Theoretical Investigation of Influence of Mechanical Stress on Magnetic Properties of Ferromagnetic/Antiferromagnetic Bilayers Deposited on Flexible Substrates

Effect of mechanical stress on magnetic properties of an exchange-biased ferromagnetic/antiferromagnetic bilayer deposited on a flexible substrate is investigated. The hysteresis loops with different magnitudes and orientations of the stress can be classified into three types. The corresponding physical conditions for each type of the loop are deduced based on the principle of minimal energy. The equation of the critical stress is derived, which can judge whether the loops show hysteresis or not. Numerical calculations suggest that except for the magnitude of the mechanical stress, the relative orientation of the stress is also an important factor to tune the exchange bias effect.

High-Quality Bi_2Te_3 Single Crystalline Films on Flexible Substrates and Bendable Photodetectors

Recently, great efforts have been made in the fabrication of arbitrary warped devices to satisfy the requirement of wearable and lightweight electronic products. Direct growth of high crystalline quality films on flexible substrates is the most desirable method to fabricate flexible devices owing to the advantage of simple and compatible preparation technology with current semiconductor devices, while it is a very challenging work, and usually amorphous, polycrystalline or discontinuous single crystalline films are achieved. Here we demonstrate the direct growth of high-quality Bi2 Te3 single crystalline films on flexible polyimide substrates by the modified hot wall epitaxy technique. Experimental results reveal that adjacent crystallites are coherently coalesced to form a continuous film, although amounts of disoriented crystallites are generated due to fast growth rate. By inserting a quartz filter into the growth tube, the number density of disoriented crystallites is effectively reduced owing to the improved spiral interaction. Furthermore, flexible Bi2 Te3 photoconductors are fabricated and exhibit strong near-infrared photoconductive response under different degrees of bending, which also confirms the obtained fexible films suitable for electronic applications.

Magnetoelastic coupling effect of Fe10Co90 films grown on different flexible substrates

The magneto-mechanical coupling effect and magnetic anisotropy of Fe10Co90(FeCo)films deposited on silicon wafer(Si),flexible polyethylene terephthalate(PET),freestanding polydimethylsiloxane(PDMS),and pre-stretched 20%PDMS substrates were studied in detail.The loop squareness ratio Mr/Ms and the coercive Hc of the FeCo film grown on a PET substrate can be obviously tuned by applying a small tensile-bending strain,and those of the FeCo film grown on a freestanding PDMS substrate can only be slightly changed when applying a relatively large tensile bending strain.For the FeCo film prepared on a 20%pre-stretched PDMS,a wrinkled morphology is obtained after removing the pre-strain.The wrinkled FeCo film can keep the magnetic properties unchanged when applying a relatively large tensile bending strain perpendicular to the wrinkles.This reveals that PDMS is an ideal substrate for magnetic films to realize flexible immutability.Our results may help for developing flexible magnetic devices.

Enhanced resonance frequency in Co2FeAl thin film with different thicknesses grown on flexible graphene substrate

The flexible materials exhibit more favorable properties than most rigid substrates in flexibility,weight saving,mechanical reliability,and excellent environmental toughness.Particularly,flexible graphene film with unique mechanical properties was extensively explored in high frequency devices.Herein,we report the characteristics of structure and magnetic properties at high frequency of Co2FeAl thin film with different thicknesses grown on flexible graphene substrate at room temperature.The exciting finding for the columnar structure of Co2FeAl thin film lays the foundation for excellent high frequency property of Co2FeAl/flexible graphene structure.In-plane magnetic anisotropy field varying with increasing thickness of Co2FeAl thin film can be obtained by measurement of ferromagnetic resonance,which can be ascribed to the enhancement of crystallinity and the increase of grain size.Meanwhile,the resonance frequency which can be achieved by the measurement of vector network analyzer with the microstrip method increases with increasing thickness of Co2FeAl thin film.Moreover,in our case with graphene film,the resonance magnetic field is quite stable though folded for twenty cycles,which demonstrates that good flexibility of graphene film and the stability of high frequency magnetic property of Co2FeAl thin film grown on flexible graphene substrate.These results are promising for the design of microwave devices and wireless communication equipment.

Thickness dependence of the properties of transparent conducting ZnO:Zr films deposited on flexible substrates by RF magnetron sputtering

Transparent conducting zirconium-doped zinc oxide （ZnO：Zr） thin films with high transparency, low resistivity and good adhesion were successfully prepared on water-cooled flexible substrates （polyethylene glycol terephthalate, PET） by RF magnetron sputtering. The structural, electrical and optical properties of the films were studied for different thicknesses in detail. X-ray diffraction （XRD） and scanning electron microscopy （SEM） revealed that all the deposited films are polycrystalline with a hexagonal structure and a preferred orientation perpendicular to the substrate. The lowest resistivity achieved is 1.55 × 10-3 Ω·cm for a thickness of 189 nm with a Hall mobility of 17.6 cm2/（V·s） and a carrier concentration of 2.15×1020 cm-3. All the films present a high transmittance of above 90% in the wavelength range of the visible spectrum.

Influence of Argon Gas Pressure on the ZnO:Al Films Deposited on Flexible TPT Substrates at Room Temperature by Magnetron Sputtering

Aluminium doped ZnO thin films（ZnO︰Al） were deposited on transparent polymer substrates at room temperature by rf magnetron sputtering method from a ZnO target with Al2O3 of 2.0 wt%. Argon gas pressure varied from 0.5 Pa to 2.5 Pa with radio frequency power of 120 W. XRD results showed that all the ZnO︰Al films had a polycrystalline hexagonal structure and a （002） preferred orientation with the c-axis perpendicular to the substrate. The grain sizes of the films were 6.3-14.8 nm.SEM images indicated the ZnO︰Al film with low Argon gas pressure was denser and the deposition rate of the films depended strongly on the Argon gas pressure, increasing firstly and then decreasing with increasing the pressure. The highest deposition rate was 5.2 nm/min at 1 Pa. The optical transmittance of the ZnO︰Al films increased and the blue shift of the absorption edge appeared when the Argon gas pressure increased. The highest transmittance of obtained ZnO︰Al films at 2.5 Pa was about 85% in the visible region. The electrical properties of the films were worsened with the increase of the Argon gas power from 1 Pa to 2.5 Pa. The resistivity of obtained film at 1.0 Pa was 2.79×10-2 Ω·cm.

A unified model for determining fracture strain of metal films on flexible substrates

Failure strain determination of polymer-supported thin films is a key for the design of the flexible devices.A theoretical model R/R0=(L/L0)2(R,L are the electrical resistance and the length of the stretched film,respectively.R0,L0 are the corresponding initial values.)has been widely used to determine the fracture strain of thin films on flexible substrates.However,this equation loses its function in some special cases.Here,a simple and universal theoretical model was proposed to determine the fracture strain of metal thin films on flexible substrates in more generally situations.With this model,we investigated the thicknessdependent failure strains of Cu-5 at.%Al films with thickness of 10 nm,200 nm,1000 nm,and Ti films with thickness of 50 nm,100 nm,300 nm.This model was also employed to study the published data available.The results showed that the new model provided a fairly good prediction of the failure strains of different films.

Mechanical and magnetocaloric adjustable properties of Fe3O4/PET deformed nanoparticle film

The flexibility of nanoparticle films is a topic of rapidly growing interest in both scientific and engineering researches due to their numerous potential applications in a broad range of wearable electronics and biomedical devices.This article presents the elucidation of the properties of nanoparticle films.Here,a flexible film is fabricated based on polyethylene terephthalate(PET)and magnetic iron oxide at the nanoscale using layer-by-layer technology.The 2D thin flexible film material can be bent at different angles from 0°to 360°.With an increase in elastic deformation angles,the magnetocaloric effect of the film gradually increases in the alternating magnetic field.The test results from a vibrating sample magnetometer and a low-frequency impedance analyzer demonstrate that the film has a good magnetic response and anisotropy.The magnetocaloric effect and magnetic induction effect are controlled by deformation,providing a new idea for the application of elastic films.It combines the flexibility of the nanoparticle PET substrate and,in the future,it may be used for skin adhesion for administration and magnetic stimulation control.

Flexible perovskite solar cells based on green,continuous roll-to-roll printing technology

By designing and fabricating thin film electronic devices on a flexible substrate instead of more commonly used rigid substrate, flexible electronics produced has opened a field of special applications. In this article, we first reviewed available products that may be used as flexible substrates, their characteristics and unique advantages as supporting material for flexible electronic devices. Secondly, flexible perovskite solar cell is examined in detail, with special focus on its potential large-scale fabrication processes. In particular, a comprehensive review is provided on low cost solution printing techniques that is viewed highly as a viable tool for potential commercialization of the perovskite solar cells. Furthermore, a summary is given on green processing for the solution printing production of flexible perovskite devices.

Influence of Different Substrates on the Electrochromic Property of Polyaniline Films

The polyaniline （PANI） films doped with complex acid （sulfuric acid and sulfosalicylic acid） were prepared using the potentiostatic method on bare nickel flake （NF） and flexible polyethylene terephthalate （PET）/indium tin oxide （ITO） substrates. The contents of the PANI films,surface elements, electrochromic property and electrical conductivity were characterized by energy dispersive X-ray spectrometer （EDS）, cyclic voltammetry （CV） and electrochemical impedance spectroscopy （EIS）. The experimental results show that differences exist among cycle stability, redox reversibility and response time of polyaniline films on these two kinds of substrates, but the electrochromic phenomenon of the PANI films is in substantial agreement. The equilibrium transmittance spectra in the visible region （400-800 nm） for the PANI film on flexible PET/ITO substrate was obtained at different applied potential from -0.4 to 1.5 V. The results show that the transmittance of the PANI film by applying voltage is adjustable in a row and has excellent electrochromic performance.

Magnetostrictive Flexible Thin Film Structure for Micro Devices

Polyimide (Kapton, Dupont Corp.) based magnetostrictive thin film structures were designed and fabricated for micro device applications. In particular the growth of films on flexible substrates was studied to allow a simple integration of the system in miniaturized magnetostrictive devices. The films were fabricated on different substrates to compare their different magnetic and structural properties. It showed much more magnetostriction and higher impact resistance results compared with traditional Si based film type actuators. In the fabrication process, amorphous TbDyFe films with thicknesses of 500 nm, 1 μm, 1.5 μm respectively, were deposited on the designed substrate by DC magnetron sputtering. During sputtering process the substrate holder was maintained at room temperature. After the sputter process, X-ray diffraction studies were also carried out to determine the film structure and thickness of the sputtered film. At last, magnetization from VSM (Vibrating Sample Magnetometer) and magnetostriction from optical cantilever method of each structure were measured to estimate the magneto-mechanical characteristics under the external magnetic field lower than 0.7 T for micro-system applications.

A PIECEWISE LINEAR MODEL FOR ANALYZING THIN FILM/SUBSTRATE STRUCTURE IN FLEXIBLE ELECTRONICS

The conventional analytical method of predicting strain in a thin film under bending is restricted to the uniform material assumption, while in flexible electronics, the film/substrate structure is widely used with mismatched material properties taken into account. In this paper,a piecewise model is proposed to analyze the axial strain in a thin film of flexible electronics with the shear modification factor and principle of virtual work. The excellent agreement between analytical prediction and finite element results indicates that the model is capable of predicting the strain of the film/substrate structure in flexible electronics, whose mechanical stability and electrical performance is dependent on the strain state in the thin film.

Highly transmitted silver nanowires-SWCNTs conductive flexible film by nested density structure and aluminum-doped zinc oxide capping layer for flexible amorphous silicon solar cells

Indium tin oxide(ITO)is widely used in transparent conductive films(TCFs);however,several disadvan-tages,such as high cost and toxicity of indium,limit its applications.Therefore,it is necessary to develop other materials that can replace ITO.Silver nanowires or single walled carbon nanotubes(SWCNTs)have attracted considerable interest owing to their unique electrical,optical,and thermal stabilities,and thus,they are ideal for transparent electrodes for flexible or stretchable devices.In this study,we develop a novel architecture of composite TCFs on a polyethylene naphthalate(PEN)flexible substrate.Herein,the silver nanowires-SWCNTs films with nested density structure were fabricated through ultrasonic spraying technology by varying the spraying width.For achieving enhanced transmittance,we combined the larger irregular grids and holes with fewer nanowires stacked in the longitudinal direction,more optical chan-nels,and good carrier transport.Thereafter,aluminum-doped zinc oxide(AZO)was used as capping to the structure for enhancing the optical properties of the TCFs.The silver nanowires-SWCNTs/AZO(ASA)bilayer was obtained in the optimized architecture,which showed superior optoelectronic performance to that shown by commercial ITO with a high optical transmittance of 92%at the wavelength of 550 nm and low sheet resistance of 17/sq.In the specially structured conductive film,the significant improvement in the transmittance and uniformity of the sheet resistance was attributed to the effective nanowire junc-tion contact compared to that in ordinary structure of silver nanowires,which reduced the mean density of small clusters of nanowires.Compared with the silver nanowires-SWCNTs films,the ASA bilayer film exhibited excellent resistance to boiling,mechanical bending(10,000 cycles),and CO_(2)plasma.Moreover,the sheet resistance of ASA changed slightly after the tape tests,thereby illustrating a strong adhesion to the PEN substrate after the enclosure of AZO.Meanwhile,the AZO capping layer can enhance the op-tical transmittance between 600 and 1500 nm.In addition,the amorphous silicon photovoltaic devices with flexible ASA TCFs exhibited a power conversion efficiency(PCE)of 8.67%.After bending for 3000 times,the PCE was decreased to 8.20%,thereby demonstrating the potential of developed films to replace traditional ITO.

An Easy Way to Quantify the Adhesion Energy of Nanostructured Cu/X(X=Cr,Ta,Mo,Nb,Zr) Multilayer Films Adherent to Polyimide Substrates

An approach based on film buckling under simple uniaxial tensile testing was utilized in this paper to quan- titatively estimate the interfacial energy of the nanostructured multilayer films （NMFs） adherent to flexible substrates. The interfacial energies of polyimide-supported NMFs are determined to be ~ 5.0 J/m2 for Cu/Cr, ~4.1 J/m2 for Cu/Ta, ~ 2.8 J/m2 for Cu/Mo, ~ 1.1 J/m2 for Cu/Nb, and ~ 1.2 J/m2 for Cu/Zr NMFs. Furthermore, a linear relationship between the adhesion energy and the interfacial shear strength is clearly demonstrated for the Cu-based NMFs, which is highly indicative of the applicability and reliability of the modified models.

Large-area, Flexible Polymer Solar Cell Based on Silver Nanowires as Transparent Electrode by Roll-to-Roll Printing

Conventional organic solar cell＇s （OSC） architectures, including rigid transparent substrate （Glass）, conductive electrode （Indium tin oxide, ITO） and small working areas, are widely utilized in organic photovoltaic fields. However, such a structure as well as conventional spin-coating method obviously restrict their industrial application. In this article, we report the deposition of silver nanowires （AgNWs） on the flexible substrate by slot-die printing. The obtained AgNWs films exhibited a high transmittance and a low resistance, and were further used as the transparent conductive electrode ofOSCs. A typical conjugated polymer, poly[（2-5-bis（2-hexyldecyloxy）phenylene）-alt-（5,6-difluoro-4,7-di（thiophen-2-yl）benzo[c] [1,2,5]thiadiazole）] （PPDT2FBT）, was used as the active material to fabricate large-area （7 cm2） solar cells by a slot-die coating process. The power conversion efficiency （PCE） could reach 1.87% initially and further increased to 3.04% by thermal annealing. Compared to the performance of reference cell on ITO substrate, the result indicated that the AgNWs could be developed as an alternative substitute of conductive electrode to fabricate the large-area flexible OSCs by roll-to-roll printing.

Fabrication of Non-woven Fabric-Based SERS Substrate for Direct Detection of Pesticide Residues in Fruits

In surface-enhanced Raman scattering(SERS),flexible substrate plays an important role in target molecular collection from various shape surfaces and increases the analytical sensitivity.In this study,silver nanoparticles(Ag NPs)were deposited on a non-woven fabric used as an SERS substrate by self-assembly,in situ growing or the self-assembly/in situ growing combination method.4-Aminothiophenol was selected as a model molecular for the evaluation of the SERS performance using the substrates.The Ag NPs substrate prepared by self-assembly/in situ growing method presented the best Raman enhancement effect and its enhancement factor was estimated as high as 3.59106.The substrate was applied to the determination of four pesticide residues on the surfaces of fruit samples through wipe sampling,and the results revealed the good reproducibility of SERS responses and high detection sensitivity.The prepared flexible substrate was simple to fabricate and environmentally friendly.It could be expected to be a useful tool in rapid on-site test of pesticide residues on fruit surfaces because of its high sensitivity,convenience and non-destructive characteristics.

In-situ growth of gold nanoparticles on electrospun flexible multilayered PVDF nanofibers for SERS sensing of molecules and bacteria

Plasmonic surface of flexible multilayered nanofibers possesses special superiority for the surface-enhanced Raman scattering(SERS)sensing of molecules and microbial cells.However,the fabrication of flexible plasmonic nanofibers with high sensitivity and reproducibility is difficult.Herein,we report a smart strategy for fabricating flexible plasmonic fibers,in which compact and homogeneous gold nanoparticles(Au NPs)are in-situ grown on the high-curvature surface of multilayered fibers of electrospun polyvinylidene fluoride(PVDF).Firstly,the surface of PVDF fibers is changed electrically,and Au seeds are deposited on the surface of PVDF fibers using electrostatic driving force.Secondly,a stable AuI_(4)−complex is formed employing coordination between I−and AuCl4−ions,which could decrease the reduction potential of AuCl4−and restrain the self-nucleation,and then the reduction reaction of AuI4−is initiated by introducing PVDF@Au seeds to pull down the barrier of potential energy.Finally,in-situ growth of AuNPs is generated on the high-curvature surface of PVDF nanofibers,and large-scale hotspots are generated by adjacent AuNPs coupling in the three-dimensional(3D)space of multilayered fibers.Membrane of PVDF@Au nanofibers also realizes the sensitive detection of thiram molecules(low limit of detection of 0.1 nM)and good reproducibility(relative standard deviation of 10.6%).Meanwhile,due to the multilayered construction of PVDF@Au nanofibers,a valid SERS signal on 3D surface of bacteria could be generated.3D distribution of hotspots on multilayered PVDF@Au nanofibers gives a clear advantage for SERS sensing of organic molecules and microbial cells.

High performance sandwich structured Si thin film anodes with LiPON coating

The sandwich structured silicon thin film anodes with lithium phosphorus oxynitride （LiPON） coating are synthesized via the radio frequency magnetron sputtering method, whereas the thicknesses of both layers are in the nanometer range, i.e. between 50 and 200 nm. In this sandwich structure, the separator simultaneously functions as a flexible substrate, while the LiPON layer is regarded as a protective layer. This sandwich structure combines the advantages of flexible substrate, which can help silicon release the compressive stress, and the LiPON coating, which can provide a stable artificial solid- electrolyte interphase （SEI） film on the electrode. As a result, the silicon anodes are protected well, and the cells exhibit high reversible capacity, excellent cycling stability and good rate capability. All the results demonstrate that this sandwich structure can be a promising option for high performance Si thin film lithium ion batteries.

Artificial neural network approach for analyzing mutual coupling in a rectangular MIMO antenna

A wideband rectangular patch antenna resonat- ing at 3.5 GHz and 8 GHz frequencies is developed on a flexible substrate, which can be used for wearable applications. The proposed antenna gives a wide impe- dance bandwidth of 116%, operating from 2.SGHz to 9.5 GHz, covering most of the ultra-wideband （UWB） operating frequency range. A two-element multiple-input multiple-output （MIMO） system is developed using the proposed antenna, and the mutual coupling between the two antennas for various separations and frequencies is analyzed by using artificial neural networks （ANNs）. The neural structure is trained by using different ANN algorithms and a comparative study is made between them. It is shown that, quasi-Newton （QN） and quasi- Newton multi layer perceptron （QN-MLP） algorithms are better in terms of training, testing errors, and correlation coefficient.