exhibited good combinations of mechanical and photovoltaic properties. In particular, we found that P3HpT exhibited the favorable photovoltaic performance of P3HT and the high mechanical compliance of P3OT. A simple theoretical model supported our experimental results. Our observations allowed us to reach two conclusions: (1) P3HpT is superior to P3HT in applications requiring significant mechanical compliance and (2) mechanical and photovoltaic properties are not inherently in competition. We believe it may be possible to use a similar analysis in the design of true rubber-like semiconductors.

Electroceramic materials have many applications such as piezoelectric sensor and electro-optical devices. In recent years, the demand for electroceramics with better performance and functionality has grown. Due to the mechanical fragility of the ceramic material, its use as sensor or other devices has some limitation. To overcome this problem, ceramic/polymer composite material has been considered. The composites combine the advantages of ceramics and polymers, presenting a novel type of material that is easy to process, and with high dielectric constant, high electromechanical efficiency, and high breakdown strength. The ceramic fillers used in the composites are usually ferroelectrics with high dielectric constant, such as barium zirconate titanate (BZT) and barium titanate (BT). The electroceramics material ensuring the good ferroelectrics properties and the flexibility was offered by the polymer. The polymeric precursor method was considered ideal for the preparation of these ceramics particles. Based on this context, this study aimed to use this method to prepare powders with perovskite structure, such as BaZrxTi1-xO3 (x = 0, 0.05, 0.08, and 0.10), with controlled particle size, good crystallinity and chemical stability.
The degree of structural order-disorder of these systems was investigated by different characterization techniques, such as: X-ray diffraction, Raman spectroscopy, and photoluminescence measurements. The XRD patterns and Raman spectra showed that the powders of BaZrxTi1-xO3 (x=0 and 0.10) have tetragonal structure and a mixture of orthorhombic and tetragonal phases were founded in the BaZrxTi1-xO3 (x=0.05 and 0.08) materials. The PL spectra showed almost null photoluminescence intensity emission.
The polymer polyvinylidene fluoride, PVDF, was used for the composite preparation. We control the size of the ceramic particles and the thickness of the composite films to establish the best condition for the piezoelectric properties. The results showed that is possible to obtain piezocomposites films with 30/70 and 50/50 v/v (ceramic/polymer). Compositefilms made with BT and BZT ceramic particles immersed in PVDF were produced by hot pressing the powder mixtures in the desired ceramic volume fraction. Composites were characterized by scanning electron microscopy, photoluminescence measurements, and the longitudinal d33 piezo coefficient response. The d33 values varied according to the Zr concentration, and according to the phases in the ceramics materials. The highest d33 value (14.3 pC/N) was found in the BaZr0.08Ti0.92O3 50/50 sample. Through the analysis of PL emission spectra was possible to verify that the presence of the PVDF and the order-disorder structural is fundamental for the presence of this property in these systems. Also, it was noted that the replacement of Ti by Zr in BT lattice promotes a considerable increase in the PL intensity and different maximum emission in the visible region of electromagnetic spectrum.

There is a rapidly increasing trend to implant medical devices directly into the body. Currently, most implantable medical devices and systems are powered by batteries, but these power sources have serious limitations due to the lifetime and the size. In general, patients require a surgical operation just to replace the battery after a few years. Therefore, to supply durable and stable power to implantable biomedical devices is one of the most challenging issues in active medical implants. Here we demonstrate the new approach for enlarging the sensing capacity and the power generation of implantable medical sensors and energy harvesters via cost effective chemical solution-derived bio-compatible piezoelectric films with conducting flexible metal foil substrates.
Chemical solution derived bio-compatible lead-free piezoelectric (Na,K)NbO3 (NKN) thin films on medial grade ultra-thin metal foil substrates were successfully prepared for vibrational energy harvesting device applications. Use of a thin conductive buffer layer led to smooth and dense microstructures of the films on these substrates. The NKN device constructed on ultra-thin flexible metal foil substrate shows dramatically enhanced remanent polarization in comparison with those on Si substrates, which is due to the large biaxial compressive strain resulting from the thermal expansion mismatch to the substrate. The device also displays improved output voltage due to the ability to achieve large flexure of the device in comparison with the device integrated with more rigid substrates. The first successful lead-free NKN thin film-based flexible energy harvesting device was fabricated for self-powered implantable medical devices using a simple process platform without complex MEMS or etch process.
Our results provide an advanced processing platform to dramatically expand the functionality and capability of high power energy scavenging systems with a long lifetime, and to explore the addition of new functionality to emerging bio-compatible and flexible electronics.

Ni-Mn-Ga intermetallic alloy has attracted researchers for more than a decade due to its diverse properties including magnetic shape memory and magnetocaloric effects. The latter effect makes this alloy, as a near room temperature solid state refrigerator, a highly potential replacement for environmentally unfriendly and expensive conventional gas-liquid refrigerators. Many investigations have been conducted on single crystal magnetic shape memory alloy addressing twin boundaries reorientation and different martensite phases. However, for magnetocaloric application, alloys in polycrystalline form are preferable, and these are lacking for reliable structural study. In this paper, effect of stress and pressure on the crystal structure by milling and diamond anvil cell (DAC) is presented, respectively.
Polycrystalline Ni2+xMn1-xGa ingots with x= 0 to 0.26 were prepared by arc melting using Ni, Mn, and Ga precursors all 99.99% pure. Samples were homogenized at 1000oC for 3 days in sealed quartz ampoules. The bulk samples were subjected to grinding/ball milling to result in powder. Energy dispersive spectroscopy (EDS), X-ray diffraction (Mo-Kα radiation), Rietveld analysis, and Transmission Electron Microscopy were performed to obtain chemical composition, structural data, and microstructure images. Results indicate that grinding by pestle and mortar as well as ball milling induce phase transformation leading to substantial change in structure and lattice parameters. Changes in the structure and lattice parameter directly affect the magnetization via changing in Mn-Mn distance and crystal symmetry. Ball milling causes decrease in magnetization and increase in atomic disorder [1]. However, DAC has the advantage of not introducing contamination and disorder unlike ball milling. Some researchers reported effect of pressure on magnetocaloric properties [2, 3], but there is not any systematic study by in-situ XRD to investigate the structural evolution and link the external pressure to magnetocaloric properties.
References:
[1] B. Tian, et al., J. All. Compd. 509, 4563-4568 (2011)
[2] K. Mandal et al., J. Appl. Phys. 105, 073509 (2009)
[3] U. Devarajan et al., J. Appl. Phys. 114, 053906 (2013)

Low-temperature thin-film processing has shown promise for integration of energy-harvesting devices and thin-film power electronics on flexible substrates. We have created non-rigid powering systems on plastic using thin-film transistors (TFTs) to build power inverters that perform DC to AC conversion from thin-film solar cells. These enable substantial wireless power delivery to other parts of a system e.g. CMOS IC. We previously illustrated how TFT characteristics (e.g. ft) impact system performance as well as how provision of output power is affected by stability of TFTs. In this work we present a means of improving these factors, primarily by reducing TFT overlap capacitances through a self-aligned fabrication process.
For this study we use TFT-based LC oscillators consisting of two cross-coupled TFTs (W/L=3600/6mu;m) with planar Cu inductors (10cm^2, L=150 mu;H, R=35#8486;) forming tanks at TFT drains. The inductors resonate with the TFT capacitances enabling wireless power transfer (>20mW) through inductive near-field coupling. Reducing TFT capacitances (Ctft) is attractive for two reasons: (1) lower capacitances result in higher oscillation frequency and inductor quality factor, enabling more efficient power transfer; and (2) lower capacitances enhance robustness of oscillations by improving the positive-feedback condition (gm/Ctft)x(L/R)>1.
For comparison, two bottom-gate TFT structures are adopted: (1) blanket-SiNx passivated back-channel etched a-Si TFTs; (2) back-channel passivated self-aligned TFTs. In the latter, first the back-channel SiNx passivation is self-aligned to the gate by backside lithographic exposure through the sample substrate and second the source/drain contacts are self-aligned to the gate. The a-Si channel island thickness of the self-aligned TFTs is thinner (~25nm), minimizing absorption through this layer during back-side exposure. Though requiring longer exposure times, this process significantly reduces source-/drain-gate overlap (~1mu;m), resulting in overlap capacitances which are typically Cgs=Cgd=2pF (TFT width=3600mu;m). The standard back-channel etched TFTs on free-standing 50mu;m polyimide (at process temperatures <180C) have source-/drain-gate overlaps of 15mu;m ensuring successful alignment over a 7x7cm sample, resulting in overlap capacitances Cgs=Cgd=9pF.
We show experimental data illustrating performance improvement using self-aligned TFTs, correlated with theory. We present a 40% increase in frequency (with coupled tank inductors), showing that tank-inductor self-capacitances (~10pF for coupled inductors, ~2pF for separate inductors) are critical with TFT capacitances minimized through self-alignment. We demonstrate that smaller inductors (with smaller L/R ratios, radius<1.5cm) can be used whilst maintaining oscillations, thanks to the improved oscillation condition. Thus the achievable reduction in both tank capacitance and inductance benefits frequency, enabling oscillations >3.5MHz with 1.5cm radius inductors.

Preform-to-fiber thermal drawing is a versatile process that allows the fabrication of polymer- or glass-based fibers with complex, multimaterial, internal structures that grant them functions ranging from optical transmission to chemical detection. However, while a wide range of materials - such as metals, semiconductors or composite and ferroelectric polymers - have been successfully drawn in various phases, the fabrication of fibers with internal porous domains remains elusive thus far. Such domains, which could be filled with various substances, would be highly desirable in applications ranging from flexible fiber batteries to chemical delivery with fibers. Because this fabrication technique intrinsically relies on the flow of the fiber materials in a low-viscosity state, direct incorporation of porous materials in the preform is inadequate, as pores tend to elongate and collapse during the process.
Here we propose a novel method based on controlled phase separation of a polymer solution, used to embed porous polymer domains inside multimaterial fibers. This is achieved by thermally drawing a hollow polymer preform filled with a liquid polymer solution in its core. The solution is tailored to be homogeneous at the elevated drawing temperature, but phase-separated at ambient temperature. Upon cooling down after the drawing process, the solution in the fiber core thus undergoes a phase separation which yields a two-phase structure made of an interconnected polymer matrix separated by a solvent rich phase. The solvent can be dried out to leave voids, thus creating a porous structure in the core of the fiber. This new method differs widely from current extrusion-based or spinning-based porous polymer fiber production methods as it allows combinations of porous domains with other materials, and thus the creation of complex architectures inside fibers. We further investigate the resulting porous domains and illustrate some potential structures and applications, including the aforementioned use of the thermally drawn porous domains as matrices for gel-polymer electrolytes inside fibers, as well as their post-draw capillary breakup into porous microspheres.

Bismuth telluride has been considered as a promising candidate for thin-film thermoelectric (TE) devices due to its superior thermoelectric properties at room temperature regime. In general, a good thermoelectrics would require large Seebeck coefficient, high electrical conductivity and low thermal conductivity. All these transport properties are strongly dependent on carrier concentration and can be modulated by doping of electron donors or acceptors. In this study, we report that a Bi-Sb-Te thin film with a step-like Ag overlayer possesses a huge voltage across the Bi-Sb-Te/Ag junction region under a temperature gradient in the direction parallel to the junction. The measured voltage is much larger than the typical Seebeck voltage generated by the temperature gradient applied on the Bi-Sb-Te thin film. The anomalous Seebeck effect occurs only if the specimen is annealed at elevated temperature. It is speculated that the enlarged Seebeck voltage is associated with the diffusion and Ag in Bi-Sb-Te film at the junction region. The effect of Ag doping on thermoelectric transport properties of Ag overlaid Bi-Sb-Te thin films will be investigated.

According to the increasing demand for portable electronics such as cell phones, laptops and mp3 players, it is strongly required to have energy storage devices integrated with other electronic components. As energy storage devices, supercapacitors have attracted considerable interest owing to their high power density, cycle efficiency and fast charge/discharge rate. Recently, high performance micro-supercapacitors were reported to be fabricated onto a chip via use of various active materials and designs. In particular, carbon based materials such as graphene, carbon nanotubes (CNTs), and activated carbon have been widely used as electrode materials since they have high electrical conductivity, light weight and high electrochemical surface area. Tailoring and engineering of those carbon materials are expected to extend the application of supercapacitors to various fields.
In this study, we report on the fabrication of all-solid-state flexible micro-supercapacitor (MSC) with patterned electrodes of graphene and multiwalled carbon nanotubes (MWCNTs) and solid-state electrolyte. First, CVD grown multilayer graphene was transferred onto a flexible substrate treated with self-assembled monolayer of (3-Aminopropyl) triethoxysilane. Then the graphene as a current collector was patterned via photolithography and reactive ion etching for the fabrication of planar-type MSC where the ion channel length was 100 mu;m. After spray deposition of functionalized MWNTs onto patterned graphene, H3PO4-PVA gel electrolyte was coated. Such fabricated all-solid-state MSC worked in the potential ranging from 0 to 0.8 V and exhibited a high areal capacitance of 2.54 mF/cm2 at a scan speed of 10 mV/s. Furthermore, it showed a stable cycle performance that 85 % of the initial capacitance was retained after 20,000 cycles at a current density of 15 mu;A/cm2. Besides, no significant change of charge/discharge characteristics was observed under bending of the whole device with both tensile and compressive stress. With a deliberate circuit design, the output voltage and the capacitance of supercapacitors could be controlled to operate micro-LEDs with various colors. This work demonstrates a high potential of planar-type flexible MSC, integrated into a electronic circuit for wearable computers and deformable electronics.

Along with the increasing interest in the wearable devices, it is strongly required to have stretchable energy storage devices as well as the stretchable active devices. Furthermore, integration of the active devices with energy storage devices on the same flexible substrate is expected to accelerate the advent of the wearable computers in daily lives.
In this study, we report on the fabrication of 3-dimensionally (3D) stretchable micro-supercapacitor (MSC) arrays on a specially designed elastomeric substrate. Our 3D stretchable substrate consists of rigid small islands (PDMS) on both-sides of the soft thin film substrate (a mixture of Ecoflex and PDMS): such design minimizes the strain in the rigid island upon stretching and the liquid metal (EGaIn) interconnections are embedded in the substrate enabling the electrical connection between the active devices on the islands.
In the planar-type MSC, Au electrodes with spray-coated multi-walled carbon nanotubes are patterned to have 12 inter-digit structure with ion channel length of 150 µm. As an electrolyte, H3PO4-PVA gel is coated. On top of the PDMS islands, planar-type all solid state MSCs are transfer printed. Such fabricated MSC arrays on the 3D stretchable substrate showed very stable device performance upon deformation by stretching up to 50%. Repetition of charge-discharge cycles by 1000 times under stretching by 30% did not deteriorate the device performance, either. The total working potential and the capacitance could be easily controlled by serial and parallel connection of MSC arrays on the substrate, which could operate devices including micro light emitting diode (LED) and nano-devices. The current and light intensity of the LED remained the same with stretching up to 50%. It is attributed to the relatively small strain applied to the PDMS islands compared to the thin film. Finite element method calculation also confirmed that the strain applied to the island was less than 7% when the whole device was stretched to 50%.
This work demonstrates noble 3D stretchable MSCs which will be widely used to operate nano-devices in various wearable electronics.

Graphene materials have high specific surface area, good chemical stability, high electrical and thermal conductivity, and excellent flexibility. Therefore, graphene-based materials can be used as free-standing and binder-free electrodes for flexible energy storage devices.
A high efficient chemical exfoliation approach was developed to synthesize large-quantity multilayer graphene materials. By using these graphene materials, a flexible graphene/polyaniline paper was prepared by in situ anodic electropolymerization of polyaniline on a graphene membrane, and it shows a stable large electrochemical capacitance and excellent cyclibility; graphene-cellulose paper membranes were fabricated as freestanding and binder-free electrodes for flexible supercapacitors with good performance; a graphene membrane was used as the current collector for Li-S batteries to trap polysulfides, which shows significant improvement in cyclability and capacity. We also developed template-directed CVD to synthesize a three-dimensional interconnected graphene framework (GF). By coating active materials on the GF, an anode and cathode were made to assemble a thin, lightweight and flexible lithium ion battery. The battery has high rate capability and capacity, and can be repeatedly bent down to <5 mm without failure and degradation of its electrochemical performance.

We created unique interconnected partially graphitic carbon nanosheets (10-30 nm in thickness) with high specific surface area (up to 2287 m2 g-1), significant volume fraction of mesoporosity (up to 58%), and good electrical conductivity (211-226 S/m) from hemp bast fiber. The nanosheets are ideally suited for low (down to 0°C) through high (100°C) temperature ionic liquid-based supercapacitor applications: At 0°C and a current density of 10 A g-1, the electrode maintains a remarkable capacitance of 106 F g-1. At 20, 60, and 100 oC and an extreme current density of 100 A g-1, there is excellent capacitance retention (72-92%) with the specific capacitances being 113, 144 and 142 F g-1, respectively. These characteristics favorably place the materials on a Ragone Chart providing among the best power - energy characteristics (on an active mass normalized basis) ever reported for an electrochemical capacitor: At a very high power density of 20 kW kg-1 and 20, 60 and 100 °C, the energy densities are 19, 34 and 40 Wh kg-1, respectively. Moreover the assembled supercapacitor device yields a maximum energy density of 12 Wh kg-1, which is higher than commercially available supercapacitors. By taking advantage of the complex multi-layered structure of a hemp bast fiber precursor, such exquisite carbons were able to be achieved by simple hydrothermal carbonization combined with activation. This novel precursor-synthesis route presents a great potential for facile large-scale production of high-performance carbons for a variety of diverse applications including energy storage.

Graphene oxide has attracted lots of attention as a promising electrode material for supercapacitor due to its large surface area and high conductivity. However, restacking of graphene oxide occurs during the reduction process due to its van der Waals force. The formation of the stacked graphenes decreases the whole surface area that is directly proportional to the specific capacitance. Many approaches has been tried to resolve this issue by adding nanostructured materials such as carbon nanotube between the graphene oxide layers. However, it still has a weakness in penetration of electrolyte into the electrode. Here, we suggest nanoporous structure of reduced graphene oxide-carbon nanotube (RGO-CNT) to enhance the ion accessibility. The nanoporous RGO-CNT is prepared using filtration system, and followed by etching the sacrifical silica ball inside of the RGO-CNT matrix. The resulting structure provides efficient transport pathway for ions, which increases the interface area between the electrode and electrolyte significantly. Additionally, we decorated RGO-CNT film with MnO2 to increase the specific capacitance, and it exhibits excellent cycle performance and high power performance that non-porous structure.

Graphene is the ultimate two-dimensional material consisting of a single layer of sp2 hybridized carbon. Chemical synthetic methods are needed in order to scale its synthesis for applications. Here we explore converting graphite into graphene oxide sheets, which readily disperse in water (1). Using a 780 nm laser in an inexpensive LightScribe dvd drive, we can convert graphene oxide into a form of graphene with both high surface area and high conductivity (2). This laser-scribed graphene can be patterned and used to make electronic devices such as sensors. When an electrolyte is combined with two pieces of laser-scribed graphene, a high performance supercapacitor is formed. These supercapacitors exhibit high power, good energy density and long cycle life (3). They can be combined in series to increase voltage or in parallel to increase capacitance. By patterning the graphite oxide deposited on a plastic substrate, flexible microsupercapacitors can be made. These microsupercapacitors exhibit very high power along with enhanced energy density (4).
References:
1. D. Li, M.B. Muller, S. Gilje, R.B. Kaner and G.G. Wallace, “Processable aqueous dispersions of graphene nanosheets”, Nature Nanotech 3, 101 (2008).
2. V. Strong, S. Dubin, M. El-Kady and R.B. Kaner, “Patterning and electronic tuning of laser scribed graphene for flexible all-carbon devices”, ACS Nano 6, 1395 (2012).
3. M.F. El-Kady, V. Strong, S. Dubin and R.B. Kaner, “Laser printing of flexible graphene-based supercapacitors with ultrahigh power and energy densities”, Science 335, 1326 (2012).
4. M.F. El-Kady and R.B. Kaner, “Scalable fabrication of high-power graphene micro-supercapacitors for flexible and on-chip energy storage”, Nature Commun. 4, 1475 (2013).

Growing demands for energy storage devices with outstanding sustainability and environmental friendliness have stimulated intensive research on electrochemical supercapacitors, which store and deliver energy at fast charging/discharging rates for high-power applications. Pseudocapacitive materials, such as metal oxides and conducting polymers, offer high levels of specific capacitance and energy storage via Faradic surface redox reactions, circumventing the key limitation of conventional electrochemical double-layer capacitors with low energy density. However, the limited cycle life and low power density, as the compromise of increased energy density, intrinsically restrict the applications of pseudo-capacitors in practical devices. In this talk, I will introduce flexible nanoporous metal based films, developed by dealloying and electrochemical plating, as robust electrodes for high-performance electrochemical supercapacitors. The hybrid materials give rise to ultrahigh energy density and power density along with excellent cycling stability. The outstanding capacitive performances result from enhanced electronic/ionic conductivity by highly conductive nanoporous metal substrates in which active oxide materials are incorporated into pore channels and chemically bond with large internal/external surfaces. The novel nanoarchitecture provides high mechanical and electrochemical stability and excellent electronic/ionic conductivity for the enhanced pseudocapacitance of oxides and conducting polymers.

Supercapacitor as an energy storage device with superior power density usually does not give high energy density comparable to that of Li-ion batteries. So, there has been extensive effort to develop novel electrode materials for improving its energy density. Furthermore, it is strongly demanded to have supercapacitors stable over mechanical deformation and integrated into electronics to operate the active devices for wearable and deformable electronics.
In this study, we report on the fabrication of flexible supercapacitors with a superior performance of high energy density as well as high power density, sufficient to operate a SnO2 nanowire (NW) UV sensor. A mixture of multi-walled carbon nanotubes (MWNTs) and V2O5 NWs was used as patterned electrodes: micro-supercapacitor (MSC) was patterned to have an ion channel length of 150 mu;m by using photolithography and then MWNT/V2O5 NW mixture with 10 vol% V2O5 NWs was spray-coated on MSC pattern. Then solid-electrolyte, PVA-LiCl was drop-coated. As a result, array of planar-type MSCs was fabricated on a flexible PET substrate via serial or parallel connection of individual MSC. Such fabricated array of MSCs exhibited a high volume capacitance of 80 F/cm3 at a scan speed of 10 mV/s. It exhibited a good cycle performance so that up to 80% of initial capacitance was maintained after 8,000 cycles at a current density of 11.6 A/cm3. Of particular interest, the MSC array also showed excellent energy density of 6.8 mWh/cm3, which is comparable to that of Li-thin film battery (1~10 mWh/cm3), as well as a high power density of 80.8W/cm3. It also showed stable device performance over deformation of both compressive and concave bending with a bending radius of up-to 1.5 mm. Even after 1000 times repetitive bending cycles with a bending radius ranging from 16 to 7 mm, 94% of the initial capacitance was kept. For the first time, we showed the operation of SnO2 NW UV sensor by MSC array integrated in the same PET substrate. This work demonstrates the potential application of all-solid-state MSC array to the operation of various nano-devices in the integrated electronic circuit of wearable devices.

Two-dimensional (2-D) materials represent a promising material platform for making powerful energy devices including flexible energy storage devices. Here we present a novel high-performance planar supercapacitor based on hybrid nanostructures of quasi-2D ultrathin MnO2/graphene nanosheets. The planar structures based on the δ-MnO2 nanosheets integrated on graphene sheets not only introduce more electrochemically active surfaces for absorption/desorption of electrolyte ions, but also bring additional interfaces at the hybrid interlayer areas to facilitate charge transport during the charge/discharge processes. The unique material and structural design enable the promising device characteristics of planar supercapacitors including high specific capacitance with excellent rate capability and cycling stability, as well as great device flexibility. The highly flexible energy storage devices based on hybrid 2-D nanomaterials can function as the power back-ups for stretchable/flexible electronic devices.

Piezotronics is a term coined in by Prof Zhong Lin Wang (Georgia Institute of Technology, Atlanta, USA) and describes the exploitation of strain and deformation internal polarization fields in polar semiconductors. Such fields already find applications in transducers and micropositioner devices but are also known to be present in GaN based light emitting diodes and lasers. Being a property of polar semiconductors piezoelectricity is present in both III-V and II-VI compounds, such as the technologically important ZnO. For many years piezoelectricity was included in the design of devices only to first order. In recent years a great deal of evidence, both model and experimental data, has been generated that such effects need to be included to at least second order.[1] The inclusion of such nonlinear effects produces surprising and non-intuitive results, notably the generation of fields of opposite polarity compared to the prediction of linear piezoelectricity and the possibility of enhancing the piezoelectric polarization by a factor of 5-10 under particular deformations. In this presentation we will show the evidence for nonlinear effects and discuss the possible applications to light emitting diodes, quantum dot emitters and energy harvesting devices. Our theory of Non Linearities, based on accurate quantum mechanical calculations and a tight binding formulation of the elastic and dielectric properties of zincblende and wurtzite crystals, is capable of highlighting and correctly predicting the polarization properties of several polar semiconductors under strain.
We observed strong non linearities in the strain dependence of both zincblende III-V&’s and wurtzite III-N and its alloys and even II-VI wurtzite semiconductors. While revealing much smaller Spontaneous Polarization effects in nitrides than previously reported, much larger values of the total (spontaneous + strain induced) polarization are observed compared to linear models, therefore highlighting the large effect of Non Linear Piezoelectricity in wurtzite semiconductors.
The significance of our work is that by incorporating our unique and well tested ability of evaluating nonlinear polarization in the design of composite semiconductor structures we will in the near future be able to propose layouts were the piezoelectric fields, i.e. the ‘engine&’ inside PEHs devices, can be suitably enhanced, greatly increasing, possibly by a factor of 2 or more, the ability of such devices to convert mechanical energy into an electrical potential difference.

[1] M. A. Migliorato et al, Phys. Rev. B 74, 245332 (2006); R. Garg et al, Appl. Phys. Lett. 95, 041912 (2009);
J.Pal et al, Phys. Rev. B 84, 085211 (2011); H.Y.S. Al-Zahrani et al, Nano Energy (2013); G Tse et al, J. Appl. Phys. 114 (7), 073515-073515-12

Owing to the coupling of semiconducting and piezoelectric properties, the piezotronic effect involves using the strain induced inner-crystal piezopotential as a “gate” to tune/control the charge carrier transport properties of a pminus;n junction or metalminus;semiconductor interface. This new concept was first proposed in 2007. A series of electronic devices have been demonstrated based on this effect, including piezopotential gated field-effect transistors, strain sensors, piezotronic logic gates, electromechanical memories, and so forth. Great potential for the application of this new effect and research field has been demonstrated. As of now, all of the experimental studies were carried out at room temperature, and there has been no investigation of temperature effects on the performance of piezotronic devices. Considering the complicated and variable working environment the devices may experience, the temperature behavior of the piezotronic device is of great significance not only for fully understanding this coupling effect between mechanical and electrical properties in nanomaterials, but also for developing new novel piezotronic devices. A comprehensive investigation was carried out on n-type ZnO nanowires for studying the temperature dependence of the piezotronic effect from 77 to 300 K. In general, lowering the temperature results in a largely enhanced piezotronic effect. The experimental results show that the behaviors can be divided into three groups depending on the carrier doping level or conductivity of the ZnO nanowires. For nanowires with a low carrier density (<10E17/cm3 at 77 K), the pieozotronic effect is dominant at low temperature for dictating the transport properties of the nanowires; an opposite change of Schottky barrier heights at the two contacts as a function of temperature at a fixed strain was observed for the first time. At a moderate doping (between 10E17/cm3 and 10E18/cm3 at 77 K), the piezotronic effect is only dominant at one contact, because the screening effect of the carriers to the positive piezoelectric polarization charges at the other end (for n-type semiconductors). For nanowires with a high density of carriers (>1018/cm3 at 77 K), the piezotronic effect almost vanishes. This study not only proves the proposed fundamental mechanism of piezotronic effect, but also provides guidance for fabricating piezotronic devices.

Polar semiconductors possess piezoelectric properties, which are often measured in both III-V and II-VI compounds, such as the technologically important ZnO. Piezoelectricity is important in the design of devices and in recent years a great deal of evidence, both model and experimental data, has been generated that such effects need to be included to at least second order.[1]
In this presentation we will show both evidence for nonlinear effects and discuss the applications to light emitting diodes, quantum dot emitters and energy harvesting devices.
In particular we will show the difference between linear and non linear piezoelectricity in ZnO and III-N nanowires, nanostructures of interest to piezotronics. We observed in ZnO nanowires, much higher positive polarization from non linear piezoelectricity compared to the linear model. This could be exploited in producing more efficient nanogenerators.
Furthermore we will discuss how management of piezoelectric fields in conventional LED devices can achieve increases in performance. We tested what would be the effect of a metamorphic buffer capable of producing additional tensile strain in the active region of the device (the multi quantum well region). The prediction is that piezoelectric polarization would be reduced and the spontaneous emission spectrum would be increased, resulting in higher performance devices.[2] Such predictions have already been verified independently by experimental groups.

[1] M. A. Migliorato et al, Phys. Rev. B 74, 245332 (2006); R. Garg et al, Appl. Phys. Lett. 95, 041912 (2009);
J.Pal et al, Phys. Rev. B 84, 085211 (2011); H.Y.S. Al-Zahrani et al, Nano Energy (2013) (http://dx.doi.org/10.1016/j.nanoen.2013.05.005); G Tse et al, J. Appl. Phys. 114 (7), 073515 (2013);
[2] J.Pal et al, J. Appl. Phys. 114, 073104 (2013)

In this paper, we will present the main results obtained in our group, both theoretically and experimentally, about the prospects of piezoelectric nanowires for mechanical transduction. Forecasted applications are in the field of sensing or energy harvesting for autonomous systems.
While piezoelectric transduction with bulk materials is usually performed using resonant MEMS structures, the scaling to nanowire-based structures may need a change in the approach, except for specific applications where high resonance frequencies are desired. Indeed, mechanical energy is mostly found in our environment in the form of vibrations, impacts or deformations which are non periodical (e. g. body movement) or at low frequency (e. g. machines vibrations). Direct conversion can then be a better approach,[1] which has indeed been followed by some groups with the realization of convincing test demonstrators. By deriving some general scaling trends,[2] we will show what the real assets of nanowire-based structures are, compared to bulk structures.
We will then detail one possible configuration, the VING,[3] which consists in converting the deformation of an array of vertical nanowires into a displacement current. Such a structure can be fabricated using low temperature processes compatible with most substrates, including non planar and flexible substrates. We will first detail its operation in compression,[4] with the trade-offs that must be realized between mechanical and electrical properties, and the influence on performance of materials and geometry. The same structure can also be used in flexion. The impact on materials and geometrical trade-offs will be discussed in view of previous conclusions. Performance will be benchmarked against that of a thin film with the same amount of piezoelectric material.
Finally, the use of nanoscale wires has the additional advantage of allowing larger piezoelectric coefficients to be obtained, even for larger diameters than predicted theoretically. AFM characterization results will be shown, where the AFM tip is used to simultaneously apply a controlled force, and measure the potential generated along a bendable nanowire. The piezopotential was found to be strongly influenced by the characteristics of the nanowire, such as its doping level, or by the introduction of a barrier along the nanowire. Although the full explanation of such improvements is not yet available, it demonstrates that nanowire-based devices are probably far from full optimization and that there is still much room for improvement. Perspectives about on-going work will conclude the talk.
[1] G. Ardila et al., IEEE-ISCDG proceedings, pp. 75-78, doi: 10.1109/ISCDG.2012.6359985 (2012)
[2] R. Hinchet et al. IEEE-IEDM'2012 Proceedings, pp. 6.2.1 - 6.2.4, doi: 10.1109/IEDM.2012.6478988 (2012)
[3] S. Xu et al., Nature Nanotechnology, vol. 5, no. 5, pp. 366-73, May 2010
[4] R. Hinchet et al., Advanced Functional Materials, accepted Sept. 2013

Targeting a variety of applications, considerable progresses have been made for wearable electronics. However, lithium rechargeable batteries do not follow such progresses due to insufficient mechanical stabilities. In this talk, I will present a textile-based lithium rechargeable battery with substantially improved mechanical stability based on unconventional material and structure for each of key cell component (current collector, binder, and separator). The final textile batteries developed for clothes and watchstraps remain fully functional even during severe in-situ folding-unfolding motions ( several hundreds folding repetitions per each battery cycle). Furthermore, the final textile full-cell was integrated with flexible and lightweight solar cells on the battery pouch to enable convenient solar-charging capabilities.

Carbon nanotubes have drawn extensive interest by researchers for their unique electrical, mechanical, and optical properties that make them attractive for a multitude of applications. A popular commercial appeal of carbon nanotubes is their ability to be dispersed in conductive inks for use in printed electronics; however, carbon nanotube inks most often depend on a surfactant to stabilize the nanostructures in suspension, or form stable suspensions through surface functionalization in aqueous solvents with low evaporation rates. We introduce a novel process to form a stable ink composed of carbon nanotubes dispersed in isopropanol alcohol that enables homogenous spray coatings of carbon nanotube films with high electrical and thermal conductivity. Various intrinsic dispersive properties of the ink are investigated, which reveal high quality carbon nanotubes that are effectively exfoliated in solution by charged surface groups on the tubes. The unique mechanical, electrical, optical, and thermal properties of the spray coated films on various substrates enforce the versatility of this ink in applications for light-weight, flexible, and stable energy devices and electronics.